Selective Methanol Carbonylation to Acetic Acid on Heterogeneous Atomically Dispersed ReO4/SiO2Catalysts

Article abstract of DOI:10.1021/jacs.0c05026

Methanol carbonylation to acetic acid (AA) is a large-scale commodity chemical production process that requires homogeneous liquid-phase organometallic catalysts with corrosive halide-based cocatalysts to achieve high selectivity and activity. Here, we demonstrate a heterogeneous catalyst based on atomically dispersed rhenium (ReO4) active sites on an inert support (SiO2) for the halide-free, gas phase carbonylation of methanol to AA. Atomically dispersed ReO4 species and nanometer sized ReOx clusters were deposited on a high surface area (700 m2/g) inert SiO2 using triethanolamine as a dispersion promoter and characterized using aberration corrected scanning transmission electron microscopy (AC-STEM), UV-vis spectroscopy, and X-ray absorption spectroscopy (XAS). Reactivity measurements at atmospheric pressure with 30 mbar of methanol and CO (1:1 molar ratio) showed that bulk Re2O7 and ReOx clusters on SiO2 (formed at >10 wt %) were selective for dimethyl ether formation, while atomically dispersed ReO4 on SiO2 (formed at <10 wt %) exhibited stable (for 60 h) > 93% selectivity to AA with single pass conversion >60%. Kinetic analysis, in situ FTIR, and in situ XAS measurements suggest that the AA formation mechanism involves methanol activation on ReO4, followed by CO insertion into the terminal methyl species. Further, the introduction of 0.2 wt % of atomically dispersed Rh to 10 wt % atomically dispersed ReO4 on SiO2 resulted in >96% selectivity toward AA production at volumetric reaction rates comparable to homogeneous processes. This work introduces a new class of promising heterogeneous catalysts based on atomically dispersed ReO4 on inert supports for alcohol carbonylation.

Full text of DOI:10.1021/jacs.0c05026

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Article
Selective Methanol Carbonylation to Acetic Acid on
Heterogeneous Atomically Dispersed ReO4/SiO2 Catalysts
Ji Qi, Jordan Finzel, Hossein Robatjazi, Mingjie Xu, Adam S.
Hoffman, Simon R. Bare, Xiaoqing Pan, and Phillip Christopher
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.0c05026 • Publication Date (Web): 21 Jul 2020
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Selective Methanol Carbonylation to Acetic Acid on Heterogeneous
Atomically Dispersed ReO4/SiO2 Catalysts
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Ji Qi1, Jordan Finzel1, Hossein Robatjazi1, Mingjie Xu2,3,4, Adam S. Hoffman5, Simon R. Bare5,
Xiaoqing Pan2,3,4, Phillip Christopher1*
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1. Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara,
California 93117, United States.
2. Department of Materials Science and Engineering, 3. Department of Physics and Astronomy,
4. Irvine Materials Research Institute (IMRI), University of California Irvine, Irvine, California
92697, United States.
5. Stanford Synchrotron Radiation Light Source, SLAC National Accelerator Laboratory, Menlo
Park, California 94025, United States
*Corresponding Author Email: pchristopher@ucsb.edu
Abstract
Methanol carbonylation to acetic acid (AA) is a large-scale commodity chemical production
process that requires homogeneous liquid-phase organometallic catalysts with corrosive halide-
based co-catalysts to achieve high selectivity and activity. Here we demonstrate a heterogeneous
catalyst based on atomically dispersed rhenium (ReO4) active sites on an inert support (SiO2) for
the halide-free, gas phase carbonylation of methanol to AA. Atomically dispersed ReO4 species
and nanometer sized ReOx clusters were deposited on a high surface area (700 m2/g) inert SiO2
using triethanolamine as a dispersion promoter and characterized using aberration corrected
scanning transmission electron microscopy (AC-STEM), UV-Vis spectroscopy and X-ray
absorption spectroscopy (XAS). Reactivity measurements at atmospheric pressure with 30 mbar
of methanol and CO (1:1 molar ratio) showed that bulk Re2O7 and ReOx clusters on SiO2 (formed
at > 10 wt.%) were selective for dimethyl ether formation, while atomically dispersed ReO4 on
SiO2 (formed at < 10 wt.%) exhibited stable (for 60 hours) > 93% selectivity to AA with single
pass conversion > 60%. Kinetic analysis, in-situ FTIR and in-situ XAS measurements suggest that
the AA formation mechanism involves methanol activation on ReO4, followed by CO insertion
into the terminal methyl species. Further, by introducing ~0.2 wt.% of atomically dispersed Rh to
10 wt.% atomically dispersed ReO4 on SiO2 > 96% selectivity toward AA production at volumetric
reaction rates comparable to homogeneous processes were observed. This work introduces a new
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class of promising heterogenous catalysts based on atomically dispersed ReO4 on inert supports
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for alcohol carbonylation.
Keywords
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single atom catalysts, atomically dispersed catalysts, methanol carbonylation, acid catalyst,
acetic acid, Rhenium
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1.
Introduction
Acetic acid (AA) is a large-scale chemical used in the synthesis of vinyl acetate monomer, as well
as many other commodity products.1 Currently, a majority of the ~15 million tons of AA produced
annually is synthesized through the carbonylation of methanol.1,2 The Monsanto and Cavita
processes are the primary commercialized routes for methanol carbonylation, which use Rh and
Ru promoted Ir homogeneous catalysts, respectively, and operate under relatively mild conditions
(180–220 ◦C, 2–4 MPa).3,4 Despite being effective and selective, these commercial processes suffer
from economic and environmental drawbacks. The former is due to the separation costs to recover
the precious metal catalysts from liquid phase processes, whereas the latter is related to the
essential role of hydroiodic acid that participates in the catalytic cycle.5 Therefore, it is desirable
to consider heterogeneous catalytic processes that rival homogeneous processes in terms of
volumetric rates and selectivity, while simultaneously removing the need for reactive halogens.
Supported transition metal heterogenous catalysts have shown promise for methanol
carbonylation.2,6 However, most processes still rely on halide additives, and/or the reported
reactivity, selectivity or stability are lower than existing homogeneous processes.7,8 For example,
atomically dispersed Iridium (Ir) on active carbon (AC) was shown to have comparable reactivity
and selectivity to that of the analogous homogeneous catalyst, but its rapid deactivation precludes
industrial viability.9,10 Co-deposition of Lanthanum (La) resulted in a three-fold increase in
reactivity compared to Ir/AC with an acetyl selectivity of 99% that remained stable for more than
1 month.11,12 Analogously, atomically dispersed Rhodium (Rh) on porous organic polymers
(POPs) has also shown good performance for methanol carbonylation, with catalyst stability for
400 hours on stream.13–15 These heterogeneous catalytic processes still use halides and water,
which minimizes the potential benefit over the already commercialized homogeneous processes.
