Ind. Eng. Chem. Res. 1994,33, 1897-1900
1897
Induction Model for the Heterogeneously-Catalyzed Liquid-Phase Oxidation of Aldehydes Bing Joe Hwang Department of Chemical Engineering, National Taiwan Institute of Technology, Taipei, Taiwan, 10672, R.O.C.
The effects of heptaldehyde concentration and inert surface on the induction period were investigated in the system of the heterogeneously-catalyzedliquid-phase oxidation of aldehydes. A theoretical model was developed, which provided a more complete insight into the induction phenomena of the heterogeneously-catalyzedliquid-phase oxidation of aldehydes. Experimental results of induction time correlated well with the theoretical induction model. Introduction One of the important processes in producing an organic peracid is the oxidation of aldehydes catalyzed by a heterogeneous Co-type resin. Production of an organic peracid from the partial oxidation of aldehyde, using a Co-type resin as catalyst, has been developed (Chou and Lee, 1985;Hwang and Chou, 1987a;Kuo and Chou, 1987). The reaction mechanism in oxidation of aldehydes using Co-type resin as catalyst for a surplus oxygen concentration (Chou and Lee, 1985; Hwang and Chou, 1987a; Kuo and Chou, 1987) is adsorption of aldehyde RCHO(b)
2 RCHO(s)
absorption of oxygen
initiation
ka
RCHO(s) + eo3+ RCO(b) + Co2++ H+ propagation RCO(b) + O,(b) RCO,(b)
-
+ RCHO
h
RCO,(b)
kS
RC0,H
+ RCO(b)
homogeneous termination
ke
2RCO,(b)
inactive species
heterogeneous termination RCO,(b)
+S
ki
inactive species
regeneration of catalyst RCO,H
-
+ 2C02+
k8
RCO;
+ 2C03++ OH-
where (b) and (s) represent the bulk solution and solid catalyst, respectively.
The reactions of heterogeneously-catalyzedliquid-phase oxidation of aldehydes in the steady-state region (Kuo and Chou, 1987; Hwang and Chou, 1987b; Hwang et al., 1991)have been sufficiently discussed. However, no paper has focused on the reaction of the heterogeneouslycatalyzed liquid-phase oxidation of aldehydes in the induction period. An induction model is developed in the present work, which should be helpful in understanding the general system in which both heterogeneous initiation and homogeneous chain reactions occur. The induction phenomenon has been observed and discussed not only in the homogeneously-catalyzed but also in the heterogeneously-catalyzedliquid-phase oxidation of hydrocarbons (Sadana and Katzer, 1974a,b;Neuberg et al., 1975; Mukherjee and Graydon, 1967; Vreugdenhil, 1973; Varma and Graydon, 1973). However, few induction models (Sadana and Katzer, 1974a,b)have been proposed for the heterogeneously-homogeneous system. An induction model is developed in this work for illustrating the induction phenomena of the heterogeneously-catalyzed liquid-phase oxidation of aldehydes. Heptaldehyde is used as a model compound to illustrate these phenomena. The theoretical induction model is also correlated with the experimental results. Experimental Details Preparation of Heterogenized Homogeneous Catalyst (Chou and Lee, 1985; Hwang and Chou, 1987a). H-form resin (Dowex 5Ow, 8% cross-linking) was used as the solid carrier to immobilize or exchange the cobalt ion for the preparation of catalyst. The amount of cobalt ion exchanged on the resin was controlled by changing the concentration of cobalt ion in the solution in which the resin was dipped. The concentration of cobalt ion in the solution was determined by measuring the UV transmittance of the solution. The exchanged resin, Le., Co-type resin, was dried or activated under vacuum at 80 "C. The color of the Co-type resin changed from red-orange to dark red and then red-violet during the activated process. Oxidation of n-Heptaldehyde. An experimentalsetup (Wang et al., 1993) is shown in Figure 1. The oxidation of n-heptaldehyde with oxygen catalyzed by Co-type resin was carried out in a cylindrical bubble type glass reactor 45 mm in diameter and 63 mm in height. A magnetic stirrer was provided for improving the mixing of the threephase system. At the bottom of the reactor, there was a side arm with a sintered glass disperser. On the top of the reactor, there was a reflux condenser that vented to the atmosphere and was maintained at -40 "C by a refrigeration system. The temperature of the reactor was controlled within dzO.1 OC by immersing the whole set of the reactor in a thermostatic bath. The oxygen flow rate was measured
0888-588519412633-1897$04.50/0 0 1994 American Chemical Society
1898 Ind. Eng. Chem. Res., Vol. 33, No. 8, 1994 E
I
] Induction region I1 : Steady-state region I
:
:
03-
8
.
