Selectivity Effects in Some Catalytic Oxidation Processes electivity, essential in all processes, is especially in oxidation reactions. Complete combustion, giving by far the most stable end products, is a vast abyss from which all reactants and products must be protected. Oxidation reactions, being in general exothermic, release so much energy that this avalanche is difficult to contain. Severtheless, there are certain catalytic oxidations which proceed with remarkable selectivity. Hardly enough information is yet available to establish an overall evaluation of all the factors that affect selectivity, but many have been recognized. Physical effects, such as heat and mass transfer, are often important and frequently override other factors inherent in the reacting system. With proper engineering, these factors can be at least approximately controlled and will not be considered here. Selectivity is determined by individual reactivities of different molecules and results from a competition of reactions. Our control of selectivity is determined by how well we control this competition. This report will concern several illustrations of selectivity behavior in the catalytic oxidation of gaseous compounds over solid catalysts and the unexpected differences which exist. The three-component interaction of reactant molecules (including products of intermediate reactions), oxidant molecules, and the molecular configuration of the solid surface will be considered. Information is almost nonexistent on the solid surface component. We can describe to some extent the reactivity of various reactant and oxidant molecules, but the controlling feature of the overall reactions, the reactivity behavior in the surface complex, remains to a great extent a black box. An established reaction, the oxidation of olefins, where the selectivity is determined both by the nature of the reactant and the solid surface, is considered first. Then, the dehydration of certain alcohols will illustrate the complexity of such a seemingly simple reaction, and finally, the author will investigate several reactions
S critical
30
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
with an unconventional oxidant, yielding a major increase in selectivity. Oxidations with Oxygen
Olefins. The catalytic oxidation of olefins has attracted widespread attention in the past few years (19). Many catalysts have been examined; among the most popular have been cuprous oxide for the oxidation of propylene to acroIein (73) and bismuth molybdate for the same reaction (17), for the ammoxidation of propylene to acrylonitrile ( 1 4 , and the oxidative dehydrogenation of olefins to dienes (12). These two catalysts offer an interesting contrast in their behavior (78). The mechanisms of the oxidation and ammoxidation of propylene (6) are the same with both catalysts, involving an initial hydrogen atom abstraction from the methyl group to form a symmetrical, adsorbed allylic intermediate, followed by a further hydrogen abstraction from one end, with finally the incorporation of the heteroatom. H H H
I l l
H-C-C=C--.H
A!
First Abstraction
Adsorbed allyl Second Abstraction 0 --+
acrolein
Third Abstraction
N
acrylonitrile
CHARLES R. ADAMS
Selectivity behavior in catalytic oxidation can be influenced by solid surface properties, molecular reactivities,changes in oxidants, and product desorption. This article investigates some of the regulating factors.
(In the preceding sketch, all species are chemisorbed intermediates. The dots do not represent free valences, but only the absence of hydrogen atoms.) Yet, the kinetics, selectivity a t high conversion, sensitivity to ammonia, and reactions of higher olefins are completely different. Table I, taken from Voge (78), gives an abbreviated comparison of the two catalysts. Here is a good example of the influence of the nature of the catalyst, where one reaction goes through the same intermediate steps, while variation in the adsorption properties of the surface results in major differences in kinetics and selectivity, sometimes to the extent that even different products are formed. TABLE I. Catalyst
coz
COMPARISON OF CUPROUS OXIDE AND BISMUTH MOLYBDATE
cuzo
Propylene Oxidation Chief product Acrolein Temperature 300-400 range for b$st selectivity, C Order in OZ 1 Order in CaH6 0 Increases Press. effect on rate None Press. effect on select. High Inhibition. by ammonia Oxidation of Other Materials Fast to HzO Hydrogen Fast to COZ Ethylene Fast to COZ Acetaldehyde Moderate to COz Ethane Isopropyl alcohol Fast to acetofie Propionaldehyde Slow to COz Acrolein 1-Butene
The oxidative dehydrogenation of the higher olefins on bismuth molybdate (7) presents a n interesting illustration of the effects of olefin structure and inhibition by product on selectivity. Table I1 gives some representative conversion-selectivity data for a number of olefins. Earlier work (8) has shown that the kinetics of propylene and the butylenes is first order in olefin and independent of oxygen. T h e selectivity can be well described by a combination of parallel and consecutive reactions : kl C3HG -C&40
Moderate to CO1 Moderate to methyl vinyl ketone others
+
Acrolein 400-525
At 460 "C, k3/kl = 0.10 and kz/kl = 0.25. The results for 1-butene a t the same temperature can be described by :
0 1 Increases Decreases None
No reaction Little reaction Fast to COZand acid No reaction Fast to propylene Moderate to COZand acids Slow to con Fast to butadiene
+
Here kS/kl = 0.05 and (ks k4)/kl = 0.05. The ratedetermining step for k l is the abstraction of a n allylic hydrogen from the olefin (7). The relative reactivity of a large number of olefins has been determined (7). The reactivity of propylene relative to 1-butene is 0.11, from which the following reactivity (or stability) parameters were estimated : Propylene (to acrolein) Propylene (combustion) I-Butene (to diene) 1-Butene (combustion) Acrolein (combustion) Butadiene (combustion
+ furan)
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31
TABLE II.
