Catalytic Hydrogenation of Carbon Dioxide to Formic Acid

CHAPTER SEVEN

Catalytic Hydrogenation of Carbon Dioxide to Formic Acid Arno Behr, and Kristina Nowakowski Technical Chemistry, Department of Bio- and Chemical Engineering, Technical University of Dortmund, Dortmund, Germany

Contents 1. Introduction 2. Hydrogenation of Carbon Dioxide 2.1 Thermodynamics and mechanism of the formic acid synthesis 2.2 Heterogeneously catalyzed hydrogenation 2.3 Homogeneously catalyzed hydrogenation 3. Continuous Hydrogenation of Carbon Dioxide in Miniplant Scale 3.1 Preliminary batch experiments 3.2 Miniplant for carbon dioxide hydrogenation 3.3 Miniplant experiments 4. Conclusions Acknowledgments References

224 226 228 230 230 247 247 250 251 252 253 254

Abstract In recent years, the utilization of carbon dioxide as alternative C1 building block has gained more and more scientific interest and has been intensely investigated. Especially the homogeneously catalyzed hydrogenation of carbon dioxide to formic acid and its derivates has been well studied. Recently, an increase in product formation was achieved by further development of the homogeneous catalysts. Currently, iridiumbased catalysts offer the highest catalytic activity known in the hydrogenation of carbon dioxide. The present chapter gives a wide overview of various catalyst systems, which have been investigated so far. In addition, current research on the continuously operated hydrogenation of carbon dioxide in miniplant scale with a promising concept for catalyst recycling is presented. Keywords: Formic acid, Carbon dioxide, Homogeneous catalysis, Hydrogena-

tion, Catalyst recycling, Miniplant technique

Advances in Inorganic Chemistry, Volume 66 ISSN 0898-8838 http://dx.doi.org/10.1016/B978-0-12-420221-4.00007-X

#

2014 Elsevier Inc. All rights reserved.

223

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Arno Behr and Kristina Nowakowski

LIST OF THE USED ABBREVIATIONS [EMIM]Cl 1-ethyl-3-methylimidazolium chloride acac acetylacetonate AcO acetoxy group bipy 2,20 -bipyridine cod cyclooctadiene Cp, Cp* cyclopentadienyl, pentamethylcyclopentadienyl CYPO Cy2PCH2CH2OCH3 DBF dibutylformamide DBU 1,8-diazabicyclo[5.4.0]undec-7-ene dcpb Cy2P(CH2)4PCy2 dcpe 1,2-dicyclohexylphosphinoethane dhbipy 4,40 -dihydroxy-2,20 -bipyridine DHphen 4,7-dihydroxy-1,10-phenanthroline dippe 1,2-(diisopropylphosphino)ethane DMSO dimethyl sulfoxide dppb 1,4-bis(diphenylphosphino)butane dppbts tetrasulfonated 1,4-bis(diphenylphosphino)butane dppe 1,2-bis(diphenylphosphino)ethane dppm bis(diphenylphosphino)methane dppp 1,3-bis(diphenylphosphino)propane EDTA ethylenediaminetetraacetic acid EMIM 1-ethyl-3-methylimidazolium hfacac hexafluoroacetylacetonate ICP inductively coupled plasma analysis mTPPMS meta-monosulfonated triphenylphosphine n.a. not available NBD bicycle[2.2.1]hepta-2,5-diene NHC N-heterocyclic carbene NMR nuclear magnetic resonance PP3 [P(CH2CH2PPh2)3] PTA 1,3,5-triaza-7-phosphaadamantane RT room temperature sc supercritical TEtA triethanolamine THF tetrahydrofuran TOF turnover frequency TON turnover number Tp hydrotris(pyrazolyl)borate TPPMS monosulfonated triphenylphosphine TPPTS trisulfonated triphenylphosphine

1. INTRODUCTION At the present time, the utilization of carbon dioxide (CO2) is of particular interest. Nature shows us the way: the carbon dioxide existing on

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225

earth’s atmosphere can be fixed very well in organic compounds by photosynthesis. Therefore, it is also conceivable to use carbon dioxide as raw material and carbon source for chemical synthesis. Carbon dioxide is an alternative and attractive C1 building block compared with other C1-starting materials (1–10). In contrast to usual carbon sources, such as carbon monoxide (CO) or phosgene (COCl2), carbon dioxide is nontoxic, affordable, and available in large quantities as by-product of various chemical and energetic processes (11–13). On the other hand, carbon dioxide is thermodynamically unfavorable, and for the conversion efficient and highly active catalysts, electrical reductive or photocatalytic methods (14–22) are required. However, the two last-mentioned methods are beyond the scope of this chapter. The amount of theoretical investigations on the homogeneously catalyzed conversion of carbon dioxide is increasing more and more (23,24). Carbon dioxide is already being used as a chemical feedstock in the industrial conversion of some organic compounds. For instance, urea, salicylic acid, and cyclic carbonates are chemical compounds based on carbon dioxide (25). However, the amount of chemically used carbon dioxide accounts only for 1% of the total quantity of carbon dioxide, whereby in 2008, the global anthropogenic carbon dioxide emission amounted to 30  109 t per year with a rising trend (12,26). Another interesting product available via carbon dioxide conversion is formic acid, a basic chemical that worldwide produced 324,000 t in 2003, whereby BASF is the largest producer (27). The applications of formic acid and its salts include a wide variety, for example, starting chemical for esters, alcohols, or pharmaceutical products. In addition, formic acid is used in the textile, leather or dye industry and as cleaning or disinfection solution. Currently, formic acid is commercially produced by the carbonylation of methanol with carbon monoxide to methyl formate and the following hydration of the ester with water (see Equation 7.1) (28): ð7:1Þ Alternative routes for the formic acid production are the following: Oxidation of hydrocarbons Hydrolysis of formamide Carbonylation of a hydroxide (e.g., sodium hydroxide) and subsequent acid hydrolysis These processes are generally depending on the toxic carbon monoxide or synthesis gas. An alternative and economical synthesis route is the • • •

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Arno Behr and Kristina Nowakowski

hydrogenation of carbon dioxide to formic acid, whereby the usage of carbon dioxide offers a one-step reaction control (Equation 7.2): ð7:2Þ The back reaction, the decomposition of formic acid to carbon dioxide and hydrogen with homogeneous or heterogeneous catalysis, is also an important topic of current research. Consequently, the resulting formic acid can be used as easily transportable hydrogen storage (29–45). However, formic acid is not necessarily an effective storage medium for hydrogen related to the mass of formic acid, but the separation offers advantages: easy decomposition, carbon dioxide as coupling product, and unproblematic handling. This chapter summarizes the studies on the homogeneously catalyzed hydrogenation of carbon dioxide with various catalysts. In addition, several separation possibilities and also our own investigations of the continuous hydrogenation of carbon dioxide in miniplant scale are presented .

