THE PURIFICATION OF BULK GASES USING CHABAZITE ADSORBENTS

Abstract: A process is provided for the selective adsorption of one or more minor constituents from a bulk gas stream using a chabazite in which the bulk gas is size excluded from the pore structure of the chabazite or the minor constituent to be separated has a heat of adsorption greater than that of the bulk gas and the process is carried out under conditions such that the partial pressure of the minor constituent is in the linear low pressure region of the isotherm for the minor constituent and the adsorbent is a dehydrated divalent cation-containing chabazite having a Si/Al ratio of 1.8 to 2.7, a cation siting, f s , of 0.7 to 1, and a cation distribution, f d , of 0.62 to 1.

Description: FIELD OF THE INVENTION This invention relates to the removal of trace impurities from bulk gases utilizing a divalent ion exchanged and dehydrated chabazite adsorbent. BACKGROUND OF THE INVENTION Recent requirements of the electronics industry for purer gases and verification of contaminant levels have increased interest in the use of zeolitic adsorbents for removing trace nitrogen from argon. Divalent chabazites have been disclosed as useful adsorbents for removing trace nitrogen from argon, for purifying methane, and for quantitatively separating oxygen from argon in chromatographic applications. The intrinsic properties of chabazites expand the range of contaminant gases which can be removed economically from bulk gases using standard adsorption techniques and the calcium form of chabazite, properly activated, is perhaps the most energetically sorbing molecular sieve for both polar and non-polar gases. However, the availability of high-grade chabazite is extremely limited. Pure chabazite exists only rarely in nature and is too expensive to be used as a commercial adsorbent. Synthetic analogues of chabazite are known and have been designated D, R, G, and ZK-14 (Breck, Zeolite Molecular Seives , John Wiley and Sons, New York, New York, p 110 (1974) and Cartleidge et al., Zeolites , 4 , 226 (1984)). However, known methods for preparing commercially useful synthetic chabazites are not practical since they suffer from low yields, poor product purity, long cystallization times, and are difficult if not impractical to scale-up. Chabazite-based adsorbents will only be fully exploitable commercially when an economic chabazite having sufficiently improved adsorbing properties for weakly interacting adsorbates is available on a large scale. SUMMARY OF THE INVENTION It has now been found that chabazites having a Si/Al ratio of 1.8 to 2.7, which have divalent cations distributed throughout the framework such that the number of large pores which contain one or two divalent cations is maximized, have improved adsorption properties over any other zeolites for weakly interacting adsorbates such as nitrogen. Heretofore, we thought that the best chabazite for scavenging weakly interacting adsorbates would depend only on the number of calcium ions and hence a chabazite with an Si/Al ratio of 1 would afford the highest capacity. Unexpectedly, it has been found that the number of adsorption sites and, consequently, the adsorption capacity, varies systematically with composition. Both the framework Si/Al ratio and cation siting and distribution in the zeolite were found to influence the nitrogen adsorption properties of the chabazite. The adsorptive capacity of calcium chabazite determined by cation siting, or fraction of cations accessible by siting requirements, (f s ), of from 0.7 to 1 and a cation distribution, and fraction of cations accessible by distribution requirements, (f d ), of 0.62 to 1. Accordingly, the invention provides an improvement in the selective adsorption of one or more minor constituents from a gas stream containing such constituents which comprises using for such sorption the dehydrated divalent cation-containing chabazite adsorbent hereinabove defined, the minor constituent to be separated having a heat of adsorption greater than that of the bulk gas or the bulk gas being size-excluded from the pore structure of the chabazite absorbent. During the sorption the partial pressure of the minor constituent is in the linear low pressure region of the isotherm for the minor constituent. By linear low pressure region is meant that portion of the isotherm in which the ratio of the change in volume adsorbed to the change in pressure is at least 200 cc(STP)/g-atm. (bar). By dehydrated is meant that the chabazite is treated until the residual water content is less than about 1.5 wt% of the chabazite, preferably by heating. