Biofuels are renewable solutions to replace the ever dwindling energy reserves and environmentally pollutant fossil liquid fuels when they are produced from low cost sustainable feedstocks. Biodiesel is mainly produced from vegetable oils or animal fats by the method of transesterification reaction using catalysts. Homogeneous catalysts are conventionally used for biodiesel production. Unfortunately, homogeneous catalysts are associated with problems which might increase the cost of production due to separation steps and emission of waste water. Inorganic heterogeneous catalysts are potentially low cost and can solve many of the problems encountered in homogeneous catalysts. Many solid acid and base inorganic catalysts have been studied for the transesterification of various vegetables oils. The work of many researchers on the development of active, tolerant to water and free fatty acids (FFA), as well as stable inorganic catalysts for biodiesel production from vegetable oils are reviewed and discussed.

Inorganic heterogeneous catalysts for biodiesel production from vegetable oils.

Catalysis in biodiesel production—a review

Biodiesel production is worthy of continued study and optimization of production procedures due to its environmentally beneficial attributes and its renewable nature. Heterogeneous transesterification is considered to be a green process. The process requires neither catalyst recovery nor aqueous treatment steps and very high yields of methyl esters can be obtained, close to the theoretical value. However, heterogeneously catalyzed transesterification generally requires more severe operating conditions, and the performance of heterogeneous catalysts is generally lower than that of the commonly used homogeneous catalysts. Heterogeneous catalysis for biodiesel production has been extensively investigated in the last few years. Many metal oxides have been studied for the transesterification process of oils; these include alkali earth metal oxides, transition metal oxides, mixed metal oxides and supported metal oxides. The use of solid metal oxides as catalysts in oil transesterification is well established, accordingly, researchers’ attempts are now focused on how to attain the highest catalyst activity. Catalyst activity is a function of its specific surface area, base strength and base site concentration. High specific surface area, strong base strength and high concentration of base sites are characteristics of an active transesterification catalyst. This review provides a brief overview of the different metal oxides frequently used in the process of transesterification of oils for the production of biodiesel with special reference to the various methods of catalyst preparation and catalyst characterization. Reaction conditions and catalyst leaching analysis are also highlighted. Finally, concluding remarks regarding catalyst selection and catalyst preparation steps are provided.

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Biodiesel production using solid metal oxide catalysts

The heterogeneous base-catalyzed transesterification for biodiesel synthesis has been studied intensively over the last decade. This review classifies the solid base catalysts for transesterification into the following six categories based on Hattori’s classification: single metal oxides, mixed metal oxides, zeolites, supported alkali/alkaline earth metals, clay minerals (hydrotalcites), and non-oxides (organic solid bases). The catalysts in each category have acceptable catalytic activities overall, and follow specific catalyst design rules, although not completely systematically, thereby drawing the best activity from them. In parallel, each catalyst is not versatile and has some limitations specifically related to its catalytic structure and properties. This review focuses on the heterogeneous base-catalyzed transesterification in terms of catalyst development, based on the published research, especially over the last decade.

Heterogeneous Base Catalysts for Transesterification in Biodiesel Synthesis

Biodiesel synthesis by TiO2-ZnO mixed oxide nanocatalyst catalyzed palm oil transesterification process.

Biodiesel produced by the transesterification of vegetable oils or animal fats with short-chain alcohols (typically methanol) is a promising alternative fuel for diesel engines, because of the limited resources of fossil fuels and environmental concerns. In this work, Li/ZnO catalysts were prepared using an impregnation method followed by calcinations, and then they were tested for soybean oil transesterification. It was determined that Li/ZnO catalysts exhibited good catalytic activities, and the catalytic performance was greatly dependent on (i) the loading amount of lithium and (ii) the calcination temperature. This Li/ZnO catalyst, at an amount of 5 wt %, resulted in a soybean oil conversion of 96.3% in 3 h using a reflux of methanol and a 12:1 molar ratio of methanol to oil. Moreover, the catalyst was characterized using X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), infrared (IR) spectroscopy, thermogravimetry−differental thermal analysis (TG-DTA), and the Hammett titration meth...

Hong Chun

Papers

Catalytic Properties of Lithium-Doped ZnO Catalysts Used for Biodiesel Preparations