Indian Journal of Chemical Technology
Vol.17, November 2010, pp. 446-450
A comparative study on basicity based on supported K-salt catalysts for
isomerization of 1-methoxy-4- (2-propene-1-yl) benzene
Baldev Singha *, Jyoti Patiala, Parveen Sharmaa, Suresh Chandraa, Sudip Maityb & N Lingaiahc
a
Indian Institute of Integrative Medicine (CSIR), Canal Road, Jammu, 180001, India
b
Central Institute of Mining & Fuel Research(CSIR), Dhanbad 826 015, India
c
Indian Institute of Chemical Technology (CSIR), Hyderabad 500 007, India
*Email: [email protected]
Received 21 April 2010; revised 5 October 2010
A comparative study on basicity of supported K-salt catalysts has been carried out for the preparation of trans-anethole
[1-methoxy-4-(1-propen-1-yl) benzene]. It is observed that strong basic sites facilitated 99% conversion of methyl chavicol
[1-methoxy-4-(2-propen-1-yl) benzene] with 89% selectivity of trans-anethole. Activation energy for isomerization of
methyl chavicol was found to be in the range of 5.65 –8.67 kJ/mol using 25% K2CO3 loaded on alumina. The catalysts were
characterized by CO2-TPD and BET surface area.
Keywords: Methyl chavicol, Trans-anethole, Basicity, Isomerization
Trans-anethole is a useful industrial chemical. Beside
wide application in perfumery chemicals1-3, transanethole also finds application in alcoholic beverage
industries, food industry, synthesis of anisic aldehyde
and synthetic intermediates for fragrance and flavor
industries4. The demand of trans-anethole in the
world market increased rapidly during last few years
due to its growing applications in various products.
The production of trans-anethole from the natural
source is not sufficient enough to fulfill the required
demand. Industrial sources of trans-anethole have
changed to address increasing demand. Therefore, the
single step preparation of anethole (mixture of cis- &
trans-isomers) from methyl chavicol via double
bond isomerization has drawn a special attention
from both industrial and academic perspective5-11.
`The current technology uses homogeneous bases
(NaOH/KOH) as catalyst10 for the production of
trans-anethole but it does have some serious
drawbacks like catalyst cannot be reconverted and
must be neutralized and separated from reaction
products and effluent disposal problems. Additionally,
the basic homogeneous process is very sensitive to
the presence of water. In fact, solid base catalysts
hold a distinct advantage over acid catalysts due
to their ability to isomerize double bond without
C–C bond interruption.
Solid base catalysts seem to be promising
candidate for replacing a homogenous process by a
heterogeneous catalytic process. In this work, loading
of different sources of K+ salts like K2CO3 and
KHCO3 as well as KNO3 on alumina are investigated
in terms of total basicity and different basic
sites which are co-related with activity for
the isomerization of 1-methoxy-4-(2-propene-1-yl)
benzene to trans-anethole. A detailed literature
review reveals that, there has been no prior study
in vapor phase in fixed bed continuous flow reactor.
Experimental Procedure
Materials
All the chemicals like potassium carbonate,
potassium bicarbonate, potassium nitrate and
aluminum oxide were purchased from Qualigens
Fine Chemicals, Bombay. Methyl chavicol (99%)
and trans-anethole (98%) were procured from
Hindustan Mint & Agro Products Pvt. Ltd. India.
Catalyst preparation
Incipient wetness impregnation method was used
to prepare the series of potassium modified alumina
catalysts. During the entire course of preparation of
different catalysts, 25 and 30 g amount of potassium
salts were dissolved in water and resulting solution
was added drop wise under vigorous stirring on 100 g
of support and temperature was maintained between
45-60°C. The resulting mixture was kept overnight
then evaporated to dryness. The dried mass was
SINGH et al.: STUDY ON BASICITY BASED ON K-SALT CATALYSTS FOR ISOMERIZATION
initially dried at 110°C for 8 h and then calcined at
different temperatures from 350 to 650°C prior to
the reaction and stored in vacuum desiccator, because
their basic sites generated in situ in tubular reactor
and CO2 contamination from atmosphere can be
avoided.
