(Products of Hydrogenation of Carbon Monoxide)
RELATION OF PRODUCT COMPOSITION TO REACTION MECHANISM A. W. WEITKAMP Research Department, Standard Oil Co. (Indiana), Whiting, Znd.
C. G. FRYE Research Department, Stanolind Oil and Gas Co., Tulsa, Okla. nism will include consideration of possible primary products of the synthesis as well as products of secondary conversions. Three aspects of the mechanism of formation of the primary products will be discussed: the initiation of chain formation, chain extension, and termination.
Evaluation of proposed reaction mechanisms for the hydrocarbon synthesis process was prompted by the availability of new product composition data. The process consists of several, perhaps many, competing and concurrent reactions. The chemical steps in the formation of primary product molecules are initiation, chain extension, and termination. The initiator of chain growth may be an adsorbed radical-possibly formed by hydrogenation of surface carbide. The adsorbed radical is enlarged by repeated addition and hydrogenation of adsorbed carbon monoxide molecules. The termination reaction produces mainly alcohols or aldehydes and olefins. Competition between growth and termination is responsible for the exponential decrease in yields of successive carbon-number fractions. The distributions of normal and branched isomers are explained on the assumption of a statistical distribution of growth on the two carbon atoms last added to the adsorbed radicals. Secondary reactions convert part of the original olefins to paraffins and may convert part of the alcohols or aldehydes to acids and ketones.
PRIMARY PRODUCTS
Alcohols, olefins, and paraffins have been suggested ( 1 1 , 23) as possible primary products of the Fischer-Tropsch process. Aldehydes, ketones, acids, and aromatic hydrocarbons are present and must also be considered. Two techniques for the identification of primary products are available. One technique is to find the products whose relative yields do not approach zero as the conversion of carbon monoxide is progressively reduced. This method is of limited applicability to heterogeneous reaction systems because mass transfer of reactants to the catalyst or of products from the catalyst may be a limiting factor at high space velocities. The other technique is to find whether possible secondary reactions are operative and, from known thermodynamic equilibria, to determine which classes of components are reactants and which are products. Possible reactions of the classes of components with hydrogen, water, and carbon dioxide are exemplified in Table I. Secondary reactions involving carbon monoxide are not included because of the close approach to the water gas shift equilibrium during synthesis. Free energies of formation, from which equilibrium constants were estimated, and partial pressures at the reactor exit, from which the degree of approach to equilibrium was calculated, are presented in Table 11. It is apparent from Table I that possible secondary reactions involving hydrocarbons are far from equilibrium. I n this way it has been established that 1-olefins are partially converted, in secondary reactions, to paraffins and %olefins. Reactions involving oxygenated compounds, except for the decarboxylation of acids to ketones, are so close to equilibrium that a choice between alcohols, aldehydes, and acids as primary products could not be made.
R
1
EACTIONS of carbon monoxide with hydrogen a t various temperatures and pressures over such different catalysts as iron, cobalt, nickel, ruthenium, zinc oxide, and thoria are unlikely to proceed by identical mechanisms. On the other hand, the mechanisms of the several reactions operative in the hydrocarbon synthesis process probably have some valid extensions t o related processes. It is a question whether differences in product composition associated with different catalyst or conditions reflect different mechanisms or merely reflect changes in the relative rates of competing reactions. An important aspect of the problem is the determination of the primary and secondary products and the consideration of the secondary conversions which the primary products may undergo. Reviews dealing with various phases of the reaction mechanism have been prepared by Kolbel ( 1 1 ) and by Storch (23). Much of the literature is about the mechanism of synthesis over cobalt catalysts. Olefins and alcohols are mentioned frequently as likely primary prodTABLE I. EQUILIBRIA AMOKG THE PRORUCTS OF SYNTHESIS ucts of synthesis with either cobalt or iron cataTemp., AF Observed lysts. Anderson (6)Qbtained experimental eviReaction K. (Reaction) Kp Ratioa dence indicating that olefins and some paraffins CzHa + HI CaHa 600 . . . . . 