Acid-Promoted Hydroformylative Synthesis of Alcohol with Carbon Dioxide by Heterobimetallic Ruthenium-Cobalt Catalytic System

Article abstract of DOI:10.1002/cctc.201802091

The acid-aided heterobimetallic ruthenium-cobalt catalytic system for the reductive hydroformylation with carbon dioxide was established. Various alkenes, including waste from biomass and petroleum industry, could be upgraded to valuable alcohols with this protocol. Acid-promoted reverse water-gas shift (RWGS), thereby accelerating the hydroformylative synthesis of alcohol. The theoretical computations revealed that acid promoted RWGS by facilitating the dehydroxylation of ruthenium hydroxy carbonyl intermediate.

Full text of DOI:10.1002/cctc.201802091

Accepted Article
Title: Acid-promoted hydroformylative synthesis of alcohol with carbon
dioxide by heterobimetallic ruthenium-cobalt catalytic system
Authors: Xuehua Zhang, Xinxin Tian, Chaoren Shen, Chungu Xia, and
Lin He
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10.1002/cctc.201802091
ChemCatChem
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Acid-promoted hydroformylative synthesis of alcohol with carbon
dioxide by heterobimetallic ruthenium-cobalt catalytic system
Xuehua Zhang,[a,c,d] Xinxin Tian,*[b] Chaoren Shen,*[a] Chungu Xia[a] and Lin He*[a]
Abstract: The acid-aid heterobimetallic ruthenium-cobalt catalytic
system for the reductive hydroformylation with carbon dioxide was
established. Various alkenes, including the wastes from biomass
and petroleum industry, could be upgraded to valuable alcohols with
this protocol. Acid promoted reverse water-gas shift (RWGS),
thereby accelerating the hydroformylative synthesis of alcohol. The
theoretical computations revealed that acid promoted RWGS by
facilitating the dehydroxylation of ruthenium hydroxycarbonyl
intermediate.
200 oC) catalysed by ruthenium carbonyl cluster anion species,[9]
ligand-modified ruthenium carbonyl clusters are employed in
reductive hydroformylation with CO2 in the presence of excess
additive.[10] However, the intricate transformations from neutral
pre-catalyst to various anionic active species are hard to be
regulated and confine the elevation of efficiency on RWGS.[11] In
comparison, [Ru(CO)3Cl3]- was an effective RWGS catalyst with
well-defined structure evolution during the reaction.[12] Its
capability in carbonyl reduction was largely unexplored. Based
on our research interest on reductive conversion of CO2,[13] we
intended to exploit its application in the reductive
hydroformylation with CO2. Inspired by the promoting effect of
acidic support on heterogeneous RWGS[14] and the benefits of
basic conditions on homogeneous WGS,[15] we conceived that
adding appropriate acid would enhance the performance of
[Ru(CO)3Cl3]- in low-temperature RWGS, thus improve the
overall efficiency of reductive hydroformylation with CO2.
As an inexpensive alternative of noble-metal hydroformylation
catalysts,[16] HCo(CO)4 from in-situ hydrogenolysis of Co2(CO)8
is active species for hydroformylation but catalytically inactive to
RWGS and carbonyl reduction.[4,17] The collaboration of
Co2(CO)8 with homogenous RWGS catalyst would expand the
applications of cobalt carbonyl catalyst in reductive
hydroformylation with CO2. Herein, by employing the
combination of Co2(CO)8 and in-situ generated [Ru(CO)3Cl3]-, an
efficient protocol with lower ruthenium loading and less additive
was established. The good performance of this binary catalyst
system allowed us to extend the method to the valorization of
industrial waste. Experiments and theoretical computations
disclosed that acid accelerated this three-step process by
promoting the ruthenium-catalyzed RWGS. Sequential
hydroformylation and aldehyde reduction respectively depended
on the cobalt and ruthenium catalysts.
