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I n d . Eng. Chem. Res. 1992, 31,119-125 Developments in Selective Oxidation; Centi, G., Trifirb, F., Eds.; Elsevier: Amsterdam, 1990a; pp 635-642. Centi, G.; TrifrB, F.; Grasselli, R. K. Design of Catalysts for Propane Ammoxidation to Acrylonitrile. Chim. Znd. (Milan) 1990b, 72, 617-624. Froment, G. F. Model Discrimination and Parameter Estimation in Heterogeneous Catalysis. AZChE J. 1976, 21, 1041-1045. Froment, G. F. The Kinetics of Complex Catalytic Reactions. Chem. Eng. Sci. 1987, 42, 1073. Froment, G. F.; Bishoff, K. B. Chemical Reactor Analysis and Design; Wiley: New York, 1976. Glaeser, L. C.; Brazdil, J. F.; Suresh, D. D.; Orndoff, D. A,; Grasselli, R. K. Catalytic Mixtures for Ammoxidation of Paraffins. U.S. Patent 4,788,173, 1988a. Glaeser, L. C.; Brazdil, J. F.; Toft, M. A. Process and Catalysts for Unsaturated Nitrile Manufacture by Alkane Ammoxidation. U.S. Patent 4,783,545, 1988b. Glaeser, L. C.; Brazdil, J. F.; Toft, M. A. Catalysts for Ammoxidation of Propane to Acrylonitrile. US. Patents 4,835,125; 4,837,191; and 4,843,055; 1988~. Guttmann, A. T.; Grasselli, R. K.; Brazdil, J. F. Ammoxidation of Paraffins and Catalysts Therefor. U.S. Patent 4,746,641,1988a. Guttmann, A. T.; Grasselli, R. K.; Brazdil, J. F.; Suresh, D. D. Catalysts for Ammoxidation of Propane and Isobutane to corresponding Unsaturated Nitriles. U.S. Patent 4,788,317, 1988b. Hatano, M.; Kayo, A. Production of Nitriles by Gas-phase Ammoxidation of Alkanes. European Patent 0,318,295, 1987. Himmelblau, D. M. Rocess Analysis and Simulation: Deterministic Systems; Wiley: New York, 1968.
Himmelblau, D. M. Process Analysis and Statistical Methods; Wiley: New York, 1970. Kim, Y. Ch.; Ueda, W.; Moro-oka, Y. Selective Oxidation of Propane to Acrolein and Ammoxidation to Acrylonitrile over Ag-doped Bismuth Vanadomolybdate Catalysts. In New Developments in Selective Oxidation; Centi, G., TrifirB, F., Eds.; Elsevier: Amsterdam, 1990; pp 491-504. Madon, R. J.; Boudart, M. Experimental Criterion for the Absence of Artifacts in the Measurement of Rates of Heterogeneous Catalytic Reactions. Zng. Eng. Chem. Fundam. 1982,21, 438-447. Mears, D. Tests for Transport Limitations in Experimental Catalytic Reactors. Znd. Eng. Chem. Prod. Res. Deu. 1971, 10, 541-547. Miyamoto, A.; Iwamoto, Y.; Matsuda, H.; Inui, T. Selective Ammoxidation of Propane on Vanadoaluminophosphate catalysts. In Zeolites: Facts, Figures, Future; Jacobs, P. A., van Santen, R. A., Eds.; Elsevier: Amsterdam, 1989; Part A, pp 1233-1242. Osipova, Z. G.; Sokolovskii,V. D. Nature of Hydrocarbon Activation in the Oxidative Ammonolysis of Propane on a Gallium-Antimony Catalyst. Kinet. Katal. 1979a, 20, 510-512. Osipova, Z. G.; Sokolovskii, V. D. Stages in the Oxidative Ammonolysis of Propane on a Gallium-Antimony Catalyst. Kinet. Katal. 1979b, 20, 512-515. Turner, J. C.R. An Introduction to the Theory of Catalytic Reactors. In Catalysis, Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1984; Vol. 1, pp 43-96.
Received for review May 3, 1991 Revised manuscript received August 3, 1991 Accepted September 20,1991
Reduction of Sulfur Dioxide over Alumina-Supported Molybdenum Sulfide Catalysts David J. Mulligan and Dimitrios Berk* Department of Chemical Engineering, McCill University, Montreal, Quebec, Canada H3A 2A7
An experimental investigation of the reduction of SO2with CHI using molybdenum sulfide supported on alumina as a catalyst was carried out. Three molybdenum loadings of 5,10, and 15% were used. In addition, a catalyst which contained cobalt ( 5 % Co-15% Mo/A1203) was evaluated. The evaluations were based on the activity as well as the yields of sulfur and carbon dioxide. Experiments were carried out at temperatures ranging from 650 to 725 OC and inlet molar feed ratios of SO2 to CH4 of 1.0 and 2.0. The 5 and 10% molybdenum loadings showed similar activities and yields to each other. The catalyst containing 15% molybdenum had the highest activity and yields (above 77% for both sulfur and COS). All catalysts tested were more effective than alumina itself. The activity of the 15% Mo/A1203catalyst was 1.5-2 times that of alumina. This catalyst was stable under all reaction conditions. The addition of cobalt reduced the activity by 20%. In order to minimize the production of undesired by-products, the reaction temperature should be less than 700 'C.
