Energy & Fuels 1994,8, 129-140
729
Highly Active Methanol Dissociation Catalysts Derived from Supported Molten Salts Andrew D. Schmitz and Darrell P. Eyman' Department of Chemistry, University of Iowa, Iowa City, Iowa 52242
Katherine B. Gloer ACCEL Catalysis, Inc., Technology Innovation Center, Oakdale Campus, University of Iowa, Iowa City, Iowa 52242 Received June 9, 1993. Revised Manuscript Received February 23, 1994"
Catalysts prepared by supporting molten CuC1-KC1 or CuCl-ZnC12-KCl on Si02, A1203,or ZnSiO3 support have been used to promote methanol dissociation to predominantly CO and H2 (Eyman et al., ref 1). The catalysts were prepared by common impregnation procedures and did not require pretreatment before use. A wide range of operating conditions was examined in continuous flow, fixed-bed reactor tests. Methanol conversion rates (mol of CH30H/L of catalyst-h) were very high up to 6300 at 400 "C, and as high as 12 000 at 500 "C. Outstanding catalyst longevity and thermal stability were also observed. Several generalizations concerning catalyst composition can be made. Catalyst activity at low reaction temperatures increases with decreasing salt fusion temperatures. Catalysts comprising salts which are stable toward reduction show the highest activity at high temperatures. Compared with CuCl-KCVSi02, catalysts containing nonacidic ZnO promoter generally had higher CO selectivity. Higher activity is achievable by switching to ZnSiO3 or A1203 support, or by adding ZnCl2 to the supported melt, but these measures also increase side reactions. Catalytic activity increased over time due to chemical transformation of the copper phase. One of the CuC1KCl/Si02 catalysts was examined before and after use by electron microscopy and X-ray powder diffraction (XRD). From these studies it is possible to conclude that copper is present in mixed oxidation states in the high-activity catalyst-metallic and copper(1) oxide. Particle size estimates for the copper phases, obtained from electron microscopy and XRD line-broadening, are compared. The ternary salt catalyst, CuCl-ZnClz-KCl/SiOz, was also tested for CO + H2 synthesis activity at 300 "C. The organic products were C1-C7 hydrocarbons. Total hydrocarbon yield as a function of carbon number was reasonably well described by the Anderson-Schulz-Flory model.
Introduction Applications for Advanced Methanol Dissociation Technology. Methanol dissociation' to retroproduce synthesis gas (eq 1)has drawn interest in many markets. CH30H F= 2H2 + CO
(dissociation)
(1)
Methanol (liquid synthesis gas) can be safely transported to remote process sites and catalytically dissociated to regenerate synthesis-gas for conversion to value-added chemicals. Srivastava has compiled an impressive list of applications for this technology.2 Use of methanol as a transportation fuel is being approached from several angles. Direct use of alcohols as fuels, either pure or blended with gasoline, has received the most attention. However, catalytic dissociation of methanol can be used on board the vehicle to generate CO/H2 for combustion. The product fuel would then have combustion enthalpy up to 90.6 kJ/mol higher than the stock-the enthalpy change for the dissociation-and the endotherm created by the reaction can be used for Abstract published in Advance ACS Abstracts, April 1, 1994. (1)Eyman. D. P.; Gloer, K. B.; Schmitz, A. D. US. Patent No. 5,-
089,245,-1992. (2) Srivastava, R. D. "State of the Art of Iron Based Catalysts for Fischer-Tropsch Synthesis." Report; DOE/PETC, DEAC22-89PCW00, July 1989.
cooling.- Overall, the energy content of liquid methanol fuel is increased by 22% through combined vaporization and disso~iation.~ Utilization of this technology overcomes a major drawback of methanol as fuel: low volumetric energy content. Fuel economy in a spark-ignition engine can be improved by 25% over liquid methanol and by 50% over g a s ~ l i n e . ~Although ?~ the potential gain from implementation of this technologyis evident, development in the automotive industry has not yet gone beyond test vehicles. Catalyst Composition and Characterization. Prior work on methanol dissociation has focused on supported Ni, Cu, or Pd metal catalysts as indicated by patent l i t e r a t ~ r e . ~ As - l ~with methanol synthesis catalysts, metal oxides are often used as activity or structural promoters. (3) Finegold, J. C.; Karpuk, M. E.; McKinnon, J. T.; Paseamaneck, R. In International Solar Energy Society (Amer. Sect.), Annual Meeting Proceedings; Glenn, B. H., Franta, G. E., EMS.; The Section: Newark, DE, 1981; Vol. 4.1, p 221. (4) Cowley, S. W.; Gebhard, S. C. Colo. Sch. Mines Q. 1983, 41. (5) Yoon, H.; Stouffer, M. R.; Dudt, P. J.; Burke, F. P.; Curran, G. P. Energy h o g . 1985,5,78. (6) Dupont, R.; Degand, P. R. Hydrocarbon Process. 1986,July, 45. (7) Eversole, J. F. U. S. Patent No. 2,010,427, 1935. (8) Holmes, P. D.; Thornhill, A. R. British Patent No. 1,010,574,1965. (9) Yokoyama, N.; Imai, T.; Fujita, H.; Murakami, M.; Miyairi, Y.; Tamai, M. U. 5. Patent No. 4,780,300,1988. (10) Cheng, W.-H. US. Patent No. 4,856,267, 1989. (11)Okada, H. U. S. Patent No. 4,865,624, 1989. (12) Isogai, N.; Takagawa, M.; Watabe, K.; Yoneoka, M.; Yamagishi, K. US.Patent No. 4,916,104, 1990. 0 1994 American Chemical Society
Schmitz and Eyman
730 Energy &Fuels, Vol. 8, No. 3, 1994
A typical catalyst preparation involves three main steps: (1)coprecipitation of metal nitrates from aqueous solution using hydroxide or carbonate; (2) calcination to generate oxides; and (3) reduction by hydrogen or synthesis gas. Each step requires careful control to achieve reproducibility. Scale-up is often difficult. Catalysts are also susceptible to deactivation by thermal shock, limiting their application. Development of new, thermal shock-resistant catalysts for vehicle on-board methanol reforming will be of significant interest with increased use of methanol as a fuel. Methanol dissociation over supported transition-metal chlorides has also been reported. In each case, the support was impregnated to achieve a 1 wt % loading of RuC13, RhCl3, PdC12, OsCla, IrC13, or H2PtC&.13 The absence of a low-melting phase and the low concentration of the catalyst on the support differentiates these catalysts from the present supported molten salt catalysis technology. Since these catalyst tests were performed in batch, it is difficult to draw comparisons with results from the present study. We report the impregnation of up to 50 wt % salts on a support. The chosen salt mixtures have fusion temperatures below the lowest reaction temperature studied, 350 "C. This technology is given the name supported molten salt catalysis (SMSC). The CuCl(68 mol % )-KC1 eutectic mixture (mp 136 "C) supported on high-surfacearea silica support showed superb activity and selectivity to the desired CO and HZproducts. Effects of different salt-phase compositions, other supports, and added promoters are discussed. One catalyst, 7.1 CuC1-23 ZnCl22.0 KCl/SiOZ, was also tested for CO + H2 synthesis activity at 300 "C. Initially, the SMSC catalyst exists as a homogeneous liquid coated on the pore walls of the support. Mass transfer within the pores is essentially unencumbered when a thin catalyst layer is used.lP16 The commercial supports were of large pore diameter, and the percentage of the pore volume filled by the molten layer was less than 30%. The estimated average liquid layer thickness on the support pore wall was 1-3 nm. The identity of the active copper phase in coppercontaining methanol synthesis, dissociation, and steam reforming catalysts has been repeatedly scrutinized. Some examples are cited here for methanol ~ y n t h e s i s l and ~-~~ d i s s o c i a t i ~ n .A~mixture ~ ~ ~ of reduced oxidation states, namely Cu(1)and Cu(O),has been implicated. Metal oxide (13) Marczewski, M.; Krzywicki, A.; Peplonski, R.; Pawula, M . React. 1983,22, 241. (14) Rony, P. R. Chem. Eng. Sci. 1968,23, 1021. (15) Datta, R.; Rinker, R. G. J. Catal. 1985, 95, 181. (16) Datta, R.; Savage, W.; Rinker, R. G. J. Catal. 1985,95, 193. (17) Mehta. S.: Simmons. G. W.: Klier. K.: Herman. R. G. J. Catal.
