CATALYTIC ACTIVATION O F TITANIA BY W. A. RUDISILL AND CARL J. ENGELDER
It is a well known fact that the activity of a catalyst is to a large extent dependent on its manner of preparation. The nature of the starting material and its subsequent chemical and physical treatment are factors in the preparation of a catalyst which have received scant attention by investigators. Adkin$ and his co-workers have done some excellent work on selective activation. A recent article by Adkins and Bischoff2on the selective activation of titania catalysts points out how the activity of titania may vary with different titanium compounds as the starting material. I n the present investigation, a study has been made of some of the factors which influence the activity of titania catalysts in decomposing ethyl alcohol. As shown in the experimental results, the treatment given the catalysts during their preparation affects to a large degree the extent of the decomposition as well as the composition of the gaseous products. The decomposition of ethyl alcohol by various catalysts has been rather thoroughly investigated by Grigoreff 3, Ipatief4, Saloatier and his colleagues5, Engelder6, Adkins and others. Oxide catalysts like Ti02, Si02,and ZrOz possess the ability to decompose C2H50Haccording to the two main reactions: C2H40 (I) CzH50H +Hz C2HjOH ----t C2H4 HzO (2) the one a dehydrogenation and the other a dehydration, both proceeding simultaneously to an extent dependent upon the specific nature of the catalyst. Titania catalysts in particular differ widely in their specificity towards these two reactions. That this specificity is dependent, to a considerable extent, on the physical factors involved in the catalyst’s preparation has been the object of this research. The factors which have been studied here are as follows:I. Variation of the Temperature of Ignition of the Catalyst. 2. Variation of the Time of Ignition of the Catalyst. 3. Variation of Anion. 4. Variation of Degree of Fineness of Catalyst.
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Experimental Apparatus and General Plan.--The general mode of procedure was to pass the vapors of ethyl alcohol a t a uniform rate over the catalyst mass contained in the decomposition apparatus and to collect and measure the gas volume 2
a
J. Am. Chem. SOC.44, 2175 (1922). J. Am. Chem. Soc. 47, 807 (1925). J. Russ. Phys. Chem. SOC. 33, 173 (1901). Ber. 34, 596 (1901); 35, 1047, (1902); 36, 1990 (1903). “Catalysis in Organic Chemistry.” J. Phys. Chem. 21, 679 (1917).
CATALYTIC ACTIVATION O F TITAKIA
107
evolved and determine its composition. The apparatus was similar to that used by Engelder with certain modifications suggested by Adkins and Kissenl. The alcohol was accurately measured and controlled by allowing mercury to fall into a burette from which the alcohol was displaced. It passed first through a preheater and then into the reaction tube of Pyrex glass. The catalyst was placed in a porcelain boht in the center of the tube. This reaction tube was maintained a t a constant temperature of 350' by being surrounded by an electrically heated furnace, the temperature of which was controlled by a Leeds and Xorthrup thermo-regulator. The reaction products passed through a spiral condenser and then into an ice-cooled receiver, to collect the liquid products, the gases passing on into a large collecting bottle containing saturated salt solution, which served as a gasometer. Provision was made for sampling the gases from the gasometer, as well as from a sampljng cock on the line leading t o the collecting bottle. Gas Analysis.-The gas mixture was analyzed according to the usual methods of gss analysis. In some cases hydrogen was determined by the fractional method over spongy palladium as well as the regular slow combustion method over mercury. Ethylene was adsorbed in bromine water, COZin KOH solution and CO in acid CuzClzsolution. All gas volumes were recalculated to N.T.P. and reported as such. Uniform Procedure.-After considerable preliminary work in which the apparatus 'was perfected, the technique standardized, and a score or so of weakly active catalysts were made and tested; the following uniform procedure for catalyst testing was adopted:All catalyst samples were ground to pass a Ioo-mesh sieve (except those whose degree of fineness was being studied). In each run 2 . 5 grams of the heated catalyst was used. The temperature for all runs with alcohol was 350°C. Absolute ethyl alcohol was used. The preheater was mainhined a t about 170'C. and the alcohol admitted a t a uniform rate of 6 cc. per hour. Readings of burette, temperature of preheater and reaction tube, and volume of gas were recorded at intervals of 3 to j minutes and slight adjustments made in the rate of flow whenever necessary. Alcohol was passed though the apparatus under the above conditions for one hour to free the train from air. At the end of the first hour the collected gas was discarded, and the run continued for another hour. The volumes of gas recorded and thei. composition are for the second hour, 1. Variation of Temperature of Ignition of Catalyst Preparation of Catalysts.-Preliminary work indicated that the temperature to which the catalyst is subjected during its preparation is an important factor in determining its specificity. Accordingly two series of titania catalysts were prepared, starting with titanium potassium oxdate, kindly furnished us through the courtesy of the Lindsay Light Co., Chicago, Ill.
