Double-supported silica-metal-organic framework palladium nanocatalyst for the aerobic oxidation of alcohols under batch and continuous flow regimes

Article abstract of DOI:10.1021/cs501573c

Stable and easily synthesized metal-organic framework MIL-88B-NH₂ represents an attractive support for catalysts employed in oxidation reactions, which are typically performed under relatively harsh conditions. However, MIL-88B-NH₂, the thermodynamic polymorph of the more popular MIL-101-NH₂, has been rarely employed in catalytic applications because of a difficult impregnation process caused by the flexible nature of the framework. We report herein a new catalyst denoted Pd@MIL-88B-NH₂ (8 wt % Pd), the first example of metallic nanoparticles successfully impregnated in the pores of MIL-88B-NH₂. Furthermore, by enclosing the MOF crystals in a tailored protective coating of SiO₂ nanoparticles, an even more enduring material was developed and applied to the aerobic oxidation of benzylic alcohols. This doubly supported catalyst Pd@MIL-88B-NH₂@nano-SiO₂ displayed high activity and excellent performance in terms of endurance and leaching control. Under batch conditions, a very convenient and efficient recycling protocol is illustrated, using a teabag approach. Under continuous flow, the catalyst was capable of withstanding 7 days of continuous operation at 110 °C without deactivation. During this time, no leaching of metallic species was observed, and the material maintained its structural integrity.

Full text of DOI:10.1021/cs501573c

Research Article
pubs.acs.org/acscatalysis
Double-Supported Silica-Metal−Organic Framework Palladium
Nanocatalyst for the Aerobic Oxidation of Alcohols under Batch and
Continuous Flow Regimes
†
,‡,#
†,‡,#
∥
‡,§
‡,§
Vlad Pascanu,
Antonio Bermejo Gom
́
ez,
̀
Carles Ayats, Ana Eva Platero-Prats, Fabian Carson,
‡
,§
‡,§
,∥,⊥ Xiaodong Zou,* and Belen
,‡,§
,†,‡
̀
Jie Su, Qingxia Yao, Miquel A. Pericas Martín-Matute*
́
†
‡
§
Department of Organic Chemistry, Berzelii Center EXSELENT on Porous Materials, and Department of Materials and
Environmental Chemistry, Stockholm University, Stockholm SE-10691, Sweden
∥
Institute of Chemical Research of Catalonia, E-43007 Tarragona, Spain
⊥
Department of Organic Chemistry, University of Barcelona, E-08028 Barcelona, Spain
S Supporting Information
*
ABSTRACT: Stable and easily synthesized metal−organic
framework MIL-88B-NH represents an attractive support for
2
catalysts employed in oxidation reactions, which are typically
performed under relatively harsh conditions. However, MIL-
8
8B-NH , the thermodynamic polymorph of the more popular
2
MIL-101-NH , has been rarely employed in catalytic
2
applications because of a difficult impregnation process caused
by the flexible nature of the framework. We report herein a new catalyst denoted Pd@MIL-88B-NH (8 wt % Pd), the first
2
example of metallic nanoparticles successfully impregnated in the pores of MIL-88B-NH . Furthermore, by enclosing the MOF
2
crystals in a tailored protective coating of SiO nanoparticles, an even more enduring material was developed and applied to the
2
aerobic oxidation of benzylic alcohols. This doubly supported catalyst Pd@MIL-88B-NH @nano-SiO displayed high activity
2
2
and excellent performance in terms of endurance and leaching control. Under batch conditions, a very convenient and efficient
recycling protocol is illustrated, using a “teabag” approach. Under continuous flow, the catalyst was capable of withstanding 7
days of continuous operation at 110 °C without deactivation. During this time, no leaching of metallic species was observed, and
the material maintained its structural integrity.
KEYWORDS: palladium nanoparticles, MIL-88B-NH , aerobic oxidation, teabag catalysis, flow chemistry
2
1
0,11
INTRODUCTION
in heterogeneous catalysis
because of their high surface
■
areas and large pore volumes, which can accommodate the
active catalytic sites. Another advantage is the possibility to
modify MOFs postsynthetically in order to tune the properties
The aerobic oxidation of alcohols is an important industrial
process. Sustained interest in the field is partially driven by the
prospect of applying aerobic oxidation to biomass conversion,
12
1
of the framework. Metallic nanoparticles can be stabilized in
the pores of MOFs through both mechanical and charge-
which is an abundant source of alcohols. Recent advances in
the area aim to use mild conditions and replace hazardous
reagents with air as the oxidant. Supported heterogeneous
oxidation catalysts have also received widespread attention
because they can be recovered and recycled, and palladium is
often the transition metal chosen to perform the oxidation
reaction. For example, Pd@ARP (amphiphilic resin) was used
for the aerobic oxidation of secondary alcohols in water, and
Pd@SBA-15 (SBA, functionalized Santa Barbara amorphous
13
2
transfer interactions. Through these mechanisms, highly
active, small nanoparticles with a narrow size distribution can
be generated, and their leaching can be avoided, facilitating the
recycling of the catalyst. On the basis of these properties,
combining the benefits of MOFs and flow chemistry can be
regarded as a natural direction for progress in the field, and we
recently reported the first example of metallic nanoparticles
3
4
5
6
11d
7
supported on MOFs for flow processes.
silica material) was used to oxidize benzylic alcohols at
Among the first MOF-supported catalysts employed for the
temperatures as low as 80 °C. Hutchings and co-workers
reported that Au−Pd bimetallic core−shell nanoparticles
supported on TiO could selectively oxidize benzylic and allylic
oxidation of alcohols, Li and co-workers developed a Pd catalyst
14
supported on MIL-101-NH2. This system displayed good
2
−1
efficiency if molecular oxygen was bubbled through the reaction
alcohols with exceptional turnover frequencies (269 000 h )
8
under neat conditions at 100−160 °C.
