PNNL

Abstract

In this work, we investigate the origin of the favorable para-selective nitration of toluene exhibited by H- beta zeolite solid acid catalyst. Periodic density functional theory calculations were performed to study the toluene nitration by acetyl nitrate within the pore system of a model H-beta structure. Firstly, energetics were calculated for each of the 32 crystallographically unique Br¿nsted acid sites of a beta polymorph B zeolite unit cell. These calculations revealed more than one favorable Br¿nsted acid site. However, steric and mechanistic considerations could be used to argue that one particular aluminum site with three favorable Br¿nsted site oxygens comprising a 12-T pore wall would be a likely site for nitration to take place. Transition state searches around this aluminum site were performed to determine the barrier to reaction for both para and ortho nitration of the toluene. A three-step process was assumed for the nitration of toluene with two organic intermediates: the ¼-complex and the ¾-complex (i.e., Wheland or arenium complex). The rate limiting step of the three-step process is the transfer of the proton from the ¾-complex to a Br¿nsted site on the zeolite cage. The barrier for this step in the ortho nitration case is shown to be nearly 2.5 times that for the para nitration case. The reason for the discrepancy appears to be a subtle steric constraint imposed by the curvature of the 12-T pore system of the beta zeolite and the methyl group of the toluene in the case of the ortho approach.