Tertiary butylation of aniline over nanosized zeolite beta catalyst

Article abstract of DOI:10.1166/jnn.2018.15538

In the current paper, we report nanosized zeolite beta to be a robust and potent catalyst for tertbutylation of aniline. Nanocrystals of beta zeolite having spherical morphology were prepared via a one-pot strategy that is based on vacuum-concentration coupled hydrothermal method. The catalyst could be used in the as-synthesized form, without any further modifications. In the presence of the as-prepared catalyst, liquid-phase tert-butylation of aniline using tert-butanol as alkylating agent proceeded successfully. Reaction parameters like duration of reaction, temperature, mole ratio of substrates and quantity of catalyst were optimized. Time-dependent data obtained was subjected to kinetic studies. Detailed studies on product selectivity were conducted. The catalyst could selectively direct the formation of C–C bonds and no N-alkylated products were detected. It also demonstrated good selectivity towards mono-substitution. The catalyst could be recovered by a simple work-up plan and could then be re-used. No appreciable loss in activity was detected in consecutive runs. Thus, the nanosized beta mediated route to introduce tert-butyl group on aniline ring can be an environment-friendly alternative to the use of homogeneous catalysts.

Full text of DOI:10.1166/jnn.2018.15538

Article
Journal of
Nanoscience and Nanotechnology
Vol. 18, 7960–7968, 2018
Copyright © 2018 American Scientific Publishers
All rights reserved
www.aspbs.com/jnn
Printed in the United States of America
Tertiary Butylation of Aniline Over Nanosized
Zeolite Beta Catalyst
N. P. Radhika1, Rosilda Selvin2ꢀ∗, Rita Kakkar1, L. Selva Roselin3, and Hsiu-Ling Hsu4
1Department of Chemistry, University of Delhi, Delhi 110007, India
2Department of Chemistry, School of Science, Sandip University, Mahiravani, Nashik 422213, Maharashtra, India
3Department of Chemistry, Faculty of Science for Girls, Rabigh Campus, King Abdulaziz University, Jeddah-21589, Saudi Arabia
4Department of Chemicals and Materials Engineering, Lunghwa University of Science and Technology,
Guishan, Taoyuan County-33306, Taiwan (R.O.C)
In the current paper, we report nanosized zeolite beta to be a robust and potent catalyst for tert-
butylation of aniline. Nanocrystals of beta zeolite having spherical morphology were prepared via a
one-pot strategy that is based on vacuum-concentration coupled hydrothermal method. The catalyst
could be used in the as-synthesized form, without any further modifications. In the presence of
the as-prepared catalyst, liquid-phase tert-butylation of aniline using tert-butanol as alkylating agent
proceeded successfully. Reaction parameters like duration of reaction, temperature, mole ratio of
substrates and quantity of catalyst were optimized. Time-dependent data obtained was subjected to
kinetic studies. Detailed studies on product selectivity were conducted. The catalyst could selectively
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direct the formation of C–C bonds and no N-alkylated products were detected. It also demonstrated
Copyright: American Scientific Publishers
good selectivity towards mono-substitution. The catalyst could be recovered by a simple work-up
Delivered by Ingenta
plan and could then be re-used. No appreciable loss in activity was detected in consecutive runs.
Thus, the nanosized beta mediated route to introduce tert-butyl group on aniline ring can be an
environment-friendly alternative to the use of homogeneous catalysts.
Keywords: Heterogeneous Catalysis, Nanosized Beta, Aniline, Zeolite, Tert-Butylation,
Re-Usable Catalyst.
1. INTRODUCTION
To bring about electrophilic substitution on aromatic
nucleus, Friedel Crafts catalysts are primarily used. But,
despite the presence of strongly activating –NH2 group,
the usual aluminium halide Friedel Crafts catalysts fail to
accomplish aromatic amine alkylation effectively. This is
because this class of catalysts forms a deactivating com-
plex by binding with –NH2 group of aromatic amines.
