C Catalyst for Methanol Carbonylation

488

Znd. Eng. Chem. Res. 1994,33, 488-492

Promoting Effect of Tin on Ni/C Catalyst for Methanol Carbonylation Tuan-Chi Liu' and Shwu-Jer Chiu Department of Chemical Engineering, National Taiwan Institute of Technology, Taipei 106, Taiwan, ROC

Vapor-phase carbonylation of methanol was investigated under atmospheric pressure on Ni/C and Sn-Ni/C catalysts. Sn-Ni/C was found as having a significantly higher activity than Ni/C in converting methanol and CO to methyl acetate and acetic acid. Product distributions of the two catalysts did not markedly differ from each other. Characterizing the catalysts by XRD demonstrated that NbSn had formed on Sn-Ni/C. The adsorption of CO became substantially enhanced by the addition of Sn to Ni/C. This addition also resulted in a slightly higher CO adsorption strength.

Introduction

Experimental Section

Methanol carbonylation is currently the most important process in producing acetic acid. The process, originally developed by Monsanto, utilizes a homogeneous catalyst for implementing the reaction in the presence of methyl iodide. The catalyst contains rhodium. The significance of acetic acid in the petrochemical industry has made the process a research subject of intensive studies. These studies have been going on for a long time, drawing attention from both industrial and academic sectors. Most research efforts are undertaken for both modifying the old catalyst and also in finding a new one. Finding a suitable heterogeneous catalyst has been previously attempted by employing various metals and supports. Liu and Wang (1992) investigated methanol carbonylation on catalysts prepared by several metals and molecular sieves. The application of molecular sieves toward the carbonylation was also reported (Andersen and Scurrell, 1983; Christensen and Scurrell, 1977; Scurrell and Howe, 1980; Takahashi et al., 1979; Yashima et al., 1979). Catalysts of activated carbon supported transition metals were also tested (Fujimoto et al., 1983,1987;Omato et al., 1985, 1988; Robinson et al., 1972; Shikada et al., 1985). The following activity order was found: Ni/C > Co/C > Fe/C (Fujimoto et al., 1982). The effects of the support on the activity were significant. Substantially higher activity could be obtained on Ni/C than on Nil Si02 or Ni/A1203 (Shikada et al., 1985;Omata et al., 1988). Polymer anchored catalysts were also explored for carbonylation (Jarrell and Gate, 1975;Hjortkjaer and Jensen, 1976; Drago et al., 1981; Shimazu et al., 1987; Webber et al., 1977, 1978). The catalysts provided evidence for the concept of heterogenizing a homogeneous catalyst. Even though considerable literature is published on promoters for the homogeneousprocess (Smithet al., 1987), only a few reports are found regarding promoters for the heterogeneous process. Liu and Wang (1992) examined the effect of adding a second metal to Rh/NaX for the reaction and found that the addition of Ni, Cu, or Co had no promoting effect. In the study of NiB for methanol carbonylation, NiB was observed by Chen and Ling (1991) as having a higher activity than Ni. This result was interpreted in a way that B served as a promoter. The promoting effect of Sn on NilC for methanol carbonylation has previously never been thoroughly explored. Original new data concerning this subject is presented in this paper. Information related to the effect, obtained by characterizing the catalysts, is also reported.

