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INDUSTRIAL AND ENGINEERING CHEMISTRY
(8) Ihid., 2,474,776 (1949). D'Alelio and Reid, J . Am. Chem. Soc., 59, 109 (1937). (10) Dillon, Physics, 7, 73 (1936). (11) Fuson, Robinson, and Behr, J . Am. Chem. Soc., 63, 623 (1941). (12) Haworth and Hey, J . Chem. Soc., 1940, 361. (13) "Hycar Blue Book," Section 11, Group C, p. 1, Cleveland, Ohio, B. F. Goodrich Chemical Co., 1943. (14) Ihid., Section VII, Group B, p. 2 (1944). (15) Medard, Bull. soc. chem., 3, S o . 5 , 1343 (1936). (16) Sanna, Gazz. c h i m . itnl., 57, 761 (1927). (17) Sherrill, Schaeffer, and Shoyei,. J . Am. Chem. Soc., 50, 474 (1928). (9)
(18) (19) (20) (21) (22) (23) (24)
Vol. 45, No. 1
Tryon, U. S. Patent 2,368,073 (1945). Ihid., Ihid., Ibid., Ibid.,
2,425,288 2,463,829 2,474,793 2,504,981
(1947). (1947). (1949). (1950).
Wenker, J . Am. Chem. Soe., 59, 422 (1937). Wojcik and Adkins, Ibid., 56, 2419 (1934).
RECEIVED for review January 2.5, 1951. ACCEPTEDAugust 20, 1952. Presented in part before the Division of Rubber Chemistry a t the 112th and 117th hfeetings of the ~ M E R I C A X c ~ E h r I c . 4 SOCIETY. ~ S e w York, September 1947, and Detroit, Blich., April 1950. respectively.
Houdriforming Reactions STUDIES WITH PURE HYDROCARBONS HEINZ HEINEYIANK, G. A. MILLS, J. B. HATTM\.IAN, AND F. W'. KIRSCH Houdry Laboratories, Houdry Process Corp., Marcus Hook, P a .
YDROCARBOS reactions occurring in the Houdriforming of pet'roleuin naphthas have previously been defined (8,11) as dehydrogenat'ion, dehydroisomerization, isomerization, hydrocracking, and dehydrocyclization. Houdriforming catalysts are polyfunctional, consisting of special preparations of precious metals of group VI11 of the periodic table on an acid support. They possess primarily dehydrogenation and isomerization properties. (Referring to these two properties, the catalysts are also defined as dual function catalysts.) Under certain operating conditions the catalyst may promote some side reactions normally associated with cracking activity. The use of complex hydrocarbon mixtures such as petroleum naphthas usually permits only group analyses and, while changes in composition and charact,er of the naphthas can be follo~ved, the ability of the catalyst to promote specific reactions is obscured. The present paper presents data obtained in the treatment of pure hydrocarbons and mixtures of pure hydrocarbons with a typical Houdriforining catalyst which throw some light on the type of reactions involved. Haensel and Donaldson (5) have recently described similar experiments for reforming with a different catalyst and a t different conditions. The data presented here were obtained wit,h hydrocarbons believed to be typical of those occurring in petroleum naphthas and at, conditions which are similar t o those used in naphtha reforming, not necessarily the optimum conditions for the specific reaction. APPARATUS AND METHODS
Esperiment,al equipment comprised an isothermal reactor consisting of a stainless steel tube in a lead bath. A stainless steel liner, 2.5 em. inside diameter and 100 em. long with a central axial thermocouple well, fit'ted snugly into the tube and contained the catalyst. The liner was welded to the reactor head and directly connected to the inlet lines t,o avoid channeling through the space between reactor and liner walls. Quick catalyst changes could be made by removing and replacing liner and reactor head with a second set. The catalyst in the reactor, which in these experiments was a typical Houdriforming catalyst, was spaced with about an equal volume of quartz chips to prevent excessive temperature drops caused by endothermic reactions. Temperature gradient,s between lead bath and center of the catalyst bed were not more than ~ k 3 F. O The catalyst section was preceded by a quartz preheater section. Oil and hydrogen feed entered this section and were mixed in it after having passed separate coil preheaters in a lead bath. Hydrocarbons were charged with a Hills-McCanna pump and hydrogen was introduced from a high pressure cylinder through a high pressure rotameter. An automatic pressure controller (Gismo, Fisher Governor Co.) maintained constant presBure, either 300 or 600 pounds per square inch gage. Reaction products were condensed in a coil condenser at operat,ing pressure and liquid products were collected in one of two