OXIDATION OF OLEFINS OVER BISMUTH MOLYBDATE AT 460 ' C Molecular
0 lefin
reactiuity
Ol$n
concersion, 70 -
0.11
Propene
IO 40 80 (1.00)
1-Butene
20 40 80 0 . 2 6 (cis) 0.19 (tranr)
2-Butene
20 40 80
0.50
2-Methylpropene
10 40 70 1-Pentene
1.38
2-Pentene
0.43
5 15
3-Methyl-I -butene
2.7
2-Methyl-I -butene
4.2
2-Methyl-2-hutene
2.0
40 35 20 40 70 30-50
Cyclopentene I-Hexene
20 40 70
...
2-Hexenc
2-Methyl-I -pentene
3.7
3-Methyl-I -penten?
1.9
4-Methyl-I -pentene
1.6
2-Methyl-2-pentene
3.7
3,3-Dirnethyl-l -butene
0.21
4,4-Dimethyl-l -pentene
0.75
3-Ethyl-I-pentene
1.4
10 50 80 2 5 10 5 10 20
10 20
...
3-Heptene
I
.
50
.
40
2.8
2-Ethyl-1-hexene
5 15
Other products are mainly CO and COz. ~~~
32
20 40 70 5 10 20
I-Heptene
a
Product Selectivitva Acrolein 90 86 73 Butadiene 95 95 90 Butadiene 90 90 85 Xfethacrolein 72 72 72 Pentadiene 93 87 38 Pentadiene 88 79 Isoprene CjHsO 80 4
~
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Isoprene CjHgO CsHsO 9 4 73 9 62 9 44 9 16 Only C O Z C O Hexadiene Hexatriene Benzene 71 2 0 7 59 5 32 7 32 Hexadiene Hexatriene Benzene 24 5 17 6 22 18 30 10 7 hlethylpentadiene CsHloO C ~ H R O 55 9 5 49 9 5 38 9 5 Methylpentadiene Vinylhutadiene 90 10 54 16 27 12 hkthylpentadiene C ~ H R O 70 6 59 6 40 5 Methylpentadiene C ~ H I OCsHaO 67 3 5 60 3 5 44 3 5 2,3-Dimethylbutadiene CsHlo CsHsO 60 3 6 56 3 8 2,3-Dimethylpentadiene 74 53
+
CiHiz 85 73
CiHio 5 8
C~HBO~ 1 2
T h e selectivity behavior for these two systems can be completely described by these molecular reactivity parameters and the assumed kinetics. The combustion resistances of 1-butene and its product are lower than those of propylene and its product, although the overall reaction of butylene is substantially more selective than that of propylene, a fact which is entirely attributable to the greater ease of allylic hydrogen abstractions in the Cd system. Another example of selectivity control by reactant reactivity is that of the isomeric n-butenes. The two 2-butene isomers are somewhat less reactive and selective than 1-butene, and these differences in behavior agree quantitatively with the differences in free energy of formation of the isomers, indicating a common intermediate for the three compounds. A perusal of the data in Table I1 shows that the olefins higher than butylene, while having high initial selectivities, decline rapidly in selectivity a t the higher conversions, even though the molecular reactivities are usually higher than the butylenes. The explanation for this behavior is the inhibition of the desired reaction by the products. The kinetics are no longer simple, but become : Rate =
k(olefin)Kl(O 2)
1
lj2
+ K1(02)'/~+ K Z(product inhibitor)
inhibition as calculated from the molecular reactivities, is shown when the product partial pressure a t the exit is 0.015 atm. The interpretation of these numbers is not exact, mainly due to product distribution effects, but they give a rough measure of the effective inhibition a t this conversion level. T h e data are not so extensive as desired, but the following conclusions can be drawn. Olefins which make similar products are similarly inhibited. Those olefins (Table 11) which produce a small amount of aldehyde as well as diene are much more inhibited than those producing only the same diene. C
