Ind. Eng. Chern. Prod. Res. Dev. 1906, 25, 257-261
Acknowledgment We are grateful to the Natural Sciences and Engineering Research Council of Canada for the financial support for this project. Also, we are grateful to Dr. W. H. Hocking of AECL, Pinawa, Canada, for recording the XPS spectra of the zeolite samples. Literature Cited Ackman, R. G. Rapeseed 011: Chemlcal and Physlcal Characterlstlcs;R a p s seed Association of Canada: Winnipeg, 1977; Bulletln No. 45, p 12. Argauer, R. J.; LandoR, G. R. “Crystalline Zeolite H-ZSM5 and method of preparing the same”; U.S. Patent 3 702 886, 1972. Borade, R. B.; Hegde, S. G.; Kulkarni, S. B.; Ratnaswamy, P. Appl. . . Catal. 1984, 13, 27-38. Chantal, P. D.; Kallaguine, S.;Grandmaison, J. L.; Mahay, A. Appl. Catal. 1984, 10(3), 317-32. Chen, N. Y.; Mlale, J. N.; Reagan, W. J. “Preparation of Zeolites. Example 5”; U.S. Patent 4 112 056, Sept 5, 1973. Dwyer, J.; Fltch, F. R.; Qln, G.; Vickerman, J. C. J. fhys. Chem. 1982, 86, 4576-78. Graille, J.; Lozano, P.; Geneste, P.; Guida, A,; Norina, 0. Rev. Fr. Corps &as 1981, 28, 421-6. Haag, W. 0 . ;Rodewald, P. G.; Weisz, P. B. frepr.-Am. Chem. SOC.,Dlv. Pet. Chem. 1980, 25, 650-6. Hidalgo, C. V.; Itoh, H.; Hattori, T.; Niwa, M.; Murakaml, Y. J. Catal. 1984, 85, 362-9.
257
Ison, A.; Gorte, R. J. J . Catal. 1984, 89, 150-8. Itoh, H.; Hidalgo, C. V.; Hattori, T.; Niwa, M.; Murakarni, Y. J. Catal. 1984. 85, 521-6. Jacob, P. A. Carbnlogenlc A c M y of Zeolites; Elsevier: New York, 1977. Nayak, V. S.; Choudhary, V. R. Appl. Catel. 19849 10, 137-45. Patterson, H. B. W. Hydrogenaflon of Fats and Oil; Applled Sclence: New York, 1983. Prasad, Y. S.;Bakhshi, N. N. Appl. Catal. 1985. 18, 71-85. Prasad, Y. S.;Bakhshi, N. N.; Mathews, J. F.; Eager, R . L. Actas Simp. Iberoam. Catal., 9th, 1984 1984, 2 , 1010. Prasad, Y. S.; Bakhshi, N. N.; Mathews, J. F.; Eager, R. L. I n “Catalysis on the Energy Scene”, presented at the 9th Canadian Symposlum on Catalysis, Quebec, 1984b; p 85-92. Rajadhyaksha, R. A.; Anderson, J. R. J. Catal. 1980, 63,510-14. Scherzer, J. I n Catalytic Materials: Relation between Structure and Reaction; ACS Syrnposlum Series 248; Amerlcan Chemical Society: Washing ton, DC, 1984; pp 157-200. Topsoe, N. Y.; Pedersen, K.; Derouane, E. G. J. Catal. 1981, 7 0 , 41-52. Vedrine, J. C.; Auroux, A.; B o k , V.; Dejalfve, P.; Naccache, C.; Wierzchowskl, P.; Derouane. E. G.; Nagy, J. B.; Gibson, J. P.; Van Hwff, J. H. C.; Van Den Berg, J. P.; Wolthulzen, J. J. Catal. 1979, 59, 248-262. Chen, T.J.; Chao, K.J.; Tsai, T . 4 . J. Catal. 1979, 6 0 , 140-7. Wang, I.; Ward, J. W. J. Catal. 1967. 9 , 225-36. Welsz, P. B.; Frllette, V. J. J. Phys. Chem. 1980, 64, 382-3. Weisz, P. B.; Haag, W. 0.; Rodewald, P. G. Science (Washington, D . C . ) 1979, 206, 57-58.
