Method for removing a harmful substance selected from the group consisting of ethylene, mercaptans, amines, and aldehydes
The present invention relates to a remover for harmful substances such as bad-smelling substances, substances of irritating smell, growth-promoting substances for garden plants, and the like, a process for producing the remover, and a method for removing a harmful substance by the use of the remover. 
     Bad smells and irritating smells generated in industries, smells emitted by wastes generated at homes and restaurants, and bad smells and irritating smells generated in cooking have become a bigger problem in recent years. These smells are composed mainly of hydrogen sulfide, ammonia, mercaptans, amines, aldehydes, etc. 
     Meanwhile, supply of vegetables, fruits and flowers in fresh states or at times other than their natural harvest times is being increasingly required. In order to keep these garden plants in fresh states, it is desired to develop a method for effectively removing growth-promoting substances (plant growth hormones) such as ethylene and the like. 
     As the conventional methods for removing these harmful substances, there are known, for example, (1) a method wherein a harmful substance is adsorbed by a porous substance such as active carbon, alumina, silica gel, zeolite or the like, (2) a method wherein an acidic (or basic) harmful substance is neutralized with a basic (or acidic) substance, and (3) a method wherein a harmful substance is chemically oxidized with potassium permanganate, chlorine dioxide or the like. 
     These conventional methods, however, are not necessarily satisfactory. That is, in the method (1), the adsorbability for basic substances (ammonia, etc.) and ethylene is not sufficient; in the method (2), the harmful substances removed are restricted depending upon the basic (or acidic) substance used; in the method (3), the removal rate is low. 
     The object of the present invention is to provide a method for harmful substance removal which is free from the above-mentioned drawbacks and which can remove various harmful substances simultaneously and efficiently. The other object of the present invention is to provide a novel harmful substance remover used in the above method and a process for producing the remover. 
     The present inventors made study in order to achieve the above objects and, as a result, found that a solid product obtained by hydrolyzing a metal alkoxide has excellent removability for harmful substances. 
     According to the present invention, there are provided a harmful substance remover which is a solid product having a specific surface area of 100 m 2  /g or more obtained by hydrolyzing a metal alkoxide in the presence or absence of a noble metal; a process for producing the above harmful substance remover, which comprises hydrolyzing a metal alkoxide and drying the resulting solid product at a temperature lower than 300° C.; and a method for removing a harmful substance by the use of the above harmful substance remover. 
     The metal alkoxide used in the present invention as a starting material for harmful substance remover (hereinafter referred to simply as remover, in some cases) has no particular restriction as to its composition or production method, as long as it is an alcohol ester of a metal. While it is generally known that metal alkoxides can be produced from most of the metals listed in the periodic table [&#34;Kogyo Zairyo (Industrial Materials)&#34;, Vol. 29, No. 5, pp. 85-89, 1989, published by the Nikkan Kogyo Shimbun, Ltd.], preferred in the present invention are alkoxides of the elements of groups IIa, IIb, IIIa, IIIb, IVa, IVb, Va, Vb and VIII of the short form of the periodic table, particularly alkoxides of aluminum, silicon, titanium, zinc and zirconium, in view of their chemical stability and availability. The alcohol component of the metal alkoxide is not particularly restricted, either; however alcohols of 1-4 carbon atoms are preferred in view of the hydrolyzability of metal alkoxide and the removability of alcohol generated in hydrolysis. 
     In the present invention, there may be used an alkoxide of two or more metals; or, there may be used two or more metal alkoxides in combination. 
     In the present invention, the method for hydrolysis of metal alkoxide is not particularly restricted. It can be effected by adding water to a metal alkoxide or by adding a metal alkoxide to water. In these methods, the metal alkoxide can be used as it is, or as a solution or dispersion in a solvent; however, the use as a solution in a solvent is preferable in order to prevent violent reaction and obtain a hydrolyzate of fine particles and uniform particle size. In the hydrolysis, water may be replaced by a mixture of water and an alcohol, and water or the water-alcohol mixture may be used in combination with a solvent. Further, water or the water-alcohol may be made acidic or basic, in advance, by the use of an acidic or basic substance in order to accelerate the hydrolysis. The solvent used above may be an inorganic compound or an organic compound; however, it is preferably the same alcohol as used in the synthesis of metal alkoxide, in view of the solubility of the metal alkoxide. In view of the removability in the drying step of the metal alkoxide or the handleability, there is preferred a solvent having a boiling point of preferably 200° C. or less, more preferably 150° C. or less. Specific examples of the solvent include alcohols such as methyl alcohol, ethyl alcohol, isopropyl alcohol, butyl alcohol, amyl alcohol and the like; aromatic hydrocarbons such as benzene, toluene, xylene and the like; and hydrocarbons of chain structure such as hexane, heptane and the like. 
     In the present invention, a harmful substance remover of high capability can be obtained by hydrolyzing a metal alkoxide in the presence of a noble metal. 
     In the present invention, the noble metal refers to a noble metal per se selected from gold, silver, copper and platinum group elements (ruthenium, rhodium, palladium, osmium, iridium and platinum), as well as to compounds containing these noble metals. Examples of said compounds include oxides, chlorides and complexes, and are not restricted to these. 
     The noble metal is allowed to be present in the reaction system at the time of hydrolysis of metal alkoxide. It is effected, for example, by (1) dissolving or dispersing, in advance, a noble metal in the solvent, water or water-alcohol mixture used in the hydrolysis, (2) mixing a noble metal with a metal alkoxide in advance and then hydrolyzing the metal alkoxide, or (3) effecting hydrolysis and simultaneously adding noble metal to the reaction system. Preferably, the noble metal is used in a solution in a solvent, to disperse the noble metal uniformly in the hydrolyzate of the metal alkoxide. 
     In the present invention, the amount of the noble metal used is not particularly restricted but is preferably 10% by weight or less based on the metal alkoxide. When the amount of the noble metal is too large, the noble metal does not uniformly mix with the hydrolyzate of the metal alkoxide, resulting in lower removability for harmful substances. 
     The hydrolyzate is then separated from the solvent or by-products by means of filtration, decantation, evaporation to dryness, or the like. In order to remove the solvent, the alcohol generated, etc. adhering to the hydrolyzate and thereby obtain improved removability for harmful substances, the separated hydrolyzate is generally dried at a temperature of lower than 300° C., preferably 60°-250° C. Drying may be effected at room temperature, but it takes a longer time. Drying at a temperature of 300° C. or more is not preferable because it brings about reduction in removability for harmful substances. The reason is not clear because the action mechanism of the harmful substance remover of the present invention is not yet clarified; however, the reason is presumed to be that drying at a temperature of 300° C. or more causes rapid reduction in specific surface area of hydrolyzate. The time required for drying is not particularly restricted and the completion timing of drying can be taken as a time when there is substantially no weight decrease. The drying time is generally 1-10 hours. Drying can be effected in air and need not be effected in an inert gas atmosphere. Further, drying can be effected in normal pressure or in reduced pressure. 
     The heating method for drying is not particularly restricted, but in practical application there is preferred heating by electricity, steam, hot water or electromagnetic wave because it provides a constant temperature uniformly and stably. 
     In the present invention, the solid product preferably has a specific surface area of 100 m 2  /g or more as measured by the BET method. The solid product, even when it has a specific surface area of less than 100 m 2  /g, can remove harmful substances although the removal rate is lower. 
     In the present invention, the solid product obtained by drying the hydrolyzate of a metal alkoxide at a temperature of less than 300° C. can be used as it is; it can also be used as a mixture with an adsorbent, or by being supported by a catalyst carrier (an inorganic porous material made of ceramic, glass or the like), a porous material made of an organic polymer, a woven cloth, an nonwoven cloth, a fiber, a paper or the like. Or, it is possible that an adsorbent, a porous material or the like be molded into a carrier of a honeycomb form, a sponge form or a more complex form and the solid product be supported on the carrier before or after said molding. Examples of the adsorbent include active carbon, natural zeolite, artificial zeolite, alumina, silica gel, allophane and clay. Examples of the catalyst carrier include silica, silica-alumina, silicon carbide, titania, zirconia, magnesia, cordierite and mullite. Examples of the organic polymer include homopolymers such as polyethylene, polypropylene, poly(vinyl chloride), polyester, polystyrene, poly(tetrafluoroethylene) and the like, and copolymers such as acrylonitrile-butadiene-styrene copolymer (ABS), acrylonitrile-methyl acrylate copolymer, ethylene-terafluoroethylene copolymer (ETFE), ethylene-vinyl acetate copolymer and the like. 
     When the solid product is used for removal of large amount(s) of harmful substance(s) or for removal of such substances in a short time, it is preferable that the solid product be filled into a reactor to form a fixed or fluidized bed, or be supported by a carrier to form a filter, in order to increase the efficiency of contact between the solid product and the harmful substance(s). In the reactor to be used as a fixed or fluidized bed, at least part of its outer wall is made of an adsorbent or porous material incapable of transmitting the solid product but capable of transmitting harmful substance(s) or a fluid containing the substance(s). When the solid product is used in a filter form, the filter is preferably such that obtained by processing an adsorbent, a porous material or the like into a honeycomb form, a sponge form or the like and allowing the resulting carrier to support the solid product. 
     The method of the present invention can be effected batchwise or continuously by introducing into a particular reactor harmful substance(s) or a gas containing said substance(s) (said gas in hereinafter referred to as &#34;gas to be treated&#34;, in some cases). 
     The harmful substance remover of the present invention can be repeatedly used by reclamation. The method for reclamation includes a method wherein a used remover is heated at a temperature of lower than 300° C., a method wherein a used remover is irradiated with an electromagnetic wave such as ultraviolet rays, visible light, infrared rays or the like, a method wherein a used remover is contacted with ozone, and a method wherein a used remover is irradiated with an ultrasonic wave. The method is not restricted to any particular one as long as it can restore the removability of the remover. 
     In the present invention, the efficiency of harmful substance removal can be enhanced by effecting the removal under the irradiation with ultraviolet rays. 
     The ultraviolet rays used in the present invention are preferably near-ultraviolet rays having a wavelength of 400-200 nm, but may contain vacuum ultraviolet having a wavelength of 200 nm of less. These ultraviolet rays can be generated by single or combination use of an ultrahigh pressure mercury lamp, a xenon lamp and a low pressure mercury lamp. The rays may be generated by using a light source improved so as to produce a light of desired wavelength distribution, by allowing a third component such as gallium, thallium or the like to be present in a discharge tube together with mercury and a rare gas. The ultraviolet rays may contain a light such as a visible light. 
     In the present invention, the site of the ultraviolet lamp provided is not particularly restricted as long as efficient irradiation of the solid product can be conducted. When the ultraviolet rays are used for irradiation of a fluidized or fixed bed, the lamp may be provided outside the reactor; however, the lamp is preferably provided inside the reactor in order to effect efficient irradiation. 
     Thus, the present invention, as compared with the prior art, enables simultaneous, rapid and efficient removal of harmful substances such as bad-smelling substances, substances of irritating smell, growth-promoting substances for garden plants, and the like. 
     Further, the method of the present invention is suitable for removal of high concentration(s) of harmful substance(s), because the solid product is effective even after the saturation with harmful substance(s). 
     The present invention is hereinafter described in more detail by way of Examples and Comparative Examples. However, the present invention is not restricted to them. In the Examples, parts and % are by weight unless otherwise specified. 
    
