Methyl Alcohol andFormaldehyde. 591 formaldehyde. The ratio of the 0 2/CH4 used up was practically the same (1*7), and of the C0/C02 formed (1*6) was almost the same in each case. The results as a whole, together with those recently obtained by Dr. Newitt and Mr. Haffner for the pressure-oxidation of methane ( cit.), have demon- strated a case of direct hydrocarbon oxidation, entirely uncomplicated by any sign of “ peroxidation,” in which (i) the most reactive mixture is that corre- sponding with the alcohol-forming proportion, (ii) substantial quantities of the alcohol have actually been isolated in circumstances (chiefly high pressure) favouring its stability and survival, and (iii) all other happenings fulfilled the predictions of the hydroxylation theory. In conclusion, we desire to thank the firm of Radiation, Ltd., of London, for their Research Fellowship, which has enabled one of us (R.E.A.) to devote his whole time to the investigation and out of which its expenses have been defrayed. The Formation of Methyl Alcohol and Formaldehyde in the Combustion of Methane at High Pressures. By D. M. N ewitt and A. B. H affner . (Communicated by W. A. Bone, F.R.S.—Received July 24, 1931.) The isolation and identification of the primary oxidation product of a hydro- carbon are so important from the point of view of the theory of hydrocarbon combustion that chemists have spared no effort to overcome the difficulties involved, but so far with incomplete success. The mechanism of the oxidation process was elucidated many years ago by the researches of Professor W. A. Bone and his collaborators as one essentially of hydroxylation, in the case of methane as involving the following stages :— Summary. CH4 -> CH3OH -* CH2(OH)2 OH OH H20 + H2:C:0^H.C:0-*H0.C:0 CO + H20 C02 + H20 on May 5, 2018 http://rspa.royalsocietypublishing.org/ Downloaded from
Transcript
Methyl Alcohol and Formaldehyde.591
formaldehyde. The ratio of the 0 2/CH4 used up was practically the same (1*7), and of the C0/C02 formed (1*6) was almost the same in each case.
The results as a whole, together with those recently obtained by Dr. Newitt and Mr. Haffner for the pressure-oxidation of methane ( cit.), have demonstrated a case of direct hydrocarbon oxidation, entirely uncomplicated by any sign of “ peroxidation,” in which (i) the most reactive mixture is that corresponding with the alcohol-forming proportion, (ii) substantial quantities of the alcohol have actually been isolated in circumstances (chiefly high pressure) favouring its stability and survival, and (iii) all other happenings fulfilled the predictions of the hydroxylation theory.
In conclusion, we desire to thank the firm of Radiation, Ltd., of London, for their Research Fellowship, which has enabled one of us (R.E.A.) to devote his whole time to the investigation and out of which its expenses have been defrayed.
The Formation of Methyl Alcohol and Formaldehyde in the Combustion of Methane at High Pressures.
By D. M. N e w it t and A. B. H a f f n e r .
(Communicated by W. A. Bone, F.R.S.—Received July 24, 1931.)
The isolation and identification of the primary oxidation product of a hydrocarbon are so important from the point of view of the theory of hydrocarbon combustion that chemists have spared no effort to overcome the difficulties involved, but so far with incomplete success.
The mechanism of the oxidation process was elucidated many years ago by the researches of Professor W. A. Bone and his collaborators as one essentially of hydroxylation, in the case of methane as involving the following stages :—
Summary.
