Platinum and cobalt intermetallic nanoparticles confined within MIL-101(Cr) for enhanced selective hydrogenation of the carbonyl bond in a,?-unsaturated aldehydes: synergistic effects of electronically modified Pt sites and Lewis acid sites

Article abstract of DOI:10.1039/d0cy02082f

Precious metals have been shown to play a vital role in the selective hydrogenation of a,?-unsaturated aldehydes, but still suffer from challenges to control selectivity. Herein, we have advanced the design of catalysts made out of Pt-Co intermetallic nanoparticles (IMNs) supported on a MIL-101(Cr) MOF (3%Pty%Co/MIL-101(Cr)), prepared by using a polyol reduction method, as an effective approach to enhance selectivity toward the production of a,?-unsaturated alcohol, the desired product. XRD, N₂adsorption-desorption, FTIR spectroscopy, SEM, TEM, XPS, CO adsorption, NH₃-TPD, XANES and EXAFS measurements were used to investigate the structure and surface properties of our 3%Pty%Co/MIL-101(Cr) catalysts. It was found that the Co-modified 3%Pty%Co/MIL-101(Cr) catalysts can indeed improve the hydrogenation of cinnamaldehyde (CAL) to cinnamyl alcohol (COL), reaching a higher selectivity under mild conditions than the monometallic Pt/MIL-101(Cr) catalysts: 95% conversion of CAL with 91% selectivity to COL can be reached with 3%Pt3%Co/MIL-101(Cr). Additionally, high conversion of furfural (97%) along with high selectivity to furfural alcohol (94%) was also attained with the 3%Pt3%Co/MIL-101(Cr) catalyst. The enhanced activity and selectivity toward the unsaturated alcohols are attributed to the electronic and geometric effects derived from the partial charge transfer between Co and Pt through the formation of uniformly dispersed Pt-Co IMNs. Moreover, various characterization results revealed that the addition of Co to the IMPs can promote the Lewis acid sites that facilitate the polarization of the charge-rich C?O bonds and their adsorptionviatheir oxygen atom, and also generate new interfacial acid sites.

Full text of DOI:10.1039/d0cy02082f

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PAPER
Platinum and cobalt intermetallic nanoparticles
confined within MIL-101(Cr) for enhanced
selective hydrogenation of the carbonyl bond in
α,β-unsaturated aldehydes: synergistic effects of
electronically modified Pt sites and Lewis acid sites†
Cite this: Catal. Sci. Technol., 2021,
11, 2433
a
a
a
Muhammad Zahid, Jiang Li, Ahmed Ismail,
b
a
Francisco Zaera * and Yujun Zhu
*
Precious metals have been shown to play a vital role in the selective hydrogenation of α,β-unsaturated
aldehydes, but still suffer from challenges to control selectivity. Herein, we have advanced the design of
catalysts made out of Pt–Co intermetallic nanoparticles (IMNs) supported on a MIL-101(Cr) MOF
(
3%Pty%Co/MIL-101(Cr)), prepared by using a polyol reduction method, as an effective approach to
enhance selectivity toward the production of α,β-unsaturated alcohol, the desired product. XRD, N
adsorption–desorption, FTIR spectroscopy, SEM, TEM, XPS, CO adsorption, NH -TPD, XANES and EXAFS
2
3
measurements were used to investigate the structure and surface properties of our 3%Pty%Co/MIL-101(Cr)
catalysts. It was found that the Co-modified 3%Pty%Co/MIL-101(Cr) catalysts can indeed improve the
hydrogenation of cinnamaldehyde (CAL) to cinnamyl alcohol (COL), reaching a higher selectivity under mild
conditions than the monometallic Pt/MIL-101(Cr) catalysts: 95% conversion of CAL with 91% selectivity to
COL can be reached with 3%Pt3%Co/MIL-101(Cr). Additionally, high conversion of furfural (97%) along with
high selectivity to furfural alcohol (94%) was also attained with the 3%Pt3%Co/MIL-101(Cr) catalyst. The
enhanced activity and selectivity toward the unsaturated alcohols are attributed to the electronic and
geometric effects derived from the partial charge transfer between Co and Pt through the formation of
uniformly dispersed Pt–Co IMNs. Moreover, various characterization results revealed that the addition of
Co to the IMPs can promote the Lewis acid sites that facilitate the polarization of the charge-rich CO
bonds and their adsorption via their oxygen atom, and also generate new interfacial acid sites.
Received 26th October 2020,
Accepted 22nd January 2021
DOI: 10.1039/d0cy02082f
rsc.li/catalysis
cinnamaldehyde (CAL) shown in Scheme 1, which include
competitive reduction steps at either of the two unsaturated
1
. Introduction
The selective hydrogenation of unsaturated aldehydes,
especially of α,β-unsaturated aldehydes, is a valuable route
for the synthesis of unsaturated alcohols, important
intermediates in the fabrication of fine chemicals like
flavours, fragrances and pharmaceutical compounds.
Nonetheless, this reaction involves complex reaction
networks, such as in the case of the hydrogenation of
groups of α,β-unsaturated aldehyde, i.e., the carbonyl bond
(CO) and the conjugated olefin bond (CC).
Thermodynamically, the hydrogenation of CC bonds is
more favorable than that of CO bonds, making it extremely
difficult to attain high selectivity toward CO bond
hydrogenation to produce unsaturated alcohols. It is quite
challenging, from either economic or scientific points of view,
1–3
but highly desirable to improve the selectivity of these
a
1,4–7
Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education
reactions to make unsaturated alcohols.
Generally, the
of the People's Republic of China, Heilongjiang University, Harbin, 150080, China.
E-mail: yujunzhu@hlju.edu.cn; Tel: +86 45186609650
hydrogenation of CO bonds is not easy to promote and
often requires supported catalysts with both mono- or bimetal
nanoparticle catalysts as well as Lewis acid sites operating
b
Department of Chemistry and UCR Center for Catalysis, University of California,
Riverside, 900 University Avenue, Riverside, California 92521, USA.
E-mail: zaera@ucr.edu
8–11
under high temperatures and high hydrogen pressures.
†
Electronic supplementary information (ESI) available: FTIR spectra, SEM
Alternatively, such a hydrogenation may be possible via
transfer hydrogenation with an alcohol as a hydride donor.
image, XPS characterization, CO adsorption, NH -TPD, EXAFS fitting results,
3
7
effects of reaction conditions on activity and comparison of the catalytic
performances with other reported Pt-based catalysts. See DOI: 10.1039/
d0cy02082f
Heterogeneous catalysts have been the most commonly
12–15
used for selective catalytic hydrogenation.
These
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Catalysis Science & Technology
ligands, forming three-dimensional well-defined zeo-type
33,34
structures.
They have attracted extensive research interest
because of their high surface area, low density, and tunable pore
size from micro- to meso-pores and 3D structure, all properties
that may be tuned for potential applications in the fields of gas
33–36
storage, adsorptive separation, sensing, and catalysis.
