Controlling catalytic activity and selectivity for partial hydrogenation by tuning the environment around active sites in iridium complexes bonded to supports

Article abstract of DOI:10.1039/C8SC05287E

Single-site Ir(CO)₂ complexes bonded to high-surface-area metal oxide supports, SiO₂, TiO₂, Fe₂O₃, CeO₂, MgO, and La₂O₃, were synthesized by chemisorption of Ir(CO)₂(acac) (acac = acetylacetonate) followed by coating with each of the following ionic liquids (ILs): 1-n-butyl-3-methylimidazolium tetrafluoroborate, [BMIM][BF₄], 1-n-butyl-3-methylimidazolium acetate, [BMIM][Ac], and 1-(3-cyanopropyl)-3-methylimidazolium dicyanamide, [CPMIM][DCA]. Extended X-ray absorption fine structure spectroscopy showed that site-isolated iridium was bonded to oxygen atoms of the support. Electron densities on the iridium enveloped by each IL sheath/support combination were characterized by carbonyl infrared spectroscopy of the iridium gem-dicarbonyls and by X-ray absorption near-edge structure data. The electron-donor/acceptor tendencies of both the support and IL determine the activity and selectivity of the catalysts for the hydrogenation of 1,3-butadiene, with electron-rich iridium being selective for partial hydrogenation. The results resolve the effects of the IL and support as ligands; for example, the effect of the IL becomes dominant when the support has a weak electron-donor character. The combined effects of supports and ILs as ligands offer broad opportunities for tuning catalytic properties of supported metal catalysts.

Full text of DOI:10.1039/C8SC05287E

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Controlling catalytic activity and selectivity for
partial hydrogenation by tuning the environment
around active sites in iridium complexes bonded to
supports†
Cite this: DOI: 10.1039/c8sc05287e
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
ab
c
d
e
Melike Babucci,
Chia-Yu Fang, Jorge E. Perez-Aguilar, Adam S. Hoffman,
e
c
e
d
Alexey Boubnov, Erjia Guan,
Simon R. Bare,
Bruce C. Gates
*
abf
and Alper Uzun
*
Single-site Ir(CO)
CeO , MgO, and La
followed by coating with each of the following ionic liquids (ILs): 1-n-butyl-3-methylimidazolium
tetrafluoroborate, [BMIM][BF ], 1-n-butyl-3-methylimidazolium acetate, [BMIM][Ac], and 1-(3-
2
complexes bonded to high-surface-area metal oxide supports, SiO
2 2 2 3
, TiO , Fe O ,
2
2
O
3
, were synthesized by chemisorption of Ir(CO)
2
(acac) (acac ¼ acetylacetonate)
4
cyanopropyl)-3-methylimidazolium dicyanamide, [CPMIM][DCA]. Extended X-ray absorption fine
structure spectroscopy showed that site-isolated iridium was bonded to oxygen atoms of the
support. Electron densities on the iridium enveloped by each IL sheath/support combination were
characterized by carbonyl infrared spectroscopy of the iridium gem-dicarbonyls and by X-ray
absorption near-edge structure data. The electron-donor/acceptor tendencies of both the support
and IL determine the activity and selectivity of the catalysts for the hydrogenation of 1,3-butadiene,
with electron-rich iridium being selective for partial hydrogenation. The results resolve the effects of
the IL and support as ligands; for example, the effect of the IL becomes dominant when the support
has a weak electron-donor character. The combined effects of supports and ILs as ligands offer
broad opportunities for tuning catalytic properties of supported metal catalysts.
Received 27th November 2018
Accepted 8th January 2019
DOI: 10.1039/c8sc05287e
rsc.li/chemical-science
Notwithstanding numerous recent investigations of structure
and performance of such catalysts on supports such as metal
Introduction
6
–14
Atomically dispersed metal catalysts on supports are drawing oxides,
there is only limited insight into how to control their
1
–5
wide attention, because they offer new catalytic properties
and an efficient use of expensive transition metals. properties by choice of ligands bonded to the metal;
catalytic properties. Some researchers have tuned the catalytic
1
5–17
the
Electron-donor properties of
supports are determined by the compositions and structures of
18–20
supports act as ligands as well.
2
1,22
a
the sites where the metals are bonded to the supports,
and
The
Department of Chemical and Biological Engineering, Koç University, Rumelifeneri
Yolu, Sariyer 34450, Istanbul, Turkey. E-mail: auzun@ku.edu.tr
2
3,24
they can be tuned by promoters nearby on the support.
b
¨
Koç University TUPRA S¸ Energy Center (KUTEM), Koç University, Rumelifeneri Yolu, effects of supports are sometimes pronounced, as illustrated by
Sariyer 34450, Istanbul, Turkey
rhodium complexes on HY-zeolite and on MgO—the strong
c
Department of Materials Science and Engineering, University of California, Davis,
electron-donor MgO induces high selectivity for 1,3-butadiene
California 95616, USA
25
hydrogenation to butenes, whereas the zeolite does not.
