(368bg) Optimizing Hierarchical Zeolite Properties By Post-Synthesis Treatments

Zeolites are nanoporous aluminosilicates with unique properties for shape-selective catalysis in numerous commercial applications. Recent efforts to optimize zeolite catalyst performance have largely focused on developing synthesis approaches to produce materials with reduced mass transport limitations. Two of the most effective approaches to address these synthetic challenges are the generation of nanosized zeolites and the introduction of mesopores and/or macropores (i.e., hierarchical zeolites). Both of these processes typically require organics as structure-directing agents. We recently demonstrate an organic-free, one-pot (direct) synthesis [1] of hierarchical ZSM-5 zeolites comprised of interconnected nanosheets, which are referred to as self-pillared pentasils (SPPs). These materials are intergrowths of two different zeolites (MFI and MEL frameworks) with five-membered rings (i.e., pentasils). These SPP zeolites have been shown to exhibit superior catalytic performance compared to traditional zeolite ZSM-5 counterparts.

In this presentation, we will describe various post-synthesis treatments that can be used to reduce defects and enhance the hydrothermal stability, which ultimately improve catalytic performance. Recent findings from our group have shown that as-synthesized ZSM-5 is prone to the formation of defects – a phenomenon that is more prevalent in nanosized and hierarchical materials. Here, we will describe the nature of these defects and highlight the most effective ways to mitigate their deleterious effects. Examples include the use of passivated siliceous exterior surfaces (core-shells) that improve mass transport properties and eliminate non-selective reactions on the exterior surfaces of zeolite catalysts. Comparisons will be made for several classes of nano-sized and hierarchical materials to demonstrate the high efficiency of SPP zeolite catalysts, using methanol to hydrocarbons as a benchmark reaction for comparing both in-house and commercial ZSM-5 catalysts. Our studies of synthesis, post-synthetic treatments, and catalytic testing are coupled with state-of-the-art characterization using techniques such as solid-state NMR and X-ray absorption to elucidate aluminum speciation in the framework. Lastly, we will demonstrate the feasibility of scale-up for this new class of zeolites to emphasize their potential for future commercialization.

References:

[1] Jain, R., Chawla, A., Linares, N., Garcia Martinez, J., Rimer, J.D.: “Spontaneous Pillaring of Pentasil Zeolites” Adv. Mater. 33 (2021) 2100897