A
particularly interesting MOF is MIL-101(Cr), which is assembled
via chromium metal ions connected by terephthalic acid. This
37
MOF was initially synthesized by Ferey et al.,
who
demonstrated its high thermal stability, chemical stability, acid–
38
base resistance, large surface area and mesoporous structure
and successfully used it as a catalyst support in heterogeneous
37–40
catalysis.
The water molecules that are coordinated to the
chromium centres in this MOF can be easily removed at elevated
temperature under vacuum, thus producing unsaturated
chromium (Cr(III)) Lewis acid sites accessible for potential
Scheme 1 Reaction pathways for CAL hydrogenation.
36,40,41
adsorption of α,β-unsaturated aldehyde molecules.
These
characteristics of MIL-101(Cr) make it an ideal host for selective
hydrogenation catalysis. MIL-101(Cr) can accommodate
intermetallic nanoparticles, and the connecting window can
provide access for the diffusion of α,β-unsaturated aldehydes to
reach the metal active sites encapsulated in MIL-101(Cr) for
catalytic reactions. There is only a limited number of citations in
heterogeneous catalysts comprise both transition metals (Co,
Sn, Cu, Ni, etc.) and noble metals (Pt, Au, Pd, etc.). Improving
the selectivity toward carbonyl (CO) bond hydrogenation can
be obtained with bimetallic catalysts, where one of the noble
metals (frequently Pt) is combined with a more electropositive,
i.e. electron deficient, transition metal (Co, Sn, Fe, Ni, Zn, etc.)
the literature on the use of MOF supports for the selective
1,8,9,13,16–18
9,10,33,40–45
to form intermetallic nanoparticles (IMNs).
These
hydrogenation of α,β-unsaturated aldehydes,
and, to
IMN catalysts have attracted extensive research interest because
of their higher catalytic selectivity towards unsaturated alcohols
under mild reaction conditions, a behavior that has been
ascribed to the synergetic effects and cooperative interactions
between the individual metal components. Previous literature
has also acknowledged that the geometric and electronic
structures of IMNs play pivotal roles in the adsorption and
activation of different functional groups during the selective
the best of our knowledge, MIL-101(Cr) as a support for metal
40–42
nanoparticles has only been reported sparingly,
in particular
for the selective hydrogenation of cinnamaldehyde (CAL) and
furfural (FFL), where the selective hydrogenation of the olefinic
(CC) bonds was reported. We hypothesized that MIL-101(Cr)
can be a great support for the dispersion of metal nanoparticles
to improve the selectivity toward unsaturated alcohols in the
hydrogenation of CAL and FFL under ambient reaction
conditions. We combine that premise with the idea of using
intermetallic nanoparticles as a way to tune the electronic
properties of the active phase.
By taking into consideration the previous findings discussed
above, in the work reported here we prepared a series of Pt–Co
intermetallic nanoparticles confined within the pores of MIL-
101(Cr) (3%Pty%Co/MIL-101(Cr) y = 1, 3, 5) made by a simple
polyol reduction method without using any additional
stabilizing agent. The 3%Pt3%Co/MIL-101(Cr) catalyst made
this way showed particularly good catalytic selectivity for the
hydrogenation of CAL and FAL, performing as an efficient and
easily recyclable catalyst for the low-cost and eco-friendly
production of unsaturated alcohols (cinnamyl alcohol (COL)
and furfural alcohol (FOL), respectively). The effect of Co in
modifying the behavior of the Pt surface and the properties of
the MIL-101(Cr) support on activity and selectivity were
investigated by using various characterization techniques.
1
,8,10,13,16–26
hydrogenation of α,β-unsaturated aldehydes.
The
active noble metal (e.g. Pt, Pd, etc.) usually facilitates the
adsorption and dissociation of both hydrogen and the
substrate (an α,β-unsaturated aldehyde), whereas the second
electron-deficient metal (e.g. Co, Sn, Zn, Ga, In, etc.) modifies
and adjusts the surface electronic structure of the noble metal
via electron transfer, and with that shifts the relative reactivity
toward the desired functional groups.
In addition to considerations regarding the active metallic
phase, the choice of support is fundamental to improve the
durability and reaction activity of these hydrogenation
catalysts and, most importantly, their selectivity toward the
desired CO hydrogenation (which they can presumably
modify via strong metal–support interactions (SMSIs)).
Typical supports used for these catalysts include metal oxides
such as acidic Al O , Fe O , and TiO , inert SiO , basic MgO,
2
3
2
3
2
2
2
,11,13,14,27–31
and carbon materials.
The specific interactions
between the supports and the IMNs may lead to distinctive
hydrogenation reaction results because of adjustments to the
electronic and geometric properties of the catalysts.
One promising family of well-defined supports are metal–
organic frameworks (MOFs). These are composed of metal ions
or metallic clusters connected by bi- or multipodal organic
2
. Experimental
3
2
2.1. Preparation of MIL-101(Cr)
MIL-101(Cr) was prepared following a HF-free hydrothermal
method reported in the literature. The HF-free method was
3
4
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chosen to avoid the toxic and highly corrosive nature of that
at 150 °C for 5 h before measurement (to remove the
adsorbed water), and the pore size distribution and surface
area were calculated from desorption isotherms by using the
Barrett–Joyner–Halenda (BJH) and Brunauer–Emmett–Teller
(BET) methods, respectively. The morphology and structure
of the catalysts were imaged by scanning electron microscopy
(SEM) (Hitachi S-4800) and transmission electron microscopy
(TEM) (JEM-2100). The surface composition was analyzed by
using X-ray photoelectron spectroscopy (XPS) (Kratos-AXIS
ULTRA DLD), employing an Al Kα radiation source. The
binding energies (BEs) were calibrated and referenced to the
C (1s) binding energy of adventitious carbon, taken to be
acid. Briefly, Cr(NO ) ·9H O (5 mmol, 2.0011 g), terephthalic
3
3
2
acid (5 mmol, 0.84 g) and deionized water (24 mL) were
mixed at room temperature for about 30 min. The mixture
was sonicated for an hour at room temperature in air. Then,
the mixture was transferred into a 50 mL stainless steel
autoclave coated with a Teflon inner layer and heated at 220
°C for 18 h without stirring. After crystallization, the resulting
solid was cooled to room temperature. Finally, the green
powder was precipitated by centrifugation, dried at 75 °C
overnight, and heat-treated directly at 150 °C for 12 h under
vacuum to obtain the MIL-101(Cr) support.
284.0 eV. X-ray absorption near-edge structure (XANES) and
extended X-ray absorption fine structure (EXAFS) spectra were
acquired at the BL14W1 station in the Shanghai Synchrotron
Radiation Facility (SSRF, 3.5 GeV, 250 mA). A Si (311) double-
crystal monochromator was used to monochromatize the
X-ray beam. The surface electronic properties of the Pt metal
species were also investigated via infrared absorption
spectroscopy by using carbon monoxide (CO) as a probe
molecule and a Nicolet 6700 FTIR spectrometer. The surface
acidity of the catalysts was confirmed by ammonia
2
.2. Preparation of MIL-101(Cr)-supported Pt and Co catalysts
A series of MIL-101(Cr)-supported Pt and Co catalysts was
prepared by using a simple ethylene glycol (EG) reduction
method. Briefly, MIL-101(Cr) (0.15 g) was put into a mixture
of 10 mL EG and 5 mL dimethylformamide (DMF) under
stirring for 5 min to obtain mixture A. 1.2 g of a pre-prepared 1
3
1
wt% H
2
PtCl
6
·6H
2
O aqueous solution and 0.74 g (2.2 and 3.7 g)
·6H O aqueous solution were
of a pre-prepared 1 wt% Co(NO
)
3 2
2
added to 10 mL EG to obtain solution B. Then, solution B was
added to mixture A dropwise while continuously stirring at
room temperature; this process took about 20 min to complete.
