Page 82 - Principles of Catalyst Development
P. 82

CATALYTIC  MATERIALS                                             69
                                TABLE 4.7.  C 2 HsOH SelectivityU

                                        Percent C 2 HsOH decomposed to

                           Oxide         H2 + C2H 4O   H2 0  + C2H 4

                           'Y- AI 2 0 3      1.5         98.5
                           Cr2 0 3           9           91
                           Ti02             37           63
                           Zr02             55           45
                           Fe2 0 3          86           14
                           ZnO              95            5
                           u  Reference  121.

            in Chapter 5.  Other correlations of this  type are given in the literature.(45)
            Data are always difficult to find and many studies were made under different
            conditions, in which dispersions were  not always measured.  Nevertheless,
            in cataloging activity  patterns for specific reactions, much more validity is
            possible within the guidelines of this electronic treatment.
                For fundamental understanding of catalytic sites, this crude treatment
            is not enough. Some consideration of surface geometry and orbital availabil-
            ity, in much the same way as in metal catalysis, will be necessary for greater
            understanding.  This  is  one  area  of  scientific  catalysis  awaiting  exploi-
            tation.
                Reactions catalyzed  by oxides and sulfides are  listed  in Table 4.8.


           4.4.  INSULATORS AND SOLID ACIDS

                Insulators  are  semiconductors  with  large  values  of Eg  and  very  low
           concentrations  of impurity  levels.  Electrons  remain  localized  in  valence
           bonds  so  that  redox-type  reactions,  found  with  better  conducting  metals
           and oxides, do not occur. However, these materials exhibit sites that generate
           protons,  thereby  promoting  important  carbonium  ion  reactions  such  as
           cracking, isomerization, and polymerization. Acid solids are characterized
           as  (1)  single  oxides, such  as  Al 20 3  and  Si0 2 ,  (2)  natural clays,  (3)  mixed
           amorphous  oxides,  for  example  Si0 2-AI 20 3 ,  and  (4)  zeolites.  Table  4.9
           shows  the  wide  range  of acid  strengths  encountered  by  these  and  other
           oxides.

           4.4.1.  Alumina

               The origin of acid sites is best illustrated with  ')'-AI 20), which is  often
           used as a support but has inherent uses of its own.(l26) Prepared as a hydrous
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