Research Article pubs.acs.org/journal/ascecg
Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11963−11969
Carbon Dioxide to Methane Using Ruthenium Nanoparticles: Effect of the Ionic Liquid Media Catarina I. Melo, Duarte Rente, Manuel Nunes da Ponte, Ewa Bogel-Łukasik,* and Luís C. Branco* LAQV-REQUIMTE, Departamento de Química, Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa, 2829-516 Caparica, Portugal
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S Supporting Information *
ABSTRACT: Carbon dioxide hydrogenation to methane is efficiently achieved using ruthenium nanoparticles (Ru NPs) prepared and stabilized in task-specific ionic liquids. 1-Octyl-3methylimidazolim perfluorobutanesulfonate [C8mim][NfO] is the best ionic liquid media producing 84% yield of methane at 150 °C. Other fluorinated anions based ionic liquids exhibited a greater influence on the methane production. In parallel, carbon monoxide is selectively produced in a small amount for the ionic liquid media containing trifluoroacetate and mesylate anions. It is known that ionic liquids strongly contribute to higher CO2 solubility as well as the formation and stabilization of Ru NPs. The catalytic system maintains some activity at least after 5 recycling studies and a promissory improvement can be achieved using a further continuous-flow system. KEYWORDS: Carbon dioxide, Hydrogenation, Ionic liquids, Methane, Ruthenium nanoparticles
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INTRODUCTION Power-to-gas (P2G) technology is a method that aims for the conversion of the surplus electricity from renewable energy sources, such as wind turbines or solar panels, into gases that can be stored and later used as fuels in periods of high power consumption.1,2 Under the frame of P2G, the power-tomethane concept is to use this energy for the electrolysis of water into H2 followed by the conversion of CO2 into methane.1,3,4 It is reported that methane as synthetic natural gas (SNG) can be easily added to the natural gas grid since it is fully compatible with the existing infrastructures.5,6 Apart from methane, CO2 hydrogenation can generate other compounds, most of which are easier to obtain: (a) CO is the simplest product which it can be used as a building block chemical manufacturing for industrial processes, (b) formic acid is a simple carboxylic acid that can be used as a raw product for commodity chemicals (with limited utilization when compared with other products), (c) methanol and higher hydrocarbons which are already used as commercial fuels, and (d) dimethyl ether is also an emergent chemical for application as an alternative fuel and synthesized from methanol produced by CO2 reduction7 However, these products would require a great investment in new infrastructures for gas or liquid storage and its large scale distribution. Carbon dioxide is an abundant, cheap, nontoxic, nonflammable molecule.8 Using it as a C1 feedstock for methane production is a very attractive strategy, not only because it can reduce fossil fuels consumption,8 but also because the process © 2019 American Chemical Society
mitigates CO2 which is known as one of the major green-house gases.6,9 Also known as the Sabatier reaction (eq 1), catalytic hydrogenation of CO2 into methane is a reversible and highly exothermic reaction.10 However, the high kinetic barriers of the eight-electron reduction process requires a considerable energy amount to start.2 CO2 + 4H 2 = CH4 + 2H 2O
ΔH298K = 165 kJ/mol (1)
