Pd(ii) immobilized on mesoporous silica by N-heterocyclic carbene ionic liquids and catalysis for hydrogenation

Article abstract of DOI:10.1039/c0cp01213k

In this work we synthesized Pd(ii) immobilized on mesoporous silica by N-heterocyclic carbene (NHC) ionic liquids (ILs) with different alkyl chain lengths. The catalysts were characterized by Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), low-angle X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and nitrogen sorption. The catalysts were used for the hydrogenation of alkenes and allyl alcohol. The results indicated that the catalysts were very active, selective, and stable. The selectivity for the hydrogenation of allyl alcohol to 1-propanol increased with the increase of the alkyl chain length of the ILs. The effect of supercritical CO₂ (scCO₂) on the hydrogenation of allyl alcohol was also studied, and it was demonstrated that scCO₂ could enhance the selectivity of the reaction considerably. The XPS study showed that the valence of Pd(ii) remained unchanged under hydrogenation conditions. the Owner Societies.

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PAPER
Pd(II) immobilized on mesoporous silica by N-heterocyclic carbene ionic
liquids and catalysis for hydrogenationwz
Gang Liu, Minqiang Hou, Tianbin Wu, Tao Jiang, Honglei Fan, Guanying Yang
and Buxing Han*
Received 16th July 2010, Accepted 24th November 2010
DOI: 10.1039/c0cp01213k
In this work we synthesized Pd(II) immobilized on mesoporous silica by N-heterocyclic carbene
(
NHC) ionic liquids (ILs) with different alkyl chain lengths. The catalysts were characterized by
Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), low-angle
X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron
spectroscopy (XPS), and nitrogen sorption. The catalysts were used for the hydrogenation of
alkenes and allyl alcohol. The results indicated that the catalysts were very active, selective, and
stable. The selectivity for the hydrogenation of allyl alcohol to 1-propanol increased with the
increase of the alkyl chain length of the ILs. The effect of supercritical CO
2
(scCO
2
) on the
could
hydrogenation of allyl alcohol was also studied, and it was demonstrated that scCO
2
enhance the selectivity of the reaction considerably. The XPS study showed that the valence of
Pd(II) remained unchanged under hydrogenation conditions.
1
. Introduction
catalysts, palladium(0)–NHC catalytic systems have been
applied for hydrogenation, but the catalytic activity was
12,14
Hydrogenation reactions are very important in the petrochemical,
pharmaceutical and food industries. Metal complexes consisting
of transition metals and ligands are commonly used homo-
relatively low.
Ionic liquids (ILs) can be considered as environmentally
more acceptable solvents for organic reactions due to their
extremely low vapor pressure. Among them, imidazolium-based
ILs have been used as ligands for palladium-mediated organic
1
–3
geneous catalysts for hydrogenation. Due to their outstanding
activity and chemoselectivity, hydrogenation by Pd-complex
4
,5
catalysts have been studied extensively.
Recently, N-heterocyclic carbenes (NHCs) as ligands in
reactions because they can form an imidazole carbene bond
15,16
with palladium.
Recently, Karimi and Enders described a
6
–8
metal-catalyzed reactions have attracted much attention.
method for simultaneous immobilization of a NHC–Pd/IL
matrix on silica. This system was effectively applied to the
Heck reaction with a wide variety of iodo- and bromo-arenes.
The catalyst showed high thermal stability and could be
The strong electron donating ability of NHCs generally is the
key factor to prepare transition-metal complexes that are
thermally-stable with high activity. The Pd–NHC complexes
have been applied in various reactions such as Heck and
17
reused. Huynh et al. presented a solvent-controlled selective
9
–11
Suzuki reactions with high catalytic activity.
But in the
synthesis and structural characterization of a trans-configured
palladium(II) bis(benzimidazoline-2-ylidene) complex, and
the results demonstrated that benzannulated N-heterocyclic
carbenes derived from benzimidazole were excellent ligands
case of hydrogenation, little success has been achieved with the
Pd-NHC catalytic system due to the fact that Pd–NHC
complexes tend to decompose because of the attack of the
1
2
hydride on the carbene. Cloke showed that the Pd complex
0
16
for catalysis.
