Applied Catalysis A: General 170 (1998) 285±296
Hydrogenation of aromatics on modi®ed
platinum±alumina catalysts
Abdel-Ghaffar A. Alia,*, Laila I. Alia, S.M. Aboul-Fotouha,
Ahmed K. Aboul-Gheit1,b
a
Chemistry Department, Faculty of Education, Roxy, Cairo, Egypt
b
Egyptian Petroleum Research Institute, Nasr City, PO Box 9540, Cairo 11787, Egypt
Received 10 November 1997; received in revised form 29 January 1998; accepted 30 January 1998
Abstract
Using a pulsed-micro catalytic reactor benzene and toluene were hydrogenated to cyclohexane and methylcyclohexane,
respectively, in a temperature range 50±2508C. Modi®ed catalysts containing 0.35% Pt±Al2O3 were investigated. Modi®cation
was carried out via (a) introducing a second metal (Ir, Rh, Re and U) and (b) ¯uorination and chlorination with different
halogen contents of 1,3 and 6 wt%. The study revealed that (1) all catalysts show good catalytic activities in the temperature
range 125±1508C, except Rh catalyst which is very active even at room temperature, (2) introducing either Ir or Re enhances
Pt activity, while U inhibits this activity, (3) halogenation promotes the catalyst activity and (4) alkyl-substitution enhanced
aromatic ring hydrogenation. # 1998 Elsevier Science B.V. All rights reserved.
Keywords: Benzene; Toluene; Hydrogenation; Pt±Al2O3 catalyst
1. Introduction over ruthenium, technetium and rhenium catalysts
[3]. Benzene ring cannot be hydrogenated in the
Hydrogenation of benzene to cyclohexane is an presence of rhenium up to 2508C, since above this
important reaction from the industrial point of view. temperature it decomposes to methane.
Cyclohexane is used in the production of caprolactam Hydrogenation of adsorbed toluene has many fea-
and as a starting material for nylon synthesis. Cyclo- tures in common with the hydrogenation of adsorbed
hexane is the only reaction product and is mainly benzene [4]. Lindfors and Salmi [5] found that methyl
obtained by platinum and palladium catalysts in the cyclohexane was the only product detected through
temperature range 300±3508C and under 1.01± toluene hydrogenation, partially hydrogenated inter-
2.02 MPa hydrogen pressure [1±3]. Above 130± mediates are not formed.
2008C normal paraf®ns of C1±C6 are also formed The adsorption of benzene and toluene on
metallic surfaces has been assumed to occur via
*Corresponding author. the interaction of -electrons of the aromatic ring
1
Fax: (202) 2747433. with d-orbitals of the metal [6±8]. Thus, these
0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved.
PII S0926-860X(98)00058-1
286 A.-G.A. Ali et al. / Applied Catalysis A: General 170 (1998) 285±296
molecules are adsorbed with the aromatic ring parallel Table 1
to the surface [9,10]. Dispersion values of the metal supported on alumina
The present work aims to promote a 0.35 Pt/Al2O3 No. Catalysts Metal dispersion
catalyst to improve its aromatics hydrogenation
Monometallic (H/M)
activity. 1 0.35% Pt/Al2O3 0.70
2 0.35% Pt (3.0% Cl)/Al2O3 0.80
