Utilization of Carbon Dioxide as a Hydrogen Storage Material

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Chapter 9

Utilization of Carbon Dioxide as a Hydrogen Storage Material: Hydrogenation of Carbon Dioxide and Decomposition of Formic Acid Using Iridium Complex Catalysts Yuichiro Himeda* National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan *[email protected]

We demonstrate the utilization of carbon dioxide (CO2) as a hydrogen storage material using organometallic complex catalysts that strongly catalyze hydrogenation of CO2 under basic conditions and decomposition of formic acid under acidic conditions. For this purpose, a sophisticated catalyst was designed, in which a phenolic hydroxyl group was introduced to a ligand. This led to pronounced catalytic activation, and control of the catalytic activity and solubility. As a first step of the hydrogen storage system, CO2 hydrogenation was achieved by increasing the catalytic activity and recycling the catalyst without waste generation––a result of automatically tuning the activity and water solubility of the catalyst via the acid-base equilibrium of its ligand. The strong electron-donating ability of the oxyanion generated from the hydroxyl group resulted in remarkably high catalytic activity—greater than 1300 times that of the unsubstituted bipyridine complex. The catalyst design concept and excellent ligand properties are expected to have broad implications for designing new homogeneous catalysts. As a second step, decomposition of formic acid using the same catalyst provides an efficient evolution of CO-free hydrogen under acidic conditions. High catalytic activity and complete consumption of formic acid were realized in water, and pressurized hydrogen was also generated. The proposed system using the iridium catalyst is promising for practical © 2010 American Chemical Society In Advances in CO2 Conversion and Utilization; Hu, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.

applications in terms of both the hydrogenation of CO2 and the decomposition of formic acid.


Introduction Massive emissions of carbon dioxide (CO2) from the combustion of fossil fuels to supply energy are the most important cause of global warming. Plants utilize CO2 as recipients of the abundant energy generated by photosynthesis, during which CO2 is reduced. Unfortunately, a suitable method of storing energy (i.e., hydrogen) based on this the concept is currently unavailable. The development of technologies for efficiently storing hydrogen in a safe and reversible manner is an important challenge for realizing a hydrogen economy. At the present time, particularly intensive efforts are being devoted to low-cost, large-scale production of CO-free hydrogen for fuel cell applications. Formic acid, which contains 4.4 wt% and 53 g/L of hydrogen, is one of the promising hydrogen storage materials, because it has low toxicity and can be easily handled and stored. Therefore, both catalytic hydrogenation of CO2 and catalytic decomposition of formic acid have recently received renewed attention for use in a hydrogen storage system (Figure 1) (1, 2). However, since the catalysts for each reaction were independently developed, to our knowledge there is no known catalyst that shows high activity for both reactions (3–5). Herein we demonstrate the utilization of CO2 as a hydrogen storage material by combining hydrogenation of CO2 and decomposition of formic acid in water. One of the remarkable features of the system is that either of the two reactions can be selected by the pH of the solution, using the same iridium catalyst that hydrogenates CO2 under basic conditions and decomposes formic acid under acidic conditions. Using a sophisticated catalyst design, we achieve hydrogenation of CO2 by high catalytic activity and recycling of the catalyst without waste generation in water. Decomposition of formic acid provides efficient CO-free hydrogen evolution with complete conversion of formic acid. The system lays out many of the elements necessary for practical application.

Figure 1. Hydrogen storage system based on interconversion between CO2 and formic acid.

142 In Advances in CO2 Conversion and Utilization; Hu, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 2. Ideal homogeneous catalytic process

Hydrogenation of CO2 As a first step, the homogeneous hydrogenation of CO2 or bicarbonate is well studied, and several highly efficient catalysts have been reported. Detailed reviews of studies up to 2006 have been published (6–9). Recently, Nozaki reported a highly active catalyst for hydrogenation of CO2 in KOH/H2O/THF solution at 5 MPa and 200 °C (10). Peris reported an interesting study on transfer hydrogenation of CO2 to formate using iPrOH as a hydrogen donor catalyzed by iridium complexes (11). However, several critical problems for practical application remain to be solved. These include the following: (1) activation of the catalyst, because the high energy demand for converting thermodynamically stable CO2 generally lowers the overall energy balance; (2) recycling of catalysts which generally contain noble metals; (3) control of catalytic activity in both the reaction and separation steps to avoid potential reverse reactions; and (4) prevention of waste generation (generally an organic solvent and additive) during the reaction. The strategic goal to overcome these problems will be to design a catalyst that is homogeneous and active during the reaction step but is heterogeneous and inactive during the separation step (Figure 2).

