Ind. Eng. Chem. Res. 1995,34, 78-82
78
Characterization of Zirconia-Based Catalysts Prepared by Precipitation, Calcination, and Modified Sol-Gel Methods Zhentao Feng, Walter S. Postula, Aydin Akgerman, and Rayford G. Anthony* Kinetics, Catalysis and Reaction Engineering Laboratory, Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843
Zirconia-based catalysts were synthesized by various methods and were evaluated for the synthesis of isobutane and isobutene from CO hydrogenation. The methods of preparation included precipitation (ZrO2 (PPT)),calcination of zirconyl salt (ZrO2 (H-0304)), and a modified sol-gel method (ZrO2 (sol-gel)). Acid-base properties of zirconia were affected by the method of preparation. The number of acidic sites on the surface follows the order of ZrO2 (PPT) > ZrO2 (sol-gel) = ZrO2 (H-0304), and the number of basic sites follows ZrO2 (PPT) > ZrOz (solgel) > ZrO2 (H-0304).Furthermore, adding aluminum increased the numbers of acidic and basic sites, and adding silicon increased the number of acidic sites but reduced the number of basic sites. The product distribution of CO hydrogenation depended on the acid-base properties of the catalyst, and the activity tests indicated that a large ratio of basic to acidic sites is a requirement for an active catalyst to produce isobutane and isobutene from CO hydrogenation.
Introduction There are strong economical incentives to develop catalysts for the hydrogenation of carbon monoxide to produce isobutane and isobutene (i.e., isosynthesis) because of a nationwide shortage of these two compounds (Sofianos, 1992). Zirconium oxide was one of the most active isosynthesis catalysts, as indicated in the work by Pichler and Ziesecke (1950)) Maehashi et al. (19841, Deflin et al. (19871, Gajda et al. (1991), and Postula et al. (1994). However, the activity of zirconium oxide depends on the method of preparation, as concluded in the review by Sofianos (1992). The primary objective of this work is to synthesize zirconia-based catalysts with different acid-base properties and to investigate the relation between isosynthesis activity and acid-base properties. Since acidbase properties are affected by the method of preparation, zirconium oxides were synthesized by three different methods in this study: precipitation, calcination of a zirconyl salt, and modified sol-gel. Precipitation, in which zirconyl in an aqueous solution is precipitated by using aqueous ammonia, is widely used in the literature to synthesize zirconia for isosynthesis. Calcination of a zirconyl salt is a commercial method to prepare zirconia. The modified sol-gel method used in this study was developed in the early 1980s by Dosch et al. (1985) to synthesize titanium-, zirconium-, niobium-, and silicon-based materials for coal liquefaction. The modified sol-gel method differs from the conventional sol-gel procedure in the hydrolysis procedure: instead of being hydrolyzed directly by an aqueous solution, alkoxide is first mixed with a basic nonaqueous solution (e.g., NaOH in methanol) to produce a soluble intermediate, and then the intermediate is hydrolyzed with acetoneiwater. Stephens et al. (1985) and Feng et al. (1992) reported that metal oxides prepared by the modified sol-gel method have several properties that make them promising catalyst materials, including dual ion-exchange capacity for cations and anions and acidbase bifunctional properties.
* Author to whom the correspondence should be directed. Telephone: (409)845-3370;Fax: (409)845-6446;e-mail address:
[email protected].
0888-5885/95/2634-0078$09.00/0
Beside the method of preparation, acid-base properties of zirconia are also affected by the presence of other metal oxides. In this work, aluminum and silicon were added to the modified sol-gel preparation to modify the acidity and basicity of zirconium oxide.
