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IhrDUSTRIAL A N D ENGINEERIXG CHEMISTRY
1'01. 18. No. 4
Increasing the Internal Volume of Silica Gels by Moist Heat Treatment' By Harry N. Holmes, R. W. Sullivan, and N. W. Metcalf OBERLINCOLLEGE, OBERLIN,OHIO
N AN earlier paper by Holmes and Anderson2 a method
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The bath in which the saturators and adsorption tube were was outlined for the preparation of silica gels of different immersed was a simple arrangement of two deep kettles with degrees of porosity. The gelatinous precipitate ob- the space between them packed with mineral wool. Beestained by mixing aqueous solutions of ferric chloride and wax was poured over the top of the wool to keep out water water glass was dried to a firm solid and treated with 6 N splashed from the bath. The outside was wrapped with ashydrochloric acid. After washing out the soluble ferric bestos paper. Since the measurements were made at 30" C., chloride formed only the hydrated silica was left. All the the temperature was easily held with less than 0.1' C. varispace occupied previously ation by radiation from by ferric oxide was now I, ; I three 45-watt carbon filain the form of coarse capilment lamps above the bath. A method is described for greatly increasing the adlaries. These lamps were turned off sorptive capacity of gels by checking the initial slow In a continuation of this and on and adjusted in disdrying at a water content of approximately 60 per cent, work by the senior author tance as was found necesallowing the partly dried gels to "sweat" or synerize and H. A. Weide the advansary. The advantage of for a week or two in closed vessels and then boiling tage of extremely slow dryheating from above was a few hours in dilute acid before washing and final ing became apparent. Inthat the exposed tubcs were drying. stead of the usual Zcm. thus kept above 3 degrees The method is general, but it is applied particularly layer of precipitate on the warmer than the bath, a to the gelatinous precipitate obtained by addition of drying racks, the authors very important point. Beferric chloride solution to water-glass solution. In this once tried an 8-cm. layer. fore this fact was learned case the boiling acid removes the ferric oxide leaving Drying was further retarded t h e r e was con si d e r a ble somewhat hydrated silica as a white, highly porous by a few weeks of wet trouble with condensation gel. weather. The h a 1 gel from of vapors in the connecting this experiment showed an tubing. It was f u r t h e r adsorptive capacity towards found -necessary to keep benzene one-third greater than the best previously obtained. doors and windows of the room tightly closed during a run Further work on the conditions of drying seemed advisable. because of cold drafts. Adequate stirring was arranged. In making a measurement the sample of gel to be tested Adsorption Train was placed in the adsorption tube and hung in an air bath The adsorption train used for the measurements given in kept a t 140" to 200" C. A stream of dried air, 250 cc. per this paper is shown in Figure 1. To secure a constant pres- minute, was then passed through the tube until it reached sure of air the well-known device represented a t the left was constant weight. This activation process required from 1 to used. Tap water enters the inverted bottle A (bottom cut 2 hours. The loss of water at 200" C. was almost exactly off) by the lower tube. The level of the water in A rises the same as that a t 150' C. After connecting the adsorption until it overflows into the sink through the upper tube. tube with the rest of the train, the air stream, saturated with Water also flows from A down the middle tube into a test benzene or other liquid contained in E , was run through a t the tube suspended in the large aspirator bottle. The water rate of 132 cc. per minute until the adsorption tube ceased to entering the aspirator bottle displaces an equal volume of gain in weight, usually a matter of 4 or 5 hours. Adsorption moist air, which passes out a tube regulated by a stopcock. under the given conditions was calculated as per cent weight The rate of flow is constant, because the head of water be- of the gel samde. tween the top of the upper outlet tube and the top of the Refluxing with Hot Hydrochloric Acid test tube is kept constant. The air stream is dried by the calcium chloride tower In the earlier work it was found advisable by Holmes 3 and is measured by the flowmeter C. The flowmeter, a and Anderson to immerse their dried red gel (hydrated ferric U-tube of 5 nun. internal diameter, is partly fUed with a oxide-silicon dioxide) in hot hydrochloric acid as the final nonvolatile liquid and is spanned a t the top by a capillary product appeared to be more adsorptive than when cold acid tube. The rate'of flow is found from the difference in levels was used. No attempt was made to keep the acid hot. It by reference to a calibration curve previously prepared. was decided to try boiling the red gel in 6 N hydrochloric After a final drying by the calcium chloride tube D the acid with a reflux condenser attached to a Soxhlet tube. air stream enters the saturators E, the spray catcher F, This device gave a hot acid treatment with intermittent reand the adsorption tube G. The saturators are specially moval of the product of reaction, ferric chloride. When the made glass bottles designed to allow the least possible con- gel was free from iron, as indicated by the change from red to tact of the vapors with rubber. A simple bulb tube, F, white, the acid was replaced by water and the gel washed filled with loose cotton, catches any droplets of spray from free from chloride ions. After drying, this gel appeared E. The usuaI form of adsorption tube G contains about extremely light and chalky in contrast to the rather hard and 10 grams of silica gel. While weighing this tube care should vitreous gels previously prepared. It adsorbed 115 per cent of its own weight of benzene from a stream of dry air saturated be taken to close both arms by rubber tips. with benzene a t 30" C., in sharp contrast to the previous 1 Received December 4, 1925. standard of 50 per cent adsorption. To learn whether the * Tms JOURNAL, 17, 280 (1925).
