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

Article abstract of DOI:10.1021/acscatal.1c01136

The selective production of C₃₊olefins from renewable feedstocks, especially via C₁and C₂platform 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 Br?nsted acidic Y sites, the latter of which have been previously uncharacterized. A secondary bed of SAA Pt-Cu/Al₂O₃selectively hydrogenates butadiene to butene isomers at a consistent reaction temperature using hydrogen generatedin situfrom 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/Al₂O₃reflects 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 onlyin situ-generated hydrogen, avoiding costly hydrogen cofeeding requirements that hinder many renewable energy processes.

Full text of DOI:10.1021/acscatal.1c01136

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Research Article
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
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*
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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 multifunc-
tional 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.
KEYWORDS: ethanol, butenes, butadiene, selective hydrogenation, catalysis, single-atom alloy, zeolite
accomplished over either Brønsted acid zeolites6,7 or metal
1. INTRODUCTION
oxides8−10 with limited olefin selectivities due to the formation
Efficient catalytic upgrading of renewable feedstocks into
carbon-neutral transportation fuels remains critically important
for combating CO2 emissions, especially for producing long-
chain (C8+) hydrocarbons required for aviation and heavy-duty
of aromatics and paraffins,11−13 ethylene, oxygenates, or other
side products including CO2.14,15 Therefore, a strong need
remains for an approach to selective C3+ olefin production for
diesel fuels. Bioethanol is a widely produced renewable
feedstock (∼86 million tons per year worldwide in 20181)
Received: March 11, 2021
Revised: April 29, 2021
Published: June 4, 2021
that can be valorized into key short-chain (C3−C6) olefin
intermediates (e.g., propene,2 1-butene, isobutene3), a class of
critical precursors for oligomerization into middle-distillate-
range hydrocarbons4 and commodity chemical production.5
Direct conversion of ethanol to C3+ olefins is typically
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Scheme 1. Proposed ETO Reaction Pathway Composed of ETB and Subsequent Butadiene Selective Hydrogenation to Form
a
Butene Isomers
a
H2 production and consumption events are highlighted in red. An additional ethanol provides the hydrogen source for the MPV reduction of
crotonaldehyde.
a
Scheme 2. Two-Bed Reactor Setup Composed of Independent ETB and Selective Hydrogenation (SH) Catalysts
a
Catalyst beds are in physical contact during reaction, with the distance here shown only to highlight key intermediates passing between catalyst
beds. Components labeled as “others” are products that are not typically observed to change significantly by the choice of SH catalyst in the
absence of hydrogen cofeeding. Single-bed ethanol valorization studies were performed over Zn−Y/Beta without an SH catalyst (vide infra Section
3.1), and single-bed selective hydrogenation reactions were performed without an ETB catalyst (vide infra Section 3.3).
economically viable liquid renewable fuel generation without
significant carbon losses, especially since carbon conversion
efficiency is a primary cost driver for renewable fuel
production.16
ETB catalysts typically possess multiple kinetically relevant
active sites for dehydrogenation and aldol condensation steps,
which are often derived from transition-metal and oxide
pairings28 such as metal-promoted MgO/SiO2.29−32 Zeolites
have also shown promising ETB reactivity through the
combination of Lewis acid sites formed through the
isomorphous substitution of framework Si centers with M4+
or M5+ heteroatoms and transition-metal sites (e.g., Ag/Zr-
Beta,33 Cu-Ta/Beta,34 Zn/Hf-MFI35), utilizing favorable
confinement effects in conjunction with dispersed active sites
to improve desired product selectivities. Recently, rare-earth
metals (e.g., Y) operating as Lewis acid sites within Beta zeolite
microporous environments have been investigated as aldol
condensation active sites and can catalyze the ETB reaction
pathway (603 K) when combined with transition metals (e.g.,
Zn).36 This suggests Zn−Y/Beta as an effective ETB catalyst
within the larger ETO reaction pathway19 to produce near-
stoichiometric butadiene and hydrogen partial pressures for
further catalyst upgrading.
Using butadiene as an intermediate, the ETO pathway
requires the selective hydrogenation of butadiene to form
butene. Butadiene selective hydrogenation reaction is generally
studied at lower temperatures (<423 K) and is often
performed with excessive amounts of hydrogen (H2/BD >
15).37 Butane formation can be observed37,38 even at such mild
reaction temperatures and with H2/BD ratios similar to those
observed in naphtha steam-cracking applications,39 situations
where the selective hydrogenation of acetylene to ethylene or
butadiene to butenes is necessary prior to downstream
operations. Traditional butadiene selective hydrogenation
catalysts include Cu/SiO2,40 Pd/Al2O3,41 and Pt-Ni/Al2O3,42
while “single-atom alloy” (SAA)-supported metal catalysts
composed of dilute solid solutions of precious metals (e.g., Pd
or Pt) within transition-metal nanoparticles (e.g., Cu) have
Ethanol conversion to 1,3-butadiene (ETB) is an econom-
ically attractive reaction for selectively generating renewable C4
monomers that can avoid the above-mentioned side product
formation17 and can serve as an important step toward the
synthesis of butene-rich olefins. Despite debates about the
specific mechanistic details, the general reaction network of
ETB (Scheme 1) consists of ethanol dehydrogenation, aldol
condensation, Meerwein−Ponndorf−Verley (MPV) reduction,
and dehydration to form 1,3-butadiene (butadiene, BD).18−20
For an ideal reaction resulting in perfect ethanol-to-butadiene
selectivity, stoichiometric amounts of butadiene and hydrogen
are formed due to hydrogen generated from ethanol
dehydrogenation (Scheme 1). However, butadiene is not a
preferred C4 olefin for middle-distillate fuel generation due to
its tendency for increased coke formation.21−24 Therefore, one
attractive approach is to directly utilize this in situ-generated
hydrogen for the selective hydrogenation of butadiene to
butenes without requiring cofed hydrogen, providing a single-
reactor approach for converting ethanol into a butene-rich
olefin stream. Such ethanol-to-C3+-olefin (ETO) valorization
without external hydrogen has not been reported in the
literature and offers the benefits of avoiding the operating and
capital costs associated with hydrogen purchase, storage,
downstream separation, and recycling. The grand challenge
of this ETO pathway is designing catalytic materials with active
sites for both high ETB reactivity and high butadiene selective
hydrogenation reactivity while mitigating butene hydrogena-
tion to butanes under stoichiometric hydrogen and butadiene
partial pressures and ETB reaction conditions (e.g., 573−673
K).25−27
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recently been studied as relevant selective hydrogenation
catalysts.37,43,44 Low-temperature (<423 K) butadiene selective
hydrogenation reactivity over Pt−Cu SAA catalysts has been
qualitatively correlated with the formation of hydrogen islands
around Pt centers through hydrogen adsorption, scission, and
subsequent adatom diffusion to proximal Cu centers;37
however, their applicability for higher temperatures has not
been investigated and particularly not for near-stoichiometric
hydrogen and butadiene partial pressures.
scanning transmission electron microscopy (HAADF-STEM)
was performed to probe bulk metal distributions for both Zn−
Y/Beta and Al2O3-supported metal catalysts with character-
ization details reported in Section S.1.2. Transmission IR
spectra were collected on Zn- and Y-containing zeolite samples
after pyridine adsorption, and the measurement details are also
reported in Section S.1.2.
XAS experiments were performed in transmission mode at
the Zn and Y K edges (9.659 and 17.038 keV, respectively, for
zeolite samples) and at the Pt L3 edge and Cu K edge (11.564
and 8.9789 keV, respectively, for supported metal catalysts) on
the Materials Research Collaborative Access Team (MR-CAT)
bending magnet beamline at the Advanced Photon Source at
Argonne National Laboratory. For samples with low Pt
loading, the Pt L3 edge XAS experiments were performed in
fluorescence mode at the MR-CAT insertion device line, 10-
ID. Detailed sample pretreatment, measurement procedures,
and data analysis methodologies are reported in Section S.1.2.
2.3. Gas-Phase Ethanol Conversion Studies. High
ethanol conversions for the two-bed reactor setup or single-
bed Zn−Y/Beta were measured using a vertically aligned
tubular quartz reactor (0.5″ OD) under ambient pressures.
