Page 117 - Principles of Catalyst Development
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CATALYST  PREPARATION                                            105
           phase  changes  occur,  simultaneously  with  loss  of hydroxyl  groups.  This
           results  in  1)-,  )1-,  and  8-phases  approximately  at  the  temperatures  shown.
           Exact  transition  points  depend  on  many  preparational  factors  and  are
           difficult  to  define.  These  oxides  are  known  as  the  ")I-group"  or  "Iow-
           temperature"  aluminas.  Structures  are  very  similar,  with  a  formula
           AI 10]nH 20, in  which  n = 0.6-0.  All  are  based  on  cubic close  packing  of
           oxygen  ions  (ABCABC)  but  are  best  described  as  a  defect  spinel,
           AI[Alsn£  )1/3)0 4 ,  where  AI  occupies  tetrahedral  or  octahedral  interstices
           and [  ] is  a cation vacancy. Different phases are distinguished by variations
           in  relative  intensities of key  diffraction  lines.
                Above  1000°C,  monoclinic  8-AI 20] forms,  transforming to  hexagonal
           (ABABAB)  a-A1 20 3  at  1200°C.  These  are  anhydrous,  low  surface  area
           oxides and are not suitable for  porous supports. They are used in applica-
           tions where mechanical strength, and not high  surface area, is  required.
               The acidity of these aluminas was discussed in Chapter 4.  In particular,
           the effect of calcination temperature on acid strength distribution is  shown
           in  Fig.  4.18.  Collapse  of smaller  pores  results  in  increasing  pore  size,  as
           shown in  Fig. 6.10. Changes in mechanical properties come from subsequent
           differences in, for example, plasticity or grain boundaries. These factors are
           important during pelleting and extrusion.
               Common  supports  such  as  AI 20 J  and  Si0 2  are  made  by  catalyst
           manufacturers using modifications of these  procedures.  Excellent  product
           quality  control  is  achieved  with  a  wide  range  of properties.  In  preparing
           catalysts, the laboratory chemist is  advised to select suitable supports from
           among  those  available,  unless  changes  in  procedure  are  essential.  Much
           labor,  time,  and  frustration  is  saved.  Suppliers  of supports  are  listed  in


                     E
                      c   10
                     (/)
                     :::J
                     D
                     «
                     cr:
                     UJ
                     cr:
                     0   5
                     Q.
                     UJ
                     C)
                     «
                     cr:
                     UJ
                     >
                     «
                         0
                          300        500         700         900
                                 CALCINATION  TEMPERATURE,  °C
                      Figure 6.10.  Effect  of calcination on the pore size of ,},-A1 20 3 •
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