Effect of pH on Kraft Lignin Depolymerisation in Subcritical Water

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EFFECT OF pH ON KRAFT LIGNIN DEPOLYMERISATION IN SUB-CRITICAL WATER Tallal Belkheiri, Cecilia Mattsson, Sven-Ingvar Andersson, Lars Olausson, Lars-Erik Amand, Hans Theliander, and Lennart Vamling Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b00462 • Publication Date (Web): 24 May 2016 Downloaded from http://pubs.acs.org on June 5, 2016

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EFFECT OF pH ON KRAFT LIGNIN DEPOLYMERISATION IN SUB-CRITICAL

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WATER

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Tallal Belkheiria*, Cecilia Mattssonb, Sven-Ingvar Anderssona, Lars Olaussonc, Lars-Erik Åmanda,

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Hans Thelianderb, Lennart Vamlinga

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a

Chalmers University of Technology, Department of Energy and Environment, SE-412 96 Gothenburg, Sweden

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b

Chalmers University of Technology, Department of Chemistry and Chemical Engineering, SE-412 96 Gothenburg, Sweden

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c

Valmet AB, Box 8734, SE-402 75 Gothenburg, Sweden

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E-mail: [email protected] (Corresponding author)

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1

ACS Paragon Plus Environment

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ABSTRACT

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Softwood kraft lignin was depolymerized using sub-critical water (623 K and 25 MPa) in a continuous

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small pilot unit. ZrO2 and K2CO3 was used as catalysts and phenol as capping agent to suppress

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repolymerisation. The effect of pH was investigated by adding KOH in five steps to the feed. The

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yield of water soluble organics increased with pH. The yield of bio-oil was also influenced by the pH

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and varied between 28 and 32% wt. The char yield on the zirconia catalyst showed a minimum at pH

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8.1. The yield of suspended solids was low under pH 8.1 but increased at higher pH’s. The oxygen

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content in the bio-oil was only 15% wt., compared to about 26% wt. in the kraft lignin.

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Keywords: Base catalyzed depolymerisation, biomass, sub-critical water, lignin valorization, kraft

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lignin

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2


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ABBREVIATIONS

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GC-MS: Gas chromatography coupled with mass spectrometry

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GPC : Gel permeation chromatography

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WSO : Water-soluble organics

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KF

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TOC : Total organic carbon

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TC

: Total carbon

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SS

: Suspended solids

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THF

: Tetrahydrofuran

: Karl Fischer

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INTRODUCTION

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1.

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One of several important methods to reduce global warming includes an increased use of renewables

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as feedstock for fuels and chemicals, and using biomass is one attractive option to achieve this. A

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large number of initiatives around the globe have attempted to develop tomorrow’s biorefineries that

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can provide fuel, energy and chemicals based on biomass and to shift progressively from a fossil-fuel-

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based economy to an economy based on biomass. In the biorefinery biomass has the potential to

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replace the use of fossil carbon sources in many scenarios. Biomass is available worldwide and is

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renewable on a short time scale compared to fossil fuels. Compared to fossil fuels, biomass does not

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contribute to greenhouse gases in the atmosphere. However, all of the advantages of biomass are

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countered with challenges that must be overcome, including the ecological concern for large-scale

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cultivation and transportation of biomass and its effect on product price.1 Additionally, the feed used

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in the biorefinery should not compete with human food supplies. Forest residues, like branches and

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stamps, are such materials that by advantage can be used as feed in the biorefinery. It consists mainly

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of cellulose, hemicellulose and lignin. Processes for taking care of cellulose and hemicellulose already

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exist but most of the lignin is still only used as fuel, so also in the kraft paper pulp mill. But energy-

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efficient measures implemented in a modern kraft paper pulp mill have made it possible to separate

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approximately 25 to 40% of lignin from black liquor without disturbing paper pulp production in the

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mill. This separation can be performed using the recently commercialized LignoBoost process. Lignin

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from this process is of high quality and can be used for further valorisation.

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Lignin, however, is a complex amorphous aromatic macromolecule built up by phenolic-propylene

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monomeric units connected by ether bonds (C-O-C) and carbon-carbon bonds (C-C). The structure of

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isolated lignin is dependent on its origin (softwood or hardwood) as well as on the cooking process

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from which it has been isolated (e.g., kraft, sulphite, soda or organosolv). For instance, kraft lignin has

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a high polydispersity (with a molecular masses between 1500 and 25000 Da). One cause of this is

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lignin condensation reactions in black liquor during the kraft process. Lignin also loses most of its β-

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O-4 bonds during the kraft process, and only a few percent of these bonds remain left in the lignin.2 4


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Kraft lignin also contains fewer methoxyl groups and more phenolic groups than native lignin. In

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addition, it contains approximately 1% to 3% sulphur. Conversely, organocell lignin has a much lower

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polydispersity (with a molecular masses between 1500 and 5000 Da), and contains more β-O-4 bonds

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and methoxyl groups. The organocell pulping process is sulphur-free.3

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An attractive opportunity presents itself when using lignin as a feedstock in a biorefinery for

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conversion into fuels and chemicals. Limited success of lignin valorisation in the past is now

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overshadowed by the significant steps to develop current and future possible technologies because

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lignin represents an important candidate for the production of chemicals and bio-fuels on a large scale.

