molecules
A icle
Fu u al P oduc ion om D-Xylose and Xylan by
Using S able Na ion NR50 and NaCl in a
Mic owa e-Assis ed Biphasic Reac ion
Sa ah Le Guenic 1, Da id Ge gela 2, Clai e Ceballos 3, F ede ic Delbecq 3and Ch is ophe Len 1,*
1Uni e si é de Technologie de Compiègne (UTC), CS 60319, 60203 Compiègne Cedex, F ance;
[email p o ec ed]
2Depa men o Chemis y, Facul y o Technology, 760 01 Zlin, Czech Republic; Da id.Ge g[email p o ec ed]
3Ecole Supé ieu e de Chimie O ganique e Miné ale (ESCOM), 1 ue du Réseau Jean-Ma ie Buckmas e ,
60200 Compiègne, F ance; [email p o ec ed] (C.C.); [email p o ec ed] (F.D.)
*Co espondence: [email p o ec ed]; Tel.: +33-344-238-828
Academic Edi o : De ek J. McPhee
Recei ed: 9 July 2016; Accep ed: 10 Augus 2016; Published: 22 Augus 2016
Abs ac :
Pen ose dehyd a ion and di ec ans o ma ion o xylan in o u u al we e pe o med in a
wa e -cyclopen yl me hyl e he (CPME) biphasic sys em unde mic owa e i adia ion. Hea ed up
be ween 170 and 190
◦
C in he p esence o Na ion NR50 and NaCl, D-xylose, L-a abinose and xylan
ga e u u al wi h maximum yields o 80%, 42% and 55%, espec i ely. The in luence o empe a u e
and eac ion ime on he eac ion kine ics was discussed. This s udy was also comple ed by he su ey
o di e en eac an a ios, such as o ganic laye -wa e o ca alys -ino ganic sal a ios. The exchange
be ween p o on and ca ion induced by an excess o NaCl was moni o ed, and a syne ge ic e ec
be ween he emaining p o ons and he eleased HCl was also disco e ed.
Keywo ds:
u u al; D-xylose; xylan; Na ion NR50; biphasic sys em; mic owa e-assis ed dehyd a ion
1. In oduc ion
Because o he deple ion o uel-based esou ces, he e is an u gen need o ind al e na i e
eeds ocks o he chemical indus y. Lignocellulosic biomass, plan was es o example, appea s o
be he bes al e na i e o ge access o a ious bio-based chemicals, such as u an de i a i es. One o
hese op- alue ma e ials is u u al, la gely p oduced om he dehyd a ion o D-xylose, he main
monome o hemicellulose- ich biomass.
Fu u al is a key compound o he p epa a ion o many in e media es [
1
]. Fo example, u u yl
alcohol is a p ecu so o comme cially a ailable esins (namely poly u u yl alcohol o PFA) and can
be o med om u u al by he educ ion o i s aldehyde unc ion. Fu u al is also use ul o he
p epa a ion o o ganic sol en s, such as e ahyd o u an (THF), me hyl e ahyd o u an (MeTHF) o
he syn hesis o bioac i e compounds. The indus ial p oduc ion o u u al is ealized by ba ch and
con inuous acid hyd olysis o lignocellulosic biomass [
2
] in o D-xylose ollowed by i s dehyd a ion
leading o u u al. Bo h acid hyd olysis and subsequen dehyd a ion o lignocellulosic biomass a e
usually ca ied ou in he same eac o , and ha dehyd a ion is ecognized as he a e-limi ing s ep.
The eac ion is usually ca ied ou in a ange o empe a u es ound be ween 170 and 185
◦
C unde
a con en ional hea ing. Pe o med in wa e , hese p ocesses equi e s ong mine al acids, such as
HCl [
3
,
4
], H
2
SO
4
[
5
] o H
3
PO
4
[
6
], causing co osion, sa e y and handling p oblems. Reco e y o
u u al equi es dis illa ion o he sol en and o he ea men s, such as neu aliza ion, which lead
o la ge was e s eams and high-ene gy consump ion. Besides, in a monophasic sys em, u u al
yields and selec i i y a e gene ally limi ed by he o ma ion o by-p oduc s, humin, a black insoluble
Molecules 2016,21, 1102; doi:10.3390/molecules21081102 www.mdpi.com/jou nal/molecules
Molecules 2016,21, 1102 2 o 11
ca bonaceous solid, de i ed om he c oss-polyme iza ion be ween u u al and pen ose in e media es,
o om he condensa ion o u u al on i sel .
Today, he use o easy- o-handle solid sul ona ed polyme s ecognized o hei s ong B øns ed
acidi y, such as p o ona ed Ambe lys [
7
–
11
] o Na ion [
12
], shows p omising esul s in he ield
o g een chemis y. Na ion NR50 is a pe luo oalkane sul onic esin ha could be an al e na i e o
o he ypes o he e ogeneous ca alys s, such as zeoli es [
13
] o he e opolyacids [
14
]. Acco ding o he
p oduce da a, Na ion NR50 has a maximum ope a ing empe a u e o 220
◦
C. Recen ly, Lam e al.
demons a ed he po en ial o he solid Na ion 117 o he C5 suga dehyd a ion [
15
]. They epo ed
he o ma ion o u u al wi h a maximum yield o 60% a 150
◦
C in dime hylsul oxide (DMSO).
Howe e , du ing he p ocess, Na ion was also subjec ed o he deposi ion o a humin laye . This laye
almos co e ed he en i e ac i e su ace o he solid ca alys and limi ed he numbe o ca aly ic uns.
Na ion NR50 esin was also epo ed as an e icien supe acid ca alys in he p esence o ionic liquids
o he hyd olysis o cellobiose in o D-glucose monome s [
16
]. To inhibi he o ma ion o side p oduc s
like humins, one o he mos p omising app oaches is o add an o ganic co-sol en , such as cyclopen yl
me hyl e he (CPME), a low wa e -miscible eco- iendly sol en [
17
]. The use o an o ganic co-sol en
allows he ex ac ion o u u al om he aqueous phase in o he o ganic phase and hus imp o es he
u u al yield by inhibi ing he gene a ion o undesi ed p oduc s. Wi h ega ds o he scale-up p ocess,
in o de o sa e ene gy and a oid undesi able deg ada ions, i is be e o use ex ac i e sol en
wi h lowe boiling poin s han ha o solubilized u u al o easily eco e he a ge compound.
