Heteropolyacid catalysts for hydroisomerization of n-hexane: Effects of alkali salt modification

Article abstract of DOI:10.1006/jcat.2001.3341

The isomerization of n-hexane over H₃PW₁₂O₄₀ and a series of its cesium, potassium, rubidium, and ammonium derivatives was studied and compared with H-mordenite. The bifunctional catalyst based on H₃PW₁₂O₄₀ was highly selective for n-hexane isomerization. The cracked products, propane, butanes and pentanes, and C₄ alkanes were formed. Little change in the distribution of the cracked products occurred as the conversion increased. The propensity for cracking of hexane into propane and n-butane increased over H-mordenite as compared with H₃PW₁₂O₄₀. The performance of the catalysts in n-hexane isomerization was represented by the yields of C₆ isomers and of dMeC₄, which have a high octane number. The formation of dibranched alkanes (dimethylbutanes) required to improve the fuel octane number, apparently not more effectively obtained over HPA than H-mordenite. The low reaction rate for the conversion of 2,3-dimethylbutane into 2,2-dimethylbutane accounted for the limited amounts of dibranched isomers.

Full text of DOI:10.1006/jcat.2001.3341

Journal of Catalysis 204, 157–162 (2001)
doi:10.1006/jcat.2001.3341, available online at http://www.idealibrary.com on
Heteropolyacid Catalysts for Hydroisomerization of n-Hexane:
Effects of Alkali Salt Modification
1
N. Essayem, Y. Ben Ta aˆ rit, P. Y. Gayraud, G. Sapaly, and C. Naccache
Institut de Recherches sur la Catalyse, CNRS, conventionn e´ UCB-Lyon, 2, Avenue Albert Einstein, 69626 Villeurbanne cedex, France
Received April 12, 2001; revised June 28, 2001; accepted June 28, 2001
tained at 523 K, 100 K higher than that over Pt/Al2O3, Cl.
1
2-Tungstophosphoric acid, H3PW12O40, and a series of its acid At 523 K the bifunctional (metal–acid) reaction mechanism
alkali salts associated with silica-supported platinum have been prevailed. The development of novel solid acid catalysts
used as catalysts in the hydroisomerization of n-hexane and com-
pared with platinum-supported, appropriately dealuminated mor-
denite. High selectivity toward isomerization was observed with
both the heteropolyacid (HPA)-type catalysts and the mordenite at
elevated conversions. The acid sites of the heteropolyacids/salts ex-
hibited an efficiency higher than that of mordenite, in line with
the higher acid strength of the former sites. Yet, despite their
higher acid strength, heteropolyacids/salts showed a lower selec-
tivity toward dimethylbutanes than mordenite. In addition, het-
such as heteropolyacids, sulfated zirconia, has initiated
additional investigations with the aim of developing new
promising catalysts for the isomerization of C5/C6 paraffins
(
1–6).
In the present study we have investigated the isomeriza-
tion of n-hexane over H3PW12O40 and a series of its cesium,
potassium, rubidium, and ammonium derivatives. Indeed,
it is now well known that the partial exchange of proton by
+
+
+
+
eropolyacids, though more active than mordenite at low temper- large monovalent cations such as K , NH , Cs , and Rb
4
ature, appeared to be only poor monofunctional isomerization may lead to solids with enhanced surface area and thus
catalysts. �c 2001 Academic Press
Key Words: hydroisomerization; acidity; heteropolyacids; alka-
nes; n-hexane.
enhanced superficial acidity. By using different exchange
levels and different cations one can monitor both textural
features and the superficial protonic densities. Therefore
one can expect different catalytic behaviors as already ob-
served (7, 8).
