Bismuth Molybdate Catalysts Kinetics and Mechanism of Propylene Oxidation George W. Keulks', Mich
I P. R
k, and Chelliah Daniel
Department of Chemistry and Laboratory f o r Surface Studies, Uniiiersity of Wisconsin-Milwaukee,Milwaukee, W i s . 53201
Oxidation of propylene to acrolein over bismuth molybdate catalysts was studied in a flow reactor a t atmospheric pressure and a t temperatures between 400" and 460°C. The kinetics were determined as first order in propylene and zero order in oxygen. Over this temperature range, the activation energy was 29 kcal per mole. The side products: acetaldehyde, formaldehyde, ethylene, carbon monoxide, and carbon dioxide, were formed almost exclusively in a consecutive pathway via acrolein or its surface species precursor when excess oxygen was present, A parallel pathway from propylene for the formation of carbon dioxide became important when there was an oxygen deficiency. A gas-phase, homogeneous oxidation of acrolein also became important when the postcatalytic volume of the reactor was increased.
T h e catalytic oxidation of olefins, using bismuth molybdate catalysts, has received considerable attention during the past decade. Studies involving olefins were principally concerned with the oxidation of propylene to form acrolein and with the oxidative dehydrogenation of n-butenes to form 1,3-butadiene. The mechanism by which propylene reacts, a t least initially, is firmly established and extensively discussed in recent reviews by Sachtler (1970), Sampson and Shooter (1965), and Voge and Adams (1967). The rate-determining step in the oxidation is the abstraction of an allylic hydrogen atom by surface oxygen t o form a symmetric allyl intermediate. A subsequent abstraction of a second hydrogen atom from either terminal carbon atom of the allyl intermediate followed by oxygen atom incorporation yields acrolein. Keulks (1970) used 1802 to show that essentially all of the oxygen atoms of the bismuth molybdate catalyst participate in the oxidation and serve as the source of oxygen for the incorporation into the products, confirming the suggestions of Batist et al. (1967) and Callahan et al. (1970). The oxidation of propylene over bismuth molybdate yields. in addition to acrolein, considerable amounts of carbon monoxide, carbon dioxide, and water, as well as lesser amounts of ethylene, formaldehyde, and acetaldehyde. Kinetic studies show the overall reaction to be first order in propylene pressure and zero order in oxygen pressure (Adams et al., 1964; Gelbshtein et al., 1965; Gorshkov e t al., 1970) which is consistent with the proposed mechanism. A topic of chief concern in several investigations is the origin of the carbon dioxide formed during the reaction. I t appears now that carbon dioxide is formed from both propylene and acrolein in a parallel-consecutive type of
' To whom correspondence 138
should be addressed.
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 2, 1971
pathway (Sampson and Shooter, 1965). The work of Gorshkov et al. (1968), however, indicates that, in addition to acrolein, other intermediates, which do not have acrolein as a precursor, produce carbon dioxide when oxidized over bismuth molybdate. Thus, carbon dioxide may be produced by a number of pathways as well as from a number of intermediates. In view of the uncertainty regarding the origin of the various side products of the reaction, the present study was undertaken with the particular objective of gaining further insight into the mechanism of the reaction. In addition, the authors decided to examine the kinetics of the catalytic reaction by carefully analyzing the product distribution and by paying particular attention to minimizing competitive, homogeneous, gas-phase reactions. Experimental
Reactor. The kinetic experiments were run in a singlepass, integral-flow reactor a t 350" to 475°C and atmospheric pressure. The reactors consisted of a length of 10-mm borosilicate glass tubing. I t was heated by a tubular furnace, whose temperature was controlled to within 10.5' by a proportional temperature controller. The catalyst was retained by borosilicate glass-wool plugs and the furnace was adjusted so that the catalyst section was in the center. T o minimize the possibility of a homogeneous reaction in the postcatalytic zone, the reactor was constructed out of 2-mm capillary tubing below the portion containing the catalyst. Oxygen (Airco, 99.6%), propylene (Baker, 95%), helium diluent (Airco, 99.99%), and ethane (Baker, 99%), which was used as an internal standard, were further purified by passage through molecular sieve traps and micron filters. The flow rates of the four gases were monitored by flow transducers (Gow-Mac Co.), and were controlled with Hoke microneedle valves. The gas mixture was sub-
