Facile reversible hydrogenation of a poly(6‐vinyl‐2,3‐dimethyl‐1,2,3,4‐tetrahydroquinoxaline) gel‐like solid

Abstract

Polymers bearing hydrogen storage units store hydrogen gas via chemical bond formation, and have inherent advantages as quasi‐solid polymeric hydrogen carriers, such as handling easiness and high safety. A recent study demonstrated that a poly(6‐vinyl‐1,2,3,4‐tetrahydroquinoxaline) gel‐like solid was dehydrogenated over 5 h under 120°C and air, and then the material was reversibly hydrogenated under 60°C and ambient hydrogen pressure in the presence of an iridium complex catalyst. The present work aimed to improve the dehydrogenation rate via the substitution of methyl groups for hydrogen atoms on carbon atoms adjacent to the nitrogen atoms in the quinoxaline unit. The dramatic improvement was attributed to the electron donating property of methyl group, and stabilization of the dehydrogenated state. The poly(6‐vinyl‐2,3‐dimethyl‐1,2,3,4‐tetrahydroquinoxaline) gel‐like solid was rapidly dehydrogenated within 2 h under the same mild conditions. This article provides a guideline for the molecular design of nitrogen heterocyclic compounds for rapid dehydrogenation.

Toward a Hydrogen Economy: Development of Heterogeneous Catalysts for Chemical Hydrogen Storage and Release Reactions

“…Typically, the substrates represent stable, non-flammable, high-boiling compounds with higher molecular weights that are able to take up and release more hydrogen molecules . Hydrogen storage and release systems based on the interconversion of N -heterocyclic aromatic compounds and their hydrogenated/saturated counterparts have been evaluated with a variety of homogeneous organometallic catalysts, e.g., Ir, − Fe, , Co, Mn (quinolines), and Ir (pyrazine), and photoredox Ru complexes with Co or Ir co-catalysts (quinolines) . It should be noted that the oxidative dehydrogenation of N -heterocycles, for example under aerobic conditions, − is not considered as a hydrogen production process, since no hydrogen molecule could be delivered.…”

Organic redox polymers as electrochemical energy materials

“…The polymer gel quickly releases two equivalents of hydrogen gas (within 2 h) under the same mild conditions (entry 3, Table 2). 129,130 Tetrahydro-/quinaldine (hydrogen density: 2.7 wt%) was introduced into poly(acrylic acid); its Ircatalyst-containing hydrogel evolved hydrogen gas at a rate of 160 mL g( polymer) −1 by warming at 80 °C for 3 h. 131 In addition to aromatic compounds, many organic liquids have been investigated for their abilities to reversibly hydrogenate. 125,132,133 Among them, aliphatic secondary alcohols are relatively easily dehydrogenated due to smaller enthalpy changes associated with dehydrogenation than those of other organic liquids (despite exhibiting larger changes than those of the aromatic compounds described above).…”

“…The polymer gel quickly releases two equivalents of hydrogen gas (within 2 h) under the same mild conditions (entry 3, Table 2). 129,130 Tetrahydro-/quinaldine (hydrogen density: 2.7 wt%) was introduced into poly(acrylic acid); its Ir-catalyst-containing hydrogel evolved hydrogen gas at a rate of 160 mL g(polymer) −1 by warming at 80 °C for 3 h. 131…”

A Bird’s-Eye View on Polymer-Based Hydrogen Carriers for Mobile Applications

“…Both the hydrogen absorption and release procedures were conducted in the presence of the same iridium complex as a catalyst. An increase in the number of nitrogen elements in the aromatic parts could increase the absorption capacity of such polymers [ 42 ]. The calculated H 2 uptake in this polymer was up to 2.6 wt% (related to the repeating unit), which is a fairly high number for a chemical solid-state hydrogen carrier so far.…”