OLEFI N-PARAFFI N SEPARATION B Y SUPPORTED CUPROUS CHLORIDE E R I C W. STERN Research & Detelopment Department, The M . 11: Kellogg Go., Jersey City 3. .V. J .
The ability of CuCl supported on Celite, Alundum, silica gel, and charcoal to separate 1 -pentene from mixtures with n-pentane was investigated a t atmospheric pressure and at several temperatures. Good olefinparaffin separations are obtainable even above room temperature and a t atmospheric pressure, provided the CuCl is highly dispersed. The degree of dispersion is influenced b y the surface area of the support, and the extent of coverage of the surface b y CuCI. The latter is determined by the method of preparation. Selectivity i s further enhanced b y decreasing the physical sorption of the paraffin. Contrary to expectation, CuCl supported on a charcoal with high surface area is not active in complex formation. A number of explanations of this phenomenon have been considered. Of these, an inactivating complex formation between CuCl and charcoal is favored.
THE fact that CuCl (either in solution or as a solid) \vi11 form complexes with various unsaturated materials has been known for some time. A survey of the literature of the past 25 years discloses wide application of these complexes for the separation of olefins from more saturated materials. Complexing ability of the dry solid salt is greatly enhanced by either mixing the CuCl with, or supporting it on, such porous materials as pumice. asbestos, sawdust. bauxite, charcoal. silica gel. and fuller's earth (4, 7, 8 ) . Solid adsorbents have been activated by a preliminary sorption-desorption of the 5.g ) , and a high degree of separation was obtained olefin (1. onl! at low temperature and high pressure ( 3 ) . It was of interest to examine the influence of various support materials on the ability of solid CuCl to complex with olefin and to determine whether these effects were due to the surface area or the nature of the support. Two supports of low surface area. Celite and Alundum, and two with high surface area. silica gel and charcoal, were chosen for this study. A silica gel with reduced surface area was also used. The separation chosen for examination was that of 1-pentene from n-pentane. T h e method of preparation of supported CuCl was such as to give presumably highly active material. Experimental
Materials. 1-Pentem, Phillips Petroleum, 95y0 material. n-Pentane. Phillips Petroleum, 99% material. Alundum, ?/ 16 x "16 inch pellets. Norton Co. Celite, 3 / 1 f i x 3 / 16 inch pellets, Johns Manville 408. Silica gel, 14-20 mesh, Fisher Scientific Co. Silica gel Lvith reduced surface area, obtained by sintering the above material under K9 for 4 hours a t 1550' F. Charcoal. 4-10 mesh, T v m BPL. Pittsburgh Coke and u Chemical Co. CuC1, reagent grade (min. 90701, Matheson, Coleman and Bell. Procedures. PREPARATION OF UXSUPPORTED CUCL. -4 saturated solution of CuCl in 1-pentene was prepared by slurrying the reagent grade material in the hydrocarbon. T h e solution was filtered and the dissolved CuCl reprecipitated by the addition of n-pentane. The purified CuCl was collected on a filter and subsequently dried a t 300" F. These operations were carried out in a dry box, under nitrogen. T h e dried poivder was stored under nitrogen and the exclusion of light. ,
,
I
Immediately prior to use, it was evenly distributed over glass helices in the reactor. PREPARATION OF SUPPORTED CVCL. All supported CuCl was prepared by Procedure A: except the material used in run 12, Table 11, which was prepared by Procedure B. PROCEDURE A. A slurry of CuCl in I-pentene was prepared and the undissolved material allowed to settle. Enough of the supernatant clear solution \cas decanted over the support material (previously heated a t 300' F. overnight in a vacuum oven) in a n evaporating dish just to cover the support with liquid. Gentle heat was applied until the solvent was evaporated and the solid material appeared dry. The support was stirred continuously by hand Lvhile the solvent was being evaporated. The procedure was repeated until it appeared that CuCl was being deposited largely on the dish rather than on the support. This usually occurred after four coatings. I n the one case where it was desired to obtain more than the usual amount of CuCl on the support, the coating procedure was continued past this point until CuCl could be flaked off the dried support by agitation. In all cases, the material was dried a t 350' F. after application of the last coat and sieved to remove loosely held CuC1. The operations described were carried out in a dry box, under nitrogen. The materials were stored under nitrogen in the absence of light and