Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979 191
centrations as high as 0.01-1% (Thomas, 1970). Therefore, it is anticipated that the Pd/A1203catalysts would be more sulfur-tolerant than nickel catalysts, thereby reducing the stringent purification procedures required for the latter catalysts. In addition, we would expect the P d catalysts to be less susceptible to permanent sulfur poisoning should a system upset occur causing an H2Sbreakthrough. These potential benefits could offset to some extent the higher cost of the P d catalyst. Summary and Conclusions Well-dispersed palladium on 17-A1203has a specific activity for CO conversion nearly 70-fold greater than large, unsupported Pd crystallites. This enhancement in activity is great enough to increase the turnover frequency for methane formation on Pd/A1203catalysts to a value within a factor of 3 of that on typical nickel methanation catalysts. If run under reactor conditions to minimize sintering, the Pd/A1203catalysts exhibit stable activity and activity per unit weight which is comparable to nickel catalysts because P d can be prepared and maintained with higher fraction exposed than Ni. This in itself does not balance out the higher cost of P d metal compared to Ni metal, but additional benefits may be gained from the use of Pd/Al2O3 catalysts including reduced C 0 2 formation, longer catalyst lifetimes, in situ regeneration, absence of carbonyl and carbide formation, and higher sulfur tolerance. The last benefit is inferred from the literature, but no direct experimental confirmation has yet been obtained. Even in
the absence of H2S tolerance data, P d catalysts appear worthy of consideration for use in methanation, although the possibility of methanol formation at high pressures must now be considered. Acknowledgment We thank Donna Piano and Larissa Turaew for their expert help in conducting the experimental work. Literature Cited Bridwr, G. W., Woodward, C., Am. Chem. SOC.Div. FuelChem. Prepr., 19, 105 (1974). Fischer, F., Tropsch, H., Diithey, P., Brennst. Chem., 8, 265 (1925). Lee, A. L., "Clean Fuels from Coal" Symposium, p 341, 1973. McKee, D. W., J. Catal., 8, 240 (1967). Mills, G. A., Steffgen, F. W., Catal. Rev., 8, 159 (1973). Poutsma, M. L., Elek, L. F., Ibarbia, P. A,, Risch, A. P., Rabo, J. A,, J. Catal., 52, 157 (1978). Schukz, J. F., Karn, F. S., Anderson, R. B., U.S. Bur. Mines Rep. No. 6974 (1967). Thomas, C. L., "Catalytic Processes and Proven Catalysts", Academic Press, Chapter 17, New York, N.Y., 1970. Thomas, W. J., Portalski, S., Ind. Eng. Chem., 50, 967 (1958). Van Harteveld, R., Hartog, F., Adv. Catal., 22, 75 (1972). Vannice, M. A,, J. Catal., 37, 449 (1975a). Vannice, M. A,, J. Catal., 40, 129 (1975b). Vannice, M. A,, J. Catal., 44, 152 (1976). Vannice, M. A., Garten, R. L., US. Patent 3941 819 (1976). Vannice, M. A., Garten, R. L., US. Patent 4093643 (1978). White, G. A., private communication, Ralph M. Parsons Co., 1974. Whne, G. A., Roszkowski, T. R., Stanbridge, D. W., Am. Chem. SOC.Div. Fuel Chem. Prepr., 19, 57 (1974). Yates, D. J. C., Sinfek, J. H., J. Catal., 8, 348 (1967).
Received for review October 20, 1978 Accepted April 2, 1979
Catalytic Conversion of Alcohols. 11 Influence of Preparation and Pretreatment on the Selectivity of Zirconia Burtron H. Davis" Potomac State College of West Virginia University, Keyser, West Virginia 26726
Pasupathy Ganesan Department of Metallurgical d Materials Engineering, University
of Kentucky, Lexington, Kentucky 40506
Zirconia has been found to resemble thoria for both dehydration and 1-alkene selectivities in 2-01 conversions. Its catalytic properties differ greatly from two other members of the group 4B family, titania and hafnia. Pretreatment and preparation play a role in determining the selectivity for dehydration and 1-alkene formation. Some zirconia samples may convert 2-01s to greater than 95% of the 1-alkene isomer. Isomerization of the pure cis-2 or trans-2-methylcyclohexanol occurred and each isomer produced the same, nonequilibrium amount of 3methylcyclohexene.