Introduction The reduction of sulfur dioxide with methane is an important reaction because of its possible use in a process for the treatment of SOz which is removed from the waste gas streams produced by the roasting of sulfide ores. The primary reaction between SO2 and CHI is 2S02 + CHI 2Hz0 + 2[S]+ COZ (1) where [SIrepresents various sulfur species in the gas phase. Along with the primary reaction products, a number of undesired by-products are also possible. These include HzS, COS, CO, and elemental carbon. Therefore, an effective catalyst for this reaction system is one that has a high selectivity for elemental sulfur as well as carbon dioxide. In the past, the reduction of SO2 was implemented in an industrial process which used alumina as the catalyst (Hunter, 1972). Mulligan and Berk (1989) examined the
-
use of pure crystalline MoS2,WSz, and FeS as catalysts for the same reaction. MoSz was found to be not only better than either of the other two metal sulfides, but also better than the commonly used alumina catalyst as MoS2 had a higher selectivity for the production of sulfur, high C02 yields, and comparable activity. Although pure crystalline MoS2is a promising catalyst, there are two problems which would have to be solved if the catalyst were to be used in a large-scale industrial process. First, the pure MoSz pellets used in the above study had a specific surface area of about 4 m2/g, which was 1125th that of alumina. This implies that a relatively large mass of MoSz would be required to obtain conversions found with much smaller amounts of alumina. The second consideration is cost, as pure MoSz is prohibitively expensive. Both of these problems can be addressed by using a catalyst support for MoS2 thus providing the required high surface area and a cost more in line with the
0888-5885/92/2631-0119$03.00/00 1992 American Chemical Society
120 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992
traditionally used alumina pellets. A sulfided cobalt-molybdenum catalyst using alumina as a support has been used for the reduction of sulfur oxides in the liquid phase (Universal Oil Products Company, 1974). An aqueous solution of sodium thiosulfate was reduced using hydrogen with reaction temperatures ranging from 125 to 175 "C. No kinetics were reported; however, conversions to sulfur were found to be as high as 98%, depending on the catalyst preparation procedure and reaction temperature. The above process is an example of the use of a supported molybdenum catalyst for the reduction of sulfur oxides at low temperature; however, such catalysts are primarily being used for hydrodesulfurization (HDS) reactions. Therefore, most of the research on the use, characterization, and preparation of this catalyst reported in the literature is based on the HDS reaction system. Because of its industrial importance, the literature abounds in information concerning the effects of process variables on the activity and selectivity of this catalyst. For example, the relationship between catalyst sulfidation and activity for thiophene hydrodesulfurization was studied by Massoth and Kibby (1977), and the effect of the preparation procedures on the performance of the catalyst was investigated by Houalla et al. (1983), Ledoux et al. (1987), and Lycourghiotis and Vattis (1982). In addition, most HDS catalysts include a promoter such as cobalt or nickel which increases significantly the activity of the supported molybdenum catalyst. Possible explanations presented in the literature for the promoting role of cobalt in HDS reactions have been summarized by Massoth (1978). The promoting role of cobalt has been ascribed to (1) an increase in Mo dispersion over the support surface by preventing the crystallization of MoS2, (2) an intercalation effect with MoS2leading to formation of Mo3+,(3) a specific kinetic effect where cobalt may affect adsorption-desorption properties, and (4) a decrease in deactivation due to decreased coking. However, for the reduction of SO2 with CH4, it is not yet known whether the net effect of the addition of a promoter is to enhance, or diminish, the performance of the molybdenum catalyst. HDS catalysts have been extensively characterized with respect to their surface area, structure, and chemical composition at temperatures below 500 "C, the upper limit for these reactions. However, the lowest temperature at which the reduction of SO2 can occur at reasonable rates is 650 "C and, hence, the stability of the supported molybdenum catalyst is not known. An important aspect of the stability of the catalyst is the crystallinity of the MoS2 phase on the surface. Crystallization of MoS2 has been observed at HDS reaction conditions and leads to catalyst deactivation (Okamoto et al., 1977). However, for the reduction of SO2 with CHI, crystalline MoS2 has been shown to be itself catalytic. Unfortunately, since the reaction temperatures are above 650 "C, and the sublimation temperature of MoS2is only 450 "C, it is possible that the MoS2 phase could be removed from the support surface. The reduction of SO2 with CHI using two HDS catalysts was studied by Sarlis and Berk (1991). One of the catalysts used was 3.5% CoO-14% MoO3/AlZO3and the other was 10-12 % Mo03/A1203. Kinetic results were reported for the reduction of SO2 with molar feed ratios of SO2 to CHI from 0.5 to 2.5 and temperatures from 650 to 750 "C. The cobalt-containing catalyst was the more active of the two. However, the molybdenum catalyst was more selective for the production of sulfur. In addition, a proposed mechanism based on the kinetic results attributed the production of the undesired by-products to the cracking of CH4. The
effect of molybdenum or cobalt loading on catalyst performance was not investigated nor was the stability of the supported catalysts. The purpose of the work presented in this paper is to evaluate various alumina-supported molybdenum sulfide catalysts for the reduction of SOz with CHI and compare them with both pure MoSz and alumina. The evaluation and comparison of the catalysts are based on the reaction rates, yields of elemental sulfur and carbon dioxide, and durability under the severe reaction conditions. In this study, three loadings of molybdenum, 5, 10, and 1596, supported on alumina are considered. In addition, a fourth catalyst containing 5% cobalt and 15% molybdenum, also supported on alumina, is evaluated. The activity and the yields of elemental sulfur and carbon dioxide were measured as functions of temperature. The effect of inlet molar feed ratio of SO2 to CH4 was measured also as a function of temperature for the 15% Mo loading. Durability was determined by observing the effect of exposure to reaction conditions on the composition of the catalyst, surface area, and, hence, activity and yields.
