n-Butane hydroconversion was studied over (Pt-loaded) molecular sieves with TON, FER, and MOR morphology. The conversion occurs via a complex interplay of mono- and bimolecular bifunctional acid mechanism and monofunctional platinum-catalyzed hydrogenolysis. Hydroisomerization occurs bimolecularly at low temperatures. This is strongly indicated by the reaction order in n-butane of 2 for isobutane formation and the presence of 2,2,4-trimethylpentane among the products. Intracrystalline diffusion limitations of the reaction rates seem to be important for TON. Due to diffusion-controlled reaction rates for TON, the presence of Pt in TON was detrimental for the isomerization selectivity. As the ratio of utilized acid sites to accessible Pt becomes low (approximately 1:75), diffusion of the feed molecules to the acid sites is too slow to prevent Pt hydrogenolysis of n-butane. Reactions on H-FER occur predominantly on the outer surface and the pore mouth of the molecular sieve, presumably owing to rapid pore filling following a transient period of single-file diffusion. Due to high intrinsic activity toward (hydro)cracking this does not lead to high selectivity toward isobutane. Addition of Pt (bifunctionality) was in this case beneficial. Reaction at the external surface is not diffusion limited, allowing bifunctional nC4 isomerization to occur. Although PtFER was found to approach selectivity levels as found for PtMOR, the latter has a significant advantage as the larger concentration of accessible acid sites leads to much higher activity.
Pieterse, J. A. Z., Seshan, K., & Lercher, J. A. (2000). Structure-activity correlations for TON, FER and MOR in the hydroisomerization of n-butane. Journal of catalysis, 195(195), 326-335. https://doi.org/10.1006/jcat.2000.3003