Preparation of heterogeneous acid catalyst from crumb rubber

Abstract

In this work, crumb rubber was chemically modified to sulfonic acid-containing rubber via a controlled oxidation with hydrogen peroxide or nitric acid at ambient pressure. The influences of oxidation conditions, including type and concentration of oxidizing of oxidizing agent were studied. The original and oxidized crumb rubbers were measured for the sulfur content by bomb calorimetry, the swelling degree according to ASTM D47 1 and the sol/gel ratio according to ASTM D47 1. Moreover, the physicochemical properties of rubber were characterized by Fourier-transform infrared spectroscopy (FTIR), thermogravimetric/differential thermal analysis (TG/DTA) and acid-base titration. The results showed that the properties of the crumb rubber oxidized with only hydrogen peroxide were not significantly changed. Nitric acid and hydrogen peroxide with formic acid oxidized the crumb rubber, giving short-chain polymer with the polar acid functions. especially sulfonic acid and carboxylic acid groups. As the nitric acid had higher oxidizing power than the hydrogen peroxide, the product generated from the oxidation with nitric acid exhibited high acidity and high stability in polar /non-polar solvents. The suitable conditions for the modification of crumb rubber to sulfonic acid-containing rubber were the nitric acid concentration of 6 M at 80 ℃ for 3 h. Then the oxidized crumb rubbers were applied as the heterogeneous acid catalysts in the esterification of octanoic acid with long chain alcohols, i.e. octanol and 2 - ethyl - 1- hexanol, 120 ℃ for 8 h. The octanoic acid conversion and catalyst recovery were analyzed by the gas chromatography and the filtration, respectively. The crumb rubber oxidized by using nitric acid was successfully applied as the catalyst in the esterification in which the octanoic acid conversion was 50.4% and 75.3% for the esterification with octanol and 2 – ethyl – 1 hexanol, respectively. However, the catalyst recovery was lower than 50% due to the dissolution of catalyst in the reaction mixture.