ZEOLITE MEMBRANE COMPOSITE, METHOD FOR PRODUCING ZEOLITE MEMBRANE COMPOSITE AND SEPARATION METHOD

Abstract

A zeolite membrane composite (1) comprises a porous support (11) and a zeolite membrane (12) formed on the support (11). The zeolite membrane (12) includes a low-density layer (13) coating the support (11) and a compact layer (14) coating the low-density layer (13). The compact layer (14) has a higher zeolite crystalline phase content than the low-density layer (13). By forming the compact layer (14) on the low-density layer (13) coating the support (11) in this way, the fine, defect-free compact layer (14) can be formed more easily than if a compact layer were formed directly on a support. As a result, it is possible to achieve the zeolite membrane composite (1) with high permeability and high selectivity.

Description

ZEOLITE MEMBRANE COMPOSITE, METHOD FOR PRODUCING ZEOLITE MEMBRANE COMPOSITE AND SEPARATION METHOD Field of invention The present invention relates to a zeolite membrane compound, a method for producing the zeolite membrane compound, and a separation method for a mixture of substances using the zeolite membrane compound. Cross-reference to related request This application claims the priority benefit of Japanese Patent Application Number 2019038444 filed with the Japanese Patent Office on March 4, 2019, the full disclosure of which is incorporated herein by reference. Background of the invention Currently, several studies and developments are underway on the formation of a zeolite membrane on a porous support to obtain a zeolite membrane composite and utilize the molecular sieving function of zeolites in applications such as the separation or adsorption of specific molecules. For example, in the separation of a mixed gas containing multiple gas types, the mixed gas is supplied to a zeolite membrane composite, and a gas with high permeability is made to permeate through the zeolite membrane composite to separate from the other gases. For example, Japanese Patent Application Number H10-57784 (Document 1) proposes a zeolite separation membrane in which a compact zeolite membrane with a thickness of 0.5 µm to 30 µm is in close contact with the surface of a porous base material. The porous base material has a porosity of 10% to 50% and an average pore diameter of 0.5 µm to 10 µm. The zeolite membrane is formed by depositing seed crystals in an amount of 0.2 mg / cm² to 3 mg / cm² on the surface of the porous base material and then hydrothermally synthesizing the seed crystals in a feedstock solution, the seed crystals having an average grain diameter that is 0.4 to 8 times the average diameter of the porous base material. In Document 1, Figure 2 discloses a cross-section of a zeolite separation membrane according to a conventional technique. In this zeolite separation membrane, a sparse zeolite layer with a bridging structure (i.e., large voids) is provided between a base material and a compact layer. Document 1 describes that the sparse zeolite layer has lower service strength because it does not cover the surface of the base material in portions that include the bridging structure and, therefore, has low strength. It also describes that this zeolite separation membrane suffers from problems of excessive membrane thickness and low permeation. Incidentally, to effectively separate a high-permeability gas from other gases in a zeolite membrane composite, it is necessary to form a compact, defect-free zeolite membrane on a porous support. Furthermore, to improve the permeability of the high-permeability gas, it is necessary to reduce the thickness of the zeolite membrane. However, reducing the thickness of the zeolite membrane increases the likelihood of defects occurring in the membrane. Therefore, it is not easy for a zeolite membrane composite to achieve both high permeability and high selectivity. Brief description of the invention The present invention relates to a zeolite membrane composite, and it is an objective of the present invention to achieve a zeolite membrane composite with high permeability and high selectivity. A zeolite membrane composite according to a preferred embodiment of the present invention includes a porous support and a zeolite membrane formed on the support. The zeolite membrane comprises a low-density layer coating the support and a compact layer coating the low-density layer, which has a higher zeolite crystalline phase content than the low-density layer. According to the present invention, it is possible to obtain a zeolite membrane composite with high permeability and high selectivity. Preferably, the compact layer contains zeolite crystals with an average particle diameter greater than the average particle diameter of the zeolite crystals contained in the low-density layer. More preferably, the average particle diameter of the zeolite crystals contained in the compact layer is 100 times or less the average particle diameter of the zeolite crystals contained in the low-density layer. Preferably, the compact layer contains zeolite crystals with an average particle diameter greater than or equal to 0.1 pm and less than or equal to 10 pm. Preferably, the compact layer has a thickness that is 0.05 times or more and 50 times or less the thickness of the low-density layer. Preferably, the content of the crystalline phase of zeolite in the compact layer is greater than or equal to 95%, and the content of a crystalline phase of zeolite in the low-density layer is greater than or equal to 5% and less than 95%. Preferably, the low-density layer has a grain boundary phase formed by an inorganic compound. Preferably, the low-density layer has a grain boundary phase that includes an amorphous phase. «can in / Lznz / B / YiAi Preferably, the compact layer and the low-density layer contain zeolite crystals of the same type. Preferably, the compact layer contains zeolite crystals that have a maximum number of 8-membered rings. The present invention also relates to a method for producing the zeolite membrane composite. A method for producing the zeolite membrane composite according to a preferred embodiment of the present invention includes: a) preparing seed crystals; b) depositing the seed crystals onto a porous support to form a seed crystal laminate on the support, the seed crystal laminate comprising a lamination of two or more layers of the seed crystals; and c) immersing the support in a raw material solution to grow a zeolite from the seed crystal laminate by hydrothermal synthesis and form a zeolite membrane on the support. The zeolite membrane comprises a low-density layer coating the support and a compact layer coating the low-density layer and having a higher zeolite crystalline phase content than the low-density layer.According to the present invention, it is possible to obtain a zeolite membrane compound with high permeability and high selectivity. Preferably, the molar ratio of a structure-directing agent to water in the raw material solution is less than or equal to 0.01. The present invention is also intended for a separation method. A separation method according to a preferred embodiment of the present invention includes: a) preparing the zeolite membrane compound according to any one of claims 1 to 10, and b) supplying a mixture of substances, including a plurality of gas or liquid types, to the zeolite membrane compound and causing a substance with high permeability in the mixture of substances to penetrate through the zeolite membrane compound to separate the substance with high permeability from other substances. Preferably, the mixture of substances includes at least one of the substances selected from a group consisting of hydrogen, helium, nitrogen, oxygen, water, steam, carbon monoxide, carbon dioxide, nitrogen oxide, ammonia, sulfur oxide, hydrogen sulfide, sulfur fluoride, mercury, arsenic, hydrogen cyanide, carbonyl sulfide, hydrocarbons from C1 to C8, organic acid, alcohol, mercaptans, ester, ether, ketone, and aldehyde. These and other objectives, features, aspects and advantages of the present invention will become more evident from the following detailed description of the present invention when taken together with the accompanying drawings. Brief description of the drawings Figure 1 is a cross-sectional view of a zeolite membrane composite; Figure 2 is a cross-sectional view of part of the zeolite membrane composite in dimensions Rcan in / ίζηζ / Β / γίΛΐ expanded; Figure 3 is an illustration of a STEM image of part of the zeolite membrane composite in enlarged dimensions. Figure 4 is a schematic diagram illustrating the proximity of an interface between a compact layer and a low-density layer in enlarged dimensions. Figure 5 is a flow diagram of a procedure for producing the zeolite membrane compound; Figure 6 is a cross-sectional view illustrating part of the zeolite membrane composite during production in enlarged dimensions. Figure 7 is an illustration of a separator. Figure 8 is a flow diagram of a procedure for separating a mixture of substances. Description of the modalities Figure 1 is a cross-sectional view of a zeolite membrane composite 1. Figure 2 is a cross-sectional view of a portion of the