2. HYDROGENATION OF CARBON DIOXIDE The catalytic conversion of carbon dioxide with hydrogen leads to a wide range of compounds and offers potential access to various basic chemicals (46–51). However, these conversions of neat CO2 have not yet been used in industrial scale. Methanol or even carbon monoxide can be produced with an excess of hydrogen (52–55). The direct hydrogenation of carbon dioxide to methanol based on the mixture CO/H2/CO2 is already performed in the industrial production of methanol (27,56). Other possible hydrogenation products, such as formaldehyde or even carbon monoxide, can be synthesized by carbon dioxide hydrogenation (see Figure 7.1). Formic acid derivates, salts, or adducts are formed in the presence of a third component (Figure 7.2). In the case of the formate esters, an alcohol, for example, methanol or ethanol, is necessary for giving methyl or ethyl formate. The yield of formate esters decreases with increasing alkyl chain. The formation occurs in two steps: the catalytic hydrogenation of carbon dioxide to formic acid and the subsequent thermal esterification (Equation 7.3). In most cases, the esterification proceeds in alkaline solution; nevertheless, the desired reaction is also possible without a base (57,58):

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227

Figure 7.1 Hydrogenation products of carbon dioxide.

Figure 7.2 Formic acid derivates based on carbon dioxide and hydrogen.

ð7:3Þ

Jessop and coworkers studied the hydrogenation of supercritical carbon dioxide (scCO2) with methanol and showed the subsequent esterification in supercritical medium (59). The synthesis of dimethylformamide in the presence of the secondary amine dimethylamine is feasible in the same solvent (60,61). Earlier, it has been reported that several catalysts, such as (Ph2PCH2CH2PPh2)2CoH, (Ph3P)2(CO)IrCl, and (Ph3P)3CuCl, achieved great turnover number (TON) in the formation of dimethylformamide under mild reaction conditions (62). In 1975, Inoue et al. reported the first synthesis of formates in the presence of alcohols, especially ethanol, and tertiary amines catalyzed by Pd(dppe)2 (63). In the following chapters, the hydrogenation of carbon dioxide to formates will be discussed in detail.

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2.1. Thermodynamics and mechanism of the formic acid synthesis Since the mid-1970s, the hydrogenation of carbon dioxide to formic acid has been intensely studied. The conversion of the comparatively unreactive carbon dioxide to pure formic acid is an endergonic process (△G0 ¼ 32.8 kJ mol1) and therefore thermodynamically unfavorable. In the presence of an additive, such as Na2CO3, triethylamine (NEt3), or ammonia, the reaction becomes exergonic (△G0 ¼ 9.5 kJ mol1) and takes place spontaneously (see Figure 7.3). Many recent studies are focused on the mechanism of the homogeneously catalyzed conversion of carbon dioxide with hydrogen (57,65,66). Several possible mechanisms are conceivable, whereby the insertion of carbon dioxide into the M–H bond is identical for all mechanisms. Carbon dioxide forms a metal formate complex via 2-coordination and migration of hydrogen. The generally accepted mechanism can be postulated as shown in Figure 7.4 (4,67). On the other hand, the insertion can theoretically occur by M–H bond breaking based on the weak H–CO2 interaction (68,69). The following elimination of formic acid can proceed in four different ways (70): • Reductive elimination of formic acid (71) • Addition of hydrogen to the formate complex (57) • Direct hydrogenolysis of the M–O bond without previous oxidative addition to hydrogen (66) • Hydrolysis of the metal formate (72) Organic solvents, water, ionic liquids, or directly supercritical carbon dioxide can be used as solvents for the conversion of carbon dioxide. One of the first investigations already demonstrated the promoting effect of water for the homogeneously catalyzed hydrogenation of carbon dioxide (72). Small amounts of water can increase the catalytic activity, whereas no

Figure 7.3 Thermodynamics of CO2 hydrogenation (64).

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229

Figure 7.4 General mechanism for the homogeneously catalyzed hydrogenation of carbon dioxide to formic acid (4).

distinct clarifications have existed until then. Yin et al. investigated and proved the positive influence of water with the metal hydride species TpRu(PPh3)(CH3CN)H (Tp ¼ hydrotris(pyrazolyl)borate) in the catalyzed hydrogenation (73). By experimental and theoretical studies, the catalytic cycle of the simultaneous transfer of a hydride and a proton from the intermediate TpRu(PPh3)(H2O)H to carbon dioxide to form formic acid was successfully proven. On the other hand, the presence of water in the rhodium-catalyzed hydrogenation in nonprotic solvents and tertiary amine has an unfavorable influence on the catalytic activity (74). Performing the reaction in water, the employed ligands are needed to be soluble in water. For this purpose, various water-soluble ligands with different substituents have been investigated, such as acidic (SO3H) or basic (NR2) functional groups (75–77). Carbon dioxide dissolved in water exhibits the following equilibrium (Equation 7.4): ð7:4Þ In addition to the hydrogenation of carbon dioxide, the reduction of HCO 3 in aqueous solution is feasible by transition metal complexes (78–81). At present, a variety of homogeneous catalysts of group VIII transition metals indicate a high activity for the synthesis of formic acid and its derivates based on carbon dioxide. In the following chapters, the first investigations of carbon dioxide hydrogenation in heterogeneous systems and subsequent the homogenous catalyst systems are presented.

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2.2. Heterogeneously catalyzed hydrogenation In 1935, Farlow and Adkins were the first to discover the synthesis of formic acid by the hydrogenation of carbon dioxide using the heterogeneous catalyst raney nickel. After 1 h reaction time at 80  C, 55% yield of formate was obtained in the presence of various amines. If the reaction temperature is above 100  C, the substituted formamide could form via dehydration of the formate (82). Previous works proposed the heterogeneously catalyzed hydrogenation, ruthenium catalysts immobilized on polystyrene resin or silica, and the use of activated carbon supported ruthenium (83–85). Ruthenium hydride species are considerably active in the hydrogenation of carbon dioxide (67,86,87). Hao et al. showed that the g-Al2O3-supported ruthenium catalyst (2.0 w%) achieved a maximal TON up to 120 at 80  C. Furthermore, a possible mechanism of the heterogeneously catalyzed hydrogenation could be demonstrated (88). A recent patent of BASF illustrates a process for the heterogeneously catalyzed hydrogenation of carbon dioxide to formic acid, in which the use of a titania-supported gold catalyst and tertiary amines are described (89). Previous investigations of Preti et al. presented the catalytic activity and high stability also towards carbon monoxide of the used gold catalyst (90,91). Great advantages of the heterogeneously catalyzed hydrogenations are the simple separation of the catalyst and the product phase, the stability and handling of the catalyst, and the easy reusability. However, the catalytic activity is limited; therefore, an enhancement of the yield can only be achieved by using homogeneous catalysis.

2.3. Homogeneously catalyzed hydrogenation In 1976, Inoue et al. reported the first homogenously catalyzed hydrogenation of carbon dioxide. Group VIII transition metal complexes, such as RhCl(PPh3)3, Pd(dppe)2, and H3Ir(PPh3)3, combined with triethylamine and water, were used under mild conditions (72). Based on these results, a variety of homogenous catalysts, in particular ruthenium- and rhodium-based systems, have been studied for the hydrogenation of carbon dioxide. At present, iridium catalysts achieved the highest known yields in the conversion of carbon dioxide. In the following, a detailed overview will be provided for the homogeneously catalyzed carbon dioxide hydrogenation.

