Alkylation of diphenylamine with α-methylstyrene and diisobutylene using acid-treated clay catalysts

Article abstract of DOI:10.1006/jcat.1996.0126

Alkylation of diphenylamine was carried out with α-methylstyrene and diisobutylene using acid-treated clay catalyst Engelhard F-24 to produce mono- and dialkylated diphenylamines. Dialkylated diphenylamines are industrially useful as antioxidants and heat stabilizers. Mono and dialkylated diphenylamines were separated in pure form from the reaction mixture and characterized by FTIR and ¹H NMR. Reaction conditions for obtaining high selectivity of each of the alkylated products were studied. A comparative study of the reaction rates, with various other grades of acid-treated clay catalysts, was done for the reaction between diphenylamine and α-methylstyrene. The reaction between diphenylamine and α-methylstyrene was also studied in a continuous mode of operation in a fixed bed reactor. Deactivation of the catalyst occurred during the course of these reactions and reuse of the catalyst posed problems; alternative schemes were considered to regenerate the deactivated catalyst. The basic reasons for the deactivation of the catalyst were investigated in the case of reaction between diphenylamine and α-methylstyrene. The fresh and used catalysts were characterized using various spectroscopic and other techniques such as X-ray diffraction, X-ray photoelectron spectroscopy, UV-visible and nuclear magnetic resonance spectroscopy, surface area (BET) and surface acidity (TPD), pore size distribution, and pore volume. These studies indicated that the deactivation of the catalyst was due to irreversible adsorption on the surface, loss of microporous structure and loss of surface acidity. The adsorbed species appeared to be diphenylamine and/or alkylated product(s), coordinated to the surface oxygen of the catalyst through nitrogen, forming a nitro kind of species.

Full text of DOI:10.1006/jcat.1996.0126

JOURNAL OF CATALYSIS 160, 84–94 (1996)
ARTICLE NO. 0126
Alkylation of Diphenylamine with �-Methylstyrene and Diisobutylene
Using Acid-Treated Clay Catalysts
1
Sandeep R. Chitnis and Man Mohan Sharma
Department of Chemical Technology, University of Bombay, Matunga, Bombay 400019
Received June 12, 1995; revised October 30, 1995; accepted January 10, 1996
process. Alkylated diphenylamines are manufactured
Alkylation of diphenylamine was carried out with � -methyl- by reacting diphenylamine (DPA) with alkenes such as
styrene and diisobutylene using acid-treated clay catalyst Engel- �-methylstyrene (AMS) and diisobutylene (DIB) over
hard F-24 to produce mono- and dialkylated diphenylamines. Di-
alkylated diphenylamines are industrially useful as antioxidants
and heat stabilizers. Mono and dialkylated diphenylamines were
acid catalysts. The products of these reactions are mono
and dialkylated diphenylamines, e.g., alkylation of DPA
with AMS produces a mixture of 4-(�,�-dimethylbenzyl)-
separated in pure form from the reaction mixture and characterized
diphenylamine, i.e., monocumyldiphenylamine (MCDPA),
1
by FTIR and H NMR. Reaction conditions for obtaining high selec-
0
and 4,4 -bis(�,�-dimethylbenzyl)diphenylamine, i.e, di-
tivity of each of the alkylated products were studied. A comparative
study of the reaction rates, with various other grades of acid-treated
clay catalysts, was done for the reaction between diphenylamine
and � -methylstyrene. The reaction between diphenylamine and � -
methylstyrene was also studied in a continuous mode of operation and 4,4 -bis(1,1,3,3-tetramethylbutyl) diphenylamine, i.e.,
in a fixed bed reactor. Deactivation of the catalyst occurred during dioctyldiphenylamine (DODPA). The reaction scheme for
the course of these reactions and reuse of the catalyst posed prob- alkylation of DPA with AMS is shown in Fig. 1 and for
lems; alternative schemes were considered to regenerate the deacti-
vated catalyst. The basic reasons for the deactivation of the catalyst
were investigated in the case of reaction between diphenylamine
and � -methylstyrene. The fresh and used catalysts were character-
ized using various spectroscopic and other techniques such as X-ray
heat aging, DCDPA is reported to be an ideal antioxi-
diffraction, X-ray photoelectron spectroscopy, UV–visible and nu-
clear magnetic resonance spectroscopy, surface area (BET) and sur-
cumyldiphenylamine (DCDPA). Alkylation of DPA with
DIB produces a mixture of 4-(1,1,3,3-tetramethylbutyl)
diphenylamine, i.e., monooctyldiphenylamine (MODPA),
0
alkylation of DPA with DIB is shown in Fig. 2.
The dialkylated diphenylamines are industrially impor-
tant as antioxidants and heat stabilizers. Due to its high an-
tioxidant activity and reduced tendency to discolor during
dant for many materials, e.g., hot melt adhesives, poly-
acetals, nylon 6, polypropylene, polyethylene, ethylene–
propylene copolymer and terpolymers, ABS, synthetic lu-
face acidity (TPD), pore size distribution, and pore volume. These
studies indicated that the deactivation of the catalyst was due to
irreversible adsorption on the surface, loss of microporous struc- bricants, polyether polyols, etc. (3). DCDPA, in combina-
ture and loss of surface acidity. The adsorbed species appeared to tionwithsomeofthephenolicantioxidants, alsoservesasan
be diphenylamine and/or alkylated product(s), coordinated to the effective antioxidant and stabilizer system for polyolefins,
surface oxygen of the catalyst through nitrogen, forming a nitro
e.g., polypropylene and polyether polyols, etc. DODPA is
kind of species.
