NICKEL CATALYSTS IN ACETYLENE AND CARBON MONOXIDE CHEMISTRY CLYDE MCKINLEY Genersl Aniline Works, Grasselli,
This paper summarizes many important applications of nickel as a catalyst in both acetylene and carbon monoxide chemistry. In certain salts, such as nickel cyanide, nickel serves uniquely in the polymerization of acetylene to form cyclo-octatetraene, a combination of four molecules of acetylene. Other nickel catalysts will effect the combination of three acetylenic molecules to form a benzene or substituted benzene structure. Examples mentioned are the formation of trimethylol- and hexamethylolbenzene, benzene, and styrene. Nickel serves, not uniquely but as do other strong hydrogenation catalysts, in the reduction of double and triple bonds. Bivalent nickel salts catalyze the reaction of acetylene and also of olefins with carbon monoxide and various reactive hydrogen compounds, such as alcohols, mercaptans, and water. Suggestions on the nature of the catalytic processes involved are made in attempting to rationalize in these fields.
ICKEL has been truly a philosopher's stone in the great catalytic developments made in the past 15 years in the fields of acetylene and carbon monoxide chemistry. Many remarkable discoveries have been made in these two fields, discoveries which rest completely or predominantly upon the role of the catalyst, nickel. Among the nickel-catalyzed reactions mentioned herein are those producing cyclo-octatetraene, trimethylolbenzene, butanediol, ethyl acrylate, and adipic acid. NICKEL I N ACETYLENE CHEMISTRY
Cyclo-octatetraene. One of the most intriguing developments in t h e acetylene field was the discovery in 1940 by Reppe (8), that in the presence of nickel cyanide, acetylene could be polymerized t o cyclo-octatetraene and other polyolefins. I n Figure 1are listed the materials which have been isolated from this reaction. The golden yellow cyclo-octatetraene predominates, with other polyolefins appearing in relatively smaller amounts. l-~henyl-l,3butadiene, vinylcyclo-octatetraene, azulene, benzene, styrene, and naphthalene have been isolated. In addition t o these a yellow hydrocarbon with the empirical formula CI~HI,has been separated, but the structure has not been established. The cyclization of acetylene to these structures is brought about by introducing acetylene a t high pressure into an anhydrous system containing a nickel salt slurried in a solvent a t a temperature around 100' C. I n a batch reaction the pressure is maintained in the range up t o 250 pounds per square inch by the continuous or intermittent introduction of acetylene to replace that which undergoes reaction. A very useful form of nickel is the cyanide. The nickel cyanide catalyst can be prepared by the addition of hydrogen cyanide to a solution of nickel chloride. The pale blue hydrated nickel cyanide which precipitates is filtered off and converted to the yellow-brown anhydrous salt by heating a t 175" C. Tetrahydrofuran is one of the best solvents for effecting the polymerization. Speculating on the role of the catalyst in the polymerization of May 1952
N. J.
acetylene may lead to useful generalizations. First, i t should be recognized t h a t an unusual catalytic process has taken place. Ordinarily a catalyst serves to bring together two particles in such a manner that an electronic rearrangement can take place. This rearrangement is followed by the escape of the newly formed entity. In the case of the nickel cyanide-catalyzed acetylene polymerization, by far the predominant reaction is the formation of cyclo-octatetraene, for which four molecules of acetylene are required. The nickel catalyst is a veritable octopus for it holds the eight carbons of four acetylene molecules in such a manner that an electronic rearrangement to the cyclo-octatetraene structure can take place. The acetylenes apparently do not escape until all four molecules have reacted. Any explanation to be considered must account for this unique catalytic action. Figure 2 shows two views of the cyclo-octatetraene molecule. The model pictured is the D4 or crown structure, the form most satisfactorily fitting the infrared and Raman evidence ( 4 ) . The model was prepared with the C-H distance of 1.07 A . , the C-C