Introduction While zirconia has been studied as a dehydration catalyst, the number of reports concerning this oxide are not nearly as numerous as those for oxides such as alumina or thoria. The preparation and pretreatment of thorium oxide have a strong influence on the catalytic character
* Address correspondence to this author at the Institute for Mining & Minerals Research, University of Kentucky, P.O. Box 13015, Lexington, Ky. 40583. 0019-7890/79/1218-0191$01,00/0
of the resulting solid (Davis, 1972; Davis and Brey, 1972). A survey of the formation and properties of zirconia has recently been published (Rijnten, 1970); however, this survey did not define the character of the calcined materials nor detail the catalytic properties. It has been reported that when 2-pentanol is passed over a zirconia catalyst at 300 "C the resulting water/hydrogen ratio is 5 and the l-pentene:2-pentene ratio is 0.4 (Freidlin et al., 1971; Sharf et al., 1972). Thus, the catalyst is selective for dehydration but not for 1-alkene formation from this 2-01. On the other hand, ZrOz at 200 "C has been 0 1979 American Chemical Society
102
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
reported to be a selective catalyst for 1-butene formation from 2-butanol and to produce only the dehydration product (Yamaguchi et al., 1973). It was concluded from IR studies that zirconia possessed both moderate acidic and basic strength (Yamaguchi et al., 1976). We have investigated the conversion of a number of alcohols over oxide catalysts with the aim of defining the selectivity of oxide catalysts, especially within a family of the periodic table. The alcohols studied have been chosen to offer a number of reaction options and to allow us to correlate catalytic selectivity and a property of the metal oxides.
Experimental Section Catalysts. Zr-2. Hydrous zirconium oxide was prepared by adding concentrated ammonia to a rapidly stirred solution of zirconium acetate (ca. 1 M). The gel was removed by filtration and washed with water to a pH of 7 . It was dried at 120 "C and then calcined at 600 "C in air. Zr-3. A portion of Zr-2 (dried at 120 "C) was dispersed in a nitric acid solution and reprecipitated by adding concentrated ammonium hydroxide to the rapidly stirred solution. It was washed to a pH of 7 and then dried at 120 "C. The material was calcined in situ at 525 "C in air. Zr-4. This was the same as Zr-3 except that the gel was calcined in air at 525 "C for 2 days prior to an in situ pretreatment in flowing hydrogen at 500 "C. Zr-5. This was the same as Zr-3 except that ammonia was added during the precipitation step to pH 8. Zr-6. This was the same as Zr-3 except that the material was heated in flowing oxygen in situ at 300 "C after drying at 120 "C. Zr-7. This was the same as Zr-6 except that flowing hydrogen was used instead of flowing oxygen. Zr-8. A sample of the ZrOz (designated sample G by Argon et al., 1975) was provided by Dr. Holmes; another portion of this sample has been used by Holmes and co-workers in their study of water adsorption (Holmes et al., 1972). Zr-10. Hydrous zirconium oxide was prepared by adding 100 mL of concentrated ammonium hydroxide to 2 L of 0.4 M zirconium solution prepared from ZrO(NO& The gel was washed five times with 1.5 L of water, dried a t 120 "C, and then calcined in air at 600 "C. Zr-58. Anhydrous ZrC1, was dissolved in water to give ca. 0.5 M solution. A large excess of concentrated ammonium hydroxide was added with rapid stirring and the resulting gel was washed with water to a negative chloride test (silver nitrate). The gel was dried at 120 "C and pretreated in situ in flowing hydrogen. Zr-Cl-1, -2, -3, -4. This was prepared the same as Zr-58 except that ZrOClzwas used to prepare the solution rather than ZrC1,. The gel, after drying at 120 "C, was pretreated in situ with the indicated flowing gas: -1, oxygen; -2, hydrogen; -3, oxygen; -4, hydrogen. Alcohols. Alcohols were purchased from commercial sources. The alcohols contained less than 1% ketone, and in most cases much less than this. However, the 2-octanol contained approximately 1.5% 2-octanone; this was taken into account in making the selectivity calculations. Procedure and Analyses. The reactor system was of conventional design with a motor-driven syringe liquid feed, an electrically heated plug flow reactor, and a liquid sample collector. The alcohol was pumped over the catalyst at atmospheric pressure without added carrier gas. The LHSV (liquid hourly space velocity in milliliters of reactant per milliliter of catalyst per hour) was varied from 0.4 to 12 in order to obtain, in most cases, a conversion less than 30%. The liquid products were collected at intervals