�c 1996 Academic Press, Inc.
reported to be a high-temperature antioxidant in petroleum
and synthetic lubricants (4). It is also used in hydraulic flu-
ids, various industrial oils, automatic transmission fluids,
and synthetic and petroleum-based engine oils. It is effec-
tive as an antioxidant and corrosion inhibitor in silane and
siloxane synthetic lubricants, both in fluids and greases. In
addition, MCDPA is useful as a rubber vulcanization sta-
bilizer, which stabilizes rubber against thermal oxidation,
dynamic fatigue, and discoloration.
The literature available on alkylation of DPA with AMS
and DIB is in the form of patents. Wheeler (5) and
Sundholm (6) have reported formation of DCDPA with
a montmorrilonite clay catalyst at 303–308 K. Goegh et al.
INTRODUCTION
The use of acid-treated clays, as heterogeneous catalysts,
offers many advantages over homogeneous acid catalysts
such as ease of separation, mild reaction conditions, better
selectivity of the desired product, and elimination of
waste disposal problems. In a large number of organic
reactions acid-treated clays have been used as catalysts on
laboratory/industrial scales (1, 2). Alkylation of diphenyl-
amine is an academically and industrially important
(
7) have reported formation of MCDPA in the presence of
1
To whom correspondence should be addressed.
clay catalyst at 353–393 K. Goegh et al. (8) have reported
84
0021-9517/96 $18.00
Copyright �c 1996 by Academic Press, Inc.
All rights of reproduction in any form reserved.

ALKYLATION OF DIPHENYLAMINE
85
Kurek et al. (13) have reported the reaction with high con-
version of DPA and minimal cracking of DIB using a rare
earth pillared clay, especially Ce and La Al chlorohydrate
pillared clay catalyst. They have reported that the catalyst
could be regenerated by heating in air at 823–873 K. DPA
and DIB were alkylated at 453 K and 67 bar H2 gas pressure
using a Ce Al chlorohydrate pillared clay catalyst and this
produced 26% MODPA and 65% DODPA, at 98% con-
version of DPA. Franklin (14) has reported formation of
DODPA at 438 K over activated Al2O3 catalyst.
Thus no systematic study appears to have been reported
in the published literature. Therefore it was decided to
study the alkylation of DPA with AMS and DIB system-
atically, where the selectivity for monoalkylated vs dialky-
lated product was explored. It was also proposed to study
the life of the catalyst and reasons for deactivation.
FIG. 1. Reaction scheme for alkylation of DPA with AMS.
ALKYLATION OF DPA WITH AMS
formation of mixture of MCDPA and DCDPA at 363–413 K
in the presence of montmorillonite or aluminosilicate cata-
lysts. A semibatch addition of AMS for 3 h, at an overall
mole ratio of DPA : AMS of 1 : 2 and 378 K produced a mix-
ture of MCDPA and DCDPA in the ratio 9 : 91. Holcik et al.
Experimental
DPA was obtained from S. d. fine chemicals Ltd. (India).
AMS was obtained from Herdillia Chemicals Ltd. (India).
The acid-treated clay catalysts Engelhard F-24, F-25, F-34,
F-44, F-54, F-124, F-224, and G-62 were obtained from En-
gelhard Corporation (USA); Tonsil K-306 and Tonsil KA3
catalysts were obtained from Sud Chemie (Germany).
The properties of various grades of acid-treated clay
catalysts used in this work are listed in Table 1 (15). All
the experiments were carried out in a 0.05 m. i.d. 100 ml
(9) have reported formation of mixture of MCDPA and
DCDPA over H3PO4/bleaching earth catalyst at 453 K in an
autoclave. Randell (10) has reported formation of mixture
of MODPA and DODPA at 381–419 K over anhydrous alu-
minum chloride catalyst at 55% conversion of DPA. Mifune
et al. (11) have reported formation of DODPA at 90% se-
lectivity at 393–403 K with active china clay catalyst. Bayha
et al. (12) have reported formation of DODPA at 373–473 K
over AlCl3 catalyst in an atmosphere of N2 at 3.4–13.6 bar.
� 6
3
(
1 ml = 1 � 10 m ) fully baffled mechanically agitated
glass reactor equipped with a stuffing box and a reflux con-
denser. A six-bladed glass disc turbine impeller was used
for agitation. The reaction was carried out at the desired
temperature by placing the reactor in a constant temper-
ature bath. The reaction was studied in the temperature
range of 363–423 K. All the catalysts used in this work were
dried under vacuum (0.008 bar) at 373 K for 6 h before use.
The analysis was done on a gas chromatograph (Chemito
8
510) equipped with a flame ionization detector. A 2-m-
long ss column packed with 5% OV-17 on Chromosorb-
WHP was used for the analysis. The injector and detector
were kept at 613 K. The oven temperature was increased
from 423 to 613 K at a ramp rate of 0.16 K/s. Nitrogen was
�
7
3
used as a carrier gas and its flow rate was 4 � 10 m /s.
X-ray diffraction patterns (XRD) were recorded on a
Jeol JDX 8030 diffractometer (with a monochromater) us-
ing Cu K� radiation at 35 KV and 25 mA. Diffractograms
�
�
were recorded between 2� value of 2 to 70 with a step
angle of 0.02 .
