Solvent-Driven Selectivity Control to Either Anilines or Dicyclohexylamines in Hydrogenation of Nitroarenes over a Bifunctional Pd/MIL-101 Catalyst

Article abstract of DOI:10.1021/acscatal.8b01834

The hydrogenation of nitroarenes is one of the most important strategies to produce the corresponding anilines and dicyclohexylamines, both of which are the fundamental raw materials in the synthesis of various pharmaceuticals and fine chemicals. Nevertheless, it is still a great challenge to develop a highly versatile and flexible catalytic system to selectively generate desired amines. Herein, we report the solvent-driven selectivity control over a bifunctional Pd/MIL-101 catalyst for the hydrogenation of nitrobenzene. An almost full selectivity of 99.9% to aniline or a surprising selectivity of 99.1% to dicyclohexylamine is achieved by using dimethylformamide (DMF, a polar solvent) or n-hexane (an apolar solvent) as the solvents, respectively. It is proposed that the polarity of solvents can effectively regulate the linkage between reactants/intermediates and Pd/MIL-101, affording controllable selectivities of aniline or dicyclohexylamine at will. In addition, the Lewis acid sites in Pd/MIL-101 can also effectively activate the aromatic ring and accelerate the cross-coupling reaction of amine. This solvent-driven catalytic system also exhibits good recyclability and compatibility for a wide substrate scope in both DMF and n-hexane, showing great promise for industrial applications. This study might open an avenue for the hydrogenation of nitroarenes to selectively produce anilines or dicyclohexylamines by simply regulating the solvent polarity over a bifunctional catalyst system.

Full text of DOI:10.1021/acscatal.8b01834

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Article
Solvent-Driven Selectivity Control to either Anilines or Dicyclohexylamines
in Hydrogenation of Nitroarenes over a Bifunctional Pd/MIL-101 Catalyst
Xiaodong Chen, Kui Shen, Danni Ding, Junying Chen, Ting Fan, Rongfang Wu, and Yingwei Li
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01834 • Publication Date (Web): 08 Oct 2018
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Solvent-Driven Selectivity Control to either Anilines or
Dicyclohexylamines in Hydrogenation of Nitroarenes over a
Bifunctional Pd/MIL-101 Catalyst
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Xiaodong Chen,† Kui Shen,*† Danni Ding,† Junying Chen,† Ting Fan,† Rongfang
Wu,†‡ and Yingwei Li*†
†State Key Laboratory of Pulp and Paper Engineering, School of Chemistry and
Chemical Engineering, South China University of Technology, Guangzhou 510640, P.
R. China.
‡ Department of Environmental Monitoring, Guangdong Polytechnic of
Environmental Protection Engineering, Foshan 528216, P. R. China.
*Eꢀmail for K.S.: cekshen@scut.edu.cn. *Eꢀmail for Y.L.: liyw@scut.edu.cn.
Abstract
The hydrogenation of nitroarenes is one of the most important strategies to produce
corresponding anilines and dicyclohexylamines, both of which are the fundamental
raw materials in the synthesis of various pharmaceuticals and fine chemicals.
Nevertheless, it is still a great challenge to develop highly versatile and flexible
catalytic system to selectively generate desired amines. Herein, we report the
solventꢀdriven selectivity control over a bifunctional Pd/MILꢀ101 catalyst for the
hydrogenation of nitrobenzene. An almost full selectivity of 99.9 % to aniline or a
surprising selectivity of 99.1 % to dicyclohexylamine is achieved by using dimethyl
formamide (DMF, a polar solvent) or nꢀhexane (an apolar solvent) as the solvents,
respectively. It is proposed that the polarity of solvents can effectively regulate the
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linkage between reactants/intermediates and Pd/MILꢀ101, affording controllable
selectivities of aniline or dicyclohexylamine at will. Besides, the Lewis acid sites in
Pd/MILꢀ101 can also effectively activate the aromatic ring and accelerate the cross
coupling reaction of amine. This solventꢀdriven catalytic system also exhibits good
recyclability and compatibility for a wide substrate scope in both DMF and nꢀhexane,
showing great promise for industrial applications. This study might open an avenue
for the hydrogenation of nitroarenes to selectively produce anilines or
dicyclohexylamines by simply regulating the solvent polarity over a bifunctional
catalyst system.
