Metal-Organic Framework-Confined Single-Site Base-Metal Catalyst for Chemoselective Hydrodeoxygenation of Carbonyls and Alcohols

Article abstract of DOI:10.1021/acs.inorgchem.1c01008

Chemoselective deoxygenation of carbonyls and alcohols using hydrogen by heterogeneous base-metal catalysts is crucial for the sustainable production of fine chemicals and biofuels. We report an aluminum metal-organic framework (DUT-5) node support cobalt(II) hydride, which is a highly chemoselective and recyclable heterogeneous catalyst for deoxygenation of a range of aromatic and aliphatic ketones, aldehydes, and primary and secondary alcohols, including biomass-derived substrates under 1 bar H2. The single-site cobalt catalyst (DUT-5-CoH) was easily prepared by postsynthetic metalation of the secondary building units (SBUs) of DUT-5 with CoCl2 followed by the reaction of NaEt3BH. X-ray photoelectron spectroscopy and X-ray absorption near-edge spectroscopy (XANES) indicated the presence of CoII and AlIII centers in DUT-5-CoH and DUT-5-Co after catalysis. The coordination environment of the cobalt center of DUT-5-Co before and after catalysis was established by extended X-ray fine structure spectroscopy (EXAFS) and density functional theory. The kinetic and computational data suggest reversible carbonyl coordination to cobalt preceding the turnover-limiting step, which involves 1,2-insertion of the coordinated carbonyl into the cobalt-hydride bond. The unique coordination environment of the cobalt ion ligated by oxo-nodes within the porous framework and the rate independency on the pressure of H2 allow the deoxygenation reactions chemoselectively under ambient hydrogen pressure.

Full text of DOI:10.1021/acs.inorgchem.1c01008

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Article
Metal−Organic Framework-Confined Single-Site Base-Metal Catalyst
for Chemoselective Hydrodeoxygenation of Carbonyls and Alcohols
Neha Antil,† Ajay Kumar,† Naved Akhtar, Rajashree Newar, Wahida Begum, and Kuntal Manna*
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ABSTRACT: Chemoselective deoxygenation of carbonyls and alcohols using
hydrogen by heterogeneous base-metal catalysts is crucial for the sustainable
production of fine chemicals and biofuels. We report an aluminum metal−organic
framework (DUT-5) node support cobalt(II) hydride, which is a highly
chemoselective and recyclable heterogeneous catalyst for deoxygenation of a
range of aromatic and aliphatic ketones, aldehydes, and primary and secondary
alcohols, including biomass-derived substrates under 1 bar H2. The single-site
cobalt catalyst (DUT-5-CoH) was easily prepared by postsynthetic metalation of
the secondary building units (SBUs) of DUT-5 with CoCl2 followed by the
reaction of NaEt3BH. X-ray photoelectron spectroscopy and X-ray absorption near-
edge spectroscopy (XANES) indicated the presence of CoII and AlIII centers in
DUT-5-CoH and DUT-5-Co after catalysis. The coordination environment of the
cobalt center of DUT-5-Co before and after catalysis was established by extended
X-ray fine structure spectroscopy (EXAFS) and density functional theory. The kinetic and computational data suggest reversible
carbonyl coordination to cobalt preceding the turnover-limiting step, which involves 1,2-insertion of the coordinated carbonyl into
the cobalt−hydride bond. The unique coordination environment of the cobalt ion ligated by oxo-nodes within the porous framework
and the rate independency on the pressure of H2 allow the deoxygenation reactions chemoselectively under ambient hydrogen
pressure.
reductants such as silane,21−26 alcohol,27 formic acid,28
INTRODUCTION
Chemoselective deoxygenation of carbonyls and alcohols to
the corresponding saturated compounds has drawn consid-
■
hydrazine,29 or hydrogen.30−37 Among all of the reductants,
H2 is the most atom-efficient and nontoxic and also produces
H2O as the only byproduct. However, the catalytic
deoxygenation reaction using H2 (hydrodeoxygenation) is
usually executed at high temperatures and pressures with the
explosion risk. Therefore, the development of heterogeneous
catalysts based on earth-abundant metals for hydrodeoxygena-
tion under mild condition is challenging but highly attractive
for economic reasons. Despite recent progress on supported
earth-abundant metal nanoparticles-based catalysts for hydro-
deoxygenation,38−42 the requirement of high catalyst loading,
high pressure of hydrogen, limited substrate scope, and ill-
defined active sites limits their practical applicability and also
complicates the investigation of the mechanism.
