Coupling MOF-based photocatalysis with Pd catalysis over Pd@MIL-100(Fe) for efficient N-alkylation of amines with alcohols under visible light

Article abstract of DOI:10.1016/j.jcat.2016.07.021

Pd nanoparticles with an average size of 1.7 nm confined inside a MIL-100(Fe) cavity (Pd@MIL-100(Fe)) were prepared via double-solvent impregnation combined with a photoreduction process. Due to efficient coupling of photocatalytic dehydrogenation and Pd-based hydrogenation, the resultant Pd@MIL-100(Fe) showed significantly superior performance in light-induced N-alkylation of amines with alcohols over Pd/MIL-100(Fe), in which larger Pd nanoparticles (6–12 nm) deposited on the external surface of MIL-100(Fe) were prepared via a conventional single-solvent impregnation followed by a similar photoreduction process. Kinetic studies and controlled experiments revealed that the whole N-alkylation reaction is limited by the photocatalytic alcohol-to-aldehyde dehydrogenation reaction. This is the first demonstration of light-induced N-alkylation of amines by alcohols over M/MOFs nanocomposites and also highlights the great potential of using M/MOFs nanocomposites as multifunctional catalysts for light-induced organic syntheses.

Full text of DOI:10.1016/j.jcat.2016.07.021

Journal of Catalysis 342 (2016) 151–157
Contents lists available at ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Coupling MOF-based photocatalysis with Pd catalysis over Pd@MIL-100
(
Fe) for efficient N-alkylation of amines with alcohols under visible light
⇑
Dengke Wang, Zhaohui Li
Research Institute of Photocatalysis, State Key Laboratory of Photocatalysis on Energy and Environment, College of Chemistry, Fuzhou University, Fuzhou 350002, People’s Republic
of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Pd nanoparticles with an average size of 1.7 nm confined inside a MIL-100(Fe) cavity (Pd@MIL-100(Fe))
were prepared via double-solvent impregnation combined with a photoreduction process. Due to effi-
cient coupling of photocatalytic dehydrogenation and Pd-based hydrogenation, the resultant Pd@MIL-
Received 19 June 2016
Revised 26 July 2016
Accepted 27 July 2016
100(Fe) showed significantly superior performance in light-induced N-alkylation of amines with alcohols
over Pd/MIL-100(Fe), in which larger Pd nanoparticles (6–12 nm) deposited on the external surface of
MIL-100(Fe) were prepared via a conventional single-solvent impregnation followed by a similar pho-
toreduction process. Kinetic studies and controlled experiments revealed that the whole N-alkylation
reaction is limited by the photocatalytic alcohol-to-aldehyde dehydrogenation reaction. This is the first
demonstration of light-induced N-alkylation of amines by alcohols over M/MOFs nanocomposites and
also highlights the great potential of using M/MOFs nanocomposites as multifunctional catalysts for
light-induced organic syntheses.
Keywords:
Pd nanoparticles
MIL-100(Fe)
Double-solvent impregnation
Multifunctional catalyst
Light-induced N-alkylation reaction
Ó 2016 Elsevier Inc. All rights reserved.
1
. Introduction
As important chemicals, N-alkyl amines are important building
It is widely accepted that the N-alkylation of amines with alco-
hols follows a hydrogen autotransfer mechanism containing three
consecutive catalytic steps: dehydrogenation of alcohols to aldehy-
des, condensation of aldehydes with amines to imines, and hydro-
genation of imines to N-alkyl amines [3–6,14–17]. To realize these
consecutive steps in one pot, multifunctional catalysts containing
blocks for the generation of agrochemicals, pharmaceuticals, and
bioactive molecules [1,2]. An attractive route for the production
of N-alkyl amines is the direct alkylation of amines with alcohols
as alkylating agents, since such a reaction has a high atom effi-
ciency and produces water as the only byproduct [3–6]. Until
now, a variety of catalysts, mostly Ru- and Ir-based homogeneous
catalysts, as well as some noble metal nanoparticle-doped hetero-
geneous catalysts, have been developed for N-alkylation of amines
with alcohols [7–10]. Unfortunately, the catalytic performance of
these systems is far from satisfactory. Usually the homogeneous
systems suffer from limited reusability and indispensable use of
co-catalysts [11,12], while most of the heterogeneous systems
are used under harsh conditions, in which overalkylation occurs.
In addition, deterioration of the performance of metal-
nanoparticle-based heterogeneous systems is unavoidable due to
the aggregation of metal nanoparticles [13]. Therefore, the devel-
opment of stable and highly efficient heterogeneous catalytic sys-
tems for N-alkylation reactions under mild conditions, in particular
under solar light irradiation, is highly desirable.
2
different catalytically active sites are required. Recently, TiO -
supported Pd, Au, and Cu–Mo nanoparticles have been reported
to show activity in this one-pot alkylation reaction under UV light
irradiation [18–20]. Although the light-induced alkylation of ami-
nes with alcohols is quite appealing, since the reaction conditions
are mild and the process requires no co-catalysts, these metal/TiO
2
nanocomposites only take effect under UV irradiation and the
aggregation of metal nanoparticles during the reactions, which
would deteriorate their activity, cannot be avoided. Additionally,
the morphology of the metal nanoparticles plays an important role
in their catalytic performance. Small nanoparticles are usually
more active, since they contain more coordinated unsaturated
sites, which are the actual active sites for catalytic reaction [18].
However, size-controllable preparations of metal nanoparticles
on the surfaces of semiconductors are usually challenging, since
aggregation of these small nanoparticles occurs readily due to their
high surface energy.
The employment of the nanopores in porous materials as
nanoreactors for direct growth of nanoparticles is a widely adopted
⇑
Corresponding author. Fax: +86 591 83779260.
E-mail address: zhaohuili1969@yahoo.com (Z. Li).
http://dx.doi.org/10.1016/j.jcat.2016.07.021
021-9517/Ó 2016 Elsevier Inc. All rights reserved.
