S abili y and pe o mance o BTC-based MOFs o
en i onmen al applica ions
Co ina Rod íguez-Es eban
a
, Regla Ayala
a
,
b
,
**
, Ca los L´
opez-Ca es
a
,
*
a
Depa amen o de Química Ino g´
anica, Facul ad de Química, Uni e sidad de Se illa, Ap do 1203, 41071, Se illa, Spain
b
Ins i u o de Ciencia de Ma e iales de Se illa, Uni e sidad de Se illa-CSIC, A da. Am´
e ico Vespucio 49, 41092 Se illa, Spain
ARTICLE INFO
Keywo ds:
HKUST-1
MIL-100(Fe)
S abili y and pe o mance
Dye adso p ion
Iodine up ake
Mois u e adso p ion
ABSTRACT
Two se ies o open me al si e MOFs, HKUST-1 and MIL-100(Fe), ha e been success ully p epa ed using di e en
me hods o syn hesis. Thei ea u es depend on he syn he ic ou e as well as hei ole play in di e en en i-
onmen al applica ions. The s abili y and pe o mance o hese BTC-based MOFs ha e been es ed bea ing in
mind Congo Red (CR) emo al, humidi y adso p ion and iodine cap u e and elease. HKUST-1 and MIL-100(Fe)
samples could o e a ema kable ole in he adso p ion o CR om aqueous solu ions. Howe e , he labili y o
HKUST-1 in wa e is e ealed as a d awback o i s eu iliza ion in bo h s a ic and agi a ion condi ions. The
o me con as s o he s abili y unde ambien mois u e. MIL-100(Fe) shows p omising p ope ies in bo h CR
adso p ion in aqueous solu ions and humidi y adso p ion. None heless, he pe o mance la gely depends on he
syn hesis condi ions. Al hough CR emo al is based on su ace in e ac ion, he e is a ela ion be ween he
adso p ed quan i y and he speci ic su ace a ea. The size and na u e o iodine allows he di usion in he po es o
bo h HKUST-1 and MIL-100(Fe) MOFs. This way, he up ake o iodine is d i ing by he po osi y and su ace a ea
o samples a he han hei inhe en na u e. As a ule, he esul s o his wo k indica e ha no only is i
impo an he speci ic na u e o he MOF chosen o a gi en applica ion bu also he way in which i has been
syn hesized and he condi ions in which hey a e used. MIL-100(Fe)-R is e ealed as he bes sui able candida e
o be used as a so ben o CR in aqueous solu ions, mois u e and I
2
gas.
1. In oduc ion
Me al O ganic F amewo ks (MOFs) cons i u e a ela i ely new class
o c ys alline hyb id ma e ials o med by me al species (a oms, ions o
clus e s) linked by mul iden a ed o ganic ligands [1,2]. The la ge
e sa ili y o hese ma e ials is due o he possibili y o selec ing all
a ailable me als unde a iable oxida ion s a es, as well as he chance o
using di e se o ganic unc ional g oups as ligands. The o me allows
uning bo h chemical composi ion and c ys alline s uc u e in o de o
design an almos in ini e numbe o membe s in MOF collec ion[3–5].
Mo eo e , pos syn he ic modi ica ions o p is ine MOFs ex en he
possibili y o ailo ing new sys ems [6,7]. Thei excep ional mic opo-
osi y and la ge speci ic su ace a eas make hem excellen candida es
o di e en echnological applica ions. MOFs ha e been employed in
gas s o age and sepa a ion [8], senso s [9], ca alysis [10–14] and d ug
deli e y [15] among o he uses. As po ous ma e ials, hey ha e also
been s udied as al e na i e sys ems o mul iple adso p ion applica ions
in gas and liquid media wi h he aim o look o imp o ed pe o mances
subs i u ing classical adso ben s.
G owing popula ion and i s associa ed human ac i i ies in e ms o
ene gy and esou ces consump ions, as well as indus ial de elopmen
ha e d ama ically inc eased pollu ion [16,17]. One o he main conse-
quences o his en i onmen al deg ada ion is he con amina ion o wa e
esou ces. A ending o hei chemical composi ion, common con ami-
nan s in wa e can be di ided in o ino ganic and o ganic species.
O ganic pollu an s can be di e si ied in de e gen s, oils, pes icides, in-
sec icides, pha maceu icals and dyes among o he s. Dyes a e commonly
used in di e en ab ica ion p ocesses such as p in ing, lea he , plas ic,
pape and ex ile indus ies. Mo e han one housand ons o dyes a e
discha ged in o wa e sys ems e e y yea [18,19]. The as majo i y o
dyes a e an impo an h ea no only o ecosys ems bu also o human
heal h, p esen ing ca cinogenic and mu agenic cha ac e s. Among he
a ailable echniques o o ganic species emo al om was ewa e ,
adso p ion based echnologies a e he mos compe i i e me hods [20,
* Co esponding au ho .
** Co esponding au ho . Depa amen o de Química Ino g´
anica, Facul ad de Química, Uni e sidad de Se illa, Ap do 1203, 41071, Se illa, Spain.
E-mail add esses: [email p o ec ed] (R. Ayala), [email p o ec ed] (C. L´
opez-Ca es).
