Biomass and Bioene gy 169 (2023) 106693
A ailable online 26 Decembe 2022
0961-9534/© 2022 The Au ho s. Published by Else ie L d. This is an open access a icle unde he CC BY-NC-ND license (h p://c ea i ecommons.o g/licenses/by-
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Elec ochemical oxida ion o lignin o he simul aneous p oduc ion o
bioadhesi e p ecu so s and alue-added chemicals
Julio J. Conde
a
,
*
, Sand a Gonz´
alez-Rod íguez
a
, Xinyi Chen
b
, Thelmo A. Lu-Chau
a
,
Gemma Eibes
a
, An onio Pizzi
b
, Ma ia Te esa Mo ei a
a
a
CRETUS, Depa men o Chemical Enginee ing, Uni e sidade de San iago de Compos ela, 15782, San iago de Compos ela, Galicia, Spain
b
LERMAB, Uni e si y o Lo aine, 88051, Epinal, F ance
ARTICLE INFO
Keywo ds:
Upcycling
O ganosol lignin
Elec ochemis y
Valo iza ion
Bio-adhesi e
ABSTRACT
Elec ochemical oxida ion o lignin has been widely ega ded as a clean and eliable al e na i e o ob ain alue-
added p oduc s om lignin, such as anillin o guaiacol. This wo k aims o go one s ep beyond he p oduc ion o
low molecula weigh molecules and explo e he possibili y o using lignin esidues om elec ochemical
ea men s in he con ex o bio e ine y. To his end, a wo-way alo iza ion o lignin by elec ochemical
oxida ion is p oposed, in o de o ob ain a liquid phase en iched in low molecula weigh o ganic oligome s and a
solid phase o modi ied lignin o be used as bioadhesi e p ecu so . Hyd oxyla ion o lignin by elec ochemical
oxida ion using bo on-doped diamond (BDD) anodes was obse ed acco ding o he FTIR and MALDI-TOF e-
sul s, concluding ha an applied cu en densi y o 10 mA cm
−2
leads o p omising modi ica ions o he
o mula ion o bioadhesi es. Fu he mo e, NIPU bioadhesi es wi h elec ochemically modi ied lignin we e
success ully p epa ed and es ed o use in pa icleboa d panels, showing sa is ac o y mechanical p ope ies, and
hus pa ing he way o mo e en i onmen ally iendly lignin modi ica ion p ocedu es o he wood indus y.
1. In oduc ion
Lignin bio e ine y app oaches ha e a ac ed eno mous esea ch
e o s in ecen yea s, o he poin ha he po en ial o lignin-based
a oma ics could be compa ed o he p oduc ion o pe oleum-based a -
oma ics [1]. Oxida i e depolyme iza ion o lignin can be ca alyzed
h ough he use o me al ca alys s, me al- ee ca alys s, acid–base
ea men s, elec ochemis y and pho oca alysis [2]. Among hese al-
e na i es, lignin oxida ion by elec oca alysis is ega ded an en i on-
men ally iendly and eliable echnique, as i allows p ecise con ol o
selec i i y by con olling elec ode po en ials, educes he need o
chemicals and allows in eg a ion wi h enewable ene gy sou ces [3].
Elec ochemical oxida ion has been mainly used o ob ain high
alue-added o ganic molecules om lignin, as i opens he possibili y o
wo king unde mode a e condi ions, such as mild empe a u e and
ambien p essu e, in con as o o he p ocesses ha equi e mo e
demanding ope a ing condi ions, such as he mal, pho ochemical and
chemical oxida ion [4]. Elec ochemical oxida ion p ocesses a e
commonly based on wa e elec olysis eac ions, using anode ma e ials
ha can p omo e eac i e oxygen species (ROS) h ough he incomple e
oxida ion eac ion o wa e by using dimensionally s able anodes (DSA),
such as RuO
2
–I O
2
, SnO
2
–Sb
2
O
3
o PbO
2
. DSA anodes ha e been e-
po ed o e ec i ely p oduce anillin and anillic acid om lignin [5],
as well as o he oligome s such as guaiacol, hyd oxybenzaldehyde o
hyd oxyme hoxyphenyl e henone [6]. Al hough wa e is he mos
commonly used eac ion medium, ionic liquids a e s a ing o be
conside ed as elec oly es o inc ease he po en ial window [7]. O he
elec ode ma e ials based on me al alloys, such as Ni- and Co-based
elec odes, ha e been in es iga ed and esul ed in high p oduc selec-
i i y o anillin and ace o anillone [8].
Al hough elec ochemical ea men s s ill need mo e esea ch o
unde s and he e ec o non-selec i e oxida ion o hyd oxyl adicals on
solid lignin a e depolyme iza ion, some s eps ha e been aken so a .
Fo ins ance, Bawa e h e al. [9] desc ibed a p elimina y app oach o
lignin depolyme iza ion in elec ochemical ea men s. The use o model
lignin compounds demons a ed ha oxida i e eac ions by elec o-
gene a ed hyd oxyl adicals e ec i ely clea e he alkyl-O-a yl bond,
wi h high selec i i y o his eac ion a low adical concen a ion,
whe eas i led o non-selec i e oxida ion a high concen a ions [10]. I
was also epo ed ha he clea age o he C–C and C–O–C bonds
* Co esponding au ho .
E-mail add ess: [email p o ec ed] (J.J. Conde).
Con en s lis s a ailable a ScienceDi ec
Biomass and Bioene gy
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h ps://doi.o g/10.1016/j.biombioe.2022.106693
Recei ed 21 June 2022; Recei ed in e ised o m 28 Oc obe 2022; Accep ed 18 Decembe 2022
Biomass and Bioene gy 169 (2023) 106693
2
be ween he C9 uni s by he combina ion o elec ogene a ed H
2
O
2
and
ROS p oduces a oma ic compounds con aining hyd oxyl, aldehyde and
ca bonyl g oups [11]. Howe e , he solid-phase lignin emaining om
he depolyme iza ion p ocess has only been analyzed o unde s and he
p oduc ion o low molecula weigh oligome s and has no been
conside ed o u he alo iza ion.