Materials and Methods Catalyst Preparation. Alumina was purchased as 2-mm-diameter spherical pellets from Alcan Chemicals, Brockville, ON. Both ammonium heptamolybdate (NH4)6Mo,024.4Hz0and cobalt nitrate (Co(N0JZ.6H20 were purchased from Johnson Matthey, Inc., Melvern, PA. Before impregnation, the alumina pellets were conditioned at 600 "C for 6 h in a flow of argon in the reactor tube and then kept dry in an oven at 125 "C. The supported molybdenum catalysts were prepared by impregnating the alumina pellets with solutions of ammonium heptamolybdate. Preliminary experiments showed that 100 g of alumina can absorb 100 cm3 of solution; thus the desired loading of the catalyst, whether 5,10, or 15% Mo, was obtained by fixing the concentration of ammonium heptamolybdate in the impregnating solution. A 15% Mo loading is defined as 15 g of the element molybdenum added to 85 g of dry alumina pellets. The molybdenum loading was based on the element, and not on the sulfide or oxide, because the oxidation and sulfidation states of the molybdenum change throughout the preparation procedure and reaction process while the quantities of both Mo and alumina remain constant throughout. Having determined the appropriate concentrations, the solutions for impregnation of the alumina pellets were prepnred by dissolving the ammonium heptamolybdate in deionized water. The solution was then mixed with the pellets and allowed to stand at room temperature for a period of 6 h. The pellets were then placed in an oven at 110 "C for 16 h to remove ammonia and water. Once dried, the pellets were calcined in a flow of air in the reactor tube at 500 "C for a period of 24 h. Following calcination, the flow of air was replaced with a flow of dilute H2S (12% H2S in argon). The temperature was raised to 600 "C. This sulfidation procedure was continued until the uptake of H2Swas completed. This was determined by gas chromatographic analysis of the reactor exit gases. Usually, a period of 5 h was required for the sulfidation of a 15-g catalyst sample. Following sulfidation, the H2Swas replaced with argon and the temperature was increased to 750 "C to remove any excess sulfur in the catalyst pores. Samples were then stored in a desiccator at ambient temperature until use. The steps involved in preparing the 5 % Co-15% Mo/ A1203catalyst pellets were essentially the same as described above. The main difference, however, was in the
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 121 Table I. Composition of Inlet Gas Mixtures at Different Feed Ratios S02/CH4feed ratio SO2, % CHI, % 1.0 25 25 2.0 30 15
Table 11. Composition of the Sulfided 15% Mo/A120s Catalyst
Ar, % 50 45
%
MoS~ Moo2
17.0 4.2 78.8
A1203
composition of the impregnating solution. An appropriate quantity of cobalt nitrate was added to the ammonium heptamolybdate solution. This solution was then mixed with the dried alumina pellets in the same proportions as described above. Experimental System and Procedure. The experimental system and procedure for determination of reaction rates were essentially the same as those described by Mulligan and Berk (1989). In the present work, experiments were carried out in the temperature range from 650 to 725 at 25 "C intervals. Two ratios of inlet SO2 to CHI concentrations were used, 1.0 and 2.0. The concentrations of each of the gases for these ratios are given in Table I. For the experiments which required differential conversions, the quantity of the catalyst charged to the reactor was adjusted to give conversions below 20% while the total feed flow rate was maintained constant. Catalyst Characterization. The characteristics of the prepared catalyst pellets were determined using a combination of several analytical methods. The surface area was measured before and after experimentation by using a Micromeritics Flowsorb Model 5200 surface area analyzer. The composition of the catalyst pellets was determined using scanning electron microscopy, X-ray diffraction (XRD), wet chemical analysis, and an elemental analyzer. X-ray mapping with a scanning electron microscope was used to determine the distribution of molybdenum and cobalt throughout the pellet. XRD analysis was used to determine the bulk composition of the crystalline phases present in the pellets. For both XRD and elemental analysis, the samples were required to be in powder form. This was accomplished by using a ball mill. Catalyst samples were pulverized under liquid nitrogen to prevent oxidation. The molybdenum content of the catalyst was verified by the following method: Catalyst samples weighing 0.1 g each were dissolved in 100 mL of aqua regia at 60 "C for a period of 72 h. A 10-mL sample of the resulting solution was then diluted to 100 mL with deionized water and analyzed for molybdenum content using an atomic absorption spectrophotometer (Therm0 Jarell Ash Corp. Model Smith-Hieftje 11). No alumina was dissolved during the digestion procedure as no aluminum was found in the solution. An elemental analyzer (Control Equipment Corp. Model 240XA) was used to determine the quantity of sulfur present in the pellets. The amount of MoSzwas calculated from the measured quantities of molybdenum and sulfur. The remainder of the molybdenum was in the oxide form as determined by XRD. The remaining portion of the pellet was alumina.