0

1
52
D. Wang, Z. Li / Journal of Catalysis 342 (2016) 151–157
and effective strategy for obtaining nanoparticles with controllable
size [21–28]. Metal–organic frameworks (MOFs), a class of hybrid
materials with highly ordered uniform nanopores, can provide
well-defined microenvironments for controllable growth of
nanoparticles [29–44]. Although still in their infancy, metal/MOFs
nanocomposites with metal nanoparticles encapsulated inside
the cavity have already shown promise in heterogeneous catalysis
and Pd/TiO
2
were prepared similarly to Pd/MIL-100(Fe) except that
UV light (254 nm) was used for the preparation of Pd/TiO
2
.
2.2. Characterizations
X-ray diffraction (XRD) patterns were collected on a D8
Advance X-ray diffractometer (Bruker, Germany) with Cu K radi-
ation. The accelerating voltage and the applied current were 40 kV
a
[
45–54]. Photocatalysis is a unique kind of heterogeneous catalysis
that involves the use of a light source [55–57]. Recently, MOFs have
emerged as a new type of promising photocatalysts due to their
tunable light absorption and the ability to organize different func-
tional components in an individual MOF material [58–64]. In addi-
tion, the highly crystalline nature of MOFs ensures rapid electron
and energy transfer from the photoexcited MOFs to the active sites
for a variety of photocatalytic reactions [65–70].
Here, we report the preparation of Pd nanoparticles encapsu-
lated inside the MIL-100(Fe) cavity (Pd@MIL-100(Fe)) via double-
solvent impregnation combined with a photoreduction process
and its application for light-induced N-alkylation of amines with
alcohols. As compared with Pd/MIL-100(Fe), in which larger Pd
nanoparticles were deposited on the surface of MIL-100(Fe),
Pd@MIL-100(Fe) showed superior catalytic activity and higher sta-
bility. For the first time, this work demonstrates the use of the
nanopores in MOFs for controllable growth of metal nanoparticles
encapsulated inside MOFs as a way to realize a highly efficient and
stable alkylation of amines via successful coupling of the MOF-
based photocatalysis and metal NPs-based hydrogenation.
and 40 mA, respectively. Data were recorded at a scanning rate of
À1
0
.02° 2h s in the 2h range of 5–30°. UV–visible diffuse reflectance
4
spectra (UV-DRS) of the powders were obtained with BaSO used
as a reflectance standard. BET surface area was determined on an
ASAP 2020M apparatus (Micromeritics Instrument Corp., USA).
The samples were degassed under vacuum at 150 °C for 10 h and
then measured at À196 °C. The transmission electron microscopy
(
(
TEM) and high-resolution transmission electron microscopy
HRTEM) images were obtained on a JEOL model JEM 2010 EX
instrument. X-ray photoelectron spectroscopy (XPS) measure-
ments were performed on a PHI Quantum 2000 XPS system (PHI,
USA). Inductively coupled optical emission spectrometry (ICP-
OES) was performed on Optima 8000 (PerkinElmer). Before the
ICP-OES experiment, the solid sample was digested in a mixture
3
of HNO and Milli-Q water.
2.3. Catalytic reactions
The N-alkylation reaction was performed in a sealed reaction
tube under visible light irradiation. The catalyst (10 mg), amine
(
(
0.1 mmol), and alcohol (3 mmol) were suspended in acetonitrile
CH CN, 2 mL) and the resultant mixture was degassed and satu-
to remove any dissolved O before the reaction.
2
. Experiments
3
rated with N
2
2
2.1. Preparations
The reaction was performed under irradiation by a 300 W Xe lamp
equipped with a UV-cut filter to remove all irradiation with wave-
lengths less than 420 nm and an IR-cut filter to remove all irradia-
tion with wavelengths greater than 800 nm. After the reaction, the
reaction mixture was filtered through a porous membrane (20
in diameter) and the products were analyzed by GC-MS and GC-FID
All the reagents were commercially available and used without
further purification. MIL-100(Fe) was prepared following the
procedures described in the literature with slight modifications
lm
[
1
71–74]. Typically, iron(III) nitrate nonahydrate (484 mg,
.2 mmol) and 1,3,5-benzenetricarboxylic acid (210 mg, 1.0 mmol)
(
Shimadzu GC-2014) equipped with an HP-5 capillary column.
Photocatalytic dehydrogenation of benzyl alcohol was con-
were dissolved in deionized water (5 mL). Then the resulting solu-
tion was stirred, transferred to a Teflon autoclave liner, and sealed
to heat at 180 °C for 12 h. The obtained yellow solid was recovered
by filtration and washed several times with deionized water and
methanol. The synthesized MIL-100(Fe) was finally dried overnight
at 60 °C in an oven.
Pd@MIL-100(Fe) was prepared by a double-solvent impregna-
tion approach combined with a photoreduction process. The
double-solvent impregnation approach was first developed by Xu
et al. for the preparation of Pt@MIL-101(Cr) and AuNi@MIL-101
ducted in a sealed reaction tube containing Pd@MIL-100(Fe)
(
10 mg), benzyl alcohol (0.1 mmol), and CH
Hydrogenation of N-benzylideneaniline with H
reaction tube containing Pd@MIL-100(Fe) (10 mg),
3
CN (2 mL).
2
was carried out
in
a
N-benzylideneaniline (0.1 mmol), benzyl alcohol (3 mmol), and
CH CN (2 mL).
Hydrogenation of N-benzylideneaniline over
Pd@MIL-100(Fe) was carried out in a Schlenk tube. Pd@MIL-100
Fe) was first activated and treated with H for 1 h at 120 °C in
the tube. Then the H was removed, followed by filling the tube
with Finally, CH CN solution (2 mL) containing
3
2
H -pretreated
(
2
(
(
Cr) [75,76]. For the impregnation, 100 mg of activated MIL-100
Fe) was suspended in 20 mL of dry n-hexane and 0.08 mL of an
2
N
2
.