C
I
I
c-c=c-c
c=c-c-c
r
I c-c=c-c '
I H\
T h e mechanistic significance is that the rate-controlling step is the abstraction of an allylic hydrogen from a gasphase or physisorbed olefin by strongly chemisorbed oxygen atoms. The product inhibitors interfere with oxygen chemsorption. At 460 "C, K1 was 21 atm-'iz and Kz was 700 atm-l for pentadiene. T h e various products from the various olefins are not equal in their inhibiting ability. A crude estimate of the amount of inhibition by product has been made in Table I11 where the rate from an integral reactor, relative to that for no
TABLE 1 1 1 . INHIBITED RATE RELATIVETO UNINHIBITED RATE FOR OLEFIN OXIDATION OVER BISMUTH MOLYBDATE A T 460 "C" Compound
Ratio, inhibited/ uninhibited
1-Pentene 2-Pentene 3-Methyl-1-butene 2-Methyl-I -butene 3-Methyl-1 -pentene 3-Ethyl-I -pentene 2-Methyl-1 -pentene 2-Methyl-2-pentene 4-Methyl-1-pentene 2-Ethyl-1 -hexene 3,3-Dimethyl-I -buteneb 4,4-Dimethyl-1-penteneb a Integral rate when exit pproduot = 0.075, pol Isomerizes to react. that f o r no inhzbition.
0.4 0.4
0.5 0.04 0.3
0.3 0.05 0.1 0.03
0.05 0.3 -0.04 =
0.72 atm relative to
''
\
Moderately C I strong inhibitor C=C-C=C
Very Jug strr-1.1innibitor
//O C
I
c=c-c=c
Very strong inhibitor
These unsaturated aldehydes therefore are much more strongly adsorbed (roughly two orders of magnitude) than the corresponding diene. The larger dienes appear to be more efficient inhibitors than the smaller ones. These effects are illustrated most clearly for the isoamylenes. 3-Methyl-1 -butene, which produces only isoprene as initial product, is only moderately inhibited and its selectivity is only moderately reduced. 2Methyl-1 -butene, which also produces a small amount of unsaturated aldehyde as initial product, is severely inhibited and the selectivity is reduced substantially. These inhibition effects are dramatically illustrated in Figures 1 and 2, where the observed conversions are plotted in comparison with the calculated conversions that would-have been obtained in the absence of inhibition. For conditions where 2-methyl-1-butene should have been converted to 80%, the actual observed conversion is only 6%. The combustion reaction is not inhibited to the same degree and hence the selectivity suffers. A later section of this report will show how the selectivity of the oxidative dehydrogenation of the higher olefins can be improved markedly with a change in oxidant. Alcohols. Bismuth molybdate has been found to be an excellent oxidation or dehydration catalyst for certain VOL. 6 1
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33
2.c
7
/
I
I I
I
I WITHOUT INHIBITION
I WITHOUT INHIBITION I I
1 .t
i
I
I
I
I
I
I
I
I
1.2
I
u
-
I
I
I
I
I
El
I
I
I
0.8
I I I 0.4 - I I I
1
I
0
1
I
2
4
1
1
6
8
m u
0
1
1
2
4
105 -
6
E
105 -
GHSV
GHSV
Oxidation of 2-methyl- 7-butene over bismuth molybdate at
Figure 7. Oxidation of 3-methyl- 7-butene ouer bismuth molybdate at 460 "C
Figure 2.
