Reduction of CO2 captured in basic solutions with biomass as reducing agent and metallic catalysts

Reduc ion o CO
2
cap u ed in basic solu ions wi h
biomass as educing agen and me allic ca alys s†
Mai a I. Chinchilla,
ab
´
Angel Ma ´
ın,
ab
J. McG ego ,
c
Fidel A. Ma o
ab
and Ma ´
ıa D. Be mejo *
ab
CO
2
cap u e and u iliza ion echnologies can make an impo an con ibu ion o he deca boniza ion o
indus y. Howe e , cap u e p ocesses en ail significan economic and ene gy cos s, mainly associa ed
wi h he pu ifica ion, comp ession and anspo o CO
2
. These cos s would be educed i cap u ed CO
2
could be ans o med in si u in o use ul p oduc s, a oiding pu ifica ion, comp ession and anspo cos s.
This wo k p esen s a hyd o he mal p ocess in which CO
2
abso bed in aqueous solu ions as bica bona e
is educed wi h biomass was e o gi e o mic acid as a join p oduc o he biomass and CO
2
ans o ma ion, and ace ic and lac ic acids as byp oduc s om he decomposi ion o he biomass.
Se e al biomass ma e ials a e applied as educ an s: so wood, suga cane bagasse, suga bee , co k, pine
needles, e micompos and pu e cellulose as e e ence ma e ial. Mo eo e , diffe en ca alys s a e es ed
o imp o e con e sion yield: Pd(5%)/C and Pd(10%)/C, Ru(5%)/C and ac i a ed ca bon. The bes esul s
(18% o mic acid yield) a e ob ained using pu e cellulose as biomass and Pd(5%)/C ca alys . The nex bes
esul s a e ob ained wi h he biomasses wi h he highes cellulose con en , such as wood (11%) and
suga cane bagasse (9%). Expe imen s pe o med wi h labelled H
13
CO
3
−
as ca bon sou ce a 300 °C
using he Pd(5%)/C ca alys demons a e ha o e 70% o he p oduced o mic acid is o med om he
ino ganic bica bona e ca bon sou ce. These high yields o con e sion using enewable biomass as
educ an can con ibu e o imp o e he echnical and economic easibili y o CO
2
cap u e echnology.
Sus ainabili y spo ligh
Cu en ly he e is g ea effo in educing CO
2
emissions, bu he cos o cap u ing and s o age is e y high. De eloping echnologies o p oducing use ul
chemicals and uels om CO
2
will con ibu e o educe hei cos . In he p esen wo k a echnology o simul aneously ans o ming CO
2
and biomass was e in
o mic acid and o he o ganic was e. The main ad an age o his echnology is ha can be di ec ly coupled wi h abso p ion o CO
2
wi h NaOH and he effluen o
he abso be can be aken di ec ly o he eac o , a oiding pu ica ion, comp ession and anspo o CO
2
ha a e he mos expensi e and ene gy consuming. I
is aligned wi h se e al UN SDG, bu mo e clea ly wi h he 13 h: clima e ac ions.
1. In oduc ion
Nowadays many effo s a e being made o limi he inc ease in
global empe a u e. In he las h ee decades, he emission o
g eenhouse gases has inc eased by 43%, wi h 65% o his being
emissions o CO
2
. To educe he emission a es o CO
2
in o he
a mosphe e, esea ch and inno a ion is being a ge ed a he
de elopmen o new echnologies p omo ing he con e sion
and u iliza ion o his compound.
1–5
The u iliza ion o CO
2
has a oused g ea in e es since his
compound can be used as aw ma e ial o manu ac u ing
p oduc s o high alue o indus y such as o mic acid,
e hylene, e hanol, e c.,
3,6–9
hus p o iding an economic e enue
o compensa e he cap u e cos s and, in a longe e m,
a sus ainable al e na i e o he cu en ossil- uel based o ganic
chemical indus y. CO
2
can be di ec ly used in ca bona a ion
p ocesses, o i can be con e ed o o he compounds by means
o elec ochemical educ ion, pho o educ ion o educ ion wi h
H
2
, among o he al e na i es. Howe e , he high he mody-
namic s abili y o he CO
2
molecule mus be o e come o ob ain
high con e sion a es.
10–14
Hyd o he mal educ ion has been conside ed as an al e -
na i e o he con e sion o CO
2
because o he peculia p op-
e ies o wa e a high empe a u e and p essu e, such as low
dielec ic cons an , densi y and iscosi y. Wa e a empe a u es
a
Resea ch Ins i u e on Bioeconomy –BioEcoUVa, P essTech G oup, Uni e sidad de
Valladolid, D Me gelina s/n., Valladolid, 47011, Spain. E-mail: ma iadolo es.
[email p o ec ed]
b
Depa men o Chemical Enginee ing and En i onmen al Technology, Uni e sidad de
Valladolid, D Me gelina s/n., Valladolid, 47011, Spain
c
School o Chemical, Ma e ials and Biological Enginee ing, Uni e si y o Sheffield,
Sheffield S1 3JD, UK
†Elec onic supplemen a y in o ma ion (ESI) a ailable. See DOI:
h ps://doi.o g/10.1039/d4su00440j
Ci e his: RSC Sus ainabili y,2025,3,
822
Recei ed 4 h Augus 2024
Accep ed 16 h No embe 2024
DOI: 10.1039/d4su00440j
sc.li/ scsus
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Sus ainabili y
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nea 300 °C and p essu es high enough o main ain i in he
liquid s a e is cha ac e ized by a high ionic p oduc and he e-
o e a high concen a ion o H
+
and HO
−
ions, which can
p omo e acid/base ca alysed eac ions.