Experimental Section Catalyst Synthesis. (A) Precipitation Method. Zirconia was prepared by slowing adding a 5 w t % aqueous zirconyl nitrate solution to a 1.5 wt % solution of ammonium hydroxide. The final pH of the mixture was adjusted to 8 with nitric acid. After being filtered and washed with acetone, the gel was dried in a vacuum a t 80 "C overnight. The catalyst is referred to as ZrO2 (PPT). (B) Calcination of Zirconyl Salt. ZrOz (H-0304). Calcination of a zirconyl salt (chloride or nitrate) at temperatures over 600 "C is a commercial method for synthesizing zirconium oxide (Clark and Reynolds, 1937; Henderson and Higbie, 1954). A sample of zirconia (98%,Catalog No. Zr-0304 T 1/8, Merck Index 11,10083) was purchased from Harshaw Chemical Co. The pellets particles and are were ground and sieved to 0.25" referred to as ZrO2 (H-0304). (C) Modified Sol-Gel Method. ZrOz (sol-gel). Zirconium oxides were synthesized by a modified solgel method. Tetramethylammonium hydroxide (0.06 mol, 25 wt % in methanol, Aldrich) was mixed with methanol (Mallinckrodt, 99.9%) to produce a solution with 10 wt 92 tetramethylammonium hydroxide in methanol. Then zirconium isopropoxide (0.2 mol, 70 wt % in l-propanol, Aldrich) was slowly added to the hydroxide solution to obtain a soluble intermediate. The intermediate was rapidly added to 200 mL of 1:lO by volume water (distillate and deionized) and acetone (Mallinckrodt,99.8%)solution. The slurry was continuously stirred until the particles were suspended in the solution. Finally, the precipitate was collected by filtering and drying in a vacuum at 80 "C overnight and is referred to as ZrO2 (sol-gel). The modified sol-gel procedure used in this work is slightly different than the original procedure proposed by Dosch et al. (1985) in that tetramethylammonium hydroxide replaced sodium hydroxide in forming the soluble intermediate. This modification is necessary
0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 79 Table 1. Surface Area, Composition, Crystal Structure, and Bulk Density of the Catalysts catalysp
SAb (m2/g)
A r o 2 (PI") ZrO2 (H-0304) ZrO2 (sol-gel) Al203-Zr02 (sol-gel) Si02-ZrOz (sol-gel)
35 26 24 51 24
compositionC crystal phased
5.0 wt % Al 5.1 wt % Si
acidic sitese (mmoVg)
basic sited (mmol/g)
densitys (g/mL)
0.609 0.165 0.176 0.453 0.255
0.194 0.087 0.148 0.271 0.042
1.9 2.4 2.2 2.0 2.3
M M T T T
a &r calcination. Surface areas after reaction determined by N2 BET. Weight % in calcined material determined by atomic absorption spectroscopy. Crystal phases determined by X-ray diffraction: M, monoclinic; T, tetragonal. e Acidic sites measured by temperatureprogrammed desorption of ammonia. f Basic sites measured by temperature-programmed desorption of carbon dioxide. 8 Bullr density.
since Feng (1994)showed that zirconias prepared by the original modified sol-gel procedure were not active for CO hydrogenation. A1203-Zr02 (sol-gel). Aluminum was added to modify the acid-base properties of zirconia. Aluminum tri-secbutoxide (0.04 mol, Aldrich, 97%) was mixed with 0.16 mol of zirconium isopropoxide, and the above procedure was followed. The catalyst is referred to as Al203-Zr02 (sol-gel). SiOz-ZrOz (sol-gel). Silicon was added to modify the acid-base properties of zirconia. Tetraethyl orthosilicate (0.04 mol, Aldrich, 97%) was mixed with 0.16 mol of zirconium isopropoxide, and the above procedure was followed. The catalyst is referred t o as SiOs-ZrO2(solgel). Characterizationof Catalysts. The catalysts were calcined in air before being evaluated for CO hydrogenation. During calcination, the catalyst was heated from room temperature to 500 "C at approximately 10 "C/min, maintained a t that temperature for 3 h, and cooled to room temperature. Similar calcination temperature was used by Pichler and Ziesecke (19501, Tseng (19881, and Feng et al. (1994) to prepare active isosynthesis catalysts. Table 1is a summary of the properties of the catalysts. Surface area and bulk density vary from 24 to 51 m2/g and from 1.9 to 2.4 g/mL, respectively. Zirconias prepared by precipitation and calcination of zirconyl salt have a monoclinic structure, whereas those prepared by the modified sol-gel method have a tetragonal structure. The catalysts in this work have a pore size of 5-20 nm. In this work, the acid-base properties of the catalysts were measured by temperature-programmed desorption (TPD)of ammonia and carbon dioxide, respectively. TPD experiments were conducted at atmospheric pressure. Typically, a 200-mg sample was loaded in a quartz reactor and activated in a helium stream by heating to 550 "C at a rate of 5 "C/min and maintaining that temperature for 30 min. After the sample was cooled to 50 "C, carbon dioxide was adsorbed. The sample was then flushed with helium at 50 "C for 2 h to remove physisorbed carbon dioxide. TPD of C 0 2 was performed from 50 to 470 "C at a heating rate of 5 "C/min, and TPD of NH3 was performed from 50 to 650 "C at 10 "C/ min. The effluent was monitored by a thermal conductivity detector. The acid and base characteristics of the catalysts are presented in Figures 1and 2 and Table 1. As expected, the number of acidic and basic sites vary with the method of preparation. The change in number of acidic sites by preparation method follows the order of ZrO2 (PFT) > ZrOs (sol-gel) FZ ZrO2 (H-03041, and the order for the number of basic sites is ZrO2 (PPT) > ZrO2 (solgel) > ZrO2 (H-0304). Furthermore, acid-base properties of the catalyst were affected by the presence of another metal oxide. Adding silica increased acidity but reduced basicity, whereas adding alumina enhanced both acidity and basicity.