distance of 1.54 A., the C=C distance of 1.34 A,, and theC-C-C angle of 127". This is the molecule that the nickel cyanide catalyst forms by effecting the combination of four acetylene molecules. It is most probable that the actual working catalyst in the polymerization reaction is an unstable complex of acetylene and nickel which is generated by the action of acetylene upon the nickel compounds present in the acetylene solution a t reaction conditions. This view is supported by the observation that only those nickel compounds in which the nickel is weakly linked to the anion are catalysts. Kickel halides and other similar salts are not catalysts. Nickel compounds in which the nickel is bound in the form of a stable complex-as in nickel phthalocyanine-are also unsuitable as catalysts. Nickel salts in which the nickel is bound very loosely, as in the case of nickel cyanide, nickel thiocyanate, and the nickel salts of acetoacetic ester and acetylatetone, are effective catalysts ( 2 ) . Anhydrous nickel cyanide slurried in tetrahydrofuran will effect cyclo-octatetraene formation. An explanation of the catalytic action may lie in the complex-forming ability of the nickel cyanide. When anhydrous nickel cyanide is placed in tetrahydrofuran it is logical to expect that coordination complexes of nickel and tetrahydrofuran will be formed. Anhydrous nickel cyanide, based on paramagnetic susceptibility measurements, is believed t o exist in a polymeric structure with the nickel atoms forming square covalent bonds with the carbon and nitrogen atoms of the qyanide groups ( 5 ) . The structure is probably something like that pictured in Figure 3. When placed in dry tetrahydrofuran it is probable that to some extent the nickel cyanide polymer will break apart as indicated by the diagonal lines to form complexes with the tetrahydrofuran. The nickel of these groups should coordinate four tetrahydrofuran molecules t o give a complex such as suggested in Figure 4. Upon introduction of acetylene into the system the tetrahydrofuran molecules may be preferentially replaced by acetylene t o give the tetraacetylene-dicyanidenickel
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NICKEL-c ATALYSTS complex of Figure 5. The acetylene molecules are pictured sharing the carbon-carbon triple bond electrons with the nickel atom. A top view, Figure 6, of this complex and of the cyclo-octatetraene molecule shows t h a t the desired cyclo-octatetraene structure has been closely approached. A nickel atom, which would be tangent
Cyclo-octatetraene
H H H C=C-C=CHz
Yellov
cia-l-Pheny1-1,3-butadiene Colorless
K
c1242
Vinyloyclo-ootatetraene
Orange
4zulene
Blue
?
Yellow
Benzene
Colorless
Styrene
Colorless
Naphthalene
Colorless
CHZCH,
Figure 1.
Products from Nickel Cyanide-Catalyzed Polymerization
Acetylene
to the carbons of the cyclo-octatetraene molecule, has a radius of 1.06 A. This appears plausible in view of the 0.78 A. radius of Ni++, the 1.03 A. radius of N i f , and the 1.24 A. radius of Nio. The complex is pictured with four acetylene molecules tangent t o a nickel atom, having a radius of 1.06 A., and with the carboncarbon distances in the acetylene molecule corresponding to the carbon double bond distances, in order t o allow somewhat for the decrease in triple bond character of the acetylene carbons caused by the complex formation. Because of the suitable proximity of the carbon atoms of adjacent acetylene molecules, it is possible that the triple bond electrons that are shared with the nickel atom rearrangewith the formation of the necessary singleC-C covalent bonds. This is suggested as the mechanism for cyclo-octatetraene formation. Considerable other circumstantial evidence supports such a picture. One bit of evidence is the poisoning effect of water and of formaldehyde. Traces of water or formaldehyde inhibit completely the reaction. It seems plausible that these materials have coordinated with nickel more strongly than does acetylene, thus preventing the acetylene molecules from attaining the cyclooctatetraene configuration. It is possible that water acts as a poison in two ways. First it may replace some of the groups in the octahedral coordination