2 . 7 8 X 1 0 6 0.5-m may be primary products over cobalt. The role 1-Butene 4- Hz e butane 600 . . . . . 1 . 2 6 X 10' 0.025-0.75 I-Butene 2-butene 600 ,.... 3.73 0.11-2.0 of olefins as intermediates in the synthesis was 600 ..... 1.70 1.0-1.5 cis-2-*utene trans-z-butene 1-Butene % isobutene - 6 0 0 ..... 1.22 0.10-0.12 studied by Smith ( 2 1 ) and Regier (19). The par617 -3.76 22 28 ticipation of alcohols as intermediates was investiCzHaOH + Hzo CaH+ ++ HzO HZ 617 -8.00 680 40 (CHs)2CHOH % CaHs HzO 617 -7.84 600 1000 gated by Kummer ( 1 4 ) . Dehydration of alcohols 617 +0.28 0.8 1.07 CzHaOH CHsCHO Hn t o olefins over a cobalt catalyst was demonstrated CHsCHO + HzO CHsCOOH + Hz 617 -2.94 11.0 7.4 bS, G ~ ](8). ] Fischer's original postulate of the in2CHaCOOH (CIIdZCO 4- coz -k HzO 617 - 11.23 9550 440 ~
+
termediacy of metal carbides ( 7 ) was studied by Kummer (") with the aid Of radioactive monoxide. The present discussion of the reaction mecha-
+
+
~
+
(CHs)zCO Hz (CI-IdpCHOH 617 +4.27 0.031 0.020 a Hydrocarbon data were obtained on product f r o m reactor A , operated at about 600' K.: equilibrium constants are from Kilpstrick ( 1 0 ) . Oxygenated compound data were obtained on product from reactor D, operated at about 617O K.; equilibrium constants were calculated from data in Table 11.
'
363
INDUSTRIAL AND ENGINEERING CHEMISTRY
364
TABLE 11. FREEENERGIES OF FORMATION AND PARTIAL PRESSVRES OF SYNTHESIS PRODUCTS Partial Pressure, AFJ a t 617' K., Kcal./Mole Atm.a 0.00 ( 1 ) 6.67 Hz co -39.73 ( 1 ) 0.344 Ha0 -50.93 ( I ) 3.40 COe -94.42 ( 1 ) 4.98 + 2 0 . 2 2 (1) 0.65 C2H4 CaHs 4-27.39 (1) 1.16 -22.716 0.056 CPHSOH CHsCHO -22.33 (18) 0,009 -72.6 (f7) 0.034 CHaCOOH -14.01 0.004 (CHx)iCHOH (CHdiCO - 18,28d 0.030 a At the reactor exit in run D-3. Calculated from AFf for ethylene and the free energy change on hydration (16). C Calculated from AFf for propylene and the free energy change on hydration (726). d Calculated from AFf for %propanol and the free energy change o n dehydrogenation (is). Substance
*
OLEFINS. 1-Olefins are the dominant hydrocarbon class in the synthesis product. Olefins have been suggested as possible intermediates in the synthesis, and a growth mechanism resembling the oxo reaction has been proposed ( 3 3 ) whereby olefins, formed by dehydration of alcohols, react with carbon monoxide and hydrogen to form alcohols with one additional carbon atom. Smith ( 2 1 ) investigated the participation of added ethylene in the formation of larger molecules; such participation seemed to occur over cobalt catalysts, but inconclusive results were obtained with iron catalysts. Regier's experiments (19) with added radioactive 1-butene under hydrocarbon synthesis conditions failed t o show any chain lengthening. It appears that olefins are not intermediates in chain formation over iron catalysts. Equilibrium considerations allow the formation of 1-olefins from 1-alcohols but not from paraffins or internal olefins; however, the formation of paraffins and internal olefins from l-olefins is possible. It has been shown by addition of olefins to the feed gas t o otherwise normally functioning reactors that hydrogenation and double bond isomerization are operative reactions ; partial hydrogenation of added ethylene has been observed in these laboratories, and formation of n-butane and 2-butene from added radioactive 1-butene has been demonstrated (19). Similarly, the formation of n-butane and 2-butene from the 1-butene which is formed in the fluid bed synthesis reaction was suggested by the correlation of the 1-butene, 2-butene, and n-butane contents of numerous product samples (66). The observation that the extent of these reactions was little affected by doubling or quadrupling residence time in the reactor by recycling precludes a rigorous kinetic interpretation of these data and suggests that mass transfer is a controlling phenomenon. However, isomerization and hydrogenation appear to be competitive reactions and extrapolation t o zero isomerization suggests that some n-butane may be formed by a reaction other than hydrogenation of 1butene, although the latter conclusion may not be justified because of the mass transfer limitation. The identification of paraffin as a primary product from data obtained with a fixed bed reactor by extrapolation of selectivit!? data to zero bed depth (9)may be questioned on the same basis although it is possible that mass transfer is less limiting in fixed bed reactors. Double bond isomerization over metal catalysts has long been known to require the presence of hydrogen. Based on this fact, and on the occurrence of hydrogen exchange during isomerisation, Twigg ( 3 4 ) proposed an "associative" mechanism Reaction is pictured as opening the double bond by the addition of an atom of hydrogen t o one of the doubly bonded carbon atoms. The double bond is subsequentlv reformed in the same or adjacent position by loss of hydrogen from the same or an adjacent carbon atom. Thus, I-butene would yield either a primary alkyl radical from which only 1-butene could be formed, or a secondary alkyl radical from which either 1-butene or 2-butene could be formed. In the event that hydrogen is lost from a nonterminal