Con en s lis s a ailable a ScienceDi ec
Jou nal o Solid S a e Chemis y
jou nal homepage: www.else ie .com/loca e/jssc
h ps://doi.o g/10.1016/j.jssc.2024.124956
Recei ed 11 June 2024; Recei ed in e ised o m 31 July 2024; Accep ed 9 Augus 2024
Jou nal o Solid S a e Chemis y 339 (2024) 124956
A ailable online 10 Augus 2024
0022-4596/© 2024 The Au ho s. Published by Else ie Inc. This is an open access a icle unde he CC BY-NC license ( h p://c ea i ecommons.o g/licenses/by-
nc/4.0/ ).
21]. MOFs ha e been p esen ed as a e y p omising al e na i e o
con en ional used adso ben s (ac i a ed ca bons and zeoli es) o e -
coming he d awbacks o hese adi ional sys ems [21–30].
Unde he inc easing ene gy demand and conside ing he g eenhouse
gas emissions gene a ed by ca bon-based sou ces, he use o nuclea
powe plan s is s ill conside ed clean and economically compe i i e. One
o he main p oblems associa ed o he ope a ion o hese acili ies
conce ns he sa e y unde an acciden and he managemen o used
nuclea uels. The isk o adionuclide emissions in he e en ual eleased
gas s eam has o be aken in o conside a ion.
129
I and
131
I, which a e
ola ile adionuclides wi h high concen a ion in nuclea uel was e,
ha e se ious e ec on human heal h and mus be e ec i ely cap u ed
om gas elease. Po ous so ben s, adi ionally being ac i a ed ca bons
and Ag-exchanged zeoli es, a e in ol ed in he main echnological
p ocesses employed in iodine emo al om gas phase [31,32]. MOFs, as
a new amily o po ous ma e ials, ha e also been p obed as good so ben
solids wi h good eco e ing and eusing pe o mances [33], o e coming
he egene a ion limi a ions o ca bons and zeoli es [21,34–36].
The adso p ion s udies o humidi y om ambien ai using po ous
ma e ials a e also o g ea in e es when cap u e and elease o wa e a e
needed. Such p ocesses a e c ucial o ene gy-e icien ha es ing o
wa e om ai in a id ea h a eas. Fo ha pu pose, some MOFs-based
adso ben de ices ha e been ecen ly de eloped [37–39]. MOFs
designed o hese applica ions mus cap u e wa e a e y low ela i e
humidi y and mus exhibi la ge up ake capaci ies. Thei s abili y unde
wa e cap u e and elease cycling condi ions, usually hea ing assis ed,
ha e o be also aken in o accoun [40,41].
In his con ex , wo se ies o well-known MOFs (HKUST-1 [42] and
MIL-100(Fe) [43]) ha e been ob ained om di e en syn he ic p o-
cedu es. Bo h MOFs a e based on 1,3,5-benzene ica boxylic acid
(H
3
BTC) and ha e he so-called coo dina i e unsa u a ed posi ions.
Acco ding o he ha d-so acid-base (HSAB) p inciple, me al–ligand
(M−L) s eng hs and s abili ies pa ly depend on he so ness o ha dness
o he componen s’Lewis acids and bases. In his sense, ha d acids like
Fe
3+
will o m he modinamically s able bonds wi h ha d bases like BTC
ligands whe eas he in e ac ion o he o me wi h Cu
2+
would no be so
s ong and he compe i ion wi h o he ha de acids could esul in
ins abili y. Ne e heless, chemical ha dness o so ness canno be
conside ed he only ac o o he s eng h o acid-base in e ac ion, o he
con ibu ions such as s e ic e ec s, medium e ec s, kine ic con ol, may
also play an impo an ole [44]. Al hough HKUST-1 is easie o syn-
hesize, MIL-100(Fe) p esen s highe chemical and he mal s abili ies
and la ge po e olumes. The o me can be e y p o i able in he
di usion p ocess needed o he in e ac ion o gues molecules o
exposed cen es. A his poin i is wo h men ioning ha o a gi en
applica ion no only is i necessa y he p esence o unsa u a ed posi ions
and high po e olumes bu also hei accessibili y ha can be uled ou
by he le -o e sol en molecules om syn hesis p ocedu e. This means
ha al hough he inhe en ea u es o a gi en MOF a e c ucial om an
Fig. 1. XRD o HKUST-1 (a), and MIL-100(Fe) (b) samples.
Fig. 2. FTIR spec a o HKUST-1 and MIL-100(Fe) samples.
C. Rod íguez-Es eban e al. Jou nal o Solid S a e Chemis y 339 (2024) 124956
2
applica ion poin o iew, i is also signi ican o ake in o accoun how i
has been syn hesized [45]. Thus, he p epa a ion ou e o MOF ma e-
ials is a key ac o o ob aining ep oducible s uc u es, opologies and
p ope ies in a sys ema ic pa e n [46]. Addi ionally, he pos -syn he ic
ac i a ion can ha e as impo an implica ions as he syn he ic me hod
i sel . I mus be also conside ed he s abili y o hese species in se e al
media and condi ions as well as he consequences o hei in e ac ion
wi h di e en solu es.