Thereby, aluminium halides are prevented from acting in
a catalytic fashion in amine alkylation. Other catalysts
like Al–AlCl3 mixture, BF3 or aluminium anilides7–9 are
usually employed to prepare tert-butylated anilines from
aniline and isobutylene. However, the use of these homo-
geneous catalysts is laden with difficulties like high capital
cost, corrosion of equipment, disposal of aggressive waste
materials, tedious product work-up and non-recovery of
the catalyst. A surge in awareness of these drawbacks cou-
pled with a drive towards enforcement of rigorous laws
regarding the disposal of industrial effluents has spurred
extensive research on re-usable heterogeneous catalysts.
Aniline is a highly versatile starting material for the man-
ufacture of several compounds of profound use to the
society. In particular, tert-butylated anilines are useful
precursors in agrochemical, pharmaceutical and polymer
production.1–3 These can be obtained from aniline through
electrophilic aromatic substitution of amine aromatic ring
by tert-butyl group, in the presence of a suitable cata-
lyst. Besides electrophilic substitution, rearrangement is
also a plausible synthetic route to procure C-alkylated
amines. Along these lines, o-allyarylamines have been
synthesized by amino-Claisen rearrangement of aromatic
N-allylamines.4–6 Ring-alkylated aryl amines were also
obtained as side products in N-alkylation reactions. As a
matter of fact, compared to such N-alkylation reactions,
there have been fewer reports on selective C-alkylation of
amines.
∗Author to whom correspondence should be addressed.
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Radhika et al.
Tertiary Butylation of Aniline Over Nanosized Zeolite Beta Catalyst
Heterogeneous catalysts like iron oxide, montmoril-
lonite clay, heteropolyacids supported on clay and microp-
orous molecular sieves like aluminophosphates have been
successfully used to C-alkylate aromatic amine nucleus
directly and selectively.10–14 The reaction conditions main-
tained in these works indicate that C-alkylation is preferred
over N-alkylation at high temperatures, in the absence of
excess water and over catalysts with strong acid sites.
Zeolite is an important class of heterogeneous catalysts
that can be an attractive alternative to homogeneous cata-
lysts. Among the zeolites, commercial zeolite Y, ZSM-5,
mordenite and faujasite have been used to C-alkylate aro-
matic amines and diamines using alkenes and alcohols as
alkylating agents.15–24 Zeolite beta is a synthetic zeolite
which offers great promise in the field of heterogeneous
catalysis. Its structure manifests a 3-dimensional 12-ring
channel system, a feature highly conducive to outstand-
ing catalytic properties.25–27 It also possesses high ther-
mal stability. Yet, despite its favorable properties, zeolite
beta has not yet been explored for selective C-alkylation
reactions of aniline. In the present work, we study the
novel applicability of zeolite beta in tert-butylation of ani-
line, a reaction that is industrially significant, but cumber-
some by homogeneous catalysis. There have been many
reports in the recent past that have decisively pointed out
that as compared to commercially available bulk zeolites;
nanozeolites have potentially augmented catalytic abili-
2. EXPERIMENTAL DETAILS
2.1. Chemicals
Commercial samples (Merck) of Aluminium iso-propoxide
(AlP), tetraethoxysilane (TEOS), tetraethylammonium
hydroxide (TEAOH, 25% aq.), tert-butanol and aniline
were used without further purification.
2.2. Synthesis of Catalyst
In a typical procedure to prepare nanosized zeolite beta
(Si/Al = 50), a three-step strategy was adopted. In the first
step, a clear solution was obtained by magnetically stir-
ring a mixture of AlP, TEAOH (25% aqueous), deionized
water and TEOS, for 2 h. The clear solution ensured that
TEOS has been hydrolyzed completely. The pH of this
solution was 11.0. In the next step, the clear sol was con-
ꢀ
centrated to a viscous sol in a rotary evaporator at 80 C,
as mentioned in ꢀthe literature.35 The concentrated sol was
then aged at 90 C for 3 days. In the final step, the aged
sample was treated hydrothermally in an autoclave at the
ꢀ
crystallization temperature of 175 C for 6 h. The sample
was then centrifuged and dried. The dried sample was then
ꢀ
calcined at 600 C for 3 h.