For comparison, H-mordenite was also included. In this
work emphasis was placed on activity measurements and
product distribution analysis. The bulk acid H3P was com-
pared with two types of porous salts (Table 1): a Cs salt
1
. INTRODUCTION
Heteropolyacids like H3PW12O40 (HPA) and their alkali
or ammonium salt derivatives show interesting properties
in acid-type catalyzed reactions. Among these reactions the
isomerization of C5, C6 linear alkanes is particularly impor-
tant in the refinery industry to increase the octane number
of the gasoline pool. Indeed, while the octane number of
n-hexane is only 29, those of 2,2-dimethylbutane and 2,3-
dimethylbutane are respectively 92 and 104. Since thermo-
dynamic data indicate that branched isomers are favored at
low temperature it is recommended that the isomerization
of n-C6 occurs at low temperature. However, the reaction
mechanism involves endothermic steps, and from the point
of view of kinetics, high reaction temperature is required.
Thus, because of the thermodynamic and kinetic limitations
only strong acid catalysts are efficient. Chlorinated alu-
mina containing platinum was found particularly efficient at
(
Cs2HPW12O40) characterized by a poor SBET but a high
protonic density and three salts with a higher exchange
level that exhibit an enhanced SBET and thus a lower
protonic density (K2.3H0.7PW12O40, Rb2.4H0.6PW12O40,
(
NH4)2.6H0.4PW12O40). The porosity of all these salts show-
ing only slight differences. Our aim was to produce a clear
picture of the isomerizing properties of HPA catalysts and
pinpoint the differences and analogies that may exist in
the various reaction steps occurring over HPA and H-
mordenite type catalysts.
2
. EXPERIMENTAL
4
23 K in the isomerization of C5/C6 paraffins, a monofunc-
2
.1. Catalysts
H3PW12O40 was prepared following the method de-
tional acid mechanism being proposed. By contrast over
Pt/H mordenite a significant isomerization rate was ob-
scribed in the literature (9). Cesium, rubidium, potassium,
or ammonium acid salts were obtained by reacting the HPA
sample in aqueous solution with metal chloride solution
1
To whom correspondence should be addressed. E-mail: essayem@
catalyse.univ-lyon1.fr. Fax: 33 4 72 44 53 99.
157
0021-9517/01 $35.00
Copyright �c 2001 by Academic Press
All rights of reproduction in any form reserved.

1
58
ESSAYEM ET AL.
TABLE 1
The zeolite-type bifunctional catalyst consisted of a
H-form mordenite (H-M) impregnated with a solution
of Pt salt (tetraammineplatinum chloride) to incorporate
Physicochemical Features of Alkali or Ammonium
Salts of H3PW12O40
0
.6 wt% Pt. H-M was partially dealuminated by hydrother-
Sample
H3P Cs2HP K2.3H0.7P Rb2.4H0.6P (NH4)2.6H0.4P
mal treatment and subsequent washing with 1 N HCl treat-
+
ment resulting in Si/Al = 10. Its H density is equal to 24 �
M/P mol. ratio
—
7
2
28
8.5
2.3
115
8.1
2.4
126
8.9
9.3
2.6
125
9.4
6.6
17
+
� 2
m .
2
� 1
10
H
SBET (m g
)
1
�
H MAS-NMR 10.5
+
17
+
� 2
m
[
H ] 10
H
20.8 20.8
12.3
2
.2. Isomerization Reaction
The reaction runs were performed using a microreactor
operating at atmospheric pressure and plug-flow mode. For
each reaction run 0.5 g of the composite catalyst was used.
Typical conditions for the reaction were: reaction temper-
ature 473–523 K, H2/n-C6 ratio 17.5, and total flow rate
ranging from 8.6 to 0.2 liters h . Prior to the reaction run
the composite catalyst was treated at 473 K in flowing H2
for a period of 2 h. The reactor effluents were analyzed with
a gas chromatograph equipped with a flame ionization de-
tector. The outlet hydrocarbon components were separated
by a CP-Sil 2CB fused silica capillary column.
containing the stoichiometric quantity of the desired cation
M . The resulting precipitate, Mx H3� x PW12O40, was cen-
trifuged and freeze-dried and the exact value of x was
+
obtained from chemical analysis of M and W. For the am-
�
1
monium salt, IR spectroscopy was applied to measure the
+
NH /P molar ratio using as reference spectrum that of
4
(
NH4)3PW12O40. Specific areas of the samples, determined
by N2 adsorption at 77 K, are given in Table 1. All het-
eropolysalts exhibit pore sizes ranging from 0.5 to 1.2 nm
with a high contribution of pores of 0.55 nm to the total
volume. At variance with their SBET, only small differences
exist in the porosity features of the salts investigated in
this study. Table 1 also summarizes two characteristics of
Preliminary experiments have shown that within 4 h time
on stream, during which WHSV has been changed to vary
the conversion of n-C6, the catalytic activity of the samples
remained unchanged.