[
Pd(ItBu)
2
]
(ItBu
=
N,N -di-tert-butylimidazol-2-ylidene)
Another attractive feature of ILs is that both the cation and
anion moieties can be designed, making it possible to modify
ILs for specific applications. A series of task-specific ionic
liquids (TSILs) have been designed to bear functional groups
that provide particular and predictable properties, such as
2
decomposed after exposure to H and formed palladium black
3
and 1,3-bis-tert-butylimidazolidine. In the case of Pd–NHC
1
Beijing National Laboratory for Molecular Sciences, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
E-mail: hanbx@iccas.ac.cn; Fax: (+86) 10-62562821
w This article was submitted as part of a special collection of papers
from the Institute of Chemistry, Chinese Academy of Sciences
18
19
20
21
22
23
amine, amide, ether, acid, phosphoryl, and thiols.
For example, Zhang et al. synthesized a TSIL which has a
tertiary amino group (N(CH ) on the cation, which was used
to promote the hydrogenation of CO to produce formic
3 2
)
2
(
ICCAS).
z Electronic supplementary information (ESI) available. See DOI:
0.1039/c0cp01213k
2
4
acid, and a satisfactory result was obtained. Zhao and
co-workers prepared a series of nitrile-functionalized ILs on
1
2
062 Phys. Chem. Chem. Phys., 2011, 13, 2062–2068
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the N-butyronitrile pyridinium based cation, which were used
as effective immobilization solvents for palladium-catalyzed
2
(0.05 eqv.). The mixture was stirred under N at room
temperature for 24 h and filtered, a yellow solid was obtained
1
after the filtrate was evaporated. H-NMR (400 MHz,
2
5
Suzuki and Stille reactions.
Controlling the selectivity of chemical reactions by steric
hindrance is an interesting topic, and some results have been
reported. For example, Uyemura and Aida succeeded in the
control of AIBN-initiated alkylation of cobalt(II) porphyrin
with an alkyne in which cobalt(II) porphyrin was encapsulated
by a radical-tolerant, large poly(aryl ester) dendritic cage.
With the largest, (m-[G3]TPP)Co(II), a single organocobalt(III)
species was selectively formed with a yield of 91% due to the
steric protection by the large dendrimer cage from the access
CDCl ): d 7.45 (s, 1H, NCHN), 7.04 (s, 1H, NCHCH), 6.89
3
(s, 1H, NCHCH), 3.90 (t, 2H, NCH CH (CH ) CH ),
3
2
2
2 13
1.75 (quint., 2H, NCH CH (CH ) CH ), 1.24 (m, 14H,
2
2
2 13
3
2 2 2 3 2 2 2 3
NCH CH (CH )13CH ), 0.87 (t, 3H, NCH CH (CH )13CH ).
2.4 Preparation of the NHC ILs with different lengths of alkyl
chain (IL-1, IL-4, IL-10, IL-16)
NHC ILs with different alkyl chain lengths were prepared.
IL-1, IL-4, IL-10, and IL-16 stand for the ILs with alkyl chain
lengths of C1, C4, C10, and C16, respectively.
2
6
of another molecule of cobalt porphyrin species. Jiang and
Gao prepared heterogeneous Pd nanoparticle catalysts
stabilized by polyamidoamine dendrimers on SBA-15, and
the selectivity of the hydrogenation of allyl alcohol could be
IL-1 was synthesized according to the method reported by
2
8
other authors. In the experiment, 1-methylimidazole (0.82 g,
0
0
.01 mol) and (3-chloropropyl) triethoxysilane (2.41 g,
.01 mol) were dissolved in toluene (20 mL), and the solution
2
7
controlled by using different generation catalysts.
Herein, we prepared a series of Pd(II)–NHC IL complexes
immobilized on mesoporous silica, which were used as
catalysts for the hydrogenation of alkenes and allyl alcohol.
The results showed that the catalysts were very active, selective
and stable. It was also shown that the Pd(II)–NHC IL structure
was the main active species for the reactions, and the selectivity
for the hydrogenation of allyl alcohol can be controlled by the
alkyl chain lengths of the ILs. As far as we know, this is
the first work to conduct hydrogenation reactions catalyzed by
the immobilized Pd(II)–NHC IL structure.
2
was refluxed at 100 1C for 24 h under N atmosphere. The
yellow IL layer was separated from the organic layer by
cooling the solution to room temperature. Then the yellow
IL layer was thoroughly washed with ether (20 mL ꢀ 3) and
1
dried at 60 1C under vacuum for 24 h. H-NMR (400 MHz,
CDCl
3
): d 10.80 (s, 1H, NCHN), 7.39 (s, 1H, NCHCH),
.30 (s, 1H, NCHCH), 4.37 (t, 2H, CH N), 4.12 (s, 3H,
7
2
NCH ), 3.80 (q, 6H, OCH ), 2.02 (q, 2H, CH CH CH ),
3
2
2
2
2
1
.21 (t, 9H, CH CH ), 0.60 (t, 2H, SiCH ).
