US2440233A - Oxidation of benzene to phenol with an organic promoter - Google Patents

Description

April 20, 1948. R. H. KRIEBLE ET AL 2,440,233

OXIDATION OF BENZENE TO PHENOL WITH AN ORGANIC PROMOTER Filed Nov. 30, 1944 l hk @Nk .El

Patented Apr. 20, 1948 UNITED STATES PATENT OFFICE OXIDATION 0F BENZENE T0 PHENGL WITH AN ORGANIC PROMOTEB liam L Denton, Woodb Socony-Vacnnm Oil Company. Incorporated. a corporation o! New York Application November so. 1944, serial No. 505m 'z claims. (ci. seo-m) This application. which is a continuation-inpart of our now abandoned copending application Serial No. 395,016, led May 24, 1941, has t0 do with the production oi' phenol by the oxidation of benzene and is more particularly concerned with that type of process wherein a reaction mixture consisting of benzene and oxygen or oxygen-containing gas such as air is passed through a reaction zone void of solid catalyst. under pressure and at elevated temperature.

We are aware of the tact that processes oi' this general character have heretofore been proposed, and in this regard reference is made to the Bone et al. Patent No. 2,199,585 and the Moyer et al. Patent No. 2,223,383. Each of these patents discloses a process of the general character above referred to, and each mentions that the process therein disclosed can be carried out by recycling. We have discovered. however, that although the process ot Bone. for example, it used with a relatively impure grade of commercial benzene, can be operated to obtain a fair yield of phenol on a small number of passes in a recycling operation, the conversion to phenol rapidly decreases with successive cycles, thus materially restricting the scope of the process and rendering it impractical as a recycle operation. We have also observed that if a chemically pure (reagent grade) oi' benzene is subjected to the process of the Bone patent, no substantial conversion to phenol is obtained.

Since the percentage conversion oi benzene to phenol in an oxidation process of the class described is relatively small. it is obviously essential from the standpoint of practlcabiiity that the operation be one in which the unconverted benzene can be continuously recycled with a substantially constant conversion and yield of phenol under a given set of operating conditions. It is the primary object of this invention to provide such a process.

This invention is predicated upon the discovery that if a minor proportion of a compound which under the conditions of reaction in the process is a better hydrogen donor than benzene-cyclohexane or cyclohexanol, for example--is continuously introduced into the stream of benzene entering the reaction mixture, the conversion and yield of phenol is substantially improved over the conversion and yield obtained without the hydrogen donor and, further. that the conversion and yield will remain substantially constant in a continuous operation for a given set of operating conditions.

For the continuous operation contemplated herein a properly proportioned mixture of bensene and oxygen. air or other oxygen-containing gas (one containing oxygen in admixture with one or more other gases). is thoroughly mixed and pre-heated tothe desired temperature and passed through a reactor tube which has been pre-heated and which for the purpose of preheating and dissipating the heat oi.' the exothermic reaction is immersed in a suitable heattransier bath such as salt. The reaction mixture. as will hereinafter be explained, is preferably maintained under pressure. With properly regulated conditions a part of the benzene is thus oxidized to phenol. This phenol and the gaseous and high-boiling products of oxidation are then separated from the benzene. which is subsequently recycled through the mixer-preheater and reactor. Additional oxygen. air or other oxygencontaining gas is of course added prior to the admission o! the benaene to the pre-heater-mixer, and in a continuous operation make-up benzene is also added to replace the benzene converted.

m such an operation we have found that where a commercial grade of benzene, such as 90% Hensel." is used as the charging stock and is recycled through the reactor with no additions of make-up stock under iixed conditions of temperature and pressure. the benzene rapidly loses its reactivity and. further. that the reactivity may be restored only in part through altering the reacting conditions by going to higher temperatures. The charge eventually becomes unreactive even at relatively high temperatures. We have also found that when the procedure just described above is altered by adding fresh make-up benzene, the reactivity and conversion to phenol is not markedly increased. However. in an operation o! the recycle type lust described. if a minor proportion oi cyclohexane is continuously added to the charge of benzene. a steady conversion performance may be obtained and the reactivity. as indicated by the per cent conversion and yield of phenol at a given temperature. is greatly increased. We have found that the reactivity in a process of the class described under a given set of conditions may be greatly increased by the incorporation in the reaction mixture of a minor proportion oi' cyclohexane. For example. under a given set oi' conditions recycle stock with 6 per cent make-up fresh benzol gave a conversion to phenol of only 0.1 per cent of the total charge. while the same recycle stock with about 1 per cent of cyclohexane gave a. conversion of 2 per cent by weight of 4phenol under the same conditions.