high pressure separators. The product stream was periodically switched between the separators, one of which was then depressurized prior t,o withdrawing liquid. Gas from the separators was depressurized in the pressure controller and met,ered in a u-et-test gas meter, and samples were collected for analysis in a Consolidated Engineering Co. mass spectrometer. Liquid product from the separators was stabilized to Cs+ naphtha in a Podbielniak Ileligrid column, 22-mm. inside diarneter and 30 em. long, a t a 10 t o 1 reflux ratio to a 75" F. overhead temperature. The stabilized product was separated into aromatic, olefinic, and paraffinic-naphthenic fractions by chromatographic adsorption and subsequent desorption on silica gel, following the principle of ASTM method D 936-49T. Both fracbions were distilled in high temperature Podbielniak columns, 13-mm. inside diameter and 45 em. long, a t a 50 to 1 reflux ratio and a 50-plate efficiency. Individual cuts were analyzed by physical properties such as refractive index, infrared spectra ohtamed with a Perkins-Elmer spectrograph, etc. Amounts of individual compounds in the product were then converted to a charge basis and conversions were calculated under proper consideration of density changes. Weight. ' balances were above 95 weight % and losses were distributed over products to obtain results on a no-loss basis. Hydrocarbons used were obtained from Phillips Petroleum Co. in technical or pure grades and hydrogen (99.770+) was procured from Liquid Carbonic, Inc. EXPERIMEUTAL RESULTS A T D DISCUSSION
The dehydrogenation of naphthenes to DEHYDROGENATION. aromatics is of prime importance in Houdriforming, permitting production of a great number of aromatics from the corresponding cyclohexanes, of which benzene, toluene, and xylene are of most immediate commercial interest Isomerization of cyclohexanes to methylcyclopentanes is undesirable for the production of aromatics because of the greater difficulty of converting methylcyclopentane to benzene, although the reverse reaction is, of course, desired and will be described later. The thermodynamic equilibrium is in favor of the alkylcyclopentanes over cyclohexanes a t all temperatures under consideration Cyclohexane and methglcyclohesane were used as typical hydrocarbons in this study. Cyclohexane is dehydrogenated over the dual function catalyst to benzene in yields which are close enough to equilibrium even a t a liquid hourly space rate of 3 to be considered equilibrium yields in view of the uncertainty of calculated equilibrium values (Table I). I n all cases thermodynamic calculations have been made on the basis of data from BPI Project 44. A t higher temperatures, such as 950" F., some cracking and isomerization of cracked products occur. Table I1 shows that a conversion of 99.7% is obtained a t 950" F , 300 pounds per square inch gage, a liquid space velocity of 3 volumes per volume per hour and hydrogen to cyclohexane feed
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INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1953
ratio of 4 moles. It is apparent that some ring splitting leading to hexanes and lower molecular weight compounds occurs as well as complex rearrangements leading t o C7 paraffins which are absent from the feed stock.
TABLE IV. DEHYDROGENATION OF METHYLCYCLOHEXANE (Liquid space velocity = 3 volumes/volume/hour, Hz-methylcyclopentane ratio = 4 moles) Pressure Aromatics, % Lb./Sq. Ih. Exptl. Calcd.' Temp., O F. Gage 85 800 300 83 92 96 300 900 45 800 600 48 '
TABLEI. CYCLOHEXANE-BENZENE CONVERSION (Hz-oil ratio = 4 moles) Pressure Lb./Sq. Ih. 300 600
Temp., 800 900 950 800 950
a
F.
