Shape selectivity of beta and MCM-49 zeolites in liquid-phase alkylation of benzene with ethylene

• H-beta zeolite with 12 MR channels has better accessibility of active centers.
• Inner channel alkylation of benzene with ethylene leads to poor EB selectivity.
• Acids in 10 MR channels of MCM-49 form little EB due to transition state selectivity.
• Better diffusion into supercages via 10 MR windows of MCM-49 improves performance.

NH3, pyridine and 2,4,6-trimethyl-pyridine were selected as the base probes to characterize the different pore acidity of H-beta and H-MCM-49 zeolites, revealing the essence in their catalytic performance from the view of shape selectivity in different pore systems. Based on the size of base probes, reactants, product, transition states and pore channels, the difference in catalytic performance over H-beta and H-MCM-49 was investigated. Easy diffusion of reactants into and long diffusion distance for ethylbenzene escaping out of confined space within beta are the main reason for superior activity and poorer ethylbenzene (EB) selectivity. As for H-MCM-49, acid sites located in 10 MR channels contributed little to the formation of EB due to transition state selectivity; Acid sites located in 12 MR cups on the outer surface with little restriction on the diffusion of reactant/product toward/from the active centers were the major active centers for EB formation, which could be detected by 2,4,6-trimethyl-pyridine; There was diffusion limitation on reactant/product in 12 MR supercages through 10 MR windows to influence activity/selectivity. The enhancement in activity and selectivity in liquid-phase alkylation of benzene with ethylene over MWW zeolites should be mainly focused on transforming confined 12 MR supercages into 12 MR cups with improved openness.

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