Applied
Catalysis, 61 (1990)
27
27-34
Elsevier Science Publishers B.V., Amsterdam -
Printed in The Netherlands
Proton Acidity in Modified y-Alumina
W. KANIA* and K. JURCZYK
Faculty of Chemistry,
A. Mickiewicz
University,
60-780 Poznari (Poland)
(Received 11 August 1988, revised manuscript received 29 November 1989)
ABSTRACT
The proton acidity of binary oxide catalysts based on y-alumina has been studied. Fe,O,, Cr,O,,
NiO, Moo3 and MgO were used as modifying agents, introduced by coprecipitation in amounts
up to 7 wt.-%. The activity in cumene cracking was determined for all catalysts. y-Alumina and
binary oxides containing the largest amounts of modifying oxide were investigated by IR spectroscopy. Arylmethanol indicators were also used in order to determine the nature of the surface
acidity of white and slightly coloured samples. A strong increase in proton acidity was found on
the surface of chromia-alumina catalysts compared with the other oxides studied.
Keywords: Acidity, alumina, cumene cracking, catalyst characterization (IR, acidity),
iron(III)oxide-alumina, chromia-alumina, nickel oxide-alumina, molybdena-alumina, magnesia-alumina.
INTRODUCTION
Catalytic activity of oxides applied in conversions of hydrocarbons can be
attributed in many instances to the acidic properties of their surface [ 11. With
pure alumina, Lewis-type acidity is observed [ 2,3]. Its inactivity for the reaction of cumene cracking [ 41 indicates the absence of strong Brsnsted acidity
on its surface. The introduction of inorganic acids onto the surface of y-alumina by impregnation has been found to change the nature of its acidity drastically [ 51 and to lead to the appearance of activity for the reaction of cumene
cracking. The nature of the surface acidity of alumina can also be changed by
preparation under appropriate conditions [ 4,6]. In a previous paper [ 71, the
study of the surface acidity of binary oxide catalysts based on y-alumina modified by the introduction of small amounts of other oxides (up to 7 wt.- % ) was
described. However, the nature of the acid centres on the surface of these oxides was not determined. This investigation was therefore aimed at determining the catalytic activity of these samples in cumene cracking. This reaction
proceeds according to a carbonium ionic mechanism on strong Bronsted acid
sites [ 8,9]. The activity of particular samples in this reaction may be considered to be a measure of the concentration of strong proton centres on their
surface. In order to corroborate the results obtained, we studied the IR spectra
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0 1990 Elsevier Science Publishers B.V.
28
of alumina and some other samples mainly with the largest amounts of modifying oxide. Moreover, for white and slightly coloured catalysts, the nature of
the surface acidity was studied using the Hirschler method [lo].
EXPERIMENTAL
Binary oxides of y-alumina containing up to 7 wt.- % of modifying oxide (Table 1) were prepared as described previously [ 71, They differed only in the
amount and type of the modifying oxide.
The activity of the catalysts for cumene cracking was measured by the pulse
method using a Perkin-Elmer Model X4-0508 microreactor gas chromatographic accessory. A 0.1-g amount of a catalyst with a particle size distribution
of 0.25-0.50 mm was used. The catalyst, calcined for 6 h at 55O”C, was subjected to reduction for 3 h with hydrogen at 500°C just before the reaction.
The cracking reaction was carried out at 370°C and the products were analysed
by gas chromatography with flame ionization detection. The flow-rate of the
carrier gas (helium ) was 20 cm3/min. The products were separated at 70” C in
a l-m column packed with 30 wt.-% Emulphor 0 supported on Chromosorb
W. Catalytic activity was expressed in terms of the JzKproduct of the BassettHabgood equation [ 111. This equation was employed in the version modified
by Yushchenko and Antipina [ 121. Apparent rate constants are presented in
Table 1.
Alumina and in particular the samples with the highest amounts of modifying oxides were subjected to IR measurements. A thin wafer of the samples
was made and placed in a vacuum cell equipped with sodium chloride windows.
The cell was then evacuated and maintained at lop5 Torr (1 Torr = 133.3 Pa)
and 500” C for 6 h. Subsequently oxygen ( 100 Torr ) was admitted twice to the
vacuum cell and the catalyst wafer was treated with oxygen at 500°C for 1 h.