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2.3.1 Precious metal catalysts for the hydrogenation of carbon dioxide First, precious metal homogeneous catalysts based on ruthenium, rhodium, and iridium will be presented, followed by the nonprecious metal catalysts. As already mentioned, the first investigation for the hydrogenation of carbon dioxide was carried out with group VIII transition metal complexes. In the first experimental studies, the yield of formic acid was very low. By continuous development of potential catalysts, an increase of the formic acid yield could be achieved. 2.3.1.1 Ruthenium catalysts in the hydrogenation of carbon dioxide

Table 7.1 gives a comprehensive overview of the ruthenium-catalyzed syntheses of formic acid and its derivates based on carbon dioxide. TpRu(PPh3)(CH3CN)H (Tp ¼ hydrotris(pyrazolyl)borate) forms different intermediates in various alcohols. Ng and Yin have suggested that the catalytic cycle of the carbon dioxide hydrogenation is the same with nonacidic alcohols as well as with acidic 2,2,2-trifluoroethanol (CF3CH2OH) (see Figure 7.5). Judging on the high promoting effect, the acidic alcohol CF3CH2OH has a high catalytic activity (97). For thermodynamically reasons, it is conventional to employ organic bases or bicarbonates as additive by using ruthenium or rhodium catalysts. However, inorganic bases can also be used. [RuCl(C6Me6)(DHphen)]Cl obtains a very high TON up to 15,400 in combination with the inorganic base potassium hydroxide (Table 7.1, entry 3). Due to the addition of base, the majority of the hydrogenation of carbon dioxide is carried out in basic solution. Hayashi observed the positive influence of a proton on the reduction of carbon dioxide under acidic condition (pH range 3–5) catalyzed by the in situ formed hydride complex [(6-C6Me6)Ru(II)(bpy) H]2þ (109). Further mechanistic investigations were carried out by Ogo and Fukuzumi with a special emphasis on the hydrogenation of carbon dioxide in the presence of water-soluble arene ruthenium and iridium aqua complexes (110,111). Thai et al. showed that the neutral and cationic complexes [(6-arene)Ru(2-N,O-L)Cl] and [(6-arene)Ru(2-N,O-L) (H2O)]þ (LH ¼ 8-hydroxyquinoline) catalyzed the hydrogenation of carbon dioxide to formate in aqueous solution (107). The group around Joo´ extensively investigated the hydrogenation of carbon dioxide in amine-free aqueous reaction solution (78,80,112). The catalytic activity depends on the pH value, whereby in certain cases, the yield increases at a pH level of 8. Especially [RuCl2(TPPMS)2]2 achieved high TON in the hydrogenation of NaHCO3 (79). Also, Gao has discovered a

Table 7.1 Ruthenium catalysts used for the hydrogenation of carbon dioxide to formic acid and their derivates Pressure CO2/H2 Time TON Temp. (bar) (h) () Entry Catalyst Solvent Base/additive ( C)

TOF (h1)

Refs.

1

[Ru(Cl2bipy)2(H2O)2] [CF3SO3]2

EtOH

NEt3

50

30/30

8

5000

625

(58)

2a

RuCl2(PMe3)4

scCO2

NEt3

50

120/80

47

7200

n.a.

(87)

3

[RuCl(C6Me6)(DHphen)]Cl

H2O

KOH

120

30/30

24

15,400 642

(92)

RuCl(OAc)(PMe3)4

scCO2

NEt3

50

120/70

0.3

31,700 95,000

(93)

[RuCl2(mTPPMS)2]2

H2O

NaHCO3

80

35/60

0.03

320

9600

(79)

RuH2(PMe3)4

scCO2

NEt3

50

120/85

1

1400

1400

(86)

7

Ru2(CO)5(dppm)2

Acetone

NEt3

RT

38/38

21

2160

103

(94)

8a

RuCl3/PPh3

EtOH

NEt3

60

60/60

5

200

40

(95)

9

CpRu(CO) (m-dppm)Mo(CO)2Cp

Benzene

NEt3

120

30/30

45

43

1

(96)

10

TpRuH(PPh3)(CH3CN)

THF

NEt3

100

25/25

16

1815

113

(73,97)

11a

RuH2(PPh3)4

Benzene

NEt3

RT

25/25

20

87

4

(72)

12

RuH2(PPh3)4

Benzene

Na2CO3

100

25/25

4

169

42

(98)

13

[(C5H4(CH2)3NMe2)Ru(dppm)] THF BF4

None

80

40/40

16

8

0.5

(99)

14

[RuCl2(CO)2]n

NEt3

80

27/81

0.3

400

1300

(100)

4

b

5 6

a

H2O, iPrOH

15

RuCl3(aq)/dppbts

H2O

HNMe2

60

25/25

15

4980

n.a.

(101)

16

K[RuCl(EDTA-H)Cl]2H2O

H2O

None

40

17/3

0.5

n.a.

250

(102)

17

RuCl2(PTA4)

H2O

NaHCO3

80

0/60

n.a

n.a

807

(78)

18

[(Cl2bipy)2Ru(H2O)2] [CF3SO3]2

EtOH

NEt3

150

30/30

8

5000

n.a.

(58)

19

[(NHC)2RuCl][PF6]

H2O

KOH

200

20/20

75

23,000 n.a.

(103)

20

[(C6Me6)Ru(dhbipy)]

H2O

KOH

120

30/30

8

13,620 4400

(104)

21

[(dppm)Ru(H)(Cl)]

H2O

NaHCO3

70

0/50

2

1374

687

(105)

22

[RuCl2(TPPMS)2]2

H2O

NaHCO3

25

5/35

2

524

262

(80)

23

[Cp*Ru(DHphen)Cl]Cl

H2O

KOH

120

30/30

24

15,400 3600

(106)

24c

[(6-arene)Ru(2-N,O-L) (H2O)]BF4

H2O

NEt3

100

50/50

10

400

n.a.

(107)

25

RuH2(PMe3)4

scCO2

NEt3

50

120/80

3

1900

630

(87)

26

RuH2(PPh3)4

DBF

TEtA

50

30/30

1

698

698

(108)

a

Water as additive. C6F5OH as additive. LH ¼ 8-hydroxyquinoline. All abbreviations can be found in “List of the Used Abbreviations.”

b c

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Figure 7.5 Mechanism of the TpRu(PPh3)(CH3CN)H-catalyzed hydrogenation in nonacidic alcohols (Tp ¼ hydrotris(pyrazolyl)borate) (97).

critical dependence on the pH value for the formation and decomposition of formic acid in acetone in the presence of Ru2(CO)5(dppm)2 (94). Nevertheless, in the absence of a base or an additive, the reaction of carbon dioxide occurs with very low TONs and TOFs (Table 7.1, entry 13, 16). By reference to these reaction conditions, the necessity of an additive is revealed. Particularly, the use of the catalyst RuH2(PPh3)4 shows the obvious influence of various additives. In comparison to triethylamine, the carbonate Na2CO3 reaches the 10-fold higher catalytic activity in terms of turnover frequency (TOF). With an increasing excess of PMe3 using RuH2(PMe3)4, the TOF is reduced from 1300 to 360 (87). Musashi and Sakaki suggested that an excess of phosphine is not advantageous for the reaction but rather the employed solvent is important for the dissociation of the phosphine (67). By the application of further theoretical investigations, it was found that the carbon dioxide insertion into the Ru(II)–H bond is the rate-determining step, whereby other reactions steps, such as isomerization of ruthenium formate and the metathesis of 1-formate intermediate, succeed faster in the experimental investigations. The use of polar solvents is recommended to overcome the rate-determining step (113).

