Catalytically active sites framework defects

The evidence reported in literature push one to think that a lower concentration of Lewis acid sites improves the catalytic capacity, and this is why the attention was focused on silicahte-1 (Frontera et al., 2012). This zeohte demonstrates good performance in other reactions that require low acidity (Bonelh et al., 2007). In fact, the presence of a large amount of defects allows obtaming a framework fiiU of sUanol group, which is responsible for a high surface density of active sites. 

Once boron-containing SSZ-24 is obtained, it can be easily converted to a form having tetrahedrally coordinated framework aluminum species by first calcining the boron-containing material to remove boron from the lattice and then incorporating A1 into the created defect sites by refluxing the material in an aluminum nitrate solution [ 101 ]. True aluminum incorporation into the framework was proven by Al MAS NMR spectroscopy and catalytic activity in Br0n-sted-acid catalyzed reactions [101]. 

The literature contains a number of examples of catalysis on active sites qualified as framework defects, for example, on open or uncoordinated carboxyUc acids. The authors cited above [93], who smdied the alkylation reaction in the presence of MOF-5, attribute the catalytic activity to encapsulated clusters of zinc hydroxide, which are formed as an impurity in the rapid synthetic routes, and may also be present in a partially hydrolyzed form of the framework structure, [Zii3(OH)2(bdc)2] [108,109]. 

MicrocrystalUne zeolites such as beta zeolite suffer from calcination. The crystallinity is decreased and the framework can be notably dealuminated by the steam generated [175]. Potential Br0nsted catalytic sites are lost and heteroatoms migrate to extra-framework positions, leading to a decrease in catalytic performance. Nanocrystals and ultrafine zeolite particles display aggregation issues, difficulties in regeneration, and low thermal and hydrothermal stabilities. Therefore, calcination is sometimes not the optimal protocol to activate such systems. Application of zeolites for coatings, patterned thin-films, and membranes usually is associated with defects and cracks upon template removal. 

The location of V-ions in vanadosilicate molecular sieves belonging to the MFI, MEL and BEA structure types synthesized by different procedures will now be discussed. It is revealed that many types of V-species are present in these metallosilicates, the relative proportions of these species being governed by the conditions of preparation. It is also confirmed that the catalytically more active vanadium ions are present in defect sites in the framework. 

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