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Lignin, however, has a highly functionalized structure compared to fossil fuels 1, and each monomeric

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unit in the lignin molecule contains a substantial number of oxygen atoms. Thus, ordinary refinery

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technologies cannot be used to convert lignin into chemicals and fuels. To achieve this, different

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thermochemical processes have been investigated; one such route is acid-catalysed depolymerisation,

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which produces low yields of monomers and promotes more condensation reactions.4 Another route is

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the oxidative depolymerisation process on the industrial scale to produce vanillin using primarily

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softwood lignin (Borregard, Norway). A third route (i.e. pyrolysis) generates a wide product portfolio

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and leads to high char formation.5 6 Other methods such as solvolysis and ionic liquids have also been

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used for lignin valorisation.7 8 9

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One common technique for the depolymerization of biomass, and lignin in particular, is the utilization

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of sub- and supercritical water. Near the critical point, water exhibits different physical properties

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compared to ambient water, enabling water to dissolve organic compounds 6 10, including lignin. In the

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sub-critical region, water also has the ability to dissolve inorganic salts. In previous studies, a small

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continuous pilot plant was used to depolymerize lignin into phenolic monomers and bio-oil.11 12 13

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Many simultaneous reactions occurred during the depolymerization of lignin, including certain

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undesirable reactions, leading to repolymerisation and char formation. To limit those repolymerisation

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reactions, many reactants have been tested as capping agents in different studies.14 15 In previous

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studies, phenol showed good results in suppressing char and minimizing condensation reactions.11 12 13

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It has been concluded in different studies that it is important to use a base to promote ether bond

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cleavages and to dissolve lignin.15 Different studies have been performed with various bases, including

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NaOH, KOH, K2CO3 and Na2CO3.16 Additionally, base-catalysed treatments of various lignin types

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were performed from which many phenolic products were obtained with different yield distributions

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17

. Roberts et. al. 18 studied the organosolv lignin degradation in a base-catalysed process at 300 °C and

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25 MPa, which showed that product yields were influenced by operating conditions like temperature,

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pressure, residence time and base concentration. Toledano et. al. 19 tested different bases (NaOH,

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KOH, etc.) using organosolv-processed olive tree pruning lignin. The results showed that catechol was

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the primary product (0.1 to 2.4% wt.) and that the oil yield was strongly dependent on the base

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concentration.

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To optimize the process and to gain a better understanding of the depolymerisation of kraft lignin

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using sub-critical water (350 °C, 25 MPa), different reaction parameters including temperature (290 to

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370 °C) and base concentration (K2CO3) have previously been investigated in the presence of phenol

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as a capping agent. These investigations were conducted and followed by many characterisations, such

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as char on the catalyst, the presence of water-soluble organics and analysis of oil products. The same

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classes of compounds (i.e., alkyl phenols, catechols, guaiacols and phenolic dimers) were identified in

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all experiments. The results showed an increase in monomeric products when the temperature and

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base concentration were increased.11 12 A marginal decrease in the char formation on the catalyst also

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occurred when the pH was increased by increasing the K2CO3 concentration. These results indicated

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that a further increase in pH should be favourable for the process. Therefore, the purpose of this study

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was to augment these important results by extending the investigation of the pH dependence of the

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depolymerisation reactions, char formation, formation of suspended solids, and yields of monomers to

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higher pH. To accomplish this goal, it was necessary to use a stronger base (potassium hydroxide,

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KOH) than potassium carbonate (K2CO3) due to the buffer capacity of lignin and phenol in the feed.

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2.

MATERIALS AND METHODS

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2.1. Lignin characterisation

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In this study, a softwood kraft lignin that was extracted with the LignoBoost process 20 in

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Bäckhammar was used. This lignin had an elemental composition (in wt. %) as follows: Carbon

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65.6%, hydrogen 5.7%, sulphur 1.85%, ash contents 0.8%, and the calculated value of oxygen content

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is approx. 26%. The moisture content was 32.6%, the HHV was 27.7 MJ/kg and the mass average

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molar-mass was 3900 g/mol.11

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2.2. Apparatus and procedure

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The experiments in this study were performed in a small continuous pilot unit (see Figure 1) equipped

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with a fixed bed reactor (500 cm3, Parr 4575). The experimental setup also included an electrical

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heating system and two high-pressure diaphragm pumps: one used as a feed pump, and the other used

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as recirculation pump. The primary purposes of the recirculation are to rapidly heat up the fresh feed

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and to allow small amounts of the product material to serve as capping agents.21 The heating system

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consisted of electrical heating jackets around the piping, the feed tank and the reactor. The feed flow

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was 1 kg/h, and the recirculation flow was 10 kg/h in all experiments. The temperatures used in the

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different parts of the pilot unit were 50 °C for the feed tank, 80 °C for the feed preheater, 350 °C for

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the heater before the reactor, and 350 °C for the reactor. The pressure used was 25 MPa. The feed

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slurry was prepared by mixing LignoBoost kraft lignin (5.5% wt.), K2CO3 (1.6% wt.), KOH and

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deionized water. This mixture was then dispersed using an Ultra Turrax disperser for approximately

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10 minutes at room temperature. Phenol (4.1% wt.) was added to the slurry after it had been heated to

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50°C in the feed tank. The pH of the feed was changed by adding KOH to the feed slurry (see Table

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1).

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Insert Figure 1 here

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Insert Table 1 here

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At start up, the system was run with deionized water as feed until a steady state was established in the

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pilot unit. The feed was then changed to the lignin feed slurry, and steady state reaction conditions

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were established within 120 minutes. A test was then conducted for 200 minutes. After the primary

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test, the experiment was run down for 120 minutes by changing the feed to the same solution with the

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exception that it did not contain lignin. At the outlet of the pilot unit, the liquid product was

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continuously collected in sampling bottles that were changed every 40 minutes. The reason for

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running down the experiment in this way was to wash out products sticking to the tube walls under the

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test. This wash-out material is referred to as ‘accumulated oil’ in this study and it has a tendency to

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form when using phenol in the system. More details about the experimental apparatus and procedure

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used in this study were described and reported in a previous publication.12

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The heterogeneous catalyst used in the reactor was made of zirconia (ZrO2) from Saint-Gobain

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NorPro, France, (length = 3 mm, diameter = 3 mm, and BET surface area = 55 m2/g). Potassium

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carbonate (K2CO3, ≥99.5%) was used as the homogeneous co-catalyst; phenol (crystallized, ≥99.5%)

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was used as the co-solvent and capping agent 15 22 23; and potassium hydroxide (KOH ≥85%) was used

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to vary the pH. All of these components were from Scharlau Chemicals, Austria, and were used as

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received.

