High-yield hydrogen production from starch and water by a synthetic enzymatic pathway

Abstract

Background: The future hydrogen economy offers a compelling energy vision, but there are four main obstacles: hydrogen production, storage, and distribution, as well as fuel cells. Hydrogen production from inexpensive abundant renewable biomass can produce cheaper hydrogen, decrease reliance on fossil fuels, and achieve zero net greenhouse gas emissions, but current chemical and biological means suffer from low hydrogen yields and/or severe reaction conditions.

Methodology/principal findings: Here we demonstrate a synthetic enzymatic pathway consisting of 13 enzymes for producing hydrogen from starch and water. The stoichiometric reaction is C(6)H(10)O(5) (l)+7 H(2)O (l)-->12 H(2) (g)+6 CO(2) (g). The overall process is spontaneous and unidirectional because of a negative Gibbs free energy and separation of the gaseous products with the aqueous reactants.

Conclusions: Enzymatic hydrogen production from starch and water mediated by 13 enzymes occurred at 30 degrees C as expected, and the hydrogen yields were much higher than the theoretical limit (4 H(2)/glucose) of anaerobic fermentations.

Significance: The unique features, such as mild reaction conditions (30 degrees C and atmospheric pressure), high hydrogen yields, likely low production costs ($ approximately 2/kg H(2)), and a high energy-density carrier starch (14.8 H(2)-based mass%), provide great potential for mobile applications. With technology improvements and integration with fuel cells, this technology also solves the challenges associated with hydrogen storage, distribution, and infrastructure in the hydrogen economy.

Figure 1. The synthetic metabolic pathway for conversion of polysaccharides and water to hydrogen and carbon dioxide.

The abbreviations are: PPP, pentose phosphate pathway; G1P, glucose-1-phosphate; G6P, glucose-6-phosphate; 6PG, 6-phosphogluconate; Ru5P, ribulose-5-phosphate; and P_i, inorganic phosphate. The enzymes are: #1, glucan phosphorylase; #2, phosphoglucomutase; #3, G-6-P dehydrogenase; #4, 6-phosphogluconate dehydrogenase, #5 Phosphoribose isomerase; #6, Ribulose 5-phosphate epimerase; #7, Transaldolase; #8, Transketolase, #9, Triose phosphate isomerase; #10, Aldolase, #11, Phosphoglucose isomerase: #12, Fructose-1, 6-bisphosphatase; and #13, Hydrogenase.

Figure 2. Hydrogen production from either 2 mM G-6-P or 2 mM starch (glucose equivalent).

The reaction based on G-6-P contained the pentose phosphate cycle enzymes (#3-12, 1 unit each), ∼70 units of P. furiosus hydrogenase (#13), 0.5 mM thiamine pyrophosphate, 2 mM NADP⁺, 10 mM MgCl₂, and 0.5 mM MnCl₂ in 2.0 ml of 0.1 M HEPES buffer (pH 7.5), at 30°C. The reaction based on starch rather than G-6-P was supplemented by 10 units of α-glucan phosphorylase (#1), 10 units of phosphoglucomutase (#2), and 4 mM phosphate at 30°C.

Figure 3. Carbon dioxide production from either 2 mM G-6-P or 2 mM starch (glucose equivalent).

The experimental conditions were the same as those in Figure 2.

Figure 4. An energy diagram showing the standard enthalpy (ΔH°) and free energy changes (ΔG°) in kJ/mol for the reactions in a renewable energy cycle operating among H₂O, CO₂, glucose, and starch.

Figure 5. The hydrogen cell system configured for monitoring H₂ with the ORNL in-house sensor based on the Figaro TGS 822 and O₂ with a modified Hersh galvanic cell .

The CO₂ analyzer (not shown) is attached between the reaction cell and the electrolysis cell.