CO
2
cap u e om wa e using a coppe /ch omium-adenine
sup amolecula ly assembled po ous me al-o ganic ma e ial
Ekain Maiza-Razkin
a
, Ga ikoi z Beobide
a,b
, Osca Cas illo
a,b,*
, An onio Luque
a,b
,
Rub´
en P´
e ez-Agui e
a,**
, Sonia P´
e ez-Ya˜
nez
a,b
a
Depa amen o de Química O g´
anica e Ino g´
anica, Facul ad de Ciencia y Tecnología, Uni e sidad del País Vasco/Euskal He iko Unibe si a ea, UPV/EHU, Leioa E-
48940, Spain
b
BCMa e ials, Basque Cen e o Ma e ials, Applica ions and Nanos uc u es, UPV/EHU Science Pa k, Leioa E-48940, Spain
ARTICLE INFO
Keywo ds:
Po ous me al-o ganic ma e ials
CO
2
cap u e
Adso p ion om wa e
Adso p ion quan i ica ion echniques
ABSTRACT
A sup amolecula me al-o ganic po ous ma e ial [C Cu
6
(
μ
-adenina o-кN3:кN9)
6
(
μ
-OH)
6
(
μ
-OH
2
)
6
](SO
4
)
1.5
(Cu
6
C ) has been es ed o CO
2
cap u e om wa e . The p ope ies o his compound, insolubili y in wa e , a
lexible sup amolecula s uc u e, and p o onable posi ions on he adenina o ligands, make i a po en ial
candida e o his ask. The expe imen al de e mina ion o CO
2
cap u e om wa e was pe o med using wo
echniques: magne ic sus en a ion, o i s ime, and g a ime ic measu emen s. Bo h echniques e i ied he
CO
2
cap u e p o iding complemen a y in o ma ion. The magne ic sus en a ion echnique measu es he mass
being inco po a ed in o he po ous ma e ial (which depends on he chemical o m in which is being cap u ed:
physiso p ion, HCO
3
-
o ca bama e), whe eas he g a ime ic measu emen quan i ies he o al mass o CO
2
being
cap u ed in he aqueous suspension o he Cu
6
C pa icles ega dless he chemical o m in which his cap u e
akes place. A e 1 hou o CO
2
bubbling (300 mL⋅min
−1
) in o he aqueous suspension o Cu
6
C pa icles,
cap u e mass alues o 22.3 % and 17.1 % a e measu ed by magne ic sus en a ion and g a ime ic echniques,
espec i ely. This di e ence is because he CO
2
is cap u ed as H
2
CO
3
ha eac s wi h he adenina o ligands o
o m adenine/HCO
3
-
pai s. These alues no malize o 5.9 HCO
3
-
and 6.4 CO
2
molecules pe Cu
6
C en i y, which
a e close o he heo e ical alue o 6, because o he six adenina o ligands pe hep ame ic Cu
6
C en i y. CO
2
adso p ion iso he ms, adso p ion/deso p ion kine ics and cycling s abili y a e also epo ed. Kine ic s udies
p o ide a ΔH
ads
=-19.4 kJ/mol, which is a signi ican ly lowe han o he CO
2
adso ben s. Howe e , he cycling
s abili y needs o be imp o ed.
1. In oduc ion
The inc ease o CO
2
in he a mosphe e seems o be he one o he
main d i ing o ce behind he empe a u e ise ha ou plane is
cu en ly expe iencing [1]. The e o e, de elopmen o ma e ials and
echnologies o he cap u e [2], s o age [3] and/o alo isa ion [4] o
CO
2
is o pa amoun impo ance. As CO
2
p oduc ion is mainly concen-
a ed on places such as powe plan s, indus ial acili ies and anspo ,
ocusing on his s age o CO
2
cap u e [5] is mo e desi able han di ec
ai cap u e (DAC) echnology [6], as he cos o cap u e inc eases as he
concen a ion o CO
2
in he lue gas dec eases [7]. The cap u e o CO
2
can be ca ied ou bo h be o e (p e-) [8] and a e (pos -) combus ion
[9], and e en in o he ypes o p ocesses such as so-called oxy--
combus ion [10]. In gene al, pos -combus ion CO
2
cap u e is he leas
dis up i e o exis ing indus ial p ocesses as he CO
2
cap u e componen
can be adap ed o he exis ing in as uc u e. In addi ion, CO
2
cap u e
can be achie ed by a a ie y o echniques, mos commonly using
abso p ion/adso p ion echniques [11] o memb ane echnology [12].
The adso p ion phenomenon o e s ad an ages such as low ene gy cos ,
high e iciency, ease o ope a ion and a ailabili y o mul iple adso ben
ma e ials [13–16]. The e o e, he adso p ion capaci y, kine ics and
en halpy o adso ben s a e widely s udied o analyse and e alua e hei
pe o mance [17]. Mos o he epo ed esea ch ocuses on he
adso p ion phenomenon om he gas phase wi h many examples o
* Co esponding au ho a : Depa amen o de Química O g´
anica e Ino g´
anica, Facul ad de Ciencia y Tecnología, Uni e sidad del País Vasco/Euskal He iko
Unibe si a ea, UPV/EHU, Leioa E-48940, Spain.
