A crystalline catalyst based on a porous metal-organic framework and 12-tungstosilicic acid: Particle size control by hydrothermal synthesis for the formation of dimethyl ether

Article abstract of DOI:10.1002/adsc.201000808

The strategy for obtaining a crystalline catalyst based on a porous copper-based metal-organic framework and 12-tungstosilicic acid with different particle sizes is reported. Through the control of hydrothermal synthesis and some simple treatments, catalyst samples with average particle diameters of 23, 105, and 450-μm, respectively, were prepared. This crystal catalyst has both the Bronsted acidity of 12-tungstosilicic acid and the Lewis acidity of the copper-based metal-organic framework, and has high density of accessible acid sites. Its catalytic activity was fully assessed in the dehydration of methanol to dimethyl ether. The effect of particle size on the catalytic activity of catalyst was studied, in order to select the particle size appropriate for avoiding the diffusion limitation in heterogeneous gas-phase catalysis. In the selective dehydration of methanol to dimethyl ether, this catalyst exhibited higher catalytic activity than the copper-based metal-organic framework, γ-alumina, and γ-alumina-supported 12-tungstosilicic acid catalysts. It showed high catalytic performances, even at higher space velocity or in the presence of excess water. In addition, the catalyst was also preliminarily assessed in the formation of ethyl acetate from acetic acid and ethylene. It also exhibited a high activity which was comparable with that of silica-supported 12-tungstosilicic acid catalyst. Copyright

Full text of DOI:10.1002/adsc.201000808

FULL PAPERS
DOI: 10.1002/adsc.201000808
A Crystalline Catalyst Based on a Porous Metal-Organic
Framework and 12-Tungstosilicic Acid: Particle Size Control by
Hydrothermal Synthesis for the Formation of Dimethyl Ether
a
a,
a
a
a
Da-Dong Liang, Shu-Xia Liu, * Feng-Ji Ma, Feng Wei, and Ya-Guang Chen
a
Key Laboratory of Polyoxometalate Science of Ministry of Education, College of Chemistry, Northeast Normal
University, Changchun, Jilin 130024, Peopleꢀs Republic of China
Fax : (+86)-431-8509-9328; e-mail: liusx@nenu.edu.cn
Received: October 26, 2010; Published online: March 10, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000808.
Abstract: The strategy for obtaining a crystalline cat- ysis. In the selective dehydration of methanol to di-
alyst based on a porous copper-based metal-organic methyl ether, this catalyst exhibited higher catalytic
framework and 12-tungstosilicic acid with different activity than the copper-based metal-organic frame-
particle sizes is reported. Through the control of hy- work, g-alumina, and g-alumina-supported 12-tung-
drothermal synthesis and some simple treatments, stosilicic acid catalysts. It showed high catalytic per-
catalyst samples with average particle diameters of formances, even at higher space velocity or in the
2
3, 105, and 450 mm, respectively, were prepared. presence of excess water. In addition, the catalyst
This crystal catalyst has both the Brønsted acidity of was also preliminarily assessed in the formation of
2-tungstosilicic acid and the Lewis acidity of the ethyl acetate from acetic acid and ethylene. It also
1
copper-based metal-organic framework, and has high exhibited a high activity which was comparable with
density of accessible acid sites. Its catalytic activity that of silica-supported 12-tungstosilicic acid catalyst.
was fully assessed in the dehydration of methanol to
dimethyl ether. The effect of particle size on the cat-
alytic activity of catalyst was studied, in order to Keywords: dimethyl ether; ethyl acetate; heteroge-
select the particle size appropriate for avoiding the neous catalysis; polyoxometalates; porous metal-or-
diffusion limitation in heterogeneous gas-phase catal- ganic frameworks
Introduction
bridging organic ligands, are emerging porous materi-
als. They combine well-defined crystalline structures,
At present, the scale of chemical production is enlarg- high surface areas, regular and tunable cavities, and
ing each year to meet the rising demands for energy high catalytic active site density. Their catalytic appli-
and chemical products. In order to reduce the corre- cations have caught the attention of researchers
[
2]
sponding economic and environmental pressure from worldwide in recent years.
the enlarged chemical production, we must develop Recently, hybrid compounds based on porous
novel environmentally benign catalysts with higher MOFs and POMs (PMOFs/POMs), which combine
performances. the properties of POMs and MOFs, have attracted
[
3]
Polyoxometalates (POMs) and metal-organic special interest. In the field of catalysis, several
frameworks (MOFs) are both environmentally benign Keggin-type POMs have been dispersed in the porous
catalytic materials. POMs, especially Keggin-type systems of HKUST-1 [Cu ACHTUGNTERNN(UNG BTC) CAHTUNGTERNNUG(N H O) , BTC=1,3,5-
3 2 2 3
POMs, have attracted considerable research attention benzenetricarboxylate] and MIL-101 {Cr F ACHTUNGRTNEUNG( H O) O-
3 2 2
for a long time. The intrinsic chemical properties of
Keggin-type POMs, such as strong Brønsted acidity high activities in some organic transformations.
