electrochemical conversion of liquefied forest biomass

Electrochemical Conversion of Liquefied Forest Biomass

Preliminary studies

Tiago André Ribeiro da Silva

Thesis to obtain the Master of Science Degree in

Chemical Engineering

Supervisors: Dr. José Augusto Dâmaso Condeço

Dr. Diogo Miguel Franco dos Santos

Examination Committee

Chairperson: Prof. Maria Joana Castelo-Branco de Assis Teixeira Neiva Correia

Supervisor: Dr. Diogo Miguel Franco dos Santos

Members of the Committee: Dr. Rui Galhano dos Santos

November 2018

I

Acknowledgments

I would like to thank my supervisors, Dr. José Condeço and Dr. Diogo Santos, for the opportunity

and guidance in the development of this work.

A word of appreciation to Professor Joana Neiva Correia for generously providing the laboratory

facilities and equipment necessary for the progress of this work.

I would also like to thank the members of the Materials Electrochemistry Group that, over the course

of this work, showed nothing but support and encouragement. A special word for Raisa Oliveira

and Aldona Balciunaite, thank you for all the time spent with me, all the assistance and, above all,

your friendship. To my colleague Inês Belo, thank you for the company during all those mornings

and afternoons we spent working in the laboratory, your presence made all the difference.

Also, a thank you to my colleagues, both past and present ones, and my friends that walked the

halls of this institute with me throughout these years.

Lastly, I would like to thank my family for everything, the support, love and all the sacrifices made.

Thank you for helping me reach this moment.

“Do not judge me by my successes, judge me by how many times I fell down

and got back up again”

- Nelson Mandela

II

Abstract

This work focuses on the use of different biomass, namely cork, pinewood and olive stones, to

produce bio-oil with the purpose of continuing further studies, which will provide preliminary information

regarding the suitability and potential towards the electrochemical conversion of bio-oils into industrially-

relevant compounds and the electrocatalytic upgrading of biomass-derived intermediates. Herein, the

liquefactions of the three biomass samples were performed and the obtained bio-oils were characterized

by several physicochemical methods (e.g., density, viscosity, conductivity and pH), followed by the

analysis of their electrochemical behavior. Both the anodic oxidation and the hydrogen evolution reaction

were evaluated at different potentials at room temperature using a Pt electrode. In the cyclic voltammetry

studies no redox peak was visible in the potential window between -2V and +2V. In the

chronoamperometry studies the current densities gradually decrease with time, stabilizing after 200

seconds. Two small-scale laboratory electrolyzers using nickel plate electrodes were assembled, one

being a single compartment cell and the other a cell with two-compartments separated by a membrane,

for the evaluation of the effect of an applied voltage of 2.5V on the composition of the bio-oils.

Electrolysis experiments were carried out up to 24 hours. The samples subjected to electrolysis were

analyzed by Attenuated Total Reflection-Fourier-transform Infrared Spectroscopy and Mass

Spectrometry, both before and after the electrolysis experiments, as to identify possible changes to the

samples chemical structure. The analyses show changes in the bio-oils composition, but the nature of

the actual changes occurring during the electrochemical process need further study.

Keywords: Biomass liquefaction; Bio-Oil; Electrolysis; Cork; Pinewood; Olive Stones.

III

Resumo

Este trabalho foca-se no uso de diferentes biomassas, nomeadamente cortiça, pinho e caroço de

azeitona, para a produção de bio-óleo com o objetivo de efetuar estudos que irão providenciar

informação preliminar em relação à usabilidade e potencial para a conversão eletroquímica dos bio-

óleos em compostos com relevância a nível industrial e o melhoramento eletrocatalítico partindo dos

compostos intermediários. Foram efetuadas liquefações da biomassa e os bio-óleos obtidos foram

caracterizados por métodos físico-químicos (i.e., densidade, viscosidade, condutividade e pH), seguida

da análise do seu comportamento eletroquímico. Tanto a oxidação anódica como a reação da evolução

do hidrogénio foram avaliadas a vários potenciais à temperatura ambiente fazendo uso dum elétrodo

de platina. Na voltametria cíclica não foi visível nenhum pico redox na janela de potencial entre -2V e

+2V. A cronoamperometria mostrou que as densidades de corrente decrescem gradualmente com o

tempo, estabilizando ao fim de 200 segundos. Um par de eletrolisadores à escala laboratorial (elétrodos

de níquel) foram montados para avaliação do efeito da aplicação de uma tensão de 2.5V na composição

dos bio-óleos: uma célula de compartimento único, e uma célula de dois compartimentos divididos por

membrana. As amostras foram analisadas por Reflexão Total Atenuada-Espectroscopia no

Infravermelho por Transformada de Fourier e por Espectrometria de Massa, antes e após eletrólise, de

modo a identificar possíveis diferenças na sua estrutura química. As análises indicam diferenças nos

bio-óleos após serem submetidos a eletrólise. Serão necessários mais estudos para interpretar as

alterações que ocorrem durante o processo eletroquímico associado.

Palavras Chave: Liquefação de biomassa; Bio-Óleo; Eletrólise; Cortiça; Pinho; Caroço de Azeitona.

IV

Table of Contents

Acknowledgments .................................................................................................................................. I

Abstract .................................................................................................................................................. II

Resumo.................................................................................................................................................. III

Table of Contents .................................................................................................................................. II

List of Figures ....................................................................................................................................... VI

List of Tables ........................................................................................................................................ IX

List of Symbols and Abbreviations ..................................................................................................... X

1. Introduction ............................................................................................................................ - 1 -

1.1. Plant Biomass ............................................................................................................. - 1 -

1.2. Thermochemical Conversion .................................................................................... - 3 -

1.2.1. Direct Liquefaction ..................................................................................................... - 4 -

1.2.2. Bio-Oil .......................................................................................................................... - 8 -

2. Electrochemistry ................................................................................................................. - 10 -

2.1. Electrochemical Cell ................................................................................................ - 10 -

2.1.1. Electrodes ................................................................................................................. - 11 -

2.1.2. Potentiostat/Galvanostat ......................................................................................... - 14 -

2.1.3. Open Circuit Potential (OCP) .................................................................................. - 15 -

2.2. Electrochemical Methods ........................................................................................ - 15 -

2.2.1. Cyclic Voltammetry (CV) .......................................................................................... - 15 -

2.2.2. Linear Scan Voltammetry (LSV) and Tafel Analysis ............................................. - 16 -

2.2.3. Chronoamperometry (CA) ....................................................................................... - 17 -

3. Experimental Methods ........................................................................................................ - 19 -

3.1. Bio-Oils and Reactants ............................................................................................ - 19 -

3.2. Solvolysis Liquefaction ........................................................................................... - 19 -

3.3. Bio-Oil Characterization .......................................................................................... - 21 -

3.3.1. Density ....................................................................................................................... - 21 -

3.3.2. Viscosity .................................................................................................................... - 21 -

3.3.3. Conductivity .............................................................................................................. - 21 -

3.3.4. pH ............................................................................................................................... - 21 -

3.4. Electrochemical Experiments ................................................................................. - 22 -

3.4.1. Cyclic Voltammetry (CV) .......................................................................................... - 23 -


3.4.3. Chronoamperometry (CA) ....................................................................................... - 24 -

V

3.4.4. Electrolysys Experiments ........................................................................................ - 24 -

3.5. Bio-Oil Analysis: Electrolysis Experiments ........................................................... - 26 -

3.5.1. Attenuated total reflection-Fourier-transform Infrared Spectroscopy (ATR-FTIR)

............................................................................................................................... ….- 26 -

3.5.2. Mass Spectrometry (MS) ......................................................................................... - 26 -

4. Results and Discussion ...................................................................................................... - 28 -

4.1. Solvolysis Liquefaction ........................................................................................... - 28 -

4.2. Bio-Oil Characterization .......................................................................................... - 29 -

4.2.1. Physicochemical properties .................................................................................... - 29 -

4.3. Electrochemical Experiments ................................................................................. - 31 -

4.3.1. Cyclic Voltammetry .................................................................................................. - 31 -


4.3.3. Chronoamperometry ................................................................................................ - 35 -

4.3.4. Electrolysis Experiments ......................................................................................... - 38 -

4.3.5. Cyclic Voltammetry: Electrolysis Experiments ..................................................... - 41 -

4.4. Bio-Oil Analysis: Electrolysis Experiments ........................................................... - 43 -

4.4.1. Attenuated Total Reflection-Fourier-Transform Infrared Spectroscopy (ATR-FTIR).

.................................................................................................................................... - 44 -

4.4.2. Mass Spectrometry (MS) ......................................................................................... - 52 -

5. Conclusions ......................................................................................................................... - 56 -

Bibliography ..................................................................................................................................... - 58 -

Appendix ................................................................................................................................................ A

A. Mass Spectrometry ........................................................................................................ A

VI

List of Figures

Figure 1. Spatial arrangement of cellulose, hemicellulose and lignin in the cell walls of lignocellulosic

biomass [1]. ......................................................................................................................................... - 1 -

Figure 2. Structure of cellulose with a 1-4-β glycosidic bond, adapted from [1]. ................................ - 2 -

Figure 3. Sugar monomers typically found in Hemicellulose, adapted from [1]. ................................ - 2 -

Figure 4. The three monomers that form the lignin polymer along with their respective subunits, adapted

from [1]................................................................................................................................................. - 2 -

Figure 5. Classification of biomass’ thermochemical conversion technology. ................................... - 3 -

Figure 6. Reaction pathway for the cellulose of an alcohol solvent and an acid catalyst [23]. .......... - 6 -

Figure 7. Reaction pathway for lignin with alcohol solvent and acid catalyst [23]. ............................. - 6 -

Figure 8. Mechanism of reaction of liquefaction fragments with alcohol solvents. In (A), the mechanism

for cellulose fragments (HO-C) with mono-, duo- and tri-alcohol solvents, and in (B) the lignin fragments

(HO-L) with the alcohol solvents [23]. ................................................................................................. - 7 -

Figure 9. Schematic of a cell’s standard setup with (A) two electrodes and (B) three electrodes [60].

....................................................................................................................................................... …- 11 -

Figure 10. Schematic of a SHE, adapted from [62]. ......................................................................... - 12 -

Figure 11. Schematic of a standard SCE, adapted from [63]. .......................................................... - 12 -

Figure 12. Schematic of a standard Ag/AgCl electrode, adapted from [63]. .................................... - 13 -

Figure 13. Example of Pt counter-electrodes in the shape of a mesh and coil. ............................... - 14 -

Figure 14. Potentiostat/Galvanostat Princeton Applied Research/EG&G Model 273A. .................. - 14 -

Figure 15. CVs at several scan rates for (A), reversible reactions and (B) irreversible reactions. .. - 16 -

Figure 16. (A), typical graph of a potential-step method and (B), example of a CA graph. ............. - 18 -

Figure 17. Glass reactor in the heating mantle, at the bottom, coupled with a Dean-Stark

separator/condenser, on the left, a mechanical stirrer, up top, and the thermocouple controller on the

right. ................................................................................................................................................... - 20 -

Figure 18. Scheme of the experimental procedure for the liquefaction’s product treatment. ........... - 20 -

Figure 19. Pt electrode, SCE and Pt coil used during the electrochemical characterization. .......... - 22 -

Figure 20. Diagram showing (A) the two emulsions, EAc with its distinct phases and EAlk, used in the

electrochemical experiments, with (B), showing the emulsions EAc’ and EAlk’, after 24 hours of

electrolysis in a single compartment cell, and (C), showing the emulsion EAc’’ and EAlk’’, after 24 hours

of electrolysis as a cathodic and anodic electrolyte, respectively, in a two-compartments cell. ....... - 23 -

Figure 21. Ni plates used as both anode and cathode in the electrolysis experiments. .................. - 25 -

Figure 22. Schematic of a two compartment cell with a membrane between each compartment, adapted

from [64]............................................................................................................................................. - 25 -

Figure 23. The effect of the addition of H2SO4 in the conductivity of sample of cork bio-oil. ........... - 30 -

Figure 24. Emulsions of cork bio-oil with (A) a 2 M H2SO4 aqueous solution, EAc, with visible separation

of the phases of the emulsion after 12 hours and an (B) a 2M KOH aqueous solution, EAlk, with no visible

phases. .............................................................................................................................................. - 31 -

VII

Figure 25. CVs of the (A) cork, (B) pinewood and (C) olive stones bio-oils, performed at 50 mV s-1.

..................................................................................................................................................... …..- 32 -

Figure 26. CVs performed at 50 mV s-1 of (A) cork bio-oil samples, pure and with concentrations of 1 M

and 3 M H2SO4, as well as the respective (B) anodic zone and (C) cathodic zone. ......................... - 33 -

Figure 27. The CVs of (A) pinewood bio-oil samples, pure and with concentrations of 1 M and 3 M

H2SO4, with (B) the anodic zone and (C) the cathodic zone. Performed at 50 mV s-1. .................... - 33 -

Figure 28. The CVs of (A) olive stones bio-oil samples, pure and with concentrations of 1 M and 3 M

H2SO4, as well as (B) and (C) the anodic and cathodic zones. Performed at 50 mV s-1. ................. - 33 -

Figure 29. The LSVs of (A) olive stones, cork and pinewood bio-oil samples with 3 M H2SO4, performed

at 50 mV s-1, with (B) Tafel plot for the respective samples with corresponding linear regression. .- 34 -

Figure 30. CAs for the pure cork bio-oil in the (A) anodic and (B) cathodic zones, as well as the CAs for

the cork bio-oil with 3 M H2SO4 in the (C) anodic and (D) cathodic zones, performed for 200 seconds.

....................................................................................................................................................... …- 36 -

Figure 31. CAs for the pinewood bio-oil in the (A) anodic and (B) cathodic zones as well as the CAs for

the pinewood bio-oil with 3 M H2SO4 for the (C) anodic and (D) cathodic zones, performed for 200

seconds. ............................................................................................................................................ - 36 -

Figure 32. CAs for the olive stones bio-oil in the (A) anodic and (B) cathodic zones as well as the CAs

for the olive stones bio-oil with 3 M H2SO4 for the (C) anodic and (D) cathodic zones, performed for 200

seconds. ............................................................................................................................................ - 37 -

Figure 33. Comparison between the CAs of the different bio-oils with 3 M H2SO4, at applied potential of

(A) 0.9 V for the anodic zone and (B) -0.9 V for the cathodic zone, performed for 200 seconds. .... - 37 -

Figure 34. Current densities observed in the first 90 minutes of electrolysis experiment n1 (see Table

III), the electrolysis of a cork bio-oil sample with 1.5 M H2SO4. ........................................................ - 38 -


III), the electrolysis of an emulsion of cork bio-oil sample with 2 M KOH aqueous solution in a single

compartment cell. .............................................................................................................................. - 39 -


III), the electrolysis of an acidic emulsion EAc as electrolyte in the cathodic side and an alkali emulsion

EAlk as electrolyte in the anodic cell, performed for 24 hours. .......................................................... - 40 -

Figure 37. CVs of (A) acidic organic phase EAc-Org, before electrolysis, (B) acidic organic phase EAc-Org’,

after electrolysis, (C) acidic aqueous phases EAc-Aq, before electrolysis and (D) acidic aqueous phase

EAc-Aq’ after electrolysis. Performed at 50 mV s-1. ............................................................................. - 41 -

Figure 38. CVs of an emulsion of cork bio-oil with an aqueous solution of 2 M KOH (A), before the

electrolysis experiments, EAlk, and (B), after electrolysis for 24 hours, EAlk’. Performed at 50 mV s-1.