3
2 3 2
aqueous solution containing PdCl (CH CN) (2.46 mg, 1 wt.% Pd)
N-benzylideneaniline (0.1 mmol) was added and stirred under
room temperature.
The catalytic reaction between aniline and benzaldehyde was
carried out in a tube containing Pd@MIL-100(Fe) (10 mg), aniline
was added dropwise within 15 min under vigorous stirring. After
the mixture was stirred for 2 h, the solid was isolated from the
supernatant by decanting, washed with ethanol, and dried under
vacuum. The as-obtained solid product was then suspended in
degassed anhydrous methanol and was irradiated under visible
light for 3 h. The resultant sample was filtered, washed with
methanol, and dried overnight at 60 °C in an oven.
For comparison, Pd/MIL-100(Fe) was synthesized employing
the direct photoreduction method via a conventional single-
solvent impregnation process. A quantity of 0.08 mL of an aqueous
(
0.1 mmol), benzaldehyde (0.5 mmol), and CH
The deuterium-labeling experiments were conducted using
benzyl alcohol and benzyl alcohol- -d as alkylating agent. The
kinetic isotope effect (KIE) is defined as k /k , in which k and k
are the rate constants for the alkylation reactions carried out over
3
CN (2 mL).
a,a
2
H
D
H
D
2
benzyl alcohol and benzyl alcohol-a,a-d , respectively.
solution of PdCl
degassed anhydrous methanol (5 mL) containing 100 mg of MIL-
00(Fe) under N . The as-obtained suspension was irradiated
under visible light for 3 h. The resultant solid was filtered, washed
with methanol, and dried overnight at 60 °C in an oven. Pd/Fe
2 3 2
(CH CN) (2.46 mg, 1 wt.% Pd) was added to the
3. Results and discussion
1
2
MIL-100(Fe) was chosen as the host matrix to encapsulate Pd
nanoparticles due to its good resistance to water and organic
2 3
O

D. Wang, Z. Li / Journal of Catalysis 342 (2016) 151–157
153
solvents [71–74]. The as-obtained product shows similar XRD pat-
terns and comparable surface area (2021 m /g) to those reported
previously (Figs. 1a and S1a), confirming the formation of MIL-
(Fe) leads to a decrease of the specific surface area from the origi-
2
2
À1
2
À1
nal 2021 m g for MIL-100(Fe) to 1562 m g for Pd@MIL-100
(Fe) (Fig. S1b). The UV–vis DRS spectrum of the Pd@MIL-100(Fe)
shows enhanced absorption in the 200–550 nm region as com-
pared with pristine MIL-100(Fe), with the adsorption edge extend-
ing to around 650 nm (Fig. 2).
To study the catalytic performance of the as-prepared Pd@MIL-
100(Fe) for N-alkylation of amines with alcohols under visible light
irradiation, the reaction between aniline (1a) and benzyl alcohol
1
00(Fe) with high quality. Pd@MIL-100(Fe) was prepared via a
double-solvent method followed by a photoreduction approach.
For the impregnation process, activated MIL-100(Fe) was sus-
pended in a large amount of dry n-hexane, followed by dropwise
2 3 2
addition of PdCl (CH CN) aqueous solution with a volume less
than the pore volume of MIL-100(Fe) under vigorous stirring. It is
believed that the hydrophobic hexane will cover the external sur-
face, which ensures the diffusion of hydrophilic Pd precursors into
the pores of MIL-100(Fe) via capillary force. The following photore-
duction resulted in the formation of Pd@MIL-100(Fe). A similar
double-solvent impregnation process was first developed by Xu
et al. to encapsulate Pt and AuNi nanoparticles inside the cavity
of MIL-101(Cr) [75,76]. As compared with the reduction process
3
(2a) was chosen and was carried out in CH CN (Table 1). It was
found that only negligible N-benzylideneaniline was obtained in
the absence of Pd@MIL-100(Fe) or without visible light (Table 1,
entries 1 and 2). In the presence of Pd@MIL-100(Fe), the N-
alkylation of aniline with benzyl alcohol proceeded smoothly and
the aniline/benzyl alcohol mole ratio played an important role in
the reaction (Table 1, entries 3–8). Among all the aniline/benzyl
alcohol ratios investigated, the catalytic system containing aniline
and benzyl alcohol in a ratio of 1:30 gave the best performance,
showing the highest conversion of aniline at 88% and a selectivity
to N-benzylaniline (3a) at 76% after being irradiated for 24 h
(Table 1, entry 6). By-products such as N-benzylideneaniline (3b)
and benzaldehyde were also produced, but no overalkylated ter-
tiary amine was detected. A prolonged reaction time of 32 h led
to an almost quantitative transformation of aniline (>99%) at an
increased selectivity to 3a (86%, Table 1, entry 9). Pd was essential
to the reaction, because no activity was observed over MIL-100(Fe)
under typical reaction conditions (Table 1, entry 10). The filtrate
reaction revealed that no further reaction occurred after Pd@MIL-
100(Fe) was removed from the reaction system (Table 1, entry
11). These results demonstrated that the alkylation of aniline with
benzyl alcohol was efficiently catalyzed by Pd@MIL-100(Fe) under
visible light irradiation. To further confirm that the current reac-
tion is really photoactivated, action spectrum analyses for amine
alkylation over Pd@MIL-100(Fe) have been studied. The conversion
of aniline over Pd@MIL-100(Fe) is observed to depend strongly on
the wavelength in a manner that correlates with their absorption
intensities in the visible light region, indicating that the amine
alkylation reaction over Pd@MIL-100(Fe) is truly induced by visible
light (Fig. 3). Pd@MIL-100(Fe) showed superior performance for
2 4
using strong reducing agents such as H and NaBH , the current
photoreduction is milder and green, since no extra reducing agent
is required, which is more favorable for preserving the MOFs
framework.
