Catalytic Oxidation of Hydrocarbons

Catalytic Oxidation of Hydrocarbons L . YA . MARGOLIS Ilzetitute of Chemical Phy@cs Academy of Sciences, MOSCOW. U.S.S.R.

. 429 437 111. Reaction Mechanism ................................................. 439 A . Oxygen Adsorption on Oxidation Catalysts ........................... 440 B. Isotopic Oxygen Exchange ......................................... 442 C. Chemisorption of Hydrocarbons on Oxidation Catalysts ................ 444 D. Charging of the Surface in Adsorption ................................ 446 E . Oxidation Schemes................................................ 448 F. Heterogeneous-Homogeneous Reaction Steps......................... 467 IV. Reaction Kinetics .................................................... 468 A. The Effect of Macroscopic Factors ................................... 468 B. Kinetics of Propene and Ethylene Oxidation over Vanadium............ 469 C. Kinetics of Naphthalene Oxidation over Vanadium Oxide .............. 470 D. Kinetics of Benzene Oxidation to Maleic Anhydride over V,O, ........... 472 E . Kinetics of Propene Oxidation to Acrolein over Cu,O ................... 473 F. Kinetics of Ethylene Oxidation to Ethylene Oxide over Silver........... 474 C-. Kinetics of High Conversion of Hydrocarbons ......................... 476 479 V. Modified Catalysts ................................................... VI . Mixed Catalysts ...................................................... 493 References .......................................................... 496 I Introduction .........................................................

I1. Catalysts ............................................................

I. Introduction Organic compounds involving oxygen are the main stock for synthesis of various plastics. lacquers. resins. and other material . Oxidative processing of hydrocarbons haa long ago attracted the attention of chemists as one of the main trends in organic synthesis. The results of investigations on the oxidation of certain hydrocarbons over various catalysts are summarized in Tables I to 111. 429

430

L. YA. MAROOLIS TABLE I Hydrocarbon Oxidationa on Metals -

~~~~

Catalyst

Platinum

Platinum

~

Support

-

Asbestos

~

~

Reaction Compound tempera- undergoing ture ("C) oxidation 800

Methane

250-600 Methane

Ethylene Acetylene Propene Pentane Benzene 150-300 Toluene

Platinum

-

Platinum

-

Platinum

Silica gel

Toluene Methane 100-300 Ethylene

Platinum

Asbestos

250-500 Methane

Platinum

Asbestos

Platinum

Asbestos

Pentane Hexane 172-700 Methane

Platinum

Silica gel

200-400 Methane

Platinum

-

150-350 Methane

Platinum

-

Platinum

Barium sulfate

Platinum

Asbestos

Platinum Palladium Palladium Palladium

800

800

700

20-180 Propene 250-500 Pentane

Heptane Octane Methane

Asbestos Asbestos Asbestos

Ethylene

Methane 450-700 Methane 150-600 Methane Ethylene Acetylene Pentme Benzene 200

Main reaction products

CO and H,O at times traces of HCOOH COPand H,O HCOOH

...

CO, and H,O . . HCOOH CO, and H,O HCOOH CO, and H,O HCOOH CO, and H,O . . HCOOH CO, and H,O . HCOOH CO, and H,O . . . HCOOH CO, and H,O HCOOH CO, and H,O . . . HCOOH CO, and H,O HCOOH CO, and H,O . . HCOOH CO, and H,O HCOOH

. ... ... . .. .. .

.. .

. ...

CO, and H,O HCOOH CO, and H,O C O , and H,O CO, and H,O

...

References

CATALYTIC OXIDATION OF HYDROCARBONS

43 1

TABLE I (cont.)

Catalyst

Support

Reaction temperature "C

Compound undergoing oxidation

Palladium Palladium Palladium Silver

Silica gel Silica gel Asbestos Silica gel

100-300 200-400 172-700 100-300

Silver

Aluminum oxide Asbestos

260-290 Ethylene

Silver

Ethylene Methane Methane Ethylene

263-293 Ethylene

Silver

-

225-325 Ethylene

Silver

-

263-293 Ethylene

Silver

Porous

220-225 Ethylene

Silver

Skeletal

220-250 Ethylene

Silver

Silica gel

200-400 Methane

Silver

Pumice

172-700 Methane

Silver

-

Silver

-

475 Methane (pressure 200 atm) 500 Methane

Pumice

263-295 Benzene 500 Methane

Copper Copper

Silica gel

100-300 150-300

Copper

Pumice

300

Copper

Silica gel

200-400

Copper Copper

Pumice

172-700 400

-

-

CO, and H,O CO, and H,O CO I and H ,O Ethylene oxide GO, and H,O Ethylene oxide CO, and H,O Ethylene oxide CO, and H,O Ethylene oxide CO, and H,O Ethylene oxide CO, and H,O Ethylene oxide CO, and H,O Ethylene oxide CO, and H,O CO,, H,O, and traces of aldehydes CO,, HzO, . . . aldehydes CO,, HzO, . . aldehydes

.

Cozy

. . . aldehydes Maleic acid CO,, H,O, and traces of HCOH CO, and H,O Ethylene CO,, H,O, and Toluene traces of benzaldeh yde Toluene Cow HzO, . . .benzaldehyde CO,, H,O, and Methane traces of CH,COOH CO, and H,O Methane 3.3-dimethyl- CO,, H,O, and pentane traces of aldehydes

Silver Copper

-

Main reaction products

Referencea

432

L. YA. MARUOLIS

TABLE I (cont.) Catalyst

Gold Gold Gold

Support

Asbestos Silica gel Asbestos

Reaction temperature "C

Compound undergoing oxidation

250-500 Methane 100-300 Ethylene 150-500 Methane

Ethylene Propene Pentane Benzene Methane

Nickel

-

Nickel

-

160-350 Methane

Nickel

__

850-900 Methane

Nickel

-

150-300 Toluene

500

Main reaction products

C O , and H,O CO, and H,O GO, and H,O

References

(6) (5)

(3)

CO,, H,O, and (24) traces of formaldehyde COB, HsO, (9) . . formaldehyde COs, H.0, (27) , formaldehyde Traces of (2) benzaldehyde

. ..

CATALYTIC OXIDATION OF HYDROCARBONS

433

TABLE I1 Hydrocarbon Oxidatwne an Metal Oxidea

Catalyst

Vanadium pentoxide Vanadium pentoxide Vanadium pentoxide Vanadium pentoxide

Support

Reaction temperature "C

-

400

Asbestos Asbestos

Vanadium pentoxide

Vanadium pentoxide

Vanadium pentoxide

Vanadium pentoxide Vanadium pentoxide Vanadium pentoxide Vanadium pentoxide

Compound undergoing oxidation Benzene

Naphthalene Phthalic anhydride 400 Ethylene Formaldehyde Acetaldehyde Acids 400 Ethylene Formaldehyde Acetaldehyde Mdeic and fumaric acids Benzaldehyde 400-600 1.3-butaMaleic acid dime Formaldehyde Glyoxal Acetic acid Methylacrolein 400-600 Isobutene 400-600 1- and 2Maleic and butene acetic acids Acetaldehyde Formaldehyde Methylvinylketone 400 Butane Maleic and metic acids Formaldehyde GIyoxal CO, CO,, and 400

Benzene

Asbestos

300

Toluene

Asbestos

Maleic acid

600

Asbestos

Asbestos

Main reaction products

H,O

Maleic anhydride Benzaldehyde

Naphthalene Phthalic anhydride 172-700 Methane CO, and H,O

460

References

434

L. YA. MAROOLIS

TABLE I1 (cont.)

Catalyst

Support

Reaction Compound tempera- undergoing ture ("C) oxidation 36&600 2-pentane

Asbestos

Asbestos Asbestos

Pumice Pumice

Trimethylethylene Amy1ene Heptane 360-400 Cyclohexane 400-450 Benzene

Vanadium pentoxide

Pumice

290-420 Naphthalene

Vanadium pentoxide Vanadium pentoxide Vanadium pentoxide Vanadium pentoxide

Pumice

Vanadium pentoxide Vanadium pentoxide Vanadium pentoxide Vanadium pentoxide Copper oxide (CUO) Copper oxide (CUO) Copper oxide (CUO) Copper oxide (CUO) Copper oxide (CUO) Cuprous oxide G O )

Silica gel Fused asbestos Alundum Silica gel Asbestos Asbestos Pumice Pumice

Asbestos Asbestos Asbestos Pumice Pumice Silicon carbide

600

Methane

Main reaction products Maleic acid Aldehydes Acids Maleic acid Maleic anhydride Phthalic anhydride Meleic acid CO, and H,O

380-480 Toluene

Benzaldehyde Maleic acid 426 Phenanthrene Phenanthrenequinone 400-600 Naphthalene Naphthaquinone Phthalic anhydride, CO,, and HSO 400-460 Propene Formaldehyde CO, and H,O 400 Toluene Benzaldeh yde Benzoic acid 400 PhenanPhenanthrenethrene quinone 400 Naphthalene Phthalic Toluene anhydride Benzoic acid 160-200 Acetylene CO, and H,O 260-360 Methane

100

Methane

360-1000 Methane

400

CO,, H,O, and formaldehyde CO, and H,O CO, and H,O

Phenanthrene Phthalic acid

360-400 Propene

Acrolein CO, and H,O

References

CATALYTIC OXIDATION O F HYDROCARBONS

435

TABLE I1 (cont.)

Catalyst

Support

Tungsten Pumice oxide (WO,) and molybdenum oxide (MOO,) WO,andMoO, Pumice W0,and MOO, Asbestos

Main reaction products

400-460 Toluene

Benzaldehyde Benzoic acid CO, and H,O

400-460 Benzene 300-426 Toluene

160-200 Acetylene

Benzaldehyde Benzaldehyde Benzoic acid CO,, CO, H,O CO, and H,O

400-460 Benzene

Benzaldehyde

400-460 Benzene

Maleic acid

160-200 Acetylene

CO, and H,O

Asbestos

250-600 Methane

CO, and H,O

Asbestos

100-350 Methane

Asbestos

360-600 Methane

Formaldehyde CO, and H,O CO, and H,O

Asbestos

360-1000 Methane

Asbestos

360-1000 Methane

Manganese oxide (MnO,) Manganese Pumice oxide (MnO,) Nickel oxide (NiO) Nickel oxide (NiO) Nickel oxide (NiO) Nickel oxide (NiO) Lead and ceric oxides Zinc and lead oxides Zinc and lead oxides Chromium oxide

Reaction Compound tempera- undergoing ture ("C) oxidation

Pumice

-

Asbestos

600

Ethylene

Formaldehyde CO, and H,O CO, and H,O Acetaldehyde Formaldehyde CO, and H,O

References

436

L. YA. MAROOLIS

TABLE I11 Hydrocarbon Oxadationa on Mixturea of Ildekzb with Certain Salts

Catalyst

Carrier

Reaction temperature ("C)

Compound undergoing oxidation

Main reaction products

References

-

Zinc Asbestos vanadate Bismuth and lead vanadates Vanadates of various metals Lead vanadate -

Chromium, tungsten, vanadium and molybdenum plumbites Uranium and berryllium oxides Bismuth and Pumice lead vanadates Manganese Pumice and lead vanadates Copper, iron, Pumice and magnesium ehromites

Zinc and Pumice manganese oxides with chromium oxide Cadmium and Pumice zinc vanadates

300-600 Toluene 200-380 Toluene

naphthalene

460

Benzaldehyde Benzoic acid Benzoic acid

Naphthalene Phthalic anhydride

(34)

153)

(35)

-

Naphthalene Phthalic acid Toluene Benzoic acid 300-400 Toluene Benzaldehyde Benzoic acid

600-000 Methane

Formaldehyde

(55)

400-460 Benzene

Benzaldehyde

(38)

290-420 Naphthalene Acetaldehyde

Cyclohexane Decalin 800-600 Pentane Heptane Ethylene Benzene Cyclohexane Tsooctane - Acetylene

-

Acetylene

Phthalic anhydride CO, and H,O

(56, 1 3 )

Acetone

(57)

Acetone

(58)

CATALYTIC OXIDATION OF HYDROCARBONS

437

Oxygen enters into diverse reactions with hydrocarbons, such as addition, substitution, and destructive and polymerizing oxidation. There are a t present three fundamental directions of pursuit in the investigation of oxidative processing of hydrocarbons : (1) heterogeneous oxidation by molecular oxygen in the gas or liquid phase; (2) homogeneous oxidation by molecular oxygen in the gas or liquid phase; (3) radiation-induced oxidation by molecular oxygen, oxidation under electric discharge, electrochemical oxidation in solutions, etc. Certain chemical features, such as the formation of peroxide compounds, of atomic oxygen, etc., are common for all the three groups of reactions. A theory of heterogeneous catalytic oxidation of hydrocarbons would be impossible without knowledge of the elementary mechanism of oxidation, of the fundamental laws governing this process, and of its rate-determining steps. Insufficient theoretical treatment of the wide amount of experimental information on the proper choice of catalysts for hydrocarbon oxidations also hampers advances in this field.

