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 H₃PO₄ molecule. Third, the extra framework Si­(OH)₄ species inserts in the framework position of a phosphorus atom while, in a concerted fashion, “kicking out” the phosphorus atom as a H₃PO₄ extra-framework species. When these exchanges of framework and extra-framework species are repeated, the isolated Si atoms may eventually cluster into Si islands

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

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

Kinetics of Zeolite Dealumination: Insights from H‑SSZ-13

When zeolite catalysts are subjected to steam at high temperatures, a permanent loss of activity happens, because of the loss of aluminum from the framework. This dealumination is a complex process involving the hydrolysis of four Al–O bonds. This work addresses the dealumination from a theoretical point of view, modeling the kinetics in zeolite H-SSZ-13 to gain insights that can extend to other zeolites. We employ periodic density functional theory (DFT) to obtain free-energy profiles, and we solve a microkinetic model to derive the rates of dealumination. We argue that such modeling should consider water that has been physisorbed in the zeolite as the reference state and propose a scheme for deriving the free energy of this state. The results strongly suggest that the first of the four hydrolysis steps is insignificant for the kinetics of zeolite dealumination. Furthermore, the results indicate that, in H-SSZ-13, it is sufficient to include only the fourth hydrolysis step when estimating the rate of dealumination at temperatures above 700 K. These are key aspects to investigate in further work on the process, particularly when comparing different zeolite frameworks

Rock ‘n’ Roll With Gold: Synthesis, Structure, and Dynamics of a (bipyridine)AuCl₃ Complex

Our previously reported microwave synthesis of (N–N)­AuCl₂⁺ complexes (where N–N = 2,2′-bipyridine (bpy) and sterically unencumbered bpy derivatives) was used to prepare derivatives where the bpy moiety was substituted in the 6,6′-positions. Instead of the square-planar complexes, these reactions produced neutral (N–N)­AuCl₃ complexes. In these, the tethered N–N ligand is bonded such that one N occupies a regular position in the square coordination plane of the Au­(III) center and the other N occupies a pseudoaxial position, interacting with Au through an elongated Au–N bond, as determined by X-ray crystallography of two complexes. Variable-temperature ¹H NMR spectroscopy reveals that the two sites of the N–N ligand undergo exchange on the NMR time scale. For N–N = 6,6′-Me₂bpy the activation parameters were determined to be Δ<i>H</i>^⧧ = 8.5 ± 0.4 kcal mol^–1 and Δ<i>S</i>^⧧ = 0.7 ± 2.0 cal K^–1 mol^–1. The dynamic behavior of (6,6′-Me₂bpy)­AuCl₃ was investigated by a DFT computational study, which detailed the in-plane <i>rocking</i> motion seen by NMR as well as decoordination of the axially bonded N with concomitant <i>rolling</i> of half of the bpy moiety by rotation around the central C–C bond of the bidentate ligand

Rock ‘n’ Roll With Gold: Synthesis, Structure, and Dynamics of a (bipyridine)AuCl₃ Complex

Our previously reported microwave synthesis of (N–N)­AuCl₂⁺ complexes (where N–N = 2,2′-bipyridine (bpy) and sterically unencumbered bpy derivatives) was used to prepare derivatives where the bpy moiety was substituted in the 6,6′-positions. Instead of the square-planar complexes, these reactions produced neutral (N–N)­AuCl₃ complexes. In these, the tethered N–N ligand is bonded such that one N occupies a regular position in the square coordination plane of the Au­(III) center and the other N occupies a pseudoaxial position, interacting with Au through an elongated Au–N bond, as determined by X-ray crystallography of two complexes. Variable-temperature ¹H NMR spectroscopy reveals that the two sites of the N–N ligand undergo exchange on the NMR time scale. For N–N = 6,6′-Me₂bpy the activation parameters were determined to be Δ<i>H</i>^⧧ = 8.5 ± 0.4 kcal mol^–1 and Δ<i>S</i>^⧧ = 0.7 ± 2.0 cal K^–1 mol^–1. The dynamic behavior of (6,6′-Me₂bpy)­AuCl₃ was investigated by a DFT computational study, which detailed the in-plane <i>rocking</i> motion seen by NMR as well as decoordination of the axially bonded N with concomitant <i>rolling</i> of half of the bpy moiety by rotation around the central C–C bond of the bidentate ligand

Rock ‘n’ Roll With Gold: Synthesis, Structure, and Dynamics of a (bipyridine)AuCl₃ Complex

Our previously reported microwave synthesis of (N–N)­AuCl₂⁺ complexes (where N–N = 2,2′-bipyridine (bpy) and sterically unencumbered bpy derivatives) was used to prepare derivatives where the bpy moiety was substituted in the 6,6′-positions. Instead of the square-planar complexes, these reactions produced neutral (N–N)­AuCl₃ complexes. In these, the tethered N–N ligand is bonded such that one N occupies a regular position in the square coordination plane of the Au­(III) center and the other N occupies a pseudoaxial position, interacting with Au through an elongated Au–N bond, as determined by X-ray crystallography of two complexes. Variable-temperature ¹H NMR spectroscopy reveals that the two sites of the N–N ligand undergo exchange on the NMR time scale. For N–N = 6,6′-Me₂bpy the activation parameters were determined to be Δ<i>H</i>^⧧ = 8.5 ± 0.4 kcal mol^–1 and Δ<i>S</i>^⧧ = 0.7 ± 2.0 cal K^–1 mol^–1. The dynamic behavior of (6,6′-Me₂bpy)­AuCl₃ was investigated by a DFT computational study, which detailed the in-plane <i>rocking</i> motion seen by NMR as well as decoordination of the axially bonded N with concomitant <i>rolling</i> of half of the bpy moiety by rotation around the central C–C bond of the bidentate ligand