Introduction
Reducing the present level of atmospheric carbon dioxide is
imperative to attain the pledge of limiting global warming within
1.5 degrees Celsius.[1] With the upgrade of carbon dioxide
capture and separation technology, utilizing carbon dioxide as
inflammable and non-toxic C1 feedstock has been regarded as a
promising approach to both CO2 emission mitigation and value-
added chemical
production.[2] In this content,
the
hydroformylative synthesis of alcohols[3] represents a highly
atom-economic and eco-friendly CO2 fixation method[4] via
tandem reverse water-gas shift[5] (RWGS, i.e. H2 (g) + CO2 (g)
→ CO (g) + H2O (g)), alkene hydroformylation[6] and carbonyl
reduction.[7] In this three-step process, intensifying CO
concentration of reaction system by driving the equilibrium of
RWGS forward is crucial to boost the yield of hydroformylation
product as well as suppress direct hydrogenation of alkene.
Owing to the endothermic nature of RWGS (ΔH298K = 41.3 kJ
mol-1), most of efficient catalysts were operated at high
temperature (> 300 oC).[8] For being compatible with
hydroformylation and carbonyl reduction, milder conditions are
demanded. Since the discovery of low-temperature RWGS (<
Results and Discussion
[a]
[b]
M. Sc. X. Zhang, Dr. C. Shen, Prof. Dr. C. Xia, Prof. Dr. L. He
State Key Laboratory for Oxo Synthesis and Selective Oxidation,
Suzhou Research Institute of LICP
Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of
Sciences
Lanzhou 730000 (P. R. China)
E-mail: helin@licp.cas.cn; shenchaoren@licp.cas.cn
Prof. Dr. X. Tian
Institute of Molecular Science, Key Laboratory of Materials for
Energy Conversion and Storage of Shanxi Province
Shanxi University
Taiyuan 030006 (P. R. China)
E-mail: tianxx@sxu.edu.cn
University of Chinese Academy of Sciences
Beijing 100049 (P. R. China)
Yancheng Teachers University
Yancheng 224007 (P. R. China)
Supporting information for this article is given via a link at the end of
the document.
To examine our tentative idea, the turnover numbers (TONs) of
RWGS by [Ru(CO)3Cl3]- from in-situ [Ru(CO)3Cl2]2 splitting with
[Bmim]Cl in the presence of different acids were measured
(Table 1). Compared with the result in acid-free conditions
(entry 1), adding acetic acid, benzonic acid, formic acid,
phosphoric acid or para-toluenesulfonic acid could significantly
elevate the RWGS efficiency of [Ru(CO)3Cl3]- catalyst at low
temperature (entries 2-6). Contrarily, the addition of hydrogen
chloride, sulphuric acid or trifluoroacetic acid inhibited CO
formation (entries 7-9). The distinct results among three
carboxylic acid additives[18] indicated that moderate acidity was
beneficial to exert promoting effect on RWGS and strong acidity
might impeded the formation of reductive ruthenium hydride
anion.[19] Additionally, the catalytic efficiency of [Ru(CO)3Cl2]2 in
the absence of chloride additive was much lower than
[c]
[d]
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10.1002/cctc.201802091
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[Ru(CO)3Cl3]-. The presence of Co2(CO)8 had little impact on the
performance of ruthenium catalyst (details see Table S4 in SI).
performances of four simple cobalt salts were tested, but none
of them was superior to Co2(CO)8 (details see Fig. S2 in SI). In
comparison, due to the phosphite-free conditions[10j,21] with lower
ruthenium loading and less chloride additive, both substrate
conversion and reaction selectivity by Ru3(CO)12 was poor (entry
19). The combination of Ru3(CO)12 and Co2(CO)8 diminished
both activity and selectivity (entry 20). Acetic acid failed to
exhibit promoting effect in the combination of Ru3(CO)12 and
Co2(CO)8 (entry 21). The evaluation on solvent showed that
aprotic polar solvent effectively suppressed the undesired
alkene hydrogenation (detailed results see Table S1 in SI). In N-
methyl-2-pyrrolidone (NMP), RWGS, hydroformylation and
carbonyl reduction were optimally compatible.