** Co esponding au ho .
E-mail add esses: [email p o ec ed] (O. Cas illo), [email p o ec ed] (R. P´
e ez-Agui e).
Con en s lis s a ailable a ScienceDi ec
Jou nal o CO2 U iliza ion
jou nal homepage: www.else ie .com/loca e/jcou
h ps://doi.o g/10.1016/j.jcou.2025.103144
Recei ed 20 Decembe 2024; Recei ed in e ised o m 30 May 2025; Accep ed 30 May 2025
Jou nal o CO2 U iliza ion 98 (2025) 103144
A ailable online 5 June 2025
2212-9820/© 2025 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-
nc-nd/4.0/ ).
adso ben ma e ials such as MOFs, zeoli es, silica es, ac i a ed ca bon,
o ganic and ino ganic polyme s, and biocha [12,18–23]. Howe e ,
he e a e no signi ican epo s o CO
2
adso p ion occu ing om an
aqueous medium [24,25]. In gene al, CO
2
molecules can be apped in a
po ous solid by physiso p ion o chemiso p ion. Bo h a e possible o
adso p ion om he gas phase, bu om he liquid phase, especially o
aqueous solu ions, chemiso p ion is mo e likely because CO
2
phys-
iso p ion compe es wi h H
2
O physiso p ion. In ac , chemiso p ion
ypically exploi s he acidic na u e o CO
2
by eac ing wi h he basic si es
p esen in he po ous ma e ial [26].
Ano he desi able p ope y o he CO
2
cap u e ma e ial, especially
when looking o he alo isa ion o he cap u ed CO
2
, is a low ene gy
penal y o he CO
2
elease. Un o una ely, he en halpy o chemi-
so p ion is usually highe han o physiso p ion (in he o de o 80–240
s. 20–40 kJ/mol) which hinde s he elease o he cap u ed CO
2
[27].
This is indeed a well-known d awback o he amine-based solu ions used
o comme cial DAC [28].
In his wo k, we ha e ocused on a p e iously epo ed [C Cu
6
(
μ
-
adenina o-кN3:кN9)
6
(
μ
-OH)
6
(
μ
-OH
2
)
6
](SO
4
)
1.5
compound (Cu
6
C ,
Fig. 1) [29]. This compound has some in e es ing ea u es ha led us o
belie e ha i could be use ul o CO
2
cap u e: (i) po osi y buil up om
disc e e hep ame ic en i ies ha a e sup amolecula ly assembled
(SMOF), (ii) adap able na u e o i s sup amolecula s uc u e, (iii) p o-
onable posi ions on he adenina o ligands ha do no al e i s hep a-
nuclea molecula s uc u e. In addi ion, he
μ
-adenina o-кN3:кN9
coo dina ion o he adenina o ligand causes i s acidi y cons an o shi
om pKa =9.8 o he uncoo dina ed adenina e anion o pK
a
≈7.2–7.4
o he coo dina ed adenina o ligand [30] which is abo e he pK
a1
alue
o CO
2
(6.35) bu close enough o he esul ing
μ
-adenine-кN3:кN9 /
HCO
3
-
adduc o be ela i ely uns able, acili a ing he CO
2
elease by he
e e se eac ion in aqueous medium. Ano he ea u e o his wo k is he
de elopmen o me hods ha allow he quan i ica ion o he adso p ion
o chemical species om an aqueous medium. This is impo an because
he g a ime ic and olume ic echniques de eloped o gas-solid
adso p ion measu emen a e no di ec ly ex apolable o liquid-solid
condi ions. In his sense, we ha e p e iously epo ed a echnique
called “magne ic sus en a ion”, which allows he quan i ica ion o he
adso bed mass pe cen age in pa amagne ic po ous compounds (mainly
MOFs and SMOFs) independen o he adso bed molecule [29,31–35].
This echnique is combined he ein wi h g a ime ic measu emen s ha
de e mine he amoun o CO
2
being inco po a ed in o he aqueous
media in which he adso ben pa icles a e suspended. Bo h measu e-
men s p o ide complemen a y in o ma ion, because he g a ime ic
measu emen p o ides he amoun o CO
2
being apped in he
wa e /po ous ma e ial mix u e, whe eas he adso bed mass measu ed
by magne ic sus en a ion depends on he chemical o m in which CO
2
is
cap u ed. The e o e, i is possible o e i y and quan i y no only he CO
2
cap u e bu also o de e mine he chemical o m in which i is inco -
po a ed (bica bona e o ca bama e) [36]. This wo k will demons a e
how new so p ion measu emen echniques, when combined wi h no el
SMOF po ous ma e ials, can shi he pa adigm o CO₂ cap u e om a
gas-based app oach o a liquid-based one.
