Oxidation of 2-Methylpropene over Copper Oxide Catalysts in the

46 Oxidation of 2-Methylpropene over Copper Oxide Catalysts in the Presence of Selenium Dioxide

Downloaded by CORNELL UNIV on September 24, 2016 | http://pubs.acs.org Publication Date: January 1, 1968 | doi: 10.1021/ba-1968-0076.ch046

R. S. MANN and K. C. YAO Department of Chemical Engineering, University of Ottawa, Ottawa, Canada

The air oxidation of 2-methylpropene to methacrolein was investigated at atmospheric pressure and temperatures ranging between 200° and 460°C. over pumice-supported copper oxide catalyst in the presence of selenium dioxide in an integral isothermal flow reactor. The reaction products were analyzed quantitatively by gas chromatography, and the effects of several process variables on conversion and yield were determined. The experimental results are explained by the electron theory of catalysis on semiconductors, and a reaction mechanism is proposed. It is postured that while at low selenium-copper ratios, the rate-determining step in the oxidation of 2-methylpropene to methacrolein is a p-type, it is n-type at higher ratios.

*nr«he selective air oxidation of 2-methylpropene (isobutene) to methacrolein (methacryl aldehyde) is of considerable importance. Though several patents ( J , 2, 4, 6, 17) have appeared during the last 20 years describing the use of various metal oxides i n the partial oxidation of 2-methylpropene, kinetic studies have been reported only recently by M a n n and Rouleau (14, 15) and Shapovalova (18). Kruzhalov et al. (13) and Kominami et al. (12) studied the air oxidation of propylene to acrolein over C u S e 0 , supported on activated alumina and silica gel. They suggested that selenium assisted the rupture of the chain reaction for the decomposition of the allyl hydroperoxide radical or as a promoter for the active centers of the catalyst during the catalytic reaction. M a r golis et al. (16) studied the effects of several additives upon the catalytic activity and selectivity of metals and semiconductors on the oxidation of 3

276

Mayo; Oxidation of Organic Compounds Advances in Chemistry; American Chemical Society: Washington, DC, 1968.


46.

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2-Methylpwpene over Copper Oxide

277

propylene and ethylene. They found that the additives having an electro negative value, E, greater than that of the catalyst decreased the activity of the catalyst and increased the work function and selectivity of the catalyst for the oxidation of ethylene and propylene. Hadley (7-10) also claimed that adding selenium to an oxidation catalyst increased its life and selectivity in the oxidation of olefins to unsaturated aldehydes. H o w ever, no mention is made in the literature of the selectivity and optimal amounts of selenium dioxide that can be used to oxidize 2-methylpropene to methacrolein under different operating conditions. This paper reports the effect of various amounts of selenium dioxide under different operating conditions on the conversion of 2-methylpropene to methacrolein and proposes a hypothesis for the hydrocarbon oxidation, which explains particularly the reactivity and selectivity of seleniumcopper oxide catalysts in oxidizing 2-methylpropene. Experimental The partial air oxidation of 2-methylpropene to methacrolein i n a constant and continuous supply of selenium dioxide was investigated i n an isothermal integral flow reactor, constructed of 316 stainless steel. The schematic diagram of the apparatus used to study the reaction is shown i n Figure 1.

Figure 1. Bi ice trap B>, liquid nitrogen trap C C>, temperature controllers D,, D>, drying tubes Ft, F», furnaces GP, gas partitioner GM, wet-gas meter M Μ·, Ms, M manometers y

h

h

h

Schematic of the apparatus ? F , P , pressure gages R, reactor Ri, R>, R , rotameters S, sampling valve Ti, air tank T 2-methylpropene tank V, selenium dioxide vaporizer VP, vapor fractometer ly