Kinet. Catal. Lett.
1979,57, 339. ' ' (18) Klier, K. Adu. Catal. 1982, 31, 243. (19) Chinchen, G. C.; Waugh, K. C.; Whan, D. A. Appl. Catal. 1986, 25, 101.
(20) HBppener, R. H.; Doesburg, E. B. M.; Scholten, J. J. F. Appl. Catal. 1986,25, 109. (21) Chinchen, G. C.; Spencer, M. S.; Waugh, K. C.; Whan, D. A. J. Chem. SOC.,Faraday Trans.1 1987,83, 2193. (22) Ghiotti, G.; Boccuzzi, F. Catal. Rev.-Sci. Eng. 1987,29 (2,3), 151. (23) Pan, W. X.; Cao, R.; Roberts, D. L.; Griffin, G. L. J. Catal. 1988, 114.440. ----
- - - 1
(24) Kohler, M. A.; Cant, N. W.; Wainwright, M. S.; Trimm, D. L. J. Catal. 1989,117, 188. (25) Robinson, W. R. A. M.; Mol, J. C. Appl. Catal. 1990,60, 61, 73; 1991. . _ 7fi. _117. (26) Kobayashi, H.; Takezawa, N.; Shimokawabe, M.; Takahashi, K. In Preparation of Catalysts; Poncelet, G., Grange, P., Jacobs, P. A., Eds.; Elsevier: Amsterdam, 1983; Vol. 111, p 697. I
. - I
Table 1. Catalyst Compositions salt-phase liquid composn, Cu/Zn, loading? catalysta mol % atomic vol % 8.4 17 CuCl-6.2 KCl/Si02 68-32 36 CuCl-13 KCl/Si02 68-32 27 68-32 1.4 27 30 CuCl-11 KC1/18 ZnO/SiOz 68-32 0.14c 11 8.6 CuC1-3.1 KCl/ZnSiO3 23 25 CuC1-3.3 KCl/A1203 85-15 85-15 27 43 CuC1-5.9 KCl/SiOZ 31 CuC1-4.2 KC1/29 ZnO/SiOz 85-15 0.86 27 36 CuC1-4.9 KC1/17 ZnO/SiO2 85-15 1.7 27 7.1 CuC1-23 ZnClz-2.0 KCl/Si02 27-63-10 0.43 13 a Formula is a listing of catalyst components with overall weight percent by formulation (format salts/promoter/support).b Based on a melt density of 2.5 g/mL. Based on total Zn(I1) in the zinc silicate support.
Table 2. S U D D O ProDerties ~~ support property Si020 ~ 2 0 s ~ ZnSi03b composition, wt % >95 99.9 46.2 surface area, m2/g 202 85 225 porosity, mL/g 1.46 0.7 0.49 av pore diameter, nm 28.8 35 na
*
a Values as reported by supplier. Values as experimentally measured; composition by formulation; na = not available.
promoters, such as ZnO, have been thought to synergistically stabilize Cu(1). The high methanol synthesis activity of unsupported, alkali-promoted, Cu(1)-containing catalysts supports the conclusion that Cu(1) is r e q ~ i r e d . ~ O In~ ~ l this work, X-ray powder diffraction (XRD), electron microscopy,energy-dispersiveX-ray microanalysis (EDX), and selected-area electron diffraction (SAED) have been used to examine the state of copper in 17 CuC1-6.2 KC1/ SiO2, both before and after use.
Experimental Section Catalysts. Table 1 provides a list of catalysts used for this work. Support properties are listed in Table 2. Silica support was granular 30/50-meshparticles (PQCorporation, Inc. 1022G). Alumina support was '/B-in. extrudates crushed and sieved to 40/60 mesh before use (United Catalysts, Inc. T-2432). Other starting materials are of the indicated purities and used as supplied unless otherwise specified. The zinc silicate suppoEt was prepared by coprecipitation of sodium silicate, NazSiO~9H20(neta,Fisher Certified Reagent), and Zn(N03)~-6H20(Fluka, purum p.a.) from aqueous solution. The two 0.5 M solutions were added, dropwise, to a rapidly stirred 20% (wt/wt) NaN03 (EM Science, GR) solution. Zinc silicate formed on mixing. The mixture was stirred for several hours until p H 6 was reached. The product was separated by filtration, Torr) overnight a t washed with methanol, dried in vacuo (le2 150 "C, and crushed and sieved to 40/48 mesh before use. For SMSC catalysts, molten salt liquid loading is the fraction of the total support pore volume occupied by the molten layer. It can be varied from 0 (no molten salt catalyst present) to 1 (support pores completely filled with molten salt). For each catalyst, the liquid loading was calculated from total salt mass and melt density. A melt density of 2.5 g/mL was estimated ~~
(27) Van Der Grift, C. J. G.; Mulder, A.; Geus, J. W. Appl. Catal. 1990, 60, 181. (28) Rodriguez-Ramos, I.; Guerrero-Ruiz,A.; Rojas, M. L.; Fierro, J. L. G. Appl. Catal. 1991,68, 217. (29) Guerrero-Ruiz, A.; Rodriguez-Ramos, I.; Fierro, J. L. G. Appl. CQtQl. 1991. 72. -, 119. ~~(30)Chu, P.-J.; Gerstein, B. C.; Sheffer, G. R.; King, T. S. J. Catal. 1989, 115, 194. (31) Sheffer, G. R.; King, T. S. J. Catal. 1989, 115, 376.
.
