Please cite this article in press as: B.L. Oliveira, V. Teixeira da Silva, Sulfonated carbon nanotubes as catalysts for the conversion of levulinic acid into ethyl levulinate, Catal. Today (2013), http://dx.doi.org/10.1016/j.cattod.2013.11.028 ARTICLE IN PRESS G Model CATTOD-8750; No. of Pages 7 Catalysis Today xxx (2013) xxx–xxx Contents lists available at ScienceDirect Catalysis Today jou rn al hom epage: www.elsevier.com/locate/cattod Sulfonated carbon nanotubes as catalysts for the conversion of levulinic acid into ethyl levulinate Bianca L. Oliveira, Victor Teixeira da Silva ∗ Universidade Federal do Rio de Janeiro, NUCAT – Programa de Engenharia Química – COPPE, P.O. Box 68502, 21941-914 Rio de Janeiro, RJ, Brazil a r t i c l e i n f o Article history: Received 4 September 2013 Received in revised form 5 October 2013 Accepted 6 November 2013 Available online xxx The authors would like to dedicate this paper to Professor Alberto Luiz Coimbra, in the 50th anniversary of COPPE (1963–2013), the Graduate School of Engineering of the Universidade Federal do Rio de Janeiro. Keywords: Levulinic acid Ethyl levullinate Sulfonated carbon nanotubes a b s t r a c t Multiwall carbon nanotubes were sulfonated at different temperatures (150, 180, 210, 230, 250 and 280 ◦ C) and used as catalysts in the esterification of levulinic acid with ethanol. The materials sulfonated between 150 and 230 ◦ C presented almost the same acidity (measured by ammonia chemisorption), which was higher than that obtained for samples sulfonated at 250 and 280 ◦ C. Raman spectroscopy revealed that the treatment with sulfuric acid did not lead to the destruction of the carbon nanotubes structure, even for the higher temperature treatment. The activity results have shown that the nanotubes sulfonated below 250 ◦ C presented a specific activity higher than those sulfonated at 250 and 280 ◦ C. The association of these results with those obtained by temperature-programmed desorption of ammonia indicate that the activity in the esterification reaction is related to the number of acidic sites that desorb ammonia in temperatures around 220 ◦ C. Temperature-programmed desorption experiments suggest that there is a strong adsorption of the levulinic acid on the active sites therefore not allowing the reuse of the materials. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The need for diversification in the global energy matrix has lead to the exploration of alternative energy sources, such as solar, wind and residual biomass. Residual biomass can be used to pro- duce energy, base chemicals and biofuels. Thus, the rational use of residual biomass not only helps to reduce the environmental prob- lems caused by the fossil fuels but also partly meets the increasing demand for energy. Levulinic acid obtained from the acid hydrolysis of lignocellu- losic residues is regarded as one of the twelve most promising molecules derived from biomass because it can be transformed into a variety of other compounds important to the chemical industry [1–4]. In particular, ethyl levulinate is a very promising compound produced via the esterification of levulinic acid with bioethanol for use as an oxygenated additive to fuels [5–7]. Esterification reactions normally use inorganic acid catalysts with H 2 SO 4 as the most widely employed. However, the use of mineral acids in industry is undesirable because they corrode equipment and require separation from the final product, which involves neutralization and waste disposal. Thus, heterogeneous ∗ Corresponding author. Tel.: +55 21 2562 8344; fax: +55 21 2562 8300. E-mail address: victor.teixeira@peq.coppe.ufrj.br (V. Teixeira da Silva). catalysts that can be easily separated from the product and reused are desirable. In particular, the synthesis of ethyl levulinate via esterification of levulinic acid with ethanol using heterogeneous catalysis has attracted the attention in the last two years with many works being published in the literature [8–10]. Pasquale et al. [9] explored the potential of Well-Dawson heteropolyacid incorporated to a silica framework by the sol–gel technique as a catalyst in the esterification of levulic acid with ethanol and observed that the material presented an excellent activity and was reusable. Fernandes et al. [8] studied the use of several zeo- lites (HBEA, HMOR, HUSY, HMCM-22, HZSM-5) and found out that the zeolite structure plays a more important role than the acidity in the esterification of levulinic acid with ethanol. These authors also studied sulfated stania as catalyst and observed that despite the remarkable performance there was leaching of the sulfate groups into the reaction medium, thus discarding this cat- alyst. Yan et al. [11] employed H 4 SiWO 40 /SiO 2 for methyl and ethyl levulinate production but the high yields obtained have to be carefully considered due to the large amounts of catalyst used. Finally, Melero et al. [12] have successfully incorporated sulfonic groups to a mesoporous silica (SBA-15) and found out that the resulting material presented an outstanding performance for the esterification of levulinic acid with ethanol. The moderate acid strength and hydrophobicity of these organosulfonic acid-modified mesoporous materials was the key of the catalytic performance, 0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2013.11.028