Abstract:
The present invention relates to a water-selective adsorbent and a method for producing the water-selective adsorbent. The water-selective adsorbent includes: a water-selective adsorbent main body that includes a porous body having a plurality of pores and at least one kind of deliquescent substance contained (supported) within the pores of the porous body; and a moisture-permeable film that is formed on the surface of the water-selective adsorbent main body so as to block up at least the pores. Consequently, loss (outflow) of the deliquescent substance such as lithium chloride can be suppressed even if adsorption and desorption of moisture are repeated, so that decrease in the moisture adsorption ability can be suppressed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a Continuation of International Application No. PCT/JP2012/078931 filed on Nov. 8, 2012, which is based upon and claims the benefit of priority from Japanese Patent Application No. 2011-247329 filed on Nov. 11, 2011, the contents all of which are incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates to a water-selective adsorbent, which is capable of adsorbing water from the atmosphere and releasing (desorbing) the adsorbed water, and further relates to a method for producing the same. 
       BACKGROUND ART 
       [0003]    Conventional water adsorbents for adsorbing water from the atmosphere (i.e., desiccant materials) include porous zeolites and silica gels. Such porous solids utilize only surface adsorption and capillary condensation phenomena for adsorbing water. 
         [0004]    A water adsorbent, which uses a deliquescent substance such as LiCl (lithium chloride) in pores of a porous solid, has been proposed (see, e.g., Japanese Patent No. 4119668 and Japanese Laid-Open Patent Publication No. 2009-226265). In the water adsorbent described in these patent documents, for example, the deliquescent substance deliquesces due to water vapor from the atmosphere, and is condensed into an LiCl solution. Therefore, the water adsorption capacity of the water adsorbent described in these patent documents is 3 to 4 times larger than the water adsorption capacity of the above-described conventional water adsorbents that make use of a porous solid. 
       SUMMARY OF INVENTION 
       [0005]    However, in the water adsorbent described in Japanese Patent No. 4119668 and Japanese Laid-Open Patent Publication No.  2009 - 226265 , etc., in a case in which water adsorption and desorption are repeatedly carried out, the deliquescent substance such as lithium chloride is lost (leaked), such that the adsorption ability of the deliquescent substance is reduced disadvantageously. 
         [0006]    In view of the above problem, an object of the present invention is to provide a water-selective adsorbent and a method for producing the same, which are capable of preventing loss (leakage) of a deliquescent substance such as lithium chloride and a reduction in the water adsorption ability, even in the case of repeated water adsorption and desorption. 
         [0007]    [1] According to a first aspect of the present invention, a water-selective adsorbent is provided comprising a water-selective adsorbent body and a water-permeable membrane. The water-selective adsorbent body contains a porous solid having a large number of pores, and at least one deliquescent substance located in the pores of a surface of the water-selective adsorbent body in such a manner that at least the pores thereof are closed. 
         [0008]    In this structure, even in a case in which water adsorption and desorption are carried out repeatedly, loss (leakage) of the deliquescent substance such as lithium chloride can be prevented, and a reduction in the water adsorption ability can be prevented. 
         [0009]    [2] In the first aspect, the water-permeable membrane preferably has a thickness of 30 to 505 μm. The thickness more preferably is 30 to 300 μm, further preferably, is 45 to 170 μm, particularly preferably, is 80 to 130 μm, and more particularly preferably, is 90 to 110 μm. 
         [0010]    [3] In the first aspect, the pores in the water-selective adsorbent body preferably have an average diameter of 2.0 to 10.6 μm. The average diameter more preferably is 3.4 to 9 μm, and further preferably, is 5 to 7.6 μm. 
         [0011]    [4] In the first aspect, the water-permeable membrane preferably includes a material containing a binding agent and a water-selective permeating agent capable of selectively permeating water molecules. 
         [0012]    [5] In the first aspect, the water-selective permeating agent preferably contains one of A-type zeolite, X-type zeolite, Y-type zeolite, P-type zeolite, mordenite, clinoptilolite, allophane, imogolite, and activated carbon. 