Typically, the range of the reaction temperatures is between 200 and 450 °C depending on the catalyst, support, and the reaction conditions.11 It is known that the reaction temperature is a critical factor due to the exothermic nature of the reaction: a lower value improves the selectivity of the process toward methane production. Nickel was the first metal catalyst used by Sabatier and Senderens,12 and it is still the most studied one due to its low cost and high activity.11 However, a Ni based catalyst can be deactivated even at low temperatures due to nickel particles sintering.10 A wide variety of other noble and non-noble metals have been widely studied as catalysts, such as Fe, Co, Cu, Pd, Rh, Ru, Pt, Mo, Re, Ag, and Au. Among these, ruthenium has been revealed to be one of the most active and stable metals requiring lower reaction Received: January 13, 2019 Revised: March 11, 2019 Published: May 28, 2019 11963
DOI: 10.1021/acssuschemeng.8b06877 ACS Sustainable Chem. Eng. 2019, 7, 11963−11969
Research Article
ACS Sustainable Chemistry & Engineering temperatures.13,14 Research on CO2 methanation with Ru mainly focuses on the catalyst support since catalytic activity greatly depends on the metal dispersion and support material.11 Ceria is one of the materials that has been widely used, improving the reaction rate at 250 °C when added to the Ru/Al2O3 catalyst.15 Methanation of CO2 with Ce0.96Ru0.04O2 and Ce0.95Ru0.05O2 catalysts reached a conversion of 55% and 99% selectivity at 450 °C reaction temperature.16 Zamani et al.17 optimized Ru/Mn/Cu-Al2O3 preparation (10.9 wt % of metal loading, 1035 °C calcination temperature, and 5 wt·g of catalyst loading) for CO2 methanation at 220 °C, obtaining a CO2 conversion value of 98.8%. Ru nanoparticles supported in TiO2 prepared in an elaborate polygonal barrel-sputtering method allowed 100% yield methanation at 160 to 180 °C in a continuous-flow reactor.18 Further DFT (density functional density) investigation has taken place for two possible adsorption structures of CO2 in this catalyst.19 Nanoparticles exhibit unique physical properties with interest in many technological applications, especially catalysis due to their high catalytic activity.20 Ionic liquid (ILs) are organic salts with low melting temperatures (liquids until 100 °C). They display several unique properties such as negligible vapor pressure, high thermal stability, high ionic conductivity, and the ability to solubilize a series of varied compounds such as CO2, for example.21 All these features can also be tuned depending of the cation and anion used. ILs can be applied in the preparation and stabilization of metal nanoparticles. They confer electrosteric (steric and electrostatic) stabilization through the formation of protective layer composed of ions surrounding the metal NPs.22 Better nanoparticle dispersion provided by ILs benefits the further application in catalysis.23 Metal nanoparticles can be synthesized by chemical and electrochemical reduction or thermal decomposition.24 Ru nanoparticle formation in IL media has been widely studied by chemical reduction of organometallic precursors such as [Ru(COD)(2-methylallyl)2] or [Ru(COD)(COT)]25 or by the decomposition of metal carbonyl compounds in IL media through microwave heating.26 They have been tested in a variety of catalytic reactions such as for Wittig olefination,27 hydrogenation of terpenes,28 N-heterocyclic compounds,29 alkenes,28,30 and arenes.30−32 Leitner group already showed a continuous-flow CO2 hydrogenation to pure formic acid using an integrated scCO2 process with immobilized catalyst and base.33 Recently, Srivastava reported the application of Ru nanoparticles in the hydrogenation of carbon dioxide to formic acid using mild reaction conditions (T < 100 °C, 50 bar of total pressure).34 Our group also reported a preliminary study about the formation of methane from the hydrogenation of carbon dioxide using Ru NPs formed and stabilized in ionic liquid media.35 Herein, a detailed study about the influence of the structure of task-specific ionic liquids in the performance of CO2 conversion to methane reaction is presented (Figure 1).
Figure 1. Scheme of methanation reaction in ionic liquid media with Ru nanoparticles.
