In situ 13C MAS NMR Study on the Mechanism of Butane Isomerization Over Catalysts with Different Acid Strength

Abstract

Reaction mechanism of skeletal isomerization of n-butane over sulfated zirconia (SZ), Cs2.5H0.5PW12O40 (Cs2.5) and H-form mordenite (H-MOR) catalysts was studied using 13C MAS NMR with 13C-labeled n-butane. The isomerization of n-butane over SZ type catalysts proceeds predominantly via a monomolecular mechanism below 333 K and gradually changes to a bimolecular alkylation-β-scission mechanism as the reaction temperature is increased to 423 K. Iron promoter in SZ catalyst facilitates the bimolecular process. The n-butane isomerization over Cs2.5 also proceeds mainly via a monomolecular mechanism below 373 K. The bimolecular mechanism becomes significant as the reaction temperature is increased to 423 K. On both SZ and Cs2.5 catalysts hydrogen inhibits the isomerization reaction, in particular the bimolecular process. In contrast, the n-butane isomerization over H-MOR with relatively moderate acid strength proceeds mainly via a bimolecular mechanism at 473 K. The kinetics of n-butane isomerization on SZ below 333 K and Cs2.5 below 373 K are well represented by the Langmuir–Hinshelwood equation for a reversible first order surface reaction, further supporting that a monomolecular mechanism proceeds primarily on SZ and Cs2.5 catalysts at early reaction stage. All results suggest that the stronger the acidity of the catalyst the lower the reaction temperature of n-butane isomerization and the more contribution of the monomolecular mechanism. The overall mechanism of 1−13C-n-butane reaction on SZ, Cs2.5 and H-MOR catalysts including 13C scrambling and butane isomerization is proposed.