    
     EXAMPLE 1 
     Run Nos. 1-3 
     1 g of deionized water was slowly added dropwise to 6 g of each metal alkoxide shown in Table 1 which was being stirred, whereby hydrolysis was effected. (Each alkoxide was used as it was when it was a liquid, and after having being dissolved in 60 g of n-hexane when it was a solid.) The resulting precipitate was collected by filtration through a glass filter, subjected to predrying at 60° C. for 8 hours in a vacuum dryer, and dried at 200° C. for 1 hour at normal pressure to obtain a harmful substance remover in a powdery state. 
     1 g of the remover was placed in an 80-ml Erlenmeyer glass flask capable of transmitting ultraviolet rays. The flask inside was purged with an air containing ethylene of an initial concentration shown in Table 1. The flask was then sealed with a silicone rubber stopper. The change of ethylene concentration in the flask with time was traced by gas chromatography. The results are shown in Table 1. 
     COMPARATIVE EXAMPLE 1 
     Run Nos. 4-6 
     The same test as in Example 1 was conducted except that the temperature of drying at normal pressure was changed to 400° C. The results are shown in Table 1. 
     COMPARATIVE EXAMPLE 2 
     Run Nos. 7-9 
     The same evaluation as in Example 1 was conducted using each 1 g of three deodorants on the market, i.e. honeycomb-shaped active carbon, potassium permanganate and titanium dioxide. The results are shown in Table 1. 
     
