University of Groningen Valorization strategies for pyrolytic lignin Bernardes Figueiredo, Monique

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University of Groningen

Valorization strategies for pyrolytic lignin

Bernardes Figueiredo, Monique

DOI:

10.33612/diss.111703614

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Bernardes Figueiredo, M. (2020). Valorization strategies for pyrolytic lignin. University of Groningen.

https://doi.org/10.33612/diss.111703614

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Catalytic hydrotreatment of pyrolytic lignins from

different sources to biobased chemicals: Identification

of feed-product relations

This chapter has been submitted to Biomass & Bioenergy in 2019.

Valorization Strategies

for

Pyrolytic Lignin

Monique Bernardes Figueirêdo

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The work described in this thesis was performed at the Department of Chemical Engineering (ENgineering and TEchnology institute Groningen - ENTEG), Faculty of Science and Engineering, University of Groningen, the Netherlands.

This research project was financially supported by the Science without Borders program from the Brazilian National Council for Scientific and Technological Development (CNPq), grant number 232493/2014-6.

ISBN (printed version): 978-94-034-2277-0 ISBN (electronic version): 978-94-034-2278-7 Cover design: Camila Schindler and Lovebird design. Layout: Lovebird design.

www.lovebird-design.com

Printing: Eikon +

Copyright © 2020 by Monique Bernardes Figueirêdo. All rights reserved. No part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, without prior permission of the author.

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Valorization Strategies for Pyrolytic Lignin

PhD thesis

to obtain the degree of PhD at the University of Groningen

on the authority of the Rector Magnificus prof. C. Wijmenga

and in accordance with the decision by the College of Deans. This thesis will be defended in public on

17 January 2020 at 14.30 hours

by

Monique Bernardes Figueirêdo

Born on 5 March 1990

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Supervisor

Prof. dr. ir. H.J. Heeres

Co-supervisor

Dr. P.J. Deuss

Assessment Committee

Prof. dr. F. Picchioni

Prof. dr. P.P. Pescarmona

Prof. dr. P.C.A. Bruijnincx

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Dedicated to my inspiring grandmothers Isabel, Luisa and Olga.

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Pyrolytic lignin: a promising feedstock for fuels

and valuable chemicals

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Abstract

Lignocellulosic biomass is a promising, widely available feedstock for the sus-tainable production of biofuels, biobased chemicals and performance materials. Biomass can be efficiently converted to pyrolysis liquids by the well-established fast pyrolysis technology. Such pyrolysis liquids may be further separated in an aqueous sugar rich phase and a water insoluble pyrolytic lignin (PL) fraction. The separation step allows the use of dedicated conversion strategies for PL, which is rich in phenolics and other aromatics, and the sugar rich phase. This is highly advantageous as these fractions are very dissimilar in composition and reactivity. Various upgrading technologies have been proposed and investigated for the sugar rich fraction, however, valorization of the PL fraction remains largely unexplored and challenging. The main issues encountered are related to the chemical structure of the PL, which is known to be oligomeric, heterogeneous and rich in stable C-C linkages between the aromatic rings. In this chapter the PL structure will be discussed in detail, followed by a state of the art regarding PL conversion by both oxidative and reductive (catalytic) strategies, as well as by a combination thereof. The possible products and related applications are explored, and recommendations for future research are provided.

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1.1. Introduction

The continuous growth in global population and improvements in living condi-tions are expected to lead to a substantial increase in the global energy demand, i.e. 32 % from 2017 to 2040 [1]. This forecast, combined with the predicted depletion of fossil resources and pressing environmental concerns related to the use of such resources, have encouraged research and development activities towards the use of renewable options like biomass. In the last decades, various strategies have been developed for biomass valorization into biofuels, biobased chemicals and performance materials. Particularly lignocellulosic biomass (plant based) is an important resource, being highly abundant and versatile. Lignocellulosic biomass consists mainly of three major biopolymers, namely cellulose, hemicellulose and lignin, each of them presenting a unique structure.

