ESTERIFICATION OF GLYCEROL WITH ACETIC ACID OVER ALUMINA BASED CATALYSTS: REACTION KINETIC STUDY

Abstract

In the future, it is expected that biodiesel production will increase to meet the growing energy demands in light of declining petroleum reserves. Glycerol is a by-product of biodiesel production and the increase in glycerol production from biodiesel refining will create a surplus in the glycerol market. Converting glycerol into valuable products is important in order to make biodiesel production more economical. Esterification of glycerol to produce glycerol acetates – Monoacetyl glycerol (MAG), Diacetyl glycerol (DAG), and Triacetyl glycerol (TAG) is one such process. MAG, DAG and TAG have a variety of uses ranging from fuel additives to ingredients in cosmetics. TAG is most attractive as an antiknock agent to gasoline and additive to biodiesel due to its viscosity reducing properties. This work studies the use of alumina based catalyst for the acetylation of glycerol. Two series of catalysts were tested; Cu/ Ni monometallic and bimetallic catalysts on γ-Al2O3 and sulfated alumina (SO42- / γ- Al2O3) catalysts. Cu/ Ni on γ-Al2O3 catalysts were prepared with varying ratios of the two metals and SO42-/ γ- Al2O3 catalysts were prepared with different concentrations of sulphuric acid. The catalysts were characterized using several techniques like X-ray diffraction, BET surface area, NH3–TPD measurements and FT-IR spectra. The impregnation of the metals in case of Cu/ Ni on γ-Al2O3 catalysts was observed by the presence of NiO and CuO peaks in the XRD patterns. The sulfate group on alumina was identified by the corresponding peaks in the FT-IR spectra and the XRD patterns. The catalytic activity of these samples for the glycerol esterification reactions were tested under standard conditions; temperature: 110 oC, atmospheric pressure, glycerol to acetic acid molar ratio: 1:9, and catalyst loading: 0.25g. Cu-Ni/ γ-Al2O3 catalysts were active for the esterification reaction and activity increased with increasing Ni content. 2M SO42- / γ- Al2O3 was found to be the most active catalyst resulting in a glycerol conversion of 97% within 2 hours of reaction and TAG selectivity of 23% after five hours of reaction. Stability of the catalysts was examined via leaching and reusability tests through three consecutive runs. With Ni/ γ-Al2O3 the glycerol conversion dropped from 97% with fresh catalyst to 88% in the third run. The loss in activity could be due to leaching of metal. With 2M SO42- / γ-Al2O3, there was no loss in activity after three consecutive runs and the catalyst can be considered stable for the esterification of glycerol with acetic acid. Arrhenius equation was employed to determine rate constant, order of equation and activation energy