1
the catalysts’ acidity: their H MAS-NMR chemical shift,
measured after treatment of the samples for 2 h at 473 K,
and their superficial protonic density deduced from their
H/P molar ratio and their BET surface area. The protonic
densities were calculated according to a model previously
3. RESULTS AND DISCUSSION
The results of the reaction of n-hexane at 498 K over
described where the porous acidic salt M3� x Hx PW12O40 is the composite catalyst Pt–H3P are given in Table 2. The n-
assumed to correspond to H3PW12O40 phase dispersed on hexane conversion was changed by varying the feed space
the neutral Cs3PW12O40 salt and the surface occupied by velocity. Table 2 lists the selectivities toward isomerization
2
each heteropolyanion is taken to be equal to 1.44 nm (7, 8). and cracking as well as the product distribution at increas-
The bifunctional heteropolyacid catalysts were prepared ingn-C6 conversion. It appearsthat the bifunctionalcatalyst
by grinding an equal quantity of HPA and Pt/SiO2 (Pt based on H3PW12O40 is highly selective for n-C6 isomeriza-
content: 1 wt% ). To obtain close contact between both tion. The cracked products consisted of propane, butanes,
components of the composite catalysts, a long grinding and pentanes, C4 alkanes predominantly being formed. In
time was allowed. The catalysts that were prepared are de- addition Table 2 shows that little change in the distribu-
noted Pt–Cs2HP, Pt–H3P, Pt–Rb2.4H0.6P, Pt–K2.3H0.7P, Pt– tion of the cracked products occurred as the conversion in-
(
NH4)2.6H0.4P.
creased. These observations suggest that C3, C4, C5 alkanes
TABLE 2
n-C6 Reaction over Bifunctional Pt–H3P a
Distribution of the cracked products
C6 isomer distribution
WHSV Conversion Selectivity
Selectivity
isomerization
�
1
(
h
)
(% )
cracking
C1
C2
C3
iC4
C4
iC5
C5
2,2-DMC4
2,3-DMC4
2-MC5
3-MC5
8
.9
.5
.2
.1
.6
.3
16
26
39
52
67
75
1.3
1.3
1.5
2.1
4.1
6.9
—
—
—
0.8 18.7 32.8 11.6 17.9 18.2
0.6 19.0 33.3 11.4 19.7 16.1
0.7 19.1 34.2 10.8 21.6 13.6
98.7
98.7
98.5
97.9
95.9
93.1
1.2
1.6
2.4
4.1
7.7
9.0
10.0
11.0
11.9
12.1
12.3
54.6
54.1
53.3
51.9
49.5
46.9
35.2
34.3
33.9
32.2
30.7
28.7
4
2
1
0
0
0.2 0.7 18.7 35.2 10.6 23.2 11.3
0.2 0.6 19.1 36.6
0.2 0.6 19.1 37.3 10.0 23.7
9.9 23.7
9.8
9.2
12.1
a
TR = 498 K; PnC6 = 41 Torr; H2/nC6 = 17.5; catalyst weight 0.2 g H3P + 0.2 g Pt/SiO2.

HYDROISOMERIZATION OVER HETEROPOLYACIDS
159
+
resulted from primary cracking reaction of large C carbe-
TABLE 4
n
+
nium ions with n > 6. The direct �-scission of n-C6H1 or
i-C6H11 carbenium ionsisnot favorable since it involvessec-
1
nC Conversion over Bifunctional Pt–H P and Its Monovalent
6
3
+
a
Acidic Salts Pt–M H
x
3� x
P
ondary → primary or tertiary → primary carbenium ions.