Mo!e % Benzene in Product Found a t 3 vol./vol./hr. Calcd. 72 70 89 90 96 93 33 31 92 94
TABLE11. CONVERSION OF CYCLOHEXANE x
3 volumes/ (960° F., 300 pounds/square inch gage, liquid space velocit volume/hour, Hz-cyclohexane ratio = 4 molesy = wt. % VOl. % of Feed of Feed 6.0 .. Ha 0.4 c1 0.4 .. Ca 0.3 Ca 1.7 C4 91.2 82:4 C6t n-Pentane and isopentsme .. 0.49 0.15 .. C clopentane n-%exane 1.66 .. 2-Methylpentane 1.17 .. 3-Methylpentane 0.90 2 2-Dimethylbutane 0.17 .. 0.10 2:3-Dimethulbutane .. Methylc yclopentane .. 1.30 .. Cyclohexane 0.30 n-Heptane 0.25 .. 0.67 2-Methylhexane .. 0.30 3-Methylhexsne 0.02 3-Ethylpentane .. 0.05 2,2-Dimethylpentane .. 0.20 2,3-Dimethylpentane 2,4-Dimethylpentane 0.15 3 3-Dimethylpentane 0.39 .. 2:2,3-Trimethylbutane 0.10 Benzene .. 91.50 0.04 Unknown
.. ..
pentane. It appears from the data in Table I11 that isomerization has caused redistribution of primary products toward equilibrium and that the dimethylbutanes are relatively slow to form. This corresponds to observations made with Friedel-Crafts type catalysts (9) and points to a similarity of the acid function of Houdriforming and Friedel-Crafts catalysts. While the analytical results are accurate, the over-all error in determining C7 isomers may be relatively large. It is, however, significant that all isomers are present in the product. Methylcyclohexane gives equilibrium yields of toluene (Table V), just as in the case of cyclohexane-benzene, DEHYDROISOMERIZATION. Alkylcyclopentanes are usually present in petroleum naphthas and frequently constitute 60 to 80% of the naphthene constituents. Their conversion to aromatics is therefore a necessity in Houdriforming for aromatics production.
..
.. .. .. .. ..
Total
100.00
4 Isomerization of cyclohexane occurs so that methycyclopentane constitutes 82% of the Cg naphthenes in the product; the equilibrium value for methylcyclopentane-cyclohexane isomerization a t these operating conditions is 91%. All of the CS and C7 paraffin isomers are present. I n Table I11 are given equilibrium distribution of these isomers along with product distributions from Table 11. n-Hexane is the primary cracking product of cyclohexane; n-hexane, Zmethylpentane, and 3methylpentane are primary cracking products of methylcyclo-
0
31
I
5
I
I
7
SPACE RATE, V/V/HR Dehydroisomerization of Methylcyclopentane
Figure 1.
Temperature, 950' F.; pressure, 300 pounds per square inch gage: hydrogen-methylcyclopentane mole ratio, 4
Methylcyclopentane (MCP) is the alkylcyclopentane most difficult to dehydroisomerize (IO) and the methylcyclopentanecyclohexane equilibrium strongly favors methylcyclopentane. Thermodynamic considerations show that benzene yields should increase with temperature. Experimental results confirm this TABLE111. PARAFFINS FROM CYCLOHEXANE HOUDRIFORMING (Table V) and also show that thermodynamic equilibrium is (950' F.; 300 pounds per square inch gage, liquid space velocity = 3 volapproached with decreasing space rate or increasing contact umes/volume/hour, Hz-cyclohexane ratio = 4 moles) time. While data in Table V, which show yields of 100% of cs, % equilibrium a t 950" F. and a space rate of 2 volumes per volume Thermodynamic equilibrium Exptl. per hour, are based on yields of aromatics and naphthenes only n-Hexane 26 40 and exclude side reactions such as cracking to paraffins, Figure 1 2-Methylpentane 28 29 3-Methylpentane 18 22 gives data for benzene formation from methylcyclopentane as a 2,2-Dimethylbutane 19 4 function of space rate, expressed as per cent of the benzene 2,3-Dimethylbutane 9 5 equilibrium concentration. The yields in Figure 1 are based on 100 100 Total total product, including paraffins. The side reactions occurring are of considerable interest. While n-Heptane 3-Methylhexane the extent of cracking and other side reactions varies with such 2-Methylhexane 2.2-Dimethvloentane factors as catalyst conditioning and cracking inhibitors present, the product distribution is quite constant in the range 950" to 975" F. I n Table VI there are presented data for the conversion _r____ 'rimethylbutane of methylcyclopentane a t three temperatures, 300 pounds per square inch gage, a space rate of 6 volumes per volume per hour Total 100 100 and hydrogen-methylcyclopentane ratio of 4,using fresh catalyst
