CATALYTIC OXIDATIOX O F ETHYL ALCOHOL n Y ALLAS R. DAY
In previous work' quantitative data were obtained on the behavior of the rare earth oxides as catalysts for vapor phase oxidation as compared with copper oxide as a catalyst. The effect of addition of various amounts of a rare earth oxide (Sm2O3)to copper oxide was also studied. In order to obtain more positive information as to the role which the rare earth oxide plays, it was decided to study the effect of samarium oxide on the catalytic properties of metallic silver in the oxidation of ethyl alcohol.
Experimental The apparatus used was essentially the same as in the previous work. The only exception being that the steam jacket which was used as a preheater was replaced by a pyrex coil enclosed in an asbestos box and maintained at a uniform temperature by means of a small burner. The burner was regulated so that the mixture of air and alcohol left the coil at a tempera1'. Preliminary x o r k showed that the temperature of this ture of I O S O C gaseous mixture was an important factor. Up to 14j'C. the yield of aldehyde was not affected but above that temperature an appreciable drop in yield was noted, even when the catalyst temperature would otherwise have been low. I t is obvious that unless the catalyst temperature can be kept constant two variables are bound to enter into work of this kind, namely, catalyst temperature and space-velocity. The catalyst temperature might be kept constant either by regulating the temperature of the incoming gases or by cooling the catalyst chamber where otherwise the catalyst temperature would be too high. -4s stated above, however, when the inlet gases mere heated above 145'C. an appreciable drop in yield of aldehyde was noted even when otherwise the catalyst temperature would hare been low. .Is a result of this preliminary work a temperature of rog0C. was adopted and used throughout the investigation. A satisfactory niethod for cooling the catalyst tube, thereby regulating the catalyst temperature, has not been found. Such a method would indeed have to be flexible in order t o keep the catalyst temperature constant while other factors such as thermostnt temperature and air rate are being varied. A further attempt TWF made t o krep the catalyst temperature constant by supplying external heat to thc tube by means of a bath or furnace, but here again difficulties were encount e r d The yields of aldehyde decreased, the yields of acetic acid increased nnd at the same time the total amount of alcohol accounted for, as aldehydp, acetic acid and unchanged alcohol, decreased.
*
J. Am. Chem. Soc., 5 2 , 3535 '19301.
CATALYTIC OXIDATION O F ETHYL ALCOHOL
3273
Since these efforts to eliminate one of the variables were unsatisfactory, usually leading to decreased yields of the desired product (acetaldehyde) they were abandoned and the following process adopted. The oxidation process was carried out by passing measured quantities of dry air through the vaporizer, containing the ethyl alcohol, maintained at constant temperature by means of a thermostat. The alcohol-air mixture passed through the preheater and then into the catalytic chamber, the catalyst having been previously heated to about 350OC. by means of a small burner. The heat of reaction was sufficient to maintain the reaction after it had once started. The catalyst temperatures were measured by means of a quartz thermometer, the bulb of which was buried in the catalyst. The same amount of catalyst was used for all of the runs. In each case the catalyst chamber (length I j mm., diameter 1 7 mm.) was filled with the impregnated 1 2 mesh pumice. The length of each run was carefully timed by means of a stop-watch. The products of the reaction were collected in several absorption flasks which were part of the cooling system. The flasks were then emptied and rinsed and the resulting solution diluted to a definite volume. Aliquots of this solution were taken for analysis. The following types of catalysts were employed : A. 2 . 7 9 2 8 g. of silver [equivalent to 3 g. of AgzO) deposited on 9 cc. of 1 2 mesh pumice. B. 2 . 7 8 j 8 g. of silver (equivalent to 2 . 9 9 2 j g. of AgzO) and 0.007 j g. of samarium oxide deposited on 9 cc. of 1 2 mesh pumice. C. 2.7788 g. of silver (equivalent to 2.985 g. of dgzO) and 0.015 g. of samarium oxide deposited on 9 cc. of 1 2 mesh pumice. D. 2.;648 g. of silver (equivalent to 2 . 9 7 g. of AglO) and 0.03 g. of samarium oxide deposited on 9 cc. of I 2 mesh pumice. E. 2.6532 g. of silver (equivalent to 2.85 g. of *igZO) and 0 . I j g. of samarium oxide deposited on 9 cc. of 12 mesh pumice. These catalysts were prepared in the same manner as the catalysts used in the previous work. Duplicate catalyst preparations, for each type of catalyst (A, B, C , D and E) were used in order to determine whether the variations in yield might be due to variations in catalyst preparation. I t was found that duplicate catalyst preparations gave excellent check results and consequently the small but constant variations recorded in Table I can scarcely be attributed to the use of various catalyst preparations. The ethyl alcohol (95 percent) which was used for the experiments was free from aldehyde in most cases. In some few cases the alcohol gave a slight test with Schiff’s reagent. Determination of Acetaldehyde.-The aldehyde was determined by Ripper’s method. This method gave good check results, usually varying at the maximum about c.3Yc for two or more analyses of the same aldehyde solution. Determination of Acetic Acid.--To a J,tinite amount of standard S a O H solution was added a little barium cf i l k solution and a few drops of phenol-
3274
ALLAX R. DAY
3275
CATALYTIC OXIDATION OF ETHYL ALCOHOL
f d 0 0 0 0 * * d d * d
$L
10 10 vi 10 vi
V'V,
10 10 v.
