Subcritical Water Reactions
of a Hardwood Derived
Organosolv Lignin with Nitrogen, Hydrogen, Carbon Monoxide, and Carbon
Dioxide Gases
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Abstract
Subcritical H<sub>2</sub>O at 365 °C is considered
for lignin
conversion, because H<sub>2</sub>O exhibits unusual properties at
higher temperatures (i.e., decreased ion product and static dielectric
constant), such that there is a high solubility for organic compounds.
This high solubility for organic compounds is expected to apply to
lignin for its conversion into high value transportation fuels, which
may prove the effectiveness of integrated biorefineries. Experiments
were conducted with hardwood derived Organosolv lignin, subcritical
H<sub>2</sub>O (defined here as H<sub>2</sub>O at 365 °C and
autogenous pressure), and various industrial gases (N<sub>2</sub>,
H<sub>2</sub>, CO, and CO<sub>2</sub> at a cold pressure of 500 psi)
for 30 min to determine both lignin’s potential to generate
value-added products (e.g., monomer compounds and methanol) without
the need for a catalyst and the roles (if any) of the H<sub>2</sub>O and the gases in the reactions. The behavior of H<sub>2</sub>O
at temperature (365 °C) and pressure within this research is
expected to be similar to the behavior of supercritical H<sub>2</sub>O (374 °C and 3205 psi), without the need to maintain supercritical
conditions. Different characterization techniques were used for the
products collected including primarily gas chromatography with flame
ionization detection and thermal conductivity detection (GC/FID-TCD)
of the evolved gases, GC/MS analysis of the organic liquids, solid
phase microextraction analysis of the recovered H<sub>2</sub>O, and
solid state <sup>13</sup>C NMR analysis of the solid residues. The
reactor pressure at temperature was shown to influence the outcome
of products, and the highest conversions (≈54–62%) were
obtained when adding gas. The collected solids from the N<sub>2</sub>, H<sub>2</sub>, and CO reactions appeared to be the most reacted
(i.e., the most changed from the unreacted lignin) according to solid
state <sup>13</sup>C NMR analysis, and the widest variety of products
(methoxy-substituted phenolic compounds) were also obtained when using
CO, according to GC/MS analysis
