Oxidation reaction mechanism of hydrothermal carbon microspheres with applications to transesterification after sulfonation

Abstract

Background An efficient sulfonic acid‐type acid catalyst with high acid site density based on hydrothermal carbon microspheres (HCMs) has been developed. The reaction by which double bonds in the HCMs were oxidized to generate carboxyl groups was evaluated based on density functional theory calculations. Results Sulfonic acid‐type HCMs (SHCMs) were prepared from HCMs through a three‐step process comprising oxidation, amidation and sulfonation. An analysis of the oxidation process indicated that the formation of carboxyl groups involved the activation of O2 molecules and CH bonds, the cleavage of CC bonds, the homolytic fission of peroxy acids and the coupling reactions of H• radicals. The formation of numerous carboxyl groups on the HCM surfaces was the key to obtaining SHCMs with a high acid density of 14.5 mmol g−1. Conclusions The SHCMs obtained exhibited good catalytic performance during the transesterification of waste frying oil with methanol. Reactions performed using a methanol/oil molar ratio of 9:1 at 90 °C for 5 h gave a fatty acid methyl ester yield of 85.56%. © 2022 Society of Chemical Industry (SCI).

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

“…Condensation reactions with and without activation by EDC and NHS were analyzed using density functional theory (DFT) calculations to better understand the activation mechanism. Compared with the results of previous work, 11 the condensation reaction temperature in the present study was lowered by 30 °C and the effect of grafting –NH 2 groups onto the OHCM surfaces was greater.…”

“…Figure 7 summarizes the effect of the reaction temperature on the yield of FAMEs when using SPEI–EN–OHCMs as a catalyst and demonstrates that the highest yield of 92.3% was obtained at 110 °C after 4 h. The yield of FAMEs obtained in the present study was higher than that of approximately 76% reported by Xue et al . after a reaction at 110 °C for 4 h 11 . This greater yield is primarily ascribed to the activating effect of EDC and NHS on the condensation reaction.…”

“…Therefore, it is important to improve both the acid site density and stability of SACSAs. In previous work by the authors, 11 SACSAs having acid densities as high as 14.5 mmol g −1 were obtained based on the low‐temperature oxidation, amidation and sulfonation of hydrothermal carbon microspheres (HCMs). In other research, Chen and Fang 12 employed a mixture of corn starch and glucose to prepare a carbon‐based solid acid via sulfonation with concentrated sulfuric acid.…”