Molecular Insights into Adsorption and Diffusion Mechanism of N-Hexane in MFI Zeolites with Different Si-to-Al Ratios and Counterions

Abstract

The effect of the silicon to aluminum ratio (SAR) and alkali metal cations on adsorption and diffusion properties of ZSM-5 and silicate-1 zeolites was investigated using n-hexane as the model probe via giant canonical Monte Carlo (GCMC) and molecular dynamics (MD) simulations. A wide range of SAR was considered in this study to explore the possible adsorption sites in the zeolites. The findings show that, at 298 K and 423 K, adsorption and diffusion of n-hexane on/in low SAR (≤50) H-ZSM-5 models were promoted due to the preferable distribution of n-hexane in straight channels and enhanced interaction between protons and n-hexane molecules (about 24 kcal·mol−1). In alkali metal cation (i.e., Na+ and K+) exchanged ZSM-5, the alkali metal cations affected transport of molecules, which led to significant differences in their adsorption and diffusion properties compared to HZSM-5. In the Na+ and K+ systems, lower saturated adsorption capacities were predicted compared to that of silicate-1, which could be attributed to the decrease in effective void size posed by alkali–metal cations. In addition, simulation results also suggested that the T9 and T3 are the most likely sites for n-hexane adsorption, followed by T2, T5, and T10. Findings of the work can be beneficial to the rational design of high-performance zeolite catalysts for n-hexane conversion.

Molecular Insights into Adsorption and Diffusion Properties of CO2/CH4 in Cation-Exchanged ZSM-5 Zeolites

“…The reason is that the adsorption of CO 2 is close to saturation, and the effect of the cations continues to weaken. The influence of spatial size dominates . At this point, the same applies to CH 4 .…”

“…The main results included the adsorption capacity and isosteric heat. Also, the simulation was based on the grand canonical Monte Carlo (GCMC) method ,, using the Metropolis algorithm . The PCFF force field was used, and its usability had been verified by previous work. ,, The partial charges of atoms were set up by the charge-equilibration (QEq) method .…”

“…21 In the model we obtained, Li + , Na + , and K + ions were located at the intersection of channels where Al atoms are located, which is consistent with the research of others. 15,18,33 Also, a supercell of 2 × 2 × 2 was further created to enhance the accuracy of simulation results, that is, the final cell size was 40.14 × 39.84 × 26.84 Å 3 . 32,34 The framework of M-ZSM-5 (M = Li, Na, and K) can be found in Figure 1, where the yellow balls, red balls, and purple balls represent the Si, O, and Al atoms, respectively.…”

“…To explore the underlying mechanisms, it is necessary to analyze the adsorption and diffusion properties of guest molecules. Also, molecular simulations, including Monte Carlo (MC) and molecular dynamics (MD) methods, are powerful tools for theoretical calculations. − It can not only explain some phenomena in experiments but also further guide the optimization and improvement of processes. In our previous work, we have analyzed the adsorption and diffusion properties of CO 2 and CH 4 in silicalite-1 (structure-type MFI) under different temperatures and pressures.…”

“…At low pressure, Na + significantly enhances the adsorption of guest molecules, especially CO 2 . 12 Zheng et al 18 used MC and MD simulations to investigate the adsorption and diffusion properties of n-hexane in Na + /K + -exchanged ZSM-5. A suitable cationic environment enhances the adsorption.…”