CA1214450A - Polyvalent ion exchanged adsorbent for air separation - Google Patents

Abstract

ABSTRACT The invention relates to faujasite-containing compositions in which the original hydrogen or monovalent forms are ion exchanged to their polyvalent form and then thermally activated to promote dehydration and dehydroxylation of the faujasite while maintaining substantially the same zeolite content. The resulting compositions containing faujasites substantially in their dehydrated/dehydroxylated state have been found to have surprisingly high selectivities and capacities for the separation of air into nitrogen and oxygen.

Description

~ P-US02875 AN IMPROVED POLYVALENT ION EXCHANGED
ADSORBENT FOR AIR SEPARATION

TECHNICAL FIELD
This invention relates to novel faujasite-containing compositions and to various methods for preparing the same. More particularly, the invention is directed to faujasite-containing zeolites which exhibit superior properties for the selective adsorption of nitrogen and which therefore provide superior utility in the separation of air into nitrogen and oxygen.

BACKGROUND OF THE PRIOR ART
Molecul~r sieve zeolites have long been observed to demonstrate selective adsorption when in contact with a variety of adsorbable mixtures. This attribute may be utilized to affect a variety of separations, as for example, the separation of n-paraffins from ~ranched chain paraffins or other well known separations using pressure swing or vacuum swing processes. The adsorptive selectivity of the zeolite towards one or more components of a mixture must be maximized to ma~imiæe the efficiency of the desired separation. Assuming all sther engineering factors remain constant, the adsorption characteristics of the material select~d for the separation process influences both the production level an~ the purity of the gas~s produced.
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The phenomenon of selective adsorption by molecular sleve zeolites may arise from one of two properties inherent to these crystalline materials. The property of molecular sieving may arise from the extremely uniform porosity demonstrated by these crystalline aluminosilicates. The size or shape of one or more components of a mix~ure may preclude its adsorption by the materials. The separation of n-paraffins from branched chain paraffins is an example of this effect.
If a zeolite with a pore opening of ~5A is employed, the n-paraffin component of a mixture is readily adsorbed, but branched chain paraffins are excluded from adsorption by virtue of their configuration, effecting a separation of the components which is the basis of several commercial processes. If, however, the molecules of the mixture to be separated are all small enough to enter the zeolite crystals, selective adsorption may none the less be demonstrated by a second mechanism. Zeolites have large quantities of exchangeable cations present within their aluminosilicate framework. These cations are situated such that a high proportion may come into contact with adsorbates small enough to enter the crystalline zeolite framework. The energetic interaction of these cations with polar or polarizable adsorbates results in these adsorba~es being selectively adsorbed from a mixture of less polar or polarizable species.
This effect allows such separations as the selective adsorption of N2 from air as demonstrated by calcium exchanged A-type zeolite and sodium mordenite by pressure swing or vacuum swing adsorption processes. A comprehen-sive summary of khe adsorptive pxoperties of prior art molecular sieve zeolites, their causes and uses is found in D. W. Breck, Zeolite Molecular Sieves, J. Wiley and Sons, New York, Chapter 8, pages 593-724 ~1974).
Nitrogen has a quadrupole of 0.31A and therefore may energetically interact more strongly with the aforementioned cations then 2~ with its quadrupole of only 0.10A3. Thermodynamics dictates that the more strongly adsorbed species will be preferentially adsorbed.
Further, this cation to N2 interaction energy, and concomitantly the adsorptive preference or selectivity, may be altered with the choice of exchangeable cations present. In general, in a given zeolite the interaction energy and thus the capacity for nitrogen rises with the charge density of the cation. Thus, it has been found in the literature, Breck, Od. Cit., pages 694-695 and H. Minato and M. Watanabe, Scientific Paper General Education, University of Tokyo, Volume 28, page 218 (1978), that for the monovalent alkali metal cations the following trend of nitrogen capacity exists:
Li~ Na+~ K+ ~ Rb+~ Cs~. Oxygen, with its smaller quadrupole and concomitantly smaller cation-quadrupole interaction energy, is much less sensitive to the cation present. N2/O2 selectivities follow the same trend as N2 capacities. One would expect that the polyvalent cations would follow a similar trend and due to their high charge density would be even more useful in the separation of nitrogen and oxygen. However, this characteristic has not been clearly demonstrated for any zeolite. In fact, it has been reported that in - the faujasite type, e.g. zeolite X, the reverse trend exists as described in U.S. Patent Nos. 3,140,932 and 3,140,933.
Water, being quite polar, is strongly bound to the aforementioned cations. It has long been xecognized that these materials must be activated at elevated temperatures to remove water, which would block adsorp-tion of such species as N2. However, previous zeolite surface scientists in the field of air separation and gas adsorption have completely failed to recognize the sensitivity of the adsorption characteristics of poly-valent-exchanged zeolites in general and calcium-exchanged faujasites in the specific, to thermal activation , procedures; see U.S. Patent Nos. 2,882,244, 3,140,932, and 3,313,091. A good example of this lack of recogni-tion is found in Milton, 2,882,244 which discloses and claims zeolite X adsorbents. It is stated at column 15, lines 23-31 of this patent that zeolite X may be activated by heating in air, vacuum or other appropriate gases at temperatures as high as about 700C, at which conditions other adsorbents have been found to be partially or completely destroyed. In fact, a well recognized ~rocedure in the manufacture of such zeslites i5 to follow the synthesis and/or ion exchange step with a drying step at temperatures of up to about 250C.
It has not been recognized previously and it has now been found through the use of zeolite content determinations, adsorption measurements, gas chromato-graphlc analyses, and infrared studies which are presented in part in the Examples below, that both cation and framework hydrolysis can give rise to reduced nitrogen capacities and selectivities for the calcium-exchanged X faujasite if the thermal history of the material is not carefully controlled after the ion exchange step has taken place. From these comparisons, it becomes evident that a significant difference in the stability e~ists between the monovalent and the polyvalent forms of faujasites. The sodium form of faujasite, a common adsorbent used in drying operations, is routinely dried at temperatures of 250C without any evidence of a decreased performance as an adsorbPnt. When one subjects the calcium form of faujasite to the same drying condi-tions, the resulting adsorbent properties can be inferior to the sodium, but are generally compar~ble to the ~odium form. It is believed that the calcium form of faujasite has been overlooked as an adsorbent for air separation because it was thought not to offer any particular advantage over the ~odium form. In ~act, out of all of the zeoli~ic prior art, only one ref~rence suggests that the calcium form has even a slight advantage over the sodium form of faujasite; see Habgood, H. W., Canadian Journal of Chemistry, Vol. 42, pages 2340-2350 (1964). It was found that when what is believed to be the Habgood heat treatment procedures were followed for preparing the X-type zeolites including a highly exchanged calcium form, e.g. greater than 90%, the nitrogen capacity extrapolated to 30C and atmospheric pressure was significantly lower than the capacity of the same highly calcium exchanged X-type of zeolite which was prepared under the carefully controlled thermal activa-tion conditions of the present invention. Although the specific drying ànd thermal activation conditions are not set forth in the Habgood reference, it is apparent that the thermal history of his zeolite has not been carefully controlled because of the significantly lower estimated nitrogen capacity and the corrected selectivity values as set forth in detail in the Examples below.
Other references which either have reported gas chromatographic selectivities and/or nitrogen capacities which are significantly lower than those obtained by the absorbent compositions of the present invention or have disclosed calcium exchanged zeolites without any suggestion as to their having any utility as adsorbents include Wolfe, E., et al., German Patent No. 110,478 (1974); Wolfe, F., et al., Z. Chem., Vol. 15, pages 36-37 (1975); Andronikasui li, T. G., et al., Izv.
Akad. Nauk. Grvz. U.S.S.R., Ser. Khim., Vol. 1, No. 4, pages 339-402 (1975); Uytterhoeven, J. B., Schsonheydt, R~, Liengme, B.V., and Hall, W. K., Journal of Catalytis 13, 425-434 (1969); Ward, J. W., Journal of ~atalysis 10, 34-46 (1968); Ward, J. W., Journal of Physical Chemistry 72, 4211 (1968); Olson, D. H., Journal of Physical Chemistry 72, 1400-01 (1968) and Bennett, J. M.
and Smith J. V., Mat. Res. ~ull. Vol. 3, 633-642 (1968).