2. Expe imen al
2.1. Syn he ic p ocedu es
2.1.1. Syn hesis o [Cu
6
C (
μ
-adenina o-κN3:κN9)
6
(
μ
3
-OH)
6
(
μ
-OH
2
)
6
]
(SO
4
)
1.5
(Cu
6
C )
The syn hesis was pe o med using a modi ica ion o he p e iously
epo ed one [29], in which ch omium(III) sul a e monohyd a e is
eplaced by ch omium(III) po assium sul a e dodecahyd a e. All he
chemicals we e o eagen g ade and we e used as comme cially ob-
ained. Adenine (0.8 mmol, 0.108 g) was dissol ed in a 20 mL wa e /-
me hanol mix u e (1:1 ol a io), and was hea ed unde con inuous
s i ing o 20 min. Then, a 20 mL aqueous solu ion o Cu(SO
4
)⋅5 H
2
O
(0.8 mmol, 0.200 g) and C K(SO
4
)
2
⋅12 H
2
O (0.2 mmol, 0.100 g) was
added. Immedia ely, a g een suspension was o med (pH =3.0). This
suspension was dissol ed h ough acidi ica ion wi h sul u ic acid un il a
ligh g een solu ion was ob ained a pH 1.5. Subsequen ly, he pH o he
solu ion was shi ed o 9.2 by adding ie hylamine, and a g een sus-
pension was ob ained, which was le in a c ys allize sealed wi h sealing
ilm (Pa a ilm M), sligh ly holed o allow he slow sol en e apo a ion.
A e 3 days, he suspension ec ys allized as g een needle-shaped
c ys als ha we e il e ed, washed 3 imes wi h wa e and d ied unde
ambien condi ions. The samples we e s o ed in a wa e sa u a ed a -
mosphe e because o i s e e sible c ys al s uc u e ansi ion om he
hyd a ed c ys alline o m o he amo phous anhyd ous. The pu i y o he
samples was e i ied by Powde X- ay Dis ac ion (PXRD) (Figu e S3.1).
Yield: 50–60 % (based on C ). Fou ie T ans o m In a ed Spec oscopy
(FTIR) (KB pelle s, cm
−1
): 3388 s, 3200sh, 1642 s, 1603 s, 1548 s,
1463 m, 1402 m, 1304 m, 1277 m, 1195 m, 1152 m, 1108 m, 1033w,
Fig. 1. The s uc u al uni s o Cu
6
C showing he
π
-
π
s acking in e ac ions ha c ea e he 3D po ous a chi ec u e.
E. Maiza-Razkin e al.
Jou nal o CO2 U iliza ion 98 (2025) 103144
2
and 935w.
2.2. CO
2
cap u e and quan i ica ion p ocedu es
Fo he de e mina ion o he CO
2
cap u e om wa e using he
magne ic sus en a ion echnique (Figu e S2.1), 50 mg o compound
Cu
6
C we e added in a glass lask wi h 50 mL o H
2
O. The lask was
sealed wi h a sep um and le o s i wi h gen le agi a ion. Two needles
we e immedia ely inse ed: one o he addi ion o he CO
2
sou ce and
he o he one as a p essu e ou le . The CO
2
was bubbled h ough he
solu ion a a low a e o 300 mL/min a 293 K o he equi ed ime.
La e he aqueous suspension was ans e ed o he magne ic sus en-
a ion measu ing de ice o de e mine he mass up ake, in e ms o
pe cen age, by compa ison wi h he alue o he p is ine po ous ma e ial
[29]. Fu he de ails on he quan i ica ion o mass adso p ion by mag-
ne ic sus en a ion can be ound elsewhe e [31–35].
A sligh ly di e en se -up was used o g a ime ically de e mine he
amoun o CO
2
cap u ed (Figu e S2.2). A Schlenk lask was used o
inco po a e a CO
2
ese oi by placing a balloon in he lask’s sidea m
ha con ains a s opcock al e. The lask was sealed wi h a sep um and a
long and a sho needle we e inse ed. Du ing he i s 10 minu es CO
2
was bubbled (300 mL/min low) h ough he long needle in o he
aqueous suspension con aining he po ous ma e ials (50 mg o Cu
6
C
and 50 mL o H
2
O) and he p essu e was eleased h ough he sho
needle. A e his ime, he sidea m s opcock al e was opened and he
sho needle (ac ing as he ou le o he sys em) was emo ed allowing
he balloon o ill. The long inle needle was hen also emo ed. The
sys em was kep gen ly s i ing o he emainde o he expe imen o
ensu e ha he CO
2
ese oi was no comple ely emp ied du ing his
ime. A e he end o he speci ied adso p ion ime, he s opcock al e
sidea m was u ned down and he Schlenk lask was placed in a poly-
lac ic acid (PLA) holde equipped wi h a wide base o enable accu a e
weigh ing o he sys em. A sho pe iod (5 min) du ing which he sys em
was exposed o CO
2
bubbling unde open-ci cui condi ions has been
in oduced o minimize wa e loss by e apo a ion, which could a ec
he accu acy o he CO
2
measu emen using he g a ime ic echnique. I
is essen ial o make a blank using only 50 mL o wa e o de e mine i s
capaci y o hold CO
2
and o calcula e by di e ence he excess o s o ed
CO
2
a ibu able o he po ous ma e ial. This p ocedu e gi es he mass
inc ease occu ing in he whole sys em.
Fo he de e mina ion o CO
2
adso p ion iso he ms, he expe imen s
we e pe o med unde di e en CO
2
/N
2
a ios (0 %, 25 %, 50 %, 75 %
and 100 %) es ablished by wo con olled lows o pu e CO
2
and
N
2
(Figu e S2.3), wi h a o al low in he gas inle o 300 mL/min. The
CO
2
bubbling was p olonged o 2 h o ensu e he equilib a ion o he
adso p ion p ocess. The adso bed mass in he po ous solid was de e -
mined by he magne ic sus en a ion echnique.