~
Highly Active Methanol Dissociation Catalysts from published density data for the CuC1-KC1 system and used for all calculation^.^^ The ternary CuC1-ZnC12-KCl mixture has a melting range of 190-220 OC (first melt to homogeneous liquid). According to phase diagrams, CuCl(85 mol %)-KC1 melts a t 350 "C and CuCl (68 mol %)-KCl, the eutectic composition, melts at 136 0C.33,34 The CuCl starting material (Fisher, A.C.S. Certified Reagent) was purified by recrystallization from HC1 to remove CuClz impurity.36 Aqueous HCl(8-15 M) solutions of the catalyst salts were impregnated on the supports by incipient wetness or dipping methods.% Excess acid solvent was most effectively removed by heating in vacuo. Some of the catalysts were either air-dried or dried using a rotary evaporator under a vacuum generated by water aspiration. Both methods caused oxidation of CuCl and were subsequently avoided. As an alternative to the use of HC1, CuCl can be impregnated from CH3CN s0lution.3~ The ZnO-containing catalysts, 31 CuC1-4.2 KC1/29 ZnO/SiOz and 36 CuC1-4.9 KC1/17 ZnO/Si02,were prepared from 43 CuCl5.9 KCl/Si02. Zinc oxide was incorporated by incipient wetness impregnation of Zn(N03)~6H20in methanol. Methanol was removed in vacuo, and the solid was heated to 360 OC under a nitrogen purge for the dissociation of Zn(NO& to ZnO which occurs rapidly a t 350 OC.38 30 CuCl-11 KC1/18 ZnO/Si02 was prepared from 36 CuC1-13 KCl/Si02, similarly. Samples which included the ZnO component were black, and those without ZnO were generally beige to brown. A commercial CuO/ZnO/AlzO3 methanol synthesis catalyst (United Catalysts, Inc., L-1968, UCZcatalyst) was also tested for activity in methanol dissociation. Before use, this catalyst was reduced in 4% H2/N2 at 250 "C for 9 h. Catalysts were sieved to 40/60 mesh and diluted with 40/60 mesh S i c (McMaster-Carr). The Sic-to-catalyst ratio was from 1to 15 (bulk v/v), with catalyst volumes 0.3-2.2 mL (0.13-1.2 9). To prepare catalyst samples for characterization by EM and XRD, two undiluted portions of 17 CuC1-6.2 KCl/SiOZ were placed in stainless steel reactor tubes. Both reactors were purged with helium during heating at 400 "C, for 10 h, to complete the drying step and to homogenize the molten salt phase on the support. Following this treatment, the first reactor was cooled and the catalyst was removed (17 CuC1-6.2 KCl/SiOz-A). The other portion was used to catalyze methanol dissociation at 400 OC and LHSV 20 (liquid hourly space velocity, L of CH30H/L of catalysteh), for 20 h. Subsequently, some brick-red catalyst was removed from this reactor (17 CuC1-6.2 KCl/Si02-B). The catalyst remaining in the reactor could not be removed easily due to coke deposits. Thus, the reactor was reassembled and purged with air during heating at 400 OC, for 4 h, to remove coke deposits. Subsequently, violet, coke-free catalyst was removed from the reactor with ease (17 CuC1-6.2 KCl/Si02-C). Catalyst Tests. Methanol was purified by distillation from Mg/12 under nitrogen according to published pr0cedures3~and was pumped directly to the reactor using a low-pressure liquid pump (FMI, QSY-2) or HPLC pump (Spectra-Physics,Isochrom LC). Mass flow controllers (Brooks Instruments) were used for blending CO and H2. The reactor system was constructed of types 304 and 316 stainless steel. The reactant preheater and reactor were heated ina fluidized sand bath (Techne). Thepreheater was constructed of a0.66 m length of 0.95 cm 0.d. tubing filled with Sic, connected (32) Sackur, 0. Z. Z. Phys. Chem. 1913, 83, 297. (33) Janz, G. J.; Tomkins, R. P. T.; Allen, C. B.; Downey, J. R., Jr.; Gardner, G. L.; Krebs, U.; Singer, S. K. J.Phys. Chem. Ref. Data 1975, 4, 871. (34) Clark, P. V., Ed. Fused Salt Mixtures: Eutectic Composition and Melting Points Bibliography 1907-1968; Sandia Laboratires, SC-R68-1680,1968. (35) Perin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals, 3rd ed.; Pergammon: Oxford, U.K., 1988. (36) Foger, K. In Catalysis: Science and Technology;Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: New York, 1984; Vol. 6. (37) Pieters, W. J. M.;Connor,W. C.; Carlson,E. J. Appl. Catal. 1989, 1 1 , 35. (38) Addison, C. C.; Logan, N. Adu. Inorg. Chem. Radiochem. 1974, 6, 71.
Energy & Fuels, Vol. 8, No. 3, 1994 731 to a 10 m long coil of 0.32 cm 0.d. tubing. The annular fixed-bed reactor was constructed from concentric tubes, 0.32 cm 0.d. inner and 0.64 cm 0.d. outer, sealed on both ends with tee compression fittings. The ca. 5-mL catalyst bed is the annulus between the two tubes, with the inner tube acting as a thermowell. Vapor-phase reactor effluent was passed to the on-line GC (Shimadzu GC-SA, with TCD) for analysis. Reactor effluent was analyzed approximately hourly. A single Porapak-QS column provided the most efficient analysis; however, a two-column arrangement, Porapak-QS and molecular sieve-l3X, gave superior resolution of the light gases. GC temperature programming for the single-column analysis was 40 to 170 OC, a t 10 "C/min. An external standard method was used for quantitative calculations from the GC data.39 No special catalyst pretreatment such as calcination,reduction, sulfiding, etc. was required. However, because the hygroscopic catalysts were exposed to air during reactor loading, the reactor was purged with helium during heat-up to the reaction temperature to redry the catalyst. Methanol feed rate was controlled by the liquid pump setting and measured at ambient temperature. Baseline reactivity of methanol in the preheater and empty reactor was measured a t 0.06 L/h methanol feed and 8.8 atm pressure, while temperature was varied 350-530 "C. To test SMSC catalyst synthesis activity, a 1/ 1mixture of CO/H2 was fed into the reactor from the mass flow controllers. Carbon mass balance across the reactor was used to calculate methanol conversion. Selectivity is defined as the ratio moles of a given product to sum of moles for all carbon-containing products. Yield is the product of methanol conversion and selectivity. Calculations for CO + H2 synthesis were done similarly. In order to compare data collected over differing space velocities, an average reaction rate was calculated as shown in eq 2, where Fo is the inlet methanol flow (mol/h), Xfis the total
methanol conversion at the reactor outlet, X' is the methanol conversion at the given temperature measured during baseline tests (vide supra), and VWtis the catalyst bulk volume (L). The rate units are therefore mol of CH30H/L of catalystah. Surface Area a n d Porosity. A Micromeretics Pulse Chemisorb 2700 analyzer was used for adsorption experiments on the zinc silicate support. The analysis gas compositions were 30% N2 in He for surface area measurements, and 5 % He in N2 for porosity measurements. All experiments were done at liquid nitrogen temperature. Electron Microscopy. A Hitachi Model 5-4000 (fieldemission gun) and a Hitachi Model S-2700 (tungsten-filament gun) scanning electron microscopes were used for imaging. The S-2700 instrument was equipped with an X-ray detectodanalyzer (Kevex) that was used for energy dispersive X-ray microanalysis (EDX). TEM, STEM, selected-areaelectron diffraction (SAED), and EDX were done on a Hitachi Model H-600 TEM/STEM electron microscope. In preparation for EM examination, samples were wetted with methanol or hexane, crushed, and subjected to ultrasound to prepare fine-particle suspensions. Mounting on carbon stubs for SEM, or on formvar-coated 200-mesh Ti grids for TEM, was accomplished by evaporating 1-3 drops of the suspensions on the surface of the support. Mounted samples were lightly coated by Au-Pd alloy sputtering (imaging only) or carbon evaporation (imaging and analysis). All samples were examined a t ambient temperature. Heating caused by adsorption of energy from the electron beam was insufficient to cause fusion. Particle size estimates for TEM and SEM images were measured on the photographic film negatives. Standard gratings were used for microscope magnification calibration. (39) Rosie, D. M.; Barry, E. F. J . Chromatogr. Sci. 1973, 1 1 , 237.
Schmitz and Eyman
732 Energy & Fuels, Vol. 8, No. 3, 1994
For semi-quantitative STEM-EDX analyses, samples were assumed to behave as thin foils so that effectsof X-rayadsorption could be neg1ected.a Calculations were done on the KEVEX quantitative EDX system software (trade name FOIL) that utilizes a Cliff-Lorimer standardless analysis with computed theoretical k factors and accounts for beam-sample-detector geometry. Precision of the quantitative data was assured by obtaining numerous analyses in each region. Relative error in the reported analyses is conservatively *lo%. X-ray Powder Diffraction. The powder patterns were obtained at ambient temperature on a Philips PS1710 powder diffractometer using nickel-filtered Cu K a radiation. Samples were mounted on aluminum sample holders. Phases were identified by comparison with powder patterns from authentic samples and d-spacing values from the literaturejl Average particle diameters d for the copper phases in the catalysts were calculated using eq 3. This equation compares
40 I
1
100
*
d=
(3)
cos 0 ( E 2- b2)l/'
the diffraction line widths at half-height (in radians) for catalyst samples E and bulk, well-crystallized Cu b. Line widths were measured by fitting Gaussian or Lorentzian functions to the diffraction lines. The calculated particle sizes were reproducible to within 20% for different diffraction lines and/or different portions of a sample.
