T H E THERMAL DECOMPOSITION O F FORMIC ACID
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BY TVM. L. NELSON A S D C RL J. ESGELDER
In some investigations on formic acid recently carried out in this laboratory, it became necessary to study the thermal decomposition of this acid without the intentional addition of catalysts, in reaction tubes of various materials, over a temperature range from 250’ to j;o°C. Formic acid in its decomposition is so sepsitive to catalytic influence that it was to be expected that the walls of the reacting vessel would reveal a specific effect and largely influence the course of the reaction. More extensive studies of this kind have recently been reported by Hinshelwood and his eo-workers, who studied the decomposition in bulbs of various glasses and with platinum, palladium, silver, and titanium dioxide as catalysts a t temperatures up to 3 50°C. Reference to their work mill be made later. Muller and Peytrall passed the vapors of formic acid through a small platinum tube a t II~OOC and concluded that the primary decomposition was into COz and H1.
Experimental The apparatus used in the experiments here recorded was substantially the same as that used in catalytic work of this kind. The vapor of formic acid was passed through the various tubes inserted in the hollow core of an electrically heated furnace, the liquid products condensed, and the gases collected over a saturated salt solution on top of which was a layer of heavy paraffin oil. The formic acid, prepared by three vacuum distillations of 85% acid over anhydrous copper sulfate, was practically anhydrous, and had a specific gravity of 1.22. The flow of acid through the reaction tubes, accurately controlled by the method of displacement by mercury2,was maintained uniformly a t the rate of three cc. per hour. The acid was vaporized in a preheater maintained at a temperature somewhat above the boiling point of the acid. The furnace was built upon a grooved silica tube, 24” long and I ” diameter, mound with nichrome wire and insulated with alundum and kieselguhr. The furnace was maintained at the desired temperatures by means of a Leeds and Northrup Thermo-Regulator. The reaction tubes were 30” long and about I ” external diameter and fitted snugly into the core of the furnace. The thermo-couple, cased in a pyrex tube, penetrated half way into the reaction tube. Beginning a t 3 50°C., the gaseous decomposition products were analyzed over mercury with the Hempel apparatus immediately after runs. I n general, COZwas determined by absorption in potassium hydroxide and H$ and C o were determined by slow combustion over mercury. Occasionally, when i t seemed desirable to check up on the presence of hydrocarbons, the CO was 2
Muller and Peytral: Bull., 29. 34 (1921). Adkins and Nissen: J. Am. Chem. SOC., 46, 140 (1924).
THERMAL DECOMPOSITION O F FORMIC ACID
471
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determined by absorption in acid cuprous chloride, and the residual gas analyzed for hydrogen and hydrocarbons by slow combustion. Only fractional percentages of saturated hydrocarbons, presumably methane, were ever found, so that it was deemed sufficiently accurate to determine, in general, only COz, CO, and H2. The apparatus was always flushed by making a run of about one hour's duration before collecting and analyzing the gas mixture, yet traces of air were present, so that all analyses and volumes recorded below have been calculated to an air-free basis and corrected to N.T.P.
Results Representative samples of gas were analyzed for the runs a t the temperatures indicated in the tables. In all cases, 3 cc. of formic acid was vaporized and passed through the reaction tube per hour. The analyses of the gas mixtures, together with the volume of gas at X.T.P. and the volumes of COz, Hz, and CO resulting from the decomposition of one cubic centimeter of liquid formic acid are here recorded for tubes of pyrex glass, unglazed porcelain, unetched and etched silica, and copper. Pyrex Glass Tube Decomposition became evident at 2 jo0C, yielding I O cc. of gas per cc. acid* At 300°C the gas evolution reached 40 cc. per cc. acid. "C
Composition of Gas Mixture 76 HZ % CO
% COa
350 400 500 600
49.1 49.6 45.3 42.5
49.1 48.0 40.3 39.4
1.8 2.4 14.4 18.1
I
cc. Total
cc. Fcrmic Acid yielded: cc.COz cc.Hz
132.6 287.7 339.7 386.7
270
580 750 910
132.6 278.4 302.2 358.5
cc.CO
4.8 13.9 108.0 164.7
Unglazed Porcelain Tube At 2 50°C. the gas evolution was 30 cc. and a t 300°C it was 50 cc. per cc. acid. "C
350 450
550
Composition of GaF Mixture %COB %Hz % C O
49.0 53.4 53.2
48.7 43.9 36.9
I
cc. Total
440 910
2.3 2.7 9.9
cc. Formic Acid yielded: cc.CO2 cc.Hz cc.CO
215.6 485.9 425.6
800
214.3 399.5 295.2
10.1 24.6 79.2
Unetched Silica Tube The gas yield a t "C
350 450
550
250OC
was 30 cc. and a t 3oo0C,
Composition of Gas Mixture %COz %Hz %CO
50.0 51.3 50.8
48.4 47.1 45.6
1.6 1.6 3.6
I CC.