II. Catalysts Metals (platinum, silver, etc.), metal oxides-semiconductors (vanadium pentoxide, molybdenum, tungsten, and copper oxides, etc.), and complex semiconductors-spinels, are the catalysts most widely used for oxidation of hydrocarbons. Oxidation catalysts almost ihvariahly contain transient metals with open inner electronic shells (Tables I V and V). The catalytic process comprises a number of consecutive elementary acts, such as the breaking and formation of bonds in reactant molecules, and electron transfers between the latter and the solid catalyst. Consequently, the electronic properties of catalyst surfaces influence the catalytic and adsorption processes. The catalytic activities of metals and semiconductors would be expected to differ due to their different electronic properties. However, under conditions of oxidation catalysis many metals become coated with a more or less thin semiconducting film of the given metal oxide, and this might be the reason why the mechanism of hydrocarbon oxidation on metals and semiconductors has much in common (59). Kalish and Burstein (60)found that the amount of oxygenadsorbed by a platinum layer adjacent to the surface was about one hundred times that required for a monolayer. Temkin and Kul’kova (61)have noted a similar phenomenon for oxygen adsorption on silver. The amount of

438

L. YA. MARGOLIS

oxygen adsorbed by the adjacent-to-surface layer of silver was equal to that for five monolayers. Hiroto and Kobajashi (62)report that even with prolonged reduction of silver in hydrogen a t 275" oxygen atoms are not removed from the metal. When the oxide film is fairly thick (about several tens of atomic layers), the chemical and electronic characteristics of the catalyst surface will be determined by the oxide film properties and the metal will exert no considerable effect on catalysis, whereas with a thin layer (of about several atomic layers) the catalytic properties will depend on the nature of the metal support. TABLE IV Characteristic Catalysts for Low Conversion of Hydrocarbons

Initial hydrocarbon

Catalyst Silver Cuprous oxide Vanadium pcntoxide

Molybdenum oxide

Ethylene Propene Propene Benzene Toluene o-Xylene Naphthalene Anthracene Butane

Main reaction products Ethylene oxide Acrolein Aldehydes Maleic anhydride Benzaldehyde Phthalic anhydride Phthalic anhydride Anthraquinone Maleic acid

TABLE V Characteristic Catalyats for High Conversion of Hydrocarbone

Catalyst Platinum, palladium Copper oxide Manganese dioxide Spinels (Co,Mn,Cr)

Initial hydrocarbon Methane, pentane, isooctane, ethylene, propene, acetylene, benzene, cyclohexane. Methane Acetylene Ethylene, propene, isooctane, cyclohexene, benzene

Recent developments in the theory of electronic structure of solids and more extensive data on the mechanism of physical phenomena in the solid phase revealed a relationship between coloration in the visible and the electronic structure of the solid. Elovich et al. (63)investigated the catalytic activities of various oxides with respect to high con-

CATALYTIC OXIDATION OF HYDROCARBONS

439

version of hydrocarbons, such as isooctane, cyclohexane, etc., as a function of the coloration of oxides and the valences of cations entering into the catalyst composition. Intensively colorated compounds display strong catalytic activity. From the standpoint of electronic concepts this dependence is indication that there is a certain class of catalytic processes occurring only with a oatalyst possessing free electrons or electrons readily passing to an excited or free state. The specificity of elementary steps of catalysts is determined above all by chemical bonds in the reactant molecule and by the structure of intermediate species appearing in the course of the reaction. There are yet no considerable achievements in this field and the choice of active oxidation catalysts is still based on extensive empirical information and on rather coarse approximations. The reactant mixture containing hydrocarbons of various structure and oxygen induces changes in the phase composition of the catalyst during the reaction. Simard et al. (64) suggested that the active surface of a vanadium catalyst represented a dynamic system involving Vs+, V4+and 0; ions. Roiter et al. (65) investigated a vanadium catalyst that was under operation in an industrial reactor for 2& years. They found that half the amount of V,O, of the upper layer was converted into V,O,. Thus the composition of a catalyst is controlled by the mixture composition. Similar results were obtained by various scientists (66, 67) in studying the behaviour of a cuprous oxide-copper oxide catalyst in the oxidation of propene into acrolein. The appearance of copper by further reduction of the catalyst results in a decreased selectivity of the process. According to Roginskii (68) such variations in the catalyst in the course of the reaction almost exclude possible modifications in the technique of preparation of active catalysts having to ensure the formation of structures with increased energy. The effect of reactants may be so great as to destroy these active structures. This is probably the reason for a certain primitivism in the choice of techniques for the preparation of oxidation catalysts, chief attention being paid to the surface area, pore sizes, etc.

111. Reaction Mechanisms The working out of an explicit scheme for hydrocarbon oxidation over various catalysts was hindered until lately due to insufficient development of the theory and to lack of extensive experimental data. Any catalytic mechanism implies that adsorption represents the

440

L. YA. MAROOLIS

primary step of catalysis and controls the transition of a reactant molecule to the active state. Molecules of oxygen or of a hydrocarbon are adsorbed on the catalyst surface during hydrocarbon oxidation. The state of these adsorbed molecules, their interaction, and their reactions with the gas phase molecules would account for different routes of the process.

A.

OXYQEN

ADSORPTION ON OXIDATIONCATALYSTS

Extensive investigations carried out by scientists of various countries (69-72) have shown that chemisorption of oxygen on metals may occur even a t relatively low temperatures ( - 80', 0'). According to Trapnell (73) the chemisorption of 'oxygen on various metals over a range of different temperatures is so fast as to make kinetic measurements impossible; this is indication that the activation energy for chemisorption is very low. Fast chemisorption is followed by slow uptake of oxygen by the metals. The dissolution of oxygen in nickel, copper and certain other metals results in the formation of oxides of these metals. With noble metals, such as platinum and silver, oxygen will be dissolved in layers adjacent to the surface, bringing about changes in the electronic properties of the latter. TABLE V I Activation Energies for the Heats of Adsorption and Kinetic Equation for Oxygen Chemieorption on Semiconductor Catalyets

Semiconductor

Heat of adsorption (kcclljmole)

Activation energy (kcal/mole)

Equation for adsorption kinetics

References

___.

-

NiO

36-62

2-8 7

43-70 25-64

16-18 32-35

Banghem RoginskiiZeldovich Bangham

(74)

~ 5 ~ 7 7 ) (76, 73)

The activation energy for adsorption of oxygen on silver and platinum varies with surface coverage, which is indication that the metal surfaces are heterogeneous and the oxygen-metal bonds have different energies. Oxygen bonds on the typical metal catalysts, namely platinum and silver, are of different strength, and two forms-molecular and atomiccoexist on the surface.

CATALYTIC OXIDATION O F HYDROCARBONS

44 1

The adsorption of oxygen on certain semiconductors, such as NiO and Cu,O was studied in fair detail, The activation energies, heats of adsorption and kinetic laws for oxygen sorption on simple semiconducting catalysts are summarized in Table VI. I n studying the chemisorption of oxygen on cuprous and copper oxides Garner et al. (77) found that this process is accompanied with slow incorporation of oxygen into the crystal lattice. Jennings and Stone (75) reported that complete coverage of Cu,O with oxygen took place at 20". The heat of oxygen chemisorption was 55-60 kcal/mole; oxygen was strongly fixed on the surface and represented a negatively charged monolayer. Adsorption of oxygen on V,O, was studied less explicitly. However, all scientists concerned report that it is insignificant. Clark and Berets (78) suggest that chemisorption of 0, on V,O, leads to a decrease in the concentration of surface defects. Margolis and Plyshevskaya (79) studied oxygen sorption on vanadium over a temperature range of 250 - 400°C. The kinetic curves obtained are described by Bangham's equation q = atl/"

(1)

Here q is the adsorbed amount, t is time, and a and n are constants. Roginskii (80) has shown that this is a typical kinetic equation for activated sorption on a heterogeneous surface characterized by the exponential shape of the distribution function with respect to activation energies for adsorption. The adsorption of oxygen on NiO occurs a t 200-360". According to Roginskii and Tzellinskaya (81)the kinetics of this process obeys the equation q =a

dT+ c

(where a and C are constants). This fact was associated with dissolution of oxygen in the NiO lattice. Keier and Kutzeva (82) have found by detailed investigation that the dissolution of oxygen in the NiO lattice may be disregarded and chemisorption may be considered as a process typical for a heterogeneous surface. With a coverage not exceeding 10% of the monolayer activation energies increase from 10 to 15 kcal/ mole. Thus oxygen is chemisorbed on all simple semiconducting catalystsmetal oxides-and in a number of cases it is partly dissolved in the lattice. Very little work on adsorption properties of complex semiconductors --spinels-was reported in literature. Magnesium and copper chro-

442

L. YA. MAROOLIS

mites, manganese cobaltite and cobalt manganite, as well as certain ferrites, were used as catalysts for high conversion of hydrocarbons. Margolis (83) studied the kinetics of oxygen adsorption at elevated temperatures close to or identical to the temperature of oxidation and found that the Bangham's law was followed in this case. An increase in coverage with oxygen from 12 to 20% resulted in a rise of activation energy from 8 to 25 kcal/mole. Linde (84)investigated oxygen adsorption on CoMn,O, and MnCo,O, spinels. The bilogarithmic law held for MnCo,O, (q = ~ t l ' ~and ) , the semilogarithmic law (q = u'e-aq) for CoMn,O,. This was another indication that the surfaces of these catalysts were heterogeneous. The activation energy for chemisorption of oxygen on MnCo,O, was found to vary exponentially with coverage, while uniform distribution of activation energies was observed for CoMn ,O,. A heterogeneous surface is characteristic of these catalysts. Thus chemisorption of oxygen on simple semiconductors and spinels involves changes in heats of adsorption and, consequently, in energies of oxygen-solid surface bonds.

B. ISOTOPIC OXYGENEXCHANGE The mobility of atoms or molecules of oxygen on the surface and in the lattice of the solids may be determined by using the isotopic exchange method. Isotopic oxygen exchange was studied in the last years by many investigators, of whom Winter (85))Vainstein and Turovskii (86, 87), and Boreskov and Kassatkina (88) have shown that oxygen of the metal oxide lattice is of a low mobility. Significant isotopic oxygen exchange on oxides occurs a t temperatures by 100-200° degrees higher than the temperature of catalysis. It may be seen from information available on isotopic oxygen exchange on silver and platinum that the values for exchange activation energies and for oxygen adsorption are close. Appreciable isotopic oxygen exchange on platinum begins only a t a temperature exceeding 350") whereas the oxidation of hydrocarbons on platinum is fast even at 20". The strongly bound oxygen does not seem to participate in catalytic oxidation. Oxygen is more mobile on silver than on platinum, since oxygen exchange on platinum sets in at 220") while catalytic oxidation may readily occur at 200". Comparison of data on adsorption and exchange on silver shows that under conditions of catalysis only a part of the oxygen adsorbed is mobile. The oxygen dissolved in the adjacent-to-surface layer does not seem to participate in catalysis.

CATALYTIC OXIDATION OF HYDROCARBONS

443

The nature of intermediates formed in adsorption may be established from investigation of oxygen exchange. The formation and breaking of chemical bonds during isotopic exchange may occur with and without unpairing of electrons (homolytic and heterolytic exchange, respectively). Investigation of homolytic oxygen exchange on various solids would show whether oxygen dissociation occurs on the surface. Margolis (74)studied homolytic oxygen exchange on platinum and silver, and on vanadium pentoxide and manganese dioxide. No homolytic exchange was observed at low temperatures for platinum, silver and semiconducting oxides (MnO, and V,O,), as well as for the spineltype compounds. Certain information on the mechanism of exchange and the nature of active intermediates may be obtained from comparison of rate constants for heterolytic and homolytic exchange involving adsorbed oxygen with the desorption constant, at the same temperature and coverage. The rate constants for homolytic exchange on silver at 232" are lower than those for heterolytic exchange by a factor of 2.5. The rate constant of desorption a t this temperature is equal t o that of oxygen exchange. This is indication that the dissociation of oxygen molecules on silver a t 232" occurs only in part. As the temperature is raised, the rate constants for homolytic isotopic exchange become close to those for ordinary isotopic exchange and desorption, and a t 290" these are almost equal. It may be inferred that under such conditions the oxygen adsorbed on silver dissociates into atoms. For platinum this equality of rate constants is observed only a t temperatures higher than 400". Phenomena similar to those observed for silver take place at lower temperatures. With semiconducting oxides such as MnO,, V,O,, etc., oxygen exchange may be observed only at elevated temperatures. Consequently, it is yet impossible to find out whether oxygen dissociates a t the temperature of catalysis. Dzisyak et al, (89) investigated homomolecular oxygen exchange on V,O, proceeding a t a temperatwe higher than 450". The initial rate of oxygen adsorption was shown to be by a power of ten higher than the rate of exchange. Correlation in rates, activation energies, and reaction orders with respect to oxygen indicates that the dissociation of oxygen into atoms is the rate-limiting step common to homomolecular and to isotopic exchange, The above mentioned scientists reported data on exchange at 500-550", when the occurrence of oxygen molecules a t the surface is scarcely probable and, consequently, only the dissociative mechanism of chemisorption and exchange would be valid under these conditions. A t lower temperatures (350-450"), such as those for

444

L. YA. MAROOLIS

catalysis on vanadium pentoxide, both the molecular and atomio oxygen forms may coexist. It may be seen from investigation of homolytic oxygen exchange on various oxidation catalysts that oxygen may be present on the surface as molecules without dissociating into atoms. The ratio of molecular to atomic oxygen content of the surface is a function of temperature. OF HYDROCARBONS ON OXIDATION CATALYSTS C. CHEMISORPTION Chemisorption of hydrocarbons on various metals, such as nickel, platinum, copper, etc., was investigated in great detail (9, 90, 91, 92). Information on chemisorption of ethylene, acetylene and methane on various metals may be found in Trapnell's review (93). However, direct application of the relations obtained to metal oxide catalysts would scarcely be justifiable, As a rule, oxygen covers the whole surface of the metal, and chemisorption of hydrocarbons occurs either on a thin layer of the given metal oxide formed as an individual phase, or on oxygen that was sorbed on the surface and has filled the adjacent-to-surface layers. Thus data on chemisorption of hydrocarbons on oxides of these metals may be of use in the above cases. Margolis (94) studied ethylene chemisorption on silver proper and on silver covered with oxygen. When the silver sudace is completely oxygen-free, ethylene sorbs in equilibrium and reversibly, and the coverage degree is low. Things are quite different when hydrocarbons are adsorbed on silver covered with oxygen, Adsorption occurs in time; adsorption kinetics follows the Roginskii-Zel'dovich equation characteristic for heterogeneous surfaces. Only a certain amount of the ethylene sorbed may be removed by outgassing, a considerable part of it becomes strongly bound with oxygen of the surface. The activation energy for the over-all process is very low, 2-3 kcal/mole. Butyagin and Elovich (12) studied propene adsorption on platinum and found that the oxygen-covered platinum surface takes up propene, and the rate of this process follows an exponential law. The effect of the metalsurface heterogeneity is very marked for oxygen-covered metals, probably due to regions with different electronic potentials. Chemisorption of hydrocarbons on semiconducting oxides was not investigated extensively. Turkevich, Howard, and Taylor (95) studied ethylene, ethane, and propane adsorption on MOO,, Cr,O, over a temperature range of 0-400" and established the boundaries between physical and chemical adsorption. They emphasized the difficulties arising in the investigation of hydrocarbon adsorption on metal oxides, due to side processes occurring at high temperatures, for instance pyrolysis and oxidation.

CATALYTIC OXIDATION OF HYDROCARBONS

445

The chemisorption of unsaturated and saturated hydrocarbons on various simple semiconducting oxides, namely vanadium pentoxide and cuprous oxide, and on spinels (CuCr,O, and MgCr,O,) was investigated by Margolis (94). The rates of hydrocarbon adsorption on all oxidation catalysts (V,O,, Cu,O, etc.) are so high as to make kinetic studies impossible. Characteristic “equilibrium” isotherms for ethylene sorption on magnesium chromite are shown in Fig. 1. This is a spurious equilibrium,

I00

200

FIQ.1. Isotherms for ethylene adsorption on magnesium chromite at 110”;

1-

primary, 2-secondary.

