Application of zone refining to the purification of organic compounds

VOL.

v,

PAGES

175-183

(1961~

Application of Zone Refining to the Purification of Organic Compounds EDWIN

A. WYNNE!

Fisher ScientQic New Jersey

Company,

Fair

Lawn,

After the first paper on zone refining t,hrough zone melting appeared in 1952,’ many others followed. A majority of these were developed for the semiconductor field. This new technique discovered by Pfann1s2 offers the chemist a means for obtaining metals, some inorganic salts, and many organic chemicals, in a state of high purity. The National Bureau of Standards and the Bureau of Mines, always on the alert for the higher purity chemicals to be used as standards, have done extensive work using the new techniques. It might be added that the International Microchemicals Standards Committee is considering the utilization of zone-refined organic chemicals as “standards” for microchemical analysis. The basis for zone melting2 is the passage in one direction of a series of molten zones through a charge. Impurities, that are present, travel with, or opposite to the zones, depending on their ability to raise or lower the melting point of the charge. Concentration of the impurities occurs in either end of the charge, the remainder being purified. Figure 1 presents a simplified picture. The optimum theoretical processing of the charge is to have the narrow zones to Idealized conditions would prevent backward pile up of impurities. be constant length and cross section of the zone, a constant distribution coefficient K, and an initially uniform composition of the charge. It is this latter condition which is least understood, in the sense which most workers will assume, that a charge can be of any purity and with just a few passes, purity of 99.99% or better can be obtained. Experience has shown that a minimum assay of the charge should be closer to 99%. Therefore, it is common practice to achieve this state of purity by accepted normal recrystallizations or distillations and 175

176

E. A. WYNNE

PURIFIED

AIR BLAST

SOLID

PIELTElD ZONE

MPIJRE SOLID

Fig. 1. Graphic representation of zone melting. then zone refine the purified charge, reaching thresholds of nearly absolute purity. Ball, Helm, and Ferrin3 have shown the requirements and equipment to produce nitrogen compounds of better than 99.9 mole-‘%, using simple equipment. The requirement of initial purity of one compound was 99.5 mole-‘% while the other was 98.5 mole-o/o. Here, again, is stressed the initial need for purification by conventional means prior to the zone-melting technique. Many of the compounds used caused breaking of the glass tubes used in the column. This was thought to be due to the difference in expansion between the liquid and solid phases. However, it is more likely that the cause is due to the tenacious adherence of the crystallized melt to the glass. For this reason, stainless steel tubes could MICROCHEMICAL

JOURNAL.

VOL.

V.

ISSUE

2

ZONE

REFIh’IKG

OF ORGANC!

COhfPOUPI;I)S

Iii

he substituted. The disadvant’ages are obvious: One rannotj see what is going on; also the ext,ract.ion of t’he refined material from the tube would be difficult, unless there was some means of disposing of the end cont,aining the impurities. At, least, wit’h glass, careful fract)uring can he accomplished without) c*ont’aminating the compound and the product removed. The lat,ter t,echnique has given good results in segregation of t,he purified from the impure fractions. Methods of heat’ing the charge pan vary from resist,ance to induction heating. The latter, more costly powerwise, is definitely more efficient for metals. Resistance heating seems to he the best procedure at, present, for organic compounds, provided control is carefully exercised in the amount of heat, necessary t’o melt t,he cross se&on. While zone refining was initially developed for semiconductor metals, many organic rompounds can he purified by t’his t,echnique. However, it has been found t,hatj the met’hod cannot he applied to all organir substances due t’o difficulGes that accompany increase in molecular size. In general, t’hese parameters include high viscosity, which hinders mixing, difirulty of nucleatjion, lark of sharp melting points and decomposition at the melting temperat)ures. In the purifirat,ion of organic compounds, several factors are en1lrltingPoint Chrmirals

TABLE I Valuw of Zone-Refined I,iterat,urc \‘alur, “C.