erable attention for its diverse applications in fine-chemical
synthesis and biofuel production. The conversion of abundant
and renewable biomass-derived carbohydrates and lignocellu-
lose to value-added chemicals and liquid fuels requires the
cleavage of strong C−O bonds and the removal of most, or all,
of the oxygen atoms in the reactants without cleaving C−C/
C−H bonds.1−8 Chemoselective deoxygenation methods also
allow late-stage modification of the highly functionalized
molecules and have been applied in the synthesis of many
pharmaceuticals such as oxandrolone9 and thromboxane
receptor antagonist.10 Classical methods for the deoxygenation
of carbonyls and alcohols to the corresponding alkanes such as
those based on the Barton−McCombie (R3SnH),11 borohy-
drides,12,13 boranes,14,15 Clemmensen (Zn/Hg, HCl),16 HI/
red-P,17 and Wolff−Kishner−Huang (N2H4, KOH) reduc-
tions18−20 are generally associated with rigorous reaction
conditions, poor functional group tolerance, the utilization of
stoichiometric amounts of reagents, and the generation of
undesirable byproduct. To improve the atom economy and
preclude hazardous reagents, transition-metal-catalyzed deox-
ygenation has been developed. Catalytic protocols for
deoxygenation typically rely on the toxic and precious late
transition metals and require a stoichiometric amount of
Metal−organic frameworks (MOFs) are an emerging class of
porous and tunable molecular material to develop heteroge-
neous and well-defined earth-abundant metal catalysts.43−49
Received: April 1, 2021
Published: June 4, 2021
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Figure 1. Hydrodeoxygenation of carbonyls and alcohols catalyzed by late transition metals21−27,29−31,33−36 and base metals.28,37−41
MOFs, built from metal-oxo cluster-based secondary building
units (SBUs) or nodes and organic bridging linkers, provide
thermally and chemically robust solid platforms to afford site-
isolated catalytically active species.50−54 Unlike the traditional
oxide supports such as silica, alumina, or other metal oxides,
MOFs offer a unique oxide support at SBUs to prepare single-
site base-metal catalysts, benefitting from their reticular
synthesis, tunable pores, and the highly disperse and uniform
hydroxyl groups of its SBUs.55−64 Moreover, MOF-supported
single-site catalysts provide both the leverages offered by
homogeneous catalysts such as homogeneity of the active sites,
reproducibility, and selectivity and those provided by
heterogeneous counterparts such as excellent robustness and
easy recycling of the catalysts.47,65−82 Herein, we report an
aluminum hydroxide SBUs in a robust MOF (DUT-5)-
supported single-site cobalt(II) hydride catalyst (DUT-5-
CoH) for chemoselective deoxygenation of a range of aromatic
and aliphatic ketones, aldehydes, and primary and secondary
alcohols, including biomass-derived substrates under 1 bar of
H2 (Figure 1).
aluminum hydroxide nodes of DUT-5. DUT-5 MOF, having
a formula of [Al(OH)(bpdc)], is constructed from Al3+ cations
that are octahedrally coordinated with μ2-hydroxide and 4,4′-
biphenyldicarboxylate (bpydc2−) bridging linkers to afford a
three-dimensional (3D) porous framework with rhombic
channels (Figure 2a).83,84 DUT-5 was easily prepared by the
solvothermal reaction of 4,4′-biphenyldicarboxylic acid
(H2bpdc) and aluminum trichloride hexahydrate in dimethyl-
formamide (DMF).83 The deprotonation of μ2-OH of SBUs by
n-BuLi followed by the salt metathesis reaction with CoCl2 in
tetrahydrofuran (THF) afforded cobalt-functionalized DUT-5
(DUT-5-CoCl) as a blue solid. Inductively coupled plasma
atomic emission spectroscopy (ICP-OES) analysis of the
digested DUT-5-CoCl showed an Al/Co ratio of 3.45,
corresponding to a formula of Al(O)(bpdc)(Li)0.71(CoCl)0.29
.