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KEYWORDS: Pd/MILꢀ101; selective hydrogenation; nitrobenzene; aniline;
dicyclohexylamine
1. Introduction
The hydrogenation of nitroarenes is one of the most important synthetic paths for
producing corresponding Nꢀcontaining products, such as anilines, azoxy and
secondary amines, which are widely used as intermediates in the synthesis of dyes,
pharmaceuticals and fine chemicals.1ꢀ7 Although extensive efforts have been made to
develop various catalytic systems for efficient transformation of nitroarenes,8ꢀ15 most
of them are focused on the hydrogenation of nitro compounds to anilines. Up to date,
there are only few reports on the reduction of aromatic ring or the cross coupling
reaction of amino groups, because harsh operation conditions or/and bifunctional or
multifunctional catalysts are usually required for these purposes.16ꢀ22 In this regard,
Muller et al. recently reported that a highly selective conversion of nitrobenzene (NB)
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to cyclohexylamine (CA) could be achieved over a Ru/CNTs catalyst under 100 bar
hydrogen pressure and 140 ˚C reaction temperature.17 Corma et al. also showed that
nitroderivatives were transformed to cyclohexylanilines in good selectivities by using
a Pd/C catalyst system with Bronsted acid as cocatalyst.18 Nevertheless, to the best of
our knowledge, there is still no catalyst system reported so far that can transform
nitroarenes into dicyclohexylamines with excellent selectivities. Furthermore, the
controllable conversion of one nitroarene compound into multiꢀtarget products via
single catalyst system remains a great challenge.
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Metalꢀorganic frameworks (MOFs) are a new class of porous materials possessing
large internal surface areas, wellꢀdefined structures, and tunable chemical
properties.23ꢀ26 Owing to these features, MOFs have received tremendous attentions in
the applications of heterogeneous catalysis, particularly when they are used as the
supports for noble metal (e.g., Pd, Au, Ru, and Pt) nanoparticles (MNPs).27ꢀ36 On the
one hand, the open and uniform channels of MOFs make them particularly suitable
for the efficient immobilization of MNPs, thus effectively avoiding the loss of MNPs
and vastly exposing the active sites.25,33 On the other hand, the organic linkers or the
metal nods of MOFs can also be used as unique functional moieties, and the
interaction between MNPs and MOFs might offer the opportunity for MNPs/MOFs to
be bifunctional even multifunctional catalysts.28,37 Based on these advantages,
MOFꢀimmobilized MNPs have been extensively studied in a wide range of chemical
transformations over the past few years.27ꢀ31 Our group has also made some
contributions in this topic, developing a series of novel metalꢀNPs@MOFs for
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efficient transformations, e.g., oneꢀstep conversion of acetone to methyl isobutyl
ketone,28 selective hydrogenation of nitriles to imines,29 effective hydrogenation of
biomass derived furfural to cyclopentanone,31 and selective oxidation of cinnamyl
alcohol.32
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In this work, as shown in Figure 1, we report the controllable selective
hydrogenation of nitroarenes over a heterogeneous catalytic system based on
ultraꢀtiny Pd NPs immobilized in a chromiumꢀbased MOF, namely MILꢀ101
(Cr3F(H2O)2O[(O2C)C6H4(CO2)]3ꢁ nH2O, n ≈ 25),38ꢀ40 by using a colloidal deposition
method. In the MILꢀ101 structure, the potential unsaturated chromium sites (up to 3.0
mmolꢁgꢀ1) can provide a large number of Lewis acidic sites after the removal of
terminal water molecules, which has been proved to play significant role in the
activation of aromatic compounds.40 Thus, after loading Pd NPs, these two types of
hydrogenation active ingredients would work cooperatively to show a bifunctional
catalytic activity. Consequently, Pd/MILꢀ101 is capable of catalyzing the selective
hydrogenation of nitroarenes to either anilines or dicyclohexylamines just by using
different solvents, whose polarity is the key factor for obtaining the controllable
selectivities to anilines vs dicyclohexylamines.
