Amorphous mesoporous materials process

This chapter discusses the synthesis, characterization and applications of a very unique mesoporous material, TUD-1. This amorphous material possesses three-dimensional intercoimecting pores with narrow pore size distribution and excellent thermal and hydrothermal stabilities. The basic material is Si-TUD-1 however, many versions of TUD-1 using different metal variants have been prepared, characterized, and evaluated for a wide variety of hydrocarbon processing applications. Also, zeolitic material can be incorporated into the mesoporous TUD-1 to take the advantage of its mesopores to facilitate the reaction of large molecules, and enhance the mass transfer of reactants, intermediates and products. Examples of preparation and application of many different TUD-1 are described in this chapter. 

Most examples discussed so far made use of amorphous inorganic supports or sol-gel processed hybrid polymers. Highly disperse materials have recently become accessible via standard processes and, as a result, materials with various controlled particle size, pore diameter are now available. Micelle-templated synthesis of inorganic materials leads to mesoporous materials such as MCM-41, MCM-48, MSU, and these have been extensively used as solid supports for catalysis [52]. Modifications of the polarity of the material can increase the reactivity of the embedded centre, or can decrease its susceptibility to deactivation. In rare cases, enhanced stereo- or even... 

Mesoporous materials of the M41S family with their regular arrays of uniform pore openings and high surface areas have attracted much attention since their first synthesis in 1992 (61), because their properties were expected to open new applications as catalysts and/or adsorbents. These materials are formed by condensation of an amorphous silicate phase in the presence of surfactant molecules (usually ammonium salts with long alkyl chains). However, the chemistry of the steps of the synthesis process is still not fully clear. Ideas put forward so far include (a) condensation of a silicate phase on the surface of a liquid crystalline phase preformed by the surfactant molecules (62) (b) assembly of layers of silicate species in solution followed by puckering of those layers to form hexagonal channels (63) and (c) formation of randomly disordered rod-like micelles with the silicate species... 

Zeolites such as Y, Beta, and ZSM-5 are widely used commercial catalysts, but their applications are strongly limited by their small pore sizes. One solution is to use ordered mesoporous materials such as MCM-41 and SBA-15. These materials exhibit good catalytic properties for the catalytic conversion of bulky reactants. Although the mesoporous silica materials made from normal synthesis processes, such as MCM-41 and MCM-48, have high thermal stability, their hydrothermal stability is poor. Calcined samples can be destroyed with moisture or water, even at room temperature. Most calcined samples became amorphous in cold water within a few minutes. The main reason is the hydrolysis reaction of the amorphous silica wall (Si—O Si bonds broken). 

The time from discovery to process for synthetic zeolites has been about ten years. This lag has already elapsed since the discovery of the ordered mesoporous materials and times seem ripe for their industrial development. The main obstacle towards viable applications is the presence on the market of much cheaper amorphous alternatives, mainly based on silica gels. Micelle-templated materials has to compete for new applications, to obtain results that can be achieved only thanks to their narrow pore size distribution. 

First obvious applications of ordered mesoporous materials were seen in catalysis, where a need for zeolite-like materials with bigger pore sizes was identified to process heavier residues more efficiently. However, since the acidity of ordered mesoporous materials does so far not substantially exceed that of amorphous aluminumsilicates, the high expectations could not be met. 

The dimensions and accessibility of pores of zeohtes and microporous solids are confined to the subnanometer scale (<1.5 run), which hmits their applications when processing bulky molecules. Mesoporous materials with pore sizes ranging from 2 to 50 nm overcome these limitations. In contrast with microporous zeolites, these materials lack atomic ordering (crystallinity) in their silica walls as these are usually amorphous. The attractive properties of ordered mesoporous materials include well-defined pore system high surface area and pore sizes narrow pore size distribution tunable up to 100 nm existence of micropores in the amorphous wall (for thicker wall materials) existence of various wall (framework) compositions obtained from direct synthesis, or posttreatment or modification high thermal and hydrothermal stabilities if properly prepared or treated and various controllable regular morphologies on different scales from nanometers to micrometers. 

The sol-gel process has been used to produce various amorphous silica structures, such as tubular organogels by molecular imprinting. The use of supramole-cules for the synthesis of crystalline mesoporous materials via sol-gel processing is also well known. Preparation of a silica sol of controlled properties was demonstrated by Stober et al. in 1968 and is discussed here. 

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