Cyclopentadienyl-based Trioxo-rhenium Complexes for the Catalytic Deoxydehydration of Diols and Bio-based Polyols to Olefins

Abstract

Renewable sources like biomass, which mainly consists of materials derived from trees and plants, are currently considered as a key and future feedstock in the chemical industry for the sustainable production of chemicals. After the pre-treatment of biomass, lignocellulosic biomass is obtained as the major component. This contains various polymeric compounds such as starch, cellulose and lignin. By breaking these polymers into smaller monomeric molecules, sugars, polyols and aromatic compounds are obtained. In particular the sugars and polyols contain many hydroxyl group functionalities and, accordingly, have a high oxygen content. In order to arrive at some of the main chemical building blocks, e.g. olefins, the challenge is to remove the hydroxyl groups (at least partly). A new type of reductive deoxygenation reaction, deoxydehydration (DODH), has recently emerged in which vicinal diols are converted to olefins. In essence, a DODH reaction involves a double dehydroxylation, which is a combination of a deoxygenation and a dehydration reaction and, therefore, requires a sacrificial reductant in combination with a catalyst.

Here in this thesis, we developed cyclopentadienyl (Cp)-ligated trioxo-rhenium (Cp’ReO3) catalysts for the DODH of different kinds of vicinal diols including bio-based polyols, converting these to olefins. The tri-tert-butylcyclopentadienyl ligated trioxo-rhenium (CptttReO3) complex was synthesized and employed as the efficient DODH catalyst (with triphenylphosphine as the reductant) to obtain moderate to excellent yields of olefins, maintaining the retention configuration of the starting vicinal diols (e.g. terminal or cyclic aliphatic, aromatic). Moreover, a record high turnover number of 1400 (per Re) was achieved, indicating substantially improved catalyst stability compared to any other previously known DODH method. The bio-based polyols were also converted to the corresponding polyene compounds. Glycerol resulted in allyl alcohol with 91% yield. Erythritol conversion to butadiene formation was optimized under different reaction conditions, resulting in 55% butadiene using PPh3/pyridine or up to 75% using 3-octanol as both solvent and reductant. Other longer chain polyols (xylitol, sorbitol) were also demonstrated to produce moderate olefinic yields.

The DODH reaction mechanism was studied to understand the above-mentioned beneficial catalytic effects of the bulkier CptttReO3 catalyst. Both kinetics and DFT studies deduced that the formation of the key dioxo-rhenium species (CptttReO2) was facilitated and its stability increased with the bulkier Cp-substituents (i.e. tert-butyl groups). Moreover, the formation of this species was substantiated by unprecedented spectroscopic characterizations and its participation in DODH reactions proven. In order to evaluate structure-activity relationships, the differently substituted Cp-ligands were used to synthesize the corresponding Cp-ligated tricarbonyl rhenium complexes followed by oxidation to the corresponding trioxo-rhenium complexes. Additionally, some novel serendipitous reactivities of the modified Cp-tricarbonyl rhenium analogues were studied. The smaller changes in the Cp-substitution were significantly reflected in the nature/stability of the high valent trioxo-rhenium complexes. The less number Cp-substituted complexes were found to be the faster DODH catalysts than the other known stable/highly substituted catalysts.

In conclusion, Cp-based Re-trioxo complexes were found to be potent deoxydehydration catalysts for the conversion of diols and bio-based polyols to olefins. Variations in Cp-ligand substitution significantly alters the stability and the catalytic activity of these complexes, allowing for further development and optimization of this Cp-based DODH system.