Page 93 - Principles of Catalyst Development
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80                                                       CHAPTER  4

                            REACTANT










                            PRODUCT
                            SELECTIVITY









                            RESTRICTED  TRANSITION
                            STATE  SELECTIVITY







                         Figure 4.25.  Shape  and  size selectivity effects  in  zeolites.


                Development of the ten-ring ZSM-5 zeolites extended these promoters
           to  more  sophisticated  selectivities.' 140)  Molecules  of  varying  degrees  of
            branching  and  chain  length  and  alkyl  aromatics  now  came  within  scope.
            Figure 4.26 shows the relative cracking rates of heptane isomers over ZSM-5,
           clearly demonstrating the effect of size exclusion. Since critical dimensions
           of alkyl  benzenes are  close to the  pore opening,  even  small  changes effect
           diffusivity,  as  demonstrated  in  Table 4.14.
                Differences such as  these are important in the second type of shape or
           size selectivity control, product exclusion. This is  the case where undesired
           products  cannot  egress  cavities,  whereas  the  desired  ones  can.  Toluene
           disproportionation  to  benzene  and  p- xylene  is  an  example  of successful
           applications of this  principle.
               A  third  type  of  control,  called  spatiospecificity,  occurs  when  both
           reactants  and  products  pass  the  opening  but  reaction  intermediates  or
           transition states are restricted by the size of the cavity. In xylene isomeriza-
           tion processes, selectivity is  lost through disproportionation to toluene and
           trimethylbenzene.  Diphenylmethane intermediates are too large for ZSM-5
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