as at the given temperature the process is only partly reversible. The hydrocarbon will not desorb from the catalyst even with repeated intermediate high-temperature training. The catalyst surface may be freed from the strongly sorbed hydrocarbon only by treating it with oxygen. Isotherms for propene adsorption on NiO, vanadium pentoxide, and cuprous oxide at 100” are shown in Fig. 2. All isotherms are described by Freundlich’s equation and correspond to a heterogeneous surface with exponential distribution as to heats of adsorption. The structures of intermediates appearing in adsorption of hydrocarbons are yet unknown. Voevodskii, Vol’kenstein, and Semenov (96) consider that the generation of free radicals in chemisorption is accounted for by free valences present on the solid surface. According to Syrkin (97)the chemisorption mechanism involves the formation of a three-center bond by sorption of unsaturated hydro-

446

L. Y A . MARGOLIS

carbon molecules on metals (Fe, Co, Ni, Pt, etc.) and semiconductors (chromium oxide). The formation of donor-acceptor bonds involves electrons of the solid and of the adsorbate, as well as the electrondonating bond formed between the antibonding orbit of the adsorbed molecule and the metal atom orbit occupied by an electron pair,

FIG.2. Isotherms for propene adsorption on NiO ( l ) ,vanadium pentoxide (2), and ouprous oxide (3) at 100".

I). CHARGINGOF

THE

SURFACE IN ADSORPTION

A number of works are concerned with charging of the surface in chemisorption (98-101).Chemisorption on metals and semiconductors is accompanied by electron transfer between the adsorbed molecule and the catalyst. The direction of electron transfer depends on the Fermi level position in the crystal, and on the energy level of the chemisorbed molecule. The Fermi level position on a semiconducting surface may be determined from the change in the electron work function. Adsorption of acceptor molecules results in an increase in the electron work function, while a fall-off is observed for donor molecules. Measurement of the electron work function during adaorption on solids permits determination of the charge sign for the adsorbed molecule (see Table VII). Thus various hydrocarbon molecules are donors when adsorbed on simple and complex semi-conductors, such as NiO, MnO,, Cr203,Cu,O, MnCo,O,, and CoMn,O,. Margolis (202) found from determination of the electron work function for silver during oxygen and ethylene adsorption that the oxygen molecule obtains a negative charge and is an electron acceptor, while the positively charged ethylene molecule is a donor. The electron

TABLE VII Charging of the Surfaces of Metal Semiconductors During Adsorptiun of Hydrocarbons and Oxygen (103) Catalyst ~

NiO

Hydrocarbon Electroconductivity Charge with respect to the work function

Propene Propane Decrease

+

p-type semiconductors Cr*O, MnO

,

cu,o

Butane

Ethane

Propene

Decrease

Decrease

Increase

+

+

+

Spinels MnC0,Ol C0Mn,Od Propene Not

measured

+

n-type semiconductors

Metals

Ag

Propene Propane Increase

+

Pt

Propene

Ethylene

Ethylene

Increase

Increase

Not measured

not determined

+

+

448

L. YA. MARQOLIS

work function values for oxygen and propene adsorption on platinum are close to those for silver. Thus for the majority of catalysts used the adsorption of oxygen molecules results in a negative charge of the surface, while a positive charge is observed for hydrocarbons irrespective of their structure.

E. OXIDATIONSCHEMES The working out of schemes for catalytic oxidation of hydrocarbons on metals and semiconductors is difficult due to the complexity of reactions, the diversity of products formed and the lack of information on the nature of elementary acts. The genetic relationships between oxidation products are much easier to determine. The high conversion to CO, CO,, and H,O hinders the formation of oxygen-containing products in direct oxidation of hydrocarbons with molecular oxygen. It is considered as firmly established that the reason for low selectivity of catalytic hydrocarbon oxidations lies in the ready subsequent oxidation and decomposition of oxygen-containing products formed by synthesis. The accepted scheme for olefine oxidation is olefine + aldehyde -+ acid -+ CO -+ CO f hefine oxide ------+ aldehyde

Pongratz (103)) Sosin (104), and Dolgov (105) suggest that the oxidation of aromatics to phthalic and benzoic aldehydes proceeds in steps. For example, naphthalene is converted into naphthol, then t o naphthahydroquinone, to phthalic anhydride, and, eventually, to CO 2. Thus optimal conditions for fast elimination of oxygen-containing compounds from the reaction zone are usually sought for in the attempts to obtain products of low conversion. Under certain conditions low selectivity may be due to unfavorable formation to conversion ratios for stable oxygen-containing intermediates, such as aldehydes, olefine oxides, acids, which seem to prevent the accumulation of valuable products. However, this reason cannot be the sole and general one. For instance, with typical catalysts for high conversion of hydrocarbons, such as magnesium and copper chromites, aldehydes undergo oxidation a t 200-400" mainly to acids and only in part to CO,. Under the same conditions hydrocarbons oxidize to water and carbon dioxide, minor amounts of aldehydes and acids being found in reaction products. Charlot (106) investigated the activity of a great number of various metal oxides in the reaction of toluene with oxygen, and stated that the high and low conversion processes are independent of each other.

CATALYTIC OXIDATION OF HYDROCARBONS

449

Marek ( 4 )believes that a catalyst accelerating the first step will accelerate all the subsequent steps as well, since it is difficult to conceive a catalyst that would accelerate the oxidation of hydrocarbons and have no effect on the oxidation of formaldehyde to water and carbon dioxide. The ratio of parallel to consecutive reaction rates is a function of the hydrocarbon structure, the catalyst type, and the reaction temperature. I n studying the kinetics of catalytic benzene oxidation to maleic anhydride Hammar (107) came to the conclusion that benzene oxidizes by two independent routes: the formation of maleic anhydride, and the complete combustion to carbon dioxide and water via unidentified intermediates. He suggested the following scheme for benzene oxidation over a vanadium catalyst:

+ 0, + X I -+ C4H,0, + CO + CO, + H,O C,H,O, + 0, -+ X, -+ CO + CO, + H,O C,H,

+

COHO 0, -+ X, -+ CO

+ CO, + H,O

where X,, X,, and X, are hypothetic intermediates. Ioffe (108) and later on d’alessandro and Parkas (42) have found from kinetic studies that heterogeneous catalytic oxidation of aromatic hydrocarbons involves a number of parallel and parallel-consecutive reactions, and that high conversion does not necessarily include the formation of low conversion products. For example

-

*

Anapht haquinone

naphthalene

C\O,

phthalic anhydride H20

+

-

maleic anhydride

Naphthalene oxidizes by three independent routes to naphthaquinone, phthalic anhydride and carbon dioxide. The latter may also be formed by oxidation of naphthaquinone and phthalic and maleic anhydrides. Ushakova, Korneichuk, and Roiter (109) suggest that various reactions of naphthalene oxidation proceed independently (Fig. 3). Unpublished results obtained by Simard et al. on the oxidation of o-xylene on V20, by a supposedly parallel-consecutive mechanism are mentioned in a review by Longfield and Dixon (110). Suvorov, Rafikov, and Anuchina (111) came to the conclusion that the first step of toluene oxidation over vanadium oxides yields hydroperoxide which may decompose by various routes. The formation of high-conversion products does not occur in parallel. Bretton, Wan, and Dodge (112) investigated the composition of products formed in the oxidation of a hydrocarbon containing four

450

L. YA. MAROOLIS

carbon atoms over V,O, as a function of the hydrocarbon structure. They suggested that the diversity of products formed is due to different structures of radicals yielded by oxidation. Thus a number of reactions proceeding a t different rates occur simultaneously on semiconductors, such as metal oxides.

+ H20

FIG. 3. Scheme of naphthalene oxidation over vanadium pentoxide, according to Roiter.

The most widely used metal oxidation catalysts are platinum, palladium, copper, and silver. With all these, except silver, hydrocarbons undergo oxidation with complete destruction of the molecular skeleton into CO, and H,O. Silver is the sole catalyst for obtaining ethylene

4

FIG.4. Scheme of ethylene oxidation to ethylene oxide over silver, according to Twigg.

CATALYTIC OXIDATION O F HYDROCARBONS

45 1

oxide from ethylene. This reaction was the object of many investigations and several oxidation schemes were proposed. Twigg (18) suggested on the basis of extensive kinetic studies that oxygen adsorbed on a silver catalyst dissociates into atoms; ethylene does not sorb on silver (Fig. 4). When ethylene comes into contact with two oxygen atoms, a number of reactions occur (Fig. 4). The two formaldehyde molecules formed undergo subsequent conversions CH,O -+ CO (ads) CO (ads) 2 H (ads)

+ 2 H (ads)

+ 0 (ads) -+

CO,

+ 0 (ads) + H,O

According to Twigg, when an ethylene molecule comes into contact with one oxygen atom, the interaction H,C=CH,

0 Ag

/ \

Ag

-

H ,C-CH

v 0

I

ethylene oxide

Ag - Ag

takes place. Ethylene oxide may isomerize to acetaldehyde which is readily oxidized on silver to CO and H ,O. The oxidation of ethylene oxide on silver yields carbon dioxide and water, but the amounts of these are not equivalent to C,H,O consumption. Twigg believes that this is accounted for by a n adsorbed organic residue formed on the catalyst surface. Ethylene also detected in oxidation products was thought to be formed by ethylene oxide decomposition to ethylene and adsorbed oxygen. Todes arid Adrianova (113)studied the kinetics of ethylene oxidation over pumice-supported silver and suggested that ethylene was converted to ethylene oxide, which yielded C O , and H,O. I n other words two consecutive reactions took place

,

and

+ 0 -+ C,H,O C,H,O + 2 0 , -+ 2 C 0 , + 2H,O

C,H,

Orzechowski and McCormak (21) proposed the following kinetic mechanism of ethylene oxidation (Fig. 5 ) . Two parallel reactions were believed to occur: the formation of ethylene oxide, and the formation of carbon dioxide and water from ethylene. These reactions would be accompanied by slow decomposition of

462

L. YA. MARGOLIS

ethylene oxide which might be sorbed on silver: C*H,Oo,

I::

+ 0 - Ag + 0

\

. . . 0 . . . Ag -+ X -+COa + Ha0 + Ag

(X is an intermediate, possibly formaldehyde). As the temperature is raised, the ratio of formation to oxidation

rates of oxygen-containing products (aldehydes, ethylene oxide) suffers a change and consecutive oxidation takes place under certain conditions.

C2H40

+ Ag

y+Ag-COz

+H20

FIG.6. Scheme of ethylene oxidation over silver, according to Orzechowski and McCormak.

The contribution of consecutive reactions is especially important when the oxygen-containing compound formed is very reactive. The kinetic reaction scheme shown in Fig. 6 was proposed by Isaev, Margolis, and Roginskii (114).The main route of the reaction was supposed to be propene -+ acrolein -+ COa

i.e., a consecutive conversion of the hydrocarbon to aldehyde and carbon dioxide. Information on propene oxidation to acrolein over a copper catalyst was published recently by Belousov et aE. (115).They consider that at a low temperature (320") acrolein and CO, are formed mainly by two parallel reaction routes acrolein propene

1 carbon dioxide

A parallel-consecutive scheme holds a t higher temperatures of 350 to 400". The relationship between different oxidation routes is a function not only of temperature, but of the catalyst surface condition as well. Isaev, Golovina, and Sakharov (116)have shown that as the surface becomes treated with the reactants, the contribution of parallel propene

CATALYTIC OXIDATION O F HYDROCARBONS

453

oxidation to the formation of CO, becomes less important. Assuming that the oxygen-containing products formed (aldehydes, ethylene oxide, etc.) readily undergo subsequent reactions with oxygen, these substances may be considered as intermediates in the formation of carbon dioxide, carbon monoxide and water.

FIG.6. Scheme of propene oxidation to acrolein over cuprous oxide.

The nature and importance of intermediates may be established by kinetic studies through comparison of formation, decomposition and oxidation rates. At present the mechanism of complex processes may be investigated by the more precise radioactive tracer technique. The genetic relationship between individual reaction products may be determined by using this technique together with kinetic studies. The origin of reaction products and the rates of individual reactions involved in the overall process may be established by C144abeling of the hydrocarbon molecules, or of the products suggested as intermediates. Neiman (117) worked out an interesting kinetic tracer method for determining the reaction kinetics from variations in radioactivity and concentration of intermediates with time. When a substance A is converted into X, and then X is converted into B (A + X + B), then the concentration and specific radioactivity of X may be determined a t any time by labeling A or X and adding the labeled substance to the reactant mixture. The rates of formation and consumption kx follow the equation

_ -aaat

-

(/3 -

a) w

z

specific activity of X; /Ithe specific activity of the initial product A; x the concentration of X.

a is the

454

L. YA. MARGOUS

WhenB = 0, then

ax - _ = W - k x

(4) at When a~ = 0, then -dx{dt = 0 and w = 0. Consequently, there is only consumption of X without its formation. When 01 changes with time, w is found before k x . 1. Ethylene Oxidation over Silver

When ethylene is oxidized over Ag, carbon dioxide may be formed either directly from ethylene, bypassing ethylene oxide, or by stepwise oxidation of ethylene oxide. To obtain confirmation for the occurrence of these reactions Margolis and Roginskii (118) carried out investigations with a mixture of C’*-labeled ethylene, ethylene oxide, and oxygen. Variations in concentrations of initial compounds and reaction products as a function of the contact time were determined, as well as the activity distribution. Direct quantitative and systematic information on the carbon dioxide formed via ethylene oxide and ethylene was obtained for the first time by means of the kinetic tracer method. Further discussion thus became unnecessary. Adrianova and Todes (113) and also Orzechowski and McCormak (21)allowed for one process of carbon dioxide generation only. This was not correct as carbon dioxide is always formed by two independent routes. However, having accepted this fact, there is no need in considering, as Twigg does (18),the predominance of one of these processes over the other, or to estimate the contribution of each process to losses of ethylene as carbon dioxide. The part played by stepwise oxidation via ethylene oxide, or by “direct” oxidation bypassing ethylene oxide, in the process of CO formation is different for various silver catalysts. For certain catalysts stepwise oxidation will be predominant a t low temperatures, and for others ‘(direct” oxidation will be more important. But this is also conditional, as the contribution of carbon dioxide formed by any route is dependent to a considerable extent on external factors, first of all on temperature. The CO Bstcp to CO edir ratio increases exponentially with temperature. Many workers consider that ethylene oxide isomerizes to acetaldehyde (18). Twigg suggests that formaldehyde and acetaldehyde are intermediates both in the stepwise and in the “direct” formation of carbon dioxide. The contribution of aldehydes to oxidation processes may be found out by oxidizing ethylene-acetaldehyde and ethylene-formalde-

CATALYTIC OXIDATION OB HYDROCARBONS

455

hyde mixtures under static and dynamic conditions and tracing the distribution of activity among various reaction products (118). The oxidation and adsorption of ethylene oxide on a silver catalyst is stronger in the presence of acetaldehyde, and the former hinders the formation of carbon dioxide from acetaldehyde. The latter fact is probably due to interaction between acetaldehyde and ethylene oxide a t the surface. A conjugated oxidation of this kind is often encountered with homogeneous reactions. The oxidation of mixtures of ethylene with ethylene oxide, formaldehyde and acetaldehyde on a stationary surface of a silver catalyst that was under operation for several hundred hours is of a different nature. The reaction rates for mixtures of radioactive ethylene with ethylene oxide, as well as with formaldehyde and acetaldehyde, are shown in Fig. 7. Oxidation proceeded under dynamic conditions a t 220".

