Biodiesel Synthesis and Glycerol Transesterification by Layered Double Hydroxides in Stirring Packed-Bed Reactor

Fossil fuel brings mankind lots of convenience and boosts the technology development. However, fossil fuel also brings crisis such as global warming and energy shortage. To deal with the problems, many scientists devote themselves into finding green and sustainable energies, and biodiesel is one of them. Biodiesel can be mixed with petro-diesel at any ratio and used in a diesel engine. Biodiesel contains no sulfur and no aromatic compounds. The surge of interest in biodiesel has also highlighted the reduction of greenhouse gas emission and pollution as well as nontoxic and biodegradable. In this research, we transesterificated (1) triglycerides to synthesize biodiesel and; (2) its byproduct glycerol to form a more valuable chemical, glycerol 1,2-carbonate, using layered double hydroxides (LDHs), specifically Mg-Al LDH. We studied the different Mg/Al ratios in catalyst preparation, calcination temperatures of LDH, reaction temperatures, catalyst loadings and the molar ratios of dimethyl carbonate/glycerol, etc., which are critical factors in both transesterification reactions. We had Mg-Al LDH coated on the 3mm spherical α-Al2O3 catalyst in the stirring packed-bed reactor. Mg-Al LDH was first calcined and then rehydrated with decarbonated water-glycerol solution to reconstruct the layer structure and also to protect the catalyst deactivation by air at the same time. In biodiesel synthesis, the yield reached 65.72% in 4 hours under 60oC, 87.45% in 3 hours under 100oC, when the methanol/soybean oil molar ratio was set to be 30, and 10wt% Mg/Al=5 LDH was used in the reaction. The glycerol transesterification was carried out by dimethyl carbonate/glycerol molar ratio 16 with the existence of solvent, dimethyl sulfoxide, using 10wt% Mg/Al=5 LDH on spherical α-Al2O3 at 95oC. The yield of glycerol 1,2-carbonate could reach up to 93.4% within 7.5 hours. When applying 10wt% catalyst of Mg/Al=5 LDH on spherical α-Al2O3 into the stirring packed-bed reactor, the yield of glycerol 1,2-carbonate reached to 80.17% within 11 hours under the residence time 22.5 hours, and dimethyl carbonate/glycerol molar ratio = 16 at 90oC.