Molecular orbital calculation of the soft-hard acidity of zeolites and its catalytic implications

The relative hardness of different compositions of model clusters of acid zeolites and the same clusters containing a metoxy group as alkylating agent were evaluated using the energy of the lowest unoccupied molecular orbital as the index. Different basis sets and pseudopotentials were used in ab initio calculations. Semiempirical MNDO-PM3 calculations were also performed. The results show that the hardness of the zeolite increases when the Si/Al ratio decreases. On the basis of Pearson's HSAB principle, the selectivity obtained during the alkylation of toluene with methanol catalyzed by acid zeolites was interpreted.