CARBONACEOUS ADSORBENTS AS HETEROGENEOUS CATALYST
SUPPORTS: APPLICATIONS AND CHARACTERIZATION
TODD JAMES LAFRENZ
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
My journey through graduate school has been long and arduous,
oft inclined to take the more difficult path to the mountain top.
as I am
However, I am
also leaving a much wealthier person in terms of knowledge and fond
During this time I have learned much about myself, my friends,
human kind, and in no way the least--chemistry.
I have realized who and what
family and friends truly are, and it is to them I express my sincerest thanks
When one looks around and tries to understand the diversity
and complexity of the world in which we live, it is only through the love and
support of family and friends that we can continue to strive for a better world,
and cope with our failures and disappointments along the way.
these same people that we share our joys and accomplishments.
It is also with
There are too
many to list by name, and I do not wish to leave anyone out, but I would like to
acknowledge one person specifically who has had the greatest impact on my
This recognition goes to one of the most important women in my life--my
mother, Joan Barnett.
It has been through her constant support and by her
own example that I have been successful in what I have undertaken.
I feel very
fortunate to have the large number of wonderful friends and family that I do, so
let me just say thank you once again to everyone.
Then where would my chemistry career be without the instructors and
advisors who had their impact? It is t
how's and why's of scientific research.
hese people who have shared with me the
First there was Dr. Sandra Etheridge at
R.K. Birdwhistle at the University of West Florida, who never allowed me to
submit or accept a "bullshit" solution to any chemical problem.
there is Professor Russell Drago, my research advisor.
Through his patience
and his never-ending excitement for chemistry, he has taught me much about
how to be a chemist.
I can only hope that when I am his age, I will still have
that same intensity and interest for research and living (I also wouldn't mind
having his hookshot and sharp elbows for 2-on-2 basketball).
Finally, let me acknowledge a few of the people who made my stay in
Gainesville so enjoyable.
First I must thank the University of Florida and its
Beyond their financial support, the opportunities and
treatment I received while there could not be matched anywhere.
I must also
thank the faculty, especially my committee members, for their support and
It was through many conversations with them that my education and
I cannot forget the support, advice and friendship I
received from the Chemistry Departments support staff, especially the guys in
the electronics, glass blowing and machine shops. Individual thanks are
extended here to Vernon Cook and Rudy Strohschein. Also included in this list
are the people at Micromeritics, especially Tommy Hill, J.R. Berrett and Tony
Thorton--it was through many visits and discussions with them that I was
introduced to much of what I know of gas adsorption theory.
Lastly, I would
like to acknowledge the Rohm and Haas Company for their financial support
during a large portion of my research.
I cannot forget the daily "behind the scenes" help of our secretaries,
Maribel Lisk and Diana Williamson.
And a sincere "Thank you!" is extended to
Ruth Drago for welcoming me as a part of the ever-growing "extended" Drano
this has been the most diverse group of people, as far as interests and beliefs,
that Ive had the pleasure to work with.
I truly believe that every one of them
helped teach me something while I was at U.F. (although I still haven't figured
out what it was for a couple of individuals).
I feel especially lucky to have had
the opportunity to meet and work with the many postdoctoral researchers that
came through our lab.
Kaufman, David Singh,
People like Drs. Robert Beer, Krzysztof Jurczyk, Phil
Garth Dahlen and Doug Bums greatly aided my
research, as well as became friends.
Finally, I must thank the Florida Men's
Rugby Club, for the great times, sportsmanship and camaraderie.
It was with
this group, and the friends I found within, that some of my fondest memories
from graduate school lie.
It is not the critic who counts; not the man who points out how the
strong man stumbles
. The credit belongs to the man who is actually
in the arena, whose face is marred by dust and sweat and blood; who
. who knows the great enthusiasm, the great
devotions; who spends himself in a worthy cause; who at the best knows
in the end of triumph of high achievement, and who at the worst, if he
fails, at least fails while daring greatly, so that his place shall never be
with those cold and timid souls who know neither victory nor defeat.
TABLE OF CONTENTS
LIST OF TABLES ...............................................................................vii
LIST OF FIGURES ..........................................................................
AN INTRODUCTION TO HETEROGENEOUS CATALYSIS .................. 1
CHARACTERIZATION OF CARBONACEOUS MATERIALS .................7
Carbons Under Study ..................... ........... .......................................9
Experimental ................... .......... ........ ................... ............... ...... 1
Results and Discussion ............... ............................................. ......19
Conclusion .......................................................... ..................40
III GAS-SOLID ADSORPTION EQUILIBRIA ......................... ........ .......... 42
Introduction ......... .........................
Experim ental .................. ..................
Results and Discussion .....................
IV HETEROGENEOUS WACKER CATALYSIS ......................................66
Background ........... ......
Results and Discussion
V CONCLUSION .................. ....................................................... ....... 90
IILIIIII~IIIIIIILII LLIIIIIIIIIIIIIIII) 1I111III11II11II1)1 11)(111111)111
................., ................... .........,......., .,......,.87
B NUMERICAL ADSORPTION ISOTHERM DATA ................... ........... 1 04
C FORTRAN PROGRAM FOR SIMPLEX DATA ANALYSIS ................... 114
D SAMPLE OF SIMPLEX DATA FITTING ROUTINE FOR
LIST OF REFERENCE
S ................... ................... ................... .........,
BIOGRAPHICAI, SI(E=TCH ................... ................... ............,., ..........
LIST OF TABLES
Elemental Analyses for Carbons Under Study
BET Surface Areas and Pore Volume Data from BJH Adsorption
Pore Volume Data from BJH Desorption Curve
Degas Temperature Effects on Surface Area and Pore Volumes for
Elemental Analyses for A-POLY and A-SO4 Polymers ....
Pyrolysis Temperature Effects on Surface Area and Pore Volumes
for A -SO 4 .............. ................... ........ ........... ................................ 35
Elemental Analyses for Pyrolyzed A-SO4
Summary of Adsorbate Gases and Associated Physical Properties .......46
Gas-Solid Adsorption Equilibria Parameters for PPAN and A-57
-AHads (kcal/mol) from InKads
1/T Plots ........................56
Summary of Heterogeneous Wacker Catalyst Conditions and
Surface Area and Pore Volume Data for Wacker Catalysts ...................84
Mass Susceptibilities of Ambersorb 563 and 572 With Dopants ........86
LIST OF FIGURES
Schematic of Glass Manifold for Adsorption Isotherm
*msm..m.eemmemm..m .mmsmmm.m..ms.mmm...e..e......mm.m.......... i...l1
Exploded View of Specific Volumes Vk,
M anifold .............................................
Vm and V,
TGA Results for A-POLY
................. m ......... C **CC***CCCCCCC*Ce....... .. m. m. .a.. a. mm.m .......3
TGA Results for A-S04 .33.........................................
Pyrolysis Temperature Affects on Pore Distributions for A-SO04 ............36
Room Temperature N2 Adsorption Isotherms for Various Carbons .......38
Room Temperature CH4 Adsorption Isotherms for Various Carbons .....39
N2 Adsorption Isotherms at Various Temperatures for (a) PPAN;
(b) A-572 .......
CO Adsorption Isotherms at Various Temperatures for (a) PPAN;
(b) A-572 ........
.....m.C...C.m. ... CCCCSCCC.mmmtmsC ....... ........*...*5
CO2 Adsorption Isotherms at Various Temperatures for A-572 .............53
InKads vs. 1/T Plots for CO2 Adsorbed on
Gas Flow Apparatus for Catalyst Screening ...............
H20 Effects on CuPd-572 Catalyst and Production of CH3CHO ............83
Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy
CARBONACEOUS ADSORBENTS AS HETEROGENEOUS CATALYST
Todd James Lafrenz
Porous materials have long been the subject for chemical research in
applications as adsorbents and as heterogeneous catalyst supports.
the traditional instrumental methods for analyzing the chemical behavior and
functionality of compounds do not readily lend themselves to the analysis of
general amorphous nature.
This is due in part to carbon's opaqueness and
Methods do exist to help probe these materials;
the most common and widely used of these is the determination and
interpretation of gas adsorption isotherms
interpret standard adsorption isotherms.
Several theories exist to help
From these analyses, surface areas
and porosity characteristics can be calculated.
Nine commercially available carbonaceous adsorbents were analyzed
using a variety of physical methods in order to further characterize these types
'rn -nZ, nl 4 nr nl r + arnl nltnhr-oal ll annlxro-So fth rrno1 he hcvuinvr card
A greater understanding of the physical properties that affect the
adsorption behavior of these types of materials was gained.
A detailed and critical analysis of the application and interpretation of
the standard BET equation was undertaken.
complementary to the existing theories was proposed.
From this analysis, a model
Derived from a multiple-
site Langmuir process, this proposed equation and its solution allow for a more
direct comparison of various adsorbents with a variety of gaseous probes.
Values for both the capacities and adsorption equilibrium constants for three
distinct adsorption processes were derived using this model.
of adsorption isotherms at a variety of temperatures allowed for the calculation
of the thermodynamic parameters associated with these adsorption processes.
The understanding gained from the analysis of these carbonaceous
adsorbents has assisted in the characterization of a heterogeneous Wacker-
type oxidation catalyst.
Salts of Pd(II) and Cu(II) were pore-filled onto a porous
and an active heterogeneous
gas flow reactor system
for the oxidation of ethylene to acetaldehyde was developed.
of the porous carbon as a support was found to provide synergistic benefits for
this type of catalysis, arising from the oxidative capability of the carbon itself
and its high adsorption capacity and affinity for various substrates.
AN INTRODUCTION TO HETEROGENEOUS CATALYSIS
From the days of the alchemists,
ideas of catalysis permeated scientific
thinking and research.
The alchemists' greatest endeavor was the
" a material which would be able to transform base
metals into gold and promote good health and long life.
Its conception was
catalytic in nature, as it was considered that one needed only a small amount
of this material to effect large changes.
Ever since these ancient times, the
roles and understanding of catalysis in chemical research and industry, as well
as the tremendous biological importance of catalysis, have continued to be
A greater understanding has been gained, but the concept of the
philosopher's stone has changed many times since.
The traditional definition of a catalyst is a compound that facilitates the
attainment of equilibrium in any chemical process much more rapidly than in
Most often this is accomplished by the lowering of the activation
energy for a reaction.
The amount of catalyst required to do this
is usually very
small in comparison to the molar amounts of reagents and products present, so
the catalytically active species itself is used over and over again.
cannot affect the position of the thermodynamic equilibrium; however, it may
change the mechanism of a reaction significantly.
Due to the inherent
reactivity of many catalyst systems, one must be careful in investigating new
T, -- +. -- i.^il: n* n1* niT n 1-. a a +11 Al e -a 4 n n stirl n^ n+ Itl n1t nt- I an s-r: 11 ^
reactivity problem in catalysis exemplifies that, in addition to present efforts in
tailoring the reactivity of systems for specific applications, there already exists
a large amount of research in the catalyst systems presently being used.
There are numerous examples in which catalytic processes have affected
the consumer market.
Included in these are hydrocarbon cracking and
reformulation catalysts used in petroleum refining, polymerization catalysts in
plastic production, hydrogenation of fats and edible oils for food-stuffs,
catalytic converters in automobiles, industrial catalysts for NH3 and SO3
and the medicinal value of understanding enzymatic catalysis
occurring in both plant and animal tissues.
Of the many types
processes that exist, all fall under the general classification of either
homogeneous or heterogeneous catalysis.
Homogeneous catalyst systems are
characterized by the existence of reactants, products and catalyst all in the
Heterogeneous catalyst systems exist in two or more phases.
most common examples of heterogeneous systems are gas phase reactions
occurring over the surface of a solid catalyst material.
Other types include two-
phase liquid mixtures (i.e., organic and aqueous phases), and liquid-solid batch
Heightened interest has been given to the heterogenizing of typical
homogeneous catalyst systems since the early 1970's.
1 Advantages of
employing heterogeneous catalysts include ease of separation of products from
the catalyst, recovery of used catalyst, and the synergistic benefits in cases
where supports are used.
The immobilization of a catalyst also inhibits its
incorporation into the product composition.
Synergistic benefits may include
r r r 'r' 1 1 1 1 1 1
acid sites (e.g., silicas, heteropolyacids), and concentration effects due to
adsorbent-adsorbate interactions and capillary condensation (e.g.,
It has also been observed that the electro-chemical
behavior of metal species is affected by doping the metal on solid supports,
which may have advantages in sensors and electrocatalytic processes.2,3,4
There exist some inherent disadvantages to employing heterogeneous
In a homogenous environment almost every active catalyst
species is capable of coming in contact with the reactant and, therefore, likely
to participate in the reaction process.
In a heterogeneous system the only
catalytic sites available are those on the surface of the solid or at the interface
between two layers.
In a heterogeneous system, therefore, it is most desirable
to suspend the catalyst in a thin film, as in a membrane type system, or
disperse the active catalyst species on a high surface area material.
this, the chemical behavior of the support must now be considered.
chemical behavior would ideally be a synergistic benefit by design, but this is
not always the case.
When employing a solid in a catalytic process, one must
also consider its mechanical strength.
This may be a solid support, as when
employing zeolites, silicas or carbons, or a solid constructed of the catalytic
species itself, as in the case of various metal oxides or nickel/platinum
There are obvious concerns if the solid decomposes
mechanically or chemically under the desired reaction conditions.
