A Heterogeneous Pt-ReOx/C Catalyst for Making Renewable Adipates in One Step from Sugar Acids

Article abstract of DOI:10.1021/acscatal.0c04158

Renewable adipic acid is a value-added chemical for the production of bioderived nylon. Here, the one-step conversion of mucic acid to adipates was achieved in high yield through deoxydehydration (DODH) and catalytic transfer hydrogenation (CTH) by a bifunctional Pt-ReOx/C heterogeneous catalyst with isopropanol as solvent and reductant. The Pt-ReOx/C catalyst is reusable and was regenerated at least five times. The catalyst exhibits a broad substrate scope of various diols. Spectroscopic studies of Pt-ReOx/C revealed ReVII and Pt0 as the relevant species for DODH and CTH, respectively. Isotope labeling experiments support a monohydride mechanism for CTH over Pt. This work demonstrates a reusable bifunctional catalyst for a one-step valorization of sugar acids to a practical monomer, which opens the door to multifunctional catalysis streamlining valorization of biomass-derived molecules.

Full text of DOI:10.1021/acscatal.0c04158

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A Heterogeneous Pt-ReOx/C Catalyst for Making Renewable
Adipates in One Step from Sugar Acids
Jun Hee Jang, Insoo Ro, Phillip Christopher, and Mahdi M. Abu-Omar*
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ABSTRACT: Renewable adipic acid is a value-added chemical for the
production of bioderived nylon. Here, the one-step conversion of
mucic acid to adipates was achieved in high yield through
deoxydehydration (DODH) and catalytic transfer hydrogenation
(CTH) by a bifunctional Pt-ReOx/C heterogeneous catalyst with
isopropanol as solvent and reductant. The Pt-ReOx/C catalyst is
reusable and was regenerated at least five times. The catalyst exhibits a
broad substrate scope of various diols. Spectroscopic studies of Pt-
ReOx/C revealed ReVII and Pt0 as the relevant species for DODH and
CTH, respectively. Isotope labeling experiments support a mono-
hydride mechanism for CTH over Pt. This work demonstrates a
reusable bifunctional catalyst for a one-step valorization of sugar acids
to a practical monomer, which opens the door to multifunctional catalysis streamlining valorization of biomass-derived molecules.
KEYWORDS: bifunctional catalysis, deoxydehydration, heterogeneous catalysis, hydrogen transfer, sugar acids
of reductants such as PPh3, Na2SO4, H2, and alcohols.12,13
High-oxidation-state rhenium catalysts and alcohol reductants
have been widely studied for the DODH of various substrates,
sustainable routes to biofuels and biochemicals.1,2 Adipic acid
1. INTRODUCTION
Lignocellulosic biomass provides a renewable resource for
including sugar alcohols and acids. Homogeneous
is a large-volume, valuable chemical (1.8 €/kg) that can be
oxorhenium(VII) catalysts with n-butanol as solvent and
obtained from biomass.3 Its primary use is in the production of
reductant convert mucic acid to muconate through DODH
nylon-6,6 with a global market size of 3.7 million tons per
(Scheme 1a). The following hydrogenation of muconate over
annum.4 The current manufacturing of adipic acid depends on
Pd/C yields 62% adipates (Table S1).14 A higher yield (99%)
petroleum-derived chemicals and emits nitrous oxide (N2O).5
of adipates can be obtained through two steps: (1) DODH by
One promising route to renewable adipic acid is from biomass-
methyltrioxorhenium (MTO) and esterification over an acid
derived C6 carbohydrates. Glucose can be converted into
cocatalyst and (2) catalytic transfer hydrogenation (CTH)
cis,cis-muconic acid via biocatalysis, followed by direct
over Pt/C.11 In 2017, it was reported that a mixture of KReO4,
hydrogenation over Pt/C to give adipic acid, as reported by
Pd/C, and phosphoric acid in combination with H2 as a
Draths et al.6,7 The conversion of glucose derivatives including
reductant yielded 86% adipates.5 Despite the high yield, the
glucaric acid8 and 2,5-furandicarboxylic acid9,10 through
reaction system (homogeneous Re + heterogeneous Pt or Pd
chemocatalytic routes has also been studied. For example,
catalysts) presents challenging and likely costly separation and
Rennovia Inc. patented a process for the conversion of glucose
recycling problems. Due to the high cost of rhenium,
to glucaric acid using Pd/SiO2 catalyst. The resulting glucaric
employing a recyclable solid rhenium catalyst for DODH is
acid underwent hydrodeoxygenation to adipic acid using a
desirable.15 Recently, a solid ReOx/ZrO2 catalyst with n-
bimetallic Pd-Rh/SiO2 catalyst and a halogen source.8
butanol as a reductant converted D-glucaric acid-1,4-lactone to
However, these pathways from glucose and its derivatives
muconate with a yield of 41% and another DODH product
either are low yielding (<30%) or employ undesirable reaction
with a five-membered ring.16 When the product stream was
conditions such as requiring the use of corrosive halogens, a
high pressure of H2 (>50 bar), or acetic acid.