SUMMARY OF THE INVE~TION
In a significant departure from the teachings of the prior art, a method of activation has been discovered whereby certain polyvalent exchanged aluminosilicate zeolites, especially those having silicon to aluminum ratios of approximately l and up to about 2 can be converted into novel highly selective adsorbents for nitrogen. It is believed that minimizing the amount of water present during the thermal activation of such materials results in both the substantial retention of accessible zeolite content and production of a preponder-ance of polyvalent cations in a dehydrated/dehydroxylated state. It has been discovered that the adsorbent compositions produced by the method of the present lnvention have substantially greater selectivities for the separation of a binary mixture of nitrogen and oxygen and higher nitrogen capacities than empirically similar materials prepared by the teachings of the prior art. This phenomenon is particularly true for compositions containing a major portion of an alumino-silicate in which the silicon to aluminum ratio is from about 1 to 2, i.e. the X-type faujasites.
In summary, the present invention is directed to a polyvalent ion exchanged adsorbent composition compris-ing at least 50% by weight fau~asite and the faujaslte portion thereof having a majority of its exchangeable ion capacity in the polyvalent form prepared by a process which comprises:
(a) ion exchanging the faujasite-containing composition with polyvalent ions, and (b) thermally activating the polyvalent exchanged composition to remove a substantial portion of its zeolitic water, which includes the hydration spheres, if any, surrounding the cations in said composition, in such a manner as to produce a preponderance of saia polyvalent ions in a dehydrated/dehydroxylated state such that the resulting nitrogen capacity and selectivity ratio of the resulting polyvalent exchanged composition is substantially greater than such a polyvalent ion exchanged composi-tion that has not undergone such a thermal activation step.
The thermal activation step can be achieved by a number of different methods in which the zeolitic water and the hydration spheres are carefully removed and the amount of water in the gaseous environment in contact with the zeolite during this step is minimized; i.e., the partial pressure of water making such contact should be less than about 0.4 atm., preferably no more than about 0.1 atm.
One method of accomplishing this is to subject the polyvalent exchanged composition which contains up to about 30% by weight of water to pressures in the range of about 0.1 to 10 atmospheres while maintaining suffi-cient molar mass velocities and residence times of aflow of a non-reactive purge gas; i.e., a molar mass velocity (G) of about 0.S to 100 kg. mole/m2~hr. (= 0.1 to 20 lbs.-mole/ ft.2-hr.) and a residence time (r) of no greater than about 2.5 minutes, heat said composition at a temperature ramp of 0.1 to 40C/min. up to tempera-tures of at least above 300~C and no greater than 650C, and maintain the composition at these temperatures for a period of at least about 12 hours. The residence time i5 defined as the volume of the column or othex unit used to thermally activate the zeolite divided by the volumetric flow rate of the purge gas at the standard temperature and pressure (STP). The molar mass velocity is simply the flow rate of the purge gas divided by the cross-sectional area of the column used for thermal activation. The purpose of the purge gas is to provide a sufficient m~ss for efficient heat and mass transfer from the surface of the adsorbent at a residence time ,o limit the water in the purge gas exiting the adsorbent bed to the desired low limits. The minimum residence time is determined by economic and process constraints, although times of less than 0.0025 minutes would appear to provide no advantages.
Another method of thermal activation is to conduct it under less than about 0.1 atmosphere vacuum without the use of a purge gas and to heat the material to the desired activation temperature at a temperature ramp from 0.1 to 40C/min.
Still another method that is available for thermal activation of the zeolitic adsorbents of this invention is the use of microwave radiatiorl at conditions that are described in H.S. Patent l~o. 4,322,394, of which the description of the mîcrowave procedure for thermally activating zeolites is referable.
Any of the above methods of activation can be employed so long as the zeolite content as determined by adsorption methods in the activated composition is maintained to at least about 70% of its initial value prior to the thermal activation step.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a plot of the data showing the effect the ion exchange loading has on gas chromatographic selectivity;
FIGS. 2 and 3 are nitrogen isotherms for various controls and examples of the present invention; and FIG. 4 is a plot of data showing the effect the initial drying temperature has on zeolite content and nitrogen capacity.
DETAILED DESCRIPTION OF THE INVENTION
The starting materials for the preparation of the compositions of the present invention include those zeolite compositions in which at least a majority is of 3~

4~0 the faujasite type, preferably those having Si/Al ratios of about 1-2, e.g. the X-type zeolites. Such base materials are exchanged usually but not neces-sarily, from their sodium form to a state containing at least a majority of their cationic content in polyvalent form. Any conventional ion exchange procedure which will produce this effect is suitable. It has been found that minor amounts of inert binders, such as clays, and/or other solid materials including other zeolitic components may be present without adversly affecting the attainment of the unexpectedly superior adsorption properties of the composition of the present invention.
In one embodiment of the process for preparing the compositions of this invention, the starting material is ion exchanged in the presence of an excess of a water soluble salt, such as a chloride, nitrate, sulfate, and the like, of any polyvalent metal. The polyvalent metals can be of the divalent and trivalent transition elements of Groups lb to 7b and Group B o~ the Periodic Table, the divalent alkaline earth metal elements of Group 2a and the lanthanide rare earth series. Prefer-ably the polyvalent metal is selected from the group consisting of magnesium, calcium, strontium, barium and mixtures thereof. Still more preferably the polyvalent metal is calcium. One example of a conventional ion exchange procedure is to repeatedly exchange the starting material with the polyvalent material in an aqueous solution at temperatures up to those at which reflux occurs, e.g. up to about 100C and about 1 atmosphere, for a period of 2 to 48 hours until at least a major portion of the original cations in the faujasitic portion of the starting material are replaced by poly-valent ions.
Once prepared ~ith at least a majority of cationic content in the polyvalent state and washed substantially free of excess, non-exchanged cations, ~he materials are ready for thermal activation.

It is known that high charge density pol~valent ions within zeolites, especially faujasites undergo a substantial degree of hydroxylation during thermal activation. This is not true of NaX type of zeolites 05 which are more s-table towards thermal activation.
Hydration is an equilibrium reaction which may be represented as:
M + XH20 = M(OH)e ( ) + eH + (X-e)H20 (1) wherein M is at least one cation having a valance n of 2 or 3, X is 1 to 6 and e is 1 or 2.
The products on the right side of the equation are detrimental to the zeolite adsorbents. Hydroxylated multivalent cations, such as Ca(OH)+, are ~nown to be, ineffective sites for the selective adsorption, especially for N2. See H. Minato and M. Watanabe, Scientific Paper General Education University of Tokyo, Vol. 28, which refers to natural zeolite, particularly mordenite page 135 (1978). Additionally, zeolite frameworks are generally unstable towards H+. This is especially true for zeolites whose Si/Al ratio is relatively low, as for example faujasites with a Si/Al ratio of 1.0-2Ø The equilibrium may be directed towards the desirable polyvalent cations and away from the destructive H-~ and ineffective hydroxylated cations by minimizing the amount of water present at any given temperature during the thermal activation, particularly temperatures above 150C.
It has been found that a lack of appropriate attention to initial drying conditions results in a substantial reduction in the subsequent adsorption properties after thermal activation at higher temperatures. This initial drying should be conducted either at temperatures ranging from ambient to no greater than 200C, preferably no greater than 120C or under a vacuum or sufficient flow of purge gas at higher temperatures.

5~

Thermal activation is achieved by raising the temperature from the level of the preactivation drying to temperatures no greater than 650C, preferably about 350 to 450~C.
At a given purge gas rate, slowing the rise of temperature increase is found to have a beneficial effect on adsorption properties. The purge gas can be any non-reactive gas such as dry air, nitrogen, neon, helium and the like. The slow tempera~ure increase allows the water in the zeolite to more nearly come to thermal eguilibrium on desorption at any given temperature.
Conversely, at a given temperature ramp, rapid purging is found to have a beneficial effect on adsorp-tion properties by more completely sweeping away wateras it is desorbed.
The exact temperature rise and purge rate necessary to produce the adsorbent compositions of this invention during thermal activation can vary and can depend on the configuration of the adsorbent bed.
In a preferred embodiment, the dried polyvalent exchanged zeolite, which has not undergone temperatures greater than 200C, preferably 120~C, is thermally activated by increasing the temperature ranging from ambient up to a maximum temperature of about 650C
while maintaining a continuous purge of non~reactive gas through the zeolite being activAted at rates in the range of 0.006 to 6 l/min., (i.e. ~ values in the range of 0.5 to 100 kg. mole/m2 hr.) until the zeolite has reached the maximum activation temperature, preferably in the range of about 350 to 450C. The zeolite is maintained at these conditions for a period of 12 to 48 ~ours. Longer periods of time can be employed, but are usually not required to obtain ~n adequate activation.
The exact temperature rise during ~he critical tempera-ture range of 120~C to 350 to 650~C depends on ~he depth and diametex of bed wi~hin the thermal activation column, the flow rate of purge gas and the temperature conditions at any given moment of the temperature rise.
However, it has been found that as long as the purge rate of the bed of material undergoing thermal activa-tion is sufficient to achieve the desired residencetimes and molar mass velocities as discussed above or the bed is maintained under vacuum of at least 0.1 atmospheres, preferably less than 100 mm Hg, the temper-ature rise can be rapid, i.e. as high as 40~C/min.
The examples below are intended to illustrate the ~oregoing methods of preparing the compositions of the present invention and to demonstrate the superiority of such compositions as selective adsorbents, especially in the separation of nitrogen from air and are not intended to restrict the scope thereof.

