Page 84 - Principles of Catalyst Development
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CATALYTIC  MATERIALS                                              71
                                TABLE 4.9.  Acidity of Catalytic
                                         Materials
                               Solid               pKa  range


                          SiOr AI 20 3               < -8.2
                          Montmorillonite clay     -5.6 to  -8.2
                          Kaolinite clay           -5.6 to  -8.2
                          r- A1 203               + 3.3  to  -5.6
                          SiOrMgO                 +3.5  to  -2.5
                          Si0 2                      -2.0
                          Ti0 2                    +6.8  to  +1.5
                          MgAI 20 4                  >7.0
                          CaO                        >7.0
                          MgO                        >7.0


           oxide,  "y-AI203  is  activated  by  calcination  above  300°c. (64 )  The  following
           sequence occurs:

                          OH      OH              0-
                           I       I   -H,O        I
                      O-Al-O-Al -- -O-Al+ -O-Al-
                                                  Lewis    Basic
                                                  site     site
                                                  OH      0-
                                                   I   H+  I
                                             -O-Al-O-Al                   (4.5)
                                                Bronsted
                                                  site

                Dehydration of the hydrous oxide gives a surface containing both Lewis
           (electron acceptor) and basic sites. However, sufficient H 20  is always present
           to  generate Bronsted sites. The proton is  hydrogen-bonded to the hydroxyl
           group with a strength related to the electronic environment of the AIH  ion.
            Factors that tend to shift the electron distribution in the Al-OH bond toward
           the AI3+  ion weaken the OH-H+ bond, making the proton more accessible,
           and increases the acidity. Aluminum ions in Al 2 0, occupy both tetrahedral
           and  octahedral  sites  with  neighboring  oxygens.  Surface  coordination
           depends on crystal faces exposed, so that a number of schemes are possible.
           One representation is shown in Fig. 4.17.(127)  Numbers 1-5 identify different
           types  of isolated  hydroxyl  ions  and  their  relationship  to  the  oxygen  and
           AlH ions in the layer below the surface. Clearly, site 1 has a weaker OH-H+
           bond  and  greater acidity  than  other sites.  A  distribution  of acidities  then
           develops,  itself an important factor in  characterizing acidic materials.
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