Table 1. The effects of various acids on homogeneous RWGS by in-situ
formed [Ru(CO)3Cl3]-
Entry
Acid
none
TON based on a Ru atom
1
2
3
4
5
6
7
8
9
14
41
40
39
28
28
12
9
AcOHa
PhCOOH
HCOOHb
Table 2. Optimization for the reductive synthesis of cyclohexylmethanol with
cyclohexene and CO2
c
H3PO4
p-TsOHd
Entry
1
Catalyst
Additive
[Bmim]Cl
1/2/3a (%)
5/4/2
HCle
[Ru(CO)3Cl2]2
f
H2SO4
2
Co2(CO)8
[Bmim]Cl
0/0/<1
CF3CCOOH
7
3
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
[Ru(CO)3Cl2]2/Co2(CO)8
Ru3(CO)12
[Bmim]Cl
17/41/4
3/87/7
Reaction conditions: [Ru(CO)3Cl2]2 (0.025 mmol), [Bmim]Cl (0.4 mmol), acid
(0.35 mmol H+), NMP (1.0 mL), H2 (3 MPa), CO2 (3 MPa), 140 oC, 10 h.
aGlacial acetic acid (0.35 mmol); b88% (w/w) HCOOH (0.35 mmol); c85% (w/w)
H3PO4 (0.12 mmol); dp-Toluenesulfonic acid monohydrate (0.35 mmol); e36
4
[Bmim]Cl/AcOHb
c
5
[Bmim]Cl/H3PO4
[Bmim]Cl/HCOOHd
[Bmim]Cl/PhCOOH
[Bmim]Cl/p-TsOHe
[Bmim]Cl/CF3COOH
[Bmim]Cl/HClf
3/85/4
f
wt% HCl (0.35 mmol); 98% (w/w) H2SO4 (0.175 mmol); [Bmim]Cl = 1-n-butyl-
6
0/77/11
3/85/7
3-methylimidazolium chloride.
7
Encouraged by the above results, the effect of acid in the
reductive hydroformylation was surveyed. The reductive
hydroformylation of cyclohexene with CO2 was selected as
benchmark reaction to test the combination of Co2(CO)8 and in-
situ generated [Ru(CO)3Cl3]- (Table 2). As expected,
[Ru(CO)3Cl3]- or Co2(CO)8 was of no avail to simultaneously
undertake the duties of RWGS, hydroformylation and carbonyl
reduction (entries 1-2). The combination of in-situ generated
[Ru(CO)3Cl3]- and Co2(CO)8 was able to achieve this tandem
process. It substantially elevated the selectivity towards
reductive hydroformylation with moderate cyclohexene
conversion (entry 3). Acetic acid, benzonic acid, formic acid,
phosphoric acid, and para-toluenesulfonic acid, which were able
to promote RWGS, drastically accelerated the conversion of
cyclohexene and retained good selectivity towards alcohol
product (entries 4-8). Oppositely, trifluoroacetic acid, hydrogen
chloride and sulphuric acid, which repressed CO formation,
dramatically reduced the selectivity towards hydroformylation or
completely inhibited the proceeding of reaction (entries 9-11).
The inferior catalytic performances in the [Bmim]Cl-absent
conditions demonstrated the essentiality of halogen anion for
splitting inactive dimer to active mononuclear ruthenium complex
(entries 12-13). Cationic counterion of chloride additive and
bromide anion subtly impacted on catalytic activity and
chemoselectivity (entries 14-21). Although the previous reports
indicated the formation of Ru-carbene complex between
Ru3(CO)12 and [Bmim]Cl via the oxidative addition of a C-H
imidazolium bond to Ru0 center[10i,20a] as well the deprotonation
of imidazolium to generate N-heterocyclic carbine ligand in the
presence of base,[20b-d] the formation of Ru-carbene complex
from [Ru(CO)3Cl3]- with [Bmim]Cl was less probable due to high
oxidation state of Ru and acidic reaction conditions. The
8
<1/70/10
3/44/45
<1/<1/<1
<1/<1/<1
2/8/43
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
g
[Bmim]Cl/H2SO4
AcOH
-
3/1/4
LiCl/AcOH
20/58/21
12/65/23
26/57/5
6/80/5
[PPN]Cl/AcOH
[Bmmim]Cl/AcOH
[Emim]Cl/AcOH
[Hmim]Cl/AcOH
[BnNMe3]Cl/AcOH
[BnNEt3]Cl/AcOH
[Bmim]Br/AcOH
[Bmim]Cl
10/71/4
24/51/18
3/47/41
15/66/4
11/31/12
<1/7/27
<1/10/18
Ru3(CO)12/Co2(CO)8
Ru3(CO)12/Co2(CO)8
[Bmim]Cl
[Bmim]Cl/AcOH
Reaction conditions: cyclohexene (5 mmol, 1.0 equiv.), NMP (5 mL), catalyst
(for ruthenium catalyst, 0.33 mol% Ru3(CO)12 or 0.5 mmol% [Ru(CO)3Cl2]2; for
cobalt catalyst, 1 mol% Co2(CO)8), additive (8 mol% halide salt with 7 mol%
acid), 3 MPa CO2, 3 MPa H2, 140 oC, 10 h. aThe yields of products were
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10.1002/cctc.201802091
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determined by GC-FID using n-dodecane as internal standard; bThe reaction
reduction were respectively inspected (details see Table S3 in
SI). The quantitative conversion of cyclohexanecarbaldehyde to
cyclohexylmethanol by [Ru(CO)3Cl3]- proved that the carbonyl
reduction of aldehyde intermediate was catalysed by ruthenium.