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By contrast, halide-free methanol carbonylation has been successfully demonstrated in the gas
phase using 8-membered rings of mordenite (MOR) zeolites.16–18 The strong acid sites in zeolites
promote the primary reaction pathway of methanol coupling to produce dimethyl ether (DME),
which then undergoes carbonylation to produce methyl acetate (MA). AA can then be produced in
a second step involving the hydrolysis of MA back to methanol and AA, although this requires an
additional water feed and a two reactor process.19,20 Alternatively, by modifying zeolites with Cu
species and operating with high (>50) CO:MeOH feed ratios, DME formation was suppressed,
resulting in high AA selectivity in one step.5,21 The requirement of either a two-step process or
operation at very high CO:MeOH feed ratios limits the advantage of the gas phase zeolite-based
AA production approaches. Hence, it remains a challenge to design catalysts that enable selective
methanol carbonylation to AA in a gas-phase, iodide free heterogeneous catalytic process while
operating close to stoichiometric CO:MeOH feed ratio.
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Recent work on Al site pairing in zeolites and Na modification of ZrO2-based catalysts
demonstrated that decreased rates of DME formation in gas phase methanol feeds correlated with
spatially separating acidic methanol adsorption and activation sites.22–24 We hypothesized that
isolated acidic sites on an inert support could minimize the on surface coupling of methanol
species, suppressing DME formation and thereby promoting the selectivity toward AA through the
carbonylation of surface methoxy species.
Here we demonstrate a heterogeneous catalyst based on atomically dispersed rhenium (ReO4)
active sites on an inert support (SiO2) for stable halide-free, gas phase carbonylation of methanol
to AA. Atomically dispersed ReO4 species and ReOx clusters were deposited on mesoporous high
surface area (700 m2/g) inert SiO2 using triethanolamine as a dispersion promoter and stabilizer.25
ReOx structures were characterized by aberration corrected scanning transmission electron
Microscopy (AC-STEM), UV-Vis spectroscopy and X-Ray absorption spectroscopy (XAS). In
methanol carbonylation reaction measurements, atomically dispersed ReO4 species (formed at <
10 wt.%) were found to exhibit stable > 93% selectivity to AA, while ReOx clusters (formed at >
10 wt.%) were primarily selective for DME formation. Kinetic analysis, in-situ FTIR and in-situ
XAS suggest that the reaction mechanism involves methanol activation on Lewis acidic ReO4
sites, followed by CO insertion into the terminal methyl species.26,27 Further, the addition of ~0.2
wt.% of atomically dispersed Rh to 10 wt.% atomically dispersed ReO4 on SiO2 increased AA
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selectivity to more than 96%, resulting in volumetric AA production rates comparable to
homogeneous processes. This work introduces a new class of promising heterogenous catalysts
consisting of atomically dispersed ReO4 on inert supports for gas phase, halide-free selective
alcohol carbonylation.
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2. Experimental Methods
2.1. Catalyst Preparation
Re catalysts were prepared by introducing water-soluble organics to induce complex formation
with metal ions,25 which may enhance the affinity between the supports and metal ions.
Triethanolamine (TEA) (Sigma-Aldrich, 90278) was chosen, because amino groups can
coordinate with metals and the hydroxyl groups can coordinate with the oxide support, which leads
to metal anchoring on the oxide support. Mesoporous SBA-15 with 4 nm pore size (Sigma-Aldrich,
7631-86-9) was used as the support and perrhenic acid solutions (HReO4) (Sigma-Aldrich, 75-80
wt.% in H2O) were used as Re precursors. SBA-15 was chosen due to its high surface area (around
700 m2/g) and chemical inertness. For the deposition of ReOx species, appropriate amounts of Re
precursor were dissolved in 10 mL of high-performance liquid chromatography (HPLC) grade
water (J.T. Baker, JT4218-3) and the desired amounts of TEA (TEA:Re molar ratio of 8:1) were
added into the solution under magnetic stirring for 30 min to ensure the solution was well mixed.
The Re precursor solution was injected via syringe pump (5 mL/hr) into a 100 mL suspension of
the SBA-15 support under magnetic stirring. After mixing for 12 hours at room temperature, the
solution was dried at 80 °C using a rotary evaporator. For the synthesis of atomically dispersed
Rh-ReO4 pair sites on SiO2, appropriate amounts of HReO4 were dissolved in 10 mL of HPLC
grade water and the desired amounts of TEA (TEA:Re molar ratio of 8:1) was added into the
solution under magnetic stirring for 30 min to ensure the solution was well mixed, following which
appropriate amounts of rhodium(III) chloride (Sigma-Aldrich, 307866) were added into the
solution and stirred for another 30 minutes. The precursor solution was injected via syringe pump
(5 mL/hr) into a 100 mL suspension of the SBA-15 support under magnetic stirring. After mixing
for 12 hours at room temperature, the solution was dried at 80 °C using a rotary evaporator. The
prepared catalysts were calcined at 450 °C for 4 hours under dry air prior to characterization and
reactivity measurements.
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Bulk Re2O7 (Sigma-Aldrich, 1314-68-7), which was used as a benchmark for the behavior of
extended ReOx domains, was physically mixed with SBA-15 prior to use.
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2.2. Catalyst Characterization
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2.2.1. Ultraviolet-visible diffuse reflectance spectroscopy
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Ultraviolet-visible (UV–vis) diffuse reflectance spectroscopy was used to characterize the
structure of ReOx species on the SiO2 support. Spectra were obtained using dehydrated MgO as a
reference with a Thermo Scientific Evolution 300 UV–Vis spectrometer equipped with a Harrick
Scientific Praying Mantis diffuse reflectance accessory. Samples were lightly ground using a
mortar and a pestle before measurements following in-situ oxidation (O2, 350 °C, 1 hour) and
reaction conditions (30 mbar methanol and 30 mbar CO at 250 °C). Calculation of the onset of
photon absorption induced by ligand to metal charge transfer (LMCT) was described
previously.28,29 For comparison, we obtained the LMCT onsets for KReO4 and Re2O7, representing
isolated ReO4 and bulk ReOx species, respectively.