Despite these apparent disadvantages, heterogeneous catalyst systems
have proven to be very useful and applicable to many commercialized
A large amount of research continues in all areas of heterogeneous
gas phase chemistry.
Analyses of the chemical structures of carbonaceous
solids and macroreticular polymers are further complicated by their
opaqueness, and their highly cross-linked network structures which render
them insoluble in most traditional solvent systems.
Methods do exist that help
characterize solid materials, including solid state nuclear magnetic resonance
(NMR), X-ray powder diffraction, X-ray photoelectron spectroscopy (XPS), magic
angle spinning NMR, diffuse reflectance infra-red fourier transform (DRIFT),
photoacoustic infra-red spectroscopy, electron paramagnetic resonance (EPR),
low energy electron diffraction spectroscopy (LEEDS), high resolution electron
energy loss spectroscopy (HREELS), scanning electron microscopy (SEM),
scanning tunneling microscopy (STM)
atomic force microscopy (AFM), and
extended X-ray absorption fine structure (EXAFS).
However, many of these
techniques only provide qualitative information, or are limited to probing the
surface or only a few molecular diameters beyond the surface.
introduction of '29Xe-NMR experiments6,7 and the combination of calorimetric
and spectroscopic data8S9 on solids have been helpful in elucidating internal
structural and surface information
, but these techniques are also limited in
It remains difficult to characterize and understand the
intricate chemistry that occurs within the internal porosity of solids employed
The ease with which gaseous or liquid reactant molecules can achieve
access to the internal surfaces and pores of a solid is often of crucial
importance in the development of a catalyst suitable for industrial applications.
If one is to be able to understand, and then further predict catalytic behavior, a
- -- .5 5 4 C .4<* i I
isotherms continues to be an elucidating method to probe the internal porosity
of these materials.
Various theories have been developed to help interpret gas
adsorption isotherms to yield surface area and pore size information of porous
It is well accepted in the literature, however, that many of these
interpretations are derived while making several assumptions, some of which
are considered to not be truly representative of the surface being probed.
qualitative discussions and direct comparisons of data under the same
conditions are considered valid.
Carbon containing polymeric adsorbents and their derivatives are
quickly expanding as the newest areas of interest for heterogeneous catalysis.
However, because most carbonaceous materials are opaque and amorphous,
they do not lend themselves readily to many of the characterization methods
Because of growing interest in the application and
characterization of carbonaceous solids as adsorbents and in catalytic
applications, it is of great interest to further quantify their chemical and
Therefore, an understanding of the interactions involved in
the physical and chemical adsorption of gases by solids is of fundamental
importance to many of the applications of these types of materials.
In this series of studies, numerous high surface area carbonaceous
adsorbents have been characterized.
Comparisons of their surface areas and
pore distributions, as well as their physical composition, have been made.
model is presented to help interpret gas adsorption isotherm data in attempts
of obtaining more quantitative measurements for comparing these types of
materials for adsorption and catalytic applications.
One of the goals of this
-,,,, I,. L -- a,.,,:, -t, lr- C .. ---- .41. .-...a .-
to assist in the choice of an appropriate solid for whatever application is
Finally, the understanding gained in characterizing carbonaceous solids
has been applied in the development of a viable, heterogeneous catalyst
This system involves a heterogeneous Wacker process, specifically
looking at the oxidation of ethylene to acetaldehyde.
Many factors have been
found to affect the activity of these types of catalysts, including the surface and
pore characteristics of the support, dopant levels, temperature and flow rates of
gaseous reactants, and the components of the active catalyst species itself.
of these parameters were considered in detail in the development of this
CHARACTERIZATION OF CARBONACEOUS MATERIALS
There exist a large variety of carbonaceous materials derived from both
natural and synthetic polymeric materials.
Natural sources include wood fiber,
coconut husks, peat moss, petroleum pitch and coal tar residues.
precursors include polymers of styrene, acrylonitrile, vinyl and furfuryl
alcohols, and phenolic resins.
Applications of the carbonized derivatives of
these materials include adsorbents for purification of gaseous and liquid
streams, pressure-swing applications for gas separations, chromatographic
stationary phases, heterogeneous catalyst supports, prosthetic devices in
medicine, and composite and electrode materials.12
Pyrolysis of many of these polymers produces carbons that can be
tailored for specific physical characteristics.
These characteristics are
determined by the original polymer structure, as well as the pyrolysis
conditions and pretreatments.
Typical carbonization conditions involve heating
the materials under an inert atmosphere (e.g.,
3000C to 30000C.
N2, Ar) from temperatures of
The porosity and extent to which these materials graphitise
(at temperatures >20000C) is largely dependent on the properties of the original
polymer and the carbonization conditions.13
Elimination of various volatile by-
products during pyrolysis produces additional porosity in the remaining solid.14
Various treatments of these polymers are employed prior to pyrolysis to
increase their thermal stability, therefore increasing carbon yields.
naturally occurring polymers, this may include chemical treatments with
agents such as ZnCla, carbonates, alkali, sulfates, and sulfuric or phosphoric
This results in dehydration and further cross-linking of the polymers,
which produce higher carbon yields unaccompanied by tarry residues from
Synthetic polymers are made as macroreticular polymers to impart
high thermal stability.
Macroreticular polymers are characterized as being
highly cross-linked, exhibiting little or no swelling upon wetting with a solvent,
and have an open, rigid macroporous structure.
Advantages of synthetic
polymer precursors include greater control over the homogeneity of the starting
, and greater reproducibility from batch to batch.
may also be imbibed or treated with chemical agents as in the case of natural
The resultant benefits of a highly cross-linked polymer system, as
well as the carbon-fixing agents previously mentioned, are to allow the carbon
to char without fusing, therefore retaining its original pore structure.
process referred to as activation may also be employed during or after pyrolysis
to impart various functionalities to the surface or increase porosity.
includes treatment with gases such as steam, air, NH3,
HC1, C12, or CO2, at
temperatures that typically range from 3000C to 10000C.
The resultant materials are generally amorphous in nature and best
described as consisting of twisted networks of defective carbon layers bridged
together with other similar layers.
Although they do not possess uniform pore
sizes as in the case of zeolites, they are found to contain unique pore shapes
a~~~~~~~I 5 0 -. W-5
structure and surface functionality.17
With this capability, carbon has
advantages as an adsorbent or support over other traditional materials like
zeolites or silica.
Advantages may include the absence of strong Lewis acid
sites as seen with aluminum in zeolites.
Additional advantages may include
solution and mechanical stability, carbon's general stability in many corrosive
environments, and high adsorption capacities for various organic substrates.
The very nature of carbon will lead to supported catalyst systems that behave
differently than other types of supports currently used.
Carbons Under Study
carbonaceous adsorbents, made by the Rohm and Haas
Company, are among the materials that have been studied.
macroreticular, sulfonated polystyrene, the pyrolyzed product has shown
interesting behavior as both an adsorbent and catalytic support.
polymer is made through a suspension polymerization method, with
divinylbenzene as a cross-linking agent.
The resultant polymer is in a bead
form, which is retained during pyrolysis and mechanically very strong.
types of Ambersorb carbons were examined Ambersorb 563, lot# 91/3298
(A-563) and Ambersorb 572, lot# 2125 (A-572). Together with differences in
pore distribution and surface area, A-563 is reported to be more hydrophobic
More information on the physical properties, adsorption
characteristics and syntheses of these materials can be found in the text by
Neely and Isacoff,is and in the patent literature.16'19
Pyrolyzed polyacrylonitrile (PPAN) was also investigated.
This material, a
- -r .---
the past several years because of its unique mechanical strength, catalytic
behavior and electronic properties.20,21,22
However, there is a concern in
developing any large scale synthesis because of HCN production during
Two different carbons were obtained from the Calgon Carbon
granular carbons are derived from bituminous coal.
Their production involves a steam activation treatment, resulting in a high
surface area, highly porous material, suitable for vapor phase adsorption and
as a heterogeneous catalyst support.
For example, these carbons are used in
the acetylene process for the production of vinyl chloride and vinyl acetate.
Filtrasorb 300, also made from coal, has been tailored to be an active
adsorbent in water purification.
Calgon BPL 4x10 (BPL) and Filtrasorb 300
8x30 (F-300) were the specific materials examined in these experiments.
A powdered carbon, AX21, was obtained from the Anderson
This material is made from a patented process
involving direct chemical activation of petroleum coke with large amounts of
KOH at elevated temperatures.23
Further details on the preparation and
properties of this material are reported elsewhere.24,25
AX21 has also been
previously studied in this research group as a catalytic support.26
Another powdered material examined was NORITTM 211 from Kodak (N-
Labeled as a decolorizing carbon, it is derived from pyrolyzed peat moss.
This is further treated with a steam activation step to increase porosity,
followed by an acid wash to leach out impurities.
was Norit N.V
The original manufacturer
., and the NORITTM line is now being distributed by Fisher
spherical bead like the Ambersorb
and PPAN materials, XUS-43493.01 (DOW
493) is designed for the concentration of organic from both air and water.
reported to possess a unique pore structure and high surface area, with good
physical strength and a hydrophobic surface.
These materials are reported as
having very little catalytic activity themselves due to extremely low ash content.
This is in contrast to some activated carbons which contain residual metal
salts from their processing or original sources.
An advantage of low ash
content would be the minimization of adsorbate decomposition, which is not
desirable in some applications.
The final carbon considered in these experiments was Kureha BAC-SP.
This is a spherical, activated carbon made by Kureha Chemical Industry Co.,
Ltd. in Japan.
These "bead activated carbons" are spherical and reported as
having a fairly uniform diameter of 0.7 mm.
They are reported to have a low
ash content and very high resistance to attrition on repeated usage.
The nine carbonaceous solids described above were chosen in order to
examine a broad range of carbon materials.
Most are commercially available,
and are derived from a variety of both natural and synthetic precursors.
wide range of composition, pore size distributions, surface areas, and
functionalities made these ideal materials for the focus of these investigations.
Characterization of Supports
All carbons were used as received without further purification or
- S t -r -r -
gases employed in adsorption studies were purchased from the Matheson Gas
Company with a minimum 99.99% purity, and were used without any further
Surface area and pore information were obtained on a
ASAP 2000 instrument.
Elemental analyses (carbon, hydrogen
and nitrogen) were performed by the University of Florida elemental analysis
Surface areas were determined using a five point Brunauer-Emmett-
Teller (BET) type analysis27 of the N2 isotherm obtained at 77K.
For this entire
range of materials, the best fit line for the BET transformation plot was found
to be at 0.01< P/Po
< 0.07 (see Results and Discussion for further details).
Micropore volumes were calculated using the Harkins-Jura (HJ) model28 with a
t-plot statistical analysis.29 The statistical thickness parameters used for the
best fit line were from 5.5-9.0A. An algorithm employing the Barret-Joyner-
Halenda30 (BJH) model was used to determine mesopore and macropore
volumes from the adsorption portion of the isotherm.
All the above
calculations were carried out using the software supplied with the
ASAP 2000 instrument, and further details can be obtained
from the instrument manual.31
The interpretation of pore volumes follows the
IUPAC recommended definitions with micropore diameters smaller than 2 nm,
mesopores between 2 and 50 nm, and macropores having pore diameters larger
than 50 nm.
Thermal gravimetric analyses (TGA) were obtained on a T.A.
Instruments, Inc., Thermal Analyst 2000 system under flowing nitrogen.
instrument was equipped with a Hi-Res TGA 2950 module, differential
ramping rate dependent on the rate of weight loss.
The maximum temperature
ramping rate was set at 100C/min, with a resolution setting of 5.00.
enhances the resolution of weight changes, while minimizing run time and
avoiding transition temperature overshoot.
Gas Adsorption Studies
Gas adsorption isotherms were obtained on the Micromeritics
2000 Chemi system, or on a glass manifold line (Figure II-1).
system consisted of a glass chamber of known volume; threaded Teflon high
vacuum stopcocks (Ace Glass, 8190 series); a Welch vacuum pump and
mercury diffusion pump; two pressure heads from MKS Baritron, type 390HA
(one capable of measurements from 1-1000 torr, the other from 10-5-1 torr);
MKS Baritron type 274 channel selector, 270B signal conditioning unit, and a
390A sensor head; and various condensers and connecting tubing.
itself was not thermostatted, but temperatures were found not to vary more
than +1C over the course of a run.
The procedure to obtain isotherm data on the glass vacuum line, known
as the successive addition technique, is a static, gas volumetric technique.
employs a known volume and changes in pressure to determine the amount of
Initially, after the sample tube has been connected to the
vacuum line via an o-ring seal, the entire system is evacuated and the sample
After closing the valve to the sample, the manifold (Vm) and known
volume (Vk) are filled with helium, a gas that is assumed to be non-adsorbing
(see Figure II-2 for further detail of specific volumes).
The valve to Vk is closed,
* I i II l i Ii llll l l li I Ii
lLI Li ll l l lii i II II i i I
L i ii i i Ii i i ii ii ii i tiil
allowing it to equilibrate with Vm yields P2, the equilibrium gas pressure.
Assuming ideal gas behavior,
Vm was then determined by the following
where ngas is the total number of moles of gas in the system, R the ideal gas
, and T the temperature (in Kelvin).