A combination of catalytic deoxydehydration (DODH) and
hydrogenation offers an efficient and green route to adipates
Received: September 22, 2020
Revised: November 7, 2020
from mucic acid. Mucic acid (also known as galactaric acid) is
an aldaric acid that can be produced by the oxidation of
galactose.11 A rhenium-catalyzed DODH reaction enables the
selective conversion of vicinal diols to alkenes in the presence
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Scheme 1. DODH and Hydrogenation of Mucic Acid by (A) a Combination of Homogeneous Re and Heterogeneous Pd or Pt
Catalysts in Multiple Steps and (B) a One-Step and Recyclable Catalyst
further processed via three different steps, including hydro-
genation over Pd/C and ring opening, adipates were produced
at 82% yield. While the yield of adipate is promising, the
multistep process requires the isolation of intermediates and
the use of different reaction conditions and catalysts.
250 °C).25 However, MTO-catalyzed DODH did not proceed
in isopropanol at 170 °C.26 Moreover, the higher polarity of
isopropanol relative to 3-octanol reduced the leaching problem
of the ReOx/support catalyst during DODH.15 Re-diolate
species, intermediate catalyst forms in the DODH cycle, are
less soluble in more polar solvents. In addition to the
advantages in DODH, isopropanol is a good hydrogen donor
for the CTH reaction. CTH reactions using isopropanol as a
hydrogen donor have garnered much attention as an
alternative to direct hydrogenation because the hydrogen
donor is easy to handle and environmentally friendly,
minimizing possible hazards.27−29 While CTH reactions have
been mostly accomplished using noble-metal complexes,
several effective heterogeneous catalysts for the CTH of
ketone, imines, and polarized alkenes have been reported.30−32
Herein, we report the catalyst Pt-ReOx/C for a one-step
tandem catalysis of DODH and CTH (Scheme 1b). An 85%
yield of adipates from mucic acid is achieved without an acid
additive. Isopropanol is an effective solvent and hydrogen
donor for both DODH and CTH reactions. On the basis of the
reactivity, spectroscopic analysis, and isotope labeling experi-
ments, a bifunctional mechanism of the tandem reaction is
proposed. Catalyst recycling and regeneration are described.
In this work, we have designed a bifunctional heterogeneous
catalyst that effectively catalyzes both DODH and CTH,
converting mucic acid to adipates in one step. High-oxidation-
state rhenium oxide on activated carbon (ReOx/C) was chosen
as a DODH catalyst. In order to catalyze the CTH reaction of
C−C unsaturated bonds produced by the DODH reaction,
metallic platinum was employed with the ReOx/C, making a
Pt-ReOx/C catalyst. In the Pt-ReOx/C catalyst, ReOx and Pt
are active sites for DODH and CTH, respectively. The role of
Re in previously reported Pt-Re bimetallic catalysts was
explained as a promoter for Pt catalysts.17 For example, in
hydrocarbon reforming reactions over bimetallic Pt-Re
catalysts, the addition of Re to Pt catalysts modified the
electronic structure of Pt. This resulted in preventing carbon
fouling and extending the lifetime of the catalyst.18−20
Furthermore, Pt-Re catalysts have shown increased activity
for aqueous phase re-forming of biomass-derived polyols in
comparison to monometallic Pt catalysts. This is attributed to
Re promotion of Pt catalysts decreasing the binding energy of
CO to the bimetallic surface and/or increasing the number of
acid sites.21−23
2. EXPERIMENTAL DETAILS
2.1. Materials. All commercial materials were used as
received. Ammonium perrhenate(VII) (Strem Chemicals) was
purchased from Strem Chemicals. Hexachloroplatinic(IV) acid
solution, nickel(II) nitrate, tetraamminepalladium(II) nitrate,
iridium(III) chloride, activated carbon (Darco 100 mesh), D-
sorbitol, D-glucaric acid-1,4-lactone, and (R,R)-(+)-hydro-
benzoin were purchased from Sigma-Aldrich. Tris-
(ethylenediamine)rhodium(III) chloride, methanol, ethanol,
isopropanol, 1-butanol, 3-pentanol, glycerol, D-sorbitol, mucic
acid, and L-(+)-tartaric acid were purchased from Alfa Aesar.
