Synthesizing a transesterification catalyst via a condensation reaction and assessment of the mechanism using density functional theory

BACKGROUND A solid acid (SPEI–EN–OHCMs) having dendritic –NH 2 groups was prepared via oxidation of hydrothermal carbon microspheres (OHCMs) followed by a condensation reaction with the –NH 2 groups of polyethyleneimine and sulfonation. The mechanism by which –COOH groups on the surfaces of OHCMs reacted with these –NH 2 groups was assessed using density functional theory (DFT). RESULT Activation using a combination of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) and N -hydroxysuccinimide (NHS) was found to promote the formation of amide bonds such that –NH 2 groups were bonded to the microsphere surfaces. This effect provided more bonding sites for –SO 3 H groups. The DFT results indicated that the energy barrier on the potential energy surface in the presence of EDC and NHS was lowered by 18.1 kcal mol −1 . X-ray photoelectron spectroscopy analyses also confirmed the introduction of –NH 2 groups followed by –SO 3 H groups onto the surfaces of the OHCMs. The concentration of surface acid sites on SPEI–EN–OHCMs was found to be as high as 11.8 mmol g −1 . Thermogravimetric data and elemental analysis results showed that the –SO 3 H groups on the SPEI–EN–OHCM surfaces were thermally stable below 180 °C. CONCLUSION The SPEI–EN–OHCMs catalyst was used to promote the transesterification reaction of waste frying oil with methanol. A trial with a methanol-to-oil molar ratio of 10:1 at 110 °C for 4 h gave a fatty acid methyl ester (FAME) yield of 92.3%. After four reuses, an SPEI–EN–OHCMs specimen provided a FAME yield of 54.3%. © 2024 Society of Chemical Industry (SCI).