Ir nanoclusters confined within hollow MIL-101(Fe) for selective hydrogenation of α,β-unsaturated aldehyde

Article abstract of DOI:10.1016/j.cclet.2021.06.047

Although the selective hydrogenation of α,β-unsaturated aldehyde to unsaturated alcohol (UOL) is an extremely important transformation, it is still a great challenge to achieve high selectivity to UOL due to thermodynamic favoring of the C[dbnd]C hydrogenation over the C[dbnd]O hydrogenation. Herein, we report that iridium nanoclusters (Ir NCs) confined within hollow MIL-101(Fe) expresses satisfied reaction activity (93.9%) and high selectivity (96.2%) for the hydrogenation of cinnamaldehyde (CAL) to cinnamyl alcohol (COL) under 1 bar H₂ atmosphere and room temperature. The unique hollow structure of MIL-101(Fe) benefits for the fast transport of reactant, ensuring the comparable reaction activity and better recyclability of Ir@MIL-101(Fe) than the counterparts which Ir NCs were on the surface of MIL-101(Fe). Furthermore, The X-ray photoelectron spectroscopy data indicates the electropositive Ir NCs, owing to the electron transfer from Ir to MIL-101(Fe), can interact with oxygen lone pairs, and Fourier transform infrared spectrum shows the Lewis acid sites in MIL-101(Fe) can strongly interact with C[dbnd]O bond, which contributes to a high selectivity for COL. This work suggests the considerable potential of synergetic effect between hollow MOFs and metal nanoclusters for selective hydrogenation reactions.

Full text of DOI:10.1016/j.cclet.2021.06.047

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Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
Communication
Ir nanoclusters confined within hollow MIL-101(Fe) for selective
hydrogenation of α,β-unsaturated aldehyde
Qinglin Liua,1, Qian Liua,1, Yurong Chen , Yinle Li , Hui Su , Qinghua Liu , Guangqin Li
a,∗
a
a
b
b
a
MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-Sen University, Guangzhou
510275, China
b
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230026, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Although the selective hydrogenation of α,β-unsaturated aldehyde to unsaturated alcohol (UOL) is an
extremely important transformation, it is still a great challenge to achieve high selectivity to UOL due
to thermodynamic favoring of the C=C hydrogenation over the C=O hydrogenation. Herein, we report
that iridium nanoclusters (Ir NCs) confined within hollow MIL-101(Fe) expresses satisfied reaction ac-
tivity (93.9%) and high selectivity (96.2%) for the hydrogenation of cinnamaldehyde (CAL) to cinnamyl
alcohol (COL) under 1 bar H2 atmosphere and room temperature. The unique hollow structure of MIL-
101(Fe) benefits for the fast transport of reactant, ensuring the comparable reaction activity and better
recyclability of Ir@MIL-101(Fe) than the counterparts which Ir NCs were on the surface of MIL-101(Fe).
Furthermore, The X-ray photoelectron spectroscopy data indicates the electropositive Ir NCs, owing to the
electron transfer from Ir to MIL-101(Fe), can interact with oxygen lone pairs, and Fourier transform in-
frared spectrum shows the Lewis acid sites in MIL-101(Fe) can strongly interact with C=O bond, which
contributes to a high selectivity for COL. This work suggests the considerable potential of synergetic effect
between hollow MOFs and metal nanoclusters for selective hydrogenation reactions.
Received 31 March 2021
Revised 8 May 2021
Accepted 17 June 2021
Available online xxx
Keywords:
Iridium
Hollow metal−organic frameworks
Selective hydrogenation
α,β-Unaturated aldehyde
Heterogeneous catalysis
©
2021 Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia
Medica, Chinese Academy of Medical Sciences.
The selective hydrogenation of α,β-unsaturated aldehyde (UAL)
to unsaturated alcohol (UOL) is a significant process in petrochem-
icals, pharmaceuticals and fragrances [1-4]. Nevertheless, since the
production of saturated aldehydes is more favorable over unsat-
urated alcohols from the view of thermodynamics [5,6], it is ex-
tremely difficult to achieve high selectivity to UOL. Significant ef-
forts have been devoted towards the development of highly UOL-
selective catalysts [7-13], among which a great amount of noble
metal based catalysts for selective hydrogenation of UAL have been
made great progress. Among the noble metals, iridium (Ir) pos-
sesses the highest intrinsic selectivity to UOL [14]. However, pure
Ir nanoparticles still showed unsatisfied selectivity in selective hy-
drogenation of UAL. In addition, the surface energy of metal nan-
oclusters (NCs) is very high, pure metal NCs are easy to aggregate
in the reaction process, leading to a loss of activity [15-18].
