Catalytic Oxidative Dehydrogenation of Butenes to Butadiene Emory W. Pitzer Research and Development Division, Phillips Petroleum Co., Bartlesville, OK 74004
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A phosphorus-tin oxide catalyst, active and selective for the oxidative dehydrogenation of butenes to butadiene, was improved in activity b y steaming at elevated temperatures. As the steaming temperature was increased from 1250" to 1 6OO0F, the reaction rate was increased about threefold. Further increases in temperatuire up to 1730°F decreased the catalytic activity. Heating in air and nitrogen did not increase the catalytic activity. O f eight chemical and physical properties determined on the treated calalysts (phosphorus content, pore volume, surface area, average pore diameter, skeletal density, tablet volume, cassiterite content, and macroporosity), only macroporosity appeared to account for the increase in catalytic activity. Steaming appeared to affect the bulk of the catalyst instead of altering only the surface of the catalyst particles.
A
major advance in the field of butene dehydrogenation has been the discovery of a,n oxidative process for this reactionthereby removing the thermodynamic limitations of conventional dehydrogenation. A halogen process, although not strictly catalytic oxidative dehydrogenation, described by Shell (1962) and others is efficient for this reaction. However, this process is not now used commercially (Weiss, 1970), possibly because of slow reaction rates and the corrosion problems associated with the use of halogens. Oxidative dehydrogenation of butenes to butadiene is described in the literature as being promoted with several different catalysts. A few serve in the dual role of catalyst and oxygen source, e.g., cobalt or nickel ferrite (Woskow et al., 1969). Also, a few function in the absence of steam, e.@.,a combination of antimony and manganese oxides (Callahan et al., 1966) and lead molybdate with cobalt or aluminum tungstate (Xolan, 1969). A larger group requires steam in the feed. Some of these are bismuth molybdate (Adams et al., 1964; Batist et al., 1966; Hearne and Furman, 1961), bismuth tungstate (-irmstrong et al., 1961), bismuth phosphate (Voge and ddams, 1961), bismuth on calcium-nickel phosphate (Alexander et al., 1968), and bismuth on calcium or magnesium phosphate with pores predominantly litrger than 1000 in diameter (llinnis e t al., 1968). Although some of these catalysts are moderately effective, they fall shoyt of the expectation for a n oxidative dehydrogenation catalyst: complete conversion of butenes to butadiene and cont,inuous operation. I n 1967 a phosphoriis-tin oxide catalyst for the oxidative dehydrogenation of butenes to butadiene was patented (Nolan, 1967). Becaus'e it offered the possibility of fulfilling the objective for an oxidative dehydrogenat'ion catalyst, developmental studies irveremade. Ot'her research a t Phillips Petroleum Co. resulted in a catalyst different from the one mentioned above. The first commercial unit has heen in operat'ion since mid-1970. .Innouncement of this commercial process was made by Huseii e t al. (1971). I n the study reported here on t'he development of the 1)hosphorus-tin oxide catalyst, met,hods of acbivating the
finished catalyst were investigated. Objectives of activation included : altered chemical coniposition and cryst'allinity of the catalyst, increased concentration of active sites, larger useful surface, and improved access to the surface. We attempted to acconiplish these objectives by heating the finished catalyst in steam, air, and nit'rogen. Experimental
The catalyst was prepared to contain phosphorus and tin in a molar ratio of about 1:1.5 by mixing aqueous solutions of stannic chloride and phosphoric acid, forming a precipitate by adding ammonium hydroxide, filt'ering, washing, drying, forming into 1/8-in. tablets, and calcining at 1100°F. Samples of this catalyst were treated further in flowing steam a t various temperatures in the range of 1250-1730°F. Steaming was performed by preheating the steam t o the intended temperature and passing it over the catalyst at 200 space velocity for 16 hr. Special precautions were baken to avoid contacting the catalyst wit'li steam when the catalyst temperature was below 900°F. Air and nitrogen treatments were performed by substituting these gases for steam. I n the measurement of oxidative dehydrogenation activity, butene-2 (Phillips pure grade) and air were metered through ceramic orifice flow meters; steam was introduced by passing the air through a water saturator controlled at the proper temperature to give the desired quantity of steam; these feed gases entered a quartz reactor and were passed through a mixing-preheating section and preheated to the reaction temperature; the preheated feed contacted the catalyst (3 cc by weight for the 300 space velocity test's) in a catalyst, section which was 0.7 em i.d. by 10 em in length. These dimensions permitted the loss of heat from the react'or and thereby afforded near-isothermal conditions. The relatively large void space in t'he reactor caused by these dimensions and the use of '/*-in. catalyst tablets could be tolerated because both the feed and product mixtures were relatively stable t'hermally at reaction temperatures. The product from the catalyst section entered a water-cooled condenser. After the reactor was in operation 15 min, a sample was t'aken of the gaseous product from the condenser. Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 3, 1972