transferred to the reactor with the exclusion of air. The copper content of the completed materials and the surface areas (BET) were determined. PROCEDURE B. Approximately 200 ml. of activated charcoal was placed in a resin kettle and heated under vacuum a t 350' F. overnight. Vacuum was maintained and the charge was cooled to dry ice-acetone temperature with stirring. Then 150 ml. of a saturated solution of CuCl in 1-pentene was added all a t once and the resulting slurry allowed to warm to room temperature gradually. Stirring and evacuation were continued during this time. \Vhen the charcoal appeared dry, the charge was again cooled and another 150 ml. of the CuCl solution was added. The entire procedure was repeated seven times. After the last coating had been applied, the charge was heated a t 350' F. under vacuum overnight. The supported CuCl prepared in this manner was sieved to remove fines and transferred to the reactor. These operations were carried out under nitrogen. OLEFIN-PARAFFIN SEPARATIONS. One hundred and twenty milliliters of supported CuCl or of the support materials (for blank runs) were charged to an atmospheric pressure vertical flow apparatus (Figure 1). Sitrogen was passed over the material in the reactor a t approximately 15 cc. per minute, and the reactor was heated to 350' F. for one hour by means VOL.
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perature until no more effluent was collected. The adsorption step was then considered complete, and the temperature of the bed was raised gradually to 350' F. to effect desorption. Results and Discussion
OAS I N L E T A D A P T E R O L A S S WOOL P L U Q
Table I details the results of experiments conducted with silica gels and CuCl supported on silica gels. and Table I 1 contains the results of experiments in ivhich charcoal and CuCl supported on charcoal were used. Calculations of selectivity and capacity were based on the weights of materials fed and desorbed and on the composition of these fractions as determined by vapor phase chromatography. Thus, the composition of the sorbed hydrocarbon was calculated by subtracting the amountS of olefin and paraffin in the effluent collected at the adsorption temperature from the amounts of these materials in the feed. Since the weights of sorbents used in the various runs differed, all results are reported on the basis of 100 grams of sorbent. The molecular weight and density of CuCl were used to calculate a molecular volume. A spherical shape was assumed in order to calculate a radius which was in turn used to calculate the area occupied by a molecule of CuC1. The result, 15.6 sq. A,, was used in calculating the per cent of support surface covered by a layer of CuCl one molecule thick. (This figure is in good agreement with the area of CuCl talculated from the ionic radii of C u + and C1-, 13.2 sq. A.) In calculating the olefin capacity of CuCl on a support, it was assumed that the entire surface area (BET) of the coated sample was due to support and that the olefin capacity per square meter of this surface \vas identical to that of uncoated support. The capacity figure, moles of olefin sorbed per was then derived from the following calculamole of Cu (X), tion :
BAS I N L E T
THERMOCOUPLE WELL B u s s W O O L PLUQ
-
L I Q U I D R E C E I V E R WATER COOLED
@AS
Figure ' * pheric pressure
'Ow
apparatus for Operation at atmos-
of closely spaced helical Nichrome winding around the outside. Temperature was controlled by a Wheelco regulator activated by a thermocouple extending to the center of the bed. After one hour, the charge was allowed to cool to the desired adsorption temperature. Temperatures below that of the room were obtained by winding the column with Tygon tubing through which cold water was circulated. Nitrogen was passed throughout the run. A mixture, approximately lOy0 by weight of I-pentene in n-pentane, was fed to the top of the column at approximately 0.8 cc. per minute. The feed material and effluent from the reactor were analyzed by vapor phase chromatography. A minimum of 50 cc. of hydrocarbon was used in each run. If no olefin was found in the effluent when 50 cc. had been passed, hydrocarbon was added until saturation with olefin appeared complete. The effluent was collected in a water-jacketed condenser maintained at ice temperature, followed by two dry ice-acetone traps. The reactor was maintained at the adsorption tem-
Table 1.
X
M where
+
S = moles of olefin sorbed per 100 grams of CuCl support So = moles of olefin sorbed per 100 grams of untreated support A = surface area of CuCl support A, = surface area of untreated support M = moles of Cu per 100 grams of sorbent
+
1
Si02
... 783
... ...