The as-prepared Pd@MIL-100(Fe) shows XRD patterns similar
to those of the parent MIL-100(Fe), indicating that the framework
of MIL-100(Fe) is well preserved (Fig. 1a). Although no characteris-
tic diffractions for Pd were detected in XRD patterns, XPS results
confirm the existence of Pd by showing two peaks at 335.4 and
0
0
3
40.7 eV, corresponding to Pd 3d5/2 and Pd 3d3/2, respectively
(
(
Fig. 1b) [77]. TEM images show that the surface of Pd@MIL-100
Fe) is smooth, confirming that there is no Pd deposition on the
external surface of MIL-100(Fe) (Fig. 1c). The HRTEM image reveals
that Pd nanoparticles with size ca. 1.7 nm are highly dispersed in
Pd@MIL-100(Fe), indicating that Pd nanoparticles have been
confined in the pores of MIL-100(Fe) (Figs. 1d and S2). The lattice
fringe of 0.224 nm matches that of the (111) plane of
face-centered cubic Pd (inset in Fig. 1d) [77]. The amount of encap-
sulated Pd in Pd@MIL-100(Fe) was determined to be 0.99 wt.% by
ICP-OES, almost identical to the amount of Pd added (1 wt.% Pd).
These demonstrate that the double-solvent approach can
effectively confine the Pd nanoparticles to the MIL-100(Fe) cavities.
The encapsulation of Pd nanoparticles inside the cavity of MIL-100
(a)
(e)
(c)
(
b)
0
(f)
(
d)
Pd 3d5/2
0
Pd 3d3/2
d=0.224 nm
Pd(111)
Fig. 1. (a) XRD patterns of Pd@MIL-100(Fe), MIL-100(Fe), and calculated MIL-100(Fe); (b) XPS spectrum of Pd@MIL-100(Fe) in Pd3d region; (c) TEM and (d) HRTEM images of
Pd@MIL-100(Fe); (e) TEM and (f) HRTEM images of Pd/MIL-100(Fe).

1
54
D. Wang, Z. Li / Journal of Catalysis 342 (2016) 151–157
with its hydrogenation to produce 3a (Fig. 4a). This assumption is
not consistent with that proposed previously, which suggests that
the whole N-alkylation reaction is limited by the imine hydrogena-
tion step [14–18]. To gain more insight into this, the kinetic isotope
effect (KIE) was examined using benzyl alcohol-
OH) as an isotopic label in the alkylation reaction. The KIE (defined
as k /k , in which k and k are the rate constants for the alkyla-
tion reaction using C CH OH and C CD OH as alkylating
2 6 5 2
a,a-d (C H CD -
H
D
H
D
6
H
5
2
6
H
5
2
agent, respectively) was determined to be 2.6 (Fig. S3a). This value
is close to the KIE value (2.8) obtained for the dehydrogenation of
benzyl alcohol over Pd@MIL-100(Fe) (Fig. S3b), suggesting that the
dehydrogenation of benzyl alcohol is the rate-limiting step for
light-induced N-alkylation over Pd@MIL-100(Fe). The dehydro-
genation of benzyl alcohol is more difficult over Pd@MIL-100(Fe)
than the condensation between aniline and benzaldehyde or the
hydrogenation of 3b can also be confirmed from the following con-
trolled experiments. Only 23% of benzyl alcohol was converted to
benzaldehyde over irradiated Pd@MIL-100(Fe) in 12 h. However,
the condensation between aniline and benzaldehyde gave an
almost quantitative transformation of aniline to 3b, and the reac-
Fig. 2. UV–vis spectra of MIL-100(Fe) and Pd@MIL-100(Fe).
the light-induced N-alkylation of aniline with benzyl alcohol as
compared with Pd/Fe and Pd/TiO . Only 26% of aniline was
converted and 19% yield to N-benzylaniline was achieved over
Pd/Fe after irradiation for 24 h, while no conversion of aniline
was observed over visible-light-irradiated Pd/TiO (Table 1, entries
2–13).
The time-dependent conversion of aniline and the formation of
2
O
3
2
tion between 3b and H gave a high yield of 93% to 3a under other-
2
2
O
3
wise similar condition. The H -pretreated Pd@MIL-100(Fe) also
2
2
showed catalytic activity for hydrogenation of 3b under N atmo-
2
1
sphere, with a yield of 14% to 3a. No reaction occurred in absence
of Pd@MIL-100(Fe) either in the dehydrogenation of benzyl alcohol
the products over Pd@MIL-100(Fe) was also investigated (Fig. 4a).
The yield of 3a as well as of the byproducts benzaldehyde and 3b
increased in the first 8 h. After that, although the yield of 3a and
benzaldehyde still increased steadily, the yield to 3b decreased.
This result confirms that the reaction between aniline and benzyl
alcohol to produce 3a over irradiated Pd@MIL-100(Fe) also pro-
ceeds through the imine as intermediate.
The observation that the amount of 3b in the reaction system
remained at a relatively low level during the whole reaction pro-
cess implies that the generation of 3b is more difficult as compared
or the hydrogenation of 3b with H . These results confirm that the
N-alkylation of amines with alcohols over Pd@MIL-100(Fe) follows
a hydrogen autotransfer mechanism, with aldehydes and imines as
intermediates.
Based on these observations, the light-induced N-alkylation of
aniline by benzyl alcohol over Pd@MIL-100(Fe) can be proposed
as follows. MOFs can be excited like conventional semiconductor
photocatalysts when irradiated to generate electrons and holes.
Loading of noble metals onto MOFs can result in efficient electron
transfer from the excited MOFs to noble metals with lower Fermi
2
Table 1
Light-induced catalytic performance for the N-alkylation of aniline with benzyl alcohol under varied conditions.