         [0013]    [6] in aspect [4] or [5], the binding agent may contain a combination of a first binder and a second binder, wherein the first binder is water glass, and the second binder is a binder corresponding to the water glass. 
         [0014]    [7] In aspect [6], the combination of the first binder and the second binder may be a combination in which the first binder Li 4 SiO 4  and the second binder is LiOH, a combination in which the first binder is Na 2 SiO 3  and the second binder is NaOH, a combination in which the first binder is K 2 SiO 3  and the second binder is KOH, a combination in which the first binder is CaSiO 3 , Ca 2 SiO 4 , or Ca 3 SiO 5  and the second binder is Ca(OH) 2 , or a combination in which the first binder is Mg 2 Si 3 O 8  and the second binder is Mg(OH) 2 . 
         [0015]    [8] In the first aspect, the porous solid preferably contains one of silica gel, calcined alumina, calcined sepiolite, calcined halloysite, calcined cellophane, calcined palygorskite, and calcined cordierite. 
         [0016]    [9] In the first aspect, the deliquescent substance preferably contains one of calcium chloride, magnesium chloride, lithium chloride, sodium chloride, potassium chloride, and lithium bromide. 
         [0017]    [10] According to a second aspect of the present invention, a method is provided for producing a water-selective adsorbent. The method comprises a body preparation step of preparing a water-selective adsorbent body containing a porous solid having a large number of pores, and at least one deliquescent substance located in the pores of the porous solid, and a membrane formation step of forming a water-permeable membrane on a surface of the water-selective adsorbent body in such a manner that at least the pores thereof are closed. 
         [0018]    [11] In the second aspect, the membrane formation step may further comprise a first drying step of drying the water-selective adsorbent body, a coating step of coating the water-selective adsorbent body with a material containing a binding agent and a water-selective permeating agent capable of selectively permeating water molecules, and a second drying step of drying the water-selective adsorbent body which is coated with the material. 
         [0019]    [12] In aspect [11], the binding agent may contain a combination of a first binder and a second binder, and the coating step may further comprise a gel preparation step of preparing a gel containing the water-selective permeating agent and the first binder, a first immersion step of immersing (including dip-coating) the water-selective adsorbent body in the gel, and a second immersion step immersing (including dip-coating) the water-selective adsorbent body in a solution of the second hinder. 
         [0020]    [13] In aspect [12], the water-selective permeating agent preferably contains one of A-type zeolite, X-type zeolite, Y-type zeolite, P-type zeolite, mordenite, clinoptilolite, cellophane, imogolite, and activated carbon. 
         [0021]    [14] In aspect [12], the first binder may be water glass, and the second binder may be a binder corresponding to the water glass. 
         [0022]    [15] In aspect [14], the combination of the first binder and the second binder may be a combination in which the first binder is Li 4 SiO 4  and the second binder is LiOH, a combination in which the first binder is Na 2 SiO 3  and the second binder is NaOH, a combination in which the first binder is K 2 SiO 3  and the second binder is KOH, a combination in which the first binder is CaSiO 3 , Ca 2 SiO 4 , or Ca 3 SiO 5  and the second binder is Ca(OH) 2 , or a combination in which the first binder is Mg 2 Si 3 O 5  and the second binder is Mg(OH) 2 . 
         [0023]    [16] In the second aspect, the porous solid preferably contains one of silica gel, calcined alumina, calcined sepiolite, calcined halloysite, calcined allophane, calcined palygorskite, and calcined cordierite. 
