SUPPORTED HETEROPOLYACIDS AS HETEROGENEOUS CATALYSTS FOR PROPENE METATHESIS

Article abstract of DOI:10.1135/cccc19950443

Catalysts containing tungsten or molybdenum in the form of Keggin type heteropolyacids, like H3PW12O40, H4SiW12O40 and H3PMo12O40, supported on silica gel were prepared.The acid and surface properties of heteropolyacids and their supported forms are discussed.Keggin type heteropolyacids supported on silica gel have been found active for propene metathesis, the activity was compared with those of the classical catalyst WO3/SiO2.The highest activity and selectivity for studied reaction showed catalyst prepared on the base of H3PW12O40.

Full text of DOI:10.1135/cccc19950443

Supported Heteropolyacids as Heterogeneous Catalysts
443
SUPPORTED HETEROPOLYACIDS AS HETEROGENEOUS CATALYSTS
FOR PROPENE METATHESIS*
Pavol HUDEC and Katarina PRANDOVA
Department of Petroleum Technology and Petrochemistry,
Slovak Technical University, 812 37 Bratislava, The Slovak Republic
Received October 27, 1994
Accepted December 16, 1994
Catalysts containing tungsten or molybdenum in the form of Keggin type heteropolyacids, like
H PW O , H SiW O and H PMo O , supported on silica gel were prepared. The acid and sur-
3
12 40
4
12 40
3
12 40
face properties of heteropolyacids and their supported forms are discussed. Keggin type hetero-
polyacids supported on silica gel have been found active for propene metathesis, the activity was
compared with those of the classical catalyst WO /SiO . The highest activity and selectivity for stu-
3
2
died reaction showed catalyst prepared on the base of H PW O .
3
12 40
In the last several years there has been an increasing interest in the catalytic properties
1
of heteropolyacids (HPA’s) and their salts . This fact partly arises from the structural
2
and chemical properties as well as from their combined acid and redox nature . HPA’s
are complexes with high molecular weight anions having from 2 to 18 hexavalent mo-
lybdenum or tungsten, or pentavalent vanadium or niob atoms around one or more
3
central atoms . The central atoms are commonly P, Ni, As, Te and Mn, although ap-
proximately 36 different elements have been used for building of the HPA structure.
3
−
For example, in [PW O ] anions each tungsten atom is at the center of an octahe-
1
2
40
dron and each oxygen atom is situated at each vertex. The phosphorus atom is located
in the center of a PO tetrahedron surrounded by WO octahedra which share corners
4
6
and edges to produce primary Keggin structure. The Keggin structure can form a rela-
tively stable secondary structure with water molecules filling the interconnected space
between the Keggin units.
4
HPA’s have been studied for use as the homogeneous catalysts in oxidation and
5
6
7
hydration of alkenes , polymerization of benzyl alcohol and alcoholysis of epoxide .
The specific properties of HPA’s give an opportunity to use them as heterogeneous
catalysts when loaded on a suitable support. A number of reaction in vapor phase have
*
Presented at the XXVIth Czech and Slovak Symposium on Catalysis, Prague, November 7 – 8, 1994.
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

