M. Yousefian and Z. Rafiee
CarbohydratePolymers228(2020)115393
MOFs is their limited chemical stability to aqueous and organic media,
and strongly acidic or alkaline environment and difficult recovery. One
of the methods to avoid the limitations of applications involving MOFs
is hybridization of MOFs with polymers to combine the benefits of both
the components. For this, different polymers such as biopolymers,
conducting polymers, polystyrene, polyimide and so forth have been
dimethylformamide (DMF), chloroform, ethanol, benzaldehyde, mal-
ononitrile, 4-nitrobenzaldehyde, 4-chlorobenzaldehyde, 2-chlor-
obenzaldehyde, 4-methoxybenzaldehyde, 4-methylbenzaldehyde, 2-
hydroxybenzaldehyde were purchased from Merck Company
(Darmstadt, Germany) and used without further purification. CS with
medium molecular weight and degree of deacetylation of 75–85% was
purchased from Sigma-Aldrich (St. Louis, MO, USA).
2014). Among the biopolymers, chitosan, a natural and low-cost bio-
polymer, is an outstanding polymeric material owing to its exceptional
properties such as biocompatibility, biodegradability, nontoxicity,
availability, exclusive three-dimensional structure, chemical reactivity
due to the presence of amino and hydroxyl groups, exceptional che-
lating properties and insolubility in most organic solvents and water
2.2. Apparatus
FT-IR spectra were recorded with a Jasco-680 spectrometer (Japan)
in the range of 4000–400 cm−1. FT-IR spectra of all compounds were
collected by making their pellets in KBr as a medium. The diffraction
pattern of related materials was recorded in the reflection mode using a
Bruker, D8 Advance diffractometer. Nickel filtered CuKα radiation
(radiation wavelength, λ = 0.154 nm) was produced at an operating
voltage of 45 kV and a current of 100 mA. The ultrasonic bath (Tecno-
GAZ SPA Ultrasonic system, Italy) was used at a frequency of 60 Hz and
power of 130 W. The morphology of catalyst was studied by scanning
electron microscopy (SEM; EM10C-ZEISS, 80 KV, Zeiss Co., Germany)
and transmission electron microscopy (TEM, EM10C-ZEISS, Zeiss Co.,
Germany). The surface area and average pore diameter for the catalyst
were carried out at 77 K using a BELSORP-mini IIREOTERM (Japan)
system, based on the Brunauer–Emmett–Teller (BET) method under the
degassing temperature of 473 K.
have been extensively applied in different reactions such as hydrogen
Knoevenagel condensation is one of the most significant and widely
employed reactions for C]C bond formation from the carbonyl com-
pounds and active methylene linkages in organic synthesis (Han et al.,
synthesis of intermediates or end products for bioactive compounds,
acids, bases and acid-base sites, and usually performed under homo-
volumes of waste produced. From the viewpoint of sustainable en-
vironment, the goal is protection of environment through use of the
cost-effective, efficient, high-yielding, simple and environmental
friendly methods. So, the recognition of the solvent-less heterogeneous
catalysts not only conduct the use of organic solvent but also side re-
actions are restricted. This leads to enhanced yields and selectivities.
Recently, the various solid-supported heterogeneous catalysts have
been applied to Knoevenagel reaction including Al-MOFs
2019). It has been well established that the growth of Cu-MOF on
chitosan (CS) surface affords stable heterogeneous catalyst with ex-
cellent recyclability. In the present work, CS-CA/Cu-MOF was fabri-
cated and utilized as a highly efficient and cost-effective mesoporous
heterogeneous catalyst for the Knoevenagel reaction under solvent-free
conditions. The synergistic effects of CS-CA and Cu-MOF is expected to
enhance catalytic performance.
2.3. Synthesis of citric anhydride
A mixture of CA (0.5 g), acetic acid (2 mL) and acetic anhydride
(1 mL) was heated at 40 °C under a nitrogen atmosphere for 20 h. After
completion of the reaction, warm chloroform (10 mL) was added to the
reaction mixture and stirred for 15 min. The resultant precipitate was
collected and washed with hot chloroform and dried at 70 °C under
vacuum.
2.4. Preparation of citrate ester of CS
CS (0.45 g) was dissolved in 10 mL of acetic acid. Then, citric an-
hydride (0.45 g) was added into the reaction mixture and stirred at
110 °C for 3 h. The resultant precipitate was collected and washed with
deionized water and dried at 70 °C under vacuum.
2.5. Synthesis of CS-CA/Cu-MOF
Citrate ester of CS (0.5 g) which was dispersed in DMF (60 mL) was
sonicated for 15 min. Then, Cu(NO3)2·3H2O (2.42 g, 10 mmol) and 1,4-
BDC (0.50 g, 3 mmol) was added into the mixture. The mixture was
transferred to a 150-mL Teflon-lined autoclave, sealed and heated in an
oven at 100 °C for 12 h. After cooling, the resulting crystalline solid was
collected by filtration, thoroughly washed by ethanol several times, and
dried overnight at room temperature.
2.6. General procedure for the Knoevenagel condensation using CS-CA/Cu-
MOF as a catalyst
A mixture of various aldehydes (1 mmol), malononitrile (1.5 mmol)
and CS-CA/Cu-MOF (10 mg) was stirred at 25 °C under solvent-free
conditions. TLC was utilized to monitor the progress of the reaction.
After completion of the reaction, warm ethanol (10 mL) was added to
the reaction mixture and CS-CA/Cu-MOF was separated. The solvent
was evaporated and the attained solid was recrystallized from ethanol
to produce the pure product. Then, the recovered catalyst was reused in
eight runs under similar conditions as the first run to exhibit the re-
cyclability and stability of the prepared catalyst.
2. Experimental
2.1. Materials
The reagents and chemicals including citric acid (CA), acetic acid,
acetic anhydride, copper(II) nitrate trihydrate (Cu(NO3)2·3H2O), 1,4-
benzenedicarboxylate
(1,4-BDC,
terephthalic
acid),
N,N-
2
Yousefian, Mahnaz
