Influence of Framework Protons on the Adsorption Sites of the Benzene/HY System

Abstract

Monte Carlo (MC) simulations were performed to study the influence of framework protons on the adsorption sites of the benzene molecule in HY zeolite with different Si:Al ratios. Eleven types of adsorption sites were observed including five reported sites (H1, H2, U4, U4­(H1), and W) and six newfound sites (W­(2H1), U4­(2H1), H1­(H2), U4­(H1,H1), H1­(H2,H1), and U4­(H1,H1,H1)), which were “supersites” with more than one proton. The stability order of the sites found in the 28Al model can be expressed as U4­(H1,H1,H1) > U4­(H1) > H1­(H2,H1) > W­(2H1) > U4­(H1,H1) > H1­(H2) > H1 > H2 > U4 > U4­(2H1) > W. Increasing number of zeolite protons resulted in an increasing proportion of supersites, which enhanced adsorption energies of sites. For HY zeolite models containing different numbers of protons with the same ratio of H1:H2, the amount of the most stable adsorption sites containing H1 proton increased, while the amount of the most stable adsorption sites containing H2 decreased, with increasing number of protons.

Molecular origin of decreased diffusivity with loading for benzene/HY

“…The negative slope of the low loading range is larger than that of the other two ranges. This result agrees with our previous result, i.e., multiple sites for benzene adsorption in HY are located at low loadings (<8 molecules/UC), with a large heterogeneity of host–guest interaction energies . A continuously increasing trend is observed for U ads‐ads , and Δ U ads‐ads (positive) is larger than the absolute value of Δ U ads‐zeo (negative) across the entire loading range, i.e., the features of U ad and Q are mainly contributed by U ads‐ads .…”

“…The zeolite models contain aluminum, and compensation hydrogen necessary for charge balance atoms have large Q at all loadings (Figure ) mainly because of attractive U ads‐zeo . The Q values of 28Al and 56Al are higher than that of 14Al because of the increasing number of zeolite protons that increases the proportion of “supersites” involving more than one proton (only existed in 28Al and 56Al) and consequently enhances the adsorption energies of benzene …”

“…Detailed information regarding the construction of HY zeolite models can be found in Ref. . The HY zeolite model used in the simulations consisted of one FAU unit cell because the systems of concern are unaffected by size.…”

“…Energies of adsorption and the initial configurations of MD simulations were obtained from fixed loading simulations through the Metropolis Monte Carlo (MMC) method in the canonical ensemble at 298 K, as previously described . For each loading, 5 × 10 6 MC steps were used to ensure equilibration, and another 5 × 10 6 MC steps were used for production.…”

“…For each loading, 5 × 10 6 MC steps were used to ensure equilibration, and another 5 × 10 6 MC steps were used for production. One MC step was attempted to move each adsorbate once, and a typical mix of MC moves was used as previously reported . Several loading points of our simulations were recomputed with 50 × 10 6 equilibration MC steps and 50 × 10 6 MC production steps to check on the convergence used.…”