Acco ding o Weinga en e al. [
18
], a biphasic sys em does no al e he undamen al kine ics o
i s en i e eac ion compa ed o he classic monophasic sys em. I s only ole is o be he s o age o
he hyd ophobic species. Se e al s udies ha e epo ed he use o MIBK, oluene, ni o oluene o
dichlo ome hane as e icien co-sol en s [
19
–
22
] o he dehyd a ion o D-xylose. In acco dance wi h a
ecen wo k o ou g oup on he dehyd a ion o D-xylose wi h homogeneous ca alysis [
23
], he p esen
wo k ocuses on he use o he Na ion NR50 as a he e ogeneous ca alys in he p esence o NaCl in a
biphasic sys em unde mic owa e i adia ion in a single mode ca i y.
2. Resul s and Discussion
2.1. In luence o he Tempe a u e on Fu u al Yield
Using he i s esul s ob ained by he g oup, he biphasic expe imen s we e pe o med by
employing wo immiscible phases, wa e -CPME wi h a a io 1:3 ( / ) [
23
]. D-xylose is no soluble in
he o ganic phase, and CPME is qui e s able a high empe a u e. Due o i s low densi y, his ex ac ion
sol en was he uppe phase. The s udies o he dehyd a ion o D-xylose we e conduc ed o 1 h
be ween 130 and 180
◦
C in he p esence o NaCl (95 mg) and wo Na ion NR50 pelle s in a biphasic
mix u e (wa e -CPME, 1:3, / ) unde mic owa e i adia ion (Figu e 1). A maximum u u al yield
o 80% was eached a 170
◦
C a e one hou o eac ion wi h a quan i a i e con e sion o D-xylose,
which is a compe i i e esul by compa ison wi h one o ou mos ecen esul s o a 74% u u al
yield ob ained wi h FeCl
3
as he ca alys in a biphasic sys em [
23
]. Be ween 130 and 160
◦
C, emaining
D-xylose loading dec eased slowly. I was no ewo hy ha Na ion NR50 pelle s a e subjec ed o a
na u al swelling in wa e a oom empe a u e [
24
]. In a p elimina y s udy, he same eac ion was
pe o med wi hou he use o NaCl. Howe e , Na ion NR50 pelle s u ned o black and exploded,
and u u al yields we e no ep oducible. As a consequence, NaCl was necessa y o keep he ac i i y
and he in eg i y o Na ion NR50 a 170
◦
C. The mechanism in ol ed du ing he addi ion o NaCl
will be desc ibed la e in he publica ion. Besides, o compa ison, he same eac ion was ca ied
ou in a sealed ube illed wi h 4.0 mL o wa e -CPME (1:3, / ) keeping unchanged he cu en
subs a e o ca alys a io in he p esence o 95 mg o NaCl. The dehyd a ion was ealized by means o
a p ehea ed oil ba h wi h con en ional hea ing and led o a u u al yield o only 55% a 150
◦
C o 2 h.
The eby, hese esul s unde line ha he ca aly ic sys em Na ion NR50-NaCl could be an al e na i e o
he common hyd ochlo ic acid aqueous solu ion unde mic owa e ac i a ion a 170 ◦C.
Molecules 2016,21, 1102 3 o 11
Molecules 2016, 21, 1102 3 o 11
Figu e 1. E ec o eac ion empe a u e on u u al yield. Reac ion condi ions: Na ion NR50 ( wo
pelle s), NaCl (95 mg), D-xylose (1.0 mmol), wa e (1 mL), CPME (3 mL) and MW, 1 h.
When he same p ocedu e was applied o L-a abinose, a lowe yield (42%) was ob ained o 1 h
a 170 °C, as has been obse ed in he li e a u e [25]. I was epo ed ha L-a abinose was
dehyd a ing less apidly han xylose a 170 °C [25,26]. This di e ence is due o he ac ha D-xylose
exp essed lowe ac i a ion ene gy o i s dehyd a ion compa ed o L-a abinose, gene ally supe io
o 112 kJ·mol−1. The s uc u e o L-a abinose was mo e s able. The con e sion o L-a abinose in o
u u al was also epo ed o be mo e empe a u e dependen and o equi e high empe a u es
(abo e 220 °C) o accele a e he p ocess [27,28].
2.2. E ec o D-xylose Loading Va ia ion
The in luence o he ini ial D-xylose concen a ion was also s udied in a mix u e o
wa e -CPME, 1:3, / , a 170 °C o 1 h unde mic owa e i adia ion (Figu e S1, ESI). Fo a D-xylose
concen a ion o 150 g·L−1 in he aqueous phase, he yield eached a alue o 80%. Then, by keeping
he op imized eac ion condi ions, he pen ose loading was g adually inc eased un il a singula
dec ease o he u u al yield was eco ded. Fo a D-xylose concen a ion o 500 g·L−1, he u u al
yield s ill eached a good alue o 72%. Howe e , i he concen a ion we e inc eased o 600 g·L−1, he
u u al yield d opped o he lowe alue o 62%. Fi s , he high pen ose concen a ion a o ed he
con ac be ween molecules and hus p omo ed condensa ion be ween pen ose in e media es and
u u al: la ge amoun s o a da k b own sy up insoluble in bo h laye s we e p oduced. Then, he
la ge amoun s o u u al p oduced om he high xylose concen a ion may also sa u a e he o ganic
phase and make he u u al ex ac ion no longe e icien . Finally, he iscosi y o he eac ion
medium also inc eased wi h inc easing subs a e concen a ion and may ha e led o a non-uni o m
hea ing in he mic owa e eac o .
2.3. In luence o he Residence Time on he Fu u al P oduc ion
A e de e mining he bes empe a u e condi ion o he dehyd a ion o D-xylose using CPME
as he ex ac ion sol en , he e ec o he eac ion ime was s udied a 160 °C, 170 °C and 180 °C
(Figu e 2). Consequen ly, di e en ime pe iods ( om 5 o 60 min) we e es ed in o de o de e mine
he op imal condi ions. The o he pa ame e s we e unchanged using D-xylose (1.0 mmol),
Na ion NR50 loading ( wo pelle s), NaCl (95 mg) and a biphasic a io wa e -CPME, 1:3, / . The
end o he D-xylose con e sion cu e did no change compa ed o hose ob ained om a ypical
monophasic ba ch ope a ed in he same ange o empe a u es epo ed in Figu e 2. A e 25 min o
as consump ion, only 5% o he D-xylose emained in he mix u e. A maximum u u al yield o
80% was eached a e 40 min o eac ion. A e his pe iod, a sligh dec ease o he yield was
obse ed. In addi ion, a 170 °C, he selec i i y o he p oduced u u al ne e exceeded 80%, and
0
10
20
30
40
50
60
70
80
90
120 140 160 180
Fu u al yield and xylose
con en s (%)
Tempe a u e (°C)
Fu u al om xylose
Remaining xylose
Figu e 1.
