(668b) Understanding Shape Selectivity Effects of Hydroisomerization Using a Reaction Equilibrium Model.

This study delves into the effects of zeolite shape selectivity on the hydroisomerization of linear alkanes, a process crucial in applications such as production of sustainable aviation fuels, where high degree of branching of isoalkanes are the preferred constituents. Understanding the equilibrium of these reversible reactions is vital for optimizing the production of branched hydrocarbons while minimizing the products formed via irreversible cracking reactions. We study shape selectivity effects of FAU-, ITQ-29-, BEA-, MEL-, MFI-, MTW-, and MRE-type zeolites on the hydroisomerization of heptane (C₇), and octane (C₈) isomers at reaction equilibrium. The Reaction Ensemble Monte Carlo (RxMC) method in conjunction with Grand-Canonical Monte Carlo (GCMC) simulations is commonly used to compute the equilibrium distribution of chemical reactions. A more efficient alternative is adopted, where the reaction equilibrium distribution is obtained by enforcing chemical equilibrium in the gas phase and phase equilibrium between the gas and the adsorbed phase components. This approach mimics the chemical equilibrium distribution of reactions in the adsorbed phase (Fig. 1). Using the Henry’s law at infinite dilution and mixture adsorption isotherm models at elevated pressures, the adsorbed loadings are calculated from the chemical equilibrium conditions in the gas phase. We find significant differences in selectivities for mono-branched and multi-branched isomers in different zeolites. Di-branched isomers are favored in cage-like pore structures such as ITQ-29-type zeolite (Fig. 2a), while mono-branched isomers are preferred in channel-like pore structures such as MEL-type zeolite (Fig. 2b). It is also observed that the gas phase pressure has a negligible effect on the reaction equilibrium of C₇ isomers, especially at high temperatures (500 K or above). This study is an initial step in understanding the reaction product distribution of hydroisomerization reactions involving long chain alkanes and to which extent chemical equilibrium is applicable.