Computational studies of acid sites in ZSM 5: dependence on cluster size

Abstract

An ab initio molecular orbital study of the proton affinities, OH stretching frequencies, and ammonia interactions of Si{sub x}TO{sub y}H{sub z} (T = Si, Al) clusters representing the ZSM-5 zeolite framework has been carried out. The dependence of these properties on cluster size was examined. The proton affinity of the central bridging Si-O-Al site was found to be slowly convergent with cluster size. In calculations on clusters containing up to two shells of silicons and two shells of oxygens beyond the bridge, the proton affinity varies by as much as 30 kcal/mol with each additional shell. Addition of a silicon shell reduces the proton affinity varies by as much as 30 kcal/mol with each additional shell. Addition of a silicon shell reduces the proton affinity and addition of an oxygen shell increases the proton affinity. This is due to the long range electrostatic interaction of the added shells on the bridging hydroxyl site. The clusters containing a central Si-O-Si bridge exhibit a similar slow convergence of the proton affinity. The OH stretching frequencies are much less dependent on cluster size than the proton affinities. The theoretical proton affinities and OH stretching frequencies are much less dependent on cluster size thanmore » the proton affinities. The theoretical proton affinities and OH stretching frequencies from the T = Al clusters are consistent with experiment. The interaction energy of the `ionic` structure that results from interaction of the ammonia with the acid site, Z{sup {minus}}...HNH{sub 3}{sup +}, has a slow convergence with cluster size similar to the proton affinity and the `ionic` structure increases instability relative to the `covalent` structure, ZH...NH{sub 3}, with increasing cluster size. The NH{sub 3} desorption energy is inversely proportional to the proton affinities of the clusters of the same size representing the ZSM-5 zeolite. 54 refs., 8 figs., 7 tabs.« less