4
44
Hudec, Prandova:
8
,9
been reported: methanol conversion , conversion of dimethyl ether into hydrocar-
1
0
11
12
bons , alcohol dehydration , decomposition of carboxylic acids , methanol addition
1
3
to isobutene . A brief review on HPA as catalyst for heterogeneous as well as homo-
geneous reactions has been published .
1
One of possible reactions, utilizing the acid properties of supported HPA heteroge-
neous catalysts, is alkene metathesis. This reaction opens new synthesis routes to typical
petrochemicals (ethylene, propene, butenes), special alkenes (neohexene, higher mole-
cular linear alkenes, α,ω-dienes) and unsaturated polymers (polynorbornene, cyclo-
octene, dicyclopentadiene) on an industrial scale.
Generally, the process is going on in the presence of heterogeneous, homogeneous or
combined type of catalyst. To the heterogeneous catalysts which are most frequently
studied, belong mainly transition metal oxides, carbonyls or sulfides of Mo, W and Re
deposited on high surface area supports (γ-Al O , SiO ).
2
3
2
Homogeneous catalysts are composed of a transition metal coordination compound
typical ones are compounds of W, Mo and Re) in combination with a cocatalyst – an organo-
(
1
4–16
metallic compound
. The activity of a given catalytic system is determined by both
the nature and concentration of the active species. There is an evidence that the active
1
7
species are metal carbenes formed from the transition metal compounds , and in the
1
8
case of catalyst of type W/SiO mainly Lewis acid centers play role .
2
In the present work, supported heteropolyacids were prepared, characterized and
tested for the metathesis of propene into ethylene and 2-butenes in vapor phase and
compared with the classical WO /SiO metathesis catalyst.
3
2
EXPERIMENTAL
Catalyst Preparation
2
Silica gel (Spolana, 383 m /g) with a particle size of 0.40 – 0.63 mm was used as a support. The
supported HPA’s were prepared by the incipient wetting of the silica with aqueous solution of
H PW O . x H O, H PMo O . x H O (both Lachema) and H SiW O . x H O (Merck). Impreg-
3
12 40
2
3
12 40
2
4
12 40
2
nation was carried out at an ambient temperature and the samples were dried at 373 K and then cal-
cined at 773 K for 4 h. By this method, samples containing from 1 to 11 wt.% of tungsten or
molybdenum were obtained.
For comparison, a classical type of catalyst for propene metathesis was prepared by the impregna-
tion of the same carrier with (NH ) WO .
4
2
4
The samples are symbolized in the text by wt.% of the metal (W or Mo) followed by W for sup-
ported classical oxide catalysts and by P–W, Si–W, or P–Mo for catalysts prepared by impregnation
with phosphotungsten acid, silicotungsten acid or phosphomolybdenum acid, respectively.
Characterization of Catalysts
Specific surface area, pore volume and pore size distribution were determined by physical adsorption
of nitrogen at 77 K (Sorptomatic 1800, Carlo Erba). The specific surface was calculated by the BET
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Supported Heteropolyacids as Heterogeneous Catalysts
445
method, pore volume was evaluated from the adsorption isotherm. Before measuring, all samples
were calcined at 770 K for 3 h and evacuated at 570 K and 2 Pa for 5 h.
The acid properties of the catalysts and their supported forms were determined by the indicator
method (titration with butylamine in benzene) with the following Hammet indicators and arylmethanols:
dimethylaminoazobenzene (pK 3.3), tolylazo-o-toluidine (pK 2.0), dicinnamalacetone (pK −3.0),
a
a
a
chalcon (pK −5.7), anthraquinone (pK −8.3), triphenylmethanol (pK −6.63), diphenylmethanol (pK −13.3),
a
a
a
a
trinitrotriphenylmethanol (pK –16.27). Samples were activated before measuring at 773 Κ.
a
X-Ray diffraction characteristics and DTA–DTG patterns of prepared catalysts were reported in
1
9
our previous paper .
Metathesis Experiments
Metathesis reaction was carried out in a quartz reactor with a fixed bed of the catalyst. Before con-
tact with propene, catalysts (0.3 – 1.4 g) were activated in a stream of air (60 ml/min) at 823 K for
.5 h and then treated1 for 15 min at 823 K with CO (40 ml/min). The catalysts were cooled to the
9
1
reaction temperature in a stream of inert gas (40 ml/min).
Feed and reaction products were analyzed by GC (Hewlett Packard 5890) with capillary columns
PONA and HP1. FID detector was used.
The experiments were carried out at reaction temperatures of 603 – 873 K, pressure 0.1 MPa, and
space velocities 5 and 10 mmol/s kg. In all experiments, the products were analyzed after 2 – 4 h of
stabilization time. Conversion as well as selectivity (production of primary reaction products of
propene metathesis – ethene and 2-butenes) were calculated on the base of total reaction products
composition in wt.%.
RESULTS AND DISCUSSION
Supported HPA’s, as well as pure carrier (SiO ) are characterized by specific surface
2
area, pore volume and pore size distribution from nitrogen adsorption.
By the loading with HPA or WO from 0 to 11 wt.% of metal, the surface area and
3
2
pore volume gradually decreased, e.g. for the P–W/SiO from 383 and 0.58 to 289 m /g
and 0.46 cm /g, respectively. The specific surface area partially depends also on the
2
3
nature of the HPA, as can be seen from examples for 5 wt.% of metal containing cata-
lysts in Table I.
TABLE I
Specific surface area (SBET) and pore volume (V ) of carrier and supported HPA’s (content of active
a
metal 5 wt.% W or Mo)
2
3
Sample
SBET, m /g
V , cm /g
a
SiO2
383
332
326
347
342
0.581
0.553
0.504
0.527
0.522
W/SiO2
P–W/SiO2
Si–W/SiO2
P–Mo/SiO2
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