E ec o eac ion empe a u e on u u al yield. Reac ion condi ions: Na ion NR50 ( wo pelle s),
NaCl (95 mg), D-xylose (1.0 mmol), wa e (1 mL), CPME (3 mL) and MW, 1 h.
When he same p ocedu e was applied o L-a abinose, a lowe yield (42%) was ob ained o 1 h a
170
◦
C, as has been obse ed in he li e a u e [
25
]. I was epo ed ha L-a abinose was dehyd a ing
less apidly han xylose a 170
◦
C [
25
,
26
]. This di e ence is due o he ac ha D-xylose exp essed lowe
ac i a ion ene gy o i s dehyd a ion compa ed o L-a abinose, gene ally supe io o 112 kJ
·
mol
−1
.
The s uc u e o L-a abinose was mo e s able. The con e sion o L-a abinose in o u u al was also
epo ed o be mo e empe a u e dependen and o equi e high empe a u es (abo e 220
◦
C) o
accele a e he p ocess [27,28].
2.2. E ec o D-xylose Loading Va ia ion
The in luence o he ini ial D-xylose concen a ion was also s udied in a mix u e o wa e -CPME,
1:3, / , a 170
◦
C o 1 h unde mic owa e i adia ion (Figu e S1, ESI). Fo a D-xylose concen a ion o
150 g
·
L
−1
in he aqueous phase, he yield eached a alue o 80%. Then, by keeping he op imized
eac ion condi ions, he pen ose loading was g adually inc eased un il a singula dec ease o he
u u al yield was eco ded. Fo a D-xylose concen a ion o 500 g
·
L
−1
, he u u al yield s ill
eached a good alue o 72%. Howe e , i he concen a ion we e inc eased o 600 g
·
L
−1
, he u u al
yield d opped o he lowe alue o 62%. Fi s , he high pen ose concen a ion a o ed he con ac
be ween molecules and hus p omo ed condensa ion be ween pen ose in e media es and u u al:
la ge amoun s o a da k b own sy up insoluble in bo h laye s we e p oduced. Then, he la ge amoun s
o u u al p oduced om he high xylose concen a ion may also sa u a e he o ganic phase and
make he u u al ex ac ion no longe e icien . Finally, he iscosi y o he eac ion medium also
inc eased wi h inc easing subs a e concen a ion and may ha e led o a non-uni o m hea ing in he
mic owa e eac o .
2.3. In luence o he Residence Time on he Fu u al P oduc ion
A e de e mining he bes empe a u e condi ion o he dehyd a ion o D-xylose using CPME as
he ex ac ion sol en , he e ec o he eac ion ime was s udied a 160
◦
C, 170
◦
C and 180
◦
C (Figu e 2).
Consequen ly, di e en ime pe iods ( om 5 o 60 min) we e es ed in o de o de e mine he op imal
condi ions. The o he pa ame e s we e unchanged using D-xylose (1.0 mmol), Na ion NR50 loading
( wo pelle s), NaCl (95 mg) and a biphasic a io wa e -CPME, 1:3, / . The end o he D-xylose
con e sion cu e did no change compa ed o hose ob ained om a ypical monophasic ba ch ope a ed
in he same ange o empe a u es epo ed in Figu e 2. A e 25 min o as consump ion, only 5% o
he D-xylose emained in he mix u e. A maximum u u al yield o 80% was eached a e 40 min
o eac ion. A e his pe iod, a sligh dec ease o he yield was obse ed. In addi ion, a 170
◦
C,
Molecules 2016,21, 1102 4 o 11
he selec i i y o he p oduced u u al ne e exceeded 80%, and only a small amoun o humin was
de ec ed in he eac ion essel. In he opposi e case, when he eac ion was pe o med a 160
◦
C,
he u u al o ma ion was accele a ed be o e 20 min, and he yield con inued o inc ease slowly un il i
eached 69% a e 60 min. A 180
◦
C, o a sho pe iod comp ised be ween 10 and 20 min, u u al was
p oduced wi h a high yield o 71%, bu beyond his ime, he u u al concen a ion s a ed o decline
due o i s esini ica ion, mo e e ec i e a his empe a u e. Thus, he op imized eac ion ime and
empe a u e we e clea ly de ined. In o de o dec ease he eac ion ime, he empe a u e was ixed a
170 ◦C o 40 min unde mic owa e i adia ion.
Molecules 2016, 21, 1102 4 o 11
only a small amoun o humin was de ec ed in he eac ion essel. In he opposi e case, when he
eac ion was pe o med a 160 °C, he u u al o ma ion was accele a ed be o e 20 min, and he
yield con inued o inc ease slowly un il i eached 69% a e 60 min. A 180 °C, o a sho pe iod
comp ised be ween 10 and 20 min, u u al was p oduced wi h a high yield o 71%, bu beyond his
ime, he u u al concen a ion s a ed o decline due o i s esini ica ion, mo e e ec i e a his
empe a u e. Thus, he op imized eac ion ime and empe a u e we e clea ly de ined. In o de o
dec ease he eac ion ime, he empe a u e was ixed a 170 °C o 40 min unde mic owa e
i adia ion.
Figu e 2. E ec o eac ion ime and empe a u e on (a) u u al yield and (b) xylose con en s.
Reac ion condi ions: Na ion NR50 (2 pelle s), NaCl (95 mg), D-xylose (1.0 mmol), wa e (1 mL),
CPME (3 mL) and MW.
2.4. E ec o he Ra io Wa e -CPME on he Fu u al Yield
In o de o e alua e he e ec o he a io wa e -CPME on D-xylose con e sion and u u al
yield, i e a ios o wa e -CPME (4:0, 3:1, 1:1, 1:3, 0:4; / ) we e s udied (Figu e 3). When he eac ion
was pe o med in pu e wa e , as expec ed, he yield o he p oduced u u al ne e exceeded 38%,
and he selec i i y was mode a e wi h a alue o 57%. Besides, he con e sion o D-xylose was no
comple e since he Na ion pelle s deac i a ed du ing he p ocess because o he deposi ion o a la ge
amoun o humins on he ca alys su ace. By inc easing he wa e -CPME a io, he selec i i y o
0
10
20
30
40
50
60
70
80
90
100
020 40 60
Fu u al yield (%)
Reac ion ime (min)
(a)
160℃
170℃
180℃
0
10
20
30
40
50
60
70
80
90
100
020 40 60
Xylose con en s (%)
Reac ion ime (min)
(b)
160℃
170℃
180℃
Figu e 2.
