Page 94 - Principles of Catalyst Development
P. 94

CATALYTIC  MATERIALS                                              81

                                                    RELATIVE
                                                    CRACKING
                                 C 7  PARAFFIN        RATE

                                                      1.00


                                                      0.52


                                                      0.38



                                                      0.09



                   Figure 4.26.  Relative  cracking  rates  of heptane  isomers over ZSM_5. i140 )

           cavities,  so  disproportionation  is  spatially  hindered  and  selectivity
           improved. Another example is aromatic formation from methanol. Although
           this reaction occurs readily  over several  acidic zeolites,  only  ZSM-5  limits
           product to  high  octane aromatics.( 140)
                When metal components are added through ion exchange, subsequent
           reduction gives new dimensions to these applications. Transition ions inside
           cavities are often difficult to reduce and are stabilized against agglomeration.
           This  is  an  advantage,  since  extremely  small,  molecular-sized  clusters  of
           metal  atoms  may  be  produced,  yielding  activities  and  selectivities  often
           found  in  organometallic  homogeneous  catalysis. (138)  As  more  is  known
           about the properties of these small metallic particles, many innovative uses
           will suggest themselves. Already, the principle of metal-zeolite combinations
           is commercially proven in hydrocracking, which combines both the features
           of hydrogenation  and  selective  cracking  to  produce  better gasoline  com-
           ponents with  less  coking.(57)
               Zeolites, with their vast possibilities of existing and potential structures,
           with shape and size selectivity, and  with  precise control  of acid and other


                      TABLE 4.14.  Relative  Diffusivities  of Xylenes  in  ZSM-S
                     Hydrocarbon           Relative  diffusivity  at 350°C

                     p-Xylene                       1000
                     m,o,m·Xylene
   89   90   91   92   93   94   95   96   97   98   99