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a plot of the nitrogen capacity of chabazite-type zeolites versus the zeolite framework Si/Al ratio measured at 30°C, 1 atm. (bar) Figure 2 correlates the actual nitrogen capacity at 30°C, 1 atm. (bar) to that predicted based solely on cation siting. Figure 3 correlates the actual nitrogen capacity at 30°C, 1 atm. (bar) to that predicted based on both cation siting and aluminum distribution. Figure 4 shows the nitrogen and argon isotherms at 30° C for the calcium form of a chabazite of the invention. Figure 5 compares nitrogen isotherms at 30°C for various highly calcium exchanged, thoroughly dehydrated chabazites against a chabazite of the invention. Figure 6 compares the nitrogen isotherms at 30°C, 1 atm. (bar) for the same degree of exchange of calcium and strontium forms of the chabazite of the invention. Figure 7 shows adsorption of nitrogen measured at 30°C, 1 atm. (bar) on chabazites of the invention as a function of activation temperature. DETAILED DESCRIPTION OF THE INVENTION U.S. Patents 4,713,362 and 4,732,584 disclose chabazites as suitable adsorbents for removing trace nitrogen from argon, for purifying methane, and for quantitatively removing oxygen from argon in chromatographic applications. Properly dehydrated forms of chabazite are disclosed in U.S. Patent 4,732,584 as displaying unusually high heats of adsorption for weakly adsorbing gases such as nitrogen, particularly at low partial pressures, for a given set of conditions. The chabazites of this invention are suitable for use as described in U.S. Notwithstanding the contribution of the broad class of chabazites disclosed as adsorbents in U.S.Patents 4,713,584 and 4,732,584, it has been found that there is a particular class of chabazites which offers significantly improved adsorption properties over any disclosed in those patents. Nitrogen isotherms for various natural and synthetic chabazites versus the nitrogen adsorption curve for chabazites of this invention as shown in Figure 5 clearly illustrate the greatly increased adsorption capacities of the chabazites of this invention. Preferred chabazites of the invention in the divalent exchanged form have the composition M 2+ / x/₂[(SiO₂) 12-x (AlO₂) x ] where x = 3.24 - 4.28 and M²⁺ is a divalent cation, preferbly calcium or strontium. The exchanged chabazites have a framework Si/Al ratio of from 1.8 to 2.7, preferably 1.9 to 2.3, most preferably 2, and demonstrate improved adsorption properties over any other zeolite for weakly interacting adsorbates such as nitrogen. Heretofore, we thought that the best chabazite for scavenging weakly interacting adsorbates would depend only on the number of calcium ions and hence a chabazite with a Si/Al ratio of 1 would afford the highest capacity as is the case with faujasites. Surprisingly, a plot of the nitrogen capacity of the chabazites of the invention versus Si/Al ratio as in Figure 1 shows a clear maximum at Si/Al = 2. Assuming that, when site II positions in chabazite contain Ca²⁺ ions (Mortier, et al. Mat. Res. Bull. , 12, 97 (1977)), one nitrogen molecule sorbs per divalent calcium ion in site II, and that when a third cation is present in the chabazite pore the two adsorption sites at position II are destroyed due to pore blockage, the sorption capacity can be predicted as a function of composition. Accordingly, the dependence of the nitrogen capacity of chabazites on framework composition should be given by the straight line in Figure 2. However, experimental data plotted in Figure 2 are all lower than the predicted values and show no apparent correlation between the predicted values and the experimental data. Therefore, the divalent cation content alone does not determine the adsorptive capacity of a chabazite adsorbent; distribution and siting of the cations in the structure are also key structural parameters. Using solid state ²⁹Si NMR (nuclear magnetic resonance) data, it is possible to determine the relative amount of silicon in each of the five chemically distinct environments, Si(nAl), where n denotes the number of aluminum atoms connected directly through oxygen to silicon (i.e., nearest neighbors) and can be 0-4. Since the divalent cations compensate for the negative charges associated with aluminum atoms in the framework structure, on the average, two divalent cations will be associated with each Si(4Al), 3/2 with a Si(3Al), one with each Si(2Al), and 1/2 with each Si(1Al) site. It has been found that only divalent cations associated with Si(2Al) and Si(1Al) sites adsorb nitrogen. When the values predicted for nitrogen adsorption using the siting hypothesis which gives the straight line of Figure 2 are scaled to reflect that only the fraction of cations associated with Si(2Al) and Si(1Al) sites contribute to the capacity of chabazite adsorbents, the predicted versus experimental values fall on a straight line as shown in Figure 3. This shows that one nitrogen molecule is adsorbed per divalent cation accessible by siting and distribution requirements. The fraction of cations accessible by distribution requirements, f d , can be calculated from NMR data using the formula: f d = R 2 (Si(2Al) + Si(1Al) 2 ) where R is Si/Al. The fraction of cations accessible by siting requirements, fs, is given