':I

3

10

FIQ. 7. Diagram of formation rates ( w ) for ethylene oxide and carbon dioxide in the oxidation of ethylene and its mixtures with aldehyde over silver at 220'; w is given in arbitrary units.

I n the presence of ethylene oxide, formaldehyde, and acetaldehyde, the rate of ethylene consumption remains unchanged. There is also no inhibition of ethylene oxide formation a t its gas-phase concentration of about 1yo.This is probably due to the blocking off of the most active surface sites. At a prolonged use of the catalyst with an ethylene-formaldehyde mixture the catalyst activity gradually decreases and complete poisoning of the catalyst takes place. The acetaldehyde effect is similar to that of formaldehyde. All oxygen-containing compounds such as formaldehyde, acetalde-

466

L. YA. MARQOLIS

hyde, and ethylene oxide are taken up by the catalyst surface to form an adsorbed residue (Twigg calls it the "organic residue"). The removal of carbon dioxide by heating the oxygen-containing compounds in the absence of gas-phase oxygen is indication that the residue contains both oxygen and carbon atoms. The silver activity varies until the complete formation of the residue, after which it becomes constant. It will be noted that inhibition of ethylene oxidation with the ethylene oxide formed is associated with the blocking off effect of the residue. No inhibiting effect of ethylene oxide is observed for stationary catalyst surfaces. Various organic compounds display a different capacity for forming the surface residue. The compounds most readily adsorbed on the surface are CH,O, CH,CHO, and C,H,O. The contribution of aldehydes to catalytic oxidation is of great interest. Aldehydes are usually considered as primary oxidation products yielding the majority of other products. Isotopic analysis of ethylene oxidation over V,O, shows that neither acetaldehyde nor ethylene oxide can be the main intermediates in the formation of carbon dioxide from mixtures of ethylene with various oxygen-containing compounds (Fig. 8).

co

-

-

7

0,

FIQ. 8. Diagram of formation rates ( w ) for aldehydes, carbon monoxide, and carbon dioxide in the oxidation of ethylene and its mixtures with ethylene oxide and acetaldehyde over V,O, at 400";w is given in arbitrary units.

When ethylene is oxidized on vanadium oxides, a considerable amount of carbon dioxide and carbon monoxide is yielded directly by the hydrocarbon, bypassing acetaldehyde and ethylene oxide. It was found by investigating the oxidation of an acetaldehyde-ethylene oxide

CATALYTIC OXIDATION O F HYDROCARBONS

467

mixture that neither of these components was formed as the main intermediate in the formation of carbon dioxide and carbon monoxide. When ethylene is oxidized over Ag or V,O, the consecutive formation of several stable oxygen-containing compounds either does not occur at all, or is of secondary importance. It may be seen from comparison of results on ethylene oxidation over silver and vanadium pentoxide that with both catalysts the oxidation of unsaturated hydrocarbons will proceed by the same mechanism. CO, generation is not accelerated in the presence of aldehydes and these cannot be intermediates in ethylene combustion. When aldehydes are introduced into the reactant mixture, the ratio of ethylene oxide to CO, formation rates undergoes a change, due to strong adsorption of aldehydes on the catalyst surface. Ethylene oxide will form on silver and is in fact absent on vanadium oxides. It was shown experimentally that the absence of acetaldehyde and formaldehyde in the products of oxidation over silver, and the low absolute content of these substances for vanadium oxides is due to the fact that they are not formed a t all, or formed at a low rate, and not to their oxidation or decomposition. 2 . Oxidation of Propene Gorokhovatskii and Rubanik (119) came to the conclusion that the catalytic combustion of propene over silver does not proceed via the propene oxide formation step, since the rate of propene oxide conversion a t 240" is considerably lower than that for propene. Zimakov (120) suggested earlier that the impossibility of obtaining propene oxide from propene over silver was due to peculiarities of the propene oxide structure and the readiness of its further oxidation. However, Gorokhovatskii and Rubanik have shown that this is not so. Adsorption of propene on the silver surface seems to be different from ethylene adsorption. De Boer, Eischens, and Pliskin (121) suggest that ethylene sorbs on a silver surface covered with oxygen to form complexes

which may readily convert into ethylene oxide (the strong reactivity of hydrogen atoms contained in the CH, group, the so-called 6 - rr

458

L. YA. MAROOLIS

conjugation, was established for propene). Thus the probability is great that propene is adsorbed on the silver surface with abstraction of a hydrogen atom and the formation of OH O-CH+2H=CH,

I I

Ag - Ag

Under the action of oxygen this compound readily oxidizes to carbon dioxide and water. This might be the main reason for absence of propene oxide in the oxidation of propene over silver. This question might have been solved by spectroscopic investigation of the adsorbed propene structure. Acrolein and carbon dioxide are formed in propene oxidation over cuprous oxide. Minor amounts of acetaldehyde and formaldehyde were found in reaction products, and this is indication that the reaction proceeds by breaking of the propene double bond. The oxidation of unsaturated hydrocarbons over other metal oxides, such as V,O,, WO,, etc., is of the same nature. Let us consider what are the intermediates in the formation of carbon dioxide. Isaev, Margolis, and Sazonova (116) attempted to elucidate the mechanism of propene oxidation to acrolein using the kinetic tracer method. The specific radioactivity of carbon dioxide was found t o be higher than that of acrolein, but lower than the propene radioactivity. If carbon dioxide and acrolein were yielded by propene separately, the specific radioactivity of CO, would be identical to that of propene. It follows from radiometric data that, within experimental error, there is no parallel route of carbon dioxide generation. It will be noted that with a fresh catalyst having a non-stationary surface up to 30% of carbon dioxide are yielded directly by propene, bypassing acrolein. Thus the parallel-consecutive scheme of propene oxidation to acrolein over cuprous oxide was revealed by the kinetic tracer method. 3. Mechanism of Catalytic Oxidation

Isotopic investigations of various reaction mechanisms yielded the scheme for catalytic oxidation of hydrocarbons shown in Fig. 9. This scheme is of great help in the proper choice of catalysts. The first task in improving the results of hydrocarbon oxidation is to find catalysts ensuring the necessary reaction route. Parallel reaction routes are to be suppressed by adjusting the chemical composition of the catalyst through addition of donor and acceptor impurities. Due to lack of experimental information on the nature of elemen-

CATALYTIC OXIDATION OF HYDROCARBONS

45 9

tary steps in the catalytic oxidation of hydrocarbons, it was difficult to establish the true mechanism of catalytic heterogeneous reactions. However, it was suggested by a number of scientists, by analogy with reactions occurring in the homogeneous phase, that catalytic oxidation of hydrocarbons proceeded via the generation and interaction of radicals. Schultze and Theile (122) pointed to the possibility of interaction between ethylene and oxygen molecules to form peroxides. Pokrovskii (123) reported a scheme of catalytic ethylene oxidation to ethylene oxide involving radicals.

Fro. 9. Scheme of Catalytic hydrocarbon oxidation; H-hydrocarbon, R, to R,-labile intermediates,probably of the peroxide type.

C-catalyst,

Lyubarskii (124) believed that upon interaction with molecular oxygen ethylene was converted to ethylene oxide. Sosin (104)suggested that in the oxidation over V,O, aromatic hydrocarbons lose hydrogen and convert into radicals to form a peroxide radical by interaction with oxygen. Bretton et al. (112) consider that the catalyst effect is one of abstracting the hydrogen atom from the hydrocarbon molecule to form a radical converting into a peroxide radical, and then into a hydroperoxide. The high rates and low activation energies of heterogeneous catalytic processes made many scientists believe long ago that active intermediates play an important part in these processes. Semenov and Voevodskii (125, 126) reported that radicals were yielded by reactions between gaseous molecules and the surface. The electronic theory of catalysis also implies that reactive radicals are generated on the catalyst surface by adsorption of molecules. Let us summarize the essential points of the hydrocarbon oxidation scheme, as derived from published data and from the electronic theory of catalysis. (1) A molecule with a double bond adsorbed on a semiconducting catalyst surface converts into a radical bound with the lattice and having a free valence. A molecule with a single bond emerging from the gas phase may react with the free valence of such a radical and dissociate. (2) An adsorbed saturated molecule with a single bond may dissociate into two radicals, one saturated with the surface valence, and

460

L. YA. MAROOLJS

the other having a free valence; free radicals will be generated by desorption of the latter radical into the gas phase. (3) It may be considered from isotopic data and electron work function measurement that negatively charged ions of molecular and atomic oxygen are present on semiconducting surfaces. The ratio of these is a function of temperature and the chemical properties of the solid. (4) Hydrocarbons are sorbed on semiconducting catalysts either weakly-reversibly, or strongly-irreversibly . The ratio of weak to strong adsorption is a function of temperature and the chemical composition of the catalyst. ( 5 ) Various types of ion-radicals are formed in adsorption of reactant molecules on the semiconducting surface; the formation of these is a function of electronic properties of the solid, and the structure and kind of bonds. (6) The catalyst surface is markedly heterogeneous both with respect to oxygen and to hydrocarbon adsorption. ( 7 ) The heterogeneous-homogeneous step occurs only for certain catalysts, such as platinum and spinels, and is not observed with oxide catalysts over the temperature range up to 400'. (8) Reaction products, such as aldehydes, olefine oxides, etc., are strongly sorbed on the catalyst surface, contributing to formation of the organic residue and representing additional sources of carbon dioxide generation. (9) It may be considered on the basis of data obtained by means of the radioactive tracer technique that the various stable oxygen-containing products on semiconducting oxides are generated by different routes, through active intermediates. (10) The oxygen in metal oxide lattices, as well as that sorbed on lattice surfaces is of a low mobility. Under certain conditions the hydrocarbon will react with oxygen of the catalyst lattice. A t low temperatures this side reaction is of small importance for oxidation. Scientists working in catalysis were long ago concerned with the theory of reduction-oxidation mechanisms of catalytic oxidation reactions. The nature of oxygen contribution to the oxidation process may be established by using the radioactive tracer technique. The reductionoxidation mechanism of reactions is a function of the lattice oxygen mobility. Vainstein and Turovskii (86) investigated the distribution of 0 1 8 in the oxidation of carbon monoxide over MnO, and CuO and found that there will be no transfer of the catalyst oxygen to the reaction producte, when water is carefully removed from the solid oxides.

CATALYTIC OXIDATION OF HYDROCARBONS

46 1

Vassiliev et al. (127)found that CO, exchanges oxygen with the MnO, surface even a t low pressures. The rate constants for CO, exchange and reduction of the surface are almost equal, whereas those for CO oxidation are by a power of ten higher. Kinetic and isotopic studies show that oxidation of carbon monoxide over V,O, cannot be considered solely as alternate reduction-oxidation of the surface. Vanadium pentoxide exchanges its oxygen with oxygen of the gas phase at temperatures above 45OoC, and catalytic oxidation proceeds within the same temperature range. Roiter et al. (128) compared the rates of isotopic oxygen exchange and catalytic oxidation of sulfur dioxide. They found that a t 500" the exchange rate is 10 times lower than that of oxidative-reductive catalysis. The rate of CO, oxidation by alternate reduction-oxidation over V,O, will be many times lower than that of oxygen exchange, and the latter is, in its turn, 10 times lower than catalysis with the given catalyst. Consequently, it may be seen from comparison of rates that the process is not a reduction-oxidation one in this case as well. Even more convincing is the comparison of exchange and catalytic oxidation rates for naphthalene. Naphthalene oxidation proceeds a t 400"; exchange over V,O, at this temperature will be considerably slower than the oxidation reaction, and the difference in rates of these processes cannot be explained on the basis of the reduction-oxidation scheme. Kinetic isotopic analysis of different reactions, such as the oxidation of CO, SO,, and hydrocarbons of various structures over MnO,, V,O,, etc., shows that the reduction-oxidation mechanism cannot be considered as the main route of these processes. Certain reaction schemes making allowance for surface charging will be discussed. Peroxidt radicals are chain-carrying centres in the homogeneous oxidation of hydrocarbons. The leading part in heterogeneous oxidation is played by peroxide ion-radicals (Margolis, 129). Let us consider three types of interaction between: (1) adsorbed hydrocarbon ion-radicals and oxygen of t h e gas phase; (2) adsorbed oxygen and hydrocarbon of the gas phase (oxygen adsorbed both with and without dissociation); (3) adsorbed oxygen and adsorbed hydrocarbon. Saturated aldehydes and acids containing less carbon atoms than in the molecule of initial hydrocarbon, as well as carbon dioxide and water, are formed in the first case. The second type of interaction yields unsaturated aldehydes, olefine oxides, carbon monoxide, carbon dioxide, and water for the oxidation of unsaturated hydrocarbons; and saturated aldehydes, carbon dioxide, carbon monoxide, and water for the oxidation of saturated hydrocarbons. The third type of reaction gives

462

L. YA. MAROOLIS

unsaturated aldehydes, carbon monoxide, carbon dioxide and water (olefine oxidation). The latter type of interaction would be impossible in the oxidation of saturated hydrocarbons. Radical oxidation schemes for unsaturated and saturated hydrocarbons may be derived on the basis of investigation of various reaction types. For instance:

+ O , + L - 0, 2L + 0 , + 2 L - it

1. L

2.

+ C,H, -+ L - H + isH, 4. L + 6 , -+ ~L ~ C,H, 6. L - 6, + C , H , - + L - 0 - O H + &H, 6. L - 0 , + CSH, -+ L - C,H7 3. L

Op

7.