       ADVANTAGE OF INVENTION 
       [0024]    As described above, in the water-selective adsorbent and the production method according to the present invention, even in a case in which water adsorption and desorption are carried out repeatedly, loss (leakage) of the deliquescent substance such as lithium chloride can be prevented, and a reduction in the water adsorption ability can be prevented. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0025]      FIG. 1A  is a cross-sectional view of the structure of a water-selective adsorbent according to an embodiment, and 
           [0026]      FIG. 1B  is a cross-sectional view of the water-selective adsorbent in which water is adsorbed; 
           [0027]      FIG. 2  is a process chart of a method for producing the water-selective adsorbent according to the embodiment; 
           [0028]      FIG. 3  is a process chart of a body preparation step S 1  shown in  FIG. 2 ; 
           [0029]      FIG. 4  is a process chart of a membrane formation step S 2  shown in  FIG. 2 ; 
           [0030]      FIG. 5  is a graph illustrating adsorbed water amounts, which vary with average pore diameters of water-selective adsorbent bodies according to samples 1 through 4; 
           [0031]      FIG. 6  is a graph illustrating differences, expressed in terms of water mass ratios (wt %), between initial adsorbed water amounts and adsorbed water amounts after an endurance test according to an Example and a Comparative Example; 
           [0032]      FIG. 7  is a graph illustrating differences, expressed in terms of densities, between initial adsorbed water amounts and adsorbed water amounts after an endurance test according to the Example and the Comparative Example; 
           [0033]      FIG. 8  is a graph illustrating differences, expressed in terms of water mass ratios (wt %), between initial adsorbed water amounts and adsorbed water amounts after an endurance test, which vary with thicknesses of water-permeable membranes, in the Comparative Example and in Examples 1 to 5; and 
           [0034]      FIG. 9  is a graph illustrating differences, expressed in terms of densities, between initial adsorbed water amounts and adsorbed water amounts after the endurance test, which vary with thicknesses of the water-permeable membranes, in the Comparative Example and in Examples 1 to 5. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0035]    An embodiment of a water-selective adsorbent and a production method according to the present invention will be described below with reference to  FIGS. 1 to 9 . 
         [0036]    As shown in  FIG. 1A , a water-selective adsorbent  10  according to the present embodiment contains a water-selective adsorbent body  18  and a water-permeable membrane  20 . The water-selective adsorbent body  18  contains a porous solid  14  having a large number of pores  12 , and at least one deliquescent substance  16  located (supported) in the pores  12  of the porous solid  14 . The water-permeable membrane  20  is formed on a surface of the water-selective adsorbent body  18  in such a manner that at least the pores  12  thereof are closed. In the example of the present embodiment, the water-permeable membrane  20  is formed over the entire surface of the water-selective adsorbent body  18  including the openings of the pores  12 . 
         [0037]    During a water adsorption and desorption cycle in the water-selective adsorbent  10 , in an initial water adsorption stage, as shown in  FIG. 1A , the deliquescent substance  16  is supported on the pores  12  of the porous solid  14 . Then, water in the air is absorbed by the deliquescent substance  16 , and wall surfaces of the pores  12  in the porous solid  14  become wetted with water. In a final stage, as shown in  FIG. 1B , the pores  12  in the porous solid  14  become filled with water  22 . Thereafter, the water  22  is desorbed, whereby the water-selective adsorbent  10  is returned to the initial state shown in  FIG. 1A . The aforementioned adsorption and desorption cycle may be repeated in this manner. Incidentally, to facilitate understanding, the sizes of the pores  12 , the deliquescent substance  16 , and the like in the water-selective adsorbent  10  are shown in an exaggerated form in  FIGS. 1A and 1B . 
         [0038]    The porous solid  14  contains one of silica gel, calcined alumina, calcined sepiolite, calcined halloysite, calcined cellophane, calcined palygorskite, and calcined cordierite. 
         [0039]    The deliquescent substance  16  contains one of calcium chloride, magnesium chloride, lithium chloride, sodium chloride, potassium chloride, and lithium bromide. 
         [0040]    The water-permeable membrane  20  includes a material containing a binding agent, and a water-selective permeating agent that is capable of selectively permeating water molecules. 
         [0041]    Specifically, the water-selective permeating agent contains one of A-type zeolite, X-type zeolite, Y-type zeolite, P-type zeolite, mordenite, clinoptilolite, allophane, imogolite, and activated carbon. 