by the formula. f s = 0 for R ≧ 5 f s = (5 - R)/3 for R = 2 - 5. f s = R - 1 for R = 1 - 2. Accordingly, a Si/Al ratio ranging from 1.8 to 2.7 with siting (f s ) and distribution (f d ) ranging from 0.7 to 1 and 0.62 to 1, respectively, will provide the unexpected improvements of this invention. The adsorptive capacity of the chabazites of the invention in moles/g is given by the equation: C = f s f d nM, where nM is the number of moles of divalent cations per gram of adsorbent. For divalent cations of suitable size to occupy either site I or II, nM = 1/2 (60.08R + A.W./2 + 58.98) where A.W. is the atomic weight of the divalent ion. For calcium chabazite, nM = 1/2(60.08R + 79.02). The volumetric adsorptive capacity expressed at STP is designated throughout as C v . C v generally ranges from 35 to 56 cc/g, preferably 45 to 56 cc/g to provide increased adsorption capacity, and most preferably 56 cc N₂/g for the best adsorption capacity. Initial attempts to correlate adsorptive capacity with composition revealed that there is a much larger increase in the adsorptive capacity than can be explained by cation content. As Si/Al ratios were decreased from 4 to 2, the calcium content of the zeolite increased by 67% while the adsorptive capacity increased by 350%. Increasing the cation content from 1.2 to 2 ions per unit cell generates many more adsorption sites than would be expected if each cation adsorbs only one nitrogen molecule. Accordingly, cation siting or the fraction of cations accessible by siting requirements (f s ) and cation distribution or the fraction of cations accessible by distribution requirement (f d ) in the zeolite are critical structural parameters for generating adsorption sites. In general, the chabazites or the invention have an f s ranging from 0.7 to 1, preferably 0.9 to 1, most preferably 1, and an fd ranging from 0.62 to 1, preferably 0.8 to 1, most preferably 1. The most preferred adsorbent of the invention has a Si/Al ratio of 2, an f s of 1, an f d of 1 and, hence, aluminum as homogeneously distributed throughout and within the crystallites as possible. Since nitrogen is too large to pass through a six-ring aperture, cations in site I (Mortier, et al., Mat. Res. Bull. 12, 97, (1977)) are inaccessible to nitrogen. Cations in site II are accessible to nitrogen since nitrogen can pass through the larger eight-ring window of the cavity. It has been hypothesized that fractional occupancy of sites I and II vary with calcium content. (Mortimer et al., Mat. Res. Bull. , 12, 97 (1977)). For several ultra-high capacity chabazites, the predicted nitrogen adsorption capacities are lower than the experimental values. It is believed that this reflects the inability of NMR to adequately resolve the Si(3Al) peak from the Si(2Al) peak. The unique chabazites of the invention require significantly lower dehydration temperatures than prior materials and have enhanced adsorption properties which make them more efficient in adsorption beds. Their higher capacity gives them longer onstream capability before regeneration is required. The reason the chabazites of the invention can achieve full capacity at much lower dehydration temperatures is not fully understood. For partially dehydrated chabazites, the chabazites of the invention having a Si/Al ratio of 1.8 to 2.7 and the siting and distribution parameters outlined above will have more dehydrated/dehydroxylated cations accessible to nitrogen and, hence, a higher nitrogen adsorbing capacity. According to the laws of adsorption technology, an adsorbent can be used to effect the separation of a gaseous mixture in two ways: (1) using a carrier gas stream (such as GC analysis), or (2) by the principle of frontal analysis. Since as taught in U.S. Patent 4,732,584, the polyvalent forms of chabazite can effectively separate oxygen and nitrogen from argon, it should be possible to remove oxygen and nitrogen from argon by the principle of frontal analysis. The efficiency of such a process will depend on a number of factors including the concentrations of the components to be removed, temperature of the process, and adsorption properties of the various gases on the particular adsorbent. Due to high heats of adsorption for nitrogen exhibited by the thoroughly dehydrated divalent forms of chabazite, these adsorbents have sufficient nitrogen capacity to have practical utility for removing nitrogen impurities from argon. This is a direct consequence of the thermodynamic adsorption properties of these adsorbents. The calcium form of chabazite has a limiting nitrogen heat of adsorption of -11.0 kcal/mole (-0.0046 j/mole) and a nitrogen/argon selectivity of 15 at 30°C. The high values for these intrinsic properties are directly responsible for the increased capacity of these adsorbents for nitrogen in the presence of argon. Even though these same adsorbents display the capability of separating oxygen from argon in a chromatographic application, both the gas