Results and Discussion Characterization of the Catalysts. Preliminary experiments showed that the procedure used for preparing catalysts was repeatable. Three samples of 15% Mo catalyst were prepared separately, and experiments showed that there was no difference in the activity and the yields of sulfur and carbon dioxide of each catalyst. X-ray mapping of a split pellet further showed that the impregnation procedure resulted in uniform distribution of molybdenum and cobalt throughout individual pellets.
Table 111. Surface Area Analssis of the Catalssts surface area, m2/g alumina 113.3 5% Mo/A1209 91.4 10% Mo/A1203 80.2 15% Mo/A1203 112.9 75.1 5% Co-15% Mo/A1203 ~
~~
Analysis by X-ray diffraction of the 5% Co-15% Mo/ Alz03pellets before use in reaction confiied the presence of MoSz, CosS8,CoMo04,and alumina. XRD analysis of the 5,10, and 15% Mo/Al203, also before use in reaction, showed that MoS2, Moo2, and y-alumina were the only species detected. These results show that complete sulfidation of all the molybdenum did not occur and that the oxide was reduced to Moo2. Thermodynamic analysis showed that the sulfidation procedure with H2S should result in the complete conversion of Moo3 to MoS,; however, some Moo3 was reduced to Moop without being sulfided. Wet chemical analysis verified that all samples contained the desired quantity of molybdenum. In order to determine the effect of long-term exposure of the supported molybdenum catalyst to the severe reaction conditions required for the reduction of SO2,the following experiment was performed. A sample of the 15% Mo/A1203 catalyst was charged to the reactor, and the reduction of SO2 with CHI was carried out at a temperature of 700 "C using a molar feed ratio (SOz/CH4)of 1.0. Steady state was achieved in 6 h. The reacting gases, SO2 and CHI, were than shut off, leaving only a flow of argon. The temperature was maintained at 700 "C for a period of 48 h. Following this period, the flow of both reacting gases was resumed, and steady state was again obtained. I t was found that all reaction rates and, hence, yields of sulfur and carbon dioxide were unchanged from the first steady state to the second. A complete analysis of the composition of the 15% Mo/A1203catalyst after use in the reaction (Table 11) shows that the species present remain unchanged by the reaction. The surface areas of each of the catalysts were measured and are presented in Table 111. These values were changed by less than 5 % during experimentation indicating that the catalysts were not significantly sintered. From this, from the kinetic results, and from the chemical analysis, it was concluded that the catalyst is stable, even after exposure to the severe reaction conditions. Reduction of SO2 over Supported Molybdena Catalysts. As stated earlier, comparison of the catalysts is based primarily on the activity and the yields of sulfur and carbon dioxide. Since SOz and CHI do not react in the absence of a catalyst below 800 "C (Sarlis and Berk, 1988), the activity is defined as the rate of consumption of SO2 in mol/(m2.s). The reaction rates are reported per unit surface area in order to take into account the surface areas of the different catalyst preparations shown in Table 111. The sulfur yield is defined as follows: Y S= ( r ~ s ~ / r s X o 2100% ) (2) where ri is the rate of reaction of species i. Similarly carbon dioxide yield is defined in the following manner: Y C O=~(rC02/rCHI)X 100%
(3)
122 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992
bi
850
725
875
750
Temperature C"
Figure 1. Effect of temperature on activity using catalysts with various molybdenum loadings.
imr
850
7FC
875
725
750
Temperature
Figure 3. Effect of temperature on rate of sulfur production using catalysts with various molybdenum loadings.
1
d .
