Cloverite zeolites

The synthesis of Cloverite is also unique in that it established the utility of HF in zeolite molecular synthesis. In the case of silicates, either hydroxide or HF serve to mineralize the silica. That is not needed in the GaP04 synthesis but the F ion plays an important role in stabilizing the structure. The fluoride ion becomes encapsulated in the D4R units. It has been suggested that fluoride ions in fact direct... 

FIGURE 7.25 The framework of the synthetic gallophosphate, cloverite, represented as shaded spheres of van der Waals radii. (From C.R.A. Catlow (ed.) (1992) Modelling of Structure and Reactivity in Zeolites, Academic... 

The most obvious approach to obtain controlled mesoporosity is the extension of zeolite-like three-dimensional framework structures to larger pore-sizes. Zeolite-like compounds in the micro- and mesoporous range are the aluminophosphate VPI-5, cloverite and ULP (MCM-41). These new materials are characterized by pore diameters between 1.3 nm and 20 nm. 

However, zeolite-like materials with ultralarge pores such as Cloverite (20 T, 0.6 x 1.32 nm), VPI-5 (18 T, 1.27 nm), AIPO4-8 (14 T, 0.79 x 0.87 nm) were recently synthesized. A comparison between the pore openings of zeolites and the kinetic diameter of guest molecules shows clearly that zeolites can be used for molecular sieving. It should however be stressed that these values are temperature-dependant temperature increases both the flexibility of the guest molecules and the breathing motions of the host zeolite pore mouth and framework. 

The active components of a present commercial FCC catalyst are zeolites Y (0.74 nm) and ZSM-5 (0.54 nm x 0,56 nm) whose pore sizes give limited access to the active centres for long chain and/or bulky molecules. The new extra-large pore crystalline materials MCM-41 (2.0-20 nm), VPI-5 (1.21 nm) or cloverite (1.32 nm) have significantly increased the restricted pore sizes encountered in zeolites Y and ZSM-5 and opened up interesting perspectives for the conversion of heavier feedstocks [1]. 

There are terminal groups in the frameworks, such as P=0, P-OH, and Al OH, which make the structures less stable than zeolites and aluminophosphate molecular sieves with (4,2) networks. These terminal groups also favor the formation of interrupted frameworks, such as cloverite and JDF-20 ... 

A. Cloverite has an extremely open structure, and its framework density is one of the lowest among the known zeolite structures. 

In recent years zeolites with very large pores (supercages) and ring openings up to 0.6x1.32 nm (cloverite) have been s3mthesised. 

Extra-large-pore zeolites (at least 14-membered rings with pore sizes around 1.0 nm)—e.g., aluminosilicates CIT-5, UTD-1. aluminophosphates (VPI-5, JDF-20, gallophosphate. cloverite). 

The available intrazeolitic space should be sufficient to accomodate a complex with a diameter of approximately 1.5 nm. This seems possible in faujasite type zeolites as weli as in very large pore AlPO -n molecular sieves or substituted anaiogs. VPI-5 is a candidate (23). For AIPO4-8 molecular graphics analysis predicts - as is confirmed by synthesis attempts - that accomodation of the complex for sterical reasons in this molecular sieve is impossible. The Cloverite material, the structure of which has been described elsewhere in this volume, also is a potentiai support. 

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