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Investigation of Hydrogenation of Formic Acid to Methanol using H or Formic Acid as a Hydrogen Source 2
Akihiro Tsurusaki, Kazuhisa Murata, Naoya Onishi, Katerina Sordakis, Gabor Laurenczy, and Yuichiro Himeda ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03194 • Publication Date (Web): 22 Dec 2016 Downloaded from http://pubs.acs.org on December 26, 2016
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Investigation of Hydrogenation of Formic Acid to Methanol using H2 or Formic Acid as a Hydrogen Source Akihiro Tsurusaki,†,§ Kazuhisa Murata,† Naoya Onishi,† Katerina Sordakis,‡ Gábor Laurenczy, ‡,
* and Yuichiro Himeda†,* †
National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565, Japan
‡
Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Avenue Forel 2, 1015 Lausanne, Switzerland
ABSTRACT
Production of methanol (MeOH) from CO2 is strongly desired as a key chemical feedstock and a fuel. However, the conventional process requires elevated temperature and pressure, and high temperature restricts the productivity of MeOH due to equilibrium limitation between CO2 and MeOH. This paper describes the efficient hydrogenation/disproportionation of formic acid (FA) to MeOH by using the iridium catalysts with electronic-tuned ligands and by optimizing reaction conditions. The iridium complex bearing 5,5′-dimethyl-2,2′-bipyridine in FA hydrogenation
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achieved the MeOH selectivity with H2 up to 47.1% for FA hydrogenation under 4.5 MPa of H2 in the presence of H2SO4. The final concentration of MeOH of 3.9 M and TON of 1314 was obtained in 12 M FA aqueous solution including 10 mol% H2SO4 at 60 ºC under 5.2 MPa of H2. Even under the atmospheric pressure without introduction of external hydrogen gas, the FA disproportionation in deuterated conditions produced MeOH with 15.4% selectivity. Furthermore, the isotope effect and NMR studies revealed the mechanistic insight of the catalytic hydrogenation of FA to MeOH.
KEYWORDS: hydrogenation of formic acid, disproportionation of formic acid, methanol production, dehydrogenation of formic acid, iridium complexes,
INTRODUCTION Methanol (MeOH), which is widely available for the production of many chemicals and fuel, is produced from syngas in industrial productions. In addition, it has been suggested as an energy carrier (12.6 wt% hydrogen content) in the “methanol economy” in recent years.1,2 CO2 and MeOH can be converted into each other by the hydrogenation/dehydrogenation via formic acid (FA) and formaldehyde according to the chemical equation (Scheme 1). Recently, several research groups intensively investigated the hydrogen production by dehydrogenation of MeOH.3-12 Although the direct hydrogenation of CO2 by H2 to MeOH is the most straightforward process (eq. 1), it requires energy-intensive conditions such as high temperature (200-250 °C) and/or high pressure (5-10 MPa), because of the extreme thermodynamic stability of CO2. In addition, the conversion of MeOH from CO2 is potentially equilibrium-limited by a high
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temperature because of the negative ∆H and ∆S of this process.13-15 Exceptionally, George Olah Renewable Methanol Plant, which went into operation in 2012, converted 5500 tonnes of CO2 to 5 million litres of MeOH by direct hydrogenation in 2015, by using the specific regional energy supply in Iceland.16 However, the MeOH production that can work at a low temperature is highly desired in most cases, to avoid the equilibrium limitation and minimization of energy consumption.13
H2
H2
H 2O HCHO
HCO2H
CO2 H2
H2
H2
H 2O
CH3OH H2
Scheme 1. Interconversion between CO2, FA, and MeOH.
