Characterizing the combined effects of mesoporosity and crystal size on conversion, product selectivity and deactivation rates on hierarchical BEA zeolite

Abstract

Zeolites are aluminosilicate, solid Brønsted acid catalysts with pores and voids of molecular dimension that sieve molecules according to size and shape. They have commercial significance in many industries such as fine chemicals, petroleum, and water treatment. A significant challenge with zeolites is their sensitivity to deactivation via formation of carbonaceous deposits (i.e., coke). During catalyst operation, pores and active sites may become blocked, which hinders reactants and desired products from diffusing into or out of pores, respectively. Varying crystal diameters and the mesoporosity can influence diffusion and thus, potentially, deactivation rates. This thesis seeks to determine if changing both parameters simultaneously in BEA (a three-dimensional, large-pore, industrially relevant zeolite) affects deactivation rates and selectivity to the desired, bulky product (1,3,5-trimethyl-2-benzylbenzene; TM2B) of Friedel-Crafts alkylation of 1,3,5-trimethylbenzene (TMB) with benzyl alcohol (BA). The effects of varying crystal diameters were tested experimentally, and then utilized to model the additional effects of induced mesoporosity. Experimental evidence confirms that the longer diffusion pathways of micro BEA (0.91 μm crystal diameter) result in a lower final BA fractional conversion (X 120 min = 0.24), as well as lower final TM2B fractional selectivity (S 120 min = 0.35) than for nano BEA (X 120 min = 0.76, S TM2B,120 min = 0.51). Subsequent reaction-diffusion models for hierarchical analogs of micro BEA and nano BEA (micro BEA-H and nano BEA-H, respectively) indicate that while all four BEA systems operate in the reaction-limited regime (η ~ 1), hierarchical systems exhibit enhanced TM2B diffusion (η hierarchical > η parent). The extent of diffusion enhancement depends on predicted reaction rates for hierarchical systems that must be verified experimentally in future work.