E~AMPLES
In these Examples prior to any of the adsorption capacity measurements, for nitrogen capacity and zeolite content by oxygen adsorption as described below, the zeolite samples were activated overnight at 400C under a vacuum of about 10 5 mm Hg. A heating rate of 1-2~C
per minute was used to obtain the 400C temperature.
After activation, the samples were handled in a nitrogen dry box; however, momentary exposure to the atmosphe~re is unavoidable when the sample buckets of the McBain-Bakr spring balance are charged.
Before the nitrogen capacity measurements were made in these Examples, the samples were outgassed in situ at 400C under 10 4 mm Hg overnight. The nitrogen capacities were measured at 30C with a nitrogen pressure of 760 torr. Adsorption was measured after an equilibra-tion time of four hours.
The samples were again outgassed overnight in situ at 400~C prior to determining zeolite rontent by oxygen adsorption. Zeolite content measurements of the Examples ~ , .

5~

were determined using a modified version of Bolton's method; A. P. Bolton, "Experimental Methods in Catalytic Research," Vol. ll, R. B. Anderson and P. T. Dawson (editors), Academic Press, New York (1976), page ll.
Instead of carrying out the experimentation at liquid oxyyen temperature and lO0 torr pressure, a similar experiment was conducted at -196C (liquid nitrogen temperature) and 20 torr. These conditions resulted in the same relative pressure, p/pO of 0.13 (pO = 155 torr at 77K for oxygen). The measurements were made on a McBain-Bakr spring balance and duplicate analyses of the same CaX adsorbent agreed within 2%.
Since the accessible micropore volume for adsorption of adsorbates is the physical quantity of interest, it is more appropriate to employ adsorption methods for determining apparent zeolite content than the more conventional X-ray line intensity measurements. The pore blockage~which can occur from a reduction in zeolite content can prevent an adsorbate from inter-acting with a portion of the micropore ~olume, whereasX-ray determination of crystallinity would include this micropore volume which is inaccessible to the adsorbates of interest. Therefore, of interest to this work and related adsorption studies is the accessibility of micropore volume to gases of interest which is most readily measured using a vapor possessing a low heat of adsorption such as oxygen at low temperatures. In addition, oxygen interacts in a more nonspecific manner with different sites in the zeolite than nitrogen or other adsorbates.
Another adsorption method for determining zeolite content; i.e., t-plot, was also employed in Examples 27-29.
This required that N2 adsorption isotherm data at -195C (obtained from an automated instrumentj be converted to a t-plot using Deboers method ~J. Colloid Interface Sci, 21, 405 (1966~). The micropore N2 uptake was estimated from the t-plot using an approach ~z~ {~

1~
put forward by Sing (Chemistry and Industry, May 20, 1967, p. 829). Since most binders and amorphous inclu-sions lack micropores, it was postulated that the zeolite content is directly proportional to the micropore-uptake. Thus, the latter can be used as it is forcomparing the zeolite content of adsorbents, or can be used to estimate the absolute value of the zeolite content by comparison with the uptake of a standard adsorbent. In all these comparisons, care was taken to compare only adsorbents that had the same cationic form and cation loading.
Zeolite content determinations using X-ray diffraction on sodium and/or calcium exchanged X zeolites were performed for Examples 27-29 and 32 using a method that invGlves addition of alpha-A12O3 to the zeolite as an internal standard and the ratioing of selected ze~olite lines to the standard using integrated peak areas.
Nitrogen and oxygen isotherms were determined in most cases using a microbalance at temperatures as indicated. In each case the activation was carried out at 400C under about 10 4 mm Hg vacuum until no detect-able weight loss accurred ( 0.004% change).
The gas chromatographic (GC) procedure used to evaluate the compositions of these Examples for selectiv-ity and indications of nitrogen capacity trends is well accepted and similar to that used by several workers;
see for example, J. R. Conder and C. L. Young, "Physio-cochemical Measuxement by Gas Chromatography", John-Wiley (1979); R. J. Neddenriep, J. Coloid Interface Science, 28, 293 (1968); D. Atkinson and G. Cur~hoys, JCS Faraday Transactions I, 77 897 (1981); and A. V. Kiselev and Y. I. Yashin, Gas-Adsorption Chromatography, Plenum Press (1969) pp. 120-125. Except where noted the samples were thermally activated in the GC column at 2C/min. to 400C undex a 100 cc/min. flow of zero grade helium and maintained at 400C for 16 hours.

Inert gas retention volumes for each column were deter-mined at 300C using 10% Ne in helium. The GC method was limited to the low pressure region of the isotherm and represents a limiting value for the =electivities that might be approached in the adsorption of nitrogen-oxygen mixtures at zero coverage. Nevertheless the method is rapid and valuable for screening and ranking adsorbent performance and is reproducible to within 10%.
The retention volume of an injected species into the adsorbent is the product of the retention time and the corrected flow rate. Thus one can calculate the retention volume, VI, of inert species (from neon retention data at 300~C), and the retention volume, Vj, of the jth adsorbate at different temperatures.
The adsorption equilibrium constant for the jth adsorbate can be calculated from the following equation:
K = i I ~II) K has the unit cc/gm or moles~gm~moles/cc. K is the initial slope of an adsorption isotherm in which the quantity on the ordinate has the units of moles/g. and that of the abcissa has the units of moles/cc. Using ideal gas law as follows, K can be converted to the initial slope Cl of the isotherm in which the quantity on the abcissa is the (partial) pressure of the adso~ate in units of pressure; see FIGS. 2 and 3, for example.
(Isotherms from gravimetric or volumetric data are usually of the latt r form.):

Cl RT column where R is the gas constant (= 82.05 cc-atm/mole-~K3 and T is the GC column temperature in K (the subscript "column" will be omitted hereinafter and T will denote the cvlumn temperature3.
Multiplication of Cl by the molecular ~ei~ht of the adsorbate give~ the guantity C2 which has ~he units of g.(adsorbate~/g.~adsorbent)~abm.

The selectivity ratio of species 2 over species 1, ~Cl, is defined as

2 nz nl (lII) 1 n~ nl where n is the number of moles in the gas phase and n is the number in the adsorbed phase.
The selectivity ratio of, for example, N2 ~ 2 separation is calculated from:
Oe N2 = N2 (IV) Several Group I and II metal exchanged X zeolites were prepared and evaluated in these Examples. The exchange procedure described in Example 1 is typical.
Though studies have shown that the procedures used are not critical as long as greater than 80% levels of exchange were achieved. The gas chromatograph was employed routinely to screen the various ion exchanged adsorbents using the procedure descr~bed in detail in Ex~mple 1 below.

ExamPle 1 An 0.45 kg. sample of 8-12 mesh beads of 13X (NaX) zeolite obtained from Davison Division of W. R. Grace having a Si/Al = 1.25 was exchanged with one liter of lM CaC12 6H2O. The lM CaC12 6H2O was added slowly to the zeolite contained in a two liter round bottom flask eguipped with a condenser and thermometer. The solution was brought to 90 to 95C over a thirty minute period and held there for an additional hour. The exchange solution was removed and the beads of 13X were washed ~hree times with approximately a liter of distilled water for each wash. After washing, the beads were allowed to soak in distilled water for approximately fifteen minutes to assure that the zeolite was completely free of any eXcess CaC12 solution. After the three washes, a fresh one liter portion of lM CaC12 6H2O was added to the zeolite beads and the flask was again brought to 95C. These exchange procedures were repeated three additional times for a total of four exchanges.
After the final washes, the resulting highly calcium exchanged zeolitic adsorbent was placed in a flat pan and dried with a current of dry nitrogen at ambient temperature. Elemental analysis showed that 95% of the ion exchange capacity had ~een converted to the calcium form.
A weighed amount of the resulting CaX composition was packed into a three foot 1/4 inch O.D. copper tube.
The packed column was weighed and placed in a GC column and was thermally activated in the manner set forth above. After thermal a~tivation, the column was brought to 300C, the flow was reduced to 30 cc/min., and e~uilibrated there for an hour, then the dead volume of the column was d~termined by injecting a 0.5 cc pulse of neon gas.
The carrier flow rate at each column outlet was measured using a bubble flow meter and corrected using standard procedures to account for ~he difference in temperature and pressure between the flow meter and column. Following t~e dead volume measurement, the retention volumes of 2% 2 and 8% N2 in helium, carrier flow rates and associated data were collected at 50~, 40 and 30C in a manner analogous to the measurement of the neon at 300DC. For each new temperature an hour equilibration time was required. Following the GC
evaluation, the column was removed from the GC and weighed again and the dry weight of zeolite was obtained by subtracting the weight of water lost. Using the retention volume data for Ne, 2 and N2 gathered and the weight of the dry adsorbent, Henry's law slope expressed as mmoles/g/atm and the N2/02 selectiYity were obtained.