Catalyst Preparation. The catalysts were prepared via impregnation. In the process, Ni(N03)~6HzOand/or SnC1~2H20(Merck, GR grade) were dissolved in deionized water. Activated carbon of 1023 m2/gfrom Strem Chemicals Inc. was then impregnated with the solution. The slurry thus obtained was then dried in vacuo at 393 K for 12 h and was then stored in a desiccator. Reducing the material from the desiccator at 673 K under hydrogen flow for 3 h results in Ni/C and Sn-Ni/C catalysts. In order to have a fresh catalyst for reaction and for characterization, the reduction was performed at a time immediately prior to usage. The catalyst prepared for this study had a nickel content of 5 w t % and a tin content ranging between 0 and 5 wt 7%. XRD. A Philips X-ray diffractometer (ModelPW 1710) was used for the analysis. The light source was CuKa. The applied current and voltage were 30 mA and 40 kV, respectively. During the analysis, the sample was scanned from 8' to 90' with a speed of 0.08O/s. BET. The surface areas of various samples were measured by an ASAP 2000 (Micromeritics Co.). Operation of the instrument was controlled via a personal computer, and the measurement was basically an automatic process. The instrument applied static physisorption of nitrogen for determining the surface area of a solid. In each measurement, 0.2 g of sample was placed in a sample cell. The sample was then degassed a t 573 K and 6.67 X lo4 kPa for 12 h. After it was degassed, the sample temperature was lowered to 77 K by liquid nitrogen. At this temperature, nitrogen was dosed into the cell, which permitted the sample to adsorb nitrogen. The quantity of nitrogen adsorbed was then converted to surface area by a BET equation. Chemisorption. In this study, chemisorption was performed on an ASAP 2000 Chemi System (Micromeritics Co.). The principal part of the instrument was the same as the ASAP 2000 for surface area measurement. The difference between the ASAP 2000 and the ASAP 2000 Chemi System was that the latter was equipped with a turbo pump and extra software. The pump permitted sample degassing to be conducted more thoroughly; consequently, the sample could be prepared with less residual contaminants. Moreover,high vacuum capability during analysis allowed more data points to be collected in the low-pressure range. This feature was especially favored for low quantity chemisorption measurement. The software was utilized for both automatically controlling operational conditions and also for data treatment. High-purity hydrogen and carbon monoxide were purchased for the experiments. Further purification of the

* To whom all correspondence should be addressed.

0888-5885/94/2633-048~~0~.50f 0 0 1994 American Chemical Society

Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 489 gases was performed by passing them through commercially available traps for removal of trace moisture and oxygen. In each experiment, 0.2g of catalyst was loaded in a cell. The adsorbed moisture and gases on the catalyst were removed by degassing the catalyst at 393 K and 6.67 X 10" kPa for 12 h. The catalyst was then reduced at 673 K under hydrogen flow for 3 h. The hydrogen, which was adsorbed on the catalyst after the reduction, was removed by heating and evacuating at 703 K and 1.32 X 10-6 kPa, respectively for 1 h. Following removal, the catalyst temperature was lowered to 308 K under vacuum. At this temperature, the gas to be adsorbed was introduced into the sample cell. The adsorbed gas, following such a procedure, contained both chemisorbed and physically adsorbed gases. To obtain the chemisorbed part, the sample was evacuated again to 6.67 X lo4 kPa and cooled to 308 K for removal of the physically adsorbed part. Quantitative removal of the physically adsorbed part was determined by a second adsorption following the evacuation. The chemisorbed part was then obtained by subtracting the physically adsorbed gas from the total adsorption. Temperature-Programmed Desorption (TPD). The sample weighed 40 mg. This sample was placed in a U-shaped pyrex tube. Quartz wool was placed on both sides of the sample so as to keep it from being carried away by the flowing gas through the sample. The experiment initiated with a reduction of the sample at 673 K under hydrogen flow for 3 h. After the reduction, the sample temperature was lowered under nitrogen flow. When the temperature reached 308 K, the flowing gas was switched to carbon monoxide for 1 h. The switch allowed the adsorption of CO to occur. Following adsorption, the flowing gas was switched to nitrogen again with a flow rate of 10 mL/min. The sample was simultaneously heated at arate of 5 K/min. The increase in the temperature resulted in the desorption of CO. A thermal conductivity detector was utilized for detecting the desorption. The results were recorded by a recorder (Shimadzu R-122T). Carbonylation. The reaction was carried out under atmospheric pressure in a 0.64 cm 0.d. U-shaped pyrex tube reactor. Quartz sands (0.2-0.8 mm, Merck) were mixed with the catalyst for reducing gas channeling and temperature gradient in the catalyst bed. In each experiment, the bed contained 0.4 g of catalyst and 0.5 g of quartz sands. Both sides of the bed were packed with quartz wool for minimizing the amount of catalyst flying off by flowing gas through the bed. The reaction temperature was measured through usage of a thermocouple. The thermocouple was inserted in the reactor with its tip positioned on top of the catalyst bed so that an accurate reaction temperature could be measured. Catalyst activation was performed in situ by reducing it under hydrogen flow at 673 K for 3 h. The catalyst temperature was then lowered to the desired reaction temperature. At the reaction temperature, methanol (MeOH) and methyl iodide (MeI) were continuously fed into the reaction system via a syringe pump. Carbon monoxide (CO) and argon (Ar) were separately fed from gas cylinders. The flow rates of the gases were controlled by means of mass flow controllers. The effluent gas from the reactor was sampled by an autosampling valve and was immediately analyzed by a gas chromatograph (China Chromatography 8900). The chromatograph was equipped with a flame ionization detector. The column for the analysis was Porapak Q, 80/lOOmesh, 2 m X 0.32 cm stainless steel. In the analysis, the column temperature was initially programmed at 423

