Aluminophosphate molecular properties

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 introduction of silicon into hypothetical phosphorus sites produces negatively charged frameworks with cation-exchange properties and weak to mild acidic catalytic properties. Again, as in the case of the aluminophosphate molecular sieves, they exhibit excellent thermal and hydrothermal stability. 

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. 

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 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. 

The catalytic properties of the aluminophosphate molecular sieves are also influenced by chemical composition. The introduction of transition metals into framework positions enhances the activity and selectivity for olefin isomerization relative to the silicoaluminophosphates. The transition metal containing aluminophosphates are also surprisingly more selective for Cs aromatic rearrangements than the corresponding SAPO molecular sieves, an effect which can not be attributed solely to improved shape selectivity. 

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 ease of the redox reactions of metal cations in the framework suggests that metal cations can be easily substituted into the framework of aluminophosphate molecular sieves. The changes in the coordination of Al ions in the framework by the adsorption of some gases such as H2 have also been reported by some researchers. This has not been observed for aluminosilicate zeolites. Although no investigation has been performed on the influence of the framework environment on the catalytic properties of aluminophosphate molecular sieves, there is a possibility that the restricted redox properties of metal cations in the framework catalyze reactions which proceed over free metal cations, as with oxides or ion-exchanged zeolites. 

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... 

Drs. Ishihara and Takita focus on a specific catalyst, the aluminophosphate molecular sieves. These materials have some useful properties that distinguish them from the more widely used aluminosilicates. These include generally milder acidity, somewhat different ion exchange properties, and higher thermal stability. This allows them to be used in a number of reactions in which aluminosilicates are not suitable, such as low-temperature catalytic combustion. 

Several aluminophosphate molecular sieves with AEL topology structure were synthesized and modified by Pd for direct transformation reaction of -butane to isobutene. The effect of pore geometry of the molecular sieves was studied. Pd modified 10-member ring SAPO-11 and metal-substituted AlPO-l 1 and SAPO-11 showed high selectivity towards isobutene. The incorporation of metal into the molecular sieves had effect on the product distribution. Catalytic properties and the result of chemical adsorption of monoxide predicted the interaction between the transition metal for substitution and the supported palladium. 

P-09 - Synthesis, characterization and properties of an anionic aluminophosphate molecular sieve with Bronsted acidity... 

The abihty of the aluminiun to attain six coordination when associated with phosphate ligands results in the aluminophosphate molecular sieves exhibiting some unusual thermal properties. As discussed previously, D Yvoire s (n = 1 -6) materials were hydrated species with some of the aliunimun in octahedral coordination, four points attached to the phosphate in the framework, and the two remaining coordination sites occupied by molecules of water. The labihty of these water molecules is dependent on the structure. AIPO4S prepared fi-om organic-containing systems can also exhibit expanded coordination around the aluminum. 

Vanadium-cobalt substituted aluminophosphate molecular sieve of AEI structure (VCoAPO-18) was found to be active and selective in the ODH of ethane. Its catalytic behavior can be related to the presence of redox (probably related to and Co " ") and acid sites (related to Co + cations) in addition to its unique structural properties. The conversion and ethene selectivity decreases in the order VCoAPO-18 >VO c/CoAPO-18 > CoAPO-18 [38]. At 873 K, the VCoAPO-18 catalyst showed a 50% ethene selectivity at 60% ethane conversion for an ethane/oxygen molar ratio of 4 8. Acid SAPO-34-based microporous catalysts with chabasite structure have been tested for the ODH of ethane in the temperature range of 823 to 973 K. Pure acid and La/Na containing SAPO-34 were catalytically active and a 75 ethene selectivity for 5% ethane conversion and a 60% ethane selectivity for 30% ethane conversion was observed [39]. 

Alumipophosphate molecular sieve membranes. In addition to zeolites, Haag Tsikoyiannis [1991] have also briefly described another type of molecular sieve membranes consisting of AIPO4 units whose aluminum or phosphorous constituent may be substituted by other elements such as silicon or metals. These membranes are made from aluminophosphates, silico-aluminophosphates, metalo-aluminophosphates or metalo-aluminophosphosilicates. Like zeolites, these materials have ordered pore structures that can discriminate molecules based on their molecular dimensions. Their separation and catalytic properties can also be tailored with similar techniques employed for zeolites. The procedures for calcining the membranes or separating them from non-porous subsuates are essentially the same as those described earlier for zeolites. 

Aluminophosphates (AIPO4S) are a class of materials which, like the aluminosilicate zeolites, assume open framework structures containing channels of molecular dimensions with molecular sieve properties. Since the AIPO4 structures contain equal numbers of AIO4 and PO4 units there is no necessity for charge-balancing extraframework cations and consequently no sites to provide acid catalytic properties. 

Aluminophosphate zeolites with interesting molecular sieve properties have also been synthesized (69) but the pentavalency and trivalency of P and Al respectively do not result in a supplementary negative charge as observed in silico-aluminate zeolite, and subsequently no acidity is expected. However, alumino-silico-phosphate zeolites with variable Al and P contents may well be of interest in the future. 

M. Hartmann L. Kevan (1999). Chem. Rev., 99, 635-663. Transition-metal ions in aluminophosphate and silicoaluminophosphate molecular sieves Location, interaction with adsorbates and catalytic properties. 

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). 

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