Vanadosilicate mesoporous SBA-15 molecular sieves incorporated with N-alkylphenothiazines

Abstract

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 V5+ ions in dehydrated VSBA-15 with Si/V greater than or equal to 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 V5+ ions by additional coordination to water molecules, suggesting an interior wall surface location for these V5+ ions. Upon CO reduction or incorporation of N-alkylphenothiazines (PCn), part of the V5+ is reduced to vanadyl VO2+ ions. While incorporation of PCn into VSBA-15 in the dark generates some PCn+ cation radicals, ultraviolet photoionization at room temperature further increases the yield for Si/V = 80 and 40. The PCn+ radicals are more stable in VSBA-15 than in siliceous SBA-15, and the photoyield increases with vanadium loading. The photoyield and stability of PCn+ also increase with increasing alkyl chain length of the N-alkylphenothiazines.