zeolite membrane composite 1 at an enlarged scale. The zeolite membrane composite 1 includes a porous support 11 and a zeolite membrane 12 formed on the support 11. The zeolite membrane 12 refers to at least one zeolite formed in a membrane on the surface of the support 11 and does not include zeolite particles that are simply dispersed in an organic membrane. In Figure 1, the zeolite membrane 12 is illustrated with bold lines. In Figure 2, the zeolite membrane 12 is hatched. In the illustration in Figure 2, the zeolite membrane 12 appears thicker than its actual thickness. Support 11 is a porous member permeable to gases and liquids. In the example illustrated in Figure 1, support 11 is a monolithic support in which a plurality of through-holes 111, each extending in a longitudinal direction (i.e., a top-down direction in Figure 1), are provided in a column-like body molded integrally. In the example illustrated in Figure 1, support 11 has a shape substantially similar to a column. Each through-hole 111 (i.e., cell) has, for example, a substantially circular cross-sectional shape perpendicular to the longitudinal direction. In the illustration in Figure 1, the through-holes 111 have a diameter larger than the actual diameter, and the number of through-holes 111 is less than the actual number.The zeolite membrane 12 is formed on the internal surfaces of the through holes 111 and substantially covers all internal surfaces of the through holes 111. Support 11 has a length (i.e., the length in the downward direction in Figure 1) of, for example, 10 cm to 200 cm. Support 11 has an outer diameter of, for example, 0.5 Rcan in / Lznz / B / YiAi cm to 30 cm. The distance between the centerlines of each pair of adjacent through-holes 111 is, for example, in the range of 0.3 mm to 10 mm. The surface roughness (Ra) of the support 11 is, for example, in the range of 0.1 pm to 5.0 pm and preferably in the range of 0.2 pm to 2.0 pm. Alternatively, the support 11 may have a different shape, such as a honeycomb shape, a flat plate-like shape, a tube-like shape, a cylinder-like shape, a column-like shape, or a polygonal prism shape. When the support 11 is tube-like or cylinder-like, the thickness of the support 11 is, for example, in the range of 0.1 mm to 10 mm. Various substances (e.g., ceramic or metal) can be used as the support material 11, provided they are chemically stable during the formation of the zeolite membrane 12 on the surface. In the present embodiment, the support 11 is a sintered ceramic compact. Examples of sintered ceramic compacts that can be selected as support material 11 include alumina, silica, mullite, zirconia, titania, yttrium, silicon nitride, and silicon carbide. In the present embodiment, the support 11 contains at least one of the following: alumina, silica, and mullite. Support 11 may contain an inorganic binder. The inorganic binder may be at least one of titania, mullite, easily sintered alumina, silica, glass frit, clay minerals, and easily sintered cordierite. Support 11 has an average pore diameter of, for example, 0.01 pm to 70 pm and preferably 0.05 pm to 25 pm. The average pore diameter of support 11 in the vicinity of the surface on which the zeolite membrane 12 forms is in the range of 0.01 pm to 1 pm and preferably 0.05 pm to 0.5 pm. Regarding the pore size distribution of support 11 as a whole, including its surface and interior, D5 is in the range of, for example, 0.01 pm to 50 pm, D50 is in the range of, for example, 0.05 pm to 70 pm, and D95 ​​is in the range of, for example, 0.1 pm to 2000 pm. The porosity of the support 11 in the vicinity of the surface on which the zeolite membrane 12 is formed is in the range of, for example, 20% to 50%. For example, support 11 has a multilayer structure in which a plurality of layers with different mean pore diameters are laminated one on top of the other in one thickness direction. A surface layer, which includes the surface on which the zeolite membrane 12 is formed, has a smaller mean pore diameter and a smaller sintered particle diameter than the other layers. The mean pore diameter in the surface layer of support 11 is, for example, in the range of 0.01 pm to 1 pm and preferably in the range of 0.05 pm to 0.5 pm. When support 11 has a multilayer structure, the material for each layer can be any of the materials described above. The plurality of layers forming the multilayer structure can be made of the same material or can be made of different materials. Rcan tn / Lznz / E / YiAi Zeolite 12 membrane is a porous membrane with small pores. It can be used as a separation membrane to separate a specific substance from a mixture of substances, employing molecular sieving. Zeolite 12 membrane is less permeable to the other substances than to the specific substance. In other words, the permeability of other substances through Zeolite 12 membrane is lower than the permeability of the specific substance described above through Zeolite 12 membrane. The zeolite 12 membrane has a thickness of, for example, 0.05 pm at 30 pm, preferably 0.1 pm at 20 pm, and more preferably 0.5 pm at 10 pm. Increasing the thickness of the zeolite 12 membrane improves selectivity. Reducing the thickness of the zeolite 12 membrane improves permeability. The surface roughness (Ra) of the zeolite 12 membrane is, for example, less than or equal to 5 pm, preferably less than or equal to 2 pm, more preferably less than or equal to 1 pm, and even more preferably less than or equal to 0.5 pm. The zeolite 12 membrane has a mean pore diameter less than or equal to 1 nm. The mean pore diameter of the zeolite membrane 12 is preferably greater than or equal to 0.2 nm and less than or equal to 0.8 nm, more preferably greater than or equal to 0.3 nm and less than or equal to 0.7 nm, and even more preferably greater than or equal to 0.3 nm and less than or equal to 0.6 nm. The mean pore diameter of the zeolite membrane 12 is smaller than the mean pore diameter of the support 11 near the surface on which the zeolite membrane 12 is formed. When n is the maximum number of ring-members in a zeolite constituting the zeolite membrane 12, the arithmetic mean of the major and minor axes of an n-member ring pore is assumed to be the mean pore diameter. An n-member ring pore refers to a small pore containing n oxygen atoms in a portion where the oxygen atoms are bonded to each other with T atoms to form a ring structure. When the zeolite has a plurality of n-member ring pores, where n is the same number, the arithmetic mean of the major and minor axes of all the n-member ring pores is assumed to be the mean pore diameter of the zeolite.Thus, the average pore diameter of the zeolite membrane is determined solely by the zeolite framework structure and can be obtained from a value presented in the zeolite structure database [online] by the International Zeolite Association on the Internet<URL:http: / / www.iza-structure.org / databases / > . There are no particular limitations on the type of zeolite that constitutes the zeolite membrane 12 and, for example, the zeolite can be of any of the following types: AEI type, AEN type, AFN type, AFV type, AFX type, BEA type, CHA type, DDR type, ERI type, ETL type, FAU type (X type, Y type), GIS type, LEV type, LTA type, MEL type, MFI type, MOR type, PAU type, RHO type, SAT type and SOD type. Rcan in / Lznz / B / YiAi Examples of zeolites that constitute zeolite membrane 12 include a zeolite in which the atoms located at the center of an oxygen tetrahedron (TO4) that constitute the zeolite (T atoms) are composed of Si and Al, an AIPO-type zeolite in which the T atoms are composed of Al and P, a SAPO-type zeolite in which the T atoms are composed of Si, Al, and P, a MAPSO-type zeolite in which the T atoms are composed of magnesium (Mg), Si, Al, and P, and a ZnAPSO-type zeolite in which the T atoms are composed of zinc (Zn), Si, Al, and P. Some of the T atoms may be replaced by other elements. Zeolite 12 membrane contains, for example, silicon (Si). It may also contain two or more of the elements Si, aluminum (Al), and phosphorus (P). Zeolite 12 membrane may also contain an alkali metal. The alkali metal may be, for example, sodium (Na) or potassium (K). When Zeolite 12 membrane contains Si atoms, the Si / Al ratio in the membrane is, for example, greater than or equal to 1 and less than or equal to 100,000. The Si / Al ratio is preferably greater than or equal to 5, more preferably greater than or equal to 20, and even more preferably greater than or equal to 100, and is preferably as high as possible. The Si / Al ratio in the Zeolite 12 membrane can be adjusted by adjusting the composition ratio of a Si source to an Al source in a raw material solution, which will be described later. From the perspective of improving CO2 permeability and selectivity, the zeolite preferably has a maximum number of rings of 8 members or fewer (e.g., 6 or 8). For example, zeolite membrane 12 can be composed of a DDR-type zeolite. In other words, zeolite membrane 12 is a zeolite membrane composed of a zeolite with a DDR frame type code assigned by the International Zeolite Association. In this case, the zeolite constituting zeolite membrane 