Selective aerobic alcohol oxidation method for conversion of lignin into simple aromatic compounds
Described is a method to oxidize lignin or lignin sub-units. The method includes oxidation of secondary benzylic alcohol in the lignin or lignin sub-unit to a corresponding ketone in the presence of unprotected primarily aliphatic alcohol in the lignin or lignin sub-unit. The optimal catalyst system consists of HNO 3 in combination with another Brønsted acid, in the absence of a metal-containing catalyst, thereby yielding a selectively oxidized lignin or lignin sub-unit. The method may be carried out in the presence or absence of additional reagents including TEMPO and TEMPO derivatives.
1. A method to oxidize lignin or lignin sub-units, the method comprising selectively oxidizing at least a portion of secondary benzylic alcohol groups in the lignin or lignin sub-unit to corresponding ketones while leaving unchanged at least a portion of unprotected primary aliphatic alcohols in the lignin or lignin sub-unit by contacting the lignin or lignin sub-unit with a catalyst comprising nitric acid (HNO 3 ) in combination with another Brønsted acid, in the absence of a metal-containing catalyst, thereby yielding a selectively oxidized lignin or lignin sub-unit, wherein the lignin or lignin sub-unit is a non-lignosulfonate lignin or lignin sub-unit.
2. The method of claim 1 , comprising contacting the lignin or lignin sub-unit with a catalyst in a polar aprotic solvent.
3. The method of claim 2 , wherein the solvent further comprises up to 20 wt % water.
4. The method of claim 2 , wherein the solvent comprises a nitrile.
5. The method of claim 2 , wherein the solvent comprises acetonitrile.
6. The method of claim 5 , wherein the solvent further comprises up to about 10 wt % water.
7. The method of claim 1 , wherein the catalyst comprises HNO 3 in combination with a Brønsted acid selected from the group consisting of hydrochloric acid (HCl), hydrobromic acid (HBr), hydrofluoric acid (HF), hydroiodic acid (HI), phosphoric acid (H 3 PO 4 ), sulfuric acid (H 2 SO 4 ), boric acid (B(OH) 3 ), tetrafluoroboric acid (HBF 4 ), perchloric acid (HClO 4 ), acetic acid (CH 3 C(O)—OH), trifluoroacetic acid (CF 3 C(O)—OH), methanesulfonic acid (CH 3 SO 3 H), solid acid resins containing sulfonic acid sites, and solid acid resins containing benzoic acid sites.
8. The method of claim 7 , wherein the catalyst comprises HNO 3 and HCl.
9. The method of claim 7 or claim 8 , wherein the HNO 3 and the Brønsted acid are each present in a concentration of from about 2 mol % to the lignin or lignin sub-unit to about 20 mol % of the lignin or lignin sub-unit.
10. The method of any one of claim 1 , 2 , 3 , or 7 , wherein the selective oxidation is conducted in contact with atmospheric air.
11. The method of any one of claim 1 , 2 , 3 , or 7 , wherein the selective oxidation is conducted in an environment comprising an O 2 partial pressure of at least about 1 atm.
12. The method of any one of claim 1 , 2 , 3 , or 7 , wherein the selective oxidation is conducted at a temperature of from about 10° C. to about 100° C.
13. The method of any one of claim 1 , 2 , 3 , or 7 , wherein the selective oxidation is conducted in the absence of a nitroxyl radical.
14. The method of any one of claim 1 , 2 , 3 , or 7 , wherein the selective oxidation is conducted in the presence of a reagent comprising a nitroxyl radical.
15. The method of claim 14 , wherein the nitroxyl radical is selected from the group consisting of T2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), 4-acetamido-(2,2,6,6-tetramethylpiperidin-1-yl)oxyl (NHAc-TEMPO), 4-C 1-6 -alkyloxy-TEMPO, 4-hydroxy-TEMPO, diphenylnitroxyl, di-tert-butylnitroxyl, 9-azabicyclo[3.3.1]nonane N-oxyl (ABNO), and 2-azaadamantane-N-oxyl (AZADO).
16. The method of claim 14 , wherein the reagent is present in a concentration of from about 2 mol % to about 10 mol % of the lignin or lignin sub-unit.
17. The method of claim 1 , comprising contacting the lignin or lignin sub-unit with a catalyst comprising HNO 3 and HCl, in a solvent system comprising acetonitrile and water, and the reaction is conducted in the presence of a reagent selected from the group consisting of 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), 4-acetamido-(2,2,6,6-tetramethylpiperidin-1-yl)oxyl (NHAc-TEMPO), 4-C 1-6 -alkyloxy-TEMPO, 4-hydroxy-TEMPO, diphenylnitroxyl, di-tert-butylnitroxyl, 9-azabicyclo[3.3.1]nonane N-oxyl (ABNO), and 2-azaadamantane-N-oxyl (AZADO).
18. The method of claim 17 , wherein the HNO 3 and HCl are each present in a concentration of from about 5 mol % of the lignin or lignin sub-unit to about 15 mol % of the lignin or lignin sub-unit.
19. The method of claim 17 , wherein the selective oxidation is conducted in contact with atmospheric air.
20. The method of claim 17 , wherein the selective oxidation is conducted in an environment comprising an O 2 partial pressure of at least about 1 atm.
21. The method of claim 17 , wherein the selective oxidation is conducted at a temperature of from about 10° C. to about 100° C.