(21g) Assessment of the Dominant Factors in Cu Catalyst Deactivation in Glycerol Hydrogenolysis

In triglyceride transesterification to fatty acid methyl
esters, 10 wt% of glycerol byproduct is formed. Upgrading the latter into a
more valuable chemical will increase the sustainability and commercial
viability of the biodiesel production process. Moreover, it will allow the
synthesis of ?green' chemicals instead of petroleum-based ones. Among other
alternatives, glycerol hydrogenolysis towards 1,2 propanediol, also denoted as
propylene glycol, is a relevant and attractive valorisation route. Supported
copper catalysts are known to efficiently catalyze glycerol hydrogenolysis with
high selectivity to the desired product propylene glycol [1]. The detailed study of the intrinsic kinetics for this
reaction has been previously studied in our group [2].

However, copper catalysts are known to be susceptible to
deactivation from sulphur and chlorine poisoning [3]. Trace amounts of sulphur can exist in
vegetable or marine oils [4] from which crude glycerol is derived.
Inorganic chlorine can also exist in the glycerol feed due to the post
treatment after transesterification. Additionally, copper catalysts are susceptible
to thermal sintering which is markedly accelerated by the presence of traces of
chlorine. In this work, medium to long term intrinsic kinetic experiments were
performed over an industrial supported copper catalyst, the objective being the
investigation of the causes of deactivation and the development of the
corresponding kinetic model. All these activities aim at a better understanding
of the deactivation kinetics and open up perspectives for rational catalyst
design.

All the reactions were performed using a high throughput
kinetic setup [5]. Pure glycerol was co-currently fed with
hydrogen over a fixed catalyst bed resulting a trickle flow behaviour. Medium
to long term intrinsic kinetic experiments (~90 hrs) performed with ultra-pure
glycerol at elevated temperatures amounting to 230 °C at a pressure of 65 bar
and hydrogen to glycerol molar ratio of 5 showed stable catalytic activity,
i.e., all variations observed could be attributed to experimental error. These
results suggest minimal sintering and coking at these idealized reaction
conditions.

Three possible contaminants in a realistic biodiesel
derived glycerol feed are sulphur, chlorine and unreacted glycerides.
Deactivation studies with all the possible impurities were performed by spiking
the ultra-pure glycerol with model molecules representing these impurities. In
depth analysis of the deactivation experiments showed significant deactivation
caused by S and Cl, that by S being most pronounced. Glycerides do not affect
the catalytic activity to the same extent when present in realistic
concentrations as in an un-purified glycerol feed.

Deactivation modelling has been performed making use of
a deactivation function. The latter is superimposed on the rate expression for
the main kinetics and represents, e.g., a loss in active sites due to
irreversible adsorption of a poison or to sintering [6]. In line with the qualitative ranking of
the deactivating effect, deactivation rate coefficients amounting to (5.5 ±
0.03) 10^-9 m⁶/kg_cat/mol_S/s and to
(17.8 ± 0.08) 10^-8 m⁶/kg_cat/mol_Cl/s
were obtained for S and Cl respectively at 230 °C. The deactivation rate
coefficient for glycerides was (5.4 ± 18.7) 10^-13 m⁶/kg_cat/mol_FFA/s
at the same temperature, signifying non-significant contribution from
glycerides to deactivation.

Insights into the causes of deactivation of Cu catalyst used
for glycerol hydrogenolysis have been gained. From the results, it is clear
that deactivation due to the presence of poisons like S and Cl in the feed are
the dominant cause of this deactivation. The effect of poison concentration on
the deactivation has been investigated and could be rationalized in terms of
physically significant parameters. The investigation will be extended towards
studying the effect of temperature on deactivation, aiming at comprehensive
understanding the deactivation kinetics.

Acknowledgments

This work was
supported by the Institute for the Promotion of Innovation through Science and
Technology in Flanders (IWT Vlaanderen)

References

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