Kinetics and process parameter studies in catalytic air oxidation of veratraldehyde to veratric acid

Article abstract of DOI:10.1021/op990018y

Kinetics and different process parameters for the air oxidation of veratraldehyde to veratric acid were studied. At a temperature of 130 °C, air pressure of 1 MPa, cobalt acetate loading of 0.03 mol/L, and an initial concentration of 30% w/v of veratraldehyde, the reaction was found to be first order with respect to veratraldehyde. In 3 h at an aldehyde conversion level of 100%, as high as 99% selectivity was achieved.

Full text of DOI:10.1021/op990018y

Organic Process Research & Development 1999, 3, 365−369
Kinetics and Process Parameter Studies in Catalytic Air Oxidation of
Veratraldehyde to Veratric Acid
Sudip Mukhopadhyay*
Chemical Engineering DiVision, UniVersity Department of Chemical Technology, UniVersity of Mumbai,
Matunga, Mumbai - 400 019, India
Scheme 1. Air oxidation of veratraldehyde
Abstract:
Kinetics and different process parameters for the air oxidation
of veratraldehyde to veratric acid were studied. At a temper-
ature of 130 °C, air pressure of 1 MPa, cobalt acetate loading
of 0.03 mol/L, and an initial concentration of 30% w/v of
veratraldehyde, the reaction was found to be first order with
respect to veratraldehyde. In 3 h at an aldehyde conversion
level of 100%, as high as 99% selectivity was achieved.
and the solution was shaken thoroughly to make it homo-
geneous. The reactor was pressurized with air to the desired
pressure. The reactor was then heated to the desired
temperature, and the flow of air was started. After allowing
the reaction to proceed for the predetermined period, the
reactor was allowed to cool to room temperature, and the
pressure was released. A complete diagram of the bubble
column reactor is shown in Figure 1.
Analytical. The reaction mixture was filtered and distilled
under vacuum to remove acetic acid and then diluted with
water. The organic compounds were extracted in toluene.
After being washed thoroughly with water the organic layer
was taken for analysis.
Estimation of Veratraldehyde. The veratraldehyde was
analyzed by using a calibrated method against standard
samples by gas chromatography. A S.S. column of 2-m
length packed with 10% SE-30 on chromosorb-W was used
along with a FID detector. GC conditions were the follow-
ing: N2 carrier gas, 30 mL/min, detector and injector
temperature, 300 °C, and oven temperature (isothermal), 235
°C.
Estimation of Veratric Acid. A measured volume of the
organic extract was washed with sodium hydroxide to extract
the veratric acid as sodium salts. The aqueous layer was
neutralized with mineral acid to precipitate the acid.The latter
was dried and weighed. The organic extract was directly
titrated with standard sodium hydroxide to estimate the
carboxylic acids when solvents other than acetic acid were
used.
Introduction
Veratric acid is an important organic intermediate with
considerable industrial significance. Usually, a costly oxidiz-
ing agent like potassium permanganate is used to synthesize
veratric acid by starting with veratraldehyde1,2 or veratryl
alcohol.3 Veratraldehyde can be synthesized from vanillin,4,5
dimethoxytoluene,6 or veratryl alcohol.7 The use of a cheaper
oxidizing agent like air is worth considering for the oxidation
of veratraldehyde to veratric acid, but there is absolutely no
information regarding the process parameters and kinetics
of this sort of air oxidation. Thus, in this study, (Scheme 1)
an attempt has been made to find out the most suitable
process conditions and the kinetics of this process.
Experimental Section
Material. The reagents for analysis and the other chemi-
cals used were of analytical reagent (A.R.) grades. Com-
pressed air, used for oxidation, was supplied from an air
compressor. A.R. grade cobalt acetate, manganese acetate,
and lithium bromide were used as the catalyst and promoter.
A technical grade of veratraldehyde was used in each
reaction.
Experimental Procedure. A predetermined amount of
the catalyst, promoter, and reactant were mixed with solvent,
* Corresponding author. Present address: C/O Professor Yoel Sasson, Casali
Institute of Applied Chemistry, The Hebrew University of Jerusalem, Givat Ram
Campus, Jerusalem 91904, Israel.
(1) Arthur, H. R.; Ng, Y. L. J. Chem. Soc. 1959, 3094-5.
(2) Edwards, G. A.; Perkin, H. W.; Stoyle, F. W. Cf. Chem. Abstr. 1925, 19,
1134.
(3) Schmitt, G.; Ozeman, S.; Klein, P. Cf. Chem. Abstr. 1981, 94, 30309.
(4) Goia, I.; Kezdi, M.; Muresan, S.; Craciun, V. Rom. Pat. RO 90634, 1984;
Chem. Abstr. 1984, 108, 55652.
(5) Huet, M.; Nobel, D. (Rhone-Poulenc Chimie), Eur. Pat., EP 434517, 1991,
Chem. Abstr. 1991, 115, 114143.
Isolation. After the stipulated reaction period, the solvent
was removed by vacuum distillation, and then water was
added to the residual mixture. Toluene was added to extract
the organic substances. The toluene layer was washed with
15% sodium hydroxide solution to extract the veratric acid
in the aqueous layer as the sodium salt. The free acid was
precipitated from the aqueous solution by addition of 10%
hydrochloric acid. The acid was then filtered, dried, and
recrystallized from methanol to obtain 99.2% pure veratric
acid.
(6) Nakamura, I.; Saito, N.; Uejima, R. (Nippon Shokubai Kagaku Kogyo Co.
Ltd.), JP 6178744, 1986, Chem. Abstr. 1986, 105, 114734.
(7) Meunier, B.; Labat, G.; Jean, L. Eur. Pat., EP 350,395, 1998.
10.1021/op990018y CCC: $18.00 © 1999 American Chemical Society and The Royal Society of Chemistry
Published on Web 08/13/1999
Vol. 3, No. 5, 1999 / Organic Process Research & Development
•
365