The mixture was further stirred for 5 h at room temperature,
after which it was treated in an ultrasonic bath for 10 min and
then transferred to a stainless steel autoclave coated with a
Teflon inner layer. The reduction reaction was performed at
3
temperature-programmed desorption (NH -TPD). The results
were recorded using an AutoChem TP5080 chemisorption
analyser (Xianquan Tianjin China) equipped with a TCD
detector, and the ammonia signal (m/z = 17) was recorded
using mass spectrometry (QIC-20, Hidden).
2.4. Liquid-phase hydrogenation reaction
150 °C for 24 h under static conditions. Finally, the resulting
The liquid-phase selective hydrogenation of CAL and of FFL was
performed in a 100 mL stainless steel autoclave. Typically, for
each reaction, CAL/FFL (0.2525 g), the catalyst (20.0 mg), and
isopropanol (15 mL), used as solvent, were introduced into the
stainless steel autoclave fitted with a temperature-controlled
furnace. Afterwards, the autoclave was flushed three times with
catalyst was separated by filtration and washed with ethanol
until it was free of Cl , as determined by testing with an
aqueous AgNO and HNO solution. The catalysts were then
3 3
dried at 60 °C under vacuum for 12 h. A series of MIL-101(Cr)-
−
supported catalysts were prepared, a monometallic 3%Pt/MIL-
101(Cr) and several Pt–Co intermetallic nanoparticle (IMN)
N at 0.5 MPa to displace the air in its volume, and then H (0.5
catalysts with various 3 :y wt% ratios, which we denote here as
2
2
MPa) was added to replace the N
2
in the reactor, flushing three
3%Pty%Co/MIL-101(Cr) (y = 1, 3, 5).
times to make sure that all of the
Subsequently, the reactor was filled with hydrogen (pressure
.6–1.2 MPa) and heated to the required temperature (30–80 °C)
under magnetic stirring (800 rpm) for the required time (10–120
min). In the stability and reusability tests, the used catalyst was
recovered by centrifugation, washed three times with
isopropanol, dried at 60 °C under vacuum, and used for the next
run under the same conditions without any further treatment.
After reaction, the solution was filtered to screen out the solid
catalyst, and the products were analyzed by gas chromatography
2
N was removed.
2
.3. Characterization
0
The contents of platinum and cobalt in the prepared catalysts
were measured by inductively coupled plasma-optical
emission spectrometry (ICP-OES), which was performed by
using
measurement, each sample was dissolved in aqua regia (HCl
HNO ). The crystallinity of the samples was assessed via
a PerkinElmer Optima 7000DV analyser. Before
+
3
X-ray diffraction (XRD) tests, conducted on a Bruker D8
Advance X-ray diffractometer using Cu Kα (λ = 1.5418 Å)
radiation (40 kV, 40 mA); the data were collected in the 5–80°
(
BFRL SP-3420, Beijing, Chain, equipped with a flame ionization
detector (FID) and a HP-5 capillary column, 30 m × 0.32 mm ×
.25 μm), using octanol as the external standard.
2
θ range. FTIR spectra were obtained by diluting the catalysts
in KBr powder and making pellets, using a Bruker Equinox
5 IR spectrometer. N2 adsorption–desorption isotherms,
used to investigate the properties of the porous structures at
196 °C, were measured using a Micromeritics Tristar II
0
5
3
. Results and discussion
3.1. Catalyst characterization
−
surface area and porosity analysis instrument. In each test,
the catalyst (more than 100 mg) was preheated under vacuum
3.1.1. Metal loading quantitation. The presence of the
encapsulated Pt and Co metal nanoparticles in 3%Pt/MIL-
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01(Cr), 3%Pt1%Co/MIL-101(Cr), 3%Pt3%Co/MIL-101(Cr),
Catalysis Science & Technology
1
and 3%Pt5%Co/MIL-101(Cr) was first confirmed by ICP-OES.
The results are listed in Table 1 (first two columns). It can be
seen here that the actual Pt metal loading attained was in all
cases close to, albeit somewhat lower than, the 3 wt% target.
The added Co also amounted to values close to those
planned for, although the Co : Pt wt% y : 3 ratios were in all
cases a little below the targeted numbers.
3
.1.2. Bulk structure analysis. Fig. 1 shows the XRD
patterns obtained for the synthesized pure MIL-101(Cr), for
%Pt/MIL-101(Cr), and for three 3%Pty%Co/MIL-101(Cr)
3
catalysts (y = 1, 3, 5). By and large, they all show the same
diffraction peaks, an observation in good agreement with the
37,38,46
Fig.
1
XRD spectra of (a) MIL-101(Cr), (b) 3%Pt/MIL-101(Cr), (c)
results of MIL-101(Cr) reported in the literature.
Specifically, peaks are seen in all cases at 2θ = 5.2°, 9° and
6.6°. This demonstrates that the crystal structure of the
3%Pt1%Co/MIL-101(Cr), (d) 3%Pt3%Co/MIL-101(Cr), and (e) 3%Pt5%Co/
MIL-101(Cr).
1
pristine MIL-101(Cr) is retained upon loading of the Pt and Co
nanoparticles. Moreover, no obvious diffraction peaks
assignable to the Pt or Co species (expected at values of 2θ =
−
1
spectra. Absorption bands are seen at 1400 and 1624 cm for
all the samples, attributed to the asymmetric and symmetric
39.7°) are observed in any of the 3%Pt/MIL-101(Cr) or
%Pty%Co/MIL-101(Cr) samples, suggesting that the Pt and
−
3
stretching vibrations of the carboxyl (COO ) groups of the
Pt–Co intermetallic nanoparticles (IMNs) are quite small and
well dispersed inside the pores of the MIL-101(Cr) support.
Nevertheless, the intensity of the diffraction peaks from the
support become partially suppressed after the addition of the
Pt and Co nanoparticles, indicating a slight decrease in the
mesoporous ordering of the pristine MIL-101(Cr). Moreover,
there are minor differences in some of the X-ray diffraction
patterns, in particular in those for the 3%Pt1%Co/MIL-101(Cr)
and 3%Pt5%Co/MIL-101(Cr) samples, as compared to the
terephthalic acid (H BDC) present in the MIL-101(Cr)
2
4
6
framework, respectively. Additional weak and narrow bands
−
1
were detected at 749, 1017 and 1160 cm , ascribed to δ(C–H)
and (two) γ(C–H) vibrations of the aromatic rings,
−
1
respectively. The weak band at 580 cm can be attributed to
−
the in-plane and out-of-plane bending modes of the COO
−
1
groups, and the intense and broad bands around 3434 cm
can be assigned to the O–H stretching vibrations, suggesting
4
6
the presence of water molecules within the catalysts. The
detection of all major vibrational modes for the organic
linkers in our MOF confirms that the host MIL-101(Cr)
structure is retained after the loading of the Pt/Pt–Co IMNs,
in agreement with the XRD results (Fig. 1).
