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

Abstract

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 Bro̷ nsted acidity causing decline in conversion or activity of the catalyst. However, pyridine FTIR shows enhancement of the Bro̷ nsted 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.

Highly selective and stable Zn–Fe/ZSM-5 catalyst for aromatization of propane

“…Zinc prevents the hydride combining with the alkenes to form alkane that is minimized backward hydrogenation reaction. So, this aided the formation of more aromatics [6,9,10,24].…”

“…Catalytic conversion of propane was performed at a reaction temperature of 540 °C and 1 atm. Conversion of propane to aromatics involves a series of reaction steps which include: dehydrogenation of propane to propene, oligomerization of formed alkenes from dehydrogenation, subsequent cracking (alkenes further conversion), new oligomers formation through alkenes alkylation followed by a hydrogen transfer and dehydrogenation, and cyclization and aromatization [6,23,29,34,40]. Dehydrogenation of propane is one of most important process of propane aromatization as it begins the reaction steps upon which subsequent reaction depends.…”

“…This occurred (working synergistically with the Bronsted sites of the HZSM-5) in propane dehydrogenation, oligomers and cyclic alkenes and dienes dehydrogenation, thus improving aromatic yield. It had been established in previous researches that aromatization on Zn/ZSM-5 catalysts occurs mainly via a bifunctional mechanism [6,23,25,27]. The species of zinc present formed the Lewis sites that catalyze the dehydrogenation of propane (activation step) to alkene.…”

“…Use of parent ZSM-5 as reported in literatures showed high coking and low selectivity towards aromatics as its limitations. Many works have been published towards mitigating this limitation via use of promoters, changing the silica alumina ratio to optimize the number and density of active sites, modifying the surface through hierarchical catalyst synthesis, shortening the path length via synthesis of nanocrystalline ZSM-5 etc [4][5][6][7][8].…”

Seed-Assisted Synthesis of ZSM-5 Aggregates Assembled from Regularly Stacked Nanosheets and Their Performance in n-Hexane Aromatization

“…The diffusion limitation can be partially solved by reducing the path length through zeolite crystals . The synthesis of nanosized crystals and introducing a hierarchical pore system into zeolite are two main strategies to reduce the diffusion path length. − However, complex synthesis conditions, high cost, and a complex product separation process are required in preparation of hierarchical zeolite and nanosized crystals.…”