Fabrication of mesoporous sorbent embedded with AuNPs for catalytic esterification of oleic acid to obtain biofuel: Textural and kinetic studies

Abstract

A mesoporous silica-based support was synthesized and named MPS. It was embedded with AuNPs through their solution dropwise addition into dispersed MPS to produce MPS-AuNPs catalyst. Textural and kinetic properties of MPS-AuNPs towards catalyzed esterification of oleic acid with methanol were investigated using different instrumental and analytical techniques. Nitrogen adsorption-desorption isotherms of MPS-AuNPs showed a typical type-IV curve with an observed hysteresis loop at P/P0 > 0.82, a high BET surface area of 879 m²/g, an average pore diameter of 11.9 nm, and total pore volume of 0.38 mL/g confirming the mesoporous structure of MPS-AuNPs. TEM image confirmed the homogeneous embedding of AuNPs into the mesoporous structure with an average size of 7.3 nm. In-situ tracking of oleic acid concentration change during the catalyzed esterification process was investigated using ART-FTIR spectrophotometer to confirm the esterification process's success and to perform kinetic measurements. Reaction conditions of 1:10 oleic acid/methanol ratio, 60 °C, and 5 % catalyst loading were selected as the optimum reaction conditions where 97.3 % conversion ratio was detected. The reaction followed pseudo-first-order kinetics with a fast equilibrium time of 70 min and a reaction rate constant K = 4.33 × 10² min^-¹ . The activation energy value of 32.51 kJ/mol indicated efficient catalysis. The catalyst showed excellent regeneration efficiency of 95 %, with only a 5 % drop in value after five consecutive cycles. The obtained methyl oleate's kinematic viscosity and flash point values were 4.51 mm²/s (at 40 °C) and 191 °C, respectively. This makes it suitable for the biodiesel industry after the recommended addition of antioxidants, as its oxidative stability of 2.5 hours is somewhat low. Compared to the batch process used, a continuous fixed-bed column reactor is recommended for large-scale industrial biofuel production