d
Department of Chemical Engineering, University of California, Davis, California
Molecules nearby metal centers on surfaces may also act as
ligands, as illustrated by electron-donor ethylenediamine coat-
Koç University Surface Science and Technology Center (KUYTAM), Koç University, ings of platinum nanowires, which inuence the selectivity of
9
5616, USA. E-mail: bcgates@ucdavis.edu
e
SSRL, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, USA
23
f
Rumelifeneri Yolu, Sariyer, 34450 Istanbul, Turkey
the platinum for catalytic hydrogenation of nitroaromatics. The
class of coatings inuencing catalytically active metals in sup-
ported catalysts has recently been extended to include ionic
liquids (ILs), which have been shown to enhance catalytic
†
Electronic supplementary information (ESI) available: Plots of residuals and
individual shells characterizing EXAFS data and ts, table summarizing
goodness of t parameters, characterization of the uncoated and IL-coated
samples by XANES and by FTIR spectroscopy, SEM images and EDX spectra of
IL-coated samples and calculated IL loadings, FTIR evidence of single-site
26
selectivity in cyclooctadiene hydrogenation. Many ILs are
iridium aer IL coating, characterization of the samples before and aer available, offering a wide range of electron-donor/acceptor
1
,3-butadiene hydrogenation reaction and time-on-stream data. See DOI:
properties. They can be quite stable on surfaces, and in
1
0.1039/c8sc05287e
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addition to acting as ligands, they can also act as “molecular Sigma-Aldrich with the highest available purities. The metal

lters” controlling the local concentrations of reactants and oxide supports were calcined in owing O with the temperature
2
26–29
ꢀ1
intermediates.
ramped at a rate of 3 K min from room temperature to
Early results show that 1,3-dialkylimidazolium-type IL coat- a selected nal temperature (K) for each support: 793 for SiO2;
ings signicantly inuence the electronic properties of iridium 673 for TiO and CeO ; 773 for Fe ; 873 for La ; and 973 for
carbonyls that are chemically bonded to alumina, Ir(CO) /g- MgO; thereaer, each sample was held at the nal temperature
and they thereby inuence the catalyst performance for for 5 h in owing O . Each IL, [BMIM][BF ], [BMIM][Ac], and
2
2
2
O
3
2 3
O
2
3
0
Al
2
O
3
,
2
4
31
partial hydrogenation of 1,3-butadiene. Owing to a wide range [CPMIM][DCA], was dried at 353 K for 6 h and kept under
of electron-donor/acceptor properties, the ILs offer broad vacuum for 12 h before being transferred into an argon-lled
prospects for tuning supported catalysts. We posit that they can Labconco glove box. The supported catalysts were synthesized
be tuned to work synergistically with supports to control the by the reaction of Ir(CO) (acac), where acac is acetylacetonate
2
ꢀ
electronic environments and thereby the catalytic properties of ([C H O ] ), (18 mg) with a sample of each metal oxide (982 mg)
5
7 2
30,31
essentially molecular species on supports.
to give an iridium loading of 1 wt% Ir in the nal uncoated
Now we report such control for a family of site-isolated sample. The precursor and support were slurried in n-pentane,
iridium complex catalysts by choice of the environment envel- which had been puried in an MBraun SBS solvent purier
oping the metal—comprising both the support and a sheath of system. The slurry was mixed for 24 h in a sealed ask, followed
IL molecules—that regulate the chemical environment, and by evacuation to remove the solvent and leave all the iridium on
thereby the catalytic properties of the metal, while allowing the the support. Each of the resultant solid catalysts was stored in
access of reactant molecules.
the argon-lled glove box. Coating of these samples with ILs
We assess the effects of the support and the IL in terms of involved impregnating the supported metal complex (200 mg)
their electron-donating/-withdrawing properties. Our data show with the IL (100 mg) to adsorb the remaining unadsorbed IL,
how to dial-in the selectivity for partial hydrogenation of 1,3- with the handling done in the glove box. Each of the resultant
butadiene to selectively give n-butenes. For example, the selec- samples prepared as a physical mixture of IL-coated supported
tivity of these isolated Ir sites on porous silica incorporating the complexes with added support adsorbing the excess IL had an
electron-withdrawing
methylimidazolium dicyanamide ([CPMIM][DCA]) was only iridium loading of 0.4 wt%. The IL loading of each sample was
2%, but this value increased to 93% when the IL was switched approximately 20 wt%, as shown by energy-dispersive X-ray
to the electron-donating 1-n-butyl-3-methylimidazolium acetate spectroscopy (EDX) measurements shown in the ESI.†
IL
1-(3-cyanopropyl)-3- average grain diameter of approximately 0.03 to 0.05 mm and an
2
(
[BMIM][Ac]). Supports separately affect the catalytic proper-
3,32–38
ties,
and we demonstrate how to measure the essential
IR spectroscopy
electron-donor/acceptor properties of the ILs and the supports
acting in concert, by using Fourier-transform infrared (FTIR)
and X-ray absorption near-edge structure (XANES) spectros-
copies. Because there is an extremely large set of available ILs,
the approach can be generalized to a large class of supported
metal catalysts ranging from atomically dispersed species to
nanoparticles.
Our goals were to learn how to control the catalytic proper-
ties of our single-site iridium catalyst by investigating a family
of supports offering a wide range of electron-donor/acceptor
A Bruker Vertex 80v spectrometer equipped with a vacuum
sample chamber was used in transmission mode to measure
FTIR spectra of solid samples with a spectral resolution of
ꢀ
1
2
cm . Samples were handled with the exclusion of moisture
and air at room temperature under vacuum. Each sample was
pressed between two KBr windows in a glove box and trans-
ferred to the sample chamber of the spectrometer immediately
prior to the measurement. The sample chamber was evacuated
within a few seconds aer the insertion of the sample holder.
