Catalyst – solid sorbent – or support therefor: product or process – Zeolite or clay – including gallium analogs – And additional al or si containing component
Reexamination Certificate
2000-02-10
2002-06-18
Griffin, Steven P. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Zeolite or clay, including gallium analogs
And additional al or si containing component
C502S064000, C502S073000, C502S078000, C502S079000, C502S086000
Reexamination Certificate
active
06407025
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to a method of producing mixed cation-containing zeolite molecular sieves, and more particularly to a method in which a parent zeolite molecular sieve is first cation-exchanged with rare earth or other appropriate polyvalent cations, then subjected to an intermediate thermal treatment, then further cation-exchanged with ammonium cations, and then cation-exchanged with lithium and/or other desired cations under conditions appropriate to replace ammonium cations in the zeolite molecular sieve by lithium and/or the other desired cations.
BACKGROUND OF THE INVENTION
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. Sand: “Molecular Sieves”, Society of Chemistry and Industry, London (1968), pages 71-76), are usually synthesized in their sodium forms, whereas zeolites LSX, i. e., zeolite X in which the atomic ratio of framework silicon-to-aluminum is approximately 1, (UK 1,580,928) and zeolite L (U.S. Pat. No. 3,216,789) are usually synthesized in their mixed sodium and potassium forms. Zeolite L may also be readily synthesized in its pure potassium form.
Although these zeolites have useful properties as synthesized, it is often 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 (D. W. Breck: “Zeolite Molecular Sieves”, Pub. Wiley, N.Y., 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 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”, pages 359-390).
Conventional exchange procedures may be economically used to prepare many single and/or mixed cation-exchanged zeolites. However, in the cases of lithium, rubidium and/or cesium cation exchange of sodium, potassium, or sodium-potassium zeolites, the original cations are strongly preferred by the zeolite; accordingly, large excesses of expensive salts of the lithium, rubidium and/or cesium cations are needed to effect moderate or high levels of exchange of the original cations. Thus, these particular ion-exchanged forms are considerably more expensive to manufacture than typical adsorbent grades of zeolites. Furthermore, to minimize the cost of the final form of the zeolite, and to prevent discharge of these excess cations to the environment, considerable effort must be made to recover the excess cations from the residual exchange solutions and from washings in which the excess cations remain mixed with the original cations that were exchanged out of the zeolite. 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 having a framework silicon-to-aluminum atomic ratio of 1 to 1.25), in which more than 88% of the original sodium cations have been replaced by lithium cations, has very good properties for the adsorptive separation of nitrogen from oxygen. In the preparation of the zeolite, the base sodium or sodium-potassium form of the X zeolite was exchanged by conventional ion-exchange procedures, using 4 to 12 fold stoichiometric excesses of lithium salts.
Additionally, a wide range of other lithium-containing zeolites allegedly exhibit advantageous nitrogen adsorption properties. For example, 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 chabazite. In each case conventional ion-exchange procedures are contemplated, involving significant excesses of lithium cations over the stoichiometric quantity required to replace the original sodium and/or potassium cations in the zeolite. In the case of the binary lithium-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 cations is still needed to achieve the desired degree of exchange of the remaining sodium and potassium cations.
U.S. Pat. No. 5,916,836, issued to Toufar et al., discloses a method of preparing lithium-exchanged or polyvalent cation and lithium cation-exchanged molecular sieves from molecular sieves that originally contain sodium cations, potassium cations or both sodium and potassium cations without requiring the use of a large excess of lithium cations. The method of Toufar et al. includes the step of exchanging the original zeolite with a source of ammonium cations prior to the lithium cation exchange. The initial molecular sieve may contain polyvalent cations in addition to sodium and/or potassium cations, or polyvalent cations may be introduced at any stage of the process.
The advantages of the ammonium intermediate exchange concept of Toufar et al. over the “classical” direct exchange method are abundant, particularly when one desires to prepare the pure lithium form of the zeolite as the product. However, the pure lithium form of these zeolites is not always the desired form for a particular application, for example, nitrogen adsorption processes, due to its relatively low thermal stability. Furthermore, because of their high lithium content, pure lithium-exchanged zeolites are considerably more costly to prepare than lithium-based mixed-cation containing zeolites. When one wishes to prepare a mixed cationic form containing, for example, lithium cations and polyvalent cations, particularly those of rare earth metals, the Toufar et al. process leaves something to be desired. In order to effect the complete release of ammonia during the lithium ion exchange, the Toufar et al. process requires the use of a high pH during this step. Unfortunately, polyvalent cations undergo a more or less extensive hydrolysis in media with high pH values, and this behavior may cause not only a higher-than-stoichiometric uptake of lithium, but also a decrease in thermal/hydrothermal stability, because of the intermediate formation of relatively unstable acidic sites within the zeolite structure.
This invention presents an efficient method of preparing a zeolite containing, as exchange cations, polyvalent cations and one or more of lithium, rubidium and cesium cations, by a method which provides more precise control of the amount of both the polyvalent cations and the lithium, rubidium and/or cesium cations introduced into the zeolite, and stabilization of the polyvalent ions within the zeolite structure.
SUMMARY OF THE INVENTION
According to a broad embodiment, the invention comprises a method of producing an ion-exchanged zeolite comprising the steps:
(a) contacting at least one synthetic zeolite selected from the group consisting of structure types FAU, EMT, LTA, CHA, MOR and combinations thereof and containing sodium cations, potassium cations or mixtures thereof with a source of polyvalent
Brandt Alfons
Bülow Martin
Fitch Frank R.
Ojo Adeola F.
Tschritter Hartmut
Griffin Steven P.
Ildebrando Christina
Neida Philip H. Von
Pace Salvatore P.
The BOC Group Inc.
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