         [0042]    Meanwhile, the binding agent contains a combination of a first binder and a second binder, in which the first binder is water glass, and the second binder is a binder corresponding to the water glass. Examples of such combinations include the following:
   (a) the first binder is Li 4 SiO 4  and the second binder is LiOH;   (b) the first binder is Na 2 SiO 3  and the second binder is NaOH;   (c) the first binder is K 2 SiO 3  and the second binder is KOH;   (d) the first binder is CaSiO 3 , Ca 2 SiO 4 , or Ca 3 SiO 5  and the second binder Ca(OH) 2 ; and   (e) the first binder is Mg 2 Si 3 O 3  and the second binder is Mg(OH) 2 .   
 
         [0048]    It should be understood that the binding agent may contain two or more of such combinations of (a) to (e), such as a combination of (a) and (b) or a combination of (a), (b), and (c). 
         [0049]    In addition to the aforementioned combinations, the binding agent may contain Ca (calcium) cement, Mg (magnesium) cement, gypsum, magnesium phosphate, etc. 
         [0050]    The water-selective adsorbent  10  according to the present embodiment contains the water-selective adsorbent body  18  and the water-permeable membrane  20 . The water-selective adsorbent body  18  contains the porous solid  14  having a large number of pores  12 , and at least one deliquescent substance  16  located in the pores  12  of the porous solid  14 . The water-permeable membrane  20  is formed on the surface of the water-selective adsorbent body  18  in such a manner that at least the pores  12  thereof are closed. Consequently, even in a case in which water adsorption and desorption are carried out repeatedly, loss (leakage) of the deliquescent substance  16  such as lithium chloride can be prevented, and a reduction in the water adsorption ability can be prevented. 
         [0051]    The thickness of the water-permeable membrane  20  may be 30 to 505 μm, preferably, is 30 to 300 μm, more preferably, is 45 to 170 μm, further preferably, 80 to 130 μm, and particularly preferably, is 90 to 110 μm. If the thickness of the water-permeable membrane  20  is excessively small, the water-permeable membrane  20  may easily become broken during adsorption and desorption of water, and the deliquescent substance  16  may be lost (leaked). On the other hand, if the thickness of the water-permeable membrane  20  is excessively large, water cannot readily be introduced into the pores  12  of the water-selective adsorbent body  18 , leading to a reduction in the amount of water that is adsorbed or desorbed per unit time. 
         [0052]    The average diameter of the pores  12  in the water-selective adsorbent body  16  preferably is 2.0 to 10.6 μm, more preferably, is 3.4 to 9 μm, and further preferably, is 5 to 7.6 μm. If the average diameter of the pores  12  is excessively small, the deliquescent substance  16  cannot be distributed sufficiently in the pores  12 . On the other hand, if the average diameter of the pores  12  is excessively large, the deliquescent substance  16 , which is located in the pores  12 , may leak out from the pores, for example, during a process of washing the water-selective adsorbent body  18  while carrying out the production method. For example, the average diameter of the pores  12  can be measured using a mercury intrusion method (mercury intrusion porosimeter). 
         [0053]    A method for producing the water-selective adsorbent  10  according to the present embodiment will be described below with reference to  FIGS. 2 to 4 . 
         [0054]    As shown in  FIG. 2 , the production method includes a body preparation step Si of preparing the water-selective adsorbent body  18 , which contains the porous solid  14  having a large number of pores  12 , and at least one deliquescent substance  16  located in the pores  12  of the porous solid  14 . The production method further includes a membrane formation step S 2  of forming the water-permeable membrane  20  on the surface of the water-selective adsorbent body  18  in such a manner that at least the pores  12  thereof are closed. 
         [0055]    During the body preparation step S 1 , in step S 11  of  FIG. 3 , for example, a porous material is crushed to prepare the porous solid  14  having a predetermined size (e.g., 3 mm square to 50 mm square). Then, in step S 12 , the porous solid  14  is introduced to an aqueous solution containing  30  to 50 wt % of the deliquescent substance  16 , and the solution is stirred for 10 to 15 hours. 
