Products of the Hydrogenation of Carbon Monoxide over an Iron Catalyst J
ALIPHATIC AND ALICYCLIC HYDROCARBONS A. W. WEITKAMP, HERMAN S. SEELIG, NORMAN J. BOWMAN, AND WILLIAM E. CADY Research Department, Standard Oil Co. (Indiana), Whiting, Znd. Early workers noted the general nature of the product from the hydrogenation of carbon monoxide over an iron catalyst, but did not investigate its composition in detail. The product consists of hydrocarbons and oxygenated compounds of a broad molecular weight range. In the present work, the hydrocarbons were analyzed by distillation, chromatography, spectrometric methods, and other means. The weight per cent yield reaches a maximum at three carbons and then declines almost exponentially with increasing carbon number. I n the aliphatic and alicyclic hydrocarbons, olefins predominate and straightchain structures are prevalent. The straight-chain content decreases exponentially with increasing carbon number. The branched isomers and the alicyclic hydrocarbons occur in increasing proportions in higher carbon-number fractions. Relationships among the structures and distributions of the open-chain and ring compounds reflect the mechanism of formation. Significant deviations from thermodynamic equilibria indicate that competing reaction rates control product composition. Knowledge of the composition not only aids in the efficient utilization of the product but also permits a better understanding of the process.
under such conditions that the principal products are hydrocarbons boiling in the liquid fuel range. Because the conditions of the Fischer-Tropsch process as practiced by the Germans have been modified so drastically by American engineers, the process as carried out in this country is referred to as the “hydrocarbon synthesis process” throughout this series of papers. The hydrocarbon synthesis process is a modification ( l a ) of the Fischer-Tropsch process which involves passing hydrogen and carbon monoxide in approximately 2 t o 1 ratio through a fluidized iron catalyst at temperatures of 280’ to 360’ C., and a t pressures of 18 to 38 atmospheres, to produce about equal volumes of oil and water and an effluent gas stream. Bruner ( 1 ) and Clark (9)have reported on some of the hydrocarbon constituents in the gasoline fraction of a hydrocarbon synthesis product and Eliot (6) on the individual water-soluble oxygenated compounds. The present work confirms and extends these analytical results and presents some aspects of the mechanism. This paper deals primarily with the composition of the aliphatic and alicyclic hydrocarbons. Four following papers discuss the aromatic hydrocarbons and oxygenated compounds, and possible steps in the reaction mechanism. EXPERIMENTAL
Since these papers are concerned with product composition, the apparatus and operating procedures used in the synthesis are briefly described. Appropriate analytical methods are mentioned incidentally with the analytical results, and new or special methods are discussed briefly as encountered. Apparatus and Procedure. The products examined were prepared on a pilot plant scale in four fluidized bed reactors having nominal outputs of 5 to 200 liters of oil per day and employing recycle operation. Reactor diameters, oil outputs, and major operating conditions are presented in Table I. Catalysts were prepared by reduction of iron oxide or iron oxide containing a small amount of potassium carbonate. The feed gas for reactors A and B was prepared by the catalytic decomposition of methanol (19). The feed for reactors C and D was obtained by the partial combustion of methane ( 2 4 ) .
NOWLEDGE of the composition of the product from the
B
Fischer-Tropsch process for the hydrogenation of carbon monoxide is essential for efficient utilization of the product and for developing improvements of the process. Such knowledge is a source of clues to the reaction mechanism and an aid in the evaluation of proposed reaction mechanisms. The heterogeneous catalysis of the hydrogenation of carbon monoxide has been accomplished with catalysts of widely differing composition, a t temperatures of 100’ to 450’ C., and a t presawes of 1 to 1000 atmospheres. Different catalysts yield radically different types of products. Ruthenium catalyst yields waxes of high molecular weight ( 1 5 ) . Thoria catalyst produces chiefly low molecular weight isoparaffins (16). Cobalt catalyst gives a product consisting mainly of liquid normal alkanes (9). The product from iron catalyst is largely olefinic and contains significant proportions of branched isomers and oxygenated compounds (1, 8). ProcTABLEI. CHARACTERISTICS AND OPERATINGCONDITIONS OF esses for the preferential production of methane, EXPERIMENTAL REACTORS methanol, higher alcohols, normal alkanes, isoTube Feed Gas alkanes, and olefins have been reported. Some Diam., Oil Output, Temp., Pressure, S ace Ratio, Recycle of these processes of carbon monoxide reduction Reaotor Cm. L./Dey O C. Atm. Vegcitya Hn/CO Ratiob A 2.6 6 816 18 4-9 2 1-4 are not ordinarily considered modifications of the B 5 20 3 16 1s 8-4 2 2 Fischer-Tropsch process; nor is it expected that C 20 200 316 18 7-9 18 18 D 20 200 280-360 18-38 7-8 1.8 1.8 the reaction mechanism is the same in all cases. Standard cubic feet per hour of oerbon monoxide per pound of iron. The Fischer-Tropsch process generally connotes b Volume of recycle g88 per volume of fresh feed gaa. reactions carried out over cobalt or iron catalysts