625
Gi-
850
e75
Temperature
7?0
725
750
C
."625
850
875
7Pc
725
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Temperature
Figure 2. Effect of temperature on sulfur yield using catalysts with various molybdenum loadings.
Figure 4. Effect of temperature on carbon dioxide yield using catalysts with various molybdenum loadings.
Both yields are essentially a measure of the degree to which the primary reaction takes place. High values of sulfur yield indicate low production rates of H2S and COS in comparison to that of elemental sulfur. Likewise, high C02 yields indicate low production rates of the undesired carbon containing species including elemental carbon, CO, and COS in comparison to that of COB. Figure 1shows the effect of temperature on the activity at a feed ratio of 1.0 for catalysts with molybdenum loadings of 5,10, and 15%. For comparison, the results for alumina and pure MoSz are also included. The highest activity is found when the 15% Mo loading is used. These results are comparable to those obtained with pure MoS2 (Mulligan and Berk, 1989). Experiments using a 21% Mo/A1203have also been performed, but the results are not shown since they are the same as those of the 15% loading. The activities of the 5% and 10% Mo/A1203are virtually equal at all temperatures, but are lower than those found for the 15% Mo/Al2O3. Alumina is the least active. In fact, the rates obtained with 15% Mo/A1203 are approximately double those with alumina. The sulfur yields and sulfur production rates for a feed ratio of 1.0 are plotted versus temperature in Figures 2 and 3, respectively. In general, the sulfur yield decreases with increasing temperature. The highest yields are found when 15% Mo/A1203is used. The lowest yields are seen with alumina. As before, the 5 and 10% Mo/Alz03 catalyst results are almost identical to each other and are between the yields of alumina and 15% Mo/A1203. The sulfur production rates plotted in Figure 3 clearly show one of the advantages of using 15% Mo/A1203for
the reduction of SOp. Since one of the primary objectives of the work was to find a catalyst for this reaction system to selectively produce elemental sulfur, a high sulfur production rate is desirable. Clearly, the 15% Mo/A1203 catalyst does not only provide the highest sulfur production rates, but it also provides the highest sulfur yield. This implies that the production of the by-products is minimized. The 5 and 10% Mo/A1203 catalysts have sulfur production ratea similar to each other, but lower than those found with the higher loading. The sulfur rates with alumina are the lowest and are approximately half those of 15% Mo/A1203. It should be noted that all of these results were obtained using differential conversions so that the reactions can be considered to take place at the average of the inlet and exit concentrations. Some integral conversion results are given later in the discussion. The effect of molybdenum loading on products containing carbon has also been considered. Figure 4 is a plot of COPyield versus temperature. Carbon dioxide yields greater than 73% were obtained with all loadings at all temperatures while alumina results, which are not included in Figure 4, ranged from only 40% to 70%. In general, the yields shown in Figure 4 decrease with increasing temperature, particularly as the temperature is increased to 725 OC. Again, 15% Mo/Alz03 was found to provide the highest yield, especially at the highest temperature tested. The decrease in the COzyield at 725 OC shown in Figure 4 is mainly due to a significant increase in the elemental carbon production at 725 OC as shown in Figure 5. Carbon deposition on the catlyst surface can also lead to the
Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 123
"
I
O.il 0.2
0.0
625
'I ~
850
875
Temperature
7?0
725
750
C
Figure 5. Effect of temperature on rate of carbon production using catalysb with various molybdenum loadings.
deactivation of the catalyst. There is only a slight difference between the three supported catalysts. On the other hand, compared to pure MoS2, the rates with the supported molybdenum are found to be approximately one-half (Mulligan and Berk, 1989) while no elemental carbon is produced with alumina even at 725 "C. Although the activity results indicated that the 15% Mo/Al2O3catalyst behaved in the same manner as pure MoS2, the carbon results show that there is a difference between the two catalysts. The effect of the support was to improve the performance of the catalyst by decreasing the production of elemental carbon while maintaining the other qualities associated with pure MoS2 such as high activity and sulfur yield. Another important observation is that no carbon monoxide was found with any supported molybdenum catalyst at temperatures below 725 "C. On the other hand, when alumina was used as the catalyst, CO was produced at all temperatures. In fact, a t 700 "C, 30% of the carbon from the reacted methane appeared as CO. Another important difference between the supported molybdenum catalysts and alumina is in the relative rates of production of H2S and COS. For the supported molybdenum catalysts, the H2Sproduction rate was 2-4 times that of COS while for alumina, the reverse was found to be true. Twice as much COS was produced in comparison to H2S. Clearly, a different reaction mechanism is involved when molybdenum is supported on alumina. There is a definite trend in the activities and yields with respect to the loadings of molybdenum. The results obtained with the 5 and 10% Mo catalysts were consistently similar to each other. The 15% Mo loading showed the highest activities and yields which were also similar to those obtained with pure crystalline MoS2. Although no positive identification of the species present on the surface was made, the 5 and 10% Mo loadings may provide only partial coverage of the alumina support surface. On the other hand, the 15% Mo loading may be sufficient to completely cover the alumina surface and allow for some crystallization. This may explain why these results are consistent with pure MoSP The experimental results discussed so far were obtained using a feed ratio of 1.0. From these results the 15% of Mo/A120, catalyst was determined to be the best of those tested. In order to investigate the effect of reactant concentration, experiments were carried out using this catalyst and a molar feed ratio of 2.0. The activity and sulfur and COz yields are given in Figures 6, 7, and 8, respectively. The effect of the feed ratio on activity is dramatic, especially at the higher tem-
625
850
875
Temperature
7FC
~~
725
750
Figure 6. Effect of temperature on activity at two feed ratios using 15% Mo/A1203. IMl
'"' 0
2.0
Feed Ratio
881
625
850
875
Temperature
7Pc
725
750
Figure 7. Effect of temperature on sulfur yield at two feed ratios using 15% Mo/A1203.
