4232
A. MUKHERJEE AND W. F. GRAYDON
indicative of capture by a neutral center and may thus be associated with F’ centers. 2. Values of 8 , for the glow peaks designated as originating from F’ center release, vary over a range of about one order of magnitude. This agrees with Lushchik’s data for s values associated with the F center in various alkali halides which demonstrate a similar variation. Such association of selected glow peaks with the F’ center, while strongly indicative, should be considered provisional until optical confirmation is made. On the basis of the above identified F’ thermal depths, it is interesting to compare E with the optical depth, hv. These are shown in Table III.l5J8 It will be noted that the Mott and Gurney’’ approximation,
E =
l/&v, applies only in the case of LiF and may not be used indiscriminately. Acknowledgment. This work was supported by the Air Force Materials Laboratory and the Air Force Avionics Laboratory under Contract AF33(615)2050. The authors appreciate the assistance of Dr. A. L. Gentile and Mr. 0. M. Stafsudd in the crystalgrowth phase of this work. (15) C. J. Delbecq and P. Pringsheim, J . Chem. Phys., 21, 794 (1953). (16) K. Konrad, M.S. Thesis, Illinois Institute of Technology, Chicago, Ill., 1966, p 10. (17) N. F, Mott and R. W. Gurney, “Electronic Processes in Ionic Crystals,” Oxford University Press, London, 1940, p 162.
Heterogeneous Catalytic Oxidation of Tetralin
by A. Mukherjee and W. F. Graydon The Department of Chemical Engineering, University of Toronto, Toronto, Canada (Received March 29, 1967)
A study has been made of the liquid-phase oxidation of tetralin (l12,3,4-tetrahydronaphthalene) with insoluble catalysts, and the reaction rates have been compared with those of the soluble ones. It was found that catalysts such as oxides of nickel, manganese, and copper were extremely active while the rest, such as oxides of aluminum and zinc were inactive. Rates of oxidation were measured in the temperature range of 45-90’. The initial product of reaction was found to be tetralin hydroperoxide which decomposed further into tetralone and tetralol. A ketone: alcohol ratio of 2 : 1 was found with most of the catalysts. It was further observed that a critical hydroperoxide to catalyst ratio existed below which the reaction did not proceed. Kinetic studies have been made with four of the best catalysts, and a reaction mechanism has been proposed.
Introduction The liquid-phase oxidation of tetralin with soluble Catalysts has been studied extensively.’-5 The mechanism is well understood. Data on heterogeneous catalysts, on the other hand, are very meagre. The only work reported so far on insoluble catalysts for the oxidation of tetralin was that of George! The catalysts that he used did not bring about great increases in rate over the thermal rate. The Journal of Phy&
Chemistry
It is known that p-type semiconductors (oxides of cobalt, nickel, manganese, copper, etc.) are good cata(1) Y . Kamiya, S. Beaton, A. Lafortune, and K. U. Ingold, Can. J . Chem., 41, 2020 (1963). (2) Y. Kamiya, S. Beaton, A. Lafortune, and K. U. Ingold, ibid., 41,2034 (1963). (3) Y.Kamiya and K. U. Ingold, ibid., 42, 1027 (1964). (4) Y. Kamiya and K. U. Ingold, ibid., 42, 2424 (1964). (5) A. E. Woodward and R. B. Mesrobian, J . Am. Chem. Soc., 7 5 , 6189 (1953).