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2.3. Product separation and analytical approach

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2.3.1. Product separation

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The collected product mixture consisted of two phases (bio-oil and aqueous phases) and was thus

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separated by centrifugation. To achieve a large recovery of bio-oil despite its high viscosity, THF

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(Tetrahydrofuran) was used to dissolve it after collecting nearly all of the water phase. Then, the two 8


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phases were characterised to determine the composition and nature of different compounds in the final

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product. To achieve those goals, measurements using different techniques [GC-MS, GPC, elemental

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analysis, KF, TC and TOC determinations] (see Figure 2) were made to characterise the recovered

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fractions. The amounts of bio-oil and aqueous phases were determined by weighing (More details are

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provided in the Supporting Information, Table S3).

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2.3.2. Analytical approach Analysis of the aqueous phase, after centrifugation, the aqueous phase was acidified and extracted

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with diethyl ether (DEE) based on the procedure described by Nguyen et. al..12 The DEE-extract was

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analysed using gas chromatography (GC) coupled with mass spectrometry (MS). To characterise the

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water-soluble organic fractions, TC and TOC analyses were performed at SP Sveriges Tekniska

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Forskningsinstitut (Borås, Sweden) based on the SS-EN 1484 method. The relative uncertainty (95%

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confidence interval) of the TOC measurement was 8%, and a detailed description of the analysis

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procedures for the aqueous phase were reported by the authors in previous publications.12

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Analysis of the bio-oil phase, GC-MS, water and suspended solids determination in bio-oil as well

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as elemental analysis were conducted based on the procedure described by Nguyen et. al..12 Briefly, a

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GC-MS analysis was performed on the DEE-soluble fraction of the bio-oil using a semi-quantitative

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method, yielding the monomeric/dimeric aromatic content in the bio-oil. The amount of suspended

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solids (i.e., the THF insoluble fraction) was determined by filtration in two steps through Duran glass

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filters with P2, which had a nominal maximum pore size of 40 to 100 µm, and P4, which had a

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nominal maximum pore size of 10 to 16 µm. The GC-MS average value of the Relative Standard

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Deviation (RSD) of the peak area was calculated to be equal to 7.1%. A molecular mass distribution

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analysis was performed by Gel Permeation Chromatography (GPC) with a PL-GPC 50 plus integrated

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system connected to RI and UV detectors (280 nm, Polymer Laboratories, Varian Inc.). A series9


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coupled Polar Gel-M column and a guard column (300 × 7.5 mm and 50 × 7.5 mm, 8 µm) were used

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with a DMSO/LiBr (10 mM) as the mobile phase (0.5 mL/min). A 10-point calibration curve with

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Pullulan standards was used to determine the apparent molecular mass (MM) and the polydispersity

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indices (PD, 708000, 375000, 200000, 107000, 47100, 21100, 11100, 5900, 667 and 180 Da,

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Polysaccharide Calibrations Kit, PL2090-0100, Varian). Data analysis was performed using Cirrus

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GPC software Version 3.2. Samples (0.25 mg/mL) were dissolved in the mobile phase DMSO/LiBr

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(10 mM) and filtered through a syringe filter (GHP Acrodisc, d =13 mm, 0.2 µm GHP membrane).

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2.4. Yield definitions

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The calculations of yields were based on dry lignin feed using mass balances and the results obtained

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from analytical techniques, including GC-MS and TOC. The monomer fractions were determined by

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the approximate quantifications provided by GC-MS. The TOC results (see Table 2) were used to

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determine the total amount of water-soluble organics present, based on the assumption that the

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primary organic compounds in the aqueous phase had phenol-like molecular structures. Finally, the

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bio-oil fraction shown in equation (2) was adjusted by the water content and included the fraction of

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suspended solids.

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The yields were defined by the following equations:

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Char yields (% wt.) = *100 Eq. (1)

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Bio − oil % wt. =

∗ 100 Eq. (2)

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WSO % wt. =

∗ 100

Eq. (3)

218 219

Monomers yield % wt. =

%&'

∗ 100

Eq. (4) 10


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Accumulated bio-oil product = 100 - (Char % wt. + Bio − oil % wt. + WSO% wt. ) Eq. (5)

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Mc : Char weight on catalyst

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ML

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MOL : Weight of recovered bio − oil after centrifugation dry, phenol free

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MPOH : Weight of reacted phenol

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MWSO : Total WSO weight in aqueous phase phenol free

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MMon : Monomer weight

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The phrase “phenol-free” indicates that the amount of phenol measured in the product material (i.e., in

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the aqueous phase and the bio-oil) reported by GC-MS is excluded. For simplification and to have a

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conservative bio-oil yield, all reacted phenol was assumed to be deposited in the bio-oil.

: Dry lignin weight

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RESULTS AND DISCUSSION

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3.

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The objective of this study was to investigate the pH dependence of lignin degradation in sub-critical

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water. The pH of the feed in the run was increased, and the products were analysed. It was found that

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the product pH was lower than the feed pH (see Table 2). The difference between the feed and the

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product pH was higher for the three lowest pH values used and lower for the three highest pH values;

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this indicates that a break point in the product yields exists between pH values of 8.1 and 8.9. Due to

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the extensive recirculation of a large portion of the outlet stream from the reactor, the reactor

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conditions are similar to the conditions of the reactor outlet stream. Therefore, the product pH was

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used as an independent variable in the following discussions instead of the feed pH.

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Insert Table 2 here

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3.1. Overall yields

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In a previous investigation 12, the char yield decreased marginally when the pH was increased. This

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indicated the possibility to further reduce the char yield on the catalyst by increasing the pH even

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more. This investigation, see Figure 3, showed that further char yield decreases could be found up to a

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pH of 8.1, after which further increases of pH instead increased the char yield. The minimum char

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yield (at pH 8.1) was approximately 13% wt., which is an improvement compared to a previous test

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series 12 that produced a char yield of approximately 20% wt. This finding is partly supported by a

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previous report 19, which showed that char could be reduced by increasing the pH. The increased char

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yield at higher pH values could be explained by an increased repolymerisation due to the increased

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pH. This explanation is supported by the observation that the yield of suspended solids (SS) also

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increased at higher pH (see Figure 5). The fact that the mass fraction of phenol in the oil decreased at

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higher pH values may also contribute to the increased char yield at higher pH values (see Figure 6)

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because its function as a capping agent is reduced.