The aim o his wo k is he compa a i e s udy o he so p ion capa-
bili ies and s abili ies o di e en HKUST-1 and MIL-100(Fe) samples
agains Congo Red (CR) emo al om wa e solu ions, humidi y cap u e
om ambien ai and iodine up ake and elease om gas phase. The
o me implies a compa ison among he capabili ies and s abili ies o he
same MOFs, di e en ly syn hesized, and di e en MOFs, BTC-based,
espec o he h ee applica ions p e iously commen ed. These appli-
ca ions accoun o di e en adso ba es species in di e en media.
Mo eo e , he implica ions o he p esence o wa e in ei he liquid o
gas phase ha e also been conside ed. This s udy will allow no only he
selec ion o he mos sui able ype o MOF o a gi en applica ion bu
also he syn hesis and ea men condi ions ha mus be conside ed o
op imize he esul s.
2. Expe imen al sec ion
2.1. Ma e ials
All he eac an s we e used as ecei ed wi hou u he pu i ica ion.
Speci ic de ails abou eac an s and sol en s a e p o ided in he sup-
plemen a y ma e ial.
2.2. Me hods
Powde X- ay di ac ion (PXRD) da a we e collec ed on a Pan-
aly ical X’PERT PRO di ac ome e wi h Cu-K
α
a oom empe a u e
using a 2θ ange 5◦-60◦using a s ep o 0.05◦and an acquisi ion ime o
200 s pe poin . Scanning elec on mic oscopy (SEM) images we e
eco ded in a Hi achi 54800 SEM-FE6 elec on mic oscope a an accel-
e a ing ol age o 5 kV. Samples we e co e ed wi h a hin ilm o
ch omium be o e analysis. Fou ie ans o med in a ed (FTIR) analyses
we e ca ied ou making KB ound pelle s om mix u es con aining a
small amoun o he sample and using a The mo Scien i ic NICOLET iS5
iD1 T ansmission spec opho ome e . The mog a ime ic analyses
(TGA) we e pe o med unde ai a mosphe e and a hea ing a e o 10 ◦C
min
−1
om oom empe a u e o 500 ◦C, using a Q600 equipmen om
TA Ins umen s. Ni ogen adso p ion analyses we e ca ied ou a liquid
ni ogen empe a u e using a Gemini V-2365//V1.00 Mic ome i ics
equipmen . The samples we e ac i a ed unde ni ogen low a 200 ◦C
o 1 h be o e he analysis. Speci ic su ace a eas we e calcula ed om
Fig. 3. SEM images o HKUST-1 samples. HKUST-1-S (a,b), HKUST-1-R (c,d), HKUST-1-RT (e, ).
C. Rod íguez-Es eban e al. Jou nal o Solid S a e Chemis y 339 (2024) 124956
3
he da a o adso p ion iso he ms using BET me hod. UV–Vis spec a
we e eco ded on a The mo Scien i ic E olu ion 60S spec ome e .
2.3. Syn hesis o MOFs
HKUST-1 was syn hesized ia sol o he mal [47], e lux [48] and
ageing a oom empe a u e me hods. Speci ic de ails abou he
Fig. 4. SEM images o MIL-100(Fe) samples. MIL-100(Fe)–S (a,b), MIL-100(Fe)-R (c,d), MIL-100(Fe) -RT (e, ).
Fig. 5. TGA analysis o HKUST-1 and MIL-100(Fe) samples. Fig. 6. Adso p ion iso he ms o HKUST-1 and MIL-100(Fe) samples.
C. Rod íguez-Es eban e al. Jou nal o Solid S a e Chemis y 339 (2024) 124956
4
syn hesis a e collec ed in he supplemen a y ma e ial.
MIL-100(Fe) was p epa ed using sol o he mal [49], e lux [50]
me hods as epo ed in bibliog aphy and ageing a oom empe a u e
ollowing he p ocedu e in he li e a u e [51] wi h sligh modi ica ions.
All de ails a e p o ided in supplemen a y ma e ial.
2.4. Congo ed emo al
Dye emo al expe imen s we e ca ied ou unde bo h s a ic and
s i ing condi ions a oom empe a u e. In gene al, CR wa e solu ions
wi h concen a ions o 50 mg/L we e used. 50 mg o he sample we e
imme sed in 50 mL o he o ganic dye solu ion and main ained unde
s a ic o so magne ically s i ing condi ions o 120 min. A e ce ain
imes 3 mL aliquo s we e aken ou and he solid was sepa a ed by
cen i uga ion. Two d ople s o NaOH aqueous solu ions 0.1 M we e
added o main ain he ini ial pH o he solu ions. The concen a ion o
CR in he liquid phase was de e mined h ough UV–Vis spec a
measu ing he abso bance a λ=497.5 nm and compa ing hese alues
wi h calib a ion ep esen a ion. The quan i y o adso bed dye, q
(mg
CR
/
g
MOF
) is di ec ly de e mined h ough g =(C0-C )⋅V/m, being C0 and C
he dye concen a ions a he beginning and a ime , V he olume o
solu ion employed o he expe imen and m he mass o MOF used as
adso ben . A e he expe imen s he solids we e eco e ed by cen i-
uga ion, washed wi h E OH and d ied o u he cycles and
cha ac e iza ion.
The iso he m CR adso p ion s udy o MIL-100(Fe) samples was
conduc ed wi h ini ial concen a ions anging om 50 o 450 mg/L and
con ac ime o 24 h wha ensu es ha he equilib ium s a e has been
eached.
