Production of renewable toluene from biomass-derived furans via Diels-Alder and dehydration reactions: A comparative study of Lewis acid catalysts

Abstract

The Diels–Alder cycloaddition of biomass-derived furans and subsequent dehydrative aromatization are potentially valuable processes for the sustainable production of aromatic chemicals from biomass. In this paper, we have investigated the catalytic activity of a series of homogeneous and heterogeneous Lewis acid catalysts in connection with the conversion of biomass-derived 2-methylfuran (MF) and ethylene to toluene. We have used a number of Lewis acid metal chlorides, cation-exchanged Y zeolites, Sn- and Zr-containing beta zeolites to understand the role of acid sites and activity correlation in the MF reaction. The Lewis acid catalysts of AlCl3 and Na-Y zeolite demonstrated superior selectivity for toluene production with regard to the Brønsted acidic zeolites of H-Beta and H-Y, which have proven to be effective for the conversion of 2,5-dimethylfuran (DMF) to p-xylene. The high toluene selectivity of the Lewis acid catalysts was mainly attributed to their ability to retard side reactions, such as oligomerization and alkylation, as well as to accelerate the cycloaddition of MF and ethylene. In the case of Brønsted acidic zeolites, the side reactions were significantly enhanced, resulting in lower selectivity to toluene. The catalytic activity of the Lewis acidic metal chloride catalysts was correlated with an empirical scale of Lewis acid strength, showing that the strongest Lewis acid, AlCl3, gave the highest toluene yield of ∼70% at 250°C after 24h. Similarly, of the alkali cation-exchanged Y-zeolites investigated here, Li- and Na-Y were more active than K- and Cs-Y due to the smaller size of the cations, resulting in stronger Lewis acidity. Na-Y was also more active than Sn- and Zr-Beta, which have framework Lewis acidic sites, demonstrating its superiority for catalyzing the cascade of Diels-Alder and dehydrative aromatization reactions of MF. A remarkable toluene yield of ∼65% was achieved with Na-Y at the nearly complete MF conversion (>96%) under optimal reaction conditions.