HETEROGENEOUS CATALYTIC OXIDATION OF TETRALIN
lysts for oxidation reactions in the vapor phase.?-9 The soluble salts of the same metals are good catalysts for liquid-phase oxidation The present work was undertaken to determine the effectiveness of insoluble oxide catalysts of these metals in the initiation of the oxidation of tetralin. Experimental Section Catalysts. Oxide of manganese (Mn203) was obtained by heating to 150" in an oven; the hydroxide was precipitated by the addition of an excess of ammonium hydroxide to manganous sulfate solution. The hydroxide precipitate was washed several times before heating. Lead sulfide (PbS) was prepared by the addition of ammonium sulfide to lead nitrate solution. The precipitate was washed several times with distilled water and then heated to 120" in an oven. Manganese and cobalt stearate were made by the addition of 1 vol. of cold 0.375 M aqueous solution of the respective sulfate to 2 vol. of hot (90") 0.25 M aqueous solution of sodium stearate. The resulting solution was stirred for 20 min at 70-80" and then was filtered under suction through a no. 1 sintered glass funnel. The precipitate was washed four times with a small quantity of hot distilled water, evacuated, and dried by keeping the precipitate at 70". The catalyst was stored in a desiccator. Manganese dioxide (MnOJ was obtained from Baker and Adamson (reagent grade). Cuprous and 'ferric oxides (Cu20, Fez03)were obtained from C. P. Baker (Analyzed, reagent grade). The cobalt and nickel oxides (COO, NiO) and the other oxides and sulfides were obtained commercially. Reagents. Tetralin (Fisher Scientific, purified grade) was washed with concentrated sulfuric acid until the washings were colorless and then washed repeatedly with distilled water until the acidity was removed. It was then distilled under vacuum in a nitrogen atmosphere; the top and bottom fractions were discarded. The fraction coming out a t 58 i 0.5" was collected and stored under nitrogen atmosphere in the cold dark. After taking a sample for oxidation, the remainder was purged with nitrogen. Occasionally, the reagent was checked for hydroperoxide content. Tetralin hydroperoxide was prepared by air oxidation of tetralin. Purified tetralin (500 ml) was placed in 1-1. flask fitted with a reflux condenser. Dry air was blown through the solution for 48 hr a t 70". The hydroperoxide content was analyzed by the method described by Wagner, Smith, and Petemlo 3,4-Dihydro-1 (2H)-naphthalelone was obtained from Eastman Organic Chemicals, Rochester, N. Y. 5,6,7,8Tetrahydro-1-naphthol was obtained from Aldrich Chemical Co., Milwaukee, Wis.
4233
Apparatus. The apparatus was similar to that used by Bolland" in the thermal oxidation of ethyl linoleate. Rates of oxidation were measured a t constant pressure. The gas buret and the controlling manometer were immersed in a water thermostat kept at 25-30", depending on external temperature (temperature was maintained within k0.2"). The gas buret was connected to the reaction vessel by means of a 2-ft long pressure tubing. Vigorous agitation of the contents of the reaction vessel was achieved by connecting it to an eccentric wheel driven a t a speed of about 1200 rpm. The reaction vessel was immersed in a subsidiary thermostat, the temperature of which was controllable to within 0.5". Prior to each run, the catalyst was weighed into the reaction vessel and the required amount of tetralin was added to it. The reaction vessel was connected to the system by a flexible rubber tube with ball and socket joint. The reactor was kept in an acetone-solid carbon dioxide mixture. The system was evacuated and oxygen was admitted. The procedure was repeated twice before the system was finally evacuated to 8-10 mm pressure. The system was left a t this stage for 10 min to check if there was any leakage. Finally, the system was filled with oxygen and the reaction vessel was dipped in an oil bath for 2 min. The shaker was started and the change in buret reading could be observed only after 2-3 min. The volume of oxygen consumed was directly measured. An n-butyl phthalate manometer was used to detect any small change in internal pressure. The upward movement of the mercury level in the gas buret was actuated by the evolution of gas from the small electrolytic cell. After a run was over, the reactor with its contents was dipped into acetone-solid carbon dioxide mixture to arrest any further oxidation prior to analysis. The reactor content was then poured into a small culture tube, and the catalyst was allowed to settle. After a few hours the clear liquid from the top was siphoned out by a capillary tube and analyzed with a Beckman Model IR9 infrared spectrophotometer. Results and Discussion Kamiya and I n g ~ l d l - have ~ done a considerable amount of work on tetralin oxidation with soluble ~~
~
(6) P . George, Trans. Faraday SOC.,42, 210 (1946). (7) B. Dmuchovsky, M.C. Freerks, and F. B. Zienty, J. Catalysis, 4, 577 (1964). (8) P. H. Emmett, Catalysis, 7, 359 (1960). (9) E. S. Stone, "Chemistry of the Solid State," WT.E. Garner, Ed., Academic Press, London, 1955, p 367. (10) C. D. Wagner, R. H. Smith, and E. D. Peters, Anal. Chem., 19, 976 (1947). (11) J. L. Bolland, Proc. Roy. SOC.(London), A186, 220 (1946).
