Nanoporous metal-containing nickel phosphates: A class of shape-selective catalyst

Article abstract of DOI:10.1002/anie.200353502

A substitute for zeolites: The nanoporous nickel phosphate VSB-1 exhibits zeolitic properties and very weak acidity and basicity (see picture). When VSB-1 is modified by metal-ion exchange or through metal incorporation into the framework, it offers promising catalytic properties, such as shape selectivity, and activity in both redox catalysis and photocatalysis.

Full text of DOI:10.1002/anie.200353502

Angewandte
Chemie
Porous Materials
Nanoporous Metal-Containing Nickel
Phosphates: A Class of Shape-Selective
Catalyst**
Jong-San Chang,* Jin-Soo Hwang, Sung Hwa Jhung,
Sang-Eon Park,* GØrard FØrey, and
Anthony K. Cheetham*
The discovery of a large family of nanoporous aluminum
phosphates in the early 1980s has generated widespread
interest in non-aluminosilicate-based microporous materi-
[
1]
als, and there has been an enormous growth in the chemical
diversity of open-framework inorganic materials in new
[
2]
compositional domains during the last decade. Commercial
applications of open-framework materials continue to be
[
3]
dominated by the aluminosilicate zeolites, but attempts are
[
*] Dr. J.-S. Chang, Dr. J.-S. Hwang, Dr. S. H. Jhung
Catalysis Center for Molecular Engineering
Korea Research Institute of Chemical Technology (KRICT)
P.O. Box 107, Yusung, Taejon 305–600 (Korea)
Fax: (+82)42-860-7676
E-mail: jschang@krict.re.kr
Prof. S.-E. Park
Department of Chemistry, Inha University
Incheon 402-751 (Korea)
Fax: (+82)32-874-7674
E-mail: separk@inha.ac.kr
Prof. A. K. Cheetham
Materials Research Laboratory, University of California
Santa Barbara, CA 93106 (USA)
Fax: (+1)805-893-8797
E-mail: cheetham@mrl.ucsb.edu
Prof. G. FØrey
Institut Lavoisier, UMR 8637
UniversitØ de Versailles Saint Quentin
45 avenue des Etats-Unis, 78035 Versailles Cedex (France)
[**] This work was partially supported by the Korean Ministry of Science
and Technology through the Research Center for Nanocatalysis (KN-
0329), one of the National Science Programs for Key Nano-
technology and Institutional Research Program, and by the MRL
Program of the National Science Foundation under Award
No. DMR00-80034. The authors thank all of the CCME members
(KRICT) related to this work for their beneficial contributions.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 2819 –2822
DOI: 10.1002/anie.200353502
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2819

Communications
being made to exploit some of the exciting properties of the
Table 1: Direct production of H
VSB-1 and zeolite catalysts.
2
O
2
from H
2
and air over Pd-exchanged
[
a]
[
4]
newly discovered systems. Recent synthetic efforts have led
[
2,5]
[
b]
[c]
to the discovery of many new phases,
for example, V-P-O,
Catalyst
Pd loading [wt%]
[H O ] [mm]
TOF
2
2
Fe-P-O, Co-P-O and Zn-P-O.However, as is the case with
most nonsilicate open frameworks, the poor thermal stability
of these systems leads to the collapse of the pore structures on
activation, thus rendering them unsuitable for applications
that require porosity.The challenge, therefore, is to design a
nanoporous system that is thermally stable with respect to
chemical or structural degradation and can be applied in
catalytic reactions that are carried out at relatively high
temperatures.In this respect, the need for new classes of
catalysts has led a quest for creating novel nanoporous
materials that might also exhibit catalytic activity.
Pd-VSB-1
Pd-BEA
Pd-L
0.72
0.70
0.95
7.1
5.6
2.7
414
338
119
[a] Reaction conditions: T=208C, P=1 atm, 0.01m HCl (400 mL),
ꢀ1
H :air=10:40 mLmin , 0.1 g catalyst, 1 h. [b] Analyzed by ICP-MS.
2
[c] TOF (turnover frequency)=moles of H O formed per mole of Pd per
2 2
hour.