Vol. 45, No. 2
carbon, either the cis or trans configuration of 2-butene would result. That the two configurations are equally probable was concluded from experiments in which the ratio of 2-butene to 1-butene u a s small ( 2 5 ) . I n other experiments in which the ratio of 2-butene t o 1-butene approached equilibrium, the cis to trans ratio also approached equilibrium. This suggests that equilibration of the cis and trans forms occurs in a reaction separate from the initial double bond shift. A 4 ~ ~ Thermodynamic o ~ ~ ~ ~data . for the dehydration of ethanol t o ethylene, presented in Tables I and 11, show that such a reaction is far from equilibrium. Evidence against the operability of the direct dehydration reaction over iron catalysts is provided by examination of the results of Kummer's experimental addition of radioactive ethanol to the feed gas entering a small fixed bed reactor ( 1 4 ) . Had the tagged ethanol been dehydrated directly, the ethylene component of the product would have contained much of the radioactive carbon. Instead, one of the reactions of the ethanol was participation with carbon monoxide and hydrogen in the formation of longer-chain hydrocarbons. The radioactive carbon was evenly distributed on a molal basis in the two-carbon to 10-carbon fractions, except for a slightly lower level in the ethylene component. Experiments of this nature should not be construed as indicating that alcohols produced from carbon monoxide and hydrogen in a normally functioning synthesis reactor are extensively consumed in the formation of larger molecules. Once desorption has taken place, mass transfer, especially in fluidized systems, may limit readsorption of alcohols, as was the case with olefins. Different behavior of the alcohols over cobalt catalysts is suggested by experiments recently reported by Gall (8). Alcohok, in the presence of hydrogen but absence of carbon monoxide, were dehydrated to olefins a t 150" C. Ethanol was appreciably less reactive than the other alcohols. From the evidence that secondary conversions of alcohols did not produce 1-olefins, it is concluded that 1-olefins are primary products. OTHER OXYGENATED COMPOUNDS.In considering whether alcohols might be primary products, their relation to other oxygenated compounds must be examined. If the last four reactions listed in Table I are operative, ethanol, acetaldehyde, and acetic acid are found in proportions too close to equilibrium with each other to indicate which ones are primary products. These thermodynamic data also indicate that acetic acid is not formed from acetone Within experimental error, 2-propanol is in equilibrium with acetone. Experimental evidence of the operability of the foregoing reactions was obtained in the present work under conditions which differed appreciably from the usual synthesis conditions. The decomposition of n-butanol, chiefly to butyraldehyde and hydrogen, occurred over reduced iron catalvst a t 400" C. Smaller but significant yields of butyric acid and of a ketone, presumably 4-heptanonej were obtained; saturated and unsaturated hydrocarbons, carbon dioxide, carbon monoxide, and water were detected The presence of butyric acid among the products of decomposition of butanol provided circumstantial evidence of the secondary oxidation of butyra1deh.i.de. The presence of the ketone suggested secondary conversion of the butyric acid. More direct evidence for the operability of the reaction of acids to form ketones was obtained by passing a mixture of acetic and butvric acids over an iron catalyst. Products of this reaction included acetone, 2-pentanone, and 4 - h ~ p t a n o n ~ . An indication of a generic relation between acids and ketones is provided by a comparison of the relative vields of the lower acid and ketone homologs. On the assumption that homologous acids are converted a t equal rates it may be concluded that the ratio of Cz to C3 acids would be equal to the corresponding ratio of C3to C4ketones, The observed acid and ketone ratios of 5.0 and 5.2, respectively ( 2 2 ) , are consistent with a mechanism of ketone formation from the corresponding acids.
