New heterogeneous catalyst for the esterification of fatty acid produced by surface aromatization/sulfonation of oilseed cake Eleonice Moreira Santos a,⇑ , Ana Paula de Carvalho Teixeira a , Flávia Gontijo da Silva a , Thérèse Ebambi Cibaka a , Maria Helena Araújo a , Willian Xerxes Coelho Oliveira a , Felipe Medeiros a , Alex Nogueira Brasil b , Leandro Soares de Oliveira a , Rochel Montero Lago a,⇑ a Departamento de Química, Instituto de Ciências Exatas, Universidade Federal de Minas Gerais, Belo Horizonte-MG 31270-901, Brazil b Biominas Engineering Industry, Itaúna, MG, Brazil highlights  Simple sulfonation of biodiesel waste cake in mild conditions produces a new and active heterogeneous acid catalyst.  The catalytic activity is comparable to H 2 SO 4 and the catalyst can be reused several times.  The catalyst combines a carbon-sulfonic acid surface with a very hydrophilic cellulose surface responsible for water removal. article info Article history: Received 30 September 2014 Received in revised form 6 February 2015 Accepted 8 February 2015 Available online 16 February 2015 Keywords: Acid catalyst Oleic acid Esterification Biodiesel cake Aromatization Sulfonation abstract In this work, an efficient heterogeneous acid catalyst for the esterification of oleic acid was prepared directly from oilseed cake by a simple sulfonation with concentrated H 2 SO 4 . Characterization by SEM/EDS, IR, Raman, TG, TG/MS, potentiometric titration showed that treatment with H 2 SO 4 for 1, 2 and 4 h at 120 °C partially dehydrates the cake to form a carbon/cellulose composite which is sulfonated to produce strong ASO 3 H acidic sites. These surface sites were active for the esterification of oleic acid with yields ca. 84%, 88% and 94% in the presence of 5, 10 and 20 wt% catalyst, respectively. These results are comparable to 98% yield obtained with 1 wt% H2SO4 and higher than 75% observed for a high surface area (880 m 2 g 1 ) sulfonated activated carbon with similar number of ASO3H active groups. These results are discussed in terms of two effects: (i) the number of sulfonic surface acidic groups and (ii) the presence of a hydrophilic cellulosic fraction in the catalyst that adsorbs/traps water formed in the reaction shifting the esterification equilibrium and improving the yield. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction The transesterification reaction to produce biodiesel in the pres- ence of basic homogeneous catalysts, e.g. KOH, NaOH or methox- ides, has been extensively investigated in the last decade [1–4]. It is well established that this alkaline catalyzed transesterification is strongly affected by the presence of free fatty acids [4,5]. The presence of these acids in concentrations higher than ca. 2% can hinder the reaction and form surface active molecules with sig- nificant complications in the purification step due to the formation of stable emulsions [6]. An alternative route to deal with acidic oils is typically a previ- ous esterification in the presence of H 2 SO 4 as catalyst [7]. However, sulfuric acid is corrosive and cannot be recovered [8,9]. In this respect, the development of an active acid heterogeneous catalyst to produce biodiesel using acidic oils is of considerable interest. Heterogeneous catalysts can be easily removed and reused avoid- ing the washing step which simplifies the process [10–13]. Different types of acidic materials, such as zeolites [14–16]; mesoporous silica [17–19], resins [20,21], oxides, e.g. zinc, titani- um, strontium oxides [22–24], zirconia [25–28], supported carbon nanotubes [29] and minerals such as a mordenite, kaolins, hal- loysite [30–32] have been investigated as catalyst for the esterifi- cation reaction. Also promising carbon based acid catalysts have been prepared by pyrolysis followed by sulfonation with sulfuric acid, using different precursors such as carbohydrates, lignin [33,34], sugar cane bagasse [35], fibers [36], biochar [37], resin http://dx.doi.org/10.1016/j.fuel.2015.02.027 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved. ⇑ Corresponding authors at: Departamento de Química, Universidade Federal de Minas Gerais – UFMG, Av. Antônio Carlos, 6627, Pampulha, Belo Horizonte-MG 31270-901, Brazil. Tel.: +55 31 3409 5719. E-mail address: rochel@ufmg.br (R.M. Lago). Fuel 150 (2015) 408–414 Contents lists available at ScienceDirect Fuel journal homepage: www.elsevier.com/locate/fuel