Catalysts were pelletized, crushed, and sieved to retain particles
within 125 and 180 μm prior to being loaded into the reactor
(Scheme 2). Catalyst beds were placed between quartz wool
beds (Chemglass, 8−15 μm) with typical bed masses of 0.250
g for Zn−Y/Beta in the first bed and 0.168 g of the desired
selective hydrogenation catalyst in the second bed, unless
otherwise stated. For single-bed operation, 0.250 g of Zn−Y/
Beta was loaded. Constant catalyst bed temperatures were
measured using a K-type thermocouple (Omega) positioned
within the first catalyst bed. Catalysts were heated to 673 K
(0.0833 K s−1) in a flowing He (0.33 cm3 s−1, Airgas, UHP,
>99.999%) for 1 h, switched to pure H2 flow (0.42 cm3 s−1,
Airgas, UHP, >99.999%) at 673 K for 0.5 h, and then switched
back to flowing He while cooling to 588 K. Catalyst beds were
then exposed to relevant H2 and He gas flow conditions (0.48
Here, this ETO reaction pathway is investigated for the first
time using a two-bed reactor setup containing Zn−Y/Beta and
Pt- and/or Cu-containing supported metal catalysts (Scheme
2) to catalyze ethanol to butadiene and subsequent butadiene
selective hydrogenation, respectively, generating C3+ olefins
from ethanol alone without external hydrogen feeding. Both
catalysts are individually characterized using a suite of
spectroscopy and microscopy techniques including X-ray
absorption spectroscopy (XAS), pyridine-adsorbed trans-
mission IR spectroscopy, and scanning transmission electron
microscopy (STEM). Catalysts are tested in both single-bed
and two-bed configurations (588 K; Scheme 2) as a function of
cofed hydrogen partial pressures (0−98.3 kPa). Steady-state
reaction rates, apparent reaction order measurements, and
density functional theory (DFT) calculations are employed to
compare selective hydrogenation catalysts (SAA 0.1Pt6Cu/
Al2O3, 0.1Pt/Al2O3, 2Pt/Al2O3, and 6Cu/Al2O3) and inves-
tigate the unique kinetic capabilities of Pt−Cu ensemble sites
under near-stoichiometric hydrogen concentrations desired for
ideal ETO operations and negligible butane formation. These
combined experimental and computational results indicate the
ETO catalytic potential of the composite catalyst composed of
Zn−Y/Beta and SAA 0.1Pt6Cu/Al2O3 utilizing stoichiometric
hydrogen partial pressures formed in situ and eliminating costly
hydrogen cofeeding and corresponding downstream hydrogen
separation and recycle steps in the middle-distillate-range
hydrocarbon generation processes.
2. EXPERIMENTAL METHODS
cm3 s−1, PH = 0−101.3 kPa with balance He, preheated to 453
2
2.1. Catalyst Synthesis. Dealuminated Beta (deAl-Beta)
was obtained using a previously reported procedure,45 with
details shown in Section S.1.1. Zn−Y/Beta, Zn/Beta, and Y/
Beta samples were synthesized from the deAl-Beta support via
solid-state grinding. In a typical Zn−Y/Beta synthesis, 0.091 g
of zinc nitrate hexahydrate (Sigma-Aldrich, 97%) and 0.345 g
of yttrium nitrate hexahydrate (Sigma-Aldrich, 97%) pre-
cursors were ground with deAl-Beta for 0.25 h. The resulting
solids were heated in a tube furnace to 823 K (0.0167 K s−1)
under airflow (10 cm3 s−1 (g catalyst)−1). Pt- and Cu-
containing supported metal catalysts were synthesized through
incipient wetness impregnation onto a γ-alumina (Al2O3)
support (Sinopharm Chemical Reagent Co., Ltd., 98.0%), with
details reported in Section S.1.1.
2.2. Catalyst Characterization. Powder X-ray diffraction
(XRD) patterns were measured on a PANalytical X-ray
diffractometer using a Cu Kα source (λ = 0.1542 nm, 45 kV,
40 mA). Diffraction patterns were collected from 4 to 40° at a
step size of 0.025° and a scan rate of 0.04° s−1. N2 adsorption
isotherms (77 K) were collected using a Quantachrome
Autosorb iQ with detailed analysis information reported in
Section S.1.2. Bulk elemental compositions were measured via
inductively coupled plasma atomic emission spectroscopy
(ICP-AES) performed at Galbraith Laboratories, Inc. Aberra-
tion-corrected high-angle annular dark-field imaging from
K). Liquid ethanol (EMD Millipore, >99%) was fed into the
heated gas mixture via a syringe pump (KD Scientific Legato
180) and vaporized prior to interacting with the catalyst beds.
No background reactions were detectable using only quartz
wool within the reactor. Products were separated and analyzed
via gas chromatography (Agilent 7820A) using a HP-PLOT-Q
column (30 m, 0.32 mm diameter, 20 μm film) and both a
flame ionization detector and a thermal conductivity detector.
Product identification was performed using a gas chromato-
graph (GC, Agilent 6850) and mass spectrometer (Agilent
5975C) with known standards. The carbon balance for all of
the reactions is typically 92−96%. Definitions of ethanol
conversions, product yields, and selectivities are reported in
Section S.1.3.
Reaction rates for Zn- and Y-containing samples were
measured in the same setup as the high conversion
experiments. Catalyst weight, ethanol flow rates, and carrier
gas flow rates were adjusted to maintain a differential
conversion regime with ethanol conversion <5%. Typically,
0.05 g of catalyst was pelletized, crushed, and sieved to retain
particles within 125 and 180 μm and then diluted with 0.80 g
silicon carbide (SiC, Alfa Aesar, 120 grit). Ethanol flow rates
were controlled via a syringe pump (1.07 mL h−1) into Ar
(1.67 cm3 s−1, 588 K, 163 kPa total pressure).
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Figure 1. (A) Product formation rates from ethanol valorization (11.1 kPa ethanol in balance Ar, 163 kPa total pressure, 543 K, WSHV = 15.9 h−1
over Zn/Beta and Zn−Y/Beta and 8.0 h−1 over Y/Beta, <5% conversion) to form butadiene (green), butenes (blue), ethylene (orange),
acetaldehyde (gray), and diethyl ether (purple) and (B) associated conversions (black) and product selectivities over Zn−Y/Beta, Zn/Beta, and Y/
Beta catalysts.
2.4. Butadiene Selective Hydrogenation Kinetic
Studies. Butadiene selective hydrogenation rates were
measured using a similar reactor setup as used for ethanol
upgrading but with only a single catalyst bed. Typically, 0.05 g
of catalyst (pelletized, crushed, and sieved to 125−180 μm)
was mixed with 0.70 g of SiC (Alfa Aesar, 120 grit) prior to
being loaded into the reactor between two quartz wool plugs.
Catalyst beds were pretreated to 673 K (0.0833 K s−1) in 10%
H2 in balance Ar (UHP, Airgas, >99.999%, 0.83 cm3 s−1) for 1
h prior to cooling to 588 K. Gas flow rates of H2 and Ar were
adjusted and butadiene (BD, Airgas, 3%, balance N2) and a
methane internal standard (CH4, Airgas, 1%, balance Ar) were
introduced to obtain a reference condition (Ftotal = 1.67 cm3
s−1, PH = 1.51 kPa, PBD = 1.51 kPa, PCH = 0.20 kPa).
Transient behavior was observed for at least 12−18 h over
each selective hydrogenation catalyst to provide sufficient data
to fit initial and steady-state rate measurements at initial and
near-infinite times on stream. Initial and steady-state rates
represent modeled regressions of transient profiles from a two-
site model with better fitting results than a one-site model as
expected for complex materials with multiple active site
configurations. Reaction orders reflect rate measurements
after catalyst stabilization at H2 partial pressures between
0.76 and 2.99 kPa and butadiene partial pressures between
0.51 and 2.53 kPa, with reported rates reflecting the average of
1 h of data collection (3 data points, <5% deactivation during
kinetic measurements at each H2/BD ratio). Flow composi-
tions were returned to the initial reference condition after each
reaction order measurement to ensure the reclamation of
steady-state reference rates. Product quantification was
performed through an identical procedure to the ethanol
conversion setup prior to normalization by the methane
internal standard.
in face-centered cubic transition-metal nanoparticles51 and was
represented by a five-layer slab (3 × 3). Studied surfaces
include a Cu(111) surface, a Pt(111) surface, and an SAA Pt−
Cu(111) surface, the latter of which reflects the substitution of
a single Pt atom into the Cu(111) surface. A 20 Å vacuum was
added between the two neighboring successive slabs for all of
the simulated surfaces. A Monkhorst−Pack k-point mesh52 of 3
× 3 × 1 (400 eV cutoff energy) was used during structure
optimization, and the optimized structures were obtained when
changes in force were less than 0.02 eV Å−1. The most stable
adsorption configurations reflect the lowest-energy structures
from several adsorption configurations. Transition states for
each reaction step were determined based on combined
climbing image nudged elastic band (CI-NEB)53 and dimer
methods.54 The vibrational frequencies of adsorbed butadiene,
1-butene, cis-2-butene, and trans-2-butene were computed on
each surface to estimate the thermodynamic properties. Only
the adsorbates were allowed to relax to accelerate the
vibrational frequency calculations. The binding energies
(BEs) of reaction intermediates, including butadiene, H2, H
adatom, 1-butene, cis-2-butene, and trans-2-butene, were
calculated based on eq 1
2
4
*
BEA* = EA* − E − EA
(1)
Here, BEA* is the total energy of adsorbate species A, E* is the
total energy of the clean surfaces, and EA is the total energy of
gas A (e.g., butadiene, H2, 1-butene, cis-2-butene, and trans-2-
butene). For gaseous H adatoms, the EA was approximated as
half of the total energy of H2 gas.
