Highly Selective Hierarchical ZnO/ZSM-5 Catalysts for Propane Aromatization

Hierarchical ZnO/ZSM-5 catalysts were prepared by desilication and impregnation with 2 wt % metallic ZnO. X-ray diffraction and Fourier transform infrared (FTIR) results showed that the structures of the hierarchical zeolites were relatively preserved despite desilication but were accompanied with sequential loss in crystallinity, likewise Bro̷nsted acidity causing decline in conversion or activity of the catalyst. However, pyridine FTIR shows enhancement of the Bro̷nsted acidic sites. Throughout the activity test, the hierarchical ZnO/ZSM-5 catalysts showed an outstanding performance within 5 h on stream with the average aromatic (benzene, toluene, and xylenes) selectivity trend, represented by their NaOH concentrations 0.3 M > 0.4 M > 0.2 M > 0.1 M corresponding to 61.0, 53.5, 40.3, and 36.8%, respectively. Their average propane conversions within the same period followed a consecutive trend 0.1 M > 0.2 M > 0.3 M > 0.4 M conforming to 34.1, 24.8, 17.3, and 10.2%, respectively. These were compared with that of the reference (ZnO/ZSM-5), which exhibited an average aromatic selectivity of 25.2% and propane conversion of 39.7%. Furthermore, the hierarchical catalyst generally displayed a low amount of C9+ heavier aromatics with the ZnO/ZSM-5(0.3 M) catalyst having the lowest C9+ selectivity of 23.7% compared to the reference catalyst with 72.7% at the same time on stream.