absorbs the acid gases resulting from the pyrolysis of halogens and sulfur while it passes ammonia. The position of the absorbent keeps it at approximately room temperature and allows the operator to watch for depletion of capacity for CO? or water. Ascarite is used without any pretreatment, and there is no water or gas equilibrium to establish or maintain. Also, it is not necessary to humidify the hydrogen to the cell and furnace. Samples can be run in any sequence, whether they are hydrocarbons, water, aldehydes, or alcohols. There
is no base carry-over to the titration cell, so Ascarite is effective whether analyzing high nitrogen samples using platinum electrodes or low level nitrogen samples using palladium electrodes (5) and very high sensitivities. The accompanying Table I illustrates some typical standard samples. RECEIVED for review November 2, 1970. Accepted December 3, 1970.
Simple Technique to Determine Vapor Adsorption Capacities of Molecular Sieves D. P. Roelofsen Laboratory of Organic Chemistry, Unicersity of Technology, Julianalaan 136, Delft, The Netherlands THE ADSORPTION ISOTHERM of a molecular sieve characteristically consists of a steep section at low relative pressure pips followed by a nearly horizontal section up to a relative pressure of about 0.8, proving complete filling of the adsorption cavities; finally the adsorption isotherm increases again because of capillary condensation on the outside surface of the crystals. Therefore the saturation capacity of a molecular sieve can be obtained satisfactorily using a onepoint method in the horizontal part of the isotherm. This paper describes a vapor adsorption apparatus of simple construction for the above purpose in which several samples can be investigated at the same time. Thus this technique offers distinct advantages over more conventional volumetric or gravimetric adsorption methods ( I ) when saturation caDacities of molecular sieves are to be determined. An adsorption apparatus with a similar aim but of a different construction has been mentioned (2) but no technical details have been published. APPARATUS
The apparatus (Figure 1) consists of two glass-bells, a coupling with Viton O-ring seals, and a holder with six adsorption tubes. In the upper glass-bell the holder with the glass adsorption tubes, each with a sealed bulb filled with activated molecular sieve, is held at a temperature TI. This is accomplished by forcing thermostated water through the jacket of the glass-bell and in addition through a circular groove in the brass coupling (Figure 2). In the lower glassbell liquid adsorbate is thermostatically controlled at a temperature T2 < TI. Using published vapor pressure data one can adjust pips above the samples by choosing TI and T,. By connecting an oil-type vacuum pump to the Serto valve, the apparatus is evacuated to remove interfering gases and to increase the adsorption velocity. On molecular sieve samples only water, of the gases possibly present, will seriously interfere with the adsorption of the vapor a t the temperatures and pressures used. To remove water completely, vacuum was applied during a few minutes, thus purging the apparatus with the vapor of the liquid adsorbate. Any last traces of water may be removed by activated 3A molecular sieve pellets in a porcelain crucible. 3A sieve has a very high (1) D. M . Young and A . D. Crowell, “Physical Adsorption of Gases,” Butterworths, London, 1962. (2) G. R . Landolt. Abstracts of Papers, TECH 007, 158th ACS National Meeting, New York, September 1969.
UPPER GLASS-BELL
IO
Crn
1
i
-
WATER 11
c G R O U H O SURFACE
--A
BAR MAGNET ADSORPTION TUBES
-
SAMPLE IN S E A L E D BULB
PERFORATED SHEET-BR4SS
VACUUM / D R Y AIR
GROUHO SURFACE
CILIHORICAL
--7
\
CLAMP
-
LOWER G L A S S - B E L L
Figure 1. Exploded view of adsorption apparatus ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
631
C R U C I B L E WITH MOL SI REMOVABLE
SH
VITON O - R I N G S
--
CIRCULAR GROUND GROOVE
13 C r n
Figure 2. Top view of coupling
affinity and selectivity for water because of the small pores which exclude nearly all compounds except water. Experiments without 3A sieve, however, gave identical results when due care was taken to purge the apparatus completely. The sealed bulbs within the adsorption tubes are broken by sharp bar magnets using a powerful horseshoe magnet. Each bar magnet moves smoothly in a glass tube provided with a small hole near the top. The vapor contacts the adsorbent through 4-mm holes bored in both parts of a 19/26 T greaseless clear precision joint. Precision joints are preferred although ordinary ground joints can also be used. At the end of each experiment, dry air is admitted through the Serto valve. The materials of construction are, unless stated otherwise, borosilicate glass and tin-soldered brass which has been nickel-plated. Apiezon grease was sparingly applied to the Viton O-rings.