Metal−organic frameworks (MOFs) are porous crystalline
Received: October 14, 2014
Revised: November 24, 2014
materials assembled from metallic ions or clusters and organic
9
multitopical linkers. MOFs have gained popularity as supports
©
XXXX American Chemical Society
472
dx.doi.org/10.1021/cs501573c | ACS Catal. 2015, 5, 472−479

ACS Catalysis
Research Article
mixture at a sufficiently high rate. The authors also suggest an
important contribution from the acid sites in the framework for
the outcome of the reaction. More recently, a modified version
of UiO-66 was also shown to tolerate a very wide scope of
substrates and to be recyclable in the presence of sacrificial
high total turnover numbers are achieved. Therefore, this is a
more durable catalyst than previously reported, synthesized in a
more economical manner, fulfilling the main directives of green
chemistry through catalysis: efficiency, sustainability, and
22−25
recyclability.
15
oxidants (e.g., TBHP and H O ). Around the same time, it
2
2
was shown that TiO2 nanoparticles incorporated in the
mesoporous HKUST-1 were capable of performing a photo-
catalytic oxidation of alcohols in moderate yields under sunlight
RESULTS AND DISCUSSION
Synthesis of Pd@MIL-88B-NH @nano-SiO . MIL-88B-
■
2
2
NH was synthesized under solvothermal conditions and
2
1
6
irradiation. However, it is well known that many MOFs suffer
activated according to an adapted procedure from Lin and
26
from poor mechanical strength compared to denser supports,
co-workers. The crystallinity of the MOF was confirmed by
X-ray powder diffraction (XRPD) and is shown in Figure 1a.
1
7
such as ceramics. On the basis of this limitation, we
envisioned that coating the MOF with a protective layer
could improve its stability and reusability for catalytic processes.
An interesting solution consists of surrounding the catalyst with
SiO2 nanoparticles that contain mesopores between the
particles, which were recently shown to improve the diffusion
1
8
into the MOF. In this way, the mechanical properties of the
MOF are improved while also allowing reactants to reach the
active sites.
Herein we report the development of a new palladium
nanocatalyst protected by an innovative double-layered MOF@
silica support. For the first time, the immobilization of metallic
nanoparticles could be achieved in the flexible functionalized
MIL-88B-NH framework. This long-elusive goal was accom-
2
plished after investigating the manner in which the material
responds to the polarity of its surrounding environment by
alternating its open/close conformations. MIL-88B-NH is the
2
denser thermodynamic polymorph of the notorious MIL-101-
NH , and thus the proposed catalyst is expected to withstand
2
much harsher reaction conditions than previously reported
1
4,19
Pd@MIL-101-type catalysts.
A good dispersion of the Pd
nanoparticles is achieved in MOFs containing nitrogen
functional groups in the organic linkers. These coordinating
groups also contribute to reduce the metal leaching during
2
0
catalysis. Importantly, the functionalized MIL-88B-NH MOF
2
Figure 1. (a) XRPD patterns of MIL-88B-NH , Pd(II)@MIL-88B-
2
could be more conveniently synthesized directly from 2-
aminoterephthalic acid, thereby avoiding wasteful postsynthetic
modification procedures required for its MIL-101 analog. The
NH , and Pd(0)@MIL-88B-NH2 and (b) schematic polyhedral
2
representation of Pd(II) impregnation in MIL-88B-NH . The purple
2
21
ball represents Pd. The blue and light green balls represent nitrogen
and chloride, respectively. The Pd(II) species lead to MIL-88B-NH2
adopting a semi-open form, highlighted in the XRPD patterns by
asterisks (*).
MOF was subsequently enclosed by SiO nanoparticles to
2
produce a more resistant material (Scheme 1) labeled Pd@
Scheme 1. Graphical Illustration of the Synthesis of Pd@
MIL-88B-NH @nano-SiO
2
2
The impregnation of this MOF with palladium can be
challenging because of the flexible nature of the framework. A
screening experiment using solvents with different polarities
confirmed that the impregnation is more efficient with polar
solvents and the best results were obtained in MeOH (ESI,
28
27
Table S1). The structure of MIL-88B expands in MeOH,
which facilitates the introduction of Pd(II) complexes into the
MOF pores. Higher loadings of palladium were achieved with
Na PdCl when compared to PdCl (CH CN) , and after 72 h,
MIL-88B-NH @nano-SiO . This material was also more
2
2
2
4
2
3
2
suitable for stabilizing small nanoparticles and preventing
loadings of between 7.15 and 8.60 wt % were achieved, as
determined by ICP-OES analysis. This corresponds to a Pd/Cr
ratio of 1:4 and ∼26% functionalization of the amino-
5c
metallic leaching than simple Pd@SiO catalysts. The catalytic
2
activity of this novel material was evaluated in the oxidation of
benzylic alcohols to their corresponding carbonyl compounds
using only air as the oxidant, at ambient pressure without using
any bubbling device. The benefits of this new support are also
demonstrated by the outstanding properties of the catalyst
under a continuous-flow regime. When packed in a microflow
reactor, the material can endure more than 7 days of
continuous operation without diminishing its efficiency. Metal
leaching is minimized throughout the whole experiment, and
29
terephthalate linkers. NaBH was then used to reduce Pd(II)
4
and form palladium nanoparticles in the MOF [Pd(0)@MIL-
88B-NH ]. The XRPD pattern matched that of MIL-88B-NH ,
2
2
confirming the integrity of the framework upon functionaliza-
30
tion (Figure 1a).