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1400 1600 STEAMING TEMPERATURE, ‘F
1800
Figure 1. Effect of steaming temperature on butadiene yield
Chemical and physical properties were determined as follows: Phosphorus was determined by activation analysis with the Cockroft-Walton positive ion accelerator. Pore volume was measured by mercury intrusion a t 15,000 psig with the dminco Digital Readout Porosimeter (American Instrument Co., Inc.). Surface area was measured by nitrogen adsorption with the Perkin-Elmer Shell Model 212C Sorptometer (Perkin-Elmer Corp.). Average pore diameter was calculated from the pore volume and surface area data. Skeletal density was measured by helium displacement. Catalyst tablet volume was calculated from the tablet diameter (determined by measuring 25 tablets) and from a length which was assumed to be equal to the diameter. Cassiterite concentration in the catalyst was estimated from the X-ray diffraction pattern. Electron micrographs m-ere made with a scanning electron microscope, the Cambridge “Stereoscan” (Cambridge Instruments Co.).
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Results and Discussion
1200
1100
#oo
1800
STEAWG TEMPERATURE. ‘F
Figure 2. Effect of steaming temperature on reaction rate
Experimental conditions for the oxidative dehydrogenation test’s consist,ed of air-to-butene-2 ratio, 4 : 1; steam-to-butene-2 ratio, 18: 1; atmospheric pressure; butene-2 space velocities of 100, 200, 300, 600, and 1200-varied by varying the quantity of catalyst in t,he reactor; 900” and l0OO’F catalyst temperatures-measured with a therniocouple in a thermon-ell which contacted bhe outlet surface of the catalyst. The gaseous products were analyzed with an Applied Automation Inc. (Phillips Petroleum Co.), ;\lode1 12 - h a lyzer-Programmer System equipped with mole sieve and ethyl adipate columns. This analysis measured the C1-CI hydrocarbons, carbon dioxide, carbon monoxide, nitrogen, and oxygen. Any other possible gaseous products and the wat’er-soluble products were disregarded in t’he calculation of the results. These typically account for a lorn rate of the butene-2 feed Because of this, the term approximat’e selectivity is used. Approximate selectivity is based on the gaseous products measured by the above analytical procedure. Other terms used here are widely accepted. Yield of butadiene (moles per 100 moles of butene-2 in the feed) and reaction rates (moles of butadiene produced per lit’er of catalyst per hour) were calculated from the analyses. Because the butadiene yield was affect’ed by the steam-t’o-butene ratio in the feed and because this ratio varied experimentally from the intended 18:1, all butadiene yields were adjusted for the deviation from 1 8 : l . Reaction rates a t a butadiene yield of 50 were obtained from plots of reaction rates vs. butadiene yield. 300
Ind. Eng. Chem. Prod. Res. Develop., Vol. 1 1 , No. 3, 1972
The treated catalysts were tested for the oxidative dehydrogenation of butenes at conditions under which the original untreated catalyst. gave only modest yields of butadiene. The yields given by the steamed catalysts are shown in Figure 1. (The points in boxes at the left side of the figure are t,lie yields given by the original untreat,ed catalyet,.) These results show that steaming t’he original catalyst at’ 1250’F reduced its activity. Increasing the steaming temperature above 1250’F increased mtalytic activity until the temperature reached about 1600°F. Higher temperatures decreased the catalytic activity. For some purposes reaction rates a t a constant yield level are more useful than yields a t a constant set of condit,ions. The rates a t a 50 yield of butadiene are given in Figure 2 . Although the shape of the curves is similar to that in Figure 1, they show that the reaction rates were increased about 150y0. This is a better measure of the change in catalytic activity t,han the 5oy0increase in butadiene yield shown in Figure I . Because approximate selectivities were 96-97 for the original cat’alyst, steaming could not improve this property very much. However, the steamed catalysts consistently gave 9798 approximat’e selectivities, and this improvement may be significant. Steaming could have improved catalytic activity by altering the composition of the catalyst; however, the phosphorus content remained unchanged a t 10.5% after even the most severe steaming-1730°F. This improvement in cat’alyticactivit’y could have resulted from changes in the skin of the cat,alyst tablet or from changes in the entire tablet. It was shown that the improvement probably came from changes it:. the entire tablet in the follo\ving manner: S e w , unsteamed tablets with only 4 ’ 3 of the original skin rrere prepared by cutting the ends from the original l/*-in. cylindrical tablets, leaving “tablet centers” in. in diameter and 1/16 in. in length (Figure 3). The performance of these “tablet centers” could not be compared directly to the performance of the original ‘’*-in. tablets because of the difference in size. To provide a direct comparison, “tablet ends” mere prepared by cutting the original tablets in half (Figure 3). These “tablet ends” were similar to the “tablet centers’’ in dimensions, but the “tablet centers” had of the original tablet skin, whereas the “t’ablet ends” had 2 / 3 of the original skin. These two samples gave essentially identical results in the oxidative dehydrogenation of butenes as shown in Table I. This indicates that the tablet skin did not impose resistance t,o diffusion which was different
“TABLET CENTERS‘
Table 1.
