Atomistic Investigations of the Effects of Si/Al Ratio and Al Distribution on the Adsorption Selectivity of n -Alkanes in Brønsted-Acid Zeolites

Journal Article · Wed Jan 03 00:00:00 EST 2018 · Journal of Physical Chemistry. C

DOI:https://doi.org/10.1021/acs.jpcc.7b11190· OSTI ID:1571101

Yang, Chi-Ta ^[1]; Janda, Amber  ^[2]; Bell, Alexis T.  ^[3]; Lin, Li-Chiang  ^[1]

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1. The Ohio State Univ., Columbus, OH (United States). William G. Lowrie Dept. of Chemical and Biomolecular Engineering
2. Univ. of California, Berkeley, CA (United States). Dept. of Chemical and Biomolecular Engineering; Stanford Univ., CA (United States). Dept. of Chemical Engineering
3. Univ. of California, Berkeley, CA (United States). Dept. of Chemical and Biomolecular Engineering

The adsorption of n-alkanes onto Brønsted-acid sites is a key step in the catalytic cracking of alkanes. Employing configurational-bias Monte Carlo simulations, we have investigated how the ratio of equilibrium adsorption constants for central C-C bonds relative to terminal bonds of n-alkanes (i.e., the adsorption selectivity ratio) in Brønsted-acid zeolites is influenced by the Si/Al ratio and the Al distribution. A new computational approach was implemented, and the developed force field was validated by a comprehensive comparison between simulation results and experimental data for a number of Brønsted-acid zeolites. While the adsorption selectivity seems to be relatively insensitive to the Si/Al ratio, our results reveal that the Al distribution plays a crucial role in determining the adsorption selectivity. Changes in the Al distribution result in a change of as much as 2-fold in the adsorption selectivity ratio for n-hexane. The selectivity generally shows larger variations with respect to Al distribution in zeolites with a larger Si/Al ratio. The two factors identified by this work that substantially influence the selectivity ratio are the siting of Al atoms among T-sites and their spatial proximity, and an atomic-level understanding of each of these effects was achieved. The siting of Al atoms at more or less selective T-sites significantly influences the overall selectivity ratio, and Al atoms in close proximity can synergistically enhance the adsorption of central C-C bonds, leading to a higher selectivity ratio relative to isolated Al atoms. Finally, we anticipate that these results will have important implications for future large-scale computational screenings and the development of advanced synthesis approaches to target certain Al distributions in zeolites.

• Research Organization: Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States)
• Sponsoring Organization: USDOE Office of Science (SC)
• Grant/Contract Number: AC02-05CH11231
• OSTI ID: 1571101
• Journal Information: Journal of Physical Chemistry. C, Vol. 122, Issue 17; ISSN 1932-7447
• Publisher: American Chemical Society
• Country of Publication: United States
• Language: English

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