under the conditions Although we do not wish to be bound by any theory as to the mechanism of this reaction or the part which the cyclohexane or other inductor or promoter plays in increasing the reactivity and conversion to phenol, it is our view that the mechanism as represented by the following equations will explain most of the observed i'acts:

Reaction I is slow and determines the rate. Reactions 1I and III. since they involve radicals. should be extremely rapid. Reaction UI is shown as a competition of phenyl and hydroxyl radi.- cals for phenyl and is probably over-simplied. The essential fact is that in the presence of a material which can be more readily dehydrogenated by Oe than benzene. H: is produced independently of reaction I and at a lower temperature. It is then capable of reacting rapidly with benzene by reactions II and III to produce the usual products.

IV. CIELI 50| CeHl BHO:

Under these conditions, far more phenol survives, less diphenyl is formed, and the yield is greatly improved.

According to the foregoing mechanism, then, an inductor or promoter for use in a process of the type contemplated herein is any compound which is a better hydrogen donor than benzene of the reaction zone; or, stated in another way, a promoter cr inductor may be deilned as a compound containing hydrogen and possessing the properties of being in the gaseous phase under the conditions of the reaction zone and, under said conditions. reacting with oxygen to form water at a lower temperature than the temperature at which benzene so reacts with oxygen.

Cyclohexane is a preferred compound for use as an inductor in this process. rst. for the reason that itis highly eiective in promoting the conversion to phenol and, second, for the reason that it is converted to benzene in the reaction of the process and thereby avoids any problem of contamination. It is to be understood, however, that the process contemplated herein is not limited to cyclohexane as the promoter or inductor but broadly comprehends any compound which under the conditions of the reaction zone is a better hydrogen donor than benzene. For example, such a compound may be obtained in the reaction mixture by partially hydrogenating the benzene. This can be conveniently accomplished, as will hereinafter appear, by passing the benzene charge through a hydrogenation chamber containing a suitable catalyst and adding hydrogen to the chamber at a metered rate in a quantity to eilect the desired amount of hydrogcnation. Other promoting compounds and materials which we have tested and found to be effective in increasing the conversion and yield in a process of the class described are: cyclohexanol, cyclohexanone, cyclohexane, tetra-hydro naphthalene, 2ethyl3propyl acrolein, octaldehyde, nhexane, diethyl ether, aniline, turpentine, n-hexene, and cracked gasoline. Each of the aforesaid promoting compounds and materials are characterized by the elements carbon and hydrogen, and several are further characterized by oxygen or nitrogen. Cyclohexanol was found to be particularly effective in promoting the reaction; and although this compound may present the same advantages as cyclchexane from the standpoint of noncontamination, it is not so desirable from an economical standpoint because of its present high cost. As classes of compounds contemplated herein for use as promoters or inductors; preference is given to t'he hydrogenated aromatic hydrocarbons typied by cyclohexane, cyclohexene, tetra-hydro naphthalene. etc., and the olens, which are typified by n-hexene and cracked gasoline.

As stated hereinabove, the promoters contem plated by the present invention are better hydrogen donors than benzene under the conditions oi the reaction zone. In other words, the prometers in our process are hydrogen-containing materials which release hyrdogen more readily than benzene under the conditions of reaction. The rates of hydrogen release from numerous materials are available in the chemical literature and may be resorted to in selecting effective promoting materials. From such data we have derived "potential hydrogen donor ability (P. H. D. A.) values for various materials and have foundthose materials having values above a certain minimum are effective in our process. The P. H. D. A. value relationship thus provides a means whereby a chemist skilled in the art may readily determine, with a high degree oi. certainty, whether or not a material will be an effective promoter.

