Vapor phase methylation of indole over nanocrystalline Cd1-xCrxFe2O4 (x = 0, 0.25, 0.5, 0.75 and 1.0) ferrospinels

Article abstract of DOI:10.14233/ajchem.2016.19721

Vapor phase methylation of indole has been carried out over nanocrystalline Cd_1-xCr_xFe₂O₄ (x = 0, 0.25, 0.5, 0.75 and 1.0) ferrospinels in a fixed bed down-flow reactor. Catalyst characterization was performed by Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy with EDX, BET surface area and temperature programmed ammonia-desorption method. Product selectivity is shown to be strongly influenced by acidic properties of the catalyst. A maximum yield of 64.29 % of 3-methyl indole with 91.46 % selectivity at 70.3 % indole conversion was obtained under optimized reaction conditions.

Full text of DOI:10.14233/ajchem.2016.19721

Asian Journal of Chemistry; Vol. 28, No. 7 (2016), 1474-1478
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2016.19721
Vapor Phase Methylation of Indole over Nanocrystalline
Cd1-xCrxFe2O4 (x = 0, 0.25, 0.5, 0.75 and 1.0) Ferrospinels
1,*
1
1
2
M. KADIAN , P. SIWACH , R.K. GUPTA and A. DUHAN
1Department of Chemistry, Guru Jambheshwar University of Science & Technology, Hisar-125 001, India
2Department of Agronomy, College of Agriculture, Chaudhary Charan Singh Haryana Agriculture University, Hisar-125 001, India
*Corresponding author: E-mail: malvikadian@gmail.com
Received: 19 November 2015;
Accepted: 13 January 2016;
Published online: 31 March 2016;
AJC-17832
Vapor phase methylation of indole has been carried out over nanocrystalline Cd1-xCrxFe2O4 (x = 0, 0.25, 0.5, 0.75 and 1.0) ferrospinels in
a fixed bed down-flow reactor. Catalyst characterization was performed by Fourier transform infrared spectroscopy, X-ray diffraction,
scanning electron microscopy with EDX, BET surface area and temperature programmed ammonia-desorption method. Product selectivity
is shown to be strongly influenced by acidic properties of the catalyst. A maximum yield of 64.29 % of 3-methyl indole with 91.46 %
selectivity at 70.3 % indole conversion was obtained under optimized reaction conditions.
Keywords: Ferrospinels, Alkylation, Indole, 3-Methyl indole.
ensuing Na[Cd(OH3)] solution was added to a 5 dm3 beaker
INTRODUCTION
containing 0.15 mol of FeCl3·6H2O in 2.5 dm3 of 0.6 M HCl
The discovery of mixed metal oxides possessing spinel
and stirred for 2 h. The resulting mixture was further heated
structure has drawn a lot of attention owing to their high thermal
at 333 K for 0.5 h. The mixture was allowed to settle down at
stability, easy separation, re-usability and environmentally
room temperature and then reacted with 2 M NaOH till a
benign nature [1-3]. In recent years, abundant research has
permanent pink colour of phenolphthalein was obtained. Then
been conducted on ferrospinels because of their fascinating
product was washed by continual decantation with distilled
water till the supernatant was freed from Cl–. The product was
structural, electrical, magnetic and catalytic properties [4-7].
They have peculiar cation distribution among the two
filtered, dried in an oven at 393 K and further calcined at 773
interstitial sites i.e., tetrahedral (Td) and octahedral (Oh) sites
of the co-ordinated oxygen atom. Indole and its derivatives
K for 16 h and finally sieved through a 6/10 mesh size sieve.
Preparation of other catalysts: Preparation of CrFe2O4
are extensively used in perfumery, in tryptophan synthesis (an
(CC-5) was similar to CC-1 as described above, except that now
amino acid commonly found in animal protein) and as an
0.075 mol of CrCl3·6H2O was used. Similarly Cd0.75Cr0.25Fe2O4
intermediate within the synthesis of plant growth hormone
(CC-2), Cd0.5Cr0.5Fe2O4 (CC-3), Cd0.25Cr0.75Fe2O4 (CC-4) was
like auxin, anti-inflammatory agents, dyes, agrochemicals and
prepared by taking 0.0562, 0.0375 and 0.0187 mol of
antibiotics. 3-Methyl indole also known as skatole have been
CdCl2·H2O and 0.0187, 0.0375 and 0.0562 mol of CrCl3·6H2O,
intensively used as an antidiuretic, stimulant, antihypertensive,
respectively.
muscular relaxant, heart stimulant and respiratory inhibitor
The vapor phase methylation of indole was carried out in
and have additionally been found to stimulate the dimerization
a continuous fixed bed down-flow glass reactor. The upper
of butadiene [8-10]. The current study is based on nanocrystalline
half worked as a pre-heater and the lower half worked as a
Cd1-xCrxFe2O4 (x = 0, 0.25, 0.5, 0.75 and 1.0) ferrospinels
reactor where 9 g of 6/10 mesh size catalyst was placed
for extremely selective synthesis of skatole via vapor phase
methylation of indole.
between two plugs of glass wool, activated at 773 K for 2 h
under a flow of air and then brought down to the desired
temperature by cooling in a current of N2 gas of 30 cm3/min.
EXPERIMENTAL
The reactants were fed from the top with a current of N2 gas of
30 cm3/min and the products were collected from an ice-cold
trap at the bottom of the reactor. Gas chromatogram was used
to determine the composition of the product mixture.
Preparation of CdFe2O4 (CC-1): A solution of 1.2 mol
of NaOH in 150 cm3 of water was allowed to react with a
solution of 0.075 mol of CdCl2·H2O in 50 cm3 of water. The

