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Keywords:

  • amines;
  • dehydrogenation;
  • hydrogen;
  • ruthenium;
  • thermodynamics

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Experimental Section
  6. Acknowledgements
  7. Supporting Information

The equilibrium position in formic acid/amine–CO2 systems has been examined as a function of pressure and temperature under isochoric conditions. The homogeneous ruthenium(II)-1,2-bis(diphenylphosphino)ethane catalyst was active in both reactions, that is, in formic acid cleavage producing pure hydrogen and CO2, as well as in carbon dioxide hydrogenation under basic conditions. High yields of formic acid dehydrogenation into H2 and CO2 are favored by low gas pressures and/or high temperatures, and H2 uptake is possible at elevated H2–CO2 pressures. These results take us one step closer to the realization of a practical H2 storage–discharge device.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Experimental Section
  6. Acknowledgements
  7. Supporting Information

A hydrogen economy, the long-term goal of many nations, can potentially confer energy security along with environmental and economic benefits.1 Hydrogen storage is an essential element in the hydrogen cycle, from production and delivery to energy conversion and applications and, so far, storage issues present a significant obstacle to the large-scale utilisation of hydrogen as an energy carrier. Improved storage technologies are needed to satisfy end-user expectations and foster consumer confidence in hydrogen-powered alternatives.2

Recently, successful attempts reported the use of CO2 as a hydrogen carrier.314 Formic acid (FA), or more precisely formates, can be produced by the reduction of CO2 in basic media (that is from bicarbonates, carbonates) with H2 in the presence of appropriate catalysts1525 and can also be converted back to H2 when needed. In this manner, FA can be regarded as a liquid carrier of H2 if H2 is combined with CO2. Therefore, the combination of CO2 reduction and FA or formate dehydrogenation under mild conditions is a CO2-neutral system.2631

Our groups have both reported the catalytic decomposition of FA using Ru-phosphine-based catalysts. However, if we address the formation of FA from gaseous H2 and CO2, it is necessary to overcome the thermodynamic barriers that result from the uphill nature of this reaction. In this sense, CO2 hydrogenation can be shifted towards the direction of FA by the addition of bases, which are often amines. These FA–amine adducts are also known to act as hydrogen donors in transfer-hydrogenation reactions.3234

Beller and co-workers demonstrated the generation of H2 from mixtures of FA and amines at room temperature.3537 Unprecedented high rates at ambient temperature were obtained by using in situ generated Ru-phosphine catalytic systems; the highest activity obtained to date for the dehydrogenation of FA was observed by applying a [RuCl2(benzene)]2 pre-catalyst and a 1,2-bis(diphenylphosphino)ethane (dppe) ligand. In addition, the hydrogenation of CO2 (more precisely bicarbonate) was also possible with this catalyst, which makes concrete applications of this system possible.

Knowledge of the position of the chemical equilibria in Equation (1) is important for the construction of a practical H2 storage–discharge device.(1)

  • equation image(1)

As a result of entropy changes during the reaction, the equilibrium is shifted largely to the direction of H2 and CO2 production, which facilitates the generation of H2 from FA. However, the reverse reaction requires a base and high pressures of H2 and CO2.22 An essential feature for practical applications, in which the gas pressure can be used as the driving force in a downstream H2 separation and purification step, is the CO2 and H2 pressure effect on the catalytic activity and on the equilibrium.38 Fellay et al.3 reported no inhibition of catalytic activity during FA decomposition (up to a pressure of 750 bar) on application of [Ru(H2O)6]2+ or RuCl3 and a water-soluble meta-trisulfonated triphenylphosphine (mTPPTS) ligand in a 4 M HCOOH/HCOONa (9:1) aqueous solution. Beller and co-workers39 presented a highly active iron catalytic system for the liberation of H2 from FA, by applying Fe(BF4)2⋅6 H2O and a tris[(2-diphenylphosphino)ethyl]phosphine [P(CH2CH2PPh2)3, PP3] ligand in propylene carbonate (PC) solvent. This system showed good tolerance towards CO2 pressure (up to 30 bar), but significant catalytic activity loss has been detected even at moderate H2 pressures. Therefore, it was considered to be of major importance to examine the stability and activity of the [RuCl2(benzene)]2 dppe catalytic system under elevated CO2 and H2 pressures.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Experimental Section
  6. Acknowledgements
  7. Supporting Information