After evacuation of oxygen, the samples were reduced with hydrogen (100 Torr )
at 500°C for 3 h. The gas phase was then evacuated at 500°C for 3 h, the IR
cell cooled to room temperature and the baseline spectrum recorded using a
Perkin-Elmer Model 580 double-beam spectrophotometer. Ammonia was then
introduced into the cell and allowed to stand overnight. Subsequently, spectra
were recorded without the evacuation of ammonia at room temperature and
after a 4-h evacuation at lop5 Torr at 20, 100, 200, 300 and 500°C. Some of
these spectra are shown in Figs. 1 and 2. The strength of Bronsted acid sites
on white and slightly coloured samples was measured by Walling’s method
[ 131 in the presence of the following Hirschler indicators [lo] : diphenylmethan01 (p&+ = - 13,3 ) , triphenylmethanol (p&+ = - 6.6) and 4,4’ ,4” -trimethoxytriphenylmethanol (pKn+ = +0.82).
29
RESULTS
AND DISCUSSION
Most samples have an intense colour, which makes the application of arylmethanol indicators to the study of the nature of surface acidity almost impossible. The Hirschler indicators could therefore be used only for the examination of white or slightly coloured catalysts. Bronsted acid sites of moderate
strength (pK,+ = - 6.6) were found on two catalysts [ molybdena-alumina and
iron (III) oxide-alumina containing 1 wt.-% of iron (III) oxide]. Moreover, the
test with 4,4’,4”-trimethoxytriphenylmethanol
demonstrated the existence of
weak protonic acidity (pK,+ = 0.82 ) on alumina and magnesia-alumina samples, but no centres of moderate strength were found on their surfaces. The
other binary oxides in this study were intensely coloured.
We have already reported ammonia chemisorption data for these oxides [ 71,
but the nature of the surface acidity (i.e., whether it is of Bronsted or Lewis
type) has not been established. Therefore, IR spectra of alumina and mainly
the samples with the largest amount of modifying oxide were studied (Fig. 1) .
Under the pretreatment conditions, well separated absorption bands were observed for all samples in the stretching vibration region of hydroxyl groups,
1/l/
z
I
K
10
I
3600
1
I
3200
WAVENUMBER[cni’
Fig. 1. IR spectra after evacuation
breviations as in Table 1.
]
in hydrogen
at 5OO’C in the hydroxyl
stretching
region. Ab-
30
which indicates the presence of isolated OH groups. In the spectrum of alumina
only three bands originating from isolated OH groups were observed (3680,
3720 and 3780 cm-‘), which is in agreement with the results reported by other
workers [ 14,151. The same bands were found in the spectrum of sample A-Mg5, but the intensity of the bands at 3680 and 3780 cm-l was lower, whereas
that of the band at 3720 cm-l was higher. In addition to these three bands, a
broad band appeared at 3570 cm-l in the spectra of A-MO-~, A-Fe-l and AFe-7. Two new bands appeared in the spectrum of A-Cr-5, a broad band at 3570
cm-’ and a narrow band at 3760 cm-l, showing the presence of additional OH
groups on its surface. For A-MO-~, A-Fe-l, A-Fe-7 and A-Cr-5, the bands at
3680,372O and 3780 cm-’ had the same intensities as those of alumina.
Adsorption of ammonia on the examined samples at room temperature causes
a change in spectral pattern of the hydroxyl groups. The bands in the stretching region shift to lower frequencies and only one band originating from the
OH groups appears (3720 cm-l), which indicates the non-acidic character of
hydroxyl groups responsible for this band. For all catalysts (Fig. 2), the adsorption of ammonia results in the appearance of bands at 1260, 1400, 1470,
1620 and 1680 cm-l. The intensities and positions of the bands clearly depend
.
A-Ma-3
A-Fe-7
-
A-Fe-l
A-Cr.5
I
I
I
'600 1400
I
1200
WAVEbiIJ%ER[
cm-']
Fig. 2. IR spectra after adsorption of ammonia and evacuation at 200°C. Abbreviations as in Table
1.
31
on the type of modifying oxide and the temperature of ammonia adsorption.
In all instances we recorded absorption bands characteristic of ammonia bound
to Lewis acid sites (1620 and 1260 cm-l ) [ 161 and an absorption band at 1470
cm-‘, which is assigned to ammonia adsorbed as NH: bound via hydrogen
bridges. The latter band has a high intensity, but disappears quickly as the
evacuation temperature of the samples increased, which indicates a low stability of such bonding. After thermal treatment at a higher temperature
(3OO”C), the bands at 1620 and 1260 cm-l in the spectrum of A-Cr-5 disappear
almost completely, whereas for other catalysts only the intensities are reduced.
