Ind. Eng. Chem. Process Des. Dev. 1980, 19, 324-325
324
COMMUNICATIONS On Critical Catalyst Concentration in Aqueous-Phase Phenol Oxidation’ The effect of temperature and reuse of catalyst on critical catalyst concentration (CCC) in aqueous-phase ph I oxidation is examined by a method utilizing induction period data. CCC is inversely proportional to an “adsorption”
type constant and is dependent on the “free” phenol hydroperoxide concentration in aqueous solution.
The critical catalyst concentration (CCC) phenomenon which exhibits a sudden and dramatic change in rate for a slight change in catalyst concentration is a characteristic of branched-chain reactions and has been observed in the heterogeneously catalyzed liquid-phase oxidation of hydrocarbons (Bacherikova et al., 1971; Evmenenko et al., 1972; Gorokhovatskii, 1973a,b, 1974; Jaki and Csanyi, 1973; Meyer et al., 1965; Mikhalovskii e t al., 1976; Mukherjee and Graydon, 1967; Neuberg and Graydon, 1972; Neuberg et al., 1975). It appears that the CCC phenomenon is related not only to either the oxidizable hydrocarbon or the catalyst concentration taken alone but, more appropriately, to the system taken as a whole (Gorokhovatskii, 1973b). The “dual function” of free-radical initiation and termination by the compounds of transition metals generally used as oxidation catalysts is presumably essential for observing CCC. This “dual function” is exhibited a t both low and high conversions in the heterogeneously catalyzed liquid-phase oxidation of cumene (Gorokhovatskii, 197313) and phenol (Sadana and Katzer, 1974a). Recently, it was shown that the CCC value obtained (Sadana, 1979) from the available length of the induction period, rI,data (Sadana and Katzer, 1974a) compared very favorably with the CCC value obtained from the expression developed for the kinetic chain length [when it equals 0.5 (Neuberg and Graydon, 1972)] in the steady-state activity regime. It is the purpose of this note to examine the effects of (a) temperature and reuse of supported CuO catalyst and (b) fresh supported M n 0 2 catalyst on CCC in aqueous-phase phenol oxidation utilizing induction period data for fresh and reused catalyst by the method linking rI with CCC (Sadana, 1979). In the initial rate regime the length of the induction period, r I , for aqueous-phase phenol oxidation at high catalyst loadings may be given by
r
1
The above form of the equation for T I was shown to fit the previously reported phenol oxidation data (Sadana and Katzer, 1974a) well except in the intermediate catalyst loading range (Sadana, 1979). The critical catalyst/initial phenol concentration ratio may be obtained from eq 1 by setting r1 = a,that is c
’ NCL Communication NO. 2330. 0196-4305/80/1119-0324$01 .OO/O
A is an “adsorption” type proportionality constant (Sadana and Katzer, 1974a) and its temperature dependence may be represented by A = A. exp(AH/RT) (34 where AH is the heat of adsorption of phenol hydroperoxide on the catalyst surface in kcal/g-mol. Since the activation energies of T I and k[= k3K9k10(Mc/Vl)/kll] are known for aqueous-phase phenol oxidation at 109.8 psi O2 pressure (Sadana and Katzer, 197413) values for A may be obtained a t different temperatures from eq 1. Then, from eq 3a one may obtain A = 5.28 X exp(l0470/RT) (3b) Equation 3b may be used to estimate A values at different reaction temperatures. On substituting for A in eq 2 the effect of temperature on the critical catalyst/initial phenol concentration ratio may be obtained. As temperature increases A decreases and the critical catalyst/initial phenol concentration ratio also increases. This is not unexpected since as A decreases, more of the phenol hydroperoxide formed remains in aqueous solution. This increase of “free” phenol hydroperoxide in aqueous solution will