Integration of Nano-Sized HZSM-5 with ZnZrO_x as a Bifunctional Catalyst to Boost Benzene Alkylation with Carbon Dioxide and Hydrogen

Benzene alkylation with CO2/H2 is a doubly beneficial option to upgrade benzene toward valuable products, in which benzene can be converted into valuable toluene and xylene, while CO2 can be utilized as an alkylation reagent to mitigate the greenhouse effect. It is quite challenging to ascertain the critical rule of zeolite regulation for the synergistic effect in oxide-zeolite composite (OXZEO) catalysts, a series of bifunctional catalysts were investigated in this work, consisting of ZnZrOx solid solution and nano-sized HZSM-5 with uniform size distribution and distinct acid properties. Key parameters affecting the catalyst performance were analyzed and optimized in this tandem reaction, namely, the mass ratio of two components, the SiO2/Al2O3 ratio of HZSM-5, and the proximity between ZnZrOx and HZSM-5. Due to the increased active site accessibility originating from nano-sized HZSM-5, the combined selectivity of toluene and xylene reaches 94.6% at 31.3% benzene conversion, respectively. Moreover, low ethylbenzene selectivity (<1%) has been achieved. It is worth noting that the long-term stability test indicates a faster deactivation in the nano-sized HZSM-5 system as compared with the micro-sized counterpart. Based on EDS mappings, it is found that nano-sized HZSM-5 experiences a more severe metal migration issue, resulting in lower catalyst stability. The reaction mechanism with formate-methoxy intermediates and the synergistic effect of CO2 hydrogenation-alkylation were further confirmed by in situ diffuse reflectance infrared Fourier transformations spectroscopy and gas chromatography–mass spectrometry. By correlating zeolite size and acidity with catalytic performances, these findings could facilitate the further rational design of OXZEO catalysts toward CO2 catalytic conversions.