ACHTUNGTREUNNGN[ (O C)-C H -(CO )] }, respectively, and they exhibited
2 6 4 2 3
[4]
and reversible oxidization properties, make them effi- Among them, only NENU-n series compounds have
cient catalysts in a large number of organic transfor- been well-defined by single-crystal X-ray diffractio-
[
1]
[4a]
mations and large-scale chemical production. MOFs, n. In NENU-n, each POM molecule is encapsulated
consisting of metal ions or metal ion clusters and in one pore of Cu (BTC) , resulting in the dispersion
AHCTUNGTRENNUNG
3
2
Adv. Synth. Catal. 2011, 353, 733 – 742
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Da-Dong Liang et al.
FULL PAPERS
of POMs molecules at the molecular level. The high and H O were eliminated, and the resulting
2
pore density of Cu
A
H
U
G
R
N
U
G
ACHTUNGTNERNUN[G Cu ACHTUNGTRNEN(GU BTC) ] [H SiW O ] (NENU-1a) was used as
3
2
2 4/3 6 4 12 40
ing. The relatively narrow windows between the pores the solid acid catalyst in the heterogeneous gas-phase
can physically prevent the agglomeration of POMs catalysis of methanol dehydration to dimethyl ether
molecules and inhibit the leaching of POMs. In catal- (DME) and the formation of ethyl acetate from
ysis, the organic transformations may take place acetic acid and ethylene (Figure 1). Through compar-
inside the porous system of the NENU-n. Each cata- ing the catalytic activities of NENU-1a with different
lytically active site of NENU-n may be utilized, and particle sizes, we selected the particle size appropriate
the single-site catalysis leads to high catalytic activity. for avoiding the diffusion limitation. In addition, the
In these NENU-n compounds, all catalytically catalytic performances of NENU-1a were compared
active sites are located inside the porous system, and with those of the catalysts used in the present large-
the internal diffusion limitation perhaps occurs in cat- scale chemical production.
alysis. An efficient method to avoid the diffusion limi-
tation is to reduce the catalyst particle size. If the par-
ticle size is reduced, the reactant molecules will dif- Results and Discussion
fuse into the porous system at a higher speed, and the
catalytic reaction will exhibit a higher rate. At the Catalyst Preparation and Characterization
same time, each catalytically active site in the small
particle where diffusion limitation is avoided may be NENU-1 was synthesized in a one-step hydrothermal
utilized in the catalytic reactions. However, smaller is reaction. The synthesis of NENU-1 to generate crys-
not always better. The smaller particle size brings tals that are suitable for single-crystal X-ray diffrac-
more difficulties in the purification of catalysts and tion study has already been reported (CCDC
[
4a]
the catalytic operations. Thus, it is necessary to devel- 686793).
Herein, the originally reported synthesis
op a synthesis method for controlling the particle size process has been modified to obtain NENU-1 with
of NENU-n and to select the optimum particle size different particle sizes.
for each catalytic reaction. Hydrothermal synthesis is
Through numerous synthesis experiments, we found
a universal method for synthesizing most PMOFs/ that the final particle sizes of NENU-1 depend on the
POMs and all NENU-n compounds. It is safe and concentrations of reactants and the time of the hydro-
convenient, and the synthesis conditions can be easily thermal reaction. Higher concentrations of reactants
controlled.
Herein,
resulted in a higher nucleation rate and provided
more nucleation sites that, in turn, facilitated the for-
we
obtained
the
[Cu ACHTUNTGNERNUG( BTC)
2 4/3
ACHTUNGTRENNUNG( H O) ] [H SiW O ]· ACHTUNGTURENNNG( C H N) (NENU-1) with dif- mation of a large number of small crystals. On the
2 2 6 2 12 40 4 12 2
ferent particle sizes, through controlling the hydro- contrary, lower concentrations of reactants facilitated
+
thermal synthesis conditions. Then, C H N cations the formation of larger crystals, although the number
4
12
of crystals was relatively small. The crystals were
grown gradually under hydrothermal conditions,
therefore, shortening the time of hydrothermal reac-
tions resulted in small crystals and vice versa.
Crystalline samples of NENU-1 with average diam-
eters of 23, 105, and 450 mm were obtained through
applying different reactant concentrations and times
of hydrothermal synthesis. Particle sizes were con-
firmed by SEM (Figure 2). The block-shaped crystals
were isolated by centrifugation. Tiny differences in
the shapes of the block-shaped crystals existed; how-
ever, these crystals had the same components and mi-
crostructures. The samples of NENU-1 were refluxed
in ethanol and then treated at 1808C under vacuum
+
to remove C H N and H O. Afterwards, NENU-1a
4
12
2
samples were obtained.
The samples of NENU-1a with different particle
sizes were identified by elemental analysis, IR, XRD,
acid-base titration, and N adsorption, respectively.
2
Elemental analysis indicates that, for these samples,
the molar ratios of Cu:Si:W are all 12:1:12, which
and ethylene (bottom) were carried out in the porous agrees with the reported molecular formula of
Figure 1. The catalytic dehydration of methanol to DME
top) and the formation of ethyl acetate from acetic acid
(
system of NENU-1a.
NENU-1a.
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Adv. Synth. Catal. 2011, 353, 733 – 742

A Crystalline Catalyst Based on a Porous Metal-Organic Framework and 12-Tungstosilicic Acid
Figure 2. SEM of NENU-1 with average particle diameters of (a) 23, (b) 105, and (c) 450 mm.
