Ind. Eng. Chem. Res. 1992,31,2475-2483
Nomenclature E = activation energy, kJ/mol k = rate constant, m3/(kmol-s) ko = preexponential factor, m3/(kmold
Ki = distribution coefficient of ith component KI= distribution coefficient of CH31 m = number of components ni = number of moles of component i p i = partial pressure of component i, MPa Pi = saturated vapor pressure for pure i, MPa r = rate of carbonylation, kmol/(m3-s) R = gas constant, 8.314 J/(mol.K) T = temperature, K V , = volume of the gas phase, m3 x i = liquid-phase mole fraction of species i r i = activity coefficient of species i Subscripts g = gas phase i = species index j = species index I = methyl iodide 1 = liquid phase 0 = initial value
Literature Cited Brodzki, D.; Lecleve, C.; Denise, B.; Pannetier, G. Catalytic Properties of the Noble Metal Complexes of Methanol Carbonylation in Acetic Acid by Rhodium Compounds. Bull. SOC.Fr. 1976,l-2, 61-65 (in French).
2476
Dake, S. B.; Kolhe, D. S.; Chaudhari, R.V. Carbonylation of Ethanol Using Homogeneous Rh Complex Catalyst: Kinetic Study. J. Mol. Catal. 1984,24,98-113. Dake, S. B.;Jaganthan, R;Chaudhari, R.V. New Trends in the Rate Behavior of Rhodium-Catalyzed Carbonylation of Methanol. Znd. Eng. Chem. Res. 1989,28,1107-1110. Eby, R.T.; Singleton, T. C. Methanol Carbonylation to Acetic Acid. In Applied Industrial Catalysis; Academic Press: New York, 1983;pp 275-296. Forster, D. On Mechanism of a Rhodium Complex Catalyzed Carbonylation of Methanol to Acetic Acid. J. Am. Chem. SOC.1976, 98,846-848. Forster, D. Mechanistic Pathways in the Catalytic Carbonylation of Methanol by Rhodium and Iridium Complexes. Adv. Organomet. Chem. 1979,17,255-267. Hjortkjaer, J.; Jeneen, V. W. Rhodium Complex Catalyzed Methanol Carbonylation. Znd. Eng. Chem. R o d . Res. Deu. 1976,15,46-49. Hjortkjaer, J.; Jensen, V. W. Rhodium Complex Catalyzed Methanol Carbonylation. Effects of Medium and Various Additives. Znd. Eng. Chem. R o d . Res. Dev. 1977,16,281-285. Paulik, F. E.; Roth, J. F. Catalysts for the Low Pressure Carbonylation of Methanol to Acetic Acid. J. Chem. SOC.,Chem. Commun. 1968,1578-1582. Roth, J. F.; Craddock, J. H.; Hershman, A.; Paulik, F. E. Low Pressure Process for Acetic Acid vis Carbonylation of Methanol. Chem. Technol. 1971,600403. Smith, B.L.; Torrence, G.P.; Murphy, M. A.; Aguilo, A. The Rhodium Catalysed Methanol Carbonylation to Acetic Acid at Low Water Concentrations: The Effects of Iodide and Acetate on Catalyst Activity and Stability. J. Mol. Catal. 1987,39,116136.
Received for review February 25, 1992 Revised manuscript received August 18, 1992 Accepted August 31, 1992
Ceric Sulfate Oxidation of p -Methoxytoluene: Kinetics and Reaction Results Theodore Tzedakis and Andre J. Savall* Laboratoire de G6nie chimique et Electrochimie, URA 192 CNRS, Universit6 Paul Sabatier, 118 Route de Narbonne, 31 062 Toulouse, France
The chemical oxidation in the indirect electrochemical process for oxidizing 4-methylanisole (MA) by a sulfuric aqueous solution of ceric sulfate was studied. The dissolution of MA into the aqueous phase precedes the pure homogeneous oxidation consecutive steps: Cmethylanisole anise alcohol anisaldehyde anisic acid. The kinetic parameters for each of the steps were deduced from the measurement of the rate of reaction by absorption spectroscopy. The MA oxidation in the two-phase medium shows that the aldehyde yield reachea 80% at 50 O C when the initial concentration of cerium(1V) is less then 40 mol m-3. Anise alcohol may be obtained with a selectivity as high as 50%. The alcohol yield decreases to 15% for a conversion of MA of 8.5%. It is shown that it is possible to achieve a MA conversion, in a liquid-liquid CSTR fed with a 300 mol m-3 C e ( W solution, of more than 50% with an aldehyde selectivity of 75 ?%
-
-
-
.
I. Introduction The indirect oxidation of substituted toluenes into aldehydes by electrochemicalmeans may be achieved by the cations, at the highest valency, of redox couples such as: Co(II)/Co(III), Mn(II)/Mn(III), Ce(III)/Ce(IV). Numerous authors present the Ce(III)/Ce(IV) couple as a mediator providing valid results concerning the selectivity of the oxidation reaction of the methyl group of the toluenes (e.g. Syper, 1966;Ho,1973;Ibl et al., 1979; Kramer et al., 1980; Torii et al., 1982;Kreysa and Medin, 1986; Kreh et al., 1987;Wendt and Schneider, 1986).