The crystallinity and structure of MOF remained intact after
metalation as evidenced by the similar patterns of powder X-
ray diffraction (PXRD) between pristine DUT-5 and DUT-5-
CoCl (Figure 2b). Thermogravimetric analysis revealed that
DUT-5-CoCl is thermally robust up to 460 °C (Figure S3,
Supporting information (SI)). DUT-5-CoCl has a BET surface
area and pore size of 730 m2/g and 1.1 nm, respectively
(Figure 2c). Spherical particles of DUT-5-CoCl with an
average diameter of 0.9 nm were observed by transmission
RESULTS AND DISCUSSION
■
Synthesis and Characterization of DUT-5-CoH. DUT-
5-CoH was prepared by postsynthetic modification of
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Figure 2. (a) Synthesis of DUT-5-CoH via postsynthetic metalation of DUT-5 MOF. (b) PXRD patterns of simulated DUT-5 (black),83 freshly
synthesized DUT-5 (blue), DUT-5-CoCl (red), DUT-5-CoH (green), and DUT-5-Co recovered after hydrodeoxygenation of 4-methoxy-
acetophenone (magenta). (c) Brunauer−Emmett−Teller (BET) nitrogen sorption isotherms of DUT-5 and DUT-5-CoCl measured at 77 K. (d)
EXAFS spectra and fits in R-space at the Co K-edge of DUT-5-CoCl(THF) with the magnitude and real component of the Fourier transformation
shown as hollow triangles and hollow squares in blue color, respectively. The fitting range in R-space is 1.1−3.5 Å (within the gray lines). (e) Co K-
edge XANES spectra of CoCl2 (black), DUT-5-CoCl (red), DUT-5-CoH (blue), and DUT-5-Co after hydrodeoxygenation of 4-methoxy-
acetophenone (green). (f) Co 2p X-ray photoelectron spectrum (XPS) of DUT-5-CoH. (g) EXAFS spectra and fits in R-space at the Co K-edge of
DUT-5-CoH(THF) with the magnitude and real component of the Fourier transformation shown as hollow triangles and hollow squares in blue
color, respectively. The fitting range in R-space is 1.1−3.5 Å (within the grey lines).
electron microscopy (Figure S2a, SI). The Co-coordination
environment in DUT-5-CoCl was investigated by extended X-
ray absorption fine structure (EXAFS) at the Co K-edge, which
was fitted with the density functional theory (DFT)-optimized
structure of SBU-supported Co moiety within DUT-5-CoCl
(Figure 2d). The DFT-optimized structure using the B3LYP
method and a basis set of 6-311G(d,p) on Gaussian 09
software suits showed a distorted square pyramidal cobalt ion,
which is coordinated with one μ2−O−, two neutral carboxylate
oxygens, one chloride molecule, and one THF molecule to give
the [(μ3-O)(carboxylate-O)2CoCl(THF)] species.63 The Co−
(μ3-O) distance is 1.90 Å, while the Co−O(carboxylate)
distances are 2.08 and 2.04 Å, respectively. Fitting the EXAFS
feature of the Co centers in DUT-5-CoCl with this DFT-
optimized structure revealed nearly identical local coordination
environment and bond lengths (Figure 2d). The similar
energies of the pre-edge and K-edge peaks of DUT-5-CoCl
and CoCl2 in X-ray absorption near-edge structure (XANES)
indicate that the Co center in DUT-5-CoCl adopts the +2
oxidation state (Figure 2e).
The reaction of DUT-5-CoCl with 1.3 equiv of NaEt3BH
with respect to cobalt in THF formed DUT-5-CoH via
halide−hydride exchange. Crystallinity and the structural
integrity of DUT-5 were maintained after the formation of
cobalt-hydride at SBUs, as indicated by the similar PXRD
patterns of DUT-5-CoH and DUT-5-CoCl (Figure 2b).
Heating a mixture of DUT-5-CoH with H2O produced nearly
1 equiv of H2 with respect to cobalt as analyzed by gas
chromatography, suggesting the formation of cobalt-hydride
after treating DUT-5-CoCl with NaEt3BH (Section 2.4, SI).
XANES analysis showed a CoII oxidation state in DUT-5-CoH
(Figure 2e). The assignment of the +2 oxidation state of cobalt
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was further established by X-ray photoelectron spectroscopy
(XPS), which displayed a 2p3/2 peak at 781.6 eV and a 2p1/2
peak at 797.8 eV (Figure 2f). Fitting of the EXAFS region of
DUT-5-CoH at the Co K-edge with DFT-calculated (μ3-
O)(O-carboxylate)2CoH(THF) structure model revealed the
coordination of each CoII center to one μ3-O−, two neutral
carboxylate oxygen atoms, one hydride, and one THF in a
distorted square pyramidal geometry, similar to DUT-5-
CoCl(THF) (Figure 2g). The calculated Co−(μ3-O−)
distance of 1.96 Å, Co−(carboxylate-O) distances of 2.01
and 1.99 Å, Co−(THF-O) distance of 2.17 Å, and Co−H
distance of 1.51 Å are similar to those measured by EXAFS
fitting (2.01, 2.04, 2.04, 2.21, and 1.45 Å, respectively).
