The Ordered Assembly Of Nano-Catalysts And Their Applications

Recently, the nanoparticles, nanoporous materials and nanoflakes with diverse compositions such as metal, oxides, zeolites, and phosphates have been synthesized accompanying with the development of nanoscience and nanotechnology. Because of their high surface areas, small sizes, special surface electronic properties and quantum effects, the application of nanomaterials in the domain of catalysis will hopefully achieve a series of the goals for an idealized highly efficient catalyst including the high reactivity, low catalyst adopting amount, special reactant/product selectivity, and so on, which will possibly initiate a technological revolution in catalysis. Thus, the catalytic behaviors of different kinds of the nanomaterials are paid special attention. However, for most of the nanomaterials, their small sizes and high surface energies often leads to the serious agglomeration during the reactions, making them unstable and sometimes difficult to be directly handled in real reactions. The common used method to stabilize the nanomaterials is to support them onto the surface of the inert inorganic porous materials such as silica and alumina, by taking use of the dispersion effect of the supports, the nanomaterials become stable and could be further applied in its further catalytic applications.'Nano-assembly' is a recently developed technique for the preparation of nanomaterials, which is based on the interactions between atoms or molecules such as chemical bond, hydrogen bond, electrostatic interactions, etc. to combine and rearrange two or more nano-building-blocks into an ordered structure to obtain the nanocompositess. With the highly demands of integration in nanosciences, such assembly method has played an important role in the preparation of novel multi-functional nanomaterials, which have found many applications in nano-optical materials, nano-electronic devices, biomimic materials or biological materials, etc. In the domain of catalysis, compared with the simple supporting method for the preparation of nanocatalyst, the adopting of nano-assembly method to design the catalyst will achieve the ordered arrangement of multi-functionalities, and such cooperative effect of the assembled multi-functionalities will not only endow the catalyst with more applications but also help the catalyst to be adapted for more complex reaction conditions. Among the various nanocatalysts with ordered structures, the 'core-shell' typed micro-capsular reactor has received much research interests. Such structure possesses the similar structure character like the natural cell and during the catalytic reaction, its shell will not only protect the inner encapsulated active species from agglomeration but more importantly, it could selectively sieve the various type of the reactants in the outer reaction environment from entering into the inner core to contact with the active species, endowing the catalyst with special selectivity. The special reactive microenvironment forms in the inside of the microcapsules during the reaction, achieving the concentration of the reactants and active species in such environment, leading to the high catalytic efficiency. On the other hand, the integration of multi-functionalities could also be realized by introducing more active sites onto the surface of its shell to adapt for the more complex real catalytic systems, behaving the functionalities like the artificial cell in some extent. Thus, the design and application of more effective 'core-shell' typed microcapsular reactor has become an important research interest in the domain of catalyst design.Based on the research status and problems of the ordered assembly of nanocatalysts, this thesis mainly focuses on the design and synthesis of 'core-shell' typed microcapsular reactor, several academic and industrial important catalytic reactions have been adopted to characterize its superior catalytic efficiency over the conventional supported type of the catalyst. The research work is categorized and discussed in the six chapters of this thesis.Chapter 2 involves the developing of an effective method for the preparation of hollow microcapsules with diverse and adjustable shell compositions. The main work is the adopting of mesoporous silica sphere as the template, after its pore channel has been incorporated with noble metal, metal oxides, sulfides, telluride nanoparticles, the template is coated with an outer layer of cationic polyelectrolyte and processed with hydrothermal treatment in alkaline solution to obtain the series of noble metal, metal oxides, sulfides and telluride hollow microcapsules. From our research, it could be revealed that both the mesoporous silica spheres and the outer polyelectrolyte make great contribution for the successful formation of hollow microcapsules. During the synthesis, the former plays a role like the reservoir for the accommodation of guest nanoparticles and offers the spherical template, while the latter acts as the protective exo-net and spherical scaffold. On the other hand, the formation mechanism of the hollow microcapsules has been studied in detail by monitoring the morphological evolution of the sample combining with the variation of silica content in the sample. Such synthetic method is a general and applicable method for the formation of alkaline-stable hollow microcapsules, the obtained microcapsules have wide application prospect in the domain of catalysis, advanced optical or electronic devices and biological materials, etc.Chapter 3 discusses the formation of mesoporous hollow noble metal or noble metal oxide microcapsules and the mesoporous carbon-platinum 'core-shell' typed microcapsules via the peripheral nano-casting method by taking the use of mesoporous silica spheres as the template. Through the change of the infiltration depth of the noble metal species in the mesoporous pore channels, a series of the hollow noble metal microcapsules with diverse shell thickness could be obtained, while the adopting of the high temperature readily oxidized noble metals such as palladium, ruthenium or iridium as the precursor for the nano-casting process leads to the noble metal oxide hollow