Diisopropyl L-(+)-tartarate, diisopropyl fumarate, and diiso-
propyl succinate were purchased from TCI Chemicals.
In addition to the separation and recycling problems, the
aforementioned reaction systems (homogeneous + heteroge-
neous catalysts) require the use of acid additives and expensive
alcohol reductants (C4 or higher) to achieve good yields of
adipates. C4−C8 alcohols have been proven as effective
reductants and solvents for the DODH reaction.13,24
Isopropanol is an attractive reductant for DODH because it
is a cheap, green, and good solvent for reactants and products
relative to the C4−C8 alcohols. For example, the ability of
isopropanol to be a reductant and solvent for DODH was
demonstrated for 1,2-hexanediol with a ammonium heptamo-
lybdate (AMT) catalyst at high reaction temperatures (240−
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Isopropanol-2-d1 was purchased from Cambridge Isotope
Laboratory.
stoichiometry between CO and surface Pt sites was assumed to
be 1:1. The catalysts after the reaction were pretreated under
an Ar or H2 flow and used for CO chemisorption.
Temperature-programmed reduction (TPR) was carried out
using a Micromeritics AutoChem II 2920 instrument. Before
TPR, samples were pretreated under an Ar flow at 400 °C for 4
h. A TPR run was carried out in a flow of 10% H2/90% Ar gas
mixture at a flow rate of 50 mL min−1 with a temperature ramp
of 10 °C min−1. The consumption of hydrogen was monitored
as a function of temperature using the TCD.
Infrared spectra of catalysts were recorded with a Thermo
Scientific Nicolet iS10 Fourier transform infrared (FTIR)
spectrometer with a mercury cadmium telluride detector
cooled by liquid nitrogen. Catalysts were diluted with KBr.
Before characterization, catalysts were pretreated in situ under
an Ar flow at 400 °C for 4 h. After pretreatment, the catalysts
were cooled to room temperature under Ar, and then a
baseline spectrum was taken before pyridine introduction.
Pyridine was introduced to the catalyst by flowing Ar through a
pyridine bubbler, and then the system was purged with 100
sccm of Ar for 10 min to remove physisorbed pyridine. For all
measurements, spectra were obtained by averaging 64
sequentially collected scans at a resolution of 4 cm−1.
2.2. Catalyst Preparation. The catalyst ReOx/C was
prepared by wet impregnation of an ammonium perrhenate
aqueous solution. A 72 mg portion of ammonium perrhenate
was dissolved in 4 mL of distilled water. A 1 g portion of
activated carbon was mixed vigorously with the perrhenate
solution overnight. After water was evaporated in a preheated
80 °C oil bath for 1.5 h and the residue dried in a 120 °C oven
overnight, the catalyst was dehydrated at 480 °C for 4 h
(heating rate of 8 °C/min) under flowing N2. Bifunctional
catalysts (M-ReOx/C, M = Ni, Pd, Ru, Rh, Ir, Pt) were
synthesized by impregnating the metal precursor solution with
the prepared ReOx/C. The M/Re molar ratio was fixed to 0.4.
