Chemoselective reduction of α,β-unsaturated carbonyl compounds with UiO-66 materials

Article abstract of DOI:10.1016/j.jcat.2016.05.013

Allylic alcohols, important intermediates in fine chemical industry, are typically obtained through chemoselective hydrogenation of α,β-unsaturated aldehydes. Here we show that UiO-66, a zirconium-based metal-organic framework can be used in the chemoselective hydrogenation of cinnamaldehyde, both under high hydrogen pressure as silver nanoparticle support, and as transfer hydrogenation catalyst in the Meerwein-Ponndorf-Verley (MPV) reduction. A recyclable 10 wt% Ag/UiO-66 catalyst reached complete conversion after 6 h and 50 bar of H₂ with 66% selectivity for cinnamyl alcohol in the inert solvent N,N-dimethylacetamide (DMA). Pure UiO-66 as MPV catalyst with isopropanol reached complete conversion with >90% selectivity after 24 h at 120 °C. The substrate scope was extended to citral and carvone, two α,β-unsaturated carbonyl compounds that are harder to reduce selectively. Introduction of a NO₂-functional group into the UiO-66 linker to increase the Lewis acidity was clearly beneficial for the conversion of carvone.

Full text of DOI:10.1016/j.jcat.2016.05.013

Journal of Catalysis 340 (2016) 136–143
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Chemoselective reduction of
with UiO-66 materials
a,b-unsaturated carbonyl compounds
⇑
Eva Plessers, Dirk E. De Vos, Maarten B.J. Roeffaers
Centre for Surface Chemistry and Catalysis, Department of Microbial and Molecular Systems, Faculty of Bioscience Engineering, KU Leuven, 3001 Heverlee, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
Allylic alcohols, important intermediates in fine chemical industry, are typically obtained through chemos-
elective hydrogenation of a,b-unsaturated aldehydes. Here we show that UiO-66, a zirconium-based metal–
organic framework can be used in the chemoselective hydrogenation of cinnamaldehyde, both under high
hydrogen pressure as silver nanoparticle support, and as transfer hydrogenation catalyst in the Meerwei
n–Ponndorf–Verley (MPV) reduction. A recyclable 10 wt% Ag/UiO-66 catalyst reached complete conversion
after 6 h and 50 bar of H2 with 66% selectivity for cinnamyl alcohol in the inert solvent N,N-
dimethylacetamide (DMA). Pure UiO-66 as MPV catalyst with isopropanol reached complete conversion
Received 29 December 2015
Revised 3 May 2016
Accepted 16 May 2016
Keywords:
Metal–organic framework
Chemoselective hydrogenation
with >90% selectivity after 24 h at 120 °C. The substrate scope was extended to citral and carvone, two a,
b-unsaturated carbonyl compounds that are harder to reduce selectively. Introduction of a NO2-functional
groupinto theUiO-66linkertoincreasetheLewis aciditywasclearlybeneficial fortheconversion of carvone.
Ó 2016 Elsevier Inc. All rights reserved.
a,b-Unsaturated aldehyde
Meerwein–Ponndorf–Verley reduction
1. Introduction
The chemoselectivity in the metal nanoparticle catalysed
hydrogenation of unsaturated carbonyl compounds to the unsatu-
Allylic alcohols, important intermediates in the pharmaceutical,
fragrance and agrochemical industry, are typically obtained
rated alcohol product depends on various factors such as the metal
used as catalyst as well as the chemical nature of the aldehyde and
reaction conditions. Various supported metal nanoparticles have
through chemoselective reduction of
a,b-unsaturated aldehydes.
Hydrogenation of the olefinic group yields saturated aldehydes
whereas hydrogenation of the carbonyl group produces allylic
alcohols. Cinnamaldehyde, citral and crotonaldehyde are widely
been reported so far aiming for the selective hydrogenation of a,
b-unsaturated aldehydes [2,5,15–22]. In these catalysts the use of
small metal nanoparticles maximises the catalyst surface area;
however, it also demands proper stabilisation by an appropriate
catalyst support [23]. A well-considered choice of support is a
key parameter in superior catalyst development as pore size, speci-
fic surface area and acidity can differ largely. These supports could
also influence the selectivity towards carbonyl bond hydrogena-
tion through metal–support interaction [24]. Typical supports are
metal oxides, such as acidic Al2O3, SiO2, TiO2 and basic MgO, as
well as active carbon [2,18,25–30]. Less studied support materials
are metal–organic frameworks (MOFs). MOFs are crystalline, por-
ous materials composed of metal (oxide) nodes interlinked by
polytactic organic ligands, thus forming three dimensional
zeolite-like structures with well-defined micropores and pore
channels. These solids have attracted researchers’ interest mainly
due to their versatility, high surface area and acid-base properties.
To the best of our knowledge there are only three reports on the
used as reference
a,b-unsaturated aldehydes [1]. The former two
are attractive since the unsaturated alcohol products are used as
raw material in e.g. the pharmaceutical and fragrance industry;
the cardiovascular drug cinnarizine is derived from cinnamyl alco-
hol [2] and nerol and geraniol derived from citral are useful for the
production of perfumes, food flavours and insecticides [3–5]. The
chemoselective reduction of carbonyl bonds in enones or enals is
however challenging. Typically, this reduction is performed either
under high hydrogen pressure conditions with supported metal
nanoparticle catalysts or via transfer hydrogenation with an alco-
hol as hydride donor and a Lewis acid catalyst. The former process
is not straightforward since olefins are often preferentially
reduced, due to thermodynamic and kinetic reasons [6–9], the lat-
ter on the contrary is selective in carbonyl-hydrogenation but also
produces by-products [10–13] and has minor atom efficiency since
besides the desired product also a ketone is formed [14].
use of a MOF-support in the chemoselective hydrogenation of
a,
b-unsaturated aldehydes. Pt nanoclusters were confined in the
cavities of amino-functionalised UiO-66 and displayed high selec-
tivity (92%) to cinnamyl alcohol [31]; MIL-101 supported Pt
⇑
Corresponding author.
E-mail address: maarten.roeffaers@biw.kuleuven.be (M.B.J. Roeffaers).
http://dx.doi.org/10.1016/j.jcat.2016.05.013
0021-9517/Ó 2016 Elsevier Inc. All rights reserved.

