Photocatalytic CO2 reduction using visible light by metal-monocatecholato species in a metal-organic framework

Article abstract of DOI:10.1039/c5cc04506a

Metal-organic frameworks (MOFs) with isolated metal-monocatecholato groups have been synthesized via postsynthetic exchange (PSE) for CO₂ reduction photocatalyst under visible light irradiation in the presence of 1-benzyl-1,4-dihydronicotinamide and triethanolamine. The Cr-monocatecholato species are more efficient than the Ga-monocatecholato species.

Full text of DOI:10.1039/c5cc04506a

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Photocatalytic CO2 reduction using visible light
by metal-monocatecholato species in a metal-
organic framework
`Cite this: DOI: 10.1039/x0xx00000x
Yeob Lee,a Sangjun Kim,b Honghan Fei,a Jeung Ku Kang,*b and Seth M. Cohen*a
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
Metal-organic frameworks (MOFs) with isolated metal- CO2 to CO under UV light irradiation.11 This pioneering work for MOF
monocatecholato groups have been synthesized via photocatalysts showed poor efficiency due, in part, to a low doping of
postsynthetic exchange (PSE) for CO2 reduction the Re catalyst into the MOF. Fu et al. synthesized visible-light
photocatalyst under visible light irradiation in the presence of sensitive NH2-MIL-125(Ti) with an amine-functionalized organic linker.
1-benzyl-1,4-dihydronicotinamide and triethanolamine
Cr-monocatecholato species are more efficient than the Ga- triethanolamine (TEOA) under visible-light irradiation.12 In addition,
.
The This material reduced CO2 to HCOO- in the presence of
monocatecholato species.
Li et al. developed a non-porous coordination polymer consisting of Y
metal ions and Ir(ppy)2(dcbpy) metalloligands; this material reduced
CO2 to HCOO- under visible light irradiation.16 Despite these
advances, the development of MOF photocatalysts for CO2 reduction
is still in its infancy.
The conversion of CO2 into hydrocarbons has attracted great
attention owing to global warming caused, in part, by CO2 from fossil
fuel combustion.1,2 Inspired by photosynthesis, development of an
artificial system that catalytically regenerates hydrocarbon fuels from
CO2, H2O, and sunlight is one very intriguing approach.3-5 Artificial
photosynthesis would consist of two reactions: water oxidation to
extract electrons from water and CO2 reduction to generate
carbonaceous radicals using electrons generated from water
oxidation. CO2 requires a large driving force to be transformed to
other compounds due to the high kinetic and thermodynamic stability
of CO2.6-8 Several photocatalytic systems for CO2 reduction, including
heterogeneous semiconductor systems and homogeneous transition
metal-based complexes have been investigated but some challenges
still remains. For example, many metal oxides are active under only
UV light, which represents only ~4% of the solar energy spectrum.
This has encouraged research on extending the light absorption edge
of metal oxides.9 Homogeneous metal complexes based on Ru, Re,
and Ir have been investigated; however, an inability to recycle and
reuse these precious metal compounds remains a limitation of these
systems.10
Herein, we report a new MOF photocatalysts that incorporate
catalytic metal sites, using postsynthetic modification methods, for
CO2 reduction to formic acid in the presence of 1-benzyl-1,4-
dihydronicotinamide (BNAH) and triethanolamine (TEOA). The
Zr(IV)-based MOF (UiO-66, UiO = University of Oslo) was subjected to
postsynthetic exchange (PSE)17-20 with
a catechol-functionalized
organic linker (catbdc, 2,3-dihydroxyterephthalic acid, to produce
UiO-66-CAT).19 Two different trivalent metal ions, Cr(III) and Ga(III),
were then incorporated into the catbdc sites to afford unprecedented
Cr- and Ga-monocatecholato species in a robust UiO-66. The catbdc
organic linkers are responsible for visible light absorption and
metalation by Cr(III) and Ga(III) facilitates electron transfer within the
MOFs.
PSE has become a facile and efficient strategy to functionalize
MOFs under mild conditions (Fig. 1).
