Polydimethylsiloxane Coating for a Palladium/MOF Composite: Highly Improved Catalytic Performance by Surface Hydrophobization

Article abstract of DOI:10.1002/anie.201600497

Surface wettability of active sites plays a crucial role in the activity and selectivity of catalysts. This report describes modification of surface hydrophobicity of Pd/UiO-66, a composite comprising a metal–organic framework (MOF) and stabilized palladium nanoparticles (NPs), using a simple polydimethylsiloxane (PDMS) coating. The modified catalyst demonstrated significantly improved catalytic efficiency. The approach can be extended to various Pd nanoparticulate catalysts for enhanced activity in reactions involving hydrophobic reactants, as the hydrophobic surface facilitates the enrichment of hydrophobic substrates around the catalytic site. PDMS encapsulation of Pd NPs prevents aggregation of NPs and thus results in superior catalytic recyclability. Additionally, PDMS coating is applicable to a diverse range of catalysts, endowing them with additional selectivity in sieving reactants with different wettability.

Full text of DOI:10.1002/anie.201600497

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Communications
Chemie
International Edition: DOI: 10.1002/anie.201600497
German Edition: DOI: 10.1002/ange.201600497
Metal–Organic Framework Composites
Polydimethylsiloxane Coating for a Palladium/MOF Composite:
Highly Improved Catalytic Performance by Surface Hydrophobization
Gang Huang, Qihao Yang, Qiang Xu, Shu-Hong Yu, and Hai-Long Jiang*
Abstract: Surface wettability of active sites plays a crucial role
in the activity and selectivity of catalysts. This report describes
modification of surface hydrophobicity of Pd/UiO-66, a com-
posite comprising a metal–organic framework (MOF) and
stabilized palladium nanoparticles (NPs), using a simple
polydimethylsiloxane (PDMS) coating. The modified catalyst
demonstrated significantly improved catalytic efficiency. The
approach can be extended to various Pd nanoparticulate
catalysts for enhanced activity in reactions involving hydro-
phobic reactants, as the hydrophobic surface facilitates the
enrichment of hydrophobic substrates around the catalytic site.
PDMS encapsulation of Pd NPs prevents aggregation of NPs
and thus results in superior catalytic recyclability. Additionally,
PDMS coating is applicable to a diverse range of catalysts,
endowing them with additional selectivity in sieving reactants
with different wettability.
functional applications in diverse fields.[2–5] Among these,
MOFs are recognized to possess the advantages of both
homogeneous and heterogeneous catalysts and are thus very
suitable for catalysis.[5] Moreover, high surface area and
porosity make MOFs excellent supports or hosts for cataly-
tically active metal NPs. There have been quite a few reports
on metal NPs@MOF in recent years, almost all of which are
focused on the confinement or stabilization of tiny metal NPs
by MOFs for catalytic purposes.[6] Investigations on the effect
of surface environment of active metal sites, and the
consequences for catalytic performance, are extremely
rare.[7] Most recently, chemical environmental control of
metal NPs in MOFs was found to regulate catalytic activity
and selectivity.[7] However, to the best of our knowledge,
modification of surface hydrophilicity or hydrophobicity with
a view to enhancing catalytic activity and/or selectivity of
metal NP/MOF composites has not yet been reported.
T
he wettability control of a catalyst surface is widely known
Herein, a typical MOF (UiO-66)[8] was employed to
immobilize Pd NPs, affording a Pd/UiO-66 nanocomposite. A
thin layer of polydimethylsiloxane (PDMS), a hydrophobic
material, was successfully coated onto Pd/UiO-66 by a facile
chemical vapor deposition (CVD) approach, thereby render-
ing the nanocomposite surface hydrophobic (Scheme 1),
based on our recent report.[9] The crystallinity, size of Pd
NPs, and Pd site accessibility of Pd/UiO-66 are retained after
PDMS coating. Remarkably, the hydrophobic coating facil-
itates the enrichment of hydrophobic substrates and thereby
promotes interaction with Pd sites, while also affording
protection and preventing aggregation of Pd NPs. As
a result, the PDMS coated Pd/UiO-66 (Pd/UiO-66@PDMS)
exhibits significantly improved catalytic efficiency in various
reactions and enhanced recyclability compared to the pristine
Pd/UiO-66. Remarkably, the PDMS coating endows a diverse
range of Pd-based catalysts with the additional ability to
differentiate hydrophilic and hydrophobic substrates.
to be of great importance in the regulation of interactions
between heterogeneous catalysts and reactants, which is
directly related to catalytic activity and selectivity.[1] Hydro-
philic or hydrophobic catalyst surfaces have particular affinity
for the corresponding substrates, with beneficial consequen-
ces for catalytic conversion. For example, hydrophobic solid
acid or base catalysts present high activity in various reactions
involving hydrophobic reactants, such as epoxidation, hydro-
genation, condensation, and esterification or trans-esterifica-
tion reactions.[1a–e] Moreover, metal nanoparticles (NPs),
which are highly active in a variety of reactions but readily
agglomerated, can be stabilized by different porous materials.
