Ruthenium-catalyzed hydrogenation of CO2as a route to methyl esters for use as biofuels or fine chemicals

Article abstract of DOI:10.1039/d0sc02942d

A novel robust diphosphine-ruthenium(ii) complex has been developed that can efficiently catalyze both the hydrogenation of CO2 to methanol and its in situ condensation with carboxylic acids to form methyl esters; a TON of up to 3260 is achievable for the CO2 to methanol step. Both aromatic and aliphatic carboxylic acids can be transformed to their corresponding methyl esters with high conversion and selectivity (17 aliphatic and 18 aromatic examples). On the basis of a series of experiments, a mechanism has been proposed to account for the various steps involved in the catalytic pathway. More importantly, this approach provides a promising route for using CO2 as a C1 source for the production of biofuels, fine chemicals and methanol.

Full text of DOI:10.1039/d0sc02942d

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Ruthenium‐catalyzed Hydrogenation of CO2 as a Route to Methyl
Esters for use as Biofuels or fine Chemicals
Received 00th xx 20xx,
Accepted 00th xx 20xx
Zheng Wang,a,b,c Ziwei, Zhao,a Yong Li,a Yanxia Zhong,d Qiuyue Zhang,b Qingbin Liu,*,a Gregory A.
Solan,*,b,e Yanping Mab and Wen‐Hua Sun*,b
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel robust diphosphine‐ruthenium(II) complex has been developed that can efficiently catalyze both the
hydrogenation of CO2 to methanol and its in‐situ condensation with carboxylic acids to form methyl esters; a TON of up to
3260 is achievable for the CO2 to methanol step. Both aromatic and aliphatic carboxylic acids can be transformed to their
corresponding methyl esters with high conversion and selectivity (17 aliphatic and 18 aromatic examples). On the basis of
a series of experiments, a mechanism has been proposed to account for the various steps involved in the catalytic pathway.
More importantly, this approach provides a promising route for using CO2 as a C1 source for the production of biofuels,
fine chemicals and methanol.
methylation of amines/amides,8 N‐methylation of imines,9
methylation of C‐H bonds (sp3)10 as well as the carboxylation of C‐H
(sp2)11 and N‐H bonds.12 Elsewhere, the non‐reductive incorporation
of a CO2 molecule into organic products, such as cyclic carbonates
and polycarbonates/polyethercarbonates has been
demonstrated.1a‐b,13
Introduction
The steadily rising levels of carbon dioxide in the atmosphere over
the last hundred years or so have made a major contribution to the
Earth’s greenhouse effect. These higher concentrations can be
attributed, in large measure, to increasing worldwide energy
consumption that is generated through power plants that make use
of fossil resources.1 However, in order to maintain the carbon
dioxide balance in the atmosphere and to reduce the dependency
on a limited fossil resource, alternative ways for the sustainable
production of fuels and chemicals represent a major global
challenge. A strategy that has been gaining attention is the use of
captured CO2 as a feedstock for the synthesis of biofuels and
chemicals.2 In recent decades, chemists have explored and
developed more than one hundred laboratory processes for using
CO2 as an alternative carbon source in fields that interface the
chemical and energy sectors (Scheme 1).1b Such processes include
the direct metal‐catalyzed hydrogenation of CO2 to formic acid,3
formides,4 methanol5 and ethanol.6 In addition, there has been the
use of CO2 as a C1 building block in hydroxymethylation,7 N‐
Scheme 1 Capturing CO2 as a carbon source for the production of fine
chemicals and biofuels
From an industrial standpoint, relatively few processes employ
CO2 as a starting material for the manufacture of organic products.
2,13e Nevertheless, those processes that do operate allow access to a
number of high demand materials including urea, methanol,
salicylic acid, organic carbonates and polycarbonates.1a‐b,13
Staggeringly however, this use of CO2 as a feedstock for chemicals
only accounts for about 0.36% of global CO2 emissions.14 Elsewhere,
it has been stated that only by completely using biomass energy
(and without fossil energy), can the concentration of carbon dioxide
reach equilibrium in the atmosphere.1a Moreover, it could be
argued that one of the best ways to reduce CO2 emissions would be
to synthesize biofuels that could be recycled.
a. Hebei Key Laboratory of Organic Functional Molecules, College of Chemistry and
Material Science, Hebei Normal University, Shijiazhuang 050024, China.
b. Key Laboratory of Engineering Plastics and Beijing National Laboratory for
Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190, China.
c. College of Science, Agricultural University of Hebei, Baoding 071001, China.
d. Department of Nursing Shijiazhuang Medical College, Shijiazhuang 050000, China.
e. Department of Chemistry, University of Leicester, University Road, Leicester LE1
7RH, UK.
*Corresponding Authors: liuqingb@sina.com (Q.L.); gas8@leicester.ac.uk (G.A.S.);
whsun@iccas.ac.cn, Tel: +86‐10‐62557955; Fax: +86‐10‐62618239 (W.‐H.S).
