A computer simulation study of distribution, structure and acid strength of active sites in H-ZSM-5 catalyst

Classical calculations using the shell model were performed to study the stability, distribution, structure and acid strength of active centers in H-ZSM-5 catalyst. The defect energies associated with Al/Si substitution, in the absence of proton or other counterions, were calculated for the 24 crystallographically nonequivalent T sites of the monoclinic structure of ZSM-5, and the lattice energies of the 96 structures obtained by periodic (Al,H)/Si substitutions in all different positions of the framework were also calculated. Proton site occupancies were estimated by considering the acid site formation as a random two-step mechanism in which Al atom is sited first and proton is then bound to one of the adjacent O atoms. This approach suggests Al14-O32-Si18 as the acid site with the largest occurrence, in contrast to previous suggestions. The electrostatic field intensities at proton sites, the O-H frequencies and the geometric features of acid sites were also obtained, and the deprotonation energies (DPE) were estimated as the defect energy of removing a proton in the periodically-protonated structures. Calculated OH frequencies are in good agreement with spectroscopic data, while the DPE are underestimated by our classical defect calculations. The contributions of short-range and long-range factors to the acidity are studied by correlating the OH frequency with the Al-O-Si angle and with the electrostatic field created by the crystal at the proton site.