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

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.

Growing concerns over the depletion of fossil fuels and anthropogenic global warming has led to the search for alternative renewable energy resources. Molecular H2 is considered as one of the perfect choices to act as a future energy source because of its high energy density and environmentally benign properties.1 Although H2 is a promising energy source, the gaseous nature of H2 makes it difficult to store, transport, and use in mobile applications.1 H2 can be stored by physical adsorption on some specific materials, by chemical bonding, or in a complexed form that is incorporated into small molecules.1a, 1c, 2 Chemical storage of H2 in liquid materials gives several advantages over other hydrogen storage materials, such as a high H2 storage capacity and easy handling and transportation with the existing infrastructures used for gasoline and diesel.2a, 3 It is necessary that both the process of releasing storage of H2 should occur at mild temperature for the reduction of material losses and energy consumption during the reactions.2c, 3 Releasing compressed H2 from a storage material for mobile applications, such as transportation, is a challenging issue because of space limitations, as typically a large volume system is required. H2‐fueled vehicles currently use H2 in the gaseous form from high‐pressure H2 gas tanks that they carry.1a, 1c, 2b The generation of high‐pressure H2 consumes a large amount of energy during compression, which corresponds to approximately 10–15 % of the H2 energy content.1a, 2b The energy consumed during the compression of H2 could be reduced by generating high‐pressure H2 through a chemical reaction.

Recently, formic acid (FA) has attracted considerable attention as a liquid hydrogen storage material because it is stable, moderately flammable, and readily biodegradable under ambient conditions.4 FA contains a relatively high content of H2, and the low reaction enthalpy permits the release of H2 at mild temperature2c, 4e through dehydrogenation, which is a thermodynamically favorable process.4e CO2, a co‐product of FA dehydrogenation, can also be converted to FA by photo‐ or electro‐chemical reduction in the presence of catalysts.4a–4d FA has been recognized as a H2 storage material since 1978.5 However, the development of a suitable process for the generation of H2 gas from FA has progressed slowly because of the requirement of severe reaction conditions, low product selectivity, and regeneration of the catalyst.2c In addition, the occurrence of CO as a by‐product deactivates the catalyst and hampers the application of the generated H2 in fuel cells.6

In the presence of homogeneous catalysts, FA can be decomposed selectively at mild temperature. Various catalysts have been developed for the selective decomposition of FA to produce H2 with a high rate at temperatures of less than 100 °C.7 Many researchers investigated the catalytic dehydrogenation of FA under atmospheric pressure conditions, which releases high‐pressure H2 gas, and although the process is energy‐efficient, it faces the problem of H2 separation. To date, there have been very few reports of the dehydrogenation of FA under high‐pressure conditions, above 10 MPa,8 because the catalyst must be capable of withstanding severe reaction conditions, especially high pressures and high concentrations of FA over long time periods. In practical applications of FA as a H2 carrier, a high concentration of FA is generally used for the fast and efficient production of large amounts of high‐pressure H2.

Recently, we developed water‐soluble Ir catalysts for fast and selective FA decomposition under mild temperatures.9 The introduction of hydroxyl groups into a bipyridine ligand activated pentamethylcyclopentadienyl Ir (Cp*Ir) complexes toward FA dehydrogenation.9a, 9e, 9f We also reported that the Cp*Ir complex containing 4,4′‐dihydroxy‐2,2′‐bipyridine (4DHBP, catalyst 1 in Figure 1) catalyzed selective FA decomposition at the high pressure of 123 MPa.10 However, the main drawback is the separation and consequent recycling of the catalyst. Even though catalyst 1 has a long lifetime of over 33 h with turnover number (TON) of 100 000 and turnover frequency (TOF) of 3100 h−1 at 60 °C under atmospheric pressure,9a both the TON and TOF decreased to 38 100 and 2510 h−1, respectively, under the high‐pressure conditions of 30 MPa and 80 °C owing to the deactivation of the catalyst (Figure S1 in the Supporting Information). As the gas pressure increases owing to the decomposition of FA, the catalyst undergoes partial hydrogenolysis as a result of the presence of high‐pressure H2 in the system, resulting in a change to an insoluble compound, which then precipitates after the reaction (Scheme S1).11 We predicted that the bipyridine ligand might be changing from its chelating conformation in 1 to another conformation under the high‐pressure H2 conditions (Figure S2 in supporting information). Thus, after precipitation, the catalyst loses its activity towards decomposition of FA.

Figure 1

Cp*Ir complexes for the development of a FA recycling system.

In this work, we have developed an effective catalyst for the dehydrogenation of FA under high‐pressure conditions, which has a long lifetime and that can be recycled several times. Here, we introduce an Ir catalyst containing 1,10‐phenanthroline‐4,7‐diol (catalyst 2) as a chelating ligand, which prevents cis/trans isomerization of the pyridine skeleton by bridging, and investigate its potential in terms of catalytic activity, durability, and reusability.