         [0056]    In step S 13 , the porous solid  14  in the aqueous solution of the deliquescent substance  16  is subjected to a boiling treatment (at a temperature of 95° C. to 105° C. for 10 to 30 minutes). By a boiling treatment, the deliquescent substance  16  is brought into a supported condition on the pores  12  of the porous solid  14 . 
         [0057]    In step S 14 , after the boiling treatment, the porous solid  14  (with the deliquescent substance  16 ) is washed with flowing water. Then, in step S 15 , the resultant product is dried (at a temperature of 110° C. to 130° C. for 45 to 50 hours) order to obtain the water-selective adsorbent body  18 . 
         [0058]    During the membrane formation step S 2 , in step S 21  (first drying step) of  FIG. 4 , the water-selective adsorbent body  18  is dried (at a temperature of 110° C. to 130° C. for 22 to 26 hours). 
         [0059]    In steps S 22  to S 24  (coating step), the surface of the water-selective adsorbent body  18  is coated with a material containing the binding agent, and the water-selective permeating agent that is capable of selectively permeating water molecules. 
         [0060]    More specifically, in step  322  (gel preparation step), a gel containing the water-selective permeating agent and the first binder is prepared. Then, in step S 23  (first immersion step), the water-selective adsorbent body  18  is immersed in the gel. During the immersion treatment, the water-selective adsorbent body  18  may be immersed in the gel for several seconds to 20 minutes. Alternatively, the immersion treatment may be a dipping treatment, in which the water-selective adsorbent body  18  is immersed in the gel for several seconds and then pulled out of the gel over a period of 3 to 20 minutes. 
         [0061]    In step S 24  (second immersion step), the water-selective adsorbent body  18  is immersed in the second binder. During the immersion treatment, the water-selective adsorbent body  18  may be immersed in a solution of the second binder for several seconds to 20 minutes. Alternatively, the immersion treatment may be a dipping treatment, in which the water-selective adsorbent body  18  is immersed in the solution of the second binder for several seconds and then pulled out of the solution over a period of 3 to 20 minutes. 
         [0062]    After the above steps S 22  to S 24  (coating step) have been carried out, in step S 25  (second drying step), the water-selective adsorbent body  18 , which is coated with the material, is dried (at a temperature of 110° C. to 130° C. for 0.5 to 2 hours). 
         [0063]    The water-selective adsorbent  10  according to the present embodiment can be easily produced in the above-described manner. The water-selective adsorbent  10 , which produced in the foregoing manner, contains the water-selective adsorbent body  18  including the porous solid  14  having a large number of pores  12 , and at least one deliquescent substance  16 , which is located (supported) in the pores  12  of the porous solid  14 . The water-selective adsorbent  10  further contains the water-permeable membrane  20 , which is formed on the surface of the water-selective adsorbent body  18  in such a manner that at least the pores  12  thereof are closed. 
       EXAMPLES 
     First Example 
       [0064]    In samples 1 to 4, the adsorbed water amounts, which varied with the average diameters of the pores  12  in the water-selective adsorbent bodies  18 , were evaluated. The adsorbed water amounts were compared in an environment most suitable for water adsorption, i.e., under a relative humidity of 90%. 
       (Sample 1) 
       [0065]    A porous solid of a calcined alumina (a porous alumina ceramic solid), which had pores  12  having an average diameter of 0.5 μm, was crushed to prepare a porous solid  14  having a size of 5 mm square and a weight of 5 g. Then, the 5 mm square porous solid  14  was introduced into an aqueous solution containing 40% by mass (wt %) of lithium chloride (LiCl), stirred for 12 hours, and boiled (at 100° C. for 20 minutes) in the 40 wt % aqueous solution of lithium chloride (LiCl). The porous solid  14  was washed with flowing water and dried (at 120° C. for 48 hours) in order to prepare the water-selective adsorbent body  18  of sample 1. 