-
I---
-
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Vol. 45, No. 1
However, aromatic distribution shifts appreciably toward benCONVERSION OF ~IETHYLCYCLOPENTANE TO zene with increasing temperature and some higher aromatics are AROMATICS~ formed a t lower temperatures, probably by alkylation and dis(300 pounds/square inch gage, Hz-methyloyclopentane ratio 4 moles) proportionation. The role of aromatics in suppressing cracking Lsv, Aromatics, Mole % Xaphthenes, Mole % will be discussed later Temp., Vol./ Calcd. Calcd. I n addition to the compoundslisted in Table VI, some pentanes F. Vol./Hr. equil. Exptl. equil. Exptl. and traces of cyclopentane and C?paraffins are formed. 900 3 79 56 21 44 It has previously been shown that, disregarding cracking, the 950 3 92.2 73 7.8 27 2 92.2 92 7.8 8 dehvdroisomerization of methylcyclopentane in Houdriforming a Formation of paraffins omitted by calculation. proceeds a t close to equilibrium values when operating a t 950" F. and 300 pounds per square inch gage. I n Figure 2 are presented the thermodynamic equilibrium yields for benzene from methylwith high cracking activity. While methylcyclopentane concyclopentane and for toluene from methylcyclohexane as a version increases with temperature, it is apparent that catalytic function of pressure. It is apparent that the production of benhydrocracking increases between 900" and 950' F., as evidenced zene in attractive yields is limited by the hydrogen partial presby the larger ratio of Cd to dry gas, and that there is no further appreciable shift in gas composition between 950' and 975' F. Hydrogen production is slightly smaller than that corresponding to the benzene production observed. This indicates that some hydrogen has been consumed in hydrocracking.
TABLE V.
-
O
TABLEVI.
COMPOSITION OF HYDROCARBON PRODUCTS FROX METHYLCYCLOPEXTANE
(300 pounds/square inch gage, liquid space velocity = 6 volumes/volume/ hour, Hz-methylcyclopentane ratio = 4 moles) Temperature, F. 900 950 975 Wt. balance Conversion, % a.2 Ha % 4. a 9.8 CI-ICI, % 24.9 25.6 26.6 Cs+ parftEns, % 33.5 38.6 40 Aromatics, % Light hydrocarbons CI wt. % of fraction 16 0 12.9 12.5 11.2 21.4 15.7 ~ 2 wt. ' % of fraotion 12.9 13.4 cI' wt. % of fraction 14.3 47.7 63 G' wt. % of fraction 58.4
-_
__
__
100 0
100.0
100.0
3. 8 4.1
Aroniatics Benzene vol. % Toluene ' vol. % xylenes' VOI. % Ethvlbehzene. vol. %
35.2 18.6 38.3
100.0
100.0
92 2.2
97.6 0.8
2.1 -
0.2
3.7
100.0
1.4
100.0
4.8 3.7
33.6 17.9 40
100.0 100 0
0 0
100.0
Cs paraffins are produced by ring splitting of methylcyclopentane. If there is random, nonpreferential cracking, the ratio of is0 t o normal CSshould be 1.5 and the ratio of 2-methylpentane to 3-methylpentane should be 2 These ratios are also close to those prevailing a t thermodynamic equilibrium. Data in Table VI show the is0 to normal ratios to be 1.6, 1.6, and 1.5 at 900°, 950°, and 975' F., respectively, and the 2- to 3-methylpentane ratios to be 1.9 a t all three temperatures, Isomerization of primary cracking products t o dimethylbutanes occurs although it does not proceed to equilibrium a t the operating conditions given. The similarity of the isomerization function of Houdriforming and Friedel-Crafts catalysts (a) is again apparent. The main aromatic product formed is, of course, benzene.