3. RESULTS
3.1. Ethanol Upgrading over Bimetallic Zn−Y/Beta. X-
ray diffraction patterns and micropore volumes determined
from N2 adsorption isotherms (77 K) collected on zeolite
samples after metal incorporation are consistent with the Beta
topology (Figures S.1 and S.2 and Table S.1). Inductively
coupled plasma atomic emission spectroscopy (ICP-AES)
measurements show no detectable Al presence within the Beta
support (Si/Al > 1300) and therefore negligible Al Brønsted
acid site densities. Zn and Y ICP-AES measurements indicate
2.5. DFT Methods. All of the calculations were carried out
using the Vienna ab initio simulation package (VASP)46,47 with
the van der Waals interaction (DFT-D3 correction)48,49
included. Exchange−correlation energies were described
using the generalized gradient approximation (GGA) Per-
dew−Burke−Ernzerhof (PBE) functional.50 The closed-
packed (111) facet was generated to reflect a common facet
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Figure 2. XAS spectra collected on Zn- and Y-containing samples after pretreatment in flowing He to 673 K, including Zn K edge XANES (A) and
EXAFS (B, k2 weightings) spectra and Y K edge XANES (C) and EXAFS (D, k2 weightings).
bulk metal loadings of 2.0 and 7.3 wt %, respectively, on
monometallic samples and are similar for Zn−Y/β (1.7 and 7.0
wt % Zn and Y, respectively; Table S.1).
formation are used to compare Zn−Y/Beta and Zn/Beta.
Measured dehydrogenation rates over Zn−Y/Beta and Zn/
Beta are similar, normalized per gram or per mol of Zn (1.0 ×
10−2 and 1.1 × 10−2 mol dehydrogenated EtOH (mol Zn)−1
s−1, respectively), which is consistent with negligible ethanol
dehydrogenation reactivity over Y centers in Y/Beta (1.2 ×
10−5 mol EtOH (mol Y)−1 s−1). Butadiene formation rates
over Zn−Y/Beta are 13× higher than over Zn/Beta at similar
Zn loadings, suggesting the catalytic need for Y sites or Zn−Y
ensemble sites for catalyzing the aldol condensation, MPV
reduction, and dehydration reaction network that converts two
acetaldehyde molecules into butadiene.
To probe the local environment of Zn and Y centers, XAS
measurements and HAADF-STEM analyses were performed
on the Zn- and Y-containing zeolite samples. Figure 2 shows X-
ray absorption near-edge structure (XANES) and extended X-
ray absorption fine structure (EXAFS) spectra of the fresh Zn-
and Y-containing samples after dehydration at 673 K (EXAFS
fits and Supporting Information in Tables S.2 and S.3 and
Figure 1 shows ethanol conversion rates measured over Zn−
Y/Beta, Zn/Beta, and Y/Beta catalysts (11.1 kPa EtOH in
balance Ar, 163 kPa total pressure, 543 K, weight hourly space
velocity (WHSV) = 15.9 g EtOH (g catalyst)−1 h−1 over Zn/
Beta and Zn−Y/Beta and WHSV = 8.0 g EtOH (g catalyst)−1
h−1 over Y/Beta, <5% conversion) to probe the catalytic
necessity of both Zn and Y species. Zn/Beta predominantly
forms acetaldehyde, a key reactant for aldol condensation into
crotonaldehyde and subsequent butadiene formation (Scheme
1). Y centers within Y/Beta yield ethylene and diethyl ether
predominantly, indicating negligible dehydrogenation reactiv-
ity under these conditions and corroborating previous
observations at 623 K.19 As the incorporation of both Zn
and Y centers produces quantifiable amounts of butadiene and
butenes over Zn−Y/Beta, apparent ethanol dehydrogenation
rates reflecting combined acetaldehyde, butadiene, and butene
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Figures S.3−S.6). At the Zn K edge, the XANES edge energies
of Zn/Beta (9.6639 keV) and Zn−Y/Beta (9.6642 keV) are
close to that of ZnO (9.6633 keV), indicating predominantly
Zn2+ centers in all three materials. The Zn K edge EXAFS
spectrum (Figure 2B) of ZnO shows two primary peaks
centered at 1.6 and 2.9 Å (phase-uncorrected distance)
reflecting Zn−O first-shell scattering and Zn−Zn second-
shell scattering, respectively. A Zn−O scattering peak centered
at 1.5 Å (phase-uncorrected distance) is observed in the
EXAFS spectra of Zn/Beta and Zn−Y/Beta. Fitting gives
coordination numbers close to 4 at a similar distance to the
Zn−O bond in ZnO. Zn−Zn scattering is not observed in the
EXAFS of Zn/Beta or Zn−Y/Beta, indicating that Zn centers
are highly dispersed on the Beta supports and that large Zn
oxide nanoparticles are absent.
At the Y K edge, the XANES edge energies of Y/Beta
(17.0402 keV), Zn−Y/Beta (17.0399 keV), and Y2O3
(17.0402 keV) are all similar, indicating Y in the 3+ oxidation
state on all three materials. The EXAFS spectrum (Figure 2D)
of Y2O3 shows scattering from Y−O at 1.8 Å (phase-
uncorrected distance) as well as two prominent peaks from
2.6 to 4.2 Å (phase-uncorrected distance), reflecting single and
multiple scatterings from Y in the second and third
coordination spheres. In contrast, Y/Beta and Zn−Y/Beta
have a Y−O peak with much lower intensity and a small
second-shell peak at a shorter distance compared to Y2O3.
Fitting the first shell gave a coordination number of close to 4
for both catalysts with a distorted environment having two
short and two long Y−O bonds. In contrast, Y2O3 is octahedral
as reflected in a much higher Y−O peak amplitude (Figure
2D). The lack of higher Y shell peaks on Y/Beta and Zn−Y/
Beta indicates the absence of significant YOx nanoparticles
occluded on the zeolite domains despite the relatively high Y
loading. HAADF-STEM images of Zn−Y/Beta (Figure S.7)
support the conclusions from EXAFS and show no discernible
Zn or Y oxide nanoparticles, suggesting the highly dispersed
nature of both Zn and Y on the zeolite support. Taken
together, monometallic Zn/Beta and Y/Beta and bimetallic
Zn−Y/Beta each consist of predominately isolated Zn2+ and
Y3+ centers on the Beta zeolite support.
The Lewis or Brønsted acidic properties of the dispersed Zn
and Y sites were probed using transmission IR. Figure 3
displays transmission IR spectra collected after pyridine
saturation of Zn−Y/Beta, Zn/Beta, Y/Beta, and deAl-Beta
materials. The spectrum collected after pyridine adsorption
onto the deAl-Beta parent material shows only minor peaks,
while adsorption onto Zn/Beta and Y/Beta gives rise to peaks
at 1491 and 1575 cm−1 and two distinct peaks each (1452 and
1612 cm−1 for Zn, 1445 and 1606 cm−1 for Y), reflecting
pyridine adsorbed onto Zn and Y sites, respectively. The four
distinct peaks match peaks for pyridine adsorption onto ZnO55
and Y2O3,56 reflecting pyridine adsorbed onto Lewis acid sites.
These peaks centered around 1450 and 1610 cm−1 and the
shared peak centered at 1491 cm−1 reflect perturbed pyridine
deformation modes upon adsorption onto Lewis acid sites,57
while the peak centered at 1575 cm−1 corresponds to
hydrogen-bonded pyridine.58 Peaks reflecting pyridine adsorp-
tion onto Zn sites in Zn/Beta match previous observations of
pyridine-bound Lewis acidic Zn species within the Beta zeolite
framework59 and, by extension, the peaks reflecting pyridine
adsorption onto Y sites in Y/Beta correspond to Lewis acidic Y
sites, as recently suggested elsewhere.60 The peaks centered at
1545 and 1637 cm−1 observed on Y-containing sites are
Figure 3. Transmission IR spectra on deAl-Beta and Zn- and Y-
containing Beta samples after pyridine saturation at 393 K.
reminiscent of previously assigned peaks for protonated
pyridine species61 derived from interactions with Brønsted
acid sites. As these peaks reflecting protonated pyridine are
absent from the deAl-Beta parent spectra, these peaks reflect
the presence of pyridine-bound Brønsted acidic Y sites on both
Y/Beta and Zn−Y/Beta. This would be consistent with Y3+
centers embedded within the zeolitic framework that require a
nearby proton for charge balance. The spectrum collected on
the Zn−Y/Beta sample after pyridine saturation has all six
distinct peaks indicative of pyridine adsorbed onto Lewis acidic
Zn and Y species and Brønsted acidic Y sites as expected for a
sample containing both metals, yet the quantification of
distinct Zn and Y active site densities is not possible due to the
lack of integrated molar extinction coefficients (integrated peak
area ratios and total acid site densities from NH3−TPD
measurements are given in Table S.4 with NH3−TPD spectra
shown in Figure S.8). However, taking these observations
together with the XAS analyses, the Zn- and Y-containing Beta
zeolites studied here are composed of predominantly site-
isolated metal centers despite high metal loadings and consist
of Lewis acidic Zn sites and both Lewis and Brønsted acidic Y
sites, respectively. Notably, Brønsted acidic Y sites have not
been previously reported for Y sites within a zeolitic
framework.