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Figure 3. Catalyst “tablet centers” and “tablet ends”
n
.d
1500
Properties of Catalyst “Tablet Centers” and “Tablet Ends”
Fraction of original tablet skin Size: length x diameter, in. Oxidative dehydrogenation, 900’F Butadiene yield a t 300 space vel. Reaction rate a t 50 butadiene yield Pore volume, ml/g, H g Surface area, X2, m2/g
“Tablet centers“
“Tab let ends”
’/3
‘/3
x
‘/I6
8!‘
‘/I6
x
Original tablets
”3
‘/a
‘/8
x
‘/8
49
52
46
6.5 0.36 28
7.0 0.35 27
6.2 0.35 28
15 I
io ..
I
I
1200 1400 1600 t800 STEAMING TEMPERATURE, ‘F Figure 4. Effect of steaming temperature on pore volume surface area, pore diameter, and reaction rate
from the bulk of the tablet. Kithout evidence of special resistance to diffusion a t the tablet skin, it seems reasonable to assume that steaming affected the total tablet. To explain the improvement in catalytic activity caused by steaming, several physical properties of the steamed catalj sts 11ere measured. Pore T olume, surface area, and average pore diameters are given in Figuie 4. Pore volume v a s slightly reduced by steaming; nonever, this reduction was completed a t 1450°F, whereas catalytic activity continued to increase as the stearning temperat are n a> increased further up to 1600°F Surface area was reduced across the entire steaming temperature range. Average pore diameter remained unchanged until the steaming temperature was incieased to 1450°F. At thiq temperature the reaction rate had been increased about 5070. As the temperature wva s increased above 1450°F, the average pore diameter increased about threefold, TI hereas the catalytic activitj increased onlj 50%. Skeletal density, tablet volume, and crystalline tin oude (casqiterite) for all of the steamed catallsts are given in Figuie 5. Skeletal densitj increased about 10% as the ternperature n a- increased to 1600”F, where the catalytic activity had been increased about I 507, Skeletal density lion remained appro~imatelyconstant, the catalytic activity dropped a significant amount Tablet volume decreased linearly nith iricreasing Steaming temperature; the catall tic actn ity increased to its mauniuni, then remained constant while the
4
1200
1400
1600
1800
STE AMlNG TEMPER AT URE Figure 5. Effect of steaming temperature on skeletal density tablet volume, cassiterite content, and reaction rate
SPLIT TABLET W M E TABLET Figure 6. Electron micrograph views on catalyst tablets
tablet volume decreased. Crystalline tin oxide (cassiterite) content increased from 0 in the new catalyst to about 15% in the catalyst steamed a t 1250°F, whereas the catalytic activity dropped significantly. .Zs the steaming temperature was further increased, the cassit’erite content increased until it reached about 457, at, 1730°F. To explain the improved activity caused by steaming, electron micrographs were made of the st,eamed t’ablets. The six different views illuslrated in Figure 6 were examined for Ind. Eng. Chem. Prod. Res. Develop., Vol. 11, No. 3, 1972
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ORIGINAL CATALYST
STEAMED A T 1730F
Figure 7. Electron micrographs: effect of steaming temperature on center of catalyst tablets
Table II.
Activity of Phosphorus-Tin Oxide Catalyst Heated in Air, Nitrogen, and Steam C4Hsyield''
ReoCti.3" rote' at
at 300 space vel.
50 yield
Original catalyst 46 Original 4-1450'F in air 39 Original 1450'F in nitrogen 36 Original 1450°F in steam 59 900'F dehydrogenation temperature.
