Heterogeneous Catalysts for Biodiesel Production

Biodiesel is an alternative fuel to crude oil-derived diesel that can be produced from renewable feedstocks (virgin vegetable oils) or waste material (used cooking oils). It is produced by the reaction of vegetable oil with methanol, usually in the presence of a liquid alkaline catalyst, to produce fatty acid methyl esters (“biodiesel”) and glycerol. Biodiesel  has lower life-cycle carbon dioxide emissions and emits fewer pollutant gases upon combustion than “petrodiesel”.

The current technology for biodiesel production has two main shortcomings. Firstly, the presence of free fatty acids and water in the feedstock causes soap to be formed, as a byproduct of the reaction. This causes a reduction in reaction yield and loss of biodiesel product via entrainment in the soap phase during the washing process. This restricts the range of feedstocks in current plants, as used cooking oil, for instance, is high in free fatty acids and water, so more expensive fresh oils must be used, or an energy intensive pre-treatment. Secondly, the alkaline catalyst must be neutralised, and the resulting salt is difficult to remove from the glycerol, making it difficult and costly to reprocess the glycerol to a saleable grade.

The use of heterogeneous catalysts would result in simpler, cheaper separation processes, a reduced water effluent load and reduced capital and energy costs. There would be fewer inputs and less waste, as no soap would be formed, and the catalyst would not have to be continuously added. Furthermore, there would be no neutralisation products, so a higher grade of glycerol could be produced that can be reused in other industries (usually the cosmetics industry).

In addition to available solid base catalysts, novel catalysts were designed for this reaction, such as skeleton polymer-supported catalysts with tuneable pore sizes to accommodate the large triglyceride molecule, and extremophilic enzymes that combine high reaction rate with specificity, leading to the possibility of combined extraction and reaction. Other catalysts included ion exchange resins and solid acid catalysts, which allow free fatty acids to be directly esterified into biodiesel. The catalyst systems were screened using the “Chemspeed Accelerator Synthesiser”. This “high throughput technology” was applied to the entire workflow, including multistep synthesis, post-reaction separations and work-up for analysis.

The catalysts were evaluated against environmental, economic and process criteria, such as timescale for reaction, robustness and practicality of the required operating conditions. Successful catalysts will be evaluated in a range of conventional and intensified, purpose-designed reactors, such as the oscillatory flow reactor, an intensified plug flow reactor.

For more information please contacted Prof Adam Harvey.