...................................................................................................................................................... ….- 42 -

Figure 39. ATR-FTIR of (A) 2 M H2SO4 cork bio-oil sample, (B) bio-oil sample after 8 hours of

electrolysis, and (C) bio-oil sample after 14 hours of electrolysis, experiment n6 (see Table III). ... - 45 -

Figure 40. ATR-FTIR of (A) pure cork bio-oil sample, (B) acidic upper emulsion phase EAc-Org, (C) acidic

middle emulsion phase EAc-Int and (D) acidic aqueous emulsion phase EAc-Aq. ................................ - 47 -

VIII

Figure 41. The ATR-FTIR of (A) acidic upper emulsion phase EAc-Org, (B) acidic emulsion phase EAc-

Org’, after the electrolysis experiment in single compartment cell and (C) acidic emulsion phase EAc-Org’’,

after the electrolysis experiment in the two-compartment cell. ......................................................... - 48 -

Figure 42. The ATR-FTIR of (A) acidic middle emulsion phase EAc-Int, (B) acidic middle emulsion phase

EAc-Int’, after the electrolysis experiment in single compartment cell, and (C) acidic middle emulsion

phase EAc-Int’’, after the electrolysis experiment in the two-compartment cell. ................................. - 48 -

Figure 43. The ATR-FTIR of (A) acidic aqueous emulsion phase EAc-Aq, (B) acidic aqueous emulsion

phase EAc-Aq’, after the electrolysis experiment in single compartment cell and (C) acidic aqueous

emulsion phase EAc-Aq’’, after the electrolysis experiment in the two-compartment cell. .................. - 49 -

Figure 44. The ATR-FTIR of (A) cork bio-oil, (B) alkali emulsion EAlk and (C) alkali emulsion viscous

residue EAlk-Res. .................................................................................................................................. - 50 -

Figure 45. The ATR-FTIR of (A) alkali emulsion EAlk, (B) alkali emulsion EAlk’, after the electrolysis

experiment in single compartment cell and (C), alkali emulsion EAlk’’, after the electrolysis experiment

in the two-compartment cell............................................................................................................... - 51 -

Figure 46. The ATR-FTIR of (A) viscous residue of the alkali emulsion EAlk-Res, (B) alkali residue EAlk-

Res’, after the electrolysis experiment in single compartment cell and (C) alkali residue EAlk-Res’’, after

the electrolysis experiment in the two-compartment cell. .................................................................. - 51 -

Figure 47. The MS with positive ESI of (A) acidic organic emulsion phase EAc-Org, (B) acidic middle

emulsion phase EAc-Int and (C) bottom aqueous emulsion phase EAc-Aq........................................... - 53 -

Figure 48. The MS with positive ESI of (A) sample of cork bio-oil, (B) acidic aqueous emulsion phase

EAc-Aq, and (C) acidic aqueous emulsion phase EAc-Aq’ after the electrolysis experiment in a single


Figure 49. MS with positive ESI corresponding to (A) sample of cork bio-oil, (B) acidic upper emulsion

phase EAc-Org, and (C) acidic upper emulsion phase EAc-Org’, after the electrolysis experiment in a single


Figure 50. The MS with positive ESI of (A) sample of cork bio-oil, (B) acidic middle emulsion phase EAc-

Int, and (C) acidic middle emulsion phase EAc-Int’, after the electrolysis experiment in a single


Figure 51. The MS with positive ESI of (A) sample of cork bio-oil, (B) alkali emulsion EAlk, (C) alkali

emulsion residue EAlk-Res, (D) alkali emulsion EAlk’ and (E) respective emulsion residue EAlk-Res’, after

the electrolysis experiment in a single compartment cell. ................................................................. - 55 -

Figure 52. The MS with negative ESI of (A) organic emulsion phase EAc-Org, (B) emulsion phase EAc-Int

and (C) aqueous emulsion phase EAc-Aq. ................................................................................................ A

Figure 53. MS with negative ESI corresponding to (A) sample of cork bio-oil, (B) organic emulsion

phase EAc-Org, and (C) emulsion phase EAc-Org’. ...................................................................................... B

Figure 54. The MS with negative ESI of (A) sample of cork bio-oil, (B) emulsion phase EAc-Int, and (C)

emulsion phase EAc-Int’. ........................................................................................................................... B

Figure 55. The MS with negative ESI of (A) sample of cork bio-oil, (B) alkali emulsion EAlk, (C) emulsion

viscous residue EAlk-Res, (D) emulsion phase EAlk’ and (E) emulsion phase EAlk-Res’. ............................. C

IX

List of Tables

Table I. Summary of some works on the liquefaction process and its working conditions. ................ - 7 -

Table II. Comparison of j0 for hydrogen evolution reaction in 1 M H2SO4 [61]. ................................ - 17 -

Table III. Electrolysis experiments run on each cell, the electrolyte used and their working conditions.

......................................................................................................................................................... ..- 25 -

Table IV. Parameters used in the MS analysis. ................................................................................ - 27 -

Table V. Working conditions and conversion of the solvolysis liquefactions of milled cork biomass…….

........................................................................................................................................................... - 28 -

Table VI. Physicochemical properties of the bio-oil samples, from the different biomasses. ........... - 29 -

Table VII. Conductivity and viscosity of the bio-oils with the addition of H2SO4 up to 3 M of concentration.

........................................................................................................................................................... - 29 -

Table VIII. Increase of conductivity with the addition of KOH in a sample of cork bio-oil. ................ - 30 -

Table IX. Current densities recorded in the CVs of the bio-oil samples at potentials of 2 V and -2 V.

..................................................................................................................................................... …..- 34 -

Table X. LSVs values of the OCP and of the current density, j, at a potential of -2 V. .................... - 35 -

Table XI. The Tafel plot values of b, the Tafel Slope, and a, the y-interception of the linear regression

from the Tafel plots for each bio-oil sample, as well as the charge transfer coefficient, α, and the

exchange current density, j0, respectively. ........................................................................................ - 35 -

Table XII. Resume of the values of jtf, the current densities detected at the end of each electrolysis

experiment. ........................................................................................................................................ - 40 -

Table XIII. Summary of the abbreviations used for each emulsion and their phases (see Figure 20).

....................................................................................................................................................... …- 43 -

Table XIV ATR-FTIR peak number, wavenumber range of values and corresponding assigned

functional groups, adapted from [70]. ................................................................................................ - 44 -

X

List of Symbols and Abbreviations

2-EH - 2-Ethylhexanol

[O] - Oxidized species

[R] - Reduced species

α - Charge transfer coefficient

A - Surface Area

ATR-FTIR - Attenuated total reflectance-Fourier-transform infrared spectroscopy

C - Molar concentration

CA - Chronoamperometry

CE - Counter-Electrode

C-OH - Cellulose fragment

CV - Cyclic voltammetry

D - Diffusivity

DEG - Diethylene glycol

DT - Dry torrefaction

E - Potential

EI - Electron ionization

ESI - Electron spray ionization

E0 - Standard potential of an electrode

EAc - Emulsion of cork bio-oil with a 2 M H2SO4 aqueous solution

EAc-Org - Upper layer of the Emulsion EAc, prominently organic

EAc-Int - Middle layer of the emulsion EAc

EAc-Aq - Bottom layer of the emulsion EAc, prominently aqueous

EAlk - Emulsion of cork bio-oil with a 2 M KOH aqueous solution

EAlk-Res - Viscous residue from the emulsion EAlk

EAc’ - Emulsion EAc after being subjected to electrolysis in a single compartment cell

EAc-Org’ - Upper layer of the Emulsion EAc’

EAc-Int’ - Middle layer of the emulsion EAc’

EAc-Aq’ - Bottom layer of the emulsion EAc’

EAlk’ - Emulsion EAlk after being subjected to electrolysis in a single compartment cell

EAlk-Res’ - Viscous residue from the emulsion EAlk'

EAc’’ - Emulsion EAc after being subjected to electrolysis as the catholyte in a two-compartments cell

EAc-Org’’ - Upper layer of the Emulsion EAc’’

EAc-Int’’ - Middle layer of the emulsion EAc’’

EAc-Aq’’ - Bottom layer of the emulsion EAc’’

EAlk’’ - Emulsion EAlk after being subjected to electrolysis as the anolyte in a two-compartments cell

EAlk-res’’ - Viscous residue from the emulsion EAlk’’

F - Faraday’s constant

HER - Hydrogen evolution reaction

HHV - Higher Heating Value

MS - Mass Spectrometry

i - Current

XI

j - Current density

jE - Current density at a specific potential, E

jp - Peak’s current density

jtf - Current density at the end of the electrolysis

j0 - Exchange current density

kB - Boltzmann’s constant

L-OH - Lignin fragment

LCO - Light cycle oil

LSV - Linear scan voltammetry

MS - Mass spectrometry

n1 - Electrolysis experiment in a single compartment cell of a sample of 1.5 M H2SO4 cork bio-oil

n2 - Electrolysis experiment in a single compartment cell of a sample of 1.5 M H2SO4 pinewood bio-oil

n3 - Electrolysis experiment in a single compartment cell of a sample of 1.5 M H2SO4 olive stones bio-oil

n4 - Electrolysis experiment in a single compartment cell of a sample of acidic emulsion, EAc

n5 - Electrolysis experiment in a single compartment cell of a sample of alkali emulsion, EAlk

n6 - Electrolysis experiment in a two-compartment cell with a sample of 2 M H2SO4 cork bio-oil as anodic electrolyte and a sample of 2 M H2SO4 aqueous solution as cathodic electrolyte.

n7 - Electrolysis experiment in a two-compartment cell with a samples of 2 M H2SO4 cork bio-oil as both anodic and cathodic electrolyte.

n8 - Electrolysis experiment in a two-compartment cell with a sample of acidic emulsion, EAc as anodic electrolyte and a sample of alkali emulsion, EAlk as cathodic electrolyte.

n - Number of electrons exchanged in an electrode reaction

na - Number of electron exchanged in the rate determining step

OCP - Open circuit potential

PEG - Polyethylene glycol

R - Universal gas constant

r - Radius of spherical particle

RE - Reference Electrode

rds - Rate-determining step

SCE - Standard calomel electrode

SHE - Standard hydrogen electrode

T - Temperature

t - Time

TsOH - p-Toluenesulfonic acid

η - Dynamic Viscosity

η - Overpotential

ν - Scan rate (mV s-1)

V - Volts

WE - Working Electrode

Wf - Final weight

Wi - Initial weight

WT - Wet torrefaction

Y - Yield

XII

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- 1 -

1. Introduction

1.1. Plant Biomass

The term biomass is used for all organic materials combustible in nature, mainly of plant and animal

origin. Plant biomass, also known as lignocellulosic biomass, encompasses residue of crop farming,

forestry industry and agro-industrial processing industries such as straw, bark, fruits and grains. This

Biomass is composed primarily of cellulose, hemicellulose and lignin, Figure 1. Dry biomass contains

roughly 40-45% cellulose, 25-35% hemicelluloses and 15-30% lignin [1–3].

Figure 1. Spatial arrangement of cellulose, hemicellulose and lignin in the cell walls of lignocellulosic

biomass [1].

Cellulose, Figure 2, is a linear polymer consisting of glucose monomers linked by 1-4-β glycosidic

bounds. The β bounds produces a linear conformation and enables the packing of numerous strands

into crystalline fibrils, as shown in Figure 1. Cellulose is the lignocellulosic polymer with highest degree

of polymerization at 10,000 or higher. The high molecular weight and low flexibility of cellulose chains

contributes to the cellulose polymer insolubility in water [1,2].

Hemicellulose is a polymer composed of C5 and C6 sugars, most typical xylose, arabinose, glucose,

mannose and galactose, Figure 3, with a degree of polymerization, around 100-200. Hemicellulose acts

as an amorphous matrix, holding the cellulose fibrils in place [1,2].

Lignin is a highly cross-linked, three-dimensional aromatic polymer consisting of three monomers,

coniferyl, sinapyl and p-coumaryl alcohols, Figure 4, mostly connected by C-C cross-links or ether

bounds. In the lignin polymer, the subunits are identified by their aromatic ring structure and are called

guaiacyl, syringyl and p-hydroxyphenyl subunits, respectively. Lignin composition differs between

- 2 -

softwood, hardwood and grasses, with softwood being composed mostly by guaiacyl while hardwood

contains a large quantity of syringyl groups [1,2].

Figure 2. Structure of cellulose with a 1-4-β glycosidic bond, adapted from [1].

Figure 3. Sugar monomers typically found in Hemicellulose, adapted from [1].

Figure 4. The three monomers that form the lignin polymer along with their respective subunits, adapted

from [1].

- 3 -

1.2. Thermochemical Conversion

Lignocellulosic biomass is an abundant resource that is commonly used as a source of energy by

direct combustion. However, several of its characteristics, such as low heating value, poor grindability

and high moisture content, are a disadvantage to its direct use as fuel [4]. Nowadays there are several

better alternatives for the efficient use of this renewable resource: sustainable production of liquid and

solid fuels, hydrogen, synthetic gases and valuable chemicals. The production of green chemicals and

clean, net zero carbon emission biofuels with an easy accessible, renewable resource has a promising

potential for development and is of extreme importance for or society and the environment. Some of the

most interesting processes to the exploitation of this resource and production of these valuable

compounds are the thermochemical conversions of biomass. These conversions consist on the

chemical transformation of organic matter at high temperatures. Thermochemical conversions, Figure

5, include processes like gasification, pyrolysis and liquefaction [5,6].

Figure 5. Classification of biomass’ thermochemical conversion technology.

Gasification is a process that can be used to produce a gas mixture from biomass such as wood or

agricultural wastes. The reaction is carried out at high temperatures in order to optimize the gas

production, resulting in small quantities of char and ash. The gasifying agent can be air, steam, air-

steam mixtures or oxygen mixtures. The gas, known as syngas, is usually a mixture of carbon monoxide,

hydrogen, methane, carbon dioxide and nitrogen, and its composition depends heavily on the

gasification process, the gasifying agent and the biomass composition [7,8]. Syngas from gasification is

part of the Indirect Liquefaction process, where liquid hydrocarbons are formed through Fischer-Tropsch

synthesis with Fe-, Co- or Ni-based catalysts [8].