Importantly, no EXAFS feature corresponding to metallic Co
nanoparticles was observed.
Scheme 1. Hydrodeoxygenation of Ketones Catalyzed by
DUT-5-CoH
a b c d
, , ,
DUT-5-CoH-Catalyzed Hydrodeoxygenation of Car-
bonyl Compounds. DUT-5-CoH is an excellent catalyst for
deoxygenation of ketones and related secondary alcohols using
H2 as the reducing agent. First, the reaction condition of
hydrodeoxygenation of benzophenone was optimized by
varying the temperature and pressure of H2 (Table S1, SI).
At a 0.4 mol % Co loading, DUT-5-Co-catalyzed deoxygena-
tion of benzophenone at 60 °C under 4 bar H2 in toluene for
22 h afforded an equal mixture of diphenylmethane and
diphenylmethanol in 25% conversion as detected by gas
chromatography−mass spectrometry (GC-MS). Upon increas-
ing the temperature to 80 °C, diphenylmethane was obtained
in quantitative yield. Interestingly, the conversion was found to
be independent of the pressure of H2. In general, the DUT-5-
Co-catalyzed deoxygenation of ketones proceeded well at 80
°C using toluene or p-xylene as a solvent under 1 bar of H2.
Under these optimized conditions, the deoxygenation of aryl
alkyl ketones such as acetophenone and 2-methylacetophe-
none with 0.4 mol % of cobalt loading gave 100% conversion
within 24 h to ethylbenzene (2a) and 2-ethyltoluene (2b),
respectively (Scheme 1). DUT-5-Co-catalyzed deoxygenation
of carbonyl-groups had a wide range of substrate scopes,
including aryl alkyl ketones, diaryl ketones, naphthone,
heterocyclic ketones, and aliphatic ketones with excellent
auxiliary functional group tolerance (Scheme 1, 2a−z2). The
deoxygenation of substituted acetophenones bearing both
electron-rich groups such as methyl (1b), methoxy (1c−e),
amino (1f−h), and hydroxo (1i-j) and electron-withdrawing
groups such as halo (1m-n), ester (1l), and amide (1k)
occurred selectively under 1 bar of H2. The reduction of
aromatic rings was not detected in all cases. The hydro-
genation of substituted benzophenones such as 2-methyl-
benzophenone (1q), 2-aminobenzophenone (1r), and 2,5-
diaminobenzophenone (1s) were deoxygenated to 1-benzyl-2-
methylbenzene (2q), 2-benzylaniline (2r), and 2-benzylben-
zene-1,4-diamine (2s), respectively, in quantitative GC-yields.
Pure methylene compounds (2q−s) were afforded by the
simple removal of solid MOF and volatiles from the crude
products.
a
Reaction conditions: DUT-5-CoH (0.4 mol % Co), 0.243 mmol of
b
ketone, 1 bar H2, 3.0 mL of p-xylene, 80 °C, 24 h. Yields were
determined by GC-MS using 4-tert-butyl-toluene as the internal
standard, isolated yields within parentheses. Reaction was carried out
at 110 °C. Substrate: 4-acetylbenzaldehyde.
c
d
However, the substrate bearing both keto and aldehyde groups
such as 4-acetylbenzalhehyde (1z2) furnished 4-ethylbenzyl
alcohol in 100% conversion at 80 °C, suggesting that the
aldehyde group is intolerable under the reaction conditions.
Several experiments were performed to examine the
heterogeneity of the MOF catalyst in the hydrodeoxygenation
of ketones. The rate of deoxygenation of 4-methoxyacetophe-
none was unchanged upon addition of the metallic mercury to
the reaction mixture, suggesting any leached metallic cobalt
was not responsible for the catalysis (Figure S7, SI). Co
nanoparticles, synthesized via reduction of CoCl2 by NaEt3BH,
were far less active and selective than DUT-5-Co, precluding
the involvement of any in situ generated Co particles as the
potential catalyst in the deoxygenation reactions (Figure S9,
The performance of DUT-5-Co catalyst was further
evaluated on other aryl alkyl phenyl ketones such as isopropyl
phenyl ketone (1t), cyclopropyl phenyl ketone (1u), benzyl
phenyl ketone (1v), and 2-acetylnaphthalene (1w). At a 0.4
mol % Co loading, the products 2t−w were afforded in
excellent yields using our standard protocol under 1 bar H2.