�
X-ray photoelectron spectra (XPS) were recorded on
VG ESCA-3 M K II spectrometer using Mg K� radiation
(
1253.6 eV) as the exciting source. All the BEs were re-
FIG. 2. Reaction scheme for alkylation of DPA with DIB.
ferred to the C1S peak at 285 eV.

8
6
CHITNIS AND SHARMA
TABLE 1
Physical Properties of Various Grades of Acid-Treated Clay Catalysts
Property
Shape
F-24
F-25
F-34
F-44
F-54
F-124
F-224
Granular
Granular
Granular Spherical Spherical
Granular Granular
beads
beads
a
a
a
Size (mm)
0.6
350
1.05
350
0.48
350
0.48
350
Internal surface
380
300
NA
0.36
NA
225
NA
0.36
NA
2
area (m /g)
Weight capacity
0.3
NA
0.26
NA
NA
0.36
NA
NA
0.32
NA
NA
0.32
NA
(
meq/g)
Void fraction
packed)
0.32
NA
(
Temperature
stability
Note. NA: data not available. Engelhard G-62, Tonsil K-306, Tonsil KA3, alumina pillared clay catalyst:
properties are not available.
a
Tylor standard sieve, passing 20 mesh% > 85.
1
UV–visible solid-state reflectance spectra were recorded by FTIR and H NMR spectra, which confirmed the purity
on a Shimadzu UV-160 spectrometer.
of these compounds.
The Fourier transform infrared (FTIR) spectra were
recorded in the region between 4000 and 400 cm at 4 cm
resolution on a Bruker IFS 88 spectrometer with a DTG
detector.
Effect of speed of agitation. The variation in stirring
speed from 1000 to 1500 rpm had no effect on the rate
of reaction, clearly indicating that external mass transfer
resistance was absent. Hence all the further experiments
were carried out at 1000 rpm.
�
1
� 1
Solid-state, magic angle spinning, nuclear magnetic res-
onance (MAS-NMR) spectra were recorded on a Varian
29
Effect of particle size of the catalyst. The effect of par-
ticle size of the catalyst on the rate of conversion of DPA
was studied in the range of particle size from 0.6 to 0.18 mm
VXR-300 S spectrometer. Si spectra were obtained at
�
5
1
9.591 MHz with a spinning rate of 4.3–4.5 k at 45 pulse;
0 s delay was used between the scans. Tetramethylsilane
�
3
(
1 mm = 1 � 10 m). The effect of particle size of the cata-
was used as the standard.
lyst on the rate of conversion of DPA is shown in Fig. 3. It
was observed that when the particle size was reduced, the
Surface area of the samples were determined by the ni-
trogen adsorption isotherms at 77 K after outgassing the
samples at 383 K. The BET method was used for surface
area calculation. Pore size distribution and total pore vol-
ume was measured using Amico’s porosimeter (4135 bar).
Total acidity of the samples were determined by temper-
ature programmed desorption (TPD) of ammonia using ni-
trogen carrier gas. Samples were heated from 393 to 823 K
�
7
3
with nitrogen flow rate of 5 � 10 m /s, at rate of 0.16 K/s
and held at 823 K for 1.3 h. Samples were then cooled
to 393 K and ammonia adsorption was carried out. Af-
ter flushing the sample with nitrogen for 2 h, temperature-
programmed desorption of ammonia was carried out.
Results and Discussions
Product separation and characterization. MCDPA was
obtained in pure form by fractional distillation of the reac-
tion mixture on a Perkin-Elmer spinning band distillation
column. DCDPA was obtained in pure form by crystalliza-
tion from a solvent by the following procedure. The reac-
tion mixture was dissolved in 96% aqueous methanol so-
lution under reflux. DCDPA was crystallized out from the
mother liquor and further purified by recrystallization from
FIG. 3. Effect of particle size of the catalyst on rate of conversion
of DPA. Reaction conditions: DPA : AMS mole ratio, 1 : 2; temperature,
methanol. Both MCDPA and DCDPA were characterized 363 K; catalyst loading, 10% Engelhard F-24.

ALKYLATION OF DIPHENYLAMINE
87
rate of conversion of DPA increased but the selectivity of
TABLE 2
DCDPA decreased, e.g., in the reaction with DPA : AMS
mole ratio of 1 : 2, 363 K, and 10% loading of Engelhard
F-24 catalyst, when the particle size was reduced from 0.6
to 0.18 mm, the rate of conversion of DPA increased, as
indicated in Fig. 3, but the selectivity of DCDPA decreased
from 65.2 to 50.8%, at 100% conversion of DPA. Since an
increase in the rate of conversion of DPA was obtained
with a decrease in the particle size of the catalyst, it can
be concluded that the intraparticle diffusion limitations are
present for the reaction between DPA and AMS. The ef-
fect of intraparticle diffusion can be correlated to the Thiele
modulus of the reaction system, which depends on radius
of the catalyst particle, the reaction rate constant, and the
effective diffusivity of the reactants.