[
4a]
Cu3
which are assigned to the Cu AHCNUTGTRNEG(UN BTC) and 12-tungsto-
3 2
ACHTUNGTNERNUNG( BTC) . The characteristic bands of NENU-1a,
2
silicic acid, are in agreement with that of NENU-1
see Figure S2 in the Supporting Information), which
demonstrates that NENU-1a has the components of
both Cu (BTC) and 12-tungstosilicic acid. There is a
(
AHCTUNGTRENNUNG
3
2
tiny difference between the IR patterns of NENU-1
and NENU-1a. In the IR pattern of NENU-1a, a
À1
peak at 1484 cm disappeared. This peak is assigned
to the CÀH vibration. Its disappearance indicates that
+
the C H N cations had been decomposed through
4
12
the vacuum heat treatment. After the decomposition
+
+
of C H N cation, H was left on the polyanion,
4
12
which could be confirmed through acid-base titratio-
[
4a]
n.
Acid-base titration results show that, in these sam-
4À
ples, each [SiW O ] corresponds to four protons.
1
2
40
Figure 3. IR patterns of NENU-1a with average particle di-
ameters of (a) 23, (b) 105, and (c) 450 mm.
Taking the NENU-1a with the average particle diam-
eter of 105 mm for example, in 0.5035 g NENU-1a, the
proton amount is 0.38 mmol.
The samples of NENU-1a with different particle
XRD patterns (Figure 4) of the NENU-1a bulk ma-
sizes showed the same characteristics in the IR pat- terials match the simulated pattern from the crystal
terns (Figure 3), which indicates that they have the structure of NENU-1, which indicates that the host
same components. The presence of Keggin-type 12- structure of NENU-1 is maintained after the elimina-
+
tungstosilicic acid and Cu
A
H
U
G
R
N
U
G
3
2
4
12
2
NENU-1a can also be confirmed through IR. As NENU-1a samples have high purity. N adsorption ex-
2
shown in Figure S1 in the Supporting Information, periments indicate that all of these samples have high
the main characteristic bands of Cu
range 1800–1090 cm and 770–450 cm region of the NENU-1a (particle diameter of 105 mm) is shown in
A
H
U
G
R
N
U
G
3
2
À1
À1
spectrum. The characteristic bands of H SiW O are Figure 5.
4
12 40
À1
in the range 1020–770 cm , and these bands are as-
On the basis of the single-crystal X-ray diffraction
[
4a]
signed to the vibration of SiÀO , WÀO ÀW, and study of NENU-1
and the above characterization
a
b/c
[
5]
WÀO , respectively. All of these characteristic bands of NENU-1a, the structure of NENU-1a is confirmed.
d
of Cu ACHTUNGTRENNUNG( BTC) and H SiW O were observed in the In NENU-1a, the MOF part is porous Cu ACHTUNGTRENNUNG( BTC)
3 2 4 12 40 3 2
IR pattern of NENU-1a, which demonstrates that 12- which consists of paddle-wheel Cu clusters and BTC
2
tungstosilicic acid and Cu ACUTHNTGRENNUN(G BTC) are both present at groups. The Cu ACHTUNGERTNNUNG( BTC) has two kinds of pores: A and
3
2
3
2
the same time in NENU-1a. This conclusion may also B (see Figure 1). Pore A has a cuboctahedral shape.
be drawn through the comparison of IR patterns of The BTC groups define 8 triangular planes of the cu-
NENU-1 and NENU-1a. Single-crystal X-ray diffrac- boctahedra and the paddle-wheel Cu clusters locate
2
tion of NENU-1 had unambiguously demonstrated 12 vertices. Pore B is surrounded by 12 Cu clusters
2
the presence of Keggin-type POM in the porous and BTC ligands. In NENU-1a, each pore A accom-
Adv. Synth. Catal. 2011, 353, 733 – 742
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asc.wiley-vch.de
735

Da-Dong Liang et al.
FULL PAPERS
Figure 4. XRD patterns of NENU-1a with average particle
diameters of (a) 23, (b) 105, and (c) 450 mm, and (d) XRD
pattern simulated from the X-ray crystal structure of
NENU-1 (CCDC 686793. These data can be obtained free
of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif).
Figure 6. XRD patterns of NENU-1a at different tempera-
tures: (a) 240, (b) 260, (c) 280, and (d) 3008C.
may be attributed to the POM encapsulation because
the size, shape, and symmetry of H SiW O highly
4
12 40
match those of the pore A of Cu ACTHUNGTRNEUNG( BTC) . Because of
3 2
such a molecular-level matching, Keggin-type POMs
2
+
and Cu ions of Cu
A
H
U
G
R
N
U
G
3
2
[
3l]
chemical interaction. Thus, when H SiW O mole-
cules are encapsulated in pores of Cu ACHTUNGTRENUNG( BTC) , they
3 2
can effectively support the framework and improve
the stability.
Catalysis
The dehydration of methanol to DME was selected to
assess the catalytic activity of NENU-1a. DME is a
non-toxic, non-corrosive, non-carcinogenic, and envi-
ronmentally-friendly chemical. It is a raw material for
making important chemicals, including dimethyl sul-
fate and high-value oxygenated compounds. It is also
used as an aerosol propellant in various spray cans,
Figure 5. N adsorption isotherm of NENU-1a (average par-
ticle diameter of 105 mm) measured at 77 K. Open symbols,
adsorption; filled symbols, desorption.