This combination of Zn and Y sites on the Beta support is
capable of catalyzing ethanol valorization to butadiene and
butenes even in low conversion rate measurements (Figure 1),
suggesting its catalytic ETB potential at more industrially
relevant conversions. High conversion measurements from
pseudo-steady-state ethanol conversion over Zn−Y/Beta (3.1
kPa EtOH, balance He, 588 K, WHSV = 0.43 h−1) show 87%
ethanol conversion with 60 and 14% selectivities to butadiene
and butene isomers (1-butene, cis-2-butene, and trans-2-
butene), respectively, along with smaller concentrations (3−
8%) of ethylene, propene, acetaldehyde, diethyl ether, and C5+
olefins (product distribution and selectivity given in Figures
S.9 and S.10, respectively). Given the high butadiene
selectivity, the possibility of using Zn−Y/Beta as a single
ETO catalyst for both ETB and subsequent butadiene selective
hydrogenation to butene isomers was investigated by cofeeding
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hydrogen. Hydrogen cofeeding with ethanol (0−98.3 kPa H2,
3.1 kPa EtOH in balance He) does not show discernible
differences in either the overall ethanol conversion or the
associated product distribution even when the carrier gas is
solely composed of hydrogen (Figure S.9). These high
conversion measurements suggest that Zn−Y/Beta alone
cannot efficiently catalyze the full ETO pathway (Scheme 1)
regardless of hydrogen partial pressure, necessitating additional
active sites for butadiene selective hydrogenation. These
selective hydrogenation active sites must catalyze selective
hydrogenation events at hydrogen and butadiene molar ratios,
reflecting gas compositions after interactions with Zn−Y/Beta
(∼1−65 H2/BD ratio) and ideally maintaining high selective
hydrogenation reactivities at stoichiometric H2/BD ratios, as
discussed next using SAA Pt−Cu-supported metal catalysts.
3.2. Multifunctional Catalyst Systems for ETO.
Supported metal catalysts consisting of dilute solid solutions
of precious metals (e.g., Pt, Pd) within larger domains of
transition metal centers (e.g., Cu) have catalytically relevant
reactivities for both low-temperature (<423 K) hydrogena-
tion37,44 and high-temperature (>573 K) dehydrogenation
reactions,62 suggesting that these materials may be relevant
butadiene selective hydrogenation catalysts at increased
reaction temperatures (>573 K). The selective hydrogenation
capabilities of these materials at ETB temperatures (>573 K)
are currently unknown though, and potential reaction concerns
include butane formation through complete butadiene hydro-
genation, minimal hydrogen coverages due to rapid hydrogen
desorption at higher temperatures, and limited reactivity at low
or stoichiometric H2/BD ratios. To probe the usefulness of
SAA Pt−Cu materials as butadiene selective hydrogenation
catalysts within the proposed ETO reaction network (Scheme
1), hydrogen scission barriers, adsorption energies of relevant
butadiene, olefin, and hydrogen surface species, and metal d-
band centers were calculated from DFT over Cu(111),
Pt(111), and Pt−Cu ensemble sites (reflecting a single Pt
site replacement into a Cu(111) surface) and are listed in
Table 1 (representative DFT images in Figure S.11).
over Pt centers in SAA materials are energetically stabilized to
similar extents on both Pt and Cu centers and, given low
diffusion barriers to and from neighboring Cu sites, suggest the
ready generation of hydrogen islands37,63 for utilization during
hydrogenation reactions. These minimal hydrogen scission
barriers are similar (0.05−0.06 eV) on Pt centers in Pt(111)
and Pt−Cu(111) ensemble sites, indicating that Pt centers can
efficiently catalyze the formation of hydrogen adatom species
necessary for most hydrogenation reaction mechanisms
regardless of Pt or Cu neighbors.
Both butadiene and butene isomer adsorption energies
decrease with decreasing d-band centers on all three
investigated surfaces, as expected for olefin adsorption onto
transition metals.64 Butadiene adsorption is stronger on all
catalytic surfaces (by −0.3 to −1.1 eV) than investigated
butene surface species, presumably due to favorable binding
interactions for both CC bonds as butadiene adsorbs flat
across multiple surface atoms in the absence of other nearby
bound molecules (Figure S.11). This adsorption configuration
has previously been observed both computationally44 and
through sum frequency generation vibrational spectroscopy
measurements.65 Butadiene adsorption modes oppose those of
butene isomers composed of one CC bond coordinated to
surface sites and alkyl chain(s) raised off the surface, effectively
yielding decreased butene adsorption strengths relative to
butadiene. Adsorption energies indicate more stable adsorption
on Pt(111) sites relative to Cu(111) for butadiene, butene
isomers, and hydrogen adatoms and are much more
pronounced for butadiene (∼−1.7 eV) and butene isomers
(∼−1.1 eV), indicating Pt centers as preferential olefin binding
sites. These significantly stronger adsorption interactions may
interfere with hydrogen adsorption and scission events over
Pt(111) centers despite having minimal hydrogen scission
barriers (0.05 eV) relative to those over Cu(111) (0.34 eV).
However, Pt sites within the SAA Pt−Cu(111) surface bind
butadiene more strongly (by ∼−0.4 eV) relative to Cu(111)
but more weakly (by ∼1.4 eV) relative to Pt(111) without
significant changes in hydrogen adsorption strengths and
scission barriers, potentially balancing butadiene adsorption
with hydrogen adatom formation steps. Taking together the
adsorption energies and hydrogen scission barriers, Pt−Cu
ensemble sites preferentially adsorb butadiene and butene
isomers relative to hydrogen intermediates due to the presence
of Pt, limit butadiene adsorption strength through weaker
interactions with neighboring Cu atoms relative to Pt
neighbors seen on Pt(111), and maintain minimal hydrogen
scission barriers, suggesting that Pt−Cu SAA materials may be
relevant catalyst candidates for high-temperature (588 K)
butadiene selective hydrogenation.
Adsorption energies for all species investigated here are
strongest on Pt(111) surfaces and weakest on Cu(111)
surfaces. Similar hydrogen adatom adsorption energies onto
Cu(111) and SAA Pt−Cu(111) surfaces indicate that
hydrogen adatoms formed through hydrogen scission reactions
Table 1. DFT-Calculated Binding Energies (0 K) for
Butadiene, Hydrogen, and Butene Isomers onto Pt(111),
Cu(111), and SAA Pt−Cu(111) Dilute Solid Solution
Surfaces, Hydrogen Scission Barriers, and Pt and Cu d-Band
Centers
Guided by these computational observations, SAA
0.1Pt6Cu/Al2O3 and related supported metal catalysts
containing Pt or Cu alone (0.1Pt/Al2O3, 2Pt/Al2O3, and
6Cu/Al2O3) were synthesized via incipient wetness impregna-
tion (IWI) of metal nitrate precursors onto a γ-alumina
support for use as catalyst candidates for butadiene selective
hydrogenation at relevant ETO conditions. ICP-AES results
indicate bulk metal compositions that match nominal IWI
solution concentrations on both monometallic materials (0.1%
Pt, 1.7% Pt, and 5.5% Cu on 0.1Pt/Al2O3, 2Pt/Al2O3, and
6Cu/Al2O3, respectively) and bimetallic 0.1Pt6Cu/Al2O3 (0.1
and 5.9 wt % Pt and Cu, respectively). This equates to a 180:1
Cu/Pt molar ratio on 0.1Pt6Cu/Al2O3, indicating the potential
for dilute solid solutions of Pt within Cu domains. HAADF-
Compound
Cu(111) Pt(111) SAA Pt−Cu(111)
Binding
butadiene
H2
Hadatom
1-butene
cis-2-butene
trans-2-butene
−1.27
−0.06
−0.35
−0.80
−0.74
−0.67
0.34
−2.93
−0.09
−0.58
−1.87
−1.82
−1.80
0.05
−1.52
−0.06
−0.38
−1.21
−1.17
−1.13
0.06
Energies (eV)
H2 Scission
Barriers (eV)
Cu d-band
Centers (eV)
Pt d-band
Centers (eV)
−2.23
−2.22
−2.06
−2.03
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Figure 4. XANES and EXAFS spectra including Pt L3 edge XANES (A) and EXAFS (B, k2 weightings) spectra and Cu K edge XANES (C) and
EXAFS (D, k2 weightings) collected on Pt- and Cu-containing supported metal samples. Samples were pretreated at 673 K (5 K min−1) in flowing
H2 and then cooled to room temperature in He prior to collecting XAS spectra.
STEM measurements show primarily isolated Pt centers
scattered throughout Cu domains, but Pt nanoparticles are
not observed (Figure S.12), suggesting the intimate mixing of
Pt and Cu species. In contrast, HAADF-STEM images of
0.1Pt/Al2O3 and 2Pt/Al2O3 show both isolated Pt centers and
Pt nanoparticles with higher Pt loadings correlating with
increased densities of Pt nanoparticles. HAADF-STEM images
of 0.1Pt6Cu/Al2O3 and 6Cu/Al2O3 suggest similar particle size
distributions with average particle sizes of 1−3 nm (Figure
S.12).