Pyrolysis is the thermal decomposition of biomass in the absence of oxygen or air, under inert

atmosphere producing a liquid oil rich in oxygenated compounds, no condensable gases and solid char.

There are several pyrolysis processes, the two main processes being the slow pyrolysis and the fast

pyrolysis. In the slow pyrolysis the biomass is heated slowly and is primary used for the production of

char or charcoal that can be used as solid fuels [9]. In the fast pyrolysis the biomass is heated rapidly

so that it reaches the peak temperature before it decomposes. The primary objective of the fast pyrolysis

process is the production of bio-oil that can be used as a replacement for fuel oil or diesel in many

applications like boilers, furnaces and turbines. Due to the bio-oil proprieties, like high oxygen and water

- 4 -

content as well as high density and viscosity, making it unfavorable to use as transportation fuel several

methods were also developed to upgrade this bio-oil to conventional hydrocarbons fuels, like filtration,

solvent addition and catalytic methods, to cite a few [9–12].

Torrefaction is also a form of pyrolysis and it is known to be an effective process to pre-treat and upgrade

the biomass, improving its fuel properties. Nowadays there are two main different torrefaction processes,

dry torrefaction (DT) and wet torrefaction (WT). DT is conventionally a thermochemical pre-treatment of

biomass carried out either in an inert gas environment or in the presence of oxygen, carbon dioxide

atmosphere or a mixture of both, and in temperature range of 200-300 ºC, obtaining a solid product,

called biochar, as the main component. WT is a pre-treatment in hot water, at a pressure superior to the

saturated vapor pressure of the water, or hydrothermal media at temperatures of 180-260 ºC. In the WT

pre-treatment the main product is a solid called hydrochar. Both these pre-treatment techniques produce

solid fuels with better chemical, physical and fuel properties than raw biomass [4,9,13,14].

1.2.1. Direct Liquefaction

Direct liquefaction thermally decomposes solid biomass into predominantly liquid products, with

some char and gases, in the presence of a liquid solvent and an adequate catalyst. Usually, it is carried

out at moderate temperatures and pressures, ranging from 150 to 400 °C and 20 to 200 bar,

respectively. The operating pressure of a given direct liquefaction system is largely dictated by the vapor

pressure of the solvent, but it can be impacted by the vapor pressures of the products, as well.

The basic reaction pathways for the liquefaction process consists on the depolymerization of the

biomass into monomer units, which decompose by cleavage, dehydration, decarboxylation and

deamination, forming light fragments of small molecules, unstable and active. These light fragments

rearrange themselves by condensation, cyclization and polymerization into more stable molecules,

leading to new compounds [15,16]

Direct liquefaction is further divided into several subcategories delineated by the primary solvent used,

such as the hydrothermal liquefaction, in which water is the primary solvent, and the solvent liquefaction,

also known as solvolysis, in which other, usually organic, solvents are used instead.

Briefly, hydrothermal liquefaction, is a process where bio-oil is formed by thermochemical conversion in

water, that acts as a solvent, at high pressure and temperature. It is generally carried out at 250-400 ºC

and between 10 and 25 MPa. Biomass is mixed with water and a catalyst like sodium carbonate and

subjected to high pressure and temperatures. It has the advantage of skipping the drying step in the

biomass pre-treatment and of recovering any inorganics present in the biomass [2,5,17,18].

Solvolysis liquefaction is a process where biomass is dissolved in an organic solvent at moderate

temperatures, 120 ºC to 180 ºC, and atmospheric pressure. The organic solvents used are usually

polyhydric alcohols such as ethylene glycol, glycerol, ethanol, 2-ethylhexanol, polyethylene glycol and

so on. Acids, both strong and weak, can be used as catalyst in solvolysis liquefaction. Examples of acids

- 5 -

used are sulfuric acid, hydrochloric acid, p-toluenesulfonic acid and oxalic acid. The yield of the chemical

reactions in the liquefaction is affected by several variables, the most important being the concentration

and chemical composition of biomass, the solvent, the concentration and type of catalyst, temperature

and reaction time.

In relation to the composition of the biomass, the amount of cellulose and hemicellulose is not as

important in the conversion as the amount of lignin due to their simpler structures and easy

decomposition. Due to its complex structure, the higher the amount of lignin the lower the conversion,

and when thermally decomposed at temperatures above 252 ºC it forms free radicals of phenol through

repolymerization and condensation reactions originating solid residues [5,19]

For the type of solvent, simpler alcohols, like methanol, ethanol and propanol originate higher yields of

liquefaction, while alcohols with longer chains and organic acids leads to a higher amount of residue.

However, simpler alcohols have lower boiling point, which can lead to evaporation before the beginning

of the liquefaction. It is recommended that the solvent should be chosen such that it strongly reacts with

cellulose and, if possible, the solvent should be a product of the liquefaction itself, such as phenol and

its derivatives, simple alcohols or polyalcohols [20,21]. Besides the type of solvent, the ratio between

the amount of solvent and biomass is very important. High amounts of biomass may lead to a large

increase of viscosity which limits the reaction rate [20–23].

As the liquefaction is held in the presence of an organic or inorganic acid catalyst, sulfuric acid being

one of the most commonly used, the choice and concentration of catalyst is an important factor. Low

concentrations, such as 3%, promote the biomass degradation, however concentrations above the

optimal value may promote repolymerization and condensation reactions [20–22].

The temperature is also a very influential variable in the yield of the reaction, as the conversion of the

reaction increases with the temperature up to a certain optimal value. However, if the reaction is held at

temperatures higher than its optimal value, the amount of residues increases due to repolymerization

and condensation reactions as well as having an impact on other proprieties like viscosity and acidity

[19–22]

According to Zou et al. [23] there are three stages during biomass liquefaction with alcoholic solvents,

including biomass dehydration, volatilization of alcoholic solvents, and biomass alcoholysis. The

mechanisms of biomass liquefaction with alcoholic solvents, as proposed by Zou et al. in [23], can be

seen in Figure 6 and Figure 7.

Figure 6 presents the reaction pathway for cellulose with an acid catalyst and an alcohol as solvent. As

shown the degradation of cellulose and hemicellulose produces simple sugars like glucose and xylose

that by further reaction steps may produce aldehydes, ketones or esters. The depolymerization of lignin

(Figure 7) forms phenols that can react with the alcohol solvent to form different substances. Figure 8

shows the possible mechanisms of the reaction of fragments of biomass liquefaction with alcoholic

solvents, in (A) the cellulose fragments (C-OH) and (B) the lignin fragments (L-OH), with mono-, di- and

tri-alcohols. The image shows that the use of poly-alcohols promotes the formation of products of higher

molecular weight, the formation of heavy oils and higher quantity of residues [23].

- 6 -

A summary of some studies focused on liquefaction and their working conditions, specifically indication

of the biomass, solvent and catalyst used, the temperature and time of reaction, and the obtained

conversion or yield, can be seen in Table I. The higher reported conversion and higher yield were

achieved when using para-toluene sulfonic acid (pTsOH) as catalyst, maintaining the reaction

temperature in the order of the 160ºC.

Figure 6. Reaction pathway for the cellulose of an alcohol solvent and an acid catalyst [23].

Figure 7. Reaction pathway for lignin with alcohol solvent and acid catalyst [23].

- 7 -

Figure 8. Mechanism of reaction of liquefaction fragments with alcohol solvents. In (A), the mechanism

for cellulose fragments (HO-C) with mono-, duo- and tri-alcohol solvents, and in (B) the lignin fragments

(HO-L) with the alcohol solvents [23].

Table I. Summary of some works on the liquefaction process and its working conditions.

Author Ref. Biomass Solvent Catalyst Temp. /

ºC Time /

min Conversion /

%

Hassan et al. (2008)

[24]

Bagasse and

cotton stalks

PEG and glycerin

3% H2SO4 150 120 79% - 88%

Wang et al. (2009)

[25] Corn

stover Glycerol 3% H2SO4 160 60-480 75% - 88%

Hu et al. (2012)

[22] Soybean

straw Crude

glycerol 3% H2SO4 240 180 -

Zhang et al. (2013)

[26] Bagasse Glycerol /

PEG 3% H2SO4 150 120 90%

Yona et al. (2014)

[27] Cork

powder Glycerol /

PEG H2SO4 and

NaOH 150-200 60-180 95%

Soares et al. (2015)

[28] Cork

powder Glycerol /

PEG 3% - 4% H2SO4 150-160 60-80 70%

Braz et al. (2015)

[29,30] Pine

Sawdust DEG / 2-EH

0.5% - 3.5% p-TsOH, H2SO4,

AlCl3, Trichlorocyanu

ric acid

120-180 120 42% - 98%

Mateus et al. (2015)

[31–33] Cork dust DEG / 2-EH 3% p-TsOH 160 90 95%

Santos et al. (2015)

[34] Cork dust DEG / 2-EH 3% p-TsOH 160 5 96%

Lee et al. (2016)

[35] Empty

fruit bunch

Glycerol / PEG

3% H2SO4 150 120 63%

Haverly et al. (2018)

[16] Southern

yellow pine

LCO - 120 - 47% - 54%

- 8 -

Huang et al. [15] also did a summary of research progress in the effect of biomass and solvent type,

such as ethanol or mixed solvents, on the liquefaction process for biomass such as pine powder and

sawdust, rice and wheat straw, cornstalk and bagasse, to cite a few.

1.2.2. Bio-Oil

Bio-oils are liquid mixtures of oxygenated compounds containing carbonyl, esters, ketones and

phenolic functional groups, derived from depolymerization and fragmentation of cellulose, hemicellulose

and lignin.

These bio-oils are advantageous due to their elevated high heating value (HHV) in comparison with the

biomass used in their production [33], despite having lower values than conventional diesel.

Nonetheless, bio-oils can be used as a fuel in boilers, diesel engines or gas turbines for heat and

electricity generation [36].

As they are also rich in polyols and can be used to produce phenolic resins as well as several other

environmental friendly polymers and derivate products [25,37–39]. Santos et al. [40] developed a novel

formulation of a natural polymeric water-based adhesive designed to glue lignocellulosic surfaces, such

as wood and cork, from cork liquefaction. Esteves et al. [41] shows that it is possible to use bio-oil from

cork biomass liquefaction for the production of polyurethane foams.

Recently, the innovative generation of syngas uses the water electrolysis process with liquefied biomass

as a carbon source, necessary to obtain carbon monoxide and carbon dioxide [42]. Syngas has many

applications like the intermediate production of transport fuels, gas fuels and various chemicals.

Liquefied biomass can also be a source of other valuable chemicals, like the levulinic acid, furfural and

5-hydroxymethylfurfural, of extreme importance as an intermediate in the synthesis of many other

compounds [43]. Nilges et al. ([44]) made use of electrochemistry for the production of renewable

chemicals and biofuels, specifically a two-step electrochemical conversion of levulinic acid to octane via

valeric acid. The conversion of levulinic acid into hydrocarbons is usually achieved via multi-step

processes (under harsh conditions of temperature and pressure, 250 – 400 ºC and 10 – 35 bar of H2),

while the electrochemical reaction is performed at room temperature and in aqueous solutions, with a

natural phase separation allowing a simple separation of the hydrocarbon. In the electrochemical

processes, the selectivity for the reaction products are dependent on the electrolyte composition,

electrode material and current density [44,45].

The use of electrochemistry in the oxidation and reduction of organic species is not new. The

electrocatalytic oxidation of glucose, a product of the depolymerization of cellulose, into compounds

such as gluconolactone, a polyhydroxy acid used in cosmetics, is a well-studied process [46–50].

Electrochemical oxidation of D-mannose on platinum electrodes to produce mannonic acid, better

known as gluconic acid and used as a food addictive, is also a known process [51]. The electrochemical

oxidation of phenol has been studied for the synthesis of hydroquinone or benzoquinone and has been

reported as a means of waste water treatment [52,53]. Both the reduction of Xylose into Xylitol, a

sweetener accepted by medical science, and its oxidation into xylonic acid [54,55] are other examples

- 9 -

of the use of electrochemical processes in the conversion of chemicals present in the bio-oil into valuable

chemicals. Weinberg et al. ([56]) presents oxidation potentials, and some potential processes of

oxidation, of many organic compounds in several different electrolytes and electrodes

These studies come as no surprise since the advantages of the electrochemical process are numerous.

These advantages include an easier handling of the reaction media since in many cases only the

removal of solvent and electrolyte is required (for there is no chemical oxidant or its products to remove

from the reactor), the low cost of the process as, neglecting the initial cost of equipment, the power is

relatively inexpensive compared to chemical reagents and the yields are often adequate [56].

Despite the bio-oil promising potential for development and its importance as a green alternative to the

production of fuels, synthetic gases and many other valuable chemicals, to the best of our knowledge,

at the time of this work no significant studies were found on bio-oil electrochemical characterization nor

of the effects of its direct electrolysis. This work is a preliminary study focused on characterizing several

different bio-oils electrochemical behavior, with the intent of identifying the biomass under study, namely

cork pinewood and olive stones, with the highest potential to continue further studies regarding the

electrochemical conversion to industrial relevant compounds and the electrocatalytic upgrading of

biomass derived intermediates.

- 10 -

2. Electrochemistry

Electrochemistry is the branch of chemistry responsible for the study of the phenomenon of transfer

of electrons for the transformation of chemical energy into electric energy and vice versa. This

phenomenon happens in the interface between an electric conductor, the electrode, and an ionic

conductor, known as electrolyte. Electrodes are usually metals, both solid (as Pt, Ni, Au) or liquid (Hg,

amalgams), carbon (graphite) or semiconductor materials (Si). In the electrolyte, the current is

transported by the movement of the ions present in the solution, therefore, the more commonly used

electrolytes are aqueous solutions containing ionic species such as H+, Na+, Cl-, in water or in a non-

aqueous solvent [57,58]. These electrochemical reactions involve the charge transfer between the

electrolyte and the electrode, being heterogeneous processes, and can be either an oxidation or a

reduction reaction, depending on the species present in the electrolyte giving or taking electrons from

the electrode, respectively. Depending on the gain or loss of electrons by the reactional species, the

electrode is called the cathode or the anode [59].

2.1. Electrochemical Cell

Electrochemical cells are devices that consist of a vessel, commonly in glass (Pyrex and quartz),

Teflon or Nylon, and a set of electrodes. The electrodes present in the cell are connected to a power

source, such as a potentiostat/galvanostat, to apply the desired current or potential. The electrochemical

cells can be classified either as galvanic cells or electrolytic cells. Galvanic cells are cells that produce

electric energy from spontaneous redox reactions through applied potential difference in the cell.

