Counter-cation effect on Keggin phosphomolybdates for deoxydehydration catalysis

Details

Supervisors

Gaigneaux, Eric M. ; Hermans, Sophie

Faculty

Faculté des bioingénieurs

Degree label

Master [120] : bioingénieur en chimie et bioindustries, à finalité spécialisée

Abstract

enPetrochemical-based chemistry still plays a central role in our modern society. However, as the Earth’s finite oil reserves are being depleted at a rate far exceeding their natural formation, alternative routes to produce key chemical building blocks have become the focus of extensive research. One promising strategy involves the use of sustainable feedstock, which offer renewable and abundant resources. Nevertheless, the high oxygen content of biomass-derived molecules represents a major challenge for their integration into existing chemical processes. Deoxydehydration (DODH) has emerged as an attractive approach to produce alkenes from polyols obtained from biomass. This reaction converts vicinal diols into olefins through the simultaneous removal of hydroxyl groups, making it a key transformation for the valorisation of bio-based resources. Most DODH catalyst reported to date are metal-oxo complexes based on midtransition metals, typically Re or more abundant alternatives metals such as Mo, W or V. In this context, Keggin polyoxometalates (POMs) are particularly attractive, as they incorporate these metals in a high oxidation state. Their structural versatility and tunability remain largely underexplored for DODH catalysis and therefore represent a promising area of investigation. Keggin POMs can undergo various modifications, including transition-metal substitution and counter-cation exchange. In particular, the nature of the counter-cation provides a means to tune both the redox properties and the solubility of the resulting salts. This work focuses on elucidating the impact of counter-cation on the DODH performances of the Keggin phosphomolybdate. Four synthesis routes for counter-cation-exchanged Keggin phosphomolybdates were investigated and characterized using IR-ATR and UV-Vis spectroscopies, PXRD, NH3TPD and H2-TPR, namely those based on metal chloride, nitrate, carbonate and sulphate precursors. The two first routes demonstrate the inability to obtain fully cation-exchanged and pure salt, whereas the carbonate and sulphate routes appear more promising. The influence of the counter-cation on reducibility is evidenced by UV-Vis spectroscopy, and significant differences in DODH performances are observed among the salts. However, as leaching of active species occurs for all samples, differentiating the respective contributions of reducibility and solubility to the observed catalytic performances was not achieved. Future research should focus on the development of reliable synthesis routes enabling complete proton substitution, as this may be desirable for catalysis where acidity should be avoided, as it the case for DODH. In parallel, heterogeneous catalytic applications should be explored by mitigating leaching through the heterogenization of Keggin salts.