Bimetallic metal–organic frameworks
(MOFs) containing two
different inorganic metal nodes exhibited enhanced properties in CO2 adsorption and catalytic conversion compared with the corresponding
monometallic MOFs. In this work, the novel bimetallic Zn/Mg-MOF-74
with different ratios of Zn/Mg was synthesized successfully by a facile
one-pot method. Powder X-ray diffraction, Fourier transform infrared,
X-ray photoelectron spectroscopy, scanning electron microscopy/transmission
electron microscopy, N2/CO2 adsorption/desorption,
and CO2/NH3-temperature-programmed desorption
techniques thoroughly characterized the structure, morphology, and
physicochemical properties of Zn/Mg-MOF-74. Besides the excellent
CO2 adsorption capacity (128.3 cm3/g at 273
K and 1 bar), Zn0.75Mg0.25-MOF-74 also showed
efficient catalytic activity for the cycloaddition reaction of CO2 and epoxides to cyclic carbonates with outstanding yield
and selectivity all over 99% under solvent-free and mild conditions
(60 °C, 0.8 MPa), outperforming the mechanical combination of
Zn-MOF-74 and Mg-MOF-74 with the same metal contents, indicating the
synergistic effect of two adjacent metals in bimetallic MOF-74. In
addition, the Zn0.75Mg0.25-MOF-74 catalyst could
be recycled for at least five runs and possess good versatility to
various substrates. Finally, a feasible mechanism of the catalytic
reaction was proposed. Thanks to the high surface area, affinity toward
CO2, and accessibility of multiple active sites of the
unsaturated metal centers as active Lewis acid sites and O atoms from
Zn–O and Mg–O as Lewis basic sites, efficient chemical
fixation of CO2 to cyclic carbonates was obtained over
the Zn0.75Mg0.25-MOF-74 catalyst. The present
facile synthesis and application of a robust bimetallic MOF catalyst
offered a competitive avenue for the integration of CO2 adsorption and CO2 catalytic conversion.
A novel Lewis acid–base
bifunctional Zn(II)-based MOF-Zn-1
[Zn2L2MA·2DMF] (MA = melamine, H2L = 2,5-thiophenedicarboxylic acid), with abundant micropores and
free -NH2 groups was facilely assembled by incorporating
zinc(II) ion with nitrogen-rich melamine and 2,5-thiophenedicarboxylic
acid ligands. The constructed MOF-Zn-1 presented an excellent affinity
toward CO2 molecules due to the Lewis-base property together
with abundant micropores. The Zn active sites could be used for epoxide
activation. The acid–base synergistic effects facilitated CO2 conversion into cyclic carbonates under ambient temperature
using the porous MOF-Zn-1 as a heterogeneous catalyst. Moreover, the
MOF-Zn-1 exhibited satisfied stability and versatility, and it was
easy to recycle with no obvious decrease of catalytic activity. Then
the feasible synergistic mechanism of MOF-Zn-1/Bu4NBr catalysts
for CO2 conversion was proposed.
A new tetratopic bridging ligand 1,1′-(propane-1,3diyl)bis(1H-pyrazole-3,5-dicarboxylic acid) (PDC) with functional pyrazole groups was designed and used for reacting with environmentally friendly zinc centers in mixed solvents; the resulting framework [Zn 3.5 (PDC) 2 (H 2 O) 10 ] (abbreviated as MOF Zn-2PDC) contained numerous organic−inorganic crablike blocks and had high density of zinc sites and coordinated water molecules. Abundant open metal sites (OMSs) were generated after the removal of coordinated water molecules by vacuum drying, which produced exceptionally catalytic performance for CO 2 cycloaddition to bulky styrene oxide, and the produced styrene carbonate yield and selectivity reached up to 86 and 97% under mild conditions of 55 °C, 12 h, and 1 MPa. Moreover, MOF Zn-2PDC also showed general applicability to other bulky epoxides. Meanwhile, the MOF Zn-2PDC catalyst remained with good catalytic performance after six cycles.
An efficient Zn(Bmic)(AT) MOF with available Lewis acid− base blocks was prepared by a solvothermal method combining the mixed ligands 5-aminotetrazole (HAT) and 1-benzimidazole-5-carboxylic acid (H 2 Bmic). The prepared Zn(Bmic)(AT) catalyst is rich in amino groups, uncoordinated oxygen atoms, and multiple open metal sites (OMSs), which contribute to CO 2 adsorption of 79 cm 3 /g at 273 K and 52 cm 3 /g at 298 K, with high adsorption enthalpy (28 kJ mol −1 ). Furthermore, its multiple functional sites facilitate CO 2 cycloaddition to various small-molecule epoxides to give five-ring cyclic carbonates with the assistance of a Bu 4 NBr cocatalyst, achieving a high TON of 170 for propylene carbonates (PC) under solventfree, mild conditions (80 °C, 5 bar). In addition, the Zn(Bmic)(AT) catalyst exhibits good recyclability for six consecutive cycles. Moreover, a feasible synergistic mechanism of Zn(Bmic)(AT)/Bu 4 NBr catalysts for CO 2 conversion into cyclic carbonate is proposed.
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