Preparation and Characterization of Mixed Metal Oxides as Solid Catalysts for Transesterification of Triglycerides

Abstract

Energy consumption is essential for the human survival and major portion of the human energy requirement is coming from the non renewable sources such as fossil fuel. In this contest biodiesel has gained significant attention in recent past as renewable alternate for conventional diesel fuel. Chemically, biodiesel is mono alkyl esters of fatty acids which are produced via transesterification of triglycerides in presence of heterogeneous, homogeneous or enzyme catalysts. Traditional homogeneous basic catalysts (NaOH and KOH) are highly effective for industrial scale biodiesel production as they could catalyze the reaction under ambient reaction conditions. However, this process yielded catalyst contaminated biodiesel and glycerol and huge quantity of effluents are generated during their purification. Additionally, homogeneous alkali catalyst due to their sensitivity towards moisture and free fatty acids (FFA) could not be used for the transesterification of waste cottonseed oil and required FFA and moisture free costlier refined oil. In order to circumvent the problem associated with homogeneous catalyst, development of reusable heterogeneous catalysts has gained significant attention in recent past. In present thesis, mixed metal oxides such as KF/CaO/NiO, Li/CaO, Li/NiO, W/Ti/SiO2 and Na/CaO/Fe3O4 were prepared and characterized by powder XRD, BET surface area, FESEM, HRTEM, TPD and Hammett indicators studies. These catalysts were successfully employed for the transesterification of waste cooking (cottonseed) oil to produce biodiesel. CaO/NiO impregnated with 20 wt% KF was found to have the basic strength of 15.0 < H_< 18.4 and required 4 h to yield the complete transesterification of waste cottonseed oil (> 98% FAMEs yield) with methanol under optimized reaction conditions. Further, reusability study of the catalyst has demonstrated that it was able to catalyze four catalytic cycles without significant loss in activity. To improve the reusability, stability and surface area, tungsten impregnated Ti/SiO2 in flower shape was prepared. The complete conversion of waste cooking oil into FAME was achieved in 3 h with 30:1 methanol to oil molar ratio. The catalyst was successfully recycled five times with lesser (< 5 ppm) metal ion leaching. In order to perform the ethanolysis as well as methanolysis of waste cooking oil and non edible oils (karanja and jatropha oil), Li/CaO and Li/NiO catalysts were prepared by wet chemical Page x method. The Li/CaO catalyst under optimal reaction conditions of ethanol/oil molar ratio of 12:1, catalyst to oil weight fraction of 5% and 65 °C reaction temperature, yielded 98 % fatty acid ethyl esters in 2.5 h of reaction duration. The catalyst was recovered and reused in four catalytic runs but with partial loss in activity during successive catalytic cycles. In order to further enhance the catalytic efficiency, Li impregnated NiO was prepared. The magnetic susceptibility measurement of Li/NiO catalyst supported the formation of Ni(III) upon lithium impregnation. The catalyst was found to be effective for the ethanolysis of vegetable oils having up to 8.4 wt% free fatty acids. During reusability experiments, Li/NiO catalyst was able to catalyze seven reaction cycles without major loss in activity. In order to achieve the easy separation of the heterogeneous catalyst from the reaction mixture, Na/CaO/Fe3O4 magnetic catalyst was prepared by wet impregnation method. The prepared catalyst has been successfully employed as heterogeneous catalyst for the transesterification of waste cotton seed oil with methanol and ethanol. The catalysts was removed from the reaction mixture by magnetic separation and reused in seven catalytic runs without significant loss in the activity. Although partial leaching of the metal ions from the catalysts was observed but leached metal ions did not show any significant homogeneous contribution, and hence, all catalysts have shown true heterogeneous mode of action. The transesterification reaction catalyzed by all catalysts has followed (pseudo) first order kinetic equation and activation energy was observed > 25 kJ/mol to support that reactions were chemically controlled and not by diffusion/mass transfer limitations. Few physicochemical properties of prepared FAMEs and FAEEs have also been studied and compared with EN 14214 and ASTM D standard values.