Since R, T and ngas stay constant,
when Vk is opened up to Vm:
which rearranges to
Vm = Vk --
This series of steps can now be repeated, where P2 becomes Pi, the valve closed
and Vm again evacuated, and then reopening Vk to Vm gives a new value for P2.
This sequence is repeated a total of five times, with progressively lower gas
pressures, to give a determination of Vm over the entire range of experimental
pressures, as well as a measure of the accuracy and reproducibility of these
Next, it is necessary to determine the free space volume (Vs), which
encompasses the volume above the sample in the sample tube and the volume
between and within sample particles (sometimes referred to as void volume).
This procedure, continued from above, is done in a similar fashion as in
determining Vm, however, this time Vm is used as the known volume to find Vs.
An initial pressure exists in the manifold (P'2) and in the sample volume (P'i).
Obviously, for the first data series P'1
pressure is obtained
Vm is then opened to Vs, and a new
The initial moles of gas present can be expressed as
-~~n l 7 jlPr..'
since n3 = ns+nm:
P'a(Vs + Vm)
which rearranges to
This series of measurements is then repeated over the entire experimental
pressure range (0-760 torr), as was done in the determination of Vm,
After closing the valve connecting Vm and V
dosed to a higher pressure yielding a new value for P'2.
the manifold is
P'3 becomes the P'1 value for this data series, and a new value for P'3 is
obtained when Vm and Vs are allowed to equilibrate.
obtained for V
As before, the values
s over this entire pressure range are averaged, giving an estimate
of the precision of these measurements.
strong adsorption of helium by the samp]
A large deviation may be indicative of
le. This was not observed for any of
the samples investigated.
With Vm and Vs now known, gas adsorption isotherms can be obtained
using the particular adsorbate of interest. Data were obtained in a similar
fashion as to that in the determination of Vs. After the manifold, known volume
and sample volume were evacuated, the sample valve was closed and Vm again
determined from Vk (Eqns. II-1 through II-3 and procedure outlined therein).
This was repeated to help minimize any errors due to non-ideal behavior of the
adsorbate gas, as well as for verification of prior results.
Any large deviation
from the value of Vm determined using helium would be indicative of the probe
gas not behaving ideally, or it strongly adsorbing to the walls and seals of the
This was not observed with any of the gases used in these
After determination of Vm with the adsorbate gas, pressures were
obtained in a fashion similar to that used for determination of Vs with helium.
The initial pressure contained in the manifold (P"2), which was filled to specific
equilibrium pressure measurement, P"3.
The number of moles of gas adsorbed
by the solid (nads) was found from the difference in the number of moles present
initially in the gas phase (nini) and that present after equilibrium was allowed to
be established (nequil):
nini = -- + --
P' '3(Vm + Vs)
Substituting equations II-10 and II-11 into II-9 gives
(P' '2Vm + P' 'iVs)
- P' '3(Vm + Vs)
This value was then divided by the mass of solid sample to give an amount
adsorbed per gram of adsorbent.
Pressure readings were recorded in torr,
yield pressures in units of atmospheres (atm).
but were all divided by 760 to
Equilibrium was considered to
be established when the pressure reading did not change 2 units over a period
of 30 seconds (values were recorded to four significant figures).
The Micromeritics ASAP 2000 Chemi system performs its analyses and
calculations in a similar fashion. Further details on the free space
determination, adsorbed gas volumes and equilibration intervals can be found
in the ASAP 2000 Chemi reference manual.34
Equilibrium was considered
attained for runs on this instrument when the pressure change per ten second
time interval was lea.a
than 0 1/% nf the average nreasire d urinu that time
and 1 atm, as most interest in these materials involves applications under
standard atmospheric pressures.
Results and Discussion
Because of the variety of sources and precursors for the various carbons
under study, an initial determination of their physical composition was
Since all of these materials have been pyrolyzed under relatively
high temperatures, it can be assumed that their chemical composition is
mainly carbon, with small amounts of hydrogen and oxygen, as well as
nitrogen if it was present in the original polymeric precursor as in the case of
Mechanistic details on the decomposition under pyrolysis conditions for
polymeric precursors of these materials are well characterized and have been
Other chemical components are assumed to have been present
in the original material as impurities, or incorporated during their preparation.
All characterizations and isotherms were obtained on the same batch and lot
number of a specific material.
Elemental analyses were done on all carbons as received, without
further washing or treatment.
The rationale was that this would most likely be
the way these materials are used in the development of large scale production
for a commercial catalyst or adsorbent system.
exhibited less than 5% weight loss upon drying.
The majority of the carbons
Exception to this was
observed for N-211
,PPAN, and AX21.
However, CHN analyses were obtained
Elemental Analyses for Carbons Under Study
a Dried under standard conditions for gas adsorption experiments,
involves degassing at 200C under a vacuum of <10-3 torr for a minimum of 8
the weight loss upon drying accounts for the majority of the remaining mass
Because of this, it was assumed that most of this weight loss occurs
simply due to loss of adsorbed water
and, to a much lesser extent, adsorbed
Any residue left after CHN analysis was attributed to metal
salt impurities, as alluded to earlier, or other functionalities existing due to
activation treatments. These may include groups containing oxygen, sulfur,
phosphorous or chlorine. It is well accepted in the literature that these types of
functionalities can exist on carbon surfaces,12 but specific analyses for other
functional groupings or metal salts were not done on these materials.
Surface Area and Pore Volumes
In employing highly porous solids as adsorbents and catalyst supports,
an accurate determination of the physical structure of the surface, internal
porosity and interconnecting channels is desirable.
One of the standard
techniques used for the determination of this information employs the N2
adsorption and desorption isotherms obtained at 77K.
Nitrogen is most often
chosen as the adsorbate in these measurements because its small molecular
size allows it access to small pores.
Furthermore, it is easily handled,
economical, approximately spherical, and inert, hence not likely to participate
Other gases can be used,
the most common of which are
krypton or argon.
Various theories exist to interpret isotherm data leading to the
determination of surface areas, pore sizes, and pore volume distributions.
Determination of surface area turns out to be very complex in the absolute
Factors that may affect the determination of surface areas
from standard methods that contribute to the inaccuracies can include
irregular surface features, the fact that most surfaces of interest are
energetically heterogeneous, resolution of the method,
molecule being used.39
and the size of the probe
Although certain assumptions have to be made in the
interpretation of gas isotherm data, the relative ease of data acquisition and the
non-destructive nature of the technique give N2 adsorption a large advantage
over other methods such as mercury porosimetry.
The BET equation (Eqn. II-13) was employed in determining surface
areas from gas adsorption data obtained on these carbons, employing N2 gas as
C nm C m
The relative pressure, X, is P/Po, where P is the measured equilibrium pressure
(in torr) and Po is the adsorbate saturation pressure at the given temperature.
The value nads is the molar amount adsorbed at the relative pressure
values of nm, the monolayer capacity, and C, a constant dependent on isotherm
shape, are both determined from the best fit linear plot of
determination of nm and C
the surface area can then be calculated from
where NA is Avagadro's number
and a is the cross-sectional area
of the probe molecule.
For N2, a value of a
= 0.162 nm2 was used.
This is yet
one more point of debate in the usage of gas adsorption data for the
determination of surface areas, and several publications have discussed how
the cross-sectional areas of adsorbed gases vary dependent on both adsorbent
The appropriate conversion factors are combined with
Equation 11-14, and the final value divided by the mass of solid in the
experiment yielding a value for the BET surface area (S.A.BET) in units of m2/g.
The multi-point methods used in these studies for surface area
determination are derived strictly from equations II-13 and 11-14.
multi-point method, analysis of the BET equation (Eqn.
plotting T versus P/Po yields a straight line, where Tis
where nads is the number of moles of gas adsorbed per gram of solid at the
mnaen1nred relative nreR'alir.
Thep lep t-snnrlrpe fit analvsis nf the line nassing
Simultaneous solution of the two equations gives nm and C, where nm is then
used to calculate the S.A.BET.42
The Micromeritics software employs this
relationship to find surface areas, employing the volume of gas adsorbed
instead of the number of moles.31
the molar gas volume.
These two quantities are related simply by
Therefore, in the case of the isotherms measured on
this instrument, Equation II-15 becomes
and the S.A.BET (in units of m2/g) is calculated from the slope and intercept of
this line by
Data points from the adsorption isotherm chosen for the determination of the
S.A.BET are typically chosen between relative pressures of 0.05 and 0.30.
this region it is found that BET theory predicts adsorption values below
Micropore filling arising from high adsorption potentials
is the major cause of this inaccuracy.
0.30, multilayer adsorption
occurs, and BET theory is found to predict an adsorption value that is too high.
In this research, five points were chosen from the region of the isotherm
that yielded the best fit line in the BET transformation plot (Eqn. 11-18).
Although the chosen data region, 0.01
for these carbons occurred
near and below the lower limit for BET analysis previously mentioned, these
limits are not exact and do vary from solid to solid. The presence of large
micropore volumes also affects the choice of this data set. In these analyses,
was found to be greater than 0.99999.
As previously mentioned, the
application of BET theory is based on several assumptions, and surface area
determination is very difficult in the absolute sense.
Therefore, for these
materials the choice of the best fit line and the use of the same data region for
each solid was assumed to be the most appropriate for making relative
The experimental data set used in applying BET and the pore-
volume theories (BJH and HJ) should always be kept in mind when making
quantitative comparisons based on these types of reported values.
BET Surface Areas and Pore Volume Data from BJH Adsorption
Pore Volumes (cc/g)
The data presented in Table II-2 shows the determined surface areas and
pore volume distributions for the carbons analyzed.
As was mentioned earlier,
pore volume distribution curves in Appendix A is made here, as well as the
data presented in Table 11-2.
From this it can be seen that a variety of
carbonaceous materials with a large diversity of relative surface areas and pore
volumes have been considered.
The initial steep slope of the experimental isotherms (see Appendix A) is
attributed to a large enhancement to the energy of adsorption due to the
presence of pores with diameters of only a few molecular diameters (with
reference to the probe molecule), and narrow openings to larger pores in the
internal volume of the porous solid.
This large interaction potential arises from
the close proximity of opposing walls, resulting in a large volume of the
adsorbed gas being concentrated in the micropore volume at pressures below
its normal saturation pressure, to the point of condensation of the adsorbate.
This large potential, which might be accounted for by the occurrence of
chemisorption, was ruled out in the case of these carbons as all of the N2
isotherms were found to be completely reversible.
Following this steep slope in the adsorption curve attributed to
micropore filling, the isotherm levels off.
The knee in the curve in this region
occurs near the monolayer coverage, or filled micropores in the case of
In this leveled region of the isotherm, monolayer and
multilayer formation occurs, which is followed by another steep increase in the
This second large increase in adsorption is attributed to mesopore filling
due to capillary condensation.
The final leveling off of the adsorption curve
represents attainment of the adsorption capacity of the porous solid.
multilayers thus formed more closely resemble the bulk liquid than the
packing of the molecules.43 For these reasons, the usage of BET t
surface area determination is not considered an absolute method.
samples run under identical conditions, however, relative comparisons are
It is on this assumption that direct comparisons were made
for the various carbons and the data presented in Table 11-2.
The overall shape of the N2 isotherms (Appendix A) can be classified as
Type II or Type IV according to the BDDT classification scheme.44
this can be seen in the case of Kureha, BPL and AX21, which more closely
resemble the Type I isotherm.
Analysis of the data in Table II-2 and the pore
distribution curves in Appendix A shows that these three solids are almost
These classifications are indicative of porous solids, and
interpretation of the various regions of these isotherms follows the description
in the previous
In general, reversible isotherms exhibiting a steep
increase in adsorbed volumes at low relative pressures are indicative of highly
porous materials and thus, the presence of micropores.37S38
This steep increase
will yield a greatly enhanced value for the BET C value (> 300), and a surface
area that may be erroneously high.
The difference between these three isotherm types is that Type I
isotherms are generally observed for solids that are mainly microporous.
steep increase is observed in the initial low pressure region of the isotherm,
with a leveling off over the remainder of the pressure range.
The shape of the
Type II and IV isotherms are very similar, where Type IV is generally seen to
level off near P/Po =
1.0 and a hysteresis often observed, yielding further insight
into the pore size distributions and shapes.
More detail on the meaning and
5 A' t 5 4 *r 4 4 it A1 flf inr 'n f
curvature on the condensed surface, and the lower the vapor pressure of the
Therefore, upon desorption of an adsorbed gas, a lower
pressure must be reached before the adsorbate desorbs and a hysteresis loop is
A similar but opposite effect would arise from narrow entrances
to larger pores (ink-bottle pores).
The restricted opening, and hence the small
diameter and greater curvature of the surface of the condensed phase at the
pore opening, would make it necessary for a greater pressure to be reached
before the pore would fill.
The rapid filling of this pore volume would result in
an erroneously high value for the volume of the smaller pore size, which was
derived from the pore entrance diameter.
This would also contribute to the
occurrence of a hysteresis in the adsorption/desorption isotherm.
The relationship between the vapor pressure and the condensed film
surface radius, in addition to the thickness of the adsorbed layers, provides the
basis of BJH theory.30o
Pore volume distribution plots of the BJH desorption
data can be found in Appendix A.
The y-axis in these plots, dV/dlog(D),
generated from the following equation:
where Vp, is the adsorbed gas volume (cm3/g @ STP) at data point I,
and Dp+i are the pore diameter range over which the BJH calculation was
done. Graphed in this fashion, the change in pore volume per change in
magnitude (change per decade
of pore diameter is represented.
