Vanadosilicate Mesoporous SBA-15 Molecular Sieves Incorporated withN-Alkylphenothiazines

Mesoporous silica SBA-15 molecular sieves have been incorporated with various amounts of vanadium via incipient-wetness impregnation with vanadium oxide triisopropoxide in hexane followed by calcination. The products (VSBA-15) have been characterized by nitrogen adsorption and diffuse reflectance UV−visible and electron spin resonance spectroscopies. The results show that the vanadium exists as isolated tetrahedral V 5+ ions in dehydrated VSBA-15 with Si/V ≥ 40 while vanadium chains develop as vanadium loading increases. Upon hydration a large fraction of the vanadium is transformed to square pyramidal and distorted octahedral V 5+ ions by additional coordination to water molecules, suggesting an interior wall surface location for these V 5+ ions. Upon CO reduction or incorporation of N- alkylphenothiazines (PC n ), part of the V 5+ is reduced to vanadyl VO 2+ ions. While incorporation of PC n into VSBA-15 in the dark generates some PC n + cation radicals, ultraviolet photoionization at room temperature further increases the yield for Si/V = 80 and 40. The PC n + radicals are more stable in VSBA-15 than in siliceous SBA-15, and the photoyield increases with vanadium loading. The photoyield and stability of PC n + also increase with increasing alkyl chain length of the N- alkylphenothiazines.