Received for review August 5 , 1985 Accepted January 27, 1986
Liquid-Phase Oxidation of p -Xylene Catalyzed by Metal Oxides Milan Hronec’ and Zden6k Hrab6 Department of Organic Technology, Faculty of Chemistry, Slovak Technical University, 8 12 37 Bratisiava, Czechoslovakia
Metal oxides were studied as catalysts for the liquid-phase oxidation of p-xylene to terephthalic acid in the presence of p-toluic acid and water. The temperature of calcination of the cobalt catalyst has a major influence on the activity that is connected with the formation of COO and Co,O, phases. However, under reaction conditions, only COO dissolves in the reaction system and produces an active homogeneous catalyst. The activity of mixed-metal catalysts is dependent on the type of metals and their mutual molar ratio. On the basis of reactivity data of competitive oxidations for different hydrocarbons and great slmilarltles between homogeneously catalyzed oxidations, we propose that the oxidation of p-xylene in the studied system proceeds through a free-radical mechanism. The homogeneous salts and not metal oxides are catalytically active.
Introduction In technical production the oxidation of alkyl aromatic hydrocarbons to the corresponding carboxylic acids is usually carried out in the liquid phase in the presence of transition-metal salts or complexes acting as homogeneous catalysts (Prengle and Barona, 1970; Lyons, 1980). Recently a considerable amount of research has been devoted to the study of the kinetics, mechanism, and various characteristics of such processes (Benson, 1976; Sheldon and Kochi, 1981; Lyons, 1984). Heterogeneous catalysts are also capable of catalyzing the liquid-phase hydrocarbon oxidation. The oxidation of secondary and tertiary hydrocarbons, e.g., tetralin, cyclohexene, and cumene, has been extensively studied (Margolis, 1973; Fukuzumi and Ono, 1976; Neuburg et al., 1975; Vreugdenhil, 1975). It was found that for the heterogeneous liquid-phase oxidation of cyclohexene, catalyzed by transition-metal oxides, the activity of the oxides is closely related to the activity of soluble acetylacetonates of the same metal ion (Gould and %do, 1969). This leads to the conclusion that the reaction mechanism is similar for both types of catalysts. Many schemes for the catalytic 0196-432 1f86f 1225-0257$O1.50fO
oxidation have been proposed (Bljumberg, 1978). Considerably less extensive was the investigation of the heterogeneously catalyzed oxidation of primary hydrocarbons, e.g., methyl aromatics. In the literature there is some information about the oxidation of p-xylene catalyzed by COYand MnY zeolites (Rouchaud et al., 1968; Landis, 1970), cobalt or manganese oxides supported on alumina (Caloyannis and Graydon, 1971), and vanadium oxide (Mruk, 1973; Plotkina et al., 1980). However, yields of terephthalic acid are very low, and generally only one methyl group in the molecule is attacked. In the present contribution we report the investigation of p-xylene oxidation in the liquid phase in the presence of p-toluic acid and water using metal oxides as heterogeneous catalysts. Particular emphasis is placed on the study of the similarities between this reaction and characteristics of oxidation catalyzed by the same metals in the form of homogeneous catalysts. Experimental Section Catalysts. Metal oxides were prepared from the corresponding pure- or mixed-metal acetates or hydroxides 0 1986 American Chemical Society
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Table I. Effect of Metal Oxides on the Oxidation of p-Xylene r,,d,e X lo3 metal oxide" reaction time, min TA yield? 0 1.73 30 Pb 0.10 40 0 Ce 0.20 50 0 Cr 0.05 30 0 Ag 40 Ni 0 f Mn 480 69.1 1.88 co 460 51.4 3.94 "Prepared at 350 "C. *Conditions: 185 "C; 2.1 MPa; air flow 30 dm3 h-l; 0.47 mol of PX; 0.15 mol of PTA; 1.11mol of H20; 0.5 g of metal oxide. 'Calculated on PX. dMaximum rate in mol of O2 kg-I s-l. 'Conditions: 150 OC; 0.9 MPa; 56.6 mmol of PX, 3.0 mmol of PTA, 111.1mmol of HzO, 70 mg of catalyst. fExplosive run.
