Page 48 - Principles of Catalyst Development
P. 48

34                                                       CHAPTER  2
                There  are  situations  where  support  acidity  has  a  positive  influence,
            influencing the  main  reaction.  The support adds  dual  functionality  to the
            overall  catalysis,  as  best  demonstrated  with  catalytic  reforming. (56)  The
            objective in this process is  to  convert  low  octane components of naphtha,
            typically  normal  paraffins  and  naphthenes,  into  high-octane  iso-paraffins
            and aromatics.  Low  loadings of Pt-type  metals on AI 20) are  used  for this
            purpose.  Metallic  Pt  dehydrogenates  naphthenes  to  aromatics  but  cannot
            isomerize  or  cyclize  normal  paraffins.  This  is  accomplished  through  the
            acidic function  of the support, as shown for  n-hexane:



                                                                           (2.4)
                                             H'
                                     n-C;;  -     i-C;;
                                            AI 2 0 J
                Neither  Pt  or Al 20 3  promotes paraffin  isomerization,  but AI20)· acid
            sites isomerize normal olefins.  In a three step sequence, n-C 6  is  dehydroge-
            nated  by  Pt  to  n-C;;, which  migrates to the  AI 20) and isomerizes to  i-C;,
            then hydrogenated by the Pt to i-Co. Platinum and AI 20) must be in intimate
            contact  and  the  AI 20)  have  sufficient  acidity.  Chlorine  is  added  during
            activation  to  generate  the  needed  acidity. (56)  Dual  functionality  is  indeed
            an elegant demonstration of catalyst design. Another important example is
            hydrocracking. (57)
                Catalytic chemists have long suspected that the properties of the active
            component may be affected by contact with the support. Indeed, as we shall
            see,  there  are  so  many  ways  in  which  supports  exert  influences  that  it  is
            not surprising that these were noted. It was not until recently that researchers
            were  able  to  unravel  some  of  the  these  effects  with  more  sophisticated
            characterization  methods.  For  example,  measured  areal  rates  of  ethane
            hydrogenolysis  showed the  patterns in  Table 2.4. t5K )
                The  most  direct  influence  of  the  support  is  on  dispersion  and
            morphology. t5~1 It is  well known that nickel is better dispersed on Si0 2  than
            AI 20), and the shape of crystallites may also be affected.  Data in  Table 2.4
            do  not  address  the  question  of crystallite  size  dependence,  which  has  a
            much firmer foundation.  t60 )  Surface contamination is always present. Com-
           mercial  reagents  used  in  support  preparation  may  contain  impurities  or
            provide ions which remain with the oxide. Subsequent deposition of active
           components  may  incorporate  these  ions  as  poisons.  Acidic  sites  vary  on
           different  supports  and  during  initial  exposure  to  reagents  produce  car-
           bonaceous  deposits  which  in  turn  deactivate  nickel  1(1  )  sites.  A  possibility
           that  cannot  be  overlooked  is  spillover,  a  phenomena  in  which  a  reactive
   43   44   45   46   47   48   49   50   51   52   53