Masayuki Iguchi

Carbon Dioxide to Methanol: The Aqueous Catalytic Way at Room Temperature

Carbon dioxide may constitute a source of chemicals and fuels if efficient and renewable processes are developed that directly utilize it as feedstock. Two of its reduction products are formic acid and methanol, which have also been proposed as liquid organic chemical carriers in sustainable hydrogen storage. Here we report that both the hydrogenation of carbon dioxide to formic acid and the disproportionation of formic acid into methanol can be realized at ambient temperature and in aqueous, acidic solution, with an iridium catalyst. The formic acid yield is maximized in water without additives, while acidification results in complete (98 %) and selective (96 %) formic acid disproportionation into methanol. These promising features in combination with the low reaction temperatures and the absence of organic solvents and additives are relevant for a sustainable hydrogen/methanol economy.

Simple Continuous High‐Pressure Hydrogen Production and Separation System from Formic Acid under Mild Temperatures

A simple and continuous high‐pressure (>120 MPa) hydrogen production system was developed by the selective decomposition of formic acid at 80 °C using an iridium complex as a catalyst, with a view to its application in future hydrogen fuel filling stations. The system is devoid of any compressing system. The described method can provide high‐pressure H2 with 85 % purity after applying an effective gas–liquid separation process to separate the generated gas obtained from the decomposition of formic acid (H2/CO2=1:1). The efficiency of the catalyst lies with its high turnover frequency (1800 h−1 at 40 MPa) to produce high‐pressure H2 with a good lifetime of >40 h. Interestingly, only very low levels carbon monoxide (less than 6 vol ppm) were detected in the generated gas, even at 120 MPa.

Development of an Iridium-Based Catalyst for High-Pressure Evolution of Hydrogen from Formic Acid

Abstract A highly efficient and recyclable Ir catalyst bearing a 4,7‐dihydroxy‐1,10‐phenanthroline ligand was developed for the evolution of high‐pressure H2 gas (>100 MPa), and a large amount of atmospheric pressure H2 gas (>120 L), over a long term (3.5 months). The reaction proceeds through the dehydrogenation of highly concentrated aqueous formic acid (FA, 40 vol %, 10 mol L−1) at 80 °C using 1 μmol of catalyst, and a turnover number (TON) of 5 000 000 was calculated. The Ir catalyst precipitated after the reaction owing to its pH‐dependent solubility in water, and 94 mol % was recovered by filtration. Thus, it can be treated and recycled like a heterogeneous catalyst. The catalyst was successfully recycled over 10 times for highpressure FA dehydrogenation at 22 MPa without any treatment or purification.

Aqueous phase homogeneous formic acid disproportionation into methanol

The catalytic activity of a homogeneous iridium complex in formic acid disproportionation into methanol was explored. Formic acid reduction to methanol proceeds efficiently in aqueous media with the methanol yield depending on the nature of the solvent, substrate concentration, applied H2 pressure and reaction temperature. The methanol yield peaked at 75% when D2O was used as a solvent at 50 °C. Increasing the reaction temperature to 80 °C and doubling the substrate concentration led to an improved methanol concentration, even though the corresponding yield dropped to 59%. Initial H2 pressures further enhanced methanol formation, affording a highly concentrated 9.8 m methanol solution or a TON of 1260 upon in situ catalyst recycling under aerobic conditions. No catalyst deactivation was observed for five cycles.

Automatic high-pressure hydrogen generation from formic acid in the presence of nano-Pd heterogeneous catalysts at mild temperatures

High-pressure hydrogen is of great interest in the industrial utilization of hydrogen energy, especially for hydrogen fuel cell vehicles. In this work, a method of automatic high-pressure H2 generation by the decomposition of formic acid, a recently renowned hydrogen storage material, in the presence of a heterogeneous catalyst (palladium nano-particles on a graphene oxide catalyst (Pd/PDA–rGO)) was proposed. This catalyst can effectively catalyze the decomposition of formic acid to produce high-pressure H2 and CO2 over 35 MPa without any detectable formation of CO or other by-products. For example, a 36.3 MPa total gas pressure was successfully achieved using an aqueous solution of 6.7 mol L−1 formic acid and 6.7 mol L−1 sodium formate at 80 °C. This research provided a preliminary study on the automatic high-pressure hydrogen gas generation by the decomposition of formic acid without any compression facilities in the presence of a heterogeneous catalyst, which can be easily separated from the reaction process, for hydrogen energy utilization.

Sequential hydrogen production system from formic acid and H2/CO2 separation under high-pressure conditions

Hydrogen (H2) production from formic acid (FA) is highly attractive as a sustainable energy source from the interconversion between CO2 and FA. Dehydrogenation of FA at high pressures has advantages over a reaction at atmospheric conditions for the separation of H2 and CO2 due to the reaction and the volumetric energy density of H2. We demonstrated the continuous production of high-pressure H2 by catalytic decomposition of FA, and subsequent separation of H2 and CO2 from FA decomposition gas (H2 : CO2 = 1 : 1) using the phase change phenomenon at low temperatures while maintaining high pressure. An iridium aqua complex coordinated with a bidentate pyridyl-imidazoline ligand catalyzed the dehydrogenation of FA with high efficiency at a pressure as high as 153 MPa. The Ir catalyst was found to be stable under continuous addition of neat FA at high pressures. The generation time and rate of high-pressure H2 were controlled by feeding neat FA to the aqueous reaction system. Using our combined system, more than 99 mol% of H2 (96 mol% of purity) and 94 mol% of CO2 (99 mol% of purity) were separately obtained from FA as a gas and liquid, respectively, under the high-pressure conditions without any mechanical compression.