3%Pt3%Co/MIL-101(Cr) sample. Extra diffraction peaks at 2θ
of 12.2°, 24.6°, 62.5° and 66° are observed for the 3%Pt1%Co/
MIL-101(Cr) and 3%Pt5%Co/MIL-101(Cr) samples that cannot
be matched with any phases by searching the PDF database
using XRD Analysis Software (Jade 5.0) and the literature. We
think extra diffraction peaks may be assigned because the MOF
structure is destroyed to some extent in the hydrothermal
reduction of Pt species using ethylene glycol as a reductant. No
such peaks are seen in the XRD pattern of 3%Pt3%Co/MIL-
3.1.3. Pore structure characterization. The surface areas
and pore volumes of the synthesized samples were determined
by N2 adsorption–desorption isotherm measurements; the
results are summarized in Fig. 2 and Table 1. As shown in
2
Fig. 2, the N adsorption–desorption isotherms of MIL-101(Cr),
1
01(Cr), indicating a more uniform distribution of Pt–Co IMNs
3%Pt/MIL-101(Cr) and 3%Pty%Co/MIL-101(Cr) all show a
combination of type I and type IV curves, according to the
IUPAC classification, characteristic of materials containing
36,38,40
confined within MIL-101(Cr).
To further characterize the structure of the MIL-101(Cr),
%Pt/MIL-101(Cr), and 3%Pty%Co/MIL-101(Cr) catalysts (y =
, 3, 5), their FTIR spectra were acquired (Fig. S1†). All the Pt/
40
3
1
both micropores and mesopores. The BET surface area and
total pore volume of the pristine MIL-101(Cr) were estimated by
2
−1
3
−1
Pt–Co supported MIL-101(Cr) catalysts exhibit similar IR
the BJH method to be 2674 m
g
and 1.40 cm g ,
Table 1 Physicochemical parameters of the catalysts made in this study
Content of Pt
species (%)
Pt
Co
Lewis acidity
BET surface
area (m g
Pore
volume (cm g
−
1
2
−1
3
−1
0
2+
4+
Catalysts
loading (wt%)
loading (wt%)
(mmol g
)
)
)
Pt
Pt
Pt
MIL-101(Cr)
—
—
—
0.90
2.87
4.61
—
0.40
0.47
0.73
0.78
0.80
—
2674
2588
2296
1930
1416
2069
1.40
1.08
1.28
1.18
1.01
1.28
—
46
45
47
42
55
—
19
21
15
10
18
—
35
34
38
48
27
3
3
3
3
%Pt/MIL-101(Cr)
2.74
2.83
2.89
2.85
—
%Pt1%Co/MIL-101(Cr)
%Pt3%Co/MIL-101(Cr)
%Pt5%Co/MIL-101(Cr)
Used 3%Pt3%Co/MIL-101(Cr)
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respectively. Both of these parameters decrease to 2588 m g
Paper
2
−1
3
−1
and 1.08 cm g , respectively, upon the addition of 3 wt% Pt,
and go down further with increasing Co content (with the
exception of the volume for the pure Pt sample that seems to
be lower than expected). The significant decrease in the values
of these physical parameters is likely the result of pore blocking
by the Pt and Co IMPs. Nevertheless, all the catalysts exhibit
similar pore size distributions, centered around ∼2.3 nm, as
indicated in Fig. 2b (except for 3%Pt5%Co/MIL-101(Cr), which
shows pore diameters centered around ∼2.0 nm). Combined
with the XRD results, we think the pore structure changes can
be assigned to the partially broken metal–organic framework to
some extent after Pt and Co addition, especially for
3%Pt5%Co/MIL-101(Cr).
3
.1.4. SEM and TEM imaging. The morphology of the as-
Fig. 3 SEM images of (a) 3%Pt/MIL-101(Cr), (b) 3%Pt1%Co/MIL-101(Cr),
synthesized 3%Pt/MIL-101(Cr) and 3%Pty%Co/MIL-101(Cr)
catalysts were characterized by SEM. Typical images are shown
in Fig. 3. They reveal that 3%Pt/MIL-101(Cr) (Fig. 3a) possesses
a similar morphology to MIL-101(Cr) (Fig. S2†): both exhibit
regular well-defined octahedral shapes with clear edges and
(
c) 3%Pt3%Co/MIL-101(Cr) and (d) 3%Pt5%Co/MIL-101(Cr).
HRTEM and by the corresponding element maps, shown in
Fig. 4d and e, respectively. In Fig. 4d, crystal fringes are
observed with calculated d-spacings of 0.229 and 0.225 nm,
which are assigned to the (111) plane of the face-centred
3
4,39
corners, consistent with previous reports.
However, it is
clear from the images in Fig. 3b–d that the surface morphology
becomes slightly rougher with the addition of Co.
2
4,47
Higher resolution TEM images are shown in Fig. 4. They
indicate that the Pt and Pt–Co IMPs are uniformly distributed
throughout the support, mostly confined inside the pores of
the MIL-101(Cr) support; no nanoparticle aggregation or their
presence on the outer surface of MIL-101(Cr) is apparent. The
average nanoparticle sizes (d) estimated from these images
are all similar, around d = 2.1 ± 0.45 nm, but perhaps
increase slightly with increasing Co content (d = 2.1 ± 0.5, 2.3
cubic Pt and Pt–Co crystals, respectively.
Fig. 4e also
shows the uniform nature of the dispersion of both Pt and
Co metals. This TEM analysis suggests that small-sized
uniformly dispersed Pt–Co IMNs confined within the pores of
MIL-101(Cr) can be prepared by the polyol reduction method.
It would also appear that the large surface area and high
porosity of MIL-101(Cr) are beneficial for the uniform
dispersion of Pt–Co IMNs.
±
0.5, and 2.3 ± 0.4 nm for 3%Pt1%Co/MIL-101(Cr),
3.1.5. Analysis of surface species. To study the surface
electronic states of the Pt species and the interaction between
Pt and Co in the Pt–Co IMNs, the 3%Pt/MIL-101(Cr),
3
%Pt3%Co/MIL-101(Cr) and 3%Pt5%Co/MIL-101(Cr),
respectively; Fig. 4). This trend is to be expected, as the total
metal content increases with increasing Co load (the Pt wt%
is approximately the same in all cases, Table 1).
