UMass-Amherst team solves mystery of cellulose chains breakdown

What exactly happens in depolymerization, as hydrogen bonds break down?

How could that revolutionize the process of making fuels, solvents, thinners, lacquers or paints out of biobased materials?

Before one is able to arrange a global bioeconomy, first there’s a little rearranging to do with the underlying biomass. – transforming it from algae, or plants or trees into products for everyday life.

There are three ways to do that.

One – fractionation – that’s dividing up something like a corn kernel into its protein, carb and lipid fractions – how you get starch for ethanol, protein for distillers grains and corn oil for food or biodiesel production. It also can be a recipe for running into the food and feed markets, which have long fractioned that same materials into their own products – and like the cheap raw material prices that came from years of low competition.

We’ve learned quite a lot about fermentation in the process – typically, fermenting starch into alcohols, alkanes and more for the production of fuels, packaging materials, chemicals and more. The move into extracting sugars from cellulose and hemicellulose – the cellulosic revolution – has given hope to the idea of utilizing parts of plants not otherwise claimed for food or feed. But there have been delays and headaches in the fantastically difficult science of extracting and fermenting those sugars, at scale, at affordable costs.

Which brings us to door number three – the thermo-chemical path. In this case, we heat up biomass until the hydrogen bonds that hold it together start to modify and break down – and from there we get a soup of material that can be repurposed into a huge range of products.

The process is called depolymerization by people who like spelling challenges. For those of us that avoid any word with more syllables than “Philadelphia”, you can think of it as something not entirely unlike melting ice. At the freezing point of water, hydrogen bonds rigidly hold the water molecules together – when heat is applied, the bonds loosen up and water molecules begin to flow as a liquid, and eventually radiate as a gas.

Those bonds can be pretty amazingly strong, as anyone who has ever foolishly stuck their tongue-tip to a freezing pole in wintertime can attest.

Melting water is pretty simple, it happens the same way every time. But the process of breaking the biomass bonds has been unpredictable, with different outcomes derived from different heating protocols.

What’s at stake? It’s a platform for really breaking open a pathway to a robust bioeconomy – with the resulting shift off imported petroleum. Not to mention job creation. Not to mention the potential impact on global warming. Basically, a paradigm shift for global civilization and trade hanging in the balance.

Good news on that front this week. Reporting in the current issue of the Journal of the American Chemical Society, theoretical chemist Scott Auerbach, chemical engineers George Huber and Paul Dauenhauer, and colleagues at the University of Massachusetts Amherst have, for the first time, modeled at the molecular level the activation energies needed for the chemical reaction known as “fast pyrolysis” to proceed in cellulose.

The model meets the tight strictures of chemical accuracy, within 5 kilojoules per mole of cellulose. “We’re quite sure that experiments testing our model will confirm it,” says Auerbach.

“No one knew how cellulose thermal depolymerization works at an atomic scale,” Auerbach says. “There had been a great deal of uncertainty and controversy about the fundamental reactions and processes. But for the first time our theoretical calculations reveal the dynamics of these bond-breaking events. Given this new knowledge, we can begin to build a picture for how cellulose depolymerizes and how it can be done better and more efficiently.”

Results of the UMass Amherst team’s modeling are presented in a table with 18 reaction pathways at two temperatures, 327 and 600 degrees C.

Overall, Auerbach says, “The key chemical feature that holds cellulose together, hydrogen bonding, can also be the seed of its own destruction.” That’s because below about 260 degrees Celsius (500 degrees Fahrenheit) hydrogen bonds hold the cellulose chains together in much the same way as they hold water molecules together in ice. But the UMass Amherst team found that above this temperature, these same hydrogen bonds begin to insert themselves between other atoms, promoting, or catalyzing, the breakup of chemical bonds.

Auerbach points out, “We modeled not only these 18 pathways, but the activation energy for each. So now we know where the transition from the intermediate liquid to vapor takes place. The table shows the different kinds of processes and transition points for each in kilocalories per mole, at a particular temperature. The amazing new thing is that hydrogen bonding keeps cellulose intact in one situation and catalyzes its own destruction in another. Cellulose carries the key to its own destruction within its chemical makeup.”

From previous experimental work, it was known that rapid heating, or fast pyrolysis, turns cellulose into a mysterious but unknown new phase dubbed “active cellulose,” with properties different from cellulose, but still not the desired vapor. Auerbach says, “This active cellulose stuff is very difficult to characterize, with different heating methods yielding different results, in some cases solid and reversible, and in others liquid and irreversible. Nobody knew what this ‘active cellulose’ really is.”

As a result of the modeling, however, he and colleagues now know cellulose’s secret, the pathways and barriers by which cellulose chains are likely to break. To make these discoveries, the researchers applied special Car-Parrinello molecular dynamics (CPMD) software, named after its developers, in a new way. This accomplishes two tasks that are usually done separately: Making accurate quantum calculations of chemical energies and performing exhaustive sampling of atomic configurations.

Auerbach explains, “The cellulose pyrolysis mystery was a classic: to engage in accurate calculations we needed to know which atomic configurations to consider, but to know which atomic configurations were important we needed accurate calculations.  CPMD allowed us to do them both in one shot.”

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Tags: pyrolysis, UMass

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