Page 91 - Principles of Catalyst Development
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78                                                       CHAPTER  4
                                TABLE 4.11.  Catalytic Zeolites

                                                 Pore opening

                     Zeolite           No.  of 0 2  ions   Dimensions  (nm)
                   Faujasite,  X, Y         12               0.74
                   Mordenite                12               0.74
                   Otfretite                12               0.64
                   ZSM-S                    10               0.54 x 0.56
                   Zeolite  A                8               0.41
                   Erionite                  8               0.36  x 0.52


                These materials  have the general formula
                                Me~;,,[(AI02USi02)v]mH20                 (4_10)

            where  Me"+  is  the  cation  that  satisfies  electric  neutrality in  the  structure_
            During catalysis, the  m H 20  molecules  are removed,  leaving a  large  cavity
           in which the reaction takes place. Cationic positions vary from one structure
           to  another,  but  some  of them  are  found  at  locations  within  the  internal
           surface of the cavities. As such, they playa vital role in establishing catalytic
           properties.  Zeolites  are  most  easily  prepared  from  sodium  silicates  and
           aluminates,  so  that  sodium  is  the  cation  in  freshly  made  material.  This  is
           readily  ion-exchanged  with  a  large  number of ions  and  the  effect  is  very
           dramatic, as demonstrated in Table 4.12. There are various models to explain
           these  results.  In  addition  to  different  valences,  the  Lennard-lones  ionic
           radius  decreases  from  Na+  to  HT.  This  suggests  that  a  good  correlation
           exists with the factor fle/ r,  the polarizing power. This polarizing effect could
           be exerted directly across the cavity or indirectly through the zeolite lattice
           to  neighboring OH groups.! 1411
               Shape and size selectivity are important when molecules approach the
           critical  dimensions  of the  pore  opening_  In  Table  4.13,  the  diameters  of
           common hydrocarbons are listed in order of size, and we expect to find  the
           largest effect  in  zeolites  with  pores and  cages  close to these  dimensions.












                                 Figure 4.24.  Structure of ZSM-5.
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