A. C. ZETTLEMOYER AND
W. C. WALKER
Lehigh University, Bethlehem, Pa.
Low-temperature nitrogen adsorption isotherms were measured on a series of commercial active magnesias. The isotherms are mixtures of types I1 and IV. The isotherm shape is explained by hypothesizing a structure consisting of plates of checkerwork of cubic holes and crystallites. The surface areas of the magnesias are shown to range from less than 1 to more than200squaremeters per gram. Somedifficulty was encountered in the use of the usual methods for calculating areas from adsorption data. The B-point areas are shown to be directly proportional to the amount of iodine adsorbed from solution, or the iodine number.-Photograph shows commercial kilns far producing active magnesia.
Suface Areas and Pore Structures from Nitrogen Adsorption
R
ECENT investigations have shown that active magnesium oxide has a place of its own in the fields of adsorption and catalysis. It is unique in being the only commercial product
low temperature nitrogen adsorption isotherms was initiated to correlate iodine adsorption with specific surface areas and to obtain information about pore structures of these grades.
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possessing high surface area and mechanical strength combined with a high degree of basicity and an ionic crystal lattice. These attributes give it unusual adsorptive and catalytic properties, only EL few of which have been investigated. The commercial active magnesias have been found to be excellent a,dsorbents for the removal of fluoride from drinking water (IO),idica from boiler water ( I ) , fatty acids from petroleum ether (8), and impurities from petroleum lubricants, dry cleaning solvents, and other nonaqueous and unreactive fluids (19). They have ,proved useful also in the chromatographic separation of the carotenes (86). Catalytically, active magnesia has been used extensively in the manufacture of synthetic rubber (16) and studied thoroughly in the dehydration of ethyl alcohol (9, 84). Most of the active commercial grades have been prepared by the light burning of magnesium hydroxide obtained from sea water bitterns. Activity is measured industrially by the adsorption of iodine from solution, The k o r k reported here on the
COMMERCIAL PRODUCTION METHODS
The essential operations for the production of the active magnesias investigated here involve first the production of lime by the calcination of oyster shells. Bitterns remaining after sodium chloride has been separated from sea water by solar evaporation are treated with this lime to precipitate the more insoluble magnesium hydroxide. Since the bitterns contain sulfate, a portion of calcium chloride obtained by the reaction of the lime with the magnesium chloride in the bitterns is recycled; this precipitates the sulfate as calcium sulfate before the reaction with the lime. The magnesium hydroxide, which is precipitated in as granular a form as possible at about 60” C. with mild mechanical agitation, is washed countercurrently in Dorr thickeners. The washed sludge is filtered in a rotary vacuum filter and fed into the stack end of a rotary kiln, The details of these operations have been adequately described by Seaton (85).
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RELATIVE
PRKSSlJRE,fi
Figure 1. Isotherms of Nitrogen Adsorption on Active Magnesias at -195'
The rotary kiln is fired countercurrent with gas or fuel oil. By trial the temperature of a thermocouple located a t a point between the feed and discharge end of the kiln is correlated to the activity of the magnesia discharged from the kiln. Subsequent production is then carried out by maintaining the temperature of this intermediate couple a t the desired level. The two factors which are correlated to the activity and ignition loss of the final product are temperature and time of retention in the kiln. Pertinent data for the commercial grades of active magnesia are given in Table I, and analyses in Table 11. Magnesias 26411 and 2642 are straight run products made by the procedure described. Magnesia 2652 is a granular variety, -8 to $28 mesh, obtained by burning magnesite ore, hydrating, and reburning; it was included in these studies for comparison with the sea water varieties. Magnesia 2652-8 is a comparable granular grade from sea water. Magnesia 2661 had been strongly burned to remove chlorides, which are highly detrimental to the production of a magnesia of high activity. It is a chemical rather than an active grade but is included for purposes of comparison. Magnesia 26611/2 is an experimental grade. Magnesia 2665 is a straight run grade which was, in this instance, air-separated to obtain a more uniformly fine particle size. The XP magnesia is one of a series of experimental grades and was made only in small batches. Production data for the series will be presented elsewhere. IODINE NUMBER T E S T FOR ACTIVITY
I n commercial practice, activity is tested by the adsorption of iodine from carbon tetrachloride solution and reported as an empirical iodine number. These iodine numbers were determined by the following procedure, which is a slight modification of the usual industrial method. As a result our iodine numbers are higher than those reported commercially, but the two are linearly related. This change was made to obtain a larger spread in iodine number. The procedure for iodine number determinations is as follows:
1. Weigh a 2-gram sample of material to the nearest milligram. 2. Transfer to a clean, dry, 200-ml., glass-stoppered bottle,
C.