E ec o eac ion ime and empe a u e on (
a
) u u al yield and (
b
) xylose con en s.
Reac ion condi ions: Na ion NR50 (2 pelle s), NaCl (95 mg), D-xylose (1.0 mmol), wa e (1 mL),
CPME (3 mL) and MW.
2.4. E ec o he Ra io Wa e -CPME on he Fu u al Yield
In o de o e alua e he e ec o he a io wa e -CPME on D-xylose con e sion and u u al yield,
i e a ios o wa e -CPME (4:0, 3:1, 1:1, 1:3, 0:4; / ) we e s udied (Figu e 3). When he eac ion
was pe o med in pu e wa e , as expec ed, he yield o he p oduced u u al ne e exceeded 38%,
and he selec i i y was mode a e wi h a alue o 57%. Besides, he con e sion o D-xylose was no
comple e since he Na ion pelle s deac i a ed du ing he p ocess because o he deposi ion o a la ge
Molecules 2016,21, 1102 5 o 11
amoun o humins on he ca alys su ace. By inc easing he wa e -CPME a io, he selec i i y o
u u al inc eased wi h alues o 56%, 71% and 79% o wa e -CPME olume ic a ios o 3:1, 1:1 and
1:3, espec i ely. Fu he mo e, he coe icien pa i ion ound in he di e en wa e -CPME a ios
abo e was also de e mined as ollow. A 170
◦
C, 1.0 mmol o u u al was pa i ioned be ween he
wo phases in he p esence o he ca aly ic sys em, and he alue was calcula ed using his o mula:
Kp = [ u u al]wa e /[ u u al]CPME
. Acco ding he HPLC measu emen , excep a sligh dec ease o
u u al concen a ion in bo h laye s, by inc easing he olume o CPME, he coe icien emained
almos cons an un il i eached a inal alue o 4.1 o he wa e -CPME (1:3, / ) olume a io.
When he eac ion was pe o med in 4 mL o pu e CPME, he selec i i y o u u al emained blocked
a 2% due o he lack o he solubili y o D-xylose in his o ganic sol en . These esul s con i med he
impo ance o a minimum o 1 mL o wa e as he eac ion sol en . A 170
◦
C, upon i adia ion in a
closed sys em, wa e emained pola enough compa ed o CPME, and D-xylose is easily in con ac
wi h HCl. O e all, wa e is he e he lowe phase, and 1 mL seems o be enough despi e he di icul y
o con ol he inne hea ing empe a u e o he essel agg a a ed by he g ea numbe o ions p esen
in solu ion due o he dissocia ion o NaCl [29].
Molecules 2016, 21, 1102 5 o 11
u u al inc eased wi h alues o 56%, 71% and 79% o wa e -CPME olume ic a ios o 3:1, 1:1 and
1:3, espec i ely. Fu he mo e, he coe icien pa i ion ound in he di e en wa e -CPME a ios
abo e was also de e mined as ollow. A 170 °C, 1.0 mmol o u u al was pa i ioned be ween he
wo phases in he p esence o he ca aly ic sys em, and he alue was calcula ed using his o mula:
Kp = [ u u al]wa e /[ u u al]CPME. Acco ding he HPLC measu emen , excep a sligh dec ease o
u u al concen a ion in bo h laye s, by inc easing he olume o CPME, he coe icien emained
almos cons an un il i eached a inal alue o 4.1 o he wa e -CPME (1:3, / ) olume a io. When
he eac ion was pe o med in 4 mL o pu e CPME, he selec i i y o u u al emained blocked a 2%
due o he lack o he solubili y o D-xylose in his o ganic sol en . These esul s con i med he
impo ance o a minimum o 1 mL o wa e as he eac ion sol en . A 170 °C, upon i adia ion in a
closed sys em, wa e emained pola enough compa ed o CPME, and D-xylose is easily in con ac
wi h HCl. O e all, wa e is he e he lowe phase, and 1 mL seems o be enough despi e he
di icul y o con ol he inne hea ing empe a u e o he essel agg a a ed by he g ea numbe o
ions p esen in solu ion due o he dissocia ion o NaCl [29].
Figu e 3. E ec o he wa e -CPME olume ic a io on dehyd a ion o D-xylose o u u al. Reac ion
condi ions: Na ion NR50 (2 pelle s), NaCl (95 mg), D-xylose (1.0 mmol), wa e -CPME, MW, 170 °C
and 40 min.
2.5. In luence o he Ra io Na ion NR50-NaCl on he Fu u al Yield
To he bes o ou knowledge, he inne s uc u e o he Na ion NR50 pelle s emains unclea ,
bu all sul onic acid unc ions we e supposed o be accessible. In ou p ocess, when NaCl was added
o he aqueous phase, a ca ion exchange be ween he Na+ o he sal and he H+ o he sul onic acid
g oups o Na ion occu ed, and HCl was eleased in he aqueous phase. Besides, he sal o med a
coa ing on he pelle s and seemed o p o ec hem om he deposi ion o humins and, hus, om
explosion. The impac o he a io o Na ion NR50-NaCl on he dehyd a ion o D-xylose p ocess was
in es iga ed using he op imized me hod desc ibed abo e: Na ion NR50 ( wo pelle s), NaCl,
D-xylose (1.0 mmol), wa e (1 mL), CPME (3 mL), MW, 170 °C and 40 min. As epo ed in Figu e 4,
he mass o NaCl was p og essi ely inc eased un il a maximum o selec i i y was ob ained wi h 95
mg o NaCl. Howe e , wi h only 50 mg o NaCl, he eac ions s a ed o u nish be e selec i i ies
(78%). This esul co obo a es a ecen s udy ha demons a es he impac o inc easing he
concen a ion o NaCl on he p oduced u u al yield [30]. The same p o ocol was ealized using
only one pelle o Na ion NR50 (45 mg), bu u u al was ob ained in a lowe yield (72%) (Figu e S2,
ESI).
0
10
20
30
40
50
60
70
80
90
100
4:0 3:1 1:1 1:3 0:4
（％）
Wa e -CPME a io ( / )
Fu u al yield
Xylose
con e sion
Fu u al
selec i i y
Figu e 3.
E ec o he wa e -CPME olume ic a io on dehyd a ion o D-xylose o u u al.
