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Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
pendent of the preparation and pretreatment; they are selective dehydration catalysts and do not show any tendency to selectively form the l-alkene from the dehydration of 2-01s. In summary, hafnia has been shown to be a selective dehydration catalyst that does not show a selectivity for any of the three alkenes formed by &elimination from 2-01s. Literature Cited Collins, D. J.; Watters, J.; Davis, 6.H.. unpublished results, 1978. Davis, 6.H. J. Org. Chem. 1972, 37, 1240. Davis, 6 . H.; Brey, W. S.J. Catal. 1972, 25, 81. Davis, 6 . H. J. Colloid Interface Scl. 1976, 111, 115. Davis, B. H. J. Catal. 1970, 52, 435.
Davis, B. H.;Cook, S.; Naylor, R. W. J. Org. Chem. in press, 1979. Davis, 8. H., unpublished results, 1979. Davis, B. H.; Ganesan, P. Id.Eng. Chem. hod. Res. Dev., preceding article in this issue, 1979. KiiDatrick, J. E.; Prosen, E. J.: Piker, K. S.;Rossini. F. D. J. Res. Net/. Bur. Stand. 1946, 36, 559. Krylov, 0. V. "Catalysis by Nonmetals", Academic Press: New York, 1970, Chapter 4. Pines, H.: Manassen, J. Adv. Catal. 1966, 16, 49. Sinfelt, J. H. J. Catal. 1973, 29, 308.
Receiued for review January 29, 1979 Accepted April 16, 1979 Acknowledgmentis made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research.
Catalytic Conversion of Alcohols. 13. Alkene Selectivity with TiO, Catalysts Dermot J. Collins and James C. Watters Department of Chemical Engineering, University of Louisville, Louisville, Kentucky 40208
Burtron H. Davis' fotomac State College, Keyser, West Virginia 26726
Anatase is a very selective dehydration catalyst and the alkenes formed from 2-01s resemble those formed with an alumina catalyst. Rutile is not a selective dehydration catalyst; the alkene selectivity depends on pretreatment and it is more selective for l-alkene formation from 2-01s than anatase. With anatase the more rapid conversion of cis-2-methylcyclohexanoI isomer and the different alkene composition from the two alcohol isomers are consistent with a contribution of an anti elimination mechanism. For the conversion of alcohols with group 4A metal oxide catalysts, anatase and hafnia resemble each other but differ markedly from zirconia and thoria.
Introduction The conversion of alcohols over alumina catalysts has been widely studied (Pines and Manassen, 1966; Knozinger, 1971). Other oxides have not been as widely studied as alumina and many of the studies that have been carried out used ethanol or 2-propanol as reactants. These two alcohols suffer the limitation that there is only one dehydration product. Much of our present knowledge of elimination reactions was deduced from alkene distributions obtained from reactants that can form more than one product (Saunders and Cockerill, 1973). We have reported results for the conversion of alcohols that form more than one alkene for members of group 3A family (Davis, 1972a; 1978; Davis et al., 1979))but the selectivity trend was complicated by the reduction of one of the members (india) during the reaction. In addition, preparation and pretreatment may influence the products even for selective dehydration catalysts such as alumina (Davis, 1972a). We have reported selectivity data for three metal oxides of group 4B: thoria (Davis, 1972a; Davis and Brey, 1972), hafnia (Al-Bahar et al., 1979), and zirconia (Davis and Ganesan, 1979). The results in the this paper provide Address correspondence to this author a t the Institute for Mining & Minerals Research, University of Kentucky, P.O. Box 13015, Lexington, Ky., 40583. 0019-7890/79/1218-0202$01 .OO/O
selectivity data for a fourth member of group 4B metal oxides, titania. Experimental Section Catalysts. Anatase. Metallic titanium was dissolved in sulfuric acid and then diluted to give ca. 25 g of Ti/L. The hydrous oxide was obtained by the addition of a large excess of ammonium hydroxide to the sulfate solution. The precipitate was washed with distilled water until peptization occurred. The solid was dried a t 120 "C and then calcined a t 600 "C in air for 24 h. The X-ray diffraction spectra agreed with the standard ASTM spectra. The surface area of the calcined material was 46 m2/g. Rutile. A portion of the rutile was calcined at 900 "C in air for 3 days (Tolstopyatova, 1969). The surface area was 1.4 m2/g. Procedure. The reaction procedure and analyses were described in the zirconia manuscript (Davis and Ganesan, 1979). After each run at a given reaction condition the catalyst was given the standard regeneration described in the zirconia manuscript. Results As shown in Table I, the anatase catalyst was selective for the dehydration of 2-01s. It appears that the dehydration selectivity may increase slightly with increasing temperature (i.e., the results with 4-methyl-2-pentanol) but even at the lowest temperature anatose is much more 0 1979 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
203
Table I. Selectivity for the Conversion of 2-01s over Anatase alcohol 2-pentanol
4-methyl-2-pentanol
pre-treat.
