Hydrogenation of carboxylic acids with a homogeneous cobalt catalyst

Article abstract of DOI:10.1126/science.aaa8938

The reduction of esters and carboxylic acids to alcohols is a highly relevant conversion for the pharmaceutical and fine-chemical industries and for biomass conversion. It is commonly performed using stoichiometric reagents, and the catalytic hydrogenation of the acids previously required precious metals. Here we report the homogeneously catalyzed hydrogenation of carboxylic acids to alcohols using earth-abundant cobalt. This system, which pairs Co(BF₄)₂·6H₂O with a tridentate phosphine ligand, can reduce a wide range of esters and carboxylic acids under relatively mild conditions (100°C, 80 bar H₂) and reaches turnover numbers of up to 8000.

Full text of DOI:10.1126/science.aaa8938

RESEARCH
| REPORTS
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ACKNOWLEDGMENTS
We thank the SENS Foundation for financial support and
G. Micalizio, A. de Grey, and W. Bains for helpful discussions.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/350/6258/294/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S5
Tables S1 to S11
References (28–32)
Spectral Data
6 July 2015; accepted 8 September 2015
10.1126/science.aac9655
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CATALYSIS
iron-based catalysts for the hydrogenation of
esters have been reported (22–24). Here we pre-
sent a cobalt-based catalytic system capable of
hydrogenating both esters and carboxylic acids
to the corresponding alcohols, using H2 as the
reductant. The catalyst is generated in situ from
a Co(BF4)2·6H2O precursor and the triphos ligand,
which are both commercially available, making
this method highly suitable for use in practical
organic synthesis and biomass conversion.
Initially, various cobalt precursors were tested,
in combination with the triphos ligand, in the hy-
drogenation of methyl benzoate to yield benzyl
alcohol and methanol. Combining methyl benzo-
ate with 10 mole percent (mol %) of the metal
precursor and the triphos ligand in distilled meth-
anol, under 80 bar of initial H2 pressure and at
a 100°C reaction temperature, produced only
minor amounts of alcohol with cobalt(III) ace-
tylacetonate, cobalt(II) acetylacetonate, and co-
balt(II) acetate (table S1). Using Co(BF4)2·6H2O as
the metal precursor resulted in almost full sub-
strate conversion within 5 hours, providing the
desired benzyl alcohol in excellent yield. De-
creasing the catalyst loading to 5 mol % resulted in
full conversion within 22 hours, with a good yield
of benzyl alcohol. Lower catalyst loadings required
very long reaction times (2 mol %, 94 hours) or led
to lower overall activity (1 mol %). Based on this
initial catalyst optimization, the substrate scope of
the Co(BF4)2·6H2Oand triphos(Co/triphos)hydro-
genation system was investigated (Table 1 and
table S2). Using tert-butylbenzoate, an ester sub-
strate that has low activity with many catalysts
due to the steric hindrance of the tert-butyl group,
full conversion was obtained within 5 hours. Also,
with aliphatic esters such as methyl butyrate and
methyl 3-trans-hexenoate, good yields of the ful-
ly reduced corresponding alcohol were obtained
within 22 hours. Hydrogenation of g-valerolactone,
a substrate that can easily be obtained from bio-
mass and, as such, is of interest for biomass val-
orization, produced 2-methyltetrahydrofuran and
1,4-pentanediol in good yields. These products
have attracted attention for their respective ap-
plications as a fuel additive and as a monomer
for polyester production (2). A long-chain fatty
acid methyl ester and even a triglyceride could
also be smoothly hydrogenated to obtain the
corresponding reduced fatty alcohol in good
yields, using the Co/triphos system.