3 2 2
IL-4 was synthesized from 1-butyl-imidazole and (3-chloro-
propyl) triethoxysilane according to the procedure given
1
2
. Experimental
above for the synthesis of IL-1. H-NMR (400 MHz, CDCl
d 10.90 (s, 1H, NCHN), 7.66 (s, 1H, NCHCH), 7.32
s, 1H, NCHCH), 4.30 (t, 2H, CH N), 4.30 (t, 2H,
NCH CH CH CH ), 3.79 (q, 6H, OCH ), 1.94 (quint., 2H,
CH CH CH ), 1.84 (quint., 2H, NCH CH CH CH ), 1.36
(sext., 2H, NCH CH CH CH ), 1.14 (t, 9H, CH CH ), 0.88
(t, 3H, NCH CH CH CH ), 0.53 (t, 2H, SiCH ).
3
):
2
.1 Materials
(
2
1
-Methylimidazole (99%), 1-butylimidazole (99%), 1-bromo-
2
2
2
3
2
decane, 1-bromohexadecane, imidazole, aqueous ammonia
25 wt%), anhydrous ether, tetrahydrofuran (THF), tetra-
ethoxysilane (TEOS), cetyltrimethylammonium bromide
2
2
2
2
2
2
3
(
2
2
2
3
3
2
2
2
2
3
2
(CTAB), Pd(OAc) , anhydrous methanol, tetrabutyl ammonium
2
IL-10 was synthesized from N-decylimidazole and (3-chloro-
iodide (Bu NI) were all analytical grade reagents and purchased
4
propyl) triethoxysilane. The procedure was also similar to that
1
for preparing IL-1. H-NMR (400 MHz, CDCl
from Beijing Chemical Company. 3-Triethoxy-silylpropyl
chloride (97%) was provided by Aldrich.
3
): d 10.72 (s,
1H, NCHN), 7.42 (s, 1H, NCHCH), 7.32 (s, 1H, NCHCH),
.32 (t, 2H, CH N), 4.32 (t, 2H, NCH CH (CH CH ), 3.75
q, 6H, OCH ), 1.95 (quint., 2H, CH CH CH ), 1.85 (quint.,
CH (CH CH ), 1.14 (m, 14H, NCH
), 1.14 (t, 9H, CH CH ), 0.88 (t, 3H, NCH
), 0.53 (t, 2H, SiCH ).
4
2
2
2
2
)
7
3
2
.2 Preparation of N-decylimidazole
(
2
2
2
2
2
H, NCH
2
2
2
)
7
3
2
CH
CH
2
-
-
In the experiment imidazole (0.68 g, 0.01 mol) was dissolved in
THF (5 mL) and then added to a solution of NaH (1.5 eqv.) in
THF (15 mL). After stirring for 1 h at room temperature,
(
(
CH
CH
2
)
)
7
CH
3
3
3
2
2
2
2
7
CH
2
IL-16 was synthesized from N-hexadecylimidazole and
1
-bromodecane (1 eqv.) was added to the solution followed by
(
3-chloropropyl) triethoxysilane according to the above
1
Bu NI (0.05 eqv.). The mixture was stirred under N for 24 h
4
2
procedures. H-NMR (400 MHz, CDCl ): d 10.60 (s, 1H,
3
at room temperature, then filtered, and a yellow liquid was
1
obtained after the filtrate was evaporated. H-NMR (400 MHz,
NCHN), 7.46 (s, 1H, NCHCH), 7.34 (s, 1H, NCHCH),
4
3
1
.28 (t, 2H, CH N), 4.28 (t, 2H, NCH CH (CH ) CH ),
2
2
2
2 13
3
CDCl
3
): 7.44 (s, 1H, NCHN), 7.02 (s, 1H, NCHCH), 6.87
s, 1H, NCHCH), 3.88 (t, 2H, NCH CH (CH CH ), 1.75
quint., 2H, NCH CH (CH ) CH ), 1.22 (m, 14H,
.72 (q, 6H, OCH
2
), 1.95 (quint., 2H, CH
CH (CH
), 1.11 (t, 9H, CH
), 0.58 (t, 2H, SiCH
2
CH
2
CH
2
),
(
(
2
2
2
)
7
3
.85 (quint., 2H, NCH
2
2
2
)
13CH
3
), 1.12 (m, 14H,
CH ), 0.79 (t, 3H,
).
2
2
2 7
3
NCH
NCH
2
CH
CH
2
2
(CH
2
)
13CH
3
3
3
2
NCH CH (CH ) CH ), 0.86 (t, 3H, NCH CH (CH ) CH ).
