Synthesis of mesoporous materials

SANS study of the mechanisms and kinetics of the synthesis of mesoporous materials from micelles of tri-block copolymers... 

In the near future, we wish to extend kinetics experiments to other synthesis of mesoporous materials, in order to determine how the self-assembly mechanisms are changing with respect to the type of synthesis. 

Nowadays synthesis of mesoporous materials with zeolite character has been suggested to overcome the problems of week catalytic activity and poor hydrothermal stability of highly silicious materials. So different approaches for the synthesis of this new generation of bimodal porous materials have been described in the literature like dealumination [4] or desilication [5], use of various carbon forms as templates like carbon black, carbon aerosols, mesoporous carbon or carbon replicas [6] have been applied. These mesoporous zeolites potentially improve the efficiency of zeolitic catalysis via increase in external surface area, accessibility of large molecules due to the mesoporosity and hydrothermal stability due to zeolitic crystalline walls. During past few years various research groups emphasized the importance of the synthesis of siliceous materials with micro- and mesoporosity [7-9]. Microwave synthesis had... 

Since the first synthesis of mesoporous materials MCM-41 at Mobile Coporation,1 most work carried out in this area has focused on the preparation, characterization and applications of silica-based compounds. Recently, the synthesis of metal oxide-based mesostructured materials has attracted research attention due to their catalytic, electric, magnetic and optical properties.2 5 Although metal sulfides have found widespread applications as semiconductors, electro-optical materials and catalysts, to just name a few, only a few attempts have been reported on the synthesis of metal sulfide-based mesostructured materials. Thus far, mesostructured tin sulfides have proven to be most synthetically accessible in aqueous solution at ambient temperatures.6-7 Physical property studies showed that such materials may have potential to be used as semiconducting liquid crystals in electro-optical displays and chemical sensing applications. In addition, mesostructured thiogermanates8-10 and zinc sulfide with textured mesoporosity after surfactant removal11 have been prepared under hydrothermal conditions. 

Behrens P., Ordered molecular arrays as templates, a new approach to synthesis of mesoporous materials, Adv. Mater. 5 (1993) pp. 127-132. 

The synthesis of mesoporous materials with metal-organoflinctional groups has attracted a great deal of interest fi-om many researchers due to their potential applicability as catalysts. MCM 41 materials, comprising hexagonally-packed arrays of one-dimensional cylindrical pores, can provide large surface areas for functionalisation with metals and/or organic species, either by postmodification or direct synthesis. ... 

In 1992, researchers of the Mobil Oil Company introduced a new concept in the synthesis of mesoporous materials. They used supramolecular arrays of surfactant molecules as templating agents in order to obtain mesostructured silicates or alumosilicates which retain after calcination an ordered arrangement of pores with diameters between 2 and 10 nm and a narrow pore size distribution comparable to that of zeolites. These materials called M41S phases give access to the regime of the mesopores which is very interesting for different kinds of new size selective applications, e.g., molecular sieves, catalysis and nanocomposites [1]. 

The synthesis of mesoporous materials has been investigated at various conditions. The nature, the pore size, the wall thickness, the channel array and the morphology of the obtained materials have been characterized by powder X-ray diffraction, BET and Scanning Electron Microscopy. The present work shows that in the studied conditions, the pore diameter remains relatively constant while the thickness of the walls increases significantly with increasing time and temperature, signifying that the condensation of the source of silicium around the micelles is enhanced From the characterization results, a synthesis mechanism is postulated. 

The Sonochemical Synthesis of Mesoporous Materials and the Insertion of Nanopartides into the Mesopores by Ultrasound Radiation... 

Mineralization on hyphae can also generate macroporous materials.[15] Using this method in the synthesis of mesoporous materials, mesoporous and macroporous composites can be obtained. The long channels in these composites are parallel to each other. The pores are at a micron level, and the thickness of the walls ranges from 50 to 200 nm. 

The typical synthesis of mesoporous material can be divided into two main stages (i) Formation of the organic-inorganic liquid-crystal phase (mesophases or mesostructure) results from the self-assembly of surfactant molecules and inorganic species which are polymerizable (or condensable) under synthesis conditions. Moreover, this mesostructure has a crystal lattice with the cell length in the nanometer range, (ii) Removal of surfactant from the mesostructure by calcination at high temperatures or other physical or chemical treatments results in the formation of mesopores (the space occupied by surfactant molecules) in the mesostructure. 

In fact, since the mesophase synthesis or reaction is a kinetically control process and the solid formed is not a thermally stable phase, the phase transformation is very common during the synthesis of mesoporous materials. The phase transformations include the transition from one structure or symmetry into another structure or symmetry, or the transition from a disordered phase to an ordered phase, or from an ordered phase to a disordered phase. The intermediate phase can be isolated as a product or be observed by analysis techniques. The phase transformation can occur during the synthesis process or in a post-synthesis treatment. The early famous example is the transition of lamellar mesophase of silicate into hexagonal mesophase in basic or near neutral media.[5]... 

Table 8.6 Synthesis of mesoporous materials with zeolite precursors as starting reactants...

The synthesis of mesoporous material should be an ecologically friendly, low toxicity, low-cost, simple procedure. A possible reuse of these materials (as adsorbent or catalyst) has to be investigated. Commercial issues such as cost and scale up of the mesoporous material have to be assessed in competition with existing materials. 

Regev, O., Nucleation events during the synthesis of mesoporous materials using liquid crystalline templating, Langmuir, 12, 4940, 1996. 

EXPLOITING SURFACTANT SYSTEM PHENOMENA IN THE SYNTHESIS OF MESOPOROUS MATERIALS... 

Another practical aspect of the synthesis of mesoporous materials concerns balancing product quality with simplicity and economics of the procedure 7J. The approaches discussed below were selected primarily because they afford high quality products (structure, stability) from common reagents and by a simple hydrothermal synthesis. 

Tanev et al. have reported the synthesis of mesoporous materials via a route which involves self-assembly between neutral primary amines and neutral inorganic framework precursors.12 The regularity of the pore structure in these materials has been illustrated by lattice images which show a honeycomb like structure. The system of channels of these molecular sieves produces solids with very high internal surface area and pore volume. This fact combined with the possibility of generating active sites within the channels produces a very unique type of acid catalyst. In the case of transition metal substituted M41S, the principal interest lies in their potential as oxidation catalysts, especially Ti and V substituted MCM and HMS type materials, and more recently synthesised large pore materials.13... 

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