Catalyst for use in producing carboxylic esters

Abstract

Disclosed is a novel catalyst for use in producing a carboxylic ester from an aldehyde, an alcohol and molecular oxygen, which comprises a carrier having supported thereon Pd and Pb and a Pd/Pb atomic ratio (S) of 3/0.7≦S≦ from 3/0.7 to 3/1.3, and which exhibits a maximum intensity peak at a diffraction angle (2θ) in the range of from 38.55° to 38.70° in a power X-ray diffraction pattern thereof, wherein the peak is ascribed to the (111) crystal face diffraction of a Pd—Pb intermetallic compound. The catalyst can be efficiently produced by subjecting a catalyst precursor comprising a carrier having supported thereon a reducible Pd compound or a mixture of a reducible Pd compound and a reducible Pb component to a reduction treatment in the presence of lead ions and at least one compound selected from a lower fatty acid, an alkali metal salt and an alkaline earth metal salt, or by subjecting a catalyst prepared by a conventional method, which comprises a carrier having supported thereon Pd or a mixture of Pd and Pb, to an activation treatment. By using the catalyst, the desired carboxylic ester can be produced with high selectivity, while suppressing the formation of by-products, even under high temperature and high aldehyde concentration conditions. Further, in the continuous production of a carboxylic ester using the catalyst, stabilization of the catalyst can be effectively performed, so that a desired carboxylic ester can be stably produced with high efficiency and in high yield for a prolonged period of time.

We claim:  
     
       1. A catalyst for use in producing a carboxylic ester from an aldehyde, an alcohol and molecular oxygen, which comprises a carrier having supported thereon palladium and lead in a Pd/Pb atomic ratio (S) of from  3/0.7 ≦S≦  to  3/1.3, 
       said catalyst exhibiting a maximum intensity peak at a diffraction angle (2θ) in the range of from 38.55° to 38.70° in a powder X-ray diffraction pattern thereof, wherein said peak is ascribed to the (111) crystal face diffraction of a Pd—Pb intermetallic compound.  
     
     
       2. The catalyst according to  claim 1 , exhibiting, in a spectrum obtained from said catalyst by X-ray photoelectron spectroscopy, a ratio of from 1/0.2 to 1/0.7 in terms of the {Pd3d(3/2)+Pd3d(5/2)}/{Pb4f(7/2)×1.75} ratio of the sum of respective intensities of two peaks respectively ascribed to the 3d(3/2) electron and the 3d(5/2) electron of metallic palladium to the intensity of a peak ascribed to the 4f(7/2) electron of metallic lead which is multiplied by 1.75. 
     
     
       3. The catalyst according to  claim 1  or  2 , wherein said carrier is an aluminum-containing silica composition comprising silica and alumina, which has an aluminum proportion of from 1 to 30 mol. %, based on the total molar amount of silicon and aluminum in said composition. 
     
     
       4. The catalyst according to  claim 1  or  2 , wherein said carrier comprises a silica-alumina-magnesia composition having silicon, aluminum and magnesium proportions of from 42 to 90 mol. %, from 5.5 to 38 mol. %, and from 4 to 38 mol. %, respectively, based on the total molar amount of said silicon, said aluminum and said magnesium. 
     
     
       5. The catalyst according to  claim 1  or  2 , wherein said carrier comprises a crystalline metallosilicate which, in the anhydride state and on a molar basis, has a composition represented by the following formula (1); 
       
         
           xM 2/n O.ySiO 2 .R 2/w O  (1)  
         
       
       wherein: 
       M represents at least one cation having the valence of n;  
       R represents at least one metal having the valence of w, which is selected from metals belonging to Groups IB, IIB, IIIA, IIIB, IVA, IVB, VA, VB, VIB, VIIB and VIII of the short-form Periodic Table;  
       n is 1 or 2;  
       w is an integer of from 1 to 4;  
       x=0.3±0.1; and  
       0.5≦y≦500.  
     
     
       6. A method for producing a catalyst of  claim 1 , which comprises subjecting a catalyst precursor comprising a carrier having supported thereon a reducible palladium compound or a mixture of a reducible palladium compound and a reducible lead compound to a reduction treatment with a reducing agent in a solvent in the presence of lead ions and at least one compound selected from the group consisting of a C 1 -C 5  fatty acid, an alkali metal salt and an alkaline earth metal salt. 
     
     
       7. The method according to  claim 6 , wherein said reducible palladium compound is at least one salt selected from palladium salts of organic acids and inorganic acids, and said reducible lead compound is at least one salt selected from lead salts of organic acids and inorganic acids. 
     
     
       8. The method according to  claim 6  or  7 , wherein said alkali metal salt is an alkali metal salt of a C 1 -C 5  fatty acid, and said alkaline earth metal salt is an alkaline earth metal salt of a C 1 -C 5  fatty acid. 
     
     
       9. The method according to  claim 6  or  7 , wherein said reducing agent comprises at least one member selected from the group consisting of formaldehyde, formic acid, hydrazine, methanol and molecular hydrogen. 
     
     
       10. The method according to  claim 6  or  7 , wherein said solvent comprises at least one member selected from the group consisting of water and an alcohol. 
     
     
       11. A method for producing a catalyst of  claim 1 , which comprises subjecting a catalyst intermediate comprising a carrier having supported thereon palladium or a mixture of palladium and lead in a Pd/Pb atomic ratio (S) of from  3/0 ≦S≦  to  3/10 to a reduction treatment with a reducing agent in the presence of lead ions in a solvent. 
     
