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Preparation of Magnesium Oxide Supports for Oxidative Coupling of Methane Catalysts

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Title:
Preparation of Magnesium Oxide Supports for Oxidative Coupling of Methane Catalysts
Creator:
Deng, Xianhe
Place of Publication:
[Gainesville, Fla.]
Florida
Publisher:
University of Florida
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Language:
english
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1 online resource (51 p.)

Thesis/Dissertation Information

Degree:
Master's ( M.S.)
Degree Grantor:
University of Florida
Degree Disciplines:
Chemical Engineering
Committee Chair:
HAGELIN,HELENA AE
Committee Co-Chair:
JIANG,PENG
Graduation Date:
5/3/2014

Subjects

Subjects / Keywords:
Betting ( jstor )
Catalysis ( jstor )
Catalysts ( jstor )
Magnesium compounds ( jstor )
Methane ( jstor )
Nanoparticles ( jstor )
Roasting ( jstor )
Surface areas ( jstor )
Surfactants ( jstor )
Temperature ratio ( jstor )
Chemical Engineering -- Dissertations, Academic -- UF
mgo -- microemulsion -- nanoparticles -- ocm
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bibliography ( marcgt )
theses ( marcgt )
government publication (state, provincial, terriorial, dependent) ( marcgt )
born-digital ( sobekcm )
Electronic Thesis or Dissertation
Chemical Engineering thesis, M.S.

Notes

Abstract:
A series of nano-sized magnesium oxide particles with both high and low surface areas were successfully prepared by a simple water/toluene reverse microemulsion method using triton X-45, 1-Dodecanol as surfactant. By controlling the amount of surfactant, it is possible to control the size of MgO nanoparticles from 20 nm to 50 nm. The size was confirmed by X-ray diffraction (XRD). ( en )
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In the series University of Florida Digital Collections.
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Includes vita.
Bibliography:
Includes bibliographical references.
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Description based on online resource; title from PDF title page.
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This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Thesis:
Thesis (M.S.)--University of Florida, 2014.
Local:
Adviser: HAGELIN,HELENA AE.
Local:
Co-adviser: JIANG,PENG.
Electronic Access:
RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2016-05-31
Statement of Responsibility:
by Xianhe Deng.

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Source Institution:
UFRGP
Rights Management:
Applicable rights reserved.
Embargo Date:
5/31/2016
Resource Identifier:
908645572 ( OCLC )
Classification:
LD1780 2014 ( lcc )

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PREPARATION OF MAGNESIUM OXIDE SUPPORTS FOR OXIDATIVE C OUPLING OF METHANE CATALYSTS By XIANHE DENG A THESIS P RESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2014

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2014 Xianhe Deng

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K eep your eyes on the stars and keep your feet on the ground. Theodore Roosevelt

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4 ACKNOWLEDGMENTS First and foremost, I would like to thank my supervisor, Dr. Helena Hagelin Weaver. When writing this thesis, I can always get support and inspiration from her Without question, the first ackn ow ledgement goes to my lab mate, T rent W. Elkins, who offered professional in sight, guidance and help to assist me finish this thesis The author owes many thanks to many without whom this thesis would not be finished. First, the committee member, Dr. Peng Jiang, provided expert opinions on this thesis. The lab mates at University of Florida Dr. Wei Cheng, H aibin Zheng and Sam antha Robert s offered useful assistance with the experiment. Also, I am thankful to those who keep inspiring me all these years: Dr. Yuan Kou, Dr. Zhirong Liu, Dr. Ding Ma, Dr. Dechun Zou, Dr. Feng Liu, as well as many others.

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5 TABLE OF CONTENTS Page ACKNOWLEDGMENTS ................................ ................................ ................................ ....... 4 LIST OF TABLES ................................ ................................ ................................ ................... 7 LIST OF FIGURES ................................ ................................ ................................ ................. 8 LIST OF ABBREV IATIONS ................................ ................................ ................................ .. 9 ABSTRACT ................................ ................................ ................................ ........................... 10 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .............. 12 Motivation ................................ ................................ ................................ ....................... 12 Oxidative Coupling of Methane ................................ ................................ ..................... 13 Background Information ................................ ................................ ......................... 13 Catalyst ................................ ................................ ................................ .................... 16 Magnesium Oxide ................................ ................................ ................................ ........... 17 Catalyst Characterization Techniques ................................ ................................ ............ 19 X ray Diffraction Analysis ................................ ................................ ...................... 19 BET Surface Area Analysis ................................ ................................ ..................... 19 2 PREPARATION OF MAGNESIUM OXIDE NANOPARTICLES ................................ .. 20 Surfactant ................................ ................................ ................................ ........................ 20 Microemulsion ................................ ................................ ................................ ................ 21 Experiment Procedure ................................ ................................ ................................ .... 23 Results ................................ ................................ ................................ ............................. 24 3 OXIDATIVE COUPLING OF METHANE REACTION ................................ ................. 29 Preparation of OCM Catalysts ................................ ................................ ........................ 29 Oxidative Coupling of Methane ................................ ................................ ..................... 29 Results ................................ ................................ ................................ ............................. 30 Brunauer Emmett Teller (BET) ................................ ................................ .............. 30 Product Analysis ................................ ................................ ................................ ...... 31 Effects of Temperature ................................ ................................ ............................ 32 Effect s of MgO S upports ................................ ................................ ......................... 37 Effects of Li Doping ................................ ................................ ................................ 41 4 SUMMARY ................................ ................................ ................................ ........................ 44 Preparation of Mag nesium Oxide Nanoparticles ................................ ............................ 44

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6 Characterization of Magnesium Oxide Nanoparticle ................................ ..................... 44 Preparation of Oxidative Coupling of Methane Catalysts ................................ .............. 44 Oxidative Coupling of Methane Reaction Over Prepared Catalysts .............................. 44 REFERENCE S ................................ ................................ ................................ ...................... 46 BIOGRAPHICAL SKETC H ................................ ................................ ................................ 51

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7 LIST OF TABLES Table page 2 1 Amount of surfactants used to form the water in oil microemulsion system ........... 24 2 2 Average size of MgO nanoparticles prepared ................................ ........................... 25 3 1 Reaction data of conversion and C2+ yield of the oxidative coupling of methane obtained from MgO supported, Li doped TbO x catalysts and pure MgO cat alysts .. 30 3 2 Reaction data of the oxidative coupling of methane obtained from MgO supported, Li doped TbOx catalysts and pure MgO catalysts after 30 minutes on stream ......... 31 3 3 Reaction data of product selectivity of the oxidative coupling of methane obtained from MgO supported, Li doped TbO x catalysts and pure MgO catalysts ................. 32 3 4 Reaction data of oxidative coupl ing of methane at 700 C ................................ ....... 38