Hence before desorption or crackingofthe C6 species, a sec- Catalyst
(NH4)2.6P-1 K2.3P-1 Rb2.4P-1 H3P Cs2HP Mordenite
27 20 16 16
+
Conversion (% )
5
3
ond C6 molecule can add to i-C6H11 to form a dimer which
will crack more effectively into predominantly isobutane.
The C6 branched isomers formed at low conversion were
predominantly 2-methylpentane (2-MeC5), 3-methylpen-
tane (3-MeC5), and 2,3-dimethylbutane (2,3-dMeC4). By
a
TR = 498 K; PnC6 = 41 Torr; H2/nC6 = 17.5; WHSV = 8.9 h� 1
.
Even at high n-C conversion (75% ), the 2,2-dMeC /2-
6
4
contrast, the 2,2-dimethylbutane containingquaternarycar- MeC ratio obtained was only 0.26 compared with 1.07
5
bonswasformed in a significant amount onlyat high conver- for the thermodynamic ratio. Dibranched C isomers for-
6
sion, the amount always being far below that expected from mation involves ring opening of the methylcyclopropyl
thermodynamics. Skeletal rearrangement of n-hexane pro- ion. Such ring opening generates either tertiary 2,3-
ceeds via a protonated cyclopropane carbenium ion inter- dimethylbutyl ion and hence 2,3-dMeC and/or secondary
4
mediate. Ring opening of the CPC intermediate results in 2,2-dimethylbutyl ion, which gives 2,2-dMeC . Since ther-
4
2
-methylpentyl and 3-methylpentyl species. As the branch- modynamically tertiary carbenium ion is more favorable
ing of the molecule at the center of the hydrocarbon chain than less stable secondary carbenium ion, 2,2-dMeC is ki-
4
is slightly kinetically more favorable, the ratio 3-MeC5/2- netically less favorable than 2,3-dMeC . At low n-C con-
4
6
MeC5 is slightly larger than expected from thermodynam- version Table 2 shows that the proportion of 2,2-dMeC4
ics although isomerization via 1,2-methyl shift may occur is considerably less than that of 2,3-dMeC , in agreement
4
simultaneously. At 16% n-hexane conversion the experi- with the low rate of 2,2-dMeC formation. As the residence
4
mental value of 3-MeC5/2-MeC5 is 0.64 compared with the time increased the conversion of n-C increased and con-
6
thermodynamic ratio of 0.67. Table 2 shows that within sequently that of 2,3-dMeC , approaching its thermody-
4
monomethylpentane isomers the distribution between 2- namic value. The isomerization reaction, which transforms
and 3-MeC5 did not vary much with the conversion, which 2,3-dMeC into 2,2-dMeC by methyl shift, is slow since it
4
4
suggests that 2-MeC5 and 3-MeC5 are both primary isomer- converts tert-carbenium ion into sec-carbenium ion. Hence
ization products.
2,2-dMeC remains below the thermodynamic value even
The formation of dibranched C6 isomers, 2,3- at high n-C conversion.
4
6
dimethylbutane (2,3-dMeC4) and 2,2-dimethybutane (2,2-
3 12 40
Apparently the acidity of H PW O is not strong
dMeC4), requires two successive “protonated cyclo- enough so that appropriate residence times of the carbe-
propane” (PCP) events. As a result the approach to the nium ion intermediates could be allowed in multistep iso-
thermodynamic value of dimethylbutanes depends on merization before desorption. In conclusion, the kinetic
the n-hexane conversion. Table 2 shows that the ratio of barrier that limits the isomerization of n-hexane into 2,2-
2
,3-dMeC4/2-MeC5 increased from 0.16 to 0.26 as n-C6 dimethylbutane is not overcome over H PW O ; the con-
3 12 40
conversion increased from 16 to 75% , at thermodynamic centration of the desired dimethylbutane isomers remains
equilibrium this ratio is 0.37. One can conclude that at high below the thermodynamic equilibrium value.
n-C6 conversion the concentrations of 2-MeC5, 3MeC5, and
,3-dMeC4 are close to their thermodynamic values. How- mordenite are summarized in Table 3. At 498 K and 42%
ever, that of 2,2-dMeC4 is far below its thermodynamic conversion the isomerization selectivity was high, 97% ,
The results of n-hexane reaction over bifunctional Pt–H
2
value.