The Ir catalysts 1 and 2 were synthesized as reported in the literature.12 First, we investigated the FA dehydrogenation using catalyst 2. When we tested the catalyst durability with 1 μmol of catalyst under atmospheric pressure using 10 m of FA, catalyst 2 continued the dehydrogenation of FA for 2600 h (about 3.5 months), and almost 100 % of FA was transformed to H2 and CO2 (Figure 2). The evolved gas volumes (H2 and CO2) increased linearly and the evolution rate was in the region of 0.11–0.12 mL h−1 for the first 1000 h. The calculated TON value of catalyst 2 was 5 000 000. Table 1 compares the results of the dehydrogenation of FA with catalysts 2 and 1. The obtained TOF value of catalyst 2 was 3010 h−1 (Table 1, entry 2), which is superior to that of catalyst 1 (Table 1, entry 1), and CO was below the detection limit (<5 ppm, Figure S3). With increasing concentration of FA, the TOF reaches its maximum between 3 and 6 m (Table 1, entries 2‐6). Interestingly, the TOF decreased when 80 % of highly concentrated FA was used, but the catalyst maintained its activity during the reaction. Moreover, the TON increased to 203 000 (Table 1, entry 6). With increasing reaction temperature, a high TOF of 62 900 h−1 and high TON of 320 000 were obtained at 98.8 °C. Hence, catalyst 2 shows comparable activity with catalyst 1 for the dehydrogenation of FA under atmospheric pressure, as well as good durability, even from a highly‐concentrated FA solution.

Figure 2

The time courses of the volume of evolved gases/rate of evolution of gases from FA decomposition in 10 mol L−1 of FA aqueous solution (500 mL) at 60 °C, catalyzed by catalyst 2 (1 μmol) in a glass autoclave [black line: volume of evolved gas (L), red cross: gas evolution rate (L h−1)].

Table 1

Dehydrogenation of FA under atmospheric pressure conditions.[a]

Entry	Cat.	Catal. Conc. [μM]	FA Conc. [M]	Bath temp. [°C]	React. time [h]	TOF [h−1][b]	TON
1	1	100	1	60	7	2020	10 000
2	2	100	1	60	5	3010	10 000
3	2	100	3	60	12	3640	30 000
4	2	100	6	60	20	3220	60 000
5	2	100	12.9[d]	60	45	2480	129 000
6	2	100	20.3[e]	60	178	910	203 000
7	2	25[c]	4	79.8[f]	12	17 000	160 000
8	2	25[c]	4	89.8[f]	7	33 800	160 000
9	2	12.5[c]	4	98.8[f]	6.5	62 900	320 000

[a] The reaction was carried out in degassed aqueous FA solution (20 mL) until gas evolution ceased; the volume of the evolved gas was temperature‐corrected. [b] The TOF was determined 30 min after beginning the reaction. [c] The reaction was carried out in a degassed aqueous 4 m FA solution (40 mL). [d] 50 wt % FA. [e] 80 wt % FA. [f] Temperature of the reaction solution was measured by a thermocouple probe.

We further evaluated the catalyst durability under highpressure conditions. Previously, we reported that catalyst 1 could generate high‐pressure gas over 100 MPa by FA dehydrogenation, but the catalytic activity gradually decreased as the reaction progressed. Catalyst 2 can also produce high‐pressure gas at 110 MPa due to the dehydrogenation of FA from a 16 m FA solution (Figure 3). It was observed that the rate of increase of pressure was comparatively faster than that of catalyst 1 (Figure S4).

Figure 3

Time course of high‐pressure evolution of gases from FA decomposition in the presence of catalyst 1 (black cross) and 2 (red circle). Reaction conditions: autoclave: 24 mL internal volume, FA aqueous solution: 16 mol L−1, 13 mL, catalyst: 2.0 mmol l −1, reaction temperature: 80 °C.

To compare the durability of catalysts 1 and 2, we recycled both of the catalysts under 22 MPa gas evolution conditions (Figure 4) by simply removing the aqueous FA under reduced pressure after the reaction. When catalyst 2 was used, the gas evolution rate remained practically unaltered in the 1st and 2nd runs. However, by the 4th run, it had decreased to 1/3 of that of the 1st run. The rate in the case of catalyst 2 was much faster (initial rate of 1.3 MPa h−1) than that of catalyst 1 (<0.1 MPa h−1). Interestingly, after the reaction, catalyst 1 was completely dissolved in aqueous solution, whereas catalyst 2 had precipitated (Figure 5 c). The precipitate could be easily separated and recovered by filtration. The structural stability of the precipitated catalyst 2 was confirmed by NMR (Figure S5). After cooling down the system to 4 °C, catalyst 2 was filtered and the Ir complex remaining in the filtrate was less than 6 mol % of the initial catalyst loading [31 ppm by inductively coupled plasma‐atomic emission spectroscopy (ICP‐AES) analysis]. Catalyst 2 in each of its conformations has a pH‐dependent solubility in aqueous medium.12a We observed that before the reaction, the pH of the 6.5 mol L−1 FA solution was 0.9, which changed to a pH of 1.9 after the decomposition of FA at 22 MPa (0.65 mol L−1). Thus, the catalyst precipitated as the reaction progresses owing to the change in pH of the system (Figure 4 and S6). As a result, catalyst 2 spontaneously precipitated and 94 mol % was recovered from the reactant without further pH adjustment.