For Pt-ReOx/C, 500 mg of the synthesized ReOx/C was mixed
with 263 mg of 8 wt % hexachloroplatinic(IV) acid solution
and 2 mL of distilled water. The catalyst Pt-ReOx/C was used
after removing water in an oil bath at 80 °C, drying at 120 °C
overnight, and dehydrating (N2/480 °C/2 h). The mono-
metallic Pt/C catalyst was prepared by the same impregnation
method with hexachloroplatinic acid. All catalysts were used
and characterized without the treatment of reduction unless
otherwise noted.
2.3. Characterization. High-angle annular dark-field
scanning transmission electron microscope (HAADF-STEM)
images were collected at 200 kV using a Thermo Scientific
Talos instrument equipped with a Super-X detector system.
The samples were diluted in ethanol and deposited on a
carbon film copper grid. In order to explore the distribution of
ReOx and Pt, energy-dispersive X-ray spectroscopy (EDX)
mapping was performed on the distribution of Re, Pt, and O
elements. The average particle size of Pt-ReOx/C was
calculated from the STEM images. Transmission electron
microscope (TEM) images of ReOx/C were obtained on an
FEI Tecnai G2 microscope.
X-ray photoelectron spectroscopy (XPS) analysis was
acquired on a ThermoFisher Escalab Xi+ instrument with a
monochromatic Al Kα X-ray source. High-resolution spectra
(20 eV) of Re 4f, Pt 4f, C 1s, and O 1s and the survey spectra
(100 eV) were recorded. All of the spectra were calibrated by
setting the binding energy of the peak of C 1s to 284.5 eV.
CasaXPS software was utilized to deconvolute the spectra. The
catalyst Pt-ReOx/C prepared by the method described above
was analyzed without further treatment. The samples after the
reaction, reduction, and regeneration were transferred to a
glovebox filled with N2, avoiding exposure to air. The samples
were mounted on a transfer vessel in the glovebox and
transferred to the XPS chamber without air exposure.
X-ray diffraction (XRD) patterns were collected from 2θ
range of 5−80° on a PANalytical Empyrean X-ray diffrac-
tometer using Cu Kα radiation. Inductively coupled plasma
(ICP) analysis was carried out to quantify the amount of metal
using a Thermo iCAP 6300 instrument. For the sample
preparation, 20 mg of catalyst was digested with 2.5 mL of
aqua regia by refluxing at 150 °C for 6 h. The solution was
cooled to room temperature, filtered, diluted, and used for ICP
analysis. Because the catalysts after reaction contain organic
deposits, they were regenerated under 5% H2 in Ar at 230 °C
for 4 h before the acid digestion.
2.4. General Catalytic Procedure. The Pt-ReOx/C
catalyst (150 mg), mucic acid (1 mmol, 210 mg), and
isopropanol (40 mL) were placed in a Parr vessel. The vessel
was pressurized with nitrogen to 15 bar and heated to 170 °C.
After 6 h of reaction, the reactor was cooled and the gases were
collected in a gas sample bag. The used catalyst was filtered,
washed with pure isopropanol (30 mL), and dried in the oven
at 120 °C overnight for recycling tests. The reaction solution
was concentrated under reduced pressure, and the products
were dissolved in d6-DMSO and analyzed by NMR with
benzaldehyde as an internal standard. 1H NMR and 13C NMR
spectra were collected on an Agilent Technologies 400 MHz,
400-MR DD2 spectrometer. The gas phase was analyzed by
GC-TCD (Shimadzu GC-9AIT with a Shincarbon ST
column), GC-MS (Shimadzu GC-2010 with a DB-1 capillary
column coupled to a QP2010 MS), and GC-FID (Shimadzu
GC-2010 equipped with a Supelco alumina sulfate column)
with 1-butene as an internal standard. The recycling test of the
Pt-ReOx/C for the 6 h DODH-CTH reaction was conducted
with or without regeneration under H2. For regeneration, the
used and recovered catalyst was treated at 230 °C for 4 h in a
flow of H2 (5%) in Ar and reoxidized in a 120 °C oven for 1 h.
During the recovery step, a slight weight loss of less than 10 wt
% was observed. Thus, the amounts of mucic acid and
isopropanol in each reuse experiment were determined on the
basis of the amount of the recycled catalyst.
