Preparation, characterization and mechanistic study on transesterification of refined used cooking oil for biodiesel production using zinc and calcium oxides-based catalysts

Abstract

Environmental concerns in fossil fuel depletion intensified the search for alternative fuel from renewable resources. Biodiesel is commonly produced by transesterification of vegetable oil in the presence of homogeneous catalyst. These catalysts, however, dissolve into the vegetable oil and large amount of water is required to clean the biodiesel that can cause saponification. Previously, extensive studies have been conducted on alkaline earth metal oxides such as calcium and magnesium oxides with manganese, iron, zirconium and cerium as the dopants. This research thus focused on the use of heterogeneous base catalysts that are easily separated and environmentally friendly for the biodiesel production. Zinc and calcium oxides-based supported on alumina were used as catalysts for the transesterification reaction of refined used cooking oil due to their highly basic characteristic. In order to improve the catalytic activity, the bimetallic and trimetallic oxides catalysts with copper, nickel, chromium and titanium as their co-catalysts were investigated. All the alumina supported catalysts were prepared by wetness impregnation method. The screening of biodiesel production using synthesized catalysts was monitored by gas chromatography-flame ionization detector (GC-FID). The two most potential catalysts were selected for the optimization and characterization study. Cu/Zn/?-Al2O3 catalyst calcined at 800°C with 10:90 wt.% dopant ratio to based and 3 times of alumina coating, exhibited the highest biodiesel production (89.50%) at mild reaction conditions (65°C, 10 wt.% catalyst loading, 1:20 oil:methanol mole ratio and 2 hours of reaction time). Cr/Ca/?-Al2O3 catalyst calcined at 700°C with 10:90 wt.% dopant ratio to based and 3 times number of alumina coating gave 86.64% of biodiesel production at 65°C, 6 wt.% catalyst loading, 1:18 oil:methanol mole ratio and 3 hours of reaction time. The physicochemical properties of the potential catalysts were accomplished using nitrogen analysis (NA) and CO2-temperature programmed desorption (CO2-TPD) that indicated high surface area (>140 m2/g) and high basicity (>3 mmol/g). X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) analysis showed the polycrystalline structure with small particles sizes (<50 nm). Energy dispersive X-ray (EDX) spectroscopy, X-ray fluorescence (XRF), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) analyses confirmed the existence of Al, O, Zn, Cu, Ca and Cr species in each potential catalyst. The optimization of catalyst preparation conditions and biodiesel production parameters were verified by response surface methodology (RSM) method and they were in good agreement with the experimental values. The mechanistic study on both potential catalysts follows the Langmuir- Hinshelwood (LH) model which involves the initial adsorption of reactants molecules on active sites of the catalyst surface. The specification analysis of produced biodiesel utilizing Cu/Zn/?-Al2O3 and Cr/Ca/?-Al2O3 catalysts showed that the refined used cooking oil has potential to be used in large-scale biodiesel production using reaction conditions found in the present study.