(195c) Oriented and Silica-Beta Zeolite Membranes for n-Butanol Recovery from Its Dilute Aqueous Solution

The microstructure and orientation manipulation of supported zeolite membranes has been a substantial challenge to chemists and chemical engineers. However, the manipulation and optimization of membrane microstructure and/or orientation are beneficial to the intensification of separation and catalytic processes. The separation and/or catalytic performances of supported zeolite membranes can be greatly enhanced by controlling the crystal orientation on top layers as well as in seed layers. Herein, we are interested in pursuing c-oriented beta zeolite membranes. The highly c-oriented beta zeolite membranes enriched with a chiral pore network of polymorph A will have application potential in membrane separation and/or catalytic reactions for chiral substance systems.

For supported zeolite films or membranes, the microstructure and orientation manipulation can be achieved using several synthetic strategies such as the designed assembly of post-synthetic crystals on a substrate surface, controlled growth over a well-oriented seed layer, in-situ oriented nucleation and growth on a structure-directing matrix, or changing synthesis conditions and the ingredient of the synthesis solution. Therefore, the highly c-oriented beta zeolite membrane is going to be synthesized by secondary growth on the α-Al₂O₃ supported seed layer. The influential factors to the orientation, thickness, continuity and surface roughness of beta zeolite membranes are to be discussed in detail.

The continuous pure-silica beta zeolite membranes of different microstructure and orientation have been synthesized by secondary growth in the fluoride media under various H₂O/SiO₂ ratios and crystallization times. It is evident that the small-sized seed layer by spin-coating instead of dip-coating can supply a suitable growth environment to achieve a nice surface smoothness and a suitable seed population, leading to the formation of the oriented beta zeolite membrane in the following seeded growth step. The microstructure and orientation manipulation of supported beta zeolite membranes can be achieved in secondary growth by changing the H₂O/SiO₂ ratio and crystallization time. At the H₂O/SiO₂ ratio of 7, the continuous and highly c-oriented zeolite beta membrane can be synthesized over the randomly oriented seed layer after secondary growth for 9 days, while the well-intergrown (h0l)-oriented beta zeolite membrane can be obtained under the H₂O/SiO₂ ratio of 4. The compactness and thickness of the synthesized beta zeolite membrane are both increased by prolonging the crystallization time. After secondary growth for 9 days, the densely-packed and 10 mm thick beta zeolite membrane with highly c-oriented truncated square bipyramidal crystals can be fabricated under the H₂O/SiO₂ ratio of 7, where the percentage of c-oriented crystals reaches to 79.9% according to the measurement of X-ray pole-figure analysis. The evolution of the highly c-oriented beta zeolite membrane follows the principle of the evolutionary selection proposed by van der Drift, based on which the fastest growth rate of truncated square bipyramidal crystals along the (00l)-direction leads to the formation of the highly oriented beta zeolite beta membranes with their c-axes perpendicular to the substrate surface. The synthesized (h0l)-oriented beta zeolite membrane exhibits a good balance between the flux and the selectivity for pervaporation of n-butanol/water mixtures, with a total flux of 1449 g·m^-2·h^-1 and a selectivity n-butanol/H₂O of 36 in pervaporation of 1 wt.% n-butanol/H₂O mixture. The availability of highly c-oriented or preferentially (h0l)-oriented beta zeolite membranes could trigger novel applications in membrane separation and/or catalytic reactions involving chiral substances.

This work was supported by the National Natural Science Foundation of China (21136008).