2.2.2. X-ray absorption spectroscopy (XAS)
XAS at the Re L3 edge (10535 eV) was executed on beamlines 4-1 and 9-3 at the Stanford
Synchrotron Radiation Lightsource at the SLAC National Laboratory using a Si(220) double
crystal monochromator. Spectra of Re reference compounds were collected by pressing finely
ground powders into self-supporting pellets, while Re metal powder was smeared as a thin film on
Kapton tape. In a typical experiment, 30 mg of catalyst was crushed, sieved (80-120 mesh), and
loaded into a 1 cm catalyst bed in a 3 mm quartz tube flow-through reactor held perpendicular to
the X-ray source.30 Samples were diluted with mesoporous silica (MCM-41) to have an absorption
length of 2.0 with a fractional absorbance of Re of about 0.2. A W foil was scanned simultaneously
for energy calibration. In-situ extended X-ray absorption fine structure (EXAFS) measurements,
consisting of 9-16 scans were collected at 50 °C following exposure to oxidizing (20% O2/He, 350
°C, 1 hour) or reaction conditions (150 mbar methanol and 200 mbar CO in He, 250 °C, 1 hour).
The raw data were processed using the Athena interface of Demeter software package.31 Spectra
were energy calibrated, merged, and normalized. The EXAFS were extracted in k-space and
Fourier transformed using the k2-weighted EXAFS function (k=3.9-14) to amplify oscillations at
high k-values. EXAFS fitting was carried out over the R range (1-2.2 Å) taking into account k1-,
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k2-, and k3-weighting using the Artemis interface of Demeter. Phase shifts and amplitude for
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relevant backscattering paths were calculated using FEFF6.32 So was determined to be 0.73 +/-
0.04 by fitting the Re metal standard (Figure S1). A multiple data set fit was performed for the 5,
10, and 20 wt.% Re loading samples simultaneously to minimize errors. Spectra were fit with two
Re-O scattering paths derived from the crystal structure of tetrahedral Re in Re2O7 (mp-1016092).
Coordination numbers to the short (double bonded) Re=O scattering path and long (single bonded)
Re-O scattering path were fixed at 3 and 1 respectively, consistent with previous EXAFS treatment
of similar samples, and the +7 Re oxidation state inferred from X-ray absorption near edge
structure (XANES) measurements.33,34
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2.2.3. High-angle annular dark field-scanning transmission electron microscopy (HAADF-
STEM)
Atomic resolution high-angle annular dark field scanning transmission electron microscopy
(HAADF-STEM) imaging was performed on a JEOL Grand ARM300CF microscope with double
aberration correctors at 300 kV. Images were collected after ex-situ calcination of samples at 450 ℃
using a convergence semiangle of 22 mrad, and inner- and outer-collection angles of 83 and 165
mrad, respectively. A beam current of 11 pA was used with pixel time of 4 μs.
2.2.4. Temperature-programmed desorption of ammonia (NH3-TPD)
NH3-TPD measurements were performed on a Micromeritics AutoChem 2920 instrument. In a
typical experiment, 0.15 g of catalyst was loaded into a U-shaped, flow-thru, quartz sample tube.
Prior to measurements, the catalyst was pretreated in O2 (30 cm3 /min) at 350 °C for 1 h to remove
adsorbed water. A mixture of 10% NH3/90% helium was flown over the catalyst (30 cm3 /min) at
50 °C for 1 h. Then the sample was flushed with pure helium (30 cm3 /min) at 50 °C for 1 h. The
TPD measurements were carried out in the range 100− 350 °C at a heating rate of 10 °C/min. NH3
concentration in the effluent was monitored with a gold-plated filament thermal conductivity
detector. The amount of desorbed NH3 was determined based on the calibrated area under the
desorption curve.
2.2.5. X-ray photoelectron spectroscopy (XPS)
High-resolution XPS measurements were performed on an Escalab Xi+ (ThermoFisher) XPS
microprobe equipped with a monochromatic Al anode X-ray source. The pressure in the XPS
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chamber was maintained below 1 × 10−9 torr during the acquisition process. XPS measurements
were performed using the vacuum/inert transfer vessel with the use of glove box for the sample
preparation and transfer, to enable analysis after pre-treatment without exposure to atmosphere.
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2.2.6. Fourier Transform Infrared Spectroscopy (FTIR)
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Catalysts were loaded into a Harrick Praying Mantis reaction chamber with ZnSe windows
mounted inside a Thermo Scientific Praying Mantis diffuse reflectance adapter set inside a Thermo
Scientific Nicolet iS10 Fourier transform infrared spectroscopy (FTIR) spectrometer. All gases
were passed across an isopropyl alcohol/liquid nitrogen cold trap and a glass trap filled with
Drierite desiccant to remove trace moisture. Before characterization, catalysts were pretreated via
in-situ oxidation at 350 °C for 1 hour under O2 flow, and a background spectrum was taken before
CO and methanol introduction. The CO partial pressure was kept as 30 mbar, and methanol partial
pressure was chosen as 42, 105 and 150 mbar with helium as a carrier gas. The spectra were
collected under methanol and CO. In all measurements, spectra were obtained by averaging 32
sequentially collected scans at a resolution of 4 cm−1.
2.3. Reactivity and kinetic measurements
Catalytic activity for methanol carbonylation was evaluated in a fixed-bed quartz reactor in the
temperature range of 220-280 °C operating at atmospheric pressure. All gas flows were controlled
by mass flow controllers (Teledyne Hastings) and an in-line bubbler, or syringe pump, was used
to deliver methanol (Fisher Scientific, HPLC Grade, LOT: 177964) to the catalyst. Helium (Airgas,
UHP, 99.999%) was used to bubble methanol and used as a diluent to control reactant partial
pressures. The reaction effluent was quantified with online mass spectrometry (HALO 201, Hiden
Analytical Inc.) and gas chromatography, which were independently calibrated. For online mass
spectrometry the following m/z values were used to detect each product: m/z = 28 for CO, m/z =
31 for methanol, m/z = 45 for dimethyl ether (DME), m/z = 60 for acetic acid, and m/z=74 for
methyl acetate. The concentrations were calibrated to the signal intensity at each mass and helium
was used as an internal standard. The mass spec results were compared to calibrated analysis via
gas chromatography, which showed quantitative agreement and the ability to close mass balances
within ~5% under all explored conditions (selected mass balances over a range of conditions are
shown in Table S1-3).
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Prior to reactivity measurements, catalysts were pre-treated by oxidation at 350 °C for 1 hour with
pure O2 at 50 mL min-1, followed by measurements at varying temperatures. The system was
maintained for 2 h at each temperature to ensure steady state was achieved. Different Re weight
loading catalysts were tested with 1, 5, 10 and 20 wt.% Re/SiO2. During kinetic measurements, the
total Re weight loading in the reactor was kept constant by using different weights of catalysts and
diluent SBA-15. Additionally, SiO2 gel was added to minimize the pressure drop (Table S4). When
analyzing the influence of methanol conversion on selectivity over the 10 wt.% Re/SiO2 catalyst,
the amount of both Re and SBA-15 were varied, while the amount of diluent SiO2 was kept
constant (Table S5). The stability measurement was executed with 600 mg of 10 wt.% Re/SiO2
diluted in 600 mg SiO2 at 280 °C.