Finally, expe imen s we e ca ied ou on he cap u e o CO
2
om a
wa e -sa u a ed CO
2
s eam on d y pa icles o he po ous ma e ial,
using he same basic se up was employed bu bubbling he pu e CO
2
s eam in liquid wa e ese oi p io o i s con ac wi h he pa icles o
he po ous ma e ial (Figu e S2.4). A speci ied imes, he pa icles we e
placed in a es ube con aining wa e and placed on he magne ic sus-
en a ion de ice o hei measu emen .
The esul s epo ed in his wo k a e based on h ee sepa a e ex-
pe imen s, and a e gi en wi h hei associa ed s anda d de ia ion (ESI).
3. Cha ac e iza ion
3.1. Gene al cha ac e iza ion echniques
The IR spec a we e eco ded on a FT/IR-8X 8400S Jasco spec-
ome e in he 4000–600 cm
−1
spec al egion wi h an a enua ed o al
e lec ance (ATR) adap e . Magne ic sus en a ion expe imen s we e
pe o med using a Newpo Pagnell England Elec omagne Type C
sou ced wi h a Hewle Packa d 6655 A Sys em DC Powe Supply o
p o ide a a iable magne ic ield unde which he pa icles o he
compound dispe sed in wa e a e a ached o he lowe end o he
elec omagne pole un il he magne ic ield dec eases o he poin whe e
g a i y p e ails and he pa icles o all down [31]. The pu i y o he
samples was assessed by powde X- ay di ac ion, he mog a ime y
and Fou ie - ans o m in a ed spec oscopy (FTIR). Powde X- ay
di ac ion (PXRD) pa e ns we e collec ed on a Philips X’PERT powde
di ac ome e using Cu K
α
adia ion (λ =1.5418 Å) o e he 5 <2θ
<70◦ ange wi h a s ep size o 0.02◦and an acquisi ion ime o 2.5 s pe
s ep a 25 ◦C. pH changes we e moni o ed using a C ison GLP 21
pH-me e .
4. Resul s and discussion
4.1. CO
2
cap u e
CO
2
and N
2
gas adso p ion expe imen s (Figu e S3.1) we e pe -
o med on degassed Cu
6
C samples showing negligible alues in bo h
cases (CO
2
: 0.12 mmol/g a 298 K; 0.5 % in weigh . N
2
: 0.11 mmol/g a
77 K; 0.3 % in weigh ). This beha iou is explained by he lexible na-
u e o i s sup amolecula po ous s uc u e, which collapses wi h he
elease o he sol en molecules p esen in he po es o he ma e ial
leading o an amo phous compac ma e ial wi h almos no ee space
emaining [29]. The esul ing compac ma e ial e e s o he o iginal
po ous sup amolecula s uc u e when exposed o a mois u e-sa u a ed
a mosphe e, bu he in e ac ion wi h pu e CO
2
in he gas phase (phys-
iso p ion) is no s ong enough as o d i e a e e sal o he po ous
s uc u e. On he con a y, he compound is e y s able when imme sed
in wa e o e a wide ange o pH (2–10) [29]. The e o e, CO
2
cap u e by
bubbling i in an aqueous medium con aining pa icles o his ma e ial
will be es ed.
This app oach equi es cha ac e iza ion echniques ha di e om
hose used o gas phase adso p ion. Two echniques will be employed
o his pu pose: (i) magne ic sus en a ion echnique, which allows he
de e mina ion o he cap u ed mass wi hin pa amagne ic po ous ma e-
ials (MOFs and SMOFs mainly), and (ii) a g a ime ic echnique which
moni o s he mass inc ease o he sys em in ol ing he aqueous medium
and he suspended po ous ma e ial pa icles. The espec i e expe i-
men al se -ups a e shown in Figu es S2.1 and S2.2.
The g a ime ic measu emen was pe o med, a 20 ◦C, by weighing
he mass inc ease o he eac o and sub ac ing he amoun o CO
2
cap u ed by he olume o wa e (50 mL; see de ails in he expe imen al
sec ion). The mass o CO
2
cap u ed by his olume o wa e is 33
±2 mg. The same expe imen ca ied ou wi h he pa icles o he
compound added should indica e a g ea e amoun o CO
2
cap u ed in
he sys em i he ma e ial is ac i e. The expe imen was ca ied ou wi h
500 mg o Cu
6
C o minimize he con ibu ion o he 50 mL o wa e .
The esul shows ha he Cu
6
C cap u ed a mass excess o CO
2
o 85.7
±4 mg, co esponding o 17.1 % o he mass o Cu
6
C adso ben used.
The no malized esul s o he CO
2
/H
2
CO
3
cap u ed by Cu
6
C o mula
co espond o 6.4 mol
CO2
/mol
Cu6C
(Table 1), which is close o he
Table 1
Magne ic sus en a ion and g a ime ic measu emen p o ided CO
2
/H
2
CO
3
cap u e alues o he Cu
6
C aqueous suspension (500 mg in 50 mL wa e ) and
wa e alone (50 mL). Expe imen s pe o med a 20ºC and 1 h equlib a ion.
