Engineering Compositional Gradients in Ti-Silicalite-1 Catalysts for Improved Olefins Epoxidation

Transitioning from conventional aluminosilicate zeolites to isostructures with alternative heteroatom compositions is a subject of growing interest because the unique properties of these materials offer advantages for relevant catalytic reactions. Replacing aluminum with different metal sites can alter the physicochemical properties of zeolites in ways that improve their catalytic performance. In this study, we focus on designing titanosilicalite-1 (TS-1) zeolite catalysts with controlled morphology and Ti atom zoning that improve performance for olefin epoxidation reactions. Here we introduce two TS-1 catalysts prepared by leveraging direct TS-1 synthesis with recent developments in the generation of MFI-type zeolites comprising finned and eggshell configurations. These synthetic approaches create silicalite-1@TS-1 materials in which the Ti active sites reside preferentially on highly accessible exterior regions of silicalite-1 crystals. Direct synthesis of nanosized TS-1 (<100 nm) is nontrivial; however, these findings demonstrate that finned materials behave as pseudo-nanoparticles (30–50 nm fins) while eggshell TS-1 materials function as pseudo-nanosheets (10–20 nm thickness). Structure–performance relationships are established by comparing finned and eggshell TS-1 catalysts against commercial and in-house TS-1 analogues using 1-heptene epoxidation as a benchmark reaction. Both finned and eggshell TS-1 reduce reactant and product contact times with Ti sites within zeolite pores, and consequently, these catalysts exhibit greater epoxide selectivities and slower catalyst deactivation. Our study reveals that the silicalite-1@TS-1 materials outperform conventional TS-1 with respect to turnover rates and 1,2-epoxyheptane selectivities. Collectively, these findings highlight alternative routes to tailor the properties of metal-substituted zeolites for improved catalytic performance in commercially relevant classes of reactions, particularly those that form products susceptible to secondary decomposition.