Iron-Modified Mesoporous Silica as Efficient
Heterogeneous Lewis Acid Catalysts
Mémoire
Wan Xu
Maîtrise en chimie
Maître ès sciences (M. Sc.)
Québec, Canada
© Wan Xu, 2018
Iron-Modified Mesoporous Silica as Efficient
Heterogeneous Lewis Acid Catalysts
Mémoire
Wan Xu
Sous la direction de :
Freddy Kleitz, directeur de recherche
Thierry Ollevier, codirecteur de recherche
Résumé
Les catalyseurs hétérogènes acides de Lewis ont principalement attiré beaucoup d'attention à cause de leurs applications dans de nombreux processus chimiques tels que le raffinage de pétrole. De plus, la facilité de séparer avec la phase liquide, et peu de déchets dangereux générés dans les processus répondent aux exigences de la chimie verte. Par conséquent, il est nécessaire de concevoir les catalyseurs acides de Lewis de façon simple, efficace, et peu couteux. Pour ce faire, la silice mésoporeuse peut agir comme support potentiel pour ce type de catalyseurs hétérogènes. La silice mésoporeuse possède les propriétés physiques et chimiques adaptées tel que la surface spécifique élevés, le volume de pore grand, la taille des pores accordable et ajustable, et la facilité de fonctionnalisation de la surface.
Par conséquent, l'objectif de cette thèse est d'explorer un procédé de synthèse facile pour préparer 'un catalyseur d'acide de Lewis hétérogène efficace en utilisant la silice mésoporeuse ordonnée fonctionnalisé par des métaux peu couteux. Pour cela, les silices mésoporeuses du type MCM-41, et SBA-15 ont été choisis comme supports de catalyseur. Par la suite, Fe-MCM-41 et Fe-SBA-15 ont été correctement synthétisé en utilisant un procédé polyvalent. Le traitement de l'ammoniac au cours de la synthèse a été trouvé être un moyen efficace d'augmenter la teneur en fer tout en préservant la dispersion convenable des cations métalliques. Les paramètres physicochimiques de la silice mésoporeuse finale contenant du fer ont été obtenus par l’analyse d'adsorption-désorption d'azote à la base température, et l'environnement de la coordination des éléments en fer a été validé par la spectroscopie de UV-Vis réflectance diffusée et la spectroscopie photoélectronique à rayons X. L'acidité de surface a été sondé à l'aide des indicateurs de Hammett. Pour distinguer en outre les sites acides de Lewis sur la surface, l’adsorption de pyridine suivie par FTIR a été mise en œuvre. Ces catalyseurs préparés ont été criblés dans la réaction d'aldolisation de Mukaiyama, qui est une réaction modèle catalysée par l’acide de Lewis. L'activité catalytique acide de Lewis des matériaux était peaufinées et les produits aldol ont été obtenus avec un bon rendement et la sélectivité. De plus, les catalyseurs hétérogènes sont très stables et peuvent être réutilisés au moins neuf fois en conservant l’activité catalytique.
Abstract
Heterogeneous Lewis acid catalysts have primarily attracted much attention for their applications in many organic processes such as petroleum refinery. Moreover, their ease of separation and no hazardous waste during the processes meet the requirements of cleaner and environmentally friendly technologies requested by public in modern society. However, in contrast to extensive studies of homogeneous Lewis acid catalysts, fewer efforts have been dedicated to the study of heterogeneous Lewis acid catalysis. It is necessary to design efficient Lewis acid catalysts through a straightforward and cost-effective method for the generation of different chemicals. Scientific interest has focused on ordered mesoporous silica because of their potential application, particularly in catalysis. Indeed, mesoporous silicas can act as potential catalysts or catalyst supports owing to their physical and chemical properties such as high surface area, larger pore size than zeolites for better support for active sites, high pore volume, tunable pore size and ease of surface functionalization.
Therefore, the objective of this thesis is to explore a facile synthetic method for preparing an efficient heterogeneous Lewis acid catalyst using ordered mesoporous silica functionalized by cheap metals. For this, MCM-41, and SBA-15 materials were chosen as catalyst supports. Subsequently, Fe-MCM-41 and Fe-SBA-15 were synthesized by using a versatile method. pH adjustment during the synthesis route was found to be an efficient way to increase the iron content while preserving suitable dispersion of the metal cations. Physicochemical parameters of the final iron-containing mesoporous silica were obtained by low temperature nitrogen adsorption-desorption equilibrium isotherms, and the bonding environment of iron elements was validated by UV–vis diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy. The surface acidity was probed by using Hammett indicators. To further distinguish the Lewis acid sites on the surface, pyridine sorption probed by the FTIR method was implemented. These prepared catalysts were screened in the Mukaiyama aldol reaction, which is a model reaction catalyzed by Lewis acid. The Lewis acid catalytic activity of the materials was fine-tuned, and the corresponding aldol products were obtained in good yield and selectivity. More importantly, the solid catalysts were very stable and could be reused at least nine times maintaining the same catalytic activity.
Table of Contents
Résumé ... iii
Abstract ... iv
Table of Contents ... v
List of Schemes ... vii
List of Figures ... viii
List of Tables ... xi
List of Abbreviations ... xii
Acknowledgement ... xv
Chapter 1 ... 1
Introduction ... 1
1.1 Need for green Lewis acid catalysts ... 1
1.2 Mesoporous solid acid catalysts ... 2
1.3 Aims and organization of the thesis ... 3
Chapter 2 ... 5
State-of-the-art ... 5
2.1 Heterogeneous catalysis ... 5
2.1.1 Fundamentals of heterogeneous catalysis ... 5
2.1.2 Supported catalysts ... 7
2.2 Synthesis of mesoporous silica supports ... 8
2.2.1 Mechanisms for formation of mesoporous silica ... 10
2.2.2 Mesostructure tailoring ... 12
2.3 Functionalization of mesoporous silica ... 15
2.3.1 Surface properties ... 16
2.3.2 Metal-modified mesoporous silica ... 16
2.3.3 The post-synthesis method using acetylacetonate-metal precursors (acac) ... 17
2.4 Mukaiyama aldol reaction ... 18
Chapter 3 ... 22
Characterization techniques for synthesized materials ... 22
3.1 Nitrogen adsorption ... 22
3.2 Powder X-Ray Diffraction (XRD) ... 26
3.3.2 Transmission electron microscopy (TEM) ... 29
3.3.3 Energy dispersive X-ray spectroscopy (EDX) ... 30
3.4 Diffuse reflectance ultra-violet visible spectroscopy (DR-UV-vis) ... 31
3.5 X-ray photoelectron spectroscopy (XPS) ... 31
3.5 Pyridine adsorption probed by FT-IR spectroscopy ... 32
3.6 Inductively coupled plasma mass spectrometry (ICP-MS) ... 33
Chapter 4 ... 35
Iron-Modified Mesoporous Silica as an Efficient Solid Lewis Acid Catalyst for the Mukaiyama Aldol Reaction ... 35
Résumé ... 36
Abstract ... 37
4.1 Introduction ... 38
4.2 Experimental section ... 40
4.3 Catalyst Characterization and Testing ... 41
4.3.1Titration of the Lewis Acid Solids with Hammett Indicators ... 43
4.3.2 Pyridine Adsorption FT-IR Experiments. ... 44
4.4 Results and Discussions ... 44
4.4.1 Synthesis and Characterization of the Materials ... 44
4.4.2 Surface Acidity ... 53
4.4.3 Catalytic Tests ... 57
4.5 Conclusion ... 67
4.6 Supporting information ... 69
4.6.1 Materials and general procedure of catalytic reactions ... 69
4.6.2 Catalytic reactions ... 69
4.6.3 Schemes of Mukaiyama aldol reactions ... 71
4.6.4 Characterization of the catalysts... 72
4.6.5 Lewis acidity tests ... 76
4.6.6 Catalytic tests ... 77 Chapter 5 ... 93 Conclusions ... 93 5.1 General conclusions ... 93 5.2 Future prospect ... 94 References ... 96
List of Schemes
Scheme 2.1 Mukaiyama aldol reaction catalyzed by a stoichiometric amount of TiCl4
Scheme S4.1 Mukaiyama aldol reaction of 1-(trimethylsilyloxy)-cyclohexene and
benzaldehyde
Scheme S4.2 Mukaiyama aldol reaction of 1-phenyl-1-(trimethylsiloxy)-propene and
List of Figures
Figure 2.1 General classification of catalysts.