BENZENE+ 3 HZ FROM CH OR MCP
I
200
Figure 2.
TABLE VII. CONVERSION OF METHYLCYCLOPEKTANE
I
I
I
boo
Thermodynamic Equilibrium
Temperature, 950' F.; hydrogen-oil mole ratio, 4
sure t o a inuch larger extent than that of toluene. On the other hand, coke formation will increase with oil partial pressure (14) and molecular size (IS), limiting the lowest pressure feasible for continuous operation a t a fixed hydrogen oil ratio. Table VI1 presents data for methylcyclopentane conversion a t four prcssures and otherwise equal operating conditions, showing that naphthene conversion and cracking as expressed as C4 and coke build-up increase with lowered pressure. Since coke build-up cannot be tolerated for continuous operation, a pressure range between 250 and 400 pounds per square inch gage appears suitable. Methylcyclohexane operation is favorable a t pressures up t o 500 pounds per square inch gage from a toluene yield consideration (Figure 2) and down to 250 pounds per square inch gage from a cracking consideration It is, therefore, possible to feed mixed benzene and toluene formers a t 300 pounds per square inch gage and obtain good yields without coke formation. ISOMERIZATION. Isomerization has to some extent been discussed in the preceding sections. It has been shown that cyclohexane forms methylcyclopentane and that paraffins generated by cracking of naphthenes isomerize. Isomerization of paraffins is secondary only to the formation of aromatics in the upgrading of naphthas for motor gasoline production. Recent data obtained with other dual function catalysts (3, 6) have shown the necessity for proper selection of catalyst activity and operating conditions to obtain maximum isomerization and minimum cracking. Since-upgrading of mixed feed stocks must take place a t rela_tively high temperatures in order to permit sufficient dehydrogenation of naphthenes, isomerization of paraffins must be carried out a t temperatures at which cracking of isomers formed is expected to reduce isoparaffin yields. With the Houdriforming catalyst employed, isomerization of normal paraffins increases with temperature until hydrocracking of isoparaffins formed overtakes isomerization (Figure 3). Reforming of typical naphthas a t high temperatures is possible with good isomer yields I
(950" F., liquid space velocity = 6 volumes/volume/hour, Ha-methylcyclouentane ratio = 4 moles) Pressuie, Lb./Sq. In. Gage 200 250 300 600 Naphthene aromatic conv., mole % 53 51 45 10 4.8 3 2 0 c4, wt. % Positive Negligible Coke build-up
I
400 600 800 TOTAL PRESSURE, PS.1.G
I N D U S T R I A,L A N D E N G IN E E R IN G C H E M I S T R Y
January 1953
133
HYDROCRACKING. Cracking occurring as a side reaction in Houdriforming at severe conditions, if closely controlled, may be utilized to supply part of the butanes for vapor pressure adjustment. Liquid products normally obtained give an increase in volume which partially compensates for the volume loss inherent in naphthene dehydrogenation. Whatever cracking occurs under these conditions is predominantly of the hydrocracking type and more closely resembles catalytic than thermal cracking ( 4 ) . No olefins or coke are formed. Data in Table IX show the moles of lighter hydrocarbons formed per 100 moles of n-heptane for the case of a severe
Figure 3.
TEMP , O F . n-Heptane Isomerization
I
Pressure, 300 pounds per square inch gage; liquid space velocity, 4 volumes/volume/hour
because aromatics formed from naphthenes act as cracking inhibitors, as will be shown later. Using n-heptane as a typical normal paraffin, results obtained a t n space rate of 4 volumes per volume per hour are presented in Figure 3. It is apparent that maximum isomerization with this catalyst occurs in a temperature range which is favorable to dehydrogenation. Further improvements in the isomer yield plotted in Figure 3 can be made by reducing space rate or increasing contact time. On the basis of results obtained in earlier work with a different catalyst (7), it was thought that the presence of naphthenes and aromatics might influence paraffin isomerization in Houdriforming. n-Heptane was therefore fed to a Houdriforming catalyst in the presence of 15% benzene after it had been shown in a blank run that benzene was recovered in equilibrium yield with regard to hydrogen and cyclohexane under these conditions. Data in Table VI11 show that a t the same operating conditions cracking is appreciably inhibited and that isomer yield is increased by this inhibitor of the cracking of C, isomers. This effect is even more pronounced a t lower space rates. Inhibition of cracking by aromatics is an effect which is also observable with Friedel-Crafts catalysts (12).
