Desilication of SAPO-34: Reaction Mechanisms from Periodic DFT Calculations

With the aim of understanding the
desilication of SAPO-34, we compared
three different reaction mechanisms for the hydrolysis of framework
silicon by use of density functional theory (DFT) calculations. All
three mechanisms are characterized by stepwise hydrolyses of Si–O–Al
bonds. In the most favorable mechanism water molecules adsorb strongly
to the Lewis acidic Al atoms neighboring the Si atom. Furthermore,
evaluation of free energies reveals that an additional water molecule
may catalyze the hydrolysis of the first Si–O–Al bond.