Heterocyclic ketones such as 4-acetylpyridine (1x), 2-
acetylpyridine (1y), and 2-acetylthiophene (1z) were also
deoxygenated without any reduction of the aromatic rings.
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SI). Furthermore, the progress of deoxygenation reaction
ceased immediately after removing the solid MOF catalyst
from the reaction mixture (Figure S6, SI). Importantly, no
apparent loss of catalytic activity of DUT-5-Co was observed
after its recycling and reuse for five consecutive runs in
deoxygenation of 4-methoxyacetophenone (Figure 3a). During
53(Al)-CoH in deoxygenation of relatively bulky and rigid
substrates such as benzophenone and 2-acetylnaphthalene. At a
0.4 mol % Co loading, DUT-5-CoH-catalyzed deoxygenation
of 2-acetylnaphathalene at 80 °C for 18 h afforded 2-
ethylnaphthalene in 97% yield. In contrast, under identical
reaction conditions, MIL-53(Al)-CoH gave only 1-(2-
naphthyl)ethanol in 36% conversion (Figure 3b). The superior
activity of DUT-5-CoH over MIL-53(Al)-CoH is likely due to
the facile diffusion of the reactant and product molecules
within its larger pores.
DUT-5-CoH-Catalyzed Deoxygenation of Aldehydes
and Primary Alcohols by Hydrogen. DUT-5-CoH was also
an active catalyst for the hydrodeoxygenation of aldehydes and
primary alcohols under 1 bar H2 (Scheme 2). However, the
deoxygenation of aldehydes requires a higher temperature at
Scheme 2. DUT-5-CoH-Catalyzed Deoxygenation of
a b c d
, , ,
Aldehydes and Primary Alcohols by Hydrogen
Figure 3. (a) % GC yield of the products at several runs in the recycle
and reuse experiment of DUT-5-Co (0.4 mol % Co) for hydro-
deoxygenation of 4-methoxyacetophenone. (b) Plots of % GC yield of
alcohol (red) and alkane (gray) after hydrodeoxygenation of
benzophenone (A, B) and 2-acetylnapthalene (C, D), using DUT-
5-Co (0.4 mol % Co, A and C) and MIL-53(Al)-Co (0.4 mol % Co, B
and D) under similar reaction conditions.
the recycling experiment, the leaching of cobalt and aluminum
from the MOF into the solution after the first run was 0.05 and
0.03%, respectively, and that was 0.26 and 0.08%, respectively,
after the fourth run. The PXRD pattern of the recovered DUT-
5-Co after run 6 was almost unchanged (Figure 2b), signifying
the retention of the crystallinity of DUT-5-Co during the
catalysis. We also compared the catalytic activity and selectivity
of DUT-5-CoH with MIL-53(Al)-supported cobalt(II)
hydride to investigate the effect of pore size on the
deoxygenation reactions (Figure 3b).63 Both DUT-5-CoH
and MIL-53(Al)-CoH have similar framework topology and
identical coordination environment of cobalt;63 however, pore
sizes of DUT-5-CoH are larger than those of MIL-53(Al)-
CoH. DUT-5-CoH was more active and selective than MIL-
a
Reaction conditions: DUT-5-CoH (0.4 mol % Co), 0.243 mmol of
b
ketone, 1 bar H2, 3.0 mL of p-xylene, 120 °C, 24 h. Yields were
determined by GC-MS using 4-tert-butyl-toluene as the internal
standard, isolated yields within parentheses. Reaction was carried out
at 140 °C. Substrate: furfural, reaction was carried out in methyl
cyclohexane at 180 °C under 40 bar H2 for 48 h.
c
d
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Figure 4. (a) Time evaluation plot of DUT-5-Co-catalyzed deoxygenation of 4-methoxyacetophenone. (b) EXAFS spectra and fits in R-space at the
Co K-edge of DUT-5-Co after deoxygenation of 4-methoxyacetophenone with the magnitude and real component of the Fourier transformation
shown as hollow triangles and hollow squares in blue color, respectively. The fitting range in R-space is 1.1−3.5 Å (within the gray lines). (c) SEM
image and SEM-EDX mapping of Al and Co in DUT-5-Co particles recovered after catalysis. (d) Plots of initial rates for hydrodeoxygenation of 4-
methoxyacetophenone versus initial average concentrations of cobalt and 4-methoxyacetophenone. (e) Proposed catalytic cycle of
hydrodeoxygenation of carbonyl compounds. (f) DFT-calculated energy profile diagram of DUT-5-Co-catalyzed deoxygenation of carbonyls
using hydrogen. See the SI for computational details.