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8 LIST OF FIGURES Figure page 1 1 Schematic diagram of integrated recycle system for con version of methane to aromatics ................................ ................................ ................................ .................... 15 2 1 Schematic representation of the most common self association structures in water, oil or a combination. ................................ ................................ ................................ ....... 20 2 2 Schematic rep resentation of three microemulsion microstructures ............................ 22 2 3 Average size of MgO nanoparticles prepared by single surfactant Tri ton X 45 at different fractions. ................................ ................................ ................................ ...... 25 2 4 Average size of MgO nanoparticles prepared by surfactant Triton X 45 and cosurfactant 1 Dodecanol. ................................ ................................ ......................... 26 2 5 XRD data obtained from MgO nanoparticles prepared by 15% Triton X 45. ........... 27 2 6 XRD data obtained from MgO nanoparticles prepared by 15% Triton X 45 and 1 Dodecanol, and Triton X 45/ 1 Dodecanol ratio is 80:20. ................................ ........ 27 3 1 Reaction data of oxidative coupling of methane over Li TbOx/s MgO as a function of temperature at a CH4:O2 ratio of 4:1 ................................ ................................ .... 33 3 2 Reaction data of oxidative coupling of methane over Li TbO x / t MgO as a function of temperature, the Li TbO x catalysts were supported on MgO nanoparticels prepared with 15% 1 Dodecanol and Triton X 45 at a CH 4 :O 2 ratio of 4:1 ............................. 34 3 3 Reaction data of oxidative coupling of methane over s MgO as a function of temperature at a CH 4 :O 2 ratio of 4:1 ................................ ................................ .......... 36 3 4 Reaction data of C2+ yield as a function of temperature at a CH4:O2 ratio of 4:1 .. 39 3 5 Reaction data of methane conversion as a function of temperature at a CH 4 :O 2 ratio of 4:1 ................................ ................................ ................................ .......................... 40 3 6 Reaction data of methane conversion and yield of C 2+ as a function of temperature (Li TbO x / s MgO). ................................ ................................ ................................ ...... 42

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9 LIST OF ABBREVIATIONS BET Brunauer Emmett Teller C 2 Ethane + E thylene C 2 + Ethane, Ethylene and other long chain compounds GC Gas Chromatography IWI Incipient Wetness Impregnation OCM Oxidative Coupling of Methane o/w O il in water TPD Temperature Program Desorption Triton X 45 4 (1,1,3,3 Tetramethylbutyl)phenyl polyethylene glycol, Polyethylene glycol 4 tert octylphenyl ether w/o Water in oil XRD X ray Diffraction

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10 Abstract of Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science PREPARATION OF MAGNESIUM OXIDE SUPPORTS FOR OXIDATIVE COUPLING OF METHANE CATALYSTS By Xianhe Deng May 2014 Chair: Helena Hagelin Weaver Major: Chemical Engineering A series of nano sized magnesium oxide particles with both high and low surface areas w ere successfully prepared by a simple water/toluene reverse microemulsion method using triton X 45, 1 Dodecanol as surfactant. By controlling the amount of surfactant, it is possible to control the size of MgO nanoparticles from 20 nm to 50 nm. The size w as confirmed by X ray diffraction (XRD). In order to investigate the reactivity of OCM reaction over the magnesium oxide support a series of catalysts were also synthesized. High surface area MgO samples prepared by microemulsion we re used as support for Li TbO x In order to get the largest coverage of the MgO support and the smallest Li TbO x particles a nitrate precursor was used during incipient wetness impregnation followed by calcination at 800 C for 4 hours Another MgO support ed Li TbO x catalyst was prepared using the same method with the low surface area MgO nanoparticles Low surface area MgO nanoparticles were also used as OCM catalysts after calcination at 800 C for 4 hours N 2 adsorption (BET) resu lts showed that the sur face area of the catalysts supported on MgO nanoparticles prepared using 15 wt% Triton X 45 was as low as 1.0 m 3 /g while the surface area of the catalyst prepared using MgO particle, synthesized with 15 wt% Triton X 45 and 1 Dodecanol mixture was 9.3 m 3 /g.

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11 Oxidative coupling of methane (OCM) catalyzed by the prepared catalysts was studied using a continuous flow quartz reactor at atmospheric pressure, 500 800 C at a CH 4 :O 2 ratio of 4:1 The methane conversion reached 21.9% at 700 C with a C 2+ selectivi ty as high as 65.5%. Thus, a maximum C 2+ yield of 13.5% was realized. However, there was a rapid deactivation during the OCM over Li doped catalysts due to the loss of Lithium.

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12 CHAPTER 1 INTRODUCTION Motivation The relative abundance of methane makes it an ideal raw material to produce more valuable or ea sily transport chemicals. Several technologies are being investigated to realize this goa l Some of the most widely studied subjects can be roughly classified into direct conversion of methane and indirect conversion of methane. Direct conversion of methane includes (1) the direct conversion from methane to formaldehyde and methanol by oxidati on, (2) the oxidative coupling of methane to generate ethane and ethylene, (3) direct conversion to hydrogen and aromatics without oxygen [1] The indirect conversion usually consists of synthesis gas production, followed by methanol or Fischer Tropsch syn thesis As the world s top commodity chemical, e thylene is the target product o f oxidative coupling of methane Ethylene derivatives are widely used in automobile manufacturing, health care devices, lubricates, industry packaging etc [2] The consumption of ethylene is massive in the chemical industry Currently, ethylene is mainly produc ed by steam cracking of naphtha, which c onsumes large amounts of energy and releases greenhouse gas heavily As the simplest and most direct method to produce ethylene t he reaction of ox idative coupling of methane offer s a promising way to take advantage of the abundance of natural gas On the other hand oxidative coupling of methane is still a big challenge for scientists nowadays After decades of i nvestigation researchers fa iled to find a way to synthesize commercial catalyst s for oxidative coupling of methane, as the required selectivity for commercial scale production is still not m et In the previous oxidative coupling of methane

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13 reaction s most of the meth ane was converted to carbon dioxide because of poor selectivity Researchers are still trying to find novel OCM catalysts with a high activity and selectivity [7] One of the major goals of modern research in oxidative coupling of methane c atalysis is to achieve high activity (conversion of methane) and selectivity to C 2+ [3,4] Scientists believe that this goal can be achieved in multiple ways. For example, changing the morphology and shape of the catalyst, try ing new catalyst materia l s contro l l ing the catalyst structure at atomic level, choosing dif f erent support for catalysts etc [5,6] To find the optimal catalysts, one must choose the right support for catalysts. This step is the focus of this study, which is to design, prepare char acterize and test an MgO support for oxidative coupling methane catalysts. Oxidative Coupling of Methane Background Information The first publication on oxidative coupling of methane came out in the early 1980s [8] For its direct conversion from natural gas to ethane oxidative coupling of methane has received a great amount of attention. However, because of the following reasons, (1) C H bonds (435 kJ/mol) are extremely stable (2) T here is no polar distributions or magnetic mom ent in CH 4 molecule (3 ) There is no functional group on CH 4 (4) N early all the conversion products of methane are more reactive than methane [9 11, 13] the oxidative coupling of methane is still one of the most challenging reactions in heterogeneous catalysis so f ar T he mechanism for oxidative coupling of methane is complex. It is commonly recognized as a heterogeneous homogeneous radical reaction induced by active surface oxygen [ 10 14]

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14 In the case of OCM reaction over Sm 2 O 3 catalysts active sites are form ed from the water desorption of two Sm 3+ OH After ad sorbing oxyg en, the active site is able to capture hydrogen. As soon as the active site is exposed to methane, CH 3 radical is generated by CH 4 giving away a hydrogen atom Ethane i s produced by the continuous collision of two CH 3 radicals The mechanism of this catalytic ethane production on surface is shown below as equations V stands for oxygen vacancy on surface [ 10, 17] 2 Sm 3+ V (s)+ O 2 2 Sm 3+ O (s) Sm 3+ O (s) + C H 3 Sm 3+ OH (s) + C H 3 2 Sm 3+ OH Sm 3+ O (s) + Sm 3+ V (s) + H 2 O 2 C H 3 C 2 H 6 The whole oxidative coupling of methane reaction path can be denoted as below. First, ethane is formed by the collision of two CH 3 radicals 4CH 4 + O 2 2C 2 H 6 + 2H 2 O H 25 = 177 kJ/mol 800 174.3 kJ/mol At high temperature, ethylene is generated by the d ehydrogenation of ethane 2 C 2 H 6 + O 2 2 C 2 H 4 + 2 H 2 O 25 = 105 kJ/mol 800 103.9 kJ/mol Besides this reaction, there are byproducts, such as CO 2 CO, H 2 generated from side reaction. CH 4 + 2O 2 2 + 2H 2 O 2 CH 4 + 3 O 2 2 CO + 4 H 2 O CH 4 + H 2 3 H 2