V o l u m e 71,Number IS December 1967
A. MUKHERJEE AND W. F. GRAYDON
4234
catalysts (decanoates of copper, manganese, nickel, and iron) a t 50 and 65". In order to compare the activity of insoluble and soluble catalysts, observations were made at 65' in this work. In Figure 1, the rates of oxygen uptake at different catalyst amounts have been plotted for different types of catalysts. It was observed that Biz03,Vz05, AlzOa,Si02, and ZnO did not increase the rate appreciably; their rates were only
1
8ot Pp
0
0.2
0.4
0.6
0.8
1.0
AMOUNT OF CATALYST (gm)
Figure 1. The measured rate of oxidation a t N T P of tetralin a t 65' as a function of catalyst amount: 0,Mn&; 0, MnOz; 0 , CUZO;A, NiO; ,. PbS. A 1-ml sample of tetralin was employed. The value for 1 ml of O2is 4.46 X IO-& mole of 02.
five to six times greater than thermal rates. A 2-7fold increase in rate was earlier reported by George6 with magnesium carbonate, keiselguhr, lead carbonate, and barium sulfate. The oxides of manganese, nickel, copper, and the sulfide of lead increased the rate to a considerably extent. Increases of the order of 40-80fold were observed. The amount of Mn20a required to achieve the maximum rate was exceedingly small compared to other catalysts. The same maximum rate was usually realized with different samples but because reproducibility of catalyst, preparation was poor, the amount of catalyst required to achieve the maximum rate differed. These differences were attributed to differences in catalyst surface area and possible variations in alkali metal impurities. There was some evidence of an inhibition resulting from sodium sulfate retention by the catalyst. The maximum rates obtained in this work with insoluble catalysts are tabulated along with those reported by Kamiya and Ingold4 for soluble salts of the same metal (Table I). Fro n the table, it is evident that the rates in the above two cases are comparable. In the case of nickel The Journal of Physical Chemistry
Table I: Comparison of Soluble Catalysts with Insoluble Ones at 65'
Metallic part
Manganese Copper Nickel Cobalt Lead sulfide
Max rate' with sol catalyst,a mole of Odhr
x
101
3.12 2.77 0.982 4.28
...
Max rate' with insol catalyst,b mole of Odhr
x
10;
3.1," 2.gd 2.7 2.0
... 1.7
Soluble salts are in the form of metal decanoates.10 * Insoluble catalysts are metal oxides. ' MnzOa. MnO,. ' Oxidation rates are reported on 1 ml of pure tetralin.
oxide, the rate was even higher than that observed with homogeneous nickel catalysts. The amount of Mnz08needed to achieve the highest rate was exceedingly small, and the rate declined catastrophically at high catalyst concentration, as shown in Figure 2. When the hydrocarbon was diluted, the break came even at lower catalyst concentration. Kamiya and Ingold4 reported a similar behavior with a cobalt decanoate-catalyzed oxidation of tetralin. Apart from showing similar rate values, these two types of catalysts, soluble and insoluble, exhibit a critical concentration above which the rate is inhibited. Such an agreement in rates in the case of homogeneous and heterogeneous catalysts may, in fact, result from either dissolution of solid catalyst into the reaction medium, or inhomogenity of so-called homogeneous catalysts. Three runs were taken in the following technique. After allowing a run to proceed until a high rate of oxidation was observed, the run was discontinued and the catalyst was allowed to settle. A measured sample from the supernatant liquid was placed into a second reactor, and the run was allowed to proceed. I t was found in every case that the rate had fallen sharply to the uncatalyzed reaction rate values. In a typical run at 65" with 0.10 g of MgzOa, an oxygen absorption rate of 55 ml at NTP/ml of tetralin-hr dropped to 1.5 ml a t NTP/ml of tetralin-hr when the catalyst was removed. This value is within the range observed for the thermal reaction (1.1-1.5 ml of oxygen/ml of tetralin-hr), The observations suggest that under experimental condition any dissolved catalyst was insufficient to affect the reaction rate significantly. Figure 3 demonstrates a few observations made with soluble cobalt stearate which are similar to the data of Kamiya and Ingold. They too observed a break at high catalyst amount. If the sample was kept for a
HETEROGENEOUS CATALYTIC OXIDATION OF TETRALIN
4235
80
90
"
0 0
0 . 0 4 0.08 0.12 0.16 0-20 0.24 AMOUNT OF CATALYST (gm )
0.28 0 3 2
w
Figure 2. The measured rate of oxidation at NTP of tetralin in chlorobenzene at 65" as a function of Mnz03concentration. The tetralin concentration is expressed in mole/l.: 0, 7.35; 0, 4.9; 0,3.7; 2.45. A 1-ml sample of tetralin was employed. The mole of 0 2 . value for 1 ml of 02 is 4.46 X
b
a
.,
a
0
Table I1 : Critical Hydroperoxide Concentration with Mn& (ROOHhl catalyst X IO', g/moleB/g
Rate, ml of 01
amt,
Catalyst,
ml
g
0.8 1.0 1.2 1.3
0.2630 0.2630 0.2630 0.2630
0.915 1.14 1.37 2.1
0 0 0 72.6
1.0 1.2
0.1000 0.1000
0.94 4.8
0 58.9
0.6 0.8 1.0
0.2793 0.2793 0.2793
0.947 1.26 2.62
0 0 73.2
1.0 1 .o
0.0967 0.0967
0.975 2.57"
0 35.6
0.8 1.0 1.2 1.4
0.0837 0.0837 0.0837 0.0837
0.90 1.13 1.35 1.575
0 0 0 0
1.0 1.2
0,0840 0.0840
1.12 2.16
0 63.5
1.0 1.1 1.3
0.0500 0.0500 0.0500
1.88 2.07 2.44
0 0
' Self-induced peroxide.
