Alcoxycarbonylation du butadiène catalysée par le Pd : aperçu mécanistique et amélioration de la réaction

Résumé de la thèse

L'acide adipique est un composé chimique clé qui peut être utilisé dans plusieurs applications telles que la synthèse du Nylon-6,6, les plastifiants, etc. La principale voie de synthèse à l'échelle industrielle de l'acide adipique est dérivée de ressources pétrochimiques et implique trois réactions séquentielles : i) l'hydrogénation du benzène en cyclohexène, ii) l'oxydation du cyclohexane en un mélange de cyclohexanol/cyclohexanone (huile KA) et iii) l'hydrolyse oxydative de l'huile KA avec du HNO3. Cependant, ce processus génère d'importantes quantités de N2O, un gaz à effet de serre qui doit être capturé et traité, ce qui augmente les émissions nettes de CO2 de l'ensemble de la transformation.

La double alcoxycarbonylation du butadiène-1,3 offre une voie plus écologique pour préparer des esters d'adipate à partir de sources potentiellement renouvelables, avec une économie d'atomes élevée et sans générer de N2O. En effet, le 1,3-butadiène peut être obtenu à partir de l'éthanol, tandis que le CO peut être obtenu à partir du CO2 par la réaction de déplacement inverse de l'eau vers le gaz. Le diester résultant peut être hydrolysé en acide adipique. Cependant, le processus catalytique présente plusieurs défis :

• L'activité et la stabilité du catalyseur doivent permettre deux alcoxycarbonylations successives.
• Une étape d'isomérisation défavorable est nécessaire pour la formation du diester linéaire.
• Des réactions secondaires telles que la télomérisation, l'hydroalkoxylation et la polymérisation peuvent se produire.
• Le mécanisme n'est pas encore entièrement compris.

Pour relever certains de ces défis, ce doctorat a combiné des approches théoriques et expérimentales afin de mieux comprendre les mécanismes, d'identifier les limites et de proposer des stratégies pour améliorer l'activité, la sélectivité et la recyclabilité du catalyseur. Ce rapport de thèse a été divisé en 5 chapitres.
L'introduction générale décrit certains aspects de la préparation industrielle de l'acide adipique et les méthodes alternatives pour sa synthèse à partir de sources renouvelables.

Pd-catalyzed butadiene alkoxycarbonylation : mechanistic insights and reaction improvement

Adipic acid is a key chemical compound which can be used in several applications such as in the synthesis of Nylon-6,6, plasticizers, etc. The main route for the industrial-scale synthesis of adipic acid is derived from petrochemical resources and involves three sequential reactions: i) benzene hydrogenation to cyclohexene, ii) cyclohexane oxidation to a mixture of cyclohexanol/cyclohexanone (KA-oil) and iii) oxidative-hydrolysis of KA-oil with HNO3. However, this process generates important amounts of N2O, which is a greenhouse gas that must be captured and treated, increasing the net CO2 emissions of the whole transformation. 
The double alkoxycarbonylation of 1,3-butadiene offers a greener route to prepare adipate esters from potentially renewable sources, with high atom economy, and without generating N2O. Indeed, 1,3-butadiene can be obtained from ethanol, whereas CO could be issued from CO2 through the Reverse Water-Gas Shift reaction. The resulting diester can be hydrolyzed to adipic acid. However, the catalytic process presents several challenges: 

• Catalyst activity and stability must support two successive alkoxycarbonylations.
• An unfavorable isomerization step is required for linear diester formation.
• Side reactions such as telomerization, hydroalkoxylation, and polymerization could take place.
• The mechanism is still not fully understood.

To address some of these challenges, this PhD combined theoretical and experimental approaches to gain mechanistic insight, identify limitations, and propose strategies to improve catalyst activity, selectivity, and recyclability. This PhD report has been divided into 5 chapters.

The general introduction describes some aspects of the industrial preparation of adipic acid and the alternative ways for its synthesis from renewable sources.

The first chapter contains a detailed bibliographic report focusing on the development of catalytic alkoxycarbonylation, paying special attention to the systems based on Pd-diphosphines, and the mechanistic aspects related to this reaction.

The second chapter of the thesis focuses on the mono-methoxycarbonylation of butadiene to methyl-3-pentenoate (3MP). Initial DFT modeling of the previously established mechanism revealed a high activation barrier (~200 kJ·mol-1), which is not reachable under our reaction conditions. By accounting for solvent effects and the role of acid-base pairs, a revised mechanism is proposed, in which the base acts as proton shuttle facilitating the reaction and reducing the barrier to ~100 kJ·mol-1, which is in line with experimental observations.

The third chapter simulates by DFT the mechanistic aspects about the isomerization steps among methyl-pentenoates (2MP/3MP/4MP) and the second alkoxycarbonylation. Base involvement is also critical for the second methoxycarbonylation, with the computed ~105 kJ·mol⁻¹ apparent barrier compatible with experimental results. Experimentally, kinetic profiles were analyzed, suggesting a kinetic bottleneck in dimethyl adipate formation due to isomerization of MP intermediates.

Thanks to this better understanding of the reaction mechanisms, we explore in chapter four strategies to improve the catalytic performances. We systematically test additives such as water, Lewis’ acids, metal co-catalysts.

The fifth chapter of this thesis explores catalyst performances in the presence of ionic liquids (ILs). We first assessed the compatibility of the catalytic system with various ILs. Among the parameters studied, the nature of the IL anion had the strongest influence on catalytic performances. Anions with a Brønsted base character were the most promising, as they can acts as proton shuttle during methanolysis, which is consistent with our earlier mechanistic findings.

In the final chapter, we provide general conclusions and open perspectives to the PhD work.