Results Products. Side reactions occurring during methanol dissociation catalysis are outlined in eqs 4-11. Formation 2CH30H d (CH3),0
+ H,O
2CH30H + HC0,CH3 + 2H,
(dehydration)
(4)
(dehydrogenation) ( 5 )
+ H, (dehydrogenation) (6) CO + 3H, + CH, + H,O (methanation) (7) (olefin synthesis) (8) CO + 2H, + (l/n)C,H,, + H,O CH30H + CH,O
+ H, + CnH2,+, (olefin hydrogenation) CO + H,O + CO, + H, (water-gas shift)
C,H,,
2CO + C + CO,
(Boudart)
(9) (10) (11)
of methyl formate over copper-containing catalysts occurs as shown in eq 5.2a29342Methyl formate is thermally unstable and decomposes to predominantly CO and H2; consequently, it was observed a t temperatures below 400 "C, and only in small amounts. Similar arguments apply to the formation of formaldehyde, eq 6, and its persistence. Dimethyl ether was also produced in small amounts, particularly at lower temperatures (eq 4). Hydrocarbons Cd and greater were rarely observed. Paraffins result from hydrogenation of the first-formed olefins as discussed elsewhere.43 Carbon dioxide was most likely the result of the reactions in eq 10 and 11 and methanol steam reforming. (40) Goldstein, J. I.; Newbury, D. E.; Echlin, P.; Joy, D. C.; Romig, A. D., Jr.; Lyman, C. E.; Fiori, C.; Lifshin, E. Scanning Electron Microscopy
and X-ray Microanalysis: A Text for Biologists, Materials Scientists and Geologists, 2nd ed.; Plenum: New York, 1992. (41) Powder Diffraction File: JCPDS, International Centre for Diffraction Data; Joint Committee on Powder Diffraction Standards
(JCPDS): Swarthmore, PA. (42) Ai, M. Appl. Catal. 1984, 11, 259. (43) Anderson,R. B. Fischer-Tropsch Synthesis; Academic Press: New York, 1984.
co rel.ctiiiy
60 40
0.9x
.
-
Ntd 0
340
390
440
490
540
Temperature (C)
Figure 1. Conversion of methanol by the preheater and empty reactor at 8.8 atm and 0.06 L of CHSOH/h. The products of the CO + H2 synthesis experiment were olefins and paraffins, CO2 and HzO. No alcohols or other oxygenated organic products were observed. Empty Reactor Test. The data in Figure 1show that the preheater and empty reactor render appreciable conversion of methanol above 450 "C. Also shown are the product selectivities and the CO yield. The oxygenated selectivity represents the sum of all oxygen-containing products (i.e., CH20, (CH3120, and HCOzCH3), and hydrocarbon selectivity represents the sum of all hydrocarbon products. Of the hydrocarbon products, greater than 90 mol % was CH4. The remainder was mostly CZ and small amounts of C3. Conversion and selectivityshow a strong dependence on temperature. Methanol conversion at 520 "C neared the calculated equilibrium value of 95.4 % .I8 From Figure 1,the overall reaction rate was 0.043 mol of CH3OH/h at 400 "C and 0.93 mol of CH30H/h at 500 "C. Catalyst Performance. Methanol dissociation catalyst performance is compared in Tables 3 and 4. As discussed below, activity increased dramatically with time for most catalysts, especially early in the runs. Consequently, Tables 3 and 4 represent data collected after several hours on-stream, generally 25 h or more, where the conversion was not changing substantially with time. As an additional measure, the results of multiple analyses under the same operating conditions were averaged. It must be pointed out that the rate calculation described in eq 2 is only rigorous at low conversion. When conversion is high, rates are to be considered approximate. All of the SMSC catalysts display a significantly higher rate a t 500 "C than at 400 "C, except 36 CuC1-13 KC1/ SO,. However, the 500 "C data for 36 CuCl-13 KCl/Si02 were obtained at 25-30 h on-stream, whereas, the 400 "C data were obtained much later in the run, after 75 h. The activity of 36 CuC1-13 KCl/Si02 increased with time causing the rates a t 400 and 500 "C to become effectively equalized. The 43 CuC1-5.9 KCl/Si02 catalyst has low activity and low CO selectivity. Incorporation of ZnO significantly CI-C7
Energy & Fuels, Vol. 8, No. 3, 1994 733
Highly Active Methanol Dissociation Catalysts
Table 3. Catalyst Performance as a Function of Composition at 400 O C catalyst 17 CuC1-6.2 KCl/Si02 36 CuC1-13 KCl/Si02 30 CuCl-11 KC1/18 ZnO/SiOz 8.6 CuCl-3.1 KCl/ZnSiO3 25 CuC1-3.3 KCl/A1203 43 CuC1-5.9 KCl/SiOz 31 CuC1-4.2 KC1/29 ZnO/SiOZ 36 CuC1-4.9 KC1/17 ZnO/SiOz 7.1 CuC1-23 ZnCl2-2.0 KCl/Si02 UCI catalyst
P,atm
LHSV
14.9 1.9 2.8 4.0 3.3 6.6 2.4 6.4 7.0 2.7
300 320 290 290 100 61 120 84 85 120
% methanol conversion 22 81 24 14 21 39 26 29 19 75
% co selectivity 88 95 92
-Fa
15 63 16 9.3 4.9 5.8 7.3 5.4 3.6 22
88
85 53 96 94 91 40
Rate as defined in eq 2, 10-2 mol of CH30H/L of catalyst-h.
Table 4. Catalyst Performance as a Function of Composition at 500 O C % co selectivity 72 94 93 90 85 57 58 92 73 68
% methanol
catalyst 17 CuC1-6.2 KCl/Si02 36 CuC1-13 KCl/Si02 30 CuC1-11 KC1/18 ZnO/SiOz 8.6 CuC1-3.1 KCl/ZnSiOa 25 CuC1-3.3 KCl/A1203 43 CuC1-5.9 KCl/Si02 31 CuC1-4.2 KC1/29 ZnO/SiOZ 36 CuC1-4.9 KC1/17 ZnO/SiOZ 7.1 CuC1-23 ZnClr2.O KCl/Si02 UCI catalyst a
Rate as defined in eq 2,
P,atm
LHSV 300 330 290 290 250 61 300 140 500 120
10.9 1.9 2.7 2.7 1.4 6.5 3.7 4.6 17.0 1.1
conversion
-f"
60 60 50 67 62 15 58 29 120 22
82 74
71 95 99.8 99.6 78 87 98 75
mol of CH,OH/L of catalysteh.
z t \ F
A
2
3
4
5
6
Carbon Number
Figure 2. Hydrocarbon yield as a function of carbon number for CO + H2 synthesis over 7.1 CuC1-23 ZnC12-2.0 KCl/Si02 at 300 " C , 1000 psig, 4.6 normal L/gof catalyst-h and H2/CO = 1.0. CO conversion is 15% .
improves CO selectivity (both 31 CuC1-4.2 KC1/29 ZnO/ Si02 and 36 CuC1-4.9 KC1/17 ZnO/SiOn) and gives a modest increase in rate for the former catalyst (Cu/Zn = 0.86). A similar comparison of 36 CuC1-13 KCl/SiOZ and 30 CuC1-11 KC1/18 ZnO/SiOz shows that the ZnO-free catalyst is superior in both rate and selectivity, especially at 400 "C. The other zinc-containing catalysts, 8.6 CuC13.1KCl/ZnSiOsand 7.1 CuC1-23 ZnClz-2.0 KCVSi02,show good CO selectivity but low activity at 400 "C. However, 7.1 CuC1-23 ZnCl2-2.0 KCI/Si02 is the most active catalyst at 500 OC, but CO selectivity is low. Catalysts having CuCl (68 mol %)-KC1 are generally superior in performance to catalysts with CuCl (85 mol %)-KCl. Among CuCl (68 mol %)-KCl/SiOz catalysts, higher salt loading gives better results (cf. 36 CuCl-13 KCl/SiOz and 17 CuC1-6.2 KCl/SiOz). The hydrocarbon product distribution for CO + H2 synthesis over 7.1 CuC1-23 ZnC12-2.0 KCl/Si02 is shown in Figure 2. Paraffin yield is the sum of straight-chain andbranched (minor) products at each carbon number.
