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

Abstract

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.

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

“…The other absorption band was between 570 and 640 nm. This is indicative of the presence of vanadyllike and oligomeric species, and the amount of which increased with increasing vanadium content in the synthesis gel [3,12,22,27].…”

“…4). The spectral similarity of (Co, V)APO-5-0.05-0.03 to VAPO-5 indicates that V sites in the (Co, V)APO-5 and VAPO-5 molecular sieves had a similar local geometry [27], viz. part of them was present in the state of isolated square-pyramidal and/or distorted octahedral vanadyl-like (VO 2+ ) species [3,12,15,22,33,37].…”

“…The electron spin resonance (ESR) spectra were measured by a Bruker ESP 300E spectrometer. 27 Al MAS NMR spectra of the as-synthesized solids were carried out on a Bruker DRX400 nuclear magnetic resonance spectrometer (9.4 T). A 0.3 ls pulse length with the rf field of %150 kHz was applied.…”

“…An aqueous solution of Al(NO 3 ) 3 was used as reference compound. The spin rate of the rotor was 20 kHz for all 27 Al NMR measurements.…”

“…In contrast, it is possible to determine whether the isomorphous substitution of transition metal ions for Al 3+ occurs by the shift of the line characteristic for tetrahedral Al 3+ [13,14,28]. Akolekar as well as Kapoor and co-workers found a significant shift of tetrahedrally coordinated 27 Al when transition metal ions were incorporated in the framework of AlPO 4 -5 and other microporous aluminophosphates [13,14]. The 27 Al MAS NMR spectra (Fig.…”