6%
850
875
Temperature
7Fc
725
750
Figure 8. Effect of temperature on carbon dioxide yield at two feed ratios using 15% Mo/A1203.
peratures at which the rate of SO2 consumption falls by over 50% when the feed ratio is changed from 1.0 to 2.0. Since the CHI concentration was decreased from 25 to 15%, while that of SO2 was increased from 25 to 30%, it can be concluded that SO2actually has little effect on the reaction rate in comparison to CH4. In addition, the effect of the molar feed ratio on sulfur yield is also significant. Increasing the feed ratio from 1.0 to 2.0 increased the sulfur yield by up to 5% at 725 "C. On the other hand, the COz yield was relatively unaffected by this change except at 725 "C. Because of the possible decreased carbon pro-
124 Ind. Eng. Chem. Res., Vol. 31, No. 1, 1992 Table LV. Effect of Cobalt on Activity and Yield of Sulfur and COz activity, x lo* g-mol/(mz.s) sulfur yield, % COz yield, % catalvst 700 O C 725 O C 700 O C 725 "C 700 O C 725 O C 91.0 77.2 87.6 6.3 12.0 92.6 15%Mo/AIz03 73.5 87.3 93.7 89.5 5% Co-15% 5.1 9.8 Mo/Alz03 Table V. Reaction Rates of Sulfur Species (Rates X lo9 g-mol/(m2 8 ) ) 15% Mo/Alz03 alumina 65OOC 700OC 65OOC 700OC SO2 10.6 26.4 8.74 28.2 14.1 8.24 8.07 9.54 [SI 0.90 11.9 0.39 19.3 HZS cos 0.03 0.39 0.11 0.83
duction at lower concentrations of CHI, higher C02yields were expected. However, COz yields obtained for a feed ratio of 1.0 were already high at temperatures less than 725 "C; hence a decrease in CHI concentration provided a significant improvement in the C02 yield only at that temperature. Reduction of SO2over the Co-Mo/A1203 Catalyst. As was stated in the Introduction, the addition of cobalt to supported molybdenum catalysts increased the activity for hydrodesulfurization reactions. Table IV shows results comparing the activity and yields of sulfur and C02for 5% Co-15% Mo/A1203and 15% Mo/Al2O3when used for the reduction of SO2 with CHI at a feed ratio of 1.0 and temperatures of 700 and 725 "C. Generally, the activity of the cobalt-containing catalyst was 20% lower than the supported molybdenum catalyst itself. Both sulfur and C 0 2 yields did not significantly change with the addition of cobalt; however, in all cases, sulfur yields were marginally higher with cobalt while C02 yields were marginally lower. One of the reasons cited in the literature for the beneficial effect of cobalt on HDS catalysts is its ability to maintain the even distribution of MoS2over the support surface and prevent MoS2 crystallization. However, pure crystalline MoSz has been shown to be an active catalyst for the reduction of SO2 with CHI (Mulligan and Berk, 1989). Therefore, a possible reason for the lower activity of the cobalt catalyst is that it inhibits the formation of MoS2 crystal clusters on the support surface. Sarlis and Berk (1991), who also investigated the effect of cobalt, reported that a 3.5% CoO-14% Mo03/A1203 catalyst was more active than a 10% M003/A1203catalyst. This increase in the activity, however, is not due to the presence of cobalt. As is shown in the present work, higher molybdenum loadings result in higher activity (Figure 1); therefore the increased activity of the cobalt-containing catalyst was due to its higher molybdenum content and not the presence of cobalt. Since there were no problems encountered with the stability of the supported molybdenum catalyst, and the addition of cobalt did not improve the characteristics and performance of the catalyst, we concluded that cobalt was undesirable for the reduction of SO2 with CHI. Integral Conversion Results. As stated above, all results presented thus far were determined from experiments where the conversions were less than 20%. For an industrial process, however, conversions as high as 100% are required. Therefore, some integral conversion results are presented in Tables V and VI for the 15% Mo/A1203 catalyst and alumina. Conversions of SO2,for both catalysts, were 30% and 100% at 650 and 700 "C, respectively.