and analyzed for conversion by temperature-programmed gas chromatography (GC) using a Carbowax 20M column. The alkene fraction was analyzed by operating the GC column appropriate for the alkenes (Carbowax 20M, @,p-oxydipropionitrile, or UC-W) isothermally. After each reaction run the catalyst was given a standard regeneration. This involved cooling the catalyst to room temperature, flushing with air, and then heating in an air flow to 250 "C; the temperature was increased to 500 "C in air and after 1 h the air was replaced by an oxygen flow. For the oxygen pretreatment, the sample was cooled to the reaction temperature in flowing oxygen. For a hydrogen pretreatment, the oxygen flow was replaced by a hydrogen flow and the heating was continued at 500 "C for four or more hours prior to cooling to the reaction temperature in flowing hydrogen. Results The conversion of 2-01s provides a number of product selectivities as outlined below. RCHzCHOHCH3
-
v
II
RCHzCCH3 RCHzCH=CHz R
>c=c
H R
H
C , H3 H'
>c=c'
H 'CH3
Thus, we have a dehydration (alkene:(alkene + ketone)) selectivity as well as a 1-alkene:(l-alkene + 2-alkene) and a cis-2-alkene:(cis-2-alkene trans-2-alkene) selectivity. The data in Table I summarize the dehydration and 1-alkene selectivities obtained with 2-octanol as reactant and with a number of zirconia preparations that had been pretreated with hydrogen and f or oxygen as catalysts. In general, it does not appear that the selectivity trend is simply related to the crystal phase, the pretreatment, or the surface area. Consider Zr-8 and Zr-10. As seen in Figure 1, both of these materials are nearly pure monoclinic; however, one (Zr-10) is selective for both 1-alkene formation and for alcohol dehydration while the other (Zr-8; Zr-9 is equivalent to Zr-8) is not. The other Zr and Zr-C1 catalysts, which were, in most cases, predominately the tetragonal form (Figure 2), showed a remarkably wide range of selectivities. Some of the catalysts maintained a nearly constant dehydration selectivity with time on stream while the selectivity of other catalysts changed significantly with time on stream. In the following sections the selectivity of some of these catalysts has been considered in more detail. The data in Table I1 demonstrate the complex influence pretreatment may have on the selectivity even when starting with different portions of the same batch of zirconia. Pretreatment with oxygen gave a catalyst (Zr-Cl-1) that was initially unselective for either dehydration or 1-octene formation from 2-octanol; as the reaction time was increased both selectivities increased. Repeated run-regeneration cycles resulted in an increase from the initial dehydration and 1-octene selectivities. Pretreating another portion of the Zr-C1 gel (dried at 120 "C) with hydrogen at 500 " C yielded a catalyst (Zr-C1-2) that was selective for dehydration but only moderately selective for 1-octene formation. On the other hand, the same pretreatment with Zr-C1-4 yielded a material that was selective for both dehydration and 1-octene formation. Regeneration of Zr-C1-4 in oxygen yielded a catalyst that was about as selective for dehydration as the hydrogen
+
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
193
Table I. Physical and Catalytic Properties of Zirconia from Various Preparations and Pretreatments for the Conversion of 2-Octanol hydrogen pretreatment oxygen pretreatment surface area, temp, time, dehydrationb 1kemp, time, dehydrationb 1sample m2/g crystal phasea “C min selectivity alkeneC C min selectivity alkeneC Zr-3 4.3 T 260 260 0 . 9 2 (0.96) 78 260 305 0.70 ( 0 . 5 8 ) 34 Zr-4 39.2 mixture (mostly T) 270 315 0.93 ( 0 . 9 9 ) 34d 270 190 ... 65 Zr-5 46.4 T 250 255 0.5 (0.6) 71e Zr-6 91.8 n o peaks; amorphous?? 250 490 0.4 (0.63) 60e Zr-7 71.6 mostly T 250 260 0 . 5 0 ( 0 . 5 2 ) 46d Zr-8 23.7 M 255 250 0.35 (0.40) 57 225 350 0.70 (0.63) 28 Zr-10 44.0 M 243 345 0.83 (0.84) 97 237 330 0.77 (0.16) 88 Zr-58 *.. 250 220 0.92 ( 0 . 9 9 ) ca. 35 Zr-C1.1 57.5 mixture (more M ) 260 360 0.62 (0.39) 88 Zr-C1-2 65.4 mixture (more T) 255 470 0.88 (0.93) 70 260 450 0 . 8 2 ( 0 . 8 1 ) 50 Zr-C1-3f 70.9 mixture (mostly T) 237 385 0.68 (0.61) 45 Zr-C1-4 72.9 mixture (mostly T ) 240 380 0.80 (0.78) 90
...
Selectivity defined by (-ene/-ene + -one); number in parentheses is for the first sample a T, tetragonal; M, monoclinic. collected after about 30-50 min; the other number represents the last sample collected after about four 4-5 h (times relaAdded heptene isomerization (see text). e No isomerizaPercent of total alkenes. tive to a LHSV = - 0 . 3 mL/mL/h). tion of added heptene (see text). f Oxygen pretreatment and regeneration prior to hydrogen pretreatment.
n
I
38
I
I
36
l
1
34
1
1
32
1
1
30 28 c
i
l
28
I
I
26
l
l
1
I
24
Figure 1. X-ray spectra for Zr-8 and Zr-10 after use as catalysts (M, monoclinic phase; T, tetragonal phase).
