Aluminophosphate molecular structure

The liquid-phase autoxidation of cyclohexane is carried out in the presence of dissolved cobalt salts. A lot of heterogeneous catalysts were developed for this process but most catalysts lacked stability. The incorporation of cobalt ions in the framework of aluminophosphate and aluminosilicate structures opens perspectives for heterogenization of this process. CoAPO (cobalt aluminophosphate) molecular sieves were found to be active heterogeneous catalysts of this oxidation.133 Site isolation was critical to get active catalysts.134... 

Aluminophosphate-Based Molecular Sieves In 1982 a major discovery of a new class of aluminophosphate molecular sieves was reported by Wilson et al. [26]. By 1986 some 13 elements were reported to be incorporated into the aluminophosphate frameworks Li, Be, B, Mg, Si, Ti, Mn, Fe, Co, Zn, Ga, Ge and As [27]. These new generations of molecular sieve materials, designated AlP04-based molecular sieves, comprise more than 24 structures and 200 compositions. 

The characteristics of aluminophosphate molecular sieves include a univariant framework composition with Al/P = 1, a high degree of structural diversity and a wide range of pore sizes and volumes, exceeding the pore sizes known previously in zeolite molecular sieves with the VPI-5 18-membered ring material. They are neutral frameworks and therefore have nil ion-exchange capacity or acidic catalytic properties. Their surface selectivity is mildly hydrophilic. They exhibit excellent thermal and hydrothermal stability, up to 1000 °C (thermal) and 600 °C (steam). 

The crystal structures of several metal aluminophosphate molecular sieves, in Innovation Zeolite Mater. Sci. (eds P.J. Grobet, W.J. Mortier, E.F. Vansant, and G. Schulz Eklofi), Stud. Surf. Sci. Gatal., vol. 37, Elsevier, Amsterdam, pp. 269-279. 

Similarly, reactive oxide mixtures are also used to synthesize aluminophosphate molecular sieves, usually starting from phosphoric acid along with the addition of alumina and silica sources analogous to those used in zeolite synthesis with a notable exception alkylammonium salts and amines were ultilized in structure-direchng and space filling to the exclusion of alkali hydroxide solutions and alkali metal salts. 

The aluminophosphate molecular sieves have an interesting property for potential use as catalyst supports, due to their excellent thermal stabilities and unique structures. AIPO4-5 is known to retain its structure after calcination at 1000°C and have uni-directional channels with pore size of 8 A bounded by 12-membered rings [2]. To utilize molecular sieves as catalyst support, chemical interactions between the molecular sieve and active component, chemical stabilities, and surface structures must be determined. However, iittle attempt has been made to clarify the surface structures or properties of catalytically active components supported on the aluminophosphate molecular sieves. 

Catalysis. - Aluminophosphate molecular sieves (A1PO) form a family of synthetic zeotypes, containing many three dimensional framework structures. Metal substituted aluminophosphates (MAPO) have important applications as catalysts and HFEPR has been used to determine the catalytically active sites. Two very detailed papers on various MAPO have been reported recently22,23 using both echo-detected HFEPR at 95 GHz and 3H and 31P ENDOR. 

Both piperidine and pyridine serve as structure-directing agents in the commercial production of Ferrierite zeolite. More recently, use of DMAP has allowed preparation of novel metallo-aluminophosphate molecular sieves with both small- <2006W02006037437> and large-pore architecture <2006USA074267>. 

These crystalline products are the aluminophosphate molecular sieves. The large number of different structure-types, as determined by X-ray diffraction, made it necessary to assign each new structure-type a number. Thus the family of A1P0 molecular sieves... 

The synthesis of the first members of a new family of aluminophosphate molecular sieves (the ALPOs) was disclosed by Union Carbide scientists in the early 1980s (Wilson et al., 1982a,b). The zeotype frameworks of the ALPO structures can be pictured as alternating [AlOJ and [P02]+ units and so are electrically neutral with both Al and P occupying adjoining T-sites. 

The zirconium fluoride phosphate has a microporous 3D structure similar to that found in some aluminophosphate molecular sieves. 

Bulk and Surface Compositions. The chemical compositions of the molecular sieves used in this study are given in Table I in terms of tetrahedral atom (T-atom) fractions, and are grouped according to structure type. The bulk compositions of AIPO4-5, AlPO -20 and VPI-5 show the ideal 1 1 ratio of A1 and P characteristic of aluminophosphate molecular sieves. The SAPO materials have frameworks consisting of Si, A1 and P T-atoms. 