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INDUSTRIAL A,VD EXGINEERING CHEMISTRY
April, 1926
improvement was due to the hot acid or merely to moist heat the ferric oxide was removed from a red gel with cold 9 N hydrochloric acid and the white product boiled jn water for 3 hours. Another sample of the same gel was immersed in 9 N hydrochloric acid at 100” C. until the iron was removed. The two gels so produced had practically the same adsorptive capacity. Evidently, the additional benefit of a hot acid treatment as compared with that by cold acid lay almost wholly in the prolonged contact with water at 100” C. or a few degrees higher. Of course, the reaction was speeded up by heat. The moist heat treatment must set the gel structure SO that it holds its form on further drying, for the product shows far less final shrinkage than that produced by any other method. Boiling with Dilute Sulfuric Acid
Since the chalky form of silica gel had obvious commercial possibilities for recovery of solvents, removal of sulfur compounds and color from petroleum oils, and for other processes, it was seen that boiling the crude gel with a volatile acid such as hydrochloric was not ideal. By substituting 6 N sulfuric acid it was possible to dispense with the troublesome reflux condenser. Dilute sulfuric acid boiling a t about 115’ C. loses no acid except by spray. The gel by hot sulfuric acid was just as adsorptive as the gel by hot hydrochloric acid. Proper Degree of Drying before Moist Heat Treatment
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hydrochloric acid, washing the gel thus formed, drying, and activating. I n the table it is designated as “Patrick’s gel.” The second gel, chemically the same but physically different, is called the “gel from iron.” It was described by Holmes and Anderson.2 In its preparation a solution of ferric chloride was added to a solution of water glass, thus securing as a red precipitate an intimate mixture of ferric oxide and silicon dioxide, considerably hydrated. The novel feature of its preparation was the treatment of the dried crude gel with hydrochloric acid to remove the ferric oxide while leaving a rigid, noncollapsing structure of hydrated silica. The third gel, termed the “heat-treated gel,” is the best of those described in this paper. It received all the benefit of very slow drying of the crude precipitate, of the sweating process after reaching a water content of about 60 per cent, and of a few hours boiling in 6 S sulfuric acid.
i Figure l--Adsorption Train
In their earlier work the writers dried the crude red gel until it had a rigid structure without any exact standard of moisture content. Later analysis showed that most of these crude gels had a water content of about 35 per cent. In an effort to determine the proper degree of drying before the application of moist heat, a comparison was made of different samples of the same gel, dried to different degrees but all refluxed with boiling 6 N hydrochloric acid for the same length of time. The results indicated clearly that these gels must be dried below a water content of 70 per cent but not lower than 50 per cent. A crude gel of 77 per cent water content yielded a product capable of adsorbing only 49 per cent of its own weight of benzene, while a gel of 55 per cent water content was converted into a product with an adsorption capacity of 115 per cent. A crude gel of 67 per cent water content yielded a product with an adsorption capacity of 110 per cent. Other results, including a short series with sulfuric acid, point to the desirability of slowly drying gels to a water content of 55 to 60 per cent before application of the moist heat treatment. Effect of Syneresis on Partly Dried Gels
On one occasion a crude gel that had been dried very sloivly to 60 per cent water content was bottled in order to hold it in that condition until needed. After it had stood in the closed bottle 8 or 10 days drops of water or water solution were observed thickly covering the solid lumps like a “sweat.” This process of syneresis without evaporation was much better than aging in the open air or than not aging a t all. I n fact, the final product after the sweating process was the best adsorbent of all. Surely, there must be a rearrangement of gel particles during this aging to yield the firmest gel structure. Three Distinct Types Compared The fundamental silica gel,, greatly improved by Patrick3 and his associates, was described in an earlier paper.2 In brief, it is prepared by mixing solutions of water glass and
* U. S. Patent
1,297,724 (March 18, 1919).
