Abstract
Bifunctional heterogeneous catalysts have a great potential to overcome the shortcomings of homogeneous and enzymatic catalysts and simplify the biodiesel production processes using low-grade, high-free-fatty-acid feedstock. In this study, we developed ZrO2-based bifunctional heterogeneous catalysts for simultaneous esterification and transesterification of microalgae to biodiesel. To avoid the disadvantage of the low surface area of ZrO2, the catalysts were prepared via a surfactant-assisted sol-gel method, followed by hydrothermal treatments. The response surface methodology central composite design was employed to investigate various factors, like the surfactant/Zr molar ratio, pH, aging time, and temperature on the ZrO2 surface area. The data were statistically analyzed to predict the optimal combination of factors, and further experiments were conducted for verification. Bi2O3 was supported on ZrO2 via the incipient wetness impregnation method. The catalysts were characterized by a variety of techniques, which disclosed that the surfactant-assisted ZrO2 nanoparticles possess higher surface area, better acid–base properties, and well-formed pore structures than bare ZrO2. The highest yield of fatty acid methyl esters (73.21%) was achieved using Bi2O3/ZrO2(CTAB), and the catalytic activity of the developed catalysts was linearly correlated with the total densities of the acidic and basic sites. The mechanism of the simultaneous reactions was also discussed.
Highlights
Biodiesel is an attractive alternative source of energy owing to its renewability, biodegradability, sustainability, and non-toxicity[1]
The optimization of the process parameters was conducted by employing response surface methodology central composite design (RSMCCD), which maintained the experimental conditions within the desired range of independent parameters
The specific values of the independent parameters used in this study, along with the surface area obtained for ZrO2(P123) and ZrO2(CTAB) are cited in Supplementary Tables S1 and S2, respectively
Summary
Biodiesel is an attractive alternative source of energy owing to its renewability, biodegradability, sustainability, and non-toxicity[1]. The most common key features of efficient and active heterogeneous transesterification catalysts are high surface area[9,10], adequate acidic[11,12,13] and basic[14,15,16,17] densities, good crystallinity[14], and -well-formed pore structure[10,12,13] Recent developments in this field have led to renewed interest in bifunctional acid–base heterogeneous catalysts for simultaneous esterification and transesterification of low-grade high-FFA model feedstock[7,18,19], such as microalgae lipid. The effect of several process parameters on the surface area of ZrO2 prepared by a surfactant-assisted sol-gel method followed by a hydrothermal treatment using non-ionic and cationic surfactants under basic conditions was investigated. The synthesized catalysts were found to be active towards the conversion of microalgae lipid to biodiesel
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