capacity and oxygen/argon selectivity are much too low to have practical utility for removing oxygen from argon. On the other hand, gases having significant quadrapole moments or permanent dipole moments should have sufficiently high adsorption properties that they can be removed from other bulk gases possessing lower adsorption properties. Because the calcium form of the chabazites of the invention is particularly useful for adsorption of weakly interacting adsorbates it is most preferred. In general, the chabazites of the invention display improved properties for removing low levels of gas contaminants having larger heats of adsorption than the bulk component. Some of the gas purifications which should be possible include removing dilute CO from hydrogen, dilute nitrogen from hydrogen, dilute methane from hydrogen, dilute methane from nitrogen, dilute nitrogen from noble gases, trace oxygen from noble gases, trace oxygen from hydrogen. Noble gases include helium, neon, argon, krypton, and xenon. Also, the combination of the shape-selective properties of the small pore chabazites and their increased heat of adsorption suggests that the adsorbents of this invention would be superior for removing nitrogen or other weakly interacting gases from bulk gases whose kinetic diameter exceeds that of the adsorbent's pore opening. For example, carbon tetrafluoride can be purified in this manner. One method for preparing the chabazites of the invention involves mixing an alumina source, sodium hydroxide, potassium hydroxide, a tetramethylammonium (TMA) reagent (hydroxide, salt and/or hydrate), and a silica source to form an aqueous gel in which the ratio of TMA₂O to alumina is 0.08 to 0.0001, crystallizing the gel by heating at from 25° to 150°C for at least one hour, separating the chabazite, and converting it into its divalent form. Details of a preferred preparation are contained in copending application Serial No. (Docket 211-P- US03871). Dehydration of the divalent cation-containing chabazite to a level of optimum performance can be carried out by any method which will remove the water from the zeolite cavity, including water surrounding the cations, without causing the collapse of the zeolite structure. To achieve this in a practical manner, the chabazite can be maintained at temperature in the range of 250°-500°C under any conditions suitable to lower the water content of the zeolite to less than about 1.5% without causing structural damage to the zeolite. The invention is further illustrated but is not intended to be limited by the following examples in which all parts and percentages are by weight unless otherwise indicated. In the examples, nitrogen and argon isotherms are measured at 30°C from 0 to 1 bar (atmosphere) using a volumetric adsorption apparatus. Prior to the uptake measurements, each adsorbent is first thoroughly dehydrated for 16 to 18 hours to a final temperature of 400°C unless otherwise indicated. In all cases the samples are heated at 1 to 2°C per minute up to the desired temperature while maintaining a pressure of less than 10⁻⁵ Torr (0.00133 Pa) in the system. All adsorption uptakes are measured at 30°C and 1 atmosphere pressure and are reported as cc(STP)/g. EXAMPLE 1 Aluminum hydroxide (60.6 g), sodium hydroxide (160.32 g), potassium hydroxide (87 g), tetramethylammonium hydroxide pentahydrate (TMA-OH) (0.56 g) and silica sol (1051.8 g) were mixed with water to produce a thick gel (composition 0.005 [TMA]₂O: 6.67 Na₂O: 2.2 K₂O: 17.5 SiO₂:Al₂O₃: 276 H₂O). The gel was stirred for 15 minutes, placed in a sealed Teflon-lined reactor, heated to 95°C, and maintained at that temperature for three days. The product was separated by filtration, washed with water, and air dried. The product had an X-ray powder diffraction pattern characterized by the following data in which I/I₀ is the relative intensity and "d" is the interplanar spacing: These x-ray data indicate that the product is essentially pure synthetic chabazite. A portion of this product (6 g) was refluxed in 100 ml of a 1 M CaCl₂ solution for two hours, filtered, and washed with 250 ml water. The procedure was repeated three times. The calcium exchange level was about 92%, the nitrogen capacity was 40.0 cc(STP)/g, the Si/Al was 2.1, the f s was 0.97, and the f d was 0.74 for the product. EXAMPLE 2 Figure 4 shows the nitrogen and argon isotherms at 30°C for the calcium form of the chabazite of Example 1. The chabazite adsorbent of the invention has about a 35% higher nitrogen adsorption capacity than prior calcium chabazites, and provides about a 50% improvement over prior chabazites in its ability to remove nitrogen from bulk gases. The preferred adsorbent of the invention in the calcium exchanged form and having a Si/Al of 2, f s of 1 and f d of 1 will have a capacity of 56.3 cc/g nitrogen at 30° C and 1 atm, or substantially 50% higher than the capacity of the best prior adsorbents known in the art. Further, in the operating