Abstract
The present contribution screens the specific activity of various inorganic and enzymatic based materials in the esterification of oleic acid that is typically used as a test reaction for the production of biodiesel from high free fatty acid feedstocks. The inorganic materials investigated in this contribution are bulk fosfotungstic heteropoly acid of the Wells Dawson structure H6P2W18O62.nH2O (HPA), as well as dispersed on titanium dioxide 18 % w/w H6P2W18O62/TiO2 and the insoluble cesium salt of the Wells Dawson heteropoly anion Cs2H4P2W18O62. Additionally, the commercial biocatalyst Novozym® 435 (immobilized lipase B of Candida antarctica) and a self-supported lipase of vegetable origin obtained from the latex Araujia sericifera (ASL) were studied among the materials of enzymatic nature. The density and accessibility of Bronsted acid sites have a key role in the specific activity of the fosfotungstic based heteropoly compounds. The HPA dispersed over an oxide support catalyzed the esterification of the fatty acid in a heterogeneous fashion with the highest activity (6.4 µmol mg−1 h−1 at 85 °C) among the inorganic materials. In contrast, the enzymatic materials are more active at lower temperature than the inorganic ones. Particularly, ASL catalyzed the homogenous methanolysis with the highest specific activity (30.7 µmol mg−1 h−1 at 40 °C) at the lowest temperature among the materials assayed.
Highlights
Biodiesel is defined as the mono alkyl ester of long chain fatty acids derived from a renewable lipid feedstock, such as vegetable oil or animal fat
The esterification of oleic acid with methanol was chosen as a reaction test in order to establish the catalytic activity of various inorganic and enzyme based materials such as: bulk H6P2W18O62.xH2O (HPA) and oxide supported (18 % HPA/TiO2) Wells Dawson heteropolyacid; the partially substituted HPA with cesium Cs2H4P2W18O62, the mechanical mixture of CsNO3 and HPA mimicking the coprecipitated cesium salt; the commercial biocatalyst Novozym® 435, and the self supported lipase of A. sericifera (ASL)
The similarity between the catalytic performance of the Keggin and Wells Dawson HPAs and the observation that the partially substituted cesium salt proved no activity are evidences of the key role of the Brønsted acid sites in the esterification of the fatty acid
Summary
Biodiesel is defined as the mono alkyl ester of long chain fatty acids derived from a renewable lipid feedstock, such as vegetable oil or animal fat. This fuel is one of the promising substitutes for the petroleum derivates fuels due to its environmental benefits and because of the decreasing amount of petroleum reserves. Acidic and enzymatic catalysts are currently investigated in order to develop new technologies in heterogeneous phase for the simplification of the production process of biodiesel [4] In this context, it is well known that heteropoly compounds (HPCs) with Keggin structure are suitable catalytic materials in esterification [5]. Matkovic et al showed that Wells Dawson type has a higher number of acid sites and acid strength than the Keggin heteropoly acid, which makes it a promising material for the production of biodiesel [6]
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