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:

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 N₂O_5(s) →2NO_2(g) + ½O_2(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

• Ye et al. (2009) Spontaneous High-Yield Production of Hydrogen from Cellulosic Materials and Water Catalyzed by Enzyme Cocktails. ChemSusChem doi: 10.1002/cssc.200900017

• The Zhang Lab

• ORNL Bioconversion Science and Technology Group