The CaX composi~ion of Example 1 had a GC selectivity (~eparation factor) of 13.1 at 30C and a 0.82 mmoles/g N2 capacity at 30C and 1 atm.

Control 1 Linde 5A (CaA) molecular sieves having a Si/Al molar ratio of about 1 in the form of 1/16 inch extrudate were packed in the same ~C column of Example 1. This calcium form of zeolite was evaluated by the GC procedure described in Example 1 and found to have a heat of adsorption of 6.1 cal./mole of nitroyen and a selectivity ratio of 4.8 and 4.0 at 30C and 50C, respectively.
This adsorbent because of its relatively good selectivity is stated in the literature as the adsorbent of ~hoice in the economically viable pressure swing adsorption process used in commercial air separation plants; see "The Properties and Applications of Zeolites", edited by R. P. Townsent, the Chemical Society, Burlington House, London (1980) pages 92-102.

Example 2 A 2,000 gram sample of synthetic sodium faujasite designated as Linde 13X obtained from the Linde Division in the form of 1/8" extrudate and having a Si/Al molar ratio of about 1.25 was exchanged with 4 liters of a~lM
CaC12 aqueous solution. The CaC12 a~ueous.solution was prepared by dissol~ing 588.1 grams of CaC12 dihydrate in 4 liters of distilled water. The slurry of zeolite in the agueous solution was refluxed for 1 hour, drained, washed with 5 liters of distilled water, soaked and drained again. The preceding procedure was repeated with an additional draining and washing step. The zeolite was soaked in the 4 liter lM CaC12 solution overnight, refluxed for 1 hour, drained, washed twice, and refluxed in 4 liters ~f 1~ calcium chloride solution for the fourth time. The xeolite was ~hen drained and washed a number of times to assure that all of the residual CaC12 had been removed. The resulting zeolite was believed to contain a high level of calcium, since similar ion exchange procedures with pure Linde X powder resulted in greater than 95~ exchange. These pellets were dried in an oven using a strong nitrogen purge at 100C overnight for about 17 hours. This sample was further dried under a mechanical pump vacuum at less than 50 mm Hg at 100C for an additional 8 hours. The zeolite was then dried for 4 more hours at 150C, 200C, 250C and finally, about 290C under this vacuum. The resulting sample contained less than 2% by weight water content before it was thermally activated in the same manner as tha-t used to thermally activate the zeolite of Example 1.
It was found that the selectivity ratio for this sample averaged 12.5 at 30C for 2 runs and the nitrogen capacity at 30C and 1 atm. was 0.64 mmoles/g.

Control 2 The procedure of Example 2 was again repeated except that the bulk of the 2 kg. sample was dried under nitrogen purge at 225C to a level of only about 10~ by weight water. The GC selectivity ratio was found to be 4.4 at 30C and the nitrogen capacity at 1 atm. was found to be only 0.16 mmoles/g.

Several Group 11 metal exchanged zeolites were prepared and evaluated by exchanging 200 grams of this material in the form of 8-12 mesh pellets obtained from Davison Division with 400 cc of aqueous solution of the appropriate metal chloride at a concen-tration so that sufficient e~uivalents were present to displace substantially all of the sodium originally present in this Na form of zeolite (5.83 Meg/gram).

5~

Each of the samples prepared in accordance with Examples 3-6 were prepared by carrying out 6 exchanges each lasting 12 hours at reflux temperatures after which each sample was thoroughly washed with distilled water and dried at ambient temperatures. Elemental analysis showed that in each example, an 80% or greater level of exchange was obtained.
Table I below summarizes the selectivities zeolite capacities obtained from the GC evaluation for Examples

3-6 and clearly shows that when th~ ion exchanged adsorbents were activated in accordance with the present invention, the opposite trend for alkaline earth çations was obtained in comparison to the results set forth in Table Il below as reported in the prior art.

Controls 4-5 13X and lithium exchanged X 2eolites of the same type used in Examples 3-6 were subjected to the same activation t~chnique as set forth under Examples 3-6 and the selectivities and zeolite capacities for the controls are summarized in Table I below.

TABLE I
EVALUAT I ON OF

GC N /0 ZEOLITE CAPACII~
EXC}1.9NCE SELEC~IV~TY FOR N (cc STP N2/g) EXAMPLE/CONTROL CATION LEVEL @ 30C _ ;2~ooC, 1 atm.

- 4 Li 81 5 . 6 0 . 34

- 5 Na 98 3.1~a) 0.25 3 - Mg 80 5 . 5 0 . 21 4 - Ca 95 13 .1 0 . 82 - Sr 88 6 . 4 0 . 62

6 - Ba 90 2.9 0.37 (a) N2jo2 selectivi'Ly was 2.6 a~ 50 with or without the ~ame theDal activation technique of Examples 3-6.

45~

Controls 6-14 Table II below summarizes the nitrogen-oxygen separation factors and zeolite capacities for various ion exchanged 13X zeolites at -78~C for a 25% 02-75% N2 gas mixture at 1 atm. total pressure. The data was taken from the prior art.

TABLE II
,SELECTIVITIES AND CAPACITIES OF ION EXCHANGED
13X-TYPE ZEOLITES AT -78~C FOR A
25~ 02-75% N~ MIXTURE AT l ATM TOTAL PRESSURE(a) ZEOLITE
EXCHANGE N /O CAPACITY FOR
CONTR0L ADSORBENT LEVEL SELE~TI~ITY N ,cc STP/g.

6 Li-X 86 7.6 111.8

7 Na-X 100 4.9 72.

8 K-X 100 2.2 58.7

9 Rb-X 56 3.5 30.0 Cs-X 50 1.5 26.3 11 Mg-X 56 2.4 50.1 20 12 Ca-X 96 4.5 37.8 13 Sr-X 96 8.6 52.7 14 Ba-X 85 15.8 62.7 (a)Data obtained from U.S. Patent ~os. 3,140,932 and 3,140,933.
The data in Table II shows that the separation factors increased for Group I cation exchanged forms, the alkali metals, as the charge density of the cation increased. While it would be expected that the same trend would hold for the Group II cation exchanged forms, the alkaline earth ions, the values presented above in Table II show the opposite trend.

r ~2~4~

Ex~
The composition of Example 7 was prepared in the same manner as set forth in Example 1 except that the wet exchanged zeolite was superficiously dried at 100C
under a nitrogen purge for two days prior to any subse-quent thermal activation step to determine adsorption properties. The GC selectivity and Henry's law slope at 30C for the resulting composition were 12.5 and 2.6, respectively. The nitrogen capacity at 1 a-tm, 3~C remained at 0.82 mmoles/g. The N2/02 selectivity at 50C and its zeolite content determined by nitrogen capacity measurements at -196C and 1 atmosphere pressure are set forth in Table III below.

Controls 15-20 The N2/02 selectivity at 50C and the zeolite capacity for nitrogen at -196C and 1 atmosphere for a calcium exchanged X zeolite containing substantially the same calcium loading as the adsorbent of Example 7 above are compared in Table III below as well as the N2/02 selectivities for other calcium loaded X zeolites of the prior art.

T ~ LE III

~I~SORPTION PROPERTIES FOR CaX- ~ PE ~ SORBE ~ S

ZEOLITE CAPACITY
CALCIUM GC N /O FOR N (cc STP/g.) EXAU~PLE/CONTROL LEVEL SELEC~IV~TY -196~C, 1 atm REFERENCE

- 15 90 10.4(a) 165 - 16 (d)3 4(a) 154 _ 17 80 4 2(b, c) NA 2 - 18 64 3.5( ) NA 3 ]O - 19 88 5.6(b) NA 3 - 20(90)( ) 6 7(b) NA 4 7 - 94 12.5( ) 149 (a) Selectivity at 50C.
(b) Selectivity at 30~C.
15(c) Value estimated from data given at 10 or 20C.
td) 13X form before exchange.
(e) Not stated but inferred from another reference by the same authors.
NA = Not available.