2

s

3

h



278

OXIDATION OF

ORGANIC COMPOUNDS

II

The flow rates of the reactants, air, and 2-methylpropene, were measured by Brooks rotameters. The reactants were mixed well before they entered the preheating section. T w o streams of air were used. While one stream passed through the selenium dioxide vaporizer and carried a certain amount of selenium dioxide vapor with it, the other stream bypassed it. The reactor was made of stainless steel tubing 6 inches long and 0.5 inch in outside diameter. The gases entered the reactor at the bottom through a porous stainless steel plate, which served as a support for the catalyst. The preheating section was made of tubing 6 feet long and 1/8 inch in outside diameter, long enough to preheat the gases entering the reactor to the desired reaction temperature. The reactor with the preheating section was immersed in a constant tempera ture liquid metal bath, consisting of a mixture of 50% bismuth and 50% lead by weight. The metal mixture was heated by an electric furnace, the temperature of which was controlled to within ± 3 ° C . by a Honeywell Pyrovane temperature controller. The flow rate of the inlet air to the selenium vaporizer was kept constant, and different amounts of selenium dioxide vapor carried by air were obtained by adjusting the temperature of the vaporizer. Thus, selenium and its compounds (oxides) were uniformly distributed over the thin layer of catalyst bed. Since the catalyst was present i n small amounts, selenium retained by the catalyst could not be determined accurately. However, most of it was recovered from the product stream by condensation. A n air condenser was installed i n the exit line of the reactor to remove the condensed selenium, thereby ensuring that selenium d i d not pollute the air. The exit gases from the reactor were first led to an air condenser to remove condensed selenium dioxide and then through an ice-cooled trap, where acids, water, and a small portion of the aldehyde condensed. The uncondensed gases were passed through a gas-sampling valve, leading to a Fisher gas partitioner, containing a hexamethylphosphoramide ( H M P A ) column and a 13 X molecular sieve column connected in series, which could determine carbon dioxide, carbon monoxide, nitrogen, oxygen, and 2-methylpropene. The off-gases from the sampling valve passed through a liquid nitrogen trap, where the remainder of the aldehydes which d i d not condense at ice temperature were condensed. The condensate, mixed with a known amount of diethyl ether, was injected with a syringe into a Perkin-Elmer Vapor Fractometer Model 154D, containing a Carbowax 1500 on Teflon column, which could separate several saturated and un saturated aldehydes and water. The pumice-supported copper oxide catalyst containing 16 weight % of copper was prepared by impregnating 20 to 40-mesh crushed pùmice stone with a copper nitrate solution and drying it at 105 °C. for 6 hours The dried catalyst was subsequently calcined at 600°C. for 6 hours and placed i n the reactor. The catalyst was activated by passing air over it for 12 hours before any experimental run was made. Results and Discussion The effect of various variables was investigated: weight ratio of selenium i n feed to copper i n the supported catalyst ( Z = 0.0067 to


46.

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2-Methylpropene over Copper Oxide

279


0.0422), oxygen ( i n the air)-2-methylpropene ratio i n the feed ( R = 0.7168 to 1.6644), and operating temperature ( Γ = 350° to 4 2 5 ° C ) , on the conversion of 2-methylpropene ( X ) , rates of formation of various products, carbon dioxide, water, and methacrolein ( Y ) , and selectivity (S) for methacrolein. While conversion is referred to as the moles of 2-methylpropene reacting (or consumed) per hour to the moles of 2-methylpropene fed per hour, the rate of formation is referred to as the moles of various products formed per hour per gram of the catalyst. Yield of methacrolein is defined as the ratio of moles of methacrolein produced per hour to the total moles of product formed per hour, and the selectivity for methacro lein formation is referred to as the moles of methacrolein produced per mole of 2-methylpropene reacting. The weights of the catalyst and 2-methylpropene charged into the feed were maintained constant during the runs. A different feed compo sition ratio was obtained by adjusting the rate of air flow. Figure 2 shows the effect of Ζ on the conversion, rates of formation, and selectivity for a W/F (reciprocal of space velocity) ratio of 1.68 and oxygen-2-methylpropene ratio, R, of 0.7168 at 425°C. The conver sion of 2-methylpropene increased rapidly with the increased amounts of selenium dioxide up to Ζ = 0.0067 and then decreased slightly with further increased amounts of selenium dioxide. Though the rate of car bon dioxide formation decreased slowly i n the beginning, it decreased rapidly with the increased amounts of selenium dioxide up to Ζ = 0.020, beyond which there was no substantial decrease i n the rate with increas ing amounts of selenium dioxide. The rate of water formation increased slowly with increasing amounts of selenium dioxide up to Ζ = 0.0067, and then decreased. The rate of methacrolein formation increased steadily with increasing amounts of selenium dioxide up to Ζ = 0.020, then de creased slightly. The selectivity for methacrolein first increased rapidly with increased amounts of selenium dioxide, and then decreased. The optimal amount of selenium dioxide giving the highest selectivity was about Ζ = 0.03 (corresponding to about 0.7% by weight of the pumice-supported cata lyst) under different operating conditions. The effect of various oxygen-2-methylpropene ratios i n the feed ( R ) on the conversion, rate of formation, and selectivity for Ζ = 0.02 at 425 °C. is shown i n Figure 3. W h i l e the conversion of 2-methylpropene and rate of formation of methacrolein increased steadily with feed ratios, the rates of formation of water and carbon dioxide increased rapidly with increasing R. However, the selectivity decreased with increased oxygen2-methylpropene ratios i n the feed.