2
2
2 7
3
2
2
2 7
3
2
(CH
2
)13CH
2
2
.3 Preparation of N-hexadecylimidazole
2
.5 Preparation of Pd–NHC IL complexes (Pd–IL-1,
Imidazole (0.68 g, 0.01 mol) was dissolved in THF (5 mL) and
then added to a solution of NaH (1.5 eqv.) in THF (15 mL).
After stirring for 1 h at room temperature 1-bromohexadecane
Pd–IL-4, Pd–IL-10, Pd–IL-16)
To prepare the Pd–IL-1 complex, IL-1 (3.23 g, 0.01 mol) and
(
1 eqv.) was added into the solution followed by Bu NI
4
Pd(OAc) (0.1 eqv.) were added to a 25 mL round-bottom
2
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flask and the air in the flask was removed by vacuum. The
mixture was heated with stirring at 60 1C for 8 h and then
heated to 100 1C for 4 h. The by-product in the reaction system
was removed using a rotary evaporator under vacuum for 2 h.
A pale yellow solution of Pd–IL-1 in IL-1 was obtained.
Pd–IL-4, Pd–IL-10 and Pd–IL-16 were synthesized from
diffractometer operated at 30 kV and 100 mA with Cu-Ka
radiation. X-ray photoelectron spectroscopy (XPS) data of the
as prepared samples were obtained with an ESCALab220i-XL
electron spectrometer from VG Scientific using 300 W Mg-Ka
ꢁ
9
radiation. The base pressure was about 3 ꢀ 10
mbar.
The binding energies were referenced to the C 1s line at
284.8 eV from adventitious carbon. Thermogravimetric (TG)
2
IL-4, IL-10 and IL-16 with Pd(OAc) using the same procedure.
1
The C-NMR spectra of these solutions all showed a new
3
measurements were performed on a thermal analyzer
peak at about 175 ppm, indicating the formation of Pd–NHC
1
IL complexes.
(NETZSCH STA 409 PC/PG) with a heating rate of 3 1C
min . The loading content of Pd in the catalysts was
5
ꢁ1
determined by ICP-AES (VISTAMPX).
2
.6 Preparation of Pd–NHC IL complexes immobilized on
silica (IMM-1, IMM-4, IMM-10, IMM-16)
2
.8 Hydrogenation reactions
In this paper, IMM-1, IMM-4, IMM-10, IMM-16 denote the
Pd–NHC IL complexes immobilized on silica, in which the
alkyl chain lengths of the ILs were C1, C4, C10, and C16,
respectively.
The stainless steel reactor of 6 mL and the experimental
3
0
procedure were similar to those reported previously. We
use the hydrogenation of allyl alcohol as an example to
describe the experimental procedure. In the experiment, allyl
alcohol (0.2 mL) and the catalyst were added into the reactor
The method was similar to that reported in the literature to
prepare porous materials by post-synthesis derivatization of
materials from Si-based MCM-41 with bidentate nitrogen
and then heated to 40 1C. After the introduction of H
desired pressure, CO was charged into the reactor with a high
2
at the
2
2
9
ligands. Here we describe the procedure to prepare IMM-1
as a representative case. In the experiment, aqueous ammonia
was added to a stirring solution of CTAB, then a mixture of
TEOS, Pd–IL-1 and neat IL-1 dissolved in methanol was added
dropwise, and the final molar ratio of TEOS/Pd–IL-1/IL-1/
pressure pump (DB-80) to the desired pressure. The reactor
was cooled in an ice bath after a suitable reaction time, and the
gases were released slowly passing through a cold trap with
DMF absorbent. The solution in the cold trap was mixed with
the reaction mixture. The product was analyzed by GC
3 2
CTAB/NH /methanol/H O was 1 : 0.01 : 0.08 : 0.14 : 8 : 2 : 100.
(
Agilent 4890 D) equipped with a flame-ionized detector. In
The mixture was stirred for 4 h under ambient conditions, then
aged at 100 1C for 24 h. The white product was obtained by
filtration, washed with water, and dried under vacuum for
the catalyst recycling experiments, the catalyst was separated
by centrifugation and was reused after washing with ethanol
and drying.
