Esterification of glycerin with fatty acids using KIT-6 acid catalysts

VERGARA LOURDES; SÁNCHEZ, BÁRBARA; MENDOW, GUSTAVO; QUERINI, CARLOS

Buenos aires

Congreso; 11th World Congress chemical Engineering; 2023

The esterification reaction between glycerin and fatty acids combines two by-products from the biodiesel industry to obtain mono, di and triglycerides (MG, DG and TG), and one molecule of water for each ester bond. Then, the oil obtained will be reinjected into the biodiesel process, increasing the added value of the plant residues. In this way, a so-called “second generation” fuel is produced. The generated water must be removed as it forms emulsions and inhibits the reaction by shifting the equilibrium to the left. The acidity needed to catalyze the reaction is very high, so KIT-6 silicas modified with sulfonic groups (-SO3H) were synthesized by different methods: co-condensation, where they are functionalized simultaneously with the formation of the support (catalysts "P” and “L” in this work) and grafting, where the functional groups are added post-synthesis of the support (catalysts “R”). With these three methodologies, catalysts with 10 and 15% contents (mol % S/Si) were prepared. The materials obtained were characterized with various techniques and were evaluated in the esterification reaction.In the synthesis of the KIT-6 materials, tetraethyl orthosilicate (TEOS) was used as silicon source, n-butanol and Pluronic P-123 as a template, in an acid medium. The precursors were dissolved at 35 °C for 24 h and the hydrothermal treatment was carried out at 130 °C for 24 h. For the incorporation of sulfonic groups in order to generate acidity, three alternatives were explored: (a) co-condensation of TEOS and Mecaptopropyl-trimethoxysilane (MPTMS) with subsequent oxidation to -SO3H groups; (b) sol-gel technique as described before with in situ oxidation; (c) grafting of KIT-6 silica with MPTMS solution in toluene and subsequent oxidation to -SO3H groups.KIT-6 silica presented high surface area and pore size, but the parameters were affected by the addition of MPTMS. It was observed that as the theoretical content of sulfonic groups increased, the BET area and Dp area decreased, while the acidity increased. The co-condensation methods (a and b) turn out to be more effective in fixing S but with grafting (c) it is possible to preserve the structure of the starting mesoporous solid. By SAXS technique, it was verified that the catalysts synthesized by co-condensation present a smaller network parameter and a decrease in the degree of ordering of the mesoporous structure, especially for the sample KIT-6 (P) 10%. In addition, the incorporation and oxidation of -SO3H was studied by TGA, finding that the oxidation of -SH groups was more efficient with the cocondensation methods.The materials were evaluated in the esterification of glycerin with fatty acids. Good conversions were observed regardless of the preparation method. The performance of KIT-6 (R) 15% stands out, as it obtained high conversion in a relatively short time (94% at 180 min). Selectivity to MG+DG+TG was determined by gas chromatography and the best result was for the aforementioned catalyst (69.5% at 180 min). At long reaction times, the conversion increases but at the expense of a greater formation of secondary products, which decrease the selectivity to the desired products