Carbon dioxide hydrogenation to formic acid with an extremely efficient basefree homogeneous catalytic system
The efficient transformation of CO2 into chemicals and fuels is a key challenge for the decarbonisation of the chemical synthetic production chain. The thermodynamic and kinetic stability of CO2 however poses great challenges for the effective reduction to useful chemicals. Formic acid (FA) represents the first product of CO2 hydrogenation by a 100% atom economy reaction and can be the precursor of subsequent transformation into higher added value products or employed as a hydrogen storage vector. The employment of bases is a typical strategy to overcome thermodynamic barriers in the synthesis of FA. However, it increases the amount of waste generated and requires post-processing of the formate salts. Base-free systems based on the employment of buffers can overcome these limitations, but their catalytic performance has so far been modest (TON 10 2 -10 3 ). Here, we present a base-free methodology to catalytically transform CO2 into FA with very high efficiency, with comparable performance to the base-assisted systems. The combination of basic ionic liquids that play multiple roles, including buffering the products of reaction, and catalyst design, achieves the synthesis of FA with unprecedented catalytic efficiency in TONs of >8*10 5 and TOFs > 2.1*10 4 h -1 , which represents a great step towards the development of cost-efficient catalytic systems.
Introduction<!>Results and Discussion<!>Conclusions
<p>Formic acid (FA) is an important basic chemical, is currently being discussed as a promising candidate for hydrogen storage and can potentially be upgraded to higher added value CO2 synthetic products. [1][2][3] Unfortunately, to date its synthesis is based on the indirect water carbonylation, using CO generated from fossil fuels. 4 Without a doubt a system using the greenhouse gas CO2 as a carbon source would be environmentally preferential. However, the direct hydrogenation of CO2 is thermodynamically unfavourable. 5 Bases are commonly added to reaction mixture to shift the thermodynamic equilibrium by the consecutive consumption of formic acid in an acid base reaction to the product side. [5][6][7] However, an important limitation of these approaches is the formation of formate adducts and salts, which would need to be tediously purified in the consecutive steps with acids, which adds to the cost and the amount of waste generated. 6,8 Therefore, the direct hydrogenation of CO2 would be preferential. Unfortunately, the hydrogenation under base free conditions is thermodynamically and kinetically difficult to operate. 5 As a consequence, considerably fewer systems have been reported for the hydrogenation of CO2 to FA under base-free conditions. Furthermore, these systems typically operate under very high pressures 9 and the catalytic activity and concentration of FA achieved are significantly lower than under basic conditions. [10][11][12][13][14][15] We have recently demonstrated that basic ionic liquids can effectively buffer the reaction, stabilise the catalysts, and they can be immobilised onto supported phases, thus facilitating the separation of the FA at the end of the process, thus shifting the thermodynamic equilibrium to the product side and in the presence of IL the reaction can be operated at relatively low pressures. 16 In our previous studies we have established the thermodynamic parameters for this transformation employing basic ILs, and therefore we can understand the maximum concentration of FA achievable as a function of the reaction conditions. Furthermore, we previously studied the effect of ligand modifications onto the hydrogenation of CO2 in the presence of ILs. Key parameters effecting the catalyst activity and stability were associated with the electron density at the metal centre and the resistance of the catalyst towards protonation. 17 Here, we present a catalytic system for the base free hydrogenation of CO2 specifically designed to work under a wide range of temperatures. This enables optimising the catalyst performance by pushing the conditions to the thermodynamic limitations imposed by the reaction and consequently balancing kinetic and thermodynamic performance and achieving high catalytic efficiency. The robustness of the catalyst enables the addition of lewis acids, which increase the performance of the catalyst in terms of activity and stability. The results obtained here are comparable to those obtained with base containing systems and represent a huge leap forward for basefree systems.</p><!><p>The design of the catalysts plays a key role in the development of efficient catalytic systems. Very recently, we have demonstrated that basic ionic liquids provide a buffering environment that enables the efficient synthesis of FA, but with much lower enthalpy than when a base (leading to a salt formation) is employed. 