G a ime ic measu emen Magne ic
sus en a ion
Δ
mass
(mg)
Δ
mass
excess
(mg)
CO
2
(%)
CO
2
/
Cu
6
C
H
2
CO
3
(%)
H
2
CO
3
/
Cu
6
C
Cu
6
C
500 mg/
50 mL
118.7
±4.0
85.7
±4.3
17.1
±0.9
6.4
±0.3
22.3
±2.6
5.9
±0.7
Wa e
50 mL
33
±1.7
- ~0 -
E. Maiza-Razkin e al.
Jou nal o CO2 U iliza ion 98 (2025) 103144
3
expec ed numbe o basic posi ions placed in he hep ame ic uni , he six
adenina o ligands (Fig. 2a). Addi ionally, Cu
6
C sample unde goes a
colo change om da k g een o a ligh e g een du ing he adso p ion
p ocess (Fig. 2b), e e ing o i s ini ial colo o e ime a e he CO
2
bubbling has ceased.
In con as , he magne ic sus en a ion echnique measu es he mass
pe cen age cap u ed di ec ly by he po ous ma e ial, unlike he g a i-
me ic echnique, which measu es he o al CO
2
s o ed in bo h he wa e
and he po ous ma e ial. Consequen ly, he esul s o he wo echniques
a e complemen a y bu no necessa ily equal. The cap u ed mass pe -
cen age measu ed by his echnique may di e om he esul ob ained
om he g a ime ic app oach because he mass inco po a ed in o he
po ous ma e ial depends on he chemical o m in which CO
2
is cap u ed.
Physiso p ion o CO
2
and ca bama e o ma ion should imply equal
alues bu he cap u e o CO
2
by he eac ion o he concomi an H
2
CO
3
wi h he basic posi ions o he adenina o ligands o gene a e an HCO
3
-
/
adenine adduc should imply a g ea e mass cap u e when measu ed by
he sus en a ion magne ic echnique han by he g a ime ic me hod.
The esul s ob ained a 20 ◦C (Table 1) a e consis en wi h he cap u e o
CO
2
in he o m o coo dina ed adenine/HCO
3
-
pai s. The cap u e mass
alue measu ed by he magne ic sus en a ion echnique (220 mg/g) is
highe han he alues ob ained by he g a ime ic measu emen (ca.
172 mg/g). Howe e , when hese da a a e no malized pe hep ame ic
en i y, he alues ob ained by bo h echniques o cap u ed CO
2
/H
2
CO
3
a e ela i ely close (6.4(3) s 5.9(7)). These alues a e, in ac ,close o
he heo e ical up ake o 6 CO
2
molecules in he o m o HCO
3
-
/adenine
pai s (Fig. 2). The combina ion o he wo echniques he e o e makes i
possible o ensu e ha CO
2
is cap u ed, bu also o disce n he chemical
o m in which i is cap u ed.
Magne ic sus en a ion measu emen s p o ide a s aigh o wa d
me hod o ob ain adso p ion kine ic cu es. Acco dingly, ime-
dependen adso p ion iso he ms (20 ◦C) we e measu ed o CO
2
in an
aqueous suspension o Cu
6
C pa icles, we CO
2
low in Cu
6
C pa icles
and d y CO
2
low in Cu
6
C pa icles (Fig. 3a). In he i s case, he CO
2
cap u e alues a e much highe , ob aining a mass inc ease o 22.3 %.
Unde hese condi ions, he adso p ion eaches he s eady s a e a e
1 hou . In compa ison, when Cu
6
C pa icles a e exposed o humid
sa u a ed and d y CO
2
luxes, he alues d op o 12.9 % and 0 %,
espec i ely, a e 180 minu es. In addi ion, he ime equi ed o each
sa u a ion inc eases g ea ly when he Cu
6
C pa icles a e exposed o a
humid sa u a ed CO
2
lux han when he pa icles a e imme sed in
wa e . These alues need o be pu in con ex wi h o he CO
2
cap u e
ma e ials [37,38]. Among hese ma e ials, amines s and ou wi h a
cap u e alue o 30 w % o MEA (monoe hanolamine), which is he
con en ional solu ion widely used a a comme cial scale [39]. Alkali and
alkali ea h me al oxides p o ides also g ea e CO
2
cap u e alues, o
example Mg
90
Fe
10
-AMS
10
cap u es 65 w % a 300 ◦C unde a lux o
pu e CO
2
[40]. Howe e , he esul s ob ained o Cu
6
C a e signi ican ly
highe han he alues epo ed o MOFs among which Mg
2
(dobpdc)
s ands ou wi h a maximum adso p ion capaci y o 12.4 %
(2.82 mmol⋅g
−1
) a 40 ◦C unde we lue gas condi ions consis ing o
15 % CO
2
, 80 % N
2
and 5 % wa e [41]. The combina ion o MOFs wi h
ca bonaceous ma e ials allow inc easing he CO
2
cap u e ( o example,
CuBTC@GO cap u es 39 w % a 273 K and 1 ba ) [42].
