Active metal supported mesoporous materials as solid catalysts for the transesterification of triglycerides

Abstract

Expenditure of energy is indispensable for the survival of mankind and the chief share of demand has been laid upon non renewable resources like fossil fuels. In this context, biodiesel has emerged as an eco-friendly green fuel substitute for the typical diesel fuel. Biodiesel is basically mono alkyl esters of fatty acids synthesised via transesterification of triglycerides in the attendance of either solid/ heterogeneous, homogeneous or enzyme catalysts. Customary and typical homogeneous basic catalysts (NaOH and KOH) are exceedingly effectual for mercantile scale biodiesel production. However, this process defers the products which are contaminated with catalyst simultaneously generating enormous quantity of waste matter during washing and refinement processes. Furthermore, homogeneous alkali catalyst owing to their kindliness for moisture and free fatty acids (FFA) cannot be employed for transesterification of waste oils and necessitate FFA and moisture liberated expensive refined oil. With a view to outwit the issues associated with uniphasic catalysts, development and expansion of mesoporous solid reusable and stable catalysts has attracted momentous interest in recent years. In present thesis, mesoporous silica based catalysts, Li+ and Ce4+ impregnated mesoporous silica, 10-Na/ZnO/SBA-15, 20-Na-K/SBA-15 and 30-Na/ZrO2/SBA-15 were prepared as well as characterised by different analytical techniques such as powder XRD, BET surface area, FE-SEM, XPS, HRTEM, and Hammett indicator studies. These catalysts were triumphantly utilised for the biodiesel production from waste cottonseed oil, virgin cottonseed oil. Li+ and Ce4+ imbued SBA-15 has been prepared under normal atmospheric condition without using harsh hydrothermal method and trailing wet incipient route. The catalyst needed 4 h to give in the complete transesterification of waste cottonseed oil (> 98% FAMEs yield) with moderate requirements of alcohol and catalyst amount at 65 °C. The catalyst demonstrated good recyclability maintaining its activity for 5 successive runs. To reduce the time and energy consumption for catalyst synthesis, Na/ZnO loaded SBA-15 was prepared by one-pot method without employing hydrothermal treatment. The catalyst was exploited for transesterification from virgin cotton seed oil. The catalyst action was found to rely more on its basic strength than on surface area. Very meagre leaching of metals ˂ 5ppm observed in 4th and 5th cycle which was within ASTM specified limitations was an important feature for application of this catalyst. In order to reduce the time of reaction, K-Na imbued SBA-15 was prepared by wet-incipient method. The catalyst was exploited for BD synthesis from virgin cotton seed oil and the activity depended on its basic strength. This catalyst could even withstand high externally added FFA content yielding upto 80% FAMEs within 8 h. In order to further develop a catalyst to execute concurrent esterification and transesterification, sodium doped zirconia impregnated SBA-15 was synthesized. It was determined that the catalyst confiscated both acidic and basic sites and hence, was victoriously exploited to carry out concurrent esterification and transesterification of VCO and high FFA holding oils. Regeneration study advocated that the catalyst may well be recycled for 5 successive cycles without significant trouncing of activity. Nevertheless, the duration of reaction entailed to achieve absolute conversion raised to 5.5 h and 8.5 h when waste soyabean oil (WSO) and jatropha oil (JO) were employed as feedstock respectively. While partial leaching of metal ions from the catalysts was detected but seeped metal ions did not show significant homogeneous contribution, and hence, all the prepared catalysts have truly depicted the heterogeneous mode of action. The transesterification catalyzed by all the prepared catalysts has tracked (pseudo) first order kinetics and activation energy was observed > 25 kJ/mol to advocate that reactions were chemically controlled, not by diffusion/mass transfer limitations. The reactions were endothermic in nature.