Electronic Supplementary Information (ESI) available: Detailed experimental
procedures, spectra (NMR, GC‐MS, LC‐MS), Figures S1 – S17, Tables S1 – S6 and X‐
ray crystallographic data in CIF for CCDC 1961855 (Ru2) and 1961856 (Ru3
available free of charge from the Cambridge Crystallographic Data Centre. See DOI:
10.1039/x0xx00000x
As part of an ongoing program, we have been interested in
developing methods of using CO2 as a feedstock to form carboxylic
acid methyl esters as their aliphatic examples could then be used as
)
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biofuels while their aromatic methyl esters as fine chemicals cymene)][ClO4] (Ru2) (Scheme 3). Alternatively, Ru2 could be
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DOI: 10.1039/D0SC02942D
(Scheme 2). In particular, a cascade strategy involving metal‐ obtained by a one‐pot reaction involving successive treatment of
catalyzed hydrogenation of CO2 to methanol and then in‐situ triphos with [RuCl2(η6‐p‐cymene)]2 and sodium perchlorate in a
condensation of methanol with a carboxylic acid has been water/toluene mixture; benzyl‐triethylammonium bromide (BTEAB)
envisioned. Of course, the carboxylic acids represent attractive was used as a phase transfer catalyst in both routes. The 31P NMR
family of reactants as they can in principle be obtained from spectrum of Ru2 in DMSO‐d6 exhibited two mutually coupled
biomass. Furthermore, the catalytic hydrogenation of methyl esters doublets at δ 26.18 and δ 25.05, which were assigned to the signals
has been shown by us15 and others,5c‐e,16 to give alkyl/aryl alcohols for the inequivalent phosphine donors; a single peak at δ 28.32 was
and methanol which can, in their own right, serve as important fuels attributed to the non‐coordinated phosphine oxide (see SI, Fig. S9).
+
-
and synthetic building blocks.15,16 Alternatively, methanol can be
produced by the hydrolysis of a methyl ester and the corresponding
carboxylic acid by‐product recycled. Overall, this sequence of
reactions could present an elegant and green sustainable route for
the production of fuels and chemicals.
Cl
[RuCl2
CH2Cl2, EtOH, 50 oC, 3 h
(
6-p-cymene)]2
Ru
Cl
Ph2P
PPh2
R1
R2
R2
Ru1 R1 = CH2PPh2, R2 = Me (92%)
NaClO4, BTEAB
CH2Cl2, H2O, RT, 5 h
R1
PPh2
R1 = CH2PPh2, R2 = Me (triphos)
PPh2
R
R
1 = CH2PPh2, R2 = Et
1 = R2 = H (dppp)
+
-
ClO4
(i) [RuCl2(
6-p-cymene)]2, toluene
(ii) NaClO4, BTEAB, H2
O
(iii) 110 oC, 16 h
Ru
Cl
Ph2P
PPh2
R1
R2
Ru2 R1 = CH2P(O)Ph2, R2 = Me (80%)
Ru3
Ru4
R
R
1 = CH2P(O)Ph2, R2 = Et (77%)
1 = R2 = H (94%)
Scheme 2 Potential strategy involving CO2 utilization to form industrially
useful methyl esters and their amenability to recycling and use in other
applications
Scheme 3 Synthetic route to Ru1 ‐ Ru4
Complex cations [RuCl{κP,κP‐(CH2PPh2)2CR1R2}(η6‐p‐cymene)]
[ClO4] [Ru3 R1 = CH2P(O)Ph2, R2 = Et; Ru4 R1 = R2 = H] could be
obtained from CEt(CH2PPh2)3 or CH2(CH2PPh2)2 (dppp), respectively,
in high yield by using a one‐pot route based on that employed for
the synthesis of Ru2. As with Ru2, Ru3 showed two doublets in the
31P NMR spectrum for the inequivalent coordinated phosphines and
a singlet for the pendant phosphine oxide group, while Ru4 showed
two distinct singlets for the bidentate dppp ligand. Besides 31P NMR
spectroscopy, all four ruthenium complexes were characterized by
1H, 13C NMR spectroscopy, elemental analysis and by ESI‐mass
spectrometry (Table S3, see SI). To confirm their structural identity,
Ru2 and Ru3 were the subject of single crystal X‐ray diffraction
studies (Figs. 1 and 2); selected bond distances and angles are given
in the Figure captions. Both structures consist of a cationic
ruthenium(II) unit and a non‐coordinating perchlorate anion. The
cationic unit adopts a three‐legged piano stool geometry comprising
To realize our goal, we herein disclose a new family of well‐
defined diphosphine‐ruthenium(II) cationic complexes that can
serve as versatile (pre)catalysts for the conversion of CO2 to a raft of
different types of methyl ester (Scheme 1). By performing the
reactions in the presence of a carboxylic acid, we show that this
acid plays two key roles; i) as a reactant in the conversion of the
methanol intermediate to the target methyl ester and ii) as a
promoter in the CO2 hydrogenation step. Unlike the ruthenium‐
triphos catalysts previously reported by Leitner and others for the
hydrogenation of CO2,5e,5j the current (pre)catalysts are based on an
organometallicη6‐arene‐ruthenium core that incorporates
a
chelating diphosphine of the type, CR1R2(CH2PPh2)2 [R1 = CH2PPh2,
R2 = Me; R1 = CH2P(O)Ph2, R2 = Me; R1 = CH2P(O)Ph2, R2 = Et; R1 = R2
= H].
