Isobutane Activation and Transformation on In-Modified ZSM-5 Zeolites: Insights from Solid-State NMR, FTIR Spectroscopy, and DFT Calculations

With respect to the potential application of indium-modified zeolites for light alkane conversion to simple aromatic hydrocarbons and their oxidation to carboxylic acids, the pathways of isobutane transformation on the In/H-ZSM-5 zeolite have been investigated by a combination of solid-state NMR spectroscopy, FTIR spectroscopy, and density functional theory (DFT) calculations. It is found that isobutane undergoes activation on InO+ sites of In/H-ZSM-5 zeolites via the “alkyl” pathway to yield isobutylindium species, which precedes the formation of isobutene at 296 K. Analysis of isobutene transformation on In/H-ZSM-5 shows the formation of allyl-like intermediates, alkoxy species, and cyclopentenyl cations from the alkene. This implies that oligomerization and aromatization of isobutene formed from the alkane occurred with the involvement of both InO+ sites and Brønsted acid sites (BASs) at 473–573 K. At higher temperatures (T ≥ 623 K), C2–C4 surface carboxylic species have been found as the products of isobutane oxidation. It is inferred that the direct oxidation of isobutane molecules to carboxylate species does not occur, contrary to the earlier found cases with propane and n-butane transformation on In-modified zeolites. Additionally, DFT calculations have been used to analyze the localization of indium species in the ZSM-5 zeolite, the energies of isobutane and isobutene adsorption on differently located active sites (InO+ and BAS), and the mechanisms of C–H bond activation in the alkane.