* Author to whom correspondence should be addressed.
The Mn3+ion in sulfuric acid has been used (Wendt and Schneider, 1986)for two-phase medium oxidation of substituted toluenes: p-chlorotoluene, p-xylene, and pnitrotoluene, for example. By operating in continuous mode, these authors were able to extract the aqueous phase and distill the organic phase. Mn3+regeneration yields obtained on a polished platinum electrode bordered on 95%. A complete kinetic study was also performed and assessed in order to anticipate the temporal distribution of oxidation products. Ceric sulfate was used in sulfuric aqueous solution to oxidize 4-methylanisole (MA) in solution in methylene chloride (Kreysa and Medin, 1986). These authors give an overall p-anisaldehyde (AA)yield of approximately 90%
0888-5885/92/2G31-2475$03.00/00 1992 American Chemical Society
2476 Ind.
Eng.Chem. Res., Val. 31, No. 11,1992 D~SSOIUUO~ of
Extraction of AO. AA. AAC ~~
Figure 1. Model for the oxidation of 4methylanisole (MA) dispersed in an aqueous solution of Ce(1V): AO, anise alcohol; AA, p-anisaldehyde; AAC,p-anisic acid.
at a temperature of 30 OC. However, the value of the current density (250 A m-z) involved was low due to the low solubility of cerium(II1) sulfate. p-Anisaldehyde (AA) was synthesized on a laboratory scale by oxidizing MA with cerium ammonium nitrate Ce(NHJ,(NO,), in solution with methanol (Torii et al., 1982). AA selectivity was assessed at 93% a t room temperature for contact times of a few minutes. However, there are several difficulties when it comes to applying this technique on an industrial scale. Using methanol as a solvent rapidly corrodes platinized titanium anodes (Mansfeld, 1971; Smith and Mansfeld, 1972). Furthermore, Ce(IV) is unstable in mediums containing methanol (Torii et aL, 1982),and regeneration of the mediator and separation of the products contained in the reactive medium are difficult operations to implement. Separating AA and regenerating the mediator involve evaporating methanol from the single-phase mixture, following which MA and AA must be separated and the solid Ce(NH,),(NO,), residue must be adequately proceased before being regenerated by electrochemical means. It has been suggested 0.7 M cerium(1V) carbonate be used to oxidize toluene and 0-chlorotoluene in solution in hexane (Ibl et al., 1979; Kramer et al., 1980). The reaction leads to aldehyde with yields or approximately80% and only 2% for the corresponding alcohol and acid. These authors have also put forward a plant project for oxidizing the above produds with continuous extraction. If Ce(1V) is not generally considered to be hazardous in oxidation of organics (Ha, 1986),on the other hand using perchloric acid in the presence of organic compounds makes this technique hazardous. Furthermore the regeneration of Ce(1V) under a current density of 250 A m-z and current yields that do not exceed 35% makes this technique nonviable. Recent work (Kreh et al., 1987; Kreh and Spotnitz, 1987) recommends the urn of cerium methanesulfonate whose relatively high solubility (0.6 M) in 6 M methnnesulfonic acid leads to higher regeneration currents. Chemical yields obtained for the oxidation of substrates such as toluene and p-chlomtoluene are high and can reach 90% in aldehydes. p-Anisaldehyde is also obtained by oxidation of MA with a selectivity of 71%. The aim of this study is to determine the oxidation kinetics of 4-methoxytoluene to 4-methoxybenzaldehyde through the action of Ce(IV) in an aqueous sulfuric solution. This reaction can be considered as a model in indirect oxidation (Tzedakis and Savall, 1991),but p-anisaldebyde is a valuable product used as an intermediate in the synthesis of a drug prescribed against cough and constitutes a classical component in cheap perfumes. This oxidation reaction is achieved in a two-phase medium, and Figure 1shows the global mechanism of the prows. Firstly, the MA substrate dssolvea in the aqueous medium containing the cerium which is not soluble in the organic phase. Then, the oxidation reaction takes place in the aqueous phase. It is a purely homogeneous reaction
m
300
400
A[nml
Figure 2. UV absorption spaetra for solutions of Ce(III),Ce(N), MA, and its denvaten m 1 M H&O, at e = 20 OC. AU concantrations are 5 X lo4 M A, Ce(1V); B,p-anisaldehyde; C,p-anisic acid; D, Ce(II1); E, 4-methylanisole; F,anise alcohol.
including three consecutive stages. The equilibrium distribution coefficients of the oxidation products (anise alcohol (AO), AA, p-anisic acid (AAC))favor their extraction by MA from the aqueous phase (Tzedakis and Savall, 1991). The sequence globally representing MA oxidation is as follows: CH,O(C,H,)CH,(organic) CH,0(C8H,)CH3(aqueous) MA or 2 CH,O(CBH,)CH, + 2Ce(IV + HzO
-
7
CH,O(C H&!HzOH or 3 CH,O(C,H,)CH,OH + 2Ce(IV)
A8
+ 2Ce(III) + 2H+ (1)
7 CH,O(C,HJCHO + 2Ce(III) + 2H+ (2)
AA or 4 CH,O(C,H,)CHO + 2Ce(IV) + H20 7 CH,O(C HJCOOH + 2
