The Decomposition of Methane and Methyl Chloride in a Microwave

27 The Decomposition of Methane and Methyl

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Chloride in a Microwave Discharge J. P. W I G H T M A N Virginia Polytechnic Institute, Blacksburg, Virginia N. J. J O H N S T O N NASA-Langley Research Center, Hampton, Virginia

Hydrogen was the major gaseous product of the decomposition of methane in a microwave discharge in the absence of a diluent. Hydrogen chloride, hydrogen, and ethane were observed as gaseous products of the decomposition of methyl chloride. The empirical formulas for the solid polymeric films obtained on decomposition of methane and methyl chloride were (CH O. ) and (CH Cl ) , respectively. Thermograms of the films were markedly different and the films differed significantly in free spin concentration. A greater part of the chlorine was displaced to the gas phase but about 30% was incorporated in the film which was hydrogen deficient. It was apparent that for a substituted parent molecule the H/C ratio cannot be used as a guide to predictfilmproperties. 1.49

0363

x

.445

.293

x

*T*he decomposition of hydrocarbons in various types of electrical disA

charges has been widely studied and examples of recent work are

cited (1, 5, 7, 9,10,11,14). However, the use of the microwave discharge to effect decomposition of hydrocarbons has been limited (6, 12, 13). Molecules in a microwave discharge are subjected to a greater degree of fragmentation than in other more commonly used discharges

thereby

dramatically altering the nature of the decomposition products. The decomposition of a series of hydrocarbons including methane, in a microwave discharge has been reported (13) and from this work, the following postulate could be advanced: if a parent hydrocarbon has a hydrogen to carbon ratio

( H / C ) greater than about 1.6 a hydrogen

saturated solid film will be produced on passage of the hydrocarbon 322 Blaustein; Chemical Reactions in Electrical Discharges Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

27.

wiGHTMAN

A N D JOHNSTON

Methane and Methyl Chloride

323

through a microwave discharge in addition to hydrogen. Conversely, if a parent hydrocarbon has a ( H / C ) ratio less than 1.6 a hydrogen deficient film will be produced and no hydrogen will be observed. The

present work is the first in a series of investigations of the

effect of functional groups on the nature of gaseous and solid decomposition products. The results of such measurements should ultimately support a model for decomposition in a microwave discharge. Further, solid polymers produced in various discharges have not in general been the


subject of more than the most rudimentary characterization.

The present

work explores possible avenues of further characterization.

Experimental Materials. Methane (ultrahigh purity grade) was obtained from the Matheson Co. and used without further purification. The following analysis was supplied with the methane: C 0 - 5 p.p.m.; 0 - 5 p.p.m.; N - 19 p.p.m.; C H - 14 p.p.m.; C H - 5 p.p.m. Methyl chloride (high purity grade) was obtained from the Matheson Co. and used without further purification. 2

2

2

6

8

2

8

Apparatus and Procedure. Reactions were carried out in a high vacuum flow system shown schematically in Figure 1. The power source for the microwave discharge was a Raytheon generator ( M o d e l KV-104) and was operated at a power level corresponding to about 40 R F . watts at 2450 M c . The generator was connected to an air-cooled cavity (Raytheon - K V series) by a coaxial cable. Pressures in the discharge region were measured with a thermocouple calibrated against a M c L e o d gage. A deposition train was used whereby a series of films were deposited sequentially. T w o procedures were used depending upon whether an analysis of the gaseous products was being made or whether solid film was being deposited for subsequent analysis. In the former case, the parent gas was passed through a variable leak valve (Veeco - V L ) . The pressure in a typical experimental run was 0.15 torr at a mass flow rate of 1 X 10~ moles/min. and a linear flow rate of 20 cm./sec. In the latter case, the parent gas was passed through a Teflon needle valve (FischerPorter). Typical pressures in the discharge region were 3 torr for methane and 1 torr for methyl chloride. F i l m deposition times varied between 5 and 20 min. The volume of the luminous discharge zone was about 4 cc. 5

The gaseous decomposition products of methane and methyl chloride were determined using a mass spectrometer (Associated Electronics Industries - MS-10). The doser (10 cc. ) was located about 140 cm. downstream from the discharge. Gas in the doser was expanded to decrease the pressure prior to introduction into the mass spectrometer. The solid polymeric films were removed mechanically from the walls of the borosilicate glass tubing. The infrared spectra of the neat films were obtained using a Perkin-Elmer 421 spectrophotometer. The electron spin resonance spectra of the neat films were obtained on a Varian E S R 6 spectrometer. Elemental analyses were made by Galbraith Labs. Ther-

Blaustein; Chemical Reactions in Electrical Discharges Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

324

C H E M I C A L REACTIONS IN E L E C T R I C A L

DISCHARGES

mogravimetric analyses were made in vacuo using a Cahn R G null type electrobalance and a van der Slice mass spectrometer to monitor volatiles.