2
[7]
such as hair spray and shaving cream. Furthermore,
DME has been identified as a promising clean-burn-
ing alternative fuel. Its cetane rating is higher than
that of conventional diesel fuel. It can be burned
modates one H SiW O molecule, and all pores B without the emission of soot, black smoke or SO ,
4
12 40
2
[
8]
are vacant. The diameter of pore B is 13 ꢂ (see Fig- and only very low amounts of NO are produced.
x
ure S3 in the Supporting Information). These vacant Therefore, there is a growing requirement for the
pores B are connected to each other, and each pore B large-scale production of DME to meet the market.
is connected to six adjacent pores B. Thus, NENU-1a In China alone, there is more than one million tons of
exhibited high porosity.
production capacity with another four million under
[9]
The samples of NENU-n with different particle construction.
sizes have no difference in thermal stability. TG (Fig-
Selective dehydration of methanol, catalyzed by
ure S4 in the Supporting Information) and variable- Brønsted acids and/or Lewis acids, is one of the
temperature XRD (Figure 6) show that NENU-1a is common industrial methods employed for DME pro-
stable up to 3008C, indicating that it has better ther- duction. Although some homogeneous catalysts have
[
9]
mal stability than pure Cu ACTHUNGTRNEUNG( BTC) which is stable up been studied in recent years, methanol dehydration
3 2
[6]
to 2408C. The improved stability of the framework to DME is mainly catalyzed over various solid-acid
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Adv. Synth. Catal. 2011, 353, 733 – 742

A Crystalline Catalyst Based on a Porous Metal-Organic Framework and 12-Tungstosilicic Acid
tion over Cu ACTHUNGTRNEUNG( BTC) cannot achieve satisfactory con-
3 2
version results by increasing the temperature.
Using NENU-1a as catalysts, DME selectivity
reached 100% and methanol conversions were largely
improved. At 2408C, methanol conversions over
NENU-1a with average diameters of 23, 105, and
4
50 mm were 60.1%, 58.0%, and 46.1%, respectively.
These values are higher than the 9.1% and 4.8% of
methanol conversion over Cu3(BTC) and g-Al O , re-
AHCTUNGTRENNUNG
2
2
3
spectively. From 250 to 3008C, NENU-1a still exhibit-
ed the highest activity among these three materials.
The temperatures of equilibrium methanol conversion
were 270, 270, and 2808C over NENU-1a, with aver-
age particle diameters of 23, 105, and 450 mm, respec-
tively. These values are lower than that obtained over
g-Al O (over 3008C), indicating that the DME pro-
2
3
duction catalyzed by NENU-1a can be performed at a
Figure 7. Conversions of methanol dehydration to DME
over catalysts of ( ) Cu
(BTC) , (*) g-Al O , and NENU-1a
with the average particle diameters of (!) 23, (~) 105, and
lower temperature than that catalyzed by g-Al O .
2
3
&
ACHTUNGTRENNUNG
3
2
2
3
The pores in NENU-1a connect each other, and
therefore the porous system is difficult to be blocked.
In the long-term experiment of methanol dehydra-
tion, which was carried out on NENU-1a with the
average diameter of 105 mm at 2708C, the conversions
(
^) 450 mm at the pressure level of 0.6 MPa and LHSV of
À1
2
h .
[10]
catalysts, because they are usually eco-friendly and of methanol were almost constant over 168 h.
can be separated easily from the reaction system. De- In the catalytic dehydration of methanol, NENU-1a
veloping novel solid-acid catalysts with better per- with the average particle diameter of 450 mm exhibit-
formances for the methanol dehydration to DME is ed lower activity than the samples with smaller parti-
being extensively pursued in both industrial and aca- cle diameters (23 and 105 mm), possibly due to inter-
demic research.
nal diffusion limitation. In addition, in the NENU-1a
The catalytic activity of NENU-1a in the heteroge- with particle diameter of 450 mm, the active sites lo-
neous gas-phase dehydration of methanol to DME cated in the deep pores of particles perhaps were not
was first compared with those of g-Al O and fully utilized. Methanol conversions over NENU-1a
2
3
Cu3
ACHTUNGTRENNUNG( BTC) . g-Al O is a porous material with Lewis with average particle diameters of 23 and 105 mm gen-
2 2 3
acidity and is commercially used for methanol dehy- erated similar results; indicating that the diffusion
[11]
dration to DME. Pure Cu ACHTNUGTRNEUNG( BTC) is also expected limitation was avoided after the particle average di-
3 2
to catalyze methanol dehydration to DME because of ameter was reduced to 105 mm.
its excellent Lewis acidity and high acid site density. The higher catalytic performance of NENU-1a than
In Cu (BTC) , all Cu are located in the paddle-wheel Cu3(BTC) is attributed to the POM encapsulation.
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
ACHTUNGTRENNUNG
3
2
2
Cu clusters. These coordinatively unsaturated metal H SiW O is an excellent Brønsted acid, and the en-
2
4
12 40
centers are Lewis acid sites as proven in several capsulation of H SiW O increases the number of
4
12 40
[12]
works.
The heterogeneous gas-phase catalyses of methanol encapsulation improved the thermal stability of the
dehydration to DME over g-Al O , Cu (BTC) , and framework, which allows the Lewis acid sites of
acid sites per unit volume. On the other hand, POM
AHCTUNGTRENNUNG
2
3
3
2
NENU-1a were performed in a fixed-bed reactor Cu(II) to improve their catalytic performances by ap-
Figure S5 in the Supporting Information) at a pres- propriately increasing the reaction temperature.
