Page 116 - Principles of Catalyst Development
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104                                                      CHAPTER  6
           give more consistent and uniform results than laboratory ovens, hot plates,
           and furnaces. m,160) Often these units are staged to achieve optimum results,
           for example, rapid, humid drying during the constant rate period, followed
           by slow control  during the  declining rate.(161)
                Dried xerogels contain 25'}'0 -30%  water, encapsulated in fine  pores or
           chemically bound to the oxide.  In this moist state the material is sometimes
           easier  to  form  into  pellets  and  extrudates.  This  is  sometimes  done  for
           convenience, provided subsequent calcination does not weaken or otherwise
           harm  the  particles.


           6.2.6.  Calcination
               Calcination is  further heat-treatment beyond drying. Several processes
           occur:  (I) loss of chemically bound water  or CO 2 ,  (2) changes in pore size
           distribution,  (3)  active  phase generation,  (4) surface conditioning, and  (5)
           stabilization of mechanical  properties.
               Alumina  is  a  good  example  with  which  to  demonstrate  all  of these
           features. (64)  Figure 6.9 shows the progression through all steps from precipi-
           tation  to calcining.  After drying, a  hydrous oxide known  as  "boehmite" is
           produced with a structure AI 20 3 nH 20. With pure boehmite n = 1, but values
           up to  1.8 are found. The structure of boehmite is distinctive and the surface
           hydrated as  shown in  Fig.  4.17.  Upon calcination above 300°C, a  series of






                                        AI  (OH) 3 gel
                                          I   pH  ::  9,  30' C
                                          t   2·10daVs
                                        . boehmite'·
                                        AI 2  03 nH 2 0   n  = 1.0  crystalline
                                                       to
                                               300  C   n  =  1.8   gelatinous
                                                            or
                                                           pseudo
                        cubic  close  packed    usually  from  higher  values  01  n
                        ABCABC
                        defect  spinel              250   m 2  Ig
                        AI  [  AI  5.'3  0  1 :)   J  04   450  C   "'/" group
                                                       low  temperature
                                                        AI 2 0 3  mH 2 0
                                                      m  ::  0  10  0.6
                          H   H                900C
                          ~   9
                          AI· 0  - AI
                         dislorled  spinel             "s  " group
                                              1,000  C   high  temperature
                                                       anhydrous  AI 2 0 3
                         monoclinic
                                              , ,200 'c
                         hexagonal
                         ABABAB
                            figure 6.9.  Preparational  steps for  aluminas.
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