Entry
1a
1a/2a ratio
Amine conv. (%)
Yield (%)
Aldehyde (lmol)
3a
3b
1:1
1:1
1:1
1:5
–c
–
–
–
5
21
42
76
53
23
86
–
–
–
–
–
b
2
3
4
5
6
7
8
20
45
65
88
74
49
>99
–
14
23
20
11
19
24
13
–
12
25
21
30
41
52
45
–
1:10
1:30
1:50
1:100
1:30
1:30
1:30
1:30
1:30
1:30
d
9
e
1
1
1
1
1
0
1
2
3
4
f
56
26
2
40
19
–
15
7
1
18
8
2
g
h
i
30
26
4
10
Notes: Reaction conditions: aniline (0.1 mmol), benzyl alcohol, CH
3
CN (2 mL), Pd@MIL-100(Fe) (10 mg), N
2
, light irradiation (800 nm P k P 420 nm), 24 h.
a
No catalyst.
Without light irradiation.
b
c
–
refers to no products or negligible products detected.
d
e
f
Irradiated for 32 h.
MIL-100(Fe) instead of Pd@MIL-100(Fe) was used as catalyst.
The catalyst was filtered after being irradiated for 12 h.
g
h
i
Pd/Fe
Pd/TiO
Pd/MIL-100(Fe) used as catalyst.
2
O
3
used as catalyst.
2
used as catalyst.

D. Wang, Z. Li / Journal of Catalysis 342 (2016) 151–157
155
Fig. 3. Dependence of the conversion of aniline on the wavelength of incident light
and the UV–vis spectra of Pd@MIL-100(Fe). The wavelength region of irradiated
light was controlled using three different bandpass filters: (a) 430–465 nm; (b)
4
85–522 nm; (c) 581–619 nm.
Scheme 1. Proposed mechanism for N-alkylation of aniline with benzyl alcohol via
hydrogen autotransfer process over Pd@MIL-100(Fe).
levels [78]. As shown in Scheme 1, when Pd@MIL-100(Fe) was irra-
diated, the photogenerated electrons were transferred to Pd
nanoparticles to form electron-rich Pd species (step i). The benzyl
alcohol introduced into the reaction system can be activated and
donates an electron to MIL-100(Fe) to form an alkoxide intermedi-
ate (step ii), which further undergoes a cleavage of the C–H bond to
form benzaldehyde. Such oxidant-free formation of benzaldehyde
from dehydrogenation of benzyl alcohol was previously reported
over hydrotalcite-supported gold catalyst [79]. In the meantime,
the hydrogen transfers to the electron-rich Pd to generate Pd
hydride (step iii). The condensation between the in situ generated
were realized after irradiation for 24 h (Table 2, entries 6–9). The
observation that substrates with different substituents on their
phenyl rings showed varied activities suggests the existence of
an electronic effect. This is reasonable, since the nucleophilic
attack toward the aldehydes by the aniline is an important step
in this reaction. Pd@MIL-100(Fe) was inactive for the reaction
between aniline and p-nitrobenzyl alcohol or that between p-
nitroaniline and benzyl alcohol (Table 2, entries 3 and 10). It is
probable that the adsorption of the strong electron-withdrawing
3
+
benzaldehyde and aniline is promoted by Lewis acidic Fe in MIL-
00(Fe) to produce imine as an intermediate (steps iv and v), which
1
2
–NO group onto the surfaces of the electron-rich Pd nanoparticles
are further hydrogenated by the active Pd hydrides to generate N-
alkyl amine (step vi).
would impede hydrogen transfer to the Pd species to form Pd
hydride.
To investigate the substrate scope of this reaction, different
functional substituted anilines were used to carry out the alkyla-
tion reactions (Table 2). Except for p-nitroaniline, all the investi-
gated aromatic amines can be transformed to the corresponding
N-alkyl anilines after irradiation for 24 h, but with different yields.
Aniline with electron-withdrawing substituents such as –Cl
showed significantly decreased conversion ratios (18%) and low
yields of N-benzyl-4-chloroaniline (11%) (Table 2, entry 2), while
The scope of the substrates can also be expanded to aliphatic
amines and alcohols (Table 2, entries 11–14). Either the reactions
of aliphatic amines with benzyl alcohols or those between anilines
and aliphatic alcohols occurred over the current Pd@MIL-100(Fe),
but with a lower conversion ratio (13–55%) and lower yield
(8–43%) than their respective N-alkyl amines.
Pd@MIL-100(Fe) showed high stability for the light-induced
N-alkylation reaction. No obvious loss of the activity was observed
after five cycling tests (Fig. 5). ICP analysis of the filtrate revealed
no detectable Pd and Fe. The Pd@MIL-100(Fe) after five runs
showed similar XRD patterns and its specific surface area
3 3
those with electron-donating groups (–OCH and –CH ) exhibited
slightly increased conversion ratios (90–92%) and comparable
yields to N-alkyl amine (73-77%) (Table 2, entries 4–5). The
reactions between benzyl alcohols with different substituents
and aniline also proceeded efficiently over Pd@MIL-100(Fe), except
for p-nitrobenzyl alcohol. Medium to high conversion ratios of ani-
line (72–85%) and reasonable yields of N-alkyl amines (51–70%)
2
À1
À1
(1392 m g
)
is comparable to that of the fresh one
2
(1562 m g ) (Figs. S4–S5). In addition, the TEM image of the used
catalyst shows that Pd nanoparticles with almost similar size
distributions were maintained (Fig. S6a).
Fig. 4. Time-dependent changes in the amounts of aniline and the three products during N-alkylation of aniline with benzyl alcohol under visible light irradiation over (a)
Pd@MIL-100(Fe) and (b) Pd/MIL-100(Fe). The ratio of Pd to aniline is 1:100.

1
56
D. Wang, Z. Li / Journal of Catalysis 342 (2016) 151–157
Table 2
Light-induced catalytic performance for N-alkylation of amines with alcohols over Pd@MIL-100(Fe).
