Challenges for Zeolites
Zeolites are aluminosilicate materials with crystalline framework structures. They are very versatile materials that are widely used by chemists both in the lab and in industry. However, their narrow micropores impose important diffusion limitations on large reactant and product molecules approaching or leaving the active sites. This leads to lowered conversion or undesirable secondary reactions which prompt coke deposition and catalyst deactivation.
Many efforts have been devoted to the synthesis of molecular sieves with larger pore sizes, especially in the mesoscale (2 nm to 50 nm), with the aim to overcome these diffusion problems. In the 90s, a new family of ordered mesoporous silica and aluminosilicate materials, M41S, was developed by the Mobil Oil Company using surfactants to produce tunable mesoporosity. Nevertheless, the amorphous frameworks and thin pore walls of this sort of material result in poorer hydrothermal stability and weaker acidity than that of zeolites. This has inhibited their industrial applications.
Surfactant-Templated Mesostructured Zeolites
It has been a long-standing goal of researchers to develop zeolites with controlled mesoporosity, i.e. mesopore walls composed of a crystalline zeolitic framework. This has been recently achieved using surfactants – like the ones used in the synthesis of amorphous M41S – for the production of surfactant-templated mesostructured zeolites. These materials represent the bridge that closes the gap between microporous crystalline zeolites and mesoporous amorphous materials. They are strongly acidic and highly hydrothermally stable while accessible to very large molecules, making them ideal catalysts for the transformation of bulky molecules.
The presence of controllable intercrystalline mesoporosity in mesostructured zeolites has been confirmed by producing a series of materials with surfactants of increasing length (quaternary ammonium surfactants with aliphatic chains ranging from C10–C22). The structure of the mesoporous zeolites can be clearly seen in transmission electron microscope (TEM) images.
|Figure 1. Transmission electron microscopy (TEM) images of (a) conventional and (b) mesostructured Y zeolites, c) density functional theory (NLDFT) pore size distribution curves calculated from Ar (87 K) isotherms, and (d) the tomogram of the mesostructured Y-zeolite crystal prepared using C16 surfactants.|
The TEM image of a conventional Y zeolite crystal shows its rows of lattice reflection lines resulting from the regular atomic structure within the crystal (Fig. 1a, a few are highlighted in red for clarity). The mesostructured zeolite crystal similarly shows crystal lattice lines, but in addition, one can see the larger, lighter colored spots corresponding to the mesopores (Fig. 1b, a few pores are circled in red). The diameter of the generated mesopores nicely correlates with the length of the surfactant molecule (Fig. 1c).
This surfactant-templating technique chemically burrows a network of surface-accessible mesopores into the crystal without materially affecting its crystal structure. By combining rotation electron diffraction and electron tomography, it was possible to directly observe the presence of mesoporosity inside the zeolite crystals. They are homogeneously distributed within the zeolite and highly interconnected (Fig. 1d).