                                           TABLE 1                                 
__________________________________________________________________________
                          Drying                                          
                               Specific                                   
                                     Initial ethylene                     
                                             Residual ethylene (%)        
       Metal alkoxide or  tempera-                                        
                               surface                                    
                                     concentra-                           
                                             After                        
                                                 After                    
                                                      After               
                                                          Half life       
Run No.                                                                   
       deodorant on the market                                            
                          ture area (m.sup.2 /g)                          
                                     tion (ppm)                           
                                             1 min                        
                                                 10 min                   
                                                      30                  
                                                          (min)           
__________________________________________________________________________
Example 1                                                                 
1      Titanium tetraisopropylate (2)                                     
                          200° C.                                  
                               335   93      44  19    1    1             
2      Zirconium tetrabutylate (2)                                        
                          &#34;    399   78      56  29    2    1             
3      Aluminum tri isopropylate (3)                                      
                          &#34;    220   106     47  18    5    1             
Comparative                                                               
Example 1                                                                 
4      Titanium tetraisopropylate (1)                                     
                          400° C.                                  
                               75    74      100 98   93  &gt;90             
5      Zirconium tetrabutylate (2)                                        
                          &#34;    83    77      100 97   94  &gt;90             
6      Aluminum tri isopropylate (3)                                      
                          &#34;    69    71      99  96   91  &gt;90             
Comparative                                                               
Example 2                                                                 
7      Honeycomb-shaped active carbon (4)                                 
                          --   --    111     81  42   38    8             
8      Potassium permanganate (5)                                         
                          --   --    113     100 99   95  &gt;90             
9      Titanium dioxide (6)                                               
                          --   51    70      100 100  90  &gt;90             
__________________________________________________________________________
 (1), (3), (5): Products of Wako Pure Chemical Industries, Ltd.           
 (2): A product of Nippon Soda Co., Ltd.                                  
 (4): A product of KYOCERA CORP.                                          
 (6): A product of NIPPON AEROSIL CO., LTD.                               
 
