n-pentane conversion on sulfated zirconia in the absence and presence of carbon monoxide - Evidence for monomolecular mechanism of isomerization fromthe C-13 MAS NMR study

In situ C-13 MAS NMR analysis of hydrocarbon products formed from the selec tively C-13-labeled n-pentane on sulfated zirconia in the absence and prese nce of carbon monoxide provide the evidence that iso-pentane forms by two p arallel processes: isomerization and conjunct polymerization. Peculiarities of the 13C label scrambling from n-pentane into iso-pentane provide unequi vocal evidence for monomolecular mechanism of the alkane isomerization. Bes ides iso-pentane, conjunct polymerization affords also the products of n-pe ntane "disproportionation," butanes and hexanes and stable cyclopentenyl ca tions; the latter are in charge of the catalyst deactivation. Carbon monoxi de suppresses completely the process of conjunct polymerization, whereas th e process of intramolecular isomerization does not. In the presence of carb on monoxide carbonylation of n-pentane occurs in parallel with its isomeriz ation, giving rise to a mixture of aldehydes, ketones, and carboxylic acids . Carbonylation of n-pentane with CO contributes to the earlier observed ne gative effect of CO on the alkane isomerization rate. (C) 2001 Academic Pre ss.