Hydrogenation of carboxylic acids
with a homogeneous cobalt catalyst
Ties J. Korstanje, Jarl Ivar van der Vlugt, Cornelis J. Elsevier,* Bas de Bruin*
The reduction of esters and carboxylic acids to alcohols is a highly relevant conversion for the
pharmaceutical and fine-chemical industries and for biomass conversion. It is commonly
performed using stoichiometric reagents, and the catalytic hydrogenation of the acids previously
required precious metals. Here we report the homogeneously catalyzed hydrogenation of
carboxylic acids to alcohols using earth-abundant cobalt.This system, which pairs Co(BF4)2·6H2O
with a tridentate phosphine ligand, can reduce a wide range of esters and carboxylic acids under
relatively mild conditions (100°C, 80 bar H2) and reaches turnover numbers of up to 8000.
here is great interest among the pharma-
ceutical and fine-chemical industries in
finding more sustainable methods to reduce
moieties such as esters and carboxylic acids
to alcohols. In the emerging field of biomass
high temperatures and pressures (4). A milder
enzymatic hydrogenation route using Pyrococcus
furiosus cells has been reported recently (5). In
recent years, however, much progress has also
been made with using homogeneous catalysts for
the hydrogenation of esters and acids under mild
reaction conditions (6–8). Thus far, primarily
ruthenium-based catalysts, in combination with
multidentate ligands such as triphos [tridentate
phosphine, CH3C(CH2PPh2)3] (9–12) or PNN pin-
cers (PNN, 2-di-tert-butylphosphinomethyl-6-
diethylaminomethylpyridine) (13–15), have been
used for the hydrogenation of esters. Carboxylic
acids, however, are difficult to hydrogenate in com-
parison with ketones and esters. Ligand protona-
tion and resultant ligand dissociation, combined
with overcoordination of the generated vacant
sites at the metal center by the (bidentate) carbo-
xylate conjugated base, result in detrimental cata-
lyst deactivation. These pathways are especially
problematic with first-row transition metals, which
typically have weaker metal-ligand bonds than
their second- and third-row congeners. To date,
homogeneously catalyzed hydrogenation of car-
boxylic acids has only been achieved with two
systems: a ruthenium/triphos system (16–19) and
an iridium/bipyridine system (20, 21). Although
these systems provide good activity and selectiv-
ity, the replacement of the scarce and expensive
noble metals by cheaper, more abundant, and
less toxic first-row transition metals, such as
iron or cobalt, would enhance the sustainability
of these hydrogenation reactions (Fig. 1). Recently,
T
feedstock conversion to produce high-value chem-
icals, the reduction of these abundantly available
oxygen-containing functional groups is also of cru-
cial importance. Traditionally, esters and carboxylic
acids are reduced using stoichiometric reagents,
such as aluminum- or borohydrides, but these pose
an inherent safety risk and produce large amounts
of waste material (1). Selective catalytic hydrogen-
ation of esters and carboxylic acids to the corre-
sponding alcohols could facilitate the synthesis of
many products in various sectors of the chemical
industry; for instance, it could facilitate the con-
version of (biomass-derived) succinic acid into the
bulk chemicals 1,4-butanediol, tetrahydrofuran
(THF), or g-butyrolactone, which can be used as
solvents and as intermediates for the production
of plastics and fibers (2). Also, numerous phar-
maceuticals require a selective hydrogenation
step of either an ester or a carboxylic acid moiety
in their synthetic route (3).
The catalytic hydrogenation of esters and car-
boxylic acids is performed industrially by using
heterogeneous catalysts that typically operate at
Van ’t Hoff Institute for Molecular Sciences, University of
Amsterdam, Science Park 904, 1098 XH Amsterdam, Netherlands.
*Corresponding author. E-mail: c.j.elsevier@uva.nl (C.J.E.);
b.debruin@uva.nl (B.d.B)
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The successful application of the Co/triphos sys-
tem for ester hydrogenation led us to investigate its
efficacy in the more challenging conversion of car-
boxylic acids (Table 2). Using benzoic acid as the
substrate, with 2.5 mol % catalyst loading and THF
as the solvent (to prevent in situ esterification and
thus to ensure that the system is truly hydrogen-
ating acids), benzyl alcohol was obtained in good
yield within 22 hours. Removal of any remaining
acid starting material and the majority of the
catalyst can be easily accomplished by washing with
a saturated aqueous sodium carbonate solution.