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46
Hudec, Prandova:
The typical examples of adsorption isotherms for pure carrier SiO and for all cata-
2
lysts containing 5 wt.% of metal loading on SiO are shown in Fig. 1. The shapes of
2
isotherms are rather similar, differing mainly in the total volume adsorbed (see Table I).
It proves that the impregnation of the carrier with quite great molecules of HPA’s in
comparison with classical (NH ) WO impregnation does not change substantially the
4
2
4
structure of the pore size distribution of SiO (main pore volume for pore radius of 2 – 4 nm).
2
The results of the acid strength determination of free HPA’s and their supported
counterparts are summarized in Tables II and III.
It is seen from Table II that H PW O was shown to be strongly acidic, with a
3
12 40
maximum pK at −16.3. H SiW O was found less acidic, with a maximum pK at
a
4
12 40
a
−
8.3. H PMo O is decomposed under the condition of thermal pretreatment preced-
3 12 40
1
9
ing butylamine titration (it was proved by DTA data ) and the anion of this acid did
not have a Keggin structure. It should be noted that the acidity was determined by both
types of indicators – Hammets and arylmethanols. Therefore, it is suggested that free
HPA’s have acid sites of both Brønsted and Lewis character.
400
V
a
300
2
1
00
00
1
2
3
4
5
0
0.2
0
0.4
0.2
0
0.6
0.4
0.2
0
0.8
0.6
0.4
0.2
1
P/P0
0.8
0.6
0.4
1
0.8
0.6
1
0.8
1
0
0.2
0.4
0.6
0.8
1
FIG. 1
3
Nitrogen adsorption (open points) and desorption (full points) isotherms (V in cm /g N ) at 77 K for:
a
2
1
5
pure SiO , 2 W/SiO (5 wt.% W), 3 Si–W/SiO (5 wt.% W), 4 P–Mo/SiO (5 wt.% Mo),
2
2
2
2
P–W/SiO (5 wt.% W)
2
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Supported Heteropolyacids as Heterogeneous Catalysts
447
The acidity of supported HPA’s were in different sequence as for free HPA’s (Table III).
The highest acidity was determined for H PMo O loaded on SiO (the acidity of pure
3
12 40
2
+
SiO was negligible – below 0.05 meq H /g). Beside this, in the case of supported
2
HPA’s only acidity detected by Hammet’s indicators was observed. This fact indicates
that the most of the acid sites are preserved as a Lewis centers on the surface of the
catalysts. The classical metathesis catalyst, containing 5% of W/SiO , showed the smal-
2
lest acidity among all studied samples.
From these results it can be said that interaction between HPA and the carrier plays
an important role. There is probably an interaction between the surface OH groups of
silica gel and the H ions of HPA on the surface of catalysts. This interaction is the most
probable explanation of the increase of the thermal stability of phosphomolybdenum
acid on SiO in comparison with its free state.
2
TABLE II
Acid sites distribution of free HPA’s of Keggin type, measured by titration method
+
Acid strength, meq H /g
Sample
pK (Hammets)
pK (arylmethanols)
a
a
3
.3
2.0
−3.0
−5.7
−8.3
−6.6
−13.3
−16.3
H PW O
2.63
2.43
0
2.42
2.22
0
2.42
1.98
0
2.18
1.76
0
0.97
0.69
0
2.18
1.94
0
0.79
0.57
3
12 40
H SiW O
0
0
0
0
4
12 40
H PMo O
3
12 40
TABLE III
Acid sites distribution of supported HPA’s of Keggin type, and classical WO /SiO catalyst (5 wt.%
3
2
W or Mo) measured by titration method
+
Acid strength, meq H /g
pK (Hammets)
Sample
pK (arylmethanols)
a
a
3
.3
2.0
−3.0
−5.7
−8.3
−6.6
−13.3
−16.3
P–W/SiO2
0.54
0.74
0.85
0.48
0.48
0.58
0.75
0.32
0.32
0.50
0.67
0.32
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Si–W/SiO2
P–Mo/SiO2
W/SiO2
0
0
0.47
0
0.37
0
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