alcohols [e.g., allyl alcohol to acrolein and crotyl alcohol and methyl vinyl carbin,ol to butadiene (41. Certain features which illustrate the effect of the nature of the surface on the reactivity and selectivity of the reaction are appropriate here. Data in Table IV show that crotyl alcohol is dehydrated to butadiene rapidly and selectively over bismuth molybdate. However, this dehydration reaction requires a t least traces of oxygen. Apparently, only the oxidized form of the catalyst surface is active. Ammonia inhibits the reaction and reduces the selectivity although no nitrogen-containing products were found. Methyl vinyl carbinol also produces butadiene selectively, and the reaction again stops when no oxygen is fed (Table V), in analogy with crotyl alcohol. However, in contrast, methyl vinyl carbinol is not inhibited by ammonia. These results illustrate how the nature of the surface, not affected by
the main reaction, can have a profound influence on the reaction. Isopropyl alcohol is dehydrated rapidly over bismuth molybdate (lOOyoconversion, 95% selectivity at 380 "C and GHSV = 15,000). This reaction apparently is not affected by the absence of oxygen. I t would then be predicted that 1,3-butanediol would react rapidly and selectively to give butadiene, by way of crotyl alcohol :
TABLE IV.
REACTION OF CROTYL ALCOHOL OVER BISMUTH MOLYBDATEe
Crotyl Cc/Min alcohol Selectivity, yo AmconverButaCrotonOxygen monia sion, Yo diene aldehyde Furan COP 89 3 3 4 40 .. 96 20 .. 90 88 6 1 4 90 5 1 3 10 .. 91 2 5 .. 83 89 7 2 2 1 .. 83 85 7 6 2 0 .. 10 (-100)b ..b .. ..b 40 10 67 72 16 4 8 40 40 24 58 16 16 10 0 40 20 86 5 5 5 a 450 'C; 2 cc catalyst; 220 cc/min helium ( S T P ) and 20 cc/mtn gaseous crotyl alcohol. N o t accurate.
34
INDUSTRIAL AND ENGINEERING CHEMISTRY
460 'C
OH
I
C-C-C-C-OH
+
C-C=C-C-OH
-+
c=c-c=c I t does react rapidly (85% conversion with complete conversion of oxygen at 408 "C and GHSV = 5400), but gives only about 10 to 15% selectivity to butadiene and to crotyl alcohol, with similar amounts of methyl vinyl ketone, propionaidehyde or acetone, acrolein, acetaldehyde, formaldehyde, CO2, etc., being formed. This is an example of a drastic change in selectivity with a seemingly minor extrapolation of the molecular structure of the reactant. Alkylaromatics. The mechanism of olefin oxidation over bismuth molybdate (7) is based on the abstraction of the weakly held allylic hydrogen in the hydrocarbon molecule. The alpha C-H bond in alkylaromatics is similar in character and bond strength to the allylic bond in olefins. Seemingly, benzaldehyde should be made readily from toluene and styrene produced from ethylbenzene. Such is not the case (4). Toluene was oxidized over bismuth molybdate at 460 "C and GHSV = 2100. Conversion was 74% and the selectivity to benzaldehyde was only 18%. Selectivity to benzene was 27% and to COZ and CO was 4373 and 18Yc, respectively (calculated on basis of total number of feed
TABLE V.
REACTION OF METHYL VINYL CARBINOL OVER BISMUTH MOLYBDATEa
MVC conver-
Partial pressure "a
0 2
0.13
0.07
a
... ... ... ... ...
