The role of surface chemistry of carbons in the catalytic production of fuel additives by glycerol etherification

Abstract

Carbon materials of various types were prepared and functionalized with acidic sites to be further used as catalysts in glycerol etherification. Two modification approaches were applied: treating the carbons with concentrated sulfuric acid or reacting them with in-situ generated diazonium salt. The obtained materials were thoroughly characterized using elemental and textural analyses, potentiometric back titration method, electron microscopy techniques (SEM and TEM), thermogravimetric (TG) analysis, and X-ray photoelectron spectroscopy (XPS). The catalytic properties of the samples were tested in the process of glycerol etherification performed with tert-butyl alcohol (TBA) under batch conditions and autogenous pressure. Based on the obtained results, it was found that the sulfonic groups inserted into the carbon matrixes played a key role in the tested process, enabling the transformation of glycerol to its ether derivatives. Mono- tert-butyl glycerol ethers (MTBGE) were the main products of the reaction. However, for each group of modified carbons (i.e., H2SO4- and BDS-functionalized), a positive correlation was also obtained between the content of -SO3H sites and the selectivity to higher-substituted (i.e., di- and tri-) tert-butyl glycerol ethers (DTBGE and TTBGE, respectively). Moreover, it was established that moderately or weakly acidic sites in the form of oxygen structures did not catalyze the etherification on their own; however, their co-presence with sulfonic groups on the carbon surface was beneficial for the process. The best-working sample gave ∼45 % yield of MTBGE and ∼10 % yield of DTBGE after 2 h. These results were comparable to those obtained with a commercial acid catalyst Amberlyst-15, proving the high potential of using carbons as catalysts for the production of glycerol ethers.