E. Plessers et al. / Journal of Catalysis 340 (2016) 136–143
137
nanoparticles on the contrary were >99.9% selective in the hydro-
genation of the C@C bond to form hydrocinnamaldehyde [15]. The
results when using Pd/MIL-53(Cr) and Ru/MIL-53(Cr) in the hydro-
genation of cinnamaldehyde and crotonaldehyde were somewhat
mixed. In the case of cinnamaldehyde the Pd-catalyst preferen-
tially hydrogenated the C@C bond while the Ru-catalyst preferen-
tially hydrogenated the C@O bond. In the case of crotonaldehyde
however, the selectivity trend was totally different: the C@C dou-
ble bond was preferentially hydrogenated on Ru/MIL-53(Cr)
whereas C@C and C@O hydrogenation occurred at a similar reac-
tion rates on Pd/MIL-53(Cr) [19].
Alternatively, unsaturated aldehydes and ketones can be selec-
tively reduced to the corresponding allylic alcohols with high
chemoselectivity in the absence of hydrogen gas via the Meer
wein–Ponndorf–Verley (MPV) reduction [10]. The earliest reported
MPV-catalysts were aluminium alkoxides [32], but in the last
15 years zirconium-based catalysts are attracting more and more
attention [12,13,33–39]. With Zr-grafted siliceous MCM-41 and
SBA-15 yields up to 95% were obtained within reasonable reaction
times with relatively small amounts of catalyst (5–8.4 mol%)
[12,33,34]. Catalytic performance was increased by incorporation
of Zr in zeolite beta [35,36,40]. This heterogeneous catalyst was
reusable, stable in water and only 1.3 mol% Zr was used. Recently
a mesoporous zirconium–phytic acid hybrid material was tested
15 mmol) and terephthalic acid (2.5 g, 15 mmol) or 2-
nitroterephthalic acid (3.2 g, 15 mmol), dissolved in N,N-
dimethylformamide (DMF) (155 mL, 2 mol). 1.5 mL of a 36 wt%
solution of HCl (17 mmol) and 20 equivalents (23 mL) of the mod-
ulator CF3COOH were also added to the mixture. UiO-66-NH2 was
synthesised without the modulator [31], also starting from an
equimolar mixture of ZrCl4 (0.48 g, 2.1 mmol) and 2-
aminoterephthalic acid (0.372 g, 2.1 mmol) dissolved in DMF
(120 mL), to which 0.15 mL of H2O was added. All synthesis mix-
tures were placed in a preheated oven at 120 °C for 21 h (modu-
lated) or 24 h (non-modulated synthesis). The powders were
collected via centrifugation (10 min, 11000 rpm) and thoroughly
washed with DMF (3 times) and methanol (3 times). The powders
were then dried at 60 °C over night and at 200 °C for 24 h to yield
3.48 g (UiO-66), 3.70 g (UiO-66-NO2) and 0.727 g (UiO-66-NH2) of
activated sample, which correspond to respectively 84%, 80% and
98% molar Zr yield.
2.1.2. Supported Ag nanoparticles
Before impregnation, the MOF-supports are activated at 150 °C
for 16 h. Silver nitrate (512 mg AgNO3, 3 mmol) is dissolved in
6 mL of a H2O:EtOH 1:5 solution. This aqueous–ethanolic AgNO3
solution is added to a vial containing 200 mg MOF; the vial is
sealed with a crimp cap and flushed with nitrogen to remove all
O2. Interaction of Ag+ ions with light is prevented by a protective
layer of aluminium foil around the vial. The MOF–AgNO3 mixture
is sonicated for 2 min and stirred continuously for 16 h at
500 rpm. The sample is thoroughly washed (5 times) with ethanol
to remove remaining AgNO3; centrifugation (8 min, 3000 rpm),
removal of the supernatant and addition of fresh EtOH are per-
formed under an inert atmosphere. Drops of a NaCl-solution are
added to the supernatant solution to check whether there is still
some Ag+ present. After washing, the sample is dried in a vacuum
oven at room temperature for 16 h.
as a MPV-catalyst for the conversion of levulinic acid into
c-
valerolactone and other carbonyl compounds [39]; high yield and
selectivity were obtained, however with 65 mol% Zr. In these
MPV catalysts the presence of Lewis acid zirconium sites is essen-
tial. Recently, Cirujano and co-workers [41] successfully showed
the presence of Lewis acid centres in UiO-66 and used these
Zr-based MOFs for the esterification of levulinic acid with various
alcohols. The catalytically active sites are coordination vacancies
of the Zr-metal, arising from crystalline defects associated with
linker deficiencies or from thermal dehydroxylation of the
Zr-cluster [41,42]. Preliminary experiments with Zr-containing
MOFs, UiO-66 and UiO-66-NO2, as MPV reduction catalysts with
tert-butylcyclohexanone as the reactant, already showed the
potential of these materials for application in more challenging
selective reductions such as those of unsaturated aldehydes and
ketones used in this work [38]. Literature suggests that the use
of HCl and trifluoroacetic acid can be used in the modulated UiO-
66 synthesis to yield a more open framework structure with a large
number of coordinative vacancies [38].
2.1.3. Reference catalysts
c
-Al2O3 (Product No. 199974), silica gel (Product No. 60752)
and ZrO2 (Product No. 230693) were obtained from Sigma Aldrich.
13 wt% Ag/SiO2 and Ag/Al2O3 were synthesised according to litera-
ture procedures [18,43].
2.2. Characterisation
In this work we explored the use of UiO-66 as support for silver
nanoparticles for the chemoselective high pressure hydrogenation
Powder X-ray diffractograms were routinely collected on a
STOE STADI COMBI P diffractometer in High-Throughput mode,
of
a,b-unsaturated aldehydes. From the literature it is known that
equipped with an image plate detector using Cu Ka radiation
the hydrogenation of C@O bonds over C@C bonds in
a,b-
(k = 1.54056 Å). Scanning Electron Microscopy (SEM) images were
unsaturated aldehydes decreases, roughly, in the order
Ag > Au > Pd > Pt > Ru [2]. Alternatively, we explored the potential
of bare UiO-66 materials without supported noble-metal particles
as catalysts in the chemoselective formation of allylic alcohols in
the MPV-reduction using isopropanol.
obtained using a JEOL SEM (JSM-6010LV). Nitrogen adsorption
and desorption isotherms at 77 K were measured using
a
Micromeritics 3Flex 3500 physisorption instrument. The sample
was degassed before measurement at 423 K for 6 h under vacuum
(10À2 mbar). The pore size distribution was calculated using the
BJH method (Harkins and Jura thickness curve and Faas correction,
3Flex 3.00 software). Diffuse Reflectance spectra (DRS) were mea-
sured on a Perkin Elmer UV/VIS spectrophotometer (Lambda 950)
2. Materials and methods
equipped with
a integrating sphere. ICP-AES measurements
2.1. Synthesis
(Varian 720-ES) were used to determine the Ag-loading. Thermal
Gravimetric Analysis (TGA) was performed under a stream of N2-
gas using a Universal V4.5A TA Instrument running from room
temperature to 800 °C with a scan rate of 3.5 °C/min.