U
a 02-
f ! . P O I -
EI V
I
'
0 9
:
00-~
= ,I
TI1 ,
,
, ,! ,,
,
,
I
Time, m i n .
Figure 2. Typical plot of concentration against time (Shih, 1986). Operating conditios: c&CHO(i,) = 1.0 M;temperature = 22 0.2 O C ; 02 flow rate; volume of solution = 150 mL; Co type of resin = 0.2 g, 3 mequivlg dry resin.
*
steady-state period have been discussed sufficiently in previous papers (Chou and Lee, 1985; Hwang and Chou, 1987a; Kuo and Chou, 1987). The formation rate of n-perheptanoic acid decreases with reaction time in region I11 as shown in Figure 2.
8
Figure 1. Experimental setup for n-heptaldehyde oxidation catalyzed by heterogenized homogeneous Cos+catalyst. (1)Nitrogen; (2) oxygen; (3) flowmeter; (4) needle valve; (5)gas mixer; (6) pair stirrer thermostat; (7) reactor; (8) magnet; (9)thermometer; (10) sampling tap; (11)reflux condenser; (12) gas dispenser; (13) cooling medium in; (14) cooling medium out.
by a gas flowmeter with a precision of 0.2%. The oxygen stream was precooled through a cooling coil in the thermostatic bath and blew into the reactor through the sintered disperser. At the beginning of a run, a desired amount of n-heptaldehyde and diethyl ketone (solvent) were fed into the reactor that was kept at a constant temperature. The reaction temperature was controlled at 20 "C. Then the required amount of Co-type resin and some inert surface (nonactive drying H-form resin) were added into the reactor and the oxygen was fed into the reactor simultaneously at a desired flow rate. The reaction time was recorded as soon as the oxygen stream started. Samples were taken periodically from the reactor by using a hypodermic syringe. The n-perheptanoic acid was analyzed by the iodometric method (Chou et al., 1985). A Typical Result. Figure 2 demonstrates the results of a typical run for n-heptaldehyde catalyzed by the heterogenized Co-type ion exchanged resin as the catalyst. The curve shown is divided into three regions. The induction region I with a induction time, td, corresponds to a relatively low concentration of n-perheptanoic acid. Only trace n-perheptanoic acid was produced in the induction period. The net formation rate of n-perheptanoic acid approaches zero in the induction period. From the coordinates of the point intersection of the two straight lines, as shown in Figure 2, the induction time (td) can be obtained (Tsepalov et al., 1977). The second straight line is the tangent to the kinetic curve at the point at which the rate of peracid formation is equal to half the rate of peracid formation in the steady-state region. Region I1 is a steady-state period. The formation rate of n-perheptanoic acid is a constant. The reaction mechanisms in the
Results and Discussion Development of the Induction Model. The heterogeneous termination of free radicals is predominant (eq 7) during the induction periods. Equation 6 can be neglected when compared to eq 7. Applying the steadystate approximation toward free-radical species, RC03(b), in the system yields
At pseudo steady state,the material balances of the species, Co3+and RC03H, in the system are (10) dCcd+/dt = ~ ~ & , ( C C , , Z - +~~CC,,S+CRCHO(~) )~
