Some nanoporous microcrystalline solids offer spaces where molecular access is controlled by various factors, mainly steric and topochemical. Indeed, the size and shape, as well as the chemical nature of the host molecules are decisive for allowing their passage through nanowindows, for the arrangement of molecules inside these solids, and in some cases for undergoing unusual chemical transformations.
Cavities in three-dimensionally organized (3D) solids, typically zeolites and related materials, as well as interlamellar spaces in two-dimensionally organized (2D) solids or tunnels in one-dimensionally organized (1D) solids, such as mesoporous silicas and fibrous clays, constitute a nanoscale region offering a chemistry different from that of conventional homogeneous media. We can speak of veritable intra-crystalline laboratories, where, for example, water molecules can be found in an anomalous state of dissociation, typically 1000 times higher than in water in its normal state, making the acidity of these environments high enough to induce unexpected catalytic transformations on the external surface of these solids.
This lecture will illustrate some examples of adsorption and selective transformation in solids with various topologies. The interaction of ZSM-5 zeolite with toluene in the presence of methanol will be considered. This MFI zeolite is widely used in the petrochemical industry due to the size of its structural channels, and the high steric selectivity which in this case leads to the formation of p-xylene.
Another family of aluminosilicates, this time displaying a lamellar-type crystalline organization, are the clay phyllosilicates. Among these solids, smectites can intercalate, i.e. adsorb a wide variety of organic species in their interlamellar space and, in some cases, transform these species in an extraordinarily selective manner. For example, we know that molecular rearrangement or transposition reactions, such as pinacol, take place in a special way in confined spaces. The migratory capacity of atoms or groups of atoms can change profoundly due to steric constraints in the confined space, and the outcome of chemical transformations can be strongly altered. This is the case for the rearrangement of glycol 2,3-diphenyl-2,3-butanol in the interlamellar space of smectites exchanged with acid cations, such as aluminum. In fact, 1,2-diphenyl-2-methyl-1-propanone is formed instead of 3,3-diphenyl-2-butanone as in conventional homogeneous media.