ally, a total pressure of 125 bar (1:1 of H2:CO2 gas ratio) and reaction time of 24 h have been used. In general, a stochiometric ratio of reactants favors the reaction; however, this ratio was selected because H2 diffusion is a rate-limiting step36 in the reaction as well as higher pressure of CO2 facilitates H2 diffusion in the ionic liquid media. In the preliminary studies, [C8mim][NTf2] (entry 4, Table 1) was selected as the best IL allowing the formation of Ru NPs with 2.5 nm average size and around 64% of methane formation. Table 1. Results of CO2 Methanation Reaction in Imidazolium Based ILsa Entry
Ionic liquid
CH4 yield (%)
TONb
TOFc (h−1)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
[C2mim][NTf2] [C4mim][NTf2] [C4dmim][NTf2] [C8mim][NTf2] [C10mim][NTf2] [C4mim][OTf] [C8mim][OTf] [C8mim][BF4] [C4mim][CTf3] [C8mim][CTf3] [C4mim][NfO] [C8mim][NfO] [C4mim][PFO] [C8mim][PFO]
26 24 58 64d 67 46 59 44 8 20 54 84 (60)e 61 50
24 22 53 67 60 42 56 42 7 18 49 78 (322)f 54 45
1.00 0.93 2.22 2.80 2.52 1.76 2.34 1.74 0.28 0.75 2.04 3.25 (2.68)e 2.24 1.88
a
Reaction conditions: 1 mL of IL; 24 h of reaction; total pressure of 80 bar at 40 °C; H2:CO2 pressure ratio of 1:1; reaction temperature of 150 °C. bCalculated as mol CH4/mol estimated surface atoms of Ru. c Calculated as TON/treaction. dValue obtained by different analytical method comparing to our previous work (69%). eSample after 5 runs of recycling. fTotal turnover number.
Experimental reactions were performed in IL/CO2+H2 biphasic media where H2 is the limiting reactant. Therefore, reaction yield, was used to do the assessment of the reaction performance instead of CO2 conversion. In the case of methane production yield (%) is calculated by the following formula:
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RESULTS AND DISCUSSION Taking advantage of our previous work, it was possible to optimize the CO2 hydrogenation reaction conditions. In particular using 150 °C, [Ru(COD)(2-methylallyl)2] as catalytic precursor solubilizes in fluorinated imidazolium ionic liquid. This catalytic system can generate the formation of Ru nanoparticles capable to produce methane.35 Addition-
CH4 yield (%) =
nCH4 × 100 ni H 2 /4
(2)
We currently aim to study the effect of the ionic liquid structure in the performance of the methanation reaction. In this context, the previous optimized reaction conditions were followed, and several task-specific ionic liquids have been tested. 11964
DOI: 10.1021/acssuschemeng.8b06877 ACS Sustainable Chem. Eng. 2019, 7, 11963−11969
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ACS Sustainable Chemistry & Engineering
Figure 2. Molecular structure of the cations and anions used in the CO2 hydrogenation reaction.
Task-specific fluorinated ILs were selected taking into account their higher capacity to solubilize CO2 already reported in the literature.37−40 Although it was not found enough data on CO2 solubility in ILs at 150 °C, we retrieved literature data of the selected ILs at 40 °C. According to this data, the CO2 solubility in the IL containing the [C4mim] cation follows this order: mesylate ([MeSO3]) < dicyanamide ([DCA]) ∼ tetrafluoroborate ([BF4]) < trifluoromethanesulfonate ([OTF]) < trifluoroacetate ([TFA]) < hexafluorophosphate ([PF6]) < bis(trifluoromethylsulfonyl)imide ([NTf2]) < tris(fluoromethanesulfonyl)imide ([CTf3]) < perfluorobutanesulfonate ([NfO]).37−40 Regarding the imidazolium cation, it is described that the solubility slightly increases with a longer alkyl side chain. Figure 2 illustrates the selected cation and anion structures of this work. After the reaction, IL media presented a black coloration as a characteristic behavior from colloidal solutions of Ru NPs. It is important to note that water is also formed in the end of the reaction. The visualization of the water phase is clear in the case of hydrophobic ionic liquid media. Gas samples were analyzed for the presence of methane and other volatile products by gas chromatography-thermal conductivity detector (GC-TCD). The liquid phase was analyzed for the presence of liquid products as well as IL stability by 1H nuclear magnetic resonance (NMR). Table 1 summarizes the CO2 methanation performance using different imidazolium based ILs. [C8mim][NTf2] as IL media showed a slight reduced reaction yield (64% CH4 yield) compared to our previous work35 (69% CH4 yield) because a different analytical method was considered (Table 1, entry 4). To keep data consistent, the values were recalculated. Cation Effect. A series of reactions using ionic liquids with the same anion, [NTf2], and different imidazolium cations were performed. 1-Ethyl-3-methylimidazolium ([C2mim]), 1butyl-3-methylimidazolium ([C4mim]), 1-octyl-3-methylimidazolium ([C8 mim]), and 1-decyl-3-methylimidazolium ([C10mim]) as organic cations were tested to check the alkyl chain cation effect. Figure 3 illustrates the comparative methane production for different imidazolium ILs. In general, the reaction yield improves for larger alkyl chain size, with the best result
Figure 3. Reaction yield of CO2 methanation with [NTf2] based ILs as stationary phase.
