Thermal Decomposition of' Simple Paraffins F. 13. FREY AND 1%.J. &PI' P h i l l i p Petroleum Company, Bartlesville, Okla.
The prinury lherniul decomposition reactions of The decornposilion of n-perilune arid n-hexane n-bu.ta.ne, isobutane, n-pentane, isopenlane, 2,2-di- was exceptional in that more pr(?found changes took methylpropane, 2,3-dimethylbulane, and n-hexane place and ?qual amounts OJ complementary products were studied by decomposing each hydrocarbon homo- were not f o r m d . geneously to a moderate extent only, and amlyzing The exceplional behaaior of n-pentune and 11the products quite completely. The primary scission hexane is in agreement with a chin-reaction mechamas accompanied by rapid hydrogenation arid de- nism involning the formation and subsequent dehydrogenation of product molecules. The breaking composition of free alkyl radicals. Where olefins existing in isomeric .forms were .f the carbon chain resulted in the formalion of a molecule of paraffin and a molecule of the com- produced, the double bond remained at the point of plementary olefin when the Darafin formed was rupture. 2-Butene was found in both isomeric methane. When a higher paraffin was produced, a forms. The normal parafins yielded only I-olefins. part of it appeared us the corresponding olefin and Where o l ~ f i n swere produced by tlte scission of a hydrogen. Carbon-carbon bonds in all positions in side chain from isopenlane and 2,3-dimethylthe paraffins studied exhibited similar tendencies to butane, the double band at the point of rupture was break in branched molecules as well as normal ones. located in the inner and not the terminal position. ITH the exception of the sinip!est para,fiu, the products of primary decomposition are numerous and difficult to determine individually. A study of the mechanism of the decomposition reaction, however, depends largely on a comp1et.e knowledge of the products formed. The object of the present investigation was to study the primary homogeneous decomposition of several paraffins complex enoogh to exhibit variety in structure, and for this purpose a rather elaborate quantitative analysis of the products was considered necessary. Familiar chemical methods of analytical separation were used, in conjunction with precise fractional distillation, both for the separation of small fractionsin a,stateofpurityand for separatingcompoouds differing in boiling point by as lit,tle as 3" C. The decomposition reactiorls of several of the paraffins includrd in this investigation-namely, ethane, propanc, n-butane, isobutane, *pentane, isopentane, and n-hexanehave interested a number of investigators (4,10, 11). The homogeneous primary decomposition takes place a t a eonvcnicnt rate within 550" to 700" C., and most experimental deeompositions h a w been carried ont in this temperature
range and a t atmospheric pressure. Ethane (G',21) has been shown to decompose into the corresponding olefin and h y d r e gen. Propane ($1, 82) decomposes in the same way, but in addition undergoes rupture of the carbon-carbon bond to form methane and ethylene. n-Butane (fa, 15, g l , %), isobiitme (18, $1, %2), and isopentane (e) break in the same way at a.ny one of the several carbon-carbon bonds; little decomposition follows the dehydrogenation route, however, except in the case of isobutane. Hague and Whceler (IO) state from experiments with the normal paraffins that "the primary decompositions can he represented by a series of equations indicating the rupture of the chain a t any position witli the production of an olefin and the complementary lower paraffin, or a t the limit, hydrogen." Burk (1) suggests R mechanism for such R decomposition. IW,h the marked exception of n-pentane a i d n-hexane, the present work confirms this rule in a general way. The two complementary hydrocarbon product molecules may, however, be olefins. The rule is applicable to the other branched paraffins studied-namely, 2,2-dirnctliylpropane and 2,3-dimethylbutane, which had not before been deeom-
42
INDUSTRIAL AND ENGINEERING
CHEMISTRY
Vol. 25, No. 4
posed, and isopentane, for which rupture of all carbon-carbon nearly the simpler mode of decomposition shown by the other bonds had not previously been shown. paraffins. n-Pentane and n-hexane, however, are sharply distinguished I n the decomposition of the higher paraffins, olefins capable from the other paraffins in that there were produced by of existing in several isomeric forms were produced. There primary decomposition more than two hydrocarbon product was detected no marked formation of isomers other than those molecules per molecule of paraffin decomposed, in exception in which the double bond is located a t the point of rupture, to the above rule. Since the decomposition is endothermic Thus, n-pentane and n-hexane yielded 1-olefins. n-Butane to the extent of about 18,000 calories when, by a single break, gave 1-butene and cis- and trans-2-butene, and isopentane an olefin and the complementary lower paraffin are produced, gave