부산대학교 도서관

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 ZrO2aggregates, 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)2species 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 D2and 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 Zr4introduction into the zeolite framework, or form residual Zr(OH)xspecies at exchange sites. Weak Raman bands in the 960−1030 cm-1region, absent in crystalline ZrO2and H-ZSM5, were detected upon exchange. The maximum attainable Zrdimer/Alfexchange ratios (0.6 ± 0.05) are consistent with Al radial structure distributions and with expected Al−Al distances required to anchor (OZr−O−ZrO)2species in ZSM-5 with a statistical distribution of Al atoms. ZrCl4species in excess of this saturation stoichiometry formed monoclinic ZrO2, detectable in Raman spectra and X-ray diffraction patterns. [ABSTRACT FROM AUTHOR]