For each reported spectrum, 256 scans were averaged, and
before each of these measurements, a background of 128 scans
was collected with the chamber under vacuum. Data charac-
terizing [CPMIM][DCA]-coated catalysts were deconvoluted
using a Voigt function to isolate the carbonyl bands from those
associated with the IL.
tendencies. The supports (SiO
2 2 2 3 2
, TiO (anatase), Fe O , CeO ,
MgO, and La O ) and the ILs were chosen to offer a wide range
2
3
of complementary electron-donor/acceptor tendencies (the ILs
were the strong electron-acceptor [CPMIM][DCA], the strong
electron-donor [BMIM][Ac], and one with intermediate electron-
donor
uoroborate, [BMIM][BF
industrially important partial hydrogenation of 1,3-buta-
character,
1-n-butyl-3-methylimidazolium
tetra-

4
]). We tested the catalysts for the
X-ray absorption spectroscopy (XAS)
39–42
diene.
Our results show how the ILs and supports act X-ray absorption spectroscopy (XAS) measurements were made
synergistically as ligands to control the catalytic properties, and at beam line 9–3 at the Stanford Synchrotron Radiation Light-
the spectroscopic data resolve the individual roles of each.
source (SSRL). Beam line 9–3 is a collimating and focusing
beam line with a 16-pole, 2 Tesla wiggler, Rh-coated harmonic-
rejection mirrors, and a double-crystal, liquid-nitrogen cooled,
Si(220) monochromator. Spectra were collected in uorescence
Experimental
Materials and methods
detection mode using N -lled ion chambers and a 30-element
2
All sample handling and preparations were done with standard Ge detector to measure signal. Approximately 250 mg of
43
air-exclusion techniques. The chemicals were purchased from a sample was pressed into an in situ XAS cell in an argon-lled
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glove box and handled in the absence of air at the beam line to of the glitch determined the error in the energy calibration of
prevent contamination. An Ir black powder (Sigma-Aldrich) was the samples to be 0.022 eV. The absorption edge energy was
scanned simultaneously with the sample for energy calibration. dened as the maximum of the rst derivative of the normal-
Extended X-ray absorption ne structure (EXAFS) spectra were ized absorbance with respect to beam energy.
measured from 200 eV below the Ir LIII edge (11 215 eV) to k ¼
ꢀ
1
1
5.3 A˚ in 20 min, with three scans collected and averaged to Point of zero charge (PZC) measurements
improve the signal-to-noise ratio.
ATHENA soware, a part of the IFEFFIT package, and the
PZC values characterizing the metal oxides were determined by
using a Zetasizer Nanoseries instrument coupled with Malvern
Multipurpose titrator. Each metal oxide (4 mg) was dispersed in
44
45
soware XDAP were used in the analysis of the EXAFS data.
Data alignment, edge calibration, and deglitching were per-
formed with Athena. The edge, determined as the rst inection
point of the absorption edge of the iridium black powder, was
calibrated to the reported Ir LIII energy, 11 215.0 eV. XDAP was
used for normalization, background subtraction, and data
a 0.1 M KNO
3
solution (20 mL), and the solutions were ultra-
sonicated for approximately 5 min. Dispersed samples were
53
titrated with 0.5 M NaOH and HNO
3
.
pH increments of the
instrument were set to 0.2 for measurements of SiO and TiO .
2
2
For back titration measurements, the pH increment was 0.1 for
4
5

tting, which allows the efficient application of a difference-
MgO, CeO , and a-Fe O . Tolerance was set to 0.2 for SiO and
2
2
3
2
46

le technique for the determination of optimized t parame-
TiO
2
and to 0.05 for MgO, CeO
2
, and a-Fe
2
O
3
. Refractive index
ters and isolation of individual shells. Reference les, used in
values used during measurements were 1.46, 2.90, 1.74, 2.20,
and 2.94 for SiO , TiO , MgO, CeO , and a-Fe , respectively.
Absorbance values used during the measurements were 0.01,
.0, 0.1, 0.01, and 0.1 for SiO , TiO , MgO, CeO , and a-Fe
respectively. The PZC characterizing La O was taken to be
4
7
the data analysis, were calculated by using the code FEFF6.0.
2
2
2
2 3
O
48
The crystal structure of Ir(CO) (acac) was used to calculate
2
the phase shis and backscattering amplitudes representing
the Ir–CCO, Ir–OCO, and Ir–O shells, as it was foreseen that the
s
1
2
2
2
2 3
O ,
2
3
structure of the supported iridium species would be similar to
that of this precursor. The Ir–CCO and multiple scattering paths
of the Ir–OCO of the carbonyl were t together to separate out the
carbon and oxygen rst shells. The phase shis and backscat-
tering amplitudes representing Ir–Si, Ir–Ti, Ir–Ce, Ir–Fe, and Ir–
Mg shells were calculated from the structural parameters
characterizing Ir–Si, Ir–Ti, Ir–Ce, Ir–Fe, and Ir–Mg alloys,
54
a literature value, 10.3 (pH).
Scanning electron microscopy coupled with energy-dispersive
X-ray spectroscopy (SEM/EDX)
A Zeiss Ultra Plus scanning electron microscope (SEM) equip-
ped with a eld-emission gun was used to collect images of the
catalyst samples in secondary-electron mode. Powder samples
consisting of IL-coated supported iridium complexes on SiO2,
˚
respectively. The Ir–Si distance (3.42 A) was found to be longer
49
˚
than typical Ir–Si (2.56 A) distance whereas the Ir–Ti distance
2 2 3 2 2 3
TiO , a-Fe O , CeO , MgO, and La O were mounted on carbon
˚
˚
(
2.56 A) and the Ir–Ce distance (2.54 A) were found to be shorter
50
˚
51
tape to minimize support charging effects. Images were
collected at magnications of 20kꢂ, 50kꢂ, and 100kꢂ with an
accelerating voltage of 3 kV for each sample. The working
distances between the probe of the microscope and samples
were in the range of 2.6–2.8 mm. Each EDX image was collected
at a magnication in the range of 10–75kꢂ with an accelerating
voltage of 10 kV and a working distance in the range of 5.2–5.4
mm.