obtained for [C10mim][NTf2] (Table 1, entry 5). This observation is in accordance with the higher CO2 solubility for longer alkyl side chains. In order to study the effect of methylation of the C-2 imidazolium ring, the ILs [C4mim][NTf2] and [C4dmim][NTf2] were selected. The replacement of the proton for a methyl group in the imidazolium ring greatly improved the yield of the reaction. This result is not in agreement with Luska et al.41 observations for hydrogenation of cyclohexane with Ru NPs formed in the same ILs and catalytic precursor. They reported higher stabilization of Ru nanoparticles in [C4mim] based IL due to its lower degree of ionicity as well as the formation of C2-carbene (NHC) can contribute to a more stable molecular arrangement of the IL. In our case, the [C4dmim] cation showed a better performance at selected pressure and temperature conditions. Anion Effect. Hydrogenation reactions with imidazolium cations combined with several anions such as [DCA], [MeSO3], [TFA], [BF4], [OTf], [NTf2], [CTf3], perfluorooctanoate ([PFO]), and [NfO] were performed (see Figure 11965
DOI: 10.1021/acssuschemeng.8b06877 ACS Sustainable Chem. Eng. 2019, 7, 11963−11969
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ACS Sustainable Chemistry & Engineering 4). All anions were tested with both [C4mim] and [C8mim] cations with the exceptions of [DCA] and [BF4], where only
Figure 4. Reaction yield of CO2 methanation reaction with imidazolium based ILs medium.
[C4mim] was tested. In all cases, apart from [PFO], [C8mim] based ILs provided higher yields than correspondent [C4mim] based ILs. In the case of the [C8mim] cation, it was observed that the reaction yield was strongly influenced by the type of anion following the order: [CTf3] < [BF4] < [PFO] < [OTf] < [NTf2] < [NfO]. The higher CO2 solubility expected for [NTf2] and [NfO] anions can account for their better catalytic performances. It is also known that a lower coordination behavior can provide smaller and more stable nanoparticles.32 The highest amount of methane was achieved with [C8mim][NfO] (84% of CH4 yield). Despite the high fluorine content as well as CO2 solubility, the examples with the [CTf3] anion provided the lowest reaction yields. Dupont et al. already studied the effect of IL hydrophobicity in these catalytic systems, using hydrophobic ILs for the formation of CH4 and higher chain hydrocarbons42 as well as hydrophilic ILs mixed with organic solvents for formic acid formation.43 In the present work, mostly due to reaction conditions such as higher pressure (80 bar before heating and 125 bar during the reaction) and 24 h reaction time, no higher hydrocarbons or formic acid were detected. It is important to note that shorter (until 12 h) or longer reaction times (more than 24 h) formed less methane and some traces of higher hydrocarbons. Transmission electron microscopy (TEM) images of [C8mim][NfO] show a low degree of aggregation (Figure 5), while [C8mim][CTf3] revealed nanoparticles sintered on a large scale (image provided in the Supporting Information). This observation can confirm the comparative catalytic performance between [NfO] and [CTf3] based ILs and indicate to us that the capacity of the IL to form and stabilize NPs is more relevant than their CO2 solubilities. The surfactant-like structure of the [NfO] anion possessing a long nonpolar chain could be more prone to produce smaller and more stabilized Ru nanoparticles. TEM images of [C8mim][PFO] reveal Ru NPs homogeneously distributed and completely dispersed along the micrograph, suggesting in this case a higher NPs stabilization. The lower yield of methane obtained with [C8mim][PFO] compared to [C8mim][NfO] or [C4mim][PFO] could be attributed to its solid state in the beginning of the reaction.
