Aluminophosphate-based molecular sieves

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. 

Virginia Polytechnic Institute number 5 (VPI-5) is a family of aluminophosphate based molecular sieves (refs. 1-5) which share a common three-dimensional topology and contain 18-membered rings (refs. 1-8). The extra-large pores of the VPI-5 materials are unidimensional channels circumscribed by rings containing 18 tetrahedral atoms and possess free diameters of approximately 12-13 A. 

The first discovery of a zeolite was recorded in 1756 (1). Since that time numerous natural and synthetic zeolites, silica polymorphs, and aluminophosphate-based molecular sieves have been reported. The largest ring in these materials consists of 12 tetrahedral (12 T) atoms. This boundary has been in existence for over 180 years since the first zeolite to contain 12 T-atom rings, gmelinite, was discovered in 1807 (1). Recently, we have... 

The search for new zeolite-like structures was initially extended to aluminophosphate-based molecular sieves,3 and these explorations produced a variety of exotic compounds with open-framework structures, which include besides metal phosphates,4 carboxylates,5 sulfates,6 selenites, and selenates.7 Growth of this area has been rapid in the past decade necessitating new editions of the Atlas of Zeolites.2 It is noteworthy that metal—organic framework (MOF) solids have been included as zeolite types in the latest edition of the Atlas of Zeolites. 

Skeletal Rearrangement Reactions of Olefins, Paraffins, and Aromatics over Aluminophosphate-Based Molecular Sieve Catalysts... 

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. 

Molecular Sieve Catalyst Preparation. The aluminophosphate based molecular sieves used in the present study were prepared according... 

Table II. Reactions of 1-Hexene over Aluminophosphate-Based Molecular Sieves...

In the present study, mixtures of supported platinum and aluminophosphate based molecular sieves have been tested for n-hexane rearrangement reactions under reforming conditions (900°... 

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. 

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). 

As for aluminosilicate zeolites, the acid catalyzed reactions of aluminophosphate-based molecular sieves have been well studied. However, the acidity of AlPO-n-based molecular sieves is generally weaker than that of zeolite as mentioned. Consequently, AlPO-n-based molecular sieves are generally less active for the reactions proceeding on acid sites such as isomerization of hydrocarbons or dehydration of alcohols. 

Aluminophosphate-based Molecular Sieves as a Thermostable Catalyst... 

Application of Aluminophosphate- based Molecular Sieves for a Thermostable Catalyst. - Although the high thermal stability is a particularly interesting property of aluminophosphate molecular sieves compared to those of aluminosilicate zeolites, the application of aluminophosphate molecular sieves as thermostable catalysts has been limited. In this section, the application of SAPO as thermostable NO reduction catalysts... 

Chapter 2 Property and Catalysis of Aluminophosphate-based Molecular Sieves... 

The first examples are devoted to zeohte synthesis (cf. also Vol. 1, Chap. 7 of this series Molecular Sieves - Science and Technology ). Aluminophosphate-based molecular sieves (acronym AIPO4) contain equimolar amounts of Al and P in lattice T sites that can both be replaced by other elements. In the case of Co insertion the acronym is changed to Co APO or more generally to MeAPO. CoAPO is usually synthesized using Et3N as template. 

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