Lignin is one of the main components of lignocellulosic biomass, and is co-produced during cellulose fragmentation in pulp industry or ethanol biorefinery but lignin is still poorly valorized. Nevertheless, its abundance and its polyaromatic structure composed of hydroxypropyl-methoxyphenols units make it the resource of choice for aromatic compounds (phenolics or BTX). Even if the conversion of lignin is not simple due to the complexity of its chemical structure, its valorization is thus highly desirable for sustainability. In order to obtain high yield of liquid from lignin, we undertook the catalytic hydroconversion of a wheat straw soda solid sample using a classical hydrotreating catalyst. A parallel study about the lignin solvolysis was undertaken in order to determine the impact of the solubility in this depolymerization reaction. A deeper understanding of this thermochemical transformation opens new perspectives for developing new selective and efficient catalysts targeting the valorization of an available resource and fitting with the need of carbon economy and more sustainability in the future especially for the biorefineries and pulp industry.
A semi-continuous reactor has been specifically designed for the catalytic hydroconversion of a technical lignin in order to get higher performances and to follow the products evolution in function of the time. A reflux system combined with separator/condensers and cold traps allow recovering water, light aromatic and naphthenic products from the reactive mixture without over-conversion. The tetraline solvent used for its H-donor properties is also recycled thanks to this reflux system. The use of a products recovery protocol and the systematic analysis of all the fractions permit to follow the evolution in function of the residence time and propose a general reaction scheme. Those experimental data combined with thermodynamics have been further used to build a kinetic model using a lumped approach. An in-situ ATR characterization was used to follow the degradation of the lignin during the lignin solvolysis.
The degradation of a wheat straw lignin was followed in function of the time and all the thermal and catalytic processes were assigned. High yield of liquid was obtained thanks to the methodology, the designed reactor and the catalysts used. The composition of liquids and solids was established thanks to the analytical strategy. A kinetic model was built on a lumped approach thanks to the experimental data and reaction scheme. The bio-liquid contains phenolic, naphthenic and benzenic monomers but oligomers still remain in a great part. The solvolysis study showed a partial thermal degradation from 240°C.
In this work we observed that the limiting reactions during the process are mainly the catalytic ones, especially the deoxygenation of phenolics to aromatics if the target is BTX with preferentially low hydrogenation to naphthenic. In addition, the oligomers are still numerous after 13h of reaction and in order to obtain more monomers, we need to increase C-C bonds inter-units cleavages. We thus need to find a catalyst with higher performances in direct deoxygenation of phenolics and catalytic cracking in those conditions. A new catalytic screening must be performed.
Besides Junjie Pu PhD manuscript available online from the 01/12/2019, we two papers: one on experimental data still under review at Appl. Catal. B, and a second one on the kinetic model, accepted after minor revision at Chem. Eng. J. 2019. Aurélie Galfré work on solvolysis was not published yet but one review on lignin solvolysis and one article on ATR characterization of lignin are planned. Numerous communications have been done by the different partners in international congresses.
Nowadays, any renewable resource able to replace fossil energies is under deep investigation due to the need of energetic independence and to environmental concerns. Among bio-resources, lignin, one of the main components of lignocellulosic biomass with cellulose and hemicellulose, is the unique precursor for aromatic compounds thanks to its particular structure. Lignin is a by-product of pulp industry that is currently valorized as a low-value-fuel or to generate electrical energy. Other lignin sources already exist and it may be assumed that the development of bio-refineries will soon provide even greater amount of lignin consequently to the production of cellulosic ethanol. The valorization of this material is thus highly desirable. Depending on the pretreatment used and on the origin of the lignin, its structure and physico-chemical properties may differ in a large extent. In consequence, the conversion process of such a complex and fluctuating feed needs to be flexible. The catalytic hydroconversion of a wheat straw lignin, under high hydrogen pressure and in presence of a hydrotreating catalyst in a batch reactor, was successfully carried out by one of the partners. Excellent mass balance and high liquid yield were reached. Analytic tools like GPC, NMR, GCxGC were extensively used to understand the reactions occurring on the lignin and to characterize the different produced fractions. The liquid fraction contained a complex mixture of alkanes, phenol, aromatic and naphthenic compounds in tetralin used as solvent. The H-donor properties of the solvent were found necessary to avoid condensation reaction and residues formation. However, the solubility of the lignin in this solvent has to be elucidated. From our knowledge, contrarily to the lignin conversion which is not really impacted, the selectivity in the liquid phase can be orientated by changing the nature of the heterogeneous catalyst. The main objective of this project is to develop a flexible process adapted to a wide range of lignins with selective catalysts in order to get mainly aromatics in the liquid effluents for a potential industrial valorization. In order to achieve this goal, the hydroconversion of, at least two different types of lignins, one from pulp industry, and one produced during cellulosic ethanol production, will be studied in addition to the wheat straw lignin used previously. The methodology planned for this project includes a study of the lignin solubility, a catalytic screening, the kinetic modeling of the hydroconversion and the evolution of the batch system into a semi-continuous reactor with further improvements, like for instance, solid injection, continuous gas phase feeding and water removal. The catalyst screening will be driven by the selective production of aromatic compounds and based on previous achievements on the study of bio-oils. A kinetic modeling study of the lignin hydroconversion, based on a stochastic approach from carbon numbers distribution, will afford a better understanding of the different steps involved according to the nature of the solvent and the temperature. All these results will be a first step towards the design of a continuous process more suitable for industrial applications. The consortium involved for this project includes academic experts in the different complementary fields such as heterogeneous catalysis, kinetic modeling, chemical engineering and one industrial partner to fit with the industrial requirements in this particular challenging scientific area.
Madame Dorothée Laurenti (Institut de recherches sur la catalyse et l'environnement de Lyon)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
LGPM Laboratoire de génie des Procédés et Matériaux
LAGEP Laboratoire d'automatique et du génie des procédés
TRTF Total Research and Technology Feluy
IRCELYON - CNRS Institut de recherches sur la catalyse et l'environnement de Lyon
Help of the ANR 469,029 euros
Beginning and duration of the scientific project: September 2014 - 42 Months