Elsevier

Molecular Catalysis

Volume 484, March 2020, 110804
Molecular Catalysis

Optimization by response surface methodology of the reaction conditions in 1,3-selective transesterification of sunflower oil, by using CaO as heterogeneous catalyst

https://doi.org/10.1016/j.mcat.2020.110804Get rights and content

Highlights

  • Transesterification of sunflower oil was carried out over a cheap and commercial calcium oxide.

  • The best reaction conditions have been obtained by Response Surface Methodology.

  • High values of temperature and amount of catalyst are required to obtain good catalytic results.

  • A biofuel with lower viscosity was obtained with a low methanol/oil molar ratio (2).

Abstract

In this work, the experimental conditions for the transesterification reaction of sunflower oil, over commercial calcium oxide (CaO) as heterogeneous catalysts, have been optimized employing a multi-factorial design based on the response surface methodology (RSM). Hence, the effects of temperature, methanol/oil molar ratio and have been considered. The partial transesterification of sunflower oil produces one mol of monoglyceride (MG) and two moles of fatty acids methyl esters (FAME). This blend constitutes a new type of biofuel called Ecodiesel which can be applicable to diesel engines. This Ecodiesel integrates the glycerol as MG removing by-products and increasing the lubricity of the biofuel. Thus, the reaction conditions, obtained by Response surface methodology (RSM), that promote the highest values of selectivity to Ecodiesel are 65 °C of reaction temperature, 0.7 g of CaO and methanol/oil molar ratio of 5.3. On the other hand, in order to get a biofuel with lower viscosity, 65 °C and 0.7 g of CaO were also employed, although a lower methanol/oil molar ratio of 2 was employed. Operating under these selected experimental conditions, a high conversion value (70 %), a selectivity value to Ecodiesel of 35 % and a viscosity value around 15 cSt can be achieved. The biofuel thus obtained can be used in blends with diesel because in order to accomplish the viscosity limits stablished in the EN 14214. Consequently, this methodology could represent an advance in the technical and economic feasibility in the process of replacing fossil fuels by renewable fuels.

Introduction

The high use of fossil fuels worldwide is causing several environmental problems, in addition to the depletion of fossil fuels supply sources, which affects the international energy security [1,2]. Thus, a smooth transition from the current scenario, in which engines work mainly with fossil fuels, to other one in which mainly renewable biofuels be used, would allow to respond the increasing demand of fuels, solving the current environmental issues as well [3,4]. That is, the replacement of fossil fuels by others of a renewable nature it is nowadays mandatory [5].

On the other hand, the current diesel engines are preferred to the spark ignition ones for heavy duty applications and power generation plants [6] because of their higher efficiency, durability and productivity [7]. In fact, they are postulated as the best option, since electric motors cannot compete with combustion ignition (C.I.) diesel engines yet, especially in the field of heavy trucks [8], aviation [9,10], or shipping sector [11,12]. Thus, in order to address the environmental and energy security challenges associated with these energy systems, an important research effort to obtain biodiesel is being carried out. Biodiesel is the most suitable biofuel for operating in diesel engines [13,14]. However, the great number of researches carried out in the last decades, to find profitable and environmentally friendly biodiesel production technologies, do not resolve yet the large-scale manufacture problems that make the biodiesel production competitive in comparison with fossil fuels [15].

Be that as it may, biodiesel is still considered as the only existing biofuel capable of replacing the fossil fuels depletion, due to its excellent properties, since it is biodegradable, sustainable, and clean, in addition to its non-toxic combustion [16]. However, its application is only possible by its implementation through direct subsidies and tax exemptions [17]. This is because, despite the efforts made in recent decades, the producing costs of biodiesel in its market and industrialization phases remains a challenge, as aforementioned [18,19]. The main handicap in the production of biodiesel at plant scale is the generation of glycerol as byproduct (approximately in a 10 % by weight of the total biodiesel produced). This glycerol usually contains various impurities, such as water, methanol, soap, fatty acids, and fatty acid methyl esters. Considerable efforts have been devoted to find applications for converting crude glycerol into high-value products, such as biofuels, chemicals, polymers, or animal feed, in order to improve the economic viability of the biodiesel industry and also to overcome environmental challenges associated with the elimination of such amount of crude glycerol [[20], [21], [22]].

One option to avoid the formation of glycerol is the design of strategies in which glycerol was a soluble derivative in the mixture of methyl or ethyl esters of fatty acids (FAME or FAEE), that constitute the conventional biodiesel. These derivatives are obtained at the same time as the transesterification process, constituting a new type of biofuel. To do so, methanol or ethanol derivatives are employed as transfer agents of the methyl groups, such as methyl or ethyl acetate as well as dimethyl carbonate [[23], [24], [25], [26]]. This methodology not only avoids the glycerol separation process, preventing the cleaning of biodiesel, but also increases the yield of the process. The atom efficiency is also improved as the total number of atoms involved in the reaction is part of the final mixture.

