Aqueous phase reforming of the residual waters derived from lignin-rich hydrothermal liquefaction: investigation of representative organic compounds and actual biorefinery streams

Secondary streams in biorefineries need to be valorized to improve the economic and environmental sustainability
of the plants. Representative model compounds of the water fraction from the hydrothermal liquefaction
(HTL) of biomass were subjected to aqueous phase reforming (APR) to produce hydrogen. Carboxylic and bicarboxylic
acids, hydroxyacids, alcohols, cycloketones and aromatics were identified as model compounds and
tested for APR. The tests were performed with a Pt/C catalyst and the influence of the carbon concentration
(0.3–1.8 wt. C%) was investigated. Typically, the increase of the concentration negatively affected the conversion
of the feed toward gaseous products, without influencing the selectivity toward hydrogen production. A
synthetic ternary mixture (glycolic acid, acetic acid, lactic acid) was subjected to APR to evaluate any differences
in performance compared to the tests with single compounds. Indeed, glycolic acid reacted faster in the mixture
than in the corresponding single compound test, while acetic acid remained almost unconverted. The influence
of the reaction time, temperature and carbon concentration was also evaluated. Finally, residual water resulting
from the HTL of a lignin-rich stream originating from an industrial-scale lignocellulosic ethanol process was
tested for the first time, after a thorough characterization. In this framework, the stability of the catalyst was
studied and found to be correlated to the presence of aromatics in the aqueous feedstock. For this reason, the
influence of an extraction procedure for the selective removal of these compounds was explored, leading to an
improvement in the APR performance.