120
cc. per cc. acid.
cc. Formic Acid yielded:
Total
CC.CO~
730 980 950
365.0 502.7
482.6
cc.Hz
353.3 461.6 432.2
CC.CO 11.7
15.7 34.2
WM. L. NELSON AND CARL J. ENGELDER
472
Etched Silica Tube The above tube was rather deeply etched with hydrofluoric acid and then used as reaction tube. Decomposition was the same a t 25ooC as with the unetched tube, but a t 3ooOC the gas evolution reached 400 cc. per cc. acid. "C
350 450 550
Composition of Gas Mixture %CO, %Hz % C O
I
cc. Total
51.1
49.5 47.5
0.3 1.4
IOIO
50.0
42.2
7.8
900
50.2
cc. Formic Acid yielded: cc.COz cc.Hz cc.CO
940
471.9 516.1 450.0
465.3 479.7 379.8
2.8 14.1 70.2
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Copper Tube "C
300 350 450
550
Composition of Gas Mixture %Cos %Hz % C O
-
-
-
48.7 49.7 49.7
47.8 49.9 49.9
3.5
I
cc. Total IIO
360
0.4
1120
0.4
1130
cc. Formic Acid yielded: cc.COz cc.H2 cc.CO -
175.3 556.6 561.6
172.0
558.9 563.9
12.6 4.5 4.5
Discussion of Results Sabatier and Mailhe', Hinshelwood2 and others, and Adkins3 have shown that formic acid undergoes decomposition according to one or more of the three primary reactions:H9 (I) HCOOH = COZ HCOOH = CO Ha0 (2) H2O HCHO (3) 2HCOOH = C02 the extent on each depending on the specific nature of the catalyst and the character of the reaction vessel. I n addition, over the temperature range covered here the decomposition of formaldehyde, according to the reaction:CO (4) HCHO = Hz must be taken into account. Consideration of these four reactions shows that the COz found may be accounted for by reactions (I) and (3); that the H I may have its origin in reactions ( I ) and (4), and that the CO may arise from reactions ( 2 ) and (4). That reaction (3) takes place to some extent is shown by the excess of the volume of C 0 2 over that of H 2 and by the further fact that formaldehyde was found in the condensate of all runs a t the lower temperatures. The thermal decomposition of formaldehyde according to reaction (4) has been studied by Bone and Smith4 and more recently by Hofmann and Schibstedj. The more
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Sabatier and Mailhe: Compt. rend., 152, 1212 (1911). Hinshelwood, Hartley and Topley: Proc. Roy. SOC.,100A, 575 (1922); Tingley and Hinshelwood: J. Chem. SOC., 121, 1668 (1922); Hinshelwood and Topley: 123, 1014; Hinshelwood and Hartley: 1333 (1923). 3Adkins: J. Am. Chem. SOC.,45, 809 (1923). 4Bone and Smith: J. Chem. SOC.,87, 910 (1905). 5Hofmann m d Schibsted: Ber., 51, 1398 (1918).
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THERMAL DECOMPOSITION O F FORMIC ACID
4 73
or less complete dissociation into Hz and CO of the formaldehyde formed will contribute its quota of these gases to those arising from the direct decomposition of formic acid itself. Turning to the data, inspection shows that by far the greater amount of gas (Hzand C O z ) is produced by reaction (I). The consistent excess of COz over that of H z must be attributed to reaction (3). This is substantiated by the finding of formaldehyde in the condensate. The volume of carbon monoxide shows a steady increase with rise in temperature for all tubes except that of copper. Its presence can be accounted for by the direct decomposition of formic acid according to ,reaction ( 2 ) , which according to Hinshelwood takes place to an equal extent with reaction (I) a t 28ooC, and is the reaction that takes place almost exclusively a t low temperatures in the presence of titanium oxide as catalyst, an observation first made by Sabatier and later confirmed by Hinshelwood and by Adkins. On the other hand, it is equally possible to account for the CO through the secondary decomposition of formaldehyde according to reaction (4), a reaction which does take place to a certain extent, This latter supposition is as consistent with our data as the former, so one cannot say with certainty which of these two reactions is the one yielding the CO. Probably both are involved. Obviously, with the data at hand, bearing in mind that each of these gases may arise from at least two separate reactions, no exact calculation can be made of the extent to which each reaction proceeds under the conditions of the experiment. Tentative calculations based on the gaseous products have, however, been made of the amount of decomposition undergone by the acid in its passage through the tubes at the rate of 3 cc. per hour, and of the extent to which each primary reaction is influenced. In the first set of such tentative calculations, the decomposition has been based on reactions ( I ) , ( 2 ) , and (3), not taking into separate account the amounts of CO and Hz which come from the partial decomposition of formaldehyde. The total H z found is thus a measure of reaction (I), the total CO a measure of reaction ( 2 ) , and the excess of COz over that of H z a measure of reaction (3). If one cc. of liquid formic acid decomposes completely according to ( I ) , there should result 1186 cc. of gas, consisting of equal volumes of COz and H z ; if according to reaction (2), 593 cc. of pure CO; if according to (31, 296 cc. of Cot, assuming here no decomposition of the HCHO. I n this way the extent to which one cc. of acid was decomposed in the direction of ( I ) , ( z ) , and (3) was computed, the total percentage decomposition being the sum of all three. Thus, for example, for the unetched silica tube at 350°C, 59.470 of the acid passed through decomposed according to ( I ) ; 2.0% according to (2); and 4.0% according to (3); giving a total decomposition of 65.4%. The extent to which the three modes of decomposition are divided among themselves gives a better approximate idea of the course of these reactions. I n the above example, for every IOO molecules decomposed, 90.8 follow reaction (I), 3.0 follow reaction ( z ) , and 6.2 follow reaction (3). Typical results computed by this method are given in the table below for the etched and unetched silica