Another strategy that avoid the production of glycerol, is the 1,3 regioselective transesterification of triglycerides. Thus, each triglyceride molecule produces only two moles of fatty acid esters, together with one mole of 2-monoacylglycerol (MG) [25]. In this way, glycerol is integrated in the monoglycerides, which are a special type of alkyl esters of fatty acids. These molecules can fit within the current definition of biodiesel, since not only methanol can be employed for the synthesis of biodiesel, but also other type of alcohols. In fact, methanol is the alcohol employed for producing biodiesel because it is the cheapest one, although other alcohols such as ethanol, iso-propanol, propanol, butanol or 2-butanol, can produce a biodiesel fuel with better fuel properties, because these branched-chain esters reduced the biodiesel crystallization temperature [27,28]. However, although other alcohols are able to produce biodiesel, the current legal regulations are designed in a way that only methyl esters can be used as biodiesel [29].

Initially, this selective process was obtained by the 1,3-regiospecific enzymatic ethanolysis of sunflower oil over some low cost 1,3-regiospecific lipases [25]. The product thus obtained was patented as Ecodiesel [30]. Ecodiesel is constituted by a mixture of two parts of FAEE and one part of MG, integrating the glycerol molecule as a soluble derivative product [31]. This biofuel not only displays a 100 % atomic efficiency, but also shows a high lubricating power, due to the presence of a great amount of monoglycerides, that enhances the biodiesel lubricity, according to recent studies [32,33].

The main handicap existing in the production of this type of biofuel is the high cost of lipases for being employed at industrial scale. Thus, to make this process viable and competitive from an economic point of view, the possibility of performing this 1,3-selective transesterification process has been investigated through kinetic control of the methanolysis process of triglycerides using commercial CaO as catalyst, attaining similar results to those previously described over lipases [34]. The high selectivity for the methanolysis in positions 1 and 3 of the glycerol molecule can be explained by the different character of the primary alcohol, while carbon 2 is of secondary character and, consequently, the rate of methanolysis is lower on this carbon (Fig. 1).

Taking into account all these premises, this research aims to get Ecodiesel over commercial CaO as alkaline heterogeneous catalyst in order to reduce the cost that lipases have associated. The best reaction conditions to have better insights into the production of Ecodiesel have been studied by Analysis of Variance (ANOVA) and by Response Surface Methodology (RSM). The application of a statistical method to optimize the experimental conditions of reaction becomes essential, since the objective is getting the experimental conditions that favor the differences in reaction rates of methanolysis between the primary alcohols (positions 1 and 3) and the secondary alcohol (glycerol position 2). Furthermore, ANOVA method is being very used for the optimization of reaction parameters, including the 1,3- transesterification reaction here studied [[35], [36], [37], [38]]. Concretely, in this study, the influence of temperature, the methanol/oil molar ratio and the catalyst amount have been considered.

Section snippets

Materials

Sunflower oil used as a reagent in the processes of selective methanolysis, is a food product, purchased in a local store. Its fatty acid profile, which was obtained from the chromatogram, is 63.5 % linoleic acid, 24 % oleic acid, 6.5 % palmitic acid, 5 % stearic acid and 2 % palmitoleic, linolenic, behenic and ketoleic acid in different percentages. Its kinematic viscosity at 40 °C is 32.0 mm2/s (or cSt). The water content was determined by Karl Fisher method and results in a negligible value,

Characterization of catalyst

Fig. 2 shows the XRD pattern of the CaO after calcination at 600 °C. As can be seen, the Ca is mainly in CaO (lime) form, with the expected cubic internal crystal structure, characterized by three strong diffraction peaks at 2θ 32°, 37° and 53° associated to the (100), (200) and (220) reflections, in agreement with the already reported literature [41]. However, calcite (CaCO3) and a small amount of Si in quartz phase were also observed in the sample.

Regarding the elemental analysis of the

Conclusions

In this work, the experimental conditions of the transesterification reaction of sunflower oil with methanol over a commercial CaO as heterogeneous catalyst have been optimized using both Analysis of Variance (ANOVA) and Response Surface Methodology (RSM). This reaction produces one mole of monoglyceride (MG) and two moles of fatty acids methyl esters (FAME), which constitutes the Ecodiesel, a new type of biofuel applicable to diesel engines, which in addition, integrates glycerol as

CRediT authorship contribution statement

Juan Calero: Methodology, Validation, Formal analysis, Investigation, Data curation, Writing - original draft, Visualization. Diego Luna: Conceptualization, Methodology, Validation, Writing - original draft, Visualization, Supervision, Project administration. Carlos Luna: Formal analysis, Investigation, Data curation. Felipa M. Bautista: Methodology, Validation, Supervision. Antonio A. Romero: Supervision. Alejandro Posadillo: Writing - review & editing. Rafael Estevez: Writing - original

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors are thankful to MINECO for financial support through project re. ENE2016-81013-R (AEI/FEDER, UE) and to Andalusian Government (UCO-FEDER Project CATOLIVAL, re. 1264113-R).

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