0.1
0.2
0.3
0.4
AMOUNT OF CATALYST ( g m )
few hours, it became a rigid gel. This behavior is indicative of the colloidal state of the original catdyst reactant systeni. In the case of Mn203,it was observed that the sample which refused to react immediately would start absorbing oxygen after standing for 3 or 4 hr. The hydroperoxide which formed after prolonged standing caused the reaction to proceed. If hydroperoxide was
Tetralin
10
(NTP/hr)
78.5
Figure 3. The measure rate of oxidation of tetralin at 65' as a function of cobalt stearate concentration. A 1-ml sample of tetralin was employed. The value for 1 ml of Oz is 4.46 X lo-&mole of 0 2 .
added immediately, the reaction proceeded at once. It was observed that there was a critical hydroperoxide to catalyst ratio below which the reaction would not proceed. The observations are summarized in Table 11. Table I1 needs some explanation. In the first set of results 0.8 ml of tetralin (initial hydroperoxide conmole/ml) was taken with 0.2630 g of tent, 3.0 X catalyst. The reaction did not proceed, so another 0.2 ml of tetralin, with the same initial content of hydroperoxide, was added to the batch. The reaction still did not proceed and yet another 0.2 ml of the above tetralin was added. This time when the reaction did not start, 0.1 ml of tetralin, initial hydroperoxide content, 19.3 X 10-6 g mole/ml, was added and the reaction started with an oxidation rate of 72.6 ml of Oz/hr. The critical ratio of hydroperoxide to catalyst amount was about 2.1 X mole/g of Mnz03. It is believed that such a critical ratio existed for the other types of catalysts, but the ratio was not determined with precision. Meyer, et u1.,12have recently reported the existence of critical hydroperoxide concentration in the chromic oxide catalyzed oxidation of cyclohexene. No data are given. It is not obvious why a critical concentration of hydroperoxide to catalyst ratio should exist. It was observed that the reaction did not proceed below the critical hydroperoxide concentration. It was further (12) C. Meyer, G . Clement, and J. C. Balaceanu, Proc. Intern. Congr. Catalysis, drd, Amsterdam, 196A 1, 184 (1965); Chem. Abstr., 63, 12362 (1965).
Volume 71. Number 19 December 1967
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A. MUKHERJEE AND W. F. GRAYDON
observed that the hydroperoxide concentration decreased immediately after the catalyst was added as shown in Table 111. The hydroperoxide, lost and presumed adsorbed, was a similar ratio to the critical ratio found in Table 11. It is assumed that the hydroperoxide was adsorbed by the catalyst surface. The amounts adsorbed are of similar magnitude to the critical ROOH needed for the reaction to start. When the hydroperoxide concentration exceeded the critical limit, the reaction proceeded. Active sites for radical production are believed to have been occupied only after saturation of inactive sites. It is assumed that there are two distinct sites on the catalyst: one of these sites preferentially adsorbs the hydroperoxide, and the other sites are responsible for the production of the radicals, ROO and "OH, which propagate the reaction. It may be possible that the active form of the catalyst is the metal in the higher oxidation state.
tion period corresponding to ROOH/catalyst of up to 4.3 x 10-5. The induction period is thus considered to be the time required to produce enough hydroperoxide to saturate the inactive sites completely and to occupy the active sites producing radicals which start the reaction. Similar observations of critical phenomenon by Kamiya and Ingold4may indicate that a similar mechanism of adsorption on colloidol catalyst is operative in the homogeneous oxidation systems. Table IV : Effect of Hydroperoxide Concentration on the Reaction Rate
Tetralin amt, ml
Catalyst (.MnzOz) amt,
Rate, moles of Oi/hr x 10'
g
Initial hydroperoxide, moles/ml x 106
Rate const, k
Catalyst A, hint03 1.0
Table 111: Absorption of Hydroperoxide by Catalyst Immediately after Catalyst Additiona
catalyst amt, g
0.0850 0.1202 a
HydroROOH~ peroxide before absbd, catalyst --Product distribution-mol@ addn ROOHb Ketoneb Alcoholb X 1Oa
11.0 11.0
6.2 4.0
2.4 3.1
0.8 1.2
1.6 2.7
A 1-ml sample of tetralin was employed.