V

0

90

75

60

45 30 3 Theta

15

0

Figure 1. XRD patterns of (A) 5% Sn-5% Ni/C, (B)5% Sn/C, (C) 5 % Ni/C, (D)activatedcarbon (0, activatedcarbon;V,nickel;0,tin; A, NiaSn).

K for 10min, then raised to 483 K a t 15 K/min, and finally held at 483 K for 16 min. Methanol conversion (X), and selectivity (Si)for the reaction are defined as

x = [ ( I - 01/11x 100% si= [(PiIvi)/(I-011 x 100% Results and Discussion

XRD. The spectra of X-ray diffraction for several samples are illustrated in Figure 1. Typical peaks of graphite, at 20 = 26.7' and 44.5O,are found with all of the samples. The activated carbon containing graphite used in this study is confirmed by the observed results. The principal peaks of Ni, in order of intensity, are at 20 = 44.5O,52.3', and 77.2O. These peaks are not easily identified inNi/C since the most intense one, 28 = 44.5O, overlaps with that of activated carbon, and the other two are too weak to be observed. Peaks of Sn are clearly seen in Sn/C. In Sn-Ni/C, two things are noticed. First, the intensities of Sn peaks in the sample are substantially lowered in comparison with those in Sn/C. Second, new peaks at 20 = 39.3O, 42.1°, 59.3O,and 72.1' appear. The powder diffraction data file indicates that these new peaks are Ni&n peaks. This observation leads to the conclusion that a major part of Sn is converted to NiaSn in Sn-Ni/C. Surface Area and Chemisorption. The measured surface areas are provided in Table 1. For the Nicontaining samples, the area decreases with Sn loading. The variation in surface area among the samples is roughly 5 % . The effect of surface area on catalyst activity is therefore limited. Chemisorptions, as exhibited by the uptakes of CO and HP,are also presented in Table 1. That the gas uptake for either activated carbon or Sn/C is little is revealed by the observed data. However, significant uptakes are discov-

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Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994