In o de o analyse he con ol mechanism o he adso p ion p ocess
pseudo- i s -o de and pseudo-second-o de kine ic models exp essed as
Eqs. (1) and (2) we e employed o es he expe imen al da a o
adso p ion kine ics.
ln(q
e
-q
)=lnq
e
-K
1
⋅ (1)
/q
=1/(K
2⋅
q
e
2
)+ /q
e
(2)
Table 1
BET Su ace a eas ob ained o HKUST-1 and MIL-100(Fe) samples.
Sample Su ace A ea (m
2
/g) Sample Su ace A ea (m
2
/g)
HKUST-1-S 1228 MIL-100(Fe)–S 920
HKUST-1-R 890 MIL-100(Fe)-R 1344
HKUST-1-RT 832 MIL-100(Fe)-RT 1291
Fig. 7. Congo Red adso p ion capaci ies o HKUST-1 (a) and MIL-100(Fe) (b)
samples unde s a ic and agi a ion condi ions.
Fig. 8. XRD o HKUST-1 (a) and MIL-100(Fe) (b) samples a e CR adso p ion
expe imen s unde agi a ion condi ions.
C. Rod íguez-Es eban e al. Jou nal o Solid S a e Chemis y 339 (2024) 124956
5

whe e, q
and q
e
(mg⋅g
−1
) a e espec i ely he masses o CR adso bed pe
uni mass o he so ben a a ce ain ime and equilib ium ime; K
1
(min
−1
) and K
2
(g mg
−1
⋅min
−1
) ep esen he pseudo- i s o de and he
pseudo-second o de adso p ion a e cons an s, espec i ely.
2.5. Humidi y cap u e om ambien ai
A ound 50 mg o MOFs we e placed in p e iously weigh ed 10 mL
emp y ials and hea ed in ai a 120 ◦C o 24 h. The weigh s o hese
ac i a ed samples we e de e mined and hen exposed o ai a oom
empe a u e o 48 h. Du ing he expe imen s he ials we e eweighed
a di e en imes and he wa e up ake e iciency we e calcula ed by
g a ime ic measu emen s. Th ee cycles o essays we e epea ed. The
samples we e ecupe a ed and cha ac e ized a e he whole p ocedu e.
2.6. Iodine cap u e om apou phase and subsequen eco e y
The iodine up ake expe imen s we e ca ied ou ollowing he p o-
cedu e epo ed in he li e a u e [34,35]. 100 mg o di e en samples
we e placed in p e iously weighed 10 mL emp y ials and hea ed in ai
a 120 ◦C o 24 h. A e ha he weigh s o hese ac i a ed samples we e
de e mined. These ials we e placed join wi h solid iodine in a closed
500 mL chambe and hea ed in o en a 75 ◦C o 24 h. The iodine cap u e
was de e mined by g a ime ic measu emen s. The p ocedu e was
epea ed 3 imes. The samples we e washed in E OH and cen i uged as
p e iously men ioned o eco e he p is ine MOFs.
To quan i y he iodine eco e y 10 mg o e e y cha ged sample was
imme sed in 10 mL o e hanol unde s a ic condi ions. UV–Vis spec a
we e acqui ed a ce ain imes be ween 5 and 120 min o ollow he
iodine eleasing om solid o liquid phase.
3. Resul s and discussion
3.1. Syn hesis and cha ac e iza ion
Di e en HKUST-1 and MIL-100(Fe) samples we e ob ained h ough
se e al syn hesis p ocedu es such as sol o he mal (-S), e lux (-R) and
ageing a oom empe a u e (-RT). Fo he sake o compa ison, Fig. 1
collec s powde X- ay di ac og ams (XRD) o all samples as well as
hose simula ed om he ideal o MOF s uc u es [42,43]. The p esence
o he cha ac e is ic e lec ions o bo h MOFs in he expe imen al e-
co ds clea ly demons a es he success ul syn hesis o all he samples.
Ne e heless, some ex a e lec ions apa om hose o he ideal
s uc u e can be obse ed in he di ac og am o HKUST-1-RT sample.
As a as we know, his p ocedu e has no been p e iously used o ob ain
HKUST-1. Acco ding o li e a u e in which mode a e s abili y o
HKUST-1 agains wa e is pos ula ed [41], and aking in o accoun ha
his sample was p epa ed using pu e wa e as sol en , he c ys alline
s uc u e o his seconda y phase could appea as a consequence o
pa ially deg ada ion o MOF s uc u e du ing he syn hesis. In ac , his
ex a e lec ions we e encoun e ed in HKUST-1 deg ada ion s udies in
wa e [52] and hey we e associa ed o a mix u e o c ys alline lamina
s uc u es o [Cu
2
(BTC)(H
2
O)
6
] and [Cu
2
(BTC)(OH)(H
2
O)] composi-
ions and an unsol ed phase coming om hyd olysis p ocesses unde -
gone by he o iginal s uc u e. Fo a be e unde s anding, he XRD o
his deg aded HKUST-1 phase was also included in Fig. 1.