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field of the nickel ion, thus preventing the coordination of four acetylene molecules in the mannernecessaryfor cyclo-octatetraene formation. I n addition to this possibility the water might cause a substantial portion of the nickel ions t o change their mode of coordination to tetrahedral. This possibility is suggested by measurement of the paramagnetic susceptibility of hydrated nickel cyanide containing between two and four molecules of water. These measurements show t h a t about half of the nickel is present as the square planar complex, Ni(CN)4--, and about half as the tetrahedral complex with water Ni(H20), (6). Cope ( 1 ) reported the preparation of substituted cyclo-octatetraene by the use of dimethylacetylene. There are in this case no reactive hydrogens. The fact that polymerization did take place is very good evidence that the acetylenic hydrogens are not involved in the mechanism of the cyclo-octatetraene reaction. In a negative way it supports the conclusion t h a t the acetvlene molecule is bound t o the nickel through the acetylenic triple bond. The relative strengths of the covalent bond energies of several materials which coordinate with nickel may be inferred from their action during catalysis. A listing in decreasing order of bond strength is given in the following table: Cyanide ion Water Ammonia Triphenylphosphine Acetylene Carbon monoxide Tetrahydrofuran, acetone, a n d acetonylacetone
Accepting the view that the nickel cyanide polymer is broken a t the nitrogen-nickel bond but not a t the carbon-nickel bond, the cyanide group heads the list. Water is a poison and so must appear high on the list. Ammonia, triphenylphosphine, acetylene, and carbon monoxide are approximately in that order; the solvents appear a t the foot of the list. This view of the cyclization indicates the necessity of using a solvent which will coordinate with nickel but will coordinate more weakly than does acetylene. The solvents, tetrahydrofuran, acetone, and acetonylacetone, work well. Trimerization of Acetylene, Propynol, Butynediol. I n the ease of the cyclo-octatetraene synthesis it was suggested that nickel was a catalyst because it had the property of bonding covalently four acetylene molecules in a plane about the nickel atom and holding them at such a distance that the cyclo-octatetraene structure could form. In considering the role of nickel in the trimerieation of propynol and butynediol ( S ) , it is suggested that the trimeriza-
Figure 2.
Scalar Views of Cyclo-octatetraene
tion of these conipounds is effected by a triphenylphosphinenickel carbonyl catalyst which coordinates acetylenic molecules favorably in three of the four tetrahedral positions in the coordination sphere of the nickel atom ( 4 ) . Nickel carbonyl is tetrahedral and melts at -25' C. Triphenylphosphine in an alcoholic solution a t room temperature will replace one carbon monoxide molecule from the tetrahedral structure to form the nickel complex containing one triphenyl-
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NICK E Lphosphine and three carbon monoxide molecules. This complex melts a t 123" C. If excess triphenylphosphine is added and the temperature raised to about 50" C. the compound containing two carbon monoxide groups and two triphenylphosphine groups is formed almost quantitatively. This nickel complex melts a little above 190" C. (a).
I \ \
C
C
CI
C
1I C
already nearly attained. The carbons are close enough together that the electron shift from the linkage a t the triple bond to form the new bond between adjacent carbons of the two adsorbed molecules can be made through the action of the catalyst. Hydrogenation. Nickel as a hydrogenation catalyst does not fill the unique role which it does in the cyelo-octatetraene and trimerization reactions. I n Reppe chemistry hydrogenation, nickel serves as do the other strong catalysts, platinum or palladium. Equations 1 through 4 illustrate some of the reductions which may be effected through the use of nickel as a catalyst. HOCH~CECCH~OH3- 2H1+
-Ni-CfW
I-C~N-MI-CsN-Ni-
j.-c:lj
I
N
N
N
N
111
IlCl
Ill
111
Ill
C
C
C
HWCCHzOH
I
N
C
cATALYSTS
HOCHzCHzCHzCH~OH (1)
+ 2Hz +CHsCHzCHzOH
OH
OH
+ HZ +CH3CHdHCH3 HOCHZCSXXECCH~OH + 4Hz + HCIC&HCHa
HOCH&HzCHzCHzCHzCHzOH
Figure 3.