120 °C compared to that of ketones. At a 0.4 mol % of cobalt
loading, DUT-5-CoH-catalyzed hydrodeoxygenation reaction
of 4-methoxybenzaldehyde at 120 °C under 1 bar H2 for 24 h
produced 4-methylanisole (4a) in quantitative yield.
Similarly, under the identical reaction conditions, substituted
benzaldehydes bearing functional groups such as methoxy
(3a−d), hydroxy (3e−g), amino (3i-j), amide (3k), and halo
(3m-n) were deoxygenated chemoselectively to afford the
corresponding methylarenes in excellent yields. Benzo[d][1,3]-
dioxole-5-carbaldehyde (3h) and 4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)benzaldehyde (3o) were transformed to 5-
methylbenzo[d][1,3]dioxole (4h) and 4,4,5,5-tetramethyl-2-
(p-tolyl)-1,3,2-dioxaborolane (4o) with 99 and 98% yields,
respectively. The deoxygenation of heterocyclic aldehydes such
as pyrrole-2-carboxaldehyde (3p), 2-pyridinecarboxaldehyde
(3q), and indole-3-carbaldehyde (3r) also proceeded well at
120 °C under 1 bar H2. Ketoprofen was deoxygenated to the
corresponding products (4s) in 67% yield keeping the
carboxylic acid group intact. The hydrodeoxygenation of
aliphatic substrates requires harsher reaction conditions. 1-
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Octanol (3t), 1-undecanol (3u), and 3-phenylpropanal (3v)
were converted to octane (4t), undecane (4u), and
propylbenzene (4v), respectively, at 140 °C. Interestingly, at
a 0.4 mol % Co loading, DUT-5-CoH was efficient to
deoxygenate biomass-derived 5-(hydroxymethyl)furfural (3w),
furfuryl alcohol (3x), and furfural (3y) to furnish the
corresponding methylfurans in excellent yields. When the
reaction with furfural was performed at 180 °C under 40 bar
H2, hydrogenation of arene also occurred simultaneously, and
2-methyltetrahydrofuran (4z) was obtained as the sole
product, which is an important organic solvent and has
potential application as a biofuel (Scheme 2).85,86
lives (Figure S11b, SI), and this is consistent with the rate to
be the first-order-dependent on substrate concentration.
However, after two half-lives, the reaction rate decreased and
deviated from the linearity, presumably due to the inhibition
by the water and alcohols. To minimize the inhibition effect,
we determined the empirical rate law by the methods of initial
rates (Section 4.2, SI). The concentration of 4-methoxyaceto-
phenone was calculated by GC-FID using 4-tert-butyl-toluene
as the internal standard. As shown in Figure 4d, the initial rate
increases linearly on increasing the substrate concentration at a
lower range of substrate concentrations (0.03−0.13 M);
however, it reaches maxima at higher substrate concentrations
(>0.13 M). The saturation kinetics thus indicate reversible
carbonyl coordination to cobalt preceding the rate-determining
step. Besides, the initial rates have a first-order rate dependence
on the concentration of cobalt (Figure 4d) but are
independent on H2 pressure ranging from 2 to 20 bar (Figure
S12, SI). The kinetic data of deoxygenation of 4-
methoxyacetophenone indicate a cobalt ion and a carbonyl
molecule participating in the rate-determining step.
We performed DFT calculations to further understand the
mechanism of the catalytic hydrogenolysis of carbonyls (Figure
4f). The coordination of 4-methoxyacetophenone to cobalt
forms a distorted square pyramidal cobalt-intermediate (INT-
1), which has a 6.8 kcal/mol higher energy than DUT-5-CoH.