3 12 40
similarly to the behavior of H PW O . However, the
TABLE 3
nC6 Reaction over 0.6% Pt/Mordenitea
Distribution of the cracked products
iC4 iC5
C6 isomer distribution
WHSV Conversion Selectivity
Selectivity
isomerization
�
1
(
h
)
(% )
cracking
C1
C2
C3
C4
C5
2,2-DMC4
2,3-DMC4
2-MC5
3-MC5
4
.5
.2
.1
.6
.3
5
10
18
29
42
0.8
1.0
1.3
1.8
2.8
—
—
4.3 41.6 18.8 11.9 17.3 6.2
4.3 43.5 18.5 13.2 14.9 5.6
99.2
99.0
98.7
98.2
97.2
3.0
3.5
3.8
4.7
6.5
12.7
12.9
13.2
13.1
13.4
52.3
51.9
51.6
51.0
49.6
32.0
31.7
31.5
31.2
30.5
2
1
0
0
0.6 3.0 43.3 18.1 14.5 14.6 5.8
0.6 3.1 42.2 18.2 15.4 14.4 6.0
0.8 3.4 38.8 18.1 16.4 15.0 7.2
a
TR = 498 K; PnC6 = 41 Torr; H2/nC6 = 17.5; catalyst weight = 0.2 g.

1
60
ESSAYEM ET AL.
TABLE 5
nC6 Reaction over Bifunctional Pt–Cs2HPa
Distribution of the cracked products
C6 isomer distribution
WHSV Conversion Selectivity
�
1
(
h
)
(% )
cracking
C1
C2
C3
iC4
C4
iC5
C5
Isomerization
2,2-DMC4
2,3-DMC4
2-MC5
3-MC5
8
1
0
.9
.1
.3
5
23
48
4.3
5.1
6.5
0.5 0.5 17.7 30.2 17.0 13.6 20.5
0.2 0.5 17.6 31.4 17.3 15.4 17.6
0.2 0.3 15.9 34.6 15.7 19.4 13.8
95.7
94.9
93.5
0.4
1.7
6.0
4.0
7.3
9.8
56.8
55.6
51.9
38.8
35.1
32.3
a
TR = 498 K; PrC6 = 41 Torr; H2/nC6 = 17.5; catalyst weight 0.2 g Cs2HP + 0.2 g Pt/SiO2.
distribution of the cracking products revealed some differ- statement:
ence. While over Pt–H3P, predominantly C4 alkanes were
1
1
. H MAS NMR chemical shift as large than 10.5 ppm
formed, over Pt/H mordenite C3 was preferentially formed;
this is particularly true at low conversion where one should
expect that almost only primary cracking is occurring. In
addition the ratio i-C4/C4 was close to 1 over H-mordenite
and 3.5 over H3PW12O40. Therefore, cracking reactions in-
volve n-hexane dimers which are less branched when the
catalyst was H-mordenite; multibranched oligomers which
contain quaternary carbons favor the formation of i-C4 by
was measured on H3PW12O40 treated at 473 K (10).
. As recently demonstrated by IR spectroscopy, 12-
phosphotungtic acid and its porous salts were able to pro-
tonate water molecules, which can be considered as a weak
base (11).
2
3
. NH3 TPD experiments showed the lack of NH3 re-
lease at temperature below 773 K (13).
�
-scission.
