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  • 1
    ISSN: 1572-879X
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Density functional theory calculations have been carried out to establish the influence of mono- and polyvalent cations on the Brønsted acidity of H-ZSM-5. The zeolite was modeled as a cluster containing 41-45 atoms, in the center of which is an Al(1)(OH)SiOAl(2)(OM)unit, where M+ = H+, Li+, Na+, K+, Ca(OH)+, AlO+, Al(OH)+ 2. The local geometry of the Brønsted acid site is affected by the nature of M+ and this in turn causes a change in the value of the proton affinity (PA) for the site. The highest value of PA is 330 kcal/mol for M+ = H+ and the lowest value of PA is 305 kcal/mol for M+ = AlO+. No correlation was found between the value of PA and the Mulliken charge on Al(1). With the exception of the case where M+ = AlO+ the binding energy of CO with the Brøsted acid proton is approximately 8.8 kcal/mol, independent of the nature of M+. When M+ = AlO+, the binding energy for CO is 11.1 kcal/mol. The present calculations suggest that different factors affect proton affinity and the binding energy for CO adsorption.
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  • 2
    ISSN: 1572-879X
    Keywords: ZSM-5 ; molecular orbital theory ; aluminum siting
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Semiempirical molecular orbital calculations have been performed to study aluminum siting in H-ZSM-5 zeolites. Local structural rearrangements upon substituting aluminum (with a charge compensating proton) for silicon are found to be important. The T12 site is found to be the most preferred site for aluminum substitution. However, the calculated energetics for substitution show that several tetrahedral sites are energetically comparable with regard to aluminum siting. Results pertaining to the electronic properties of the acidic site upon aluminum substitution at each of the twelve distinct tetrahedral sites are presented. The acidic center is found to be a rather soft species, with the HOMO-LUMO energy gap being roughly 8 eV.
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  • 3
    ISSN: 1572-9028
    Keywords: Ga/H-ZSM-5 ; H2/D2 exchange ; density functional theory
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Abstract Density functional theory calculations have been carried out to determine the thermodynamic stability of various Ga species in gallium-exchanged ZSM-5, the thermodynamics of H2 adsorption, and the most favorable pathway for H2/D2 exchange. The portion of the zeolite associated with Ga was represented by a cluster containing 7, 21, or 33 atoms. The B3LYP hybrid method was used to account for the effects of electron exchange and correlation. The most likely form of Ga expected in freshly exchanged and calcined ZSM-5 is ZGa(OH)2. H2 reduction of this species is projected to produce ZGa(H)(OH) and ZGa(H)2. While the thermodynamics of H2 desorption from ZGa(H)2 are favorable, the process is projected to be slow because of a high activation barrier. The most favorable pathway for H2/D2 exchange over ZGa(H)2 proceeds via Z(D)(Ga(H)2(D)) as an intermediate. Similar calculations have been carried out for H2/D2 exchange over H-ZSM-5.
    Type of Medium: Electronic Resource
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