2.3. Characterization of the Catalyst
Powder X-ray diffraction patterns were recorded using a
Rigaku 2000 diffractometer equipped with a Cu-Kꢁ radi-
ation (1.5418 Å) from 2.5ꢀ to 60ꢀ at a scan rate of 2ꢀ/min
using a step size of 0.04ꢀ. Morphology of the zeolite nano-
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ties. This has been attributed to the presence of greater
Copyright: American Scientific Publishers
fraction of active sites on the nano-catalyst surface and
crystals was studied with SEM (JEOL JSM-6500f).
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higher diffusion mobility of molecules to and fro the active
sites in nanozeolites.28–31 Specifically, nanomorphic zeo-
lite beta has been shown to be exceptionally better than
its bulk counterpart in the test reaction, benzylation of
benzene.32 Hence, it deem fit to choose and prepare nano
forms of zeolite beta over the commercial bulk samples.
Along these lines, nanoparticles of beta were prepared by
adopting a relatively quick vacuum-concentration coupled
hydrothermal method. The efficiency of this nano catalyst
was then tested and compared with commercial beta in
selective C-alkylation of aniline with tert-butanol. Prelim-
inary kinetic studies were done to determine the apparent
activation energy. To the best of our knowledge, this is
the first report on the applicability of a nanozeolite in tert-
butylation of aniline.
Even though isobutylene is generally the most com-
mon tert-butylating agent, there are difficulties associated
with its availability in high degrees of purity. It is usually
contaminated with other C4 isomers such as 1-butene or
2-butene.13ꢀ33 Also, it is a highly explosive and flammable
gas. Alternatively, isobutylene can be generated in situ
from tert-butanol or methyl tert-butyl ether (MTBE). How-
ever, the use of MTBE has been restricted in many parts
of the world as it is known to contaminate ground water
reserves.34 For these reasons, in this study, tert-butanol was
chosen as the alkylating agent over isobutylene or MTBE.
2.4. Catalytic Studies
The test reaction, tert-butylation of aniline in the pres-
ence of beta zeolite was carried out in a 100 ml stain-
less steel Parr autoclave under autogenous pressure in
the temperature range 140 C–170 ꢀC for 6 h. A pre-
ꢀ
determined quantity of reactants and catalyst were charged
into the autoclave and the temperature was raised to the
desired value. A typical experiment consisted of a reac-
tion mixture of 0.01 mol aniline, 0.05 mol tert-butanol and
ꢀ
0.20 g of beta catalyst at 140 C for 6 h. The products
were analyzed by gas chromatographic analysis (Hewlett
Packard 5890, OV-101 column) using flame ionization
detector (FID). Product identification was done by gas
chromatography-mass spectrometry (GC-MS). The prod-
ucts were determined to be 2-tert-butylaniline (2-TBA),
4-tert-butylaniline (4-TBA) and 2,4-di-tert-butylaniline
(2,4-DTBA) (Scheme 1).
The extent of reaction is expressed in terms of the per-
centage of aniline converted into alkylated products. The
influence of reaction parameters such as reaction time,
catalyst quantity, aniline to tert-butanol mole ratio and
reaction temperature was studied. Kinetic data was ana-
lyzed to determine the apparent activation energy for the
reaction. Also, a mechanistic route is suggested for the
reaction.
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Tertiary Butylation of Aniline Over Nanosized Zeolite Beta Catalyst
Radhika et al.
NH2
NH2
NH2
NH2
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
Nanosized zeolite
beta
HO
+
+
+
H3C
CH3
CH3
H3C
CH3
CH3
Aniline
tert-butanol
2-tert-butylaniline
(2-TBA)
4-tert-butylaniline
(4-TBA)
2,4-di-tert-butylaniline
(2,4-DTBA)
Scheme 1. Product distribution in tert-butylation of aniline over nanosized zeolite beta.