Anhydrous Nickel Cyanide
If a mixture of these complexcs is placed in an anhydrous solvent such as benzene or tetrahydrofuran, and then refluxed while adding propargyl alcohol stepwise, the propargyl alcohol will form about a 50-50 mixture of 1,3,5-and of 1,2,4-trimethylolbenzene. The observed 1 to 1 ratio of the two isomers suggests that steric hindrance is a factor in the complex formation which leads t o the trimerization. Random arrangement of the complex propynol molecules should result in a 3 to 1 ratio of the 1,2,4- to the 1,3,5isomer. With acetylene rather than propynol, the same catalyst has been reported by Reppe to yield 88% benzene and 12% styrene (6). The catalyst for these reactions probably is triphenylphosphine-nickel tricarbonyl. The tetracarbonyl nickel complex will have been distorted by the replacement of one carbon monoxide molecule by triphenylphosphine. This distortion may render the remaining three carbon monoxide groups particularly susceptible to replacement by carbon-carbon triple bond groupings. This catalyst will work only in a n anhydrous system. If water is present it apparently enters the coordination sphere and prevents the formation of the necessary configuration of triple bonds. I n an aqueous system, Raney nickel will trimerize propynol or butynediol. The same configuration is brought about with Raney nickel as with the triphenylphosphine-nickel carbonyl catalyst, but here the explanation of the catalysis must be different since the reaction is heterogeneous rather than homogeneous. Figure 7 shows on the left the 1,3,5-trimethylolbenzenemolecule and on the right a possible adsorption arrangement that would lead to trimerization. The nickel lattice background is drawn with nickel-nickel spacings of 2.49 A. The nickel atoms indicated b y the corners of the triangles will have unsatisfied surface forces. This figure suggests that the acetylenic group of the propynol molecule shares electrons with the nickel atom above which it is situated, and, in addition, may form weak bonds with the two nickel atoms at either end, thus effecting some degree of orientation. Since the adsorption would result in some lowering of the electron density between the acetylenic carbons the figure is drawn with the double bond carbon distance of 1.34 A. used for the unsaturated carbons of the propynol molecule rather than the triple bond distance of 1.20A. If three propynols are thus adsorbed on the nickel lattice, the trimethylolbenzene structure is May 1952
(2)
(3) (4)
The outstanding German wartime use was the reduction of butynediol in aqueous solution to butanediol, a n intermediate in the synthesis of butadiene from acetylene. The particular catalyst used for this reduction was a silica gel-supported nickel-coppermanganese catalyst, with nickel being the major catalytic e l e ment (7). Nickel serves the same purpose in present production of butanediol, an intermediate in the production of polyvinylpyrrolidone. With suitable conditions and the proper catalyst the hydrogenation of propynol may be selectively carried out t o allyl alcohol, propionaldehyde, acrolein, or n-propanol. With a nickel catalyst the reduction is complete t o n-propanol. In a like manner 3-butyne-2-01 yields 2-butanol. Nickel also effectively r e
Figure 4.
Proposed Tetrahy drofuran-Nickel Cyanide Coordination Complex
duces the diynediols to the corresponding dkanediol. Equation 4 pictures the reduction of the diynediol, which is obtained by oxidative coupling of propynol in the presence of cuprous chloride catalyst. These reductions are carried out under a pressure of 100 to 400 atmospheres and with the relatively mild temperature of 50"to 200" c. NICKEL IN C A R B O N M O N O X I D E CHEMISTRY
I n tho catalysis of reactions involving carbon the case in many acetylene reactions, nickel Equations 5 through 7 show three general cases oxide reactions which are catalyzed by nickel. -S-, or -NH--. tions, X represents -&,
co + H
+
monoxide, aa is also is unique. of carbon monI n these equa0 II
~ C H RXH -+H ~ C = C H ~ X R 0
co + H ~ C = C H ~+ RXH --t C H ~ C H ~ C ~ R
INDUSTRIAL AND ENGINEERING CHEMISTRY
(5)
(6)
997
NICKEL-CATALYSTS H&-CH2 2CO
I
+ II,C
1
CH,
The action of this catalyst may be understood by considering the nature of the nickel complex. Nickel carbonyl, the carbon monoxide transferring agent, is tetrahedral. The active catalysts are those complexes of nickel in the presence of the halides or of triphenylphosphine. These are strong coordinating agents
+ HOH +
'0'
0
0
'I
I/
HO-CCH~CH~CHLCHG-OH( 7 ) Carbon monoxide reacts with acetylenic materials and a reactive hydrogen compound t o yield acrylic acid derivatives. Carbon monoxide also reacts with olefins and a reactive hydrogen compound to produce saturated carboxylic acid derivatives, and with aliphatic and cyclic ethers. The reaction illustrated in Equation 7 is that for carbon monoside with tetrahydrofuran a n d water t o yield adipic acid.
Figure 6.
Figure 5 ,
Proposed Acetylene-Nickel Cyanide Coordination Complex
Acrylic Acid Derivatives. Various reactions of carbon monoxide and acetylene with reactive hydrogen compounds are shown in Equations 8 through 11.