for the reaction substrates/products [19-26]. Moreover, compared
with other porous materials such as zeolites and porous carbons,
MOFs contain a large number of metal centers, which may in-
teract with reactants to improve selectivity or catalyze the re-
action directly [27-35]. Tang’s group synthesized MIL-101(Fe)@Pt
and achieved 86.4% selectivity to UOL, further suggested that MIL-
101(Fe) can serve as effective selectivity regulator [28]. Hence, with
MIL-101(Fe) as a protective layer and selectivity regulator, Ir NCs
hybridized with MIL-101(Fe) were expected to exhibit an excel-
lent activity and selectivity for hydrogenation of UAL to obtain high
yield of UOL.
Herein, we have synthesized the hollow MIL-101(Fe) on pre-
formed Ir NCs to generate Ir@MIL-101(Fe) composite. The obtained
Ir@MIL-101(Fe) exhibits excellent performance in selective hydro-
genation of a model molecule of α,β-unsaturated aldehyde, cin-
namaldehyde (CAL), with 93.9% conversion in 4 h and 96.2% se-
lectivity towards cinnamyl alcohol (COL), as well as high durability
after 5 successive cycles, all which are much better than those of
Ir NCs and physical mixture Ir/MIL-101(Fe) catalysts. On one hand,
the Lewis acid sites in MIL-101(Fe) are capable to interact with the
aldehyde group in CAL and electropositive Ir NCs might preferen-
tial absorb the electronegative oxygen atom in C=O group, both of
which can promote the selectivity for COL. On the other hand, the
Recently, metal-organic frameworks (MOFs) supported metal
nanoparticles have attracted tremendous research interest because
the micropores of MOFs can not only prevent the nanoparticles
from aggregation and growth but also serve as a transfer path
∗
Corresponding author.
E-mail address: liguangqin@mail.sysu.edu.cn (G. Li).
1
These authors contributed equally to this work.
https://doi.org/10.1016/j.cclet.2021.06.047
1001-8417/© 2021 Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Please cite this article as: Q. Liu, Q. Liu, Y. Chen et al., Ir nanoclusters confined within hollow MIL-101(Fe) for selective hydrogenation of
α,β-unsaturated aldehyde, Chinese Chemical Letters, https://doi.org/10.1016/j.cclet.2021.06.047

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face of Ir@MIL-101(Fe) was a little bit rough compared with MIL-
01(Fe) (Figs. S2b and S3 in Supporting information), owing to the
1
existence of Ir NCs may slightly affecting the nucleation process
of MOFs. The HRTEM images showed a lattice fringe of 0.22 nm
corresponding to the (111) plane of the Ir metallic, and no observ-
able aggregation on the external surface of MIL-101(Fe) (Figs. 1a
and b). To test the element distribution of Ir@MIL-101(Fe), energy-
dispersive X-ray spectroscopy (EDS) elemental mappings were car-
ried out, and the Ir element was found to be mainly distributed
on the near-surface of MIL-101(Fe) (Fig. 1c), which might shorten
the diffusion distance of reactants on the surface of MIL-101(Fe) to
the metal active site and thus facilitate the reaction rate. The ac-
tual Ir content in Ir@MIL-101(Fe) was quantitatively determined to
be 5.3 wt% by inductively coupled plasma mass spectrometry (ICP-
MS), listed in Table S1 (Supporting information). According to the
N2 adsorption isotherms (Fig. S4a in Supporting information), the
Brunauer–Emmett–Teller (BET) surface area and total pore volume
Scheme 1. Synthetic route to generating hollow Ir@MIL-101(Fe).
