n-Butane Transformation on In-Modified ZSM-5 Zeolite: A Case Study by ¹³C MAS NMR and FTIR Spectroscopy

In-modified ZSM-5 zeolites are promising catalysts for light alkane dehydrogenation and aromatization. The mechanism of alkane activation and transformation on In-containing zeolites is still under discussion. In this work, n-butane transformation on H-ZSM-5 zeolite modified with InO+ sites has been investigated by 13C magic-angle spinning NMR and Fourier transform infrared (FTIR) spectroscopy at 296–673 K. It is established that n-butane conversion occurs by two parallel pathways: dehydrogenation to n-butene, followed by the formed alkene aromatization to simple aromatic hydrocarbons, and the direct oxidation of the alkane to C2–C6 carboxylate surface species. For the dehydrogenation pathway, n-butane activation has been established by FTIR spectroscopy to occur through C–H bond dissociation on the reactive In═O bond of the InO+ site. The oligomerization of the initially formed n-butene occurs by two pathways: (1) on InO+ sites via the formation of the n-butene π-complex and carbanionic allyl-like species as the intermediates; (2) with the involvement of Brønsted acid site (BAS). The formed oligomers convert to aromatics on the BAS with the formation of methyl-substituted cyclopentenyl cations as the intermediates. As for the oxidation pathway, the formation of the carboxylate surface species implies the dissociative adsorption of both the C–C and C–H bonds of the alkane on InO+ sites to give C2–C4 oxyindium-alkyl species. The latter are oxidized by InO+ to oxyindium-carboxylates, which can be further transformed to carboxylic acids by hydrolysis.