The hyd o he mal educ ion o CO
2
has been pa icula ly
success ul in o ming o mic acid when s a ing wi h CO
2
dis-
sol ed in sligh ly basic solu ions as bica bona es o ca bama es,
such as hose o med when CO
2
is cap u ed by amines o
ammonia. These p ocesses ha e he ad an age ha he well-
known abso p ion CO
2
cap u e p ocess can be associa ed wi h
an in si u eac ion p ocess in which he eed o he eac o is he
effluen o he abso p ion column, wi hou he in e media e
sepa a ion s eps and hei associa ed cos s.
The eac ion can be pe o med using me als such as Al, Fe o
Zn as educ an s
14–17
o di ec ly wi h gaseous H
2
.
13,18
Yields o
o mic acid can be highe han 60% in esidence imes o 1–2h
using bica bona e a empe a u es be ween 250 °C and 300 °C. In
combina ion wi h ca alys s and using gaseous H
2
as educ an ,
eac ion empe a u e can be educed o empe a u es nea 200 °C
( e . 14,17) o e en o 120 °C in he case o ammonium ca bama e
o CO
2
cap u ed by amines.
13,18
Among he ca alys s s udied, Pd/C
ca alys s ha e p esen ed an especially good pe o mance.
13,14
Jin e al. we e he  s o p o e ha CO
2
cap u ed as bica -
bona e could also be educed o o mic acid using o ganic
subs ances wi h alcohol g oups such as glyce ine o
isop opanol.
19–21
This opened he possibili y o simul aneously
ans o m CO
2
and biomass, as i is well known ha lignocel-
lulosic biomass can be dissol ed and hyd olysed in hyd o-
he mal media o o m suga s such as glucose and o he
subp oduc s wi h alcohol g oups.
22–27
In 2018, And´
e ez e al.
showed ha a numbe o compounds de i ed om he hyd o-
he mal decomposi ion o lignocellulosic biomass could eac
wi h bica bona e a 300 °C, p essu es abo e 20 MPa o ensu e
liquid phase condi ions and a esidence ime o 2 h o p oduce
o mic acid.
28
Fu he s udies made wi h glucose as model
o ganic educ an showed ha pa o he o mic acid p oceeded
om ino ganic CO
2
and he o he was o igina ed by glucose
decomposi ion, and ha his combined eac ion inc eased he
selec i i y o he ans o ma ion o glucose o o mic acid.
29
In
2022, Chinchilla e al.
30
showed he effec o se e al ca alys s in
he p oduc ion o o mic acid in solu ions o sodium bica -
bona e. Yields o o mic, lac ic and ace ic acid we e highe using
sodium bica bona e as ino ganic ca bon sou ce han wi h
ammonium ca bama e, and he global yield was no imp o ed o
only sligh ly imp o ed wi h he ca alys s es ed. Howe e , wi h Pd
and Ru suppo ed in ca bon, he p opo ion o o mic acid
coming om ino ganic CO
2
inc eased.
The possibili y o pe o m he educ ion o CO
2
wi h biomass
esidues allows o alo ise lignocellulosic ma e ial ha is
commonly conside ed as esidue. To con e hese esidues in o
p oduc s o high added alue p omo ing he eu iliza ion o
biomass ma e ial and he educ ion o CO
2
emissions in o he
a mosphe e is becoming o g ea in e es . Some au ho s a e
implemen ing mic oalgae esidues om biodiesel ex ac ion o
molecules de i ed om p o eins o be u ilized as educing
agen s o he con e sion CO
2
cap u ed.
31,32
And´
e ez e al.
33
p esen ed esul s o he simul aneous con e sion o CO
2
and
biomass (pine needles and suga cane bagasse) a empe a u es
up o 275 °C, esul ing in low yields o o mic, lac ic and ace ic
acid. Ne e heless, he e is possibili y o imp o e he yields o
eac ion o o mic by adding ca alys ha p omo e he hyd o-
gena ion o he CO
2
cap u ed. Also, in he p e ious wo k he
o igin o he o mic acid was no in es iga ed. Thus, o ad ance
in his in es iga ion, a sys ema ic s udy encompassing a wide
ange o biomass educ an s and ca alys s and assessing he
o igin o he o ganic p oduc s (ei he he ino ganic CO
2
sou ce
o biomass decomposi ion) is equi ed.