by calcination in an electric furnace at the desired temperature, usually at 350 OC for 7 h in air. The calcined material was finely pulverized. Cobalt sulfide (COS)and cobalt(I1) 3,5-diisopropylsalicylatewere prepared according to the literature (Lutz and Haeussler, 1971;Dixon et al., 1972). Procedure, kinetic measurements, competitive rate studies, and analysis of products were the same as we described previously (Hronec et al., 1985). Since in the reaction system p-toluic acid is always present at the beginning of the reaction, the yield of terephthalic acid is calculated only on p-xylene. Solubility of Metal Oxide Catalysts. Dissolution of catalysts (ca. 70-mg samples) calcined at different temperatures was studied at 150 "C and 0.8 MPa of oxygen in solution, prepared from 10 mL of H 2 0 and 0.70 g of p-toluic acid. The solid catalyst was separately weighed on a Teflon foil placed inside the pressure vessel, and after heating it in a thermostated oil bath (10min), the catalyst was mixed with the mentioned solution by vigorous shaking of the vessel using a vibrator. After 30 min the vessel was rapidly cooled, and the amount of dissolved cobalt was analyzed by chelatometric titration. X-ray Study of the Catalysts. The catalysts were shown to be sufficiently crystalline. In the quantitative x-ray diffraction measurement of cobalt oxide catalysts C O K ~ ,radiation ,~ at 30 kV and 10 mA was scanned over a 2 0 range at 28O-8Oo;TiOz (rutile) was used as an internal standard (dlIo= 0.324 nm). The mean surface areas of catalysts measured by the BET method with nitrogen were 1.2-3.1 m2 8-l.
Results and Discussion Preliminary experiments reveal that various metal oxides are almost inactive as catalysts for the oxidation of undiluted p-xylene (PX) to p-toluic acid (PTA) or terephthalic acid (TA). Their activity considerably increases in the presence of p-toluic acid (Table I); however, for the oxidation to terephthalic acid only cobalt and manganese oxides are active. In the case of other metal oxides the oxidation ceases at 5-10% conversion of p-xylene. The present study shows that the catalytic activity of cobalt and manganese oxides is strongly dependent on the calcination temperature. Thus, cobalt acetate calcined at 350 "C produces cobalt oxide possessing the highest activity for the oxidation of the mixture of p-xylene and p-toluic acid to terephthalic acid (Figure 1). The yields of terephthalic acid are very high already with catalytic concentration of the mentioned oxide (Figure 2). However, yields increase abruptly in a very narrow concentration interval, above the critical concentration of the catalyst. Catalyst calcination ranging from 4 to 24 h does not influence the activity. The presence of a certain amount of water in the
6ol
mol% TA
0
d O'
\-0
400 -4;O
3;O
3d0
."L
Figure 1. Effect of temperature of C O ( O A C ) ~ . ~ H calcination ~O on TA yield. Conditions: 185 O C ; 2.1 MPa; air flow 30 dm3 h-'; 50 g of PX + 20 g of PTA + 20 g of HzO; 0.5 g of catalyst.
mol
6ol
,o 0 1.0 wt% coo, Figure 2. Dependence of TA yield on "concentration" of cobalt oxide calcined a t 350 " C . Conditions as in Figure 1.
0
10
20
30
40 W!D/~P$
Figure 3. Influence of water on production of TA. Conditions: 185 OC; 1.9 MPa; air flow 30 dm3 h-l; 30 g of PX + 50 g of PTA; 1.5 g of cobalt oxide calcined at 350 OC.
