This work demonstrates the development of mesoporous
trimetallic
oxide scaffolds (TOSs) as heterogeneous catalysts for the solvent-free
transformation of CO2 into various value-added products.
The TOS catalyst was prepared using a solution-combustion protocol
as a time- and energy-saving method using cobalt, magnesium, and cerium
metal nitrate salts as precursors and ethylene glycol as the fuel
system and compared with their monometallic counterparts. Characterization
suggested strong metal–metal and metal–oxygen interactions
in the 3D-interconnected hierarchical porous network, which resulted
in substantial alteration in the electronic, structural, and physicochemical
properties. This resulted in an appreciable surface area, acid–base
cooperative sites, and a larger pore volume in the catalyst. Thereafter,
the CoMgCe-TOS catalyst was first used for the solvent-free cyclization
of o-phenylenediamines and CO2 to produce
benzimidazoles. In the presence of dimethylamine borane as a reductant,
benzimidazoles were obtained in 94% yield at 100 °C under pressurized
conditions, along with good recyclability for 12 cycles. It was established
that selectivity toward benzimidazole improved upon the incorporation
of basic metals (Mg and Ce), with Co aiding in the formation of cooperative
Lewis acid–base sites. A plausible mechanism was also predicted,
wherein the overall conversion of reactants for the N-formylation
step was influenced by the number and strength of basic sites, and
the cyclization step for selectively obtaining benzimidazole was affected
by the acidic site strength. Besides the production of benzimidazole,
the catalyst was also highly active for the synthesis of cyclic carbonates
using epoxides and CO2 under optimized reaction conditions.
This work therefore provides a new and greener route for the synthesis
of benzimidazoles and organic carbonates, which can be easily adapted
for scale-up applications.
The excess anthropogenic CO 2 depletion via the catalytic approach to produce valuable chemicals is an industrially challenging, demanding, and encouraging strategy for CO 2 fixation. Herein, we demonstrate a selective one-pot strategy for CO 2 fixation into "oxazolidinone" by employing stable porous trimetallic oxide foam (PTOF) as a new catalyst. The PTOF catalyst was synthesized by a solution combustion method using transition metals Cu, Co, and Ni and systematically characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), N 2 sorption, temperature-programmed desorption (TPD), and X-ray photoelectron spectroscopy (XPS) analysis. Due to the distinctive synthesis method and unique combination of metal oxides and their percentage, the PTOF catalyst displayed highly interconnected porous channels along with uniformly distributed active sites on its surface. Well ahead, the PTOF catalyst was screened for the fixation of CO 2 into oxazolidinone. The screened and optimized reaction parameters revealed that the PTOF catalyst showed highly efficient and selective activity with 100% conversion of aniline along with 96% selectivity and yield toward the oxazolidinone product at mild and solvent-free reaction conditions. The superiority of the catalytic performance could be due to the presence of surface active sites and acid−base cooperative synergistic properties of the mixed metal oxides. A doubly synergistic plausible reaction mechanism was proposed for the oxazolidinone synthesis experimentally with the support of DFT calculations along with bond lengths, bond angles, and binding energies. In addition, stepwise intermediate formations with the free energy profile were also proposed. Also, the PTOF catalyst displayed good tolerance toward substituted aromatic amines and terminal epoxides for the fixation of CO 2 into oxazolidinones. Very interestingly, the PTOF catalyst could be significantly reused for up to 15 consecutive cycles with stable activity and retention in physicochemical properties.
The discharge of CO2 into the atmosphere has become a crucial issue for mankind and a great threat to the environment due to escalating consequences of global warming. This grants...
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