Results and Discussion
Synthesis and characterization of Ru1 ‐ Ru4
Cationic [RuCl(κP,κP‐triphos)(η6‐p‐cymene)][Cl] (Ru1) was prepared
in excellent yield by the reaction of [RuCl2(η6‐p‐cymene)]2 with
1,1,1‐tris(diphenylphosphinomethyl)ethane (triphos) in a mixture of
o
CH2Cl2 and EtOH at 50 C (Scheme 3, see SI); related ruthenium(II)
complexes and reaction conditions have been reported elsewhere.17
The 31P NMR spectrum of Ru1 exhibited two singlet peaks at δ 26.02
and δ 24.81, corresponding to the inequivalent phosphine donors,
as well as one single peak at δ ‐29.77 for the pendant phosphine
(see SI, Fig. S3).
Counteranion exchange and oxidation of the pendant
phosphine arm in Ru1 and could be readily achieved by its reaction
with sodium perchlorate in a mixture of dichloromethane and water
at room temperature to give [RuCl(κP,κP‐triphos(O))(η6‐p‐
Fig. 1 ORTEP representation of Ru2. Thermal ellipsoids are shown at the
30% probability level; hydrogen atoms have been omitted for clarity.
Selected bond distances (Å): Ru1‐Cl1 = 2.4086(11), Ru1‐P2 = 2.3276(12),
Ru1‐P1 = 2.3286(12), Ru1‐C47 = 2.221(4), Ru1‐C46 = 2.269(4), Ru1‐C45 =
2.322(4), Ru1‐C43 = 2.254(4), Ru1‐C44 = 2.260(4), Ru1‐C48 = 2.295(5),
P3‐O1 = 1.480(3); Selected angles (deg o): P1‐Ru1‐Cl1= 85.07(3), P2‐Ru1‐
Cl1 = 87.77(4), P2‐Ru1‐P1 = 86.85(2)
2 | J. Name., 2012, 00, 1‐3
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present in Ru2 and Ru3 derives from a ruthenViuiemw A‐mrticeledOiantleinde
oxidation of the corresponding –CH2PPh2 unDitO. I: 10.1039/D0SC02942D
Catalytic hydrogenation of CO2 to give carboxylic acid methyl
esters
(a) Optimization of conditions
To allow an optimization of the conditions, Ru2 was selected as the
(pre)catalyst and decanoic acid (1) as the carboxylic acid substrate
with 30 mL of 1,2‐dimethoxyethane (DME) as solvent (Table 1). With
o
the reaction temperature set at 160 C, the combined pressure of
CO2 and H2 at 80 bar (12:68 ratio) and the substrate to catalyst
molar ratio (S:C) at 100:1, a 28% conversion was observed after 20
hours of which 25% constituted methyl decanoate (2) and 3% n‐
decanol (3) (entry 1, Table 1). By slightly increasing the hydrogen
pressure in the gaseous mixture to 12:70, the conversion decreased
to 14% with a larger proportion now being 3 (entry 2, Table 1).
Under the same conditions but with the volume of solvent reduced
to 10 mL, to increase the substrate concentration, a 91% conversion
was achieved with 83% being 2 (entry 3, Table 1). To our delight, by
reverting to the pressure ratio of CO2:H2 = 12:68 and maintaining
the higher concentration of substrate, a high conversion and
selectivity (conv. = 99%, selectivity = 100%) were realized (entry 4,
Table 1).
With a view to establishing the most compatible reaction
medium for the hydrogenation using Ru2, four related solvents
were screened, namely, DME, 1,2‐diethoxyethane, diglyme and
triglyme (entries 4 – 7, Table 1). On examination of the data, DME
was the standout performer in terms of the conversion (99%) and
selectivity for 2 (100%). When 1,2‐diethoxyethane was employed as
solvent, a lower conversion (76%) and selectivity (92%) was
observed while with the longer chain solvents, diglyme and triglyme,
Fig. 2 ORTEP representation of Ru3. Thermal ellipsoids are shown at the
30% probability level; hydrogen atoms have been omitted for clarity.
