Synthesis of biodiesel from cottonseed oil and methanol using a carbon-based solid acid catalyst
Abstract
A carbon-based solid acid catalyst was prepared by the sulfonation of carbonized vegetable oil asphalt and used to catalyze the transesterification of methanol with cottonseed oil. This catalyst was characterized by scanning electron microscopy/energy dispersive spectroscopy, BET surface area and pore size measurement, thermogravimetry analysis and Fourier transform infrared spectroscopy. The sulfonated multi-walled carbon nanotubes (s-MWCNTs) was also prepared and used to catalyze the same transesterification as the asphalt catalyst. The asphalt-based catalyst shows higher activity than the s-MWCNTs for the production of biodiesel, which may be correlated to its high acid site density, its loose irregular network and large pores can provide more acid sites for the reactants. The conversion of cottonseed oil 89.93% was obtained (using the asphalt-based catalyst) when the methanol/cottonseed oil molar ratio was 18.2, reaction temperature at 260 °C, reaction time 3.0 h and catalyst/cottonseed oil mass ratio of 0.2%. Also, it can be re-used. The sulfonated polycyclic aromatic hydrocarbons provide an electron-withdrawing function to keep the acid site stable. The catalyst can substantially reduce energy consumption and waste generation in the production of biodiesel.
Introduction
Due to the potential exhausting and increasing price of petroleum together with environment concerns caused by the combustion of fossil fuels, the search for alternative fuels has gained much attention [1], [2], [3]. Biodiesel (fatty acids methyl or ethyl esters, FAME or FAEE) can be derived from either the transesterification of triglycerides (the main component of vegetable oils or animal fats) or the esterification of free fatty acid (FFA) with a short chain alcohol (mainly methanol). It has became popular as a possible alternative to fossil fuels. The main advantages of this fuel are that its properties and performance are similar to conventional diesel fuels [4], [5].
Waste cooking oils, soapstock (byproduct of vegetable oil refinery) and non-edible oils, which are available cheaply, are attractive starting materials for biodiesel [6], [7]. Currently, most commercial processes used for biodiesel synthesis employ a homogeneous catalyst, such as NaOH or H2SO4 [8], [9], [10]. Georgogianni et al. [9] reported the production of biodiesel from the soybean frying oil using 2% NaOH as catalyst. It can give a high yield of methyl esters (95%) after a short reaction time (20 min) due to the homogeneous base catalyst can significantly accelerate the transesterification reaction. However, the commonly practiced technology based on base catalysts requires the use of refined vegetable oils that contain no more than 0.5% of FFAs and anhydrous conditions. When waste oils are used as the raw material, if the FFA content is more than or equal to 1%, an acidic catalyst has to be used first to reduce the FFA content. The pretreatment and product purification processes add neutralization and separation steps to the overall process, and it will lead to a series of environmental problems related to the use of high amounts of energy and solvent [11]. The homogeneous acid H2SO4 shows a better performance with FFAs than base catalysts, and can catalyze esterification and transesterification simultaneously [9]. However, it suffers from several drawbacks, such as equipment corrosion and the need to deal with the waste from the neutralization of H2SO4. The consumption of sulfuric acid, separation of sulfates (from neutralization processing) and purification of the product also involve substantial energy and material use.
The use of heterogeneous catalysts to replace homogeneous ones can be expected to eliminate the problems associated with homogeneous catalysts [12], [13], [14], [15], [16], [17], [18]. The solid base catalysts are easily regenerated and have a less corrosive nature, leading to safer, cheaper and more environment-friendly operations [12], [13], [14], [15]. However, they only can be directly used as catalysts to prepare biodiesel when the FFA content (in a raw material) is not more than or equal to 1%. Hence, they are not suitable for the waste cooking oils. Heterogeneous acid catalysts can simultaneously catalyze the transesterification of triglycerides and the esterification of FFA. Hence, the use of heterogeneous acid catalysts to produce biodiesel is becoming more popular [16], [17], [18]. However, they also have some common problems: low acid site concentrations, microporosity, and hydrophilic character of catalyst surfaces, active site leaching and high cost.