12 has an intrinsic pore diameter of 0.36 nm x 0.44 nm and an average pore diameter of 0.40 nm. The CO2 permeability of the zeolite 12 membrane at a temperature of 20°C to 400°C can be, for example, greater than or equal to 100 nmol / m²s Pa. A ratio (permeability ratio) between the CO2 permeability and the CH4 (leakage) permeability of the zeolite 12 membrane at a temperature of 20°C to 400°C can be, for example, greater than or equal to 100. The CO2 permeability and permeability ratio are values ​​for the case where there is a difference of 1.5 MPa in the partial pressure of CO2 between the supply side and the permeation side of the zeolite 12 membrane. Figure 3 is an illustration (i.e., STEM image) obtained by observing a cross-section of a soil surface of the zeolite membrane composite 1 near the zeolite membrane 12 with a scanning transmission electron microscope (STEM). The zeolite membrane 12 includes a low-density layer 13 and a compact layer 14. The low-density layer 13 is in direct contact with the surface of the support 11 and covers the surface of the Rcan in / ίζηζ / ε / γίΛΐ support 11. The compact layer 14 is in direct contact with the surface of the low-density layer 13 and covers the surface of the low-density layer 13. The compact layer 14 is not in direct contact with the surface of the support 11, but is indirectly in contact with the surface of the support 11 through the low-density layer 13. The compact layer 14 has a higher content of a crystalline zeolite phase than the low-density layer 13. In Figure 3, an interface between the compact layer 14 and the low-density layer 13 (i.e., the surface of the low-density layer 13) is indicated by a solid line 15. The zeolite crystalline phase content in the compact layer 14 is preferably greater than or equal to 95% and more preferably greater than or equal to 96%. The zeolite crystalline phase content in the low-density layer 13 is preferably greater than or equal to 5% and less than 95% and more preferably greater than or equal to 20% and less than or equal to 90%. The zeolite crystalline phase content in the compact layer 14 is obtained by dividing the volume of the zeolite crystalline phase in the compact layer 14 by the total volume of the zeolite crystalline phase and the grain boundary phase in the compact layer 14. The zeolite crystalline phase content in the low-density layer 13 is also obtained in the same way. The aforementioned grain boundary phase is a region between a plurality of zeolite crystals. For example, the grain boundary phase may be a phase that includes non-crystalline (i.e., amorphous) crystals other than zeolite crystals and / or voids. The grain boundary phase has a lower density than the density of the crystalline zeolite phase. The zeolite crystalline phase content in compact layer 14 is obtained using the aforementioned STEM image. Specifically, in the STEM image, an arbitrary zeolite crystal is selected, and a region encompassing grain boundary phases between the selected zeolite crystal and all adjacent zeolite crystals is selected. This region is then binarized using a predetermined threshold value. The threshold value is appropriately determined to allow for the recognition of the selected zeolite crystal and the grain boundary phases. Subsequently, the areas of the portions with concentrations below the threshold value (e.g., zeolite crystalline phase) and the areas of the portions with concentrations greater than or equal to the threshold value (e.g., grain boundary phase) are obtained based on the binarized image of the aforementioned region.The zeolite crystalline phase content in this region is then obtained by dividing the area of ​​the zeolite crystalline phase by the total area of ​​the zeolite crystalline phase and the grain boundary phase. In this modality, the zeolite crystalline phase content is obtained for each of the 10 regions of compact layer 14 in the STEM image, and an average of these contents is considered the zeolite crystalline phase content in compact layer 14. The zeolite crystalline phase content in the low-density layer 13 is also obtained in the same way as for compact layer 14. Rcan in / ίζηζ / ε / γίΛΐ The grain boundary phases in the low-density layer 13 preferably include an amorphous phase. More preferably, the grain boundary phases in the low-density layer 13 include 10% or more by weight of an amorphous phase. The grain boundary phases in the low-density layer 13 are preferably composed of inorganic compounds. In other words, the grain boundary phases in the low-density layer 13 are preferably composed only of inorganic compounds and substantially do not include any organic compounds. The phrase "substantially do not include any organic compounds" refers to not including 5% or more by weight of organic compounds. The interface 15 between the compact layer 14 and the low-density layer 13 is obtained using the aforementioned STEM image. Specifically, first, an approximate surface 142 is obtained in the STEM image by linear approximation of a surface 143 of the compact layer 14 (i.e., the outer surface on the side opposite the interface 15) using least squares. In Figure 3, the approximate surface 142 of the compact layer 14 is indicated by a double-dashed line chain. Then, a plurality of straight lines (not shown) parallel to the approximate surface 142 are placed at regular intervals in a direction perpendicular to the approximate surface 142 (i.e., a downward direction in Figure 3). The interval between the straight lines is a distance that is sufficiently smaller than the thickness of the compact layer 14. Next, for each of the straight lines that do not include the background of the STEM image, the proportion of grain boundary phases on the straight line (i.e., the grain boundary phase occupancy) is obtained. Then, a straight line that is parallel to approximate surface 142 and located at the midpoint between two straight lines in a direction away from approximate surface 142 is determined as interface 15, one of the two straight lines being a straight line on which the proportion of grain boundary phases first reaches or exceeds 5%, and the other straight line being located adjacent to one of the two straight lines on the side of approximate surface 142. The thickness of the compact layer 14 is obtained as the distance between the approximate surface 142 and the interface 15 in the direction perpendicular to the approximate surface 142 of the compact layer 14. The thickness of the low-density layer 13 is obtained as the average distance between the interface 15 and an approximate surface 112 of the support 11 in the direction perpendicular to the approximate surface 142 of the compact layer 14. In Figure 3, the approximate surface 112 of the support 11 is indicated by a double-dashed line. The approximate surface 112 of the support 11 is obtained by linear approximation of the surface of the support 11, using least squares on the aforementioned STEM image.The average distance between interface 15 and approximate surface 112 of support 11 refers to the arithmetic mean of the distances between interface 15 and approximate surface 112 of support 11 at positions in the right-to-left direction of the STEM image (i.e., the right-to-left direction in Figure 3). The thickness of the zeolite membrane 12 is equal to the total thickness of the... Rcan in / Lznz / B / YiAi compact layer 14 and the thickness of the low-density layer 13. Note that the STEM image used to obtain the thicknesses of the compact layer 14 and the low-density layer 13 is acquired in such a way that the total thickness of the zeolite membrane 12 and the support 11 is approximately three times the thickness of the zeolite membrane 12 and that the width of the zeolite membrane 12 in the right-left direction is approximately four times the thickness of the zeolite membrane 12. As described above, the thickness of the zeolite membrane 12 is, for example, in the range of 0.05 pm to 30 pm, preferably in the range of 0.1 pm to 20 pm, and more preferably in the range of 0.5 pm to 10 pm. The thickness of the compact layer 14 is, for example, in the range of 0.01 pm to 20 pm, preferably in the range of 0.03 pm to 10 pm, and more preferably in the range of 0.05 pm to 8 pm. The thickness of the low-density layer 13 is, for example, in the range of 0.01 pm to 20 pm, preferably in the range of 0.05 pm to 10 pm, and more preferably in the range of 0.1 pm to 5 pm. The thickness of the compact layer 14 is preferably greater than or equal to 0.05 times and less than or equal to 50 times the thickness of the low-density layer 13, and more preferably greater than or equal to 0.1 times and less than or equal to 30 times the thickness of the low-density layer 13. In the zeolite membrane 12, the compact layer 14 and the low-density layer 13 contain zeolite crystals of the same type. In the example in Figure 3, the compact layer 14 and the low-density layer 13 each contain primarily zeolite crystals of one type, and the zeolite crystals contained in the compact layer 14 and the zeolite crystals contained in the low-density layer 13 are of the same type. The zeolite crystals contained in the