Aluminophosphate surface

According to Table VIII, P-OH groups (Fig. 21b) represent the most stable Bronsted acid sites on the aluminophosphate surfaces. At the same time, P-OH-A1 sites (bridged hydroxyl groups) exhibit the strongest acidity. Their relative surface concentration with respect to P-OH could be estimated... 

New aluminophosphate oxynitrides solid basic catalysts have been synthesised by activation under ammonia of an AIPO4 precursor. When the nitrogen content increases, XPS points out two types of nitrogen phosphorus bonding. The conversions in Knoevenagel condensation are related to the surface nitrogen content. Platinum supported on aluminophosphate oxynitride is an active catalyst for isobutane dehydrogenation. 

The addition of aluminium to the liquid slowed down the reaction. An amorphous cement was formed and there was no crystallization in the bulk of the cement. However, after some time crystallites were formed at the surface. Thus, the presence of aluminium exerts a dedsive influence on the course of the cement-forming reaction. This effect is to be attributed to the formation of aluminophosphate complexes (see Sections 6.1.2 and 4.1.1). These complexes may delay the predpitation of zinc from solution and also introduce an element of disorder into the structure, thus inhibiting crystallization. It is significant that zinc, which does not form complexes, has little effect on the nature or speed of the reaction. 

It is interesting that this cement has been known for over 100 years and yet certain features of its chemistry remain obscure. The exact nature of the matrix is still a matter for conjecture. It is known that the principal phase is amorphous, as a result of the presence of aluminium in the liquid. It is also known that after a lapse of time, crystallites sometimes form on the surface of the cement. A cement gel may be likened to a glass and this process of crystallization could be likened to the devitrification of a glass. Therefore, it is reasonable to suppose that the gel matrix is a zinc aluminophosphate and that entry of aluminium into the zinc phosphate matrix causes disorder and prevents crystallization. It is not so easy to accept the alternative explanation that there are two amorphous phases, one of aluminium phosphate and the other of zinc phosphate. This is because it is difficult to see how aluminium could act in this case to prevent zinc phosphate from crystallizing. 

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). 

Beale, A.M. and Weckhuysen, B.M. (2007) Understanding the crystallisation processes leading to the formation of microporous aluminophosphates, in Zeolites to Porous Materials-The 40th Anniversary of International Zeolite Conference, Studies in Surface Science and Catalysis, vol. 170, Elsevier Science Publishers B V, Amsterdam,... 

Many books, reviews and treatises have been pubUshed on related subjects [1-7]. Thus the objective of this chapter is the deUneation of the key features of the catalytic surface and the process conditions which enable better control of the reaction pathways for more efficient and environmentally friendly processes and minimal utiHzation of precious natural resources. As it stands today, hundreds of known framework types have been synthesized and scaled-up [8], but only a handful have found significant application in the hydrocarbon processing industries. They are zeolite Y and its many variants, ZSM-5, Mordenite and zeohte Beta. Other very important crystalline materials (including aluminophosphates (ALPOs),... 

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. 

Oliver, S., Kuperman, A., Coombs, N., Lough, A., Ozin, G. A., Lamellar aluminophosphates with surface patterns that mimic diatom and radiolarian microskeletons. Nature 1995, 378, 47-50. 

One more type of cluster can be obtained by intracluster saturation of the broken bonds (41). This model was used in the comparative description of the surface centers of aluminophosphates and aluminosilicates (see Section V). 

Naturally, structures (d) and (f) do not exhaust all possible states of low-coordinated A1 atoms on the surface of the oxides considered. The calculations, however, seem quite sufficient to suggest that water molecule coordination by a LAS is energetically less favorable for aluminophosphate than for aluminosilicate surfaces. This conclusion is also in accordance with IR data, which indicate that LASs of the both oxides quite similarly interact with pyridine, whereas the LASs of aluminophosphates do not coordinate C02 molecules (136). Indeed, in the case of a sufficiently strong base (pyridine), adsorption interaction appears stronger than the structural coordination and therefore stabilizes the A1 atom in the adsorption state. On the contrary, for C02, which is certainly a very weak base, the interaction is strong enough in the case of aluminosilicates but is insufficient for the adsorption stabilization of aluminum in aluminophosphates. 

Bulk and Surface Compositions. The chemical compositions of the molecular sieves used in this study are given in Table I in terms of tetrahedral atom (T-atom) fractions, and are grouped according to structure type. The bulk compositions of AIPO4-5, AlPO -20 and VPI-5 show the ideal 1 1 ratio of A1 and P characteristic of aluminophosphate molecular sieves. The SAPO materials have frameworks consisting of Si, A1 and P T-atoms. 

I he recent literature related to selective skeletal isomerization of -butenes catalyzed by medium-pore zeolites and Me-aluminophosphates is reviewed. In the presence of medium-pore molecular sieve catalysts, o-butenes are selectively transformed into isobutylene via a monomolecular mechanism. This is an example of restricted transition state shape selectivity, whereby the space available around the acidic site is restricted, constraining the reaction to proceed mainly through a monomolecular mechanism. Coking of (he ciitalysl that leads to poisoning of (he acidic sites located on the external surfaces and to a decrease in the space around the acidic sites located in the micropores renders the catalyst more selective. 

Figure 16. Model for vesicle and bilayer control of macroscopic morphology and surface patterning, and mesolamellar structure of the aluminophosphates [206].

Kramer and co-workers used ab initio calculations of H4TO4 (T = Si, Al, P) clusters to derive parameters for the rigid ion potential model. The potential energy surface of the clusters was scanned along two modes of distortion, and the resulting potential curves were fitted using Eq. [15]. The set of parameters was refined by the use of experimental data on a-quartz. This procedure resulted in a parameterization that well reproduced both structure and elastic moduli of silicates, aluminosilicates, and aluminophosphates. Subsequently, this approach was extended to protonated forms of zeolites. ... 

Stereoselective catalysis in zeolites is still one of the ultimate goals in zeolite science. Earlier work in this field was summarized recently [4]. More recently, Mahrwald et al. [95] reported that the addition of aluminophosphate molecular sieves in the liquid phase alkylation of a-chiral benzaldehydes by butyllithium results in an increased proportion of the so-called Cram product in the diastereomeric mixture. It is argued that in this Grignard type reaction the adsorption of the reactants on the molecular sieves favors the attack at the sterically less hindered position of the molecule. This shape selectivity effect is even observed when the reactant is adsorbed at the outer crystal surface, as demonstrated for the case of the small-pore AIPO4-I7. 

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