Synthesis of Medium-Pore Brønsted-Acid Zeolites with Tailored Active Site and Crystallite Properties and their Application for Propene Oligomerization Catalysis

Elizabeth E Bickel, Purdue University

Abstract

Brønsted acid zeolites can be synthesized in a wide range of topologies, each characterized by diverse void sizes, shapes, and micropore connectivity. The location of Brønsted acid sites (H+-sites) within microporous voids of different size and shape, and the relative proximity of H+-sites influences their reactivity. Additionally, the diffusion of reactant and product molecules through a given zeolite topology depends on micropore size, tortuosity, and connectivity. The coupled influences of reaction kinetics and intrazeolite reactant and product diffusion govern rates and selectivity for a plethora of zeolite-catalyzed reactions and underlie the well-established effects of “shape-selectivity”. The independent effects of reaction and diffusion on rates and selectivity for a given reaction are often obfuscated by concomitant changes in the zeolite properties governing diffusion (e.g., crystallite size) and reactivity (e.g., H+-site density or proximity) in zeolite materials synthesized with conventional methods. Herein, we develop synthetic methods to decouple H+-site density, proximity and crystallite size in medium-pore, 10-membered ring (10-MR) zeolites, and evaluate the independent effects of these material properties on the kinetic and transport phenomena that govern propene oligomerization catalysis. Among synthetic methods to influence H+-site proximity in zeolites, varying the charge-density and ratio of structure directing agent (SDA) cations that compensate anionic charges in frameworks at Al centers has been reported to influence H+-site proximity in MFI and CHA zeolites of fixed H+-site density. Changes in H+-site proximity can be evaluated using Co2+ cations to selectively titrate and quantify subsets of proximal H+-sites (H+-site pairs); conditions to perform such titrations were identified for MEL zeolites. The fraction of paired H+-sites changed concurrently with changes in framework Al content in MEL zeolites synthesized using a single organic SDA (OSDA), tetrabutylammonium hydroxide (TBA+). Synthesis of MEL with mixtures of TBA+ and Na+ as an inorganic SDA (ISDA), at fixed total SDA and Al content, allowed the fraction of paired H+-sites to be systematically varied in MEL zeolites of fixed H+-site density, reflecting changes in the location and quantity of charge-balancing SDAs with Na+/TBA+ ratio. The energetic favorability of SDA occlusion in MEL was also evaluated with density functional theory (DFT). In contrast to MEL, occluded SDA content in TON zeolites crystallized with varied OSDA (1,6-diaminohexane, or 1,8-diamooctane) and K+ content, at fixed total SDA content, was invariant with K+/OSDA ratio, reflecting a different mechanism of SDA occlusion in TON. These findings provide an approach to influence H+-site pairs in 10-MR zeolites of fixed H+-site density and demonstrate the dependence of SDA occlusion on zeolite topology. The independent influences of H+-site and crystallite properties on rates and selectivity of propene oligomerization to heavier alkenes in a representative medium-pore zeolite topology (MFI) were explored by interrogating suites of samples crystallized with independently varied H+-site density (0.3–5.7 H+/u.c.), proximity, and crystallite size (0.03–2.65 μm) over a wide range of reaction conditions (483–523 K, 7–615 kPa C3H6). Dimerization rates (per H+) decreased with increasing crystallite size among MFI materials synthesized with fixed H+-site density (0.3 or 1.3 H+/u.c.), revealing the strong and ubiquitous influence of intrazeolite diffusion limitations on measured dimerization rates.

Degree

Ph.D.

Advisors

Ribeiro, Purdue University.

Subject Area

Inorganic chemistry|Chemical engineering

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