Catalysis Science & Technology
Paper
Moreover, a particle size reduction may be applied to
avoid mass-transfer limitations. Thus, for catalysts with
mesopores of 10 nm, to achieve effectiveness factors ≥0.9 in
toluene hydrogenation, the particle size would have to be
below 25 μm, which is in the range of fine powders. If cata-
lyst particles of larger diameters are to be applied, the only
option for prohibiting mass-transfer-limited conditions for
liquid-phase hydrogenations is, thus, to introduce sufficiently
large pores, ideally with widths of 100 nm or beyond.
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The effective diffusion coefficient of toluene for the cata-
lysts with 4 and 10 nm pore width was determined from the
initial reaction rates using the Thiele concept. The diffusion
coefficient obtained from PFG-NMR is, however, an order of
magnitude larger. Apparently, the Thiele concept is too sim-
plified to assess the intraparticle transport of the reactants
only. Rather, additional transport hindrances such as surface
barriers may affect the diffusivity determination via the
Thiele concept. It presents a challenge for further studies to
identify more precisely these additional phenomena influenc-
ing diffusivity determination under conditions of catalytic
reactions.
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Acknowledgements
The authors would like to thank the “Deutsche
Forschungsgemeinschaft” for funding within the Interna-
tional Research Training Group GRK 1056 “Diffusion in
Porous Materials”.
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