SAPO-37: the implications of structure flexibility on acidity

When tetrapropylammonium cations are removed from the α cavities of SAPO 37, but tetramethylammonium cations are still in the β-cages, a shift in the vibrational frequency of hydroxy groups to lower frequencies, with the corresponding appearance of stronger acid sites is observed; this is a consequence of the increase of the T–O–T′ angle dimensions owing to the presence of the bulky tetramethylammonium cations.