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J. Am. Chem. SOC.46, 140 (1924).
IO8
W. A . RUDISILL AND CARL J. ENGELDER
Catalysts designated by G.-The “G”series was made as follows: Titanium potassium oxalate was dissolved in water and precipitated with ammonium hydroxide. The precipitate was washed four times by decantation, filtered, and then washed on the filter. The precipitate was dissolved in dilute HC1 andreprecipitated withNH40H. It w m then washed again as described above, The precipitate was dried at 15o’C. and then heated at the same temperature for three hours. It was then ground to pass a Ioo-meFh sieve. Separate portions of this were heated for periods of two hours each at temperatures of 300°, 350°, 450’) 550°, 650°, and 7 jo’c., giving respectively catalysts GI, G2, G3, G4, G6,and G6. A portion of the roo-mesh, dried batch was reserved for later work. Catalysts designated by H.-The “€1” series was prepared in the same way, with the exception that the first precipitate of Ti(OH), was redissolved in dilute H,SOI instead of dilute HC1. Separate portions of the dried, Ioo-mesh batch were heated for two hours each a t temperatures of 2 5 0 ° , 300°, 350°,4500, j50°, and 65ooC, giving respectively the catalysts “I,HI, H2,H3, H4, and H6. Other portions of this preparation were reserved for later work. The activity of these “G” and “H” samples was then determined in the uniform manner adopted. The results are tabulated below in Table I. The results clearly show the imporiance of proper temperature of drying the catalyst. With increasing temperature of ignition, for both series, beyond 3 j0’C the total gas evolution falls off rapidly, and there is a corresponding decrease in the volume of C2H4 per cc. of alcohol, particularly with the “H” series. In general the “H” series of titanias is more active, especially in regard to dehydration (ethylene production). The best catalyst is that precipitated from the sulfate solution and ignited at 300’C. Quite apparently, changes have been brought about by the higher temperatures of ignition modifying the surface conditions, especially as regards the dehydration ability of the catalyst. Small amounts of carbon dioxide and carbon monoxide were always found, doubtless arising from secondary decomposition. Considerable amounts of ethane were always present, amounting to 23.8% in the case of the “H” catalyst ignited to 650’. The particular mechanism by which this is formed has not been definitely determined. Adkins’ believes that the ethane is produced probably according to the reaction: 2CHaCH20H +CHSCH3 CH3CHO HzO one molecule pf the alcohol being reduced to the saturated hydrocarbon, the other oxidized to acetaldehyde. It is possible, on the other hand, to assume hydrogenation of the ethylene, yet no evidence here or in previous work is available to suhtantiate this.
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2. Variation in Time of Ignition of Catalyst To determine whether the length of time of igniting the catalyst is an important factor, two series of runs were made with portions of the “G” and “H” lots reserved for this purpose, Separate portions of the reserve supply 1
J. Am. Chem. Sot. 47, 817 (1925).
CATALYTIC ACTIVATION O F TITANIA
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'LOCI
H
H
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W. A. RUDISILL AND CARL J. ENGELDER
I10
were heated, a t 3 jo°C. for I , 4, and 6 hours giving respectively the samples designated as H6,H’, H8,and runs made under identical conditions as with H2. A similar series was made with the “G” lot precipitated from chloride solution. The results for one set are here reported.
TABLE I1 All runs made at 350°C. a t the constant rate of 6 cc. alcohol per hour with grams of catalyst, precipitated from sulfate solulion. Catalyst Time of Gas evolvcd 7% 7% 7% 970 5% 2.5
Ignition
HE
I
€12
2
H7 H*
4 6
hr
,, ’, ”
per cc. alcohol 1st 2d hr. hr.