Following catalysts were prepared using alumina
oxide as support and designated as A, B, C, D,
E and F
A)
B)
C)
C-I)
D)
E)
F)
Alumina oxide (purchased from Qualigens
Fine Chemicals, Bombay, India)
Alumina oxide modified with (25%)
K2CO3 and calcined at 450°C
Alumina oxide modified with (25%)
K2CO3 and calcined at 550°C
Alumina oxide modified with (30%)
K2CO3 and calcined at 550°C
Alumina oxide modified with (25%)
K2CO3 and calcined at 650°C
Alumina oxide modified with (25%)
KHCO3 and calcined at 550°C
Alumina oxide modified with (25%) KNO3
and calcined at 550°C
Evalution of catalytic activity and product analysis
Activity of the prepared catalysts for the vapor
phase reaction was evaluated in fixed bed catalytic
reactor, under atmospheric pressure at wide range
of temperature from 250 – 350°C under the flow
of nitrogen gas. The arrangement for testing was
employed with some modifications as described
elsewhere12, here instead of top feeding bottom
feeding was employed.
Analytical technique
GC Analysis
GC analysis with respect to trans-anethole
conversion was carried out on Nucon 5765 gas
chromatograph equipped with FID and AIMIL
chromatography data processor. The separation
was achieved using a FFAP, SE-30 fused-silica
capillary column (20 m × 0.25 mm i.d., 0.25 µm
film thickness); column temperature, 90°C (2 min) to
220°C at 4°C/min; injection temperature, 240°C;
detector temperature, 260°C; mode, split; carrier gas,
helium at column flow rate of 1.05 mL/min (100 kPa).
Retention indices (RI) of the sample components
and authentic compounds were determined on
the basis of homologous n-alkane hydrocarbons
447
(n-nonane to n-nonadecane) under the same conditions.
The quantitative composition was obtained by peak
area normalization, and the response factor for each
component was considered to be one.
CO2 TPD and surface area
CO2-TPD and surface area were determined by
CHEMBET-3000
(TPR/TPD/TPO)
instrument,
containing a quartz reactor (i.d. = 4 mm) and a T.C.D.
detector. Prior to CO2 adsorption, catalyst was
pretreated in He gas at 300°C for 2 h to remove any
adsorbed impurities. Subsequently, the sample cooled
to room temperature in helium gas flow. The
adsorption of CO2 is carried out by passing a mixture
of 10% CO2 balanced He gas over the catalyst for one
hour. Finally the system was heated from 80 to
1200°C at the rate of 10°C/min and the desorbed gas
is monitored with a T.C.D. detector. All the flow rates
were maintained at normal temperature and pressure
(NTP).
Results and Discussion
An important step in isomerization reactions is the
interconversion of π- and σ-bonded organometallic
complexes13. Earlier workers have shown that the
double bond isomerization is highly influenced by
the nature of the metals and ligands14. In the
present study, conversion and selectivity data
show that basicity facilitate the catalytic activity
and subsequently, different basic sites resulting
from different K-salts impregnation show higher
probability of exposure and react with molecules
of methyl chavicol yielding higher conversion and
selectivity of desired product.
The K+ ions replace the protons of isolated hydroxy
groups of alumina to form Al-O-K+ groups, which are
considered to be the active species of the catalyst and
likely related to the maximum activity. Table 1 shows
the BET surface area of three different samples.
Sample A shows surface area of 88.9789 m2/g, when
modified with 25% potassium carbonate (sample C)
surface area is decreased to 59.9403 m2/g, on further
increasing the load on sample A (sample C-I)
the surface area sharply decreased to 25.2237 m2/g.
It seems that higher loading amount of potassium
salt hinders the dispersion of K+ and leads to
agglomeration of bulk carbonate and as a result
surface area reduces sharply.