80
70
Temperature, C Figure 1. Acidity of catalyst. Temperature-programmed desorption O f NH3,50-650 "C at 10 "C/min: (a) ZrOz (PI"); (b)Al203ZrOz (sol-gel); (c) SiO2-ZrO2 (sol-gel); (d) ZrOz (H-0304);(e) ZrOz (sol-gel).
Kl
Temperature, C Figure 2. Basicity of catalyst. Temperature-programmed desorption of COz, 50-470 "C at 5 '%/mix (a)ZrO2 (PI");(b) 4 2 0 s ZrOz (sol-gel); (c) Si02-ZrOz (sol-gel); (d) ZrO2 (H-0304);(e) ZrO2 (sol-gel).
Apparatus and Procedure. The reactor unit used in this work is a CDS 900 fully automated reactor-GC system manufactured by Autoclave Engineers. A 20 in. long 1/2 in. i.d. stainless steel tubular reactor was located in the reactor oven which was heated to 180 "C during reactions. A 6-in. section of the reactor was heated separately by a three zone heater to maintain reaction temperature. Reactants were fed to the reactor system in separate lines and then preheated and mixed in a mixing chamber. Products were analyzed by an on-line GC equipped with parallel FID an TCD systems. The TCD system included two Porapak Q columns, a Molecular Sieve 5A column, and a palladium hydrogen transfer tube, while the FID system was equipped with
80 Ind. Eng. Chem. Res., Vol. 34,No. 1, 1995
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Space Time, s Figure 3. Catalytic activity of the catalysts, 450 "C, 70 atm: ( 0 ) ZrO2 (PPT);(v)ZrO2 (sol-gel); (+I Al203-Zr02 (sol-gel); (4ZrOz (H-0304); (0)SiOz-ZrOz (sol-gel).
tco (mmol h-' mL-l)
ZrOz (PPT) ZrO2 (H-0304) ZrOz (sol-gel) Al203-Zr02 (sol-gel) SiOz-ZrO2 (sol-gel)
25.3 6.6 13.4 12.6 2.3
a
TOF (s-l) 4.4 x 5.4 x 5.2 x 2.5 0.94
3 4 5 6 Carbon Number n
7
8
Figure 4. Anderson-Schdz-Flory plot for hydrocarbon distribution, 450 "C, 70 atm, conversions 11-12%: (VI ZrOz (sol-gel); (+) A1203-Zr02 (sol-gel); (A)ZrOz (H-0304).
Results and Discussion
Table 2. Initial Rate of CO Reaction and Turnover Frequency catalyst
2
'
10-3 10-3 10-3 10-3 10-3
Calculated at CO conversion of zero.
a 50-m BP-1 capillary column. The separation was complete, except for 1-butene and isobutene, which were separated by an off-line Carle TCD/FID GC equipped with five separate columns and a hydrogen transfer system. Molecular sieve (5-A)beds were installed in both carbon monoxide and hydrogen feed lines to remove residual moisture, and a zirconium oxide filter and an activated charcoal filter were also in the carbon monoxide line to remove carbonyls. Carbon monoxide (Matheson, 99.5%) and hydrogen (Airco, 99.995%)were stored in gas cylinders located in a ventilation hood. Roughly 10 mL of catalyst was charged into the reactor. The reactor was pressured to reaction condition at room temperature with nitrogen, then heated to reaction temperature at a rate of 5 "C/ min, and maintained at that temperature for 4 h before nitrogen was discontinued and reactants were continuously fed into the system. A base condition of 450 "C and 70 atm was chosen in this work since early research by Pichler and Ziesecke (1950)showed increased alcohol and ether production at lower temperatures or higher pressure and increased methane production at higher temperatures. Equal moles of carbon monoxide and hydrogen were fed, and data were taken at 2 h of time on stream. No deactivation was observed over 20 h time on stream.
