Page 120 - Principles of Catalyst Development
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108                                                      CHAPTER  6
            compounds. This  is  because the  defect  spinel structure of y-AI 20]

                                                                           (6.4 )
            favors  solid  solution  with  transition  oxides  such  as  FeO,  CoO,  NiO,  and
            CuO. Cations, M2+,  do not necessarily occupy the octahedral vacancies but
            migrate  to  positions  with  higher  stabilization  energy.  Thus  we  tind
            AI[AIFe]04, Co[AI 2]04, AI[AINij04, and Cu[AI2J04' The cations occupy
            stable positions and are difficult to reduce. These same procedures are used
            to prepare catalysts for low temperature shift and alcohol synthesis reactions,
            CuO-ZnO-AI 20,. In this case, mixed gels of CuO-AI 20J are easily precipi-
            tated,  with  ZnO added at this  or some later point. (166)
                Coprecipitated  oxides  are  the  precursors  to  metal  catalysts  in  Table
            6.1  that  are  produced by extraction.

            6.3.3.  Zeolites
                Zeolite synthesis  was  first  attempted  by  duplicating the  hydrothermal
            genesis  of  naturally  occurring  minerals.  Mostly  these  were  unsuccessful.
            Breakthroughs  came  with  the  realization  that  cogels  of aluminosilicates,
            like  those  in  SiOrAI,Ol preparations, are  depolymerized by  OH- ions  to
            yield crystalline nucleii.  Depending upon composition, temperature, press-
            ure, and pH, these nuclcii develop into a vast number of zeoli tic types.( 11'.136)
            With  correct  conditions  for  supersaturation,  crystalline  seeds  grow  into
            microcrystalline powders of appropriate material. (351  For example, Faujasite
            Y is  made by a  procedure similar to that for silica-alumina. Sodium silicate
            and  sodium  aluminate solutions  in  the  correct  proportions  (SiO l /  AI 20 l  =
            3-30)  are  mixed,  not  with  an  acid,  but  with  sodium  hydroxide  at  a  pH  of
            12.  Finely  divided  silica  is  added  as  a  seeding agent, and  the temperature
            raised  from  100  to 400°C,  with  pressure  necessary at  higher temperatures.
            Nucleation  and  crystallization  commence  at  rates  determined  by  the  tem-
            perature  and  pressure.  This  digestation  continues  until  the  reaction  is
            complete,  taking  a  period  of time  that  may  vary  from  hours  to  days.,lb71
            Other zeolites are made in similar ways, with essential differences in starting
            conditions  and  compositions. I 1381


            6.4.  DEPOSITION  OF  THE  ACTIVE  COMPONENTS

                Dispersion  of oxides  on  high-area  supports  is  carried  out  by  one  of
            four methods:  (J)  precipitation,  (2)  adsorption,  (3)  ion  exchange, and  (4)
            impregnation.  Each  technique  has  advantages  and  disadvantages.  Often
            preference  for  one  method  over  another  is  a  matter  of compromise.  The
            supports are either in powder or particle form. Depositing active components
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