0
vi
0
0
vi vi
0
vi
d
b
d
f
d
d
d
10 10 vi vi 10 vi
3276
A L L A S R. DAY
CATALYTIC OXIDATION OF ETHYL ALCOHOL
3277
phthalein. The resulting solution was titrated with a portion of the condensate until the red color u a s discharged. This adaptation of Kinkler's method gave excellent results. Determination of Unchanged Ethyl Alcohol.-A suitable aliquot of the condensate was treated with a n excess of ammoniacal silver nitrate and heated on a steam bath for 2-3 hours under a slight pressure. The resulting mixture was distilled until about two-thirds of the solution passed over. The distillate was just acidified with sulphuric acid and again distilled. This method yielded a distillate which gave no tests for aldehyde or acetic acid. The alcohol content of this solution was determined by means of a dipping refractometer. The results obtained checked fairly well, usually within o.;-o.s';.
The results obtained for duplicate runs under various conditions are given in Table I. Space does not permit recording all the data obtained. Only the more important results are included. The process yields were based upon the total amount of alcohol used, while the material yields were based upon the actual amount of alcohol oxidized. A minimum catalyst temperature of about 370°C. was used throughout. Below that temperature the reaction was more difficult to maintain, but apparently the reaction will take place as low as 320°-3300C., although with somewhat lower yields due to the irregularity of the reaction. At a temperature of about 370'C. glowing was often not apparent, even in a darkened room, but the reaction seemed to maintain itself uniformly.
Discussion of Results Fig. I and A in Table I show the effect of various alcohol-oxygen ratios on the yields of acetaldehyde when silver was used as the catalyst (catalyst A). It will he noted that the curve for a thermostat temperature of 54°C shows the highest yields. At a bath temperature of j4'C. the ratio, molesCZ€150H,' moles 02,was 1/0.492 t o 0. j o g . The theoretical ratio calculated for the same conditions is 1 i 0 . 5 . It is interesting to note that with metallic silver the highest yields were obtained using the theoretical oxygen supply, while with copper (in previous work) considerably more than the theoretical oxygen supply was necessary. The yields of aldehyde were appreciably lowered by working a t thermostat temperatures higher than 54°C'. Lowering of the thermostat temperature also produced a marked decrease in the yields of aldehyde. Moreau and Xlignonac' have used silverized asbestos as a catalyst for the oxidation of ethyl alcohol to acetaldehyde. They reported a yield of acetaldehyde, using reduced silver as the catalyst, which corresponded to an 8+ percent conversion of the alcohol used. They added the theoretical amount of air in two installments to the alcohol vapor, the outlet gases from the first reaction tube being cooled before adding the second installment of air and passing into the second reaction tube. 1
Moreau and Mignonac: Compt. rend., 170,
2j8 (1920)
3278
ALLAN R. DAY
Simington and Adkins' using silver gauze and working at a bath temperature of 46OC. obtained a 43.7 percent yield of acetaldehyde. They obtained a 76 percent yield, however, when they used a catalyst composed of 90% Cu and 10% Ag. Most of the earlier work has been carried out at lower thermostat temperatures and higher air rates, and as a result there would have been available more oxygen per gram of alcohol. The present investigation shows rather definitely that a bath temperature of 54OC. represents the optimum temperature.
0
FIQ.I (I)
Bath at 54';
(2) Bath
at jo"; ( 3 ) Bath at 47';
(4) Bath a t 45".
The results recorded in Table I (B and C) show that the presence of small amounts of samarium oxide (.z5ycand .soycrespectively) with silver slightly increases the process yields of aldehyde. These results are more readily interpreted from a study of Fig. 2. It is interesting to compare the slopes of curves ( I ) and ( 2 ) with the slope of (3). At a temperature of about 370'C. the differences in process yields are quite small but as the catalyst temperature rises the differences become larger. This is just the opposite of what one would expect from previous experience. The material yields for the optimum conditions (catalyst temperature about 37oOC.) remain about the same as with the pure silver catalyst, but as the catalyst temperature rises they fall off much more rapidly than in the case of the silver catalyst. These variations in process and material yields remained consistent throughout the work even when duplicate catalyst preparations were used. It would seem that the presence of small amounts of the rare earth oxide does increase the process yields of acetaldehyde and a t the same time exerts a stabilizing influence on Sirnington and Adkins: J. Am. Chem. SOC., 50, 1449 (1928).
CATALYTIC OXIDATION O F ETHYL ALCOHOL
3279
the process yields of aldehyde as the catalyst temperature rises. The same cannot be said of the material yields for, except at the optimum conditions, they are materially decreased. The results recorded in Table I (D and E) show that the presence of larger amounts of samarium oxide (1% and 5CCrespectively) causes a marked lowering of the yields of acetaldehyde. Here again, however, it will be noted that the decrease in process yields was not so rapid as in the case of the silver catalyst. I t was thought a t first that this rather marked lowering of process and material yields was due to further oxidation and posvibly to increased decomposition of the aldehyde. Other facts, however, do not confirm this
FIG.2 (I)
Catalyst, B;
( 2 ) Catalyst,
C; (3) Catalyst, A. Bath at 54" C
explanation. I t was found, a t a thermostat temperature of 54OC., that when the Smz03 content reached 5 percent, the amount of unsaturated compounds produced during the reaction became somewhat larger. This was determined by bromine absorption. KOeffort was made to identify the products, only the relative amounts being measured. This might indicate that when the rare earth oxide occupies a sufficient area of the contact surface i t may, in small part, function as a dehydrating agent, especially if an excess of air for the oxidation of alcohol to aldehyde is not present. The exact role played by the rare earth oxides can not be based upon one set of results. Further work is needed on the higher alcohols and other compounds before any general conclusions can be made.
summary Data have been obtained showing the effect produced on the catalytic activity of silver by the addition of small amounts of samarium oxide. 2. I t has been shown that the presence of small amounts of samarium oxide (at a low catalyst temperature, about 37oOC.) slightly increases the yields of acetaldehyde. 3. Higher yields of acetaldehyde have been obtained by a one step vapor phase oxidation of ethyl alcohol than have been previously reported from similar methods. I.