Cellulose is a linear homopolymer consisting of β-D-glucopyranose (also known as D-glucose) units, linked together by covalent β-1,4-glycosidic bonds and with different degrees of polymerization (up to 10,000 Da) [2]. These long glucose chains are arranged in a parallel mode and held together by hydrogen bonds that are responsible for the longitudinal strength and toughness of the leaves, roots and stems of plants. Hemicellulose also consists of polysaccharides, however, its chemical composition is more diverse and the backbone consists of shorter chains of both pentoses and hexoses (primarily xylose and mannose) with various substituents. It binds tightly, though non-covalently, to the surface of the cellulose microfibril, contributing to the cell wall strength. Lignin is pres-ent as an amorphous three-dimensional, highly branched interlocking network, constituted by phenylpropane units connected by C-O-C and C-C bonds with a variable degree of methoxylation [3]. Lignin’s main precursors are p-coumaryl, coniferyl and sinapyl alcohols (see Figure 1), further denominated p-hydroxy-phenyl (H), guaiacyl (G) and syringyl (S) units when incorporated into the macromolecule. The structure is overall complex and highly variable depending on the plant species, and even between seasons and age [4–7]. Lignin plays an important role in the cell wall, being for instance involved in water regulation processes and pathogen resistance, while also providing mechanical support, strength and rigidity to the plants tissues.

The amounts of the individual biopolymers, as well as moisture content and amounts of minor compounds, i.e. alkali metals and silica, vary significantly and are a function of the type of biomass, climate and harvesting conditions. Cellulose

p-coumaryl alcohol

(H) coniferyl alcohol (G) sinapyl alcohol (S)

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Py ro lyt ic lig nin: a p ro mi sin g f ee ds to ck f or f ue ls a nd va lu ab le c hemic al s

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and hemicellulose together comprise about 40–75 wt% of lignocellulosic biomass, while lignin corresponds for 15–40 wt% and minor compounds such as organic extractives and inorganic materials occur in between 2 and 10 wt% [8,9].

Due to the chemical heterogeneity and structural complexity of lignocellulosic biomass, its efficient fractionation and conversion to valuable chemical products is a major challenge. Nonetheless, extensive research exploring a wide range of processes over the past two decades have generated promising results, e.g. strategies for the complete transformation of biomass into fuels, hydrogen and polymer building blocks [10,11]. A conceptual approach was then introduced for lignocellulosic biomass conversions, the so-called biorefinery concept. It involves a refinery in which biomass feedstocks are processed with a minimum of waste and energy to obtain a combination of valuable chemicals, materials and energy.

1.1.1. Fast pyrolysis and the pyrolysis liquid biorefinery

Within the biorefinery concept, fast pyrolysis stands out as an attractive primary thermochemical process to liquefy biomass due to the flexibility of feedstock and process conditions, relatively low cost and high energy conversion efficiency [12,13]. For instance, yields of up to 75 wt% of pyrolysis liquid (also known as pyrolysis oil and bio-oil) can be achieved, with char and gaseous compounds as by-products. During fast pyrolysis, thermal conversion of biomass takes place at elevated temperatures and in the absence of oxygen. The process requires rapid heating of the biomass particles along with rapid cooling of the hot vapors (residence times < 10 seconds) in order to maximize the liquid yield [14]. The product distribution and composition vary with feedstock properties, process conditions and the reactor type. A number of reactor technologies (e.g. rotating cone, fluidized bed) have been developed, being currently close to commercial cost-effective operation [15].

The obtained pyrolysis liquid is a dark brown, free flowing liquid with a pungent odor. Compared to the solid biomass feed, it is more easily transportable and stored. Nonetheless, besides the direct use for heat, power generation and/or gas-ification, pyrolysis liquids properties hinder higher-value end uses. For instance, pyrolysis liquids have a relatively poor higher heating value (HHV) of about half of that of crude oils. Furthermore, they are hydrophilic (thus not miscible with crude oil or crude oil fractions), acidic and present limited thermal and storage stability (i.e. ageing) [16,17]. The high oxygen content (up to 40 wt% [18]) is the primary reason for such differences, as pyrolysis liquids are a complex mixture of water (up to 30 wt% [18]) and oxygenated compounds. Actually, instead of an ‘oil’, it is considered a micro-emulsion in which the continuous phase is an aqueous solution of cellulose and hemicellulose decomposition products that stabilizes the discontinuous phase of lignin fragments, for example through hydrogen bonding [19]. Oxygen is present in most of the +300 compounds already identified in the product [20], which include, among others, phenols and methoxy-phenols, anhydrosugars, ketones, aldehydes and furans. Notwithstanding all analytical efforts, still only about 40–50 wt% of the components that make up the pyrolysis liquid has been completely characterized, since the large, non-volatile molecules present are more difficult to characterize.