Electrolytic cells are cells where the introduction of electric current enables the non-spontaneous

chemical reactions, inducing the conversion of electric energy in chemical energy [60]. Electrochemical

cells might present a varied number of arrangements. In this work will be used mainly arrangements

with two or three electrodes. Figure 9 displays both the referred arrangements, in (A) the common two

electrodes arrangement used in the electrolyzer, and in (B), the three electrodes arrangement frequently

utilized for fundamental studies. In the (A) arrangement with two working electrodes, the potential

difference is applied between both electrodes where, in a galvanic cell, the electrode with a more positive

charge is the cathode, and the other is the anode. These electrodes can be of the same material or of

different materials, being that their surface areas can be the same or vary, depending on the objective

and purpose. The (B) arrangement consists in the working electrode, the reference electrode and the

counter-electrode, also known as auxiliary electrode. In (B), the working electrode can work as either

anode or cathode, according to the applied potential difference between it and the reference electrode.

In this arrangement, the potential difference is recorded between the working electrode and the

reference electrode, and the electrons transfer happens between the working electrode and the counter-

electrode, avoiding possible disturbances to the reference electrode’s potential [58,61].

- 11 -

Figure 9. Schematic of a cell’s standard setup with (A) two electrodes and (B) three electrodes [62].

2.1.1. Electrodes

As previously mentioned, the electrodes used during this work can be divided in three groups, the

working electrodes, the reference electrodes and the counter-electrodes.

Working Electrodes

Working electrodes are the electrodes where the redox reaction of interest happens. Those can be made

of a varied number of materials, for instance noble metals, like Pt and Au, due to their natural resistance

to corrosion and oxidation, pure metals such as Ni or Hg, alloys like steel, graphite or semiconductor

materials like Si. Besides the material used, the electrode’s surface area is an important factor in

electrochemistry and both may vary depending on the purpose of the electrode [62].

Reference Electrodes

Reference electrodes are electrodes with a well-known, stable potential, used as a reference point in an

electrochemical cell for the control and measurement of the cell’s potential. The stability of the

electrode’s potential comes from the use of a redox system where the concentration of the components

or elements involved is constant. There are many different reference electrodes that can be used

depending on the characteristics of the cell, the working electrode and the electrolyte. The most

commonly used are: Standard Hydrogen Electrode (SHE), the Saturated Calomel Electrode (SCE) and

the Silver-Silver Chloride Reference Electrode [58,62,63].

Standard Hydrogen Electrode is used as standard reference point for standard electrochemical

reduction potentials, with its potential, Eº, assigned as zero volts at all temperatures by convention,

allowing the measure of the potential of any other electrode with respect to the SHE. The electrode is

composed of a Pt foil, connected to a Pt wire, immersed in a 1 M H+ aqueous solution, as exemplified

in Figure 10. The process starts when the electron charged Pt foil attracts a H+ from the solution to its

surface, forming a hydrogen atom, that combines with other hydrogen atom to create H2(g), that is

- 12 -

released from the system. For the system to work the surface of the Pt foil needs to be able to catalyze

Eq. [1] and a constant flow of H2 gas is required.

2 H3O+ + 2 e− → H2 + H2O Eq. [1]

Figure 10. Schematic of a SHE, adapted from [64].

Saturated Calomel Electrode is composed of solid Hg2Cl2 and liquid Hg immersed in a saturated KCl

solution, as shown in Figure 11. It is necessary to have the solution saturated as it allows for the

concentration of Cl- to be fixed by the KCl solubility, even if part of the solution is lost by evaporation,

ensuring the potential of the SCE remains constant. The potential is determined by the activity of Cl- in

equilibrium, according to:

1 2⁄ Hg2Cl2(s) + e− ⇌ Hg(l) + Cl

−(aq) Eq. [2]

Figure 11. Schematic of a standard SCE, adapted from [65].

- 13 -

Silver/Silver Chloride Electrode consists of solid silver wire coated in AgCl and submerged in a KCl

and AgCl solution, as shown in Figure 12. The electrode is based on Eq. [3], the redox couple between

AgCl and Ag, where the activity of the Cl- determines the potential. This a widely used reference

electrode because it is inexpensive and not as toxic as the SCE, which contains mercury.

AgCl(s) + e- ⇌ Ag+(aq) + Cl

-(aq) Eq. [3]

Figure 12. Schematic of a standard Ag/AgCl electrode, adapted from [65].

Counter-Electrodes

Counter-electrodes, also known as auxiliary electrodes, are used to close the electric circuit of the cell

by promoting the transfer of electrons between itself and the working electrode, through the external

circuit, avoiding any interference with the potential of the reference electrode provoked by said transfer.

As the electron transfer happens between it and the working electrode, its surface area is usually much

larger than the surface area of the working electrode to avoid it being a limiting factor. The substance

used in these electrodes is usually inert, such as Pt, Au or graphite, to avoid interfering in the process,

presenting itself in the shape of a mesh or coil to maximize its surface area, as shown in the figure

below:

- 14 -

Figure 13. Example of Pt counter-electrodes in the shape of a mesh and coil.

2.1.2. Potentiostat/Galvanostat

Potentiostat is a device used to control the potential difference between two electrodes, usually

between the working and the reference electrodes, according to a set-point value. The galvanostat is a

device used to control the flux of current in a cell. Usually, these devices already have coupled the

needed devices to measure both electric current and potential, also working as galvanometer and

potentiometer, respectively [59,62]. The following Figure 14 shows, as an example, the

potentiostat/galvanostat used in this work.

Figure 14. Potentiostat/Galvanostat Princeton Applied Research/EG&G Model 273A.

- 15 -

2.1.3. Open Circuit Potential (OCP)

The open circuit potential (OCP), is the potential of the working electrode measured in relation to

the reference electrode when the current is zero, this is, when no potential or current is being applied.

From this value it’s possible to find the ratio between the oxidized ([O]) and reduced ([R]) species of a

known redox reaction by applying the Nernst equation, Eq. [4], where R is the perfect gas constant, T

the absolute temperature, F the Faraday constant and n the number of electrons traded in the reaction

[57,58,63].

E = E0 −

RT

nFln (

[R]

[O]) Eq. [4]

2.2. Electrochemical Methods

In electrochemistry, there are numerous methods of analysis of a system. In this work, the

electrochemical methods addressed were the cyclic voltammetry (CV) and linear scan voltammetry

(LSV), where both methods study a system by applying a voltage scan, along with chronoamperometry

(CA) which studies the evolution of the current density with time at a fixed potential. The following topics

present a brief description of these methods.

2.2.1. Cyclic Voltammetry (CV)

Cyclic voltammetry (CV) is a technique that applies a linear voltage scan in both positive and

negative directions, at a specific scan rate, from an initial potential (E0) to a certain set point potential

(E1) and back again (E0 → E1 → E0). Thought the test, the current density for each potential is recorded,

tracing a current (i) vs. potential (E) curve and redox reactions appear as peaks in the curve at certain

potentials. CVs can be done at different scan rates, where each one will get a curve of similar shape,

with the difference being the total current, which increases with increasing scan rates. In slower scan

rates the diffusion layer has time to grow further from the electrode, and the flux to the electrode surface

is considerably smaller compared with faster rates. Figure 15 shows an example of a voltammogram

for the CV at different scan rates. In reversible reactions, Figure 15 (A), the peaks appear at a specific

potential, independently of the scan rate, as the electron transfer rate is superior to the mass transfer

rate. In irreversible reactions, Figure 15 (B), the opposite happens, showing an increase of the peak

potential with the scan rate.

- 16 -

Figure 15. CVs at several scan rates for (A), reversible reactions and (B) irreversible reactions.

2.2.2. Linear Scan Voltammetry (LSV) and Tafel Analysis

A similar method to CV, Linear Scan Voltammetry (LSV) scans the potential from an initial value to

a setpoint potential and records the resulting currents. In this work, the LSV is used for the analysis of

the cathodic zone where reduction reactions occur, such as the hydrogen evolution reaction (HER). The

analysis of the HER in bio-oils is important since it is one of the possible pathways for the valorization

of bio-oils while using the process of electrolysis.

Developed by Tafel, the Eq. [5], can be used for the analysis of the kinetics of reactions with a single

rate-determining step (rds). The equation relates the rate of an electrochemical reaction to the

overpotential, where a, the zero intercept, and b, the slope, are constants obtained from the linear

regression of the graphic representation of the overpotential, η, versus the logarithm of the current

density, log(-j), known as the Tafel Plot. The equation is usually used on high overpotential, in a range

between 100 mV and 300 mV, and in a region of the LSV where the current densities present a so-

called Tafel behavior, i.e., a linear relation between η and log(-j). The slope, b, and y-intercept, a,

obtained from Eq. [5] can be used to calculate the charge transfer coefficient, α, and the exchange

current density, j0, using Eq. [6] and Eq. [7], respectively, where R is the ideal gas constant, T is the

temperature in K, and F corresponds to the Faraday constant. The use of these parameters makes it

possible to compare the HER in different media.

η = E - E0 = a + b log(-j) Eq. [5]

b = 2.3 RT

αF Eq. [6]

a = b log(-j0) Eq. [7]

- 17 -

The charge transfer coefficient, α, defined as the fraction of the potential used in the electrochemical

reaction, this value can be used to determine the number of electrons exchanged in a reaction step

of a known system. For this it is necessary to resort to a linear regression, Eq. [8], where jp is the

current density of the peak, C is the concentration of the reduced species, D is the diffusion

coefficient and n is the number of electrons exchanged [57].

jp =

i

A = 2.99×10

5[(1-α)na]

1/2nC(Dν)

1/2 Eq. [8]

The exchange current density, j0, corresponds to the current where the forward and reverse

reactions are at state of equilibrium. It is a background current to which the net current observed at

various overpotentials is normalized, being an important variable in the rate of hydrogen evolution

reaction on metallic surfaces. Table II shows j0 values for different metals in a 1M H2SO4 solution,

where platinum shows to be the better metal for hydrogen evolution reaction, while the mercury is

the worst [57].

Table II. Comparison of j0 for hydrogen evolution reaction in 1 M H2SO4 [63].

Electrode material

-log (j0 / A cm-2)

Platinum 3.1

Iridium 3.7

Nickel 5.2

Gold 5.4

Titanium 8.2

Cadmium 10.8

Lead 12.0

Mercury 12.3

2.2.3. Chronoamperometry (CA)

Chronoamperometry (CA) is a potential-step method used to analyze the behavior of the current

density with time at a specific potential. In this method, the potential of the working electrode changes

instantly from an initial potential (E1) to a set potential (E2), as shown in Figure 16 (A). The

chronoamperogram corresponds to the current intensity registered with time, usually until it stabilizes,

Figure 16 (B). The zone of the graph where the current stabilizes is usually known as the diffusion-

controlled zone as in this zone the reaction is controlled by the diffusion of the species from within the

solution to the interface of the electrode.

- 18 -

Figure 16. (A), typical graph of a potential-step method and (B), example of a CA graph.

Chronoamperometry is a useful method to complement with other methods. It should only be used alone

when the process of the system in study is well known. Limited information about the identity of the

electrolyzed species can be obtained, from the ratio of the peak oxidation current versus the peak

reduction current, if present.

- 19 -

3. Experimental Methods

3.1. Bio-Oils and Reactants

All the bio-oil samples characterized in this study came from the liquefaction of ground plant

biomass, more specifically of cork, pinewood and olive stones biomass. In the solvolysis liquefaction,

the solvent used was 2-ethylhexanol (2-EH, 99%), from Acros Organic and the catalyst used was p-

toluenesulfonic acid (p-TsOH, 99%) from Acros Organic. Acetone (99.7%), from Sigma-Aldrich was

used in the treatment of the liquefaction solid residue. The reactants used in the electrochemical studies

was sulfuric acid (H2SO4, 95%), from Sigma-Aldrich, and potassium hydroxide (KOH, 90%), from Sigma-

Aldrich.

3.2. Solvolysis Liquefaction

The solvolysis liquefactions were performed with a procedure similar to ones found in the literature

[33]. The liquefactions were carried out in a glass reactor of 2 L equipped with mechanical stirring, a

thermocouple connected to the heating mantle, and a Dean-Stark separator/condenser. Figure 17,

below, displays the assembly used in the liquefaction. The solvolysis was performed with a weight ratio

of 1:2 biomass to solvent and 3 wt.% of catalyst, Eq. [9]. The reaction was run for four hours at 160 ºC,

after which the reactor was left to cool down at room temperature.

% catalyst = mcatalyst

msolvent

× 100 Eq. [9]

The procedure for the product treatment is exemplified in Figure 18. When cold, the reaction mixture

was subjected to centrifugation and sieving to separate the bio-oil from the solid residues. The solid

residues were thoroughly washed with acetone, subjected to centrifugation, filtration and then dried at

55 ºC in a hoven, and weighted. The bio-oil recovered from the solid residues was treated in a rotary

evaporator, at 40 ºC and low pressure, to remove the acetone from the bio-oil, and stored in separated

containers along with the remaining bio-oil for future use. The reaction conversion, defined in terms of

mass change, according to [21,29], is:

% C = (𝑊𝑖 – 𝑊𝑓)

𝑊𝑖

× 100 Eq. [10]

where Wi and Wf are the initial and final mass of the solid fraction, respectively.

- 20 -

Figure 17. Glass reactor in the heating mantle, at the bottom, coupled with a Dean-Stark

separator/condenser, on the left, a mechanical stirrer, up top, and the thermocouple controller on the

right.

Figure 18. Scheme of the experimental procedure for the liquefaction’s product treatment.

- 21 -

3.3. Bio-Oil Characterization

The bio-oils obtained by liquefaction were subject to several physicochemical methods to better

understand their characteristics and plan the following electrochemical studies with better rigor. The

characteristics analyzed were the density, viscosity, conductivity and pH, each briefly addressed in the

next topics. All samples subjected to electrochemical experiments were also subjected to attenuated

total reflection-Fourier-transform infrared spectroscopy (ATR-FTIR) and mass spectrometry (MS) with

the purpose of identifying possible changes in the bio-oils chemical composition related to said

experiments.

3.3.1. Density

Density or, to be more precise, the volumetric density of a substance is its mass for each unit of

volume. Density was verified as to obtaining a detailed characterization of the bio-oil. It was measured

with the aid of a pycnometer at room temperature.

3.3.2. Viscosity

Dynamic viscosity refers to the fluid’s internal resistance to flow when force is applied. It is a

characteristic taken in consideration as it directly affects the diffusivity of the particles in the bio-oil, as

shown by the well-known Stokes-Einstein equation, Eq. [11], where kB is the Boltzmann’s constant, T

is the absolute temperature, r is the radius of the spherical particle and η is the dynamic viscosity. This

has a direct impact on the electrochemical studies and electrolysis experiments when the reaction in the

interface of the electrode is controlled by the diffusion of particles in the electrolyte. The viscosity was

measured with a cone-and-plate viscometer from Research Equipment (London) Ldt., at 25 ºC.