compact layer 14 and the low-density layer 13 are formed, for example, by a DDR-type zeolite. The compact layer 14 contains zeolite crystals that have a maximum number of 8-membered rings. The low-density layer 13 also contains zeolite crystals that have a maximum number of 8-membered rings. The zeolite crystals contained in the compact layer 14 and the low-density layer 13 have inherited pore diameters of 0.36 nm x 0.44 nm. Figure 4 is a schematic diagram illustrating the proximity of the interface 15 between the compact layer 14 and the low-density layer 13 of the zeolite membrane 12 in magnified dimensions. As illustrated in Figure 4, the zeolite crystals 141 contained in the compact layer 14 have a larger average particle diameter than the average particle diameter of the zeolite crystals 131 contained in the low-density layer 13. In the illustration in Figure 4, the zeolite crystals 141 contained in the compact layer 14 are the same size, but the zeolite crystals 141 can be of different sizes. In the illustration, the zeolite crystals 131 contained in the low-density layer 13 are the same size, but the zeolite crystals 131 can also be of different sizes. The average particle diameter of the zeolite crystals 141 contained in the layer Rcan in / Lznz / B / YiAi compact layer 14 is preferably 100 times or less the average particle diameter of the zeolite crystals 131 contained in the low-density layer 13, and more preferably 80 times or less the average particle diameter of the zeolite crystals 131. The average particle diameter of the zeolite crystals 141 contained in the compact layer 14 is also preferably twice or more the average particle diameter of the zeolite crystals 131, and more preferably three times or more the average particle diameter of the zeolite crystals 131. The average particle diameter of the zeolite crystals 141 contained in the compact layer 14 is preferably greater than or equal to 0.01 pm and less than or equal to 10 pm, and more preferably greater than or equal to 0.03 pm and less than or equal to 5 pm.The average particle diameter of the zeolite crystals 131 contained in the low-density layer 13 is preferably greater than or equal to 0.01 pm and less than or equal to 10 pm and more preferably greater than or equal to 0.05 pm and less than or equal to 5 pm. The average particle diameter of the zeolite 141 crystals contained in the compact layer 14 is calculated by capturing an image of the surface of the zeolite membrane 12 with a scanning electron microscope (SEM) to select a 15 pm x 15 pm interval, calculating the number of zeolite 141 crystals included in the image and the area of ​​the zeolite 141 crystals, and then dividing the area of ​​the zeolite 141 crystals by the number of zeolite 141 crystals, assuming that the zeolite 141 crystals have circular shapes. The average particle diameter of the zeolite 131 crystals contained in the low-density layer 13 is calculated from a cross-sectional STEM image that includes the zeolite membrane 12 and the support 11.Specifically, the average particle diameter of zeolite 131 crystals is calculated by selecting, in the cross-section image, a screen of an area within a distance where the zeolite 12 membrane and the support 11 have equal thickness in the direction of the thickness of the zeolite 12 membrane, calculating the number of zeolite 131 crystals included in the image and the area of ​​the zeolite 131 crystals after binarization using the method mentioned above, and dividing the area of ​​the zeolite 131 crystals by the number of zeolite 131 crystals assuming that the zeolite 131 crystals have circular shapes. The following describes an example of a procedure for producing zeolite membrane composite 1 with reference to Figure 5. In the production of zeolite membrane composite 1, seed crystals are first prepared for use in the production of zeolite membrane 12 (Sil stage). For example, the seed crystals are obtained from DDR-type zeolite powder synthesized by hydrothermal synthesis. This zeolite powder can be used as is, as seed crystals, or it can be processed into seed crystals by sputtering or other methods. Next, the seed crystals are deposited onto support 11 (stage S12). In stage S12, a seed crystal laminate 125, which includes a laminate of two or more layers of seed crystals, is Rcan in / Lznz / B / YiAi forms on the internal surfaces 115 of the through holes 111 in the support 11 as illustrated in Figure 6. In this way, a support for seed crystal depositing is prepared. In the illustration in Figure 6, the seed crystals are circular in shape and larger than the actual size. In the example illustrated in Figure 6, the seed glass laminate 125 comprises a first seed glass layer and a second seed glass layer. The first seed glass layer is in direct contact with the inner surfaces 115 of the support 11 and covers the inner surfaces 15, and the second seed glass layer is in direct contact with the surface of the first seed glass layer and covers the surface of the first seed glass layer. The second seed glass layer is not in direct contact with the inner surfaces 115 of the support 11, but is in indirect contact with the inner surfaces 115 of the support 11 through the first seed glass layer. Preferably, the seed glass laminate 125, the first seed glass layer, and the second seed glass layer each have a substantially uniform thickness on the support 11.The 125 seed glass laminate may also include another layer of seed glass laminated onto the second seed glass layer. The deposition of the seed crystal laminate 125 onto the support 11 in step S12 is carried out by immersing the porous support 11 in a solution in which the seed crystals are dispersed. In this case, to form the seed crystal laminate 125 on the support 11, the immersion of the support 11 in the solution and the drying of the support 11 can be repeated a plurality of times. Alternatively, the deposition of the seed crystal laminate 125 onto the support 11 can be carried out by placing a solution containing the dispersed seed crystals in contact with the internal surfaces 115 of the through-holes 111 of the support 11. As another alternative, the seed crystal laminate 125 can be deposited onto the support 11 using other techniques. Support 11, with the despotted seed crystal laminate 125 on it, is immersed in a raw material solution. The raw material solution is prepared, for example, by dissolving components such as a Si source and a structure-directing agent (hereafter also referred to as SDA) in a solvent. For example, the raw material solution has a composition of 1.0 S1O2: 0.015 SDA; 0.12 (CEEHNELX). The solvent in the raw material solution can be, for example, water or an alcohol such as ethanol. When water is used as the solvent for the raw material solution, the molar ratio of SDA to water in the raw material solution is preferably less than or equal to 0.01. The molar ratio of SDA to water in the raw material solution is also preferably greater than or equal to 0.00001. The SDA in the raw material solution can be, for example, an organic compound.For example, 1-adamantanamine can be used as SDA. The DDR-type zeolite membrane 12 is then formed on the support 11 by growing a DDR-type zeolite by hydrothermal synthesis using several crystals from the aforementioned seed crystal laminate 125 as cores (step S13). The hydrothermal synthesis temperature is preferably in the range of 120 to 200°C and can, for example, be 160°C. The hydrothermal synthesis time is preferably in the range of 10 to 100 hours and can, for example, be 30 hours. Once the hydrothermal synthesis is complete, the support 11 and the zeolite membrane 12 are rinsed with deionized water. After rinsing, the support 11 and the zeolite membrane 12 are dried, for example, at 80°C. After the support 11 and the zeolite membrane 12 have dried, the zeolite membrane 12 is subjected to heat treatment to burn off and almost completely remove the SDA from the zeolite membrane 12 and to make the micropores in the zeolite membrane 12 pass through the membrane. In this way, the zeolite membrane composite 1 mentioned above is obtained. The separation of a mixture of substances using the zeolite membrane composite 1 will now be described with reference to Figures 7 and 8. Figure 7 is an illustration of a separator 2. Figure 8 is a flow diagram of a procedure for separating a mixture of substances, carried out by the separator 2. Separator 2, in which a mixture of substances including a plurality of fluid types (i.e., gases or liquids) is supplied to the zeolite membrane compound 1, separates a substance with high permeability from the mixture of substances by causing the substance to penetrate through the zeolite membrane compound 1. For example, separation by separator 2 can be carried out in order to extract a substance with high permeability from the mixture of substances, or in order to condense a substance with low permeability. The mixture of substances (i.e., mixed fluid) can be a mixed gas that includes a plurality of gas types, or it can be a mixed solution that includes a plurality of liquid types, or it can be a two-phase gas-liquid fluid that includes both gases and