Structure-activity correlations for TON, FER, and MOR in the hydroisomerization of n-butane

n-Butane hydroconversion was investigated over (Pt-loaded) molecular sieves with FER, TON, and MOR morphology. The conversion occurred via a complex interplay of mono- and bimolecular bifunctional acid mechanism and monofunctional Pt-catalyzed hydrogenolysis. Hydroisomerization occurred bimolecularly at low temperatures, which was evident from the reaction order in n-butane of 2 for isobutane formation and the presence of 2,2,4-trimethylpentane in the products. Intracrystalline diffusion limitations of the reaction rates were important for TON. The presence of platinum in TON was detrimental for isomerization selectivity as diffusion of the feed molecules to the acid sites was too slow to prevent platinum from hydrogenolysis of n-butane. Reactions on HFER occurred predominantly on the outer surface and the pore mouth of the molecular sieve. Because of high intrinsic activity towards (hydro)cracking, this did not result in high selectivity toward isobutane. In this case, adding platinum (bifunctionality) was beneficial. The reaction at the external surface was not diffusion limited, allowing bifunctional nC₄ isomerization to occur. Although PtFER approached selectivity levels as found for PtMOR, the latter had a major advantage as the larger accessibility of acid sites resulted in a much higher activity.