D = kB × T

6 π × η × r Eq. [11]

3.3.3. Conductivity

Conductivity is a propriety that should be taken in consideration for electrolytes submitted to

electrochemical studies. As the redox reactions take place in the interface between the electrode and

the electrolyte, the contact between both phases should be adequately promoted for the efficient transfer

of the electrical currents. Conductivity was measured with a conductivimeter from HANNA Instruments,

model HI8733, at room temperature.

3.3.4. pH

The pH of the electrolyte is another propriety of extreme importance as certain electrodes can work

better or worse depending on the pH of the medium. Another factor is its effect on the redox reactions

in study, which may either be favored, or hindered by the pH of the medium. The pH value was measured

with a pH meter form HANNA Instruments, model HANNA pH20 at room temperature.

- 22 -

3.4. Electrochemical Experiments

The electrochemical experiments were performed at room temperature using a

potentiostat/galvanostat from Princeton Applied Research/EG&G, model 273A, shown in Figure 19,

controlled by PowerSuite software package. A platinum (Pt) electrode, from Metrohm, model 60305100

(A = 1 cm2) was used as the working electrode, a saturated calomel electrode (SCE) from HANNA,

model HI5412, was used as reference electrode and the auxiliary electrode was a Pt coil (A = 9.5 cm2),

as shown:

Figure 19. Pt electrode, SCE and Pt coil used during the electrochemical characterization.

A couple of small-scale laboratory electrolyzers were assembled with a couple of identical Ni electrodes

(A = 22 cm2), where it was used as both electrodes. The electrolyzers were a simple single compartment

cell and a two-compartment acrylic cell with an industrial membrane between compartments. The

samples studied where the cork, pinewood and olive stones bio-oil samples, both pure and with 3 M

H2SO4.

Another approach tried for the electrochemical experiments was adding an acid or alkaline aqueous

solution to the bio-oil, as the diagram in Figure 20 demonstrates. These mixtures formed emulsions of

bio-oil with aqueous solutions, in a 1:1 ratio. The diagram in Figure 20 (A) shows the two emulsions,

EAc being the emulsion formed by mixing the bio-oil with a 2 M H2SO4 aqueous solution, and EAlk, formed

by mixing the bio-oil with a 2 M KOH aqueous solution. The emulsion EAc was composed by three distinct

phases, the upper layer hereby known as EAc-Org, the middle layer EAc-Int and the lower layer EAc-Aq. The

emulsion EAlk did not form distinct layers, but formed a viscous residue, EAlk-Res. Part (B) of the diagram

in Figure 20 shows the emulsions EAc’, EAlk’ and their respective phases after 24 hours of electrolysis

in a single compartment cell with Ni plates as electrodes. And part (C), showing the emulsion EAc’’ and

EAlk’’ after 24 hours of electrolysis as cathodic and anodic electrolytes, respectively, in a cell with two

compartments divided by a porous membrane.

- 23 -

Figure 20. Diagram showing (A) the two emulsions, EAc with its distinct phases and EAlk, used in the

electrochemical experiments, with (B), showing the emulsions EAc’ and EAlk’, after 24 hours of

electrolysis in a single compartment cell, and (C), showing the emulsion EAc’’ and EAlk’’, after 24 hours

of electrolysis as a cathodic and anodic electrolyte, respectively, in a two-compartments cell.

All electrochemical experiments referred are addressed in the next topics.

3.4.1. Cyclic Voltammetry (CV)

Cyclic voltammetry was used to assess the liquefied biomass anodic oxidation as well as to

evaluate the HER potential in the bio-oil. The CV scans were performed using a Pt electrode in the

potential window between -2 V and +2 V at a scan rate of 50 mV s-1. The CVs were run using bio-oil

samples of cork, pinewood and olive stones bio-oil samples, both pure and with 3 M H2SO4 as well as

the acidic emulsion EAc and the alkali emulsion EAlk.

- 24 -


Linear scan voltammetry was used specifically to assess the samples cathodic processes, more

specifically, the HER. The LSV scans were performed in the samples of cork, pinewood and olive stones

with 3 M H2SO4, from the OCP up to potential of -2 V, at a scan rate of 50 mV s-1. These LSVs were

used to get the Tafel plots for the study of the HER.

3.4.3. Chronoamperometry (CA)

Chronoamperometry measurements were made by applying a range of potentials from 0.7 V to

1.3 V in the anodic zone and from -0.5 V to -1.3 V in the cathodic zone, for 200 seconds. The samples

subject to analysis were the cork, pinewood and olive stones bio-oil samples, both pure and with 3 M

H2SO4.

3.4.4. Electrolysis Experiments

Small-scale laboratory electrolyzers were assembled using a pair of identical Ni plates (A = 22 cm2)

as electrodes, see Figure 21. The experiments were performed using cork, pinewood and olive stones

bio-oil samples and an applied potential of 2.5 V at room temperature. It was used both a simple single

compartment cell with a volume of 150 cm3 and a two-compartment acrylic cell, each compartment with

a volume of 80 cm3, with an industrial membrane separating both compartments, as seen in Figure 22.

The electrolysis experiments were performed up to 24 hours using a BK Precision (model 1621A), and

a summary of the different electrolysis experiments run on each cell can be seen on Table III, below.

In the single compartment cell, the samples subjected to electrolysis were a sample of cork bio-oil with

1.5 M H2SO4, n1, a sample of 1.5 M H2SO4 pinewood bio-oil, n2, a sample of 1.5 M H2SO4 olive stones,

n3, an emulsion EAc, of cork bio-oil with a 2 M H2SO4 aqueous solution, n4, and an emulsion EAlk, of

bio-oil with 2 M KOH aqueous solution, n5.

In the two-compartment cell, Figure 22, several different electrolytes were used in both the anodic and

cathodic compartment. In experiment n6, an aqueous solution of 2 M H2SO4 was used as electrolyte in

the cathodic compartment, and a sample of cork bio-oil with 2 M H2SO4 as electrolyte in the anodic

compartment. In electrolysis experiment n7, was using a sample of cork bio-oil with 2 M H2SO4 as

electrolyte in both compartments to try to isolate the possible redox reactions happening at each

electrode.

In experiment n8, the emulsion EAc was used as electrolyte in the cathodic compartment and the

emulsion EAlk was used in the anodic compartment. The electrolysis experiments of the emulsions were

performed with continuous mechanical agitation to ensure the emulsion remained as “homogeneous”

as possible.

- 25 -

Table III. Electrolysis experiments run on each cell, the electrolyte used and their working conditions.

Single Compartment Cell

Electrolysis Electrolyte Duration / h Agitation

n1 1.5 M H2SO4 cork bio-oil 14 No

n2 1.5 M H2SO4 pinewood bio-oil 14 No

n3 1.5 M H2SO4 olive stones bio-oil 14 No

n4 Emulsion EAc 24 Yes

n5 Emulsion EAlk 24 Yes

Two Compartments Cell

Electrolysis Anodic Electrolyte Cathodic electrolyte Duration / h Agitation

n6 2 M H2SO4 cork bio-oil 2 M H2SO4 aqueous solution 14 No

n7 2 M H2SO4 cork bio-oil 2 M H2SO4 cork bio-oil 14 No

n8 Emulsion EAlk Emulsion EAc 24 Yes

Figure 21. Ni plates used as both anode and cathode in the electrolysis experiments.

Figure 22. Schematic of a two-compartment cell with a membrane between each compartment, adapted

from [66].

- 26 -

3.5. Bio-Oil Analysis: Electrolysis Experiments

The next sub-chapters focus on the bio-oil analysis before and after the electrochemical studies.

The analysis used in this approach were the attenuated total reflection-Fourier-transform infrared

spectroscopy (ATR-FTIR) and the mass spectrometry (MS). The ATR-FTIR and MS analysis were used

in several cork bio-oil samples subjected to electrolysis, both before and after, with the purpose of

identifying possible changes in the functional groups present.

3.5.1. Attenuated total reflection-Fourier-transform Infrared Spectroscopy (ATR-FTIR)

Fourier-transform infrared spectroscopy (FTIR) is a technique used to obtain an infrared spectrum

of absorption or emission of a solid, liquid or gas. The infrared spectrum absorption peaks correspond

to the frequencies of vibrations between the bonds of the atoms making up the material. As each

different material is a unique combination of atoms, each compound produces a unique infrared

spectrum. Therefore, infrared spectroscopy can result in a positive identification, a qualitative analysis,

of every different kind of material. In addition, the size of the peaks in the spectrum is a direct indication

of the amount of material present. An attenuated total reflection accessory operates by measuring the

changes that occur in a totally internally reflected infrared beam when the beam comes into contact with

a sample [67–69].

Several cork bio-oil samples were subjected to ATR-FTIR analysis to identify the possible changes in

the samples after being subjected to electrolysis. The samples analyzed, before and after the

electrolysis experiments, were a cork bio-oil sample with 2 M H2SO4 as well as samples of the emulsions,

EAlk’, EAlk’’, EAc-Org’, EAc-Org’’, EAc-Aq’ and EAc-Aq’’.

ATR-FTIR spectroscopic analysis was performed using a Thermo Nicolet Nexus apparatus with an ATR

accessory. Each spectrum was obtained by the average of 32 scans with a resolution of 8 cm -1.

3.5.2. Mass Spectrometry (MS)

Succinctly, mass spectrometry (MS) is an analytical technique that ionizes chemical compounds

and sorts the ions formed based on their mass-to-charge ratio. The mass spectrum is a plot of the ions

signal as a function of the mass-to-charge ratio and is used to determine the masses of the present

species. The atoms or molecules in the sample can be correctly identified by correlating known masses

to the masses determined by MS. Depending on the nature of the ionization process, and the nature of

the atoms and molecules in the sample, different ion types can be formed. The most common way in

which ions are produced in a mass spectrometer is through the loss of an electron by the initial collision

of a gaseous atom or molecule with an electron in a process known as electron impact or electron

ionization (EI). However, the ionization method used in this MS is the electron spray ionization (ESI), in

- 27 -

which a high voltage is applied to a liquid to create an aerosol. Simultaneously, increasing the

temperature originates desolvation. This method is especially useful in producing ions from

macromolecules because it overcomes the propensity of these molecules to fragment when ionized. As

the polarity of the voltage applied to the detector is opposite to that of the ions in order to attract them

to the ion detector, the detection of both positive and negative ions simultaneously is not possible, each

mass spectrum being either positive or negative [70].

The first samples subjected to MS were the original cork bio-oil, the emulsion phases EAc-Org, EAc-Int and

EAc-Aq, as well as the emulsion EAlk and the residue EAlk-Res. The emulsion phases EAc-Org’, EAc-Int’, EAc-

Aq’, EAlk’ and EAlk-Res’, were also analyzed by MS and compared with the originals to identify possible

changes caused by said electrolysis experiment.

The instrument used in the Mass Spectrometry was a detector Quatromicro / Micromass from Waters,

a triple quadrupole with electron spray ionization (ESI). The parameters used were according to the

table:

Table IV. Parameters used in the MS analysis.

Instrument Parameters

Polarity ESI NEGATIVE

Capillary / kV 3.00

Cone / V 20.00

Extractor / V 3.00

RF Lens / V 0.50

Source Temperature / °C 140

Desolvation Temperature / °C

220

Cone Gas Flow / L h-1 60

Desolvation Gas Flow / L h-1

600

Infusion

Cycle time / s 10.100

Scan duration / s 10.000

Inter Scan Delay / s 0.100

Start and End Time / s 0.00 to 120.00

Ionization mode ESI NEGATIVE

or ESI POSITIVE

Data type Accurate Mass

Function type Scan

Mass range 20 to 1980

Syringe Pump Flow / µL min-1

20

- 28 -

4. Results and Discussion

4.1. Solvolysis Liquefaction

In order to have bio-oil for the electrochemical studies, three solvolysis liquefactions of cork

biomass were performed. The conditions of the liquefactions and their percentage of conversion, Eq.

[10], are presented in Table V:

Table V. Working conditions and conversion of the solvolysis liquefactions of milled cork biomass.

Liquefaction nº 1st. 2nd. 3rd.

Biomass / g 100 200 300

Solvent / g 200 400 600

Catalyst / g 6 12 18

Time / h 4 4 4

Solid Residue / g 33 64 98

Conversion / % 67 68 67

When compared the degree of conversion obtained in these liquefactions with similar ones found in

literature, such as Mateus et al. [33], the values are lower than expected. The reason might be related

to the ratio of biomass to solvent, as in the literature is usually 1:9, much higher than the ratio used in

this work. Another factor to have in consideration is the differences between cork dust or powder and

the grinded cork granules used during this work. The smaller cork particles combined with the higher

ratio of biomass to solvent may explain the higher conversion rates. Overall, the conversion values seem

to be consistent between the liquefactions produced during this work.

The bio-oils from olive stone and pinewood were produced with yields of around 60%, by a previous

master student colleague Sriram Hariharakrishnan [71] and gently provided for this study.

- 29 -

4.2. Bio-Oil Characterization

4.2.1. Physicochemical properties

The bio-oils characterization began by determining the values of density, viscosity, conductivity and

pH for each sample, which can be seen in Table VI. The value of viscosity was not properly determined

as the instrument was designed to measure the viscosity of very viscous materials, not measuring values

below the 0.1 Poise. It is also evident the low conductivity of all samples, which was predictable since

the media is an organic mixture of fragments from the depolymerization of cellulose and lignin.

Table VI. Physicochemical properties of the bio-oil samples, from the different biomasses.

Bio-Oils Density / g cm-3 Viscosity / P Conductivity / mS cm-1 pH

Cork 0.88 <0.1 0.5 x 10 -3 1.6

Pinewood 0.89 <0.1 1.0 x 10 -3 -0.5

Olive Stones 0.93 0.1 0.8 x 10 -3 0.4

Since the objective is to perform electrochemical studies, there is a need to increase the bio-oils

conductivity. Therefore, to increase the conductivity, it was added H2SO4 acid to the samples, see Table

VII. The value of the conductivity increased as expected, however the viscosity of the samples also

increased considerably with the addition of H2SO4. This increase of viscosity might suggest the

repolymerization of the bio-oil and may hinder the electrochemical studies. Comparing the different

samples, the cork bio-oil presents itself as the sample with the highest increase in conductivity and

lowest increase in viscosity. Figure 23 presents a graph of the increase of conductivity with the addition

of up to 3 M H2SO4 in a cork bio-oil sample.

Table VII. Conductivity and viscosity of the bio-oils with the addition of H2SO4 up to 3 M of concentration.

Bio-Oils H2SO4 / M Conductivity / mS cm-1 Viscosity / P

Cork

0 0.5 × 10-3 <0.1

1 1.23 0.3

3 5.34 1.5

Pinewood

0 0.8 × 10-3 <0.1

1 1.17 1.4

3 4.80 2.6

Olive Stones

0 1.0 × 10-3 0.1

1 0.74 11.0

3 1.85 19.1

- 30 -

Figure 23. The effect of the addition of H2SO4 in the conductivity of sample of cork bio-oil.