This indicates that the strength of Lewis acid centres of A-Cr-5 is lower then
that of other catalysts. All the catalysts investigated also absorb IR radiation
at 1400 cm-l, which suggests the presence of proton acidity [ 161. Although the
existence of proton acidity on y-alumina heated at 500’ C has been questioned
by some workers [ 2,3], in this study we observed a band originating from ammonium ion. The presence of a band at 1680 cm-’ (Fig. 2 ), characteristic of
the bending vibration of ammonia bound to Bronsted acid centres, additionally
proves the existence of proton-donor centres on alumina [ 171. The formation
of NH,+ on the surface of alumina was also detected by Peri [ 181 and Dunken
and Fink [ 17,191. Its concentration is significantly affected by the degree of
hydration, which depends, in turn, on the temperature of catalyst activation.
The IR spectra of molybdena-alumina and chromia-alumina samples correspond with literature data [ 20-231.
In order to obtain an insight into the strength of proton acid centres, we
measured the catalytic activity in cumene cracking. This reaction may proceed
according to two mechanisms giving different types of products: on catalysts
having strong acid centres of Brarnsted type [ 8,9] it proceeds via an intermediate carbonium ion, yielding benzene and propene, whereas when it proceeds
according to the free-radical mechanism it yields styrene, a-methylstyrene and
methane [ 241. It should be added that on free-radical cracking initiated on
dehydrogenation sites, more cr-methylstyrene is formed, whereas on thermally
initiated free-radical cracking, mainly styrene is produced. Hence the activity
of catalyst in conversion towards benzene and propene may be assumed to be
a measure the concentration of strong proton centres. As shown in Table 1,
alumina is almost inactive (K,,=0.35*10-‘1mol
g-l s-’ Pa-‘), which leads to
the conclusion that virtually no strong acids are present on its surface. Binary
oxides with molybdena as well as iron ( III ) oxide ( 1 wt.- % ) show a low cracking
activity, which supports the presence of a certain amount of Bronsted acid
centres, in accordance with the results of tests using arylmethanol indicators.
However, catalysts with larger amounts of iron (III) oxide show reduced activity in this reaction. Nickel oxide-alumina catalysts are only slightly active,
whereas magnesia-alumina samples are almost inactive. On the other hand,
chromia-alumina catalysts show the highest activity for cumene cracking. With
the latter catalysts, significant amounts of benzene and propene were formed,
32
TABLE 1
Catalytic activity in cumene cracking at 370-G
No.
Abbre-
Composition
viation
(wt.-%)
expressed as apparent rate constants,
“First slug” activity
Activity
kK
after ten pulses
kK (mol g-‘s-’
kK (mol m-2s-1
kK (mol g-‘s-l
kK (mol m-2s-1
Pa-‘X10”)
Pa-’
Pa-IX
Pa-IX
X 1013)
10”)
Al,03
AlSO3 + 1% Fe,O,
0.35
0.16
0.33
0.15
A-Fe-1
1.02
0.45
0.78
0.34
A-Fe-3
Al,O, + 3% Fe203
0.75
0.34
0.58
0.26
A-Fe-5
A1,0,+5%
0.56
0.25
0.48
0.22
A-Fe-7
Al,O,+
7% Fe,O,
0.46
0.20
0.25
0.11
A-0-1
Al,O,+
1% Cr,03
34.50
15.61
2.69
1.22
A-Cr-3
A1,03+3%
Cr,O,
64.62
26.16
5.68
2.30
Cr,O,
A
FezOs
8
A-G-5
A1203+5%
87.98
33.97
8.69
3.35
9
A-Ni-1
Al,O,+
1% NiO
0.95
0.43
0.88
0.39
10
A-Ni-3
A1,03 + 3% NiO
0.60
0.33
0.59
0.33
11
A-Ni-5
AlsO, + 5% NiO
0.79
0.39
0.67
0.33
12
A-MO-~
AI,O,+3%
Moo3
1.14
0.53
0.76
0.35
13
A-Mg-1
A1,03 + 1% MgO
0.32
0.14
0.28
0.12
14
A-Mg-3
A&O, +3%
MgO
0.36
0.15
0.31
0.13
15
A-Mg-5
A1,03+5%
MgO
0.27
0.11
0.24
0.10
lOIS)
indicating the presence of strong proton acids. The highest activity was observed for the samples with the largest amount of chromia, which suggests that
the number of strong centres of the Brranstedtype rises as the content of chromia
increases.
