Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2000-09-15
2002-05-28
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S049000, C095S051000, C095S052000, C095S054000, C096S004000, C096S011000
Reexamination Certificate
active
06395067
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to processes for fluid separation utilizing membranes formed from crystalline titanium silicate molecular sieves.
BACKGROUND OF THE INVENTION
Since the discovery by Milton and coworkers (U.S. Pat. Nos. 2,882,243 and 2,882,244) in the late 1950's that aluminosilicate systems could be induced to form uniformly porous, internally charged crystals, analogous to molecular sieve zeolites found in nature, the properties of synthetic aluminosilicate zeolite molecular sieves have formed the basis of numerous commercially important catalytic, adsorptive and ion-exchange applications. This high degree of utility is the result of a unique combination of high surface area and uniform porosity dictated by the “framework” structure of the zeolite crystals coupled with the electrostatically charged sites induced by tetrahedrally coordinated Al
+3
. Thus, a large number of “active” charged sites are readily accessible to molecules of the proper size and geometry for adsorptive or catalytic interactions. Further, since charge compensating cations are electrostatically and not covalently bound to the aluminosilicate framework, they are generally base exchangeable for other cations with different inherent properties. This offers wide latitude for modification of active sites whereby specific adsorbents and catalysts can be tailor-made for a given utility.
In the publication “Zeolite Molecular Sieves”, Chapter 2, 1974, D. W. Breck hypothesized that perhaps 1,000 aluminosilicate zeolite framework structures are theoretically possible, but to date only approximately 150 have been identified. While compositional nuances have been described in publications such as U.S. Pat. Nos. 4,524,055; 4,603,040; and 4,606,899, totally new aluminosilicate framework structures are being discovered at a negligible rate.
With slow progress in the discovery of new aluminosilicate based molecular sieves, researchers have taken various approaches to replace aluminum or silicon in zeolite synthesis in the hope of generating either new zeolite-like framework structures or inducing the formation of qualitatively different active sites than are available in analogous aluminosilicate based materials.
It has been believed for a generation that phosphorus could be incorporated, to varying degrees, in a zeolite type aluminosilicate framework. In the more recent past (JAC 104, pp. 1146 (1982); proceedings of the 7
th
International Zeolite Conference, pp. 103-112, 1986) E. M. Flanigan and coworkers have demonstrated the preparation of pure aluminophosphate based molecular sieves of a wide variety of structures. However, the site inducing Al
+3
is essentially neutralized by the P
+5,
imparting a +1 charge to the framework. Thus, while a new class of “molecular sieves” was created, they are not zeolites in the fundamental sense since they lack “active” charged sites.
Realizing this inherent utility limiting deficiency, for the past few years the research community has emphasized the synthesis of mixed aluminosilicate-metal oxide and mixed aluminophosphate-metal oxide framework systems. While this approach to overcoming the slow progress in aluminosilicate zeolite synthesis has generated approximately 200 new compositions, all of them suffer either from the site removing effect of incorporated p
+5
or the site diluting effect of incorporating effectively neutral tetrahedral +4 metal into an aluminosilicate framework. As a result, extensive research has failed to demonstrate significant utility for any of these materials.
A series of zeolite-like “framework” silicates have been synthesized, some of which have larger uniform pores than are observed for aluminosilicate zeolites. (W. M. Meier, Proceedings of the 7
th
International Zeolite Conference, pp. 13-22 (1986)). While this particular synthesis approach produces materials which, by definition, totally lack active, charged sites, back implantation after synthesis would not appear out of the question although little work appears in the open literature on this topic.
Another and most straightforward means of potentially generating new structures or qualitatively different sites than those induced by aluminum would be the direct substitution of some charge inducing species for aluminum in a zeolite-like structure. To date the most notably successful example of this approach appears to be boron in the case of ZSM-5 analogs, although iron has also been claimed in similar materials. (EPA 68,796 (1983), Taramasso, et. al.; Proceedings of the 5
th
International Zeolite Conference; pp. 40-48 (1980)); J. W. Ball, et. al.; Proceedings of the 7
th
International Zeolite Conference; pp. 137-144 (1986); U.S. Pat. No. 4,280,305 to Kouenhowen, et. al. Unfortunately, the low levels of incorporation of the species substituting for aluminum usually leaves doubt if the species are occluded or framework incorporated.
In 1967, Young in U.S. Pat. No. 3,329,481 reported that the synthesis of charge bearing (exchangeable) titaniumsilicates under conditions similar to aluminosilicate zeolite formation was possible if the titanium was present as a “critical reagent” +III peroxo species. While these materials were called “titanium zeolites” no evidence was presented beyond some questionable X-ray diffraction (XRD) patterns and his claim has generally been dismissed by the zeolite research community. (D. W. Breck, Zeolite Molecular Sieves, p. 322 (1974); R. M. Barrer, Hydrothermal Chemistry of Zeolites, p. 293 (1982); G. Perego, et. al., Proceedings of 7
th
International Zeolite conference, p. 129 (1986)). For all but one end member of this series of materials (denoted TS materials), the presented XRD patterns indicate phases too dense to be molecular sieves. In this case of the one questionable end member (denoted TS-26), the XRD pattern might possibly be interpreted as a small pored zeolite, although without additional supporting evidence, it appears extremely questionable.
A naturally occurring alkaline titanosilicate identified as “Zorite” was discovered in trace quantities on the Siberian Tundra in 1972 (A. N. Mer'kov, et. al.; Zapiski Vses Mineralog. Obshch., pp. 54-62 (1973)). The published XRD pattern was challenged and a proposed structure reported in a later article entitled “The OD Structure of Zorite”, Sandomirskii, et. al., Sov. Phys. Crystallogr. 24(6), November-December 1979, pp. 686-693.
No further reports on “titanium zeolites” appeared in the open literature until 1983 when trace levels of tetrahedral Ti(IV) were reported in a ZSM-5 analog. (M. Taramasso, et. al.; U.S. Pat. No. 4,410,501 (1983); G. Perego, et. al.; Proceedings of the 7
th
International Zeolite Conference; p. 129 (1986)). A similar claim appeared from researchers in mid-1985 (EPA 132,550 (1985)). The research community reported mixed aluminosilicate-titanium (IV) (EPA 179,876 (1985); EPA 181,884 (1985) structures which, along with TAPO (EPA 121,232 (1985) systems, appear to have no possibility of active titanium sites. As such, their utility has been limited to catalyzing oxidation.
In U.S. Pat. No. 4,938,939, issued Jul. 3, 1990, Kuznicki disclosed a new family of synthetic, stable crystalline titanium silicate molecular sieve zeolites, which have a pore size of approximately 3-4 Angstrom units and a titania/silica mole ratio in the range of from 1.0 to 10. The entire content of U.S. Pat. No. 4,938,939 is herein incorporated by reference. Members of the family of molecular sieve zeolites designated ETS-4 in the rare earth-exchanged form have a high degree of thermal stability of at least 450° C. or higher depending on cationic form, thus rendering them effective for use in high temperature catalytic processes. ETS zeolites are highly adsorptive toward molecules up to approximately 3-5 Angstroms in critical diameter, e.g. water, ammonia, hydrogen sulfide, SO
2
, and n-hexane and are essentially non-adsorptive toward molecules, which are larger than 5 Angstroms in critical diameter.
A large pore crystalline titanium silicate mole
Bell Valerie A.
Jacubinas Richard M.
Kuznicki Steven M.
Engelhard Corporation
Frenkel Stuart D.
Spitzer Robert H.
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