XAS measurements were performed at the Pt L3 and Cu K
edges to characterize the local structures of Pt and Cu in these
materials, as seen in Figure 4. XANES edge energies,
coordination numbers, and EXAFS fitting parameters for Pt-
and Cu-containing materials are listed in Tables S.5 and S.6
and Figures S.13−S.17. XANES spectra collected at the Pt L3
edge on 0.1Pt6Cu/Al2O3 after reduction at 673 K show a 0.4
eV edge energy increase over Pt foil and a concomitant
decrease in white line intensity, consistent with platinum
alloys. The corresponding EXAFS spectrum collected on
0.1Pt6Cu/Al2O3 shows only a single R space peak, contrasting
the 3 R space peaks of the Pt foil reflecting first-shell Pt−Pt
scattering. Based on the STEM analysis and the catalyst
composition, the spectrum was modeled using a Pt−Cu
scattering path. Fitting gives a coordination number of 8 at a
bond distance of 2.56 Å (Table S.6). This result shows that the
local environment of platinum in 0.1Pt6Cu/Al2O3 consists
exclusively of copper. Additionally, the Pt−Cu bond distance is
equal to that of the Cu−Cu bond distance in pure copper,
which would be expected in a dilute solid solution alloy
following Vegard’s law. EXAFS fits of the spectra collected on
0.1Pt/Al2O3 and 2Pt/Al2O3 also have unfilled first-coordina-
tion spheres with 7.7 and 6.9 Pt neighbors, respectively.
Consistent with the small particle sizes suggested by the
coordination numbers, the Pt−Pt bond distances in 0.1Pt/
Al2O3 and 2Pt/Al2O3 are contracted to 2.70 and 2.68 Å,
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Figure 5. (A−D) Ethanol upgrading product distributions obtained through the two-bed catalytic system composed of Zn−Y/Beta followed by Pt-
and/or Cu-containing supported metal catalysts as a function of cofed hydrogen partial pressure at the reactor inlet (3.1 kPa EtOH, 0 (∼1 H2/BD
ratio), 17.2 (∼12 H2/BD), 50.7 (∼34 H2/BD), or 98.3 (∼65 H2/BD) kPa H2 in balance He, 588 K, 0.26 h−1 WHSV with respect to the two beds,
0.43 h−1 with respect to Zn−Y/Beta alone). Product distributions measured over 0.1Pt6Cu/Al2O3-PM reflect a physical mixture of Zn−Y/Beta and
0.1Pt6Cu/Al2O3 into a single catalyst bed. Residual product distribution percentages reflect the remaining ethanol at a sub-100% conversion. Errors
in product distributions are 15% under these conditions. (E−H) Corresponding differences in observed product yields as a function of secondary
catalyst selection at each cofed hydrogen partial pressure. Differences reflect product yield percentages (normalized by ethanol conversion)
between each two-bed catalyst setup and the corresponding single-bed product stream over Zn−Y/Beta alone.
respectively, whereas bulk Pt has a Pt−Pt bond distance of
2.77 Å. EXAFS fits (Figures S.16 and S.17) require Pt−O
scattering paths on both 0.1Pt/Al2O3 and 2Pt/Al2O3 (1.99 and
1.97 Å, respectively), indicating a subset of nonreduced Pt
sites. Pt−O scattering in the EXAFS is reflected in the XANES
as an increase in the edge energy and white line intensity
relative to the Pt foil (0.2−0.5 eV) and is suspected to
correlate with isolated Pt sites observed in STEM images
(Figure S.12) that cannot be readily reduced under the
conditions studied here.
XAS measurements collected at the Cu K edge on
0.1Pt6Cu/Al2O3 and 6Cu/Al2O3 describe the local environ-
ment of Cu centers within nanoparticles. Fits of single-shell
Cu−Cu scattering paths indicate coordination numbers of 7.3
and 7.2 on 0.1Pt6Cu/Al2O3 and 6Cu/Al2O3, respectively, and
equivalent Cu−Cu bond lengths. This suggests similar metal
nanoparticle sizes between the two samples and, by extension,
similar bulk surface Cu site distributions. Coordination
numbers for Cu (7.3) and Pt (8) on 0.1Pt6Cu/Al2O3 are
also similar as expected for metal centers evenly distributed
throughout occluded nanoparticles. These observations from
STEM and XAS suggest the formation of alloyed Pt−Cu
nanoparticles with single-site Pt centers distributed within the
Cu domains.
The catalytic capabilities of the SAA 0.1Pt6Cu/Al2O3
catalyst and the monometallic 0.1Pt/Al2O3, 2Pt/Al2O3, and
6Cu/Al2O3 control materials were studied in conjunction with
Zn−Y/Beta for catalyzing the complete ETO reaction network
using a layered two-bed setup (Scheme 2), a strategy
commonly used to develop multifunctional catalysts for a
variety of reactions such as the Fischer−Tropsch synthesis of
syngas to liquid fuels66 or light olefins.67 Figure 5 depicts
ethanol conversions and associated product distributions at 0,
17.2, 50.7, and 98.3 kPa of H2 (3.1 kPa EtOH in balance He,
588 K; more detailed product distributions and product
selectivities are shown in Figures S.18−S.29) over the two-bed
catalyst system composed of Zn−Y/Beta and a selective
hydrogenation catalyst. Figure 5E−H also depicts product
yield difference plots normalized by ethanol conversion at each
measured hydrogen partial pressure to highlight product
distribution differences between each selective hydrogenation
catalyst relative to Zn−Y/Beta alone. Here, 17.2 kPa of cofed
hydrogen reflects similar hydrogen partial pressures for
acetylene selective hydrogenation within steam-cracking
product streams39 and low-temperature (313−383 K)
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butadiene hydrogenation studies over Cu and SAA Pt−Cu
catalysts.37 Product distributions formed at 588 K are
composed of a mix of butadiene, butenes (1-butene, trans-2-
butene, cis-2-butene), C3,5+ olefins (predominantly propene,
pentenes, and hexenes), oxygenates (acetaldehyde and diethyl
ether), and other short-chain hydrocarbons (methane, ethane,
ethylene, and propane). Ethanol conversions are consistent
(above 80%) at all cofed hydrogen partial pressures and are
primarily due to interactions between ethanol and Zn−Y/Beta
(Figure S.9).
98.3 kPa hydrogen. Therefore, Pt centers within the Pt−Cu
domains are critical for butadiene selective hydrogenation at
low hydrogen and butadiene ratios characteristic of ETB outlet
streams (∼1.5 kPa each) and are primarily responsible for
complete butadiene conversion even at 17.2 kPa of cofed
hydrogen, catalyzing the final reaction steps of the ETO
pathway (Scheme 2).
The combination of the bimetallic Zn−Y/Beta composed of
at least three distinctive active site types with the bimetallic
SAA 0.1Pt6Cu/Al2O3 catalyst capable of catalyzing high-
temperature hydrogenation reaction at near-stoichiometric
hydrogen and butadiene partial pressures yields a butene-rich
C3+ olefin stream. These observations regarding the site and
structural properties of both catalysts, as well as the high
conversion product distributions, provide significant evidence
of the potential industrial viability of the ETO pathway,
yielding desirable olefin precursors for applications including
jet fuel production. To further probe the reactivities of these
hydrogenation catalysts, single-bed butadiene selective hydro-
genation kinetic measurements over Pt- and Cu-containing
catalysts are combined with DFT calculations to correlate the
unique catalytic behavior of alloyed Pt−Cu active site
ensembles with the observed product distributions in the
two-bed ETO measurements over 0.1Pt6Cu/Al2O3 at the near-
stoichiometric H2/BD ratio (Figure 5A).
3.3. Butadiene Selective Hydrogenation over Mono-
metallic and SAA Pt- and Cu-Containing Catalysts.
Butadiene hydrogenation rates obtained over SAA 0.1Pt6Cu/
Al2O3, 0.1Pt/Al2O3, 6Cu/Al2O3, and 2Pt/Al2O3 catalysts were
measured at similar hydrogen and butadiene partial pressures
to those expected under high conversion ETB operations. With
3.1 kPa ethanol, an ETB reaction pathway with complete
ethanol conversion and perfect butadiene formation selectivity
generates a maximum of 1.55 kPa each of hydrogen and
butadiene (Scheme 1). Steady-state butene formation rates per
gram of catalyst (588 K, 1.51 kPa H2, 1.51 kPa BD, 0.20 kPa
CH4, SVtotal = 100 cm3 s−1 (g catalyst)−1) over SAA 0.1Pt6Cu/
Al2O3 are approximately 20×, 3×, and 0.6× higher than rates
over 0.1Pt/Al2O3, 6Cu/Al2O3, and 2Pt/Al2O3, respectively
(Table S.7), while reaction rates measured on the γ-alumina
support alone are not detectable (<10−7 mol butenes (g
catalyst)−1 s−1). Therefore, butadiene selective hydrogenation
events are catalyzed by both Cu and Pt sites. These results are
consistent with the observations from ETO experiments
(Figure 5A), where significantly higher butene formation was
observed over SAA without hydrogen cofeeding when
compared with 0.1Pt/Al2O3 or 6Cu/Al2O3. Selective hydro-
genation rates (per gram catalyst) over 0.1Pt6Cu/Al2O3 are
slightly lower than those over 2Pt/Al2O3 (∼20× Pt loading)
under purer flow conditions established during single-bed
kinetic measurements but not during two-bed ETO reaction
measurements at stoichiometric H2/BD ratios. This may
suggest that the Cu-free Pt/Al2O3 catalysts suffer inhibition
caused by other compounds formed over the Zn−Y/Beta bed,
which includes water and other oxygenates (Scheme 2).