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15 OCM reaction is a highly exothermic reaction. Along with other side reactions, there is a large amount of heat released. Therefore, the temperature in the catalyst bed can be more than 150 C higher tha n the outside temperature [15,16 ], making it is a challenge to control the heat. Figure 1 1 Schematic diagram of integr ated recycle system for conver sion of methane to aromatics. C) Mass flow controller. F) F low me ter. P ) Gas sampling port. R) P ressure regulator [17] As a derivative of the OCM reactor researchers have designed a recycle system to convert methane to aromatics. In this system, a Ga/H ZSM 5 zeolite catalyst was used to convert

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16 the C 2 H 4 from the OCM reactor, and benzene and toluene were the target product [17 ]. A scheme of this recycle is presented in Fig. 1 1 Catalyst Since 198 0s, there are hundreds of OCM catalysts that have been studied. Generally speaking, they can be classified into four categories: 1) reducible metal oxides, 2) nonreducible metal oxides, 3) halogen containing oxide materials, and 4) solid electrolytes [13]. Rare earth metals, such as La 2 O 3 and Sm 2 O 3 are widely used a s catalysts fo r oxidative c oupling of methane. Among many rare earth metal catalysts, SrO/ La 2 O 3 was studied at 800 ~ 850 C by L. Yu, etc. This catalyst can achieve a 30.1% conversion at 85.6% selectivity [18, 19] For Re 2 O 3 and ReOF (Re could be any rare earth element) catalysts, the highest yield could reach 19% [20]. Also, extensive research has been done on alkaline earth metals. SiO 2 supported catalysts Mn(NO 3 ) 2 and Y 2 WO 4 or X 2 MoO 4 (X=Li, Na, K, Rb, Cs; Y=K and Ca), were prepar ed [ 10 ,21, 22] The ef fects of reaction temperature ( 700 and 750 C ) reactant flow rate ratio and other factors have been investigated on a lkali metal doped CaO catalysts [23] One of the most active and selective OCM catalysts was found to be Mn/Na 2 W 4 /SiO 2 which was also proved to be long time stable At 800 C the selectivity to C 2+ products was 25%, but the conversion was only 5%. In the OCM reaction over Mn/Na 2 WO 4 /SiO 2 catalysts, the mechanism of deep oxidation with CO 2 participation was proposed: CO 2 O* + CH 4 CH 3 + OH* 2 CH 3 2 H 6 O* + C 2 H 6 2 H 4 + H 2 O + (S)*

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17 C 2 H 6 2 H 4 + H 2 The symbol denotes the active site on the surface of Mn/Na 2 WO 4 /SiO 2 [24] Oxidative coupling of methane was carried out over the catalyst PrOx/Mg, while d oped Na could increase the C 2+ selectivity at the expense of a decrease in conversion [25] Na/CaO was also chosen as a MCO catalyst. The selectivity to eth ane and ethylene could be as high as 80% at 10.5% conversion with the addition of Cerium [26]. WO 3 /ZrO 2 impregnated with Ce or Mn together with NaCl also has high activity [27] Li doped MgO was proved to be an active catalyst for OCM. An important finding about Li doped M gO was that the loss of activity was fast during OCM reaction. However, the selectivity was not effected [28] This phenomenon could be explained as a rapid loss of active sites on the surface of Li doped MgO catalysts. Regardless of its own OCM activity MgO was selected as an ideal support for rare earth metal catalysts in the OCM reaction, due to its basic properties and effectiveness. To activate the C H bond OCM reaction requires high temperatures ( ), and the melting point for MgO is 2 Moreover MgO was inexpensive and readily available [29 32] Magnesium Oxide Nanostructured magnesium oxide (MgO) has attracted tremendous research interests because of its excellent properties, such as high melting point (2830 ), low heat capacity, high catalysis activity, high secondary electron emission, as well as strong microwave adsorption ability [37 39] With these properties, MgO is an ideal material for various fields. For instance, MgO is mainly employed in areas like superconduct or production, catalysis, refractory material toxic waste remediation, pharmaceutical industry and paint production [35 37] In literature, various route s were reported to obtain MgO successfully As one of the most popular methods, MgO is usually obtained from the calcination of Mg(OH) 2

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18 Mg ( OH ) 2 2 O Decomposition of MgCO3 is also frequently used to prepare MgO [41,42 ] Mg CO 2 2 Many other strategies are also be used to synthesize MgO nanoparticles, such as microwave assisted aqueous wet chemical, surfactant method, laser vaporization, sol gel ch emical gas phase deposition polyol mediated thermolysis process, etc [40, 42 44] Diverse MgO nanstructures including nanowire, nanoporous nanorods, nanoflower, nanorods, nanoplate, w ere obtained through different methods with controlled morphologies [45] Ma et al. fabricated MgO nanobelts by the evaporation of Mg metals The process began with the evaporation of pure Mg metals under N 2 atmosphere at 650 C, followed by switching to N 2 /O 2 mix gas at 800 C [45] According to the research of Yu et al. commercial bulk MgO crystals were used to prepare porous MgO nanoplates via h ydrothermal recrystallization [47, 48]. The effect of precursors and solvents on this process was also investi gated [46] Zhu et al. prepared fishbone like MgO nanos sized rom [ 49 ] Zhao et al. synthesized rectangular MgO nanoparticles by a chemical vapor deposition method which was simple and quick [50] Because of its simplicity low cost and effectiveness, the sol gel process was chosen to synthesize MgO nanoparticles from magnesium ethoxide. Hydrolysis catalysts, such as HCl, C 2 H 4 O 2 C 2 H 2 O 4 or N H 4 OH were selected to control the particle size [51] After t hermal decomposition treatment at 450 560 C MgO could be obtained from magne sium alcoholate vi a alcoxy/hydroxy intermediates. The particle size could be controlled by using precursors with different a lcohol chain length [ 52 ]

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19 Catalyst Characterization Techniques X ray Diffraction Analysis The XRD data was gathered on a Phillips APD 3720 X ray diffr action with Cu radiation. The XRD spectrum was recorded in a 2 range of 20 to 80 at ambient conditions A verage crystallite size was estimated with the Scherrer equation. In the Scherrer equation, is the crystallite size is the shape factor is the Cu radiation wavelength, is the peak width at half the maximum intensity in radians, and is the Bragg angle, which is h alf of the diffraction angle (2 ). e qual s to 0.154 nm and is a constant generally taken as unity. BET S urface Area Analysis Brunauer Emmett Teller (BET) surface area measurements were performed on a Quantachrome NOVA 1200.

