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Enzymatic Process Converts Cellulosic Materials and Water into Hydrogen at Low Temperature; Close to Theoretical Yield of H2 From Glucose

Zhang
Hydrogen production from cellodextrin and water by a synthetic enzymatic pathway. Ye et al. (2009) Click to enlarge.

Researchers at Virginia Tech, Oak Ridge National Laboratory (ORNL), and the University of Georgia have produced hydrogen gas in a spontaneous, “one-pot” process using an enzyme cocktail, cellulosic materials from non-food sources, and water. The hydrogen yield was 11.2 moles per mole of anhydroglucose unit of cellobiose, corresponding to 93.3% of the theoretical yield of 12 moles. A paper on the work was published online in the journal ChemSusChem on 2 February.

In 2007, the researchers had reported the development of a novel method using a combination of 13 enzymes to form an unnatural enzymatic pathway to completely convert starch and water in one reactor into hydrogen. (Earlier post.)

Only approximately half of a glucose equivalent of soluble starch can be converted into glucose-1-phosphate mediated by starch phosphorylase for hydrogen production. The reported yield of hydrogen in this earlier process was only 5.19 moles of hydrogen per glucose equivalent of starch consumed, largely due to incomplete reaction. The non-efficient conversion of starch and its limited supplies would have economically prohibited large-scale hydrogen production through that approach, the researchers concluded.

In the new process, the team targeted the production of hydrogen in high yield from cellulosic materials and water. Cellobiose is a dominant product of primary enzymatic cellulose hydrolysis, and cellodextrins are prepared in high yields from cellulose or biomass by using mixed acid hydrolysis.

The team designed a new synthetic enzyme pathway containing five sub-modules. These spontaneous reactions are conducted under modest reaction conditions of 32 °C (90 °F) and atmospheric pressure. The overall cellobiose-to-hydrogen reaction is summarized as:

C12H22O11(aq) + 7H2O(l) → C6H12O6(aq) + 12 H2(g) + 6CO2(g)

Thermodynamic analysis clearly suggested that this cellobiose- to-hydrogen reaction was spontaneous but endothermic. To our limited knowledge, this reaction was the first chemical reaction that can absorb ambient-temperature heat and convert it into chemical energy that we can utilize, that is, the output/input (chemical energy) ratio is greater than 1. This reaction was spontaneous (ΔG<0) when the reaction temperature was higher than 0 °C, because both gaseous products were released from the aqueous solution under the modest conditions of less than 100 °C and about 1 atm, accompanied with an entropy gain (ΔS>>0). Although spontaneous endothermic (entropy-driven) chemical reactions are rare, several examples are reported, such as N2O5(s) →2NO2(g) + ½O2(g).

—Ye et al. (2009)

Dr. Percival Zhang at Virginia Tech attributed the less than 100% yield to running the process in a batch reaction. “If we run it in continuous mode, the yield should be 12.

The group announced three advances from their new process:

  1. A novel combination of enzymes,
  2. An increased hydrogen generation rate to as fast as natural hydrogen fermentation; and
  3. A chemical energy output greater than the chemical energy stored in sugars due the to process converting low-temperature thermal energy into high-quality hydrogen—“like Prometheus stealing fire,” according to Zhang.

This could be the first chemical reaction that can convert ambient-temperature heat to chemical energy (hydrogen). Most people dd not realize how important it is, stealing energy from environment that we cannot utilize before.

—Percival Zhang

The researchers used cellulosic materials isolated from wood chips, but crop waste or switchgrass could also be used. According to Zhang, if around 10% of US annual biomass production (some 1.3 billion tons) were used for sugar-to-hydrogen fuel cells transportation, “we could reach transportation fuel independence.”

The process would need to overcome two major obstacles to be viable commercially: the high cost of the enzymes, and slow reaction rates. The authors suggest that the costs associated with the enzymes can be decreased through a combination of approaches, including the use of (hyper)thermostable enzymes, enzyme immobilization, simple enzyme purification, and large-scale production of recombinant protein.

