134
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
January 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
predicted: n-hexane 26%, 2-methylpentane 28%, 3-methylpentane IS%, 2,2-dimethylbutane lS%, and 2,3-dimethylbutane
10%. C6 paraffin molecules are very unstable toward simple cracking to one paraffin and one olefin molecule or toward hydrocracking to two paraffin molecules. The Ca paraffin is relatively stable thermodynamically with respect to formation of naphthene plus hydrogen. The extent to which cyclohexane will form benzene and methylcyclopentane is illustrated in Figure 6. This has been calculated on the basis of no olefin formation. The total olefins found experimentally are 1 to 2%. I n fact, an olefin content of 1 to 2% in the product has been observed consistently in extensive work on Houdriforming (8). Since the exact free energies of the diolefins are not available, only approximate quantities can be calculated by the following method. If the free energies of formation of cyclohexane, cyclohexene, and benzene are plotted against temperature, it is possible to estimate the free energy of formation of cyclohexadiene. I n the absence of other evidence, this has been done by taking the free energy to fall midway between that of cyclohexene and benzene; then the calculated equilibrium mole per cent concentrations are (for 950' F., 300 pounds per square inch gage, and 4 to 1 hydrogen to cyclohexane mole ratio) cyclohexane 0.4, cyclohexene 4.4, cyclohexadiene 21.2, and benzene 74.0. This computation may indicate too high a concentration of cyclohexadiene. If the free energy of formation of cyclohexadiene were as large as that of cyclohexene a t this temperature, the calculated mole per cent concentration under the same conditions given would be cyclohexane 0.45, cyclohexene 5.6, cyclohexadiene 0.3, and benzene 93.6. However, even this predicted olefin concentration is much larger than the observed olefin concentration. CATALYTIC INFLUENCE O N EXTENTOF ACTUALREACTIONS. From the considerations presented in the previous section, it is recognized that there are several highly desirable reactions, notably isomerization of paraffins and alkylcyclopentanes and .dehydrogenation of cyclohexanes, as well as undesirable reactions, for example, cracking and coke formation. However, many of the undesirable reactions are thermodynamically feasible under conditions favoring the desired reactions. Moreover, it is known that certain catalytic properties necessary to hasten the desired reactions may also promote undesirable reactions. For example, a catalyst acid property functions to catalyze not only isomeriza-
135
tion but also cracking, polymerization, and hydrogen transfer, which reaction may lead to coke formation. It is thus necessary t o balance the dual function catalyst in properties and to utilize this catalyst under conditions of optimum compromise. I
I
I
~
I
I I
!
I
I
I
I
I
I
I
c
I
k METHVLCYCLOPENTAHE
I--/ I
Ii
I
1
IP
'J
CYCLOHEXAH
0
P(PS1A)
Figure 6. Calculated EQuilibrium Composition us. Pressure for Benzene, Cyclohexane, and Methylcyclopen tane Hydrogen-hydrocarbon mole ratio, 4 to 1; temperature, 950' F.
Turning now to the experimental data, Table X summarizes the results obtained using the four key C6 cyclic hydrocarbons under the influence of three catalyst types. I n each of the experiments the total liquid product was over 90 weight 70 of charge. I n anticipation of the mechanism of reaction which is believed to occur, reference is also made to Figure 7 where, as will be discussed later, the possible reactions occur according to the arrows shown, with separate types of reactions occurring on isomerization and dehydrogenaTABLEX. EFFECTOF CATALYST TYPEAND HYDROCARBON STRUCTURE ON tion catalyst sites. Since catalysts promote both PRODUCT forward and backward reactions, it is to be under(950O F., 300 pounds/square inch gage, liquid hourly space velocity = 3, Hs-hydrocarbon = stood that, where only dehydrogenation is referred 4 moles) Vol. 9'0 of Liquid Product to, hydrogenation can likewise be catalyzed. Catalyst In considering the data it should be remembered ' IsomerisaDehydroDual that the results on pure compounds do not exactly Charge Product tion genation function predict the behavior of mixtures, because of the Cyclohexane Aromatic 2 92 92 Olefin 0 1 2 influence of one compound on another a t the Naphthene 98 2 1:5 catalyst surface. Paraffin 0 5 . 4.6 Ca/Cs ring ratio" >50 to 1 >25 to 1 1 to 4 Returning again to Table X and following the Methylcyclopentane Aromatio 3 7 49 data on each hydrocarbon, it is seen that cycloOlefin 0 4 2 Naphthene 95 80 23 hexane is not altered in the presence of an isomeriParaffin 2 9 26 zation catalyst but is rapidly converted to equilibCs/Cs ring ratio5 Traces of Traces of 1 to 4 cyclohexane cyclohexane rium yield of aromatics by both the single dehyCyclohexene Aromatic 8 92 83 drogenation and dual function catalyst. Also Olefin 86 3 2 Naphthene 5 0 11 listed are the CS-C~ring ratios in the naphthene Paraffin 7 5 4 plus olefin products. It is significant that only Cs/Cs ring ratioa Approx. >50 to 1 Approx. 1 to 10 1 to 4 small amounts of Cg ring compounds from isomeriMethylcyclopentene Aromatic 7 16 48 zation were found in the product from either of the Olefin 74 2-3 1 Naphthene 19 48-70 13 single function catalysts. I n contra&, the Cg ring Paraffin 0 34-11 38 compounds predominated in the naphthene plus Ce/Ca ring ratio" 1 to 14