All chemicals and solvents used in the syntheses were of
reagent grade and used without further purification.
2.1.1. UiO-66 materials
2.3. Catalytic experiments
All UiO-66 MOFs were made in a closed Schott DURANÒ pres-
sure plus bottle with a volume of 1 L under static conditions.
UiO-66 and UiO-66-NO2 were synthesised in the presence of a
modulator, starting from an equimolar solution of ZrCl4 (3.5 g,
2.3.1. High H2 pressure hydrogenation
For the high pressure hydrogenation reaction, 25 mg of catalyst,
1.15 mmol (145 lL) of cinnamaldehyde, 0.95 mmol n-tetradecane

138
E. Plessers et al. / Journal of Catalysis 340 (2016) 136–143
(245
l
L, internal standard) and 3.3 mL of N,N-dimethylacetamide
(DRS) (Fig. 1); the extra absorption maximum around 400 nm
can be assigned to surface plasmon resonance (SPR) of small Ag
nanoparticles (<10 nm) [46]. The absence of larger silver particles
was further supported by powder XRD of the Ag/UiO-66 which
would give rise to 38.1° reflection of Ag [111], and further no sig-
nificant framework decomposition was observed (Fig. S1). ICP-AES
measurements show a Ag-loading of 10–14 wt%.
(DMA, solvent) were introduced into a 15 mL autoclave. After
flushing with N2, the autoclave was pressurised with H2 to 20–
50 bar of H2 and stirred with a magnetic stirring bar at 500 rpm.
The hydrogenation reaction was performed at 140 °C for 6 h, unless
stated otherwise. For analysis of the liquid products, the catalyst
was removed by centrifugation (3000 rpm, 8 min) and analysis of
the reaction products was carried out using a gas chromatograph
(Shimadzu 2010 GC, CP-Sil 8, FID detector). n-Tetradecane was
added as internal standard for quantitative GC analysis. Identifica-
tion of the compounds was carried out using GC–MS.
For the recycling test, the used catalyst was isolated from the
reaction mixture by centrifugation (3000 rpm, 10 min) and thor-
oughly washed with DMA (at least three times) until the super-
natant was purely solvent, as confirmed by GC. The recovered
catalyst was reactivated overnight (16 h) at room temperature in
a vacuum oven (10 mbar) and reused under the same reaction
conditions.
Chemoselective hydrogenation of cinnamaldehyde (CALD) was
carried out at 140 °C and 30 bar of H2 in N,N-dimethylacetamide
(DMA) as the solvent. Catalytic activity of UiO-66 supported silver
is clearly visible; in contrast to the thermodynamically favoured
C@C hydrogenation, cinnamyl alcohol (CALH) is the preferentially
formed product with a selectivity of 66 3% (Table 1, entry 1). As
expected, the hydrogenation rate increases with higher hydrogen
pressures, increasing pressure from 30 to 40 and 50 bar of H2
increases the turn-over frequency to respectively 3.5 0.1 and
5.5 0.1 mol cinnamyl alcohol per mol Ag per hour (Table 1, entry
1–3). Interestingly this pressure increase has no significant effect
on the selectivity towards cinnamyl alcohol, which remains 65–
70%. Reference Ag/SiO2 and Ag/Al2O3 catalysts with similar Ag-
loadings were used under the same reaction conditions (Table 1,
entry 5–6). The Ag/SiO2 powder shows a comparable reaction
selectivity (71 3%), however with a slightly higher hydrogenation
rate (TOF of 6.1 0.1 mol CALH/mol Ag/h). On the other hand,
strongly reduced CALH selectivity of 47 5% was obtained with
Ag/Al2O3. This is most probably related to the formation of larger
Ag nanoparticles as seen on powder XRD of this black powder
(Fig. S1). The choice of the support has thus an important influence
on the formation and stabilisation of the Ag nanoparticles and the
resulting catalytic properties. The chemoselective formation of
CALH has been reported before for PVP-stabilized Ag nanocolloids,
with up to 93% selectivity after 48 h [17]. Although these stabilized
nanoparticles were successfully recycled, heterogenisation of
nanoparticles on solid supports typically facilitates synthesis and
removal of the reaction medium, and improves thermal and chem-
ical stability [47]. In contrast, the use of silica supported copper
catalysts resulted exclusively in the undesired HCAL during the ini-
tial stages of the reaction [27].
2.3.2. MPV-reduction
Before reaction, each catalyst was dried at 200 °C to remove
residual solvent molecules; catalytic reactions were carried out
in 10 ml glass crimp cap vials loaded with 20–30 mg catalyst and
a magnetic stirring bar. A solution of the substrate in 3.3 mL iso-
propanol (IPA) was added; n-tetradecane was added as internal
standard. For each catalyst, a substrate to Zr ratio of 7.8 was used
as to compare the activity of each catalyst. After introduction of the
reaction mixture, the vials were placed in an aluminium heating
block (at 120 °C) and stirred. Reaction samples were filtered
through a 0.2 lm filter and analysed with gas chromatography
(Shimadzu 2010 GC, CP-Sil 8, FID detector). Reaction products were
identified using GC–MS. Reactions were performed in duplo, and
the results shown are averaged.
For the recycling test, the used catalyst was isolated from the
reaction mixture by centrifugation (3000 rpm, 10 min) and thor-
oughly washed with isopropanol (at least three times) until the
supernatant was purely solvent, as confirmed by GC. The recovered
catalyst was reactivated at 60 °C (2–4 h) and 150 °C (overnight,
16 h) and reused under the same reaction conditions.