Entry
Substrates
Amine conv. (%)
Yield (%)
3a
Aldehyde (lmol)
1a
2a
3b
1
2
3
4
5
6
7
8
9
Aniline
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
88
18
10
90
92
81
85
78
72
6
76
11
–
73
77
63
70
62
51
-
11
7
8
30
40
54
25
23
28
30
24
20
5
p-Chloroaniline
p-Nitroaniline
p-Methoxyaniline
p-Touidine
Aniline
Aniline
Aniline
Aniline
Aniline
15
13
18
13
16
20
6
11
14
6
Benzyl alcohol
p-Methoxybenzyl alcohol
p-Methylbenzyl alcohol
m-Methylbenzyl alcohol
o-Methylbenzyl alcohol
p-Nitrobenzyl alcohol
Benzyl alcohol
Benzyl alcohol
Cyclohexanol
Butyl alcohol
1
1
1
1
1
0
1
2
3
4
Cyclohexylamine
n-Butylamine
Aniline
55
34
18
13
43
20
11
8
21
27
12
8
Aniline
4
3 2
Notes: Reaction conditions: amine (0.1 mmol), alcohol (3 mmol), CH CN (2 mL), catalyst (10 mg), N , light irradiation (800 nm P k P 420 nm), 24 h.
poor stability, as evidenced from the decreased activity during
the cycling reactions (Fig. S8). Although the used Pd/MIL-100(Fe)
showed XRD patterns similar to those of the fresh Pd/MIL-100
(
Fe), its TEM image shows larger Pd nanoparticles (10–15 nm),
implying that Pd nanoparticles aggregated during the reaction
Figs. S6b and S9).
(
4
. Conclusions
In summary, Pd@MIL-100(Fe), with Pd nanoparticles confined
inside the cavities of MIL-100(Fe), was successfully obtained via
double-solvent impregnation combined with a photoreduction
approach and acted as a superior catalyst for the light-induced
N-alkylation reaction of amines by alcohols. This is the first
demonstration of M/MOFs nanocomposite catalysts for the light-
induced N-alkylation of amines with alcohols. This study not only
provides a green method for the production of N-alkyl amines, but
also highlights the great potential of using MOFs for light-induced
organic syntheses. It is anticipated that the combination of the
diversified MOFs and the metal nanoparticles will bring about
almost infinite possibilities for developing multifunctional cata-
lysts for light-induced organic transformations.
Fig. 5. Cycling of Pd@MIL-100(Fe) for the N-alkylation of aniline with benzyl
alcohol under visible light irradiation. Each run was irradiated for 24 h.
For comparison, Pd nanoparticles deposited on the external sur-
face of MIL-100(Fe) (Pd/MIL-100(Fe)) were also prepared via a con-
ventional single-solvent impregnation followed by
a similar
photoreduction process and was used in the reaction between ani-
line and benzyl alcohol. TEM/HRTEM images of Pd/MIL-100(Fe)
show that Pd nanoparticles at a size of 6–12 nm were deposited
on the surface of MIL-100(Fe) (Figs. 1e, f, and S7). It was found that
Pd/MIL-100(Fe) exhibited inferior catalytic activity for the N-
alkylation reaction to Pd@MIL-100(Fe) under similar conditions
Acknowledgments
This work was supported by the 973 Program (2014CB239303),
the NSFC (21273035), the Specialized Research Fund for the Doc-
toral Program of Higher Education (20123514110002), and an
Independent Research Project of State Key Laboratory of Photo-
catalysis on Energy and Environment (2014A03). Z. Li thanks the
Award Program for Minjiang Scholar Professorship for financial
support. We also thank Professor Masakazu Anpo for helpful
discussions.
(
Fig. 4b). Only about 30% of aniline was converted and a lower yield
of 3a (26%) was achieved over Pd/MIL-100(Fe) after irradiation for
4 h (Table 1, entry 14). The lower activity for N-alkylation over
2
Pd/MIL-100(Fe) can be attributed to its inferior activity for alcohol
dehydrogenation, since the conversion of benzyl alcohol to ben-
zaldehyde over Pd@MIL-100(Fe) (23%) was much higher than that
over Pd/MIL-100(Fe) (7.7%) in a similar irradiation time (12 h). It is
believed that due to their small size, the cavity-encapsulated Pd
nanoparticles contain more active unsaturated Pd atoms than
those supported on the surface of MIL-100(Fe), and the dehydro-
genation of alcohol to generate aldehydes, which proceeds via
the cleavage of the C–H bond, is catalyzed by Pd nanoparticles. In
addition, with the cavities of the MIL-100(Fe) acting as nanoreac-
tors, the encapsulated Pd nanoparticles and the substrates con-
fined in close proximity can ensure a rapid electron transfer from
the excited Fe–O clusters to Pd nanoparticles, which favors the
activation of the substrates to realize an efficient light-induced
multistep reaction. It was also found that Pd/MIL-100(Fe) showed
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.2016.07.021.
References
[
1] S.A. Lawrence, Amines: Synthesis, Properties, and Application, Cambridge
University Press, Cambridge, 2006.
[2] R.N. Salvatore, C.H. Yoon, K.W. Jung, Tetrahedron 57 (2001) 7785.

D. Wang, Z. Li / Journal of Catalysis 342 (2016) 151–157
157
[
[
3] G. Guillena, D.J. Ramón, M. Yus, Chem. Rev. 110 (2010) 1611.
4] M. Ousmane, G. Perrussel, Z. Yan, J.-M. Clacens, F. De Campo, M. Pera-Titus, J.
Catal. 309 (2014) 439.
[45] A. Dhakshinamoorthy, H. Garcia, Chem. Soc. Rev. 41 (2012) 5262.
[46] A.H. Chughtai, N. Ahmad, H.A. Younus, A. Laypkov, F. Verpoort, Chem. Soc. Rev.
44 (2015) 6804.
[
[
5] S. Sithambaram, R. Kumar, Y.-C. Son, S.L. Suib, J. Catal. 253 (2008) 269.