36
l
l
l
34
l
l
32
l
l
30
l
l
28
1
1
26
I
1
24
Figure 2. X-ray spectra for Zr-C1 catalysts after use for catalytic conversions (M, monoclinic phase; T, tetragonal phase). 35,
pretreated one but was not selective for 1-octene formation. Zr-C1-3 had two runs and oxygen regenerations prior to a hydrogen pretreatment; after the hydrogen pretreatment the catalyst was not selective for either dehydration or 1-octene formation. A low surface area zirconia, Zr-3, was prepared by precipitation with concentrated ammonium hydroxide from a “zirconyl nitrate” solution which, in turn, had been prepared by disolving the “hydrous oxide” in nitric acid. The results in Figures 3 and 4 show that the hydrogen pretreated sample was a selective dehydration catalyst which was reasonably selective for the formation of 1octene from 2-octanol. In contrast, the same sample subjected to an oxygen pretreatment was nonselective; dehydration and dehydrogenation occurred to an equal extent and nearly equal amounts of the three octene isomers were produced. The dehydration activity appears to be about the same after either pretreatment; the selectivity difference is due to the low dehydrogenation activity of the hydrogen pretreated sample. The first and third run using hydrogen pretreatments show that both
l
38
I
Clygen
I
I
I
I
I
\
I
251
11me.Min
Figure 3. Dehydration and dehydrogenation of 2-octanol over hydrogen and oxygen pretreated Zr-3.
dehydration and 1-alkene selectivity are imparted by the pretreatment and that there are only minor changes in selectivities after two run-regeneration cycles when these are followed by the appropriate pretreatment.
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
Table 11. Dehydration and Alkene Selectivitv for the Conversion of 2-Octanol with Zr-C1 Catalvsts
catalyst
pretreatment
reaction temp, " C
time:
min
conversion. mol %
octenes, mol %
selectivityb
1-
trans-2-
cis-2-
31 0.39 47 23 30 80( 1) 165(2) 22 0.45 63 16 21 245(3) 22 0.53 80 7.5 12 360(4) 23 0.62 87 5.5 7.0 0,(500)c 228 lOO(1) 9 0.49 59 14 27 210(2) 6 0.51 79 8.5 13 320( 3 ) 6 0.58 83 6.5 11 410(4) 5 0.64 84 5.8 10 O,(5 0 0 y 265 llO(1) 29 0.63 74 12 14 410(4) 25 0.70 88 4.7 7 .O Zr-Cl-2 H2(500) 255 105(l)d 56 0.93 56 18 16 470(5) 30 0.88 78 15 17 0,(500) 255 90(l)d 43 0.81 45 16 39 450( 5 ) 31 0.81 49 20 31 Zr-C1-3e 02(500) 23 5 105(l)d 28 0.61 41 16 33 02(500) 320(4) 32 0.68 47 21 32 Zr-cl-4f H2(500) 245 85(l)d 13 0.78 87 7.3 5.8 380( 4 ) 10 0.87 89 5.6 5.2 a Number in parentheses is the sample number. Defined as -en/(-ene + -one). A standard regeneration preceded each of these runs. Very little change in selectivity or octene distribution during these runs. e Two run-regenerations preceded the hydrogen pretreatment. f Reactant was an equal molar mixture of 2-methyl-2-butanoland 2-octanol. Zr-C1-1
70
0,(500)
255
Scheme I
c /
-1
alkene( g) -+
f.l
-+
alcohol(g) -+ alcohol(a) + alkene(a) +- isomerized alkene(a) + isomerized alkene(g) s
J
50t
L i
3ot
Scheme I1 alkene(g)
? - +
alcohol(g)
- 7loo h h k 7 200
; 1
2k
,bo
Time. Min
Figure 4. Octene distribution from the conversion of 2-octanol over 1-octene;0,cis-2-octene; hydrogen and oxygen pretreated Zr-3 (0, A, trans-2-octene).