As can be seen in Table 1 the main products are cinnamyl alco-
hol (CALH, 66–69% selectivity) and hydrocinnamaldehyde (HCAL,
27–30% selectivity). The consecutive hydrogenation product of
both compounds, hydrocinnamyl alcohol (HCALH), is only formed
in small amounts, even at high conversions. One other by-product
was detected in small amounts, typically 1–2%, and identified by
GC–MS as N,N-dimethylcinnamylamine (DMC). As shown in
Scheme 1, in the presence of an acid catalyst cinnamaldehyde
3. Results and discussion
3.1. Chemoselective reduction of cinnamaldehyde under high H2
pressure
UiO-66 is
a zirconium-terephthalate based metal–organic
framework (MOF) which is best known for its high chemical and
thermal stability and easy functionalisation. When HCl and trifluo-
roacetic acid are added to the MOF synthesis mixture, terephtha-
late linkers are partially replaced by trifluoroacetate, resulting in
a more open framework with a large number of open sites [38]. Tri-
fluoroacetate acts as a modulator in this case since it has a similar
chemical functionality as the terephthalate linker, but has only one
functional group. Since modulated synthesis also increases physi-
cal stability [44], we followed this approach to synthesise our
catalyst support. Characterisation was performed with powder
X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), N2-
physisorption and Fourier Transform Infrared (FTIR) spectroscopy
(see supplementary information, Figs. S1–4). Ag nanoparticles
were introduced into the porous MOFs via impregnation with an
aqueous-ethanolic silver nitrate solution. Further, addition of a
reductant or inclusion of an additional reduction step was not nec-
essary since Ag(I) is effectively reduced in the presence of EtOH to
Ag(0) [45]. The formation of Ag nanoparticles was evidenced by
diffuse reflectance spectroscopy of the yellow Ag/UiO-66 powder
Fig. 1. Diffuse reflectance UV/visible spectra of UiO-66 and 14 wt% Ag-loaded UiO-
66 after Kubelka–Munk (K–M) correction.

E. Plessers et al. / Journal of Catalysis 340 (2016) 136–143
139
Table 1
Conversion and selectivity in the reduction of cinnamaldehyde (CALD) in N,N-dimethylacetamide, catalysed by UiO-66, Ag/UiO-66 and a Ag/SiO2 reference catalyst.
Entry
Catalyst
H2 (bar)
Wt% Aga
X (%)
TOFb
SCALH (%)
SHCAL (%)
SHCALH (%)
1
2
3
4
5c
6
Ag/UiO-66
Ag/UiO-66
Ag/UiO-66
UiO-66
Ag/SiO2
Ag/Al2O3
30
40
50
30
30
50
14
10
10
0
13
13
82
64
>99
<1
90
96
3.3
3.5
5.5
/
6.1
2.9
66
69
66
/
71
47
30
29
27
/
28
36
2
1
6
/
1
18
CALD = cinnamaldehyde, CALH = cinnamyl alcohol, HCAL = hydrocinnamaldehyde, HCALH = hydrocinnamyl alcohol; reaction conditions: CALD (1.15 mmol), n-tetradecane
(0.95 mmol), solvent N,N-DMA (3.3 mL), 25 mg catalyst, 140 °C, 6 h, 500 rpm.
a
Determined via ICP-AES.
mol CALH per mol Ag per h.
4 h.
b
c
can react with a secondary amine, like dimethylamine, the ther-
mal decomposition product of N,N-dimethylacetamide, to form
an enamine [48,49]. After selective hydrogenation catalysed by
the Ag/UiO-66 catalyst, N,N-dimethylcinnamylamine (DMC) is
formed, which proves that the Ag catalyst typically also prefers
C@N over C@C hydrogenation. N,N-DMA was chosen as solvent
since the amide group will coordinate with free Zr-sites of the
MOF framework, avoiding the influence hereof (vide infra) and
thus only the catalytic selectivity of the Ag nanoparticles is
observed.
To confirm that the observed catalytic activity can solely be
attributed to the Ag nanoparticles, the reaction was also performed
with pure UiO-66. As can be seen in Table 1 (entry 4) no significant
conversion of CALD was detected in this blank reaction after 6 h. A
known problem with supported silver catalysts is the facile silver
aggregation during the catalytic experiment resulting in a rapidly
decreasing catalytic performance. The aggregation of silver in lar-
ger clusters typically induces a colour change in the material. Upon
visual inspection no significant colour changes occurred with the
yellow Ag/UiO-66 powder during the catalytic test which shows
that the UiO-66 support strongly suppresses Ag nanoparticle
aggregation. This is further supported by powder XRD of the Ag-
loaded materials before and after CALD hydrogenation (Fig. S5).
Powder XRD of the Ag-loaded UiO-66 after catalytic reaction indi-
cates that the crystallinity of the UiO-66-framework remained
intact and no additional diffraction peaks were observed. In the
case of Ag-loaded silica on the contrary, an additional diffraction
at 38.3° can be attributed to Ag [111] reflections from larger aggre-
gated Ag particles. Ag leaching from the UiO-66 support to the
reaction mixture during the course of the reaction was furthermore
not detected by ICP-AES on the product mixture (<0.05 ppm, detec-
tion limit). To further test the stability of the Ag/UiO-66 catalyst, a
recycle study was conducted. The used catalyst was isolated from
the reaction mixture by centrifugation (3000 rpm, 10 min) and
thoroughly washed with DMA until the supernatant was purely
solvent, as evidenced by GC. The recovered catalyst was reacti-
vated overnight at room temperature in a vacuum oven (10 mbar)
and reused under the same reaction conditions. Powder XRD and
SEM showed that the crystallinity was retained after the washing
and reactivation (see supplementary information, Fig. S5). In reac-
tions with the recycled catalysts, the original catalytic activity was
retained at >99% up to 5 times recycling with only minor loss in
chemoselectivity which in the last reaction was still 60 3% to cin-
namyl alcohol (see supplementary information, Fig. S6). These
recycling experiments evidence the durability and reusability of
the catalyst. The conversion and selectivity obtained with these
recyclable UiO-66 supported silver catalysts are however still
lower than the results for the reference silver on silica catalyst.
Under these high pressure and temperature conditions with an
inert solvent such as DMA, higher selectivity can probably only
be achieved by the use of multi-component metal nanoparticles
[6,25].
3.2. Transfer hydrogenation with UiO-66 and analogues
High conversions in the cinnamaldehyde (CALD) chemoselec-
tive hydrogenation are often reported when using isopropanol
(IPA) as a solvent. Under such conditions, using IPA instead of
DMA as solvent and with 30 bar of H2, the Ag/UiO-66 catalyst
shows a remarkably higher hydrogenation selectivity of 83 2%
and a conversion of 81 3% after 6 h at 140 °C (Table 2, entry 1).