LHSV
temp, "C
time, mina
air
0.23
214
0.46
23 5
0.90
255
0.046
200
0.90
225
85(1) i45i2j 310(4) 365( 5) loo( 1) 185(4) 25(1) 125(5) 45(1) 320(4) llO(1) 235(4) 55(1) 120(4) 185(4) 245(5)
air
1.8 2-octanol
air
H, 4,4-dimethyl-2-pentanol
a
air
250 205 220 228 24 5 265 210 225 220
conv., mol %
-
445(4) 62(6) 180(4) 210( 3 ) 35 52 80 95
alkene, mol % selectivity
1-
trans-2
cis-2
0.66 0.17 0.79 0.79 0.91 0.98 0.93 0.96 0.66
24 30 32 32 30 31 31 35 34 43 39 40 41 41 28 34. 34 36 32 36 41 56 56 60 59
24 23 21 21 25 24 28 24 66 51 61 60 59 59 25 21 21 24 20 26
52 47 47 47 45 45 41 41 66 57 61 60 59 59 47 45 45 40 48 38 32 9
8 8 8
I 14 12 18 20 4 4
0.18
I
0.90 0.90 0.94 0.94 0.91
8 13 12 14 9
-
-
60 88 12 9 11 10 9 9
0.99 0.99 0.96 0.81 0.61 0.76 0.80 0.80
21
35 31 31 32
I 9 9
Number in parentheses is the sample number collected.
selective for dehydration than for dehydrogenation. Slightly lower amounts of the 1-alkene isomer and slightly more of the cis-2-alkene isomer were usually obtained at the early time on stream and/or the lower reaction temperature than a t the higher temperatures or later reaction times. The alkene distribution quickly became constant with increasing time on stream. A similar alkene distribution was obtained from 2-pentanol and 2-octanol. In addition, essentially the same octene distribution was obtained from 2-octanol using an anatase catalyst that had been pretreated with hydrogen or with air. With 2pentanol and 2-octanol the cis-2-alkene was formed in a larger amount than either of the other two alkenes allowed by a direct /3-elimination. The amount of 1-alkene and obtained from 4-methyl-2-pentanol was about the same as with 2-pentanol and 2-octanol. The alkenes from 4,4-dimethyl-2-pentanol differed markedly from the other 2-01s since the 1-isomer accounted for about 60% of the alkenes formed. Furthermore, the cis:trans 2-alkene ratio was 0.28 for the alkenes from 4,4-dimethyl-2-pentanol in contrast to a ratio greater than 2 obtained for 2-pentanol and 2-octanol. The cis:trans alkene ratio for both 2pentanol and 2-octanol resembled that obtained with a similar carbon number secondary alcohol that could only form internal alkenes by a @-elimination (Table 11). Tertiary alcohols are dehydrated more rapidly than the isomeric secondary alcohol (Table 111). The amount of the 1-alkene from the dehydration of 2-methyl-2-butanol depends on the reaction temperature; an increase in temperature produces an increase in the amount of 2methyl-1-butene. In contrast, 1-alkene was the major product from the dehydration of another tertiary alcohol, 2,3-dimethyl-2-butanol. For comparative purposes, results from the conversion of 3-methyl-2-butanol are included in Table I11 rather than Table I. The major dehydration product from 3-methyl-2-butano1, in contrast to 2,3-dimethyl-2-butanol, is the 2-alkene. In addition to the two alkenes that form by @-elimination from 3-methyl-2butanol, about 10% of 2-methyl-1-butene was present in the dehydration products. This contrasts to the other methyl butanol isomer, 2-methyl-2-butano1,where only the
Table 11. Alkenes from the Conversion of Acyclic Secondary Alcohols over Air-Pretreated Anatase alcohol
temp, "C
2-pentanol 3-pentanol 2-octanol 4-octanolb 3-heptanolb
235 232 223 235 220
alkene, mol
131.4 trace 34.5
-
%a
cis-2-a/ trans-2- cis-2- trans-223.3 39.1 20.0 46.5 50.3
45.4 60.3 45.5 53.5 49.7
1.96 1.43 2.27 1.15 0.99
a Average of the four to six samples collected during the course of a run. For 3-heptanol the products are cisand trans-3-heptene; for 4-octanol trans- is a mixture of trans-3- and trans-4-octene and cis- is a mixture of cis-3and cis-4-octene.