Catalyst Design A half-sandwich bipyridine rhodium complex [Cp*Rh(bpy)Cl]+ (1), which serves as an efficient catalyst for transfer hydrogenation using formate as a hydrogen source in water (12, 13), was chosen as the prototype catalyst (Figure 3). Preliminary studies showed that 1 interconverted between CO2/H2 and formic acid (eqs. 1 and 2) (14). However, the abovementioned problems remained unresolved. Before designing the catalyst, it was first considered that CO2 reaches an equilibrium with bicarbonate and carbonate––one that is strongly dependent on the pH, temperature, and CO2 pressure (eq. 3). In addition, changes in the pH of the solution during the reaction (i.e., from basic to acidic) due to the consumption of bicarbonate and production of formic acid were considered. 143 In Advances in CO2 Conversion and Utilization; Hu, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


Figure 3. Prototype catalyst for hydrogenation of CO2 and decomposition of formic acid in water.

Scheme 1. Design of the catalyst for hydrogenation of CO2 in water. In light of these considerations, the design of the catalyst for hydrogenation of CO2 was elaborated via the route shown in Scheme 1. Namely, (i) catalyst activation was achieved by the introduction of a hydroxyl group to the bipyridine ligand, during which a strong electron-donating oxyanion generated by its deprotonation plays a significant role; and (ii) catalyst recycling was achieved by replacement of the bipyridine ligand with the phenanthroline analog, which shows negligible water solubility under weakly acidic conditions.



Catalyst Activation The first focus of the catalyst design is on the enhancement and control of the catalytic activity. The most important development in this regard is based on the acid-base equilibrium of the phenolic hydroxyl group in a bipyridine ligand (Figure 4). Namely, the hydroxyl group shows moderate electron-donating ability and polarity, while its deprotonated oxyanion shows strong electron-donating ability and polarity. Therefore, 4,4’-dihydroxy-2,2’-bipyridine (DHBP) is devised from bipyridine as a ligand. The acid-base equilibrium of DHBP leads to simultaneous changes in both the electronic and polar properties of the catalyst, which are attributed to the properties of the hydroxyl and oxyanion groups. In fact, the oxyanion at the beginning of the reaction will be protonated to generate the hydroxyl group at the end of the reaction, since the basic reaction solution changes to a weakly acidic one. Thus, the strong electron-donating ability of the oxyanion should lead to the activation of the catalyst, and the significant differences in electronic properties between the hydroxyl and the oxyanion groups should lead to the control over the catalytic properties. We examined the acid–base equilibrium of 2 (X = H2O), which has two types of proton-donating groups (i.e., two phenolic hydroxyl groups and an aqua ligand). Figure 5 shows the absorption spectra of 2 (X = H2O) as a function of pH (adjusted by H2SO4/KOH) (15). The percentage change in the optical density of 2 (X = H2O) at 265 nm plotted against the pH is shown in Figure 6. From our previous studies, the change from pH 3 to 6 can be attributed to the acid–base equilibrium of the hydroxyl groups in the bipyridine ligand. The change from pH 8 to 10 may be attributed to that of the aqua ligand. Thus, the complex appears to exist in a hydroxyl form at pH 2.6, in an oxyanion form at pH 7.3, and as a hydroxo complex at pH values above 10.

Figure 4. Acid-base equilibrium between hydroxyl and oxyanion forms.



Figure 5. Absorption spectra of 2 (X = H2O) (0.1 µM) in an aqueous solution as a function of pH. The values indicate the value of pH.

Figure 6. Absorbance change of 2 (X = H2O) at 265 nm as a function of pH. The electronic effect of the substituent on the hydrogenation of CO2 was then investigated using a series of iridium complexes (2 and 4a-d). The good correlation between the substituent (-R) and the initial TOF can be explained on the basis of Hammett’s rule (Figure 7). Note that the phenolic hydroxyl group 146 In Advances in CO2 Conversion and Utilization; Hu, Y.; ACS Symposium Series; American Chemical Society: Washington, DC, 2010.


(σp+ = –0.92) and the carboxyl group (+0.42) are deprotonated to generate the oxyanion (–2.30) and carboxylate (–0.02) under basic reaction conditions (16). The large σp+ value of –2.30 and the ρ value of –1.3 resulted in a remarkably high initial TOF for 2, which was over 1300 times greater than that of the unsubstituted complex 4a. It is clear that enhancement of catalytic activity is achieved by the strong electron-donating effect of the oxyanion generated by the deprotonation of the hydroxyl group (17). Table 1 summarizes the results for the iridium DHBP catalyst 2. High activity was obtained at 6 MPa and 120 °C (entry 1). Compared to the previously reported catalyst [RuCl2(tppms)2]2 (TOF = 30–50 h–1 at 1 MPa and 50 °C) (18), 2 showed far superior catalytic activity under the same conditions (entry 2). Furthermore, the hydrogenation of CO2 proceeded at atmospheric pressure (entry 3). The result shows that the corresponding hydride complexes can easily be generated as active species at atmospheric pressure. This is presumed by the observation that the color of the solution changed immediately from pale yellow to deep yellow when H2 was admitted (8). In addition, the control of the catalytic activity, which is attributed to the acidbase equilibrium, resulted in activation of the catalyst in the reaction step and its deactivation in the separation step, because of a change in the pH of the solution. Therefore, the undesirable reverse reaction, i.e., the decomposition of formic acid, was prevented in the separation step.