The crystal structure of the Pt–Co IMNs in 3%Pt3%Co/
MIL-101(Cr) and their uniform dispersion were confirmed by
3%Pt1%Co/MIL-101(Cr),
3%Pt3%Co/MIL-101(Cr)
and
3%Pt5%Co/MIL-101(Cr) samples were probed by XPS. Survey
XPS spectra are provided in Fig. S3,† and the Pt 4f spectra,
deconvoluted into the different valence states by fitting
Fig. 2 (A) N
2
-adsorption desorption isotherms and (B) pore size distributions for (a) MIL-101(Cr), (b) 3%Pt/MIL-101(Cr), (c) 3%Pt1%Co/MIL-101(Cr),
(
d) 3%Pt3%Co/MIL-101(Cr), (e) used-3%Pt3%Co/MIL-101(Cr) and (f) 3%Pt5%Co/MIL-101(Cr).
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Fig. 4 (a) TEM image and (g) particle size distribution for 3%Pt/MIL-101(Cr); (b) TEM image and (h) particle size distribution for 3%Pt1%Co/MIL-
01(Cr); (c) TEM image, (d) HETEM image, (e) element maps (scale = 300 nm) and (i) particle size distribution for 3%Pt3%Co/MIL-101(Cr); (f) TEM
1
image and (j) particle size distribution for 3%Pt5%Co/MIL-101(Cr).
Gaussian peaks after Shirley-background subtraction (and by
fixing the peak separation and area ratios between the 4f7/2
and 4f5/2 peaks due to spin splitting), are shown in Fig. 5 (the
resulting Pt 4f7/2 peak positions are shown in Table S1†).
This suggests that there is a significant net charge transfer
from Pt to Co in the bimetallic IMPs. The effect reverses
somewhat upon further addition of Co (BE(Pt 4f7/2) = 71.15
eV and 71.05 eV for 3%Pt3%Co/MIL-101(Cr) and 3%Pt5%Co/
MIL-101(Cr), respectively), possibly because of charge
saturation of the Co centers. Perhaps the important
observation here is that the electronic properties of the
metallic Pt atoms can be fine-tuned via the controlled
addition of Co.
0
2
,24,48
According to previous literature,
binding energies (BEs)
for the Pt 4f7/2 feature at 70.8–71.4 eV, 73.5–73.7 eV, and
0
7
5.5–75.7 eV are characteristic of zero-valent metallic Pt (Pt )
2
+
4+
and oxidized Pt and Pt species, respectively.
0
2+
4+
The percentages of Pt , Pt , and Pt surface species in
the samples calculated from these data are summarized in
Table 1. In general, the percentage of metallic Pt remains
0
approximately constant (within experimental error) with
increasing Co content; values between 42% and 47% were
measured for all 3%Pt/MIL-101(Cr), 3%Pt1%Co/MIL-101(Cr),
3
%Pt3%Co/MIL-101(Cr) and 3%Pt5%Co/MIL-101(Cr). On the
4
+
other hand, the fraction of Pt increases with Co content, at
2
+
the expense of the amount of Pt . It would seem that the
presence of Co facilitates the deep oxidation of Pt atoms. It
should be noted, though, that these distributions of Pt
oxidation states correspond to the as-synthesized samples;
they are expected to change upon preconditioning of the
catalysts and after their use for the promotion of
hydrogenation reactions (which are carried out in a strongly
reducing environment) (more on this below). Also noteworthy
0
is that the position of the Pt XPS peak blue-shifts
significantly upon the addition of Co atoms to the IMNs
Fig. 5 Pt 4f XPS spectra of (a) 3%Pt/MIL-101(Cr), (b) 3%Pt1%Co/MIL-
101(Cr), (c) 3%Pt3%Co/MIL-101(Cr), (d) 3%Pt5%Co/MIL-101(Cr), and (e)
used 3%Pt3%Co/MIL-101(Cr).
0
(
BE(Pt 4f /2) = 70.8 eV vs. 71.35 eV for the 3%Pt/MIL-101(Cr)
7
vs. 3%Pt1%Co/MIL-101(Cr) samples, respectively; Table S1†).
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The corresponding Co 2p XPS data are shown in Fig. S4 .† It
orbitals. Clearly, the picture on the modification of the
electronic properties of Pt upon the addition of Co is nuanced
and complex. Nevertheless, it is important in terms of the
is seen there that the Co species are typically in an oxidized
2+
3+
0 48
state on the surface as Co and Co rather than Co .
Unfortunately, given the low signal-to-noise ratio of the spectra,
it is not possible to reliably deconvolve these spectra into their
10,49
tuning of catalytic properties.
NH -TPD experiments were also carried out to characterize
3
2+
3+
Co and Co constituents, and therefore no quantitative
analysis can be provided (a tentative set of fitted curves is
shown in Fig. S4†). However, the data confirm that the cobalt
any potential acidic sites on the surface of the catalysts. The
data are reported in Fig. S6 .† In that figure, the appearance of
broad features in the 150–390 °C range can be assigned to
δ+
3+
species are in a partially charged state (Co ) and that they are
3
desorption of NH bonded to the open Cr Lewis acid sites,
3
6
more oxidized (there are larger components in the spectra for
the higher Co oxidation states) in the samples with higher Co
content. This parallels the trend seen with Pt. However, as
confirming the presence of moderate Lewis acid sites. In
terms of the quantitation of those sites, Table 1 indicates that
the coverage of Lewis acids increases significantly with Co
−1
discussed above, the presence of oxidized CoO surface species
addition, going from 0.40 mmol g on the pure support to
x
−1
because of exposure of the catalysts to air cannot be ruled out;
these data are not representative of the nature of the working
catalyst and can only be used to extract general trends in terms
of ease of oxidation and charge transfer.
0.73 mmol g for 3%Pt1%Co/MIL-101(Cr). However, the Lewis
−1
acid amount only increases to 0.80 mmol g as the Co content
is 5%, which can be ascribed to the significant decrease in the
2
−1
specific surface area from 2296 m
g
for 3%Pt1%Co/MIL-
2
−1
3
.1.6. CO-FTIR and NH
3
-TPD titration of surface sites. The
101(Cr) to 1416 m g for 3%Pt5%Co/MIL-101(Cr) (Table 1).
The shape of the desorption traces also becomes more
complex, indicating the presence of new adsorption sites. It is
quite possible that in addition to the Lewis sites of the support,
new adsorption sites are generated at either the IMPs or,
perhaps more likely, at the interface between the metal
nanoparticles and the support. Because the ammonia uptake
increases with Co content, it appears that it is the Co species
that participate in this new ammonia adsorption.