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The distribution of products between the WSO and bio-oil phases was found to vary with product pH,

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as shown in Figure 3, where a clear trend can be seen: an increase in the pH of the product from 7.5 to

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9.7 moved more of the products into the water phase (WSO). The WSO yield increased from

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approximately 24% wt. (pH = 7.7) to a maximum of 40% wt. at the highest pH used (pH = 9.7). One

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possible explanation for this movement of small organic compounds to the WSO phase could be due

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to the pKa values of the phenolic derivatives, which have been reported to be between 9.5 and 10.5.

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Increasing the pH shifts the acid-base equilibrium to the right, which moves more ionized phenolic

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compounds into the water phase (WSO).

267 268

The determination of the yields of char and WSO shown in Figure 3 are relatively straight-forward

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from determinations of mass and TOC. The evaluation of bio-oil yields is, however, more complex.

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The majority of the bio-oil comes out as expected together with the product stream and the bio-oil

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analysis is based on this measurable part of the bio-oil. But some part got stuck to the tube walls in the 12


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pilot unit and accumulated bio-oil could only partly be recovered during washing out of the pilot unit

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after the test. Both the measurable part, 32% wt, and the accumulated part, 28% wt, of the bio-oil are

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fairly constant when the pH increases to the pH of the catalyst char minimum. This observation is in

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line with the analysis results who showed that the amount of suspended solids and its average

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molecular mass were low for these pH’s too, see Figure 5. For pH’s above the char minimum pH, the

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measurable bio-oil decreased slightly, from 32% wt. at pH 8.1 to 28% wt. at pH 9.7. One contribution

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to this might be the fact that the bio-oil is getting heavier and contains more suspended solids when the

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pH increases. Some of this very heavy bio-oil might also form char on the catalyst. At the same time,

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smaller phenolic compounds in the bio-oil were transferred to the WSO phase. This might be one

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explanation to why the accumulated bio-oil diminished even more than the measurable bio-oil at pH

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9.7. The accumulated bio-oil went down from 28% wt to 16% wt when the pH increased from 8.1 to

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9.7.

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An analysis of possible error sources in determination of product yields showed that two major ones

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were present. The first one was the separation of water and oil from the centrifuge bottles and the use

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of THF in this operation to transfer the bio-oil from the centrifuge bottle to the storage bottle. THF

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evaporate very easily and this must be prevented in this operation so that the mass balance will not be

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influenced by large errors. The yield calculation method can be chosen to minimize the influence of

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this uncertainty. The second source of error is minor undetectable leaks in the pilot unit, undetectable

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because the entire pilot unit is insulated. Under the assumption that a small leak will have the same

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composition as the feed or product, a carbon balance calculation showed that this discrepancy cannot

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have any considerably influence on the calculated yields.

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3.2. Molecular mass determinations of bio-oil

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The molecular distributions of the bio-oils were measured using GPC (see Figure 4). Four primary

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regions can be observed in the diagrams. Regions 1 and 2 represent the monomers and dimers in the

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bio-oil that were also identified by GC-MS and have an average molecular mass of approximately 60

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to 200 Da; region 3 represents heavier compounds with a molecular mass of approximately 800 to

302

2000 Da; and region 4 represents the heaviest materials in the bio-oil (i.e., the suspended solids) and

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showed a peak value of approximately 7-10 kDa. The distributions of the bio-oil were similar for all

304

the tested pH values; however, as shown in the figure 4, the bio-oil contained more high-molecular-

305

mass materials (i.e., suspended solids) at higher pH values (see Figure 5).

306 307


308 309

3.3. Analysis of suspended solids in bio-oil

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As shown in Figure 5, the amount of suspended solids is lower than 10% wt. when the pH is in the

311

range of 7.5 to 8.1. However, when the pH value is higher, there is a large increase in the yield of

312

suspended solids (e.g., up to nearly 35% wt. at pH 9.7). Additionally, a similar increase in the average

313

molecular mass is observed. The NMR analysis shows that there are no typical lignin linkages in the

314

suspended solids 2; this indicates that the suspended solids (in the same way as the char) are primarily

315

produced via condensation and repolymerisation reactions of intermediate products and not from

316

unreacted lignin. The pH at the breakpoint where the suspended solids begin to increase is the same as

317

the pH where the minimum char yield on the catalyst occurs and the same pH as where the alkyl

318

phenols start to increase in the bio-oil.

319 320


321

14


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Energy & Fuels

322

3.4. Phenol consumption

323

Phenol was added to the feed to serve as a capping agent during the depolymerisation of lignin in the

324

reactor. Analysis of the product from the pilot unit showed that approximately 40 ± 4% wt. of the

325

added phenol had reacted and that the pH had no influence on the consumed phenol. However, the

326

growth of the amount of suspended solids at higher pH values (see Figure 5) showed that its

327

effectiveness as a capping agent declined at higher pH values. One reason for this phenomenon might

328

be that phenol is also used in other reactions and not only in the capping reactions. For example, in

329

Figure 6, alkyl phenols are shown to increase at the same time as the phenols decrease at pH values

330

above 8.1. This connection is interesting because alkyl phenols are assumed to be formed from phenol.

