Optimization for esterification of saturated palm fatty acid distillate by D-optimal design response surface methodology for biolubricant production

Abstract

This work presents a synthesis of palm fatty acid distillate (PFAD)-based esters to produce biolubricant oils through the esterification reaction between saturated palm fatty acid distillate (SFA-PFAD) with different types of high degree polyhydric alcohols such as trimethylolpropane (TMP), di-trimethylolpropane (Di-TMP), pentaerythritol (PE), and di-pentaerythritol (Di-PE) in the presence of sulfuric acid as catalyst. The chemical structures of synthesized SFA PFAD-based esters were characterized and confirmed by using FTIR, NMR $(^1H and ^{13}C)$ spectroscopies and GC-FID chromatography. The FTIR spectra of SFA PFAD-based ester products clearly showed the peaks of C=O and C–O of ester group at $1732–1740 cm^{−1} and at 1239–1162 cm^{−1}$, respectively. Furthermore, $^1H$ NMR spectra confirmed the proton chemical shift $(-CH_2-O-)$ of the ester group at 3.80–4.01 ppm. The 13C NMR spectra confirmed the carbon chemical shifts of ester carbonyl signals at 171.09–174.07 ppm and secondary carbons (CH2-C = O) at 40.57–42.44 ppm. The results showed that the optimum conditions for the esterification of SFA-TMP was obtained at acid catalysts of 5%, esterification time and temperature of 6 h and 150 °C, respectively. The results have shown the ester products yields have been significantly increased up to 93% with selectivity of 99% SFA-TMP tri-ester after the optimization process by using D-optimal design. The results for lubrication properties have shown that the SFA PFAD-based esters have low-temperature properties with pour points value in the range of 18–35 °C, flash point (270–310 °C), onset oxidative stability temperature (251–322 °C) and viscosity indices (115–131), respectively. The results showed that the presence of many esters functional groups in the molecule structure of SFA PFAD-based esters provides a positive impact on the lubrication properties. Overall, the results indicated that the SFA PFAD-based esters can be used as biolubricant base oils with pour point depressants.