In addi ion o alue-added chemicals, lignin has been also alo ized
in o aluable ma e ials such as ca bon ibe s, esins, plas ics, adso ben s
and ene gy s o age de ices [1]. Mo eo e , lignin has been ex ensi ely
s udied as a bioadhesi e p ecu so o se e al decades, wi h he aim o
educing wood indus y dependence on oil. The e m bioadhesi e, as
de ined by Pizzi [12], e e s o “ hose ma e ials o na u al, non-mine al
o igin, which can be used as such o a e mino modi ica ions o
ep oduce he beha io and pe o mance o syn he ic esins”. Nowa-
days, o maldehyde-based adhesi es a e commonly used as binde s o
he manu ac u e o composi e ma e ials. Howe e , hese adhesi es a e
p oduced om non- enewable ossil sou ces and en ails a se ious en i-
onmen al isk due o emissions o o maldehyde and ola ile o ganic
compounds du ing he li e cycle o his ype o composi e p oduc s [13].
Simila ly, polyu e hane adhesi es pose en i onmen al and heal h isks
due o he oxici y o he isocyana es used o adhesi es o mula ions,
such as diphenylme hane diisocyana es [14].
Se e al s udies ocused on li e cycle analysis demons a ed ha bio-
based adhesi es om di e en sou ces p esen s be e en i onmen al
pe o mance han pe ochemical adhesi es in wood panel
manu ac u ing, al hough lignin-based adhesi es ha e no ye imp o ed
hei en i onmen al bu den ela ed o ene gy consump ion [15,16].
Al hough he chemical s uc u e o lignin a ies depending on he
sou ce, he eac i i y o lignin is usually limi ed due o a low concen-
a ion o phenolic hyd oxyl g oups, high ing subs i u ion and s e ic
hind ances due o i s s uc u e [17]. Among he a ious modi ica ion
me hods conside ed o inc ease lignin eac i i y, deme hyla ion has
been widely used o con e me hoxy g oups o phenolic hyd oxyl
g oups, including se e al chemical and mic obial-based me hods [18].
Fo ins ance, he inclusion o modi ied lignins has been epo ed as a
sui able subs i u e o phenol in he syn hesis o phenol- o maldehyde
adhesi es [19], as pa o non-isocyana e-based polyu e hane (NIPU)
adhesi es [20] o e en as adhesi e o he p oduc ion o high-densi y
ibe boa ds [21].
Wi h he dual objec i e o inding a use o lignin esidues a e
elec ochemical ea men s and imp o ing he en i onmen al p o ile o
lignin modi ica ion o use as a bioadhesi e, he p esen s udy analyses
he elec ochemical modi ica ion o o ganosol lignin using bo on-
doped diamond (BDD) elec odes ocusing on he solid ac ion o
lignin as well as on he oligome s p oduced in he eac ion. BDD elec-
odes we e selec ed o p oduce hyd oxyl adicals due o hei high
po en ial window, educed ouling, and excellen s abili y. To he bes o
ou knowledge, his is he i s app oach o lignin modi ica ion o
bioadhesi e p oduc ion using elec ochemis y as an al e na i e me hod
o lignin ans o ma ion. Size exclusion ch oma og aphy (SEC), Fou ie -
ans o m in a ed spec oscopy (FTIR) and ma ix assis ed lase
deso p ion/ioniza ion (MALDI-TOF) cha ac e iza ions we e pe o med
o analyze he size, deg ee o deme hyla ion as well as he inc ease o
hyd oxyl g oups, among o he s, ha a e s ablished as key p ope ies ha
allows he c oss-linking o lignin componen s in bioadhesi es. In pa -
allel, gas ch oma og aphy-mass spec ome y (GC-MS) was conduc ed
o e alua e he p oduc ion o high-added alue oligome s. Addi ionally,
NIPU adhesi es we e o mula ed and es ed by means o he mo-
mechanical analysis (TMA) and a s anda dized In e nal Bond (IB)
s eng h es o e alua e he iabili y o he modi ied lignin as a bio-
adhesi e p ecu so .
2. Ma e ials and me hods
2.1. Elec ochemical modi ica ion o lignin
O ganosol lignin om beech wood was p o ided by he F aunho e
Cen e o Chemical-Bio echnological P ocesses (Leuna, Ge many). The
concen a ion o o ganosol lignin was ixed a 10 g L
−1
in a 0.2 M NaOH
solu ion (J.T. Bake , ACS specs). The expe imen s a e conduc ed in a
150 mL non-di ided cell using a wo-elec ode se up wi h pa allel pla es
a oom empe a u e (20 ±2 ◦C) and gen le agi a ion wi h a magne ic
s i e . The sys em consis s o a bo on-doped diamond (BDD) anode
(NeoCoa ®, Swi ze land) and a 316 s ainless s eel coun e elec ode
wi h iden ical dimensions (50x25x2 mm) moun ed on a homemade
holde . The ac i e a ea o he wo king elec ode was main ained a 7.5
cm
2
wi h an in e elec ode spacing o 5 mm. Oxida ion expe imen s
we e pe o med a cu en densi ies up o 60 mA cm
−2
using a bench op
DC powe supply (TENMA 72–2710) o ea men imes ixed a 180
min. The elec ochemical se up was scaled up o p oduce la ge quan-
i ies o lignin a he op imized eac ion condi ions o he o mula ion o
bioadhesi es. Fo his pu pose, a CONDIACELL® s ack ype EAOP® es
ki was used, consis ing o h ee DIACHEM® BDD anodes and wo 316
s ainless s eel ca hodes wi h dimensions o 230x26x2 mm. The expe i-
men s we e pe o med in a 5 L glass eac o main aining a cons an
elec oac i e a ea pe olume o ea ed solu ion o he selec ed cu en
densi y.