2
4 h. The resulting solid was added to a solution of 250 mL
methanol and 6 mL HCl (37%) at 50 1C and the mixture was
stirred for 6 h to remove the surfactant. The white solid was
then filtered and washed with methanol (20 mL ꢀ 3) and dried
at 60 1C for 24 h. Similarly, IMM-4, IMM-10 and IMM-16
were synthesized from Pd–IL-4, Pd–IL-10, Pd–IL-16 using the
same procedure. The Pd contents of IMM-1, IMM-4, IMM-10
and IMM-16 determined by ICP-AES were 0.19 wt%, 0.15 wt%,
3. Results and discussion
3
.1 Synthesis and characterization of the catalysts
The routes to synthesize IMM-1, IMM-4, IMM-10 and
IMM-16 are presented in Scheme 1 and the detailed
procedures are described in the Experimental section. The
ILs were synthesized by the reaction of (3-chloropropyl)
triethoxysilane and the corresponding N-alkylimidazole in
0
.17 wt% and 0.087 wt%, respectively.
2
.7 Characterization of the catalysts
toluene. After Pd(OAc) was added to the ILs, the mixture
2
The FT-IR spectra were collected on a Bruker Tensor 27
spectrometer in KBr pellet form. TEM observation was
performed on a transmission electron microscope (JEOL,
JEM 1011) at an operating voltage of 100 kV, and the images
were electronically captured using a CCD camera. X-ray
powder diffraction was performed on an X’PERT SW X-ray
was heated at 60 1C for 8 h and then 100 1C for 4 h to form a
Pd(II)–NHC complex which was confirmed by C NMR. The
corresponding Pd(II)–NHC immobilized on mesoporous
1
3
SiO
2
was formed by the reaction of the resulting IL
containing Pd(II)–NHC complex and TEOS with CTAB as
surfactant.
Scheme 1 Synthesis procedures of IMM-1, IMM-4, IMM-10 and IMM-16.
064 Phys. Chem. Chem. Phys., 2011, 13, 2062–2068
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2

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Fig. 3 Low-angle XRD patterns for IMM-1, IMM-4, IMM-10 and
IMM-16.
Fig. 1 The FT-IR spectra of IMM-1, IMM-4, IMM-10, IMM-16 and
SiO
2
.
TEM images of IMM-10 are given in Fig. 4, which show
two dimensional hexagonal symmetry structures. The other
catalysts had similar structures. The TEM images demon-
strated that the catalysts had highly ordered pore structures.
IMM-1, IMM-4, IMM-10 and IMM-16 were characterized
by TEM, FT-IR, low-angle XRD, TGA and nitrogen
sorption, respectively. Fig. 1 gives the FT-IR spectra of
2
IMM-10 was characterized by N sorption analysis to determine
IMM-1, IMM-4, IMM-10, IMM-16 and SiO
the spectrum of SiO , a new weak absorption peak at about
560 cm appeared in the spectra of the four catalysts. It can
be attributed to the CQC stretching of the imidazole ring,
2
. Compared to
the surface area and porous characteristics (Fig. 5). The pore
size was about 3.27 nm, which agreed with that obtained from
the TEM image. The specific surface area of the catalyst
2
ꢁ
1
1
2
ꢁ1
1
7
was determined to be 187.2 m
g
. On the basis of these
indicating the existence of the ILs in the catalysts. The peaks
ꢁ
1
ꢁ1
characterizations, we can deduce that IMM-n has the structure
schematically shown in Fig. 6.
at about 2927 cm and 2855 cm are attributed to aliphatic
C–H stretching, and as expected the intensity increased as the
length of alkyl chains of the ILs increased, suggesting that the
ILs were immobilized on the silica.
3
.2 Hydrogenation reactions
The four catalysts were examined by TGA. The results in
Fig. 2 demonstrated that all four catalysts had weight losses
below 180 1C due to desorbed water and inner water molecules.
Further weight loss at higher temperature was attributed to
We studied the catalytic activity of the catalysts for the
hydrogenation of different alkenes and allyl alcohol. The
results of the reactions are shown in Table 1. The catalytic
activity of IMM-1, IMM-4, IMM-10 and IMM-16 for allyl
alcohol was similar to that of commercial Pd/C. However, the
activity of IMM-1 for 1-hexene and cyclohexene was lower
than that of Pd/C. This can be explained by the steric effect.
Compared to allyl alcohol, 1-hexene and cyclohexene have
larger molecular volume, which leads to low TOF.
the elimination of the ILs immobilized on the SiO , and the
2
loading of the ILs for the four catalysts were about 9.3 wt%,
9
.6 wt%, 11.2 wt%, 10.2 wt%, respectively.
Powder low-angle XRD patterns of the catalysts are shown
in Fig. 3. All the samples exhibited peaks at about 2.21 that
3
1
corresponded to 100 reflections of MCM-41. This indicates
that the periodic channels were formed. Meanwhile, it can be
noted that the intensities of diffraction peaks become weaker
from IMM-1 to IMM-16 as the length of the alkyl chain is
increased.