In such circumstances, hydrophobic modification of the pore
surface can boost catalytic performance.[1f–h]
Emerging as a relatively new class of porous materials,
metal–organic frameworks (MOFs) have captured wide
research interest in recent years because of their crystalline
nature, tailorable structures, and most importantly, multi-
Microporous UiO-66 (Zr6O4(OH)4(BDC)6, BDC =
1,4-benzenedicarboxylate) is one of the most studied MOFs.
It possesses an intersecting 3D structure, high thermal and
[*] G. Huang, Q. Yang, Prof. Dr. S.-H. Yu, Prof. Dr. H.-L. Jiang
Hefei National Laboratory for Physical Sciences at the Microscale,
CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innova-
tion Center of Suzhou Nano Science and Technology, Department of
Chemistry, University of Science and Technology of China
Hefei, Anhui 230026 (P.R. China)
E-mail: jianglab@ustc.edu.cn
Homepage: http://staff.ustc.edu.cn/~jianglab/
Prof. Dr. Q. Xu
National Institute of Advanced Industrial Science and Technology
Ikeda, Osaka 563-8577 (Japan)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201600497.
Scheme 1. Preparative route for Pd/UiO-66@PDMS.
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chemical stability, and high porosity and surface area, which
are desirable attributes amenable to immobilization of metal
NPs.[8] The incipient wetness method was adopted to stabilize
Pd NPs with UiO-66. Typically, Pd2+ was absorbed into
evacuated UiO-66, followed by Pd2+ reduction in an H2
atmosphere at 2008C to yield Pd/UiO-66 composite with
a 0.71 wt% Pd loading. To coat the PDMS thin layer onto the
Pd/UiO-66 composite, a facile CVD process was used at
2008C with varying lengths of coating time (T), affording
Pd/UiO-66@PDMS-T (Scheme 1).
A
powder X-ray diffraction (PXRD) pattern of
Figure 1. Static water contact angle of a) Pd/UiO-66,
Pd/UiO-66 displays sharp characteristic peaks indexed to
UiO-66, demonstrating retention of UiO-66 crystallinity and
structure upon Pd NP loading (Supporting Information,
Figure S1). The absence of a diffraction peak for Pd species
infers the low Pd content and/or the formation of small Pd
NPs. The PDMS coating does not cause a change in the
PXRD pattern, further supporting a preserved UiO-66
crystalline structure (Supporting Information, Figure S1).
The N2 physisorption isotherms of Pd/UiO-66 and
Pd/UiO-66@PDMS are similar in shape and the BET surface
areas of the materials present a slight decrease with respect to
the parent UiO-66 (Supporting Information, Figure S2). The
high surface area of Pd/UiO-66@PDMS suggests that the thin
PDMS coating is permeable and does not affect the trans-
portation of molecules.
b) Pd/UiO-66@PDMS-10, and c) Pd/UiO-66@PDMS-20. d) Photo-
graph of Pd/UiO-66@PDMS-T samples obtained after different dura-
tions of PDMS coating time dispersed in a water–ethyl acetate
(1:1 v/v) biphasic solution.
The contact angle of a water droplet on Pd/UiO-66 is
approximately 258, which quickly increases to 1158 and 1408
upon PDMS coating by CVD (10 and 20 min, respectively;
Figures 1a–c), clearly suggesting that the PDMS coating
transforms the surface character of the nanocomposite from
hydrophilic to hydrophobic. Although a longer coating time
(> 20 min) results in a plateau contact angle of about 1408
(Supporting Information, Figure S3), Pd/UiO-66@PDMS
obtained with increasing lengths of coating time displays
gradually enhanced hydrophobicity that is discernible in
a water–ethyl acetate biphasic mixture: the nanocomposite
transfers gradually from the aqueous to the organic phase
with longer PDMS coating time (Figure 1d).