343
INDUSTRIAL AND ENGINEERING CHEMISTRY
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AND OXYGEN BALANCES TABLE 11. CARBON
R u n D-1 Product Type Hydrocarbons total Aliphatic add alicyclic Aromatic Oxygenated compounds, total Water soluble Oil soluble Carbon dioxide Water Total ~~~
Per Cent of CO Consumed Carbon Oxygen 66 0 62 0 4 0 18 G 10 4 8 2 16 32 62 0 100 100
~
The effluent from the reactors was separated into several product streams by successive cooling steps. The total effluent was cooled from reaction temperature to 100' C.; a waxy hydrocarbon product and some water were condensed. Further cooling to 20" C. condensed additional hydrocarbons and most of the water. In some runs, the remaining gases were further cooled, under reaction pressure, to -40' C.; a liquid phase consisting mainly of hydrocarbons of low molecular weight was obtained. The uncondensed phase consisted of hydrocarbons, carbon dioxide, and unconverted reactants. I n each experiment] 8 portion of the uncondensed gas was recycled to the synthesis reactor; the remainder was metered, sampled for analysis] and vented. Analyses of the condensed and uncondensed streams were combined proportionally to obtain the composition of the total product. Product samples obtained a t different times or under different conditions are identified by a serial number following the reactor designation. Analytical Results. So complex is the product that analysis for all the individual components is impossible by available methods. Hydrocarbons comprise a major portion of the product. Distributions by carbon number have been determined through (216. Compositions of the carbon-number fractions in terms of the paraffin] olefin, and aromatic classes were determined through Cg. Increasing complexity limited analysis of the C , fraction to identification of carbon skeletons. In higher fractions certain structural types were identified, but the picture is far from complete. Fortunately, hydrocarbon synthesis product is ao constituted that knowledge of the simpler molecules can be extrapolated to provide a better understanding of the more complex molecules. DISTRIBUTION ACCORDING TO TYPE OF PRODUCT. For orientation with respect to the distribution of hydrocarbons and oxygenated compounds, carbon and oxygen balances on the carbon monoxide consumed in a typical hydrocarbon synthesis experiment are presented in Table 11. The aliphatic and alicyclic hydrocarbons,
Vol. 45, No. 2
which are the subject of this paper, accounted for 62YGof the converted carbon monoxide. DISTRIBUTION OF PARAFFINS, AROMATICS, A N D OLEFIXSBY CARBONNUMBER. Yields of individual carbon-number fractions and distributions of paraffins] aromatics, and olefins in the carbon-number fractions are presented in Table 111. The distribution of hydrocarbons by carbon number in a typical product was determined by fractional distillation of the liquid streams and mass-spectrometric analysis of the vent gas produced during 72 hours of steady operation. Paraffin, aromatic, and olefin classes weredeterniined throughCs by analysis for individual hydrocarbons, and in C Sand higher fractions by a micropercolation technique using fluorescent dyes to mark interfaces on silica gel ( 3 ) . The concentrations of olefins above CSwere also estimated from bromine numbers determined by an electrometric titration procedure (6). ANALYSISFOR INDIVIDUAL HYDROCARBONS. Complete component analysis was accomplished from C1 through Cg, although with diminishing precision in the Ce fraction. Because of increasing complexity, the C7 fraction was hydrogenated before analysis. The compositions of the C,, CZ, and Ca fractions are completely expressed in terms of paraffin and olefin contents in Table 111. Analyses of other C2 and C) fractions from a typical iron catalyst and from an unalkalized iron catalyst are given in Table I\'. OF Two-, THREE-, AKD FOUR-CARBON TABLE IT'. COMPOSITION
HYDROCARBONS
Component Two-carbon hydrocarbons Ethane Ethene Three-carbon hydrocarbons Propane ProDene
Composition, Weight % Sample Sample Bruner A-1 A-2U (1) 72 28
'
28 72 2.0
8.4
12.2 69.8 4.1 3.5 _
_
T h ~ ~ ~ f a t 600' K. ( l J ) , Mole %
100 0
... ...
99.9999 0.0001
100 0
20.2 79.8
99.999 0.001
8.5 3.0 74.5 4.4 -
I
1.9 8.7 13.6 64.3 11.5
... ... , . .
_
100.0 100.0 100.0 Total Four-carbon types Isohydrocarbons 10.4 11.5 10 6 45 Paraffins 14.2 83.0 15.5 99. 999 2-Butene in n-butenes 9.8 68.6 15.2 79 cis-%Butene in 2-butenes 46.1 39.5 ... 37 a Catalyst contained no potassium carbonate. b The equilibrium isohydrocarbon content was calculated on t h e basis of 100% olefins: for paraffins the value would have been 40%.