Table VI. Yields of Sulfur and C 0 2 sulfur yield, % 65OOC 700OC 15% Mo/Al,O? 91.1 53.4 - " alumina 94.3 28.9
COz yield, % 65OOC 70O0C 99.4 97.7 88.5 91.7
The higher conversions for these experiments in comparison to the differential experiments were obtained by simply increasing the catalyst loading in the reactor. Again, the two catalysts are compared with respect to their activities and to the yields of sulfur and COBfor a molar feed ratio of 1.0 at the temperatures of 650 and 700 "C. The rates of SO2 consumption of both catalysts are comparable to each other at both temperatures with the rates with alumina being slightly higher at 700 "C. It is also seen that the sulfur yields for both catalysts decrease significantly with the increase in temperature. A comparison of the two catalysts shows that, at 700 "C, the sulfur yield obtained with 15% Mo/Al2O3was 25% higher than with alumina. The higher rate of consumption of SO2 with alumina at this temperature, therefore, was the result of the increased production of H2S and not that of elemental sulfur. Clearly, if reasonably high sulfur yields are to be maintained, the reaction temperature must be kept below 700 "C. The rate of COS production increased approximately by an order of magnitude with the increase in temperature for both catalysts; however, these rates remain relatively low compared to those of H2S. Temperature had little effect on the C02yields for either catalyst. However, the supported molybdenum catalyst results were at least 6% higher than the C02 yields obtained with alumina and remained above 97%, even with the high conversions. Conclusions All catalysts containing molybdenum showed higher activities and higher sulfur and CO, yields than alumina. The 5 and 10% Mo/A12Q3catalysts were similar in all three aspects of catalyst performance considered. However, the 15% Mo/A1203catalyst was found to have activity 1.5-2 times those of the other loadings, higher sulfur yields, and comparable C02yields. This catalyst was also found to be stable under the severe reaction conditions. The major side product was H a , but ita rate of production could be minimized by keeping the reaction temperature below 700 "C. The addition of cobalt to this catalyst had a detrimental effect on its performance. From these results, and from the results of the integral experiments, it was concluded that 15% Mo/A1203 was the best of the catalysts tested. Acknowledgment We thank the Natural Sciences and Engineering Research Council of Canada for the financial support of this project. Registry NO. SOZ, 7446-09-5; CHI, 74-82-8; MoS~,1317-33-5; CO, 7440-48-4; S, 7704-34-9; COz, 124-38-9. Literature Cited Houalla, M.; Kibby, C. L.; Petrakis, L.; Hercules, D. M. Effects of Impregnation pH on the Surface Structure and Hydrodesulfurization Activity of Mo/Alz03Catalysts. J. Catal. 1983, 83, 50-60. Hunter, W. D.; Wright, J. P. Sulfur Dioxide Converted to Sulfur in Stackgas Cleanup Route. Chem. Eng. 1972,79,50-51. Ledoux, M. J.; Hantzer, S.; Guille, J. A Comparative Study of the Influence of the Preparation on the Activity of NiMo and NiW HydrodesulfurizationCatalysts. Bull. SOC.Chim. Belg. 1987,96, 855-863.
I n d . Eng. Chem. Res. 1992,31, 125-130 Lycourghiotis, A.; Vattis, D. Hydrodeeulfurizationof Thiophene over Na-Doped CoMo/Alp03 Catalysts Prepared by Inverse Impregnation. React. Kinet. Catal. Lett. 1982,21, 23-27. Massoth, F. E. Characterization of Molybdena Catalysts. In Advances in Catalysis; Eley, D. D., Pines, H., Weisz, P. B., Eds.; Academic: New York, 1978; Vol. 27. Massoth, F. E.; Kibby, C. L. Studies of Molybdena-Alumina Catalysts V. Relation Between Catalyst Sulfided State and Activity for Thiophene Hydrodesulfurization. J. Catal. 1977,47,300-315. Mulligan, D. J.; Berk, D. Reduction of Sulfur Dioxide with Methane Over Selected Transition Metal Sulfides. Znd. Eng. Chem. Res. 1989,28,926-931.
Okamato, Y.; Nakano, H.; Shimokawa, T.; Imanaka, T.; Teranishi, S. Stabilization Effect of Co for Mo Phase in Co-Mo/A1203Hy-
125
drodesulfurization Catalysts Studied with X-Ray Photoelectron Spectroscopy. J. Catal. 1977,50,447-454. Sarlis, J.; Berk, D. Reduction of Sulfur Dioxide with Methane over Activated Alumina. Znd. Eng. Chem. Res. 1988,27, 1951-1954. Sarlis, J.; Berk, D. Reduction of Sulfur Dioxide by Methane over Transition Metal Oxide Catalysts. Chem. Eng. Commun. 1991, in press. Universal Oil Products Company. Catalyseurs Bimetalliques Utilisable Notamment pour la Reduction de Composes Oxysoufres. Fr. Patent 2223081,1974.