280

OXIDATION O F O R G A N I C C O M P O U N D S

II

GM- SELENIUM / GM-COPPER Figure 2.

Effect of selenium dioxide on conversion, rate of forma tion, and selectivity

The effect of temperature on conversion and selectivity for R = 0.9910 and Ζ = 0.020 is shown i n Figure 4. W i t h increase in temperature from 350° to 4 2 5 ° C , although the conversion increased, the selectivity remained nearly constant. Bretton, W a n , and Dodge (3) and several other workers (19) have treated heterogeneous oxidation of hydrocarbons as a homogeneous reac tion, neglecting the effect of solid catalysts. They suggest that the oxidation reaction involves a free radical mechanism (22) i n which the first step is the removal of a hydrogen atom from the hydrocarbon, forming a free radical, which then can react with a molecule of oxygen


46.

M A N N A N D Y AO


281


to form a peroxide radical. The radical then gains a hydrogen atom, becoming a peroxide, which later decomposes. This mechanism does not take into consideration the physical properties of the solids and the surface effects. W e therefore prefer to use the electron theory of catalysis (21), and modify it accordingly to explain our results.

Figure 3.

Effect of feed composition on conversion, rate of formation, and selectivity

W e treat the free electrons or positive holes on the surface as one of the reactants or products and propose that a surface reaction between charged adsorbed particles, both reactants and products, is the essential



282

OXIDATION OF ORGANIC COMPOUNDS

II

step, and that the heterogeneous catalytic reactions traditionally described as taking place: (1) diffusion (external and internal) of reactants and products. (2) adsorption and desorption of reactants and products (physical adsorption and weak chemisorption ), (3) surface reaction be modified to take place as follows: (1) electron transfer from or to the reactants (strong chemical band formation) (2) homogeneous reaction (formation of activated complex and rearrangement of the charged particles on the surface) (3) Electron transfer from or to the products.

350

400

375

425

TEMPERATURE Figure 4,

Effect of temperature on conversion and selectivity

The formation of methacrolein by the partial oxidation of 2-methyl propene, based on the above modification, can be visualized to take place according to the following scheme. Reaction I: Partial Oxidation C H 4

0

2

8

(g) ^ ( C H ) * + pL ^ ( C H * ) * 4

8

(g) ^ ( 0 ) * + 2eL^ 2

4

(2 0 - ) *

8

(1) p-type (2) n-type


46.