liquids. The mixture of substances includes, for example, one or more types of substances selected from the group consisting of hydrogen (H2), helium (He), nitrogen (N2), oxygen (O2), water (H2O), water vapor (H2O), carbon monoxide (CO), carbon dioxide (CO2), nitrogen oxide, ammonia (NH3), sulfur oxide, hydrogen sulfide (H2S), sulfur fluoride, mercury (Hg), arsenic (AsHs), hydrocyanic acid (HCN), carbonyl sulfide (COS), C1 to C8 hydrocarbons, organic acid, alcohol, mercaptans, ester, ether, ketone, and aldehyde. Nitrogen oxide is a compound of nitrogen and oxygen. The nitrogen oxide mentioned above can be, for example, a gas called NOx such as nitrogen monoxide (NO), nitrogen dioxide (NO2), nitrous oxide (also known as dinitrogen monoxide) (N2O), dinitrogen trioxide (N2O3), dinitrogen tetroxide (N2O4), or dinitrogen pentoxide (N2O5). Sulfur dioxide is a compound of sulfur and oxygen. The sulfur dioxide mentioned Rcan in / ίζηζ / ε / γίΛΐ previously can be, for example, a gas called SOx such as sulfur dioxide (SO2) or sulfur trioxide (SO3). Sulfur fluoride is a compound of fluorine and sulfur. The sulfur fluoride mentioned above can be, for example, disulfide difluoride (FSSF, S = SF2), sulfur difluoride (SF2), sulfur tetrafluoride (SF4), sulfur hexafluoride (SFe), or disulfide decafluoride (S2F10). Hydrocarbons from C1 to C8 are hydrocarbons that contain one or more and eight or fewer carbon atoms. Each of the C3 to C8 hydrocarbons can be any of a linear chain compound, a side-chain compound, or a cyclic compound. Each of the C3 to C8 hydrocarbons can be a saturated hydrocarbon (i.e., the absence of a double and a triple bond in a molecule) or an unsaturated hydrocarbon (i.e., the presence of a double and / or a triple bond in a molecule). C1 to C4 can be, for example, methane (CH4), ethane (C2H6), ethylene (C2H4), propane (C3H8), propylene (C3H6), normal butane (CH3(CH2)2CH3), isobutane (CH(CH3)3), 1-butene (CH2 = CHCH2CH3), 2-butene (CH3CH = CHCH3) or isobutene (CH2= C(CH3)2). The organic acid mentioned above can be, for example, a carboxylic acid or a sulfonic acid. A carboxylic acid can be, for example, formic acid (CH₂O₂), acetic acid (C₂H₄O₂), oxalic acid (C₂H₂O₄), acrylic acid (C₃H₄O₂), or benzoic acid (CeF₂COOH). A sulfonic acid can be, for example, ethanesulfonic acid (C₂H₆O₃S). An organic acid can be a chain compound or a cyclic compound. The alcohol mentioned above can be, for example, methanol (CH3OH), ethanol (C2H5OH), isopropanol (2-propanol) (CFLCHfOHjCFL), ethylene glycol (CH2(OH)CH2(OH)), or butanol (C4II9OII). Mercaptans are organic compounds with a terminal sulfur hydride (SH) group and are also called thiols or thioalcohols. Examples of mercaptans include methyl mercaptans (CH3SH), ethyl mercaptans (C2H5SH), and 1-propanethiol (C3H7SH). The ester mentioned above can be, for example, a formic acid ester or an acetic acid ester. The ether mentioned above can be, for example, dimethyl ether ((CH3)2O), methyl ethyl ether (C2H5OCH3) or diethyl ether ((C2H5)2O). The aforementioned ketone can be, for example, acetone ((CH3)2CO), methyl ethyl ketone (C2H5COCH3) or diethyl ketone ((C2H5)2CO). The aldehyde mentioned above can be, for example, acetaldehyde (CH3CHO), propionaldehyde (C2H5CHO) or butanal (butyraldehyde) (C3H7CHO). The following description takes the example of the case in which the mixture of substances to be separated by separator 2 is a mixed gas that includes a plurality of gas types. Rcan in / ίζηζ / ε / γίΛΐ The separator 2 includes the zeolite membrane compound 1, sealants 21, an outer cylinder 22, sealing elements 23, a supply part 26, a first collection part 27, and a second collection part 28. The zeolite membrane compound 1, the sealants 21, and the sealing elements 23 are housed in the outer cylinder 22. The supply part 26, the first collection part 27, and the second collection part 28 are positioned outside the outer cylinder 22 and connected to the outer cylinder 22. Sealants 21 are members mounted on both ends of support 11 in the longitudinal direction (i.e., the left-to-right direction in Figure 6). They cover and seal both end faces of support 11 in the longitudinal direction and the outer surface of support 11 near both end faces. Sealants 21 prevent the entry and exit of gases from both end faces of support 11. Sealants 21 can be, for example, plate-like elements made of glass or resin. The material and shape of the sealants 21 can be appropriately changed. Since the sealants 21 have a plurality of openings that overlap with the through holes 111 of support 11, both ends of each through hole 111 of support 11 in the longitudinal direction are not covered by the sealants 21. This allows the entry and exit of gases or similar substances from both ends to the through holes 11. The outer cylinder 22 is a tube-like member, generally cylindrical. The outer cylinder 22 can be made, for example, of stainless steel or carbon steel. The longitudinal direction of the outer cylinder 22 is substantially parallel to the longitudinal direction of the zeolite membrane composite 1. The outer cylinder 22 has a supply port 221 at one end in the longitudinal direction (i.e., the left-hand end in Figure 1) and a first exhaust port 222 at the other end. The outer cylinder 22 also has a second exhaust port 223 on its side face. The supply port 221 is connected to the supply portion 26. The first exhaust port 222 is connected to the first collection portion 27. The second exhaust port 223 is connected to the second collection portion 28. The outer cylinder 22 has an internal space, which is an enclosed space isolated from the space surrounding the outer cylinder 22.The two sealing elements 23 are positioned around the entire circumference between the outer surface of the zeolite membrane composite material 1 and the inner surface of the outer cylinder 22, near opposite ends of the zeolite membrane composite material 1 in the longitudinal direction. Each sealing element 23 is generally ring-shaped and made of a gas-impermeable material. For example, the sealing elements 23 are O-rings made of flexible resin. In the example illustrated in Figure 1, the sealing elements 23 are in close contact with the outer surfaces of the sealants 21 and indirectly in close contact with the outer surface of the zeolite membrane composite 1 through the sealants 21.A space between the sealing elements 23 and the outer surface of the zeolite membrane composite 1 and a space between the sealing elements 23 and the inner surface of the outer cylinder 22 are sealed in such a way as to almost or completely disable the passage of gases. The supply section 26 delivers a mixed gas to the internal space of the outer cylinder 22 through the supply port 221. For example, the supply section 26 could be a fan or a pump that delivers the mixed gas to the outer cylinder 22 under pressure. The fan or pump includes a pressure regulator that regulates the pressure of the mixed gas delivered to the outer cylinder 22. For example, the first collection section 27 and the second collection section 28 could be tanks that store gases from the outer cylinder 22, or they could also be fans or pumps that transfer the gases. In the case of separating a mixed gas, the separator 2 mentioned above is provided to prepare the zeolite membrane compound 1 (step S21). The supply section 26 then supplies a mixed gas to the internal space of the outer cylinder 22. This mixed gas comprises several types of gases with varying permeability to the zeolite membrane 12. For example, the mixed gas may consist predominantly of CO2 and CH4. It may also include gases other than CO2 and CH4. The pressure of the mixed gas supplied from the supply section 26 to the internal space of the outer cylinder 22 (i.e., the supply pressure) may be, for example, in the range of 0.1 MPa to 20.0 MPa. The separation temperature of the mixed gas may be, for example, in the range of 10°C to 250°C. The mixed gas supplied from supply part 26 to the outer cylinder 22 is introduced from the left end of the zeolite membrane composite 1 in the drawing into each through-hole 111 of the support 11, as indicated by arrow 251. A gas with high permeability (e.g., CO2 and hereafter referred to as the high-permeability substance) in the mixed gas penetrates through the zeolite membrane 12 provided on the inner surfaces of the support 11 and through the support 11, and is then emitted from the outer surface of the support 11. Consequently, the high-permeability substance is separated from a gas with low permeability (e.g., CH4 and hereafter referred to as the low-permeability substance) in the mixed gas (step S22).The gas emitted from the outer surface of the support 11 (hereafter referred to as permeate substance) is collected by the second collection part 28 through the second exhaust port 223, as indicated by an arrow 253. The pressure (i.e., permeation pressure) of the gas collected by the second collection part 28 through the second exhaust port 223 can be, for example, approximately atmospheric pressure (0.101 MPa). In the mixed gas, a gas other than the gas that has penetrated through the zeolite membrane 12 and the support 11 (hereafter