2
3
SiOZ/CuCl SiOz/CuC1 0.048 688a 12.1
0.11 644 17.8
5.8
13.2
82-98
71-80
80-90
Selectivity 70olefin in feed 7; olefin in sorbed hydrocarbon
8.6 19.2
9.2 32.2
11.8 53.9
Capacity Grams olefin sorbed/100 g. sorbent Grams paraffin sorbed/100 g. sorbent Mole olefin sorbed/100 g. sorbent Moles olefin sorbed/mole Cu
3.29 13.84 0,047
6.68 14.09 0.095 1.1
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A S--& A,
Separation of 1-Pentene from n-Pentane by CuCl on Silica Gel
Run Yo. Sorbent Mole Cu/100 g. sorbent Surface area, sq. m./g. yo of surface lost by coating yo of surface covered by layer of CuCl 1 molecule thick Sorption temp., F.
a
=
...
8.42 7.19 I3.12 0.74
4
5
6
7
Si02
SiO2/CuC1
Si02
SiOt/CuC1
... ... ...
0.072
... 150 9.1 41.8 2.03 2.83 0.029
...
Determined f o r sample containing 4.370 Cu. Results f o r run 2 obtained with sample containing 3.376 Cu.
l&EC PROCESS DESIGN AND DEVELOPMENT
... 12.1
... 276
0.064
...
125 54.8
8.6
...
21.8
149-155
84-87
80-87
9.6 56.2
10.8 16.9
10.8 23.5
3.10 2.39 0.044 0.26
1.33 6.52 0.019 ...
0.99 3.23 0.014 0.084
Sorption temperatures reported are average temperatures maintained during sorption. With high surface area supports, initial sharp temperature rises, due presumably to high heats of absorption, were noted. Since. however, the bulk of sorption was carried out a t the reported temperatures, the maximum initial temperatures reached have not been reported. Unsupported CuCl and CuCl Supported on Low Surface Area Materials. Unsupported CuCl and CuCl supported on Celite and Alundum appear to be unable to complex with 1-pentene a t room temperature and atmospheric pressure. Only very small quantities of hydrocarbon are retained by these sorbents and no selectivity for olefin is evident. Detailed results of these experiments are not reported: because the errors possible in the collection and weighing of volatile materials and in calculating the per cent composition of mixtures by measurement of peal\ areas in vapor phase chromatograms are great. re1:itive to the amounts of hydrocarbon involved. The total amounts of hydrocarbon sorbed can be increased by lowering the sorption temperature to 40’ F. However, this change in conditions does not further preferential olefin sorption. Silica Gel and Silica Gel-CuC1. The capacity of silica gel for hydrocarbons is considerably greater than that of the lower surface area supports discussed above. Moreover. olefin is sorbed on this material in preference to paraffin. This preferential sorption is enhanced by applying a coating of CuCl to the silica gel. Thus, CuCl dispersed on silica gel appears to be a good complexing agent for olefin. A comparison of r u r s 1 and 2 (Table I ) shows that the material containing CuCl is considerably more selective in separating 1-pentene from n-pentane than the support alone and that this selectivity is due to increased olefin capacity rather than decreased paraffin capacity. Indeed, it appears that the CuCl present is active on a mole for mole basis in complex formation. An increase in the extent of coverage of the silica surface lvith CuCl (run 3) increases the selectivity of the separation still further. Here, the added selectivity appears to stem from both the presence of additional CuCl \vhich is active as a coniplesing agent (although not quite as active as in run 2) and the decreased capacity of the material for paraffin. Similarly, if tke adsorption is carried out a t a higher temperature (150’ F.), as shown in runs 4 and 5, the total amount of hydrocarbons sorbed on silica gel alone becomm less: but paraffin sorption is affected to a greater extent than olefin sorption. The coating of CuCl on the silica surface further accentuates this trend and indicates complex formation even under these conditions.
