The selective conversion of biomass derived coumalic acid to functionalized aromatics and novel intermediates

Pfennig, Toni
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The heavy reliance on petroleum as the raw feedstock for the production of chemicals imparts negative environmental effects and exerts economic pressure due to the diminishing availability of natural crude oil reserves. A change in the raw material supply can already be observed in the U.S., where petroleum is increasingly being replaced by inexpensive shale gas. The increased shale gas dependence, however, reduces the availability of >C4 chemicals (e.g. aromatics). Since existing alternatives, such as selectively accessing aromatics from renewable carbon sources (e.g. biomass), are still facing significant limitations, there is a need to develop new technologies.

An innovative approach to selectively access bio-based aromatics in high yield is provided herein via a Diels-Alder/decarboxylation/palladium catalyzed dehydrogenation domino sequence starting from coumalic acid (or methyl coumalate) in conjunction with the inexpensive and easy to separate/recycle ethylene (or propylene). This approach is guided by an in-depth reaction network analysis, solvent and kinetic studies, and complemented by density functional theory (DFT) calculations with the goal of providing key insights into the formation of intermediates and by-products on the pathway to aromatics.

The solvent studies show that polar aprotic solvents including 1,4-dioxane, y-valerolactone and acetone lead to excellent aromatic yield and selectivity (78 - 91 mol%) when starting with coumalic acid, whereas non-polar toluene provides only poor solubility of coumalic acid which is associated with by-product formation and, therefore, poor aromatic yield (< 55 mol%). Starting with methyl coumalate, however, improves yield and selectivity significantly (up to 99 mol%) across all solvents tested, indicating that the starting substrate (ester vs acid) considerably impacts the formation of aromatics.

The kinetic analysis of relevant steps reveals that decarboxylation is the rate limiting step, which is in agreement with DFT calculations. This critical information enables the development of a tailored catalyst through which significant process optimizations are achieved, including the selective access to dihydrobenzenes from bicyclic lactones in high yield (> 98 mol%). Novel dihydrobenzene structures have interesting dual functionality and further expand the diverse coumalate platform with species that are challenging to access via conventional petroleum routes.

Further diversification of bicyclic lactones is achieved through the choice of solvent and catalyst (e.g. Brà ¸nsted and Lewis acids) by enabling highly selective pathway modifications. Lewis acids enhance decarboxylation, whereas Brà ¸nsted acids enable ring-opening of bicyclic lactones to produce novel chemical species. Ring-opening is also achieved when bicyclic lactone conversion is mediated in methanol, a polar protic solvent. Cleavage of the lactone bridge is, thereby, induced through methanolysis in the absence of a Brà ¸nsted acid, therefore providing another environmentally benign pathway.

Lastly, the combination of kinetic studies, DFT calculations and high resolution magic angle spinning NMR characterization reveals important insights into the Lewis acid (y-Al2O3) catalyzed decarboxylation mechanism at the solvent-catalyst-interface and provides evidence of the catalytic active site responsible for decarboxylation. This knowledge can be broadly transferred for decarboxylation of a range of 2-pyrone derived bicyclic lactones and improve access to novel dihydrobenzenes and aromatics from biomass.

Aromatics, Biorenewable, Catalysis, Diels-Alder, Diversification, Novel intermediates