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

Abstract

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 report a multifunctional catalyst system composed of Zn–Y/Beta and “single-atom” alloy (SAA) Pt–Cu/Al2O3, which selectively catalyzes ethanol-to-olefin (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 upgrading steps (588 K, 3.1 kPa ethanol, ambient pressure) regardless of cofed hydrogen partial pressure (0–98.3 kPa H2), forming butadiene as the primary product (60% selectivity at an 87% conversion). The Zn–Y/Beta catalyst possesses site-isolated Zn and Y Lewis acid sites (at ∼7 wt % Y) and Brønsted acidic Y sites, the latter of which have been previously uncharacterized. A secondary bed of SAA Pt–Cu/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, highlighting these operating conditions as a critical SAA catalyst application area for conjugated diene selective hydrogenation at high reaction temperatures (>573 K) and low H2/diene ratios (e.g., 1:1). Single-bed steady-state selective hydrogenation rates, associated apparent hydrogen and butadiene reaction orders, and density functional theory (DFT) calculations of the Horiuti–Polanyi reaction mechanisms indicate that the unique butadiene selective hydrogenation reactivity over SAA Pt–Cu/Al2O3 reflects lower hydrogen scission barriers relative to monometallic Cu surfaces and limited butene binding energies relative to monometallic Pt surfaces. DFT calculations further indicate the preferential desorption of butene isomers over SAA Pt–Cu(111) and Cu(111) surfaces, while Pt(111) surfaces favor subsequent butene hydrogenation reactions to form butane over butene desorption events. Under operating conditions without hydrogen cofeeding, this 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 using only in situ-generated hydrogen, avoiding costly hydrogen cofeeding requirements that hinder many renewable energy processes.

Selective direct conversion of aqueous ethanol into butadiene via rational design of multifunctional catalysts

“…Compared with H-beta and Si-beta, other synthesized catalysts exhibited lower peak intensity, suggesting that the Zn, Zr, and Y species were grafted into the Si-beta zeolite. 49 The EDX results disclosed that the composition of each element was close to the theoretical value (Table S11†).…”

“…suggesting that the Zn, Zr, and Y species were grafted into the Si-beta zeolite. 49 The EDX results disclosed that the composition of each element was close to the theoretical value (Table S11 †).…”

Isolated Metal Sites in Cu–Zn–Y/Beta for Direct and Selective Butene-Rich C3+ Olefin Formation from Ethanol

“…, B 3+ , Al 3+ , and Ga 3+ ) into zeolite frameworks via isomorphous substitution with a silicon atom generates an exchangeable Brønsted acid sites to balance the framework charge. Here, the introduction of Y 3+ into silanol nests formed during dealumination could also generate the Brønsted acid sites observed in IR spectra at 1637 cm –1 over Y 7 /Beta . The addition of Zn and/or Cu sites onto Y 7 /Beta further reduces the amount of Brønsted acid sites, as indicated by the decreased IR peak area at 1637 cm –1 , as shown in the pyridine-saturated transmission IR analysis for Zn–Y 7 /Beta and Cu–Zn–Y 7 /Beta, which is potentially due to ion exchange of Y-induced Brønsted acid sites with either Cu or Zn sites.…”

“…Here, the introduction of Y 3+ into silanol nests formed during dealumination could also generate the Brønsted acid sites observed in IR spectra at 1637 cm −1 over Y 7 /Beta. 48 The addition of Zn and/or Cu sites onto Y 7 /Beta further reduces the amount of Brønsted acid sites, as indicated by the decreased IR peak area at 1637 cm −1 , as shown in the pyridinesaturated transmission IR analysis for Zn−Y 7 /Beta and Cu− Zn−Y 7 /Beta, which is potentially due to ion exchange of Yinduced Brønsted acid sites with either Cu or Zn sites. In addition, the peak intensity of the O−H stretching region (3800−3200 cm −1 ) decreases after incorporating Y (Figure S11), which suggests metal interactions with the silanols, either Beta (100% conversion).…”

Low-temperature selective ethanol upgrading to primary or secondary alcohols by homogeneous catalysis

“…Moreover, these hydrocarbons are also essential feedstock for the production of high-value-added products, such as rubbers, polymers, and synthetic oils 28 . Indeed, recently the production of butenes from bioethanol has attracted more attention in sustainability research elds, with yields exceeding 60% at reaction temperatures between 300-400 C 29,25 . As a signi cant component of lique ed petroleum gases (LP gases), butane is industrially derived from natural gas and crude oil 30 .…”