Simultaneous esterification and transesterification of andiroba oil using niobium oxide-sulfate as catalyst
Abstract
The rising market in biofuels has been encouraging researchers to develop new routes for biodiesel production in order to increase the yields of the process. Heterogeneous acid catalysts represent an alternative to avoid the traditional alkaline pathway for producing alkyl esters from oils with a high content of free fatty acids. In this study, sulfated - niobium oxide catalyst was developed by impregnating Nb2O5 with H2SO4 (0.5 mol/L) and used to carry out simultaneous esterification and transesterification of andiroba (Carapa guianensis) oil with high free fatty acid content (acid value: 33.1 mg KOH/g). Reactions were performed with anhydrous ethanol under different conditions of temperature and molar ratio of oil to ethanol, using the SO4 2- /Nb2O5, catalyst at 5% (w/w reaction mixture). The oil conversion was determined by the 1HNMR spectroscopy method and the maximum conversion of andiroba oil into ethyl esters was 90.9%, which was achieved in a pressurized reactor.
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Introduction
A large expansion of the world’s biodiesel industries along with the increasingly demand for raw-materials has been stimulating biofuel companies to seek new alternatives in order to increase the profitability of the biodiesel production process, either through the use of non-edible vegetable oil as raw material or in the development of new and efficient catalysts.
The most commonly used technologies for ester production are based on a chemical transformation of vegetable oils with alcohol by using homogeneous catalysts (basic or acid) to promote the cleavage of triglyceride molecules and generate a mixture of esters with fatty acids [1]. The search for sources of triglycerides and more efficient and low-cost continuous production processes are key factors for a sustainable biodiesel industry [2]. Oilseed sources, including oils from unusual kinds of fruit such as andiroba (Carapa guianensis), macaw palm (Acrocomia aculeate) and jatropha (Jathropha curcas), many of them with high acidity levels, represent an excellent source of raw material for the production of biodiesel [3].
The high concentration of free fatty acids (FFA) in vegetable oils that are used as raw material in transesterification processes can negatively affect the ester production, since FFAs can react with the base homogeneous catalyst (traditionally being NaOH or KOH) to form metallic esters (soap). This secondary reaction decreases the selectivity for esters, interferes with glycerol removal, contributes to the formation of emulsion during washing, promotes low ester productivity, and increases the overall cost of the downstream processes [4 – 5]. Nonetheless, to solve this problem, several studies have proposed a treatment step prior to the alkaline transesterification based on FFAs esterification with alcohol in the presence of an acid catalyst, typically being sulfuric acid. The major disadvantage of this catalyst is its corrosive force and the difficulty in being separated from the product at the end of the reaction. In addition, a considerable amount of alkaline is required to neutralize the acidic catalyst [1 – 2].
Conclusion
The development of a heterogeneous catalyst-based acid niobium oxide-sulfate SO4 2- /Nb2O5 allowed the conversion of highacidity oil (andiroba) into ethyl esters, achieving a maximum yield of 90.9% and viscosity level below 6.0 mm2 /s. Such results were obtained at reactions conditions of 120:1 molar ratio of ethanol to andiroba oil at 260ºC. The niobium oxidesulfate hereby developed is a promising catalyst for the simultaneous esterification and transesterification into ethyl esters without a previous hydrolysis of andiroba oil, well as other oils with high free fatty acid content.