Pd K-edge extended X-ray absorption fine structure
(EXAFS) spectroscopy was performed on Pd(II)@MIL-88B-
NH (Cr) to determine the coordination environment of
2
4
73
dx.doi.org/10.1021/cs501573c | ACS Catal. 2015, 5, 472−479

ACS Catalysis
Research Article
palladium (Figure 2a). The data were best fit using a cis square-
planar geometry at the Pd(II) center, with the coordination
which implies that the MOF is partially locked in a semi-open
30
form (ESI, Section S4).
The reduction of Pd(II), following a procedure similar to
18
that described for Pd(0)@MIL-101-NH2, afforded Pd(0)@
MIL-88B-NH . A second layer of protection was achieved by
2
coating the Pd(0)@MIL-88B-NH crystals with nanosized SiO
2
2
particles (ESI, Section S1 for the detailed synthesis procedure).
The material was washed with HCl (aq, pH 2) to remove
residual TEOS, which can affect its catalytic performance.
Traces of acid were neutralized with an aqueous solution of
Et N. The integrity of the MOF framework was confirmed by
3
XRPD, which contained a large background hump attributed to
the silica (Figure 3a). This new material, denoted Pd@MIL-
8
8B-NH @nano-SiO , contained 0.51 wt % Pd, as measured by
2 2
ICP-OES analysis. It is noteworthy that a similar coating
strategy applied to Pd@MIL-101-NH2 led to the decom-
position of the MOF and the formation of a completely
amorphous material.
The Pd nanoparticles were evenly distributed in the MOF
with an average particle size of 2 to 3 nm, as shown by
transmission electron microscopy (TEM; ESI, Section S5).
Furthermore, the presence of nanosized SiO2 particles
surrounding the Pd@MOF crystal surface is evident in Figure
3
b. Some larger Pd agglomerates were visible on the MOF
surface subsequent to silica coating (Figure 3b).
N sorption experiments on Pd(0)@MIL-88B-NH showed
2
2
very low gas uptake, indicating that the closed form of the
MOF is predominant. In contrast, the SiO -supported material
2
displayed a type IV isotherm with significant N uptake (Figure
3c). This hysteresis is due to mesopores between the silica
2
Figure 2. (a) Pd K-edge XAFS spectra for Pd(II)@MIL-88B-NH and
34
2
3
k-weighted XAFS, χ(k)k (detail). (b) Experimental (solid line) and
3
nanoparticles. The BET surface area and pore volume for Pd@
fitted (dashed line) FT[χ(k)k ] functions (bottom) with Δk ≈ 2−14
2
−1
MIL-88B-NH @nano-SiO were calculated to be 603 ± 4 m /g
Å . The palladium coordination environment was used to fit the
EXAFS data (hydrogen atoms omitted for clarity). The purple ball
represents palladium. The blue and light-green balls represent nitrogen
and chloride, respectively.
2
2
3
and 1.82 cm /g, respectively (ESI, Section S4).
The TGA analyses of all materials showed a considerably
improved thermal resistance (ca. 60 °C) of the doubly
supported silica-MOF compared to that of the original Pd@
MOF catalyst (ESI, Section S6).
environment comprising chloride anions and two amino groups
CATALYTIC PROPERTIES
(
Figures 1b and 2b). The cis conformation appears to be more
■
likely than the trans (ESI, Section S2), which agrees with
palladium(II) complexes located in the pores of the MOFs’
closed form (Figure 2). The data fitting indicated Pd bonds to
two N atoms at 2.06(1) Å and two Cl atoms at 2.307(5) Å.
These distances are in agreement with the crystallographic data
The properties of the new material were then investigated in
terms of robustness and catalytic activity in the oxidation of
secondary benzylic alcohols. Pd@MIL-88B-NH @nano-SiO
2
2
was demonstrated to be highly active and selective, using air
as the oxidant in p-xylene at 150 °C, tolerating a wide variety of
substitution patterns (Table 1). With typical substrates such as
1-phenylethanol (1a), 1-(4′-methylphenyl)ethanol (1b), 1-(4′-
methoxyphenyl)ethanol (1c), and 1-phenylpropanol (1d),
Pd@MIL-88B-NH @nano-SiO (2 mol % Pd) returned the
31
of reported [Pd(II)(ArNH ) Cl ] complexes. The nearest-
2
2
2
neighbor carbon atom from the phenyl ring was determined at
a distance of 3.38(5) Å, which is consistent with a semi-open
form of this MOF (3.052 Å crystallographic value for the closed
2
2
3
2
form).
corresponding ketones (2a−2d) in excellent yields within
reasonably short reaction times (Table 1, entries 1−4).