On he o he hand, he adso p ion o Cu
6
C unde di e en CO
2
concen a ions (CO
2
/N
2
mix u es, Fig. 3b) shows ha he adso p ion
achie ed a e 1 h o expe imen dec eases wi h he dec easing o CO
2
concen a ion om a 22.3 % o mass cap u e o 16.2 % a 1:1 CO
2
/N
2
lux.
Fig. 4a summa ises he CO
2
adso p ion kine ic cu es o aqueous
suspensions o Cu
6
C pa icles ca ied ou a di e en empe a u es (20,
30 and 40 ◦C). These expe imen al da a se s show ela i ely good i o
he Langmui model wi h co ela ion coe icien s o 0.993, 0.981 and
0.975, espec i ely (Fig. 4b). No e ha he sa u a ion implies he same
amoun o adso bed mass which u he suppo s chemiso p ion ia
o ma ion o HCO
3
–
/adenine ensembles, a he han CO
2
physiso p ion,
o which he sa u a ion alue would dec ease wi h he empe a u e.
The appa en ac i a ion ene gy (E
a
) o adso p ion can be calcula ed
using he A henius equa ion (Eq. 1):
lnkads = − Ea(ads)
RT +C(1)
whe e k
ads
is he adso p ion a e cons an , and C is a cons an . As shown
in Fig. 4c, E
a(ads)
is ob ained om he slope o he ln k s 1/T plo and
gi es a alue o 21.8 kJ/mol. A compa ison wi h o he CO
2
adso ben s
will be gi en la e .
Fig. 2. (a) Schema ic ep esen a ion o he CO
2
cap u e in he o m o HCO
3
-
/adenine pai s in ol ing he [Cu
6
C (
μ
-adenina o-κN3:κN9)
6
(
μ
3
-OH)
6
(
μ
-OH
2
)
6
]
3+
en-
i ies. (b) Colou change o Cu
6
C sample du ing CO
2
bubbling. The pic u es we e aken while he pa icles we e in he aqueous medium.
E. Maiza-Razkin e al.
Jou nal o CO2 U iliza ion 98 (2025) 103144
4
4.2. CO
2
elease
As men ioned abo e, i is desi able o he CO
2
adso ben o be able
o elease he cap u ed CO
2
wi hou a la ge ene gy penal y. The e o e,
he CO
2
elease beha io o his compound was also measu ed unde wo
condi ions: (i) while in he aqueous media and (ii) a e emo al o he
compound pa icles om he aqueous media. The elease kine ic s udy
was ca ied ou by keeping he pa icles in he aqueous media ( es ube)
exposed o he ai unde cons an empe a u e condi ions using a he -
mos a ic wa e ba h and pe o ming successi e magne ic sus en a ion
measu emen s a di e en imes which does no equi e he pa icles o
be emo ed om he aqueous media. These deso p ion measu emen s
we e pe o med a 20, 30 and 40 ◦C (Fig. 5a). The deso p ion kine ic
cu es can again be i ed o a Langmui model (Fig. 5b). Based on his
empe a u e dependence, an E
a(des)
alue can be calcula ed o his
p ocess (41.2 kJ/mol; Fig. 5c). The ob ained E
a(ads)
and E
a(des)
alues a e
simila o hose ound o amines loaded in di e en adso ben s: 12.6
and 54.2 kJ/mol o hyd oxyl e hylenediamine (AEEA) suppo ed in
HZSM-5 zeoli e [43], 7.4 and 41–47 kJ/mol o e ae hylenepen amine
(TEPA) suppo ed on ca bon nano ubes [44], and 19.6 and 51.1 kJ/mol
o TEPA suppo ed in TiO(OH)
2
[45].
Fu he mo e, he eac ion hea in he adso p ion p ocess can be
calcula ed using Eq. 2:
ΔHads =Ea(ads)−Ea(des)(2)
In Cu
6
C compound, E
a(ads)
and E
a(des)
alues a e 21.8 and 41.2 kJ/
mol, espec i ely, gi ing an adso p ion hea (ΔH
ads
) o −19.4 kJ/mol,
co esponding o an exo he mic p ocess. The alue ob ained is lowe
han hose co esponding o he op imized sys ems desc ibed abo e, in
which di e en amines a e inco po a ed in po ous ma ices o c ea e
he e ogeneous adso p ion de ices, which show ΔH
ads
alues anging
om −31.5 o −41.6 kJ/mol. The adso p ion en halpies o non-
encapsula ed amines a e e en highe : a ound (-80)-(-100) kJ/mol o
p ima y and seconda y amines, and (-50)-(-65) kJ/mol o e na y
amines in which ca bama e o ma ion is o bidden, chemiso p ion
elying on he o ma ion o ammonium hyd ogenca bona e pai s [46].
The e o e, Cu
6
C o e s a signi ican ene ge ic imp o emen o e hese
ma e ials.
In addi ion o measu ing he CO
2
emaining in he po ous ma e ial,
he pH alue o he aqueous medium was also moni o ed. The ini ial pH
o he CO
2
-sa u a ed aqueous medium is app oxima ely 3.5 and ises o
6.0 e y as a e he CO
2
bubbling s ops. Howe e , he suspension
con aining 500 mg o Cu
6
C in 50 mL o wa e gi es an ini ial pH alue
o 5 al hough CO
2
bubbling is main ained o 1 h, which indica es he
p esence o HCO
3
-
in he media. In addi ion, when he CO
2
bubbling is
s opped, he inc ease in pH is much slowe , eaching a alue o 5.6
(Figu e S9.1).
