(204e) Quantification of the Thermodynamic Effects Regarding Alkane Adsorption within ZSM-5 Using Advanced Sampling Grand Canonical Monte Carlo Simulations

With an abundance of biomass within the US, it has been suggested to use its olefin extracts to synthesize valuable aromatics.[1] Current synthesis methods require significant energy and produce poor selectivity, however a more selective Diels-Alder mechanism has been proposed.[1] Such reactions offer near 100% atom economy while limiting unwanted byproducts, additionally, Lewis/Brönsted acids with confinement affects have shown to facilitate Diels Alder reactions.[1][2] Zeolites are of particular interest as appropriate catalysts, since they contain unique microporous networks and offer acidity through metallic substitution of tetrahedral sites.

Despite having been exploited by the chemical industry for years, accurate experimental quantification of the thermodynamics behind shape selective zeolite adsorption remains elusive[2]. Furthermore, the interaction of both the adsorbent pore structure and adsorbate molecular structure on entropic transfer quantities is largely unexplored and, consequently, significant opportunities exist for the development of systems or correlations for relating the adsorption-thermodynamics to fundamental and more easily measured thermophysical properties.

In our work, we implement flat-histogram Monte-Carlo sampling methods to investigate the adsorption of real and model alkanes in zeolites. Such techniques provide the flexibility necessary to efficiently compute thermodynamic properties both in and out of confinement and at multiple thermodynamic state-points. Specifically, we present the use of The Free Energy and Advanced Sampling Simulation Toolkit [3] to facilitate Grand Canonical Transition Matrix Monte Carlo simulation within the siliceous and acidic framework of ZSM-5. Additionally, the adsorption of alkanes are modeled to examine the association between pore geometry, molecular shape, and entropy. The ultimate aim of generating accurate thermodynamic calculations is in the development of microkinetic models for chemical reaction pathways, enabling better understanding of such materials as catalysts for hydrocarbon transformations.

[1] Settle, A. E., Berstis, L., Rorrer, N. A., Roman-Leshkóv, Y., Beckham, G. T., Richards, R. M., & Vardon, D. R. (2017). Heterogeneous Diels-Alder catalysis for biomass-derived aromatic compounds. Green Chemistry, 19(15), 3468–3492.

[2] Smit, B., & Maesen, T. L. M. (2008). Towards a molecular understanding of shape selectivity. Nature, 451, 671.

[3] Hatch, H. W., Mahynski, N. A., and Shen, V. K. (2018) FEASST: Free Energy and Advanced Sampling Simulation Toolkit. J. Res. Natl Inst Stan, 123, 123004.