In he p esen wo k, he con e sion o sodium bica bona e a
300 °C using diffe en biomass educ an s (mic oc ys alline
cellulose, sowood, pine needles, suga cane bagasse, e mi-
compos and suga bee and co k was e) and ca alys s (g anula
and powde ac i a ed ca bon, Pd(5%)/C and Pd(10%)/C and
Ru(5%)/C) is s udied. The o igin o o mic acid p oduced in
eac ions a 200 °C and 300 °C is e alua ed pe o ming expe i-
men s wi h labelled H
13
CO
3
−
as ino ganic ca bon sou ce, wi h
Pd(5%)/C as ca alys and he diffe en biomasses es ed. This
esea ch aims a assessing he bes p ope ies o biomass o ac
as CO
2
educ an in he hyd o he mal p ocess, as well as he
bes combina ions o educ an and ca alys .
2. Ma e ial and me hods
2.1. Ma e ials
All solu ions we e p epa ed wi h deionized wa e (conduc i i y 2
mScm
−1
). Sodium bica bona e (99.7%), sodium bica bona e
13C (98.5%), ace ic acid (99.7%), lac ic acid (88%), o mic acid
(97.5%), galac u onic acid (99%), D-glucose (99%), glycolic acid
(99%), glyce in (99.5%), o maldehyde (37%), e hyleneglycol
(99.8%), 1–2 p opanediol (99.5%), ac ylic acid (99%), e hanol
(99.8%), ace one (99.5%), 5-HMF (99%), mic oc ys alline
cellulose, lignin (99%), u u al (99%) and sul u ic acid (96%)
we e pu chased om Sigma-Ald ich. Ac i a ed ca bon and
me al suppo ed ca alys s (Pd(5%)/C, Pd(10%)/C) we e acqui ed
om Sigma-Ald ich, Ru(5%)/C (50% we pas e) was pu chased
om S em Chemicals.
Xylose (99%), a abinose (99%) and glucose (99%) we e used
(Sigma-Ald ich) as s anda ds o measu ing suga s ca bohy-
d a es and lignin in biomass.
Biomass ma e ials used as educing agen s we e ob ained
om diffe en sou ces: pine needles and suga cane bagasse
(kindly p o ided by p o . James McG ego , Uni e si y o She -
eld), suga bee (kindly p o ided by AB Azuca e a Ibe ia
S.L.U.), e micompos (elabo a ed and kindly p o ided by Eyup
Yildi i om Usak Uni e si y, Tu key) and sowood chips
(pu chased om Vi ak aS.L.).
2.2. P epa a ion o biomass
All biomasses we e g ounded and sie ed in a mesh o 100 mm.
The siing ob ained was used o ca y ou he expe imen s.
2.3. Humidi y
The mois u e con en o all samples was measu ed wi h a Sa o-
ius Model MAT 160. 80 °C was se as he d ying empe a u e.
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2.4. Ba ch hyd o he mal eac ions
Solu ions o sodium bica bona e (SB) 0.5 M and SB
13
C 0.5 M
we e p epa ed g a ime ically in deionized wa e . 4.5 cm
3
o
his solu ion was cha ged in each high-p essu e ba ch eac o .
In expe imen s, addi ionally 0.1 g o ca alys and 0.1 g biomass
we e cha ged in he eac o . Mass a io ca alys o biomass was
always 1 : 1, excep in he expe imen s in which he inuence o
he amoun o biomass was s udied in which amoun s o 1, 0.05
and 0.025 g o biomass we e used, bu he amoun o ca alys
was kep cons an in 0.1 g.
The eac o s used consis o s ainless-s eel high-p essu e
ubing o 1
2
00 diame e and 10 cm
3
o inne olume, closed wi h
high p essu e caps in he end.
14,28,30
A he mocouple was
ins alled in he eac o o measu e he inne empe a u e o he
sys em. Fo sa e y easons and o a oid o e p essu e du ing
hea ing, eac o s we e lled up wi h liquids only up o 45% o
hei olume. The nal p essu e is hus he apo p essu e o
he wa e o he solu ion, plus he p essu e gene a ed by he
gases eleased. Thus, a ac ion o eac o olume no lled wi h
liquid is necessa y o a oid o e p essu e due o his possible gas
elease. P essu e was no measu ed inside o he eac o . The
p essu e inside he eac o s was es ima ed as he sa u a ion
p essu e o wa e a he wo king empe a u e.
Once lled and closed, he eac o s we e in oduced in
a ho izon al posi ion in a cus om-made uidized bed o en
al eady p ehea ed a he ope a ional empe a u e. This o en
consis s o a bed o alumina uidized by ai and con aining
elec ical esis ances. I can wo k a empe a u es as high as
400 °C, and he uidiza ion allows a apid and uni o m hea ing.
Expe imen s we e ca ied ou a 200 °C, 250 °C and 300 °C. In
expe imen s pe o med in a eac o equipped wi h a he mo-
couple i has been de e mined ha he in e io o he eac o
eaches he ope a ion empe a u e in 6–7 minu es. The eac o s
a e kep inside he o en o 2 h and hen, he eac ion was
s opped by quenching he eac o in cold wa e a 10 °C.
Once cooled, he eac o s we e opened. No app eciable amoun
o gases was collec ed, hus he ini ial assump ion ha he inne
p essu e o he eac o is he apo p essu e o wa e is co ec .
All expe imen s we e epea ed a leas wice. The unce ain y
in he yields was es ima ed wi h he s anda d de ia ion o
eplica ions.