CH4 -> CH3OH -* CH2(OH)2 OH OH
H20 + H2: C : 0 ^ H . C : 0 - * H 0 . C : 0
CO + H20 C02 + H20
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Although it is generally agreed that the main course of hydrocarbon combustion is in accordance therewith, during recent years there has been some discussion as to whether the initial product is an hydroxylated molecule or a peroxide, although the cumulative weight of experimental evidence, both direct and indirect, has been overwhelmingly in favour of the former. Thus, the initial formation of ethyl alcohol by the interaction of ethane and ozone at 100° C., the results of recent studies of the slow oxidation of methane and ethane in Professor Bone’s laboratories—showing that much the fastest reacting mixtures are 2CH4 -f- 0 2 and 2C2H6 + 0 2 and not equimolecular ones—the formation of acetaldehyde (by intermolecular change from vinyl alcohol) during the initial stages of the slow combustion of ethylene, and experiments upon the explosive combustion of methane at high initial pressures, have all pointed unmistakeably in that direction. Nevertheless, up to now, all efforts to isolate the corresponding alcohol from the products of the slow combustion of one of the simpler paraffins at ordinary pressures have been frustrated, apparently because under such conditions the further oxidation of the alcohol to the di-hydroxy stage occurs so rapidly. Hence upholders of the hydroxyla- tion theory have always postulated an initial “ non-stop ” run through the mono-hydroxy to the di-hydroxy stage.
Now, however, that facilities are available at the Imperial College of Science and Technology for the systematic investigations of reactions at high pressures, it was thought desirable to study the slow oxidation of methane under such conditions. Accordingly, the experiments described herein were undertaken, with results which are decisively in favour of the hydroxylation theory, inasmuch as large proportions of methyl alcohol, but never a trace of any peroxide, have been isolated and identified from the oxidation products.
Before proceeding to describe the experiments in detail it may be well to consider the experimental conditions most likely to favour the survival of the three intermediate products. The stoichiometric and thermodynamic aspect of their formation would be as follows :—
It will be seen that while methyl alcohol and formic acid are formed with a contraction in volume, formaldehyde involves an expansion. Hence the effect of increasing the pressure in a reacting system of methane and oxygen at a given temperature would be to favour the survival of alcohol and acid at the
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expense of the formaldehyde. Moreover, from free-energy considerations, an increase of temperature would accelerate all three reactions, and tend to promote the complete oxidation of the methane. A combination of high pressure with as low a temperature as possible was suggested as favouring most the survival of the alcohol.
From the kinetic standpoint, the effect of increase of pressure would be to speed up the reaction, and an additional advantage is thereby gained since it becomes possible to work at much lower temperatures than would otherwise be convenient. Precautions should be taken, however, to dissipate the heat evolved during the combustion at such a rate that no considerable rise in temperature occurs during the experiment. In the present instance this was done by diluting the theoretical methane-oxygen mixture with either a large excess of the combustible gas, or with a corresponding quantity of nitrogen carbon dioxide, or steam. The initial mixtures employed have usually had the composition 2CH4 + 0 2 + (6 to 6-5) X, where X — one or other of such diluents.
E x p e r im e n t a l .
Apparatus and Method.The apparatus consisted essentially of a reaction vessel heated electrically
and connected (1) with cylinders containing the initial gases mixed and compressed to a suitable pressure, and (2) with an expansion chamber into which the hot products of combustion could be suddenly released at the conclusion of the oxidation or at some predetermined intermediate stage. The general arrangement of the apparatus will be clear from the diagram (fig. 1).
The reaction vessel F was constructed of nickel-chromium-molybdenum steel and was capable of withstanding a working pressure of 300 atmospheres at a temperature of 500° C. ; its internal capacity was 500 c.c. The cylinder A and reserve cylinder B containing the initial gas mixture communicated through the control valve C with the inlet valve I of the reaction vessel. The expansion chamber P, having a capacity of 2 litres, was fitted with two valves K and N, the former being connected directly with the outlet valve J of the reaction vessel and the latter being used to “ blow off ” the wash water contained in P. The Bourdon gauge D recorded the initial pressure of the reactants at the beginning of an experiment.