We recently showed that the open-framework nickel(ii)
phosphate, VSB-1 (Versailles/Santa Barbara-1), is sufficiently
stable to be rendered nanoporous and exhibits typical zeolitic
properties, making it the first example of a nanoporous
[
6]
nickel(ii) phosphate material. The VSB-1 structure is based
on a 3D network of NiO and PO tetrahedra defining a large,
6
4
unidimensional channel, with an estimated free diameter of
[
6]
8
.8 Š. Structural analysis indicates that HPO and extra-
4
+ +
framework cations (NH4 or K ) line the channel in VSB-1.
2
ꢀ1
Its surface area and chemical composition are 183 m g
after dehydration) and Ni (HPO ) (OH) F (H O/
NH ) ·12H O, respectively.We also discovered a second
(
1
8
4
14
3
9
3
4
4
2
nickel phosphate molecular sieve (VSB-5); this has several
[
7]
applications such as hydrogen storage and selective hydro-
[
8]
Figure 1. Competitive hydrogenation of an olefin mixture over Pd-VSB-
containing 0.72 wt% Pd. Reaction conditions: substrate=1 mol%
each of 1-octene, cyclohexene, and cyclododecene in n-hexane (90 mL),
genation and base catalysis. Catalytically active centers in
porous oxides can give rise to highly selective new catalysts
that are of great importance in the development of clean
1
T=308C, H pressure=2 atm, catalyst weight=0.2 g.
2
[
9]
technology. Here, we describe a new class of shape-selective
catalyst based on nanoporous nickel phosphates.
We first tested the acid–base properties of VSB-1 by
examining its reactivity with 2-methyl-3-butyn-2-ol (MBOH)
as a model reaction for acid–base catalysis, showing that VSB-
VSB-1 containing 0.72 wt% Pd exhibits shape selectivity for
competitive hydrogenation of 1-octene, cyclohexene, and
cyclododecene.Negligible hydrogenation rates are found for
larger substrates such as cyclododecene, clearly demonstrat-
ing the sieving properties of the catalyst.Furthermore, it
proves that the active hydrogenation centers are mainly
located within the micropores of Pd-VSB-1, which indicates
the potential of the noble metal in the VSB-1 system to act as
a new type of shape-selective catalyst.
1
possesses only weak acidity and basicity (see Supporting
Information).
To examine the possible accessibility of Pd species in the
pores during catalytic reactions, we have explored catalytic
II
activities of Pd -exchanged VSB-1 (Pd-VSB-1) for the direct
formation of H O from H and air in aqueous media.Such a
2
2
2
[
10]
process is an attractive alternative to the current technol-
ogy that involves the hydrogenation of an alkylanthraquinone
to the corresponding hydroquinone, followed by the reaction
of the hydroquinone with oxygen to yield H O .We pre-
Catalytic hydroxylation of phenol to dihydroxybenzenes
such as catechol and hydroquinone is an important industrial
reaction for the production of fine chemicals and has been
[
13]
extensively investigated. Microporous titanosilicates with
large surface areas have proved to be the most important
catalysts for phenol hydroxylation with hydrogen peroxide,
2
2
II
viously demonstrated that Pd -exchanged zeolite catalysts
Pd-BEA) exhibit more effective performance relative to
(
[
11]
[14]
II
those prepared from other types of zeolites. In this work,
Pd-VSB-1 containing 0.72 wt% Pd is fairly active for the
reaction, yielding more H O than Pd-BEA, as illustrated in
mainly in organic solvents. However, in this work, Cu -
exchanged VSB-1 (Cu-VSB-1) exhibits promising catalytic
activity for phenol hydroxylation with H O in an aqueous
2
2
2
2
Table 1.It is evident that the nanoporous structure of VSB-1
is suitable as a catalyst support of palladium to produce H O .
solution.Cu-VSB-1 was prepared by ion-exchange from VSB-
1 and was applied to phenol hydroxylation in an aqueous
solution.As shown in Table 2, it is found that Cu-VSB-1 is
very active for phenol hydroxylation.Phenol conversion with
Cu-VSB-1 increases up to 40.1% after 3 h when increasing
the copper content to 2.9 wt% (corresponding to 70% ion
exchange).It gives high catechol selectivity (67–70%)
regardless of the copper content, which is higher than that
2
2
Employing the nanoporous VSB-1 material as a support
for a noble metal enables the shape-selective properties of the
nanoporous structure to be conferred on the hydrogenation
function.For instance, the combination of noble metal/
nanoporous material is expected to be beneficial for selective
[
12]
hydrogenation of cycloolefins. As shown in Figure 1, Pd-
2
820
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Angew. Chem. Int. Ed. 2004, 43, 2819 –2822

Angewandte
Chemie
[
a]
Table 2: Activity of Cu-VSB-1 for phenol hydroxylation with H O , in terms of copper content.