and
where the subscript x represents the species RC03H. Substituting eq 9 into eq 11 yields
dB/dr = dl2YX(1dY,ldr = #:YRB/B,
- 4228YR
- d12/3Y,(1 - e)2
(13) (14)
The initial conditions of eqs 13and 14 in the dimensionless form are Yx=O
at t = O or
e=eo
at t
= or~
r=O T = O
(15) (16)
where 80 is initial coverage of active sites. In the induction period, dYJdr approaches zero at pseudo-steady-state assumption. Equation 14 can be simplified to
Ind. Eng. Chem. Res., Vol. 33, No. 8,1994 1899
- el2
(17)
(42/(0,8) - 4:)eY~
(18)
&YRe/e, = 4;8yx(1 Substituting eq 17 into eq 13 yields dO/dr
The dimensionless concentration of aldehydes, YR,can be considered as 1 since the reaction species is slightly consumed during the induction period. Integrating eq 18 with initial condition provides
e = eo exp(B7)
(19)
where
02
B = ($:/(e$)
- $22)
(20)
YR
= (kd(1k3/k7Cs- k3)CORCHO(b)td
(21)
The induction period happens due to the transformation of Co2+to Cos+ within the Co-type resin. The definition of induction time, t d , is the time required for 8 approaching 1. From eq 19,it obtains ~~
10
= [l/h(l/@)I(kd(1k$k7C, - k3)CORCHO(b)
The total outer surface area can be obtained from
C, = 6W/pD where p and D are density and averaged diameter of resin, respectively. W is the total weight of all resin added. In cm, this case, p and D are 1.43 g/cm3 and 4.48 X respectively. Since p and D are constant, eq 24 can be expressed as
(25)
in which W is the total weight of resin (Co-typeresin and H-type resin) added in the solution. Substituting eq 25 into eq 23 yields
Equation 26 is a theoretical induction equation which can be applied to predict the induction phenomena of the heterogeneously-homogeneoussystem. When 6 approaches 1, the reaction transfers from the induction period to the steady-state period. At the steadystate period, 0 approaches 1 and dCJdt is a positive constant. The assumption of dCJdt = 0 cannot be applied in the steady-state period. The concentration of peracid in the solution increases with time in the steady-state region. Both the homogeneous termination reaction and heterogeneous termination reaction are important in the steady-state period. In the steady-state region, the formation rate of peracid has been discussed adequately in previous studies (Chou and Lee, 1985;Hwang and Chou, 1987a;Kuo and Chou, 1987). Effect of Heptaldehyde Concentration on Induction Time. The oxidation of n-heptaldehyde catalyzed by the heterogenized Co-type resin was carried out at various n-heptaldehyde concentrations when other operation conditions were maintained constant (Shih, 1986).
14
16
Table 1. Effect of Heptaldehyde Concentration on the Induction Period (Shih,1986) (Operating Conditions: h O ( b ) = Variable; Temperature = 22 f 0.2 O C ; 0, Flow Rate; Volume of Solution = 150 mL; Co Type of Resin = 0.2 g, 3 mequiv/g dry resin) CRCHOt M
td,mh
A combination of eqs 21 and 22 produces
12
M
Figure 3. Inverse of induction time against concentration of heptaldehyde. Operating conditions: h o & ) = variable; temperature = 22 f 0.2 "C; 0%flow rate; volume of solution = 150 mL; Co type of resin = 0.2 g, 3 mequiv/g dry resin.