@-eliminationproducts were obtained. The other tertiary alcohol in Table 111,3-methy1-3-pentanol,was unique since the alkene distribution was nearly the equilibrium composition. The catalytic property of the rutile form differed markedly from anatase. Whereas anatase was a selective dehydration catalyst that gave essentially the same selectivity whether it was pretreated with air, oxygen, or hydrogen, rutile was considerably more active for dehydrogenation of 2-octanol than for dehydration. In addition, hydrogen pretreated rutile was 3-4 times as active as the air pretreated material and was a less selective dehydration catalyst than the air pretreated material. With air pretreated rutile, 1-octene accounts for about 50% of the alkenes and about equal amounts of the cis-2-octene and trans-2-octene were formed. Hydrogen pretreated rutile yielded an alkene distribution that, based on published thermodynamic data for the alkenes allowed by P-elimination from C4-C6 2-01s (Kilpatrick et al., 1946),was near the equilibrium composition. (See Table IV). Anatase is a selective dehydration catalyst for the cyclic alcohol, 2-methylcyclohexanol (Table V). Under similar reaction conditions, it appears that the cis isomer is converted about three times as rapidly as the trans isomer. The amount of cis-trans isomerization of the alcohol reactant is not significant with respect to the rate of the
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
Table 111. Alkenes from the Conversion of Tertiarv Alcohols over Air-Pretreated Anatase alkenes, mol % alcohol
temp, "C
LHSV
2-methyl-2-butanol
145
0.50
162
1.9
200
1.9
220
3.6
time, min 52( l)= 180(4) 14(1) 52(5) 85(1) llO(4) 14(1) 3U5)
conv., mol %
2-methyl1-butene
2-methyl2-butene
3 3 5 3 18 17 29 27
34 39 38 42 44 47 50 50
66 61 62 58 56 53 50 50
2-Et-l-Bub t-3-Meb 3-methyl-3-pentanol
20 5
3.6
30( 3 ) 45(5) . ,
18 16
4W) 125(5)
6.5 7.3
21 22
c-3-Meb
46 47
33 31
3-Me-l-BuC2-Me-l-BuC2-Me-2-BuC 3-methyl-%butanol
220
2,3-dimethyl-2-butanol
4.5
2 20
3.0
65(1) 105(2) i35(3j 165(4)
24 29
53 23 17 15
12 12
64 59
2,3-DMel-Bud
2,3-DMe2-BU
60 72 73 74
40 28 27 26
a-Et-l-Bu,2-ethyl-1-butene;t-3-Me, trans-3-methyl-2-pentene; Number in parentheses is the sample number collected. c-3-Me, cis-3-methyl-2-pentene. 3-Me-l-Bu, 3-methyl-1-butene; 2-Me-l-Bu, 2-methyl-1-butene; 2-Me-2-Bu, 2-methyl-2butene. 2,3-DMe-l-Bu,2,3-dimethyl-l-butene; 2,3-DMe-l-Bu,2,3-dimethyl-2-butene. Table IV. Dehydration and Alkene Selectivities for the Conversion of 2-Octanol over Rutile at 270 "C, and LHSV = 1.1 pre-treat. air, 500°C
H,, 500°C
time, conv., dehyd. min mol % select. 130 225 250 325 75 185 230
7 5 4 5 19 19 16
0.089 0.069 0.059 0.069 0.037 0.018 0.031
alkenes, mol % 1- trans-2 cis-2
40 46 51 51 28 23 25
31 27 22 22 41 44 44
29 27 27 27 25 33 31