February 1953
INDUSTRIAL A N D ENGINEERING CHEMISTRY
The operability of the last reaction in Table I-the interconversion of ketones and secondary alcohols-can be assumed from the known operability of the dehydrogenation of alcohols t o aldehydes. From the present data it is not possible to determine which of the equilibrium oxygenated forms is the precursor of the others. However, i t is unlikely that either alcohols or aldehydes are formed from acids. The observation of a much greater incidence of branching in the structures of the higher acids than in the higher alcohols (6) suggests t h a t alcohols are not derived from acids and that a t least part of the acids are not derived directly from alcohols. Additional data on the structure of the higher aldehydes may further clarify the relationships between the different oxygenated compounds. From the present data it is concluded that both alcohols and aldehydes may be primary products. Operative primary and secondary reactions may be summarized as follows: -1-Olefins ---+ 2-Olefins
\J.
Paraffins l-A41cohols% Aldehydes % Acid
Ketones
CHAIN EXTENSION
Explanations of the shape of the carbon-number yield curve based on equilibrium of structures already formed (16) or on extensive degradation of very large molecules (6) are unattractive for iron catalysts because of the dearth of evidence for cracking reactions. I n particular, tagged molecules, such as ethanol (16)or 1-butene (19),have suffered negligible cracking. Herington (9) visualized synthesis over cobalt catalyst as a growth process in which carbon atoms were added in a one-by-one sequence. A definite probability of desorption following each addition accounted for the exponential decrease of hydrocarbon yield with increasing carbon number, except for anomalously low yields a t CZto C5 hydrocarbons. Because most molecules produced over iron catalysts contain a single terminal functional group, it has been generally assumed that growth occurs a t only one end of the molecule. Additional support of the one-ended nature of growth is given by the structures of the three-carbon hydrocarbons obtained from tagged ethanol (16). It was shown that 88 to 90% of the entering carbons were joined t o the carbons originally bearing the hydroxyls. Furthermore, distributions of aliphatic isomers are more accurately predictable on a statistical basis when one-end addition is assumed (66).
TABLE
111. DISTRIBUTIONO F HYDROCARBONS AND CHEMICALS Run D - l
Component Carbon dioxide Water-soluble chemicals Acetaldehyde Propionaldehyde 2-Propanone Methanol Butyraldehyde 2-Butanone Ethanol 2-Pentanone Propanol Butanol Acetic acid Propionic acid Butyric acid Wash from oil stream Oil-soluble chemicals (Ca+) Gaseous hydrocarbons Methane Ethylene Ethane Propy 1ene Propane Butenes Butanes Liquid Hydrocarbons ( C s f ) Total
yo of Converted CO 0 52 0 18 0 96 0 04 0 28 0 34 3 50
16 1 10 2
TABLE IV.
