D. Wang et al. / Journal of Catalysis 361 (2018) 248–254
249
MNPs/photocatalysts nanocomposites, including MNPs/TiO
2
(M =
approach combined with a photoreduction process as reported
previously [27,56]. Bimetallic PdAu@MIL-100(Fe) with a fixed total
noble metal doping of 1.0 wt%, but different Pd/Au ratio (denoted
Pd, Au, Cu-Mo alloy) and PdNPs@MIL-100(Fe), have also been
applied to realize the N-alkylation reactions by coupling the photo-
catalytic dehydrogenation of alcohols to form aldehydes with the
MNPs-based hydrogenation of imines [27,40–42].
x y
as Pd Au @MIL-100(Fe)), were prepared in similar procedures as
those in the preparation of Pd@MIL-100(Fe), except that different
Our previous study reported that PdNPs@MIL-100(Fe) act as a
multifunctional catalyst for light induced N-alkylation reaction,
in which MIL-100(Fe) acts as a photocatalyst for the dehydrogena-
tion of alcohols to generate aldehydes, and the Lewis acidic
Fe promotes the condensation between aldehydes and amines
to form imines, while PdNPs catalyze the hydrogenation of imines
to form N-alkyl amines [27]. Although the use of PdNPs@MIL-100
ratio of HAuCl
4
and PdCl
2
(CH
3
CN)
2
were used instead of bare
@MIL-100(Fe) for example,
PdCl (CH CN) . Take 1.0 wt% Pd
2
3
2
1
Au
1
MIL-100(Fe) was activated at 120 °C under vacuum to remove
the residual solvent in the pores. 100 mg of activated MIL-100
(Fe) was suspended in 20 mL of hydrophobic anhydrous n-
hexane. The as-obtained suspension was sonicated for 1 h before
3
+
a hydrophilic solution (0.08 mL) containing PdCl
2 3 2
(CH CN) (1.3
(
Fe) for the N-alkylation reactions is attractive since the reactions
mg, 0.5 wt% Pd) and HAuCl O (1.1 mg, 0.5 wt% Au) was added
4
Á4H
2
can be realized under visible light irradiations, the yield to the tar-
geted products over Pd@MIL-100(Fe) for some N-alkylation reac-
tions are still not satisfactory. Kinetic studies on the reaction
between benzyl alcohol and aniline over PdNPs@MIL-100(Fe)
under visible light revealed that the dehydrogenation of benzyl
alcohol to benzaldehyde is the rate-limiting step for the whole
N-alkylation reaction. This finding points out a strategy to improve
the whole N-alkylation reaction by accelerating the alcohol-to-
aldehyde transformation, the rate-limiting step in the tandem
reaction. Considering that photocatalysis has become important
for homogeneous gold catalysis and metallic AuNPs are active
catalysts for abstracting hydrogen from alcohols to form aldehydes
drop-wise into the above suspension under vigorous stirring. After
stirring for 2 h, the solid was isolated, washed with ethanol and
dried under vacuum. The as-obtained solid was suspended in
degassed anhydrous methanol and irradiated under visible light
for 3 h. The resultant solid product was filtered, washed with
methanol and dried overnight at 60 °C in oven.
2.2. Characterizations
X-ray diffraction (XRD) patterns were collected on a D8
Advance X-ray diffractometer (Bruker, Germany) with Cu K radi-
a
ation. The accelerating voltage and the applied current were 40 kV
[
43–54], we proposed that the introduction of metallic Au into
Pd@MIL-100(Fe) would promote the photocatalytic dehydrogena-
tion of alcohols, which in turn would result in an improved
efficiency for the whole tandem reaction to yield N-alkyl amines.
In addition, it is preferred that the resultant Au-H species by
abstracting hydrogen from the alcohols should be recovered by
hydrogen transfer to Pd sites to form Pd-H, which acts as the active
species for the hydrogenation of the imines. To facilitate the hydro-
gen transfer from Au to Pd, Au and Pd should be in close proximity.
