The use of alkaline earth oxides as pH modifiers for selective glycerol oxidation over supported Au catalysts

The activity and selectivity of an Au/TiO2 catalyst in a reaction mixture which has had its pH adjusted by the addition of NaOH (aq) is compared to that of the same catalyst in reaction mixtures whose pH was adjusted using suspensions of BaO and CaO. It was found that the suspensions acted as effective bases and reactions in their presence showed similar conversions to those where NaOH(aq) was used. The selectivities of the reactions were different in each case. In all cases dihydroxyacetone, glyceric acid, glycolic acid and formic acid was produced but more glyceric acid was produced when alkaline earth oxide suspensions were used (at the expense of glycolic and formic acid) than when NaOH(aq) was used. Given glycolic and formic acids are produced from oxidation of glyceric acid, it seems that glyceric acid is somewhat stabilized against further oxidation in the presence of the group II oxides. One explanation for this is the formation of glyceric acid hemicalcium salts (and analogous Ba2+ species) in solution.