The products from the same typical and unalkalized iron catalysts were analyzed by infrared for the C4 hydrocarbon components. The r e s u l t s of TABLE 111. HYDROCARBON DISTRIBUTION BY CARBON NUMBER AND CLASS the analyses are summarized Sample B-1 in Table I V to show some reRatio of Hydrocaroon a s s by oy Hydrocarbon u Glass Olefins by Trans-Internal lationships of the Ca hydrocarCarbon Yield Percolation, Vol. 70 Bromination, t o Terminal bons to pertinent thermodyNumber Wt. yo Mole yo Paraffins Aromatics Olefins nit. Vo Double Bonds 28.7 100a 0 namic equilibria (IS). 1 8.9 0 ... 13.7 2 76 85 23.2a 7.8 ... 0 ... The analysis for the 10 com19.4 18 5w 81.5" 3 15.9 0 ... 13 4y 13.9 12.7 0 0:oiz 4 86. 6a ... ponents of a Cg fraction re12 7a 11.5 0 8.47 87.45 5 ... 0.022 5.26 quired fractional distillation 12 8 8.6 86.9 0 3 6 87.5 12 4 3.46 83.9 3.7 83.2 6.6 7 0:04s into narrow boiling cuts and 2.30 5.0 13 2 81.4 5.4 82.1 8 0.051 1.60 77.5 3.9 15 0 7.5 80.9 0.049 9 water washing of the cuts t o 1.11 15 1 12.2 72.7 10 3 .O 0.066 77.8 remove o x y g e n a t e d c o m 15 4 73.6 11 2.4 0.81 11.0 74.2 0.061 12 0.55 16 2 15.6 68.2 73.2 1.8 0.078 pounds, so that isomers of 20.40 21.7 1.4 15 4 62.9 73.4 13 0 094 0.29 1.1 75.3 19.9 14 16 5 63.6 0.11 pentene could be determined 0.25 1.0 17 4 75.9 15 by infrared and the other com0.9 0.21 75.6 16 0:it!, 306 6.3 0.79 .". 75.7 ponents by mass spectrometry. Total 100.0 100.80 This analysis and published a Determined by hydrocarbon component analysis, wt. %. results ( 1 , 2 ) are presented in b Average carbon number of distillation residue. Table V. >~~
-
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345
to include toluene, is shown in Table VI1 along with literature OF FIVE-CARBON HYDROCARBONS data (1). Each of the indicated structures is undoubtedly TABLE V. COMPOSITION
Thermodynamic Equilibrium Composition, Weight % a t 600' K. (19). Sample B-1 Bruner ( I ) Clark (9) Mole % 3.5 0.5 ... 4.3 7.9 10.9 ... 8.3 67.2 45.4 68.6 . 5.8 24.0 3.5 7.3 4.9 11.1 ... 9.8 0.7 5.9 .,. 0.6 0.1 0.1 0.2 __ ... 0.4 100.0 100.0 100.0 19 6 IS. 8 19.7 800 12.6 11.5 17.4 99 99
Component Isopentane n-Pentane 1-Pentene trans-2-Pentene cia-2-Pentene 2-Methyl-1-butene 3-Methyl-1-butene 2-Methyl-2-butene Cyclopentane Cyclopentene Totd Isohgdrocarbons Paraffins 2-Pentene in n-pentenes 4.2 8.0 cis-2-Pentene in 2pentenes 50 ... 5 Calculated on the basis of 100% olefins.
E}
*
-
I ,
... ... I
...
.
.
...
,..
.
I
.
...
34.5
87
...
38
A C6 fraction was distilled and narrow boiling cuts were water washed. The cuts were analyzed by means of refractive index calculations, supplemented in certain cases with infrared and mass spectrometry. The results of this analysis and fragmentary literature data (8) are presented in Table VI. This fraction was so complex that the presence, in traces, or total absence of certain paraffin or olefin isomers could not be rigorously established. A C, fraction was dearomatized by silica gel percolation so that hydrogenated toluene would not interfere with the determination of methylcyclohexane. The toluene-free sample was hydrogenated and fractionated. Individual cuts were analyzed by refractive index calculations and mass spectrometry. The composition of the hydrogenated C? fraction, calculated
E}
.5r
I44
I 0
25
I x,
I 75
100
WEIGHT % DESORBED
Figure 1.
