Production of monoaromatics from lignin

Abstract

Monomeric aromatics can be applied as transportation fuel components. This work aims to contribute to developing a sustainable, carbon-neutral alternative to petroleum-derived aromatics by tapping into the most abundant biological source of aromatics; lignin. The nature of lignin poses a number of challenges to its utilization in the fuel supply by virtue of its recalcitrance and oxygen content. One possible strategy to overcome such challenges is to reduce isolated lignins by exposing them to hydrogen in presence of chemical catalysts. Such process can be termed hydrotreatment, hydroprocessing, or hydrogenolysis.

In this work, the hydrotreatment of organosolv lignin in presence of a 3% Pd/C heterogeneous catalyst was studied under different combinations of reactions parameters; initial hydrogen pressure, reaction temperature, and reaction time. The products of hydrotreatment were analyzed by a number of methods. Gravimetric analyses were used to calculate the yields of bio-oils and of char. Elemental analyses were applied to determine the levels of deoxygenation and hydrogenation, as well as to estimate the higher heating value of the products. Gel permeation chromatography was used to quantitate the decrease in molar mass of the products with respect to the feedstock, thereby representing the degree of depolymerization. Finally, gas chromatography with mass spectroscopy was used to identify monoaromatic components in the products.

Oxygenated monoaromatics such as guiacols and syringols were identified in hydrotreatment products. On the other hand, the best results in terms of deoxygenation, heating value, and depolymerization were attained in an experiment carried out at 400°C, which is considered a high temperature in the framework of this study. Higher initial hydrogen pressures (50 bar) were observed to improve yields of bio-oil and to hamper char formation. On the other hand, the reaction times tested (30-180min) did not display any marked effects on the products.