Characterisation of Co<sup>II</sup>and Co<sup>III</sup>in CoAPO molecular sieves

Kraushaar‐Czarnetzki, Bettina; Hoogervorst, W.G.M.; Andréa, R. R.; Emeis, C.A.; Stork, W.H.J.

doi:10.1039/ft9918700891

Cited by 113 publications

(36 citation statements)

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“…27 The amounts of H 2 O formed were not measured, making it difficult to confirm the exclusive presence of cations at framework positions in these samples. One of these studies proposed that the reduced species resided at extraframework positions and resembled Co oxides, 26 a proposal inconsistent with the absence of H 2 O evolution during reduction reported here.…”

Section: Chemical and Structural Changes Inmentioning

confidence: 99%

“…Recent reviews have described spectroscopic data (UVvisible, X-ray absorption, 27 Al-and 31 P NMR, electron-spin resonance) that probe the structure and electronic properties of cations in AlPO materials. [2][3][4] Active cations isomorphously substituted in AlPO frameworks have not yet been directly or unequivocally connected with specific structures involved in redox cycles required for oxidation turnovers, even after significant attention and continuing debate.…”

Section: Introductionmentioning

confidence: 99%

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Structural and Functional Characterization of Redox Mn and Co Sites in AlPO Materials and Their Role in Alkane Oxidation Catalysis

et al. 2004

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The number of redox-active and inactive Mn and Co species in MeAPO-5 and MeAPO-18 (Me ) Mn, Co) was measured from H 2 consumption rates during H 2 temperature-programmed reduction (H 2 -TPR), and their structure and oxidation state were probed at identical conditions by UV-visible and X-ray absorption spectroscopies. H 2 consumption, loss of Me 3+ UV-visible features, and a decrease in X-ray absorption edge energy occurred concurrently and at similar rates, indicating excellent agreement between these techniques. No H 2 O or CO 2 were detected during treatment in H 2 or CO, respectively, indicating that reduction from Me 3+ to Me 2+ occurred by introduction of protons. These protons are fully removed by treatment in O 2 to 773 K, and O 2 -H 2 redox cycles involving reversible proton formation suggest that cations reside within AlPO framework positions, in which protons reside as charge balancing cations at Me 2+ -O-P bridges. H 2 consumption rates measured during H 2 -TPR could be accurately described by Arrhenius-type behavior, and H 2 /Me ratios showed that only a fraction of all Me cations undergo reversible redox cycles. This fraction was 0.86 for MnAPO-18 (atomic Mn/P ) 0.05); it decreased from 0.68 to 0.40 for MnAPO-5 as Mn/P ratios increased from 0.028 to 0.10. For CoAPO-18 with 0.028 Co/P and CoAPO-5 with 0.40 Co/P, the fractions of redox sites were 0.64 and 0.40, respectively. UV-visible spectra showed no detectable Me 3+ features after thermal treatment in H 2 . Thus, cations that do not undergo redox cycles remain as permanently divalent cations throughout O 2 -H 2 cycles. The redox-active fraction also decreased during repeated H 2 -O 2 redox cycles above 773 K, indicating that redox cations convert into permanently divalent sites. This conversion coincides with the evolution of H 2 O during H 2 treatments above 773 K. These findings together with the effects of Me/P ratio on the redox fraction are consistent with a mechanism in which OH groups at divalent framework cation sites recombine to form H 2 O and an oxygen vacancy. Cyclohexanol and cyclohexanone formation rates during liquid-phase cyclohexane oxidation with O 2 on MnAPO-5 samples increased linearly with the number of redox-active sites, suggesting that elementary steps of cyclohexane oxidation involve cycling between Me 2+ and Me 3+ , and require cation sites able to reversibly form charge balancing cationic species.