Reac ion condi ions: Na ion NR50 (2 pelle s), NaCl (95 mg), D-xylose (1.0 mmol), wa e -CPME, MW,
170 ◦C and 40 min.
2.5. In luence o he Ra io Na ion NR50-NaCl on he Fu u al Yield
To he bes o ou knowledge, he inne s uc u e o he Na ion NR50 pelle s emains unclea ,
bu all sul onic acid unc ions we e supposed o be accessible. In ou p ocess, when NaCl was added
o he aqueous phase, a ca ion exchange be ween he Na
+
o he sal and he H
+
o he sul onic acid
g oups o Na ion occu ed, and HCl was eleased in he aqueous phase. Besides, he sal o med
a coa ing on he pelle s and seemed o p o ec hem om he deposi ion o humins and, hus,
om explosion. The impac o he a io o Na ion NR50-NaCl on he dehyd a ion o D-xylose p ocess
was in es iga ed using he op imized me hod desc ibed abo e: Na ion NR50 ( wo pelle s), NaCl,
D-xylose (1.0 mmol), wa e (1 mL), CPME (3 mL), MW, 170
◦
C and 40 min. As epo ed in Figu e 4,
he mass o NaCl was p og essi ely inc eased un il a maximum o selec i i y was ob ained wi h 95 mg
o NaCl. Howe e , wi h only 50 mg o NaCl, he eac ions s a ed o u nish be e selec i i ies (78%).
This esul co obo a es a ecen s udy ha demons a es he impac o inc easing he concen a ion
o NaCl on he p oduced u u al yield [
30
]. The same p o ocol was ealized using only one pelle o
Na ion NR50 (45 mg), bu u u al was ob ained in a lowe yield (72%) (Figu e S2, ESI).

Molecules 2016,21, 1102 6 o 11
Molecules 2016, 21, 1102 6 o 11
Figu e 4. E ec o he amoun o NaCl on he dehyd a ion o D-xylose o u u al. Reac ion
condi ions: Na ion NR50 (2 pelle s), NaCl, D-xylose (1.0 mmol), wa e (1 mL), CPME (3 mL), MW,
170 °C and 40 min.
To comple e ou s udy, NaCl was eplaced by o he ino ganic sal s, such as KCl o LiCl. In
ela ion o he ca ion size and hei ela i e mobili y inside he Na ion pelle s, a compe i ion be ween
wa e and he posi i ely-cha ged species was expec ed o ha e an in luence on he elease o he
p o ons in he eac ion. Ne e heless, by using he same sal mola a io, no a ia ion o pH was
eco ded, and he yield o p oduced u u al was cons an wi h a alue o 80%.
Na ion NR50 has a capaci y o 0.8 mmol o H+ pe g am. Fo wo pelle s and an excess o NaCl,
i co esponded o a pH o 1.1 in he ial i he o ali y o he p o ons was eleased in he medium.
The pH o ou eac ion media eached a alue o 1.3 on a e age, which means ha a pa o he
sul onic g oups g a ed on he esin emained p o ona ed. To e i y i he D-xylose con e sion was
only ca alyzed by he HCl eleased in he medium o also by he esin, a eac ion wi h an HCl
solu ion a a pH o 1.3 (0.05 mol·L−1) was pe o med o 40 min in he p esence o 90 mg o NaCl a
170 °C in wa e -CPME,1:3, / . A lowe u u al yield o 73% was ob ained ins ead o he 80%
eached wi h he mix u e o Na ion and NaCl. All o hese obse a ions pe mi ed es ablishing ha a
new acid ca aly ic sys em was ob ained ha ing a syne ge ic e ec be ween Na ion NR50 and NaCl.
2.6. Reusabili y o he Na ion NR50
Se e al consecu i e eac ions we e pe o med wi h he same Na ion NR50 pelle s. A e each
eac ion, he esin pelle s we e eco e ed om he eac ion medium and egene a ed in an acidic
HCl ba h o e nigh . The eac ions we e pe o med o 40 min a 170 °C in wa e -CPME, 1:3, / . The
ac i i y o he Na ion NR50 pelle s was kep cons an o h ee consecu i e cycles (Figu e 5). A e
he ou h cycle, he u u al yield began o dec ease. This obse a ion can be explained by he
g adual deac i a ion o he pelle s by humin deposi ion.
0
10
20
30
40
50
60
70
80
90
100
10 20 30 50 70 95
（％）
Mass o NaCl (mg)
Fu u al yield
Xylose
con e sion
Fu u al
selec i i y
Figu e 4.
E ec o he amoun o NaCl on he dehyd a ion o D-xylose o u u al. Reac ion condi ions:
Na ion NR50 (2 pelle s), NaCl, D-xylose (1.0 mmol), wa e (1 mL), CPME (3 mL), MW, 170
◦
C and
40 min.
To comple e ou s udy, NaCl was eplaced by o he ino ganic sal s, such as KCl o LiCl. In ela ion
o he ca ion size and hei ela i e mobili y inside he Na ion pelle s, a compe i ion be ween wa e
and he posi i ely-cha ged species was expec ed o ha e an in luence on he elease o he p o ons
in he eac ion. Ne e heless, by using he same sal mola a io, no a ia ion o pH was eco ded,
and he yield o p oduced u u al was cons an wi h a alue o 80%.
Na ion NR50 has a capaci y o 0.8 mmol o H
+
pe g am. Fo wo pelle s and an excess o NaCl,
i co esponded o a pH o 1.1 in he ial i he o ali y o he p o ons was eleased in he medium.
The pH o ou eac ion media eached a alue o 1.3 on a e age, which means ha a pa o he
sul onic g oups g a ed on he esin emained p o ona ed. To e i y i he D-xylose con e sion was
only ca alyzed by he HCl eleased in he medium o also by he esin, a eac ion wi h an HCl solu ion
a a pH o 1.3 (0.05 mol
·
L
−1
) was pe o med o 40 min in he p esence o 90 mg o NaCl a 170
◦
C in
wa e -CPME,1:3, / . A lowe u u al yield o 73% was ob ained ins ead o he 80% eached wi h he
mix u e o Na ion and NaCl. All o hese obse a ions pe mi ed es ablishing ha a new acid ca aly ic
sys em was ob ained ha ing a syne ge ic e ec be ween Na ion NR50 and NaCl.
2.6. Reusabili y o he Na ion NR50
Se e al consecu i e eac ions we e pe o med wi h he same Na ion NR50 pelle s. A e each
eac ion, he esin pelle s we e eco e ed om he eac ion medium and egene a ed in an acidic
HCl ba h o e nigh . The eac ions we e pe o med o 40 min a 170
◦
C in wa e -CPME, 1:3, / .