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Figur e 2. Th e p yr ol ysi s liq uid b io refin er y [28].

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Pyrolysis liquids require further processing to convert them into value-added products. An increasingly popular strategy consists in fractionating the pyrol-ysis liquid by adding water, which leads to the separation of a sugar (aqueous) fraction and a water-insoluble fraction comprised mostly of the lignin-derived aromatic fragments [21]. The latter is referred to as pyrolytic lignin (PL). The two fractions, intermediates in a pyrolysis liquid biorefinery scheme, can be processed independently by strategies tailored to their nature and inherent properties into a wide range of valuable products, e.g. alkylphenolics [22,23], biofuels [24,25], hydroxymethylfurfural (HMF) [26], as well as feedstocks suitable for co-feeding traditional refineries [27]. The pyrolysis liquid biorefinery is illustrated in Figure 2. While upgrading of the sugar fraction has been studied in detail, e.g. to ob-tain biobased sugars (e.g. fermentation), efficient conversion of the PL fraction remains a challenge due to its complex structure. In detail, research on the PL fraction is still on its infancy (vide infra), and its potential as feedstock for the production of valuable chemicals needs to be explored in depth to increase the techno-economic feasibility of pyrolysis liquid biorefineries, thus enabling their realization in a near future.

1.1.2. PL formation and properties

The thermal decomposition of lignin in lignocellulosic biomass is a complex pro-cess that involves numerous pathways, including competitive and/or consecutive reactions throughout a wide temperature range [29,30]. During pyrolysis, exten-sive cleavage of C-C and C-O-C linkages occurs. In particular, the β-O-4 linkages that make up 60 % of the interaromatic linkages in native lignin are fully broken down [31,32]. Cleavage of the propanoid side chains leads to the formation of considerable amounts of monomeric and oligomeric alkylated methoxyphenols, which can be further converted to alkylated phenols [30,33]. The biomass source determines the product composition to a large extent. Softwood lignins form mainly guaiacols, whereas hardwood lignins are decomposed in both guaiacols and syringols [34]. A large fraction of the PL consists of substituted phenolic

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oligomers, for which two main routes of formation are proposed (see Figure 3): i) direct thermal ejection into the gas phase [35,36]; ii) repolymerization of highly reactive monomeric species during pyrolysis and/or during condensation of the pyrolysis vapors [30,37–39]. Gaseous compounds (e.g. CO, CO2, H2 and CH4) and char are also produced from reactions taking place during lignin pyrolysis,

i.e. decarbonylation, decarboxylation, dehydration, cracking and

repolymeriza-tion [29,34,39–41].

The visual appearance of PL is a function of the biomass source, the pyrolysis process and the water fractionation protocol. Regarding the latter, a brown hy-groscopic powder is obtained when using an elaborated water wash protocol [42], while a viscous dark brown oil is obtained when using a simplified fractionation procedure using less water [21,43]. Previous literature on PL characterization provides some insight on its main structure, showing that it consists mainly of trimers and tetramers of HGS units (proportions depend on the biomass), as anticipated from the harsh pyrolysis conditions leading to thermally driven depolymerization reactions [35,44–46]. Compared to other technical lignins obtained from industrial processes (e.g. Kraft, Alcell), PL has a substantially lower molecular weight (Mw) [42,47]. New types of inter-unit linkages different

from the typical alkyl-aryl-ether in native lignins are formed, particularly car-bon-carbon linkages (e.g. diphenyl, diaryl methine) and saturated aliphatic side chains [35,44,48–50]. Residual alkyl-aryl-ether linkages might be present as a result of the thermal ejection of (less modified) lignin oligomers [36]. Importantly, thermal splitting during pyrolysis is also reported to generate unconjugated car-bonyl groups and C-C double bonds [35,46,49], while the amount of methoxy groups and aliphatic hydroxy groups decreases substantially when compared to native lignin [46]. Figure 4 shows different proposed molecular structures for PL oligomers obtained from various biomass sources and pyrolysis conditions, based on different analytical observations.