1.0 1.0 1.0 ROOH/ catalyst, moles/g X 104
1.88-2.67 2.26
Moles/ml X
106.
When there is a small amount of hydroperoxide, all of it is used to saturate the inactive sites. Since inactive sites do not contribute to radical formation, the reaction does not proceed. This explains the long induction period with a large amount of catalyst. The increase in induction period was observed with both soluble4 and insoluble catalysts. Calculations made on observed induction periods, obtained using Mnz03heterogeneous catalysts, indicated that in every case the thermal rate of hydroperoxide products would be sufficient to provide an excess of hydroperoxide over the required critical amount. The measurements of induction periods are subject to considerable deviation. However, in several cases the ratio of the thermal products of hydroperoxide during the inhibition period to the mass of catalyst present gave results very similar to those observed for the critical ratio by analysis of solutions. Thus, for example, inhibition times corresponding to ROOH/catalyst ratio of 1.78, 2.02, and 2.03 were observed although some runs showed inhibiThe Journal of Physical Chemistry
0.0838 0.0838 0.0838 0.0838
1.5 1.47 1.39 1.185
0.2 4.0 9.0
0.208 0.207 0.198 0.170
7.5 1.5
0.141 0.144
2.5
Catalyst B, MnOz 1.0
1.0
0.3800 0,3800
1.28 1.28
A critical amount of hydroperoxide was required to initiate the reaction. An excess of peroxide, however, did not enhance the reaction rate as shown in Table IV. It was observed that the rate of reaction was independent of the amount of hydroperoxide present in the reaction mixture, if more than the critical amount was present. In fact, a slight decrease in rate constant is observed. This decrease might be attributable to a known inhibiting effect of alcohol p r 0 d u ~ t . l ~The liquid-phase oxidation reaction is a free-radical initiated reaction. The hydroperoxide is the source of free radicals, while the catalyst particle is responsible for the creation of these radicals from hydroperoxide stock. The concentration of free radicals in the reaction medium will be proportional to the active surface area of the catalyst and not to the amount of hydroperoxide present. Observations of the hydrocarbon order of reaction were made with four types of catalysts: Mn203,Mn02, NiO, and CuzO. Chlorobenzene was used as diluent. A large number of runs were made with 3InzO3 with different catalyst amounts and at various hydrocarbon concentrations. Runs were made a t only one catalyst (13) K. U.Ingold, Can. J. Chem., 34, 600 (1956).
HETEROGENEOUS CATALYTIC OXIDATION OF TETRALIN
amount for the other catalysts. The orders with respect to hydrocarbon concentration are tabulated in Table V. With cobalt acetate catalyzed oxidation of tetralin, Woodward and Mesrobians seemed to have obtained an order of 1.7-1.8 with respect to tetralin while Kamiya and Ingold got a second order.2 George6
4237
Apparent Activation Energy. Oxidation rates were measured at four different temperatures in the range 45-90'. Plots of rate us. 1/T gave apparent activation energies in the range of 9-10 kcal/mole. These rates were measured at 760 mm oxygen pressure and at various catalyst amounts (0.05-1.2 g/ml of tetralin). The slopes of the lines obtained were independent of catalyst amount. (See Table VII.)