The yield o he desi ed p oduc s was calcula ed by di iding
he mola concen a ion o he p oduc by he ini ial concen-
a ion o SB o SB
13
C as indica ed in eqn (1).
YP oduc ¼CP oduc
CNaHCO3;ini ial
100% (1)
Al hough only pa o he o mic acid comes om bica -
bona e and he lac ic and ace ic acid come en i ely om
biomass, he yields ha e been exp essed wi h espec o bica -
bona e o acili a e compa ison.
2.5. Analysis o solid samples o biomass
2.5.1. To al solids con en . To de e mine he o al solid
con en o biomass samples, 2 g o solids suppo ed in an
aluminum pan we e placed in a con ec ion o en a 105 °C o 24
hou s. The weigh o solids was eco ded be o e and ae he
ea men . The pe cen ages o o al solids (%TS) we e calcula ed
wi h eqn (2), Min all equa ions ep esen mass weigh ed in
g ams.
%TS ¼MD y panþd y sampleMD y pan
MSample as ecei ed100% (2)
2.5.2. S uc u al ca bohyd a es and insoluble lignin.
300 mg o he biomass sample p e iously d ied a 105 °C we e
used. Each o he weigh ed samples we e pu in p essu e ubes.
Then, 4.92 g o H
2
SO
4
72% we e added in o each ube o each
sample. These ubes we e closed and hen placed in o a wa e
ba h a 30 °C o 1 hou . Ae his, 84 mL o wa e we e added
in o each ube, and hen hey we e pu in a silicon ba h a 120 °C
o 1 h. The same p ocedu e was ca ied ou o analyze he
s anda ds. 60 mg o xylose, a abinose and glucose we e used as
s anda ds. Once hyd olysis o samples ook place, he solid and
liquid phase we e sepa a ed by l a ion. 5–10 mL o he liquid
phase we e aken o neu aliza ion o pH 6–7 wi h CaCO
3
.2mL
o he liquid phase we e hen aken and l e ed wi h 0.22 mm
nylon sy inge l e and la e analyzed in HPLC.
The solid phase was collec ed and placed in c ucibles and
loca ed in he u nace a 575 °C. Samples we e ea ed o 24
hou s. Samples we e weighed be o e and ae he ea men .
The ash con en was calcula ed as indica ed in eqn (3).
%Ash ¼MD y panþashMD y pan
MD y panþd y sampleMD y pan100% (3)
The pe cen age o acid insoluble lignin (%AIL) was calcu-
la ed as shown in eqn (4).
%AIL ¼MSample as ecei ed MD y panðMC ucibleþAsh MC ucibleÞ
MD y panþd y sampleMD y pan
100%
(4)
The pe cen age o s uc u al ca bohyd a es wi h  e and six
ca bon a oms ae he hyd olysis o biomass was calcula ed as
shown in eqn (5) and (6), espec i ely.
%C5 ¼ MA abinose om HPLCþMXilose om HPLC
MD y panþd y sampleMD y pan!100% (5)
%C6 ¼MGlucose om HPLC
MD y panþd y sampleMD y pan100% (6)
2.6. Analysis o solid samples o ca alys s
2.6.1. X- ay diff ac ion es (XRD). X- ay diff ac ion es s o
diffe en ca alys s we e ca ied ou wi h a B uke D8 Disco e
A25 diff ac ome e , equipped wi h a 3 kW gene a o and a 2.2
kW coppe ce amic ube ype FFF. The de ec o used was he
LynxEye 40 kV 30 mA.
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2.6.2. Scanning elec on mic oscope (SEM) images. SEM
images we e aken in o de o obse e he ex e nal s uc u e o
he ma e ial. A Hi achi FlexSEM1000 equipmen was used.
Samples we e p e iously p epa ed by co e ing he su ace wi h
a 10 nm laye o gold by using a Balze s SCD004 gold e apo a o
ope a ed a 30 mA. The samples we e in he e apo a o o 40
seconds.
BSEs (Backsca e ed elec ons) o SEM images was ca ied
ou a 15 kV and a low acuum o 30 Pa in a “FlexSEM1000”o
he HITACHI b and.
2.6.3. A omic spec oscopy. To e alua e he eu iliza ion o
he me al suppo ed ca alys s, he concen a ion o he me al in
he ca alys was measu ed wi h a omic spec oscopy by ICP-
OES. 50–100 mg o pul e ized sample was diges ed wi h 4 mL
HNO
3
and 1 mL HF. Samples we e diges ed in an E hos Sel
diges e . The p ocess was ca ied ou in wo s eps: a  s s ep a
25 min, 250 °C, and 1500 W; ollowed by a second diges ion a
10 min, 250 °C, and 1500 W. Subsequen ly, 350 mg o H
3
BO
3
is
added and ca ied again in E hos Sel diges e unde he
ollowing condi ions:  s a 10 min, 160 °C, and 1500 W; and
hen a second diges ion o 5 min, 160 °C, and 1500 W.
These las s ages allow he o al libe a ion o F
−
o he
solu ion o a oid he o ma ion o insoluble uo ides and
co osion o he qua z pa s o he ICP used. Finally, ae
diges ion, he sample is dilu ed wi h MQ wa e and l e ed wi h
a 0.45 mml e .