P ecipi a ion o lignin was ca ied ou by acidi ica ion wi h sul u ic
acid (J.T. Bake , 95–97%) a pH 2.5. Then, o ensu e comple e sepa a-
ion o he phases, cen i uga ion was pe o med o 5 min a 7000 pm.
A e sepa a ion, he liquid phase was s o ed a −20 ◦C, while he solid
phase was washed wice wi h dis illed wa e and eeze-d ied o 48 h
and s o ed in he da k a oom empe a u e o u he analysis.
2.2. Size exclusion ch oma og aphy (SEC)
An HP-1100 HPLC sys em equipped wi h a HP-1047A e ac i e
index de ec o was used o measu e he molecula weigh s o lignin,
using a 10 mM NaOH solu ion as mobile phase on MCX-100A (5
μ
m, 8 ×
300 mm) and MCX-1000A (5
μ
m, 8 ×300 mm) columns coupled in
se ies wi h a MCX p e-column (all supplied by PSS GmbH). Fo d ied
lignin, 20
μ
l o dilu ed samples (p epa ed by dissol ing 1 g o d y lignin
in 1 mL o 0.1 M NaOH) we e injec ed a a low a e o 1 mL min
−1
. The
liquid ac ion was di ec ly measu ed a e he sepa a ion o he supe -
na an using he same p ocedu e a e adjus ing pH o 12. The sys em
was calib a ed using pullulan s anda ds (ReadyCal-Ki Pullulan-
pulki 1, PSS GmbH). Fo he de e mina ion o molecula weigh s, an
R sc ip based on he algo i hm epo ed by L´
opez-Abelai as e al. [22]
was used.
2.3. Ma ix-assis ed lase deso p ion ioniza ion ime-o - ligh (MALDI-
TOF)
MALDI-TOF spec a we e ob ained using a Shimadzu Bio ech Axima
mass spec ome e and eco ded using a posi i e pola i y linea uning
mode by pe o ming 1000 p o iles pe sample wi h wo cumula i e sho s
pe p o ile, esul ing in an accu acy o ±1 Da. D ied samples we e
dilu ed in 1 mL in a 50:50 ace one/wa e solu ion o a concen a ion o
5 mg mL
−1
. Sample pla es we e p epa ed by imme sing and e apo a ing
2
μ
L o 0.1 M NaCl solu ion in 2:1 / me hanol/wa e . A e wa ds, 1
μ
L
o he sample (p epa ed by adding 10
μ
L o a 2,5-dihyd oxy benzoic acid
ma ix o he ace one/wa e solu ion) is placed on he pla e and d ied
again be o e measu emen s.
2.4. Fou ie - ans o m in a ed spec oscopy (FTIR)
FTIR spec a o he d y lignin samples we e eco ded on a VARIAN
FT-IR 670 spec ome e equipped wi h a GladiATR accesso y (Pike
J.J. Conde e al.
Biomass and Bioene gy 169 (2023) 106693
3
Technologies). The samples we e measu ed on a diamond c ys al in
e lec ion mode using ni ogen as a pu ge gas. Each spec um (and
backg ound spec a) was eco ded using a spec al wid h om 400 o
4000 cm
−1
wi h 64 scans and a esolu ion o 4 cm
−1
.
2.5. Gas ch oma og aphy–mass spec ome y (GC-MS)
Samples o GC-MS analysis we e ob ained by liquid-liquid ex ac-
ion wi h chlo o o m (J.T. Bake , HPLC g ade) om he liquid phase
ob ained a e lignin p ecipi a ion. The ex ac ion was pe o med in 500
μ
l o liquid phase by h ee consecu i e ex ac ions wi h 150
μ
l o chlo-
o o m. Measu emen s we e pe o med on a B uke 451-GC equipped
wi h a 30 m Rxi-5Sil capilla y column and coupled o a SCION iple
quad upole de ec o . Sample injec ion was pe o med wi h a cons an
low a e o 1 mL min
−1
in spli less mode using an injec o empe a u e
o 250 ◦C and He as ca ie gas.
2.6. Fo mula ion and es ing o he bioadhesi e
The bioadhesi e was p epa ed using 25.9 g o modi ied lignin (d y
lignin om acid p ecipi a ion modi ied using 10 mA cm
−2
o 3 h) dis-
sol ed in 21.6 g o deionized wa e , ollowed by he addi ion o 17.5 g
dime hyl ca bona e (DMC, 99 w %) while hea ing he mix u e o 50 ◦C
o 1 h. A e wa ds, 35.1 g o hexame hylenediamine (HMDA, 70 w %)
we e added and empe a u e was inc eased o 90 ◦C o ano he 2 h.
Finally, he mix u e was cooled o oom empe a u e. Biosou ced glyc-
e ol diglycidyl e he (GDE) was used o ein o ce he lignin-NIPU ad-
hesi es by adding a 15% based on he d y weigh o lignin-NIPU, o be
p essed and es ed unde he same condi ions. All he chemicals o
bioadhesi e p epa a ion we e pu chased om Sigma-Ald ich.
Samples o he momechanical analysis (TMA) we e p epa ed by
applying he bioadhesi e be ween wo beech wood plies, o be es ed in
non-iso he mal mode be ween 25 ◦C and 250 ◦C a a hea ing a e o
10 ◦C min
−1
wi h a Me le Toledo 40 TMA measu ing module, using a
p ocedu e p e iously desc ibed by Chen e al. [23]. Mechanical p op-
e ies o he adhesi e we e e alua ed using a s anda dized In e nal
Bond (IB) s eng h es . One-laye pa icleboa d panels using non-GDE
and 15% GDE lignin NIPU adhesi es o 50x50x12 mm p essed a a
maximum p essu e o 28 kg cm
−2
, ollowed by a p essu e-dec easing
p essing cycle a 220 ◦C o a 10 min p essing cycle. The panels we e
ligh ly sanded on he su ace and es ed in quad uplica e o he EN-312
d y IB s eng h es [24].