The ac i i y o he Na ion NR50 pelle s was kep cons an o h ee consecu i e cycles (Figu e 5).
A e he ou h cycle, he u u al yield began o dec ease. This obse a ion can be explained by he
g adual deac i a ion o he pelle s by humin deposi ion.
Molecules 2016,21, 1102 7 o 11
Molecules 2016, 21, 1102 7 o 11
Figu e 5. Reusabili y o he aqueous phase con aining Na ion NR50 and NaCl o he D-xylose
dehyd a ion o u u al. Reac ion condi ions: Na ion NR50 (2 pelle s), NaCl (95 mg), D-xylose (1.0 mmol),
wa e (1 mL), CPME (3 mL), MW, 170 °C and 40 min.
2.7. SEM Obse a ions EDX and Analysis o Na ion NR50 Pelle s
Fo each s age o ou s udy, he mo phology o he Na ion esins was obse ed by SEM. The
pho og aphs o hese obse a ions a e p o ided (Figu e 6) in associa ion wi h a se ies o EDX
analysis spec a (Figu e S3, ESI). Figu e 6a shows he ine po osi y o he na i e Na ion NR50
su ace. The EDX spec um S3a ga e he main cons i uen s o Na ion: luo ine, ca bon, oxygen and
sul u . The pelle analyzed in Figu e 6b was used in a eac ion wi hou NaCl. The su ace o he
deg aded ca alys was cha ac e ized by a ne wo k o c acks o a ious sizes and by he deposi ion o
an o ganic esidue on he su ace. Fo his sample, he EDX spec um S3b showed an en ichmen o
he ca alys wi h ca bon and oxygen, e idence o he humin deposi ion. When he eac ion was
conduc ed in a NaCl aqueous solu ion, he eco e ed Na ion NR50 pelle s (Figu e 6c) appea ed o
emain in ac . Howe e , he su ace o he ca alys was en i ely coa ed by NaCl c ys als, an
obse a ion con i med by he EDX analysis S3c. When hese pelle s we e used wi hou ea men in
an aqueous HCl solu ion, a u he ca aly ic un in he p esence o D-xylose ga e only u u al in a
21% yield. Indeed, because o he pa ial exchange o p o ons H+ wi h Na+ ca ions, he ca alys was
pa ially deac i a ed. Finally, he Na ion pelle can be egene a ed in an acidic ba h o HCl o eco e
i s ac i i y. As we can see in he image (Figu e 6d), he ino ganic sal coa ing was comple ely
emo ed by his ea men , and i was p o en by he supp ession o bo h he Na and Cl peaks on he
co esponding EDX spec um S3d.
Figu e 6. FE-SEM images o he Na ion NR50 su ace mo phology ob ained om: (a) p is ine Na ion;
(b) he eac ion wi hou sal ; (c) wi h sal a e one ca aly ic un; (d) a e egene a ion in
concen a ed HCl solu ion (scale ba = 50 µm).
0
20
40
60
80
100
1
2
3
4
Fu u al yield (%)
Cycle
Figu e 5.
Reusabili y o he aqueous phase con aining Na ion NR50 and NaCl o he D-xylose
dehyd a ion o u u al. Reac ion condi ions: Na ion NR50 (2 pelle s), NaCl (95 mg), D-xylose
(1.0 mmol), wa e (1 mL), CPME (3 mL), MW, 170 ◦C and 40 min.
2.7. SEM Obse a ions EDX and Analysis o Na ion NR50 Pelle s
Fo each s age o ou s udy, he mo phology o he Na ion esins was obse ed by SEM.
The pho og aphs o hese obse a ions a e p o ided (Figu e 6) in associa ion wi h a se ies o EDX
analysis spec a (Figu e S3, ESI). Figu e 6a shows he ine po osi y o he na i e Na ion NR50 su ace.
The EDX spec um S3a ga e he main cons i uen s o Na ion: luo ine, ca bon, oxygen and sul u .
The elle analyzed in Figu e 6b was used in a eac ion wi hou NaCl. The su ace o he deg aded
ca alys was cha ac e ized by a ne wo k o c acks o a ious sizes and by he deposi ion o an o ganic
esidue on he su ace. Fo his sample, he EDX spec um S3b showed an en ichmen o he ca alys
wi h ca bon and oxygen, e idence o he humin deposi ion. When he eac ion was conduc ed in a NaCl
aqueous solu ion, he eco e ed Na ion NR50 pelle s (Figu e 6c) appea ed o emain in ac . Howe e , he
su ace o he ca alys was en i ely coa ed by NaCl c ys als, an obse a ion con i med by he EDX
analysis S3c. When hese pelle s we e used wi hou ea men in an aqueous HCl solu ion, a u he
ca aly ic un in he p esence o D-xylose ga e only u u al in a 21% yield. Indeed, because o he pa ial
exchange o p o ons H
+
wi h Na
+
ca ions, he ca alys was pa ially deac i a ed.
Finally, he Na ion
pelle can be egene a ed in an acidic ba h o HCl o eco e i s ac i i y. As we can see in he image
(Figu e 6d), he ino ganic sal coa ing was comple ely emo ed by his ea men , and i was p o en by
he supp ession o bo h he Na and Cl peaks on he co esponding EDX spec um S3d.
Molecules 2016, 21, 1102 7 o 11
Figu e 5. Reusabili y o he aqueous phase con aining Na ion NR50 and NaCl o he D-xylose
dehyd a ion o u u al. Reac ion condi ions: Na ion NR50 (2 pelle s), NaCl (95 mg), D-xylose (1.0 mmol),
wa e (1 mL), CPME (3 mL), MW, 170 °C and 40 min.
