Bioinduced Room‐Temperature Methanol Reforming

Abstract

Imitating nature'sapproach in nucleophile-activated formaldehyde dehydrogenation, air-stable ruthenium complexes proved to be exquisite catalysts for the dehydrogenation of formaldehyde hydrate as well as for the transfer hydrogenation to unsaturated organic substrates at loadings as low as 0.5 mol %. Concatenation of the chemical hydrogen-fixation route with an oxidase-mediated activation of methanol gives an artificial methylotrophic in vitro metabolism providing methanol-derived reduction equivalents for synthetic hydrogenation purposes.M oreover,f or the first time methanol reforming at room temperature was achieved on the basis of this bioinduced dehydrogenation path delivering hydrogen gas from aqueous methanol.Inlight of anthropogenic global warming and the depletion of ecologically and economically reasonable sources for fossil fuels,inrecent years,one-carbon molecules have emerged as highly attractive renewable hydrogen carriers. [1][2][3][4][5][6][7][8][9][10][11] Owing to difficulties in the selective activation of methane at low temperatures and the rather poor energy balance of formic acid, methanol is currently discussed as most promising C 1 unit.[12] Combining ahigh hydrogen content (12.5 wt %) with the convenience of al iquid fuel, catalytic methods for the dehydrogenation of methanol as well as its production from CO 2 and H 2 have attracted major interest from the scientific community over the past decade to establish acarbon-neutral methanol economy.[13] However,d espite an umber of successful examples on the methanol-to-H 2 conversion, in particular this process is still in great need for further improvement. Herein we present ac onceptually unprecedented strategy for the hydrogen generation from aqueous methanol based on am ulticatalytic system implementing enzyme catalysis as new player in the field.[14] Theinterplay of methanol-activating biocatalysts with H 2 -liberating metal complexes is opening up new opportunities en route to ambient-temperature hydrogen-production systems or enzyme-driven hydrogen fuel cells.In contrast to the recent history of ex vivo approaches utilizing the C 1 feedstock, millions of years of evolution have provided solutions to the problems of the activation and use of organic one-carbon entities by creating well-defined lowtemperature pathways for methane-or methanol-feeding organisms. [15,16] Interestingly,a so pposed to the chemical objective aiming for full dehydrogenation processes (e.g. aqueous methanol yielding the maximum three equivalents of H 2 ), in aerobic methylotrophic yeasts and bacteria, formaldehyde is found to play acentral role in the cells metabolism. Once formed by oxidase/catalase-catalyzed formal oxygenation of methanol, formaldehyde hydrate serves both as source of reduction equivalents,such as NAD(P)H, as well as the actual carbon feedstock for the generation of carbohydrates.[17] Dehydrogenation generally proceeds by nucleophilic activation of formaldehyde through enzyme cofactors, such as pterins (e.g. tetrahydrofolate) or me...

Future perspectives for formaldehyde: pathways for reductive synthesis and energy storage

“…In vast contrast, many reports Please do not adjust margins Please do not adjust margins on CO2 hydrogenation to formic acid and methanol are available, likewise reports where methanol and formic acid are described as hydrogen source. 1, 14,15,34,35,37,[41][42][43][44][45][46][47][48][49][50][51] However, there are reports of trapping formaldehyde in form of derivatives such as acetals. An overview about those chemical transformations is given in another section of this article.…”

“…formaldehyde proceeds under mild conditions in a temperature range between room temperature and 95°C using a molecular ruthenium catalyst under base-free, respectively pH neutral conditions, without any preventions to exclude air. 1, 14,15 As catalyst precursors a wide range of [Ru(arene)Cl2]2 complexes are suitable (arene = benzene, toluene, p-cymene, etc.). The catalyst is in situ transformed into a cationic ruthenium hydride formiato species [Ru(arene)(µ-H)µ-Cl(µ-HCO2)Ru(arene] + X -(X: Cl, BF4) which plays a major role in the catalytic cycle.…”

“…1, 14,15 The pathways in green are realized and the pathway in red remains the missing puzzle piece in this network.…”

“…1, 14,15,22 New formaldehyde production methods are also required to feed the potential future application in the energy sector in the large scale because the currently produced formaldehyde is already reserved for the established synthetic applications. Likewise the current annual global methanol production capacity (~110 Mt) cannot serve solely the energy sector owing the requirements for methanol (~80-90 Mt per year) in the chemical and pharmaceutical industries.…”

Homogeneously catalysed conversion of aqueous formaldehyde to H2 and carbonate

“…Maitlis and coworkers have reported the conversion of acetaldehyde to acetic acid promoted by half-sandwich complexes of Ru25. More recently, Prechtl et al 26. described the dehydrogenation of aqueous methanol to formaldehyde under the release of H 2 under mild conditions (298–368 K).…”