The values given in the table represent practical saturation of the gels, although in some instances this required a run of 18 hours, in others only 2 hours. An air stream saturated a t 30” C. was used except in the case of amylene, where the temperature was 25” C. The rate of flow was 60 cc. per minute for amylene, ether, and ethyl bromide; 132 cc. per minute for carbon disulfide, carbon tetrachloride, and benzene; and 260 cc. per minute for toluene, gasoline, and oxylene. With the exception of the gasoline the materials used were pure research chemicals. Adsorptlon of Vapors a t 30’ C. (From a stream of air saturated with vapor) Patrick’s gel Gel from iron He:at-treated VAPORADSORBED Per cent Per cent Per cent 126.0 Benzene 32.2 62.0 Toluene 33.1 83.0 119.0 120.0 o-Xylene 33.4 62.0 222.0 Carbon tetrachloride 57.7 120.0 Carbon disulfide 44.0 91.5 182.0 202.0 Chloroform 55.7 107.0 Ethyl bromide 57.1 107.0 208.0 Ether 30.5 54.3 108.0 69.0 Gasoline, 80’ to 90’ C. 27.6 96.7 Amylene 25.9 53.4
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The results in this table are not only more numerous but more accurate than those in Table IV of the paper by Holmes and Anderson.2 Following the earlier work it was learned that a determination must be carried considerably past the slowing-up stage to afford the best comparison. I n each case “Patrick’s gel” was purchased from the Silica Gel Corporation but in different years. The gel from iron of the present paper was much superior to the gel from iron of the earlier work due to much slower drying. The low value for carbon disulfide in the case of the gel from nickel2 was undoubtedly due to solvent effect on a rubber connection. The present technic safeguards against such an error. From the table we must conclude that there is not an optimum size of capillary for each vapor. In Figure 2, per cent adsorption of chloroform is plotted against the time in hours. Curve 1corresponds to the writers’ heat-treated gel, while Curve 2 corresponds to Patrick’s gel. The flattening of the curve due to near-saturation is
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Vol. 18, No. 4
INDUSTRIAL AND ENGINEERING CHEMISTRY
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U
/
2
3
4
3
-7me /i7 hoursFigure 2-Chloroform Adsorption Curves for a Heat-Treated Gel (1) and for the Common Silica Gel (2). Air Stream Saturated a t 30° C.
very differently placed for the two. The three curves for amylene in Figure 3 are still more significant. Patrick’s gel is glassy and contains the smallest capillaries; the gel from iron is not glassy but is opaque and has somewhat of a vitreous appearance with larger capillaries; and the heat-treated gel is absolutely chalky because it contains the largest capillaries of the three. It is not claimed that the relative adsorption as given in the table for air saturated with vapors will hold a t low partial pressures. No single type of gel can be the most efficient for all purposes. Final Procedure in Preparation of a Heat-Treated Gel
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/D
/5
2.0
2.5
-7mein hums Figure 3-Amylene
Adsorption Curves for a Heat-Treated Gel
(1) for a Simple Gel from Iron (2) and for the Common Silica
Gei (3).
Air Stream Saturated at’25’ C.
sulfates the gel is dried a t approximately 150’ C. for eight hours. It is then reduced to the mesh desired and activated when needed at 140’ to 200’ C. in a stream of dry air. Discussion
The process described above permits the regulation of capillary size. For example, if a highly hydrated gel is dried slowly to a water content of 35 per cent before treating with hot acid, the shrinkage will be considerable and the capillaries smaller; if the gel is dried to a water content of 55 per cent before acid treatment, the gel structure will be set by the moist heat so as to permit much less shrinkage and the capillaries will be larger. Treating a gel with hot water, steam, or aqueous solutions is the only way to apply heat while interrupting evaporation. The various steps in the improvement of the gel prepared It may be applied with benefit to other highly hydrated gels by the writers may well be summarized here for working or precipitates than the one described here. I n some indirections. Slow addition, with violent stirring, of 8 liters of stances water will be preferable to acid. Variations in the rate of drying have decided influence 2 N ferric chloride to 2.5 liters of sodium silicate which has previously been diluted with 50 liters of tap water yields a on the quality of the product. The writers’ record sample red-brown precipitate. The density of the water glass before gel, unfortunately too small for general research, showed an diluting should be near 1.375 and the ratio of NazO to SiOz adsorptive capacity for benzene (from an air stream saturated should be very nearly 1:3.5. The mixture is allowed to a t 30” C.) of 133 per cent. As noted in a previous paper from this laboratory,2 some stand a t least 60 hours and is then filtered through fine cheesecloth supported on coarse galvanized iron wire. The other salts can be used instead of ferric chloride for precipilayers of precipitate on the filter rack should be 5 cm. deep tation of sodium silicate. The final products from the to secure the best quality of gel. As soon as the mass can use of aluminium chloride and calcium chloride were inferior. be handled a t all it should be broken up into uniform lumps However, by the use of the new moist heat treatment their about 2 cm. in diameter. When the water content of the capacity was greatly improved. The earlier gel from calcium gel drops to about 60 per cent, or a little lower (preferably chloride adsorbed only 21 per cent of benzene while moist requiring 2 weeks or longer), the whole mass is bottled to heat treatment and the sweating process increased this to prevent evaporation and allowed to “sweat” for at least a 101 per cent. In commercial practice a countercurrent series of gels and week. To remove the ferric oxide the partly dried lumps are hot acid might be arranged with economy. The gel nearly broken into pieces less than 1 cm. on an edge and then boiled exhausted of its iron should meet the strongest acid. gently with 6 N or 9 N sulfuric acid for 1 hour after all red color has disappeared from the center of each lump. The Theoretical and Recorded Pressures in Oxygen acid should be changed two or three times. After draining Bomb Determinations-Correction. off the acid each time it is advisable to rinse the gel a few In Figure 2 of the article under this title, THISJOURNAL, times with hot tap water to hasten the removal of ferric 18, 307 (1926), the maximum pressures should read 0 to 600 kg. sulfate formed in the reaction. When freed from soluble per sq. cm., a t intervals of 100 kg.
