Structure of Zirconium-Exchanged H-ZSM5 Prepared by Vapor Exchange of ZrCl4

Abstract

Zr dimers (Zr2O32+) interacting with two exchange sites and uniform in structure were prepared by grafting anhydrous ZrCl4(g) onto H-ZSM5 followed by hydrolysis and dehydration. This method led to selective exchange, without concurrent formation of crystalline ZrO2 aggregates, ubiquitously formed via aqueous exchange because of the slow diffusion of aquo oxo-ions within small channels. ZrCl4(g) reacts with acidic OH groups to form HCl and ZrCl3+-ZSM5 species. This exchange stoichiometry was confirmed by the number of HCl molecules evolved during hydrolysis, a process that forms ZrO(OH)+ species. These species then dehydroxylate via condensation with vicinal ZrO(OH)+ to form (OZr−O−ZrO)2+ species interacting with two exchange sites and containing one bridging O-atom. This exchange process is consistent with the number of residual OH groups titrated by D2 and with the intensity of O−H infrared bands. X-ray diffraction, 27Al nuclear magnetic resonance, and infrared spectra showed that the exchange process did not influence zeolite crystallinity, led to Zr4+ introduction into the zeolite framework, or form residual Zr(OH)x species at exchange sites. Weak Raman bands in the 960−1030 cm-1 region, absent in crystalline ZrO2 and H-ZSM5, were detected upon exchange. The maximum attainable Zrdimer/Alf exchange ratios (0.6 ± 0.05) are consistent with Al radial structure distributions and with expected Al−Al distances required to anchor (OZr−O−ZrO)2+ species in ZSM-5 with a statistical distribution of Al atoms. ZrCl4 species in excess of this saturation stoichiometry formed monoclinic ZrO2, detectable in Raman spectra and X-ray diffraction patterns.