T H E CATALYTIC DECOMPOSITION O F FORMIC ACID BY B. B. WESCOTT AND CARL J. ENGELDER
The experiments of Eelson, reported in the preceding paper, have shown that when formic acid is slowly passed through tubes of various materials heated to temperatures of 350' to 550°C, the decomposition is mainly in the direction of COZ and Hz, according to the reaction:Hz (I) HCOOH = COz The excess of C02 over H2, together with the production of formaldehyde shows that the following reaction takes place to a certain extent :HzO HCHO (3) 2HCOOH = COS The decomposition of formaldehyde into CO and Hz a t these temperatures accounts for a t least some of the CO and H z found in the gaseous decomposition products. The dehydration of formic acid according to the reaction:HzO (2) HCOOH = CO undoubtedly takes place to some extent. That these reactions are greatly influenced by dehydrating and dehydrogenating catalysts is well known from the work of Sabatierland the recently published papers of Hinshelwood2 and Adkins3. These investigators worked a t temperatures up to 350'C. The former used a layer of catalyst 50 cm. long, heated in a Jena glass tube. Hinshelwood employed bulbs coated with the metallic catalyst or partly filled with the powdered material. The effect produced by relatively small amounts of alumina, titania and nickel as catalysts on the decomposition of formic acid over a temperature range from 200' to 500'C has been studied by MTescottand recorded in the present paper. The extent to which the wall reactions have been modified by the insertion of the catalysts is shown by the data here presented.
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Experimental Procedure The apparatus consisted of a burette for measuring the acid used, a vaporizer, an electrically heated and controlled furnace containing the reaction tube, a suitable condensing train, and a gas collection vessel. The acid was practically anhydrous, obtained from Kahlbaum's and from Merck's. The acid was admitted at a uniform rate of about I O cc. per hour. The gaseous products were analyzed by the usual methods of gas analysis. The undecomposed formic acid and formaldehyde were removed in the condensing train. The reaction tube was an unglazed clay Johnson combustion tube such as used in steel mill laboratories for carbon determinations. It was 40 cm. long and about one and a half cm. internal diameter. It decomposed formic acid, when passed through it, a t the rate of I O cc. per hour, mainly into Hz and COZ. Sabatier and Mailhe: Compt. rend., 152, 1212 (1911). 2Hinshelwood and Topley: J. Chem. Soc., 123, 1014(1923). SAdkins: J. Am. Chem. SOC., 45,809 (1923).
CATALYTIC DECOMPOSITION O F FORMIC ACID
47 7
The decomposition became apparent at 294'C yielding 7cc. gas per cc. acid. At 400' the gas evolution reached 320 cc. gas per cc. acid, of which 166.7 cc. were C02, 1 6 3 . 3 ~ H2, ~ . and 14.7 cc. CO. At 484' the gas yield was 274.7 cc. COZ,271.6 cc. Hz, and 36.7 cc. CO, or a total of 582 cc. per cc. acid. These values are lower than those obtained by Nelson with other tubes at a slower rate of passage but they are similar in general characteristics. To get the superimposed effect of the catalysts, these were spread in a thin layer in a combustion boat about I O cm. long and placed in the middle of the tube. T i f a n i a Catalyst A sample of ignited titanium dioxide ground to pass a zoo-mesh sieve when used in a shallow layer about I O CG. long gave the results shown in Table I. Another sample made by precipitation from titanium tetrachloride yielded similar results. TABLE I I cc. Formic Acid yielded: "C 202
234 2 80 3 40 375 398 498
cc. Total Gas
38.8 63.4 177.6 439.0 657.0 820.0 777.5
cc.
coz 2.7
4.9 19.5 128. 241. 274. 248.
CC.
Hz
1.8 3.8 16.0 112.3 222.0 246. 211,
cc.
co
33.7 54.6 142. 197. 189. 293 * 308.
A l u m i n a Catalyst The alumina catalysts were prepared in general by dissolving aluminum sulfate in water, largely diluting, and precipitating the aluminum hydroxide with dilute ammonium hydroxide. The precipitate was washed several times by decantation, filtered, dried a t 250' and ground to pass a zoo-mesh sieve. Portions of the dried A1203from the same preparation were used in the series of temperature runs. The results of a typical series of runs are given in Table 11. These results as well as those with titania have been confirmed in a general way by another student, Mr. E. H. Ohl, working in the same laboratory. TABLE I1 I cc. Formic Acid yielded: "C 287 311 338 3 73 417 491 503
cc. Total Gas
178. 243. 422.5 619.0 850.0 763 . o 754.5
cc.
coz
9.4 12.4 91.2 215.5
300. 221.