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For methanol and CO pressure-dependent experiments, 10 wt.% Re/SiO2 was pretreated by
oxidation at 350°C for 1 h. After the pretreatment, while maintaining a constant temperature of
280°C and methanol partial pressure at 30 mbar, the CO partial pressure was varied from 200 mbar
to 600 mbar with Ar as a diluent. Alternatively, while maintaining a constant temperature of 280°C
and CO partial pressure at 30 mbar, the methanol partial pressure was varied from 30 mbar to 330
mbar and Ar acted as a diluent. The reaction system was held at each condition for 2 h to ensure
steady state conditions.
3. Results
3.1 Characterization of ReOx structure
Figure 1. Representative HAADF-STEM images of (a) 5 wt% Re/SiO2, (b) 10 wt% Re/SiO2 and (c) 20 wt%
Re/SiO2. Samples were pretreated via calcination at 450 °C for 4h in dry air prior to measurements.
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Re/SiO2 catalysts at various Re weight loadings (1, 5, 10, and 20%) were synthesized through
impregnation of aqueous perrhenate (HReO4) with the addition of 8:1 molar ratio of
triethanolamine (TEA) to promote Re dispersion.25 HAADF-STEM images of the calcined (450
°C, dry air) samples are presented in Figure 1(a-c), where the higher atomic number of Re as
compared to Si allows identification of Re domains via increased scattering of the electron beam.
While atomically dispersed Re species were primarily observed for the 5 wt% Re sample, an
increase in the density and existance of Re clusters was observed with increasing Re loading to 10
and 20 wt.%. However, the fraction of Re clusters in each sample is likely an over estimation of
the as-prepared state due to the electron beam induced coalescense during image acquisition.35
Previoius analysis of WOx speceis on ZrOx noted that analysis by STEM imaging suggested higher
concentrations of WOx clusters than atomically dispersed species as compared to correlated
spectrscopic measurements.36
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To provide sample-averaged information on the ReOx structure without influence of the elctron
beam, UV-vis and XAS were performed. In-situ UV-Vis spectroscopy measurements of the edge
energy (Eg) for the LMCT electronic transition were executed under an oxidative environment,
and reaction conditions. Eg provides useful information regarding the electronic structure of the
ReOx species, which is sensitive to the ReOx domain size and structure (moving from discrete
electronic structure of monomeric species to bands existing in bulk structures).29 Figure 2a shows
a decrease in Eg with increasing Re weight loading from ~4.45 eV for 1 and 5 wt% Re to ~4.1 eV
for 20 wt% Re following in-situ calcination in O2 at 350 °C and exposure to reaction conditions at
250 °C. A comparison of the Eg values to that of control samples, dispersed (KReO4, Eg = 4.1 eV)
and bulk (Re2O7, Eg = 2.8 eV) ReOx species, suggests that at lower Re weight loadings (1, 5 and
10 wt%) ReOx exists predominantly in the form of atomically dispersed species, while ReOx
clusters are present at higher Re weight loadings and their size increases with increasing Re
loading. The similar Eg following calcination and reaction conditions suggests that ReOx domains
remain stable under reaction conditions. This is in contrast to a previous report in which ReOx on
SiO2 was unstable after 2 hours during 2-butanol dehydration,37 suggesting that the use of TEA
during synthesis promotes the stability of a highly dispersed ReOx species on SiO2.
XAS was used to further characterize the oxidation state and structure of ReOx species on the
support. Re/SiO2 samples of various weight loadings showed nearly identical XANES spectra
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following oxidation (Figure 2b), with edge energies correlating to Re in +7 oxidation state (Figure
S2), which was also comfirmed by XPS (Figure S3). EXAFS spectra were fit with a proposed
ReO4 model that is consistent with the Re +7 oxidation state observed from XANES, in which Re
makes a single bond to the support via oxygen (Re-O 1.77 ± 0.04 Å), and is coordinated to three
additional Re=O double bonds (Re-O 1.69 ± 0.01 Å). The fit yielded a good match to the measured
EXAFS spectrum for the 5 wt% Re sample (Figure 2c). The EXAFS data and fit (Figure S4, fit
parameters in Table S6) are also in agreement with previously reported structures of atomically
dispersed ReO4.33,34 Notably, the EXAFS data for the 5, 10 and 20 wt% Re samples did not vary
considerably (Figure S4), suggesting that this sample averaged technique may not be able to
unambiguously identify small, sub-nanometer ReOx clusters within a sample containing
atomically dispersed ReO4 species.38 This is not unsuspected, as even bulk crystalline oligomeric
Re species (Re2O7) show Re-O-Re scattering at long bond lengths (Figure S5), which is thus more
sensitive to destructive interference from competing multiple scattering paths. Therefore EXAFS
likely, in contrast to STEM, underestimates the existence of ReOx clusters present. Based on a
combination of STEM, UV-Vis and XAS, it is concluded that ReOx was primarily atomically
dispersed as ReO4 species for 1 and 5 wt% Re, a mix of atomically dispersed ReO4 and small ReOx
clusters for 10 wt% Re, and primarily in the form of ReOx clusters for 20 wt% Re.
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Figure 2. (a) Onset of LMCT of dehydrated Re/SiO2 measured by in-situ UV−vis spectroscopy as a function of Re
loading. Measurements were performed after catalysts were oxidized at 350 °C for 1h (red dots) and after the catalysts
were exposed to 30 mbar methanol and 30 mbar CO at 250 °C for 1 hr (black dots). Measurements for the LMCT for
reference compounds (KReO4 and Re2O7) were included for comparison. (b) Re L3-edge XANES spectra
characterizing various Re weight loading samples after 1h oxidation in 20% O2/He at 350 °C. The slightly lower
absorbance for the 10 wt% Re sample is likely due to absorbance saturation by the detector. The edge energy was
consistent with other samples supporting consistent +7 oxidation state for Re. (c) EXAFS of 5 wt% Re/SiO2 after 1h
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oxidation in 20% O2/He at 350 °C. The magnitude and imaginary portion of the Fourier transformed EXAFS are
shown by the solid black and dashed blue lines, respectively while the fit to the imaginary portion is shown in red.
The inset shows the model used to fit the EXAFS spectrum.
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3.2 Influence of Re Structure on Methanol Carbonylation
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On the basis of the UV-vis, STEM and XAS, the influence of ReOx structure on reactivity and
selectivity for methanol carbonylation was examined by comparing reactivity as a function of Re
weight loading. Bulk Re2O7 was also used to calibrate the reactivity of extended ReOx domains.
Catalysts were pre-oxidized at 350 °C in O2 before being exposed to a mixture of methanol and
CO at a molar ratio of 1:1 (at 30 mbar of each) with a balance of helium at atmospheric pressure.