References:
1. Habgood, H. W., '~Adsorptive and Gas Chromato-graphic Properties of Various Cationic Forms of Zeolite X"; Canadian Journal of Chemistry, Vol. 1964, pages 2340-2350.
252. Friedrich Wolf, Peter Konig, E. German Patent ~o. 110,478 (1974~.
3. F. Wolf, P. Ronig, and K. Gruner; Z Chem, Vol. 15, 36-37 ( 1975 ) .
4. T. G. Andronikashuili, T. A. Chumbridze and 30G. V. Tsitsishvili; Izv. Akad. Nauk, Grvz.

S5R, Ser. Khim., Vol. 1, No. 4, pages 339-402 (1975) (Translated Russian Article). , From the data set forth in Table III above, the only adsorbent that appears to even approach ~hat of the present invention is disclosed in reference 1. The commercial zeolite that was used in the reference experiment was the Linde Division NaX molecular sieve in the form of 1/16" pellets having approximately 20%
clay binder which is stated in the reference to be assumed to have negligible adsorptive properties. The zeolite used in Example 7 was a Davison 13X zeolite having approximately 30% binder which had negligible adsorption properties.
As shown in Table I, the N2/02 selectivity determined from EC and limited to the low pressure region for the 13X zeolite at 50C is 2.6, which is the accepted value in the prior art. These values are in sharp contrast to the selectivity of 3.4 at 50C for the 13X zeolite reported in reference 1. A consis~ent over-estimate of the VI (see equation II above) will result in a consis-tently higher value in the calculation for the N2/O2 selectivity. Based on the accepted value of 2.6, the over-estimate in reference 1 for the 13X selectivity is about 24%. Therefore, assuming the same over-estimate of~VI in the calculation of CaX zeolite, the selectivity is calculated to be no higher than 8.2 in contrast to the 10.4 reported in reference 1. The 8.2 value is substantially the same as that obtained for a CaX
zeolite in which the initial drying was done at 250C
before conducting the thermal activation of 16 hours at 400C. The foregoing is believed to support the fact that the CaX zeolite of reference 1 was subjected to a conventional drying step.

Examples 8-9 The same procedures in Examples 2 and 1, xespectively were follo~ed to prepare the Examples 8-9 compositions except that Linde NaX powder was used as the starting material. The nitrogen uptake capacity or ze~lite 4~i~

capacity at 77K (-196C) obtained on the Example 9 composition and the nitrogen capacities at 303K (30~C) obtained on both of the compositions as well as those for the Examples 2 and 7 compositions are summarized in Table IV below.

Controls 21-23 Zeolite capacities at 77K were obtained for the NaX powder received from Linde (Control 21) and the NaX
8-12 mesh pellets from Davison (Control 22) and the nitrogen capacities at 303K were obtained for the Control 22 and 23 compositions. The latter was obtained by subjecting the Control 22 composition to the same type of ion exchange procedure as set forth in Example 1 to load the material with 95% of its ion capacity in the calcium form and then drying it in a conventional manner at 250C before determining its relative zeolite capacity and ni~rogen capacity ~y the procedures set forth above.
Table IV compares the capacity data obtained from Examples ~-9 and Controls 21-23 with that obtained from Example 7 and Controls 15-16.
A 24-26% increase in zeolite capacity at 77K was obtained in converting the sodium form to the calcium form using the process of the present invention; compare Controls 21 and 22 with Examples 9 and 7, respectiveiy.
There was over 240% increase in nitrogen capacity at 303K in making this conversion; compare Control 22 with Example 7.
The fact that there was only a 7% increase in zeolite capacity in converting from the sodium to the calcium form upon comparing the data reported from reference 1 (Controls 15-16) is further evidence that the CaX zeolite was subjected to a conventional drying ~.tep. The nitrogen capacity data obtained on Control 23 dried at such conven~ional drying conditions indicates some increase occuxs, but nowhere near the increase of 240% obtain d when the present process is followed.

2~

TABLE IV
ZEOLITE AND NITROGEN CAPACITIES
FOR VARIOUS Na AND CaX SA~PLES
ZEOLI~E
CAPACTIY
FOR N N CAPACITY
(cc STP2N2/g. ) (2mole N2/g) EXAMPLE/CONTROL SAMPLE INITIAL DRYING -196DC, 1 atm. 30C, 1 atm.
- 15 LiDde Unknown pro- 154 NA
1/16" NaX cedure of reference 1 - 16 Linde ~nknown pro- 165 NA
1/16" CaX cedure of reference 1 - 21 Linde NaX As received169 powder 8 - Linde CaX Dried progres- NA 0.93 powder sively to 300C
under 50 mmHg 9 Linde CaX Superiicially 209 1.15 powder dried at ambient temp.
- 22 Davison As received 118 0.24 8-12 mesh NaX
2 - Davison Dried progres- 139 0.64 8-12 mesh sively to 300C
CaX under 50 mmHg 7 - Davison Superficially 149 0.82 8-12 mesh dried at ambient CaX temp.
- 23 Davison 250C 107 0.42 8-12 mesh CaX
NA = Not available Examples 10-16 Adsorbents were prepared in the manner set forth under Example 2 except that in each case the samples were subjected to the specific type of drying and activation conditions as set forth in Table V below.

Controls 24-27 Adsorbents were preparecl in the same manner as set forth under Examples 10-16 and were subjected to the specific drying and activation conditions summarized as noted in Table V below.

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The data of Table Y above indicates that when the CaX adsorbents were dried and activated under the mild conditions, of the process of the present invention, high selectivities and capacities were realized. These results were obtained when the adsorbent was dried and activated either under vacuum or high flows of purge gas to keep the partial pressure of water in the gas phase low. The exact conditions reguired to achieve an acceptable partial pressure vary depending on the configuration of the adsorbtion bed.
One will note that even when the material was first dried at the substantially ambient conditions of 25C and then rapidly activated at 40C/min. and a linear velocity of purge gas of 16.1 kg. mole/m2-hr and a residence time of 0.15 min. (Example 10), there was a reduction in the Henry's law slope indicating a reduc-tion in nitrogen capacity, assuming the isotherms do not cross. This assumption is believed to be valid based on the fact that six isotherms for CaX samples obtained from various drying procedures have been measured and found not to cross; see the data plotted in FIG. 2. This reduction in Henry's law slope at 40C/min. is to be compared to the slope obtained with a temperature ramp of 2C/min. (Example 11). This loss in capacity can be explained by the fact that even though the initial drying temperature was maintained l~w and no loss of adsorption properties should have occurred, the rapid temperature ramp of 40C/min. was a little too rapid for a G of 16.1 and 0.15 min. residence time. This fact is confirmed in Example 12 in which practically the same loss in adsorption properties including a destruction of zeolite content (as evidenced by the same Henryls law slope) occurred as in Example 10, because the rise of 2DC~min. ~as a little too rapid for a G of 4.39 and a residence time of 0.45 min. The purge rate must be rapid en~ugh for any given temperature , s~

ramp to prevent a build-up of water partial pressure in the drying/activation system.
Similarly, the combination of an initial drying in the range of 100 to 150C and a rapid temperature rise of 40C/min. either at G's of 16.1 or 4.39 and residence times of 0.15 or 0.45 min., respectively, resulted in irreversible damage to the desired adsorbent ~Controls 24-25). This is especially the case when one compares these results with those of Example 13. These data further confirm that if the temperature ramp is maintained at a rapid pace without a corresponding increase in the purge gas flow rate, there is a substantial loss in nitrogen capacity due to a build up of water partial pressure in the gas phase at high temperatures which destroys a substantial number of the dehydrated/
dehydroxylated sites of the adsorbent and siynificantly reduces the zeolite content.
Finally, the results of Control 26 dramatically illustrates when compared to the results of Example 14 that initially drying the zeolite at temperatures as high as 250~C in a conventional deep bed results in a sufficient immediate loss of the dehydrated/dehyroxyl-ated sites and zeolite content in the adsorbent such that subsequent activation under very controlled condi-tions of temperature ramp and flow rate does not resultin any improvement in nitrogen capacity. On the other hand if the bed of ads~rbent were maintained shallow or ur.der a high enou~h vacuum, there will be no harmful buildup of water vapor pressure. To summarize, the best mode of operating from a practical standpoint is to remove the superficial or surface water of the adsorbent at temperatures no higher than 150C using a s~eady nitrogen purge and follow the drying steps with thermal activation with a substantial purge of inert gas as the temperature is increased to temperatures in the range of 350-450C to prevent the partial ~ressure of water in the gas phase from increasing beyond tolerable limits.