Comparison of Performance of Various Grades
of Acid-Treated Clay Catalysts
% Conversion of
DPA per unit area
of the catalyst
Type of
catalyst
% Conversion % Selectivity
of DPA
of DCDPA
F-24
F-25
F-34
F-44
100
85.6
78
100
74.9
93.2
95.2
71.2
74.5
65.2
50.9
24.8
55
0.285
0.244
0.205
0.333
0.332
0.266
0.272
—
F-54
32
F-124
F-224
G-62
55.3
61.6
28
Tonsil K-306
45.6
—
Note. Reaction conditions: DPA : AMS mole ratio, 1 : 2; tempera-
ture, 363 K; catalyst loading, 10%; period of reaction, 1.5 h.
Effect of various catalysts. It was observed that the re-
action was unsuccessful with a macroporous ion exchange
resin catalyst Amberlyst-15 upto 393 K, even in the pre-
sence of a solvent, e.g., cumene. The reaction was also un-
successful with an alumina pillared clay catalyst which was
calcinated upto 773 K. The reaction occurred with other
grades of acid-treated clay catalysts such as F-25, F-34,
F-44, F-54, F-124, F-224, G-62, and Tonsil K-306. However,
the reaction did not occur with Tonsil KA3 catalyst. The
Engelhard G-62 catalyst was in the pelleted form, whereas
Tonsil K-306 and Tonsil KA3 catalysts were in the form
of spherical beads. The Engelhard G-62, Tonsil K-306, and
Tonsil KA3 catalysts were crushed to 0.6 mm size before
using in the reaction. All the experiments were done with
the fresh catalysts. The results of this study are reported in
Table 2. Thus the reaction did not occur with some of the
catalysts, whereas it occurred with the other catalysts at dif-
ferent rates of conversion of DPA and different selectivity
of DCDPA with each of the catalyst. This difference can be
attributed to the difference in the physical properties of the
catalysts, e.g., acidity and surface area of these catalysts.
Catalyst regeneration. Various alternative schemes were
tried for the regeneration of the deactivated catalyst.
Effect of reflux methanol wash on catalyst regeneration.
It is known that methanol has a great affinity for the clay
surface (16). Therefore, it was thought that the deactivated
catalyst could be regenerated by refluxing with methanol.
The deactivated catalyst was refluxed with methanol for
2
h, dried at 373 K under vacuum for 6 h, and then reused
in the next reaction. The extent of catalyst regeneration
was studied for three consecutive batches by refluxing the
catalyst with methanol each time followed by drying. The
percentage conversion of DPA was 100, 100, and 93.2%
and the selectivity of DCDPA was 56.2, 48.5, and 42.7%
Catalyst reusability. The effect of repeated use of the
catalyst on the rate of conversion of DPA and selectivity
of DCDPA was studied. All the reactions for the catalyst
reusability and regeneration study, in the further discussion,
are carried out with DPA : AMS mole ratio of 1 : 2, 10%
(w/w) loading of Engelhard F-24 catalyst, and at 363 K and
the results of percentage conversion of DPA and selectivity
of DCDPA after 1.5 h of reaction are reported. The effect
of repeated use of the catalyst on the rate of conversion
of DPA is shown in Fig. 4. The percentage conversion of
DPA with the fresh catalyst was 100% and the selectivity
of DCDPA was 65.2%. The percentage conversion of DPA
decreased to 74.7, 62.2, and 58.2% and the selectivity of
DCDPA decreased to 38.8, 34.7, and 32.5% for the first,
second, and third reuse of the catalyst, respectively. Thus,
thisstudyrevealedthatdeactivationofthecatalystoccurred
during the course of the reaction.
FIG. 4. EffectofrepeateduseofcatalystonrateofconversionofDPA.
Reaction conditions: DPA : AMS mole ratio, 1 : 2; temperature, 363 K;
catalyst loading, 10% Engelhard F-24.

8
8
CHITNIS AND SHARMA
for the first reuse after methanol wash, second reuse af-
ter methanol wash, and third reuse after methanol wash,
respectively. Thus, for first two batches of the regenerated
catalyst, the rate of conversion of DPA was the same as the
rate with the fresh catalyst, whereas for the third batch it
dropped marginally. The selectivity for DCDPA, even with
the first batch of the regenerated catalyst, was lower than
that with the fresh catalyst and it dropped significantly with
every batch of the regenerated catalyst. This kind of re-
generation of the catalyst, where the rate of conversion of
DPA and/or the selectivity of DCDPA with the regenerated
catalyst are lower than that with the fresh catalyst will be
termed hereafter as partial regeneration of the deactivated
catalyst.
Effect of use of a solvent in the reaction on catalyst re-
generation. The extent of catalyst regeneration by the use
of a solvent in the reaction was studied separately for two
solvents, i.e., ortho-xylene and 1,2-dichlorobenzene. In the
reaction with ortho-xylene solvent, the rate of conversion
FIG. 6. Effect of nitrogen atmosphere on catalyst regeneration. Reac-
tion conditions: DPA : AMS mole ratio, 1 : 2; temperature, 363 K; catalyst
of DPA with fresh catalyst was lower than the rate without loading, 10% Engelhard F-24.
any solvent. In the reaction with 1,2-dichlorobenzene sol-
vent, the rate of conversion of DPA with fresh catalyst was
higher than the rate without any solvent. However, when
these catalysts were reused in the respective reactions, the
rates of conversion of DPA were drastically lower as com-
pared to the rates with the fresh catalysts. Thus, the use
of a solvent in the reaction produced an adverse effect on
the extent of regeneration of the catalyst. The rates of con-
version of DPA with ortho-xylene and 1,2-dichlorobenzene
solvents for the fresh and used catalysts are shown in Fig. 5.
