Much has been discussed concerning the effect of diffusion in the heterogeneous catalytic systems with macromolecules as reactants. However, it’s challenging to clarifying the effect of diffusion because the practical catalysts within the multihole environment are accompanied with increased complexity.
On the other hand, the Weisz-Prater number is often applied to assess the internal diffusion effect in heterogeneous catalytic reactions. The values of most physical quantities can be measured experimentally except the effective diffusion coefficient D_eff which is associated with the tortuosity factor τ of the catalyst. The tortuosity τ is ever-changing because there are many random and irregular channels inside the catalyst, making it difficult to obtain the accurate values of the tortuosity.
To obtain a clue about the effect of diffusion, a series of Pt/HPMo/SBA-15 catalysts with the pore size as a single variable were prepared. Different from the metal-acid microporous zeolites, the above catalysts can greatly simplify the influence factors in the reaction system. The diversity of diffusion performance over different catalysts was characterized by TG-TPD of n-C16 and Zero-Length Column (ZLC) of n-C8. Utilizing dimensionless variables, internal effectiveness factor ( ) and Thiele modulus ( ), and the apparent activities over catalysts with different diffusion capacity, combining the diffusion-reaction kinetic method, N_W-P is obtained with improved accuracy.
Using the novel method, a conclusion is drawn that the formation of m-i-C16 is more susceptible to internal diffusion than the consumption of n-C16 in the hydrotreatment of n-C16. The provided novel method to calculate the N_W-P can be extended to other heterogeneous catalytic systems with macromolecules as reactants to discuss the diffusion effect and design the catalyst more accurately.

Junhao Sun

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