Molecular reactors, zeolite structures

Zeolite membranes have attracted a lot of interest for their uniform pore size at molecular scale, which allows the separation of liquid and gaseous mixtures in a continuous way. Because of their thermal and chemical stability, they can also be used in processes at high temperatures and in the presence of organic solvents where polymeric membranes fail. In addition, zeolite materials exhibit intrinsic catalytic properties which clearly suggests the use of zeolite membranes as catalytic membrane reactors (CMRs). In the last two decades, enormous progress on zeolite membrane synthesis has been made, but only 20 structures are used for membrane preparation even if 170 zeolitic structures are available today (Baerlocher et al, 2007). The high cost and poor reproducibility in the synthesis step hinder the application of the zeolite membranes at industrial level (Caro et al, 2005 Mcleary et al., 2006). Until now, only NaA and T-type zeolite membranes... 

In the isomerization of styrene oxides in a fixed bed reactor under gas phase conditions, the catalytic performance of various catalysts on the activity, selectivity and service time was screened at 300°C and WHSV = 2-3h" . As shown in Fig. 15.1, zeolites with MFI-structure are superior to other zeolite types and non zeolitic molecular sieves, as well as greatly superior to amorphous metal oxides. 

Zeolite membranes indicate inorganic membranes with a selective/cata-lytic layer composed of a zeolite which is crystalline aluminosilicate with the feature of a high ordered porous structure with size comparable to molecular dimension. An example of the use of zeolites as a catalyst in a multi-phase membrane reactor can be found in Shukla and Kumar (2004) who have immobilized a lipase on a zeolite-clay composite membrane by using glu-taraldehyde as a bifunctional ligand in order to carry out the hydrolysis of olive oil. An application of a zeolite-based membrane in a three-phase membrane reactor has been reported by Wu et al. (1998), where TS-1 zeoUte crystallites were embedded in a polydimethylsiloxane (PDMS) membrane in order to catalyse the oxyfunctionalization of n-hexane (from a gas phase) with hydrogen peroxide (from a liquid phase). 

Zeolite membranes are promising for the development of a reactor because they are characterized by efficient thermal resistance, an inherent catalytic activity, and a complex microporous structure, which guarantees high-specific surface. This is a very captivating property because it partially solves the problem of low surface that is typical of most common commercial membranes. The potential of this kind of material does not stop here they are also able to distinguish between species with similar molecular weight, and this is possible because their channels remain accessible only for species with acceptable steiic hindrance. 

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