365
DISTRIBUTION OF HYDROCARBONS AND CHEMICALS (EXCEPTCOz) BY CARBON NUMBER Run D-1 Moles/100 Moles of GO Converted HydroHydrocarbons carbons and chemicals 9 0 9 04 4 3 7 85 4 2 4 92 2 6 2 92 3 5 4.62
yo of CO Converted Carbon Atoms
Hydrocarbons
Chemicals
9 0 8 6 12.6 10 4 25.1
0 04 7 10 2 16 1 30 8 0
1
2 3 4 5+
The anomalously low yields of C2 hydrocarbons occurring in products from synthesis with iron catalysts is analogous to the low yields of Cs to C5 hydrocarbons which occur in producta from cobalt catalysts. However, in the case of synthesis with fluid bed iron catalyst, a compensatory yield of CZoxygenated compounds is obtained. Product obtained in a typical fluid bed synSecondary thesis experiment was analyzed for hydrocarAlcohols bons and oxygenated compounds, and analyses representing 28 test periods from this experiment were averaged to obtain the results in Table 111. Distributions of hydrocarbons and oxygenated compounds by carbon number are presented in Table IV. The oxygenated compounds were allocated to corresponding carbon-number fractions, except that the ketones were allocated on the assumption that each molecule of ketone was derived from two molecules of acid. Yields of
-
n
10
I
W
i-
a: W >
z
0 0
0
u
HYDROCARBONS CHEMICALS
v)
+
WI
0
H
0
0
0.5
I
I
I
I
I
I
u
I
2
3
4
5
6
7
I
CARBON NUMBER
Figure 1. Distribution of Hydrocarbons and Chemicals Run D-1
hydrocarbons and yields of hydrocarbons plus oxygenated compounds are plotted in Figure 1 as functions of carbon number. The distribution curves drawn as solid lines are calculated curves such that
0 12
1 00 0 38 1.18 0 33 0.17 1.20 8 0 40 6 9.0 3.9 4.7 9.9 2.7 7.8 2.6
25.1 100.0
‘wi = an = k1-t- 0.012n cn where was approximated from the molal ratio of Ca to Ca hydrocarbons, and was adjusted so that ZCsf was equal to the observed molal yield of five-carbon and higher hydrocarbons. Oilsoluble oxygenated compounds were allocated to the five-carbon and higher fractions in the same relative proportions as the hydrocarbons. The concept of a constant ratio of the probabilities of growth and termination-originally applied by Herington t o
366
INDUSTRIAL AND ENGINEERING CHEMISTRY
the CS and higher fractions of the hydrocarbons from cobalt cata!yst-is applicable to the Cs and higher fractions of hydrocarbons from iron catalyst, and also to the C , fraction when the oxygenated compounds are appropriately allocated. MECHANISM OF SYNTHESIS
The primary synthesis reaction appears t o involve one-by-one addition of carbon atoms to one end of a growing chain. Chains are considered to be generated by an adsorbed-radical mechanism. Although experimental proof is lacking, such a mechanism offers a reasonable explanation of many of the features of product coinposition. The nature of chain initiation, chain extension, and chain termination is open to speculation. CHAINFORMATION. Numerous variations of the mechanism of chain growth have been proposed. Fischer ( 7 ) suggested that carbon chains were formed by the polymerization of methylene groups formed by hydrogenation of surface carbides. Craxford (6) viewed the polymerization as yielding macromolecules R hich later decomposed; whereas Herington (9) proposed that a terminating reaction was competitive with methylene polymerization. Brotz ( 5 ) visualized an intermediate biradical attached to the catalyst by the two carbon atoms a t the end and adjacent t o the end of the chain; growth by addition of methylene groups led to formation of a methyl branch whenever addition involved the adjacent-to-end carbon. Gall (8) suggested a variation in which alcohols were formed by addition of methylene groups to an adsorbed methyl01 radical. Distinctly different was the vien held by 0. A. Beeck of Shell Development Co. that chains, initiated by amethylene or possiblyameth~-lradical, were enlarged by the one-by-one addition and reduction of carbon monoxide molecules. A variant of this hydropolymerization process is the view that olefins (25) rather than adsorbed radicals are the intermediate stage. The nature of the growth initiating group has not been established experimentally. A basis for speculation is provided by the finding that only one ethanol molecule may participate in the synthesis of a molecule of a higher hydrocarbon. In the experiments with tagged ethanol ( 1 5 ) , the nearly even distribution of radioactivity on a molal basis showed that ethanol was not involved randomly in extension of the chain. Similarlv, it has been argued that iron carbide may participate but once in the synthesis of a product molecule (90). Experimental investigation of the role of radioactive iron carbide showed that surface carbide, while participating to some extent, was not serving as an intermediate ( 1 4 ) . Indeed, the intermediacy of bulk Hagg iron carbide was shown to be thermodynamically unfavoi able ( 1 3 )on the basis of experimental free energy data. However, a mechanism in which one caibon atom in each molecule was derived