Lately, the production of MeOH from CO2 using homogenous catalysts has received considerable attention.12,17-19 In 1993, Sasaki reported production of MeOH by CO2 hydrogenation using Ru3(CO)12-KI in N-methyl-2-pyrrolidone at 240 °C and 9 MPa.20 Recently, Sanford and Leitner have demonstrated the successful example of cascade/tandem catalysis, which involves (i) hydrogenation of CO2 to FA, (ii) esterification to formate ester, and (iii) hydrogenation of the ester to MeOH using three compatible catalysts in each steps.21,22 It was also found that the MeOH production was promoted by just adding acid.23 Very recently, it was reported that the modified catalysis using amine led to improvement of productivity.24,25 Sanford and Milstein independently reported the indirect hydrogenation of the CO2 to MeOH via hydrogenation of CO2 derivatives, such as carbonate,26,27 carbamates,28 formates,26 and urea,29
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which can be easily available from CO2 at relatively milder reaction conditions. Even through the cutting-edge catalytic systems, elevated reaction temperature (> 100 °C) has been still required. In the previous communication, we have reported the direct CO2 hydrogenation to MeOH using the proton-responsive iridium catalyst, [Cp*Ir(4DHBP)(H2O)](SO4) (1a, 4DHBP = 4,4′dihydroxy-2,2′-bipyridine) under acidic conditions.30 In that study, it was clarified that the addition of sulfuric acid (H2SO4) remarkably improved the MeOH productivity in the catalysis.30 However, there is still room for improvement of the catalysis, which consists of hydrogenation of CO2 to FA and hydrogenation of FA to MeOH. The hydrogenation of CO2 to FA (not formate) without base (eq. 2), which has been reported by Ogo,31 Laurenczy,32 and Li33, is strongly governed by the equilibrium. Furthermore, the following hydrogenation of FA (eq. 3) competes with dehydrogenation of FA as a reverse reaction of hydrogenation of CO2, while several efficient homogeneous catalysts for hydrogenation of acid were reported recently.34-39 2+
Me Cp*
N Ir
(OTf)2
H 2O
Ru
2–
N Me + I PPh2
OC OC
Mo
PPh2 PPh2 H CO
II
III
Chart 1. Homogeneous catalysts for the FA disproportionation
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CO2 (g) + 3 H2 (g) → CH3OH (l) + H2O (l)
(eq. 1)
H2 (g) + CO2 (g) → HCO2H (aq)
(eq. 2)
HCO2H (l) + 2 H2 (g) → CH3OH (l) + H2O (l)
(eq. 3)
3 HCO2H(aq) → CH3OH (aq) + H2O (aq) + 2 CO2 (g)
(eq. 4)
However, the disproportionation of FA (i.e., the hydrogenation of FA to MeOH using hydrogen source from dehydrogenation of FA) is also one of the promising methods for the indirect production of MeOH from CO2 (eq. 4),40-42 because formate production by hydrogenation of CO2 have been significantly improved in the last two decades.43-46 Although the FA disproportionation was previously regarded as the side reaction of dehydrogenation of FA,47 the possibility of the MeOH production by the reaction catalysed by iridium catalyst I was re-recognised by Goldberg and co-workers in 2013 (Chart 1). The maximum values of turnover number (TON) and turnover frequency (TOF) were 156 and 6.5 h−1, respectively, in 12 M FA aqueous solution at 80 °C for 24 h in a closed vessel. In the system, MeOH was produced with only 12% selectivity, while residual FA was dehydrogenated to H2 and CO2 in a 12 M FA aqueous solution at 60 °C in a closed vessel.40 Cantat significantly improved the MeOH selectivity of 50.2% with a ruthenium catalyst and CH3C(CH2PPh2)3 (II), in THF at 150 °C in a sealed vessel for 1 h in the presence of methanesulfonic acid,41 whereas Neary and Parkin reported the MeOH selectivity of 21% in benzene at 100 °C with the molybdenum catalyst III.42 However, the improvement of MeOH selectivity and the necessity of high temperature are still
