Conversionof n-Heptane to lsobutane e METAL-ALUMINUM CHLORIDE CATALYSTS Experiments with water- or hydrogen chloride-promoted aluminum chloride and n-heptane, with and without the addition OF sodium, magnesium, or aluminum, have been run at 95-100" C. and atmospheric pressure to determine the yields OF isobutane, light paraffins (G-C,), heavy paraffins (C, and higher), and the oil From the catalyst complex. These metals exert a very marked influence on the promoted catalyst as shown b y decreased conversions, increased isomerization t o light paraFfins, and decreased crackingisomerization to isobutane and oil. Because aluminum has the greatest effect in retarding o i l Formation, which inactivates the catalyst, it i s the bestof the three metals tested For controlling this reaction as a route to the production of isobutane and OF light paraffin hydrocarbons.
UMEROUS studies of the action of aluminum halides on paraffin hydrocarbons have shown that isomerization and cracking may occur; the relative extent of these reactions depends on the nature of the hydrocarbon, activity of the catalyst, and other experimental conditions. Isomerization, for example, predominates in the n-butane-aluminum chloride reaction, if hydrogen chloride or other promoter is present, and this process is now commercially important as a source of isobutane for aviation gasoline manufacture. Higher paraffin hydrocarbons may undergo both isomerization and cracking, and isobutane has frequently been a prominent product from these reactions (1, 2, 3, 11, 19). Recently, conditions were studied that will convert a single normal paraffin, n-heptane, to isobutane in the presence of aluminum chloride (6). With water or hydrogen chloride as catalyst promoters, a maximum yield of 76% isobutane was obtained by the action of aluminum chloride on n-heptane a t 95-100" C. and a t atmospheric pressure in an apparatus which permitted the isobutane to distill as rapidly as formed; the reaction mixture was thus kept concentrated with respect to catalyst, intermediate products, and n-heptane. Complete conversion of the heptane and the maximum yield of isobutane were obtained with a heptane/aluminum chloride mole ratio of 3/1, plus the addition of about 7.570 water with respect to the aluminum chloride. The remainder of the heptane is converted to a highly unsaturated mixture of hydrocarbons which forms a catalytically inactive complex with aluminum chloride. Less vigorous conditions and, therefore, less severe cracking-isomerization result from higher reactant ratios and less intensive promotion of the catalyst. Such conditions produce smaller conversions, lower yields of isobutane, but higher yields of C6-C, paraffins. The purpose of the present investigation was to determine the effect of certain added metals on the heptane-aluminum chloride reaction when carried out under conditions which give relatively high isobutane yields. Combination metal-aluminum chloride catalysts have already been studied in several hydrocarbon reactions and Friedel-Crafts syntheses. I n the catalytic cracking of gas oil with aluminum chloride, the addition of 2070 aluminum with respect to the catalyst is claimed to increase by 50% the yield of hydrocarbons boiling to 200" C. and to decrease coke formatjion (14). I n the cracking of heavy naphtha to isobutane and a light isoparaffinic naphtha by aluminum chloride, the addition of 2070 aluminum to the catalyst decreases the iso1 2
d e* 9.M M P WESTERN RESERVE UNIVERSITY, CLEVELAND, OHIO
butane/light naphtha ratio from 0.5-1.0 to 0 (16). Cracking of paraffin wax by aluminum and hydrogen chloride a t 210-260" C. to high yields of gasoline is effectively catalyzed by aluminum chloride generated in situ, but until all the aluminum is converted, the catalyst is actually a mixed one (8). For the conversion of propane to isobutane a mixture of aluminum, iron, or other metal with aluminum chloride is said to be a particularly effective catalyst combination (18). I n the polymerization of ethylene by aluminum chloride, Hall and Nash ( 7 ) observed that added aluminum inhibited cracking and the formation of a catalyst-hydrocarbon complex without hindering polymerization. An improved process for the polymerization of tertiary olefins to lubricating oils by Friedel-Crafts catalysts plus 25-10070 of an alkali metal is claimed ( I S ) . Metal-aluminum chloride catalysts have been most frequently proposed for paraffin hydrocarbon isomerization