˚
than typical Ir–Ti (2.69 A) and typical Ir–Ce (3.05 A) distances,
respectively. These contributions might also represent an Ir–O*
contribution (where O* is oxygen at long distance); however, the
data quality was not sufficient to identify the back-scatterer or
even to determine this contribution with condence.
Iterative tting was carried out for various structural models
until the best agreement was attained between the calculated
0
1
2
k -, k -, and k -weighted EXAFS data and the postulated model.
Plots were made to show the residuals remaining aer the
Catalyst performance measurements

tting; representative results are shown in the ESI.†
The tting ranges in both k- and R-space, where k is the wave To evaluate the performance of each catalyst, uncoated and IL-
vector and R the distance from the absorbing Ir atom, in the coated supported iridium complexes (each including 300–
analysis of the data characterizing the as-prepared sample were 350 mg of the supported iridium gem-dicarbonyl complex,
determined by the data quality; the range in k was in general independent of whether an IL coating was present) were each
ꢀ1
˚
between 3.5 ꢁ 0.4 and 12.0 ꢁ 0.6 A , and the range in R was placed in a 1/4-inch (OD) stainless-steel once-through tubular
˚
0
–3.5 A for each sample. These values were used with the ow reactor. A Thermcra three-zone resistively heated furnace
52
Nyquist theorem to estimate the justied number of tting (Model number XST-3-0-18-3V) equipped with PC-operated
parameters. The accuracies of the parameters are estimated to temperature controllers was used for temperature control of
be as follows (except for Ir–Si, Ir–Ti, Ir–Ce, Ir–Fe and Ir–Mg the reactor. Electronic mass ow controllers (Aalborg, Model
contributions): coordination number N, ꢁ20%; distance R, GFC17) were used to control the ow rates of feed gases. Before
2
˚
ꢁ
0.02 A; disorder term Ds , ꢁ20%; and inner potential each measurement, the catalysts were treated in owing
correction DE
0
, ꢁ20%.
ATHENA using standard procedures. The data for each sample complexes into mixtures of Ir(CO) (C H ) and Ir(CO) (C H )
4 2
ethylene at 373 K for 1 h as the temperature was ramped at a rate
ꢀ1
Data analysis for XANES spectra was performed with of 3 K min to convert the almost inactive supported Ir(CO)2
44
2
2
4
2
2
were rst calibrated using a known glitch in the mono- complexes, as reported for the isostructural HY-zeolite-
37
0
chromator observed in the I signal. A least-squares Gaussian t supported iridium complexes. Then, the ow of the reaction
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55
2
mixture with a molar ratio of H (Linde, 99.99 vol%) to 1,3- catalyst. The best-t EXAFS model characterizing each sample
butadiene (Linde, 99.6 vol%) of 2 : 1 was started through the includes an Ir–O scattering path (the subscript refers to the
s
catalyst bed, operated at atmospheric pressure and 333 K. support) with a coordination number of 2.2 ꢁ 0.2 (Table 1) and
˚
Products were analysed with an online gas chromatograph a bonding distance ranging from 2.01 to 2.07 A, as expected for
4
6
(
5
Agilent 7890A) equipped with a GS-alumina column (50 m ꢂ positively charged group-8 metals on metal oxides. The data in
30 mm) and a ame-ionization detector. Conversions of 1,3- Table 1 indicate that each Ir atom was bonded to approximately
butadiene were in the differential range, <2%. Reaction rates two carbon atoms (indicated by a coordination number of 2.1 ꢁ
˚
are reported per Ir atom in terms of turnover frequency (TOF), 0.2) at a distance ranging from 1.90 to 2.02 A, which is typical of
56
assuming that all of the Ir atoms were accessible for reaction. Ir–C bonding in iridium dicarbonyl complexes.
Repeat measurements showed that the TOF values are repro- IR spectra (Fig. 1) conrm the identication of iridium gem-
ducible within ꢁ2%. Control experiments done with catalysts dicarbonyls. We observed no bridging CO bands that would
containing double the loading of IL were performed to verify have indicated the presence of iridium clusters (details in the
57
that the intraparticle mass transfer limitations were not ESI, Section S2.1†). A comparison of the IR spectra of the
signicant.
Ir(CO)
2
(acac) precursor and the supported samples (Fig. 1,
(acac) was adsorbed,
Tables 2 and S2†) shows that when Ir(CO)
2
56
some bands shied or were replaced, as expected, for the
formation of the supported iridium gem-dicarbonyls (details in
the ESI, Section S2.1†).
Results
Atomically dispersed supported iridium complexes
Supported iridium complexes were synthesized by the reaction
Supports as ligands
of Ir(CO) (acac) with freshly calcined metal oxides. The samples
2
incorporated 1 wt% Ir, giving an average distance between Ir The n(CO) spectra (Table 2) distinguish one support from
atoms of at least 1 nm, depending on the support surface area, another as electron-donor/acceptor ligands. The major nsym(CO)
56
to assure site-isolation. Consistent with the atomic dispersion, peaks indicate Ir(CO)
2
on the surface.