1 = h(i/eo)/B
Cs = k'W
08
CQ,,
Equation 20 can be restated as
l/td
08
04
0.3 78
0.5 57
0.6 49
0.75 46
1
35
1.25 31
1.5 28
Table 2. Effect on Inert Surface on the Induction Period (Shih,1986) (Operating Conditions: f h C H O ( b ) = 1.0 M, Temperature = 22 f 0.2 O C ; 01Flow Rate; Volume of Solution = 150 mL; Co Type of Resin = 0.2 g, 3 mequiv/g dry resin; Inert Resin = Variable) weight of inert particles,g 0.0 0.02 0.05 0.10 0.30 td, min 35 39 46 58 108
The induction time decreases with the increase in the concentration of n-heptaldehyde, as listed in Table 1. When the concentration of n-heptaldehyde increased from 0.3 to 1.5 M, the induction time decreased from 78 to 28 min. The plot of the inverse of induction time against the concentration of n-heptaldehyde indicated in Figure 3 results in a straight line with slope 1.92 X 1k2min-l M-l. The relationship of the concentration of n-heptaldehyde and the inverse of induction time can be expressed as
(1.2 = 0.99) (27) The results correlate well with eq 26 in the theoretical consideration. Effect of Inert Surface on Induction Time. The effect of inert surface on the induction time was determined on the basis of those results coming from several runs by adding some nonactive drying H-form resin into 150 mL of n-heptaldehyde solution containing 0.2 g of 3 mequiv/g Co-type resin (Shih, 1986). The induction time increases with the increasingthe weight of the H-form resin as shown in Table 2. The plot of the inverseof induction time against the inverse of total weight of resin yields a straight line with a slope equal to 6.47 X 103 as listed in Figure 4.The effect of the total surface area of resin on the induction time can be expressed as l/t, = -3.94 X 10"
+ 6.47 X lo4/ W
(28)
1900 Ind. Eng. Chem. Res., Vol. 33, No. 8, 1994
Ci = concentration of species indicated by the subscript, M C, = total outer surface area of the catalyst and inert particle,
0.030
g
Ct = total (COS+and Co2+) ion concentration, M D = averaged diameter of Co-type resin, cm K1 = saturated adsorption constant of eq 1 kz, ..., k8 = rate constant of eqs 2-8, respectively t = reaction time, min t d = induction time, min YX = defined by CRCOsH(b)/C'RCHO(b) YR = defined by CRcHo(b)/CoRcHo(b) Greek Letters 20
25
30
35
40
45
6 = defined by ct/CoRCHO(b)
50
1/cs,g" Figure 4. Inverse of induction time against inverse of total weight of resin. Operating conditione: (?+&CHO(b) = 1.0 M temperature = 22 & 0.2 "C; 02 flow rate, volume of solution = 150 mL; Co type of resin = 0.2 g, 3 mequiv/g dry resin; inert resin = variable.
The expression of eq 28 was found to correspond with the theoretical analysis of eq 26. The experimental results correlate well with the theoretical induction model, as indicated from a comparison of the experimental results and theoretical equation. Comparing eqs 26 and 28 gives [l/ln(1/6°)](kd(,k3/(k,lz'))CO~~~~(b) = 6.47 x ([l/ln(l/6°)]lz3)CORCHO~~)= 3-94 x
io-3
(29) (30)
Solving eqs 29 and 30 with operating parameters ( W = 0.2 g and CRCHO(b) = 1 M), the kinetic parameters can be obtained as [l/ln(1/6~)]k,= 3.94 x 10-~ k,K,(k,k') = 1.64
(31) (32)