2.5. Stability Test. Diisopropyl L-(+)-tartarate and
diisopropyl fumarate were used as substrates for the stability
tests due to their good solubility in isopropanol. DODH of
diisopropyl L-(+)-tartarate gave diisopropyl fumarate and CTH
of diisopropyl fumarate produced diisopropyl succinate. These
diester compounds in the product solution were analyzed with
GC-FID (Agilent Technologies 6890N instrument with a DB-
5 capillary column) with mesitylene as an internal standard. A
4 mmol amount of the substrate, 100 mg of Pt-ReOx/C
catalyst, 200 mg of activated carbon, and 40 mL of isopropanol
were loaded into the reaction vessel, and the vessel was
pressurized with 15 bar of N2. The reaction started once the
temperature reached 170 °C. After 20 min, the reaction was
stopped and quenched in an ice/water bath. The used catalysts
The CO chemisorption studies were carried out using a
Micromeritics AutoChem II 2920 instrument. The as-prepared
catalysts were pretreated under an Ar flow at 450 °C for 2 h,
and CO adsorption was performed at 35 °C. The adsorption
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a
Table 1. DODH and CTH Tandem Reaction of 1 over ReOx/C
b
product selectivity (%)
b
entry
reductant
T (°C)
t (h)
conversn (%)
2
3 and 4
5
others
1
2
3
4
5
6
isopropanol
isopropanol
isopropanol
methanol
ethanol
3-pentanol
170
170
230
170
170
170
6
12
24
12
12
12
77
94
100
88
94
83
86
75
0
60
66
87
2
13
19
13
14
0
0
12
11
46
27
20
13
1
35
0
0
0
a
b
Reaction conditions: ReOx/C (150 mg, 4.5 wt % Re), 1 (1 mmol), i-PrOH (40 mL), and N2 (15 bar). Conversion and selectivity were calculated
1
by H NMR.
were separated and washed with 30 mL of isopropanol and
dried overnight in a 120 °C oven. The amounts of mucic acid
and isopropanol in each reuse experiment were determined on
the basis of the amount of the recycled catalyst. The
regeneration step (5% H2/230 °C/4 h) including reoxidation
was employed after deactivation was observed.
resulting in the enhanced solubility of mucic acid.11 In the
absence of p-toluenesulfonic acid, the turnover frequency per
Re atom in the homogeneously catalyzed system decreased to
0.9 h−1. While the addition of a Brønsted acid to ReOx/C
increased the number of ester groups, it did not increase the
conversion or selectivity of 2 (Figure S2 and Table S2). This
indicates that the reactant solubility has little effect on the
turnover frequency per Re atom of DODH over the solid
ReOx/C catalyst. Weak Lewis acid sites in ReOx/C were
detected by pyridine probe-molecule FT-IR (Figure S3).
However, addition of exogenous Lewis acids such as ZnCl2 had
no effect on the conversion or selectivity, indicating negligible
involvement of the Lewis acid sites of ReOx/C in the DODH
reaction (Table S2).
Despite the high conversion and selectivity of DODH over
ReOx/C with isopropanol, CTH was quite slow at 170 °C
(Table 1, entry 2). Four different temperatures from 170 to
230 °C were investigated to convert trans,trans-muconic acid
(2-diacid) to 5 through CTH (Figure S4). The highest yield of
5 (43%) was obtained at 230 °C. However, the elevated
temperature also resulted in 35−50% byproducts including
oxepane (6), alkylated adipates (7), and other unidentified
products. As a result, the higher temperature conversion of 1
led to a moderate yield of 5 (35%) and several other products
(entry 3 in Table 1).