from a Hagg iron carbideintermediate was considered ( 9 0 ) t o be thermodynamically possible and in accord with the resultof the experiments with radioactive iron carbide. The participation of ethanol or possibly of carbide but oncr i n the synthesis of a molecule may be associated with chain initiation or with chain termination, but not with chain growth. The assumptions that added ethanol initiates chains and that carbon monoxide initiates chains via a carbide can be rationalized on the basis that the initiating group is a reduced group. If this is trur, the growth group, being different, mould have a higher oxidation state. Such a picture is essentially in accord with Beeclt’s view. Kone of the many proposed reaction mechanisms is fully consistent with all available experimental data, although some have attractive features. Many details of the reaction can be explained in terms of a mechanism R-hPreby a single reduced carbon is coupled with carbon monoxide. The resulting ta-o-carbon complex is reduced and again coupled with carbon monoxide. This process can repeat itself many times to yield long chains of carbon atoms. The nearly exponential decrease in yields of hydio-
Vol. 45, No. 2
carbons plus oxygenated compounds w-ith increasing carbon number indicates an approximately constant ratio of the probabilities of chain termination and chain growth. That the yield decreaseis exponential despite the abnormal ratio of C Joxygenated compounds to Cp hydrocarbons suggests that the product type is determined following the termination of chain grol?th. The abundance of primary alcohols relative to secondary alcohols and the ocrurrence of other classes of terminally oxygenated compounds suggest that, when oxygen is retained in the surface complex, it is associated mith the last-added carbon atom. That branching of the carbon chain I s o h i E R DISTRIBUTION. is an integral part of the growth mechanism and is not the result oi a secondary chain-isomerization reaction is suggested by the constancy of the branched-isomer content of the Ca fraction over the range of synthesis conditions and by the predictability of isomer distributions in higher fractions (26). Regardless of the nature of the adsorbed radical or radicals, branching can be rationalized by assuming that the growing complex is subject to further growth, primarily on the two carbon atoms last added to the chain. In the case of tagged ethanol, analysis showed that 90% of the C3 product u as formed by addition to the carbon atom originally bonded to the hydroxyl. Similarly, isomer distribution in the C4 and higher aliphatic fractions can be explained on the assumption that addition on the first and qecond positions takes place in a constant 90 to 10 ratio, u ith the exception that quaternary structures are not observed. The corresponding ratio for cobalt catalyst is approximately 97 to 3 The presence of small amounts of such compounds as 2-ethyl-1-butene in the product from synthesis over iron suggests that there is a small probability for addition on the third carbon of the chain. CYCLIZATION. The formation of cyclic and aliphatic structures from identical precursors was deduced from observed dietributions of aromatic isomers ( 4 ) and from the occurrence of certain alicyclic structures ( 2 5 ) . The mechanism of cyclization, like the mechanism of chain growth, can be visualized as involving the one and two positions of the adsorbed radical. Quaternary structures are excluded on the assumption that any reacting carbon atom must have a t least one of its four valences directed toward the catalyst. In cyclization, the carbon being coupled with the growing end of the chain is already attached to one or two other carbon atoms, whereas, in chain growth the coupling involves a separate carbon atom. Following ring closure, desorption yields a cyclo-olefin, which may be hydrogenated to a cycloparaffin or-if the ring contains six carbons-dehydrogenated t o a benzene ring. The occurrence of cyclic ketones ( d d ) , like the occurrence of aliphatic oxygenated compounds, is consistent with the idea that oxygen is closely associated with thr adsorbed complex. Each homologous complex may be the precursor of several different product types, and an indication of mechanism may be gleaned from their distributions. Thus, the olefin to alcohol ratio is abnormally low in the two-carbon fraction. The first members of the cycloaliphatic, cyclic ketone, and aromatic series are relatively less abundant than the higher homologs. A common factor relating the low yields of unsubstituted rings to the low yield of ethylene may be that in each case the formation of the carbon-carbon bond involved the carbon atom at the remote end of the chain. If the nature of the complex is such that the end carbon is a methyl group, the low yield may be attributed to the greater difficulty of abstracting hydrogen from the terminal (methyl) group than from a nonterminal (methylene) group.