surface electronic properties of the Pt surface species were also
investigated by characterizing the adsorption of carbon
monoxide, a molecule often used as a probe, using in situ FTIR
spectroscopy (Fig. S5 and Table S2†). As expected, no band for
the linear adsorption of CO atop Pt metal sites, expected
−1
between 2000 and 2090 cm , was observed for the pristine
10
MIL-101(Cr). Instead, two weak bands are evident between
−1
2
100 and 2200 cm , assigned to the physisorption of gaseous
3
6
CO molecules. In contrast, atop CO adsorption is clearly
evident in the spectra with metal nanoparticles. On 3%Pt/MIL-
3.1.7. XANES and EXAFS results. The electronic structure
and local coordination of the Pt species in 3%Pt/MIL-101(Cr)
and 3%Pt3%Co/MIL-101(Cr) were also examined by XANES
and EXAFS. Representative data are shown in Fig. 6. As seen
1
2
01(Cr), the C–O stretching vibration of that species is seen at
−1
068 cm . Interestingly, the IR peak red shifts slightly upon
the addition of Co, although the effect is quite subtle and not
from the normalized Pt L -edge XANES curves shown in
3
−1
fully consistent: no variations larger than 10 cm are seen, and
the sample with the largest Co content shows a reverse in this
trend (Table S2†). This result is at first sight difficult to square
with the XPS results, because a C–O stretching frequency red
shift implies an increase in electron density in the d orbitals
that participate in the π* CO bond. However, it should be
noted that the electron transfer in IMPs tends to involve mainly
the delocalized sp electronic band, not these localized d
Fig. 6A, the 3%Pt/MIL-101(Cr) and 3%Pt3%Co/MIL-101(Cr)
samples show identical absorption threshold energies and
peak shapes, starting at higher energies than that seen in the
reference data obtained for a Pt metal foil. This indicates the
δ+
presence of some of the Pt in oxidized form (Pt species), in
agreement with the XPS results (Fig. 5).
Fig. 6B shows the radial distributions calculated from
analysis of the EXAFS data. They reveal the presence of not
3
Fig. 6 (A) Normalized XANES spectra at the Pt L -edge of (a) 3%Pt/MIL-101(Cr) and (b) 3%Pt3%Co/MIL-101(Cr). (B) Radial distributions calculated
from EXAFS data for (a) 3%Pt/MIL-101(Cr) and (b) 3%Pt3%Co/MIL-101(Cr).
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0
only Pt–Pt bonds (Pt species; RPt–Pt 2.74 ± 0.01 and 2.73 ±
.01 Å for 3%Pt/MIL-101(Cr) and 3%Pt3%Co/MIL-101(Cr),
selectivity to 89% over 3%Pt5%Co/MIL-101(Cr). Similar
trends were seen for the hydrogenation of FFL: as listed in
Table 2, a 97% conversion with a 94% selectivity to FOL was
seen with 3%Pt3%Co/MIL-101(Cr), much higher values than
those measured with the other catalysts.
0
δ+
respectively) but also Pt–O bonds (Pt species, RPt–O = 2.00 ±
.02 Å and 1.98 ± 0.02 Å); this is the case in both 3%Pt/MIL-
01(Cr) and 3%Pt3%Co/MIL-101(Cr) samples. The Pt–Pt
0
1
bonds seem to be slightly shorter than those in the reference
Pt foil, and the coordination numbers much smaller, likely a
reflection of the small size of the IMNs. Furthermore, a small
shoulder peak at 2.7 Å confirms the formation of Pt–Co
Based on the above results, it can be concluded that the
best catalyst in our set for selective hydrogenation is
3%Pt3%Co/MIL-101(Cr). Accordingly, we chose to perform
more detailed kinetic measurements for the selective
hydrogenation of CAL and FFL with that sample.
5
0–52
IMNs.
In addition, the coordination number (CN) may
also be employed to recognize the formation and existence of
the Pt–Co bond. As shown in Table S3,† the CN values for
3.2.2. Effect of reaction time. The hydrogenation of both
CAL and FFL was followed as a function of reaction time by
sampling and analyzing the reaction mixtures after 10, 15,
20, 25 and 30 min of the start of the conversion using the
3%Pt3%Co/MIL-101(Cr) catalyst. These experiments were
performed at different temperatures, at 30, 45, 60, 70 and 80
°C, as discussed in the next section. As can be seen in Fig.
S7–S9,† the primary selectivity towards the unsaturated
alcohols (COL and FOL, respectively) is higher than those of
any of the other products at all times and temperatures.
Examples of the kinetic data acquired in these runs over a
longer time period (several hours) is provided in Fig. 7 for
the hydrogenation of both CAL and FFL at 60 °C. It is seen
there that total conversion of both CAL and FFL is reached
within 3 h, slightly faster with FFL, and that the initial
selectivity toward the unsaturated alcohols (COL and FOL) is
about 90%. The selectivities toward COL (Fig. 7A) and FOL
(Fig. 7B) are quite high at all times, starting above 90%, but
decrease slightly with time because of the subsequent
hydrogenation of those primary products to the saturated
alcohols, as indicated in Scheme 1.
3
%Pt/MIL-101(Cr) and 3%Pt3%Co/MIL-101(Cr) are much
smaller than those in the reference Pt-foil, confirming the
formation of the Pt–Pt bond. Moreover, the CN value (for the
Pt–Pt bond) for the 3%Pt3%Co/MIL-101(Cr) sample is higher
than that for the 3%Pt/MIL-101(Cr) sample. Also, the CN
value for the Pt–O bond is less for 3%Pt3%Co/MIL-101(Cr)
compared with the 3%Pt/MIL-101(Cr) sample, suggesting that
Pt is more reduced in the 3%Pt3%Co/MIL-101(Cr) sample
(consistent with XPS results).
3
.2. Catalytic activity results
.2.1. Activity over 3%Pty%Co/MIL-101(Cr). The selective
3
hydrogenation of CAL to produce COL was chosen as a model
reaction to evaluate the catalytic performance of our different
3
%Pty%Co/MIL-101(Cr) catalysts. In Table 2, we report the
basic kinetic results from such study. The blank kinetic run,
without a catalyst, resulted in almost no conversion of CAL
(data not shown), a result that not only highlights the role of
the catalyst but also confirms that transfer hydrogenation is
insignificant under our reaction conditions. One conclusion
from the data in Table 2 is that the catalytic performance is
significantly influenced by the introduction of Co in the
In the case of CAL, the hydrogenation of COL to the
corresponding saturated alcohol (hydrocinnamyl alcohol,
HCOL) is much easier than in the case of the secondary
hydrogenation of the FOL produced via hydrogenation of FFL
(Fig. 7), which indicates that FOL is more stable than COL
under our reaction conditions. A 2 h reaction time was
selected for the subsequent studies to minimize the effect of
this secondary reaction. Critically, no significant amounts of
the saturated aldehyde, the competing primary product, were
detected at any time during the course of these reactions.
3.2.3. Effect of reaction temperature over 3%Pt3%Co/MIL-
101(Cr). The effect of reaction temperature on the CAL and FFL
hydrogenation was investigated in the 30–80 °C range under a 1
3
%Pty%Co/MIL-101(Cr) catalysts. Indeed, while the 3%Pt/
MIL-101(Cr) catalyst yields a 53% conversion of CAL with
0% selectivity toward COL after 5 h under 1 MPa hydrogen
2
and at 60 °C, those numbers increase significantly, to 73%
and 86%, respectively, after only 2 h with the 3%Pt1%Co/
MIL-101(Cr) catalyst, and to 95% and 91% in 2 h over
3
%Pt3%Co/MIL-101(Cr) (under similar reaction conditions).