331


332 333

334

3.5. Bio-oil and WSO characterisation

335

3.5.1. Bio-oil analysis with GC-MS

336

Softwood lignin is composed of 95% guaiacyl-propane units (e.g., coniferyl alcohol); therefore, high

337

levels of released guaiacol monomers are expected when kraft lignin is depolymerized by sub-critical

338

water. However, using GC-MS analysis on the DEE-extractable organic compounds (i.e., the light oil)

339

from the bio-oil, only small amounts of guaiacols were found, and the amount decreased from 8.6 to

340

3.3 wt. % as pH increased in this study (Figure 6). The decreases in guaiacol yields are an indication

341

of increased conversion reactions to other more stable monomeric compounds and/or repolymerisation

342

to higher molecular mass structures (e.g., char and suspended solids).22 23 24 Other compound groups

343

like phenolic dimers (Ar-CH2-Ar and Ar-CH2-CH2-Ar), xanthenes, catechols, acetyl phenols and

344

retenes (7-isopropyl-1-methylphenanthrene) were found to be rather stable over the investigated pH

345

range of 7.5 to 9.7 (Figure 6) (More details are provided in the Supporting Information, Table S1). In

346

the light oil fraction (i.e., the DEE fraction), GC-MS analysis showed that the anisoles (17 to 20.5 wt.

347

%) and the alkyl phenols (17.8 to 28.4% wt.) were the major compound classes formed during the sub-

348

critical depolymerisation process (see Figure 6). The yields of anisole and alkyl phenol compounds 15


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Page 16 of 42

349

increased in the total bio-oil fraction with increasing pH (e.g., from 7.5 to 9.7). With alkyl phenols, the

350

yield increased by a factor of 1.6 from 17.8 to 28.4% wt., which indicated that the production of alkyl

351

phenols was promoted by an increased base level. This trend of an increasing formation of alkyl

352

phenols was also established in a previous study at a lower pH range (e.g., pH = 7.1 to 8.2).11 12

353

However, alkyl phenols can also be formed from several different chemical pathways (i.e., from

354

demethoxylation of guaiacol and from capping of reactive intermediates by phenol).14 The high level

355

of anisole is quite surprising and has not been detected as one of the major monomers in other sub-

356

critical water depolymerisation processes with base catalysis.17 18 19 25

357

counterparts have only been detected when phenol/p-cresol was present under sub- and super-critical

358

water conditions of lignin and lignin model compounds. The current thought regarding this

359

phenomenon is that methanol or formaldehyde/acetaldehyde reacts with phenol/p-cresol to form these

360

anisole derivatives.27 Phenolic dimers (Ar-CH2-Ar and Ar-CH2-CH2-Ar) and xanthenes are likely

361

products derived from phenol capping reactive intermediates (i.e., formaldehyde and acetaldehyde)

362

that are formed during the depolymerization process.14 The capping mechanism of phenol is known to

363

produce alkyl phenols, xanthenes and phenolic dimers 14; in the absence of phenol, catechol typically

364

dominates the product mixture.17

26

Anisole and its alkylated

19

365 366

3.5.2.

WSO analysis with GCMS and TOC

367

The total amount of organic material in the water phase was determined by the TOC analysis. About

368

54 to 70 % wt were then identified by the GC/MS analysis. The largest groups in the water phase were

369

alkyl phenols and catechols (0.18 - 0.29% wt. and 0.20 - 0.31% wt., respectively). These results are in

370

good agreement with earlier studies in continuous reactor systems 11

371

the Supporting Information, Table S2). However, catechols were primarily found in the WSO phase

372

due to their hydrophilic nature at alkaline conditions. In addition, the less polar compound classes

373

were found in lower amounts or not detected at all (i.e., anisoles, xanthenes and polyaromatic

374

hydrocarbons). Lastly, guaiacols followed the same trend as they did in the bio-oil fraction; their

375

quantity decreased in the WSO (0.07 to 0.03% wt.) as pH increased from 7.5 to 9.7.

12

16


(More details are provided in

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Energy & Fuels

376 377

3.5.3.

Total yield of organic compounds in the bio-oil and WSO fractions

378

To provide a better overview of the total yield of released phenolic monomers and dimers in the bio-

379

oil and WSO fractions, the GC-MS mass fraction yields were recalculated based on dry lignin

380

excluding phenol (see Figure 8). The major compound groups formed during the sub-critical

381

depolymerisation process were alkyl phenols > catechols > anisoles (Figure 7). The most dominant

382

structures formed were alkyl phenols, which resulted from several different chemical pathways for

383

instance demethoxylation of lignin guaiacol, and from capping of reactive intermediates by phenol

384

derivatives (Figure 8). However, cathecol, guaiacol and acetyl phenols are believed to be the major

385

products from the depolymerisation of lignin (Figure 7). In addition, one possible explanation for the

386

high yields of alkyl phenols, cathecols and anisoles could be the increased stability under sub-critical

387

conditions compared with guaiacol-like structures.28

388

model compounds, that guaiacol is a reactive substance with low thermal stability under

389

sub/supercritical water conditions and forms products like catechol, phenol and ortho-cresol together

390

with high-molecular weight compounds (i.e., char).22

391

compound, been shown to have relatively high thermal stability under the same conditions (370 °C, 25

392

MPa). The main depolymerization product of catechol in sub/super critical water (without catalyst)

393

was found to be phenol by hydrolysis of a hydroxyl group.31 32 In addition, previous in house studies

394

(250 °C, 25 MPa) have shown high stability of phenol in presence of potassium carbonate12.

29

It has previously been shown, in studies of

23 30

On the other side catechol has, as model

395


396 397


398 399

Analysis of the elemental composition of the bio-oil

400

3.5.4.

401

To use bio-oil with ordinary refinery processes, its oxygen content must be as low as possible.33 It is

402

therefore important to investigate the elemental composition of the bio-oil to verify the oxygen 17


Energy & Fuels

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403

content. The oxygen content of the processed bio-oils is approximately 16% wt. (More details are

404

provided in the Supporting Information, Table S5), which is a substantial decrease from the original

405

value and may make the bio-oil suitable for further upgrades using existing refinery processes. This

406

should be compared with pyrolysis bio-oil, which typically has an oxygen content of approximately 22

407

to 25% wt..34 35 When the elemental composition of the bio-oils are compared to that of LignoBoost

408

kraft lignin and native11 36 lignin in a van Krevelen diagram, it is shown that the bio-oils had lower

409

O/C and H/C ratios than the untreated materials (see Figure 9). For native lignin, the O/C ratio ranged

410

from 0.47 to 0.56, while for the bio-oil, the range was lower (e.g., from 0.14 to 0.25).