The same approach was tried with the addition of KOH to a stored sample of cork bio-oil, Table VIII.

Although the conductivity increased, the increase of viscosity was much higher than that observed

during the addition of acid to the cork bio-oil, with a viscosity of 9 P at 0.5 M KOH, thus the KOH addition

approach was abandoned.

Table VIII. Increase of conductivity with the addition of KOH in a sample of cork bio-oil.

KOH / M Conductivity / mS cm-1 Viscosity / P

0 1.8 0.8

0.05 2.4 ̶

0.10 2.8 ̶

0.15 3.4 ̶

0.30 3.7 ̶

0.50 4.9 9.0

- 31 -

4.3. Electrochemical Experiments

The two emulsions created for the electrochemical experiments consisted of a mixture of the cork

bio-oil with a 2 M H2SO4 aqueous solution, the emulsion EAc, and of a mixture with a 2 M KOH aqueous

solution, emulsion EAlk, according to Figure 20. In the case of the acidic emulsion it progressively

separated in several phases over a period of 12 hours, as seen in Figure 24 (A). On the other hand, in

(B), the emulsion EAlk did not form visible phase separation, instead, it contained a homogeneous

viscous residue at the bottom of the funnel. This lack of formation of visible phases in the alkali emulsion

was possibly due to the formation of phenolates, which are hydrophilic in nature; while in the acidic

emulsion the neutralization of such phenolates leads to a phase separation.

Figure 24. Emulsions of cork bio-oil with (A) a 2 M H2SO4 aqueous solution, EAc, with visible separation

of the phases of the emulsion after 12 hours and an (B) a 2M KOH aqueous solution, EAlk, with no visible

phases.

4.3.1. Cyclic Voltammetry

Cyclic voltammetry (CV) was used to assess the bio-oil samples anodic oxidation and cathodic

reduction at Pt electrodes. The CVs were performed in samples of cork, pinewood and olive stones bio-

oil with up to 3 M H2SO4 as well as in a mixture of cork bio-oil with an aqueous solution of 2 M H2SO4

and in a mixture of cork bio-oil with an aqueous solution of 2 M KOH.

The CVs for the samples of bio-oil of the different biomasses with 3M H2SO4 can be seen in Figure 25.

From the graphs, it is evident the low current densities as well as the lack of peaks across all samples,

indicating that no specific redox reaction was occurring at those potentials.

A B

- 32 -

Figure 25. CVs of the (A) cork, (B) pinewood and (C) olive stones bio-oils, performed at 50 mV s-1.

The CVs for the samples of cork bio-oil with 1 M and 3 M H2SO4 are present in Figure 26, where it is

observable an increase of current densities with the concentration of H2SO4, especially in the cathodic

zone. The current density for the sample with 3 M reaches maximum values of 1.1 mA cm-2 at 2 V, in

Figure 26 B, and -2.2 mA cm-2 at -2 V, in Figure 26 C. The increase of current densities in the cathodic

zone is expected as the addition of acid promotes the HER, the only reaction visible in this zone. The

maximum current density for each sample, in the CVs’ anodic and cathodic zones, can be seen in Table

IX.

Figure 27 shows the CVs of the pinewood bio-oil, pure and with concentrations of 1 M and 3 M H2SO4.

The CVs display similar behavior to the cork bio-oil CVs, with no visible peaks in the anodic zone, in

Figure 27 B, nor in the cathodic zone, in Figure 27 C. It displays an increase in the current densities

with the addition of H2SO4, with the highest current densities for the sample with 3 M being 1.74 mA

cm-2 at a potential of 2 V and -4.87 mA cm-2 at -2 V, (see Table IX), revealing an increase in the current

densities of over 3 orders of magnitude regarding the values for the pure pinewood bio-oil sample.

Figure 28 presents the CVs of the olive stones bio-oil sample, both pure and with concentrations of 1

M and 3 m H2SO4. The behavior is similar to the other bio-oils, having an increase in current densities

with the acid concentration. The maximum current densities in the 3 M H2SO4 sample, in Table IX, are

between 0.64 mA cm-2 for the potential of 2 V, in Figure 28 B, and -1.15 mA cm-2 for -2 V, in Figure 28

C.

Comparing the current densities between all bio-oil samples, visible in Table IX, one can ascertain that

the pinewood bio-oil samples show higher values across all concentrations of acid. At 3 M H2SO4, the

olive stones sample shows the lowest current densities for both anodic and cathodic zones. Only

considering the current densities shown in the CVs, the pinewood bio-oil shows better potential for the

electrolysis experiments. However, the cork bio-oil samples had a higher value of pH and conductivity,

while having a much smaller increase in viscosity with the addition of H2SO4.

- 33 -

Figure 26. CVs performed at 50 mV s-1 of (A) cork bio-oil samples, pure and with concentrations of 1 M

and 3 M H2SO4, as well as the respective (B) anodic zone and (C) cathodic zone.

Figure 27. The CVs of (A) pinewood bio-oil samples, pure and with concentrations of 1 M and 3 M

H2SO4, with (B) the anodic zone and (C) the cathodic zone. Performed at 50 mV s-1.

Figure 28. The CVs of (A) olive stones bio-oil samples, pure and with concentrations of 1 M and 3 M

H2SO4, as well as (B) and (C) the anodic and cathodic zones. Performed at 50 mV s-1.

- 34 -

Table IX. Current densities recorded in the CVs of the bio-oil samples at potentials of 2 V and -2 V.

Bio-Oils H2SO4 / M jE = 2 V / mA cm-2 jE = -2 V / mA cm-2

Cork

0 0.24 x 10-3 -0.27 x 10-3

1 0.33 -0.58

3 1.10 -2.23

Pinewood

0 1.09 x 10-3 -1.45 x 10-3

1 0.52 -0.79

3 1.74 -4.87

Olive Stones

0 0.99 x 10-3 -1.23 x 10-3

1 0.23 -0.34

3 0.64 -1.15


The results obtained can be seen in Figure 29 where (A) is the LSVs of the cathodic zone,

performed at 50 mV s-1, for the olive stones, cork and pinewood bio-oils with 3 M H2SO4 and (B) is the

Tafel plot for the respective LSVs. The values of the OCP in the LSVs and the current density, j, at a

potential of -2 V are presented in Table X. The values of the linear equation, obtained from the linear

regression on the Tafel plot, Eq. [5], as well as the charge transfer coefficient, α, and the exchange

current density, j0, calculated from Eq. [6] and Eq. [7] respectively, are presented in Table XI. In Figure

29 (A) no reduction peak is visible in any of the LSVs, indicating that the only reduction reaction

happening in the cathode is the HER. Can also be seen that pinewood bio-oil sample has the highest

current densities at the same potential of all samples and olive stones having the lowest. From Figure

29 (B), the pinewood bio-oil has the smallest overpotential for comparable current densities, whereas

olive stones has the highest overpotential of the three samples. The Tafel slopes are between 0.59 V /

decade and 0.95 V / decade for pinewood and olive stones samples respectively.

Figure 29. The LSVs of (A) olive stones, cork and pinewood bio-oil samples with 3 M H2SO4, performed

at 50 mV s-1, with (B) Tafel plot for the respective samples with corresponding linear regression.

- 35 -

Table X. LSVs values of the OCP and of the current density, j, at a potential of -2 V.

Bio-Oils OCP / V jE=-2 V

/ mA cm-2

Cork 0.56 -2.23

Pinewood 0.24 -4.87

Olive Stones 0.67 -1.15

Table XI. The Tafel plot values of b, the Tafel Slope, and a, the y-interception of the linear regression

from the Tafel plots for each bio-oil sample, as well as the charge transfer coefficient, α, and the

exchange current density, j0, respectively.

Bio-Oils b / V dec-1 a / V α j0 / mA cm-2

Cork 0.59 1.25 0.10 7.4 x 10-3

Pinewood 0.56 0.73 0.11 49.4 x 10-3

Olive Stones 0.95 1.82 0.06 12.1 x 10-3

4.3.3. Chronoamperometry

The CAs for the cork bio-oil samples, both pure and with 3 M H2SO4 are present in Figure 30. For

the study of the behavior and stability of the samples, it was applied potentials 0.7 V and 1.3 V for the

anodic zone and -0.7 V and -1.3 V for the anodic zone, performed for 200 seconds.

In Figure 30 (A) and (B), the sample used was cork bio-oil and, due to the current densities in the order

of 10-4 mA cm-2, the CAs display some noise and are not well defined. Nonetheless, the behavior is the

expected one, with higher current densities for higher potentials and the current densities stabilize during

the 200 seconds run time. For the CAs in Figure 30 (C) and (D), it was used a sample of bio-oil with 3

M H2SO4. It shows the increase in the current densities, in comparison with the previous sample, while

displaying similar behavior, confirming that the addition of acid does not affect the behavior nor the

stability of the current densities.

For the samples of pinewood, the CAs present in Figure 31 (A) and (B), corresponds to the bio-oil

sample for the anodic and cathodic zones, respectively, together with (C) and (D), corresponding with

the sample with 3 M H2SO4, for the anodic and cathodic zones. The CAs are very similar between this

and the previous cork samples. The CA’s seem to be better defined for the anodic zone than for the

cathodic zone, where the fall of current densities with time is not as sharp.

For the olive stones bio-oil samples, the Figure 32 (A) and (B) shows the CAs for the anodic and

cathodic zones of the bio-oil, while Figure 32 (C) and (D) shows the CAs for the anodic and cathodic

zones for the bio-oil sample with 3 M H2SO4. The behavior of this bio-oil is similar to the other bio-oils

shown before, with (A) and (B) displaying low values of current density and poorly defined curves with

some noise. (C) and (D) show current densities about over 3 orders of magnitude superior to the ones

in the pure sample, also showing better defined curves, stabilizing at the 200 seconds of run time.

- 36 -

Figure 30. CAs for the pure cork bio-oil in the (A) anodic and (B) cathodic zones, as well as the CAs for

the cork bio-oil with 3 M H2SO4 in the (C) anodic and (D) cathodic zones, performed for 200 seconds.

Figure 31. CAs for the pinewood bio-oil in the (A) anodic and (B) cathodic zones as well as the CAs for

the pinewood bio-oil with 3 M H2SO4 for the (C) anodic and (D) cathodic zones, performed for 200

seconds.

- 37 -

Figure 32. CAs for the olive stones bio-oil in the (A) anodic and (B) cathodic zones as well as the CAs

for the olive stones bio-oil with 3 M H2SO4 for the (C) anodic and (D) cathodic zones, performed for 200

seconds.

The Figure 33, below, shows a comparison between the cork, pinewood and olive stones bio-oil

samples with 3 M H2SO4 in (A), the anodic zone and (B), the cathodic zone, at the same potential of

|0.9| V. It clearly shows the similar behavior between bio-oil samples, also shows the higher values of

current density of the pinewood bio-oil for the same potential. The fall in the current densities with time

in the anodic zone seems to be better defined than in the cathodic zone, where the characteristic curve

is barely visible.

Figure 33. Comparison between the CAs of the different bio-oils with 3 M H2SO4, at applied potential of

(A) 0.9 V for the anodic zone and (B) -0.9 V for the cathodic zone, performed for 200 seconds.

- 38 -

4.3.4. Electrolysis Experiments

The electrolyzes where performed for up to 24 hours in both a single compartment cell and a two-

compartment cell, as shown in Table III. As the low-precision power source used only shows currents

above 10 mA, it was not possible to record the electrolysis currents for the full time of the experiments.

Figure 34 presents the currents detected in the first 90 minutes of electrolysis experiment n1 of Table

III, of a cork bio-oil sample with 1.5 M H2SO4, in a single compartment cell with a couple of Ni plates of

22 cm2 of surface area as working electrodes. It shows a sharp decline of currents in the first few

minutes, stabilizing at values of 1.5 mA cm-2 and keeping those values for most of the registered time

until around 90 minutes of electrolysis where the current falls below the minimum value detected by the

power source.


III), the electrolysis of a cork bio-oil sample with 1.5 M H2SO4.

The electrolysis experiments n2 and n3 (Table III) with pinewood and olive stones, respectively,

followed the same procedure used on electrolysis experiment n1, however, the currents produced were

below the detection range of the power source, which is 10 mA. The same behavior was shown in the

electrolysis experiment n4, of the emulsion EAc, the currents produced were below the detection range

of the power source.

The currents observed for the first 25 minutes of the electrolysis experiment n5, the emulsion of cork

bio-oil with a 2 M KOH aqueous solution, emulsion EAlk (see diagram in Figure 20), performed with

continuous mechanical agitation in the single compartment cell, can be seen in Figure 35. The currents

stabilized in the first 25 minutes and maintained the current densities of around 0.9 mA cm-2 for the

entire duration of the 24 hours electrolysis.

- 39 -


III), the electrolysis of an emulsion of cork bio-oil sample with 2 M KOH aqueous solution in a single

compartment cell.

The two electrolysis experiments n6 and n7 of the Table III, with cork bio-oil sample with 2 M H2SO4 as

the anodic electrolyte in the two-compartment cell were performed for 14 hours. In these electrolysis

experiments n6 and n7 it was used a 2 M H2SO4 cork bio-oil sample and a 2 M H2SO4 aqueous solution

as electrolytes in the cathodic compartment, respectively, with an industrial membrane separating the

two compartments. Just like several electrolysis experiments in the single compartment cell, these

experiments did not produce currents above the minimum value detectable by the power source.

The electrolysis experiment n8 from Table III, used emulsion EAc, as electrolyte in the cathodic side and

emulsion EAlk as electrolyte in the anodic cell was performed for 24 hours, with continuous mechanical

agitation in both compartments, and the currents detected in the first 10 minutes can be seen in Figure

36. The currents stabilized at values around 2.2 mA cm-2 in the first 10 minutes of electrolysis and

maintained that values for the entire duration.

- 40 -


III), the electrolysis of an acidic emulsion EAc as electrolyte in the cathodic side and an alkali emulsion

EAlk as electrolyte in the anodic cell, performed for 24 hours.

The summary of the value of the current, at the end of each electrolysis experiment can be seen on

Table XII. From the table, it is evident the higher currents obtained during the electrolysis experiment

n8. It is also visible the low currents for all electrolysis experiments performed with the addition of H2SO4

to the samples, where the currents are below the detection range of the power source.

The samples subjected to electrolysis were then analyzed by ATR-FTIR and MS to determine possible

changes in their composition. By the analysis of the currents during electrolysis, it would be expected

for the samples of the electrolysis experiments n5 and n8 to show higher degree of difference between

their initial and final compositions.

Table XII. Resume of the values of jtf, the current densities detected at the end of each electrolysis

experiment.