The results of activity tests are shown in Figure 3. The experiments were conducted over equal volumes of catalyst. Catalytic activity varies with the method of preparation and composition of the catalyst and follows the order of ZrO2 (PPT) > ZrO2 (sol-gel) > Al203-Zr02 (sol-gel) > ZrO2 (H-0304) > SiOz-ZrOz (sol-gel). Anthony et al. (1993) and Postula (1994)reported the following empirical expression for CO reaction rate:
They are reported that the value of k varies with catalysts, whereas the value of K remains fairly constant. In this work, a K value of 0.331 atm-l is used for all the catalysts. By assuming plug flow, CO reaction rate was calculated as a function of CO conversion, and the initial rate of CO reaction was obtained by assuming CO conversion 0. Table 2 lists the initial reaction rates of CO per volume of catalyst and the turnover frequencies based on the initial reaction rate and the total number of acidic and basic sites. The method of preparation does not affect the turnover frequency significantly, whereas adding aluminum and silicon reduces the turnover frequency. Under the reaction conditions in this study, the products consisted primarily of carbon dioxide and hydrocarbons. Water, alcohol, and ethers were not produced in measurable amounts. Table 3 summarizes the hydrocarbon distributions. The distributions over ZrO2 (PPT), ZrO2 (H-0304), ZrOa (sol-gel), and A l 2 0 3 ZrOz (sol-gel) are compared at almost equal conversions. Because of the low activity of SiOz-ZrO2 (solgel), hydrocarbon distribution is reported for a lower CO conversion. Isosynthesis is different from Fischer-
-
Table 3. Hydrocarbon Distributions over Catalysts at 460 "C, 70 atm, and 111 CO/H2 hydrocarbon distributionC(wt %) ZrOz (PFT) ZrOz (H-0304) ZrO2 (sol-gel) A l 2 0 3 - Z r 0 2 (sol-gel) SiO2-ZrO2 (sol-gel)
space time" (5) 25 70
CO conv (%)
50
12 12 3.0
60 80
14 11
base/acidb 0.32 0.53 0.84 0.60 0.16
C1 27 31
28 31 50
CZ
c3
c4
4.1(24%) 8.3(48%) 6.5 (34%) 6.2 (47%) 17 (31%)
2.6(38%) 6.6(60%) 4.0 (44%) 3.6 (68%) 7.8 (57%)
14 24 25 23 13
c5 20 20 20 18 1.0
CS
cS+
18 6.1
15 4.7
6.4 8.7 1.3
10
10
9.4
a Space time is defined as Vho, where V is volume of catalyst and uo is inlet volumetric flow rate at reaction 2' and P. The ratio of number of basic sites to number acidic sites on the surface. e Numbers in parenthesis are the wt % of olefins.
Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 81 Table 4. Distributions of C4 Hydrocarbons at 450 "C, 70 atm, and 111 COD32 distribution of C4 hydrocarbon (wt %) ZrOz (PPT) ZrOz (H-0304) ZrOz (sol-gel) AlZ03-ZrOZ (sol-gel) SiOz-ZrOz (sol-gel)
space time (s)
CO conv (%)
baselacid
i-Ca10
i-C4Hs
n-C4Hlo
1-C4Hs
cis-Cas
trans-C4Hs
25 70 50 60 80
14 11 12 12 3.0
0.32 0.53 0.84 0.60 0.16
28 22 22 13 12
44 58 67 71 28
6.6 3.5 2.6 3.4 12
5.9 4.4 3.3 4.3 17
9.1 6.7 3.2 4.3 17
7.1 5.6 2.6 3.4 13
Tropsch synthesis, as shown in an Anderson-SchulzFlory plot (Figure 4). None of the distributions follow a straight line, which is characteristic of a standard polymerization reaction, such as F-T synthesis. Another characteristics for the hydrogenation of CO over zirconia-based catalysts is the large amount of branched C4 hydrocarbons in the products, as shown in Table 4. The distribution of products varies with acid-base properties. The ratio of the number of basic sites to the number of acidic sites follows the order of ZrO2 (solgel) > Al203-Zr02 (sol-gel) > ZrO2 (H-0304) > ZrOz (PPT) > SiOz-ZrOz (sol-gel), which coincides with the order of the ratio of isobutane and isobutene in C4 hydrocarbons, as shown in Table 4. Therefore, from the results of this work, the ratio of i-Cdtotal C4 increases with increasing ratios of basic to acidic sites. Furthermore, the weight percent of C4 in hydrocarbons is generally higher if the ratio of basic to acidic sites is larger, as shown in Table 3. That the formation of isobutane and isobutene is favorable over a catalyst with a high ratio of basic to acidic sites can be explained by the reaction mechanism. i-C4 hydrocarbons are formed via