Recently, a new class of sulfonated carbon-based solid acid catalysts was reported as promising catalysts for the production of biodiesel [19], [20]. The catalyst can be produced either from the carbonization of sulfopolycyclic aromatic hydrocarbons (such as the sulfonate derivative produced by the reaction of naphthalene with concentrated H2SO4) or the sulfonation of a carbonized inorganic/organic compound [21], [22], [23]. The carbonization of sulfonated naphthalene (at 200–250 °C) has been reported to give an active and stable solid catalyst that can catalyze the esterification of ethanol and acetic acid to form ethyl acetate [24]. However, this catalyst is only a soft aggregate of polycyclic aromatic hydrocarbons rather than a rigid carbon material. The sulfonic acid group (−SO3H) are easily leached from the solid when it is used in liquid-phase reactions over 100 °C or by fatty acids used as surfactants, resulting in the rapid loss of catalytic activity [25]. However, the protonation of the carbonyl group of a triglyceride to start the transesterification for the production of biodiesel is difficult, and a high temperature is generally required [26]. The carbon material that is a soft aggregate of polycyclic aromatic hydrocarbons will not be suitable under this reaction condition.
Carbon-based solid acid catalysts prepared from the sulfonation of carbonized inorganic/organic compounds, such as sugar, can form a rigid carbon material composed of small polycyclic aromatic carbon sheets (a three dimensional sp3-bonded structure) by carbonization [25]. Sulfonation of such carbon materials can be expected to give a highly stable solid with a high density of active sites.
The use of soapstock for biodiesel production generates many solid residues (vegetable oil asphalt). This presents an environmental problem in terms of adequate disposal. In order to make the preparation of biodiesel from soapstock more environmentally friendly, alternative uses for such waste biomass are proposed. Since the main components of vegetable oil asphalt are large organic hydrocarbons, we carbonized these hydrocarbons and then treated these with concentrated H2SO4 to prepare a solid acid catalyst for the synthesis of biodiesel. The strong solid acid obtained probably consisted of a flexible carbon-based framework decorated with highly dispersed polycyclic aromatic hydrocarbons containing sulfonic acid groups. It can reduce the waste generated from biodiesel production and can help make biodiesel competitive in price with petroleum diesel.
The potential of vegetable oil asphalt as the raw material in the preparation of a carbon-based solid acid catalyst for the transesterification of cottonseed oil with methanol was investigated. The reaction conditions were optimized by separately changing the experimental conditions of reaction temperature, reaction time, catalyst/cottonseed oil mass ratio, and methanol/cottonseed oil molar ratio.
Section snippets
Catalyst preparation
The carbon carrier was obtained by carbonizing vegetable oil asphalt. Firstly, the vegetable oil asphalt from a biodiesel plant (Linyi Qingda New energy Co., Ltd, China) was pre-treated to remove water and residual esters. Secondly, batches of 10.0 g of extracted vegetable oil asphalt were oxidized for 1.0 h at 280 °C in a stream of air (300 mL min− 1). Thirdly, these were heated to 500–700 °C at a rate of 2 °C min− 1 under an argon atmosphere (300 mL min− 1).
The sulfonation of the carbon carrier
Characterization of the catalyst
The morphology of the carbon catalyst is shown in Fig. 2. The carbonized vegetable oil asphalt exhibited a loose irregular network structure (Fig. 2A). Part of the particles had agglomerated, and several pores with sizes of micrometers can be found. Also, mesopores and micropores existed in the carbon, but these are not clearly shown in the SEM images due to the limited resolution. Fig. 2B shows that after the sulfonation treatment by concentrated H2SO4, the particle agglomerates had
Conclusions
A novel solid acid catalyst with good activity for transesterification has been prepared by sulfonating a composite material formed by the incomplete carbonization of vegetable oil asphalt. The resulting strong solid acid consists of a flexible carbon-based framework decorated with highly dispersed polycyclic aromatic hydrocarbons containing sulfonic acid groups. The high activity can be ascribed to the high acid site density, loose irregular network and large pores that can provide more acid
Acknowledgements
The work was supported by the Foundation for the Natural Scientific Foundation of China (No.20736004, No. 20736007, No. 20606021), and by Specialized Research Fund for the Doctoral Program of Higher Education (No. 20050003030).
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