Electrolysis Experiment jtf / mA cm-2

n1 < 0.4

n2 < 0.4

n3 < 0.4

n4 < 0.4

n5 0.9

n6 < 0.4

n7 < 0.4

n8 2.2

- 41 -

4.3.5. Cyclic Voltammetry: Electrolysis Experiments

The emulsions EAc and EAlk (see Figure 20) were analyzed both before and after electrolysis

experiments with the intent of observing differences in the emulsion behavior provoked by electrolysis.

For the emulsion EAc, the emulsion phases analyzed were the EAc-Org and EAc-Aq.

In the Figure 37 (A), the CV of the emulsion phase EAc-Org, it can be seen the low current densities,

lower than the ones found in the cork bio-oil samples, presented in Figure 26. In (B), the CV of emulsion

phase EAc-Org’, showing similar behavior and current densities as before. In (C), the CV of the emulsion

phase EAc-Aq, with low current densities in the anodic zone and large current densities in the cathodic

side, reaching densities of over 1200 mA cm-2 for a potential of -2 V. In (D), the emulsion phase EAc-Aq’,

showing the same behavior shown in (C), with very low current densities in the anodic zone and large

current densities in the cathodic zone.

In Figure 38 (A), the CV of the emulsion EAlk, shows a reduction peak at a potential of -0.6 V. In (B),

the emulsion EAlk’, the reduction peak disappears, with the cathodic zone showing higher current

densities for high potentials.

Figure 37. CVs of (A) acidic organic phase EAc-Org, before electrolysis, (B) acidic organic phase EAc-Org’,

after electrolysis, (C) acidic aqueous phases EAc-Aq, before electrolysis and (D) acidic aqueous phase

EAc-Aq’ after electrolysis. Performed at 50 mV s-1.

- 42 -

Figure 38. CVs of an emulsion of cork bio-oil with an aqueous solution of 2 M KOH (A), before the

electrolysis experiments, EAlk, and (B), after electrolysis for 24 hours, EAlk’. Performed at 50 mV s-1.

- 43 -

4.4. Bio-Oil Analysis: Electrolysis Experiments

The next chapters focus on the bio-oil analysis after the electrochemical experiments and their

possible differences when compared with before the electrolysis experiments. The analysis applied

during this work were the ATR-FTIR and the MS. These analyses were applied to samples of pure cork

bio-oil, samples of 2 M H2SO4 bio-oil, and samples of the emulsions EAc and EAlk, with their respective

phases (see Figure 20), before and after the electrolysis experiments. Table XIII, below, shows a

resume of the abbreviations used for each emulsion and their phases, as these abbreviations are used

extensively during the next chapters.

Table XIII. Summary of the abbreviations used for each emulsion and their phases (see Figure 20).

Abbreviation Emulsion

EAc Emulsion of cork bio-oil with a 2 M H2SO4 aqueous solution.

EAc-Org Upper layer of the Emulsion EAc, prominently organic.

EAc-Int Middle layer of the emulsion EAc.

EAc-Aq Bottom layer of the emulsion EAc, prominently aqueous.

EAlk Emulsion of cork bio-oil with a 2 M KOH aqueous solution.

EAlk-Res Viscous residue from the emulsion EAlk.

EAc’ Emulsion EAc after being subjected to electrolysis in a single compartment cell.

EAc-Org’ Upper layer of the Emulsion EAc’.

EAc-Int’ Middle layer of the emulsion EAc’.

EAc-Aq’ Bottom layer of the emulsion EAc’.

EAlk’ Emulsion EAlk after being subjected to electrolysis in a single compartment cell.

EAlk-Res’ Viscous residue from the emulsion EAlk’.

EAc’’ Emulsion EAc after being subjected to electrolysis as the catholyte in a two-compartments cell.

EAc.Org’’ Upper layer of the Emulsion EAc’’.

EAc-Int’’ Middle layer of the emulsion EAc’’.

EAc-Aq’’ Bottom layer of the emulsion EAc’’.

EAlk’’ Emulsion EAlk after being subjected to electrolysis as the anolyte in a two-compartments cell.

EAlk-Res’’ Viscous residue from the emulsion EAlk’’.

- 44 -

4.4.1. Attenuated Total Reflection-Fourier-Transform Infrared Spectroscopy (ATR-FTIR)

The ATR-FTIR spectra of the samples subjected to electrolysis experiments were made with the

purpose of identifying the samples’ functional groups and the identification of possible conversions

amongst functional groups during the electrolysis process. However, notice that in order to have a

positive identification of functional groups conversion, some conditions must be met:

- either the electrolysis is rather extensive so that the amount of species generated overcome the

initial ones, and also

- that the wavenumbers of the radiation influencing the new functional group (either originating

stretching, vibrations etc.) cannot overlap with the wavenumbers of the radiation responsible for

the fingerprint of the initial functional group.

In each of the following figures representing ATR-FTIR spectra of several different samples, the main

peaks were numbered. To each corresponding wavenumber of a main peak, it was assigned a functional

group, which can be seen in Table XIV. Therefore, the information of Table XIV is used several times

in the discussion of the results.

Table XIV ATR-FTIR peak number, wavenumber range of values and corresponding assigned

functional groups, adapted from [72].

Peak nº Wavenumber / cm-1 Assigned Functional Group

1 3600-3100 -O-H stretching vibration

2 2949 -2850 -C-H stretching vibration

3 1750-1735 -C=O stretching vibration in esters

4 1725-1705 -C=O stretching vibration in ketones

5 1648-1638 -C=C stretching vibration in alkenes

6 1650-1561 -C=C stretching vibration in cyclic alkenes

7 1465-1462 -C-H bending vibration

8 1420-1330 -O-H bending vibration in alcohols

9 1390-1310 -O-H bending vibration in phenols

10 1275-1200 -C-O stretching vibration in alkyl aryl ethers

11 1234-1230 -C-O-C stretching vibration in alkyl aromatics

12 1225-1200 -C-O stretching vibration in ethers

13 1205-1087 -C-O stretching vibration

14 1075-1020 -C-O stretching vibration

15 980-960 -C=C bending vibration in alkenes

16 880 ± 20 -C-H bend vibration

17 700 ± 20 -C-H bend vibration

- 45 -

Figure 39 (A) shows the ATR-FTIR of a 2 M H2SO4 cork bio-oil sample. In Figure 39 (B), the same 2

M H2SO4 cork bio-oil sample used as electrolyte in the anodic side of an electrolysis experiment of 8

hours, in the two-compartment cell, electrolysis experiment n6 (see Table III). Figure 39 (C) shows the

ATR-FTIR spectrum of the same sample after 14 hours of electrolysis. The cathodic side being a 2 M

H2SO4 aqueous solution. The objective is to identify differences in the bio-oil caused by the electrolysis

in the anodic zone, due to possible oxidations of species in the bio-oil. The main differences between

each AT-FTIR seems to be in the intensity of the peaks (10-12), (14) and (15). The two peaks (10-12),

situated around 1230-1220 cm-1, show a small increase in intensity, which correspond to -C-O stretching

vibrations. The peak at (14), just like the ones before corresponds to -C-O stretching vibrations. The

peak at (15), corresponding to -C=C bending vibrations also show increases in their signal over the

duration of the electrolysis experiment. The increase of -C-O and -C=C vibrations may indicate the

oxidation of the species in solution.

Figure 39. ATR-FTIR of (A) 2 M H2SO4 cork bio-oil sample, (B) bio-oil sample after 8 hours of

electrolysis, and (C) bio-oil sample after 14 hours of electrolysis, experiment n6 (see Table III).

The next ATR-FTIR, from Figure 40 to Figure 43, correspond to the acidic emulsion EAc and from

Figure 44 to Figure 46 the alkali emulsion EAlk, with their respective phases, both before and after the

electrolysis experiments (see Table XIII).

- 46 -

Figure 40 presents the ATR-FTIR spectra of (A), the pure cork bio-oil sample, and the spectra of the

three phases of the acid emulsion EAc, (B), the organic emulsion phase EAc-Org, (C), the emulsion phase

EAc-Int and (D), the aqueous emulsion phase EAc-Aq. The purpose of the analysis is to evaluate the

differences between the original bio-oil and the bio-oil emulsions with acid aqueous solution. In Figure

40 (B), the ATR-FTIR corresponding to the emulsion phase EAc-Org, it shows an increase in the intensity

of the peaks:

• (3), related to the -C=O stretching vibration;

• (8-9), assigned to -O-H bending vibrations;

• (11), the -C-O-C stretching vibration in alkyl aromatics;

• (15), -C=C bending vibration in alkenes.

Figure 40 (C), the ATR-FTIR of the emulsion EAc.2, shows no visible differences between its peaks and

the peaks from emulsion EAc-Org. This comparison shows the similarities between the composition of

both phases, demonstrating that both phases are mainly organic, with the same functional groups.

Figure 40 (D) shows relative fewer peaks, with peaks at:

• (1), related to the -O-H stretching vibrations;

• (5), related to -C=C stretching vibrations;

• (12) and (14), both related to -C-O stretching vibrations.

The large peak at (1) is attributed to the water in this phase, and the remaining peaks may be related to

soluble sugars, fragments of the decomposition of cellulose and hemicellulose.

Figure 41 presents the spectra of the organic phase of the initial emulsion, of the equivalent organic

phase after electrolysis in one cell compartment and in two-compartment cell. In (B) there are no

relevant differences when compared to (A). A possible explanation is due to the low currents generated

during the electrolysis, preventing any reaction to occur at the interface. In Figure 41 (C), it is visible a

decrease of intensity of the peaks:



• (11), the -C-O-C stretching vibration in alkyl aromatics.

This phase of the emulsion EAc’’ was used as a cathodic electrolyte in a two-compartment cell, as such,

the visible decrease of these peaks might indicate the reduction of the species in solution by the

electrolysis.

Figure 42 deals with the same sequence (before the electrolysis process and after electrolysis

experiments) for the middle phase separated from the emulsion. The middle phase of the acid emulsion

shown few to no differences in the functional groups when compared to the top, predominantly organic

phase. When comparing (B) with (A), it is visible few differences, possibly due to the low currents

generated during the electrolysis, preventing any reaction to take place at the interface. When

comparing (C) with (A), it is visible the disappearance of the peaks:

- 47 -



• (11), the -C-O-C stretching vibration in alkyl aromatics.

As this phase was used as a cathodic electrolyte in the electrolysis experiment with the two-

compartment cell, the decrease of the peaks related with the oxygen might indicate the reduction of the

species in solution provoked by the electrolysis.

In Figure 43 it is shown the ATR-FTIR of (A), the acid emulsion phase EAc-Aq, (B) the emulsion phase

EAc-Aq’, and (C), the emulsion phase EAc-Aq’. In Figure 43 (B) it is visible the appearance of the peak (3),

related to the -C=O stretching vibration and an increase of the intensity of the peaks:


• (12-13), related to -C-O stretching vibration;

• (16), -C-H bending vibrations.

In Figure 43 (C) it is visible a small increase of intensity of the peaks (5), -C=C stretching vibration in

alkenes and in (12-13), related to -C-O stretching vibrations. This might indicate that, for the aqueous

phase, there are some changes happening during the single compartment electrolysis experiment, but

the same is not visible in the sample used in the two-compartment electrolyzer.

Figure 40. ATR-FTIR of (A) pure cork bio-oil sample, (B) acidic upper emulsion phase EAc-Org, (C) acidic

middle emulsion phase EAc-Int and (D) acidic aqueous emulsion phase EAc-Aq.

- 48 -

Figure 41. The ATR-FTIR of (A) acidic upper emulsion phase EAc-Org, (B) acidic emulsion phase EAc-

Org’, after the electrolysis experiment in single compartment cell and (C) acidic emulsion phase EAc-Org’’,

after the electrolysis experiment in the two-compartment cell.

Figure 42. The ATR-FTIR of (A) acidic middle emulsion phase EAc-Int, (B) acidic middle emulsion phase

EAc-Int’, after the electrolysis experiment in single compartment cell, and (C) acidic middle emulsion

phase EAc-Int’’, after the electrolysis experiment in the two-compartment cell.

- 49 -

Figure 43. The ATR-FTIR of (A) acidic aqueous emulsion phase EAc-Aq, (B) acidic aqueous emulsion

phase EAc-Aq’, after the electrolysis experiment in single compartment cell and (C) acidic aqueous

emulsion phase EAc-Aq’’, after the electrolysis experiment in the two-compartment cell.

The studies of the addition of a basic aqueous solution (KOH) are discussed next. Figure 44 presents

the ATR-FTIR of (A), the cork bio-oil, (B), the alkali emulsion EAlk and (C), the emulsion viscous residue

EAlk-Res. The purpose of these analyses is to compare and understand the effect of the alkali aqueous

solution on the bio-oil. Comparing (B) with (A), we can see an appearance of a couple of small peaks

in (5) and (6), both related to -C=C stretching. It is also visible the increase of the intensity of the peak

at (8-9), related to -O-H bending, and the appearance of a large peak at (11), related to -C-O stretching

vibration. The peaks related to -C=C stretching, and -C-O stretching may indicate a deprotonation of the

compounds. Figure 44 (C) is the viscous residue EAlk-Res, visible on the containers of the emulsion. The

residue shows few defined peaks, the better visible ones being the peak at (1), related to -O-H stretching

vibrations and at (5), related to -C=C stretching vibrations.

Figure 45 shows and compares the alkali emulsion before and after the electrolysis experiments. It is

shown, in (A), the alkali emulsion EAlk, in (B), the alkali emulsion EAlk’ and in (C), the emulsion EAlk’’. In

Figure 45 (B), there can be seen an increase of intensity in several peaks. The major peak increase

seen is in the peaks:

• (3), associated with -C=O stretching vibration;

• (7), associated with -C-H bend vibration;

• (8-9), -O-H bending vibrations;

• (11), -C-O-C stretching vibration.

- 50 -

In Figure 45 (C) it can be seen an increase in the intensity of the peak (7), associated with -C-H bend

vibration, while getting a decrease in the peaks (3), associated with -C=O stretching vibration. There is

also visible a decrease of intensity of the peaks at (8-9), related to -O-H bending, and (11), -C-O-C

stretching vibration. The decrease of the functional groups related to oxygen in EAlk’’ may possibly be

related to an oxidation reaction during the electrolysis as the emulsion was used as the anodic

electrolyte in the two-compartment cell.

Figure 46 shows the ATR-FTIR of (A), the viscous residue of the alkali emulsion EAlk-Res, (B), the residue

EAlk-Res’ and (C), the residue EAlk-Res’’. These residue displays similarities between all samples, with

minor changes to the peaks intensity, showing that it did not suffer any visible change to their functional

groups during the electrolysis experiments.

Figure 44. The ATR-FTIR of (A) cork bio-oil, (B) alkali emulsion EAlk and (C) alkali emulsion viscous

residue EAlk-Res.