(
sure level of 0.6 MPa and the liquid hourly space ve-
locity (LHSV) of 2 h . The respective performances acidity, NENU-1a has additional Brønsted acidity. To
Compared with g-Al O , which has solely Lewis
2 3
À1
of these catalysts are summarized in Table S1 in the assess NENU-1a further, a series of g-Al O -support-
2
3
Supporting Information and depicted in Figure 7. ed H SiW O catalysts with different POM loadings
4
12 40
Over g-Al O , the methanol conversion was 4.8% at was prepared through impregnation. The samples
2
3
2
408C; this gradually increased with increasing tem- with H SiW O loadings of about 5, 10, 15, 20, and
4 12 40
perature. The conversion of methanol over 30 wt% are denoted as H SiW O (n%)/g-Al O ,
4
12 40
2
3
Cu3
water as the sole products. However, Cu AHCUTNGTRENUNGN( BTC) is de- methanol dehydration over g-Al O and g-Al O -sup-
3 2 2 3 2 3
ACHTUNGTRENNUNG( BTC) was 9.1% at 2408C, resulting in DME and where n=5, 10, 15, 20, and 30, respectively. The
2
activated at higher temperatures due to the collapse ported H SiW O exhibited different conversions
4
12 40
of framework. These results indicate that although (Figure 8 and Table S2 in the Supporting Informa-
Cu3(BTC) is active and selective, methanol dehydra- tion). When the H SiW O loading is in the range of
ACHTUNGTRENNUNG
2
4
12 40
Adv. Synth. Catal. 2011, 353, 733 – 742
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737

Da-Dong Liang et al.
FULL PAPERS
Figure 8. Conversions of methanol dehydration to DME
over (!) NENU-1a with the average particle diameter of
Figure 9. Conversions of methanol dehydration to DME
over catalysts of (!
g-Al
, and (^) H
) NENU-1a with average of 105 mm, (*)
1
05 mm, (*) g-Al O , and g-Al O -supported H SiW O
2
O
3
4
SiW12O
10 (15%)/g-Al
2
O
3
at the pressure
2
3
2
3
4
12 40
À1
catalysts with the H SiW O loading of (*) 5, (^) 10, (~)
level of 0.6 MPa and LHSV of 10 h .
4
12 40
1
5, (~) 20, and (!) 30% at the pressure level of 0.6 MPa
À1
and LHSV of 2 h .
LHSV, and more DME will be produced in any spe-
cific unit time.
The high catalytic performances of NENU-1a are
5% to 15%, the following sequence of catalytic activi- associated with its structural features that not only
ty has been observed: g-Al O <H SiW O (5%)/g- allow high catalytic active site density, but also allow
2
3
4
12 40
Al O <H SiW O
(10%)/ g-Al O <H SiW O
accessibility of all active sites. In NENU-1a, the
2
3
4
12 40
2
3
4
12 40
(
15%)/g-Al O . This result indicates that the introduc- H SiW O loading (54 wt%) is much higher than the
2
3
4
12 40
tion of H SiW O improves the catalytic perfor- optimal loading (15 wt%) of g-Al O -supported
4
12 40
2
3
mance of g-Al O . With a POM loading higher than H SiW O catalysts. Furthermore, all catalytic active
2
3
4
12 40
1
5%, the sequence of catalytic activity is H SiW O
sites in NENU-1a are accessible. Each H SiW O
4
12 40
4 12 40
(
15%)/g-Al O >H SiW O
(20%)/g-Al O > with Brønsted acidity can access 6 vacant pores B
2
3
4
12 40
2 3
H SiW O (30%)/g-Al O . This indicates that excess through windows between pores A and B; each
4
12 40
2
3
POM in g-Al O may decrease the catalytic activity. copper, which is a Lewis acid site, is located in the
2
3
When the H SiW O
loading is excessive, nodes of framework and points to the center of pore
4
12 40
H SiW O molecules are easily agglomerated by B (see Figure 1). Each catalytically active site in
4
12 40
high surface energy, resulting in the partial blockage NENU-1a is freely accessible to reactant molecules,
of the cavities of g-Al O . Therefore, in order to and the single-site catalysis ultimately results in high
2
3
obtain satisfactory conversion results, the H SiW O
methanol conversion.
4
12 40
loading over g-Al O must be limited at a reasonable
In this work, NENU-1a was also assessed in the
2
3
level. Based on the results of the above experiments, catalytic dehydration of methanol in the presence of
the optimum POM loading should be set at 15%. excess water. Water is one of the products of metha-
NENU-1a exhibited higher catalytic activity in nol dehydration, and the presence of excess water
comparison with various g-Al O -supported may decrease methanol conversion. In addition, water
H SiW O catalysts (see Figure 8). Further compari- competes with methanol for the same Lewis acid sites
2
3
4
12 40
son of the catalytic performance at higher LHSV on the catalyst, dramatically decreasing the Lewis
[
7a]
demonstrates the apparent advantages of NENU-1a. acid activity. At 2708C, when the crude methanol
The conversions over pure g-Al O and H SiW O with 20 wt% water was fed, methanol conversion over
15%)/g-Al O dramatically decreased at an LHSV of NENU-1a only slightly decreased, demonstrating the
2
3
4
12 40
(
1
2
2
3
À1
À1
0 h , compared with the conversions at LHSV of high water resistance of the NENU-1a (Figure 10).
h
(Figure 9 and Table S3 in the Supporting Infor- This result may be due to the high density of Brønst-
mation). In contrast, methanol dehydration over ed acid sites of NENU-1a as supported by the fact
NENU-1a maintained high conversions. If NENU-1a that water only reduces the activity of Lewis acid sites
[
7a]
is used to replace g-Al O3 or g-Al O -supported but has little effect on Brønsted acid sites.