2
3
of Ir@MIL-101(Fe) decreased to 42.2 m /g and 0.09 cm /g, com-
2
3
pared to pure MIL-101(Fe) (664.1 m /g and 0.38 cm /g), attributed
to the formation of mesoporous in MIL-101(Fe). Furthermore, the
pore-size distribution curve in Fig. S4b (Supporting information)
also displayed the existence of mesoporous [30,37]. These results
confirmed that most of the Ir NCs were indeed encapsulated in-
side the MOFs [38]. Moreover, Ir@MIL-101(Fe) with different hy-
drothermal times were synthesized. As shown in Fig. S5 (Support-
ing information), with the increase of hydrothermal time, the inner
part of MIL-101(Fe) gradually became transparent, indicating hol-
low MOFs were formed. Since there is PVP on the surface of Ir NCs,
PVP will dissolve in the growth solution during the hydrothermal
reaction, and then act as metal-coordinating bulky polymer dur-
ing the formation of MIL-101(Fe) domains to induce partial dis-
ruption of imidazole-Fe(III) catenation process and generate high
mesoporosity in the resulting MIL-101(Fe) [39], which was consis-
tent with pore-size distribution curve of Ir@MIL-101(Fe) (Fig. S4b
in Supporting information).
To demonstrate the generality of our approach, Rh NCs confined
within MIL-101(Fe) were also synthesized. The synthesis method
of Rh@MIL-101(Fe) was similar to that of Ir@MIL-101(Fe), in which
MIL-101(Fe) was in situ growth with the Rh NCs in the precur-
sor solution to build hollow Rh@MIL-101(Fe) structures. The XRD
patterns, SEM and HRTEM images, and N2 adsorption isotherms of
Rh@MIL-101(Fe) were shown in Figs. S6, S7 and S10 (Supporting
information). Resembled to that of Ir@MIL-101(Fe), the BET surface
areas of Rh@MIL-101(Fe) (Table S2 in Supporting information) was
far less than those of the parent MIL-101(Fe), due to the forma-
tion of mesoporous MIL-101(Fe), which illustrated the generality of
our new strategy for preparing the hollow metal@MIL-101(Fe) cat-
alysts. In order to explore the role of MOFs, Ir@UIO-66(Zr) was syn-
thesized for comparison [40]. The detailed information of Ir@UIO-
66(Zr) was shown in Figs. S8-S10 (Supporting information).
Fig. 1. (a, b) HRTEM images of Ir@MIL-101(Fe) samples. (c) HAADF-STEM image and
the corresponding elemental mapping images of Ir@MIL-101(Fe).
special hollow structure of MIL-101(Fe) shortens the diffusion dis-
tance of reactants and remained the excellent hydrogenation.
As schematically illustrated in scheme 1, the Ir@MIL-101(Fe)
was synthesized by two processes. Firstly, the metal precursor salt
was reduced using PVP as stabilizer and ethylene glycol as re-
ductant and solvent to obtain Ir NCs, with the average size of
Selective hydrogenation of α,β-unsaturated aldehyde is very
important in chemical industry. The hydrogenation of an α,β-
unsaturated aldehyde, CAL, is selected to explore the possible cat-
alytic performance of Ir@MIL-101(Fe). As illustrated in Fig. 2a, CAL
is hydrogenated to COL or hydrocinnamaldehyde (HCAL), and sub-
sequently further hydrogenated to hydrocinnamyl alcohol (HCOL).
Here, the hydrogenation of CAL was carried out under 1 bar H2
pressure at room temperature. The conversion of CAL and selectiv-
ity for COL as a function of reaction time over Ir@MIL-101(Fe) were
shown in Fig. 2b. After 4 h reaction, the conversion of CAL achieved
93.9%, with high selectivity of desired product COL (96.2%). When
the reaction time was prolonged, the overhydrogenated product
HCOL could not be observed, confirming the extraordinary prod-
uct selectivity. As shown in Table S3 (Supporting information), the
selectivity of Ir@MIL-101(Fe) is better than that of the Ir-based cat-
alyst reported previously, which is comparable to the most ad-
1.7 nm (Fig. S1 in Supporting information). Then the MIL-101(Fe)
was grown in situ with the addition of Ir NCs in the precursor
solution to obtain Ir NCs confined into hollow MIL-101(Fe) [29].
The structures and morphologies of as-prepared specimens were
examined by powder X-ray diffraction (XRD), scanning electron
microscopy (SEM) and high-resolution transmission electron mi-
croscopy (HRTEM). As shown in Fig. S2a (Supporting information),
the XRD pattern of Ir@MIL-101(Fe) matched well with the MIL-
101(Fe), no clear diffraction peaks of the Ir NCs were observed,
due to the ultrafine size of the nanoclusters, which was consistent
with HRTEM images (Fig. 1a) [30,36]. As for morphology, the sur-
2

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Fig. 3. XPS profiles of Ir and Fe in different catalysts. (a) Ir 4f level. (b) Fe 2p level.