CO
//O + H C s C H + HOH +H~CZCHC-OH
(8)
CO
No + H C S C H + XOH +HaC=CHC-OE
(9)
for nickel and will have a strong tendency to enter into the tetrahedral coordination sphere of the nickel. It'is reasonable t,o expect that a halide or a triphenylphosphine replacing one of the four coordinated carbon monoxides will distort the structure more or less and shift the remaining t'hree positions with respect to each other. ' If an acetylene molecule now replaces one of the remaining carbon monoxide molecules, it may be held in a position suitable for combination t o form the intermediate adduct of carbon monoxide and acetylene, which immediately reacts with an active hydrogen compound from the solvent. Carboxylic Acid Derivatives. The reaction of an olefin with carbon mouoxide and a reactive hydrogen compound is very similar to the same reaction with acetylene in place of the olefin. The same catalysts are used and it appears reasonable that they serve in the same way in the two cases. The temperature and pressures used are somewhat higher but otherwise the carboxylic acid synthesis xith olefins is the same as the acrylic acid eynt.hesis with acetylene. The react,ions of Equations 12 through 15 are carried odt in the neighborhood of 200 at,mospheres of carbon monoxide.
CO
CO
CO
+ H C E C H + IZSH --+
//O H,C=CHC--SR
No
+ H C z C H + H2SR +HZC=CHC--PI"X
P + HjC=CHR + HOH ---+RCH,CH?C-OH
01
C&
(10)
(11)
With water acrylic acid is obtained, with alcohol the corresponding acrylic ester, with a mercaptan the thioester, and lvith an amine the acrylamide ( 7 ) . For these reactions, nickel carbonyl can be used in a stoichiometric manner as the carbon monoxide transferring agent. The reaction of ethanol, acetylene, and nickel carbonyl takes place in the presence of an acid a t 40" C. to produce ethyl acrylate. The nickel salt produced during the reaction may be converted separately to the carbonyl and so recovered for re-use. This process is not considered catalytic since the carbonyl charged is used up during the reaction. In a truly catalytic process, the carbon monoxide which is used up from the carbonyl must be replaced. The reaction conditions, therefore, must be suitable for the formation of nickel carbonyl. h-ickel carbonyl alone does not effectively serve as a catalyst but a mixture of nickel carbonyl-nickel halide works well. Nickel halide-carbonyltriphenylphosphine complexes also are catalysts. These catalysts are used a t about 150" C. and a t a pressure of 30 atmospheres with acetylene and carbon monoxide introduced in a 1 to 1 ratio.
998
Comparison of Proposed Cyclo-octatetraene and Acetylene-Nickel Cyanide Complex
P
XCHC-OH
+ RqC=CHR + R'OH +RCHqCHZC-OR'
(12)
I/
CO
01
CII, I NO RCHC-OR' CO
+ H?C=CIIR + R'SH
(18)
//O
+RCH2CH2C-StZ'
01'
CH, I //O XCHC-SRI
(14)
CHI
CO
H?C=CHR
+ 132 +RCH,CH,CI-IO
I
01
RCIICHO
(15)
The carbon monoxide can add to either side of the olefin doublr bond, so that with the olefinic group in the end position as pirtured, either the n-carboxylic or methyl carboxylia derivative is produced. With water carboxylic acid is the product, with alcohol the ester, and with mercaptan the thioester ( 2 ) . I n the special case, known as the OXO process, where the active hydrogen com-
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Vol. 44, No. 5
NICKEL-CATALYSTSpound is hydrogen itself, cobalt carbonyl rather than nickel carbonyl is the catalyst. For these reactions, as for the acrylate synthesis with acetylene, the catalyst appears t o be a complex such as nickel iodotricarbonyl. Nickel carbonyl alone is ineffectual but in the presence of halides its reactivity increases as the series progresses from fluoride t o iodide. This supports the view that the active catalyst is a deformed complex containing one or two halides, because the largest deformation of the original carbonyl tetrahedral structure has been effected with t h e largest halide ions as they are the strongest coordinators. It is most probable that the nickel halide complex, tetraiodo nickel, is planar. If this is the case, the
The reactants, whose electronic formulas are listed below, have in common the ability t o share electrons with the nickel atom which needs them to complete its coordination sphere-the acetylene through its triple bond, the ethylene through its double bond, and the ether through its oxygen atom.
H
H:C: ::C:H Acetylene
H
H H
*.
*.
H : C : :C:H Ethylene
H
..O ..