As a
2
2
3
H SiW O as the catalyst in the production of DME, result, when NENU-1a is used as the catalyst for
4
12 40
methanol dehydration may be performed at higher DME production, cheaper crude methanol with a
738
asc.wiley-vch.de
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 733 – 742

A Crystalline Catalyst Based on a Porous Metal-Organic Framework and 12-Tungstosilicic Acid
The dehydration of mixed alcohols was performed
in a modified fixed-bed reactor. The mixture, consist-
ing of 10 atomic% methanol and 90 atomic% n-buta-
nol, was introduced to the reaction zone by bubbling
N (99.999%) through a glass saturator filled with the
2
mixture maintained at 258C. The effluent was moni-
tored by GC.
When the reactor was at 258C, the content of meth-
anol largely decreased once the alcohol solution
passed through the reactor filled with NENU-1a, and
no ethers were observed in the effluents. This phe-
nomenon indicates that methanol molecules are selec-
tively adsorbed on the surface of NENU-1a. They
either coordinate to the copper of Cu clusters or
2
form hydrogen bonds with the POM molecules. The
reaction of alcohol dehydration did not take place be-
cause of the low temperature.
The reactor was heated to 2708C, and the dehydra-
tion reaction was carried out. In the production, n-
butyl methyl ether was dominant and small amounts
of DME and dibutyl ether were also present, as
shown in Figure 11 and Table S4 in the Supporting In-
formation. This can be explained in that the alcohol
Figure 10. Water addition effect over NENU-1a. Anhydrous
methanol was fed at the first 5 h, and then methanol with
2
0 wt% water was fed instead of anhydrous methanol. Con-
À1
ditions: 2708C, 0.6 MPa, LHSV=2 h .
higher water content can be used as starting material dehydration over NENU-1a is a surface reaction. One
instead of expensive anhydrous methanol. The high alcohol molecule is first adsorbed on the inner surface
catalytic activity of NENU-1a in the presence of of NENU-1a, and a second alcohol molecule from the
excess water also allows NENU-1a to be used in the gas-phase interacts with the absorbed alcohol mole-
syngas-to-DME process, which is another important cule. Methanol was the dominant adsorbed alcohol on
[
13]
industrial process involved in DME production.
the surface of NENU-1a; n-butanol is the dominant
An important property of the PMOFs/POMs cata- alcohol in the gas phase. Thus, n-butyl methyl ether is
lysts is that their catalytic performances may be fine- the main product. Because the contents of methanol
turned by encapsulating different POMs. A previous and n-butanol are less in the gas phase and on the sur-
work has reported that NENU-3a [with H PW O
face of NENU-1a, respectively, only less DME and di-
(BTC) ] showed high activity in butyl ether exist in the effluent.
3
12 40
encapsulated in Cu
A
H
U
G
R
N
N
3
2
[
4a]
the heterogeneous liquid-phase hydrolysis of esters.
NENU-1a was also assessed in the formation of
However, when NENU-3a was assessed in the cataly- ethyl acetate. Ethyl acetate is an important chemical
sis of methanol dehydration, the highest selectivity of product which is broadly employed as a solvent in
DME only reached 75%, which may be attributed to paints, inks, enamels, adhesives, and the pharmaceuti-
the strong acidity of H PW O . DME is an inter-
3
12 40
mediate of methanol dehydration; if the acidity of the
catalyst is too strong, the methanol tends to further
dehydrate to form olefins or coke, thereby decreasing
the selectivity of DME. H SiW O has milder acidity
4
12 40
and more protons than H PW O . Thus, NENU-1a is
3
12 40
more suitable for catalyzing the selective conversion
of methanol to DME than NENU-3a.
The mechanisms of catalytic methanol dehydration
[
14]
to DME differ over different solid acids.
In this
work, an experiment of mixed alcohol dehydration
was designed, and the mechanism of alcohol dehydra-
tion over NENU-1a was explored.
In the vapor adsorption experiments, NENU-1a ad-
sorbed a large amount of methanol and only adsorbed
a small amount of n-butanol (data not included in this
work). Thus, NENU-1a may have the ability of ad-
sorbing methanol selectively from the gas-phase mix-
tures of methanol and n-butanol.
Figure 11. Dehydration of mixed alcohols, composed of 10%
methanol and 90% butanol, over NENU-1a in a fixed-bed
at 2708C. The effluent consisting of (^) C H OH, ( )
!
4
9
CH OC H , (*) CH OCH , and (
&
) C H OC H was ana-
3
4
9
3
3
4 9 4 9
lyzed by GC every 5 min.
Adv. Synth. Catal. 2011, 353, 733 – 742
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
739

Da-Dong Liang et al.
FULL PAPERS
Table 1. Catalytic data for the formation of ethyl acetate comprising H PW O for some selective acid catalyt-
3
12 40
from acetic acid and ethylene over various solid acid cata-
lysts.
ic reactions.