reaction system has a highly positive effect on catalytic activity (Figure 3). Yields of terephthalic acid (calculated only on p-xylene) above 100 mol % are in that case caused by the oxidation of a part of p-toluic acid present in the reaction system at the beginning of the reaction. The activities of cobalt and manganese oxides are influenced also by other metals used in the catalyst preparation. Thus, the majority of binary mixtures of Co-M and Mn-M oxides are catalytically less active in terms of both maximum reaction rate and yield of terephthalic acid (Tables I1 and 111). Only mixtures of Co-Cr and Mn-Ce oxides in the metal molar ratio of 20:l are slightly more active. In addition to the type of bimetallic mixture, the mutual ratio of these metals is very important. As can be seen from Figure 4,the influence of such catalysts can be either synergistic or antagonistic, depending on the ratio of metals in mixed Co-Mn oxide. At 185 "C and at the mentioned composition of the reaction medium, strong
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986 259
Table 11. Oxidation of p-Xylene Catalyzed by Mixed Co-M Oxides" TA mixed reaction yield:d oxideb time, min mol % rmmeJX lo3 CoO/Co3O, co 460 51.4 3.94 1.39 250 14.4 2.74 0.77 Co-Ni 300 44.9 0.09 0.34 Co-Pb 0.08 0.25 Co-Ce 125 4.3 330 48.4 1.31 Co-Ag 300 32.6 1.69 0.55 Co-Mn 0.81 Co-Cr 350 51.8 a 201 molar ratio of meals. *Prepared a t 350 OC. Conditions: 185 OC; 2.1 MPa; air flow 30 dm3 h-l; 0.47 mol of PX; 0.15 mol of PTA; 1.11mol of H20; 0.5 g of metal oxide. dCalculated on PX. eMaximum rate in mol of O2 kg-' 5-l. fconditions: 150 OC; 0.9 MPa; 56.6 mmol of PX; 3.0 mmol of PA; 111.1 mmol of H,O; 70 mg of catalyst.
2'o
7
4
1
60
2o
n 0
270
370
'C
470
Figure 5. Effect of temperature of catalyst calcination on the ratio of formed cobalt oxides and on the solubility of corresponding catalysts in reaction system.
Table 111. Oxidation of p-Xylene Catalyzed by Mixed Mn-M Oxides" TA mixed reaction yield,',d oxideb time, min mol % F,,~J x io3 69.1 1.88 Mn 480 62.1 1.87 Mn-Ni 450 53.4 0.19 Mn-Pb 480 70.4 2.24 Mn-Ce 450 62.3 1.86 Mn-Ag 480 63.5 1.85 Mn-Co 375 43.9 1.82 Mn-Cr 360 a
2 0 1 molar ratio of metals. "See Table 11.
Figure 6. Dependence of maximum rate of p-xylene oxidation on temperature of catalyst preparation. Conditions: 150 "C; 0.8 MPa oxygen; 7 cm3 of P X + 2 cm3 of H 2 0 + 0.4 g of PTA; 70 mg of cobalt oxide catalyst.
I
m o I Yo TA
20,
0
0
0,5
CO
1.0
CotMn
Figure 4. Effect of mixed Co-Mn oxides on TA yield. Conditions as in Figure 1; catalyst 0.7 g.
antagonism was observed in two concentration regions. This may suggest formation of intermetallic compounds at Co/Mn molar ratio 1:l and 1:20, respectively. They probably act as scavengers of peroxy radicals and terminate a chain (Hronec et al., 1985). The catalytic effect of cobalt oxides has also been studied in detail from the mechanistic viewpoint. First of all, we investigated the influence of catalyst calcination temperature on the structure of the metal oxide catalyst. Thermogravimetric analysis has shown that cobalt acetate loses its acetate ligands at temperatures above 260 "C. X-ray analysis proves (Figure 5) that the temperature of calcination influences the ratio of cubic (COO)and spinel (Co304)phases in the resulting oxide catalyst. A t a calcination temperature of about 350 "C, the COO phase predominates, and when catalyst prepared at this temperature is used, the highest yield of terephthalic acid is achieved (Figure 1). Catalysts prepared at 350 "C are also highly soluble in the reaction medium formed from the mixture of p-toluic acid and water (Figure 5). It is known
that the spinel Co304is in general slightly soluble in many acids (Brauer, 1954), and also in the studied reaction system at temperatures of 150-180 "C it dissolves very slowly. Hence, catalysts with a high content of the spinel Co304are slightly active. The surface area of the oxide catalyst, which might influence the reaction rate, changes very little with the calcination temperature. The above-mentioned results indicate that the catalytically active species for the oxidation of p-xylene in mixture with p-toluic acid and water are not heterogeneous cobalt oxides but their soluble salts, probably in the form of toluates. Therefore, in the absence of p-toluic acid at the start of the reaction, the oxidation of p-xylene with cobalt oxide catalyst proceeds very slowly. No oxidation is observed also with