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235

The bimetallic complexes of Ru–Mo and Ru–W show low catalytic activity in the formation of formic acid, which is due to the lacking formation of the required hydride intermediate (96). Instead of organic solvents, a conceivable alternative for the hydrogenation of carbon dioxide is supercritical carbon dioxide (scCO2) (114,115). Jessop et al. described the first successful use of supercritical solvents for the production of formic acid. The desired reaction occurs in a supercritical mixture of CO2 and H2 in the presence of ruthenium(II) phosphine complexes. Under these conditions, a TON of 1400 was obtained, whereas organic solvents provided a lower activity and a slower reaction (86). In contrast to the chlorine-containing ruthenium phosphine complex RuCl2(PMe3)4, lower TONs are achieved by using RuH2(PMe3)4 (87). Both theoretical and experimental investigations were carried out by Baiker and coworkers (116). In addition, Jessop et al. confirmed that an enhancement of the TON is possible by using scCO2 as solvent. In the presence of triethylamine, RuCl(OAc)(PMe3)4 achieved the highest TON in ruthenium systems up to 31,700 (Table 7.1, entry 4) (93). It is supposed that the higher yield is attributed to the increased miscibility of hydrogen in scCO2 (117). Mechanistic investigations of the ruthenium trimethylphosphine-catalyzed hydrogenation, especially with RuCl(OAc) (PMe3)4, were carried out using high-pressure NMR spectroscopy (118). The active catalyst in the reaction is an unsaturated, cationic ruthenium complex [(PMe3)4RuH]þ. Getty et al. suggested that the used base DBU is involved in the generation of the active catalyst species. Leitner et al. have studied the continuous-flow hydrogenation of supercritical carbon dioxide containing the immobilized catalyst [Ru(cod)(methallyl)2] with [EMIM]Cl and [PBu4][TPPMS] as ligand. The biphasic system contains a mobile (scCO2) and a stationary phase (ionic liquid). The scCO2 is simultaneously reactant and extractive phase for the pure formic acid. By a continuous removal of formic acid, the reaction takes place with a TOF of 1089 at 50  C and 0.5 h of reaction (119,120). Based on these results, supercritical carbon dioxide seems to be a promising, alternative solvent for carbon dioxide hydrogenation. A further ionic liquid system with an immobilized ruthenium catalyst containing water and a tertiary amine was applied by Zhang et al. (121). In 2010, Yasaka et al. reported the use of ionic liquids based on imidazole for the hydrogenation of carbon dioxide to formic acid. The product is stabilized by the formate anions of the ionic liquid (122).

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2.3.1.2 Rhodium catalysts in the hydrogenation of carbon dioxide

The rhodium catalysts, which are described in literature for the hydrogenation of carbon dioxide, are summarized in Table 7.2. In 1981, Yamaji examined the hydrogenation of carbon dioxide with the Wilkinson complex RhCl(PPh3)3 and Na2CO3 as additive in benzene (123). After 3 h reaction time, a TON up to 173 was reached (Table 7.2, entry 6). In addition, Ezhova et al. have reported that RhCl(PPh3)3 with triethylamine and PPh3 as added ligands achieved a distinct higher TON by increasing the P:Rh ratio (124). The group of Leitner has extensively investigated the hydrogenation of carbon dioxide using rhodium complexes with bidentate phosphine ligands (74,125,126). The in situ formed rhodium complexes are active systems for the direct formation of formic acid. The generated complex [RhCl (cod)]2/dppb in dimethyl sulfoxide (DMSO) and under mild reaction condition achieved a TOF up to 52 (Table 7.2, entry 12). The steric and electrochemical properties of the phosphorous ligands have a considerable influence on the catalytic activity. This is particularly the case in the direct comparison with dppb and dippe with [RhCl(cod)]2. By using dippe as ligand, the TOF is lower than with dppb (Table 7.2, entry 5). Kinetic measurements of the rhodium–cyclooctadiene complexes have shown that the neutral rhodium(I) hydride complexes are the active intermediates in the rhodium-catalyzed hydrogenation (74). Furthermore, Schindler has studied the kinetic of the hydrogenation of carbon dioxide in detail using the formed complex [(dppp)2RhH] and various solvents (127). The cationic formate complex [(dppp)2Rh]HCOO is generated in DMSO; therefore, the insertion is only possible in DMSO. At the same time, Tsai and Nicholas have identified the detailed mechanism of the hydrogenation of carbon dioxide catalyzed by [Rh(NBD) (PMe2Ph)3]BF4 in tetrahydrofuran (THF) (see Figure 7.6). In the presence of hydrogen, the catalytic active rhodium dihydride complex [H2Rh (PMe2Ph)3(S)] [S ¼ H2O, THF] is formed. Via kinetic and spectroscopic investigations, it was experimentally proven that the reductive elimination step is the limiting factor (65). By theoretical investigations, Hutschka and Dedieu were able to show the same results (128,129). Pomelli et al. investigated the elimination step of formic acid in the presence of rhodium complexes by density functional theory and figured out that the dissociation of formic acid is thermodynamically preferred in scCO2 (130).

Table 7.2 Rhodium catalysts used for the hydrogenation of carbon dioxide to formic acid and their derivates Entry Catalyst Solvent Base Temp. ( C) Pressure CO2/H2 (bar) Time (h) TON () TOF (h1) Refs.

1

Rh(hfacac)(dcpb)

DMSO

NEt3

25

20/20

5

n.a

1335

(132)

2

RhCl(TPPTS)3

H2O

NHMe2

RT

20/20

12

3439

n.a.

(131)

3

[RhH(cod)]4/dppb

DMSO

NEt3

RT

20/20

18

2200

375

(74)

4

RhCl(TPPTS)3

H2O

NHMe2

81

20/20

0.5

n.a.

7260

(133)

5

[RhCl(cod)]2/dippe

DMSO

NEt3

24

40 total

18

205

11

(126)

6

RhCl(PPh3)3

benzene Na2CO3

100

55/60

3

173

58

(123)

7

[Rh(2-CYPO)2]BPh4

MeOH

NEt3

55

25/25

7

1000

n.a

(134)

8

RhCl3/PPh3

H2O

NHMe2

50

10/10

10

2150

215

(135)

a

9

[Rh(NBD)(PMe2Ph)3]BF4 THF

None

40

48/48

48

128

3

(65)

10

RhCl(PPh3)3/PPh3

MeOH

NEt3

25

40/20

20

2700

125

(124)

11

RhCl3(aq)/dppbts

H2O

HNMe2

60

25/25

15

3910

n.a.

(101)

12

[RhCl(cod)]2/dppb

DMSO

NEt3

RT

20/20

22

1150

52

(125)

13

RhCl(TPPTS)3

H2O

NEt3

23

20/20

n.a.

n.a.

