Page 21 - Principles of Catalyst Development
P. 21

CATALYTIC  FUNCTIONS                                               7
                Catalysis  occurs  through  complexing  and  rearrangement  between
           molecules and ligands of the catalyst.  Reactions can be very specific, with
           high  yields  of  desired  products.  Since  the  mechanisms  involve  readily
           identified species, these reactions are easily studied in  the laboratory with
            the techniques of organometallic chemistry. They are, however, difficult to
           operate commercially.  Liquid  phase  operation  places restrictions  on tem-
            perature and pressure, so  equipment is  complicated. The catalyst must be
            separated from  the products, imposing additional difficulties.
                For  these  reasons,  homogeneous  catalysis  is  found  only  in  limited
            industrial use, appearing usually in the manufacture of specialty chemicals,
            drugs,  and food.  Exceptions  are  acetic  acid  production, olefin  alkylation,
           and hydroformylation.



            1.2.2.  Heterogeneous Catalysis

                In  these  systems  the  reactants  and  catalyst  exist  in  different  phases.
            Most commonly, solid catalysts are used with gaseous or liquid reactants,
           sometimes both. Other permutations are possible but less often encountered.
                Introducing  a  separate  catalyst  phase  immediately  complicates  the
            mechanism.  Interfacial  phenomena  now  become  important.  Diffusion,
           absorption,  and  adsorption  all  play  critical  roles  in  establishing  the  rate.
           These additional steps become difficult to separate from surface chemistry.
           Accordingly, heterogeneous systems are difficult to study in the laboratory.
            Disappearance of reactants and appearance of products are easily followed,
           but  important  features  such  as  the  rates  and  energetics  of  adsorption,
           structure of the active surface, and nature of reactive intermediates, require
           separate experimentation using a constantly changing arsenal of techniques.
           Often, the sequence of surface steps can only be deduced from accumulated
           evidence,  with  many  uncertainties.  In  every  important  application  of
           heterogeneous  catalysis  there  is  much  controversy on  the  exact details of
           chemistry. Ammonia synthesis, for example, was the first important catalytic
           process,  emerging  over  70  years  ago.(13)  Yet  the  nature  of the  catalytic
           surface  is  still  debated.(14)  Another  case  is  hydrodesulfurization.  Process
           and catalyst development dates from the early 1950s(15) but a large amount
           of scientific research is  now appearing in  the  current literature.(J6)
                Nevertheless  heterogeneous  catalysts  are  convenient to  use  commer-
           cially. Easily prepared solid catalyst pellets, packed in tubes through which
           reactants  flow,  satisfy  process  requirements  for  simple  construction  and
           dependable  operation.  Control  is  good,  product  quality  high.  It  is  not
           surprising that the vast majority of industrial catalytic processes adopt this
           approach.
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