Effect of nitrogen atmosphere on catalyst regeneration.
The effect of nitrogen atmosphere on the rate of conver-
sion of DPA for both fresh and used catalyst was studied.
The rates of conversion of DPA, for both the fresh and the
used catalyst, were higher in nitrogen atmosphere than the
rates of corresponding reactions in air. From the compar-
ative study of reaction rates, it was thought that nitrogen
atmosphere suppressed the formation of product(s) of ei-
ther of the reactants, which deactivated the catalyst. The
rates of conversion of DPA for reactions in nitrogen atmo-
sphere and air with the fresh and used catalysts are shown
in Fig. 6.
Effect of ultrasonic irradiation on catalyst regeneration.
Ultrasonic irradiation is used for a wide variety of applica-
tions such as cleaning of surfaces, erosion, and sterilization.
The formation and collapse of microbubbles, i.e., cavita-
tion, is responsible for the chemical effects of ultrasonic
irradiation. Cavitation causes the local temperatures at the
4
6
center of the bubble to rise up to 10 –10 K and pressures to
several thousands of bars, which promotes the cleaning pro-
cess. Therefore, the effect of ultrasonic irradiation on the
extent of catalyst regeneration was studied. Ultrasonic ir-
radiation of the deactivated catalyst was carried out by sus-
pending it in methanol solvent. The irradiated catalyst was
then dried at 373 K under vacuum for 6 h and then reused.
The time of ultrasonic irradiation was increased from 0.16
to 0.5 h. The percentage conversions of DPA were 100 and
1
1
00%, respectively, and the selectivities of DCDPA were
8.2 and 52.7%, respectively, for the catalysts which were
FIG. 5. Effect of use of a solvent in the reaction on catalyst regene-
ration. Reaction conditions: DPA : AMS mole ratio, 1 : 2; temperature,
63 K; catalyst loading, 10% Engelhard F-24.
reused after ultrasonic irradiation for 0.16 and 0.5 h. Thus,
the catalyst could be partially regenerated with the help of
3

ALKYLATION OF DIPHENYLAMINE
89
ultrasonic irradiation. Also, there was no further increase
TABLE 4
in the extent of regeneration, when the time of ultrasonic
irradiation was increased beyond 0.5 h.
Physical Characterization of Fresh and Used Catalyst
BET surface Pore volume
Reaction in a fixed bed reactor (FBR). As the reaction
was found to be appreciably fast, it was studied in a con-
tinuous mode of operation in a shell and tube type of fixed
bed reactor (FBR), having a tube diameter of 0.017 m and a
length of 0.3 m. The tube side of the reactor was packed with
Engelhard F-25 catalyst, in which the length of the catalyst
bed was 0.25 m and a top portion of the bed (0.02 m) was
filled with a layer of inert packings to ensure uniform distri-
bution of the reactants in the bed. Hot water was circulated
on the shell side of the reactor, as a heat transfer medium. A
controlled feeding of the mixture of DPA and AMS (mole
ratio 1 : 2; preheated to 393 K) was done from the top of
the reactor and the reaction products were withdrawn at
the bottom; i.e., the reactor was operated in a downflow
manner. The percentage conversions of DPA and selecti-
vity of DCDPA at residence times (RT) of 0.015, 0.066, and
2
3
Status of catalyst
area (m /g)
(m /g)
� 6
Fresh catalyst (size, 0.6 mm)
Used catalyst (size, 0.6 mm)
Regenerated catalyst by reflux
methanol wash (size, 0.6 mm)
Fresh catalyst (size, 0.18 mm)
277
27.1
254.8
0.207 � 10
0.164 � 10
—
�
6
6
225
0.857 � 10�
0
.25 m for both the catalysts. When the FBR was operated
in a downflow manner, insufficient wetting of the catalyst
bed was observed for both these catalysts. Therefore, the
FBR was operated in an upflow manner; i.e., addition of
the mixture of DPA and AMS (mole ratio, 1 : 2; preheated
to 393 K) was done from the bottom of the FBR through a
tube of diameter 0.015 m, under the hydrostatic head of the
reactants and the reaction samples were collected from an
overflow point at the top of the reactor. With the Engelhard
G-62 catalyst, 33% conversion of DPA and 51% selectivity
of DCDPA was obtained at a RT of 0.9 h, whereas with the
Tonsil K-306 catalyst, 25% conversion of DPA and 63% se-
lectivity of DCDPA was obtained at a RT of 1.16 h. Thus,
appreciably low conversion of DPA and moderate selec-
tivity of DCDPA were obtained at high residence time of
reactants in the FBR, with these catalysts.
0
.17 h were measured. The percentage conversions of DPA
at these RTs were 47, 70, and 100%, respectively, and the
corresponding selectivities of DCDPA were 38, 44.3, and
5
8.2%, respectively. Thus, it was possible to obtain appre-
ciably good conversion of DPA and selectivity of DCDPA
at short residence times of reactants in the reactor.
The extent of deactivation of the catalyst in the FBR was
studied by carrying out a reaction continuously for a period
of 12 h, with Engelhard F-25 catalyst. During this reaction,
there was no deactivation of the catalyst for a period of first
5
h. Beyond this period, the catalyst deactivated at a very
Catalyst Characterization2
slow rate and a marginal decrease in the percentage con-
version of DPA and selectivity of DCDPA was observed.
