Page 47 - Principles of Catalyst Development
P. 47

STRUCTURE  OF  CATALYSTS                                          33
            resistance is  present. (52)  The best supports are those that are easily manipu-
            lated to produce optimum texture properties. Alumina and silica are good
            in this regard. (53.54)  Much experience has been accumulated on the hydro-
            thermal  treatment  of hydrous  alumina  and  silica  gels.  Catalyst  manufac-
            turers  put  this  to  good  use  in  producing  supports  with  controlled  pore
            shapes and sizes.  Examples are  given in Chapter 6.
                Mechanical  strength  and  thermal  stability  of  catalyst  particles  are
            always of concern to  process designers.  In some cases it  may be the  most
            critical  feature.  This  was  emphasized,  for  example,  in  steam  reforming.
            Strong pellets with good thermal resistance are required. Catalyst designers
            use  mixed oxides  fired  at high temperatures to  form  ceramic compounds.
            Particles must be preformed and active components added later.
                Ideally, support materials should have  no catalytic activity leading to
            undesirable side reactions. This is usually true for high melting oxides fired
            to  give  low  surface  area  supports.  However,  colloidal  hydrous  oxides  are
            usually acidic in nature. Alumina, for example, is dehydrated during prepar-
            ation:


                             OH     OH                     0-
                             I       I               +      I
                       -O-AI-O-AI -            O-AI  -O-AI-O-              (2.2)
                                         -H,O
                                                   Lewis   Basic
                                                   acid    site
                                                  site

                However,  there  is  always  sufficient  adsorbed  water  present  on  the
           catalyst surface to give




                                     0-                OH      0-
                               +      I      +H,O       I       I
                       -O-AI  -O-Al-O -             O-Al-O-Al-            (2.3)
                                                     Bronsted
                                                      acid
                                                      site

            Bronsted sites, that initiate carbonium ion reactions, and Lewis sites, giving
           ion radical reactions, coexist, althouth it appears that in practical usage the
           Bronsted  acidity  predominates. (55)  When  using  y-A120 3  as  a  support,
           undesirable  side  reactions  such  as  cracking,  isomerization  and  "coke"
           formation always exist. These give unwanted products and lead to catalyst
           deactivation.
   42   43   44   45   46   47   48   49   50   51   52