The present application is a U.S. National Phase Application of International Application No. PCT/CN2014/094509, filed Dec. 22, 2014, which claims the benefit of Chinese Application No. 20130751115.X, filed Dec. 31, 2013, both of which are hereby incorporated by reference in their entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a technical field of catalysis, in particular to a catalyst for catalyzing and oxidizing methanthiol gas and methods for preparation and application thereof. 
     BACKGROUND OF THE INVENTION 
     Dimethyl disulfide is a widely used chemical raw material, which can be used for synthesizing methanesulfonyl chloride and methanesulfonic acid products. In addition, dimethyl disulfide is an organic solvent often used in polymerization and cyanation reactions. In the food industry, dimethyl sulfide is allowed to be used as an edible flavor. Besides, dimethyl disulfide can be used as a catalyst and passivating agent in petroleum industry, an odor agent for city gas, an industrial cleaning agent, a pesticide penetrating agent and the like. Therefore, it is a research hotspot for the skilled person in the art to study the preparation technology of dimethyl disulfide. 
     In theory, it is an available way for preparing dimethyl disulfide by catalytic oxidation of raw material methanthiol (4CH 3 SH+O 2 →2CH 3 SSCH 3 +2H 2 O). However researches of industrial production process of preparing dimethyl disulfide by catalytic oxidation of raw material methanthiol haven&#39;t been under way. Chinese patent application CN102816093A discloses a method for preparing dimethyl disulfide by oxidization of methanthiol, comprising introducing a mixture gas of methanthiol, oxygen and nitrogen dioxide with molar ratio 4:1.25:0.2 into a tower reactor to perform oxidation reaction in the presence of an emulsifying agent by controlling the molar ratio of methanthiol to emulsifying agent to 1:10-30 at a reaction temperature of 10-150° C. and a pressure of 0.01-0.1 MPa to produce a reaction product, then after 10 to 20 minutes&#39; standing, dimethyl disulfide is separated from the reaction product. In this method a qualified dimethyl disulfide product is prepared by using nitrogen dioxide as a catalyst and by reaction, separation and rectification processes. 
     Although researches of industrial production process of preparing dimethyl disulfide by catalytic oxidation of raw material methanthiol haven&#39;t been under way, and there lacks of intensive study on catalyst for catalytic oxidation of methanthiol, the process of preparing dimethyl disulfide by catalytic oxidation of methanthiol still has vast development potential because it has some advantages such as simple and safe. In order to realize its industrial production, the problem to be solved in the prior art is to develop new catalysts to improve the catalytic conversion rate of methanthiol. 
     SUMMARY OF THE INVENTION 
     In order to solve the problem that there are fewer kinds of catalysts for preparing dimethyl disulfide by catalytic oxidation of methanthiol in the art, the present invention provides a catalyst for oxidization of methanthiol which has a high catalytic conversion rate, a low cost and is suitable for industrial production, and also provides methods for preparation and application. 
     In one aspect, the present invention provides a new use of magnetic iron oxide red Fe 21.333 O 32  as a catalyst in a method for preparing dimethyl disulfide by oxidization of methanthiol. 
     In a class of embodiments, the catalyst consists of magnetic iron oxide red Fe 21.333 O 32  in an amount of 80-95 wt %, and the balance is a binder. 
     In another aspect, the present invention provides a method for preparing the catalyst, comprising: 
     (1) preparing a solution of a soluble carbonate and a soluble ferrous salt by controlling a molar ratio of the carbonate to the ferrous salt to 1:0.8-1.5, stirring the solution to allow the carbonate reacting with the ferrous salt in the solution to form a first mixture, and filtrating the first mixture to obtain a filter cake; 
     (2) calcining the filter cake at 250-400° C. for 2-5 h, then washing with water and drying the filter cake to yield the magnetic iron oxide red Fe 21.333 O 32 ; 
     (3) mixing the magnetic iron oxide red Fe 21.333 O 32  obtained in step (2) and a binder to form a second mixture, followed by roll molding at room temperature and drying the second mixture to produce the catalyst. 
     In a class of embodiments, the soluble carbonate is sodium carbonate or potassium carbonate, and the soluble ferrous salt is ferrous sulfate. 
     In a class of embodiments, the ferrous sulfate has a concentration of 1.5-3.0 mol/L in the solution. 
     In a class of embodiments, the ferrous sulfate has a concentration of 2.2 mol/L in the solution. 
     In a class of embodiments, calcining the filter cake at 300° C. in the step (2). 
     In a class of embodiments, the binder is polyvinyl alcohol or red clay. 
     In a class of embodiments, drying the second mixture at a temperature no more than 100° C. in the step (3). 
     In a further aspect, the present invention provides a method for preparing dimethyl disulfide by catalytic oxidation of methanthiol using the catalyst of the present invention, comprising, 
     introducing a gas mixture of a methanthiol gas and an oxygen containing gas, wherein a molar ratio of the oxygen in the oxygen containing gas and the methanthiol gas is equal to or more then 1:3, into a fixed bed reactor for reacting in the presence of the catalyst by controlling a reaction temperature to 10-150° C., an space velocity of the gas mixture to 500-2000 h −1 , and a reaction pressure to atmospheric pressure. 
     In the method for preparing the catalyst for preparing dimethyl disulfide by oxidization of methanthiol of the present invention, the step (1) comprises preparing a solution of a soluble carbonate and a soluble ferrous salt by controlling a molar ratio of the carbonate to the ferrous salt to 1:0.8-1.5, stirring the solution to allow the carbonate reacting with the ferrous salt in the solution to form a first mixture, and filtrating the first mixture to obtain a filter cake. By specially controlling the molar ratio of the carbonate to the ferrous salt to 1:0.8-1.5, the used raw materials can react sufficiently and thus the yield of magnetic iron oxide red is high, and in addition the used raw materials are low in cost. The step (2) comprises calcining the filter cake obtained in the step (1) at 250-400° C. for 2-5 h, then washing with water and drying the filter cake to yield the magnetic iron oxide red Fe 21.333 O 32 . The step (3) comprises mixing the magnetic iron oxide red Fe 21.333 O 32  obtained in step (2) and a binder to form a second mixture, followed by roll molding at room temperature and drying the second mixture to produce the catalyst. In a preferred embodiment, the present invention employs polyvinyl alcohol or red clay as the binder, which can impart the catalyst a high mechanical strength, and can benefit the molding operation. 