referred to as the non-permeated substance) passes through each through-hole 111 of the Rcan in / Lznz / B / YiAi support 11 from left to right in the drawing, and is collected by the first collection part 27 through the first exhaust port 222, as indicated by arrow 252. The pressure of the gas collected by the first collection part 27 through the first exhaust port 222 may, for example, be substantially the same as the supply pressure. The non-permeable substance may also include a high-permeability substance that has not passed through the zeolite membrane 12, in addition to the low-permeability substance mentioned above. The relationship of the zeolite membrane structure 12 to the zeolite membrane composite 1, the method for producing the zeolite membrane 12, and the permeability and selectivity of the zeolite membrane composite 1 will now be described with reference to Tables 1 and 2. The permeability and selectivity in Tables 1 and 2 were obtained from a permeate substance (i.e., permeate gas) collected by the second collection part 28 as a result of a mixed gas of CO2 and CH4 supplied from the supply part 26 to the zeolite membrane composite 1 in the outer cylinder 22 and made to penetrate through the zeolite membrane composite 1 in the separator 2 described above. It was assumed that each of the volume fractions of CO2 and CH4 in the mixed gas supplied from supply part 26 was 50%, and the partial pressures of CO2 and CH4 were each assumed to be 0.3 MPa. The permeability in Tables 1 and 2 indicates the ratio of CO2 permeability obtained based on the CO2 permeability collected by the second collection part 28. When the CO2 permeability in comparative example 1 is assumed to be 1 in Table 1 and the CO2 permeability in example 8 is assumed to be 1 in Table 2, the CO2 permeability ratio indicates the ratio of CO2 permeability in the other comparative examples to the CO2 permeability in the reference example. The selectivity in Tables 1 and 2 indicates the value obtained by dividing the CO2 permeability collected by the second collection part 28 by the CH4 leakage collected by the second collection part 28 (i.e., the CO2 / CH4 permeability ratio). «can m / ίζηζ / ε / γίΛΐ Table 1 Zeolite Membrane Thickness (pm) Zeolite Membrane Structure Compact Layer Thickness / Low Density Layer Thickness Permeability Selectivity CO2 Permeability Ratio CO2 / CH4 Permeability Ratio Example 1 4.5 Compact Layer Low Density Layer 0.04 15 102 Example 2 4.0 Compact Layer 0.3 10 203 Example 3 4.2 Low Density Layer 1 8 166 Example 4 3.2 Compact Layer 3 7 172 Example 5 3.0 Low Density Layer 13 3 163 Example 6 2.0 Compact Layer 20 10 204 Example 7 3.0 Low Density Layer 60 1.3 231 Comparative Example 1 3.0 Compact Layer - 1 (reference) 161 Comparative Example 2 3.0 Low Density Layer - 67 11 Rcan in / Lznz / B / YiAi In Examples 1–7 shown in Table 1, the zeolite membranes 12 each included the low-density layer 13 and the compact layer 14. Examples 1–7 differed in the ratio of the thickness of the compact layer 14 to the thickness of the low-density layer 13. In Examples 2–6, the thicknesses of the compact layers 14 ranged from 0.05 to 50 times the thickness of the low-density layer 13. In Comparative Example 1, the zeolite membrane included only a compact layer and did not include a low-density layer. In Comparative Example 2, the zeolite membrane included only a low-density layer and did not include a compact layer. The zeolite membranes 12 in Examples 1–7 had thicknesses from 2 µm to 5 µm. The zeolite membranes in Comparative Examples 1 and 2 had a thickness of 3 µm. In example 5, the compact layer 14 had an average particle diameter of 0.In Example 5, the compact layer 14 had an average particle diameter of 0.05 pm, and the low-density layer 13 had an average particle diameter of 0.05 pm. In Example 8, the compact layer 14 had an average particle diameter of 0.2 pm, and the low-density layer 13 had an average particle diameter of 0.05 pm. In Example 10, the compact layer had an average particle diameter of 0.4 pm, and the low-density layer 13 had an average particle diameter of 0.1 pm. In Example 5, the crystalline layer content in the compact layer 14 was 99%, and the crystalline layer content in the low-density layer 13 was 51%. In Examples 8 through 10, the crystalline layer content in the compact layer 14 ranged from 96% to 98%, and the crystalline layer content in the low-density layer 13 ranged from 59% to 80%. In comparative example 1, the content of a crystalline phase of zeolite in the zeolite membrane was 98%.In comparative example 2, the content of a crystalline phase of zeolite in the zeolite membrane was 90%. In examples 1–7, where CO2 permeability ratios ranged from 1.3 to 15 and CO2 / CH4 permeability ratios ranged from 102 to 231, the zeolite membranes exhibited high permeability and high selectivity. Specifically, in examples 2–6, where CO2 permeability ratios ranged from 3 to 10 and CO2 / CH4 permeability ratios ranged from 163 to 204, the zeolite membranes favorably exhibited high permeability and high selectivity. On the other hand, in comparative example 1 where the CO2 permeability ratio was 1, the zeolite membrane exhibited lower permeability than in examples 1 to 7. In comparative example 2 where the CO2 / CH4 permeability ratio was 11, the zeolite membrane exhibited lower selectivity than in examples 1 to 7. Table 2 Rcan in / Lznz / B / YiAi Production Method Zeolite Membrane Thickness (pm) Zeolite Membrane Structure Permeability Selectivity Seed Crystals SDA Molar Ratio CO2 Permeability Ratio CO2 / CH4 Permeability Ratio Example 8 Seed Crystal Laminate 0.00072 3.0 Tight Layer Low Density Layer 1 (reference) 194 Example 9 Seed Crystal Laminate 0.00021 2.4 Tight Layer Low Density Layer 1.7 181 Example 10 Seed Crystal Laminate 0.00009 1.8 Tight Layer Low Density Layer 1.8 206 Comparative Example Single Layer 0.00072 3.0 Tight Layer 0.2 162 In Examples 8 to 10 shown in Table 2, the seed crystal laminate 125, which includes a lamination of two or more layers of seed crystals, was deposited onto the support 11 in step S12 during the production of the zeolite membrane 12. In step SI3, different values ​​were used for the molar ratio of SDA to water contained in the raw material solution. The zeolite membrane 12 became thinner with decreasing molar ratios. The zeolite membrane 12 included the low-density layer 13 and the compact layer 14. In Examples 8 to 10, where the CO2 permeability ratios were in the range of 1 to 1.8 and the CO2 / CH4 permeability ratios were in the range of 181 to 194, the zeolite membranes exhibited high permeability and high selectivity. In comparative example 3, a single layer of seed crystals (i.e., seed crystals that are not laminated) was deposited onto the support during the production of the zeolite membrane. The molar ratio of SDA to water in the raw material solution was set to be the same as in example 8. In comparative example 3, the zeolite membrane consisted of only a compact layer and did not include a low-density layer. In comparative example 3, the CO2 permeability ratio was 0.2, and the zeolite membrane exhibited lower permeability than in examples 8 through 10. As described above, the zeolite membrane composite 1 comprises the porous support 11 and the zeolite membrane 12 formed on the support 11. The zeolite membrane 12 comprises the low-density layer 13 coating the support 11, and the compact layer 14 coating the low-density layer 13. The compact layer 14 has a higher content of a crystalline zeolite phase than the low-density layer 13. By forming the compact layer 14 on top of the low-density layer 13 coating the support 11, it is possible to form the thin, defect-free compact layer 14 more easily than if the compact layer were formed directly on the support. As a result, it is possible to achieve the zeolite membrane composite 1 with high permeability and high selectivity. As described above, the zeolite 141 crystals contained in the compact layer 14 preferably have a larger average particle diameter than the average particle diameter of the zeolite 131 crystals contained in the low-density layer 13. By causing the zeolite 141 crystals to grow large in the compact layer 14, it is possible to favorably increase the zeolite crystalline phase content in the compact layer 14. More preferably, the zeolite crystals 141 contained in the compact layer 14 have an average particle diameter that is 100 times or less the average particle diameter of the zeolite crystals 131 contained in the low-density layer 13. This allows both the permeability and selectivity of the zeolite membrane composite 1 to fall within favorable ranges. As described above, the zeolite crystals 141 contained in the compact layer 14 Rcan tn / Lznz / B / YiAi preferably have an average particle diameter greater than or equal to 0.1 pm and less than or equal to 10 pm. This allows both the permeability and selectivity of the zeolite membrane composite 1 to fall within favorable ranges. As described above, the thickness of the compact layer 14 is preferably greater than or equal to 0.05 times and less than or equal to 50 times the thickness of the low-density layer 