Mesoporous cavities

This chapter deals with the selective preparation, TEM/EXAFS/XPS characterization and catalysis of mono- and bimetallic nanowires and nanoparticles highly ordered in silica FSM-16, organosilica HMM-1 and mesoporous silica thin films. The mechanism of nanowire formation is discussed with the specific surface-mediated reactions of metal precursors in the restraint of nanoscale void space of mesoporous silica templates. The unique catalytic performances of nanowires and particles occluded in mesoporous cavities are also reviewed in terms of their shape and size dependency in catalysis as well as their unique electronic and magnetic properties for the device application. 

Figure 15.21 shows a schematic representation of the SCCO2 treatment effect for promoting the internal diffusion of metal ions to prepare Rh and RhPt alloy nanoparticles in mesoporous FS-16 and HMM-1. The supercritical phase displays both liquid and gas properties at the same time. SCFs can also dissolve various metal precursors, which promotes their mobiUty and surface-mediated reaction to form nanoparticles by the hydrogen reduction in the mesoporous cavities of... 

Nano-Confinement. There are limited, but interesting studies, regarding the confinement in ordered mesoporous materials. First observations were made on nematic liquids within mesoporous SBA-15 host materials which showed a change in the phase transition, when confined within the mesoporous cavities. To evidence also that there are many studies of confinement in mesoporous materials in the polymer diffusion and membrane literature, but they refer essentially to entropic effects due to restricted motion of these materials inside the ordered mesoporous materials which in enhanced by more hydrophobic and less polar surfaces. This is especially true as the molecules become larger, because the number of conformations the molecule can adopt in a confined space is limited. We refer here, on the contrary, to aspects relevant for catalysis and in which thus the dimensions of the molecules (of the order of 0.1 nm) is far below the dimensions of the cavities (around 5 nm for SBA-15, for example). 

The first demonstration, to our knowledge, that the properties of a gas or liquid within a mesoporous cavity change was made by Dosseh et al. studying the properties of cyclohexane and benzene confined in MCM-41 and SBA-15. The effect was related to the influence of the inner surfaces of mesoporous silica. Other authors have further demonstrated the influence of confinement on the adsorption and properties of fluids within an ordered mesoporous material. Flowever, Fajula et were the... 

Produced by the above means, probabiy in a confidential scheme, the activated carbon must have the following characteristics (1) adequate mesopores (cavities) which constitute pathways for iarge and small molecules to access the inner porous structure, (2) a surfeit of micropores (less than 2 nm in major diameter) which contain the primary adsorption sites, and (3) nearly complete modification of the internal surface structure to make it capable of attracting and bonding to chemicals via van der Waals forces (weak electrostatic forces between molecules), not the covalent or ionic forces bonding the atoms of molecules together. 

First of all, we have to define rigorously the different volumes (porosities) which are going to be characterized. The column volume (Vc) is composed of the interparticle volume (Vj) and the particle volume (Vp). The particle volume consists in the solid volume (volume of the atoms, Vs), the macroporous volume (Fw), the microporous volume (f ) and the mesoporous volume (V ). The latter can be divided in two volumes the mesoporous cavities volume, called internal mesoporous volume (V,m), and the cylindrical mesoporous volmne, called external mesoporous volume (Vem) ... 

For this determination, the molecular probe must not penetrate into the zeolite micropores (and so not in the mesoporous cavities). Therefore, we must choose a bulky probe molecule whose size is larger than the zeolite micropore dimensions. In these conditions, the concentrations of the probe molecule in the micropores and in the internal mesopores equal zero, so the equation (4) becomes ... 

The previous determination allowed to determine Ve . The determination of the other mesoporous volume, Vm, is much more complicated, since this volume is related to the microporous volume F (the mesoporous cavities are only accessible via the micropores). To obtain Vm and F separatly, we have to employ two molecular probes at the same time these two molecules A and B must have a similar structure to prevent an adsorption selectivity in... 

The sintering behavior of an amorphous zircon powder synthesized by sol-gel processing has been studied by Veytizou et al. (2002). Gel samples were prepared from a reactive precursor solution of zirconyl nitrate and tetraethoxy silane. N2-adsorption-desorption measurements on the porous precursor powder of surface area 412 m /g indicated the presence of spheroidal mesoporous cavities with narrow necks created by a packing of spherical particles. The precursor powder with irregularly shaped and different sized agglomerates or aggregates had a microporous volmne of 27 mm /g and a... 

