Basicities and transesterification activities of Zn–Al hydrotalcites-derived solid bases

Abstract

Solid bases were prepared by calcining as-prepared Zn–Al hydrotalcites at different temperatures. The evolutions of their structure and basicity were characterised using X-ray diffraction, thermogravimetric analysis, infrared spectroscopy of CO₂ adsorption and temperature programmed desorption of methanol. With an increase in the calcination temperature, the as-prepared Zn–Al hydrotalcites were firstly converted into dehydrated Zn–Al hydrotalcites and next converted into Zn–Al oxides. The basic sites of these solid bases changed from OH groups to Mⁿ⁺–O²⁻ (M = Zn or Al) pairs and isolated O²⁻ ions. These solid bases were evaluated in transesterification reactions used for biodiesel production. The dehydrated Zn–Al hydrotalcites obtained at 473 K exhibit the highest activity, with a biodiesel yield of approximately 76% at 413 K, 1.7 MPa and 1.0 h⁻¹. This catalyst exhibits no deactivation after about 150 h. The structure–reactivity relationship of the catalyst is also discussed.