Results Gaseous Products. Selected peaks from the mass spectra of methane and methyl chloride obtained with the discharge on and off are shown in Figure 2 ( A and R ) .

The most significant features in the methane

spectra (Figure 2A) were the increase in the m/e Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 23, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0080.ch027

and the decrease in the m/e

=

15 ( C H ) 3

+

=

2 (H ) 2

+

peak

peak when methane was

passed through the discharge. The most significant features of the methyl chloride spectra (Figure 2R) were the increase in the m/e peak, the decrease in the m/e the m/e

=

30 ( C H 2

G

+

=

=

2 (H ) 2

+

15 ( C H ) peak, the appearance of 3

+

) peak and the disappearance of the m/e

=

50

( C H C 1 ) peak. 3

+

Ballast ->Vacaum Mass ^ s

Γ®4-< Inlet r

c

I Doser

TC gage Expansion

Mano

Mcleod Deposition gage train

Figure 1.

/\ Vacuum Mcleo d gage Trap

Schematic of microwave discharge apparatus

Results cited below suggest that HC1 is a decomposition product yet was not observed in the mass spectra. The disappearance of HC1 is presumed to be attributable to the reaction of HC1 with stainless steel in the inlet system of the mass spectrometer.

The reaction would give

hydrogen in addition to that produced in the discharge. Independent evidence for the presence of HC1 was obtained by trapping the down stream gases at liquid air temperature, allowing the trap to warm-up and bubbling the gas (es)

through water.

The resultant solution was

titrated with a standardized N a O H solution. Solid Films.

A solid polymeric film was observed to form in the

discharge region when either methane or methyl chloride was passed through the discharge. Roth films were characterized in several ways.


27.

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A N D JOHNSTON

325

Β before di s c h a r g e 10000 during


discharge

> Ω

ξιοοο ο UJ I < u CL

100

10

15

2 15 M A S S / C H A R G E RATIO

50

30

Figure 2. (A) Selected peaks from mass sepectra of methane in a microwave discharge and (B) Selected peaks from mass spectra of methyl chloride in a discharge The

results of elemental analysis of the polymeric films produced

from methane and methyl chloride are shown in Table I. Both films were heated at 60 ° C .

under reduced pressure prior to analysis to

remove

adsorbed water. Table I Gas Methane Methyl chloride

% C

% Η

% Cl

85.35 53.40

10.61 1.98

— 44.7

% Ο 4.13 None

Total 100.00 100.15

Empirical Formula ( C H ! 0.0363 )x 49

(CH.445^.293 ) x


326

C H E M I C A L REACTIONS IN E L E C T R I C A L DISCHARGES

The infrared spectrum of a neat sample of freshly prepared film from methane is shown in Figure 3 and is almost identical to a thin film of polyethylene (2)

with one notable exception:

the absence

of strong

absorption bands in the 700-800 cm." region characteristic of ( C H ) 1

2

n

groupings where η ^ 2. T h e broad weak absorption centered at 3420 cm.

-1

is attributed to adsorbed water and not to alcoholic O - H stretching

frequencies since the film for infrared analysis, contrasted to elemental analysis, was not handled under anhydrous conditions.

T h e lack of

absorption in the 1075-1170 cm." region characteristic of the high inten Downloaded by UNIV OF CALIFORNIA SAN DIEGO on July 23, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0080.ch027

1

sity C - O H stretching vibrations supports the presence of water. The broad absorption at 1700 cm." which increases in intensity on aging is 1

assigned to carbonyl groups probably formed by reaction of radical sites with oxygen after the film was removed from the reaction tube. This is consistent with the fact that a small percentage of oxygen was found on analysis of the methane film (Table I). Jesch et al. (3) reported similar observations with polymer produced in an ethylene glow discharge. 100

3000

Figure 3.

