Page 86 - Principles of Catalyst Development
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CATALYTIC  MATERIALS                                              73


                        -
                        I
                         0)   0.4
                         <I)
                         0
                         E
                         E   0.3
                        ..:
                         z
                        ::>
                        0   0.2
                         ~
                        c(
                        0
                        U   0.1
                        c(
                             0
                             +4                             -5
                                          ACIDITY,  p  Ka

                         Figure 4.18.  Acid  strength distribution  for  y-AI20, .'SS)

           4.4.2.  Natural Clays

                Highly acidic natural clays, montmorillonite are complex layers of Si0 4
           and Al0 4  tetrahedra. They also contain small amounts of MgO and Fe 20 3 •
           These impurities are leached with sulfuric aid, which also adds protons to
           increase maximum pKa  values from  -3.0 to -8.2. These clays were the first
           cracking  catalysts  used  with  fixed  and  moving  beds.  However,  they  were
           quickly replaced by the superior synthetic silica-aluminas that were ideal for
           fluidized beds. (128.129,130)  Today, they are used as the matrix in zeolite-based
           cracking catalyst.

           4.4.3.  Silica-Alumina

                Clearly, single oxides such as Al 20 3  and Si0 2  are much less acidic than
           combinations of the two. In fact, most highly acidic materials are luminosili-
           cates.  The  reason  for this  is  seen in  the structure
                                      H         H
                                       1   H+   1
                                   o  0     0   0    0                    (4.9)
                                    ".1   /   ".1  /
                                      AlH       Si 4 +

           in  which  Si +  replaces tetrahedral  A1 +  with  a  more electropositive center,
                                             3
                      4
           thereby  weakening  the  O-H  bond  and  increasing  acidity.  Silicon  and
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