A batch of zirconia (Zr-10) which was prepared by precipitating the hydrous oxide from a zirconyl dinitrate (0.5 M) solution by adding an excess of concentrated ammonium hydroxide was very selective for 1-octene formation after either a hydrogen or oxygen pretreatment. The hydrogen pretreated material appeared to be slightly more selective (Figure 5). The results shown in Figure 5 are for the first two runs using Zr-10 as catalyst. The oxygen pretreated sample was not selective for dehydration initially but as the reaction continued the selectivity increased to approach that of the hydrogen pretreated sample. Unlike the dehydration selectivity, the alkene selectivity did not undergo a large change with time. The total conversion was about the same regardless of pretreatment and was nearly constant with time on stream. A portion of the same batch of gel (dried at 120 "C) used to prepare Zr-10 was calcined at 525 "C in air for 4 h; X-ray examination of this material showed it to be a mixture of the tetragonal and monoclinic crystalline forms. For the five 2-01s in Table 111, Zr-10 was very selective in forming the 1-alkene. There was no preference for the cis-2- alkene; nearly equal amounts of the cis-2- and trans-2-alkene isomers were formed. After about 25
-+
alcohol(a)
-+
alkene(a) + isomerized alkene(a) isomerized alkene(g)
-+
run-regeneration cycles with Zr-10, the pretreatment became inconsequential and dehydration was favored over dehydrogenation by a factor of ca. 1.5 to 2.0. The catalyst retained its selectivity for 1-alkene formation throughout the run-regeneration cycles. After approximately 30 runs the catalytic material, as shown by the X-ray diffraction spectra in Figure 1, contained only the monoclinic crystalline form. This suggests that initially Zr-10 was a mixed crystal phase which changed to monoclinic during use. The surface area of the Zr-10 apparently increased slightly during use from 36 m2/g to 44 m2/g. The nitrogen isotherm at -195 "C (Figure 6) is typical of that for a material with limited pore volume. The pore volume (Figure 7) for the fresh catalyst showed a marked maximum at an average pore diameter in the range of 135 A from the adsorption isotherm or about 100 A from the desorption isotherm; the maximum did not change appreciably following use of the material as a catalyst. A sample of zirconia (designated Zr02-G in Argon, 1975) was obtained from Dr. H. F. Holmes. The adsorptiondesorption isotherm was essentially the same for the fresh (Zr0,-G) and the used zirconia catalyst (Zr-9) in Figure 8, and the hysteresis loop suggests a complex variety of pore shapes and sizes (Argon, 1975; deBoer, 1958). Five run-regeneration cycles resulted in a small surface area loss (from 24 to 19 m2/g). With this sample the oxygen pretreatment gave a catalyst that was selective for dehydration whereas the hydrogen pretreatment did not (Figure 9). This is just the opposite of that of Zr-3 and Zr-10. With these catalysts the hydrogen pretreated sample was selective for 1-octene formation from 2-octanol and the oxygen pretreated sample was not (Figure 10).
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
I x
Conv x
x - l x
x
1
195
Average Pore Diameter, P
Figure 7. Pore volume distribution as a function of the average pore diameter for Zr-10 prior to use as a catalyst (0,from the adsorption isotherm; X, from the desorption isotherm). IO0
200
300
IO0
200
303
T i m e , Min
80
Figure 5. Conversion of 2-octanol over oxygen and hydrogen pretreated Zr-10 ( X , conversion; 0,dehydration selectivity; 0,1-octene; 0,cis-2-octene; A, trans-2-octene). BO
I
1
I
I
I
c
I.
1
70t t
6o
Y
I
1
I
I
1
02
04
06
08
10
P/Po
I
I
I
1
I
02
04
36
08
Figure 8. Adsorption ( 0 )and desorption ( X ) of nitrogen at -195 O C on Zr02-G (top) and catalyst Zr-9 (bottom). Note that the amount adsorbed is offset 20 mL (STP)/g upward for sample Zr02-G. IO BO
P/Po
Figure 6. Adsorption ( 0 )and desorption on Zr-10.
(X)
I
1
I
of nitrogen at -195 "C
The alkenes in the liquid products may differ from the primary alkene distribution due to isomerization to secondary alkene products. Two reaction pathways that may operate are outlined in Schemes I and 11. In the schemes (a) refers to the adsorbed phase and (g) to the gas phase. In Scheme I the alkene adsorption equilibrium is established. Thus, an alkene that closely resembles the alkene dehydration product in chemical and physical properties should undergo isomerization when it is added to the alcohol charge. Therefore, the addition of a heptene isomer to the 2-octanol reactant should be a definitive test of isomerization which alters the primary alkene product distribution. On the other hand, the addition of an alkene would not exclude the operation of Scheme 11. The results in Table IV show that with the oxygen pretreated Zr-8 catalyst Scheme I probably operates to some extent since the 2-heptene added to the 1-octanol charge underwent isomerization. However, the extent of
7 oI t
6ot t
I
I
1
1
i
6o
&& 2-cclanol
I
1 -1
I
T i m e , Min
Figure 9. Dehydration and dehydrogenation of 2-octanol with hydrogen or oxygen pretreated Zr-8.
isomerization of the added heptene declined with time; a corresponding decrease in the amount of 2-octene for-
196
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
Table 111. Alkene Distribution from the Conversion 2-01s with Zirconia Derived from Zirconyl Nitrate by Ammonia Precipitation (Zr-10) reaction teomp, C
time on stream, min
2-butanol
290 235
2-pentanol
240
2-hexanol
240 235
130 135 325 250 325 450 220 280 210 270 90 180 330 465 145 290 345 100
reactant
pretreatment
4-methyl-2-pentanol
0 2
245
2-octanol
OZd
237 265 243
4,4-dimethyl-2-pentanol
air
250
conversion,a mol % selectivity 44 8
alkene, mol % 1-
0.66 0.2c 0.2 0.42 0.50 0.54 0.55 0.56 0.5 0.59 0.47 0.61 0.77 0.73 0.82 0.82 0.84 0.71
8 11 12 16 18 20 16 17 21 19
18 27 28 31 29 21
trans-2-
96 97 98 89 92 94 98
cis-2-
2.2 1.7 1.2 5.2 3.6 3.0 1.1 1.1
98 81 88
2.2 1.7 1.2 6.0 4.3 3.0 1.o 1.1 19 12 15 11 7.0 4.8 1.1 5.8 2.3 1.1
11 5.7 6.0 5.8 1.2 1.9 2.7 5.8
74 81 87 89 98 92 95 93
a Conversion is for the sample collected at the time-on-stream shown in this table, Defined as (-ene)/(ene t -one). After approximately 30 run-regeneration cycles preceding these runs. Probably low because of small gas flow.