25
26
27
28
29
30
31
32
33
Time on-Stream (h)
Figure 3. Performance of 36 CuCl-13 KCl/Si02 in methanol dissociation catalysis as function of time on-stream at 500 OC,1.8 atm, and 330 LHSV.
The main olefin products were always a-olefins. Total hydrocarbon yield as a function of carbon number is reasonably well described by the Anderson-Schulz-Flory mode1,2943144 with chain growth probability factor a = 0.5 from the line-fit slope. Influence of Time On-Stream. Changes in methanol dissociation catalyst performance over time for several SMSC catalysts are shown in Figures 3-6. Figure 3 represents data for supported CuC1-KC1 alone, whereas, (44) Henrici-Oliv6,G.; Oliv6, S. Chemistry of the Catalyzed Hydrogenation of Carbon Monoxide; Springer-Verlag: Berlin, 1984; p 143.
Schmitz and Eyman
734 Energy & Fuels, Vol. 8, No. 3, 1994
#
20
20
15
15
#
10
5
5
0
0 100
90
60
70
*
10
*
50
60
40 30
20 0
10 10
20
30
49
50
60
70
80
93
1W
0
10
Time on-Stream (h)
20
30
40
50
Time on-Stream (h)
Figure 4. Performance of 7.1 CuC1-23 ZnClz-2.0 KCl/SiOz in
Figure 6. Performance of 30 CuCl-11 KC1/18 ZnO/SiOz in
methanol dissociation catalysis as function of time on-stream a t 400 O C , 24 atm, and 58 LHSV.
methanol dissociation catalysis as function of time on-stream at 400 "C, 1.4 atm, and 290 LHSV.
. ..
60
50
# 30 40
IP'
omammated
t
\
I
.-
0 1
70
mnveraon
1
60 50
* 40 2o
tl
40 30
20
tl
10
0
0 0
10
20
30
40
50
Time on-Stream (h)
Figure 5. Performance of 8.6 CuC1-3.1 KCl/ZnSiOa in methanol dissociation catalysis as function of time on-stream a t 385 "C, 9.0 atm, and 290 LHSV.
the following four figuresrepresent data for zinc-containing catalysts. Considering data for 36 CuC1-13 KCl/Si02 (Figure 3), conversion increases steadily with time up to the equilibrium value. Concurrent with the increase in conversion is a decrease in CO selectivity at the expense of increased hydrocarbon production. Hydrocarbon selectivity rises sharply when conversion exceeds 90%. 7.1 CuC1-23 ZnCl22.0KCl/Si02 and 8.6 CuC1-3.1 KCl/ZnSiOa (Figures 4 and 5) also show increasing amounts of hydrocarbons and COP with time, while oxygenated organics remain a very small percentage of the products. 8.6 CuC1-3.1 KCl/ZnSiOa (Figure 5) shows a decrease in conversion over time, but
0
10
20
30
40
50
Time on-Stream (h)
Figure 7. Performance of CuO/ZnO/A1203, UCI catalyst, in methanol dissociation catalysis as function of time on-stream a t 380 "C, 1.5 atm, and 120 LHSV.
not before a rapid increase in conversion and CO selectivity in the first 4 h. Figure 6 shows data for 30 CuCI-11 KC1/ 18 ZnO/SiOz, which was prepared from 36 CuC1-13 KC1/ Si02 (see data in Figure 3). Initial CO selectivity for 30 CuC1-11 KC1/18 ZnO/SiO2 is approximately 80% but increases with time to greater than 90 % The UCI catalyst deactivates rapidly with a concurrent increase in CO selectivity (Figure 7). Operating Conditions. The effect of temperature on methanol dissociation catalysis is shown in Figures 8 and 9, for 36 CuC1-4.9 KC1/17 ZnO/SiO2 and 25 CuC1-3.3 KC1/ A1203, respectively. All of the catalysts had similar performance vs temperature trends, although the unique
.
Energy & Fuels, Vol. 8, NO.3, 1994 735
Highly Active Methanol Dissociation Catalysts 10
I
1
. .. lSa.-PraduaS.llctrvh.a
L
15
1
I
ill;
oaygenald
+I
co S d O d i V i t y
x
60
so
;JI
CO yield
I
0' 400
420
440
460
480
40
10
500
20
30
50
60
70
80
90
Pressure (atm)
Temperature (C)
Figure 8. Effect of temperature for 36 CuC1-4.9 KC1/17 ZnO/
Si02 in methanol dissociation catalysis at 5.9 atm and 110LHSV.
I/ - / /l I .
40
Figure 10. Effect of pressure for 17 CuC1-6.2 KCl/SiOz in methanol dissociation catalysis at 450 "C and 740 LHSV.
..
CO selectwily
hydroarbon.
15
0
+-+ *'.
co +-q-cp-o
conversion
"i/t * 40
-
2
4
6
8
10
1OS/LHSV (L-CHsOH/L-cat.h)-'
Figure 11. Effect of reciprocal space velocity (contact time) on conversion and CO selectivity for 7.1 CuC1-23 ZnCl2-2.0 KCl/ Si02 in methanol dissociation catalysis at 450 "C and 10 atm.
" 390
410
430
450
470
490
510
530
Temperature (C)
Figure 9. Effect of temperature for 25 CuC1-3.3 KCl/A120s in methanol dissociation catalysis at 1.2 atm and 100 LHSV.
properties of the particular catalysts are also shown. For example, the ZnO-containing catalysts often displayed higher CO selectivity, with lower amounts of COZand hydrocarbons, than their CuCl-KCl/SiOZ parent catalysts. As seen in Figure 8, CO selectivity is high, even a t 500 "C. Very little COZwas produced. An appreciable amount of (CH3)zO is produced below 490 OC over the alumina-supported catalyst. This is most likely a consequence of higher surface acidity for alumina as compared to silica support. Silica-supported catalysts gave low (CH3)20selectivity at all temperatures. Reaction temperatures above 500 OC cause a significant increase in hydrocarbon production, due mostly to methanation. This behavior is not unique to alumina-supported catalysts but is common to all catalysts tested.