Figure 5. TEM micrographs and histogram with size distribution of Ru NPs generated in [C8mim][NfO] ionic liquid.
In general, the Ru NPs average size is around 2.5 nm for most of the selected ILs. For this reason, we concluded that the performance of the CO2 hydrogenation reaction depends more from Ru NPs stabilization than NP size. Additionally, the type of IL influences the methane production: in general, more hydrophobic ILs are more favorable. The anion basicity seems to reduce the methane formation. Comparing [OTf] and [TFA], both anions possess a similar structure apart from the carboxylate group from TFA that confers a higher basic nature. A similar comparison could be considered between [NfO] (sulfonate anion) and [PFO] (carboxylate anion). Two ILs possessing sulfonate anions ([OTf] and [NfO]) allowed more methane production. Apart from the reactions where methane was formed, other tested ILs provided different results. After reaction using [C4mim][DCA], the IL showed a yellowish and foamy appearance and no hydrogenation was detected. It was assumed that the strong coordinating nature of dicyanamide prevented the reduction of the metal precursor.31 It is particularly interesting that a change in the reaction selectivity using [TFA] and [MeSO3] based ILs was obtained. For these ILs, no methane was formed but a small amount of CO was produced (2 to 8% yield). According to one of the most accepted mechanisms for CO2 methanation, CO is an intermediate product of the reaction. For these cases, it is possible that [TFA] and [MeSO3] are not so efficient to produce and stabilize Ru NPs compared to other anions (e.g., [OTf], [NTf2], and [NfO]). TEM images showed a lower 11966
DOI: 10.1021/acssuschemeng.8b06877 ACS Sustainable Chem. Eng. 2019, 7, 11963−11969
ACS Sustainable Chemistry & Engineering
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CONCLUSION Task-specific ionic liquids can greatly influence the CO2 hydrogenation conversion to methane. Depending on the IL structure as media, it is possible to change the activity and selectivity of the reaction. It is observed that longer alkyl chains from the imidazolium cation as well as the replacement of the acidic proton at the C-2 position of the imidazolium ring improve the reaction yield. Fluorinated anions based ILs exhibited a greater influence on the methane production. Methane formation is dependent on several parameters including CO2 solubility in the IL media and the further capability of each IL to form and stabilize the Ru NPs. The size of NPs it seems not crucial for the catalytic performance while the presence of small amounts of water from IL media improve the reaction. [C8mim][NfO] is the best IL media, producing 84% yield of methane at 150 °C. The catalytic system remains active at least after 5 recycling runs, and a promissory improvement can be achieved using a further continuous-flow system.
concentration of Ru NPs in the cases of [C8mim][TFA] and [C8mim][MeSO3] (see the Supporting Information). Nanoparticles from both samples were more dispersed than the samples of ILs that produce methane. Recycling Study. [C8mim][NfO] as the best IL was selected for the recycling study (Figure 6). The system was
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Figure 6. Recyclability study of [C8mim][NfO]/Ru(0) NPs system.