isobutylene and cis- and trans-Zbutene. In neither it is not improbable that the decomposition takes place in case did the relative proportions correspond to the maximum more than one step. thermodynamic staThe i n t e r m e d i a t e "I" bility. However, the molecule formed must T-FRACTIONATION cis and trans isomers be highly r e a c t i v e , APPARATUS were present in apsince increasing the proximately equilibextent of decomposirium proportions, tion affects the relaand isomerization in t i v e proportions of this case may h a v e t h e several products taken place. only slightly. Several mechaEXPERIMENTAL nisms for the decomPROCEDURE p o s i t i o n of paraffins D EcoM P o sI T I o N h a v e been proposed METHOD. Since which i n v o l v e the fairly large quantities formation of reactive of reaction products intermediates. Hurd were usually needed (11) discusses the for analysis, in most dissociation into D cases a flow m e t h o d free alkyl r a d i c a l s FIGURE 1. DECOMPOSITION APPARATUS was used in preference which are s u b s e to a t i m e d exposure quently converted into the corresponding paraffins or ole- for the thermal decomposition. The apparatus was designed fins. Kassel (13) suggests that the primary process may be to permit fairly accurate control of reaction time and temperature. Since it was also desirable to minimize surface catalyof the type sis, an empty silica reaction tube was chosen. It has been CsHs = CHd CHsCH shown that silica exerts little catalytic effect on the primary which would be about 40,000 calories endothermic, a value decomposition of propane (6). much lower than the rather high one required for dissociation In spite of the low temperature and low extents of deinto free alkyl radicals. A chain mechanism for the de- composition it is possible that a slight carbon deposition took composition of paraffins has been proposed recently by Rice place in the silica tube. Carbon formed in this way, however, (34) who postulates the formation of the corresponding free does not affect the over-all decomposition rate appreciably. alkyl radicals which may decompose into olefin molecules and Though the absence of a slight catalytic effect detectable by simpler complementary alkyl radicals. This mode of scis- the analytical method used has not yet been demonstrated, sion, when applied exhaustively to represent the decompo- it is probable that such an effect if present is a minor one. sition of each of the paraffins studied, leads to the formation of CALCULATION OF VELOCITY CONSTANTS.Since all the more than two hydrocarbon product molecules per molecule decomposition products were collected, the fraction of the of n-pentane or n-hexane decomposed, and just two hydro- hydrocarbon decomposed was most conveniently calculated carbon product molecules by rupture of the carbon-carbon from the quantity of the hydrocarbon surviving and the bond from each of the other paraffins, in agreement with the quantity of products. The time was calculated from the present experimental results. This is discussed in detail in volume of the decomposition tube and the average flow rate of connection with the data. the gas entering and leaving. The first-order velocity conThe formation of reactive intermediates was also indicated stants shown in the tables were calculated by the equation by the distribution of hydrogen among the product molecules. kl = 2 303 log,, 100 At the higher temperature of decomposition, 575" C., a t t 100 - x which paraffins in equilibrium with the corresponding olefins and hydrogen should be largely dissociated, there were often where t = time, seconds z = percentage decomposed produced chiefly complementary olefins and hydrogen instead of an olefin and a complementary paraffin, even though a DECOMPOSITION APPARATUS. The apparatus is shown correspondingly higher endothermic heat of reaction was required. At low decomposition temperatures (400 " to in Figure 1. 425 " C.) hydrogen appeared almost wholly in combination Pressure controller A delivered the gas or vapor to be dewith the olefins to form paraffins. Ordinary hydrogenationcomposed to flowmeter B at constant pressure. The gas leaving dehydrogenation takes place too slowly t o account for this the flowmeter passed through drying tube C containing magadjustment. nesium perchlorate trihydrate, into a preheating spiral, F , conThe decomposition a t 400" to 425" C., like that a t 575" C., sisting of 300 mm. of 3-mm. silica tubing heated by chrome1 coil resulted in the rupture of the carbon-carbon bonds a t all G within 50' C. of the decomposition temperature. The preheated gas then entered silica tube D where it was decomposed. positions, but the relative tendencies of the several bonds to D was an empty helical silica tube, 10 mm. in inside diameter break was affected slightly by change in temperature. At the with a capacity of 166 cc. From D the decomposed gas passed lower temperatures n-pentane and n-hexane approached more through pressure controller 0 to the analytical apparatus.
