In situ 13C MAS NMR study on the mechanism of butane isomerization over catalysts with different acid strength

Reaction mechanism of skeletal isomerization of n-butane over sulfated zirconia (SZ), Cs2.5H0.5PW12O40 (Cs-2.5) and H-form mordenite (H-MOR) catalysts was studied using C-13 MAS NMR with C-13-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-beta-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 Cs-2.5 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 Cs-2.5 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 Cs-2.5 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 Cs-2.5 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-C-13-n-butane reaction on SZ, Cs-2.5 and H-MOR catalysts including C-13 scrambling and butane isomerization is proposed.