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.

Greener Cement

One of the challenges in attempting to reduce emissions of greenhouse gases (such as carbon dioxide and methane) is the considerable variety of emission sources.  Some, like the burning of fossil fuels for power generation and transportation, are reasonably obvious, but there are others that aren’t.   For example, it has been estimated that, in some California counties, cattle flatulence produces more greenhouse gas than motor vehicles.

Another source that might not spring immediately to mind is the production of cement, which is estimated to account for 5-6% of man-made greenhouse gas emissions.  Conventionally, cement is made by heating crushed limestone (which is basically calcium carbonate, CaCO₃) to a temperature of about 1500°C, where it breaks down to give calcium oxide (CaO, also called quicklime), the key ingredient of cement, and carbon dioxide.

CaCO₃ -> CaO + CO₂

About 60% of the carbon dioxide produced in the production of cement comes from this reaction; the balance comes from the fossil fuel burned to heat the limestone.

According to an article at Technology Review, a team at George Washington University has developed a new process that produces calcium oxide from limestone without the emission of carbon dioxide.

The new process changes the chemistry. Rather than emitting carbon dioxide, it converts the gas, using a combination of heat and electrolysis to produce oxygen and either carbon or carbon monoxide, depending on the temperatures employed.

In this process, the crushed limestone is mixed with lithium carbonate and heated to about 900°C.  The application of a relatively small electric current causes calcium oxide to form as a precipitate.

The research team believes that the heating required for the process could be provided by solar energy, and has constructed a proof-of-concept apparatus to demonstrate this.  The device uses two large Fresnel lenses to concentrate sunlight on the mixture of limestone and lithium carbonate, and a third to focus sunlight on a solar cell, which provides the electricity required.   The device makes use of about 50% of the available solar energy, which compares favorably with the ~15% efficiency of solar cells.

The technique is still at a very early stage of development, and considerable work remains to check that the process can be scaled up to industrial size.  Nonetheless, it is an intriguing idea, and another example of the value of encouraging research into alternative technologies.  Cement is not very exciting or sexy, after all; but I suspect that progress in reducing greenhouse gases will, in the end, involve a number of changes like this.