We have presented the particle size control of
NENU-1 in hydrothermal synthesis for the catalytic
applications. Through applying different reactant con-
centrations and hydrothermal synthesis times, we ob-
tained the NENU-1 with the average particle diame-
ters of 23, 105, and 450 mm, respectively.
Entry
Catalyst
Yield of ethyl acetate [%]
[
a]
[16]
1
2
3
4
5
6
7
8
H PO /SiO
2
1
3
0
0
3
4
[
a]
[16]
Nafion-SiO
2
[
a]
[16]
WO -ZrO2
3
[a]
[16]
H SiW O
4
12 40
NENU-1a has both the Brønsted acidity of
[a]
[16]
H SiW O /SiO
50
4
12 40
2
[
b]
H SiW O and the Lewis acidity of Cu3 ACHTUTGNENRN(UG BTC) , and
4 12 40 2
NENU-1a (450 mm)[
b]
55
67
67
it has a high density of accessible acid sites. Thus, it
can be used as an effective single-site catalyst in acid
catalysis. The internal diffusion limitation in the heter-
ogeneous gas-phase catalysis was avoided, when the
average particle diameter was reduced to 105 mm. In
the catalytic dehydration of methanol to DME,
NENU-1a exhibited higher catalytic performances
NENU-1a (105 mm)
[
b]
NENU-1a (23 mm)
[
a]
Reaction conditions: reactant gas: acetic acid (8%), ethyl-
À1
ene (78.5%), water (4.5%), and N (9%); SV=1500 h ,
2
reaction temperature 438 K, and pressure 0.8 MPa.
Reaction conditions: reactant gas, acetic acid (8%), ethyl-
[
b]
À1
ene (78.0%), water (5.0%), and N (9%); SV=1500 h ,
2
than pure Cu ACTHUNGTRENUNG(N BTC) , g-Al O , and g-Al O -supported
reaction temperature 453 K, and pressure 1.0 MPa.
3 2 2 3 2 3
H SiW O catalysts. If NENU-1a is used as the cata-
4
12 40
lyst in the DME production, the catalytic reaction can
be carried out at lower temperature, and the cheap
methanol with high water content can be used as
cal industry. It is a green alternative to aromatic com- starting material instead of expensive anhydrous
pounds which cause serious damage to human beings methanol. In addition, the study of mixed alcohols de-
[
15]
and the environment. The current annual demand hydration indicates that the dehydration takes place
of ethyl acetate approaches 1200 kilotons throughout between the alcohol molecule adsorbed on the
the world, and the demand is still increasing each NENU-1a and a second alcohol molecule from the
[16]
year.
The direct reaction of ethylene with acetic gas-phase. In the formation of ethyl acetate, the
acid is one important industrial method for ethyl ace- NENU-1a exhibited high catalytic activity, which was
tate production. SiO -supported H SiW O catalyst comparable with that of SiO -supported H SiW O
12 40
2
4
12 40
2
4
is highly active for this reaction, and it has higher ac- catalyst.
tivity than H-montmorillonite, XE386 resin, nafion, NENU-n series compounds are just the tip of the
H-zeolite Y, and H-theta-1. Thus, it has been used PMOFs/POMs iceberg. There are infinite possibilities
[
17]
[
18]
in the present industrial ethyl acetate production.
for the combination of porous MOFs and POMs, es-
The activities of a section of solid-acid catalysts for pecially when the diversity of the sizes and shapes of
the reaction of acetic acid with ethylene to form ethyl the MOFs cavities, as well as the diversity of the
acetate are listed in Table 1, illustrating that NENU- structures and components of POMs are considered.
1a is a highly active catalyst for this reaction. For Continued engagement in the synthesis of PMOFs/
NENU-1a, the selectivity of the ethyl acetate was POMs for catalytic applications will be encouraged in
more than 98%, while the activities were different, future work.
depending on the particle sizes. The sequence of yield
is NENU-1a (450 mm)<NENU-1a (105 mm)=
NENU-1a (23 mm). Thus, the NENU-1a with average
particle diameter of 105 mm is the optimum for the
Experimental Section
catalytic reaction, because of the combination of high
catalytic activity and appropriate particle size. In com-
Characterization Methods
parison with literature reports of the catalytic per- Using KBr pellets, IR spectra were recorded in the range
À1
formances of catalysts should be treated with caution 400–4000 cm on an Alpha Centaurt FT/IR spectrophotom-
eter. XRD measurements were performed on a Rigaku D/
because of different experimental parameters.
MAX-3 instrument with Cu Ka radiation at 293 K. Element
analysis was performed with a PLASMA-SPEC(I) ICP
atomic emission spectrometer. Morphological characteristics
of the samples were determined by scanning electron mi-
Conclusions
croscopy (SEM) using a Philips XL 30 microscope and a
Shimadzu SSX-550 microscope. TG analysis was performed
The NENU-n series compounds are promising single-
site catalysts. Their catalytic performances can be
on a Perkin–Elmer TGA7 instrument in N with a heating
2
À1
turned by encapsulating different POMs. The NENU- rate of 108Cmin . The products of the catalytic experi-
1
a comprising H SiW O is better than NENU-3a ments were analyzed with a GC (Agilent 6890N instru-
4 12 40
740
asc.wiley-vch.de
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2011, 353, 733 – 742

A Crystalline Catalyst Based on a Porous Metal-Organic Framework and 12-Tungstosilicic Acid
ment). Hiden Isochema Intelligent Gravimetric Analyser
IGA-100) was used to measure N sorption; the NENU-1a
ACHTUNGTNERNUNG[ Cu AHTCUNGTERUNN(NG BTC) ] [H SiW O ] (NENU-1a)
2 4/3 6 4 12 40
(
2
All the crystals of NENU-1 produced from the processes de-
scribed above were refluxed in ethanol and then treated at
sample (85Æ1 mg) was used for the gas sorption measure-
ment at 77 K.
