Redox-Acid Bifunctional Catalysis. Applications in Refining and Petrochemicals

By definition, a bifunctional catalytic process involves two types of catalyst function (or site), either in concert (e.g. acid-base catalysis) or consecutively (e.g. redox-acid catalysis). Only redox-acid catalysis is involved in the transformation of paraffinic, naphthenic and aromatic hydrocarbons from petroleum cuts. The discovery of a synergy between the hydrodehydrogenating and acid functions of reforming catalysts dates back to the 1950s, and a reaction scheme was proposed shortly afterwards to explain its origin. This scheme comprises a number of successive stages:

• catalysis on hydrodehydrogenating sites (formation of highly reactive unsaturated intermediate species) and acidic sites (rearrangement, cracking, cyclization, etc. of these species);

  transport of reagent molecules, products and reaction intermediates.

Although complex, this bifunctional transformation occurs very rapidly and in an apparent single step: the unsaturated intermediates, which are thermodynamically very unfavorable, only appear in trace form in the final products.

Although certain transformations implemented industrially using bifunctional redox-acid catalysis can also be carried out using monofunctional acid (or redox) catalysis, operating using bifunctional catalysis is often economically preferable. This is shown by the comparison of hydrocracking (bifunctional) and acid cracking (FCC, Fluid Catalytic Cracking) processes for heavy feedstocks (T _eb > 350 °C):

• These feedstocks are processed at lower temperatures: 380 to 420°C for hydrocracking, 480 to 580°C for FCC;

• this transformation is easily directed towards more valuable products: oil bases, diesel fuel instead of gasoline, and the selectivity of desired products is greater;

• since hydrocracking units operate under hydrogen pressure, catalyst deactivation by coke deposition is much slower: hydrocracking catalysts are regenerated after several years of operation (1 to 3 years), while FCC catalysts must be regenerated after a few seconds.

The additional advantage of bifunctional catalysis is that it enables refractory compounds to be converted using monofunctional acid catalysis. For example, catalytic cracking catalysts (FCCs) are unable to transform polyaromatic hydrocarbons from heavy feedstocks, which is what bifunctional hydrocracking catalysts enable. This is also the case for the isomerization of the ethylbenzene component of aromatic C ₈ cuts, only achievable by bifunctional catalysis.