For the hydrogenation of allyl alcohol catalyzed by IMM-1,
IMM-4, IMM-10 and IMM-16, the selectivities for 1-propanol
were 80.0%, 80.5%, 84.1% and 84.0%, respectively, which are
all higher than that obtained using the commercial Pd/C
catalyst, as can be seen from Table 1. This indicates that the
ILs can not only immobilize the metal catalyst, but also
can enhance the selectivity of the reaction. As reported
1
7
previously, the key factor in controlling the selectivity of
the reaction is the diffusion rate of the substrate to the Pd
center, which is controlled by the steric hindrance. The
selectivity for the hydrogenation product (1-propanol)
increases with increasing length of alkyl chain of the ILs up
to C10, but the effect of chain length on the selectivity was not
obvious as the size of the chain was increased further. This can
be explained by the proposed reaction mechanism shown in
3
2
Fig. 7. There are two catalytic routes in the hydrogenation of
allyl alcohol: the hydrogenation reaction to produce 1-propanol
catalyzed by hydrogen-saturated Pd and isomerization of allyl
alcohol to form the by-product propanal catalyzed by bare Pd.
The steric hindrance results in a more significant decrease in
Fig. 2 TGA diagram of IMM-1, IMM-4, IMM-10 and IMM-16.
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Fig. 4 TEM images of IMM-10.
Fig. 5 Nitrogen sorption isotherms (a) and pore size distribution (b) of IMM-10.
Fig. 6 Schematic structure of the catalyst IMM-n.
the diffusion rate of allyl alcohol than that of hydrogen
because of its larger molecular volume. Obviously, the steric
hindrance of the catalysts prepared in this work should
increase with the length of the alkyl chain of the ILs, leading
to an increase of selectivity. But when the chain length was
larger than C10, the selectivity of 1-propanol was not changed
noticeably because the steric hindrance was large enough.
Fig. 8 shows the XPS spectra of IMM-10 before and after
the reaction (entry 7 of Table 1). The binding energies of Pd(II)
intensity of the peak at about 335.0 eV for Pd(0) is very low,
indicating that nearly no Pd(0) nanoparticles formed during
the reaction. The HRTEM image in Fig. 9 shows that there
were no visible palladium nanoparticles on the wall of the
silica after the reaction, and the highly ordered pore structure
is unchanged. This also suggests that Pd(II) was the main
active species for the hydrogenation reaction. The main reason
may be that the Pd(II) in the carbene structure is stable and
hydrogen could not reduce it effectively. The detailed reason
should be studied further.
3
d
5/2 in the catalyst both before and after the reaction were
3
37.5 eV, which are very close to 337.9 eV, the typical value of
2
The effect of adding CO on the reaction was also studied
3
Pd(II). The value is also consistent with that of a Pd(II)–NHC
3
using IMM-10 as the catalyst, and the results are presented in
Table 2. The selectivity increased from 84.1% to 90.2% as the
3
complex reported before. According to the XPS spectra the
4
2
066 Phys. Chem. Chem. Phys., 2011, 13, 2062–2068
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a
Table 1 Results of catalyst performance for different hydrogenation reactions
b
ꢁ1
Entry
Catalyst
IMM-1
Pd/C
Substrate
Time/min
Conversion (%)
Selectivity (%)
TOF/h
1
2
3
4
5
6
7
8
9
1-Hexene
1-Hexene
100
50
140
80
35
37
40
41
100
100
100
100
100
100
100
100
100
100
100
100
100
80.0
80.5
84.1
84.0
74.0
6000
c
12000
4285
7500
17142
16216
15000
14634
15000
IMM-1
Pd/C
Cyclohexene
Cyclohexene
Allyl alcohol
Allyl alcohol
Allyl alcohol
Allyl alcohol
Allyl alcohol
c
IMM-1
IMM-4
IMM-10
IMM-16
c
Pd/C
40
a
b
, 4.00 MPa; the molar ratio of substrate to Pd was 10000. The by-product is propanal. Commercial Pd/C, 5%
c
Reaction conditions: 40 1C; H
2
30
Pd on carbon powder from Baoji Rock Pharmachem Co., Ltd., China. The detailed characterization was provided previously.
Fig. 9 HRTEM image of recovered IMM-10 after the reaction.