       (Samples 2 to 4) 
       [0066]    The water-selective adsorbent bodies  18  of samples 2, 3, and 4 were prepared in the same manner as the water-selective adsorbent bodies  18  of sample  1 , except that the pores  12  in the porous solids  14  had diameters of 3.4 μm, 8.9 μm, and 13.8 μm, respectively. 
       (Evaluation) 
       [0067]    After water was adsorbed at a relative humidity of 90% under a saturated water vapor pressure at 25° C. for 48 hours, the adsorbed water amount of each of samples 1 to 4 was measured. The mass ratio of the adsorbed water to the water-selective adsorbent body  18  was obtained as the adsorbed water amount. Thus, the mass percentage (wt %) of the adsorbed water was obtained based on 100% by mass of the water-selective adsorbent body  18 . The evaluation results are shown in  FIG. 5 . 
         [0068]    As made clear from the evaluation results, the average diameter of the pores  12  in the water-selective adsorbent body  18  preferably is 2.0 to 10.6 μm, more preferably, is 3.4 to 9 μm, and further preferably, is 5 to 7.6 μm. 
       Second Example 
       [0069]    In the Example and the Comparative Example, a difference between the initial adsorbed water amount and the adsorbed water amount after an endurance test were evaluated. 
       Comparative Example 
       [0070]    A porous solid of calcined silica (a porous silica ceramic solid), which had pores  12  with an average diameter of 5.1 μm, was crushed in order to prepare a porous solid  14  having a size of 5 mm square. The 5 mm square porous solid  14  was introduced into an aqueous solution containing 40% by mass (wt %) of lithium chloride (LiCl), stirred for 12 hours, and boiled (at 100° C. for 20 minutes) in a 40 wt % aqueous solution of lithium chloride (LiCl). The porous solid  14  was washed with flowing water and dried (at 120° C. for 43 hours) in order to prepare the water-selective adsorbent of the Comparative Example. 
       Example 
       [0071]    In the same manner as the Comparative Example, a porous solid of calcined silica (a porous silica ceramic solid), which had pores  12  with an average diameter of 5.1, was crushed in order to prepare a porous solid  14  having a size of 5 mm square. Then, the 5 mm square porous solid  14  was introduced into an aqueous solution containing 40% by mass (wt %) of lithium chloride (LiCl), stirred for 12 hours, and boiled (at 100° C. for 20 minutes) in the 40 wt % aqueous solution of lithium chloride (LiCl). The porous solid  14  was washed with flowing water and dried (at 120° C. for 48 hours) in order to prepare the water-selective adsorbent body  18  of the Example. 
         [0072]    Thereafter, the water-selective adsorbent body  18  was dried (at 120° C. for 24 hours) Meanwhile, 20 ml of lithium silicate (Li 4 SiO 4 ) and 15 g of A-type zeolite (molecular sieves 4A available from Tosoh Corporation) were mixed and stirred for 30 minutes in order to prepare a gel. The water-selective adsorbent body  18  was subjected to an immersion treatment in the gel (an immersion treatment for several seconds to 10 minutes or a dip coating treatment). The water-selective adsorbent body  18  was taken out from the gel and subjected to an immersion treatment in a solution containing 2 to 3 wt % of lithium hydroxide (LiOH) (an immersion treatment for several seconds to 10 minutes or a dip coating treatment). Thereafter, the water-selective adsorbent body  18  was separated from the lithium hydroxide (LiOH) solution and dried (at 120° C. for 0.5 to 1 hour) in order to produce the water-selective adsorbent of the Example having a 100 μm thick water-permeable membrane  20  formed on the water-selective adsorbent body  18 . 
       (Measurement of Initial Adsorbed Water Amount) 
       [0073]    The initial adsorbed water amount of the water-selective adsorbent of the Comparative Example was measured while increasing the relative humidity by 5% within a range of 5% to 90%. 
