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.

Figure 1: Glycerol conversion at 230 °C and 65 bar and space time 1 a.u. a) versus time  for various poisons (♦) ultra-pure glycerol, (●) glycerides, (▲) Cl and  (■) S b) versus  concentration of poisons in the glycerol feed.
a)      Rate equation for main kinetics b)     Rate equation for 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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