The method of carrying out an experiment was briefly as follows. The reaction vessel having been brought to the desired temperature, the whole system was evacuated. About 50 c.c. of distilled water were then drawn into
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the expansion vessel and the valves J and K closed. By means of the control valve C the reactants were rapidly filled into the vessel F up to the desired pressure and the valve I immediately closed. During the filling process, which occupied only a few seconds, there was usually a temperature rise of about 3° due to the adiabatic compression of the gases ; the temperature then fell to its original value and remained constant until the reaction started. The progress of events was followed by means of the temperature change as indicated by
the readings of a platinum rhodium couple situated in a tube traversing axially the reaction chamber. When it was desired to stop the reaction, the valves J and K were opened and the contents of F rapidly expanded into the vessel P, which was then disconnected from the system. The gases and vapours remaining in the reaction vessel at a considerably reduced pressure were then passed through two cooling coils surrounded by ice and through a Jena-glass scrubber containing distilled water. Gas samples for subsequent analysis were taken at a point between the cooling coils and the scrubber, due allowance being made in the results for their alcohol content. The contents of the
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cooling coils, scrubber and expansion chamber were mixed, diluted and aliquot parts used in the estimation of methyl alcohol, formaldehyde and other products.
Preparation of Gases ; Detection and Estimation of Intermediate Oxidation Products.
Gases.—The methane used had been prepared in the Laboratory and purified both chemically and by liquefaction and fractional distillation; after compression its purity was usually about 99*5 per cent., the remainder being nitrogen. Oxygen was purchased in cylinders from the British Oxygen Company and was used without further purification.
The methane-oxygen mixtures were made up at atmospheric pressure in a 10 cubic foot gas-holder and were compressed in cylinders to 200 atmospheres.
Methyl Alcohol.—In the early stages of the investigation the condensable products from a number of experiments were collected and used for qualitative tests. Methyl alcohol was identified by preparing therefrom (1) methyl salicylate and (2) a specimen of methyl p-nitrobenzoate. For the latter preparation the liquid from the cooling coils, after three successive experiments, was dried over quicklime for 12 hours and distilled below 90° C. The distillate was treated with p-nitrobenzoyl chloride, gently warmed and the methyl derivative precipitated by pouring into water. After repeated recrystallisation it gave a melting point of 95° C.
The quantitative estimation was carried out by the method of Fischer and Schmidt, slightly modified in detail to accelerate the reaction. The method consists essentially in converting the alcohol to methyl nitrite, hydrolysing the ester in the presence of acid potassium iodide, and estimating the iodine liberated by means of a standard sodium thiosulphate solution in the usual way.
Formaldehyde was detected by Schiff’s reagent, and estimated by Romijin’s potassium cyanide method. In a few cases the total aldehydes were also estimated by Ripper’s bisulphite method ; but, as no appreciable difference was obtained by the two methods, higher aldehydes were assumed to be absent.
Formic Acid.—The condensate was warmed to liberated carbon dioxide and titrated with N/100 alkali. Only traces of formic acid, too small to estimate with accuracy, were found.
Peroxides were tested for by the titanic sulphate reaction ; but, although the method is sensitive to 1 part in 500,000, no trace of their presence was ever detected.
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Special tests were also made with the object of detecting any (i) dimethyl- ether, possibly formed by the dehydration of methyl alcohol, and (ii) acetals, but with entirely negative results in each case.
The General Characteristics of the Reaction at High Pressures.
As an example of the behaviour of methane-oxygen mixtures undergoing reaction at high pressures, the course of an experiment at 106*4 atmospheres pressure may be described, as being typical of all those included in the present paper. In this case the initial mixture consisted of the two gases in the proportion of 8 • 1 parts of methane to 1 part of oxygen, the reaction temperature being 34-1° C. The curves in fig. 2 show the rate of disappearance of oxygen and the rate of change of temperature from the moment of filling until the reaction was substantially complete.
FillingCo m p l e t e
Co m m e n c e m e n t < of F illing
T i m e (Minutes)
F ig. 2.
Considering first the temperature-time curve, it will be seen that an initial rise of some 3° took place due to the adiabatic compression of the gases on filling ; after 3 minutes the temperature had fallen nearly to its original value, but almost immediately thereafter it commenced to rise, and between 6| and 8 minutes increased rapidly. At 12 minutes it had reached a maximum and was again diminishing. The oxygen-time curve indicates the existence
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of an induction period of some 3 minutes, during which no detectable diminution in the oxygen content of the mixture took place ; this was succeeded by a period of rapid reaction during which the rate of oxygen disappearance showed a rough parallelism with the rate of temperature rise ; at maximum temperature no uncombined oxygen remained. The occurrence of this induction period, similar to that found by Bone and Hill in the oxidation of ethane and by Fort and Hinshelwood for ethylene, methane, and methyl alcohol, is typical of the high pressure reaction and under certain conditions may amount to as much as 80 minutes.