The reaction was executed in the kinetically limited regime by keeping the methanol conversion
below 10% (Figure S6). Arrhenius plots of the reactivity for AA and DME production as a function
of Re loading are shown in Figure 3a and b with production rates normalized to the Re weight
loading. The results for MA and formaldehyde production rates are provided in Figure S7. The
bare SBA-15 support showed no measurable reactivity under these conditions.
For 1 and 5 wt% Re/SiO2 catalysts, the AA and DME production rates were indistinguishable,
showing 0.038 mmol s-1 gRe-1 and 0.0005 mmol s-1 gRe-1, respectively, at 280 °C. For 10% Re/SiO2,
-1
the AA production rate decreased slightly to 0.0285 mmol s-1 gRe at 280 °C and the DME
production rate increased slightly to 0.0006 mmol s-1 gRe-1. However, increasing the Re loading to
20 wt% resulted in a 30-fold increase in the DME production rate at 280 °C, while the AA
production rate decreased by 5-fold compared to that of the 1 and 5 wt% Re/SiO2. Compared to
the 20 wt.% Re sample at 280 °C, bulk Re2O7 showed a DME production rate of only two-fold
higher and a 50% lower AA production rate. The similar reactivity of 20 wt.% Re/SiO2 and Re2O7,
and their stark difference to the behavior of samples with 10 wt.% Re or less, substantiate the use
of Re weight loading variation for comparing the behavior of atomically dispersed ReO4 and ReOx
clusters.
The measured apparent activation energies (Eapp) for AA and DME formation were similar for 1
and 5 wt.% Re/SiO2, showing Eapp of ~130±30 kJ/mol and ~100±40 kJ/mol for AA and DME
formation, respectively. On 10 wt.% Re/SiO2 the Eapp for AA and DME formation were 155±20
and 112±28 kJ/mol, respectively. The slight deviation of the Eapp on 10 wt% Re/SiO2 from 1 and
5 wt.% Re/SiO2, suggests relatively consistent active site distributions with a small increase in
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ReOx cluster formation at the higher weight loading, in agreement with inferences from catalyst
characterization. On the 20 wt.% Re/SiO2 the Eapp for AA and DME were much higher, 259±64
and 257±9 kJ/mol, respectively. Bulk Re2O7 exhibited Eapp of 272±33 and 273±32 kJ/mol for the
formation of AA and DME, respectively, which are statistically similar to the 20 wt.% Re/SiO2
sample. We note that there is some apparent deviation from Arrhenius like behavior at lower
temperatures in Figure 3a. This likely arises from the low analytical signals at these temperatures,
rather than a true deviation from exponential behavior.
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We hypothesize that differences in the measured Eapp for AA and DME formation over atomically
dispersed ReO4 versus ReOx clusters on SiO2 and bulk Re2O7 derives from differences in acidity,
as measured by NH3 temperature programmed desorption shown in Figure S8 structure.
Relationships between ReOx weight loading on oxide supports, acid strength and reactivity have
previously been observed.28,39,40, 41 Together with our results, this suggests that ReOx domain size
dictates acid strength.
The product formation rates and Eapp on the 1, 5, and 10 wt.% Re/SiO2 catalysts provide evidence
that a consistent active site exists with a close to linear relationship between site density and
reactivity. Previous work on Re species for olefin metathesis showed that not all the Re sites are
active and the reactivity was not linearly related to weight loadings.42–44 Thus, the reactivity results
here suggest that either the majority of sites on Re/SiO2 are active, or the same fraction of active
sites are produced for different weight loadings.45 Figure 3c shows the measured selectivity for
AA production in the range of 220-280 °C, where samples at lower weight loadings (1%, 5% and
10 wt.% Re/SiO2) exhibit an AA selectivity of ~93% at > 250 °C, whereas 20 wt% Re/SiO2 and
bulk Re2O7 exhibited AA selectivity < 20% across all temperatures. It is noted that at above 280
°C a significant increase in the rates of byproduct formation were observed, resulting in a decrease
in AA selectivity.
Based on the characterization and methanol carbonylation reactivity results it can be concluded
that atomically dispersed ReO4 species on SiO2 are selective for AA formation, whereas ReOx
clusters and bulk Re2O7 with stronger acid sites primarily drive DME production. This suggests
that communicating acidic sites existing on the surface of ReOx clusters facilitate the rate of
methanol coupling to DME, as initially hypothesized.46,47 This will be discussed further below.
The DME carbonylation reaction was tested on 5 wt.% Re/SiO2 using 30 mbar DME and 30 mbar
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CO in the feed, Figure S9. The DME carbonylation reaction to MA proceeded with 12 times higher
rate than methanol carbonylation to AA under identical conditions. This highlights the critical role
of minimizing DME formation pathways for obtaining high selectivity to AA. We also note that
water, which is a product during DME formation, may compete with other reaction intermediates
at Lewis acid sites26,48 and may poison active sites that contribute to the production of AA. The
influence of water on AA formation will be analyzed in future work.
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Figure 3. Arrhenius plots showing the (a) AA formation rate and (b) DME formation rate on a per-gram Re basis
(mmolꢀs−1 gRe−1). The error bars on each data point represent the standard deviation calculated from 2 independently
run reactions. The error on Eapp estimates is the 90% confidence interval of the fit. (c) AA selectivity during methanol
carbonylation from 200 to 280ꢀ°C under 30 mbar methanol and CO (balance He).
We investigated the influence of methanol conversion on AA selectivity and catalyst stability for
the 10 wt.% Re/SiO2 catalyst, which exhibited the highest volumetric AA production rates still
with good selectivity (Figure 4a). To analyze the influence of methanol conversion on selectivity,
the loading of 10 wt.% Re/SiO2 in the reactor was increased, while the amount of diluent SBA-15
was decreased such that the space velocity was identical in all experiments. Increasing the catalyst
loading resulted in increasing methanol conversion to ~60%, which was limited from further
increases based on the reactor size. The measured AA selectivity was independent of methanol
conversion, showing ~93% selectivity for different methanol conversions (Figure 4b). Catalyst
stability measurements were performed at 280 °C in 30 mbar of MeOH and CO with 600 mg of
10 wt.% Re/SiO2 catalyst (the highest catalyst loading we used here). Figure 4c shows a stable
~60% methanol conversion and 93% AA selectivity over 60 hours on stream, suggesting that the
atomically dispersed ReO4 active sites on SiO2 remain quite stable under the explored reaction
conditions.
