Synergetic catalysis between ZnZrO_x and silanol nest-enriched silicalite-1 in CO₂ hydrogenation coupled with toluene alkylation

Abstract

CO₂ hydrogenation coupled with toluene alkylation is a promising route for application in greenhouse gas utilization and para-xylene (PX) production; however, the strong Brønsted acid sites of the conventional ZSM-5 catalyst lead to uncontrollable (de)alkylation and xylene isomerization. In this study, a silanol nest-enriched silicalite-1 zeolite (AS-1) with a moderate acidity was employed as an alkylation catalyst to suppress side reactions. Dealkylation and xylene isomerization were significantly suppressed compared to those observed using ZSM-5, which reduced the benzene selectivity from 6.04% to 0.5% and increased the PX selectivity from 23.6% to 33%. An epitaxial silicalite-1 shell was introduced to passivate the external sites to enhance the shape-selective effect of AS-1, thus improving the PX selectivity from 36.2% to 44.4%. The intimate contact established by optimizing the spatial distribution facilitated the conversion of the methanol intermediates, increasing the toluene conversion to 16.3%. A deposition-precipitation strategy was adopted to load ZnZrO_x particles onto silicalite-1 zeolite-coated AS-1, which increased the ZnZrO_x dispersion and minimized the transfer distance for the methanol intermediates. Consequently, considerable levels of toluene conversion and a high PX selectivity of 15% and 42.7%, respectively, were maintained, while the CO₂ conversion significantly increased from 5.9% to 9.5%. Additionally, ammonium hexafluorosilicate treatment generated small mesopores within the AS-1 zeolite, which enhanced diffusion and further increased the toluene conversion to 20.9%. This study provides novel insights into the design of bifunctional catalysts comprising silanol nest-enriched silicalite-1 and ZnZrO_x for use in CO₂ hydrogenation coupled with toluene alkylation.

Graphical abstract

Keywords

silanol nest / CO₂ hydrogenation / bifunctional catalysts / silicalite-1

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