Separationof CISto C30ProduOt on Silica Gel
The C7 to (2x6 fractions were isolated by distillation. Ratios COMPOSITION OF SIX-CARBON HYDROCARBONS of trans-internal double bonds to nontertiary terminal double Sample '2-2, Clark (D, bonds, presented in Table 111, were estimated from the intensiComponent B.P., C. Vol. 5% '01' % ties of infrared absorption at 10.35 and 10.06 microns. The 3,3-Dim~thyl-l-butene 41 2 None-trace0 ... 2,2-Dimet hylbutane 49 7 None-tracea ... ratios were estimated on the assumption that the relative intensi3-Methyl-1-pentene 53 8 6 0 ties of these bands are independent of molecular weight. 4-Methyl-1-pentene 54.0 trans-4-Methyl-2-pentene 55 0.0 .... .. Information on the types of hydrocarbons present in the Cts 2,3-Dimethyl-l-butene 55 6 0 9 2,3-Dim~thylbutane 58 0 0 9 ... to CBO range was obtained by examination of a waxy hydrocarbon czs-4-Methyl-2-pentene 58.4 .. ... product (sample C-4). The crude wax was alkali extracted t o 2-Methylpentane 60 3 1.4 1.4 2-Methyl-1-pentene 62 2 2.8 ... remove 7 weight Yo acids. Vacuum distillation yielded a Cli 3-Methylpentane 63 3 4 5 1.4 1-Hexene 63.6 62.2 53.1 to CSO distillate representing 75% of the nonacid material. The 2-Ethyl-1-butene 65 0 0 5 ... 25% residue was not analyzed. Percolation of the CIS to C*O 3- and 4-Methyl-1-cyclopenteneb 65 0 0.9 ... 2-Methyl-2-pentene 67 2 1.6 ... portion through silica gel yielded cuts whose refractive indices and trans-3-Methyl-2-pcntene 67 8 Trace .. .... ozs- and trans-, 2- and 3-Hexene 68 6 1.3 bromine-number unsaturations (calculated as Czl aliphatics or n-Hexane 68 7 8.1 11.3 aromatics) are plotted in Figure 1. The type composition cm3-Methyl-Z-pentene 70 5 1.0 ... Met ylcyclopentane 71 8 0 1 .. .... of the CIS to Cao product, expressed on a basis including a pro2 3-hnethyl-Z-butene 73 2 1 4 1: and 4-Methyl-1-cyclopenteneb 75 0 6 ... portional share of the acids, was 47 weight % nonaromatic Diolefins so:i 0.1 1 0 ... 0 2 hydrocarbons, 30% aromatics, and 23% oxygenated compounds. Benzbne 80.7 C yclghexane 0 2 . . . The nonaromatic hydrocarbon portion consisted of mono-oleTrace Cyclohexene 83 2 26'6 fins with minor amounts of paraffins and diolefins. Infrared High index hexenes Total analysis showed that about 85% of the double bonds in the monoThe presence in traces (or absence) could not be rigorously established olefins were of the nontertiary terminal variety (R-CH =CH2), by infrsred. * The boiling point of 4-methyl-l-c~clo~enteneis not definitely estab- and that, on the average, only one of the double bonds in the lished. diolefins was terminally located. Weak absorption due to transvII. CoMPosITIoN OF SEVEN-CARBON internal double bonds indicated that most of the internal unsatuHYDROCARBONS ration had the cis configuration. Such a distribution suggests Sample C-1, Brgyg), ring unsaturation. Component Vol. % Analysis for structural types in the nonaromatic hydrocarbons n-Heptane 63 0 60.2 required hydrogenation of the olefins, extraction of the normal 2-Methylhexane 9.4 29 3 3-Methylhexane 15.9 alkanes with urea (%'a),and fractional distillation. Hydro2 3-Dimethylpentane 1.7 genation lowered unsaturation of the nonaromatic hydrocarbons 2'4-Dimethylpentane 1:2- and 1,3-Dimethylcyolopentane from 95% to,about 1%. Extraction with urea separated the Ethylcyclo entane 1 0 8 8 Meth yloycfohexane hydrogenated mixture int! 46 weight % extract and 54 Qeight yo 1.7 5 5 Toluene __ raffiate. The extract and ra&ate were each vacuum distilled; Total 100.0 100.0 r &e extract and part of the boiling point, TABLEVI,
+
derived from several different olefin isomers contained in the original sample. Similar analyses of hydrogenated Cg and Cs fractions have been published (1). ANALYSISFOR HYDROCARBON TYPES. Although 'individual paraffihs and olefins were not determined in fractions above Cs, some aspects of composition were revealed by infrared examinacarbon-number fractions and by a more tion of the C, to C I ~ detailed examination of a C15to Caoproduct.
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-
1oo.o
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Vol. 45, No. 2
The regular decline in yield with increasing carbon number is characteristic of products from both iron and cobalt catalysts. The plot in Figure 4 of the mole per cent yield, on a logarithmic scale, of C3 and higher fractions gave a nearly linear relation with carbon number. A similar relation was observed for product from a cobalt catalyst by Herington ( I O ) , who proposed the explanation that a constant fraction, an,of chains of any length greater than about five carbons will remain on the catalyst and will continue t o grow by adding another carbon atom. Thus, an
= Cn+l - is
C,
a constant.
