Regulating the Atypical Brønsted Acidity of Zr-SBA-16 Catalysts for Selective Ethanol Conversion to Butene-Rich C ₃₊ Olefins

Lewis-acid molecular sieves are essential catalysts for transforming biomass-derived platform molecules into valuable chemicals. However, the Brønsted acidity arising from incorporated Lewis metals has been rarely reported and remains poorly understood. Using Zr-SBA-16 as a model catalyst, we discovered that the open site Zr centers ((Si–O)3–Zr–OH) exhibit an atypical Brønsted acidity, distinct from the conventional Brønsted acidity arising from Al atoms in the Si–OH–Al framework of zeolites. In a model reaction of ethanol conversion to butenes, these unique Zr centers concurrently catalyze the dehydration of ethanol and promote the C–C coupling of acetaldehyde. Upon selective poisoning with potassium acetate, these open site Zr species were converted into enclosed (Si–O)3-Zr-OK configuration, which suppressed both ethanol dehydration and acetaldehyde C–C coupling activities. Thus, an optimum level of potassium doping was found to achieve the balance of ethanol dehydration and C–C coupling activity. With this, 85% C3+ olefin selectivity at ∼98% ethanol conversion was achieved over the Cu-loaded Zr-SBA-16 catalyst in ethanol upgrading to butene-rich C3+ olefins. The discovery of the Brønsted acidity over Zr-SBA-16 provides a new perspective on the acidity of heteroatom-incorporated molecular sieve materials.