To measu e concen a ion he diges ed sample was in o-
duced in a Mic owa e-ULTRAWAVE (Miles one) and 725-ES ICP-
Op ical Emission Spec ome e (Agilen ).
2.6.4. BET su ace. The su ace a ea o he ca alys s was
de e mined wi h N
2
adso p ion–deso p ion iso he ms in an
Au oso b IQ (Quan ach ome Ins umen s) a 1 ba .
2.6.5. T ansmission elec on mic oscope (TEM) images.
TEM images we e aken wi h a JEOL JEM-FS2200 HRP equip-
men ea u ed wi h a 200 kV Field Emission TEM mic oscope.
Wi h accele a ing ol ages: 80, 100, 120, 160 and 200 kV, and
high esolu ion HRP pola pa (spo : 0.23 nm, line: 0.1 nm).
2.7. Analysis o liquid samples
Liquid samples coming om he hyd o he mal eac ion we e
collec ed and l e ed wi h 0.22 mm nylon l e o be u he ly
analyzed by HPLC o NMR. The gas phase was no analyzed, as
in all expe imen s no gas o a e y small amoun o gas was
p oduced, and i was no possible o collec i .
2.7.1. High p essu e liquid ch oma og aphy (HPLC). All
liquid samples we e analyzed in an HPLC Wa e s® (sepa a ion
module Aliance e2695) wi h RI de ec o (Wa e s, module 2414)
using a Phenomex Rezex ROA O ganic Acid H
+
column.
Tempe a u es o column and de ec o we e 40 °C and 30 °C,
espec i ely. As mobile phase, a 0.5 mL min
−1
ow o 0.25 mM
H
2
SO
4
was used.
Fo s uc u al ca bohyd a es analysis, a Shodex® column
was used. Tempe a u es o column and de ec o we e 60 °C and
30 °C, espec i ely. As mobile phase, a 0.8 mL min
−1
ow o
0.25 mM H
2
SO
4
was used.
2.7.2. Nuclea magne ic esonance (NMR). The samples
ob ained wi h SB
13
C we e analyzed wi h a 500 MHz Agilen
ins umen equipped wi h OneNMR p obe. The acquisi ion
pa ame e s o
13
C NMR spec a we e: 25 °C, 70 s elaxa ion
delay be ween ansien s, 45° pulse wid h, spec al wid h o 31
250 Hz, a o al o 16 ansien s and 1.048 s acquisi ion ime. The
in e se ga ed decoupling echnique o supp ess he nuclea
O e hause effec (NOE) was used o ob ain quan i a i e
measu emen s. The acquisi ion pa ame e s o
1
H NMR spec a
we e: 25 °C, 70 s elaxa ion delay be ween ansien s, 90° pulse
wid h, spec al wid h o 8012.8 Hz, a o al o 4 ansien s and
2.044 s acquisi ion ime. The PRESAT sequence was used in
o de o supp ess he s ong signal o wa e .
1
H and
13
C NMR chemical shis(d) we e epo ed in pa s
pe million (ppm) and e e enced o e ame hylsilane (TMS).
Fo all he samples, a capilla y ube lled wi h chlo o o m was
inse ed in o he NMR ubes o assu e lock condi ions.
3. Resul s
3.1. Biomass cha ac e iza ion
In Table 1, he measu ed p ope ies o he biomass samples
be o e he eac ion a e epo ed. Table 2 p esen s he esul s o
he elemen al analysis o hese samples.
3.2. Ca alys s cha ac e iza ion
In Fig. 1, he SEM images o he physical s uc u e o he ca a-
lys s is p esen ed. I can be obse ed ha C g anula (Fig. 1(a)),
C powde (Fig. 1(b)), Pd(5%)/C (Fig. 1(c)), and Pd(10%)/C
(Fig. 1(d)) p esen ed an inne ubula s uc u e. On he o he
hand, Ru(5%)/C (Fig. 1(e)) p esen s an amo phous s uc u e.
In Fig. 2 XRD pa e ns o all he ca alys s be o e eac ion
expe imen s a e p esen ed. In Fig. 2(a), he ypical XRD pa e n o
ac i a ed ca bon in powde o m p esen ing wo b oad diff ac-
ion peaks a ound 24°–26° and 42°–47° a e obse ed.
34
All ca a-
lys s a e ca bon-based ma e ials, hus hese wo wide peaks a e
epea ed in all he XRD pa e ns o all s udied ma e ials. XRD
Table 1 P ope ies o biomass be o e eac ion
Biomass % Humidi y % To al solid con en % Ash % Acid insoluble lignin % C5 % C6
Sowood 4.8 95.2 0.6 30 18 33
Suga cane bagasse 6.6 93.4 1.1 25 23 31
Suga bee 8.1 91.9 1.8 36 16 23
Co k 3.7 96.3 0.1 75 6 4
Pine needles 6.3 93.7 0.1 44 10 16
Ve micompos 4.6 95.4 38.7 29 2 2
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pa e n o C g anula (Fig. 2(b)) p esen s se e al na ow peaks
ha co espond o c ys alline s uc u es ha a e iden ied as
es s o calci e and qua z ha can come om he o igin sou ce
o his ype o ca bon. Pd(5%)/C and Pd(10%)/C (Fig. 2(c) and (d))
ca alys s p esen ed wo na ow peaks a ound 40° and 47° ha
co esponded o he ace-cen e ed cubic s uc u e o Pd sup-
po ed.