13. Consequently, the thin compact layer 14 can be favorably formed without defects in the low-density layer 13. As a result, it is possible to favorably obtain the zeolite membrane composite 1 with high permeability and high selectivity. In the zeolite membrane composite 1, the zeolite crystalline phase content in the compact layer 14 is preferably greater than or equal to 95%, and the zeolite crystalline phase content in the low-density layer 13 is preferably greater than or equal to 5% and less than 95%. Therefore, it is possible to favorably obtain the zeolite membrane composite 1 with high permeability and high selectivity. As described above, the grain boundary phases in the low-density layer 13 are preferably composed of inorganic compounds. In general, inorganic compounds have, for example, greater corrosion resistance (e.g., resistance to water or an organic solvent), greater pressure resistance, and greater heat resistance than organic compounds. Therefore, if the grain boundary phases are substantially made to exclude organic compounds, it is possible to give the low-density layer 13 greater durability (e.g., corrosion resistance, heat resistance, and pressure resistance). As a result, it is possible to improve the durability of the zeolite membrane 12. The grain boundary phases in the low-density layer 13 preferably include an amorphous phase. This reduces the stress caused by differences in thermal expansion between zeolite crystals with different crystal orientations and thus suppresses the occurrence of damage to the zeolite membrane 12, such as cracking, during heating of the zeolite membrane 12 (e.g., heat treatment during the production of the zeolite membrane composite 1). In other words, it is possible to further improve the durability of the zeolite membrane 12. The amorphous phase included in the grain boundary phases in the low-density layer 13 is preferably 10% or more by weight. As described above, the compact layer 14 and the low-density layer 13 preferably contain zeolite crystals of the same type. This simplifies the production of the zeolite membrane 12 and, consequently, simplifies the production of the zeolite membrane composite 1. As described above, the zeolite crystals 141 contained in the compact layer 14 preferably have a maximum number of 8-membered rings. This favorably allows the selective permeation of a target substance with relatively small molecular diameters into the zeolite membrane composite 1. Rcan in / Lznz / B / YiAi As described above, the method for producing the zeolite membrane composite 1 includes the step of preparing seed crystals (step SI 1), the step of depositing the seed crystals onto the porous support 11 to form the seed crystal laminate 125, which includes laminating two or more layers of the seed crystals onto the support 11 (step S12), and the step of immersing the support 11 in the raw material solution to grow a zeolite from the seed crystal laminate 125 by hydrothermal synthesis and form the zeolite membrane 12 onto the support 11 (step S13). The zeolite membrane 12 includes the low-density layer 13 coating the support 11, and the compact layer 14 coating the low-density layer 13. The compact layer 14 has a higher content of a crystalline zeolite phase than the low-density layer 13.Therefore, it is possible to easily produce the zeolite membrane compound 1 with high permeability and high selectivity. In the method for producing the zeolite membrane composite 1, the molar ratio of SDA to water contained in the raw material solution is preferably less than or equal to 0.01. By laminating the seed crystals into two or more layers and maintaining a low molar ratio of SDA to water, it is possible to favorably obtain the zeolite membrane 12 comprising the compact layer 14 and the low-density layer 13. It is also possible to reduce the thickness of the zeolite membrane 12 that will form on the support 11. The separation method described above includes the step of preparing the zeolite membrane compound 1 (step S21) and the step of supplying a mixture of substances, including a plurality of gas or liquid types, to the zeolite membrane compound 1 and producing a substance with high permeability within the mixture to permeate through the zeolite membrane compound 1, thus separating the highly permeable substance from the other substances (step S22). As described above, since the zeolite membrane compound 1 has high permeability and high selectivity, this separation method allows for the efficient separation of a mixture of substances. The separation method is particularly suitable for use in separating a mixture of substances that include one or more types of the following substances: hydrogen, helium, nitrogen, oxygen, water, steam, carbon monoxide, carbon dioxide, nitrogen oxide, ammonia, sulfur oxide, hydrogen sulfide, sulfur fluoride, mercury, arsenic, hydrogen cyanide, carbonyl sulfide, hydrocarbons from C1 to C8, organic acid, alcohol, mercaptans, ester, ether, ketone, and aldehyde. The zeolite membrane compound 1, the method for producing the zeolite membrane compound 1, and the separation method, described above, can be modified in various ways. For example, the maximum number of member rings in the compact layer 14 of the zeolite membrane 12 can be less than 8 or it can be greater than 8. The same applies to the maximum number of Rcan in / Lznz / B / YiAi member rings in the low density layer 13 of the zeolite membrane 12. The components of the grain boundary phases in the low-density layer 13 can be modified in various ways. For example, the grain boundary phases in the low-density layer 13 may consist solely of an amorphous phase. Alternatively, the grain boundary phases do not necessarily have to include an amorphous phase. The grain boundary phases may include crystals other than zeolite crystals, or they may not necessarily include crystals at all. The grain boundary phases do not necessarily have to be composed solely of an inorganic compound and may include an organic compound. The thickness of the zeolite membrane 12 is not limited to the ranges described above and can be modified in various ways. The thickness of the compact layer 14 can be greater than 50 times the thickness of the low-density layer 13, or it can be less than 0.05 times the thickness of the low-density layer 13. The average particle diameter of the zeolite crystals contained in the compact layer 14 and the low-density layer 13 can be modified in various ways. For example, the average particle diameter of the zeolite crystals contained in the compact layer 14 can be less than 0.1 pm or greater than 10 pm. The average particle diameter of the zeolite crystals contained in the compact layer 14 can also be more than 100 times the average particle diameter of the zeolite crystals contained in the low-density layer 13. Alternatively, the average particle diameter of the zeolite crystals contained in the compact layer 14 can be less than or equal to the average particle diameter of the zeolite crystals contained in the low-density layer 13. The zeolite crystalline phase content in the compact layer 14 may be less than 95% provided that it is greater than the zeolite crystalline phase content in the low-density layer 13. In the zeolite membrane 12, the zeolite crystals contained in the compact layer 14 can be zeolite crystals of a different type than the type of zeolite crystals contained in the low-density layer 13. The zeolite membrane composite 1 may further include a functional or protective membrane laminated onto the zeolite membrane 12, in addition to the support 11 and the zeolite membrane 12. Such a functional or protective membrane may be an inorganic membrane, such as a zeolite membrane, a silica membrane, or a carbon membrane, or it may be an organic membrane, such as a polyimide membrane or a silicone membrane. Furthermore, a substance that readily absorbs CO2 may be added to the functional or protective membrane laminated onto the zeolite membrane 12. In separator 2 and the separation method, substances other than those exemplified in the description above can be separated from a mixture of substances. Rcan in / Lznz / B / YiAi The constitutions of the preferred modalities described above and the variations may be appropriately combined provided there are no mutual inconsistencies. Although the invention has been shown and described in detail, the foregoing description is in all respects illustrative and not restrictive. It should therefore be understood that numerous modifications and variations may be devised without departing from the scope of the invention. Industrial applicability The zeolite membrane composite according to the present invention is, for example, applicable as a gas separation membrane, or it can be applicable in various fields using the zeolite as a separation membrane to separate a substance other than gases or as an absorbent membrane to absorb various substances. List of reference signs Zeolite membrane composite Medium Zeolite membrane Low-density layer Compact layer 131 Zeolite crystals (in low-density layer) 141 Zeolite crystals (in close-packed layer) Sil Stage aS13,S21 to S22 Rcan in / Lznz / B / YiAi

