Tuning the framework polarity in MFI membranes by deboronation: Effect on mass transport

The polarity of a zeolite is an important parameter determining the transport and separation properties in zeolite packed beds and membranes. This study focuses on the systematic variation of the zeolite polarity for membrane applications by varying the amount of silanol groups within the framework of the MFI zeolite by deboronation and consecutive heat treatment from 9 to 4 silanol groups per unit cell. The decrease in the number of silanol groups after heat treatment is confirmed by diffuse reflection infrared Fourier transform (DRIFT) and ²⁹Si Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). The permeation of pure water through the modified silicalite membranes is insensitive to the number of silanol groups, attributed to a decreased adsorption compensated by an increased diffusivity. The ethanol selectivity in water/ethanol mixtures shows an increase with decreasing amount of silanol groups. Residual boron in the zeolite may react with ethanol and decrease its adsorption capacity. A reduction in flux through the debronated MFI membranes was not observed after ethanol/water permeation.