Sol-gel process for the production of silica aerogels

Abstract

A improved sol-gel process is described for the production of silica aerogels, in particular from plant raw materials, in particular from rice processing waste.

1 - 13 . (canceled) 
     
     
         14 . A process for the production of a silica aerogel or xerogel comprising the following steps:
 preparing an aqueous solution of an alkali metal silicate;   separately preparing a solution of a concentrated inorganic acid selected from sulfuric, hydrochloric, nitric and phosphoric acid;   combining the aqueous solution of an alkali metal silicate and the solution of a concentrated inorganic acid under conditions effective to form a wet gel;   washing the wet gel formed by said combining step with a wash solvent to exchange a liquid phase present in pores of the wet gel;   drying the wet gel to obtain an aerogel or xerogel;   wherein said combining step is carried out by pouring the aqueous solution of an alkali metal silicate into the solution of a concentrated inorganic acid, and operating so that the pH of the system remains below 1.   
     
     
         15 . The process according to  claim 14 , wherein the wash solvent comprises at least a first wash solvent that is water, and at least a second wash solvent that is a mixture of water and acetone. 
     
     
         16 . The process according to  claim 15 , wherein the wash solvent further comprises at least a third wash solvent that is a hypercritical extraction liquid. 
     
     
         17 . The process according to  claim 14 , wherein during said washing step, the wash solvent is admixed with an alkylchlorosilane (R 3 —Si—Cl, wherein the three R substituents, equal to or different from each other, are alkyl radicals), thereby introducing R 3 —Si— groups on the surface of the pores of the wet gel. 
     
     
         18 . The process according to  claim 14 , wherein during said combining step, the pH of the system remains at values equal to or lower than 0. 
     
     
         19 . The process according to  claim 14 , wherein said aqueous solution of an alkali metal silicate contains a composition M 2 O×nSiO 2  (M=Na, K, Li), wherein n is in the range from 0.67 to 4, and from 50 to 150 mg/mL SiO 2 . 
     
     
         20 . The process according to  claim 14 , wherein said aqueous solution of an alkali metal silicate is prepared by dissolving a silica-containing material with a solution of an alkali hydroxide at a concentration of between 10% and 20% by weight. 
     
     
         21 . The process according to  claim 20 , wherein said silica-containing material is an ash resulting from combustion of a plant biomass containing at least 10% by weight of silica. 
     
     
         22 . The process according to  claim 21 , wherein said ash is obtained from combustion of rice husk or rice straw. 
     
     
         23 . The process according to  claim 14 , wherein said concentrated inorganic acid is selected from a solution of HCl 37% by weight and a solution of HNO 3  from 40 to 65% by weight. 
     
     
         24 . The process according to  claim 14 , wherein said drying step is carried out by evaporation, and the product formed is a xerogel. 
     
     
         25 . The process according to  claim 14 , wherein said washing step includes a washing period using a hypercritical extraction liquid and said drying step is carried out under supercritical conditions, and the product formed is an aerogel. 
     
     
         26 . The process according to  claim 25 , wherein amounts of the aqueous solution of an alkali metal silicate and the solution of a concentrated inorganic acid used during said combining step yield a final density of the aerogel that is between 0.01 and 0.3 g/mL. 
     
     
         27 . The process according to  claim 26 , wherein the final density of the aerogel is between 0.05 and 0.12 g/mL. 
     
     
         28 . The process according to  claim 25 , wherein the hypercritical extraction liquid is carbon dioxide. 
     
     
         29 . The process according to  claim 25 , further comprising:
 subjecting the resulting aerogel to a heat treatment in an oxidizing atmosphere consisting of oxygen, air or synthetic oxygen/nitrogen mixtures, at a temperature of between 300 and 800° C.   
     
     
         30 . A silica aerogel or xerogel product formed according to the process of  claim 14 .