Page 87 - Principles of Catalyst Development
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74                                                       CHAPTER  4

            aluminum ions must be intimately mixed during preparation. This effect is
            shown  in  Fig.  4.19,  where  we  see  that optimum  acidity  develops  at  about
            30%  AI~Ol'  Synthetic  silica-alumina  catalysts  led  to  fluidized  catalytic
            cracking,  the  largest  and  most  important  process  in  the  refining  industry.
            Other  dual  oxide  systems  display  degrees  of  acid  behavior,  but  the
           superiority of silica-alumina justifies its  role  in  catalytic cracking. mll
                Carbonium ion-type reactions initiated by these acidic oxides are shown
            in  Table 4.10.
                Activity  patterns  follow  acid  strength,  as  shown  in  Fig.  4.20.  Parallel
           trends  are  obeyed  for  gasoline  production  and  cumene  dealkylation,  and
           the  latter  is  commonly  used  as  a  "model"  reaction  for  catalytic cracking.

           4.4.4.  Zeolites

               Zeolites, like clays and synthetic SiO-AI 2 0 3  catalysts, are aluminosili-
           cates.  Unlike  these  materials,  they  have  three  properties  that  make  them
           unique and deserving of a separate category. First, they are highly crystalline
           with  well-defined  structures.(1321  The  aluminosilicate  framework  encloses
           cavities occupied by large ions and water molecules. Access to these cavities
           of various sizes is through a network of openings ranging from 0.3 to 1.0 nm
           in diameter, which are of the order of molecular dimensions. Size and shape
           of these pores determine which molecules enter the cavities and which are



                                   BRONS TED  ACIDITY  p  Ka ~ 1.5
                        '-t
                         o   5                       o
                         ><




                         o
                         E   3
                         E
                        ~-  2
                        ;:)
                        °
                        ~
                        «
                        a
                        o
                        «
                                   20     40     60     80    100

                                          Si0 2 ,  wt%
                          Figure 4.19.  Acidity  in  silica-alumina catalysts.'!3I)
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