Calculation of the Structural, Energetic, and Spectral Properties of Alumoxane and Aluminosilicate “Drum” Molecules

Tossell, J. A.

doi:10.1021/ic970674j

Cited by 9 publications

(5 citation statements)

References 23 publications

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“…Small aluminosilicate anion species are technologically significant, which are proposed as the nucleation precursors and growth units for zeolite crystal, and thus understanding the formation mechanism of Si-O-Al linkage is of scientific importance. There have been some experimental [6][7][8][9][10][11][12][13][14][15] and theoretical [16][17][18][19][20][21][22] studies on the aluminosilicate oligomers in solution and colloid.…”

Section: Introductionmentioning

confidence: 99%

Theoretical study of formation mechanism of aluminosilicate in the synthesis of zeolites

Jiao

Chen

2008

Struct Chem

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The condensation of silicic acid with aluminate in alkaline environment, the essential reaction of zeolite synthesis, is studied using the density functional theory, with the hybrid functional B3LYP in conjunction with the 6-311++G(d, p) basis set. The Si(OH) 4 monomer and Al(OH) 4 -anion are used as the reactant models to study the condensation pathway in basic solution. The solvent effect is included by the COSMO-RS model. The study includes the complete geometry optimization and frequency calculation of reactants, products, reaction intermediates, and transition states, as well as the calculation of the activation energy of the different pathways involved. The intrinsic reaction coordinate method is used to verify the reactant and product corresponding to the transition state. The calculation shows that the formation of Si-O-Al linkage can proceed via two possible reaction pathways. The first is a single-step process, in which the formation of SiOÁÁÁAl bond and removal of water are synchronous, with the activation energy of 83.7 kJ/mol. The second is a stepwise route, in which the AlOÁÁÁSi bond is first formed to give a 5-coordianted Si intermediate, and then water is removed to yield a dimer aluminosilicate, with the barriers of 62.7 and 69.3 kJ/mol for the two steps, respectively.

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Section: Introductionmentioning

confidence: 99%

Theoretical study of formation mechanism of aluminosilicate in the synthesis of zeolites

Jiao

Chen

2008

Struct Chem

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“…This work is a continuation of our previous research on DnR silsesquioxane and aluminosilicate species. 14,15 Those studies primarily examined structures, relative stabilities of different isomers of the Al-containing species, IR spectra, and Al, Si, and O NMR. The field of silsesquioxanes has recently become a very active one for theoretical study.…”

Section: Introductionmentioning

confidence: 99%

Calculation of ¹⁹F and ²⁹Si NMR Shifts and Stabilities of F^- Encapsulating Silsesquioxanes

Tossell

2007

J. Phys. Chem. C

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Hartree-Fock and density functional theory techniques have been used to calculate the structures, the stabilities in aqueous and toluene solutions, and the 19 F and 29 Si NMR shifts of Fencapsulating double-ring geometry silsesquixoanes DnR with n ) 3-6. We find for the fluorides that the encapsulation free energy is most negative for the double four-ring (D4R) cage, in which the most stable geometry has Fin the center. For n ) 5, 6 (and for an opened version of the double five-ring, D5R, cage), the most stable geometry has Fbonded to a single apex Si atom, with a bond distance around 1.76 Å. The 19 F NMR is somewhat deshielded relative to that of the Si(OH) 4 Fmonomer for all of the apex bonded F -DnR species and is very strongly deshielded for the central Fencapsulating DnR species with n ) 3, 4. The Si is slightly deshielded by the presence of Ffor the DnR with central Fions, while in the apex-bonded larger double rings, n ) 5, 6, the Si directly bonded to Fis substantially shielded. The energetic results help to explain why fluorides, as mineralizing agents, increase the yield of zeolites and mesoporous silicas with D4R rings, and the NMR results provide a means for ascertaining double-ring size and the position of Fwithin the double-ring cage.