Figure 2.2 Representation of hurdles in a heterogeneous catalyzed reaction (center); potential
energy diagram (left); volcano plot of catalyst activity and adsorption forces (right).
Figure 2.3 Two pathways for the synthesis of ordered mesoporous silica: A, cooperative
self-assembly; B, true liquid-crystal templating.
Figure 2.4 Schematic representation of different types of cooperative interactions for the
inorganic-organic hybrid mesophase.
Figure 2.5 Representations of the pore topology with symmetries of (A) p6mm, (B) Ia3d, (C)
Pm3n, (D) Im3m, (E) Fd3m, and (F) Fm3m.
Figure 2.6 Micropore formation in mesoporous SBA-15 silica where steps (i) and (ii)
correspond to treatment with sulfuric acid and calcination, respectively.
Figure 2.7 Schematic routes for the functionalization of mesoporous silica via various
methods.
Figure 2.8 Various types of surface silanols/siloxanes on mesoporous silicas.
Figure 2.9 Hydrolysis constants and water-exchange rate constants for determining Lewis
acidity.
Figure 3.1 Different types of the physisorption isotherms as classified by IUPAC. Figure 3.2 Classification of adsorption-desorption hysteresis loops.
Figure 3.3 Typical low-angle powder XRD patterns obtained for MCM-41(Left) and
SBA-15(right), showing reflections of the hexagonal plane group.
Figure 3.4 The main types of the signal generated by the electron beam-specimen interaction. Figure 3.5 TEM images of (a) Fe-MCM-41 (b) Fe-SBA-15.
Figure 3.6 Schematic representation of an ICP-MS instrument.
Figure 4.1 N2 adsorption-desorption isotherms measured at 77.4 K (-196 °C) for (A)
Fe-MCM-41 and (C) Fe-SBA-15 with various iron contents and the corresponding NLDFT pore size distributions for (B) Fe-MCM-41 calculated from the adsorption branch of the isotherm and (D) Fe-SBA-15 calculated from the desorption branch of the isotherm.
Figure 4.2 High-resolution transmission electron microscopy images of (A) Fe-MCM-41
(10%); (B) Fe-MCM-41 (10%, pH 10) and (C) Fe-SBA-15 (10%); (D) Fe-SBA-15 (10%, pH 10) and their corresponding energy dispersive X-ray spectroscopy data.
Figure 4.3 XPS spectra of (A) Si 2p, (B) O 1s and (C) Fe 2p for the Fe-SBA-15 (20%) sample
(taken as representative).
Figure 4.4 UV-vis absorption spectra of different indicators and the catalysts with the
indicators adsorbed.
Figure 4.5 FT-IR spectra for Fe-MCM-41 (20%) and Fe-SBA-15 (20%) materials with and
without pyridine materials.
Figure S4.1 Low-angle XRD patterns for Fe-MCM-41 (A) and Fe-SBA-15 (B) with various
iron contents, as indicated.
Figure S4.2 Wide-angle XRD patterns for Fe-MCM-41 (A) and Fe-SBA-15 (B), with various
iron contents, as indicated, and the reference pattern of Fe2O3 is also shown.
Figure S4.3 SEM images of (A) MCM-41, (B) Fe-MCM-41 (10%), (C) Fe-MCM-41 (10%,
pH=10), and (D) SBA-15, (E) Fe-SBA-15 (10%), (F) Fe-SBA-15 (10%, pH=10), and their corresponding EDX spectra, with surface Fe/Si molar ratio of (G) 2%, (H) 13%, (I) 1.7% and (J) 11%, for samples in (B), (C), (E) and (F), respectively.
Figure S4.4 UV–vis diffuse reflectance (DR-UV–vis) spectra of calcined Fe-MCM-41 and
Fe-SBA-15 samples.
Figure S4.5 1H NMR spectrum of 1-(trimethylsilyloxy)-cyclohexene
Figure S4.6 1H NMR spectrum of 1-phenyl-1-(trimethylsiloxy)-propene
Figure S4.7 1H NMR spectrum of 2-(hydroxyphenylmethyl)-cyclohexanone
Figure S4.8 13C NMR spectrum of 2-(hydroxyphenylmethyl)-cyclohexanone
Figure S4.9 1H NMR spectrum of 3-hydroxy-2-methyl-1,3-diphenylpropan-1-one
Figure S4.10 13C NMR spectrum of 3-hydroxy-2-methyl-1,3-diphenylpropan-1-one
Figure S4.11 1H NMR spectrum of
3-hydroxy-3-(4-methoxyphenyl)-2-methyl-1-phenyl-1-propanone
Figure S4.12 13C NMR spectrum of
3-hydroxy-3-(4-methoxyphenyl)-2-methyl-1-phenyl-1-propanone
Figure S4.13 1H NMR spectrum of
3-hydroxy-3-(4-chlorophenyl)-2-methyl-1-phenyl-1-propanone
Figure S4.14 13C NMR spectrum of
Figure S4.15 1H NMR spectrum of
3-hydroxy-3-(4-cyanophenyl)-2-methyl-1-phenyl-1-propanone
Figure S4.16 13C NMR spectrum of
3-hydroxy-3-(4-cyanophenyl)-2-methyl-1-phenyl-1-propanone
Figure S4.17 1H NMR spectrum of
3-hydroxy-3-(4-nitrophenyl)-2-methyl-1-phenyl-1-propanone.
Figure S4.18 13C NMR spectrum of
3-hydroxy-3-(4-nitrophenyl)-2-methyl-1-phenyl-1-propanone
Figure S4.19 1H NMR spectrum of
3-hydroxy-3-(2-nitrophenyl)-2-methyl-1-phenyl-1-propanone
Figure S4.20 13C NMR spectrum of
3-hydroxy-3-(2-nitrophenyl)-2-methyl-1-phenyl-1-propanone
Figure S4.21 1H NMR spectrum of 3-hydroxy-2-methyl-1-phenyl-1-hexanone
List of Tables
Table 2.1 Parameter to be considered for the selection of catalyst supports. Table 4.1 Indicators Used for Acid Strength Measurements.
Table 4.2 Chemical Composition and the Structural Properties of All Prepared Materials. Table 4.3 Effect of Catalyst Amount for Mukaiyama Aldol Reaction over Fe-MCM-41 (20%,
pH 10) Catalyst.
Table 4.4 Effect of Catalyst Amount for Mukaiyama Aldol Reaction over Fe-MCM-41 (20%,
pH 10) Catalyst.
Table 4.5 Effect of the Solvent for Mukaiyama Aldol Reaction over Fe-MCM-41(20%, pH
10)
Table 4.6 Effect of the Iron Content in the Mukaiyama Aldol Reaction of
1-phenyl-1-(trimethylsiloxy)propene with Benzaldehyde.
