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Selective Butene Formation in Direct Ethanol-to-C3+-Olefin Valorization over Zn–Y/Beta and Single-Atom Alloy Composite Catalysts Using In Situ-Generated Hydrogen

Publication Type
Journal Name
ACS Catalysis
Publication Date
Page Numbers
7193 to 7209

The selective production of C3+ olefins from renewable feedstocks, especially via C1 and C2 platform chemicals, is a critical challenge for obtaining economically viable low-carbon middle-distillate transportation fuels (i.e., jet and diesel). Here, we investigate multifunctional catalysts for direct ethanol-to-olefins (C3+, ETO) valorization in the absence of cofed hydrogen, forming butenes as the primary olefin products. Beta zeolites containing predominately isolated Zn and Y metal sites catalyze ethanol (EtOH) upgrading steps (588 K, 3.1 kPa EtOH, ambient pressure) regardless of cofed hydrogen partial pressure (0-98.3 kPa H2), forming butadiene as the primary product (60% selectivity at 87% conversion). A secondary bed of “single-atom alloy” (SAA) Pt-Cu supported on Al2O3 selectively hydrogenates butadiene to butene isomers at a consistent reaction temperature using hydrogen generated in situ from ethanol-to-butadiene (ETB) conversion. This unique hydrogenation reactivity at near-stoichiometric hydrogen and butadiene partial pressures is not observed over monometallic Pt or Cu catalysts, indicating the necessity of alloyed Pt-Cu ensemble sites for high temperature selective hydrogenation reactivity. Steady state selective hydrogenation rates over these supported metal catalysts and DFT calculations of the corresponding Horiuti-Polanyi reaction mechanisms indicate that Pt-Cu ensemble sites catalyze hydrogen scission with lower barriers than over Cu sites while limiting strong olefin binding onto Pt sites and subsequent butene hydrogenation reactions. The combination of Zn-Y/Beta and SAA Pt-Cu catalysts can selectively form butenes (65% butenes, 78% C3+ selectivity at 94% conversion) and avoid butane formation with in situ generated hydrogen while avoiding costly hydrogen cofeeding, a significant expense for many renewable energy processes.