Dehydrocyclization of diphenylamine to carbazole over platinum-based bimetallic catalysts

Article abstract of DOI:10.1016/S1872-2067(10)60129-5

Gas phase dehydrocyclization of diphenylamine (DPA) to carbazole over monometallic and bimetallic 0.4 wt Pt-based catalysts in a fixed bed reactor was studied in the presence of hydrogen at a temperature of 550 °C. Alumina and carbon supported Pt catalysts showed very high initial activity (> 95). The selectivity for carbazole over carbon supported Pt catalysts was slightly lower. Doping of the catalyst with potassium led to an increase in the selectivity for carbazole by 15. Bimetallic Pt-Sn catalysts prepared by co-impregnation were less selective than catalysts prepared by successive impregnation. The selectivity for carbazole over bimetallic Pt-Sn catalysts prepared by successive impregnation was 75, but their activity decreased with increased Sn loading. Highly active and reasonably selective catalysts were Ir-doped bimetallic Pt-based catalysts. The conversion of diphenylamine over Pt-Ir catalysts was above 98 and the selectivity for carbazole was nearly 55, while the lifetime was much longer.

Full text of DOI:10.1016/S1872-2067(10)60129-5

CHINESE JOURNAL OF CATALYSIS
Volume 31, Issue 12, 2010
Online English edition of the Chinese language journal
Cite this article as: Chin J Catal, 2010, 31: 1439–1444.
RESEARCH PAPER
Dehydrocyclization of Diphenylamine to Carbazole over
Platinum-Based Bimetallic Catalysts
1
,
1
2
1
Miroslav VLýKO *, Zuzana CVENGROŠOVÁ , Zuzana CIBULKOVÁ , Milan HRONEC
1
Institute of Organic Chemistry, Catalysis and Petrochemistry, Faculty of Chemical and Food Technology, Slovak University of Technology,
Radlinského 9, Bratislava 812 37, Slovakia
2
Institute of Physical Chemistry and Chemical Physics, Faculty of Chemical and Food Technology, Slovak University of Technology,
Radlinského 9, Bratislava 812 37, Slovakia
Abstract: Gas phase dehydrocyclization of diphenylamine (DPA) to carbazole over monometallic and bimetallic 0.4 wt% Pt-based cata-
o
lysts in a fixed bed reactor was studied in the presence of hydrogen at a temperature of 550 C. Alumina and carbon supported Pt catalysts
showed very high initial activity (> 95%). The selectivity for carbazole over carbon supported Pt catalysts was slightly lower. Doping of the
catalyst with potassium led to an increase in the selectivity for carbazole by 15%. Bimetallic Pt-Sn catalysts prepared by co-impregnation
were less selective than catalysts prepared by successive impregnation. The selectivity for carbazole over bimetallic Pt-Sn catalysts pre-
pared by successive impregnation was 75%, but their activity decreased with increased Sn loading. Highly active and reasonably selective
catalysts were Ir-doped bimetallic Pt-based catalysts. The conversion of diphenylamine over Pt-Ir catalysts was above 98% and the selec-
tivity for carbazole was nearly 55%, while the lifetime was much longer.
Key words: diphenylamine; carbazole; dehydrocyclization; platinum; tin; iridium; bimetallic catalyst
Carbazole (CB) belongs to a group of heterocyclic com-
The common way of metal loading onto the support is by
impregnation. Suitable supports are oxides of Al, Zn, Mg, and
Si. Carbazole can be also obtained by non-catalyzed cycliza-
pounds with increasing fields of utilization developed during
the last decade. Insecticides, pigments, drugs [1,2], polymers
o
[
3], dentistry, holography, microelectronics [4], and organic
tion of 2-aminodiphenyle (500–800 C) in the presence of air.
light emitting diodes [5] are areas where carbazole and its
derivatives have been utilized. Carbazole can be obtained from
crude oil and coal [6]. However, the utilization of such carba-
zole can be problematic due to contamination by impurities
with similar properties. For fine chemicals manufacture, car-
bazole with a purity > 98% is required.
However, this produces many byproducts, mainly tars [8].
