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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Publication date:

2020

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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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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) Figure 1. Major building blocks of lignin.

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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 (M

w

) [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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