1364

(133)

[Cp*Rh(DHphen)Cl]Cl

H2O

KOH

80

20/20

32

2400

270

(106)

15

b,c

Rh(NO)(dcpe)

n.a.

DBU

50

1.5/1.5

16

106

n.a.

(136)

16

d

[RhCl(mTPPMS)3]

H2O

HCOONa 50

50/50

20

65

n.a.

(137)

17

d

[RhCl(mTPPMS)3]

H2O

CaCO3

20/80

24

300

n.a.

(138)

14

a

Water as additive. Rh(III)(NO)(dcpe)Cl2 achieved similar results. Addition of the cocatalyst C6F5OH shows no influence. d TON was calculated by the given data. All abbreviations can be found in “List of the Used Abbreviations.” b c

50

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Arno Behr and Kristina Nowakowski

Figure 7.6 Mechanism of the rhodium-catalyzed carbon dioxide hydrogenation investigated by Tsai and Nicholas (65).

In addition, efficient water-soluble rhodium phosphine complexes, such as RhCl(TPPTS)3, showed a high catalytic activity in water (Table 7.2, entry 2). In the presence of dimethylamine and 12 h reaction time, a TON of 3439 was reached at room temperature (131). Joo´ showed that the formation of free formic acid in aqueous solution is possible in the presence of CaCO3 (138) or HCOONa by using [RhCl (mTPPMS)3] (137). In the CaCO3 system, the rhodium catalyst achieved higher yields of formic acid. A direct comparison of ruthenium- and rhodium-based catalysts shows that the ruthenium catalyst RuCl3(aq) is significantly more active than the rhodium analogous (Table 7.1, entry 15 and Table 7.2, entry 11). The same applies for the half-sandwich complexes [Cp*M(DHphen)Cl] Cl (M ¼ ruthenium or rhodium) (Table 7.1, entry 23 and Table 7.2, entry 14). The previous theoretical investigations by Musashi and Sakaki have suggested that the CO2 insertion into M–H bonds will depend on the metal and its oxidation level. Therefore, the CO2 insertion into the Rh(I)–H and Ru(II)–H bonds is associated with no barrier and a moderate activation barrier, while for the insertion into the Rh(III)–H bond, a distinctly higher activation is necessary (139). In the majority of reactions, the relatively strong base triethylamine is being used as additive. Triethylamine is inexpensive, is available in large

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239

amounts, and is obtaining higher yields compared to other bases. Leitner and coworkers have demonstrated this effect by comparing a variety of bases in the aqueous carbon dioxide hydrogenation with RhCl(TPPTS)3 (133). Munshi and Jessop investigated various organic and inorganic Brønsted and Lewis bases in the hydrogenation of carbon dioxide catalyzed by RuCl(O2CMe)(PMe3)4 and methanol as additive (93). In this context, it was discovered that DBU achieved distinctly higher yields in contrast to triethylamine. In the presence of bases with a weak basic character, no formic acid was formed. The product is a formic acid–amine adduct, whereas various ratios of 1:1, 2:1, and even 3:1 are possible. The common ratios of HCOOH/NEt3 adducts are 1.3:1 till 1.8:1 using ruthenium(II) or rhodium(I) complexes (140), for example, the RuCl2[PMe3]4 complex leads to a ratio of 1.7:1 (Table 7.1, entry 2). Among others, the solvent is decisive for this ratio: in nonprotic solvents, ratios higher than 1 are possible due to the formation of stable 2:1 HCOOH/NEt3 adducts (141). Adduct ratios of 1.6:1 are possible in scCO2 (87). Wagner has indentified that 3:1 adducts are the most stable adducts, and a thermal separation is possible for the longerchain alkyl amines (142). Furthermore, a direct application of the synthesized formic acid–triethylamine adducts as selective hydrogen-transfer reagents in organic synthesis is feasible (143). However, the formation of pure formic acid–triethylamine adducts is realized by the conversion of carbon dioxide and hydrogen in triethylamine at 40  C under 120 bar CO2/H2 with [RuCl2(PMe3)4] as catalyst. After 1 h reaction time, the 1.78:1 adduct is formed. By distillation of the solution, the ratio of the adduct changes into 2.35:1, a stable azeotrope. Before the distillation has been performed, the catalyst is deactivated (140). 2.3.1.3 Separation of the formic acid–amine adducts

To obtain pure formic acid, the product has to split off the salt and the reaction solution. Among others, the formic acid is isolated from the catalyst, which in most cases could also catalyze the decomposition of formic acid depending on the reaction conditions. Consequently, the separation of the formed salts has economical and ecological reasons due to the recovery of the expensive catalysts. One possibility for the separation is the addition of another acid to liberate the formic acid or via base exchange with highboiling amines, such as imidazole (100,144). The use of a high-boiling amines includes a thermal decomposition of the adduct, realizing the amine, which can be recycled again. The negative aspects of the imidazole application are an additional and undesired separation step and the need of further

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solvents (145,146). An additional alternative is the use of multiphase reaction systems, consisting of easily separable catalyst and product phase. Certainly, the phase containing formic acid has to be purified by subsequent processing. Behr et al. showed that in aqueous solutions containing RhCl3 or RuCl3 and the water-soluble ligand dppbts (tetrasulfonated 1,4-bis (diphenylphosphino)butane), an extraction of the resulting formic acid is technically feasible by using N,N-dibutylformamide (101). Another process for the separation of formic acid–amine adducts has been investigated by Paciello and coworkers of BASF. The longer-chain amine trihexylamine is used as base for the lipophilic [Ru(H)2(PnBu3)4]-catalyzed reaction in diols. The formic acid–trihexylamine adduct is not miscible with the free amine but with the polar diol phase, whereas the catalyst remains in the amine phase. The resulting solution of the salt and the diol can be easily separated by distillation under mild conditions (147,148). Other researchers have reported the formation of noncomplexed formic acid in an aqueous solution (137,138). However, conventional distillation of the mixture is not economical since large amounts of energy are required due to the azeotrope formation. Several patents of BASF demonstrate the removal of formic acid from aqueous solution by extraction with secondary formamides, such as N,N-dibutylformamide (149). This method is, among others, applicable to quaternary ammonium formates (150). Subsequently, the formic acid can be isolated from the amine by distillation. 2.3.1.4 Other transition metals in the hydrogenation of carbon dioxide

As can be seen from Table 7.3, also other transition metals, such as palladium, have been investigated in the hydrogenation of carbon dioxide. In general, it can be concluded that palladium shows a lower catalytic activity in the hydrogenation of carbon dioxide. For the formation of potassium formate with the catalyst PdCl2, Kudo achieved a TON up to 1580 at 160  C within 3 h reaction time (Table 7.3, entry 1). Since many years, ruthenium- and rhodium-based catalysts with phosphine ligands achieved the best yields in the hydrogenation of carbon dioxide. However, current research demonstrates certain iridium-based systems with very impressive high TONs, whereby the reaction temperature (120–150  C) is very high in contrast to the conventional ruthenium and rhodium systems (25–50  C). In 2004, the half-sandwich iridium(III)complexes with 4,4-dihydroxy-2,2bipyridine (dhbipy) or 4,7-dihydroxy-1,10-phenanthroline (DHphen) ligands were designed by Himeda et al. (104,106,153–155) (Figure 7.7).