Metal aluminophosphates (MeAPO) contain framework metal (Me), aluminum, and phosphorus. When the metal is divalent (e.g., Zn +, Co +, and Mg +) and substitutes for aluminum, a negatively charged framework results, with H+, for example, serving to compensate the charge. Many aluminophosphate molecular sieves have been synthesized. SAPO-11 and MeAPO-11 have interesting catalytic properties. Their structures have onedimensional 10-ring channels. The 10-ring pore aperture is elliptical with dimensions 0.39 x 0.63 nm. Table 1 is a summary of the characteristics of the molecular sieves which have been used for the skeletal isomerization of n-butenes. 

The difficulty of incorporating metal ions into the molecular sieve lattice results from the fact that actually two requirements have to be fulfilled, i.e., (i) the metal cation must have approximately the size of the atom it replaces (Si, A1 or P) and (ii) it must be able to coordinate in a tetrahedral position in the firamework. Fiuthermore, to function as a successful redox catalyst, a change in the valency and/or the coordination of the oxidant must be realized via reversible change of the coordination of the metal cation. Only a limited number of cations have been reported to be incorporated in the fiamework of zeolite and metal-aluminophosphate molecular sieves. These cations include Co, V, Mn, Cr. Ti [158,159] and a short compilation of the structures available (isomorphously substituted molecular sieves) is compiled in Table 1. Generally, it seems that aluminophosphate lattices are more easily adaptable for isomorphous substitution, but that the resulting materials have a lower stability than the corresponding zeolite frameworks [160]. 

There are terminal groups in the frameworks, such as P=0, P-OH, and Al OH, which make the structures less stable than zeolites and aluminophosphate molecular sieves with (4,2) networks. These terminal groups also favor the formation of interrupted frameworks, such as cloverite and JDF-20 ... 

J.M. Bennett and B.K. Marcus, The Crystal Structures of Several Metal Aluminophosphate Molecular Sieves. Stud. Surf. Set Catal., 1988, 37, 269-279. 

J.W. Richardson and E.T.C. Vogt, Structure Determination and Rietveld Refinement of Aluminophosphate Molecular Sieve AIPO4-8. Zeolites, 1992, 12, 13-19. 

M. Tiemann and M. Froba, Mesostructured Aluminophosphates Synthesized with Supra-molecular Structure Directors. Chem. Mater., 2001, 13, 3211-3217. 

The aluminophosphate molecular sieves (AiP04 s) consist of aluminum and phosphorus linked by oxide ions. In the larger family of aluminophosphate based molecular sieves with three or more framework cations an additional 13 elements have been incorporated with a variety of crystal structures. The whole aluminophosphate based molecular sieve family comprises more than two dozen crystal structures and about two hundred compositions. While AIPO4 molecular sieves with only two framework elements are catalytically inactive, most of the three or multi-component aluminophosphate based molecular sieves possess cation exchange capacity, and in the protic form they display carboniogenic catalytic activity. 

Aluminophosphate based molecular sieves are known to exist in a wide range of structural and compositional diversity . Substitution of silicon in the framework of aluminophosphate molecular sieves (SAPO) imparts acidity to the material and thus makes it active for acid catalyzed reactions. Through controlled substitution of the amount of Si in aluminophosphate, the catalytic activities due to its acidic properties can be altered. The extent of Si substitution in the aluminophosphates is however limited and is determined by the topology of the structure. 

Aluminum-27 Double Rotation NMR spectroscopy (DOR) has been used to investigate framework ordering in the aluminophosphate molecular sieves VPI-5, AIPO4-5, and AIPO4-8. Well-resolved peaks in the Al DOR spectra of both hydrated and dehydrated VPI-5 allow isotropic shifts to be conelated with local framework structure. More distorted aluminum environments are reflected by broader lines in Al DOR spectra of AIPO4-5 and AIPO4-8. 

An important related class of materials are the aluminophosphate molecular sieves, designated AIPO4S and originally reported by Wilson and coworkers [12]. These have exactly equal numbers of Al- and P-based T-sites and exact alternation of the Al and P atoms. Because of this structure, the frameworks are neutral, and they have no inherent catalytic capabilities. However, they do show molecular-sieving characteristics and can be made acidic by the introduction of a proportion of other elements into the framework. For example. Si substitution produces silicon aluminophosphates (SAPOs) with mild acidity. One recent AIPO4 of special interest is VPl-5 [13] (Fig. 3). This sieve has a unidimemsional channel system with a particularly large diameter. It is made up of 18-membered... 

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