Although under the foregoing conditions the reaction in a steel vessel is predominantly heterogeneous, its rate being increased several times by increasing the surface/volume ratio of the reaction vessel, indications were obtained of its becoming homogeneous at higher temperatures. Such was manifested by an unusually high temperature increase and by an abrupt fall in the amount of methyl alcohol surviving in the product (see Table II) with probable occurrence of flame in the mixture, although subsequent examination showed no carbon to have been deposited during the reaction. In this connection it may be recalled that Bone, Newitt and Smith* found that a methane-air mixture containing 41-4 per cent, of methane could be exploded at an initial pressure of 134 • 1 atmospheres without any liberation of carbon, although a 38-1 per cent, mixture under identical conditions gave a copious deposit.
Three series of experiments were carried out to determine (1) the quantity of alcohol present in the system at various times during the reaction, and (2) the effect of initial pressure and temperature upon the yields of alcohol obtained in the products when the free oxygen present had completely disappeared.
Detailed Study of the Initial Stages of the Oxidation.Before proceeding to investigate the effect of pressure and temperature on
the course of the combustion, experiments were carried out with a 8 • 1CH4 + 0 2 mixture with a view to ascertaining the change in the rates of formation of methyl alcohol and formaldehyde, respectively, as the relative concentration of oxygen in the reacting system progressively diminished ; for, since the disappearance of oxygen is accompanied by a corresponding increase in the ratio CH4/0 2, the conditions for the formation of the primary product would appear to become increasingly favourable as combustion proceeds.
* “ Gaseous Combustion at High Pressures ” (Longmans, Green & Co.), 1929.
2 RVOL. CXXXIV.— A.
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The method employed was to carry out a number of experiments at constant temperature and pressure and to stop the reaction after various time intervals by suddenly expanding and cooling the system in the manner already described. Two series of experiments were made commencing in each case with a 8 • 1CH4 + 0 2 mixture at initial pressures of 48-0 and 106*4 atmospheres respectively. The results are summarised in Table I and, in the case of the series at the higher pressure, are illustrated by means of curves in fig. 2, above.
Table I.—The Amounts of Methyl Alcohol and Formaldehyde present atvarious Stages during the Slow Oxidation of Methane at High Pressures.
DurationComposition of gaseous medium, c.c. at N.T.P. Ratios.
ofexperiment. o 2. c h 4. c o 2. CO. CH3OH. H.CHO CO/
c o 2.CHsOH / H.CHO.
Initial pressure = 106*4 atmospheres. Temperature == 341° C.
From an inspection of the figure it will be seen that, following an induction period of about 3 minutes, during which time no detectable change in the composition of the mixture took place, the oxidation commenced, at first slowly and then with increasing velocity, until after the lapse of 12 minutes the whole of the oxygen had been consumed. Both methyl alcohol and formaldehyde were detected at a very early stage and the methyl alcohol increased in concentration with time, its actual rate of formation (as derived from the concentration-time curves) showing a progressive increase as the ratio CH4/0 2 in the system increased, until towards the end when there was a
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marked slowing down. Peroxides and hydrogen were tested for with negative results.
Both carbon monoxide and dioxide were found in quantities representing in all cases upwards of 70 per cent, of the methane burnt. The C0/C02 ratios are somewhat irregular, but in general showed an increase with time ; thus, in the series at 106 A atmospheres, the ratio after 6*5 minutes is 0*31 and after 12 minutes is 1-45 and a similar result is observed at the lower pressure.
In two experiments at 106*4 atmospheres the products were allowed to remain in the reaction vessel for 18 minutes and 16| hours, respectively, after all the oxygen had disappeared. During this time some interaction occurred, both the amounts of alcohol and aldehyde diminishing, although they never entirely disappeared ; the C0/C02 ratio showed a rapid fall to 0*63 in the former case and 0*13 in the latter.