     
       12. The method according to  claim 11 , wherein said reduction treatment is conducted in the presence of at least one compound selected from the group consisting of a C 1 -C 5  fatty acid, an alkali metal salt of a C 1 -C 5  fatty acid and an alkaline earth metal salt of a C 1 -C 5  fatty acid. 
     
     
       13. The method according to  claim 11  or  12 , wherein said reducing agent comprises at least one member selected from the group consisting of formaldehyde, formic acid, hydrazine, methanol and molecular hydrogen. 
     
     
       14. The method according to  claim 11  or  12 , wherein said solvent comprises at least one member selected from the group consisting of water and an alcohol. 
     
     
       15. A method for producing a catalyst of  claim 1 , which comprises oxidizing a catalyst intermediate comprising a carrier having supported thereon palladium and lead in a Pd/Pb atomic ratio (S) of from  3/0.7 ≦S≦  to  3/1.3 with a gaseous oxidizing agent and subsequently reducing the resultant oxidized catalyst intermediate with a gaseous reducing agent, wherein a sequence of the oxidation and the reduction is conducted at least once. 
     
     
       16. The method according to  claim 15 , wherein said gaseous oxidizing agent comprises molecular oxygen. 
     
     
       17. The method according to  claim 15  or  16 , wherein said gaseous reducing agent comprises at least one member selected from methanol gas, molecular hydrogen and C 2 -C 4  olefin gas. 
     
     
       18. A method for producing a catalyst of  claim 1 , which comprises oxidizing a catalyst intermediate comprising a carrier having supported thereon palladium and lead in a Pd/Pb atomic ratio (S) of from  3/1.3 ≦S≦  exclusive to  3/10 with a gaseous oxidizing agent and subsequently reducing the oxidized catalyst intermediate with a gaseous reducing agent, wherein a sequence of the oxidation and the reduction is conducted at least once; and, subsequent to said sequence of the oxidation and the reduction which sequence is conducted at least once, treating the resultant catalyst intermediate with a solution of at least one compound selected from the group consisting of a C 1 -C 5  fatty acid, an alkali metal salt of a C 1 -C 5  fatty acid and an alkaline earth metal salt of a C 1 -C 5  fatty acid in a solvent. 
     
     
       19. The method according to  claim 18 , wherein said gaseous oxidizing agent comprises molecular oxygen. 
     
     
       20. The method according to  claim 18  or  19 , wherein said gaseous reducing agent comprises at least one member selected from methanol gas, molecular hydrogen and C 2 -C 4  olefin gas. 
     
     
       21. The method according to  claim 18  or  19 , wherein said solvent comprises at least one member selected from the group consisting of water and an alcohol. 
     
     
       22. A method for producing a catalyst of  claim 1 , which comprises treating a catalyst intermediate comprising a carrier having supported thereon palladium and lead in a Pd/Pb atomic ratio (S) of from  3/2.5 ≦S≦  to  3/10 with a solution of at least one compound selected from the group consisting of a C 1 -C 5  fatty acid, an alkali metal salt of a C 1 -C 5  fatty acid and an alkaline earth metal salt of a C 1 -C 5  fatty acid in a solvent. 
     
     
       23. The method according to  claim 22 , wherein said solvent comprises at least one member selected from the group consisting of water and an alcohol. 
     
     
       24. A method for producing the catalyst of  claim 1 , which comprises reacting, in a reactor, a catalyst intermediate comprising a carrier having supported thereon palladium or a mixture of palladium and lead in a Pd/Pb atomic ratio (S) of from  3/0 ≦S≦  to  3/10 with an alcohol or a mixture of an alcohol and an aldehyde in the presence of lead ions, wherein said reaction is conducted under conditions such that the partial pressure of oxygen at the outlet of the reactor is from 0 to 0.8 kg/cm 2 . 
     
     
       25. A method for producing the catalyst of  claim 2 , which comprises reacting, in a reactor, a catalyst intermediate comprising a carrier having supported thereon palladium or a mixture of palladium and lead in a Pd/Pb atomic ratio (S) of from  3/0 ≦S≦  to  3/10 with an alcohol or a mixture of an alcohol and an aldehyde in the presence of lead ions, wherein said reaction is conducted under conditions such that the partial pressure of oxygen at the outlet of the reactor is from 0 to 0.4 kg/cm 2 . 
     
     
       26. A method for producing the catalyst of  claim 2 , which comprises reacting, in a reactor, the catalyst of  claim 1  with an alcohol or a mixture of an alcohol and an aldehyde in the presence of lead ions, wherein said reaction is conducted under conditions such that the partial pressure of oxygen at the outlet of the reactor is from 0 to 0.8 kg/cm 2 . 
     
     
       27. A method for continuously producing a carboxylic ester, which comprises continuously reacting an aldehyde, an alcohol and molecular oxygen in a reactor in the presence of the catalyst of  claim 1 , while adding to the reactor a substance capable of forming lead ions in a reaction system comprising said aldehyde, said alcohol and said molecular oxygen. 
     
     
       28. The method according to  claim 27 , wherein said continuous reaction is conducted under conditions such that the partial pressure of oxygen at the outlet of the reactor is 0.8 kg/cm 2  or less. 
     
     
       29. The method according to  claim 27 , wherein said molecular oxygen is in the form of air. 
     
     
       30. A method according to  claim 27 , wherein said aldehyde is selected from the group consisting of methacrolein, acrolein and a mixture thereof, and said alcohol is methanol.