Many industrially utilized zeolites are most economically synthesized in their sodium, potassium or mixed sodium-potassium cation forms. For example zeolites A (U.S. Pat. No. 2,882,243), X (U.S. Pat. No. 2,882,244) and mordenite (L. B. Sand: "Molecular Sieves", Society of Chemistry and Industry, London (1968), p71-76) are usually synthesized in their sodium forms, whereas zeolites LSX (X in which the ratio of silicon to aluminum is approximately 1, UK 1,580,928) and L U.S. Pat. No. 3,216,789) are usually synthesized in their mixed sodium and potassium forms. Zeolite L may also readily be synthesized in its pure potassium form.
Although these zeolites have useful properties as-synthesized, it may be preferred to ion-exchange them to further enhance their adsorption and/or catalytic properties. This topic is discussed at length in chapter 8 of the comprehensive treatise of Breck (Donald W. Breck: "Zeolite Molecular Sieves", Pub. Wiley, New York, 1973). Conventional ion-exchange of zeolites is carried out by contacting the zeolite, in either powdered or agglomerated form, using batch-wise or continuous processes, with aqueous solutions of salts of the cations to be introduced. These procedures are described in detail in Chapter 7 of Breck (See Above) and have been reviewed more recently by Townsend (R. P. Townsend: "Ion Exchange in Zeolites", in Studies in Surface Science and Catalysis, Elsevier (Amsterdam) (1991), Vol. 58, "Introduction to Zeolite Science and Practice", p 359-390). Conventional exchange procedures may be economically used to prepare many single and/or mixed cation exchanged zeolites. However, in the cases, particularly, of lithium, rubidium and/or cesium exchange of sodium, potassium, or sodium-potassium zeolites, not only are the original cations strongly preferred by the zeolite (meaning that large excesses of the lithium, rubidium and/or cesium cations are needed to effect moderate or high levels of exchange of the original cations), but the salts themselves are expensive. This means that these particular ion-exchanged forms are considerably more expensive to manufacture than typical adsorbent grades of zeolites. Great efforts must be made to recover the excess ions of interest from the residual exchange solutions and washings in which the excess ions remain mixed with the original ions exchanged out of the zeolite, in order to minimize the cost of the final form of the zeolite, and to prevent discharge of these ions to the environment. Since lithium-containing zeolites have great practical utility as high performance adsorbents for use in the noncryogenic production of oxygen, and rubidium and cesium exchanged zeolites have useful properties for the adsorptive separation of the isomers of aromatic compounds and as catalysts, this problem is of significant commercial interest.
U.S. Pat. No. 4,859,217 discloses that zeolite X (preferably with a silicon to aluminum ratio of 1 to 1.25), in which more than 88% of the original sodium ions have been replaced by lithium ions, has very good properties for the adsorptive separation of nitrogen from oxygen. The base sodium or sodium-potassium form of the X zeolite was exchanged, utilizing conventional ion-exchange procedures and 4 to 12 fold stoichiometric excesses of lithium salts.
Additionally, a wide range of other lithium-containing zeolites have been claimed to exhibit advantageous nitrogen adsorption properties: U.S. Pat. Nos. 5,179,979, 5,413,625 and 5,152,813 describe binary lithium- and alkaline earth-exchanged X zeolites; U.S. Pat. Nos. 5,258,058, 5,417,957 and 5,419,891 describe binary lithium- and other divalent ion-exchanged forms of X zeolite; U.S. Pat. No. 5,464,467 describes binary lithium- and trivalent ion-exchanged forms of zeolite X; EPA 0685429 and EPA 0685430 describe lithium-containing zeolite EMT; and U.S. Pat. No. 4,925,460 describes lithium-containing zeolite chabazite. In each case conventional ion-exchange procedures are contemplated, involving significant excesses of lithium over the stoichiometric quantity required to replace the original sodium and/or potassium ions in the zeolite. In the case of the binary-exchanged zeolites, it may sometimes be possible to slightly reduce the quantity of lithium salt used by carrying out the exchange with the second cation before the lithium ion-exchange step (U.S. Pat. No. 5,464,467) or by carrying out both exchanges simultaneously (EPA 0729782), but in either case a large excess of lithium ions is still needed to achieve the desired degree of exchange of the remaining sodium and potassium ions.
The properties and uses of alkali metal exchanged zeolites are reviewed by D. Barthomeuf in the learned paper "Basic Zeolites: Characterization and Uses in Adsorption and Catalysis", published in "Catalysis Reviews, Science and Engineering, 1996, Vol. 38, N4, p.521.
U.S. Pat. No. 4,613,725 teaches a process for separating ethylbenzene from xylenes using a rubidium-substituted X-type zeolite.
JP A 55,035,029 describes a cesium- and lithium- and/or potassium-exchanged zeolite L with useful properties for the separation of p-xylene from mixtures of xylene isomers.
U.S. Pat. No. 5,118,900 describes a catalyst for the dimerization of olefins comprising a low sodium natural faujasite or zeolite Y and at least one alkali metal hydroxide, preferably KOH, wherein the metal hydroxide is supported on the zeolite and is present in the range of 1 to 25 per cent by weight. DE 3330790 describes a catalyst for the preparation of ethyltoluene from the corresponding xylenes and methanol using an alkali metal-exchanged form of zeolites X or Y prepared by exchange of the zeolite with a cesium salt (preferably the hydroxide, borate or phosphate) and optionally a lithium salt (preferably LiOH).
Cation exchange of zeolites has also been demonstrated to occur when the base zeolite is brought into intimate solid-state contact with salts of the desired cations, and, if necessary, heating the mixture. This subject is discussed in detail by Karge (H. G. Karge: "Solid State Reactions of Zeolites", in Studies in Surface Science and Catalysis, Vol. 105C, Elsevier (Amsterdam) (1996), "Progress in Zeolite and Microporous Materials" (H. Chon, S.-K. Ihm and Y. S. Uh (Editors) p1901-1948). The solid-state ion-exchange between zeolite sodium Y and metal chlorides (including lithium and potassium chlorides) is described by Borbely et al. (G. Borbely, H. K. Beyer, L. Radics, P. Sandor, and H. G. Karge: Zeolites (1989) 9, 428-431) and between NH.sub.4 Y and metal chlorides (including those of lithium and potassium) by Beyer et al. (H. K. Beyer, H. G. Karge and G. Borbely: Zeolites (1988) 8, 79-82). The main problem with the solid-state ion-exchange procedures of the prior art is that the exchanged zeolite is produced in admixture with salts of the original cations. Washing of the resulting exchanged zeolite to remove the salts of the cations originally contained in the zeolite can often lead to at least partial back exchange of the original cations into the zeolite. There is a need for a process for the production of lithium-, rubidium- and/or cesium-exchanged zeolites which does not require the use of large excesses of expensive salts of these cations and which has the added advantage that it does not require the use of expensive and energy intensive cation recovery schemes. This invention provides such a process.