Substrates containing cyclic motifs (Table 1, entry 6, 2f) and
bulky substituents on either side of the hydroxyl group (Table
1, entries 5 and 7−9, 2e and 2g−2i) were easily converted to
the desired carbonyl compounds in reaction times ranging from
15 to 20 h and were isolated in excellent yields. More
functionalized substrates, such as diol 1j (Table 1, entry 10)
and benzoin 1k (Table 1, entry 11), were also transformed into
the corresponding ketones (2j−2k) with no difficulties and in
very good isolated yields. Although these last two substrates
with electron-withdrawing groups afforded good results (1j−
1k), other benzylic alcohols substituted with nitrogen-
containing electron-withdrawing groups in the para position
(2l and 2m) could not be oxidized (Table 1, entries 12 and 13).
To further study the functionalization of MIL-88B-NH with
2
palladium complexes, we performed N sorption experiments
2
and LeBail refinements of the XRPD patterns (ESI, Figure S3).
Pd(II)@MIL-88B-NH contains two forms of MIL-88B: a
2
closed form and a semi-open form in a ratio of ∼70:30 (V =
3
1
976(1) and 2099(1) Å , respectively, see Table S4). We
attribute the presence of this semi-open form to Pd(II)
attached to the amino groups in the MOF, which agrees with
the results from EXAFS and ICP-OES (Figure 2). The
33
underlying net of MIL-88B is acs-a, which consists of packing
six-membered rings in a chair conformation. MIL-88B is
flexible, and palladium incorporation into the MOF changes the
degree of pore opening. In addition, the BET surface area
2
increases from 7 to 30 m /g after palladium impregnation,
4
74
dx.doi.org/10.1021/cs501573c | ACS Catal. 2015, 5, 472−479

ACS Catalysis
Research Article
Table 1. Substrate Scope
Figure 3. (a) XRPD patterns of Pd(0)@MIL-88B-NH and Pd(0)@
2
MIL-88B-NH @nano-SiO , (b) TEM micrograph of Pd(0)@MIL-
2
2
8
8B-NH @nano-SiO , and (c) N adsorption (filled symbols) and
2 2 2
desorption (open symbols) isotherms of Pd(0)@MIL-88B-NH and
2
Pd(0)@MIL-88B-NH @nano-SiO .
2
2
However, alcohol 1n, with a pyridyl substituent, afforded the
product in good yield when 4 mol % Pd was used (Table 1,
entry 14). Importantly, when 1-phenylethanol (1a) was reacted
under neat conditions on a 10 mmol scale, the catalyst loading
could be lowered to 0.04 mol % Pd and a full conversion could
still be achieved within 12 h with complete selectivity toward
the oxidation product (Table 1, entry 1). A blank silica material
synthesized in a similar manner without the addition of Pd@
MIL-88B-NH2 was employed under the standard reaction
conditions, but no conversion was observed. When the reaction
a
Reaction run under neat conditions, with no byproduct formation.
Yield estimated by H NMR spectroscopy.
b
1
4
75
dx.doi.org/10.1021/cs501573c | ACS Catal. 2015, 5, 472−479

ACS Catalysis
Research Article
of 1a was carried out without catalyst under an atmosphere of
air at 150 °C for 20 h, 2a was obtained in less than 5% yield.
Although the teabag approach led to a convenient handling
of the material, its rather rapid deactivation under these
conditions was not satisfactory. One of the main limitations of
the reaction under the conditions described above is the low
solubility of air in BTX solvents (benzene, toluene, and xylene).
This limitation was aggravated by the fact that stirring was
significantly impeded, and no additional air-bubbling device was
used.
RECYCLING EXPERIMENTS USING A CATALYTIC
TEABAG
The beneficial effect of the innovative silica protective layer
■
over the recyclability of the new material was demonstrated in a
“teabag” experiment (Figure 4, Movie S1) showing that Pd@
AEROBIC OXIDATION UNDER CONTINUOUS
FLOW
■
We predicted that a better mixing of the liquid and gaseous
phases would drastically improve the reaction rate and would
allow us to decrease the reaction temperature and prolong the
lifetime of the catalyst. This hypothesis was implemented and
evaluated in a continuous-flow setup, which led to an
exceptional improvement over the reaction outcome.
1
-Phenylethanol (1a) was chosen as a model substrate for
oxidation in a continuous-flow regime, and synthetic air (20.9 ±
% pure O ) was used as the oxidizing agent. The instrumental
1
2
setup was adapted for a three-phase reaction at high
temperature, as schematically represented in Figure 5 (detailed
Figure 4. (a) Ingredients for the preparation of the catalytic teabag,
b) catalytic teabag before use, (c) reaction setup using a Radley
(
carousel for reactions open to air, (d) catalytic teabag after five runs,
and (e) change in color upon deactivation of the catalyst.
Figure 5. Instrumental setup for aerobic oxidation under continuous
flow.