Ir LIII-edge EXAFS spectra of the samples (Tables 1 and S1, Electronic environments of the supported iridium were
Fig. S1–S36 in the ESI†) give no evidence of an Ir–Ir scattering probed with XANES spectroscopy at the Ir LIII edge (Fig. S37–S43
path that would represent metal clusters/nanoparticles, in the ESI†). Table S3 in the ESI† is a summary of the absorption
consistent with atomically dispersed iridium in each edge energies of each uncoated sample. This energy decreased
Table 1 Summary of EXAFS fit parameters characterizing samples prepared by adsorption of Ir(CO)
electron-donor characteristics (quantified by the their corresponding PZC values)
2
(acac) on metal oxides having various
a
b
2
3
2
ꢀ1
˚
˚
k range (A˚
R range ( A˚ )
0.5–3.50
Sample (PZC of support, pH)
Shell
N
R (A)
Ds ꢂ 10 (A )
DE
0
(eV)
)
Ir(CO)
Ir(CO)
Ir(CO)
Ir(CO)
Ir(CO)
Ir(CO)
2
2
2
2
2
2
/SiO
2
(1.7)
Ir–O
s
2.25
2.25
2.00
2.50
2.40
2.20
2.10
1.40
2.40
2.14
1.90
1.12
2.30
2.10
1.90
2.20
2.40
2.20
2.00
1.40
2.00
1.89
2.14
2.10
2.07
2.00
3.28
3.42
2.03
1.92
2.95
2.65
2.01
1.90
2.91
2.52
2.02
1.90
2.83
2.54
2.06
2.02
2.95
2.76
2.02
1.93
2.92
2.46
3.46
3.87
7.78
5.94
0.10
1.55
5.00
6.21
0.10
2.80
6.89
7.69
0.10
3.73
6.54
10.17
0.10
0.62
5.78
0.10
0.10
3.16
5.98
0.10
ꢀ3.42
ꢀ1.61
ꢀ5.12
5.01
3.82–12.43
Ir–CCO
Ir–OCO
Ir–Si
/TiO
2
(4.5)
Ir–O
s
4.21
3.17–12.10
3.92–12.30
3.92–12.60
4.02–11.30
3.15–11.30
0.5–3.00
0.5–3.20
0.5–3.2
Ir–CCO
Ir–OCO
Ir–Ti
ꢀ0.64
5.66
ꢀ8.00
3.54
8.00
/Fe
2
O
3
(6.8)
Ir–O
s
Ir–CCO
Ir–OCO
Ir–Fe
ꢀ1.70
ꢀ5.48
ꢀ3.75
8.00
ꢀ7.73
ꢀ8.00
ꢀ6.98
0.29
/CeO
2
(8.7)
Ir–O
s
Ir–CCO
Ir–OCO
Ir–Ce
/MgO (9.8)
Ir–O
s
0.5–3.40
0.5–3.20
Ir–CCO
Ir–OCO
Ir–Mg
ꢀ8.00
7.36
/La
2
O
3
(10.3)
Ir–O
s
1.76
4.26
8.00
0.21
Ir–CCO
Ir–OCO
Ir–O
l
a
2
Notation: N, coordination number; R, distance between the absorber and backscatterer atoms; Ds , mean square relative displacement; DE
0
, inner
denote surface oxygen atoms
of supports; CCO and OCO denote carbon and carbon and oxygen atoms of carbonyl ligands bonded to iridium, respectively.
2
b
˚
potential correction. Error bounds are estimated as follows: N, ꢁ20%; R, ꢁ0.02 A; Ds , ꢁ20%; DE
0
, ꢁ20%.
s l
O and O
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example, a red shi in the nsym(CO) band position indicates an
58
increase in the electron density on the iridium sites. The data
show variations in broadness of these n(CO) bands on various
supports, originating from differences in the degrees of surface
uniformity of these metal oxides. The broad bands and shoul-
ders that were observed for Ir/CeO
2 2 3
and Ir/Fe O indicate
iridium carbonyls on minority surface sites.
ILs distributed uniformly on supports
IL loadings were chosen to be 20 wt%, sufficient to provide
a multilayer covering of all Ir sites in each sample but not so
much as to present a signicant transport limitation to the
26
reactant molecules in catalysis. The loading corresponds to 45
3 IL molecules per Ir site, on average. The IL was present in
ꢁ
layers with a thickness on average of several molecules. On
a macroscopic scale, at least, the ILs were uniformly distributed
over the supported species, as shown by SEM/EDX images
(Fig. S44–S50 and Table S4 in the ESI; details in the ESI†).
ILs as ligands
FTIR spectra of the various samples and the bulk ILs (Fig. 1 and
S51–S53 in the ESI†) show that the supported complexes were
still present as site-isolated iridium gem-dicarbonyls aer
addition of the IL, as shown by the two distinct carbonyl
stretching bands of the iridium gem-dicarbonyls in each sup-
ported catalyst. The IL coatings shi these bands. For example
(
Fig. 1B and Table 2) the nsym(CO) and nasym(CO) bands of
ꢀ
1
2 2
Ir(CO) /SiO , at 2081 and 2008 cm , were shied to 2052 and
1
ꢀ
1
981 cm , respectively, upon coating with [BMIM][BF
4
]. Simi-
larly, when the complexes were coated with [BMIM][Ac], the
ꢀ
1
bands were red-shied by 32 and 55 cm , respectively. These
2
8,29,59
shis conrm
ligands. In contrast, the electron-withdrawing IL [CPMIM]
DCA], with its cyano group on the cation, blue-shied the
that these ILs donate electrons, acting as
[
ꢀ1
corresponding bands to 2098 and 1974 cm . Doubling the IL
loadings led to no signicant changes in the band shis,
demonstrating that ILs were present at saturation loadings in
30
terms of their ligand effects.