To assess the plausibility of producing CO via the hydrogenation
of CO2 to formic acid followed by the dehydration of formic acid
during RWGS. The dehydrative decomposition of formic acid to
CO under reaction conditions was examined (Fig. S5 in SI). The
sluggish CO production via the dehydration of formic acid with or
without [Ru(CO)3Cl3]- implied that the reduction of CO2 to CO via
formic acid intermediate was much less feasible under such
conditions.
Tracking the fluctuation of RWGS turnover frequency (TOF)
over time showed that TOF increased with the transformation of
[Ru(CO)3Cl2]2 to [Ru(CO)3Cl3]- at the initial stage (Table 4,
entries 1-4).[13b] TOF was sustained in the circumstance of
longer reaction time (entry 5). The influence of hydroformylation
on TON and TOF of RWGS was also probed (entries 6-7). In
accordance with Le Chateliers principle,[22] the CO consumption
by hydroformylation prompted both TON and TOF of RWGS.
When both Co2(CO)8 and cyclohexene were added, TOF of
RWGS was raised to 9.9 h-1 with the occurrence of
hydroformylation and it was higher than TOF of hydroformylation
(4.6 h-1) in this cascade process (entry 7).
time was prolonged to 20 h; bGlacial AcOH; c85% (w/w) H3PO4; d88% (w/w)
f
HCOOH; ep-Toluenesulfonic acid monohydrate; 36% (w/w) HCl; g98% (w/w)
H2SO4. NMP
methylimidazolium chloride; [PPN]Cl
chloride; [Bmmim]Cl = 1-n-butyl-2,3-dimethylimidazolium chloride; [Emim]Cl =
1-ethyl-3-methylimidazolium chloride; [Hmim]Cl 1-n-hexyl-3-
1-n-butyl-3-methylimidazolium
=
N-methyl-2-pyrrolidone; [Bmim]Cl
=
1-n-butyl-3-
=
bis(triphenylphosphine)iminium
=
methylimidazolium chloride; [Bmim]Br
=
bromide; p-TsOH = p-toluenesulfonic acid.
Afterwards, the loadings of acetic acid and Co2(CO)8 as well
as the substrate concentration were optimized (details see Figs.
S3 and S4 in SI). When acetic acid was more than 7 mol%, the
yield was unable to rise. When Co2(CO)8 loading was over 1
mol%, the yield remained above 85%. Neat conditions or more
diluted concentration undermined the chemoselectivity on
cyclohexylmethanol. The outcomes of few hydroformylation
products under the CO2-free conditions verified CO2 as the
source of most implanted hydroxymethyl groups (Table S2 in SI).
