Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
199
Catalytic Conversion of Alcohols. 12. Selectivity of Hafnium Oxide Fawriyah AI-Bahar, Dermot J. Collins, and James C. Watters Department of Chemical & Environmental Engineering, University of Louisville, Louisville, Kentucky 40208
Burtron H. Davis" Potomac State College, Keyser, West Virginia 26726
Hafnia is a selective dehydration catalyst when pretreated with either hydrogen or oxygen. The catalyst does not show a preference for any of the three alkenes allowed by P-elimination from 2-01s. Alkenes formed from the dehydration of pure cis- and trans-2-methylcyclohexanol indicate that an anti elimination is an important reaction pathway. Hafnia resembles titania a s an alcohol conversion catalyst but differs greatly from other members of the 48 family, zirconia and thoria.
Introduction Study of a family of metals, group 8B, has resulted in correlation for catalytic activity and selectivity for hydrocarbon conversions (Sinfelt, 1973). In many cases, the catalytic property changed uniformly with position within the family. The ability to obtain a reproducible surface by hydrogen reduction has been a major factor in establishing these correlations. Metal oxide catalysts have not been as well defined as metals have. One problem has been a variable catalyst activation. It has been shown that for many metal oxides the pretreatment with gases such as hydrogen or oxygen determines the selectivity for the conversion of alcohols (Davis, 1976). Undoubtedly, the lack of attention to catalyst preparation and pretreatment conditions has contributed to some of the conflicting reports of catalytic properties (Krylov, 1970). Results with three members of group 3A metal oxide catalysts showed that the catalytic selectivities changed abruptly between Ga and In rather than uniformly with position in the periodic table (Davis, 1972, 1978a; Davis et al., 1979). However, this comparison suffered because indium oxide was reduced during contact with the alcohol to an undetermined oxide. Metal oxides of group 4B should allow a comparison of the catalytic properties within a family without the complication of bulk oxide reduction. The present study provides data to define the selectivity of hafnia for the conversion of several alcohols. Experimental Section The reaction procedure and analysis are the same as was used with zirconia (Davis and Ganesan, 1979). The hafnia (Hf-1) used in most of the runs and for all of the data reported in the tables and the figures was obtained from Alfa Inorganics, Inc. The BET surface area of the fresh catalyst was 8.7 m2/g; after use as a catalyst it was 8.9 m2/g. Hf-2 was prepared by dissolving metallic Hf in concentrated H F and the hydroxide then precipitated by adding concentrated ammonium hydroxide. This hydroxide was dissolved in a mixture of concentrated HC1 and HNOB and reprecipitated by adding concentrated ammonium hydroxide. The precipitate was washed ten
* Address correspondence to this author at the Institute for Mining & Minerals Research, University of Kentucky, P.O. Box 13015, Lexington, Ky. 40583.
Table I. Representative Dehydration Selectivity Data for the Conversion of 4-Methyl-2-Pentanolwith Hafnia pretreatment
temp, "C
LHSVa
conversionb range, mol %
H, H, H, H, air air air air
240 260 270 290 250 260 270 290
0.59 1.16 2.25 8.70 0.59 1.16 2.25 8.70
14-20 11-16 14-17 15-20 16-23 17-25 7-25 10-14
length alkene/ of run, (alkene + min ketone) 260 70 110 40 270 65 90 95
0.95 0.89 0.95 0.94 0.98 0.89 0.95 0.97
a LHSV = liquid hourly space velocity in milliliters of reactant per milliliter of catalyst per hour, High and low conversion for the four to six samples collected during the run.
times by repeated filtration-dispersion cycles. After calcination a t 600 OC the surface area was 63 m2/g. Results A stable activity was maintained for an air or a hydrogen pretreated catalyst during a 5-h run (Figure 1). The data in Figure 1 show that the dehydration selectivity for 4methyl-Zpentanol did not change with time-on-stream for either the hydrogen or air pretreated catalyst. Either hafnia pretreatment produced a catalyst that was selective for dehydration; the dehydration selectivity of the hydrogen pretreated sample (96-98 5% ) was slightly higher than with the air pretreated material (92-9470). At the end of a 5-h run, 0.31 mol of alcohol had passed over each square meter of surface; this corresponds to ca. 0.02 mol of conversion/m2. The data in Figure 2 from the hydrogen pretreated hafnia show that there is an excellent linear relationship between the reciprocal flow and the conversion as is required for a zero-order reaction. Each data point in Figure 2 corresponds to the average conversion during a run at each temperature. After completing a run at a temperature, the catalyst was given a standard regeneration followed by a hydrogen pretreatment; thus the regeneration reproduced the original catalyst state. The data for the air (or oxygen) pretreated material fit the same curve as the hydrogen pretreated hafnia but there is a greater scatter of the data for these pretreatments than there is for hydrogen.