Figure 2. Effect of air pressure on the rate of reaction.
Figure 1. Experimental set up for the liquid-phase air
oxidation of veratraldehyde to veratric acid.
Results and Discussions
In the present investigation, oxidation of veratraldehyde
to veratric acid, a significant part of the work was carried
out to study the parameters to establish the simplest kinetic
interpretation from the process development and research
point of view.
Figure 3. Effect of air flow rate on the rate of oxidation.
Table 1. Time vs conversion data for different air-flow
Definitions.
ratesa
%
%
%
%
%
conversion conversion conversion conversion conversion
at 20 mL/s at 30 mL/s at 40 mL/s at 45 mL/s at 50 mL/s
time, air-flow
air-flow
rate
air-flow
rate
air-flow
rate
air-flow
rate
min
rate
0
30
60
0
29
52
66
69
72
78
0
40
63
76
80
86
89
0
48
72
86
89
93
95
0
52
76
89
93
97
100
0
53
78
90
95
97
100
Process Parameter Studies. Effect of Air Pressure. In
a liquid-phase oxidation by air, usually the reaction is carried
out at a high pressure. Such a high pressure is required to
keep reactants in liquid phase and to maintain sufficient
partial pressure of oxygen.
90
120
150
180
In this work, the pressure was varied from 0.4 to 1.0 MPa
(Figure 2). At a pressure of 0.4 MPa, the rate of reaction
was too slow. When pressure was increased to 0.8 MPa, a
marked improvement in the rate of reaction was noticed.
However, a further increase in pressure did not have any
influence on the rate of reaction.
Effect of Air-Flow Rate. The reactor employed in this
investigation was a typical bubble column reactor where no
mechanical agitator was provided for agitation. However,
in such reactions adequate mixing can be attained if a
significantly high linear superficial velocity of the gas is
a Reaction conditions: veratraldehyde concentration, 30% w/v; cobalt acetate,
0.03 mol/L; manganese acetate, 0.03 mol/L; lithium bromide, 0.005 mol/L; air
pressure, 1 MPa; temperature, 130 °C; solvent, acetic acid; total reaction volume,
300 mL.
maintained. To ascertain whether mass transfer effects have
been eliminated, air flow was varied from 20 mL/s to 50
mL/s (Figure 3); between 45 and 50 mL/s, there was no
change in the conversion obtained (Table 1). Hence, in all
of the subsequent runs air-flow rate was maintained at 50
mL/s.
366
•
Vol. 3, No. 5, 1999 / Organic Process Research & Development