In order to better understand this tandem process, the
scenario of replacing CO2 with CO was investigated (Table 3). In
the atmosphere of 0.2 MPa CO, the conversion of alkene and
yield of alcohol declined to 72% and 57% (entry 1). Once the CO
pressure was raised to 0.5 MPa, both the conversion of
substrate and yield of alcohol were nearly same as the scenario
under 3.0 MPa CO2 (entry 2). This trend demonstrated that
sufficient CO produced from RWGS could enhance
hydroformylation and restrain the hydrogenation of alkene. The
low yield of hydroformylation products under the Co2(CO)8-
absent conditions indicated that the hydroformylation was mainly
responsible by cobalt carbonyl catalyst (entry 3). From the
unaffected conversion and selectivity in the absence of acetic
acid, we concluded that the acid was uninvolved into the
hydroformylation and carbonyl reduction (entry 4). No
conversion of cyclohexene was observed in the absence of
ruthenium catalyst (entry 5). Given that the necessity of typical
high CO and H2 total pressure (> 20 MPa) for Co2(CO)8- or
[Co(CO)4]--catalyzed hydroformylation,[6a,16a] this phenomenon
suggested that ruthenium catalyst could facilitate the
hydrogenolysis of Co2(CO)8 to active HCo(CO)4 under low H2
pressure.
Table 4. Comparison of over time in the presence of AcOH.
Entry
Variation of conditions
TON (TOF/h-1)a
5 (1.7)
1
2
3
4
5
6
7
3 h
6 h
18 (3.0)
8 h
29 (3.7)
none
20 h
41 (4.1)
92 (4.6)
Adding Co2CO8 (0.05 mmol)
44 (4.4)
Table 3. Conditions-control experiments in the CO atmosphere.
Adding Co2CO8 (0.05 mmol) and
cyclohexene (5 mmol)
99 (9.9)b
Reaction conditions: [Ru(CO)3Cl2]2 (0.025 mmol), [Bmim]Cl (0.4 mmol),AcOH
(0.35 mmol), NMP (1.0 mL), H2 (3 MPa), CO2 (3 MPa), 140 oC, 10 h. aCO
contents were determined by GC-FID equipped with a methanizer, TONs and
TOFs of RWGS were calculated based on a Ru atom. b95% conversion of
cyclohexene; 1% yield of cyclohexanecarbaldehyde; 90% yield of
cyclohexylmethanol; 0.39 mmol CO in the gas phase after reaction and the
total amount of RWGS-generated CO was 4.94 mmol.
Entry
Variation of conditions
none
Conv.a (%)
1/2/3a (%)
3/57/12
2/93/1
4/21/6
2/56/12
-/-/-
1
2
3
4
5
72
96
31
70
<1
0.5 MPa instead of 0.2 MPa CO
without Co2(CO)8
To the best of our knowledge, theoretical studies of
homogeneous RWGS remain sparse.[23,24] Herein, density
functional theory (DFT) computations at M06 level were
conducted to gain more understanding about the observed
acceleration of RWGS by acid. Since the reactions were
conducted at 413.15 K, only the results at 413.15 K were
presented and the solvent effects of N-methyl pyrrolidone (NMP)
were included by using the polarizable continuum model
(PCM).[25] The results at 298.15 K (Figs. S6-9) were listed in SI
for comparison. The computations are divided into two parts; the
formation of ruthenium hydride via hydrogen splitting and the
reduction of CO2 with ruthenium hydride. Based on the previous
spectra evidence,[12,14] the hydrogen splitting process undergoes
without AcOH
without [Ru(CO)3Cl2]2 and [Bmim]Cl
Reaction conditions: cyclohexene (5 mmol, 1.0 equiv.), NMP (1 mL), 0.5
mmol% [Ru(CO)3Cl2]2, 1 mol% Co2(CO)8, [Bmim]Cl (8 mol%), AcOH (7 mol%),
3 MPa CO2, 3 MPa H2, 140 oC, 10 h. aThe conversion of cyclohexene and
yields of products were determined by GC-FID using n-dodecane as internal
standard.
By replacing cyclohexene with cyclohexanecarbaldehyde, the
catalytic activities of ruthenium and cobalt catalysts on carbonyl
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10.1002/cctc.201802091
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the formation of [Ru(CO)2Cl3]- (2) and generating [RuH(CO)2Cl2]-
(4) (Fig. 1). The generation of 2 and 4 are correspondingly via
the CO elimination from [Ru(CO)3Cl3]- (1) and the heterolysis of
H2 on 2 succeeded by releasing HCl. Calculation results
exhibited the following patterns: i) CO elimination (13.45 kcal
mol-1) was indeed more favored than eliminating Cl- from
[Ru(CO)3Cl3]- (18.60 and 27.10 kcal mol-1); ii) eliminating CO
from [Ru(CO)3Cl3]- was much more endothermic (13.45 kcal mol-
1 for 1 to 2) than the formation of [RuH(CO)2Cl2]- (1.12 kcal mol-1
for 2 to 4), albeit the activation energy of 13.22 kcal mol-1 for the
formation of [RuH(CO)2Cl2]- (3 to TS1). The energy patterns
involving H2 splitting on the ruthenium catalyst resembled the
very recent report.[26]
RuII in [Ru(CO)3Cl2(η1-O2CMe)]- was endothermic (Fig. S10 in
SI). The resulted [Ru(CO)3Cl2(η1-O2CMe)]- could also be
converted to 1 through the reaction with HCl (Table S6 in SI).
