(39f) Multiscale Simulation of Heterogeneous Catalysis: Butadiene Hydrogenation from Molecular Reaction Steps to the Fluid Flow

The selective hydrogenation of poly-unsaturated organic molecules is of high industrial interest in the area of fine and polymer chemistry. In order to have a good control of the reaction, it is necessary to gain a detailed understanding of all its stages and scales. On the smallest atomic scale, the reaction is governed by the interactions of the olefin with the metal catalyst surface. At a larger scale, the reaction is governed by temperature and pressure conditions and reactant and product flow in the reactor. Nevertheless the two scales are closely interlinked since local conditions at the surface of the catalyst's particle have a direct impact on the atomistic environment of the metal surface. In particular, hydrogen is known to easily cover the palladium and platinum surfaces that are used in such reactions.¹The coverage may depend on the local temperature and pressure condition but also on the kinetics of the reaction in those conditions. In this work we have studied the reaction with a multi-scale approach, the atomic scale with electronic quantum chemical methods, and the macroscopic scale with Computational Fluid Dynamics (CFD) simulations, both scales being coupled by a mesoscopic stage using mean-field kinetic simulations.

Using periodic plane wave density functional theory calculations,² the hydrogenation reactivity of the simplest poly-unsaturated hydrocarbon, butadiene, on the Pd(111) surface has been investigated. The effect of the hydrogen coverage on the Pd(111) surface has been accounted for in order to model different macroscopic reaction conditions. The most important energy barriers have been exactly determined using the Nudge Elastic Band method³ while transition states suspected to have a higher potential energy have been estimated using a Bronsted-Evan-Polanyi relationship.⁴Using transition state theory, the hydrogenation energy profile at low hydrogen coverage, developed in an earlier stage of the project, has been used to build a first implementation of a micro-kinetic model. The PeliGRIFF CFD code developed at IFP Energies Nouvelles has been upgraded in order to include for the first time a reaction term in the catalyst grain. The obtained kinetic law has been included in PeliGRIFF and it was shown that with intra-grain diffusion coefficient one order of magnitude lower than the gas phase diffusion, the reaction mainly happens in a shell surrounding the grain.

References

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