1.2. PL valorization

The low amount of monomers and chemical heterogeneity requires further de-polymerization so that PL can be used as a source for biobased chemicals and fuels. To that end, both reductive and oxidative approaches can be applied [23]. Catalytic hydrotreatment is a well-explored reductive strategy for the upgrading of pyrolysis feedstocks [52,53], and the state of the art on PL hydrotreatment is presented in section 1.2.1. Subsequently, potential oxidative approaches for the valorization of PL are explored in detail in section 1.2.2., and combined multistep systems are disclosed in section 1.2.3.

Importantly, the development of processes able to break down the complex lignin structure represents an important contribution towards sustainability and the establishment of a circular economy. Such processes, in line with the biorefinery concept, may open pathways for obtaining drop-in chemicals that can be easily incorporated in the current petrochemical-based industries and markets, as well as be used as biobased energy carriers, innovative performance materials and platform molecules [54]. As typical examples, the phenolic and aromatic lignin building blocks can serve as high-value platform chemicals, (di)

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Figure 4. Structures of PL oligomers proposed in the literature. (A) G-based tetramers from pine sawdust PL [36] (B) Pentamer from red oak PL [48] (C) Oligomers from beech wood PL [35]

(D) Oligomer from switchgrass PL [51].

carboxylic acids formed during oxidation are of interest for various applications in the polymer and food industries [23,55,56] and esterified mixtures may be targeted as biobased fuels or (fuel) additives [57]. Furthermore, residue streams can be used to supply energy to the process.

1.2.1. Reductive approaches for PL valorization

Catalytic hydrotreatment is a known upgrading strategy for pyrolysis liquids, which involves the catalyzed reaction of pyrolysis liquids with molecular hy-drogen. Upon this treatment, hydrodeoxygenation and hydrocracking reactions occur, and valuable monomers can be obtained. Hydrodeoxygenation (HDO) is, in theory, closely related to the hydrodesulphurization (HDS) process performed in refinery industries for the removal of sulfur from petrochemical-based mix-tures [58,59]. Both processes use molecular hydrogen to eliminate the undesired heteroatom, forming respectively H2O and H2S. However, despite the conceptual similarity, petrochemical-based and biobased feedstocks are extremely different in terms of structure and properties, and the oxygen content of the latter is much higher than the sulfur content in fossil fuels. This makes hydroprocessing of pyrolysis feedstocks challenging, as will be shown below.

During catalytic hydrotreatment, the highly complex chemical character of pyrolysis feedstocks typically leads to low product selectivity, as different starting materials combined with multiple reaction pathways occur simultaneously (see Figure 5) [60]. Hydrogen consumption is related to the conversion of various compound classes (acids, aldehydes, ketones, double bonds, etc), and com-plex molecules can be accompanied by (often undesired) saturation [61]. Ide-ally, the hydrogen use during hydroprocessing should be minimized, as it is an important cost contributor, and carbon losses to the gas phase resulting from

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decarbonylation, decarboxylation and methanation pathways must be suppressed to enhance the overall carbon yields. Furthermore, thermally-induced repolymer-ization pathways [62] must be prevented, as they ultimately lead to char formation. Catalyst selection is of paramount importance for the process. For instance, good catalyst stability, selectivity and activity under the harsh conditions applied and in the aggressive reaction medium (which contains water and acidic com-pounds) are fundamental, yet difficult to achieve [53,63]. Various deactivation mechanisms have been described in the literature, e.g. blocking of active sites, poi-soning, sintering and coking [64–66]. Despite the drawbacks observed, promising results have been reported for the catalytic hydrotreatment of biobased streams [53,67,68]. Experimental studies have been extensively performed with whole pyrolysis liquids at different conditions and using a range of catalysts, which in-clude, among others: i) typical HDS catalysts, e.g. NiMo/Al2O3 and CoMo/Al2O3 [69–73], which need an external sulfur addition to improve activity and stability [60,74]; ii) supported noble metal catalysts, e.g. Pd/C, Pt/C, Ru/C [62,67,75,76],

iii) inexpensive Ni-based supported catalysts [68,77–81]. There is still great

potential for further improvements through the use of tailored bimetallic and bifunctional supported catalysts [82–84].