According to these traits, we reasonably inferred that adding
acetic acid would properly enhance the Brønsted acidity of
reaction system and assisted the dehydroxylation of Ru-COOH
complex without hindering the formation of anionic ruthenium
hydride.
Figure 2. Free energy profile for the CO2 reduction of six-coordinated
ruthenium hydride complex [RuH(CO)2Cl3]- (pathway A) at 413.15 K. The
relative free energies (kcal mol-1) were calculated using 4 as the reference.
Figure 1. Free energy profile for the hydrogen splitting to afford the active
ruthenium hydride at 413.15 K. The relative free energies (kcal mol-1) were
calculated using 1 as the reference.
In the following CO2 reduction by ruthenium hydride complex,
two plausible pathways respectively involving five-coordinated 4
(pathway B, Fig. 3) and six-coordinated 5 (pathway A, Fig. 2)
were considered. 5 could be formed via the addition of chloride
anion onto 4 without hindrance. Free energy profiles illustrated
that inserting CO2 into Ru-H bond was the critical step for the
CO2 reduction, and the activation energy in pathway B was
lower than that of in pathway A. The instantaneous formation of
9 was also reported.[27] Therefore, the pathway B is more likely
to occur. The obtained activation energy (47.31 kcal mol-1 from 8
to TS3) and endothermic character (7.21 kcal mol-1 from 8 to 9)
of pathways
B were consistent with the high reaction
temperature not only employed in our reaction but also reported
in the literature.[27a] Since the addition of ruthenium hydride onto
CO2 (4 to 9 via TS3) has the highest activation energy among all
the elementary steps of RWGS, it can regarded as the rate-
determining step in RWGS. The dehydroxylation of 7 and 9
respectively to [Ru(CO)3Cl3]- (1) and [Ru(CO)3Cl2] (10) by
hydronium were both exothermic without evident energy barrier
(details see Fig. S13 in SI). In contrast, the acid-absent
intramolecular dehydroxylation of 9 to fac-[Ru(CO)3Cl2(OH)]- (12)
through 11 and TS4 was endothermic. It had to go through
isomerization (9 to 11) and overcome significant energy barrier
(13.26 kcal mol-1 from 11 to TS4). The energy barrier for the
reaction between [RuH(CO)2Cl2(η1-O2CMe)]2- and CO2 was even
higher than pathways A and B (Fig. S11 in SI). The coordination
between AcO- and 10 was the plausible competing reaction
during the transformation from 10 to 1. Compared with the
coordination between Cl- and 10, it was more favored (Fig. S9 in
SI). But the further bidentate coordination between acetate and
Figure 3. Free energy profile for the CO2 reduction of five-coordinated
ruthenium hydride complex [RuH(CO)2Cl2]- (pathway B) at 413.15 K. The
relative free energies (kcal mol-1) were calculated using 4 as the reference.
With the optimized conditions in hand, the scope of substrates
was explored (Table 2). Applying cyclic olefins, for example
cyclopentene (2b), cycloheptene (2c), cyclooctene (2d) as well
as indene (2e) led to alcohol products in moderate to good
yields. For terminal alkenes (2f-k), the chemoselectivity was
disturbed by the occurrence of alkene hydrogenation. It resulted
in the moderate to low yields of alcohol products. Mono-
substituted alkenes (2f-i) afforded a mixture of the branched and
linear alcohols was formed. Tests on 1-octene with shorter
reaction time (i.e. 6 hours) or replacing CO2 with CO clarified
that the regioselectivity of product 2g remained constant both
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10.1002/cctc.201802091
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over time and in the more CO-abundant atmosphere. Internal
octenes were detected by GC-FID in the reaction mixture after 6
hours. α-Substituted styrenes (2j-k) gave the alcohols with good
regioselectivity. Some isolated dienes were tested as well. In
cases of 1,5-cyclooctene (2l) and 1,7-octadiene (2m), products
with one double bond being reduced were formed.
disclosed the assist of proton on dehydroxylation of Ru-COOH
intermediate during RWGS. Notably, the valorization of industrial
waste and the chemical fixation of CO2 could be implemented
simultaneously with this protocol.