cause critical phenomena to occur a t a higher catalyst/initial phenol concentration ratio. Increases in “free” hydroperoxide concentration of the hydrocarbon being oxidized in the liquid phase have caused critical phenomena to occur a t a higher catalyst/ initial hydrocarbon concentration ratio for other liquidphase hydrocarbon oxidations (Evmenenko et al., 1972; Gorokhovatskii, 1973a; Meyer et al., 1965; Mikhalovskii et al., 1976; Mukherjee and Graydon, 1967; Neuberg and Graydon, 1972; Varma and Graydon, 1973). For example, Evmenenko et al. (1972) noted an increase in the CCC of cobalt oxide from 21.6 g/L at 296 K to 60 g/L a t 303 K in the heterogeneously catalyzed liquid-phase oxidation of isopropylbenzene. For a corresponding increase in CCC in phenol oxidation at 370 K the temperature would need to be increased by 28.7 K (eq 3b). The data obtained by Mikhalovskii et al. (1976) in the Co203-catalyzedliquidphase oxidation of isopropylbenzene at 303, 318, and 333 K also indicate that CCC increases on increasing temperature from 303 to 333 K, though the differences obtained in CCC are relatively small. Reused CuO catalyst exhibited a significant reduction in T~ when compared with T~ for fresh CuO catalyst although the rate of phenol oxidation in the initial rate regime was apparently unchanged under comparable reaction conditions (Sadana and Katzer, 197413). Let us now examine, more quantitatively, the effect of reused catalyst on rI and hence on CCC. Table Ia gives induction period values experimentally observed for fresh and reused CuO 0 1980 American
Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 19, No. 2, 1980
325
Tabie I initial phenol
run no.
1 2 3 4
concn, g/L
4.56 4.64 4.64 4.54
1/A = 1/A,= t(Mc/ VI )/ [(MCIVI )I
0 2
cat. concn, g/L
temp,
K
part. induction press., period,
psi
T,,
min
A
A,
c
h,olcrit Qfresh)
cph,oIcnt (reused)
( a ) Estimated Values of A , A , , and [(M,/V,)/C,h o ] d t in Aqueous-Phase Phenol Oxidation over Fresh and Reused CuO Catalyst 6.42, fresh 387 122 218 0.043 na 23.26 na 6.09, reused 387 124 13 nab 0.00282 na 354.60 6.00, fresh 369 100 643 0.084 na 11.90 na 6.40,reused 369 145 135’ na 0.014 na 71.43
A (from eq 3b)
0.0452 na
0.0787
na
( b ) Estimated Values for A and [ ( M , / V , ) / C p ~ , u ] cin ~ t Aqueous-Phase Phenol Oxidation over Fresh MnO, Catalyst 0.0295 na 33.90 na na 5 4.71 6.00, fresh 40 1 96 240 a T~
reported corrected t o 100 psi.
na:
not applicable.
catalyst a t 369 and :387 K along with other reaction conditions. Since, for the reused CuO catalyst the phenol oxidation curve exhi bits typical free-radical characteristics (for example, a slow initial rate regime along with an induction period and a more rapid steady-state period) as shown by fresh CuO catalyst (Sadana and Katzer, 1974b) the following form for the length of the induction period (similar to eq 1) may also be suggested for the reused catalyst (4)
where the subscript r denotes reused catalyst and k = [k3K&1O(Mc/VJ/kll]. A (fresh catalyst) values at 369 and 387 K may be determined from experimentally observed rI and k values (Sadana and Katzer, 1974b) from eq 1. A , (reused catalyst) values a t 369 and 387 K are determined from experimentally observed T I and ~ 1 values , ~ and by taking the ratio of eq 1and 4. Note that for reused catalyst [(Mc/Vl)/Cph,o]cr~t = i[l/Ar) by setting T ~ =, ~ in eq 4. The A values shown in Table Ia compare favorably well a t 369 and 387 K with the estimated A values from eq 3b. It may be noted that in both the above cases the critical catalyst/initial phenol concentration ratio increases by more than one-half order of magnitude on reusing the catalyst. In fact, a t the higher temperature the critical catalyst/initial phenol concentration ratio increases by a factor of 15.25 on reusing, the