Oxidation of 1,2-propanediol to methyl lactate over hydroxyapatite-supported Au–Cu catalysts with varying Ca/P ratios†

Abstract

The oxidative esterification of 1,2-propanediol to methyl lactate was catalyzed using Au–Cu catalysts supported on hydroxyapatite (Au–Cu/HAP) with varying Ca/P ratios in an alkali-free methanol system, and the influence of these ratios on the surface properties and the catalytic performance of the catalysts was explored. The presence of basic sites in the Au–Cu/HAP catalysts contributes positively to their catalytic activity. However, an excess of basic sites does not automatically translate to superior catalytic performance. This could be attributed to the activation and oxidation of hydroxyl groups, which are the result of the combined action of basic sites and Au active sites, and the basic sites are sufficient in all catalysts. Meanwhile, an increase in oxygen vacancies within the catalysts is advantageous as it enhances the catalysts' capacity to adsorb and activate oxygen, which in turn improves their ability to catalyze the oxidation of 1,2-propanediol. The conversion of 1,2-propanediol and the selectivity of methyl lactate reached 71% and 70.6% over the optimized Au–Cu/HAP-2 catalyst.