16,18 Under these conditions, optimal electrondonation properties of the ligand are key to balance reactivity and the resistance to deactivation via protonation of the catalyst (Figure 1). Furthermore, complexes with high chemical and thermal stability lead to catalysts with large operational windows and an increased compatibility with other components, such as co-catalysts. NHC based pincer architectures fulfil all these criteria, hence being ideal candidates for this transformation. 19 Furthermore, pincer ligand architecture have already been demonstrated to be active in the hydrogenation of CO2. However, pincers normally exhibit a co-operative mechanism in the activation of hydrogen 20,21 which leads to the deactivation of the catalyst (Figure 1, substrate poisoning). 22 Consequently, a suitable catalyst should prevent the coordination of substrates in undesired positions. In this study this was achieved by a direct connection of the aromatic pyridine with the imidazole side arms. The silver trans metalation route employing [Ru(CO)2(Cl)2]n as the Ru source, and L-1 as the ligand was chosen to synthesize the desired Ru-CNC-complex. The reaction was followed by the disappearing signal for the C2-H in the 1 H-NMR spectra and the appearance of a new peak at 181 ppm in the 13 C spectra (see ESI, Figure S1 and Figure S2). In the IR spectra two significant bands at 1993 cm -1 and 2051 cm -1 (see ESI, Figure S3), corresponding to two asymmetric CO stretching vibrations can be observed. The general structure was confirmed with Single X-ray crystal analysis.</p><p>Minor scrambling is observed with Clbond to Ru, i.e. replacement of Br -. In a similar fashion the counterion distributed 1:1 (Br:Cl), giving an overall molecular sum formula of C21H25Br0.63Cl1.29N5O2Ru. The major halide bond to the metal centre was chloride, as confirmed with ESI-ms. ESI-ms analysis gave a mass of 516 m/z and a minor product with 562 m/z confirming (see ESI, Figure S4). N5O2Ru. This in bromide to chloride ratio of 30:70. Indeed, the observed mass peaks for the M-Cl + to M-Br + of ca. 73:27 indicate that the ratio determined by single crystal analysis is valid. Further evidence for this ratio was found by ICP-OES analysis. Here a Ru content of 19.2 mg L -1 was found in a 0.199 mM solution (for 100% Cl expected Ru content is 20.8 mg L -1 and for 100% Br expected Ru content is 18.2 mg L -1 ), which indicate a molar ratio between 1-Br and 1-Cl of 70:30. In this way a molecular mass of 578.11 g/mol was used to determine the amount of catalyst in solution. Furthermore, neither the addition of Clor Brshowed a difference in catalytic activity or stability in comparison to the non-modified system, indicating that the halide plays an insignificant role onto the catalytic activity, see ESI for more detail.</p><p>In an initial screening of catalytic activity, it was observed that 1 achieves a concentration of 0.5 M solution of FA at 80°C in DMSO: water (5 v/v% water) in the presence of 1-butyl-2,3-dimethylimidazolium acetate (BMMI.OAc) under 60 bar pressure (H2:CO2 = 1:1). According to the thermodynamic parameters previously calculated for this system, 23 the concentration of FA observed is close to the thermodynamic equilibrium at approximately 120°C (see ESI Figure S5). Above 80°C the decomposition of DMSO was observed. Fortunately, when BMMI.OAc is employed as an additive the reaction can be performed in a broad range of solvents. 16 In dioxane the catalytic system achieves similar performance than in DMSO (0.54 M at 100°C).</p><p>It is important to note that no evidence of nanoparticle formation was observed under the reaction conditions assayed. Even at high temperatures and pressures (120°C and 60 bar (H2:CO2 = 30 bar)), TEM and DLS analysis showed no evidence of nanoparticles (see ESI for more details). Furthermore, the catalyst was not active in a reaction typically catalysed by Ru nanoparticles, [24][25][26][27] such as the hydrogenation of benzene at 120°C under 50 bar H2 in the presence of BMMI.OAc even when high catalyst loadings are used, see ESI for more details.</p><p>In the next step the catalyst activity and stability was explored. Lowering the concentration of catalyst showed that increasing the temperature led to increased kinetics, represented by increasing TOF values. However, this comes at the expense of reduced achievable concentration of FA (and consequently TON) due to the thermodynamic limitations at temperatures above 120°C, see table 1 entry 4 and 5. Plotting the concentration of FA generated after 72 h allows (Figure 3A) allows for a better understanding of the efficiency of the catalytic system. Here a maximum concentration of FA can be observed at 120 °C. Below that temperature, the system is controlled by kinetic limitations of the catalyst, whilst at higher temperatures the reaction is limited by approaching the thermodynamic equilibrium inherent to the reaction under the studied conditions. Our previous report shows the thermodynamic equilibrium curve calculated employing Van't Hoff plots for the CO2 hydrogenation to FA employing BMMIOAc as a buffering system (see ESI for more details). 