2. Experimental
2.1. Catalyst preparation
All chemicals were purchased from commercial sources and used without further
treatments. All solvents were analytical grade and distilled prior to use.
2.1.1 Synthesis of Pd/MILꢀ101
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MILꢀ101 was prepared according to the previous reported procedures.38 Typically,
the hydrothermal reaction of Cr(NO3)3⋅9H2O (0.80 g, 2 mmol), terephthalic acid (0.33
g, 2 mmol), HF (48 wt %, 2 mmol), and 10 mL deionized water were carried out at
220 ˚C for 8 h. The mixture was cooled first to 150 ˚C in 1 h, and then slowly to room
temperature in 12 h. Subsequently, the mixture was first filtrated by using a large pore
fritted glass filter (2, Schott) to isolate large terephthalic acid crystal, and then
filtrated in a small pore paper filter (1°, Whatman) to remove the free terephthalic
acid. The obtained green MILꢀ101 powder was soaked in ethanol (95 % EtOH with
5 % water) at 80 ˚C for 24 h followed by filtrating the hot solution. The solid was
finally dried at 150 ˚C under vacuum for 24 h.
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MILꢀ101ꢀsupporting palladium catalyst was prepared via a colloidal deposition
method.41 In a typical synthesis, a methanol solution of Pd(NO3)2·2H2O (0.001 M)
was first prepared with PVP (polyvinylpyrrolidone) as a protecting agent (Pd:PVP
monomer = 1:10, molar ratio) and vigorously stirred for 1 h. Then, a desired amount
of NaBH4 (Pd:NaBH4 = 1:5, molar ratio) was added to the solution to obtain a dark
brown sol. After few minutes sol generation, an appropriate amount of activated
MILꢀ101 powder was added to the colloidal solution, which was stirred for 24 h. The
asꢀsynthesized sample was dried at 100 ˚C under vacuum for 2 h and then treated in a
stream of H2 at 200 ˚C for 2 h to yield Pd/MILꢀ101. The Pd loading on the sample
was 2 wt %.
2.1.2 Synthesis of Pd/ZIFꢀ8, Pd/HKUSTꢀ1, and Pd/UIOꢀ66
ZIFꢀ8, HKUSTꢀ1, and UIOꢀ66 were prepared according to precious reports.33, 42, 43
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And Pd/ZIFꢀ8, Pd/HKUSTꢀ1, and Pd/UIOꢀ66 were prepared via the same path and
with the same loading amount as the preparation of Pd/MILꢀ101 except using
different supports.
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2.1.3 Synthesis of Pd/SiO2 and Pd/γꢀAl2O3
SiO2 and γꢀAl2O3 were obtained from commercial source. Pd/SiO2 and Pd/γꢀAl2O3
were prepared by an impregnation method. Typically, a certain amount of support
(SiO2 and γꢀAl2O3) was dispersed in 20 mL of acetone, which was stirred for 30 min
at room temperature. Subsequently, a methanol solution of Pd(NO3)2·2H2O (0.001 M)
was added dropwise to the above solution under vigorously stirring for 10 min. After
being subjected to ultrasonication for 20 min, the slurry was then stirred at room
temperature for 24 h. The impregnated samples were washed with acetone for a few
times and dried slowly at room temperature followed by treating at 150 °C for 8 h
under vacuum. The Pd2+ in asꢀsynthesized samples was subsequently reduced to Pd0
in a stream of H2 at 200 °C for 2 h to obtain Pd/SiO2 and Pd/γꢀAl2O3. The Pd loading
on both samples were 2 wt%.
2.2. Characterization
Powder Xꢀray diffraction patterns of the samples were obtained by a Rigaku
diffractormeter (D/MAXꢀIIIA, 3 kW) using Cu Kα radiation (40 kV, 40 mA, λ =
0.1543 nm). The N2 adsorption/desorption isotherms were measured by a
Micromeritics ASAP 2020M instrument at 77 K, and the samples were evacuated at
150°C overnight before the analysis. The palladium loadings of the samples were
measured quantitatively by atomic absorption spectroscopy (AAS) on a Hitachi
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Zꢀ2300 instrument. The morphology and nanoparticle size of samples as well as the
element mapping were investigated by using a scanning electronic microscope (SEM,
Hitachi SU8220) and a transmission electron microscope (TEM, JEOL, JEMꢀ2100F
and JEMꢀ1400) with energyꢀdispersive Xꢀray spectroscopy (EDS) analysis at 200 kV.