cobalt sulfide or cobalt diisopropylsalicylate catalysts, also in the presence of p-toluic acid, as a result of non exchange reaction of toluate ligands under reaction conditions. Other metal oxides (Table I), which under reaction conditions are transformed to homogeneous forms, are catalytically active, but, similar to their homogeneous analogues (Hronec and Ilavsky, 1986), they are very rapidly deactivated at very low conversion of p-xylene. The dependence of maximum oxidation rates on the temperature of preparation of cobalt oxides has an effect other than the dependence of the yield of T A (Figures 1 and 6). Differences were observed with catalysts prepared by calcination at temperatures below 350 "C. With these catalysts the maximum reaction rates are equally high; however, the yields of T A are different. These differences are probably caused by various active forms of soluble catalysts produced from cobalt oxides calcined at temperatures below 350 "C. Maximum rates are measured at
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
Table IV. Competitive Study of Alkyl Groups Attack in
,
-
- 2.0 I
p -Cymene
catalvst CoO,-H20" CO"'(OAC)~*H~O~ CoBr,Py2-H2Oc
Royd CO"'( OAc)yAcOHe Mn"'(OAc),.AcOHf
reactivity. isoDrouv1:methvl 2.8:l 3.7:l 2.6:1 3.2:l 1:19 5.2:l
OConditions: 115 OC; 1.25 MPa; 22.4 mmol of p-cymene; 0.7 mmol of p-toluic acid: 55.5 mmol of HzO; 0.10 g of cobalt oxide prepared at 350 OC. bHronec and Ilavskjl (1982). eHronec and Ilavskjl (1983). dRussell (1956). eOnopchenko et al. (1972). fonopchenko and Schulz (1972).
Table V. Relative Reactivities (per Molecule) of Aromatics toward Metal Catalysts and Some Radicals Co(II1)- Co(II1)hydrocarbon COO," HzOb AcOH' RO,-d~e t-BuO.'ff 1 1 1 1 1 to1u en e 7.8 4.02 9.4 4.7 1.2 ethylbenzene 5.3 0.1 13.3 6.41 cumene 12.1 "Conditions: 160 "C; 1.25 MPa; 18.8 mmol of each hydrocarbon; 14.7 mmol of PTA; 111.1mmol of HzO; 0.10 g of cobalt oxide prepared at 350 "C. bConditions same as for COO,. (Hronec and Ilavskjr, 1982). e Conditions: 105 OC (Onopchenko, 1972). Conditions: 90 "C (Russell, 1956). e Per active hydrogen. fconditions: 110-160 'C (Brooks, 1957).
20-30% conversion of p-xylene, which under experimental conditions is reached in ca. 30 min of reaction. However, at higher conversions and longer reaction times, differences in activity due to deactivation can become evident, and the result is a lower yield of terephthalic acid. In order to find out the similarities between oxidation of alkyl aromatics catalyzed by cobalt oxides and by homogeneous cobalt salts (Hronec and Ilavsky, 1982,1985, 1986), oxidation of different hydrocarbons was investigated. Under comparable conditions, oxidation of p-cymene with oxygen, catalyzed by cobalt oxide, proceeds with the formation of p-methylacetophenone as the major product, and the reactivity of the isopropyl group vs. the methyl group is 2.8:l (Table IV). Obtained selectivities are analogous to those observed with homogeneous cobalt catalyst in an aqueous reaction system, with manganese catalyst in acetic acid solvent, or in noncatalytic freeradical oxidation. Reactivities for toluene, ethylbenzene, and cumene obtained by competitive oxidation using cobalt oxide prepared at 350 "C follow the order of hydrogen abstraction in reasonable agreement with the homogeneous cobalt catalyst operating in aqueous systems or with peroxy radicals in the noncatalyzed reaction (Table V). As indicated by the data summarized in Tables IV and V, the reactivities of studied hydrocarbons toward Co(II1) in acetic acid solvent differ from that obtained with heterogenous cobalt oxide catalyst in an aqueous system. The data suggest that different mechanisms or rate-determining steps are operative. In acetic acid solvent the electrontransfer path is generally accepted for the cobalt(II1)catalyzed oxidation of aromatics (Sheldon and Kochi,1981; Eberson, 1983). The mechanism involves an initial reversible electron-transfer step by a rate-determining proton-transfer step: RCH3 + Co(II1) e RCH3+ + Co(I1) The distinguishing feature of this system is the requirement of a high concentration of metal ions in their higher valence state. In contrast to the oxidation of hydrocarbons
I
I
I
2.3
2.4
215
vT.103
Figure 7. Arrhenius plot for the oxidation of p-xylene in the pres100 mg ence of).( 45 mg of cobalt oxide calcined at 350 OC and (0) of Co(I1) acetate. Conditions: 6 g of P X + 2 g of PTA + 2 g of H,O; oxygen pressure 1.3 MPa.