3.2. Bifunctional Catalysis. A bifunctional catalyst
provides two different catalytic sites for a consecutive reaction
(A → B → C).39,40 To realize high efficiency for both DODH
and CTH, several bimetallic catalysts (M-ReOx/C) were
prepared by a sequential impregnation method and evaluated
for mucic acid conversion (Figure 1 and Table S3). Among
them, Pt-ReOx/C showed the highest conversion (79%) and
selectivity for adipates (82%, entry 1 in Table 2). The
bimetallic catalysts containing Ni and Pd did not promote the
CTH reaction. Some hydrogenated products (3/4 and 5) were
obtained by adding Ru, Ir, and Rh to ReOx/C, but the reactant
conversion was low. The addition of Pt to ReOx/C
significantly enhanced the selectivity of 5 while maintaining
the excellent DODH ability of ReOx/C (Figure 1 and entries 1
and 7 in Table S3). The carbon balance after 6 h of the
DODH-CTH reaction over Pt-ReOx/C catalyst is described in
Table S4. The sum of C6 reactant and products (1−5)
exhibited 92% carbon balance, and remaining 8% could be
explained by organic deposits, which are discussed in a later
3. RESULTS AND DISCUSSION
3.1. DODH Reaction. The heterogeneous oxorhenium-
catalyzed DODH reaction, as an alternative to homogeneous
DODH, has attracted much attention due to process
advantages.15,33−37 Oxorhenium supported on activated carbon
(ReOx/C) is an active catalyst for DODH with 3-octanol as a
reductant.35 Recently, this catalyst also promoted the CTH
reaction with isopropanol for C−O cleavage of lignin model
compounds.38 In this study, the ability of isopropanol to act as
a reductant and solvent for both DODH and CTH catalyzed
by a heterogeneous ReOx/C catalyst was investigated. The
ReOx/C catalyst with 4.5 wt % of Re was prepared by a wet
impregnation method with ammonium perrhenate as a
precursor. The ReOx nanoparticle size ranges from 1 to 4
nm (Figure S1). In 6 h, ReOx/C with isopropanol converted
77% of mucic acid (1) to muconic acid and its esters (2) with a
high selectivity of 86% (entry 1 in Table 1), showing the high
DODH ability of ReOx/C and isopropanol. To the best of our
knowledge, this is the first report to demonstrate isopropanol
as an effective solvent and reductant for the rhenium-catalyzed
DODH reaction. After 12 h, conversion reached 94%,
producing not only 2 but also hydrogenated products (3−5).
CTH proceeded to some extent (14% selectivity of 3/4 and 5),
even with ReOx/C alone (entry 2). DODH in 3-pentanol also
displayed high selectivity for 2, but further hydrogenation was
not observed (entry 6). Lower selectivity for 2 was obtained in
methanol and ethanol (entries 4 and 5). Thus, among the
tested C1−C5 secondary alcohols, isopropanol was found to
be the most effective hydrogen donor for DODH and CTH
over ReOx/C.
The heterogeneous catalyst ReOx/C with isopropanol did
not require an acid additive and showed a higher DODH
turnover frequency per Re atom (3.1 h−1) in comparison to
MTO and p-toluenesulfonic acid with 3-pentanol (1.7 h−1,
entry 2 in Table S1). It was previously reported that the
addition of a Brønsted acid promotes the esterification of 1,
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S4). Gas product analysis by GC-MS and GC-FID with 1-
butene as an internal standard can quantify propene (0.12
mmol), propane (0.32 mmol), and isopropanol (0.06 mmol)
in the gas phase. On the basis of the liquid- and gas-phase
analysis and the conversion of isopropanol, the C3 carbon
balance reached 96% (Table S4).
Monometallic Pt/C by itself was inactive for DODH (entry
6 in Table 2), but with 2-diacid as a substrate, quantitative
conversion to 5 was observed (99% yield). The product
selectivity was sensitive to Pt loading (entry 7). A higher Pt
loading (4.5 wt %) was detrimental because ReOx active sites
were blocked, lowering conversion of 1 (entry 8). A 1.8 wt %
Pt loading along with 4.5 wt % Re was identified as an optimal
composition for DODH and CTH. CTH over Pt-ReOx/C in
isopropanol was observed to be superior to that in 3-pentanol
(entry 9).