411


412 413 414

It was concluded that the high heating value (HHV) of the bio-oil was higher than that of the different

415

lignin types. However, one more valorisation step is required to reach the level of higher heating value

416

products, such as benzene (O/C = 0) or kerosene. This goal can be reached by a hydro-deoxygenation

417

treatment process.37 The bio-oil product was shown to have low sulphur (less than 0.5% wt.) and low

418

nitrogen (less than 0.1% wt.) contents, which indicate good bio-oil quality. Sulphur and nitrogen were

419

effectively removed from the organic materials by this process and ended up in oxidized forms in the

420

water phase. With these low values, further desulphurization of the bio-oil is not required to use the

421

product in traditional refinery processes. There are uncertainties regarding the oxygen content

422

determination used in this study because it was determined by calculations from experimental

423

determinations of elemental compositions (e.g., carbon, nitrogen, hydrogen, potassium etc.). However,

424

it was observed that over the investigated pH range, the O/C ratios for all tests were below 0.2,

425

indicating that the bio-oil oxygen content is acceptable or near acceptable to be used directly in

426

existing refinery processes.33

427 428 429

18


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Energy & Fuels

430

4.

CONCLUSIONS

431

The main conclusion of this work, based on the observations that

432

•

the product yields are pH dependent,

433

•

the amount of char formed on the catalyst showed a minimum at pH 8.1,

434

•

at the same pH, the yield of suspended solids in the bio-oil also had a breakpoint. Below this

435

pH value, the yield of suspended solids was low and above it the yield of suspended solids

436

increased considerably, •

437

and the yield of Water Soluble Organics showed a clear increasing trend, from 24% wt. at pH 7.5 up to 40% wt. at pH 9.7,

438 439

is that it is important to control the pH for optimal depolymerisation of softwood kraft lignin in sub-

440

critical water.

441

Another important finding is that the oxygen content in the bio-oil was about 16 %, a clear decrease

442

compared to the lignin feed (26 %).

443

ACKNOWLEDGEMENTS

444

This study was supported by grants from the Chalmers Energy Initiative, Valmet AB, and the Swedish

445

Energy Agency (LignoFuel Project).

446 447

5.

REFERENCES

448

(1) Strassberger, Z.; Tanase, S.; Rothenberg, G., The pros and cons of lignin valorisation in an

449

integrated biorefinery. RSC Advances 2014, 4, (48), 25310-25318.

450

(2) Mattsson, C.; Andersson, S.-I.; Belkheiri, T.; Åmand, L.-E.; Olausson, L.; Vamling, L.;

451

Theliander, H., Subcritical water de-polymerization of Kraft lignin: a process for future biorefineries.

452

Structural characterization of bio-oil and solids. In 6th Nordic Wood Biorefinery Conference, Helsinki,

453

Finland, 2015.

454

(3) Vishtal, A.; Kraslawski, A., Challenges in industrial applications of technical lignins. BioResources

455

2011, 6, (3), 3547-3568. 19


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(4) Hepditch, M. M.; Thring, R. W., Degradation of solvolysis lignin using Lewis acid catalysts. The

457

Canadian Journal of Chemical Engineering 2000, 78, (1), 226-231.

458

(5) Yang, H.; Yan, R.; Chen, H.; Lee, D. H.; Zheng, C., Characteristics of hemicellulose, cellulose and

459

lignin pyrolysis. Fuel 2007, 86, (12–13), 1781-1788.

460

(6) Azadi, P.; Inderwildi, O. R.; Farnood, R.; King, D. A., Liquid fuels, hydrogen and chemicals from

461

lignin: A critical review. Renewable and Sustainable Energy Reviews 2013, 21, (0), 506-523.

462

(7) Dorrestijn, E.; Kranenburg, M.; Poinsot, D.; Mulder, P., Lignin depolymerization in hydrogen-

463

donor solvents. Holzforschung 1999, 53, (6), 611-616.

464

(8) Zakzeski, J.; Jongerius, A. L.; Bruijnincx, P. C. A.; Weckhuysen, B. M., Catalytic Lignin

465

Valorization Process for the Production of Aromatic Chemicals and Hydrogen. ChemSusChem 2012,

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5, (8), 1602-1609.

467

(9) Binder, J. B.; Gray, M. J.; White, J. F.; Zhang, Z. C.; Holladay, J. E., Reactions of lignin model

468

compounds in ionic liquids. Biomass and Bioenergy 2009, 33, (9), 1122-1130.

469

(10) Kumar, S.; Gupta, R. B., Biocrude Production from Switchgrass Using Subcritical Water. Energy

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& Fuels 2009, 23, (10), 5151-5159.

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(11) Nguyen, T. D. H.; Maschietti, M.; Åmand, L.-E.; Vamling, L.; Olausson, L.; Andersson, S.-I.;

472

Theliander, H., The effect of temperature on the catalytic conversion of Kraft lignin using near-critical

473

water. Bioresource Technology 2014, 170, (0), 196-203.

474

(12) Nguyen, T. D. H.; Maschietti, M.; Belkheiri, T.; Åmand, L.-E.; Theliander, H.; Vamling, L.;

475

Olausson, L.; Andersson, S.-I., Catalytic depolymerisation and conversion of Kraft lignin into liquid

476

products using near-critical water. The Journal of Supercritical Fluids 2014, 86, (0), 67-75.

477

(13) Belkheiri, T.; Vamling, L.; Nguyen, T. D. H.; Maschietti, M.; Olausson, L.; Andersson, S.-I.;

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Åmand, L.-E.; Theliander, H., Kraft lignin depolymerization in near-critical water: Effect of changing

479

co-solvent Cellulose Chemistry and Technology 2014, 48, (9-10), 813-818.

480

(14) Toledano, A.; Serrano, L.; Labidi, J., Improving base catalyzed lignin depolymerization by

481

avoiding lignin repolymerization. Fuel 2014, 116, 617-624.

