Selective oxidation of biorenewable glycerol with molecular oxygen over Cu-containing layered double hydroxide-based catalysts

To convert glycerol, a by-product of biodiesel, into value-added fine chemicals is of great industrial and economic importance and represents a great challenge. This paper reports the influence of various metal elements (M = Ni, Zn, Cu, Co, etc.) incorporated into MgAl layered double hydroxide (LDH) materials used as catalysts for the liquid phase oxidation of glycerol to fine chemicals. The catalytic behaviour was also studied and discussed in terms of structure and texture of the synthesized materials that were determined using PXRD, FE-SEM, TEM, TG-DTA, FTIR, XPS and nitrogen adsorption-desorption isotherms. The results revealed that CuAlMg LDH materials are active in converting glycerol to glyceric acid (GLYAC) and formic acid, while yielding oxalic and hydroxyehthanoic acid. The selectivity to glyceric acid remarkably increased over Cu-containing catalysts, compared with those over similar materials with the addition of Ni, Zn, Co metal elements. The increase of Cu content not only affects the catalyst activity but also the selectivity. Moreover, an improvement of activity and selectivity was found on calcined CuAlMg samples. The calcined catalysts, with a Cu: Al: Mg molar ratio of 20: 100: 200, exhibited superior performances with a 97.3% conversion of glycerol and a 70% glyceric acid selectivity. It was concluded that the nature of the metal added to the LDH structure, such as Cu as active sites, in combination with the chemical state and microenvironment in calcined and uncalcined materials significantly affects the catalytic performance of the synthesized catalysts in the liquid phase oxidation of glycerol in terms of product activity and distribution.