Solvent effects in the synthesis of CoAPO-5, -11 and -34 molecular sieves

Fan, Wen; Li, Ruifeng; Dou, Tao; Tatsumi, Takashi; Weckhuysen, Bert M.

doi:10.1016/j.micromeso.2005.04.025

Cited by 22 publications

(11 citation statements)

References 35 publications

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“…We note that the role of both the template molecule and the solvent is not considered in this model. However, since many different templates and a variety of solvents (both protic and aprotic) have been used to synthesize AFI and since the same template can be used to crystallize different aluminophosphate structures, we surmise that their role is not specific for AFI crystallization and therefore the mechanism proposed can be considered to be generally applicable. , The Co 2+ ions influence the crystallization process by hindering and therefore retarding it. It is most likely that this is related to the necessity of the Co 2+ ions to transform from octahedral into a tetrahedral geometry before being incorporated into the Al−O−P network.…”

Section: Discussionmentioning

confidence: 95%

Unraveling the Crystallization Mechanism of CoAPO-5 Molecular Sieves under Hydrothermal Conditions

et al. 2005

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The hydrothermal crystallization of CoAPO-5 molecular sieves has been studied using time-resolved in-situ SAXS/WAXS, UV-vis, Raman, and XAS. Data collected during heating to 180 degrees C allowed the observation of different steps occurring during the transformation of the amorphous gel into a crystalline material from a macroscopic and atomic perspective. Raman spectroscopy detected the initial formation of Al-O-P bonds, whereas SAXS showed that these gel particles had a broad size distribution ranging from ca. 7 to 20 nm before crystallization began. WAXS showed that this crystallization was sharp and occurred at around 160 degrees C. Analysis of the crystallization kinetics suggested a one-dimensional growth process. XAS showed that Co(2+) transformed via a two-stage process during heating involving (i) a gradual transformation of octahedral coordination into tetrahedral coordination before the appearance of Bragg peaks corresponding to AFI, suggesting progressive incorporation of Co(2+) into the poorly ordered Al-O-P network up to ca. 150 degrees C, and (ii) a rapid transformation of remaining octahedral Co(2+) at the onset of crystallization. Co(2+) was observed to retard crystallization of AFI but provided valuable information regarding the synthesis process by acting as an internal probe. A three-stage, one-dimensional crystallization mechanism is proposed: (i) an initial reaction between aluminum and phosphate units forming a primary amorphous phase, (ii) progressive condensation of linear Al-O-P chains forming a poorly ordered structure separated by template molecules up to ca. 155 degrees C, and (iii) rapid internal reorganization of the aluminophosphate network leading to crystallization of the AFI crystal structure.

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Section: Discussionmentioning

confidence: 95%

Unraveling the Crystallization Mechanism of CoAPO-5 Molecular Sieves under Hydrothermal Conditions

et al. 2005

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“…Despite of this, it was found that all reflections in the XRD patterns of Co-containing samples shifted to lower positions, as compared to the materials free of this element. This reveals that addition of Co in the synthesis gel led to an enlargement of the unit-cell volume as a result of the unit-cell expansion [8][9][10][11], showing that Co 2+ ions were easily incorporated in the framework. This is further confirmed by the data shown in Table 1.…”

Section: Catalyst Synthesis and Characterizationmentioning

confidence: 89%

Synthesis, characterization and catalysis of (Co, V)-, (Co, Cr)- and (Cr, V)APO-5 molecular sieves