con-cLus~onThe present investigation of the composition of hydrocarbon synthesis product has provided new data that are useful in the evaluation of proposed mechanisms of the reaction. The data are insufficient for identification of all the primary products of the synthesis. Although it appears that olefins, alcohols, and aldehydes are primary products, it is not possible to conclude
February 1953
INDUSTRIAL A N D ENGINEERING CHEMISTRY
that paraffins are primary products. Additional data on the structures of the higher aldehydes may clarify the relationships between the different classes of oxygenated compounds and indicate their roles in the synthesis. LITERATURE CITED
(1) Am. Petroleum Inst., Research Project 44, “Selected Values of Properties of Hydrocarbons,” Washington. D. C., U. S.Govt. Printing Office (November 1947). (2) Anderson, R. B.,Krieg, A., Friedel, R. A., and Mason, L. S., ISD.ENQ.CHEW,41,2189 (1949). (3) Brotz, W., Electrochem., 53, 301 (1949). (4) Cady, W.E.,Launer, P. J., and Weitkamp, A. W., IND.ENG. CHEIM., 45,350 (1953). (.5,) Cain. D. G., Weitkamp, A. lv.,and Bowman, N. J., Ibid., 45, 359 (1953). (6) Craxford, S. R.,Trans. Faraday SOC.,35, 946 (1939). (7) Fischer, F., and Tropsch, H., Brennstof-Chem., 7,97 (1926). (8) ~, Gall. D., Gibson, E. J., and Hall, C . C., J. Applied Chemistrg, 2, 371 (1952). (9) Herington, E.F. G., Chemistry and Industry, 65, 346 (1946). (10) Kilpatrick, J. E., Prosen, E. J., Pitzer, K. S.,and Rossini, F. D., J . Research Natl. Bur. Standards, 36, 559 (1946). (11) Kolbel, H., and Engelhardt, F., Chem.-Ing.-Tech., 22, 97 (1950). (12) Kolb, H. J., and Burwell, R. L., Jr., J. Am. Chem. Soe., 67, 1084 (1945).
367
(13) Kummer, J. T.,Browning, L. C., and Emmett, p. H., J. Chem. Phys., 16,739 (1948). (14) Kummer’ J* T*’ Dewit’’ T ’w ’ 3and Emmett, p. H., J. Am. Chem. Soc., 70,3632 (1948). (15) Kummer, J. T., Podgurski, H. H., Spencer, W. B., and Emmett, P.H., Ibid., 73,564 (1951). (16) Montgomery, c. W., and Weinberger, E. B., J . Chem. phys., 16, 424 (1 948). (17) Parks, ’G. S.‘, and Huffman, H. M., “Free Energies of Some Organic Compounds,” New York, Chem. Catalog Co., 1932. (18) Piteer, K. S.,and Weltner, W., Jr., J . Am. Chem. SOC.,71, 2842 (1949). (19) Regier, R.S.,and Blue, R. W., paper presented at the Oklahoma Tri-Section Meeting, AM. CHEM.SOC., Bartlesville, Okla., October 1950. (20) Schuman, S. C., J. Chem. Phys., 16, 1175 (1948). (21) Smith, D. F a , Hawk, C. O., and Golden, P. L., J . Am. Chem. SOC., 52,221 (1930). 45, 353 (22) Steitz, A., Jr., and Barnes, D. K., IWD.ENG.CHEIM., (1953). (23) Storch, H. H., Golumbic, N., and Anderson, R. B., “The Fischer-
Tropsch and Related Syntheses,” New York, John Wiley &
Sons, 1951.