    
     As is clear from Table 1, the harmful substance removers of the present invention are far superior in ethylene removability to typical deodorants on the market. It is also clear that ethylene removability is lower when specific surface area is smaller than 100 m 2  /g. 
     EXAMPLE 2 
     Run Nos. 10-13 
     The same test as in Example 1 was conducted except that the deionized water was replaced by an deionized water solution containing 10% of a noble metal shown in Table 2. 
     The results are shown in Table 2. 
     As is clear from Table 2, the removers obtained from a product obtained by hydrolyzing a metal alkoxide in the presence of a noble metal compound, have a higher ethylene removability. 
     
                                           TABLE 2                                 
__________________________________________________________________________
                               Specific                                   
                                     Initial ethylene                     
                                             Residual ethylene (%)        
      Metal alkoxide or                                                   
                      Noble    surface                                    
                                     concentra-                           
                                             After                        
                                                 After                    
                                                      After               
                                                          Half life       
Run No.                                                                   
      deodorant on the market                                             
                      metal    area (m.sup.2 /g)                          
                                     tion (ppm)                           
                                             1 min                        
                                                 10 min                   
                                                      30                  
                                                          (min)           
__________________________________________________________________________
Example 2                                                                 
10    Titanium tetraisopropylate (1)                                      
                      Silver nitrate                                      
                               319   112     1.4 0.2  0.0 &lt;1              
11    Zirconium tetrabutylate (2)                                         
                      &#34;        307    90     3.1 0.7  0.0 &lt;1              
12    Aluminum tri isopropylate (3)                                       
                      &#34;        211   104     3.8 0.9  0.0 &lt;1              
13    Titanium tetraisopropylate (1)                                      
                      Palladium nitrate                                   
                               356   118     1.8 0.1  0.0 &lt;1              
__________________________________________________________________________
 (1), (2), (3): The same as in Table 1                                    
 
    
     EXAMPLE 3 
     Run Nos. 14-16 
     The same remover as used in Example 2 Run No. 10, prepared from titanium isopropylate and silver nitrate was measured for removability for each of harmful substances (shown in Table 3) present in air, in the same manner as in Example 1. 
     The results are shown in Table 3. It is clear from Table 3 that the harmful substance remover of the present invention is effective for removal of various harmful substances. 
     