Experimental Section
Table 5. Scope of substrates
General procedure for measuring CO generated from RWGS
under different conditions: A 10-mL glass vial was charged
with [Ru(CO)3Cl2]2 (12.8 mg, 0.025 mmol, 1 mol% Ru), [Bmim]Cl
(69.6 mg, 0.4 mmol, 8 mol%), acid (0.35 mmol H+), NMP (1 mL)
and a magnetic stirring bar. The vial was fixed in an aluminium
alloy plate and transferred into a 300-mL autoclave under N2
atmosphere. After subtracting the volume occupied by the vial,
plate and stirring bar, the remaining volume of 300-mL autoclave
measured by water displacement was 270 mL. Then at room
temperature, CO2 (3 MPa) and H2 (3 MPa) were introduced
2b, 93%
2f, 61%c
2c, 93%
2d, 89%
2h, 67%f
2e, 58%b
2i, 64%g
2g, 79%d,e
o
before the autoclave was heated to 140 C. After 10 hours, the
2j, 68%
2k,38%
2l, 53%h
reaction was cooled down to room temperature. Gas was
collected by gas sampling bag after reaction at room
temperature and then transferred into the gas sampling loop of
gas chromatography (Agilent 7890B) equipped with Porapak Q
polymers packed GC columns, a 5 Å molecular sieve column,
thermal conductivity detector (TCD), methanizer and flame
ionization detector (FID). Gas products were analyzed by TCD
for the H2 content and then studied by FID with a methanizer for
CO. Based on the quantification from the integral area ratio of
the reduction products to the certified standard gas, the molar
contents of gaseous products were determined with the peak
areas on GC. The single standard gas (from Wujiang Messer
Industrial Gas Co., Ltd., certified contents: 0.499% CO, 1.242%
H2, 0.124% CH4, 0.049% C2H4 and 0.051% C2H6) data was
adopted to quantify the test gas.[13a] The mole amount of
generated CO was calculated from multiplying the molar
concentration of CO by 270 mL (the real capacity of 300-mL
autoclave).
2m, 63%i
2n, 75%
2o, 80% (61%)j
Reaction conditions: alkene (5 mmol, 1.0 equiv.), NMP (1 mL), 0.5 mmol%
[Ru(CO)3Cl2]2, 1 mol% Co2(CO)8, [Bmim]Cl (8 mol%), AcOH (7 mol%), 3 MPa
CO2, 3 MPa H2, 140 oC, 10 h. Isolated yield. Conversion and regioselectivity
were respectively determined by GC-FID and 1H NMR. bα: β = 1:1.5; cn : iso =
1:1; dn : iso = 1:0.4; en : iso = 1:0.4, 6 h with 54% yield of 2g and 76%
conversion of 1-octene or 10 h with 88% yield of 2g and 96% conversion of 1-
octene when the mixture of CO and N2 (0.5 MPa + 2.5 MPa) replaced CO2; fn :
iso = 1:0.25; gn : iso = 1:0.5; h1,5-cyclooctadiene (5 mmol), the bold bond
represents the hydrogenation of the double bond; i1,7-octadiene (5 mmol), n :
j
iso = 1:0.3; Diisobutylene (5 mmol), the mixture of 2,4,4-trimethyl-1-pentene
and 2,4,4-trimethyl-2-pentene with the molar ratio of 3:1, 82% conversion of
DIB and 2% yield of 2,2,4-trimethylpentane by-product. In the parentheses
was the yield of 3,5,5-trimethylhexanol in the reaction scale of 30 mmol
conducted in 300-mL autoclave.