Table 4.7 Effect of Iron Content for Mukaiyama Aldol reaction of
1-(trimethylsilyloxy)cyclohexene with Benzaldehyde.
Table 4.8 Catalytic performances of Fe-SBA-15 (5%)(A) and Fe-SBA-15 (20%)(B) in
Mukaiyama aldol reactions (substrate scope)
Table 4.9 Reusability of catalysts for Mukaiyama aldol reaction of
1-(trimethylsilyloxy)-cyclohexene with benzaldehyde over Fe-SBA-15 (20%) catalyst
Table 4.10 Catalytic test of Fe-MCM-41-HMDS (20%) and Fe-SBA-15-HMDS (20%)
catalyst.
Table S4.1 Results of titration of Fe-modified mesoporous materials with Hammet indicators. Table S4.2 Effect of the addition of the surfactant (SDS) in the Mukaiyama aldol reaction. Table S4.3 Mukaiyama aldol reaction using FeCl3 as catalyst
Table S4.4 Reusability of catalysts for Mukaiyama aldol reaction of
List of Abbreviations
2D 2-dimensional 3D 3-dimensional BE Binding energy BET Brunauer–Emmett–Teller BJH Barrett–Joyner–HalendaCMC Critical micellar concentration
CTAB Cetyltrimethylammonium bromide
DMC Dimethyl carbonate
EDX Energy-dispersive X-ray spectroscopy
FT-IR Fourier transform infrared spectroscopy
GNP Gross national product
HRTEM High-resolution transmission electron microscopy
HMS Hexagonal mesoporous silica
HMDS Hexamethyldisilazane
IUPAC International Union of Pure and Applied Chemistry ICP-MS Inductively coupled plasma mass spectrometry ICDD International Center for Diffraction Data
KE Kinetic energy
KIT-6 Mesoporous silica Korean Institute of Technology number 6
MCM-41 Mobil Composition of Matter Number 41
MCM-48 Mobil Composition of Mater number 48
MCM-50 Mobil Composition of Mater number 50
MOF Metal-organic framework
NLDFT Nonlocal density functional theory
NMR Nuclear Magnetic Resonance
Pluronic P123 Triblock copolymer, PEO20PPO70PEO20
PEO Poly(ethylene oxide), -(CH2CH2O)-
PPO Poly(p-phenylene oxide)
SBA-15 Santa Barbara Amorphous Number 16
SEM Scanning electron microscope
SDS Sodium dodecyl sulfate
TON Turnover number
TLCT True liquid-crystal templating
TEOS Tetraethyl orthosilicate
TEM Transmission electron microscopy
TS-1 Titanium silicalite-1
UV-vis DRS Ultraviolet-visible diffuse reflectance spectra
WERCs Water exchange rate constants
XRD X-ray diffraction
Acknowledgement
I would first like to give my sincere gratitude to my supervisor Prof. Freddy Kleitz for giving me the opportunity to work in his laboratory. His guidance and support throughout my entire MSc. study and research also helped me a lot. I am truly grateful to him for his patience, insightful comments, and encouragements on the thesis as well as on all the time of the research. I also would like to thank to my co-supervisor Prof. Thierry Ollevier. He is always willing to spend a lot of time to give me advices and share his immense knowledge and skills with me. I would like to express my deepest thanks to both of my supervisors, my research and related works would not have been possible without their guidance and help.
My sincere thanks also go to professors, administrative staff and technicians in Laval University for their assistance during my MSc life. I would like to thank Prof. Peter Mcbreen for his help in my course and seminar. Great thanks to André Ferland, for his kind help for SEM analysis and interesting talks. I would also like to thank Jean Frenette for XRD, Alain Adnot for XPS analysis and Serge Groleau for ICP-MS analysis. I would like to acknowledge Jongho Han and Prof. Ryong Ryoo from KAIST, Korea for providing the data of XRD measurement images of high-resolution TEM and element mapping.
I am also very appreciating and grateful to all the members in Kleitz group and Ollevier group, for their working experience, stimulating discussions and friendship. It is my honor and pleasure to work with them all. I would like to thank to Dr. Justyna Florek, Dr. Angela Jalba, Dr. Maria Zakharova and Estelle Juère for their training, supports, and friendship. I would like to thank Dr. Nima Masoumifard, for his great help and suggestions at the beginning of my research. He is a good friend, teacher and researcher because of his earnest, patience, and diligence. Thanks to Yimu Hu, her intelligence, and hardworking always inspires me a lot. In addition, the abstract in French of this thesis was completed with her great help. Great appreciation also goes to Dazhi Li, Di Meng, Dandan Miao and Mao Li for all their help both in my research and my life.
Finally, I need to thank my dearest sister, parents, grandma, and my boyfriend, who give me endless love, inspiration, and power to help me go through all the difficulties.
Chapter 1
Introduction
1.1 Need for green Lewis acid catalysts
Catalysis played an important role in the chemical industry over the past century. It now continues to be essential to economic growth, contributing about 20% of the world GNP in total.3,4 Since the beginning of the 21st century, catalytic technologies have been widely
studied to meet the requirements of greener production of chemicals and sustainable fuels. For a greener process to be carried out in a reactor, the catalysts must be designed for ease of separation and the ability to reuse. Also, the design of a catalyst requires the high productivity as well as the reduction of waste.5,6
Lewis acid catalyzed reactions are of high importance in catalytic technologies for green chemistry as many fine and pharmaceutical chemicals manufacturing processes depend on the application of homogeneous Lewis acid catalysts. Since many of these industrial processes were invented almost more than one hundred years ago, their only goal was to improve the productivity of the catalysts, neglecting the influence of the hazardous and toxic waste on the environment. Most of the waste is produced during the isolation step of the reaction mixture and therefore it is necessary to add a step to break the bond existing between the products and the catalysts to destroy the acid-base adduct. Lewis acid catalysts usually decompose completely during this step, not only producing undesired by-products but also making it hard to regenerate catalytic activity.7-9 In addition, it is common for a Lewis acid catalyzed reaction to employ a greater amount of catalyst than the stoichiometric equivalents of Lewis acid because the product usually has stronger bases than the reactants. Therefore, the percentage of the waste and by-products are mostly derived from the conventional Lewis acid catalysts. In summary, there is no surprise that the reactions catalyzed by Lewis acids are considered as processes that are less environmentally-safe in the chemical industry.6
Therefore, it is necessary and urgent to design a heterogeneous solid Lewis acid catalyst that could be employed in this kind of reactions.The efficient use of solid acid catalysts facilitates the separation of the catalysts, simplifies the isolation of the products and improves the
selectivity. Meanwhile, only conventional organic solvents or water are required to manipulate the reagents and products in the processes using the solid acid catalysts.3,10,11
1.2 Mesoporous solid acid catalysts
One way to synthesize environmentally friendly Lewis acid solid catalysts is supporting corrosive Lewis acids or introducing metal species on solids such as zeolites, silica, graphites or alumina, which exhibit high specific surface areas. The catalytic performance of the obtained supported materials is strongly affected by the type of material used as support. When a material is employed as a support, there are two critical factors that it should satisfy. First, as a catalyst, it should be stable when it is used in a reaction process, both thermally and chemically. Second, good accessibility and homogeneous dispersion for active sites are necessary for a support material. Regarding this, nanoporous materials have been considered as very suitable supports in recent years.