The catalytic hydrotreatment of the PL fraction of pyrolysis liquids in particular has been less explored. Nonetheless, interesting results were reported in studies using a range of different catalysts, set-ups, reaction conditions and solvents. Re-garding the latter, as some studies explored the use of hydrogen donors (e.g. alco-hols, tetralin) instead of molecular hydrogen, the term catalytic depolymerization

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is used here to refer to both catalytic hydrotreatment and transfer hydrogenation strategies. Table 1 shows an overview of the literature regarding the catalytic depolymerization of PL. Importantly, solvent-free processes are generally pre-ferred, as PL is a liquid at relatively low temperatures and can therefore act as a liquid reaction medium itself. This prevents extra separation steps and other issues related to the use of solvents, such as participation in reaction pathways. In some studies, the pyrolysis step was performed with technical lignins (e.g. Kraft, Alcell) instead of a lignocellulosic biomass, and the resulting lignin oil was then hydrotreated [85,86]. The vast majority of the literature regarding PL hydrotreatment consists of exploratory studies performed in batch set-ups. While these studies have given insights in reaction pathways, continuous processes suitable for a future scale-up need to be investigated in the future.

Monomer yields vary substantially, but it should be noted that a comparison is difficult due to differences in process conditions, particularly in the approach to determine the yield of such monomeric products (e.g. based on input, based on product oil, based on GC detectables, etc). Even so, advanced analytical techniques such as NMR and GC×GC recently provided valuable information regarding the product composition and the amounts of alkylphenolics and ar-omatics obtained from PL hydroprocessing [42,43,47]. Advantageously, these monomers are present in considerable amounts, and the compounds typically identified can be used in various existing applications, e.g. polymers, dyes, resins, fine chemicals, fuels and fuel additives [23,87]. Noble metal catalysts were shown to favor the over-reduction of aromatic rings, leading to higher yields of (cyclo) alkanes [43,86]. The aqueous phase formed due to oxygen removal contains residual polar compounds and can be further converted to hydrogen and small hydrocarbons via aqueous phase reforming [88].

So far, most research is exploratory in nature and typically performed in batch set-ups. Further process development is required, among others the use of con-tinuous set-ups. Important aspects to be explored are: i) detailed catalyst studies aimed at the selection of improved catalysts regarding activity, selectivity, stability and recyclability ii) the biomass source, which has a direct impact on the PL struc-ture and properties; iii) the pyrolysis conditions, which also have a direct impact on the PL structure and properties; iv) the hydrotreatment conditions, e.g. pressure, temperature, (residence) time; v) the hydrogen donor, e.g. H2, methanol, formic acid; vi) the reactor configuration; vii) new fractionation approaches for PL [99].

1.2.2. Oxidative approaches for PL valorization

Oxidative lignocellulosic biomass conversion processes have been performed industrially for many decades, and a representative example is the pulp bleaching process [100]. Here, residual highly colored lignin fragments are oxidized to low molecular weight water soluble compounds. Several oxidants are used (e.g. chlorine, oxygen, hydrogen peroxide), which react via electrophilic and radical mechanisms with aromatic chromophores and promote delignification [101,102]. An oxidative strategy similar to the one used in pulp industries has been recently proposed [103–107] for the deconstruction of lignin prior to enzymatic digestion of cellulose, i.e. a first step able to significantly decrease biomass recalcitrance

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and, consequently, increase the availability of biomass derived sugars for fur-ther processing. This strategy takes advantage of the fact that lignin structures are highly prone to oxidation, which potentially leads to many possibilities for converting lignin into valuable products via oxidative strategies [102,108–110]. Studies regarding the oxidation of PL in particular are nearly absent, with the only single example being an oxidation system with oxygen and polyoxometalates as the catalysts, in which PL showed the highest product yields compared to other feeds [111]. The oxidation of other lignin types (e.g. alkali, Alcell), however, as well as of lignin model compounds, has been extensively investigated with oxygen,

Table 1. Overview of literature for the catalytic depolymerization of PL and lignin oils.

Biomass source T (°C) P (bar) Catalyst(s) Solvent _{yield (wt%)}Monomer Ref.