Even higher reaction selectivity is obtained in the absence of the
Scheme 1. Reaction products of the hydrogenation of cinnamaldehyde with UiO-66 supported Ag nanoparticles, determined via GC–MS (CALD = cinnamaldehyde,
CALH = cinnamyl alcohol, HCAL = hydrocinnamaldehyde, HCALH = hydrocinnamyl alcohol).

140
E. Plessers et al. / Journal of Catalysis 340 (2016) 136–143
Table 2
Conversion and selectivity in the reduction of cinnamaldehyde in isopropanol, catalysed by Ag-loaded UiO-66 or by UiO-66 as such.
a
b
Entry
Catalyst
H2 (bar)
X (%)
SCALH (%)
SHCAL (%)
SHCALH (%)
SEther (%)
SAcetal (%)
1
2
3
Ag/UiO-66
UiO-66
UiO-66
30
30
0
81
>99
>99
83
93
92
5
0
0
11
6
6
<1
1
1
1
<1
<1
CALD = cinnamaldehyde, CALH = cinnamyl alcohol, HCAL = hydrocinnamaldehyde, HCALH = hydrocinnamyl alcohol; reaction conditions: CALD (1.15 mmol), n-tetradecane
(0.95 mmol), solvent IPA (3.3 mL), 25 mg catalyst, 500 rpm, 140 °C, 6 h, 30 bar of H2.
a
1-Cinnamyl-2-propyl ether.
Cinnamaldehyde diisopropyl acetal side products (Scheme 2).
b
Table 3
Conversion and selectivity in the CALD transfer hydrogenation catalysed by UiO-66, NO2- and NH2-functionalised UiO-66 and a reference ZrO2 catalyst in isopropanol.
a
b
Entry
Catalyst
T (°C)
t (h)
X (%)
SCALH (%)
SEther (%)
SAcetal (%)
1
2
3
4a
4b
5a
5b
6a
6b
7
UiO-66
82
82
82
24
24
24
8
24
8
24
8
24
24
17
18
7
82
>99
52
84
10
17
2
68
78
28
95
94
94
93
87
90
0
2
4
2
1
2
1
1
1
1
0
29
17
66
<1
0
1
<1
7
UiO-66-NO2
UiO-66-NH2
UiO-66
120
UiO-66-NO2
UiO-66-NH2
ZrO2
120
120
120
5
100
CALD (1.15 mmol), n-tetradecane (0.95 mmol), solvent IPA (3.3 mL), 7.8 mol% Zr, 500 rpm.
a
1-cinnamyl-2-propyl ether.
Cinnamaldehyde diisopropyl acetal side products (Scheme 2).
b
supported silver nanoparticles; pure UiO-66 displays over 90 2%
selectivity towards cinnamyl alcohol (CALH) at complete conver-
sion (Table 2, entry 2). This can be explained by the fact that zirco-
nium is known to be a transfer hydrogenation catalyst, often in
combination with IPA as hydride donor, a mechanism called the
Meerwein–Ponndorf–Verley (MPV) reduction [50]. MPV-
reduction with modulated UiO-66 and UiO-66-NO2 as catalysts
and with tert-butylcyclohexanone as the reactant obtained
93 2% yield with 10 mol% Zr within 24 h [38]. The excellent cat-
alytic performance of UiO-66, even in the absence of H2 (Table 2,
entry 3), can thus be explained by the MPV-reduction mechanism
[10,51].
As mentioned above MPV-reduction of unsaturated aldehydes
and ketones to the corresponding alcohols is an alternative route
to obtain allylic alcoholic intermediates with high chemoselectiv-
ity and without the use of hydrogen gas. MPV-reductions in iso-
propanol (IPA) are typically performed at reflux conditions, more
specifically at 82 °C with IPA in large excess, to force the equilib-
rium reaction towards the allylic alcohol product. With Zr-
grafted siliceous SBA-15 cinnamyl alcohol yield of 56% was
obtained at 82 °C within 5 h with 8.4 mol% Zr [34]. Catalytic perfor-
mance was increased by incorporation of Zr in zeolite beta, after
3 h at 82 °C, and 96% yield was obtained with 1.3 mol% Zr [36].
When UiO-66 is used in these conditions, conversion is relatively
low (17 2%) and the selectivity for the desired CALH drops back
to 68 3% after 24 h (Table 3, entry 1). The reported selectivity in
this case however does not reflect the intrinsic hydrogenation
selectivity towards the carbonyl group as the unwanted hydrocin-
namaldehyde (HCAL) is not formed, but rather reflects the forma-
tion of other side products. Based on GC–MS these compounds
were identified as 1-cinnamyl-2-propyl ether and cinnamaldehyde
diisopropyl acetal (Scheme 2) which form as a result of etherifica-
tion with an excess of isopropanol. These side products are also
reported in the literature when hydrous zirconia is used in the
MPV-reduction of cinnamaldehyde [11]. After 24 h of reaction at
80 °C, the selectivity to 1-cinnamyl 2-propyl was 7.8% with this
hydrous Zr catalyst.
Electron withdrawing groups (NO2) on the organic UiO-66 lin-
ker are known to enhance the Lewis acid strength of coordination
vacancies of the Zr atoms in the adjacent node [52], while intro-
ducing a basic amino site creates an acid-base catalyst [53]. When
UiO-66-NO2 is used (Table 3, entry 2) conversions only slightly
increase from 17% to 18%, but the selectivity clearly increases to
78 3% due to decreased side product formation. As expected
amine-functionalisation of the linker has the opposite effect; it
slows down the reaction with a conversion of only 7% after 24 h
and has a dramatic impact on selectivity, that decreases to
28 5% (Table 3, entry 3). However, the decreased performance
can also partly be explained by the fact that the UiO-66-NH2 sam-
ple was prepared without modulation. As mentioned before, when
HCl and trifluoroacetic acid are used in the modulated UiO-66 syn-
thesis, terephthalate linkers are partially replaced by trifluoroac-
etate, resulting in a more open framework with a large number
of coordinative vacancies.
Recently a mesoporous zirconium–phytic acid hybrid material
was tested as a MPV-catalyst for the conversion of carbonyl
Scheme 2. Side products during transfer hydrogenation of cinnamaldehyde in
isopropanol: 1-cinnamyl 2-propyl ether and cinnamaldehyde diisopropyl acetal.