Examination of the mole per cent
yield data in Table I11 and Figure 4 and of the data used by Herington reveaIed a gradual increase in an with increasing carbon number which is approximated by the linear equation 25
im
7s
M
VOLUME % DISTILLED
Figure 2. Distillation of Urea Extract and Raffinate of Hydrogenated Nonaromatic Hydrocarbons from CIS to C ~Product O Numbers on curves indicate carbon number
and refractive index plots for the raffinate are shown in Figure 2. Through C I S , the normal alkanes were relatively pure; higher fractions were contaminated with branched alkanes. The contaminants probably have a branching methyl group near one end of a long chain ( 1 8 ) . The complexity of the raffinate is revealed by the absence of boiling point plateaus. The presence of naphthenes in the raffinate is indicated by the fact that the refractive indices are higher than the indexes of known branched paraffins, Ring contents of carbon-number fractions of the raffinate mere estimated by means of a correlation of refractive index and molecular weight with ring content for known compounds (11 ). Values ranging from an average of two ring-arbon atoms per molecule a t C1, to nearly five a t C28 are plotted in Figure 3. Also shown are the ring contents of the Cb, GO,and C7 fractions calculated from data in Tables I V , V, and VI.
5 -
Y
0
B
4 -
0
z
PL1: 3
3 -
W
r
P
2 -
I -
O
1
5
I
I
IO
15
I eo
I 25
30
CARBON NUMBER
Figure 3. Ring Content of Branched Paraffin and Naphthene Carbon-Number Fractions CHAIN GROWTH
Two noteworthy features of hydrocarbon distribution with respect to carbon number are revealed by the yield data from Table I11 plotted in Figure 4. One is the anomalously low yield of Cz hydrocarbons, and the other is the regular decline in yield with increasing carbon number. I n products from iron catalysts, high yields of C2 oxygenated compounds nearly compensated the low yields of C2 hydrocarbons. Published data (9, 1 0 ) on products from cobalt catalysts indicate that this anomaly extends from CZ to Cg or Ce and that the yields of oxygenated compounds are very low.
an =
kl
+hn
+
For the product from iron catalyst, = 0.606 0.012 n, and for the product from cobalt catalyst, an = 0.670 f 0.012 n. One can, therefore, speculate that the adhesive force between the molecule and the catalyst consists of a small force operating on each carbon atom in the molecule as well as a larger force possibly operating a t a single point. The latter force, which depends on the catalyst and operating conditions, largely controls the probability of desorption and establishes the slope of the carbon-number distribution curve. The distribution of hydrocarbon product to the three major classes, olefins, paraffins, and aromatics, was shown in Table 111. With increasing carbon number above Cs, there is (1) a slight increase in paraffin content, (2) an increase in aromatic content, and (3) an increase in internal olefins relative to terminal olefins. These trends help to differentiate between primary and secondary reactions. SECOND 4RY CONVERSIONS
The initial products of the synthesis are exposed to secondary conversions before they issue from the reactor. I t appears that olefins are hydrogenated to paraffins and that double bonds are shifted from terminal to internal positions. An interesting aspect of double bond isomerization is the interconversion of the cis and trans configurations. The extent of secondary conversion is dependent on operating conditions and, under a given set of conditions, varies with molecular weight. ISOMERIZATION AND HYDROGENATION. The variability of the ratios of paraffins to olefins and of terminal to internal unsaturation in product samples prepared under different operating conditions is evidence of the secondary nature of hydrogenation and double bond isomerization. Other evidence was provided in the present work by an experiment in which ethylene, added t o the feed gas, was partially converted to ethane. Similarly, Regier ( 1 7 ) added 1-butene and observed its partial conversion to 2-butene and n-butane. Catalyst composition is one of the factors affecting the extents of these secondary conversions. Cobalt catalyst gives a more paraffinic product than is normally obtained with iron catalysts, and the residual olefins are largely internally unsaturated ( 8 ) . Nevertheless, iron catalysts can be made t o yield products that are extensively isomerized and hydrogenated if the usual trace of potassium carbonate is omitted. A good example of this is shown in Table IV. Numerous analyses of C, hydrocarbons from reactor A revealed differences in composition which may be attributed to the specific conditions of operation. A ternary plot in Figure 5 of the 1-butene, 2-butene, and n-butane contents of these samples suggested a relationship between the rates of hydrogenation and double bond isomerization. Efforts to evaluate such a relation have not been entirely successful, inasmuch as a precise mathematical treatment of the kinetics of reactions in fluidized bed reactors is not available. Of the other possible approaches to the mathematical problem-the use of a batch or a stirred-flow reactor (4)as the model-the stirred-flow reactor was selected since
February 1953
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100
TABLEVIII.