Examples 17-26 and Controls 27-28 These examples are designed to show the effect of ha~ing the majority of the calcium ions in a dehydrated/
dehydroxylated form and maintaining at least 70% of the initial zeolite content if superior adsorbent character-istics are to be realized. In all of the examples, a Dav,ison 13X zeolite in the form of ~-12 mesh particles was exchanged in aqueous lM CaCl2 solutions at either ambient or reflux conditions as set forth in Table VI
below using the number of exchanges noted to achieve the exchange level set forth in Table VI below, dried at 25C and activated in a GC column as described in Example l above.
The procedure as used in Examples 17-26 were used to pr~pare the controls in which the calcium loading was maintained at less than a majority of the original cations present in the zeolite.
In Table VI below, the exchange level and conditions are compared with the N2/O2 5C selectivities at 30C.
These data indicate that below 50% exchange level, there is a small positive correlation between selectivity as determined by the GC method and the level of calcium exchanged in the 13X zeolite. The exact exchange conditions are not critical since adsorbents obtained by ion exchanging at ambient temperatures over a period of days have similar ~C selectivities and nitroyen capacities as those having the same calcium content and prepared at reflux temperatures. The data of Table Vl is plotted on Figure l and a dramatic change in the slope is noted at calcium loadings greater than 50% if all other steps of the method of the present invention are f~,llowed.

TABLE VI
DA~A PLO~TED ON FIG. 1 EXC} ~ GE EXCHANGE N2/02 GC SELECTIvITy EXAMPLE/CONTROL LEVEL CONDl~IONS AT 30C
- 27 26 Reflux3.1 .
- 28 44 Reflux 3.6 17 - 60 Ambient5.3 18 - 64 Ambi~nt5.7 19 - 66 Ambi~nt5.2 ~O 20 - 64 Reflux 6.4 21 - 67 Ambient7.1 22 - 71 Reflux 7.8 23 - 70 Ambient8.3 24 - 75 Ambient8.3 - 92 Reflux11.8 26 - 95 Ambient12.6 Examples 27-29 and Controls 29-31 Davison 13X zeolites having the ~eolite content set forth in Table VII below were calcium exchanged using the same procedure as Example 1 and subjected to a variety of drying conditions. The isotherm data at 1 atm. which is summarized in Table VII and which is plotted against pressure in FIG. 2 were obtained by activating each sample to 400C under 10 5~m Hg of vacuum until no detectable weight loss occured ~less than 0.004% change) with one exception. The adsorbent used to obtain the data for curve D shown in FIG. 2 was activated to 400C under nitrogen flow to a dew point of -37C.. The different activation procedure does not account for the large loss of adsorption characteristics noted in a comparison between curves A, B, C, C' and D.
The data of Example 27 used for the Curve A isotherm, were obtained on a sample ~ith no prior thermal treatment, i.e. the drying was accomplished by setting ~he sample in a hood at ambient temperature. The sample of Fxample 28, the data of which is also shown in Curve A, was super-ficially dried by being placed in an oven at 100-120C
and then activated under a vacuum of 10-5 mmHg and a temperature ramps of 1.5 and 15C/min., respectively.
The isotherms for all three sets of conditions were the same.
The adsorbent of Example 29 was dried progressively in a deep bed (14 cm) configuration to 300C under a mechanical pump vacuum at less than 50 mm Hg and the resulting isotherm after careful activation is shown in Curve B in FIG. 2. The Control 29 adsorbent was heated to 400C at 40C/min. in a GC column with a 30 cc/min.
helium purge (G = 4.39 Kg./hole/m2-hr. and r = 0.45 min.) and the isotherm data was plotted as Curve C of FIG. 2. A comparison of the nitrogen capacities at 1 atmosphere show a 96~ improvement between the data of Curves C and A resulting from the different thermal treatments. The 22% decrease in nitrogen capacity observed between the samples of Examples 27-28 and of Example 29 depicted by Curves A and B is largely due to some hydroxylated calcium ions being present in the æeolite. This is the case because the zeolite contents shown in Table VII below as determined by adsorbtive and X-ray methods were found to be substantially ~he same in Examples 27-29. The small difference in the degree in crystallinity cannot account for the reduction in nitrogen capacity and is further evidence that such a loss in capacity is due to incomplete cation dehydra-tion/ dehydroxylation rather than loss of zeolite content.
In Control 31, a sample of the CaX used to obtain Curve A was placed in a laboratory oven at 250C for two hours. A procedure typically used to activate a NaX zeolite before use. The nitrogen isotherm determined for thi~ material is ~hown ~y Curve C' and is very ~4~

similar to th~t obtained from rapid activation in the GC column.
Finally, a comparison between Curves A and D in FIG. 2 show a five-fold change in nitrogen capacity in which the Control 30 adsorbent was dried in a deep bed configuration in an oven at 225C under nitrogen purge.
The resulting adsorbent whose isotherm is shown in Curve D was found to be inferior to the NaX zeolite.
This result is consistent with the data reported in the literature for CaX and thus it is believed that this is the drying technique that was used in the prior art.

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In the presence of high vacuum or a high purge rate, an increase in the temperature during thermal activation from 1.5 to 15C/min. did not alter the isotherm as shown in Curve A of FIG. 2. This suggests that as long as the water is being removed efficiently, the production of any significant quantity of steam is avoided and therefore the framework hydrolysis is not possible. The data for selective examples and controls for zeolite content were determined by both adsorptive and X-ray diffraction methods and the results summarized in Table VII above indicate that significant decreases of the zeolite content occurs as more severe drying and thermal activation conditions are employed.
Estimations of nitrogen-oxygen separation factors were calculated using the Ideal Adsorbed Solution Theory as described by Meyers and Prausnitz for the two highly exchanged calcium X samples described in Example 28 and (J. AIChE 11, 121 (1965)) Control 30. Nitrogen and oxygen isotherms at 32C were obtained for both materials using a volumetric adsoprtion apparatus after activation under flowing nitrogen to 400C and maintained at 400C
until a dew point lower than -35C was achieved. The CaX sample superficially dried at 100C (similar to curve A in FIG. 2) resulted in a calculated selectivity ~5 at one atmosphere of 8.4 whereas the CaX initially dried to 225C (curve D in FIG. 2) resulted in a selec-tivity of 3.9. Therefore, ~he large decrease in adsorptive nitrogen capacity is accompanied ~y a large decrease in selectivity resulting in an adsorbent that is very similar to the NaX adsorbent. It is believed that cation hydrolysis and considerable reduction in zeolite content occur if samples of CaX are more than superfici-ally dried.
In comparison to NaX zeolite, the stability of the zeolitic framework in the CaX form is reduced causing a higher loss of zeolite content to occur when ex~osed to the severe drying and activation conditions that do not s~

alter the sodium form. The hydrolysis of the framework and calcium ions (resulting in Ca-OH species possessing much lower charge density) contributes to the decreased nitrogen capacities which were obtained in these examples and controls.

Examples 30-31 The same procedure as set forth under Example 2 above was used to prepare the adsorbents of Examples 30 and 31 which were each dried under ambient conditions.
The adsorbent of Example 30 was placed under 10 5 mm Hg of vacuum as the temperature was increased to 400C.
The adsorbent of Example 31 was heated to 400C under a nitr~gen flow until the dew point reached less than _35C.
The nitrogen isotherms for Examples 30 and 31 are plotted in FIG. 3.

Contr~l 32 The procedure of Example 31 was repeated except that the maximum activation temperature was only 300C.
The isotherm for this control is also plotted in FIG. 3.
The isotherms of FIG. 3 illustrate the considerable improvement that is realized in thermally activating at temperatures of 400C rather than at 300C. The impr~ve-ment shown in nitrogen capacity is not simply the result of removing bound water, since no water band~
w~re detected upon measurement and by infrared spectros-copy, but rather the result of dehyroxylation of some of the Ca-OH species formed by cation hydrolysis.
Infrared spectra that had been gathered in connection with Examples 30 to 31 and Control 32 show qualita-tively a decrease of the hydroxyl band assigned to the Ca-OH and an increase in the integrated intensity of the nïtrogen band when the activation temperature of the CaX sample is increased from 300~ to 400C. The infrared data also showed that some hydrolysis does occur suggesting '.hat even milder or more controlled activation conditions will result in further improvements in the adsorption characteristics.

