Shape Effect of ZnO Crystals as Cocatalyst in Combined Reforming–Hydrogenolysis of Glycerol

Abstract

Disk- and rod-shaped hexagonal ZnO crystals with various length-to-diameter aspect ratios were controllably synthesized via a facile solution route by adjusting the precursor concentration. The shape and the dimension of the synthesized ZnO crystals were observed by scanning electron microscopy (SEM). The wurtzite structure and the growth habit were determined by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM) coupled with selected-area electron diffraction (SAED). It is found that with the lowering of the precursor concentrations, the ZnO crystals were elongated along the c-axis, and the diameter of the {001} planes was reduced, leading to shape evolution from hexagonal disk to prismatic rod. As a result, the ZnO crystals were different from each other in the proportion of the {100} nonpolar planes and the {001} polar planes. The well-defined ZnO crystals were used as the cocatalyst with skeletal Ni40Mo10 in the combined reforming–hydrogenolysis (CRH) of glycerol in the absence of adventitious H2. A remarkable shape-dependent effect on the selectivity to the C3 hydrogenolysis products and the production rate of 1,2-propandiol (1,2-PDO) was identified. ZnO with a larger proportion of the nonpolar planes was more effective in the CRH of glycerol to the C3 products. An excellent linear relationship between the surface area of the {100} nonpolar planes and the production rate of 1,2-PDO was identified. This is attributed to the in situ enhancement of the Lewis acidity of the nonpolar planes of ZnO by chemisorbed CO2 from the reforming of glycerol, which greatly accelerates the dehydration of glycerol to acetol, the intermediate to 1,2-PDO.