Zeolites: Large molecules welcome.

Many eff orts to improve the performance of heterogeneous catalysts and adsorbents focus on creating materials that are highly active and selective for their target reactants and products. Th is typically involves the use of porous solids with high surface areas, for instance crystalline or amorphous inorganic compounds, whose diff usional access, adsorption, and reaction properties depend strongly on their compositions and porous structures. Oft en, particularly in the case of large molecules or macromolecules, this means making a compromise between competing properties, such as catalytic activity versus mass-transport of reactants and products in and out of the structure. A polycrystalline zeolite, prepared by Choi et al. and presented on page 718 of this issue, with its high catalytic activity and mesoscopic pores (as large as 8 nm in diameter) signifi cantly reduces the need for such compromise1. Zeolites are crystalline aluminosilicate materials that are technologically important for their applications in catalysis (for gasoline production), radioactive ion sequestration, purifi cation systems (for water soft ening) and many more. In catalytic processes, they typically off er high activities, but their nanoporous structures (pores approximately 1 nm in diameter) rather limit their uses to reactions involving reactants and products that are small enough to diff use in and out of nanosized pores. A great deal of eff ort has been devoted to overcoming this pore-size limitation of zeolites by preparing porous inorganic solids with high surface areas and pores in the mesoscopic regime (about 2–50 nm) from gel mixtures containing zeolite ‘seeds’ (that is, zeolite crystals at a very early stage of nucleation). Such materials, however, have tended to have amorphous frameworks with low catalytic activities, although their pore dimensions are large enough to allow high-molecular-weight molecules to diff use in and out of the framework structure. Where molecular order has been imparted by co-assembly with surfactant, polymeric, or carbon species to generate porosity at the mesoscale, the resulting frameworks have been composed of zeolite nanoparticles that displayed catalytic activity inferior to those of bulk zeolites2–5, or have been thin fragile lamellae that lose their mesoporosity when calcined6–8. Very recently, a more advanced strategy for controlling the aggregation of zeolite seeds, by functionalizing their exterior surfaces with organosilane species, has led to zeolite structures with bimodal nano/mesoporosities and favourable catalytic properties9. An organosilane moiety is also the key to the success of Choi and colleagues’ synthesis, which represents an important advance because the material obtained combines high catalytic activity with low resistances to mass-transport, and moreover off ers the opportunity of tuning the size of the mesoscopic pores1. In their synthesis, the authors combined zeolite-directing agents and meso-structure directing agents. In particular, the latter is a surfactant molecule rationally designed by Choi and colleagues to have multifunctional character (Fig. 1): a positively charged quarternary ammonium group that may facilitate zeolite-like surface structural ordering, an alkyl tail that makes the molecule amphiphilic (hydrophilic on the ammonium side and hydrophobic on the alkyl side), and an organosilane moiety linked to the ammonium group that enhances surfactant interactions with the zeolite as it crystallizes. Th e long alkyl tail promotes the formation of mesopores with relatively uniform size. Th us the synthesis enables hierarchical control of the dimensions of the nanoand mesopores. Choosing a porous solid for catalysis usually involves a trade-off between