Synthesis of sodium zincsilicate (Na₂ZnSiO₄) and heterogeneous catalysis towards biodiesel production via Box-Behnken design

This study synthesizes a Na₂ZnSiO₄ phase as efficient and low-cost heterogeneous catalyst for biodiesel production. X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) along with Energy Dispersive X-Ray Spectroscopy (EDS), Brunauer–Emmett–Teller (BET) surface area analysis isotherms are herein conducted to characterize the structure, texture, composition and surface area of the catalyst, respectively. A Box-Behnken design (BBD) is performed to screen out: the methanol: oil ratio (6:1 to 14:1), catalyst load (0.5 to 5.5 wt%) and stirring rate (500 to 800 RPM), affecting the transesterification reaction using commercial soybean oil as a triglyceride source while fixing the temperature reaction at 65 °C. The oil and its fatty acid methyl esters (FAME) are characterized using Fourier Transform Infrared Spectroscopy with an Attenuated Total Reflectance modulus (FT-IR-ATR). XRD and SEM confirm the successful synthesis of the monoclinic Na₂ZnSiO₄ phase with low traces of ZnO (<5 wt%) and undetectable for Na₂SiO₃; presenting a BET surface area around 1.9 m² g⁻¹. The material is quite active towards the biodiesel production since a conversion superior to 99% is achieved in 45 min, using the statistical optimization conditions estimated with the BBD, 500 RPM, 5.30 wt% and 14:1 MeOH: Oil ratio. According to the response surface methodology, the sensitivity of the parameters increases in the following order: catalyst load > methanol: oil ratio > stirring rate. The FAME conversions drop to 48.4 and 62.42% after 5 cycles of continuous catalyst reuse, according to FT-IR-ATR and proton nuclear magnetic resonance (¹H NMR) measurements, respectively.