Authors: Chaves, TF; Passos, AR; Briois, V; Martins, L; Pulcinelli, SH; Santilli, CV; Perez-Pariente, J
Microporous Mesoporous Mat.. vol: 279. page: 1387-1811.
Date: may-01. 2019.
Aluminum-free MWW structure containing vanadium was easily prepared from a mixture of suitable templates (N,N,N-trimethyl-1-adamantamonium hydroxide (TMAdaOH) and hexamethyleneimine (HMI)). Besides, the use of VCl3 as vanadium source, in which this metal is in low oxidation state (V3+) and the preparation of the synthesis gel under inert atmosphere are crucial to get an efficient incorporation of vanadium into the zeolitic framework. UV-Vis spectrum of the as-synthesized material whose oxidation was prevented by working under inert atmosphere reveals the presence of isolated V3+ sites isomorphically coordinated. Besides, the Si-29 MAS NMR spectra and the V K-edge X-ray Absorption Spectroscopy (XAS) analysis show that the vanadium was incorporated into the zeolite framework with formation of V-O-Si linkages. C-13 MAS NMR evidences that both templates are present in the as-synthesized material, showing that the mixture of TMAdaOH and HMI is crucial for obtaining pure crystalline vanadosilicate phase. XAS measurements of the V-containing zeolite during calcination under air showed that initially the sample contains V4+ sites in a square pyramidal geometry, which were completely oxidized to isolated V5+ in T-d coordination after calcination..