Table IV. Alkene Products from the Conversion of a Mixture of (cis + trans)-2-Heptene and 1-or 2-Octanol with Zirconia Catalysts
catalyst
pretreatment
Zr -8
02C500)
Zr-8
HA500)
Zr-4
Hz(500)
Zr-5
HA5001
Zr-6
H2(300)
Zr-7
Oz(300)
time, min
teomp, C
conversion, mol%
heptenes 1-
75P) 140(2) 200(3) 460(4) 7W) 155(2) 230(3)
260 260 260 260 260 260 260
1-0ctanol 75 18 73 10 73 7.6 72 4.8 15 0.3 10 tr 8 0.9
76 315 120(2 ) 255(4) 80(1) 200( 3 ) 490(5) 85(1) 145(2) 312(4)
265 265 250 250 250 250 250 250 250 250
2-octanol 85 19 80 3.8 12 12 37 tr 28 28 18 2.9 12 7
octenes
trans-2-
cis-2-
1-
trans-2-
cis-2-
26 25 22 19 18 15 20
56 65 70 76 82 85 79
53 53 56 56 67 60 62
25 23 22 21 18 25 24
22 23 23 23 15 15 14
30 17 18 16 15 14 13 18 14 16
51 79 82 84 85 86 87 79 86 84
47 31 73 71 42 55 61 36 36 47
29 30 11 11 23 17 17 25 23 10
24 39 16 18 35 28 22 39 42 33
Reaction mixture contained 16.4% trans-2- and 83.6% cis-2-heptene.
mation from the 1-octanol was not observed. In contrast, the added 2-heptene, mixed with 1-octanol, did not undergo isomerization over the hydrogen pretreated Zr-8 even though 2-octenes were formed. The results with the 2heptene/2-octanol mixtures suggest that isomerization, as shown in Scheme I, does not occur with Zr-5, -6, or -7. Consequently, the alkene distribution we observe is the primary, gas phase distribution. Since all of the reactions depicted in Scheme I1 occur on the same active site, isomerization should be viewed as a part of the dehydration rather than as a separate isomerization reaction. Scheme I1 should only be viewed as a distinct mechanistic step when surface migration of the adsorbed primary alkene occurs prior to isomerization. Alumina dehydrates internal secondary alcohols such as 3-pentanol or 4-octanol to alkenes which have a high cis-trans ratio (Pines and Manassen, 1966). Hydrogen pretreated Zr-C1-4 selectively formed 1-octene from 2octanol; cis-trans ratios for the small amount of 2-octenes formed were in the range of 0.8-1.0. Both 3-pentanol and
80
1
I
I
' KO'
I
I
I
za-8
2-0claml Hydrqen
a
h
100
100
200
200
300
Trons-2
400
500
Time, Min
Figure 10. Octene distribution from the conversion of 2-octanol with hydrogen or oxygen pretreated Zr-8 (distributions correspond to the dehydration-dehydrogenations shown in Figure 9;symbols are the same as for Figure 4).
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
197
Table V. Comparison of the Alkenes from the Conversion of 2-01s and Internal Secondary Acyclic Alcohols
alcohol 2-octanol 3-pentanol
catalyst Zr-C1-4 500) Zr-C1-4 (H2, 500)
w2,
4-octanolb
Zr-C1-4 (H*, 500)
2-pentanol
Zr-10 (H2,500) Zr-10 (H2, 500)
3-pentanol
selectivity" 0.79 0.87 0.14 0.18 0.19 0.27 0.21 0.24 0.54
conversion, mol % 13 10 10 8 8 18 12 11 15
0.09
Selectivity defined as (-ene)/(-ene t -one). octenes.
