First Insights into the Use of Activated Carbons as Catalysts for Selective Oligomerization of Glycerol

Glycerol oligomerization is one of the alternatives to add commercial value to surplus glycerol from biodiesel industries. Also known as glycerol self-etherification, the oligomerization process aims to produce short-chain polyglycerols, mainly diglycerol. Short-chain polyglycerols are suitable for the manufacture of food, cosmetics, and polymers. Selective glycerol oligomerization can be achieved using basic and form-selective catalysts. Due to their outstanding physical and chemical properties, activated carbons (ACs) were evaluated as catalysts for the selective oligomerization of glycerol. Different methodologies were used to modify a commercial H 3 PO 4 -AC: thermal treatment, hydrothermal treatment with NH 4 OH, and chemical treatment with KOH. The study mainly focused on investigating the catalytic activity of different AC structures: (1) oxygen-containing functional groups (OCFGs); (2) phosphorus-containing functional groups (PCFGs); (3) nitrogen-containing functional groups (NCFGs); and (4) K-intercalated species. The synthesized ACs were characterized through CHN elemental analysis, X-ray fluorescence (XRF), X-ray diffraction (XRD), N 2 physisorption, Fourier Transform Infrared Spectroscopy (FTIR), thermogravimetry (TG), and acid-base titration. The catalytic tests were carried out at 230 °C for 8, 16, and 24 h and with catalyst loadings of 2 wt%, 4 wt%, and 6 wt%. OCFGs and NCFGs did not exhibit significant catalytic activity during glycerol oligomerization tests. For the acid-activated carbons, PCFGs were found to be active sites producing both polyglycerols and dehydration products. The intercalated potassium of the basic carbon leached completely and selectively produced short-chain polyglycerols through homogeneous catalysis (X G 24 h, 2 wt% = 29%, Y DG 24 h, 2 wt% = 24%).