c
I
20
i
/A'/
/ '
+O
J'O
1
~z+.-;2Lo, o - o - o ~ o 0
20
40 m i n
60
Figure 8. Yields of PTA (a),p-tolualdehyde (e),p-toluoyl alcohol ( O ) ,and PX conversion (A) during oxidation of a mixture of 7 cm3 of P X + 2 cm3 of H 2 0 + 0.4 g of PTA at 140 "C and 0.8 MPa. Catalyst: 45 mg of cobalt oxide prepared at 350 "C.
in acetic acid solvent, the studied reaction catalyzed by cobalt oxide in aqueous systems proceeds through a freeradical mechanism, where an initial abstraction of hydrogen from alkyl aromatics involves peroxy radicals or some radical species. The reactivity data obtained with cobalt oxide are almost the same as we reported previously with homogeneous cobalt salts (Hronec and Ilavsky, 1982). Also, the apparent activation energy (94.0 kJ mol-') and distribution of oxidation products were found to be almost the same for both homogeneous and heterogeneous cobalt catalysts (Figures 7 and 8). Depending on the type of metal and its concentration, cobalt and manganese oxides doped by other metals display a change in their activity. On the basis of the results in Table I1 it can be considered that the cocatalytic activity of added metals is derived from an ability to change the ratio of COOand Co304phases in the prepared catalyst, Le., solubilities of resulting oxide catalysts. However, this effect is not the only one because x-ray studies of mixed cobalt-manganese oxides proved that there is not an exact relation between catalytic activity of the mentioned mixed-metal oxides and the amount os spinel phase in the catalyst. Disagreement is particularly in regions where the activity is low (Figures 4 and 9). At a nearly equimolar ratio of metals in mixed Co-Mn oxide catalysts differences in catalytic activity, expressed as maximum reaction rate and yield of terephthalic acid, were also observed. They are probably caused by the different activities of the formed homogeneous catalysts in the first stage of reaction, i.e., at lower p-xylene conversion (20-30%), where maximum rate is measured, and at high conversion, where partial catalyst deactivation can take place. Another explanation could be the different
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986
mol O,.kg.s 3,G r
co 0
I '
A'
0
0.5
co
1.0
Co+Mn
Figure 9. Dependence of maximum reaction rate (0) and ratio of formed cobalt oxides (A) on composition of mixed Co-Mn oxide catalysts. Conditions as in Figure 6.
rates of dissolution of corresponding cobalt and manganese oxides, but the results with homogeneous mixed cobaltmanganese catalysts do not support this assumption (Hronec and Ilavskjr, 1986). Moreover, our investigations of p-xylene oxidation in the same system but with homogeneous mixed cobalt-manganese catalysts show that the dependence of terephthalic acid yield on the ratio of Co/Mn is significantly different than that observed with heterogeneous Co-Mn oxides. These results suggest that during the dissolution of mixed-metal oxides in the reaction medium other types of homogeneous catalysts are formed, which possess different activities and probably also higher stability against deactivation during the oxidation process. This assumption is supported by the results of oxidation using a mixture of two metal oxide catalysts, cobalt oxide and manganese oxide in 1:l molar ratio (metal) prepared by separate calcination of corresponding metal acetates at 350 OC. In comparison with mixed CoMn oxide catalyst prepared by simultaneous calcination of binary mixtures of acetates, different results were obtained. Conclusions In view of what we have shown about the behavior of metal oxides, it is possible to comment on the similarities between the catalytic action of heterogeneous metal oxides and their homogeneous salts in the oxidation of p-xylene in the presence of p-toluic acid and water. It is well recognized that the following experimental results are common for both types of catalysts: (a) the catalytic oxidation of p-xylene proceeds only when p-toluic acid is present at the beginning of the oxidation; (b) the reaction requires only a catalytic amount of catalyst (