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Figure 4. (a) Methanol conversion as a function of temperature for various amounts of 10 wt% Re/SiO2 added to the
reactor (balance was SiO2 to give a total amount of material added to the reactor of ~1.2 g in all cases) at 30 mbar
methanol and 30 mbar CO (with balance of He). (b) AA selectivity as function of methanol conversion measured at
280 °C from the data in (a). (c) Methanol conversion and AA selectivity as a function of time on stream with 600 mg
of 10 wt% Re/SiO2 catalyst at 280ꢀ°C with 30 mbar methanol and CO (balanced with He).
3.3 Reaction pathway for methanol carbonylation to AA on ReO4/SiO2
Mechanistic insights into the observed AA formation on atomically dispersed ReO4 species and
ReOx clusters were developed by executing methanol carbonylation on 10 and 20 wt.% Re/SiO2
at 280 °C with independently varied methanol and CO partial pressures in the range of 30 to 330
mbar and 200 to 600 mbar, respectively (Figure 5). Figure 5a shows that for 10 wt.% Re/SiO2
(mostly atomically dispersed ReO4) a first-order dependence of AA production on methanol partial
pressure was observed in the range of 30-90 mbar, which transitioned to a zero-order dependence
at higher methanol pressure. This indicates that methanol, or a methanol derived intermediate, is
first-order in the rate determining step (RDS) and saturates the active site at methanol partial
pressures > ~120 mbar. For 20 wt.% Re/SiO2 that contained a mix of atomically dispersed ReO4
and ReOx clusters, a ~0.6 reaction order for methanol partial pressure in the AA production rate
was observed from 30-85 mbar, with an apparent decrease in methanol reaction order at ~85 mbar,
followed by a small increase in methanol reaction order up to ~120 mbar, and finally a reaction
order of 0 above ~120 mbar. The distinct kinetic behavior of the two types of sites is consistent
with the differences in Eapp for AA formation observed in Figure 3 and may also be influenced by
H2O produced due to DME formation.
In contrast to methanol, the AA production rate exhibited nearly a first-order (0.8-0.9) dependence
on CO partial pressure for both catalysts across a broad partial pressure range from 200 to 600
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mbar CO (Figure 5b). This suggests that CO is involved in the RDS, and that CO does not exist as
an adsorbed species at an appreciable coverage on the active site.18,48,49
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Figure 5. AA production rates as a function of (a) methanol partial pressure and (b) CO partial pressure over 10
wt% Re/SiO2 and 20 wt% Re/SiO2. In both sets of experiments, the non-varied reactant partial pressure was held
constant at 30 mbar. Before reactivity experiments, the catalyst was oxidized at 350 °C for 1 h.
The nature of bound species during AA formation on atomically dispersed ReO4 species was
analyzed via in-situ FTIR measurements executed under varying methanol partial pressure (at a
constant CO partial pressure of 30 mbar) at 280 °C on 5 wt.% Re/SiO2 (Figure 6a). Independent
of the methanol partial pressures, two strong bands at 2954 and 2854 cm-1 were observed that
correspond to physiosorbed methanol on SiO2, as they were also seen on pure SiO2 (Figure
S10).50,51 Three bands at 2979, 2922 and 2822 cm-1 (marked with the dashed lines) were also
observed. We assign the 2979 and 2922 cm-1 bands to the asymmetric and symmetric stretches of
CH3 in -OCH3 bound to ReO4, respectively, while 2822 cm-1 is assigned as the first overtone of
the symmetric stretch.52
Alcohol activation on the homogeneous methyltrioxorhenium complex (MTO, CH3ReO3), in
which Re exists in a +7 oxidation state, has been studied previously.53–55 Based on theoretical
analysis and isotopic labelling experiments it was proposed that dissociative alcohol adsorption
occurs through proton transfer from the alcohol to an oxygen native of MTO, resulting in the
coordination of -OR group to the Lewis acidic Re center, thus a maintained +7 oxidation state of
Re. This is analogous to the mechanism for ethanol activation over Lewis acidic Al2O3 surfaces,
where -OCH2CH3 coordinates to the Lewis acidic Al center, while the proton transfers to a
neighboring oxygen.56–60 Thus, it is hypothesized that methanol adsorption onto ReO4 occurs
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dissociatively at the O-H group, resulting in the formation of Re(-O-Si)(-OH)(=O)2(-OCH3),
where Re remains in a +7 oxidation state.
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With increasing methanol partial pressure in the feed from 42 mbar to 105 mbar an increase in the
intensity of the bands assigned to -OCH3 stretches were observed, indicating an increase in the
coverage on ReO4 sites. However, the -OCH3 stretch intensities remained unchanged with further
increase in the methanol partial pressure to 150 mbar, suggesting saturation of adsorption sites on
ReO4 species. The in-situ FTIR results are consistent with the kinetic measurements shown in
Figure 5a, where a saturation in the methanol reaction order in AA formation rate correlates with
a saturation in -OCH3 adsorption sites. This suggests that the resting state or most abundant surface
intermediate (MASI) for the ReO4 species at methanol partial pressure > ~120 mbar is Re(-O-Si)(-
OH)(=O)2(-OCH3), containing dissociatively adsorbed methanol.
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The oxidation state of atomically dispersed ReO4 species was probed with in-situ XANES
measurements for pre-oxidized 5 wt.% Re/SiO2 and under methanol carbonylation reaction
conditions where kinetic measurements and in-situ FTIR suggest that ReO4 species should be
saturated with -OCH3 (150 mbar methanol, 200 mbar CO and 250 °C). As observed in Figure 6b,
the XANES region did not change due to exposure to reaction conditions suggesting that Re was
in a +7 oxidation state in the most occupied state in the reaction cycle, consistent with the proposal
above. While the XANES region of 5 wt.% Re/SiO2 was indistinguishable comparing pre-oxidized
and under reaction conditions, changes in the EXAFS region were observed (Figure 6c). The
intensity of the scattering feature associated with scattering from oxygen in the first coordination
shell of Re decreased under reaction conditions. These changes could be consistent with a
decreased coordination number to shorter Re=O bonds and increased coordination to longer Re-O
bonds that have weaker scattering intensity, (Figure S11).53–55 We do not rule out the possibility
of CO-induced restructuring of ReOx species. Additional quantitative analysis is beyond the scope
of this work, but could provide further insights. The in-situ XANES and EXAFS under reaction
conditions are consistent with the proposed MASI described above.
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Figure 6. (a) In situ FTIR spectra for 5 wt% Re/SiO2. The samples were oxidized at 350 °C for 1h before being
exposed to 42, 105 and 150 mbar methanol with a CO partial pressure of 30 mbar (balanced by He). (b) Re L3-edge
XANES spectra collected under oxidation condition (under O2 at 350 °C) and reaction condition (150 mbar methanol
and 200 mbar CO diluted in He, 250 °C, 1 hour) for 5 wt% Re/SiO2. (c) EXAFS spectra collected at 50 °C after 350
°C oxidation and 250 °C reaction condition for 5 wt% Re/SiO2. (d) Proposed mechanism for AA formation on
atomically dispersed ReO4 species.