To rule out the possibility that the iridium complex was
removed from the surface and dissolved in the IL coating, we
washed some IL-coated samples in methanol and checked the
FTIR spectra of both the ltrates and the washed samples. No
vibrational bands associated with iridium carbonyls were
detected in the ltrates, and the spectra of the washed samples
still indicated the presence of the iridium carbonyls (although
the peak positions and shapes were changed by the treatments,
4
Fig. 1 IR spectra of uncoated (black) and IL-coated ([BMIM][BF ]
(
blue), [BMIM][Ac] (red), and [CPMIM][DCA] (green)) Ir(CO)
2
complexes
supported on SiO
of (A) 1300–1700 cm and (B) 1900–2200 cm
2
, TiO
2
, Fe
2
O
3
, CeO
2
, MgO, and La
O
2 3
in the region
ꢀ
1
ꢀ1
.
from 11 218.0 to 11 216.0 eV in the order SiO , TiO , Fe O , as illustrated in Fig. S54 in the ESI† for [BMIM][BF
4
]-coated
2
2
2 3
CeO , MgO, and La O indicating the increasing electron Ir(CO)
2
/MgO). These results show that the iridium complexes
2
2 3
37
remained bonded to the support even aer the samples were
coated with the ILs and did not dissolve in the IL.
density on iridium donated by the support. This order is
consistent with the PZC values characterizing the metal oxides
53,54
(Fig. 2A).
The IR nsym(CO) band positions of the iridium
Catalytic hydrogenation of 1,3-butadiene
carbonyls provide still another, easily determined, measure of
the electron-donor strengths of the supports, as conrmed by
the strong correlation between the edge energies of the iridium
carbonyls and their nsym(CO) band positions as shown in Fig. 2B
The data presented above show that the electronic environ-
ments of the Ir centers are inuenced both by the support and
the IL. Thus, we tested the inuence of each one on the catalytic
properties for the hydrogenation of 1,3-butadiene. First, each
(or between nsym(CO) and PZC of the support (Fig. 2C)). Thus, for
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Table 2
2 2
nsym(CO) and nasym(CO) band positions characterizing Ir(CO) (acac) and uncoated and IL-coated metal-oxide-supported Ir(CO)
complexes
ꢀ1
ꢀ1
Supported Ir(CO)
2
complex
nsym(CO) (cm
)
nasym(CO) (cm
)
Ir(CO)
Ir(CO)
Ir(CO)
Ir(CO)
Ir(CO)
Ir(CO)
Ir(CO)
Ir(CO)
2
2
2
2
2
2
2
2
(acac)
(acac) in hexane
/SiO
/TiO
/Fe
/CeO
/MgO
/La
2049
2074
2081
2076
2073
2071
2069
2066
2052
2055
2056
2057
2058
2055
2049
2052
2053
2050
2056
2053
2098
2084
2081
2074
2064
2061
1985
2000
2008
1988
1981
1982
1985
1977
1981
1971
1969
1974
1960
1970
1953
1962
1947
1966
1952
1970
1974
1956
1973
1955
1959
1991
56
2
2
2 3
O
2
2 3
O
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
[
BMIM][BF
4
4
4
4
4
4
]-coated Ir(CO)
]-coated Ir(CO)
]-coated Ir(CO)
]-coated Ir(CO)
]-coated Ir(CO)
]-coated Ir(CO)
2
2
2
2
2
2
/SiO
/TiO
/Fe
/CeO
/MgO
2
BMIM][BF
BMIM][BF
BMIM][BF
BMIM][BF
BMIM][BF
BMIM][Ac]-coated Ir(CO)
BMIM][Ac]-coated Ir(CO)
BMIM][Ac]-coated Ir(CO)
BMIM][Ac]-coated Ir(CO)
BMIM][Ac]-coated Ir(CO)
BMIM][Ac]-coated Ir(CO)
CPMIM][DCA]-coated Ir(CO)
CPMIM][DCA]-coated Ir(CO)
CPMIM][DCA]-coated Ir(CO)
CPMIM][DCA]-coated Ir(CO)
CPMIM][DCA]-coated Ir(CO)
CPMIM][DCA]-coated Ir(CO)
2
2
O
3
2
/La
2
O
3
2
2
2
2
2
2
/SiO
2
/TiO
/Fe
/CeO
/MgO
/La
2
2
O
3
2
2
O
3
2
2
2
2
2
2
/SiO
/TiO
/Fe
/CeO
/MgO
/La
2
2
2
O
3
2
2
O
3
catalyst was activated by removal of at least one of the CO bands as well as the formation of new bands, but none indi-
ligands per Ir atom by treatment in owing ethylene at 373 K; cating the formation of carbonyls bonded to neighbouring
this treatment incorporated ethylene ligands in place of CO, iridium centres, which would have indicated iridium cluster
31
31
which is a strong catalytic reaction inhibitor, as reported for formation. New bands located between the two carbonyl
similar metal carbonyl complexes supported on metal oxides stretching bands are consistent with the partial replacement of
60,61
and zeolites. The relative concentrations of species and the CO with ethylene.
