Recently, a zeolite shaped body composed of particles of zeolite has been widely used for catalysts, catalyst carriers, adsorbents and the like in addition to molecular sieve membranes (gas separation membranes, pervaporation membranes). Also, this zeolite membrane may be used after having been formed on a porous substrate as a zeolite layered composite. In these situations, there have been proposed methods for producing various zeolite layered composites.
For example, proposed are methods wherein glass, mullite, cordierite type ceramic, alumina, silica or the like is used, or the ones wherein a metal or another substrate coated with an inorganic material is used as the substrate of the zeolite membrane (JP-A-59-213615).
Moreover, composites which thin membrane of cage-type zeolite is united with one surface of porous support body of a metal, inorganic, or polymer substance, are proposed (JP-A-60-28826). Among them, those having a high affinity to a gel material are proposed as especially preferable ones for the support body to be used and practically, it has been proposed to use the one called with tradename: No. 7930 manufactured by Coning Glass Works Co., generally called as Vycor glass, as especially preferable one.
Furthermore, a method proposed (JP-A-1-148771) relates to a method for crystallization of zeolite on the surface of a monolithic ceramic support body as a substrate which may have an oxide composition containing 45 to 4% by mass of silica, 8 to 45% by mass of alumina, and 7 to 20% by mass of magnesia; and practically proposed is a sintered monolithic support body of cordierite, glass, or glass ceramic.
Furthermore, another proposed production method (JP-A-6-32610) relates to a production method for an A type or faujasite type zeolite membrane using a substrate of a material mainly containing silicon dioxide. This method aims to solve the problem of inferior adhesion of the zeolite membrane to a substrate, wherein a zeolite membrane is used as a substrate itself and the surface of the substrate is made to be a zeolite membrane from its constitution, synthesis and adhesion can simultaneously be carried out to simplify the processes. To be practically, a substrate made of borosilicate glass, quartz glass, silica alumina, mullite or the like is proposed.
As described above, various zeolite layered composites each obtained by layering or forming a zeolite membrane on a substrate have heretofore been proposed, but these composite have the following problems.
That is, as shown in FIG. 2, a thermal expansion coefficient of zeolite shows a quite complicated behavior. It is an extremely lower value at a temperature to about 200° C., and it is a negative coefficient value at temperature further higher than that. Therefore, when the zeolite membrane is used at a temperature exceeding 200° C., its thermal expansion difference from the substrate, for example, alumina-based substrate becomes extremely large, resulting in the formation of cracks from the thermal stress in the zeolite membrane.
Moreover, depending on the type of the zeolite membranes, when the zeolite membrane is synthesized, a template or a crystallization promoting agent is required to be added. In the case of zeolite membrane containing the template, the template is removed by calcining it at about 500° C., however, as shown by a thermal expansion curve of MFI type zeolite, the thermal expansion behavior (thermal expansion curve before calcining in FIG. 3) of the zeolite membrane containing the template is significantly different from that (thermal expansion curve after the calcining in FIG. 3) of the zeolite membrane without template, so that the thermal expansion difference becomes extremely wide between a substrate such as an alumina substrate and the zeolite membrane, and cracks are caused in the zeolite membrane by the thermal stress during calcining.
To such problems, above proposed examples cannot be sufficient countermeasures.
Moreover, as a double layer structure of the substrate and the zeolite membrane, there has been proposed an asymmetric membrane comprising: a macro porous layer substantially formed only of a molecular sieve crystal having a prescribed thickness; and an upper layer for molecule separation, formed substantially only of the same type of molecular sieve crystal as that of the material of the macro porous layer, having a prescribed thickness and prescribed effective diameter of fine pores (JP-K-7-505333). There has been proposed a structure composed of three layers; a carrier, an intermediate layer, and an upper layer and in which the intermediate layer and the upper layer contain prescribed crystalline molecular sieves (JP-K-11-511685). Moreover, there has been proposed a zeolite composite membrane produced by coating a zeolite membrane containing the template on a porous zeolite shaped body containing the template, and thereafter calcining the resultant to remove the template therefrom and form the membrane simultaneously (International Laid-open Pamphlet No. WO 00/23378). These membranes and structures are superior in that a pore size can be precisely adjusted, and the formation of cracks can be effectively prevented.
However, when these membranes or structures (zeolite layered composite) are used as a substrate for a gas separation membrane such as a molecular sieve membrane or a pervaporation membrane, it is required to improve the efficiency of the use by reducing pressure loss of during gas or liquid pass through the membrane or the substrate. However, it is extremely difficult to obtain the one capable of satisfying both reduction of pressure loss and maintenance or improvement of mechanical strength of the substrate for supporting the zeolite membrane since the mechanical strength is decreased if one makes the dimension of the substrate particles larger which is a main cause for bringing an increase of the pressure loss (The reduction in the pressure loss of the substrate has an antinomic relation to the improvement in the mechanical strength of the substance). Thus, no one has hitherto succeeded in obtaining the membranes or the structures capable of satisfying both reduction of pressure loss and maintenance or improvement of mechanical strength.
Moreover, in the Proceedings of the 30-th Autumn Annual Meeting for presenting Researches by the Society of Chemical Engineers, p. 164 (1997), a production method for zeolite powder has been proposed in which a desired prepared solution is aged at a prescribed temperature for a prescribed time, so that the amount of obtained zeolite powder can be increased. By S. Shimizu et al., Chem. Lett., 1996, 403, it has been proposed that the prepared solution is allowed to stand at room temperature overnight before obtaining dry gel, and in order to promote crystallization of zeolite. However, the proposals cannot be applied to the zeolite substrate (shaped body) which satisfies the reduction of the pressure loss and the improvement of the mechanical strength in the present situations.
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