Bifunctional catalysts are the preferred choice for hydrocracking processes. Ideal hydrocracking performance is defined by quasi-instantaneous (de)hydrogenation reactions over the metal sites and limited acid catalyzed side reactions, particularly the secondary cracking of desired products. Although their performance is heavily determined by the balance in activity between metal and acid sites, the spatial arrangement between both functions also holds a significant influence. Distance represents the simplest measure of the spatial distribution of active material. Various authors have explored the influence of the distance between sites, challenging the assumption that a minimal separation is necessary for ideal hydrocracking. Experimental evidence shows the important role of diffusion in the discussion of an intimacy criterion. The balance between site dispersion and mass transport properties within the porous catalyst can serve as a guideline in the tuning or design of ideal bifunctional catalysts.

The influence of the spatial distribution of catalyst particles in a fixed bed reactor on the observed reaction rates has historically been of central interest in catalysis and chemical physics. Results from both fields suggest that the apparent catalyst activity is determined by the distance between active particles and transport phenomena, although a universal understanding of their combined influence remains incomplete. In this study, pulse experiments under contrasting transport regimes show that a given amount of catalyst particles exhibits higher activity with increasing degrees of separation. This is attributed to a shadowing effect induced by the competition between closely neighboring active particles, which limits their combined apparent activity. Experimental results also suggest the dependence of this effect on mass transport phenomena within the reactor space.

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