Increasing the rate of biohydrogen production could be accelerated “by several orders of magnitude” by using a combination of technologies, such as (hyper)thermophilic enzyme replacement, elevated reaction temperatures, optimization of key enzyme ratios, higher substrate concentration, higher enzyme loading, and even metabolite channeling.

In this study, we have increased the hydrogen production rate by 8.2-fold as compared to our previous results on starch by increasing the rate-limiting hydrogenase concentration, increasing the substrate concentration, and by elevating the reaction temperature slightly from 30 to 32 8C. An overall rate enhancement by about 20-fold has been implemented in the past two years.

—Ye et al. (2009)

The research is supported by the Air Force Office of Scientific Research; Zhang’s DuPont Young Professor Award, and the US Department of Energy.

Resources

Comments

Alex Kovnat

The process developed by Virginia Tech, ORNL and U. of Georgia to produce hydrogen from cellulose is all well and good but we must remember, hydrogen is difficult to transport and store. If we must use a light gas, I'd rather it be methane. The latter, must be stored under high pressure to get a decent range if used as vehicular fuel, but it doesn't have to be as high a pressure as one needs with hydrogen.

I still believe the big prize in cellulose research is finding economical ways to break it down to glucose for subsequent fermentation to ethanol. But then, I'm willing to look at all alternatives to fossil fuels.

Sam

Nice point about H2 vs. CH4 Alex, and of course H2 is currently being produced from natural gas (largely CH4) so methane production from anaerobic digestion can also provide H2 if necessary for other bioprocesses.

ai_vin

Hydrogen, as a vehicle fuel, is not necessarily a bad choice but there are better options. However the fact remains hydrogen IS in wide use around the world (it is used in industrial chemistry of all kinds) and it is produced from fossil fuels. So this process could be of huge benefit.

Alain

Since in this reaction H2O (instead of air) is used as oxygen source for the production of CO2, the gasses comming out of it are only H2 and CO2. Separating these gasses is very easy, so it is easy to produce concentrated CO2 as 'waste gas', which can then be sequestered. Burning the H2 would be much cleaner than burning methane.

sulleny

At some point further down the road this discovery will be recognized as groundbreaking in accepting the role of low temp reactions releasing matter greater than their direct inputs.

A very good step.

G.

[i]At some point further down the road this discovery will be recognized as groundbreaking in accepting the role of low temp reactions releasing matter greater than their direct inputs.[/i]

First, it can't be greater than ALL its direct inputs (this would include the low grade heat). That would violate thermodynamics. It's just that it is spontaneous AND endothermic... rare but doable if the entropy delta is high enough. But this still requires some low grade heat.

And it won't matter if the reaction speed is very slow, as seems to be the case here (they have compared it to the speed of natural degredation).

There's more to chemistry than a reaction being spontaneous or not. Reaction rates matter. Scalability matters.

Probably the rate can be increased as they suggest, but that will likely require higher temps etc.

Finally, the hydrogen doesn't have to be used directly as a fuel for cars... it can be run through a fuel cell to create electricity. This could be used to power cars or simply to displace coal generated electricity, etc.

Interesting though.

sulleny

@ G:

Thanks for that clarification. I expect that some part of the automotive industry is still invested in H2 for FCs. Whether they can ever compete with battery storage on cost is a major factor.

But this, and other processes could be applied to home CHP units which would help remove some of the residential load on the grid. Provided economic scalability.

Engineer-Poet

About the only thing we can really use gaseous hydrogen for at the moment is chemical feedstock.  This isn't bad, but today's push is to make motor fuel, and hydrogen makes a pretty poor motor fuel for today's vehicles.  It would make more sense to take the products of this reaction, remove some of the CO2 and react it to methanol:

12 H2(g) + 4 CO2(g) → 4 CH3OH(l) + 4 H2O(l)

Voila, motor fuel.  It would probably work even better than ethanol in the Ford/MIT boosted engines.

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