+
+
6.2 4.5 3 9 9 1
about 50 tablets to find one meaningful view which could be used to compare different catalysts. View 5, the center of the split tablet, appeared to be satisfactory for this purpose. Micrographs of the center of split tablets on Figure 7 show that steaming a t elevated temperatures increased the roughness of the surface of the split tablet. This appeared to show that streaming created macropores i n the range of 1000-10,000 in diameter. These were not obvious at 1250°F; however, they clearly were present a t 1450"F, where the average pore diameter had changed little. This macropore size appeared to continue increasing as the temperature was raised up to 173OOF. Heating the original catalyst in air and nitrogen (instead of steam) at 1250'F reduced the catalytic activity much like the reduction which occurred with steam. However, in contrast to steam, increasing the heating temperature i n air and
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nitrogen t.o 1450'F did not increase catalytic activity (Table 11).All of the physical properties of the catalysts heated in air and nitrogen were similar to those for the catalyst steamed a t the same temperature (1450OF) except the apparent macroporosity. Electron micrographs showed that the surfaces of the split tablets were relatively smooth and similar to the original catalyst instead of being rough like the steamed catalysts. An explanation for the improvement in catalytic activity caused by steaming was not obvious from any of the eight chemical and physical properties measured on each catalyst. However, some insight into the possible mechanism is provided. The reduction in activity which occurred ab 1250°F shows that Steaming damaged the intrinsic activity of the catalytic surface. Further increases i n steaming temperature may have caused additional damage to the intrinsic activity of the catalyiic surface; however, this surface was made more readily available by the formation of macropores and possibly by an increase in average pore diameter. The net result was a n increase in catalytic activity until the steaming temperature reached about 1600'F. Further increases i n steaming temperature gave a net decrease in catalytic activity, apparently because the reduction in intrinsic activity was faster than the increase in availability of the surface. Heating the phosphorus-tin oxide catalyst in air and nitrogen gave only a decrease in catalytic activity, probably hecause these heat treatments did not cause the formation of macropores. Thus, air and nitrogen treatments caused only a reduction in intrinsic activity of the surface without a compensating increase in the availability of the surface, Literature Cited
Adams, C. R., Voge, H. H., Morgan, C. Z.,Armstrong, W. E., J . Catal., 3, 37946 (1964). Alexander, D. S., Minnis, H., Oliver, B. H. (to Polymer Corp., Ltd.), U.S. Patent 3,396,205 (August 6, 1968). Armstrong, W. E., Voge, H. H., Adams, C. R. (to Shell Oil Co.), U S . Patent 2,991,322 ( J d y 4, 1961). Batist, PH. A,, Lippens, B. C., Schuit, C. A,, J . Catal., 5 , 55-64 ,,Off,
,lJY",.
Callahan, J. L., Gertisser, B., Gra:iselli, R. (to the Standard Oil Co.), U.S. F'atent 3,257,474 (June 21, 1966). Hearne, G. p!'., Furman, K. E. (tu Shell Oil Go.), U.S. Patent 2,991,320 (July 4, 1961). Husen, P. C., Deel, X. R., Peters, W. D., Oil Gas J., 69 (31), 60-61 (Aogwi 9 1071\ .""I,_"._,. Minnis, H., Ch e r , B. H., Rolston, J. H. (to Polymer Corp., Ltd.:1, U.S. Patent 3,409,696 (November 5, 1968). Nolan. G. J. (to Phillips Petroleum Co.), U.S. Patent 3,320,329 (May 16, 1967). Nolan, G. J . (to Phillips Petroleum Co.), U.S. Patent R,446,X69 (May 27, 1969). Shell Internationale Research Maatschbppij N.V., French Patent 1,306,223 (September 3, 1962). Voge, H. IT., Adams, C. R. (to Shell Oil Co.), U.S. Patent 2,991,321 (July 4, 1961). Weiss, A. H., in "Refining Petroleum for Chemicals," Advan. Chem. Sei., No. 97, p 153, R. F. Gould, Ed., American Chemical Society, Washington, DC, 1970. WoskoN, M. Z., Coiling, P. M., Karkalits, 0. C. (to Petra-Tex Chemical Corp.), U.S. Patent 3,428,703 (February 19, 1969). RECEIVED for review November 15, 1971 ACCEPTEDApril 17, 1972 Presented at the Division of Physical Chemistry, 159th Meeting, ACS-CIC, Toronto, Canada, May 1970.