34
Fo Ru(5%)/C na ow peaks a ound 44° co espond o
Ru diff ac ion planes, he o he na ow peaks co esponded o
es s o qua z emaining in he ca bon suppo .
35,36
In Table 3 he diame e and BET su ace o he ma e ials is
p esen ed. The C g anula p esen ed he lowe su ace a ea in
compa ison wi h he es o he ma e ials.
3.3. Inuence o he ca alys in he simul aneous eac ion o
biomass and CO
2
The main p oduc s ob ained in all eac ions a e o mic acid (FA),
ace ic acid (AA), lac ic acid (LA). Glyce aldehyde, glycolaldehyde,
o maldehyde, e hylene glycol, ace one, py u aldehyde, galac u -
onic acid, 5-HMF, among o he s, we e de ec ed o a lesse ex en .
All hese componen s a e ypical p oduc s de i ed om biomass
decomposi ion in hyd o he mal media.
22,23
The expe imen s o
de e mine he inuence o he diffe en ca alys s o he diffe en
biomass we e ca ied ou a a empe a u e o 300 °C wi h 0.1 g o
ca alys and 0.1 g o biomass and 2 h o eac ion ime. The 2 h
esidence ime was selec ed based on p e ious li e a u e esul s
which sugges ha sho e eac ion imes a e less effec i e in
enhancing o mic acid p oduc ion.
19,20,28–30,33
The FA yields ob-
ained unde hese condi ions a e shown in Fig. 3(a) (go o
Sec ion 1.1, 2.1, and 3.1 o he ESI o see he abula ed da a†).
I is obse ed ha , using Pd(5%)/C ca alys , o mos samples
o biomass, he global FA yield was highe han wi hou ca alys ,
in con as o he esul s obse ed by Chinchilla e al.
30
in
expe imen s wi h glucose as model o ganic educ an a 200
and 250 °C. In he cases o g anula and powde ca bon, he
yields o o mic acid a e o he same o de han wi hou ca alys
and in he case o Ru(5%)/C hey a e much lowe . The bes
esul s we e ob ained wi h Pd(5%)/C ca alys , wi h yields ha
we e e en highe han hose using Pd(10%)/C. The highes yield
o FA ob ained was 18%. This yield was ob ained using pu e
cellulose as a biomass de i a i e (black ci cles), and wi h he
Pd(5%)/C ca alys ; ollowed by a 16% yield also ob ained wi h
cellulose and Pd(10%)/C (black ci cles). Wi h he sowood
biomass, a maximum FA yield o 12% and 10% was ob ained
using Pd(5)%/C and Pd(10%)/C (na y blue c oss), espec i ely.
The yield o o mic acid is lowe o co k (less han 6%) and pine
needles (less han 7%). I we compa e i wi h he composi ion o
he diffe en biomass p esen ed in Table 3, i is obse ed ha
samples o biomass ich in cellulose p esen ed highe yields o
Table 2 Elemen al analysis o biomass be o e eac ion
Biomass
Elemen
H/C O/C%N %C %H %O
Sowood 0.00 48 6.1 43.3 1.5 0.7
Suga cane bagasse 3.4 44.9 5.6 42.2 1.5 0.7
Suga bee 0.2 46.9 5.7 41.5 1.4 0.7
Co k 0.4 60.5 7.6 27.5 1.5 0.3
Pine needles 2.0 48.1 5.9 37.7 1.5 0.6
Ve micompos 1.8 24.1 2.6 20.8 1.3 0.6
Fig. 1 SEM o ca alys be o e eac ion: C g anula (a), C powde (b),
Pd(5%)/C (c), Pd(10%)/C (d), Ru(5%)/C (e).
Fig. 2 XRD pa e ns o ca alys s be o e use: C powde (a), C g anula
(b), Pd(10%)/C (c), Pd(5%)/C (d), Ru(5%)/C (e).
Table 3 Physical p ope ies o ca alys s be o e eac ion
Ca alys A e age diame e BET su ace (m
2
g
−1
)
C g anula 2 mm 75
C powde 55 mm 650
Pd(5%)/C 45 mm 948
Pd(10%)/C 45 mm 880
Ru(5%)/C 50 mm 844
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con e sion o o mic acid. The biomass wi h he lowes yield o
FA was e micompos (less han 2% and no imp o ed by using
any ca alys ), p obably due o he high amoun o ashes and he
low amoun s o C5 and C6 compounds (see Table 2).
The esul so heyieldso ace icacidob ainedin he
expe imen s a e p esen ed in Fig. 3(b). I is obse ed ha , in
mos cases, he yield o ace ic acid imp o ed wi h he use o
ca alys s and in gene al p esen ed simila alues o he
diffe en biomass samples, wi h he bes esul s again being
achie ed wi h Pd(5%)/C. I is obse ed ha he suga bee
p oduced lowe ace ic acid yields wi h powde and g anula C
ca alys s. Co k p esen ed lowe alues o Pd and Ru ca alys s
and sowood p esen ed lowe yields o ace ic acid wi h
Ru(5%)/C ca alys . The biomasses wi h which he highes
amoun s o ace ic acid we e ob ained a e suga cane bagasse
(4–6%), suga bee (3.5–5%) and pine needles (3–4%). The
Fig. 3 Effec o diffe en ype o ca alys s in he yields o o mic acid (a); ace ic acid (b) and lac ic acid (c) in he eac ion o HCO
3
−
wi h biomass.