Selected bond distances (Å): Ru1‐Cl1 = 2.4249(7), Ru1‐P1 = 2.3333(7), Ru1‐
P2 = 2.3193(6), Ru1‐C43 = 2.322(2), Ru1‐C44 = 2.285(2), Ru1‐C45 =
2.224(2), Ru1‐C46 = 2.298(2), Ru1‐C47 = 2.241(2), Ru1‐C48 = 2.248(2), P3‐
o
O1 = 1.4893(15); Selected angles (deg ): P1‐Ru1‐Cl1 = 82.55(3), P2‐Ru1‐
Cl1 = 90.39(3), P2‐Ru1‐P1 = 86.76(2)
a
6‐p‐cymene,
a monodentate chloride and a bidentate
CR2(CH2P(O)Ph2)(CH2PPh2)2 (R2 = Me Ru2, Et Ru3) ligand which binds
through the diphenylphosphine phosphorus atoms while the
oxidized phosphine arm remains uncoordinated; the bond
parameters around ruthenium are not exceptional and indeed
similar to related half‐sandwich structures. To try and explain the
origin of the phosphine oxide units in Ru2 and Ru3, the four
different ruthenium complexes were separately evaluated as
catalysts for the oxidation of PPh3 in a toluene‐water mixture at 100
oC over 24 hours (Table S2, SI). Inspection of the results revealed
conversions to triphenylphosphine oxide of 24% for [RuCl2(η6‐p‐
cymene)]2, 94% for Ru1, 43% for Ru2 and 97% for [[RuCl2(η6‐p‐
cymene)]2 + triphos]. Evidently, the uncoordinated phosphine oxide
Table 1 Catalytic evaluation of the hydrogenation of CO2, in the presence of decanoic acid (1), to give methyl decanoate (2).a
Entry
1c
S:C
100:1
100:1
[Ru]
Ru2
Ru2
Solvent
DME
DME
CO2:H2 (bar)
12:68
Conv. (%)b
2/3/4 (% conv. to each)b
25/3/0
28
14
2c
12:70
12/2/0
3
4
100:1
100:1
Ru2
Ru2
DME
DME
12:70
12:68
91
99
83/8/0
99/0/0
5
6
100:1
100:1
Ru2
Ru2
1,2‐diethoxyethane
12:68
12:68
76
36
70/6/0
30/6/0
Diglyme
7
8
100:1
200:1
Ru2
Ru2
Triglyme
DME
12/68
12:68
65
96
60/5/0
94/2/0
9
500:1
1000:1
100:1
100:1
100:1
100:1
100:1
Ru2
Ru2
Ru2
DME
DME
DME
DME
DME
DME
DME
12:68
12:68
12:68
12:68
12:68
12:68
12:68
100
42
100
3
76
67
56
100/0/0
40/2/0
100/0/0
0/2/1
76/0/0
67/0/0
56/0/0
10
11d
12
13
14
15
16e
17f
[RuCl2(p‐cymene)]2
Ru1
Ru3
Ru4
100:1
100:1
Ru2
Ru2
DME
DME
00:80
12:68
n.d.
n.d.
n.d.
n.d.
a Conditions: decanoic acid (1.0 mmol), [Ru] (1.0 ‐ 10.0mol), solvent (10 mL), PH2 = 68 ‐ 70 bar (at RT), PCO2 = 0 ‐ 12 bar (at RT), Temp. = 160 oC, Time
= 20 h, S:C = the molar substrate to catalyst ratio, DME is 1,2‐dimethoxyethane; bThe conversion, with reference to decanoic acid (1), was determined
by GC (using mesitylene as the internal standard) and by GC‐MS; c 30 mL of DME in place of 10 mL; d DME (2.5 mL), the ester products are methyl
decanoate (95%) and ethyl decanoate (5%);e In the absence of CO2; f In the absence of decanoic acid (1), no CH3OH was observed.
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the conversions observed were markedly less at 36% and 65%, hexanoate, 12% of hexyl hexanoate and 33% 1‐hexanol (entry 4,
respectively. Table 2).
