Aromatisation of bio-derivable isobutyraldehyde over HZSM-5 zeolite catalysts

Abstract

An efficient route to obtain aromatic products based on biomass feedstock is a challenge for future biorefineries. Isobutyraldehyde is a promising feedstock available via biotechnological routes based on both carbohydrates and the direct bioconversion of CO₂. Herein, we report an efficient process for the aromatisation of isobutyraldehyde over zeolite catalysts in a continuous fixed bed reactor to provide value-added aromatic compounds with a yield of 93%. Benzene, toluene and xylenes are the major compounds formed with 79% yield and a productivity of 65 mmol g_cat⁻¹ h⁻¹. Zeolite Y, Beta and Mordenite and ZSM-5 of different modules were studied. Comprehensive catalyst characterisation using XRD, NH₃-TPD, N₂-physisorption, and ICP-OES enabled establishing structure–performance relationships with a major role of zeolite structure, density of strong acid sites and mesoporosity, respectively. HZSM-5 with Si/Al ratios of 15 to 49 proved effective at the aromatisation possessing a comparable density of strong acid sites of 0.37–0.52 mmol g⁻¹. Superior stability was observed for HZSM-5 zeolites with higher mesopore volumes. The reaction is suggested to mainly proceed via catalytic cracking to C₁–C₄ alkanes/alkenes, which undergo further oligomerisation, cyclisation, and aromatisation to form the observed alkyl-aromatics. Emphasizing the potential of the concept, long term continuous operation was carried out by alternating 10 hours time on steam operation and catalyst regeneration via in situ calcination.