(activated complex) ?± (C H O )* + (H 0 )*

( C H ) * + (2 Ο")* 4

8

283


M A N N A N D YAO

+

4

+

e

(3)

+

2

( C H 0 ) * + eh τ± ( C H 0 ) * ^ C H 0 (g)

(4) n-type

( H 0 ) * + eh τ± ( H 0 ) * ^ H 0 (g)

(5) n-type

4

+

6

4

+

2

6

4

2

Over-all Reaction: C H 4

6

2

+ 0 ^± C H 0 + H 0

8

2

4

6

(6)

2

Reaction II: (Further Oxidation) (activated complex) ( C H 0 ) * + (10 O-)* ^± (4 C 0 ) * + (3 H 0 ) *

(7)

( C Q ) * + eh ?± ( C 0 ) * *± C 0 (g)

(8)

( H 0 ) * + eh ?± ( H 0 ) * ç± H 0 (g)

(9)


4

+

6

2

2

+

2

+

2

2

+

2

2

2

Over-all Reaction: C H 0 + 5 0 ^ 4 C 0 4

+

6

2

+ 3H 0

2

(10)

2

Reaction III: Complete Oxidation (activated complex) ( C H ) * + (12 Ο")* (4 C 0 ) * + (4 H 0 ) *

(11)

( C 0 ) * + eh ^ ( C 0 ) * ^± C 0 (g)

(12)

( H 0 ) * + eh ^ ( H 0 ) * ^ H 0 (g)

(13)

4

8

2

2

+

2

+

2

+

4

8

2

+

2

2

Over-all Reaction: C H

+

2

+ 6 0 ^ 4 C0 + 4 H 0 2

2

(14)

2

Each step in the above reaction may contribute a certain resistance. The over-all rate is usually determined by the so-called rate-controlling step. The sign of electrical charge on each adsorbed particle is deter mined by measuring work function changes. Reactions I, II, and III can also be expressed in terms of the "power rate l a w " as follows: m

1

f\=

ra

2

= h CC

4

H O 6

m ^C4H

+

3

8

+

n C --

*

0

n • C o -

^l' C

C 4

0

H60

+

' C

ra ' k' C 2

C 0 2

' C

(15)

+

0

H

2

(16)

0

n'

3

+

2

2

+ +

ra ' ^3' Cco2

x

H

n'

2

2

ns C --

n

m{

x

*1 Cc4H 8 + '

3

* C

H 2

o

(17)

+

Here the evaluation of the surface concentration of charged particles during the reaction is important for investigating the conversion and selectivity i n the heterogeneous catalytic reaction. Using a treatment similar to Hauffe's (11), the concentration of charged particles on the surface can be evaluated at equilibrium as: C C

C H 4

+ 8

+

—_

K

Γ - Ρ

C H 4

8

.— f~ Ρ —Jtf

M^Hs

2 ? r g 2

β Χ

(CC4H *) 1 8

^(H)

2

J V((+H) )


n

8

x

284

OXIDATION O F O R G A N I C C O M P O U N D S

II

and Co- = Κ

·Ρ

θ2

θ 2

- · exp { 2 sinh-

}

^ (19)

The surface concentration of 2-methylpropene and oxygen ions is not a function of F C H and ? o only but also depends on the positive hole concentration i n the valence band of the bulk catalyst, ?/( ) . A n i n crease i n the positive hole concentration [r/(+) ] w i l l therefore increase C H and decrease C - . 4

8

2

1 / 2

+

(H)

(H)


C 4

8

+

0

Extending the definition of η-type and p-type reactions, as defined by Vol'kenshtein (21) to the electron transfer step, it would seem that the only reaction given by Equation 1 is a p-type reaction. This reaction would be accelerated by the increase i n the value of free hole concentra tion. O n the other hand, all other reactions besides the one given b y Equation 1 are η-type and would be accelerated by the increase i n free electron concentration. Hydrocarbon oxidation reactions catalyzed b y solid oxides are accompanied by oxidation and reduction of the catalyst and the degree of the stoichiometric disturbance i n the semiconductor changes. The catalytic process i n the oxidation of 2-methylpropene over copper oxide catalyst i n the presence of S e 0 can be visualized as: 2

CuO

+ slight amount of C u 0

activated in air

2

C,H

Se0

2

> CuO

A

->Se

(21)

Se + 4 CuO ^± S e 0 + 2 C u 0 (stoichiometric) 2

C u 0 (stoichiometric)

(20)

2

(22)

diffusion of small amount of Oo

2

> C u 0 (nonstoichiometric) (23) 2

excess Oo

Cu 0 >CuO (24) A certain amount of selenium may be considered as an acceptor impurity to copper oxide since the Fermi level of copper oxide catalyst is lowered or its p-typeness is increased. This agrees with the observation of Margolis (16). 2

M a n n and Rouleau (14) and Shapovalova et al. (18) studied the oxidation of 2-methylpropene to methacrolein i n the absence of selenium dioxide, and they found that the oxidation of 2-methylpropene was a surface reaction controlling between adsorbed 2-methylpropene and weakly adsorbed oxygen. However, no theoretical explanation is given for this.



