Transesterification of waste cooking oil to biodiesel using Ca and Zr mixed oxides as heterogeneous base catalysts Asghar Molaei Dehkordi ⁎, Mohammad Ghasemi Department of Chemical and Petroleum Engineering, Sharif University of Technology, P.O. Box 11155-9465, Tehran, Iran abstract article info Article history: Received 27 September 2011 Received in revised form 4 January 2012 Accepted 11 January 2012 Available online 4 February 2012 Keywords: Biodiesel Waste cooking oil Transesterification Mixed oxide catalyst CaO-ZrO 2 Heterogeneous solid catalysts comprising CaO and ZrO 2 mixed oxides with various Ca-to-Zr molar ratios were synthesized by means of coprecipitation method. These synthesized mixed oxide catalysts were used for the transesterification of waste cooking oil (WCO) as feedstock with methanol to produce biodiesel fuel (BDF) at 65 °C and 1 atm. The influences of Ca-to-Zr molar ratio, catalyst loading, methanol-to-oil molar ratio, and the reaction time on the BDF yield were carefully investigated. In addition, the stability of prepared solid catalysts was studied. These catalysts were characterized by using techniques of X-ray diffraction, X-ray florescence, X-ray photoelectron spectroscopy, surface area measurement (BET method), and temperature- programmed desorption. Moreover, the obtained experimental results clearly indicate that the activity of synthesized catalysts increases with increasing the Ca-to-Zr molar ratio but the stability of the catalysts de- creases as well. Under the appropriate transesterification conditions at 65 °C (i.e., catalyst loading = 10 wt.%, methanol-to-oil molar ratio = 30:1, and reaction time = 2 h), a BDF yield of 92.1% could be achieved over CaO-ZrO 2 catalyst with a Ca-to-Zr molar ratio of 0.5. Furthermore, the obtained experimental results clearly indicate that synthesized catalysts can be used as recyclable, stable, and active catalysts for the production of BDF from WCO. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The world has been confronted with environmental and energetic crises due to the depletion of natural resources and increased popula- tion as well as environmental pollution. Therefore, it is important to search for an alternate low cost fuel, which should be sustainable and also friendly to the environment. Biodiesel fuel (BDF) is a prom- ising nontoxic and biodegradable substitute of fossil diesel, which could be used directly or as diesel mixture in engines with little change [1,2]. Transesterification is the most common method for BDF preparation. Transesterification of vegetable oils to BDF with methanol can be carried out using both homogeneous and heteroge- neous catalysts. Most biodiesel today is produced in the presence of homogeneous catalysts such as sodium methoxide, sodium or potas- sium hydroxide [3]. However, homogeneous base catalysts are more effective and provide much faster reaction rates than those of hetero- geneous catalysts. But using homogeneous base catalysts lead to seri- ous contamination problems that require the implementation of nearly perfect separation and product purification processes, result- ing in increased production costs [4]. Heterogeneous catalysts have a number of advantages such as noncorrosive, environmentally be- nign, etc. as well as present fewer disposal problems. In addition, they are also much easier to separate from liquid products and can be designed to give higher activity, selectivity, and longer catalyst lifetimes [5]. Numerous investigations have been conducted on the BDF pro- duction using heterogeneous base catalysts [6–10]. Since the catalytic activity of basic catalysts is higher than that of acid solids, they have been preferably studied. Various heterogeneous solid base catalysts have been used such as metal oxides CaO [11,12], SrO [13], MgO [14], etc.; mixed oxides Ca/Zn [15], Ca/Mg [16], etc.; alkali-doped metal oxides CaO/Al 2 O 3 [17], MgO/Al 2 O 3 [18], Li/CaO [19], etc.; Al 2 O 3 -supported alkali metal oxide catalysts Na/NaOH/γ-Al 2 O 3 [20], K 2 CO 3 /Al 2 O 3 [21], and KF/γ-Al 2 O 3 [22]; hydrotalcites [23–25], and magnetic composites such as Ca/Al/Fe 3 O 4 [26]. Although these het- erogeneous catalysts have advantages such as catalyst separation and pollution reduction, most of them have also some limitations in catalytic activity and stability. CaO as an active catalyst has been widely used in the transesteri- fication studies [11,12]. In comparison with pure CaO catalyst, several researchers have demonstrated that the combination of CaO with other metal oxides such as MgO and ZnO can provide higher BDF yield [15,27]. Ngamcharussrivichai et al. [15] synthesized a high ac- tive Ca/Zn mixed oxide catalyst to reach BDF yield of 94% from palm kernel oil at 60 °C and a reaction time of 1 h. They reported that the mixed oxides compared to pure CaO and ZnO have relatively small particle sizes and high surface areas, [15]. Thus, the BDF formation could be enhanced via a bifunctional catalysis route. ZrO 2 as a support Fuel Processing Technology 97 (2012) 45–51 ⁎ Corresponding author. Tel.: + 98 21 66165401; fax: + 98 21 66022853. E-mail address: amolaeid@sharif.edu (A. Molaei Dehkordi). 0378-3820/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2012.01.010 Contents lists available at SciVerse ScienceDirect Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc