Mesoporous active redox framework

Dye-sensitized solar cells (DSSCs) are photoelectrochemical solar devices, currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. DSSCs are based upon the sensitization of mesoporous nanocrystalline metal oxide films to visible light by the adsorption of molecular dyes.5"7 Photoinduced electron injection from the sensitizer dye (D) into the metal oxide conduction band initiates charge separation. Subsequently, the injected electrons are transported through the metal oxide film to a transparent electrode, while a redox-active electrolyte, such as I /I , is employed to reduce the dye cation and transport the resulting positive charge to a counter electrode (Fig. 17.4). 

The development of mesoporous materials with more or less ordered and different connected pore systems has opened new access to large pore high surface area zeotype molecular sieves. These silicate materials could be attractive catalysts and catalyst supports provided that they are stable and can be modified with catalytic active sites [1]. The incorporation of aluminum into framework sites of the walls is necessary for the establishment of Bronsted acidity [2] which is an essential precondition for a variety of catalytic hydrocarbon reactions [3], Furthermore, ion exchange positions allow anchoring of cationic transition metal complexes and catalyst precursors which are attractive redox catalytic systems for fine chemicals [4]. The subject of this paper is the examination of the influence of calcination procedures, of soft hydrothermal treatment and of the Al content on the stability of the framework aluminum in substituted MCM-41. The impact on the Bronsted acidity is studied. 

Porous oxide catalytic materials are commonly subdivided into microporous (pore diameter <2nm) and mesoporous (2-50 nm) materials. Zeolites are aluminosilicates with pore sizes in the range of 0.3-1.2 nm. Their high acidic strength, which is the consequence of the presence of aluminium atoms in the framework, combined with a high surface area and small pore-size distribution, has made them valuable in applications such as shape-selective catalysis and separation technology. The introduction of redox-active heteroatoms has broadened the applicability of crystalline microporous materials towards reactions other than acid-catalysed ones. 

Zeolites are microporous frameworks, and all of the ET chemistry that we have discussed is with molecules smaller than 13 A. The unique features of zeolites are their ion-exchanging ability, a stable structure upon dehydration and a pore/chan-nel structure that allows for a well-defined arrangement of molecules in space and the fact that redox-active atoms can be substituted on the framework. In most cases, the zeolite is an active host, influencing ET reactions via electrostatic fields or steric effects, a feature that is not found with the mesoporous and sol gel materials. Packing of molecules/ions in the intrazeolitic space with very high densities is also possible and was found to be important in charge propagation and electrochemistry. 

Composition of the inorganic framework can be varied just like the surfactant use has been expanded beyond the original cationic species. The number of explored element combinations has been enormous and often driven by specific catalytic needs and prior experience with amorphous or crystalline compositions [37]. One of the leading approaches was doping of silica formulation with appropriate activating clement, such as aluminum (lo impart acidity) and titanium or vanadium for redox potential. Based on analogy of AlPO s and SAPO s to zeolites these compositions were also synthesized in the mesoporous form as were other non-silica inorganic oxides and their combinations [38]. Pure metal mesoporous product was obtained via the pre-formed liquid crystal route 1391. 

The success of TS-1 sparked a flourish of activity, which still continues today, on the synthesis and application of other redox molecular sieves [21], This was driven by the expectation that TS-1 was the progenitor of a new class of novel catalytic materials. For example, titanium has been incorporated in the framework of wide variety of molecular sieves including mesoporous silicas, such as MCM-41, with pore diameters ranging from 20 to 50 A (see below). 

---