The catalysts investigated were pure Ir NCs, MIL-101(Fe) and Ir@MIL-101(Fe). (c)
HCAL and COL hydrogenation performances of Ir@MIL-101(Fe). (d) FT-IR spectra for
CAL adsorption on Ir@MIL-101(Fe).
and Ir/MIL-101(Fe) outperformed pure Ir NCs (Fig. S12), which may
be attributed to hydrogen spillover effect on the surface of MIL-
101(Fe) [44]. To investigate the universality of Ir@MIL-101(Fe), hy-
drogenation of different α,β-unsaturated aldehydes were tested. As
shown in Table 1, almost all the substrates can reach over 90%
selectivity to corresponding UOL. Above results fully suggest that
Ir@MIL-101(Fe) is an excellent catalyst for selective hydrogenation
of α,β-unsaturated aldehydes.
Since surface chemical structure of catalysts have a considerable
influence on the catalytic behavior [45,46], the electronic proper-
ties of Ir and Fe were characterized by X-ray photoelectron spec-
troscopy (XPS). Ir 4f spectrum of Ir@MIL-101(Fe) and Ir NCs clearly
demonstrated that most of Iridium remained Ir0 valance but a por-
Fig. 2. (a) Schematic CAL hydrogenation. CAL hydrogenation performances of (b)
Ir@MIL-101(Fe) and (c) Ir/MIL-101(Fe). (d) Product selectivity (Sel.) and conversion
(
Conv.) at 4 h for the CAL hydrogenation over different catalysts. (e) Recyclability
of Ir@MIL-101(Fe) and (f) Ir/MIL-101(Fe) for hydrogenation of CAL. Reaction condi-
tion: Cinnamyl aldehyde (0.1 mmol), catalyst (15 mg), isopropanol (2.5 mL), water
(2.5 mL), 30 °C, 1 bar H2 atmosphere, 4 h.
vanced catalyst reported recently. To investigate whether the Ir ac-
tive site was decreased after covering MOFs, Ir/MIL-101(Fe) was
prepared by just mechanically mixing the pre-synthesized Ir NCs
and MIL-101(Fe) (Fig. S11 in Supporting information). As observed
in Fig. 2c, the similar catalytic activity of Ir/MIL-101(Fe) compared
to Ir@MIL-101(Fe) inferred that the Ir NCs inside hollow MIL-
tion of Ir3 material existed on the surface, due to the oxidation
in the air (Fig. 3a) [47]. The Fourier transforms of extended X-
ray absorption fine structure (FT-EXAFS) further verified the ex-
istence of Ir metallic and oxide, since the peak of Ir-Ir bond and
+
Ir-O bond were clearly found in the Ir L -edge (Fig. S13 in Sup-
3
1
01(Fe) maintained instinct catalytic ability, which attributed to
porting information). Compared with the binding energy of Ir0 in
Ir NCs (63.1 eV, 4f5/2), the higher binding energy in Ir@MIL-101(Fe)
(64.1 eV, 4f5/2) indicated the lower electron density of Ir after con-
fining within the hollow MIL-101(Fe). As for the binding energy
the fasten transport speed and the shorten diffusion distance of re-
actants to the metal active site [36,41]. Moreover, Ir@MIL-101(Fe)
and Ir/MIL-101(Fe) gave the higher selectivity for COL over pure
Ir NCs (47.2%), Rh@MIL-101(Fe) (40.3%) and Ir@UIO-66(Zr) (4.6%)
of Fe3 in Ir@MIL-101(Fe), the peak of Fe 2p3/2 obviously shifted
from 716.0 eV to 714.5 eV after immobilization of Ir NCs, confirmed
the electron transfer from Ir to MIL-101(Fe) (Fig. 3b). Since the O
atom in C=O was electronegative, electropositive Ir NCs in Ir@MIL-
101(Fe) may be able to absorb the C=O bond to activate it [48,49],
resulting in high selectivity towards COL.
+
(
Fig. 2d). Besides, less than 1% conversion of CAL was detected
when catalyzed by MIL-101(Fe), which could be considered as a
negligible influence on the hydrogenation reaction. The detailed
catalysis results were listed in Table S4 (Supporting information).