Xꢀray photoelectron spectroscopy (XPS) measurement was performed on a Thermo
Scientific KꢀALPHA system with a base pressure of 2 × 10ꢀ9 mbar.
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2.3. Catalytic Reaction
The nitrobenzene hydrogenation process was conducted in a 10 mL Teflonꢀlined
stainless steel autoclave equipped with a pressure gauge under the magnetic stir.
Typically, nitroarenes (0.5 mmol), catalyst (1.0 mol%, based on Pd content), and 4 ml
of solvent were added to the autoclave and the mixture was stirred at 600 rpm. The
autoclave was purged ten times with H2, and then pressurized with H2 to the required
pressure and heated up to the target temperature. After reaction, the products were
quantified and identified by a GCꢀMS (Agilent Technologies 7890Bꢀ5977A equipped
with a 0.25 mm × 30 m HPꢀ 5MS capillary column) with dodecane as an internal
standard (Figure S1ꢀS3).
The conversion of NB and the selectivity and yield of the products (aniline (AN),
cyclohexylamine (CA), phenylcyclohexylamine (PC) and dicyclohexylamine (DA))
were calculated based on following equations:
xC(i)
∑
Con.(NB) =
×100 %
C(NB) +
xC(i)
∑
xC(i)
Sel.(i) =
× 100 %
xC(i)
∑
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Yield(i) = Sel.(i) × Con.(NB) × 100 %
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where C represents the concentrations of the products, i represents one of AN, CA,
PC and DA, and x means the benzene ring numbers in one product molecule.
For the recyclability test, the catalyst was separated from the reaction solution,
washed with acetone, and dried for the next run.
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Figure 1. Schematic illustration of the selective hydrogenation of nitrobenzene over
the bifunctional Pd/MILꢀ101 containing Lewis acid sites and Pd NPs.
Figure 2. (a) PXRD patterns of simulated MILꢀ101, asꢀsynthesized MILꢀ101, and
Pd/MILꢀ101; (b) N2 adsorptionꢀdesorption isotherms of asꢀsynthesized MILꢀ101 and
Pd/MILꢀ101.
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3. Results and discussion
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MILꢀ101 was synthesized and activated under vacuum according to the reported
procedure with some modifications.38 Subsequently, Pd NPs were generated and
immobilized on MILꢀ101 to prepare the Pd/MILꢀ101 catalyst by using a colloidal
deposition method. Here, the loading amount of Pd was fixed to 2 wt % and the actual
value was measured by atomic absorption spectrum (AAS) (Table S1). Powder Xꢀray
diffraction (PXRD) was used to characterize the crystallinity of the prepared catalyst.
As shown in Figure 2a, Pd/MILꢀ101 matches well with both the simulated and
asꢀsynthesized MILꢀ101, indicating the crystallinity and structure of MILꢀ101 are
well retained after loading of Pd NPs.41
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Figure 3. (a, c) TEM, (b) STEM and (d) HRꢀTEM images of the Pd/MILꢀ101. (e)
HAADFꢀSTEM image and the corresponding elemental mappings of the Pd/MILꢀ101.
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The inset of (c) is the size distribution of Pd NPs.