214.
Hz 6.0
CC.
10.5
85.0 199' 287. 189. 183.
cc.
co
164.0 223. 242. 205.
259. 354. 354.
478
B. B. WESCOTT AND CARL J. ENGELDER
Nickel Catalyst The nickel catalysts were prepared by ignition of the nitrate to the oxide followed by reduction to the finely divided metallic form in a stream of hydrogen in a separate furnace and subsequently transferred as needed to the catalytic apparatus without contact to the air. The results are given in Table 111.
TABLE I11 I
"C
cc. Total Gas
2IO
257 3 53 45 5
327, 490. 873. 814.
524
801.
cc. Formic Acid yielded: cc. COZ
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157.6 235. 453. 36;. 283.
cc. R z 152. 220.5
395.5 311. 257 *
cc.
co
18.6 34.6 36.7 138. 262.
Discussion of Results The results show that with titania and alumina the dehydrating reaction (HCOOH = CO HzO) greatly predominates up to temperatures somewhat above 3oo0C, reactions ( I ) and (3) being of minor importance. This is what would be expected of these dehydrating catalysts and the results confirm in general the work of earlier investigators. With nickel, the dehydrogenating reaction (HCOOH = COz Hz) takes place almost exclusively at the lower temperatures, as indicated by the large volume of C02 and Hz evolved, reactions ( 2 ) and (3) taking place to a minor extent. That these minor reactions with all three catalysts are not solely the effects of the tube can be deduced from the blank runs made under otherwise identical conditions and from Eelson's data on other tubes where the wall decompositions were studied under more nearly equilibrium conditions. At the higher temperatures the course of the main reaction is more difficult to follow and the volume relationships become complicated. With the titania catalyst a t 340' reaction ( 2 ) comes to the front, accounting for about jI% of the gas yield; a t 398' it accounts for about 63% of the total gas evolved. This reaction is the one taking place selectively at the walls of the tube but the wall reaction accounts for only about one-half the amounts of C 0 2 and Hz found here, hence the titania itself catalyzes this reaction to some extent as well. The alumina catalyst, like TiOz, catalyzes the dehydrating reaction almost exclusively a t lower temperatures. At 3 73' the dehydrogenating reaction becomes of equal importance. At the highest temperatures observed, 491' and 503'C, the CO production is again in excess of that of COZ and Hz. Adkinsl found that the extent to which these two reactions are selectively catalyzed depends upon the method of preparation of the alumina catalysts.
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'Adkins: J. Am. Chem. SOP.,45, 809 (1923).
CATALYTIC DECOMPOSITION O F FORMIC ACID
479
The abnormally large amounts of CO found in the nickel runs a t 4 j j' and It seems unlikely that the primary decomposition of the acid into CO and HZO should show this sudden increase at these temperatures with nickel as catalyst. The tube reaction at these temperatures follows this direction to only a minor extent. Formaldehyde, decomposing into CO and Hz, cannot yield these large amounts of CO consistently with the other data. The possibility of nickel catalyzing the reaction: Hz +CO HzO ( j ) COn presents itself. This reduction of COz by Hz was noted by Sabatier and Senderensl for an active copper catalyst, but with nickel, hydrogenation to methane is the usual result: C02 4Hz =: CH, 2Hz0 With the nickel used by Rescott, no methane mas found, The corresponding decrease in the volumes of Hz and COz favor this explanation. The effect of active metal catalysts on the water gas equilibrium is under investigation and may throw light on the results here presented. Reaction (3) takes place to a small extent in all the runs and can be attributed solely to the effect of the walls of the tube. The extent to which the primary decompositions are influenced by the introduction of the catalysts is readily seen at the lower temperatures. The wall-reactions and secondary effects partly obscure the specific catalytic effect at higher temperatures. Calculations based on the effect of the catalysts would, therefore, be misleading. 524' are difficult to account for.
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Conclusions The catalytic effect of small amounts of alumina, titania, and nickel on the decomposition of formic acid between 200' and ;oo'C has been studied. The reactions taking place a t the walls of the reaction tube have been greatly modified through the presence of added catalysts. Alumina and titania catalyze the dehydration reaction:HCOOH = COz Hz The tube itself decomposes the acid chiefly according to this reaction, with the dehydrating reaction and the reaction: 2HCOOH COz HzO HCHO playing minor roles. Formaldehyde decomposes partially into H2 and CO. The effect of the tube becomes marked above 350'.
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University oj" Pittsburgh, Pittsburgh, Pa. December, 1925. Sabatier and Senderens: Ann. Chim. Phys. ( S ) , 4, 426 (1905).
