HIGH-RESOLUTION SOLID-STATE NMR-STUDIES OF SULFATE-PROMOTED ZIRCONIA IN RELATION TO N-PENTANE ISOMERIZATION

Combined H-1, N-15 and C-13 NMR studies have been performed to elucida te the role of proton donating (Bronsted) and electron accepting (Lewi s) sites in isomerization and disproportionation of n-pentane catalyze d by sulfate-promoted zirconia. H-1 NMR MAS of proton sites in a serie s of sulfate-promoted zirconia as well as in ZrO2 and Zr(SO4)(2) has r evealed a variety of surface OH groups that differ in their chemical s hifts. Their amounts as function of sample preparation procedure and e vacuation temperature have been measured. Lewis acidic sites have been characterized by N-15 NMR of adsorbed N-2. The results have shown tha t the most active zirconia catalysts have very strong Lewis acidic sit es with a concentration of several mu mol/g. Their amount has been fou nd to decrease on increase of an evacuation temperature of samples up to 600 degrees C. The numbers of Bronsted and Lewis sites have been co rrelated with the catalytic activity in reaction of n-pentane isomeriz ation measured by in situ H-1 MAS NMR. Effects of catalyst treatment t emperature and addition of CO and H2O have been studied. No correlatio n between the catalyst' s activity in the reaction of n-pentane isomer ization and the number and types of surface OH groups has been found. At the same time, the proportionality between the activity and the amo unt of the most strong Lewis sires detected by N-15 NMR of adsorbed N- 2 has been revealed. The products of n-pentane conversion over sulfate -promoted zirconia have been identified by C-13 MAS NMR. The role of L ewis and Bronsted acidic sites in n-pentane isomerization and dispropo rtionation is discussed.