CO insertion in an q3enolate (CH3-CH--CH-O),and the enolate is stabilized on basic sites, as suggested by Mazanec (1986). Another possible route for isosynthesis is the aldol condensation type reaction over basic oxides, as observed by Hindermann et al. (1993) and Maruya et al. (1994). Previous research by Pichler and Ziesecke (1950), Kieffer et al. (19831, and Maruya et al. (1988, 1989) also indicated that most of the active isosynthesis catalysts are basic metal oxides, such as ThO2, La203, Y203, and Dy203. A large number of basic sites also enhances production of heavy hydrocarbons (CS'), as indicated Table 3: Zr02 (PPT), Al203-Zr02 (sol-gel), and ZrO2 (sol-gel) have a larger number of basic sites than ZrOz (H-0304) and SiOs-ZrOz(sol-gel), and the weight percents of C6' are also higher. The increased formation of heavy hydrocarbon over catalyst with strong basicity was also observed by Pichler and Ziesecke (1950) over alkalipromoted catalysts. Enhanced acidity, on the other hand, is not favorable in i-C4 formation. In this study, the catalyst with strong acidity but minimal basicity, i.e., SiOs-ZrOz (sol-gel), is more selective t o methane and less selective t o i-C4's. Jackson and Ekerdt (1986) observed that methoxide is formed on acidic sites of zirconia. Since methane is produced from methoxide as proposed by Mazanec (1986) and Jackson and Ekerdt (19861, production of methane is enhanced when a large amount of acidic sites but a minimal amount of basic sites are present on the surface. However, if the number of basic sites increases, methane production is reduced, as observed over ZrO2 (PPT).
Conclusions Zirconium oxide-based catalysts were synthesized by different methods, including precipitation, calcination, and a modified sol-gel. The method of preparation
affects the acid-base properties of the catalyst and, consequently, affects the activity and product distribution for CO hydrogenation. Though turnover frequency, calculated from the initial reaction rate of CO and the total number of acidic and basic sites, does not vary significantly with the method of preparation, zirconia prepared by precipitation method shows the highest CO conversion because of the large number of acidic and basic sites on the surface. The distribution of hydrocarbons is related to the ratio of basic to acidic sites on the surface. Catalysts with a higher ratio of basic to acidic sites, such as ZrO2 (solgel) and Al203-Zr02 (sol-gel), are more selective to isobutane and isobutene. On the other hand, catalysts with a lower ratio of basic to acidic sites, such as ZrO2 (PPT) and SiOz-ZrOz (sol-gel), are less selective to isobutane and isobutene. Therefore, a requirement for a catalyst to selectively produce isobutane and isobutene from CO hydrogenation is a high ratio of basic to acidic sites on the surface.
Acknowledgment This work was performed as a part of the project funded by the U.S.Department of Energy on Catalysts and Process Development for Synthesis Gas Conversion to Isobutylene, Contract DE-AC22-90PC90045, Texas A&M Research Foundation Project 6722.
Literature Cited Anthony, R. G.; Akgerman, A.; Postula, W. S.; Feng, Z.; Philip, C. V.; Erkey, C. Catalyst and Process Development of Synthesis Gas Conversion to Isobutylene. Presented at the DOE Contractor's Meeting, Pittsburgh, PA, 1993. Arai, T.; Maruya, K. I.; Domen, K.; Onishi, T. Selective Formation of Ethene from CO Hydrogenation Reaction over Inz03-Ce02, -La203, and -Y203 Mixed Oxide Catalysts. Bull. Chem. SOC. Jpn. 1989,62,349-353. Clark, G. L.; Reynolds, D. H. Chemistry of Zirconium Dioxide. Znd. Eng. Chem. 1937,29,711-715. Deflin, M.; Lambert, J. C.; Dejaifve, P. E. Process for the Production of Hydrocarbons. Canadian Patent 1,226,875, 1987. Dosch, R. G.; Stephens, H. P.; Stohl, F. V. Catalysts Using Hydrous Metal Oxide Ion Exchanges. US.Patent 4,511,455, 1985. Dosch, R. G.; Stephens, H. P.; Stohl, F. V.; Bunker, B. C.; Peden, C. H. F. Hydrous Metal Oxide-Supported Catalysts. DOE Report Sand89-2399; Department of Energy: Washington, DC, 1989.