Examples 32 and Controls 33-43 Seventy gram samples of CaX prepared as described in Example 1 and samples of the starting NaX zeolite after being saturated with water were dried in a laboratory oven in a deep bed (14 cm) at dif~erent temperatures as detailed in Table Vlll. For each experiment, a sample of NaX and CaX was placed directly into the oven at the desired temperature and remained there for two hours. After the drying procedure, N2 capacities at 30C, atmosphere and zeolite conten-t determinations using both adsorption and X-ray methods were made for each of the samples.
The results are tabulated in Table Vlll and graphically illustrated in FIG. 4 for Example 32 and Controls 33-39 and show that there is definitely a difference appearing in the relative stabilities of the sodium and calcium forms of X zeolite. As the drying temperature increases; the percent decrease in nitrogen capacity of the sodium form is much less -than that of -the corresponding calcium form. Comparison of the absolute values of nitrogen capacity show that CaX dried 250C is only slightly higher than the sodium control sample.
In controls 40-41, samples of NaX and CaX were placed in a shallow bed (1 cm) and dried at 250C in the manner described above. As seen in Table Vlll, even in the shallow bed configuration there was a substantially greater loss of nitrogen capacity for CaX sample~
In controls 42-43, the samples were dried in the deep bed (14 cm) as described above with a vigorous dry nitrogen purge ranging from 2-10 liters/min. for the t~o-hour period. As seen in Table Vlll, there is still ~4~

a significant loss of nitrogen capacity for the calcium X sample for such a deep bed configuration.
Comparison of the two independent methods for ~ measuring apparent zeolite content show that a large decrease in accessible micropore volume to oxygen results from drying CaX at high temperature whereas the reduction in zeolite content is indicted to be much less by X-ray methods.
FIG. 4 graphically illustrates the data of the relative change in the nitrogen capacity and zeolite content for NaX and CaX dried in a deep bed configuration without purge; see Example 32 and Controls 33-39. The relative change shown in the ordinate is either the nitrogen capacity or zeolite content for a given control referenced to the corresponding value for the CaX or NaX for Example 32 and Control 33. As seen in Figure 4, drying temperatures less than about 150C do not substan-tially alter the observed nitrogen capacity after subsequent proper activation nor the zeolite content initially present in the adsorbent composition. For drying temperatures above about 150C as shown in FIG.
4, NaX reduction in zeolite content and nitrogen capacity are substantially the same within experimental error and are considerably less than that sbtained for the calcium form. Further, the calcium form has substan-tially lower nitrogen capacities than can be accounted for solely by the loss in apparent zeolite content as measured by oxygen adsorbtion.
Apparently an appreciable quantity of the pore volume of the zeolite becomes inaccessible to adsorbates of interest after drying at tPmperatures between 200 and 300C, whereas, the zeolite content determined by accepted X-ray methods fails to give a quantitative indication of this effect. The X-ray and adsorption methods for apparent zeolite content are in excellent agreement for NaX or CaX zeolite which have not been altered by ~hermal heating at temperatures ahove 200C, see Table VII above. Yet for CaX that has been exposed to temperatures in excess of 200C, therP is a significant departure in the zeolite contents determined by the two methods; compare the results of Examples 27-29 with that of Control 31 in Table VII and Example 32 with Control 43 in Table VIII. This difference may help explain why previous workers, if they used X-ray methods routinely to characterize the zeolite, did not discover the high nitrogen capacities associated with CaX.

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Example 34 and Control 44 Another more rigorously controlled comparison drying study was performed under a constant 300 torr ~0.39 atm.) water pressure performed using samples of CaX (Example 34) and NaX (Control 44) which were saturated with water similar to the samples used for Examples 32 and Controls 33-43. Forty ~ram guantities of these Na and Ca samples were placed in a 250 cc round bottom flask equipped with stopcocks and connected to a vacuum manifold. A 250 cc flask containing distilled water was also linked to the manifold and maintained at 75 to 80C where the vapor pressure of water approximately equals 300 torr. The manifold was maintained at about 100C so that the flask containing the water would control the total pressure of water in the system.
Prior to exposure of the zeolite samples, the manifold was evacuated to about 10 4 mm Hg. The system was then isolated from the vacuum pump and the samples heated to 250C and held there for four hours during which time the total pressure and water pressure over the zeolites was defined by the temperature of the water in the reservoir flask and fluctuated between 289 (0.38 atm.) and 355 torr (0.47 atm.) The CaX and NaX samples treated in the manner described above were subjected to - 25 the GC evaluation set forth in Example 1. The resulting Henry's law slope for nitrogen indicates that the CaX
sample lost about 70% of its initial nitrogen capacity while the corresponding NaX only lost about 10%.
The comparison between the estimated N2 capacities obtained in Example 34 and of Control 44 again demon-strates that, under very controlled conditions, there is a substantial difference in the stability between C~X and NaX.

5~

_ ample 35 The adsorbent prepared in accordance with the procedures of Example 1 was evaluated ~sing the gas chromatographic method previously described. The only difference being that the activation was carried out using dry air (dew point of -52C) instead of helium.
Using this procedure a selectivity at 30C of 12.9 was achieved. This value is well within the experimental error of the measurement and not significantly different from the 13.1 value obtained for the same adsorbent activated under helium flow. The above example shows that dry air can be used to activate the adsorbents and suggests that any nonreactive dry gas can be used for the thermal activation step.
The data set forth above based on the zeolite content measurements have shown that any time the CaX
is exposed to appreciable quantities of water vapor at high temperature a reduction in accessible micropore volume; i.e., apparent æeolite content, was observed.
It is believed that the protons produced from the hydrolysis of water by the calcium ion during the drying and thermal activation procedures, at conditions where NaX zeolite is stable, attack the zeolitic frame-work producing hydroxylated silicon and aluminum atoms resulting in the destruction of some of the zeolite.
It is also believed that alternative pathways to dehy-droxylation of polyvalent cations and more particularly the divalent cations such as calcium, magnesium, strontium and barium, become more accessible with an increase in the initial conc~ntration of the hydroxylated cation species due to concomitant formation of a larger number of protons. Finally, it is believed that the previous scientists ~nd other workers in the field did not recognize the foregoing phenomenon and the fact that the drying and thermal activation must be done in the manner herein described so as to promote dehydroxylation , ~2~

of the divalent ion exchanged faujasites by using higher temperatures while at the same time carefully controlling the attainment of these higher temperatures so as to eliminate or minimize d~mage to the framework structure which is induced by cation hydrolysis of water.

,

Claims (55)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A polyvalent ion exchanged adsorbent composition com-prising at least 50% by weight faujasite and the faujasite por-tion thereof having a majority of its exchangeable ion capacity in the polyvalent form prepared by a process which comprises:
(a) ion exchanging said faujasite-containing composi-tion with polyvalent ions; and (b) thermally activating the polyvalent exchanged composition to remove a substantial portion of its zeolitic water in said composition in such a manner as to produce a pre-ponderance of said polyvalent ions in a dehydrated/dehydroxy-lated state under conditions to minimize the water vapor pres-sure throughout this step by subjecting the polyvalent exchange composition containing up to about 30% by weight of water to pressures in the range of about 0.1 to 10 atmospheres while maintaining a flow of a non-reactive purge gas at a molar mass velocity of about 0.5 to 100 kg. mole/m2-hr. and a residence time of no greater than about 2.5 minutes, heating said composi-tion at a temperature ramp of 0.1° to 40°C./min. up to temper-atures at least about 300°C. and no greater than 650°C. and maintaining said composition at these temperatures for a period such that the resulting nitrogen capacity and selectivity ratio of the resulting polyvalent exchanged composition for the separ-ation of a binary mixture of oxygen and nitrogen is substan-tially greater than such a polyvalent ion exchanged composition that has not undergone such a thermal activation step.

2. The composition of claim 1 wherein said ion exchange step is carried out in an aqueous medium and the partial pres-sure of water in the gaseous environment in contact with said composition during said thermal activation step is maintained less than 0.4 atmospheres.

3. The composition of claim 1 wherein said polyvalent ion is divalent.

4. The composition of claim 3 wherein said divalent ion is selected from the group consisting of magnesium, calcium, strontium, barium and mixtures thereof.

5. The composition of claim 4 wherein the divalent ion is calcium.

6. The composition of claim 1 wherein said molar mass velocity is at least about 10 kg. mole/m2-hr. and a residence time of at least about 0.1 minute.

7. The composition of claim 1 wherein said polyvalent exchanged composition is subjected to an initial drying step at temperatures of no greater than 200°C. to remove a substantial portion of the surface water before carrying out said thermal activation step.

8. The composition of claim 7 which contains no more than about 2% by weight of water after said drying step.

9. The composition of claim 8 wherein said molar mass velocity is in the range of about 0.5 to 10 kg. mole/m2-hr.
and said residence time is in the range of about 0.0025 to 0.1 min.

10. The composition of claim 8 wherein said linear velo-city is in the range of about 10 to 100 kg. mole/m2-hr. and said residence time is in the range of about 0.1 to 2.5 min.

11. The composition of claim 1 wherein said thermal acti-vation step is carried out to temperatures of at least 350°C.

12. The composition of claim 1 wherein the polyvalent ion is calcium and the exchange step is carried out so that the fau-jasitic portion has at least 80% of its exchangeable ion capa-city in the calcium form.

13, The composition of claim 12 wherein at least 70% by weight of the material is a faujasite having a silicon to alumi-num ratio of approximately 1 up to about 2 and the balance of the composition is either a non-faujasite zeolite or an inert solid material.