Table 2 The conversion and selectivity of hydrogenation of allyl
alcohol catalyzed by IMM-10
Fig. 7 The possible mechanism of the hydrogenation of allyl alcohol.
pressure of CO changed from 0.0 to 7.0 MPa at a fixed H
2
2
a
pressure of 4.0 MPa. When the pressure of CO
2
exceeded
b
7
.0 MPa, the selectivity was nearly independent of CO
2
2
Catalyst Time/min PCO2 (MPa) Conversion (%) Selectivity (%)
pressure. It is very difficult to give the exact reasons for CO
IMM-10 40
IMM-10 40
IMM-10 40
IMM-10 40
0.0
3.0
7.0
10.0
7.0
7.0
7.0
7.0
100
100
100
100
100
100
100
100
84.1
85.6
90.2
89.3
89.0
90.1
88.8
74.0
enhancing the selectivity of the reaction. One of the main
reasons may be that H is miscible with gaseous or super-
critical CO , and CO can enhance the miscibility of gases and
imidazolium based ionic liquids. Therefore, it can be
2
2
2
c
3
5
IMM-10 40
IMM-10 40
d
e
IMM-10 40
deduced that the concentration of H near the Pd(II) in the
2
f
ionic liquid layer of the catalyst can be higher than that in
Pd/C
40
a
the absence of CO , and thus more H can be adsorbed by the
2
2
Reaction conditions: 40 1C; the molar ratio of allyl alcohol to Pd was
b c
10000. The by-product is propanal. The catalyst IMM-10 was
catalyst. As discussed above, the hydrogenation reaction to
produce the product 1-propanol is catalyzed by hydrogen-
saturated Pd and isomerization of allyl alcohol to form the
d
reused for the first time. The catalyst IMM-10 was reused for the
e
second time. The catalyst IMM-10 was reused for the third time.
f
Commercial Pd/C, 5% Pd on carbon powder from Baoji Rock Co.
Ltd, China.
by-product propanal is catalyzed by bare Pd. Therefore, CO
can enhance the selectivity for the desired product.
2
Fig. 8 XPS spectra of IMM-10 before (A) and after (B) the reaction (entry 7 of Table 1).
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Phys. Chem. Chem. Phys., 2011, 13, 2062–2068 2067

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The reusability of the catalyst IMM-10 was tested. The
results are also given in Table 2. The catalytic activity and
selectivity did not change after the catalyst was reused three
times. The Pd content of fresh catalyst and recycled catalyst
were determined by ICP-AES, The Pd content of the catalyst
remained almost unchanged after being reused three times.
9 S. Sakaguchi, K. S. Yoo, J. ONeill, J. H. Lee, T. Stewart and
K. W. Jung, Angew. Chem., Int. Ed., 2008, 47, 9326.
1
0 P. Hauwert, G. Maestri, J. W. Sprengers, M. Catellani and
C. J. Elsevier, Angew. Chem., Int. Ed., 2008, 47, 3223.
1 J. Schwarz, V. P. W. Bohm, M. G. Gardiner, M. Grosche,
W. A. Herrmann, W. Hieringer and G. Raudaschl-Sieber,
Chem.–Eur. J., 2000, 6, 1773.
1
1
1
1
1
1
1
2 J. W. Sprengers, J. Wassenaar, N. D. Clement, K. J. Cavell and
C. J. Elsevier, Angew. Chem., Int. Ed., 2005, 44, 2026.
3 P. L. Arnold, G. N. Cloke, T. Geldbach and P. B. Hitchcock,
Organometallics, 1999, 18, 3228.
4
. Conclusions
4 V. Jurcik, S. P. Nolan and C. S. J. Cazin, Chem.–Eur. J., 2009, 15,
2
Pd(II) supported on mesoporous silica by NHC ILs with
different alkyl chain lengths has been prepared. The catalysts
have high activity and selectivity for the hydrogenation of
alkenes and allyl alcohol. The XPS study indicates that the
valence of Pd(II) remained unchanged after the hydrogenation
reaction. For the hydrogenation of allyl alcohol to 1-propanol,
the selectivity is considerably higher than that of the
commonly used Pd/C catalyst, and the selectivity increased
with an increase in the length of the alkyl chain of the ILs. This
can be explained by the fact that the steric effect of an IL with
a longer hydrocarbon chain is more pronounced, which is
favorable to increasing the selectivity for the desired product.
509.
5 L. J. Xu, W. P. Chen and J. L. Xiao, Organometallics, 2000, 19,
1123.
6 H. V. Huynh, J. H. H. Ho, T. C. Neo and L. L. Koh,
J. Organomet. Chem., 2005, 690, 3854.
7 B. Karimi and D. Enders, Org. Lett., 2006, 8, 1237.
18 W. A. Herrmann, C. Kocher, L. J. Goossen and G. R. J. Artus,
Chem.–Eur. J., 1996, 2, 1627.
