Zeolites are crystalline alumino silicates which embrace pores of the order of molecular sizes, and membranes made up of zeorites are widely used as molecular sieves because of their property of selectively allowing molecules to pass through themselves depending on the molecular size or shape. Particularly, their use as membranes for separating water from organic solvents, especially, as membranes for separating water from alcohols has attracted considerable attention these days. Zeolite membranes, which function as separation membranes, do not have sufficient mechanical strength in themselves, and therefore they are usually used in form of supporting with a porous support which is made of ceramics etc.
Zeolite membranes supported by porous supports have been manufactured by hydrothermal synthesis method in such a manner that porous supports are immersed in the respective raw materials that contain a silica source and an alumina source as main ingredients. Once a porous support is immersed in a slurry of a raw material that contains a silica source and an alumina source, a zeolite membrane is grown and formed with the aid of fine zeolite seed crystals, as nuclei, in the slurry. Thus, it is necessitated that the slurry is to be supersaturated with the raw material zeolite.
However, when a porous substrate is immersed in a slurry supersaturated with a zeolite raw material, fine zeolite seed crystals are attached to the surface of the porous substrate to cause the growth of a zeolite membrane, and, large zeolite crystals which have been hugely grown in the slurry are also attached to the surface of the porous support and from where the zeolite membrane are also grown. The zeolite membrane thus formed is not uniform in pore size and thickness, and it gives rise to a problem of being apt to have pinholes. Thus, when intending to prepare a zeolite membrane on a porous substrate by hydrothermal reaction, it is preferable to carry the zeolite seed crystals on the porous substrate of ceramics, etc., in advance, and to set the concentration of the zeolite raw material in the slurry to a low level.
Japanese Patent No. 3272119 (Patent literature 1) discloses a process for manufacturing a zeolite crystal membrane which comprises the steps of: impregnating an alumina substrate with a slurry where zeolite crystals are suspended in a binder solution; rinsing and drying the alumina substrate in order to obtain the alumina substrate of which the surface and pores' inside have zeolite crystals attached, and then immersing the obtained alumina substrate in a reaction solution containing a zeolite precursor, which is followed by proceeding the hydrothermal synthesis in order to grow the zeolite crystals as a crystalline film. When preparing an A type zeolite membrane by this process, the alumina substrate is held in an autoclave at 70 to 90° C. for about 15 minutes to 12 hours. And when forming a ZSM-5 type zeolite membrane by this process, the alumina substrate is held in an autoclave at 160 to 200° C. for about 24 to 72 hours.
In this preparation process, a filter of alumina is used as a substrate and zeolite crystals attached on the surface of and on the pores' inside of the substrate are allowed to grow in the reaction solution. Thus, the concentration of the zeolite raw material in the slurry can be set low, and the growth of the zeolite crystals in the slurry can be decreased. The process, however, might cause the formation of zeolite membranes on both sides of the substrate. A separation membrane which has zeolite membranes formed on both sides thereof will bear a problem that its permeation rate is too low to have adequate separation ability.
JP-9-202615A (Patent literature 2) discloses a process for manufacturing a zeolite membrane which comprises the steps of: immersing in a sol for zeolite synthesis a cylindrical porous support of which outer side surface is sealed so that the outer side surface does not come into contact with the sol; evacuating a vessel containing the sol to 10 mmHg with a vacuum pump; keeping the container in such a state for 6 hours; and placing the porous support together with the sol in an autoclave to subject the same to hydrothermal synthesis at 170° C. for 72 hours. Since this process allows a zeolite membrane to be formed in the inside of the porous support, the zeolite membrane formed is hardly exfoliated from the support. However, when the zeolite membrane goes into the pores of the porous support, substantial thickness of the zeolite membrane becomes too large, which brings an excessive pressure loss.
JP-2001-240411A (Patent literature 3) discloses a process for manufacturing a zeolite membrane which includes the steps of: coating the surface of a porous substrate with a gel solution of mordenite (MOR) type zeolite crystals; placing the gel solution in a pressure container; and placing the tubular porous substrate vertically in the gel solution to subject the same to hydrothermal synthesis. When the porous substrate is placed in the gel solution so that the surface on which a zeolite membrane is to be formed is horizontal, an oriented MOR type zeolite membrane is hard to form on the top of the substrate, though it is formed on the underside of the substrate. In this process, however, the porous substrate is placed in the gel solution so that the surface on which a zeolite membrane is to be formed is vertical, a MOR type zeolite membrane whose b axis and c axis are oriented parallel to the substrate can be formed on the overall surface of the porous substrate.
In the meantime, part of the zeolite crystallites formed in the gel solution during hydrothermal synthesis is settled on the bottom of the container. And when sediment thus formed is attached locally on the porous substrate, a uniform zeolite membrane cannot be obtained. The process described in JP-2001-240411A, in which a porous substrate is placed vertically in a gel solution of a zeolite raw material, is likely to cause the attachment of such sediment on the bottom of the substrate. Thus, it gives rise to a problem of being unable to form a uniform zeolite membrane.
Besides the problems described as above, all the above described processes give rise to a problem of increased manufacturing cost when used to produce a large number of zeolite membranes or large-sized zeolite separation membranes, because a pressure container such as an autoclave is indispensable to them.
Patent Literature 1: Japanese Patent No. 3272119
Patent Literature 2: JP-9-202615A
Patent Literature 3: JP-2001-240411A