+
1
808C under vacuum for 12 h to remove C H N and H O.
4
12
2
Acid-base titration and back titration: Crystalline solid
NENU-1a (0.5035 g) was placed in carbonate-free, sodium
hydroxide solution (50.00 mL, 0.0125M), which was standar-
dized by titration with a standard potassium hydrogen
phthalate solution. The mixture was sealed and stirred for
Afterwards, NENU-1a samples were obtained.
Catalytic Studies
A fixed-bed reactor was used to perform the heterogeneous
gas-phase catalysis of methanol dehydration to DME and
the formation of ethyl acetate from acetic acid and ethylene.
The catalyst (NENU-1a) was packed in a steel reactor
10 h at 258C, filtering off the solids. Afterwards, the filtrate
was back-titrated using hydrochloric acid (0.0118M) stand-
ardized by titrations with the standardized sodium hydrox-
ide solution. The number of acidic protons of NENU-1a was
calculated from the amount of consumed sodium hydroxide.
(
inner diameter=0.8 cm, length=40 cm). Quartz wool was
placed on the top and the base of the bed. The gas flow rate
was controlled by a mass flow controller. Liquid was fed
into the reactor by an ISCO liquid pump. In the catalytic de-
hydration of methanol, N was used as a carrier gas, and the
2
Materials
pressure drop over the reactor was 0.6 MPa. In the forma-
tion of ethyl acetate, the pressure is 1.0 MPa. The products
were analyzed with GC.
Experimental H SiW O and Cu ACHTUGNTRENN(NUG BTC) were synthesized
3 2
[19,12a]
4
12 40
based on the procedures described in the literature;
they were identified by IR and XRD. Other chemicals in-
cluding those used for catalytic application were commer-
cially purchased and used directly without additional purifi-
cation.
In the study of the mechanism of alcohol dehydration, the
reaction was performed in a modified fixed-bed reactor. The
mixture, consisting of 10 atomic% methanol and
90 atomic% n-butanol, was introduced to the reaction zone
by bubbling N (99.999%) through a glass saturator filled
2
with the mixture maintained at 258C. The effluent was
monitored by GC.
ACHTUNGTRENNUNG[ Cu ACHTUNGTRENNUNG( BTC) ACHTUNGTRENNUNG( H O) ] [H SiW O ]· ACHTNGUETRNNUG( C H N)
2 4/3 2 2 6 2 12 40 4 12 2
(
NENU-1)
Supporting Information
Preparation of a sample with an average particle diameter of
3 mm: A mixture of Cu(NO ) ·3H O (0.97 g, 4.0 mmol) and
H SiW O (0.80 g, 0.28 mmol) in distilled water (10 mL)
The preparation of g-Al O -supported H SiW O catalysts,
2
3
4
12 40
2
ACHTUNGTRENNUNG
3
2
2
confirmation of the pore diameter in NENU-1a, IR of
NENU-1a compared to that of Cu (BTC) , H SiW O , and
4
12 40
AHCTUNGTRENNUNG
3
2
4
12 40
was stirred for 15 min, after which H BTC (0.84 g,
3
NENU-1, TG analysis curve of NENU-1a, schematic view
of experimental equipment, and tables of results of the cata-
lytic transformations are available as Supporting Informa-
tion.
4
.0 mmol) and C H NCl (0.44 g, 4.0 mmol) were added in
4 12
succession. The pH of mixture was adjusted to 2–3 using
C H NOH. After stirring for 2 h, the mixture was sealed in
4
12
a Teflon-lined autoclave and heated at 1808C for 6 h fol-
lowed by rapid cooling to room temperature. The crystallites
of NENU-1 were separated from the mixture with a yield of
4
5% based on Cu.
Preparation of a sample with an average particle diameter
Acknowledgements
of 105 mm: A mixture of Cu
ACHTUNGRTNNGE(U NO ) ·3H O (0.48 g, 2.0 mmol)
3 2 2
This work was supported by NSFC (Grant Nos. 20871027
and 20973035), Program for New Century Excellent Talents
in University (NCET-07-0169), Fundamental Research Funds
for the Central Universities, and the Program for Changjiang
Scholars and Innovative Research Team in University.
and H SiW O (0.40 g, 0.14 mmol) in distilled water
4
12 40
(
10 mL) was stirred for 15 min, after which H BTC (0.42 g,
3
2
.0 mmol) and C H NCl (0.22 g, 2.0 mmol) were added in
4 12
succession. The pH of the mixture was adjusted to 2–3 using
C H NOH. After stirring for 2 h, it was sealed in a Teflon-
4
12
lined autoclave and heated at 1808C for 15 h followed by
rapid cooling at room temperature. The crystallites of
NENU-1 were separated from the mixture with a yield of References
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asc.wiley-vch.de
741

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