3. Resul s and discussion
3.1. Analysis o he un ac iona ed s eam
As a i s app oach o e alua e he e iciency o lignin modi ica ion
using BDD elec odes, se e al es s o he elec oly ic p ocess we e
pe o med in basic medium using h ee di e en cu en densi ies: 20,
40 and 60 mA cm
−2
. In his p elimina y analysis, he eac ion medium
was no ac iona ed, and he measu emen s we e pe o med di ec ly in
he basic medium.
Lignin clea age was analyzed by SEC, which allowed he molecula
weigh o be moni o ed a di e en eac ion imes. This is a widely used
echnique o analyze he molecula weigh s o polyme s and has also
been used o examine di e en ypes o lignin [25]. Fig. 1 shows he size
dis ibu ion o he ea ed lignin a e 180 min o elec ochemical
ea men , as well as he e olu ion o weigh -a e age molecula weigh
(M
w
) e sus ime o he di e en cu en densi ies applied.
The esul s show ha he adicals gene a ed by BDD elec odes can
in lic a d as ic depolyme iza ion o he lignin molecule, e idenced by
he appea ance o a dis ibu ion o low molecula weigh agmen s
ep esen ed in he peak cen e ed in 1.2 kDa. Non- ea ed lignin shows a
small shoulde in 1.5 kDa, ha is ans o med in na ow peaks a 1.2
kDa, g owing almos linea ly as he applied cu en densi y inc eases. In
he ange om 2 o 20 kDa, he mos signi ican d op in in ensi y is
obse ed be ween 2 and 6 kDa, while abo e 6 kDa a na ow peak ap-
pea s cen e ed a 8.5 kDa, indica ing ha o e -oxida ion o he lignin
agmen s is mo e likely han he clea age o he molecule. Fo ins ance,
o an applied cu en densi y o 40 mA cm
−2
in he high molecula
weigh egion abo e 6 kDa is i ually he same, indica ing ha excess o
hyd oxyl adical p oduc ion only a ge s he low molecula weigh
lignin agmen s.
Addi ionally, in o de o quan i y he se e i y o he elec ochemical
ea men using BDD elec odes, lignin mine aliza ion was moni o ed by
he quan i ica ion o he o al o ganic ca bon (TOC) o he un ac io-
na ed lignin a e he elec ochemical ea men s. Table S1 in he sup-
plemen a y in o ma ion shows ha an inc easing cu en densi y
g adually leads o mine aliza ion o he low molecula weigh lignin
agmen s p oduced. Consequen ly, ino ganic ca bon inc eases in he
same o de as o ganic ca bon dec eases, due o he o ma ion o ca -
bona es by CO
2
gene a ion in alkaline media. F om hese esul s, i can
be con i med ha cu en densi ies abo e 40 mA cm
−2
can mine alize
almos 10% o he o al lignin o ganic ca bon. Taking hese esul s in o
accoun , 40 mA cm
−2
is se as he maximum wo king cu en densi y
using BDD anodes o a oid o e oxida ion.
MALDI-TOF has been p oposed as a sui able me hod o e alua e he
p esence o di e en oligome s in he lignin s uc u e due o he abili y
o ob ain s uc u al in o ma ion in la ge molecula mass egions. Fig. 2
shows he di e en condensed s uc u es iden i ied by MALDI-TOF in
he ea ed lignin a 40 mA cm
−2
o 3 h o eac ion. This in o ma ion
was ex ac ed om he MALDI-TOF spec a included in he supple-
men a y in o ma ion (Figs. S1 and S2). The p esence o hese oligome s
by he elec ochemical ea men is con i med by compa ison wi h he
spec a o un ea ed lignin (Figs. S2 and S3).
The in e p e a ion o he MALDI spec a o he elec ochemical
ea ed lignin indica es ha he lignin has been s ongly deme hyla ed,
almos selec i ely, making i po en ially mo e eac i e and has unde -
gone some adi ional ype ea angemen s. The e idence om he
Fig. 1. a) Ch oma og ams depic ing he dis ibu ion o molecula weigh s and hei associa ed in ensi y a 180 min and b) e olu ion o he a e age molecula weigh
o he samples in he di e en expe imen s.
J.J. Conde e al.
Biomass and Bioene gy 169 (2023) 106693
4
s uc u es iden i ied and shown in Fig. 2 is ha he –OCH
3
g oups ha e
massi ely deme hyla ed o ans o m o –OH g oups inc easing wo
ypes o eac i i y o he lignin. Fi s , he eac i i y o he a oma ic ings
owa ds eac ion wi h aldehydes, namely owa ds aldehyde addi ion
and subsequen polycondensa ion is inc eased. Second, he inc ease o
he p opo ion o –OH g oups ende possible much mo e ex ensi e e-
ac ions o he lignin wi h isocyana es o o m classical polyu e hanes
and, mo e impo an ly, i would esul in an enhancemen o c oss-
linking o lignin o o m NIPU esins o a a ie y o applica ions. This
is a de ini e imp o emen o e he o iginal lignin, u ning he elec o-
chemical modi ied lignin in o an app op ia e candida e o he o mu-
la ion o bioadhesi es.
3.2. Lignin p ecipi a ion owa ds wo-way alo iza ion
A e con i ma ion o he elec ochemical modi ica ion in he
un ac iona ed expe imen s, lignin was p ecipi a ed by acidi ica ion o
ob ain wo ac ions and u he analyzed by SEC and MALDI-TOF.
Addi ionally, he samples we e cha ac e ized by in a ed spec oscopy
o gain u he insigh s on he e olu ion o he unc ional g oups as a
unc ion o he applied cu en densi y. Based on he abo e esul s, he
ange o applied cu en densi ies was educed o a oid lignin o e -
oxida ion, so he selec ed cu en densi y alues we e 5, 10, 20 and
40 mA cm
−2
.