EFFECT OF RECYCLE RATIO ON OLEFIN AND ISOMERIZATION HYDROGENATION Paraffins. Mole %
Sample
Recycle Ratio
C2
Ca
C4
2 ~in ~ %-Butenes, %
t
paraffins are formed by a route other than hydrogenation of olefins. CIS-TRANS ISOMERIZATION. A relationship between the ratio of cis to trans olefin isomers and the ratio of internal t o terminal unsaturation was indicated b y the analyses of the C4 hydrocarbons in Table IV. A ternary plot in Figure 6 of many analyses for 1-butene, cis-2-butene, and trans-2-butene showed trans- 2- BUTENE
0.1
I
I
I
I
4
8
I2
CARBON NUMBER
Figure 4.
Hydrocarbon Distribution
0 Experimental values run B-1 Solid line caleulated’for
a n = 0.606
+ 0.012 n
it more nearly approximates the fluidized bed reactor. When the results in Figure 5 were analyzed on this basis, it was found t h a t the apparent rate of isomerization is approximately 10 times the rate of hydrogenation. The inadequacy of the stirredflow reactor as a model was revealed by experimental evidence, presented in Table VIII, t h a t the extent of conversion was not proportional t o residence time. This evidence was obtained from
I-
Figure 6. 2-BUTENE
I-’BUTENE
50 75 n-BUTANE WT. % Figure 5. Composition of n-C1 Hydrocarbons A. Thermodynamic equilibrium 25
B.
“Best” curve
experiments in which residence time was increased by a factor of 2 and decreased by a factor of 4 as a result of abrupt changes in recycle ratio. It follows t h a t the kinetics of secondary conversions in the fluidized catalyst bed are influenced by the rates of transfer of reactant and product molecules between the gas phase and the catalyst. Because of this mass transfer limitation, the backward extrapolation of the curve in Figure 5 t o zero 2butene content may not provide a valid argument t h a t some
A. B. C. D.
WT. % Composition of n-Butenes
1-Butene-2-butene equilibrium
eis-2-Butene-trans-2-butene equilibrium
Random distribution “Best” curve
t h a t in samples having high 1-butene contents the cis- and trans2-butenes were present in nearly equal proportions, while samples in which the 1-butene to 2-butene ratio was near equilibrium the cis t o trans ratios were near the 37 to 63 equilibrium ratio (IS). This showed that the initial shifting of the double bond from the terminal position yielded cis and trans isomers with equal probability. Equilibration of the cis and trans isomers must have occurred in a second step. Inasmuch as the maximum cis to trans ratio in the aliphatic olefins in synthesis product is near unity, the high ratio noted 1 6 t o Cao olefins is very likely due t o cyclic olefins. in the c RELATION OF MOLECULAR WEIGHTTO EXTENT OF SECONDARY CONVERSION.The analyses in Table 111 showed that transinternal unsaturation increased a t the expense of terminal unsaturation with increasing molecular weight. Paraffin content decreased in the CZt o c6 range and then slowly increased with increasing molecular weight. The latter effect was paralleled by an increase in an,and may indicate that the larger molecules tend increasingly to linger on the catalyst. The declining paraffinicity of the CZto Cg homologs very probably reflects declining hydrogenation rates. Emmett ( 7 ) found the relative rates of hydrogenation of CZ, CB, and Cd olefins t o be 6, 3, and 1 at -20’ C. over an iron Fischer-Tropsch catalyst. A similar relation is reflected in the composition of synthesis product. I n order to estimate the apparent relative rates of hydrogenation necessary to produce the observed degrees of hydrogenation, the paraffin contents of numerous Cp, C3,and straight-chain C,
~
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INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
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-n 0
I
I
I
ll
20
40
60
80
100
WT. '/e 2-BUTENE IN n-BUTENES
Figure 7. Relation between Hydrogenation of CI to C4 Olefins and Isomerization of 1-Butene
fractions were plotted in Figure 7 as functions of the degree of 1-butene isomerization. Since each gas sample was analyzed for all CZ, C S ,and C4 hydrocarbons, the content of 2-butene in the n-butenes is an appropriate coordinate for all three hydrogenation reactions. It was calculated by use of the mathematics of the stirred-fiow reactor ( 4 ) that the apparent isomerization rate of 1-butene was about half the apparent hydrogenation rate of ethylene, about three times that of propylene and, as previously indicated, about ten times that of 1-butene. Again, there are large uncertainties regarding the actual reaction times because the limited transfer between gas phase and catalyst obscures the relation between reaction time and residence time. Two- to fourfold differences in residence time of the homologous olefins resulted from the characteristics of the product-condensing and gas-recycling systems. Thus, ethylene was in the reactor about twice as long as propylene and four times as long as butylene. These differences may not be very significant, in view of the low significance of two- to fourfold changes of residence time due to changes in recycle ratio. However, they cannot be ignored since the observation that added olefins were partially converted showed that readsorption may occur. CHAIN BRANCHING
