Nitric Acid-Oxidized Carbon for the Preparation of Esters Under Ultrasonic Activation

Abstract

Temperature-programmed desorption, X-ray photoelectron spectroscopy (XPS) spectroscopy and catalytic test reaction have been applied to investigate acid sites in a microporous activated carbon (Norit RX 1 Extra). Treatment with HNO₃ generates acid sites in the pristine carbon. XPS data show an increase of the amount of carboxyl and hydroxyl groups on the activated carbon surface. The thermodesorption profiles of the material corroborate the XPS data and indicate the presence of hydroxyl groups of carboxylic acids. The results obtained by these methods indicate the presence of acid sites. Thus, the acidity of the carbon was tested in the esterification of organic acids with alcohols. The pristine carbon presents no activity. By contrast, the treated carbon presents enough acidity to carry out the reaction achieving conversion values of around 45% for the reaction of benzoic acid with ethanol in a batch reactor. Such a value increases if the reaction is carried out under ultrasound activation, affording conversion values of around 85%. The selectivity of the reactions is high, around 95%. Only in the case of 1-octanol are some side-products observed, due to the oxidation of the alcohol.