Introduction of Zn, Ga, and Fe into HZSM-5 Cavities by Sublimation:  Identification of Acid Sites

M/ZSM-5 zeolites with M = Ga, Fe, or Zn were prepared by subliming volatile compounds onto HZSM-5. They were characterized by XRD, H2-TPR, CO-TPR, FTIR, adsorption of pyridine and acetonitrile-d3 and, in the case of Fe/ZSM-5, also by ESR spectroscopy. Immediately after sublimation, the concentration of Brnsted acid sites is low, but with Ga/ZSM-5 and Fe/ZSM-5, it rises again after hydrolysis. This dissipation of multipositive ions to monopositive ions, each located near the negative countercharge in the zeolite matrix, is favored by a considerable gain in Coulomb energy. In Ga/ZSM-5, some of the regenerated Brnsted sites are stronger than those in the original HZSM-5. In Fe/ZSM-5 and Zn/ZSM-5, such Brnsted sites are weaker, as evidenced by CD3CN adsorption. However strong Lewis acid sites are formed, they are characterized by NH3-TPD, and pyridine and CD3CN adsorption. Zeolites with a high Si/Al ratio contain lattice defects with internal silanol groups. These defects disappear when metal ions are added by sublimation, and some of the ions end up in distorted tetrahedra carrying their own Brnsted acid sites. For Ga/SM-5 and Fe/ZSM-5, FTIR and TPR evidence shows the presence of at least three kinds of species: (i) (GaO)+ and binuclear Fe3+ ions located in cationic exchange positions and contributing to the perturbation of T-O-T vibrations; (ii) neutral clusters of Ga2O3 and Fe2O3; (iii) Ga3+ or Fe3+ ions in tetrahedral coordination. In the case of Fe/ZSM-5, the presence of binuclear ions with bridging oxygen is also identified by ESR spectroscopy. In contrast to Fe and Ga, Zn seems to be stable in cationic exchange positions either compensating the charge of two Al-centered tetrahedra or ligated to one internal silanol or OH group.