Nitrogen-containing porous cerium trimetaphosphimate as a new efficient base catalyst

Article abstract of DOI:10.1039/c1jm10669d

A new layer nitrogen-containing porous cerium trimetaphosphimate with pendant NH groups projecting into the channels has been synthesized and characterized. Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate is carried out with this complex a

Full text of DOI:10.1039/c1jm10669d

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Nitrogen-containing porous cerium trimetaphosphimate as a new efficient
base catalyst†
a
a
Jianglong Yi, Zhiyong Fu, Shijun Liao,a Desheng Songa and Jingcao Daib
*
Received 15th February 2011, Accepted 9th March 2011
DOI: 10.1039/c1jm10669d
doping techniques. As hundreds of metal phosphates have been
A new layer nitrogen-containing porous cerium trimetaphosphimate
with pendant NH groups projecting into the channels has been
synthesized and characterized. Knoevenagel condensation of benz-
aldehyde and ethyl cyanoacetate is carried out with this complex as
support in ethanol at 333 K. Experimental results indicate that it is
an efficient base catalyst. The NH groups are incorporated into the
host framework and provide the base sites for catalysis. This stable
solid base catalyst can be reused without obvious loss of its activity
for three cycles.
prepared by the principle of crystal engineering,1b it is possible to
build nitridate metal phosphates in a chimie douce approach. Until
now, no structurally characterized nitridate zeolite or zeolite-like
complexes have been reported. The position and ratio of NH groups
in the open frameworks remain unknown. These factors prohibit
further investigation of structural design and reaction mechanism for
these species. Herein we report the first synthesis and structural
characterization of nitrogen-containing cerium trimetaphosphimate
Ce(PO2NH)3$5H2O 1, and its catalytic behavior in a Knoevenagel
condensation reaction. In the structure of 1, NH groups are
embedded into the cavities and provide base sites for catalysis. The
complex is reusable, and retains its activity and framework stability
for the next catalytic cycles.
Crystalline metal silicates and phosphates have been widely use in the
acid catalytic processes.1 Their acid character comes from the metal
centers and oxygens incorporate into the open frameworks. Recently,
the industrial requirement of base solid catalyst has initiated research
for zeolite and zeolite-like complexes with basic properties.2 By
comparison with liquid bases, basic zeolites and zeotypes show high
efficiency for basic catalysis reactions and they simplify the separation
process for organic preparations. In the past decades, various strat-
egies have been developed for the exploration of new families of basic
porous materials. Alkaline exchange is a common way to modify
zeolite materials. However, their sensitivity to air or moisture for
embedded alkaline complexes limits the routes for further applica-
tions.3 Recently, a useful approach to prepare basic porous materials,
decoration of the open framework with NH groups, has been
reported.4 According to the basis of the isoelectronic principle, the
replacement of oxygen atoms by isoelectronic NH groups will not
alter the charge of the fundamental building unit.5 This introduction
provides the basicity of the nitrogen sites in the open-framework
materials. Extensive framework nitridation reactions have been
carried out for silica and metal phosphates.6 The treatments involve
heating of zeolite analogues in a stream of ammonia gas at high
temperature or doping NH species to reactive frameworks under mild
reactions.7 In many cases, this approach can easily induce structural
collapse during the nitridation process at high temperature.8 Hence,
there is a great need for research and development of alternative NH
With the addition of Na3(PO2NH)3 to a solution of Ce
(NO3)3$6H2O in a molar ratio of 1 : 2 under mild hydrothermal
conditions, colorless crystals of a new nitrogen-containing cerium
phosphate Ce(PO2NH)3$5H2O 1 were obtained.9 Single crystal data
reveal that it has a layer structural motif.10 Each cerium atom is
surrounded by eight oxygen atoms with a distorted bicapped trigonal
aKey Laboratory of New Energy Technology for Guangdong Universities,
School of Chemistry and Chemical Engineering, South China University
of Technology, Guangzhou, 510640, China
bInstitute of Materials Physical Chemistry, Huaqiao University, Quanzhou,
Fujian, 362021, China
Fig. 1 The asymmetric unit of compound 1, showing the atom-labeling
† CCDC reference numbers 794008. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/c1jm10669d
scheme.
