Nanocrystals of Zeolite ZSM-5 as Catalysts for the Liquid Phase Benzylation of Anisole with Benzyl Alcohol
Radhika et al.
radiation (1.5418 Å) from 2.5ꢀ to 30ꢀ at a scan rate of
2ꢀ/min using a step size of 0.04ꢀ. Morphology of the
zeolite nanocrystals was determined with SEM (JEOL
JSM-6500f). FT-IR spectrum of the catalyst was recorded
using the KBr wafer technique (1.2% w/w) on a Perkin
Elmer FT-IR instrument. The spectrum was recorded from
zeolites in micro-regime. Also, diffusion resistance offered
to the reactant molecules within the micropores is consid-
erably reduced in nanocrystals. Yet, the main obstacle in
the wide-spread use of synthetic nanozeolites as catalysts
is the long time required to synthesize them. A relatively
new method, vacuum concentration coupled hydrothermal
synthesis has been reported to considerably lower the time
required to prepare nanocrystalline zeolites.20 Nanocrys-
talline zeolite ZSM-5 was prepared by vacuum concen-
tration coupled hydrothermal synthesis and was tested for
its activity in benzylation of anisole. To the best of our
knowledge, this is the first report on the use of a nanocrys-
talline ZSM-5 for catalyzing liquid phase benzylation of
anisole. The effects of reaction parameters such as catalyst
quantity, reaction time, temperature and anisole to benzyl
alcohol ratio were investigated and best conditions were
obtained. Extensive data obtained was subjected to kinetic
studies to test and arrive at a suitable mechanistic model
for the nano-zeolite catalyzed benzylation of anisole.
400 to 4000 cm−1
.
2.4. Catalyst Studies
The liquid-phase benzylation reaction of anisole was con-
ducted in a two-necked glass reactor of 100 mL capacity.
The reaction set up comprised of a round bottom flask
fitted with a reflux condenser and CaCl2 guard tube. A typ-
ical reaction mixture consisting of 93.3 mmol of anisole,
31.1 mmol of benzyl alcohol and 0.25 g of catalyst was
charged into the round bottom flask. The contents were
ꢀ
heated to 140 C for 60 min accompanied by magnetic
stirring. The temperature was maintained by immersing
the reaction system in thermostated oil bath. The aliquots
of reaction mixture were withdrawn periodically. After
centrifugation, these samples were analyzed by China
Chromatography 8900, using a SE-30 column and a FID
detector. The products were confirmed by GC-MS (Var-
ian star 3400 GC, Column: DB-5). The products of ben-
zylation of anisole using benzyl alcohol were determined
to be o-benzyl anisole, p-benzyl anisole and dibenzyl
ether (Fig. 1). The conversions into the desired benzy-
2. EXPERIMENTAL DETAILS
2.1. Chemicals
Commercial samples (Merck) of Aluminium isopropox-
ide (AlP), tetraethoxysilane (TEOS), tetrapropylammo-
nium hydroxide (TPAOH, 40% aq.), anisole and benzyl
alcohol were used without further purification.
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lated products and the byproduct dibenzyl figure ether are
reported as the percent of benzyl alcohol consumed in the
reaction.
2.2. Preparation of Catalysts
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In a typical procedure to prepare nanocrystalline ZSM-5
(Si/Al = 100), AlP was and TPAOH (40% aq) were trans-
ferred into a polypropylene (PP) bottle. The solution was
stirred by a magnetic stirrer for about 30 min to get
a clear solution. Into this, TEOS was added dropwise
together with deionized water. The resulting solution was
stirred vigorously at room temperature for about 2 h till a
clear solution was obtained. The clear solution indicated
that TEOS had been hydrolyzed completely. The molar
composition of the resultant solution was 1SiO2/4EtOH/
0.25TPAOH/0.01AlP/20H2O. The pH of this solution was
11.8. The clear sol was then concentrated to a viscous sol
3. RESULTS AND DISCUSSION
3.1. XRD
The XRD pattern of the synthesized catalyst is shown in
Figure 2. The peaks are characteristic of zeolite ZSM-5 of
MFI framework type. The broadening of the peaks is due
to the scattering effects of the nanoscale crystals.
3.2. SEM Analysis
ꢀ
The SEM micrograph of the synthesized nanocrystalline
zeolite ZSM-5 is shown in Figure 3. The particles show
spherical morphology and the particle size is approxi-
mately 50 nm. Small crystals are produced due to aging of
in a rotary evaporator at 80 C, as mentioned in the litera-
ture [20]. It had a H2O/SiO2 molar ratio of 3.55. The con-
ꢀ
centrated sol was then aged at 80 C for 24 h. The bluish
appearance of the sol suggested the initiation of crystal-
lization. The aged sample was then treated hydrothermally
in an autoclave at the crystallization temperature of 175 ꢀC
for 1 h. Following the hydrothermal treatment, the sample
obtained was translucent. This suggests that the product of
synthesis route is in the nanocrystallite form. The sample
was then centrifuged and dried.ꢀThe dried sample was then
ꢀ
the precursor sample at 80 C for 24 h. This step of aging
has increased the number of viable zeolite germ nuclei,
thereby resulting in smaller crystals. Also, the inclusion
of the concentration step in rotary evaporator creates more
supersaturated precursor and thus increases the rate of
nucleation and crystallization. It is our general observation
that ZSM-5 zeolites with higher Si/Al ratio are more fea-
sible to be synthesized. However, despite the high Si/Al
ratio, the catalytic activity of the zeolite prepared is not
compromised as it possess spherical morphology and small
particle size.
ꢀ
calcined in an air oven at 550 C for 6 h (2 C/min).
2.3. Characterization
Powder X-ray diffraction patterns were recorded using
a Rigaku 2000 diffractometer equipped with a Cu-Kꢁ
1330
J. Nanosci. Nanotechnol. 17, 1329–1337, 2017