Biodiesel production via transesterification of palm oil by using CaO–CeO2 mixed oxide catalysts
Introduction
In recent studies, researches have shown great interests in the field of biodiesel. Biodiesel is believed to serve as an alternative for petroleum, to overcome the scarcity of petroleum-based resources and the price fluctuation related to these resources. Biodiesel is a mixture of methyl esters produced via transesterification of animal fats or vegetable oils [1]. Since it is derived from biomass, it is known as renewable energy source.
Calcium oxide, CaO, is the most commonly utilized heterogeneous catalyst in transesterification reactions [2], [3], [4]. Although CaO is a promising catalyst in transesterification reactions, the active phase CaO component tends to leach into the methanolic solution [5], [6]. The soluble Ca2+ was generated from the dissociation of CaO during the reaction with methanol [7]. The leached CaO active phase will react with the free fatty acid and result in soap formation and deactivation of the catalyst. Hence, the stability of bulk CaO can be further improved by supporting CaO onto carriers such as transition metals oxides.
Cerium (IV) oxide, CeO2, is a basic catalyst [8]. Although it is not very reactive in transesterification reactions, it is a good candidate to improve the properties of bulk CaO. However, the studies on transesterification process using CaO–CeO2 mixed oxide catalyst are scarce. Yu et al. [8] reported the use of CaO–CeO2 mixed oxide catalyst with Ce/Ca molar ratio of 0.15 for transesterification of Pistacia chinensis oil with methanol. The optimum yield of 91% was achieved under reaction conditions of methanol to oil molar ratio of 30:1, catalyst amount of 9%, reaction time of 6 h and reaction temperature of 110 °C. The catalyst can be reused 5 times with biodiesel yield of more than 70%. Kawashima et al. [9] carried out transesterification reaction on rapeseed oil with the aid of CaO–CeO2 catalyst. The optimum biodiesel yield obtained was 89% at 60 °C with a 6:1 molar ratio of methanol to oil and a reaction time of 10 h. It was relatively durable with reusability of 7 times with biodiesel yield more than 80%.
In this study, CaO–CeO2mixed oxide catalysts were prepared in order to improve the physico-chemical properties, catalytic activity and reusability of the bulk CaO in transesterification of palm oil. CaO–CeO2 mixed oxide catalysts with various CaO compositions were prepared via wet impregnation method. Effects of various CaO loadings, the correlation between basicity and BET surface area with biodiesel yield were studied. Deactivation of catalyst in transesterification reaction was also investigated.
Section snippets
Preparation and characterization of catalysts
The catalysts with composition 10–70 wt.% CaO, were prepared via wet impregnation method and labeled as xCa–Ce, where x = 10, 20, 30, 40, 50, 60 and 70 wt.% CaO. An appropriate amount of calcium nitrate tetrahydrate, Ca(NO3)2 ⋅ 4H2O (Sigma–Aldrich, 99%) was dissolved in 100 ml distilled water. Next, a complementary amount of cerium (IV) oxide, CeO2 (Sigma–Aldrich, 99.9%) was added slowly into the solution and continuously stirred to obtain homogenous solution. The solution was gently heated to
Characterization of catalyst
The phase identity of the prepared catalysts was studied by using XRD. The XRD patterns for various compositions CaO–CeO2 mixed oxide catalysts are depicted in Fig. 1. All catalysts showed similar patterns where two distinct metal oxide crystalline phases, calcium oxide (CaO) and cerium (IV) oxide (CeO2) were identified, without formation of new binary phases (CaCeO solid solutions). CaO peaks were observed at 2θ = 32.3°, 37.3°, and 53.8° (JCPDS file No. 37-1497) whereas CeO2 peaks at 2θ = 28.5°,
Conclusion
The CaO–CeO2 mixed oxide catalysts prepared via wet impregnation method was successfully applied in transesterification of palm oil. Among these catalysts, 50Ca–Ce showed the highest catalytic activity with biodiesel yield, 95%, under the reaction conditions of 5 wt.% of catalyst, methanol to oil molar ratio of 12:1, reaction temperature of 65 °C and reaction time of 4 h. The high catalytic activity of the 50Ca–Ce catalyst was governed by both basicity and high BET surface area, in which more base
Acknowledgement
The authors acknowledge the financial supports of the National Science Fund (NSF) from the Ministry of Science, Technology and Innovation (MOSTI), and University Putra Malaysia (UPM), as well as the palm oil provided by IOI Corporation Berhad.
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