dehydration reaction. The two 2-methylcyclohexanol isomers yielded a different alkene distribution; however, in both cases the more stable 1-methylcyclohexene is formed in larger amounts than the less stable 3-methylcyclohexene isomer. In general the anatase retained a similar catalytic selectivity after a regeneration and pretreatment from one run to the next; however, a comparison of the initial results with those obtained after 30-40 regenerations showed that there was a small decrease in activity. The conversions, corrected to the same flow rate for 2-pentanol, 2-octanol,
and 4-methyl-2-pentanol were the same within f10% and the "apparent activation energy" calculated from these rates was 28 kcal/mol. Discussion We found anatase to be a very selective dehydration catalyst whether the sample was pretreated with air or hydrogen. The material may show a slight change in dehydration selectivity with increasing temperature. The selectivity for dehydration/dehydrogenation of alcohols over TiOz (Ferrier, 1977) follows a similar trend with temperature as we reported for Y203(Davis, 1978) although with TiOz the ratio was always much greater than 1. The alkene selectivity for the conversion of 2-01s with TiOz is similar to that obtained with alumina (Pines and Manassen, 1966). However, TiOz produced slightly less of the 1-alkene, and correspondingly more of the trans2-alkene, than alumina. The alkene selectivity for the oxides of group 4B is one of contrasts as the alkene distributions presented in Table VI show. Thoria and zirconia proved to be an extremely complicated catalyst system whose dehydration and alkene selectivities depend on preparation and pretreatment. For these two catalyst systems the variables which determine the selectivity have
Table V. Conversion of cis- and trans-2-Methylcyclohexanolover TiO, (Air Pretreatment at 525 "C)
reactant
LHSVa
temp, "C
cis-
3.6
265
trans-
0.25 0.97 3.6 0.25
250 250 250 240
1.9
265
(cis
+ trans)c
time, min
conv., mol %
selectivityb
200 210 24 0 265 280 195 315 160
22 22 65 24 7.7 39 37 23
0.94 0.94 0.99 0.99 0.97 0.99 0.99 0.99
alcohol, mol %' cistrans99 99 2.1 0.10 0.61 48.4 47.5 51.0
0.56 0.49 98 99 99 51.6 52.5 49.0
methylcyclohexenes, mol % 3-
1-
25 24 43 40 38 31 31 32
75 76 51 60 62 69 69 68
Alcohol composition in liquid Selectivity = (alkene)/(alkene + ketone). LHSV = cm3 of liquid/cm3 of catalyst h. product. Less than 1.5% of any other methylcyclohexene isomers. e Alcohol reactant contained 53.1% of the cis isomer.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
Table VI. Representative Alkene Distributions from the Conversion of 2-Octanol over Group 4B Metal Oxide Catalysts
catalyst TiO, ZrO, HfO,
pretreat. air H, 02' H2' air
H, CeO,
Tho,
0, HZa
02b HZb
react. teomp, C
1-
220 225 237 243 250 250 250 250 250 250
34 41 87 95 36 36 29 99 34 50
octene, mol % trans-2 21 27 6 27 22 25 24
1.0 36 26
cis-2 45 32 7 2.3 42 39 47 1.0 30 24
a Sample prepared from thorium hydroxide obtained by Prepared by deprecipitation from a nitrate solution. composition of the carbonate.