Table V : Order with Respect t o Hydrocarbon Concentration Catalyst
Order
MnzOa MnOz cuzo NiO
1.8 1.4 1.5 0.8
and Meyer, et a1.,l2 obtained a first-order dependence on hydrocarbon with insoluble catalysts. George chose tetralin while .Meyer, et al., used cyclohexene in their investigation. Catalyst Amount. The rate constants for the reaction did not show a simple proportionality to the catalyst amount as noted earlier. In fact, the dependence could be fitted by a simple power relationship which could be considered empirically as an order with respect to catalyst. A 0.5 order with respect to benzoyl peroxide Table VI: Rate Dependence on Catalyst Amount Catalyst
Exponent
Mnz03 MnOz CUZO NiO
0.65 0.615 0.73 0.795
and azobisisobutyronitrile was reported by Woodward and Mesrobian.6 I n the steady-state oxidation reaction catalyzed by cobalt acetate, the order was zero. George' did not report the order with respect to catalyst m o u n t , but his data show an exponent of 0.62 for blanc fixe. Meyer12 obtained an order of 0.7 with respect to metal oxide catalyst. In the heterogeneous systems, the reduction in the effectiveness of the catalyst may result in part from a less effective surface exposure a t high catalyst densities. (See Table VI.) Oxygen Concentration. A general agreement exists on zero order with respect to oxygen concentration at an oxygen pressure above 100 mm using soluble catalysts. The present study was made in the range of oxygen pressures of 500-800 mm, and the rate was found to be independent of oxygen Concentration.
Table VI1 : Activation Energies Mode of Initiation
E, kcal/mole
MnOz CuzO N io MnzOa
9.05 9.55 9.92 9.94
The values for activation energies in the present work agree well with the values reported by Bamford and Dewar14 for photosensitized oxidation of tetralin. The values reported by Woodward and Mesrobiad are twofold greater. Kamiya and Ingold4 measured the temperature effects on the rate of oxidation of pure tetralin and 3.67 moles/l. of tetralin in acetic acid with five soluble catalysts (cobalt, nickel, manganese, copper, and iron) over a temperature range of 26-95'. Over the entire temperature range, cobalt acetate gave an over-all activation energy of 13.8 f 0.4 kcal/ mole. This value is smaller than the one reported by Woodward, et aL5 Kamiya and Ingold believed that this difference arose from the fact that Woodward and Mesrobian did not use a sufficiently large cobalt acetate concentration to reach the limiting rate. The data in the present study show no change in activation energy with a catalyst-hydroperoxide ratio. Product Distribution. No attempt has been made in the past to measure the products of the tetralin oxidation reaction quantitatively. It was reported that the initial reaction product was tetralin hydroperoxide which under extensive oxidation broke down predominantly to tetralone. In heterogeneous catalysis, the above-mentioned two compounds could not account for all the oxygen consumed. The departure was approximately 15-20%. On further analysis, a new product of reaction was identified as tetralol. The yield of tetralol was not unexpected, as the works of Robertson and Waters,15 and Pritzkow and Muller16 (14) C. H. Bamford and M. J. S. Dewar, Proc. Roy. Soe. (London), A198, 252 (1949). (15) A. Robertson and W. A. Waters, J. Chem. soc., 1574, 1578. 1585 (1948).
Volume 71.Number 19 December 1967
A.
4238
demonstrated that when tetralin hydroperoxide was decomposed, an appreciable quantity of tetralol was found along with tetralone. Tables VIII-XI show the distribution of reaction
Table VIII: Distribution of Products of Tetralin Oxidation with MnOz Catalyst Run no.
Hydroperoxide"
Ketonea
Alcohol"
264 239 240 24 1 242 243 244 13 91 290 291 294 298 300
0.0 7.8 6.0 6.0 5.7 4.0 4.0 18.0 11.0 1.5 5.5 1.7 2.5 -4.0
12.6 11.5 4.4 2.3 11.4 6.0 1.8 3.6 17.8 11.6 21.7 18.2 19.0 19.0
5.2 5.9 2.5 1.4 6.3 3.8 1.1 2.1 8.0 6.5 9.4 9.9 9.5 10.0
Moles/ml X 106.
Ketone/ alcohol ratio"?)
2.4 2.0 1.8 1.7 1.8 1.6 1.6 1.7 2.3 1.8 2.3 1.8 2.0 1.9
'Average ratio of ketone/alcohol
= 2.0.
Table IX: Distribution of Products of Tetralin Oxidation with CuzO Catalyst Run no.
68 50 262 263 234 236 237 a
Hydroperoxide"
Ketone"
Alcohol"
Ketone/ alcohol ratioat*
30.0 38.0 11.0 7.5 16.8 17.0 22.5
2.7 2.8 3.2 2.5 5.6 2.9 0.8
1.7 1.9 1.95 2.1 2.1 1.4 0.5
1.6 1.5 1.6 1.2 2.7 2.1 1.6
Moles/ml X 106.
