Conversion Reactions in Surface-Functionalized Mesoporous Materials: Effect of Restricted Transport and Catalytic Site D
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Conversion Reactions in Surface-Functionalized Mesoporous Materials: Effect of Restricted Transport and Catalytic Site Distribution Jing Wang1,2, David M. Ackerman1,3, Kapil Kandel1,3, Igor I. Slowing1, Marek Pruski1,3 and James W. Evans1,2,4 1 Ames Laboratory – USDOE, and Departments of 2Mathematics, 3Chemistry, and 4Physics & Astronomy, Iowa State University, Ames, IA 50011, U.S.A. ABSTRACT We analyze the interplay between anomalous transport and conversion reaction kinetics in mesoporous materials functionalized with catalytic groups. Of primary interest is functionalized mesoporous silica containing an array of linear pores with diameters tunable from 2-10 nm, although functionalization can produce smaller effective diameters, d. For d < 2 nm, transport and specifically passing of reactant and product species within the pores can be strongly inhibited (single-file diffusion). The distribution of catalytic groups can also vary depending on the synthesis approach. We apply statistical mechanical modeling (utilizing spatially discrete stochastic lattice-gas models) to explore the dependence of reactivity on the extent of inhibition of passing of species within the pore, as well as on the distribution of catalytic sites. INTRODUCTION Functionalized mesoporous materials integrate the selectivity of homogeneous catalysts with the stability and separability of heterogeneous catalysts [1,2]. In the case of mesoporous silica, nanospheres with diameters of the order of 100 nm are synthesized via a soft-templating technique wherein a silica precursor (TEOS) aggregates around a self-assembled array of cylindrical CTAB micelles. Removal of CTAB produces mesoporous silica nanospheres (MSN) with a hexagonal arrangement of parallel linear nanopores with nominal pore diameters of around 2 nm or larger [3]. Control of surface properties is achieved by functionalization with suitable anchored groups serving as catalysts, and sometimes by additional groups modifying selectivity by acting as “gatekeepers” near the pore openings [4], or tuning activity (e.g., by strongly interacting with one of the products to alter the reaction equilibrium [5]). Functionalization by grafting of these groups after formation of the MSN should produce a higher loading of catalytic groups near the pore openings and likely also populate the exterior surface. In contrast, co-condensation during nanosphere formation produces a more uniform distribution within the pores. For MSN, it is significant to note that functionalization can reduce the effective diameter, d, below 2 nm. Then, transport can be strongly inhibited. The extreme case of “no passing” of reactants and products corresponds to so-called single-file diffusion [6]. There have been extensive studies of single-file diffusion systems often motivated by studies of transport and catalytic reaction in zeolites [7]. Typically, these studies emphasize the anomalous nature of tracer- or self-diffusion, this anomaly being reflected in a sub-linear increase (vs. a conventional linear increase) with time i
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