Furthermore, the capability of valorizing wastes from biomass
and petroleum industry by the chemical fixation of CO2 was
demonstrated. Limonene (2n), one of the abundant terpenes
derived from biomass[28] and prominent waste stream of the
citrus industry,[29] was converted to unsaturated alcohol. The tri-
substituted double bond in the ring was retained, which would
facilitate the further functionalization of products. Diisobutylene
(DIB, 2o), the inexpensive mixture of 2,4,4-trimethyl-1-pentene
and 2,4,4-trimethyl-2-pentene generated via the irreversible
exothermic dimerization of isobutylene over acid catalyst during
the refining process of C4 fraction,[30] was transformed to
fragrance ingredient 3,5,5-trimethyl-1-hexanol[31] with 97%
selectivity. The yield of 3,5,5-trimethylhexanol reached 80%. The
chemoselectivity towards alcohol product was maintained in the
scale-up (30 mmol) reaction.
General procedure for the reductive hydroformylation of
olefins with CO2 and H2: A 10-mL glass vial was charged with
[Ru(CO)3Cl2]2 (12.8 mg, 0.025 mmol, 1 mol% Ru) or Ru3(CO)12
(10.7 mg, 0.017 mmol, 1 mol% Ru), [Bmim]Cl (69.6 mg, 0.4
mmol, 8 mol%) and a magnetic stirring bar. Then in the
atmosphere of N2, cyclohexene (508 μL, 5 mmol, 1 equiv.),
acetic acid (20 μL, 0.35 mmol, 7 mol%) and NMP (1 mL) were
successively injected into the vial by syringe.
A capsule
containing Co2(CO)8 (17.1 mg, 0.05 mmol, 2 mol% Co) was
added. The vial was fixed in an alloy plate and transferred into a
300-mL autoclave under N2 atmosphere. At room temperature,
the autoclave was flushed with carbon dioxide for three times
and 3.0 MPa CO2 and 3.0 MPa H2 were consecutively charged.
o
The reaction was heated at 140 C for 10 hours. Afterwards, the
autoclave was cooled to room temperature and the pressure
was carefully released. Internal standard dodecane (40 μL) was
added into the reaction mixture. The resulted reaction mixture
was diluted with methanol and filtered through a short pad of
Celite. The Celite pad was washed with methanol and
tetrahydrofuran. The combined solution was sampled and
analyzed by gas chromatography. Pure product could be
isolated by column chromatography on silica gel (eluent gradient
from petroleum ether/ethyl acetate = 1:0 to petroleum ether/ethyl
acetate = 10:1).
Conclusions
In this study, the acid-aid heterobimetallic ruthenium-cobalt
catalytic system exhibited good performance in the reductive
hydroformylation of alkene with CO2. Experiments revealed that
adding acetic acid can effectively accelerate the reductive
hydroformylation by promoting the ruthenium-catalysed RWGS.
Hydroformylation and aldehyde reduction respectively depended
on the cobalt and ruthenium catalysts. Theoretical computations
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10.1002/cctc.201802091
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Acknowledgements
L.H. thanks OSSO in LICP, CAS for start-up support. The work
was financially supported by the National Natural Science
Foundation of China (no. U1510103 for X.T., 91645118 for L.H.,
21633013 for C.X. and L.H.), Key Research Program of Frontier
Sciences of CAS (QYZDJ-SSW-SLH051) and the Natural
Science Foundation of the Jiangsu Higher Education Institutions
of China (18KJB150033 for X.Z.). The calculations were
performed at the Shanghai Supercomputing Centre and the
Supercomputing Centre of Shanxi University. We thanked Dr.
habil. Haijun Jiao of Leibniz-Institut für Katalyse and Prof. Yan-
Bo Wu of Shanxi Univeristy for their suggestion and discussion.
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Acid-promoted hydroformylative
synthesis of alcohol with carbon
dioxide by heterobimetallic
Acceleration by acid: The acid-aid heterobimetallic ruthenium-cobalt catalytic
system exhibited good performance in the hydroformylative synthesis of alcohols
with carbon dioxide. The applicability of valorizing industrial waste was highlighted.
Experiments and theoretical computations demonstrated that adding acetic acid
could effectively accelerate the reductive hydroformylation by promoting reverse
water-gas shift.
ruthenium-cobalt catalytic system
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