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

Abstract

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.