Structure directing agents , large pore

Both piperidine and pyridine serve as structure-directing agents in the commercial production of Ferrierite zeolite. More recently, use of DMAP has allowed preparation of novel metallo-aluminophosphate molecular sieves with both small- <2006W02006037437> and large-pore architecture <2006USA074267>. 

MCM-22 (or PSH-3) contains two independent (i.e., not interconnected) pore systems and large (7.1 A x 7.1 A x 18.2 A) supercages [29]. The structure-directing agent (i.e., template) was hexamethyleneimine. Mobil workers prepared both the aluminosilicate and borosilicate versions of MCM-22 [30,31]. The acidic properties of the aluminosilicate version are close to... 

The mesoporous materials discussed here comprise silicates and aluminosilicates that are formed from synthesis conditions comparable to zeolites. The main difference with the latter is the use of supra-molecular assemblies of surfactant molecules, e.g. alkyltrimethylammonium ions, as the structure directing agents. Due to the large dimensions of these spherical or cylindrical micelles, the framework of the silicates is not so well crystallized as in the case of zeolites. This causes lower framework stability as well as weaker Bronsted acidity. Upon calcination, large pores of uniform diameter (say 4-10 nm, depending on the surfactants used) become accessible. 

Since the phosphate-based, extra-large pore materials do not reveal stability that is likely sufficient for many commercial applications, I will limit my discussions below to silica-based materials. As I have discussed elsewhere, the lack of stability in phosphate-based materials may not be due to the presence of extra-large rings, but rather to the nature of the structural units [9], With silica-based, extra-large pore materials, it appears that bulky structure directing agents will be required for their preparation. I will make this assumption in further discussions below. 

We compare the structure of Fe(III) active sites in the small pore FeAlPO-18 and large pore FeAlPO-5 catalysts prepared using appropriate structure directing agents. The present study clearly points out that it is possible to substitute Fe(III) ions for tetrahedrally coordinated Al(IlI) in the framework of AlPO-5 and AlPO-18 to yield an active and selective oxidation catalysts. The use of atomistic simulations is again proven to provide accurate geometries which, when combined with the analysis of the EXAFS data, yield accurate models for the active sites. 

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