Abstract
Design and fabrication of a catalyst with the highest activity can be achieved by optimising the synthesis conditions. In this study, the sol-gel synthesis conditions of citric acid concentration, gelling temperature, complex time, and calcination temperature were studied for the preparation of a novel semi-hexagonal calcium/titania-zirconia nano-catalyst used in the esterification reaction. After synthesis of around 24 samples at various conditions, their activity was tested in the esterification reaction and the results were analysed by multi-layer perceptron (MLP) and support vector machine (SVM) models. Both models predicted the actual data with high coefficients of determination, and indicated that the calcination temperature has the most influence on the activity of the prepared semi-hexagonal calcium/titania-zirconia nano-catalyst for the esterification reaction. Moreover, the genetic algorithm (GA) was utilised for optimising the preparation conditions based on the SVM model, due to its higher generalisation capability for prediction. The prepared nano-catalysts under the optimum conditions of 1.42 acid ratio, gelling temperature of 72 °C, complex time of 2.65 h, and calcination temperature of 487 °C showed good crystalline structure and metal–metal and metal–oxygen cation bonding. Finally, the fabricated catalyst had a high surface area (276.5 m2/g) with 3.5 nm pore diameter and almost uniform particle size (80–110 nm) distribution, leading to a high conversion of 97.6% in the esterification reaction, with good catalytic stability up to five times.
Highlights
The problems of burning petroleum fuels for production of energy, including air pollution, global warming, climate change, a variety of human diseases, etc. are forcing governments to invest in renewable and biodegradable fuels
When the complex time increased to 2 h, the catalytic activity around 10% improved the connection between the metal cations, due to sufficient duration
Higher gelling temperature led to fast evaporation of the solvent and the formation of some clusters, which reduced the catalytic activity
Summary
The problems of burning petroleum fuels for production of energy, including air pollution, global warming, climate change, a variety of human diseases, etc. are forcing governments to invest in renewable and biodegradable fuels. Al-Qaysi et al [31] synthesised a sulfated SiO2–TiO2 nano-catalyst via a sol-gel method, and investigated the preparation conditions, including Si/Ti ratio, pH of the preparation medium (nitric acid concentration), and the organic concentration (sulfate group) on the surface of the support They reported that Si/Ti ratio of 0.8, a nitric acid concentration of 4.0 M and a loading of 1.0 M sulfuric acid solution on the SiO2–TiO2 surface can provide a catalyst with the highest activity. According to the importance of each parameter to the properties of the final designed catalyst, accurate optimisation would require obtaining maximum catalytic efficiency of esterification and transesterification reactions The statistical methods, such as response surface method (RSM) [38,39], Taguchi [40], and Box-behnken [41] have rarely been studied in the context of optimising the catalyst preparation conditions. The prepared catalysts under various conditions of the sol-gel method were examined in the esterification reaction of oleic acid to form biodiesel. The stability of the catalyst was tested for several uses in the esterification reaction
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