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 C-2-C-6 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 p-complex and carbanionic allyl-like species as the intermediates; (2) with the involvement of Bronsted 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 C-2-C-4 oxyindium-alkyl species. The latter are oxidized by InO+ to oxyindium-carboxylates, which can be further transformed to carboxylic acids by hydrolysis.