1
2
2
2
9 K. M. Lee, Y. T. Lee and Y. J. B. Lin, J. Mater. Chem., 2003, 13,
1
079.
0 L. C. Branco, J. N. Rosa, J. J. M. Ramos and C. A. M. Afonso,
Chem.–Eur. J., 2002, 8, 3671.
1 Z. Fei, D. Zhao, T. J. Geldbach, R. Scopelitti and P. J. Dyson,
Angew. Chem., Int. Ed., 2005, 44, 5720.
2 Z. Mu, W. Liu, S. Zhang and F. Zhou, Chem. Lett., 2004, 33, 524.
In addition, scCO
2
can also improve the selectivity of the
23 H. Itoh, K. Naka and Y. Chujo, J. Am. Chem. Soc., 2004, 126,
3026.
reaction considerably.
2
2
2
2
4 Z. F. Zhang, Y. Xie, W. J. Li, S. Q. Hu, J. L. Song, T. Jiang and
B. X. Han, Angew. Chem., Int. Ed., 2008, 47, 1127.
5 D. B. Zhao, Z. F. Fei, T. J. Geldbach, R. Scopelliti and
P. J. Dyson, J. Am. Chem. Soc., 2004, 126, 15876.
6 M. Uyemura and T. Aida, J. Am. Chem. Soc., 2002, 124,
Acknowledgements
The authors are grateful to the National Natural Science
Foundation of China (20973177, 20932002), the Ministry of
Science and Technology of China (2009CB930802) and the
Chinese Academy of Sciences (KJCX2.YW.H16).
1
7 Y. J. Jiang and Q. M. Gao, J. Am. Chem. Soc., 2006, 128, 716.
1392.
28 S. Brenna, T. Posset, J. Furrer and J. Blumel, Chem.–Eur. J., 2006,
12, 2880.
2
9 P. D. Vaz, C. D. Nunes, M. Vasconcellos-Dias, M. M. Nolasco,
P. J. Ribeiro-Claro and M. J. Calhorda, Chem.–Eur. J., 2007, 13,
7874.
0 H. Z. Liu, T. Jiang, B. X. Han, S. G. Liang and Y. X. Zhou,
Science, 2009, 326, 1250.
References
3
1
C. A. Sandoval, T. Ohkuma, K. Muniz and R. Noyori, J. Am.
˜
Chem. Soc., 2003, 125, 13490.
B. Breit, Angew. Chem., Int. Ed., 2005, 44, 6816.
I. D. Gridnev, T. Imamoto, G. Hoge, M. Kouchi and
H. Takahashi, J. Am. Chem. Soc., 2008, 130, 2560.
31 J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz,
C. T. Kresge, K. D. Schmitt, C. T. W. Chu, D. H. Olson and
E. W. Sheppard, J. Am. Chem. Soc., 1992, 114, 10834.
32 A. K. Zharmagambetova, E. E. Ergozhin, Yu. L. Sheludyakov,
S. G. Mukhamedzhanova, I. A. Kurmanbayeva, B. A. Selenova
and B. A. Utkelov, J. Mol. Catal. A: Chem., 2001, 177, 165.
33 K. R. Priolkar, Parthasarathi Bera, P. R. Sarode, M. S. Hegde,
S. Emura, R. Kumashiro and N. P. Lalla, Chem. Mater., 2002, 14,
2120.
34 V. Calo, R. D. Sole, A. Nacci, E. Schingaro and F. Scordari, Eur.
J. Org. Chem., 2000, 869.
35 D. G. Hert, J. L. Anderson, S. N. V. K. Aki and J. F. Brennecke,
Chem. Commun., 2005, 2603.
2
3
4
5
6
D. A. Liprandi, E. A. Cagnola, M. E. Quiroga and
P. C. L’Argentiere, Catal. Lett., 2009, 128, 423.
Y. Z. Hao, Z. X. Li and J. L. Tian, J. Mol. Catal. A: Chem., 2007,
265, 258.
E. A. B. Kantchev, C. J. Obrien and M. G. Organ, Angew. Chem.,
Int. Ed., 2007, 46, 2768.
7
8
W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41, 1290.
N. Marion, O. Navarro, J. Mei, E. D. Stevens, N. M. Scott and
S. P. Nolan, J. Am. Chem. Soc., 2006, 128, 4101.
2
068 Phys. Chem. Chem. Phys., 2011, 13, 2062–2068
This journal is c the Owner Societies 2011