     The present invention has the following advantages: 
     Under a circumstance that there lacks of intensive study on catalyst for catalytic oxidation of methanthiol in the prior art, the inventors of the present invention carried out a lot of long-term researches and surprisingly found that when the magnetic iron oxide red Fe 21.333 O 32  prepared by inventors themselves is used as the catalyst for preparing dimethyl disulfide by oxidization of methanthiol, a very high catalytic conversion rate can be achieved. In addition, since the magnetic iron oxide red Fe 21.333 O 32  is prepared using carbonates and ferrous salts as raw materials, the present invention has advantages of low cost and simple preparation technology, and is very suitable for industrial production. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a XRD pattern of the magnetic iron oxide red Fe 21.333 O 32  prepared in the present invention; 
         FIG. 2  shows a gas chromatogram of CH 3 SSCH 3  standard sample; 
         FIG. 3  shows a gas chromatogram of outlet gas sample. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Example 1 
     A method for preparing a catalyst for oxidization of methanthiol in the present example comprises: 
     (1) adding water into a beaker, placing the beaker in a water bath at 40° C., putting FeSO 4 .7H 2 O solid into the beaker, followed by stirring until the FeSO 4 .7H 2 O solid is completely dissolved in the water to obtain a FeSO 4  solution having a concentration of 1.5 mol/L; then slowly adding Na 2 CO 3  solid into the FeSO 4  solution by controlling a molar ratio of the Na 2 CO 3  to the FeSO 4  to 1 to form a first mixture; stirring the first mixture for 2 h, followed by suction filtration to obtain a filter cake; 
     (2) putting the filter cake prepared by step (1) into a muffle furnace, calcining the filter cake at 300° C. for 3 h, then washing with water and filtering for 3 times, followed by drying the filter cake at 100° C. to yield the magnetic iron oxide red Fe 21.333 O 32 ; and 
     (3) mixing 80 g of the magnetic iron oxide red Fe 21.333 O 32  obtained in step (2) with 80 g of polyvinyl alcohol as a binder to form a second mixture, followed by roll molding at room temperature and drying the second mixture at room temperature to produce the catalyst A. 
     The catalyst in the present example consists of magnetic iron oxide red Fe 21.333 O 32  in an amount of 80 wt %, and the balance is the binder. 
     Example 2 
     (1) Adding water into a beaker, placing the beaker in a water bath at 40° C., putting FeSO 4 .7H 2 O solid into the beaker, followed by stirring until the FeSO 4 .7H 2 O solid is completely dissolved in the water to obtain a FeSO 4  solution having a concentration of 3 mol/L; then slowly adding Na 2 CO 3  solid into the FeSO 4  solution by controlling a molar ratio of the Na 2 CO 3  to the FeSO 4  to 0.8 to form a first mixture; stirring the first mixture for 2 h, followed by suction filtration to obtain a filter cake; 
     (2) putting the filter cake prepared by step (1) into a muffle furnace, calcining the filter cake at 300° C. for 3 h, then washing with water and filtering for 3 times, followed by drying the filter cake at 120° C. to yield the magnetic iron oxide red Fe 21.333 O 32 ; and 
     (3) mixing 85 g of the magnetic iron oxide red Fe 21.333 O 32  obtained in step (2) with 15 g of red clay as a binder to form a second mixture, followed by roll molding at room temperature and drying the second mixture at 90° C. to produce the catalyst B. 
     The catalyst of the present example consists of magnetic iron oxide red Fe 21.333 O 32  in an amount of 85 wt %, and the balance is the binder. 
     Example 3 
     (1) Adding water into a beaker, placing the beaker in a water bath at 40° C., putting FeSO 4 .7H 2 O solid into the beaker, followed by stirring until the FeSO 4 .7H 2 O solid is completely dissolved in the water to obtain a FeSO 4  solution having a concentration of 2.2 mol/L; then slowly adding Na 2 CO 3  solid into the FeSO 4  solution by controlling a molar ratio of the Na 2 CO 3  to the FeSO 4  to 1.5 to form a first mixture; stirring the first mixture for 2 h, followed by suction filtration to obtain a filter cake; 
     (2) putting the filter cake prepared by step (1) into a muffle furnace, calcining the filter cake at 300° C. for 3 h, then washing with water and filtering for 3 times, followed by drying the filter cake at 190° C. to yield the magnetic iron oxide red Fe 21.333 O 32 ; and 
     (3) mixing 85 g of the magnetic iron oxide red Fe 21.333 O 32  obtained in step (2) with 15 g of polyving akohol as a binder to form a second mixture, followed by roll molding at room temperature and drying the second mixture at 90° C. to produce the catalyst C. 
     The catalyst of the present example consists of magnetic iron oxide red Fe 21.333 O 32  in an amount of 85 wt %, and the balance is the binder. 
       FIG. 1  shows the XRD pattern of the magnetic iron oxide red Fe 21.333 O 32  prepared by the above examples. 
     Test Example 
     In order to demonstrate the catalytic effect of the magnetic iron oxide red Fe 21.333 O 32  for oxidization of methanthiol, the present invention provides the test example to evaluate the performance of the catalysts. 
     The evaluation test is performed in a quartz fixed bed reactor of Ø10 mm×100 mm, filled with the catalyst to reach a height of 40 mm. A gas mixture of methanthiol gas and an oxygen with a molar ratio of 3:1-5:1 is introduced into the quartz fixed bed reactor under the condition of a reaction temperature of 10-100° C., a space velocity of 500-2000 h −1 , and a reaction pressure of atmospheric pressure. The specific parameter settings are listed in the following table. 
     The catalytic conversion rate X of the catalyst of each example is calculated according to the formula below:
 
