Page 90 - Principles of Catalyst Development
P. 90

CATALYTIC  MATERIALS                                              77
           are combined, as  in  Fig.  4.22,  a  cavity with 0.7-nm  openings is  produced.
           This is the zeolite type A structure, designed mostly for separation processes
           in which molecules less than 0.4-0.5 nm in size are preferentially adsorbed.
               On the other hand, if the sodalite units are bridged across six-membered
           faces,  the resulting structure  (Fig.  4.23)  has a  0.73-nm opening to  a  12-nm
           cavity, forming faujasite.  Still another arrangement gives the parallel pores
           of mordenite (Fig. 4.23).
               There  are  over  70  known  different  kinds  of building  arrangements,
           each resulting in a distinct structure. Some of these occur as natural minerals;
           some are synthetic. The search for new combinations goes on. Catalytically
           important zeolites are listed in Table 4.11.  More details on exact structures
           will  be  found  in  the  references.  (137.138.139)  In  recent  years,  much  attention
           has  been directed toward a  new zeolite, ZSM-5,  whose structure is  shown
           in  Fig.  4.24.  This  is  a  zeolite  with  openings  of 0.54-0.56 nm,  intermediate
           between  the  zeolite  A  (0.4 nm)  and  faujasite  X  and  Y  (0.74 nm).  It has
           been successfully applied to the production of gasoline from  methanol and
           for other specialized purposes.(140)






                                                    0.6  nm









                                            (a)


















                        Figure 4.23.  Structure of (a) faujasite  and  (b)  mordenite.
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