In order to determine the optimal S:C ratio, four different Intriguingly, decanoic acid (1) can also be used as additive for
combinations, 100:1, 200:1, 500:1 and 1000:1, were screened using adjusting the activity and selectivity of the ruthenium catalyst. For
Ru2 (entries 4 and 8 ‐ 10, Table 1). It was found that with ratios example, using undecanoic acid as the substrate, the selectivity for
between 100:1 and 500:1, high conversions (96 ‐ 100%) could be the methyl ester increased from 82% to 100% and the conversion
achieved. By contrast with the ratio at 1000:1, the CO2 from 96% to 99% when 10 mol% decanoic acid was introduced
hydrogenation was incomplete with only 42% conversion achievable (entries 8 vs. 9, Table 2). The longer carbon chain aliphatic acids (12
(entry 10, Table 1). With the volume of DME decreased to 2.5 mL ≤ C ≤ 19) also gave excellent results in the presence of decanoic acid
and a 100:1 ratio re‐deployed, 100% conversion was obtained, but as an additive (entries 9 – 13, Table 2). Significantly, many of the
the ester products consisted of 2 (95%) and ethyl decanoate (5%) resulting aliphatic esters are in the range of biodiesels.18
(entry 11, Table 1, see SI). By contrast, when [RuCl2(p‐cymene)]2 was
For dicarboxylic acids incorporating aliphatic linkers of a range
employed as catalyst, no methyl decanoate was obtained instead of chain lengths, all underwent 99% conversion with 95‐100%
minor amounts of 3 and decyl decanoate (4) were detected (entry selectivity. However, it was noted that longer reaction times (up to
12, Table 1). In addition, when the catalysis was conducted using 24 hours) were needed to complete the transformation when
Ru2 but in the absence of decanoic acid, no CH3OH was observed compared to those containing only one carboxyl group (entries 14 –
(entry 17, Table 1), which suggests that the decanoic acid acts as a 16, Table 2). It was also apparent that the shorter carbon chain
promoter.
dicarboxylic acid, adipic acid, was at the lower end of the selectivity
Using the best overall set of conditions established for Ru2 [T (95%) range with 94% dimethyl adipate and 5% of 6‐methoxy‐6‐
= 160 oC, CO2/H2 = 12:68 (80 bar in total), S:C = 100:1, 20 h, DME (10 oxohexanoic acid produced (entry 14, Table 2).
mL)], the remaining ruthenium complexes Ru1, Ru3 and Ru4 were
In the case of aliphatic carboxylic acids incorporating some
also evaluated. Indeed, all of these complexes were active and unsaturation, methyl ester formation was also achievable but at the
selective catalysts for methyl decanoate with their conversions, expense of double bond (C=C) hydrogenation. For example, 99% of
when compared to Ru2, falling in the order: Ru2 > Ru1 > Ru3 > Ru4 methyl benzenepropanoate was obtained when using cinnamic acid
(entries 4, 13‐15, Table 1). As a control, no conversion to 2 was while 98% of methyl octadecanoate were observed from octadec‐9‐
achieved when the hydrogenation was performed in the absence of enoic acid (entries 17 and 18, Table 2).
CO2 under the same conditions (entry 16, Table 1).
The hydrogenation capacity of Ru2 is not limited to aliphatic
carboxylic acids. Indeed, a wide variety of aromatic examples
differing in their ortho‐, meta‐ and para‐substitution patterns, also
showed excellent conversions and selectivity for methyl esters (6a –
6r) (Table 3). For example, benzoic acid (5a) underwent 93%
conversion to methyl benzoate (6a) with a S:C ratio of 100:1 that
only reduced to 69% (99% selectivity) when the ratio was increased
to 500:1. Interestingly, benzoic acids (5b – 5e, 5f – 5h, 5j – 5l)
incorporating electron withdrawing groups such as halides (F, Cl, Br)
or CF3 at the ortho‐ (5b – 5e), meta‐ (5f – 5h) or para‐positions (5j –
5l) showed little variation in conversion (73 ‐ 99%) and selectivity
(almost 100%) for the methyl ester. However, dehalogenation was
an outcome of all benzoic acids containing either Cl or Br
substituents resulting in methyl benzoate as the sole methyl ester
product in these cases (see substrates 5c, 5d, 5g, 5h, 5k, 5l); this
finding can likely be attributed to the high temperature and the
reactivity of Ru2 towards the Caryl‐X bonds present in these
particular substrates. In comparison, the electron‐rich benzoic acids
with methyl or methoxy substituents (5n and 5o) at the para‐
positions gave 100% conversions to their methyl esters (6n and 6o).
Interestingly, p‐methoxybenzoic acid (5o) produced two kinds of
esters, methyl 4‐methoxybenzoate (6o, 82%) and ethyl 4‐
methoxybenzoate (6o', 18%). The presence of 6o' would suggest
(b) Exploring the substrate scope of Ru2
With the aim to explore the scope and functional group tolerance of
Ru2 as a catalyst for the hydrogenation of CO2 to methyl esters, a
broad range of aliphatic carboxylic acids was investigated as
substrates. The effects of carbon chain length of both mono‐ and
dicarboxylic acids as well as unsaturation within the aliphatic chain
were all factors to be evaluated using the optimal conditions
established using decanoic acid. In addition, the effect of using
decanoic acid as an additive was also investigated. The complete set
of results are listed in Table 2.
