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Hydrothermal Liquefaction of Enzymatic Hydrolysis Lignin – Biomass Pretreatment Severity Affects Lignin Valorization Mads M. Jensen, Demi T. Djajadi, Cristian Torri, Helene B. Rasmussen, Rene Bjerregaard Madsen, Elisa Venturini, Ivano Vassura, Jacob Becker, Bo B. Iversen, Anne S Meyer, Henning Jørgensen, Daniele Fabbri, and Marianne Glasius ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04338 • Publication Date (Web): 18 Mar 2018 Downloaded from http://pubs.acs.org on March 19, 2018
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ACS Sustainable Chemistry & Engineering
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Hydrothermal Liquefaction of Enzymatic Hydrolysis Lignin –
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Biomass Pretreatment Severity Affects Lignin Valorization
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Mads M. Jensen1, Demi T. Djajadi2, Cristian Torri3, Helene B. Rasmussen1, René B. Madsen1, Elisa
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Venturini4, Ivano Vassura4, Jacob Becker1, Bo B. Iversen1, Anne S. Meyer2, Henning Jørgensen2,5,
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Daniele Fabbri3, Marianne Glasius1*
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Department of Chemistry and iNANO, Aarhus University, Langelandsgade 140, 8000 Aarhus C,
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Denmark
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Plads Building 229, 2800 Kgs. Lyngby, Denmark
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Bologna, Italy
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Department of Chemical and Biochemical Engineering, Technical University of Denmark, Søltofts
Dipartimento di Chimica “Giacomo Ciamician”, University of Bologna, Via Selmi 2, 40126
Department of Industrial Chemistry “Toso Montanari”, University of Bologna, Viale Risorgimento
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4, 40146 Bologna, Italy
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Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark
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*corresponding author:
[email protected] Present address: Department of Plant and Environmental Sciences, University of Copenhagen,
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Abstract
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Alkaline hydrothermal liquefaction (HTL) of lignin-rich enzymatic hydrolysis residues (EnzHR)
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from wheat straw and Miscanthus x giganteus was performed at 255, 300, and 345 °C to investigate
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valorization of this side-stream from second-generation bioethanol production. The EnzHR were
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from biomass hydrothermally pretreated at two different levels of severity (190 °C, 10 min or 195
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°C, 15 min), and HTL at 300 °C of these EnzHR showed the most effective lignin depolymerization
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of the low severity EnzHR for both wheat straw and Miscanthus. The degree of depolymerization
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during HTL was temperature dependent and was not complete after 20 min at 255 °C, most
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distinctly for the Miscanthus EnzHR. The yields of 128 monomeric products quantified by gas
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chromatography – mass spectrometry were up to 15.4 wt% of dry matter. Principal component
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analysis of the quantified compounds showed that non-lignin HTL products are main contributors to
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the variance of the HTL products from the two biomasses. The chemically modified lignin polymer
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was found to have increased thermal stability after HTL. Analytical pyrolysis was applied to
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investigate the chemical composition of a larger fraction of the products. Analytical pyrolysis
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contributed with additional chemical information as well as confirming trends seen from quantified
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monomers. This work is relevant for future lignin valorization in biorefineries based on current
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second-generation bioethanol production.