In our previous studies [10,11], Pt catalysts were found to be
the best for carbazole synthesis, but further catalyst develop-
ment was needed to maximize the yield of carbazole and pro-
long the lifetime of the catalyst. The literature revealed that
carbon supported Pt catalysts are efficient for dehydrogenation
reactions [12]. Much data are available for alumina and carbon
supported bimetallic catalysts consisting of platinum as the
primary metal, and rhenium, iridium, or tin as the second metal
that gave improved activity and selectivity in many reactions
[13,14]. In Pt-Sn catalysts, the main effects due to Sn intro-
duction is increase in catalyst stability, decrease of the rate of
deactivation, change in selectivity, and inhibition of hydro-
cracking and coke deposition on the catalyst [15]. Iridium
deposition into a Pt catalyst led to the destruction of coke
There is not much data on carbazole synthesis in the litera-
ture. According to the patent literature, carbazole can be syn-
thesized by the dehydrogenation of aromatic amines in the
presence of hydrogen over supported noble metal catalysts
[
7–9] based on Pt, Ir, Rh with loadings of 0.1–2.0 wt%:
H
H
N
N
catalyst
+
H2
ꢀ
o
4
80 580 C
Received date: 5 June 2010.
Corresponding author. Tel: +421-908-697-695; E-mail: miroslav.vlcko@gmail.com
Foundation item: Supported by Slovak Science Grant Agency (VEGA).
*
Copyright © 2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(10)60129-5

Miroslav VLýKO et al. / Chinese Journal of Catalysis, 2010, 31: 1439–1444
precursors by hydrogenolysis [16,17]. Moreover, in a hydrogen
was prepared by dissolution of an appropriate amount of
H PtCl and SnCl ·2H O in acetone (with the concentration to
atmosphere, iridium can remove coke deposits [18]. In the
present work, platinum based bimetallic catalysts for the de-
hydrocyclization of diphenylamine (DPA) to CB were studied.
2
6
2
2
give 0.4 wt% Pt and 0.4 or 1.0 wt% Sn in the catalyst) and then
10 g of support (alumina) was added to this solution. After 1 h
of impregnation at room temperature, the catalysts were dried
o
o
1
Experimental
at 40 C and then at 120 C overnight. The catalysts were de-
noted as PtSn0.4 and PtSn1.0.
1
.1 Catalyst preparation
The Pt-Ir catalysts were prepared by co-impregnation using
hexachloroplatinic acid (H PtCl ) and hexachloroiridic acid
2
6
o
In our previous study [10], Ȗ-alumina calcined at 1 050 C
(H IrCl ). The concentration of the impregnation solution was
2 6
for 5 h was found to be a suitable support for the dehydro-
cyclization of diphenylamine. Ȗ-Alumina (Eurosupport Lit-
vínov, Czech Republic) was calcined under these conditions to
adjusted to give a loading of 0.4 wt% Pt and 0.1 wt% Ir. The
procedure of Pt-Ir catalyst preparation was the same as for the
monometallic catalysts. The catalysts were marked PtIrA
(alumina support) and PtIrC (carbon support).
2
give an Į-phase of 30 m /g specific surface area. This was
crushed and sieved to a grain size of 0.315–0.6 mm. For
comparison of the support, activated carbon Gac 1240 supplied
by NORIT was used. Since carbon can contain metals, the
The Pt-Sn bimetallic catalysts prepared by successive im-
pregnation were prepared using the following procedure. To
the appropriate amount of 1 mol/L HCl solution of SnCl ·2H O
2
2
carbon support was treated with HNO solution (10 wt%) for
was added 10 g of alumina. The obtained slurry was left to
stand at room temperature for 1 h and then it was heated at 80
3
24 h and subsequently washed with deionised water to pH 7 to
o
o
prevent their potential influence [19].