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20 CHAPTER 2 PREPARATION OF MAGNESIUM OXIDE NANOPARTICLES Surfactant The conventional surfactant molecule generally has a polar (hydrophilic) head group region and a n aliphatic (hydrophobic) tail region, which usually has a long chain. Once dispersal in water/oil mixtures, surfac tant molecule s concentrate at the oil water interface to reduce the interfacial tension. In this process, the orientation of the surfactant molecules are reversed compared to the situation in aqueous solution [53]. Figure 2 1. Schematic representation of the most common self association structures in water, oil or a combination A) Surfactant molecules. B) Spherical micelle. C) Rod shaped micelle. D) Hexagonal phase. E) Lamellar phase. F) Reverse hexagonal phase. G) Reverse micelle [53]. Surfactants can form a series of phase structures. Fig. 2 1 shows schematic representation of the most commonly encountered self association structures in water, oil or a combination

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21 Generally speaking, surfactants can be grouped into two categories, whi ch are ionic and non ionic. Ionic surfactants includes sodium bis(2 ethylhexyl) sulfosuccinate (AOT), Cetyl trimethylammonium bromide (C 16 H 33 (CH 3 ) 3 N + Br CTAB), perfluoropolyether carboxylic acid (PFPE), sodium lauryl sulfate (SLS), etc. Some commonly us ed nonionic surfactants are Triton X 100 [Po lyoxyeth ylene(9)4 (1,1,3,3 tetramethyl butyl)phenyl ether], Poly(oxyethylene) 5 nonylphenol ether (NP 5), pol y(oxy ethylene) 9 nonylphe nol ether (NP 9), poly(oxyethy lene) 12 nonylphenol ether (NP 12), etc. CeO 2 nano particles, Ce 1 x Zr x O 2 bimetal nanoparticles, Ce T b mixed oxides nanoparticles have been obtained in microemuls ion with Triton X 45 as surfactant [54 57]. M icroemulsion According to the theory of Danielsson and Lindman m icroemulsion was defined as a system of water, oil and amphiphile which is a single optically isotropic and thermodyna mically stable liquid solution. Surfactants were usually used to stabilize the single phase microemulsion systems [58] In as early as 1940s, Hoar and Schulman clai med to find a clear si n gle phas e solution by titratin g hexanol to a milky emulsion [59 ]. Since then, the properties of microemu l sion have been extensively investigated. There are multiple microemulsion microstructures, and some of the most frequently occur red microstructures are: (1) oil in water microemulsion, (2) water in oil microemulsion, (3) bicontinuous microemulsion. Fig. 2 2 shows schematic representations of t hem. It is widely accepted that the microemulsion system contains numerous microstructur es, which allows microemulsion to perform as chemical reactors. The special interfacial properties of microemulsion system make the intimate contact between hydrophilic phase and hydrophobic

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22 phase possible. Usually, the water phase is dispersed in the cont inuous oil phase. Surfactants are relocated to the interface between the aqueous phase and organic phase. The water phase exists as microdroplet, which acts like a microreactor. If there are reactants dissolved in the aqueous phase, synthesis of nanoparti cles is available. While collisions among these microdroplets happens continuously during reaction, coalescence and fusion take place because of the dynamic nature of microdroplets. Finally, products are prepared in the scale of nanometer [60] Figure 2 2 Schematic representation of three microemulsion microstructures A) O il in water microemulsion. B) Bicontinuous microemulsion. C) W ater in oil microemulsion [53] No matter the microemulsion contains oil in water (o/w) microdroplets or water in oi l (w/o) micro droplets there is always an interfacial surfactant between the aqueous and hydrocarbon phases. Microemulsion with high oil/water ratio usually contains water in oil (w/o ) microdroplets, and it is named reverse microemulsion. On the contrary if the oil fraction is low, oil in water microemulsion is likely to occur When the amount of water is close to that of oil, there is a great chance to obtain a bicontinuous microemulsion. In order to prepare metal oxides nanoparticles, reverse micro emul sion is frequently selected because reactants are usually dissolved in the aqueous phase. Therefore, aqueous phase should be dispersed in the continuous hydrocarbon phase, which requires the microemulsion to be reverse, not the other way around.

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23 Instead of using just one kind of surfactant, medium chain length alcohols which are named cosurfactant s are usually employed to reduce the i nt e r facial tension [ 61 ]. Despite of increasing the entropy, adding m edium chain length alcohols not only increas es the m obility of the hydrocarbon tail, but also makes it easier for the oil to penetrate into the interface [62 ] Experiment Proce dure MgO nanoparticles were obtained by calcination of MgOH, which was first prepared by a water in oil microemulsion method. The re were two different microemulsion system s used. One of them was made up of toluene, water, Triton X 45 [ 4 (1,1,3,3 Tetramethylbutyl)phenyl polyethylene glycol, Polyethylene glycol 4 tert octylphenyl ether ]. The other one was made up of toluene, water, Triton X 45 [ 4 (1,1,3,3 Tetramethylbutyl)phenyl polyethylene glycol, Polyethylene glycol 4 tert octylphenyl ether ] and 1 Dodecanol. Mg(NO 3 ) 2 was selected as the precursor. First of all, the toluene and surfactant ( Triton X 45 or 1 Dodecanol) were w ell mixed. The micro emulsion system was formed by titrating Mg(NO 3 ) 2 solution to the organic phase at a steady and appropriate rate Then NaOH solution was introduced to the microemulsion system by titration. The reaction system was sealed for 2 hours und er stirring. Ethanol was used to terminate the reaction. The product MgOH was washed with ethanol and water. A sonicator bath was used in the washing procedure to disperse Mg(OH) 2 products. When dispersed in the solvent completely, products were sent to c entrifuge. After the washing procedure was over, MgOH products were left in oven at 105 C overnight. Finally, MgO nanoparticles were synthesized from the calcination of MgOH for 4 hours at 400 C In o rder to study the effects of surfactants Triton X 45 and 1 Dodecanol on microemulsion formation different amounts of these two surfactants were used to form the

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24 microemulsion system ( Table 1). Triton X 45 was chosen to be the main surfactant, and 1 Dodecanol acted as the cosurfac tant. Table 2 1. Amount of surfactants used to form the w ater in oil microemulsion system Surfactant Total amount b wt% X 45 b wt.% 1 Dodecanol b wt.% Ratio a 10 .0 10 .0 0 15 .0 15 .0 0 18 .0 18 .0 0 20 .0 20 .0 0 15 .0 14.25 0.75 95:5 15 .0 13.5 1.5 90:10 15 .0 12.0 3.0 80:20 a the ratio is the weight of X 45 over the weight of 1 Dodecanol b the total amount is calculated from the total weight of surfactants divided by the total weight of microemulsion system The MgO nanoparticles prepared were characterized by BET Results In the current reverse microemulsion system, toluene act ed as a continuous hydrocarbon phase, in which Triton X 45 and 1 Dodecanol were dissolved. Nano size water droplets containing the rea ctants were stabilized by Triton X 45 and 1 Dodecanol A Phillips APD 3720 X ray diffractometer with Cu K radiation was used for X ray diffraction analysis on the dry MgO powders. is equal to 0.154 nm. A n average crystallite size was estimated with the Scherrer equation In the Scherrer equation, is the crystallite size K is the shape factor is the Cu K radiation wavelength, is the peak width at half the maximum intensity in radians, and is the Bragg angle, which i s half of the diffraction angle (2 ) [63]