The time profile of the DODH-CTH reaction over Pt-
ReOx/C is shown in Figure 2 (see also Figure S6). It follows a
sequential kinetic scheme of DODH followed by two steps of
CTH: 1 → 2 → 3/4 → 5. It is notable that both intermediates
2 and 3/4 were observed along the path of the reaction,
indicating that CTH proceeded at rates similar to those of
DODH. After 24 h, an 85% overall yield of 5 was attained. 5
contains adipic acid (30%) and isopropyl esters (70%),
Figure 1. Conversion of 1 over several bimetallic catalysts. Reaction
conditions: 1 (1 mmol), M-ReOx/C (150 mg), isopropanol (40 mL),
N2 (15 bar), 170 °C, 6 h.
section. During the 6 h reaction, 5.5% of isopropanol was
converted, offering hydrogen for both DODH and CTH and
producing C3 products, including acetone. The higher amount
of acetone (5.2 mmol) produced in comparison to the
theoretically required amount of H2 (2.8 mmol) indicated the
independent dehydrogenation of isopropanol by Pt-ReOx/C. A
1.2 mmol amount of molecular hydrogen was detected by GC-
TCD analysis of the gas phase (Figure S5). The produced
acetone can be rehydrogenated to isopropanol with hydro-
genation catalysts. For example, Raney nickel catalyst showed
99.9% conversion of acetone and 99.9% selectivity of
isopropanol in the liquid phase.41 In addition to the conversion
of isopropanol to acetone, dehydration of isopropanol also
occurred, producing some of propene (0.2 mmol) and
diisopropyl ether (1.8 mmol) in the reaction solution (Table
1
quantified by H and 13C NMR (Figure S7). Acid hydrolysis
of the products gave isolated adipic acid in 77% yield (Figure
S8). The reaction concentration increased from 0.025 to 0.1 to
0.25 M (entries 1−3 in Table 3). In DODH-CTH with the
higher concentration solution, the selectivity of 5 decreased
mainly due to the side reactions and byproducts. The most
probable side reaction was intramolecular dehydration of 1 to
cyclic compounds and following DODH-CTH could make the
cyclic byproducts shown in Scheme S1. The side reactions
could be suppressed by using diisopropyl mucate (13) as a
substrate, showing a high selectivity of 5 (83%) even in the
higher concentration solution of 0.25 M (entry 4 in Table 3).
a
Table 2. DODH-CTH Tandem Reaction of 1 over Pt-ReOx/C
b
product selectivity (%)
b
entry
1
catalyst
Pt-ReOx/C
Re/Pt amount (wt %)
4.5/1.8
conversn (%)
2
3 and 4
5
others
79
64
77
72
0
7
50
10
19
6
82
40
82
77
83
11
9
8
4
11
c
2
2nd cycle
3rd cycle
4th cycle
5th cycle
1
0
0
0
d
3
4
d
e
5
81
6
7
8
9
10
Pt/C
0/1.8
no reaction
Pt-ReOx/C
Pt-ReOx/C
Pt-ReOx/C
ReOx/C + Pt/C
2nd cycle
4.5/0.9
4.5/4.5
4.5/1.8
4.5/1.8
78
44
73
78
48
42
0
90
0
44
9
7
3
4
5
71
0
83
85
9
20
3
14
11
f
g
e
11
0
a
b
Reaction conditions unless specified otherwise: 170 °C, 6 h, 1 (1 mmol), catalyst (150 mg), i-PrOH (40 mL), N2 (15 bar). Conversion and
c
d
1
selectivity were calculated by H NMR. Without regeneration. Regeneration conditions: H2/230 °C/2 h and reoxidation (air/120 °C/1 h).
e
f
g
Regeneration conditions: H2/230 °C/4 h and reoxidation (air/120 °C/1 h). 3-pentanol (40 mL). 1.8 wt % Pt/C (150 mg) + 4.5 wt % ReOx/C
(150 mg).
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studies of the Pt/C catalyst, prepared from H2PtCl6, showed a
uniform peak around 200−300 °C, which was accompanied by
2−
HCl emission due to the decomposition of PtIVCl6 and
PtIICl4
.
42,44 Two different peaks at 335 and 417 °C observed
2−
in the profile of ReOx/C indicate the heterogeneous nature of
Re on carbon support. The addition of Pt to ReOx/C resulted
in the shift of the first reduction peak to 275 °C in the profile
of Pt-ReOx/C. This shift can be attributed to the hydrogen
spillover from metallic Pt to Re.43,45 Moreover, the reduction
behavior of Pt-ReOx/C catalyst starts at a lower temperature
(125 °C) in comparison to ReOx/C (200 °C). Shifts in both
the reduction peak and the reduction starting temperature
exhibit the close interaction between Pt and Re in the Pt-
ReOx/C catalyst, which is consistent with STEM and EDX
analysis.