The carbon skeletons in the saturated and unsaturated hydrocarbons include straight chains, branched chains, five-carbon
z
-0I-
rings, and six-carbon rings. Straight-chain structures decrease in abundance with increasing carbon number but are more abundant than branched-chain structures up to 10 carbons. Chain branches are limited essentially to methyl branches, randomly distributed, with not more than one methyl branch per chain carbcn. The Cd through C, fractions contain all possible nionomethyl- and dimethyl-branched structures except neopen tane and its olefin or paraffin homologs. Ethyl side chains weie not reported by Bruner ( 1 ) in the C7 and Cs fractions, nor u a s 3ethylpentane found in the hydrogenated C, fraction (Table V I l ) . However, 2-ethyl-l-butene, identified in the Cg fraction by infrared, could be considered a terminal olefin with an ethyl side chain. The isohydrocarbon content of the many Cq fractions, plotted in Figure 8 as a function of the 2-butene content, reveals that chain branching does not vary in the same way as double bond isomerization or hydrogenation. This insensitivity of chain structure t o variations in synthesis conditions is an important characteristic of hydrocarbon synthesis product. Von ITeber ( 2 0 ) was the first t o note a regular increase in branched iEomer content with increasing carbon number and to propose the idea of a definite probability of branching a t each carbon atom addition. \Teller and Friedel ( 2 1 ) reported excellent systems for predicting isomer distribution in product from cobalt catalyst on a statistical basis. They assumed that an entering carbon atom mould add to a terminal or penultimate carbon atom a t either end of a growing chain with a fixed ratio of probabilities. Addition a t tertiary carbon atoms or to side chains a t other than penultimate positions mas excluded on the basis of analytical data showing the absence of quaternary structures and of ethyl side chains. Structure distributions for CB to C S aliphatic hydrocarbons in products from iron catalysts are summarized in Table IX. In Figure 9 a plot of the straight-chain content, on a logarithmic scale, against carbon number gave a straight line. This straight line requires, for product from iron catalyst, the further restriction of addition a t only one end of the chain. The assumptions of no quaternary structures, only methyl side chains, and addition a t only one end of the chain and the successive applications of the probabilities of terminal or penultimate addition were used, as illustrated in Figure 10, to obtain a simplified set of equations for predicting isomer distribution in product from iron catalyst. These equations are useful for predicting relative yields of individual olefin isomers. For calculation of structure distribution as in the last column of Table IX, the following generalized equation is convenient:
F =
(n - b
-
3)! (1
- a)bab-Zb-3) - 3)! (n - b
+
b! (n - 26
- 3)! (1 - a)ba(n-Zb-Z) ( b - l ) ! ( n - 26 - 2)!
2c
TABLEIX.
0
STRUCTURE DISTRIEUTIOK OF ALIPHATIC HYDROCARBONS
a a
Carbon Atoms
LL
Experimental
0
100 89.6
z z
IC
a
100 89.4
80.4
81.2
74.1 69.9
78.8 66 0 58.7
...
a
Per Cent Monomethyl
0 0
a
4 5 6 7 8
n 2-
I
8 + L
Bruner ( 1 )
Clark ( 8 )
Calculated
Per Cent Normal
0 lJY
Vol. 45, No. 2
50
I00
WT. % e-BUTENE i N ~ - E U T E N E S Figure 8. Isohydrocarbon Content as a Function of Double Bond Isomerization
6 7 8
10.4 19.6 22.7 28.1
...
10.6
IS. s
20.8
32.1
38.8
Per Cent Dimethyl 3.2 0.4 2 0 1 9
...
2.5
100
...
80.3
... ... ...
... 19.7
... ...
...
100 90 81 72.9 65.6 59.0
10
19 -
10.1
31.6 35.7 1. o 2.8 5.1
INDUSTRIAL AND ENGINEERING CHEMISTRY
February 1953 loo
a W
-
Isomer distributions of the cyclic hydrocarbons have not been 9 determined precisely. Of the three possible monomethylcyclopentenes, the most abundant isomer apparently is 3-methyl-l-
90.
I
8 z-a
cyclopentene, C-C-C-C-C=C. The similarity of its structure to that of the abundant 1-hexene, C-C-C-C-C=C, suggests a generic relation. Similarly, cyclopentene can be related to 1-pentene, and ethylcyclopentane, C-C--
80.
r
0
r
70
r '3 a
a
3
349
C-C-C-C-C,
6ol CARBON NUMBER
The hydrocarbons produced by the hydrogenation of carbon monoxide are a complex mixture, characterized by extreme deviations from thermodynamic equilibria. The structure of the product is definable by two functions-a,,, the probability that another carbon atom will be added to the growing chain, and a, the probability t h a t it will be added t o the end of the chain rather than adjacent to the end. Cyclization and the secondary occurrence of double bond isomerization and hydrogenation are minor but significant features of the process.
