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.