Aluminophosphate molecular crystal structure

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. 

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

VPI-5 is an aluminophosphate framework with very large one-dimensional pores defined by 18-member ring[22]. The crystal structure report[7j of synthesized VPI-5 revealed the possible role of water molecules as templates. The use of VPI-5 as a molecular sieve and as a catalyst primarily depend on the removal of the occluded water molecules without the destruction of the framework structure. Prasad et al.[23] reported from their TGA experiment that the seven distinct types of water molecules could be desorbed from VPI-5 in a step-wise fashion, in the temperature range of 35 to 60°C. The cluster model calculations[24] have revealed the actual electronic interaction of water molecules with VPI-5 framework. CG technique also indicated that the void volume in VPI-5 could be controlled by the partial removal of water molecules. 

Medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and Cg aromatic reactions. With olefin feeds, they promote isomerization with little loss to competing hydride transfer and cracking reactions. With Cg aromatics, they effectively catalyze xylene isomerization and ethylbenzene disproportionation at very low xylene loss. As acid components in bifunctional catalysts, they are selective for paraffin and cycloparaffin isomerization with low cracking activity. In these reactions the medium pore aluminophosphate based sieves are generally less active but significantly more selective than the medium pore zeolites. Similarity with medium pore zeolites is displayed by an outstanding resistance to coke induced deactivation and by a variety of shape selective actions in catalysis. The excellent selectivities observed with medium pore aluminophosphate based sieves is attributed to a unique combination of mild acidity and shape selectivity. Selectivity is also enhanced by the presence of transition metal framework constituents such as cobalt and manganese which may exert a chemical influence on reaction intermediates. 

A large family of novel aluminophosphate based molecular sieves has recently been described in the literature(l-3). The individual crystal species of these molecular sieves represent a wide variety of crystal structures and chemical compositions. Structures include several novel crystal types and various intracrystalline pore sizes. Thus aluminophosphate-based molecular sieves have been... 

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. 

The changes of the acid - base properties of metal substituted aluminophosphate based molecular sieves (MeAPO) as function of the chemical composition and the crystal structure are proposed to be complicated and to be substantially different compared to zeolites (1,2). 

Aluminophosphates denoted as AlPO-n (n means the framework structure) are a new family of molecular sieves with unique zeolitic pore structure. Since the first report on the synthesis of AlPO-n by Wilson et al., synthesis of various types of AlPO-n as well as the isomorphous substitution of metal cation into the AlPO-n structure have been reported. At present, more than 50 different crystal structures of AlPO-n have been reported, which is comparable to that of aluminosilicate zeolites, making the aluminophosphates an important class of microporous crystals." ... 

More recently, the conversion of methanol to C2 C olefins has been also reported using aluminophosphate-based molecular sieves. Surprisin y, in contrast to zeolite catalysts where best results were obtained with the medium pore ZSM-5, the best results with aluminophosphate catalysts have been described with the small pore SAPO-34 as catalyst. This molecular sieve, with a crystal structure belonging to the chabazite family, produces ethylene, propylene, and butenes with 90% or even higher selectivity. According to the data, methanol can be converted with emphasis to ethylene or to propylene as principal products by using an appropriate choice of reaction conditions (Table 6). Practical process development efforts for the conversion of methanol to C2-C4 olefins have been reported using SAPO-34 catalyst in a fluid-bed configuration. 

Recently fluoride ions were introduced in the starting mixtures of ionothermal syntheses in order to prepare (cobalt)aluminophosphate molecular sieves [200, 201]. In these systems, the solvent is an ionic liquid or eutectic mixture and, in many cases, can be considered as a precursor of the structure directing agent. It seems that F also plays important roles for the crystallization in these media containing few water molecules. 

ABSTRACT. Progress in molecular sieve science and technology is reviewed in the fields of synthesis, catalyst characterization, and catalysis. In synthesis, the molecular sieve crystal types and compositions have been greatly expanded from zeolites and pure silica materials to aluminophosphate-based molecular sieves representing more than two dozen crystal structures and many more chemical compositions, with up to six framework metals chosen from 13 elements. These new molecular sieves represent a variety of chemical properties, and they display weak to medium-strong acid catalytic activity. 

Several families of new molecular sieves based on a novel aluminophosphate family have been recently described by Flanigen al. [1]. These materials may consist of Al and P as the only tetrahedral framework atoms (T), or they may include additional T atoms chosen from 13 elements, with valence from one to five. These new compositions may contain up to six different T elements. The whole aluminophosphatebased molecular sieve family represents more than two dozen crystal structures and about 200 chemical compositions. 

In addition to the large new families of aluminophosphate-based molecular sieves reported from Union Carbide s laboratories, recent reports by Bond et al. [2], Tapp et al. [3] and Pyke al [4] also describe aluminophosphate-type compositions containing aluminum, phosphorus, and additional elements. These molecular sieves represent ternary and occasionally quaternary aluminophosphate compositions with the -5 or the -11 type crystal structures identified in Table I, and they contain Zn [2], Co " " [3], and Si, Ti, Zn, V, Fe, or Mg [4] as additional T elements. An interesting theoretical discussion of isomorphic metal substitution in molecular sieves has been published recently by Tielen et al. [5]. 

Crystalline microporous aluminophosphates containing framework metals, that is, MeAPO, constitute an interesting group of molecular sieves [29,137], These materials are synthesized hydrothermally between 100°C and 250°C using organic templates, and crystallize into different structure types. 

A novel class of crystalline, microporous aluminophosphate phases has been discovered. It represents the first class of molecular sieves with framework oxide compositions free of silica. The new class of materials encompasses some fourteen reported three-dimensional microporous framework structures, and six two-dimensional layer-type structures. The three-dimensional structures include structural analogues of the zeolites sodalite and erionite-offre-tite. The novel phases can be synthesized hydro-thermally in the presence of organic amines and quaternary ammonium templates. The template is entrapped or clathrated within the crystallizing aluminophosphate network. After thermal decomposition of the template the three-dimensional molecular sieves have the general composition of Al303 1.0 ... 

Silicoaluminophosphates (SAPOs) are a new generation of crystalline microporous molecular sieves. They have been discovered by incorporating Si into the fr unework of the aluminophosphates (AIPO4) molecular sieves. Several small-pore SAPO crystals have been synthesized. SAPO-17, SAPO-34 and SAPO-44 have pore openings of about 0.43 nm. SAPO-17 has an erionite-like structure, while SAPO-34 and SAPO-44 have a chabazite-like structure. 

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