Cellulose acetate aerogels

Abstract

Mechanically strong, biodegradable and reusable aerogels are disclosed, which can be made with a cross-linked cellulose ester, and which exhibit a low density and high porosity. The aerogels disclosed herein may be used as sorbent materials and can be modified with a hydrophobic and/or oleophilic agent.

What is claimed is: 
     
         1 . An aerogel comprising a cross-linked cellulose ester having a degree of substitution of from about 1.0 to about 2.8,
 wherein the aerogel has a bulk density of from about 1 mg/cc to about 500 mg/cc, and   wherein the cellulose ester is a (C 1 -C 10 )alkyl-C(O)-functionalized cellulose.   
     
     
         2 . The aerogel of  claim 1 , wherein the degree of substitution of the cellulose ester is from about 1.8 to about 2.6. 
     
     
         3 . The aerogel of  claim 1 , wherein the cellulose ester is cellulose acetate, cellulose acetate propionate, cellulose acetate butyrate or a combination thereof. 
     
     
         4 . The aerogel of  claim 1 , wherein the cellulose ester is cross-linked via an ester, ether or urethane linkage. 
     
     
         5 . The aerogel of  claim 4 , comprising a cross-linking agent at from about 1 wt. % to about 15 wt. %. 
     
     
         6 . The aerogel of  claim 4 , comprising the cellulose ester at from about 85 wt. % to about 99 wt. %. 
     
     
         7 . The aerogel of  claim 1 , wherein the cross-linking agent is selected from at least one of a C 7 -C 20  aromatic anhydride, a (C 5 -C 20 )cycloalkyl anhydride, a bis(halo(C 1 -C 3 )alkyl)-(C 5 -C 20 )aromatic, a bis(halo(C 1 -C 3 )alkyl)(C 3 -C 7 )cycloalkyl, a bis(isocyanato(C 1 -C 3 )alkyl)(C 5 -C 20 )aromatic, or a bis(isocyanato(C 1 -C 3 )alkyl)(C 3 -C 7 )cycloalkyl. 
     
     
         8 . The aerogel of  claim 7 , wherein the cross-linking agent is PMDA. 
     
     
         9 . The aerogel of  claim 1 , wherein the aerogel has the capacity to absorb at least about 20 grams of water per gram of aerogel. 
     
     
         10 . The aerogel of  claim 1 , further comprising a hydrophobic agent. 
     
     
         11 . The aerogel of  claim 10 , wherein the hydrophobic agent is selected from at least one of a C 2 -C 20  alkyl silane, a C 3 -C 7  cycloalkyl silane, an aromatic silane, an (C 2 -C 20 )alkyl, a (C 3 -C 7 )cycloalkyl, an (C 2 -C 20 )alkyl urethane, a (C 3 -C 7 )cycloalkyl urethane, an (C 2 -C 20 )alkyl ester, a (C 3 -C 7 )cycloalkyl ester, an (C 2 -C 20 )alkyl carbonate, a (C 3 -C 7 )cycloalkyl carbonate or any mixture thereof. 
     
     
         12 . The aerogel of  claim 11 , wherein the hydrophobic agent is a C 5 -C 10  alkyl silane. 
     
     
         13 . The aerogel of  claim 10 , wherein the bulk density of the aerogel is from about 15 mg/cc to about 85 mg/cc. 
     
     
         14 . The aerogel of  claim 10 , wherein the aerogel exhibits at least one of the following:
 (a) a porosity equal to or greater than 90%,   (b) a maximum compressive stress of from about 50 kPa to about 400 kPa,   (c) a maximum strain of from about 15% to about 100%,   (d) a compression modulus of from about 20 kPa to about 500 kPa,   (e) a pore size average diameter of from about 20 μm to about 100 μm,   (f) a BET of from about 2 m 2 /g to about 5 m 2 /g, and   (g) a surface tension of from about 30 mN/m to about 60 mN/m.   
     
     
         15 . The aerogel of  claim 10 , wherein the aerogel adsorbs at least about 20 grams of oil per gram of aerogel. 
     
     
         16 . The aerogel of  claim 15 , wherein the aerogel adsorbs at least about 10 grams of oil per gram of aerogel after 5 cycles of adsorption and compression. 
     
     
         17 . The aerogel of  claim 15 , wherein the ratio of oil:water capable of being adsorbed per gram of aerogel is from about 1:5 to about 1:50. 
     
     
         18 . A method of adsorbing a non-polar liquid from a mixture of a non-polar liquid and a polar liquid, the method comprising contacting the mixture with the aerogel of  claim 15 . 
     
     
         19 . The method of  claim 18 , wherein the non-polar liquid is hexane or oil and the polar liquid is water. 
     
     
         20 . The method of  claim 18 , wherein the aerogel can adsorb from about 25 grams to about 60 grams of non-polar liquid per gram of aerogel. 
     
     
         21 . A method for preparing an aerogel comprising a cross-linked cellulose ester, the method comprising the steps of:
 a) forming a homogenous solution of a cellulose ester having a degree of substitution of from about 1.0 to about 2.8 in a solvent;   b) adding a cross-linking agent to the homogenous solution to form a cellulosic intermediate cross-linked via an ester, ether or urethane linkage;   c) exchanging the solvent with water to obtain a hydrated cellulosic intermediate;   d) freezing the hydrated cellulosic intermediate; and   e) lyophilizing the frozen cellulosic intermediate to obtain the aerogel, wherein the aerogel has a bulk density of from about 1 mg/cc to about 500 mg/cc.   
     
     
         22 . The method of  claim 21 , further comprising the step of
 f) functionalizing the aerogel with a hydrophobic agent to form a hydrophobic aerogel.   
     
     
         23 . The method of  claim 22 , wherein the hydrophobic agent is selected from at least one of a C 2 -C 20  alkyl silane, a C 3 -C 7  cycloalkyl silane, an aromatic silane, an (C 2 -C 20 )alkyl, a (C 3 -C 7 )cycloalkyl, an (C 2 -C 20 )alkyl urethane, a (C 3 -C 7 )cycloalkyl urethane, an (C 2 -C 20 )alkyl ester, a (C 3 -C 7 )cycloalkyl ester, an (C 2 -C 20 )alkyl carbonate, a (C 3 -C 7 )cycloalkyl carbonate or any mixture thereof. 
     
     
         24 . The method of  claim 22 , wherein the functionalizing step is performed by chemical vapor deposition. 
     
     
         25 . The method of  claim 22 , wherein the method does not comprise sonication. 
     
     
         26 . The method of  claim 22 , wherein the bulk density of the aerogel is from about 15 mg/cc to about 150 mg/cc. 
     
     
         27 . The method of  claim 21 , wherein exchanging the solvent with water is done at least 2× at a solvent volume fraction of from about 1 to about 0. 
     
     
         28 . The method of  claim 27 , wherein the first exchange of solvent with water is done for a time of equal to or less than 48 hours. 
     
     
         29 . The method of  claim 21 , wherein the hydrated cellulosic intermediate is frozen in one direction. 
     
     
         30 . The method of  claim 21 , wherein the cross-linking agent is added to the homogenous solution at from about 0.1 w/v % to about 0.5 w/v %. 
     
     
         31 . The method of  claim 21 , wherein the cellulose ester is present in the homogenous solution at from about 1 w/v % to about 15 w/v %.