The deso p ion kine ics we e analyzed by he magne ic sus en a ion
echnique a e he c ys als we e emo ed om he aqueous media by
as il e ing he pa icles and keeping hem a ambien condi ions
(20 ◦C). A e his ime, he CO
2
apped by he pa icles is measu ed
using he magne ic sus en a ion echnique and ollowing o he p o-
cedu e desc ibed in he expe imen al sec ion. The minimum ime o his
p ocedu e is 10 min and p o ides an almos comple e elease o he
cap u ed CO
2
. Consecu i e FTIR-ATR measu emen s (Fig. 6) we e also
ca ied ou on a eco e ed sample o Cu
6
C which had been subjec ed o
1 h o CO
2
bubbling in wa e (300 mL/min). The i s measu emen
shows he
υ
as
(C
–
–
O) and
υ
s
(C
–
–
O) o he HCO
3
-
anion a 1575 and
1325 cm
−1
, espec i ely, which pa ially o e lap wi h he signals o he
p is ine SMOF (see ESI). The second measu emen , aken 5 min a e he
i s , shows a signi ican dec ease in bo h signals and a e 20 min i can
be said ha bo h signals a e no longe isible. These ac s seem o
indica e a as e elease kine ic when he CO
2
molecules a e ans e ed
di ec ly om he po e sys em o he ma e ial o he su ounding ai han
when his elease akes place in he aqueous media.
4.3. S abili y upon CO
2
cap u e/ elease cycling
Up o ou successi e cycles o CO
2
cap u e and deso p ion s eps we e
pe o med, wi h magne ic sus en a ion measu emen s aken a e each
s ep (Fig. 7). Each deso p ion s ep in ol ed keeping he aqueous sus-
pension in an open a mosphe e a 40 ◦C o 2 h. The ze o mass gain
ob ained indica e ha he deso p ion was comple e unde hese condi-
ions. The adso p ion s ep was allowed o equilib a e o 1 h (o 30 min).
In bo h cases, he adso p ion capaci y is educed wi h each new cycle,
bu he decline o he adso ben is mo e acu e o 1 h adso p ion ime
han when i was kep o 30 min. A e 4 cycles o 1 h bubbling Cu
6
C
can ha dly be conside ed as a CO
2
adso ben ma e ial, while educing
Fig. 3. (a) CO
2
adso p ion kine ic cu es by Cu
6
C unde di e en condi ions: bubbling CO
2
in an aqueous suspension o Cu
6
C pa icles (g een), exposu e o Cu
6
C
pa icles o humid sa u a ed CO
2
low (blue), and exposu e o Cu
6
C pa icles o d y CO
2
lux (blue). (b) Bubbling CO
2
adso p ion iso he m cu e by Cu
6
C as a
unc ion o CO
2
pe cen age (CO
2
dilu ed wi h N
2
; 2 h equilib a ion ime). Tempe a u e: 20 ◦C; o al lux: 300 mL/min.
E. Maiza-Razkin e al.
Jou nal o CO2 U iliza ion 98 (2025) 103144
5
he bubbling ime o 30 min, he adso p ion capaci y is sligh ly be e
e ained, al hough a e 4 cycles i was almos negligible. Bea ing in
mind ha he pH o he aqueous media eaches a ela i ely acidic alue
du ing CO
2
bubbling, he g ea e decay obse ed wi h longe bubbling
imes has been a ibu ed o a chemical ins abili y o Cu
6
C unde hese
condi ions. In ac , as s a ed in he seminal wo k on his compound, he
obse ed chemical s abili y is due o he kine ic ine ness o he C (III)
me al cen e and no due o he modynamic s abili y. The e o e, p o-
longed exposu e o hese acidic condi ions will esul in a p og essi e
collapse o i s c ys al s uc u e. A inal a emp o ensu e he cyclabili y
o he ma e ial du ing he CO
2
adso p ion/deso p ion cycles was made
using a 0.1 M NaHCO
3
solu ion ins ead o pu e wa e . Fi s ly, he s a-
bili y o Cu
6
C was checked by imme sing a sample o his compound in
he solu ion o 18 h a ambien empe a u e. No signi ican solubiliza-
ion was obse ed, and he PXRD pa e n showed no signi ican changes
wi h espec o he p is ine compound (ESI). The esul s a e ou cycles
show a clea imp o emen in he s abili y o he sample, which s ill
e ains a 50 % o i s ini ial adso p ion capaci y a he las ou h cycle,
Fig. 4. (a) CO
2
adso p ion kine ic cu es o compound Cu
6
C a di e en
empe a u es. (b) Nume ical i ing o he Langmui model (see able o he
i ing pa ame e s). (c) A henius plo .
Fig. 5. (a) CO
2
deso p ion kine ic cu es o compound Cu
6
C a di e en
empe a u es. (b) Nume ical i ing o he Langmui model (see able o he
i ing pa ame e s). (c) A henius plo .
