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

n-Butane hydroconversion was studied over (Pt-loaded) molecular sieves with TON, FER, and MOR morphology. The conversion occurs via a complex interpla y of mono- and bimolecular bifunctional acid mechanism and monofunctional p latinum-catalyzed hydrogenolysis. Hydroisomerization occurs bimolecularly a t low temperatures. This is strongly indicated by the reaction order in n-b utane of 2 for isobutane formation and the presence of 2,2,4-trimethylpenta ne among the products. Intracrystalline diffusion Limitations of the reacti on 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 isomeriza tion selectivity. As the ratio of utilized acid sites to accessible Pt beco mes low (approximately 1:75), diffusion of the feed molecules to the acid s ites is too slow to prevent Pt hydrogenolysis of n-butane. Reactions on H-F ER occur predominantly on the outer surface and the pore mouth of the molec ular sieve, presumably owing to rapid pore filling following a transient pe riod 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 exter nal surface is not diffusion limited, allowing bifunctional nC(4) isomeriza tion to occur. Although PtFER was found to approach selectivity levels as f ound for PtMOR, the latter has a significant advantage as the larger concen tration of accessible acid sites leads to much higher activity. (C) 2000 Ac ademic Press.