The 3%Pt3%Co/MIL-101(Cr) catalyst reaches the optimum
performance as the activity declines to 67% and the
Table 2 Catalytic performance of our catalysts for the conversion of CAL and FFL
Selectivity (%)
CAL
e
FFL
conversion (%)
FOL
selectivity (%)
TOF
1
−
Catalyst
conversion (%)
COL
HCAL
HCOL
Others
(h )
a
c
3
3
3
3
%Pt/MIL-101(Cr)
53
73
95
67
20
86
91
89
59
5
6
15
8
2
6
1
1
7
37
69
97
69
66
83
94
90
111
397
546
377
b
d
%Pt1%Co/MIL-101(Cr)
%Pt3%Co/MIL-101(Cr)
%Pt5%Co/MIL-101(Cr)
b
d
b
d
3
2
a,b
c,d
a
b
Reaction conditions: 60 °C, 1 MPa H
2
pressure, 0.2525 g CAL /FFL , 20 mg catalyst, 15 mL isopropanol. 5 h reaction times. 2 h reaction
c
d
e
times. 3 h reaction times. 2 h reaction times. TOF = [moles of COL yield]/[(moles of Pt loading) × (reaction time) × (% dispersion/
molecular weight of Pt)].
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Fig. 7 Selective hydrogenation of (A) CAL and (B) FFL as a function of reaction time over the 3%Pt3%Co/MIL-101(Cr) catalyst (reaction conditions:
0 °C, 1 MPa H pressure, 0.2525 g CAL, 0.2525 g FFL, 20 mg catalysts, 15 mL isopropanol, 800 rpm).
6
2
MPa H
2
pressure. The results, presented in Fig. 8, indicate the
unsaturated alcohols, i.e., toward COL and FOL, is >90% in the
whole 30–60 °C range. That selectivity declines slightly at the
higher reaction temperatures (70 and 80 °C), but such a trend
can again be explained by an enhancement in the further
hydrogenation of the primary products, as discussed above.
excellent catalytic activity of 3%Pt3%Co/MIL-101(Cr) in this
entire temperature range, even at room temperature (30 °C). As
expected, increasing the reaction temperature leads to an
increase in CAL (Fig. 8A) and FFL (Fig. 8B) conversions (in this
case reported for a fixed 2 h reaction time), as in any activated
reaction. Importantly, the selectivity toward the desired
Additional
time-dependent
kinetic
studies
were
performed for the CAL hydrogenation at five different
Fig. 8 Conversion and selectivity after 2 h for the hydrogenation of (A) CAL and (B) FFL vs. temperature over 3%Pt3%Co/MIL-101(Cr). (C) Plot
of ln C vs. time for the conversion of CAL at different temperatures. (D) Arrhenius plot of the first-order rate constants extracted from the data
shown in (C).
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temperatures i.e. 30, 45, 60, 70 and 80 °C, the results of
which are shown in Fig. 8C and S7.† As seen in Fig. 8C,
semilogarithmic plots of the percentage of conversion versus
time show good linear relationships, a hallmark of first-
order kinetics. This is in agreement with past literature
3.2.5. Reusability and recyclability of 3%Pt3%Co/MIL-
101(Cr). One of the most important concerns with
heterogeneous catalysts is their stability and reusability. To
assess this issue, recycling experiments were carried out for
the hydrogenation of CAL over 3%Pt3%Co/MIL-101(Cr) at 60
3
,40
reports.
Subsequent analysis of the resulting reaction
2
°C and 1 MPa H . The data are shown in Fig. 10. No
rate constants (k), which are plotted in an Arrhenius form
appreciable loss in activity or selectivity was seen after up to
five runs (the conversion decreased slightly in the fifth run).
Further characterization by XPS of the 3%Pt3%Co/MIL-
101(Cr) after five cycles indicated a significant increase in
(
ln(k) versus 1/T) in Fig. 8D, yielded a value for the apparent
−
1
a
activation energy (E ) of 18 kJ mol . There are very limited
studies on the estimation of Ea for the hydrogenation of
CAL to use a reference: our value, obtained with the
0
the fraction of metallic Pt on the surface of the IMNs (from
3
%Pt3%Co/MIL-101(Cr) catalyst, is comparatively lower than
47% to 55%; data for used 3%Pt3%Co/MIL-101(Cr), Fig. 5
and Table 1), a result that is easily understood by the fact
that the reaction conditions are highly reducing. The large
percentage of Pt that is in a reduced state under the
−
1
that for 1.5% Pd–WN/SBA-15 (23.2 kJ mol ), Pt/BN (35 kJ
mol ), Pt/MIL-101 (42.8 kJ mol ), or Pd–0.3Ag/MCM-41 (75
−
1
−1
−
1 3,32,40,53
kJ mol ).
%Pt3%Co/MIL-101(Cr) can reduce the activation energy
barrier for the CAL hydrogenation reaction.
.2.4. Effect of hydrogen pressure. The effect of H
It would appear that the addition of Co in
0
3
reaction conditions suggests that metallic Pt species (Pt )
may play a pivotal role in the hydrogenation reaction. This
conclusion is tentative, however, as the XPS analysis was
carried out ex situ; exposure of the catalyst to ambient
conditions after reaction can affect these results.
3
2
partial
pressure was also investigated: the H2 pressure was varied
from 0.6 MPa to 1.2 MPa in kinetic studies of the
hydrogenation of both CAL and FFL with 3%Pt3%Co/MIL-
1
01(Cr) at relatively low temperatures. It was found that the
overall conversion of both CAL and FFL is linearly enhanced
with increases in the H partial pressure at both 30 (Fig.
3
.3. Relationship between structure and catalytic performance
2
The hydrogenation of α,β-unsaturated aldehydes, i.e. CAL and
FFL, was carried out in the liquid phase for the optimization
of the performance of HF-free MIL-101(Cr)-supported Pt/Pt–
Co IMN catalysts. As described above, selectivity toward COL
is poor when using the monometallic 3%Pt/MIL-101(Cr)
catalyst, but it is significantly enhanced, to above 90%, over
the 3%Pty%Co/MIL-101(Cr) catalysts (Table 2). These changes
in catalytic performance can be related to the nature of the
surface Pt species as they are modified by the addition of Co
as well as to the special structure of the MIL-101(Cr) support.
The various characterization results indicated the formation
of well-mixed and well-dispersed Pt–Co IMNs. Moreover, a
strong synergistic effect was identified on the MIL-101(Cr)-
based catalysts between the two metals, resulting in a net
charge transfer, the extent of which can be tuned by tuning
the Pt : Co ratio.
S11B and S10B†) and 60 °C (Fig. 9), while the selectivity
toward COL and FOL is maintained above 90%. In the case
of the CAL hydrogenation, the selectivity toward HCOL is also
enhanced slightly at higher H partial pressures (1.2 MPa at
2
6
0 °C, Fig. 9A), pointing to the limitation of using high
pressures to enhance activity in these reactions. Luckily, the
%Pt3%Co/MIL-101(Cr) catalyst shows acceptable activity
3
and excellent selectivity for CAL and FFL hydrogenation to
the corresponding unsaturated alcohols (COL and FOL)
under mild reaction conditions such as 1 MPa H2 pressure
and 60 °C. Table S4† provides data for other Pt-based
bimetallic (or IMN) catalysts used in the past for CAL
selective hydrogenation for comparison: the catalytic
performance of our 3%Pt3%Co/MIL-101(Cr) is comparable to
or better than most of those other examples.