Acetanilide Anthracene Anthraquinone Benzoic acid p-Bromacetanilide p-Chloracetanilidc p-Fluorbenzoic arid o-Iodohenzoir arid Xaphthalene Salicvlic acid Succinic acid Stearic arid Vanillin Sulfanilamidc ~AS dekrminrd stem-rorrcct~rd.

b,v Thick

I 13-114 216-218 286-288 122-l 23 165-1617 li8-180 184-186 269-2iO 79-80 159-161 185-18i 68- 69 .5 82-83 .5 164-165 t.ulw trrhniqlw

Chemicn.!s

--

Obwrved L

2 “C.:i

114.8 219.1 288.0 124 16i.R 181.2 187 2iO.8 82.0 161 .!) 18T.8 70 1 84.0 165.2

rising short, mngcl Ihcwnomtxtcw,

E. A. WYNNE

178

Elemental

Compound Acetanilide Anthracene Benzoic acid Naphthalene Succinic acid Vanillin

c

07 f 10

il.30 94.46; 68.82 93.44 40.98 63.16

TABLE II Analysis of Zone-Refined --Found

Chemicals%

____Theory

H,Yt

N, 40’

c, yb

H, o/;

N, ‘5,

c: .59 5.78 4.85 6.23 5.07 5.20

10.15 -

Tl .Ol 94.34 68.84 93.71 40.68 63.15

ti.il 566 4.95 6.29 5.12 5.30

10

a Analyses by Dr. A. Sjteyermark,

-

35

-

Hoffman-LaRoche.

Fig. 2. Fisher Zone Refiner

with safety shield removed.

MICROCHEMICAL

JOURNAL,

VOL.

V,

ISSUE

2

ZONE REFINING

OF ORGANIC

COMPOUNDS

179

countered which differ from those of metals. Low thermal diffusivity usually requires the use of artificial cooling between zones. Additionally, the low chemical diffusivity makes agitation desirable which Unfortunately, such agitation sysmakes for better heat transfer. tems are not available on any commercial refiners as yet. One suggestion is the use of “ultrasonics” which could be set at the impure end of the charge tube. Magnetic stirring has been designed for metals. However, by controlling the heat and rate of travel, this factor may be overcome to a moderate degree. In the present work several organic compounds were investigated, and a number were found not to be adaptable for zone melting. Table I lists those we have successfully refined along with the differences between the observed and literature values of their melting points. Table II gives the elemental analyses of some zone-refined chemicals. Purification of these compounds was done on a protot’ype of the Fisher Zone Refiner shown in Figure 2.

Reagents and Apparatus Organic chemicals as listed in Table I (Eastman Kodak and Fisher Scientific Co.). Apparatus and reagents were: acetone, ACS (Fisher Scientific Co., catalog #A-18); methyl alcohol, ACS (Fisher Scientific Co., catalog #A-412) ; isopropanol, ACS (Fisher Scientific Co., catalog #A-416); Fisher Zone Refiner; Modified Thiele Tube Melting Point Apparatus; Pyrex glass tubes, 22 mm. o.d. X 24 inches and sealed on one end; plastic head hammer; dry box; silimanite mortar and pestle.

Description of Zone Refiner The apparatus is self-contained having a variable speed heater drive with a fast reverse motor. Heating wires are controlled by a powerstat. The carriage contains two sets of cooling tubes and resistance heaters set in tandem. Operating on a down pass, the heaters are always below the cooler, as shown in Figure 1. For an upward pass, the positions are reversed. Operation is both automatic and single pass. On automatic, the carriage reaches a limit switch, set according to the length of the tube. This switch stops the slow motor, actuates the fast motor and cuts off the current to the heaters. The carriage then moves to the upper

180

E. A. WYNSE

limit switch which shuts off the fast motor, starts the heaters and the slow motor. In order to determine the number of passes an elect#ric counter was installed on the lower limit switch. This has a microswitch placed so that the carriage arm touches it as it comes in contact, with the limit switch. The apparatus can accommodate tubes from 4 t,o 36 mm. (0.d.). Chemicals with melting points of 50%300°C. can be refined.