3 0.35% Pt (3.0% F)/Al2O3 0.86
2. Experimental 4 0.35% Ir/Al2O3 0.68
5 0.35% Ir (3.0% Cl)/Al2O3 0.78
6 0.35% Ir (3.0% F)/Al2O3 0.78
2.1. Catalyst preparation 7 0.35% Rh/Al2O3 0.67
8 0.35% Rh(3.0% Cl)/Al2O3 0.79
10 g of Al2O3 produced by ``Rhone Poulenc- 9 0.35% Rh(3.0% F)/Al2O3 0.72
Chimie Fine'' with the following speci®cations: white Bimetallic (H/Ms)
extrudates (2.53 to 5 mm); surface area 216 m2/g, 1 0.35% Pt±0.35% Ir/Al2O3 0.65
grain density 1.24 g/cm2, structural density 3.0 g/cm3 2 0.35% Pt±0.35% Ir(3.0% Cl)/Al2O3 0.58
and total pore volume 0.47 cm3/g, were impregnated 3 0.35% Pt±0.35% Ir (3.0% F)/Al2O3 0.75
4 0.35% Pt±0.35% Rh/Al2O3 0.66
in an aqueous solution of hexachloroplatinic acid, 5 0.35% Pt±0.35% Rh (3.0% Cl)/Al2O3 0.62
0.019 M, such as to obtain a catalyst containing 6 0.35% Pt±0.35% Rh (3.0% F)/Al2O3 0.58
0.35% Pt/Al2O3. Citric acid was added to the platinum 7 0.35% Pt±0.35% Re/Al2O3 0.64
precursor solution to improve the penetration and 8 0.35% Pt±0.35% Re (3.0% Cl)/Al2O3 0.68
dispersion of Pt into the catalyst pores [11,12]. After 9 0.35% Pt±0.35% Re (3.0% F)/Al2O3 0.72
10 0.35% Pt±0.35% U/Al2O3 0.62
drying the catalyst was calcined in air at 5308C for 4 h. 11 0.35% Pt±0.35% U (3.0% Cl)/Al2O3 0.69
Reduction of the catalyst was then carried out in a ¯ow 12 0.35% Pt±0.35% U (3.0% F)/Al2O3 0.65
of dry hydrogen (20 cm3/min) for 8 h in situ at 5008C.
The same procedure was repeated for the preparation
of 0.35 Ir/Al2O3, 0.35 Rh/Al2O3, 0.35 Re/Al2O3 and
0.35 U/Al2O3 using the precursors 0.033 M irridium
chloride, 0.038 M rhodium chloride, 0.037 M ammo- 2.2. Hydrogenation reactor, conditions and analysis
nium perrhenate and 0.02 M uranyl nitrate, respec-
tively. A micro-catalytic stainless steel reactor of 10 cm
2 g of 0.35 Pt/Al2O3 was impregnated in a solution length and 6 mm external diameter was used. The
containing the required quantity of the precursor of the catalyst bed contains 0.2 g of a catalyst in an internal
second metal, i.e., 0.35% (Ir, Rh, Re and U) [13]. The stainless steel tube inserted into the micro-catalytic
produced catalysts were then dried, calcined and reactor. The reactor was electrically heated and elec-
reduced as mentioned above. tronically thermostated to 1.08C. The micro reactor
For promotion with Clÿ and Fÿ ions, ammonium was ®xed at the inlet of a gas chromatograph type 3400
chloride and ammonium ¯uoride, respectively, were Varian, for direct analysis of the reaction ef¯uents
added in the impregnation solutions. The required using a separation column of 4 m length packed with
quantities of these salts were dissolved in distilled 10% didecylphthalate supported on chromosorb W-
water to produce catalysts containing 1,3 and 6 wt% of HP of 80±100 mesh. The reaction conditions were as
Cl or F. Drying was carried out very slowly in order to follows: reaction temperature 50±2508C, ¯ow rate of
prevent removal of Cl or F. Then, the catalysts were H2 20 cm3/min and reactant pulse1 ml.
calcined and reduced as discussed above.