-
+
~
April, 1933
443
INDUSTRIAL AUD ENGINEERING CHEMISTRY
Controller 0 consisted of a slightly tapered glass needle, seated in a tapered tube through which the gas had to pass. The needle was supported on a float resting in mercury, which formed one arm of a manometer, the other arm of which was an adjustable leveling bulb. The leveling bulb was set at such a height that the needle unseated a little to release gas at t'he pressure desired for the decomposition. A operated on the same principle, controlling the pressure on the inlet side of the silica tube. No fluctuations in pressure were observed during norma,l operation. Decomposition tube D was heated t o a uniform temperature in a copper box, H , with walls 18 mm. thick, heated by a resistance wire coil, I, covering 6he top, sides, and bottom of the cop er box. The preheat coil and copper box were insulated wit% powdered Sil-0-Cel, J. Chromel-alumel ther~nocouplesK and L were mounted in different parts of the box. Protected thermocouples M and N were located in the gas stream. The thermocouples indicated the same temperature during operation. The chromel-alumelcouples were checked at the solidificationpoint of antimony and were found to agree with the calibrtition curve. The temperatures were uniform and reproducibleto l o and correct within 3" C. All connections were glass-to-glass seals except at the ends of the silica tube, where rubber connections covered with De Khotinsky cement were used. ORIGIN AND PURITYOF HYDROCARBONS. Propane, nbutane, isobutane, n-pentane, and isopentane were prepared by careful fractionation from natural gas condensate obtained from virtually sulfur-free natural gas. By an accurate analytical fractionation the propane, n-butane, n-pentane, and isopentane were shown to contain less than one per cent of homologs or isomers. Other impurities were present in no more than traces. The isobutane contained 1.5 per cent of n-butane. 2,2-Dimethylpropane (neopentane) was prepared by treating tert-butyl bromide Tnth methylmagnesium iodide. The reaction product was fractionated to separate the hydrocarbon, which was then agitated with concentrated sulfuric acid and refractionated carefully (boiling point 9.5"; melting point, -20" C.). 2,3-Dimethylbutane was prepared by treating isopropyl iodide with sodium in ether. The hydrocarbon was purified by fractionating carefully, heating with alcoholic potash to insure the absence of iodides, washing repeatedly with water and concentrated sulfuric acid, and refractionating (boiling point 57.5" C.; n2:, 1.3770). n-Hexane was obtained from the Eastman Kodak Company. It was purified in the same way as the dimethylbutane, except that the first fractionation was omitted (boiling point, 69" C.; n2t, 1.3750). ANALYTICAL METHODS.The method of analysis used depended chiefly on precise fractional distillation, in conjunction with which the familiar methods of bromination and selective absorption by sulfuric acid were used ( 1 2 ) . The Bureau of Mines precision Orsat apparatus ( l a ) was used for the determination of oxygen, nitrogen, hydrogen, carbon monoxide, carbon dioxide, and paraffins, and for the determination of olefins by acid absorption. A mercury-filled gas pipet, into which one-cc. portions of the acid could be introduced, was used for this purpose. The pipet was always rinsed Kith fresh acid before use to remove any mercury sulfate which might activate the acid. The ethane-ethylene, propane-propylene, and butanebutene fractions obtained in the course of the analysis were analyzed in the same Orsat apparatus. Ethylene, propylene, n-butenes, and isobutylene were determined by absorption in fuming (15 per cent sulfur trioxide) 85 and 65 per cent sulfuric acid, respectively. The purity of the unabsorbed paraffin was usually confirmed by slow combustion with oxygen. The examination of the pentane fractions is described in connection with the experiments in which they were obtained. Significant amounts of allene and acetylenes were not formed. Only traces of products of higher molecular weight than the paraffin decomposed were found.