TPD profiles of CO2 adsorbed on alumina, as
well as modified with various potassium salts
derived catalysts are shown in Fig. 1, quantified in
INDIAN J CHEM. TECHNOL., NOVEMBER 2010
448
Table 2 and different concentration of sites are
presented in Table 3. The Fig. indicates that different
basicity and basic sites are present on the surface.
To facilitate the discussion, the basic sites have
been divided into weak, medium and strong sites.
The total basicity refers to the total amount of CO2
desorbed.
It is revealed from CO2 –TPD data that basicity
changed with nature of potassium salt and calcination
temperature. Table 2 shows that unmodified sample
A exhibit different CO2 desorption peaks with total
basicity being 6.8 µmolg-1 but there is no major
variations in basic sites. After modification of sample
A with potassium carbonate and calcination at 450,
550 and 650°C (designated as sample B, C, and D),
noticeable changes are observed in basic sites as well
as in total basicity (Table 2). Sample B possesses total
basicity of 17.0 µmolg-1 comprising concentration
of strong basicity predominate i.e. 7.2 µmolg-1. When
sample C calcined at 550°C, total basicity increased
to 25.7 µmolg-1 and amount of medium and strong
basicity is 2.8 and 22.6 µmolg-1 respectively.
Whereas, after loading 30% K2CO3 on alumina
(sample C-I) total basicity decreased to 20.9 µmolg-1,
this might be due to residual K2CO3 on the surface
support. However, sample D exhibits maximum
total basicity of 26.7 µmolg-1, due to calcinations at
higher temperature i.e. at 650°C.
Samples E and F (modified with KHCO3 and
KNO3 respectively), showed maximum total basicity
of 19.0 µmolg-1 and 32.3 µmolg-1. Besides total
basicity measurement, TPD curves show the
distribution of strong, medium and weak basic
sites. Sample A indicates (Table 3) greater number
of strong basic sites up to 37.0%, with weak
and moderate basic sites as 30.5% and 32.5%
respectively. In sample B change in the distribution
of strong, medium and weak basic sites is observed,
with increase of total basicity up to 17.0 µmolg-1,
under marked variation in concentration of basic sites
Fig. 1—CO2 -TPD of samples A-F
Table 1—BET surface area of catalysts
S. no.
1
2
3
Catalyst
BET surface area (m2g-1)
A
C
C-I
88.9789
59.9403
25.2237
Table 2—Distribution of basicity and conversion/selectivity
S. no.
1
2
3
4
5
6
7
Basicity (µmol g-1)
Catalyst
A
B
C
C-I
D
E
F
Weak
2.1
2.9
0.3
0.8
5.34
0.2
0.2
Moderate
2.2
6.9
2.8
1.4
6.1
8.8
-
Strong
2.50
7.20
22.6
18.7
15.26
10.0
32.1
Conversion
Total
6.80
17.0
25.7
20.9
26.7
19.0
32.3
60
98
99
94.4
98.0
95.0
96.5
Selectivity
trans
21.0
43.0
89.0
61.5
45.2
63.7
77.4
cis
10.0
40.0
10.0
21.4
39.0
18.1
19.0
SINGH et al.: STUDY ON BASICITY BASED ON K-SALT CATALYSTS FOR ISOMERIZATION
17.1, 40.8 and 42.1% respectively. Samples C and
D showed noticeable results with respect to total
basicity but major concentrations are of strong basic
sites with 85.2 and 57.2% respectively.
The sample E possesses the total basicity of
19.0 µmolg-1, comprising 46.3% of medium and
53.0% strong basic sites and concentration of weak
basic sites are very less compared to strong and
medium basic sites. Interesting changes are evident
in case of sample F; total strength of basicity is
32.3 µmolg-1, with greater number of strong basic
sites up to 99.5% and weak basic sites around
0.5%. Thus, it may be inferred that the Al2O3
modified either with K2CO3, KHCO3 or KNO3, all
have shown marked variation of total basicity and
distribution in concentration of basicity.
