Lignin Depolymerization into Aromatic Monomers over
Solid Acid Catalysts
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Abstract
It is imperative to develop an efficient
and environmentally benign
pathway to valorize profusely available lignin, a component of nonedible
lignocellulosic materials, into value-added aromatic monomers, which
can be used as fuel additives and platform chemicals. To convert lignin,
earlier studies used mineral bases (NaOH, CsOH) or supported metal
catalysts (Pt, Ru, Pd, Ni on C, SiO<sub>2</sub>, Al<sub>2</sub>O<sub>3</sub>, etc.) under a hydrogen atmosphere, but these methods face
several drawbacks such as corrosion, difficulty in catalyst recovery,
sintering of metals, loss of activity, etc. Here we show that under
an inert atmosphere various solid acid catalysts can efficiently convert
six different types of lignins into value-added aromatic monomers.
In particular, the SiO<sub>2</sub>–Al<sub>2</sub>O<sub>3</sub> catalyst gave exceptionally high yields of ca. 60% for organic solvent
soluble extracted products with 95 ± 10% mass balance in the
depolymerization of dealkaline lignin, bagasse lignin, and ORG and
EORG lignins at 250 °C within 30 min. GC, GC-MS, HPLC, LC-MS,
and GPC analysis of organic solvent soluble extracted products confirmed
the formation of aromatic monomers with ca. 90% selectivity. In the
products, confirmation of retention of aromatic nature as present
in lignin and the appearance of several functional groups has been
carried out by FT-IR and <sup>1</sup>H and <sup>13</sup>C NMR studies.
Further, isolation of major products by column chromatography was
carried out to obtain aromatic monomers in pure form and their characterization
by NMR is presented. A detailed characterization of six different
types of lignins obtained from various sources helped in substantiating
the catalytic results obtained in these reactions. A meticulous study
on fresh and spent catalysts revealed that the amorphous catalysts
are preferred to obtain reproducible catalytic results