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Keywords: Base-catalyzed depolymerization, biorefinery, hydrothermal pretreatment, principal
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component analysis, PARAFAC2, thermogravimetry, size exclusion chromatography
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Introduction
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Lignocellulosic side-streams from food production, paper pulp production, forestry and agriculture
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are good feedstock candidates for sustainable fuels and chemicals. One example is wheat, which
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can be utilized to produce food and straw for biorefining into fuel (bioethanol)1 and potentially
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chemicals.2 Additionally, energy crops such as Miscanthus are candidates for production of liquid
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fuels and chemicals.3 Lignocellulosic biomass primarily consists of cellulose, hemicelluloses,
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lignin, and extractives.4 Lignin makes up 15–35 wt% of lignocellulosic biomass and is the largest
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renewable source of aromatics.5-6 Only about 2% of industrially extracted lignins are used as
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chemicals, e.g., as dispersants.7 Lignin-rich enzymatic hydrolysis residue (EnzHR) from second-
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generation bioethanol production is an emerging type of industrial lignin and only about 40% of this
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lignin side-stream is needed for internal energy use.1,6
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Biomass in aqueous slurry can be converted into valuable products by hydrothermal liquefaction
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(HTL) at 200–370 ˚C and 4–20 MPa, where the changes in the physicochemical properties of water
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result in unique solvation and reaction conditions.8-9 The reaction chemistry of lignin HTL is
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dominated by ionic rather than radical reactions.10 The utilization of water for HTL also means that
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energy-intensive drying of wet biomasses is avoided. This makes HTL an ideal match in a
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biorefinery setup with the utilization of the wet lignin residue resulting from the ethanol production,
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which is currently dried and burned.1 Products from HTL of lignin have been shown to have
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antifungal properties11 and can partly substitute bisphenol A in an epoxy resin.12
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The plant species, lignin extraction method (kraft, soda, etc.), biomass pretreatment method
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(hydrothermal, dilute acid, etc.), and pretreatment severity affect the structure and chemical
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composition of the lignin, and can thus be expected to affect the HTL products.13-14 Differences in
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lignin HTL products due to plant species are easily evident by comparing softwood lignin (guaiacyl
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(G)
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hydroxyphenyl/guaiacyl/syringyl (HGS) type) also producing syringol.15-17 Effects of type of
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pretreatment on lignin HTL products have also been investigated, e.g., choice of organosolv
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solvent18 and mechanical pretreatment.19 Hydrothermal pretreatment is used in recent second-
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generation bioethanol plants of demonstration1 and full scale.20 Hydrothermal pretreatment is the
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treatment of biomass with steam without additives.1 Increasing the severity of hydrothermal
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pretreatment of wheat straw from a severity factor of log R0 2.77 to 3.65 results in a large reduction
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of β-O-4 ether bonds and likely formation of new β-5 C-C bonds in the lignin.21 Pretreatment of
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aspen wood with steam explosion in the log R0 range 3.2–4.5 cause an almost linear decrease of β-
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O-4 bonds.22 The change of lignin inter-unit bonding is likely to affect subsequent valorization of
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the lignin in the EnzHR from second-generation bioethanol production. Yet, the effects of varying
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the severity of hydrothermal pretreatment have, to our knowledge, not received any attention in
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HTL literature.
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In this work, alkaline HTL was performed on EnzHR from two grass species (Miscanthus and
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wheat straw) and two different and industrially relevant hydrothermal pretreatment severities (log
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R0 3.65 and log R0 3.97) after initial screening of HTL temperature (255 °C, 300 °C, and 345 °C), to
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investigate the effects of these parameters on HTL product composition. The composition of
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feedstock and HTL products, monomeric as well as polymeric compounds, was characterized using
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mainly size exclusion chromatography (SEC) and gas chromatography – mass spectrometry (GC-
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MS) combined with pyrolysis techniques. The study is subdivided into effects of varying HTL
type)
producing
phenols/guaiacols/catechols
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and
grass
lignin
(p-
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temperature for both biomasses (Part I) and effects of varying pretreatment severity for both
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biomasses (Part II).
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Materials and methods
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Detailed descriptions of materials and experimental methods can be found in the Supporting
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Information. Results are reported as mean and standard deviation of data obtained from replicate
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HTL experiments, i.e., covering uncertainties of the entire procedure.
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Enzymatic hydrolysis residues
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EnzHR used as feedstocks for HTL were prepared by subjecting Miscanthus x giganteus stalks and
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wheat straw to hydrothermal pretreatment (steam pretreatment with autohydrolysis) at severity log
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R0 3.65 (190 °C, 10 min) or log R0 3.97 (195 °C, 15 min) as detailed elsewhere.23 The pretreated
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biomasses were then cut to