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Then, the surface areas and pore volumes of MILꢀ101 and Pd/MILꢀ101 were
measured by N2 adsorption/desorption at 77 K (Figure 2b). Both samples exhibit
similar type Ι isotherms with high surface areas of 2658 m2gꢀ1 and 2092 m2gꢀ1 for
MILꢀ101 and Pd/MILꢀ101, respectively, further indicating the ordered porous
structure of Pd/MILꢀ101 are well inherited from the MILꢀ101 support.41 The
appreciable decrease in N2 adsorption amount after the introduction of Pd NPs can be
attributed to the partial blockage of the cavities in Pd/MILꢀ101 by the deposited Pd
NPs. SEM image (Figure S4) also reveals that the octahedral shape of MILꢀ101 can
be well preserved after the loading of Pd NPs. The detailed structure of Pd/MILꢀ101
was further investigated by transmission electron microscopy (TEM) and scanning
transmission electron microscopy (STEM). As shown in Figure 3aꢀc and S5, a large
number of small NPs are homogeneously distributed on an octahedron matrix with a
mean diameter of 2.56 ± 0.72 nm, where no remarkable formation of aggregates can
be observed. Highꢀresolution TEM (HRꢀTEM) image shows that the interplanar
spacing of the particle lattices is about 0.225 nm, corresponding to the (111) plane of
faceꢀcentered cubic Pd (Figure 3d).41 A representative highꢀangle annular
darkꢀfieldꢀscanning transmission electron microscopy (HAADFꢀSTEM) image and
the corresponding EDS mappings are shown in Figure 3e. From the selected area, the
uniform distributions of Cr, C and Pd elements are observed in the same area, further
confirming that Pd NPs are effectively deposited on MILꢀ101 matrix with potential
interaction between Pd NPs and the support. Xꢀray photoelectron spectroscopy (XPS)
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was further used to study the chemical state of Pd in Pd/MILꢀ101. As shown in Figure
S6, Pd/MILꢀ101 shows two obvious peaks at 335.5 and 340.8 eV, which can be
ascribed to the binding energies of Pd 3d5/2 and Pd 3d3/2 of zeroꢀvalent Pd,
respectively, indicating that Pd in this sample is in its metallic state.44
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The hydrogenation reaction of NB was subsequently conducted over Pd/MILꢀ101
(reaction conditions: 120 ˚C, 0.6 MPa of H2 and 4 ml of selected solvents). In order to
investigate the influence of solvent on the reaction, a series of solvents with different
polarities were first used to perform the reaction. The solvents we chose here are
methanol (MeOH), ethanol (EtOH), N, Nꢀdimethylformamide (DMF), tetrahydrofuran
(THF) and nꢀhexane. The polarity of these solvents is estimated by the dielectric
constant listed in Table 1,45, 46 whose order is DMF > MeOH > EtOH > THF >
nꢀhexane. A complete conversion of NB is achieved in all solvents listed in Table 1
(entries 1ꢀ6), confirming the high catalytic activity of Pd/MILꢀ101 for the reduction of
aromatic nitro group. Note that aniline (AN) is the only product when using DMF as
the solvent (Table 1, entry 1). The kinetic curves of Pd/MILꢀ101 for this reaction in
DMF solvent show that AN is the only detected product and diazo intermediates are
not detected in the whole reaction process, indicating the aromatic nitro compound is
reduced via a direct route (Figure 4a).47 However, with a decrease in the polarity of
solvents, the selectivities of AN over Pd/MILꢀ101 also decrease sharply from ＞
99.9 % in DMF to 64.2 % in MeOH, and then 28.8 % in EtOH (Table 1, entries 2ꢀ3).
Besides AN, three other products, namely cyclohexylamine (CA),
phenylcyclohexylamine (PC) and dicyclohexylamine (DA), are also observed in these
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systems. Further decreasing the polarity, the selectivity of AN can be further
decreased to almost 0 % in both THF and nꢀhexane (Table 1, entries 4ꢀ5).
Interestingly, in contrast to AN, the selectivities of DA over Pd/MILꢀ101 increase with
decrease in solvent polarity, thus following the order of MeOH < EtOH < THF <
nꢀhexane (Figure 5 and entries 2ꢀ6 of Table 1). It is noteworthy that 99.1 % selectivity
of DA can be obtained when using nꢀhexane as the solvent, which is also the highest
value among all the previously reported systems.4, 18, 48, 49 Clearly, the high activity
and solventꢀdriven selectivity of our system show great promise for industrial
applications.
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Figure 4. The kinetics curves for the hydrogenation of NB under 120 °C and 0.6 MPa
of H2 with (a) DMF and (b) nꢀhexane as the solvents.