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25 Table 2 2 Average size of MgO nanoparticles prepared A mount b wt% Surfactant Size [nm] c X 45 a wt .% 1 Dodecanol a wt.% Ratio b 10.0 10.0 0 50.6 15.0 15.0 0 42.0 18.0 18.0 0 32.8 20.0 20 .0 0 34.7 15.0 14.25 0.75 95:5 32.8 15.0 13.5 1.5 90:10 27.2 15.0 12.0 3.0 80:20 23.6 a T he total amount is calculated from the total weight of surfactants divided by the total weight of microemulsion system b The ratio is the amount of X 45 over the amount of 1 Dodecanol c Determined using XRD Figure 2 3 Average size of MgO nanoparticles prepared by single surfactant Triton X 45 at different fractions A series of MgO nanoparticles were obtained from the calcination of MgOH prepared by microemulsion and the size of MgO nanoparticles was estimated by the results of XRD ( Table 2 2 ). By using different amount s of s urfactants used in the microemulsion, it was possible to control the size of MgO nanoparticles. When the surfactant Triton X 45 only accounted 10 wt% of the total microemulsion, the size of MgO nanoparticles obtained was around 50.0 nm. While 20 25 30 35 40 45 50 55 8 10 12 14 16 18 20 22 Size [nm] Amount of Triton X 45 [% ]

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26 increasing the fraction of Tr iton X 45 to further stabilize the microemulsion system, the size of Mg O nanopa rticles decreased fro m 5 0nm to 3 0 nm as expected Higher fraction of surfactant le d to form smaller nano sc ale aqueous microdroplets, thus smaller MgO nanoparticles were synthe siz ed Figure 2 4 Ave rage size of MgO nanoparticles prepared by surfactant Triton X 45 and cosurfactant 1 Dodecanol T he total fraction o f surfactants was fixed at 15 wt% in order to learn the role of cosurfactant 1 Dodeconal In this case, Triton X 45 was still used as the primary surfactant, and 1 Dodecanol was expected to act as the cosurfactant. According to the results from table 2 2 even though only a small amount of cosurfactants 1 Dodecanol were added to the microemulsion system, it seemed to make a big difference t o the synthesis of MgO nanoparticles. If only 5% of the total surfactants were cosurfactants 1 Dodecanol, the average size of M gO nanoparticles dropped from 40 nm close to 25 nm which was much smaller than the MgO nanoparticles prepared by 15% pure Triton X 45 surfactan ts. The size of MgO nanoparticles could be controlled within around 20 nm by increasing the fraction of 1 Dodecanol In addition of 20 25 30 35 40 45 0 5 10 15 20 25 Size [nm] Fraction of 1 Dodecanol [%]

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27 cosurfactant 1 Dodecanol, the oil/water interfacial tension was further reduced to form a more stable microemulsion. Figu re 2 5 XRD data obtained from MgO nanoparticles prepared by 15% Triton X 45. Figure 2 6. XRD data obtained from MgO nanoparticles p repared by 15% Triton X 45 and 1 Dodecanol, and Triton X 45/ 1 Dodecanol ratio is 80:20. The XRD patterns of selected MgO n anoparticles are shown in Fig. 2 5 and 2 6 The main phases present in both figures are detected to be cubic MgO. By comparing Fig. 2 5 and F ig 2 6 there are a litt le more impurity peaks in Fig. 2 -5 More than one type of crystalline existed in p roducts prepared by simple Triton X 45. This difference might be caused by the 20 25 30 35 40 45 50 55 60 65 70 Intensity ( Arbitrary Unitd) 2 (degrees) 20 25 30 35 40 45 50 55 60 65 70 Intensity (Arbitrary Unit) 2 (degrees)

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28 different stabilities in two microemulsion systems In the microemulsion system with both Triton X 45 and 1 Dodecanol, the interfac e between oil and water phases was reduced to a more stable level. Thus, the aqueous microdroplets in this microemulsion system were likely to be more uniform in size an d volume. Because of this, produ cts prepared by this method were more uniform.

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29 CHAPTER 3 OXIDATIVE COUPLING OF METHANE R EACT ION Preparation of OCM Catalysts Nano size MgO supported Li TbO x catalysts were prepared via incipient wetness impregnation method Terbium nitrate hydrate (99.9%, Alfa Aesar) and Lithium nitra te hydrate (99.9%, Alfa Aesar) were used as precursors A precise amount of Terbium nitrate hydrate and Lithium nitrate hydrate was dissolved into a certain amount of DI water Most importantly, the volume of the DI water used to dissolve precursors should be extre mely equal to the total volume of the pores of the nano size MgO support s Then, this solution was added to the nano size d MgO support s under continuous mixing until incipient wetness. The catalysts were dried in oven at 80 o C for 2 hours and kept in oven overnight at 105 o C, followed by calcination at 800 o C for four hours. The weight percentage of the Terbium oxide was 20% and the lithium was 2.5% Finally, the composition of catalysts was 2.5/20/77.5 Li Tb O x / MgO. Two kinds of MgO nano particles prepared by microemuls ion were both used in this procedure. The single MgO OCM catalysts were obtained aft er calcination at 800 o C for four hours [10] Oxidative Coupling of Methane Catalytic activity behaviour was investigated in a quartz tube reactor at atmosphere pressure First, catalyst powders were com pressed into a pellet with a Carver pellet press. The pellet was crushed and sieved to collect catalysts from 180 to 250 microns. 0.05 g of the sieved catalyst was then loaded in the tube reactor betwe en two pieces of quartz wool A mixture of methane, oxygen and nitrogen was fed thr ough the system at a rate of 100 standard cm 3 (with N 2 constant at 23.2 standard cm 3 ) using th ree mass flow controllers The nitrogen served as an internal standard. The CH 4 :O 2 feed ratio was fixed at 4:1 all the time as this is the stoichiometric

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30 amount needed for conversion of methane to ethane and water, and it was also shown to give the highest CH 4 conversion at a reasonable C 2 selectivity [10]. The catalytic activity and selectivity were evaluated as a function of temperature. Starting at 5 00 C, the temperature was increased by intervals of 50 C and measur ements were taken up to 800 C Results Brunauer Emmett Teller (BET) The surface area of Li doped catalysts w ere measured by Brunauer Emmett Teller (BET) with a 6 point isotherm on a Quantachrome NOVA 1200 This instrument operat es at a liquid nitrogen environment All correlation values were greater than 0.999. Table 3 1. Reaction data of conversion and C2+ yield of the oxidative coupling of methane obtained from MgO supported, Li doped Tb O x catalysts and pure MgO catalysts Catalyst a Teperature [ C ] [%] b [%] c [%] d [%] e Li TbO x / s MgO s 650 0.9 0.9 0.0 0.0 700 21.9 20.6 65.5 13.5 750 16.5 15.4 63.2 9.7 800 15.4 14.1 60.8 8.6 Li TbO x / t MgO t 650 1.0 0.6 0.0 0.0 700 4.2 3.4 36.7 1.3 750 21.3 19.2 63.6 12.2 800 18.1 16.1 60.3 9.7 s MgO s 650 1.0 0.3 0.0 0.0 700 2.1 1.7 0.0 0.0 750 3.3 2.8 40.5 1.1 800 16.6 14.3 61.3 8.7 a Terbia weight fraction is 20% Li weight fraction is 2.5%. b O verall methane conversion. Calculated as ( CH 4in CH 4out )/ CH 4in c G as phase CH 4 conversion to CO x and C 2 products only d Percent of CH 4 converted to C 2 products e C alculated as s MgO nanoparticles prepared with single Triton X 45 as surfactant ( 15 wt.%) t MgO nanoparticles prepared with Triton X 45 and 1 Dodecanol (15% total amount, the ratio of Trito n X 45 : 1 Dodecanol is 90:10