* Average ratio of ketone/alcohol
= 1.75.
products for the catalysts MnOz, CuzO, NiO, and MnzOa. One striking feature is the constancy of the ratio of ketone to alcohol value, irrespective of the catalyst amount and its type. With the exception of Mn203, this ratio is about 2, while that for MnzOa is about 3. Reaction Mechanism. A reaction mechanism proposed should explain the following observations : (a) There is a critical hydroperoxide/catalyst ratio of 2.1 X lo-' mole/g in the case of MnzOa, indicating that hydroperoxide saturates the catalyst surface, after The Journal of Physical Chemietra,
hIUKHERJEE AND
w. F. GRAYDON
Table X : Distribution of Products of Tetralin Oxidation with NiO Catalyst Ketone/ alcohol
Run no.
Ilydroperoxide
Ketonea
Alcoholn
ratios**
260 261 224 225 226 227 228 229 69 46 44
5.0 7.5 22.4 14.3 16.5 16.8 14.0 22.4 21.0 21.0 21.5
9.2 8.7 4.64 13.53 2.0 8.4 15.6 6.07 6.70 3.6 6.3
5.8 4.5 2.08 5.45 1.0 3.0 5.2 2.08 4.00 2.3 3.6
1.6 1.9 2.2 2.5 2.0 2.8 3.0 2.9 1.7 1.6 1.8
Moles/ml
x
106.
* Average ratio of ketone/alcohol
=
2.15.
which active sites are filled. (b) An excess of hydroperoxide does not affect the reaction rate, indicating that hydroperoxide is active only on the catalyst surface. (c) There is a long induction period with a large amount of catalyst. This induction period can be eliminated by the addition of hydroperoxide. Again, the critical value of the order of 2.1 X mole/g for MnzOI is obtained. (d) The order with respect to hydrocarbon concentration should vary between 1 and 2, while that with respect, to catalyst amount between 0.5 and 1.0, depending on the particular catalyst used. (e) The ratio of ketone concentration to alcohol concentration was approximately 2 for most of the catalysts, The ratio was nearly 3 in the case of Mnz03. Empirical Rate Law. The empirical rate of oxygen absorption was found to be
-dOz - - K (RH)"M~ dt where a R H is tetralin concentration in moles/l. ; M is the mass of catalyst per ml of tetralin; and a,p, and k are constants for a particular type of catalyst. The values for the different catalysts are summarized in Table XI1 along with the ketone/alcohol ratio figures. Theoretical Derivation of the Surface-Catalyzed Oxidation Rate. From the evidence accumulated in this study, it is possible to speculate on the nature of the initiation step in the metal oxide catalyzed oxidation of tetralin. The function of the metal catalysts is to break the RO-OH bond and thus to create free radicals ROO and "OH. This happens at active sites on the surface of the catalyst. These ROO and "OH radicals (16) W. Pritzkow and K. A. Muller, Chem. Ber., 89, 2321 (1956).
HETEROGENEOUS CATALYTIC OXIDATION OF TETRALIN
Table XII : Experimental Q and 0 Values with Ketone/Alcohol Ratio Figures
Table XI : Distribution of Products of Tetralin Oxidation with Mn20sCatalyst Run no.
200 201 202 203 204 205 206 207 209 210 258 259 292 298 108 and 109 110 111 128 129 94 95 96 97 98 99 100 101 102 103 104 105 107 304 305 306 307 308
Hydroperoxide”
Ketonea
Alcohola
Ketone/ alcohol rstioasb
16.0 13.0 20.0 6.0 2.5 10.7 19.8 2.2 23.0 31.5 8.0 0 2.5 2.5
21.4 14.6 9.6 14.0 31.6 3.6 24.2 35.2 15.4 33.8 22.0 13.7 23.0 14.6
7.0 4.0 3.2 4.8 9.25 1.4 7.7 10.35 5.05
3.0 3.6 3 2.9 3.4 2.6 3.1 3.5 3.1
17.0 14.0 8.4 4.0 10.0 7.0 3.0 13.0 4.0 4.0 11.0 13.5 5.5 17.0 4.0 5.0 20.0 10.8 2.5 9.0 4.0 3.5 3.5
21.0 28.8 30.7 12.6 18.1 8.6 14.2 8.0 10.2 17.6 10.1 8.0 12.4 20.0 21.8 11.9 8.6 27.6 8.7 9.2 10.8 5.3 3.8
5.5 10.4 7.9 3.5 5.9 3.4 5.3 2.8 3.4 5.6 3.3 2.8 4.5 5.7 6.6 4.3 3.3 7.2 3.3 3.8 4.0 2.3 1.8
Moles/ml X 1Db.