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Hudec, Prandova:
The catalytic activity of supported HPA’s were tested for propene metathesis. Con-
version of propene is expressed as an amount of converted propene and selectivity as
the conversion to ethylene and 2-butenes (trans and cis).
CH −CH=CH
CH −CH
CH2
CH2
3
2
2
3
+
+
(
A)
CH −CH=CH
CH −CH
3
3
The influence of the content of the active metal loaded on SiO is demonstrated on
2
Fig. 2 for P–W catalyst. This figure shows the production of primary metathesis pro-
ducts over loading of H PW O heteropolyacid at 773 K and space velocity of 4.98
3
12 40
mmol/s kg. The production of ethylene and 2-butenes increases up to about the 5 wt.%
of tungsten sharply, further twofold increase of the tungsten content increased the yield
of main product only by 2 wt.%.
From Fig. 3 it can be seen the temperature influence on the production of primary
metathesis products at space velocity of 9.96 mmol/s kg for 5 wt.% of metal loaded
catalysts. An intense increase of the production of ethylene and 2-butenes starts about
the temperature of 753 K and reaches 32% at 833 K for P–W/SiO catalyst. In compari-
2
son with classical W/SiO catalyst, the P–Mo catalyst has only little higher activity at
2
the temperature up to 800 K, further increase of temperature has no possitive effect on
desirable hydrocarbons yield, probably for the reason of greater coking by side poly-
TABLE IV
Products of propene metathesis (in wt.%) over W or Mo catalysts with 5 wt.% of the metal (T = 773 K,
F/W = 9.96 mmol/s kg
a
a
b
a
Products
P–W/SiO2
Si–W/SiO2
P–Mo/SiO2
W/SiO2
Ethylene
10.4
5.7
6.7
5.1
2.2
5.3
3.6
3.9
2.8
8.4
3.2
2.0
2.3
1.7
1.1
4.0
2.1
2.8
2.1
1.2
1
2
2
-Butene
-trans-Butene
-cis-Butene
Pentenes
Conversion, wt.%
Selectivity, wt.%
30.1
73.8
24.0
50.0
10.3
69.9
12.2
73.0
a
b
After 2 h of the activity stabilization. After 4 h of activity stabilization.
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Supported Heteropolyacids as Heterogeneous Catalysts
449
merization reaction as a result of great acidity. For Si–W/SiO and P–W/SiO catalysts
2
2
the desired temperature for the same yield of ethylene and 2-butenes is lower by 20 °C
and 40 °C, respectively. At the higher temperature the production of these alkenes
decreases because more side reactions occurs.
Beside ethylene and 2-butenes, 1-butene is also formed as a consequence of double
bond isomerization. The mutual reaction of these two products gives 2-pentenes, that
undergo double bond isomerization and results in the formation of 1-pentenes or 3-pen-
tenes. By the metathesis of 1-pentenes with 2-butenes, 3-hexenes are produced. All
these substances can be mutually metathesized and then isomerized. All these products
are summarized in Table IV as pentenes. This table illustrate the total composition of
propene metathesis products at the 773 K and 9.96 mmol/s kg together with the conver-
sion and selectivity of the reaction.
The selectivity of the process over all the catalyst, except Si–W/SiO vary between
2
7
0 – 75%. Different behavior of Si–W/SiO catalyst (only 50% of selectivity) is prob-
2
ably due to the interaction between Si from carrier and Si from HPA. Consequently,
sites more suitable for isomerization and dimerization reactions are formed, because a
rather high amount of pentenes and hexenes was obtained.
The advantage of using the catalysts of heteropolyacid basis for propene metathesis
in the temperature range 753 – 793 K is in the higher production of the primary meta-
thesis products over HPA-catalysts. From this point of view the highest activity has the
P–W/SiO catalyst, Si–W/SiO had lower selectivity to the primary products of propene
2
2
metathesis, and P–Mo/SiO catalyst with stronger acidity showed high side-reaction
2
activity with coking.
16
1
3
0
Y
2
Y
3
4
20
8
10
0
0
0
2
4
6
8
10
653
703
753
803
853
T, K
W
FIG. 2
FIG. 3
Influence of the metal content (W, wt.%) in
P–W/SiO2 catalyst on the yield (Y, wt.%) of
primary products of propene metathesis
Temperature dependence of the yields (Y,
wt.%) of primary products of propene meta-
thesis (ethylene and 2-butenes) over: 5 wt.%
P–W/SiO (1), 5 wt.% Si–W/SiO (2), 5 wt.%
W/SiO2 (3) and 5 wt.% P–Mo/SiO2 (4) cata-
lysts. (F/W = 9.96 mmol/s kg)
(
ethylene and 2-butenes) at 773 K and F/W =
.98 mmol/s kg
2
2
4
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

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Hudec, Prandova:
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