Aluminophosphates adsorption

Electron Spin Resonance Spectroscopy. Several ESR studies have been reported for adsorption systems [85-90]. ESR signals are strong enough to allow the detection of quite small amounts of unpaired electrons, and the shape of the signal can, in the case of adsorbed transition metal ions, give an indication of the geometry of the adsorption site. Ref. 91 provides a contemporary example of the use of ESR and of electron spin echo modulation (ESEM) to locate the environment of Cu(II) relative to in a microporous aluminophosphate molecular sieve. 

The amount of theoretical and experimental research focused on the interaction, equilibrium and dynamical properties of noble, simple and polyatomic gases within quasi-one-dimensional nanotubes is still limited [6-13]. Experimental adsorption isotherms have been reported for simple gases (Ar,N2) and alkanes (methane [11], ethane [12], propane-butane-pentane [13]) in monodisperse nanotubes of aluminophosphates. It is expected that similar experiment could be carried out soon in bundles of monodispersed carbon nanotubes. 

P30s. The various structures exhibit intracrystalline adsorption pore volumes from 0.04 to 0.35 cm3/g, and pore sizes from 0.3 to 0.8nm. The aluminophosphate frameworks are hydrophilic. 

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. 

Many new adsorbents have been developed over the past 20 years including carbon molecular sieves, new zeolites and aluminophosphates, pillared clays and model mesoporous solids. In addition, various spectroscopic, microscopic and scattering techniques can now be employed for studying the state of the adsorbate and microstructure of the adsorbent. Major advances have been made in the experimental measurement of isotherms and heats of adsorption and in the computer simulation of physisorption. 

Molecular simulation of the adsorption of gases by the ALPOs was pioneered by Cracknell and Gubbins (1993), who pointed out that the aluminophosphates should be easier to model than the aluminosilicates. There are two important advantages first, the charge neutrality of the framework means that there are no exchangeable cations to be taken into account (this is, of course, also true for pure Silicalite) and second, the modelling is simpler because the pores are unidirectional with no interconnections. 

The most common kind of porous materials are the ones possessing a disordered or random porous network [1,2]. In this class of solids belong various silicas [3] aluminas [4] aluminophosphates [5] as well as a large number of aluminosilicate solids encountered in geological forms [6]. The pore size distribution of such disordered or random porous materials is usually characterized on a routine basis by N2 adsorption at 77K. From such measurerrient the following characteristic parameters of porous solids can be calculated ... 

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. 

In the field of transition metal catalysis, zeolites may offer opportunities for uniform active sites. With the discovery of both aluminosilicate and aluminophosphate, zeolites with a variety of transition-metal ions in tetrahedral firework positions may offer new possibilities. On the basis of existing zeolite chemistry dealing vrith aluminum hydrolysis and the formation of adsorption adducts in the zeolite pores, chemists may envision strategies aimed at the activation of tetrahedral transition metal ions, either by lattice oxide replacement or by the application of strong donor ligands. The demonstrated... 

Consider now real materials with model micropores, that is to say with regular dimensions and whose pore walls consist of well-defined crystalline adsorption sites (including possible cationic sites). Such solids can be found within the realm of zeolites and associated materials such as the aluminophosphates. One can imagine the probability that a fluid adsorbed within such micropores may be influenced by the well-defined porosity and thus become itself "ordered. Such phenomena have already been highlighted with the aid of powerful but heavy techniques such as neutron diffraction and quasi-elastic incoherent neutron diffusion. The structural characterisation of several of the following systems was carried out with the aid of such techniques in collaboration with the mixed CNRS-CEA Leon Brillouin Laboratory at Saclay (France). 

The structures of typical aluminophosphates, namely A1P04-11[6] and VPl-5[7] were generated from the reported crystal structures, adsorption of probe molecules such as water and ammonia, as well as the template organic molecules and larger molecules were... 

Optimization of the synthesis of an ultra-large pore aluminophosphate VPI-5 using di-n-butylamine (n-DBA-VPI-5) has been done. The sample synthesized was characterized by XRD, SEM, FTIR, TGA-DTA, NMR and sorption studies. The extra-large pores (larger than lOA) in VPI-5 were confirmed by adsorption measurements. N2 adsorption at 77K shows mesopores with pore diameter of 36A alongwith the micropores of n-DBA-VPI-5. 

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. 

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