- 51 -

Figure 45. The ATR-FTIR of (A) alkali emulsion EAlk, (B) alkali emulsion EAlk’, after the electrolysis

experiment in single compartment cell and (C), alkali emulsion EAlk’’, after the electrolysis experiment

in the two-compartment cell.

Figure 46. The ATR-FTIR of (A) viscous residue of the alkali emulsion EAlk-Res, (B) alkali residue EAlk-

Res’, after the electrolysis experiment in single compartment cell and (C) alkali residue EAlk-Res’’, after

the electrolysis experiment in the two-compartment cell.

- 52 -

4.4.2. Mass Spectrometry (MS)

The samples subjected to the electrolysis experiments were analyzed using Mass Spectrometry in

order to verify the possible formation of new compounds during the electrolysis, by comparing the MS

samples before and after. The samples subjected to MS were the pure bio-oil and the emulsion phases

EAc-Org, EAc-Int, EAc-Aq, as well as EAlk and EAlk-Res (see Table XIII).

The MS corresponding with Figure 47 up to Figure 50 are relative to the acidic treatment to the bio-oil.

Figure 47 show the MS with positive ESI, respectively, of the different phases of the emulsion EAc,

namely the emulsion phases EAc-Org, EAc-Int and EAc-Aq. Figure 47 (A) being the MS of the organic

emulsion phase EAc-Org, (B), the MS of the emulsion phase EAc-Int and (C), the MS of the aqueous

emulsion phase EAc-Aq. The MS of the emulsion phases EAc-Int, Figure 47 (B), shows a higher number

of peaks, with larger mass and higher intensity, such as a large peak at 315 m / z that do not appears

in the other emulsion phases. A peak at around 90 m / z and another between 115 and 120 m / z appear

in all emulsion phases. A peak at around 340 m / z appears in (A) the organic emulsion phase EAc-Org

and in (B), the middle emulsion phase EAc-Int, but not in (C), the lower aqueous phase EAc-Aq. The MS

show the differences between each phase, with emulsion phase EAc-Int having compounds with higher

mass and the lower aqueous phase EAc-Aq having compounds with lower mass.

Figure 48 show the MS with positive ESI of (A), the sample of cork bio-oil, (B) the aqueous emulsion

phase before electrolysis EAc-Aq, and (C) the aqueous emulsion phase after electrolysis EAc-Aq’. Figure

48 (C) shows larger peaks, especially at around 120 m / z, 210 m / z and 340 m / z, somewhat similar

to ones found in the MS of the original bio-oil, (A). It is also visible the disappearance of the peak visible

in (B) at around 310 m / z. Another visible difference in (C) is the appearance of a couple of small, but

well-defined peaks at around 460 m / z and around 660 m / z. These peaks show the appearance of

new compounds soluble in the aqueous phase after electrolysis.

Figure 49 shows the MS with positive ESI of (A) the sample of cork bio-oil, of (B) the organic emulsion

phase before the electrolysis experiment EAc-Org, and of (C) the organic emulsion phase after the

electrolysis experiment, EAc-Org’. Comparing (B) with (A), it is seen a reduction of the intensity of the

peaks around 90-150 m / z, as well as at 200-230 m / z. While there is a visible increase of the intensity

of the peaks around 275-330 m / z, again showing an increase of the mass of the compounds present

in the bio-oil with the addition of the acid aqueous solution. A peak at around 340 m / z, some peaks

between 400-475 m / z, 525-625 m / z and a peak at 700 m / z are present in all MS samples. These

compounds show very high stability and considering their high mass-to-charge ratio, these may be

fragments from the lignin depolymerization. Comparing (C) with (B) it is visible the intensity reduction

and disappearance of the peaks around 300-325 m / z and the reappearance of several peaks of lower

mass around 90-150 m / z and around 210-230 m / z.

- 53 -

Figure 50 shows the MS with positive ESI of (A) of the sample of cork bio-oil, (B) of the middle emulsion

phase before the electrolysis experiment EAc-Int, and (C) of the middle emulsion phase after the

electrolysis experiment EAc-Int’. Comparing the MS (B), from the emulsion phase, with (A), the bio-oil, it

is visible a decrease of the intensity of several peaks below 250 m / z, and an increase of several peaks

above 275 m / z, with large increases around 275-330 m / z, 400-475 m / z and around 575 m / z. These

peaks put in evidence the increase in the mass of the compounds of the emulsion phase EAc-Int in

comparison with the initial bio-oil. In the MS (C), relative to the emulsion phase EAc-Int’, subjected to

electrolysis, it is evident the peaks relative to compounds with smaller mass than the ones in (B). Further

investigations are needed in order to fully understand what it is happening during electrolysis.

Figure 51 displays the MS with positive ESI of the emulsion EAlk and the viscous residue EAlk-Res. With

Figure 51 (A) being the MS relative to the cork bio-oil, (B) being the MS relative to the alkali emulsion

EAlk, (C) being relative to the emulsion viscous residue EAlk-Res, (D) being relative to the emulsion EAlk’

and (E) being the viscous residue EAlk-Res’. Comparing the positive ESI MS of Figure 51 (B) with (A),

there is a well-defined peak at 340 m / z in (A) that does not appear in (B), while a new peak appears

between 310-320 m / z. The MS (C) of the viscous residue shows large peaks between 280-400 m / z.

Observing the MS (D) it is visible the appearance of peaks at 340 m / s, between 400-480 m / s, 540-

600 m / s and at 700 m / s, previously not visible in (B), hinting at the occurrence of reactions during

electrolysis. The MS (E), relative to the viscous residue EAlk-Res’, is similar to (C), where a high

concentration of large peaks between 280-400 m / z may indicate some degree of repolymerization of

the emulsion components.

Figure 47. The MS with positive ESI of (A) acidic organic emulsion phase EAc-Org, (B) acidic middle

emulsion phase EAc-Int and (C) bottom aqueous emulsion phase EAc-Aq.

Cone 20V

m/z100 150 200 250 300 350 400 450 500 550 600 650 700 750

%

0

100

08102018_CORK_MIX_ACET_AMON_ESIPOS_01 3 (0.505) Cm (2:11) Scan ES+ 1.00e7

B

A

C

- 54 -

Figure 48. The MS with positive ESI of (A) sample of cork bio-oil, (B) acidic aqueous emulsion phase

EAc-Aq, and (C) acidic aqueous emulsion phase EAc-Aq’ after the electrolysis experiment in a single

compartment cell.

Figure 49. MS with positive ESI corresponding to (A) sample of cork bio-oil, (B) acidic upper emulsion

phase EAc-Org, and (C) acidic upper emulsion phase EAc-Org’, after the electrolysis experiment in a single

compartment cell.

Cone 20V

m/z100 150 200 250 300 350 400 450 500 550 600 650 700 750

%

0

100


Cone 20V

m/z75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 500 525 550 575 600 625 650 675 700 725 750

%

0

100


B

C

A

B

C

A

- 55 -

Figure 50. The MS with positive ESI of (A) sample of cork bio-oil, (B) acidic middle emulsion phase EAc-

Int, and (C) acidic middle emulsion phase EAc-Int’, after the electrolysis experiment in a single

compartment cell.

Figure 51. The MS with positive ESI of (A) sample of cork bio-oil, (B) alkali emulsion EAlk, (C) alkali

emulsion residue EAlk-Res, (D) alkali emulsion EAlk’ and (E) respective emulsion residue EAlk-Res’, after

the electrolysis experiment in a single compartment cell.

Cone 20V

m/z100 150 200 250 300 350 400 450 500 550 600 650 700 750

%

0

100


C

A

B

C

A

E

R D

B

- 56 -

5. Conclusions and Future Work

5.1. Conclusions

The liquefaction of the cork biomass reached conversion rates of around 65% of the biomass weight.

This value is slightly below the ones found in the literature, possibly due the difference between cork

powder and the larger particles of grinded cork used in this work. The effect of the solvent and its quantity

in relation to the biomass used is another variable to take in consideration, as in the literature the ratio

of biomass to solvent is 1:9, and in this work it was used a ratio of 1:2 biomass to solvent.

It is shown that the conductivity of the bio-oils is low and the addition of H2SO4 to the bio-oil is an effective

way to increase these values. Of all the bio-oil samples, the cork bio-oil was the one showing the highest

increase in conductivity for the same molar concentration of acid, while olive stones showed the smallest

increase in conductivity. However, the addition of strong acid or alkali compounds, such as H2SO4 or

KOH also leads to an undesirable increase of viscosity of the bio-oil, possibly due to the repolymerization

of the unstable polymer fragments. Inversely to the increase of conductivity, the olive stones bio-oil was

the one that showed the highest increase of viscosity for the same molar concentration of acid, while

the cork bio-oil was the one showing the smallest increase of viscosity. The addition of KOH was shown

to have a large effect on the viscosity of the bio-oil when compared with the acid addition, with a cork

bio-oil sample having a value of 9 P for a concentration of 0.5 M KOH, therefore the approach was

abandoned. Another approach studied was the mixture of an acid or alkali aqueous solution with the

bio-oil, forming an emulsion. The acid emulsion EAc showed the separation into three distinct phases,

the top layer being predominantly organic and the bottom layer being predominantly aqueous. The alkali

emulsion of 2 M KOH with cork bio-oil, formed no distinct phases visible. The effect of acid or alkali

addition, both directly and in an aqueous solution, to the bio-oil as well as other possible alternative

methods to increase the bio-oil conductivity, need to be further studied.

The CVs of the different bio-oil samples show no visible redox peaks from an applied potential of 2 V up

to -2 V. These charts also show a notable increase in the current densities of the samples with addition

of H2SO4 in comparison with the samples without any addition. The addition of H2SO4 also helps with

the hydrogen evolution reaction (HER), studied by the application of the Tafel analysis to the samples

cathodic LSVs. The CA exhibited the expected behavior, with higher currents being recorded for higher

overpotentials.

The electrolyzes of the bio-oil samples were performed both in a single and a two-compartment cell,

with Ni plates as electrodes. The potential applied was of 2.5 V and the experiments were run for 24

hours. The electrolysis experiment n1 (Table III) of the sample of 1.5 M H2SO4 cork bio-oil produced

current densities that stabilized around the 1.5 mA cm-2, keeping those values for most of the registered

time until around 90 minutes of electrolysis where the current falls below the minimum value detected

by the power source. In the electrolysis experiments n6 and n7, both in the two-compartment cell using

2 M H2SO4 cork bio-oil, the currents produced were below the minimum values detected by the power

source.

- 57 -

The electrolyzes of the emulsions were done with constant mechanical agitation to ensure the proper

mixture of the emulsion. The electrolysis experiment n5, of the alkali emulsion EAlk, the current densities

stabilized around 0.9 mA / cm-2. In the electrolysis experiment n8, with acidic emulsion EAc as cathodic

electrolyte and alkali emulsion EAlk as anodic electrolyte, the current densities visible during the

experiment were of around 2.2 mA / cm-2.

The ATR-FTIR confirm changes in the functional groups of the species during electrolysis, such as

electrolysis experiment n6, where it is visible an increase in the signal of the peaks corresponding to the

vibration of functional groups with oxygen. The ATR-FTIR of the emulsion phases also shows the

possibility of reactions during the electrolysis experiments n4, n5 and n8.

The MS shows several differences between the peaks of the bio-oils before and after the electrolysis

experiments. The spectra show the appearance and an increase of intensity of peaks with higher mass

with the addition of acid and alkali compounds to the bio-oil, which may in part be related to some

repolymerization reactions of the bio-oils. They also show a several differences in the peaks present

after electrolysis. These changes in the bio-oil after the electrolysis experiments, visible in the MS, need

further studies.

5.2. Future Work

For future work, improving the low conductivity, and low currents produced during electrolysis, may be

the best approach. Potential good results may be obtained by studying the solvent extraction of relevant

compound from the bio-oil may show potential good results, for example with water. Doing so, may solve

the problem related with the lack of conductivity and high viscosity of the bio-oil. Other possibility is the

addition of ashes from the burning of either the biomass or of the solid residues of the liquefaction, that

add inorganic ions to the solution, possibly increasing its conductivity without adding new compounds

to the bio-oil. The introduction of CO2 gas to the bio-oil during the electrolysis may also be a possibility,

as it works as both an acidifying agent and in the carbonation of the bio-oil components. The advantage

of using CO2 is being able to capture and use this gas that is an undesired product of many processes.

Another possibility is the study of different processes of liquefaction to produce bio-oils. An example of

that is the use of ionic liquids as solvents in the liquefaction of lignocellulosic biomass [1, 73-75] as these

ionic liquids have also been studied as additives in electrochemical processes. In the electrochemical

processes the ionic liquids are shown to improve the electrolyte solution properties and enhance the

HER kinetics, making them attractive and safe for industrial applications [76, 77].

The reactions happening during the electrolysis experiments, as shown both by ATR-FTIR and MS,

need to be subjected to further studies. Understanding the processes and mechanisms of the

electrolysis of bio-oil is a key point in the use of this resource for the production of industrial relevant

compounds.

- 58 -

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A

Appendix

A. Mass Spectrometry

This appendix shows the MS with the negative ESI of the same samples discussed in the

subchapter 4.4.2. Mass Spectrometry (MS).

Figure 52. The MS with negative ESI of (A) organic emulsion phase EAc-Org, (B) emulsion phase EAc-Int and (C) aqueous emulsion phase EAc-Aq.

Cone 20V

m/z100 150 200 250 300 350 400 450 500 550 600 650 700 750

%

0

100

16102018_AMOSTRA_8DOT2_ACET_AMON_ESINEG_01 11 (1.854) Cm (2:11) Scan ES- 5.00e5

A

B

C

B

Figure 53. MS with negative ESI corresponding to (A) sample of cork bio-oil, (B) organic emulsion phase EAc-Org, and (C) emulsion phase EAc-Org’.

Figure 54. The MS with negative ESI of (A) sample of cork bio-oil, (B) emulsion phase EAc-Int, and (C) emulsion phase EAc-Int’.

Cone 20V

m/z75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 500 525 550 575 600 625 650 675 700 725 750

%

0

100

08102018_CORK_MIX_ACET_AMON_ESINEG_01 8 (1.348) Cm (2:11) Scan ES- 5.00e5

Cone 20V

m/z100 150 200 250 300 350 400 450 500 550 600 650 700 750

%

0

100


A

B

C

A

B

C

C

Figure 55. The MS with negative ESI of (A) sample of cork bio-oil, (B) alkali emulsion EAlk, (C) emulsion viscous residue EAlk-Res, (D) emulsion phase EAlk’ and (E) emulsion phase EAlk-Res’.

Cone 20V

m/z75 100 125 150 175 200 225 250 275 300 325 350 375 400 425 450 475 500 525 550 575 600 625 650 675 700

%

0

100


A

B

C

D

E

electrochemical conversion of liquefied forest biomass

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