Then, 1,2-insertion of the coordinated carbonyl into Co−H
bond in the transition state-1 (TS-1), which requires an
activation energy of 3.5 kcal/mol, gives cobalt-alkoxide
intermediate (INT-2). In the next step, INT-2 is transformed
to 1-(4-methoxyphenyl)ethanol and DUT-5-CoH via σ-bond
metathesis of Co−O bond with H2 in TS-2 requiring an
activation free energy of 10.9 kcal (Figure 4f). The regenerated
DUT-5-CoH further catalyzes the deoxygenation of 1-(4-
methoxyphenyl)ethanol to produce 4-ethylanisole and water as
shown in Cycle-2 in Figure 4e. The coordination of 1-(4-
methoxyphenyl)ethanol to cobalt of DUT-5-CoH gives INT-3,
which undergoes σ-bond metathesis of Co−H bond and C−O
bond of the coordinated alcohol in TS-3 to afford cobalt-
hydroxide intermediate (INT-4). The transformation of INT-3
to INT-4 has an energy barrier of 2.8 kcal/mol. In addition, the
formation of INT-4 and 4-ethylanisole from INT-3 is
exergonic by 29.9 kcal/mol. Finally, σ-bond metathesis
between the Co−O bond of INT-4 and dihydrogen releases
water and regenerates DUT-5-CoH. The DFT-calculated
energy profile diagram identifies the conversion of cobalt-
hydride of INT-1 to the corresponding cobalt-alkoxide
intermediate (INT-2) via TS-1 as the rate-determining step
of the catalytic cycle. The calculated structure of TS-1 reveals a
four-member cyclic transition state involving the insertion of
the carbonyl group into the Co−H. The nature of the plot of
initial rates versus substrate concentrations is also consistent
with our theoretical studies, which displayed a first-order rate
dependency at lower substrate concentrations followed by rate
independency at a higher substrate concentration suggesting
the reversible carbonyl coordination to cobalt to form INT-1
preceding the rate-determining step (Figure 4e,f).
Mechanistic Investigation of DUT-5-CoH-Catalyzed
Hydrodeoxygenation of Carbonyl Compounds. The
reaction of DUT-5-CoH (0.4 mol % Co) and 4-methox-
yacetophenone under 1 bar of H2 at 80 °C first produced 1-(4-
methoxyphenyl)ethanol without a detectable induction period.
The time evaluation studies of the same reaction showed the
formation of 1-(4-methoxyphenyl)ethanol as the major
product in the first half-life, and no trace of 4-vinylanisole or
any other side product was observed during the course of the
reaction (Figure 4a). We thus infer that the deoxygenation
reactions occur in two sequential pathways, where the
carbonyls are first reduced to alcohols, and the subsequent
deoxygenation of alcohols produces the corresponding
methylene compounds. The XANES analysis of DUT-5-Co
recovered after deoxygenation of 4-methoxyacetophenone
suggests the existence of CoII species after catalysis (Figure
2e). EXAFS studies of the recovered DUT-5-Co were also
conducted to identify the cobalt species as the catalyst resting
state. The EXAFS at the Co K-edge was well fitted with the
DFT-calculated coordination environment of a cobalt−
hydroxide complex, indicating the formation of (μ3-O−)-
(Ocarboxylate)2Co(OH) species after catalysis (Figure 4b). We
observed a significant misfit of EXAFS data using the model of
DUT-5-Co(OH) including 5% of cobalt nanoparticles, which
suggests that cobalt nanoparticles were not formed during the
catalysis. In addition, the PXRD of DUT-5-CoH and DUT-5-
Co after catalysis displayed no characteristic reflection peaks at
higher 2θ angles, ruling out the formation of Co nanoparticles
upon reaction with NaEt3BH and during the catalysis (Figure
2b). Furthermore, scanning electron microscopy (SEM)-
energy-dispersive X-ray (EDX) mapping of DUT-5-Co
recovered after catalysis indicated that Co and Al are uniformly
dispersed throughout the MOF particle (Figure 4c). Therefore,
we surmise that the node-supported CoII hydride in DUT-5-
CoH was the active catalytic site for the hydrodeoxygenation
of carbonyl compounds, and the proposed catalytic cycle is
displayed in Figure 4e. We propose that the coordination of
carbonyl to the cobalt center followed by 1,2-insertion of the
carbonyl into cobalt−hydride bond generates cobalt-alkoxide
intermediate, which then undergoes σ-bond metathesis with
H2 to give the alcohol (Cycle-1, Figure 4e). Subsequently, the
coordination of this alcohol to the cobalt center of DUT-5-
CoH followed by σ-bond metathesis between Co−H and C−
O bond generates the deoxygenated product and the Co-
hydroxide intermediate (Cycle-2, Figure 4e). The subsequent
σ-bond metathesis of Co-hydroxide and H2 regenerates the
cobalt-hydride catalyst with the production of water.
The DFT studies indicate that the coordination environ-
ment of cobalt at the SBUs within the pores of MOF lowers
the activation energy of the binding and the interaction with
small H2 molecule (TS-2 and TS-4, Figure 4) compared to
those with relatively much bulkier carbonyl substrates resulting
in a zeroth-order reaction with respect to the pressure of H2.