Table 1. Surface Area and ChemisorDtion Results

~

~~~

1023 959 997 968 951 943

activated carbon 5% Sn/C 5% Ni/C 1% Sn-5% Ni/C 3% Sn-5% Ni/C 5 % Sn-5 % Ni/C

0.34 0.00 2.07 2.12 2.59 2.72

22.10

8.10 6.85 192.90 207.80 264.00 298.70

CD

93.10 98.20 102.10 109.80

Adsorption conditions: P = 13.8 kPa, T = 308 K. Adsorption conditions: P = 42.1 kPa, T = 308 K. 369

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275 300 325 350 375 400 425 450 475 500 525 Temperature( K ) Figure 2. Temperature-programmed CO desorption of (A) 5% Sn5% Ni/C, (B) 5% Sn/C, (C) 5% Ni/C, (D) activated carbon (heating rate, 5 K/min; carrier gas flow rate, 10 mL/min).

ered for samples containing Ni. Of the Sn-Ni/C samples, with various Sn loading, the uptake increases with Sn loading. The adsorption of CO is also found to be substantially higher than that of hydrogen. Several possibilities can be adopted in accounting for the situation. First of all, the adsorption pressure of CO is higher than that of Hz. Higher adsorption pressure leads to higher adsorption. Spillover is another possibility. Spillover causes the adsorbed gas molecules to migrate from catalyst metal sites to the support. Consequently, enhanced adsorption can be obtained. The third possibility is that a reaction between CO and Ni occurs (Bartholomew and Pannell, 1980).The reaction enables the adsorbed CO to penetrate deep into the metal particles, which is unlike the case of adsorption that involves only the surface Ni of the particles. The penetration results in an excess CO consumption. The formation of Ni3Sn on Sn-Ni/C may also have played a role in causing the discrepancy between CO and Hz adsorptions. Temperature-ProgrammedDesorption (TPD). The desorption patterns of carbon monoxide from activated carbon, Ni/C, Sn/C, and Sn-Ni/C are all illustrated in Figure 2. The desorption occurs over a broad range of temperatures from roughly 300 to approximately 400 K. That CO desorption from both activated carbon and Sn/C is insignificant is indicated in this figure. Appreciable CO desorption occurs only with the presence of Ni in the sample. Of the two Ni-containing samples, Sn-Ni/C has a substantially higher quantity of CO desorption than Ni/ C.

Table 2. Methanol Carbonylation. MeOH selectivity (% ) catalvst conversion (%) CHA DME AcOMe AcOH .~ 5% Sn/C 7.9 34.8 63.4 1.8 0.0 21.0 4.6 62.6 11.8 5% Ni/C 43.3 20.7 5.1 65.1 9.2 1% Sn-5% Ni/C 53.8 19.3 68.6 6.6 3% Sn-5% Ni/C 55.5 5.5 5% Sn-5% Ni/C 58.8 18.2 3.5 70.0 8.3 Contact time, 10.6 g of cat h/mol; feed molar ratio, CO/MeOH/ MeI/Ar = 10/9/1/5; reaction conditions, T = 573 K,P = 100 kPa; time on stream, 4 h.

The temperature at which the peak desorption rate occurs relates to the adsorption strength. The higher desorption temperature occurs for the stronger adsorption. For the four samples, the temperature is in the following sequence: Sn/C > Sn-Ni/C > Ni/C > C. However, the discrepancies in the temperature among the samples are minor. The difference in electronegativity between Sn and Ni has apparently played a vital role in the amount and strength of CO adsorption. The electronegtivity for Sn is 2.0 and for Ni is 1.9 (Petrucci and Harwood, 1993). Since CO is an electron-donating species, the addition of higher electronegative Sn to Ni/C tends to cause the adsorption of more CO and with greater strength. The enhancement in CO adsorption by the addition of Sn to Sn-Ni/C is significant despite the small variation in electronegtivity between Sn and Ni. The small variation is, however, reflected only in the adsorption strength. Carbonylation. Low activity can be obtained for Sn/ C. Methanol conversion using Sn/C is only 7.9% (Table 2). The principal productsare dimethyl ether (DME) and methane. Selectivities toward carbonylation products, methyl acetate (AcOMe)and acetic acid (AcOH),by using Sn/C are low, as revealed in the table. The Sn-Ni/C catalysts have higher activity than Sn/C as indicated in Table 2. The promoting effect of Sn on Ni/C for the carbonylation is clearly observed in this table. For example, with the addition of 1wt % Sn to Ni/C, the Sn-Ni/C catalyst produces a 24% increase in methanol conversion. The increase is in proportion to the Sn loading within the experimental range. Using either Ni/C or Sn-Ni/C results in high selectivities toward methyl