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major problems. Thus, development of efficient catalysts are highly desirable from the point of view of not only the mitigation of the energy consumption but also thermodynamics. In here, we comprehensively investigated FA hydrogenation/disproportionation to MeOH, using the iridium catalyst with substituted 2,2′-bipyridine derivatives under various reaction conditions, since the substituent and positional effect of the bipyridine ligand were not sufficiently clarified.48,49 We found that the iridium catalyst bearing 5,5′-dimethyl-2,2′-bipyridine, which is commonplace but has never been tried, showed the high MeOH selectivity for FA hydrogenation/disproportionation. In addition, the isotope effect and NMR studies revealed the mechanistic insight of FA disproportionation to MeOH. It was elucidated by the NMR studies that the abundance ratio of the intermediates, such as iridium-hydride or iridium-dihydrogen, depended on H2SO4 and H2. Furthermore, MeOH selectivity of 15.4% was obtained even under the atmospheric pressure in 4 M DCO2D including 5% D2SO4 (0.05 M as solution) in D2O without introduction of hydrogen. The results provided further motivation for the improvement of selectivity of MeOH production from FA. Results and Discussion Ligand Effect for FA hydrogenation We investigated the catalytic activities of Cp* iridium aqua complexes with various 2,2′bipyridine ligands (Chart 2), which were prepared according to an usual procedure (See Experimental Section in Supporting Information). The hydrogenation of FA to methanol was carried out under a constant pressure of mixture gases (H2/CO2 (1/1)) in 4 M FA (4 mL, 16 mmol) in H2O including 10 mol% H2SO4 (1.6 mmol, 0.4 M as solution: where the term of ‘X mol% H2SO4’ is expressed as a percentage of FA) and a catalyst (5 µmol) with 1500 rpm at 50
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ºC for 20 h in a stainless autoclave. Initially, a mixed gases of H2/CO2 (1/1) was charged, and the pressure was kept at 3.0 MPa during the course of the reaction by equipped with a back-pressure regulating valve in order to exclude the effect on the increasing H2/CO2 gases by the FA dehydrogenation. MeOH and methyl formate were observed by the GC analysis after the reaction because the equilibrium exists between FA/MeOH and methyl formate/H2O in acidic conditions in water.50 In all reactions, the term, “the MeOH yield”, expresses the sum of MeOH and methyl formate in the manuscript. On the other hand, formaldehyde, which is considered as one of the possible reduction products of FA, was not detected. The result was consistent with that of the previous report, because the hydrogenation of formaldehyde to MeOH was much faster than that of FA to formaldehyde.40 We note that the MeOH selectivity in the case of the introduction of external H2 was calculated as the moles of the produced MeOH divided the amount of the consumed FA, because the initially introduced H2 gas can serve as the reducing agent of FA (See Supporting Information).
R
2+
N
Cp* Ir
SO42-
H 2O
2+
HO
N
N
Cp*
SO42-
Ir H 2O
R
N
Me
H 2O
2-
SO4 N Me
4a: R = OH 4b: R = Me
N
Cp*
Ir
SO4
+
2+
N
Cp* 2-
R
N
Me
Me
N N
H 2O
3
2+
H 2O
OH SO42OH
Ir
2
R
Ir
N
Cp*
HO
+ 1a: R = OH (σp = -0.91) + 1b: R = OMe (σp = -0.78) + 1c: R = Me (σp = -0.31) + 1d: R = H (σp = 0) + 1e: R = CO2H (σp = 0.42)
Cp*
2+
Me 5
Ir Cl
Cl
-
N
Me 6
Chart 2. Iridium complexes for FA hydrogenation
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Table 1. Ligand Effect in FA Hydrogenationa Entry
Cat.
Yield of MeOH MeOH Selectivity TONb with H2 (%)b (µmol)b,c
TOF (h−1)b FA Conv. (%)d
1
1a
170
1.3
34.1
1.7
80
2
1b
225
2.0
45.0
2.2
69
3
1c
358
6.5
71.5
3.6
35
4
1d
191
10.3
38.3
1.9
12
5
1e
81.7
57.9
16.3
0.8