Encouraged by these results, we further investigated the kinetic curves of
Pd/MILꢀ101 in this reaction using nꢀhexane as solvent. As shown in Figure 4b, four
products, AN, CA, PC and DA are all detected in the reaction process. In the initial
stage, AN is emerged as the main product, but rapidly decreases as the reaction
progresses, indicating it can be further hydrogenated easily when using nꢀhexane as
the solvent. In the meantime, a small amount of CA, generally regarded as the product
from the hydrogenation of aromatic ring in AN, is observed although its selectivity is
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always below 10 % with the reaction going on, implying that CA is also an active
intermediate product. In addition, a certain amount of PC is also formed in the early
stage, which is originated from the cross coupling reaction of AN and CA. The
amount of PC is cumulative to the maximum at ~1 h, and then gradually decreases
due to its transformation to DA, which also confirms that DA mainly results from the
hydrogenation of PC rather than the cross coupling reaction of two molecules of CA.
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Figure 5. Selective hydrogenation of nitroarenes to either aniline (AN) or
dicyclohexylamine (DA) over a bifunctional Pd/MILꢀ101 catalyst.
For comparison purposes, some other MOFs supported catalysts, such as Pd/ZIFꢀ8,
Pd/HKUSTꢀ1, and Pd/UiOꢀ66, and traditional porous materials supported Pd NPs,
like Pd/SiO2 and Pd/γꢀAl2O3 were also prepared (their Pd loadings are also in the
range of 1.7ꢀ1.9 wt %) and their catalytic performances for this reaction were then
investigated (Figure S7ꢀS11, Table S1). As shown in Table 1 (entries 6ꢀ15), full
conversion of NB can be achieved over these catalysts in both DMF and nꢀhexane,
and AN is also the only product when the reaction is conducted in DMF (entries 6, 8,
10, 12 and 14). Unfortunately, when using nꢀhexane as the solvent, Pd/SiO2 provides
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no selectivity of DA (Table 1, entry 7) and Pd/γꢀAl2O3 shows only 19.5 % selectivity
to DA (Table 1, entry 9), although both of them also contain solid acid. ZIFꢀ8 and
7
8
9
HKUSTꢀ1 are widely used as supports due to their high specific surface area and pore
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volume.33,
However, Pd/ZIFꢀ8 and Pd/HKUSTꢀ1 just exhibit very poor
solventꢀdriven performances with only 11.1 % and 5.2 % selectivities to DA observed
in nꢀhexane, respectively (Table 1, entries 11, 13). Furthermore, Pd/UiOꢀ66 shows a
moderate catalytic performance, giving 56.0 % selectivity of DA in nꢀhexane (Table 1,
entry 17), which might be attributed to its analogous construction with Pd/MILꢀ101
with slightly lower surface area and Lewis acidic sites derived from unsaturated Zr4+
sites.43 These results directly demonstrate that only by integrating both of high surface
area and Lewis acid can the Pd/MILꢀ101 catalyze the solventꢀdriven selective
hydrogenation of NB to either AN or DA through controlling the aromatic ring
hydrogenation and cross coupling reaction in different solvents.
Table 1. The catalytic results of the hydrogenation of NBa
Entry
Catalyst
Solvent Dielectric Time
Con. (%)
Sel. (%) b
constant
(ε)
(h)
AN CA PC DA
1
2
3
4
5
6
7
8
Pd/MILꢀ101
Pd/MILꢀ101
Pd/MILꢀ101
Pd/MILꢀ101
Pd/MILꢀ101
Pd/SiO2
DMF
MeOH
EtOH
38.3
33.6
25
6
100
100
100
100
100
100
100
100
>99.9
ꢀ
ꢀ
ꢀ
10
10
15
8
64.2 1.3 13.4 21.1
28.8 0.7 29.3 41.2
THF
7.58
1.89
38.3
1.89
38.3
ꢀ
4.3 10.2 85.5
nꢀhexane
DMF
ꢀ
0.9
ꢀ
ꢀ
ꢀ
ꢀ
99.1
12
8
>99.9
>99.9
>99.9
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Pd/SiO2
nꢀhexane
DMF
Pd/γꢀAl2O3
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Pd/γꢀAl2O3
Pd/ZIFꢀ8
nꢀhexane
DMF
1.89
38.3
1.89
38.3
1.89
38.3
1.89
15
10
12
10
12
8
100
100
100
100
100
100
100
49.6 4.2 26.7 19.5
>99.9
64.7 5.6 18.6 11.1
>99.9
76.5 4.8 13.5 5.2
>99.9
20.9 3.3 19.8 56.0
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12
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ꢀ
ꢀ
ꢀ
Pd/ZIFꢀ8
nꢀhexane
DMF
Pd/HKUSTꢀ1
Pd/HKUSTꢀ1
Pd/UiOꢀ66
Pd/UIOꢀ66
ꢀ
ꢀ
ꢀ
nꢀhexane
DMF
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ꢀ
ꢀ
ꢀ
nꢀhexane
15
aReaction conditions: 0.5 mmol NB, Pd catalysts (1.0 mol% of Pd relative to NB), 4 ml of solvents,
120 ˚C, and 0.6 MPa of H2.
