Group 4 metal halides/alumina solid superacids: Relation between electronegativity, acid strength and catalytic properties

Article abstract of DOI:10.1039/a707395j

The catalytic properties of solid superacids obtained by the action of the vapour of Group 4 metal halides on alumina have been related to the electronegativities of the halides used. It was found that the acid strength of the Lewis superacid centres and the reaction pathways of low-temperature pentane isomerization depend on the electron acceptor ability of the metal halide species bonded to the alumina carrier.

Full text of DOI:10.1039/a707395j

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Group 4 metal halides/alumina solid superacids
Relation between electronegativity, acid strength and catalytic properties
Marek Marczewskia* and Wojciech Juszczykb
a W arsaw University of T echnology, Faculty of Chemistry, Koszykowa 75, 00-662 W arsaw,
Poland
b Institute of Physical Chemistry of the Polish Academy of Science, Kasprzaka 44/52, 01-224
W arsaw, Poland
The catalytic properties of solid superacids obtained by the action of the vapour of Group 4 metal halides on alumina have been
related to the electronegativities of the halides used. It was found that the acid strength of the Lewis superacid centres and the
reaction pathways of low-temperature pentane isomerization depend on the electron acceptor ability of the metal halide species
bonded to the alumina carrier.
Acids with acid strength higher than pure sulfuric acid (H \
be formed. However, the strength of such superacid sites
should depend on the electron-withdrawing ability of the
metal halide species bonded to the alumina surface. The
electron-withdrawing ability is determined by the electron-
acceptor properties of the metal halides which are, in turn,
related to their electronegativities. The aim of this work was
to study the inÑuence of the electronegativity of the Lewis
acid used to modify the alumina surface on the catalytic activ-
ity of the resulting superacid systems. The Group 4 metal
halides: ZrF , TiCl and HfCl , with electronegativities, cal-
0
[11.4) are classiÐed as superacids.1,2 This deÐnition is some-
times difficult to apply to solid systems whose physical
properties, e.g. colour, make impossible the use of indicators
for acid strength measurements. In such cases catalytic activ-
ity in low-temperature alkane transformation can be applied
as a criterion for the presence of superacid centres since only
these sites are able to convert such weak bases as alkanes into
reactive carbocations and then catalyse their isomerization at
room temperature.3
4
4
4
In spite of many fundamental studies of superacids,4 no
general rules have been found which relate the properties of
the species used in the solid superacid synthesis to the acid
strength and catalytic activity of the complex superacid
system formed. This is probably due to the existence of a
variety of surface centres responsible for the superacid proper-
ties of solids. The choice of superacids characterised by the
same type of active sites should facilitate the study of the
inÑuence of superacid centre structure on acidity and catalytic
activity. The family of superacids obtained by the adsorption/
reaction of metal halides on an oxide support would appear to
be suitable for such a study. We found that treating c-alumina
with the vapour of metal halides (MeX ), such as AlCl or
culated according to Sanderson,7 of 4.771, 4.221 and 4.157,
respectively, were chosen. The catalysts obtained were tested
for the presence of superacid sites in the n-pentane isomer-
ization reaction at 298 K.
Experimental
The synthesis of the superacid catalysts and the experimental
details of the IR and EPR measurements and catalytic activity
study have been described in detail elsewhere5,6 and thus will
only be described brieÑy here.
Aluminium oxide was prepared by calcination of aluminium
hydroxide at 823 K in a stream of air. The syntheses of the
superacids involved the following steps: alumina calcination
for 1 h at 773 K under reduced pressure (1.3] 10~2 Pa);
contact of the cooled sample with water vapour; calcination
for 2 h in vacuo at the desired temperature; contact of the
sample with TiCl , HfCl and ZrF (all Alfa Ventron
n
3
ZrCl produces a superacid system with Lewis acid centres.5,6
4
We postulated that the metal chlorides react with both surface
hydroxy groups and oxygen O2v electron-donor sites inÑu-
encing the charge density on the Al3` electron-deÐcient site of
aluminium oxide.
4
4
4
products) vapour at 373, 623 and 798 K, respectively, followed
by 1 h evacuation. The resulting catalyst was then transferred
to the reactor.
IR measurements of ammonia adsorption on alumina and
on the synthesised catalysts in situ in the IR cell were carried
out, according to the procedure described elsewhere,6 in the
frequency regions characteristic for vibrations of both surface
OH groups (3750È3500 cm~1) and adsorbed ammonia (1600È
1400 cm~1).8 The catalytic system ZrF /Al O was not exam-
The resulting surface Lewis superacid centre is able to
abstract hydride anion from n-pentane and then to catalyse its
isomerization at temperatures as low as 298 K. The other
4
2 3
ined since the IR cell used could not be heated to 798 K, the
product of reaction (1), HX is adsorbed by the MeX -
y
temperature needed for ZrF sublimation.
containing surface site, forming a BrÔnsted centre of low acid
4
EPR measurements of tetracyanoethylene (TCNE) adsorp-
strength:
tion were used to study the ability of the catalysts to donate
one electron.9 The procedure has been described in detail pre-
viously.6
The number of OH groups, determined thermogravimetri-
cally, on alumina calcined at 773, 873 and 973 K is equal to
0.53, 0.35 and 0.17 mmol g~1, respectively.
Isomerization of n-pentane was studied in a batch reactor
(volume 61 cm3, reactant pressure 24.4 kPa). The initial reac-
tion rate (i.e. the rate extrapolated to zero contact time) was
that can not catalyse n-alkane isomerization.5,6
One can assume that, for catalysts obtained by the action of
metal halides belonging to the same group of the periodic
table, on alumina, the same type of active Lewis centres would
J. Chem. Soc., Faraday T rans., 1998, 94(5), 719È722
719