Steady-state reaction rates measured over 0.1Pt/Al2O3 or
6Cu/Al2O3 reflect minimum rates per mole of Pt or Cu,
respectively, to compare against rates measured over the SAA
catalyst. Here, dilute Pt incorporation within Cu domains
(∼180:1 mol Cu/mol Pt) is assumed to insignificantly affect
Cu active site densities due to similar Cu particle sizes and bulk
Cu coordination between 6Cu/Al2O3 and 0.1Pt6Cu/Al2O3
from Cu edge XAS measurements (Figure 4 and Table S.5).
Product distributions measured without cofed hydrogen at
588 K show an ∼3× higher butene yield over SAA 0.1Pt6Cu/
Al2O3 than over any other selective hydrogenation catalysts
without observable butane formation (Figure 5A). Other Pt-
and Cu-containing catalysts display minimal differences in
butadiene and butene yields under identical conditions,
indicating that Pt or Cu species alone possess insufficient
selective hydrogenation reactivity without hydrogen cofeeding
at ETO temperatures. Therefore, increased butadiene selective
hydrogenation over SAA 0.1Pt6Cu/Al2O3 reflects increased
reactivity catalyzed by alloyed Pt−Cu ensemble sites relative to
Pt or Cu sites alone, which cannot be easily overcome by
increasing the bulk Pt loading (by 20× on 2Pt/Al2O3). This
increased reactivity is accomplished at stoichiometric hydrogen
concentrations generated in situ from ethanol dehydrogen-
ation, indicating the SAA Pt−Cu catalyst’s selective hydro-
genation efficacy for the targeted ETO pathway (Scheme 2)
relative to other Pt- or Cu-containing catalysts studied here.
Further conversion of butadiene to butene isomers was
investigated by cofeeding hydrogen. At all three cofed
hydrogen partial pressures studied here (17.2, 50.7, and 98.3
kPa), no residual butadiene is observed over any Pt-containing
catalysts. Increased butene and subsequent butane yields are
derived from butadiene converted over Pt-containing catalysts
with saturated butane formation becoming more pronounced
with increasing hydrogen partial pressure. Increased saturated
butane yield with increasing cofed hydrogen partial pressure
occurs simultaneously with decreased ethylene and propene
selectivities (Figures S.26−S.29) and increased C1−C3 paraffin
formation, indicating that the observed hydrogenation
reactivity over Pt centers is indiscriminate over the range of
short-chain olefin compounds as expected over Pt-containing
catalysts. Decreased butane and C1−C3 paraffin formation are
observed over 0.1Pt6Cu/Al2O3 relative to 2Pt/Al2O3 (2.3×)
even under a 98.3 kPa hydrogen flow, potentially reflecting the
decreased butene adsorption strength onto Pt−Cu ensemble
sites relative to Pt centers, as indicated in DFT calculations
(Table 1). Complete hydrogenation of butadiene to saturated
hydrocarbons reduces the desired C3+ olefin yield, indicating
the need for minimal or no external hydrogen cofeeding (near-
stoichiometric H2/BD molar ratio) at ETO reaction temper-
atures for maximum C3+ olefin recovery.
The role of Pt−Cu ensemble sites can be further
investigated by comparing product distributions measured
over the SAA 0.1Pt6Cu/Al2O3 and 6Cu/Al2O3 catalysts. At all
hydrogen partial pressures, product distributions measured
over 6Cu/Al2O3 show no butane formation, while minimal
changes in butene formation are observed at low hydrogen
partial pressures (0−17.2 kPa) relative to Zn−Y/Beta alone.
Residual butadiene conversion over 6Cu/Al2O3 increases
directly with hydrogen partial pressure to form butene isomers
at high hydrogen partial pressures (50.7−98.3 kPa), but
complete butadiene conversion is still not observed even at
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Figure 6. Steady-state combined butene formation rates (588 K, 100 cm3 s−1 (g catalyst)−1) collected over 0.1Pt6Cu/Al2O3 (blue), 6Cu/Al2O3
(orange), 0.1Pt/Al2O3 (black), and 2Pt/Al2O3 (red) as a function of (A) butadiene partial pressure and (B) hydrogen partial pressure. Dashed lines
reflect linear regression best fits.
The linear combination of steady-state rates of the 0.1Pt/Al2O3
and 6Cu/Al2O3 catalysts remains significantly lower than the
rate measured on 0.1Pt6Cu/Al2O3 (Table S.7), indicating that
Pt−Cu ensemble sites have increased butadiene selective
hydrogenation reactivity relative to Cu domains or Pt sites
alone at low hydrogen partial pressures.
The larger apparent butadiene reaction order over 6Cu/Al2O3
relative to Pt-containing catalysts would then reflect a lower
coverage on active Cu sites due to 0.25−1.66 eV less favorable
butadiene binding enthalpies relative to Pt sites (Table 1).
Further, Pt sites embedded within Cu(111) surfaces or
Pt(111) also display increased butene adsorption enthalpies
relative to Cu sites alone, providing energetically favorable
binding sites for butene adsorption and subsequent hydro-
genation. The resulting butane formation would increase with
hydrogen partial pressure as observed during ETO experiments
(>17.2 kPa; Figure 5B−D) over Pt-containing catalysts.
The kinetic implications of Pt−Cu ensemble sites can be
evaluated by comparing the SAA catalyst with the mono-
metallic analogues using apparent reaction orders and DFT-
calculated adsorption enthalpies, hydrogen scission barriers,
and energy barriers through a commonly utilized Horiuti−
Polanyi reaction mechanism for selective hydrogenation.44
Figure 6 shows steady-state butene formation rates as a
function of butadiene and hydrogen partial pressures for the
four hydrogenation catalysts studied here (isomer-specific
results are given in Figure S.30 and Table S.8). Butene
formation rates over both 0.1Pt6Cu/Al2O3 and 0.1Pt/Al2O3
follow a nearly zero-order dependence on butadiene partial
pressure (0.1 and 0.3, respectively), while 2Pt/Al2O3 has a
lower yet still approximately zero-order dependence (−0.2)
and 6Cu/Al2O3 has a significantly higher apparent order (0.7).
Assuming that butadiene selective hydrogenation follows a
Langmuir−Hinshelwood kinetic model over these catalysts as
previously modeled over other butadiene selective hydro-
genation catalysts (e.g., Pt(111), Pt2Sn/Pt(111),68 and Pd/
Al2O369), differences in apparent butadiene reaction orders
most likely reflect different coverages of reactive intermediate
species on each catalyst surface. Apparent butadiene reaction
orders decrease as calculated butadiene adsorption energies
become more negative (Table 1). These adsorption energies
indicate that bound butadiene species on kinetically relevant
active sites are adsorbed more strongly than hydrogen- or
butene-bound surface species and presumably yield higher
coverages. Butadiene-covered active sites would corroborate
the apparent zero-order butadiene dependence on Pt-
containing catalysts where butadiene adsorption is energeti-
cally favorable enough to completely saturate kinetically
relevant active sites and even becomes inhibitory as seen
through the negative apparent reaction order on 2Pt/Al2O3.
Apparent hydrogen reaction orders are approximately first-
order over 0.1Pt6Cu/Al2O3 (1.0), 0.1Pt/Al2O3 (0.9), and
6Cu/Al2O3 (0.8) and considerably higher (1.6) over 2Pt/
Al2O3. These apparent reaction orders and associated
increased butadiene selective hydrogenation rates with
increasing hydrogen partial pressure are consistent with the
increased butene and butane yields observed in the two-bed
system product distributions (Figures 5A−D and S.19−S.22),
the largest of which is observed over 2Pt/Al2O3. These
apparent hydrogen reaction orders trend opposingly with
apparent butadiene reaction orders, which may reflect the
competitive adsorption of butadiene and hydrogen onto
kinetically relevant sites. Further, DFT calculations of Pt
centers embedded in both Cu and Pt nanoparticles show ∼0.3
eV lower hydrogen scission barriers than Cu sites alone,
indicating essentially barrierless formation of hydrogen
adatoms and matching previously reported observations over
Pt−Cu SAA materials.70 These lower hydrogen scission
barriers calculated over Pt sites then suggest the formation of
bound hydrogen adatom species on either Pt or neighboring
Cu centers as previously observed via scanning tunneling
microscopy on SAA Pt−Cu materials at 85 K.37 These
hydrogen adatom species would then be capable of facilitating
selective hydrogenation reactions with bound butadiene
species. Further, these Pt−Cu ensemble sites remain catalyti-
cally active significantly longer than isolated Pt sites or small Pt
nanoparticles on 0.1Pt/Al2O3 (Figure S.31) while limiting
inhibitory butadiene interactions seen through the negative
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apparent reaction order (and subsequently the higher hydro-
gen order) over 2Pt/Al2O3.