For aliphatic carboxylic acids containing carbon chain lengths
of C ≤ 5, low conversions and low selectivity were evident after a 20
hour run time. For example, only 10% of methyl acetate was
observed when using acetic acid and 6% of methyl pentanoate
when using pentanoic acid (entries 1 and 2, Table 2). By contrast,
the longer chain lengths (6 ≤ C ≤ 10) showed excellent conversions
and high selectivity for the corresponding methyl ester (entries 3 – 7,
Table 2). In the case of hexanoic acid, it was shown that in the
absence of solvent the selectivity dramatically reduced [100% (with
DME) to 54% (without), entries 3, 4, Table 2] producing 54% methyl
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that the hydrogenation of CO2 produced a small amount of the and 100% selectivity (6p). Notably, the difluoro‐substVitieuwtiAornticloenOntlhinee
higher alcohol ethanol which then underwent condensation with p‐ aryl ring did not affect the reaction with 3,4‐DdOifIl:u1o0r.o10b3e9n/zDo0iScCa0c2id94(25Dq)
methoxybenzoic acid (5o) to give 6o'; the mechanism of ethanol also producing 99% of methyl 3,4‐difluorobenzoate (6q). In addition,
production from CO2 is assumed to be similar to that reported by the dicarboxyl‐substituted isophthalic acid (5r), though reaching 100%
Han.6 Furan‐2‐carboxylic acid (5p) also resulted in 99% conversion conversion, displayed a slight difference in reactivity when
Table 2 Hydrogenation of carbon dioxide to aliphatic estersa
Entry
1
Substrate
S:C
t (h)
20
Conv. (%)b
16
ester/alcohol (%conv. to each)b
10/ 6
100:1
2
3
100:1
500:1
20
20
10
6/4
100
100/0
4c
500:1
20
99
66(54/12)/33
5
6
500:1
500:1
500:1
500:1
500:1
500:1
500:1
100:1
100:1
500:1
20
20
20
20
20
20
20
20
20
24
100
99
100/0
99/0
100/0
78/18
99/0
99/0
99/0
99/0
99/0
94/5
7
100
96
8
9d
99
10d
11d
12d
13d
14e
99
99
99
99
99
15
16
500:1
500:1
500:1
24
24
20
99
99
98
99/0
99/0
98/0
17d,f
18g
500:1
20
99
99/0
o
a Reaction conditions: substrate (1.0 mmol), Ru2 (2.0 ‐ 10.0mol), DME (10 mL), PH2 = 68 bar (at RT), PCO2 = 12 bar (at RT), Temp. = 160 C, S:C = the
molar substrate to catalyst ratio; b The conversion with reference to the carboxylic acid was determined by GC (using mesitylene as the internal standard)
and by GC‐MS; c In the absence of solvent; 54% of methyl hexanoate and 12% of hexyl hexanoate were produced; d In the presence of decanoic acid (0.1
mmol) as additive;e 5% of the product is 6‐methoxy‐6‐oxohexanoic acid; f The product is methyl stearate;g The product is methyl benzenepropanoate.
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DOI: 10.1039/D0SC02942D
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ARTICLE
Table 3 Conversion of aryl carboxylic acids (5) to aromatic methyl esters
(6) via the Ru‐mediated hydrogenation of carbon dioxide a
O
O
Ru2 (cat.), CO2/H2
aryl
O
DME, 160 oC, 20 h
OH
aryl
5
6
Substrate
Product
Substrate
Product
(Conv. %)b
(Conv. %)b
6a R = H, 93%
6a 69%c
6b R = F, 99%
6a 99%
6a 99%
6e R = CF3,99%
6f R = F, 99%
5a R = H
5f R = F
5b R = F
5g R = Cld
5h R = Brd
5i R = CF3
6a 99%
6a 99%
6i R = CF3, 99%
5c R = Cld
5d R = Brd
5e R = CF3
Scheme 4 Proposed mechanism for the Ru2 catalyzed hydrogenation of CO2
to methyl decanoate in the present of decanoic acid via Ru(II) intermediates
derived from formic acid, formaldehyde and methanol
6j R = F, 78%
6a 99%
5j R = F
5k R = Cld
similar to I have been reported based on the interaction of a
5l R = Brd
5m R = CF3
5n R = Me
6a 99%
6m R = CF3, 73%
6n R = Me, 100%
precatalyst with a carboxylic acid.5j,19e Moreover, examination of the
1
solid species formed following work‐up of the catalytic run by H
NMR spectroscopy showed no evidence for an 6‐coordinated para‐
cymene (see Fig. S18, SI). In the next step of the catalytic cycle, CO2
insertion can occur to give formate II, which is then reduced to the
hydroxymethanolate species III. Hydrogenation of III and
concomitant loss of water gives methoxide IV which can then
eliminate methanol and regenerate I following hydrogenolysis. The
methanol produced during conversion of IV to I can then undergo a
ruthenium‐catalyzed condensation with the carboxylic acid
(decanoic acid) to form the methyl ester (methyl decanoate) and
water. Notwithstanding the pathway outlined above, we cannot
fully rule out the role, at some level, of ruthenium nanoparticulate
species in the catalysis. However, it should be pointed out only
signals corresponding to unidentified Ru‐phosphine species could
be detected post‐operando in the 31P NMR spectrum (see Fig. S19,
SI).
