The process was inspired by an explosives making method

What can't palladium do?  Named after the mythological statue erected by Greek goddess Athena in honor of Pallas, the daughter of her cousin whom she slew in a friendly fight, the platinum group metal (PGM) is used in everything from catalytic converters to explosives making.

I. From Explosives to Biofuel

Researchers at U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (referred to as LBNL or Berkeley Lab) -- located on the campus of the University of California, Berkeley -- have devised a way to use palladium to catalyze the production of gasoline, diesel, or jet fuel from fermented biomass.

Traditionally biomass, fermented by genetically engineered bacterium, produces a slurry of low-carbon byproducts -- acetone and the alcohols butanol and ethanol (known collectively as ABE).

The LBNL researchers were inspired to use palladium as a means of producing higher-carbon byproducts based on a similar technique used to produce cordite, a type of smokeless explosive.  While largely made obsolete by newer formulations in the improved military rifle (IMR) nitrocellulose smokeless powders, cordite played a key role in military history, being used heavily by the British in World War II.

Palladium bar
The new work is one more example of the value of multi-functional palladium.
[Image Source: Unknown]

Co-author Harvey Blanch describes the pickup of this throwback catalysis process, remarking, "In some ways, this work is a step back in time in which a very old fermentation process is being used with some new engineering and chemistry.  While there has been some progress in engineering microbes to produce advanced biofuels, the quantities produced thus far – technically, the solution’s titer – tend to be very limited. A hybrid method, combining microbial production with chemical catalysis, might provide a pathway to more efficient production of these advanced biofuels."

Biomass bacteria
The process begins by bacterial fermentation. [Image Source: Toste Group]

From a layman's perspective, the bacteria takes biomass -- say yard waste -- and "digests" it to produce a byproduct that is three parts acetone, six parts n-butanol, and one part ethanol.  The acetone is the key to move to longer chains, as it can be used to "tack" on carbons onto the short-chain alcohols, building long carbon chains that mirror those found in traditional fossil fuel-derived gasoline.

The acetone's nucleophilic alpha-carbon undergoes alkylation reactions to produce longer chain carbons.  But the process is slow and energetically unfavorable, so a catalyst is needed to lower the energy barrier.

Many catalysts were tested, but the LBNL team found one worked much better than the rest -- palladium.  Researchers say the process, which they tested in small vats, is amenable to commercial production.

II. Challenges Remain

One limiting factor will be the cost of palladium.  While fuel is valuable as anyone who drives a car recognizes, palladium is an even more expensive resource.  It sells for approximately $20,500 USD per kilogram [source].  Therein lies one problem; the longer the ABE spends on the palladium, the more long-chain byproducts are produced.  So if you had more palladium and more vats, you could spread the ABE out for more palladium TLC.  But the high costs are somewhat limiting to that approach.  That said, process engineers could optimize the process to maximize yields and minimize cost.

Still, the process is much better than other ABE techniques, such as hydrogenation from a yields perspective.  States Professor Blanch, "Integrating chemistry and fermentation is a useful way to capitalize on the best of both worlds. The chemistry described in our Nature paper is exciting because new carbon-carbon bonds are being formed between molecules and oxygen is being rejected without the need of hydrogenation. This results in very high yields."

The researchers have published their work in the prestigious peer-reviewed journal Nature.  The paper's co-authors include Professor Blanch, plus Pazhamalai AnbarasanZachary BaerSanil SreekumarElad GrossJoseph BinderDouglas Clark, and corresponding author Dean Toste..

The research was funded by the Energy Biosciences Institute.

lawn waste
Coming up with a viable biomass supply change is a key challenge.
[Image Source: DeCamp Trucking]

Looking ahead, while the researchers exhaustively tested a series of PGM catalysts, they are hopefully that they may discover even better novel catalysts.  Comments Professor Toste, who splits his time between UC Berkley and LBNL, "While palladium on carbon was the best catalyst in these tests, we have already identified other transition metal catalysts that could be even better."

Tough challenges for a biomass-based fuel economy remain -- including how to funnel/transport common biomass sources (yard waste, forestry byproducts, farm waste, etc.) into a steady supply chain to fuel production facilities.  But work like this gives biomass fermentation a leg up over other struggling biofuel offerings like the highly unattractive corn ethanol.

"We can't expect users to use common sense. That would eliminate the need for all sorts of legislation, committees, oversight and lawyers." -- Christopher Jennings

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