The P. H. D. A. values obtained for various materials are derived in the following manner. Hydrogen atoms are classified here into four categories: primary, secondary, tertiary and hydrogens attached to an olefinic bond or an aromatic ring. No attempt has been made to evaluate bonds other than the carbonhydrogen bond. With this as a starting point and taking activation energy values froli the chemical literature, it is possible to evaluate relative ratios of release of the diil'erent hydrogen atoms from hydrogencontaining materials. Average activation energy values taken from "The Aliphatic Free Radicals (F. O. Rice and O. K. Rice, Johns Hopkins Press; 1935) are as follows:

A verage Relative rates of hydrogen release under conditions typical of our process, as at 427 C.. are calculated from the well-known relationship [Arrhenius equation (Getman and Daniels: Outlines of Theoretical Chemistry, 5th ed., pages 336 and 344, John Wiley & Sons, Inc., N. Y.) l.

wherein K is the specific a constant; E represents calories); R is a constant (1.987 calories); T is absolute temperature (in degrees Kelvin). The relative rate of a, primary hydrogen atom as velocity constant; C is activation energy (in compared with an oleiinic hydrogen atom is calculated as follows:

In this manner it is found that the several hydrogen atoms have the following K values at 427' C.

Olennie (aromatic) 1 Primary 'I3 Secondary 215 Tertiary 1,280

It will be recognized that the foregoing values have been assigned to the so-called normal hydrogen atoms. Hydrogen atoms attached to carbon atoms which are in the alpha position to an olenlc bond or some electronegative group such as keto C=O). ether (0) or ester (-COOR) are released from the molecule at an increased rate. It is diilicult to evaluate this eiect since it depends largely upon the structure of the molecule. It is recognized that different substituent groups such as olen, ketone, ester. ether, etc., groups may activate hydrogen atoms to different degrees. For convenience, however, a constant value has been assigned to such activation. Accordingly, whenever a primary hydrogen is in an activated position, it is assigned a value equivaient to a normal secondary hydrogen. Similarly, an activated secondary hydrogen atom is assigned a value equivalent to a normal tertiary hydrogen atom; and for an activated tertiary hydrogen atom, the normal rate is doubled. The P. H. D. A. of a compound, on a weight basis, is the sum of all the evaluated hydrogen atoms multiplied by a factor of ten and divided by the molecular weight of the compound.

From the Km' c. values o! 0. 1, 3 and 15 shown above, the P. H. D. A. values of hydrogen-containing materials are easily calculated. By' way of illustration, cyclohexane has twelve secondary hydrogen atoms and. therefore, has a P. H. D. A. value, on a weight basis, of

Illustrative of materials found to be inellective experimentally and so indicated by its P. H. D. A. value is ethyl alcohol. This compound has three primary and two secondary hydrogen atoms, and has a P. H. D. .L value, on a weight basis. of 2.0.

The P. H. D. A. values for materials tried in our process as promoters are in substantial agreement with the experimental results obtained. However. in the case o! ilrst members oi' the various chemical classes which often behave diiferently than all other members of their classes. some discrepancies obtain between tho P. H. D. A.

4.4 (from 6 values and the experimental results. In general, then. those materials which have P. H. D. A. values. on a weight basis. of from 0 to about 2.6 are ineffective. and those which have values greater than about 3.5 are edeetive; however, those having values greater than 4 are generally characterized by a high degree of eilectiveness. lThist'I relationship is shown in the following tabua on:

P. n. n. A. Cmm'md (wenn) ylacrolein ne taken asa-pinene) ethyl xanth disulilde As will be readily apparent to those skilled in the art, the apparatus used in carrying out a process ot the type contemplated herein may take various forms. In the accompanying drawings, however, we have shown diagrammatically one form of apparatus which may be satisfactorily used in carrying out an operation for the continuous conversion of benzene to phenol. As will hereinafter appear. the conditions of operation embody a number of variables which may be changed with respect to one another over relatively wide limits. and no attempt will be made in describing the apparatus to take account of these possible changes in variables.

Referring now to the drawing. reference numerals il' and il' indicate conduits which carry benzene and are connected through suitable valves I2 and I2' with the inlet I3 of a pump Il. The pump Il delivers the benzene through a conduit il to a 1'connection i6. where it is introduced into a mixing conduit II leading to the coils Il of a mixer and pre-heater mounted in an insulated case Il which is nlled with a suitable heat-exchange medulm.

Reference numeral 20 indicates an air-compressor which discharges into pipe 2i connected through the connection 22 to a compressed-air storage reservoir 2l. The pipe Il discharges through a pressure-reducing valve Il and an orice flow-control 25 into the conduit I1. through which the air-and-benzene reaction mixture is conducted to the mixing and preheating coil lli. The inductor or promoter is introduced into the reaction mixture at any suitable polnt, preferably in the benzene stream. through a conduit as indicated by numeral 2l, such conduit being provided with a metering valve 29 to control the quantity of an inductor, such as cyclohexane. which is introduced.

