545687 Effects of BrøNsted Acid Site Density and Proximity in MFI and CHA Zeolites on Light Alkane Activation and Light Alkene Oligomerization Catalysis

Wednesday, June 5, 2019: 2:42 PM
Texas Ballroom EF (Grand Hyatt San Antonio)
Philip M. Kester, Young Gul Hur, Elizabeth E. Bickel, Fabio H. Ribeiro, Jeffrey T. Miller and Rajamani Gounder, Charles D. Davidson School of Chemical Engineering, Purdue University, West Lafayette, IN

Effects of Brønsted acid site density and proximity in MFI and CHA zeolites on light alkane activation and light alkene oligomerization catalysis

Philip M. Kester, Young Gul Hur, Elizabeth E. Bickel, Fabio H. Ribeiro, Jeffrey T. Miller, and Rajamani Gounder*

Charles D. Davidson School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907, USA


The activation of light alkanes to form alkenes, and subsequent oligomerization to heavier molecular weight hydrocarbons, provide a strategy for converting gases into liquids useful as transportation fuels. In the case of Brønsted acid zeolite catalysts (e.g., MFI), the proximity of framework Al atoms, which generate active H+ sites, has been proposed to influence monomolecular alkane activation turnover rates via polarization of kinetically-relevant carbonium ion transition states [1]. The sizes of primary crystallites and the density of framework Al atoms also influence the rates, selectivity, and deactivation during alkene oligomerization in Brønsted acid zeolites because longer diffusion paths and higher active site densities preferentially increase intracrystalline residence times of larger products [2]. The combined effects of crystallite size and Al content are reflected in a characteristic diffusion parameter, proportional to the volumetric density of H+ sites and the square of the characteristic diffusion length [2]. Although these structural properties are typically correlated in aluminosilicate MFI zeolite samples synthesized hydrothermally, they can be manipulated independently through alternative synthesis routes, such as discussed here through addition of B heteroatoms into synthesis solutions containing tetra-n-propylammonium (TPA+) and ethylenediamine (EDA) as structure directing agents (SDAs).

Crystallite sizes (0.3 – 10 μm) and Al density (0.1 – 0.2 × 10-3 mol Al g-1) of B-Al-MFI zeolites are independently varied by the addition of B to synthesis mixtures (Si/B = 2.6 – 13) containing TPA+ and EDA. The presence of different SDAs during crystallization also influence the siting of Al in zeolite frameworks [3]. Here, we provide evidence that the synthesis of B-Al-MFI zeolites (Si/Al = 50) from mixtures containing only TPA+ as the SDA contain both proximal and isolated Al, while those made from mixtures containing EDA and TPA+ contain predominantly isolated Al. Protons compensating framework B are weaker in acid strength (higher deprotonation energy [4]) than those compensating framework Al, requiring methods to discriminate and quantify protons at framework Al prior to interpreting catalytic phenomena on boroaluminosilicates. Temperature programmed desorption (TPD) of ammonia of B-Al-MFI after liquid-phase NH4+-exchange quantify all framework B and Al heteroatoms, but TPD performed after NH4-form B-Al-MFI were purged in flowing He (433 K), or after gas-phase NH3 adsorption (433 K) onto H-form B-Al-MFI, quantify only protons at framework Al atoms (H+Al) [5]. Methanol dehydration turnover rates on B-Al-MFI zeolites (415 K, per H+Al) measured in zero-order kinetic regimes, which are insensitive to acid strength and confinement, are identical to those measured on Al-MFI zeolites, validating these NH3 site titration protocols. Turnover rates (per H+Al) measured in first-order kinetic regimes, which are sensitive to differences in both acid strength and confinement [6], are lower on B-Al-MFI zeolites than those previously reported for Al-MFI zeolites [7], suggesting that Al is preferentially sited within larger MFI intersections (~0.7 nm) in boroaluminosilicate compositions. Among CHA zeolites, monomolecular propane cracking rates (748 K, per H+Al) increase with the fraction of proximal H+ sites, even among samples of fixed Al composition. We will also discuss the dependences of monomolecular propane activation rates (718-778 K) and of propene oligomerization rates (473-573 K) on H+Al proximity in MFI zeolites.


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[4] Jones, A. J., Carr, R. T., Zones, S. I., Iglesia, E. J. Catal. 312 (2014) 58-68.

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[7] Di Iorio, J. R., Nimlos, C. T., Gounder, R. ACS Catal. 7 (2017) 6663-6674.

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