Mass transfer of guest molecules in nanoporous crystalline materials is significant in spanning heterogenous catalysis and gas separation. Intracrystalline diffusion is intrinsic to molecular properties and material structures, and surface barriers represent another important transport mechanism.
In many processes, surface barriers of guest molecules may dominate the overall mass transport rate in zeolite catalysts. Although the importance of surface resistance in mass transfer over zeolite crystals is considered, controlling the performance of catalytic reaction by direct modulation of surface barriers remains challenging.
Recently, a research group led by Prof. YE Mao and Prof. LIU Zhongmin from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences studied the mass transfer in the zeolite surface. They regulated the catalytic lifetime and the light olefins selectivity of Methanol-to-olefins (MTO) process by controlling the surface barriers of SAPO-34 zeolites.
The study was published in Angew. Chem. Int. Ed. on Sept. 3.
The reduction of surface barriers could prolong catalyst lifetime and promote light olefins selectivity (Image by LI Hua)
"We modified SAPO-34 zeolites by chemical liquid deposition and acid etching," said Prof. YE.
According to the surface permeability formula, surface permeabilities of guest molecules for these zeolites could be directly measured. Meanwhile, the morphology, pore structure, acid properties and intracrystalline diffusivity were almost unchanged during the modification.
The results showed that a decrease in surface barriers could speed up the transfer of guest molecules inward and outward the zeolite crystal, and improved the catalytic lifetime and the selectivity of light olefins.
This work demonstrated that the controlling of surface barriers in SAPO-34 zeolites could be applied to modulate catalytic performance. It opens up a way to improve reaction performance by controlling mass transport of guest molecules in zeolite catalysis.
The research was supported by the National Natural Science Foundation of China. (Text by LI Hua)