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31 Product Analysis Table 3 2. Reaction data of the oxidative coupling of methane obtained from MgO supported, Li doped TbOx catalysts and pure MgO catalysts after 30 minutes on stream Product Distribution [%] Catalyst a Reaction Temp [ o C] C 2 H 4 C 2 H 6 CO 2 CO C 2 Yield [%] [%] Li TbO x / s MgO s 650 b 0.0 0.0 100.0 0.0 0.0 1.0 650 c 0.0 0.0 100.0 0.0 0.0 0.9 700 b 28.6 37.2 33.8 0.4 15.7 24.9 700 c 26.6 37.5 35.0 0.9 11.2 18.9 7 50 b 29.1 33.8 35.3 1.7 10.2 17.5 750 c 28.7 33.9 35.7 1.7 9.1 15.5 800 b 32.7 28.5 35.3 3.5 9.1 16.1 8 00 c 14.8 32.6 27.1 36.6 3.8 7.9 Li TbO x / t MgO s 650 b 0.0 0.0 96.9 3.1 0.0 1.0 650 c 0.0 0.0 96. 6 3.4 0.0 0.9 700 b 3.7 33.1 60.8 2.5 1.3 4.3 700 c 3.8 32.9 60.8 2.6 1.3 3.8 7 50 b 27.9 35.5 35.9 0.8 14.3 25.0 750 c 27.7 35.4 35.4 1.5 10.0 17.6 800 b 31.6 28.7 36.5 3.3 10.2 19.1 8 00 c 29.9 29.6 36.9 3.7 9.0 17.0 s MgO s 700 b 0.0 0.0 85.7 14.3 0.0 2.2 70 0 c 0.0 0.0 85.4 14.6 0.0 2.0 800 b 34.9 26.1 34.2 4.7 9.6 18.1 8 00 c 33.7 26.9 34.1 5.3 7.8 15.2 a Terbia weight fraction is 20% Li weight fraction is 2.5%. b Initial reaction data. c Reaction data after 30 minutes on stream. d Percent of CH 4 converted to C 2 products. e C alculated as s MgO nanoparticles prepared with single Triton X 45 as surfactant ( 15 wt.%). t MgO nanoparticles prepared with Triton X 45 and 1 Dodecanol (15% total amount, the rati o of Triton X 45 : 1 Dodecanol is 90:10.

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32 The products of OCM were detected and analyzed b y an on line gas chromatograph. Table 3 1 gives some major statistics, such as methane conversion, C 2+ selectivity, C 2+ yield, of the OCM over Li doped catalysts. Table 3 2 gives information about the product distribution at different temperatures, and the ratio of ethane and ethylene was also calculated. Table 3 3. Reaction data of product selectivity of the oxidative coupling of methane obtained from MgO support ed, Li doped Tb O x catalysts and pure MgO catalysts Catalyst a Teperature [ C ] CO 2 / CO C 2 H 4 / C 2 H 6 Product Selectivity(%) C 2 H 4 C 2 H 6 CO 2 CO Li TbO x / s MgO s 650 0.0 0.0 100.0 0.0 700 57.2 0.7 27.8 37.3 34.3 0.6 750 20.9 0.8 29.0 33.9 35.5 1.7 800 10.0 1.1 32.6 27.9 35.9 3.6 Li TbO x / t MgO t 650 0.0 0.0 96.8 1.6 700 24.3 0.1 3.7 33.0 60.8 2.5 750 32.5 0.7 27.8 35.4 35.7 1.1 800 10.5 1.1 30.8 29.1 36.7 3.5 s MgO s 650 0.0 0.0 85.3 7.3 700 5.9 0.0 0.0 85.5 14.5 750 5.1 0.2 6.1 33.7 50.3 9.8 800 6.8 1.3 34.4 26.5 34.2 5.0 a Terbia weight fraction is 20% Li weight fraction is 2.5%. s MgO nanoparticles prepared with single Triton X 45 as surfactant ( 15 wt.%) t MgO nanoparticles prepared with Triton X 45 and 1 Dodecanol (15% total amount, the ratio of Trit on X 45 : 1 Dodecanol is 90:10. Effects of T emperature F or convenience, MgO nanoparticles prepared with single Triton X 45 ( 15 wt% ) as surfactant is denoted as s MgO, and MgO nanopart icles prepared with Triton X 45 ( 13.5 wt% ) and 1 Dodecanol ( 1.5 wt% ) is denoted as t MgO. The CH 4 conversion, C 2 + selectivity, and C 2 + product yield (defined as the product of the CH 4 conversion multipl ied by the C 2+ and C 2 + selectivity) as a funct ion of reaction temperature for Li TbO x /s MgO, Li TbO x /t MgO and s

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33 MgO are presented in Figure 3 1, Figure 3 2 and Figure 3 3 separately. The three catalysts have quite different performances along with reacti on temperature. Figure 3 1 Reaction data of oxidative coupling of methane over Li TbOx/s MgO as a function of temperature at a CH4:O2 ratio of 4:1. A) Methane conversion ( ). B) Methane conversion to C 2+ products ( ). C) Selectivity to C 2+ products ( ). D) C 2+ product y ield ( ). In literature, there are very few publications about the performance of T erbia as an OCM catalysts In the previous study of our lab, it was found that Li doped Terbia catalysts are promising OCM catalysts with relatively high activity and selectivity From Fig. 3 1, t he Li TbO x /s MgO catalyst has low activity below 700 C Only very little methane is converted to CO 2 and there are n o other products like ethane or ethylene The temperature must be above 65 0 C for this catalyst to exhibit a significant activity However, once the catalyst is activated above 65 0 C the activity and selectivity increase very rapidly between 650 C and 700 C. The slope of methane conversion and C 2+ selectivity is sharp 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 500 550 600 650 700 750 800 850 Conversion rate/Yield [%] Temperature [ C ] A B C D

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34 between 650 C and 700 C in Fig. 3 1. The maximum C 2 + yield (13.5%) from the Li TbO x /s MgO catalyst is obtained at 700 C where the CH 4 conversion is 21.9% and the C 2 + selectivity 65.5 %. If the temperature is increased above 700 C there is a slight decrease in selectivity, while the CH 4 conversion drops rather rapidly to 16.5%. The decrease of methane conversion is likely due to the removal of Li from the ca talyst surface, caused by the extended time on stream and the increasing temperature. Figure 3 2 Reaction data of oxidative coupling of methane over Li TbO x / t MgO as a function of temperature, the Li TbO x catalysts were supported on MgO nanoparticels p repared with 15% 1 Dodecanol and Triton X 45 at a CH 4 :O 2 ratio of 4:1. A) Methane conversion ( ). B) Methane conversion to C 2+ products ( ). C) Selectivity to C 2+ products ( ). D) C 2+ product y ield ( ). At low temperature, ethane is more thermally preferable to form by oxidative coupling of methane. 0.00 10.00 20.00 30.00 40.00 50.00 60.00 70.00 500 550 600 650 700 750 800 850 Methane conversion/Yield [%] Temperature [ C ] A B C D