Acidic micro- and mesoporous materials, and in particular USY type zeolites, are widely used in petroleum refinery and petrochemical industry. Dealumination treatment of Y type zeolites referred to as ultrastabilisation is carried out to tune acidity, porosity and stability of these materials [1]. Dealumination by high temperature treatment in presence of steam creates a secondary mesoporous network inside individual zeolite crystals. In view of catalytic applications, it is essential to characterize those mesopores and to distinguish mesopores connected to the external surface of the zeolite crystal from mesopores present as cavities accessible via micropores only [2]. Externally accessible mesopores increase catalytic effectiveness by lifting diffusion limitation and facilitating desorption of reaction products [3], The aim of this paper is to characterize those mesopores by means of catalytic test reaction and liquid phase breakthrough experiments. 

It has been previously reported [21, 22] that metal colloids are formed by radiochemical reactions in water/alcohol solutions, in which the reduction of metal salts takes place by solvated electrons and free radicals produced under UV or y-ray irradiation. Ichikawa et al. have applied this photoreduction method to the surface-mediated reaction of metallic ions and succeeded in synthesizing metal/aUoy nanowires in the constrained cavities of mesoporous supports such as FSM-16 and MCM-41 [18-20, 23-25]. The adsorbed water and alcohol work not only as solvents in the nanoscale silica void space but also as a source of reducing species for metallic ions to metals under UV-vis and y-ray [11, 18, 19] irradiation. The results indicate the dense formation of Pt nanowires inside the charmels of mesoporous supports, such as FSM-16, which act as the templates. In fact, no any Pt wire is observed on the external surface of FSM-16 or amorphous silica surface. Short wires, 10 nm long, are also observed as a minor species in the samples in the initial stage of UV and y-ray irradiation. 

Research in zeolites has also branched out to try to prepare new materials by incorporating various molecules and ions in the cages of these microporous and mesoporous structures. An early example of this was the preparation of the pigment ultramarine used in many paints and colourants. It is based on the zeolite sodalite (SOD) structure and contains 83 ions trapped in the cages this is the same anion found in the mineral lapis lazuli, to which it imparts the beautiful deep blue colour. Treatment of zeolites such as Na-zeolite Y with sodium vapour traps Na4 ions in the cavities, which impart a deep red colour. 

The second example demonstrated immobilization via ship in a bottle , ionic, metal center, and covalent bonding approaches of the metal-salen complexes. Zeolites X and Y were highly dealuminated by a succession of different dealumi-nation methods, generating mesopores completely surrounded by micropores. This method made it possible to form cavities suitable to accommodate bulky metal complexes. The catalytic activity of transition metal complexes entrapped in these new materials (e.g, Mn-S, V-S, Co-S, Co-Sl) was investigated in stereoselective epoxidation of (-)-a-pinene using 02/pivalic aldehyde as the oxidant. The results obtained with the entrapped organometallic complex were comparable with those of the homogeneous complex. 

Porphyrin-based self-assembled molecular squares 389 can form mesoporous thin films in which the edge of a square, thus the size of the cavity, can be adjusted by appropriate choice of substituents [8]. Fibers that form coil-coiled aggregates with distinct, tunable helicity are built from crown ethers bearing porphyrins 390 [9]. In addition to the porphyrin applications discussed in Sections 6.3.2.2 and 6.4, dendrimer metalloporphyrins 391 to be applied in catalysis [10] and the water-soluble dendritic iron porphyrin 319 modelling globular heme proteins [11] can be mentioned. 

The individual pores may vary greatly in both size and shape within a given solid, and between one solid and another. As presented in Chapter 2, the pores can be classified as (a) macropores, for diameters above 50 nm, (b) mesopores, for diameters in the range 2-50 nm, and (c) micropores, for diameters below 2 nm. It has to be mentioned that in a solid, a wide and continuous range of pore sizes is found, from macropores to micropores. A pore can also be an open pore, seen as a cavity or channel communicating with the surface of the particle, or a closed pore that is not connected to the surface. 

Transition metal complexes encapsulated in the channel of zeolites have received a lot of attention, due to their high catalytic activity, selectivity and stability in field of oxidation reactions. Generally, transition metal complex have only been immobilized in the classical large porous zeolites, such as X, Y[l-4], But the restricted sizes of the pores and cavities of the zeolites not only limit the maximum size of the complex which can be accommodated, but also impose resistance on the diffusion of substrates and products. Mesoporous molecular sieves, due to their high surface area and ordered pore structure, offer the potentiality as a good host for immobilizing transition complexes[5-7]. The previous reports are mainly about molecular sieves encapsulated mononuclear metal complex, whereas the reports about immobilization of heteronuclear metal complex in the host material are few. Here, we try to prepare MCM-41 loaded with binuclear Co(II)-La(III) complex with bis-salicylaldehyde ethylenediamine schiff base. 

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