E. Plessers et al. / Journal of Catalysis 340 (2016) 136–143
141
Fig. 2. MPV reduction of CALD with UiO-66-NO2 (j) and hot filtration of the
catalyst (h).
compounds in isopropanol [39]; high yield and selectivity were
obtained, however with 65 mol% Zr and at 100 °C instead of
82 °C. When increasing reaction temperature to 120 °C for all three
UiO-66 materials, conversion and selectivity are considerably
improved and side product formation markedly decreases, espe-
cially for UiO-66 and UiO-66-NO2. After 24 h respectively 94 4%
and 93 1% selectivity to cinnamyl alcohol is reached at complete
conversion for UiO-66 and 84 2% conversion for UiO-66-NO2. At
low conversion cinnamaldehyde diisopropyl acetal was the major
side product, when conversion increased also the formation of
1-cinnamyl-2-propyl ether increased. However the side products
only account for maximum 2 mol% of the product mixture after
24 h in the case of UiO-66 and UiO-66-NO2. In the MPV-
reduction with UiO-66-NH2 at 120 °C, the selectivity to cin-
namaldehyde diisopropyl acetal and 1-cinnamyl-2-propyl ether is
respectively 7% and 1%, considerably less than the reduction at
82 °C where selectivity was respectively 66% and 2%. All reactions
were carried out in duplo and after hot filtration of the reaction
mixture after 5 h to remove the catalyst, conversion did not
increase after 24 h, indicating that there is no leaching of Zr into
the solution and the catalyst is truly heterogeneous (Fig. 2). To test
the recycling of the UiO-66 catalyst, the used catalyst was isolated
from the reaction mixture by centrifugation and thoroughly
washed with isopropanol until the supernatant was purely solvent,
as evidenced by GC. The recovered catalyst was reactivated at
150 °C and reused under the same reaction conditions. Every recy-
cling step causes a loss in activity of about 35%; however, selectiv-
ity to the alcohol is retained at >90%. After three recycling steps
about 25% of the initial cinnamyl alcohol yield is preserved. Pow-
der XRD confirms that recycling does not cause a measurable loss
in crystallinity (see supplementary information, Fig. S5). Further,
thermal gravimetric analysis (TGA) shows an increase of about
50 °C in the temperature at which the BDC linker is lost from the
UiO-66 framework and decomposed (see supplementary informa-
tion, Fig. S7). This indicates stronger bonding of the remaining link-
ers as the result of a decrease in linker molecules per Zr-cluster
during reaction and washing [54]. In a UiO-66 metal–organic
framework, the Zr-metal is theoretically 8-fold coordinated and
possesses no free coordination sites. But as the result of thermal
activation and linker deficiency, coordinatively unsaturated
Zr-sites (cus Zr) arise turning the Zr-metal into Lewis-acid catalyt-
ically active site. The further BDC linker loss during recycling
makes the Zr-sites more accessible and also more prone for poison-
ing, which is confirmed by a visual colour change to light yellow.
Linker loss can also lead to leaching of zirconium and empty micro-
domains [55,56]. However, hot filtration of the catalyst (Fig. 2)
indicates that leached species are not active as MPV catalyst.
Fig. 3. MPV reduction of
cinnamaldehyde (A), cis,trans-citral (B) and carvone (C) catalysed by UiO-66 (j),
UiO-66-NO2 (h) and UiO-66-NH2 (d) (120 °C, 3.3 mL IPA, substrate:Zr = 7.8).
3
different
a
,b-unsaturated carbonyl compounds:
Clearly UiO-66 is a very active and selective cinnamaldehyde
MPV catalyst, although there is still room for improvement in recy-
clability of the material.
With the reference ZrO2 catalyst no MPV-reduction activity is
measured (Table 3, entry 7), indicating the successful development
of catalytically active cus Zr-sites by incorporation in a metal–or-
ganic framework.
UiO-66 was thus successfully used as heterogeneous MPV
reduction catalyst for the
more challenging is the selective reduction of the
unsaturated linear aldehyde citral and the ,b-unsaturated ketone
a,b-unsaturated aldehyde CALD. Even
a,b-
a
carvone (Fig. 3B and C). The unsaturated alcohol product of the lat-
ter is also an important component in the flavour industry.
After 24 h more than 90% citral conversion is reached, with both
UiO-66 and UiO-66-NO2 and selectivities of respectively 93 1%
and 91 2%. The negative impact of amine-functionalised linkers
is also clearly observed in hydrogenation of both citral and car-
vone; in all reactions lower conversions and selectivities were
reached. Unsaturated ketones are typically harder to selectively
reduce than aldehydes, so it is no surprise carveol yields are lower.

142
E. Plessers et al. / Journal of Catalysis 340 (2016) 136–143
In this more demanding reduction reaction higher the stronger
acidic UiO-66-NO2 outperforms UiO-66 (Fig. 3C), both catalysts
display an excellent hydrogenation selectivity of 92 3% towards
the desired carveol. Infrared spectroscopy with CD3CN as probe
[27], confirmed that the NO2-functionalisation enhances the Lewis
acidity of coordination vacancies of the adjacent Zr-metal. There-
fore, the catalytic performance in MPV reactions can be expected
to decrease in the following order: UiO-66-NO2 > UiO-66 > UiO-
66-NH2. This trend is not fully obeyed in the MPV-reduction of
CALD (Fig. 3A) and citral (Fig. 3B). This can be due to several
reasons: (1) the already high transfer hydrogenation activity of
UiO-66 with these substrates might mask the enhancement by
the NO2-group, (2) NO2-functionalisation not only induces the
electronic effect of increased Lewis acidity, but may also increase
steric hindrance, (3) UiO-66 and UiO-66-NO2 do not necessarily
have exactly the same amount of active sites. However, with more
difficult substrates such as carvone the beneficial effect of NO2-
functionalisation on the conversion is clearly observed in the trans-
fer hydrogenation.
thank the Fonds Wetenschappelijk Onderzoek (FWO) (Grant
G.0990.11, Ph.D. scholarships to E.P.), BELSPO (IAP-VII/05), and
the Flemish government (Long term structural funding-
Methusalem funding CASAS METH/08/04).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2016.05.013.
These data include MOL files and InChiKeys of the most important
compounds described in this article.
References
[1] S. Nishimura, Handbook of Heterogeneous Catalytic Hydrogenation for Organic
Synthesis, John Wiley and Sons, New York, 2001, p. 178.
[2] Y. Yuan, S. Yao, M. Wang, S. Lou, N. Yan, Curr. Org. Chem. 17 (2013) 400–413.