L - 0, - CsH,

=

(a) CH,CHO (b) C H I 0 ( 0 ) COs, H , O

- 0 , + L - C,H7 -+ L - O O H + L - C,H, 9. L - CSH, + 0 , -+ L - C,H, - 0 , 8. L

10. L

- C,H, - 0, -+ (a) CH,CHO

(b) CH*O CH,COOH (d) H C O O H (8) CO, CO,, Ha0

(c)

11. L

- C,H, - 0 , + C3H, -+

polymerization

All types of reactions between oxygen and hydrocarbons yield oxygen-containing compounds, such as aldehydes, acids, etc., present together with the products of complete combustion, i.e., with carbon monoxide and water. The reaction selectivity seems to be determined by the strength of bonding between the surface and the ion-radicals formed, and may be increased solely by changing the chemical composition of the catalyst. Let us consider schemes for certain oxidation reactions over metals and semiconducting catalysts. 4. Metals a. Ethylene Oxidation to Ethylene Oxide over Silver. The scheme for ethylene oxidation over silver suggested by Margolis in 1957 (118, 129) is based on the following considerations. It was found by investigating homolytic exchange on silver a t temperatures of catalytic ethylene oxidation that the surface-adsorbed oxygen may be present as mole-

CATALYTIC OXIDATION OF HYDROCARBONS

463

cules and atoms. Measurement of electrical conductivity and the electron work function of silver in oxygen adsorption shows that the surface oxygen is of a negative charge. Ethylene will not sorb chemically on a silver surface free of oxygen and its electrical conductivity will remain unchanged. Oxygen-containing surface ethylene will be sorbed obtaining a positive charge, and sorption kinetics will follow an equation characteristic of a heterogeneous surface. It was found by isotopic studies that ethylene and carbon dioxide were formed independently and by parallel reactions, unstable oxygencontaining products being intermediates. Aldehydes cannot be the main intermediates in the generation of CO, and H,O. Due to strong adsorption of ethylene oxide, an organic film will be formed on the silver surface, blocking off the active centers. I n accord to Enikeev et al. (102) variations in the electron work function ( A 4 ) in adsorption of the above compounds on silver are indication that ethylene and ethylene oxide are electron donors, and oxygen and CO, electron acceptors. Water would slightly decrease the +value. Following reactions occur a t the Ag surface : C,H,

-e

CaH40

-e

+e

CO,

-+ (CaH4)+ -+ (C,H40Jf -+ (C0,)-[possibly (CO,)-]

Carbon dioxide may be sorbed reversibly only on an oxygen-covered surface, probably to form a (C0,)- complex. Twigg has shown that a t temperatures about 200 to 250°C ethylene oxide will decompose in part to ethylene and adsorbed oxygen. Variations in the contact potential difference in adsorption of ethylene oxide at the above temperatures are indication that the electron work function will continuously increase with surface coverage. The following scheme of ethylene oxidation over silver may be proposed on the basis of experimental information available: Ag

+ 0 , -+ Ag - ( 0 a ) -

2Ag Ag

+ 0,-+ 2Ag - (0)-

- Oa + CaH4 -+ Ag - (0, - CaH4) Charged complex 1.

- 0 , - CaH4 -d(COa) + (HaO)++ Ag Ag - 0 , - CZH4 --+ (CaHaO)' + Ag - (0)Ag - 0 , - CaH4 + CaH, + Ag - 0 , - (CZH,) Ag - 0 + CSH,+=Ag - (0 - CaH4) Ag

*

+o

464

L. YA. MAROOLIS Charged complex 2.

- 0 - CaH, + 0,+ (COJ + 2 (HnO)' Ag - (C,H,O)+ + e + A g + C,H,O Ag - (COJ - e - + A g + CO, Ag - (HpO)+ + e + A g + H,O Ag

Hayes (130) proposed in 1959 a scheme for ethylene oxidation over silver. He believed that ethylene oxide might be yielded only by interaction between ethylene and atomic oxygen, while carbon dioxide and water would be formed by a reaction of ethylene with molecular oxygen. If this were consistent with reality, the marked selectivity of silver with respect to ethylene oxidation to ethylene oxide would be inexplicable. Atomic oxygen is present on the surface of other metals as well, for instance on platinum and palladium, but no ethylene oxide is formed by reaction of these with ethylene. b. Olejine Oxidation over Platinum. The oxidation of hydrocarbons over platinum is of a markedly different nature compared with that observed for silver. Solely carbon dioxide and water are always present in reaction products in the oxidation over platinum, within a very wide range of experimental conditions, such as temperature, concentration of components, pressure. It was shown by Elovich and Butyagin (12) that the propene sorbed becomes strongly bound with platinum and may be eliminated only by treatment of the surface with oxygen, after which propene is taken up in greater amounts. I n oxygen adsorption on platinum oxygen may be dissolved in the adjacent-to-surface layers in an amount equal to that required for several tens of monolayers. I n accord to Frumkin (70) the binding of oxygen with platinated platinum becomes stronger with longer contact times and its reduction is then hindered. It was shown by investigating the electron work function in the adsorption of oxygen on platinum than on the platinum surface oxygen is an electron acceptor. It may be seen from results on isotopic exchange than 0; ions of molecular oxygen might be present a t the surface. Oxygen was found to be strongly sorbed on the platinum surface. Isotopic oxygen exchange becomes important at temperatures by 200-250" higher than the temperature of catalysis. Thus the binding of oxygen and hydrocarbons on the platinum surface is different from that for silver. Due to the great strength of oxygenhydrocarbon bonds, the formation of ethylene oxide on platinum seems to be scarcely probable. It was found, moreover, that the propene oxidation reaction passed into the gas phase even a t 70". With other metals, such as Ni and Cu, a new phase, i.e., a film of NiO

CATALYTIC OXIDATION O F HYDROCARBONS

465

and Cu,O oxides will be formed during catalytic oxidation and, consequently, the latter should be considered as a reaction proceeding on semiconductors. 5 . Semiconductors Various reactions, such as high conversion, low conversion with partial destruction of hydrocarbon molecules, and without essential distortion of the molecular structure may occur in the oxidation over semiconducting catalysts. Let us consider several reactions of propene oxidation over various semiconductors. a . Propene Oxidation over Cuprow Oxide. The cuprous oxide surface becomes charged by adsorption of oxygen, propene, and acrolein. The charge of molecules adsorbed may be determined from the electron work function. At the surface of CuO propene and acrolein are electron donors, like the majority of organic compounds. Adsorbed water slightly decreases the electron work function. Consequently, water also is an electron donor. The following reactions occur in the adsorption of various gases and vapors on cuprous oxide: 0,

+ e-+(O&

0

+e-+(O)-

C,H, - e -+ (C,H,)+ CHO H,O

- CH = CH, - e -+ (C,H,O)+ (acrolein) - e -+ (HnO)+

These data make possible the determination of a number of steps and elucidation of the nature of electron transfers in adsorption. Propene oxidation to acrolein over cuprous oxide seems to follow the scheme: 0, 0

+ 20

+ e + (0)-

0,

+

CsH6

e

-+

(0,)-

+ (0s)-

-+ (CaHsOOH)

charged complex 1 (hydroperoxide)

(CaHsOOH)+3 (CJH40)++ (H,O)+

+e (CsH@) + 0 , (C8H40)+

--+

-+

(CsH40)gas (CsH40

- 0,)

charged complex 2.

466

L. YA. MARGOLIS

+ H,O + (RH)’ (RH)’ + 0, -+ CO, + H,O + (R‘H)’ - 0,)-+

(C,H,O

- e -+

C,H,

(CsH,)+

CO,

(C,H,)+

+ 2(0)-

-+

(CsH, 00) a

charged complex 3.

(C,H,

- 00)-+ C,H,O + HCHO

(C,H,

*

(RH)’

00) -+ ROO

+ 0, -+

+ (RH)’

R’OO

+ (R”H)+

etc.

b. Propene Oxidation over Spinels (MgCr,O,,CoMn,O,,MnCo,O,, etc.). Hydrocarbons are oxidized over these catalysts to carbon dioxide and water, i.e., hydrocarbon molecules become completely destructed. No acids are found in reaction products. It was suggested above that a chain reaction occurred a t the surface resulting in complete oxidation of hydrocarbon molecules. The charge signs of the components adsorbed measured from the electron work functions are similar to those for simple oxides. The reaction scheme may be written as follows: 20

0,

$

0

+ e -+ (0)-

Oa

+e

C,H,

-+

(0,)-

- e + (CsH,)+

+

(0,)- C,H,

--f

(CSH600) Charged complex I.

(C,H,)+

+ 0,

-+

(C,H,OO) Charged complex 11.

(C,H,

- 00),-+ (R’H) + RO,

(C3HeOO)11+ 0 , -+ (HOa)

+ ROa

+ 0, -+ RO, -t(R”H) etc. (RO,) + O,+R‘O, + (R”H) eto. (R’H)

The charged complex I (C,H,O,) is bound with the spinel lattice through oxygen. The oxygen-surface bonding is known to be very strong for these compounds; oxygen desorption is not observed even a t temperatures above 5OO0C. Thus complex I may suffer destruction

CATALYTIC OXIDATION O F HYDROCARBONS

467

only by further interaction with oxygen. Cationic sites of the Mn3+ and other types seem to be the active centers for oxygen adsorption on spinels. These sites are located at a considerable distance from each other and, consequently, a reaction of the complex with the oxygen adsorbed will be much less probable. It may be suggested that complex I involving an unpaired electron may enter into reaction with oxygen of the gas phase. The charged complex I1 is bound with the spinel lattice through carbon. The bonding is weak; hydrocarbon adsorption is reversible at 100200’. Thus the decomposition of such a complex or its desorption and subsequent oxidation in the gas phase are probable.

F. HETEROGENEOUS-HOMOGENEOUS REACTION STEPS The various hydrocarbon oxidation schemes discussed above were believed to proceed a t the catalyst surface only. The present concepts accept the occurrence of complex heterogeneous-homogeneous reactions proceeding in part at the solid surface and in part in the gas or liquid phase. Many catalytic oxidation processes considered recently as purely heterogeneous appeared to proceed by the heterogeneoushomogeneous mechanism. Such are the oxidations of hydrogen, methane, ethane, ethylene, propene, and ammonia over platinum a t elevated temperatures, as studied by Polyakov et al. (131-136). When hydrocarbons are oxidized over platinum the reaction sets in on the catalyst surface and terminates in the gas phase. However, until recently, the homogeneous continuation of the reaction could be established only by indirect means, using kinetic and other methods. In 1946 Koval’skii and Bogoyavlenskaya (137)proposed a method of differential calorimetry permitting a more accurate and unambiguous solution of this problem. With catalysts for high conversion the process of oxidation will proceed by the heterogeneous-homogeneous mechanism, while surface reactions only will occur with low conversion catalysts, at ordinary temperature of catalysis. However, the classification of catalysts as belonging to two groupsfor low and high conversion is not always justifiable with respect to surface and heterogeneous-homogeneous .processes. Thus, in using cobalt-manganesespinels catalyzing propene oxidation to carbon dioxide and water only, Linde (138)did not observe the passing of the reaction to the gas phase. With these catalysts the reaction will proceed on the surface only. Popova et al. (139) have shown that the conversion of carbonyl compounds in the oxidation of propene to acrolein over a copper catalyst involves a heterogeneous-homogeneous step.

468

L. YA. MARGOLIS

IV. Reaction Kinetics It is very difficult to establish kinetic laws for hydrocarbon oxidation first of all due to the high endothermicity of this reaction resulting in sintering of the catalyst, in surface changes, and in the intensification of side processes. This is probably the reason why the kinetics of a number of hydrocarbon oxidation reactions is insufficiently known, and the data reported in literature are scarce. A. THE EFFECTOF MACROSCOPICFACTORS

A number of physical side processes, such as the diffusion of initial compounds and reaction products, the liberation and distribution of heat, the dynamics of gases and liquids exert an influence on hydrocarbon oxidation under working conditions. All these factors are of prime importance for the design of catalytic apparatus, and moreover, may bring a change in the main oxidation characteristic, i.e., in the selectivity. The diffusion coefficient for catalyst pores is usually calculated approximately. Roiter et al. (140, 141) worked out a method for experimental determination of the diffusion coefficient and calculation of reaction rates without errors induced by diffusion. A catalyst possesses a considerable inner surface the access to which is difficult for gases, due to windings and small diameters of pores. Oxidation proceeds mainly in the internal diffusion region and as the temperature is raised to 400", it passes into the external diffusion region, with sharp heating up of the catalyst to 100-1 10" (at a reaction temperature 420"). The phthalic anhydride yielded by naphthalene inside the pores oxidizes to the end-products, C,O and H,O, due to hindered diffusion and the increased contact time (Fig. 10); this results in lower selectivity. Boreskov (142) found that the contribution of the inner surface is different for various industrial catalysts. Inside the catalyst grains the reaction is several times slower than at the external surface. Kholyavenko and Rubanik (143) investigated the effect of internal diffusion on the ethylene oxidation rate, using the diaphragm method, and calculated the effective diffusion coefficients for ethylene and carbon dioxide diffusing through a silver diaphragm. Ethylene oxidation on a porous silver catalyst proceeds over a wide temperature range in the transient internal region (190-260"). The selectivity of ethylene oxidation a t 190-250" is the same inside the grain and a t its external surface. The nature and structure of the support exert a considerable effect

CATALYTIC OXIDATION O F HYDROCARBONS

469

on the efficiency and selectivity of reactions. For example, Gorokhovatskii et al. (144)studied the oxidation of propene to acrolein over cuprous-copper oxide catalysts on various supports, namely on glass, aluminum oxide, and silicon carbide. The supports differed in specific activities. The selectivity of these catalysts was a function of the support structure. For aluminum oxide-supported catalysts the Selectivity decreased with increasing of the inner surface of the siipport.

Fro. 10. The efficiency ( U )of catalytic naphthalene oxidation over V,O, as a function of temperature; 1-in naphthalene; 2-in phthalic anhydride. U in molelmin g X lo6.

Frank-Kamenetzkii (145) suggested that theoretical analysis of exothermic regimes would make possible the proper choice of conditions ensuring the reaction development with weak heating up in the kinetic and strong heating up in the diffusion region.

B. KINETICSOF PROPENE AND ETHYLENE OXIDATIONOVER VANADIUM Roginskii et al. (146) found that the rate of olefine oxidation is proportional to oxygen concentration, and almost independent of hydrocarbon concentration. The activation energy values for olefine oxidations are summarized in Table VIII. The activation energy for carbon dioxide generation, Eco,, is higher than those for carbon monoxide and aldehyde (Eco and E,). E,, increases with an increasing number of carbon atoms in the hydrocarbon molecule. The activation energies for the formation of aldehydes from propene and isobutene are low and close to each other.