acetate and acetic acid. Both of the products come from the carbonylation reaction. These products require the insertion of a CO molecule into a methanol molecule. Product distribution is not significantly affected by the addition of Sn. A slight increase in methyl acetate selectivity and a slight decrease in acetic acid selectivity are also observed. The addition of Sn to Ni/C promotes the activity of Ni/C. This result can be interpreted as Sn being capable of increasing the number of active sites for carbonylation. This interpretation is supported by the CO chemisorption data. An increase in CO adsorption is indicated by this data to occur as Sn is added to Ni/C. Another interpretation for the increase in the activity of Ni/C by Sn is that Sn can cause the sites to become more active. The formation of Ni3Sn on Sn-Ni/C may play a role in this aspect. The variation in electronegtivity between Sn and Ni may also affect the site activity. Making a site more active essentially requires a change in the chemical properties of the site. If the change is significant enough, the mechanism of the reaction occurring on the site would change. In turn, the reaction selectivity changes. The selectivity in this study is not substantially altered by the addition of Sn to Ni/C. This result reveals that the changes in site properties are insignificant. Evidence can also be obtained via the TPD data (Figure 2). Little difference

Ind. Eng. Chem. Res., Vol. 33,No. 3,1994 491 3,aMethane I,O:DME

50

80

0

3 4 5 6 7 8 Time on stream(hr) Figure 5. Effect of time on stream on methane and dimethyl ether selectivities (reaction condition: T = 573 K;P = 100 kPa; contact time = 10.6 g of cat h/mol; molar ratio in feed, CO/MeOH/MeUAr = 10/9/1/5). 100

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Temperature(K) Figure 7. Effect of temperature on methyl acetate and acetic acid selectivities (reaction condition: P = 100 kPa; time on stream = 4 h; contact time = 10.6g of cat h/mol; molar ratio in feed, CO/MeOH/ MeI/Ar = 10/9/1/5).

are illustrated in Figures 7 and 8. In Figure 7,the decrease in methyl acetate selectivity is accompanied by an increase in acetic acid selectivity. Similar results have also been obtained by Fujimoto et al. (1983) for the reaction employing Ni/C as the catalyst. Acetic acid is a secondary product from methyl acetate as is clearly indicated by the changes in methyl acetate and acetic acid selectivities. The temperature effects on the rates of methanol to methyl acetate and methyl acetate to acetic acid are not equal. The latter is affected more than the former. As a result, increasing the temperature increases the selectivity of acetic acid. Comparing the two catalysts, Sn-Ni/C has a higher methyl acetate selectivity than Ni/C. The selectivities for acetic acid are approximately equal for the two

492 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 2 ,$Methane C ,S:DME

---:5%Sn-5%Ni/C --5%Ni/C

3ot

"

500 520 540 560 580 600 620

Temperature(K) Figure 8. Effect of temperature on methane and dimethyl ether selectivities (reaction condition: P = 100 kPa; time on stream = 4 h; contact time = 10.6 g of cat h/mol; molar ratio in feed, CO/MeOH/ MeI/Ar = 10/9/1/5).

catalysts. A substantial amount of methane is also produced. Its selectivity increases with the reaction temperature (Figure 8). A low reaction temperature should be applied in limiting its production. Higher selectivities of methane and dimethyl ether are observed for Ni/C than for Sn-Ni/C. This is obvious since Ni/C has a lower methyl acetate selectivity than Sn-Ni/C.

Conclusion Significant promoting effects could be obtained by adding Sn to Ni/C for methanol carbonylation. This addition was found to significantly increase the conversion of methanol. The selectivities toward carbonylation products, i.e., methyl acetate and acetic acid, were only slightly altered. These promoting effects could possibly related to: (1)the significant increase in CO adsorption on Sn-Ni/C compared to that on Ni/C, (2) the slightly greater CO adsorption strength on Sn-Ni/C than that on Ni/C, or (3) the formation of NisSn on Sn-Ni/C.

Acknowledgment Financial support from the National Science Council of the Republic of China under Contract No. NSC 82-0402E-011-036 is gratefully acknowledged.