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35 4CH 4 + O 2 2C 2 H 6 + 2H 2 O, H 25 = 177 kJ/mol, 800 174.3 kJ/mol At high temperature, ethylene is generated by the d ehydrogenation of ethane [15]. 2 C 2 H 6 + O 2 2 C 2 H 4 + 2 H 2 O, 25 = 105 kJ/mol, 800 103.9 kJ/mol As expected, the C 2 H 4 :C 2 H 6 ratio increases with temperature from table 3 3 and more C 2 H 4 is formed than C 2 H 6 above 800 C What s more, the C 2 H 4 :C 2 H 6 ratio in crease s from 1.3 to 0.9 from 700 C to 800 C. Despite the maximum C 2+ yield is obtained at 700 C, the production of C 2 H 4 which is a more valuable products than ethane in industry, increases with temperature The C 2 H 4 :C 2 H 6 ratio in crease s from 0.7 to 1.1 from 700 C to 800 C. The activity of the Li TbO x /t MgO catalyst is quite different from the s MgO support ed catalysts. Similar to the Li TbO x /s MgO catalyst, it requires a high temperature to activate. There is almost no OCM products below 700 C. A temperature above 700 C is str ictly required for this catalyst to exhibit a significant methane conversion. Compared to the Li TbO x /s MgO catalyst, the increase of activity and selectivity is relatively slower above 700 C. Between 70 0 C and 7 50 C the CH 4 conversion increases from 4. 2% to 21.3 %, and the C 2 + selectivity increases from 36.7% to 63.6 %. T he maximum C 2 + yield (12.2%) is obtained at 750 C Above 7 50 C there is a slight decline in the C 2+ yield and methane conversion due to an increase in both CO and CO 2 product formation. The decrease in C 2+ selectivity with temperature is likely due to the rather low CH 4 :O 2 ratio, as at higher CH 4 :O 2 ratios rare earth catalysts are more likely to exhibit an increase in C 2+ selectivity with temperature since the O 2 conce ntration is low and can limit the CO 2 or Co formation [65 67]

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36 For Li TbO x /t MgO catalyst s, the C 2 H 6 :C 2 H 4 ratio heavily relies on the reaction temperature. This C 2 H 6 :C 2 H 4 ratio increases almost 10 times from 700 C to 800 C which corresponds to the fact that the formation of ethylene is high temperature favorable. When the temperature is increased above 750 C, there is a gradual decrease in selectivity, while the CH 4 conversion drops rather rapidly by nearly 4%. The decrease in selectivity is likely due to removal of Li from the catalyst surface, from the extended time on stream as well as the increasing temperature. Figure 3 3 Reaction data of oxidative coupling of methane over s MgO as a function of temperature at a CH 4 :O 2 ratio of 4:1. A) Methane conversion ( ). B) Methane conversion to C 2+ products ( ). C) Selectivity of C 2+ products ( ). D) C 2+ product y ield ( ). For undoped MgO catalysts, both the methane conversion and selectivity increase with the reac tion temperature. Unlike the Li TbO x /MgO catalysts, it requires a even high er temperature to activate. Surprisingly, the MgO cat alysts become relatively active and selective at 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 450 550 650 750 850 Methane conversion, Yield [%] Temperature [ C ] A B C D

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37 800 C The conversion rat e reached as high as 16.6%. There is also a sharp increase of selectivity above 750 C. There is no deactivation between 700 C and 800 C Early investigators found that doping lithium on magnesium oxide suppor t could produce an active catalyst under OCM conditions with a relatively high C 2 product selectivity [68, 69] Pure basic oxides have been found to be effective catalysts, and the basicity i s commonly regarded as an indicator in the OCM catalyst activity [70]. Researcher s have claimed to increase the C 2 + selectivity and yield of t he catalysts in the OCM process by doping alkali metal to CaO. They believed that doped alkali metals caused a significant increase in the surface basicity and formed strong basic sites. more, they also proved that the addition of alkali metals caused a decrease in the surface area [71] Among the alkali metal doped CaO catalysts, Na CaO (Na/Ca = 0.1, before calcination) catalyst (calcin ation at 750 C), showed best performance (C 2 + selec tivity of 68.8% with 24.7% methane conversion). Because NaOH solution is used to synthesize the MgO with precursor, so sodium exists in the MgO nanoparticles. Thus, the simple MgO catalysts may be regarded as sodium doped MgO catalysts This could explain why MgO catalysts have such a high activity and selectivity at 800 C Effects of MgO S upports T h e size of two different MgO nanoparticles has been confirmed by both XRD and BET. The average size of s MgO nanoparticle is larger than that of t MgO. The effects of MgO support particle size on the Li TbO x catalysts were investigated at a reaction temperature of 700 C (Table 3.4) The activity, selectivity and product distribution are given in Table 3 1. Both Li TbO x / s MgO and Li TbO x / t MgO catalysts are prepared by the same incipient wetness impregnation method The main differences between Li TbO x / s MgO and Li TbO x / t MgO

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38 catalysts are their MgO support s Fig. 3 5 clearly presents that Li TbO x / s MgO catalysts have a much higher activity than Li TbO x / t MgO catalysts at 700 C. Fig 3 4 also indicates that Li TbO x / s MgO catalysts have a much better selectivity and yield than Li TbO x / t MgO catalysts at 700 C Both the maximum methane conversion and C 2+ yield of oxidative c oupling of methane are achieved over Li TbO x / s MgO at 700 C. It seems that larger MgO nanoparticles are more favorable by the oxidative coupling of methane. Table 3 4 Reaction data of oxidative coupling of methane at 700 C Temperature: 700 C Product Selectivity (%) Catalyst a [%] b [%] c [%] d [%] e C 2 H 4 C 2 H 6 CO 2 CO Li TbO x / s MgO s 21.9 20.6 65.5 13.5 27.8 37.3 34.3 0.6 Li TbO x / t MgO t 4.2 3.4 36.7 1.3 3.7 33.0 60.8 2.5 s MgO s 2.1 1.7 85 .5 14 .5 a Terbia weight fraction is 20% Li weight fraction is 2.5%. s MgO nanoparticles prepared with single Triton X 45 as surfactant ( 15 wt.%) t MgO nanoparticles prepared with Triton X 45 and 1 Dodecanol (15% total amount, the ratio of Triton X 45 : 1 Dodecanol is 90:10. According to the XRD measurement, the average size of s MgO is around 40 nm, and the average size of t MgO is close to 20 nm. This estimation corresponds to the BET results. The surface area of Li TbO x / s MgO is 1 m 2 /g, and the surface area of Li TbO x /t MgO is 9 m 2 /g. Not surprisingly, smaller MgO nanoparticles have larger surface area. In the oxidative coupling of methane, higher surface area might lead to low C 2+ selectivity and yield. 4CH 4 + O 2 2C 2 H 6 + 2H 2 O 2 CH 4 + O 2 C 2 H 4 + 2 H 2 O 2 CH 4 + 3 O 2 2 CO + 2 H 2 O CH 4 + 2 O 2 CO 2 + 2 H 2 O The complete oxidation of methane, whose products are CO 2 and H 2 O is what researchers tried to limit for the oxidative coupling of methane. The higher ratio of O 2 : CH 4

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39 reacts the lower C 2+ selectivity will be When oxidation of methane takes place over the of Li TbO x / s MgO catalyst the chance for methane to get complete oxidation is relatively low. Because this catalyst has rather low surface area, the contact time between catalysts and feed gas is instant. Most likely, the methane will be converted to ethane or ethylene instead of CO 2. However, the situation is different for Li TbO x / t MgO Because of its relatively high surface ar ea, the contact time between Li TbO x / t MgO catalysts and feed gas is much longer. Thus, complete oxidation of methane is likely to occur. From Table 3 4, the selectivity of C 2+ of Li TbO x / s MgO is 60.8% at 700 C while the selectivity of C 2+ of Li TbO x / t MgO is only 36.7%. Figure 3 4 Reaction data of C 2+ yield as a function of temperature at a CH4:O2 ratio of 4:1. A) Li TbO x / s MgO B) Li TbO x / t MgO C) s MgO It is also easy to tell the minimum temperature required to activate these two Li doped cataly sts. From Fig. 3 5, Li TbO x / s MgO exhibits a significant conversion above 650 C. 0.0 5.0 10.0 15.0 20.0 25.0 500 550 600 650 700 750 800 850 Conversion rate [%] Temperature [ C ] A B C