The results of this study are reported in Table 3.
The reactions with the pelleted catalyst Engelhard
G-62 and the spherical bead-like catalyst Tonsil K-306, were
carried out in a FBR having a tube diameter of 0.034 m
and a length of 0.3 m. The length of the catalyst bed was
Measurement of surface area, pore volume, and pore size.
The results of surface area and pore volume of fresh and
used catalysts are reported in Table 4. The results of pore
size distribution of the fresh and used catalyst of 0.6 mm
size and 0.18 mm size are shown in Fig. 7. From these re-
2
sults, for the 0.6 mm size catalyst surface area is 277 m /g,
�
6
with pore size in the range of 0.003–0.5 � (1 � = 1 � 10 m)
�
6
3
and the total pore volume is 0.207 � 10 m /g. Maximum
pores are in the range of 0.003 to 0.01 �, which contributes
TABLE 3
�
6
3
to a total volume of 0.129 � 10 m /g. This suggests a mi-
Extent of Deactivation of Engelhard F-25
Catalyst in FBR for the 12-h Reaction
croporous structure for the catalyst. After the reaction, the
2
surface area of the catalyst reduced to 27.1 m /g (9.78%
of the original), but the loss of pore volume was marginal
Period of
reaction (h)
% Conversion
of DPA
% Selectivity
of DCDPA
� 6
3
(
0.164 � 10 m /g), 79% of the original pore volume being
retained. However, this is due to the creation of larger pores
of 0.01 to 60 �, as evident from the pore size distribution
curve. It would therefore seem that the deactivation of the
catalyst is due to loss of micropores and subsequent reduc-
tion in surface area. The surface area of the regenerated
0
6
7
8
9
0
1
2
to 5
100
58.2
57.6
57.2
57.2
57.2
53.5
53.5
53.3
92.3
91.6
90.3
90.3
86.6
86.6
86.1
1
1
1
2
catalyst, by reflux methanol wash, is 254.8 m /g, indicating
2
Note. Reaction conditions: Residence time of re-
The catalyst characterization study has been carried out for Engel-
actants in the reactor, 0.17 h.
hard F-24 catalyst.

9
0
CHITNIS AND SHARMA
FIG. 7. Results of pore size distribution. (a) Fresh catalyst, (b) used catalyst, both of 0.6 mm size, (c) fresh catalyst of 0.18 mm size. Y1, axis for (a)
and (b); Y2, axis for (c).
that there is a large recovery in the surface area after the peak at � 101.6 ppm was assigned to OH containing sil-
reflux methanol wash of the catalyst.
icon unit, i.e., (SiO)3 Si OH (17). Relative peak height,
linewidths, and peak areas of the three peaks for fresh
and deactivated catalysts are reported in Table 5. Although
there is line broadening in case of deactivated catalyst, it
is clear from individual component plots that the peak at
Measurement of surface acidity. Total acidity of the
fresh and used catalysts was determined by temperature
programmed desorption (TPD) of ammonia. The chro-
matograms of ammonia TPD are shown in Fig. 8. There
were two major peaks centered around 483 and 703 K rep-
resenting weak and strong acidic sites, respectively, with
the high temperature peak being more intense in the deac-
tivated catalyst. The low temperature peak is due to des-
orption of ammonia from Brønsted acid sites and the high
temperature peak is due to desorption of ammonia from
Lewis acid sites. The total acidity, in terms millimoles of
ammonia/g, for the fresh catalyst was 2.48 (100%) and for
the used catalyst was 2.25 (90.7%). Although the reduction
in the total acidity of the deactivated catalyst is only 9%, the
loss of Brønsted acid sites is quite significant. This should
be responsible for the deactivation of the catalyst.
�
101.6 ppm looses intensity after reaction. This could be
due to possible interaction of the OH group with reactant
and/or product species.
UV–visible spectra. From the UV–visible solid-state
spectra of the fresh and the deactivated catalyst, the deac-
tivated catalyst (which had an intense green colour) has an
�
9
absorption band at 678 nm (1 nm = 1 � 10 m), in contrast
to 800 nm in the case of the fresh catalyst. The intense dark
green coloration of the deactivated catalyst is thought to be
due to the formation of a charge transfer complex by the
interaction of adsorbed species with the surface functional
groups of the catalyst.
X-ray diffraction of fresh and used catalysts. From the
small angle X-ray powder diffraction patterns of the fresh
and used catalysts, basal spacing in the both the catalyst
samples, i.e., fresh and used, was found to be the same,
i.e., 16.6 A˚ (1 A˚ = 1 � 10 m) at 2� = 5.3 . This suggested
that there is no expansion in the interlayer spacing; hence
therewasnoevidenceofintercalationofthereactantand/or
product into the clay lattice.
XPS studies. Results of XPS studies are shown in
Fig. 10. The XPS of the deactivated catalyst shows two
peaks at binding energies (BE) of 401 and 406 eV in the
N1S region. The peak at 401 eV can probably be assigned
to adsorbed DPA, since DPA has a N1S BE of 400 eV, and
the positive shift of 1 eV could be due to the drain of elec-
trons away from the nitrogen of DPA due to adsorption on
the catalyst. This was confirmed by recording XPS spectra
�
10
�
29Si MAS-NMR.
