MFI Zeolite with Small and Uniform Intracrystal Mesopores

Abstract

The catalytic activity of zeolites [1] often is limited by the diffusion of reagents and reaction products through the framework pore network. Recently, several efforts have been made to reduce the diffusion path length in zeolites by confining crystal growth to a nanometer length scale [2][3][4][5][6][7][8] or by providing intracrystal mesopores during synthesis.[9-13] The latter strategy is intrinsically appealing, in part, because it precludes the need for colloidal crystal formation and avoids the drawbacks associated with nanoparticle processing. Moreover, the presence of mesopores in zeolite crystals offers the possibility of improving product selectivity in the catalytic cracking of polymeric molecules. [14] The embedding of carbon nanoparticles and certain polymers into zeolites crystals during synthesis has been shown to be an effective means of generating intracrystal mesopores.[9-13] These templating methods provide much better control of pore size than conventional steaming and chemical leaching approaches to mesopore formation in zeolites.[15] However, particle templates typically afford average pore sizes that are too large (! 10 nm) and pore size distributions that are too broad (> 10 nm widths at half maximum) for catalytic cracking reactions with high product selectivity.Herein we report a method to prepare templated zeolites with small intracrystal mesopores (average pore size 2.0-3.0 nm) and narrow pore size distributions (ca. 1.0-1.5 nm width at half maximum). We selected the MFI zeolite ZSM-5 to illustrate our approach, in part, because it is widely recognized for its unique properties as a catalyst for NO x reduction, Fisher-Tropsch chemistry, and toluene disproportionation, and as an additive for petroleum cracking. Scheme 1 illustrates our synthetic strategy for templating uniform mesopores within a zeolite matrix. In this scheme, a silane-functionalized polymer is used as a porogen for the formation of intracrystal mesopores. The presence of -SiO 3 units on the polymer allows it to be integrated into a silicaalumina sol-gel reaction mixture. Nucleation of the zeolite phase in the presence of the silylated polymer is accompanied by the grafting of the polymer to the zeolites surface through covalent Si-O-Si linkages. As the zeolite crystal grows, the

Zeólitas hierarquicamente estruturadas

“…Outros materiais zeolíticos, ou com características zeolíticas, foram preparados através desta metodologia, tais como a ZSM-5, zeólita Y e TS-1 (titanossilicato). 66 No mesmo período, Pinnavaia e colaboradores 18 empregaram polímeros funcionalizados com grupos silanos, na preparação das zeólitas hierárquicas. A principal vantagem desse método consiste na obtenção de cristais zeolíticos com mesoporos intracristalinos com diâmetros pequenos e uniformes, na faixa de 2,0 a 3,0 nm.…”

“…[9][10][11][12][13][14][15][16][17][18][19] As metodologias de síntese mais bem sucedidas envolvem o uso de agentes geradores de mesoporosidade (agentes orgânicos e nanopartículas) ou nanomoldes (moldagem em nanoespaços), que geram sólidos com mesoporosidade intracristalina com uma estreita distribuição de tamanho de poros; isto resulta em sólidos contendo mesoporos, além dos microporos intrínsecos das zeólitas. Além disso, é gerada uma mesoporosidade intercristalina, resultante da aglomeração dos cristais de zeólita com tamanhos nanométricos.…”

Metal oxide–zeolite composites in transformation of methanol to hydrocarbons: do iron oxide and nickel oxide matter?

“…7 In recent years several new avenues were developed to prevent coke formation and/or increase the catalyst activity. These avenues include the development of hierarchical zeolites, i.e., zeolites containing both micro-and mesopores, [8][9][10][11][12][13][14][15][16] minimizing internal framework defects in the zeolite structure, 17 formation of nanosized ZSM particles, 18,19 etc. The other approach to improve catalyst stability of ZSM-5 is its modification with various metals.…”

Investigating the Influence of Mesoporosity in Zeolite Beta on Its Catalytic Performance for the Conversion of Methanol to Hydrocarbons

“…[9][10][11] Conventional methods for preparing hierarchical zeolites are based on various postsynthesis processes such as steaming, 12 acid (or base) leaching, [13][14] and chemical treatments. [15][16][17] Recently, more efforts have been made towards the "direct synthesis" of hierarchical zeolites through soft-templating routes, [18][19][20][21] which can achieve greater mesoporosity, more precise control of pore size, and higher zeolite structure integrity compared with "post-treatment" methods. Hierarchical zeolites prepared by different methods have been tested as catalysts for various reactions, and typically, they exhibit higher catalytic activities (especially when the reactant molecules are bulky) , [22][23] longer lifetimes, [24][25] slower coke formation rates, 26 and altered product selectivities 27 compared to conventional zeolites with the same framework types.…”