Fan

Song

et al. 2006

Microporous and Mesoporous Materials

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Synthesis, characterization and catalysis of (Co, V)-, (Co, Cr)-and (Cr, V)APO-5 molecular sieves ) molecular sieves have been hydrothermally synthesized and characterized. The as-synthesized and calcined samples were investigated with XRD, ICP, DRS UV-vis spectroscopy, ESR, 27 Al MAS NMR techniques to explore the valence, coordination and location of the incorporated transition metal ions. It was shown that isomorphous substitution of Co 2+ for Al 3+ convincingly occurred, and V 4+ also anchored to the sample by coordinating to two framework oxygen atoms and most probably occupying P 5+ sites, while no unambiguous spectroscopic evidence was provided for the incorporation of Cr 3+ in the lattice, although chemical analysis suggests this possibility in the case that Co 2+ or V 4+ are added to the synthesis gel. The preferential sequence for the incorporation of these transition metal ions in the AFI framework is: Co 2+ > V 4+ > Cr 3+ . After calcination, almost all V 4+ ions were oxidized to V 5+ , whereas Cr 3+ ions were oxidized to Cr 5+ and Cr 6+ . As catalysts, the (M, N)APO-5 molecular sieves are highly active for the selective oxidation of cyclohexane with a high cyclohexanone/cyclohexanol ratio in the presence of H 2 O 2 . The reaction temperature and solvent significantly influenced the catalytic behavior. Although some leaching of the transition metal ions was observed, the catalytic performance remained stable at least within five repeated runs.

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“…A divalent Co(II) cation may be accommodated in SAPOs either on a substitutional site (replacing a trivalent Al ion) and forming a Broensted acid site after protonation or as an extraframework cation forming a Lewis site . The ratio of substitutional and extraframework Co(II) depends strongly on the preparation conditions.…”

Section: Ab Initio-optimized Crystalline Geometriesmentioning

confidence: 99%

Application of Hybrid Functionals to the Modeling of NO Adsorption on Cu−SAPO-34 and Co−SAPO-34: A Periodic DFT Study

et al. 2009

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The structural and chemical properties of Cu- and Co-exchanged silicoaluminophosphates with a chabazite-type structure (SAPO-34) and the adsorption of NO molecules on the extraframework cations have been studied using periodic density functional (DFT) calculations using both convential gradient-corrected exchange-correlation functionals and hybrid functionals mixing exact (i.e., Hartree−Fock) and DFT exchange. To assess the importance of the cation-support interaction, comparative studies of NO adsorption on isolated metal atoms and ions have been performed. We find that whereas the geometrical properties for stoichiometric aluminophosphate (ALPO-34) and protonated silicoaluminophosphate (H−SAPO-34) calculated using both types of functionals are essentially on par, the hybrid functionals predict much wider energy gaps in better agreement with experiment. Significant differences arise for the coordination of extraframework cations in the metal-exchanged silicoaluminophosphates (Cu−SAPO-34 and Co−SAPO-34) and, in particular, for the electronic eigenstates of the cations. The different metal−framework coordination and the differences in the electronic properties lead to markedly different predictions of the chemical reactivities: hybrid functionals predict a weaker binding of molecular adsorbates such as NO. The reduction of the adsorption energy is more pronounced for the magnetic Cu(II) and Co(II) cations than for the nonmagnetic Cu(I) cation. This is a consequence of a much larger exchange splitting of the d states of the cation and also of an enhanced ligand-field splitting due to the binding to the framework. The occupied d states of the transition-metal cations are shifted to larger binding energies, and the empty d states are located in the upper part of the energy gap. Hybrid functionals tend to overestimate the N−O stretching frequency even for the isolated molecule and for gas-phase nitrosyls, whereas conventional functionals predict eigenvalues that are too low. This tendency is also reflected in the vibrational spectra of the Cu(Co)−NO adsorption complexes, whereas the adsorption-induced frequency shifts predicted by both types of functionals are in reasonable agreement with experiment. Significant differences are found in the prediction of the spin states of the adsorption complexes. Altogether, we conclude that conventional gradient-corrected DFT functionals describe these complex materials with good accuracy, whereas the inclusion of exact exchange in the hybrid functionals does not lead to improved performance for the metal-exchanged systems.

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Solvent effects in the synthesis of CoAPO-5, -11 and -34 molecular sieves

Cited by 22 publications

References 35 publications

Unraveling the Crystallization Mechanism of CoAPO-5 Molecular Sieves under Hydrothermal Conditions

Unraveling the Crystallization Mechanism of CoAPO-5 Molecular Sieves under Hydrothermal Conditions

Synthesis, characterization and catalysis of (Co, V)-, (Co, Cr)- and (Cr, V)APO-5 molecular sieves

Application of Hybrid Functionals to the Modeling of NO Adsorption on Cu−SAPO-34 and Co−SAPO-34: A Periodic DFT Study

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