         [0074]    In addition, the initial adsorbed water amount of the water-selective adsorbent of the Example was measured while increasing the relative humidity by 5% within a range of 5% to 90%. 
         [0075]    The mass ratios of the adsorbed water to the water-selective adsorbent, and densities, which were converted from the mass ratios, were obtained as initial adsorbed water amounts of the Comparative Example and the Example. Thus, based on 100% by mass of the water-selective adsorbent, the mass percentage (wt %) of the adsorbed water was obtained as the mass ratio. The density was calculated using the formula (water mass ratio (percentage)/100)×(weight of water-selective adsorbent/volume of water-selective adsorbent). Evaluation results of the initial adsorbed water amounts of the Comparative Example are shown by the plotted black triangles ▴, and evaluation results of the initial adsorbed water amounts of the Example are shown by the plotted black circles  in  FIGS. 6 and 7 . 
       (Endurance Test) 
       [0076]    Each of the water-selective adsorbents of the Comparative Example and the Example was stored at 25° C. for 48 hours (in a water adsorption process), and then was stored at 120° C. for 43 hours (in a water desorption process) under a saturated water vapor pressure. The water adsorption and desorption cycle was repeated 10 times. 
       (Measurement of Adsorbed Water Amount After Endurance Test) 
       [0077]    The adsorbed water amounts of the water-selective adsorbent of the Comparative Example after the endurance test were measured while increasing the relative humidity by 5% within a range of 5% to 90%. 
         [0078]    In addition, the adsorbed water amounts of the water-selective adsorbent of the Example after the endurance test were measured while increasing the relative humidity by 5% within a range of 5% to 90%. 
         [0079]    The adsorbed water amounts (water mass ratios and densities) of the Comparative Example and the Example after the endurance test were obtained in the same manner as the initial adsorbed water amounts. Evaluation results of the adsorbed water amounts after the endurance test of the Comparative Example are shown by the plotted white triangles Δ, whereas evaluation results of the adsorbed water amounts after the endurance test of the Example are shown by the plotted white circles ◯ in  FIGS. 6 and 7 . 
       (Evaluation) 
       [0080]    As shown in  FIGS. 6 and 7 , in the Comparative Example, the adsorbed water amounts were reduced by the endurance test to about 10% of the initial amounts in a relative humidity range of 0% to 10%, to about 12.5% to 39% of the initial amounts in a relative humidity range of 20% to 85%, and to about 40% of the initial amounts at a relative humidity of 90%. 
         [0081]    In contrast, in the Example, the adsorbed water amounts were reduced by the endurance test to about 71% and 78% of the initial amounts at relative humidity of 5% and 10%, respectively, and to about 82% to 96% of the initial amounts at other relative humidity. On average, the adsorbed water amounts were reduced to about 90% of the initial amounts. 
       (Consideration) 
       [0082]    It is presumed that the amount of water reduction observed after the endurance test in the Second Example was caused by leakage of the deliquescent substance  16  (LiCl) from the pores  12  in the water-selective adsorbent body  18 . As made clear from the evaluation results of the Example, leakage of the deliquescent substance  16  can be prevented by the water-permeable membrane  20  that is formed on the surface of the water-selective adsorbent body  18 . 
       Third Example 
       [0083]    In the Comparative Example and in Examples 1 to 5, differences between the initial adsorbed water amounts and the adsorbed water amounts after an endurance test were evaluated using various thicknesses of the water-permeable membranes. 
       Comparative Example 
       [0084]    The water-selective adsorbent of the Comparative Example was produced in the same manner as in the Second Example. Thus, the water-permeable membrane was not formed on the surface of the water-selective adsorbent body. 
       Examples 1 to 5 
       [0085]    The water-selective adsorbents of Examples 1, 2, 3, 4, and 5 were produced in the same manner as in the Second Example, except that the water-permeable membranes had thicknesses of 45 μm, 105 μm, 220 m, 390 μm, and 505 μm, respectively. 