WM. L. NELSOR AND CARL J. ENGELDER
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474
tube. These figures can only be taken as approximations, but they are useful in showing the differences obtainable with smooth and rough surfaces of the same material. I n another tentative method of calculation, the decomposition has been based on reactions ( I ) and (3), assuming that all the CO and an equal volume of H z have been derived from the secondary partial decomposition of the formaldehyde formed in reaction (3). The apparent extent to which reaction ( I ) proceeds can then be taken as the total H 2 formed, less a volume equal t o that of the CO found. The excess of COz, as a measure of (3), will thus be somewhat larger than by the first method of computation, The volume of CO gives a measure of the gaseous decomposition of'formaldehyde; the C02 attributed t o reaction (3) is always in excess of the CO indicating that the formaldehyde does not decompose completely into gaseous products, but forms the oily and tarry polymerization products found in the condensate. The data for the tubes have been calculated in this manner and compared with the first method. The results for the two silica tubes are given below.
Unetched Silica Tube "C
Total
5%
Decomp.
350 450
550
65.4 94.3 95.7
Method I Extent followed by reaction (1) (2) (3) per IOO molecules decomposed
90.8 82.5 76.2
3.0 2.8 6. I
6.2 14.7
17.7
Method 2 Extent followed by reaction (1) (3) per IOO molecules deccmposed
87.9 79.8 70.I
12.1
20.2
29.9
Etched Silica Tube 'C
Total
5%
350 450
550
Method I Extent followed by reaction (1)
Decomp.
per
81.2 95.6 99.6
96.7 84.7 64.3
(2)
100 molecules
0.6 2.5
11.9
(3)
decomposed 2.7 I2
.g
23.8
Method 2 Extent followed by reaction (1) (3) per 100 molecules decomposed
96.0 79.0 52.5
4.0 21.0
47 * 5
Comparable results were obtained with the other tubes. In the case of the porcelain tube a t 5 joo, the abnormally large excess of COz gives an apparent total decomposition of over one hundred percent; either the data for this run are a t fault, or the inadequacy of this tentative method of calculation is here revealed. The slight deficiency of C02 for the copper tube a t the higher temperatures is well within the limits of error of the gas analysis. Obviously, such calculations are only approximations and sweeping conclusions cannot be drawn from them. The matter becomes still more complicated when one considers the possibility of secondary reactions taking place, such as the reduction of carbon dioxide by hydrogen and the formation of methyl alcohol. The reaction:H 2 - t CO 3. H20 ( 5 ) COz according to Muller and Peytral accounts for the presence of CO a t I I 5o°C, these investigators concluding that the only primary reaction in the decom-
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THERMAL DECOMPOSITION O F FORMIC ACID
47s
position of formic acid a t this high temperature is the one giving COZ and Hz. To what extent this reduction takes place over the temperature range covered has not been investigated by us. In some cases, equilibrium concentrations for the water gas equilibrium are reached, as shown by calculations of the equilibrium constant CO x H 2 0 / C 0 2 X Hz = k, yet no definite conclusions can be drawn from the data available here. This point requires more investiga tion.
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Conclusions Formic acid when slowly passed through tubes of silica, pyrex, porcelain, and copper, heated to tenlperatures of from 350' to j5ooC, decomposes principally into COz and Hz, according to the reaction:HCOOH = CO, H2 A certain, usually small, amount decomposes according to the reaction:2HCOOH = COz HzO HCHO The two reactions :HCOOH = CO HzO HCHO = CO Hz account for the CO found. The decomposition is 90% or more complete a t 45oOC. The pyrex glass tube is least active; the copper tube most active. The silica tube after being etched shows a much greater activity than it did in its unetched condition, showing the effect of increased active surface. With the copper tube reaction ( I ) takes place almost exclusively and brings about a decomposition of over 99% at 4 j O o and 550'C. No exact calculation can be made of the extent to which each reaction proceeds on account of the complications arising in the gas volume relationships. The instability and sensitiveness of formic acid to the catalytic influence of the walls of the reacting tube are shown by the results here recorded. These results have an important bearing on the work of Wescott reported in the next paper.
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University of Pzttsburgh, Pittsburgh, Pa. December, 19%.