8
Figure 9. Straight-Chain Isomer Content of Aliphatic Hydrocarbon Fractions 0 Thiswork
a
to n-heptane. CONCLUSION
50
0
i
(1) (2)
ACKNOWLEDGMENT
I n this equation F is the fraction of C, hydrocarbons containing b methyl branches, and a is the probability of terminal addition. The value of a is 0.90 for products from iron catalysts and 0.967 for products from cobalt catalysts.
The authors express thanks to the Research Laboratories of the Stanolind Oil and Gas Co. for preparing some of the samples that were analyzed and for providing certain analytical and operating data, t o R. H. Burwell, Jr., of Northwestern University, and t o H. M. Grubb and R. F. Marschner of the laboratories of this company for valuable suggestions. LITERATURE CITED
c
(1) Bruner, F. H., IND.ENQ.CHEM.,41, 2511 (1949). (2) Clark, A,, Andrews, A., and Fleming, H. W., Ibid., 41, 1527 (1949). (3) Criddle, D. W., and LeTourneau, R. L., Anal. Chem., 23, 1620 (1951). (4) Denbigh, K. G., Trans. Faraday SOC., 40, 352 (1944); Denbigh, K. G . , Hicks, M . , and Page, F. M., I b i d . , 44, 479 (1948);
Stead, B., Page, F. M., and Denbigh, K. G., Discussions Faraday SOC.,2 , 2 6 3 (1947). (5) DuBois, H. D., and Skoog, D . A., Anal. Chem., 20, 624 (1948). (6) Eliot, T. Q., Goddin, C. S., and Pace, B. S., Chem. Eng. Progr., 45.
I
cc Figure 10.
Prediction of Isomer Distribution
Equations for one-end addition satisfactorily predict isomer distribution in product from both catalysts, whereas equations for either-end addition are suitable for product from cobalt catalyst but not for product from iron catalyst. This can be explained on the basis t h a t product from cobalt catalyst is more straight-chained and the correspondingly small absolute differences between the two methods of predicting isomer distribution fall within the range of analytical error. Thus, on the basis of isomer distribution data, it is unnecessary t o seek a fundamental difference in mechanisms of chain branching on iron and cobalt catalysts, for both may involve one-end addition. CYCLIZATION
Cyclic hydrocarbons were found in Cs and higher fractions. Hydrocarbon component analyses of the Cs and Cs fractions reveaIed t h a t both saturated and unsaturated rings of five and of six carbon atoms were present. Similar structures are believed t o occur in higher fractions; possibly some molecules contain more than one ring. Polycyclic molecules having condensed alicyclic ring systems are likely, as such systems were found in the aromatic fraction.
532, (1949).
(7) Emmett, P. H., and Gray, J. B., J . Am. Chem. SOC.,66, 1338 (1944).
(8) Fischer, F., and Tropsch, H., Brennstof-Chem., 4, 276 (1923). (9) Friedel, R. A., and Anderson, R. B., J. Am. Chem. SOC., 72, 1212 (1950). (10) Herington, E. F. G., Chemistry & I n d u s t r y , 65, 346 (1946). ( 1 1 ) Hersh. R. E., Fenske, M . R., Booser, E. R., and Koch, E. F., J . PetroleumInst., 32, 624 (1950). (12) Keith, P. C., Oil Gas J . , 45, No. 6, 102 (1946). (13) Kilpatrick, J. E., Prosen, E. J., Pitzer, K. S., and Rossini, F. D., J . Research Natl. B u r . Standards, 36,591, 603 (1946). (14) Mungen, R., and Krataer, R. B., IND. ENG.CHEM.,43, 2782 1951. (15) Pichler, H., and Buffleb, H., Brennstof-Chem., 21, 257 (1940). (16) Pichler, H., Ziesecke, K. H., and Titzenthaler, E., Ibid., 30, 333 (1949). (17) Regier, R . S., and Blue, R. W., paper presented a t the Oklahoma Tri-Section Meeting, AM. CHEM. Soc., Bartlesville, Okla., October 1951. (18) Schlenk, W., Jr., Ann., 565,204 (1949). (19) Seelig, H. S., and Marschner, R. F., IND. ENO.CHEM.,40, 583 (1948). (20) Weber, Ulrich von, Angew. Chem., 52, 607 (1939). (21) Weller, S., and Friedel, R. A., J . C h m . Phys., 17, 801 (1949); 18,157 (1950). (22) Zimmerschied, W. J., Dinerstein, R. A., Weitkamp, A. W., and Marschner, R. F., IND.ENQ.CHEM.,42, 1300 (1950). RECEXVED for review January 2, 1952. ACCEPTED September 8,1952. Presented in part before the American Association for the Advancement of Science, Gordon Research Conference on Catalysis, New London, N. H. June 1949.