105 112
95 150
98 99 90 91
Hz
CQH,
CZHG
7.9
74.9 73.3 74.8 68.3
16.1
0.9
0.8
17.1
0.8 0.7
0.7
8.5 8.4 6.9
15.3 21.8
CO
1.0
COQ
0.8 1.3
The results show that the time factor of ignition is of little importance, the yield of gas being almost the same after the first hour and its composition practically the same for the several catalysts. Somewhat lower yields were obtajned with the corresponding “G” catalysts, but they were practically constant both as regards total volume and percentage composition, 3. Influence of Anion The series of experiments recorded in Table I indicated that the catalysts made by reprecipitation from a sulfate solution gave in general a more active catalyst than that series prepared by reprecipitation from a chloride solution. Both series otherwise received the identical treatment, such as uniformity in concentration, quantity of wash water, etc. The cbserved differences in behavior might be ascribed to the influence of the anion in the solution from which precipitation mas made, or to adsorbed anions. To obtain further data on this point, some additional catalysts were prepared and two series of runs on alcohol made. The catadysts designated as “J” were prepared in the same way as the “H” series except that the final precipitate was purposely not washed free from sulfate. One portion, J1, was heated to 3ooOC and the second portion, J2,to 350°C. These unwashed samples were compared in their effect on alcohol a t 3 j0”C. wiih the thoroughly washed samples, H1 and H2. Catalysts “K” were made by dissolving potassium titanium oxalate in water and precipitating by ammonium hydroxide. The precipitate was washed four times by decantation, filtered and then washed on the filter. Up to this point, the treatment was identical as that given to the lot from which the “G” and “H”series were derived. The mass was then dried at I 50°C for three hours. One portion was heated for two hours a t 3oo0C, giving K1; another portion was heated to 3 50°C for two hours, giving K2. Runs were made with alcohol at 350°C by the standard procedure. Still another series of catalysts was made by precipitating the hydroxide, filtering, dissolving in hydrochloric acid and then largely diluting and pre-
CATALYTIC >4CTIVATION O F TITANIA
I11
cipitating the Ti(0H) 4 by boiling. The hydrolyzed precipitate was washed, dried, heatzedto 15o'C €or three hours and separate portions, passing a IOOmesh sieve, were heated to 300'C and 3 5o°C, giving respectively the catalysts designated as L1 and L2. The tabulated results are given in the table below.
TABLE I11 All runs made a t 350'C a t the constant rate of 6 cc. alcohol per hour with 2.5 grams of catalyst. Time of ignition two hours. Catalyst
H1 J' K1 G1 L1 H2
J2 K2 G2 L2
Composition of Gas Mixture Gas evolved Method of Temp. of Preparation Ignition per cc. Slcohol 70 7% % % % 1st 2d C2Hh HP CzH, Co COP hr. hr.
SulfateWashed Sulfate not washed Oxalate Chloride Hydrolysis of Chloride Sulfate Washed Sulfate not washed Oxalate Chloride Hydrolysis of Chloride
300'
150
140
67.8
300'
7.7
22.8
1.2
1.4
4.8
23.8
0.9
1.1
117
85 103
40.9 46.4 76.0
20.6 38.7 4.6
3 soo
112
99
73.3
3 50'
150
81
3 50' 3 50' 3 soo
89
25
111
92 128
300' 300' 300'
84 I04
139
27
35.7
1.4
1.1
14.6 18.0
0.9
0.5
0.4
1.5
8.5
17.1
0.8
0.7
66.0
3.1
29.1
0.9
1.0
31.0 48.9 70.8
27.9 29.8 4.7
36.9 18.3 22.9
0.9 0.7 1.0
3.0 1.1 1.1
That thorough washing is necessary to give an active catalyst and one that does not depreciate with prolonged use is shown by a comparison of H' with J1 and H2 with J2. The unwashed samples rapidly lost their activity. Reprecipitation is very important, as shown by the runs with the samples preparecl from the oxalate without reprecipitation. A rather inactive catalyst, particularly in its dehydrating ability results from only one precipitation. The unusually high values of ethane are significant. Preparation of samples by hydrolysis yields a very active catalyst. This bears out the results of other investigators who have found that in general hydrolytically prepared catalysts are very efficient. 4. Variation in Degree of Fineness
It is to be expected that for the same weight of catalyst, the finer particle size and hence the more surface exposed, the greater will be the activity, other conditions being the same. Since titania influences both decomposition reactions of ethyl alcohol, it was t,hought interesting to study the variation of
W. A. RUDISILL AND CARL J. ENGELDER
I12
particle size with the extent to which these simultaneous reactions are effected. Accordingly, a series of samples was prepared as follows:-The method of preparation of the "G" catalysts was followed closely. The lot dried a t 150' for three hours was divided. One portion was ground to pass a 20-mesh sieve, but withheld by a 40-mesh sieve; this is designated as MI. Another portion was ground to pass a Ioo-mesh sieve; this is called M2. A third portion passed a zoo-mesh sieve; this is M3. Each portion was then heated for two hours a t 35oOC. Runs by the uniform procedure were then made with alcohol, using as always, 2 . 5 grams of the catalyst. The data are given in Table IV.
