In hot water heated up to 250 °C and 4 MPa, methanol and formic acid are produced from the Cannizzaro-type reaction of formaldehyde without a catalyst, although this disproportionation reaction is well-known to occur in the presence of a large amount of base catalysts in ambient conditions. Formic acid further undergoes the hydride transfer reaction with formaldehyde, and the final yield of methanol exceeds 50%.

Hydrothermal reaction of formic acid has been studied by varying the following conditions: temperature (275 to 350 °C), concentration of formic acid (0.1 to 1 M), addition of HCl, and presence of metal alloy powders. In hot water, formic acid decomposes to CO and CO2. The product ratio of CO to CO2 strongly depends on the conditions. At higher concentrations and lower temperatures, the yield of CO gets larger. In most of the conditions studied here, the main product is not CO2 but CO in contrast to previous works. Acid enhances the CO production and metal powders catalyze the decarboxylation pathway.

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New reaction mechanism has been found for the pyrolysis of anisole according to the total product analysis based on the liquid- and gas-phase 1H and 13C NMR spectroscopy. Formaldehyde and benzene are produced in the first rate-determining step by the intramolecular proton transfer, and phenol, methane, and carbon monoxide in equal amounts successively through the fast intermolecular proton and hydride transfers from hot formaldehyde to anisole.

It is important to investigate the kinetics and mechanisms of the thermal reaction process of ethers in order to understand more deeply high-temperature reactions and to develop more efficient high-temperature cracking and processing of fossil (oil and coal) and biomass fuels. In the previous letter, 1 we reported a kinetic study on the noncatalytic pyrolysis of the asymmetric ether, anisole (methyl phenyl ether), at 1 M and at 400430°C. By the product analysis with the gas-and liquid-phase NMR we have succeeded in revealing that the thermal fragmentation of the ether is induced by not homolytic (radical) but heterolytic (ionic) CH bond fission, and that the reaction consists of the slow and the fast elementary steps: (1) The slow step is the unimolecular CH bond fission that is initiated by the intramolecular proton transfer from the methyl to the phenyl group to generate the reactive intermediate formaldehyde (HCHO*). (2) The fast step is composed of the successive intermolecular proton and hydride transfers over the electronegative oxygen atom from the thermally excited intermediate HCHO*, respectively, to the phenoxy and methyl groups of the parent anisole molecule. They are expressed as:C 6 H 5 OCH 3 ! C 6 H 6 þ HCHO Ã ðslowÞ ð 1ÞAs represented here, the major products of the high-pressure (concentration) pyrolysis are benzene, phenol, methane, and carbon monoxide, all in almost equal amounts. The intramolecular proton transfer induced by the COC bending mode was also observed for acetaldehyde, 2 and dimethyl and diethyl ethers; This is called "hinge reaction" at high temperature. 3,4When the elementary steps assumed previously (see below) were the case, CO carbon should come from the methoxy but not from the phenoxy. To test this, here we have selectively labeled the asymmetric ether, anisole, by 13 C as C 6 H 5 O 13 CH 3 . In all of the earlier papers, 512 the following radical CO pathway was postulated:The homolytic bond breakage is assumed to take place between the ether oxygen (eq 3) and the methyl carbon to generate the methyl and phenoxy radicals, and furthermore, the ring-size reduction is presumed to give rise to CO as in eq 4. The thermal fragmentation of dimethyl ether, which is regarded as the prototype of the "unimolecular reaction," has been believed to have the radical mechanism since the pioneering work by Hinshelwood and co-workers in the 1920s. 13 The methoxy group is a key characteristic of these symmetric and asymmetric ethers, and the reactive intermediate formaldehyde* is expected to be generated for both cases. The asymmetric ether can be a promising candidate to distinguish the reaction pathways for the generated hydrocarbon fragments: benzene and methane for anisole and only methane twins for dimethyl ether. Convincing evidence is wanted to resolve the discrepancy in the mechanism of the fundamental thermal reaction. To this end we have scrutinized the CO generation pathway by applying highresolution NMR spectroscopy to the labeled anisole studied in the dark to carefully avoid the ...

Chemical reactions in super-and subcritical water have been studied in our group over the decades, and recent advances are reviewed. The reaction mechanism in hydrothermal conditions is disclosed for ether and aldehyde in general form, and a new type of C C bond formation is discovered in connection to the chemical evolution on primitive earth. Toward a new-generation hydrogen-fuel technology, it is proposed on the basis of physicochemical reaction properties of formic acid that formic acid acts as a chemical tank for hydrogen storage and transportation. The hot-water chemistry is further discussed in the contexts of energy and environmental concerns, and its role in establishing green chemistry is stressed.