3. Add 100 A 0.5 ml. of 0.1000 N iodine in carbon tetrachloride. The carbon tetrachloride must be free from traces of sulfur or carbon disulfide. 4. Stopper the bottle and shake vigorously in a suitable shaking device for exactly 30 minutes. (The test is relatively insensitive to temperature so that no temperature controls are employed.) 5. Allow t o settle 5 minutes and pipet 20 ml. of the clear solution into a 250-ml. Erlenmeyer flask containing 50 ml. of 0.03 N potassium iodide in 75% ethanol. 6. Titrate with standard 0.05 N sodium thiosulfate. A sharp end point can be obtained without the use of starch indicator. 7. Calculate the iodine number by subtracting the milliliters of thiosulfate used from the milliliters of thiosulfate required for 20 ml. of the original iodine solution, and multiply the results by 250 times the normality of the sodium thiosulfate.
IodineNo. = 250 X NthiOX (ml. thio for blank
- ml. thio for titration)
This value is reported to the nearest unit only. The isotherms of this adsorption are of the Langmuir type (type I) in which adsorption does not proceed past the formation of the first unimolecular layer (19). The point determined for the iodine number is always on the horizontal portion of the isotherm and is thus indicative of the amount of iodine in the monolayer. This test has been extensively used for rapid determination of the activity of magnesia. APPARATUS AND PROCEDURE
The apparatus and procedure were essentially the same as described by Emmett and Brunauer (11, 12) and others (23'). Nitrogen was adsorbed a t liquid nitrogen temperatures in a constant volume apparatus. Known volumes of nitrogen were added and the resulting pressures noted. The nitrogen added was either adsorbed on the chilled sample (0.1 to 0.3 gram) or occupied the dead space above it. The dead space volumes were measured a t liquid nitrogen temperatures with helium. Since the accuracy of the adsorption measurements depends on the precision with which the dead space is kept constant throughout the experiment, the end of the dead space at the pressure manometer was equipped with a tungsten contact. When the mercury was raised to the proper level in the dead space arm of the manometer, a circuit was completed so that, through a relay, a onewatt neon bulb flashed on. Corrections to the dead space volumes for the deviation of nitrogen from the ideal gas laws were determined experimentally. The temperature of the bath surrounding the sample tube was measured with a nitrogen vapor pressure thermometer. The vapor pressure-temperature relations were taken from the measurements of Giauque and Clayton (14). A vacuum pump evacuating the space above the liquid nitrogen bath made possible the use of temperatures below the normal boiling point of nitrogen. The pump was controlled through a relay by contacts in the nitrogen thermometer arranged as described by Orr (81). *
January 1947
Selection of optimum degassing conditions for active magnesia presented a serious problem. The temperature could not be so high as that generally used for other adsorbents, since active magnesia sinters readily. The conditions finally chosen were a temperature of 250' C. and a pressure of 10-6 mm. mercury for 1 hour. Maintenance of these conditions for longer periods led to a decrease in the surface area, 1.6% after 4 hours and 3.2% after 20 hours. At higher temperatures the area increased initially, but the loss due to sintering was more pronounced. Under the selected conditions the samples lost from 0.4 to 2.5% by weight during the degassing period. Thus it is believed that these degassing conditions produce the maximum cleaning of the adsorbent surface without destroying the structure of the sample. A gas microanalysis was performed on the gases removed from the XP sample under the standard degassing conditions. Before the sample was subjected to the oven temperature (250" C.), it was evacuated for 10 minutes a t 22' C. This gas was entirely carbon dioxide and water; consequently it may be concluded that these compounds, rather than adsorbed air, contaminate the surface of active magnesia. The imole ratio of carbon dioxide to water was approximately 5:3.