16 In the next step the effect of pressure was investigated. Here a linear correlation in between the achieved concentration of FA and pressure can be observed (Figure 3B).</p><p>This in consistent with the Henry's law, predicting a linear relationship in between the amount of dissolved gas and the applied pressure of the bespoke gases. As the gases represent the reactive species a reduced pressure leads inevitably to a lower concentration of FA (Figure 3B). Mechanistically, the hydrogenation of CO2 to FA can be divided into two main steps.</p><p>In the first step, a hydride is transferred onto CO2. In the second step, H2 is activated releasing a proton and formate, with a simultaneous regeneration of the active hydride (figure 4A). In order to determine the rate determining step, various gas compositions were tested (maintaining constant the total pressure). The observed reaction rate decreased significantly at low partial pressure of CO2 (Figure 4B). Indeed, at 20 bar CO2 and 40 bar H2 a TOF of 2700 h -1 was obtained. On the contrary an increased partial pressure of CO2 led to increased rates, i.e. at 45 bar CO2 and 15 bar H2 a TOF of 9700 h -1 is determined, (c.f. 4360 h -1 at PCO2 = PH2 = 30 bar). These experiments clearly show a strong dependence on CO2, thus indicating the CO2 insertion to be rate determining steps.</p><p>An activation energy of 93 kJ mol -1 ± 7 kJ mol -1 was determined from the Arrhenius plot (figure 4C). The activation energy can either be decreased by increasing the electron donation onto the metal, leading to a more nucleophilic hydride, decreasing hydricity (∆𝐺 𝐻 − ). 28 Based on our recent studies, a low hydricity will ultimately lead to a more nucleophilic/ basic hydride, rendering the catalyst prone protonation and hence deactivation.</p><p>It has been demonstrated that charged intermediates and transition state formed during the CO2 insertion can be stabilised with lewis acids. [29][30][31][32] Therefore Sc(OTf)3</p><p>was tested due to its high solubility in organic solvents, tolerance towards water and due to its relatively weak acidic nature. 33 It was found that Sc(OTf)3 significantly increased the reaction rate. However, smaller amounts have only a minor effect on the rate (Figure 4D). At an optimal concentration of 20 mM for Sc(OTf)3 was found a TOF of 21200 h -1 , which corresponds to 4.9 times the activity without using the lewis acid.</p><p>Above this concentration a drop in rate was observed, presumably due to catalyst poisoning as already observed in previous studies. 34 However, in the presence of Sc(OTf)3 the concentration of FA generated decreased slightly to 0.32 M, yielding a TON of 114400, see table 2 entry 2. Increasing, the hydrogen to carbon dioxide ratio the achieved did not show any pronounced effects. Under these conditions, the amount of catalyst was reduced to 1.7 nmol, which led to an unprecedented TON of 833800, while the TOF was maintained at 20600 h -1 (table 3, entry 4). In this case, there is no conversion without BMIMOAc or with Sc(OTf)3 only as catalyst. Reaction conditions 0.017 µmol catalyst, T = 120°C in 6 mL 1,4-dioxane:H2O (5 v/v% H2O) in the presence of 3.3 mmol BMMI.OAc; TON and [FA] determined after 72h using BMMI.OAc as an internal standard, TOF determined after 4h; a) 0.0017 µmol catalyst, TON and [FA] determined after 5 days using BMMI.OAc as an internal standard, TOF determined after 18h.</p><p>In summary we have developed a catalyst system, which has an up to a 50 times higher value in TON and 23 times higher value in TOF in the hydrogenation of CO2 to FA then ever reported under base free conditions. [9][10][11]14,23 Furthermore, the TON and TOF towards FA are comparable to values obtained under basic conditions. [35][36][37] Interestingly, the here reported CNC-Ru pincer outperforms its previously reported analogue with a six membered bite angle ring, even though the previous complex was reported to operate under basic conditions. 38</p><!><p>To conclude, we have presented a catalytic system capable of highly efficiently transform CO2 into FA under base-free conditions. The combination of basic ionic liquids, highly robust catalysts based on Ru pincer N-heterocyclic carbenes and lewis acid catalysts showed high efficiency at elevated temperatures, balancing kinetic and thermodynamic performance. The catalytic activity observed is the highest observed to date for base-free systems and comparable to literature reports employing basic conditions that lead to the formation of thermodynamically stable formate salts. This work will open new avenues in the hydrogenation of CO2 to chemicals and for the storage of hydrogen in liquid energy vectors.</p>