Inorganic Chemistry
Article
For sustainable growth and mitigating water pollution,
development of new smart materials with efficient reversible
dye adsorptive removal with good recyclability has great
promise with real-world applications. Besides, traditional
porous functional materials such as zeolite, charcoal, activated
carbon, etc., MOFs have been receiving much attention as
adsorbents for hazardous molecules including dyes in recent
years.16−25 The primary requirement for MOF as a dye
adsorbent is the thermodynamic stability of the MOF in
aqueous dye solutions. The mechanistic aspects of dye
adsorption on MOF surfaces mainly hinge on various inherent
elements such as porosity and pore geometry, MOF linkers
with specific functional groups, and suitable donor−acceptor
pairs for hydrogen-bonding interactions of the framework and
dye molecules. Moreover, the charge of dye and adsorbent
material also plays a crucial role in governing the dye
adsorption phenomenon. For example, anionic frameworks
tend to adsorb cationic dyes and vice versa, and neutral
frameworks can show selectivity for dyes of either charge,
recognized by various noncovalent interactions such as
hydrogen bonding, π···π stacking, as well as pore dimensions
and steric factors.26−28 Hence, MOFs as celebrated materials
have been used for adsorptive removal/separation of dyes from
wastewater/textile effluent due to their tunable structure−
property features.
Alternatively, MOFs are also well-known for their hetero-
geneous and recyclable catalytic properties for chemical
productions based on their high surface area, tunable pore
sizes, abundant Lewis/Brønsted acid/base sites, and control-
lable structures. Catalytic transformation of fructose and
glucose into high-value chemical intermediates including 5-
hydroxymethylfurfural (HMF) is one of the important
reactions with industrial significance. HMF is a promising
building block for chemical industries, and the selective C6
sugar-to-HMF catalytic transformation is an industrially
important reaction in the context of sustainable development.
HMF is a versatile intermediate in the production of value-
added chemicals such as alternative fuels, diesel fuel additives,
industrial solvents, bioderived polymers, etc. A typical strategy
for the synthesis of HMF is the dehydration reaction of
fructose/glucose using a variety of catalytic materials such as
metal complexes and oxides, metal halides, ion-exchange
resins, zeolites, functionalized carbonaceous materials, func-
tionalized mesoporous materials, acidic ionic liquids (ILs)
including organic inorganic acids, etc.29−33 The main drawback
for some of these catalytic systems such as equipment
corrosion and difficult product isolation, low product yield,
longer reaction times in the case of solid acid catalysts, and the
low thermal stability of the resin-based catalysts hampering
reaction efficiency. Even though a handful of MOF-based
catalysts for fructose to HMF production is available in the
literature, the design and development of stable and efficient
catalysts are in demand considering the industrial importance
of HMF.34−41 MOFs with a highly porous and ordered nature
with coordinatively unsaturated metal sites and an organic
ligand component with inherent functional groups or the
introduction of desired functional groups into MOFs through
postsynthetic modification (PSM) can probably promote
substrate transfer within the MOF catalyst in facilitating
HMF production by an efficient catalytic reaction.
and selective detection/sensing of hazardous pollutants.9,42−47
Herein, we synthesized two water-stable, isostructural mixed-
ligand two-dimensional (2D) MOFs {[Zn(5OH-IP)(L)]}n
(ADES-4) and {[Cd(5OH-IP)(L)]}n (ADES-5) involving
(E)-N′-(pyridin-3-ylmethylene)nicotinohydrazide) (L) and 5-
hydroxyisophthalic acid (5OH-H2IP) as ligands. Both MOFs
were crystallized by the self-assembly of 5OH-IP and L with
the respective metal salt in a MeOH/H2O medium by slow
diffusion at room temperature, and the bulk phase-pure
materials were synthesized by different routes. The present
work reports on the preparation and comprehensive character-
ization of two isostructural MOFs, by various physicochemical
methods as well as SXRD analysis. The application of these
smart materials toward reversible adsorptive removal of dyes
and recyclable heterogeneous catalysts for the conversion of
C6 sugar to high-value chemical intermediate HMF under
moderate reaction conditions is discussed.
RESULTS AND DISCUSSION
■
Characterization of ADES-4 and ADES-5. Crystals and
bulk phase-pure product of the isostructural compounds
ADES-4 and ADES-5 obtained by different methods have
been characterized by SXRD analysis and various physico-
chemical methods. The experimental PXRD profile of the
synthesized materials is in good agreement with SXRD
patterns of the crystals establishing the bulk phase purity of
both MOFs (Figure S1). Further, detailed characterization of
both MOFs was established from experimental data such as
FTIR, TGA, SXRD, and elemental analysis. Both MOFs
revealed numerous characteristic absorption bands of (E)-N′-
(pyridin-3-ylmethylene)nicotinohydrazide) (L) and the corre-
sponding dicarboxylate anion in the FTIR spectra. The
presence of carboxylate groups appearing as strong bands at
1545 and 1390 cm−1 for ADES-4 and at 1554 and 1379 cm−1
for ADES-5 respectively can be ascribed to the ν(COO)as and
ν(COO)s vibrations. The difference in carbonyl symmetric and
antisymmetric stretching frequencies Δν = 155 and 175 cm−1
indicates the carboxylate moieties with chelating and bidentate
coordination modes in MOFs. The amide N−H and CO
bands of L appeared at 3197 (w) and 1666 cm−1 (s) for ADES-
4 and at 3203 (w) and 1667 cm−1 (s) for ADES-5, which were
observed at 3202 and 1676 cm−1 in the free ligand L (Figure
S2).46 The red shift of the N−H bands in the spectra of the
MOFs can be explained by the formation of N−H···O
hydrogen bonds between the amide functionality of L with
dicarboxylate oxygen atoms as observed from SXRD analysis.
The nonbonded hydroxyl stretching frequency of 5OH-IP
appeared as broad medium bands at 3421 and 3423 cm−1
corresponding to ADES-4 and ADES-5. Thermal and chemical
stability is a highly desirable property of MOFs toward
practical applications. As depicted in Figure S3, TGA data
revealed good thermal stability for both MOFs, and the
degradation of the organic ligand moiety followed by the
structural decomposition commences after 350 °C in both
cases. Further, the hydrolytic stability of both MOFs was also
established by PXRD data of the dried, recovered MOF
material soaked in water for 72 h, showing an identical pattern
with simulated PXRD data (Figure S1). The chemical stability
of ADES-4 in MeOH and DMSO solvent was also check by
dispersing the MOF material in the respective solvent for 72 h
as well as refluxing for 24 h in the case of DMSO, and the
PXRD patterns of recovered materials matched well with
simulated PXRD data confirming the stability of the material in
Our research activities are focused on the design, synthesis,
and practical utility of mixed-ligand MOFs/luminescent MOFs
(LMOFs) as heterogeneous catalysts for CO2 sequestration
9182
Inorg. Chem. 2021, 60, 9181−9191
Patel, Unnati