Vol. 28, No. 7 (2016)
Vapor Phase Methylation of Indole over Nanocrystalline Cd1-xCrxFe2O4 Ferrospinels 1475
Catalyst characterization: The different compositions
of the Cd1-xCrxFe2O4 ferrospinels were characterized by various
physico-chemical techniques i.e., FTIR, XRD, SEM with EDX,
BET surface area and ammonia desorption methods. FTIR
spectra for all the compositions of Cd1-xCrxFe2O4 ferrospinels
consist of two significant bands around 700 cm-1 and 500 cm-1
due to stretching vibration of the tetrahedral M-O group and
the octahedral M-O group respectively [11]. The difference in
the band position is due to the difference in the Fe3+–O2-
distance for tetrahedral and octahedral sites. X-ray diffraction
pattern of Cd1-xCrxFe2O4 ferrospinels was studied by Rigaku
X-ray diffractometer in the 2θ range of 10-80° using Cu Kα
radiations (λ = 1.54 Å) at a scan rate of 2°/min as shown in
Fig. 1. All the peaks in the XRD analysis match well with the
characteristic reflections of corresponding ferrites and this
reveals a single phase spinel structure.
CC-5
CC-4
CC-3
CC-2
CC-1
20
40
60
80
2θ (°)
Fig. 1. X-ray diffractogram of Cd1-xCrxFe2O4 ferrospinels
The BET surface area of Cd1-xCrxFe2O4 ferrospinels was
determined with the help of OMNISORP 100 CX instrument
and the results are represented in Table-1. The micro-structure
and the suface morphology of the Cd1-xCrxFe2O4 ferrospinels
were investigated by using scanning electron microscopy
(SEM model HITACHI S-3700 N). SEM micrographs are
shown in Fig. 2. From the micrographs, it is clear that the
obtained ferrospinels are highly dense and have almost
spherical structure with different grain size. The particle size
calculated using scanning electron microscopy was found to
be in the nanometer range. The compositional analysis of the
obtained Cd1-xCrxFe2O4 ferrospinels was carried out using EDX
as shown in Fig. 3. The results of EDX confirm the formation
TABLE-1
COMPOSITIONAL PARAMETER ‘x’, ACIDITY, CATALYTIC
ACTIVITY IN TERMS OF % INDOLE CONVERSION AND BET
SURFACE AREA OF Cd1-xCrxFe2O4 FERROSPINELS
Acidity
(Total NH3
uptake in
mmol/g)
Catalytic
activity (%
Indole
BET
X
Cd1-xCrxFe2O4
surface
area (m2/g)
conversion)
0.00 CdFe2O4
1.14
1.31
1.47
1.65
1.73
49.8
53.1
56.9
62.4
70.3
65.52
61.09
56.63
49.84
43.26
0.25 Cd0.75Cr0.25Fe2O4
0.50 Cd0.5Cr0.5Fe2O4
0.75 Cd0.25Cr0.75Fe2O4
1.00 CrFe2O4
CC-1
CC-2
CC-3
CC-4
CC-5
Fig. 2. SEM images of Cd1-xCrxFe2O4 ferrospinels

