Role of silanol groups in the incorporation of V in beta zeolite

Aqueous solutions of ammonium metavanadate of increasing concentrations hav e been contacted with a beta zeolite before (sample AIP, Si/Al = 11) and af ter (sample Si beta, Si/Al > 1000) dealumination in a 13 N HNO3 solution. U V-visible and V-51 NMR characterizations of the V-loaded zeolites show that mainly octahedral V species are formed in AIP whereas, up to a loading of 1.75 V atoms per unit cell (2.3 V wt.%), only pseudotetrahedral V species ( nonhydroxylated (SiO)(3)V=O and hydroxylated (SiO)(2)(HO)V=O) are observed in Si beta. XRD and FT-IR results indicate that the V atoms present in Si b eta are incorporated in the framework in a pseudotetrahedral environment. T hese V species are resistant to washing either with water or with an aqueou s solution of NH4OAc (I mol 1(-1), 12 h). In contrast, the octahedral V spe cies present in VA1 beta (whatever the V loading) and in VSi beta (at loadi ngs higher than 1.75 V per unit cell) are eliminated upon washing showing t hat these species are loosely bound to the zeolite. As demonstrated by FT-I R, the incorporation of V in the Si beta framework involves silanol groups formed upon dealumination. This incorporation generates new Bronsted acidic sites. New FT-IR bands (3650, 3620, 980, 950 cm(-1)) are formed whose attr ibution is proposed, The maximum amount of V atoms incorporated in Si beta (1.75 per unit cell) is lower than the potential amount of framework vacant T-sites generated by dealuminating the beta framework (5.3 per unit cell). However, this V amount is 20 times larger than that usually found in V-loa ded zeolite prepared by hydrothermal synthesis. Therefore, this study confi rms that dealuminating a zeolite is an attractive method to generate silano l groups which can be used to react with vanadium complexes and allow the i ncorporation in the zeolitic walls of high amounts of V atoms resistant to washing treatments. (C) 2000 Elsevier Science B.V. All rights reserved.