The independency of initial rate on H2 pressure allows all of
The reaction pathway of DUT-5-CoH-catalyzed deoxygena-
tion of 4-methoxyacetophenone to 4-ethylanisole was further
characterized by kinetic and DFT studies. Plots of ln[4-
methoxyacetophenone] versus time were linear up to two half-
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Inorganic Chemistry
pubs.acs.org/IC
Article
Authors
the deoxygenation reactions to occur under 1 bar of H2.
Furthermore, NBO population analysis identifies electron-rich
hydride and μ3-oxo sites with charges of −0.535 and −1.40,
respectively, in DUT-5-CoH (Table S5, SI). We thus assume
that the reactive and highly polar Co hydride facilitates the
insertion of the unsaturated carbonyl bonds, leading to
excellent catalytic performance. Interestingly, our DFT
calculation suggests that the neutral O(carboxylate) ligands
at SBUs weakly coordinate to the cobalt ion to stabilize the
active cobalt-hydride species within the MOF. Furthermore,
the labile Co−O(carboxylate) bonds also dissociate in the
sterically congested turnover-limiting step (TS-1) to create
vacant coordination sites at the cobalt center for substrate
binding and activation. We surmise that the high catalytic
efficiency of DUT-5-Co under mild reaction conditions is
originated due to the presence of single-site and reactive Co−
H bonds, the unique coordination environment of cobalt(II)
ion at SBUs ligated by both anion and neutral oxo ligands, and
the uniform distribution of cobalt-hydride species through the
large MOF channels.
Neha Antil − Department of Chemistry, Indian Institute of
Technology Delhi, New Delhi 110016, India
Ajay Kumar − Department of Chemistry, Indian Institute of
Technology Delhi, New Delhi 110016, India
Naved Akhtar − Department of Chemistry, Indian Institute of
Technology Delhi, New Delhi 110016, India
Rajashree Newar − Department of Chemistry, Indian Institute
of Technology Delhi, New Delhi 110016, India
Wahida Begum − Department of Chemistry, Indian Institute
of Technology Delhi, New Delhi 110016, India
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c01008
Author Contributions
†N.A. and A.K. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The Science and Engineering Research Board (SERB), India
(project ECR/2017/001931), has funded this research. N.A.
and R.N. acknowledge CSIR, and W.B. acknowledges UGC for
financial support. The authors thank Prof. B. Jayaram for
helping computational studies. They acknowledge Raja
Ramanna Centre for Advanced Technology-Indore
(RRCAT) for providing Indus-2 SRS facility for XAFS
measurement. Dr. S. N. Jha and Babita are gratefully thanked
for their support during XAFS measurement. The authors
acknowledge the Central Research Facility, IIT Delhi, for
instrument facilities.
CONCLUSIONS
■
In conclusion, a mild and cost-effective method for the
deoxygenation of a wide range of aromatic and aliphatic
aldehydes, ketones, and alcohols using an easily affordable
MOF-supported cobalt catalyst is described. This cobalt
catalyst was synthesized by metalation of aluminum hydroxide
nodes of the DUT-5 with CoCl2 followed by the reaction with
NaEt3BH. Our synthetic protocol displayed excellent tolerance
for a wide range of functional groups such as aryl, amines,
amides, esters, and hydroxyl, and also utilized cheap and atom-
economical reductant hydrogen. The mechanism of the
catalytic hydrodeoxygenation was investigated in detail by
spectroscopic, kinetic, and computational studies. The unique
coordination environment of cobalt ion ligated by both neutral
and monoanionic oxo ligands of SBUs within the porous
framework and the rate independency on the pressure of H2
bring the chemoselectively of deoxygenation reactions under 1
bar of H2. This work highlights the development of novel
catalytic technologies based on earth-abundant metal-function-
alized MOFs for efficient hydrodeoxygenation of C−O bonds
in biomass and organic molecules for sustainable synthesis of
chemical feedstocks and biofuels.
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01008.
Synthesis and characterization of DUT-5-MOF and
DUT-5-Co; experimental details for catalytic reactions;
details of kinetics, EXAFS, and GC analysis; and DFT
calculations (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Kuntal Manna − Department of Chemistry, Indian Institute of
Technology Delhi, New Delhi 110016, India; orcid.org/
0000-0002-2924-0353; Email: kmanna@
chemistry.iitd.ac.in
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