2.7. SEM Obse a ions EDX and Analysis o Na ion NR50 Pelle s
Fo each s age o ou s udy, he mo phology o he Na ion esins was obse ed by SEM. The
pho og aphs o hese obse a ions a e p o ided (Figu e 6) in associa ion wi h a se ies o EDX
analysis spec a (Figu e S3, ESI). Figu e 6a shows he ine po osi y o he na i e Na ion NR50
su ace. The EDX spec um S3a ga e he main cons i uen s o Na ion: luo ine, ca bon, oxygen and
sul u . The pelle analyzed in Figu e 6b was used in a eac ion wi hou NaCl. The su ace o he
deg aded ca alys was cha ac e ized by a ne wo k o c acks o a ious sizes and by he deposi ion o
an o ganic esidue on he su ace. Fo his sample, he EDX spec um S3b showed an en ichmen o
he ca alys wi h ca bon and oxygen, e idence o he humin deposi ion. When he eac ion was
conduc ed in a NaCl aqueous solu ion, he eco e ed Na ion NR50 pelle s (Figu e 6c) appea ed o
emain in ac . Howe e , he su ace o he ca alys was en i ely coa ed by NaCl c ys als, an
obse a ion con i med by he EDX analysis S3c. When hese pelle s we e used wi hou ea men in
an aqueous HCl solu ion, a u he ca aly ic un in he p esence o D-xylose ga e only u u al in a
21% yield. Indeed, because o he pa ial exchange o p o ons H+ wi h Na+ ca ions, he ca alys was
pa ially deac i a ed. Finally, he Na ion pelle can be egene a ed in an acidic ba h o HCl o eco e
i s ac i i y. As we can see in he image (Figu e 6d), he ino ganic sal coa ing was comple ely
emo ed by his ea men , and i was p o en by he supp ession o bo h he Na and Cl peaks on he
co esponding EDX spec um S3d.
Figu e 6. FE-SEM images o he Na ion NR50 su ace mo phology ob ained om: (a) p is ine Na ion;
(b) he eac ion wi hou sal ; (c) wi h sal a e one ca aly ic un; (d) a e egene a ion in
concen a ed HCl solu ion (scale ba = 50 µm).
0
20
40
60
80
100
1
2
3
4
Fu u al yield (%)
Cycle
Figu e 6. FE-SEM images o he Na ion NR50 su ace mo phology ob ained om: (a) p is ine Na ion;
(
b
) he eac ion wi hou sal ; (
c
) wi h sal a e one ca aly ic un; (
d
) a e egene a ion in concen a ed
HCl solu ion (scale ba = 50 µm).
Molecules 2016,21, 1102 8 o 11
2.8. Fu u al P oduc ion om Xylan
The p oduc ion o u u al om xylan in ol es a wo-s ep eac ion: a pseudo- i s o de
i e e sible depolyme iza ion and a pseudo- i s o de dehyd a ion. In ou condi ions op imized o
he dehyd a ion o D-xylose (Na ion NR50 (2 pelle s), NaCl (95 mg), wa e (1 mL), CPME (3 mL), MW,
170
◦
C and 40 min), he xylan oligome u nished he a ge u u al in a 19% yield. Thus, an inc ease o
he ime pe iod (60 min s. 40 min) and he a ia ion o he empe a u e (130, 140, 150, 160, 170, 180 and
190
◦
C) we e ealized (Figu e 7). As empe a u e ose om 130
◦
C o 160
◦
C, hyd olysis o xylan in o
D-xylose was accele a ed, and he amoun o suga eleased in he media inc eased o each a maximum
o 42%. Then, om 140
◦
C, D-xylose was dehyd a ed in o u u al, in a ange o empe a u es om his
empe a u e o 170
◦
C, bu he u u al concen a ion emained unde he concen a ion o he eleased
xylose. The yield inc eased om 30% o 55% wi h an inc easing empe a u e om 170
◦
C o 190
◦
C.
In ega ds o ou esul s, he op imized condi ions o u u al o ma ion om xylan we e iden i ied:
Na ion NR50 (2 pelle s), NaCl (95 mg), wa e (1 mL), CPME (3 mL), MW, 190 ◦C and 60 min.
Molecules 2016, 21, 1102 8 o 11
2.8. Fu u al P oduc ion om Xylan
The p oduc ion o u u al om xylan in ol es a wo-s ep eac ion: a pseudo- i s o de
i e e sible depolyme iza ion and a pseudo- i s o de dehyd a ion. In ou condi ions op imized o
he dehyd a ion o D-xylose (Na ion NR50 (2 pelle s), NaCl (95 mg), wa e (1 mL), CPME (3 mL),
MW, 170 °C and 40 min), he xylan oligome u nished he a ge u u al in a 19% yield. Thus, an
inc ease o he ime pe iod (60 min s. 40 min) and he a ia ion o he empe a u e (130, 140, 150,
160, 170, 180 and 190 °C) we e ealized (Figu e 7). As empe a u e ose om 130 °C o 160 °C,
hyd olysis o xylan in o D-xylose was accele a ed, and he amoun o suga eleased in he media
inc eased o each a maximum o 42%. Then, om 140 °C, D-xylose was dehyd a ed in o u u al, in
a ange o empe a u es om his empe a u e o 170 °C, bu he u u al concen a ion emained
unde he concen a ion o he eleased xylose. The yield inc eased om 30% o 55% wi h an
inc easing empe a u e om 170 °C o 190 °C. In ega ds o ou esul s, he op imized condi ions o
u u al o ma ion om xylan we e iden i ied: Na ion NR50 (2 pelle s), NaCl (95 mg), wa e (1 mL),
CPME (3 mL), MW, 190 °C and 60 min.
Figu e 7. Fu u al yield om xylan as a unc ion o empe a u e o 60 min. Reac ion condi ions:
Na ion NR50 (2 pelle s), NaCl (95 mg), xylan (1.0 mmol), wa e (1 mL), CPME (3 mL), MW, 170 °C
and 60 min.
3. Expe imen al Sec ion
3.1. Gene al In o ma ion
Subs a es we e pu chased om Ac os O ganic (D-xylose ≥ 99%, L-a abinose ≥99%), xylan om
beech wood ≥90% om Sigma Ald ich (Sain -Quen in Falla ie , F ance). Ca alys (Na ion NR50)
was pu chased om Sigma Ald ich. Sol en s we e pu chased om Ac os (cyclopen yl me hyl e he ,
Illki ch, F ance) and Fishe Scien i ic (ace oni ile, Illki ch, F ance). The s anda d ( u u al 99%) was
ob ained om Ac os. All ma e ials we e used wi hou u he pu i ica ion. The wa e used in all
expe imen s was Millipo e Milli-Q g ade.
Each sample was analyzed sepa a ely by means o a Shimadzu P ominence HPLC. Pen oses
we e de ec ed wi h a low empe a u e e apo a i e ligh sca e ing de ec o (ELSD-LTII, Shimadzu,
Ma ne_la-Vallée, F ance), and u u al was de ec ed wi h a UV-VIS de ec o (SPD-M20A, Shimadzu,
Ma ne-la-Vallée, F ance) a a wa eleng h o 275 nm. The column used was a G ace P e ail C18
column (250 × 4,6 mm, 5 µm). The mobile phase was a MeOH-H2O (9:1) solu ion lowing a a a e o
0.5 mL·min−1. The column o en was se a 40 °C. D-Xylose con e sion (X), u u al yield (Y) and
u u al selec i i y (S) we e calcula ed by he ollowing equa ions:
0
10
20
30
40
50
60
70
80
90
100
130 150 170 190
Fu u al yield and xylose
con en s (%)
Tempe a u e (°C)
Fu u al om
xylose
Xylose eleased
om xylan
Figu e 7.