However, the performance of the catalysts in n-hexane
On H-mordenite, the dimerization reaction of C6 carbe- isomerization is represented not only by the yields of total
nium ions occurs within the narrow zeolite channels. The C6 isomers but particularly by the yields of dMeC4 iso-
steric constraint imposed on the oligomer prevents the for- mers, which have a high octane number. Table 3 shows the
mation of multibranched C12 carbenium ions, thus limiting distribution of C6 isomers at increasing conversion over
the cracking into iC4. By contrast, over H3PW12O40, the re- H-mordenite. It appears that at high conversion the iso-
action occurs at the grain external surface; multibranched mer distribution ressembles that found over H3PW12O40. In
C12 intermediates form easily, favoring the cracking into Fig. 1 the dMeC4/MeC5 ratios are plotted versus n-hexane
i-C4.
conversion for various composite HPA catalysts and for
In conclusion, the propensity for cracking of hexane into H-mordenite. At a conversion of n-hexane less than 40% ,
propane and n-butane increases over H-mordenite as com- the yield of dimethylbutanes, for a given conversion, ap-
pared with H3PW12O40.
parently was slightly higher over H-mordenite. However,
Tables 2 and 3 give the conversion of n-hexane at 498 K as n-C6 conversion increased to reach the expected ther-
over Pt–H3P and Pt–H-mordenite at identical WHSV. The modynamic value the ratio tended to 0.40 for all catalysts
activity of H3PW12O40 catalyst was about four times higher which is considerably less than that expected from ther-
than that of H-mordenite. Since the isomerization selec- modynamics. Although clear indications were given in the
tivity was comparable over the two catalysts it is clear literature that H3PW12O40 is more acidic than H-mordenite
that the yield of C6 isomers will be four times higher (10–12), apparently the acidity of H3PW12O40 is not enough
over H3PW12O40. More likely, the catalytic differences be- to increase substantially the mean lifetime of the carbenium
tween these two bifunctional catalysts should be ascribed ion intermediates, as multiisomerization events can occur
to the highest acid strength of the heteropolyacid. Several before desorption, such multiisomerization events being re-
spectroscopic data and other experiments argue for this quired to approach the equilibrium value of dMeC4/MeC5.
TABLE 6
nC6 Reaction over Bifunctional Pt–K2,3H0.7P a
Distribution of the cracked products
iC4 iC5
0.4 14.9 38.4 8.2 26.3 11.7
C6 isomer distribution
WHSV Conversion Selectivity
�
1
(
h
)
(% )
cracking
C1
—
C2
C3
C4
C5
Isomerization
2,2-DMC4
2,3-DMC4
2-MC5
3-MC5
8
2
0
.9
.2
.3
20
45
79
1.7
2.9
14.2
98.3
97.1
85.8
2.0
3.9
14.8
11.6
12.4
12.5
53.3
51.9
45.0
33.1
31.8
27.7
0.1 0.3 16.1 40.0 7.1 28.1
0.1 0.3 17.8 41.6 7.2 25.0
8.2
8.0
a
TR = 498 K; PrC6 = 41 Torr; H2/nC6 = 17.5; catalyst weight 0.2 g K2.3H0.7P + 0.2 g Pt/SiO2.

HYDROISOMERIZATION OVER HETEROPOLYACIDS
161
TABLE 7
nC6 Reaction over Bifunctional Pt–Rb2.4H0.6HP a
Distribution of the cracked products
C6 isomer distribution
WHSV
Conversion
(% )
Selectivity
cracking
�
1
(
h
)
C1
C2
C3
iC4
C4
iC5
C5
Isomerization
2,2-DMC4
2,3-DMC4
2-MC5
3-MC5
8
2
0
.9
.2
.3
17
44
79
1.2
2.0
9.3
—
0.2
0.1
0.6
0.5
0.3
18.7
18.9
20.3
38.2
39.5
40.9
6.0
5.7
6.0
31.0
30.4
24.5
5.4
4.9
7.8
98.8
98.0
90.7
2.9
4.5
15.2
11.9
12.7
12.5
52.7
51.2
44.8
32.6
31.6
27.5
a
TR = 498 K; PrC6 = 41 Torr; H2/nC6 = 17.5; catalyst weight 0.2 g Rb2.4H0.6P + 0.2 g Pt/SiO2.