2
For example, in the spectrum of the TiO -
FTIR spectra before and aer ethylene exposure are provided in supported iridium complex coated with [BMIM][BF ], the
4
ꢀ
1
Table S5 and Fig. S55–S77 in the ESI.† FTIR spectra recorded intensity of the nasym(CO) band at 1988 cm decreased slightly
before and aer ethylene exposure (Fig. S55–S77 in the ESI†), relative to that of the nsym(CO) band, accompanied by the
ꢀ1
indicate changes in the intensities and positions of the n(CO) appearance of two new bands, at 2030 and 2048 cm , assigned
Fig. 2 Measures of the electron-donor strengths of the supports: variation of (A) Ir LIII edge energy of the uncoated supported iridium carbonyls
with PZC of the support; (B) Ir LIII edge position with the corresponding nsym(CO) band positions of uncoated supported iridium carbonyls; and (C)
nsym(CO) band positions of uncoated supported iridium carbonyls with PZC of the support.
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3
1
+
2 4 2 4 2
to Ir(CO)(C H ) and Ir(CO)(C H ) , respectively. The areas of when the ILs incorporated [BMIM] -ions and decreased when
+
the n(CO) bands indicate that the supported complexes were the IL incorporated [CPMIM] -ions (Fig. 3). For example, under
converted into mixtures of Ir(CO)2 (ꢃ74%), Ir(CO)(C H ) our conditions, the TOF characterizing the SiO -supported
2
4
2
ꢀ
3
ꢀ3
(
ꢃ16%), and Ir(CO)(C H ) (ꢃ10%) on TiO , for example. Data iridium complex decreased from 9.6 ꢂ 10 to 1.2 ꢂ 10 (mole
2
4 2
2
ꢀ1
characterizing other samples indicate the presence of mixtures of 1,3-butadiene converted) ꢂ (Ir site ꢂ s) , whereas the partial
of Ir(CO) , Ir(CO)(C ), and Ir(CO)(C in varying concen- hydrogenation selectivity increased from 42 to 73% as a result of
trations (Table S5†). Following the activation of each supported the deposition of [BMIM][BF ]. Data presented in Table S6† and
2
2
H
4
2 4 2
H )
4
iridium complex by reaction with ethylene, it was cooled to 333 Fig. 3 also indicate that the effect of the IL coating on the
K in owing helium followed by the start of ow of the reactant catalytic performance depends strongly on the type of support.
stream consisting of H + 1,3-butadiene in a molar ratio of 2 : 1
2
at a space velocity chosen to ensure differential conversions of
the 1,3-butadiene (<2%). Typical data (Fig. S78 in the ESI†)
Discussion
characterizing [BMIM][BF
stream show that the product distribution and 1,3-butadiene as ligands determining the electronic properties of the metal,
conversion of ca. 1.5% were essentially constant.
which is represented simply by the nsym(CO) band positions,
4 2
]-coated iridium on SiO for 3 h on The essential results are: (a) that both the support and the IL act
Table S6† is a summary of the TOF and selectivity data and (b) that the electronic properties control the catalytic
characterizing each catalyst, and Fig. 3 illustrates the changes in selectivity and activity (TOF) for partial hydrogenation of 1,3-
total partial hydrogenation selectivity of the catalysts with butadiene. All data are accounted for by the correlations of
changes in electron-donor/acceptor properties of the supports Fig. 4. Thus, when the electron density on the iridium is the
and the ILs; the catalyst performance data were measured at greatest, the selectivity for partial hydrogenation of 1,3-buta-
differential conversions, <2%. The data representing uncoated diene is maximized, and vice versa (Fig. 4A). On the other hand,
2
SiO -supported iridium complexes, for example, indicate a TOF when the electron density on iridium is the greatest, the TOF is
ꢀ
3
ꢀ1
of 9.6 ꢂ 10 (mol of 1,3-butadiene converted) ꢂ (Ir site ꢂ s)
minimized, and vice versa (Fig. 4B).
(
(
Table S6†) with a selectivity for butenes (sum of 1-butene
21%), trans-2-butene (11%), and cis-2-butene (10%)) of from (i) non-uniformity of the active sites associated with the
The deviations from the trend lines in Fig. 4 might originate
approximately 42% (Fig. 3). Such a low selectivity for partial heterogeneity of the support surfaces and/or (ii) undetermined
hydrogenation is typical of iridium complexes on supports such differences in the behavior of the ILs on various supports.
as SiO that are not good electron-donor ligands. In contrast, Presence of this second effect was illustrated in a report
2
when the iridium complexes were supported on good electron- focusing on the interactions of [BMIM][BF ] with various metal
4
2 3
donor supports such as La O , the selectivity for butenes was oxides in the absence of any active metal. These interactions
approximately twice as high (81%) (Fig. 3). For comparison, vary considerably with the changes in the electron-donor char-
Table S6† and Fig. 3 also include the corresponding perfor- acter of the supports and lead to variations in the thermal
53
mance data characterizing the IL-coated catalysts—showing stability of [BMIM][BF ] on these metal oxides. The interac-
4
that the IL layer strongly inuences the catalytic performance. tions can also alter other physical and chemical characteristics
The TOF values characterizing the coated catalysts were lower of the IL when it is in contact with a metal oxide. Consequently,
than those characterizing the respective uncoated catalysts. The solubilities of guest molecules in a specic IL—thus, a lter
26
partial hydrogenation selectivity, on the other hand, increased effect —can vary signicantly when an IL is dispersed on
Fig. 3 Partial hydrogenation selectivity (sum of all butene isomers: trans-2-butene, 1-butene, and cis-2-butene) (colored bars) and TOF (black
symbols) characterizing each catalyst, measured at steady state and under differential conversion conditions (<2%) at an H /BD feed molar ratio
2
of 2.0 at 333 K and 1 atm catalyzed by activated uncoated and IL-coated iridium complexes supported on metal oxides with various electron-
donor characteristics.
