Identifying the key steps determining the selectivity of toluene methylation with methanol over HZSM-5

Qingteng Chen,Jian Liu,Bo Yang

doi:10.1038/s41467-021-24098-5

Qingteng Chen, Jian Liu + Show 1 more

Open Access

https://doi.org/10.1038/s41467-021-24098-5

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Journal: Nature Communications	Publication Date: Jun 17, 2021
Citations: 27	License type: open-access

Affiliation: ShanghaiTech University

Abstract

Methylation of toluene with methanol to produce p-xylene has been investigated for decades, but the origin of selectivity is still under debate. Here we report computational studies based on ab initio molecular dynamics simulations and free energy sampling methods to identify the key steps determining the selectivity. The steps of toluene methylation to protonated-xylene, deprotonation of protonated-xylenes, and diffusion of xylene in HZSM-5 channels are compared. We find the pathways of formation for protonated p-/m-xylenes have similar free energy barriers. Meanwhile, the methylation is found rate-determining, thus the probability to generate p-/m-xylenes at the active site are similar. We then find that the diffusion for m-xylene along the zigzag channel is more difficult than its isomerization to p-xylene, which in turn further promotes the selectivity of p-xylene formation. These insights obtained at the molecular level are crucial for further development of high-performance zeolite catalysts for toluene methylation.

Highlights

The free energy barriers of (de)protonation reactions are much lower compared with those of methylation and isomerization
For p-xylene, we find that the isomerization showed a much higher barrier than diffusion in HZSM-5, it tends to diffuse away from zeolite rather than undergo isomerization
In a recent experiment, Wang et al improved the selectivity of p-xylene significantly through an approach of increasing the ratio of sinusoidal pore openings in the HZSM-5catalyst[7]

Summary

Introduction

The free energy barriers of (de)protonation reactions are much lower compared with those of methylation and isomerization. Upon the formation of p-xylene and m-xylene molecules at the active sites within HZSM-5, there are two competing processes, namely diffusion out of the zeolite, and the isomerization between p-xylene and m-xylene. The free energy barriers of xylene diffusion across the channel intersections through these channels were calculated.

Results

Conclusion