The effect of lanthanum doping on activity of Zn-Al spinel for transesterification

The transesterification of soybean oil with methanol to biodiesel using lanthanum doped zinc aluminate with spinel structure as catalyst is studied. The catalyst was characterized by X-ray diffraction (XRD), N2 adsorption/desorption, temperature programmed desorption of CO2 (CO2-TPD) and infrared spectroscopy of CO2 adsorption (CO2-IR). The results of CO2-IR show that the basic sites of La doped zinc aluminate are surface isolated O2− anions and OH− groups. The results of CO2-TPD show that the total amount of basic sties of the catalyst improves with the increase of the La doping amount until which is to 18.5 wt%. Doping La also enlarges the pore volume and diameter of zinc aluminate. It gets the largest pore volume (0.30 cm3 g−1) and pore diameter (7.9 nm) when doped with 5.5 wt% La. Zinc aluminate doped with 5.5 wt% La (5.5% La/ZnAl2O4) exhibits the highest activity, with which the biodiesel yield exceeds 95% at 160 °C, 2.0 MPa and 0.9 h−1. Furthermore, the transesterification reaction of about 500 h in a fixed-bed reactor was carried out over 5.5% La/ZnAl2O4 catalyst. No deactivation and leaching of catalyst components were happened on the catalyst in reaction.

Graphical abstractZinc aluminate doped with 5.5 wt% lanthanum (5.5% La/ZnAl2O4) was a high active catalyst for the synthesis of biodiesel by transesterification of soybean oil with methanol. With this catalyst, the biodiesel yield exceeded 95% at 160 °C, 2.0 MPa, 0.9 h−1, and nmethanol/noil of 24:1.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Lanthanum doped zinc aluminate shows high activity to the synthesis of biodiesel. ► Doping La increases the basicity and pore diameter of Zn-Al spinel. ► The basic sites of the catalyst are surface OH groups and isolated O2− anions. ► No deactivation is happened on the catalyst in the reaction of about 500 h.