UiO-66 was prepared
solvothermally in DMF containing 1:1 molar ratio mixture of ZrCl4 and
H2bdc with acetic acid as a modulator at 120 °C. UiO-66 was then
exposed to DMF/H2O solution containing 2 equiv catbdc2- at 85 °C to
achieve PSE into UiO-66.19 This gave a UiO-66 derivative that
contained ~34% catbdc and ~66% bdc ligand. The metalation of the
catechol functionality in UiO-66-CAT was conducted using aqueous
K2CrO4 under weakly acidic conditions (pH = 3). After incubation at
room temperature (1 h) the pale yellow UiO-66-CAT changed to a
dark brown color. Similarly, an aqueous solution of Ga(NO3)3(H2O)x
Metal-organic frameworks (MOFs) are hybrid materials that
consist of secondary building units (SBUs) and organic linkers. The
rational design of MOFs with tunable properties through a selective
combination of metal ions and organic ligands has produced materials
useful for various applications. Photocatalytic applications of MOFs
also have been studied.11-15 Lin and co-workers reported a MOF
photocatalyst doped with Re(bpy)(CO)3Cl complexes that reduced
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was used to achieve metalation with Ga(III). After metalation, the GaCAT also consists of two peaks at 1144.79 eV and 1117.89 eV
MOFs were isolated by centrifugation, washed extensively with corresponding to 2p1/2 and 2p3/2 energy levels, respectively. These
deionized water and MeOH, and dried under vacuum. Inductively values match well to binding energies in Ga2O3 (1144.67 eV and
coupled plasma mass spectrometry (ICP-MS, Table S1) confirmed the 1117.79 eV). Therefore, XPS of UiO-66-CrCAT and UiO-66-GaCAT
atomic ratio of 0.25 (Cr/Zr) and 0.26 (Ga/Zr), indicating that ~80% of confirm the trivalent oxidation state of Cr and Ga in these MOFs.26,27
the available catbdc ligands were metalated by both the Cr and Ga
procedures based on three independent samples.
Fig. 2. (a) PXRD patterns of UiO-66 and functionalized UiO-66 derivatives; (b)
UV-Vis spectroscopy for UiO-66, UiO-66CAT, UiO-66-CrCAT, and UiO-66-GaCAT.
Fig. 1. Preparation of MOF photocatalysts through postsynthetic exchange (PSE)
F(R) were calculated from diffuse reflectance measurement; X-ray photoelectron
and metalation.
spectroscopy analysis of (c) UiO-66-CrCAT and (d) UiO-66-GaCAT.
The MOFs formed as nanocrystallites ~150 nm on an edge, with
These M(III)-monocatecholato functionalized MOFs were
an octahedral morphology. The crystallinity of the MOFs did not
investigated for their photocatalytic CO2 reduction activity. The
change upon PSE or metalation as evidenced by the PXRD patterns as
MOFs were introduced into a mixed solution of 4:1 (v/v) MeCN and
shown in Fig. 2a. Scanning electron microscopy (SEM) images
TEOA, which contained BNAH (0.1 M). In this photocatalytic reaction,
showed that the morphology and crystal size of UiO-66-CrCAT and
BNAH serves as
a reductant for CO2 to produce carbonaceous
UiO-66-GaCAT were also unchanged from the parent UiO-66 material
(Fig. S1, S2). The characteristic M(III) signals was detected using
energy dispersive X-ray spectroscopy (Fig. S1).
radicals28,29 and TEOA acts as a sacrificial base to capture protons
from BNAH.30 The product solutions were found to consist of water,
ethyl acetate, MeCN, and HCOOH. The photocatalytic activity of
each UiO-66-M(III)CAT are shown in Fig. 3a. Turnover numbers were
calculated from the amount of HCOOH produced versus the number
of M(III)-catecholato sites in each MOF. Turnover numbers were
calculated as 11.22±0.37 for UiO-66-CrCAT and 6.14±0.22 for UiO-66-
GaCAT, respectively. UiO-66-CrCAT and UiO-66-GaCAT produced
51.73±2.64 μmoles and 28.78±2.52 μmoles of HCOOH from CO2
photocatalysis, respectively (6 h of visible light irradiation). These
numbers indicate that each Cr-catecholato species catalyzed the
conversion of ~11 CO2 molecules, while each Ga-catecholato species
catalyzed the conversion of ~6 CO2 molecules over the 6 h reaction
time. UiO-66-CrCAT proved to be a more efficient catalyst than UiO-
66-GaCAT under these reaction conditions. Both UiO-66-M(III)CATs
produced negligible amount of H2 and CO that can be generated from
photocatalysis of CO2 in the presence of TEOA and BNAH. This
indicates that metalated catecholato species are suitably selective for
CO2 reduction to formate.