The dispersion of the metals with catalysts under
study was determined using a pulsed technique of 3. Results and discussion
hydrogen chemisorption, based on 1:1 stoichiometry
(H/M) according to Freel [14]. The data obtained are Benzene and toluene are ideal probe molecules
given in Table 1. for studying the interaction of aromatics with
A.-G.A. Ali et al. / Applied Catalysis A: General 170 (1998) 285±296 287
transition metal surfaces. Furthermore, they are of Rh active sites by the nonreactive species of the
involved in many catalytic processes in the dehydrogenated residue or by reaction products
petrochemical industry as intermediates or ®nal pro- may contribute to the decrease of the hydrogenation
ducts. rate at higher temperatures [5,15±17]. Graydon and
Langon [18] assume that the hydrogenation of
3.1. Hydrogenation activity on monometallic benzene over the Rh-catalyst requires more than a
catalysts single Rh-atom. They conclude that at higher reaction
temperatures some changes in the metal crystallinity
The conversion of benzene and toluene into cyclo- may result in decreasing the particle size. However,
hexane and methylcyclohexane, respectively, as a Re and U supported Al2O3 are catalytically inert
function of reaction temperature up to 2508C over under the same experimental conditions. The
alumina supported 0.35% wt% Pt, Ir, Rh, Re and U has hydrogenation activities of most of the catalysts
been investigated as shown in Fig. 1. It is observed pass through a maximum at various reaction tempera-
that from 508C to 1708C, the hydrogenation activity tures, which may be due to the equilibrium set up
obeys the following order: between hydrogenationdehydrogenation. Above
these maxima, the rate of hydrogenation decreases
Rh > Pt > I > Re U; with temperature due to shift in the equilibrium to
the left-hand side:
whereas above 1708C the activity of Pt-catalyst sur- Low t0 8C
passes that of the Rh catalyst. At higher temperatures, Aromatics H2 Cycloparaffins
High t0 8C
the hydrogenation activity of Rh starts to decrease
whereas that of Pt increases. This behaviour may be It has been reported that alkyl-substituent in the
due to thermodynamic limitations. At lowest tempera- aromatic ring reduces the rate of hydrogenation either
tures Rh which is most active starts hydrogenation due to its increased strength of adsorption or steric
at a higher rate than that of Pt. Similarly at higher effects [3,19]. In the present study, the situation is
temperatures the dehydrogenation activity of Rh different because the catalyst was saturated with
has also taken place faster than that of Pt, therefore hydrogen before feeding with the hydrocarbon.
the reverse reaction (hydrogenation) should take Fig. 1 shows that toluene hydrogenation activity sur-
place at a lower rate than Rh. Moreover, the blocking passes that of benzene, which appears against the
Fig. 1. Conversion of benzene ( ) and toluene (ÐÐÐ) over monometallic catalysts.
288 A.-G.A. Ali et al. / Applied Catalysis A: General 170 (1998) 285±296
thermodynamic feasibility. The higher hydrogenation which differs from benzene adsorption via the aro-
activity of toluene in the present study can be attrib- matic ring with higher electronegativity.
uted to (1) the adsorption±desorption rate of toluene is The behaviour is evident from the hydroconversion
much faster than that of benzene since toluene is more of p-xylene to dimethylcyclohexane, its hydrogena-
voluminous. The retention of toluene on the catalyst is tion rate is greater than toluene, Fig. 2. At temperature
larger than benzene [4,5,20] and (2) the suggestion of above 2008C, the adsorption limitation will be
Orozco and Webb [4] that toluene adsorbs via its less effective and thus the hydrogenation rates
methyl group yielding a species of the type proceed according to thermodynamic and kinetic
limitations whereby benzene hydrogenates faster
than toluene, which by its turn hydrogenates faster
than p-xylene.
Fig. 2. Conversion of bezene, toluene and p-xylene over 3.0% fluorinated (a) Pt±Re/Al2O3 and (b) Pt±U/Al2O3 catalysts.
A.-G.A. Ali et al. / Applied Catalysis A: General 170 (1998) 285±296 289
Fig. 3. Conversion of benzene (a) and toluene (b) over bimetallic catalysts.