MIL-88B-NH @nano-SiO is remarkably easy to handle and
2
2
procedure in Supporting Information). One of the essential
components in the system is a microchip mixing device (1 mL
volume, 1.6 m length microchannel) where the liquid phase
recycle. In contrast, the separation of the particles of Pd@MIL-
8B-NH (i.e., without silica) from the reaction mixtures is
8
2
tedious, which is due to the small size of the silica-free particles
and can be achieved only through centrifugation. The catalyst
(
(
0.1 M, 1-phenylethanol in toluene) and gaseous phase
synthetic air) were pumped simultaneously (Movie S2). By
(
200 mg of 0.51 Pd wt %, 1 mol %) in a teabag firmly tied with
a Teflon wire was inserted into a solution of 1-phenylethanol
1a) in p-xylene. After 16 h at 150 °C, the catalyst was pulled
precisely regulating the flow rates of the two components, very
effective mixing is achieved before the reaction mass reaches the
catalyst. This was decisive in order to achieve a satisfactory
conversion in one pass through the reactor at only 110 °C,
compared to 150 °C under batch conditions (vide supra).
The packed-bed reactor consisted of a fritted-glass
chromatography column loaded with 0.51 wt % Pd(0)@MIL-
(
out using the attached Teflon wire and subsequently immersed
in a fresh reaction batch. This facile recycling process could be
repeated four times to obtain very good yields (Table 2) until a
significant drop in activity was observed, which was
accompanied by an intense yellow coloration of the solution.
8
8B-NH @nano-SiO (800 mg, 0.0385 mmol of Pd, ca. 40 mm
2 2
bed height) and kept at a constant temperature of 110 °C. To
avoid toluene evaporation, the reactor outlet was connected to
a supplementary pressure controller that provided a back
pressure of 0.9 bar. From the pressure controller, the reaction
mixture was directed further to a collection flask. The residence
time was determined to be ca. 10 min. Samples were collected
regularly and analyzed by achiral gas chromatography to
determine the conversion and by ICP-OES to measure the level
of leached metallic species.
Table 2. Conversion and Metallic Leaching over Five Runs
during the Catalyst Recycling Experiment in the Oxidation
of 1a
run
conversion (%)
Pd leaching (ppm)
Cr leaching (ppm)
1
2
3
4
5
85
86
82
71
38
0.2
0.1
0.8
11.1
0.8
0.3
0.3
0.3
0.8
0.4
Initially, toluene was passed through the column for several
minutes until the pressure was stabilized, followed by a mixture
4
76
dx.doi.org/10.1021/cs501573c | ACS Catal. 2015, 5, 472−479

ACS Catalysis
Research Article
toluene/MeOH (100:1) (details in Supporting Information,
Section S1.5). It was found that this treatment inhibits the
formation of acetals as byproducts, presumably by methanol
coordination to the free Lewis acid sites of the chromium
clusters. Subsequently, the column was washed again with
toluene and the reaction mixture was pumped in the microchip
where it was intensively mixed with air, and the experiment was
allowed to run continuously at 150 μL/min (50 μL/min and
where only the Pd(II) species can act as an oxidant. The
limitations of NaBH as a reducing agent for the synthesis of Pd
4
nanoparticles supported on MOFs were recently highlighted in
36
the literature, and a detailed investigation of this scenario is
currently underway in our laboratories.
3
5
CONCLUSIONS
■
We have developed a double-supported nanopalladium catalyst
and reported the first successful immobilization of catalytically
active metallic nanoparticles in the pores of the flexible and
100 μL/min for liquid and gaseous phases, respectively) for 1
week during which the catalyst displayed surprisingly stable
behavior. The conversion stabilized after the first 6 h and
remained almost constant at around 80% (∼75% acetophenone
robust MIL-88B-NH framework. The resulting Pd@MOF
2
material is coated with a customized protective layer of
mesoporous silica through an innovative approach that
drastically increases the durability and recyclability of the
catalyst.
+
∼5% ethylbenzene) until the experiment was stopped after 7
days (168 h). The evolution of the yields of acetophenone and
ethylbenzene in time is plotted in Figure 6 (all data points in
Supporting Information, Table S5).
Pd(0)@MIL-88B-NH @nano-SiO can catalyze the aerobic
2
2
oxidation of benzylic alcohols with very good efficiency while
completely avoiding the formation of byproducts by simply
performing the reaction in tubes open to air, avoiding the
hazardous yet common bubbling of oxygen in a reaction
mixture containing Pd and refluxing organic solvents.
Importantly, no additional oxidants or additives were required.
Pd(0)@MIL-88B-NH @nano-SiO was also used to prepare a
2
2
catalytic teabag, which allowed us to recycle and reuse the
catalyst in an extremely simple procedure that would be very
easily scaled up. Even though the catalyst was deactivated fairly
quickly when reused for five batches at 150 °C, the catalytic
teabag method is extremely efficient at recovering all of the
catalyst from the reaction mixture with minimal leaching of
metallic species and is a promising solution for large-scale
processes.
Figure 6. Conversion and metallic leaching in the aerobic oxidation of
1
-phenylethanol catalyzed by 0.51 wt % Pd(0)@MIL-88B-NH @SiO
2 2
over 7 days.
By adapting the reaction to a continuous-flow process in
which a much better mixing of air and starting materials was
achieved, we were able to decrease the temperature to 110 °C
while maintaining the same performance of the catalyst. Under
these conditions, Pd(0)@MIL-88B-NH @nano-SiO could be
The product was collected between 6 and 168 h and isolated
by distillation from toluene using a Vigreux column, followed
by column chromatography to yield 31.02 mmol (3.722 g) of
isolated acetophenone corresponding to a TOF of 4.97 h .