bThe selectivity was calculated based on benzene ring.
On the basis of the above results, we propose a plausible reaction mechanism to
explain the solventꢀdriven hydrogenation of nitrobenzene over Pd/MILꢀ101 (Figure 6).
First, because of its strong electronꢀwithdrawing behavior, the nitro group of
nitrobenzene can strongly coordinate with the surface of Pd NPs, and then is directly
reduced to aminobenzene (AN) by dissociated H2 absorbing on Pd NPs.17 It is worth
to note that the aromatic ring in nitrobenzene is electron deficient, but become
electron rich in AN.50 Thus, it is presumed that the linkage between Pd/MILꢀ101 and
the electronꢀrich aromatic ring can be tremendously influenced by the polarity of the
solvent. When the reaction proceeds in solvents with strong polar (like DMF), the
asꢀobtained AN can easily desorb from the catalyst surface and be released as the final
product. On the contrary, when using nonpolar nꢀhexane as the solvent, the
initiallyꢀproduced AN can adsorb strongly on Pd NPs. Consequently, the aromatic
ring of AN can be activated by the Lewis acidic sites (i.e., Cr3+) in MILꢀ101,51 which
accelerate the reduction of AN to CA. Then, the cross coupling reaction of AN and
18, 22, 52, 53
CA is motivated to produce PC by following a “hydrogenꢀborrowing”
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mechanism: firstly, the dehydrogenation of CA to an imine would occur; then the
resultant imine can react with AN by nucleophilic attack to generate an unstable
aminoaminal; finally, after the release of an ammonia (Figure S12), the corresponding
secondary imine is formed, which is subsequently hydrogenated to produce PC. From
the kinetic curve in Figure 4b, we can see that this process is quick and efficient,
which might be attributed to the synergistic effect of Pd NPs and MILꢀ101. On the
one hand, Pd NPs have been previously demonstrated to be able to stimulate some
dehydrogenation and hydrogenation reactions.8, 54 On the other hand, the Lewis acidic
sites in MILꢀ101 can not only activate the imine, but also quench the released
ammonia and therefore promote the cross coupling reaction.3 Finally, the generated
PC is further hydrogenated to produce DA with very high yield.
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Figure 6. The proposed routes for the hydrogenation of NB in polar or apolar solvent
over Pd/MILꢀ101.
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Figure 7. Reuses of the Pd/MILꢀ101 for the hydrogenation of NB in (a) DMF and (b)
nꢀhexane by using short reaction times of 20 and 10 min, respectively.
The recyclability of the Pd/MILꢀ101 was further investigated in both DMF and
nꢀhexane under the identical reaction condition. After reaction, the catalyst was
separated from the reaction solution, washed and dried for the next run. As shown in
Figure 7a, only a slight decrease of NB conversion is observed in DMF after the 6th
run, while its selectivity to AN remains nearly unchanged. Similarly, the conversion
of NB can be also well retained after the 6th run when using nꢀhexane as the solvent
(Figure 7b). PXRD patterns show that the diffraction peaks of the used Pd/MILꢀ101
are all well matched with the fresh one (Figure S13), suggesting that the structural
integrity of Pd/MILꢀ101 can be well preserved after recycling tests. Moreover, the
octahedral shape of Pd/MILꢀ101 is also well preserved regardless of the solvents used
as exhibited by SEM images (Figure S4 and S14a, d). In addition, only slight
aggregation of Pd NPs is observed in the TEM images of reused catalysts even after
six catalytic runs (Figure S14b, c, e and f). These results further confirm the good
stability and recyclability of the Pd/MILꢀ101 for this reaction.
