Self-assembly of ionic liquids and metal complexes in super-cages of NaY: Integration of free catalysts and solvent molecules into confined catalytic sites

Article abstract of DOI:10.1016/S1872-2067(09)60090-5

The integration of a free metal complex, Pd(phen)²⁺, and a room temperature ionic liquid (IL) solvent molecule, 1-decyl-3-methyl imidazolium bromide, into a complete catalytically active site and confined in a NaY zeolite super-cage with an appropriate spatial arrangement was investigated. This integration was achieved through a molecular self-assembly method. A preliminary test of this catalyst system for the carbonylation of aniline to methyl phenyl carbamate indicated that far higher catalytic activity could be achieved (TOF increased from 3 000 to 23 000 h^-1) with far lower amounts of the IL solvent and the Pd complex in comparison with a simple mixture of Pd(phen)Cl₂/IL/NaY as the catalyst. This new system can also be applied to other areas of catalysis.

Full text of DOI:10.1016/S1872-2067(09)60090-5

CHINESE JOURNAL OF CATALYSIS
Volume 31, Issue 8, 2010
Online English edition of the Chinese language journal
Cite this article as: Chin. J. Catal., 2010, 31: 933–937.
SHORT COMMUNICATION
Self-Assembly of Ionic Liquids and Metal Complexes in
Super-Cages of NaY: Integration of Free Catalysts and
Solvent Molecules into Confined Catalytic Sites
MA Yubo1,2, HE Yude1, ZHANG Qinghua1, SHI Feng1,*, MA Xiangyuan1, LU Liujin1, DENG Youquan1,#
1Center for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000,
Gansu, China
2Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Abstract: The integration of a free metal complex, Pd(phen)2+, and a room temperature ionic liquid (IL) solvent molecule,
1-decyl-3-methyl imidazolium bromide, into a complete catalytically active site and confined in a NaY zeolite super-cage with an appro-
priate spatial arrangement was investigated. This integration was achieved through a molecular self-assembly method. A preliminary test of
this catalyst system for the carbonylation of aniline to methyl phenyl carbamate indicated that far higher catalytic activity could be achieved
(TOF increased from 3 000 to 23 000 h–1) with far lower amounts of the IL solvent and the Pd complex in comparison with a simple mixture
of Pd(phen)Cl2/IL/NaY as the catalyst. This new system can also be applied to other areas of catalysis.
Key words: active site; ionic liquid; metal complex; carbonylation; aniline; phenyl methyl carbamate; turnover frequency
A typical liquid homogeneous catalysis system generally
consists of a transition metal complex and a solvent, in which
the solvent as a reaction medium plays a very important role in
achieving satisfactory catalytic performance. These solvent
molecules may participate directly in the catalytic reaction as
‘ligands’ of the metal complex and this may establish an inte-
grated or whole catalytically active site in addition to its role as
a reaction medium. This means that only those solvent mole-
cules located in an appropriate spatial position can participate
in the formation of active sites with metal complexes (Fig.
1(a)). Large amounts of solvent molecules, however, are not a
part of these active sites. Far fewer metal complexes and sol-
vent molecules would be required if they are fixed at spatially
appropriate positions (Fig. 1(b), or part of the dashed line in
Fig. 1(a)).
homogeneous catalysis systems [1]. Recently, the immobiliza-
tion of RTILs onto solid supports [2,3] and the confinement of
RTILs in porous materials [4] have been reported.
Herein, we report a new system wherein the integration of
the free metal complex and the RTIL solvent molecule into a
complete catalytically active site inside the super-cage of a
(a)
(b)
Catalyst
Reactant
Solvent
Room temperature ionic liquids (RTILs) possess peculiar
physicochemical properties such as very low vapor pressures
and strong electrostatic fields in comparison with molecular
liquids and they offer great opportunities to develop novel
Fig. 1. Illustration of (a) a whole catalytically active site in a homoge-
neous reaction and (b) an isolated catalytically active site confined in a
nano-pore.
Received date: 14 March 2010.
*Corresponding author. Tel/Fax: +86-931-4968116; E-mail: fshi@licp.cas.cn
#Corresponding author. Tel/Fax: +86-931-4968116; E-mail: ydeng@licp.cas.cn
Foundation item: Supported by the National Natural Science Foundation of China (20533080).
Copyright © 2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(09)60090-5