Most importantly, throughout the whole experiment (samples
measured at 6, 24, 96, and 168 h) the level of leached palladium
remained below the detection limit of the ICP-OES technique
−1
2
2
used for at least 7 days, showing no signs of deactivation in
−1
continuous operation at a turnover frequency of 5 h .
The extraordinary stability of Pd(0)@MIL-88B-NH @nano-
2
(
<0.1 ppm), whereas the chromium levels in solution were
SiO under these conditions represents conclusive proof that
2
constant at 0.2 ppm.
the deposition of the MOF in the silica matrix has a
tremendous beneficial effect on the framework. The protective
The recovered catalyst was washed with toluene and twice
with ethanol and then characterized. The XRPD pattern
properties of the SiO coating together with the adaptation of
2
2
showed that the recycled Pd@MIL-88B-NH @nano-SiO2
the catalytic system to operation under continuous flow lead to
a radical improvement in the methodology for the aerobic
oxidation of benzylic alcohols. This reaction was redesigned as a
safer and cleaner process, which could be more attractive for
scale up.
material remained crystalline (ESI, Figure S4). The ICP-OES
analysis revealed a limited degradation of the framework. The
C, H, and N contents were slightly lower in the recycled
material whereas the content of metallic species increased from
0.51 to 0.62 wt % for palladium and from 0.96 to 1.22 wt % for
chromium (ESI, Table S6). This result suggests that the silica
matrix is very efficient at retaining the metallic species and
preventing the contamination of the final product, even upon
MOF degradation. This special property of the silica is also
confirmed by the TEM images of the recycled material, which
show a tendency of Pd to agglomerate at the interface between
ASSOCIATED CONTENT
■
*
S
Supporting Information
The following files are available free of charge on the ACS
Publications website at DOI: 10.1021/cs501573c.
Further experimental details and spectroscopic measure-
ments and analysis (PDF)
the MOF and the SiO particles (ESI, Figure S9).
2
The activation period observed during the experiments in a
continuous-flow regime may be explained by an incomplete
reduction of Pd(II) complexes upon treatment with NaBH4.
This was further supported by running a control experiment
performed under anaerobic conditions. A conversion of 7% 1-
phenylethanol to acetophenone was measured after running the
reaction for 16 h at 150 °C under an inert argon atmosphere,
Movie S1; Beneficial effect of the innovative silica
protective layer over the recyclability of the new material
demonstrated in a teabag experiment (MPG)
Movie S2; Microchip mixing device in which the liquid
phase and gaseous phase are pumped simultaneously
(MPG)
4
77
dx.doi.org/10.1021/cs501573c | ACS Catal. 2015, 5, 472−479

ACS Catalysis
AUTHOR INFORMATION
Research Article
Gascon, J., Eds.; RSC Publishing: Cambridge, U.K., 2013; pp 237−
■
4
(
24.
Corresponding Authors
11) (a) Gustafsson, M.; Bartoszewicz, A.; Martín-Matute, B.; Sun, J.;
Grins, J.; Zhao, T.; Li, Z.; Zhu, G.; Zou, X. Chem. Mater. 2010, 22,
316−3322. (b) Carson, F.; Agrawal, S.; Gustafsson, M.; Bartoszewicz,
A.; Moraga, F.; Zou, X.; Martín-Matute, B. Chem.Eur. J. 2012, 18,
5337−15334. (c) Platero-Prats, A. E.; Bermejo Gomez, A.; Samain,
L.; Zou, X.; Martín-Matute, B. Chem.Eur. J. 2014, DOI: 10.1002/
chem.201403909. (d) Pascanu, V.; Hansen, P.; Bermejo Gomez, A.;
, M. A.; Martín-
*
E-mail: xzou@mmk.su.se.
E-mail: belen@organ.su.se
*
3
Author Contributions
#
1
́
V.P. and A.B.G. contributed equally.
Notes
́
The authors declare no competing financial interest.
Inquiries related to the work on continuous flow should be
addressed to mapericas@iciq.es.
̀
Ayats, C.; Platero-Prats, A. E.; Johansson, M. J.; Pericas
̀
Matute, B. ChemSusChem 2014, 7, DOI 10.1002/cssc.201402858. For
a related article, see (e) Tao Yang, T.; Bartoszewicz, A.; Ju, J.; Sun, J.;
Liu, Z.; Zou, X.; Wang, Y.; Li, G.; Liao, F.; Martín-Matute, B.; Lin, J.
Angew. Chem., Int. Ed. 2011, 50, 12555−12558.
ACKNOWLEDGMENTS
■
(
2
2
12) (a) (Review) Tanabe, K. K.; Cohen, S. M. Chem. Soc. Rev.
This project was supported by the Swedish Governmental
Agency for Innovation Systems (VINNOVA) and AstraZeneca
through the Berzelii Center EXSELENT, by the Swedish
Research Council (VR) through project grants to B.M.-M and
011, 40, 498−519. (b) Song, Y.-F.; Cronin, L. Angew. Chem., Int. Ed.