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taken as a measure of the catalytic activity. Reaction products
were analysed by gas chromatography using a squalane capil-
lary column (30 m).
acid properties of the catalysts studied, as found previously for
the ZrCl /Al O system. More information on the structure of
4
2 3
the active centres can be obtained using alumina calcined at
di†erent temperatures. Surface dehydration, which results on
calcination, exposes Al3` cations of acid character and
electron-donor O2~ anions and leaves some isolated OH
groups.10
Results and Discussion
The reaction of Group 4 metal halides with c-alumina was
followed by IR, EPR and catalytic measurements. The IR
studies allowed us to observe the changes in alumina OH
band intensity upon reaction with the metal halides, EPR
examination of TCNE adsorption allowed us to evaluate the
concentration of electron-donor O2~ centres before and after
the reaction, while n-pentane isomerization was the test reac-
tion for the presence of superacid centres. The results obtained
(Table 1) indicate that the metal halides react with surface
hydroxy groups and one-electron donating centres of alumina,
completely using up these sites. Simultaneously, all the synthe-
sised systems were able to catalyse low-temperature n-pentane
isomerization indicating the formation of new superacid
centres not present on the unmodiÐed alumina surface. A
One can predict, on the basis of the theoretical studies of
c-Al O dehydration by Peri11 and Zamora and Cordoba,12
2
3
that, for alumina calcined at 873 and 973 K, isolated OH
groups prevail on the surface while OH groups located in
close proximity should characterise the alumina calcined at
lower temperatures, 773 or 798 K. Such surface centres react
with halide vapour. Some halides, e.g. ZrF , form polymeric
4
structures in the solid state but at elevated temperatures close
to their sublimation point they exist as monomolecular MeX
species13 which can react with alumina. Our catalyst prep-
aration procedure, i.e. a long evacuation period after the reac-
4
similar phenomenon was reported for the action of ZrCl on
4
alumina6 where the active superacid centres were identiÐed as
tion of MeX with alumina avoids physical adsorption and
strong electron-acceptor sites. It seems plausible that similar
active centres will be present in the other Group 4/alumina
superacids. IR measurements of ammonia adsorption on
TiCl - and HfCl -modiÐed alumina demonstrated the exis-
4
condensation of the reactants. Hence, the structures a and b
can be considered as superacid centres, formed according to
reaction (1), on alumina calcined at higher (873 and 973 K)
and lower (773 and 798 K) temperatures, respectively.
4
4
tence of two types of acid centres, i.e. BrÔnsted, which form
surface NH ` cations (1410 cm~1) and Lewis, which can
4
bond NH coordinatively (1610 cmv1).8 The acid strength of
3
these centres can be evaluated on the basis of ammonia
desorption at elevated temperatures followed by IR spectros-
copy. The results, presented in Fig. 1, indicate that Lewis acid
centres bond ammonia at higher temperatures than BrÔnsted
sites. Hence, they can be considered as the source of the super-
The method proposed by Sanderson7 can be used to evalu-
ate the acid strength of both structures. It is based on the
principle of equalisation of atom electronegativities within a
molecule and allows one to calculate an average charge on all
atoms forming a molecule of given chemical composition.
Such calculations have already been used to describe acidÈ
base properties of binary oxides14 and zeolites.15,16 The value
of the charge on hydrogen was used to evaluate the acidity of
zeolites with di†erent Al/Si ratio15 while the partial charge on
oxygen was adopted as a measure of the base strength of
alkali-metal exchanged X zeolites.16 If the hypothetical com-
pound:
Fig. 1 Ammonia adsorbed by Lewis [(K) TiCl /Al O , (=)
4
2 3
is taken as a simpliÐed model of the dehydrated surface of
alumina then sites with structures a and b can be obtained
HfCl /Al O ] and BrÔnsted [(L) TiCl /Al O , (…) HfCl /Al O ]
4
2
3
4
2
3
4
2 3
acid centres as a function of desorption temperature
Table 1 Physicochemical and catalytic properties of MeX /c-Al O superacids
x
2 3
one-electron
donor groups
/10~15 spin g~1
initial
reaction rate
/min~1 g~1
T
OH groups
/mmol g~1
Al3` charge
/e
calc.
catalyst
c-Al O
/K
structure
798
798
873
873
973
973
773
773
773
773
0.51
0.00
0.35
0.00
0.17
0.00
0.53
0.00
0.00a
0.00a
430
0
550
0
615
0
425
0
0.57
0.72
0.57
0.76
0.57
0.76
0.57
0.0000
0.0168
0.0000
0.0212
0.0000
0.0095
0.0000
0.0166
0.0017
0.0014
2
3
ZrF /c-Al O
b
a
a
4
c-Al O
2
2
3
3
3
3
ZrF /c-Al O
4
c-Al O
2
2
3
ZrF /c-Al O
4
c-Al O
2
2
3
ZrCl /c-Al O a
b
b
b
4
2 3
TiCl /c-Al O
0
0
0.65
0.64
4
2 3
HfCl /c-Al O
4
2 3
a Results taken from ref. 6. b Results from IR measurements.
720 J. Chem. Soc., Faraday T rans., 1998, V ol. 94