Fig. 2 shows he FTIR spec a o he HKUST-1 and MIL-100(Fe)
samples. Fo he sake o comple eness, he spec um o he co e-
sponding o ganic ligand was also included. The ypical bands o he
isubs i u ed a oma ic ing (730 and 1278 cm
−1
), he C–H and C=C
s e ch ib a ions o he a oma ic ing (1450-1400 cm
−1
), he C–H
bending modes (760 and 707 cm
−1
) appea in bo h HKUST-1 [53] and
MIL-100(Fe) [54] samples. The cha ac e is ic band o he C=C
s e ching o he a oma ic ing is also obse ed a 1583 cm
−1
as long
Fig. 9. PH e olu ion o suspensions o HKUST-1 and MIL-100(Fe) samples in
wa e unde agi a ion condi ions.
Fig. 10. Reusabili y o MIL-100(Fe) samples in Congo Red adso p ion unde
agi a ion condi ions.
Fig. 11. Congo Red adso p ion capaci y o MIL-100(Fe) s ini ial dye
concen a ion.
C. Rod íguez-Es eban e al. Jou nal o Solid S a e Chemis y 339 (2024) 124956
6
wi h he C=O symme ic (1450 cm
−1
) and asymme ic (1374 cm
−1
)
s e ching. The na owing o he wide band a 1710 cm
−1
in he p e-
pa ed samples sugges s he lack o un eac ed o ganic linke and,
he e o e, a good pu i ica ion o he inal p oduc s. The signal ela ed o
he Cu-OOC isobiden an e in e ac ion (1700-1300 cm
−1
) and he Cu–O
s e ching (491 cm
−1
) a e well- ecognized in HKUST-1 samples. Addi-
ionally, he Fe–O s e ching (464 cm
−1
) is p esen o MIL-100(Fe)
p epa a ions.
SEM images co esponding o HKUST-1 samples a e shown in Fig. 3.
As e ealed by he di ec inspec ion o hese pic u es, pa icle mo -
phologies p esen a clea dependence on he syn hesis p ocedu e. Well-
shaped oc ahed al pa icles a e ob ained when sol o he mal syn hesis is
used while qui e i egula and smalle ounded pa icles a e obse ed on
he images o he sample p epa ed by e lux me hod. Two ypes o
pa icles a e ound in he images coming om he sample p epa ed a
oom empe a u e in wa e . On one hand, well-shaped pa icles ha
could be assigned o he HKUST-1 ac ion o his sample, and on he
o he hand, needle-like pa icles ha should co espond o he second-
a y c ys alline phase iden i ied by p e iously commen ed X-Ray
di ac ion s udies. SEM images o MIL-100(Fe) samples a e shown in
Fig. 4. In gene al, he samples a e mo e homogeneous and no di e ences
associa ed o he syn hesis can be iden i ied. In all cases, no well- ace ed
pa icles wi h ounded shapes a e obse ed. Howe e , he sol o he mal
sample p esen s smalle pa icle sizes han he e lux and oom em-
pe a u e p epa a ions.
Fig. 5 shows he TGA analyses o all he samples. Bo h, HKUST-1 and
MIL-100(Fe) samples, show TGA p o iles simila o hose ound o hese
MOFs in li e a u e [50,55]. This means a i s d op a a empe a u e
lowe han 200 ◦C co esponding o he emo al o physically adso bed
moie ies and a second s age a ibu ed o he collapse o he s uc u e
a ound 300 and 350 ◦C o HKUST-1 and MIL-100(Fe), espec i ely.
These esul s indica e he highe he mal s abili y o MIL-100(Fe) s.
HKUST-1 ma e ials [56].
The ype o po osi y and su ace a ea a e impo an p ope ies o
adso p ion applica ions. Fig. 6 collec s he N
2
adso p ion iso he ms o
HKUST-1 and MIL-100(Fe) samples. The BET su ace a eas a e displayed
in Table 1. Bo h HKUST-1 and MIL-100(Fe) samples show ype I
iso he m and high su ace a ea in acco dance wi h li e a u e [50,57].
The main con ibu ion in such a high su ace a ea is due o mic opo es.
HKUST-1 samples p esen a wide ange o su ace a eas ha can be
ela ed o he di e en mo phologies ound in he SEM images. As ex-
pec ed HKUST-1-RT sample has he lowes po osi y in ag eemen wi h
he idea o pa ial deg ada ion o he MOF s uc u e du ing he syn-
hesis. As a ule, he la ge su ace a eas a e shown by he MIL-100(Fe)
samples.
3.2. Congo ed emo al
The e olu ion o he UV–Vis spec a wi h ime o samples con aining
aqueous CR solu ions in con ac wi h HKUST-1 and MIL-100(Fe) sam-
ples unde agi a ion condi ions a e included in he Supplemen a y Ma-
e ial (Fig. S1). In all cases, a dec ease o abso bance wi h ime e idences
he adso p ion o dye by MOFs. As men ioned in he Ma e ial sec ion,
aqueous NaOH 0.1 M was added a e cen i uga ion. Wi hou he
o me addi ion, a shi o he maximum owa ds highe wa eleng hs (λ
=570 nm) was obse ed as ime p oceeds wha implies a change in he
colou o he samples om ed o iole (see Fig. S2 in Supplemen a y
Ma e ial). This change is due o a a ia ion o he solu ion pH. One
should ecall ha CR is an acid-base indica o ha emains blue below
pH 3.0 and g adually u ns o a b illian ed abo e pH 5.2. The acid
condi ions can be ela ed o MOF hyd olysis[58–60] wha can p o oke
he deg ada ion o i s s uc u e. Thus, i is impo an o s udy in de ail
he consequences o hyd olysis o es ablish he con enience o using his
ype o ma e ials o adso p ion p ocesses in aqueous solu ions.
