About this Research Topic
In view of the rise in frequency of meteorological extremes and related natural disasters in the Third Millenium, there can be little doubt that anthropogenic climate change or “global warming”, driven mainly by accumulating levels of atmospheric carbon dioxide, could lead to dire consequences to civilized life. Substitution of fossil fuels by sustainable chemical alternatives (whose use is carbon-neutral) on a global scale has never been more urgent. Pure hydrogen generated from water electrolysis by renewable primary sources like solar-, wind- and hydro-power would be an ideal transportation fuel if its long-term storage in energy-dense form ever becomes viable. Alternative research started in the early 1990s into hybrid energy cycles based on compounds that effectively couple the advantage of clean-burning hydrogen with the high energy density, efficient storage and handling of conventional carbon-based fuels. All hydrocarbons are effectively carriers of incipient hydrogen but their use is only carbon-neutral provided they are derived from sustainable biomass, where Nature closes the carbon cycle by the ingenious process of photosynthesis. Unfortunately, their direct combustion also generates a range of pollutants such that various forms of pre- or post-processing are mandatory. Simple oxygenates, such as methanol, ethanol, formic acid, etc., occupy a unique position because, as compared to their hydrocarbon analogues, they are much more readily transformed by catalytic steam reforming (CSR) releasing hydrogen:
CnHmOk + (2n − k) H2O → n CO2 + (2n + m⁄2 − k) H2
CSR is a highly endothermic process driven by the entropy increase. In creating H2-rich gas mixtures, not only is there a substantial gain in calorific value (exergy), but the emission of pollutants post-combustion is drastically reduced. Dumesic and colleagues have shown that it even works on aqueous extracts of polyols and carbohydrates. CSR is also amenable to both thermal- and photo-catalytic activation. Closure of the cycle by oxygenate synthesis (the reverse of the equation above) faces fundamental and technical hurdles related to catalyst performance. Methanol is one notable exception, as its synthesis from “renewable syngas” proceeds under kinetic control – one basis for Olah’s visionary Methanol Economy. Mechanistically, C-C bond activation in CSR and uncontrollable chain growth in the synthesis are obstacles in energy cycles based on specific oxygenates of carbon number > 1, for example ethanol, ethylene glycol, glycerol, etc.
In this cross-cutting Research Topic focused on carbon-neutral oxygenates as renewable hydrogen carriers, the Topic Editors invite submissions on recent advances in:
• Fundamental and applied catalysis (thermal-, photo-, electro-) and reaction engineering in:
- steam reforming of oxygenates (H2 release)
- oxygenates synthesis by CO2 hydrogenation (including electrochemical CO2 reduction).
• Schemes (actual or virtual) describing the techno-economics and logistics of:
- sourcing renewable (solar, electrolytic, etc.) hydrogen from water, and
- potential synergies in link-up with biorefineries based on fermentation ethanol.
• CO2 recycling from other industrial sources (power station flue gas, etc.) and/or air capture for supply to dedicated synfuel installations.
In steam reforming, studies in combined photo-/thermal-activation, stationary and on-board reformers linked to fuel cells, and novel upgrading processes (for example, conversion of glycerol to methanol or formic acid) are especially welcome.
Keywords: oxygenates, carbon-neutral, hydrogen energy, steam reforming, CO2 hydrogenation
Important Note: All contributions to this Research Topic must be within the scope of the section and journal to which they are submitted, as defined in their mission statements. Frontiers reserves the right to guide an out-of-scope manuscript to a more suitable section or journal at any stage of peer review.