Page 118 - Principles of Catalyst Development
P. 118

106                                                      CHAPTER  6
            Appendix  7.  For  do-it-yourself enthusiasts,  detailed  recipes  are  given  in
            Appendix 6.



            6.3.  DUAL  OXIDES

                Dual  oxides  are  intimate  combinations  of two  oxides,  resulting  in
            synergistic  catalytic  properties.  For  high  dispersion  and  interaction,  best
            results  are  obtained  through  co precipitation  and  subsequent  treatment  of
            the two gels. Common examples are SiOr AI 20 3 ,  NiO-AI 20 3 ,  and zeolites.
            Dual oxides are not separate compounds, since they exist over a wide range
            of compositions,  but  are  more  like  solid  solutions.  This  is  especially true
            for  low  calcination  temperatures.  With  high  heat  treatments,  solid  state
            reactions  occur and  identifiable compounds "precipitate" out.





                The best known dual oxide is  SiOr AI 20 3,  used as a  cracking catalyst
            or acidic support. The preparation is  very similar to that described for Si0 2 •
            Two  solutions  are  prepared,  the  first  a  30%  mixture  of water  glass  and
            water,  the  second  a  mixture  of 4N  HCI  and  0.5N  AI 2(S04)3.  These  are
           cooled to  5°C  and mixed  rapidly in a  2: 1 ratio.  As  the pH of the acidified
            water glass  reaches  6,  precipitation  of both  silica  and  alumina  sols  begin
           and  gelation  takes  about  45 sec.  The  combined  gel  is  aged  for  two  days,
           treated  with  2%  AI 2(S04)3  solution  to  ion-exchange  Na,  and  washed  free
           of sulfate  ions.  Preparation  is  completed  with  drying  and  calcination  at
           5500C. (163)
               The  resulting  catalyst  contains  about  25%  Al 20 3  and  is  amorphous,
           i.e., has no distinctive x-ray diffraction pattern indicating long-range crystal-
           linity.  Acidic  properties are  described  in  Chapter 4.





               Coprecipitated  NiO-AI 20 J  samples  are  typical  of a  wide  variety  of
           dual hydrous oxide gels. They are precursors to reduced Nij AI 20 3  catalysts
           used  in  hydrogenation and methanation.  Although these catalysts are pro-
           duced by other routes, the intimate interactions fostered by coprecipitation
           are  believed  to  impart improvements in  activity and stability to  the  nickel
           crystallites,  11641  although mechanical strength is  lower.  It may also be poss-
           ible that irreducible compounds of NiO and A1 20 3 ,  facilitated by coprecipi-
           tation,  play some role in  forming stable  promoters.
   113   114   115   116   117   118   119   120   121   122   123