The UV-visible spectroscopy of the MOFs was altered upon PSE
and metalation as illustrated in Fig. 2b. The diffuse reflectance of
samples was measured and the reflectance values were subjected to
the Kubelka-Munk function ((1-R)2/2R) to quantify the light absorption
ability of samples.
H2bdc derivatives with electron donating
functionality such as NH2, OH, and SH are known to increase the
HOMO level of H2bdc.21-23 Thus, UiO-66-CAT is expected to absorb
some visible light as a result of the catechol groups. A color change to
dark brown was observed upon metalation with Cr(III). As expected,
Cr(III) binding to catbdc results in the generation of ligand-to-metal
charge transfer (LMCT).24,25 A similar color change was not observed
upon metalation with Ga(III), as expected for this closed-shell ion.
X-ray photoelectron spectroscopy (XPS) of UiO-66-CrCAT and
UiO-66-GaCAT was carried out to determine the oxidation states of Cr
and Ga. The 2p orbital information was obtained and each spectrum
exhibits two peak contributions. Chromium oxide (Cr2O3) and gallium
oxide (Ga2O3) were selected as references for both UiO-66-M(III)CATs.
UiO-66-CrCAT shows two peaks at 586.10 eV and 576.58 eV
corresponding to 2p1/2 and 2p3/2 binding energies, respectively. These
values compare well with energy levels in Cr2O3 (586.13 eV and 576.33
eV). This indicates that the Cr(VI) in K2CrO4 was reduced to Cr(III)
upon metalation, as previously observed.19 The XPS for UiO-66-
Both MOFs were tested over three catalytic cycles to investigate
the stability and reusability of the MOF photocatalysts. Samples were
recovered through isolation by centrifugation, washed with copious
amounts of MeOH, and activated under vacuum after each cycle. The
numbers of M(III)-catecholato sites were also redetermined for each
cycle to obtain accurate turnover numbers. The catalytic activities of
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both MOFs were relatively unchanged over the three cycles (Fig. 3a, instead of 13CO2, was also carried out but no H13COO- peak was
Fig. S4). However, a small amount of M(III) ions leached from the detected (Fig. S6). The strong resonance peaks observed for H13COO-
MOFs based on an decreasing M(III)/Zr(IV) ratio as determined by ICP- indicate that the carbon source is primarily from CO2 gas, not from
MS (Table S1) after each reaction. The photocatalysis results in Fig. 3a decomposition of the MOFs.
were reproducible based on findings from three independent samples
MOFs containing M(III)-monocatecholato species after three
(Fig. S5). Quantum yields for both UiO-66-M(III)CAT MOFs were cycles of photocatalysis were characterized using PXRD and SEM
obtained under monochromatic light irradiation using a band-pass measurements to evaluate their structural and chemical integrity,
optical filter (450 nm). UiO-66-CrCAT showed a higher quantum yield which is another essential characteristic feature for an optimal
value (1.83±0.16%) when compared to UiO-66-GaCAT (1.17±0.11%). photocatalyst. Both UiO-66-CrCAT and UiO-66-GaCAT maintained
The Fe(III) metalated UiO-66-CAT was also prepared following a their crystalline structure over 18 h of exposure to the photocatalysis
previous report.19 Under identical photocatalytic conditions, UiO-66- conditions (although some Cr and Ga leaching was observed, Table
FeCAT (Zr:Fe = 1:0.27, ~80% of catbdc was metalated) produced little S1). Crystal size, morphology, and surface (Fig. S7) remained intact
HCOOH (1.47 μmoles, Table S2). The redox potential for Fe(III) is not throughout photocatalysis, indicating the SBUs and organic linkers
suitable (0.77 V vs. SHE) for CO2 reduction (unlike Cr(III) and Ga(III)), were not significantly degraded. Furthermore, the crystallinity was
and hence this MOF derivative is not a suitable photocatalyst.
maintained in both UiO-66 derivatives during 1 week of light
irradiation (Fig. S8). Lastly, XPS results show that both Cr and Ga
remained in their trivalent oxidation states after photocatalysis (Fig.