Water-extracted PL from pyrolysis liquid, solvent used as hydrogen donor

Mixed maple wood

25–150 50 Ru/TiO2 Ethanol 15–16.3a _[89]

Rice husk 260 20 Ru/ZrO2/SBA-15 Ethanol — [90]

Red oak 150 35 Ru/C Ethanol — [48]

Maple wood 340–415 1 HZSM-5 Tetralin 22.2–31.3b _[91]

Water-extracted PL from pyrolysis liquid, H2 used as hydrogen donor

Pine wood 340 35 HZSM-5, α-Al2O3,

MoO3

— 3.1–17.1c _[92]

Hog fuel 230–415 140 CoMo — 50d _[93]

Forestry residue 220–310 190 Ru/C — — [94]

Pine wood 450 100 Limonite — 23.4e _[47]

White oak 150–400 69–167 NiMo/Al2O3,

Pd/C, Pt/C

— — [95]

Pine wood 300–400 190–200 Ru/C, NiMo/Al2O3 — — [96]

Pyrolyzed lignin oils, solvent used as hydrogen donor

Rice huskf _150–170 ₄₀ _Ru/SBA-15 _Isopropanol _83–85c _[97]

Alcell ligning ₃₅₀ ₁₀₀ _Ru/C _Dodecane ₂₆h _[85]

Pyrolyzed lignin oils, H2 used as hydrogen donor

Kraft and

Orga-nosolv ligning 350–400 100 Ru/C, 20NiMoP/_{AC, CoMo/Al2O3} — 28.8–81.9 h _[86] Organocell

ligning 400 1 Fe/SiO2, Fe/AC — 5.7–6.1

i _[98]

a _{‘Volatile liquids’ based on PL intake, obtained from vacuum distillation of the organic product}

(55 °C, 170 mbar, 1 h).

b _{‘Organic distillate’ based on PL intake, obtained from vacuum distillation of the organic layer}

(200 °C, 1.7 mbar, 30 minutes).

c _{Sum of total hydrocarbons and total phenolics as determined by GC-MS, based on PL intake.} d _{Fraction of the organic product (≈ 60–65wt% of PL intake) boiling within gasoline range, estimated by}

simulated distillation.

e_{Monomer yield as determined by GC×GC-FID, based on PL intake.}

f_{Phenolic fraction separated by glycerol-assisted vacuum distillation of the pyrolysis liquid}

(≈10 wt% yield).

g_{Lignin (instead of biomass) used as the pyrolysis feedstock.}

h_{Monomer yield as determined by GC×GC-FID, based on the hydrotreated organic product.} i_{Mass yield of the GC-analyzed oils after pyrolysis and hydrotreatment.}

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air and hydrogen peroxide as oxidants. A broad range of catalytic systems has been reported, in which homogeneous catalysts are most often employed, e.g. TEMPO [112–114], oxovanadium complexes [115–118], metallosalen complexes [119,120], POMs [111,121]. Heterogeneous catalysts (e.g. chalcopyrite [122], metal-supported [123–125], metal oxides [126–128]) and innovative approaches using biomimetic catalysts [129–131] and ionic liquids [132–134] have been studied as well. Products derived from lignin oxidation include aromatic acids and aldehydes such as vanillin [135–137], phenolic building blocks [138–141] and di-carboxylic acids (DCAs) [142,143] with several potential applications.

Ozone is relatively less explored as an oxidant for lignin, despite its uses in wastewater treatment as a disinfectant [144], in the paper and pulp industry as a bleaching agent [109,145], as a pretreatment to improve the enzymatic hydrolysis of biomass [105,106,146–149] and to upgrade vegetable oils [150]. Ozone (O3) is much less stable than the diatomic allotrope oxygen (O2), and safety is an important aspect to consider when handling it. For instance, ozone’s instability is such that concentrated ozone may decompose explosively at elevated temperatures. Therefore, it is commercially used in low concentrations and mild conditions. Other safety issues are related to the chronic exposure and exposure to high concentrations of ozone, which may cause respiratory difficulties and eye irritation. Ozone is a powerful oxidant and has a high reactivity towards both phenolic and non-phenolic nuclei at mild conditions, and neither chemi-cal additives nor catalysts are typichemi-cally needed [102]. Advantageously, it can be easily generated in situ by a high-voltage electric corona discharge, either from oxygen or dry air, and safe ozone generation technologies are well-established and available at all scales. Furthermore, ozone has a short half-life of less than one hour when dissolved [151], thus any residual ozone in the system quickly decomposes to O2, providing an overall clean process with no need of extra separation steps [152]. Previous works showed that ozonated solutions from biomass and/or lignin contain a range of oxygenated aromatics, quinones and carboxylic acids with potential as fuel additives [152], polyurethanes [153] and fine chemicals for the food and pharma industries [122,154] (see Table 2).