470

L. YA. MAROOLIS

The low activation energies for aldehydes and carbon dioxide formation might be due t o the fact that these reactions occur in the diffusion or transient regions. The activation energies for the formation of aldehydes and acids from propene over V,O, were measured in the kinetic region by Kutzeva and Margolis (147) and were found t o be 13 and 14 kcal/ mole, respectively. T A B L E VIII Activation Energies for the Formation of Aldehydes, Carbon Monoxide, and Carbon Dioxide from Various Hydrocarbona Reactions

Hydrocarbon ~

_

_

~

Activation energies References kcal/mole ____ _ _ __

Formation of aldehydes Formation of aldehydes Formation of acids Formation of CO, Formation of CO

Propene Propene Propene Propene Propene

22 2

Formation of CO, Formation of CO

Ethylene Ethylene

21 5

Formation of CO Formation of formaldehyde

Isobutene Isobutene

7

-

4

13 14

5

(146)

(30)

(314

It was noted before by the same workers that acrolein was formed along with saturated aldehydes in propene oxidation over metal oxides of the Vth and VIth groups of the Periodic System of Elements ( 5 1 ) . The kinetics of acrolein generation from propene over V,O, was also studied recently, and the activation energy for this reaction was found to be 12 kcal/mole. C. KINETICSOF NAPHTHALENE OXIDATION OVER VANADIUM OXIDE Ushakova et al. (148) studied the kinetics of naphthalene oxidation over V,O, under conditions preventing the effect of macroscopic factors on the reaction rate. Experiments were made over a temperature range of 380 to 410" with large crystals of nonporous V,O, in a flow-circulating system. As the gas circulation rate was 3.5 liters/min/cycle, there were no variations in naphthalene concentration in the catalyst layer. Calculation of experimental information gave following equations for

CATALYTIC OXIDATION OF HYDROCARBONS

47 1

individual reaction rates in the oxidation of naphthalene over V,O,: formation of phthalic anhydride Wpht.an.

= kph. Cnaph.

formation of maleic anhydride 0.5

w m a ~ a n .= kmal. Cnaph.

(6)

formation of naphthaquinone Wnaphthaquin.

2

= knq. Cnaph.

(7)

high conversion wCO,

= kCO, *Cnaph.

(8)

The activation energies for these reactions were found to be for phthalic anhydride 37.4 kcal/mole for maleic anhydride

31.6 kcal/mole

for naphthaquinone

32.7 kcal/mole

for carbon dioxide

37.2 kcal/mole

Ioffe and Sherman (149)studied the kinetics of naphthalene oxidation to phthalic anhydride on a more complex vanadium-potassium-sulfate catalyst over a wide range of conversions and temperatures. The naphthalene oxidation was found to be independent of naphthalene concentration, This reaction is first order with respect to oxygen concentration and is inhibited with reaction products.

Here [O,] is the oxygen concentration in the reactant mixture; [A,] is the concentration of reaction products expressed as the amount of oxygen consumed for their formation; k , and k , are constants. The activation energy for this process is 27 kcal/mole; the preexponential factor 5 x 109. The zero reaction order obtained earlier by Calderbank (150) is jmitated for low conversion. The kinetic laws for naphthalene oxidation over a mixed vanadium catalyst and pure vanadium pentoxide are different. The adsorption capacities of naphthalene and oxygen seem to suffer a change when potassium sulfate is added to V,O,. According to Boreskov and Kassatkina (88) the rate of isotopic oxygen exchange on V,O, increases in

47 2

L. YA. MARQOLIS

the presence of potassium sulfate, while the activation energy for exchange decreases. The rate-limiting step for this catalyst probably differs from that for V,O,. Ioffe and Sherman suggest that the rate of naphthalene oxidation is limited by that of desorption of reaction products. Roiter et al. (148)studied the kinetics of naphthalene oxidation to 1.4-naphthaquinone over a mixed vanadium-potassium-sulfatesilica gel catalyst over a wide range of naphthalene and oxygen concentrations at 330-360". Certain information on the kinetics of naphthalene oxidation over various vanadium catalysts is given in the review by Dixon and Longfield (110).This reaction was found to vary from zero to first order with respect to naphthalene, and be close to first order for oxygen.

D. KINETICSOF BENZENE OXIDATIONTO MALEIC ANHYDRIDE OVER V20, Hammar (107) studied benzene oxidation over catalysts consisting of V,O, on supports, and found that the rates of maleic anhydride, CO, and CO formation were proportional to benzene concentration (the reaction order was close to first, as calculated from the degree of conversion aa a function of the contact time). The activation energies were equal and amounted to 2 4 f kcal/mole. The reaction order with respect to benzene, as determined from initial rates, varied from zero to second order. I n studying the kinetics of benzene oxidation to maleic anhydride and in order to eliminate diffusion hindrance, Ioffe and Lyubarskii (151) used the flow-circulating method. The rate of this reaction was found to be proportional to benzene concentration to the power of 0.78, and that of high conversion to the power of 0.71. The rate of maleic anhydride oxidation followed a first order equation. Kinetic equations were derived from experimental results : mole/liter: For oxygen concentrations lower than 4 x The rate of maleic anhydride formation (MA)

(C6HB)0-7fJ w 1 = k,(O2)Z - -____ (MA)OS~~ The rate of maleic anhydride oxidation w2 = k, (MA) The rate of high conversion of benzene wg

=

(C H )O.'l k3(0,)2S _ _ B - _ (MA)O*'O

CATALYTIC OXIDATION OF HYDROCARBONS

473

The over-all rate of benzene conversion (13)

For oxygen concentrations higher than 4 x lo-, molejiter: w1

=

(C8H6)0'78 k1 ( m ) 0 . 7 4

w 2 = k,(MA) (C6H6)O*'11 W, = k, __(MA)o*74

(15)

w,, = k 1(C6H6)0*78 + k3(C8H6)0*71 (MA)O.74 1

Activation energies: For the formation of maleic anhydride El = 22.6 kcal/mole For the oxidation of maleic anhydride E , = 12.6 kcal/mole For high conversion of benzene E , = 37.0 kcal/mole Ioffe and Lyubarskii believe that the main amount of benzene is decomposed by the oxygen adsorbed on V,O,. The lattice oxygen is responsible for the decomposition of a small amount of C,H,, but the rate of this reaction is considerably lower. Maleic anhydride is readily sorbed on V,O,. No allowance is usually made in kinetic studies for the side reaction of decomposition by the lattice oxygen. Ioffe and Lyubarskii (151) derived an equation for the benzene oxidation rate, allowing for phase transitions in the V,O, lattice.

E. KINETICS OF PROPENE OXIDATIONTO ACROLEIN OVER Cu,O Almost no papers were published on the kinetics of acrolein formation from propene. Those available report that cuprous oxide on silicon carbide or on pumice was used as catalyst, Isaev and Margolis (152) studied the kinetics of acrolein synthesis from propene under dynamic conditions a t atmospheric pressure. The rates of acrolein and carbon dioxide formation were found t o be proportional to oxygen concentration in the gas phase and independent of propene concentration. It will be of interest to note that the two reactions are of the same order. A similar result was obtained earlier for the formation of aldehydes, CO and CO, in the oxidation of propene over vanadium pentoxide.

474

L. YA. MARGOLIS

The activation energy for C 0 2 formation is 28-30 kcal/mole, and for acrolein it amounts to 12-14 kcal/mole. With acrolein transition to the external diffusion region occurs a t a lower temperature than that for CO,. The selectivity of propene oxidation to acrolein is determined by the ratio of the reaction rate constants for acrolein and carbon dioxide. Over the temperature range lower than 320' the reaction rate constant for acrolein formation may be higher than that for carbon dioxide, while the reverse is observed a t temperatures higher than 350". Belousov et al. (115,153) studied the kinetics of propene oxidation t o acrolein on a cuprous-copper catalyst, using the flow-circulating method. The reaction products were shown to exert a considerable effect on the reaction rate, and this permitted the derivation of a more precise kinetic equation for this reaction. Over the temperature range of 305-365" the rates of propene oxidation to acrolein (wacr,) and carbon dioxide (wco,)are fairly well described by equations

k , [O 21'

wco, = [C,H40]o*7[CsHal0*2 Here 0,, C3H40,and C3H, are the oxygen, acrolein, and propene concentrations per cycle in volume percentage; A, is the concentration of products expressed as the amount of oxygen consumed for their formation; k,, k , are constants that may be obtained from kinetic data. The activation energies obtained are:

E,,,

=

30 & 2 kcal/mole,

Eco, = 36 & 2 kcal/mole

By calculating the heats of adsorption of products it is possible to obtain the activation energies for these reactions under conditions ruling out inhibition by products. Then E,,, = 20 & 1 kcal/mole, and Eco, = 26 & 1 kcal/mole, and this is in agreement with results obtained by other workers. The kinetic equations derived by using the flow and flow-circulating method show good agreement.

F. KINETICSOF ETHYLENE OXIDATIONTO ETHYLENE OXIDE OVER SILVER The data on kinetics of ethylene oxidation to ethylene oxide are summarized in Table IX. I n all papers published the rate of ethylene oxidation is but slightly

CATALYTIC OXIDATION OF HYDROCARBONS

475

dependent on ethylene concentration (the reaction order within zero to 0.45) and proportional to oxygen concentration (first order in oxygen). Temkin et al. (159) studied the kinetics of ethylene oxidation over a stationary silver surface. It was shown by means of the flow-circulating method that the rate of ethylene oxide and carbon dioxide formation was proportional to ethylene concentration in the gas phase, and that there was inhibition with reaction products. TABLE IX Kineties of Ethylene Oxidation to Ethylene Oxide over Silver Catalyata

Reaction equation

Reaction

Oxidation of.eth~lene d[C&I to ethylene oxide -__ dt

=

Activation energy (kcal/mole)

k l [ C p H 4 1 0 . 4 S [ O ~ 1 0 . ~ ~ 16

Formation of CO, from d[CO ] 2=k n[C,H4]o*S [0n]'" ethylene dt

16

Ethylene oxide oxidation

20

- [o,x& dCC,H,OI -dt

Oxidation of ethylene d [ C 2 H 4 1= k , [ C p H 4 ] [ C , H 4 ] o [ 0 , ] 1 ~ U13 7 to C , H 4 0 and C O , Ethylene oxide oxidation

d[C,H,Ol dt

=

k,[C8H4]1[Oz]1

17-19

Oxidation of ethylene d [ C d h I = k[C,H410.8[0,10., to ethylene oxide - dt-

19

-d[C,H,I ~--__ dt

-

Ethylene oxidation Ethylene oxide oxidation Ethylene oxidation

--

d[C&O] dt

- [o,ll

-

C0,l'

H ' kL [C,H,I'[O*I0 -d[C 2 =---dt [C$&Ol k[COJ

+

15(C8H40)

19 (CO,)

The regularities observed experimentally may be classified as characteristic of three cases, when the rates of ethylene and carbon dioxide formation are proportional to: (1) oxygen concentration (being independent of ethylene concentration) ; (2) ethylene concentration (being

476

L. YA. MAROOLIS

independent of oxygen concentration); and (3) ethylene and oxygen concentrations to fractional powers. These dependences seem t o be determined by the surface content in molecular and atomic oxygen, and by the rates of oxidation and decomposition of peroxide radicals.

G. KINETICSOF HIGH CONVERSION OF HYDROCARBONS Almost no data were reported in literature on the kinetics of the high conversion of hydrocarbons t o carbon dioxide and water. This is probably due to the fact that the strong exothermicity of high conversion processes makes difficult the obtaining of reliable kinetic characteristics. Margolis and Todes (13)have studied the kinetics of catalytic oxidation of various hydrocarbons a t an excess of oxygen, using a number of catalysts, such as magnesium chromite, copper, and platinum under isothermal conditions. For hydrocarbons of aliphatic isostructure (2.2.4-dimethylpentane),and for naphthene hydrocarbons (cyclohexane and methylcyclohexane) the reaction kinetics is second order. For other classes of normal paraffins, such as n-pentane and n-heptane, and for unsaturated hydrocarbons (C,H,, C,H,) the reaction rates are proportional to the first power of hydrocarbon concentrations. For unsaturated and normal aliphatic hydrocarbons the kinetics of oxidation is proportional to the hydrocarbon concentration to the first power. At 200’ the specific catalytic activities will be in the order: platinum > copper chromite > magnesium chromite. As the temperature is raised to 400°,this order changes. Platinum remains the most active catalyst, while the catalytic activity of magnesium chromite becomes almost by 3 powers of ten higher than that of copper chromite. It will be of interest to note that over certain tem’perature ranges catalysts of higher activation energies are less active than those of lower activation energies. Thecurveslog k v s 1/Tfor two different catalysts may intersect (Fig. 11) due to the fact that E and K Ovalues change in the same sense. This was found to be characteristic of all catalysts used in the oxidation of various hydrocarbons. Simultaneous changes in E and K O were studied for a large number of organic reactions, such as hydrogenation and dehydrogenation (160). The activation energies and preexponential factors for the oxidation of saturated and unsaturated hydrocarbons over various catalysts are summarized in Table X. I n the oxidation of hydrocarbons of various structure over CuCr,O, and MgCr aO,, the activation energies and preexponential factors

TABLE X

The E and K O Valuesfor Catalytk Combvatbn of Hydrocarbons over Various Catalysts E (cal/mole) Hydrocarbon Platinum n-pentane n-heptane n-octane Benzene Ethylene

7800 8000 7300 -

Ka

Manganesechromium

Copper chromium

19,300 2700 46,000 4700 8100

18,300 3900 33,000 6400 25,000 20,000 5300 17,500

-

5500 28,000

Platinum

lo5..‘ 1054 105

Manganesechromium

Copper chromium

108.4 102.2 1019.5 102.7 104 108.1 1011.6

107 102 10’2 108 109 109.8

1074

Temperature range

280-350 350-550 300-330 330-500 300-500 300-400 400-550 280-350

0

ki

t P

l

4

478

L. YA. MAROOLIS

increase with the number of carbon atoms in the hydrocarbon suffering oxidation. This dependence is not observed for the oxidation of normal saturated hydrocarbons, such as pentane, heptane, and octane over platinum. Linde et al. (138) have studied propene oxidation under static conditions at low pressures over manganese-cobalt spinels. The reaction rate was found to be proportional to coverage of the surface with oxygen and independent of the hydrocarbon concentration. bk

-I

.o

'

20040-8

I

150.10-5

I

1o040-8

FIG. 11. The rate constant logarithm as a function of the temperature reciprocal in the oxidation of iso-octane over various catalysts: 1-copper chromium, 2-copper chromite, 3-platinum, 4-copper aluminum and iron-aluminum, 5-silver manganate, 6-iron-chromium, 7-magnesium chromite, B-copper chromium with addition of PbO.