formation of the corresponding
charge-transfer excited state,
2
2
[
b]
[e]
Catalyst
Phenol
conv. [%]
H O2
efficiency [%]
Product selectivity [%]
2
2
+
ꢀ
[
c]
[d]
[Fe -O ]*.Moreover, UV irradi-
ation for methanol-adsorbed Fe-
VSB-1 also decreases the intensity
CAT
HQ
BQ
Cu-VSB-1(14)
Cu-VSB-1(39)
Cu-VSB-1(50)
Cu-VSB-1(70)
17.2
25.1
28.0
40.1
61.9
65.8
64.8
73.4
68.7
67.6
68.3
69.8
28.3
30.0
29.7
30.2
3.0
2.4
2.0
–
2
ꢀ
3+
of the O to Fe charge-transfer
band due to electron transfer from
3
+
MeOH to Fe .These results
encouraged us to utilize Fe-VSB-1
as a photocatalyst.As an example,
we have investigated UV photo-
chemical oxidation of ammonia in
an aqueous solution using Fe-VSB-
[
a] Reaction conditions: 0.2 g catalyst, solvent: water (60 mL), phenol/H O molar ratio=1:1, amount
of phenol used=22 mmol, 3 h, T=608C. [b] Numbers in parenthesis denote the degree of ion-
exchange, as determined by ICP-MS. [c] Phenol conversion [%]=100”[phenol before reaction
2 2
(
moles)ꢀphenol after reaction (moles)]/phenol before reaction (moles). [d] H O efficiency [%]=
2
2
1
00”[products (CAT+HQ+BQ) (moles)]/total H O (moles) converted. [e] Product selectivity was
2 2
calculated for the dihydroxybenzenes and benzoquinone. CAT=catechol, HQ=hydroquinone, BQ=
1
, assisted by H O .It follows the
2 2
benzoquinone.
pathway of the photo-Fenton reac-
tion for the degradation of pollu-
[
14]
of the titanosilicate TS-1 (49%) reported in the literature,
tants, in which photoreduced transition-metal species lead to
C
[18]
revealing that the large-pore VSB-1 material exhibits reduced
pore limitation in the formation of diphenols, compared to
TS-1.No catalyst deactivation or leaching of copper species
from the substrate was observed when catalytic measure-
ments were twice repeated over a catalyst that had been
filtered after being used for a first run.XP spectra of Cu-VSB-
the formation of HO radicals and thereby convert ammo-
nia to nitrate or nitrite ions.VSB-1 itself shows negligible
activity for this reaction, but Fe-VSB-1 exhibits very good
activity to oxidize ammonia into nitrate and nitrite ions under
UV irradiation (Table 3).The activity is much better than that
1
indicates that the VSB-1 structure produces copper oxide
Table 3: Activities of TiO , FeCl , and Fe-VSB-1 in the photo-Fenton
oxidation of ammonia in water.
2
2
[
a]
clusters in the pore together with the exchanged copper
cations upon ion-exchange of copper (see Supporting Infor-
mation), which suggests that not only copper cations but also
copper oxide species within the pore of VSB-1 are effective
for the hydroxylation of phenol with H O .These two species
might provide a beneficial effect to improve the catalytic
activity.In the H O /Cu oxidation system, cupric ions (Cu )
added to hydrogen peroxide were found to generate hydroxyl
radicals (HO) capable of benzoate hydroxylation. In the
[b]
Catalyst
NH conversion [%]
TOF
3
TiO (P-25)
11
18
43
1.8
4.7
7200
2
FeCl2
2
2
[c]
Fe-VSB-1
II
2+
[a] Reaction conditions: NH OH (758 ppm), 30% H O (20 mL), H O
4 2 2 2
2
2
(660 mL), 0.5 g catalyst, UV wavelength: 200 nm, UV power: 450 W,
irradiation time: 10 min. [b] TOF (turnover frequency) is moles of NH3
C
[15]
decomposed per mole of Ti (or Fe) per hour. [c] Fe content=0.69 wt%.
