CO₂ cycloaddition with epoxides to produce cyclic carbonates is a promising route with 100 % atom economy, offering both environmental sustainability and economic feasibility. Cyclic carbonates are high-value products used as electrolytes in lithium-ion batteries, polar aprotic solvents, and intermediates in fine chemical synthesis. However, conventional homogeneous catalytic systems face challenges in catalyst separation and recycling, high energy consumption, and substantial waste generation. In a study published in ENG. Chem. Eng., researchers at Tianjin University report a series of Ce-doped BiOI catalysts that overcome these limitations through a unique oxygen vacancy-mediated Lewis acid-base pair mechanism.
The team synthesized CeₓBi₁₋ₓOI catalysts via a one-pot solvothermal method. TEM images revealed that Ce₀.₁Bi₀.₉OI and Ce₀.₁₅Bi₀.₈₅OI maintained a nanoflower-like architecture assembled from nanosheets, while excessive Ce doping (Ce₀.₂Bi₀.₈OI) disrupted this structure. HRTEM analysis showed gradual contraction of the (110) interplanar spacing from 0.2832 nm in BiOI to 0.2768 nm in Ce₀.₂Bi₀.₈OI, confirming successful Ce incorporation into the [Bi₂O₂]²⁺ lattice via isomorphous substitution.
XPS analysis revealed that Ce₀.₁Bi₀.₉OI exhibited the highest Ce³⁺ proportion (48.2 %) and the highest oxygen vacancy concentration (54 %) among all samples, confirmed by EPR analysis showing a stronger signal at g = 2.003. Pyridine-IR spectroscopy demonstrated enhanced Lewis acidity on Ce₀.₁Bi₀.₉OI, while low-pressure CO₂ adsorption showed improved CO₂ uptake, attributed to increased surface oxygen vacancies.
Under solvent- and cocatalyst-free conditions (2 MPa CO₂, 120 °C, 12 h), Ce₀.₁Bi₀.₉OI achieved a butylene carbonate yield of 91 %, significantly outperforming undoped BiOI (65 %). The catalyst also delivered 99 % propylene carbonate yield and 93 % (chloromethyl)ethylene carbonate yield. Activity across samples showed a clear positive correlation with surface oxygen vacancy concentration. Oxygen vacancy repair experiments (O₂ treatment at 200 °C) reduced BC yield from 91 % to 43 % for Ce₀.₁Bi₀.₉OI–O₂, confirming the critical role of oxygen vacancies.
Kinetic analysis revealed a decreased reaction order for butylene oxide and a substantially lower apparent activation energy (70.67 kJ·mol⁻¹ vs. 108.09 kJ·mol⁻¹), providing strong evidence that Ce doping facilitates epoxide adsorption and reduces the energy barrier for epoxy ring-opening.
DFT calculations showed that BO adsorption on the Oᵥ–Ce³⁺ site is stronger than on the Oᵥ–Bi³⁺ site, with a shorter adsorption bond length. In situ DRIFTS revealed that PO molecules are strongly adsorbed onto the catalyst surface, and product propylene carbonate forms continuously with no detectable linear carbonate intermediates, confirming that ring-closure is not rate-determining.
The proposed mechanism involves: BO adsorption onto Oᵥ–Ce³⁺ sites; adjacent I⁻ nucleophilic attack on the less sterically hindered carbon, initiating ring-opening; CO₂ insertion at Bi–Oᵥ sites to form a linear alkyl carbonate intermediate; and intramolecular cyclization to yield the cyclic carbonate product.
This work demonstrates that constructing oxygen vacancy-mediated Lewis acid-base pairs via Ce doping provides a feasible strategy for designing efficient heterogeneous catalysts for CO₂ conversion.
DOI
10.1007/s11705-026-2677-z