 X =dimethyl disulfide actual output /dimethyl disulfide theoretical output ×100%
 
wherein dimethyl disulfide theoretical output is calculated according to the methanthiol introduced into the fixed bed reactor.
 
     In the present example, gas chromatograph is used for qualitative and quantitative detection of the dimethyl disulfide in the products. The chromatograms of the standard sample and outlet gas sample of the dimethyl disulfide are shown in  FIGS. 2 and 3 . According to the chromatogram, under same conditions, the peak appearance time of the CH 3 SSCH 3  standard sample is almost the same with that of the sulfur containing compound in the outlet gas sample, so it can be determined that the sulfur containing compound in the outlet gas is CH 3 SSCH 3 . 
     After detection, the catalytic conversion rates of the catalysts of examples 1-3 are summarized in the table below: 
     
       
         
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                   
                   
                 gas 
                   
                   
                 gas 
                   
                   
                 gas 
                   
                   
                 gas 
                   
               
               
                   
                 reaction 
                 space 
                   
                 reaction 
                 space 
                   
                 reaction 
                 space 
                   
                 reaction 
                 space 
               
               
                   
                 temperature 
                 velocity 
                 molar 
                 temperature 
                 velocity 
                 molar 
                 temperature 
                 velocity 
                 molar 
                 temperature 
                 velocity 
                 molar 
               
               
                   
                 ° C. 
                 h −1   
                 ratio 
                 ° C. 
                 h −1   
                 ratio 
                 ° C. 
                 h −1−1   
                 ratio 
                 ° C. 
                 h −1   
                 ratio 
               
               
                 Catalyst 
                 10 
                 500 
                 3:1 
                 25 
                 1000 
                 4:1 
                 70 
                 2000 
                 4:1 
                 100 
                 700 
                 5:1 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Catalyst A 
                 92.1% 
                 93.3% 
                 90.6% 
                 94.9% 
               
               
                 Catalyst B 
                 91.9% 
                 92.6% 
                 90.2% 
                 88.7% 
               
               
                 Catalyst C 
                 96.4% 
                 96.8% 
                 93.7% 
                 95.2% 
               
               
                   
               
             
          
         
       
     
     From the above results it is known that the magnetic iron oxide red Fe 21.333 O 32  has an excellent catalytic oxidation effect when used as a catalyst for oxidization of methanthiol. 
     Obviously, the above embodiments are merely examples for clear illustration, rather than limitation for the application. For those skilled in the art, changes and modifications may be made on the basis of the above description, and it is not necessary and could not exhaust all embodiments, thus obvious changes and modifications derived from the above embodiments still fall within the protection scope of the invention.
The present invention relates to a novel use of magnetic iron oxide red Fe 21.333 O 32 as a catalyst in oxidizing methanthiol to prepare dimethyl disulfide. The magnetic iron oxide red Fe 21.333 O 32 according to the present invention has extremely high catalytic selectivity in catalyzing and oxidizing methanthiol to prepare dimethyl disulfide. The magnetic iron oxide red Fe 21.333 O 32 is prepared with a carbonate and a ferrite as raw materials, has advantages of low cost and simple preparation process, and is suitable for industrial production.