Materials chemistry
Zeolites, mesoporous silica
Explore how ordered pores in zeolites and mesoporous silica enable molecular sieving, catalysis, and adsorption, and how framework chemistry governs performance.
IntuitionA solid with molecular-sized corridors
A porous crystal can act like a building with uniform doorways: molecules that fit and interact favourably enter, while others are excluded or move more slowly. The pore wall chemistry matters as much as the empty space.
SchoolFrameworks and pore classes
Definition: Zeolite
A crystalline microporous aluminosilicate (or related framework) built from corner-sharing TO₄ tetrahedra, commonly SiO₄ and AlO₄. Framework Al introduces negative charge balanced by extra-framework cations, which can exchange or provide acid sites.
The IUPAC pore-width classes are micropores below 2 nm, mesopores from 2 to 50 nm, and macropores above 50 nm. Many zeolites are microporous; MCM-41 and SBA-15 are examples of ordered mesoporous silicas templated by surfactant assemblies.
| Class | Width / implication |
|---|---|
| Micropore | <2 nm; strong overlap of adsorption potentials |
| Mesopore | 2–50 nm; capillary condensation may occur |
| Macropore | >50 nm; facilitates transport |
Example: Charge balance in a zeolite
Solution
Each Al-for-Si substitution leaves one negative charge on the framework relative to the all-silica network. A monovalent extra-framework cation such as Na⁺ compensates one such charge; a divalent cation can compensate two, subject to site and composition constraints.
UndergraduateAdsorption and transport
Gas physisorption isotherms probe accessible surface and pores. BET analysis estimates an apparent specific surface area over a selected pressure range; pore-size distributions require a model (for example, NLDFT/QSDFT) appropriate to adsorbate, geometry, and surface.
AdvancedTopology, acidity, and stability
Framework topology sets channel intersections and cage connectivity; Si/Al ratio, cation identity, and extra-framework species tune hydrophilicity and acid-site strength. Hydrothermal dealumination, steaming, or framework collapse can change porosity and catalytic selectivity, so characterization after use matters.
ResearchResearch frontier
References
- A New Family of Mesoporous Molecular Sieves Prepared with Liquid Crystal Templates · J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T.-W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins, J. L. Schlenker, 1992
- Ordered Mesoporous Molecular Sieves Synthesized by a Liquid-Crystal Template Mechanism · C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, 1992
- Intracrystalline and Molecular-Shape-Selective Catalysis by Zeolite Salts · P. B. Weisz, V. J. Frilette, 1960