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Laminated zeolite composite and method for production thereofRelated Patent Categories: Catalyst, Solid Sorbent, Or Support Therefor: Product Or Process Of Making, In Form Of A MembraneLaminated zeolite composite and method for production thereof description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070184967, Laminated zeolite composite and method for production thereof. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application is a division of U.S. Application Ser. No. 10/797,833, filed Mar. 10, 2004, which is a continuation of International Application No. PCT/JP02/09317 having an international filing date of Sep. 12, 2002, which designated the United States, the entireties of which are incorporated herein by reference. [0002] This application also claims the benefit of Japanese Application No. 2001-281675, filed Sep. 17, 2001, the entirety of which is incorporated herein by reference. TECHNICAL FIELD [0003] The present invention relates to a laminated zeolite composite and a method for producing the laminated zeolite composite. BACKGROUND ART [0004] Zeolite composite membranes obtained by forming a zeolite membrane on a substrate have been known and have been used as a gas separation membrane or a liquid separation membrane. With respect to such zeolite composite membranes, it is known that when the SiO.sub.2/Al.sub.2O.sub.3 (molar ratio) of the membrane (hereinafter, it means a molar ratio when simply "SiO.sub.2/Al.sub.2O.sub.3" is mentioned) varies, the interaction between the pore surface of zeolite and a molecule passing therethrough varies and the characteristic of the membrane when used as a separation membrane varies as well. [0005] For example, p. 25 of an article by Tsuneji Sano and Yusuke Kawakami in Kagaku Kogyo, Feb. 1995 discloses a finding on the influence of SiO.sub.2/Al.sub.2O.sub.3 on the permeation and separation characteristic of MFI type zeolite membrane (hereinafter referred to also as "MFI membrane") when water and alcohol are separated from each other by pervaporation using the membrane. It is known that the membrane shows striking alcohol selectivity when the SiO.sub.2/Al.sub.2O.sub.3 thereof is increased. [0006] As a specific example of the generally used MFI membrane, there can be mentioned a zeolite composite membrane obtained by forming a MFI membrane on an alumina substrate. With respect to such a zeolite composite membrane, it is known that, during the formation of the MFI membrane, aluminum in the alumina-made substrate dissolves into the MFI membrane and is taken into the skeleton of the MFI membrane and, as a result, the MFI membrane becomes a SiO.sub.2/Al.sub.2O.sub.3-reduced MFI membrane (hereinafter this membrane is expressed also as "low-silica MFI membrane"). [0007] Also, JP-A-6-127937 discloses a self-supported MFI membrane not formed on any substrate (hereinafter this membrane is referred to as a "MFI self-supported membrane"), into which aluminum is taken and wherein the SiO.sub.2/Al.sub.2O.sub.3 is reduced, and a method for the production thereof. [0008] In the zeolite composite membrane obtained by forming a MFI membrane on an alumina substrate, however, it is impossible to strictly control the SiO.sub.2/Al.sub.2O.sub.3 of the MFI membrane and, therefore, it is difficult to steadily synthesize a low-silica MFI membrane exhibiting uniform separation characteristic. Further, in synthesis of the MFI membrane, a structure-directing agent is added generally and it is necessary to remove the structure-directing agent by high-temperature calcination; however, since the substrate alumina and the MFI membrane (zeolite) differ in thermal expansion coefficient, the MFI membrane may generate cracks during calcination. [0009] In the zeolite membrane described in JP-A-6-127937, since it is a MFI self-supported membrane, problems such as crack generation in MFI membrane due to difference in thermal expansion coefficient between substrate and zeolite can be avoided. Even in the method for the production of MFI membrane according to the above literature, however, it is described therein that the MFI membrane formed at the initial stage of synthesis has a large SiO.sub.2/Al.sub.2O.sub.3 and the SiO.sub.2/Al.sub.2O.sub.3 tends to become smaller with the growth of the membrane; therefore, a low-silica MFI membrane having small value in SiO.sub.2/Al.sub.2O.sub.3 is obtainable only when the thickness of the membrane is made large. Consequently, the MFI membrane obtained exhibits permeation and separation characteristic as a low-silica MFI membrane, but has a large thickness and accordingly a small permeation factor and has a problem of low permeability. [0010] The present invention has been made in view of such problems possessed by the prior art, and aims at providing a laminated zeolite composite having high separation characteristics and a high permeability, and a method for producing such a laminated zeolite composite. SUMMARY OF THE INVENTION [0011] According to the present invention, there is provided a laminated zeolite composite characterized in that it comprises a MFI membrane constituted by a MFI type zeolite having a SiO.sub.2/Al.sub.2O.sub.3 (molar ratio) of 40 to 100, and a porous substrate constituted by a MFI type zeolite having a SiO.sub.2/Al.sub.2O.sub.3 (molar ratio) of 20 to 400, wherein the MFI membrane is formed on the porous substrate. [0012] In the present invention, the MFI membrane preferably has a thickness of 25 .mu.m or less. Incidentally, the SiO.sub.2/Al.sub.2O.sub.3 (molar ratio) of the MFI membrane may gradually decrease from the side of the membrane contacting the porous substrate toward the other side thereof. [0013] The laminated zeolite composite of the present invention is suitably used for the separation of butane isomers or for the separation of propane and propylene. [0014] According to the present invention, there is also provided a method for producing a laminated zeolite composite comprising the steps of immersing a porous substrate in a silica sol-containing sol for membrane formation, and forming a MFI membrane on the porous substrate under heating conditions; the method being characterized in that a porous substrate constituted by a MFI type zeolite having a SiO.sub.2/Al.sub.2O.sub.3 (molar ratio) of 20 to 400 is immersed in a sol for membrane formation having a SiO.sub.2/Al.sub.2O.sub.3 (molar ratio) in a range of 40 to 150 and a Na.sub.2O/Al.sub.2O.sub.3 (molar ratio) of 15 or less. [0015] In the present invention, it is preferred to form a MFI membrane constituted by a MFI type zeolite having a SiO.sub.2/Al.sub.2O.sub.3 (molar ratio) of 40 to 100. BRIEF DESCRIPTION OF THE DRAWINGS [0016] FIG. 1 is a schematic drawing showing a mode for carrying out the permeation and separation test of butane isomers. [0017] FIG. 2 is a graph wherein the separation factor of butane isomers is plotted against the SiO.sub.2/Al.sub.2O.sub.3 of MFI membrane. DETAILED DESCRIPTION OF THE INVENTION [0018] Modes for carrying out the present invention are described below. However, the present invention is not restricted to the following modes and it should be construed that design change, improvement, etc. may be made appropriately based on the ordinary knowledge of those skilled in the art as long as there is no deviation from the gist of the present invention. Continue reading about Laminated zeolite composite and method for production thereof... 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