6] M.H.S.A. Hamid, C.L. Allen, G.W. Lamb, A.C. Maxwell, H.C. Maytum, A.J.A.
Watson, J.M.J. Williams, J. Am. Chem. Soc. 131 (2009) 1766.
7] Q. Yang, Q.F. Wang, Z.K. Yu, Chem. Soc. Rev. 44 (2015) 2305.
8] A. Corma, T. Ródenas, M.J. Sabater, Chem. Eur. J. 16 (2010) 254.
9] O. Saidi, A.J. Blacker, M.M. Farah, S.P. Marsden, J.M.J. Williams, Chem. Commun.
[47] L.Y. Chen, B.B. Huang, X. Qiu, X. Wang, R. Luque, Y.W. Li, Chem. Sci. 7 (2016)
228.
[48] X.L. Li, Z.Y. Guo, C.X. Xiao, T.W. Goh, D. Tesfagaber, W.Y. Huang, ACS Catal. 4
(2014) 3490.
[49] P. Hester, S.J. Xu, W. Liang, N. Al-Janabi, R. Vakili, P. Hill, C.A. Muryn, X.B. Chen,
P.A. Martin, X.L. Fan, J. Catal. 340 (2016) 85.
[
[
[
4
6 (2010) 1541.
[50] J. Juan-Alcañiz, J. Ferrando-Soria, I. Luz, P. Serra-Crespo, E. Skupien, V.P. Santos,
E. Pardo, F.X. Llabrés i Xamena, F. Kapteijn, J. Gascon, J. Catal. 307 (2013) 295.
[51] Z.C. Zhang, Y.F. Chen, X.B. Xu, J.C. Zhang, G.L. Xiang, W. He, X. Wang, Angew.
Chem. Int. Ed. 53 (2014) 429.
[
[
[
[
[
10] J.W. Kim, K. Yamaguchi, N. Mizuno, J. Catal. 263 (2009) 205.
11] S. Röler, M. Ertl, T. Irrgang, R. Kempe, Angew. Chem. Int. Ed. 54 (2015) 15046.
12] C. Gunanathan, D. Milstein, Acc. Chem. Res. 44 (2011) 588.
13] T. Ishida, N. Kawakita, T. Akita, M. Haruta, Gold Bull. 42 (2009) 267.
14] A. Nandakumar, S.P. Midya, V.G. Landge, E. Balaraman, Angew. Chem. Int. Ed.
[52] Y.B. Huang, S.J. Liu, Z.J. Lin, W.J. Li, X.F. Li, R. Cao, J. Catal. 292 (2012) 111.
[53] Y.L. Liu, Z.Y. Tang, Adv. Mater. 25 (2013) 5819.
5
4 (2015) 11022.
[54] A. Aijaz, Q. Xu, J. Phys. Chem. Lett. 5 (2014) 1400.
[55] X.J. Lang, X.D. Chen, J.C. Zhao, Chem. Soc. Rev. 43 (2014) 473.
[56] H. Tong, S.X. Ouyang, Y.P. Bi, N. Umezawa, M. Oshikiri, J.H. Ye, Adv. Mater. 24
(2012) 229.
[
[
[
15] Y.S. Zhao, S.W. Foo, S. Saito, Angew. Chem. Int. Ed. 50 (2011) 3006.
16] H.H. Liu, G.-K. Chuah, S. Jaenicke, J. Catal. 292 (2012) 130.
17] Y. Zhang, C.-S. Lim, D.S.B. Sim, H.-J. Pan, Y. Zhao, Angew. Chem. Int. Ed. 53
(
2014) 1399.
18] Y. Shiraishi, K. Fujiwara, Y. Sugano, S. Ichikawa, T. Hirai, ACS Catal. 3 (2013)
12.
[57] W.G. Tu, Y. Zhou, Z.G. Zou, Adv. Mater. 26 (2014) 4607.
[58] T. Zhang, W.B. Lin, Chem. Soc. Rev. 43 (2014) 5982.
[59] D.K. Wang, R.K. Huang, W.J. Liu, D.R. Sun, Z.H. Li, ACS Catal. 4 (2014) 4254.
[60] Y.H. Fu, D.R. Sun, Y.J. Chen, R.K. Huang, Z.X. Ding, X.Z. Fu, Z.H. Li, Angew. Chem.
Int. Ed. 51 (2012) 3364.
[
3
[
[
[
19] L.N. Zhang, Y. Zhang, Y.Q. Deng, F. Shi, Catal. Sci. Technol. 5 (2015) 3226.
20] D. Stíbal, J. Jacinto Sá, J.A.V. Bokhoven, Catal. Sci. Technol. 3 (2013) 94.
21] J.W. Liu, L.F. Chen, H. Cui, J.H. Zhang, L. Zhang, C.-Y. Su, Chem. Soc. Rev. 42
[61] M.A. Nasalevich, M. van der Veen, F. Kapteijn, J. Gascon, CrystEngComm 16
(2014) 4919.
(
2014) 6011.
[
[
[
[
22] A. Stein, Adv. Mater. 15 (2003) 763.
[62] M. Alvaro, E. Carbonell, B. Ferrer, F.X. Llabrés i Xamena, H. Garcia, Chem. Eur. J.
13 (2007) 5106.
[63] T. Tachikawa, J.R. Choi, M. Fujitsuka, T. Majima, J. Phys. Chem. C 112 (2008)
14090.
[64] D.S. Sun, W.J. Liu, M. Qiu, Y.F. Zhang, Z.H. Li, Chem. Commun. 51 (2015) 2056.
[65] D.K. Wang, Z.H. Li, Catal. Sci. Technol. 5 (2015) 1623.
[66] J.-L.T.H. Zhou, Y.H. Du, A. Borgna, J.D. Hong, Y.B. Wang, J.Y. Han, W. Zhang, R.
Xu, Energy Environ. Sci. 6 (2013) 3229.