From this graphical data, it was observed that the majority of the
carbons analyzed generally have a bi-modal distribution of pore size ranges,
-t r.. ..~ a_~
n_ t.~_r_.. __ n r~n
In the case of Kureha, almost all of its pore volume is contained
in the micropore region (< 20A), with a small peak at -40A.
The BJH pore
volume distribution plots compliment the data reported in Table 11-2, and show
these materials have a limited
, well defined pore structure.
can be used to further help in determining which adsorbent would be the most
appropriate choice for a specific application.
In general, it is proposed that the BJH desorption data is more
representative of the surface than the adsorption data.
However, both data
sets are subject to error from effects of narrow pore openings into larger pores
and failures in the assumptions made by the model.
The BJH model assumes
a cylindrical pore shape, which obviously cannot be rigidly correct as the
majority of these pores consist of irregular spacings between carbon particles.
Consequently, the reported pore sizes and volumes cannot be taken as
absolute, but may be interpreted as a statistical average of irregular pore
It is therefore more important in making comparisons of a variety of
solids to be consistent in data manipulation, and for these experiments the
BJH adsorption curve was used to calculate mesopore and macropore volumes.
For point of interest, the mesopore and macropore volumes determined from
the BJH desorption curve are reported in Table 11-3.
later discussion the values reported in Table 1I-2, bE
However, for purposes of
ised solely on the BJH
adsorption curve, will be the only values used.
The BET C constant is related to the increased effect on the heat of
adsorption for these systems.
For the determination of a reliable value of nm,
the monolayer coverage, it is beneficial to have a sharp knee in the initial
Pore Volume Data from BJH Desorption Curve
Pore Volumes (cc/.)
spherically symmetrical and tend not to localize on the surface.
localization demands a low value for C, as a large value is indicative of strong
interactions between the adsorbate and adsorbent.
The stronger the
interaction between the two, the more an effect the adsorbent lattice has on the
orientation and geometry of the adsorbed molecule.45
The BET C value has
also been related to the enthalpy of interaction by the following equation:46
= (AHads- AHi)/RT
where AHads is the adsorption energy and AHi the heat of liquefaction of the
However, it is generally accepted that, although the value of C may
give an indication of the magnitude of the adsorbent-adsorbate interaction and
the isotherm shape, this value is not usually directly translatable to adsorption
Because of this, degassing at elevated temperatures under vacuum before
isotherms are obtained is necessary.
The data in Table II-4 illustrates the effect
of degas temperatures on surface area and pore volume for PPAN.
It has been
reported that for degassing of microporous, carbonaceous materials
temperatures of at least 3000C should be employed.
analyses included some of the polymeric precursors to the carbons reported
that were known to be thermally unstable at these higher temperatures (see
Figures 11-3 and 1I-4).
For this reason, a lower temperature of 2000C was
chosen for these gas isotherm studies.
From the data presented in the
following table, it was observed that increasing the degas temperature had its
greatest effect on increasing the surface area and micropore volume.
Degas Temperature Effects on Surface Area and Pore Volumes
TGA, Pyrolysis and Pore Volumes
Samples were obtained, courtesy of the Rohm and Haas Company, that
were similar in chemical composition to the precursor materials for the
syntheses of Ambersorb
adsorbents, for the purpose of studying the thermal,
surface area, and pore volume characteristics of the polymeric materials used
Pore Volumes Ice
considered representative of this class of compounds.
These analyses were
found to yield interesting information about the effects of functionality and
thermal treatments on the resultant pyrolyzed solids.
Two solids were analyzed with a variety of elemental analyses, surface
area and pore volume studies, and thermal gravimetric analyses (TGA).
these materials, referred to as A-POLY, was a macroreticular copolymer of
styrene and divinylbenzene, with no additional functionality.
SO4, was a similar polysulfonated copolymer.
The other, A-
Both of these materials were in
bead form, made through suspension polymerization, and were used as
received without any
Elemental Analyses for A-POLY and A-SO4
loss upon drying
a The CHN results for the pyrolyzed samples were obtained from the chars
resulting from the TGA analyses (Figs. II1-3 and II-4) on the original polymeric
The two polymeric materials were both subjected to pyrolysis conditions,
and weight loss as a function of temperature was analyzed employing TGA.
Note that the major difference between these two materials is that A-SO4 has
been sulfonated, where A-POLY has not.
A... n) + l;n~:n
* 1 ** l***III UIli ~li ~ ll
It was observed that without
thiQ tvne rfnrrlU-rrn.r .qvten, rPedilv dea-rlde.s nt ternnern tliire
C (D CUl
0 0 0 0
CD(0 C "U
sulfonation of these polymers greatly increases their thermal stability, where
-45% of the original mass still remains at 6000C (Figure II-4).
a 94.2% carbon yield.
Elemental analyses for these two polymers and their
resulting chars are presented in Table 11-5.
One can infer from this information
that approximately 44% of the mass of A-SO04 was due to the presence of the
The data from the last column in Table II-5 shows that
functionalization of this polymer greatly enhances its tendency to adsorb
The initial weight loss in Figure II-4 between room
temperature and 2000C is attributed to the loss of this moisture, and this value
agrees well with the observed weight loss reported in the table.
phase of weight loss, observed above
2000C, is reported to be due to the loss of
the sulfonate functional groups,36 and this is in agreement with the results
observed in the TGA.
At 6000C, approximately 55% of the original mass has
volatilized, which is in agreement with the loss of sulfonate functionality and
adsorbed moisture inferred from the CHN data.
To observe how pyrolysis temperatures affect pore distributions and
surface areas, a series of experiments was performed with A-S04 varying the
maximum temperature by increments of 500C up to 6000C.
These runs were
done by placing -1.5g of A-SO4 in a porcelain boat, placing this inside a quartz
tube in a tube furnace
, and performing the pyrolysis under flowing N2 gas.
tube furnace (Hevi-Duti Electric Company, type M-3012) was controlled with
an Omega CN-2042 programmable temperature controller.
purged with nitrogen before elevating the temperature, anm
All samples were
I the ramp rate was
constant for all, at
The sample was held at the maximum
F (r r r 11 lr 11 1 1 11 _r
However, the degas temperature for all these samples was lowered
due to the thermal instability of A-POLY and A-SO4.
For comparison purposes
this temperature was kept constant for all samples at 1100C.
Pyrolysis Temperature Effects on Surface Area and Pore Volumes
a Degas temperature changed from normal for these samples.
kept at 1100C under
<10-3 torr vacuum for a minimum of 8 hours.
b Unpyrolyzed polymers as received.
Table II-6 shows the changes in surface areas and pore features from the
original polymeric materials as a function of the pyrolysis temperature.
readily be seen how increases in temperature increased all of these parameters
up to about 5500C.
After this point, increases were no longer observed,
for the BET C value
, and changes occurring above this temperature were much
definite trend of rising %C and falling %H was seen as pyrolysis temperatures
Further illustration of these changes in pore volumes is given in
Elemental Analyses for Pyrolyzed A-S04
A-S04 300 350 400 450 500 550 600
Vr *ui -. V v v* ***
Gas Adsorption Isotherms
To gain further understanding of the adsorption characteristics of these
carbonaceous materials, a series of gas adsorption isotherms was obtained at
room temperature employing a variety of gases.
The tabular data for these
adsorption isotherms, as well as isotherms obtained at other temperatures, are
summarized in Appendix B.
Figure II-6 compares the N2 adsorption isotherms
obtained on all nine carbons at room temperature.
Relative comparisons of the
affinity and capacity of these materials can be made from this data.
to correlate the adsorption capacities with any of the previously reported
surface area, pore volumes or the BET C constants yielded no apparent trends
for this data.
Similar data is reported in Figure II-7 for room temperature CH4
adsorption, except for data for F-300 and a limited data set for PPAN.
capacity of all of these solids was higher for CH4 than N2, yet again no direct
correlation for the relative ordering could be made with any of the physical
parameters previously found.
In comparing the N2 adsorption data with that of CH4, the relative
affinities and capacities of some of the solids change as a function of both the
adsorbate and equilibrium pressure.
adsorption data for BPL and N-211.
This is most obvious in the CH4
Since no physical parameter, such as
micropore volume or surface area, can be employed to adequately explain this
shift in adsorption behavior, the chemical functionality of the surface and the
adsorbate must have decided effects on the adsorption process.
It is of interest
in comparing these adsorbents to be able to better quantify the adsorption
affinity and capacity they would have under various conditions.
Therefore, it is
- 4 t 4. ,,,- 4 1. .-, -,,,,j, -- -- .Z -, t. -- A 1- S 4 4. ,-
flTTYrf(lE q I rirt rTflYTY
I~frE* 1* CI
I I I T I 2 3 I Q
nune 9 Ione corarry
Referring to Table II-1, despite the large variety of source materials and
the pyrolysis and treatment conditions the original polymers were exposed to,
the elemental compositions of most of the resultant carbonized materials were
However, it can be seen from the data presented in Table 11-2, all
the variables in chemical treatments and pyrolysis conditions have a decided
impact on the porosity and surface features of these carbons.
From this it was
concluded, as stated in the introduction, that through varying a large number
of parameters a carbonaceous adsorbent can be tailored to have specific
capacities and affinities for a wide variety of chemical compounds, both
dependent on their shape and size, and chemical functionality
. Varying these
parameters would have a large impact on how these materials behave as
adsorbents, catalysts, and catalyst supports.
In addition to the surface area and porosity differences observed in these
, their affinities and capacities for various adsorbates is also
dependent on the synthesis of these materials.
It was found that for two fairly
non-interactive probes, N2 and CH4, differences in adsorption characteristics
were observed that cannot be accounted for simply by the measured physical
parameters of these adsorbents.
It is therefore likely that the adsorption
character of these materials, although affected by parameters such as surface
area and pore volumes and distributions, is highly dependent on their chemical
properties such as surface functionality, elemental composition and acidity.
This observation has been observed in the development of heterogeneous
It would therefore be desirable to develop another
and predictive ability of the adsorption characteristics of these types of solids.
Such a model is developed in the following chapter.
The composition of the remaining mass of these solids,
additional functionality and residual metal salts (ash content), was not well
characterized in these studies.
From the CHN analyses of the
(Table II-1) it was found that several have significant amounts (>5% by mass) of
other elements incorporated in their structure or on their surface.
detailed analysis for trace metals is warranted, as it has already been surmised
that the chemical functionality of these materials is very important in their
The presence of trace metals would also have a large
impact on the catalytic behavior of these solids.
The shape of the isotherms and hysteresis loops can help in elucidating
information about the structure, shapes and sizes of the pores in these
Although pore shapes and sizes have not been fully developed
here, a large amount of information has been gained about the relative
surface area and adsorption characteristics of the nine carbons
The BET theory and interpretation of isotherms have been discussed
in great detail to understand the large amount of information gained from gas
Further analysis of the BJH model and Harkins-Jura theory
employed to determine pore size distributions is beyond the scope of this work.
However, a good understanding of the BET equation (Equation II-13) is helpful
in understanding the informative value of the interpretation developed from the
isotherms presented in Figures II-6 and II-7 in Chapter III.
GAS-SOLID ADSORPTION EQUILIBRIA
Porous supports have been examined in great detail for applications in
the area of heterogeneous catalysis.
Porosity, pore size, surface area, and the
accessibility to the internal surface area are all important factors facilitating
reaction pathways on solids. As was found in Chapter II
correlation of a solid's adsorption behavior cannot be del
physical structure alone. Certain pore structures may a
reactants near active catalyst sites. The same concentra
with reaction products, which could inhibit the reaction.
understanding and quantitative measure of a solid's cap
:, however, direct
termined from its
effect concentration of
Ition effect may occur
city and affinity for
various adsorbates are essential in understanding and developing a rational
selection process for solid materials.
The general phenomenon of adsorption involves the enrichment of one or
more components at an interfacial layer.
Gas-solid equilibria are typically
characterized by three types of adsorption interactions:
chemisorption and physisorption.
Absorption occurs when the adsorptive
molecules penetrate the surface layer of the solid and enter the bulk structure.
Chemisorption involves irreversible bond formation to the solid, and is
generally associated with large enthalpies of adsorption (10-50 kcal/mol or
- .t- ~ _._--.l *. --~1-,- a- ...i -t: L..L U.....ts.....
may evolve chemically different than the original chemisorbed compound.
example of this is seen when 02 strongly binds to charcoal at room
at elevated temperatures, but as CO and CO2.
Physisorption, which will include the process of reversible chemisorption for
this discussion, involves the reversible adsorption of molecules on a surface.
This adsorption process is dependent mainly on intermolecular interactions
such as induced dipoles, permanent dipoles, quadrupolar interactions, and
Interactions between the interface and adsorbent are
usually found to be most significant for the first monomolecular layer
Therefore, it is found that most physisorption processes involve
adsorption enthalpies that approach the enthalpy of condensation for the
As described earlier in Chapter II, up to three processes may be present
in the physisorption isotherm.
These are micropore filling, monolayer and
multilayer adsorption, and capillary condensation.
pores in the solid,
The size and shape of the
the adsorbate diameter, temperature, and the equilibrium
gas pressure of the adsorbate all have an effect on which process is occurring.