[3] L.A. Saudan, Acc. Chem. Res. 40 (2007) 1309–1319.
[4] H.-D. Belitz, W. Grosch, P. Schieberle, Food Chemistry, Springer, Berlin, 2009.
[5] J. Mikkola, P. Virtanen, H. Karhu, T. Salmi, D.Y. Murzin, Green Chem. 8 (2006)
197–205, http://dx.doi.org/10.1039/b508033a.
4. Conclusion
[6] P. Gallezot, D. Richard, Catal. Rev. Sci. Eng. 40 (1998) 81–126, http://dx.doi.org/
10.1080/01614949808007106.
[7] P. Claus, Top. Catal. 5 (1998) 51–62.
[8] V. Ponec, Appl. Catal. A Gen. 149 (1997) 27–48.
Zr-based UiO-66 metal–organic frameworks were successfully
used for the chemoselective hydrogenation of cinnamaldehyde,
both in high pressure hydrogenation as supports for Ag nanoparti-
cles, and as transfer hydrogenation catalyst in the Meerwein–Ponn
dorf–Verley reduction. A 10 wt% Ag/UiO-66 catalyst reached com-
plete conversion after 24 h at 50 bar of H2 with 66% selectivity for
cinnamyl alcohol in the inert solvent N,N-dimethylacetamide
(DMA). Recycling experiments evidenced the durability and
reusability of the catalyst since the original catalytic activity was
retained up to 5 times recycling with only minor loss in chemose-
lectivity. Compared to silver loaded alumina and silica, the UiO-66
framework offers a more efficient stabilisation of the Ag nanopar-
ticles during catalytic reaction as evidenced by powder XRD. The
conversion and selectivity obtained with these UiO-66 supported
silver catalysts are however still lower than the reported results
for other supported metal catalysts. If high pressure hydrogen dri-
[9] B. Corain, G. Schmid, N. Toshima, Metal Nanoclusters in Catalysis and Materials
Science: The Issue of Size Control, Elsevier, Amsterdam, 2007, p. 167.
[10] G.K. Chuah, S. Jaenicke, Y.Z. Zhu, S.H. Liu, Curr. Org. Chem. 10 (2006) 1639–
1654.
[11] S.H. Liu, S. Jaenicke, G.K. Chuah, J. Catal. 206 (2002) 321–330, http://dx.doi.org/
10.1006/jcat.2001.3480.
[12] M. De bruyn, M. Limbourg, J. Denayer, G.V. Baron, V. Parvulescu, P.J. Grobet,
et al., Appl. Catal. A Gen. 254 (2003) 189–201, http://dx.doi.org/10.1016/
S0926-860X(03)00483-6.
[13] Y. Zhu, S. Liu, S. Jaenicke, G. Chuah, Catal. Today 97 (2004) 249–255, http://dx.
doi.org/10.1016/j.cattod.2004.07.002.
[14] J. Hagen, Industrial Catalysis: A Practical Approach, WILEY-VCH, Weinheim,
Germany, 2006.
[15] H. Liu, Z. Li, Y. Li, Ind. Eng. Chem. Res. 54 (2015) 1487–1497, http://dx.doi.org/
10.1021/ie504357r.
[16] G. Neri, I. Arrigo, F. Corigliano, L. De Luca, A. Donato, Catal. Lett. 141 (2011)
1590–1597, http://dx.doi.org/10.1007/s10562-011-0688-x.
[17] P.G.N. Mertens, P. Vandezande, X. Ye, H. Poelman, I.F.J. Vankelecom, D.E. De
Vos, Appl. Catal.
A Gen. 355 (2009) 176–183, http://dx.doi.org/10.1016/j.
apcata.2008.12.013.
ven reduction of
a,b-unsaturated carbonyl compounds is com-
[18] M. Steffan, a. Jakob, P. Claus, H. Lang, Catal. Commun. 10 (2009) 437–441,
http://dx.doi.org/10.1016/j.catcom.2008.10.003.
[19] T. Sawai, T. Yonehara, A. Yonezawa, M. Sano, T. Suzuki, T. Miyake, J. Jpn. Petrol.
Inst. 57 (2014) 58–64.
[20] H. Wei, C. Gomez, J. Liu, N. Guo, T. Wu, R. Lobo, et al., J. Catal. 298 (2013) 18–26,
http://dx.doi.org/10.1016/j.jcat.2012.10.027.
[21] M.S. Ide, B. Hao, M. Neurock, R.J. Davis, ACS Catal. 2 (2012) 671–683.
[22] Z. Tian, Q. Li, J. Hou, L. Pei, Y. Li, S. Ai, J. Catal. 331 (2015) 193–202, http://dx.
doi.org/10.1016/j.jcat.2015.08.020.
pared to Meerwein–Ponndorf–Verley (MPV) reduction, MPV is
much more selective in the hydrogenation of C@O bonds when
pure UiO-66 was used as transfer hydrogenation catalyst. Com-
plete conversion was reached with >90% selectivity without the
need of a precious metal. The substrate scope was further extended
to citral and carvone, two other
a,b-unsaturated carbonyl com-
pounds that are harder to selectively reduce. Although there are
side products, the higher selectivity obtained at milder conditions
and excellent Lewis acid properties of the Zr-metal in the modu-
lated UiO-66 framework make transfer hydrogenation with this
material preferable over high H2 pressure hydrogenation. The suc-
cessful use of one material, such as UiO-66, for the same reaction
both as support and as catalyst demands critical interpretation of
experimental results when using a Lewis acid support material in
the high pressure hydrogenation of carbonyl compounds. The cat-
alytic performance of the supported metal nanoparticles in the H2-
driven hydrogenation could be strongly biased by the support-
catalysed transfer hydrogenation. Advances in both H2-driven
hydrogenation and MPV-reduction are reported on a regular basis;
however, reports investigating both mechanisms at the same time
are rare.
[23] R.J. White, R. Luque, V.L. Budarin, J.H. Clark, D.J. Macquarrie, Chem. Soc. Rev. 38
(2009) 481–494, http://dx.doi.org/10.1039/b802654h.
[24] K. Furman, D. Baudouin, T. Margossian, K.D. Sabnis, Y. Cui, F.H. Ribeiro, et al., J.
Catal. 324 (2015) 9–13, http://dx.doi.org/10.1016/j.jcat.2015.01.006.
[25] P. Mäki-Arvela, J. Hájek, T. Salmi, D.Y. Murzin, Appl. Catal. A Gen. 292 (2005) 1–
49, http://dx.doi.org/10.1016/j.apcata.2005.05.045.