As was previously pointed out. the inductor or promoting compound may be formed by hydrogenating a portion of the benzene prior to its introduction through the reaction zone, and means for carrying out this alternative procedure may be embodied in an apparatus of the type illustrated by placing a valve lll in the benzene conduit i5 and providing a connection Il on the up-stream side of the valve Il through which a conduit il on the side of the valve I0.

The pipes Il and I! are shown as being provided with control-valves ll' and Il'. drogenation chamber. es aforesaid, contains a suitable hydrogenation catalyst such as nickel and is maintained in any suitable manner at an elevated temperature oi' from about 250 F. to 400 F. Hydrogen is introduced through pipe 31 provided with a meter IB.

The pre-heating and mixing coil il, wherein the benzene-promoter-air mixture is intimately mixed and pre-heated to a temperature below the temperature at which reaction will take place, discharges into a header In, which connects with a series oi' reactor tubes 4l. The reactor tubes are suspended in a suitable heat-transfer bath. such as a i'used salt bath, capable ot maintaining a close temperature control, such bath being contained in the insulated case l2, which is provided with an inlet conduit 43 having a pump I4 the a constant temperature. 46 receives the heat-exchange medium from the tank Il through a discharge conduit 41.

In the form of the apparatus shown in the drawing, the reactor tubes are illustrated as be- For the thermal balance oi' the process this heat-exchanger 52 is shown as being connected through conduits Il and il with the mixer and pre-heater i8 so that the heat-exchange medium is circulated by means of pump 65 from and pre-heater I9 to the bottom o! the heat heater through the pipe 53.

The cooled reaction mixture containing the phenol and other products of the reaction dis charges from the heat-exchanger through pipe 5F50' and tilter 5l into a, high-pressure mist-breaking tower 6l having a high-pressure gas-discharge valve 59 which may lead to a turbine. Discharge valve I9 is controlled by prince accumulating in the bottom oi' the high-pressure tower Il is conducted through a pressure-reducing valve Il into a low-pressure-packed tower 0l provided with a valved vent B2 to release gaseous products. The liquid product phenol, high-boilers," is delivered to the benzene-recovery still i2. In the still as illustrated the liquid products pass through a preheater Il and are discharged through the discharge pipe 6I' into the bottom o! the stili 62, where the benzene is distilled olf by a steam ooil reboiler 04. The still B2 is shown as being equipped with bubble plates 65 and a water coil redux-condenser 65'. The benzene vapors are discharged through conduit 6i into a benzene condenser 66', from which the liquid product discharged through conduit 01 ls pumped by means oi' pump 68 into a benzene-washing tower a heat-exchanger i 8 Il, from which the recycle benzene enters the ene conduit il'. The bottom of ugh oxidation, are referred to as hi boilers," are discharged from the still 'il through the conduit 8l by means of a pump Il.

As has been previously pointed out. the procused as the proportions in which it is used.

In order that some indication may be afforded as to the ei'i'ect of these variables, each o1' them will now be discussed individually.

Reaction mixture The proportion of air that can be used to advantage in the reaction mixture has an upper of the reactor tube (4 i-M In tubes having an internal diameter of from .2 inch to .35 inch operating at a pressure of 1000 pounds per square inch, good yields o! phenol are Mixtures oi suit in a greatly increased production of "highboilers, which greatly lowers the yield ot phenol even though the conversion to phenol may be moderately greater. Also, good yields of phenol phenol over the use of the 4:1:4 mixture rst referred to. Under the conditions ,lust described o! from 8 to 4 molecular proportions of benzene to 1 molecular proportion of oxygen.

With a smaller reactor tube having an internal diameter o! say .086

oxygen is consumed. Under these conditions a higher yield o! phenol is obtained than with the mixture SCHs-i-Oz-i-m: although the conversions to phenol are not greatly changed.

With a small tube o! this character higher pressures in the neighborhood i' from 2000 to 3000 pounds per square inch are also desirable.