Figure 5. Effect of cobalt acetate loading on the rate of
oxidation of veratraldehyde.
Figure 4. Effect of reactant concentration on the rate of
reaction.
Table 2. Time vs conversion data for different initial
concentrations of veratraldehydea
% conversion at
10 % w/v initial
% conversion at
20 % w/v initial
% conversion at
30 % w/v initial
time, concentration of concentration of concentration of
min
veratraldehyde
veratraldehyde
veratraldehyde
0
30
60
0
50
74
0
51
76
0
53
78
90
88
90
94
100
89
92
96
100
90
95
97
100
120
150
180
Figure 6. Effect of temperature on the rate of reaction.
Table 4. Time vs conversion data for different
temperaturesa
a Reaction conditions: temperature, 130 °C; air-flow rate, 50 mL/s; cobalt
acetate, 0.03 mol/L; manganese acetate, 0.03 mol/L; lithium bromide, 0.005 mol/
L; air pressure, 1 MPa; solvent, acetic acid; total reaction volume, 300 mL.
time, % conversion % conversion % conversion % conversion
min
at 110 °C
at 120 °C
at 130 °C
at 140 °C
0
30
60
0
16
29
40
50
58
64
0
36
59
73
83
88
92
0
53
78
90
95
97
100
0
67
89
96
100
Table 3. Time vs conversion data for different cobalt acetate
catalyst loadinga
90
% conversion % conversion % conversion % conversion
at 0.01 mol/L at 0.02 mol/L at 0.03 mol/L at 0.04 mol/L
time, cobalt acetate cobalt acetate cobalt acetate cobalt acetate
120
150
180
min
loading
loading
loading
loading
a Reaction conditions: veratraldehyde concentration, 30% w/v; air-flow rate,
50 mL/s; cobalt acetate, 0.03 mol/L; manganese acetate, 0.03 mol/L; lithium
bromide, 0.005 mol/L; air pressure, 1 MPa; solvent, acetic acid; total reaction
volume, 300 mL.
0
30
60
0
28
47
53
61
71
78
0
41
67
79
84
89
91
0
53
78
90
95
97
100
0
55
79
91
96
97
100
90
120
150
180
Effect of Cobalt Acetate Loading on the Rate of
Reaction. The reaction was studied for different cobalt
acetate catalyst loading using manganese acetate as the
cocatalyst (Table 3). It was observed that with an increase
in cobalt acetate loading the conversion increased signifi-
cantly but the conversion was almost leveled off beyond a
catalyst loading of 0.03 mol/L (Figure 5).
Effect of Temperature. The reaction temperature was
varied from 110 to 140 °C (Figure 6). At 110 °C the rate of
oxidation was very poor (Table 4). A temperature of 130
°C was found to be the best under the reaction conditions to
a Reaction conditions: veratraldehyde concentration, 30% w/v; air-flow rate,
50 mL/s; manganese acetate, 0.03 mol/L; lithium bromide, 0.005 mol/L; air
pressure, 1 MPa; temperature, 130 °C; solvent, acetic acid; total reaction volume,
300 mL.
Effect of Initial Reactant Concentration. The initial
concentration of the aldehyde was varied from 10 to 30%
w/v (Figure 4). It was observed that the effect of concentra-
tion had very little influence on the rate of oxidation of
veratraldehyde (Table 2).
Vol. 3, No. 5, 1999 / Organic Process Research & Development
•
367