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from its reaction with MeX4. They are approximated by the
following formulae:
ences not only the rate of the reaction but also the reaction
pathway. The changes in reactant composition with time for
two model systems: ZrF /Al O (873) [the most acidic catalyst
4
2 3
(structure a)] and HfCl /Al O (773) [the less acidic one
4
2 3
(structure b)] are presented in Fig. 2. For both catalysts the
n-pentane skeletal isomerization product 2-methylbutane
results from the primary reaction. The strong superacid
centres of ZrF /Al O (873) catalyse methylpropane formation
4
2 3
in
a
consecutive reaction while the weaker sites of
catalyse methylpropane and 2-
HfCl /Al O (773)
4
2 3
methylbutane formation in parallel. We have already observed
suitable for calculation of the positive charge on the alu-
minium cation. The results of the calculation are presented in
Table 1. The calculated values of the positive charge located
on the aluminium cation can be treated as an indirect measure
of the strength of the Lewis superacid model centres. The
lowest acid strength has the Al3` of the model alumina
surface which is inactive in n-pentane isomerization. Reaction
with metal halides enhances the acid strength of the Al3`. The
sites with structure a are of higher acid strength than those
with structure b. The catalytic properties of the superacids
with structures a and b were examined in order to investigate
the inÑuence of acid strength on n-pentane isomerization. n-
Pentane reacts in the presence of all the studied catalysts
giving 2-methylbutane as the primary product and methyl-
propane as the secondary product. The initial rate of reaction
depends on the acid strength of the Lewis superacid centres
(see Table 1), those with higher acid strength having the
higher rate. The strength of the active superacid centres inÑu-
a similar phenomenon with alumina modiÐed with SbF ,
5
AlCl , BF and ZrCl .6,17h19 The strongest superacid,
3
3
4
SbF /c-Al O 17 catalysed methylpropane formation accord-
5
2 3
ing to pathway 1,3 in the reaction scheme below while the
weaker superacids6,18,19 follow pathway 2.
A similar phenomenon has been also discussed by Fabre20
for liquid protic superacids. Based on thermodynamic con-
siderations, Fabre explained the di†erences in the action of
strong and weak superacids on alkanes in terms of di†erent
initiation reaction mechanisms. A strong superacid such as the
HFÈSbF system is able to protonate alkanes thus catalysing
5
the isomerization while weaker acids such as CF SO H initi-
3
3
ate only alkane radical dimerization followed by decomposi-
tion.
For both model catalysts studied, ZrF /Al O (873) and
4
2 3
HfCl /Al O (773), the total n-pentane conversion did not
4
2 3
reach an equilibrium value close to unity, even after a long
reaction time. This is characteristic for decaying systems.21
Applying
a previously described procedure, based on
LangmuirÈHinshelwood formalism and time-on-stream (TOS)
theory of catalyst decay,21 to describe the reaction kinetics in
a batch reactor, one can obtain the general rate equation for
n-pentane uptake:17
A(1[ x)
(B ] Cx)(1] Gt)N
r \
(I)
where x and t denote n-pentane conversion and reaction time.
We found for both, i.e. the strongest (SbF /c-Al O ) and
5
2 3
weaker (ZrCl /c-Al O ) superacids17,22 that B is close to
4
2 3
unity and the Cx term being less than unity can be neglected.
Additionally, the rate equation for the ZrCl -containing
4
system is of zero order in respect to n-pentane in the applied
region of substrate pressure.22 Applying these simpliÐcations
to the ZrF /Al O (873) and HfCl /Al O (773) systems, one
4
2
3
4
2 3
can describe the rate of 2-methylbutane formation for the
ZrF /Al O (873) system as:
4
2 3
r
\ k (1[ x)(1] G t)~N [ k exp([G t)
(II)
MB
1
1D 3D
3
where r is the rate of 2-methylbutane formation and k ,
MB
, N and t denote the rate constant for the ith reaction
1
G
1D
step, the decay rate constant for the ith reaction step, the
decay constant and the reaction time, respectively. The second
term in eqn. (II) describes the rate of methylpropane consecu-
tive formation. The active centres responsible for this reaction
step undergo faster, exponential decay.23
Fig. 2 Calculated reactant and product vs. time evolution: pentane
(1), 2-methylbutane (2) and methylpropane (3) for A, ZrF /Al O (873)
4
2 3
and B, HfCl /Al O (773). Points represent experimental data. Batch
4
2 3
reactor, reaction temperature 298 K, pentane pressure 24.4 kPa, 1 g of
catalyst.
Table 2 Kinetic constants of n-pentane reactions
catalyst
k /min~1
k /min~1
G
/min~1
1D
G
/min~1
3D
N
1
3
ZrF /Al O (873)
0.022
0.0008
0.055
0.0550
0.0136
0.02
2
2
4
2 3
HfCl /Al O (773)
4
2 3
Batch reactor, 1 g of catalyst, temperature 298 K, initial pentane pressure 24.4 kPa.
J. Chem. Soc., Faraday T rans., 1998, V ol. 94
721