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Fig. 4 Variation of (A) selectivity for partial hydrogenation (sum of all butene isomers) and (B) TOF of 1,3-butadiene with the nsym(CO) band
position of the corresponding coated and uncoated iridium complexes supported on metal oxides, following activation. Black symbols indicate
4
uncoated samples, and red, dark red, and blue symbols represent [BMIM][BF ]-, [BMIM][Ac]-, and [CPMIM][DCA]-coated supported complexes,
respectively.
various supports. Because the IL lter effect controls the strength of the ILs' ligand effects compared with those of the
concentrations of the reactants and intermediates interacting supports.
with the active sites, it inuences the catalytic performance, and
To further investigate the ligand effects on catalysis, we
this effect explains the deviations from a perfect correlation in considered the change in catalytic selectivity for butenes as
Fig. 4. Thus, we recognize the potential inclusion of an addi- a function of the electron-donor character of the support.
tional structural parameter that could be tuned to improve the Fig. 5B shows that the increase in partial hydrogenation selec-
+
catalytic performance.
tivity upon the deposition of [BMIM] -containing IL sheaths
The electronic effects shown in Fig. 4 are substantial, but, on becomes dominant when the support is a weak electron-donor,
some supports the effects of the electron-donor/acceptor prop- but when the IL incorporates a cyano group, the catalyst
erties of the IL are small, whereas on others they are at least becomes less selective for partial hydrogenation, because (as
twice as large. For example, when the iridium complex is sup- illustrated in Fig. 5A), this IL withdraws electrons from iridium.
2 3
ported on the strongly electron-donating La O , the effect of the As shown in Fig. 5B, this effect is dominant when the support is
ꢀ
1
+
IL is indicated by shis <10 cm in the nsym(CO) band position a weak electron-donor, as in the case of [BMIM] -containing ILs,
Fig. 5A). In contrast, when the support is the weak electron- but presenting a correlation with a reverse trend.
donor SiO , the magnitude of the shi indicating the ligand Although there is more to learn about the interactions
effect of the IL exceeded 20 cm when the IL sheath contained between the IL and the catalytic species, the correlations shown
(
2
ꢀ
1
+
+
the [BMIM] ion (a red shi) and the [CPMIM] ion (a blue shi). here make clear that the IL, like a support, acts as a ligand.
Fig. 5A illustrates the magnitudes of these shis, and thus the Moreover, the data conrm that these ligand effects are
Fig. 5 (A) Variations in the magnitudes of nsym(CO) band shifts observed upon coating the supported metal complexes with various ILs. The x-axis
quantifies the electron-donor character of the supports, and the vertical axis represents the magnitude of the ligand effect of the IL; (B) variation
4
in the change of partial hydrogenation selectivity upon the deposition of various ILs, [BMIM][BF ], [BMIM][Ac], and [CPMIM][DCA] with nsym(CO)
band positions on the corresponding uncoated supported iridium complexes, quantifying the electron-donor character of the support.
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additive, providing a high degree of exibility for controlling the This work was supported by the TUBITAK National Young
electronic structures of catalytic sites with an enormous Researchers Career Development Program (CAREER)
¨
number of possible combinations presenting opportunities for (113M552), by Koç University TUPRA ¸S Energy Center (KUTEM),
tuning the electronic environments of supported metals over and by the U.S. Department of Energy (DOE), Office of Science,
wide ranges. We posit that the opportunities offered by envel- Basic Energy Sciences (BES), Grant DE-FG02-04ER15513 (C-YF,
oping the catalytically active species with IL sheaths/supports JEP-A, EG). AU acknowledges the TUBA-GEBIP Award, TARLA,
for tuning the catalytic properties extend to many metals, and Koç University Summer Research Program and JEP-A
supports, and ILs, and recent results characterizing nickel acknowledges a Chevron Fellowship. We acknowledge the
42
nanoparticles on supports incorporating ILs indicate that the Stanford Synchrotron Radiation Lightsource (SSRL) for access
opportunities extend beyond atomically dispersed metals to to beam time on Beamline 9-3. SSRL, SLAC National Accelerator
metal nanoparticles.
Laboratory, is supported by DOE, BES, under Contract No. DE-
AC02-76SF00515, and the DOE BES-funded Consortium for
Operando and Advanced Catalyst Characterization via Elec-
tronic Spectroscopy and Structure (Co-ACCESS) at SLAC
Conclusions
Atomically dispersed supported iridium complexes were National Accelerator Laboratory.
synthesized on metal oxides having a wide range of surface
electron-donor characteristics, SiO
and La , and coated with various ILs, [BMIM][BF
Ac], and [CPMIM][DCA]. The electron density on the iridium
2
, TiO
2
, Fe
2
O
3
, CeO
2
, MgO,
2
O
3
4
], [BMIM]
Notes and references
[
was probed with FTIR spectroscopy determining the n(CO)
frequencies. The results show that the electron-donor/acceptor
character of the support and that of the IL sheath together
determine the electronic structures of the iridium complexes
and their catalytic properties for hydrogenation of 1,3-buta-
diene, for which high selectivities for butenes were obtained.
Thus, the metal oxide support works as a ligand that inuences
the electron density on the metal sites. As the electron-donor
strength of the metal oxides increases, as probed by an
increase in the corresponding PZC, the isolated supported
iridium sites become more electron-rich and consequently
become more selective as catalysts for the formation of butenes
from 1,3-butadiene. The data illustrate a similar ligand effect of
the IL sheaths, which becomes dominant when the support is
a weak electron-donor. Among the ILs considered, those
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There are no conicts to declare.
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