Figure 4

Time course of high‐pressure evolution of gas from FA in the presence of catalyst (a) 1 and (b) 2. The reaction was carried out at 60 °C in an autoclave (internal volume is 7 mL) with FA aqueous solution (7 mol L−1, 4 mL) and catalyst (0.1 mmol l −1). The catalyst was recycled 4 times (red: 1st run, black: 2nd run, blue: 4th run) without any purification.

Figure 5

Images of the reactant (catalyst 2: 8 μmol, water: 3 mL, FA (100 %): 1 mL) during the reaction at different stages: (a) before the reaction at RT (20 °C), pH 6.8; (b) during reaction after addition of FA at 50 °C under high‐pressure condition (22 MPa), pH 0.9; and (c) after the reaction and precipitation of catalyst after cooling down to RT (20 °C), pH 1.9.

Recycling of catalyst 2 was conducted for high‐pressure dehydrogenation of FA at a pressure above 22 MPa (Figure 6). Catalyst 2 can be successfully recycled over 10 times while maintaining its activity, and it has a durability of over 200 h of total reaction time under 22 MPa. In each experiment, all the evolved gases were confirmed as H2 and CO2 without any detectable amount of CO (<6 ppm, Figure S7). Catalyst 2 is homogeneous but can be recycled like a heterogeneous catalyst many times under high‐pressure conditions.

Figure 6

Recycling experiments of high‐pressure gas evolution from FA in the presence of catalyst 2. The catalyst was recycled after the former reaction without any purification except filtration. Reaction conditions: 50 °C, 2 MPa He, FA aqueous solution (7 mol L−1, 4 mL), catalyst (2 mmol l −1, 8 μmol). FA was added after depressurization (1 mL). The upper horizontal axis represents times of high‐pressure gas release.

In conclusion, we have developed an Ir catalyst (catalyst 2) bearing the 1,10‐phenanthroline‐4,7‐diol ligand, which can be used for the generation of high‐pressure hydrogen from FA with an obtained highest TON value of 5 000 000. It can be successfully recycled over 10 times without losing any catalytic activity. Catalyst 1 with a 4,4′‐dihydroxy bipyridine ligand has the ability to generate high‐pressure gas from FA aqueous solution effectively, but it is not suitable for the development of a high‐pressure H2 evolution system for practical use. The developed catalyst 2 has comparable activity with the catalyst 1 with the advantage that after the reaction, it can be easily separated owing to its pH‐dependent solubility properties. This work will be extended to develop a high‐pressure H2 evolution system from highly‐concentrated FA and to confirm the practical application of FA as a H2 storage material.

Experimental Section

FA (>99.0 %) was used as received from Wako Pure Chemical Industries, Ltd. Deionized water was prepared through a filtration system (EMD Millipore Corp., ZFSQ240P4) and distillation system (Toyo Roshi Kaisha, Ltd., GS‐590). All liquid reagents were degassed to remove air by bubbling helium gas (He, >99.995 %) before use. The Ir catalysts, [Cp*Ir(4DHBP)(H2O)][SO4] (1) and [Cp*Ir(DHPT)(H2O)][SO4] (2) were synthesized according to the literature.12 A GC‐μTCD system (Agilent Technologies, 3000A Micro GC) was used to determine the concentrations of H2, CO2, and CO in the released gas. The concentration of FA in the reaction solution was measured by a HPLC‐UV system on an ion exclusion column (Showa Denko K. K., KC‐811) with phosphoric acid aqueous solution. The pH value of the reaction solution was determined with a glass electrode (DKK‐TOA Corp., HM‐25R). The chemical structures of the catalysts were analyzed by 1H NMR in [D6]DMSO (Bruker Corp., AVANCE III 400). The catalyst concentration in solution after the reaction was monitored by inductively coupled plasma atomic emission spectroscopy (ICP‐AES, SPS3100, SII Nano Technology Inc.).

A reactor (7–24 mL) was equipped with a pressure transducer (Kyowa Electronic Instruments Co., Ltd., PGM‐500 KE or PG‐2TH) and a stop valve. In a typical experiment, the catalyst aqueous solution and FA were loaded into the reactor at room temperature. After purging air in the reactor with He, the reactor was pressurized to the desired pressure with He and then heated to the desired temperature. When the reaction was completed, the reactor was cooled and then depressurized to atmospheric pressure. The released gas was collected during depressurization.

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Supplementary

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