It is proposed that AA formation on atomically dispersed ReO4 active sites on SiO2 occurs through
dissociative methanol adsorption to form Re(-O-Si)(-OH)(=O)2(-OCH3), see Figure 6d. Based on
the partial pressure dependent methanol reaction order for AA formation, in-situ FTIR and in-situ
XAS it is proposed that this species is the MASI when the methanol partial pressure is > 120 mbar.
Because the AA formation rate is first order in methanol and CO at lower methanol partial
pressures, it is proposed that the RDS is CO insertion into the terminal CH3 group of the Re(-O-
Si)(-OH)(=O)2(-OCH3) species. Typically, for methanol carbonylation on homogeneous Rh
catalysts CO insertion is a migratory process into the M-C bond of Rh-CH3 species.4,61,62 However,
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for the halide-free methanol carbonylation process in zeolites where an Al-O-CH3 intermediate
participates in the CO insertion reaction, extensive DFT calculations have shown that the
energetically feasible pathway is CO insertion into the terminal CH3.26,49,63 Given the similar
proposed M-O-CH3 structure for ReO4 active sites, it is hypothesized that a similar CO insertion
into the carbon end of the bound -O-CH3 species followed by the rearrangement to acetyl group
occurs for ReO4 active sites, although further analysis is needed to confirm this. Finally, it is
proposed that the proton transfers back to form AA that desorbs, leaving ReO4 intact, Figure 6c.
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The rate law for AA formation within this mechanism can be written as (see SI for derivation):
(1)
Where, K1 is the equilibrium constant for dissociative methanol adsorption, and k2 is the rate
constant for the gas phase attack of CO on surface bound methoxy. At lower methanol partial
pressure, vacant sites are abundant, and the denominator of the rate law is approximately equal to
1, thus, the rate law appears as a first-order with respect to methanol and CO partial pressures. In
contrast, at higher methanol partial pressures, active sites are saturated with dissociatively
adsorbed methanol and the denominator becomes K1 PCH3OH, which results in a zero-order
dependence of AA formation on the methanol partial pressure.
3.4 Influence of ReOx structure on DME formation.
There are two proposed reaction pathways for DME formation: (i) through the interaction of gas
phase methanol with surface methoxy species (the dissociative mechanism),26 and (ii) through the
coupling of two adsorbed methanol species to produce DME (the associative mechanism).22,64 In
zeolites, the reaction pathways and rates of DME formation vary based on whether acid sites are
isolated or paired.24,46,47 Similar to recent analysis of the influence of acid site pairing in zeolites
on the DME formation rate, we observed an order of magnitude increase in DME production rate
comparing atomically dispersed ReO4 and ReOx clusters, where contiguous acidic sites likely exist
on the cluster surfaces, Figure 3b.
On 10 wt% Re/SiO2 the DME formation rate exhibited a second-order dependence on methanol
up to ~100 mbar and a transition to a zero-order at higher partial pressures, Figure 7a. Conversely,
on ReOx clusters (20 wt% Re) the DME formation rate was inhibited at methanol partial pressures
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> 85 mbar. The second order dependence on methanol partial pressure is consistent with the
bimolecular reaction leading to DME formation. Inhibition of the DME formation rate at higher
methanol pressures has recently been ascribed to the formation of supramonolayer coverages of
methanol in the case of DME formation in small pore zeolites.24 In this context, the data in Figure
7a suggests that ReOx clusters can support supramonolayer methanol or methoxy coverage, which
cannot occur on atomically dispersed ReO4 active sites, although this requires further analysis.24
Both catalysts exhibited a negative CO reaction order for DME formation, which suggests a
competition between CO and methanol in reactions with adsorbed -OCH3 species, Figure 7b.
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Figure 7. DME production rates as a function of (a) methanol partial pressure and (b) CO partial pressure over 10
wt.% Re/SiO2 and 20 wt.% Re/SiO2. In both sets of experiments, the non-varied reactant partial pressure was held
constant at 30 mbar. Before reactivity experiments, the catalyst was oxidized at 350 °C for 1 h.
3.5 Promoting ReO4/SiO2 reactivity with atomically dispersed Rh
It was hypothesized from the kinetic data that critical features of the ReO4/SiO2 active site that
enable high AA selectivity are (i) reduced rates of DME formation, which may result from the
effective blocking of the associative DME formation pathway by the isolated active sites, and (ii)
the ability of CO to outcompete methanol in the attack on the terminal CH3 in bound -OCH3
species.26,27 We previously showed that atomically dispersed Rh can promote CO insertion into
methoxy species bound to neighboring acidic sites on a ZrO2 support.22 Therefore, we postulated
that the formation of Rh-ReOx pair-sites on SiO2 could further promote the AA production rate on
atomically dispersed ReO4 active sites. Atomically dispersed Rh-ReOx pair-sites catalysts were
prepared and characterized by CO probe molecule FTIR, as we recently reported on Al2O3.65
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Figure 8a shows CO probe molecule FTIR spectra for 0.2 wt.% Rh/SiO2 and 0.2 wt.% Rh/10 wt.%
Re/SiO2 at saturation coverage and room temperature. Two strong bands centered at 2091 and
2030 cm-1 were observed for 0.2 wt.% Rh/SiO2, which are assigned to the symmetric and
asymmetric stretches of the Rh(CO)2 gem-dicarbonyl species, demonstrating the predominant
existence of atomically dispersed Rh species.66,67 Another CO stretch at 2107 cm-1 was observed,
which has been assigned to an atomically dispersed Rh species that is simultaneously coordinated
to CO and an additional O.68 A blue-shift of ~6 cm-1 in the CO stretch frequency for Rh(CO)2
species was observed comparing Rh/ReOx-SiO2 and Rh/SiO2. The CO stretch frequency in
Rh(CO)2 depends on the supports ability to withdraw or donate charge to Rh. Thus, the blue shift
is evidence that Lewis acidic ReOx species reduced the charge transfer to CO species bound to
Rh.28,65,69 We recently correlated a similar blue shift in CO frequency with the formation of Rh-
Re atomically dispersed pair sites on Al2O3 through correlation with HAADF-STEM imaging,
providing confidence in the formation of Rh-Re atomically dispersed pair sites in the current
catalyst. However, we note that the coordination environment in this interaction and its dependence
on reaction conditions is yet to be unraveled.
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Figure 8. (a) CO probe molecule FTIR spectra of Rh/SiO2 and Rh/ReOx−SiO2 catalysts. The catalysts were reduced
at 250 C under CO and spectra were collected under Ar. (b) AA production rate comparison between 10 wt.%Re/SiO2
and 0.2 wt.% Rh/10 wt.% Re-SiO2. The catalysts were oxidized at 350 °C for 1h before being exposed to the reaction
condition (30 mbar methanol and CO).