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Table 7.3 Other metals for the conversion of carbon dioxide to formic acid and their derivates Pressure Time TON TOF Temp. CO2/H2 (bar) (h) () (h1) Refs. Entry Catalyst Solvent Base ( C)

PdCl2

H2O

KOH

160

n.a./110

3

1580 530

(151)

PdCl2

H2O

KOH

240

40/106

3

340

n.a.

(151)

Pd(dppe)2 Benzene NEt3

110

25/25

20

62

3

(72)

4

Pd(dppe)2 Benzene NaOH RT

24/24

20

17

0.9

(152)

5a

PdCl2[P Benzene NEt3 (C6H5)3]2

50/50

n.a.

15

n.a.

(144)

1 2 3

a

RT

a Water as additive. All abbreviations can be found in “List of the Used Abbreviations.”

Figure 7.7 Half-sandwich iridium(III)complexes (left: 4,4-dihydroxy-2,2-bipyridine ligand (dhbipy); right: 4,7-dihydroxy-1,10-phenanthroline ligand (DHphen)) (104,106,153–155).

In other works, Joo´ proposed that the formed formic acid could exceed the amount of base. The resulting solution changes its pH value from alkaline to acidic (138). This given fact is used for the performance of the cationic Cp*iridium(III)complexes in aqueous solution. Depending on the pH value, the properties of the dhbipy- or DHphen-containing catalyst change from being dissolved homogeneously in alkaline solution to being precipitated heterogeneously during the reaction. This effect is explained by the electronic effect and polarity of the oxyanion caused by the phenolic hydroxyl group. By the use of these iridium complexes, a high catalytic activity and the possibility of catalyst separation can be achieved. The advantages of homogenous and heterogeneous catalysis are thus combined. Through modification of [Cp*Ir(dhbipy)Cl]Cl, the iridium-based catalyst [Cp*Ir(dhbipy)(H2O)]2þ is obtained, which is applicable for the synthesis of formic acid as well as for the formic acid decomposition in the absence of organic additives. The reaction can be controlled by regulating the pH

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value (156). Moret et al. investigated the influence of pH level and temperature both in the hydrogenation of carbon dioxide and in the decomposition of formic acid via NMR spectroscopy. Important data for the development of new catalysts for the CO2–formic acid cycle were identified (157). Two further iridium-based systems are the [IrI2(AcO)(bis-NHC)] complexes containing heterocyclic carbene ligands (NHC) (see Figure 7.8A and B). Because of the sulfonate functionalities, the water solubility of the catalyst is increased. Additionally, the systems are more active in the reduction of carbon dioxide to formate as in absence of the sulfonate groups. The complex with the bis-abnormal coordination of the NHC ligand currently shows the best catalytic activity in the reduction with isopropanol to formate by transfer hydrogenation (158). Compared with other iridium-based systems, these catalysts reached similar TON in the hydrogenation of carbon dioxide with potassium hydroxide (Table 7.4, entry 5). In 2009, the Ir(III)–pincer complex [(PNP)IrH3] containing two diisopropylphosphanyl substituents was announced by Nozaki et al. (Figure 7.8C) (159). In aqueous solution and with KOH as additive, the Ir(III)–pincer complex achieved a remarkable TON up to 3,500,000 and a TOF of 73,000 after a reaction time of 48 h at 120  C (Table 7.4, entry 4). It has to be mentioned that the basicity has a major influence on the yield. With weaker bases, such as K2PO4, the yield decreases. Currently, this iridium catalyst is the most active system for the hydrogenation of carbon dioxide with an impressive high TON. The mechanism of the iridium(III) trihydride was investigated by means of density functional theory by Ahlquist (160). The computational studies

Figure 7.8 Iridium-based catalysts: (A) and (B) [IrI2(AcO)(bis-NHC)] complexes (C) Ir(III)– pincer complex [(PNP)IrH3] (158,159).

Table 7.4 Iridium complexes used for the hydrogenation of carbon dioxide Entry Catalyst Solvent Base Temp. ( C) Pressure CO2/H2 (bar)

Time (h)

TON ()

TOF (h1)

Refs.

1

[Cp*Ir(dhbipy)Cl]Cl

H2 O

KOH

120

30/30

57

42,000

190,000

(154)

2

[Cp*Ir(DHphen)Cl]Cl

H2 O

KOH

120

30/30

48

222,000

33,000

(106)

3

[Cp*IrCl(DHphen)]Cl

H2 O

KOH

120

30/30

10

21,000

2100

(92)

4

[(PNP)IrH3]

H2O

KOH

120

30/30

48

3,500,000

73,000

(159)

5

[IrI2(AcO)(bis-NHC)]

H2 O

KOH

200

30/30

75

190,000

n.a.

(158)

All abbreviations can be found in “List of the Used Abbreviations.”

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were simplified by replacing the isopropyl groups through hydrogen. The formate complex is formed via two steps. Initially, a H-bound formate complex and a subsequent more stable O-bound complex are formed. The formate complex is replaced by hydrogen coordinating to the iridium(III) center. It was found that the iridium(I) hydride is highly active and thermodynamically advantageous. Yang complemented this research work and showed that the analogous catalysts based on iron and cobalt are promising alternatives (161). The disadvantages of using precious metals without catalyst recycling are the high costs of catalyst and the low availability of some metals. The employment of nonprecious catalysts is an interesting alternative. The first investigations of iron- and cobalt-based systems are presented in section 2.3.2.

2.3.2 Nonprecious metal catalysts for the hydrogenation of carbon dioxide In addition to the active ruthenium, rhodium, and iridium systems, several nonprecious homogenous catalysts are known for the hydrogenation of carbon dioxide. In previous works, Evans and Newell presented the iron catalyst [HFe3(CO)11] for the synthesis of methyl formate (162). Significantly higher activities can be achieved by the combination of FeCl3 (Table 7.5, entry 1) with bidentate phosphine ligands (163). However, for the desired reaction, the expensive base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 100 bar absolute pressure are required. In addition to the iron-based systems, nickel, titanium, and molybdenum are investigated in the hydrogenation of carbon dioxide (see Table 7.5). As an example, NiCl2(dcpe) reaches a comparable TON to ruthenium- or rhodium-based systems, however only after 216 h of reaction (Table 7.5, entry 5). Since 2010, the group of Beller has been investigating new nonnoble homogeneous catalyst systems based on iron and cobalt. The iron catalyst Fe(BF4)26H2O can reduce carbon dioxide and even bicarbonates. Combined with the phosphorous ligand [P(CH2CH2PPh2)3], abbreviated as PP3, the system Fe(BF4)26H2O can hydrogenate sodium bicarbonate to sodium formate with a TON of 610 (Table 7.5, entry 2) (164). In addition to this, the same catalyst can be used for the hydrogenation of carbon dioxide to methyl formate (TON 585), dimethylformamide (TON 727), or formylpiperidine (TON 373). With the assistance of

Table 7.5 Nonprecious metals for the hydrogenation of carbon dioxide Entry Catalyst Solvent Base/additive Temp. ( C) Pressure CO2/H2 (bar) Time (h) TON () TOF (h1) Refs.

