4 research outputs found
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
Mechanism of Si Island Formation in SAPO-34
With
the aim of understanding the Si island formation in SAPO-34, we have
carried out a computational mechanistic study. Briefly, the Si island
formation in SAPO-34 is explained by three successive reactions. First,
the framework Si atom is removed from the framework through the action
of four water molecules. Second, the hydrogarnet defect generated
by the desilication is healed by an available H<sub>3</sub>PO<sub>4</sub> molecule. Third, the extra framework SiÂ(OH)<sub>4</sub> species
inserts in the framework position of a phosphorus atom while, in a
concerted fashion, “kicking out” the phosphorus atom
as a H<sub>3</sub>PO<sub>4</sub> extra-framework species. When these
exchanges of framework and extra-framework species are repeated, the
isolated Si atoms may eventually cluster into Si islands
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
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