For our investigations, 5 HCOOH/2 triethylamine (NEt3) was chosen as the substrate. It was observed that with the [RuCl2(benzene)]2/dppe catalyst, FA alone does not produce H2 and CO2; a base as additive is required, in agreement with Beller’s findings. The decomposition of an FA/amine mixture under atmospheric pressure, catalysed by [RuCl2(benzene)]2 and six equivalents of dppe, yielded 100 % FA conversion into H2 and CO2 after heating at 40 °C. Even at 30 °C, no traces of FA could be detected by using 1H and 13C NMR spectroscopy on the equilibrium position. However, on performing the reaction under isochoric conditions, in a closed reactor, the FA dehydrogenation decreases to 44 %, which indicates a shift of the chemical equilibrium position towards the direction of FA formation with increasing gas pressures. The pressure inside the vessel increased rapidly and levelled off at 40 bar (Table 1, entry 1). The release of the gas pressure (to 1 bar) triggered further FA decomposition until the dehydrogenation reached 100 %, which indicates that the catalytic activity was not lost with increasing pressure. To examine the effect of CO2 and H2 pressure on the equilibrium position, several experiments with different initial CO2 and H2 pressures were performed (Table 1, entries 2–5). It was found that the equilibrium position is shifted towards the direction of FA formation because of the presence of both H2 and CO2 pressures, although the effect of the latter was more pronounced. The equilibrium position was determined by recording multiple quantitative 13C NMR spectra (using appropriate delay times for complete relaxation40 during the measurements) after the reaction was initiated. If the concentrations of the various species in solution (and also the pressure) were found to be stable over an adequate period of time, chemical equilibrium was considered to be attained. In all the cases, the experiments could be reproduced with a maximum relative error of ±10 % (however, in about 90 % of the experiments conducted, the deviation was in the range of relative error ±5 % for the average values given in Table 1).

Table 1. Effect of initial gas pressure on FA/amine decomposition under isochoric conditions[a] with [RuCl2(benzene)]2 and a dppe ligand.[b]
Entrypinline image [bar]pinline image [bar]Final pressure [bar]Conversion [%]
  1. [a] 1.5 mL of 5 HCOOH/2 NEt3 mixture, experiments performed in sapphire NMR tubes, data correspond to the average of several experiments. [b] 4.775 μmol [RuCl2(benzene)]2, Ru/P=1:6, pre-treatment of the catalyst 2 h in 0.5 mL DMF. Reproducibility was ±10 % relative to the given values.

1004044
22006040
34007536
40205032
50406425
610105838
720207736

To realise a multiple-use H2 storage device, only closed vessels that contain the catalyst and the chemical(s) used for storage are appropriate. Charging would then be possible at elevated pressure and discharging (H2 release) at low pressure.22 The thermodynamic requirement of such a device is that H2 must be involved in an equilibrium sufficiently mobile in the expected pressure (and temperature) range.41

To determine the position of the equilibrium in Equation (1), the reaction was allowed to proceed from both directions in presence of the Ru-dppe catalyst. For the FA decomposition reaction, the volume of the 5 HCOOH/2 NEt3 mixture was varied between 0.05 and 3.3 mL, keeping the catalyst concentration and the temperature constant (Figure 1), and the pressure increase as a result of gas evolution was monitored. The measured pressures were then converted to percentage FA conversion by normalisation to the calculated pressure yielded by 100 % FA decomposition. An equimolar mixture of H2 and CO2 was assumed to be produced by the FA decomposition reaction according to previous investigations by Beller et al. with the same catalytic system.37 As CO2 is soluble in amine solutions and as the FA conversion was calculated based on the measured pressure, the amount of dissolved CO2 was determined by quantitative 13C NMR spectroscopy by using appropriate relaxation delays.40

thumbnail image

Figure 1. Equilibrium position for FA decomposition/formation in an amine solution at 40 °C with [RuCl2(benzene)]2 (4.775 μmol) and dppe (6 equiv.) in DMF (0.5 mL). FA decomposition reaction (□), CO2 hydrogenation reaction (▴).

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The CO2 concentration in the liquid phase varies according to the applied temperature and pressure. High pressures and low temperatures promote the dissolution of CO2. The concentration of dissolved CO2 was evaluated by using NEt3 as an internal reference and then used to correct the conversions calculated based on the pressure increase. The decomposed FA quantities were confirmed, and the conversions were in good agreement (±5 %) with those determined by the integration of the FA peaks obtained by quantitative NMR spectroscopy, relative to those of NEt3. For the reverse reaction, a solution that contained the catalyst and the amine was pressurised with 50 bar H2 and 50 bar CO2, then heated and left to equilibrate.

Low pressures allow the almost complete decomposition of FA into H2 and CO2 (Figure 1), the rate of which depends on the applied temperature as described below. However, an increased pressure leads to a shift of the equilibrium position towards the direction of FA formation. Hence, the higher the applied pressure, the lower the FA conversion into H2 and CO2. It can also be seen that the reverse reaction, that is, the production of FA through the hydrogenation of CO2, is subject to thermodynamic barriers as discussed above.

The equilibrium composition is dependent on the temperature. Hence, the effect of temperature on the equilibrium position in Equation (1) was examined (Figure 2). As the dehydrogenation of FA is a slightly endothermic reaction (ΔH°=+31.2 kJ mol−1), an increase in temperature is expected to promote the production of gases and, therefore, the decomposition of FA. Indeed, an increase in temperature increases the rate of FA decomposition as well as the overall FA conversion into H2 and CO2. The increase in FA conversion is more pronounced at lower temperatures (Le Chatelier–Braun principle). Despite the shift of equilibrium towards the direction of FA formation with increasing pressure, high temperatures allow high FA dehydrogenation yields.