Table 2. Catalytic hydrogenation of various substituted nitrobenzenes over the Pd/MILꢀ101
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catalyst with DMF or nꢀhexane as solvents.a
5
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Number
Substrates
Solvents
Products
Reaction Con. (%) Sel. (%)
time (h)
DMF
6
8
100
100
100
100
100
100
>99.9
99.1
1
nꢀhexane
6
>99.9
94.5
DMF
nꢀhexane
DMF
2
3
12
6
>99.9
94.1
12
nꢀhexane
DMF
6
100
100
>99.9
92.1
4
5
6
nꢀhexane
12
8
95.4
100
>99.9
86.7
DMF
nꢀhexane
DMF
15
8
15
8
97.3
100
95.4
100
>99.9
88.5
nꢀhexane
DMF
>99.9
88.1
7
nꢀhexane
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DMF
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DMF
8
91.2
100
90.9
100
>99.9
83.8
8
9
20
15
30
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>99.9
79.4
nꢀhexane
aReaction conditions: 0.5 mmol of substrates, Pd/MILꢀ101 (1.0 mol% Pd relative to substrates), 4
ml of selected solvents, 120 ˚C, and 0.6 MPa of H2.
Finally, several aromatic nitro compounds containing various alkanes were also
investigated over Pd/MILꢀ101 with the optimized conditions in both DMF and
nꢀhexane. As summarized in Table 2, all the investigated nitroarenes can be
selectively hydrogenated to their corresponding anilines or dicyclohexylamines by
simply using DMF or nꢀhexane as the solvent, demonstrating the good tolerance of
Pd/MILꢀ101 to different substrates. Nevertheless, the yields of both anilines and
dicyclohexylamines decrease slightly with the complexification of alkanes, which
might be ascribed to the influence of steric effect.
4. Conclusions
In summary, we have demonstated that Pd/MILꢀ101 featuring encapsulated Pd NPs
in the pores of MILꢀ101 with abundant Lewis acidic sites can be used as a
bifunctional catalyst for the solventꢀdriven selective hydrogenation of nitrobenzene to
either aniline or dicyclohexylamine at will. The key to success for this controllable
system is the use of the solvent polarity as an effective strategy to regulate the linkage
between reactants/intermediates and Pd/MILꢀ101. In addition, the synergetic effect of
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Pd NPs and MILꢀ101 can also effectively elevate the aromatic ring hydrogenation as
well as the cross coupling reaction of amines. Consequently, an almost full selectivity
to aniline is obtained in DMF (a polar solvent), and a surprising selectivity of 99.1 %
to dicyclohexylamine is achieved in nꢀhexane (an apolar solvent). Furthermore, this
solventꢀdriven catalytic system is also confirmed for a wide substrate scope and thus
provides a facile strategy for the hydrogenation of nitroarenes to selectively produce
anilines or dicyclohexylamines.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications
website at DOI:
Additional SEM and TEM images, XPS spectra, XRD patterns, AAS results,
optical photographs, and MS spectra.
AUTHOR INFORMATION
Corresponding Author
*Eꢀmail for K.S.: cekshen@scut.edu.cn.
*Eꢀmail for Y.L.: liyw@scut.edu.cn.
ORCID
Kui Shen: 0000ꢀ0002ꢀ4132ꢀ4533
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Ting Fan: 0000ꢀ0001ꢀ9710ꢀ9966
Yingwei Li: 0000ꢀ0003ꢀ1527ꢀ551X
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China
(21436005, 21576095, 21606087), Guangdong Natural Science Funds for
Distinguished Young Scholar (2018B030306050), the Fundamental Research Funds
for the Central Universities (2017PY004, 2017MS069), the Science and Technology
Program of Guangzhou (201804020009), the Pearl River S&T Nova Program of
Guangzhou (201806010140), the State Key Laboratory of Pulp and Paper
Engineering (2017ZD04, 2018TS03), and the Natural Science Foundation of
Guangdong Province (2016A050502004, 2017A030312005).
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