(496c) Lignin Depolymerization into Aromatic Monomers over Acidic Mesoporous Silicates

Lignin
Depolymerization into Aromatic Monomers over Acidic Mesoporous Silicates

Kakasaheb Y. Nandiwale^a,b,
Andrew M. Danby^a, Anand Ramanathan^a, Raghunath V.Chaudhari^a,b,
and Bala Subramaniam^{a, b}

^aCenter for Environmentally
Beneficial Catalysis, The University of Kansas, Lawrence, KS 66047, USA.

^bDepartment of Chemical and
Petroleum Engineering, The University of Kansas, Lawrence, KS 66045, USA.

Keywords: Biomass, Lignin depolymerization,
mesoporous silicates

We
report on novel applications of Zr-incorporated mesoporous silicates for the
one-pot conversion of lignin into value added aromatic monomers. Quantitative
analysis of the depolymerized products along with mass balance based on the
overall solid and liquid products are reported.

Cubic
mesoporous silicates, KIT-6 and KIT-5, modified with different Zr loadings (1-6
wt%) display dominant Lewis acidity. These materials display superior lignin
depolymerization activity compared to commercial zeolites that are typically
Brønsted acids. Briefly, batch depolymerization
of dealkaline lignin (TCI chemicals) was carried out in a 300 mL Parr reactor
at 250°C, 100 psi N₂, 1000 RPM for 30 min in a methanol + water
solvent mixture.  The performances of 12
commercially available zeolite catalysts were evaluated. H-ZSM-5 (Si/Al = 11.5)
was found to be the best catalyst among the commercial zeolites evaluated,
providing 92% lignin conversion. Approximately 54 wt.% of the products are
soluble in THF while the lignin substrate is insoluble in THF. The identified
aromatic monomers in the THF-soluble fraction represent 4.4 wt. % of the
initial lignin. These results with H-ZSM-5 are consistent with those reported
in the literature. Commercial H-ZSM-5 catalyst was therefore selected for
further comparative study with Zr-KIT-5 and Zr-KIT-6 catalysts. Zr-KIT-5(Si/Zr
= 20) provided the best catalytic performance with 97% lignin conversion,
yielding 65% of THF soluble products containing 5.5% identified monomers. Gel
permeation chromatography (GPC) analysis of depolymerized product from both
runs indicates a decrease in the molar mass when compared to the lignin
substrate, clearly implying lignin deconstruction. Approximately 25 monomers
were identified (by GC-FID, GC-MS) in the THF-soluble product mixture and 15
monomers were identified using external standards. Most of the identified
products are typically expected from lignin depolymerization as reported in the
literature.  These results, along with fundamental
insights into the correlation between catalyst activity and total acidity of
catalyst are presented.