Taking the apparent first-order hydrogen reaction orders
and nearly zero-order apparent butadiene reaction orders over
0.1Pt6Cu/Al2O3 and 0.1Pt/Al2O3, a Horiuti−Polanyi reaction
mechanism can be proposed starting from a bound butadiene
intermediate to form 1-butene and trans-2-butene, matching
previous assertions.44,71,72 A detailed description is provided in
Section S.2.5 and Scheme S.1. Figure 7 shows DFT-calculated
energies throughout the investigated reaction mechanism
(Scheme S.1) over Cu(111), Pt(111), and SAA Pt−Cu(111)
surfaces reflecting 6Cu/Al2O3, 2Pt/Al2O3, and 0.1Pt6Cu/
Al2O3 catalysts, respectively62 (calculated activation energies
and energy differences between elementary steps are listed in
Table S.9, and butadiene hydrogenation pathways to butane
through diradical intermediates (I3 and I4, Scheme S.1) are
shown in Figure S.32). The addition of two hydrogen adatoms
yields the formation of either butene isomers or two reduced
intermediate species (I3 and I4), the latter of which possesses
significantly higher energies than 1-butene or trans-2-butene
and reflects significantly less stable intermediates due to the
absence of conjugated bonds. These intermediates must then
undergo further hydrogenation to butane prior to desorption
from catalytic surfaces or isomerize into more stable butene
configurations, the latter of which may consist of significant
activation energy barriers that were not investigated here.
After 1-butene or trans-2-butene formation, olefin selectivity
for butenes relative to butane can be assessed by comparing
butene desorption energies with butene hydrogenation
activation energy barriers. Over Cu(111) and SAA Pt−
Cu(111) surfaces, butene isomer desorption energies are
lower than calculated activation energy barriers to form butane
(Table S.9), indicating a preference for butene desorption and
concomitant termination of the hydrogenation reaction. This
difference between desorption energy and butene hydro-
genation activation energy is larger over Cu(111) than over
SAA Pt−Cu(111). This corroborates observed product
distributions from two-bed measurements over 6Cu/Al2O3
and 0.1Pt6Cu/Al2O3, where negligible butane formation is
observed over both catalysts at near-stoichiometric H2/BD
ratios but is never observed over 6Cu/Al2O3 at all studied
hydrogen partial pressures (Figure 5). The limited difference
between butene desorption energy and hydrogenation
activation energy over SAA Pt−Cu(111) further corroborates
selective butene formation at the near-stoichiometric hydrogen
partial pressures in single- or two-bed measurements and could
explain the decreased butene selectivity with increasing
hydrogen partial pressures given the first-order apparent
reaction order with respect to hydrogen partial pressure from
experimental measurements (Figure 6). These observations,
both experimental and computational, are inconsistent with
previously reported calculations indicating that selective
hydrogenation reactivities are similar over Cu(111) and Pt−
Cu(111) surfaces when accounting for relative surface
concentrations of Cu and Pt species.44 Similar computational
observations over Pt(111) indicate that butene hydrogenation
activation energy barriers are lower than butene desorption
energies (Table S.9). This suggests that hydrocarbon
selectivities should heavily favor butane formation over
Pt(111), as seen at all cofed hydrogen partial pressures (Figure
5). The above observations on Pt(111) and Cu(111) reflect
the potentially highest butane and butene selectivities,
respectively, while the reaction energy profile over Pt−
Figure 7. Reaction coordinate diagrams reflecting butadiene selective
hydrogenation pathways to form 1-butene and trans-2-butene and
subsequent butene hydrogenation pathways to form butane over (A)
Cu(111), (B) Pt(111), and (C) Pt−Cu(111) surfaces. Distinct
pathways are depicted here using the designations presented in
Scheme S.1: butadiene → I2 → 1-butene → I6 → butane (black),
butadiene → I1 → trans-2-butene → I5 → butane (blue), and
butadiene → I1 → 1-butene → I5 → butane (red). Diagonal dashed
lines reflect desorption energies for bound butenes. Energy differences
and activation energies are listed in Table S.9.
Cu(111) contains Pt−Cu active site ensembles possessing
both higher hydrogenation reactivity than Cu sites alone and
energetic barriers that favor butene desorption over subsequent
butene hydrogenation to form butane.
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The energy differences between butene desorption and
hydrogenation phenomena should be treated generally for
comparing these Pt- and Cu-containing catalysts. Increased
temperatures will destabilize adsorbed species and decrease
desorption energies through entropic contributions, while
activation energies will generally be less affected by temper-
atures for consistent most abundant surface intermediates and
kinetically relevant transition states. Therefore, energy differ-
ences are maximized at 0 K and will decrease with increasing
temperatures, indicating that the selectivity toward butene
formation without overhydrogenation to butane should be
more pronounced at increased temperatures. The observed
trend favoring butene desorption relative to hydrogenation
over Cu(111), Pt−Cu(111), and Pt(111) is significantly more
obvious with binding energies calculated at reaction temper-
atures (Table S.9) due to destabilized bound butene
intermediates (>1 eV higher binding energies). This trend
indicates that the butadiene hydrogenation reaction network
should favor butene desorption over further hydrogenation to
butane in order of Cu(111), Pt−Cu(111), and Pt(111) with
Pt(111) favoring further hydrogenation. This trend should
hold at all hydrogen partial pressures as observed in product
distributions from two-bed experiments but should shift
toward complete butadiene hydrogenation given high hydro-
gen partial pressures.
Taking the experimental and computational findings
together, overall butene formation rates over the Pt−Cu SAA
catalyst are higher than those measured over 6Cu/Al2O3 or
0.1Pt/Al2O3 at stoichiometric butadiene and hydrogen partial
pressures and are similar to rates over 2Pt/Al2O3 yet with
significantly lower Pt metal concentrations. Increased butene
formation rates relative to 6Cu/Al2O3 indicate that incorpo-
rated Pt centers within Cu domains either generate additional
kinetically relevant active sites or shift the overall selective
hydrogenation reaction mechanism with an associated higher
reactivity. The former may occur by promoting nearby Cu
atoms through Cu−H adatom formation due to negligible
hydrogen scission barriers over Pt sites. These same Pt sites
generate energetically favorable butadiene or butene binding
sites while stabilizing butadiene to a far greater extent than
butene isomers. In either case, the incorporation of Pt centers
into Cu nanoparticle domains within the SAA catalytic material
generates active site ensembles capable of catalyzing selective
hydrogenation reactions at low relative hydrogen partial
pressures with rates that surpass those on monometallic Pt
or Cu catalysts. This combination of DFT calculations and
apparent kinetic measurements supports the selection of SAA
Pt−Cu materials as a critical catalytic component for the final
ETO reaction steps, namely, the selective hydrogenation of
butadiene at stoichiometric hydrogen and butadiene partial
pressures (H2/BD = 1). While Zn−Y/Beta containing
Brønsted acidic Y sites and Lewis acidic Zn and Y sites
catalyzes ETB conversion, the selective hydrogenation of
butadiene at low hydrogen partial pressures requires Pt−Cu
ensemble sites at stoichiometric hydrogen and butadiene
partial pressures and ETB-relevant reaction temperatures.
However, the applicability of this ETO pathway (achieved
over the two-bed composite catalyst of Zn−Y/Beta and SAA
0.1Pt6Cu/Al2O3) must be further demonstrated via optimizing
the selectivity of butene-rich C3+ olefins.
0.1Pt6Cu/Al2O3 catalyst systems, many opportunities exist to
maximize butene and C3+ olefin selectivities at high ethanol
conversions (e.g., tuning active site distributions, space
velocities, and reaction temperatures). Since optimizing
product distributions toward specific olefins is not the primary
goal of this work as noted by the singular elemental
composition of the multifunctional catalyst and the fixed
reaction conditions, only a couple of small steps are explored
here to enhance the ETO pathway viability. As shown in
Figure 5A, ∼20% of the observed product distribution over the
composite Zn−Y/Beta and SAA 0.1Pt6Cu/Al2O3 catalyst
system (∼88% conversion) is butadiene, indicating that butene
yields can be increased simply by balancing butadiene
formation with selective hydrogenation reactivity. As expected,
increasing the catalyst loading of SAA 0.1Pt6Cu/Al2O3 to
equivalent catalyst loadings of Zn−Y/Beta and SAA 0.1Pt6Cu/
Al2O3 yields increased butadiene conversion over SAA and
selectivity to butenes (54%) and C3+ olefins (65%) at slightly
higher ethanol conversions (95%; Figure S.33).
Stoichiometric H2/BD ratios are necessary to avoid
overhydrogenation of butadiene to butane, as observed in
Figure 5 and DFT energy profile calculations (Figure 7).