S9). However, the BET specific surface area of both UiO-66-CrCAT
and UiO-66-GaCAT was slightly decreased after three cycles of
photocatalysis, which indicates that some pores were blocked or
collapse occured during intense photoirradition; however, the surface
areas indicated that the MOFs were still highly porous (Fig. S10). The
data suggest that isolated M(III)-monocatecholato functionalities
residing in MOFs acted as independent photocatalytic sites.
Fig. 3. (a) Photocatalytic ability of UiO-66-CrCAT (red) and UiO-66-GaCAT (green)
over three cycles (6 h/cycle); (b) 13C liquid NMR spectra of photocatalytic CO2
reduction by UiO-66-CrCAT (red) and UiO-66-GaCAT (green) after 13 h reaction.
The use of UiO-66-CAT prior to metalation as a photocatalyst did
not produce HCOOH as measured by GC-MS (and 13C NMR, see
below). This rules out the catbdc ligand alone or the Zr6 SBU clusters
as the catalytic sites for reduction. UiO-66-CAT is not suited to accept
photo-generated electrons from the catbdc ligand because the redox
potential of Zr6 SBU is higher than the LUMO of bdc linkers.15,31 The
control reactions with UiO-66-CAT support that photocatalytic
reduction of CO2 is dependent on the M(III)-catecholato species in the
MOF.
Fig. 4. (a) Photoluminescence (PL) spectra of UiO-66-CAT series; (b) Solid-state
fluorescent lifetime of excited states by time-correlated single photon counting
(TCSPC) method.
13C liquid NMR spectrum was acquired using a 13C isotope of CO2
Photoluminescence (PL) spectroscopy was carried out to
characterize the charge transfer between catbdc and M(III) in UiO-66-
M(III)CAT. As shown in Fig. 4a, the emission intensity of UiO-66-CAT
is significantly reduced after metalation. This indicates that the
recombination rate of photogenerated electron-hole pairs in the
organic linkers was markedly decreased upon metalation, suggesting
that charges were transferred to other sites in the MOFs. The
decreases in emission intensity were different for UiO-66-CrCAT and
UiO-66-GaCAT, suggesting that the metalated catbdc centers are the
acceptors. This indicates that LMCT occurred between the catechol
units and metal elements, as expected.34 In particular, UiO-66-CrCAT
quenched ~80% of the photo-generated charges in catbdc and this
value is close to the ratio of Cr-bound catechol ligands in UiO-66-
CrCAT. This increased electron transfer ability may explain the
greater photocatalytic efficiency of UiO-66-CrCAT over UiO-66-
GaCAT. The charge accepting ability of these trivalent ions are
expected to be quite different due to differences in their outer shell
electronic configurations (Cr(III) [Ar]3d3 versus Ga(III) [Ar]3d10).35,36
More energy is needed to accept electrons for Ga-species because the
as a substrate, in order to confirm the origin of the carbon source for
HCOOH upon photocatalysis with MOFs.
The photocatalytic
conditions used were identical to those described above except for
the use of CD3CN and with the reactor purged by 13CO2 (~99% of 13C)
before light irradiation. After 13 h, the reaction mixture was
transferred to an NMR tube without further treatment (i.e. no H2SO4).
Reference solutions were prepared for H13COOH and the results are
shown in Fig. S5. The chemical shift for H13COOH in CD3CN was found
at 162.73 ppm; however, this value was increased to 169.86 ppm in 4:1
CD3CN:TEOA solution due to deprotonation to H13COO-.30,32 In
2-
-
addition, it was expected that 13CO2 could form 13CO3 and H13CO3
under the alkaline reaction conditions.33
-
Several signals corresponding to H13COO-, 13CO2, 13CO32-, H13CO3 ,
MeCN, and other small signals were found in 13C NMR spectrum of
product solution from both UiO-66-M(III)CAT reaction mixtures (Fig.
3b). Small signals were also observed from BNAH isotopes and these
signals were also found in reference solution of CD3CN:TEOA with
BNAH. Moreover, 13C NMR for experiment using unlabeled 12CO2,
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redox potential of Ga(III)/Ga(II) is higher than Cr(III)/Cr(II). This is
consistent with the ~2× higher turnover number demonstrated by
3
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5
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a LMCT
mechanism (Fig. 4b, see details in ESI). This result also suggests that
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electron transfer to CO2.
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