Method of making highly porous, stable aluminum oxides doped with silicon

Abstract

The present invention relates to a method for making high surface area and large pore volume thermally stable silica-doped alumina (aluminum oxide) catalyst support and ceramic materials. The ability of the silica-alumina to withstand high temperatures in presence or absence of water and prevent sintering allows it to maintain good activity over a long period of time in catalytic reactions. The method of preparing such materials includes adding organic silicon reagents to an organic aluminum salt such as an alkoxide in a controlled quantity as a doping agent in a solid state, solvent deficient reaction followed by calcination. Alternatively, the organic silicon compound may be added after calcination of the alumina, followed by another calcination step. This method is inexpensive and simple. The alumina catalyst support material prepared by the subject method maintains high pore volumes, pore diameters and surface areas at very high temperatures and in the presence of steam.

1 . A method for preparing a silicon-doped alumina comprising:
 a) Bringing together as reactants at least one aluminum salt from the group consisting of an aluminum alkoxide, aluminum phenoxide and combinations thereof and water in an amount sufficient to hydrolyze without dissolving the reactants to produce an alumina nanoparticle precursor;   b) calcining the precursor of step a) at a temperature of from about 300-1200° C. to produce an aluminum oxide;   c) mixing the aluminum oxide of step b) with water and at least one of polydimethylsiloxane or an organic silicon compound of the structure   
       
         
           
           
               
               
           
         
          wherein R is selected from the group consisting of C 1 -C 12  alkyl, C 5 -C 12  cycloalkyl, aryl, a polyalkyl siloxane radical and combinations thereof, said water being present in an amount sufficient to hydrolyze without dissolving the reactants, and 
         d) calcining the product of step c) at a temperature of from about 300-1200° C. 
       
     
     
         2 . The method of  claim 1  wherein the aluminum salt is represented by the formula Al(O—R) 3  wherein R is C 1 -C 12  alkyl, C 5 -C 12  cycloalkyl, aryl and combinations thereof. 
     
     
         3 . The method of  claim 1  wherein the aluminum salt is selected from the group consisting of aluminum isopropoxide, aluminum sec-butoxide, aluminum phenoxide, aluminum ethoxide, aluminum tert-butoxide, and aluminum hexoxide. 
     
     
         4 . The method of  claim 1  wherein the aluminum salt is aluminum isopropoxide. 
     
     
         5 . The method of  claim 1  wherein the organic silicon compound is at least one compound having the following structure: 
       
         
           
           
               
               
           
         
         wherein R is selected from the group consisting of C 1 -C 12  alkyl, C 5 -C 12  cycloalkyl, aryl and a polyalkyl siloxane radical and combinations thereof. 
       
     
     
         6 . The method of  claim 5  wherein R is C 1 -C 12  alkyl. 
     
     
         7 . The method of  claim 6  wherein the organic silicon compound is selected from the group consisting of tetraethyl ortho silicate, tetra-n-butyloxysilane, tetra n-propoxy silane, polydimethyl siloxane and triethoxy methyl silane and combinations thereof. 
     
     
         8 . The method of  claim 1  wherein the organic silicon compound is employed in a proportion to provide about 1%-30% silica by weight of the final product. 
     
     
         9 . The method of  claim 1  wherein the reaction of step a) is carried out in the presence of a diluent selected from the group consisting of an alcohol, a ketone, an ether or a combination thereof to adjust the pore characteristics of the final product. 
     
     
         10 . An organic silicon-doped alumina composition having the following properties:
 BET surface are at 1000° C. of >100 m 2 /g;   a mesopore volume at 1000° C. of at least 0.3 cm 3 /g; and   wherein said composition contains about 1-30 weight % silica and is substantially in the gamma form at a temperature of 1200°.