Nevertheless H3PW12O40, although it has a low surface area
First, it is important to recall that Cs2HPW12O40 salt, with
resulting in low accessibility of the acid sites to the reactant, this precise composition, was observed to be more active
showed considerably higher activity than H-mordenite. It than the bulk acid for n-C4 isomerization, a reaction that
is clear that the isomerization reaction could be carried requires a high density of strong acid sites (7, 8, 13). The
out at lower temperature than over H-mordenite, which highest activity of Cs2HPW12O40 was simply ascribed to
is thermodynamically better for reaching a high yield of its highest superficial acidity with respect to the acid H3P.
dimethylbutanes. Moreover, at laboratory scale, under the On the other hand, the almost totally exchanged porous
experimental conditions employed, the catalytic stabilities salts exhibit only poor activity in n-C4 isomerization due
ofH-mordenite and HPA were similar. Within 4h on stream to their low protonic density compared with bulk H3P and
the loss of activity was less than 2–5% for both samples.
The present results clearly demonstrate that heteropoly-
Cs2HPW12O40 (7, 8, 13).
Partial exchange of H3PW12O40 with Cs ions, to form
acid catalysts are better candidates for isomerization of Cs2HPW12O40, has no beneficial effects either on activ-
n-hexane, not in terms of their better selectivity to form ity or on the yield of dimethylbutane. By contrast, par-
+
+
+
dimethylbutanes or their stability, but rather in terms tial exchange with K , Rb , and NH to reach a com-
4
of their efficiency even at relativity low temperature of position near M2.5H0.5P considerably increased not only
reaction.
the specific surface area of the solids but also the catalytic
As the number of acid sites accessible to n-hexane is activity for n-hexane isomerization (Table 4), the highest
+
small on bulk H3PW12O40, partially exchanged porous promoting effect being obtained with NH . Apparently, a
4
salts have been used for n-C6 isomerization, namely, high protonic density does not favor the activity of HPA
Cs2HPW12O40, K2.3H0.7PW12O40, Rb2.4H0.6PW12O40, and in n-C6 isomerization (Table 4). However, this observa-
(
NH4)2.6H0.4PW12O40. The first sample is characterized by tion is not coherent with the activity level of bulk H3P.
2
� 1
a low BET surface area, 28 m g , and a protonic den- That is why it is believed that a dispersion of H3P over
sity equivalent to that of the bulk acid (Table 1). This Cs Pt/SiO2 might have occurred through the mechanical mix-
salt corresponds to more than one-monolayer coverage of ing applied to obtain the composite bifunctional catalyst.
H3P over the Cs3P, assuming that these samples correspond Indeed it has previously been shown that dispersed H3P
to nonacidic high-surface-area salt Cs3PW12O40 (or K, Rb, over neutral Cs3P can be prepared by grinding (14). The
NH4 ) over which highly dispersed H3PW12O40 is supported isomerization selectivity and the isomer product distribu-
(
7–13). The latter three samples, with a higher exchange tion at identical n-C6 conversion were close to those ob-
level, exhibit a higher BET surface area and therefore a served with pure H3PW12O40 catalyst (Table 5 and Fig. 1).
lower protonic density (Table 1). However, all these salts In Tables 5–8 are reported the product distributions for
exhibit quite equivalent porosity; they are mainly microp- the cracking reaction as well as the C6 isomer distribution.
orous with pore apertures centered at 0.55 nm.
One can observe that Cs2HP salt shows a slightly higher
TABLE 8
nC6 Reaction over Bifunctional Pt–(NH4)2.6H0.4P a
Distribution of the cracked products
C6 isomer distribution
WHSV
Conversion
(% )
Selectivity
cracking
�
1
(
h
)
C1
C2
C3
iC4
C4
iC5
C5
Isomerization
2,2-DMC4
2,3-DMC4
2-MC5
3-MC5
8
2
0
.9
.2
.3
27
61
82
0.9
2.2
12.4
0.5
0.2
0.1
1.0
0.5
0.3
17.8
19.0
20.5
37.0
39.0
41.1
6.5
5.9
6.2
31.9
30.5
24.4
5.3
4.9
99.1
97.8
87.6
2.0
4.7
17.3
11.5
13.0
11.9
53.5
50.9
44.0
33.0
31.4
26.8
a
TR = 498 K; PnC6 = 41 Torr; H2/nC6 = 17.5; catalyst weight 0.2 g (NH4)2.6H0.4HP + 0.2 g Pt/SiO2.