Figure 7. Hammett plot of initial TOF vs. σp+ value of substituent (R) in the catalyst (0.02–0.2 mM) at 80 °C in 1 M aqueous KOH solution at 4 MPa. 4a (R = H: σp+ = 0.0), 4b (R = CO2H: see text), 4c (R = Me: σp+ = –0.31), 4d (R = OMe: σp+ = –0.78)


Table 1. Hydrogenation of CO2 using iridium-DHBP catalyst 2a Entry

Conc. of 2 / µM

Temp. / °C

Time /h

P / MPa

1

0.5

120

57

6

42,000

190,000

2

20

50

30

1

820

9,500

3

200b

60

50

0.1

33

376

Initial TOF / h–1

TON

a


The reaction was carried out in an aqueous KOH solution (1 M) at the desired CO2/H2 (1:1) pressure. b The reaction was carried out in an aqueous K2CO3 solution (0.1 M).

Table 2. Batch-wise catalyst recycling in the conversion of CO2 into formate using iridium-DHPT catalyst 3a Cycle

Loaded/recovered iridium / ppm

Recovery Efficiency / %

Leaching iridium / ppmb

Final conc. of formate / Mc

0.11

0.105

1

9.0

2

8.4

93

0.22

0.104

3

7.7

92

0.42

0.103

4

7.0

91

0.61

0.103

a

The reaction was carried out using catalyst 3 (2.5 µmol) under 6 MPa of H2:CO2 (1:1) in 0.1 M KOH solution (50 mL) at 60 °C for 20 h. b The Ir concentration was measured by ICP-MS analysis. c No bicarbonate was detected in any of the four cycles.

Figure 8. Solubilities of 2 (open circles/dotted line) and 3 (filled circles/solid line) in a 1 M aqueous formate solution.



Catalyst Recycling The final goal in designing the catalyst is to recover and recycle it efficiently. The abovementioned acid-base equilibrium leads to changes not only in the electronic properties of the catalyst but also in its polarity, thus enabling tuning of its water-solubility. Figure 8 shows the solubilities of the complexes at various pH values in a 1 M formate solution. The DHBP complex 2 is soluble (albeit slightly, ca. 1 ppm) under neutral conditions. For satisfactory catalyst recovery, the ligand was changed from DHBP to the less water-soluble phenanthroline analog 4,7-dihydroxy-1,10-phenanthroline (DHPT) (Scheme 1). The negligible water solubility of 3 (ca. 100 ppb) under weakly acidic conditions can lead to efficient recovery of the catalyst by simple filtration. Recycling of the catalyst 3 was examined in a batch-wise cycle (Table 2) (19). When the reaction was carried out under optimized conditions, the catalyst precursor spontaneously precipitated and the added KOH was completely consumed. The catalyst was recovered by filtration, and the iridium leaching into the filtrate was found to be 0.11 ppm ( 99

5

2·SO4

90

2 M HCO2H

14000

> 99

6

2·SO4

60

1 M pH 3.0

750

46

7

2·SO4

60

1 M pH 4.0

111

16

8

2·SO4

60

4 M HCO2H

3100

> 99

9

2·SO4

60

8 M HCO2H

3000

> 99

Solution

Initial TOF / h–1

Conversion / %

a

The reaction was carried out using the catalyst (2 µmol) in an aqueous formate solution (10 ml). b catalyst (5 µmol).



Figure 9. Hammett plot of initial TOF vs. σp+ value of substituent (R) in the catalyst (2.0–20 µmol) at 60 °C in 1 M aqueous formic acid solution (10 mL).

Figure 10. Utilization of CO2 using a combination of the preparation and decomposition of formic acid catalyzed by iridium catalyst.

Catalytic decomposition of formic acid proceeded in a closed system. The gas smoothly evolved in an autoclave and the pressure exceeded 4 MPa. It was possible to control the rate of gas evolution by the reaction temperature. These results indicated that the catalytic system could provide the desired amount of COfree hydrogen gas for an extended period of time. We have achieved the highest catalytic activity for the decomposition of formic acid in water, in which CO-free and pressurized hydrogen could be evolved. These results suggest that a procedure for producing CO-free hydrogen from formic acid can serve as the basis for generating electricity by fuel cells. In the near future, formic acid obtained from biomass may be of importance as hydrogen source (31).


Summary


We have demonstrated a hydrogen-storage system using a combination of the preparation and decomposition of formic acid under mild conditions in water (Figure 10). One of two reactions can be selected by the pH of the solution using the same iridium catalyst, which hydrogenates CO2 in a basic solution and decomposes formic acid in an acidic solution. Overall, the system should be environmentally benign and highly efficient. More importantly, the acid-base equilibrium of the hydroxyl group on the catalyst ligand offers a novel tool for simultaneous control over the activity and water solubility of the catalyst. The remarkable catalytic activity is due to the strong electron-donating oxyanion generated from the hydroxyl group.

Acknowledgments Financial support from the Ministry of Economy, Trade, and Industry is gratefully acknowledged.

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Utilization of Carbon Dioxide as a Hydrogen Storage Material

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