Methanol carbonylation reactivity measurements at 30 mbar methanol and CO as a function of
temperature are shown in Figure 8b for 10 wt.% Re/SiO2 and 0.2 wt.% Rh/10 wt.% Re-SiO2. This
comparison showed an order of magnitude increase in the rate of AA formation, without promoting
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DME formation (Figure S12), with the addition of 0.2 wt.% Rh. This results in an increase of AA
selectivity to ~96%, even at the stoichiometric, low pressure feed conditions used here. It is noted
that 0.2 wt.% Rh/SiO2 showed no measurable methanol conversion under these conditions. Eapp
for AA production was measured to be 155±20 and 133±27 kJ mol-1 for 10 wt.% ReOx/SiO2 and
0.2 wt.% Rh/10 wt.% ReOx-SiO2, respectively (Figure S13). This difference in Eapp is consistent
with the difference in Eapp for AA production on the 1, 5, and 10 wt.% Re/SiO2 catalysts, Figure
3a, suggesting Rh may be selectively promoting AA formation on ReO4 species rather than
modifying the inherent barrier. CO preferentially adsorbs on atomically dispersed Rh compared to
methanol or methanol-derived intermediates.70,71 Thus, it is postulated that the formation of
Rh(CO)2 species promotes the attempt frequency of CO insertion into neighboring Re-OCH3
species, although the nature of this interaction deserves further exploration.22
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4. Discussion and Conclusion
We calculated the volumetric and per metal site reaction rates of AA production on atomically
dispersed ReO4/SiO2 and Rh-ReO4/SiO2 and compared these with other heterogeneous and
homogeneous systems (see Table 1). The AA production rates on a per g of metal basis observed
for the Re-based catalysts reported here are competitive with Cu modified zeolites,72,73 atomically
dispersed Rh on polymer supports,13 and atomically dispersed Ir-La site pairs,11,74 albeit in the
absence of high operating pressure, high CO:MeOH feed ratios, and halide co-feeds. The
volumetric AA production rates on our Rh-ReO4/SiO2 catalyst in a fixed bed process is on par with
the Monsanto process and a few-fold lower than the Cavita process.62
This comparison demonstrates the substantial potential and operational advantages of the Rh-
ReO4/SiO2 catalyst reported here. Given that the reaction rates reported here are from
measurements at ~30 mbar CO and methanol, and that we observed first order behavior in CO up
to 600 mbar partial pressure, it is expected that operating at higher pressure equimolar CO and
methanol feeds may significantly further promote the volumetric AA production rates. Further,
optimization of the Rh and Re loadings on the basis of volumetric production rate per catalyst cost
is likely to enable further optimization of catalyst performance.
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Table 1. Comparison with other systems for methanol carbonylation to acetic acid.
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* unit conversion from mmol/g/s to mmol/cm3/s was estimated using catalyst densities and volume fractions.
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Acetic acid
production rate
Acetic acid
production rate
(mmol dm-3 s-1) *
CO: methanol
molar ratio
Reactor
pressure
Reaction
temperature
Iodide
promotor
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10%Re/SiO2
0.2%Rh/SiO2
0.2%Rh/
10%Re-SiO2
1.3%Cu-MOR26
0.02
0.13
1:1
1 atm
280 °C
-
mmol gRe-1 s-1
0
0.2
0
1.3
1:1
1:1
1 atm
1 atm
280 °C
280 °C
-
-
mmol gRe-1 s-1
0.07
0.2
1.4
1.8
0.3
1.5
5.5
200:1
1:1
50 atm
25 atm
270 °C
230 °C
-
mmol gCu-1 s-1
1%Ir-
0.15
CH3I
CH3I
CH3I
HI
0.7%La/AC11
mmol gIr-1 s-1
2.5%Ni/AC75
1.47
1:1
11 atm
300 °C
mmol gNi-1 s-1
0.265%Rh/
3.9
1:1
35 atm
Below 200 °C
Below 200 °C
190 °C
POL-2BPY13
mmol gRh-1 s-1
Monsanto Process
(Homogenous)62,76
2:1
30-60 atm
28 atm
Cativa Process
HI
(Homogenous)61
In conclusion, atomically dispersed ReO4 species on inert SiO2 supports were observed to be
effective heterogeneous methanol carbonylation catalysts without the need for a halide co-feed.
The catalytic process exhibited mechanistic similarities to zeolite-based carbonylation catalysts,
although the Re-based catalysts promote CO insertion into methoxy species to produce AA, while
zeolite catalysts favor methanol insertion into methoxy species prior to carbonylation. Through
promotion of the atomically dispersed Re/SiO2 with atomically dispersed Rh, AA production rates
rivaling reports of the Monsanto process were achieved. This work introduces a new class of
promising methanol carbonylation catalysts and raises questions about how atomically dispersed
pairs sites could act cooperatively to drive catalytic processes.
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ASSOCIATED CONTENT
Supporting Information
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The Supporting Information is available free of charge online.
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EXAFS fitting for Re metal, XANES edge energy for Re standards, calibration curve for Re
oxidation state, EXAFS data in k- and R-space, mass transport limitation test, reaction kinetic data,
tables for selected mass balance, rate expression derivation, characterization of samples by CO
probe molecule FTIR and XPS.
AUTHOR INFORMATION
Corresponding Author
*pchristopher@ucsb.edu
Notes
The authors declare the following competing financial interest(s): J.Q. and P.C. have filed for a
patent based on the findings reported in this paper.
ACKNOWLEDGEMENTS
P.C. acknowledges funding from the Air Force Office of Scientific Research MURI Grant
FA9550-15-10022. P.C. also acknowledges S. Soled for helpful discussions regarding catalysts
synthesis on SiO2 supports. J.Q. acknowledges funding from the Mellichamp Initiative for
Sustainability at the University of California, Santa Barbara. H.R. acknowledges the Postdoctoral
Fellowship support by Arnold O. and Mabel Beckman Foundation. The UCSB MRL Shared
Experimental Facilities are acknowledged for use of the X-ray Photoelectron Spectroscopy. TEM
experiments were conducted using the facilities in the Irvine Materials Research Institute (IMRI)
at the University of California-Irvine. Part of this work was also performed at Stanford
Synchrotron Radiation Lightsource (SSRL) of SLAC National Accelerator Laboratory and use of
the SSRL is supported by the U.S. Department of Energy, Office of Science, Office of Basic
Energy Sciences under Contract No. DE-AC02-76SF00515 and by Co-ACCESS supported by the
U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences, Geosciences &
Biosciences Division.
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