3.2. Hydrogenation activity on bimetallic catalysts However, for toluene the activity order of the catalysts
is as follows:
The results obtained for the hydrogenation of
benzene and toluene over bimetallic catalysts at Pt Rh > Pt Re > Pt Ir > Pt > Pt U:
different reaction temperatures are represented in
Fig. 3. In the temperature range 50±2508C, the It is also found that the pro®le of conversion
benzene hydrogenation activity of the catalysts temperature is the same as that obtained for mono-
follows the order: metallic catalysts, where the conversion increases
with reaction temperature and reaching a maximum
Pt Rh > Pt Ir > Pt Re > Pt > Pt U: at a plateau in the range 150±2008C depending on
290 A.-G.A. Ali et al. / Applied Catalysis A: General 170 (1998) 285±296
the type of catalyst and the aromatic molecule, to that of Pt and Ir separately, Figs. 1 and 3. The
beyond which the activity decreases. These results same behaviour has been found similar for Pt
indicate that Rh, Ir and Re enhance the hydrogena- Re/Al2O3 catalyst. It was reported that Re possesses
tion activity of Pt/Al2O3 catalyst. The higher hydro- an aromatic hydrogenation activity approximately
genation activity of Rh is due to its higher ability equal to that of Pt if both metals are combined
to adsorb more hydrogen due to its characteristic on alumina support, whereas Re alone on Al2O3 is
d-bond character [16,21,22]. The Ir/Al2O3 catalyst inactive [23,24].
has a very low hydrogenation activity, whereas In case of combining U with Pt, it is found that
the Pt Ir/Al2O3 exhibits higher activity compared the hydrogenation activity of Pt U/Al2O3 is lower
Fig. 4. Conversion of benzene (a) and toluene (b) over Pt/Al2O3 catalyst with different fluorine content.
A.-G.A. Ali et al. / Applied Catalysis A: General 170 (1998) 285±296 291
than that of Pt/Al2O3 but higher than U/Al2O3. 3.3. Chlorination and fluorination of Al2O3
This indicates an inhibiting effect by U when supported Pt, Pt Re and Pt U for benzene and toluene
combined with Pt±U-crystallites which may have hydrogenation
weakened the accessibility of Pt sites by hydrogen
due to the decrease in M±M bond produced and/ The effect of including 1, 3 and 6 wt% F and Cl on
or some change in phase has occurred. The low Al2O3 supported Pt, Pt Re and Pt U catalysts for the
electronegativity of U (1.38) compared to that hydrogenation of benzene and toluene is given in
of Pt (2.28) may lead to the formation of Ptdÿ Figs. 4±6. These results show that catalysts with 3.0%
and Ud. F or Cl exhibit highest activities for benzene and toluene
Fig. 5. Conversion of benzene (a) and toluene (b) over Pt±Re/Al2O3 catalyst with different chlorine content.
292 A.-G.A. Ali et al. / Applied Catalysis A: General 170 (1998) 285±296
Fig. 6. Conversion of benzene (a) and toluene (b) over Pt±U/Al2O3 catalyst with different chlorine content.
hydrogenation. These higher activities may be due to 3.3.1. Effect of 3% halogen on the activity of
improving hydrogen spillover as well as to increasing monometallic catalysts for benzene and toluene
the dispersion of the metal into the support [25±27], hydrogenation
Table 1. Over the catalysts containing 6.0% F or Cl The results obtained for the effect of 3% halogen
the activity might have decreased the hydrogen treatment on the hydrogenation activity of Al2O3-sup-
spillover by decreasing the number of -OH groups ported Pt, Ir and Rh are represented in Fig. 7(a)±(c). It
whereupon H2 moves to the active metal crystallites is observed that both halogens enhance the catalyst
[28]. activity which may be attributed to increasing the
A.-G.A. Ali et al. / Applied Catalysis A: General 170 (1998) 285±296 293
for benzene and toluene hydrogenation is generally
low (4.2% and 5.4%) at 1508C. Addition of Cl or F
enhances the activity to 21% and 18% for benzene
hydrogenation, 25% and 30% for toluene hydrogena-
tion, respectively. The hydrogenation maxima using F
and Cl promotion are reached at lower temperatures
for toluene than for benzene, which may be attributed
to the strong adsorption of the large toluene molecule
than that of benzene.
The results obtained for Rh/Al2O3, Fig. 7(c), show
that Rh is highly active for aromatics. Chlorine is more
active than ¯uorine which appears in accordance with
Rh dispersion by Cl and F (0.79 and 0.72), respec-
tively. On the other hand, Re and U/Al2O3 exhibit
negligible activities, whether halogenated or not, for
hydrogenation of aromatics up to 2508C.