9
temp, "C
time, min
H 2
220
H2
210
H2
255 210 240
85 275 375 375 445 205 490 285
0 2 0 2
LHSV 0.6
0.3 0.3 0.3 3.2
2con- methylver l-pension, tene, mol mol % % alkenes 28 33 31 26 29 30 50 33
temp, "C 24 1 245 245 243 243 24 7 247 247 240
190
237
187 89 tr tr tr
94
trans-27.3 5.6 53 54 55 56 58 58 3.0 63
cis-2-
5.8 5.2 47 46 45 44 42 42 3.0 37
cis-2/ trans-2 0.8 0.94 0.91 0.84 0.81 0.78 0.73 0.73 1.0 0.59
The chromatographic method was only able t o separate cis- and truns-
Table VI. 1-Alkene Formation from the Conversion o f 2-Methyl-2-pentanolwith Zr-10
pretreatment
alkenes, mol %
time, min 85 380 80 150 21 5 100 160 24 0 450
42 46 47 44 40 47 29 40
4-octanol gave products with a cis-trans ratio similar to that obtained from 2-octanol (Table V). With Zr-10 this ratio is lower from 3-pentanol than from 2-pentanol. However, it is apparent that neither catalyst has the strong cis selectivity that alumina has. While zirconia is a selective catalyst for the formation of the 1-alkene from secondary 2-01s it is not selective for the formation of the 1-alkene from the tertiary 2-01s, 2-methyl-2-butanol (Table VI). The amount of 2methyl-1-butene formed is about the same as has been obtained with a number of catalysts. The selectivity of metal oxide catalysts for 1-alkene formation from secondary 2-01s is not a factor that influences the product distribution in 2-methyl-2-butanol dehydration. Essentially the same, nonequilibrium mixture of 3- and 1-methylcyclohexenewas obtained from pure cis-2- or pure trans-2-methylcyclohexanoland the same mixture was obtained with either pretreatment (Table VII). Isomerization of each alcohol charge occurred. The cis-trans alcohol isomerization, relative to the amount of dehydration plus dehydrogenation, was about three times
greater with the cis-2 alcohol than with the trans-2 alcohol; it did not depend on pretreatment. The dehydration selectivity was the same for both pretreatments and for both isomers. After completing one of the runs with methylcyclohexanol,the reactor was flushed with hydrogen at the reaction temperature (250 "C) to remove this alcohol charge from the preheater and reactor section; after about 10 min the hydrogen flow was stopped and 2-octanol was passed over the catalyst a t 250 O C . The 2-octanol dehydration selectivity was 0.7 and greater than 95% of the octenes were the 1-octene isomer. This suggests that isomerization of methylcyclohexene isomers did not occur after desorption. The activity at later time-on-stream was used to calculate the temperature coefficients in Table VIII. It should also be noted that a chromatographic effect, where the alcohol experiences a longer hold-up time in the catalyst bed than the alkene product does, contributes to the apparent high conversion for the first sample collected; thus, the activity decline was not as severe as might be deduced from a cursory examination of the conversion figures. The dehydration and dehydrogenation reactions have been taken to be zero order in alcohol. The change in conversion with flow indicated that the conversion of 2-methyl-2-butanol was zero order up to 3 0 4 0 % conversion. The temperature coefficient for dehydration is slightly lower than the 26-30 kcal/mol reported for alumina as catalyst (Pines and Manassen, 1966). Discussion The results obtained in this study make it clear that the catalytic properties of zirconia are quite complicated. The runs were carried out at 1 atm alcohol pressure and lasted for 4-6 h. Thus, the variety of selectivities cannot be due to the presence of a few sites with special selectivity that quickly poison or deactivate as might occur in a pulse reactor.
Table VII. Product Selectivities for the Conversion of cis-2 or trans-2-Methylcyclohexanol over Zr-10 (LHSV, 0.3; Temperature, 25 C )
pretreatment air air air (repeat run) hydrogen hydrogen
reactant
conversion, mol %
time, minQ
dehydration selectivityb
alcoholC cis trans
meth ylcyclohexene 431-
total
trans 9 175(5) 9.9 0.52 90.1 0.7 56 43 1.1 cis 11 105(5) 0.39 64 36 1.6 50 48 3.3 cis 9 92(3) 0.31 69 1.5 54 44 3.4 31 trans 7 90(5) 0.51 9 91 0.8 60 39 1.2 cis 12 70(4) 0.33 60 40 1.9 57 41 3.3 ' First number gives the time that the sample (sample number in parenthesis) was collected. Defined as (alkene/ alkene + ketone). Composition of the alcohol in the liquid products. (Amount of cis-trans alcohol isomerization)/ (amount of dehydration + dehydrogenation).