Porous materials, solids with channels or cavities that are deeper than they are wide,12 have been widely studied regarding applications as heterogeneous catalysts or catalyst supports. On the basis of different pore size, the classification of porous materials defined by IUPAC has three types: microporous materials with a pore size less than 2 nm, macroporous materials with a pore size larger than 50 nm, and mesoporous materials with a pore size between 2 nm and 50 nm.13 The most famous members among the class of microporous materials are zeolites, which exhibit small molecular size pores, have been extensively studied in the literature.14-16
For instance, zeolites as heterogeneous acid catalysts, are nowadays considered as a substitute for conventional homogeneous acid catalysts. However, zeolite-type catalysts could prevent the accessibility of reactants with sizes larger than the dimensions of the micropore openings (most often <0.8 nm), especially in the case of liquid-phase reactions. Taking the Mukaiyama-aldol reaction as a representative example, Corma17 and co-workers
reported that Ti-containing mesoporous silica catalysts showed an excellent yield up to 98% in a solvent-free system. They demonstrated that well-prepared Ti-MCM-41 could be a better catalyst for the Mukaiyama aldol reaction compared to Ti-zeolites, owing to the
less diffusional restriction for reactants. Inthis respect, for carrying out catalytic reactions involving larger molecules, larger pore sizes are necessary. Therefore, attempts were focused on broadening the pore diameter of zeolites into the mesoporous range, which can also maintain the porous structure and allow the entrance of large molecules into the pore system. Meanwhile, the diffusion of the reactants to the active sites could be facilitated because of the presence of mesoporous system.
It has been found that several factors have impacts on the catalytic activity of mesoporous solid acid catalyst, such as their Brønsted/Lewis acidity, their strength and number of introduced active sites and the surface area/porosity of the support.3 By fine-tuning these properties, a range of novel heterogeneous mesoporous acid catalysts, which possess high product selectivity could be obtained and employed in various reactions that require acidity. For example, a novel solid acid catalyst, zinc triflate supported hexagonal mesoporous silica (HMS) was synthesized by Wilson and co-workers for the rearrangement of α-pinene oxide.18 This solid acid catalyst showed excellent catalytic activity and selectivity and also exhibited high stability for their high selectivity in the in the recycling experiments (no ie).18 Meanwhile, the mesoporous solid acids have been proven to be effective for a typical acid catalyzed reaction, Friedel-Crafts reaction. Armengol found that mesoporous aluminosilicate MCM-41 could be employed as a catalyst for Friedel-Crafts alkylation of cinnamyl alcohol and a bulky aromatic compound. This discovery exemplified the application of MCM-41 as a catalyst.19 It was found that different metal-incorporated mesoporous silica-containing acid sites, especially Fe-MCM-41, showed excellent catalytic activity in the Friedel-Crafts benzylation of benzene.20 It was also reported that several metal modified mesoporous materials could be used as catalysts for the Mukaiyama aldol reaction, which is usually catalyzed by homogeneous Lewis acid catalysts, such as TiCl4 and Sc(OTf)3.21-23 However,
studies of mesoporous Lewis acid catalysts are still limited. Thus, it is of particular importance to develop a straightforward and cheap method for the synthesis of this type of catalyst, which enables the diffusion of bulkier substrates.
1.3 Aims and organization of the thesis
reactions. To reach this goal, two typical mesoporous silicas, MCM-41 and SBA-15, were firstly synthesized. The powder mesoporous silicas were then used as supports for the introduction of metal sites through the post-grafting method. The resulting metal-modified mesoporous silicas were applied as heterogeneous catalysts in a model Lewis acid catalyzed carbon-carbon bonding reaction, i.e., the Mukaiyama aldol reaction. The catalytic activity, selectivity as well as the stability of the catalysts were examined. Furthermore, the influence of mesopore size, as well as that of the surface hydrophobicity of the mesoporous support on the catalytic activity, were also be investigated.
The organization of this thesis consists in five parts, introduction, state-of-the-art, experimental section, result and discussion and conclusions, as follows:
Chapter 2 introduces the state-of-the-art including the fundamental principles of heterogeneous catalysis. This chapter also explains the basic concepts of the catalyst supports as well as the principles behind the synthesis of the mesoporous materials and the final catalysts. Chapter 3 presents the experimental techniques applied in the thesis for the characterization of the synthesized materials. In Chapter 4, a simple approach based on the post-grafting method to synthesize iron modified mesoporous silica is introduced in detail. The resulting solids were used as catalysts for Mukaiyama aldol reaction to investigate their catalytic activity. Finally, the last chapter presents conclusions of the work and indicates some perspectives.
Chapter 2
State-of-the-art
2.1 Heterogeneous catalysis
2.1.1 Fundamentals of heterogeneous catalysis
In general, a substance that can participate in a chemical reaction to convert the reactants into products and increase the reaction rate is the catalyst. The catalyst may be transformed into some other different entities during the reaction, but it will regenerate to its original form after each reaction complete catalytic cycle. There are three types of catalysts: heterogeneous, homogeneous and biological catalysts, respectively. Figure 2.1 shows the range of the classification.
Figure 2.1 General classification of catalysts.24
Heterogeneous catalysts are estimated to be used in 90% of all the chemical processes. Raw materials can be converted into beneficial compounds in an efficient and environmentally friendly way by using heterogeneous catalysts, which is of great importance for numerous traditional industrial applications such as chemical production, food, pharmaceutics, and oil
industries.25,26 Also, heterogeneous catalysts can be applied in novel areas, e.g., green chemistry27, biotechnology, fuel cells28, and nanotechnology.29
There are several advantages of using a heterogeneous catalyst for a reaction. One of the most important ones is that the separation step for a solid catalyst removal from either gas or liquid reactants and products is very straightforward. Active sites on the solid surface are the most important part of heterogeneous catalysts because they are the active ingredients for the reactions.
In general, there are some activation barriers and hurdles which need to be overcome for the reactants and products of a heterogeneous catalyzed reaction (Figure 2.2). First, the reactants are diffusing to the surface of the catalyst, and then adsorption at the active site occurs. Once the active site is reached, the surface reaction takes place, the products are formed. The products are finally transported away from the catalyst particle through desorption and diffusion.30 Thus, in a heterogeneous catalytic reaction cycle, the rate of the reaction is not
only affected by the actual chemical reaction but may also be influenced by the diffusion rate of the reactants and products, the adsorption and desorption processes.30 If the adsorption
force is too weak, the catalyst will have little capability to break the bond, resulting in a reaction of slow rate. On the contrary, if the interaction is too strong, the desorption of the products will be very difficult, and the active sites will be “poisoned”, a “volcano” effect on activity may be expected 30 (Figure 2.2 (inset)).