3.3.2. Effect of 3% halogen on the activity of
bimetallic catalysts for benzene and toluene
hydrogenation
3.3.2.1. Pt Ir/Al2O3 catalyst. It is observed in
Fig. 8(a) that chlorination and fluorination can
reduce or improve the hydrogenation activities of Pt
Ir/Al2O3. For benzene hydrogenation, it is found that
the activity of Pt Ir>Pt Ir 3% F>Pt Ir 3% Cl; 57.2, 52.3
and 50.8, respectively. For toluene hydrogenation: Pt
Ir 3% Cl>Pt Ir 3% F>Pt Ir; 59.3, 57.4 and 48.6,
respectively, at 1758C. Improvement of the metals
dispersion in the support (Table 1) may contribute
to this difference in activity. The dispersion of
metals in the presence of F and Cl is 0.75 and 0.58,
respectively, compared to 0.65 for the unhalogenated
catalyst.
Fig. 7. Conversion of benzene ( ) and toluene (ÐÐÐ) over
nonhalogenated and 3.0% halogenated (a) Pt/Al2O3, (b) Ir/Al2O3 3.3.2.2. Pt Rh/Al2O3 catalyst. Fig. 8(b) shows that the
and (c) Rh/Al2O3 catalysts.
Pt Rh/Al2O3 catalyst whether fluorinated or
chlorinated possesses an exceptionally higher
activity for benzene and toluene hydrogenation. At
acidity of the support which facilitate the adsorption of a temperature as low as 508C, benzene hydrogenation
relatively basic aromatic molecules [29,30]. reaches >90%, whereas toluene hydrogenation
In case of Pt/Al2O3, Fig. 7(a) it is found that reaches 100%. This higher hydrogenation activity
¯uorination promotes the catalyst activity higher than persists with increasing the reaction temperature up
chlorination for benzene and toluene hydrogenation. to 1758C, beyond which the activity decreases slowly
This may be due to the higher dispersion value of to reach 88.4%, 79.1% and 55.8% for Pt Rh, Pt Rh 3%
¯uorinated Pt/Al2O3 (0.86) than the chlorinated one Cl and Pt Rh 3% F, respectively.
(0.80), Table 1. In ¯uorine and chlorine inclusion in Ir/ Moreover, beyond 1758C, the activities are appar-
Al2O3 it is found that the activity of Ir/Al2O3 catalyst ently reversed such that the produced cycloparaf®ns
294 A.-G.A. Ali et al. / Applied Catalysis A: General 170 (1998) 285±296
Fig. 8. Conversion of benzene ( ) and toluene (ÐÐÐ) over nonhalogenated and 3.0% halogenated (a) Pt±Ir/Al2O3, and (b) Pt±Rh/Al2O3
catalysts.
are lower on the halogenated catalysts than on the These activities are highly improved in the presence
unhalogenated one. This is attributed, in fact, to of F and Cl compared to unhalogenated catalysts. The
enhancing the dehydrogenation activities of the halo- results appear to be compatible with the values of
genated catalysts since the equilibrium is shifted from dispersion of metals in the support, Table 1. The
hydrogenation to dehydrogenation by increasing ¯uorinated Pt Re/Al2O3 catalyst, Fig. 9(a), has higher
reaction temperature. The superior activity of Pt Rh activity than the chlorine promoted version, which
catalyst may be due to the distinct d-bond character may be attributed to increase in the H2 spillover
[21]. through the reaction. Concerning the catalyst Pt U/
Al2O3, chlorination exhibits higher promotion than
3.3.2.3. Pt Re and Pt U/Al2O3 catalysts. The results ¯uorination for the hydrogenation of benzene and
obtained for benzene and toluene hydrogenation are toluene. Fluorine may have produced a less active
shown in Fig. 9(a) and (b). uranium ¯uoride species.
A.-G.A. Ali et al. / Applied Catalysis A: General 170 (1998) 285±296 295
Fig. 9. Conversion of benzene ( ) and toluene (ÐÐÐ) over nonhalogenated and 3.0% halogenated (a) Pt±Re/Al2O3, and (b) Pt±U/Al2O3
catalysts.
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