Page 53 - Principles of Catalyst Development
P. 53

STRUCTURE  OF  CATALYSTS                                          39
           effectively  "doping"  the  electronic  structure  and  producing  a  "CoMoS"
           active  site. (73)
                The  promotion  effect  of the  CoMoS  site  appears  to  occur  through
           increased  hydrogen  activation,  which  facilitates  removal  of sulfur  atoms
           after  cleavage  of  C-S  bonds  on  exposed  molybdenum  ions. (74)  Cobalt
           incorporated  into  the  support  may  also  act  as  a  structural  promoter  by
           enhancing dispersion of the sulfided species. (75)
                With heavier feeds, potassium is added to poison acid sites and inhibit
           coking.  Phosphorous is  also used, presumably to inhibit acidity but also to
           promote dispersion of the  molybdenum compounds. (76)
                One additional feature occurs when this catalyst is used with extremely
           heavy  residua feedstocks.  With  proper preparational techniques,  bimodal
           pore  distributions  are  produced  in  which  constrictions  at  the  openings
           inhibit  diffusion  of large  asphaltenic,  coke-producing  molecules,  whereas
           smaller  sulfur  compounds  have  easy  access. (77)  This  effectively  reduces
           excessive coking which  otherwise occurs.  (69)
                In  this  example,  we  see  how  the  catalyst designer "tailors" the  com-
           ponents  of the catalyst to  meet process  needs. This is  not usually  a  spon-
           taneous,  creative  event  but  one  that  evolves  with  experience.  How  this
           occurs and how catalysts are developed is  considered in the  next chapter.
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