Most of the studies on lignin oxidation using ozone have been performed in batch set-ups. However, the use of continuous processes for instance by using microreactors for ozonation is very attractive, as higher ozone mass transfer rates from the gas to liquid phase are attainable, much smaller volumes are used and a superior control over the reaction conditions can be achieved. Accordingly, the microreactor technology is considered a sustainable solution from both safety and energy saving aspects as it offers a substantial process intensification due to the enhanced mass and heat transfer rates, as well as the ease of upscaling by num-bering-up [165,166]. Ozonation of a range of alkenes, aromatics and amines were successfully performed in such devices [167–171]. For lignin oxidation in general, the development of continuous flow processes in microreactors is still on its infancy, and only a few reports are currently available. Promising results were reported on the photocatalytic degradation of lignin model substrates [172], ultrafast hydro-thermal [173] and copper chloride mediated oxidative [174] depolymerization of Kraft lignin, and supercritical extraction of Kraft lignin oxidation products [175].

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Similar to the reductive approaches, the development of feasible systems that minimize reagents consumption while maximizing product yields is fundamental for future oxidative upgrading processes for PL. This includes tuning process conditions according to the characteristics of the feedstock and targeted products, optimizing the process set-up and, in the case of catalytic systems, improving the catalyst stability and selectivity.

1.2.3. Combined approaches for PL valorization

The previous sections showed that reductive and oxidative routes have great potential for PL valorization. In addition to that, a combination of both an ox-idative and reductive step may also be of interest for improving product yields. For instance, the use of an oxidative pre-treatment step to reduce the molecular weight of PL before hydrotreatment may have a positive effect on monomeric product yields and lead to lower levels of char/coke formation in the hydrotreat-ment step by reducing the rate of repolymerization pathways.

Successful combined oxidative-reductive approaches were recently demon-strated for whole pyrolysis liquids [176–178]. For instance, a sequence of an oxidation reaction based on H2O2/oxone and a subsequent hydrotreatment

Table 2. Literature overview on the ozonation of biomass and lignin.

Biomass source(s) Type(s) of lignin _(°C)T Solvent(s) Identified products Ref.

Ozonation of Lignin

NA Alkali RT Ethanol Oxygenated aromatics,

aliphatic esters

[152] Corn stalk, poplar

wood

Steam-exploded RT Water Aromatic acids and

aldehydes, aliphatic (di) acids, quinones

[155]

Corn stalk Steam-exploded RT Water Oxygenated aromatics,

aliphatic acids

[156]

NA Kraft 30 Water Acids [157]

NA Organosolv RT — Lignin with higher

hydroxyl content for polyurethane production

[153]

Grass Organosolv and

alkali

RT Aqueous acid solution

Aromatic aldehydes [158] Corn stover, wheat

straw and white oak

Organosolv 70 Acetic and

formic acid

Aromatic aldehydes [159]

Almond shells Organosolv RT Water and

acetone

Aromatic and aliphatic acids

[160] Ozonation of biomass

Aspen sawdust — 25 Water Oxygenated aromatics,

aliphatic acids

[161]

Wheat straw — RT Water Acids, methoxy

aromatics, aromatic esters, phenols

[162]

Pine wood — 25 Water Acids [163]

Grasses — RT Water Aromatic and aliphatic

acids

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ultimately gave a product enriched in hydrocarbons. Compared to the direct hydrotreatment, products from the 2-step system were obtained in higher yields and had improved HHV, as well as lower acid values, char and oxygen contents. A follow-up study achieved similar promising results using syngas (from biomass gasification) instead of pure hydrogen in the hydrotreatment step [57].

Next to a reductive approach, esterification of the oxidized products has been explored to stabilize pyrolysis liquids and to prevent aging and corrosion prob-lems due to the present of organic acids. In this concept, the organics acids are converted to esters. In addition, as boiling points of esters are lower than those of their parent acids, separation through reactive distillation is possible [179–181]. Various catalysts have been investigated, i.e. ion-exchange resins [182], ionic liquids [183], zeolites [184,185], homogeneous/solid acids [186–189] and mixed oxides [190]. Previous studies showed that aldehydes can negatively affect the conversion of acids into esters and promote repolymerization pathways during esterification. As such, the oxidation of aldehydes in a preceding step may be a convenient solution to improve product properties [181,191]. Accordingly, oxidation-esterification systems using both ozone and H2O2/ozone for the first step were reported and showed potential to upgrade pyrolysis liquids into fuels [181,192].

