Straightforward Processing Route for the Fabrication of Robust Hierarchical Zeolite Structures

Strong hierarchical porous zeolite structures are prepared by a sol-gel method using freeze gelation. Instead of conventional binders in powder form, such as bentonite or kaolin, it has been proven that using a freeze gelation method based on a colloidal silica sol is a more straightforward and easier-to-use-approach in fabricating highly mechanically stable zeolite monoliths. The resulting zeolite slurries possess superior rheological properties (not being pseudoplastic) and show low viscosities. This low viscosity of the slurry enables an increase in the solid content without compromising the extraordinary good flow behavior for casting applications. Additionally, in comparison to conventional powdery binders, zeolite samples prepared by using a colloidal silica sol exhibit a significantly higher mechanical strength. This mechanical strength can be further improved by either increasing the zeolite content or increasing the silica to zeolite ratio. Increasing the zeolite content leads to an increased volumetric adsorption capacity for CO₂ as the test gas, resulting from the increased amount of zeolite particles per unit volume. In addition, a higher solid content of the zeolite monoliths leads to higher compression strengths, while showing the same elastic deformation and brittle failure characteristics. In turn, increasing the silica to zeolite ratio does not affect the volumetric adsorption capacity for CO₂. Nevertheless, higher silica contents lead to a significant increase in the elastic deformation and absorbed work until failure. Therefore, the proposed processing route based on freeze gelation presents an easy and unique tool to tune the mechanical and gas adsorptive properties of hierarchically structured zeolite monoliths, according to the application requirements.