(445e) Increasing Btp-X and C2-C3 Olefins in Methanol to Aromatics over Shape-Selective Zn-Si-HZSM-5

Recently, due to the increase in shale gas supply, C₁ chemistry to convert methane into methanol and methanol to valuable products has generated significant interest. Valuable aromatics hydrocarbons like benzene (B), toluene (T), p-xylene (p-X) and olefins like C₂-C₃ are the key building blocks in chemical industries. For example, synthesis of phenol and synthesis of cumene requires benzene (or toluene as a source of benzene) and propylene as key intermediates, C₂ is used as an intermediate in polypropylene and p-X is used as intermediate in polymer, dye industries etc. One potential route to obtain these valuable products is the conversion of methanol to aromatics (MTA) over ZSM5 or metal modified HZSM5^1-3. Considering that both BTp-X and C₂-C₃ olefins belong to the most valuable products in hydrocarbons, obtaining a high selectivity of them is a good choice.

In MTA reaction over ZSM5 catalyst, benzene and toluene and xylene can undergo secondary alkylation reaction to form undesired A9+ aromatics, while p-X can isomerize to form m-X and o-X isomers on the external surface of the catalyst in presence of external surface acid sites. In addition to this, olefins can undergo hydrogen transfer reactions to form alkanes. With an objective to increase the selectivity of BTp-X and C₂-C₃ olefins, the catalysts should have characteristics like high shape selectivity, absence of external acid sites, aromatization ability and low hydrogen transfer activity.

In order to achieve these characteristics, first HZSM5 was doped by silica using chemical liquid deposition of polyphenyl methyl siloxane homopolymer to enhance shape selectivity and eliminate external surface acid sites to stop secondary alkylation, isomerization as well as hydrogen transfer reactions occurring on the external surface of the HZSM5. Further to increase aromatization activity and decrease hydrogen transfer reaction, surface passivated Si/HZSM5 was impregnated with Zn which is well known for its associated dehydrogenation capability. As a result, 1.5 wt% Zn/Si/HZSM5 exhibited a high selectivity of BTp-X and C₂-C₃ of 53% far higher than that of parent HZSM5 (23 %), Si/HZSM5 (40%), and 1 wt% Zn/HZSM5(28%). The obtained values of BTp-X+C₂-C₃ (53%) on 1.5 wt% Zn/Si/HZSM5 are higher in contrast to 29 % reported by Hu et al. (2017)³ in similar study.

Reference

1. Conte, M.; Lopez-Sanchez, J. A.; He, Q.; Morgan, D. J.; Ryabenkova, Y.; Bartley, J. K.; Carley, A. F.; Taylor, S. H.; Kiely, C. J.; Khalid, K.; Hutchings, G. J., Modified zeolite ZSM-5 for the methanol to aromatics reaction. Catalysis Science & Technology 2012, 2 (1), 105-112.
2. Niu, X.; Gao, J.; Miao, Q.; Dong, M.; Wang, G.; Fan, W.; Qin, Z.; Wang, J., Influence of preparation method on the performance of Zn-containing HZSM-5 catalysts in methanol-to- aromatics. Microporous and Mesoporous Materials 2014, 197, 252-261
3. Hu, Q.; Huang, X.; Cui, Y.; Luo, T.; Tang, X.; Wang, T.; Qian, W.; Wei, F., High yield production of C₂-C₃ olefins and para-xylene from methanol using a SiO₂-coated FeO_x/ZSM-5 catalyst. RSC Advances 2017, 7 (46), 28940-28944.