Aluminium species in activated zeolites: solid-state NMR spectroscopy of the active sites

摘要

The acid catalytic activity of zeolites is usually assumed to reside in sites involvingtetrahedrally coordinated framework aluminium (proton-donating or Br?nsted acid sites).Acid-catalysed reactions may also proceed at electron-pair accepting (Lewis acid) sites which are connected with the presence of non-framework aluminium (NFAl) species with an octahedral coordination. For a complete characterisation of the acid properties of a catalyst it is necessary to: (i) identify the different sorts of acid sites; (ii) determine their concentration; (iii) determine their acid strength. The idea to link acidity and Al coordination has been fully exploited in the application of the ~(27)Al MAS NMR method. The 60 ppm signal in the ~(27)Al MAS spectrum is attributed to tetrahedrally coordinated Al species, the 0 ppm band resides from octahedrally coordinated Al atoms. From a combination of ~(29)Si and ~(27)Al MAS NMR, taking into account the Loewenstein rule, one obtains in principle the concentration of framework and non-framework acid sites. On the other hand, a direct measure of the active sites and their acidity is possible by 1H MAS NMR [1].