Reaction time For the purpose of this discussion the reaction time may be defined as the time taken for a molecule ot the reactance mixture to pass through the reactor tube under operating conditions. The reaction time is not an important variable provided it exceeds a certain minimum value which varies with the apparatus and the conditions of use. Above this so-called critical minimum value an increase in reaction time can be compensated for by a small decrease in reaction temperature. However, for reaction times below the minimum value there is a sharp rise in the production of high-boilers and a corresponding decline in phenol yield although the actual conversion to phenol is not greatly aiected. The value of this so-called critical minimum reaction time, as aforesaid. is dependent upon the apparatus and conditions of use, and it is probable that the controlling factor rests in the ability of the apparatus to dissipate heat. The critical minimum reaction time is about seconds under the following conditions:

Charge: Bakers reagent benzene+l% cyclohexane. Reactor: .35 inch internal diameter stainless steel tube l5' long. Mixture: 8CeHs+O:l-4Nz. Pressure: 1000 pounds per square inch.

Ii the reaction time is between 13 and 140 seconds, the optimum reaction temperature lies between 700 F. and 750 F.. the conversion to phenol between 2 and 3 per cent, and the yield of phenol from 55 to 65 per cent.

The critical minimum reaction time is about 2 seconds under the following conditions:

Charge: 1 thiophene-free benzene (containing 1% cyclohexane).

Reactor: .086 inch internal diameter iron tube l5 long.

Pressure: 1000 pounds per square inch.

With a 2.3 second reaction time, phenol is produced in 3 per cent conversion and 50 per cent yield at 955 F. Better operation is obtained, if in the above conditions, the pressure is raised to 2000 pounds per square inch. Then phenol is produced in from 3 to 4 per cent conversion and from 60 to 65 per cent yield over a range of reaction times from 5 to 35 seconds. The optimum reaction temperatures are from 790 F. to 840 F.

Pressure We have obtained phenol in goed conversion at pressures of from 60 pounds per square inch to 3000 pounds per square inch. The higher the pressure, the lower the optimum reactor temperature, other conditions being the same. Thus, while at 60 pounds per square inch, temperatures of the order of 1200 F. are required, at 2000 pounds pressure good results have been obtained at 675 F. Since ferrous metal surfaces strongly catalyze total combustion at temperatures above 1000o F., it is necessary to use inert surfaces such as glass, enamel, silica, etc., in the reactor when operating at low pressures and the concomitant high temperatures. In this connection, good results have been obtained in nickel tubes at a pressure of 350 pounds per squ.' `e inch where the optimum reaction temperature was about 1030 F.. and probably still lower pressures and higher temperatures could have been successfully used. With iron tubes, sufilcient pressure to keep the reaction temperature below 1000 F. is required, usually at least 500-1000 pounds per square inch However, at 1000 pounds pressure, the use oi nickeltubes instead of iron or stainless steel tubes presents no advantage.

The use oi high pressure has certain fundamental advantages, however. At low pressures, even though a glass-lined reactor be used, the loss to oxides of carbon is materially higher than at high pressures in stainless steel equipment. Furthermore, the minimum critical reaction time apparently increases with decreasing pressure: and, since for a constant reaction time the rate of feed varies directly with the pressure (by deflnition), the maximum permissible rate of throughput falls of! very rapidly with decreasing pressure.

There is also an upper useful limit of pressure. As the pressure is raised, the temperature required to initiate reaction decreases. as explained above. Furthermore, the boiling point of benzene ls raised until the critical temperature of liquid benzene (550 F.) is reached, where it remains unchanged as the pressure is further increased. Slnce it is advantageous to mix thoroughly the benzene vapor and air before initiating reaction, in order that local regions of undesirably high oxygen-concentration be eliminated, it is not desirable to indennitely lower the reaction temperature by the use of extremely high pressures. There ls a relationship between the maximum useful pressure and the internal diameter of the reactor tube, since we have found that the smaller the internal diameter of the reactor tube. the higher the temperature necessary to initiate reaction. ThusI pressures at least as high as 3000 pounds per square inch and probably higher, can be used to advantage in a .086 inch internal diameter reactor tube, while in a .35 inch internal diameter reactor tube 2000 pounds per square inch appears to be about the upper useful limit, and we prefer a pressure of 1000 pounds per square inch. Because of this same relationship between internal diameter and reaction temperature and the catalytic etlect of the iron tube walls discussed above, a .086 inch internal diameter tube requires a pressure of at least 1000 pounds per square inch, while a .35 inch luternal diameter tube has given fairly good results at 500 pounds per square inch.

Temperature The optimum reaction temperature can be varied from 650-1225" F. by changing the operating conditions, among the most effective of which are pressure, Inductor concentration and internal diameter of reactor tube. This is discussed in the sections entitled Pressure and Inductor.