3. RESULTS AND DISCUSSION
3.1. Characterizations of the Catalyst
morphology. The formation of spherical particles can be
traced to Ostwald ripening effect.38 The particle size is
approximately 30 nm. This shows the success of the syn-
thesis protocol in directing the formation of small-scale
zeolite nanoparticles. The use of TEAOH as the organic
structure-directing agent (SDA) and the incorporation of
concentration and aging steps lead to nanomorphic zeolite
beta. Because of the addition of vacuum concentration step
and low-temperature aging prior to the hydrothermal crys-
tallization, more number of viable nuclei could be created
which subsequently developed into small crystals. These
steps thus boosted the rate of nucleation over crystalliza-
tion which ultimately facilitated nanoparticle formation in
a relatively short period of time.
XRD pattern of the synthesized catalyst is shown in
Figure 1. Good crystallinity is indicated as the diffrac-
tograms show the characteristic peaks at 2ꢂ = 6ꢃ98ꢀ–8.31ꢀ
and 22.46ꢀ–23.86ꢀ, which are characteristic of zeolite beta
of BEA∗ framework.36ꢀ37 The broad peak at low angle
region (6.98–8.31ꢀ) is due to the overlap of four peaks at
2ꢂ = 6ꢃ98ꢀ ((100) plane, polymorph A), 7.74ꢀ ((101) plane,
polymorph A), 7.34ꢀ ((110) plane, polymorph B) and 8.31ꢀ
ꢀ
¯
((111) plane, polymorph B). The broad peak at 22.46 –
23.86ꢀ can be assigned to the overlap of peaks at 2ꢂ =
ꢀ
ꢀ
¯
22ꢃ46 ((311) plane, polymorph A), 22.64 ((422) plane,
polymorph B), 22.65ꢀ ((240) plane, polymorph B), 23.03ꢀ
ꢀ
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((222) plane, polymorph B) and 23.86 ((113) plane, poly-
Copyright: American Scientific Publishers
3.2. Catalytic Activity
morph B). Thus, the crystals obtained can be rationalized
to contain both polymorphs A and B.
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Systematic investigation was conducted to test the effi-
ciency of the synthesized nanosized zeolite beta in tert-
butylation of aniline with tert-butanol, under autogenous
autoclave pressure. The effects of various parameters
like reaction time, catalyst quantity, reactant mole ratio
and temperature were studied. The optimized conditions
that have been so arrived at are set forth in following
sub-sections.
The SEM micrograph of the synthesized zeolite
is shown in Figure 2. The particles show spherical
Figure 1. XRD profile of nanosized zeolite beta.
Figure 2. SEM image of nano sized zeolite beta.
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Radhika et al.
Tertiary Butylation of Aniline Over Nanosized Zeolite Beta Catalyst
Figure 3. Effect of reaction time on aniline percent conversion in tert-
butylation of aniline (catalyst, 0.20 g nanosized zeolite beta; aniline, 0.01
mol; tert-butanol, 0.05 mol; aniline:tert-butanol, 1:5; temperature, 150 ꢀC;
pressure, autogenous pressure in 100 ml autoclave).
3.2.1. Effect of Reaction Time
In order to investigate the effect of reaction time, series
of experiments under a specified set of conditions were
conducted and reactant conversions were tracked at regular
intervals. With increase in the duration of reaction, the
change in extent of reaction becomes progressively less
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Copyright: American Scientific Publishers
This is because as time passes and reactants get con-
pronounced (Fig. 3).
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sumed in the reaction, the rate of adsorption of the reac-
tant molecules on the catalyst surface decreases. Also with
time, more and more product molecules get accumulated
in the reactant mixture and there is a decrease in their rate
of desorption from the catalyst sites. In fact, most of the
aniline conversion was obtained in the first 6 h (Fig. 3).
Accordingly, further conditions were monitored by allow-
ing the reaction to occur for 6 h.
