Converting CO₂ into value added chemicals is a key strategy for reducing greenhouse gas emissions. Coupling CO₂ hydrogenation with toluene alkylation offers an atom economic route to produce para xylene (PX), an important platform chemical for synthetic fibers, resins, and rubbers. However, conventional ZSM 5 catalysts possess strong Brønsted acidity that triggers uncontrollable side reactions including dealkylation, deep alkylation, and xylene isomerization, limiting PX selectivity.
In a study published in ENG. Chem. Eng., researchers at Taiyuan University of Technology report a bifunctional catalyst system that addresses these challenges using silanol nest enriched silicalite 1 (AS 1) as the alkylation component, combined with ZnZrOₓ (ZZO) as the CO₂ hydrogenation catalyst.
The AS 1 zeolite exhibits a total acidity of only 38 μmol·g⁻¹, far lower than that of ZSM 5 (443 μmol·g⁻¹). This moderate acidity originates from silanol nests rather than framework aluminum, effectively suppressing side reactions. Compared to ZSM 5, AS 1 reduces benzene selectivity from 6.04 % to 0.5 % and increases PX selectivity from 23.6 % to 33 %. In situ DRIFTS and GC MS analyses confirmed that the mild acidity of AS 1 significantly reduces coke formation, as the spent AS 1 catalyst showed minimal soluble coke compared to the dark, heavily coked ZSM 5.
Systematic optimization of the ZZO:AS 1 mass ratio revealed a volcano shaped trend in toluene conversion, with the best performance at a 1:4 ratio (toluene conversion: 13.2 %). Optimizing the particle sizes (ZZO: 20–40 mesh, AS 1: 80–100 mesh) further increased toluene conversion to 16.3 %. Proximity studies showed that powder mixing enhanced contact and shortened diffusion paths, yielding superior performance compared to physically separated dual bed configurations.
To enhance shape selectivity, an epitaxial silicalite 1 shell was grown on AS 1. The AS 1@25 %S 1 sample achieved a PX selectivity of 44.4 %, while maintaining a toluene conversion of 15.2 %. However, thicker shells (50 % and 100 %) introduced excessive diffusion limitations, reducing toluene conversion.
Ammonium hexafluorosilicate (AHFS) treatment was employed to generate mesopores and improve diffusion. The 16 h treated sample showed increased mesopore volume (from 0.02 to 0.08 cm³·g⁻¹) and enhanced toluene conversion to 20.9 %, though PX selectivity decreased to 31.3 % due to promoted isomerization.
Finally, the deposition precipitation method was used to load ZZO onto AS 1@S 1, maximizing dispersion and minimizing intermediate diffusion distances. The integrated ZZO/AS 1@25 %S 1 catalyst (1:2 loading ratio) achieved a CO₂ conversion of 9.5 %, toluene conversion of 15 %, and PX selectivity of 42.7 %. Under reduced pressure (0.5 MPa), PX selectivity reached 57.5 %. Stability tests over 48 h showed that the integrated catalyst maintained stable performance, with toluene conversion decreasing only from 16.2 % to 13 %, attributed to the S 1 shell mitigating Zn migration poisoning.
This work provides novel insights into designing bifunctional catalysts for CO₂ hydrogenation coupled with toluene alkylation, offering a sustainable route for both greenhouse gas utilization and high value aromatic production.
DOI
10.1007/s11705-026-2674-2