Carbon as a catalyst: Esterification of acetic acid with ethanol

Multiwalled carbon nanotubes (CNTs) and carbon xerogels prepared by the sol-gel process (CXs) were modified by different chemical and thermal treatments with sulphuric and nitric acid in order to produce materials with different textural properties and acidic nature. The presence of sulphonic acid groups was identified by X-ray photoelectron spectroscopy on the sulphuric acid treated samples. Carboxylic acids and anhydrides, phenol groups, some lactones and carbonyl-quinone groups were incorporated during the nitric acid treatment, as shown by temperature programmed desorption. Oxidation with nitric acid affects more the textural properties (the effect is more pronounced in the case of CNTs than in CXs) promoting an increase of surface area and a decrease of total pore volume, while a decrease of surface area was observed with the sulphuric acid treatment. The prepared materials were tested in the esterification of acetic acid with ethanol as a model acid-catalyzed reaction. The rate of formation of ethyl acetate was found to depend strongly on the chemical properties of the carbon materials, the presence of sulphonic acid groups being more adequate for this reaction. A good correlation was obtained between the rate of formation of ethyl acetate and the concentration of sulphonic groups. (C) 2013 Elsevier B.V. All rights reserved.