35
I5
30
251
20 ML. N V
15
IO
ADSORPTION ISOTHERMS
5
The nitrogen a d s o r p t i o n i s o t h e r m s a t -195.8' C. for three active grades, 2642, 2652-5, and 2652, are given in Figure 1, and for 2642 and XP, in Figure 2. Careful study was made of the high prcssure points. For the powdered grade, equilibrium was usually attained in 30 minutes, but the pressure readings were taken after 1 hour. The nitrogen was introduced slowly and removed slowly (coming down the hysteresis loop) over a period of at least 15 minutes, For the granular grades somewhat longer periods of time were required for equilibrium re?dings. The isotherms in Figures 1
I
RELAT I V E
Intermediate Cou le, % of Kiln Len& from Discharge E n d
Temp.,
c.
750 700 625 625 800-900 600-700 300-400
K
PRESSURE
Figure 2. Isotherms for Nitrogen Adsorption on XP Magnesia and Magnesia 2642 Compared with Corresponding Brunauer-EmmettTeller Isotherms for n = m
TABLE I. PRODUCTION CONDITIONS Grade 2641 2642 2652 26524 2661 266 11/z 2655
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Retention Time, Hr. 0.5 0.5 0.5 0.6 5-6 2 4
and 2 distinctly show a second plateau in the neighborhood of 0.8 relative presmre. The isotherms appear to be mixtures of the sigmoid type I1 and the much rarer type IV isotherms, according to the widely used classifhation of Brunauer (6). Type IV isotherms possess second plateaus generally attributed to the filling of pores of uniform radius. From the shape of the isotherms, then, one would suspect a large number of pores of uniform size as well as a considerable number with random size distribution. The adsorption isotherms possess definite hysteresis loops (Figures 1 and 2). Hysteresis is usually attributed to a narrowing of the pores at the openings-bottle-neck picture (17, 18)or to the presence of-pores with two open ends (IS).
TABLE 11. ANALYSESOF SAMPLES Grade 2641 2642 2652 2652-5 2661 26611/z2665 XP Iodine No. 76 4 130 146 150 44 25 210 Ignition loss, % 8.7 6.8 7.0 7.2 1 . 4 0.1 6.5 4.9 Typical chem. analysis, ignite d bas/s, Yo b Chloride as NaCl 0.3 a ... 0.02 0 . 3 0.07 b SiOn 0.8 a 0.8 a 0.7 0.6 0.9 b Fen03 0.3 = 0.4 a 0.2 0.2 0.3 b AlnOs 0.4 0 0.3 0.3 0.2 0.8 b CaO 2.6 a 1.3 1.5 1.2 1.5 b MzO 9 6 . 5 (1 96.9 a 97.4 97.2 96.4 Approx. particle 2 6 (8to28mesh) 2 3 2 3 size, microns Bulk density, Ib./ 27 30 46 42 22 5 2 . 5 14.5 6 cu. f t . a Same grade as 2641. b Same grade as 2661.
AREA DETERMINATION
The first method for area determination to be applied to the adsorption of nitrogen on active magnesia was the method of Brunauer, Emmett, and Teller (6). According to their theory the volume of nitrogen required to complete a unimolecular layer Vm can usually be obtained from the slope and intercept of the linear plot of p / v ( p ~- p ) . V P ( p =0
-L p +)
C vmc - -
l vmc
( PEo )
where p = measured pressure pa = condensation pressure of the adsorbate u = measured volume of gas adsorbed c = constant related to heat of adsorption
(1)
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