46.

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285

W h e n 2-methylpropene ( a donor-type gas ) is adsorbed on the surface of cuprous oxide (a p-type semiconductor), because of the electropositive nature of 2-methylpropene, electrons from it flow to the catalyst surface, and pass through the acceptor level. Since the acceptance of these elec trons is limited, an exhaustion boundary layer is formed, and the chemisorption of 2-methylpropene ceases at low coverage and far from equi librium. O n the other hand, when oxygen (an acceptor-type gas) is adsorbed on the surface, because of its electronegative nature, electrons flow from the catalyst to it and are removed from the valence band, creating free holes. Since the supply of these electrons from the valence band is great, the concentration of free holes is increased greatly, and an inundation boundary layer is formed. Chemisorption of oxygen on the surface is fast and proceeds easily, forming nearly a complete monolayer of it on the surface. The adsorption of oxygen on the surface of the catalyst is thus relatively much faster than the adsorption of 2-methyl propene on it. The adsorption of 2-methylpropene can, therefore, be considered as a next slower step than the surface reaction. A n y increase in the adsorption rate of 2-methylpropene would, therefore, result in an increase in the over-all Reaction I (Equation 6). A t this moment, the decrease in the available electrons in the valence band is still compara tively negligible in its effect on the concentration of holes, and the de creases i n oxygen adsorption and product desorption rates are insignifi cant. Therefore, while 2-methylpropene conversion ( Reaction I ) increases rapidly at the beginning of the introduction of selenium dioxide ( Z < 0.0067), the rate of carbon dioxide formation is influenced slightly. This is i n agreement with the findings of Voge et al. (20) and Enikeev et al. (5), who observed that in propylene oxidation, at temperatures higher than 3 5 0 ° C , carbon dioxide was mainly produced from Reaction II (Equation 10) and not from Reaction III (Equation 14). Introducing a slightly larger amount of selenium dioxide ( Z = 0.0067 to 0.02) decreases the available electrons in the valence band appreciably, resulting in the slowing down of the rate of oxygen as the rate-determin ing step for Reaction II. The "power rate l a w " also indicates that the surface concentration of the charged adsorbed oxygen ion has a pre dominating influence on Reaction II. This results in the slowing down of the further oxidation reaction ( Reaction II ) much more rapidly than partial oxidation (Reaction I ) . For higher amounts of selenium dioxide in the feed ( Z > 0.02), while the surface concentration of adsorbed oxygen ions gradually de creases, the surface concentration of adsorbed products becomes signifi cant, and possibly results in the modifiers entering the catalyst lattice substitutionally, rather than interstitially, C u , C u , or O " is replaced by selenium, forming a small unit of a covalent compound. Thus, both free 2 +

+



286

OXIDATION O F ORGANIC COMPOUNDS

II

electrons and holes are annihilated, and active centers for 2-methylpropene and oxygen are blocked, decreasing the conversion and formation of carbon dioxide. The rate of formation of water would increase with increased rates of conversion of 2-methylpropene and decrease with decreased rate for mation. The yield of water, therefore, increased first and then decreased because of the combined effects of these two processes. O n the other hand, if methacrolein is formed by Reaction I and subsequently further oxidized by Reaction II, its yield w i l l depend on the extent of Reactions I and II, and an optimum w i l l exist. In agreement with this, it was found that the optimal amount of selenium dioxide giving the highest selectivity under operating conditions was about 0.7% by weight of the catalyst. The effect of the oxygen-2-methylpropene ratio, R, on conversion, rate of formation and selectivity can be visualized easily from the fact that at a constant rate of 2-methylpropene flow, an increase i n oxygen2-methylpropene ratio would mean an increase i n the partial pressure of oxygen ( Ρ ) or 2-methylpropene in the surface concentration of adsorbed oxygen ion ( C - ) . Increased surface concentration of adsorbed oxygen ions would have a predominant influence on the course of the reaction, increasing the conversion and rates of formation and decreasing the selectivity with increased oxygen—2-methylpropene ratios i n the feed. θ 2