As we have demonstrated previously [ 71, chromia-alumina catalysts show a
higher ability than alumina to adsorb ammonia, so their total acidity is also
higher. The high activity for cumene cracking, or in other words the presence
of a large number of strong proton acid sites, indicate that this type of centre
contributes significantly to the total acidity. These results are in agreement
with the work of Paukshtis et al. [20], who also found strong proton acidity
for a commercial chromia-alumina catalyst. In the latter catalyst the concentration of Lewis acid centres is lower than that in y-alumina [ 201. Moreover,
as shown in Table 1, after ten O.l+l cumene pulses the activity of poorly active
catalysts decreases to a lesser extent than that of samples of high activity. A
large amount of coke is formed, which blocks active centres.
With chromia-modified samples, in addition to benzene andpropene, a large
amount of a-methylstyrene was formed as a result of the increase in the number of dehydrogenation centres which takes place in mixed catalysts of the
A1,03-Crz03 type [25]. The formation of a-methylstyrene probably results
from a free-radical mechanism of cumene decomposition initiated by dehydrogenation centres [24]. The temperature of the studied reaction is relatively
low (37O”C), so it can be assumed that thermally initiated free-radical crack-
33
ing does not contribute to the course of the process and the radical mechanism
can also be generated as a result of the contribution of free radicals present in
coke substance to the catalytic process [ 261. As far as other catalysts are concerned, the formation of Lu-methylstyrene was observed only on A-Fe-7 and ANi-5 and only during the first pulses and in small amounts.
Comparing the results of infrared measurement with the activity of the catalysts in cumene cracking, it should be added that the weak Brensted acidity
observed for magnesia-alumina and alumina (positive result of the test with
4,4’,4”-trimethoxytriphenylmethanol)
plays almost no role in cracking. Hence
these samples showed the lowest activity. Further, the spectrum in the range
of stretching vibrations of hydroxyl groups (Fig. 1) clearly indicates decreased
intensities of the bands at 3680 and 3780 cm-’ and increased intensity of the
band at 3720 cm-’ for sample A-Mg-5 in comparison with alumina. The first
two bands are assigned to the most acidic OH groups, so the concentration of
Bronsted acid centres on y-alumina (which are known for their low acid strength
[7,27] ] ) decreases after the introduction of magnesia.
The proton acid centres are slightly stronger on molybdena-alumina and
iron (III) oxide-alumina. With catalysts A-MO-~ and A-Fe-l, the presence of
Bronsted acidity of moderate strength was detected by using the triphenylmethanol indicator test. Moreover, IR spectroscopic measurements on both
samples demonstrated the presence of additional acidic hydroxyl groups. This
explains their higher activity in cumene cracking in comparison with alumina
and magnesia-alumina.
As has already been mentioned, chromia-alumina catalysts show the highest
activity. Hence, the additional hydroxyl groups detected on their surfaces by
IR spectroscopy are proved to be strong acid centres of the Bronsted type which
are active in catalysing cumene cracking. Such behaviour of chromia-alumina
catalysts seems to result from differences in electronegativity between cations
of both metals. According to literature data, under the conditions of the applied
hydrogen treatment, alumina present in the sample strongly stabilizes Cr3+
ions and prevents their reduction below stoichiometric Cr20, [ 281. The electronegativity of Cr3+ ions is higher than that of A13+ ions [29]. It is possible
that coordinatively unsaturated Cr3+ ions in addition to A13+ ions may also
chemisorb water in a dissociative way. As a result, on the chromia-alumina
catalysts additional OH groups may be formed which may be able to interact
as acid sites of the Bronsted type.
The remaining oxides used to modify y-alumina have a metal either of lower
valency (NiO, MgO) or easily undergoing strong reduction under the conditions of the applied hydrogen treatment (Fe203, Mo03) [29]. As a consequence, cations of lower electronegativity in comparison with AP+ ions may
be formed.
In conclusion, metal oxides added to y-alumina in small amounts have different influences on the nature of its acidity. Alumina has only weak acid centres
34
of the BrBnsted type, hence its acidity must be primarily attributed to strong
sites of the Lewis type. Iron (III) oxide (except for A-Fe-l), nickel oxide and
magnesia when added to y-alumina do not change the nature of its acidity to
any significant extent. These samples have only weak Brernsted acid centres
and consist mainly of Lewis acid centres of different strength. Bransted acidity
of moderate strength was found on A-Fe-l and A-MO-~ samples.
On the other hand, addition of chromia in small amounts (up to 5 wt.-% ) to
alumina causes a considerable change in the nature of its acidity. Chromiaalumina is characterized by strong Bronsted acidity. The concentration of these
sites rises as the amount of chromia increases. It also contains acid centres of
the Lewis type, but their strength decreases with increasing chromia content.
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