n-Pentane Conversion on Sulfated Zirconia in the Absence and Presence of Carbon Monoxide: Evidence for Monomolecular Mechanism of Isomerization from the 13C MAS NMR Study

In situ 13C MAS NMR analysis of hydrocarbon products formed from the selectively 13C-labeled n-pentane on sulfated zirconia in the absence and presence of carbon monoxide provide the evidence that iso-pentane forms by two parallel processes: isomerization and conjunct polymerization. Peculiarities of the 13C label scrambling from n-pentane into iso-pentane provide unequivocal evidence for monomolecular mechanism of the alkane isomerization. Besides iso-pentane, conjunct polymerization affords also the products of n-pentane “disproportionation,” butanes and hexanes and stable cyclopentenyl cations; the latter are in charge of the catalyst deactivation. Carbon monoxide suppresses completely the process of conjunct polymerization, whereas the process of intramolecular isomerization does not. In the presence of carbon monoxide carbonylation of n-pentane occurs in parallel with its isomerization, giving rise to a mixture of aldehydes, ketones, and carboxylic acids. Carbonylation of n-pentane with CO contributes to the earlier observed negative effect of CO on the alkane isomerization rate.