Sulfonated carbons from olive stones as catalysts in the microwave-assisted etherification of glycerol with tert-butyl alcohol

Abstract

Sulfonated carbons have been prepared from an important waste in Andalusia, the olive stones. The sulfonic acid groups incorporation has been made by sulfuric acid treatment and different conditions have been tested, i.e., temperature, carbon/sulfuric acid weight ratio, sulfonation time, as well as two ways of heating, either conventional heating or microwave irradiation. The morphology of the solids has been characterized by XRD and Raman Spectroscopy and their textural properties evaluated by N2 adsorption- desorption of nitrogen at 77 K. The incorporation of sulfur in the carbons has been verified by elemental analysis, TGA, XPS and ATR-IR. The acidity of the solids was evaluated by elemental analysis (sulfur content) and compared with the results obtained from acid-base titration. The sulfonation time needed to attain an identical sulfur incorporation percentage was considerably reduced under microwave irradiation in comparison to conventional heating (30 min vs. 2 h). The highest yield obtained in the microwave-assisted etherification of glycerol with tert-butyl alcohol at autogenous pressure, 75 °C and 15 min of reaction (∼21 %) is comparable to those reported on other sulfonated carbons but using higher temperatures (100−120 °C) and reaction times (6 h). Furthermore, the sulfonated carbons exhibited high stability, maintaining their activity after being used in several consecutive reactions.