1476 Kadian et al.
Asian J. Chem.
Cd
O
Cd
150
100
50
O
Cr
CC-2
CC-1
CC-3
150
O
150
100
50
Fe
Fe
Cd
Fe
100
Fe
Fe
Fe
50
Cr
Cr
Fe
Fe
7
Fe
7
0
0
0
1
2
3
4
5
keV
6
8
9
10
1
2
3
4
5
keV
6
8
9
10
1
2
3
4
5
6
7
8
9
10
keV
O
Cr
Fe
400
O
Cr
500
400
300
200
100
0
CC-4
CC-5
300
200
100
0
Fe
Fe
Fe
Cr
Cd
Cr
Fe
Cr
6
Fe
Cr
2
4
8
keV
10
12
14
2
4
6
8
10 12 14 16 18
keV
Fig. 3. EDX Spectra of Cd1-xCrxFe2O4 ferrospinels
TABLE-2
of ferrites with desired composition. Acidity values of all the
Cd1-xCrxFe2O4 ferrospinels were determined by temperature
programmed ammonia-desorption method and the values are
shown in Table-1.
PERFORMANCE OF VARIOUS CATALYSTS IN
METHYLATION OF INDOLE AT 598 K, WHSV 0.5 h-1
AND INDOLE/METHANOL MOLAR RATIO OF 1:6
3-Methyl-
Indole
conversion
(%)
3-Methyl-
indole
yield (%)
indole
selectivity
(%)
Others
yield (%)
Catalyst
RESULTS AND DISCUSSION
The present study involves the vapor phase alkylation of
indole with methanol as the alkylating agent for the highly
selective synthesis of 3-methyl indole. The surface area data,
acidity values and performance of various catalysts in the
alkylation of indole is presented in Table-1. It has been found
that acidity of the system follows the order CC-5 > CC-4 > CC-
3 > CC-2 > CC-1. The order of catalytic activity of ferrospinels
toward overall conversion was found to be CC-5 > CC-4 > CC-
3 > CC-2 > CC-1. An examination of Table-1 reveals that the
acidity and surface area of Cd1-xCrxFe2O4 ferrospinels increases
with increasing ‘x’ values and the better performance of CC-5
as compared to CC-1 can be ascribed to its higher acidity.
Effect of catalyst composition: Catalytic activity as a
function of composition of catalyst is shown in Table-2.Among
the various composition of the system, the catalytic activity
with reference to indole conversion increases with increase in
‘x’ value of Cd1-xCrxFe2O4 ferrospinels. Systems possessing
higher ‘x’ values were found to be more active and selective
for 3-methyl indole formation since acidic sites favour 3-methyl
indole formation.
CC-1
CC-2
CC-3
CC-4
CC-5
49.8
53.1
56.9
62.4
70.3
34.45
38.76
45.84
53.25
64.29
69.18
72.95
80.57
85.34
91.46
15.35
14.34
11.06
9.15
6.01
maximum yield of 3-methyl indole was obtained at 598 K.
Indole conversion and selectivity for 3-methyl indole formation
increases with rise in temperature from 548 to 598 K and above
598 K, indole conversion increases but selectivity of 3-methyl
indole decreased due to the generation of strong lewis acid
sites at higher reaction temperature [12] and such sites enhance
the formation of polyalkylated product, hence decreasing the
selectivity of 3-methyl indole.
Effect of molar ratio of indole to methanol: The effect
of molar ratio of reactants on the vapor phase methylation of
indole was carried out over CC-5 at temperature 598 K and
0.5 h-1 weight hour space velocity (WHSV) and the results are
represented in Fig. 5. It has been found that the indole conver-
sion and the selectivity of 3-methyl indole increases with increase
in indole to methanol molar ratio and reaches a maximum
at 1:6. At higher molar ratio selectivity of 3-methyl indole
decreased due to unavailability of active sites.
Effect of reaction temperature: A series of indole alky-
lation reactions were performed in the temperature range of
548-673 K over CC-5 and the results are shown in Fig. 4. The

