On the reactivity and selectivity trends in the tandem Diels-Alder cycloaddition and dehydrative aromatization between dimethylfuran and ethene over solid acids

Tandem Diels-Alder (DA) cycloaddition and dehydration reactions between bio-derived furans and dienophiles offer a sustainable route potentially able to meet the growing demands of aromatic chemicals worldwide. In particular, efforts to maximize the production of p-xylene from dimethylfuran (DMF) and ethene have led to an expanding list of solid acids with, reportedly, outstanding catalytic performances. In this work, we have revisited the kinetic complexities of this reaction that are characterized by a distinct shift in the rate-limiting step depending sensitively on catalyst mass, and addressed the possible origins of reactivity and selectivity trends across a range of solid acids that hold promise for the target reaction, including aluminosilicates and a series of mesoporous metal phosphate catalysts (MOPOx; M = Sn, Ti, Zr, Hf, Nb and Ta), in both dehydration- and cycloaddition-limiting regimes. In contrast to initial p-xylene formation rates in the dehydration-limiting regime, which benefit from an increased acid strength of Brønsted acid sites, p-xylene formation rates within the cycloaddition-limiting regime are essentially unaffected by variations in acid site densities, Lewis and Brønsted acid strength, and pore confinement. Such weak dependences of DA reactivity on a multitude of acid properties provide strong hints that DA cycloaddition between DMF and ethene follows an inverse-electron-demand [4 + 2] route, most likely as a consequence of a more favorable adsorption of DMF than that of ethene on the acid sites enhancing so the electrophilicity of DMF, but not that of ethene as required by the normal-electron-demand cycloaddition. Product selectivities and carbon balances were tracked as functions of reaction time, conversion and reactant pressure or concentration to identify the side reactions and acid properties responsible for p-xylene selectivity losses and the ultimate formation of carbonaceous deposits. These findings deepen our understanding of the roles of Lewis acid sites and Brønsted acid sites for the chemical reactivities toward the DA cycloaddition, the consecutive cycloadduct dehydrative aromatization and undesired acid-catalyzed side reactions, and inform strategies to enhance DA reactivities and suppress deleterious side reactions from the standpoint of both materials and process design.