       (Measurement of Initial Adsorbed Water Amount) 
       [0086]    The initial adsorbed water amount of the water-selective adsorbent of the Comparative Example was measured at a relative humidity of 90%. 
         [0087]    In addition, the initial adsorbed water amounts of the water-selective adsorbents of Examples 1 to 5 were measured at a relative humidity of 90%. 
         [0088]    Mass ratios of the adsorbed water to the water-selective adsorbent, and densities, which were converted from the mass ratios, were obtained as the initial adsorbed water amounts of the Comparative Example and Examples 1 to 5, in the same manner as in the Second Example. Evaluation results of the initial adsorbed water amounts of the Comparative Example and Examples 1 to 5 are shown by the plotted black triangles ▴ in  FIGS. 8 and 9 . 
       (Endurance Test) 
       [0089]    Each of the water-selective adsorbents of the Comparative Example and Examples 1 to 5 was stored at 25° C. for 48 hours (in a water adsorption process) and then was stored at 120° C. for 48 hours (in a water desorption process) under a saturated water vapor pressure. The water adsorption and desorption cycle was repeated 10 times. 
       (Measurement of Adsorbed Water Amount After Endurance Test) 
       [0090]    The adsorbed water amount of the water-selective adsorbent of the Comparative Example after the endurance test was measured at a relative humidity of 90%. 
         [0091]    In addition, the adsorbed water amounts of the water-selective adsorbents of Examples 1 to 5 after the endurance test were measured at a relative humidity of 90%. 
         [0092]    The adsorbed water amounts (water mass ratios and densities) of the Comparative Example and Examples 1 to 5 after the endurance test were obtained in the same manner as the initial adsorbed water amounts. Evaluation results of the adsorbed water amounts after the endurance test of the Comparative Example and Examples 1 to 5 are shown by the plotted white triangles Δ in  FIGS. 8 and 9 . 
       (Evaluation) 
       [0093]    As shown in  FIG. 8 , the adsorbed water amount after the endurance test of the Comparative Example was 89 wt %, while the adsorbed water amounts after the endurance test of Examples 1 to 3 were more than 150 wt %. The adsorbed water amount after the endurance test of Example 2 was twice as large as that of the Comparative Example. Also, in the case of being converted into densities, similar results were obtained. As shown in  FIG. 9 , the density of the Comparative Example was 0.99 g/cm 3 , while the densities of Examples 1 to 4 were greater than 1.5 g/cm 3 . The adsorbed water amounts of Examples 1 and 3 were approximately 2 times greater, the adsorbed water amount of Example 2 was approximately 2.5 times greater, and the adsorbed water amount of Example 4 was approximately 1.5 times greater than the adsorbed water amount of the Comparative Example. 
         [0094]    Furthermore, as shown in  FIGS. 8 and 9 , in the Comparative Example, the adsorbed water amount was reduced by the endurance test to about 40% of the initial amount. In contrast, in Examples 1 to 5, the adsorbed water amounts were reduced only slightly by the endurance test to about 82% to 96% of the initial amounts. 
         [0095]    Although the adsorbed water amount after the endurance test of Example 5 was approximately equal to that of the Comparative Example, the adsorbed water amount of Example 5 was reduced only slightly by the endurance test to about 71% of the initial amount. Thus, Example 5 was superior to the Comparative Example in terms of maintaining the water amount. 
       (Consideration) 
       [0096]    As made clear from the Third Example, in view of preventing leakage of the deliquescent substance  16 , the thickness of the water-permeable membrane should be 30 to 505 μm. The thickness of the water-permeable membrane preferably is 30 to 300 μm, more preferably, is 45 to 170 μm, even more preferably, is 80 to 130 μm, and particularly preferably, is 90 to 110 μm. 
         [0097]    It should be understood that the water-selective adsorbent and the production method of the present invention are not limited to the above embodiment, and various changes and modifications may be made to the embodiment without departing from the scope of the invention.