It was envisioned that surface hydrophobization of
Pd/UiO-66 by PDMS coating would affect catalytic per-
formance significantly. Styrene hydrogenation, a classical
reaction performed over Pd NPs, was initially employed to
evaluate the effect of PDMS coating on the catalytic
performance of Pd/UiO-66. Under the given reaction con-
ditions (Supporting Information), it took 255 min to realize
complete hydrogenation of styrene over Pd/UiO-66. In
comparison, the catalyst exhibited greatly improved effi-
ciency after PDMS coating. Activity increased in tandem with
catalyst coating time, up to 60 min, at which point the
Pd/UiO-66@PDMS-60 composite achieved 100% conversion
in 65 min (reducing reaction time by 75% compared to
pristine Pd/UiO-66). A coating time longer than 60 min leads
to decreased catalytic efficiency (Figure 2a). This fact
suggests that, to achieve efficient conversion, coating time
should be optimized to generate an ideal PDMS layer
Figure 2. a) Catalytic hydrogenation of styrene over UiO-66@PDMS,
Pd/UiO-66, and Pd/UiO-66@PDMS (1:150 Pd:styrene molar ratio).
TEM images of b) Pd/UiO-66 and c) Pd/UiO-66@PDMS-60.
d) HAADF-STEM image of Pd/UiO-66 (inset: Zr and Pd elemental
mapping for the selected rectangular area).
To determine the underlying reasons for such distinct
differences in activity before and after PDMS coating,
transmission electron microscopy (TEM) was conducted on
the materials. Only a few Pd NPs (ꢀ 5 nm in size) were
observed in Pd/UiO-66 (Figure 2b), which seems unreason-
able and might be caused by the similar contrast of Pd NPs
and Zr-oxo clusters in UiO-66 upon exposure to an electron
beam.[6h] To determine the true nature of the system,
elemental mapping was performed and clearly indicates that
a
mass of tiny Pd NPs, not distinguishable in the
aforementioned TEM image, are indeed evenly dispersed
within UiO-66 (Figure 2d; Supporting Information, Fig-
ure S4). It is worth noting that, although a small fraction of
Pd NPs are larger than the pore sizes of UiO-66, the pore
surface structure of the MOF offers spatial restriction to Pd
NPs to some extent, in accordance with recent reports
describing metal NPs/MOFs.[6] The PDMS coating does not
affect the size and distribution of Pd NPs (Figure 2c;
Supporting Information, Figure S5), confirming that Pd NPs
remain stable during PDMS encapsulation upon thermal
thickness that matches the wettability of
substrate.
a particular
treatment.
The
elemental
mapping
images
for
Pd/UiO-66@PDMS-60 show a uniform Si distribution with
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a slightly more intense signal at the edges (Supporting
Information, Figure S5), which exactly matches our
expectations.
X-ray photoelectron spectroscopy (XPS) revealed a clear
signal for Si 2p after PDMS coating (Supporting Information,
Figure S6), further demonstrating the successful surface
modification of Pd/UiO-66. We speculate that the volatile
short PDMS chains first produce a conformal layer on the
Pd/UiO-66 surface and subsequently crosslink during heat
treatment to generate PDMS coatings on the catalyst.[10] The
PDMS coating is assumed to contain defects that enable the
transportation of catalytic substrates and products. Ar+
sputtering shows a peak shift in Si 2p, implying that inter-
actions occur between PDMS and the pristine catalyst. The
persistent XPS signals for Zr and Pd suggest no change of
their electronic states after PDMS coating (Supporting
Information, Figure S6).
Figure 3. a) Catalytic activity over three consecutive styrene hydrogena-
tion runs using Pd/UiO-66 in the absence or presence of PDMS
coating (65 min for each catalytic run). TEM images for b) Pd/UiO-66
and c) Pd/UiO-66@PDMS after recycling experiments.
The size and electronic state of Pd NPs remained after
PDMS coating, as indicated above, suggesting that they will
not affect catalytic activity. Given the good permeability of
the PDMS layer, the H2 absorption ability of the MOF is
similar in both Pd/UiO-66 and Pd/UiO-66@PDMS (Support-
ing Information, Figure S7).[11] Therefore, the enhanced
styrene hydrogenation mediated by Pd/UiO-66@PDMS
should primarily be attributed to the hydrophobic PDMS
modification on the surface of Pd sites. A hydrophobic surface
enables more efficient accumulation of hydrophobic substrate
(styrene) and boosts the interaction of that substrate with Pd
sites, thereby accelerating catalytic conversion. To some
extent, longer coating time leads to a catalyst with a more
hydrophobic surface (Figure 1; Supporting Information, Fig-
ure S3) and higher activity. Despite this, excessive coating
time (> 60 min) produces a thick PDMS layer (Supporting
Information, Figure S8), which might block some Pd sites
and/or impede the transportation of substrate and product,
thus lowering the activity.