From Table 2, it is observed that sample C shows
the maximum activity and yields of trans-and
cis-isomers are 89 and 10% respectively. Table 3
indicates that the concentration of strong basic sites
is 85.2% and amount of basicity is 22.6 µmolg-1.
Noticeable results are observed when basicity is
increased beyond 25.7 µmolg-1 i.e., 32.3 µmolg-1,
which mainly dominated strong basicity in sample
F, there is variation in the yield of trans- and
cis-isomers viz 77.4 and 19.0% respectively. When
basicity decreased to 17.0 µmolg-1 (sample B)
comprising strong basicity 7.2 µmolg-1 the yield
of trans-isomer sharply decreased to 43.0% and
cis-isomer increased up to 40.0%. The results are
very well correlated in term of total basicity
and activity as seen from Table 2. When total
basicity is slightly increased to 19.0 µmolg-1,
comprising strong basicity 10.0 µmolg-1, the yield
of trans– isomer is increased to 63.7% and cis-isomer
is decreased to 18.1%
It appears that the strong basic sites, which provide
reversible adsorption sites for reactant and products
of catalytic reactions are very important in improving
the yield.
Medium basic sites also play a very important
role. Table 2 showed that lower basicity gave better
yield of trans-anethole. However, beyond 2.8 µmolg-1
conversion of methyl chavicol has no significant
change; whereas yield of trans-anethole gradually
decreases (Table 2). It also appears from Table 2 that
if weak basic sites increased beyond 0.3 µmolg-1,
there is decrease in the yield of trans-anethole.
Thus, number and amount of the basic sites
are very important to understand the catalytic
449
phenomenon of the solid basic catalyst reaction.
To synthesize an active, selective and stable
catalyst for the conversion of methyl chavicol to
trans-anethole, the amount and concentration of
optimum weak, medium and strong basic sites
is very essential.
Kinetic analysis
It was observed that the rate of reaction depends
on the reaction temperature. At lower temperature,
below the boiling point of methyl chavicol,
conversion as well as selectivity of desired
product was decreased. A rate equation for
the first order irreversible reaction, when the
concentrations are expressed in terms of conversion,
in the integrated form is
KP T W/F = -X – 2ln (1 – X)
RT
…(1)
W/F = g cat/g mole feed/h and X = mole converted/
g mole of feed.
The reaction velocity constant (k) is related to
the reaction temperature by Arrhenius equation,
the constant of which are obtained by a plot
of lnk against 1/T as shown in (Fig. 2). Activation
energies for the isomerization of methyl chavicol
under experimental conditions were found between
Fig. 2—Arrhenius plot for the isomerization of methyl chavicol
to trans-Anethole
Table 3—Distribution of base concentration (%)
S. no
1
2
3
4
5
6
7
Catalyst
A
B
C
C-I
D
E
F
Basicity
Weak
Moderate
Strong
30.5
17.1
4.1
3.8
20.0
00.7
00.5
32.5
40.8
10.7
6.7
22.8
46.3
-
37.0
42.1
85.2
89.5
57.2
53.0
99.5
450
INDIAN J CHEM. TECHNOL., NOVEMBER 2010
5.65 to 12.48 kJ/mol. Whereas, it was observed
that 25% K2CO3 loaded on alumina showed
the higher conversion of methyl chavicol and
for reasonably excellent selectivity of transanethole, activation energy for isomerization
of methyl chavicol was found to be in the range
of 5.65 – 8.67 kJ/mol.
References
Conclusion
The nature, number and strength and distribution
of basic sites are very important for understanding
the catalytic phenomenon for the conversion of
methyl chavicol to trans-anethole reaction.
7
8
9
10
Acknowledgements
Authors wish to express their sincere gratitude
to the Director IIIM, Jammu for his keen interest
and permission to publish the paper.
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