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“…Cage-shaped hydrosilasesquioxanes are more stable than the ring fragments and they exist in much more rigid architectures. , They are of the general formula (HSiO 3/2 ) 2 n , where n = 2, 3, etc., and those with n = 4, 6, and 8 correspond to zeolite secondary building units: double four-member rings (D 4 R) with eight tetrahedral atoms are secondary building units in LTA zeolites and the mineral gismondine. , An alumosilasesquioxane D 4 R with Si/Al = 1 and 4Na + at positions nearly centered opposite to a four-member ring window has been synthesized . The cage-shaped structures have been subjected to many theoretical and experimental studies. − The reactivity of substituted double four-member rings (X 8 Si 8 O 12 ; X = H, Cl, CH 3 ) has been modeled by the Extended Huckel method, 9 and the stability of anionic species with different levels of Si/Al substitution has been studied by the semiempirical AM1 method . Modeling of the cage-shaped polysiloxanes at the Hartree−Fock level revealed the high stability of cages containing rings with 4, 5, and 6 tetrahedral atoms. , DFT studies of H-silsesquioxane cages pointed that nonlocal density approximation is required for reliable comparison of isomers …”

Section: Introductionmentioning

confidence: 99%

“…The cage-shaped structures have been subjected to many theoretical and experimental studies. − The reactivity of substituted double four-member rings (X 8 Si 8 O 12 ; X = H, Cl, CH 3 ) has been modeled by the Extended Huckel method, 9 and the stability of anionic species with different levels of Si/Al substitution has been studied by the semiempirical AM1 method . Modeling of the cage-shaped polysiloxanes at the Hartree−Fock level revealed the high stability of cages containing rings with 4, 5, and 6 tetrahedral atoms. , DFT studies of H-silsesquioxane cages pointed that nonlocal density approximation is required for reliable comparison of isomers …”

Section: Introductionmentioning

confidence: 99%

“…10 Modeling of the cage-shaped polysiloxanes at the Hartree-Fock level revealed the high stability of cages containing rings with 4, 5, and 6 tetrahedral atoms. 11,12 DFT studies of H-silsesquioxane cages pointed that nonlocal density approximation is required for reliable comparison of isomers. 14 The aim of the present study is to examine the Si,Al ordering in double four-member rings H 8 T 8 O 12 (T ) Si, Al) with different Al/Si ratios in the presence of charge-compensating cations, Na + or H + .…”

Section: Introductionmentioning

confidence: 99%

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DFT Study of Zeolite LTA Structural Fragments: Double Four-Member Rings of Oxygen-Bridged Silicon and Aluminum Atoms

Uzunova

Nikolov

2000

J. Phys. Chem. A

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The electronic structure and relative stability of [H 8 Si 8-x Al x O 12 ] xstructural fragments with Si and Al atoms located at tetrahedral sites and connected by bridging oxygen atoms so as to form a tetragonal prism, together with oxygen-coordinated charge-compensating cations, are studied by the B3LYP/6-31G(d) method. Clusters with Al/Si ratios in the 0-1 range and different orderings of the extraframework cations and Si and Al at tetrahedral sites are proper models for the double four-member rings, which are secondary building units of the LTA structure. The stability of the structure fragments is higher when the framework's negative charges are at maximum distance, in agreement with Dempsey's rule. The HOMO orbitals of H 8 Si 8 O 12 , the anionic clusters [H 8 Si 8-x Al x O 12 ] x-, and the proton-balanced ones are mainly O 2p nonbonding AO. In fragments with Al/Si > 0 and Na + at extraframework cation positions, the oxygen nonbonding nature of the HOMO orbital decreases and certain electron density is shifted to the Al centers. The LUMO orbitals of Na x H 8 Si 8-x Al x O 12 clusters consist of the charge-balancing cations' AO.

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Calculation of the Structural, Energetic, and Spectral Properties of Alumoxane and Aluminosilicate “Drum” Molecules

Cited by 9 publications

References 23 publications

Theoretical study of formation mechanism of aluminosilicate in the synthesis of zeolites

Theoretical study of formation mechanism of aluminosilicate in the synthesis of zeolites

Calculation of ¹⁹F and ²⁹Si NMR Shifts and Stabilities of F^- Encapsulating Silsesquioxanes

DFT Study of Zeolite LTA Structural Fragments: Double Four-Member Rings of Oxygen-Bridged Silicon and Aluminum Atoms

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Calculation of the Structural, Energetic, and Spectral Properties of Alumoxane and Aluminosilicate “Drum” Molecules

Cited by 9 publications

References 23 publications

Theoretical study of formation mechanism of aluminosilicate in the synthesis of zeolites

Theoretical study of formation mechanism of aluminosilicate in the synthesis of zeolites

Calculation of 19F and 29Si NMR Shifts and Stabilities of F- Encapsulating Silsesquioxanes

DFT Study of Zeolite LTA Structural Fragments: Double Four-Member Rings of Oxygen-Bridged Silicon and Aluminum Atoms

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Calculation of ¹⁹F and ²⁹Si NMR Shifts and Stabilities of F^- Encapsulating Silsesquioxanes