1

FeCl3/dcpe

DMSO

DBU

50

60/40

7.5

113

15.1

(163)

2

Fe(BF4)26H2O, PP3

MeOH

NaHCO3

80

0/60

20

610

n.a

(164)

Co(BF4)26H2O, PP3

MeOH

NaHCO3

80

0/60

20

3877

n.a

(165)

4

trans-[(tBu-PNP)Fe(H2) (CO)]

H2O

NaOH

80

3.33/6.66

5

788

n.a

(166)

5

NiCl2(dcpe)

DMSO

DBU

50

160/40

216

4400

20

(163)

6

Ni(dppe)2

Benzene NEt3

RT

25/25

20

7

0.4

(72)

7

TiCl4/Mg

THF

None

RT

1/1

24

15

n.a.

(167)

8

MoCl3/dcpe

DMSO

DBU

50

60/40

7.5

63

8

(163)

3 a

a THF as additive. All abbreviations can be found in “List of the Used Abbreviations.”

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Figure 7.9 Mechanism of the carbon dioxide hydrogenation with Fe(BF4)26H2O, PP3 (164).

in situ high-pressure NMR spectroscopy and X-ray analysis, the mechanism of the iron-catalyzed reaction could be verified for the first time (Figure 7.9). The active and well-defined iron hydride complexes [FeH(PP3)]BF4 and [FeH(H2)(PP3)]BF4 are formed under these reaction conditions (Table 7.5, entry 2). Under carbon dioxide pressure, the intermediate [FeH(CO2)(PP3)] is formed. Subsequently, the insertion of carbon dioxide into the M–H bond occurs, and after a further intermediate stage, formic acid is eliminated. Apart from investigations on the Fe(BF4)26H2O system, an analogous cobalt precursor was developed and tested (165). As a result of this examination, sodium formate was obtained in 94% yield by the hydrogenation of sodium bicarbonate with the cobalt complex Co(BF4)26H2O and the ligand PP3. No activity was observed with various other common ligands, for example, xantphos, triphenylphosphine, and 1,2-bis (diphenylphosphino)ethane. By increasing temperature to 120  C and catalyst loading of 3.49  106 mol, a remarkable TON of 3877 was reached. A variation of cobalt precursors can be performed in this case, because several cobalt complexes, such as Co(acac)2, Co(acac)3, and CoCl2 in combination with the phosphorous ligand PP3, reached similar results. As with the previously presented iron catalyst, also, derivates of formic acid can be synthesized. A further iron-based catalyst is the iron–pincer complex trans-[(tBuPNP)Fe(H2)(CO)] developed by Milstein et al. (see Figure 7.10) (166). Under low pressures (6–10 bar), the hydrogenation of carbon dioxide and sodium bicarbonate occurs in aqueous solution at 80  C with a TON of 788 (Table 7.5, entry 4). The investigations of the mechanism by NMR showed that the iron hydride directly attacks the carbon dioxide and then the formate compound is replaced by water.

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247

Figure 7.10 Iron–pincer complex (166).

3. CONTINUOUS HYDROGENATION OF CARBON DIOXIDE IN MINIPLANT SCALE In the previous chapters, the optimization of catalyst systems in batch operation mode was described. In contrast, our research work is focused on the homogeneously catalyzed hydrogenation of carbon dioxide to formic acid performed in a continuously operated miniplant (168). In general, miniplants are used in process development for the preparation of feasibility studies of new processes. By employing the miniplant technique, important information of the reaction can be attained, such as the long-term stability of the catalyst, the accumulation of by-products, and the catalyst losses in the product phase. Further applications are the examination of the recycling concepts and process optimization. Additionally, miniplants are used in industry for the production of small production volumes, sample quantities, and fine chemicals (169–171). When using homogeneous catalysis, the recycling of the homogeneous transition metals without a loss of activity or the resynthesis of the nonactive catalyst is a possible pathway for an economical process. To accomplish the catalyst recycling, several reaction pathways are possible, for example, the use of liquid–liquid multiphase systems, the reaction with in situ, or the postextraction of the product or the catalyst or thermomorphic multicomponent solvent systems (172–176). The most important and industrially used application of the liquid–liquid multiphase concept is the hydroformylation of propene to butanals in the Ruhrchemie/Rhoˆne– Poulenc process. The aqueous phase contains the water-soluble catalyst, while the butanals and the propene exist in the organic phase (177,178).

3.1. Preliminary batch experiments Prior to transferring the hydrogenation of carbon dioxide into the miniplant technique, the reaction has been investigated and optimized in laboratory scale via batch experiments. Various homogeneous transition metals with

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different oxidations levels, especially ruthenium-, rhodium-, and iridiumbased catalysts, were applied in the hydrogenation of carbon dioxide to formic acid in aqueous solution. The strong base triethylamine serves as additive, and further longer-chain amines were also deployed in the hydrogenation of carbon dioxide, in order to find out in what way the basicity of the amine influences the catalytic activity of the catalyst (see Table 7.6). Ruthenium(III)tris(acetylacetonate) (Ru(acac)3, Figure 7.11) proved to be a robust and highly active catalyst in the hydrogenation of carbon dioxide. The formed product is the salt consisting of formic acid and triethylamine. Performing the hydrogenation of carbon dioxide in the presence of Ru(acac)3, a TON up to 73,000 is achieved after a reaction time of 3 h at 25  C under 30 bar CO2/H2 pressure (1:1). These reaction conditions were optimized for the application in the continuously operated miniplant. In addition, the optimization of the reaction pressure was performed in the miniplant, where a continuous gas dosing is possible. As shown in Table 7.6, the TON of the hydrogenation of carbon dioxide decreases with increasing alkyl chain length of the amine. The effect observed can be ascribed to the lower solubility of carbon dioxide, the decreasing pH level, and the lower solubility of the longer-chain amines in water. Table 7.6 Tertiary amines in the hydrogenation of carbon dioxide Base TON []

pH []

1

Triethylamine

73,000

12.7

2

Tripropylamine

21,365

11.4

3

Tributylamine

11,500

10.6

4

Trihexylamine

2250

7.2

5

Trioctylamine

75

7

VReactor ¼ 300 ml, Vl ¼ 75 ml, p(H2) ¼ p(CO2), p ¼ 30 bar, cat: Ru(acac)3, T ¼ 25  C, c(cat.) ¼ 0.5 mmol l1, t ¼ 3 h, solvent ¼ water, D ¼ 700 rpm.

Figure 7.11 Ruthenium(III)tris(acetylacetonate).