catalyst. This is not unexpected since for a reused catalyst before the reaction starts there is probably already some phenol hydroperoxide adsorbed on the catalyst surface from the previous run, and now much less phenol hydroperoxide needs to be adsorbed from the aqueous solution as the run proceeds to come to the end of the induction period. Once again, this increase in “free” phenol hydroperoxide in aqueous solution would increase the critical catalyst / initial phenol concentration ratio. Finally, the fresh Mn02-catalyzedaqueous-phase phenol oxidation curve also shows similar free-radical characteristics as exhibited by the fresh CuO-catalyzed phenol oxidation curve (Sadana and Katzer, 1974b). One may, then, once again assume the form of the equation for rI for CuO catalyst to apply for MnO, catalyst. Thus
r
1
available for M n 0 2 a t 401 K (Sadana and Katzer, 1974b), a value of A = 0.0295 may be obtained from eq 1 (Table Ib). At 401 K an estimated value of A = 0.030 is obtainable from eq 3b for CuO which gives a 1.7% difference in A values between CuO and M n 0 2 at 401 K. In the heterogeneously catalyzed liquid-phase oxidation of isopropylbenzene, M n 0 2 (13.2 g/L) exhibited a lower CCC value than Co203 (21.5 g/L) with their difference being 38.6% at 296 K. In tetralin oxidation catalyzed by NiO catalyst pretreated at 773 and 873 K for 5 h duration each, Jaki and Csanyi (1973) noted a critical catalyst/initial tetralin concentration ratio of 4.63 and 13.89, respectively, giving a 66.7% difference in CCC. Neuberg et al. (1975) studied the liquid-phase oxidation of cyclohexene catalyzed by /3-Mn02 with a relatively low and high specific surface and y M n O , a t 333 K. The critical catalyst/initial cyclohexene concentration ratios observed by them were 6173, 11.1,and 18.5, respectively, thus giving a very large difference in the observed critical catalyst/initial cyclohexene concentration ratios. Thus, there may be a small (this work) to a very large difference (Neuberg et al., 1975) in the CCC values of different catalysts used in the liquid-phase oxidation of the same hydrocarbon under comparable reaction conditions. Besides the different elements involved it is possible that the very large differences in CCC are due to the differences in surface areas. Nomenclature A , A , = proportionality constant C = concentration, g-mol/cm3 K i= equilibrium constant for step i k , = rate constant for step i (s)-I (other units dependent on step) M , = mass of catalyst, g Po, = oxygen pressure, atm Literature Cited Bacherikova, I. V., Gorokhovatskii, Ya. B., Evmenenko, N . P., Kinet. Katal.. 12, 1437 (1971). Evmenenko. N . P., Gorokhovatskii, Ya. B., Pylenko, Yu. I., DOH. Akad. Nauk SSSR, 202, 1117 (1972). Gorokhovatskii, Ya. B., Kinet. Katal., 14, 83 (1973a). Gorokhovatskii, Ya. 6..Proc. 5th Int. Congr. Catal., 879 (1973b). Gorokhovatskii, Ya. B., Pyatnitskaya, A . I., Kinet. Katal.. 13, 1527 (1972). Jaki, K., Csanyi, L. J., Proc. 5th Int. Congr. Catal., 888 (1973). Meyer. C., Clement, G., Balaceanu, J. C., Proc. 3rd Int. Congr. Catal., 1, 184 (1965). Mikhalovskii, S. V., Gorokhovatskii, Ya. B., Evmenenko, N . P., Kinet. Katal., 17, 1058 (1976). Mukherjee. A., Graydon, W. F., J . Phys. Chem., 71, 4232 (1967). Neuberg, H. J., Graydon, W. F., J . Catal., 25, 425 (1972). Neuberg, H. J., Phillips, M. J., Graydon, W. F., J . Catal., 38, 33 (1975). Sadana, A,, Katzer, J. R.. J . Catal.,35, 140 (1974a). Sadana, A., Ind. Eng. Chem. Process Des. Dev., 18, 50 (1979). Sadana, A., Katzer, J. R., Ind. Eng. Chem. Fundam., 13, 127 (1974b).
National Chemical Laboratory Poona 41 1 008, India
for MnOz catalyst over a comparable temperature range and reaction conditions. Then, since T I , k and Po, are
Ajit Sadana
Received for review March 30, 1979 Accepted December 20, 1979