was used in the kinetic experiments. Before any kinetic data were obtained, the catalyst was lined-out for 18 hr a t 425'C with a flow of propylene, oxygen, and helium in the ratio 40:40:80 cm' (STP) per min. Reaction Kinetics
I
I
1
1
I
I
I
I
I
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 P ,atm 02 Figure 1. Percentage conversion of propylene vs. oxygen partial pressure Temp, 425°C; pressure (CIH~), 0.25 atm; total flow, 160 cm' (STP) per
min; catalyst wt, 3 grams
j
sequently homogenized prior to entering the reactor. Ethane was used as an internal standard because of its demonstrated inertness under reaction conditions and its convenient chromatographic retention time. I t was used in conjunction with known thermal response factors (Dietz, 1967) to obtain absolute calibrations of the gas chromatograph for each component of the reaction effluent in terms of peak areas vs. micromoles. Gas chromatographic analysis of the reactor exit gas was accomplished by diverting the effluent through a 0.6cm3 loop of a gas sampling valve (Carle Instruments Co., No. 2014). The gas chromatograph contained two columns: silanized Porapak Q (Waters Assoc.) which provided the separation for C O P ,methane, ethylene, ethane, propylene, water, formaldehyde, acetaldehyde, and acrolein; and molecular sieve 5A, which provided the separation of CO and 0'. The propylene conversion was calculated using the internal standard according to Hall et al. (1960), and by using the sampling valve to analyze the feed mixture prior to its entrance into the reactor, and then comparing the peak areas of propylene before and after reaction. The consistent agreement of these two methods in calculating the various conversions was indicative of the lack of an appreciable volume change during the reaction. Catalyst Preparation and Pretreatment. The bismuth molybdate catalyst used in the experiments was prepared by coprecipitation from solutions of bismuth nitrate and ammonium molybdate, according to the method of Adams et al. (1964). I t had a surface area of 2.5 meters' per gram, as determined by the B E T method using nitrogen at -195" C as the adsorbate. Approximately three grams of 40- to 100-mesh particles
The kinetic order with respect to oxygen was determined by varying the oxygen partial pressure while maintaining the propylene partial pressure at 0.25 atm, and the total flow at 160 cm' (STP) per min. As shown in Figure 1, the oxidation of propylene is independent of the oxygen partial pressure over a range of 0.15 to 0.8 atm. At partial pressures of oxygen below 0.15 atm, the catalyst rapidly lost activity. This is in agreement with the reported results of Callahan et al. (1970), Dalin et al. (1962), and Gelbshtein et al. (1965) who observed a similar loss in activity at low oxygen partial pressures. This rapid loss in activity in a reducing atmosphere is owing to the reduction of Mo6- to Mo4 which results in the depletion of active lattice oxygen (Callahan et al., 1970). Because the catalyst rapidly lost activity in the presence of excess propylene, the kinetic order with respect to propylene was determined in excess oxygen. This was accomplished by varying the contact time while maintaining the propylene partial pressure at 0.25 atm and the oxygen : propylene : helium ratio at 1.5:1:1.5.The first-order rate plots of -In (1 - x ) , where x is the fraction of propylene converted vs. reciprocal space velocity, are shown in Figure 2. As indicated, the reaction conformed to first-order kinetics over the temperature range 400" to 460" C. The apparent activation energy for the reaction was computed as 29 f 5 kcal per mole from the Arrhenius equation using the integral reaction rates determined from the first-order rate plots. Considering the complexity of the reaction along with the experimental error involved in such measurements, this value is in reasonable agreement with those reported by Adams et al. (1964) and more recently by Peacock et al. (1969).