Further purification using column chromatogra-
phy yielded the isolated alcohol product. Increasing
the catalyst loading to 5 mol % led to full conversion
after 4 hours, compared with 22 hours for methyl
benzoate. This indicates that hydrogenation of
carboxylic acids with the Co/triphos system out-
competes the conversion of the corresponding
carboxylic esters, which is surprising, consider-
ing the difficulties associated with hydrogenating
carboxylic acids in most systems. The system tol-
erates several functional groups, such as chlor-
ides, fluorides, trifluoromethyls, and hydroxides,
which can be used for further synthetic process-
ing, such as C-C cross-coupling. More labile groups
such as bromide, iodide, boronate, and methoxy
functionalities undergo (partial) defunctionalization,
but they do not hamper the reactivity (table S3).
Furthermore, the catalytic system is capable of
hydrogenating a wide scope of carboxylic acid sub-
strates, including aromatic and long- and short-
chain aliphatic acids, all of which are smoothly
hydrogenated to the corresponding alcohols.
From a biomass viewpoint, the direct hydrogen-
ation of levulinic acid to 2-methyltetrahydrofuran
and 1,4-pentanediol is relevant, because it elim-
inates the need to isolate g-valerolactone as the
cyclic ester intermediate. The hydrogenation of
succinic acid is also of interest, because it
produces 1,4-butanediol, which is a comonomer
in the production of polyesters such as polybutylene
terephthalate. Furthermore, polymerization of 1,4-
butanediol with (potentially biomass-derived)
succinic acid constitutes a completely biomass-
derived route to polybutylene succinate. In our
system, short-chain acids are hydrogenated more
easily than long-chain acids, which is in agree-
ment with previous research on acid hydrogen-
ation (20). Moreover, it is possible to reduce the
liquid acids under solventless conditions, allow-
ing low catalyst loadings (down to 0.1 mol %) for
butyric acid and acetic acid, while still giving
good yields of 1-butanol and ethanol, respectively.
Under these conditions, esterification of the car-
boxylic acid starting material by the alcohol pro-
duct also takes place, as demonstrated by the
observation of butyl butyrate and ethyl acetate,
respectively. When using formic acid or trifluoro-
acetic acid, neat conditions did not produce any
alcohol, probably because of the high polarity of
these acids. With THF as the solvent, however, a
very low catalyst amount of 125 parts per million
can be used for trifluoroacetic acid, while still
reaching full conversion in 22 hours and giving
a good yield of 50% of trifluoroethanol (the
remaining mass balance consists of trifluoroethyl
trifluoroacetate and an unidentified side prod-
uct). Hydrogenation of formic acid in THF also
proceeds smoothly, but it requires a somewhat
higher catalyst loading of 0.25 to 0.5 mol %.
Table 1. Hydrogenation of esters using Co(BF4)2·6H2O and triphos. General conditions used, unless
otherwise noted, were 0.15 M substrate, 1:1 ratio of Co(BF4)2·6H2O and triphos (mol % loading shown),
distilled MeOH, 80 bar initial H2 pressure, 100°C, 22 hours. In the substrate columns, catalyst amounts
are given per ester group, and in the product columns, conversions are given with gas chromatography
(GC) yields in parentheses.
In comparison with the reported P. furiosus
biocatalytic system (5), which uses a reaction tem-
perature of 40°C and 5 bar H2 pressure, our sys-
tem generally has a lower activity and requires
harsher reaction conditions; however, it is able to
convert a much wider substrate scope and can
obtain higher conversions of the carboxylic acid
within the same period of time. Relative to our
Fig. 1. Hydrogenation of carboxylic acid to alcohol. Advantages and drawbacks of previous
approaches are compared with those of our catalytic system.