Similar to pyrolysis liquids, such an innovative multistep approach may also be promising for the depolymerization of PL into valuable monomers. Yields and distribution of the desired products may be optimized and tuned by for instance process conditions, catalyst and solvent choice. An overview of the valorization possibilities of PL and possible low molecular weight products including fields of applications are presented in Figure 6.

1.3. Thesis outline

In this thesis, experimental studies are described to characterize different PLs and to evaluate strategies for the efficient depolymerization and conversion of PL into value-added monomers. In particular, oxidation using ozone, solvent-free catalytic hydrotreatment and a combination thereof are studied. In addition, the oxidative-reductive strategy, here proposed for PL, was also tested for the conversion of other lignin types.

In Chapter 2, a commercially available pine-derived PL is characterized in detail by chromatographic and spectroscopic techniques, with an emphasis on advanced NMR protocols for a clear elucidation of the main structural motifs (particularly in the oligomeric fragments) and monomers present.

In Chapter 3, the catalytic hydrotreatment of a pine-derived PL in a batch set-up is described. A catalyst screening was performed with a range of car-bon-supported noble metal catalysts commercially available, as well as with sul-phided catalysts typically used in the hydroprocessing of petro-based feedstocks. All gaseous, aqueous and organic product fractions were characterized in detail. Different reaction temperatures were evaluated and the main reaction pathways involved in the process are proposed.

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In Chapter 4, six PLs obtained from the fast pyrolysis of different biomass sources were hydrotreated with Pd/C, i.e. the catalyst of best performance observed in Chapter 3. Different reaction conditions (i.e. reaction time and temperature) were applied in order to assess their effects on product properties. All gaseous, aqueous and organic product fractions were characterized in detail. From the statistical modelling of the experimental dataset obtained, relevant feed-product relations were identified.

In Chapter 5, the valorization of PL by a catalyst-free, innovative oxidation approach using ozone as oxidant is explored. Experiments were performed in a semi-batch set-up with methanol as solvent, and varying ozonation times were evaluated. Thirteen representative lignin model compounds, pine-derived PL and a well-characterized biosynthetic lignin were used as feeds. The ozonated products were quantified and characterized in detail to allow for a better un-derstanding of the main reaction pathways involved in the process, as well as the most reactive chemical functionalities present within the lignin-derived structures.

Following the promising results observed in Chapter 5, in Chapter 6 the ozo-nation of a lignin model compound (i.e. vanillin), PL and two organosolv lignins in a continuous flow microreactor under ambient conditions is evaluated. The advantages of such set-up regarding mass transfer, ease of operation and safe han-dling of ozone are demonstrated, and the ozonated products were characterized in detail and compared with the semi-batch results. A two-step system consisting of an ozone pretreatment and the subsequent catalytic hydrotreatment of a PL feed was also successfully demonstrated, having as objective the achievement of higher depolymerization degrees (i.e. higher monomer yields) with the combined oxidative-reductive strategy.

In Chapter 7, ozonation studies were extended to other lignins besides PL, particularly to readily available and degraded technical lignins. All semi-batch experiments were performed under ambient conditions with ethanol, thus being highly advantageous and in line with green chemistry principles. Two groups of lignins were tested: i) Kraft lignin and ball-milled Kraft lignin; ii) Alcell lignin and a mild organosolv lignin. While the former group provided insights on the

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1

effect of particle size on the product yields, the latter group provided valuable insights on the effect of lignin condensation on its reactivity towards ozone. Finally, in Chapter 8 the two-step oxidative-reductive approach for an im-proved depolymerization of technical lignins is applied in a larger (batch) scale, varying both the lignin feed (i.e. Kraft lignin, PL and organosolv lignin) and the solvent system (i.e. methanol, ethanol and 1,4-dioxane). The (ozonated) lignins were hydrotreated at fixed conditions, and all gaseous, aqueous and organic product fractions were characterized in detail and compared to the direct hydrotreatment.

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University of Groningen Valorization strategies for pyrolytic lignin Bernardes Figueiredo, Monique