  The dispersion and coordination of aluminium species on the surface of
silica-alumina based materials are essential for controlling their catalytic
activity and selectivity. Al(IV) and Al(VI) are two common coordinations of Al
species in the silica network and alumina phase, respectively. Al(V) is rare in
nature and was found hitherto only in the alumina phase or interfaces
containing alumina, a behavior which negatively affects the dispersion,
population, and accessibility of Al(V) species on the silica-alumina surface.
This constraint has limited the development of silica-alumina based catalysts,
particularly because Al(V) had been confirmed to act as a highly active center
for acid reactions and single-atom catalysts. Here, we report the direct
observation of high population of homogenously dispersed Al(V) species in
amorphous silica-alumina in the absence of any bulk alumina phase, by high
resolution TEM/EDX and high magnetic-field MAS NMR. Solid-state 27Al
multi-quantum MAS NMR experiments prove unambiguously that most of the Al(V)
species formed independently from the alumina phase and are accessible on the
surface for guest molecules. These species are mainly transferred to Al(VI)
species with partial formation of Al(IV) species after adsorption of water. The
NMR chemical shifts and their coordination transformation with and without
water adsorption are matching that obtained in DFT calculations of the
predicted clusters. The discovery presented in this study not only provides
fundamental knowledge of the nature of aluminum coordination, but also paves
the way for developing highly efficient catalysts.