Feng, Z. Formation of Isobutane and Isobutene from Synthesis Gas over Zirconia Catalysts. Ph.D. Dissertation, Texas A&M University, College Station, TX, May 1994. Feng, Z.; Liu, L.; Anthony, R. G. Reactions of Propane on Modified Metal Oxides. J . Catal. 1992, 136,423-431. Feng, Z.; Postula, W. S.; Erkey, C.; Philip, C. V.; Akgerman, A.; Anthony, R. G. Selective Formation of Isobutane and Isobutene from Synthesis Gas over Zirconia Catalysts Prepared by a Modified sol-gel Method. J . Catul. 1994,148, 84-90. Gajda, G. J.;Barger, P. T.; Piasecki, C. A. Synthesis to Isobutylene. Resented at the DOE Contractor's Review Meeting, Pittsburgh, PA, 1991. Henderson, A. W.; Higbie, K. B. An Improved Method for Obtaining High Purity Zirconium and Hafnium Oxide. J . Am. Chem. SOC.1954,76,5878-5879.
82 Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995 Hindermann, J. P.; Hutchings, G. J.; Kienemann, A. Mechanistic Aspects of the Formation of Hydrocarbons and Alcohols from CO Hydrogenation. Cutul. Rev. Sci. Eng. 1993,35,1-127. Jackson, N. B.; Ekerdt, J. G. Methanol Synthesis Mechanism over Zirconium Dioxide. J . Cutul. 1986,201,90-102. Jackson, N.B.; Ekerdt, J. G. Isotope Studies of the Effect of Acid Sites on the Reactions of C3 Intermediates during Isosynthesis over Zirconium Dioxide and Modified Zirconium Dioxide. J . Catul. 1990, 126,46-56. Kieffer, R.; Varela, J.; Deluzarche, A. Reaction of Carbon Monoxide and Hydrogen on Rare Earth Metal Oxide Catalysts. J . Chem. SOC.,Chem. Commun. 1983,763-764. Maehashi, T.;Maruya, K. I.; Domen, K; Aika, K. I.; Onishi, T. Selective Formation of Isobutene from Carbon Monoxide and Hydrogen over Zirconium Oxide Catalyst. Chem. Lett. 1984, 747-748. Maruya, K. I.; Fujisawa, T.; Maehashi, T.; Haraoka, T.; Narui, S.; Asakawa, Y.; Domen, K; Onishi, T. The CO-H2 Reaction over ZrOz to Form Isobutene Selectively. Bull. Chem. Soc. Jpn. 1988,61,667-671. Maruya, K. I.; Fqjisawa, T.; Takasawa, A.; Domen, K.; Onishi, T. Effect of Additives on Selective Formation of Isobutene from the CO-H2 Reaction over ZrOz. Bull. Chem. SOC.Jpn. 1989, 62, 11-16. Maruya, K. I.; Takasawa, A.; Aikawa, M.; Haraoka, T.; Domen, K.; Onishi, T. Mechanism of Branched Carbon-Chain Formation from CO and Hz over Oxide Catalysts. 1. Adsorbed Species
on ZrOz and CeO2 During CO Hydrogenation. J. Chem. SOC. Faraday Trans. 1 1994,90,511-917. Mazanec, T. J. On the Mechanism of Higher Alcohol Formation over Metal Oxide Catalysts. J. Cutul. l986,98,115-125. Pichler, H.; Ziesecke, K H. The Isosynthesis. Bulletin 488; Bureau of Mines: Washington, DC, 1950. Postula, W. S. Conversion of Synthesis Gas to Isobutylene over Precipitated Zirconia Based Catalysts. W.D. Dissertation, Texas A&M University, College Station, TX, May 1994. Postula, W.S.; Feng, Z.; Philip, C. V.; Akgerman, A.; Anthony, R. G. Conversion of Synthesis Gas to Isobutylene over Zirconium Dioxide Based Catalysts. J. Cutul. 1994,145,126-131. Stephens, H. P.;Dosch, R. G.; Sbhl, F. V. Hydroua Metal Oxide Ion Exchangers for Preparation of Catalysts for Direct Coal Liquefaction. Znd. Eng. Chem. Prod. Res. Dev. 1986,24,1519. Tseng, S. C.; Jackson, N.B.; Ekerdt, J. C. Isosynthesis Reactions of COD32 over Zirconium Dioxide Oxide. J . Catul. 1988,109, 284-197.
Received for review May 16,1994 Accepted September 21, 1994@ I39403129 @
Abstract published in Advance ACS Abstracts, November
15,1994.
