Combining Catalysts for Biofuels

Back in September of last year, I wrote about a process, developed by scientists at Tulane University, that used bacteria (of genus Clostridium) to produce butanol [C4H9OH] from cellulose.   Ars Technica now has a report on some further research along the same lines by a group of researchers at the University of California, Berkeley [UCB].  The process uses a combination of bacterial fermentation and metal catalysts to produce longer-chain hydrocarbons (~11 carbon atoms); the resulting mixture has characteristics similar to petroleum-based diesel fuel.  The paper describing this process has been published in Nature [abstract]; UCB has also issued a news release.

The first stage of the process involves a fermentation originally described by the chemist Chaim Weizmann, in which the bacterium Clostridium acetobutylicum ferments sugars into a mixture of butanol, ethanol [C2H5OH], and acetone [CH3-CO-CH3].  (Weizmann, born near Pinsk in what is now Belarus, emigrated to Britain, where he became a lecturer in chemistry at the University of Manchester.  Later in life, he would become the first president of Israel.)   The process, discovered at the beginning of World War I, was originally valued for the acetone produced, which was needed to produce cordite, a replacement for gunpowder.  Left to its own devices, the reaction shuts down in time, because these metabolic products are harmful to the bacteria.

The UCB scientists have found that a class of organic solvents, in particular glyceryl tributyrate, can be used to extract the butanol and acetone from the fermentation mixture, leaving most of the ethanol behind in the original, water-based solution.  The researchers then used a catalyst of potassium phosphate [K3PO4] and palladium [Pd] metal in a condensation reaction, in which the acetone and butanol combine to produce a longer-chain ketone.  Further condensation produces a mixture of ketones, about half of which is an 11-carbon compound.  This mixture, although not chemically the same as conventional diesel fuel, has similar properties, so that it can be used as a feedstock for fuel production.

At present, this process is not economically competitive with producing fuel from petroleum.  One issue is the cost of the palladium catalyst; however,  the researchers feel that alternative catalysts, less expensive but equally effective, can be found.  The fermentation and extraction process is already fairly efficient, compared to conventional distillation.

As with the previous work in this area, there is a good deal of work to be done before the research leads to a commercially viable process.  Nonetheless, it is encouraging that different avenues are being explored.  After all, petroleum and other fossil fuels were formed by chemical transformations of organic materials, albeit over long time spans.  We just need to speed things up a bit.

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