14. The composition of claim 13 which has a selectivity ratio at 30°C. of at least 9 in the limit of low partial pres-sure of about 80% nitrogen and about 20% oxygen adsorbate and a nitrogen capacity at 30°C. of greater than about 0.4 millimols per gram of said composition.

15, The composition of claim 1 wherein its zeolite con-tent is maintained to at least about 70% of its initial value which it had prior to said thermal activation step.

16, A method for the separation of air into nitrogen and oxygen comprising providing a source of air and treating the air with the composition of claim 1.

17. A method for the separation of nitrogen from a mix-ture of gases containing nitrogen comprising adsorbing nitrogen from said mixture with the composition of claim 1.

18. The composition of claim 1 wherein said zeolite is X
type of zeolite.

19. A calcium ion exchanged composition comprising at least 70% by weight faujasite having a silicon to aluminum ratio of approximately 1 up to about 2 and the faujasite por-tion thereof having at least 80% of its exchangeable ion capa-city in the calcium form prepared by a process which comprises:
(a) exchanging said material with calcium ions in an aqueous system, and (b) thermally activating the calcium exchanged compo-sition to remove a substantial portion of the zeolitic water and the hydration spheres surrounding the cations in said mate-rial by heating said composition at a temperature ramp of no greater than 40°C. per minute while maintaining a flow of a nonreactive gas through said composition at a linear velocity of about 0.5 to 100 kg. mole/m2-hr. and a residence time of about 0.0025 to 2.5 minutes up to a temperature in the range of 400° to 500°C. and maintaining the composition at these tem-peratures for a period of at least 12 hours.

20. The composition of claim 19 wherein the calcium ex-changed composition is first subjected to a drying step at tem-peratures no greater than 200°C. to remove a substantial por-tion of the water before the thermal activation step.

21. The composition of claim 20 wherein the ion exchange step is carried out in an aqueous solution containing an excess of a water soluble salt of calcium.

22. The composition of claim 21 wherein the calcium salt is calcium chloride, calcium nitrate, calcium oxide or mixtures thereof.

23. The composition of claim 19 wherein the pressure of the thermal activation step is in the range of about 1 to 10 atmospheres, said molar mass velocity is in the range of about 1 to 10 kg. mole/m2-hr. and said residence time is in the range of about 0.01 to 0.1 min.

24. The composition of claim 19 wherein the pressure of the thermal activation step is in the range of about 1 to 10 atmospheres, said molar mass velocity is in the range of about 10 to 100 kg. mole/m2-hr. and said residence time is in the range of about 0.1 to 2 min.

25. A polyvalent ion exchanged adsorbent composition for the separation of a binary mixture of nitrogen and oxygen com-prising at least 50% by weight faujasite and the faujasite por-tion thereof having a majority of its exchangeable ion capacity in the polyvalent form prepared by a process which comprises:
(a) ion exchanging said faujasite-containing composi-tion with polyvalent ions in an aqueous medium, and (b) thermally activating the polyvalent exchanged composition to remove a substantial portion of its zeolitic water in said composition in such a manner as to produce a pre-ponderance of said polyvalent ions in a dehydrated/dehydroxy-lated state under conditions to minimize the water vapor pres-sure in the gaseous environment in contact with said composi-tion throughout this thermal activation step to less than about 0.4 atmospheres at temperatures above 150°C. such that the resulting nitrogen capacity and selectivity ratio of the result-ing polyvalent exchanged composition for the separation of a binary mixture of oxygen and nitrogen is substantially greater than such a polyvalent ion exchanged composition that has not undergone such a thermal activation step.

26. The composition of claim 25 wherein said polyvalent ion is a divalent ion selected from the group consisting of mag-nesium, calcium, strontium, barium and mixtures thereof.

27. The composition of claim 26 wherein the divalent ion is calcium.

28. The composition of claim 25 wherein said polyvalent exchanged composition is subjected to an initial drying step at temperatures of no greater than 200°C. to remove a substantial portion of the surface water before carrying out said thermal activation step.

29. The composition of claim 25 wherein said thermal acti-vation step is carried out to temperatures of at least 350°C.

30. The composition of claim 25 wherein the polyvalent ion is calcium and the exchange step is carried out so that the faujasitic portion has at least 80% of its exchangeable ion capacity in the calcium form.

31. The composition of claim 30 wherein at least 70% by weight of the material is a faujasite having a silicon to alumi-num ratio of approximately 1 up to about 2 and the balance of the composition is either a non-faujasite zeolite or an inert solid material.

32. The composition of claim 25 wherein its zeolite con-tent is maintained to at least about 70% of its initial value which it had prior to said thermal activation step.

33. The composition of claim 25 wherein said zeolite is X
type of zeolite.

34. The composition of claim 33 wherein said polyvalent ions are selected from the group consisting of magnesium, cal-cium, strontium, barium and mixtures thereof.

35. The composition of claim 34 wherein said polyvalent ions are calcium ions.

36. The composition of claim 25 wherein said zeolitic com-ponent content is at least 75% by weight X type of zeolite and the balance is a material selected from the group consisting of an A-type of zeolite, and inert clay binder, other inert solid materials and mixtures thereof.

37. A method of preparing a crystalline aluminosilicate zeolite which comprises:
(a) selecting a zeolite, the major portion of which is a faujasite having a silicon to aluminum ratio in the range of approximately 1 and up to about 2 in either its hydrogen or its metallic form;
(b) exchanging at least a major portion of said hydrogen or metal cations of a polyvalent metal, and (c) thermally activating the exchanged zeolite to remove a substantial portion of the zeolitic water and the hydration spheres surrounding the zeolite cations therein by subjecting said exchanged zeolite containing up to about 30% by weight water to a vacuum of less than about 0.1 atmospheres and heating said composition at a temperature ramp of 0.1° to 40°C./min. up to temperatures in the range of about 400° to 500°C. and maintaining said composition at these temperatures for a period of at least about 6 hours.

38. The method of claim 37 wherein said polyvalent metal is selected from the group consisting of magnesium, calcium, strontium, barium and mixtures thereof.

39. The method of claim 37 wherein step (b) is carried out until at least 90% of the exchangeable cations in the zeo-lite are in the calcium form.

40. The method of claim 37 wherein said vacuum is less than 100 mm Hg.

41. The method of claim 37 wherein the zeolite from step (b) is dried to remove a substantial portion of its surface water at temperatures of no greater than 200°C. and thermally activating the dried zeolite as set forth in step (c).

42. The method of claim 41 wherein the drying temperature is no greater than 120°C.

43. The method of claim 37 wherein the dried zeolite is placed in an adsorption zone for the adsorption of nitrogen from a mixture of gases containing nitrogen and said zeolite in the adsorption zone is thermally activated in accordance with the procedures of step (c).

44. The method of claim 43 wherein said mixture of gases is air.

45. The method of claim 37 wherein said zeolite contains at least 75% by weight X type of zeolite and the balance mate-rial is selected from the group consisting of an A-type of zeo-lite, and inert clay binder other inert solid materials and mix-tures thereof.

46. In a process for the adsorption of nitrogen from nitrogen-containing gases, the improvement which comprises con-tacting said nitrogen-containing gases with a polyvalent cation exchanged zeolitic adsorbent containing at least 50% by weight faujasite and the faujasite portion thereof having a majority of its exchangeable cations present in the polyvalent cation exchanged form, said adsorbent having been dehydrated in such a manner as to minimize the amount of water in the gaseous envir-onment in contact with said adsorbent during this dehydration step to a partial pressure of water of less than about 0.4 atmospheres at temperatures above 150° up to about 650°C.
thereby resulting in a preponderance of its polyvalent ions in a dehydrated/dehydroxylated state.

47. The process of claim 46 for the separation of air into nitrogen and oxygen.

48. The process of claim 46 for the adsorption of nitro-gen from a mixture of gases containing nitrogen.

49. The process of claim 46 wherein said faujasite has a silicon to aluminum ratio in the range of approximately 1 up to about 2.

50. The process of claim 46 wherein said faujasite is an X-type.

51. The process of claim 46 wherein said adsorbent con-tains at least 75% by weight of an X-type of faujasite and the balance is an adsorbent selected from the group consisting of an A-type of zeolite, an inert clay binder, other inert solid materials and mixtures thereof.

52. The process of claim 51 wherein said polyvalent ca-tions are calcium ions.

53. The process of claim 46 wherein said polyvalent ca-tions are divalent.

54. The process of claim 46 wherein said polyvalent ca-tions are selected from the group consisting of magnesium, cal-cium, strontium, barium, and mixtures thereof.

55. The process of claim 46 wherein said polyvalent ca-tions are calcium ions.