General Procedure The tubes are filled, after preliminary purification of the chemical to be refined, by carefully controlled melting of the compound and pouring it, in the t,ube. After solidification, t#he upper end may be sealed off, either wit,h vacuum or under slight positive pressure of an inert gas. The switch is flipped a.nd temperature slowly taken up t,o approximately 3-5°C. above the melting point of the compound. Rate of It has been found that having pass is adjustfed by t,he timer control. a travel speed of l/i6 in./min. produces a molten zone between I/&/? in. The smaller the zone, the more efficient the refining. The heater wires are chrome1 ribbon 3 mm. wide. For smaller zones, a ribbon 1.5 mm. is recommended. Air from a pump or air line is passed through the coolers. Generally 5 t’o 10 passes are sufficient, for purification. The glass tube is removed from the apparatus and after cooling to room temperature, it is carefully fractured, using a plastic hammer, and the solidified column is removed. The solidified compound is then broken up so that it can be ground in an agate or sillimanite mortar. This step should be done in a dry box. Moisture of the at,mosphere tends to accumulate on the surfaces of the ground partiCare should be cles if the grinding is done under room conditions. exercised to prevent dust and fumes, generally encountered in a laboratory. Horizontal and vertical movement of the heaters and coolers were attempted. With the latter, both downward and upward passes were made depending on the chemical. It was noted in the purification of stearic acid containing about 15% oleic acid and other fatty acids that a downward passing did absolutely nothing, other than give a more t,horough mixing of the components. When the pass was MICROCHEMICAL

.TOURNAL,

VOL.

V.

ISSUE

2

ZONE: REFIXING

OF ORGANIC

COMPOUNI)S

181

reversed, three zones resulted. The topmost consisted of a clear oleic acid zone, the middle, a slush consisting of oleic, myristic, lauric, and stearic acids, and a bottom zone of solid stearic acid of high purity. Forty-five passes at the rate of about l/l; in./min. were necessary to obtain this fractionation.

Fig. 3. Zone melting with focused radiat,ion (after Handley and Herington).

At present, there is now commercially available apparatus in this country for zone refining of organic chemicals on a large scale. Pfann has a patent describing a continuous process for metals.4 Also, there are several types of apparatus for batch processing used in the semiconductor field. For the production of highly refined organic chemicals, a refiner has been developed for the production of pure chemicals by the company with which the author is associated. It is hoped that this will be capable of purifying a kilogram each of three different organic substances at the same t’ime. Each will have independent control of the heater elements. A modificat,ion of the Fisher Zone-Refiner by General Electric permits control of each individual heater, rather than the present series hookup of most refiners. This modification gives better control over the size of the molten zone.

182

E. A. WYNNE

In England, a model capable of refining kilogram quantities is already available commercially. Handley and Herington* in England, have a refiner for purifying gram quantities by means of a focused beam from a tungsten lamp (Fig. 3).

Conclusion It is reasonable to expect that the zone refiner described will find extensive application in the purification of many organic substances which are stable at their melting temperatures. The important parameters to be considered are composition or eutectics, thermal conductivity, and diffusion of the molecules to and from the solid interface. It is highly probable that through continued research and apparatus refinement, those organic substances which have been difficult to purify at present, will be available for purification by the zone-melting technique. On the other hand, zone-freezing,5-g production equipment may be soon available for obtaining purity of 99.999999 mole-% of organic solvents. Indications from t,he National Research Council and the Xational Bureau of Standards seem to predict such ultrapurity. But obviously this is only the first of perhaps many applications of this method. Indeed, the possibilities for inorganic salts have not been discussed; conceivably, these could result in even more ext#ensive applications.

Summary Zone refining of organic chemicals to a purity higher than that obtained through customary refining technique is described. Tables showing the resultant increase of melting points and elemental analyses are included. The apparatus used is shown and the conclusion made that it can be considered a valuable adjunct to the microchemist.

References 1. Pfann, W. G., Trans. AIME, 194, 747 (1952). 2. Pfann, W. G., Zone Melting, Wiley, New York, 1958. 3. Ball, J. S., R. V. Helm, C. R. Ferrin, Petrol. Eng., 30, C36 (1958). 4. Pfann, W. G., U. S. patent P,739,045 (March 20, 1956). 5. Schwab, F. W., and E. Withers, J. Research Natl. Bur. Standards, 32, 253 (1944). MICROCHEMICAL

JOURNAL,

VOL.

V, ISSUE

2

ZONE

REFINING

OF ORGANIC

COMPOUNDS

183

6. Herington, E. F. G., Analyst, 84, 680 (1959). 7. Wolf, H. C., and P. H. Deutsch, Naturwissenschaften, 41, 425 (1954). 8. Rock, H., Naturwissenschuften, 43, 81 (1956). 9. Parr, N. L., Royal. Inst. Chevn. London, Lectures, Monographs Repts., 1957, no. 3.

Received December 13, 1960