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system, noble metal–based heterogeneous cata-
lysts such as Ru-Sn/Al2O3 (25) and Pt-Re/TiO2 (26),
operating at temperatures of 240° and 130°C
and pressures of 80 and 20 bar, respectively, give
slightly lower conversions (82%), higher turnover
numbers of stearic acid (512 and 97, respectively),
and a lower selectivity due to the formation of
decarboxylation and dehydration products. Fur-
thermore, leaching of tin occurs when using the
former catalyst, which is highly undesirable for a
pharmaceutical application, and the Pt-Re/TiO2
catalyst suffers from aromatic ring hydrogenation
when using aromatic substrates. In comparison
with previously reported homogeneous catalysts,
our cobalt-based system outperforms both the
ruthenium- and iridium-based systems for acid
hydrogenation (19, 20): The ruthenium-based
system operates at much higher temperatures
(140° to 220°C) and with higher catalyst loadings
(>2 mol %) than our system, and the iridium-
based system requires a higher reaction tem-
perature (120° versus 100°C) and much longer
reaction times (65 versus 22 hours) to produce
similar turnover numbers with acetic acid (777
for iridium versus 780 for cobalt).
further research is required to completely explain
the observed counterion effect.
as the active catalyst, because this would require a
much lower amount of poison for full inhibition of
the catalysis (29). This result is consistent with a
mercury poison test, which did not show any
considerable inhibition of the catalysis.
We studied the reaction further by means of
high-resolution electron spray ionization mass
spectrometry (ESI-MS). Using benzoic acid as
the substrate, a catalytic sample was analyzed
after 1 hour of reaction time; the major signal de-
tected (mass/charge ratio = 804.1875) has a mass
consistent with that of a [Co(triphos)(benzoate)]+
species (fig. S2). The same species was detected
upon separately mixing the cobalt precursor, tri-
phos, and benzoic acid in a 1:1:1 ratio and heating
To further investigate the reaction mechanism,
we performed partial poisoning studies using TEMPO
(2,2,6,6-tetramethylpiperidine-1-oxyl) as a radical
inhibitor and TMTU (tetramethylthiourea) as a
strongly binding poison (28). With both reagents,
the reaction was completely inhibited by one
equivalent per catalyst, and for TMTU, only half
an equivalent was sufficient (fig. S1). This suggests
that TMTU deactivates two cobalt centers, either
by acting as a bridging poison ligand or by acting
on a dinuclear cobalt species, whereas TEMPO
poisons a monomeric cobalt species. Furthermore,
this result excludes the formation of nanoparticles
To investigate the specificity of the triphos lig-
and, different ligand systems were tested. The te-
tradentate analog tetraphos [P(CH2CH2PPh2)3],
known to be active in CO2 hydrogenation in com-
bination with cobalt (27), did not show any activity
in ester hydrogenation. In addition, the combina-
tion of various bidentate phosphines with mono-
dentate phosphines did not show any reactivity
for either esters or acids. The influence of additives
was investigated, but neither acid nor base had a
positive effect on the reactivity in the ester hydro-
genation reaction, nor did an additional reducing
agent, such as zinc dust (10). The analogous cobalt
precursor [Co(NCCH3)6](BF4)2, in combination
with triphos, also did not show any activity in
ester or acid hydrogenation under dry conditions.
However, when methyl benzoate was used as the
substrate, the activity was restored by adding six
equivalents of water (relative to the catalyst), and
[Co(THF)6](BF4)2 showed carboxylic acid conver-
sion under dry conditions. The inactivity of the
[Co(NCCH3)6](BF4)2 precursor in the absence of
water was probably caused by the stronger co-
ordination of the acetonitrile ligands, which ham-
per reactivity under dry conditions.
The known dinuclear cobalt trishydride com-
plex [Co2(triphos)2(m-H)3]BF4 also was found to
be inactive in this reaction. The inactive cobalt
precursor Co(OAc)2·4H2O (OAc, acetate) could
be activated by the addition of two equivalents
of BF3·OEt2 (OEt2, diethylether) or by the addition
of various noncoordinating salts, such as HBF4,
NaSbF6, or LiB(C6F5)4. In contrast, the addition
of NaBF4, NaPF6, or NaBPh4 did not provide this
activating effect. The addition of BF3·OEt2 or HBF4,
however, does not improve the activity in the
case of the Co(BF4)2·6H2O precursor. Furthermore,
the addition of other Lewis acids, such as scandium
(III) triflate or iron(III) chloride, had a detrimental
effect on the reactivity of the Co/triphos system.