In this manuscript, we reported the preparation of small
bimetallic PdAu alloy NPs encapsulated inside the cavities of
MIL-100(Fe) (PdAu@MIL-100(Fe)) via a double-solvent impregna-
tion followed by a photoreduction method, and their application
in tandem reactions between amines and alcohols to realize the
N-alkylation under visible light irradiation. As compared with bare
Pd NPs encapsulated inside MIL-100(Fe), the as-formed bimetallic
PdAu@MIL-100(Fe) showed superior performance for the
generations of desired N-alkyl amines. In addition, the Pd/Au
ratio in PdAu@MIL-100(Fe) influences its catalytic performance
for N-alkylation reaction. The comparison of the catalytic perfor-
mance of PdAu@MIL-100(Fe) with different Pd/Au ratio indicates
that the reaction rates of two consecutive steps in the
N-alkylation reaction, i.e., photocatalytic dehydrogenation of alco-
hols and hydrogenation of imines, can be tuned by changing the
Pd/Au ratio. This study not only provides an efficient way to realize
a highly efficient and stable alkylation of amines via a successful
coupling of the MOF-based photocatalysis and MNPs-based hydro-
genation, but also demonstrates that in a cascade/tandem reaction,
the reaction rates of different catalytic steps can be tuned for an
efficient cascade/tandem reaction via a rational design of the
multifunctional catalyst.
and 40 mA, respectively. Data were recorded at a scanning rate of
À1
0
.02° 2h s in the 2h range of 5° to 30°. UV–visible diffuse reflec-
tance spectra (UV-DRS) of the powders were obtained with BaSO
4
used as a reflectance standard. BET surface area was carried out on
an ASAP 2020M apparatus (Micromeritics Instrument Corp., USA).
The samples were degassed in 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) measurements were per-
formed on a PHI Quantum 2000 XPS system (PHI, USA). Inductively
Coupled Optical Emission Spectrometer (ICP-OES) was performed
on Optima 8000 (PerkinElmer). Before ICP-OES experiment, the
3
solid sample was digested in mixture of HNO and milli-Q water.
2.3. Catalytic reactions
The N-alkylation reactions were performed in a sealed schlenk
tube under visible light irradiation. The catalyst (10 mg), amines
(0.1 mmol) and alcohols (3 mmol) were suspended in CH
mL) and the resultant mixture were degassed and saturated with
to remove any dissolved O before reaction. The reactions were
3
CN (2
N
2
2
performed under the irradiation of a 300 W Xe lamp equipped with
a UV-cut filter to remove irradiation with wavelengths less than
420 nm and an IR-cut filter to remove irradiation with wavelengths
>800 nm. After reaction, the reaction mixture was filtered through
a porous membrane (20 lm in diameter) and the products were
analyzed by GC–MS and GC-FID (Shimadzu GC-2014) equipped
with a HP-5 capillary column.
Photocatalytic dehydrogenation of benzyl alcohol was con-
ducted in a sealed reaction tube containing catalyst (10 mg), ben-
2
. Experimental
3
zyl alcohol (0.1 mmol) and CH CN (2 mL).
2.1. Preparations
The reaction between aniline and benzaldehyde was carried out
in a tube containing catalyst (10 mg), aniline (0.1 mmol), benzalde-
All the reagents were commercial available and used without
hyde (0.5 mmol) and CH
benzylideneaniline with H was carried out in a reaction tube con-
taining catalyst (10 mg), N-benzylideneaniline (0.1 mmol), benzyl
alcohol (3 mmol) and CH CN (2 mL).
3
CN (2 mL). Hydrogenation of N-
further purifications. MIL-100(Fe) was prepared following previ-
ously reported procedures with slight modifications [55].
Pd@MIL-100(Fe) was prepared by a double-solvent impregnation
2
3