MA Yubo et al. / Chinese Journal of Catalysis, 2010, 31: 933–937
NaY zeolite and with an appropriate spatial arrangement is
prepared using a similar procedure.
achieved through molecular self-assembly (MSA) [5]. Ini-
tially, the IL (1-methyl-imidazole and decyl bromide) and
metal complex (Pd2+ and phenanthroline (phen)) fragments are
successively introduced into the NaY super-cage (super-cage
with a 1.2 nm diameter and channels with a 0.74 nm diameter
[6]). The corresponding metal complex (Pd(phen)2+) and the
IL (Im, 1-decyl-3-methyl imidazolium bromide) are then syn-
thesized in situ, as shown in Fig. 2. Recent studies have sug-
gested that synergistic interactions between self-organizing
components and a self-assembling material can lead to hier-
archically ordered structures; therefore, these Pd complexes
and IL molecules are distributed in an orderly fashion in the
NaY super-cage [7].
Analysis by atomic emission spectrometry (3520 ICP AES)
showed that Pd was successfully introduced into the NaY with
varied loadings of 0.07%–0.09% for the different assembled
samples. FT-IR characterization (Nicolet Nodule) showed that
three characteristic peaks are present for the pure Im IL and two
of them at 1 466 and 1 571 cm–1 were also observed for
Im-NaY. The peak at 1 169 cm–1 was overlapped by the strong
Si–O–Si peak. This result indicated that an in situ formation of
the Im ionic liquid occurred and this probably took place within
the NaY super-cage. Raman characterization (Nicolet 5700) of
the Im-NaY showed abnormal Raman spectra in comparison
with the pure and bulk Im IL (Fig. 3).
This phenomenon was also observed when an organometal-
lic complex [8] or an IL [9] was confined in nanoporous ma-
terials, which resulted from unusual changes in the symmetry
and coordination geometry of the molecules confined in a
nanoscale space. The FT-IR and Raman spectra of
Pd-phen-NaY are quite different from that of free phenan-
throline, Pd(phen)Cl2, and the assembled phen-NaY, which
suggests that a different Pd complex forms in the NaY but its
structure is not clear at this stage. For Pd-phen-Im-NaY, the
same FT-IR and Raman spectra as for Im-NaY are observed
and the characteristic peaks of the Pd complex could not be
observed because of the overlapping IL peaks and the low
loading of Pd. The Raman spectrum of Pd-phen-NaY-Im is
almost the same as that of pure Im IL but quite different from
that of Pd-phen-Im-NaY suggesting that the Im IL in
Pd-phen-NaY-Im was not incorporated into the super-cage of
the NaY zeolite.
For comparison, several samples with slightly different
loadings of Pd, ligand, and IL were prepared by MSA and these
are denoted as Pd-phen-NaY, Im-NaY, Pd-Im-NaY, and
Pd-phen-Im-NaY. Pd-phen-NaY-Im was also prepared by the
impregnation of the assembled Pd-phen-NaY with the Im IL.
A typical procedure for the preparation of the catalyst
Pd-phen-Im-NaY is as follows. A 10 g sample of NaY (Aldrich,
aluminum content = 6.8%) and 25 ml methylimidazole were
added to a 250 ml round bottom flask and stirred at RT for 12 h.
Then, 75 ml of decyl bromide was added and stirred at 60 °C
for 12 h. The methyl imidazole and decyl bromide were dis-
tilled before use. The mixture was filtered and washed 3 times
(30 ml × 3) with hot ethanol. A white solid powder (~11 g) of
Im-NaY was thus obtained and the Im-NaY was transferred
into a 100 ml conical flask. Then, 30 ml H2O and 4.8 ml
H2PdCl4 solution (0.028 mol/L) were added and stirred at RT
for 12 h. Finally, 1 g of phen was added and stirred at RT for
another 12 h. The reaction mixture was filtered and washed
with ethanol and acetone, upon which it was dried at 180 °C for
3 h to obtain ~10 g Pd-phen-Im-NaY. The other catalysts were
(1)
(2)
(3)
(4)
4000
3500
3000
1500
1000
500
Raman shift (cm−1)
Fig. 2. The process for the self-assembly of IL and Pd complex in the
Fig. 3. Raman spectra of different samples. (1) Pd-phen-Im-NaY; (2)
super-cage of NaY.
Im-NaY; (3) Pd-phen-NaY-Im; (4) Im.