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Section: Chemical and Structural Changes Inmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Structural and Functional Characterization of Redox Mn and Co Sites in AlPO Materials and Their Role in Alkane Oxidation Catalysis

et al. 2004

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“…There is patent literature claiming that Co 2+ , Cr 3+ , Mn 2+ , and V 4+ ions can occupy framework positions in the AST structure, [29][30][31][32] but research papers dealing with the characterization of these materials are very scarce. 33, 34 Here, we report on the hydrothermal synthesis and the characterization of CoAPO-16, MnAPO-16, CrAPO-16, and VAPO-16 molec-ular sieves. The aim of this study was to distinguish framework and extraframework metal ion species, to monitor changes in the oxidation state of the metal ions during heating, and to study the thermal stability of the AST molecular sieves.…”

Section: Introductionmentioning

confidence: 99%

Incorporation of Transition Metal Ions in Aluminophosphate Molecular Sieves with AST Structure

et al. 2001

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Transition metal ion containing aluminophosphate molecular sieves with AlPO 4 -16 (AST) structure have been synthesized using the classic template, quinuclidine, and the corresponding salts of V 4+ , Co 2+ , Cr 3+ , and Mn 2+ . The cubic AST form was the main product, but in the presence of fluoride ions, the tetragonal AST form could also be obtained. The AST materials were characterized with X-ray diffraction, thermal analysis, scanning electron microscopy, electron spin resonance, infrared spectroscopy, and diffuse reflectance spectroscopy in the UV-vis-NIR region. Spectroscopic measurements showed that small amounts of Co 2+ and Mn 2+ can be incorporated in the framework. The diffuse reflectance spectra are indicative for the presence of two types of Co 2+ , most probably taking the two framework positions T 1 and T 2 with a T 1 :T 2 occupation ratio of 3-8. In contrast, V 4+ is present in defect sites by coordination to two (or three) framework oxygen atoms. The incorporation of Cr 3+ in the AST framework is also uncertain and two surface Cr 3+ species were observed: clusters similar to the bulk Cr 2 O 3 oxide and isolated Cr 3+ ions in strongly distorted octahedral coordination. These spectroscopic observations were confirmed by thermal analysis results, which showed that the degree of metal ion incorporation is inversely proportional with the phase transition temperature of AST to berlinite. Furthermore, Co 2+ and V 4+ are very stable in the AST structure and difficult to oxidize upon heating.

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“…No appreciable differences are observed among the three different loadings. Upon calcination in O 2 the color of the samples changes to green indicating the formation of Co 3+ sites as proposed in the literature [32][33][34][35][36].…”

Section: Resultsmentioning

confidence: 76%

[M]-CAL-2: MeAPSO-34-like molecular sieves using a lamellar aluminophosphate as precursor

Strauss

Martins

Berlier

et al. 2014

Microporous and Mesoporous Materials

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Characterisation of Co^IIand Co^IIIin CoAPO molecular sieves

Cited by 113 publications

References 1 publication

Structural and Functional Characterization of Redox Mn and Co Sites in AlPO Materials and Their Role in Alkane Oxidation Catalysis

Structural and Functional Characterization of Redox Mn and Co Sites in AlPO Materials and Their Role in Alkane Oxidation Catalysis

Incorporation of Transition Metal Ions in Aluminophosphate Molecular Sieves with AST Structure

[M]-CAL-2: MeAPSO-34-like molecular sieves using a lamellar aluminophosphate as precursor

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Characterisation of CoIIand CoIIIin CoAPO molecular sieves

Cited by 113 publications

References 1 publication

Structural and Functional Characterization of Redox Mn and Co Sites in AlPO Materials and Their Role in Alkane Oxidation Catalysis

Structural and Functional Characterization of Redox Mn and Co Sites in AlPO Materials and Their Role in Alkane Oxidation Catalysis

Incorporation of Transition Metal Ions in Aluminophosphate Molecular Sieves with AST Structure

[M]-CAL-2: MeAPSO-34-like molecular sieves using a lamellar aluminophosphate as precursor

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Characterisation of Co^IIand Co^IIIin CoAPO molecular sieves