processes. Undesirable side reactions are suppressed by the presence of aluminum in the hydrogen chloride-promoted isomerization of n-butane (16) and n-pentane (I?). Isomerizations, in which the aluminum halide is generated in situ by the action of a hydrogen halide, alkyl halide, or halogen on aluminum, gave better yields of isoparaffins than comparable reactions with the aluminum halide (9, 12). The catalyst for Friedel-Crafts reactions may advantageously be made in the reaction mixture from aluminum metal and hydrogen chloride ( I O ) , or the mixed catalyst itself may be used, as in the preparation of dibenzyl from benzene and ethylene chloride (20). The presence of aluminum in the Friedel-Crafts synthesis of benzophenone from benzene and benzoyl chloride caused consecutive reactions of reduction. Hydrogen, formed by the action of hydrogen chloride on aluminum, the hydrogen chloride being a product of the acylation, was the reducing agent (6). From these examples of cracking, polymerization, and isomerization, i t appears that the effect of added aluminum or other metal is to diminish the severity of cracking and thereby to increase the yields of light naphtha, polymer, or isoparaffin, and extend the life of the catalyst through diminished rate of complex formation. EXPERIMENTAL PROCEDURE
n-Heptane was the California Chemical Company's knock-rating grade: boiling point (760 mm.) 98.42' C., freezing point - 90.64" C., nZ,O 1.38776, di0 0.68367. Reagent-grade aluminum chloride was weighed into glass ampoules holding enough for one run (33.3 grams, 0.25 mole) in a dry box so that the purity and, therefore, activity of the catalyst would be constant in the various experiments. The aluminum and magnesium metals were 100mesh powders thoroughly washed and dried before using. Sodium metal was converted into sodium shot in the usual way. The apparatus consisted of a 500-ml. three-neck flask fitted with a water-addition pipet or gas inlet tube, stirrer, and Friedrich's reflux condenser, a t the top of which was a lowtemperature, glass-helix-packed, jacketed, fractionating column. By means of circulating pumps and cooling baths the column was automati-
Present address, Sherwin-Williams Company, Chicago, Ill. Present address, U. S. Navy.
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All of the experiments wcre run with 125 grams (1.26 moles) of n-heptane and 33.3 grams (0.25 mole) of aluminum chloride. When the reaction time was 4 hours, the promoter was uniiormly added during the first 2 1 / 2 hours. Otherwise, the promoter was added during Ghe whole time of reaction.
ISOBUTANE/OIL RATIO w N O AL AL
-WITH
17
z > W
IO
8
8
ISOBUTANE /LIGHT Figure 1
PARAFFIN
RATIO
I
Conversion and Relative Yields from AluminumAluminum Chloride Experiments
cally maintained a t a temperature so that isobutane was s l o ~ l y and continuouslg distilled during the reaction, higher-boiling hydrocarbons being returned to the flask. Isobutane vaporized a t the head of the column, passed through soda lime and calcium chloride, and was collected in carbon dioxide-cooled ampoules. Water for catalyst promotion n-as uniformly added directly to the flask from a specially designed pipet a t thc rate of 0.05 ml. per 3 minutes. When hydrogen chloride was used it as passed from a Harshaw Chemical cylinder directly into the reaction mixture. An experiment, was carried out by first carefully displacing all air and nioisture from the whole apparahs by purified nit,rogen. The aluminum chloride, heptane, and metal were then added to the flask, and the reaction was carried out under the conditions indicated in Table I. Refractionation of the isobutane in a lowtemperature Podbielniak column showed it t o be a t least 957, pure. Smallamounts of propane 11-erefound in a f a y experiments; any material boiling above isobutane was added to t,lie liquitllayer products. The residual reaction mixture was vacuum-distilled a t 95-100 C. and 10-15 mm. to collect higher. hydrocarbons, which were then fractionated into a light pa,rafin cut (‘2C7), n-heptane, and a heavy paraffin cut (C, and abov dark red catalyst, lower layer was decornposed by the ad ether. Filtration of the et,her solution to remove unreacted metal, hydrolysis of the filt’rate,and distillation of the ether gave the unsaturated oil. The ether probably reacts with the complex t’o form an ether-aluminum chloride addition compound which i.; then hydrolyzed: O
