Zeolites are crystalline alumino-silicates with a three dimensional framework structure that forms uniformly sized pores of molecular dimensions. Zeolites adsorb molecules that fit inside the pores and exclude molecules that are too large, i.e. they act as sieves on a molecular scale. Thanks to their unique characteristics, which include ion-exchange and adsorption properties as well as catalytic activity, they are efficiently used in various applications and industries. For instance, they are utilized as highly selective adsorbents and/or heterogeneous catalysts in a broad range of applications in the oil refining and petrochemical industries, but have also found use in other fields such as gas separation, and are even used in laundry detergents.
The performance of conventional zeolites is sometimes limited due to the fact that their pores are usually only in the micropore size range, ultimately leading to major steric and diffusional restrictions in reactions where bulky molecules are involved. In order to overcome this problem, major progress has been made concerning the synthesis of nanoporous materials with tailored pore size and structure. The most prominent examples are mesoporous silica molecular sieves such as MCM-41, MCM-48, SBA-15, KIT-6 and others, which were considered the solution to the diffusion problem existing in zeolites. However, it turned out that these mesoporous molecular sieves exhibit only little catalytic activity and limited hydrothermal stability due to their amorphous nature. As a consequence, the focus has shifted towards the development of mesoporous zeolites exhibiting a hierarchical pore structure with an appropriate balance of micropores (pore width < 2nm) and mesopores (2-50 nm). Hierarchically structured zeolites combine advantages of both conventional zeolites and mesoporous materials:
|Chemisorption techniques are used to assess catalytic activity (see topic CATALYSIS at Quantachrome’s Website) of zeolitic materials. In addition, physisorption (based on nitrogen and argon adsorption at 77 K and 87 K, respectively) allows one to obtain a comprehensive, textural characterization (with regard to surface area, micro- and mesopore volume, pore size distribution, pore volume, and pore structure). This is crucial for optimizing the application of zeolites and mesoporous zeolites because textural properties control transport phenomena (e.g. diffusion rates) and govern selectivity in catalyzed reactions. High resolution adsorption experiments, particularly with argon at 87 K (which, unlike nitrogen, does not exhibit quadrupole interactions with polar surface functionalities) coupled with dedicated, theoretical approaches based on density functional theory (DFT) lead to an accurate pore size/volume analysis over the complete range of micro-and mesopores. Furthermore, advanced experimental investigations of the adsorption hysteresis observed in the pressure range of mesopore filling (using hysteresis scanning experiments) allows one to obtain detailed information of the pore structure and connectivity. Complimentary surface chemistry information (such as hydrophilicity and hydrophobicity) can be obtained by adsorption of polar molecules such as water.
Instruments: Autosorb IQ, Autosorb IQ-MP/XR, Quadrasorb-evo, V-star
Thommes, M.: Textural characterization of zeolites and ordered mesoporous materials by physical adsorption. Stud. Surf. Sci. Catal. 168, 495-523 (2007)
Zhang, X., Liu, D., Xu, D., Asahina, S., Cychosz, K.A., Agrawal, K.V., Al Wahedi, Y., Bhan, A., Al Hashimi, S., Terasaki, O., Thommes, M., Tsapatsis, M. : Synthesis of self-pillared nanosheets by repetitive branching. Science 336, 1684-1687 (2012)
Garcia-Martinez, J., Xiao C., Cychosz K., Li K., Wan W., Zou*, Thommes M, “Evidence of Intracrystalline Mesostructured Porosity in Zeolites by Advanced Gas Sorption, Electron Tomography and Rotation Electron Diffraction” , ChemCatChem (2014), DOI: 10.1002/cctc.201402499
Thommes, M.; Mitchell, S.; Pérez-Ramírez, J.: Surface and pore structure assessment of hierarchical MFI zeolites by advanced water and argon sorption studies. J. Phys. Chem. C 116, 18816-18823 (2012)