E. Maiza-Razkin e al.
Jou nal o CO2 U iliza ion 98 (2025) 103144
6
whe eas he use o wa e implies ha he compound loses i s CO
2
adso p ion capaci y and amo phizes.
5. Conclusions
To he bes o ou knowledge, his is he i s ime ha a po ous
sup amolecula me al-o ganic ma e ial (SMOF) has been used o cap u e
o CO
2
om wa e [41]. This ma e ial composed o hep anuclea
[C Cu
6
(
μ
-OH)
6
(
μ
-adenina o-кN3:кN9)
6
(
μ
-OH
2
)
6
]
3+
en i ies coun e -
balanced by sul a e anions is no able o cap u e CO
2
in he absence o
wa e , bu in aqueous media i is e y e icien a s o ing almos up o 6
CO
2
molecules (in he o m o adenine/HCO
3
-
pai s; he heo e ical
maximum) pe hep anuclea en i y. The pK
a
alue o he coo dina ed
adenina o ligand (7.2–7.4) and he pK
a1
alue o H
2
CO
3
(6.4) allow he
acid-base eac ion o ake place, bu a he same ime hey a e close
enough o a oid he esul ing adenine/HCO
3
-
pai being oo s able as o
equi e high ene gy o he CO
2
elease. The ob ained hea o adso p ion
(ΔH
ads
) o −19.4 kJ/mol and he deso p ion media ed by ambien
condi ions suppo his hypo hesis.
We ha e used a no el echnique o quan i y CO
2
cap u e: magne ic
sus en a ion in combina ion wi h a con en ional g a ime ic measu e-
men . Whe eas he g a ime ic app oach ells he mass amoun o
gaseous CO
2
being cap u ed, he magne ic sus en a ion echnique
measu es he mass being inco po a ed in o he po ous ma e ial pa icles.
The esul s o he la e echnique depend on he chemical o m (phys-
iso bed, HCO
3
-
, ca bama e) in which he CO
2
is being cap u ed.
Compa ing he alues ob ained by he wo echniques helps o iden i y
he chemical o m in which CO
2
is being cap u ed. Magne ic sus en a-
ion measu emen s also made i possible o ob ain he adso p ion and
deso p ion kine ic cu es a di e en empe a u es. This capabili y is
c ucial o de e mine he co esponding ac i a ion ene gies o adso p ion
and deso p ion h ough he A henius equa ion.
O e all, he manusc ip p o ides bo h an inno a i e ype o ma e ial
and echniques ha may be aluable in he sea ch o mo e e icien CO
2
Fig. 6. (a) FTIR spec a o p is ine Cu
6
C ( ed) and Cu
6
C subjec ed o CO
2
bubbling o 1 h (300 mL/min) du ing he CO
2
deso p ion ou side he aqueous medium.
Magni ica ions o he
υ
s
(C
–
–
O) (b) and
υ
as
(C
–
–
O) (c) signals o HCO
3
-
a 1325 cm
−1
and 1575 cm
−1
, espec i ely.
E. Maiza-Razkin e al.
Jou nal o CO2 U iliza ion 98 (2025) 103144
7
cap u e p ocesses ha can cope wi h he p esence o wa e . In he nea
u u e, we a e in e es ed in de eloping Cu
6
C -like ma e ials ha
inco po a e addi ional basic posi ions in he coun e ion, which could
boos CO₂ cap u e o highe le els.
CRediT au ho ship con ibu ion s a emen
Sonia P´
e ez-Ya˜
nez: W i ing – e iew & edi ing, P ojec adminis-
a ion, Me hodology. Ekain Maiza-Razkin: W i ing – e iew & edi ing,
W i ing – o iginal d a , Me hodology, In es iga ion, Fo mal analysis.
Osca Cas illo: W i ing – e iew & edi ing, W i ing – o iginal d a ,
Supe ision, Funding acquisi ion, Concep ualiza ion. Rub´
en P´
e ez-
Agui e: W i ing – e iew & edi ing, W i ing – o iginal d a , Fo mal
analysis, Concep ualiza ion. An onio Luque: W i ing – e iew & edi -
ing, Valida ion, Fo mal analysis. Ga ikoi z Beobide: W i ing – e iew &
edi ing, Me hodology, Funding acquisi ion.
Decla a ion o Compe ing In e es
The au ho s decla e ha hey ha e no known compe ing inancial
in e es s o pe sonal ela ionships ha could ha e appea ed o in luence
he wo k epo ed in his pape .
Acknowledgemen s
This wo k has been unded by Eusko Jau la i za/Gobie no Vasco
(IT1722–22) and by he Spanish Minis y o Science and Inno a ion
(PID2022–138968NB-C22 p ojec unded by MCIN/AEI /10.13039/
501100011033/ and by FEDER A way o make Eu ope and
TED2021–129810B-C22 unded by MCIN/AEI/10.13039/
501100011033 and Nex Gene a ion EU/PRTR). Technical and human
suppo p o ided by SGIke (UPV/EHU, MICINN, GV/EJ, ESF) is also
acknowledged.
Appendix A. Suppo ing in o ma ion
Supplemen a y da a associa ed wi h his a icle can be ound in he
online e sion a doi:10.1016/j.jcou.2025.103144.
Da a A ailabili y
Da a will be made a ailable on eques .
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