(535a) Dual Functional Zr-KIT-5 Shows Remarkable Activity for Depolymerization of Corn Stover Lignin into Stable Phenolic Monomers

Kakasaheb Y. Nandiwale^1,2, Andrew M. Danby¹, Anand Ramanathan¹, Raghunath V. Chaudhari^1,2, and Bala Subramaniam^1,2

¹Center for Environmentally Beneficial Catalysis, University of Kansas, Lawrence, Lawrence, Kansas 66047, United States

²Department of Chemical and Petroleum Engineering, University of Kansas, Lawrence, Kansas 66045, United States.

Keywords: lignin depolymerization, Zr-KIT-5, Lewis acid, acetic acid

We previously showed that Zr-KIT-5 exhibits superior performance than H-ZSM-5 for depolymerization of dealkaline lignin yielding up to 77wt% of THF-soluble products (monomers and dimers). Structural degradation and loss of acidity occur in spent Zr-KIT-5 catalyst primarily due to a lack of hydrothermal stability in aqueous methanol solvent.¹

These insights motivated us to screen various non-aqueous solvents such as 1,4-dioxane, acetic acid, and γ-valerolactone (GVL) for their ability to solubilize lignin and to avoid the hydrothermal degradation of Zr-KIT-5 catalyst. As expected, the organosolv (corn stover-derived) lignin was completely soluble in acetic acid, the solvent used for separating the lignin from cellulose fraction. Significantly, the solvent exhibited thermal stability at typical depolymerization temperatures. Furthermore, Zr-KIT-5 catalyst showed much better physical stability in acetic acid solvent compared to aqueous (water + methanol) solvent. About 12 different phenolic monomers (including a few that are acetylated) were identified in the depolymerized product mixture by GC-MS/FID. The Lewis-acid sites of Zr-KIT-5 catalyst appear to facilitate not only depolymerization of lignin, but also acetylation of the monomers, thus preventing monomeric condensation/repolymerization. High yield of identified monomers (28 wt%), comparable to those reported to date, and ethyl acetate soluble aromatic species (43 wt%) were obtained with thermo-solvolytic treatment of lignin followed by catalytic depolymerization over Zr-KIT-5 catalyst. The Zr-KIT-5 was found to be a promising solid Lewis acid catalyst displaying stable activity for up to five catalytic lignin depolymerization cycles.

Reference:

(1) K.Y. Nandiwale, A.M. Danby, A. Ramanathan, R.V. Chaudhari, B. Subramaniam, ACS Sustainable Chem. Eng. 2017, 5, 7155.