Sulfite-based processes for producing nanocellulose, and compositions and products produced therefrom
Processes disclosed are capable of converting biomass into high-crystallinity nanocellulose with low mechanical energy input. In some variations, the process includes fractionating biomass with sulfur dioxide or a sulfite compound and water, to generate cellulose-rich solids and a liquid containing hemicellulose and lignin; and mechanically treating the cellulose-rich solids to form nanofibrils and/or nanocrystals. The total mechanical energy may be less than 500 kilowatt-hours per ton. The crystallinity of the nanocellulose material may be 80% or higher, translating into good reinforcing properties for composites. The nanocellulose material may include nanofibrillated cellulose, nanocrystalline cellulose, or both. In some embodiments, the nanocellulose material is hydrophobic via deposition of some lignin onto the cellulose surface. Optionally, sugars derived from amorphous cellulose and hemicellulose may be separately fermented, such as to monomers for various polymers. These polymers may be combined with the nanocellulose to form completely renewable composites.
1. A process for producing a nanocellulose material, said process comprising:
(a) providing a lignocellulosic biomass feedstock;
(b) fractionating said feedstock in the presence of a solution consisting of sulfur dioxide or a compound derived therefrom and water, wherein said solution does not contain a solvent for lignin, wherein said compound is selected from the group consisting of sulfurous acid, sulfite ions, sulfite salts, lignosulfonic acid, and combinations thereof, and wherein said fractionating is conducted at a pH selected from about 0 to about 6, to generate cellulose-rich solids and a liquid containing hemicellulose and lignin;
(c) mechanically treating said cellulose-rich solids to form cellulose nanofibrils and/or cellulose nanocrystals, thereby generating a nanocellulose material having a crystallinity of at least 70%; and
(d) recovering said nanocellulose material,
wherein no enzyme treatment is employed during said process,
wherein no acid hydrolysis of said cellulose-rich solids is employed during said process following step (b), and
wherein said nanocellulose material is at least partially hydrophobic via deposition of at least some of said lignin onto a surface of said cellulose-rich solids during step (b) and/or via deposition of at least some of said lignin onto a surface of said nanocellulose material during step (c) or step (d).
2. The process of claim 1 , wherein said compound is selected from calcium sulfite or bisulfite, magnesium sulfite or bisulfite, sodium sulfite or bisulfite, potassium sulfite or bisulfite, ammonium sulfite or bisulfite, or combinations thereof.
3. The process of claim 1 , wherein during step (c), said cellulose-rich solids are treated with a total mechanical energy of about 1000 or less kilowatt-hours per ton of said cellulose-rich solids.
4. The process of claim 1 , wherein during step (c), said cellulose-rich solids are treated using a disk refiner, conical refiner, cylindrical refiner, homogenizer, microfluidizer, sonicator, or a combination of thereof.
5. The process of claim 1 , said process further comprising bleaching said cellulose-rich solids prior to step (c) and/or as part of step (c).
6. The process of claim 1 , said process further comprising bleaching said nanocellulose material during step (c) and/or following step (c).
7. The process of claim 1 , wherein said crystallinity of said nanocellulose material is at least 80%.
8. The process of claim 1 , wherein said nanocellulose material comprises nanofibrillated cellulose.
9. The process of claim 1 , wherein said nanocellulose material comprises nanocrystalline cellulose.
10. The process of claim 1 , wherein said nanocellulose material comprises nanofibrillated cellulose and nanocrystalline cellulose.
11. The process of claim 1 , said process further comprising hydrolyzing amorphous cellulose into glucose in step (b) and/or step (c), recovering said glucose, and optionally fermenting said glucose to a fermentation product.
12. The process of claim 1 , said process further comprising recovering, fermenting, or further treating hemicellulosic sugars derived from said hemicellulose.
13. The process of claim 12 , said process further comprising fermenting said hemicellulosic sugars to produce a monomer or precursor thereof; polymerizing said monomer to produce a polymer; and combining said polymer and said nanocellulose material to form a polymer-nanocellulose composite.
14. The process of claim 1 , said process further comprising chemically converting said nanocellulose material to one or more nanocellulose derivatives selected from the group consisting of nanocellulose esters, nanocellulose ethers, nanocellulose ether esters, alkylated nanocellulose compounds, cross-linked nanocellulose compounds, acid-functionalized nanocellulose compounds, base-functionalized nanocellulose compounds, and combinations thereof.