Lignin clea age was analyzed by SEC o de e mine he di e ences in
he e olu ion o he molecula weigh o he p ecipi a ed ac ion
conside ing he applied cu en densi ies. In addi ion, Fig. 3b depic s he
pe cen age mass loss o he ea ed p ecipi a ed ac ion compa ed o he
un ea ed p ecipi a ed sample. In his analysis, he d y lignin mass in a
known olume a e eeze-d ying is compa ed o he d y mass o he
un ea ed lignin a e dissolu ion and subsequen p ecipi a ion, o ac-
coun o he solid phase mass loss ha can be a ibu ed o he elec-
ochemical ea men .
F om he esul s depic ed in Fig. 3b i can be concluded ha he mass
loss abo e 20 mA cm
−2
is oo high o be conside ed o lignin alo i-
za ion as a bioadhesi e, as he elec ochemical ea men is causing o al
mass losses a e p ecipi a ion abo e 15% when applying cu en den-
si ies abo e 20 mA cm
−2
o 60 min. This end indica es ha he gen-
e a ion o acid soluble ac ions inc eases s ongly wi h inc easing
cu en densi ies. In Fig. 3a, an inc ease anging om 1 o 3% o he
a e age molecula weigh o he p ecipi a ed lignin is ob ained a he
end o he eac ion, p obably caused by he sepa a ion o low molecula
weigh agmen s in o he liquid phase a e acidi ica ion. The ch o-
ma og ams o he molecula weigh dis ibu ions, howe e , show a
Fig. 2. Condensed s uc u es iden i ied by MALDI-TOF in he ea ed lignin a 40 mA cm
−2
o 3 h (MALDI-TOF spec a can be ound in he Supplemen a y
in o ma ion).
Fig. 3. a) E olu ion o he a e age molecula weigh o he samples in he di e en expe imen s and b) e olu ion o mass loss o d y lignin using di e en cu -
en densi ies.
J.J. Conde e al.
Biomass and Bioene gy 169 (2023) 106693
5
small shi owa ds he high molecula weigh agmen s (see Fig. S5),
sugges ing a small deg ee o c oss-linking o he lignin chains. This e ec
has been p e iously epo ed by Bawa e h e al. [9] using nickel elec-
odes wi h cu en densi ies in he ange o 40 mA cm
−2
, hypo hesizing
ha elec ochemical clea age o he e he bonds could be ollowed by
subsequen eac ions such as phenolic condensa ion and adical
coupling. Howe e , i should be no ed ha he SEC calcula es he mo-
lecula weigh om he hyd odynamic olume o he analy e compa ed
o linea polyme s anda ds. In his line, Rinaldi e al. [26] claim ha he
ea angemen o lignin molecules could in luence he appa en mo-
lecula weigh wi hou a ec ing he eal molecula weigh , as s e ic
e ec s canno be simula ed by he model polyme s used o calib a ion.
The e olu ion o he molecula mass o he sepa a ed agmen s
wi hin he liquid ac ion is p esen ed in Fig. S6 in he Supplemen a y
In o ma ion, in which he ch oma og ams a he e alua ed cu en
densi ies o ea men imes o 3 h a e compa ed. In he liquid phase o
non- ea ed lignin, h ee dis inc peaks (cen e ed in 225, 500 and 900
Da) appea in low molecula weigh egions, wi h no molecules de ec ed
abo e he weigh o 2 kDa. A e he oxida ion ea men , an inc ease
p opo ional o he applied cu en densi y o he peak cen e ed on 500
Da is clea ly obse ed, bu he peak a 225 Da emains p ac ically un-
changed. This e ec is a ibu ed o he p e iously discussed o e -
oxida ion e ec s, which esul in he mine aliza ion o he smalle
molecules gene a ed in he ea men . Howe e , he mos signi ican
e ec o he elec ochemical oxida ion is he gene a ion o soluble lignin
agmen s wi h molecula weigh s in he ange o 2–12 kDa, especially
when using cu en densi ies abo e 10 mA cm
−2
. This e ec migh be
caused by he o ma ion o hyd oxyl and ca boxylic g oups ac ing as
wa e so p ion si es, which di ec ly a ec s he hyg oscopic p ope ies o
he lignin [27].
Fig. 4 ep esen s he e olu ion o he selec ed signals o he in a ed
spec a o d y lignin, using he ATR echnique. F om he ob ained
in a ed spec a, he peaks a 1213, 1708, 2841 and 3388 cm
−1
we e
selec ed as ep esen a i e o he phenolic hyd oxyl, ca bonyl, me hoxy
and phenolic +alipha ic hyd oxyl unc ional g oups espec i ely [28],
no malizing he alues by he a oma ic g oups o lignin. The ib a ions
o he a oma ic skele on be ween 1505 and 1515 cm
−1
we e aken as a
e e ence o no malize he alue o he di e en peaks [29]. Al hough
many au ho s ha e used he a oma ic skele al ib a ions band a 1600
cm
−1
o no maliza ion, his wa eleng h is also a ec ed by he C
–
–
O
s e ch, ha could be po en ially modi ied in he elec ochemical
oxida ion.
Func ional g oup analysis shows a clea inc ease o o al hyd oxyls
(phenolic +alipha ic) wi h inc easing cu en densi y. Howe e ,
inc easing he cu en densi y abo e 20 mA cm
−2
does no ha e a la ge
impac on he gene a ion o mo e hyd oxyl g oups a 180 min. In
con as , changes in phenolic hyd oxyl g oups does no e eal a ele an
impac , as he no malized signals a 1213 cm
−1
ha e no clea end and
he signal in ensi y a ies less han 5%. F om hese esul s, i can be
concluded ha alipha ic-OH is he cause o he signal inc ease as he
signal o phenolic-OH emains p ac ically unchanged. Assuming he
inc ease o alipha ic-OH, he solid phase is sui able o bioadhesi e
o mula ion, as i has been iden i ied as a key ea u e, inc easing he
eac i i y o lignin, in he o mula ion o lignin-based polyu e hane
adhesi es [30]. Howe e , he inc ease o hyd oxyl g oups is no
accompanied by a educ ion o me hoxy g oups, as e idenced in Fig. 4c,
showing a beha io ha esembles he e olu ion o o al hyd oxyl
signal. On he o he hand, he p esence o ca bonyl g oups inc eases
wi h applied cu en and ea men ime o all cases. The appea ance o
ca bonyl g oups has been ela ed o he clea age o C–C and C–O bonds
in lignin [31], bu may also be ela ed o he oxida ion o hyd oxyl o
ca bonyl-con aining g oups, such as quinones [32], and hus indica ing
he o e oxida ion o he gene a ed hyd oxyl g oups.