29Si MAS-NMR spectra of the fresh of only DPA adsorbed on the catalyst. The higher binding
and used catalyst is shown in Fig. 9. The large peak at � 95 energy peak at 406 eV is in the region of nitro compounds,
ppm and relatively smaller peak at � 111.9 ppm were as- suggesting therefore the formation of an adsorbed species
3
4
signed to Q (O Al) and Q (O Al) units, respectively. The with the nitrogen bound to the surface oxygen.

ALKYLATION OF DIPHENYLAMINE
91
FIG. 8. Chromatograms of ammonia TPD. (a) Fresh catalyst—weight of the catalyst taken for analysis: 0.0819 g. (b) Used catalyst—weight of the
catalyst taken for analysis: 0.1115 g.
ALKYLATION OF DPA WITH DIB
Experimental
DIB was obtained from Maruzen Petrochemicals
(
Japan), through the courtesy of Herdillia Chemicals Ltd.
The experiments at 373 K were carried out in a 0.05-m-
i.d., 100-ml fully baffled mechanically agitated glass reactor
equipped with a stuffing box and a condenser. All the ex-
periments above 373 K were carried out in a 0.051-m-i.d.,
1
00-ml Parr autoclave fitted with a stirrer, a sampling port,
a pressure gauge, and a gas vent. The temperature was con-
trolled by temperature controlling device. The reaction was
studied in the temperature range of 373 to 453 K.
Analysis was done on a gas chromatograph (Chemito
8
3
510) equipped with the flame ionization detector. A
-m-long ss column packed with 5% SE-30 supported on
TABLE 5
Relative Linewidths and Peak Areas of 29Si MAS-NMR
Peak height
Linewidth (Hz)
Peak area
Peak Fresh Deactivated Fresh Deactivated Fresh Deactivated
1
2
3
53.9
22.9
39.5
38.1
14.2
31.6
170
229
310.6
251.9
206.4
373.3
9162.4
5233.5
12270.6
9588.2
2930
10855.7
FIG. 9. 29Si MAS-NMR. [A] Fresh catalyst, [B] used catalyst. (a)
Note. Weight of fresh catalyst sample: 0.2911 g. Weight of deactivated
Actual sprectrum, (b) full-fit, and (c) individual component plots.
catalyst sample: 0.2803 g.

9
2
CHITNIS AND SHARMA
FIG. 10. X-ray photoelectron spectra of used catalyst.
Chromosorb-WHP was used for analysis. The injector and
Effect of particle size of catalyst. The effect of particle
detector were kept at 573 K. The oven temperature was size of the catalyst on the rate of reaction was studied in the
kept at 573 K isothermal. Nitrogen was used as a carrier range of particle size from 0.6 to 0.18 mm. It was observed
�
7
3
gas and its flow rate was 5 � 10 m /s.
that there was no effect of particle size of the catalyst on
the rate of conversion of DPA and selectivity of DODPA.
Results and Discussions
Catalyst reusability. The effect of repeated use of the
Product separation and characterization. MODPA and catalyst on the rate of conversion of DPA and selectivity of
DODPA were obtained in pure form by fractional distil- DODPA revealed that deactivation of the catalyst occurred
lation of the reaction mixture on a Perkin Elmer spin- during the course of the reaction. All the reactions for the
ning band distillation column. Both MODPA and DODPA catalyst reusability and regeneration study, in the further
1
were characterized by FTIR and H NMR spectra, which discussion, are carried out, with DPA : DIB mole ratio of
confirmed the purity of these compounds. In the range of 1 : 4, 10% (w/w) loading of Engelhard F-24 catalyst, and at
mole ratios of DPA : DIB studied in this work, a part of 423 K and the results of percentage conversion of DPA and
DIB cracked into isobutylene gas (IB) at temperatures selectivityofDODPAafter6hofreactionarereported. The
of 423 K and above. IB reacted with a part of DPA to effect of repeated use of the catalyst on the rate of conver-
form 4-tert-butyl diphenylamine and with a part of MODPA sion of DPA is shown in Fig. 11. The percentage conversion
0
to form 4-(1,1,3,3-tetramethylbutyl), 4 -tert-butyl dipheny- of DPA with the fresh catalyst was 100% and the selectiv-
lamine. The formation of 4-tert-butyl diphenylamine was ity of DODPA was 45.7%. The percentage conversion of
confirmed by reacting DPA with IB in the presence of DPA decreased to 79, 73.8, and 71.1% and the selectivity of
Engelhard F-24 catalyst in an autoclave at 383 K un- DODPA decreased to 25.5, 22.7% and 21.3% for the first,
der 3.4 bar pressure of IB; whereas the formation of 4- second, and third reuse of the catalyst, respectively.
0
(
1,1,3,3-tetramethylbutyl), 4 -tert-butyl diphenylamine, was
Effect of reflux methanol wash on catalyst regeneration.
The deactivated catalyst could be partially regenerated by
refluxing with methanol for 2 h followed by drying at 373 K
under vacuum for 6 h. The extent of catalyst regeneration
was studied for three consecutive batches by refluxing the
confirmed by reacting MODPA with IB in the presence of
Engelhard F-24 catalyst in an autoclave under same condi-
tions of temperature and pressure. n-heptane was used as a
solvent for both these reactions.