0

Nomenclature C = e €

=

k = k' = K(H)

m η Ρ r R S Τ W/F X Y ζ

=

= =

= =

=

eh = pL Ψ —

surface concentration of charged adsorbed particles per sq. cm. unit electrical charge dielectric constant rate constant, forward reaction rate constant, reverse reaction adsorption constant concentration of free holes i n the bulk of catalyst/cc. reaction order reaction order partial pressure reaction rate, gram moles/hr. gram catalyst feed composition ratio (oxygen-2-methylpropene) selectivity absolute temperature reciprocal of space velocity percentage conversion of 2-methylpropene rate of formation of products, g. moles/hr. g. catalyst weight ratio of selenium i n feed to copper i n supported catalyst free electrons holes yield


46.

MANN

A N D YAO

2-Methylpwpene

over Copper Oxide

287

Acknowledgment The authors are indebted to the Selenium-Tellurium Development Association, N e w York, for financial support of the project, and a fellow ship to one of the authors ( K . C . Y . ) .


Literature Cited (1) Barclay, J. L., Bethell, J. R., Bream, J. M., Hadley, D. J., Jenkins, R. H., Stewart, D. G., Wood, B. W , British Patent 864,666 (April 6, 1961). (2) Barclay, J. L., Hadley, D. J., Stewart, D. G., British Patent 873,712 (July 26, 1961). (3) Bretton, R. H., Wan, S. U., Dodge, B. F., Ind. Eng. Chem. 44, 594 (1952). (4) Dowden, D. Α., Caldwell, A. M. U., British Patent 828,812 (Feb. 24, 1960). (5) Enikeev, E. Kh., Isaev, Ο. V., Margolis, L. Ya, Kinetika i Kataliz 1, 431 (1960). (6) Farbenfabriken Bayer, A. G., British Patent 839,808 (June 29, 1960). (7) Hadley D. J., British Patent 694,354 (July 22, 1953). (8) Ibid. 727,318 (March 30, 1955). (9) Hadley, D. J., U. S. Patent 2,716,665 (Aug. 30, 1955). (10) Ibid. 2,810,763 (Oct. 22, 1957). (11) Hauffe, K., Advan. Catalysis 7, 213 (1955). (12) Kominami, N., Shibata, Α., Minekawa, S., Kogyo Kagku Zasshi 65, 1510 (1962). (13) Kruzhalov, B. D., Shestukhin, E.S.,Garnish, A. M. Kinetika i Kataliz 3, 247 (1962). (14) Mann, R.S.,Rouleau, D., ADVAN. CHEM. SER. 51, 40 (1965). (15) Mann, R. S., Rouleau, D., Ind. Eng. Chem. Prod. Res. Develop. 3, 94 (1964). (16) Margolis, L. Ya, Enikeev, E. Kh., Isaev, Ο. V., Krijlova, Α. V., Kinetika i Kataliz 3, 181 (1962). (17) Montecatini, British Patent 847,564 (June 29, 1960). (18) Shapovalova, L. P., Gorokrovastiski, Ya B., Rufanik, M. Ya, Kinetika i Kataliz 5, 330 (1964). (19) Tipper, C. F. H., "Oxidation and Combustion Reviews," Vol. 1, p. 225, Elsevier, New York, 1965. (20) Voge, Η. H., Wagner, C. D., Stevenson, D. P., J. Catalysis 2, 58, (1963). (21) Vol'kenshtein, F. F., "Electron Theory of Catalysis on Semiconductors," Macmillan, New York, 1963. (22) Waters, W. Α., Trans. Faraday Soc. 42, 184 (1946). RECEIVED October 16, 1967.


Oxidation of 2-Methylpropene over Copper Oxide Catalysts in the

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