Brønsted acidity of zeolites and SAPOs

A long standing research project has been carried out by our group on this subject by using several computational techniques. A remarkable finding has been made on several zeolites and related microporous materials which seems to be of general application and it is the capability of atomistic forcefield methodologies to simulate OH stretching frequencies, that characterise Brønsted acidity. One of the examples considered has been the zeolite ITQ-7 shown below.

View of ITQ-7 zeolite (whose IZA code is ISV)

In a purely siliceous structure an Aluminium atom has been introduced in each of the crystallographic T-sites, and for each of them the four bridging protons have been considered. A model like that was not able to reproduce the experimental infrared spectrum for ITQ-7. When the template was considered into the structure, the Aluminium distributions changed dramatically their relative stability. Considering first the Al incorporation (with the template) and then -second- the proton incorporation, the populations of the acid centres were recalculated and from that the infrared calculated bands were recalculated. The resultant comparison with experiments shows a much better agreement and it can be seen that both infrared bands are reproduced by our calculations as well as their relative populations.

Experimental infrared bands of H-ITQ-7. The deconvolution of the experimental is also shown. The calculations give two bands of 3577 and 3624 wavenumbers, in close agreement with the experimental at 3595 and 3624.