Fu u al yield om xylan as a unc ion o empe a u e o 60 min. Reac ion condi ions:
Na ion NR50 (2 pelle s), NaCl (95 mg), xylan (1.0 mmol), wa e (1 mL), CPME (3 mL), MW, 170
◦
C and
60 min.
3. Expe imen al Sec ion
3.1. Gene al In o ma ion
Subs a es we e pu chased om Ac os O ganic (D-xylose
≥
99%, L-a abinose
≥
99%), xylan om
beech wood
≥
90% om Sigma Ald ich (Sain -Quen in Falla ie , F ance). Ca alys (Na ion NR50)
was pu chased om Sigma Ald ich. Sol en s we e pu chased om Ac os (cyclopen yl me hyl e he ,
Illki ch, F ance) and Fishe Scien i ic (ace oni ile, Illki ch, F ance). The s anda d ( u u al 99%) was
ob ained om Ac os. All ma e ials we e used wi hou u he pu i ica ion. The wa e used in all
expe imen s was Millipo e Milli-Q g ade.
Each sample was analyzed sepa a ely by means o a Shimadzu P ominence HPLC. Pen oses we e
de ec ed wi h a low empe a u e e apo a i e ligh sca e ing de ec o (ELSD-LTII, Shimadzu,
Ma ne_la-Vallée, F ance), and u u al was de ec ed wi h a UV-VIS de ec o (SPD-M20A, Shimadzu,
Ma ne-la-Vallée, F ance) a a wa eleng h o 275 nm. The column used was a G ace P e ail C18
column (250
×
4.6 mm, 5
µ
m). The mobile phase was a MeOH–H
2
O (9:1) solu ion lowing a a a e
Molecules 2016,21, 1102 9 o 11
o 0.5 mL
·
min
−1
. The column o en was se a 40
◦
C. D-Xylose con e sion (X), u u al yield (Y) and
u u al selec i i y (S) we e calcula ed by he ollowing equa ions:
X=(Ini ial xylose amoun (mol)−Final xylose amoun (mol))
Ini ial xylose amoun (mol)×100
Y=Final u u al amoun (mol)
Ini ial xylose amoun (mol)×100
S=Fu u al yield
Xylose con e sion ×100
Regula ly, he calib a ion cu e was checked in o de o a oid e en ual expe imen al e o s
associa ed wi h all measu emen s epo ed abo e. The esul s epo ed in all igu es a e a e aged
alues calcula ed om h ee measu emen s.
SEM (scanning elec on mic oscopy)-EDX (ene gy dispe si e X- ay di ac ion) analysis o Na ion
pelle s a di e en s ages o he expe imen s was pe o med on a Quan a FEG 250 (FEI) equipped
wi h a mic oanalysis de ec o o EDX (B ucke , Wissembou g, F ance). SEM mic og aphs acqui ed in
seconda y elec on mode we e ob ained a low acuum, 15 kV o accele a ing ol age wi h a 10-mm
wo king dis ance. EDX spec a we e collec ed a a 30
◦
angle, a 15-kV accele a ing ol age and a 10-mm
wo king dis ance.
3.2. Gene al P ocedu e o he Syn hesis o Fu u al in a Wa e -CPME Biphasic Media
In a ypical expe imen , a 10-mL glass essel was cha ged wi h wa e (1 mL), CPME (3 mL),
he subs a e (1.0 mmol), Na ion NR50 (2 pelle s,
≈
95 mg) and NaCl (1.62 mmol). The essel was sealed
wi h a sep um, placed in he mic owa e appa a us (An onPaa Monowa e 300) and g adually hea ed
o 5 min o he desi ed empe a u e ia a esonan single mode unde magne ic s i ing (600 pm) o
he desi ed ime. Tempe a u e in he essel was moni o ed by means o an IR senso . A he end o
he eac ion, he essel was cooled down o 40
◦
C using comp essed ai . Then, he wo phases we e
sepa a ed. The aqueous phase was dilu ed in 200 mL o dis illed wa e and il e ed p io o analysis
h ough a il e pape (10–20
µ
m, VWR, Fon enay-sous-Bois, F ance). The o ganic phase was dilu ed
in 200 mL o ace oni ile and il e ed p io o analysis h ough a sy inge il e (PTFE, 0.45
µ
m, VWR).
All expe imen s we e epea ed a leas h ee imes, and he de ia ion was lowe han 5%.
4. Conclusions
In summa y, his wo k epo s a new e icien p ocess o he p oduc ion o u u al in an 80%
yield. The me hod used a mix u e o Na ion NR50 and NaCl in a wa e -CPME biphasic sys em
a 170
◦
C unde mic owa e i adia ion. The associa ion o Na ion NR50 and NaCl gene a ed an
unusual s abili y o he esin a 170
◦
C and a syne ge ic e ec in acid ca alysis. Rega ding hese
p omising esul s, he ansposi ion o his me hodology o cellulose o s a ch in o de o p oduce
5-hyd oxyme hyl u u al (HMF) will be de eloped in due cou se.
Supplemen a y Ma e ials:
The Supplemen a y Ma e ials a e a ailable online a : h p://www.mdpi.com/
1420-3049/21/8/1102/s1.
Acknowledgmen s:
S.L.G. would like o hank he Minis è e de l’Educa ion Na ionale e de la Reche che o
he inancial suppo . This wo k was pe o med in pa ne ship wi h he COST Ac ion FP 1306 Valo isa ion
o ligno-cellulosic biomass s eams o sus ainable p oduc ion o chemicals, ma e ials and uels using low
en i onmen al impac echnologies.
Au ho Con ibu ions:
F ede ic Delbecq and Ch is ophe Len concei ed and designed he expe imen s;
Sa ah Le Guenic and Da id Ge gela pe o med he expe imen s; Sa ah Le Guenic, Clai e Ceballos
and F ede ic Delbecq analyzed he da a; Ch is ophe Len con ibu ed eagen s/ma e ials/analysis ools;
F ede ic Delbecq and Ch is ophe Len w o e he pape .
Con lic s o In e es : The au ho s decla e no con lic o in e es .