TABLEVIII.
3 5 7 C ATOMS PER MOL OF PARAFFIN
Figure 4. Distribution of Cracked Products during Houdriforming of n-Heptane Charge, 0 = n-heptane; A = n-heptane plus 15 % benzene
reaction with fresh catalyst and a mild reaction in the presence of an inhibitor a t the same operating conditions. Figure 4 shows that product distribution is not affected by the presence of aromatics, even though cracking is inhibited. Assuming that primary cracking may lead to the following pairs per mole of n-heptane: C, plus CS, Cg plus CZ,Ca plus CI, it is surprising to note the large amount of CCformed relative to CSor even relative to secondary decomposition products of Ca. While no conclusive explanation of this can be advanced a t this time, it is quite obvious that butanes are the main cracking products.
INHIBITION OF CRACKING
925' F., 300 pounds/square inch gage, liquid space velocity = 6 volumes/
volume/hour)
5
Charge Liquid recovery, 5% C7 paraffin recovery, % iso-C7, 5% of feed Results corrected for 100% CT.
76.0
85.0 79.8 44.2
70.5 36.0
TABLEIx. CRACKING PRODUCTS (9Z0
n-Cr plus 15% Benzene5
n-C7
FROM
?+HEPTANE
F., 300 pounds/square inch gage, liquid space velocity = 6 volumes/ volume/hour)
Moles/100 moles feed
n-Heptane
Results corrected for 100% C7,
n-C7
+
1570 Benzene=
TEMP, O F
Figure 5. Houdriforming n-Heptane to Aromatics
AROMATIZATION. Production of aromatics from paraffins is most likely to occur as dehydroaromatization. I n the Houdriforming of naphthas for motor gasoline production higher aromatic contents have often been observed in the product than can be accounted for on the basis of naphthene dehydrogenation and selective cracking to gas of nonaromatic compounds. Figure 5 shows aromatic yields (volume per cent of feed) for n-heptane operation a t 300 and 600 pounds per square inch gage. It is obvious that aromatization increases with increasing
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INDUSTRIAL AND ENGINEERING CHEMISTRY
temperature and decreasing pressure and that in commercial operation it is necessary to weigh and balance the relative position of isomerization, cracking, and aromatization for optimum operating conditions and for each type of feed stock. As has been discussed earlier in this and in another paper ( 7 ) , the presence of aromatics in the feed modifies appreciably results obtainable with pure paraffins. ACKNOWLEDGNIEKT
The authors wish to acknowledge the many valuable suggestions made and the assistance given by members of the staff of Houdry Process Corp., particularly A. G. Oblad and J. J. Donovan.
(Houdriforrning Reactions)
CATALYTIC MECHANISM G. A. MILLS, HEINZ HEINEMANN, T. H. MILLIKEN, AND A. G. OBLAD
T
H E hydrocarbon reactions in Houdriforming are complex inasmuch a8 consecutive reactions of various hydrocarbon types occur under the influence of a polyfunctional catalyst. The over-all chemical reactions which take place in Houdriforming petroleum naphthas and certain pure compounds have been discussed in the first section of this paper and in earlier papers ( 8 , l l ) . However, in order to deal adequately with catalyst behavior, as well as with the influence of operating variables, it is essential to understand the mechanism of the hydrocarbon reactions. This mechanism necessarily involves the particular action of the catalyst. Houdriforming catalysts possess what is regarded as two different types of catalytic properties-namely, an “acidity,” the main present function of which is t o effect isomerization and, secondly, a property probably associated with an electron defect structure which confers, in the present use, the ability to dehydrogenate and hydrogenate. Houdriforming catalysts consists of special supported preparations of precious metals of group VIII of the periodic table. Other reforming catalysts having dual properties have been described ( S , 5 ) . The determination of reaction mechanism has been approached in the present investigation by studying experimentally and theoretically the reactions of four selected c6 hydrocarbons in the presence of three catalysts representing the three significant catalyst types. The hydrocarbons chosen were cyclohexane, cyclohexene, methylcyclopentane, and methylcyclopentene. These hydrocarbons were tested using a single function isomerization catalyst, a single function dehydrogenation catalyst, and a dual function isomerization-dehydrogenation Houdriforming catalyst. The same components used in the two single function catalysts were used in combination in preparation of the dual function catalyst. From the information developed and a general knowledge of the Houdriforming reactions of aromatics and paraffins, a coherent concept of the reactions occurring in Houdriforming has been deduced. EXPERIMENTAL
Cyclohexane, cyclohexene, and methylcyclopentane were obtained commercially in pure grade. Methylcyclopentene was obtained by isomerizing cyclohexene (1) and purifying by distillation. A mixture of about equal parts of 1- and 2-methylcyclopentene was used.