008, 47, 4635−4637. (c) Savonnet, M.; Cockrick, E.; Camarata, A.;
Bazer-Bachi, D.; Bats, N.; Lecocq, V.; Pinel, C.; Farrusseng, D. New J.
Chem. 2011, 35, 1892−1897.
X.Z. and the MATsynCELL project through the Ro
Ångstrom Cluster, and by the Knut and Alice Wallenberg
Foundation (Catalysis in Selective Organic Synthesis and
DEM-NATUR). The work on flow chemistry was funded by
MINECO (grant CTQ2012-38594-C02-01), DEC (grant
009SGR623), and the ICIQ Foundation. ICIQ authors also
thank EU-ITN network Mag(net)icFun (PITN-GA-2012-
90248) for financial support. B.M.-M. was supported by
̈
ntgen
(
13) (a) (Review) Dhakshinamoorthy, A.; Garcia, H. Chem. Soc.
̈
Rev. 2012, 41, 5262−5284. (b) Zhu, Q.-L.; Li, J.; Xu, Q. J. Am. Chem.
Soc. 2013, 135, 10210−10213. (c) Zahmakiran, M. Dalton Trans.
2012, 41, 12690−12696. (d) Jiang, H.-L.; Akita, T.; Ishida, T.; Haruta,
M.; Xu, Q. J. Am. Chem. Soc. 2011, 133, 1304−1306.
14) Chen, G.; Wu, S.; Liu, H.; Jiang, H.; Li, Y. Green Chem. 2013,
5, 230−235.
15) Fei, H.; Shin, J.; Meng, Y. S.; Adelhardt, M.; Sutter, J.; Meyer,
3
(
1
(
2
2
K.; Cohen, S. M. J. Am. Chem. Soc. 2014, 136, 4965−4973.
VINNOVA through a VINNMER grant. V.P. is grateful to the
EU-ITN network Mag(net)icFun (PITN-GA-2012-290248) for
appointment at ICIQ. C.A. thanks MICINN for a Juan de la
Cierva postdoctoral fellowship.
(
(
16) Abedi, S.; Morsali, A. ACS Catal. 2014, 4, 1398−1403.
17) Tan, J. C.; Cheetham, A. K. Chem. Soc. Rev. 2011, 40, 1059−
1
080.
(
(
18) Li, Z.; Zheng, H. C. J. Am. Chem. Soc. 2014, 136, 5631−5639.
19) (a) Pascanu, V.; Yao, Q.; Bermejo Gomez, A.; Gustafsson, M.;
Yun, Y.; Wan, W.; Samain, L.; Zou, X.; Martín-Matute, B. Chem.Eur.
J. 2013, 19, 17483−17493. (b) Pan, Y.; Yuan, B.; Li, Y.; He, D. Chem.
Commun. 2010, 46, 2280−2282. (c) Yuan, B.; Pan, Y.; Li, Y.; Yin, B.;
Jiang, H. Angew. Chem., Int. Ed. 2010, 24, 4054−4058.
́
REFERENCES
■
(
(
1) Davis, S. E.; Ide, M. S.; Davis, R. J. Green Chem. 2013, 15, 17−45.
2) For a review on the scope and limitations of oxidation reactions
using O as oxidant, see Shi, Z.; Zhang, C.; Tanga, C.; Jiao, N. Chem.
2
(
3
20) (a) Chen, L.; Chen, H.; Luque, R.; Li, Y. Chem. Sci. 2014, 5,
708−3714. (b) Zhang, D.; Guan, Y.; Hensen, E.; Xue, T.; Wang, Y.
Catal. Sci. Technol. 2014, 4, 795−802. (c) Huang, Y.; Zheng, Z.; Liu,
T.; Lu, J.; Lin, Z.; Li, H.; Cao, R. Catal. Commun. 2011, 14, 27−31.
21) Bernt, S.; Guillerm, V.; Serre, C.; Stock, N. Chem. Commun.
011, 47, 2838−2840.
22) (a) Poliakoff, M.; Fitzpatrick, J. M.; Farren, T. R.; Anastas, P. T.
Science 2002, 297, 807−810. (b) Collins, T. Science 2001, 291, 48−49.
23) (Review) Anastas, P.; Eghbali, N. Chem. Soc. Rev. 2010, 39,
01−312.
24) (a) Russell, D. A. M.; Shiang, D. L. ACS Sustainable Chem. Eng.
013, 1, 2−7. (b) Hansen, K. B.; Hsiao, Y.; Xu, F.; Rivera, N.; Clausen,
A.; Kubryk, M.; Krska, S.; Rosner, T.; Simmons, B.; Balsells, J.;
Ikemoto, N.; Sun, Y.; Spindler, F.; Malan, C.; Grabowski, E. J. J.;
Armstrong, J. D., III. J. Am. Chem. Soc. 2009, 131, 8798−8804.
(c) Calvo-Flores, F. G. ChemSusChem 2009, 2, 905−919. (d) Sheldon,
R. A. Green Chem. 2008, 10, 359−360. (e) Horvath, I.; Anastas, P.
Chem. Rev. 2007, 107, 2169−2173.
Soc. Rev. 2012, 41, 3381−3430.
(
3) For a comprehensive and comparative review on aerobic
oxidation of alcohols catalyzed by various transition metals, see
̈
Parmeggiani, C.; Cardona, F. Green Chem. 2012, 14, 547−564.