present case, copper(ii) species dispersed in the pores of VSB-
1
might also generate the hydroxyl radicals from H O that
2 2
are active for the phenol hydroxylation.
of anatase TiO (P25) as a typical photocatalyst and pure
2
We prepared Fe-doped VSB-1 (Fe-VSB-1) and explored
the framework iron species as a chromophore and a redox site
for photoinduced chemistry or catalysis.We believe that the
unique and fascinating properties of zeolites involving
transition-metal ions within the zeolite frameworks or cavities
open up new possibilities for light-induced applications, not
only in photocatalysis but also for various photochemical
FeCl as a photo-Fenton reagent, and its superiority is evident
2
when the activity is displayed in terms of turnover frequency
III
(TOF).This result indicates that highly dispersed Fe sites in
the framework of VSB-1 can behave as active centers for
intrazeolite photocatalysis.
In summary, the novel material, VSB-1, is found to have
important properties such as nanoporosity, zeolitic properties,
and very weak acidity and basicity.When VSB-1 is modified
with metal ion-exchange or metal-incorporation into the
framework, it can offer promising catalytic properties: e.g.,
shape selectivity, redox catalysis, and photocatalysis.Indeed,
the catalytic results exemplified in this work clearly demon-
strate that nanoporous metal-containing nickel phosphates
can be classified as a new class of shape-selective catalyst.
[
16]
processes.
This is because the micropore of a zeolite
confines the organic reactant in a nanoscopic vessel and,
moreover, the transition-metal ions in metal-containing
porous materials are considered to be highly dispersed at
the atomic level and well defined, existing in a specific
structure within the zeolite framework.The UV/Vis DRS
spectrum of Fe-VSB-1 shows a strong absorption in the
2
ꢀ
2
50–350 nm interval, which is typically assigned to an O to
3
+
3+
!
2ꢀ
3+
Fe charge-transfer transition ([Fe
O ]) of isolated Fe
species. It is confirmed that the framework Fe species,
possibly octahedral iron species, are sensitive to UV irradi-
[
17]
III
Experimental Section
II
Syntheses of VSB-1 and metal-modified VSB-1 materials: VSB-1 was
initially prepared in a hydrothermal reaction at 1808C for 6 days
between nickel(ii) chloride hexahydrate and phosphoric acid (85% by
weight) in the presence of tris(2-aminoethyl)amine in a pyridine/HF/
ation, which leads to photoreduced Fe species (see Support-
ing Information).Thus, UV irradiation for Fe-VSB-1 con-
taining 0.69 wt% Fe gives rise to a decrease in the intensity of
3
+
!
2ꢀ
[
6]
the charge transfer ([Fe
O ]) band centered at 280 nm
water solvent system; further details are given elsewhere. Iron-
incorporated VSB-1 was synthesized likewise using an aqueous
due to enhancement of charge separation, and leads to the
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Communications
II
II
solution of FeCl ·4H O (Aldrich, 99%).For the Pd and Cu ion-
2
2
exchanged materials, palladium and copper were introduced by ion
exchange of the proton form of VSB-1 using a 0.01m aqueous solution
of [Pd(NH ) Cl ] and a 0.02m aqueous solution of Cu(NO ) ,
3
4
2
3 2
respectively.The resulting metal-loaded materials were dried at
208C overnight and calcined at 3508C for 4 h in air.For catalytic
1
measurements, Pd-VSB-1 was reduced for 4 h in 5% H at 3008C.
2
Metal loading in the final products was determined with ICP-MS.
Catalytic measurements were conducted with a plug-flow reactor
(
for MBOH decomposition) or a glass reactor (for H O production,
2 2
olefin hydrogenation, phenol hydroxylation, and ammonia decom-
position).Detailed methods for catalytic measurements and analysis
are described in the Supporting Information.
Received: December 10, 2003 [Z53502]
Keywords: heterogeneous catalysis · microporous materials ·
.
nickel · phosphates · zeolite analogues
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