Selective oxidation of toluene to benzaldehyde in liquid phase over CoAl oxides prepared from hydrotalcite-like precursors

摘要

CoAl oxides were prepared from the calcination of CoAl layered double hydroxides (LDHs, Co/Al = 2, 3 and 4) and applied to the selective oxidation of toluene to benzaldehyde using tert-butylhydroperoxide as the oxidant. These samples were characterized by XRD, SEM, BET, DR-UV-Vis, XPS, FTIR and CO2-TPD techniques to explain their catalytic properties. The influence of various reaction parameters, such as reaction temperature, reaction time, content of catalyst, substrate/oxidant mole ratio and nature of solvent on the activity and selectivity of aldehyde was evaluated. Co4Al oxide showed a maximum conversion of toluene of 8.2 % with 77.6 % selectivity towards benzaldehyde under the optimized conditions. The moderate basic sites on the CoAl oxides and the increased Co3O4 content may be responsible for the catalytic activity and selectivity. Repeated runs showed that the catalyst was stable for at least five cycles without obvious changes of catalytic activity and selectivity.