Received for review April 25, 1991 Revised manuscript received September 11, 1991 Accepted September 24, 1991
Methyl tert -Butyl Ether Synthesis over Titanium-Silicalite I Catalysts Kyung-Ho Chang, Geon-Joong Kim, and Wha-Seung Ahn* Department of Chemical Engineering, Znha University, Incheon 401 - 751, Korea
The vapor-phase reaction of methanol with isobutene to form MTBE (methyl tert-butyl ether) was carried out using titanium-modified silicalites at 70-110 "C under atmospheric pressure, and the results were compared with those obtained using HZSM-5. The acid sites responsible for MTBE synthesis were mainly of weak to medium acid strength. MTBE synthesis reaction kinetic data could be fit with the Langmuir-Hinshelwood mechanism, which assumes the reaction between adsorbed methanol molecules with isobutene adsorbed at two different acid sites is the rate-determining step.
Introduction Methyl tert-butyl ether (MTBE) has been regarded as a promising alternative octane booster to toxic lead additives in gasoline (Reynolds et al., 1975). The reaction for obtaining MTBE could also be of interest as a means of quantitative separation of isobutene from 1-butene in C4 petroleum cuts (Fattore et al., 1981). MTBE can be formed by addition of methanol to the double bond of isobutene using acid catalysts. Commercially, MTBE is obtained in the liquid phase below 100 "C at 200 psig using a cation-exchanged resin (Amberlyst) catalyst (Pecci and Floris, 1977). The ion-exchanged resin is,however, not stable above 90 "C; overheating causes the release of acid materials from the catalyst (Takesonoand Fujiwara, 1980), and there exists a need for developing a stable alternative catalyst. According to Chu and Kiihl(1987), zeolites ZSM-5 and ZSM-11 produced both high conversion and high selectivity to MTBE. Furthermore, it is expected that isomorphous substitution of Ti into the zeolite framework would produce mild strength acid sites over the catalyst surface, which may be effective for MTBE reaction. The objective of the present study is to prepare a series of isomorphously substituted Tiailicalites and to evaluate their catalytic activities and selectivities to MTBE in the vapor-phase reaction of methanol with isobutene. Experimental Section Materials. Fine amorphous silica powder (KoFran Co., 91.8% sio2-8.2% H20), sodium hydroxide (Junsei Co., 95% 1, tetrapropylammonium bromide (Fluka Co., TPABr), and Ti(OC3H7I4(Aldrich, 99%) were used for Ti-silicalite preparation. Methanol (James Burrough Ltd., Witham), with a minimum purity of 99% containing less than 0.1% water, and isobutene (Korea Standard Research Institute), with a minimum purity of 99%, were used without further
purification for the MTBE synthesis reaction. Zeolite Synthesis and Characterization. Tiailicalite samples were prepared from the substrate compositions of 0.13Na20Si02-(0.0014-0.014)Ti02-39H20-0.12TPABr. The reaction mixture was stirred for 30 min, and the hydrothermal synthesis reaction was carried out in a stainless steel tube (100-mL capacity) at 140-170 OC without agitation. The solid products were washed, filtered, and dried at 120 "C for 12 h. The crystalline phase obtained was identified using an X-ray diffractometer (Philips, PW-1700, Cu Ka, Ni filter), and the relative crystallinity of the sample was determined by comparing the peak areas at 28 = 20-25O with those of the best crystallized silicalite I. Morphologies were examined with a scanning electron microscopy (Hitachi, X-650), and IR spectra were obtained with a Nicolet lOOMX spectrometer. The acid type and acid strength of the zeolite catalysts used in the MTBE synthesis reaction were determined by and means of temperature-programmed desorption (TPD) IR analysis after pyridine preadsorption at room temperature. Reactant adsorption characteristics on different acid sites have also been examined. MTBE Synthesis Reaction and Analysis. The vapor-phase MTBE synthesis reaction were carried out in a f=ed bed reactor at atmospheric pressure. N2carrier gas was bubbled through a wash bottle filled with methanol, and the methanol-saturated N2 was then mixed with isobutene gas before the reactant mixture was introduced into the Pyrex reactor. Lines suspected of condensation were all electrically heated. The inside/outside temperature gradient of the reactor was measured by means of two chromel-alumel thermocouples. Catalysta were pretreated in a N2environment at 500 OC for 4 h. A sampling valve at the reactor outlet allowed on-line analysis of the reactor effluent using Hitachi 263-30 gas chromatography equipped with a thermal conductivity detector. A 2 m X 0.0032 rn O.D.stainless steel GC column packed with Porapak Q (80/100 mesh) was used for product separation. The
OSSS-5SS5/92/2631-0125$03.Q0~0 0 1992 American Chemical Society