Selectivity,
sion, %
Butadiene
99 99 98 35 13 99 99
85 87 86
Furan
Acrylic acid
4 3 6 15 16 8
0.3 0.3 0.3 6
0.03 0.01 72 0.00 42 0.13 0.03 76 0.13 0.13 85 0.00 0.13 6 27 450 "C, G H S V = 18,000, MVCpartialpressure = 0.14 atm.
molecules to each product). Ethylbenzene a t 490" C and GHSV = 4200 was 64% converted, and the selectivity to styrene was 30y0,while that to benzaldehyde, benzene, COz, and CO was 23, 9, 27, and 12%. The expected reactions of these alkylaromatics occur, but in reduced selectivity, due to secondary reactions of dealkylation and combustion. It is not clear why this system responds differently from the aliphatic system. Steric and adsorption factors in the surface complex can certainly be conjectured, but the known tendency of alkylaromatics to crack to the phenyl radical is undoubtedly of great importance. a-Methylstyrene can be considered as either a substituted aromatic or a substituted propylene. Substituted aromatics give benzaldehyde, benzene, and COZ as principal products while substituted propylenes give substituted acroleins. At 494 "C, and GHSV of 2200, the conversion of a-methylstyrene was 45%. The carbon selectivity to various products was as follows: Phenyl acrolein Benzaldehyde Benzene c02
co
Per cent 7 25 27 36
G
Even less phenyl acrolein was made under other conditions, I n air oxidation over bismuth molybdate, amethylstyrene thus behaves as a substituted aromatic rather than as a substituted propylene. This illustrates how selectivity behavior is affected by different components within the same reacting molecule. Oxidations with Sulfur Dioxide
There are three components in a catalytic oxidation system: reactant, oxidant, and catalyst. Two of these components have been fairly well explored; but relatively little attention has been paid to oxidants other than molecular oxygen. The author has found that the selectivity of certain oxidation reactions is very substantially improved when sulfur dioxide is used instead of oxygen (5). Thermodynamics are still favorable for high conversion, when hydrogen sulfide and water are
yo
MVK
co + co, 8 7 6 5 10
16 0.40 0.2 4
2 27
12 11 39
80 -
&?
60-
g 2
t; Y
500°C 55OoC 6OO0C 40-
A
BISMUTH PHOSPHATE-TUNGSTATE
0
0
20
PRESULFIOED BISMUTH MOLYBDATE
40 60 ISOAMYLENE CONVERSION,
80
11 0
%
Figure 3. Oxidation of 2-methyl-2-butene with SO2 ouer bismuth phosphate-tungstate and presulfidd bismuth molybdate
formed. Another advantage is that with SO2 the oxidation reactions are usually mildly endothermic, thus eliminating the disruptive effect of release of large amounts of energy. Details of catalyst preparation and behavior have been given (5). Some results are cited below concerning selectivity effects in this system. Olefins and paraffins. I t will be recalled that, in the oxidative dehydrogenation of olefins over bismuth molybdate with oxygen, butadiene is formed in high selectivity a t high conversion, while the higher olefins suffer a substantial loss in selectivity at high conversion. Figure 3 gives the conversion-selectivity curve for 2methyl-2-butene, where the curve without points represents the system with oxygen. When bismuth molybdate is used in the oxide form with sulfur dioxide as oxidant, its initial behavior (first hour) is very similar to that with oxygen. The selectivity is essentially identical to that obtained with oxygen, other oxygenated products (C5HsO and C5H30) are formed, and large VOL. 6 1
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35
amounts of COZare produced. However, if the catalyst is presulfided with H2S at 500 "C, the data given in Figure 3 are obtained. Selectivity is improved over that of the oxide form; isoprene plus methylthiophene account for about 90% over the entire conversion range. Practically no oxygenated CEH10
+ 1/3 SO2
-.t
ChHs
+ 1/3 HZS + 2/3 HzO
compounds are formed, not even Con. Use of sulfur dioxide instead of oxygen results in much higher selectivity to isoprene. Methylthiophene is the only major by-product, and it is undoubtedly produced in a subsequent reaction of the isoprene. The behavior of bismuth molybdate in the oxide form probably is due to high bulk oxygen mobility in this catalyst, so that the surface continues to act as if it were seeing oxygen. Bismuth phosphotungstate exhibits a different behavior. Figure 3 shows the results when this catalyst is used in the oxide form with sulfur dioxide. Selectivity is the same as for the presulfided bismuth molybdate. Certain other metal phosphate catalysts give essentially identical results to that of the tungstate.