10 -
9 -
a 7 -
I
/
f
0.2 0.4 0.6 0.8 A/Fi
1.0
1.2
14
rnin-m2/rnoles
1.6 x
1.8
2.0
lo2
Figure 2. First-order rate plots
0 460"C 450"C
A 425" C 400°C
+
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 2, 1971
139
Table I. Effect of Oxygen Partial Pressure on the Product Distribution
[Temp, 425°C; pressure (CiH6),0.25 atm; total flow; 160 em' (STP) per min; catalyst wt, 3 grams] Moles per 100 moles C I H ~converted
Pressure
(0.).atm
YO
CO
CO,
C,H,
0.125 0.250 0.312 0.375 0.500 0.625 0.750
19 22 20 21 20 23 24
13 19 20
23 24 22 25 24 23 26
1 2 2 2 2 2 2
17 18 25 26
C~HIO
CiHiO
D
73 80 76
7 7 7 7 6 7
rr
ID
nr
ii
73 73
Table I I . Pulse Reactor Studies of Propylene Oxidation Components C
Pulse components
Temp,
C Hi, CIHi + 01 C ,HI0 GH,O + 02 CHiCHO CH iCHO + 0,
300-500 300-500 300-485 300-500 200-500 200-500
Observed products
None None None
CO, Cor, CPH,,H>O,CHqCHO CO + CH, (traces) CO, COP, HCHO, H20, CH,
Product Distributions
The product distribution, in terms of moles per 100 moles of propylene converted, for various oxygen partial pressures is given in Table I. These results show that the product distribution does not vary significantly with the oxygen partial pressure. The amount of acrolein formed as a function of temperature and contact time is shown in Figure 3. The corresponding changes in the product distribution are shown in Figure 4. The yield of acrolein decreases with both an increase in temperature and an increase in contact time while an increase is observed in the yield of the various products. These results suggest that the various products are produced from an intermediate in a reaction step that kinetically does not depend upon the gaseous partial pressure of oxygen. Thus, as indicated earlier, the reactive oxygen in this reaction step probably comes from the bismuth molybdate catalyst itself. I n addition, these results suggest that acrolein, or some surface species which is a precursor t o acrolein, is the intermediate responsible for the formation of the various products observed in the oxidation.
products are allowed to remain a t elevated temperatures in the postcatalytic zone, they can undergo homogeneous reactions. In an effort to gain further insight into the thermal stability of the reactants and some of the products, a reactor was constructed of 10-mm borosilicate glass tubing. I t contained no catalyst and was packed with 4-mm borosilicate glass beads. As expected, under conditions similar to those used for catalytic studies, propylene (40 cm' per rnin in 160 cm' per min helium) and propylene : oxygen : helium mixtures (40:60:60 cm' per min) underwent no homogeneous reaction a t 425" C. Similarly, acrolein (40 cm3 per rnin in 180 cm3 per min helium), in the absence of oxygen, was inert under these conditions. Mixtures of oxygen and acrolein, however, exhibited extensive reaction a t 425" C, in some cases with explosive force. A qualitative study of this process was made by passing a 1:l mixture of oxygen and acrolein through the reactor a t varying contact time. As expected, the extent of conversion of acrolein decreased with decreasing contact time. More significantly, however, although the relative amounts of carbon dioxide, ethylene, and water also decreased with increasing flow rate, the amounts of formaldehyde and acetaldehyde exhibited corresponding increases with decreasing contact time, suggesting their roles as intermediates in the overall homogeneous decomposition of acrolein (Kusuhara, 1961). The thermal stability of the various products observed in the oxidation of propylene in a flow reactor also was studied in a pulse reactor. The helium flow was adjusted to give contact times in the hot zone comparable to those in the flow reactor. The results of these studies are given in Table 11. The results obtained with a reactor having a large postcatalytic zone, the results obtained with a flow reactor containing glass beads, and the pulse reactor studies clearly indicate that homogeneous reactions between oxygen and
z_ Lu
Homogeneous Reactions and Thermal Stability of the Products
The product mixtures resulting from the oxidation of propylene conceivably may undergo a variety of homogeneous reactions in the postcatalytic zone of the reactor (McCain and Godin, 1964). T o examine the importance of such homogeneous reactions in more detail, a number of experiments were conducted with a reactor in which the volume of the postcatalytic zone was increased by replacing the capillary tubing with 10-mm tubing. The amount of acrolein decreased and also decreased significantly with an increase in the oxygen partial pressure as shown in Figure 5 . Also, the decrease in the amount of acrolein observed with increasing partial pressure of oxygen in the feed was accompanied by concomitant increases of all of the various side products. Thus, if the 140
Ind. Eng. Chem. Prod. Res. Develop., Vol.
10, No. 2, 1971
I
I
I
I
0.2 0.4 0.6 0.8 A/Fi
1
1.0
I
1
1.2 1.4
min-m2/rnoles
1
1
1.6
1.8
:0
x102
Figure 3. Effect of contact time and temperature on yield of acrolein 0 425" C W 460" C X 400°C A 450 C
100
90 a0
1 tL
i9
4 8
- 1 I
l
l
1
0.2 0.4 0.6 0.8 A/Fi,
1
1
1
1
1.0
1.2
1.4
1.6
1
1
1.8 20
min-mymoles x 1 0 2
Figure 4. Variation in the product distribution at 425°C as a function of the space velocity 0 Acetaldehyde A CO
+ CO?