The Effect of Temperature upon the Survival of Methyl Alcohol and Formaldehydein the Products.
In the experiments of this series pressures of 150, 106 • 4 and 48 • 2 atmospheres were employed, the initial mixture having the same composition as before.
The method consisted in charging the reaction vessel, previously heated to a selected temperature, with the 8*1CH4 -j- 0 2 mixture and then following the progress of events by the temperature change. Immediately after the temperature had reached a maximum the contents of the vessel were rapidly expanded and cooled and the products treated in the manner already indicated.
The main features brought out by the experimental results, which are summarised in Table II and illustrated by means of curves in fig. 3, are as follows :—
(1) At each pressure there was a definite temperature at which maximum survivals of alcohols and aldehydes were obtained. This was by no means the lowest temperature at which oxidation would take place, but was usually one giving a medium time of reaction. For example, at 106*4 atmospheres a yield of methyl alcohol corresponding with 22*3 per cent, of the methane burnt was obtained at 341° C., at which temperature there was an induction period of 2*5 minutes followed by a reaction time of 9*5 minutes. When the induction period was extended to 53 minutes and the reaction time to 35 minutes, the yield was only 10*1 per cent., although the temperature was some 3° lower. A similar diminution was noticed when the temperature was teo high and the duration of reaction correspondingly short.
M ethyl Alcohol and Formaldehyde. 599
2 r 2
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(2) The ratio of C0/C02 in the products remained fairly constant with temperature in the series at 48*2 atmospheres, but showed a definite increase at the remaining two pressures. In two experiments at 106*4 atmospheres pressure the ratio showed an exceptionally large value, the figures being 3 • 9 for 352° C. and 4*9 for 355° C. ; this increase corresponded with an abrupt fall in the alcohol and aldehyde figures and probably indicated the formation of flame in the vessel; no carbon was deposited in either case nor was any hydrogen found in the products.
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(3) A comparison of the ratios of alcohol/aldehyde at the various temperatures showed that whilst temperature had no definite influence, there was a marked
Formaldehyde x S
T e m p e r a t u r e ( ° C )
F ig. 3.
increase as the initial pressure was raised. Thus, taking the mean of all the results at each pressure, the figures were :—
Initial pressure. Ratio CH3OH/H . CHO.48-2 13-0
106-4 31-5150-0 41-6
The Effect of Initial Pressure on the Survival of Methyl Alcohol and Formaldehyde.
A consideration of the results included in Table II showed that to obtain comparative figures for the effect of pressure it would be necessary to vary the temperature so that in every case the maximum survivals of alcohol and aldehyde wTere obtained ; this involved the carrying out of a large number of experiments at different temperatures and pressures on the lines of those recorded in the preceding section.
In the results, which are set forth in Table III for a 8 • 1CH4 + 0 2 mixture, the pressure range 10 to 149 atmospheres has been covered ; and, with the
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possible exception of experiments at 40 and 149 atmospheres, the data may be taken as fairly representing the maximum survival under the given experimental conditions, at the pressures in question.
The figures require little comment; in general, the effect of pressure was not only to increase the survival of alcohol, and, to a lesser extent of aldehyde, but also to increase the ratio CH3OH/H, CHO. It will be noticed that in all cases the best results were obtained when the time of oxidation was between 4 and 12 minutes and when the temperature rise during the experiment did not exceed 14° C.
The Effect of Diluents upon the Combustion.
Experiments were next carried out to determine the effect (1) of inert diluents, and (2) of the non-combustible products of combustion (steam and carbon dioxide) on the reaction. For this purpose mixtures of composition 2CH4 + 0 2 + 6-5X, where X = C02, H20 or N2, were employed, and in each case the reaction was allowed to proceed to completion and the products analysed. A control experiment in which the diluent was replaced by its equivalent of methane was made for purposes of comparison.