This indicates that a noncoordinating anion is
required to provide catalytic activity, although
Fig. 2. Proposed mechanism for the Co/triphos–catalyzed hydrogenation of carboxylic acids
to alcohols. BF4 anions are omitted for clarity. The central area shows the crystal structure of the
proposed resting state, [Co(triphos)(k2-OOCPh)]+. The numbers below the structures indicate the
relative enthalpies (kcal mol−1) at 373 K, obtained from dispersion-corrected density functional theory
(DFT-D3), using the BP86 functional, def2-TZVP basis set, and Grimme's version 3 dispersion cor-
rections (disp3). Colors in the central structure correspond to cobalt (blue), oxygen (red), phosphorus
(yellow), and carbon (gray).
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this mixture briefly under an inert atmosphere.
Because ESI-MS does not provide information
on the oxidation state, we also performed in situ
electron paramagnetic resonance (EPR) spectros-
copy. The catalytic reaction was carried out in a
high-pressure EPR tube, using 2-methyltetrahy-
drofuran as the solvent (to obtain a good glass)
and 15 bar of H2 pressure, followed by rapid cool-
ing of the reaction mixture from 100° to –198°C
(76 K) after 2 hours of reaction time. The EPR tube
was subsequently cooled to 20 K in the EPR cavity
to record the EPR spectrum. Two signals were ob-
served: a broad signal, arising from a high-spin
species (electron spin, S = 3/2) and a well-defined
signal from a low-spin species (S = 1/2) with a com-
plex hyperfine coupling pattern due to coupling
with cobalt (nuclear spin, I = 7/2) and three phos-
phorus atoms (I = 1/2) (fig. S3). The high-spin spe-
cies originated from a small excess amount of
Co(BF4)2·6H2O, whereas the low-spin species
produced an identical signal to that of an inde-
pendently prepared 1:1:1 mixture of Co(BF4)2·6H2O,
triphos, and benzoic acid (fig. S2). This confirms
the results obtained by ESI-MS and also shows
that the cobalt center in this species remains in
the +2 oxidation state. Based on these results,
we propose the mechanism depicted in Fig. 2.
The proposed mechanism starts with the
formation of a precatalytic species, [Co2(triphos)2
(m-OH)2](BF4)2, which is known to form spontane-
ously when Co(BF4)2·6H2O and triphos are mixed
(30). This dimer is split by the substitution of the
hydroxyl ligand for an alkanoate, producing the
catalytically active species [Co(triphos)(alkanoate)]
BF4, which is observed in EPR and ESI-MS and is
known to be a stable species (30). By means of slow
vapor diffusion of diethyl ether into a CH2Cl2 so-
lution of the complex, we were also able to ob-
tain single crystals that enabled x-ray crystal
structure determination of this species as the
BPh4 salt. The complex was prepared by mixing
Co(BF4)2·6H2O, triphos, and benzoic acid in a
1:1:1 ratio in THF, followed by the addition of
NaBPh4 in a 10-fold excess amount. The x-ray crys-
tal structure (Fig. 2 and table S5) shows the expec-
ted bidentate (k2) binding mode of the benzoate
and a slightly distorted square pyramidal structure
around the cobalt center [t = 0.11 (31)], as expected
for a low-spin cobalt(II) species. This species pro-
duces the same EPR spectrum as the catalytic
mixture, confirming the Co(triphos)(k2-alkanoate)
species as the resting state, but it was found to be
inactive in catalysis because of the presence of the
BPh4 anion [which has a deactivating effect on
catalysis, as we also observed when NaBPh4 was
added to the catalytically active 1:1 mixture of tri-
phos and Co(BF4)2·6H2O].