Figure 2.2 Representation of hurdles in a heterogeneous catalyzed reaction (center); potential energy
diagram(inset, left); volcano plot of catalyst activity and adsorption forces (inset, right).31
The performance of a catalyst is determined by a lot of parameters. First, it is obvious that a good catalyst should exhibit high activity and high selectivity. This means that desired products need to be obtained at a required conversion on a period of time. The elimination of by-products should be easy and minimal purification cost required. Good selectivity is the primary objective compared with high activity/conversion in catalyst development. It represents the ability of a catalyst converting the starting substance to the desired products. Furthermore, a catalyst should exhibit sufficient stability under the reaction conditions over an extended period of time (lifetime), or it should be possible to regenerate high activity and selectivity by appropriate treatment of the deactivated catalyst.32
2.1.2 Supported catalysts
One of the largest series of heterogeneous catalysts in the chemical industry are supported catalysts. Supported catalysts can be obtained by introducing small amounts of active sites, such as metal ions, onto the surface of a porous material, i.e., supports. The reasons for the predominant role of supported catalysts in heterogeneous catalysis are the low costs, high activity, good selectivity and ability to maintain activity and selectivity after several cycles
of reaction. The main factors that would affect these properties are the type of support materials and the arrangement of the active sites in the pore structure of the supports.33 In general, the catalytic activity of a catalyst improves as the specific surface area increases. Higher surface area of a support can increase the accommodation of the active sites on the support surface. However, the relationship between the catalytic behavior (reaction rate) and the surface area is not always linear since the catalytic activity may also be influenced by the structure of the support. The diffusion of the reactants can be highly affected by various pore structure of the catalyst, leading to a different reaction rate. Moreover, the catalytic activity may also be affected by the pore size of the catalyst because different pore sizes may have different impact on the dispersion of the active sites on the surface of the catalyst. In summary, the catalytic activity of a supported catalyst is highly depended on its surface area, pore structure, and pore sizes. Therefore, ordered mesoporous materials which have various pore structures, high specific surface area, and different pore sizes, are of great interest in catalyst field and there are a lot of challenges and possibilities left to explore since it was discovered only two decades ago. In this thesis, ordered mesoporous materials will be used as the catalyst supports.
2.2 Synthesis of mesoporous silica supports
In the early 90s, scientists at Mobile Oil Corporation prepared a novel class of ordered mesoporous aluminosilicate materials named as M41S.2,34 Among these materials, MCM-41 (Mobil Composition of Matter No. 41) shows one-dimensional (1D) cylindrical pores accompanied with a relatively narrow pore size distribution. The pores are highly ordered and hexagonally arranged. However, the walls of this new type of material are amorphous silica. Other related materials with well-defined structures: MCM-48 with cubic mesostructure and MCM-50 with lamellar mesostructure, were also reported in these early publications.2,34 Shortly after the discovery of these mesoporous siliceous materials, the
synthesis of various non-siliceous mesoporous oxides was studied under different conditions.35 Since then, numerous non-siliceous mesostructured materials, such as oxides,
metals, and phosphates were discovered.36-38 However, compared to non-siliceous materials,
thermal stability of the amorphous wall and a great variety of available methods for functionalization.39 Over the recent years, great progress has been obtained in the synthesis of mesoporous silica.40 A large variety of synthetic approaches have been discovered for the formation of mesoporous silica, e.g., SBA-15,41 SBA-16,42 KIT-6,43 etc., with different morphology, structure, and texture.
Most of the synthetic routes for mesoporous silica are based on the use of organic amphiphilic molecules acting as a template or structure-directing agent, around which the silica precursors can condense and thus result in the formation of an open framework. After the organic template is removed by calcination or solvent extraction, a cavity or a pore, which retains the structure and the morphology of the template, forms.
A large variety of synthetic approaches have been developed for the formation of different mesoporous silicas by using various surfactants and synthesis methods; both can be controlled and altered. Different reagent ratios, organic additives as well as the synthesis conditions (time, temperature, pH) and the nature of surfactants are being used to control the structural properties and morphology of the obtained materials. Basically, for mesoporous materials, there are three main features involved in their synthesis: the specific synthesis mechanism, the combination of surfactant and solvent type, the interactions between the inorganic compounds and the template molecules.40,44 In general, several steps are involved for the preparation of the mesoporous silicas: first, the molecules used as a template are dissolved in the solvent (pH, temperature, and co-solvents, additives could be altered or added in this step), and the silica source is then added. The mixture is stirred for a period of time at a certain temperature to allow hydrolysis and condensation of the silica source. The temperature is increased for aging (sometimes microwave synthesis or pH adjustment or hydrothermal treatment could be used as assistant method). The products will be subsequently recovered by filtration, washing, and drying. In the final step, the porous structure of the final materials could be opened after removal of the template by calcination or solvent extraction. Calcination is the most common way used in practice to eliminate the template of the as-synthesized materials. The materials are heated to a given temperature often under a flow of nitrogen, oxygen or air, in the calcination step. The solvent extraction method may be used as an alternative method to calcination. For instance, the block
copolymer template of SBA-15 silica can easily be extracted using a mixture of acid and ethanol solutions at low temperature.45
2.2.1 Mechanisms for formation of mesoporous silica
Two different pathways have been proposed for the synthesis of mesoporous silica, that is,
true liquid-crystal templating (TLCT) route and cooperative self-assembly route. The TLCT
mechanism was originally suggested by Beck and co-workers.2,34 It suggested that the liquid crystal mesophase can be formed without the presence of the inorganic species if the concentration of the surfactant was high enough (above CMC). The LC could serve as the template for the construction of the mesoporous structure (Figure 2.3, pathway B). On the other hand, another mechanism pathway, i.e., the cooperative self-assembly route, was plausible to explain the formations of the mesophase at lower concentration of the surfactant (Figure 2.3, pathway A). The cooperatively self-assembly of the silica species and the surfactant micelles results in the inorganic-organic hybrid micelles, which further aggregate into silica-surfactant rods through inorganic-organic interactions. These rods subsequently
Figure 2.3 Two pathways for the synthesis of ordered mesoporous silica: A, cooperative
assemble into hexagonal mesostructure with further condensation and polymerization of inorganic networks around them through hydrothermal or ammonia treatment.46,47
Following the cooperative self-assembly route, the formation of the mesostructure is strongly dependent on the inorganic-organic interactions between the inorganic precursor (I) and the head group of the surfactant (S). As shown in Figure 2.4, there are six plausible cooperative interactions for the inorganic-organic hybrid mesophase.44,48,49 When the synthesis takes place under alkaline conditions, the silicate present as anions I−, interact with the oppositely charged surfactant cations S+ (S+I− mechanism). These electrostatic interactions are applicable for the synthesis of MCM-41 and MCM-48, which are both carried out in basic conditions. On the other hand, if the synthesis is carried out under acidic medium, S−I+ mechanism should be proposed since the silicate species are positively charged cations. In the case of the inorganic species and the surfactant both positively charged or negatively charged, two other indirect routes are proposed. It is necessary to introduce a counter-ion to make the interaction possible. Under the acidic media, the use of halide anions (X− = Cl–, Br–) make the S+ X− I + route possible. Conversely, for a base catalyzed synthesis, S–M+I– route is plausible with the assistance of alkali metal ion (M+ = Na+, K+). For synthesis using a neutral surfactants (S0) or non-ionic surfactant (N0) in neutral media, new assembly routes denoted as S0I0 and N0I0 were proposed.48 In these cases, the driving force of the interaction is supposed to be hydrogen bonding. In 1998, the synthesis of a new ordered mesoporous silica named SBA-15 was reported by Zhao et al.41 The synthesis was performed in acidic conditions, and non-ionic triblock copolymers were used as the surfactant. Since the silica species are present as cations under acidic condition (I+) and the triblock copolymer could also be positively charged, a more realistic pathway (N0H+)(X−I+) derived from N0I0 was proposed.
Figure 2.4 Schematic representation of different types of cooperative interactions for the
inorganic-organic hybrid mesophase.44
2.2.2 Mesostructure tailoring
Concerning the prospects of applications of ordered mesoporous silica in catalysis, sensing, sorption and so on, diversity in mesostructure is essential. Figure 2.5 displays the most common mesostructures synthesized. Among these, the 2D mesostructures of mesoporous silica consist of cylindrical pore channels that are hexagonally close-packed, with the p6mm symmetry. The two most important representatives among the hexagonal phases of mesoporous silica are MCM-41 and SBA–15.