... 7.0 5.28 6.6 7.5
3.1 2.6 3.5 2.0 3.8 2.8 3.9 3.6 3.1 2.5 2.7 2.9 3.0 3.1 3.1 2.9 2.8 3.5 3.3 2.8 2.6 3.8 2.6 2.4 2.7 2.3 3.2
Average ratio of ketone/alcohol = 3.12.
on the catalyst surface can abstract hydrogen from the surrounding hydrocarbon medium. Workers agree on the production of R o radical in the initiation step. The general scheme ROOH catalyst. . Ro has been previously postulated. The production of alcohol in the initiation step is a new postulation made in this study. The initiation steps will be, accordingly
+
-
ROOH*M -% ROO M+ “OHM ROOM
+ RH -% ROH + R o
4239
Catalyst
a
8
Ketone/ alcohol ratio
MnzOa MnO2
1.80 1.40 0.8 1.5
0.65 0.615 0.795 0.73
3.2 2.0 2.15 1.75
NiO cu20
“OHM
Rate constant (k)
0.196f0.012 0.178 & 0.036 0.555 f 0.135 0.097&0.021
+ R H -% H 2 0 + R o
(C)
The amount of ROOH*M will be proportional to M , the amount of catalyst present,. Because of the dispersion of the catalyst, the amount of ROOH*M is equivalent to concentration. The propagation steps for the oxidation are well understood and are believed to be operative in the liquidphase oxidation reactions in general. The scheme is as follows
Ro ROO2
+
0 2
k’,ROO2
(D)
+ RH -% ROOH + R o
(E)
The bimolecular termination of radicals
2ROo2-% 2R”CO
+ H20 +
0 2
(F)
as shown by step F has frequently been used in the past. Workers have differed in opinion regarding the products formed by such bimolecular termination reaction. In this study, it is postulated that along with such bimolecular termination, monomolecular termination can occur (G). Often this step is associated with chain transfer as shown in step H
+ O2 + H2O ROOz -% R”C0 + MOOH
ROO2 -% R”C0
(GI (HI
When steps G and H are eliminated, the rest of the reaction sequence yields two solutions, depending on the existence of equilibrium or steady-state concentration of “OHM and ROOM radicals. Under equilibrium condition
“OHM
=
Kp%.f = ROOM
The find form is
(A)
(B) Under steady-state conditions, i.e. Volume 71, Number I d
December 1067
A. MUKHERJEE AND W. F. GRAYDON
4240
d(O0HM) d(ROoM) = Oa =O dt dt then
With monomolecular termination along with chain transfer, under equilibrium condition, the final form of oxygen adsorption is
-do2 - - k(k2
+ k3)KiM(RH)a
+
h+ka
dt
The orders with respect to hydrocarbon concentration (a) and metal amount @) along with ketone/alcohol ratio derived from the proposed reaction mechanisms are summarized in Table XIII. Comparing the values in Tables XI1 and XIII, it is observed that eq 1 explains well the MnOg and CUZOTable XIII: Theoretical CY and j3 Values with Ketone/Alcohol Ratio Figures Ketone/ alcohol Equation
1 2 3
(I
B
1.5
0.5
1.0 1-2
0.5 1
ratio
2.0 2.0 2.0f
catalyzed runs. Mnz03-catalyzed runs are consistent with eq 3, while eq 2 fits the experimental data obtained for the NiO catalyst,. One interesting observation made in the present study, which sheds much light on the initiation step, is the ketone/alcohol ratio. In this study the ratio was found to be approximately 2 for most of the catalysts as shown earlier. In the decomposition studies of tetralin hydroperoxide at 76" with and without a soluble catalyst,'S the ratio of ketone/alcohol ratio was found to be 1.6. It was also reported that figures for alcohol included the phenolic acids, like v-o-hydroxphenylbutyric acid and P-o-carboxyphenylpropionic acid, which were supposed to have formed from tetralone by overoxidation. It is understood that while the ketone value is slightly reduced, the alcohol 'value has been correspondingly increased, so the ratio of ketone/alcohol is expected to be greater than 1.6. The Mesrobians mechanism of soluble catalysis gives a ketone/alcohol ratio of unity. The proposed mechanism can explain the ketone and alcohol stoichiometry for soluble salt catalyzed reactions of tetralin in liquid phase. It is interesting to observe that the form of the eq 2 is identical with the oxygen absorption equation proposed by Woodward and Mesrobians for benzoyl peroxide and azobisisobutyronitrile-initiated oxidation of tetralin.
Acknowledgments. Financial assistance received from the University of Toronto and the National Research Council of Canada is gratefully acknowledged.
