New heterogeneous catalyst for the esterification of fatty acid produced by surface aromatization/sulfonation of oilseed cake

• Simple sulfonation of biodiesel waste cake in mild conditions produces a new and active heterogeneous acid catalyst.
• The catalytic activity is comparable to H2SO4 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.

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 H2SO4. Characterization by SEM/EDS, IR, Raman, TG, TG/MS, potentiometric titration showed that treatment with H2SO4 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 SO3H 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 m2 g−1) sulfonated activated carbon with similar number of SO3H 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.