Positive apparent hydrogen reaction orders over SAA
0.1Pt6Cu/Al2O3 indicate that selective hydrogenation reaction
rates can be further increased by increasing hydrogen partial
pressures. Increasing the ethanol partial pressure from 3.1 to
6.1 kPa over equivalent Zn−Y/Beta and SAA 0.1Pt6Cu/Al2O3
catalyst loadings maintains a consistent near-stoichiometric
H2/BD ratio while yielding significantly higher butene (65%)
and C3+ (78%) selectivities at a similar ethanol conversion
(94%; Figure S.34). However, these product selectivities
remain relatively unoptimized due to no optimization of active
site distributions or other reaction conditions responsible for
side product formation. Further, these results are expected to
hold even when Zn−Y/Beta and 0.1Pt6Cu/Al2O3 are
physically mixed into a single bed of composite catalyst
based on relatively small changes in nonoptimized product
distributions at all studied cofed hydrogen partial pressures
(Figure 5; 0.1Pt6Cu/Al2O3-PM).
Despite the need for further catalyst optimization, C3+ olefin
selectivities observed here at high ethanol conversions (>85%)
exceed other published ethanol valorization pathways (Figure
S.35 and Table S.11) such as ethanol to olefins via an ethylene
intermediate over Brønsted acid zeolites, Guerbet coupling of
ethanol and subsequent dehydration to olefin products, and
ethanol condensation to form isobutene or propene over oxide
materials (e.g., ZnZrOx)5. The utility of this two-bed catalytic
system is process intensification by performing cascade
reactions at a constant temperature and pressure despite the
presence of coproducts (e.g., water, oxygenates, ethylene).
Further, this two-bed composite Zn−Y/Beta and SAA
0.1Pt6Cu/Al2O3 catalyst setup can operate without cofed
hydrogen, a significant advantage from both process design and
economic perspectives.
4. CONCLUSIONS
Ethanol valorization to C3+ olefin (ETO) is a promising
reaction pathway for generating key oligomerization precursors
by leveraging butadiene as a critical intermediate. This ETO
pathway was investigated over a composite catalyst composed
of a bimetallic Zn−Y/Beta zeolite and SAA 0.1Pt6Cu/Al2O3-
supported metal catalyst, yielding a high C3+ olefin selectivity
(78% at 94% ethanol conversion, 588 K) without hydrogen
3.4. Preliminary ETO Reaction Pathway Optimization.
As minimal efforts have been taken here to tune olefin
distributions over the composite Zn−Y/Beta and SAA
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cofeeding. XAS measurements and HAADF-STEM images
identified the isolated nature of Zn and Y on the Beta zeolite
support despite the high Y loading (∼7 wt %) and isolated Pt
within Cu nanoparticles, while transmission IR spectra
collected after pyridine adsorption onto Zn−Y/Beta indicated
the presence of both Lewis acidic Zn and Y sites and previously
unreported Brønsted acidic Y centers, identifying various active
metal species to match with ETO kinetic measurements.
Bimetallic Zn−Y/Beta catalyzes the multistep formation of
butadiene from ethanol with the concomitant formation of
near-stoichiometric hydrogen partial pressures, while SAA
0.1Pt6Cu/Al2O3 selectively hydrogenates butadiene to form
butene products at this stoichiometric H2/BD ratio and ETB
reaction temperature over Pt−Cu ensemble sites. DFT
calculations indicate that increased selective hydrogenation
reactivity over Pt−Cu(111) surfaces combines low hydrogen
scission barriers and favorable butadiene binding energies with
preferential butene desorption energies relative to further
hydrogenation activation energies forming butane. Together,
this multifunctional catalyst enables direct bioethanol valor-
ization using only in situ-generated hydrogen to form butene-
rich C3+ olefin streams, which are readily oligomerized into
longer-chain olefin products necessary for renewable jet fuel
production.
Laboratory, Oak Ridge, Tennessee 37830, United States;
orcid.org/0000-0002-9870-1029
Evan C. Wegener − Chemical Sciences and Engineering
Division, Argonne National Laboratory, Lemont, Illinois
60439, United States
Kinga A. Unocic − Center for Nanophase Materials Sciences,
Oak Ridge National Laboratory, Oak Ridge, Tennessee
37830, United States
Lawrence F. Allard − Material Science and Technology
Division, Oak Ridge National Laboratory, Oak Ridge,
Tennessee 37830, United States
Mingxia Zhou − Materials Science Division, Argonne National
Laboratory, Lemont, Illinois 60439, United States;
orcid.org/0000-0002-1896-1024
Rajeev S. Assary − Materials Science Division, Argonne
National Laboratory, Lemont, Illinois 60439, United States;
orcid.org/0000-0002-9571-3307
Jeffrey T. Miller − Department of Chemical Engineering,
Purdue University, West Lafayette, Indiana 47907, United
States; orcid.org/0000-0002-6269-0620
Theodore R. Krause − Chemical Sciences and Engineering
Division, Argonne National Laboratory, Lemont, Illinois
60439, United States
Fan Lin − Institute for Integrated Catalysis, Pacific Northwest
National Laboratory, Richland, Washington 99354, United
States
Huamin Wang − Institute for Integrated Catalysis, Pacific
Northwest National Laboratory, Richland, Washington
99354, United States; orcid.org/0000-0002-3036-2649
A. Jeremy Kropf − Chemical Sciences and Engineering
Division, Argonne National Laboratory, Lemont, Illinois
60439, United States
Ce Yang − Chemical Sciences and Engineering Division,
Argonne National Laboratory, Lemont, Illinois 60439,
United States
Dongxia Liu − Department of Chemical and Biomolecular
Engineering, University of Maryland, College Park, Maryland
20742, United States; orcid.org/0000-0001-8712-2219
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c01136.
XRD patterns, N2 adsorption isotherms, XAS fitting
details and associated figures, STEM images, NH3−TPD
spectra, additional product distributions and selectivity
plots, DFT-calculated adsorption images, additional
plots of butene formation rates as a function of
butadiene and hydrogen partial pressures, transient
butadiene selective hydrogenation profiles, DFT-calcu-
lated energy differences and activation energy barriers,
additional reaction coordinate diagrams, product dis-
tribution plots of preliminary optimization strategies,
and comparison of presented ETO pathway relative to
literature observations (PDF)
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c01136
Author Contributions
M.J.C. and J.Z. contributed equally. The manuscript was
written through contributions of all authors. All authors have
given approval to the final version of the manuscript.
AUTHOR INFORMATION
■
Corresponding Author
Funding
Zhenglong Li − Manufacturing Science Division, Oak Ridge
National Laboratory, Oak Ridge, Tennessee 37830, United
States; orcid.org/0000-0001-8811-8625; Email: liz3@
ornl.gov
Microscopy research, K.A.U., M.J.C., J.Z., S.C.P., D.L., and
Z.L., were sponsored by the U.S. Department of Energy
(DOE), Office of Energy Efficiency and Renewable Energy
(EERE), Bioenergy Technologies Office (BETO), under
Contract DE-AC05-00OR22725 (ORNL) with UT-Battelle,
LLC, and in collaboration with the Chemical Catalysis for
Bioenergy (ChemCatBio) Consortium, a member of the
Energy Materials Network. Microscopy research was also
supported by the Center for Nanophase Materials Sciences
(CNMS), which is sponsored by the Scientific User Facilities
Division, Office of Basic Energy Sciences, U.S. DOE. MR-CAT
operations are supported by the DOE and the MR-CAT
member institutions. This research used resources of the
Advanced Photon Source, a U.S. DOE Office of Science User
Facility operated for the DOE Office of Science by Argonne
National Laboratory under Contract No. DE-AC02-
Authors
Michael J. Cordon − Manufacturing Science Division, Oak
Ridge National Laboratory, Oak Ridge, Tennessee 37830,
United States
Junyan Zhang − Manufacturing Science Division, Oak Ridge
National Laboratory, Oak Ridge, Tennessee 37830, United
States; Department of Chemical and Biomolecular
Engineering, University of Maryland, College Park, Maryland
20742, United States
Stephen C. Purdy − Department of Chemical Engineering,
Purdue University, West Lafayette, Indiana 47907, United
States; Neutron Scattering Division, Oak Ridge National
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National Science Foundation under Cooperative Agreement
No. EEC1647722. The computing resource was provided on
“BEBOP”, a computing cluster operated by the Laboratory
Computing Resource Center at Argonne National Laboratory.
M.Z. and R.S.A. acknowledge Computational Chemistry
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Bioenergy Technologies Office of Energy Efficiency &
Renewable Energy. F.L. and H.W. were financially supported
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The views and opinions of the authors expressed herein do not
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Notes
The authors declare the following competing financial
interest(s): Patent application is pending for the inventor Z.L.
This manuscript has been authored in part by UT-Battelle,
LLC, under Contract DE-AC05-00OR22725 with the U.S.
Department of Energy (DOE). The U.S. government retains
and the publisher, by accepting the article for publication,
acknowledges that the U.S. government retains a nonexclusive,
paid-up, irrevocable, worldwide license to publish or reproduce
the published form of this manuscript, or allow others to do so,
for U.S. government purposes. DOE will provide public access
to these results of federally sponsored research in accordance
with the DOE Public Access Plan (http://energy.gov/
downloads/doe-public-access-plan).
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ACKNOWLEDGMENTS
■
The authors acknowledge Dr. Meijun Li for additional
experimental assistance with the XRD analysis. The authors
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