TABLE VI. SUMMARY OF PARAFFIN AND OLEFIN OXIDATION W I T H SULFUR DIOXIDE Results and Discussion
Compound Isoamylene Butylene Propylene Propane n-Butane
Fast and selective to diene: 80% S at 40y0 C ; niethylthiophene formed sequentially from diene Fast and selective to diene: 857&.S at 4Oy0 C ; thiophene formed sequentially from diene Moderately slow and unselective. Mainly combustion products Very slow, very poor selectivity toTopy1ene (11% C, 54% S ; 16% C, 30 S ) Moderately slow, forms unsaturates but rapidly cyclizes to thiophene, best CdHB CIHB: 48Y0 S at 52y0 C, best thiophene: 64ro S at 71% C Moderate rate to isobutylene: 7oy0 S at 26y0 C, 61% S at 50% C Practically no reaction, only combustion products Moderately fast, at 4870 C : 6170 S to methylthiophene, 12y0 S to isoamylene and 16y0 S to isoprene Moderately fast, at 6870 C : 74y0 S to benzene, 14y0 S to linear unsaturates Moderate rate to coke Moderately fast, low selectivity to unsaturates and dimethylthiophene, surprisingly large amounts of dealkylation and combustion Fast, >4Oy0 S to vinylthiophene at 50 to 70% c Fast to diene, 37y0 S to vinylthiophene at 95% C, appreciable amounts of dimethylthiophene Large amounts of coke and about 3070 S to toluene; methyl ethyl and methylvinylthiophene low Fast, very little thiophenes or combustion, at 36% C : 31% S to olefins, 137, S to diolefins, 29q-b S to aromatics, and 28% to cracked products
Thus, selectivity can be influenced markedly by such solid-state properties of the catalyst as oxide mobility. The use of sulfur dioxide as oxidant has produced a number of interesting reactions. A brief summary of results for the various paraffins and olefins is given in Table VI. Reaction rates and product selectivities are very dependent on hydrocarbon feed structure. Branching and increasing chain length increase reactivity substantially. Dehydrogenation appears to be the first reaction in all cases. Olefins are dehydrogenated selectively to dienes, but at high conversions, these dienes are converted to thiophene. If there is a linear Cq or Cs chain, thiophenes are produced so fast that it is difficult to obtain large yields of olefin or diene intermediate from paraffins. If there is a C g or longer chain, aromatization becomes very rapid and thiophenes formation is quite low. Cyclic Cg rings go to coke. Certain reactions stand out as interesting: olefins to dienes, dienes to thiophenes, isobutane to isobutylene, butane and isopentane to the corresponding thiophenes, hexane to benzene, and 3-methylpentane to vinylthiophene. Certain surprises also occurred : poor selectivity for propane to propylene, presumably due to low propane reactivity, a large amount of dealkylation and combustion for 2-methylpentane, and large amounts of coke and toluene from 3-ethyl-lpentene. Catalytic oxidation with sulfur dioxide certainly holds promise for certain reactions, but the selectivity behavior of various hydrocarbons is quite dependent on individual structure. An interesting illustration of the influence of competing and consecutive reactions is given by the formation of 3-vinylthiophene from 3-methylpentane ( 3 ) . The relative complexity of such a reaction may be visualized from the following sketch, all steps of which are significant:
+
Isobutane Isobutylene Isopentane n-Hexane Methylcyclopentane 2-Methylpentane
3-Methylpentane 3-Methyl-1 -pentene 3-Ethyl-1 -pentene n-Dodecane
~~
36
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Paraffin
4
Olefin Di:ne Triene
\
thiophene 3-Vinylthiophene
2,3-Dimethylthiophene
\Methylthiophene T'hiokene
j.