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Figure 2. Effect of temperature on FA decomposition under isochoric conditions with [RuCl2(benzene)]2 (4.775 μmol) and dppe (6 equiv.) in 5 HCOOH/2 NEt3 solution (0.3 mL). Total solution volume=1.5 mL.

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At a reaction temperature of 40 °C, a long induction period is observed (Figure 3). This is in agreement with the findings of Beller et al., who reported the same behaviour during the first catalytic cycle of a 5 FA/4 N,N-dimethyl-n-hexylamine decomposition reaction on application of a catalytic system that contained [RuCl2(benzene)]2 and dppe.37 The reason is probably the slow formation of the catalytically active species during the first run.

thumbnail image

Figure 3. Effect of temperature on the rate of FA decomposition under isochoric conditions with [RuCl2(benzene)]2 (4.775 μmol) and dppe (6 equiv.) in 5 HCOOH/2 NEt3 solution (0.3 mL), T=40 (⧫), 50 (◊), 70 (○), 80 (▵), 90 (□), 100 °C (•).

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The composite equilibrium constant at a given temperature is defined by Equation (2).

  • equation image(2)

Based on Equation (2) and by taking into account the gas–liquid distribution of CO2, hence using the corrected CO2 pressure, the enthalpy of reaction can be determined by combination of Equations (3) and (4) as usual(3), (4):

  • equation image(3)
  • equation image(4)

The enthalpy of reaction was calculated from the slope of the plot shown in Figure 4 and was found to be (+32.6±0.5) kJ mol−1.

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Figure 4. Temperature dependence of the composite equilibrium constant for FA/amine decomposition under isochoric conditions in the temperature range of 10–110 °C with [RuCl2(benzene)]2 (4.775 μmol) and dppe (6 equiv.) in DMF (0.5 mL).

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From a practical point of view, it is essential that the desirable FA dehydrogenation yield can be obtained under isochoric conditions. Therefore, the catalytic system needs to remain active under elevated gas pressures that result from FA decomposition. We have determined the equilibrium position in Equation (1) as a function of pressure and shown that high yields of FA dehydrogenation can be obtained even under elevated pressures by increasing the reaction temperature.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Experimental Section
  6. Acknowledgements
  7. Supporting Information

[RuCl2(benzene)]2 was prepared according to a literature method42 and checked by 1H NMR spectroscopy. The amine was distilled, and DMF and the 5 HCOOH/2 NEt3 solution were degassed before use. All other chemicals were commercial products and were used as received. NMR spectra were recorded by using a Bruker DRX 400 MHz spectrometer.

All catalytic experiments were performed in 10 mm external diameter, medium pressure sapphire NMR tubes.43 In a typical experiment, [RuCl2(benzene)]2 (4.775 μmol) and dppe (6 equiv.) were dissolved in DMF (0.5 mL) and stirred for 2 h at RT under a N2 atmosphere. The pre-mixed solution of FA and amine was heated to 40.0 °C in a Schlenk tube and then transferred to an NMR tube. The reaction was initiated by adding the catalyst to the mixture in the sapphire NMR tube.

The tube was then thermostatted either with an electric heating jacket or directly in the spectrometer, the temperature of which was determined before and after measurement by using an external temperature probe. The reaction was followed by monitoring the pressure increase as a result of gas formation as a function of time by using a pressure transducer connected to the tube via a high pressure capillary, either manually or with an in-house LabView 8.2 program with a NI USB 6008 interface, and/or in situ by 1H NMR spectroscopy. To verify that the kinetics of the reaction can be followed by recording the change in pressure, FA decomposition was followed by 1H NMR spectroscopy and pressure increase simultaneously. The kinetic curve obtained from the 1H NMR spectra that shows the decomposition of FA was compared to the data obtained from pressure measurements. The measured pressures were converted to percentage FA conversion by normalisation of the final pressure to the final conversion determined by quantitative NMR spectroscopy. As the data are in good agreement, it is a valid method to follow the reaction only by recording the change in pressure and recording NMR spectra before and after the reaction to determine the conversion.

The integrals of the NMR spectroscopic signals that correspond to FA and CO2 are proportional to their concentrations if appropriately long delay times (D1>5 T1) were chosen in the experiments. The relaxation times (T1) of the CO2, HCOOH and NEt3 solutions were determined by using the inversion recovery method.41

For reactions under various pressures, the sapphire tube that included all reagents was pressurised at room temperature with CO2 and/or H2. High pressure techniques, which include pressurisation and depressurisation with H2 and CO2 were performed in a fume hood with appropriate precautions. All experiments were performed at minimum in triplicate.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Experimental Section
  6. Acknowledgements
  7. Supporting Information

The Swiss National Science Foundation and EPFL are thanked for financial support.

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Experimental Section
  6. Acknowledgements
  7. Supporting Information

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