The well-established, cost-competitive ethanol market provides an opportunity to shift the composition of jet fuel and other fuel products away from petroleum. In the first step of a multi-step ethanol-to-jet-fuel process earlier developed by DOE’s Oak Ridge National Laboratory (ORNL’s), a catalyst is used to convert ethanol into butene-rich C₃₊ olefins, important intermediates that can then be processed into aviation fuels. Two more steps—oligomerization and hydrotreating—convert these intermediates into the liquid hydrocarbons used as fuels.

A team led by ORNL’s Zhenglong Li, tasked with improving the current technique for converting ethanol to C₃₊ olefins, has demonstrated a novel composite catalyst that upends current practice and drives down costs. The research was published in ACS Catalysis.

There are two challenges that hinder current conversion techniques from wider adoption: low olefin yield and high production costs. Also, recent approaches to conversion require additional hydrogen, another cost burden. As a result, the cost of upgrading ethanol needs to be lowered significantly to compete with petroleum.

While we think of this as one process, from the chemistry side when you zoom in, there are several elementary steps. In the first step, we internally generate hydrogen—can we use that low concentration of hydrogen downstream where it is needed and avoid using additional hydrogen? To do this, we need to develop new catalysts; the current standards cannot do this conversion at the relative high temperature required.

—Zhenglong Li

The team developed and tested a composite catalyst—a zinc-yttrium beta catalyst combined with a single-atom alloy catalyst. ORNL materials scientists, including Li’s co-author Lawrence Allard, pioneered the use of single-atom catalysts, which was introduced in a 2011 Nature Chemistry paper.

Single-atom alloys are used for low-temperature selective hydrogenation, but no one has yet reported its use in this kind of high temperature reduction. We also know that we could easily over-hydrogenate these molecules, which would not be usable. The critical thing here was modulating the ratio of hydrogen and butadiene generated during the reaction.

—Zhenglong Li

The multifunctional catalyst system, which selectively catalyzes ethanol-to-olefin (C₃₊) valorization in the absence of cofed hydrogen, forms 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).

… 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).

… 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.

—Cordon et al.

The 78% selectivity at 94% ethanol conversion is the highest reported among the literature, Li said.

The research is a first for combining these catalysts and provides new fundamental understanding of how these materials work. Li’s team will push the technique further.

This research was sponsored by the DOE Office of Energy Efficiency and Renewable Energy’s Bioenergy Technologies Office and conducted through the multi-laboratory Chemical Catalysis for Bioenergy Consortium. Microscopy was performed at the Center for Nanophase Materials Sciences, a DOE Office of Science user facility, and supported by the DOE Basic Energy Sciences program and in collaboration with the Advanced Catalyst Synthesis and Characterization project. This research used resources of the Advanced Photon Source, a DOE Office of Science user facility at Argonne National Laboratory.

Resources

• Michael J. Cordon, Junyan Zhang, Stephen C. Purdy, Evan C. Wegener, Kinga A. Unocic, Lawrence F. Allard, Mingxia Zhou, Rajeev S. Assary, Jeffrey T. Miller, Theodore R. Krause, Fan Lin, Huamin Wang, A. Jeremy Kropf, Ce Yang, Dongxia Liu, and Zhenglong Li (2021) “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” ACS Catalysisdoi: 10.1021/acscatal.1c01136