To examine the effect of pore sizes on adsorption, Everett and Powl54 calculated
interaction energies as a function of pore size based on the Lennard-Jones
In cylindrical pores five adsorptive diameters or less in size,
the model predicts increasing adsorption energies with decreasing pore size.
Preferential micropore adsorption occurs because of these higher adsorption
Furthermore, if the temperature is below the critical temperature of
the adsorbate, capillary condensation is possible in pores larger than four
from the Langmuir and Kelvin equations, it better accommodates multilayer
formation and porous surfaces.
As discussed previously, this model does
have limitations on its applicability and the assumptions made in its
In addition, the value of the BET C constant and its physical
meaning have often been debated.
This constant appears to be a complex
quantity related to the isotherm shape, surface coverage, the ratio of
equilibrium constants for the various adsorption sites and processes, and
contributions from the enthalpies and entropies of adsorption of the monolayer
Literature attempts to interpret the BET C value have led to
various proposals,56 but none appear universal in obtaining thermodynamic
information about gas-solid interactions.
The focus of this research was to develop a method complementary to
existing theory for the analysis of adsorption data.
The goal of this analysis
was to obtain thermodynamic data for the interaction of the adsorptive
molecules with the strongest binding sites on the solid surface.
studies, the adsorption isotherms of two carbonaceous adsorbents that had
been previously well characterized in this research group were focused upon.
Pyrolyzed polyacrylonitrile (PPAN) and Ambersorb 572, lot#
2125 (A-572) were
More information on the physical characteristics and adsorption
behavior of these two solids can be found in the previous chapter.
All gas adsorption isotherms were obtained in the exact manner as that
described in Chapter II, employing the glass vacuum line (Fig. II-1) or the
-. -. *5 -
obtained by immersing the sample in a Dewar filled with a slush made from
liquid N2 and an appropriate solvent. Toluene was the solvent for the -93C
isotherms, and acetonitrile for the -420C runs. An ice/water mix was used for
isotherms obtained at 0C. Variance of less than +1C was observed for these
systems during the course of a run. For isotherms obtained at room
temperature (25C) or higher, a heating mantle was employed to regulate the
For these systems temperature control was not as precise, with
variations of +20C observed.
The numerical data for all the obtained isotherms
for the various gases and temperatures are tabulated in Appendix B.
Results and Discussion
Supports and Adsorptives
Two chemically different porous carbonaceous adsorbents and a series of
different gases were focused on to better characterize the processes involved in
gas-solid adsorption equilibria.
polystyrene, and PPAN, an ads<
where characterized in detail in Chapter II.
A-572, derived from sulfonated, macroreticular
orbent derived from pyrolyzed polyacrylonitrile,
It was observed that these two
materials differ in both their physical composition (Table II-1) and surface
areas/pore distributions (Table 11-2). 1
towards N2 and CH4 (Figs. II-6 and II-7
additionally, their adsorption behavior
at room temperature were observed to
differ in a fashion not solely accounted for by the physical characterizations
The gases used in the adsorption measurements discussed in this
quadrupolar CO2 were chosen to investigate the influence of polarity on the
application of the developed adsorption model and to provide an indication of
the importance of these properties on adsorption by these two solids.
CO are non-condensible gases in this study because their isotherms were
obtained above their critical temperatures.
These adsorptives will therefore be
subject only to gas-solid, micropore filling, and multilayer interactions, with no
possibility of capillary condensation occurring.
CO2 is, however, a condensible
adsorptive at room temperature, so there also exists the possibility of capillary
condensation with this adsorbate.
Summary of Adsorbate Gases and Associated Physical Properties57
a Molar volumes of the liquids at their normal boiling point (unless otherwise
Equilibrium Analysis and Adsorption Model
The measured isotherms were analyzed using an equilibrium model
derived by analogy to a multiple site Langmuir adsorption model.
equation is given below:
i (1 + Ki,adsatm )
where ntit is the total number of moles of gas adsorbed per gram of solid; ni the
available canacitv of process i
K1 ,. the eauilibrium constant for the adsorption
routine (Appendix C) was used to solve equation III-1 for the values of ni and
It has previously been found in using this data fitting routine that a very
shallow minimum in the solution series results from solving simultaneous
equations for data involving even two processes.58
In addition, it is an accepted
limitation of the simplex approach that misleading data may arise due to the
minimization of a complex equation in local or relative minima.59
experience in solving Equation III-1 for room temperature gas adsorption data
found a shallow surface and uncertain quantities for ni's
and Ki's for up to
Employing six parameters to fit these relatively simple curves
yields values for ni and Ki,ads, but one must critically evaluate the physical
meaning of the determined values.
In the Cal-Ad method,sso60 adsorption and
calorimetric data are solved simultaneously to better define the minimum.
adsorption enthalpies of physisorption processes for most carbon adsorbents
are so small that this approach cannot be employed.
isotherms were measured at various temperatures.
Instead, a series of
Assuming the ni values are
temperature independent, each isotherm at a different temperature introduces
only new Ki's as unknowns, leading to a better definition of the minimum in the
solution of the combined data sets.
The reproducibility and precision in the adsorption measurements were
determined from three successive experiments in which N2 adsorption by A-572
at 250C was followed by evacuating and repeating the N2 adsorption experiment
on the same sample.
The adsorption isotherms were found to be very
At higher equilibrium pressures (Patm > 0.05), the relative error
- --- ---
small volumes of gas adsorbed and the limits of sensitivity of the pressure
Two successive adsorption isotherms were also obtained for CO
on A-572 at T = -930C.
At this low temperature, CO leads to much higher
volumes of gas adsorbed at lower pressures.
However, as observed in the N2
data, the greatest differences in successive runs were observed at the lower
For the two CO adsorption isotherms a 6
< 4% was
found for Patm < 0.05.
In this type of data analysis, it is essential to add
processes only until the adsorption data is fit to the precision and accuracy to
which it is known.
The relative errors discussed above were used as the
criteria for the minimum number of processes needed to fit the adsorption
The procedure for obtaining ni's and Ki's for the gases N2 and CO in this
study is outlined as follows:
The isotherms for N2 and CO at -420C were fit to two processes using
the above criteria, leading to values for all four parameters (ni, n2, Ki,
At this temperature, the value for nl is best defined61 as the data
points range from zero to near full capacity for process 1.
explanation of this assumption will follow, along with an interpretation
of the physical meaning of the different processes.
, the Ki's
sufficiently differ so that the two processes are felt to be well
(2) The -93C isotherms required three processes to fit the data to
within experimental error.
obtained in Step 1. This l
The value for ni was fixed to the value
power temperature data provides a better
definition of n2, so this value was not fixed, although the value of n2
data points over the majority of their capacity range.
values of n2, Ki and K2 are felt to be well defined for this data set.
Process 3 is not as well characterized as there exist a limited number of
data points in the region where process 3 is occurring.
n3 and K3 are not believed to be as accurately known.
Because of this,
This is a limit
imposed by the apparatus and experimental data set, as it was not
possible to take measurements above Patm =
1.0, where process 3 is
occurring to the largest extent.
The above two isotherms (at -42 and -93C) were then reanalyzed,
employing a total of three processes.
Since ni, n2 and n3 have already
been determined, these values were not allowed to vary.
was also used on the room temperature data.
values for all Ki's
This final step yields
at the three different isotherm temperatures.
In addition to finding the Ki,ads's at various temperatures, Step 3
constitutes a final check to determine if meaningful parameters for ni's
If the values found for ni, n2 and n3 from Steps 1 and
2 are well
defined, it should be possible to fit all the data sets to within the precision of
the experimental measurements.
A second check results from plotting the
natural logarithm of the Ki values versus the reciprocal temperature (in Kelvin).
This relationship, known as the van't Hoff equation, should yield a straight line
whose slope is proportional to the enthalpy of adsorption.
temperatures were used for the CO2 adsorption data, the same basic procedure
was followed for fitting this data.
In this case, however, a good data fit was
- Cr C~ *6 *1
Further detail and discussion of this method, and determination of what
is considered a good fit of experimental data, are found in Appendix D.
exemplified and discussed with a sample set of data taken from Appendix B.
It is important to realize that the Ki,ads values found from this procedure
were derived from static adsorption data, where the established criteria for
equilibria (see Chapt. II, Experimental
- Gas Adsorption Studies section)
generally observed 4-5 minutes passing between each obtained data point. I
these materials are allowed to stay in contact with the adsorbates for several
hours, the reported adsorption capacities are found to increase, but by a
relatively small amount
However, this rapid equilibrium process is of
greatest interest for these types of adsorbents when considering separation and
Figures III-1 through III-3 show the adsorption isotherms at various
temperatures for PPAN and A-572 with N2, CO and CO2, respectively.
cases the adsorption isotherms were observed to become more linear, and the
capacities of the adsorbents decrease, with increasing temperature.
numerical data in all the isotherms were fit to Equation III-1 as previously
, producing the best values for nl, n2, and n3, and the different values
of Ki, K2,
and K3 for all the measured temperatures.
The isotherms for all three
adsorbates, on both solids at all the temperatures measured, could be fit within
the accuracy of the experimental measurements using three processes.
previously mentioned, the numerical data for these, as well as several other
n ~a-I ,*- -- C-h l a nnjt: a .n jh A4 0A n fl-, on nr- t+ TT cr0 +0ht11i~i a + iA
bO 3.0E-03 -
0.2 0.4 0.6 0.8 1.0
0.2 0.4 0.6 0.8 1.0
0.2 0.4 0.6 0.8 1.0
c 2.0E-03 -
0.2 0.4 0.6 0.8 1.0
o 0 C
IIII I I I
rM~Z 1) :LJ
Table III-2 summarizes the ni and Ki,ads values determined for the
isotherms shown in Figures III-1 through III-3.
In the table, ni is the capacity
of the solid for the specific adsorption process i, expressed in millimoles of gas
adsorbed per gram of solid
The ni values are a measure of the
capacity of each process, and are functions of the pore size and pore
distribution of the solid, as well as the adsorbate size.
The reported Ki values
measure the affinity of a specific process on the solid for the gaseous probe.
is well known that for solid materials, surface interactions are not easily
characterized due to the solid being energetically heterogeneous.56,62 This is
especially true for highly porous materials such as those studied here. With
this in mind, it is important to recognize that the Ki's
found are likely to be
average values for adsorption processes at different solid sites whose Kads'S can
not be differentiated by this type of data analysis.
It is also likely that although
the Ki's are significantly different as to separate them into three different
processes, the second and third processes are occurring before adsorption
process 1 is complete.
The enthalpies of adsorption for the three processes on the individual
solids have been determined using the well known relationship between the
thermodynamic parameters of enthalpy (AH
and entropy (AS), and the
equilibrium constant K, which in this case will all be adsorption values:
= AHads -
where R is the gas constant and T the temperature in Kelvin.
sides of Equation III-2 by -RT yields
and intercept is AS/R.
The results are summarized in Table III-3, and a
graphical plot of this analysis for CO2 on A-5
72 is shown in Figure III-4.
the points shown are the experimental data, and the lines generated
from the least-squares fit analyses.
The least-squares fit of the resultant plots
are given as a footnote to Table III-3.
The proposed model (Eqn. III-1
verified by the linearity of these data series, as shown in Figure III-4.
Additionally, the values calculated for AHads are consistent with those expected
a Process 1
-AHads (kcal/mol) from InKads VS.
- 8.64( 11
70 ( 158)/T
InK at 250C was omitted
b Process 1:
c Process 1:
it is poorly defined.
K = 1509 ( 27)/T 8.54(+
K = 2562 (+ .1)1/T 9.00(
= 2548 (
1492 ( 154)1/T
.04); InK at 250C was omitted.
- 8.0(+ .
- 9.7( .
d Process 1:
= 3196 ( 163)1/T- 9.8(+
= 2705 ( .59)1/T- 10.6(
= 2665 (+ 81)1/T 10.1
= 1892 ( 1
e Process 1:
= 1808 ( 119)1/T
+ 39)1 /T
= 2695 ( 238)1/T
= 1512 ( 188)1/T
The determination of accurate and meaningful numbers for the three
processes is in part based on the ability of Equation III-1 to predict the
Additionally, measuring a large number of data points
Process Na COb CO2c N2d COe
1 4.7 + 0.1 5.1 + 0.04 7.4 + 0.6 5.3 + 0.2 5.3 + 0.1
2 4.5 + 0.3 5.1 +0.4 6.3 + 0.3 3.8 + 0.2 5.3 + 0.5
3 3.0 + 0.1 3.0 + 0.3 5.4 0.3 3.6 + 0.2 3.0 + 0.4
important to note that the final ni's
solely on the -93C isotherm. It ha
and Ki's were within 10% of those based
s already been noted that the ni values for
CO2 were derived exclusively from the 0C isotherm.
Both these results
indicate that if enough adsorbate is adsorbed by a solid in the region of the
isotherm where a specific process is dominant, reasonable estimates of ni are
possible from measurements at only one temperature.
If this is not the case,
as seen with the N2 and CO data, isotherms at various temperatures must be
obtained to better isolate the individual processes.
Interpretation of the Processes Involved
The equilibria, enthalpies, BET surface areas, and porosimetry values of
A-572 were utilized to provide an interpretation of the physical nature of the
Because it is the most thoroughly characterized of the two
, the interpretation of the processes involved is illustrated with A-572.
PPAN was studied with N2 and CO to afford a comparison of adsorbents.
different values of K1,ads, K2,ads, and K3,ads for A-572 are consistent with different
interaction potentials for different adsorption processes.