The ability of CuCl supported on silica to complex with olefin appears to be a function of the degree to which CuCl is dispersed on the support surface. The lower the coverage, the greater the olefin capacity per mole of copper. (In all cases reported here, the per cent of surface lost on covering silica and charcoal with CuCl is greater than the coverage to be expected from a monomolecular layer. This must mean that CuCl is covering pore mouths on the support.) However, since in no case was enough CuCl present to cover the support surface completely with a monomolecular film, such differences \voulcl not be expected, if the CuCl actually were distributed evenly on the surface. That they exist, would argue for increased clumping with increased coverage. This effect seems to be even more pronounced as the support area is decreased by sintering. In run 7: only one molecule of CuCl in 10 retains the ability to form a complex, although a layer one molecule thick would be expected to cover less than one fourth of the surface. The acidic nature of the particular silica gel used is indicated by the degree to which sorbed I-pentene is converted to 2pentene. The isomerization is retarded by repeated adsorptions and desorptions carried out on the same material and by covering the surface with CuC1. I n run 1, 41.3y0 of the olefin fed \vas recovered as 2-pentene. I n a previous run carried out on the same material, 50% isomerization was noted. I n r u n 2. only 10.5% of the I-pentene fed was isomerized. This observation is consistent urith a formulation for C u t - olefin complexes in which there is no net displacement of charge ( 7 ) . The capacity of coated samples for olefin does not appear to be changed by repeated ad- and desorptions. The reported activation may well have been due to a solution and redeposition of CuCl on the support surface, leading ultimately to en improved dispersion. Charcoal and Charcoal-CuCl. Charcoal is a high surface area material, and coating it with CuCl does not affect the surface area greatly. It is surprising, therefore, in view of the results obtained lvith silica gel, that when CuCl is dispersed on charcoal, no complex-forming ability is indicated. In all runs made under comparable conditions, the olefin capacity of coated samples is less than that of the support blank. (It is not possible to interpret the results obtained a t 40’ F. because no support blank \vas run at this temperature.) The total hydrocarbon capacity of these systems is decreased in proportion to the amount of CuCl on the charcoal surface, indicating that hydrocarbon sorption is a function of the remaining uncoated charcoal surface. Thus, the slight degree of selectivity shown in runs 9 and 10 (Table 11) is more likely
Table It. Separation of 1-Pentene from n-Pentane by CuCl on Charcoal Run No. 8 9 10 11 Sorbent Charcoal Charcoal/CuCl Charcoal/CuCl Charcoa1;’CuCl Mole Cu/100 g. sorbent ... 0,083 0.047 0.083 Surface area, sq. m./g. 1042 ... 999 ... of surface lost by coating .. ... 4.5 ... 70 of surface covered by layer of CuCl 1 molecule thick .. 7.5 4.2 7.5 Sorption temp., ’ F. 84-87 68-80 77-84 40-50 Selectivity 7~ olefin in feed 10.9 11 . 0 10.7 10.8 70 olefin in sorbed hydrocarbon 9.7 11.2 11.7 13.1 Capacity Grams olefin sorhed/100 g. sorbent 2.54 1.68 2.40 3 27 Grams paraffin sorbed/100 g. sorbent 23.59 13.36 18.10 21.70 hfole olefin sorbedjlO0 g. sorbent 0,036 0.024 0,034 0,047
VOL.
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NO.