Above results indicated that synergistic effect of Ir NCs and the
MIL-101(Fe) support may be the key factor for high yield of COL.
For the recyclability was another critical factor to evaluate the
catalytic performance of catalysts, the stability tests of Ir@MIL-
In order to deeply investigate the reason for excellent COL se-
lectivity, hydrogenation using COL and HCAL as the substrate were
performed over Ir@MIL-101(Fe). As expected, the hydrogenation
rate of C=O bond was clearly faster than that of C=C bond (Fig. 3c),
attributing to enhance the selectivity of COL and avoid the over
hydrogenation to a great extent. To find out the mechanism for se-
lective hydrogenation of CAL, the Fourier transform infrared (FT-
IR) spectroscopy was used to examine the adsorption of CAL on
Ir@MIL-101(Fe). As shown in Fig. 3d, FT-IR survey showed an ob-
101(Fe) and Ir/MIL-101(Fe) for selective hydrogenation of CAL were
carried out. As seen in Fig. 2e, no appreciable loss in activity
and selectivity was observed in Ir@MIL-101(Fe) in up to five runs.
On the contrary, when it came to the third cycle, the activity
of Ir/MIL-101(Fe) declined sharply and the selectivity dropped to
8
1.0% (Fig. 2f). This is because the Ir active sites in Ir/MIL-101(Fe)
probably leached and aggregated during the reaction [42], while
the Ir NCs confined within MIL-101(Fe) were immune to this prob-
lem, leading to great recyclability. It is worth mentioning that con-
fining Ir NCs within MIL-101(Fe) cavities did not sacrifice their ac-
tivity, since the turnover frequency (TOF) values of Ir@MIL-101(Fe)
and Ir/MIL-101(Fe) were similar (Fig. S12 in Supporting informa-
tion). The mesopores in MIL-101(Fe) provided enough space for the
transport and diffusion of substrates and products, thus ensuring
the efficient reaction [39]. Another factor attributed to high activ-
ity was that Ir NCs were mainly loaded in the near surface of MIL-
vious redshift of the υC
=
bond of CAL after mixing with Ir@MIL-
O
101(Fe), confirming strong interaction between the C=O bond of
CAL and Ir@MIL-101(Fe) [28,50]. Similar adsorption behaviors were
also observed when mixing CAL with Ir/MIL-101(Fe) and MIL-
101(Fe) (Fig. S14 in Supporting information). These revealed that
the synergistic effect of electropositive Ir NCs and the aldehyde ac-
tivator MIL-101(Fe) was key to preferentially hydrogenate aldehyde
group and maintaining high selectivity towards COL.
In summary, we have successfully developed a new method
for confining metal NCs within hollow MOFs via in-situ growth of
MOFs with metal NCs in the precursor solution. Interestingly, as-
prepared Ir@MIL-101(Fe) exhibited impressive activity (93.9%), se-
lectivity (96.2%) as well as great recyclability for the hydrogena-
tion of CAL to COL under mild conditions. The electropositive Ir
1
01(Fe), which afforded a short diffusion distance of the reactants
from the MIL-101(Fe) surface to the highly exposed NCs active sites
43]. In addition, though some active sites of Ir NCs were covered
when loaded in or on the MIL-101(Fe), the TOF of Ir@MIL-101(Fe)
[
3

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Table 1
Hydrogenation of α,β-unsaturated aldehyde catalyzed by Ir@MIL-101(Fe).
Entry
Substrate
Time (h)
Conv. (%)
90.2
Sel. of UOL (%)
Yield of UOL (%)
1
4
98.0
88.4
2
3
5
8
94.1
88.7
83.5
>99.9
>99.9
>99.9
4
8
92.7
99.0
91.8
5
6
8
8
83.1
83.2
>99.9
83.1
81.4
97.8
7
8
97.9
90.8
88.9
Reaction condition: Substrate (0.1 mmol), catalyst (15 mg), isopropanol (2.5 mL), water (2.5 mL), 30 °C,
bar H2 atmosphere.
1
NCs, owing to the electron transfer from Ir to MIL-101(Fe), prefer
to interact with oxygen lone pairs of aldehyde group and the Lewis
acid sites in MIL-101(Fe) can strongly interact with C=O bond, both
factors are attributed to high selectivity to COL. This work provides
an efficient strategy to prepare promising materials for enhanced
selective hydrogenation of α,β-unsaturated aldehyde.
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