Figure 2.5 Representations of the pore topology with symmetries of (A) p6mm, (B) Ia3d, (C) Pm3n, (D)
Im3m, (E) Fd3m, and (F) Fm3m. Adapted from reference 50
MCM-41. MCM-41 was first synthesized by using cationic surfactants as templates and
tetraethyl orthosilicate (TEOS) as the silica source under alkaline conditions. MCM-41 materials consist of an amorphous-silicate framework with a highly ordered hexagonal array of cylindrical mesopores. The thickness of the pore wall usually varies from 0.7 to 1.1 nm. MCM-41 has high BET surface area up to 1000 m2/g and pore volumes exceeding 1 cm3/g.45 The high surface area and large pore volume of MCM-41 make it a possible material for catalysis and sorption applications.51 In addition, it presents a quite narrow pore size distribution due to the relatively uniform mesopores.52 In general, the pore sizes of MCM-41 can be finely tuned by changing the chain length of the surfactant, and a larger pore size could be obtained by using surfactants with a longer hydrophobic chain. The conventional material with the highly ordered arrangement and pore size of 2-10 nm can be easily obtained.
SBA-15 (Santa Barbara acids No 15). SBA-15 is the most important and extensively studied hexagonal mesoporous silica after MCM-41. The synthesis of SBA-15 involves the non-ionic triblock copolymer P123 (EO20PO70EO20) as the surfactant, which consists of large
polypropylene oxide (PO)m and polyethylene oxide (EO)n blocks, combined with silica
molecules such as TEOS or tetraethyl orthosilicate (TMOS) under strongly acidic aqueous condition. The cationic silica species can interact with EO units of P123, resulting in the formation of the mesostructure. SBA-15 exhibits 2D hexagonally tunable uniform mesopores
Impéror-Clerc and co-workers,53 the presence of micropores within the walls results from penetration of the hydrophilic PEO part of the surfactant into the wall (Figure 2.6). With
different synthesis conditions, the size of the micropores differs from 0.5 to 3 nm. The use of triblock copolymers as a surfactant improves the available pore size range of the mesopores. In contrast to MCM-41, SBA-15 exhibits larger pore sizes adjustable from 6 to15 nm and thicker pore walls of 3−7 nm. The large mesopores of SBA-15 make it a favorite in the catalysis field because of the increase in the diffusion and accessibility of any active sites in the pores. The thermal stability and hydrothermal stability could be significantly improved because of the thicker pore wall of SBA-15, compared to other mesoporous silica exhibiting thinner pore walls (about 1 nm). The characteristic of the large specific surface area and pore volume also make it a promising support in catalysis field. Because of these desirable features, in addition to the simple synthesis route, SBA-15 attracted massive attention and many publications about its applications in heterogeneous catalysis have appeared.54,55,56
Figure 2.6 Micropore formation in mesoporous SBA-15 where steps (i) and (ii) correspond to
2.3 Functionalization of mesoporous silica
The use of pristine mesoporous silicas in the chemical industry, especially in heterogeneous catalysis, was restricted by their poor chemical activity due to the lack of active sites to the amorphous silica wall. In the past years, numerous efforts have been dedicated to the functionalization of mesoporous silicas, of which the chemical properties could be significantly improved, and therefore the field of application could be broadened. There are two primary methods to incorporate functionalities into mesoporous silicas (Figure 2.7). The first one is “direct synthesis” which is also known as co-condensation. Namely, a given functional organosilane is added into the silica precursor (tetraalkoxysilanes)/surfactant sols, and the final functionalized mesoporous material can be obtained by the co-condensation of organosilane and silica precursors. In this way, the functional group can be partly incorporated into the framework of the silica walls while most of them were dangling on the surface. The other method is based on the post-grafting or impregnation or adsorption, involving a grafting reaction after the first formation of the mesoporous silicas.
Figure 2.7 Schematic routes for the functionalization of mesoporous silica via various methods. Adapted
2.3.1 Surface properties
For the mesoporous silica, there are abundant silanol groups on the surface that are accessible as anchoring sites silane coupling or metal species. (Figure 2.8) Even though the silanol density of pristine mesoporous silica (MCM-41 or SBA-15) is relatively low (1-3 SiOH/nm)58,59 compared to the regular hydroxylated silica (4-6 SiOH/nm),60 some silanol groups are still available for surface modification after calcination. Surface modification of the mesoporous silica is always performed by mixing the silica powder and the organic solutions containing silylating agents, such as hexamethyldisilazane or trimethylchlorosilane,61,62 under reflux for a period of time. Therefore, the silanol groups are passivated by the introduction of the alkylorganosilanes on the surface of the mesoporous silica, which can be used as a strategy to provide the walls more hydrophobicity, and thus to improve the structural stability of the materials.
Figure 2.8 Various types of surface silanols/siloxanes on silica. 2.3.2 Metal-modified mesoporous silica
Much effort has been focused on the activation of mesoporous silica, and one facile pathway is inclusion of heteroatoms such as boron, aluminum, and different transition metals, into the silica frameworks for the modification of the composition of the siliceous walls. These metal-modified mesoporous silicas are of particular importance in regards to applications in catalysis,63 Because in this process, a vast number of acidic sites could be formed because of the substitution of the silicon in the siliceous frameworks with ion-exchange capacity, and thus resulting in a high catalytic activity, in a similar pathway as for the amorphous silicates and zeolites. An element of the third main group, for example, boron, aluminum, gallium, etc. and some of the transition metals, such as titanium, iron, and copper, are the most favorable substitution elements for mesoporous silica.63
2.3.3 The post-synthesis method using acetylacetonate-metal precursors (acac)
Essentially, one can obtain metal-modified mesoporous silica through two different pathways. Namely, the position of the inorganic components are preferentially located on the surface of the mesopores, mostly as metal oxides, or silicon atoms in the frameworks are replaced by metal ions. In general, the modification of the siliceous frameworks can be obtained by mixing the heteroelements, e.g., Al, Ti, Cr, and Fe, with the synthesis sols containing silica source and surfactants. In this way, the silicon atoms in the framework can be substituted by tetrahedrally coordinated trivalent or tetravalent element, leading to homogeneously incorporated heteroatoms in the materials. In another way, pore surface functionalization can be carried out through the post-synthesis grafting of metal elements on the silanol groups, resulting in a higher heteroatom concentration on the surface without altering the structure of the mesophase. For instance, iron oxide could be deposited on M41S materials using iron(III) nitrate as a metal precursor through the incipient wetness method. The presence of iron oxide nanoparticles was reported by different authors.64,65 However, even though the
pathway based on the post-synthesis can benefit from the high concentration of metal species on the silica surface, it also suffers from aggregation and formation of metal oxide, leading to a low dispersion of heteroatoms on the silica surface.66 Since the dispersion of the metal sites on the silica surface can affect the performance of the catalyst, it is important to develop a grafting method in order to obtain a catalyst with high dispersion of the metal atoms. To reach this goal, it is crucial to select a proper metal precursor, which could have adequate interactions with the silanol groups on the silica surface, and therefore generate a catalyst with highly dispersed metal atoms.