Fig. 5 shows a selec ion o condensed s uc u es iden i ied by MALDI-
TOF o all cu en densi ies e alua ed a 180 min. The in ensi y o he
di e en peaks was ex ac ed om he spec a collec ed in he supple-
men a y in o ma ion (Figs. S9–S16). Th ee di e en se o s uc u es
we e selec ed: a se o low molecula weigh s uc u es p e iously p e-
sen in he un ea ed lignin (177, 199 and 361 Da), he s uc u e a
575.5 Da along i s associa ed p o ona ed signals (a 576.5 and 577.5 Da)
and new s uc u es gene a ed a e he elec ochemical ea men (459,
490 and 520 Da). In con as o he non-sepa a ed lignin, he spec a o
he d y lignin we e compa ed wi h hose o he un ea ed p ecipi a ed
lignin, o conside possible modi ica ions caused by alkaline dissolu ion
Fig. 4. E olu ion o he no malized in a ed signal in ensi ies o a) alipha ic +phenolic hyd oxyl, b) phenolic hyd oxyl, c) me hoxy and d) ca bonyl unc ional
g oups a di e en cu en densi ies.
J.J. Conde e al.
Biomass and Bioene gy 169 (2023) 106693
6
and subsequen acidi ica ion and ac iona ion (Figs. S7–S8).
F om he assigna ion o s uc u es in he e alua ion o MALDI-TOF
spec a i can be concluded ha lignin is deme hyla ed and he in-
c ease o hyd oxyl g oups in he analyzed agmen s is ema kable. As
depic ed in Fig. 5, he s uc u e assigned o peaks in he ange o
575.5–577.5 is hea ily me hoxyla ed. The signals o he unp o ona ed
and p o ona ed s uc u e s eadily dec ease wi h inc easing cu en
densi y, gi ing ise o he o ma ion o deme hyla ed s uc u es. The low
molecula weigh s uc u es (177, 199 and 361 Da) achie e a maximum
a 10 mA cm
−2
while he high molecula weigh s uc u es (459, 489 and
519 Da) achie e a maximum a 5 mA cm
−2
o all cases, al hough he
alues eco ded a 10 mA cm
−2
a e s ill no ewo hy. Fo in ensi ies
abo e 10 mA cm
−2
he p esence o he iden i ied agmen s is educed,
excep o 459 and 489 Da agmen s, and hus con i ming he o e -
oxida ion o lignin a he highe ange o he s udied cu en densi ies.
Finally, he quali a i e s udy o he p oduced oligome s o in e es
was pe o med by GC-MS. As he p oduc ion o high alue-added
o ganic compounds is widely desc ibed in he li e a u e, only a i s
sc eening o p oduc s o in e es is pe o med o con i m he p oduc ion
o high alue-added molecules using BDD-based elec ochemical p o-
cess. In Table 1, a sc eening o he mos ele an o ganic molecules (wi h
no malized a eas highe han 0.5% o o al a ea o spec um) ex ac ed
wi h chlo o o m a 10 mA cm
−2
and 3 h o eac ion a e p esen ed. F om
he GC-MS sc eening i can be concluded ha he liquid ac ion is
subjec o se e e oxida ion, as mos o he de ec ed molecules a e linea
hyd oca bons be ween 13 and 30 ca bon a oms.
Fig. 5. E olu ion o s uc u es o in es es iden i ied by MALDI-TOF a he e alua ed cu en densi ies a 180 min o p ecipi a ed lignin (MALDI-TOF spec a can be
ound in he Supplemen a y in o ma ion).
Table 1
Rele an o ganic compounds on he GC-MS sc eening o samples aken a 3 h o eac ion a 10 mA cm
−2
Name S uc u e Name S uc u e
4-Hyd oxy-3,5-dime hoxybenzaldehyde (Sy ingaldehyde)
4-Hyd oxy-3-me hoxybenzaldehyde (Vanillin)
Hexadecanoic acid (Palmi ic acid) Oc adecanoic acid (S ea ic acid)
7,9-Di- e -bu yl-1-oxaspi o[4,5]deca-6,9-diene-2,8-dione 2,4-Di- e -bu ylphenol
J.J. Conde e al.
Biomass and Bioene gy 169 (2023) 106693
7
Sy ingaldehyde and anillin a e common monome s iden i ied in
lignin elec ochemical oxida ion eac ions due o depolyme iza ion e-
ac ions. These molecules a e g ea ly app ecia ed in ood and pha ma-
ceu ical indus ies [7]. Di- e -bu ylphenol has been iden i ied as an
oxida ion compound p oduced elec ochemical-gene a ed hyd oxyl
adicals in aqueous alkaline media and can be u ilized as an i- ungal, UV
s abilize o an an ioxidan o hyd oca bons [32]. The gene a ion o he
di- e -bu ylphenol molecules could be ela ed o he hyd ogen gas
p oduced in he coun e elec ode. I has been p e iously epo ed ha
hyd ogen pe o ms hyd ogenolysis eac ions ha allow he b eaking o
he in amolecula bonds o lignin, ob aining a high yield o mono-
phenols as eac ion p oduc s [33]. The oxaspi o[4,5]decane molecule
has no ye been epo ed as an elec ochemical oxida ion p oduc in he
li e a u e, bu i is p obably o igina ed om he oxida ion o
spi o-dienone s uc u es ha has been p e iously obse ed in lignins
de i ed om sp uce and aspen ees [34]. Palmi ic acid and s ea ic
acids, which a e used as p ecu so s o bio uels, we e epo ed as he
majo con e sion p oduc s using elec o-Fen on elec olysis, also ca a-
lyzing lignin depolyme iza ion h ough he p oduc ion o hyd oxyl
adicals [35]. Acco ding o he au ho s, he p esence o hese a y acids
indica es ha lignin unde wen deme hoxyla ion, opening ing, and
couple eac ions. In his line, he oxida ion o seconda y alcohols o
ke ones using elec ochemical me hods was p oposed by Wang e al.