Table I. Mol. sieve 13X 13X 13X 4A 4A
Adsorbate Cyclohexane Cyclohexane ( 2 ) Cyclohexane (3) Methanol Methanol (4)
RESULTS AND DISCUSSION
This technique was used successfully to determine, for instance, the adsorption capacities of type X and Y sieves for cyclohexane and of type A and AW sieves for water and different alcohols. The precision was checked as shown by the examples on Linde sieve powders given in Table I. The first measurement of Table I was repeated on the same 13X sample under identical conditions using an electrobalance. A value of 18.3 gjl00 g of sieve was found as compared with a value of 18.9 gjl00 g of sieve in Table I. Although obtained under different experimental conditions and on different batches of molecular sieves, literature data are included in Table I for comparison.
Adsorption Capacities
"C
PIP8
45
0.50 0.20 0.46 0.50 0.03
25 25 45 25
EXPERIMENTAL PROCEDURE
The samples were activated as follows. A borosilicate glass tube of 8 mm o.d., blown out at one end and with a ground glass cone at the other end, was weighed, filled with 0.5-1.5 grams of molecular sieve, reweighed, a plug of glass wool inserted, the tube weighed again and connected to a high vacuum line together with five similar tubes. After activation at 400 "C and mm, the bulb was sealed off, the two parts were weighed, and the amount of activated molecular sieve was calculated. A sealed bulb and a bar magnet were inserted into a carefully cleaned adsorption tube. After a short evacuation to remove traces of volatile material, the stoppers were turned to exclude moisture. To minimize magnetic disturbance, the assembled adsorption tubes were weighed while suspended in a loop of nylon. The stoppers were turned again so as to get a free passage through the sccket and cone of each joint. After pouring a small portion of (3) D. W. Breck and E. M. Flanigen, Papers Conference on "Molecular Sieves," 1967 (1968), SOC.Chem. Ind., London, p 47. (4) D. W. Breck, W. G. Eversole, R. M. Milton, T. €3. Reed, and T. L. Thomas,.I. Amer. Chem. SOC.,78,5963 (1956).
632
pure adsorbate in the lower glass-bell, the apparatus was assembled (Figure l), vacuum applied for a few minutes, and after closing the Serto valve the sealed bulbs were broken magnetically. After a convenient adsorption time (24 hours was chosen although a few hours will be sufficient in most cases) dry air was admitted at such a low rate that no condensation occurred. The upper glass-bell and the holder were removed and, after cooling for a few minutes, the stoppers were turned. After equilibration in the balance room, the tubes were reweighed. By including a blank with an empty sealed bulb in each experiment, a correction for the dead volume was obtained. The adsorbed amount was expressed in grams of adsorbate per 100 grams of activated molecular sieve. When changing from one adsorbate to the other, the Viton O-rings were degassed at 150 "C.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
~~~h~ of expts 17 22 1 5
1
Grams adsorbed/100 grams Std dev Mean value 18.90 0.08 15.3 0.22 19.6 ... 19.37 0.05 17 ...
The large difference in the mean adsorption capacity of 13X for cyclohexane, found in reference (2) and in this work, is presumably due to a different amount of amorphous material in the 13X samples. An important advantage of the present method over that described in reference (2) is that the activated samples were contained in sealed bulbs which were broken magnetically, thus largely eliminating any problem of moisture uptake. In conclusion, the present technique allows a rapid and reliable determination of adsorption capacities of molecular sieves. ACKNOWLEDGMENT
Thanks are due to H. van Bekkum, E. R. J. Wils, and L. Maat for helpful discussions, to the general service of the chemistry department for the construction of the apparatus, and to Miss H. C. Niedeveld and J. Medema of the Chemical Laboratory, National Defense Research Organization TNO, for performing the electrobalance adsorption measurement. RECEIVED for review September 4,1970. Accepted December 21, 1970.