Reactor tube As we have previously indicated, it is highly desirable that the reactor tube be void of solid catalyst and that it be of a material which will have no substantial catalyzing effect in accelerating the oxidation of benzene. We have observed in this connection that the chief eiect of a solid catalyst is to increase the loss to total-combustion 11 products and therefore to be avoided in the reactor tube (II-II') At the relatively high temperature encountered in low-pressure oxidation the use of extremely inert (non-catalytic) inner surfaces of the reactor tubes. such as glass, enamel, glazed porcelain. or silica, is imperative.

At moderate pressures (several hundred pounds per square inch) nickel is satisfactory. At pressures in the neighborhood of 1000 pounds per square inch low-carbon steel and stainless steel are satisfactory, and nickel presents no advantage.

As to the internal diameter of the reactor tube (II-4l successful operation has been obtained in tubes varying from .086 inch to .875 inch in internal diameter. For iron tubes, the smaller the internal diameter the higher the required pressure. We prefer to use a tube having an internal diameter in the neighborhood of .35 inch and to operate with pressures in the range of from 500 to 2000 pounds per square inch. Smaller tubes are equally satisfactory. but they require higher operating pressures. For a tube having an internal diameter of .086 inch. pressures o! from 2000 pounds to 3000 pounds per square inch are recommended.

The length of the reactor tube (4I-4V) apparently determines the rate of throughput. and the existence of a critical minimum reaction time for a tube of nxed length and internal diameter has already been discussed. Satisfactory results in the operation of a process of the class described have been obtained with tube lengths of 15 feet, and tubes of at least this length are recommended.

Mizer-pre-heater tube As to the mixer-pre-heater tube (Il) any material having suitable mechanical properties such as stainless steel is satisfactory. The internal diameter of this tube should be sumciently small to prevent reaction taking place therein. and the tube should be sumciently long to provide the heating surface necessary for a heatexchange that will maintain the feed temperature within 25 F. to 50 F. of the temperature of the pre-heater bath. The temperature of the pre-heater bath should be such that the reaction mixture discharged from the pre-heater coll Il is at a temperature of about 50-150 l". below the temperature maintained in the reactor bath in the case 42.

Inductor The required properties of the lnductors and the theory of their action. together with certain preferences therein to cyclohexane and the like. have already been discussed.

The concentration of the inductor may be varied depending upon the character of the make-up stock, the conditions ol' operation. etc.: but we have found that this concentration should be maintained in the neighborhood of from about 0.2 per cent to about 3 per cent. by volume, based upon the quantity of benzene in the reaction mixture. We prefer to use a concentration of inductor in the neighborhood of from 0.5 per cent to 1 per cent, by volume, and have found that increasing the concentration of the inductor greatly lowers the optimum reaction temperature.

It will be seen from the foregoing discussion that a process of the type contemplated herein is susceptible of numerous operating conditions through manipulation oi' the variables discussed. and the present invention is not concerned with any particular set of operating conditions but. as aforesaid, is predicated upon tbe discovery that greatly improved results and a continuous recycling operation may be obtained by introducing into the reaction mixture an inductor or promoting compound of the type hereinabove discussed. It is to be understood, therefore. that although we have described and illustrated a specific form of apparatus and have discussed in considerable detail various reaction conditions which may be employed in the operation of such apparatus, the invention is not limited to this apparatus or to any particular set of operating conditions but includes within its scope such changes and modincations as fairly come within the spirit of the appended claims.

We claim:

l. In the method of making phenol wherein a reaction mixture ot benzene vapor and oxygencontaining gas is passed under a pressure in excess oi about 500 pounds per square inch through a reaction zone at a temperature between about 650 F. and 1000 F.. said reaction zone having a. metallic surface and being void of solid catalyst. the step oi.' incorporating in the reaction mixture a minor proportion, from about 0.2 per cent to about 3.0 per cent by volume based upon the quantity of benzene in the reaction mixture, of cyclohexane.

2. In a method for the continuous manufacture of phenol from benzene wherein a reaction mixture of benzene vapor and oxygen-containing gas is passed under a pressure in excess of about 500 pounds per square inch through a reaction zone at a temperature between about 650 F. and about 1000 F'. to convert a part of the benzene to phenol and other oxidation products and wherein unconverted benzene is separated from the phenol and other oxidation products and is returned for recycling in the reaction mixture, said reaction zone having a metallic surface and being void of solid catalyst: the improvement which comprises continuously incorporating in the benzene so recycled, prior to its admission to the reaction zone, a minor proportion, from about 0.5 per cent to about 1.0 per cent by volume, of cyclohexane.