MA Yubo et al. / Chinese Journal of Catalysis, 2010, 31: 933–937
Table 1 Oxidative carbonylation of aniline over NaY confined Pd-ionic
C
liquid catalysts to phenyl methyl carbamate
H
NHCO Me
N
NH
2
2
MeOH
+ CO + O
2
O
catalyst
Br
Pd
Conversion
Selectivity
TOFc
(h−1)
Entry
Catalyst
(%)
27
21
81
95
10
67
73
(%)
~99
~99
~99
~99
~99
~98
~99
Fig. 4. The spatial arrangement of the ionic liquid molecule and the
Pd(phen)2+ complex in the super-cage of NaY.
1
Pd-phen-NaY
Pd-Im-NaY
8500
6600
23000
13000
2400
3060
20000
2
3
4a
Pd-phen-Im-NaY
Pd-phen-Im-NaY
Pd-phen-NaY-Im
ergism between the Pd complex, the IL, and NaY is necessary,
and this spatial synergism is required for enhanced catalytic
performance. The reaction results also suggest that in addition
to the introduction of Pd, phenanthroline, methyl imidazole,
and decyl bromide into the NaY and the in situ formation of
Pd(phen)2+ and Im IL, both the Pd(phen)2+ and the Im IL can
incorporate into the same NaY super-cage with an appropriate
spatial arrangement although these integrated catalytic sites are
few (less than 2% of NaY super-cages).
5
6b
7
Pd(phen)Cl2/Im/NaY
Pd-phen-Im-NaY
Reaction conditions: catalyst 50 mg, aniline 1 ml, MeOH 5 ml, CO 5.5
MPa, O2 0.5 MPa, 150 oC, 1 h; acatalyst 100 mg; bPd(phen)Cl2 1 mg, IL 1
ml, NaY 50 mg.
cDefined as the moles of substrate converted per mole of Pd per hour.
Computer modeling (Forcite Plus/Materials Studio, Accel-
rys) indicated that the self-assembly of an Im IL molecule and a
Pd(phen)2+ complex in the same super-cage of NaY was pos-
sible and a suitable spatial arrangement was also calculated
(Fig. 4). The Im IL and the Pd(phen)2+ were located in parallel
positions within the super-cage. The computer model also
revealed that some space (0.5–0.6 nm between the planes of the
phenanthroline and the imidazolium rings) exists for reactant
transportation.
Elemental analysis (Elementar Vario EL) showed that the IL
concentration in all these samples was between 8.7% and
11.2% (or ~(1.7–2.2) × 1020 IL molecules/g). The phenan-
throline content in Pd-phen-NaY was ~0.2% (or 6 × 1018 phe-
nanthroline molecules/g). Because there are 2.6 × 1020 su-
per-cages per gram of Pd-phen-Im-NaY (pore volume ≈ 0.34
ml/g, 1.3 nm diameter pores) [6], the ratio of IL mole-
cules/super-cages ≈ (65–85)/100 and the ratio of Pd at-
oms/super-cages ≈ 2.1/100.
In conclusion, a novel method for the integration of a free
metal complex and a RTIL molecule into a NaY zeolite su-
per-cage, which formed an effective catalytic site, was devel-
oped by molecular self-assembly. Far higher catalytic activity
was achieved for the carbonylation of aniline using much lower
amounts of ionic liquid as a solvent and less metal complex as
well. This new system can be applied to other areas of cataly-
sis.
The carbonylation of aniline is an important process for the
phosgene free synthesis of isocyanate, and RTILs is effective in
these reactions [10,11]. Therefore, aniline carbonylation was
selected as a model reaction to test the catalysts (Table 1). For
the synthesis of methyl phenyl carbamate (MPA) from aniline,
the Pd-phen-NaY and Pd-Im-NaY catalysts had lower activity
(entries 1 and 2). Pd-phen-Im-NaY greatly increased the cata-
lytic activity and the TOF reached 23 000 h−1 (entry 3), while
the conversion reached 95% when more catalyst was used
(entry 4). However, only ~10% conversion was obtained for
Pd-phen-NaY-Im (entry 5). The catalysts prepared by
self-assembly had much higher catalytic activity than the sim-
ple mixture of Pd(phen)Cl2/Im/NaY (entry 6) although ~200
times more ionic liquid and ~20 times more Pd were used in
Pd(phen)Cl2/Im/NaY. Furthermore, the reuse of Pd-phen-
Im-NaY gave a 73% conversion (entry 7), which indicated that
the reuse of this catalyst system is possible.
Acknowledgements
We thank NeoTrident Technology Limited for helping with
computer modeling.
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