At low pressures,
adsorption of the gas into the small micropores, and the most energetic sites of
the solid, should be the dominant adsorption process.
As a measure of the
affinity of the adsorbate for these sites, Ki,
increases in the order N2
CO2 at any given temperature (Table III-2).
The adsorption enthalpies increase
in the same order (Table III-3) and are directly proportional to the polarizability
of the gas molecules (Table III-1).
This result suggests that the predominant
interaction of the adsorbates studied with this solid involves dipole and
The assignment of process 1 to adsorption in the smallest pores is
supported by the literature,54,63'64 and by pore resolved NMR porosimetry
In the NMR spectra, it is observed that the smallest pores of A-572
fill first when CH3CN is adsorbed from a dilute CC14 solution.
This is a direct
observation that shows the equilibrium constant for this process is the largest.
For all of the systems in Table III-2, K1 is almost an order of magnitude larger
2 is attributed to adsorption in the larger micropores and
mesopores of the solid.
For N2 and CO on A-572, the enthalpies for processes 1
2 are the same, within experimental error.
The predominant contribution
to the enthalpy in process 2 is the solid-adsorbate dispersion interaction.
and CO are similar in size, and therefore it is reasonable to assume they have
access to the same internal pore volume.
This then indicates that the
increased interaction potentials arising from the close proximity of pore walls in
the microporous region have only a small enthalpic, but a large entropic
With the larger molecule CO2, the enthalpic contribution to
process 1 is 1.1 kcal/mole larger than that for process 2.
The small pores
involved in the solids studied apparently have dimensions that lead to a larger
enthalpic interaction for the CO2 molecule.
molecules N2 and CO
For the smaller, less polarizable
, the enthalpic contribution is small and the process is
driven more by entropic contributions.
The predominant contribution to the enthalpy of process
2 for CO, N2,
and CO2 arises from the dispersion force interactions of these molecules with
the carbon surface.
The values of both ni and n2 are larger for CO than for N2.
r II* I
more strongly than N2.
If this consideration is limited to process 1 as
previously defined, this would result in larger values of ni and KI for CO over
N2, as observed.
The Ki and K2 values for CO2 are much larger than those
found for the other two gases, which is consistent with the above arguments.
However, the ni and n2 values for CO2 are smaller than those reported for both
CO and N2.
The smaller ni and n2 values are accounted for by CO2 being a
much larger molecule and,
therefore, not having access to the same micropore
volume as does N2 or CO.
Process 3 includes adsorption on the remaining surface and larger
pores, multilayer formation, and capillary condensation in cases where
isotherms are obtained below the critical temperature of the gaseous adsorbate.
As was previously mentioned, there exist a limited number of measured data
points under the temperature and pressure conditions where process 3 is
thought to be dominant for these solids.
Additionally, since the measured
isotherms were never observed to level off, the total capacity of the solid was
Because of this
, the values for n3 and Ka are considered
approximations, and not as well defined as those for processes 1 and
A polarized surface molecule could lead to multilayer adsorption even
though isotherm temperatures are above the critical temperature of the
Whether the adsorbed molecules are all on the surface or on
surface and bilayer sites is often of minor concern.
Therefore, in processes
where the solid-adsorbate interactions are comparable to the adsorbate-
adsorbate interactions, it is difficult to distinguish the contribution from
multilayer adsorption and surface bound interactions.
In this case, the two
Using an adsorbate cross-sectional area of 16.2 A2 for nitrogen, the
values of ni and n2 for A-572 correspond to a surface area of 45 and 155 m2/g,
The value of na corresponds to
A similar analysis for
CO using an adsorbate area66 of 15.0 A2 gives 48,
values of nli, n2 and n3, respectively.
164, and 446 m2/g from the
In both these cases, process
attributed to adsorption by the larger pores and remaining surface.
the results from this data analysis with those from BET theory suggests that
only a small portion of the total surface area for A-572 (S.A.BET =
occupied under these experimental conditions.
1159 m2/g) is
Knowing that a large amount of
the reported S.A.BET is because of the large micropore volume of this solid, it
appears that all of the adsorption occurring under these conditions is in the
The Micromeritics software reports a value of 1080 m2/g
for the micropore area of A-572.
This number is calculated based on the
difference between the reported total surface area (S.A.BET) and that calculated
for the external surface area.
This external area is defined as the surface area
of the solid excluding that of the micropores.
The value for the external area is
calculated from the slope of the Harkins-Jura t-plot analysis,
including what is
referred to as surface area and density correction factors.67
Comparing the ni values to the total amount of CO and N2 adsorbed for
the various isotherms (e.g.,
-1.7x10-3 moles/g at Patm=1.0 and -42C on A-572)
shows that up to an equilibrium pressure of 1.0 atm all of the adsorbate can be
accommodated by the ni and n2 values.
At the lowest temperature,
is not the case.
However, the ni and n2 processes still have the capacity for
over half of the amount of gas adsorbed.
These observations were found to be
-~~~ ~~ 1 -- U. -I ru.- -. 'V -C ,r1 .
seen to be poorly defined at the temperature and pressure regions studied.
This is caused by only a fraction of na's capacity actually being occupied.
When similar calculations are done with the CO2 data, using a cross-
sectional area66 of 21.8A2
m2/g for processes 1 and
, the corresponding surface areas are 24 and 181
An area of 1140 m2/g is calculated based on the
The total area thus calculated from ni, n2, and na for these three
processes exceeds the reported S.A.BET.
Thus, CO2 must involve both
adsorption by the solid surface and multilayer processes.
Both ni and n2 are
smaller for CO2 than for either N2 or CO, as expected from its larger size.
However, n3 is much larger, and provides the only clear manifestation of a
multilayer process in the systems reported in Table III-2.
It is of interest to note that the enthalpies of process 3 are larger than
the enthalpies of vaporization of the liquids.
From this observation, the
polarization of a surface bound molecule must increase the enthalpy of
interaction of a second layer with the surface layer. In general, if multilayer
formation occurs in each of the processes, the Ki,ads's are comparable to those
for surface adsorption of those processes.
The interaction potentials that give
rise to the different processes influence not only the surface interactions, but
also the multilayer interactions.
However, this multilayer interaction beyond
the monolayer coverage has been found to decrease rapidly with distance,51,52
and therefore it is not likely that many areas on the surface of these solids
contain more than a bilayer at the pressures studied.
Comparison of PPAN and A-572
-. -- ~ 4IF 44 4*tj U 4 .4 ..
A-572 in its adsorption characteristics towards the gases studied.
values for ni, n2, Ki, and K2 where found to be very similar for the two solids.
Thus, the nitrogen functionality of PPAN appears to do little to change the
interaction of the solid surface with the adsorbed N2 or CO.
before, the adsorption characteristics, and more specifically the trends
observed for these two solids ni and Ki,ads values, do not appear to have any
direct correlation to any of the surface area or porosity values previously
determined using standard gas adsorption theory.
The BET model was developed to account for multilayer adsorption by
assuming that the Langmuir equation, which involves only monolayer
adsorption, applies to each layer.
56 the BET C constant is
related to the ratio of equilibrium constants for both the monolayer and
It is also related exponentially to the difference in the
enthalpies of adsorption of the monolayer and the multilayer.
encompasses several processes and thus is difficult to relate to the affinity or
strength of interaction of an adsorbate with the solid.
Although BET deals with
multilayer formation, it assumes the surface is energetically homogeneous.
previously discussed, this is not the case with most solids.
In the multiple process equilibrium model presented here (Eqn. III-1),
the BET C constant is essentially being separated into components for surface
adsorption in pores of different sizes and multilayer formation. By interpreting
the isotherms as a combination of energetically different processes, distinction
is also being made between energetically different sites on the surface of the
A significant advantage of this model is that Ki,ads for micropore filling is
a true thermondnamic roantitv and can be related to nhvsical nrnnerties. (e.g..
From this analysis, a thermodynamic measure of gas-solid equilibria and a
thermodynamic basis for understanding adsorption processes have been
It is of interest to note that both adsorbents were cycled through
adsorption experiments and degassing at 2000C up to fifteen times. No
changes were observed after repeated analysis on the same sample in the
overall surface area or pore size distribution from the original sample.
thermal stability and physical integrity is important when considering
separation and catalytic applications.
The affinities of three gases for two porous solids have been compared
using a multiple process equilibrium analysis.
For the non-condensible
adsorbates N2 and CO, a multiple process adsorption model is able to separate
the processes of adsorption in small micropores, adsorption by larger
micropores and mesopores, and adsorption by the remaining surface.
enthalpies of interaction for condensible adsorptives indicate that multilayer
condensation may accompany surface adsorption for each process and is
clearly involved for CO2 in process 3.
Comparison of the affinity of the solids
for large and small probe molecules is complicated,
however, by the fact that a
different distribution of pore sizes comprises processes 1 and 2 for different size
The Ki,ads and ni values for different adsorptives provide a quantitative
characterization of the forces involved in porous solid-gas equilibria.
4 4 C zI. tr ,.. rr 11 ilL.. t
of the solid surface.
Comparison of the Ki,ads and the AHi,ads values provides
insight into the fundamental nature of the interaction of the adsorbent and
adsorptive molecules, and allows a quantitative means of selecting solids for
adsorption or catalytic applications. The gas-solid interactions studied here
were found to be predominantly physisorption. Accordingly, the carbonaceous
adsorbents studied here are expected to be effective in the separation of
molecules that differ in polarizability.
The excellent fit of the data for the non-condensible gases to the three
process model, using the same nl, n2, and ns values for isotherms at all
suggest that the ni and n2 values obtained are accurate
measures of the available capacity for these different processes.
It should also
be considered that the values found from this data analysis may prove to be
more useful in comparing the adsorption behavior of different solids.
applications as adsorbents, catalysts, or catalyst support materials, the
absolute surface area and pore structure of a solid are not necessarily the most
important parameters to be measured.
Instead, the effective or accessible
surface area and pore volumes for a specific adsorbate are of greater interest.
The ni values are a measure of the capacity of this effective pore volume. The
Ki,ads and subsequent AHi,ads are measures of the strength of these interactions.
Kinetic factors are also important in the practical applications of solids
in separation and catalytic processes.
However, this aspect of the problem was
not addressed in these studies.
HETEROGENEOUS WACKER CATALYSIS
Palladium catalyzed oxidation of olefins is a well known synthetic
approach in homogeneous catalysis.68,69
One of the better known applications
arose from the discovery by Smidt et al.70 of the homogeneous catalytic
oxidation of ethylene to acetaldehyde.
Most often referred to as the Wacker
process after the German company that first commercialized the process, it
involves an aqueous co-catalyst system of [PdCl4]-2 and CuCl2.
to Pd(0) upon the oxidation of ethylene to acetaldehyde, and is then reoxidized
The catalytic cycle is completed by the rapid oxidation of Cu(I) back
to Cu(II) by 02.71.72
The overall process is represented by the following
C2H4 + PdC12 + H20 -> CH3CHO + Pd(0) + 2HC1
Pd(0) + 2CuC12
CuCI + 2HC1 + 02
-> PdC12 +
2CuCl2 + H20
C2H4 + O2
The proposed mechanism involves the displacement of a Cl- from the
palladium complex followed by coordination of ethylene.
olefin for hydration by the solvent, water.
This activates the
Addition of another H20 molecule to
-. ~L ..Y a:.. '. gA l aIM -~n -rr thn.1 an t a nn nA in CE '1a*\f W rr'-
1r- 11 -1 -- 1 -
U`IYln ilr +Hcnrre
nCH n "
One of the major disadvantages to the industrial homogeneous system is
the production of chlorinated oxygenates as unwanted by-products.73
addition to the presence of chloride as the counterion, it has been suggested
that the presence of copper is a major contributor to the formation of these
A variety of research has been done investigating the
replacement of both copper and chloride ion in this catalyst system.
been reported that this system is very susceptible to the counterion and
oxidant, and that different product distributions are obtained with changes in
As shown in Eqn. IV-2, the importance and function of
copper in this catalytic system is in the regeneration of the active palladium
Halide free catalysis systems have been reported, and other species
have been found that can serve the function of Cu(II) in this system.
include vanadium oxides,77
78 peroxides, benzoquinone
derivatives,79 and activated charcoal.80
Considering that the function of copper
may be met by carbonaceous materials, it is feasible that doping a specific
adsorbent, like those discussed in Chapter II, with Pd(II) salts would yield a
functioning catalytic system for Wacker-type catalysis.
Due to carbon's unique
chemical and physical properties, it may also prove advantageous over more
traditional supports (e.g., zeolites or silica) to employ a high surface area
carbonaceous support as a heterogeneous oxidation catalyst and/or catalyst
For this study Ambersorb
carbonaceous adsorbents were chosen.
adsorbents have previously shown catalytic activity towards both
oxidation and reduction applications,8182,83 as well as resistance to self-
volumes that would be advantageous for catalytic applications (see Chapter II).
All the reactants and products for the Wacker oxidation can be easily handled
in the gas phase, and therefore lend themselves well to a gas flow system with
an immobile Pd(II) solid catalyst.
For these reasons it was felt that Ambersorb
adsorbents would be excellent candidates for the support of Pd(II) salts that
could lead to a heterogeneous Wacker catalyst system.
Ambersorb 572 (lot# 2201) was employed as the carbonaceous support
for this series of experiments.