C and finally placed in an oven at 120 C overnight. To the
Monometallic catalysts containing Pt were prepared by the
conventional wet impregnation method. The support (10 g)
was added to an appropriate volume of an aqueous solution of
H PtCl with the concentration to give a Pt loading of 0.4 wt%.
dried catalyst was added the solution of hexachloroplatinic acid
and the obtained slurry was treated as before. The concentra-
tion of impregnation solutions was adjusted to give the load-
ings of 0.4 wt% Pt and 1 wt% Sn. The same procedure was
used for the preparation of similar catalysts but the Pt metal
was impregnated first. The catalysts were denoted as SnPtA
and PtSnA, where the order of the metals denoted the order of
metal introduction onto the support (alumina). The catalysts
with their characteristics are listed in Table 1.
2
6
After 1 h of impregnation at room temperature, the catalysts
o
o
were dried at 80 C and then at 120 C overnight. These cata-
lysts were denoted as PtA (A for alumina) and PtC (C for
NORIT active carbon). The catalyst prepared with HNO3
treated carbon as support was marked as PtC*.
The positive effect of potassium addition on the Pt catalyst
described in our previous study [10] was also applied to the PtC
catalysts. The PtC catalysts after drying were doped with an
appropriate volume of aqueous solution of KOH (5 wt%) [7].
After this procedure, water was evaporated and the catalyst
dried as before (the catalyst was denoted as PtCK).
1.2 Catalyst characterization
The specific surface area of the support was measured with a
Micromeritics ASAP 2020 using nitrogen. Prior to the meas-
o
urement, the sample was outgassed at 400 C for 2 h.
Bimetallic catalysts were prepared by co-impregnation and
successive impregnation. The co-impregnated Pt-Sn catalyst
The Pt dispersion was measured by CO chemisorption. The
o
measurement was performed at 25 C using He as carrier gas.
Table 1 Physicochemical properties of the catalysts and their catalytic performance for dehydrocyclization of DPA to CB
c
c
Metal loading Specific surface area
Acidity
(mmol/g)
0.225
N/A
Dispersion (%)
Fresh Deactivated
DPA conversion
(%)
CB selectivity
(%)
TOF
a
Catalyst
Metal
2
ꢀ3
(
wt%)
(m /g)
30
(10 )
PtA
PtC
Pt
Pt
0.4
59.0
72.5
66.8
64.3
32.2
28.4
29.9
25.7
51.2
54.3
10.6
12.3
12.6
13.1
—
99.0
75.1
8.41
6.83
0.4
1150
1150
1150
29
98.8
58.5
PtC*
Pt
0.4
N/A
97.4
57.9
7.31
b
PtCK
Pt
0.4
N/A
98.5
74.2
7.67
PtSn0.4
PtSn1.0
SnPtA
PtSnA
PtIrA
Pt-Sn
Pt-Sn
Pt-Sn
Pt-Sn
Pt-Ir
Pt-Ir
0.4+0.4
0.4+1
1+0.4
0.4+1
0.4
0.225
0.208
0.196
0.201
0.225
N/A
91.4
28.1
14.22
13.00
10.29
9.14
28
—
73.7
31.5
26
—
61.4
75.7
26
—
46.9
75.7
30
12.6
16.2
98.6
53.4
9.65
PtIrC
0.4
1150
99.0
55.7
9.13
a
Catalysts prepared by impregnation.
b
Catalyst doped with KOH.
c
o
Reaction conditions: 550 C, hydrogen flow 30 ml/min, WHSV 0.1 g/(g·h).

Miroslav VLýKO et al. / Chinese Journal of Catalysis, 2010, 31: 1439–1444
The catalysts (100 mg) were reduced in a flow of hydrogen at
conversion = (m(DPA
d p d
) – m(DPA ))/m(DPA ) and CB
o
400 C for 2 h. After reduction, hydrogen was replaced by
selectivity = m(CB)/(m(DPA ) – m(DPA )), respectively,
d
p
helium and the catalyst was purged at the same temperature for
where DPA is the weight of DPA dosed into the reaction and
d
1
3
h. CO was dosed into the system as pulses (0.5 ml). Typically,
–5 pulses were required for saturation. The fraction of
DPA is the weight of DPA in the product (unconverted DPA).
p
non-adsorbed CO molecules was detected by a thermal con-
ductivity detector (TCD). The adsorbed CO amount was cal-
culated from the consumption of the peaks. A CO/Pt ratio of 1
was used for the calculation of Pt dispersion [20]. Since the
2 Results and discussion
2.1 Monometallic catalysts
o
reaction temperature was 550 C, the catalysts were reduced at
The physicochemical properties of the catalysts are pre-
sented in Table 1. Catalysts based on the carbon support
showed a higher dispersion of the active metal. This may be
attributed to the higher specific surface area of the carbon. The
influence of the acidity of the supported catalysts on di-
phenylamine dehydrocyclization was discussed in our previous
paper [10].
this temperature and chemisorption measurements were carried
o
out as described above and compared with those at 400 C. The
differences in dispersion values were negligible.