A maximum in the CO yield versus pressure curve for 17 CuC1-6.2 KCl/SiOz occurs between 20 and 40 atm (Figure 10). Methanol conversion increases with pressure and CO selectivity decreases as both hydrocarbons and COZbecome more prevalent. Catalyst tests at low pressure (1-10 atm) show the same trends excepting higher selectivity to oxygenated products (1-3 mol% of the product) below 5 atm. The data for the other catalysts strongly resemble the effect of pressure trends shown in Figure 10. Changes in conversionand CO selectivity with reciprocal space velocity (contact time) are shown in Figure 11.For this catalyst, the reaction becomes diffusion limited at high conversion: increasing the contact time from 3.3 X 103to 1C2does little to increase conversion. CO selectivity scarcely changed over the range. Admittedly, more data points are required for exact descriptions of these curves. Nearly all of the experimental data were obtained at contact times ranging from 3 X 103to 10-2. Many of the catalysts gave equilibrium, or near equilibrium, conversions within this range. It seems that diffusion limitations are only of consequence for some of the catalysts. Further
Schmitz and Eyman
736 Energy & Fuels, Vol. 8, No.3,1994
I:
0COseleC.
Rate
-
0 Y
380
530
I I AI I
3
1
0 I
1 -
0
380
Temperatwe (C)
Figure 12. Effect of thermal shock for 31 CuC1-4.2 KCV29 ZnO/ Si02 in methanol dissociation catalysis. Test conditions as follows: (a) initial testing at 380 "C, 4 atm and 120 LHSV, (b) 530 O C high-temperature test, 1.4 atm and 300 LHSV, and (c) retest at 380 OC.
testing is required to define diffusion regimes for the preferred SMSC catalysts. Thermal Stability. All of the SMSC-based catalysts showed superb resiliency when subjected to thermal extremes. This catalyst property is particularlyimportant for on-board methanol dissociation in vehicle fuel applications requiring operation over a broad temperature range, with excursions up to 550 "C under high load.4 In Figure 12, rate and CO selectivity over 31 CuC1-4.2 KC1/ 29ZnO/Si02are compared at 380 "C, both before and after testing at 530 "C for 3.5 h. Following catalyst testing a t high temperature (thermal shock), the CO selectivity is unaffected and the rate nearly doubles. Normal catalyst activation over time under isothermalconditions was likely accelerated by operation at 530 "C. X-ray Powder Diffraction. 17 CuC1-6.2 KCl/Si02-A gave weak and broad diffraction lines representative of poorly crystallized, dispersed phases. This observation is consistent with the supposition that, upon melting, the salts form a thin layer on the high surface area support and do not agglomerate. Upon resolidification, the salts form poorly diffracting microcrystallites. Low diffraction intensitiesand multiple phases made precise identification difficult. Some CuCl and KC1 remained, but the most intense lines (28 = 16.3,22.0, 28.8,34.1,44.6, and 57.4') correlate with double salts of KC1with CuCl or CuC12, and possibly a copper silicate phase. The predominant copper phase in the powder patterns of 17CuC1-6.2 KCl/Si02-Band -C is clearly Cu(0)asshown in Figure 13. There is no evidence for the persistence of a copper chloride or complex chlorocupratephase. Broadening of the Cu(ll1) reflection at 43.30°, and Cu(200) at 50.43", indicates 30-45 nm copper particles, with no discernible particle size difference between the catalysts within the error of measurement. The other prominent lines in the patterns are attributed to KC1 (detected in all catalyst samples), aluminum in the sample holder, and silica support which gives a broad line at 22.2' in Figure 13c. Weak, broadened lines for Cu20 appear in both parts c and d of Figure 13, but are more prevalent in part d. The strongest Cu20 line is the (111)at 36.48'. It is poorly resolved from a peak at 35.62' that comes from another phase. In Figure 13c, a single, nearly symmetrical peak at 35.90' results from the two overlapped peaks. The half-
10
20
30
40
50
60
70
80
90
100
AI@e (28) Figure 13. X-ray powder diffraction patterns for (a) CuzO, (b) Cu, (c) 17 CuC1-6.2 KCl/SiOrB, and (d) 17 CuC1-6.2 KCVSiOr C. Table 5. XRD Data for Contaminant Phase anale.28 Ill0 d.A annle,28 1.604 57.39 30.16 30 62.8 100 1.478 35.62 ~~
d.A 2.961 2.518
Itlo 30 40
width at half-height of this peak indicates 13-nm Cu20 particles. In Figure 13d,the same feature is resolved into two peaks, leading to a calculated Cu20 particle size of 23 nm. Other diffraction lines for Cu20 were not amenable to line-broadening measurements due to low intensities and overlapping peaks. Diffraction peaks listed in Table 5 are due to an ironcontaining contaminant in 17 CuC1-6.2 KClISiOrB and -C, assumed to be a single phase with its most intense peak a t 35.62'. The cited line positions were measured a t the center of the broadened peaks; Kal and Ka2 were not resolved. Corrosion of the stainless steel reactor in a high-temperatureenvironment of HC1 (byproductof CuCl reduction in hydrogen) and H20 was probably the source of the contaminant. The diffraction lines due to the contaminant phase increased in intensity following the high-temperatureoxidative treatment used to remove coke from the reactor which is consistent with contamination by corrosion. Electron Microscopy. A series of scanning electron micrographs of the silica support and catalysts 17 CuC16.2 KCl/Si02-A and -B are presented in Figures 14 to 18. The cavernous silica surface (Figure 14) consists of cross-
Highly Active Methanol Dissociation Catalysts
Energy & Fuels, Vol. 8, No. 3, 1994 737
Figure 14. Secondary electron scanning electron micrograph of the silica support, bar = 290 nm, and Vam = 15 keV.
Figure 15. Secondary electron scanning electron micrograph of 17 CuC1-6.2 KClISiOrA, bar = 150 nm, and Vam = 20 keV.
Figure 16. Secondary electron scanning electron micrograph of 17 CuC1-6.2 KCl/Si02-A, bar = 590 nm, and V,, = 3.0 keV.
linked spheres producing pores 10-30 nm in diameter and large pockets 50-300 nm in diameter. Examination of several different areas of 17 CuCl-6.2 KCVSiOrA revealed a nonuniform loading of the salts. Most regions had moderate loading, as in Figure 15. However, some regions of the support surface were nearly covered with salt crystals, while other regions had no catalyst. Large salt crystals protrude from the pore structure of the support in Figure 15(100-150 nm diameter) and rest on the support surface in Figure 16 (150-600 nm diameter). These salt crystals are ca. 5-20 times the diameter of the pores. The dimensions of subsurface salt crystals, unobservable by
Figure 17. Scanning electron micrograph of 17 CuC1-6.2 KC1/ SiOz-B, bar = 800 nm, and V,, = 15 keV; (a, top) secondary electron image; and (b, bottom) backscattered electron image of the same region as in (a).
Figure 18. Backscattered electron scanning electron micrograph of 17 CuC1-6.2 KCl/Si02-B, bar = 1.3 pm, and V , = 15 keV.