range of this invention (i.e., the low pressure region), based on experimental adsorption studies, the preferred chabazite of the invention should have about twice the gas capacity for the minor constituent being separated from bulk gas. Figure 4 also shows that the chabazites of the invention have a large nitrogen capacity and high selectivity for nitrogen over argon at these conditions. Carrying out the purification at lower temperatures will further increase the capacity of the adsorbent of the invention for minor constituents in bulk gases. The top three curves of Figure 5 compare the nitrogen isotherms at 30° C for a Nova Scotia chabazite (Si/Al = 2.6) and a prior synthetic chabazite prepared by the procedure described in British Patent 841,812 against the isotherm for the composition of Example 1. The prior synthetic chabazite is fully calcium exchanged and thoroughly dehydrated by heating at 2° C/minute to a final temperature of 400° C and held there for 16-18 hours under less than 10⁻⁵ Torr Hg (0.00133 Pa) vacuum. The large enhancement in nitrogen capacity in the low pressure region for the chabazites of the invention is clearly evident. EXAMPLE 3 The chabazite of Example 1 was ion-exchanged into the strontium form from the pure sodium form of the chabazite. Four exchanges with 1 M SrCl₂ (0.5 1/10 g) produced a highly strontium exchanged sample. Figure 6 provides a comparison between the nitrogen isotherms for calcium and strontium forms of the same chabazite. These data indicate that the strontium and calcium forms behave similarly. EXAMPLE 4 The effect of dehydration temperature on nitrogen adsorption for a natural chabazite and the chabazite of Example 1 is given in Figure 7. These data show that the synthetic chabazites of the invention reach their ultimate nitrogen adsorbing capacity at significantly lower dehydration temperatures compared to Nova Scotia (NS) chabazite. The chabazites of the invention reach essentially full capacity at 350° C compared to about 500° C for the Nova Scotia chabazite. At a commercially practical temperature range of from 300-375° C for dehydration, the chabazites of the invention provide a 33%-100% improvement in nitrogen adsorption over Nova Scotia chabazite. EXAMPLE 5 The thoroughly dehydrated, highly calcium exchanged samples of Examples 1 and 2 of U.S.Patent 4,732,584 and the chabazite of Example 1 were tested to determine their nitrogen adsorption capability. The resultant nitrogen isotherms at 30° C over a range of pressures, 0-1 atm, (bar) illustrate that the nitrogen capacity of the cabazites of the invention is significantly higher at any pressure. This effect is particularly striking in the low pressure region as shown in Figure 5. While the examples illustrate some preferred embodiments of the invention, similar results can be obtained using other embodiments suggested herein.

Claims: Use, for the selective adsorption of one or more minor constituents from a bulk gas stream on the basis of size exclusion or heat of adsorption, of a dehydrated divalent cation-containing chabazite having a Si/Al ratio of 1.8 to 2.7, a cation siting value, f s , of 0.7 to 1, and a cation distribution value, f d , of 0.62 to 1. Use according to claim 1 wherein the minor constituent is nitrogen and the bulk gas is argon, hydrogen, helium, krypton, neon, xenon, tetrafluoromethane or mixtures thereof. Use according to claim 1 wherein the minor constituent is carbon monoxide, nitrogen, methane or mixtures thereof and the bulk gas is hydrogen or helium. Use according to any preceding claim wherein the cation is an alkaline earth metal. Use according to claim 4 wherein the cation is calcium. Use according to claim 4 wherein the cation is strontium. Use according to any preceding claim wherein the chabazite has a Si/Al ratio of 1.8 to 2.7, a cation siting, f s , of 0.7 to 1 and a cation distribution, f d , of 0.62 to 1. Use according to claim 7 wherein the chabazite has an adsorptive capacity, C v , of 35 to 56 cc N₂/g at 30° and 1 atm. (bar). Use according to any preceding claim wherein the chabazite has a Si/Al ratio of 1.9 to 2.3, a cation siting, f s , of 0.9 to 1 and a cation distribution, f d , of 0.8 to 1. Use according to claim 9 wherein the chabazite has an adsorptive capacity, C v , of 45 to 56 cc N₂/g at 30° and 1 atm. (bar). Use according to any preceding claim wherein the chabazite has a Si/Al ratio of 2, a cation siting, f s , of 1 and a cation distribution, f d , of 1. Use according to claim 11 wherein the chabazite has an adsorptive capacity, C v of about 56 cc N₂/g at 30°C and 1 atm. (bar). Use according to any preceding claim wherein the chabazite has the composition M 2+ / x/2[(SiO₂) 12-x (AlO₂) x ] wherein x = 3.24 - 4.28 and M²⁺ is a divalent cation.

Inventors: COE, CHARLES GARDENER, GAFFNEY, THOMAS RICHARD
Applicants: AIR PRODUCTS AND CHEMICALS, INC.
Application year: 1990
Main classification: B01J20
IPC codes: B01D53, C01B23, B01J20, C01B3