3. In a method for the continuous manufacture of phenol from benzene wherein a. reaction mixture oi' benzene vapor and oxygen-containing gas is passed under a pressure in excess of about 500 pounds per square inch through a. reaction zone at a temperature between about 650 F. and about 1000 F. to convert a part of the benzene to phenol and other oxidation products and wherein unconverted benzene is separated from the phenol and other oxidation products and is returned for recycling in the reaction mixture. said reaction zone having a metallic surface and being void of solid catalyst; the improvement which comprises continuously hydrogenating a minor proportion of the benzene so recycled prior to its admission to the reaction zone, thereby incorporating in the benzene a. minor proportion, from about 0.2 per cent to about 3.0 per cent by volume, of hydrogenated benzene.

4. In the method of making phenol wherein a. reaction mixture oi benzene vapor and oxygencontaining gas is passed under a pressure in excess oi' about 500 pounds per square inch through a reaction zone at a temperature between about 650 F. and about 1000 F., said reaction zone having a metallic surface and being void of solid catalyst, the step of incorporating in the reaction mixture a minor proportion, from about 0.2 per cent to about 3.0 per cent by volume based upon the quantity of benzene in the reaction mixture, of an organic compound selected from the group consisting of those consisting oi' carbon and hydrogen and those consisting of carbon, hydrogen, and oxygen. which compound is in the gaseous phase under the conditions of the reaction zone and which, under said conditions, is a better hydrogen donor than bensene and which has a "potential hydrogen donor ability" value greater than about 3.5.

5. in a method for the continuous manufacture of phenol from benzene wherein a reaction mixture of benzene vapor and oxygen-containing gas is passed under a pressure in excess of about 500 pounds per square inch through a reaction sone at a temperature between about 650 F. and about 1000 F. to convert a part of the benzene to phenol and other oxidation products and wherein unconverted benzene is separated from the phenol and other oxidation produc'ts and is returned for recycling in the reaction mixture. said reaction sone having a metallic surface and being void oi' solid catalyst; the improvement which comprises continuously incorporating in the benzene so recycled. prior to its admissionto the reaction sone, a minor proportion. from about 0.2 per cent to about 3.0 per cent by volume, of an organic compound selected Vfrom the group consisting of those consisting of carbon and hydrogen and those consisting of carbon, hydrogen. and oxygen. which compound is in the gaseous phase under the conditions ot the reaction sone and which, under said condi- `tions, is a better hydrogen donor than bensene -and which has a upotential hydrogen donor ability" value greater than about 2.6.

6. The continuous method for the manufacture of phenol from benzene which comprises: passing a reaction mixture of benzene and oxygen-containing gas through a reaction sone having a metallic surface and being void of solid catalyst. under a pressure in excess of about 500 pounds per square inch and at a temperature between about 650 Il'. and about 1000' 1".. said benzene containing a minor proportion. from about 0.2 per cent to about 3.0 per cent by volume. of an organic compound selected from the group consisting of those consisting of carbon and hydrogen and those consisting of carbon, hydrogen. and oxygen, which compound is in the gaseous phase under the conditions of the reaction sone and which, under said conditions is a better hydrogen donor than benzene and which has a potential hydrogen donor ability value greater than about 2.5 separating from the products of reaction unconverted benzene and phenol substantially free of unconverted benzene: recycling unoonverted benzene with a quantity of fresh benzene suiiicient to replace bensene converted in the previous passage through said reaction sone. and with oxygen-containing gas: and continuously incorporating with the recycle benzene charge. prior to its admission in said reaction sone, said minor proportion of said organic compound.

7. In the method of making phenol wherein a reaction mixture of benzene vapor and oxygencontaining gas is passed under a pressure in excess of about 500 pounds per square inch through a reaction zone at a temperature between about 650 F. and about 1000 1"., said reaction sone having a metallic surface and being void of solid catalyst, the step of incorporating in the reaction mixture a minor proportion, from about 0.2 per cent to about 3.0 per cent by volume based upon the quantity of benzene in the reaction mixture. oi cyclohexenc.

ROBERT H. KRIEBIE. WILLIAM I. BENTON.

REFERENCES CITED The following references are of record in thel ille of this patent:

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