Suppression of aldehyde formation in xylene
FIELD OF THE INVENTION 
     This invention relates to the suppression of the formation of tolualdehyde in xylene contacted with air at temperatures upward from 50° C. and more specifically pertains to such suppression of tolualdehyde formation in xylene contacted with air at a temperature of from 50° C. up to its normal boiling point (138.5° to 144° C. at 760 mm Hg) by dissolving in the xylene a small amount of thiodipropionic acid or one of its dialkyl esters soluble in the xylene. 
     STATE OF THE ART 
     Little or no concern has been expressed in the art pertaining to the use of xylenes with respect to the formation of tolualdehyde in small or even large amounts as an impurity which might convey undesirable properties or conditions to other materials, for example, to dimethyl terephthalate or maleic anhydride, or phthalic anhydride, with which such aldehyde contaminated xylene may contact. Xylenes have been proposed as solvents for the purification of dimethyl terephthalate (DMT) by extraction and/or recrystallization and any xylene remaining in the DMT is removed as a forecut of reduced pressure fractionation. 
     Recently, concern has arisen for the deliberate formation of tolualdehyde because its further oxidation under liquid phase conditions to a phthalic acid apparently avoids co-production of one or more undesirable and difficultly removable impurities. 
     In any event the state of the art is rather silent with respect to the formation of small amounts, for example 70 to 150 ppm, o-tolualdehyde in o-xylene upon casual contact with air even at moderate temperature conditions; e.g. at a temperature upward from 50° C. 
     We have discovered that o-xylene at a temperature of 60° C. can by merely being contacted with air in storage develop 55 to 60 weight parts o-tolualdehyde per million weight parts of o-xylene in 72 hours and develop 20 to 25 additional weight parts o-tolualdehyde per million weight parts of o-xylene at 60° C. in 45 hours when air is injected into the liquid o-xylene. We have also discovered that the rate of formation of p- or m-tolualdehyde in p-xylene or m-xylene is faster than the rate of formation of o-tolualdehyde in o-xylene. For example, at 60° C. quiescent p-xylene in contact with air will in 72 hours have a 111 ppm increase in p-tolualdehyde content. 
     In addition to the use of a xylene as solvent for purification or partial purification of dimethyl terephthalate by recrystallization, other uses for xylenes have been proposed. For example, xylenes have been proposed for use in the removal of water by azeotropic distillation of aqueous acetic acid containing 6 to 15 weight percent water. Also, xylene has been proposed for use in the recovery of maleic anhydride or phthalic anhydride from an aqueous solution of maleic acid or o-phthalic acid by forming a water-xylene azeotrope for distillative removal of solvent water and water split out by the formation of the anhydride. In those uses of a xylene its tolualdehyde can contaminate and discolor the diester or the anhydride product. 
     Treatment of the xylene containing tolualdehyde with a solid absorbent such as a molecular sieve having pore sizes upward from about 9 A diameter can remove the tolualdehyde. However, the absorbent would, when used commercially in a bed of one meter in diameter and 10 meters long, have to be regenerated every 8 to 10 hours. But such removal of tolualdehyde would not prevent the tolualdehyde from forming again when the solid absorbent treated xylene was exposed to air. 
     However, we have discovered a relatively simple means for treating a xylene which is or may be contacted with air in its use so that the tolualdehyde does not form and hence need not be removed so the tolualdehyde does not contaminate the compound it is being used with to purify or form. 
    
    
     STATEMENT OF THE INVENTION 
     The formation of a tolualdehyde in a xylene can be suppressed by dissolving in the xylene a small amount of thiodipropionic acid or a dialkyl ester thereof. The &#34;small amount&#34; of thiodipropionic acid or its xylene soluble diester is upward from 100 weight parts of the dibasic acid per million weight parts of xylene or the chemical equivalent thereof as the diester. 
     The diester can be any one from dimethyl up to the dialkyl ester formed from the highest carbon content alkanol currently available which diester is soluble in a xylene. Preferably, the diester is a di(C 6  to C 18  alkyl)ester of thiodipropionic acid for the suppression of tolualdehyde formation in a xylene. 
     The minimum use of thiodipropionic acid is 100 ppm based on the xylene. The upper limit, as a practical economic matter, is 400 ppm based on the weight of xylene but larger amounts can be used where process economics are of no concern. The use of such larger amounts, e.g., above 400 ppm based on the weight of the xylene will not have any disadvantageous technical effects. 
     The chemically equivalent amounts of the dialkyl esters of thiodipropionic acid useful according to this invention can be readily ascertained by dividing the molecular weight of the diester by the molecular weight of thiodipropionic acid and multiplying the resulting quotient by the foregoing 100 to 400 ppm range for the free dibasic acid. For example, said quotient for dilauryl thiodipropionate (a di-C 16  alkyl ester) is 3.5. Thus the range for using the dilauryl ester is from 350 up to 1400 ppm based on the xylene. 
     The following examples will illustrate the ease of tolualdehyde formation in a xylene and how the present inventive concept suppresses the tolualdehyde formation. 
     Firstly, in the following three comparative examples fresh xylene is heated to 60° C. either in contact with air or with air injected into the heated xylene at 181 nl/hr. The tolualdehyde content of the xylenes before and after contact with air are shown in TABLE I. 
     
                       TABLE I                                                     
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FORMATION OF TOLUALDEHYDE IN XYLENE                                       
Initial                                                                   
Alde-                        Final                                        
hyde      History of Heating Aldehyde                                     
Xylene                                                                    
      Conc.   Temp.,   Time,         Concentration,                       
Type  ppm.    ° C.                                                 
                       hr     Aerated                                     
                                     ppm                                  
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ortho 14      60       72     No     70                                   
ortho 19      60       89.5   No     46                                   
ortho 44      60       45     Yes    67                                   
ortho 23      60       48     No     540                                  
ortho 23      60       48     Yes    750                                  
ortho 540     60       264    No     685                                  
ortho 750     60       264    Yes    944                                  
para  308     60       72     No     419                                  
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     The data in TABLE II will illustrate the benefits of the present inventive concept for suppressing the formation of tolualdehyde in xylene. 
     
                       TABLE II                                                    
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SUPPRESSION OF TOLULALDEHYDE FORMATION                                    
IN ORTHO-XYLENE                                                           
                                Final                                     
                                Aldehyde                                  
Initial History of Heating      Concen-                                   
Aldehyde,                                                                 
        Temp.,  Time,   Addi-      Aerated                                
                                          tration                         
ppm     ° C.                                                       
                hr      tive, ppm  181 nl/hr                              
                                          ppm                             
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15      60      72      di(c16)                                           
                               100 No     49                              
                        ester                                             
19      60      89.5    di(c16)                                           
                              1000 No     21                              
                        ester                                             
19      60      89.5    di(c16)                                           
                               500 No     21                              
                        ester                                             
19      60      45      di(c16)                                           
                              1000 Yes    34                              
                        ester                                             
17      60      45      di(c16)                                           
                               500 Yes    32                              
                        ester                                             
373     60      92      di(c16)                                           
                              1000 Yes    341                             
                        ester                                             
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