Mechanistic Comparison of the Dealumination in SSZ-13 and the Desilication in SAPO-34

With
the purpose of understanding the behavior of aluminosilicate
zeolites and silicoaluminophosphates (SAPOs) in the presence of steam,
we carried out a computational density functional theory (DFT) study
on the desilication of SAPO-34. The mechanism studied was a stepwise
hydrolysis of the four bonds to the Si heteroatom. An analogous process
to the desilication of SAPO-34 is the dealumination of SSZ-13. To
investigate possible mechanistic differences between the two processes,
we compared the results of this study with the results of a previous
study on dealumination in SSZ-13. We found that the intermediates
along the dealumination path of SSZ-13 have one of the protons bonded
to a bridging oxygen atom. In the corresponding intermediates of the
desilication path in SAPO-34, the same proton prefers to be part of
an aqua ligand coordinated to an Al atom. The principal factor determining
the different proton locations is the electronic requirement of the
atoms surrounding the proton. The different proton locations in SSZ-13
and SAPO-34 put clear conditions on possible mechanisms, thus causing
them to be different for the two materials. We expect the principles
determining the proton location also to be valid for other mechanisms
of dealumination in SSZ-13 and desilication in SAPO-34.