Electron spin resonance and electron spin-echo modulation evidence for theisomorphous substitution of titanium in titanium aluminophosphate molecular sieves

A series of titanium aluminophosphate molecular sieves with four different structure types including TAPO-5 (AFI topology), TAPO-11 (AEL topology), TA PO-31 (ATO topology), and TAPO-36 (ATS topology) have been studied to deter mine the titanium location in the aluminophosphate framework. Some Ti(IV) w as reduced to paramagnetic Ti(LII) by gamma-irradiation and CO reduction to serve as a spin probe. The electron spin resonance (ESR) parameters confir m Ti(III) formation but do not clearly confirm a tetrahedral environment. H owever, P-31 and Al-27 electron spin-echo modulation of Ti(III) provide evi dence based on phosphorus distances for titanium substitution into a tetrah edral phosphorus site of the aluminophosphate framework in all four structu res. When the TAPOs are treated with NO, an NO radical is observed by ESR b ut Ti(III) is not formed.