2000

1000

FREQU ENCY(cm.

Infrared spectrum of polymer produced from methane

The presence of C H and C H groups is shown by the strong absorp tions at 2960 (vasCH ), 2930 (v sCH ), 2870 ( v C H , C H ) , 1450 ( 8 C H , 8 C H ) , and 1375 ( 8 C H ) cm." . This latter band does not appear to be split showing the absence of geminal dimethyl units. The very weak absorption at 1630, 965, and 880 c m . can be assigned to d i - and trisubstituted alkene moieties. The infrared data indicate that the film produced from methane is comprised mainly of highly branched saturated carbon chains containing pendant and terminal C H groups and little, if any, ( C H ) „ units where η ^ 2. T h e highly cross-linked nature of the polymer is consistent with its solubility behavior (see below). Tetrasubstituted alkenes are difficult to detect since they possess weak C=C stretching frequencies and no C - H out-of-plane deformations. Thus, the percentage double bond character in the methane film might be somewhat greater than indicated by its spectrum. 3

2

3

S

2

a

B

3

2

s

3

2

1

vl

3

2


M

3

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A N D JOHNSTON

327

The infrared spectrum of a neat film obtained from methyl chloride is shown in Figure 4a.

The spectrum is appreciably less definitive than

the methane film spectrum ( Figure 3 ). The spectrum of a more concen trated powdered sample contained in a potassium bromide disc is shown in Figure 4b.

The insert at the low frequency end was made with a

different interchange.


100

3000

2000 F R EQ U Ε N C Y (c m" )

1000

1

Figure 4a.

Infrared spectrum of neat polymer from methyl chloride

100 80 60 40h 20 1000 FREQUENCY(cmJ)

2000

Figure 4b.

400

600

Infrared spectrum of polymer (in KBr) from methyl chloride

Carbon-hydrogen stretching frequencies are present in the charac teristic 2870-2960 cm." region; however, the C - H bending mode at 1430 1

cm."

1

is shifted to a lower frequency from its position observed in the

methane film spectrum attributable to the influence of adjacent ethylenic bonds (8).

The weak 1370 cm." band is the symmetrical methyl bending 1

vibration and the stronger absorption centered at 1595 cm." is assigned 1

to the C=C stretching vibration of a conjugated polyene structure

(8).

The substitution of halogen for ethylenic hydrogen tends to lower the frequency of this vibration (4).

The relative intensities of these absorp

tions and the elemental analysis clearly indicate the presence of a high degree of unsaturation and relatively small amounts of C H

3

and C H 2

groups.


328


DISCHARGES

The lack of strong well resolved absorptions in the fingerprint and far infrared regions make the C - C l stretching frequency difficult to assign. Most of the chlorine atoms probably are attached to unsaturated carbons; this may result in low intensity C - C l band and a shift of its frequency to higher wavenumbers than normally observed for saturated C - C l stretching modes.

Consequently, the weak peaks in the 750-870 cm."

1

region may be assigned to C - C l and/or ethylenic absorptions. The

electron spin resonance

spectra

of the films produced from


methane and methyl chloride are shown in Figures 5a and 5b. A significantly larger free spin concentration on the order of one thousand times greater was noted in the case of the methyl chloride film. Thus, the methyl chloride film contains a greater number of unsaturated valences than the methane film. This is consistent with the results obtained from the infrared spectra of both films. N o fine structure was observed in the case of the methyl chloride film. The g-values of both films were essentially identical to the value for pitch (g = 2.000).

H

pitch(g = 2X»0)

Gain= 2.5x10'

25 gauss

Figure 5a.

ESR spectrum of polymer produced from methane

A n extensive series of liquids were tested as possible solvents for the films prior to N M R measurements.

Partial solubility was noted in hot

hexamethylphosphoramide and hot coned. H0SO4.

E v e n in these two



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329

Gain= 1.25x10-

Figure 5b. ESR spectrum of polymer produced from methyl chloride cases the concentration of the resultant solution was not sufficient to obtain an N M R spectrum. Thermograms of the methane and methyl chloride films and a commercial sample of polyethylene are shown in Figure 6.

T h e latter is

presented

between

for comparative

purposes.