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strength acid sites obtained by the action of ZrF4 on alumina
pretreated at 873 K are able to catalyse the consecutive trans-
formation of 2-methylbutane which is not possible in the pres-
ence of the slower deactivating but initially less populated and
weaker centres of HfCl /Al O (773).
4
2 3
Conclusions
The catalyst synthesised by the reaction of Group 4 metal
halides vapour with c-alumina possesses Lewis superacid
active centres. There is a correlation between: (i) the halide
electronegativity and the acid strength of the superacid sites;
(ii) the acidity of the superacid sites and the initial rate of
n-pentane isomerization; (iii) the acid strength of the superacid
centres and the n-pentane reaction pathway; and (iv) the acid
strength and the deactivation rate. This correlation conÐrms
the simple model of active superacid centres formation
according to reaction (1) and allows us to predict how di†er-
ent modiÐers can inÑuence the strength, catalytic activity and
stability of Lewis superacid centres.
Fig. 3 Active centre decay for 1, ZrF /Al O (873) and 2,
4
2 3
HfCl /Al O (773)
4
2 3
For the HfCl /Al O the rate of formation of 2-
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4
2 3
methylbutane is given by:
\ k (1] G t)~N
1
2
R. J. Gillespie, Can. J. Chem. Educ., 1969, 4, 9.
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(III)
MB
1
1D
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4
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1D
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G
10 H. Kno
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1D
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2
3
4
2 3
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(IV)
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1D
are presented in Fig. 3.
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4
2 3
decay faster, probably owing to extensive coking. The initial
concentration of superacid centres formed according to reac-
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of electron-accepting sites surrounded by OH groups and
electron-donating centres. Flockhart et al.9,24 have shown
that on increasing alumina calcination temperature from 773
to 873 K the number of one-electron acceptor and donor sites
increases owing to surface dehydration. Hence, one can
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form to the more acidic a form. Thus, in spite of the higher
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Paper 7/07395J; Received 14th October, 1997
722
J. Chem. Soc., Faraday T rans., 1998, V ol. 94