The rates of high conversion of hydrocarbons over simple metal and oxide catalysts were studied by few workers. Pigulevskii ( 4 3 ) has shown that the rate of propene combustion over V,O, was proportional to oxygen concentration. Reyerson and Swearingen ( 9 ) found that the rate of ethylene oxidation over platinum was directly proportional to oxygen concentration and inversely proportional to ethylene concentration. Elovich and Butyagin (12) investigated the high conversion of

CATALYTIC OXIDATION OF HYDROCARBONS

479

propene on platinum and found that at a moderate surface coverage with propene the rate of carbon dioxide formation was independent of oxygen concentration, while with extensive coverage it was independent both of propene and oxygen concentrations. Branson, Hanlon, and Smythe (161)have studied recently the oxidation of methane, ethane, propane, butane, and isobutane on copper oxide over a temperature range of 330-650" and found that a pseudoinduction period was observed almost for all hydrocarbons. The rate of the first oxidation step follows the equation

w=

kb

(U - X )

1 - b ( a -x)

where lc and b are constants. The rate of the second step obeys the Zeldovioh-Roginskii equation. Thus the formal kinetics of high conversion of hydrocarbons is primarily a function of molecular structure and is but slightly affected by the nature of catalysts (chromites and platinum). The greater the number of carbon atoms in a molecule the higher are the preexponential factor and the activation energy for high conversion. This regularity holds both for saturated and unsaturated, as well as for simple cyclic hydrocarbons. Change in the order of the kinetic equation as a function of the molecular structure of a hydrocarbon provides evidence for a rate-determining step that seems to be related to the nature of hydrocarbon radicals formed in adsorption, I n certain cases the rate-determining step is the chemisorption of oxygen. The above considerations on reaction kinetics seem to show that surface charging must exert an effect on the kinetics of catalytic and adsorption processes. Inhibition of the reaction rate with reaction products is one of the characteristic features of hydrocarbon oxidation. Oxygen-containing compounds, such as aldehydes, olefine oxides, etc., are strongly sorbed on various catalyst surfaces. Measurement of the electron work function in the adsorption of these compounds showed that all these were electron donors, like hydrocarbons, and were probably sorbed at the same surface sites, thus inhibiting the reaction rate.

V. Modified Catalysts The phenomenon called promotion was discovered long ago. This is the increase in catalytic activities of various semiconductors and metals by addition of certain impurities. Margolis and Todes (162) studied the effect of impurities on the rate

480

L. YA. MARGOLIS

of high conversion. They found that the main kinetic characteristics, i.e., the activation energy and the preexponential factor, change in the same direction depending upon concentration of impurities in the catalyst. This and the different effect of impurities on the catalytic activity induced Roginskii to discard the old definitions (“promotion,” “poisoning”) and to introduce the word ‘(modification” meaning a dual change in the cataIytic activity. Impurities added to a solid may either fill the lattice vacancies, or substitute certain ions in the lattice, or else become diluted in the solid. Adsorption of impurities on the solid surface or substitution of sodium

: :-

1 2 3 4 5 6 7 8 9

0.7

15%

r

Li,O

FIG.12. Variations in the electron work function (A+ in ev) various impurities (I’in atomic yo)in ZnO.

VB

the concentration of

or potassium ions for hydrogen ions (the so-called ion-exchange adsorption) are possible. Of especial interest are semiconducting systems with a part of atoms (ions) replaced by atoms (ions) of about the same dimensions, but of a different valence or charge. According to de Boer and Verwey (163, 164) considerable modifications of the electron properties of such systems may be obtained in this way. The electron theory of catalysis established the relationship between chemisorptive and catalytic activities of solids and the Fermi level position. The Fermi level of a solid will be shifted in different directions with respect to the conduction band, depending upon the nature of impurities added, i.e., upon their being electron donors or acceptors.

CATALYTIC OXIDATION OF HYDROCARBONS

48 1

The electrical conductivity of the solid would also vary as a function. of the impurity nature and the chemical effect of these must be different. However, experimental results show that the relationship between conductivity and catalytic activity is much more complex. This is probably due t o the fact that the conductivity o! polycrystalline semiconductors often is not affected by changes in the Fermi level of the surface. Thus there must be another connection between changes in electron work functions of modified catalysts and their adsorptive and catalytic activities. The effects exerted by surface impurities, and by impurities located in the adjacent-to-surface layer at a depth exceeding the Debye length, on electrical conductivity and the electron work function will be different.

201 --

-A

E

FIO. 13. Variations in the activation energies for chemisorption calculated in kcel/ mole from initial rates vs the electron work function in kcal; ZnO containing various impurities.

Zhabrova et al. (165) report that with a modified zinc oxide catalyst a lithium cation impurity, located at a depth having no effect on the electron work function, is an electron acceptor and raises the activation energy for conduction. The same impurity on the grain surface is an electron donor and decreases the electron work function. Thus the Fermi level shift on the surface of modified catalysts cannot be determined unambiguously from change in the electrical conductivity. According to Enikeev, Margolis, and Roginskii (166), lithium and thorium cations added to zinc, copper, and nickel oxides change the

482

L. Y A . MAROOLIS

electron work function, and this change is a function of their concentration (Fig. 12). I n other words, modification of semiconductors makes possible a wide range of variations in the electron work function (d4). On the other hand, the same workers (167) found that A+ was a linear function of the activation energy for chemisorption of oxygen (Fig. 13). The kinetics of the over-all process of catalytic hydrocarbon oxidation will be considerably affected by impurities. An important part in the catalytic oxidation of various compounds is played by the mobility of the surface-adsorbed oxygen; it may be determined from oxygen exchange. Margolis and Kisselev (168) have studied isotopic oxygen exchange on typical oxidation catalysts such as silver (catalyzing the oxidation of ethylene to ethylene oxide) in the presence of halides, and copper oxide (a catalyst for propene oxidation to acrolein) in the presence of lithium, chromium, and bismuth oxides, and of copper sulfate. The logarithmic dependence of variations in isotopic exchange with electron work functions are shown in Fig, 14. The exchange rate in.o

-0.5L

FIG.14. Variations in the rate logarithm (w,) for isotopic oxygen exchange VE varistions in the electron work function ( A $ in ev). Exchange on CuO; additions made at 412'.

creases with A$. This is indication that the surface concentration of oxygen is controlled by the electron work function. The addition of donor or acceptor impurities to CuO, V,O,, or Ag changes the activities of the latter substances, as well as the selectivity of reactions. To find out the nature of the effect of impurities on the selectivity of hydrocarbon oxidation reactions, Enikeev, Isaev and Margolis (102) attempted to find out the relationship between the electron work function of a modified catalyst and the reaction rates and activation

CATALYTIC OXIDATION O F HYDROCARBONS

483

energies for two reactions: the formation of acrolein from propene, and of ethylene oxide from ethylene and carbon dioxide. Three major steps are observed for these reactions W1

C,H,

-+

WI

acrolein -+

L

W8

CO,

CJ

To a first approximation, without allowing for variations in the true activation energy with surface charging, the rate of acrolein formation may be expressed as w1 = KolCo,exp

( - E o l +RT

-

A+

Here K O ,is the preexponential factor, C,, is the surface concentration of oxygen; Eol the true activation energy for acrolein formation; Q the heat of oxygen adsorption on CuaO; A+ the change in the electron work function with addition of impurities: ( + A + ) for acceptors, and ( - A + ) for donors. Variations in the rates of CO, formation from acrolein and propene with surface charging may be represented as:

w3 = KO3*CO, exp

It was necessary to determine experimentally the nature of E and K O variations with A + . The relationship between variations in the electron work function of a modified copper oxide and the activation energies for CO, and acrolein formation relative to and E values for pure cuprous oxide are shown in Fig. 15. An increase in the electron work function brings about a decrease in the activation energy for carbon dioxide formation. The variations in activation energies for acrolein and CO, formation are linearly dependent upon the electron work function

A compensation effect is observed for propene oxidation on modified catalysts, as well as for other catalytic reactions, The dependence of the preexponential factor K O on is shown in Fig. 15. For acrolein forma-

+

484

L. YA. MARGOLIS

tion E,, increases with decreasing 4; the increase in w is slight. This seems to be due to the compensation effect of K O . The dependence of two rates (wzand w3)on the surface potential is essential for CO, formation. The rate of its formation from acrolein is a function of acrolein concentration and must decrease with increasing 4. The rate ( w J of

FIQ. 15. Variations in activation energies (LIE kcal/mole) end in the preexponential factor logarithm (logKO) for the formation of acrolein and carbon dioxide in the oxidation of propene over copper catalysts with admixtures vs variations in the electron work function (44 in ev). 1-CuO; 2-CuO Fe,O,; 3-CuO Cr,O,; 4-CuO+ Li,O; 6-CuO Ci-; 6-CuO SO:-.

+

+

+

+

parallel CO formation from propene, bypassing acrolein, is proportional to C,, and must increase with 4. It was shown experimentally (Fig. 15) that the activation energy Eco, builds up with 4 and, consequently, the rate of acrolein oxidation controls the rate of CO, formation, The may be expressed as the wllw,ratio reaction selectivity (8)

The rate of acrolein formation (wl)increases with 4, and that for CO, (wz)falls down. Variations in the selectivity of acrolein synthesis as a function of those in the electron work function ( A $ ) are shown in

Fig. 16. Acceptor impurities added to CuO raise the selectivity by 15 to 20%, and the electron work function by 0.2 to 0.3 ev. To obtain a still greater increase in selectivity it would be necessary to investigate

CATALYTIC OXIDATION OF HYDROCARBONS

486

impurities capable of inducing a considerable increase in the electron work function. Extensive investigations on the effect of diverse impurities added to CuO were carried out by Margolis et al. (169). The effect of these substances was found to proceed by an electronic mechanism. The results obtained are summarized in Table 11. The elements are given in the order of increasing electronegativity . Those involving atoms with

-AS LO.I FIG. 16. Variations in the selectivity of propene oxidation ( A S )over CuO vs variations 3-CuO+ Cr,O,; P C u O in the electron work function. d-$/ev. 1-CuO; 2-Cu0+Fe,O3; Li,O; 5-CuO C1-; 6-CuO SO:-.

+

+

+

an electronegativity lower than that of CuO decrease the electron work function, and those with E exceeding that of CuO increase it (see Table XI). The qualitative relationship between electronegativity values for impurity atoms, and the increase or decrease in the electron work function of a semiconductor reveals that the solid solutions or . microheterogeneous systems formed exert a prevailing effect on L I ~It was established by electron diffraction of a number of systems that the impurities added form a separate phase on the CuO surface. This is indication that the effect of microheterogeneous inclusions on the electronic properties of the surface is predominant. Modifying additions increasing the A $ of CuO raise, and those decreasing it lower the selectivity of oxidation reactions. The electronic mechanism of semiconductor modifications associating variations in activation energy with A $ was investigated by Hauffe ($70)’ Vol’kenstein (274,and Roginskii (272). The oxidation of ethylene to ethylene oxide is a typical process occurring by a parallel scheme. Kummer (173) reports that A 4 is increased by adsorption of oxygen on silver. A similar effect is exerted on C$ by acceptor impurities C1, I , S, Se, etc. Similar variations in $ for silver catalysts were observed by Wilson

486

L. YA. MAROOLIB

et al. (174) upon addition of metalloids, such as chlorine, sulfur, and phosphorus. with adsorption of the components of ethylene The changes in oxidation are indication that on the catalyst surface these substances possess a charge. As the electron work function is related to the heat of adsorption, it may be supposed that with modified silver it will increase for acceptor gases (02,CO,) and decrease for donors (ethylene). It may be seen from results on isotopic exchange on silver samples modified with ion-chlorine, that with increased chlorine concentration in Ag (increased A + ) the amount of surface oxygen will fall off. Kinetic studies of oxidation over silver revealed discrepances in the data obtained by various scientists (Table IX). The rates of C,H40 and CO formation are proportional to oxygen and ethylene concentrations, but the reaction order with respect to these components varies from zero to one. The dependence of reaction rates on C,H4 and 0, concentrations is expressed more often than not in fractional powers. Temkin et al. (158) consider that this diversity in kinetic laws is due to varying oxygen content in the adjacent-to-surface layer. The dependence of partial surface concentrations of 0, and C2H4on variations in 4 of the catalyst seems to explain the unsteady nature of reaction kinetics. It will be expected, moreover, that addition of acceptor impurities will raise the reaction order for oxygen and decrease it for ethylene. The zero reaction order with respect to oxygen in the formation of C,H40 and CO,, as found by Temkin et al., may apparently be explained as follows. The electron work function is related to oxygen pressure A+ = ylogC,, Substituting this value into the equation for the rate of ethylene oxide formation, we obtain

+

,

w ~ , =~KolCo, , ~ exp

RT

Then

r

Y

=

Y

RT

When y' = 1 the reaction rate will be independent of oxygen concentration. It is also necessary to take into account the effect of reaction products on kinetics of the reaction. The rate of ethylene oxide formation is known to be inhibited both by ethylene oxide and CO,. The measured shifts of C$ with adsorption of these substances on silver have shown

TABLE XI Changes in the Electron Work Fu7aetwn of Mali$& Coppw O d d 8 and in the Selectivity of Propene Ozidotion to Acrolein

wu 5

0

Elements

Ba

Electronegstivity 0.85 Changes in the electron work function: (mv) in vacuum in 6 mixture of C,H, 0, ( 1 : l ) - 120 Changes in the selectivity of propene oxidation to acrolein -10

+

Li

Cr

Pb

Fe

Mo

CuO

Bi

P

S

1.0

1.6

1.8

1.8

1.9

2.0

2.0

2.1

2.5

3.0

- 150

-400

-150

+400

+300

+lo0

4

-5

-3

-3

+5

+20

+24

3U TI

- 400 -12

- 70 41

0

0

+300 +3

Modified CuO samples were reduced to cuprous oxide during the catalytic process. Pure CuO and Cu,O samples differed in the electron work function but slightly. Thus Ad brought about by modification should be comparable. The E-value for metal copper was taken as the CuO electronegativity.