                                           TABLE 3                                 
__________________________________________________________________________
                        Residual ethylene (%)                             
      Kind of harmful                                                     
               Initial concentra-                                         
                        After                                             
                            After                                         
                                After                                     
                                    Half life                             
Run No.                                                                   
      substance                                                           
               tion (ppm)                                                 
                        1 min                                             
                            10 min                                        
                                30 min                                    
                                    (min)                                 
__________________________________________________________________________
Example 3                                                                 
14    Methyl mercaptan                                                    
               100      2   0.5 0.0 &lt;1                                    
15    Trimethylamine                                                      
               123      0.0 0.0 0.0 &lt;1                                    
16    Formaldehyde                                                        
                92      11  5   0.0 &lt;1                                    
__________________________________________________________________________
 
    
     EXAMPLE 4 
     Run Nos. 17-20 
     9 g each of the titanium alkoxides shown in Table 4 was placed in an enameled vat of 15 cm x 15 cm x 2 cm (depth). Thereinto was immersed a nonwoven cloth with electrostatic charge of 10 cm×10 cm, and the cloth was impregnated with the titanium alkoxides. The impregnated nonwoven cloth was taken out and uniformly sprayed with deionized water by means of hand spraying. The resulting cloth was subjected to predrying at 60° C. for 8 hours in a vacuum dryer and then dried at 150° C. for 1 hour at normal pressure to obtain electrostatic nonwoven cloths each supporting a different harmful substance remover. 
     Two test pieces of 1 cm×5 cm were prepared from each of the above cloths. (The amount of harmful substance remover supported on these two test pieces was 0.8 g in total.) The two test pieces were placed in an 80-ml Erlenmeyer glass flask capable of transmitting ultraviolet rays. The flask inside was purged with an air containing ethylene of an initial concentration shown in Table 4. The flask was then sealed with a silicone rubber stopper. The flask was irradiated with ultraviolet rays emitted from an ultrahigh pressure mercury lamp (illiminance=10 mW/cm 2 , main wavelength=365 nm) placed at a distance of 20 cm from the flask. The change of ethylene concentration in the flask after the start of irradiation was traced by gas chromatography. The results are shown in Table 4. 
     The same test as above was conducted except that no irradiation with ultraviolet rays was effected. The results are also shown in Table 4. 
     COMPARATIVE EXAMPLE 3 
     Run Nos. 21-22 
     The same test as in Example 4 was conducted using 1 g each of two deodorants on the market, i.e. silver-supporting zeolite [Bactekiller (trade name) manufactured by KANEBO, Ltd.] and titanium dioxide. The results are shown in Table 4. 
     As is clear from Table 4, the present invention method enables rapid removal of ethylene while the removal rate is low in Comparative Example 3 using deodorants on the market. 
     
                                           TABLE 4                                 
__________________________________________________________________________
                     Drying                                               
                           Specific                                       
                                  Ultraviolet                             
                                         Initial ethylene                 
                                                 Residual ethylene (%)    
       Titanic acid ester or                                              
                     tempera-                                             
                           surface area                                   
                                  irradia-                                
                                         concent-                         
                                                 After                    
                                                      After               
                                                          Half life       
Run No.                                                                   
       deodorant on the market                                            
                     ture (°C.)                                    
                           (m.sup.2 /g)                                   
                                  tion   ration (ppm)                     
                                                 1 min                    
                                                      10                  
                                                          (min)           
__________________________________________________________________________
Example 4                                                                 
17     Titanium tetraisopropylate                                         
                     200   316    Yes    99       6   0.22                
                                                          &lt;1              
18     Titanium tetra-n-butylate                                          
                     200   367    Yes    91       7   0.22                
                                                          &lt;1              
19     Titanium tetraisopropylate                                         
                     200   316    No     98      12   0.77                
                                                          &lt;1              
20     Titanium tetra-n-butylate                                          
                     200   367    No     95      15   1.11                
                                                          &lt;1              
Comparative                                                               
Example 3                                                                 
21     Silver-supporting zeolite                                          
                     --    --     Yes    106     90   88  &gt;180            
22     Titanium dioxide                                                   
                     --     52    Yes    103     93   67   15             
__________________________________________________________________________
 
    
     EXAMPLE 5 
     Run Nos. 23-24 
     A solution of 14 g of titaniunm tetraisopropylate dissolved in 50 ml of n-hexane, contained in a dropping funnel was slowly added dropwise to a mixture consisting of 50 ml of n-hexane, 10 ml of ethanol and a 10% aqueous silver nitrate solution, which was being stirred. The resulting precipitate was collected by filtration through a glass filter, subjected to predrying at 60° C. for 8 hours in a vacuum dryer, and dried at 200° C. for 1 hour at normal pressure to obtain a solid product. 1 g of the solid product was subjected to the same test as in Example 4. 
     The same test as above was conducted except that the silver nitrate was replaced by paladdium nitrate. 
     The result of these tests are shown in Table 5. 
     