Vol. 28, No. 7 (2016)
Vapor Phase Methylation of Indole over Nanocrystalline Cd1-xCrxFe2O4 Ferrospinels 1477
Indole conversion (%)
Mechanism: Indole is a nucleophilic heterocycle and its
excessive π-character makes indole more susceptible to
electrophilic substitution reactions at 3-position of indole ring
to form 3-methyl indole as the major product. The preferred
site for electrophilic substitution is C-3 rather than C-2, since
the cation formed by attack at C-3 is more stable as compared
to C-2 because the positive charge can be delocalized without
involving the benzenoid part of the molecule [13,14]. Further
alkylation results in the formation of polyalkylated products
as shown in Scheme-I.
3-Methyl indole yield (%)
3-Methyl indole selectivity (%)
Others yield (%)
90
80
70
60
50
40
30
20
10
0
540 560 580 600 620 640 660 680
N
Temperature (K)
H C O
3
H
H
Indole/Methanol
Fig. 4. Effect of reaction temperature on indole methylation (Catalyst: CC-
5, WHSV: 0.5 h-1, indole/methanol molar ratio 1: 6)
Step-1
Step-2
Acidic sites
Basic sites
H
H
Indole conversion (%)
3-Methyl indole yield (%)
3-Methyl indole selectivity (%)
Others yield (%)
100
90
80
70
60
50
40
30
20
10
0
N
N
H
H C O
H
H C O H
3
3
H
H
CH
3
Step-3
N
N
H C O
3
H
H
H
+
H O
2
CH
3
N
H
Step-4
Polyalkylated products
H C O
3
H
1:2
1:4
1:6
1:8
+
Molar ratio
Scheme-I
Fig. 5. Effect of molar ratio on indole methylation (catalyst: CC-5, WHSV:
0.5 h-1, Temperature: 598 K)
Conclusion
Effect of weight hour space velocity (WHSV): The effect
of WHSV on methylation of indole was studied over CC-5 at
598 K, indole to methanol molar ratio of 1:6 in the range of
0.1 to 0.7 h-1 WHSV and the results are shown in Fig. 6. Indole
conversion increased with increase in WHSV and reaches a
maximum at 0.5 h-1 and thereafter decreased. The high contact
time causes charring over active sites, therefore decreasing
the indole conversion below 0.5 h-1 and above 0.5 h-1 indole
conversion further decreases due to reduced contact time.
Nanocystalline Cd1-xCrxFe2O4 (x = 0, 0.25, 0.5, 0.75 and
1.0) ferrospinels prepared via low temperature coprecipita-
tion method were studied for the alkylation of indole using
methanol as the alkylating agent. It was found that these
nanocrystalline ferrospinels can effectively alkylate indole to
give a maximum yield of 64.29 % of 3-methyl indole with
91.46 % selectivity at 70.3 % indole conversion over CrFe2O4
(CC-5) at 598 K, WHSV 0.5 h-1 and indole/methanol molar
ratio of 1:6. Indole conversion increases with progressive
substitution of Cd by Cr ions due to increase in the acidity of
the catalyst.
90
80
70
60
50
ACKNOWLEDGEMENTS
The authors are highly thankful to Delhi Technological
University, Delhi, India for SEM and EDX analysis of ferro-
spinels.
40
Indole conversion (%)
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30
20
10
0
3-Methyl indole yield (%)
3-Methyl indole selectivity (%)
Others yield (%)
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