Incorporation of vanadium in mesoporous MCM-41 and microporous AFI zeolites

Vanadium-containing silicate MCM-41 (V-MCM-41) zeolite and aluminophosphate AFI (VAPO-5) zeolite were synthesized and characterized by spectroscopic techniques. In as-synthesized form, the vanadyl ions (V^IV=O)²⁺ were found to be the major vanadium species in the form of atomic dispersion on AFI by EPR and to exist simultaneously with tetrahedral (T_d) V⁵⁺ in MCM-41 by UV-vis. ²⁹Si MAS NMR investigations suggested that the vanadium ions might attach to MCM-41 through interaction with the silanol groups on the internal wall of hexagonal tubes. The V⁵⁺ (in T_d) ions are incorporated into the lattice of MCM-41 during synthesis, while the VO²⁺ (in T_d) is the loosely bound V species. The results of Raman spectroscopy indicated that the rodlike aggregation of cationic surfactant (cetyltrimethylammonium bromide, CTAB) was encapsulated in the intrachannel space of synthetic MCM-41 as in an aqueous solution. After calcination and hydration, the V⁴⁺ species in as-synthesized V-MCM-41 was totally oxidized to +5 as shown by UV-vis and EPR spectroscopies, and they further aggregated as two-dimensional vanadate chain species that were nonuniformly deposited on the wall of MCM-41 channels as verified by Raman and HREM with EDS spectroscopies, while the V⁵⁺ species of synthetic V-MCM-41 remains stable in a tetrahedral coordination. Comparatively, two types of VO²⁺ ions were observed in as-synthesized VAPO-5 by EPR and they could be oxidized by calcination treatment. The presence of water vapor facilitates the oxidation of (V^IV=O)²⁺ and the formation of V₂O₅ cluster instead of isolated (V^V=O)³⁺ species.