Condi ions o eac ion: mass a io ca alys : biomass 1 : 1, 0.1 g o ca alys , 0.5 M NaHCO
3
, 45% filling olume o eac o , 300 °C, 2 h.
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biomass ha p oduced he lowes yield o ace ic acid was, in
gene al, cellulose (2%). This can be explained because ace ic
acid is p oduced om he hyd olysis and oxida ion o he
suga s gene a ed by he hyd olysis o hemicellulose and
cellulose, bu , addi ionally, he dissolu ion o hemicelluloses
eleases ace yl g oups ha a e pa o he hemicellulose
s uc u e.
37,38
Fu he mo e, he elease o acid inc emen s he
hyd olysis eac ion a e due o an au oca aly ic effec .
39
Pd/C
and Ru/C a e some o he he e ogeneous ca alys s mos used
o hyd ogena ion applica ions. The high su ace a ea (a ound
850 o 950 m
2
g, see Table 3) han allows palladium and
u henium o dispe se uni o mly on he solid suppo , his
p o ide high ac i i y o he ca alys , which can explain he high
yields o FA ob ained when Pd(5%)/C is used in he eac ion.
Howe e , Ru/C decompose o mic acid and hyd ogena e o he
compounds such as le ulinic acid in acid media,
36
his
decomposi ion can be p omo ing he lowe yields o o mic
acid ob ained in he eac ion a 300 °C wi h his ca alys .
Fig. 4 Effec o a ia ion o empe a u e in he yields o o mic acid (a); ace ic acid (b) and lac ic acid (c) in he eac ion o HCO
3
−
wi h biomass.
Condi ions o eac ion: mass a io Pd(5%)/C : biomass 1 : 1, 0.1 g o ca alys , 0.5 M NaHCO
3
, 45% filling olume o eac o , a 200 °C, 250 °C,
300 °C, 2 h.
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Rega ding C powde and C g anula , hese ma e ials ha e low
BET su ace (75 and 650, espec i ely, see Table 3) which
educe he ac i i y o he ma e ial. Besides he low BET su ace,
i is no ed ha he addi ion o he me al in he ca bon suppo
enhances he ac i i y o he ca bon ma e ials.
Fig. 3(c) shows he lac ic acid yields. I can be obse ed ha
he highes lac ic acid yields we e ob ained when using g an-
ula C as ca alys and suga cane bagasse (15%) and bee oo
(16%) as biomass. I is obse ed ha he LA yield sligh ly
inc ease when using g anula C in biomasses ich in suga
such as bee ( om 14 o 16%) and suga cane bagasse ( om 9 o
15%). This is possibly due o he p esence o saccha ose (a
dime made o glucose and uc ose), which decomposes as e
han s uc u al ca bohyd a e, and uc ose eleased, ha is
apidly con e ed o lac ic acid p omo ed by he sligh ly basic
media.
40
In he case o e micompos and co k i is obse ed
ha he LA yield did no change signican ly when using
adiffe en ca alys .
Fig. 5 Effec o diffe en amoun o biomass in he yields o o mic acid (a); ace ic acid (b) and lac ic acid (c) in he eac ion o HCO
3
−
wi h
biomass. Condi ions o eac ion: mass a io Pd(5%)/C : biomass 1 : 1, 2 : 1, and 4 : 1, 0.1 g o ca alys , 0.5 M NaHCO
3
, 45% filling olume o eac o ,
300 °C, 2 h.
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3.4. Inuence o he empe a u e in he simul aneous
eac ion o biomass and CO
2
The expe imen s o de e mine he inuence o he eac ion
empe a u e o he diffe en biomass we e ca ied ou using
Pd(5%)/C as ca alys , as his ga e he highes yield o o mic
acid, as epo ed in Sec ion 3.2. The aim was o de e mine i he
use o he ca alys could make possible he ope a ion a lowe
eac ion empe a u es. Tempe a u es o 200, 250 and 300 °C
we e conside ed. 0.1 g o ca alys and 0.1 g o biomass we e used
and he eac ion ime was xed again in 2 h (go o Sec ion 1.3,
2.3 and 3.3 o he ESI o see he abula ed da a†).
The yields o o mic acid a diffe en empe a u es a e shown
in Fig. 4(a). I can be obse ed ha he ha he highe he
eac ion empe a u e, he highe he yield o o mic acid ob-
ained. Fo example, using pu e cellulose, which is he biomass
de i a i e ha p oduces he highes FA yield, a yield o FA o
18% a 300 °C is ob ained, a alue ha d ops down o almos 4%
a 200 °C. In he case o sowood, he yield alls om 11% a
300 °C o 1% a 200 °C. Simila beha io was obse ed in
expe imen s wi h o he biomass educ an s.