The usual procedure for analysis was to condense the products of decomposition directly in a bulb a t the base of an analytical fractionating column of the type described by Oberfell and Alden (20) and Podbielniak (23). The refluxing zone a t the top of the column was kept constantly a t a temperature low enough to condense hydrocarbons boiling higher than methane; methane and less condensable gases passed out of the column through a trap immersed in liquid nitrogen and into a storage bulb to be analyzed subsequently by the Orsat apparatus. The liquid caught in the trap was later, after the removal of methane, returned to the column. A t the conclusion of the decomposing operation the condensate was fractionated, and fractions containing 1, 2 , 3, 4, etc., carbon atoms per molecule were taken off consecutively and analyzed as described. Between neighboring homologs boiling over 25" C. apart, the completeness of separation by the column was limited only by the amount of material residing as a mixture in the upper part of the column when passing from one fraction to the next, which makes it im-
l l l l l l
1
1
1
/
1
1
GAS DISTILLED IN C C
FIGURE2. DISTILLATION OF BUTENES FROM
n-BUTANE
possible to select a point to interrupt the collecting of a fraction a t which no overlap with the next fraction will occur. With a difference in boiling point of 10" C. or less, the efficacy of the column became the limiting factor. For the isolation of fractions so small that overlapping, due to column holdup, was excessive, a smaller column of similar design was available. For purifying fractions too small to separate on the small column, the Shepherd-Porter apparatus was used (8, 26).
All the analytical apparatus was connected by glass-toglass seals, and for stopcock lubricant a solution of castile soap in glycerol was used in place of ordinary rubber stopcock grease when necessary to avoid errors due to absorption of the less volatile hydrocarbons. ANALYBISOF BUTENESBY FRACTIONAL DISTILLATION After isolating the butane-butene fraction from the products of decomposition by fractional distillation, the butenes were separated from butanes by bromination. The unsaturated hydrocarbons were converted into bromides by titrating the butane-butene fraction with liquid bromine a t -40" to -60' C. The addition was discontinued when a decided bromine coloration persisted after several minutes a t a temperature of -10" to -20' C. The butane was then distilled off without ebullition a t -75" C., an excess of one per cent of bromine was added, and the mixture kept a t 0" C. for 15 hours to complete the addition of bromine to 1,3butadiene, since the formation of the tetrabromide from the dibromide takes place somewhat slowly. The excess bromine was then removed by aqueous sodium sulfite, after which the bromides were distilled a t 40" to 60" C. and 0.5 mm. pressure. The distillation residue of semi-crystalline 1,3-butadiene tetrabromide was estimated by weighing. Under these conditions substitution reactions, even with isobutylene present, did not take place to a serious degree. The distillate containing only the dibromides was then converted back into
I N DUSTR I A L A N D E N GI N EER I N G CHE M I ST R Y
444
butenes by treating with zinc dust in ethyl alcohol. To prevent loss of isobutylene through polymerization, the addition of 15 per cent of water to the alcohol was necessary. The butenes were distilled from the alcohol through a calcium chloride tube into a fractionating column. Yields of 90 to 97 per cent were obtained. By analyzing synthetic mixtures, it was shown that the composition of a mixture of the four butenes was changed little by this sequence of operations. The normal butenes differ so little in boiling point and chemical properties that their individual determination is somewhat difficult. The boiling points are as follows: 1-butene, -6.5" to -6.7" C. (748 mm.); cis-Bbutene, +2.95" to $3.05" C. (746 mm.); trans-2-butene, +0.3" to +0.4" C. (744 mm.) (3). The lower boiling isomer is hereafter designated the trans form. The boiling point differences