TABLE IV 2. j
All runs made a t 350' at the constant rate of 6 cc. alcohol per hour with grams of catalyst.
Catalyst
Temp.
of
Time
of. Ignition
Ignition 2 0-40
350'
2
mesh (M') Ioo-mesh
350'
2
350'
2
hrs.
Gas per Composition of Gas Mixture cc. CC. cc. alcohol o/o % % 70 7, C Z H ~ Ht 1st 2d C Z H ~ HZ CZH, CO COZper cc. per cc. hr. hr. alcohol. alcohol.
106
91 4 9 . 2 34.0 15.1 0 . 8
"
120
95
"
130
115
50.0
1.044.6
30.9
29.5 18.4 1 . 5 1 . 3 4 7 . 5
28.0
20.9 1.3 1.3 59.2
28.9
(M2) 200-mesh
51.5 2 5 . 2
(Ma) An increase in activity is shown for an increase in the degree of subdivision. More interesting, however, is the increase in dehydration with the finer sized contact masses, as indicated in the increased amounts of ethylene obtained. An increase in the amounts of ethane is also observed. Degree of fineness is thus another factor influencing the specificity of catalysts.
5. Stability of Catalyst The relative stability of the different catalysts used is indicated by a cornparison of the volume of gas during the first and second hours. Of the many catalysts prepared and studied in this investigation the ones designated as H' and L2 were the most active. In order to determine the life of the cataIyst, i.e., itsactivitywith prolonged use, a catalyst WAS made according to theH series with a preliminary ignition of two hours a t 3oo0C., giving catalyst Hg, and subjected to a ten-hour run. The results of this run are given in Table V. Samples of the gas were analyzed a t the end of the second, fourth, seventh and tenth hours. Catalyst H9depreciated very slowly with continued use and the composition of the gas after the first three hours changed very little. There was a slight increase in ethylene and in ethane between the second and fourth hours. The percentage of hydrogen was low and showed very little change.
CATATYTIC ACTIVATION O F TITANIA
Time
1st zd 3d 4th 5th 6th 7th 8th 9th 10th
Gas Envolved prr cc. Alcoho!
%
C&s
I54 I39 132 136
22.3 25.9
I3 5 126
I28
26.5
I20 I22
I I8
26.8
Summary I. Some of the factors which influence the activity of a catalyst have been studied in detail by preparing a large number of titania catalysts, passing the vapors of ethyl alcohol over them a t a temperature of 350°C., and noting the volume of gas produced as well as its composition. It was found in the first place, that the temperature of ignition of the 2. catalyst greatly influenced both the yield of gas and its composition. In general, it was found that with increase in temperature of ignition there was a decrease in activity, and further, a decided decrease in ethylene production. In order to produce a good dehydrating catalyst the temperature must be low. 3. The preliminary ignition period of two hours is sufficient for the preparation of an efficient catalyst. Heating beyond one hour has little effect. 4. It was found that the character of the anion present during precipitation is important. Those catalysts made by precipitating from sulfate and chloride solutions and by hydrolysis were the most efficient. Those prepared from the sulfate and by hydrolysis gave the highest percentage of ethylene and the lowest percentage of hydrogen. Catalysts prepared by precipitation from the oxalate gave the lowest activity, t.he lowest percentage of ethylene, and the highest percentage of ethane. Catalysts prepared from thoroughly washed precipitates were the more active. 5 . The degree of fineness of a catalyst influenced to some extent its activity. As the degree of fineness was increased, the activity was increased. Grinding the catalyst to fine mass before final ignition gave a more active catalyst . 6. Some catalysts were prepared which depreciated very slowly with use. A catalyst prepared from the sulfate solution was very active after ten hours use.
University of Pittsburgh, Pittsburgh, Pa. Oct. 1925.