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prismatic coordination environment (Fig. 1). The Ce–O connections
ꢀ
ꢀ
range from 2.418(3) A to 2.571(3) A, involving two coordination
water molecules and six bidentate PO2 groups of the trimetaphos-
phimate anions. The O–Ce–O bond angles vary from 70.58(10) to
81.79(19)ꢀ. These structural parameters are typical for eight-coordi-
3ꢁ
nate cerium in phosphate compounds.11 The rings of the (PO2NH)3
ligand exhibit a twist-boat conformation. All the bridging PON
ligands engage in Ce–O linkages with vertex-sharing modes (P–O–Ce
bond angle ¼ 126.06(12)–155.49(19)ꢀ) (Fig. 2). The P–O bond
ꢀ
ꢀ
distances range from 1.487(3) A to 1.504(3) A and the P–N distances
ꢀ
fall between 1.662(3) to 1.681(3) A. These geometrical parameters are
in agreement with previous reports for related metal phosphates and
metal phosphimates.12 The connections between the CeO8 bicapped
3ꢁ
trigonal prismatic polyhedra and P2N2 tetrahedra in the (PO2NH)3
ligands result in a two-dimensional structural motif with three-
membered and five-membered rings as subunits. The layer of the
novel nitrogen-containing cerium trimetaphosphimate displays
undulated micro-cavities due to the boat conformation of the
bridging (PO2NH)33ꢁ ligands. The pendant NH groups attach at the
frame and point to the center of the cavities, providing base sites for
the microporous material. Guest water molecules are located in the
space between the layers and they form hydrogen bonds with the
ꢀ
oxygen atoms of the host framework (O5/O4 ¼ 2.76 A, O6/O2 ¼
ꢀ
2.92 A). BET measurements were determined by N2 adsorption on
a Micromeritics porosimeter. The sample was degassed at 100 ꢀC for
12 h to remove the lattice water molecules. The results (average pore
diameter: 0.57 nm, pore volume 0.1097 cm3 gꢁ1, surface area 291.43
m2 gꢁ1) indicate that it is a microporous phosphate material.
Thermogravimetric analysis (TGA) was performed on a poly-
crystalline sample of 1 (4.54 mg) from 25 ꢀC to 800 ꢀC. The diagꢀram
(Fig. 3) shows an obvious weight loss of 11.50% (50 ꢀC–100 C),
corresponding to loss of three lattice water molecules per formula
unit (anal. calcd 11.64%). Weight loss of 7.51% starts at 110–280 ꢀC,
attributed to loss of two coordinated molecules per formula unit
(anal. calcd 7.75%). The XRD pattern shows no phase transition with
the removal of lattice water molecules. After heating to 280 ꢀC for 60
min, the XRD pattern indicates that the network collapses with the
loss of coordinating water molecules and the sample begin to lose its
crystallinity.
Fig. 2 (a) Layer structure of compound 1. (b) Polyhedral view of the
layer. (c) Packing diagram viewed along the [100] direction.
Fig. 3 The TGA diagram for compound 1.
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amount. The total surface area of the catalyst is increased and also
more catalytic active sites are available. Fig. 4b shows that 0.75 mmol
catalyst (pretreated for 2 h at 100 ꢀC) gives yields of the condensation
product up to 95%. The condensation product was analyzed by
a GC-MS-QP2010 system equipped with a Zebron ZB-5ms capillary
GC column (0.25 mm ꢂ 30 m; Phenomenex, Torrance, CA, USA)
(gas chromatograph coupled to a mass spectrometer; carrier gas ¼
helium; flow, 1 ml minꢁ1; oven, 40–400 ꢀC; injector, 150 ꢀC; detector,
Scheme
1 Knoevenagel condensation of benzaldehyde and ethyl
cyanoacetate with compound 1 as catalytic support.
