Separation of Oxygenated Compounds and Hydrocarbons by Adsorption Compounds Typical of Certain Coal Degradation Products JACOB ENTEL, CLARENCE H. RUOF, AND H. C. HOWARD Coal Research Laboratory, Carnegie Institute of Technology, Pittsburgh, Pa. Adsorption was investigated as a supplementary process for the separation of close boiling mixtures of aromatic and saturated hydrocarbons and oxygenated compounds such as those present in the hydrogenolysisproducts of the esters of the aromatic acids recovered from the oxidation of coal. Adsorption isotherms of indene, indan, cis-hexahydroindan, phthalan, hexahydrophthalan, tetrahydrofuran, o-xylene, and methylcyclohexane in pentane solutions each on Florisil, magnesia, Nuchar C, bentonite, alumina, and silica gel indicate that bentonite is the best adsorbent for the separation of phthalans from aromatic and saturated hydrocarbons. This has been confirmed by quantitative resolution of test mixtures of phthalan and indan, of hexahydrophthalan and indene, and of phthalan, indan, and cis-hexahydroindan. The separation of the aromatic from the saturated hydrocarbons was readily effected by silica gel.
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N RECEST studies on the nuclear structure of the water-
solution by any of the adsorbents employed and therefore the data are not included in the figure. The isotherms presented in Figure 1 indicate a variety of possible separations of the adsorptives from pentane solutions, but further discussion here is limited t o the most effective means of separation of the phthalans and aromatic and saturated hydrocarbons. For example, the specific adsorption capacity of silica gel for phthalans is shown in Figure 1 t o be much higher than that of bentonite; nevertheless, on this scale of operation at least, bentonite is indicated to be the better adsorbent for separation of the phthalans as it selectively adsorbs the phthalans but not aromatic or saturated hydrocarbons, whereas silica gel adsorbs both phthalans and aromatics. Silica gel is indicated to be the best adsorbent for the separation of the aromatics from the saturates. The flow rate through the bentonite is slow, but it can be increased by increasing the particle size-e.g., pelletizing-by increasing the pressure drop through the column with pressure on the head or vacuum on the bottom of the column, and by diluting the bentonite x i t h a filler such as Celite. Hydroxylated adsorptives and developers, hoxr-ever, should be avoided whenever pos-
soluble polycarboxylic acids from the oxidation of bituminous coals, the esters of the coal acids have been subjected t o hydrogenolysis to convert the carbalkoxy groups to methyl groups (6). The resulting methylated nuclei have been fractionally distilled in precision columns and the expected di-, tri-, and tetramethylbenzenes have been readily isolated and characterized. However, the bulk of the material boils above the tetramethylbenzenes and consists of close-boiling mixtures of aromatic and saturated hydrocarbons and oxygenated compounds which appear t o be mainly methylated phthalans. The present 1%-orkTTas undertaken to assess the possibility of further separation of these classes of compounds by means of adsorption processes. In order to determine the most effective adsorbent for the desired separations, experiments have been conducted on the quantity of different types of compounds adsorbed per unit quantity of adsorbent by a method similar to that used by llair and Forziati (4). Indene, indan, cis-hexahydroindan, phthalan, hexahydrophthalan, tetrahydrofuran, o-xylene, and methylcyclohexane were chosen as adsorptives [the term “adsorptive” to designate the substance to be adsorbed was suggested by Weil-Ualherbe (711 because they are available in this laboratory and because they are representative of the types of compounds which other studies have shown might be expected in the hydrogenolysis products ( I , $ , 6). Florisil, magnesia, Nuchar C, bentonite, alumina, and silica gel were chosen as typical of the available commercial adsorbents. Each of the adsorptives, in an accurately known concentration in pentane, was mixed intimately and allowed to come to equilibrium with an accurately known weight of adsorbent a t the prevailing room temperature. The volume of adsorptive adsorbed on a unit weight of adsorbent was then calculated from the change in refractive index of the pentane solution, with the assumptions that no pentane was adsorbed and that the refractive indices are a linear function of volume concentration. The data are plotted as adsorption isotherms in Figure 1; the methylcyclohexane was not preferentially adsorbed from the pentane
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pentane was first passed down a bentonite column to remove the phthalan and then down one of silica gel to separate the indan from the cis-hexahydroindan (Figures 4 and 5 ) . This procedure represents a most effective scheme for the resolution of mixtures of phthalans, aromatics, and saturated hydrocarbons such as constitute the hydrogenolysis products from the esters of the coal acids. In order to obtain information on the use of bentonite as an adsorbent for the separation of other types of oxygenated compounds which might be present in the hydrogenolysis products of the esters of the coal acids, adsorption isotherms of dibutyl phthalate, 2-niethylcyclo-
and the benzofurans show very little. It thus appears that bentonite is a selective adsorbent for those molecules in which an oxygen atom is in an unhindered position as in the phthalan type. While the dihydrobenzofurans and the phthalans are isomeric cyclic ethers x i t h close-boiling points, densities, and refractive indices, the adsorption of phthalans on bentonite and the nonadsorption of their isomers permit their separation. I t has been shown that phthalans form crystalline adducts with anhydrous stannic chloride in pentane ( 2 , 5 ) ; this property serves a3 a very sensitive qualitative test for the presence of even minute concentrations of phthalans and was used on the pentane effluent t o follow the elution process. The benzofurans do not form adducts nith stannic chloride and are much more readily ruptured on hydrogenolysis over copper chromium oside catalyst ( 1 , 2 ) . Thus, adsorption on bentonite, adduct formation with stannic chloride, and behavior on hydrogenolysis all are methods of demonstrating the difference in character of the oxygen atom and of effecting characterization of the isomers.