Since carbon has an electrostatic charge, measurements with
He as carrier gas were not reproducible. For the carbon sup-
ported catalysts, chemisorption measurements were carried out
in a hydrogen atmosphere. The temperature was raised to 400
The catalytic results obtained over the monometallic cata-
lysts are shown in Fig. 1. From the comparison of the activity
and selectivity of the carbon supported catalysts, it was obvious
o
o
o
C at a rate of 10 C/min. After treatment at 400 C for 2 h, the
o
catalyst was cooled in hydrogen to 25 C. Subsequently, CO
pulses of 0.5 ml were injected into the carrier gas (H ) [21–23].
that the treatment of the carbon support with HNO solution
2
3
Typically, 3–5 pulses were required for saturation.
(residual metals removal) had no influence on catalyst per-
formance. The conversion of DPA decreased with time on
stream, but the conversion was still > 80% after 5 d. A similar
trend was observed with the alumina supported catalysts, but
these catalysts were more selective. The selectivity for carba-
zole on the carbon supported catalysts after 6 d of testing de-
creased from 60% to 45%. Over the alumina supported plati-
num catalyst, the selectivity was 70% and decreased slightly to
65%. A positive effect on the activity and selectivity of the
carbon supported catalysts was due to the doping of the catalyst
with potassium (PtCK catalyst). Using this catalyst, DPA
conversion of after 8 d of testing remained more than 85%
while the selectivity for carbazole decreased from 75% to >
50%.
Support acidity (for alumina) was determined from ammo-
nia temperature-programmed desorption measurements (TPD)
o
in the temperature range 100–600 C in a nitrogen atmosphere.
Measurements were carried out in a conventional flow appa-
o
ratus at a heating rate of 20 C/min.
DTA-TG measurement using a SHIMADZU DFG60 was
employed to study oxidation of the deactivated catalysts.
Analyses were carried out under an oxygen atmosphere (flow
rate 50 ml/min). The samples of 2–3 mg were placed onto
o
standard aluminium pans and heated to 500 C (heating rate 10
o
C/min).
1
.3 Catalytic tests
As compared to the PtC catalysts, the selectivity improve-
ment was 15%. A positive effect of potassium was also ob-
served in our previous experiments with alumina supported
catalysts [10]. The effect of potassium on the selectivity im-
provement can be explained by a weak interaction of potassium
with platinum [24] or by structural changes and changes in
electronic properties [25]. Juan-Juan et al. [26] ascribed
changes in the structure of the metal in the presence of potas-
sium to the modification of metal-support interactions and
consequently, the reducibility of the metal species. A similar
behavior of potassium was also observed by other authors
[27,28]. Since potassium is a good catalyst for carbon gasifi-
cation [29,30], it cannot be excluded that the increase of the
potassium/metal ratio decreased the rate of coke deposition
[26].
Catalytic tests were carried out in a stainless steel fixed-bed
reactor. Prior to the reaction, the catalyst was reduced in a flow
o
of hydrogen (30 ml/min) at 400 C for 2 h. Then, the tem-
o
perature was increased to 550 C and catalytic tests were per-
formed. DPA was dosed in the form of a solution in aniline (50
wt%). The weight hour speed velocity was 0.1 g of di-
phenylamine per 1 g of catalyst per hour. The flow of hydrogen
(
30 ml/min) was measured by a mass flowmeter. The products
were collected in an air cooled tube made of stainless steel and
the product gas was absorbed in methanol. After 6 h on stream,
the experiment was stopped for sampling and continued the
next day starting with the reduction of the catalyst. The prod-
ucts and byproducts were analyzed by gas chromatography
(
Hewlett Packard 5890) using FID and a SE 30 column and
confirmed by GC/MS. The main byproducts were benzene,
ammonia, and trace amounts of biphenyl, dimethylindoles, and
other hydrocarbons.