not been completely defined. The data shown are for T h o 2 and ZrOz prepared by precipitation from a nitrate solution; we have found that this procedure produces catalysts that have similar selectivity from batch to batch. T h o z and Zr02 are unique in the selective formation of 1-octene. While both of these catalysts catalyze dehydrogenation as well as dehydration, the selective catalysts in Table VI show greater than 80% dehydration. On the other hand, titania and hafnia were selective dehydration catalysts whether pretreated with air, hydrogen, or oxygen. In addition, the alkene distribution differed greatly from T h o 2 and ZrO, and only slightly more of cis-2-octene than the other two isomers. The results for CeOz are preliminary ones; the catalyst gave only about 10% dehydration but an alkene distribution that resembled TiO, and HfOz. These results with CeOz are in contrast to an earlier report (Sharf et al., 1972) that ceria was selective for 1-alkene formation. In any case, it appears that the selectivity for the conversion of 2-01s does not change uniformly in going down the group 4B metal oxides. Our alkene selectivities differ markedly from those reported earlier for TiO, (Carrizosa and Munuera, 1977) where it was stated that there was a rapid decrease in the amount of the 1-alkene and an increase in the cis-2-alkene at temperature greater than ca. 225 OC. Our results with 2-01s do agree more closely to the ones that these authors reported in a later communication (Munuera and Carrizosa, 1979). Even in the latter communication the amount of the trans-2-butene appears to be higher than in our study with 2-01s. It appears that additional work will be
205
required to put the conclusions concerning the relative contributions of an E-1 or E-2 mechanism on a firmer foundation. The similarity and contrast of the pairs are also evident in the conversion of 2-methylcyclohexanol. Ti02and Hf02 do not cause the isomerization of the pure cis- or trans2-methylcyclohexanol reactant whereas ZrOz and T h o z do; the latter two oxides also catalyze both dehydration and dehydrogenation whereas the former two only catalyze dehydration. Both T i 0 2 and Hf02 produce a different alkene mixture from the dehydration of the trans alcohol isomer than from the cis isomer; however, the alkenes from the trans isomer do not contain the high percentage of the 3-methylcyclohexene that would be expected for the anti elimination mechanism. With T h o 2and ZrOz both alcohol isomers produced a similar alkene distribution which was predominately the 3-methylcyclohexene isomer. It appears that the cis alcohol was converted to an intermediate that resembled the trans alcohol during the dehydration step. The results with the group 4B, as well as the results with group 3A, indicate that the selectivity for alcohol conversion is not simply related to the position within a family of the periodic table. Acknowledgment Acknowledgment is made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, for support of this research. Literature Cited AI-bahar, F.; Collins, D. J.; Watters, J. C.; Davis, B. H. Ind. Eng. Chem. Prod. Res. Dev. accompanying paper in this issue, 1979. Carrizosa, I.; Munuera, G. J . Catal. 1977, 49, 189. Davis, B. H. J . Org. Chem. 1972a, 37, 1240. Davis, B. H. J . Catal. l972b, 26,348. Davis, B. H. J . Colloid Interface Sci. 1976, III, 115. Davis, B. H. J . Catal. 1978a, 52, 435. Davis, B. H. J . Catal. 1978b, 52, 176. Davis, B. H.;Brey, W. S. J . Catal. 1972, 25, 81. Davis, B. H.;Cook, S.;Naylor, R. W. J . Org. Chem. 1979, in press. Davis, B. H.; Ganesan, P. Ind. Eng. Chem. Prod. Res. Dev. accompanying paper in this issue, 1979. Kilpatrick, J. E.; Prosen, E. J.; Pitzer, K. S . ; Rossini, F. D. J . Res. Natl. Bur. Stand. 1946. 36. 559. Knozinger, H. "The Chemistry of the Hydrozyl Group", S. Patai, Ed.; Interscience: New York, 1971. Munuera, G., Carrizosa, I.J . Catal. 1979, 56,299. Pines, H.; Manassen, J. Adv. Cafal. 1966, 16,49. Saunders, W .H.,Jr.; Cockerill, A. F. "Mechanism of Elimination Reactions", Wiley: New York, 1973. Sharf, V. 2 . ;Freidin, L. Kh.; Abdumavlyanova, V. Sh. Izv. Akad. Navk SSSR, Ser. Khim. 1972, 1059. Tolstopyatova. A. A.; Filatova, T. N.; Korytnyi, E. F.; Balandin, A. A. I z v . Akad. Nauk SSSR, Ser. Khim. 1969, 1439.
Received f o r review May 7 , 1979 Accepted May 24, 1979