Ethylene
the various side products can occur and, indeed, contribute significantly to the observed product distribution if proper precautions are not taken to minimize these reactions. The decrease in the amount of acrolein observed with increasing oxygen partial pressure (Figure 5 ) and the concomitant increases for all of the various side products indicate that acrolein may serve as the precursor for these products in the homogeneous reaction.
their relative amounts were essentially identical t o those observed for the oxidation of propylene. These results are in agreement with those reported by Gorshkov et al. (1968), except that in the case of acrolein they observed acrylic acid and did not observe acetaldehyde. Their study of a number of other side products observed in the oxidation of propylene also showed that the oxidation of these side products occurs by consecutive-parallel pathways. These authors conclude that the main sources of CO? formation are acids and that the main sources of CO formation are aldehydes. The results shown in Figures 3 and 5 , together with those of Gorshkov et al. (1968), and those from the acrolein oxidation experiments offer strong evidence for the conclusion that all of the side products observed during the oxidation of propylene, including virtually all of the CO and CO,, arise from subsequent oxidation of acrolein or its surface species precursor. However, this appears . t o be true only when propylene is reacted with excess oxygen. In the absence of excess oxygen, a pathway for CO, formation directly from propylene apparently becomes available. This is suggested by the observation that when the oxygen partial pressure is greatly reduced, the formation of COYpredominates. Unfortunately, the rapid loss of catalytic activity in the highly reducing atmosphere prevented us from obtaining any quantitative results. However, this observation is in agreement with the results of Peacock et al. (1969). They report that in the absence of oxygen, the reaction of propylene with bismuth molybdate produces CO, predominately. They have also shown that acrolein, on the other hand, in the absence of oxygen produces twice as much CO as CO? and in
Catalytic Reactions
The results presented in Table I , in which a reactor having a minimal postcatalytic volume was used, are believed to be indicative of the product distribution arising mainly from the catalytic reaction. As shown in Figures 3 and 5, the curves for acrolein extrapolate to 100 moles of acrolein formed per 100 moles of propylene reacted a t infinite space velocity. Thus it appears that the primary catalytic process is
CH,=CHCH,
+ 2 O(cata1yst)
ao
c T~ 100
l
70
k
c \
-+
CH,=CHCHO
+ H20
As noted earlier, the bismuth molybdate catalyst serves as the source of oxygen for the reaction. T o gain further insight regarding the formation of the various side products, a series of experiments was performed in an effort t o determine the relative reactivity of some of the observed products under our reaction conditions. When a 1.5:1.0:3.0 mixture of oxygen : ethylene : helium (total flow = 180 cm' per min) was passed over the bismuth molybdate catalyst a t 425" C, no conversion to any products was observed. Similar experiments with an oxygen : carbon monoxide mixture indicated that the rate of oxidation of carbon monoxide over the catalyst is also negligibly slow a t 425°C. When a 2:1:3 mixture of oxygen : acrolein : helium (total flow approximately 180 cm' per min) was passed over the catalyst a t 425"C, however, 85% conversion of acrolein was observed. The oxidation products (carbon monoxide, carbon dioxide, water, ethylene, formaldehyde, and acetaldehyde) and
2oiY ' OOOL '
0.1
0.2
0.3
0.4
0.5
Partiol Pressure of Oxygen
Figure 5. Effect of oxygen partial pressure on acrolein yield. Large postcatalytic volume in reactor Temp, 425°C; pressure (CyHs), 0.167 atm; total flow, 240 cm (STP) per min; catalyst wt, 3 grams
Ind. Eng. Chern. Prod. Res. Develop., Vol. 10, No.2, 1971
141
the presence of excess oxygen only nearly equal amounts of CO and CO?. Thus, the enhanced COz production under reducing conditions cannot be explained by an enhanced consecutive reaction of acrolein. Instead, it is suggested that under these conditions a parallel pathway for complete combustion directly from propylene becomes important. I n conclusion, it may be stated that the primary catalytic reaction of propylene oxidation over bismuth molybdate is the formation of acrolein. The side products formed result from a subsequent oxidation of the acrolein or its surface species precursor. The selectivity for acrolein is also complicated by the reactor design and the propylene : oxygen ratio: a large postcatalytic volume increases significantly the occurrence of homogeneous reactions; and a deficiency of oxygen enhances the parallel pathway from propylene for complete combustion to COz. These factors need t o be carefully considered in obtaining the maximum selectivity for acrolein formation over bismuth molybdate catalysts. Literature Cited
Adams, C. R., Voge, H. H., Morgan, C. Z., Armstrong, W. E., J . Catal., 3, 379 (1964). Batist, Ph. A., Kapteijns, C. J., Lippens, B. C., Schuit, G. C. A,, ibid., 7, 33 (1967). Callahan, J. L., Grasselli, R . K., Milberger, E. C., Strecker, H. A., Ind. Eng. Chem. Prod. Res. Develop., 9, 134 (1970).