The differences

the

methane and methyl chloride films can be seen readily from their thermal stabilities. The behavior of the methane film is similar to polyethylene, losing weight rapidly above 300 ° C . The methyl chloride film by contrast loses weight slowly above 3 0 0 ° C . and the loss levels off above 5 5 0 ° C . About 81% of the sample is left at 800 ° C . Polyethylene degraded into hydrogen and low molecular hydrocarbons (m/e

^

58)

accounting for 95%

The fragments from the methane film having m/e only 55% of the total pressure.

weight

of the total pressure. ^

58 accounted for

Such decomposition behavior would be

expected from highly branched cross-linked materials.

The methyl chlo-

ride film displayed no detectable fragmentation in the m/e

=

30 to 38

range. This implies that the hydrogen and chlorine atoms in the polymer are stable as in vinyl type compounds with little tendency to form hydrogen chloride during the thermal treatment.

Discussion The equation for the observed decomposition of methyl chloride in the discharge may be written xCH Cl 3

(CH. C1 ) 4

H

x

+ .7xHCl + .9xH

2



330


DISCHARGES

800 T E M P E RAT URE(°C)

Figure 6. Comparison of thermograms obtained with polyethylene and with polymerfilmsproduced from methane and methyl chloride neglecting ethane which does not subtract from the present argument since it was not formed in high concentrations.

Chlorine introduced into

the discharge by dissociation of methyl chloride thus appeared in two decomposition

products.

Approximately 70

mole percent of CI was

accounted for as HC1 and the balance is incorporated into the polymer. The

inclusion of CI produced a polymer having characteristics of a

hydrogen deficient film similar to the films reported previously on decomposition of acetylene, benzene, and naphthalene in the discharge.

The

color of the transparent methyl chloride film was dark ( brownish-black ) characteristic of hydrogen deficient films in contrast to the light yellow methane film characteristic of hydrogen saturated films. The high electron spin concentration

of the methyl chloride film had also been ob-

served for the hydrogen deficient film from acetylene. In conclusion, the ( H / C ) ratio of a parent molecule will not necessarily be a valid guide to predictions of substituted decomposition products.

Extension to other functional groups is expected to

determine

whether this is a general scheme.

Acknowledgments The authors would like to acknowledge the help of R. E . Dessy in obtaining the E S R spectra and the experimental

assistance of R. O .

McGuffin and S. H . W u . The authors wish to thank R. A . Jewell and


27.

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A N D JOHNSTON


331

H . D . Burks of N A S A - Langley Research Center for the thermograms and the infrared spectra.


Literature Cited (1) Borisova, Ε. N . , Eremin, Ε. N . , Zh. Fiz. Khim. 40, 2366 (1966). (2) Haslam, J., Willis, Η. Α., "Identification and Analysis of Plastics," p. 409, D. van Nostrand Co., Princeton, N . J., 1965. (3) Jesch, K., Bloor, J. E . , Kronick, P. L . , J. Poly. Sci.,A-14, 1487 (1966). (4) Jones, R . N . , Sandorfy, C., "Chemical Applications of Spectroscopy," Chap. 4, Vol. IX of "Techniques of Organic Chemistry," A. Weissberger, E d . , Interscience, New York, Ν. Y., 1956. (5) Kraaijveld, H . J., Waterman, H . I., Brennstoff-Chem. 42, 369 (1961). (6) McCarthy, R . L., J. Chem. Phys. 22, 1360 (1954). (7) Mignonac, G . , Miquel, R . , Lecouls, H . , Bull. Soc. Chim. 1966, 2161. (8) Nakanishi, K., "Infrared Absorption Spectroscopy—Practical," HoldenDay, San Francisco, Calif., 1962. (9) Ponnamperuma, C., Woeller, F., Nature 203, 272 (1964). (10) Schuler, H . , Prchal, K., Kloppenburg, Ε., Z . Naturforsch. 15a, 308 (1960). (11) Stille, J. K., Sung, R . L., van der Kooi, J., J. Org. Chem. 30, 3116 (1965). (12) Streitwieser, Jr., Α., Ward, H . R . , J. Am. Chem. Soc. 85, 539 (1963). (13) Vastola, F. J., Wightman, J. P., J. Appl. Chem. 14, 69 (1964). (14) Williams, T., Hayes, M . W., Nature 209, 769 (1966). RECEIVED

May 8,

1967.


The Decomposition of Methane and Methyl Chloride in a Microwave

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