Methods for preparing thermally stable lignin fractions

1. A method of fractionating a lignin sample into stable high purity lignin fractions having a consistent glass transition temperature, the method comprising:

heating the lignin sample in an organic solvent for 0.5 to 24 hours to form a resulting biphasic mixture, wherein the mixture comprises:

a) a remainder solid designated as solvent insoluble (SI) lignin fraction comprising a first fraction of the lignin sample, and

b) a liquid solution comprising the organic solvent and a second fraction of the lignin sample, wherein the second fraction is designated as solvent soluble (SS) lignin fraction;

spatially separating the SI lignin fraction from the SS lignin fraction;

evaporating the organic solvent from the SS lignin fraction; and

drying each fraction to obtain a dry solid SS lignin fraction and a dry solid SI lignin fraction;

wherein the two dry solid lignin fractions have different mass average molar masses (M W ) as measured by gel permeation chromatography (GPC), and

wherein at least one of the lignin fractions has the consistent glass transition temperature (Tg) as determined by two differential scanning calorimetry (DSC) runs of a same lignin portion in a single day according to DIN 53765-1994,

wherein a first Tg is measured during the first DSC run, a second Tg is measured during the second DSC run, and the difference between the first Tg and the second Tg is less than 5° C.

2. The method of claim 1 , wherein both the dry solid SS lignin fraction and the dry solid SI lignin fraction have consistent glass transition temperatures.

3. The method of claim 2 , wherein the glass transition temperatures of the dry solid SS lignin fraction and the dry solid SI lignin fraction are different.

4. The method of claim 1 , wherein the consistent Tg is above 160° C. as determined using DSC according to DIN 53765-1994.

5. The method of claim 1 , wherein at least one of the two dry solid lignin fractions has a Mw greater than 10,000 Da as measured by GPC.

6. The method of claim 1 , wherein the organic solvent comprises at least one organic molecule having up to 5 carbon atoms and at least one heteroatom.

7. The method of claim 1 , wherein the organic solvent is selected from the group consisting of methanol, ethanol, isopropanol, ethyl acetate, ethyl formate, dichloromethane, and any mixture thereof.

8. The method of claim 1 , wherein the spatial separating comprises filtration or decantation of the organic solvent from the insoluble lignin.

9. The method of claim 1 , wherein at least one of the two dry solid lignin fractions is characterized by at least one of the following properties:

i) a polydispersity less than 7.00 as measured by GPC;

ii) a formula of C 9 H X O Y ; wherein X is less than 12 and Y is less than 3.5;

iii) a degree of condensation less than 0.8 as determined by NMR;

iv) methoxyl content (#/aryl group) as determined by NMR of less than 1.4;

v) aliphatic linkages (β-O-4′) (#/aryl group) less than 0.6;

vi) ratio of Aromatic C—O:Aromatic C—C:Aromatic C—H (#/aryl group) 1.6:2.3:2.1 or 1.6:2.2:2.2;

vii) an amount of aromatic C—O bonds (#/aryl group) less than 2.1;

viii) elemental composition of greater than 61% carbon, less than 27% oxygen, and less than 0.5% nitrogen by mass as measured by elemental analysis;

ix) a marker molecule at a concentration of at least 100 ppb;

x) less than 0.1 times the volatile sulfur compounds found in Kraft lignin;

xi) an ash content of less than 0.5%;

xii) a sulfur content of less than 700 PPM;

xiii) a phosphorus content of less than 100 PPM; and

xiv) a soluble carbohydrate content of less than 0.5%.

10. The method of claim 9 , wherein at least one of the two dry solid lignin fractions is characterized by at least three of the characteristics.

11. The method of claim 9 , wherein at least one of the two dry solid lignin fractions is characterized by at least five of the characteristics.

12. The method of claim 9 , wherein at least one of the two dry solid lignin fractions is characterized by at least eight of the characteristics.

13. The method of claim 9 , wherein at least one of the two dry solid lignin fractions is characterized by at least ten of the characteristics.

14. The method of claim 9 , wherein at least one of the two dry solid lignin fractions is characterized by at least twelve of the characteristics.

15. The method of claim 11 , further comprising, prior to the contacting, removing hemicellulose sugars from a biomass, thereby obtaining a lignin-containing remainder, wherein the lignin-containing remainder comprises lignin and cellulose.

16. The method of claim 15 , further comprising contacting the lignin-containing remainder with a lignin extraction solution to produce a lignin extract and a cellulosic remainder, wherein the lignin extraction solution comprises a limited-solubility solvent, an organic acid, and water, and wherein the limited-solubility solvent and water form an organic phase and an aqueous phase.

17. The method of claim 16 , further comprising separating the lignin extract from the cellulosic remainder; wherein the lignin extract comprises lignin dissolved in the limited-solubility solvent.

18. The method of claim 17 , further comprising one, two, or three additional step(s) selected from:

(iv) distilling or flash evaporating the lignin extract, thereby removing the bulk of the limited-solubility solvent from the lignin extract to obtain the lignin sample;

(v) heating the lignin sample, thereby removing trace limited-solubility solvent or water from the lignin sample; and

(vi) applying a vacuum to the lignin sample, thereby removing trace limited-solubility solvent or water from the lignin sample.

19. The method of claim 1 , wherein the heating comprises agitating and/or pressurizing the lignin and organic solvent.