Vol. 45, No. 1
The apparatus used in carrying out the catalytic reactions was of the isothermal type and has been described. Runs weie made under conditions in the range used in Houdriforming: 950” F., 300 pounds per square inch gage, a liquid hourly space rate of about 3 volumes per volume per hour, and a hydrogen to hydrocarbon mole ratio of 4. Analysis of products was obtained by use of silica gel adsorption, distillation, and chemical, infrared, refractive index, and mass spectrometer examination, as described in the first section of this paper. RESULTS AND DISCUSSION
It is convenient to consider the reactions which occur from the following viewpoints: the types of reactions possible; the types of reactions occurring to classes of hydrocarbons; the thermodynamic equilibrium limitation of reactions; and the catalytic influence on extent to which reactions actually occur. TYPESOF REACTIONS POSSIBLE. The hydrocarbon reactions which are important here may be classified as hydrogenationdehydrogenation (1) involving molecular hydrogen and (2) not involving molecular hydrogen-Le., disproportionation; and skeletal isomerization (1) without ring formation and (2) with cyclization. I n addition, change in carbon number can occur with a decrease by cracking or an increase by alkylation or polymerization. TYPESOF REACTIONS OCCURRING TO CLASSESOF HYDROCARBONS. The main reactions which may be expected t o occur a t the conditions obtained in Houdriforming with the different types of hydrocarbons are: 1. Paraffins may undergo dehydrogenation to form olefins, skeletal isomerization to isoparaffins and cracking. 2. Xaphthenes may undergo dehydrogenation to form cycloolefins, reversible isomerization of alkyl Cj rings to Cg rings, and cracking. 3. Aromatics may be considered to undergo only hydrogenation. 4. Olefins may hydrogenate to paraffins (or cyclo-olefins to naphthenes); dehydrogenate to diolefins and, in case of Ce ring diolefins, to aromatics; transfer hydrogen by disproportionation producing molecules of more and less saturation; isomerize; cyclize; polymerize; alkylate; and crack. It is evident that the list enumerated for olefins is very extensive in accordance with the well-known reactivity of olefins. THERMODYNAMIC EQUILIBRIUM LIIIITATIONS O F REACTIONS. It is next logical to consider from a thermodynamic equilibrium viewpoint the extent to which the projected reactions can possibly proceed. Here, as in all catalytic reactions, it should be kept in mind that the catalyst will determine which of the thermodynamically possible reactions are predominant. Also, even small concentrations of certain components such as olefins may be important, since they may constitute important intermediates in the instance that they are in rapid dynamic equilibrium. A set of operating conditions lying within Houdriforming operating range can be considered-namely, 950’ F.,300 pounds per square inch gage, and a 4 to 1 hydrogen to hydrocarbon mole ratio. Obviously more extensive calculations would show the influence of temperature, pressure, and hydrogen on the reaction but are beyond the scope of this report. For C6 paraffin molecules the extent of dehydrogenation to straight-chain olefins is small, for example:
n-hexane
+1-hexene + Hs;
C6H12 CiHii ~
0,003
These and subsequent data were calculated using results of API Project 44. On the other hand, isomerization of normal paraffins should proceed extensively, since at 950’ F. the following equilibrium is