1
62
ESSAYEM ET AL.
+
face area and, consequently, enhanced H dispersion. The
HPA-based catalysts apparently are not active enough to
be employed for monofunctional acid-catalyzed isomeriza-
tion of n-hexane. Addition of a dehydrogenating function,
Pt for example, is still needed to obtain significant conver-
sion levels at relatively low temperature. As a result the
isomer product distributions depend mainly on the conver-
sion level at a given reaction temperature. The formation
of dibranched alkanes, i.e., dimethylbutanes, required to
improve the fuel octane number, apparently is not more ef-
fectively obtained over HPA than over H-mordenite. The
reason is that the formation of 2,2-dimethylbutane which
contains a quartenary carbon is kinetically limited even
over HPA. The low reaction rate for the conversion of
2
,3-dimethylbutane into 2,2-dimethylbutane accounts for
FIGURE 1
the limited amounts of dibranched isomers. On this aspect
heteropolyacids and H-mordenite behave similarly. Finally
although there are some advantages, higher activity, in car-
rying out the isomerization of C5/C6 alkanes over het-
eropolyacid catalysts, one needs to involve bifunctional
mechanisms to obtain subtantial isomer yields, and as such
the benefit and/or interest with respect to chlorinated alu-
mina remain questionable.
selectivity to cracked products than the other salts, caused,
more likely, by its higher protonic density. The compari-
son with the results observed on pure H3PW12O40 indicates
clearly that little difference exists when the comparison is
made at the same hexane conversion. However, one can ob-
serve a slightlyhigher formation ofdibranched alkanesover
the exchanged monovalent salts Rb2.4, (NH4)2.6, and K2.3.
We do not believe that the acidic features of the salts are at
the origin of this result since spectroscopic data are rather
in favor of sites of higher strength over the bulk acid. It
is more reasonable to refer to the microporosity of these
salts which can favor the formation of dibranched alkanes,
which are secondary products. This reinforces the idea that
the reaction occurs over quite identical acid sites whatever
the nature of the exchange.
REFERENCES
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(
1994).
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2000).
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3
(
4
5. Vaudagna, S. R., Comelli, R. A., Canavese, S. A., and Figoli, N. S.,
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CONCLUSIONS
6
7
Consistent with other studies the results presented here
show that the protons in heteropolyacid bifunctional cata-
lysts are more efficient that those of zeolites such as mor-
denite probably because they are more acidic but also
because the reaction is less affected by diffusion since, on
heteropolyacid materials only, nonconstraint surface pro-
1
8. Guisnet, M., Bichon, Ph., Gnep, N. S., and Essayem, N., Top. Catal.
11/12, 247 (2000).
9
. Pope, M. T., “Heteropoly- and Isopolyoxometallates,” Inorganic
Chemistry Concepts, Vol. 8, Springer-Verlag, Berlin, 1983.
10. Essayem, N., Tong, Y. Y., Jobic, H., and V e´ drine, J. C., Appl. Catal. A:
General 194/195, 109 (2000).
tons intervene in the reaction. During n-hexane conversion 11. Essayem, N., Holmqvist, A., Gayraud, P. Y., V e´ drine, J. C., and Ben
the selectivity toward isomerization is similar when the re-
action is carried out over heteropolyacid catalysts and over
T aˆ arit, Y., J. Catal. 197, 273 (2001).
2. Essayem, N., Frety, R., Coudurier, G., and Vedrine, J. C., J. Chem. Soc.
Faraday Trans. 93, 3243 (1997).
3. Essayem, N., Coudurier, G., Fournier, M., and V e´ drine, J. C., Catal.
Lett. 34, 223 (1995).
1
1
1
+
H-mordenite. The partial exchange of H with cations such
+
+
+
as K , Rb , and NH to form porous salts near the com-
4
position M2.5H0.5PW12O40 has some beneficial effect on
the activity due more likely to their enhanced specific sur-
4. Gayraud, P. Y., Essayem, N., and V e´ drine, J. C., Catal. Lett. 56, 35
(1998).