Page 92 - Principles of Catalyst Development
P. 92

CATALYTIC  MATERIALS                                              79
                                TABLE 4.12.  Effect  of Faujasite
                                    Cation on Activity  for
                                    Cumene  Dealkylation"

                             Cation              Relative  activity
                          Na+                       1.0
                          BaH                       2.5
                          Sr H                      2.0 x  10
                          Ca 2 +                    5.0 x  10
                          Mg2+                      1.0  X  10'
                          NiH                       1.1  X  10 3
                          La'                       9.0  X  10 3
                          H+                        8.5  X  10 3
                          SiOr AI,03 =  1.0

                          "  Reference  141.
               There  are  three possible types  of shape and  size  selectivity effects,  as
           shown in  Fig.  4.25.  First, the  reactant molecules  may be too large to enter
           the  cavities.  Comparison  of Tables  4.11  and  4.13  shows  that  all  of the
           molecules  access  faujasite  structures.  Only  molecules  larger  than  penta-
           methyl  benzene are excluded.  Early examples of shape and size selectivity
           were  almost  completely  limited  to  small  openings  and  normal  versus
           branched  paraffins.  For example,  n-hexane was  selectively  cracked  in the
           presence  of 3-methylpentane over zeolite  A catalysts.  Other, more  subtle,
           effects may occur. The diffusivity of trans-butane-2 is 200 times larger than
           that of cis-butene-2 in zeolite CaA. By adding Pt to CaA, selective hydroge-
           nation of trans-butene-2  occurs.(t34)

                         TABLE 4.13.  Critical  Diameter of Hydrocarbons
                         Hydrocarbons                           Size (nm)

               n-Paraffins                                        0.45
               Methyl-paraffins                                   0.57
               Dimethyl paraffins
                 Benzene
                Toluene                                           0.63
                p-Xylene
               o,m-Xylene
               1,2,4-Tri-methyl  benzenes
               1,2,4,5-Tetra-methyl benzene naphthalene           0.69
               1,3,5-Tri-methyl  benzene
               Penta-methyl benzene                               0.78
   87   88   89   90   91   92   93   94   95   96   97