Figure S11). As expected, Pd/UiO-66@PDMS exhibited
superior activity compared to that of Pd/UiO-66 in both
reactions. For the selective hydrogenation of cinnamaldehyde,
pristine Pd/UiO-66 took as long as 7200 min to reach 100%
conversion (1:50 Pd:substrate molar ratio), while the catalyst
coated with PDMS for 10–60 min offered enhanced activity to
varying degrees. Pd/UiO-66@PDMS-30 exhibited the best
activity and promoted completion of the reaction in 2250 min,
a third of the reaction time compared to the PDMS-free
catalyst. The optimized PDMS coating time for the
Pd/UiO-66 catalyst was found to be 20 min in the case of
nitrobenzene hydrogenation. The resulting catalyst,
Pd/UiO-66@PDMS-20, promoted full conversion in 60 min
(1:150 Pd:NB molar ratio), when compared to pristine
Pd/UiO-66, which only yielded about 5% under identical
conditions and required 460 min to reach complete conver-
sion (eight-fold of the reaction time required by the PDMS-
coated catalyst).
Encouraged
by
the
improved
activity
of
The recycling stability of a catalyst is important in
practical applications. Pristine Pd/UiO-66 displayed deceas-
ing activity over three repeated runs of styrene hydrogenation
because of the aggregation of Pd NPs (Figures 3a,b). As some
Pd NPs with sizes larger than the MOF pores remain on the
MOF surface, the migration and growth of Pd NPs is
unavoidable during the reaction. In sharp contrast, the
Pd/UiO-66@PDMS composite possesses stable activity over
three cycles (Figure 3a; Supporting Information, Figure S9).
We believe that the PDMS layer plays a critical role in the
stabilization of Pd NPs by encapsulation, which is further
supported by good retention of Pd NP size (see TEM images
taken after cycling, Figure 3c). Moreover, the XPS signal for
Si 2p in the catalyst does not decay after recycling (Support-
ing Information, Figure S6c), suggesting that the coating layer
does not peel off during the reaction. PXRD and N2 sorption
experiments further support retention of porosity and stabil-
ity of Pd/UiO-66@PDMS after catalysis (Supporting Infor-
mation, Figure S10).
Pd/UiO-66@PDMS, PDMS coating was attempted to boost
the performance of other Pd-based catalysts. Representative
examples include, commercial Pd/C and classical Pd/SiO2,
which were coated with thin PDMS layers using a similar
approach (Supporting Information, Figure S12). The com-
mercial Pd/C catalyst required 120 min to fully convert
styrene into ethylbenzene, while only 60 min was needed for
Pd/C@PDMS-10. The difference in activity was even larger
for Pd/SiO2 with or without PDMS coating: Pd/SiO2 required
150 min to achieve 100% conversion, whereas catalysis
mediated by Pd/SiO2@PDMS-60 was complete after 70 min.
Similar to Pd/UiO-66, the additional stabilization of Pd NPs
acquired
upon
PDMS
encapsulation
endowed
Pd/SiO2@PDMS with remarkably improved recyclability
when compared to pristine Pd/SiO2 (Supporting Information,
Figure S13).
The above results unambiguously demonstrate that the
hydrophobic PDMS layer promotes the accumulation and
penetration of hydrophobic substrates and thus greatly
improves catalytic activity. However, the PDMS coating
may hamper the transportation of hydrophilic molecules. To
investigate this possibility, hydrophilic 2-butene-1,4-diol was
evaluated as a representative substrate in hydrogenation. All
To determine whether the Pd/UiO-66@PDMS catalyst is
extendable to other reactions with hydrophobic substrates,
cinnamaldehyde and nitrobenzene (NB) were selected as
reactants for hydrogenation (Supporting Information,
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substrate was converted in 240 min with Pd/UiO-66 (1:500
Pd:substrate molar ratio). In contrast, PDMS-coated
Pd/UiO-66 reached full conversion after 480 min, twice the
reaction time needed by the pristine catalyst (Supporting
Information, Figure S14). The results suggest that the hydro-
phobic PDMS coating can boost catalytic performance in
reactions where hydrophobic substrates are involved, while it
is detrimental to catalysis involving hydrophilic reactants. The
mechanism underlying this diametric response lies in the
wettability interaction between the catalyst surface and
substrates.