Nomenclature I feed rate of methanol (mol/min) Ni: number of methyl groups in product i 0: efflux rate of methanol (mol/min) P: reaction pressure Pi: efflux rate of product i (mol/min) Si: selectivity toward product i T reaction temperature (K) X conversion of methanol Compounds

AcOH: acetic acid AcOMe: methyl acetate DME: dimethyl ether MeI: methyl iodide MeOH: methanol

ylation of Methanol. J.Mol. Catal. 1983,18, 375 -380. Bartholomew, C. H.; Pannell, R. B. The Stoichiometry of Hydrogen and Carbon Monoxide Chemisorption on Alumina- and SilicaSupported Nickel. J. Catal. 1980,65,390-401. Chen, Y. Z.; Ling, K. S. Carbonylation of Methanol over Supported Nickel Boride Catalysts. J.Chin. Inst. Chem. Eng. 1991,22,103108. Christensen, B.; Scurrell, M. S. Selectivity of a Heterogeneous Rhodium Catalyst for the Carbonylation of Monohydric Alcohols. J.Chem. Soc., Faraday Trans.1 1977,73,2036-2039. Drago, R. S.;Nyberg, E. D.; Amma, A. E.; Zonbeck, A. Ionic Attachment as a Feasible Approach to Heterogenizing Anionic Solution Catalyst. Carbonylation of Methanol. Inorg. Chem. 1981, 20,641-644. Fujimoto, K.;Shikada, T.; Omata, K.; Tominaga, H. Vapor Phase Carbonylationof MethanolwithSupported NickelMetalCatalysta. Ind. Eng. Chem. Prod. Res. Dev. 1982,21,429-432. Fujimoto, K.;Shikada, T.; Omata, K.; Tominaga, H. Vapor Phase Carbonylation of Organic Compounds over Supported Transition Metal Catalysts. 2. Synthesis of Acetic Acid and Methyl Acetate from Methanol with Nickel-Active Carbon Catalyst. Ind. Eng. Chem. Prod. Res. Dev. 1983,22,436-439. Fujimoto, K.;Omata, K.; Shikaka, T.; Tominaga, H. Catalytic Features of Carbon-Supported Group VI11 Metal Catalysts for Methanol Carbonylation. ACS Symp. Ser. 1987,328,208-219. Hjortkjaer, J.; Jensen, V. W. Rhodium Complex Catalyzed Methanol Carbonylation. Ind. Eng. Chem. Prod. Res. Dev. 1976,15,46-49. Jarrell, M. S.;Gates, B. C. Methanol Carbonylation Catalyzed by a Polymer-Bound Rhodium(1) Complex. J. Catal. 1975,40, 255267. Liu, T. C.; Wang, C. H. Methanol Carbonylation on Molecular-Sieve Supported Catalyst. J. Chin. Inst. Chem. Eng. 1992,23,95-102. Omata, K.;Fujimoto, T.; Shikada, T.; Tominaga, H. Vapor Phase Carbonylation of OrganicCompounds over Supported TransitionMetal Catalyst. 3. Kinetic Analysis of Methanol Carbonylation with Nickel-Active Carbon Catalyst. Ind. Eng. Chem. Prod. Res. Dev. 1985,24,234-239. Omata, K.; Fujimoto, T.; Shikada, T.; Tominaga, H. Vapor Phase Carbonylation of OrganicCompounds over Supported TransitionMetal Catalyst. 6.On the Character of NickeVActive Carbon as Methanol Carbonylation Catalyst. Ind. Eng. Chem. Res. 1988,27, 2211-2213. Petrucci, R. H.; Harwood, W. S. General Chemistry-Principles and Modern Applications, 6th. ed.; Macmillan: New York, 1993;pp 363. Robinson, K. K.;Hershman, A.; Craddock, J. H. Kinetics of the Catalytic Vapor Phase Carbonylation of Methanol to Acetic Acid. J. Catal. 1972,27,389-396. Scurrell, M. S.; Howe, R. F. Highly Active Rhodium-Zeolite Catalyst for Methanol Carbonylation. J. Mol. Catal. 1980,7,535-537. Shikada, T.; Yagita, H.; Fujimoto, K.; Tominaga, H. Vapor Phase Carbonylation of Methyl Acetate, Methanol and Dimethyl Ether with Molybdenum-ActiveCarbon Catalyst. Chem.Lett. 1985,547550. Shimazu, S.; Ishibashi, Y.; Miura, M.; Uematsu, T. Methanol Carbonylation Catalyzed by Polymer-Supported Rhodium Complexes. Appl. Catal. 1987,35,279-288. Smith, B. L.; Torrence, G. P.; Murphy, M. A.; Aguilo, A. The RhodiumCatalyzed Methanol Carbonylation to Acetic Acid at Low Water Concentrations: The Effect of Iodide and Acetate on Catalyst Activity and Stability. J. Mol. Catal. 1987,39,115-136. Takahashi, N.; Orikasa, Y.; Yashima, T. Kinetics and Mechanism of Methanol Carbonylation over Rh-Y Zeolite. J. Catal. 1979,59, 61-66. Webber, K.M.;Gates, B. C.; Drenth, W. Design and Synthesis of a Solid Bifunctional Polymer Catalyst for Methanol Carbonylation. J. Mol. Catal. 1977178,3, 1-9. Yashima, T.; Orikasa, Y.; Takahashi, N.; Hara, N. Vapor Phase Carbonylation of Methanol over Rh-Y Zeolite. J. Catal. 1979,59, 53-60.

Received for review September 13, 1993 Accepted December 13, 1993'

Literature Cited Anderson, S. L. T.; Scurrell, M. S. Observations on an Alternative Route for the Preparation of Rh-Zeolites Active in the Carbon-

* Abstract published in Advance ACS Abstracts, February 1, 1994.