Adsorbent aluminophosphates

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. 

One-electron Fe redox catalysts may also be immobilized by incorporation into aluminophosphate frameworks. Dugal el al. (143) reported the oxidation of cyclohexane to give adipic acid with air as the oxidant in the presence of Fe-AlPO-31. This molecular sieve has narrow pores, with a 0.54-nm diameter. Cyclohexane is easily adsorbed in the micropores, but desorption of initial products such as cyclohexyl hydroperoxide or cyclohexanone is slow. Consequently, subsequent radical reactions occur until the cyclohexyl ring is broken to form linear products that are sufficiently mobile to diffuse out of the molecular sieve ... 

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. 

As the shortcomings of the traditional preparative methods outlined above became apparent, it was realized that alternative procedures were required to produce uniform or tailor-made adsorbents and shape-selective catalysts. As we saw in Chapter 11, one major route was opened up by the Linde synthesis in 1956 of the crystalline molecular sieve zeolite A. The search for new microporous crystalline materials has continued unremittingly and has resulted in the synthesis of novel zeolitic structures including the aluminophosphates, which are featured in this chapter. 

In reporting the results of a spectroscopic study of aluminium phosphate in 1971, Peri drew attention to the isostructural nature of A1P04 and Si02 and the likely value of A1P04 as an adsorbent and catalyst support. Stable high-area A1P04 gels could readily be prepared in 1971, but at that time there was no indication in the open or patent literature that zeolitic forms of aluminophosphate could be synthesized. 

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. 

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). 

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. 

AJ. Ramirez-Cuesta, P.C.H. Mitchell S.F. Parker (2001). J. Mol. Cat. A Chemical, 167, 217-224. An inelastic neutron scattering study of the interaction of dihydrogen with the cobalt site of a cobalt aluminophosphate catalyst. Two-dimensional quantum rotation of adsorbed dihydrogen. 

The Cr/aluminophosphate catalysts exhibit a "fast kinetics profile relative to the profile of Cr/silica. That is, the polymerization rate of Cr/AIPO4 develops almost immediately with no induction time [637], The polymerization rate rises for about 10-20 min, and then declines during the rest of the 1-2-h run. Figure 168 shows an example of the reaction kinetics, when the catalyst had a P/Al atomic ratio of 0.8 and was activated at 700 °C. The rapid development of polymerization suggests that the initiation steps are faster on Cr/aluminophosphate catalysts than on Cr/silica catalysts. This difference could indicate faster reduction, or alkylation, or that the redox by-products, such as formaldehyde, are more quickly removed from the reaction diluent. These aluminophosphate supports are usually better adsorbents for polar compounds than silica activated at 700 °C, and this difference may contribute to the kinetics profile. 

Davis and coworkers [104] studied " Xe NMR of xenon adsorbed in several SAPOs, ALPOs, and Y zeolites. From a comparison of the xenon chemical shift extrapolated to zero pressure, these authors concluded that Xe atoms feel significantly smaller electrostatic fields and field gradients in the aluminophosphates compared to aluminosilicates. The extrapolated chemical shift decreased from 97 ppm in erionite to 60 ppm in Y zeolite and to 27 ppm in AIPO4-5, with the values for SAPOs being intermediate to Y zeolites and AlPOs as would be expected from the acidity trends. They concluded as well that SAPO-37 does not contain separate aluminophosphate and aluminosilicate islands. Dumont et al. [105] also carried out xenon NMR experiments in SAPO-37. From xenon sorption capacity and the decrease in the chemical shift, their conclusion was that the framework of calcined SAPO-37 is unstable when exposed to moist air. 

DOR and DAS open up whole new areas of research into the solid state, but there have so far been very few published applications of these techniques [163-167]. Chme]ka et al. [164] used O DOR and DAS to distinguish the three inequivalent oxygen sites known to be present in the mineral diopside, CaMgSi20(,. The remaining applications all involve Al in aluminophosphate molecular sieves. Wu et al. [ 165] showed that Al DOR is capable of resolving discrete framework aluminum sites in VPI-5, permitting quantitative investigation of site-specific adsorbate interactions (typically H2O interactions) with the... 

Molecular ordering of the adsorbed phase within the microporous model aluminophosphate AIPO4-II at cryogenic temperatures... 

Xe continues to be an excellent probe in characterizing porous materials. Applying the trends predicted by Jameson and de Dios, Koskela et have interpreted the differences seen in variable temperature Xe NMR spectra of Xe adsorbed in aluminophosphate molecular sieves with circular and elliptical cross sections at two different Xe loading values. Autsbach and Zurek have repeated the calculation of the chemical shift of Xe in of a Xe atom trapped inside C60. 

Other families of microporous solids, such as the aluminophosphates, are much more reactive in water, even in vapour form. Aluminium in these structures readily coordinates water to adopt five- or six-fold coordination, and exposure of many calcined AlPOs to moisture results in hydrolysis of framework bonds and loss of crystallinity. One way to avoid this, and thereby to enable handling, is to adsorb volatile organic compounds upon cooling after calcination. This protects the most reactive parts of the framework against... 

A number of molecular sieves ean be prepared as defect-free, neutral frameworks. Traditional, inorganic examples inelude pure silicas and aluminophosphates most of the organic-inorganie MOFs also fall into this eategory. In these cases the interactions of adsorbed moleeules are dominated by dispersive interactions of the adsorbed molecules with the atoms of the framework and so can be modelled by interatomic potential models that have been parametrised according to experimental data. Because these inorganic systems are so well defined and well characterised, they make exeellent model systems for ealculations. From an... 

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