12
Charcoal/CuCl 0.11 824 21 .o
8.6 79-90 10.1
9 8 1.40 12.97 0.020
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due to a relatively greater decrease in paraffin sorption than to a n increase in olefin sorption. The following explanations of this phenomenon have been considered : The behavior of CuCl on charcoal is “normal” and a synergistic interaction of silica with CuCl is responsible for enhancement of complex formation. This does not appear likely but cannot be rejected merely because it would be difficult to devise an esthetically pleasing mechanism for such an interaction. Loss of complexing ability of CuCl by a decrease of the surface area of the supporting silica gel (run 7, Table I) is not sufficient evidence against this explanation either, since the active centers necessary for interaction might have been lost in sintering the gel. S o evidence for interaction between CuCl and silica was obtained from a comparison of the ultraviolet a n 3 near-infrared spectra of CuCl on silica gel with those of uncoated gel (6). However, the interaction in question may be undetectable in this fashion. The CuCl occupies a much smaller portion of the total charcoal surface than that of silica gel, which might occur if most of the CuCl were deposited in a small number of large pores in the charcoal while being more evenly dispersed throughout the silica. Since the charcoal used has a n extremely narrow pore size distribution, very similar to that of the silica gel, with a major portion of its pore volume in micropores, it appears unreasonable that all of the CuCl on charcoal would be unavailable, while most of it is available on silica. Moreover, a total hydrocarbon sorption in inverse proportion to the CuCl coverage (compare runs 8, 9, 10. and 12, Table 11) favors a fairly even distribution of CuCl on charcoal, and coating of the charcoal under vacuum (run 12) did not lead to enhancement of selectivity. Cuprous copper is converted to cupric copper on the charcoal surface by chemisorbed oxygen or by disproportionation according to the equations:
+ o* = CUO + CuCle 2 c u c 1 = c u + CuC12 2cuc1
Accurate determination of the oxidation state of copper on charcoal by chemical means was not possible because of interference by the charcoal. However, the presence of substantial amounts of C u + was indicated, as was the absence of metallic copper. The electron spin resonance spectra of coated and uncoated charcoal showed no unpaired electrons (70). Since Cu and Cu+* have unpaired electrons, their presence on the coated samples in significant quantity is unlikely. The CuCl forms a complex with the charcoal of sufficient stability that complexing with 1-pentene becomes impossible. The charcoal used has a high degree of unsaturation (iodine S o . 1050 to 1200). T h e extreme difficulty encountered in removing well dispersed CuCl from the charcoal surface
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P R O C E S S DESIGN A N D DEVELOPMENT
(as was found during attempts to determine the Cu present) makes it appear that formation of a strong complex between CuCl and electron pairs in the charcoal is. at least, a good possibility. Unfortunately, no direct evidence is available for further substantiation. Summary
A high degree of dispersion of CuCl on a support surface is favorable to CuC1-olefin complex formation, and good olefinparaffin separations can be obtained even abo\e room temperature and a t atmospheric pressure. The degree of dispersion is influenced by the surface area of the support and the extent of coverage of the surface by CuC1. The latter is dependent on the method of preparation. The reported activation of CuCl by repeated ad- and desorption of olefin may be merely a device for obtaining a more eben coating of CuCI on a support surface by dissolving and redepositing the CuCI. Selectivity in a n olefin-paraffin separation on supported CuCl can also be improved by decreasing the ph) sical sorption of paraffin. Lowering the sorption temperature. which enhances CuC1-olefin complex formation, is not as effective in improving selectivity as raising the temperature, since it also results in increased paraffin sorption. The nature of the support appears to influence the ability of CuCl to complex with olefin. Thus, contrar) to eupectation, CuCl supported on a charcoal tvith high surface is not active in complex formation. .4 number of explanations of this phenomenon have been considered. Of these. an inactivatinq complex formation between CuCl and charcoal is favored. Acknowledgment
T h e author is grateful to A. S. Logiudice for performing the bulk of the experimental work and to H. Heinemann, H. P. Leftin, and J. Turkevich for helpful discussions. Literature Cited
(1) Dewar, M. J. S.:Bull. SOC. Chim. France 18, C79 (1951). (2) Ferber, E., Anders, L., Angew. Chem. 57, 119 (1944). (3) . , Gilliland, E. R., Seebold, J. E., ISD. ENG. CHEM.33, 1143 (1941). (4) Gilliland, E. R., Seebold, J. E., Fitzhugh, J. R., Morgan, R. S., J . Am. Chem. Soc. 61,1960 (1939). (5) Hillyer, J. C., Dees, A. C. (to Phillips Petroleum Co.), U. S. Patent 2,756,267 (July 24, 1956). (6) Leftin, H. P., M. W. Kellogg Co., Jersey City, Ii.J., private ._ communication. (7) Schulze, W. A Hillyer, J. C., Drennan, H. E. (to Phillips Petroleum Co.), Co.), S.Patent 2,386,354 (Oct. 9, 1945). (8) Schulze. W.‘ W. A.. Morris. L. ’C. C. ?to (to Phillips .Petroleum Petroleum Co.), Ibid., 2,386,358(Oct. 9, 1945). (9) Tropsch, H., Mattox, W. J., J . A m . Chem. Soc. 57, 1102 (1935). (10) ,Turkevich, J., Princeton University, Princeton, N. J., private communication.
e, e.
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RECEIVED for review November 2, 1961 . ~ C C E P T E D May 8, 1962
