Theoretical study of the mechanism behind the para-selective nitration of toluene in zeolite H-Beta

Periodic density functional theory calculations were performed to investigate the origin of the favourable para-selective nitration of toluene exhibited by zeolite H-beta with acetyl nitrate nitration agent. Energy calculations were performed for each of the 32 crystallographically unique Brnsted acid sites of a beta polymorph B zeolite unit cell with multiple Brnsted acid sites of comparable stability. However, one particular aluminum T-site with three favourable Brnsted site oxygens embedded in a straight 12-T channel wall provides multiple favourable proton transfer sites. Transition state searches around this aluminum site were performed to determine the barrier to reaction for both para and ortho nitration of toluene. A three-step process was assumed for the nitration of toluene with two organic intermediates: the - and -complexes. The rate limiting step is the proton transfer from the -complex to a zeolite Brnsted site. The barrier for this step in ortho nitration is shown to be nearly 2.5 times that in para nitration. This discrepancy appears to be due to steric constraints imposed by the curvature of the large 12-T pore channels of beta and the toluene methyl group in the ortho approach that are not present in the para approach.