[26] C. Volckmar, M. Bron, U. Bentrup, A. Martin, P. Claus, J. Catal. 261 (2009) 1–8,
http://dx.doi.org/10.1016/j.jcat.2008.10.012.
[27] A. Chambers, S.D. Jackson, D. Stirling, G. Webb, J. Catal. 168 (1997) 301–314.
[28] P. Claus, H. Hofmeister, J. Phys. Chem. B 103 (1999) 2766–2775, http://dx.doi.
org/10.1021/jp983857f.
[29] S. Handjani, E. Marceau, J. Blanchard, J.-M. Krafft, M. Che, P. Mäki-Arvela, et al.,
J. Catal. 282 (2011) 228–236, http://dx.doi.org/10.1016/j.jcat.2011.06.017.
[30] H. Bernas, I. Simakova, I.P. Prosvirin, P. Mäki-Arvela, R. Leino, D.Y. Murzin,
Catal. Lett. 142 (2012) 690–697, http://dx.doi.org/10.1007/s10562-012-0809-
1.
[31] Z. Guo, C. Xiao, R.V. Maligal-Ganesh, L. Zhou, T.W. Goh, X. Li, et al., ACS Catal. 4
(2014) 1340–1348.
[32] A.L. Wilds, Org. React. 2 (1944) 178–223.
[33] M. De bruyn, D.E. De Vos, P.A. Jacobs, Adv. Syn. Catal. 344 (2002) 1120–1125.
[34] Y. Zhu, S. Jaenicke, G.K. Chuah, J. Catal. 218 (2003) 396–404, http://dx.doi.org/
10.1016/S0021-9517(03)00160-X.
[35] Y. Zhu, G. Chuah, S. Jaenicke, J. Catal. 227 (2004) 1–10, http://dx.doi.org/
10.1016/j.jcat.2004.05.037.
Acknowledgements
The authors thank Dries De Sloovere and Xian Gu for helping
with the experimental work. For financial support, the authors
[36] Y. Zhu, G. Chuah, S. Jaenicke, J. Catal. 241 (2006) 25–33, http://dx.doi.org/
10.1016/j.jcat.2006.04.008.

E. Plessers et al. / Journal of Catalysis 340 (2016) 136–143
143
[37] A. Ramanathan, M.C. Castro Villalobos, C. Kwakernaak, S. Telalovic, U.
Hanefeld, Chem. Eur. J. 14 (2008) 961–972, http://dx.doi.org/10.1002/
chem.200700725.
[38] F. Vermoortele, B. Bueken, G. Le Bars, B. Van de Voorde, M. Vandichel, K.
Houthoofd, et al., J. Am. Chem. Soc. 135 (2013) 11465–11468.
[39] J. Song, B. Zhou, H. Zhou, L. Wu, Q. Meng, Z. Liu, et al., Angew. Chem. Int. Ed. 54
(2015) 9399–9403, http://dx.doi.org/10.1002/anie.201504001.
[40] H.Y. Luo, D.F. Consoli, W.R. Gunther, Y. Román-Leshkov, J. Catal. 320 (2014)
198–207, http://dx.doi.org/10.1016/j.jcat.2014.10.010.
[46] M. Quinten, Appl. Phys. B Lasers Opt. 73 (2001) 317–326, http://dx.doi.org/
10.1007/s003400100666.
[47] D. Astruc, F. Lu, J.R. Aranzaes, Angew. Chem. Int. Ed. 44 (2005) 7852–7872,
http://dx.doi.org/10.1002/anie.200500766.
[48] J. Clayden, N. Greeves, S. Warren, Organic Chemistry, Oxford University Press,
New York, 2012, p. 353.
[49] P.Y. Bruice, Organic Chemistry, seventh ed., Pearson, New York, 2014, p. 814.
[50] R.A. Sheldon, H. van Bekkum, Fine Chemicals through Heterogeneous Catalysis,
WILEY-VCH Verlag, GmbH Weinheim, 2001, p. 438.
[41] F.G. Cirujano, a. Corma, F.X. Llabrés i Xamena, Chem. Eng. Sci. 124 (2015) 52–
60, http://dx.doi.org/10.1016/j.ces.2014.09.047.
[42] L. Valenzano, B. Civalleri, S. Chavan, S. Bordiga, M.H. Nilsen, S. Jakobsen, et al.,
Chem. Mater. 23 (2011) 1700–1718.
[51] M. Boronat, A. Corma, M. Renz, J. Phys. Chem. B 110 (2006) 21168–21174.
[52] F. Vermoortele, M. Vandichel, B. Van de Voorde, R. Ameloot, M. Waroquier, V.
Van Speybroeck, et al., Angew. Chem. Int. Ed. 51 (2012) 4887–4890, http://dx.
doi.org/10.1002/anie.201108565.
[43] K. Shimizu, K. Sawabe, A. Satsuma, Catal. Sci. Technol. 1 (2011) 331, http://dx.
doi.org/10.1039/c0cy00077a.
[44] B. Van de Voorde, I. Stassen, B. Bueken, F. Vermoortele, D. De Vos, R. Ameloot,
[53] J. Hajek, M. Vandichel, B. Van de Voorde, B. Bueken, D. De Vos, M. Waroquier,
et al., J. Catal. 331 (2015) 1–12, http://dx.doi.org/10.1016/j.jcat.2015.08.015.
[54] C. Trickett, K.J. Gagnon, S. Lee, F. Gándara, H.-B. Bürgi, O.M. Yaghi, Angew. Chem.
Int. Ed. 54 (2015) 11162–11167, http://dx.doi.org/10.1002/anie.201505461.
[55] S. Ling, B. Slater, Chem. Sci. (2016), http://dx.doi.org/10.1039/C5SC04953A.
[56] M.J. Cliffe, W. Wan, X. Zou, P. Chater, A.K. Kleppe, M.G. Tucker, et al., Nat.
Commun. 5 (2014) 4176, http://dx.doi.org/10.1038/ncomms5176.
et al., J. Mater. Chem.
C4TA06396A.
A 3 (2015) 1737–1742, http://dx.doi.org/10.1039/
[45] R.J.T. Houk, B.W. Jacobs, F. El Gabaly, N.N. Chang, A.A. Talin, D.D. Graham, et al.,
Nano Lett. 9 (2009) 3413–3418, http://dx.doi.org/10.1021/nl901397k.