Conclusion The induction model for the heterogeneously-catalyzed liquid-phase oxidation of aldehydes was developed. The heterogeneous termination reaction of free radicals is dominant during the induction period. Both the homogeneous termination reaction and heterogeneous termination reaction are important in the steady-state period. The effects of concentration of aldehyde and inert surface area on the induction period could be described qualitatively and quantitatively by the proposed model. The induction period happens due to the transformation of Co2+to C O ~ + within the resin catalyst in this reaction system. The induction model could be applied to the systems of homogeneous chain reactions which are initiated by a heterogenizing homogeneous catalyst or a heterogeneous catalyst. Acknowledgment The authors would like to thank Professor Tse-Chum Chou of the National Cheng-Kung University for his kind provision of the data on the heterogeneously-catalyzed liquid-phase oxidation of heptaldehyde. The financial support of the National Science Council (NSC 80-0402E001-11) and National Taiwan Institute of Technology is also acknowledged. Nomenclature B = defined by eq 20 Cc0s+= Co3+ion concentration, M
9 = defined by Ccos+/Ct e0 = dimensionless group of initial active sites BS = defined by C,/Ct p = density of Co-type of resin, g/cm3 T = defined by t / t d 91' = defined by (k8CoRCHO(b)Ct)td 92' = defined by (k3CoRCHo(b)td
432 = defined by (Klk3kdk~)td Literature Cited Chou, T.-C.; Lee, C.-C. Heterogenizing Homogeneous Catalyst 1: Oxidation of Acetaldehyde. Znd. Eng. Chem. Fundam. 1985,24, 32. Hwang, B. J.; Chou, T.-C. Heterogenizing Homogeneous Catalyst 2: Effect of Particle Size and Two-Phase Mixed Kinetic Model. Ind. Eng. Chem. Fundam. 1987a,26,1132. Hwang, B. J.; Chou, T.-C. Overall Effectiveness Factor of the Heterogeneous-HomogeneousChain Reactions with One Limited Species in a Catalytic Slurry Reactor. Can. J.Chem. Eng. 1987b, 65,935. Hwang, B.J.; Yeh, H. J.; Chou, T. C. Using Inhibitor Method to Study the Effect of Cation on the Initiation Catalyzed by Organ0 Metallic Ion. J. Chem. 1986,44,101. Hwang, B. J.; Do, J.-S.; Chou, T.-C. Catalytic Effectiveness Factor of the Heterogeneously-Catalyzed liquid Phase Oxidation of Aldehydes. J. CZChE 1991,22,157. Kuo, M. C.; Chou, T.-C. Heterogenizing Homogeneous Catalyst 3 Oxidation of Benzaldehyde in a Semi-batch Tubular Wall Reactor. Znd. Eng. Chem. Fundam. 1987,26,1140. Mukherjee, A.;Graydon, W. F. Heterogeneous Catalytic Oxidation of Tetralin. J. Phys. Chem. 1967, 71,4232. Neuberg, H. J.; Philips, M. J.; Graydon, W. F. Kinetic Study of the Liquid Phase Oxidation of Cyclohexane Catalyzed by Manganese Dioxide. J. Catal. 1975,38, 33. Sadana, A.;Katzer, J. R. Involvementof Free Radicals in the AqueousPhase Oxidation of Phenol Over Copper Oxide. J. Catal. 1974a, 35,140. Sadana, A,; Katzer, J. R. Catalytic Oxidation of Phenol in Aqueous Solution Over Copper Oxide. Znd. Eng. Chem. Fundam. 1974b, 13, 127. Shih, C. C. Comparison the Oxidation of n-Heptaldehyde Initiated by Thermal and Co-type Resin Catalyst. M.S. Thesis of National Cheng Kung University, Tainan, Taiwan, ROC, 1986. Tsepalov, V. F.; Kharitonova, A. A.; Gladyshev, G. P.; Emanuel, N. M. Determination of the Rate Constants and Inhibition Coefficients of Phenol Antioxidants with the Aid of Model Chain Reactions. Kinet. Katal. 1977,18,1261. Varma, G. R.; Graydon, W. F. Heterogeneous Catalytic Oxidation of Cumene in Liquid Phase. J. Catal. 1973,28,236. Vreugdenhil, A. D. Mechanism of the Silver-on-Silica Catalyzed Oxidation of Cumene in the Liquid Phase. J. Catal. 1973,28,493. Wang, J.-G.; Chou, T X . ; Hwang, B. J. Adsorption and Kinetics of Benzaldehyde Oxidation Catalyzed by Heterogenized Homogeneous Cos+ Catalyst. J. CZChE 1993,24,127. Received for review June 29, 1993 Revised manuscript received February 22, 1994 Accepted May 12,1994' e AbstractpublishedinAdvance ACSAbstracts, June 15,1994.