a Conditions: substrate (1.0 mmol), Ru2 (10.0 mol), S:C = 100:1, DME
(10 mL), PH2 = 68 bar (at RT), PCO2 = 12 bar (at RT), Temp. = 160 oC, Time =
b
20 h; The conversion, with reference to the carboxylic acid, was
determined by GC (using mesitylene as the internal standard) and by GC‐
MS; c Ru2 (2.0 mol), S:C = 500:1; d A dehalogenation reaction occurs
with the product being methyl benzoate (6a).
compared to benzoic acid by producing 65% of the dimethylester
(6r) and 35% of the methyl ethyl ester (6r'). Once again the
hydrogenation of CO2 to form a small amount of ethanol seems
likely to account for the formation of 6r'.
In attempt to shed some light on this proposed mechanism,
1
the following set of experiments were conducted. Firstly, H NMR
(c) Mechanistic pathway
spectroscopy was used to detect for methanol in the catalytic
system. Using the optimal operating conditions established in entry
4 (Table 1), free methanol was indeed detected by 1H NMR
spectroscopy after 20 hours (Fig. S16, see SI). Furthermore, when
the volume of DME was decreased from 10 mL to 2.5 mL a small
amount of ethanol was also observed. This finding can explain the
formation of methyl decanoate (95%) and ethyl decanoate (5%) that
was observed in entry 11 (Table 1).6 On the other hand, there was
no methanol observed when the hydrogenation was conducted in
the absence of CO2 (entry 16, Table 1, see SI). More importantly, in
Based on the recent work concerning the ruthenium‐catalyzed
hydrogenation of CO2 to methanol,3,5j‐k,19 an acid additive can act as
promoter in a carboxylate assisted proton transfer.5j Hence, a
plausible catalytic cycle is shown in Scheme 4 involving Ru(II)
intermediates derived from formic acid, formaldehyde and
methanol. Firstly, the neutral carboxylate complex, [L'Ru(η2‐O2CR)H]
(I, L' = κP,κP‐triphos(O)), is formed through the heterolytic cleavage
of hydrogen and substitution of p‐cymene for decanoate. Despite
multiple attempts, crystallization of
I proved unsuccessful.
Nonetheless, several examples of penta‐coordinate intermediates
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the absence of decanoic acid, no CH3OH was observed (entry 17,
Table 1), which would imply decanoic acid acts as a promoter.
smoothly formed. However, the different carbon sources required
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different run times to complete the transDfoOrIm: 1a0t.i1o0n39(/TDa0bSlCe0529, 4S2ID
6.1). For example, the use of one or fifty equivalents of methanol
needed less time (2 – 4 hours) to be converted to methyl
decanoate than that using formaldehyde and formic acid. Indeed,
using formic acid took close to 20 hours to allow a comparable
conversion. Although uncertain at this stage, it would seem likely
that the rate‐determining step in the catalytic cycle is located in
the sequence of steps leading from HCOOH to CH3OH.3h,5j‐k,19 As a
final point, only 5% conversion to methyl decanoate was observed
in the absence of Ru2 (entry 4, Table 5).
Table 4 The amount of methanol produced by Ru1 ‐ Ru4a
e
Entry
[Ru]
Conv. (%)b
Conv.
(mg/mL)c
90.03
MeOH
(n mmol)d
28.12
TONMeOH
1
2
3
4
Ru1
Ru2
Ru3
Ru4
76
99
67
56
2812
3262
3094
1949
104.4
32.62
99.31
30.94
62.36
19.49
a Conditions: decanoic acid (1.0 mmol), [Ru] (10.0mol), DME (10 mL),
PCO2 = 12 bar (at RT), PH2 = 68 bar (at RT), Time = 20 hours, Temp. = 160 oC,
b
S:C = 100:1; The conversion with reference to decanoic acid was
determined by GC (using mesitylene as the internal standard) and by GC‐
MS; c The conversion to methanol was determined by GC (using a standard
concentration curve of methanol as the external standard (see SI 6.2);
d The total volume of the reaction solution was 10 mL; e TON is the ratio of
the moles of methanol (n) to the moles of catalyst (10 mol).
Conclusions
We have developed a novel route to prepare a wide range of methyl
esters from CO2 via a ruthenium catalyzed hydrogenation to
methanol and in‐situ condensation with a carboxylic acid. By
evaluating four structurally related diphosphine‐ruthenium
(pre)catalysts, complex [RuCl(κP,κP‐triphos(O))(6‐p‐cymene)][ClO4]
(Ru2) has shown the greatest efficiency for the transformation.