SEM, would be limited to the diameters of the pores in which they are contained, unless the structure of the support is disrupted by salt crystallization. The sample removed from the reactor following methanol dissociation catalysis, 17 CuC1-6.2 KCl/Si02-B, shows the catalyst phase intertwined with the support (Figures 17 and 18). EDX results confirm that high-intensity regions of the backscattered electron images, Figure 17b and 18,represent high concentrations of copper. Similar intensity gradients are observed in secondary electron images but with less contrast (seeFigure 17). Best depicted in Figure 17 is the biphasic distribution of the copperenriched component: 20-500 nm diameter spheroid
738 Energy & Fuels, Vol. 8, No. 3, 1994
Schmitz and Eyman Table 6. X-ray Microanalysis Results for Regions of Figure 19
atom ?6 cu 90
K
C
41 40
D
15
E
6
14 15
sample A
B
Figure 19. Transmission electron micrograph of 17 CuC1-6.2 KCl/SiOrB, bar = 520 nm, and Vaw= 75 keV. particles in regions marked A, and the tube-shaped phase of region B. The spheroid particles are also especially visible in the backscattered electron image in Figure 18. Overall, the majority of spheroid particles were 20-40 nm. Particles smaller than 10 nm could not be resolved. 17 CuC1-6.2 KCl/Si02-B was also examined by TEM and STEM. Copper (or copper-enriched) particles of diameter 100-2000 nm were readily observed; however, smaller particles were not distinguishable. Figure 19 is a representative TEM from one region on the sample grid. The corresponding microanalyses (from the STEM), excluding counterions, are given in Table 6. The large particle (ca. 340 X 500 nm) of region A, Figure 19, gives an EDX spectrum dominated by copper, with a weak signal from silicon in the support. Regions B and C are also rich in copper, again indicative of a multiphasic distribution of copper in the catalyst. The iron-containing contaminant (see XRD results) is also detected in the microanalyses. It was not possible to assign a discernible feature in the TEM or STEM to the contaminant phase. Spectra free of characteristic iron X rays were obtained when the electron beam was focused onto large copper particles as in Figure 19,region A. Thus, the contaminant phase is probably not associated with large-particle copper. No correlation between the concentrations of Fe and Si or C1could be established; however, an Fe/K atomic ratio of 0.9 f 0.2 was typically observed in the microanalyses. SAED of 17 CuC14.2 KCl/Si02-B in the transmission electron microscope gave weak, overlapping patterns formed from multiple crystal diffractions. Severalpatterns indexed to Cu20, but more work is required to substantiate these findings. It was not possible to discern a unique catalyst feature that gave rise to these interestingpatterns.
Discussion MethanolDissociationCatalyst Activity Patterns. Rate of methanol conversionis used to establish a relative scale of catalyst activity. Calculation of rate according to eq 2 is only rigorous under differential reactor conditions (low conversion, isothermal). The conversion was rarely less than 30% so some error was introduced by this approximation. However, the differences in rate between the catalysts are of sufficient magnitude to allow a general description of the trends in activity.
Si
Fe
0
10
12
38 41 55
0 9.3 8.8
10
64
16 15
36 CuC1-13 KCl/Si02 was the superior catalyst at 400 "C and afforded superb activity and CO selectivity at 500 "C. Typically, basic or amphoteric oxides (such as ZnO) enhance the activity of copper catalysts toward methanol dissociation (see refs 29 and 42). However, the activity of 36 CuC1-13 KCl/Si02 was not increased by ZnO incorporation. In apparent contrast, both the low activity and low CO selectivity of 43 CuC1-5.9 KCl/Si02 were bolstered by ZnO, especially for Cu/Zn = 0.86. Neither the incorporation of zinc by addition of ZnCl2 to the molten phase (7.1 CuC1-23 ZnC12-2.0 KCl/Si02) nor the use of zinc silicate support (8.6 CuC1-3.1 KC1/ ZnSiO3) resulted in a catalyst superior to 36 CuCl-13 KCl/ Si02. Although 7.1 CuC1-23 ZnC12-2.0 KCl/Si02 was very active a t 500 "C, the increased Lewis acidity from added ZnClp led to increased hydrocarbon production. The use of zinc silicate or alumina support, rather than silica support, can offer a marginal increase in catalytic activity, but CO selectivity is compromised. Physical differences between the supports could account for differences in activity, but selectivity was also effected. A significant portion of the support surface must remain exposed (not covered by catalyst, molten or otherwise) so that differences in support surface acidity influence the course of reaction. The UCI catalyst deactivated within 15h and gave low CO selectivities. This catalyst, which was tested for activity comparison, is not marketed as a methanol dissociation catalyst, but as a medium-pressure methanol synthesis catalyst. However, Cu/ZnO catalysts have been proposed as reasonable methanol dissociation catalysts and, by this test, are not good catalysts for this process. Cowley and Gebhard have also studied the use of traditional methanol synthesis catalysts (Cu/ZnO and supported palladium) for methanol diss~ciation.~ Their reaction conditions were chosen to simulate methanol dissociation in a reactor heated by automobile engine exhaust: temperatures of 300 or 550 "C for normal or high-load driving, respectively, and 1.56 g of CH30H/g of catalyst-hmethanol feed rate. Activity and CO selectivity were dependent. on the support composition. Acidic supports gave higher activity and increased catalyst thermal stability but also promoted side reactions. Addition of basic oxides to catalysts with acidic supports suppressed side reactions, but thermal stability was lost. Palladium on acidic alumina had the highest thermal stability but only moderate CO selectivity. They were unable to achieve high CO selectivity at all temperatures, while maintaining thermal stability, with a single catalyst. However, it is shown in the present report that high conversionand high CO selectivity, together with thermal stability, are achievable with SMSC catalysts. Space velocities up to 1000 L of CH30H/L of catalystoh, equivalent to 2000 g of CH30H/g of catalystoh, have been possible with SMSC catalysts as shown by the data given in Table 7.
Highly Active Methanol Dissociation Catalysts Table 7. High Space Velocity Data for 17 CuC1-6.2 KCl/SiOf
Energy & Fuels, Vol. 8, No. 3, 1994 739
during the catalytic process. Also, although molten mixtures containing CuCl are known to chlorinate product distribution, hydrocarbon^,^^^ chlorination products have not been % methanol mol % b detected in our GC analyses. P? T,OC atm LHSV conversion CO COz CHdC (CH&CO Time On-Streamand OperatingConditions. There can be little doubt that the initial catalyst rapidly reacts 450 13.6 lo00 63 76 3.6 20 0.4 to produce a superior catalyst, considering the data a Methanol feed rate 0.32 L/h at 25 "C. Catalyst amount: 3.2 X presented in Figures 3-5. Concurrent with the changing lo-'L, 0.13 g. b Including carbon-containing products only, Hz and conversionswere small changes in the product distribution. HzO excluded. Greater than 99% of the hydrocarbon product was methane. Generally,hydrocarbon and C02 production increased with time. This was probably due to increasing concentrations Extensive effort has been made at Conoco, Inc., to of metallic copper. Adsorption of H2 and CO on metallic develop improved methanol dissociation catalysts for copper in turn promotes the reactions of eq 7-11. automobile fuel appli~ation.~ Catalysts significantly more Hydrocarbon synthesis consumes H2 and produces H20, active than others reported in the literature were claimed further increasing COa production through water-gas shift to have been developed, although compositions were not and methanol steam reforming. Added zinc oxide minirevealed. Since the catalyst amounts used in their tests mizes the decrease in the CO yield over time and enhances were not specified, it is not possible to compare their results thermal stability, perhaps by stabilizing Cu(1)as postulated with the present work. It is clear, however, that the Conoco for Cu/ZnO methanol synthesis catalysts. catalysts deactivate very slowly and give CO selectivity Operating conditions that maximized CO yield were 90% or greater at low temperatures. Catalyst thermal approximately 500 "C and 20-45 atm (Figures 8-10). stability above 300 "C was not addressed. Compared to Methane production increased significantly above 500 "C, other patented catalysts for methanol d i s s o ~ i a t i o n , ~ ~ ~ Jwhich ~ is typical of reactions catalyzed by supported metals. SMSC catalysts afford comparable CO selectivity but at Characterization Studies. An uneven distribution rates 100-3000 times faster. of the salt components on the support is observed in the An important question remains unanswered: Is the high SEM. Catalyst distribution