Metal salts were dispersed onto the support by
pore-filling, also known as the incipient wetness method.
This involves making
a concentrated solution of a soluble salt of the metal species in an appropriate
solvent, usually deionized water.
Aliquots of the solution are added to the solid
until its surface appears wetted, avoiding excess liquid so as to not suspend
the solid in the solution.
The wetted solid is then dried under vacuum until it
This procedure is repeated until the entire salt solution has been
The final catalyst is then dried under vacuum at 100C.
is used in hopes of concentrating the active catalytic species in the internal
volume and/or microporosity of the support.
As was previously discussed, the
micropore regions in these types of solids are thought to be very important in
Catalyst systems for the oxidation of ethylene to acetaldehyde were
evaluated in a gas flow system (Figure IV-1).
Reactant gases, which included
moist air and an ethylene/nitrogen mix (ranging from 10-20% C2H4 in N2), were
placed on top of the catalyst bed to fill the remainder of the reactor tube.
Water was introduced into the system by passing the air stream through a
glass bubbler fitted with a coarse air diffuser.
The amount of moisture in the
air stream was regulated by either placing the bubbler in a Dewar containing
ice water, leaving it at room temperature (RT
~ 250C), or heating the bubbler
slightly with thermal tape to increase the vapor pressure of water in the
Catalyst temperature was regulated with an Omega solid state
temperature controller (model CN310) and variable transformer (Staco, Type
3PN1010) connected to an oven that surrounded the reactor tube.
Temperatures did not vary more than 2C from the set-point on the
temperature controller during the course of a run.
constructed entirely of glass.
a 10 mm I.D. glass tube. Co
ground glass joints, and a co
the catalyst bed was supported.
The reactor flow system was
The reactor tube was 40 cm long, constructed of
nnections were made at both ends via 10/30
,arse glass frit was mounted in its center on which
A thermocouple, surrounded by a glass jacket,
was mounted directly below the glass frit and used for oven temperature
control and monitoring reactor temperature.
The reactor gas flow was in a
downward direction, as diagrammed in Figure IV-1.
A Hewlett-Packard 5890 gas chromatograph (g.c
equipped with an
-160 capillary column and flame ionization detector were used in
the analyses and identification of organic reactants and products from the
catalyst flow system.
This non-polar, 100% dimethylsiloxane polymeric phase
separates organic analytes mainly on the basis of their boiling points.
product line no longer used by Alltech.
This is a
The same phase can now be found sold
" TO FLOWMETER
Gas samples were taken both before and after the catalyst bed, which
allowed for calculation of conversions and selectivities.
A Varian 3700
equipped with a CarboxenTM 30 foot, 60/80 mesh packed column and a thermal
conductivity detector was also employed for monitoring N2, 02, CO and CO2.
This was done for purposes of mass balance and to monitor any possible over
oxidation of ethylene or the carbon support under reaction conditions.
Preparation of H21PdC141 Catalyst:
To 0.31g of PdCl2 was added 1.0 mL concentrated HCl(aq and 4.0 mL
The resultant solution was pore-filled onto 5.0g of Ambersorb
The resultant solid was dried under vacuum at 100C for one hour before
use in any catalysis runs.
The doping level in the resultant catalyst was
3.5x10-4 moles Pd(II)/g of carbon (3.7 wt%).
Preparation of Na2 PdC141 Catalyst:
To 0.26g of PdC12 was added 1.0 mL concentrated HCl(aq) and 1.0 mL
To the resultant brown solution was added 0.18g NaC1 (-2 moles
Na(I) per mole Pd(II)), dissolved in
-2 mL H20.
This solution was evaporated to
dryness over a steam bath, the resultant solid (presumed to be Na2[PdCl4])
dissolved in a water/ethanol mix (1/1), and then this solution was pore-filled
onto 5.0g of Ambersorb 572.
The resultant solid was dried under vacuum at
100C for one hour before use in any catalysis runs.
The doping level in the
resultant catalyst was 3.0x10-4 moles Pd(II)/g of carbon (3.2 wt%).
Preparation of Na2 PdC14 + CuC12 Catalyst:
To 1.04g of the NaPd-572 catalyst was added 0.5 Ig of CuCl2-2H20,
dissolved in 3 mL of a water/ethanol mix (1/1), by the
The resultant solid was dried under vacuum at 1000C for one hour before use
in any catalysis runs.
Preparation of NHa/Na2aPdC14/ CuC12 Catalyst:
Prior to pore filling, the carbon was washed with 6M NH3(aq),
to stay in contact with this solution for -1 hour.
This was followed by repeated
rinsing of the solid with distilled H20 until a pH-neutral effluent was observed,
dried under vacuum at 1000C, and pore-filled with a solution containing both
Pd(II) and Cu(II) salts.
The Naa[PdC14] was made in similar fashion as in the
preparation of NaPd-572.
3g of PdCl2 was added 1.0 mL concentrated
HCl(aq) and 1.0 mL distilled H20. To t
0.15g NaC1, dissolved in 2.0 mL H20.
the resultant brown solution was added
This solution was evaporated to dryness
over a steam bath, the resultant solid dissolved in water,
CuCl22H20 was added to this solution.
This liquid mixture was then pore-
filled onto 5.0g of the NH3 washed Ambersorb 572.
The resultant solid was
dried under vacuum at 1000C for one hour before use in any catalysis runs.
The doping level in the resultant catalyst was 2.5x10-4 moles Pd(II)/g of carbon
and 1.3x10-3 moles Cu(II)/g of carbon (8.2 wt%).
The Cr2H4 gas employed (Matheson) was C.P. grade (99.5% pure).
and used as received.
All other reagents used were of common laboratory
grade, obtained from Fisher Scientific, and used without any further
purification unless otherwise noted.
Typical catalyst run conditions used between 0
1.0g of catalyst in the
flow reactor system.
Reaction temperatures were varied between
It was found that above 1250C, significant amounts of CO2 were
produced, arising from either oxidation of the support, or over-oxidation of the
The best reaction temperature for these catalysts was
found to be around 750C.
Total gas flows ranged between
2.0 and 10.0
mL/minute, with C2H4 concentrations typically ranging from 1-3% of the total
Results and Discussion
HPd-572 Catalyst Results
Initial catalysis runs employed the H2[PdCl4] doped Ambersorb 572
(HPd-572). Reaction conditions were varied in attempts to yield the most active
catalyst system. Using 1.0g of the HPd-572 catalyst with a bed height of 2.0
cm, flow rates were varied over ranges from 2-10 mL/minute.
temperatures were varied from 50-150C, and the moisture content from the
described method with the water bath temperature ranging from RT
humidity) to a
heated bubbler at 750C
Relative amounts of
C2H4 did not exceed 20%, and the selectivity to CHaCHO was less than 25%.
Several additional products were found by g.c. analysis.
However, since the
desired activity was not observed and the production of chlorinated alkanes
and ethers is known to occur from acid catalyzed reactions with olefins, no
further attempt was made at identification or quantification.
homogeneous industrial process it is reported76 that small amounts of acetic
acid, 2-chloroethanol, chloroethane, chloroacetaldehydes and acetaldehyde
condensation products are the most often observed by-products.
It has been
assumed that the minor by-products formed in this reaction are also composed
of these compounds.
Appreciable activity was not observed in this system until temperatures
exceeded 1250C, and this also resulted in the production of large amounts of
Analyses of g.c. data showed almost complete consumption of 02 at
temperatures above 1250C, unaccompanied by a proportional increase in
aldehyde or other oxygenated organic products.
Since no noticeable decrease
in the carbon mass of the support was noticed over the course of the run, the
production of CO2 at these elevated temperatures is attributed to complete
oxidation of the organic substrates.
It was also observed that turning off the
C2H4 in the gas feed caused an increase in the amount of CO2 observed in the
Again, since no measurable decrease in the support mass was
observed over the course of a run, this was assumed to be due to the complete
oxidation of adsorbed organic on the support.
Returning the C2H4 feed to its
original levels resulted in an immediate decrease in CO2 production, back to its
original levels at a given temperature.
Based on calculated turnover numbers
i ... .. t
not found to be catalytic.
The low activity it did exhibit was found to quickly
taper off within three hours of the beginning of the reaction.
overall activity of HPd-572 was very low, the catalyst system was found to be
more active at the slower flow rates (2-3 mL/min).
Initial observations found the support to have a high affinity for
adsorbing organic substrates, as it was at least 1 hour before any organic
analytes were observed in the post-catalyst gas stream for all of the catalyst
Additionally, the adsorption capacities and affinities of
these supports84 indicate they will not only adsorb and concentrate organic
reactants, but will have even higher affinities for the products of this reaction.
Due to this observation, no quantitative data for reported conversions or
selectivities were obtained until adsorption equilibrium was considered to be
This was determined from an approximate mass balance of the
amount of organic reactants measured in the pre-catalyst gas sample,
compared to that measured after the catalyst bed.
During the initial stages of a
catalyst run, where adsorption was occurring to a large extent,
it was obvious
that the majority of the organic reactant was not accounted for in post-gas
NaPd-572 Catalyst Results
Because of the low activity of HPd-572 and the apparent production of
acid-catalyzed products, the next catalyst screened was designed to eliminate
possible effects due to the presence of Brensted-Lowry acidity.
In this system
dividing this by the number of moles of active catalyst species.
A T.O.N.< 1
. I.- I- --:I j." C .. : u -------------- ----- ..... -----1 L -- ------ "
the sodium salt of tetrachloropalladate was employed as the pore-filling salt
Reaction temperatures were generally kept below 1000C, since
results with HPd-572 had shown no benefit in going to higher temperatures,
and flow rates were also kept low (3.0 mL/min).
Using 0.5g of the catalyst,
with a bed height of 1.3 cm and the water bubbler kept at RT, this system was
run under the typical reaction conditions described earlier (see Run
Initially, the NaPd-572 catalyst showed activity towards acetaldehyde
production at 500C.
The activity of this system was found to maximize around
75C, with -70% conversion of C2H4 and >90% selectivity towards CHaCHO.
Other products included very small amounts of CO2, and trace organic
Determination of original conversions and selectivities were initially
calculated as much lower, as large amounts of ethanol were observed in the
It was later determined that the observed ethanol was not
the product of a reaction of C2H4, but residual adsorbed solvent from the pore-
filling preparation of the catalyst.
This was further verified by the drying of a
batch of this catalyst overnight at 2000C under flowing air, and then subjecting
to normal run conditions with wet air and C2H4/N2 mix.
At temperatures below
1000C, CH3CHO was the only observed organic product, and no ethanol was
observed in the post-gas of the air dried NaPd-572.
To further verify that all the acetaldehyde produced originated from the
ethylene, a mixture of air and ethanol vapors were passed over the catalyst
under standard run conditions.
ethanol was observed. This con
Under these conditions, no reaction with
firmed the observation that all the CH3CHO
.,','ri rnnA ar,-'.a f-rnrn,-, Alr-nn+ ar4Alntn ,f f-l--l. .m'tiltr-h fha ma4i A', l a h art nl fanm
The large amount of ethanol remaining in this catalyst once again points to the
strong adsorptive capabilities of this support.
When the reaction with NaPd-
was carried out above 1000C, a variety
of heavier organic components (determined from g.c.
elution times) were
observed, along with a decrease in conversions and selectivity to acetaldehyde.
No further analyses to identify these trace compounds were undertaken.
was previously observed in the HPd-572 catalyst, lower temperatures are more
desirable for minimizing CO2 production.
Lower reaction temperatures are also
advantageous when considering overall costs for industrial scale processes.
Although this system was found to be catalytic, exhibiting 3 T.O.N. in optimal
conditions, its activity was observed to decline rapidly after a period of 5 hours,
leading to conversions of C2H4 of
A series of eight runs on the same sample of the NaPd-572 catalyst
(0.50g, under standard reaction conditions:
3.0 mL/min gas flow, 750C) were
evaluated in an attempt to regenerate the catalyst to initial reactivity levels.
These included a variety of overnight treatments with air at elevated
temperatures (2000C), in hopes of reoxidizing the palladium and/or carbon
surface, as well as extended exposure to a humid air stream at room
temperature. Results of the air treatments saw a partial return of catalyst
activity, with about 5-10% conversion of C2H4, 100% selective to CH3CHO, but
this activity steadily declined over a period of 4 hours.
Moist air treatments
were more successful, leading to a return of -40% conversion, again with 100%
selectivity towards CH3CHO.
As was observed previously, however, catalyst
activity declined steadily after the first few hours of the reaction, dropping to
conversions after -5 hours.
Heating the water bubbler to 750C to increase the overall
humidity in the system prolonged catalyst life to -5 hours, yet conversions still
dropped off to
<5% after this period of time.
From these results, it would
appear that the reoxidation of the active palladium species by the carbon
support is not totally efficient under reaction conditions.
some measure of activity was observed to return after the various attempts at
regeneration, none restored the catalyst to its original conversion levels.
this catalyst system was started, even when conversions were very low (<5%),
CHaCHO was always observed in the post-gas stream.
However, instead of
continued catalytic activity, this could simply be from the desorption of
adsorbed product, which is feasible considering the previously observed
adsorptive capacity of the support.
