Chitosan with Sulfonic Groups: A Catalyst for the Esterification of Caprylic Acid with Methanol.

José Castanheiro

doi:10.3390/polym13223924

Abstract

Esterification of caprylic acid with methanol was performed over chitosan with sulfonic acid groups, as a catalyst, at 60 °C. The sulfonic acid groups were introduced into chitosan (CH) by using chlorosulfonic acid. Catalysts were characterized by scanning electron microscopy (SEM), elemental analysis, thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and acid–base titration. Catalytic activity increased with the amount of sulfonic acid groups present on chitosan. The 4–CH–SO3H catalyst (chitosan with sulfonic acid groups—sample 4 prepared) showed the highest activity of all materials. The esterification of caprylic acid with methanol was optimized using a 4–CH–SO3H catalyst. Under optimized reaction conditions, it was found that, at 60 °C, with 0.2 g of catalyst loading and with a molar ratio methanol to caprylic acid equal 1:95, a caprylic acid conversion of about 83%, after 4 h could be obtained. Catalytic stability of the 4–CH–SO3H material was evaluated through consecutive batch runs. After the second batch, the catalytic activity stabilized.

Highlights

Biodiesel is a mixture of fatty acid methyl ester
It is very important to reduce the amount of the free fatty acids present in raw material so as to not influence the transesterification reaction
To overcome the problems associated with the homogenous catalyst, esterification reactions have been carried out using solid acid materials, which can be removed from the mixture and reused [5,6,7,8,9,10,11]

Summary

Introduction

Biodiesel is a mixture of fatty acid methyl ester. It can be obtained by esterification of fatty acids with an alcohol (methanol, ethanol) and by transesterification of triglycerides with an alcohol (methanol, ethanol, propanol, or butanol) [1,2,3,4]. When the amount of free fatty acids present in vegetable oil or animal fats is high, the transesterification reaction cannot occur due to the formation of soap. The fatty acids present in vegetable oil or animal fats can be reduced by an earlier reaction (an esterification reaction) between the fatty acids and an alcohol to produce biodiesel. This reaction is carried out in the presence of homogenous catalysts, such as H2SO4. After 8 h of reaction, under optimized conditions (molar ratio of acid to alcohol 1:20; m = 0.1 g; T = 60 ◦C), the oleic acid conversion was 95% [7]. The starch-derived catalyst showed the highest catalytic activity [11]

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Results