2.2 Bimetallic catalysts
DPA conversion and CB selectivity were calculated as DPA
As seen from the results summarized in Table 1, bimetallic

Miroslav VLýKO et al. / Chinese Journal of Catalysis, 2010, 31: 1439–1444
100
PtC
PtC*
80
60
40
20
DPA conversion
CB selectivity
DPA conversion
CB selectivity
1
2
3
4
5
6
1
2
3
4
5
Time on stream (d)
Time on stream (d)
PtCK
PtA
100
80
60
40
20
DPA conversion
CB selectivity
DPA conversion
CB selectivity
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
Time on stream (d)
Time on stream (d)
o
Fig. 1. Dehydrocyclization of DPA to CB over monometallic catalysts at 550 C.
Pt-Sn catalysts prepared by co-impregnation were active
conversion of DPA 91.4% and 73.7%) but their selectivity for
The results of the catalytic tests using Pt-Ir alumina and
carbon supported catalysts are shown in Fig. 2. From the de-
pendences of the conversion of DPA and the selectivity for
carbazole versus time on stream, it is clear that the presence of
iridium very positively influenced the lifetime of both catalysts
tested. Over the alumina supported Pt-Ir catalyst, the conver-
sion of DPA remained more than 98% during 9 d of testing. The
selectivity for carbazole after 9 d decreased from the initial
53% to 30%. The lifetime of this catalyst was prolonged almost
twice in comparison to the monometallic Pt/alumina catalyst.
Even better results were obtained with the carbon supported
Pt-Ir catalyst. Over this catalyst the conversion of DPA after 18
d remained more than 92% and the selectivity for the desired
product varied in the range of 45%–63%. Barbier et al. [34]
have proposed that the positive effect of iridium addition to
platinum catalysts is related with the hydrogenolysis ability of
these bimetallic catalysts. Iridium-doped Pt catalysts have the
ability to destroy coke precursors by hydrogenolysis [16,17]. In
a hydrogen atmosphere, Ir can destroy even the coke deposits
on the surface of platinum [18]. Also, an effect of iridium on
the breaking up of large platinum ensembles into smaller ones,
which gave less coking has been reported [35].
(
the desired product was low at 30%. The increase of the tin
loading negatively influenced the activity of the catalyst. In
contrast, the catalysts with the same Pt/Sn ratio prepared by
successive impregnation gave very high selectivity but their
activities were relatively poor (conversions of DPA were only
4
5% – 60%). A decrease in catalyst activity with increasing tin
loading was also observed in other types of reactions [31]. It is
probable that at higher tin loadings, tin gets spread on the
support and cover the alumina [32]. The alumina surface tin
species would decrease the amount of active alumina sites
responsible for catalyst activity. It is known that the adding of
tin to a Pt/alumina catalyst prevented Pt particles from sinter-
ing [32], which resulted in more active metal sites available for
reaction and decreased the amount of carbon deposited on the
metallic phase. Lieske and Volter [33] have shown that the
addition of tin to platinum catalysts led to the formation of
more mobile coke precursors, which can more easily migrate to
the alumina to leave more platinum sites coke-free.
A very positive effect on the performance of the Pt-based
catalysts was the doping of the catalysts with iridium. Using
the same Pt/Ir atomic ratio and procedure of preparation, bi-
metallic Pt-Ir catalysts supported on thermally treated alumina
and activated carbon were prepared and tested. As seen in
Table 1, despite the enormous difference between the specific
surface areas of the alumina and carbon support, the overall
dispersions of the metals over these supports were similar. As
was observed in our previous work [10], the specific surface
area did not have a significant influence on the catalyst per-
formance.