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(15) Yuan, Z.; Cheng, S.; Leitch, M.; Xu, C. C., Hydrolytic degradation of alkaline lignin in hot-

483

compressed water and ethanol. Bioresource Technology 2010, 101, (23), 9308-9313.

484

(16) Guo, D.-l.; Wu, S.-b.; Liu, B.; Yin, X.-l.; Yang, Q., Catalytic effects of NaOH and Na2CO3

485

additives on alkali lignin pyrolysis and gasification. Applied Energy 2012, 95, 22-30.

486

(17) Wang, H.; Tucker, M.; Ji, Y., Recent Development in Chemical Depolymerization of Lignin: A

487

Review. Journal of Applied Chemistry 2013, 2013, 1-9.

488

(18) Roberts, V. M.; Stein, V.; Reiner, T.; Lemonidou, A.; Li, X.; Lercher, J. A., Towards quantitative

489

catalytic lignin depolymerization. Chemistry 2011, 17, (21), 5939-48.

490

(19) Toledano, A.; Serrano, L.; Labidi, J., Organosolv lignin depolymerization with different base

491

catalysts. Journal of Chemical Technology & Biotechnology 2012, 87, (11), 1593-1599.

492

(20) Tomani, P., The Lignoboost Process. Cellulose Chemistry and Technology 2010, 44, (1-3), 53-58.

493

(21) Mueller-Andersen, R.; Andersen, K., Facilitating unstable reaction at high temp.-pressure - by

494

removing side stream for extra heating before return to main stream. In Google Patents: 1991.

495

(22) Yong, T. L. K.; Yukihiko, M., Kinetic Analysis of Guaiacol Conversion in Sub- and Supercritical

496

Water. Industrial & Engineering Chemistry Research 2013, 52, (26), 9048-9059.

497

(23) Lawson, J. R.; Klein, M. T., Influence of water on guaiacol pyrolysis. Industrial and Engineering

498

Chemistry Fundamentals 1985, 24, (2), 203-208.

499

(24) Wahyudiono; Machmudah, S.; Goto, M., Utilization of Sub and Supercritical Water Reactions in

500

Resource Recovery of Biomass Wastes. Engineering Journal; Vol 17, No 1 (2013): Regular Issue

501

2012.

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(25) Lavoie, J.-M.; Baré, W.; Bilodeau, M., Depolymerization of steam-treated lignin for the

503

production of green chemicals. Bioresource Technology 2011, 102, (7), 4917-4920.

504

(26) Wahyudiono; Sasaki, M.; Goto, M., Recovery of phenolic compounds through the decomposition

505

of lignin in near and supercritical water. Chemical Engineering and Processing: Process

506

Intensification 2008, 47, (9–10), 1609-1619.

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507

(27) Okuda, K.; Ohara, S.; Umetsu, M.; Takami, S.; Adschiri, T., Disassembly of lignin and chemical

508

recovery in supercritical water and p-cresol mixture: Studies on lignin model compounds. Bioresource

509

Technology 2008, 99, (6), 1846-1852.

510

(28) Pińkowska, H.; Wolak, P.; Złocińska, A., Chemical Engineering Journal 2012, 187, (0), 410-414.

511

(29) Huang, X.; Korányi, T. I.; Boot, M. D.; Hensen, E. J. M., Catalytic Depolymerization of Lignin in

512

Supercritical Ethanol. ChemSusChem 2014, 7, (8), 2276-2288.

513

(30) Wahyudiono, n.; Kanetake, T.; Sasaki, M.; Goto, M., Utilization of Sub and Supercritical Water

514

Reactions in Resource Recovery of Biomass Wastes. Chem. Eng. Technol. 2007, 30, (8), 1113.

515

(31) Wahyudiono; Sasaki, M.; Goto, M., Conversion of biomass model compound under hydrothermal

516

conditions using batch reactor. Fuel 2009, 88, (9), 1656-1664.

517

(32) Brunner, G., Chapter 8 - Processing of Biomass with Hydrothermal and Supercritical Water. In

518

Supercritical Fluid Science and Technology, Gerd, B., Ed. Elsevier: 2014; Vol. Volume 5, pp 395-

519

509.

520

(33) Marker, T. L., Opportunities for biorenewables in oil refineries. Report No DOEGO15085 Final.

521

UOP LLC 2005.

522

(34) Wang, Y.; He, T.; Liu, K.; Wu, J.; Fang, Y., From biomass to advanced bio-fuel by catalytic

523

pyrolysis/hydro-processing: Hydrodeoxygenation of bio-oil derived from biomass catalytic pyrolysis.

524

Bioresource Technology 2012, 108, (0), 280-284.

525

(35) Kosa, M.; Ben, H.; Theliander, H.; Ragauskas, A. J., Pyrolysis oils from CO2 precipitated Kraft

526

lignin. Green Chemistry 2011, 13, (11), 3196-3202.

527

(36) Sabawi, H.; Ahmad, M., New Van Krevelen diagram and its correlation with theheating value of

528

biomass. Research Journal of Agriculture and Environmental Management 2013, Vol 2(10) pp 295-

529

301

530

(37) E. de Jong, E.; Gosselink, R. J. A., Lignocellulose-Based Chemical Products. In Bioenergy

531

Research: Advances and Applications, 2014; pp 277-313.

532

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Energy & Fuels

533

Table captions

534

Table 1. pH in the feed after the addition of KOH.

535

Table 2. Measurements of the feed and product pH values, and the TC and TOC values in the aqueous

536

phase.

23


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

537

Figure captions

538

Figure 1. Schematic diagram of the pilot plant.

539

Figure 2. Analysis and separation sequences of the product mixture. Abbreviations: GC-MS, gas

540

chromatography mass spectrometry; GPC, gel permeation chromatography; TOC, total organic

541

carbon; TC, total carbon.

542

Figure 3. Yields of bio-oil, water-soluble organics (WSO), char and accumulated bio-oil products

543

calculated on a lignin basis (based on eq. 1 to 5).

544

Figure 4. A representative molecular mass distribution curve from gel permeation chromatography of

545

the bio-oil with product pH 7.7 (normalized response relative to peak 1).