catalyst complex $- (CJ-T6)ZO ----‘+- A1C1,.(C211J,0 4- oil Addition compounds with ether are l m o ~ n(4).Tho weight of oil obhined by direct hydrolysis of the complex or by reaction witli ether first was the samc, but direct hydrolysis had the serious disadvant,ageof allowing the metal to displace hydrogen from the aqueous acid layer, which made it difficult to separate the oil. I S Q B U T A N E / O I L RATIO
f
m - N O
METAL
-WITH
17
17
ISOBUTANE/LIGHT Figure
METAL 17
PARAFFIN R A T I O
2. Conversion and Relative Yields from Metal-Aluminum Chloride Experiments
VOl. 48, No. 2
‘I’he whole proccdure was standardized so that fairly satiafutory checlis, considering the nature of the reaction and analysis, could be obtained from duplicate experiments; e.g., two expcriments similar l o 17 (Tablc I) go,w 84.0 and 83.57, conversions> compared to 84.7%; 69.0 and 66.77, isobutane, compnred to 67.8%,. The results of several experiments TT-ith water or hydrogen chloride-promoted aluminum chloride, with and ivithout aluminum, magnesium, or sodium, are summarized in Tablc I and Figures 1 and 2. All reactions were run at 95-100” C. and at alrnospheric pressure with a mole ratio 07 tieptancialuIninum chloride of 5 and a reaction time 01 4 hours, unless othcrn-isc indicated. The amount of metal prcsent was 5070 with rcspc:cL to the weight of aluminurn chloride. The products from t h e cxperiments were placed in four groups: isobutane, light paraffins (C&,), heavy paraffins (C, nntl higher), and the unsaturated oil obtained by hydrolysis of thc catalyst complex. I n Table I the percentage yields 01 thcsc products arc calculated, on the basis both ol heptane taken and of total products. From thcsc data the relative yields of isobutane, oil, and light paraffine are shown by means of the ra,tios: isobutane/oil and isobutanc/light paraffins. The conversions and relative yields are given in Figures 1 and 2 by bloclts w h o x heights are proportional t o per cent conversion, top widths to isobutane/oil ratio, and bottom n-idths to isobutane/light p a r ~ i i h rat,io. ALUMINUM C H L O R I D E A C T I V I T Y
ICxperimciit I v a s a blank to dctermiiie t h e act ivity of aluniinurn chloridc without the addition of water or hydroym chloride. Since the same cat’alyst was used throughout and since relat.ivcly large amounts of promoters were to be added, the fact that tlie duminum chloride \\-a&riol absolutely pure and therefore inactive n-as considered unimportant. The low ratio of isobiitam!! light, paraffins shows that tho extent of cracking-isomeriaation (or “destructive isomerizatiou”) to form isobutane is s o m c ~ h o lehs t than the isomerization reaction, the latter tern being uscti broadly to cover the formation of light paraffins. I n experiment 9 the same catalyst, was modified by the addition of metallic aluminum. It was surprising to learn that there was no grcnt changc in the conversion and in t,hc over-all distribution of products. Thc greatest effect was the diminished oil yield which thcroby in.. crcnsed the isobutane/oil ratio (Figure 1). Io experiments 10 and 11 the catalyst \vas modemtely promoted by the addition of 1.5% water. T n the absence of aluminun1 this brought the conversioii to and increascd both the isobutanc/light parafin ratio and the isobut,ane/oil ratio: that is, the over-all reaction was increased, especially crnrliirrgisomcrization to form isobutane, but the isomerization t o iiglit paraffins was decreased. The addition of aluminum to tlris pro-. moted catalyst in experiment 3 1 had a marked effect: convcrsion W R S decreased, the formation of both isobutane and nil R-U decreased, with the reduction in oil yield being slightly greater, and the yield of light parafins Tras greatly increased (Figure 1). Comparison of experiments 1 and 9 with 10 and 11 indicat,cs that aliirninum exerts a much greater influence on a prcrnoted react,ioii, suoh as 10, where the cracking-isomerization reaction is relatively more important, than the isomerization t o light paraffins. Thc sniallcr yield of oil in 11 probably permits the catalyst lower layer to retain greater catalytic activity than that from the otherexperiments. I n view of this, the high yield of light paraffins, ant1 t,he very substantial yield of isobutane, experiment 11 is probably the most efficient of this group of four experiments.
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OF HEPTANE-METAL-ALUMINUM CHLORIDE EXPERIMENTS TABLEI. SUMNARY Experiment No. Added t o AICla, %"
n;i