[67] S.Y. Jin, H.-J. Son, O.K. Farha, G.P. Wiederrecht, J.T. Hupp, J. Am. Chem. Soc. 135
(2013) 955.
[68] V. Stavila, A.A. Talin, M.D. Allendorf, Chem. Soc. Rev. 43 (2014) 5994.
[69] Z.C. Hu, B.J. Deibert, J. Li, Chem. Soc. Rev. 43 (2014) 5815.
[70] S. Natarajan, P. Mahata, Chem. Soc. Rev. 38 (2009) 2304.
[71] P. Horcajada, S. Surblé, C. Serre, D.-Y. Hong, Y.-K. Seo, J.-S. Chang, J.-M.
Grenèche, I. Margiolaki, G. Férey, Chem. Commun. (2007) 2820.
[72] D.K. Wang, M.T. Wang, Z.H. Li, ACS Catal. 5 (2015) 6852.
[73] K.G.M. Laurier, F. Vermoortele, R. Ameloot, D.E. De Vos, J. Hofkens, M.B.J.
Roeffaers, J. Am. Chem. Soc. 135 (2013) 14488.
23] T. Uemura, N. Yanai, S. Kitagawa, Chem. Soc. Rev. 38 (2009) 1228.
24] N. Yi, Y. Cao, Y. Su, W.-L. Dai, H.-Y. He, K.-N. Fan, J. Catal. 230 (2005) 249.
25] J.J. Zhou, P. Wang, C.X. Wang, Y.T. Goh, Z. Fang, P.B. Messersmith, H.W. Duan,
ACS Nano 9 (2015) 6951.
[
[
26] Y.-H. Kang, X.-D. Liu, N. Yan, Y. Jiang, X.-Q. Liu, L.-B. Sun, J.-R. Li, J. Am. Chem.
Soc. 138 (2016) 6099.
27] G. Lu, S.Z. Li, Z. Guo, O.K. Farha, B.G. Hauser, X.Y. Qi, Y. Wang, X. Wang, S.Y. Han,
X.G. Liu, J.S. DuChene, H. Zhang, Q.C. Zhang, X.D. Chen, J. Ma, S.C.J. Loo, W.D.
Wei, Y.H. Yang, J.T. Hupp, F.W. Huo, Nat. Chem. 4 (2012) 310.
28] L.Y. Chen, H.R. Chen, R. Luque, Y.W. Li, Chem. Sci. 5 (2014) 3708.
29] Y.J. Cui, Y.F. Yue, G.D. Qian, B.L. Chen, Chem. Rev. 112 (2012) 1126.
30] W.-Y. Gao, M. Chrzanowski, S.Q. Ma, Chem. Soc. Rev. 43 (2014) 5841.
31] H.-C. Zhou, S. Kitagawa, Chem. Soc. Rev. 43 (2014) 5415.
32] W.M. Xuan, C.F. Zhu, Y. Liu, Y. Cui, Chem. Soc. Rev. 41 (2012) 1677.
33] A. Corma, H. García, F.X. Llabrés i Xamena, Chem. Rev. 110 (2010) 4606.
34] O. Shekhah, J. Liu, R.A. Fischer, Ch. Wöll, Chem. Soc. Rev. 40 (2011) 1081.
35] J.Y. Lee, O.K. Farha, J. Roberts, K.A. Scheidt, S.T. Nguyen, J.T. Hupp, Chem. Soc.
Rev. 38 (2009) 1450.
[
[
[
[
[
[
[
[
[74] J.-W. Zhang, H.-T. Zhang, Z.-Y. Du, X.Q. Wang, S.-H. Yu, H.-L. Jiang, Chem.
Commun. 50 (2014) 1092.
[
[
[
[
[
[
[
[
36] S.M. Cohen, Chem. Rev. 112 (2012) 970.
37] S.L. Qiu, M. Xue, G.S. Zhu, Chem. Soc. Rev. 43 (2014) 6116.
38] Q.-L. Zhu, Q. Xu, Chem. Soc. Rev. 43 (2014) 5468.
39] H.R. Moon, D.-W. Lim, M.P. Suh, Chem. Soc. Rev. 42 (2013) 1807.
40] H.-L. Jiang, Q. Xu, Chem. Commun. 47 (2011) 3351.
41] H. Furukawa, U. Müller, O.M. Yaghi, Angew. Chem. Int. Ed. 54 (2015) 3417.
42] Z.Q. Wang, S.M. Cohen, Chem. Soc. Rev. 38 (2009) 1315.
43] W.N. Zhang, G. Lu, C.L. Cui, Y.Y. Liu, S.Z. Li, W.J. Yan, C. Xing, Y.R. Chi, Y.H. Yang,
F.W. Huo, Adv. Mater. 26 (2014) 4056.
[75] Q.-L. Zhu, J. Li, Q. Xu, J. Am. Chem. Soc. 135 (2013) 10210.
[76] A. Aijaz, A. Karkamkar, Y.J. Choi, N. Tsumori, E. Rönnebro, T. Autrey, H.
Shioyama, Q. Xu, J. Am. Chem. Soc. 134 (2012) 13926.
[77] X.M. Chen, G.H. Wu, J.M. Chen, X. Chen, Z.X. Xie, X.R. Wang, J. Am. Chem. Soc.
133 (2011) 3693.
[78] D.R. Sun, W.J. Liu, Y.H. Fu, Z.X. Fang, F.X. Sun, X.Z. Fu, Y.F. Zhang, Z.H. Li, Chem.
Eur. J. 20 (2014) 4780.
[79] W.H. Fang, J.S. Chen, Q.H. Zhang, W.P. Deng, Y. Wang, Chem. Eur. J. 17 (2011)
1247.
[
44] Y.K. Hwang, D.-Y. Hong, J.-S. Chang, S.H. Jhung, Y.-K. Seo, J. Kim, A. Vimont, M.
Daturi, C. Serre, G. Férey, Angew. Chem. Int. Ed. 47 (2008) 4144.