0019-7890/79/1218-0199$01.00/00 1979 American Chemical Society
200
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979
Table 11. Representative Alkene Distributions Obtained from the Conversion of 2-01s with Hafnia pretreatment
alcohol
H, H, H, H,
2-pentanol 2-hexanol
air
H, H, H, H, H, H,
2-octanol
a Average for the four the run.
6
0 091
0
1- trans-2 cis-2
"C
250 29 26 45 270 31 25 44 290 30 26 44 250 32 29 39 250 31 29 40 250 36 25 39 270 35 29 36 35 30 35 284 35 29 36 288 33 30 37 307 32 35 33 340 to six samples collected during
c-2
1
0
alkene, mol %
temp,
0 08
$, 0
07
o
5 0 06
36
o 0
0
GO
0 05
I
1
1
IO0
m
200 Time. Min
Figure 1. Activity and selectivity for the conversion of 4-methyl2-pentanol with an air or hydrogen pretreated hafnia catalyst.
3029 -
Pretreat 0 Air Pretrsut 0 0 2 Pretreat
ae
w
!.o
o
'
HP
31
I
I
I
I
10
20
30
40
50
'/Flow
Figure 2. Conversion vs. reciprocal flow for the conversion of 4methyl-2-pentanolwith an air, oxygen, or hydrogen pretreated hafnia catalyst.
The dehydration selectivities presented in Table I for the conversion of 4-methyl-2-pentanol with an air or with a hydrogen pretreated hafnia are representative of the results obtained with acyclic alcohols (3-methyl-2-butano1, 2-pentanol, 2-hexanol, and 2-octanol) in the temperature range 220-350 OC. 3-Pentanol appeared to be an exception since dehydration accounted for only about 50% of the total conversion; however, 3-pentanol has given this unique selectivity, due to a greater contribution of the dehydrogenation reaction, with other metal oxide catalysts (Davis, 1979). Representative alkene distributions obtained with 2-01s are presented in Table 11. The alkenes obtained with 2-octanol did not change with time-on-stream (Figure 3); the data presented in Figure 3 are representative of all of the runs included in Table 11. The same alkene distribution, within 4 mol % , was obtained with either an air or a hydrogen pretreatment in the reaction temperature range 250-350 "C. About equal amounts of each of the three alkenes allowed by P-elimination were formed; this distribution is not close to the equilibrium distribution (Kilpatrick, 1946). It was established that the alkenes, once they desorb to the gas phase, did not undergo
1
0
1
1
,
Y I
1
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 3, 1979 201 Table 111. Alkene Product Composition from the Conversion of Tertiary Alcohols with Hafnia alkenebpc alcohol
pretreatment
2-methyl-2-pentanol
H, air H2 air H, air
temp, C 200 200 220 220 240 240
LHSVa 0.59 0.59 2.25 2.25 8.7 8.7
conversionb 29 37 50 50 33 36 1
260 62 60 61 55 56
1-
40 38 40 39 45 44 trans-2
cis-2
200 2.25 23 20 51 29 3-methyl-3-pentanol H, air 200 2.25 50 16 53 31 See footnote a , Table I. Values are for steady-state activity; i.e., the average of the conversion for all samples except the first one collected. Alkenes from 2-methyl-2-butene are 2-methyl-1-butene and 2-methyl-2-butene; from 3-methyl-3pentanol are 2-ethyl-l-butene, cis- and trans-3-methyl-2-pentene. Table IV. Selectivities for t h e Conversion of 2-Methylcyclohexanol Isomers with Hafnia
charge cis
+ transd
cis trans
pretreatment
temp, "C
LHSV," h-
conversion,bsc mol %
HZ
250 270 290 250 270 290 270 270 270 270
0.59 2.25 8.7 6.59 2.25 8.7 2.25 2.25 2.25 2.25
22 16 13 37 23 15 20 27 9 5
H2 H, air air air HZ air H2 air
alcohol,b mol % cis
trans
52 53 51 50 51 52 98 99t