Figure 4. Effect of the amount of catalyst on (a), aniline percent con-
version (b), product selectivity in tert-butylation of aniline (catalyst, nano-
sized zeolite beta; aniline, 0.01 mol; tert-butanol, 0.05 mol; reaction time,
6 h; aniline:tert-butanol mole ratio, 1:5; temperature, 140 ꢀC; pressure,
autogenous pressure in 100 ml autoclave).
the catalyst mass is nearing the superfluous value. Cat-
alyst quantity of 0.20 g seems to provide enough active
sites to well-accommodate most of the reactant molecules
available at the chosen concentration of reactants. Besides,
when catalyst quantity is increased to 0.25 g, less selec-
tivity is displayed and di-substituted product is also pro-
duced (3%) (Fig. 4(b)). This is in tandem with a decrease
in the yield of 2-tert-butylaniline. It is thus implicit that
as the number of active sites becomes progressively more,
they tend to become less discriminatory and catalyze di-
substitutions alongside mono-substitutions. Hence, taking
into the account the selectivity and economic factors, fur-
ther studies were conducted using 0.20 g of the catalyst.
3.2.2. Effect of Catalyst Quantity
The influence of the amount of catalyst on percent conver-
sion of aniline after a fixed reaction time (6 h) was exam-
ined. Sets of experiment were conducted by varying the
catalyst amount in the range 0.05 g to 0.25 g in a reaction
mixture containing 0.01 mol of aniline and 0.05 mol of
tert-butanol. The temperature was kept constant at 140 ꢀC.
GC-MS studies confirmed that no product formation took
place in the absence of the catalyst. Thus the nanosized
zeolite beta plays a pivotal role in driving the reaction
by providing a surface and active sites to the reactants
to undergo a series of bond-breakings and bond-makings.
With an increase in the catalyst amount, initially, the per-
cent conversion of aniline increased (Fig. 4(a)). This is due
to a concomitant increase in the number of active sites.
But beyond 0.20 g, the increase in percent conversion is
only marginal. This trend towards a plateau suggests that
3.2.3. Effect of Mole Ratio
The effect of aniline to tert-butanol mole ratio was studied
by keeping the total volume of the reactant mixture con-
stant. An increase in the amount of alkylating agent aug-
mented the reactant conversion, but diminished the product
selectivity (Fig. 5(a)). Beyond aniline: tert-butanol mole
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Tertiary Butylation of Aniline Over Nanosized Zeolite Beta Catalyst
Radhika et al.
Figure 5. Effect of mole ratio of reactants on (a), aniline percent conversion (b), product selectivity in tert-butylation of aniline (catalyst, 0.20 g
nanosized zeolite beta; reaction time, 6 h; temperature, 140 ꢀC; pressure, autogenous pressure in 100 ml autoclave).
ratio of 1:5, there is not much increase in the percent con-
version of aniline. At aniline to tert-butanol ratio 1:10, the
aniline amount has approached the limiting value. More-
over, higher concentrations of alkylating agent are promot-
ing di-substitution at the cost of 2-tert-butylaniline yield.
This is detrimental for the selectivity towards the mono-
substituted product (Fig. 5(b)). Hence, it can be concluded
All other reaction parameters were kept constant. Data
collected indicates that temperature affected the rate of
conversion markedly, particularly in the lower ranges
(Fig. 6(a)). Temperature also had an influence on the prod-
uct selectivity ((Fig. 6(b)). Apparently, percentage yield
of 2,4-DTBA increased, at the cost of reduction of selec-
tivity towards mono-alkylated products, with an increase
in reaction temperature. This takes place despite the fact
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that for the reaction under study, optimum aniline to tert-
Copyright: American Scientific Publishers
butanol mole ratio is 1:5.
that high temperature conditions can cause faster des-
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orption of mono-substituted products from catalyst sites,
thereby preventing their further alkylations. It can thus
be elucidated that at high temperatures, the fraction of
molecules having energy equal to or greater than the
3.2.4. Effect of Temperature
The reaction was conducted by varying the temperature
in the range of 140 ꢀC to 170 ꢀC, in steps of 10 ꢀC.
