High-resolution solid-state NMR studies of sulfate-promoted zirconia in relation to n-pentane isomerization

Abstract

Combined 1H, 15N and 13C NMR studies have been performed to elucidate the role of proton donating (Brønsted) and electron accepting (Lewis) sites in isomerization and disproportionation of n-pentane catalyzed by sulfate-promoted zirconia. 1H NMR MAS of proton sites in a series of sulfate-promoted zirconia as well as in ZrO 2 and Zr(SO 4) 2 has revealed a variety of surface OH groups that differ in their chemical shifts. Their amounts as function of sample preparation procedure and evacuation temperature have been measured. Lewis acidic sites have been characterized by 15N NMR of adsorbed N 2. The results have shown that the most active zirconia catalysts have very strong Lewis acidic sites with a concentration of several μmol/g. Their amount has been found to decrease on increase of an evacuation temperature of samples up to 600°C. The numbers of Brønsted and Lewis sites have been correlated with the catalytic activity in reaction of n-pentane isomerization measured by in situ 1H MAS NMR. Effects of catalyst treatment temperature and addition of CO and H 2O have been studied. No correlation between the catalyst's activity in the reaction of n-pentane isomerization and the number and types of surface OH groups has been found. At the same time, the proportionality between the activity and the amount of the most strong Lewis sites detected by 15N NMR of adsorbed N 2 has been revealed. The products of n-pentane conversion over sulfate-promoted zirconia have been identified by 13C MAS NMR. The role of Lewis and Brønsted acidic sites in n-pentane isomerization and disproportionation is discussed.