(
2
(
(
4) Arends, I. W. C. E.; Sheldon, R. A. Appl. Catal., A 2001, 212,
1
75−187.
5) (a) Shokouhim, M.; Shin, K.-Y.; Lee, J. S.; Hackett, M. J.; Jun, S.
W.; Oh, M. H.; Jang, J.; Hyeon, T. J. Mater. Chem. 2014, 2, 7593−
599. (b) Karami, K.; Ghasemi, M.; Haghighat Naeini, N. Catal.
Commun. 2013, 38, 10−15. (c) Johnston, E. V.; Verho, O.; Karkas, M.
D.; Shakeri, M.; Tai, C.-W.; Palmgren, P.; Eriksson, K.; Oscarsson, S.;
Bac
kvall, J.-E. Chem.Eur. J. 2012, 18, 12202−12206. (d) Wu, H.;
(
(
3
(
2
7
̈
̈
̈
Zhang, Q.; Wang, Y. Adv. Synth. Catal. 2005, 10, 1356−1360.
(
(
6) Uozumi, Y.; Nakao, R. Angew. Chem., Int. Ed. 2003, 42, 194−197.
7) Karimi, B.; Abedi, S.; Clark, J. H.; Budarin, V. Angew. Chem., Int.
Ed. 2006, 45, 4776−4779.
8) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espriu, B.;
(
Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.;
Hutchings, G. J. Science 2006, 311, 362−365.
(25) Office of Pollution Prevention and Toxics, The Presidential
Green Chemistry Challenge Awards Program, Summary of 2008
Award Entries and Recipients, U.S. Environmental Protection Agency:
Washington DC, EPA 744R08002, 2008.
(
9) (a) Metal-Organic Frameworks: Design and Application; Eddaoudi,
M., Eubank, J. F., MacGillivray, L. R., Eds.; John Wiley and Sons, 2010;
p 37. (b) For a dedicated issue comprising 18 review articles covering
MOF structures, applications, synthesis, and properties, see Chem.
Rev. 2012, 112, 673−674.
(26) Shih, Y.-H.; Lo, S.-H.; Yang, N. S.; Singco, B.; Cheng, Y.-J.; Wu,
C.-Y.; Chang, I.-H.; Huang, H.-Y.; Lin, C.-H. ChemPlusChem. 2012,
77, 982−986.
(
10) (a) For a concise description of the fundamental principles of
catalysis with MOFs, see Ranocchiari, M.; van Bokhoven, J. A. Phys.
Chem. Chem. Phys. 2011, 13, 6388−6396. (b) For a comparative
review on MOF and zeolite catalysts, see, Dhakshinamoorthy, A.;
Alvaro, M.; Corma, A.; Garcia, H. Dalton Trans. 2011, 40, 6344−6360.
(27) Nondry methanol was used as the solvent for the impregnation
of Pd(II) in MIL-88B-NH
(28) Serre, C.; Mellot-Draznieks, C.; Surble,
Filinchuk, Y.; Ferey
.
2
́
S.; Audebrand, N.;
́
, G. Science 2007, 315, 1828−1831.
(
c) For a comprehensive manual on all aspects of MOFs synthesis,
characterization, and applications in catalysis, see Metal Organic
Frameworks as Heterogeneous Catalysts; Llabres i Xamena, F. X.,
(29) Elemental analysis: Found: Cr, 14.15; Pd, 7.40. Calculated: Cr,
15.45; Pd, 7.38. Based on [Cr O(O C-C H NH -
3
2
6
3
2
́
CO ) F(H O) (PdCl ) ·CH CN·4H O].
2 3 2 2 2 0.7 3 2
4
78
dx.doi.org/10.1021/cs501573c | ACS Catal. 2015, 5, 472−479

ACS Catalysis
Research Article
(
30) No peaks corresponding to crystalline palladium could be
observed in the diffraction pattern.
31) Search on the Cambridge Crystallographic Database over 1372
(
reported crystal structures of palladium complexes with the proposed
coordination environment [Pd(II)(ArNH ) Cl ] gave average values
2
2
2
of Pd−N = 2.03(6) Å and Pd−Cl = 2.30(2) Å.
32) Horcajada, P.; Salles, F.; Wuttke, S.; Devic, T.; Heurtaux, D.;
Maurin, G.; Vimont, A.; Daturi, M.; David, O.; Magnier, E.; Stock, N.;
Filinchuk, Y.; Popov, D.; Riekel, C.; Ferey, G.; Serre, C. J. Am. Chem.
Soc. 2011, 133, 17839−17847.
33) Carson, F.; Su, J.; Platero-Prats, A. E.; Wan, W.; Yun, Y.;
Samain, L.; Zou, X. Cryst. Growth Des. 2013, 13, 5036−5044.
34) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.;
Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem.
985, 57, 603−619.
35) When methanol was used as a cosolvent (10% v/v) throughout
(
́
(
(
1
(
the whole experiment, a quick deactivation of the catalyst was observed
within the first 4 to 5 h.
(
36) Zhang, D.; Guan, Y.; Hensen, E. J. M.; Xue, T.; Wang, Y. Catal.
Sci. Technol. 2014, 4, 795−802.
4
79
dx.doi.org/10.1021/cs501573c | ACS Catal. 2015, 5, 472−479