Table 5. Effect of temperature on conversion and selectivitya
temperature, °C
% conversion
% selectivity
110
120
130
140
50
83
95
99
99
99
92
100
a Reaction conditions: veratraldehyde concentration, 30% w/v; air-flow rate,
50 mL/s; cobalt acetate, 0.03 mol/L; manganese acetate, 0.03 mol/L; lithium
bromide, 0.005 mol/L; air pressure, 1 MPa; solvent, acetic acid; total reaction
volume, 300 mL; reaction time, 2 h.
Figure 9. -ln(1 - XA) vs t at different initial veratraldehyde
concentrations.
Figure 7. Effect of promoter on the rate of reaction.
Figure 10. -ln(1 - XA) vs t at different cobalt acetate loading.
solvent, but the rate of reaction was very slow and only 11%
conversion was obtained in 3 h.
Kinetics. The reactions were carried out at a high air-
flow rate to eliminate mass transfer effects. There was
absolutely no increase in the conversion level when air
pressure was increased from 0.8 to 1.0 MPa. It indicates that
the reaction is insensitive with a further increase in pressure.
Thus, in the rate equation the pressure term can be considered
as constant, and the simplest rate equation of this catalytic
air oxidation can be written as
Figure 8. Effect of solvent on the conversion of veratraldehyde.
achieve maximum conversion and selectivity to veratric acid
(Table 5).
-dCA/dt ) kCAWpair
(1)
Effect of Promoters. Different types of promoters like
paraldehyde, sodium bromide, and lithium bromide were
used. It was observed that under the reaction conditions,
lithium bromide was found to be the best promoter (Figure
7). However, lithium bromide is costlier than sodium
bromide; hence, 30-70% (w/w) a mixture of lithium bromide
and sodium bromide proved to be the best to achieve a
satisfactory reaction rate under the reaction conditions. A
promoter concentration of 0.005 mol/L was used in each
experiment.
or
-ln(1 - XA) ) k1t
(2)
Where, XA ) moles of veratraldehyde consumed/moles
of veratraldehyde taken, W ) g of catalyst taken, t ) time,
pair ) partial pressure of air ) 1 MPa ) constant, k1 ) kWpair,
and k ) rate constant.
Figure 9 shows that the reaction is first order with respect
to a given initial veratraldehyde concentration at an air
pressure of 10 atm, air-flow rate of 50 mL/s, and a cobalt
acetate catalyst loading of 0.03 mol/L. The initial rates at
different catalyst loading are found from the slopes of Figure
10. It is observed that (Figure 11) the initial rate increases
Effect of SolWent. The effect of different solvents on the
rate of reaction was studied, and it was observed that when
acetic acid was used as a solvent, the rate of reaction was
most satisfactory (Figure 8). Water was also used as a
368
•
Vol. 3, No. 5, 1999 / Organic Process Research & Development

Figure 11. Initial rate at different cobalt acetate loading.
Figure 13. Arrhenius plot for the air oxidation of veratral-
dehyde.
At different temperatures, -ln(1 - XA) vs t is plotted
(Figure 12) and from the slopes, k at different temperatures
are found to be 4.3 × 10-6, 1.07 × 10-5, 1.87 × 10-5, and
2.8 × 10-5 s-1 MPa-1 g-cat-1 under the reaction conditions.
The energy of activation is found to be 60 kJ/mol from the
slope of the Arrhenius plot (Figure 13).
Conclusions
The air oxidation of veratraldehyde to veratric acid follows
a first-order kinetic. The energy of activation was found to
be 60 kJ/mol.
Under the best suitable reaction conditions (air-fow rate,
50 mL/s; air pressure, 1 MPa; cobalt acetate, 0.03 mol/L;
manganese acetate, 0.03 mol/L; promoter, lithium bromide,
0.005 mol/L; temperature, 130 °C; time, 3 h.), veratric acid
could be obtained with 99% selectivity at 100% overall
conversion level of veratraldehyde.
Figure 12. -ln(1 - XA) vs t at different temperatures.
linearly with the increase in cobalt acetate loading. The
diffusion is estimated to be unimportant due to the homo-
geneity of the reaction mixture and the conditions employed.
Thus, the data represent the true kinetics of the process at a
catalyst loading of 0.03 mol/L.
Received for review March 16, 1999.
OP990018Y
Vol. 3, No. 5, 1999 / Organic Process Research & Development
•
369