Effect of speed of agitation. The variation in stirring catalyst with methanol each time followed by drying. The
speed from 1000 to 1500 rpm had no effect on the rate percentage conversion of DPA was 100, 100, and 100% and
of reaction, indicating that the external mass transfer resis- the selectivity of DODPA was 37.8, 36.6, and 34% for the
tance was absent. Hence, all the further experiments were firstreuseaftermethanolwash, secondreuseaftermethanol
carried out at 1000 rpm.
wash, and third reuse after methanol wash, respectively.

ALKYLATION OF DIPHENYLAMINE
93
out the reaction in semibatch mode of addition. In the case
of reaction of DPA with DIB, the extent of dimerization of
DIB was negligible.
To summarize, under otherwise identical conditions,
AMS is a more reactive olefin than DIB for the alkylation of
DPA. Also in the case of reaction between DPA and AMS,
high selectivity of DCDPA could be realized at low tem-
perature, e.g., 363 K, and atmospheric pressure; whereas in
the case of reaction of DPA and DIB, to obtain the same
selectivity of DODPA, the reaction had to be carried out at
a higher temperature, i.e., 423 K, in an autoclave.
CONCLUSIONS
Acid-treated clay, Engelhard F-24, was found to be a very
effective catalyst for the alkylation of DPA with AMS and
DIB, to produce mono- and dialkylated diphenylamines.
It was possible to obtain high selectivity of each of the
FIG. 11. Effect of repeated use of catalyst on rate of conversion of
DPA. Reactionconditions:DPA : DIBmoleratio, 1 : 4;temperature, 423K; alkylated products by a proper selection of the reaction
catalyst loading, 10% Engelhard F-24.
conditions. The various other grades of acid-treated clays
were also found to be useful catalysts for the reaction be-
tween DPA and AMS. The reaction was unsuccessful with
Comparison of the alkylation data for AMS and DIB. the macroporous ion exchange resin catalyst, Amberlyst 15.
The similar features of alkylation of DPA with AMS and A comparison of the alkylation data for the two olefins indi-
DIB, were that the alkylation occurred at both the para cated that, for the alkylation of DPA, AMS is more reactive
positionsofDPAforboththeolefinsandtherewasnoortho, as compared to DIB.
meta, orNalkylationineithercase. Deactivationoftheacid-
treated clay catalyst occurred during the course of both the in a continuous mode of operation in a FBR, where high
alkylations. Also both these reactions were unsuccessful conversion of DPA and selectivity of DCDPA could be re-
with the macroporous ion exchange resin catalyst.
The rate of alkylation of DPA with AMS, was much
The reaction between DPA and AMS can be conducted
alized.
Deactivation of the catalyst occurred during the course
higher than that of DPA with DIB, e.g., in a reaction be- of these reactions. The characterization of the fresh and
tween DPA and AMS, with a mole ratio of DPA : AMS of used catalysts, of the reaction between DPA and AMS, by
1
: 2, 10% (w/w) loading of Engelhard F-24 catalyst, 100% various techniques indicated that the fresh catalyst had a
conversion of DPA was obtained, at 363 K after 1.5 h of re- microporous structure with a high surface area. The deacti-
action; whereas in a reaction between DPA and DIB, even vation of the catalyst was due to adsorption of DPA and/or
with a mole ratio of DPA : DIB of 1 : 3, 10% loading of En- thealkylatedproduct(s)onthesurfaceofthecatalyst, which
gelhard F-24 catalyst, only 55.1% conversion of DPA was led to a decrease in the surface area of the catalyst. The de-
obtained, at 373 K even after 6 h of reaction. A compari- activation was also due to loss of microporous structure of
son of the selectivities of dialkylated products of both the thecatalystandduetoadecreaseinthenumberofBrønsted
reactions, i.e., DCDPA and DODPA, indicated that high acid sites, which were responsible for the reaction, since the
selectivity (� 65%) of DCDPA was obtained even at low reaction is acid catalyzed.
temperature like 363 K, whereas the selectivity of DODPA
It was possible to regenerate the deactivated catalyst par-
wasverylow(� 12%)atatemperatureof373Kunderother- tially, by washing it with methanol under reflux and also by
wise identical conditions. At high temperatures, e.g., 423 K, the use of ultrasonic irradiation. Use of a solvent in the
the selectivity of DODPA reached a maximum (� 62%), at reaction produced an adverse effect on the catalyst regene-
a mole ratio of DPA : DIB of 1 : 5 and then decreased due ration.
to cracking of DIB into IB. In the case of reaction between
DPA and AMS, as there was no question of cracking of
ACKNOWLEDGMENTS
AMS, very high selectivity (� 95%) could be realized at a
mole ratio of DPA : AMS of 1 : 4 and 423 K. Dimerization of
SRCthanksU.G.C., NewDelhi, for awardingsenior researchfellowship
during tenure of this work. Thanks are due to Dr. Monika Datta for her
assistance in the study of the characterization of the catalyst by different
AMS occurred in the presence of acid-treated clay catalyst
even at low temperatures like 333 K. The extent of unde-
sired AMS dimerization could be suppressed by carrying methods.

9
4
CHITNIS AND SHARMA
Hrabinova, J., and Durmis, J., Czech, patent CS 261,547 (1989).
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1
2
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(1985).
3
4
5
. Uniroyal Chemical Company brochure, “Speciality Chemicals.”
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7
8
to Uniroyal, Inc.
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