The Promiscuous, the Selective, and Biofuels

May 3, 2013 |

PromiscuousA new, potentially game-changing path to biofuels?

And what exactly does promiscuity have to do with it?

There’s been some potentially world-changing work going on of late at the University of Wisconsin, the University of Guelph and Los Alamos National Laboratory that it’s time for you to know about.

And if phrases like “enantioselectivity” or “aldol chemistry” are in your everyday vocabulary, you can skip the next nine paragraphs, where we attempt to reduce a thousand years of chemical history & practice into a quick backgrounder.

Here’s what you need to know, upfront, about Biomass and Promiscuity

If you think about it, when we say that biomass is renewable and fossil fuels are not, what we are really saying is that biomass is made from a promiscuous and fast set of reactions under everyday conditions of heat, light and pressure.

Promiscuous, because a zillion organisms know how to use photosynthesis, under a zillion set of circumstances, to turn simple and abundant molecular compounds — like carbon dioxide and water — into longer-chain and useful molecules for structure, protection or energy.

Fast, because biomass can double in 24 hours with a given organism — and in a growing season you can turn a brown field and some seeds, nutrients and rainfall into food, fiber, and fuel for the world. In fact, most of what we understand as the Neolithic Agricultural Revolution is taking a promiscuous field and turning it into a selective one.

You might know it as monoculture, or making a field produce as much of one target plant by applying genetics, pesticides, herbicides and so on in order to keep Nature from doing what it would normally do — create a promiscuous set of reactions that result in a whole bunch of biodiversity and not very much of the plant that we want to harvest in bulk.

Here’s what you need to know, upfront, about Fossil Fuels and Selectivity

By contrast, when we say that fossil fuels are a finite and non-renewable resource, what we are really saying is that fossil fuels are made from a selective and slow set of reactions under rare conditions of heat, light and pressure.

You see, if you toss a bunch of seeds on a field, there’s a high probability it’ll turn into biomass. If you toss a whole bunch of biomass onto a field, and come back in 60 million years, there’s a certain probability that it will have turned into the soup of hydrocarbons you and I know as fossil fuels. But not a high probability.

With fossil fuels — just like biomass, Nature will do the work for free. Just not at the speed we need, nor with a particularly high yield (consider, for example, the amount of biomass created by Nature over the history of the plant, and how little of that actually became a fossil fuel – the yield is something like 1 ton of hydrocarbons for every 100 million tons of biomass, and Nature is on average producing something on the order of 150,000 tonnes of fossil fuels per year, enough to run one city the size of West Covina, California on a renewable basis).

The problem with fossil fuels is that we’ve had no Neolithic Revolution — we’re still hunting and gathering hydrocarbons, and we’ve figured out no particularly effective way to speed up, industralize, or densify the way in which Nature produces them. Something that was worked out with, say, wheat and barley several thousands of years ago. That’s why we’re running low — the problems of selectivity and rate.

So, promiscuous and selective, fast and slow — these are the parameters that define the energy equation.

Promiscuous and selective catalysts

(OK, those of you who were skipping paragraphs — dive in right here.)

Newt Gingrich once famously pointed out, you can’t fill up a car and run it on algae; less usefully, he neglected to mention that you can’t fill up a car and run it on kludgy Venezuela crude, either.

We take those biomass or fuel precursors and turn them into useful products — one way is through fermentation, but the more traditional path is through chemistry and catalysis — the science of making useful reactions happen better, faster and cheaper.

Here are the challenges with biomass. First, you have to get the oxygen out in order to get a hydrocarbon. Second, you want the molecular chains to be longer so that they have the boiling points that work in combustion engines and they have sufficient energy density.

Here’s another set of problems. Catalysts are designed, traditionally, to be selective, and you lose a lot of yield that way. Worse, the reactions that result in hydrocarbons work better under high temp, high pressure conditions (one reason why you find hydrocarbons forming underground where those conditions are abundant, and why oil refineries need special permits for the intense conditions).

So, what’s the good news?

Well, making furan aldehydes from C6 and C5 sugars (e.g. glucose and xylose) is a known art.

Better? It’s been known for some time that under certain intense conditions (e.g. 300 degrees celsius, and about 50 times the standard atmospheric pressures), you can use catalysis to make hydrocarbons, non-selectively, from tetrahydrofuran intermediates.

So, there’s a pathway there. Convert a biomass to an intermediate which can be elongated into the required long-chain molecule. All that is known stuff. In fact that is, in part, how Lipitor in made.

But how how do you achieve this without blowing out the energy input budget with a whole bunch of high temperatures and high pressures? Until now, impossible.

But there’s been, ahem, a change.

A group of collaborators from Los Alamos and The University of Guelph in Canada (Andrew Sutton, Fraser Waldie, Rulian Wu, Marcel Schlaf, Pete Silks and John Gordon) published an article in Nature Chemistry last month — titled “The hydrodeoxygenation of bioderived furans into alkanes“.

(We wish articles weren’t titled this way — it’s been known to frighten children and animals, and guarantees a limited readership. Sigh.)


But it describes how to take building blocks that are derived from glucose or cellulose and couple them with other bio-derived building blocks to give new molecules that have between eight and fifteen carbons in a row. This work describes a completely new approach, an alternative route to convert this class of molecules to hydrocarbons that uses much less energy and has a very high degree of conversion to provide pure products.

Here’s what happened, The researchers spotted new, promising work on elongating the molecules using a high-pressure approach, reported out of Jim Dumesic’s lab at the University of Wisconsin — and dreamed up a way to get the same type of results using low-energy organocatalysts.

Add in a second set of steps to get the oxygen out, thereafter (for chemistry lovers, they are using ring opening of furans, hydrogenation of olefins and hydrodeoxygenation of ketones). Voila, a new path to diesel and jet fuel.

Why it’s important

Remember our example, above — you toss biomass on the ground and come back in 60 million years and some tiny portion has become a fossil fuel — after being dragged down into an unworldly cave filled with high temperatures and crushing pressures?

Well, imagine accomplishing the same result without the 60 million years, the high temps and pressures, or having to frack your way to recovering hydrocarbons from the shales Nature is known to hide them in. You make the biomass in a season (or less, if you are using photobioreactors) and make hydrocarbons on the spot.

Next steps?

The team is looking to improve catalyst recyclability and scale-up methods — which is to say, don’t expect the fuel to show up at a pump near you in the next few weeks.

But it’s a fascinating step forward — especially for those who believe that catalysis offers more predictable, scalable processing conditions for industrial scale than fermentation.

We’ll stand by for more.

READ MORE: The Nature Chemistry article is fee-based, but you can rent 48-hour access for $2.99, here.

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