[36], due o he unspeci ic oxida ion o elec ochemical eac ions ha
allow he o e oxida ion o he hyd oxyl g oups p oduced.
These esul s con i m he po en ial o BDD elec odes on lignin
ans o ma ion o p oduce chemicals o in e es , as well as he ini ial
hypo hesis o he double alo iza ion o employ bo h liquid and solid
phases a e sepa a ion. Howe e , he p oduc ion o high-added alue
molecules could be u he imp o ed by he op imiza ion o he elec-
olysis condi ions. Se e al au ho s poin ed ou in he li e a u e ha
o e oxida ion should be conside ed and s udied di e en app oaches o
inc ease he p oduc ion and selec i i y o high-added alue o ganics by
pu ing ou he desi ed molecules by di e en me hods. This was
con i med by Zhu e al. [11], as hey obse ed ha sepa a ion a hal o
he eac ion ime is a o able o a oid u he oxida ion o p oduc s. Fo
ins ance, S ie el e al. designed a couple o eac ion/sepa a ion uni s,
in eg a ing ei he a nano il a ion memb ane [37] o an anion exchange
memb ane [38], o emo e he gene a ed p oduc s om he eac ion
media, while Di Ma ino e al. [39] de eloped a new p ocess o he
elec ochemical depolyme iza ion combined in-si u ex ac ion using an
emulsion o deep eu ec ic sol en s oge he wi h an ex ac an phase.
Using such app oaches, he p oduc ion o oligome s o in e es s could be
enhanced while he lignin emains could be alo ized o bioadhesi e
p oduc ion.
3.3. Bioadhesi e o mula ion and es ing
In he ligh o he esul s o lignin cha ac e iza ion, 10 mA cm
−2
was
selec ed as he mos app op ia e cu en densi y o p oduce lignin han
can be used as bioadhesi e p ecu so . The p ocess was scaled up
main aining he same expe imen al condi ions used in he p e ious
sec ion, using a 5 L glass eac o while main aining a cons an elec o-
ac i e a ea pe olume o ea ed solu ion. The o mula ion o modi ied
lignin NIPU adhesi es was based on he syn he ic ou e published by
Sa aˇ
zin e al. [20], in which he common app oach o using cycles
o ganic ca bona es and diamines ha e been simpli ied by using a cheap
and non- oxic alipha ic ca bona e such as dime hyl ca bona e. The
in oduc ion o biosou ced glyce ol diglycidyl e he , which is an
alipha ic epoxy monome used as a diepoxy c ossslinke , has been p e-
iously used o dec ease he s a ing cu e empe a u e o adhesi es and
elimina e he excess o ee HMDA because o i s high eac i i y wi h
epoxy g oups, hus educing he adhesi es po en ial en i onmen al
impac [40].
The momechanical analysis was pe o med o un a el he cu ing
beha io o he o mula ed bioadhesi es, as a cha ac e iza ion o he
cu ing p ocess is equi ed o op imize p essing pa ame e s, which a e
essen ial o he manu ac u e o wood-based composi es. This analysis
allows he calcula ion o he modulus o elas ici y (MOE) o moni o he
ha dening o he adhesi e wi h espec o empe a u e, indica ing he
ha dening speed and join ing s eng h o an adhesi e sys em. The ele-
an cu e o MOE as a unc ion o empe a u e o elec ochemical
ea ed lignin NIPU adhesi e wi h and wi hou he addi ion o GDE is
shown in Fig. 6.
Fo non-GDE adhesi e, wo peaks a e clea ly obse ed, as i has been
he case wi h all p e iously biomass NIPU adhesi es epo ed in he
li e a u e [20,23,41]. The i s peak occupies he 75–125 ◦C ange while
he o he de elops abo e 200 ◦C. The e o e, a e an ini ial MOE
dec ease due o he lowe iscosi y when he empe a u e s a s
inc easing, he MOE g adually inc eases because o he o ma ion o
linea oligome s o ming a physically en angled ne wo k. A e ha , he
MOE dec eases due o he pa ial disen angling o he linea oligome s
o med due o he inc ease in chain mobili y as he empe a u e in-
c eases. Finally, he eal chemical c oss-linking s a and he MOE
g adually inc eases o a peak indica ing he o ma ion o a chemically
c oss-linked ne wo k. Howe e , he lignin adhesi e s a s cu ing a a
a he high empe a u e, app oxima ely 200 ◦C, which is also simila o
esul s ob ained wi h o he biomass based NIPU wood adhesi es
[41–43]. The maximum MOE is eached a be ween 230 and 250 ◦C.
Howe e , his a he high empe a u e limi s he expansion o NIPU
adhesi es o indus ial applica ions such as pa icleboa d, making i
necessa y o minimize his d awback and o y o dec ease cu ing
empe a u e. By in oducing GDE as a eac ion enhance , he s a ing
cu e empe a u e o he adhesi e dec eases he s a ing cu e empe a-
u e o 185–195 ◦C. P e ious esea ch has indica ed ha he s a ing
cu e empe a u e o he esul an adhesi es dec eases e en u he wi h
inc easing GDE le els, inally ending owa ds he 145–155 ◦C ange
[40]. Thus, i can be no iced om he pa icleboa d esul s ha he
modi ied lignin adhesi es show a be e bonding pe o mance wi h
espec o he nea annin-based NIPU adhesi e [42,43].