The adso p ion capaci y o he samples is plo in Fig. 7. I can be seen
ha he amoun o CR adso bed pe g o MOF is a he simila in all
cases. The high adso p ion a e a he beginning can be a ibu ed o he
la ge numbe o accessible ac i e si es on he su ace o MOFs. Then, as
he adso p ion p ocess p oceeds, he as majo i y o ac i e si es on he
su ace we e occupied by he CR molecules, and he adso p ion a e
slowed down g adually un il an equilib ium adso p ion s a e was ach-
ie ed. Addi ionally, he as majo i y o he CR in solu ion was ans-
e ed o he MOFs a e only 120 min. The FT_IR spec a eco ded a e
he expe imen con i m he adso p ion by bo h HKUST-1 and MIL-100
(Fe) samples (Fig. S3 in he supplemen a y ma e ial). The shi ing o
he peaks in he ange 1582-1562 and 1391-1368 cm
−1
indica es he
π
-
π
Fig. 12. The pseudo- i s -o de plo o Congo Red adso p ion o MIL-100(Fe) and HKUST-1 samples unde s a ic and agi a ion condi ions.
C. Rod íguez-Es eban e al. Jou nal o Solid S a e Chemis y 339 (2024) 124956
7
in e ac ion be ween he benzene ings o he MOF amewo k and he
a oma ic backbone o he o ganic dye. Al hough he e is a ela ion be-
ween he speci ic su ace a eas and he adso p ion, he size o CR
molecule (a ound 21 Å) makes di icul he di usion along he po ous.
Thus, i can be assumed ha he adso p ion akes place in he su ace o
he MOFs. This way he samples wi h la ge pa icle sizes as es ima ed
by SEM p o ide be e adso p ion capabili ies. Mo eo e , i is wo h
men ioning he appea ance o a p ecipi a e in he solu ion in acid con-
di ions ela ed o he dye. This could be in e p e ed as CR is ex ac ed
om he aqueous solu ions no only adso bed on he MOF bu also as an
independen solid.
A his poin , i is necessa y o s udy he chemical s abili y o he
samples a e CR adso p ion o e alua e hei eusabili y. To asses his
p oposal, Fig. 8 shows he XRD o HKUST-1 and MIL-100(Fe) a e he
adso p ion in compa ison wi h he simula ed diag am o each MOF and
he deg aded HKUST-1 phase [52]. The esul s indica e ha HKUST-1 is
spoil and new peaks assigned o lamina s uc u es o [Cu
2
(BTC)
(H
2
O)
6
] and [Cu
2
(BTC)(OH)(H
2
O)] composi ions appea in he diag am
whe eas no ob ious changes in s uc u e o he MIL-100(Fe) samples
we e app ecia ed.
The change o colou o he aqueous solu ions o CR in con ac wi h
MOFs and he deg ada ion o HKUST-1 mo i a e he s udy o aqueous
suspensions o MOFs. Fig. 9 displays he e olu ion o wa e pH in con ac
wi h HKUST-1 and MIL-100(Fe). In all cases, he e is a dec ease o he
wa e pH bu he acidi y o he media is MOF dependan . This way,
HKUST-1 samples diminish wa e pH in mo e han 2 uni s, being mo e
p onounced in he case o HKUST-1 –R and -RT. Fo MIL-100(Fe), he
dec ease o he wa e pH is almos immedia ely and keeps cons an
along he whole expe imen . The acidi y o he samples jus i ies he
change o colou s obse ed in he CR adso p ion. MOFs we e also
cha ac e ized a e he s udy o pH e olu ion (Fig. S4 in supplemen a y
ma e ial). MIL-100(Fe) samples keep hei iden i y while HKUST-1
samples a e deg aded in he same way as in he p esence o CR.
Hence, i can be concluded ha , he loss o HKUST-1 s uc u e is no
esponsibili y o he dye bu i is due o he hyd olysis p ocesses mo i-
a ed by he MOF p esence in aqueous medium. I is also wo h es i-
ma ing i he agi a ion has any implica ion in he deg ada ion o HKUST-
1, he CR emo al we e epea ed in s a ic condi ions (Fig. 7). As ex-
pec ed, he abso p ion o CR in s a ic condi ions is smalle due o a lesse
con ac be ween MOF and dye. The XRDs o he HKUST-1 samples a e
CR adso p ion in s a ic condi ions (Fig. S5 in supplemen a y ma e ial)
show di e ences om he as-p epa ed samples, namely, he appea ance
o new peaks ha emind hose ound in he assays unde agi a ion.
These esul s sugges ha al hough HKUST-1 is as good adso ben o CR
in aqueous solu ions as MIL-100(Fe), i s in eg i y is no main ained due
o hyd olysis e en s no ma e he condi ions s ablished in he
expe imen .
MIL-100(Fe) samples we e ecycled and eu ilised in se e al
adso p ion cycles. The esul s shown in Fig. 10 indica e, on one hand,
ha a he simila beha iou appea s no ma e he sample conside ed.