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As can be seen from Figure 7.12, the yield of formic acid can be increased by the variation of the base concentration. The maximal TON is achieved with a base concentration of about 3 mol l1. Higher base concentrations result in a slight decrease of the TON. The reduction of the yield can be explained by the lower water concentration, which stabilizes the formed product. In addition, a two-phase system based on water and triethylamine, which can be separated easily after the reaction, is obtained above a certain concentration of triethylamine (5 mol l1) (see for the principle Figure 7.13). Investigations of the catalyst distribution in the aqueous and organic phase by

55,000 50,000 45,000

TON [–]

40,000 35,000 30,000 25,000 20,000 15,000 10,000 5000 0

0.1

0.5

1.0

1.1

2.1

3.1

4.1

5.1

c(NEt3)[mol/l]

Figure 7.12 Formation of formic acid–triethylamine salt depending on the base concentration. VReactor ¼ 30 ml, Vl ¼ 7,5 ml, p(H2) ¼ p(CO2), p ¼ 30 bar, cat: Ru(acac)3, T ¼ 25  C, c(cat.) ¼ 0.5 mmol l1, t ¼ 6 h, solvent ¼ water, D ¼ 700 rpm.

Figure 7.13 Principle of the liquid–liquid multiphase systems.

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means of inductively coupled plasma analysis (ICP) show that the majority of the catalyst (95 w%) is dissolved in the organic phase. A remarkable advantage of this reaction control is that no additional solvents are required for the formation of a secondary liquid phase. However, the phase ratio is depending on the yield of product. Before starting the reaction in batch, the (NEt3)/(water) mass ratio is 1.6; during the reaction, it decreases to 0.4. Performing the reaction in the miniplant scale, constant phase ratios are absolutely required. The existing azeotrope between water and formic acid can be avoided by extraction. For the extraction of formic acid from aqueous solution, different aliphatic, long-chain amines can be used (179,180). For example, trioctylamine is particularly suitable for the extraction due to the nonmiscibility with water and excellent extractive properties towards formic acid (181–183). For the hydrogenation of carbon dioxide in the presence of triethylamine, an extraction of the product is not possible due to the formation of a formic acid–triethylamine salt, which is only soluble in a protic solvent.

3.2. Miniplant for carbon dioxide hydrogenation In addition to the investigations in laboratory scale, a continuous and adjustable miniplant with three unit operations, reaction, liquid–liquid separation, and liquid–gas separation, has been developed and built (Figure 7.14). The reaction proceeds in a continuously stirred tank reactor, where higher gas solubility is realized via a newly developed gas-entry stirrer. Dosing the gaseous and liquid educts is performed by a gas sparger and two piston pumps for water and triethylamine. After adjusting the

Figure 7.14 General flow sheet of the miniplant.

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residence time in the reactor, the two phases will be separated in a liquid– liquid separator. The organic amine phase containing the ruthenium catalyst is recycled by means of a gear pump. These two unit operations are held on the reaction pressure to enable the recirculation of the catalyst phase. The miniplant is equipped with several pressure and temperature sensors and is regulated via a fully automatic control system. Hereby, a high operational safety is achieved. The reaction progress and the formation of gaseous by-products can be traced via online gas chromatography.

3.3. Miniplant experiments First, a successful scale-up of the hydrogenation of carbon dioxide was executed into the miniplant technique. Similar results were achieved in the miniplant with the laboratory reaction conditions. An increased yield can be achieved by continuous gas supply as well as higher initial pressure (see Figure 7.15). The semicontinuous reaction control at 30 bar has been carried out; however, a white solid precipitated due to the high product formation. Based on this result, no TON could be calculated. In consideration of 80,000 70,000 60,000

TON [–]

50,000 40,000 30,000 20,000 10,000 0

10

20

30

Initialpressure [bar] Semi-continuous

Batch

Figure 7.15 Comparison of different initial pressures and reaction operations in the hydrogenation of carbon dioxide in miniplant scale. VReactor ¼ 2 l, Vl ¼ 500 ml, p(H2) ¼ p(CO2), cat: Ru(acac)3, T ¼ 25  C, c(cat.) ¼ 0.5 mmol l1, c(NEt3) ¼ 5 mol l1, t ¼ 3 h, solvent ¼ water, D ¼ 700 rpm.

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Arno Behr and Kristina Nowakowski

50,000 45,000 40,000

TON [–]

35,000 30,000 25,000 20,000 15,000 10,000 5000 0

Run 1

Run 2

Run 3

Run 4

Run 5

Figure 7.16 Recycling of the catalyst phase in the hydrogenation of carbon dioxide in miniplant scale via batch experiments. VReactor ¼ 2 l, Vl ¼ 500 ml, p(H2) ¼ p(CO2), p ¼ 10 bar (semi-continuous), cat: Ru(acac)3, T ¼ 25  C, c(cat.) ¼ 0.5 mmol l1, c(NEt3) ¼ 5 mol l1, t ¼ 3 h, solvent ¼ water, D ¼ 700 rpm.

the changing phase ratio depending on the product yield, the continuous hydrogenation of carbon dioxide is carried out at 10 bar. For a continuously operated process, constant activity of the catalyst is desirable. Therefore, the recycling of the catalyst was examined. As can be seen in Figure 7.16, the catalyst phase could be recycled in five consecutive runs in batch experiments with continuous gas dosing, and the activity of the ruthenium catalyst decreases only slightly. With regard to the recycling experiments, it was found that the catalyst leaching depends on the yield of product. The more the product is formed, the lower is the catalyst leaching into the aqueous product phase. The miniplant concept developed could be verified by the first continuous operations. In further investigations, the long-term stability of the catalyst, the catalyst losses, and the accumulation of by-products will be examined in more detail.

4. CONCLUSIONS In the present chapter, a comprehensive overview of the homogenous hydrogenation of carbon dioxide to formic acid and its derivatives has been given. Various noble metal complexes based on rhodium, ruthenium, and

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253

Figure 7.17 Formic acid–carbon dioxide cycle.

iridium were summarized and compared as catalyst. In recent years, the catalytic activity of these homogeneous catalysts has been increased successfully. In addition, intensive research in the hydrogenation of carbon dioxide with nonprecious metals has been carried out. The iron and cobalt catalysts developed show in part similar results compared with the precious metal-based catalysts. Latest research results present that the highest catalytic activity is achieved by the Ir(III)–pincer complex (Figure 7.8C). In most cases, formic acid derivates or formic acid salts are synthesized. Although several separation methods to obtain pure formic acid are pointed out, the separation of the resulting products still remains a great challenge. The catalysts used for the conversion of carbon dioxide also catalyze the decomposition of the formed product. This result is on the one hand negative considering the separation; on the other hand, it is highly favorable in the terms of the hydrogen storage in the carbon dioxide–formic acid cycle (see Figure 7.17). As mentioned in the introduction, formic acid can store - relating to the mass - less hydrogen, but it is definitely a feasible possibility to store hydrogen. However, the hydrogenation of carbon dioxide is only economical, if the required hydrogen is produced by regenerative processes on the basis of water. The presented concept is the research topic in the German joint research project CO2RRECT (“CO2-Reaction using Regenerative Energies and Catalytic Technologies”), in which the main focus of investigation is on the catalytic conversion of carbon dioxide using regenerative energies to form formic acid and the subsequent decomposition to hydrogen as well as carbon monoxide. By this means, the produced formic acid is a compound, which can easily be stored and transported for important basic chemicals.

ACKNOWLEDGMENTS This research work is part of the German project “CO2RRECT,” in which 18 research groups work together for an efficient use of carbon dioxide as a carbon building block. The project is sponsored by the German Federal Ministry of Education and Research. In addition, we thank the Umicore AG & Co. KG for catalyst donation.

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