H
This similarity must explain the success of the nickel iodocarbonyl catalyst in effecting the reaction of carbon monoxide with these three widely different materials. ACKNOWLEDGMENT
The helpful discussions with J. P. Brusie, L. E. Craig, R. E. Dial, D. L. Fuller, S.T. Gross, and H. B. Hass of General Aniline & Film Corp. and with J. C. Bailor of the University of Illinois are gratefully acknowledged. /
,
’
L ,
j d
Figure
7.
Adsorption of Propynol
Y
\
’
on Nickel
entrance of one or two iodine atoms into a tetrahedral nickel carbonyl complex will greatly distort it. It seems likely that this distorted hybrid structure, neither planar nor tetrahedral, is the catalyst for these syntheses. Adipic Acid. Carbon monoxide, by means of the same catalysts used for the acrylic and carboxylic acid syntheses, will also react with ethers. The synthesis t h a t Reppe was especially interested in was that of adipic acid. Tetrahydrofuran was reacted with carbon monoxide and water at the somewhat more drastic conditions of 270” C. and 200 atmospheres of carbon monoxide to produce adipic acid and valerolactone and valeric acid as by-products (6).
LITERATURE CITED
(1) Cope, paper presented before the 12th Natl. Organic Chemistiy Symposium, Division of Organic Chemistry, A 4 ~ CHCM. . SOC.,
Denver, Colo., June 1951. (2) Copenhaver and Bigelow, “Acetylene and Carbon Monoxide Chemistry,” New York, Reinhold Publishing Corp., 1949. (3) Xleinschmidt, R. F., U. S. Patent 2,542,417 (1951). (4) Lippincott, E. R., Lord, R. C., and McDonald, R. S.,tech. repts. 1 and 2 of Contract N5,,1-07810, Spectroscopy Laboratoxy, Mass. Inst. Technol., 1950. ( 5 ) Pauling “Nature of the Chemical Bond,” Ithaca, N. Y . ,Cornell University Press, 1948. ( 6 ) Reppe, J. W., “Chemie und Technik der Acetylen-DruckReactionen,” Weinheim, Germany, Verlag Chemie, G.m.b.H., 1951. (7) Reppe, J. W., PB Rept. 18852-s (1949). (8) Reppe, Schlichting, Klager, and Topel, Ann., 560, 1 (1948). RECEIVED for review October 17, 1961. ACCEPTEDN a r c h 8, 1952.
NICKEL CATALYSTS FOR HYDROCARBON-STEAM REACTION M. R. ARNOLD, KENTON ATWOOD, H. M. BAUGH, AND H. D. SMYSER The Girdkr Corp., Louisville, K y . T h e work reported here is part of a program of detailed laboratory investigation on the properties of catalysts for the reaction of hydrocarbons with steam. The program, which is being continued, will include investigation of the surface area, porosity, pore size, and particle size of the catalysts. Most of the experimental work relating to the life and activity of the catalysts was conducted in semimicro reactors. Results show that, in general, the catalysts with higher nickel contents have higher activities. The exceptions indicate, however, that other factors are important in determining catalyst activity. Rapid inactivation of the commercial catalysts takes place above 1900 F.
HE majority of the many catalysts which have been proposed for the reaction of hydrocarbons with steam have contained nickel as the principal active constituent. Mond and Langer ( 7 ) made hydrogen in 1888 by passing coal gas and steam over re-
T
May 1952
duced nickel or cobalt supported on pumice. Dieffenbach and Moldenhauser ( 3 ) suggested use of wire gauzes of nickel, cobalt, etc., as catalysts in 1909, and Rfittasch and Schneider (6) produced hydrogen (1915) by reacting methane and steam over a nickel catalyst supported on a refractory such as magnesia. I n 1930 the Standard Oil Co. of New Jersey installed a large plant at Bayway, N. J., using nickel catalysts for the methanesteam reaction, A larger plant t o produce hydrogen for t h e hydrogenation of petroleum was started b y the same company in 1931 a t Baton Rouge, La. Plants producing hydrogen for t h e same purpose were installed prior t o 1940 a t Port Arthur, Tex., Richmond, Calif., and Whiting, Ind. (8). I n 1940 the Hercules Powder Co. a t Hercules, Calif., completed a plant t o use natural gas and steam as a source of hydrogen for ammonia synthesis ( 2 ) . By 1946 seven plants with a total capacity of 3 billion cubic feet per month, about three fourths of
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