on the support is influenced activity of these SMSC-derived catalysts attributable to by uniformity of impregnation, and salt-phase migration the presence of fused salts? Clearly, CuCl is transformed and agglomeration occurring during the 400 "C heating during methanol dissociation to a more active phase. step. It was thought that heating at 400 "C would enable However, at 400 "C, the catalysts with the lowest saltthe molten fluid to distribute evenly over the surface of phase melting points were the most active. CuCl(68 mol the support. But there appears a tendency toward %)-KC1 has a melting point approximately 200 "C lower agglomeration on the support surface. Still, the broadthan CuCl (85 mol %)-KC1, and 150 "C lower than the line XRD data indicate that the salt phase is well dispersed. CuCl-ZnC12-KCl mixture. Salt fusion was observed at Some of the salt crystals observed by SEM are much 180 "C for 36 CuC1-13 KCl/Si02 catalyst before use, larger than the support pore diameter. I t is difficult to showing that the bulk- and supported-salt melting points discern whether the large crystals simply occupy the are similar. If the salt phase does not melt, the surface pockets seen in Figure 14 within the support structure or available for reaction would be small. Perhaps low activity if the support structure is disrupted by the large crystals. at 400 "C is a consequence of high salt fusion temperature. Further investigation of salt-phase redistribution during Catalysts having CuCl(85 mol % )-KC1 show intermediate fusion and prolonged heating could be advantageously activity at 500 "C. pursued by SEM equipped with a sample heating stage. It is interesting that 7.1 CuC1-23 ZnC12-2.0 KCl/Si02 The XRD data show that potassium persists as KC1; is the most active of the group at 500 "C. Even if all of however, only small amounts of chlorine are detected by the CuCl in the molten mixture was transformed (reduced, EDX. When chlorine is detected, at least an equivalent hydrolyzed, etc.), the remaining ZnCl2 (86 mol %)-KC1 amount of potassium is also detected. XRD lines for KCl would probably still be molten. Mixtures of ZnClz with decrease in relative intensity by approximately 15% after 0-55% KC1 have fusion temperatures less than or equal the oxidative treatment. The oxidative treatment also to 300 XRD of 17 CuC1-6.2 KCl/Si02 shows that at results in disappearance of the silica support peak at 22.2". least some of the original KC1 persists throughout the Unloaded silica support, calcined under similar conditions, catalytic process. The fate of ZnClz in the ternary-salt still displayed the peak at 22.2". Consequently, KC1 may catalyst has not been investigated, but it is reasonable to be involved in the reaction of the support, perhaps forming assume that it is not reduced to zinc metal. a highly dispersed phase such as K2O that is XRD If high-temperature catalyst activity is dependent on transparent. There is also a correlation between the preservation of the molten salt, the eutectic components concentration of iron contaminant and potassium: Fe/K should remain chemically unchanged during the catalytic atomic ratio of 0.9 f 0.2 is typical. reaction. Ternary, or even quaternary, mixtures can offer The d spacings and the intensity ratios for the conflexibility in the design of new SMSC systems. A binary taminant correspond well with reported data for magnetite or ternary salt eutectic could be used as a solvent for (Fe304);however, other spinel structures of iron oxide such another catalytically active component that may be present as trevorite (NiFepO4)provide close matches with the data in variable oxidation states. The solvent components may and cannot be ruled No other corresponding themselves be catalysts or cocatalysts. Further examinacomponents of type 316 stainless steel from the reactor, tion of CuCl-ZnC12-KCl and other new ternary catalysts will help discern the role of the molten layer in producing (45) Fontana, C. M.; Gorin,E.;Kidder, G. A.;Meredith, C. S.Ind. Eng. Chem. 1952,44,363. highly active catalysts. (46) Little, J. A.; Kenney, C. N. J. Catal. 1985, 93, 23. Concerns about the volatility of CuCl or ZnClz proved (47) Garcia, C. L.; Resasco, D. E. Appl. Catal. 1987,29,55. (48) Garcia, C. L.; Resasco, D. E. Appl. Catal. 1989,46, 251. to be unwarranted as sublimation has not been observed
740 Energy &Fuels, Vol. 8, No. 3, 1994
such as Cr or Ni, were detected by EDS, although the signals from these components should have been above the limits of detection. Both the XRD and preliminary selected area electron diffraction data support the existence of cuprous oxide in 17CuC1-6.2KCl/SiOp-B. The most plausible explanation for the formation of CupO is the hydrolysis of CuCl by byproduct water. Most of the CuCl is ultimately reduced to Cu(O),however. Neither CuCl nor a complexinvolving CuCl is observed in the XRD of the used catalyst. The most likely reducing agents are Hp and CO. The relative amounts of CupO metallic copper were not quantitatively measured. These data are necessary to correlate catalyst activity with the concentration of a given copper phase. Particle size information gained from EM and XRD line-broadening indicates copper dispersed primarily as 20-50 nm particles. The oxidative procedure causes a 200% increase in the relative XRD peak intensities for CupO. A t the same time, the average CupO particle size determined from the line-widths increases from 13 to 23 nm. The different copper phases were not discernable using imaging and EDX in the electron microscope. Success in this area will require more effective application of electron diffraction and other surface analysis techniques. Carbon Monoxide Hydrogenation. The Cu/Zn ratio in 7.1 CuCl-23 ZnClz-2.0 KCl/SiOp is that found optimum for coprecipitated Cu/ZnO methanol synthesis catalysts.18 The salt melting point was depressed by adding 10 mol % KC1. Yet, this catalyst produces only hydrocarbons, even at 250 OC. The absence of alcohol products is likely due to Lewis acid-catalyzed alcohol dehydration. The olefin yield is low, but we have since found that the percentage of olefin products increases significantly a t higher space velocities and Hp/CO C 1. Even the weakly acidic CuC1KC1 eutectic supported on silica does not afford alcohols, only hydrocarbon^.^^ Conclusions
A series of novel catalysts has been prepared that render very high methanol dissociation activity, high selectivity to CO and Hp, and excellent thermal stability. Various zinc compounds were examined as promoters: dispersed (49) Schmitz, A. D.; Eyman, D. P. Manuscript in preparation.
Schmitz and Eyman ZnO, zinc silicate support, and ZnClp as a eutectic component. Effects of these promoters on activity are mixed and depend on reaction temperature. Zinc silicate and ZnClp can bolster activity but do not increase CO selectivity, whereas, ZnO can increase activity, CO selectivity, and thermal stability. The alumina-supported catalyst shows lower CO selectivity due to side reactions catalyzed on acidic sites. Operating conditions that optimize the CO/Hp yield have been established. Insight into the chemical and physical nature of the active catalyst centers has been gained by XRD and EM techniques. In the before-use catalyst, the CuC1-KC1 eutectic forms large particles on or near the support surface that are visible by SEM. However, XRD shows that the salt catalyst is well dispersed and poorly ordered, overall. Bulk and supported CuC1-KC1 have similar melting ranges, showing that CuCl and KCl are in intimate contact on the support and not segregated. Supported molten salt catalyst activity increases dramatically with time. A multiphasic distribution of copper, in multiple oxidation states, exists on the active catalyst. X-ray powder diffraction patterns are dominated by metallic copper with weak lines for Cup0 and other phases. It has not yet been possible to correlate activity with the presence or concentration of a given phase. As we seek to reduce our dependence on oil and turn to alternate fuel possibilities, commercialization of methanol dissociation technology may become very important. The results herein describe catalysts with very high methanol dissociation activity. Considering potential applications, the demonstrated catalyst thermal stability is also very important and unprecedented. It is thought that SMSC catalyst composition can be optimized to improve performance still further. Acknowledgment. The authors gratefully acknowledge the receipt of funds from the US. Air Force (administered by Wright Patterson AFB on a subcontract from ACCEL Catalysis, Inc.) and the Graduate College of the University of Iowa. We thank the staff of The Center for Electron Microscopy Research at the University of Iowa for their technical assistance. Gratitude is also extended to RavindraDatta (Department of Chemical and Biochemical Engineering, the University of Iowa) and to Tracy Bell (NSF-REU fellow).