Table 2. Study of [C8mim][NTf2] Water Content in CO2 Methanation Reactiona
Vacuum dried IL IL no H2O addition 0.25 mL H2O addition 0.50 mL H2O addition
Water content % (w/w)
CH4 yield (%)
TONb
TOFc (h−1)
0.01 0.15 16.08
0 63 60
0 56 54
0 2.33 2.25
27.51
41
36
1.50
EXPERIMENTAL SECTION
Materials. All compounds were used as supplied. Hydrogen and carbon dioxide were purchased from Air Liquide with a stated purity of 99.998 mol %. The precursor, [Ru(COD)(2-methylallyl)2], was supplied by Sigma-Aldrich. The ionic liquids [C4mim][Cl], [C8mim][Cl], [C2mim][NTf2], [C4mim][NTf2], [C10mim][NTf2], [C8mim][BF4], [C8mim][PF6], and [C4mim][DCA] were supplied by Solchemar with a purity >98% while [C8mim][NTf2], [C4dmim][NTf2], [C4mim][OTf], and [C8mim][OTf] were supplied by Iolitec with a purity of 99%. In the case of ILs, the reactants potassium nonafluoro-1-butanesulfonate (98%) and methanesulfonic acid (≥99%) were purchased from Sigma-Aldrich; potassium tris(trifluoromethanesulfonyl)methide (98%) was purchased from SynQuest Laboratories; sodium trifluoroacetate, sodium perfluorooctanoate, and sodium hydroxide (98%) were purchased from Alfa Aesar. The solvent ethanol absolute anhydrous (≥99.9%) was purchased from Carlo Erba. The solvents acetone (≥99.8%), methanol (≥99.8%), and dichloromethane (≥99.9%) were purchased from Sigma-Aldrich. Ionic Liquid Synthesis. The method for ILs synthesis was adapted from the literature46 with minor changes. Solvents used for ionic exchange were ethanol for [CTf3] and [TFA], water for [MeSO3], and acetone for [PFO] based ILs. [C4mim][NfO]: colorless liquid, 82.3%. [C8mim][NfO]: white solid, 67.9%. [C4mim][CTf3]: colorless liquid, 73.3%. [C8mim][CTf3]: colorless liquid, 86.4%. [C4mim][TFA]: colorless liquid, 68.9%. [C8mim][TFA]: yellowish liquid, 75.2%. [C4mim][MeSO3]: colorless liquid, 73.4%. [C8mim][MeSO3]: colorless liquid, 78.1%. [C4mim][PFO]: colorless liquid, 74.6%. [C8mim][PFO]: white solid, 79.0%. All experimental details and characterization of the prepared ILs are in the Supporting Information. Catalytic Experiments. The reduction reaction took place in a stainless-steel autoclave connected to a hydrogen and carbon dioxide supply. In a typical experiment 1 mL of the selected IL was charged in the autoclave together with 0.04 g of [Ru(COD)(2-methylallyl)2]. At 40 °C, hydrogen (40 bar) was admitted to the system followed by carbon dioxide up to a total pressure of 80 bar. Temperature was increased to 150 °C, and the mixture was stirred for 24 h reaching a total pressure around 125 bar. After the reaction the reactor was set again to 40 °C and gas sample was taken for GC-TCD analysis. The ionic liquid phase was analyzed by 1H NMR to check IL stability. Analytical Methods. Identification and quantification of the compounds found in the gas phase were performed by GC. After each reaction, gas from the reactor was flushed through a trap in order to equilibrate the gas with atmospheric pressure, and samples were taken with a VICI precision sampling gas syringe, A-2 series, and injected in a GC Thermo Trace GC Ultra instrument, equipped with a 1 mL
active after 5 cycles without any washing or treatment between each run. During these cycles the activity dropped at a constant rate, with a total activity drop of around 20%. Nanoparticles kept the same size (2.4 nm) after the 5 runs. The activity decrease can be attributed to the water accumulation after each reaction as well as a possible agglomeration of Ru nanoparticles suggested by TEM images. Water Presence. It has been reported that the presence of water during Ru(0) NPs formation leads to agglomeration44 of the nanoparticles; however, in some hydrogenation reactions the activity of these catalysts improves.28,45 To study the water effect in CO2 hydrogenation, four reactions were made using [C8mim][NTf2] with different water contents (Table 2).
Sample
Research Article
a
Reaction conditions: 1 mL of [C8mim][NTF2]; 24 h of reaction; total pressure of 80 bar at 40 °C; H2:CO2 pressure ratio of 1:1; reaction temperature of 150 °C. bCalculated as mol CH4/mol Ru catalyst. cCalculated as TON/treaction.
Reaction yield lowered when water was added to the reaction; this is an expected outcome since water is one of the reaction products. However, no CO2 conversion was observed in the case where IL was completely dried (