NaPd-572+Cu Catalyst Results
Unsuccessful attempts to regenerate the NaPd-572 system and its
limited lifetime suggested the addition of a better oxidant for Pd(II)
Cu(II) salt, to this catalyst system may improve its overall activity.
such as a
resultant catalyst, NaPd-572+Cu, was screened in the same fashion as the
A total gas flow of 3.0 mL/min, and a reaction temperature of
75C were used on 0.50g of catalyst with a bed height of 1.1 cm.
showed almost no activity initially (<5% conversion to CHaCHO).
after an overnight exposure to moist air at 2000C activity was observed.
Acetaldehyde production occurred at temperatures as low as 500C.
Selectivities greater than 98% were observed up to 1250C.
The conversion of
the activity of this system was found to rapidly decline after -4hrs.
system was found to be catalytic, producing 17 T.O.N. of acetaldehyde.
NaPd-572 Without Ethanol Catalyst Results
To further characterize and determine the effects ethanol may have on
the reactivity of this catalyst system, another batch of the NaPd-572 catalyst
was made using methanol and water in place of the ethanol/water mix used as
the pore-filling solvent (see Preparation of Na2[PdCl4] Catalyst).
the catalyst was dried in a vacuum oven overnight at 1000C prior to use, to
ensure removal of the pore-filling solvents. The standard reaction conditions
and setup were used, as previously described. Two different runs were done
using this prepared catalyst.
The first experiment on this catalyst involved heating the water reservoir
to 500C, as it was previously noted that not enough water was being retained
by the system to remain active if the water bubbler was not heated.
activity of this system was quite high, with conversions of -80% and
selectivity to CH3CHO at 750C.
As before, however, the activity of this catalyst
was observed to rapidly decline after -6 hours. This system was found to be
more active than the catalyst made with ethanol, exhibiting -10 T.O.N.
The second experiment with this batch of catalyst was meant to
determine ethanol's effect on the reactivity of the system.
In this case, a 1/1
ethanol/water mix was placed in the bubbler through which air passed,
instead of distilled water only.
This introduced both water and ethanol vapors
to the catalyst bed during the course of the reaction.
Using the standard run
appearance of other organic products was also observed.
This is not completely
surprising, as the existence of another organic substrate would obviously allow
for the production of other compounds in this catalyst system.
was observed from pregas and postgas analyses that a large amount of the
ethanol in the feed gas stream was simply being adsorbed by the support.
significant evolution of ethanol was noticed until the catalyst temperature was
taken above 1000C.
Both of these experiments point to the presence of ethanol
being undesirable in this catalyst system.
CuPd-572 Catalyst Results
In an effort to combine all of the observations previously seen with the
various Wacker-type catalysts supported on Ambersorb 572, a base washed
catalyst was made containing both copper and palladium salts (CuPd-572).
Base washing with aqueous ammonia was done in hopes of removing any
residual acidity from the solid that might exist, considering its precursor is a
sulfonated ion-exchange resin.
And as observed in the case of the NaPd-
572+Cu catalyst, although this system was observed to be catalytic without
copper present, addition of this ion greatly increased its activity.
Under standard reaction conditions, which included total flow rates of
3.0 mL/min, temperatures ranging from room temperature to 1000C, C2H4
concentrations of -2%, and employing a heated water reservoir at 50C
(resulting in -5-10% humidity after considering dilution of air stream with
C2H4/N2 gas mix), experiments were done with 0.50g of catalyst, and a bed
height of 0.95 cm.
This catalyst system was found to be the most active of all
10 days, and during this time activity was never observed to decline.
Conversion of C2H4 was found to be >90%, with greater than 98% selectivity
towards CH3CHO production.
During this time period, it was found that the
catalyst produced more than 120 T.O.N.
Small amounts of CO2 were observed, but no other organic products
were indicated by g.c. analysis.
was lost during the reaction. T
However, further tests indicated that chlorine
'his is surmised from elemental analyses of the
Results for the catalyst before use found it to contain
9.83% Cl- by mass (theoretical value is 10.2%).
Analysis of the catalyst after
the 10 day experimental run found it to contain 6.24% Cl-
. This is a 37%
decrease in the amount of chloride ion, and is not accounted for in the detected
This is assumed to occur either because the chlorine-containing by-
products are in such low concentrations that they are below the detection
limits of the g.c.
detectors, or that these products are adsorbed and retained on
Summary of Heterogeneous Wacker Catalyst Conditions and Results
Catalyst HPd-572 NaPd-572 NaPd-572+Cu CuPd-572
Dopant Level 0.0037g 0.0032g (see prep. of 0.0027g Pd(II)
_(g metal9_carbon Pd II) ___Pd(II ___catalyst __ 0.0082g CueII)
Total Gas Flow 2-10 3.0 3.0 3.0
mLminLml ______n ________
Catalyst Temp. (0C) 75-150 50-125 50-125 25-75
C2H4 Conversion 15% max. 70% max. 80% max >90%
Selectivity to <20% >90% >98% >98%
the carbon surface under reaction conditions.
Regardless, only small amounts
of materials were made, and this loss of C1- did not appear to affect catalyst
More long-term studies of this catalyst are warranted to further
understand this observation.
Table IV-1 summarizes the results for the various catalyst systems
studied, excluding the observations from the second sequence of experiments
done on the NaPd-572 catalyst made without ethanol.
In an effort to further qualify the effect of moisture content on this
system, and gain further evidence that a traditional Wacker-type mechanism
was being followed, the water feed to the system was turned off for a brief
period of time. This was done by simply routing the air flow around the
aqueous bubbler. The effects of this, which are further detailed in Figure IV-2,
show a decrease in the conversion of C2H4 concurrent with a drop in the
humidity of the system.
As soon as the water feed was reintroduced to the
system, the catalyst returned to its original activity level.
Porosimetry and Surface Area Studies
The presence of micropores in these types of carbonaceous supports is
thought to be advantageous for heterogeneous catalysis applications.
discussed in Chapter II, these microporous regions allow for areas of
concentration of reactants.
Ideally, these are localized around active catalyst
Accessibility to these areas is also of great importance.
size of the substrates must be kept in mind,
as well as the presence of
mesoporous and macroporous regions on the solid that would facilitate
"l)' TTfT 7rT^l T n*"
Ambersorb 572 as a support for this heterogeneous catalyst system facilitates
these important processes.
This was determined from the large adsorption of
ethylene that was initially seen in the beginning of a reaction, as well as the
retention of ethanol by this support that was observed in the case of the NaPd-
572 catalyst system.
The development of an active catalyst system (CuPd-572)
also indicates these processes are occurring with some degree of efficiency.
In an effort to further determine where the pore-filled salts reside on this
support, a series of surface area and porosity studies were done on these
This data is summarized in Table IV-2.
These values were obtained
in the same manner as those reported in Chapter II, with the only difference
being the degas temperature for all the samples listed was chosen to be 1100C.
Surface Area and Pore Volume Data for Wacker Catalysts
a This is a different lot number
Pore Volumes (ce/g)
.es mesopore ma
and degas temperature
As can be seen from the data, a decrease in all the physical parameters is
observed upon doping A-572 with the various salts.
not specific to just the micropores, it is
undergoes the largest change. This is (
Although this decrease is
this region of the pore structure that
evidence that the majority of the pore-
filled salts are residing in, or blocking the entrances to the micropore volume of
Further Characterization of Ambersorb Carbonaceous Adsorbents
Magnetic susceptibilities and electron paramagnetic resonance (EPR)
spectra were obtained on two different Ambersorb
carbons in efforts to further
understand the activity of these materials as catalysts and catalyst supports,
and gain further insight into the interaction of the metal dopants with the
Magnetic susceptibilities were obtained using a Johnson Matthey
magnetic susceptibility balance (model MSB-1).
This employs a method where
the magnetic susceptibility of a sample is determined by the force the sample
exerts on a suspended permanent magnet, which is the equal but opposite
force determined by the traditional Guoy balance method where the force
exerted on the sample by a fixed permanent magnet is measured.
summarizes the results of the measured mass susceptibilities (Xg) of the two
carbons with and without metal dopants.
No corrections were made to the measured values for the volume
susceptibility of air or diamagnetic contributions from chloride or water present
in the samples.
Close analysis of the data in Table IV-3 does yield some
A negative value for Xg is found for A-563/2144,
- -r ,
Mass Susceptibilities of Ambersorb 563 and 572 With Dopants
A-563 (lot #2144)
A-563/2144 with 5% Cu(II)b
A-572 (lot #2201)
A-572/2201 with 5% Cu(II)b
a All samples were ground to a fine powder using a Wig-L-Bug (Crescent), to
minimize errors from inhomogeneous packing and trapped air volume between
b The 5% doping level of Cu(II) was a weight percent pore-filling value, using
CuCl2-2H20 as the dopant.
reported Xg is so small, it is most likely that the measured diamagnetic
character of A-563/2144 is a bulk property arising from either diamagnetic
contributions from the matrix, magnetic anisotropy of the sample, or perhaps
anti-ferromagnetic coupling of the carbon free radicals to some trace metal in
The existence of a definitive EPR signal shows that this solid is
paramagnetic, however, and the g value is typical of carbon free radicals.
Three EPR spectra were obtained at 190K for pure CuC12l2H20, a 5%
Cu(II) glass made from a 2:1 ethylene glycol:water mix, and for A-563/2144
with 5% Cu(II).
As would be expected, a very broad, unrefined peak is observed
for the pure copper salt, and the hyperfine splitting typical of Cu(II)-d9 ion is
not seen until the sample is dispersed in the glass and is magnetically dilute.86
The hyperfine structure of Cu(II) is observed in the case of the doped A-
563/2144, along with a single sharp peak associated with the pure carbon.
between the metal ion and the carbon support, the appearance of the hyperfine
splitting does indicate that Cu(II) is well dispersed on the solid.
The A-572/2201 sample has a fairly large and positive Xg, typical of
Its value of 4.82x10-6 c.g.s. is comparable to that of
Of further interest here is that upon doping
A-572/2201 with 5% Cu(II) ion, the magnitude of Xg decreased.
for this occurrence is not obvious.
also reported here.
The Xg value for the CuPd-572 catalyst is
No EPR was obtained on the A-572 carbons as it was not
possible to obtain a lock on the instrument for this sample.
Surface area and micropore volume are considered to be very important
factors in heterogeneous catalysis.
It stands to reason that the greater the
exposed surface area for the solid catalyst, the more likely the possibility of
reactants coming in contact with an active site on the catalyst surface.
Maximizing exposed catalyst surface area is the major limitation in developing
a heterogeneous catalyst from a homogenous system.
Employing a support
that has synergistic effects on the desired reaction is of even greater benefit,
and this was observed in the catalyst systems described.
Included in these
observed synergistic effects are the ability of the support to adsorb and
concentrate organic reactants in the internal pore structure of the carbon,
where most of the catalyst is believed to be present, and the support's
serve as an oxidative compound for the catalytic cycle.
'ri-. *: +. 1^ 4f C-.n trfltnfl ,n; d1,.n nrtn, fi n r Et n I4tn 117rn +trr1n nfI* 4+rnt flltVC 4%
This in turn yields countless possibilities and parameters to be
modified in tailoring a heterogeneous catalyst system for specific reactivities
and reaction conditions.
Included in these are the possibilities of replacing the
chloride counterion with others, such as nitrate or acetate, which eliminates
the possibility of forming undesired chlorinated by-products, and application of
this catalyst to other systems of environmental and commercial interests.
Although no chlorinated by-products were observed in the most active catalyst
system, CuPd-572, evidence did exist that chloride was somehow being lost
during the reaction.
Changing the counter-ion in such a way would also have
some effect on the overall reaction and product distribution, which could also
The preferential oxidation exhibited by these catalysts for an
organic substrate versus the support itself would also allow employing these
supports at higher temperatures, without concern for their combustion.
It is often observed in dispersed heterogeneous catalyst systems like
those reported here that the actual percentage of active catalyst species is
much less than 100%.
reported here were based on the assumption
that all of the Pd(II) was active, which is probably not a true representation of
this catalyst system.
Further studies involving chemisorption and other
surface methods would help in gaining a more complete picture of the actual
catalyst surface and active sites.
The possible error arising from this
assumption is advantageous, however, as it shows that the reported catalytic
activity of these catalysts is a conservative estimate of their minimum activity
Results of the magnetic susceptibility measurements and EPR studies
and III, they provide another characterization of the behavior of these systems.
The curious results from the EPR and susceptibility measurements definitely
warrant further investigation into determining the effect these supports have
on metallic species doped onto their surface, and the nature of the chemical
interactions between the two.
The purpose of the research presented was to develop a greater
understanding of the behavior and properties of carbonaceous adsorbents.
Through the employment of a variety of physical methods such as elemental
analyses, gas adsorption studies, and the determination of surface areas and
porosities, a better understanding of the physical characteristics of porous
carbonaceous materials has been gained.
This included the determination of
what have proven to be the most important factors of the adsorption
characteristics of porous carbonaceous solids.
It was found that the
adsorption behavior of a solid is governed by a complex series of parameters.
Surface area and pore distributions of a porous solid are definitely important in
facilitating transport and adsorption behavior, but the chemical functionality of
the carbon supportlO0,1112,37,48 plays a much more significant role.
determination of what these various functionalities may be, where they exist on
the surface of a solid, and their relative concentrations have proven to be
difficult to determine by most modem instrumental methods.
advances are being made in methods to help characterize these types of
materials, driven by their importance and successes in adsorbent and catalytic
As observed in these studies, a large effort in the characterization of