Epoxidation of cyclohexene with H2O2 over efficient water‐tolerant heterogeneous catalysts composed of mono‐substituted phosphotungstic acid on co‐functionalized SBA‐15

A series of Keggin-type heteropolyacid-based heterogeneous catalysts (Co-/Fe-/Cu-POM-octyl-NH3-SBA-15) were synthesized via immobilized transition metal mono- substituted phosphotungstic acids (Co-/Fe-/Cu-POM) on octyl-amino-co-functionalized mesoporous silica SBA-15 (octyl-NH2-SBA-15). Characterization results indicated that Co-/Fe-/Cu-POM units were highly dispersed in mesochannels of SBA-15, and both types of Bronsted and Lewis acid sites existed in Co-/Fe-/Cu-POM-octyl-NH3-SBA-15 catalysts. Co-POM-octyl-NH3-SBA-15 catalyst showed excellent catalytic performance in H2O2-mediated cyclohexene epoxidation with 83.8% of cyclohexene conversion, 92.8% of cyclohexene oxide selectivity, and 98/2 of epoxidation/allylic oxidation selectivity. The order of catalytic activity was Co-POM-octyl-NH3-SBA-15 > Fe-POM-octyl-NH3-SBA-15 > Cu-POM-octyl-NH3-SBA-15. In order to obtain insights into the role of -octyl moieties during catalysis, an octyl-free catalyst (Co-POM-NH3-SBA-15) was also synthesized. In comparison with Co-POM-NH3-SBA-15, Co-POM-octyl-NH3-SBA-15 showed enhanced catalytic properties (viz. activity and selectivity) in cyclohexene epoxidation. Strong chemical bonding between -NH3+ anchored on the surface of SBA-15 and heteropolyanions resulted in excellent stability of Co-POM-octyl-NH3-SBA-15 catalyst, and it could be reused six times without considerable loss of activity.