Catalyst for biodiesel production

A solid catalyst being developed by a cross-disciplinary UK team could allow farmers to produce biodiesel from a variety of plant oils in a single step.

Because plant-oil feedstock contains small amounts of fatty acids, conventional biodiesel production requires a large reactor and a number of steps. These fatty acids can corrode vehicle parts, so need to be removed or neutralised before the biodiesel is produced.

BP Biofuels is a project partner in the EPSRC-funded scheme, where researchers from York,  Manchester and Newcastle universities are designing a catalyst and associated reactor that could neutralise the acids and produce the biodiesel in a single step.

Dr Adam Lee from York University’s Department of Chemistry is working alongside the project’s principal investigator, Dr Karen Wilson. He explained: ‘Synthetic chemistry has dominated in the area of transportation fuels. Manufacturers add large amounts of chemicals such as sodium hydroxide or sodium methoxide, which are soluble materials that can catalyse reactions, but these catalysts can’t be recovered.

‘We’re trying to combine our materials knowledge with reactor designs using flow systems rather than the batch reactors currently used for biodiesel production. These have several disadvantages: you can’t easily separate the products until the end of the reaction so you can’t have continuous production, the mixing can be poor and the materials can pose hazards.’

The researchers aim to develop porous solid catalysts with high surface areas. Computational engineering will help ensure the design of interconnecting pores, which will allow for the viscosity of plant oils, which are much thicker than mineral oils. These are known as second-generation mesostructure solids, meaning they have big pores to handle the long-chain molecules involved. The target substrate material is silica, which is easy to work with, reproducible and has well-researched structural properties. Because silica is thermally and chemically stable, strongly basic and acidic sites will be introduced to make it into an active catalyst.

The materials will be tested in oscillatory baffle reactors at Newcastle. ‘As biodiesel is viscous, it needs a long time in the reactor bed to get a decent reaction,’ added Lee. ‘We will use a reactor that has a continuous flow, but using a pulse system, so it oscillates two steps forward and one step back as it passes down the catalyst bed.’

The idea is for the catalyst to perform two chemical reactions in one — neutralise the free fatty acids while the esterification of the plant oil triglycerides into biodiesel takes place. The acids will be converted into an ester, a non-corrosive molecule that would be burnt off in the vehicle engine.

One solution would be to set up a two-bed system, where the plant oils are pushed through a catalyst bed to deal with the fatty acids before moving on to biodiesel creation. But ideally the researchers hope to devise catalysts that have basic sites and acidic sites in the same material, so the acidic sites will esterify the undesired contaminants and the basic sites will perform the transesterification to make the biodiesel.

‘With these porous silicas, you could put down nanoparticles of something such as magnesium oxide, which is a nice solid base,’ said Lee. ‘Alongside those, you could anchor a range of solid acids, such as sulphated zirconias or polyoxyl methylates. These could be grafted into the same particle so you can have a crystallite of silica that contains solid base and acid sites geographically separated so they can attack each kind of molecule coming through.’

One aim of the project is to make a catalyst tailored to feedstock such as jatropha, which grows on scrubland in arid conditions and does not compete with food crops.

‘The oil composition will vary with the type of plant it comes from, so the trick will be to find a catalyst that enjoys working with jatropha oil,’ explained Lee.

By the end of the project the team hopes to have a suite of catalysts tuned to handle different feedstocks, an operating reactor system in which to use the catalyst and a semi-pilot scale operation to make the catalysts.

The goal is to produce portable reactors, as this would allow farmers to make biofuels. The solid-catalyst systems would make possible a reactor with a 2m-long pipe and a few of grams of catalyst bed, where plant oils could be poured in at the top and biodiesel would come out at the bottom.

Berenice Baker