198
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
Table VIII. Temperature Coefficients (Apparent Activation Energy) for the Conversion of Alcohols with . Zirconia e x . Zirconyl Nitrate (Zr-10) temp coeff, kcal/mol
H2
0 2
pretreatment pretreatment dehydehydehy- dro- dehy- drodra- gena- dra- genation tion tion tion
alcohol 2-methyl-2-butanol 2-octanol 4-methyl-2-pentanol 2-pentanol Scheme 111 CIS
-
28 21 19 21
-
26 22
17 19 21
9
or trans alcohol I
trans a l c o h o l ( g )
ketone(g1 intermediate 1.0-1.2
cis a l c o h o i ( g )
alkene(g)
J
c/>-c
t
e
c
Zirconia may be a very selective dehydration catalyst. However, the selectivity is not simply related to the bulk crystal phase. In many instances the hydrogen pretreated sample was a more selective dehydration catalyst than the oxygen pretreated material. There are exceptions to this, such as Zr-8, where just the opposite pretreatment-selectivity relationship was observed, the oxygen pretreated sample being the selective dehydration catalyst. Both the preparation and pretreatment influence the dehydration selectivity but other factors must also be involved. More of the catalysts were selective for dehydration than were selective for 1-alkene formation. With oxygen pretreated Zr-8, isomerization of the alkene product, after it desorbed to the gas phase, did occur. However, this appears to be an exception and, as shown by the data in Table IV, a number of hydrogen pretreated samples did not catalyze secondary reactions. Added 2-heptene did not isomerize even when the catalyst was not selective for 1-alkene formation from either 1- or 2-octanol. We conclude that with most of the runs the alkene distribution is representative of the initial gas phase composition. For the catalysts that were selective for 1-alkene formation from 2-01s about equal amounts of the cis-2 and trans-2 isomers were formed. Zr-10, prepared from the zirconyl dinitrate solution, was very selective for 1-alkene formation following either air or hydrogen pretreatments. This catalyst is recommended for studies where 1-alkene selectivity is important. The same products were obtained from the conversion of either 2-methylcyclohexanol isomer with air or hydrogen-pretreated Zr-10. The results from the conversion
are suggestive of a common intermediate for all three reactions: dehydration, dehydrogenation, and cis-trans isomerization of the alcohol charged. Dehydration of the cis alcohol normally results in the formation of 15-20 mol 70 of the 3-methylcyclohexene isomer (Pines and Manassen, 1966; Davis, 1978) but with Zr-10 the amount of 3-methylcyclohexene is much higher than this. Thus, the conversion of the cis isomer over Zr-10 differs from other catalysts and the results suggest that the cis isomer is converted to some intermediate which resembles the trans alcohol. Scheme I11 with a common intermediate would account for the conversion data (slowest rate for the three reactions was assigned a relative rate of 1.0). In general, our results agree with earlier observations (Rijnten, 1970; Holmes et al., 1972) since the tetragonal form predominated, in most cases, after calcination at 500 or 600 O C and use as a catalyst. However, ZR-10 appeared to differ appreciably since the material was the monoclinic form after about thirty regenerations at 500 "C. The gel used to prepare Zr-10, after calcination in air a t 525 "C for 4 h, was a mixture which appeared to be comprised of about equal amounts of the tetragonal and monoclinic form. A crystallite size effect can operate to decrease the tetragonal-monoclinic transformation temperature; in addition, the critical crystallite size at a given temperature is strongly dependent on any stresses present (Garvie, 1978). Clearly, the influence of the preparation and activation of zirconia catalysts on the crystal phase is a subject that requires more study. With respect to the influence of preparation and pretreatment on the dehydration and 1-alkene selectivities, zirconia resembles thoria (Davis and Brey, 1972) as a catalyst for alcohol conversions. It is quite different from two other members of the group 4B family, titania (Davis, 1978) and hafnia (4-Bahar et al., 1979) which are selective for dehydration whether pretreated with air or hydrogen and resembles alumina (Davis, 1972; Pines and Manassen, 1966) in alkene selectivity. Literature Cited Ai-Bahar, F., Collins, D. C., Waters, J., Davis, B. H., Ind. Eng. Chem. Prod. Res. Dev., following article in this issue, 1979. Argon, P.A., Fuller, E. L., Holmes. H. F., J. Cdloid Interface Sci.,52, 553 (1975). Davis, B. H., unpublished results, 1978. Davis, B. H., J . Org. Chem., 37, 1240 (1972). Davis, B. H., Brey, W. S., J . Catal., 25, 81 (1972). deBoer, J. H., in "The Structwe and Properties of Porws Materials", D. H. Everett and F. S. Stone, Ed., p 68, Academic Press, New York, N.Y., 1958. Akad. Nauk SSSR, Freidiin, L. Kh., Sharf, V. Z.,AWumaviyanova, V. Sh., IZY. Ser. Khim., 2308 (1971). Garvie, R. C., J . Phys. Chem., 82, 216 (1978). Holmes, H. F., Fuller, E. L., Jr., Gammage, R. B., J . Phys. Chem., 78, 1497 (1972). Pines, H., Manassen, J., Adv. Catal., 16, 49 (1966). Rijnten, H. Th., in "Physical and Chemical Aspects of Adsorbents and Catalysts", B. G. Linsen, Ed., Chapter 7, Academic Press, New York. N.Y., 1970. Sharf, V. Z.,Freidiin, L. Kh., AWumaviyanova, V. Sh., Izv. Akad. Nauk SSSR, Ser. Khim., 1059 (1972). Yamaguchi, T., Nakano, Y.. Iizuka, T., Tanabe, K., Chem. Lett., 677 (1976). Yamaguchi, T., Sasaki, H., Tanabe, K., Chem. Lett., 1017 (1973).
Received for review January 29, 1979 Accepted April 16, 1979 Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
