Exploring the Myth of Nascent Hydrogen and its Implications for Biomass Conversions

Abstract

Iron (and to a lesser extent manganese) in the wall of a 316 stainless steel (SS) reactor is responsible for the hydrogenation of cyclohexanone to cyclohexanol when using an aqueous formic acid solution under high temperature and pressure water (HTPW) conditions. However, not only dilute formic acid but also aqueous solutions of several other organic and mineral acids in the presence of iron are active in this reaction covering a range of aldehydes and ketones, even under ambient conditions. The stoichiometry, kinetics, and the possible mechanisms of both dihydrogen production as well as of the hydrogenation of the model compound cyclohexanone were examined. The reduction is essentially stoichiometric with respect to metallic iron, and the conversions are highly dependent on the speed of stirring as well as temperature and reactant concentrations. Importantly, it is established unequivocally that water participates in dihydrogen gas formation (hydrogen atoms originate from both the acid and water molecules) and facilitates substrate reduction.

Mechanism of the Fe-Assisted Hydrothermal Liquefaction of Lignocellulosic Biomass

“…In our previous work, we confirmed that pyruvaldehyde could be hydrogenated under hydrothermal condition in the presence of Fe to produce hydroxyacetone . According to the literature, − there are a few possible hydrogenation pathways using zerovalent metal in water (Scheme ). Pathway (i) is initiated by the attack of a highly active hydrogen radical, generated from metallic Fe and a proton, on the carbonyl group.…”

“…Pathway (iii) involves single-electron transfer from metallic Fe to the substrate as the key step. The hydrogen radical in pathway (i) was originally assigned as the so-called “nascent hydrogen”; however, its existence was rejected by several succeeding studies. , Fábos et al proposed pathway (ii) as an alternate explanation for the nascent hydrogen in their work on the mechanism of hydrogenation using metallic Fe and acids . The most general explanation for hydrogenation reactions using zerovalent metals is pathway (iii), which is analogous to the Bouveault–Blanc reduction in which esters are converted into alcohols by metallic Na. , Reduction of aldehydes had also been achieved using Fe, Zn, Mg, Al, and Mn as a reductant. − A mechanism has been proposed in which aldehydes adsorbed on the surface of Fe are reduced by single-electron transfer from Fe to form radical intermediates .…”

“…In our previous work, we confirmed that pyruvaldehyde could be hydrogenated under hydrothermal condition in the presence of Fe to produce hydroxyacetone. 13 According to the literature, [29][30][31] there are a few possible hydrogenation pathways using zero-valent metal in water (Scheme 1). Pathway (i)…”

“…29,30 Fabos et al proposed pathway (ii) as an alternate explanation for the nascent hydrogen in their work on the mechanism of hydrogenation using metallic Fe and acids. 31 The most general explanation for hydrogenation reactions using zerovalent metals is pathway (iii), which is analogous to the Bouveault−Blanc reduction in which esters are converted into alcohols by metallic Na. 32,33 Reduction of aldehydes had also been achieved using Fe, Zn, Mg, Al, and Mn as a reductant.…”

Role of geochemical protoenzymes (geozymes) in primordial metabolism: specific abiotic hydride transfer by metals to the biological redox cofactor NAD+

“…Fe and Co powder are able to produce nascent H 2 : freshly synthesised H 2 from two hydrogen atoms. The intermediates of this reaction are surface‐bound hydrides [ 57 ] which could, in theory, be directly transferred to NAD + . It is also known that Fe readily produces H 2 from water under mild alkaline conditions [ 58 ].…”