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Gaining a better understanding of diffusion phenomena within catalysts

January 2018

As part of its research in the field of vacuum distillate desulfurization, IFPEN is studying the impact of the porous structure of catalysts on their performance.

Hydrotreatment is a refining process aimed at removing the sulfur contained in hydrocarbon fractions, derived from vacuum distillation, which are used to produce fuels. Alumina hydrotreatment catalyst supports are granular solids - known as mesoporous - whose pores characteristically measure around ten nanometers. However, vacuum distillates are oil cuts presenting characteristic molecule sizes of similar dimensions (closer to a nanometer). With a view to improving catalyst performance, the issue of the diffusion of these molecules within the mesoporous network during the hydrotreatment process is thus of interest: is it a question of free diffusion (molecular diffusion) or are there phenomena that restrict the accessibility of the molecules to the porosity at the location of the active phases?

To answer this question, research has been conducted [1-2] on the characterization of the diffusive properties of catalyst supports, in other words, their capacity to bring reagents and catalytic sites into contact easily. The research focused on the development of an inverse liquid chromatography technique, adapted to the characterization of transport phenomena within real extruded catalyst supports.

Focusing on the diffusion of molecules of different sizes, varying between 7 and 30 carbon atoms, representative of “gasoline” and “distillate” cuts , a first study using this technique showed that for hydrotreatment catalyst supports, molecules have access to the entire porous volume and are not restricted by adsorption phenomena. The diffusion of vacuum distillate molecules is thus of the molecular type, implying that, for macro/mesoporous catalysts, diffusive capacities, as measured by inverse liquid chromatography, are directly related to the available pore volume and the tortuosity of the porous network.

Subsequent research then revealed considerable disparities between the networks of the various mesoporous solids. A sample of aluminas presenting a variety of pore size distributions (Figure 1) was thus studied, revealing that the tortuosity values obtained can vary by 50% while traditional texture characterizations (nitrogen adsorption and mercury intrusion porosimetry tests, helium pycnometry) did not identify these differences. Such results demonstrate that it is possible to adjust the diffusive properties of alumina catalyst supports and that it is vital to intervene on their internal organization to do so.

Lastly, the measurements taken provided tortuosity values that are not consistent with the correlations in the literature concerning the random packing of objects. The varied - and above all higher - tortuosity values obtained may be due to a porous alumina organization made up of two networks: a distinction is thus made between porosities within crystallite aggregates and those located between the aggregates, as shown in Figure 2.

The hypothesis of a link between a dual distribution of pore sizes, obtained by physisorption, and diffusive properties was thus proposed.

The working method developed, combining conventional textural analyses with innovative approaches, made it possible to examine the detailed architecture of mesoporous solids, opening up avenues of potential interest for gaining a better understanding of the relationship between the conditions for the synthesis of materials and the organization of their hierarchized porous structure.

In this respect, the prospect of being able to determine mass transfer properties from simple nitrogen adsorption experiments should be of interest to the heterogeneous catalysis community.

Lastly, the new inverse liquid chromatography technique, developed in the context of the hydrotreatment of oil cuts and that has been used to describe and characterize diffusion phenomena within mesoporous solids, can be applied to other catalytic systems involving large-sized molecules, such as bio-based molecules.
   

Scientific contact:  antoine.hugon@ifpen.fr

  1. [1] Svetan Kolitcheff ; Approche multitechnique des phénomènes de diffusion en hydrotraitement de distillats, [Multi-method approach to diffusion phenomena in distillate hydrotreatment], PhD thesis, 2017.
    >> https://tel.archives-ouvertes.fr/tel-01581795/
       
  2. [2] Svetan Kolitcheff, Elsa Jolimaitre, Antoine Hugon, Jan Verstraete, Pierre-Louis Carrette, Melaz Tayakout-Fayolle ; Tortuosity of mesoporous alumina catalyst supports: Influence of the pore network organization ; Microporous and Mesoporous Materials 248 (2017) 91-98.
    >> DOI: 10.1016/j.micromeso.2017.04.010

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