The mechanical p ope ies o he adhesi es we e es ed by means o
he In e nal Bond (IB) s eng h es on pa icleboa d panels. Table 2
p esen he esul s o pa icleboa d es s o elec ochemically modi ied
lignin NIPU adhesi es, wi h and wi hou he use o GDE o enhance
c osslinking. The in e nal bond s eng h o he pa icleboa d panels, o
a panel densi y o 0.70 g cm
−3
, shows an a e age alue o 0.35 MPa,
which is sligh ly abo e he es ablished h eshold in he Eu opean s an-
da d o in e io pa icleboa ds [24]. This esul is a be e han he
esul s ob ained o he modi ied lignin NIPU esin wi hou any GDE
ein o cemen , ha s and a 0.16 MPa, o a panel densi y o 0.67 g
Fig. 6. Modulus o elas ici y (MOE) e sus empe a u e o join s bonded wi h
modi ied lignin NIPU adhesi e.
J.J. Conde e al.
Biomass and Bioene gy 169 (2023) 106693
8
cm
−3
.
P e ious esul s using un ea ed o ganosol lignin as p ecu so
showed ha IB s eng h o lignin-based adhesi es anges om 0.12 o
0.35 MPa o non- ein o ced NIPU esin, depending on p essing em-
pe a u e. Howe e , p e ious s udies showed ha an IB as high as 0.60
MPa o a densi y o 0.70 g cm
−3
could be ob ained o panels ein o ced
wi h an epoxy silane [20]. The ela i ely lowe IB ob ained in his s udy
migh be caused by lignin ea angemen while eeing a g ea e numbe
o –OH eac i e g oups o o m he u e hane, esul ing in a lowe le el o
c oss-linking o he p opo ion o dime hyl ca bona e and diamine
used. This p oblem will be add essed in u u e esea ch, as an inc ease o
he p opo ions o DMC and diamine a e expec ed o inc ease he ela-
i e p opo ion o u e hane c oss-linking in he ha dened lignin adhesi e
ne wo k and hus inc easing he IB s eng h o he panel. Fu he mo e,
as wi h he o he biobased NIPU esins, he high cu ing empe a u e
indica es ha he adhesi e in he inne mos co e o he pa icleboa d
will no be comple ely cu ed, hence dec easing he appa en IB s eng h
o he panel. This also indica es ha such a p oblem will mos likely no
occu in hin boa ds (3–5 mm hickness), such as ha dboa ds o pa i-
cleboa ds, whe e he inne mos co e empe a u e eaches much highe
le els.
4. Conclusions
The deme hyla ion and subsequen hyd oxyla ion o he solid ac-
ion o o ganosol lignin and he simul aneous p oduc ion o high-added
alue o ganics by elec ochemical oxida ion was p o en o be easible,
hus con i ming he ini ial hypo hesis o double-way alo iza ion o
lignin. Howe e , he p oduc ion o oligome s o in e es using BDD
elec odes should be u he in es iga ed o inc ease he yields o mol-
ecules o in e es and o a oid o e oxida ion. In his line, he op imi-
za ion o bo h p oduc s should be ca e ully s udied o op imize he yields
and hei p ope ies and composi ion. As o he p oduc ion o modi ied
lignin, conside ing he esul s o FTIR and MALDI-TOF, he mos
app op ia e cu en densi y using BDD anodes o he p oduc ion o a
bioadhesi e p ecu so is 10 mA cm
−2
. The elec ochemical se up was
scaled up o he o mula ion and es ing o elec ochemical modi ied
lignin NIPU bioadhesi e, which showed p omising mechanical p ope -
ies complying wi h he Eu opean h eshold alues o pa icleboa d
panels.
Funding
This esea ch has been inancially suppo ed by an ERA CoBioTech
p ojec (PCI2018-092866) P og amaci´
on Conjun a In e nacional 2018 –
WooBAdh p ojec , unded by MCIN/AEI/10.13039/501100011033 and
co unded by he Eu opean Union.
Da a a ailabili y
Da a will be made a ailable on eques .
Acknowledgmen s
JJC acknowledges inancial suppo om Galician Go e nmen
hough a pos doc o al ellowship (ED481B-2021/015). SG-R and GE
p edoc o al and pos doc o al ellowships (BES-2017-081677 and RYC-
2018-024846-I, espec i ely) we e unded by MCIN/AEI/10.13039/
501100011033 and by “ESF In es ing in you u u e”. JJC, SG-R, TAL-C,
GE and MTM belong o he Galician Compe i i e Resea ch G oup (GRC)
ED431C-2021/37. LERMAB is inanced by he F ench Agence Na ionale
de la Reche che (ANR) as pa o he labo a o y o excellence (LABEX)
ARBRE. The au ho s would also like o hank he use o he analy ical
acili ies o IR-Raman Spec oscopy Uni and Mass Spec ome y Uni
om RIAIDT-USC.
Appendix A. Supplemen a y da a
Supplemen a y da a o his a icle can be ound online a h ps://doi.
o g/10.1016/j.biombioe.2022.106693.
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Table 2
Resul s o pa icleboa d es s wi h elec ochemical modi ied lignin NIPU
adhesi es.
Pa icleboa d Densi y/g
cm
−1
A ea/
mm
2
S eng h/
kN
IB/
MPa
Re .
15% GDE 0.70 2532.4 0.88 0.35
0.71 2507.3 0.92 0.37
0.70 2522.5 0.85 0.34
0.74 2502.4 0.88 0.35
A e age 0.70 0.35 This
wo k
No epoxy 0.65 2504.9 0.44 0.18
0.65 2504.8 0.37 0.15
0.69 2524.9 0.36 0.14
0.67 2517.4 0.39 0.15
A e age 0.67 0.16 This
wo k
Non- ea ed
(p essed@180 ◦C)
0.70 – – 0.12 [20]
Non- ea ed
(p essed@230 ◦C)
0.70 – – 0.35 [20]
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