Thiophthene
Of course, all species are also susceptible to cracking and combustion. Conversion-selectivity data for this reaction are shown in Figure 4. A broad maximum in selectivity (at slightly above 407,) to vinylthiophene occurs between 50 and 707, paraffin conversion. Some of the
products are precursors of vinylthiophene, and when these are considered to be recycle feed material, the maximum selectivity to vinylthiophene is slightly over 50%. For such an assembly of competing and consecutive reactions, selectivity to a particular intermediate is quite sensitive to catalyst and conditions. Alkylaromatics. The catalytic oxidation of ethylbenzene to styrene using sulfur dioxide as oxidant proceeds with remarkably high yields ( 2 ) . CsHlo
+ 1/3 SO2
+
CsHs
+ 1/3 HzS + 2/3
100
+
HzO
Several metal phosphates may be used as catalysts. Figure 5 gives a typical conversion-selectivity .curve for calcium-nickel phosphate. Selectivity to styrene is over 90y0at conversions of more than 9070. Benzothiophene is formed from the styrene only a t very high conversions. Once-through yields of styrene of 86y0have been obtained (2). The increase in selectivity due to the use of sulfur dioxide instead of oxygen as oxidant is outstanding. Diethylbenzenes are oxidized in a typical consecutive fashion (Figure 6), but o-divinylbenzene is very rapidly cyclized to naphthalene (Figure 7). No ethylbenzothiophene was detected, and selectivity to vinylbenzothiophene was only 5% at 90% conversion for the metapara mixture, while vinylbenzothiophene was not detected with the o-diethylbenzene feed. The oxidative dehydrogenation of isopropylbenzene to a-methylstyrene was not as clean as that of ethylbenzene. Selectivity was only 60 to 70%, at conversions of 40 to 8OY0, primarily due to losses to acetone and phenol. Toluene was converted to an unidentified solid containing both sulfur and oxygen. These results are consistent with those with olefins and paraffins, indicating that the sulfur dioxide system is good only for oxidative dehydrogenation and not for oxidation to aldehydes. Summary
Several selected examples have been given of some of the factors that affect selectivity in the catalytic oxidation process. The importance of the subtleties of the properties of the solid surface is illustrated by a comparison of cuprous oxide and bismuth molybdate, where the same formal steps in the multistep reaction occur, but the variation in the adsorption properties of the surface results in major differences in kinetics, selectivity, and even in different products being formed. The oxidation of olefins over bismuth molybdate offers an excellent illustration of the effect of individual molecular reactivities on selectivity, pointing out clearly the importance of relative reactivities in competing reactions rather than absolute reactivities. The oxidation of higher olefins in this system shows how the last step, desorption of product, can markedly influence the selectivity of the overall reactions. The oxidation/dehydration of certain alcohols over bismuth molybdate provides examples of how the changing steady-state nature of the surface affects the reaction and how extensions to molecules of very similar structure may fail to give the predicted results. The oxidation of alkylaromatics over bismuth
8o
'S
-
60 -
0
+
A: OLEFIN DIENE TRIENE B: COMBUSTION PRODUCTS C: VINYLTHIOPHENE D: DIMETHYL-, METHYL-, AND UNSUBSTITUTED THIOPHENE E: THIOPHTHENE F: ETHYLTHIOPHENE
20
40 60 PARAFFIN CONVERSION,
80
1 0
%
Figure 4. Oxidation of 3-methylpentane over calcium-nickel phosphate at 550 "C
8ol
60
40
20 0
0 0
+..@is
BENZOTHIOPHENE
40 60 ETHYLBENZENE CONVERSION,
80
11 0
%
Figure 5. Oxidation of ethylbenzene with sulfur dioxide over caIciumnickel phosphate at 550 'C
R. Adams is with the Shell Development Co., Emeryville, Calif. 94608. T h e author thanks H. H . Voge, T. J . Jennings, and P. H. Deming of the Emeryville Research Center for collaboration and helpful discussions. AUTHOR Charles
VOL. 6 1
NO. 6 J U N E 1 9 6 9
37
I
l0Ol
80 -
’S
60-
Ez c
u --I w
40-
20-
Ob
_g.?pI-