On he o he hand, he e is no a signi ican pe o mance decay along he
cycles. In addi ion, Fig. S6 illus a e ha he e is no change in he
s uc u e o he MOF due o he ecycling and eu iliza ion. The eby
hese esul s con i m he good esponse o epea abili y and good long-
e m s abili y o MIL-100(Fe) samples espec o CR adso p ion in
aqueous solu ions.
To e alua e in mo e de ail, he sensi i i y o he di e en MIL-100
(Fe) samples owa ds dye elimina ion, he e ec o he ini ial concen-
a ion ac oss he ange o 50–450 mg/L was s udied. Fig. 11 depic s he
amoun o adso bed CR pe g o MOF .s. he CR ini ial concen a ion in
solu ion. Up o 100 mg/L he adso p ion capaci y o MIL-100(Fe)
inc eased wi h he ini ial concen a ion o all he samples. A highe
concen a ions, he end depends on he me hod o syn hesis. MIL-100
(Fe)-R ollows a non-s op inc easing beha iou un il a lowe ing is
obse ed. This endency can be explained in e ms o exchange ligands
o he MOF wi h dissol ed species in he medium [61]. Hence, CR may
eplace BTC ligands due o he amino sulpha e g oups p esen in he dye
[62]. MIL-100(Fe)–S and MIL-100(Fe)-RT p esen a lowe pe o mance
o he CR elimina ion. None heless, acco ding o he o me easoning
hei s uc u al in eg i y seems o be main ained a high concen a ions.
These esul s poin ou ha he pe o mance o MIL-100(Fe) is a he
dependan on how i has been syn hesized. A conclusion based only on
he ype o MOF should be misleading.
Recen ly, kine ic s udies ha e been widely pe o med o he
adso p ion mechanism o dyes om aqueous solu ions on o adso ben s
Table 2
Pa ame e s o he kine ic models used o i he adso p ion o CR on o MIL-100
(Fe) and HKUST-1 samples.
Pseudo i s -o de kine ic
q
exp
(mg/
g)
in e cep q
cal
(mg/g)
slope k
1
(min
−1
)
R
2
MIL-100
(Fe)–
S_Agi .
38.6 4.443 85.0 −0.0422 0.0422 0.5720
MIL-100
(Fe)-
R_Agi .
46.7 4.357 78.0 −0.0460 0.0460 0.7666
MIL-100
(Fe)-
RT_Agi .
40.3 2.194 9.0 −0.0179 0.0179 0.2461
MIL-100
(Fe)–
S_S a .
15.4 1.570 4.8 −0.0167 0.0167 0.5305
MIL-100
(Fe)-
R_S a .
10.8 3.268 26.3 −0.0453 0.0453 0.7671
MIL-100
(Fe)-
RT_S a .
29.9 4.773 118.3 −0.0640 0.0640 0.5367
HKUST-1-
S-Agi .
47.0 4.742 114.7 −0.0560 0.0560 0.6679
HKUST-1-
R_Agi .
46.9 0.404 1.5 −0.0034 0.0034 −0.1086
HKUST-1-
RT_Agi .
47.0 3.067 21.5 −0.0334 0.0334 0.2826
HKUST-1-
S_S a .
21.0 3.225 25.2 −0.0460 0.0460 0.6596
HKUST-1-
R_S a .
28.1 4.066 58.3 −0.0558 0.0558 0.8216
HKUST-1-
RT_S a .
25.3 3.917 50.2 −0.0442 0.0442 0.6281
Pseudo second-o de kine ic
in e cep k
2
(g mg
−1
min
−1
)
slope q
cal
(mg/
g)
R
2
MIL-100(Fe)–
S_Agi .
1.042 0.0004 0.0207 48.40 0.8280
MIL-100(Fe)-
R_Agi .
0.448 0.0007 0.0183 54.73 0.9831
MIL-100(Fe)-
RT_Agi .
0.073 0.0085 0.0249 40.16 0.9930
MIL-100(Fe)–
S_S a .
0.320 0.0137 0.0661 15.13 0.9879
MIL-100(Fe)-
R_S a .
10.567 0.0000 −0.0144 −69.25 −0.1346
MIL-100(Fe)-
RT_S a .
1.293 0.0005 0.0258 38.83 0.8872
HKUST-1-S-
Agi .
0.527 0.0006 0.0172 58.00 0.9626
HKUST-1-
R_Agi .
−0.020 −0.0255 0.0229 43.73 0.9974
HKUST-1-
RT_Agi .
0.164 0.0027 0.0209 47.89 0.9724
HKUST-1-
S_S a .
0.498 0.0040 0.0445 22.47 0.9944
HKUST-1-
R_S a .
0.740 0.0012 0.0292 34.20 0.9744
HKUST-1-
RT_S a .
1.615 0.0005 0.0271 36.95 0.8011
C. Rod íguez-Es eban e al. Jou nal o Solid S a e Chemis y 339 (2024) 124956
8
[63]. In o de o ob ain in o ma ion abou he adso p ion kine ic, he
expe imen al da a we e i ed using he analysis model de ailed in he
Ma e ial Sec ion. The esul s a e collec ed in Fig. 12 and Table 2. The
co ela ion coe icien s (R
2
) o pseudo i s -o de kine ic model was in
he ange [0.24–0.77] wha indica es ha i is no app op ia e o p edic
he adso p ion o CR on MIL-100(Fe) o HKUST-1.