H

d

C

0

2

488

L. YA. MARBOLIS

that at low temperatures C,H,O is a donor, and at high temperatures an acceptor of electrons. The shift of d, in the adsorption of C,H40 is greater than with carbon dioxide. Inhibition of the reaction by its products cannot be explained solely by blocking off of the surface, the change in with adsorption should also be taken into account. Water does not considerably decrease at the temperature of reaction and probably only blocks off certain surface sites. The effect of C,H40 and C 0 8 on the electron work function is similar to that of oxygen and metalloid impurities. Thus in the presence of these substances the surface activity would be expected t o fall off and the reaction selectivity to raise. However, the effect of these products on is less marked than that of metalloids and, consequently, the selectivity would scarcely be considerably increased by admission of C O , into the reactant gas. The presence of sulfur impurities in silver results in an increased ethylene oxide yield; further increase in sulfur concentration would lead to poisoning of the catalyst. Roginskii et al. (167)found that the same was observed for silver samples modified with chlorine. The work function of sulfur-containing samples, as well as of those containing chlorine, will increase. The activity maximum is controlled by the ratio of ethylene to oxygen surface concentrations. The reaction order with respect to ethylene will decrease with increasing sulfur concentration, i.e., with +, while that for oxygen will buiId up. At a certain value the rate to1 = K,,C$C&, will be maximum, when n = m. This is apparently the reaaon why silver is modified with sulfur. When chlorine is added to silver, no increase in the ethylene oxide yield is observed, as “pure” silver involves ion-chlorine8 in excess over the optimum amount. It was shown above that the activation energy is related to +. There is almost no information available on as a function of activation energies and rates of catalytic reactions over silver. According to Hayes (130)the activation energy for N,O decomposition on alloys of silver with various +decreasing metals will be low. Sosnovsky (175)has investigated the catalytic activities ( E and K O )for different planes of silver crystals, with respect to the decomposition of formic acid. E and K O were found to increase with plane indexes. The relation between and the rate of ethylene oxidation to ethylene oxide waa not established. With modified silver the activation energy for ethylene oxidation to ethylene oxide will not suffer considerable changes. This may be explained by increased concentration of donor molecules compensating the change in under the action of metalloids. The differences in activation energy values for ethylene oxidation, as reported by various

+

+

+

+

+

CATALYTIC OXIDATION OF HYDROCARBONS

489

scientists (Table IX) did not exceed 4 kcal/mole. Different reaction rates observed upon modification are probably due to changes in surface concentrations of the reaction components. The reaction selectivity (8) is determined by the ratio of C,H,O (wl) to CO, (wz) formation rates. The relationship between the electron work function and the over-all activity and selectivity of oxidation may be seen from Fig. 17. The A S 100

r

1

0

I

0.I

I

0.2

I

0.3

tA4

FIG. 17. Variations in the activity ( A ) and selectivity (8)of ethylene oxidation to ethylene oxide vs variations in the electron work function (A+ in ev).

activity of silver decreases and the reaction selectivity increases with increasing electron work function. Changes in reaction rates with the electron work function are different for low and high conversions. is proportional to oxygen concenThe rate of C,H,O formation (wl) tration to the power of 0.4-0.7, and that of CO, formation t o the power of 1.1. Thus the changes in w,and w,with increasing work function will be different. The higher is the reaction order with respect to oxygen, the greater will be the decrease in reaction rate with 4. Higher 4 results in greater selectivity of the reaction. Analysis of data available seems to reveal that the modification of silver by addition of acceptor impurities proceeds by an electronic mechanism. As no experimental data on the effect of alkali and earth alkali impurities on electrical and catalytical properties of silver were available, corresponding studies were undertaken with a silver catalyst. Elements of the 1st and IInd groups of the Periodical System of Elements decrease, and those of the VIth and VIIth groups usually increase the electron work function for metals. The mechanism of 4 variations with adsorption of atoms and molecules of various substances on metal surfaces is due to formation of a double electrical layer

490

L. YA. MAROOLIS

accounted for by dipoles of adsorbed particles (121).The relationship values for metals, the ionization potential, and electron between affinity of adsorbed atoms was reported by Mikhailov et al. (176).With a great number of elements added the electronegativity ( E ) would be a more convenient value than the ionization potential or the electron affinity for investigating in metals. Experimental data on variations, and also on the selectivity of silver in the presence of various impurities are summarized in Table XII. Alkali and earth-alkali metal impurities decrease, while metalloids increase the electron work function for silver. The elements studied are given in the order of increasing electronegativity. I n the presence of elements with a n electronegativity lower than that of silver the value falls off, while high-electronegativity elements raise it. This law holds for measured in vacuum and in a hydrocarbon-oxygen mixture. Only the sign of change in the electron work function dependent on c should be taken into account. I n many cases the decrease in heat of adsorption with greater coverage may be accounted for by interaction of dipoles in the adsorbed layer. Consequently, the effect of impurities on the reaction rate may be explained as follows. The impurities localized on the silver surface and displaying an electronegativity higher than that of silver, thus increasing the work function, will form dipoles with the negative charge outwards. Measurement of in the adsorption of oxygen and ethylene has shown that such is the case for oxygen, while for ethylene outwards is the positive charge. As a result of an electrostatic reaction of 0, and C,H, dipoles with a metalloid, the heats of adsorption become lower and, consequently, there is a decrease in the surface coverage with oxygen; the reverse is observed for coverage with ethylene. The probability of complete combustion of an ethylene molecule will be considerably lower in this case. Modification of V,O, was investigated for propene oxidation t o saturated aldehydes, acrolein, etc. The impurities added to V,O, may be classified as falling into two groups: acid (metalloid) anions of SO,, P,O,, and other alkali cations such as Na, K, etc. The additions of SO:- results in a sharp rise in the activation energy for the generation of reaction products, while in the presence of K and Na the activation energy for the formation of acrolein, aldehydes, and acids falls off. Sodium was found to decrease and acids to increase the electron work function. Activation energies for the formation of acrolein, acetaldehyde, acid, and carbon dioxide change with A + . A characteristic feature of hydrocarbon oxidation over Cu ,O, V,O,,

+

+ +

+

+

+

d

Elements

'

Electronegativity Changes in the electron work function (mv) in vacuum (20") in a mixture of C,H, 0,(1:l) ( 1800)

+

K

BE

Na

Ca

Be

Ag

Mo

Bi

S

J

C1

0,

0.8

0.85

0.9

1.0

1.5

1.8

1.9

2.0

2.5

2.6

3.0

3.5

-50

-80

-300

-130

-70

0

+200

+lo0

+200

+300

+600

+300

-100

-60

-40

0

-3

0

+300

Changes in the selectivity of

ethylene oxidation 60 ethylene oxide

14

-3

-2

-11

+3

Poisoning

+15

+18

+23

0 M

492

L. YA. MARQOLIS

and Ag is the possibility of suppressing complete combustion by increasing the electron work function. Thus choice can be made of substances changing the reaction selectivity. It is apparent from qualitative data that alkali cations, such as Na, K, Ba that lower the work function, increase the rate of high conversion to COa and HaO and decrease the selectivity. All acid impurities, such as C1, I, Br, SO,, PO,, etc., cause an increase in the electron work function, a decrease in the rate of CO, formation, and a rise in selectivity, but also exert an effect on the formation of oxygen-containing products, such as ethylene oxide, acrolein, acetaldehyde. Thus high activity and selectivity would be ensured only at a strict optimum concentration of the compound added. Complex semiconducting spinels of a defect structure are catalysts for high-degree oxidation of hydrocarbons. Transfer of charge in the spinel lattice seems to occur by a relay mechanism and, consequently, of importance is the spacing of cations that are responsible for charge transfer. Anyway the electrical and catalytic properties of these compounds would be changed only by addition of considerable amounts of other substances. This is indication that the collective effects observed for simple semiconductorswill be negligible with the above systems. Notwithstanding the great amount of work concerned with the effect of modifying impurities, there is no information on their behavior in catalysis. Stepanov, Margolis, and Roginskii (177) used the radioactive tracer technique in studying the mobility of impurities in metals. Labeled chlorine ( C P ) , iodine (P), and sulfur (SsS)were added to silver, which was then heated in a flow of various gases. It was shown that in a reducing medium the concentration of modifying impurities rapidly falls off. Reduction of silver halide in ethylene and hydrogen yielding hydrogen halide detected in exit gases occurs at the catalyst surface. As a result of similar experiments with a single crystal of silver containing C P as KCP, it was established that chlorine does not diffuse over the silver lattice at 300'. The same conclusion was drawn by Mikulski and Werber (178) in studying the diffusion of sulfur in silver at 400'. A great amount of work was reported in literature on the effect of various organic halides on the selectivity of ethylene oxidation to ethylene oxide (123), but the mechanism of this effect was not studied. Organic impurities may sorb on the catalyst surface and react with oxygen in the gas phase, or else decompose. Stepanov et at?. (177)found that the rate of oxidation of metalloid-

CATALYTIC OXIDATION OF HYDROCARBONS

493

containing organic impurities over silver is a function of the molecular structure of these compounds. The sequence of processes occurring on a catalyst containing a metalloid impurity follows the scheme RHsl

4

reduction Ag +------

Ag

+ AgHal+

CO,

+ H,O

adsorption

!CHd

oxidation

4% The difference in activation energies for the oxidation of organic halides (Box= 4.6 kcal/mole) and the reduction of AgCl (Bred= 15 kcal/mole) is indication that the rates of oxidation and reduction may vary for different silver samples, depending upon the ion-chlorine wntent of the surface. Changes in the ratio of these processes accounted for by different temperature and the structure of organic volatile compounds result in different distribution of impurities over the catalyst layer, and this may exert a marked effect on the selectivity.

VI. Mixed Catalysts Various solid mixtures that are either converted into solid solutions or remain polyphase are used, along with simple and complex semiconducting oxides, for conducting different reactions. Information was reported by Rienacker (l79),and in patent literature, on the catalytic activity and the selectivity of mixed catalysts with respect to dehydrogenation of alcohols, oxidation of hydrocarbons, etc. More recent work on the activity of such catalysts (180,181) revealed that the change in specific surfaces with increasing content of one metal oxide in the other is not additive. Mixtures involving oxides of molybdenum and vanadium, molybdenum and cobalt, iron and chromium, etc., were used as catalysts for hydrocarbon oxidation. The concentration of defects may increase a t the interphase boundary during the preparation of mixed catalysts, and these would then display higher catalytic activity. Frequently the changes in catalytic activity of mixed contacts are due to formation of spinels. The effect of a polyphase system of this kind, consisting of metal oxide spinels (often as solid solutions), on the rates of various reactions is a complex problem. Various mixed catalysts were used for the catalytic oxidation of hydrocarbons. Variations in activity and selectivity of benzene oxida-

494

L. YA. MARQOLIS

tion to maleic anhydride as a function of the composition of vanadiummolybdenum catalysts, as established by Ioffe et al. (180)are shown in Fig. 18. The activity maximum coincides with the limit on the dissolution of MOO, in V,O,. The effect of MOO, dissolved in V,O, is suggested to result from the appearance of defects in the V,O, lattice. Voevodskii et al. (182) have shown by ESR studies that for samples with low MOO, content (up to 30%) V4+ and Mae+ ions form solid solutions in the V,O, lattice. Compounds containing four-valent vanadium ions are formed a t a considerable MOO, content.

MO0 3

"2 '5

FIG. 18. Selectivity of benzene oxidation to maleic anhydride as a function of the composition in etomic yo of vanadium-molybdenum catalysts. I-over-all conversion of C,H,; 2-conversion in ?/, of C,H, to C4H8O4.

Mixed catalysts are complex polyphase systems, the electron work function of which may vary either a t the interphase boundaries or due to formation of solid solutions by substitution and incorporation. The effect of these catalysts on high conversion will differ with preparation conditions. The activation energies for the formation of saturated aldehydes, acids, and CO, decrease with propene oxidation on a catalyst MOO, solution. consisting of a solid V,O, With a mixed molybdenum vanadium catalyst the selectivity of acrolein formation remains the same, while decreasing on a solid solution of these oxides. The formation of oxygen-containing products (acrolein plus saturated hydrocarbons and oxides) is more selective in the oxidation of propene over a mixed catalyst.

+

CATALYTIC OXIDATION OF HYDROCARBONS

495

at. % Bi at. 'lo Mo;W

A4

0.5

0.3

Mo

IOOat. X Bi

0.20.3 0.4 0.1

\

0.4

(b) Fro. 19. (a)The selectivity of propene oxidation to acrolein as a function of the composition of bismuth-molybdenumand bismuth-tungstencatalysts. (b) Variations in the electron work function (A+ in ev) vs the composition of mixed bismuth-molybdenumand biemuth-tungsten Catalysts.

496

L. YA. MARGOLIS

It was shown before by Margolis et al. (51) that propene will suffer oxidation over molybdenum oxides at a high temperature (560') yielding acrolein, carbon dioxide, and water. Variations in the selectivity of propene oxidation as a function of the catalyst composition are shown in Fig. 19a and b. If the suggested electronicmechanism of the action of mixed catalysts is true, the electron work function (4) of mixtures should be higher than that of pure molybdenum and bismuth oxides. The dependence of A# on the composition of a molybdenum-bismuth catalyst is shown in Fig. 19b. The maximum change in the electron work function corresponds to highest selectivity. Such a proportional change in catalytic and electronic properties seems to provide evidence for the electronic mechanism of the effect of these mixed catalysts. Tungsten yields compounds displaying chemical properties approaching those of molybdenum. Like molybdenum, WO, is a mild catalyst for low conversion of hydrocarbons. Variations in electron work functions of mixed tungsten-bismuth catalysts of various composition are shown in Fig. 19, also. A relationship was observed between selectivity and the electronic properties of these catalysts. * According to Rienacker (179) either the electron mechanism of the effect of mixed catalysts, or a change in the strength of oxygen bonds in metal oxides are possible. Under the assumption that the oxygen bond strength is important for catalysis, the oxidation-reduction mechanism will have to be accepted. However, data were given above (Section 11, E, 3) showing the invalidity of this scheme. Consequently, the changes in reactiop rates in the presence of mixed catalysts are probably due to changes in the amount and nature of the defeots at the phase boundary, Further experimental inveatigatiov of the relationship between wtalytio aptivity and electronjp propsrties of mixed catqlyets would Qoptrfbutato the duoidation ~f the e#e& of bhse sptems on oatal$rt;te

reactlone.

REF~EREYCEO 1: pavy, H., Phil. Tram. Roy. Soc. (1817). 2. F o b , S . A., Zhur. R u e . F k-Khim . Obahchlva 40,216 (1908). 3. Philips. O.,J . Am. Ohm. Soc. 16, 104,266 (1894). *New reauh were obtained reoently by the author of this paper. Theee will be published in the near future.

CATALYTIU OXIDATION OF HYDROCARBONS

497

4. Marek, L. F., and Hahn, D. A., “The Catalytic Oxidation of Organic Compounds 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 16. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 310. 32. 33. 34. 35. 36. 37. 38. 39, 40. 41. 42. 43.

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