Fig. 9 Selective hydrogenation of (A) CAL and (B) FFL over 3%Pt3%Co/MIL-101(Cr) vs. H
2
pressure. Reaction conditions: 60 °C, 0.2525 g CAL,
0
.2525 g FFL, 20 mg catalysts, 15 mL isopropanol, 2 h, 800 rpm.
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lone electron pair at the oxygen atom (electron-donating group).
In fact, it is interesting to note that the NH -TPD results (Fig.
3
S6† and Table 1) also identify additional base (ammonia)
uptake upon the addition of Co to the IMPs, suggesting that
new interfacial sites may add to the improved performance of
the catalyst. In summary, our 3%Pty%Co/MIL-101(Cr) catalysts
provide both the electronically modified Pt sites and the Lewis
acid sites that have been proposed to enhance activity in
unsaturated aldehyde hydrogenation reactions. It may also
display new IMN–support sites to better direct adsorption and
selective reactivity.
Additionally, the metal nanoparticles confined within the
narrow pores/channels of the MOF facilitate a strong steric
Fig. 10 CAL hydrogenation performance vs. recycling run number
10,45
hindrance (steric effect).
The Pt–Co IMNs confined within
with 3%Pt3%Co/MIL-101(Cr) (reaction conditions: 60 °C, 1 MPa H
pressure, 0.2525 g CAL, 20 mg catalyst, 15 mL isopropanol, 2 h,
2
the pores of MIL-101(Cr) generate a strong steric effect and
change the adsorption geometry of α,β-unsaturated aldehyde
molecules (CAL and FFL) over 3%Pty%Co/MIL-101(Cr). The
α,β-unsaturated aldehyde molecules adsorbed on the surface
of Pt–Co IMNs confined within the pores of MIL-101(Cr)
vertically via the CO bond end (located at the tail end)
while they are sterically hindered the horizontal adsorption
via the CC bond end (located in the middle). Thus, the
CO bond reaches the active sites on the catalysts, which is
selectively activated and hydrogenated.
Hence, due to the combined synergistic effects of the
electronically modified Pt sites and the Lewis acid sites, and
the steric effect, the catalytic performance as well as the
selectivity toward unsaturated alcohols (COL and FOL) is
superior over 3%Pt3%Co/MIL-101(Cr) catalysts (Table 2).
800 rpm).
Generally, in heterogeneous IMN catalyst systems, it has
been proposed that electron transfer from the second
(
electropositive) metal to the first (the main active component)
10,13,54,55
may enhance the catalytic performance of the latter.
As
a result, the electron-enriched active metal shows less probability
towards CC bond adsorption (because of d-band electron
repulsive forces) and exhibits a decreased heat of adsorption as
well as a lower activation barrier for CO bond activation.
These two trends result in the enhanced hydrogenation of the
56–60
CO bond.
In our specific case, the data reported above
from the various characterization studies indicate a strong
interaction between Pt and Co atoms in 3%Pty%Co/MIL-
Conclusion
101(Cr). The charge transfer that occurs between the two is,
however, somewhat nuanced and difficult to fully understand.
On the one hand, the XPS signal for the metallic Pt(0)
component shifts toward higher BEs upon the addition of Co, a
sign of a loss of electron density. On the other hand, the
addition of Co also results in more extensive oxidation (more
Pt formation). There is a minor effect on the strength of
adsorption of carbon monoxide as well, as identified by the IR
data. In the end, all this characterization was carried out ex situ,
under conditions much different from those used during
catalysis. In fact, the XPS data for the used catalysts showed that
much of the initially oxidized Pt becomes reduced after
A series of Pt/Pt–Co nanoparticles supported on MIL-101(Cr)
(3%Pt/MIL-101(Cr), 3%Pty%Co/MIL-101(Cr)) were synthesized
via a polyol reduction method and used to promote the
selective hydrogenation of CAL and FFL. It was shown that
selectivity toward COL and FOL production can be
successfully tuned by designing a homogeneously dispersed
Pt–Co intermetallic nanoparticle system confined within the
pores of MIL-101(Cr). From the collection of catalysts made
here with various Pt : Co content ratios, the 3%Pt3%Co/MIL-
101(Cr) catalyst turned out to exhibit the best catalytic
performance for the hydrogenation of CAL and FFL, better
than that of the rest of the catalysts tried here and also
relative to other bimetallic catalysts reported in the literature.
4+
2
exposure to high H pressures. What is clear is that the addition
of Co to Pt does affect its electronic structure (and its ability to
be reduced) and that that in turn affects catalytic performance.
Although arguments can be put forward about how these charge
transfers may favour CO versus CC adsorption, a more
specific model on how that takes place may require electronic
characterization of the IMNs in situ under reaction conditions.
The other factor that may help with catalytic performance in
CO-FTIR, XPS, XANES, EXAFS and NH -TPD studies revealed
3
that Co addition to Pt-based catalysts can help tune the
surface electronic structure of the Pt active sites via the
formation of Pt–Co intermetallic nanoparticles and also
enhance the surface Lewis acidity of the catalyst. The
formation of additional Lewis acid sites at the MOF–metal
interfaces provides a third factor that helps with the
improved catalytic performance. The current work opens a
new exciting way to attain high selectivity in hydrogenation
reactions toward unsaturated alcohols by pairing the use of
well-defined supports based on metal organic frameworks
3
+
our samples is the presence of surface Lewis acid, i.e., the Cr
centers in MIL-101(Cr).
36,40,61
3+
The Cr of MIL-101(Cr) and the
electron-modifying Co metal species can act as electrophilic or
Lewis acid sites for the adsorption, polarization, and activation
of the electron-rich CO bond (which is a Lewis base) via their
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MIL-101(Cr)) with the dispersion of intermetallic
nanoparticles to rely on the synergistic interaction between
precious noble metals and other less precious transition
metals confined within the pores of MOFs.
Catalysis Science & Technology
(
20 L. Yang, Z. Jiang, G. Fan and F. Li, Catal. Sci. Technol.,
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2 Z. Guo, Y. Chen, L. Li, X. Wang, G. L. Haller and Y. Yang,
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Conflicts of interest
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3 I. Ro, I. B. Aragao, Z. J. Brentzel, Y. Liu, K. R. Rivera-Dones,
M. R. Ball, D. Zanchet, G. W. Huber and J. A. Dumesic, Appl.
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The authors declare no competing financial interest.
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4 X. Wang, Y. He, Y. Liu, J. Park and X. Liang, J. Catal.,
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YZ acknowledges the Natural Sciences Fund of Heilongjiang
Province (LH2020B021). FZ acknowledges funding from the US
National Science Foundation under grant no. CHE-1953843.
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6 Y. Chen, C. Li, J. Zhou, S. Zhang, D. Rao, S. He, M. Wei,
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