Page 52 - Principles of Catalyst Development
P. 52

38                                                       CHAPTER  2

            complexity, from fixed to trickle bed reactors and even ebullating processes.
            Catalyst  particles  are  available  in  a  wide  range  of sizes  and  shapes  to  fill
            these  needs. (70)
                The fundamental chemistry, however, remains the same. The preferred
           support is  'Y-AI203  prepared by  precipitation and preformed into particles
           with  designed  pore  size,  shapes,  and  distribution.  Details  are  given  in
                                             2
           Chapter 6.  High surface area (~250 m g -I) is achieved during preparation.
           Small amounts  (~1 %) of SiO~ are added to stabilize the  high area phase.
                The active component is  a sulfided molybdenum compound promoted
           with cobalt. Cobalt and molybdenum salts are deposited onto the alumina
           and calcined to a mixture of MoO), CoO (C0 30 4 )  and CoAl 20 4 YI) Molyb-
           dena is  the precursor of the active component. Cobalt aluminate originates
           from  solid state  reactions  between  CoO and A1 20 4 •  Surface studies reveal
           that this compound forms a substrate on which a monolayer of MoOrtype
           structures deposits.  Some cobalt is  incorporated within this  monolayer, as
           shown in  Fig.  2.8. (72)
               The  catalyst  is  activated  through  sulfiding,  either by  high-sulfur feed
           or treatment. During this process the Mo0 3  layer breaks up into microcrys-
           tals  of MoS 2 ,  into  which  small  amounts  of cobalt  ions  are  incorporated,







                       ~-

                       /




                                       Q      SULFIDING






                                                                 2+
                                                                o







                        Figure 2.8.  Surface structure of CoMo/ Al 20, catalysts.
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