Metal-Substituted Microporous Aluminophosphates

This chapter aims to present the zeotypes aluminophosphates (AlPOs) as a complementary alternative to zeolites in the isomorphic incorporation of metal ions within all-inorganic microporous frameworks as well as to discuss didactically the catalytic consequences derived from the distinctive features of both frameworks. It does not intend to be a compilation of either all or the most significant publications involving metal-substituted microporous aluminophosphates. Families of AlPOs and zeolites, which include metal ion-substituted variants, are the dominant microporous materials. Both these systems are widely used as catalysts, in particular through aliovalent metal ions substitution. Here, some general description of the synthesis procedures and characterization techniques of the MeAPOs (metal-contained aluminophosphates) is given along with catalytic properties. Next, some illustrative examples of the catalytic possibilities of MeAPOs as catalysts in the transformation of the organic molecules are given. The oxidation of the hardly activated hydrocarbons has probably been the most successful use of AlPOs doped with the divalent transition metal ions Co2+, Mn2+, and Fe2+, whose incorporation in zeolites is disfavoured. The catalytic role of these MeAPOs is rationalized based on the knowledge acquired from a combination of the most advanced characterization techniques. Finally, the importance of the high specificity of the structure-directing agents employed in the preparation of MeAPOs is discussed taking N,N-methyldicyclohexylamine in the synthesis of AFI-structured materials as a driving force. It is shown how such a high specificity could be predicted and how it can open great possibilities in the control of parameters as critical in catalysis as crystal size, inter-and intracrystalline mesoporosity, acidity, redox properties, incorporation of a great variety of heteroatom ions or final environment of the metal site (surrounding it by either P or Al)

Nanoscale Dynamics and Transport in Highly Ordered Low Dimensional Water

Highly ordered and highly cooperative water with properties of both solid and liquid states has been observed by means of neutron scattering in hydrophobic one dimensional channels with van der Waals diameter of 0.78 nm. We have found that in initial stages of adsorption water molecules occupy niches close to pore walls, followed later by the filling of the central pore area. Intensified by confinement intermolecular water interactions lead to the formation of well ordered hydrogen bonded water chains and to the onset of cooperative vibrations. On the other hand, the same intermolecular interactions lead to two relaxation processes, the faster of which is the spontaneous position exchange between two water molecules placed at 3.2 4 from each other. Self diffusion in an axial pore direction is the result of those spontaneous random exchanges and is substantially slower than the self diffusion in bulk wate

Carbon Nanotube Selective Membranes with Subnanometer, Vertically Aligned Pores, and Enhanced Gas Transport Properties

Membranes consisting of ultrathin, oriented, single-wall carbon nanotube (SWCNT) micropores with a diameter of ∼4 Å were developed. c-Oriented AFI-type aluminophosphate (AlPO) films (AlPO4-5 and CoAPO-5), consisting of parallel channels 7.3 Å in diameter, were first fabricated by seeded growth on macroporous alumina supports, and used as templates for synthesis of CNTs inside the zeolitic channels by thermal treatment, utilizing the structure directing agent (amine) occluded in the channels as carbon source. Incorporation of CNTs inside the AFI channels altered the transport mechanism of all permeating gases tested, and imposed a substantial increase in their permeation rates, in comparison to the AlPO4-5 membrane, despite the pore size reduction due to nanotube growth. The enhancement of the permeation rates is attributed to repulsive potentials between gas molecules and occluded nanotubes, which limit adsorption strength and enhance diffusivity, coupled to the smooth SWCNT surface that enables fast diffusion through the nanotube interior. Separation ability, evaluated with respect to H2 and CO2 gases, was enhanced by using polysterene as defect-blocking medium on both AlPO and CNT/AlPO membranes and was preserved after CNT growth. © 2015 American Chemical Society