Tuning the Activity of Zeolite Catalysts: From Defect Engineering to the Design of Bifunctionality
Jeffrey D. Rimer
University of Houston
Zeolites are commonly used catalysts in both traditional and emerging (petro)chemical processes owing in large part to their tunable acidity, which can be engineered through the incorporation of various framework and extra-framework Brønsted and Lewis acid sites. This talk will describe recent efforts by our group to understand the nature of Al incorporation as well as its substitution with heteroatoms (e.g., Ga, B, Ti, etc.). The first part of the talk will focus on zeolite ZSM-5, which is highly prone to contain defects in the form of residual amorphous material, crystallographic dislocations, and non-framework or extra-framework Al species. Minimizing these defects is a subject of interest owing to their correlation with the onset of catalyst deactivation. Here, we will discuss how defects are more prevalent in hierarchical ZSM-5 materials than previously reported in literature, focusing on five classes of nanosized/hierarchical zeolites: nanosheets (ca. 3 nm thickness), self-pillared pentasils (ca. 3 nm thickness), nanoparticles (ca. 20 nm), finned zeolites (ca. 30 nm fins), and coreshells (ca. 10 nm shell thickness). Using methanol to hydrocarbons (MTH) as a benchmark reaction, we have shown that as-synthesized materials are universally less active than either commercial or conventional ZSM-5; however, our studies reveal that catalyst performance can be dramatically improved via post-synthesis treatments to reduce defects, which include hydrothermal annealing wherein crystals are subjected to a saturated siliceous solution at high-temperature. Our findings reveal that the removal of defects from ZSM-5 catalysts result in as low as 3-fold to as high as 10-fold increases in turnover number compared to their as-synthesized counterparts. In the second part of the talk, we will show how Al can be substituted with other heteroatoms that either reduce the overall acidity or add Lewis acids as bifunctional catalysts. Examples span Ti-zeolites for epoxidation reactions to Ga-zeolites for dehydration reactions. We have also been exploring the use of metal-substituted zeolites for oxygen-assisted oxidative dehydrogenation of propane (ODHP) to produce propylene as well as the upgrading of model biomass compounds to aromatics. Here, we will discuss methods of synthesizing zeolites catalysts with different active site speciation as a means of developing structure-performance relationships. Our collective findings aim to better understand the impact of active site identity (speciation) on the performance of zeolite catalysts for diverse applications.