Aluminophosphates catalytic activity

On nitrided aluminophosphates, AlPON, Massinon et al. [206] observed on a series of six samples with increasing nitrogen contents a good correlation between the catalytic activity in the Knoevenagel condensation reaction and the amount of surface NH, species (1 < x < 4) quantified by the Kjeldahl method. The authors suggest that those species are not the only active species and evoke an additional role of the nitride ions in the reaction [206] on the other hand, Benitez et al. [207] suggest hydroxyls linked to aluminum cations in the vicinity of terminal P-NH2 groups as basic centers. 

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. 

The catalytic activity of aluminophosphates has been discussed by Tada et al. (133-135). It seems also of interest to perform a theoretical investigation of possible active centers (both BAS and LAS) in these systems and to compare them with the respective centers in aluminosilicates. Such a comparison implies certain requirements both to the scheme of computations and to the choice of the cluster models. Most important is that the procedure of saturating the dangling bonds of a cluster should affect the results to a minimal extent. A simple way of attaining this aim is to construct closed clusters with terminal bonds mutually saturating each other (41). 

In the development of novel catalysts for the cracking of heavy fractions in crude oils, the focus has been on aluminosilicate and aluminophosphate materials with very large pore openings. Until recently, VPI-5 was the material with the largest known pore openings (ca 12 A) [1]. Consequently, a thermally stable VPI-5 appeared to be of considerable interest in catalyst development, provided that catalytic activity can be obtained. 

Amorphous alumincphosphates have been used as catalysts or catalyst supports for a number of years. Canpelo and his coworkers have shewn that they can be promoted by alkali metals or F ions (Ref. 25-26), but they behave like the amorphous aluminosilicates in being non-selective. The unsubstituted aluminophosphates have essentially no catalytic activity although they do dehydrate methanol to dimethyl ether (ref. 11). They can, hewever, be used as catalyst supports, and Coughlin and Rabo (ref. 27) have considered irtpregnated AlP04 s as supports for Fischer-Tropsch catalysts. 

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 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. 

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. 

Figure 17 (a) Dependence of catalytic activities of aluminophosphate catalysts on the P/Al ratio (I) reaction rate constant for isobutanol dehydration, (II) reaction rate for I-butene isomerization, (b) H MAS-NMR spectra of aluminophosphate catalysts with different P/Al ratios A 1.6 B 1.4 C 1.0 D 0.5 E model compound AI(H2P04)2. Spinning sidebands are indicated by asterisks. Centerband chemical shifts are indicated in the plot. (From Ref. 14.)... 

P-07 - High catalytic activity of Fe (Ill)-substituted aluminophosphate molecular sieves (FeAPO) in oxidation of aromatic compounds... 

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. 

Titanium. In recent years, titanium-modified open-framework materials have been extensively stndied. Tetrahedral titanium has been incorporated into several zeolites silicalite-1 (169), silicalite-2 (170), MCM-41 (171), zeolite-Beta (172), and ZSM-5 (173). These titan-modified materials show remarkable reactivity in the oxidation reactions using diluted aqueous hydrogen peroxide nn-der mild conditions. The catalytic activity is mostly related to the tetrahedrally coordinated titanium incorporated in the framework (174-179). It is worth noticing that the titanium-incorporated silicalite-1 (TS-1) has been one of the most relevant industrial catalysts in the past two decades. These facts have directed research to investigate the possibility of the incorporation of titanium into the aluminophosphate framework. 

The presence of the framework titanium sites in the aluminophosphate molecular sieves (TAPO-5, -11, -31 and -36) were proved indirectly by the catalytic activity of these materials in the liquid-phase oxidation and epoxidation reactions by hydrogen peroxide. The incorporation of Ti(IV) centers in mesoporous hexagonal alnminophosphates was determined by catalytic activity in the oxidation of phenols at room temperature, where remarkable paraselectivity was achieved in TiHMA (188) and TAP (189). 

Table 5. Ti Structure in Aluminophosphates and Test of Catalytic Activity... 

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