OIVINYLBENZENES 1
1
I
OIETHYLBENZENES CONVERSION,
$1
%
Figure 6. Oxidation of meta-para mixture ( 7 ; 7) of diethylbenzenes with suvur dioxide ouer calcium phosphate at 500 “C
1YLVINYLEENZENE
N A PUTU 1
bp
40
REFERENCES
20 N
A
0 0
’
I
20
*
-
Y
I
h
-
I
I
40
60
A
DIETHYLBENZENE CONVERSION,
I
80
J
100
%
Figure 7. Oxidation of o-diethylbenzene with sulfur dioxide over calcium phosphate
38
molybdate also illustrates the inability of a mechanism, established in fair detail, to cover adequately the behavior of seemingly similar systems. The use of sulfur dioxide instead of oxygen gives an excellent example of how a change in oxidant, here providing the hydrogen abstracting agent, may profoundly influence the selectivity of oxidative dehydrogenation. Several examples are given of selectivity behavior in consecutive reactions. The use of sulfur dioxide also offers an unusual way of eliminating one of the most common disruptive problems in catalytic oxidation-the release of large quantities of energy. There are, of course, many other examples of selectivity behavior in the literature, and it is appropriate to ask whether the time has come when some allencompassing theories can be set forth for the selectivity behavior of catalytic oxidation reactions. I t would not yet appear so; the most important missing areas are the oxidant-catalyst reactions, and the reactant-solid surface interactions. Some good starts have been made. Margolis (75) and coworkers have found interesting reationships, for cuprous oxide, between the catalyst work function, oxygen exchange rates, and catalytic behavior, as modified by various additives. ATevertheless, the electronic theory of catalysis is not able to handle adequately the behavior of transition metal oxides in oxidation, beyond a superficial level (17). The bond strength and availability of oxygen on the surface are obviously important and this problem is being attacked (10, 761,but much remains to be done in this area. Surface activity in the bismuth molybdate system, on the other hand, is not simply related to any bulk compositions ( 9 ) . The reactivity behavior of various reactants is becoming fairly well established, but secondary effects, such as adsorption phenomena, are not well understood. Finally, a quantitative comparison theory of catalysts with entirely different chemical compositions is completely lacking. The time has not yet come when the behavior of an unknown reaction can be obtained without actually carrying out the reaction.
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
(1) Adams, C. R., “Proceedings Third International Congress on Catalysis,’ p. 240, North-Holland Publishing, Amsterdam, 1965. (2) Adams, C. R., U.S. Patent 3,299,155(January 17, 1967). (3) Adams, C. R., ibid., 3,309,381 (March 14,1967). (4) Adams, C. R., J. Cutolyrir 10, 355 (1968). ( 5 ) Adams, C. R., ibid., 11, 96 (1968). (6) Adams, C. R., and Jennings, T. J., ibid., 2, 63 (1963). (7) Adams, C. R., ibid., 3, 549 (1964). (8) Adams, C. R., Voge, H. H., Morgan, C. Z . , and Armstrong, W. E., ibid., 3, 379 (1964). (9) Batist, P. A., Lippens, B. C., and Schuit, G. C. A,, ibid., 5 , 55 (1966). (10) Callahan, J. L., and Grasselli, R. K., J. A.I.Ch.E., 9, 755 (1963). (11) Cullis, C. F., IND. END.CHEM.,59 (12), 18 (1967). (12) Furman, K. E., and Hearne, G. W., U.S.Patent 2,991,320 (July 4, 1961). (13) Hearne, G. W., and Adams, M. L., ibid., 2,451,485 (October 19, 1948). (14) Idol, J. D., Jr., ibid., 2,904,580 (September 15, 1959). (15) Margolis, L. Ya., Aduancerin Catdyris, 14, 429 (1963). (16) Sachtler, W. M.,H., and de Boer, N. H., “Proceedings Third Internationa Congress on Catalysis,” p. 252, North-Holland Publishing, Amsterdam, 1965. (17) Veatch F Callahan, J. L., Milburger, E. C., and Forman R. \V “Proceedings Secodd ‘international Congress on Catalysis,” p. 2647, ’Editio& Technip, Paris, 1960. (18) Voge, H . H., “Oxidation of Organic Compounds,” Vol. 11, p. 242, Aduancer zn Chemwlry Series 76, ACS, Washington, D. C., 1968. (19) Voge, H. H., and Adams, C. R.,Adooncesin Catalyrir, 17, 151 (1967).