Figure 6. Effect of temperature on (a), aniline percent conversion (b), product selectivity in tert-butylation of aniline (catalyst, 0.20 g nanosized
zeolite beta; aniline, 0.01 mol; tert-butanol, 0.05 mol; reaction time, 6 h; aniline:tert-butanol mole ratio, 1:5; pressure, autogenous pressure in 100 ml
autoclave).
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Radhika et al.
Tertiary Butylation of Aniline Over Nanosized Zeolite Beta Catalyst
Figure 9. SEM image of nanosized zeolite beta after the fourth cat-
alytic run.
Figure 7. Comparison of efficiency of nano-beta with commercial beta
in tert-butylation of aniline (catalyst amount, 0.20 g; aniline, 0.01 mol;
tert-butanol, 0.05 mol; aniline:tert-butanol mole ratio, 1:5; reaction time,
6 h; temperature, 140 ꢀC; pressure, autogenous pressure in 100 ml
autoclave).
3.2.5. Comparison with Bulk Beta Catalyst
A comparative study was done to assess the efficiency
and product selectivity that can be achieved in the pres-
ence of nanosized zeolite beta relative to commercial bulk
beta in tert-butylation of aniline, under identical reaction
activation energy for di-substitutions must have increased
and this factor outweighs the desorption effects. Signifi-
cantly, at 140 ꢀC (413 K), there is an exclusive production
of mono-alkylated products. Our findings also reveal that
Table I. Percent conversion of aniline in tert-butylation of aniline at
different temperatures.
2-tert-butylaniline is more prone to di-substitution than the
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Copyright: American Scientific Publishers
corresponding para isomer. The increase in percent con-
Temperature (K)
423 433
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version to 2,4-DTBA was accompanied by a conspicuous
Time (h)
413
443
decrease in the selectivity of 2-TBA. The effect on selec-
tivity towards 4-TBA was less evident. This trend was the
same for all reaction parameters studied (Figs. 4–6).
0
2
4
6
0
0
0
0
40.3
52.1
74.3
43.5
72.1
85.8
56.9
82.1
93.0
75.2
91.0
96.7
Notes: Reaction condition: Catalyst, 0.20 g nanosized zeolite beta; aniline, 0.01 mol;
tert-butanol, 0.05 mol; tert-butanol:aniline mole ratio, 5:1; pressure, autogenous
pressure in 100 ml autoclave.
Figure 8. Reusability of nanosized zeolite beta in tert-butylation of ani-
line (reaction conditions: Catalyst, 0.20 g nanosized zeolite beta; aniline,
0.01 mol; tert-butanol, 0.05 mol; aniline:tert-butanol mole ratio, 1:5; reac-
tion time, 6 h; temperature, 140 ꢀC; pressure, autogenous pressure in
100 ml autoclave).
Figure 10. Kinetic plot for tert-butylation of aniline.
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Tertiary Butylation of Aniline Over Nanosized Zeolite Beta Catalyst
Radhika et al.
conditions. As expected, due to shorter diffusion pathways
and greater density of accessible acid sites, nano-form
showed significantly much better performance than the
bulk beta, both in terms of yield and selectivity (Fig. 7).
As the products could desorb faster from nanosized zeo-
lite beta, they were less available for di-substitution which
consequently improved selectivity towards monoalkylated
products.39
3.2.6. Catalyst Reusability
The recoverability and reusability of the catalyst was tested
for upto four runs. The catalyst after each use was cen-
trifuged and was washed with ethanol. It was then dried
ꢀ
at 120 C for 24 h, and was reused under optimum reac-
tion conditions. There was no appreciable decline in the
percent conversion of aniline in consecutive runs after the
fresh use (Fig. 8).
Figure 11. Arrhenius plot.
