Solvent-free toluene aerobic selective oxidation over Co(OH)2/Cr2O3：The effect of calcination temperature on product selectivity

Series of Co(OH)2/Cr2O3 catalysts before and after calcination (denoted as CoCrOx) were prepared and used for the selective oxidation of toluene using oxygen in the absence of solvent. Co(OH)2/Cr2O3 catalyst before calcination has exhibited excellent catalytic activity for the oxidation of toluene, conversion of toluene reaches 37.8% and selectivity of benzoic acid is up to 94.4%. Interestingly, calcined catalyst CoCrOx mainly produces benzaldehyde, and the selectivity for benzaldehyde can be as high as 63.6% (conversion of toluene remains high at 14.5%). A series of characterizations including XRD, TEM, FT-IR, XPS, H2-TPR and CO2-TPD illustrate that calcination makes phase transformation occur and leads to the change of acid-basic sites and reactive oxygen species, which affect the activity and selectivity of toluene oxidation. After calcination，Co2+ migrates to vacant sub-surface position of surface oxygen (Osur) and remains on the surface. The formation of Co-Co bond widens defect area so that activated Osur induces oxidation of toluene, leading to an increase in the amount of adsorbed oxygen, which becomes the decisive factor for high selectivity of benzaldehyde. [Display omitted] •The catalysts before and after calcination showed high selectivity for benzoic acid and benzaldehyde, respecvitvly.•The interaction of Co and Cr species in Co(OH)2/Cr2O3 promotes the production of lattice oxygen.•The strong basic sites provided by lattice oxygen is a determinant of the high selectivity of benzoic acid.•The formation of oxygen vacancies in calcined catalyst results in the migration and enhanced activity of oxygen species.