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ADSORBENTS
The adsorbents used were as follows: Grade F-20, 80-200-mesh Alcoa activated alumina (bluminum Ore Co., East St. Louis, 1ll.j; B237/2 bentonite powder, U.S.P. (Fisher Scientific Co., Pittsburgh, Pa.); W-5657,30-60-mesh Florid (Floridin Co., Inc., PVarren, P a . ) ; SO. 2641 adsorptive powdered magnesia (Westvaco Chemical Division, Food Machinery and Chemical Carp., iYew York, N. Y , ) ; Suchar C unground activated carbon (Industrial Chemical Sales Division, West Virginia Pulp and Paper Co., New
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refractive index of the solution, which was determined to zkO.0002. Ten milliliters of the solution was then pipetted into a bottle containing the adsorbent. Approximately 2 grams of adsorbent accurately weighed was used except in the case of the Nuchar, where only 1 gram was used because of its bulk. The bottle was tightly covered by a screw cap, shaken vigorously for 5 minutes, and allowed to settle for 15 minutes. The amount of adsorptive remaining in solution in the pentane was then computed from the refractive index. The amount remaining on the adsorbent was determined by difference and plotted in Figure 1 in terms of milliliters of adsorptive adsorbed per gram of adsorbent. The remainder of the original 10% solution of adsorptive in pentane was then successively diluted to approximately 8, 6, 4, 2, and 1%, and the process was repeated for each concentration. SEPARATION OF TEST MIXTURES
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Phthalan Hexahydrophthalan Tetrahydrofuran Dibutyl phthalate 2-Methylcyclohexanemethanol Anisole 2-Methyl-2,3-dihydrobenzofuran Benzofuran
Indan, boiling point 176’ C. a t 736 mm. and 1.5360, &-as obtained by hydrogenation of the pure indene over palladium on charcoal catalyst a t 25’ C. followed by fractional distillation. cis-Hexahydroindan, boiling point 166’ C. a t 740 mm. and n q 1.4698, was prepared by hydrogenation of the purified indene over a Universal Oil Products nickel catalyst a t 195’ t o 200’ C. followed by a careful fractionation in a column of 50 theoretical plates to remove the trans isomer. Phthalan (2),hexahydrophthalan ( d ) , 2-methylcyclohexanemethanol ( d ) , 2-methyl-2,3-dihydrobenzofuran( 1), and benzofuran ( 1 ) were pure samples described in the indicated references. +Xylene, methylcyclohexane, and anisole (Eastman Organic Chemicals Nos. 276, 946, 468, respectively), dibutyl phthalate (Fisher Scientific Co. No. D-30), and tetrahydrofuran (Du Pont Elchem) were commercial chemicals used without further fractionation except in the case of the tetrahydrofuran. ISOTHERMS
Approximately 5 ml. of adsorptive w m dissolved in 50 ml. of faactionated pentane and the composition was computed from the
support, were packed by adding small amounts of the adsorbent and tamping it tightly with a blunt glass rod. Pentane from a dropping funnel attached to the top of the column was then added
The progress of the separation Tyas readily followed by plotting the refractive index of the effluent against the weight of effluent. -4lthough large quantities of pentane were eluted first, only the last several fractions of pentane prior to the elution of the solution of the adsorptives were plotted; this was sufficient to establish the “solvent line.” With the test mixtures shown in Figures 2, 3, 4,and 5 a very high degree of separation of the components was effected, as demonstrated by the deviation of the refractive index from, and return to the solvent line when the first component was eluted and before the second component appeared. The mixtures employed for the test separations in this work included: 6 ml. of phthalan and 6 ml. of indan in 120 ml. of pentane on 120 grams of bentonite (Figure 2); 2 ml. of hexahydrophthalan and 3 ml. of indene in 100 ml. of pentane on 120 grams of bentonite (Figure 3); and 7 ml. of cis-hexahydroindan, 7 ml. of indan, and 7 ml. of phthalan in 125 ml. of pentane on 133 grams of bentonite (Figure 4). I n this last case the effluent was passed again through a column of 120 grams of silica gel to separate the cis-hexahydroindan from the indan (Figure 5). Recovery of the adsorptives ranged from 95 to 103qb, measured either by integration of the area under the refractive index curve or by desorption with ether followed by evaporation of the solvent. ACKNOWLEDGMENT
The authors wish to express their appreciation to Joseph B. Simsic and Daniel T. Muth for their assistance. LITERATURE CITED
( 1 ) Entel, J., Ruof, C. H., and Howard, H. C., J . Am. Chem. Soc., 73, 4152 (1951). (2) Ibid., 74,441 (1952). (3) Gauger, A. TV., and Breston, J. N., J . I m t . Petroleum. 33, 68 (1947). (4) llair, B. J., and Forziati, A. F , J . Research Natl. Bur. Standards, 32, 165 (1944). (5) Ruof, C. H., and Howard, H. C., ”Stannic Chloride Adducts with
Coal Degradation Products,” presented before Division of Gaa and Fuel Chemistry, 118th Meeting, AMERICAN CHEMICAL SOCIETY, Chicago, Ill.. 1950. (6) Ruof, C. H., Savich, T. R., and Howard, H. C., J . Am. Chem Soc., 73, 3873 (1951). (7)Weil-Malherbe, H., J . Chem. Soc., 1943, 303.
RECEIVEDfor review November 10, 1952.
Accepted November 28, 1952.
