Synthesis, Characterization And Applications Of Hierarchical Porous Materials

According to the IUPAC, porous materials can be classified into threetypes: microporous, mesoporous, and macroporous. Microrous materials arecharacterized by pore sizes of < 2 nm. Pores are in the range of 2-50 nm formesoporous structures, and those of macroporous materials are generallyregarded as being > 50 nm in diameter. The microporous materials have beenwidely used in many fields such as ion-exchange, adsorption and separation,and industrial catalysis, because of their unique pore structures and characters.However, microporous materials cannot effectively deal with large moleculesdue to the limitation of the pore size. For example, when microporous zeolitemolecular sieves are used as catalysts, the size of the reactants is limited to 1.2nm, which greatly restrains their applications in catalysis of large molecules.The discovery of mesoporous molecular sieves overcomes the problem ofmicroporous zeolite molecular sieves. They show great potential applicationsin catalysis of larger molecules and attract much attention because of theiruniform and larger mesopores. However, compared with microporous zeolite, mesoporous materials show poor hydrothermal stability and weaker catalyticactivities, which seriously limit their extensive uses. Generally, themacroporous materials cannot sieve molecules, since their pore size exceeds 50nm. The applications of porous materials are sometimes significantlyinfluenced by their sole porosity. It is highly desirable to obtain facile routes tohierarchical porous materials, combining advantages of multi-porosity. Thehierarchical porous structure with high diffusion, surface area, and porevolume will lead to their high accessibility and high storage capacity in manypossible applications. In this thesis, we focused on design, preparation andrelated application of hierarchical porous materials, investigating theirstructures, physic-chemical characters and their catalysis.It is well known, the polystyrene spheres are used as the template forcreating large pore. Therefore, in chapter three, the noncross-linked polymerparticles with different and well-defined particle size were prepared viaemulsifier-free emulsion polymerization and dispersion polymerization. Inaddition, the noncross-linked polymer particles with functional groups on thesurface were also prepared via emulsifier-free emulsion polymerization. TheSEM was used to characterize the morphology. Then, we selected the suitablePS to the template for creating macropores.The reaction of phenol with tert-butyl alcohol is typical of Friedel-Craftsalkylation. The alkylated products of Para-tert-butyl phenol (4-TBP) and 2,4-di-tert-butyl phenol (2,4-di-TBP) are important intermediates widely used inindustry as raw material for preparation of a variety of resins, antioxidants,varnishes, surface-active agents, ultraviolet absorbers, and as petroleumadditives, etc. Therefore, investigating this alkylation reaction has great commercial interests as well as academic interests. It is well known that thephenol tert-butylation is a kind of acid-catalyzed reaction. The catalysts used inthe alkylation reaction include liquid acid catalyst (Lewis acids or Br?nstedacids), heterogeneous ion-exchange resins and solid acid catalysts (zeolites ormesopore molecular sieves), etc. Solid acid catalysts to catalyze alkylation ofphenol with tert-butanol have been widely studied due to their inherentadvantages such as large surface, strong acidity, high thermal stability, andrecyclability. Additionally, zeolites catalysts are environment friendly, whilethe limited pore size makes it difficult for substrates and products to diffusefrom the narrow channel of zeolite. However, when classic mesoporousaluminosilicates are used as catalysts, both phenol conversion and 2,4-di-TBPselectivity are very low due to the weak acidity. In the chapter four, hierarchicalporous materials were prepared under hydrothermal conditions throughpolystyrene (PS) colloidal spheres and tetrapropylammonium hydroxide(TPAOH) dual templates method. The physical and acid properties of thehierarchical porous materials were characterized by XRD, SEM, N2 adsorption,27Al-MAS NMR and NH3-TPD techniques. Catalytic tests showed that thehierarchical porous catalysts exhibited high catalytic activity for alkylation ofphenol with tert-butanol than those of conventional ZSM-5 zeolite and classicmesoporous Al-MCM-41. The high phenol conversion and 2,4-di-TBPselectivity are mainly assigned to the presence of the hierarchical porosity andstrong acidity. The macropores offer easier transport and access to the activesites.In the chapter five, hierarchical porous materials were prepared usingLayer-by-Layer self-assembly method via templating with PDDA-modified polystyrene (PS) colloidal spheres. The physical and acid properties of thehierarchical porous materials were characterized by XRD, FT-IR, SEM, N2adsorption/desorption, techniques. In conclusion, PDDA has an important effecton the preparation of hierarchical porous materials.In the chapter six, we showed that the assembly of AlPO precursor andsilicate-1 zeolite nanocluster with the surfactant of cetyltrimethylammoniumbromide (CTAB) could synthesize mesoporous SAPO, which contains MFIprimary building units in its mesoporous walls. SAPO were characterized byXRD, FT-IR, TG, N2 adsorption/desorption, TEM, etc. Similarly to othermesoporous materials prepared from zeolite zeolite nanoclusters, SAPOexhibited relatively strong acidity and might therefore be used as catalyst incertain catalytic process.Above all, by the different method, hierarchical porous solid materialshave been developed and these new materials with advanced performanceswould play important roles in many applications. We are confident that newsurprises in material synthesis and properties will emerge and that theexcitement of the field will continue in the upcoming years.