Claims

1. A zeolite membrane composite comprising: a porous support; and a zeolite membrane formed on said support; wherein said zeolite membrane includes: a low-density layer coating said support; and a compact layer coating said low-density layer and having a higher content of a crystalline zeolite phase than said low-density layer.

2. The zeolite membrane composite according to claim 1, wherein said compact layer contains zeolite crystals having an average particle diameter greater than the average particle diameter of the zeolite crystals contained in said low-density layer.

3. The zeolite membrane composite according to claim 2, wherein the average particle diameter of the zeolite crystals contained in said compact layer is 100 times or less the average particle diameter of the zeolite crystals contained in said low-density layer.

4. The zeolite membrane composite according to any of claims 1 to 3, wherein said compact layer contains zeolite crystals with an average particle diameter greater than or equal to 0.1 pm and less than or equal to 10 pm.

5. The zeolite membrane composite according to any of claims 1 to 4, wherein said compact layer has a thickness that is 0.05 times or more and 50 times or less than the thickness of said low-density layer.

6. The zeolite membrane composite according to any of claims 1 to 5, wherein the crystalline phase content of zeolite in said compact layer is greater than or equal to 95%, and a crystalline phase content of zeolite in said low-density layer is greater than or equal to 5% and less than 95%.

7. The zeolite membrane composite according to any of claims 1 to 6, wherein said low-density layer has a grain boundary phase formed by an inorganic compound.

8. The zeolite membrane composite according to any of claims 1 to 7, wherein said low-density layer has a grain boundary phase that includes an amorphous phase.

9. The zeolite membrane composite according to any of claims 1 to 8, wherein said compact layer and said low-density layer contain zeolite crystals of the same type.

10. The zeolite membrane composite according to any of claims 1 to 9, wherein said compact layer contains zeolite crystals having a maximum number of 8-membered rings.

11. A method for producing a zeolite membrane composite, comprising: a) preparing seed crystals; b) depositing said seed crystals onto a porous support to form a seed crystal laminate on said support, said seed crystal laminate including a lamination of two or more layers of said seed crystals; and c) immersing said support in a feedstock solution to grow a zeolite from said seed crystal laminate by hydrothermal synthesis and form a zeolite membrane on said support, wherein said zeolite membrane includes: a low-density layer coating said support; and a compact layer coating said low-density layer and having a higher zeolite crystalline phase content than said low-density layer.

12. The method for producing the zeolite membrane compound according to claim 11, wherein a molar ratio of a structure-directing agent to water in said raw material solution is less than or equal to 0.

01.

13. A separation method comprising: a) preparing the zeolite membrane compound according to any of claims 1 to 10; and b) supplying a mixture of substances including a plurality of gas or liquid types to said zeolite membrane compound and causing a substance with high permeability in said mixture of substances to permeate through said zeolite membrane compound to separate said substance with high permeability from other substances.

14. The separation method according to claim 13, wherein said mixture of substances includes at least one of the substances selected from a group that Rcan in / Lznz / B / YiAi consists of hydrogen, helium, nitrogen, oxygen, water, steam, carbon monoxide, carbon dioxide, nitrogen oxide, ammonia, sulfur oxide, hydrogen sulfide, sulfur fluoride, mercury, arsenic, hydrogen cyanide, carbonyl sulfide, C1 to C8 hydrocarbons, organic acid, alcohol, mercaptans, ester, ether, ketone, and aldehyde.