NH
H C
2
3
CH
3
H C
NH
H
3
2
H
CH
NH
3
2
H
O
H
O
CH
On zeolite surface
(a)
3
CH
3
OR
H
H
+
CH
3
H C
OH
3
CH
3
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(b)
Copyright: American Scientific Publishers
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NH
2
H C
NH
3
2
CH
3
CH
OR
3
CH
H C
+
+
H O
2
3
3
CH
CH
3
OH
H O
CH
2
3
H C
3
3
Disubstitution
(c)
H C
3
NH
2
CH
3
CH
3
H3C
CH3
NH
CH
CH3
2
CH
CH
3
NH
NH
3
2
2
CH
CH
3
3
CH
CH
+
+
H O
3
3
2
+
H C
CH
3
3
H3C
CH3
3
CH3
(d)
Scheme 2. Mechanistic pathway for tert-butylation of aniline (a) adsorption of both reactants on the catalyst surface (b) protonation from the
catalyst and attack of tert-butanol on aniline (c) formation of mono-substituted products which may undergo further substitution (d) desorption of the
products.
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Radhika et al.
Tertiary Butylation of Aniline Over Nanosized Zeolite Beta Catalyst
Though insignificant, the slight decrease in the percent
conversion observed in subsequent runs can be attributed
to the particle agglomeration that took place, as also indi-
cated in the SEM image of the used catalyst (Fig. 9).
The spherical morphology of the nano-catalyst remains
unaltered even after repetitive use (Fig. 9). It follows
that the catalyst is quite sturdy to be reused. After the
removal of the catalyst by centrifugation, the reaction was
conducted in the supernatant for another 2 h, under the
optimized reaction conditions. No further change in the
product concentration was observed in this duration. This
indicates the easy recoverability of the catalyst and its true-
heterogeneous character.
4. CONCLUSIONS
Nanosized zeolite beta was fabricated successfully by a
one-pot, facile, vacuum concentration-hydrothermal-based
process. The as-synthesized catalyst proved to be effi-
cient in direct and selective tert-butylation of aniline.
It could selectively direct the formation of C–C bonds,
as opposed to N-alkylation. Additionally, it showed high
selectivity towards mono-tert-butylated aniline relative to
the di-substituted products. The effects of various reac-
tion parameters were investigated and optimum conditions
were derived. Under optimum conditions, aniline percent
conversion of 74% and 100% selectivity towards mono-
substituted products were recorded.
The as-synthesized catalyst also demonstrated superior
performance (1.6 times more conversion under identical
conditions) when compared to commercial beta sample.
Thus the nano-scaling proved to be vital in enhancing
the catalytic ability of zeolite. The nanocatalyst could be
recovered easily and re-used without any appreciable loss
of activity. This renders the whole process economically
reasonable and environmentally benign. Additionally, only
water was produced as the by-product, which makes the
process even more eco-friendly. From the fitting of time-
dependent data, apparent activation energy in the presence
of catalyst was determined to be 51.3 kJ/mol. The reaction
is thus essentially in the kinetic regime.
3.3. Kinetic Studies
Depending on whether one or both reactants are
adsorbed, reactions on heterogeneous catalysts may follow
Eley–Rideal or Langmuir–Hinshelwood–Hougen–Watson
(LHHW) mechanism. Yet, LHHW mechanism is more
probable as the time-scale for a gas-surface collision
is extremely short, in orders of pico-second.40 In an
attempt to obtain the apparent activation energy and fre-
quency factor for the reaction between aniline (A) and
tert-butanol, the fractional conversion of aniline as a
function of time was obtained at different temperatures
(Table I). The data was tried to be fitted into inte-
In conclusion, our findings imply that nanosized zeolite
grated rate equation for second-order kinetics on the
IP: 81.22.46.145 On: Wed, 23 Jan 2019 01:09:25
beta can be an environment-friendly alternative to homoge-
neous catalysis in the challenging reaction, tert-butylation
of aniline.
Copyright: American Scientific Publishers
surface of solid-acid catalysts, as has been previously
reported.13
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ꢄM −XAꢅ
ln
= ꢄM −1ꢅkRꢆAꢇ0t
(1)
References and Notes
Mꢄ1 −XAꢅ
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C
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7967

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Received: 17 July 2017. Accepted: 29 October 2017.
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7968
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