Benzene Methylation Catalysed By Hierarchically Porous Zeolite: An Effective Way to Promote Xylene Selectivity and Catalyst Lifetime for Large Scale Commercial Use

Benzene Methylation Catalysed by Hierarchically Porous Zeolite: An Effective Way to Promote Xylene Selectivity and Catalyst Lifetime for Large Scale Commercial Use

Xuan He¹, Kake Zhu¹, Xuedong Zhu*²

1 College of Chemical Engineering, East China University of Science and Technology, Shanghai, China, 2 Engineering Research Centre of Large Scale Reactor Engineering and Technology, East China University of Science and Technology, Shanghai, China

Abstract

Methylation of benzene is an alternative low-cost route to produce xylenes, but selectivity to xylene remains low over conventional zeolitic catalysts. In this work, a combined dry-gel-conversion and steam-assisted-crystallization method is used to synthesize hierarchically porous zeolite ZSM-5 with varied Si/Al ratios. N₂ physisorption, NH₃- TPD, SEM and FT-IR are employed to characterize the texture and acidity of both hierarchically porous zeolites and their conventional counterparts. The method is found to be applicable to ZSM-5 with Si/Al ratios from 20 to 180. Catalytic properties are test on benzene methylation at 460 ^oC to investigate the effect of additional porosity and Si/Al ratios. At low Si/Al ratios, the benzene conversions over conventional and hierarchical ZSM-5 are close, and selectivity to toluene is high over hierarchical ZSM-5. It is found that hierarchical porosity markedly enhances the utility of zeolite and selectivity towards xylenes via improved mass transport at higher Si/Al ratios. Under optimized hierarchical ZSM-5 catalyst, xylene selectivity reaches 34.9 % at Si/Al ratio of 180, and a remarkable lifetime of 3000 h is accomplished in industrial sideline evaluation.

Key words: benzene methylation, hierarchical porous zeolite, process intensification

 

 

References

[1]Van der Mynsbrugge, J., Visur, M., Olsbye, et al. Methylation of benzene by methanol: Single-site kinetics over H-ZSM-5 and H-beta zeolite catalysts[J]J. Catal., 2008, 292: 201-212

[2]Mirth, G., Lercher, J.A.. In Situ IR spectroscopic study of the surface species during methylation of toluene over HZSM-5[J]J. Catal., 1991, 132: 244-252

[3]Odedairo, T., Al-Khattaf, S.. Comparative study of zeolite catalyzed alkylation of benzene with alcohols of different chain length: H-ZSM-5 versus mordenite[J]Catal. Today, 2013, 204: 73-84

Fig.1 Crystallization mechanism on hierarchical porous ZSM-5 in this work

Fig.2 MFI topology and para-xylene

Fig.3 Mechanism on benzene alkylation with methanol

Fig.4 SEM image of conventional ZSM-5 (a, c, d) and hierarchically porous (b, e, f) ZSM-5-HTS with Si/Al ratios of 20 (a, b) and 180 (c-f)