From this Co(triphos)(k2-alkanoate) species, we
propose that initial heterolytic hydrogen splitting
occurs over the Co-O bond, followed by the mi-
gratory insertion of the hydride into the carbonyl
carbon and a second heterolytic hydrogen split-
ting step over the cobalt(II) hydroxyalkanolate
intermediate, thereby expelling one equivalent of
water and producing the [Co(triphos)(H)(aldehyde)]+
species (32). A final hydride migration step, followed
by proton transfer and ligand exchange with
another carboxylic acid substrate, gives the desired
alcohol and regenerates the active catalyst. This
mechanism fits the ESI-MS and EPR results that
show [Co(triphos)(OOCR)]+ as the resting state
under catalytic conditions, and it also correlates
well with the quantitative poisoning studies. The
proposed mechanism is also supported by density
functional theory (DFT) calculations (performed
on the complete system, using acetic acid as the
substrate; Fig. 2 and figs. S4 and S5), showing
that the (lowest-energy) resting state is the Co
(triphos)(k2-alkanoate) species. The overall calcu-
lated reaction barrier is 18.6 kcal mol−1, which is
a reasonable energy barrier for a reaction proceed-
ing smoothly at 100°C. The rate-determining tran-
sition state involves hydride migration to the
coordinated aldehyde, whereafter energetically
favorable steps occur involving proton transfer
and exchange of the product for a carboxylic acid
substrate, thus resulting in an overall mildly exo-
thermic reaction (–3.8 kcal mol–1).
Table 2. Hydrogenation of carboxylic acids using Co(BF4)2·6H2O and triphos. General conditions
used, unless otherwise noted, were 0.15 M substrate, 1:1 ratio of Co(BF4)2·6H2O and triphos (mol %
loading shown), distilled THF, 80 bar initial H2 pressure, 100°C, 22 hours. In the substrate columns,
catalyst amounts are given per acid group, and in the product columns, conversions are shown with
GC yield/isolated yield in parentheses.
The results presented here describe the first
catalytic system based on a cheap and abundant
first-row transition metal that is capable of hydro-
genating carboxylic acids to the corresponding
alcohols, using molecular hydrogen as the reduc-
tant. Insight into the mechanism of this system
was obtained by means of ESI-MS, in situ EPR,
x-ray crystallography, and DFT calculations, all of
which support a Co(triphos)(k2-alkanoate) resting
state. The DFT calculations suggest a mechanism
that involves heterolytic H2 splitting over cobalt
(II) alkanoate and cobalt(II) hydroxyalkanolate
intermediates as key steps in the catalytic cycle.
..
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*Neat conditions.
†36% GC yield and 24% isolated yield of butyl butyrate obtained. Isolated yields
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but does not have H2O/CO2-sharing sites. Therefore, it readily adsorbs both H2O
and CO2 from the humid flue gases and atmosphere, but the adsorbing H2O does
not interfere with the adsorption of CO2. It is also highly stable after adsorption
of H2O and CO2 because it was synthesized hydrothermally.
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fforts to curtail the increase in atmospheric
CO2 concentrations rely on the development
of economical methods of capturing CO2
from flue gas and the atmosphere (1–5). One
possible approach involves capture of CO2
Using a gel consisting of sodium silicate and
copper sulfate, we synthesized microporous cop-
persilicate crystals of uniform size and shape (see
supplementary materials). The crystals, which we
call SGU-29, have a square bipyramid crystal mor-
phology, which suggests that each crystal has
pseudo–four-fold symmetry along the axis (as-
signed later as the c axis) from the center of the
square to the top of the pyramid. Two typical
scanning electron microscopy (SEM) images with
different crystal sizes are shown in Fig. 1A and
the inset. The determined chemical formula was
Na2CuSi5O12. Almost all reflections observed in
the x-ray powder diffraction pattern matched well
with those of ETS-10 (22, 23) and AM-6 (fig. S1)
(24, 25). The crystals are stable in air up to 550°C
(fig. S2). The effective magnetic moment of Cu
(fig. S3A) confirmed that the oxidation state of Cu
is 2+. The electron spin resonance spectrum of
SGU-29 showed that the electron spins on Cu2+
ions are strongly coupled (fig. S3B). Character-
istic features observed by transmission electron
microscopy (TEM) can be summarized as follows:
(i) In the high-resolution TEM (HRTEM) image
taken along the 110 direction (parallel to the chan-
nel direction; Fig. 1B), large bright ellipses are ar-
ranged horizontally with a period of 14.7 Å with a
dark contrast observed between any two neigh-
boring ellipses. Simultaneously, a horizontal row
of small white dots arranged in a zigzag manner
can also be noticed between the successive arrays
of ellipses. The bright contrast of ellipses and small
dots corresponds to large and small pores, respec-
tively. The large pores resemble channels formed
by 12-membered rings judging from their sizes.