                                           TABLE 5                                 
__________________________________________________________________________
               Specific               Residual ethylene (%)               
               surface area                                               
                      Ultraviolet                                         
                            Initial ethylene                              
                                      After                               
                                          After                           
                                              Half life                   
Run No.                                                                   
      Noble metal                                                         
               (m.sup.2 /g)                                               
                      irradiation                                         
                            concentration (ppm)                           
                                      1 min                               
                                          10 min                          
                                              (min)                       
__________________________________________________________________________
Example 5                                                                 
23    Silver nitrate                                                      
               322    Yes   102       4   0.0 &lt;1                          
24    Palladium nitrate                                                   
               359    Yes    99       3   0.0 &lt;1                          
__________________________________________________________________________
 
    
     EXAMPLE 6 
     Run Nos. 25-27 
     The solid product prepared from titanium tetraisopropylate and silver nitrate in the same manner as in Example 5, was measured for removability for harmful substances (shown in Table 6) present in air, in the same manner as in Example 5. 
     The results are shown in Table 6. Table 6 indicates that the present invention method enables rapid removal of various harmful substances. 
     
                                           TABLE 6                                 
__________________________________________________________________________
                       Residual proportion (%) of                         
                Initial                                                   
                       harmful substance                                  
                concentration                                             
                       After                                              
                           After                                          
                               After                                      
                                   Half life                              
Run No.                                                                   
      Harmful substance                                                   
                (ppm)  1 min                                              
                           10 min                                         
                               30 min                                     
                                   (min)                                  
__________________________________________________________________________
Example 6                                                                 
25    Methyl mercaptan                                                    
                 97    1   0.4 0.0 &lt;1                                     
26    Trimethylamine                                                      
                102    0.0 0.0 0.0 &lt;1                                     
27    Formaldehyde                                                        
                106    3   1.1 0.0 &lt;1                                     
__________________________________________________________________________
 
    
     EXAMPLE 7 
     Run Nos. 28-29 
     7 g of zinc chloride was dissolved in isopropanol dehydrated with a molecular sieve. Through the resulting solution being ice-cooled was bubbled ammonia gas supplied through a glass tube, for 1 hour. Ammonia gas was removed from the resulting solution by using an evaporator. The ammonia-removed solution was subjected to centrifugation. To the resulting supernatant solution was dropwise added 10 ml of deionized water. The resulting precipitate was collected by filtration, subjected to predrying at 60° C. for 8 hours in a vacuum dryer, and dried at 200° C. for 1 hour at normal pressure to obtain a solid product. The solid product was subjected to the same test as in Example 5. The results are shown in Table 7. 
     
                       TABLE 7                                                     
______________________________________                                    
Specific   Ultra-  Initial-  Residual ethylene (%)                        
      surface  violet  ethylene            Half                           
Run   area     irradia-                                                   
                       concent-                                           
                               After After life                           
No.   (m.sup.2 /g)                                                        
               tion    ration(ppm)                                        
                               1 min 10 min                               
                                           (min)                          
______________________________________                                    
Exam-                                                                     
ple 7                                                                     
28    345      Yes     104      5    0.15  &lt; 1                            
29    345      No      102     51    21     1                             
______________________________________                                    
 