Besides the improvements in catalytic hydrogenation
activity and recyclability attained with hydrophobic sub-
strates, we were delighted to learn that PDMS coating on Pd
catalysts offered wettability-selective catalytic behavior. The
hydrogenation of hydrophobic nitrobenzene (NB) and hydro-
philic 4-nitrophenol (4-NP) were chosen as model reactions.
Under hydrogenation conditions, Pd/UiO-66 was able to
reduce both NB and 4-NP (Pd:4-NP (1:10), Pd:NB (1:20)
molar ratios) into their respective products, and the complete
conversion of 4-NP was faster (6.5 min) than that for NB
(180 min; Figure 4a; Supporting Information, Figure S15).
substrates. This pronounced size selectivity behavior has been
further demonstrated in the hydrogenation of diverse olefins
over Pd@ZIF-8 and Pd@UiO-66 with different MOF pore
sizes (Supporting Information, Figure S21).
In summary, modification of surface hydrophobicity of Pd
NP catalysts was developed using a facile PDMS coating
approach. The MOF-stabilized Pd NPs are encapsulated by
a thin hydrophobic PDMS layer, which makes the surface of
the catalyst hydrophobic and thus creates enhanced affinity
for hydrophobic substrates. As a result, Pd/UiO-66@PDMS
shows significantly improved activity compared to pristine
Pd/UiO-66 in various reactions. PDMS coating leads to
improved catalytic recyclability as well as novel wettability
selectivity for hydrophobic and hydrophilic substrates.
Further experiments indicate that the PDMS-coating
approach is not confined to Pd/MOF, being an ideal candidate
to demonstrate the applicability of surface hydrophobization
(Supporting Information, Section 4), and can be extended to
various Pd nanoparticulate catalysts, such as commercial Pd/C
and Pd/SiO2. We envision that this facile surface hydro-
phobization PDMS coating approach could be extended to
other types of catalysts and that this work would open a new
avenue to enhanced activity and stability, as well as additional
selectivity in catalysis.
Acknowledgements
We are grateful to the reviewers for valuable suggestions, and
Prof. Zhiyong U. Wang at Troy University for helpful
discussions. This work is supported by the NSFC (21371162,
51301159 and 21521001), the 973 program (2014CB931803),
NSF of Anhui Province (1408085MB23), the Recruitment
Program of Global Youth Experts and the Fundamental
Research Funds for the Central Universities (WK2060190026,
WK2060190065).
Figure 4. A comparison of catalytic reduction of nitrobenzene
*
(c c) and 4-nitrophenol (c&c) over a) Pd/UiO-66 and
b) Pd/UiO-66@PDMS-60.
Upon PDMS coating, the same reaction with the hydrophobic
NB was completed in 60 min (Figure 4b; Supporting Infor-
mation, Figure S16a), inferring that hydrophobic NB can pass
through the PDMS layer. In stark contrast, the reduction of
hydrophilic 4-NP over Pd@UiO-66@PDMS did not proceed,
as evidenced by a negligible change in the characteristic
UV/Vis peak of 4-NP during 24 h of reaction (Figure 4b;
Supporting Information, Figure S16b). Given the high activity
of Pd NPs toward the reduction of 4-NP reported else-
where,[12] this result indicates that the hydrophobic PDMS
shell of the catalyst impedes the transportation of hydrophilic
4-NP and thus the Pd active sites become unavailable to 4-NP.
Therefore, the PDMS coated Pd-based catalyst promotes high
catalytic selectivity for hydrophobic molecules but excludes
hydrophilic molecules. Further to this, the PDMS coated
Pd@UiO-66 catalyst, where Pd NPs are incorporated inside
UiO-66,[13] not only offers wettability-selective catalytic
behavior (Supporting Information, Figures S17–19), but also
possesses intrinsic size selectivity for sieving catalytic sub-
strates of different sizes (Supporting Information, Fig-
ure S20).[11a,13a] Therefore, Pd@UiO-66@PDMS possesses
dual wettability- and size-selectivity toward different reaction
Keywords: heterogeneous catalysis · metal–organic framework ·
microporous materials · nanoparticles · surface hydrophobicity
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Received: January 16, 2016
Revised: March 24, 2016
Published online: May 4, 2016
Angew. Chem. Int. Ed. 2016, 55, 7379 –7383
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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