Mixed metal oxide as catalysts for acetin synthesis from glycerol

Abstract

In the present thesis four different heterogeneous acidic catalysts viz., SO42-/SiZrO4, Fe3O4@SiO2@SO42-, Nb@Fe3O4 and Ce/TiZrO4@SO42- were synthesized by chemical method. The prepared catalysts were employed for the glycerol acetylation with acetic acid to yield the diacetin or triacetin as exclusive products. The structural property of all the prepared catalysts was determined with the help of powder X-ray diffraction pattern, and FTIR analysis. X-ray photoelectron spectroscopy also revealed that the active species was incorporated over the matrix to impart the Brønsted as well as Lewis acidity to the catalyst which is vital for the acetylation activity. The surface morphology and elemental distribution were studied by SEM-EDX, and HRTEM analysis. The BET analysis of the catalysts reveals the surface porosity and pore size of the prepared samples. The acidic strength of the catalysts was quantified by the NH3-TPD analysis while nature (Brønsted versus Lewis) of those sites was determined by analysing the FTIR of pyridine adsorbed catalyst samples. The reaction parameters were optimized to obtain a specific acetin as an exclusive product. Under the optimized reaction parameters of 9:1 AcA/GL molar ratio, 3 wt% catalyst, 80 °C reaction temperature, and 40 min of reaction duration, a 93 % triacetin yield was obtained during SO42-/SiZrO4 catalyzed glycerol acetylation. The catalyst was recovered from the reaction mixture and reused during six consecutive reaction runs while retaining 54 % triacetin selectivity in the last cycle. A plausible mechanism suggests the heterogeneous catalyst-assisted protonation of carbonyl group of acetic acid to initiate the stepwise acetylation of the hydroxyl groups of glycerol. A magnetic catalyst was prepared by embedding magnetite (Fe3O4) within the silica shell on which sulphate ions were anchored (Fe3O4@SiO2@SO42-). The catalyst was successfully employed for glycerol acetylation with acetic acid to obtain 100 % selectivity towards triacetin within 45 min of reaction duration at 80 °C. The catalyst was successfully isolated from the reaction medium magnetically and reused for 6 consecutive reaction cycles while retaining 90 % glycerol conversion levels and 50 % triacetin selectivity in the last reaction cycles. In another approach, niobium oxide was incorporated over magnetic Fe3O4 nanoparticles, to impart the acetylation activity to the catalyst. The prepared catalyst (Nb@Fe3O4) in 7 wt% amount was found to yield 92 % triacetin within 80 min of reaction duration. The glycerol acetylation with acetic acid was found to follow the pseudo first-order kinetic equation. The thermodynamic parameters viz., enthalpy (ΔH⧧), entropy (ΔS⧧), and Gibbs free energy (ΔG⧧) of the reaction were also determined to suggest the endothermic and unspontaneous nature of the reaction. Upon completion of the reaction, the catalyst was recovered from the reaction mixture under the influence of an external magnetic field and reused during seven consecutive reaction cycles. To prepare diacetin molecule as an exclusive product, Ce/TiZrO4@SO42- was prepared and employed as a heterogeneous acid catalyst for the acetylation of glycerol with acetic acid. The prepared catalyst, Ce/TiZrO4@SO42-, demonstrated almost 96 % diacetin selectivity in 60 min by employing acetic acid to glycerol molar ratio of 6:1 at 120 °C reaction temperature. The catalyst was recycled in five successive reaction cycles while retaining ~36 % activity during the last cycle.