Fig. 4(b) shows ha in mos cases he AA yield inc eases
sligh ly by inc easing empe a u e. In he case o pine needles,
he yield o AA emained app oxima ely cons an in he
empe a u e ange es ed. In he case o suga bee esidues and
suga cane bagasse, i was obse ed ha he AA yield was highe
when inc easing empe a u e om 200 °C o 300 °C (inc easing
om 3 o 5% and om 3 o 4% espec i ely). The same esul
was obse ed wi h cellulose, sowood, co k, and e micompos
(inc easing om 0 o 2%, 1 o 6%, 2 o 3%, 1 o 2%).
Wi h espec o he yield o lac ic acid (Fig. 4(c)), i was
obse ed in mos cases ha his yield inc eased by inc easing
empe a u e, excep when using suga bee , wi h which he
maximum yield o lac ic acid was achie ed a 250 °C. This
inc ease was conside able in he cases o suga cane bagasse and
pine needles ( om 2 o 9% and om 1 o 6.5%, espec i ely).
Wi h e micompos , cellulose, co k and sowood, he LA yield
inc eased sligh ly by inc easing empe a u e ( om 0 o 1.5%,
om 3 o 4%, om 1.5 o 3%, and om 1 o 4%, espec i ely).
3.5. Inuence o he amoun o biomass in he simul aneous
eac ion wi h CO
2
The effec o he amoun o biomass added in he expe imen ,
o a cons an amoun o bica bona e was s udied wi h 0.1 g
Pd(%5)/C as ca alys a 300 °C, using 0.025, 0.05 o 0.1 g o
biomass, a eac ion imes o 2 h. The yields o o mic, lac ic and
ace ic acids ob ained in hese condi ions a e shown in Fig. 3 (go
o Sec ion 1.2, 2.2, and 3.2 o he ESI o see he abula ed da a†).
Fig. 5(a) shows ha , o cellulose, pine needles and co k, he
yield o o mic acid inc eased wi h he amoun o biomass
added. Fo example, wi h 0.1 g o cellulose, a yield o 18% was
p oduced, while wi h 0.025 g he yield was 7%. In he case o
bee and suga cane bagasse, he FA yield inc eased sligh ly
when using 0.05 g o biomass ins ead o 0.1 g ( om 9.3% and
9.5%, o 8.5 and 9.1%, espec i ely). Fo e micompos , he
highes yield (4%) was ob ained when a smalle amoun o
biomass o 0.025 g was added in he eac o . And´
e ez e al.
29
ound ha he yield o he o mic acid dec eases when
inc easing he biomass (glucose), ne e heless he same
au ho s
33
ound his effec less impo an when using as
biomass suga cane bagasse in amoun s simila o ha used in
his wo k a 250 °C.
Fig. 5(b) shows ha , using Pd(5%)/C as a ca alys , in he case
o cellulose, sowood, pine needles, and e micompos , he AA
yield inc eases sligh ly ( om 0 o 2%, 1 o 2%, 1 o 3%, and 0 o
2%) as he amoun o biomass inc eases. In he case o bee
esidues, suga cane bagasse, and co k, a highe p oduc ion o
AA is obse ed wi h he inc ease in he amoun o biomass
( om 1 o 5%, om 1 o 6% and om 1 o 4% espec i ely).
In Fig. 5(c), i is obse ed ha when using Pd(5%)/C as
a ca alys , he LA yield inc eased as he amoun o biomass
inc eased in he expe imen s. The inc emen was highe when
using pine needles, suga cane bagasse and suga bee ( om 0 o
9%, om 1 o 7.5% and om 1 o 6.5% espec i ely). The
inc emen in he LA yield o co k and cellulose was lowe ( om
0 o 4% and om 0 o 2.5%), while i was negligible o e mi-
compos samples ( om 0 o 1.5%).
3.6. O igin o he o mic acid a diffe en empe a u es and
ype o biomass
Acco ding o p e ious s udies,
29,30
FA can be o med ia wo
ou es, om he oxida ion o biomass compounds o om he
educ ion o sodium bica bona e (SB) (CO
2
sou ce). NMR
spec oscopy was used o de e mine how much o mic acid
comes om each species. The expe imen s we e ca ied ou
wi h NaH
13
CO
3
(SB
13
C), which is an iso ope o SB. The NMR
ca bon spec a in a ypical expe imen is shown in Fig. 6 (go o
Sec ion 4 o he ESI o see he NMR spec a†).
In he
13
C NMR spec um only he compounds de i ed om
he educ ion o CO
2
cap u ed as SB
13
C a e obse ed. The
p oduc s obse ed we e ace ic acid (d=183) and o mic acid (d
=173). The peak a d=163 co esponds o SB
13
C ha did no
eac and he peak a d=127 co esponds o
13
CO
2
dissol ed in
he sample. The peak a d=183, co esponding o ace ic acid,
does no indica e ha ace ic acid is a p oduc o eac ion o
Fig. 6 NMR ca bon spec a. Condi ions o eac ion: mass a io Pd(5%)/
C : cellulose 1 : 1, 0.1 g o ca alys , 0.5 M NaH
13
CO
3
, 45% filling olume
o eac o , 300 °C, 2 h.
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