    
     EXAMPLE 8 
     30 kiwi fruits on the market were placed in a 12-liter desiccator provided in a constant low-temperature test chamber. Using a tube, there were connected, in the following order, (1) the above desiccator, (2) a 300-ml flask for photochemical reaction which contains 5.0 g of a solid product prepared from titanium tetraisopropylate and silver nitrate in the same manner as in Example 2 and to which a low-pressure mercury lamp (illuminance=3 mW/cm 2 , main wavelength= 254 nm) was attached in order to enable irradiation of the solid product, and (3) a diaphragm type air pump. Incidentally, the flask (2) and the air pump (3) were installed outside the constant low-temperature test chamber. The desiccator inside was kept at 10° C. and a humidity of 90% or more, after which the air in the total system was circulated by the diaphragm type air pump [Test A (present invention)]. The same test was conducted except that no ultraviolet irradiation was made [Test B (present invention)]. Also, the same test was conducted except that no remover (no solid product) was used [Test C (Comparison)]. 
     Neither ethylene nor acetaldehyde was detected in any of Tests A, B and C, at the start of air circulation. After four weeks, the concentrations of ethylene and acetaldehyde in the desiccator were measured by gas chromatography; neither ethylene nor acetaldehyde was detected in Tests A and B, while 7 ppm of ethylene and 2 ppm of acetaldehyde were detected in Test C. Measurement was made again after 60 days; any of the two substances was detected in Tests A and B, while 15 ppm of ethylene and 4 ppm of acetaldehyde were detected in Test C. 
     The kiwi fruits were also measured for saccharose concentration, using a refractometer manufactured by Atago. The saccharose concentration indicated in refractometer index after four weeks was 16 in Tests A and B and was not much different from the concentration (17) before the test, while the concentration after four weeks was low (12) in Test C. Moreover, the kiwi fruits of Test C after four weeks had soft skins generally and shrivelled, as compared with those of Tests A and B. Six of the Test C kiwi fruits showed oozing-out of fruit juice. 
     It is appreciated from the above results that the present invention method is effective to keep garden plants fresh. 
     EXAMPLE 9 
     10 g of a harmful substance remover prepared from titanium tetraisopropylate and silver nitrate in the same manner as in Example 2 was placed in a cyclindrical glass reactor capable of transmitting ultraviolet rays, of 35 mm in inside diameter and 300 mm in length, having an inlet and an outlet (both of 5 mm in inside diameter and 40 mm in length) at the both ends via respective glass filter portions. 
     The reactor was kept vertically. Through the lower leg portion of the reactor was introduced into the reactor an air containing about 100 ppm of ethylene, methyl mercaptan, trimethylamine or formaldehyde, at a rate of 0.5-10/min, whereby the part or whole portion of the remover of the present invention was kept floating; the air was allowed to leave out through the upper leg of the reactor. The resulting gas was measured for concentration harmful substance by gas chromatography. The concentration was below the detection limit in all cases. 
     It is clear from the above test that the harmful substance remover of the present invention when used as a fluidized bed shows a striking effect for harmful substance removal. 
     EXAMPLE 10 
     Run Nos. 30 
     1 g of a solid product prepared from titanium tetraisopropylate and silver nitrate in the same manner as in Example 2, was placed in an 80-ml Erlenmeyer glass flask capable of transmitting ultraviolet rays. The flask inside was purged with an air containing 10,000 ppm of ethylene. The flask was sealed with a silicone rubber stopper and allowed to stand for 8 hours. The stopper was removed, and the flask inside was exposed to air for 1 hour to allow the ethylene remaining in the flask to leave. Then, the solid product in the flask was irradiated with ultraviolet rays for 2 hours using an ultrahigh pressure mercury lamp (illuminance=10 mW/cm 2 , main wavelength=365 nm) provided at a distance of 20 cm from the flask. Then, the flask inside was purged with an air containing about 100 ppm of ethylene. The flask was sealed with a silicone rubber stopper and subjected to the same test as in Example 1. The results are shown in Table 8. 
     COMPARATIVE EXAMPLE 4 
     Run Nos. 31 
     The same test as in Example 10 was conducted except that the ultraviolet irradiation after leaving of ethylene was not conducted. The results are shown in Table 8. 
     
                       TABLE 8                                                     
______________________________________                                    
       Initial Residual ethylene (%)                                      
         ethylene                       Half                              
         concent-  After   After  After life                              
Run No.  ration(ppm)                                                      
                   1 min   10 min 30 min                                  
                                        (min)                             
______________________________________                                    
Example 10                                                                
         101        91     70     38      15                              
30                                                                        
Comparative                                                               
          99       100     97     91    &gt;180                              
Example 4                                                                 
31                                                                        
______________________________________                                    
 
    
     As is clear from Table 8, the harmful substance remover of the present invention can be repeatedly used by reclamation.