Sodium modified hydroxyapatite: Highly efficient and stable solid-base catalyst for biodiesel production

Highlights: • Sodium modified hydroxyapatite is a promising solid base catalyst for biodiesel production. • Experiments were conducted in a laboratory-scale batch reactor. • The methyl ester yield can reach 99% over 50-NaHAP-800 solid base catalyst at optimized reaction parameter. • The catalyst has good stability and strong ability to be reuse for more than five cycles. • Minor leaching concentrations of calcium and phosphorus species were detected in the product. - Abstract: The present study focuses on the transesterification of rapeseed oil into biodiesel using sodium-modified hydroxyapatite (NaHAP) as a new highly efficient solid base catalyst. The catalyst was prepared by a simple impregnation of NaNO₃ on the HAP support followed by the calcination at different temperatures. The prepared solid-base catalysts were characterized using the X-ray powder diffraction, thermogravimetric analysis, Fourier transform infrared (FTIR) scanning electron microscopy (SEM), ³¹P solid-state NMR, BET, and basicity measurement by phenol adsorption to determine their physical and chemical properties. Characterization results revealed that the catalyst loaded with 50 wt% of NaNO₃ and calcined at 800 °C exhibited the highest amount total basicity, which is 121 μmol/g. The suitable reaction condition for maximum biodiesel yield up to 99% were methanol to oil molar ratio of 6:1, 4 wt% of catalyst and reaction temperature of 100 °C. The catalyst has good stability and strong ability to be reuse for more than five cycles. Moreover, some of the most important physicochemical properties of the produced biodiesel fuel were determined according to the European standard and were found to be within the recommended EN14214 specifications.