Transition Metal Ions in Microporous Crystalline Aluminophosphates: Isomorphous Substitution

Weckhuysen, Bert M.; Rao, R. Ramachandra; Martens, Johan A.; Schoonheydt, Robert A.

doi:10.1002/(sici)1099-0682(199904)1999:4<565::aid-ejic565>3.0.co;2-y

Cited by 122 publications

(91 citation statements)

References 79 publications

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“…This allows the isomorphous substitution of Al 3+ and/or P 5+ by transition metal ions. This process is of considerable interest since it gives rise to materials with acid and/or redox sites and consequently leading to interesting catalytic properties [1][2][3][4][5]. In this respect, Co-, Cr-, Fe-, Mn-, Ti-and V-containing aluminophosphate molecular sieves have been extensively studied .…”

Section: Introductionsupporting

confidence: 87%

“…MAPO-5 and (M, N)APO-5 molecular sieves were synthesized with bidistilled water, pseudo-boehmite (70% Al 2 O 3 , Catapal), H 3 PO 4 (85 wt.% in aqueous solution, Acros), trimethylamine (TEA, 99%, Acros), and one or two types of transition metal ion salts (Co(CH 3 -COO) 2 4 was first dissolved in water, followed by the addition of the transition metal ion salts. Then, pseudo-boehmite was added to the solution at 0°C under continuous stirring condition after the transition metal ion salts were dissolved.…”

Section: Catalyst Synthesismentioning

confidence: 99%

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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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Section: Introductionsupporting

confidence: 87%

Section: Catalyst Synthesismentioning

confidence: 99%

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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“…1,2 Many transition metal ions are thought to be incorporated in the framework of these molecular sieves. Only for Co 2+ has unambiguous evidence been presented for isomorphous substitution.…”

Section: Introductionsupporting

confidence: 86%

Electron Spin Resonance of High-Spin Cobalt in Microporous Crystalline Cobalt-Containing Aluminophosphates

et al. 1999

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Four highly crystalline, cobalt-containing microporous aluminophosphates have been investigated by using liquid He X-and/or Q-band electron spin resonance (ESR) spectroscopy in order to investigate the coordination of high-spin cobalt before and after calcination. The ESR spectra of the four zeolite structures are characterized by an axial signal with an effective g ⊥ ≈ 5.80-5.44 and g || ≈ 2.00. Quantitative temperature dependence measurements of this axial signal in the temperature range 4-30 K reveal a Curie-Weiss behavior for both as-synthesized and calcined samples confirming (a) the m s ) ( 1/2 ground state of magnetically isolated high-spin cobalt and (b) a zero field splitting ∆ > 0 cm -1 . Quantitation of the ESR signals indicated that most of the Co 2+ is ESR active and that only about 30% of this Co 2+ can be oxidized to the ESR-inactive Co 3+ after calcination. The spin Hamiltonian parameters of as-synthesized and calcined CoAPO-5 material, as determined by spectrum simulation and the microwave power saturation technique, support the presence of framework Co 2+ in a flattened or elongated tetrahedron (D 2d ).

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“…Each stream was individually purified using a 13X molecular sieve at ambient temperature to remove H 2 O and hydrocarbon traces. The effluent was analyzed using a differentially pumped mass spectrometer (MKS Instruments, Orion Compact Residual Gas Analyzer) as the sample temperature was linearly increased while flowing H 2 In Situ UV-Visible Spectroscopy. UV-visible spectra were measured using a Cary 400 Bio spectrophotometer (Varian Corp.) and an in situ flow cell (Harrick, Inc.) with a diffuse reflectance accessory.…”

Section: Synthesis and Characterization Of Meapo Samplesmentioning

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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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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Transition Metal Ions in Microporous Crystalline Aluminophosphates: Isomorphous Substitution

Cited by 122 publications

References 79 publications

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

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

Electron Spin Resonance of High-Spin Cobalt in Microporous Crystalline Cobalt-Containing Aluminophosphates

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

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