(434b) Heterogeneously Catalysed Benzyl Alcohol Oxidation in a Three-Phase Micro-Packed Bed Reactor: Influence of Hydrodynamics and Reactor Design on Reaction Performance and Catalyst Deactivation

Micro-packed bed
reactors are being increasingly used in multiphase catalysis due to their
enhanced heat and mass transfer and well-controlled environments
[1]. Understanding the
prevailing hydrodynamics and mass transfer behaviour is pertinent to utilising
the advantages of microreactors to their full potential. The aims of this study
are to identify influences of hydrodynamics and reactor design on mass transfer,
reaction rate and catalyst deactivation for the Au-Pd catalysed benzyl alcohol
oxidation. The catalyst used is of particular interest due to its superior
activity in catalysing alcohol oxidations to aldehydes [2], a class of
reactions ubiquitous in the pharmaceutical and speciality chemicals industries.

The
hydrodynamics had implications on the inter- and intra-phase mass transfer
characteristics as well as the stability of the catalyst. Operating in a
gas-continuous flow regime led to enhanced external mass transfer, resulting in
high reaction rates with respect to the desired reaction pathway (TOF of 4500
hr^-1) [3]. Two limiting cases were found: (i) gas-limited operation
that occurs at low G:L (gas:liquid flow rate) and (ii) liquid-limited operation
that prevails at high G:L (Figure 1). The gas-liquid mass transfer
coefficient under the liquid-dominated flow regime was experimentally measured
to be 23.6 s^-1, a value that is two orders of magnitude higher than
that commonly observed in conventional trickle bed reactors. The catalyst
stability was drastically influenced by the hydrodynamics – possibly linked to
the tendency of by-products to deposit on the catalyst surface at high G:L
ratios, the extent of which was dependent on the degree of catalyst wetting.

Variations
in reactor inlet design had implications on the observed reactant conversion
through its influence on the hydrodynamics that developed downstream in the
packed bed region. Superior performance (94% conversion in comparison to 80%
conversion) was observed when the liquid was introduced directly into the
catalytic bed, precluding the formation of gas-liquid slugs upstream, thus
influencing the mass transfer within the packed bed. 

Figure 1. Benzyl alcohol
conversion with increasing G/L Molar Ratio.

References

1.   Al-Rifai, N., Cao, E.,
Dua, V., Gavriilidis, A. Microreaction technology aided catalytic process
design. Current Opinion in Chemical Engineering 2013, 2(3),
pp. 338-345.

2.   Enache, D., Edwards, J.,
Landon, P., Solsona-Espriu, B., Carley, A., Herzing, A., Watanabe, M., Kiely,
C., Knight, D., Hutchings, G. Solvent-free oxidation of primary alcohols to
aldehydes using Au-Pd/TiO₂ catalysts. Science 2006,
311, pp. 362-365.

3.   Al-Rifai, N.,
Galvanin, F., Morad, M., Cao, E., Cattaneo, S., Sankar, N., Dua, V., Hutchings,
G., Gavriilidis, A. Hydrodynamic effects on three phase micro-packed bed
reactor performance – Gold-palladium catalysed benzyl alcohol oxidation. Chemical
Engineering Science 2016, 149, pp. 129-142.