These pores belong to single layers that are
marked as A, B, C, and D. (ii) There is a horizontal
shift by one-quarter of the ellipses between suc-
cessive layers either to the right in the upper part
of the image forming ABCD stacking sequence or
E
by physical adsorption on microporous materials
that have high surface areas. To date, various ma-
terials that have high CO2 sorption capabilities at
298 K have been developed. They include zeolites
(6–10), metal-organic frameworks (MOFs) (11–16),
and zeolitic imidazolate frameworks (17, 18). How-
ever, these materials require the incoming gas
stream to be completely dehydrated, as water
causes a drastic reduction in the CO2 sorption
capabilities (19, 20) or may even promote their
decomposition (13, 14). Although such moisture-
sensitive CO2 sorbents can still be used to capture
CO2 directly from nonpretreated humid flue gases
by charging the column with a water-sorbing layer
before the CO2-sorbing layer, the use of a single
moisture-insensitive layer would be preferable
(4, 5, 21).
Only a limited number of materials meeting this
requirement have been discovered, and these sub-
stances can only adsorb small to moderate amounts
of CO2 from humid flue gas (13, 14, 18–20). Further-
more, most flue gases are hot, with temperatures
ranging between 373 and 403 K at the point they
are released into the atmosphere. Therefore,
thermal stability under humid conditions is
another key property.
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acetaldehyde, [Co(triphos)(H)(PEt3)]BPh4 is described in (33).
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ACKNOWLEDGMENTS
This research was performed within the framework of the CatchBio
program. The authors gratefully acknowledge the support of the
Smart Mix Program of the Netherlands Ministry of Economic Affairs
and the Netherlands Ministry of Education, Culture and Science. We
thank E. Zuidinga for ESI-MS analysis and L. Lefort and P. Alsters
for useful discussions. The supplementary crystallographic data
for this structure can be obtained free of charge from The Cambridge
Crystallographic Data Centre (CCDC) at www.ccdc.cam.ac.uk/
getstructures (CCDC number 1420869).
SUPPLEMENTARY MATERIALS
1Korea Center for Artificial Photosynthesis, Department of
Chemistry, Sogang University, Seoul 121-742, Korea. 2Pohang
Accelerator Laboratory, Pohang University of Science and
Technology, Pohang 790-784, Korea. 3Department of Materials
and Environmental Chemistry, Stockholm University, SE-106 91
Stockholm, Sweden. 4Graduate School of EEWS, KAIST,
Daejeon 305-701, Korea. 5School of Physical Science and
Technology, ShanghaiTech University, Shanghai 201210, China.
*Corresponding author. E-mail: yoonkb@sogang.ac.kr
www.sciencemag.org/content/350/6258/298/suppl/DC1
Materials and Methods
Figs. S1 to S39
Tables S1 to S5
References (34–46)
Cartesian Coordinates (XYZ Format) for All Calculated Structures
3 April 2015; accepted 8 September 2015
10.1126/science.aaa8938
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Hydrogenation of carboxylic acids with a homogeneous cobalt
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Ties J. Korstanje et al.
Science 350, 298 (2015);
DOI: 10.1126/science.aaa8938
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