microcapsules. By applying the electrooxidation of methanol as the model reaction, the Pt-series materials including Pt hollow microcapsules and mesoporous carbon/Pt core shell behaved higher reactivity than that of the conventional discrete noble metal nanoparticles such as the PVP-protected Pt nanoparticels or the nanoparticles obtained via crushing the shell structure of Pt hollow microcapsules. Furthermore, the mesoporous carbon-platinum 'core-shell' typed microcapsules behaves the highest reactivity, which is probably derived from its highly porous core/shell or the cooperative effect between the carbon core and the porous Platinum shell.Chapter 4 studies the assembly of zeolitic microcapsulized palladium nanocatalyst and its further use in quasi-homogeneous fine chemical Heck coupling reactions between the aryl iodides and olefins. Because of the quasi-homogeneous mechanism consisting of the active palladium species dissolution, the conventional supported type of the catalyst such Pd/C and etc. has the high tendency of Pd leaching into the reaction solution, which leads to the dissolution of the Pd species from the catalyst and in consequence, the loss of the reusability of the catalyst and the increasing of the total production cost accompanying with the inevitable heavy-metal pollution in the product. The zeolitic microcapsulized catalyst prepared in this work behaves the high reusability during the reaction, which retained most of its reactivity after 10 times of reuse. By a series of the characterization on its textural properties and reactivity, we have found that the zeolitic shell in the catalyst could effectively prevent the soluble palladium species from being leached into the reaction solution because of the special molecular sieving effect of the zeolitic shell, endowing the catalyst with high reusability.Chapter 5 involves synthesis of zeolitic microcapsulized platinum and silver nanocatalyst, and applied the obtained catalyst in the molecule oxygen catalyzed aryl alcohol oxidation in liquid phase and aliphatic alcohol with low carbon numbers oxidation in gas phase. Such type of the reaction fulfill the requirements of the green chemistry and economics with its high selectivity, low cost, which aroused much research interest both in academic and industrial aspect. However, for most of the noble metal participated reactions, since the trace amount of sulphor or nitrogen containing heterocyclic compounds in the reactant, such molecules strongly bonds onto the active surface of noble metals during the reaction, which often leads to the poisoning of the catalyst. On the other hand, the life of the catalyst and its reusability also influence its large scale application in real catalytic process. Thus, the obtaining of the noble metal catalyst with high poison-resistance, high stability and good reusability is the key factor of the application of such type of catalyst. In the systematical works of comparing the different textural property, adsorption behavior of the guest molecules and the catalytic behaviors of zeolitic microcapsulized catalyst, shell-crushed zeolitic microcapsulized catalyst and commercial supported type of the catalyst in this chapter, the contribution of the zeolitic shell for the superior reactivity, reactant selectivity, stability, poison-resistance and reusability has been concluded. Only the samples with the complete zeolitic shell could behave the above advantages. Furthermore, we have also found that such thin zeolitic shell not only make little influence on the reactant/product diffusion properties but endow the catalyst with good dispersion ability in liquid phase reaction and enough mechanical strength for the pretreatment of gas phase reaction, which will largely widen the application fields of such zeolitic microcapsulized catalyst.Chapter 6 discusses the synthesis and functionalization of mesoporous zirconium(â…£) phosphate-phenylphosphonate materials. This chapter and the following chapter are both extension works. In this work, the synthesis and functionalization process of the non-silica based mesoporous zirconium(â…£) phosphate-phenylphosphonate materials have been introduced in detail. Through a series of the characterization method, the formation of mesostructure has been proved. As a new type of the non-silica based mesoporous materials, it possesses high surface areas and thermal stability. Furthermore, the phenyl groups in the pore channel could be functionalized through a series of the electrophili-substitution reaction to further incorporate the functional groups such as sulphonic, bromo and nitro groups., The samples after sulphonation and brominating well retained its mesostructure, however, the nitro-substituted materials behaves partial destruction of the pore structure. Based on the results of the ammonia microcalometric, thermal gravimetric and esterification reaction between benzoic acid and methanol, it could be clearly seen that the sulphonated samples not only behaves the good thermal stability, but the increasing amount of acid sites endowed it with higher reactivity. The functionalized mesoporous zirconium(â…£) phosphate-phenylphosphonate materials will find more applications in the domain of catalysis, adsorption and separationChapter 7 involves the synthesis of layered silver(â… )-long chain alkylamine complex and its in-situ reduction. The common method for the synthesis such kind of complex is based on the coordination of silver ions with long chain alkylamine and the toxicity of the solvent and the nonspecific morphology of the product often inhibit the application of such kind of material. In this work, a series of layered silver(â… )-long chain alkylamine complex has been synthesized in the alcohol/water system for the first time, and such compound possesses the special sponge-like macroporous structure. Its spaces between each layer could be adjusted by changing the length of the carbon chain in the alkylamine compounds. Through the in-situ reduction with KBH₄, a series of the silver(0) -alkylamine complexes has been obtained, the special structure and morphology of such kind of complexes will find wide application in the construction of catalysis, photonic or electronic materials.