546

Figure 5. Mass percent and average molecular mass of suspended solids in bio-oil for different pH

547

values. Abbreviations: SS, suspended solids.

548

Figure 6. Distribution of phenolic compounds in light oil (DEE-soluble fraction of the bio-oil)

549

measured by GC-MS analysis, calculated on light oil basis.

550

Figure 7. Major compound groups formed during the sub-critical depolymerisation process: alkyl

551

phenols, catechols, anisoles, guaiacols, acetyl phenols, phenolic dimers and xanthenes.

552

Figure 8. Total monomer yields in bio-oil and WSO fractions calculated based on dry lignin.

553

Figure 9. Elemental composition ratios of the bio-oil compared to benzene, kerosene and lignin.

24


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Energy & Fuels

Tables

554 555 556

Table 1. pH in the feed after the addition of KOH.

KOH (% wt.) pH in feed

A

B

C

D

E

F

0

0.2

0.4

0.8

1.6

2.0

8.9

9.3

9.5

9.8

10.1

10.4

557 558

Table 2. Measurements of the feed and product pH values, and the TC and TOC values in the aqueous

559

phase.

560

Experiment

A

B

C

D

E

F

pH pH in feed

8.9

9.3

9.5

9.8

10.1

10.4

pH in product

7.5

7.7

8.1

8.9

9.5

9.7

Aqueous phase

561

TC (mg/L)

NA

27000

27000

29000

33000

33000

TOC (mg/L)

NA

24000

24000

25000

28000

29000

NA: Not analysed

562

25


Energy & Fuels

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563

Supporting Information. Supplementary material for publication has three pages, includes

564

five tables and one figure namely:

565

Table S1, Table S2, Table S3, Table S4, Table S5 and Figure S1.

566

26


Page 26 of 42

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Energy & Fuels

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Page 28 of 42

Lignin

Product mixture

Centrifugation

Aqueous phase (WSO)

TC

TOC

GC-MS

Elemental analysis

GPC

Bio-oil

Char

Karl Fischer

Suspended solids (SS)


GC-MS

Page 29 of 42

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Energy & Fuels

100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 8.9 pH (feed) pH (product) 7.5 WSO (wt.%)

9.3

9.5

9.8

10.1

10.4

7.7

8.1

8.9

9.5

9.7

Accumulated bio-oil product (wt.%)

Char (wt.%)

Bio-oil (wt.%)


Energy & Fuels

1 1.0

0.8

Normalized response

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 30 of 42

0.6

4 2

0.4 3

0.2

0.0 0.0

0.0

0.1

1.0 Molecular mass (kDa)


10.0

100.0

1000.0

Page 31 of 42

40%

30000

35% 25000 30% 20000 25% 20%

15000

15% 10000 10% 5000 5% 0 7.0

7.5

8.0

8.5

9.0

9.5

pH product Molecular mass of SS

SS wt.% on total oil basis


0% 10.0

Suspended solids

Molecular mass (Da)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

Energy & Fuels

45% 40% Distribution of light oil

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 32 of 42

35% 30% 25% 20% 15% 10% 5% 0% 7.0

7.5

8.0

8.5 pH product

9.0

9.5

Anisoles

Phenol

Alkyl phenols

Guaiacols

Catechols

Phenolic dimers


10.0

Page 33 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels


Energy & Fuels

12%

10% Yields on dry lignin basis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 34 of 42

8%

6%

4%

2%

0% 7.0

7.5

8.0

8.5

9.0

9.5

pH product Anisoles

Catechols

Alkyl phenols

Guaiacols


Acetyl phenols

10.0

Page 35 of 42

LignoBoost Kraft lignin Lignin (native, softwood) Lignin (native, hardwood) Bio-oil Benzene Kerosene

0.20 0.18 0.16 Hydrodeoxygenation

0.14 0.12 H/C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

0.10

Depolymerization

0.08 0.06 0.04 0.02 0.00 0.0

0.1

0.2

0.3 O/C

0.4


0.5

0.6

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Page 36 of 42

Page 37 of 42

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 8.9 pH (feed) pH (product) 7.5 WSO (wt.%)

9.3

9.5

9.8

10.1

10.4

7.7

8.1

8.9

9.5

9.7

Accumulated bio-oil product (wt.%)

Char (wt.%)

Bio-oil (wt.%)


Energy & Fuels

1 1.0

0.8

Normalized response

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 38 of 42

0.6

4 2

0.4 3

0.2

0.0 0.0

0.0

0.1

1.0 Molecular mass (kDa)


10.0

100.0

1000.0

Page 39 of 42

40%

30000

35% 25000 30% 20000 25% 20%

15000

15% 10000 10% 5000 5% 0 7.0

7.5

8.0

8.5

9.0

9.5

pH product Molecular mass of SS

SS wt.% on total oil basis


0% 10.0

Suspended solids

Molecular mass (Da)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

Energy & Fuels

45% 40% Distribution of light oil

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 40 of 42

35% 30% 25% 20% 15% 10% 5% 0% 7.0

7.5

8.0

8.5 pH product

9.0

9.5

Anisoles

Phenol

Alkyl phenols

Guaiacols

Catechols

Phenolic dimers


10.0

Page 41 of 42

12%

10% Yields on dry lignin basis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Energy & Fuels

8%

6%

4%

2%

0% 7.0

7.5

8.0

8.5

9.0

9.5

pH product Anisoles

Catechols

Alkyl phenols

Guaiacols


Acetyl phenols

10.0

Energy & Fuels

LignoBoost Kraft lignin Lignin (native, softwood) Lignin (native, hardwood) Bio-oil Benzene Kerosene

0.20 0.18 0.16 Hydrodeoxygenation

0.14 0.12 H/C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Page 42 of 42

0.10

Depolymerization

0.08 0.06 0.04 0.02 0.00 0.0

0.1

0.2

0.3 O/C

0.4


0.5

0.6

Effect of pH on Kraft Lignin Depolymerisation in Subcritical Water

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