Heterogeneous catalysis of plant derived oils to biodiesel

Abstract

Heterogeneous catalytic transesterification of plant derived oils (PDO) i.e. soybean oil, and microalgae derived oil (MDO) to fatty acid methyl esters (FAME) i.e. biodiesel was investigated. Calcium, magnesium and zinc based catalysts were screened for their catalytic activity, where calcium methoxide was found to have superior conversion efficiency of PDO to FAME. Calcium methoxide, as prepared by reacting calcium metal and methanol, resulted in a catalyst with a surface area of 32 m2/ g and a pore volume of 0.19cm3/g. From the TGA analysis, the molecular formula for the catalyst was determined as CaO0.13(OCH3)1.74. SEM and FTIR analysis confirmed the presence of surface methoxide groups on the surface of the catalyst. In the first phase of investigation, the conversion efficiency of the catalyst was evaluated using triglycerides and polar lipids. Reaction variables including: reaction temperature; amount of catalyst; and methanol-to-oil molar ratio were optimized using response surface methodology at 64oC, 4% and 9:1, respectively. No lixiviation of the catalyst into methanol was observed. The heterogeneous catalyst can be re-used for at least ten times without significant loss of activity. When polar lipid was used as the feedstock, the fate of organic phosphorus was determined. Phosphorus content of the FAME layer was 0.081% (w/w) which was only 1.26% of the total phosphorus, with the remainder concentrated in the polar layer. The removal of residual phosphorus from biodiesel was investigated using adsorption and water-washing techniques. Three different adsorbents i.e. silica gel, magnesol and magnesium silicate were tested with biodiesel spiked with phosphatidylcholine (PC). Silica gel has the maximum adsorption capacity (i.e. 0.60 mmol/g) and a greater affinity for PC than magnesol and magnesium silicate; it is possible to bring the P content in the FAME below 0.001% using all three adsorbents. Water-washing was not an effective method to remove PC from biodiesel, where removal efficiency was less than 10% at room temperature. In the second phase of investigation, extraction and conversion of MDO was investigated, together with factors affecting intracellular lipid extraction from microalgae. Chlorinated solvent systems including chloroform-methanol and dichloromethane-methanol resulted in higher lipid extraction efficiencies than other solvent systems. Hexane, when used alone, had a poor lipid extraction efficiency at 16.4%, but improved when the polar solvents iso-propanol and methanol were added to 19.1 and 25.5% respectively. The moisture content of the microalgae biomass affected both lipid extraction efficiency and FFA content of the extracted lipid. Above a 20% moisture content, lipid yields were significantly reduced. When the moisture content was increased from 20% to 85% lipid yield dropped from 25.4 to 13.0%, and FFA content of the lipid increased from 1.5% to 7.8%. Reaction variables for conversion of MDO including: temperature; amount of catalyst; and methanol-to-oil molar ratio were optimized at 80oC, 5% and 22:1, respectively. Lipids with less pigment are most suited for biodiesel production via transesterification using heterogeneous catalysis. Lipid, with an 8% pigment content extracted from Nannochloropsis, gave a FAME yield of 60% compared to a 13% for lipid extracted from Chlorella with an 18% pigment content. Finally, a novel method that combines the extraction of lipid from microalgae and its conversion to FAME was developed using a hexane-methanol azeotrope as the solvent, and calcium methoxide as the catalyst. Heterotrophically cultured Chlorella gave a maximum FAME yield of 25% as dry weight of biomass followed by mixotrophically cultured Nannochloropsis (13% dry weight of biomass); then photoautotrophically cultured Chlorella (6% dry weight of biomass).