Vol. 25, No. 4
INTERPRETATION OF RESULTS PROPANE AND ISOBUTANE. These paraffins have been shown to decompose both by loss of hydrogen to form the corresponding olefin and by breaking of the carbon chain to form methane and the complementary olefin (6, 12, 16, 16, 21, 22, 25). The data of Table I confirm the stoichiometric relations called for. The hydrogenation of ethylene produced from propane and of propylene from isobutane takes place more rapidly than would be expected in the absence of a concurrent primary decomposition (6, 11b). n-BUTANE. This substance (12, 15, 21, 22) has been shown to decompose chiefly into propylene and methane and into ethylene and ethane, the former reaction predominating. Dehydrogenation into butene and hydrogen also occurs t o a
TABLEI. DECOMPOSITION OF' LOWERPARAFFINS AT 575" C. Pressure, mm. Hg Time, sec. % decomposed
ki
Analysis of products, mole %: N2 H2 CO
coz
CHI
CZHP C2Ha CaHa C3Hs EH2: CHCH: CHn CHa)zC:CHz zHaCH: CHI CHaCH: CHCHa (trans) CHnCH: CHCHs (cis) C4HlO C(CHa)a
0.50 4.10
.. ....
4.63 4.03 0.49 4.50 81.75
....
.... ....
747 39.6 17.4 0.0048
77
...
752 25.2 11.3 0.0048
0.95 3.65 0.09
0.0 0.84 0.0 0.0 6.38 3.14 2.25 5.85 0.29 0.14
6.7
...
0.35 6.9s 0.13 0.43 6.26 0.33 0.35 4.64 1.47
2.55 0.0 0.13 1.94 0.24
8.61
.... ....
3.85
70.45
S0:00
...
....
...
... ...
....
....
100.00 100.00 This figure includes the butylenes and unchanged neopentane.
Total
*
PROPANB 739 74 10 0.0015
... 100.00
are small, but, since a fractionating method capable of giving a n analytical separation was available, it was used for the purpose. The column was a modified form of the type referred to, with an inside diameter of the column proper of 2.5 mm. operating a t a constant reflux temperature of 1" C. Excellent insulation of the column proper and a steady correct rate of boiling were essential to obtaining a true separation. Since the intervals between boiling points were small, a fractionation required the rather long time of 6 t o 20 hours. However, the column needed little attention during operation. Figure 2 shows a typical distillation curve of the butenes, in this case produced by the decomposition of n-butane. The percentage composition was determined from the curve. In some cases isobutylene was present. Since this olefin boils a t nearly the same temperature as 1-butene, they could not be differentiated on the distillation curve; thus, the butadiene having been previously removed, isobutylene was determined in the Orsat apparatus by selective absorption in 65 weight per cent sulfuric acid. ACCURACY OF ANALYTICAL METHOD. The accuracy of the analysis is affected by many factors. Neighboring homologs may be determined by the column fractionation under reasonably favorable conditions with an error of less than one per cent of the amount of the fraction. Sample size, fraction size, boiling point intervals, and chemical examination affect the precision of determining different constituents to different degrees. The authors believe the error was usually less than 5 per cent of the amount of a constituent present to the extent of one per cent or more, and somewhat greater for less than one per cent. I n the analysis of butenes and pentenes, while the error of determination of each total fraction is of this order, the error in the determination of each individual butene or pentene may be 3 to 5 per cent of the fraction in which it occurs, or, in a few cases, greater. Where the error has a bearing on the interpretation, it has been indicated.
....
0.67 0.33 0.16 79.95
0.0 0.3 0.2 0.0 18.3
0.2 0.87 0.0 0.0 5.58 2.84 2.30 6.41 . 0.39
I ....
182.41
....
100.00
~,~-DXMETRYLPROPANE Static Flow 400-600 66 95 20.5 22 4.0 0.0026 0.0020
745 24.3 10.5 0.0046
100.00
0.65
....
0.0 0.0
3.81
.... .... .... ....
[
16.0