1
250 ꢀC). MS, FT-IR, H NMR spectra and melting point of the
condensation product indicate that it is ethyl (E)-a-cyanocinnamate.
The peak at 2264 cmꢁ1 is attributed to the n(C^N) stretch, while the
peaks at 1747 cmꢁ1 and 1601 cmꢁ1 are assigned to the n(C]O) and n
(C]C) stretch vibrations respectively. The data represent the char-
acteristic bands of cyanocinnamate. The 1H NMR (CDCl3) spectrum
was recorded on a Brucker Ultrashield 400 Plus (Fig. 4d): 1.306 (t,
J ¼ 7.2 Hz, 3H, ethyl), 4.301 (q, J ¼ 7.2 Hz, 2H, ethyl), 7.275–7.4849
(m, 3H, m- and p-aromatic protons), 7.886–7.904 (m, 2H, o-aromatic
protons), 8.157 (s, 1H, alkene proton). The product shows a melting
The basic catalysis performance of compound 1 was tested with the
Knoevenagel condensation reaction of benzaldehyde and ethyl cya-
noacetate in ethanol at 333 K (Scheme 1). In a batch experiment, Ce
(PO2NH)3$5H2O was added to a solution of 20 mmol benzaldehyde
and 20 mmol ethyl cyanoacetate in 5 ml of ethanol. The mixture was
then stirred vigorously at 333 K for two hours. The solid material is
a heterogeneous catalyst for this reaction. At the end of the reaction,
it was removed by filtration, and solvent ethanol was taken away by
vacuum distillation. Since the external and cavity active surface areas
are important for the catalyst, there are two factors affecting the
performance, as follows. a) The guest water molecules. From the
single crystal data, it can be viewed that the lattice water molecules
form hydrogen bonds with the framework. Their removal helps to
release the base sites for the reaction. b) The amount of catalyst. The
conversion of condensation product increases with increasing catalyst
ꢀ
point of 51 C. These properties are comparable to the previously
report for the (E) type of ethyl cyanocinnamate.13 The base catalytic
activity is related to the crystallinity of the solid during the reaction.
The crystalline solid shows clearly higher activity than the amorphous
solid (about 30%). With malononitrile as an active methylene
compound, the condensation reaction shows a higher conversion
Fig. 4 GC-MS and 1HNMR spectra of the condensation product: (a) blank (without catalyst); (b) with 0.75 mmol compound 1 (pretreated at 100 ꢀC) as
1
solid catalyst, catalytic yield 95%; (c) MS spectrum; (d) H NMR spectrum.
6146 | J. Mater. Chem., 2011, 21, 6144–6147
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been made by rare-earth forms of metal phosphates, such as func-
tional materials in catalysis, photonic device and trapping of radio-
active elements,14 it can be expected that this kind of open framework
may find special applications in base-catalyzed reactions.
Acknowledgements
The authors thank the Natural Science Foundation of China
(21053001, 20701014, 50971063), Natural Science Foundation of
Fujian Province of China (No.2010J01042), Fundamental Research
Funds for the Central Universities (2009ZM0030) and SRP program
for financial support, and Prof. Tong Chun Kuang (Analytical and
testing center of SCUT) for the collection of crystal data.
Notes and References
1 (a) W. H. Flank, T. E. Whyte, Perspectives in Molecular Sieve Science,
American Chemical Society, Washington DC, 1988; (b)
ꢁ
A. K. Cheetham, G. Ferey and T. Loiseau, Angew. Chem., Int. Ed.,
1999, 38, 3268.
2 H. Hattori, Chem. Rev., 1995, 95, 537.
3 (a) A. Corma, V. Fornes, R. M. Martinaranda, H. Garcia and
J. Primo, Appl. Catal., 1990, 59, 237; (b) D. Barthomeuf, Catal.
Rev., 1996, 38, 521.