On he o he hand, as shown in Fig. 13 he pseudo-second-o de
model was i ed well o he kine ic da a. Addi ionally, he calcula ed
alue o he q
e
pa ame e om he pseudo-second-o de kine ic model
exhibi ed g ea acco dance wi h he expe imen al alues. The esul s o
kine ic analysis showed ha he adso p ion kine ics con o med o he
pseudo-second o de kine ic model. K
2
cons an s collec ed in Table 2
show a a ie y o alues ha no only depend on he ype o MOF bu
also on he p epa a ion me hod. Again, i is highligh ed ha he pe -
o mance o a gi en MOF does no only elies on i s inhe en na u e.
3.3. Humidi y adso p ion
The esul s o mois u e adso p ion a e collec ed in Fig. 14. MIL-100
(Fe) samples p esen a simila adso p ion capaci y whe eas he beha -
iou o HKUST_1 samples is mo e unalike. On one hand, he in e ac ion
wi h he wa e molecules in bo h cases is mainly h ough he open me al
cen e sa u a ing he coo dina ion sphe e o he me al cen e. In gene al,
he a ini y o MIL-100(Fe) o wa e molecules is highe and can be
explained in e ms o la ge bond en halpy (−75 kJ/mol s. −47.3 kJ/
mol o MIL-100(Fe) and HKUST-1, espec i ely) [64,65]. On he o he
hand, wa e molecules also in e ac wi h ligands and wi h o he wa e
molecules [66]. To do so, hey need o di use h ough he s uc u e, so
ha po e sizes and speci ic su ace a eas a e ea u es ha play a sig-
ni ican ole in mois u e adso p ion.
In all cases, he adso p ion p ocess is e e sible in he sense ha he
samples eco e hei ini ial weigh a e hea ing a 120 ◦C o 24 h. Two
addi ional cycles we e pe o med in o de o e alua e he ecycling and
euse aspec s o he p ocess. The analysis o Fig. 14 b) and c) shows ha
MIL-100(Fe) samples a he main ain he adso p ion p ope ies whils
he pe o mance o HKUST-1 ma e ials dec eases in each cycle. The
cha ac e iza ion o he samples a e mois u e adso p ion indica es ha
he e is no any change in he di ac og ams o all he samples espec o
hose o he p is ine MOF (Fig. S7). In o de o analyse in a g ea e ex en
he s abili y o HKUST-1 espec o mois u e adso p ion, he HKUST-1
samples we e exposed o ambien condi ions o 7 days. XRDs pa e ns
we e eco ded a di e en ime in e als and collec ed in Fig. S8 o he
supplemen a y ma e ial. The analysis indica es ha he s uc u e o
HKUST-1 emains a e 7 days o exposu e wi hou he appea ance o
new phases due o he hyd olysis o he MOF. Ne e heless, i is clea
ha he diminishing in he adso p ion could be in e p e ed as a i s s ep
in he deg ada ion o hese MOFs, especially in he case o HKUST-1 –R.
3.4. Iodine cap u e
Table 3 collec s he esul s o he adso p ion capaci y o apou
Iodine. The samples show a g ea a ini y o he Iodine wi h an
adso p ion la ge han 100 w % in 24 h. This up ake is an o de o
magni ude la ge han adi ional zeoli e so p ion ma e ials [34]. The
size o he I
2
molecules (2.6 Å) allows he di usion h ough he po es o
bo h MOFs. In his case, he adso p ion capaci y depends on he su ace
a eas being highe o HKUST-1-S and MIL-100(Fe) samples. When he
adso p ion is conside ed in e ms o mole o I
2
pe mole o MOF (see
Fig. S9 in he supplemen a y ma e ial), he beha iou o MIL-100(Fe) is
in ag eemen wi h he p esence o la ge po es in his MOF.
The eu iliza ion o he samples a e hei washing and d ying gi es
ise o he main enance o he le el o adso p ion. The PXRDs o samples
a e h ee cycles o iodine adso p ion expe imen s do no show any
e idence o s uc u al changes (see Fig. S10 in he supplemen a y ma-
e ial). The posi ions o he main e lec ions om MIL-100(Fe) and
HKUST-1 s uc u es emain unchanged in he diag ams. I is well-known
he s abili y o hese MOFs in e hanol media [27,52,67]. Howe e , i is
wo h no ing ha in he HKUST-1 diag ams, he in ensi y o he e-
lec ions is highly modi ied compa ed o hose in he p is ine samples
(see Fig. 1). This may be ela ed o he plausible o de ed iodine
adso p ion inside he mic opo e s uc u e, using he MOF s uc u e as a
empla e. Mo eo e , he cha ac e iza ion o he ma e ials using XRD
echniques indica e ha he emo al o I
2
does no a ec MOF s uc u es
in bo h MIL-100(Fe) and HKUST-1 samples.
The deso p ion o I
2
was s udied in a quali a i e way because he
ex ac ion wi h e hanol gi es ise o he o ma ion o di e en species
Fig. 13. The pseudo-second-o de plo o Congo Red adso p ion o MIL-100(Fe) and HKUST-1 samples unde s a ic and agi a ion condi ions.
C. Rod íguez-Es eban e al. Jou nal o Solid S a e Chemis y 339 (2024) 124956
9