Pore size, aluminophosphate molecular sieves

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 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. 

Aluminophosphate molecular sieves with uniformly sized pores are a new porous material composed of A1P04 and called ALPO. Recently much attention has been... 

The present paper reports on the catalytic properties of selected aluminophosphate molecular sieves in model hydrocarbon reactions. The molecular sieves were selected to represent large and medium pore sizes with a variety of framework elements including transition metals, in addition to aluminum and phosphorus. Model reactions were chosen to explore catalytic performance in paraffin, olefin and aromatic rearrangement reactions to probe molecular sieve character, shape selectivity and catalytic activity, particularly for reactions involving olefins or olefin reaction intermediates. 

Cfi Aromatic Reactions Without Hydrogen. In the present study, tire aluminophosphate molecular sieves have been used alone and with added platinum and hydrogen to isomerize Cs aromatic feeds. In an initial screening study, a series of large to medium pore size molecular sieves were evaluated for catalytic activity for m-xylene rearrangements at 1000° F without added metal and hydrogen. 

Table IV. m-Xylene/Ethylbenzene Reactions with Medium Pore Size Aluminophosphate Based Molecular Sieves...

Iron substituted aluminophosphate molecular sieves (Fe-AlP04-l 1, Fe-AlP04-5 and Fe-VPI-5) are catalytically active in oxidations of aromatic compounds such as hydroxylation of phenol, benzene, and naphthol, as well as epoxidation of styrene. Catalytic data show that the activities of Fe-AlP04-l 1, Fe-AlP04-5 are comparable with that of TS-1 in the oxidation of aromatic compounds. Furthermore, Fe-VPI-5 shows high activity in naphthol hydroxylation by H2O2, while TS-1 is completely inactive due to the small pore size. By comparison of various catalysts, Fe (III) in the framework is considered to be the major active site in the catalytic reactions. 

In catalysis, the size of the pores is crucial for the application of aluminophosphate molecular sieves in processes, where bulkier molecules are involved. Attempts to improve the diffusion of reactants to the catalytic sites in aluminophosphates and porous materials in general have so far focused on (1) increasing the pore sizes of microporous catalysts, (2) decreasing the catalyst crystal size, as well as (3) providing new mesoporous materials with pore diameters form 2 to 50 nm, the latter being most intensively investigated in the past decade (65,66). 

Aluminophosphate molecular sieves exhibit a number of novel crystalline structures as well as analogues of topologies found in zeolites, with a large variety of pore sizes and compositions. 

The extensive size of organic amine as structure-directing templates or pore filling agents, coupled with a new gel chemistry resulted in the discovery of a third generation of zeolites containing Al3+ and P5+ as lattice atoms (1982). These aluminophosphate materials are a family of molecular sieves as shown in figure 7.10. 

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... 

Microwave heating has also been used to prepare other molecular sieves such as aluminophosphate aud silicoaluminophosphate materials. A comparison between conventional and nticrowave heating in the syntheses of SAPO-11 has been performed, with particular phasis on study effects on nncleation and crystal growth. Both were enhanced nsing microwave heating, narrower pore size distributions and more uniform crystal morphologies being observed. 

In particular, Alberti et al. (1991) proposed zeolite-based sensors for detection of hydrocarbons such as butane and Balkus et al. (1997) used thin film aluminophosphate (AlPO)-5 molecular sieve as the dielectric phase in a capacitance-type chemical sensor for CO and CO. AlPO-n is a family of phosphorus molecular sieves which, similar to zeolites, have ordered molecular-sized pores. The AlPO-5 structure used for the dielectric layer consists of four- and six-membered rings of alternating phosphate and aluminum ions bridged by oxygen. These rings are arranged to produce one-dimensional channels 0.73 nm in diameter. The properties of AlPO-n are reviewed in detail by Ishihara and Takita (1996), and one of the attractive properties of these materials is their heat stability. The properties of zeolites as they relate to zeolite-based gas sensors are discussed in a special section in Vol. 2. 

The adsorption of water vapour has been studied with a range of microporous carbons, zeolites and aluminophosphates in order to elucidate the relative influence of surface chemistry, pore size and pore shape upon the form of the water isotherm. It was possible to separate the adsorbents into three groups on the basis of their affinity and capacity for water vapour. The porous carbons were further examined using the BET and Dubinin-Serpinsky equations. The results show that the adsorption of water vapour at low p/p° is largely dependent upon specific adsorbent-adsorbate interactions whilst at higher relative pressures the micropore size and shape control the extent of adsorption. It is proposed that hydrogen-bonded layers of water can be more readily accommodated in the narrow slit shaped pores (-0.5nm) of molecular sieve carbons than in tubular pores of similar width (e.g. Silicalite/ZSM-5). 

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