Solvent molecules can act as gatekeepers inside zeolite catalysts, limiting the space available for larger reactants to reach internal active sites. A recent study has shown that controlling this solvent occupancy can substantially improve catalytic performance while preserving the material's intrinsic micropore structure.
Led by researchers Assoc. Prof. XING Jiacheng, Prof. XU Yunpeng and Prof. LIU Zhongmin from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences, and published in the Journal of the American Chemical Society, the study reveals how relatively bulky molecules gain access to active sites within narrow zeolite channels.
Zeolites contain networks of nanoscale channels that regulate the adsorption and conversion of molecules. In titanium silicalite-1 (TS-1), the low activity toward cyclohexene has often been attributed to the narrow MFI channels. Efforts to improve the conversion of bulky molecules have therefore frequently focused on enlarging pores or shortening diffusion pathways.
Using cyclohexene epoxidation as a model reaction, the researchers found that cyclohexene conversion was below 6.9% in methanol but reached as high as 88.8% without added reaction solvent. The selectivity toward the desired epoxide remained above 89%. This improvement was achieved without modifying the original micropore structure of TS-1.
Furthermore, the researchers reveled the underlying mechanism. Cyclohexene can enter the MFI channels when they are empty or only weakly occupied. In contrast, methanol preferentially occupies confined space at pore entrances and within the channels, reducing access for cyclohexene. The results link solvent-dependent catalytic behavior to competition for limited pore space and access to titanium active sites.
The researchers describe this behavior as "solvent-gated accessibility," highlighting solvent occupancy as an additional factor governing molecular access in porous catalysts, alongside pore geometry, framework motion and molecular conformation. Similar solvent-dependent effects were observed in other titanosilicate catalysts and with larger cyclic alkenes, suggesting that the mechanism may have broader relevance.
"Our study demonstrates a practical strategy for improving the utilization of existing microporous catalysts by regulating solvent competition within their pores," said Prof. XU. "Reducing the use of added solvent could also simplify solvent recovery and product separation, offering opportunities for more resource-efficient oxidation processes."