(24) Twigg, G. H., Trans. Faraday Soc., 35,934 (1939). A. W., Seelig, H. S., Bowman, N. J., and Cady, 45,343 (1953). W. E., IND.ENG.CHEM., (26) Wenner, R. R., Chem. Eng. Proor., 45, 194 (1949). (25) Weitkamp,
RECEIVED f o r review January 2, 1952.
ACCEPTED September 8, 195-0.
Reactions of Vinvltrichlorosilane J
and Vinyltriethoxvsilane G. H. WAGNER, D. L. BAILEY, A. N. PINES, M. L. DUNHAM, AND D. B. McINTIRE Research Laboratory, Linde Air Products Co., Tonawanda, N . Y .
S
ILICOS compounds containing the vinyl group are of
il
interest in the field of silicon chemistry because of the reactivity of the vinyl group. Cohydrolysis of vinylchlorosilanes with alkyl or aryl Substituted chlorosilanes has given silicone resins which exhibit good thermal life and rapid curing properties (6). Copolymers have been prepared from vinylpolysiloxanes and vinyl monomers, such as methyl methacrylate, having desirable electrical properties (8, 6, 10). Recent work has shown that the wet strength of polyester laminates is improved by a vinylpolysiloxane size on the glass cloth. Undoubtedly, new applications for vinyl silicon compounds will arise based on the unique chemistry of these compounds. Although a number of preparations of vinylchlorosilanes have been reported ( 4 , 11, 14, 16), little has been published concerning the chemical reactivity of vinyl groups attached to silicon. This paper reports the results of an investigation of some reactions of vinyltrichlorosilane and vinyltriethoxysilane. A number of interesting reaction8 of wide utility were found, which are summarized in Table I. HYDROLYSIS OF VINYLTRICHMROSILANE
Vinyltrichlorosilane is readily hydrolyzed by addition of an ether solution of the compound to ice water. The resulting polysiloxane is a brittle, white solid with a softening point of 85’ to 90’ C., soluble in alcohol, ether, acetone, and benzene, but insoluble in hydrocarbons such as heptane and cyclohexane. Judged by the usual rules of polymer chemistry, it is rather unusual to obtain a soluble polymer from a trifunctional monomer. However, this appears to be a general characteristic of RSiCla compounds and is not unique for vinyltrichlorosilane. It is dependent on the cyclicieing tendencies of siloxanes.
Obtaining a solid, soluble polymer by the above technique requires the action of a small amount of weak base such as ammonium hydroxide or triethylamine on the hydrolyzate. This part of the reaction, which amounts to a partial silanol condensation, is carried out in the ether solution of vinylpolysiloxane after washing out the hydrochloric acid liberated during the hydrolysis. The concentration of the polysiloxane hydrolyaate in the soIution should not be too high. Without this treatment of the solution of hydrolyzate with a weak base, it gummy-type product is obtained which slowly sets to a gel on long storage. A typical polymer prepared by this method contained 9.53% OH (as SiOH), a number average molecular weight of 3800, and the formula [ CH2=CHSiOl.Zr The above type of polymer will form copolymers with other vinyl monomers such as styrene, vinyl acetate, etc. In such a case the above polymer must be considered aa having an average functionality of 46, since all the vinyl groups are in one molecule. If each vinyl group were a reactive center, this could lead to highly branched, or cross-linked molecules of very high molecular weight. Experience has indicated that not all, but a reasonable fraction, of these vinyl groups are reactive. The high funbtionality of such molecules has made them useful as cross-linking agents, as, for example, in unsaturated alkyd resins. Where a lower vinyl functionality is desired, siloxane copolymers from vinyltrichlorosilane and other RSiCls molecules may be made. Such copolymers have been prepared from a number of RSiCla-type compounds such as phenyltrichlorosilane, ethyltrichlorosilane, cyclohexyltrichlorosilane,and amyltrichlorosilane. The preparation conditions were the same as those for the siloxane polymers from vinyltrichlorosilane a8 outlined above.