Recent studies in our laboratory have shown that chelated metal complexes are among the most promising metal precursors for the post-grafting of metal atoms on the silica surface. Conventional metal precursors, such as metal chlorides or metal alkoxides, can easily aggregate during the synthesis process, resulting in large clusters. On the contrary, due to the relatively stability of the metal chelated complexes, and therefore limited hydrolysis and condensation, various metals such as V, Cu, Co, Fe, etc.67 have been successfully grafted on the silica surface via a generalized post-grafting method using corresponding metal acetates or acetylacetonates as precursors, and the thus-synthesized materials showed potential
catalytic activity.68 For example, our group demonstrated that the titanium-modified SBA-15 exhibited extensively higher catalytic activity for the epoxidation of cyclohexene compared to materials synthesized by co-condensation method.69 Besides, the resulting Ti-SBA-15 material showed a high stability during the recycling tests, indicating high stability of the active sites.70 The main advantages of this generalized post-grafting method using metal acetates or acetylacetonates as precursors compared to traditional post-grafting methods using conventional metal precursors are: (1) It facilitates the synthesis procedure, and (2) it is highly tunable by modifying the related grafting parameters, e.g., pH, temperature, and acac/metal ratio.67-70
Therefore, in this thesis, the metal-modified mesoporous materials were prepared according to the generalized post-grafting method, whereby the MCM-41 and SBA-15 were used as silica supports and Fe(acac)3 as the metal precursor.
2.4 Mukaiyama aldol reaction
The Mukaiyama aldol reaction was discovered by Mukaiyama and co-workers more than four decades ago.71,72 It is a directed cross-aldol reaction in the presence of a Lewis acid. The formation of the bond between an aldehyde and a preformed silicon enolate (a silyl enol ether derived from a ketone or a ketene silyl acetal derived from an ester), gives rise to the final aldol adduct.73,74 The Mukaiyama aldol reaction has greatly inspired the development of various related carbon–carbon bond-forming reactions in organic chemistry, for instance, the Sakurai–Hosomi allylation reaction75 and hetero-Diels–Alder reactions of Danishefskys dienes.71,76 For electrophiles such as acetals, ketimines, thioacetals and imines and for nucleophiles such as allylsilanes, silyl cyanides and silicon dienolates have both been studied as substrates under acidic conditions.73 In a word, the Mukaiyama aldol reaction has extended the organic synthesis into a larger scope. Moreover, classical aldol reactions performed under alkaline media generally suffer from side reactions such as polymerization, dehydration and self-condensation, leading to a low yield, and selectivity of the reactions.77 On the contrary,
Scheme 2.1 Mukaiyama aldol reaction catalyzed by a stoichiometric amount of TiCl4
The catalyst applied for the Mukaiyama aldol reaction in the first report published in 1973 was a stoichiometric amount of TiCl4 (scheme 2.1).78 Since then, a series of metal halides
used as conventional homogeneous Lewis acid catalysts such as BF3, AlCl3 and FeCl3 were
also developed as the catalysts by the same authors.23 However, strictly anhydrous conditions
and quite low temperatures were always necessary, otherwise moderate or low yields were obtained. On the other hand, organic reactions in water are of current interests for environmental and economic concerns. However, during a long period of time, it was considered that the Lewis acid catalysts were incompatible with water because of their moisture sensitivity, thus being easily decomposed or deactivated during the processes. Therefore, much effort has been made to produce water-tolerant catalysts for the Mukaiyama aldol reactions. In the 1990s, rare earth triflates were revealed to exhibit excellent performance in the Mukaiyama aldol reactions in the water/organic solvents systems.79-82 For
instance, it was discovered that Sc(OTf)3 was efficient in the Mukaiyama aldol reactions of
different silicon enolates with aldehyde in water-THF mixture.81 The discovery of rare earth
metal triflate as catalysts for the Mukaiyama aldol reaction has inspired extensive research to expand the availability of Lewis acid catalysts. Heterogeneous catalysts were then employed as alternatives for going further toward a greener process due to their easy recovery and recycling. The most prominent and early developed examples of heterogeneous catalysts comprising Lewis acidic sites are based on microporous zeolites.83-85 Kumar found that
TS-1 and Ti-Beta zeolites could be used as active solid catalysts in the Mukaiyama aldol reaction.86,87 However, the low yields of reaction led to the speculation that the too small
micropore environment in zeolites prevents the contact of the reactants with the Lewis acid sites, decreasing the ability for the chemical transformation. Therefore, the pores of supports for the acid sites were expanded to the meso region. For example, Ti-containing mesoporous materials showed satisfactory yields as high as 98% in solvent-free systems.17 According to
mesoporous silica functionalized with sodium-benzenesulfonate for the Mukaiyama aldol reaction, the thus-prepared materials displayed a much higher reactivity than that of homogeneous catalysts, such as Sc(OTf)3.88-89 Surprisingly however, there is still only very
few supported or heterogeneous catalysts reported for the Mukaiyama aldol reactions. More scientific work needs to be done to expand the use of the heterogeneous catalysts in the Mukaiyama aldol reactions, and further in other organic synthesis.
It should be highlighted that Kobayashi and co-workers have established criteria to understand the Lewis acidity of different metals, which are efficient for Mukaiyama aldol reactions in the aqueous medium. The catalytic activities of metal cations in water could be somehow determined by their hydrolysis constants (Kh) and water-exchange rate constants
(WERCs). WERCs correspond to the exchange rate constants for the substitution of water ligands. The pKh and WERC values of different ions are shown in Figure 2.9. The range of
pKh of these active metal cations varied from 4.3 (Sc (III)) to 10.08 (Cd(II)) while the WERC
values are all larger than 3.2 × 106 M–1 s–1. In general, cations with small pK
h values tend to
be efficiently hydrolyzed. For pKh values less than 4.3, the metal cations undergo easy
hydrolysis and the protons produced from hydrolysis result in the decomposition of the silyl enol ethers. On the contrary, if the pKh values are larger than 10, the metal cations will be not
able to catalyze the aldol reaction because of its weak Lewis acidity. The concept of the hydrolysis constant and WERC can help us understand the catalytic activity and Lewis acidity of different metals in an aqueous medium, thus choosing an appropriate metal to be introduced into the support for Mukaiyama reaction.90
Figure 2.9 Hydrolysis constants (Kh) and water-exchange rate constants for determining Lewis acidity.
Chapter 3
Characterization techniques for synthesized materials
The materials synthesized in this thesis were characterized by various techniques to understand their physical and chemical properties thoroughly.
3.1 Nitrogen adsorption
Adsorption can be categorized into two types: physisorption and chemisorption. When a gas adsorptive contacts the surface of an adsorbate, physisorption occurs. The interaction involved in physisorption is mainly the van der Waals forces, while chemical bonding occurs in them in the chemisorption process. Gas (commonly nitrogen, argon or krypton gas) adsorption is one of the most commonly used and efficient experimental methods for measuring the pore information of the porous materials including specific surface area, pore size distribution, pore volume, and porosity. The processes of physisorption are usually presented as graphs known as adsorption isotherms, which illustrate the relationship between the gas amount adsorbed on the surface of the adsorbate and the relative pressure P/P0 at a
given temperature, where P0 is the saturation pressure of the gas used in the measurement.
The shape of the adsorption isotherms is strongly affected by the porous structure of the measured materials as well as the strengths of fluid-wall and fluid-fluid interactions.91 Therefore, one can deduce the porous structure and some other pore-related information on the basis of the adsorption isotherms. According to the notation of the IUPAC, the adsorption isotherms (Figure 3.1) can be classified into six shapes or types, and the pores are proposed to be categorized by their pore width.