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40 However, Li TbO x / t MgO increases to 750 C. In table 3 2, initial reaction data and reaction data after 30 minutes on stream are collected. It is obvious that a decrease in activity happens to occur over all catalysts at any temperature after a certain period of time. At 700 o C the methane conversion of Li TbO x / s MgO drops from 24.9% to 18.9% in just 30 minutes. This situation also applies to Li TbO x / s MgO at other temperature. Unlike Li TbO x / s MgO, Li TbO x / t MgO catalysts seem to be more stable at these high reaction temperatures. Figure 3 5. Reaction data of methane conversion as a function of temperature at a CH 4 :O 2 ratio of 4:1. A) Li TbO x / s MgO B) Li TbO x / t MgO C) s MgO On the hand, nanoscale supports create catalysts with more edges and corners, which can lead to higher performance of the catalyst. However, there is a balance bet ween surface area and activity in this case [72]. The influences of the corners and edges on OCM reaction needs further 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 500 550 600 650 700 750 800 850 Conversion rate [%] Temperature [ C ] A B C

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41 investigation. A high surface area is determined since the selectivity will decrease. Some surface area is required to abstract the hydrogen from methane. Effects of Li Doping In table 3 2, initial reaction data and reaction data after 30 minutes on stream are collected. It is interesting to compare these two sources of data. By comparing the methane conversi o n rate and product selectivity be tween Li TbO x catalyst s and the MgO nanoparticles it is easy to see that doped Li catalysts are active for oxidative coupling of methane from 650 o C to 800 o C A lmost every methane conversion rate and C 2+ yield collected after 30 minutes are smaller than that of the initial reaction data from able 3 2 which is a sign of deactivation. In Fig. 3 6, t he conversion line and C 2+ yield line of initial reaction data are always above that of reaction data after 30 minutes. The loss of Lithium might be the cause of this rapid deactivation The mechanism of oxidative coupling of methane over Lithium is given below [73 75] 2 Li + [*] + O 2 + 2 Li + O 2 2 Li + O Li + O + CH 4 Li + OH + CH 3 Li + OH 2 Li + O 2 + Li + [*] + H 2 O 2 CH 3 2 H 6 According to this mechanism, t he catalysis activity of oxidative coupling of methane is highly relative to the number of active centers on the catalysts surface. It is safe to conclude that more Lithium atoms per surface area th ere are, the higher activity of the catalysts will be. By correl ating electron paramagnetic reso nance (EPR) signals with the methyl radical formation rate, Lunsford et al. proposed th at the active centers for OCM over Li doped catalysts are those [Li + O ] defect s which is a generally accepted mechani sm [73 75]

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42 Li doped MgO was claimed to lose catalysis activity quickly by Ross et al. [ 76 ]. It has also been proved that the catalyst can be regenerated by treatment with CO 2 with the presence of Lithium The deactivation could be slowed down by adding low concentration CO 2 to the reaction system They furthermore concluded that CO 2 temporarily poisons the active site and simultaneously stab ilized it against deactivation [ 76, 77 ]. The selectivity did not change so much in their long term experiments, ther efore the con clusion was that the nature of the active site has not changed much but the number decreased. Figure 3 6 Reaction data of methane conversion and yield of C 2+ as a function of temperature (Li TbO x / s MgO) A ) Methane conversion ( calculated from i nitial reaction data B ) Methane conversion ( calculated from reaction data after 30 minutes on stream C) C 2+ product yield calculated from initial reaction data D) C 2+ product yield calculated from initial reaction data reac tion data after 30 minutes on stream However, Kimble and Kolts showed that Li is lost from the catalysts after calcination at 850 C [78]. Moreover, Mirodatos and co workers showed that Li doped MgO suffers from severe deactivation due to sintering and loss of Li. 0.0 5.0 10.0 15.0 20.0 25.0 30.0 600 650 700 750 800 850 Conversion rate/Yield [%] Temperature [ C ] A B C D

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43 However, Kimble and Kolts showed that Li is lost from the catalysts after calcination at 850 C [78]. Moreover, Mirodatos and co workers showed that Li doped MgO suffers from severe deactivation due to sintering and loss of Li. Another important find ing was that the use of experi mental equipment made of quartz glass is detrimental to the stability of Li doped MgO [ 79,80 ]. It was s hown that the catalyst deactivates due to a loss of Li as the volatile LiOH or as Li 2 SiO 3 due to the fact that many laboratory reactors are made of quartz glass. However, this is not limited to quartz devices, the Li caused problems in reactors made of Al 2 O 3 Al 2 SiO 5 and ZrO 2 (stabilized with Ca) [ 81,82 ]. Generally it seems a problem that currently no material exist which is stable against the highly mobile Li. A detailed structural analysis of 0.5 wt% Li doped MgO showed heavy losses of Li, reduced surfa ce area and grain growth. A correlation between these factors and the deactivation could not be found. The reaction temperature and the flow rate were found to be the main deactivation parameters [83]

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44 CHAPTER 4 SUMMARY Preparation of Magnesium Oxide Nanoparticles A series of nano sized magnesium oxide particles were successfully prepared by a simple water/toluene reverse microemulsion method using triton X 45, 1 Dodecanol as surfactant. By controlling the amount of surfactant, it is possible to contro l the size of MgO nanoparticles from 20 nm to 50 nm. It has also been proved that single surfactant is no t enough to stabilize mocroemuls ion system in some cases. Thus, a medium length alcohol is required as a cosurfactant for smaller particle sizes Cha racterization of Magnesium Oxide Nanoparticle The size of the magnesium oxide nonaparticles prepared by microemulsion method was estimated by XRD measurement. The XRD measurement also showed that the major component of the magnesium oxide nanoparticles is cubic MgO. Preparation of Oxidative Coupling of Methane Catalysts Both high surface area and low surface area MgO samples prepared by microemulsion we re used as support for Li TbO x In order to get the largest coverage of the MgO suppo rt and the smallest Li TbO x particles a nitrate precursor was used during incipient wetness impregnation followed by calcination at 800 C for 4 hours N 2 adsorption (BET) resu lts showed that the surface area of MgO nanoparticles prepared with 15 wt% Triton X 45 was as low as 1.0 m 3 /g and that of MgO powders prepared with 15 wt% Triton X 45 and 1 Dodecanol mixture was 9.3 m 3 /g. Oxidative Coupling of Methane Reaction Over Prepared Catalysts Oxidative coupling of methane (OCM) driven by catalysts prepar ed was studied using a continuous flow quartz reactor at atmospheric pressure, 500 800 C at a CH 4 :O 2 ratio of 4:1

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45 The methane conversion reached 21.9% at 700 C with a C 2+ selecti v ity as high as 65.5%. Thus, a m aximum C 2+ yield of 13.5% was realized. However, there was a rapid deactivation during the OCM over Li doped catalysts due to the loss of Lithium.

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51 BIOGRAPHICAL SKETCH Xianhe was born in Chongqing, China in August 1989 He graduated fr om Nankai Middle School in Chongqing in 2007. He entered in material chemistry with a focus in catalysis from College of Chemistry and Molecular Engineering of Peking University, Beijing, in July of 2011. During 2011 and 2012, Xianhe worked at China Petroleum & Chemical Corporation. Xianhe ent ered the University of Florida Department of Chemical E ngineering gradu ate school in August of 2012. He joined Dr. Helena Ha gelin of 2012. His research was focus on the development of oxidative coupling of methane catalysts. Xianhe served as the officer for the Graduate Association of Chemi cal Engineeing from 2012 to 2014.