Dalin, M. A., Lobkina, V. V., Abaev, G. N., Serebryakov, B. R., Plaksunova. S. L., Dokl. Akad. Nauk SSSR, 145, 1058 (1962). Dietz, W. A., J . Gas Chromatogr., 5, 68 (1967). Gelbshtein, A. I., Bakshi, Yu. M., Stroeva, S. S., Kulkova, N. V., Lapidus, V. L., Sadovskii, A. S.,Kinet. Katal., 6, 1025 (1965). Gorshkov, A. P., Gagarin, S.G., Kolchin, I. K., Margolis, L. Ya., Neftekhimiya, 10, 59 (1970). Gorshkov, A. P., Kolchin, I. K., Gribov. A. M., Margolis, L. Ya., Kinet. Katal., 9, 1086 (1968). Hall, W. K., MacIver, D. S., Weber, H. P., Ind. Eng. Chem., 52, 421 (1960). Keulks, G. W., J . Catal., 19, 232 (1970). Kusuhara, S.,Reu. Phys. Chem. (Japan), 31, 34 (1961). McCain, C. C., Godin, G. W., Nature, 202, 692 (1964). Peacock, J. M., Parker, A. J., Ashmore, P. G., Hockey, J. A., J . Catal., 15, 398 (1969). Sachtler, W. M. H., Catal. Reo., 4, 27 (1970). Sampson, R. J., Shooter, D., in “Oxidation and Combustion Reviews,” C. F. H. Tipper, Ed., Vol. 1, Elsevier, Amsterdam, The Netherlands, 1965, pp 223,256. Voge, H. H., Adams, C. R., Aduan. Catal. Relat. Subj., 17,151 (1967). RECEIVED for review July 23, 1970 ACCEPTED January 28, 1971 Partial financial assistance for this work was provided by a National Science Foundation-Departmental Science Development Program Grant. Presented at the Division of Physical Chemistry, 159th Meeting, ACS, Toronto, Canada, May 1970.
Hydroformylation of 1 -Pentene and 1-Hexene in Presence of Phosphine-Modified Ca ta Iys t s Wolfgang Rupilius, John J. McCoy, and Milton Orchin’ Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221
I t is now well-known that the addition of trialkylphosphines to the conventional hydroformylation catalyst can lead to as much as 88 to 90% of straight alcohol (Slaugh and Mullineaux, 1966, 1968; Tucci, 1968) compared to the 60 to 70% in the absence of phosphines. I t is also well-established that the presence of phosphines retards the rate of hydroformylation. The early claim (Tucci, 1968) that the hydroformylation of terminal olefins in the presence of n-BusP leads to high proportions of straight-chain isomers independent of temperature (150” to 180”C ) , catalyst concentration, or carbon monoxide partial pressure now appears to be valid only when large concentrations of the bisphosphine complex, C O ? ( C O ) ~ -
‘ To whom correspondence should be addressed. 142
Ind. Eng. Chem. Prod. Res. Develop., Vol. 10, No. 2, 1971
( P R J ? , are present, or, when excess butyl phosphine is present with small concentrations of cobalt. I n fact, the controlling conditions determining the selectivity to straight-chain isomer are probably the positions of the equilibria (Bianchi et al., 1969; Piacenti et al., 1970; Szabo et al., 1968)
C O ~ ( C O ) ~ ( P R+~CO )Z 2 C O ~ ( C O ) ~+ PR PR, ~ (1) HCo(C0)3PR, + CO 2 HCo(C0)d + PR, (2) In the presence of an excess of P R I , the active catalyst under most conditions is H C O ( C O ) ~ P R +This catalyst is responsible for high selectivity to straight-chain product. Under conditions where H C O ( C O )may ~ be present, considerably more branched-chain product is formed. This work shows that the relative amounts of the different catalysts