Figure 3.1 Different types of the physisorption isotherms classified by IUPAC.92
Type I is the characteristic isotherm of microporous materials such as zeolites whereas, type II and III usually refer to the physisorption isotherm of macroporous or non-porous materials. Type IV and V isotherms corresponds to mesoporous materials. Type VI is an unusual type of isotherm, representing a layer by layer adsorption, which is rarely observed. The presence of a hysteresis loop in type IV and V isotherms can be seen only for mesoporous materials. For microporous materials, the adsorption behavior is mainly determined by the fluid-wall interactions, resulting in a continuous filling of gases in the pore without phase transition. However, the fluid-fluid interactions also exist during the adsorption process of gases into the mesopores, leading to multilayer adsorption, in other words, pore condensation, which is often accompanied by the hysteresis loop.93-95 Therefore, the pore size range can be determined based on the presence or the absence of the hysteresis loop. In a word, when using nitrogen as the adsorptive at 77 K, no hysteresis loop will be observed if the pore size was below 4 nm.96
The pore structure can also be identified based on the shape of the hysteresis loop in the isotherms. It was classified into four different types by IUPAC. (Figure 3.2) Type H1 loop represents highly uniform cylindrical pores with a narrow pore size distribution. Type H2 corresponds to the presence of ink bottle mesopores. H3 type hysteresis is associated with
adsorption-desorption process in narrow slit-like mesopores, can also be caused by the structural defects in mesoporous materials.63
Figure 3.2 Classification of adsorption-desorption hysteresis loops.97
3.1.2. The BET specific surface area
The specific surface area is increased if a particle exhibits pores. Surface area is one of the most significant parameters when a porous material or functionalized porous material is designed as a catalyst. Hence, to evaluate the catalytic activity of a material, its surface area should be measured. The most commonly used method is the BET method proposed by Brunauer, Emmett, and Teller.98 The BET equation (3.1) is frequently applied for the evaluation of specific surface area:
1/(𝑛(𝑝0/𝑝) − 1)) = 1/𝑛𝑚𝐶 + (𝐶 − 1)/𝑛𝑚𝐶 × 𝑝/𝑝0 (3.1)
in which n is the total gas amount adsorbed at p/p0; nm is the monolayer (saturated) capacity
of the adsorbate, C is the BET constant which gives some indication of the heat of adsorption and condensation in the first adsorption layer.99
Usually, there are two stages involved in the calculation of the BET specific surface area. First, the monolayer capacity of the adsorbate, namely nm in the BET equation, should be
determined. It can be obtained by transforming a physisorption isotherm into a plot. In general, the BET equation could be expressed as a linear equation f(p/p0) = a(p/p0) + b. A
linear line, which is called BET plot, could be observed for most mesoporous materials in the relative pressure region of 0.05-0.3.93,94 The monolayer capacity nm and BET constant C
can both be calculated from the slope of the plot and the Y-intercept. The specific surface area is obtained from the following equation (3.2):
𝑆 = 𝑛𝑚× 𝑁𝑎× 𝜎 (3.2)
in which σ is the cross-sectional area of the adsorbate (16.2 Å2 if nitrogen was used as an adsorbate), Na is the Avogadro constant (6.02 x 1023mol-1). It should be noted that the BET
equation is applicable in most cases such as mesoporous and nonporous materials, however, is not suitable for the microporous adsorbents because of the difficulty to distinguish the multilayer adsorption from the micropore filling process.100
3.1.3. Pore size analysis
Different theoretical methods are applied to determine the pore size of mesoporous materials. The Barret-Joyner-Halenda (BJH) method101 which originates from Kelvin equation is used for the mesopore size analysis.
ln(𝑝/𝑝0) = −2γcosθ/RTρ(𝑟𝑝− 𝑡𝑐) (3.3)
p, equilibrium vapor pressure, P0, the saturated pressure, γ, the surface tension of the liquid
condensate generated from nitrogen gas, R, the gas constant, and T, the temperature. θ is contact angle of the liquid meniscus against the pore wall, r is the radius of the pore, and tc
the thickness of an adsorbed multilayer film formed before the pore condensation.
However, the BJH method fails to assess the thermodynamic and thermophysical properties of the confined pre-adsorbed fluid, leading to an underestimation of the pore size, especially for those materials with pores under 10 nm.93,94,96 Therefore, nonlocal density functional
suitable method for the pore size analysis. The details of the local fluid structure near the solid wall of the surface can be provided by NLDFT, as well as the arrangement of the gas molecules adsorbed in pores. The advantage of the DFT method is that it considers the characteristics of the hysteresis, which is determined by the pore shape of the porous materials. Consequently, one can choose corresponding theoretical isotherms or kernels for pores of respective pore geometry with different pore sizes to obtain the pore size distributions. The NLDFT method could be employed for analyzing the pore size of materials in all the range when an appropriate kernel model is chosen.45,102
3.2 Powder X-Ray Diffraction (XRD)
Powder XRD analysis is one of the most widely used and powerful techniques for the characterization of solid materials. When a monochromatic X-ray beam with a wavelength of λ strikes a solid crystalline sample, the scattered X-ray will interfere constructively, which gives rise to a diffraction peak. The geometry of the diffractogram can be described by using the Bragg law. Since atoms in a single crystal present excellent periodicity, the distance d between two lattice planes (dhkl) can also be calculated based on the Bragg law.
nλ = 2dsinθ (3.4)
where n is an integer number of wavelengths also called, λ is the wavelength of the X-ray radiation, and θ is the diffraction angle.
The powdered XRD diffraction patterns can be classified into low-angle XRD and wide-angle XRD according to the diffraction wide-angles. For the crystalline materials, due to the small distance between lattice planes, the diffraction peaks are usually collected in the 2 theta region between 10-80º. On the other hand, in the case of ordered mesoporous silica, which is amorphous at the atomic scale, the reflected intensities in the diffraction patterns are not from the ordered arrangement of atoms, but from the periodicity of mesopores arrays. Therefore, the diffraction peaks of are usually observed at low diffraction angles, typically in the 2-theta range of 0.8 and 5 º. From these reflection peaks determined by the unit cell parameters and symmetry of the investigated solids, on can reduce the size and symmetry of the lattice. Like liquid phases, mesoporous silica exhibits hexagonal, lamellar, or cubic phases. Figure 3.3
hexagonal symmetry. The typical XRD diffraction patterns of MCM-41 and SBA-15 usually exhibit four or five reflections indexed to (100), (110), (200), (210) and (300), corresponding to the hexagonal space group. Furthermore, the unit cell parameter in a hexagonal lattice can be calculated as follow:
1 𝑑2 = (
4
3𝑎2) (ℎ2+ 𝑘2+ ℎ𝑘) + (𝑙2/𝑐2) (3.5)
Hence, the unit cell parameter can be obtained as k = 0 and l = 0, 𝑎 = 2𝑑100/√3(𝑤𝑖𝑡ℎ𝑙 = 0) (3.6)
In summary, powder XRD is a powerful method for the investigation of mesoporous materials. It provides the necessary information of the mesostructures, as well as the domain size of the crystals.
Figure 3.3 Typical low-angle powder XRD patterns obtained for MCM-41(A)34 and SBA-15(B)41,
3.3 Electron microscopy
Electron microscopy is one of the most useful methods to determine the mesostructure. There are two typical electron microscopy techniques for imaging the siliceous materials, namely, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In principle, an electron microscope uses a focused beam of electrons to examine the materials on a very fine scale. As illustrated in Figure 3.4, when the electron beam hits a specimen, different energy signals can be produced depending on various interactions between an electron beam and a sample.
Figure 3.4 The main types of the signal generated by the electron beam-specimen interaction.103
3.3.1 Scanning electron microscopy (SEM)
Scanning electron microscopy (SEM) characterization is a powerful technique primarily for the study of the morphology and the topography of the materials in the form of a solid on a scale down to 10 nm. The information of topographical features, particle aggregation, could be imaged by SEM. When it is coupled with energy-dispersive X-ray spectroscopy (EDX), the compositional difference within the material could also be obtained by SEM.