2.3 Deactivation of the catalysts
After catalyst testing, samples of deactivated catalysts were
taken and CO chemisorption measurements were carried out to
evaluate the active metal dispersion. As seen from Table 1, the
effect of metal dispersion on the catalyst performance showed a
nonlinear dependence. Over carbon supported catalysts after 6
or 8 d of testing, the conversion of DPA dropped 20% but the

Miroslav VLýKO et al. / Chinese Journal of Catalysis, 2010, 31: 1439–1444
100
90
80
70
60
50
40
30
PtIrA
PtIrC
DPA conversion
CB selectivity
DPA conversion
CB selectivity
1
2
3
4
5
6
7
8
9
2
4
6
8
10
12
14
16
18
Time on stream (d)
Time on stream (d)
o
Fig. 2. Dehydrocyclization of DPA to CB over Pt-Ir catalysts at 550 C.
metal dispersion decreased from the initial 64%–72% to 12%–
3%. On the other hand, a significant decrease in Pt dispersion
support sites [38] and blocked the acid sites resulting in the
decrease of the activity of PtA catalysts. However, irid-
ium-doped Pt catalysts exhibited a high activity as reported
above. We assumed that over the Pt-Ir catalysts, iridium con-
tributed to the very high DPA conversion due to its ability to
catalyze hydrogenolysis reactions.
1
caused a decrease of selectivity for carbazole of only 15%. The
nonlinear Pt dispersion dependence was more obvious in the
case of the alumina supported catalysts. After 7 d of testing, the
conversion of DPA decreased by 15% and the selectivity only
by 5% despite a sharp decrease in Pt dispersion (from 59.0% to
In the TG curves, it can be seen that the TG curve for the
spent PtA catalyst exhibited an almost linear decrease, which
suggested that the weight loss (approximately 8%) was uni-
form and consequently the amounts of coke on metal and
support sites were approximately the same. In contrast, the
TGA curve of the PtIrA catalyst weight loss (11%) exhibited a
1
0.6%). A similar nonlinearity between Pt dispersion and
catalysts performance was observed also in our previous ex-
periments [10]. The explanation of this phenomenon and
comparison with other studies is difficult due to a lack of in-
formation on other types of chemical reactions. In the litera-
ture, only data from model dehydrocyclization reactions can be
found.
o
larger decrease at 320 C, when the oxidation of condensed
o
coke species started. The loss of weight in the 250–320 C
The study of deactivated Pt-Ir catalysts revealed that the
metal dispersions were comparable to those of the monome-
tallic catalysts. Thus, we conclude that the modification of the
properties of the metal phase by doping with iridium gave a
decrease in the deactivation of the metal phase, resulting in a
significant increase in catalyst lifetime.
region was 4%, which showed that 36% of the coke were on
metal sites. The rest of the coke (64%) were located on support
sites. This suggested that in case of Pt catalysts, coke deposi-
tion was random. In case of Pt-Ir catalysts, the deposition of
coke was higher over support sites and left more metal sites
available for dehydrocyclization, which resulted in prolonged
catalyst lifetime.
Since catalyst deactivation can be related also with coke
formation, DTA-TG measurements were used to study the
carbonaceous deposits formed on the catalysts during the
catalytic reaction. DTA-TG curves of PtA and PtIrA catalysts
are shown in Fig. 3. In the case of Pt catalysts supported on
1
5
0
0
1
00
o
activated carbon, the support itself starts to burn at 300 C,
9
5
0
which makes DTA-TG analysis impossible. Thermograms of
coked samples of alumina supported catalysts (coke content
8
%–10%) exhibited two distinct coke oxidation temperature
9
-5
regions corresponding to the metal and support sites, in
agreement with those observed by Barbier [36]. The 250–300
-10
-15
o
85
C region consisted of coke deposited on metal sites, which
PtA
PtIrA
were oxidized at lower temperatures. This type of coke has a
low molecular weight and low density, hence it occupies more
80
-20
volume and block more active sites by being more extended
o
100
200
300
400
500
[
37]. The 400–450 C region corresponded to the oxidation of
o
Temperature ( C)
condensed/polymerized coke species, which were oxidized at
higher temperatures. This type of coke accumulated over
Fig. 3. DTA-TG profiles of deactivated catalysts.

Miroslav VLýKO et al. / Chinese Journal of Catalysis, 2010, 31: 1439–1444
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1
5
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