Indeed using Ru2, both aromatic and aliphatic carboxylic acids can
be converted with high selectively to the corresponding carboxylic
acid methyl ester. Furthermore, we have also probed the
mechanism through a series of experiments, which has shown that
the pathway involves the direct hydrogenation of CO2 to form
consecutively formic acid, formaldehyde and methanol which then
finally undergoes condensation with carboxylic acids to the form the
corresponding methyl ester. Moreover, this work also presents an
original and effective ruthenium catalyzed process for producing
methanol from CO2 with the TON for the CO2 to methanol step up to
3262; the role of the carboxylic acid as a promoter in this reaction
has been demonstrated. More importantly, this work provides an
attractive route for the utilization of CO2 as a carbon source for the
production of biofuels and fine chemicals. We therefore hope that
this report will inspire the development of more efficient metal‐
catalyzed hydrogenation reactions of CO2 and ultimately reduce the
demand for fossil resources.
Secondly, for each ruthenium catalyst (Ru1 ‐ Ru4), the amount
of methanol produced during the hydrogenation of CO2 when in the
presence of decanoic acid (1 mmol), was quantitatively determined:
o
conditions, 10 mL of DME at 160 C with PCO2 = 12 bar (at RT) and
PH2 = 68 bar (at RT) (Table 4, SI 6.2). To our surprise, the turnover
number (TON) of Ru2 for the direct transformation of CO2 to
methanol was up to 3262 which noticeably exceeds that previously
reported by Leitner et al. (TON = 442, 2‐MTHF as solvent, at 140 oC
with PCO2 = 20 bar (at RT) and PH2 = 60 bar (at RT)).5e,5j Although the
origin of this increase remains uncertain, it does support the
assertion that decanoic acid present in the catalytic system can act
as a promoter in the conversion of CO2 to methanol. It was also
found that the amount of methanol produced using Ru2 and Ru4
proved consistent with the percentage conversions to the methyl
decanoate. On the other hand, that observed for catalysts Ru1 and
Ru3 showed some discrepancy (entries 1 and 3, Table 4), which may
be due to the structural differences of the ligands leading to the
different activities for catalytic esterification.
Table 5 Using MeOH, HCHO, HCOOH as the C1 carbon source for the
production of methyl decanoatea
Experimental Section
Catalytic study
Conv.
(%)b
99
ester/alcohol
(Yield. %)b
99 / 0
Entry
C1 carbon source
S:C
t (h)
Under an atmosphere of argon, a stainless steel 100 mL autoclave,
equipped with a magnetic stir bar, was charged with Ru1 – Ru4 (2.5
‐ 10 mol) and the solvent to be used (2.5 – 5 mL). A solution of the
carboxylic acid (1 mmol) in the solvent (5 – 25 mL) was then added
via a syringe. The autoclave was purged by three cycles of
pressurization/venting with CO2 (5 ‐ 10 bar), and then pressurized
with the desired mixture of CO2 and H2. The autoclave was heated
to the desired temperature and the contents stirred. After the pre‐
determined reaction time, the autoclave was cooled to room
temperature and the pressure slowly released. The reaction mixture
was filtered through a plug of silica gel and then analyzed by GC and
GC‐MS.
1
2
3
4
MeOH (50 eq.)
MeOH (1 eq.)
HCHOc (1 eq.)
HCO2H (1 eq.)
MeOH (1 eq.)
100:1
100:1
100:1
100:1
100:0
2
4
10
20
4
99
99
99
5
99 / 0
99 / 0
99 / 0
5 / 0
5
a
Reaction conditions: decanoic acid (1 mmol), Ru2 (10 μmol), DME (10
mL), PH2 = 80 bar (at RT), Temp. = 160 oC, S:C = the molar substrate to
catalyst ratio; b The conversion with reference to the decanoic acid was
determined by GC (using mesitylene as the internal standard) and by GC‐
MS; c 37% wt aqueous solution
Finally, we set about attempting to confirm that the catalytic
mechanism proceeded via three steps involving intermediates
derived from formic acid, formaldehyde and methanol.3h,5j‐k,19
Hence, we used MeOH, HCHO (37% wt aqueous solution) and
HCOOH independently as the C1 sources in the absence of CO2
under the same conditions. In all cases methyl decanoate was
Conflicts of interest
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Journal Name
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There are no conflicts to declare.
Acknowledgements
We acknowledge the support from the National Natural Science
Foundation of China (21871275), the Nature Science Foundation of
Hebei Province (B2019205149) and Talent Introduction Foundation
of Agricultural University of Hebei (YJ201931). GAS thanks the
Chinese Academy of Sciences for a President’s International
Fellowship for Visiting Scientists.
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DOI: 10.1039/D0SC02942D
Journal Name
ARTICLE
GRAPHICAL ABSTRACT
Ruthenium‐catalyzed Hydrogenation of CO2 as a Route to Methyl Esters for use as Biofuels or
fine Chemicals
A novel robust diphosphine‐ruthenium(II) complex has been developed that can efficiently catalyze both the
hydrogenation of CO2 to methanol and its in‐situ condensation with carboxylic acids to give methyl esters.
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