4 Y. D. Xia and R. Mokaya, Angew. Chem., Int. Ed., 2003, 42, 2639.
5 Z. Fu and T. Chivers, Inorg. Chem., 2005, 44, 7292.
Fig. 5 PXRD patterns for compound 1: (a) simulated from the single-
crystal data; (b) sample before catalytic process; (c) sample after catalytic
process.
ꢁ
€
6 (a) M. J. Climent, A. Corma, V. Fornes, A. Frau, R. Guil-Lupez,
S. Iborra and J. Primo, J. Catal., 1996, 163, 392; (b) M. Folman,
Trans. Faraday Soc., 1961, 57, 2000.
than that of ethyl cyanoacetate. This indicates the existence of shape
and size selectivity for the catalysis. The solid also show catalytic
activity for other basic catalytic reactions, such as the trans-
esterification reaction of ethyl acetate with methanol.
7 R. Conanec, R. Marchand and Y. Laurent, J. High Temp. Chem.
Processes, 1991, 1, 157.
8 L. Regli, S. Bordiga, C. Busco, C. Prestipino, P. Ugliengo,
A. Zecchina and C. Lamberti, J. Am. Chem. Soc., 2007, 129, 12131.
9 A solution of Na3(PO2NH)3 (0.25mmol, 0.08g) was added dropwise
to a solution of Ce(NO3)3$6H2O (0.5 mmol, 0.21 g) in 10 mL H2O
and stirred for 10 min at 25 ꢀC. The solution was transferred to
a 23 mL Teflon-lined steel autoclave and heated at 80 ꢀC for 36 h.
Colorless crystals of 1 were obtained with 56% yield. Anal. Found
(calcd) for H13N3O11P3Ce: H, 2.73(2.80); N, 9.17(9.05%).
X-ray powder diffraction patterns for the thermal stability studies
of 1 show phase purity of the bulk samples by comparison of the
observed and simulated patterns (Fig. 5). Before and after the base
catalytic process, no phase transition is observed for the XRD pattern
indicating the stability of the Ce(PO2NH)3$5H2O framework struc-
ture. After washing with with CH2Cl2, the base catalyst is reused
without loss of its catalytic activity for the second and third runs.
In summary, the first rare-earth nitrogen-containing metal phos-
phate has been synthesized and characterized, demonstrating a new
method to get nitrogen-containing crystalline zeolite-like micropo-
rous materials. With trimetaphosphimate as bridging ligand, the NH
groups are introduced into the open framework and provide the basic
sites for the catalyst. It shows activity for the Knoevenagel conden-
sation reaction and retains the framework stability and activity for
subsequent catalytic recycles. By analogy with the progress that has
10 Crystal data for 1: CeH13N3O11P3, M ¼ 464.16, orthorhombic,
ꢀ
ꢀ
ꢀ
P21212, a ¼ 7.3040(15) A, b ¼ 9.1675(18) A; c ¼ 9.5663(19) A,
V ¼ 640.6(2) A , Z ¼ 2, m(Mo-Ka) ¼ 3.982 mmꢁ1, qmax ¼ 24.95ꢀ,
3
ꢀ
4785 reflections measured, 1081 unique (Rint ¼ 0.0412) and used to
refine 110 parameters. R1(2s) ¼ 0.0191.† CCDC reference number
794008.
ꢁ
11 M. A. Salvado, P. Pertierra, C. Trobajo and J. R. Garcia, J. Am.
Chem. Soc., 2007, 129, 10970.
ꢁ
12 (a) M. Nazaraly, M. Quarton, G. Wallez, C. Chaneac, F. Ribot and
J. Jolivet, J. Solid State Chem., 2007, 9, 672; (b) S. Correll,
S. Sedlmaier and W. Schnick, Solid State Sci., 2005, 7, 1261.
13 Y.-Q. Li, J. Chem. Res., 2000, 524.
14 M. Nazaraly, G. Wallez, C. Chaneac, E. Tronc, F. Ribot, M. Quarton
and J. P. Jolivet, Angew. Chem., Int. Ed., 2005, 44, 5691.
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