The principal results of one such series at an initial pressure of 50 atmospheres are summarised in Table IV. It will be seen from a comparison of the figures with those in Table II that no matter whether the diluent was an inert gas or one of the non-combustible products of combustion, the effect was to slow down the rate of reaction to such an extent that, in order to obtain
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Table IV.—The Survival of Methyl Alcohol and Formaldehyde in the Slow Combustion of 2CH4 -j- 0 2 Mixtures diluted with Nitrogen, Carbon Dioxide and Steam.
Initial pressure = 50 atmospheres.
Initial mixture 2CH4 -f- 0 2 + .
Temperature.
Temperature
Timeof
Amount of CH3OH and H. CHO surviving
in products, c.c.’s at N.T.P.
Ratio CH3OH / H. CHO.
rise. reaction.c h 3o h . H . CHO.
6 -5 N 2 6-5 H20
°c.397
°c.3
mins.16 48 2-7 17-9
400 2 2 43 1-6 26-96-5 C02 390 4 8 50 3 0 16-7
Control experiment 6-5 CH4 397 12
1________1 80 4-6 15-7
times of reaction comparable with those for the simple CH4 — 0 2 mixture, it was necessary to employ temperatures nearly 20° higher. As might be expected, the higher temperatures led to a diminution in the concentrations of methvl alcohol and formaldehyde surviving in the products ; but, in addition, the control experiment shows that the observed diminution in yields is not entirely a temperature effect, but arises partly from the substitution of the excess methane in the standard 8 • 5CH4 + 0 2 mixture by the diluents.
Under the conditions of our experiments methyl alcohol, carbon monoxide and hydrogen all oxidise readily and only survive in the products of combustion of methane when oxygen is in defect of that required for its complete oxidation to carbonic anhydride and steam.
Summary.It has been shown th a t :—
(1) In the slow combustion of methane at high pressures considerable quantities of the primary product, methyl alcohol, survive and can be isolated.
(2) As combustion proceeds in an 8 • 1CH4 + 0 2 mixture the rate of formation of methyl alcohol increases with the CH4/0 2 ratio until a point is reached when the concentration of steam and carbon dioxide in the products begin to exert a retarding effect.
(3) At any particular pressure there is a definite temperature at which optimum amounts of methyl alcohol and formaldehyde survive.
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(4) The effect of increasing pressure is not only to increase the amounts of both products surviving, but also to increase the ratio CH3OH/H . CHO.
(5) The oxidation of methane under the experimental conditions described is mainly a surface effect and is characterised by a marked induction period.
In conclusion, one of us (A.E.H.) desires to express his thanks to the Gas Light & Coke Company, whose Gas Research Fellowship has enabled him to devote his whole time to the work.
The Third Spark Spectrum of Arsenic (As IV).
By K. R. R a o , D .S c., Madras Government Research Scholar, ImperialCollege, London.
(Communicated by A. Fowler, F.R.S.—Received July 16, 1931.)
Introductory.
The third spark spectrum of Arsenic has been investigated in the extreme ultra-violet by Sawyer and Humphreys,* who have found the three triplet combinations of the deep term 4 p3P with 5s 3S, 4 3D and 4 3P and the singlet resonance line 4s1S0 — 4p 1P1. Two more triplets involving the term 5 p3P were discovered later by the writer.^ The present paper deals with the identification of the higher members of the triplet system as well as the singlets and inter-combination lines of the spectrum. Most of the lines which could be assigned to the trebly-ionised atom of Arsenic have been classified and it is now possible to estimate the absolute values of the characteristic terms.
Experimental.The general experimental methods employed in the present investigation
on the spectrum of Arsenic have been briefly described already in two previous communications^ dealing with the arc and the second spark spectrum of Arsenic. For the excitation of the third spark spectrum, the chief source
* Sawyer and Humphreys : ‘ Phys. Rev.’ vol. 32, p. 583 (1928).t K. R. Rao : ‘ Nature,’ Feb. 16 (1929).% K. R. Rao : As I, ‘ Proc. Roy. Soc.’ A, vol. 125, p. 238 (1929), and As III, ‘ Proc.
Phys. Soc.’ vol. 43, 68 (1931).
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