The as-prepared Pt-ReOx/C has both high-valent Re and
metallic Pt, as evidenced by XPS (Figure 4a,b and Table S5).
In the curve-fitted results of the Re 4f spectrum, the major
oxidation states at binding energies of 44.7 and 47.1 eV
represent ReVII (72%), followed by ReIV (27%) and a very
small amount of metallic Re0 (1%). Pt consists of 85% metallic
Pt0 with a 4f7/2 peak at 71.8 eV and 15% PtII. The reported
binding energy for the Pt 4f7/2 peak of bulk metallic Pt0 ranged
between 71.0 and 71.3 eV.46 The shift toward higher binding
energy indicates smaller size and high dispersity of Pt
particles.46,47 The thermal treatment step under N2 at 480
°C during synthesis results in PtIV precursor reduction to
metallic Pt0 due to the reductive properties of the carbon
support.48 Similar oxidation states of Pt and Re were observed
in monometallic Pt/C and ReOx/C samples (Figure S10 and
Table S5).
XRD patterns of bare activated carbon, ReOx/C, Pt/C, and
Pt-ReOx/C are shown in Figure 3f. Pt0 and ReVII, major species
detected by XPS analysis, did not appear in the XRD patterns,
suggesting a high dispersion of particles in Pt/C and Pt-ReOx/
C. This is consistent with the XPS analysis: the higher Pt0 4f7/2
peak position of the catalysts in comparison to that of bulk Pt.
While the particles are small and well-dispersed, ReO2 peaks in
the pattern of ReOx/C and Pt-ReOx/C suggest the existence of
larger particles containing the ReO2 phase.
Figure 2. Reaction profile of DODH-CTH tandem reaction of 1 over
Pt-ReOx/C. Reaction conditions: 1 (1 mmol), Pt-ReOx/C (150 mg),
isopropanol (40 mL), N2 (15 bar), 170 °C. The solid lines are to
guide the eye. Because the reaction time was counted after the inside
temperature reached 170 °C, some amount of 1 was converted to 2
before the reaction time started.
3.3. Catalyst Characterization. Pt-ReOx/C was charac-
terized by microscopy- and spectroscopy-based methods.
STEM combined with EDX analysis showed that Pt and Re
existed on the same particles (Figure 3 and Figure S9). Even
though each particle contains both Re and Pt, the Pt/Re peak
ratio is not uniform, suggesting different compositions within
particles (Figure S9). The average particle size of Pt-ReOx/C
calculated from STEM images is 3.2 1.5 nm (Figure S9c),
which is larger than that of ReOx/C (2.2 0.4 nm, Figure S1).
This indicates the formation of larger particles when Pt is
added to ReOx/C.
H2-TPR profiles of monometallic and bimetallic catalysts
were analyzed to investigate the interaction between Re and Pt
(Figure 3e). The peak at a high temperature of around 600 °C
is due to methanation of the carbon support with H2.42,43 The
TPR result of Pt/C exhibits a uniform reduction peak at 303
°C, which is assigned to the reduction/decomposition of PtII
species shown in XPS results of Pt/C below. Previous TPR
3.4. Proposed Reaction Mechanism. As discussed in
Bifunctional Catalysis, ReOx/C was active for DODH but
a
Table 3. Effect of Concentration of Reaction Solution
b
product selectivity (%)
b
entry
substrate
concn (M)
conversn (%)
2
3 and 4
5
others
c
1
1
1
1
13
0.025
0.1
0.25
0.25
95
100
100
90
0
0
0
0
0
2
5
0
89
77
65
83
11
21
30
17
2
3
d
4
a
Reaction conditions unless specified otherwise: 170 °C, 24 h, Pt-ReOx/C (3.6 mol % Re and 1.4 mol % Pt relative to substrate), i-PrOH (10 mL),
b
c
d
1
N2 (15 bar). Conversion and selectivity were calculated by H NMR. i-PrOH (40 mL). 12 h reaction.
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Figure 3. Characterization of as-prepared Pt-ReOx/C catalyst. (a) STEM high-angle annular dark-field (HAADF) analysis. (b) Line scan elemental
analysis on a particle in (a). (c) EDX mapping of Pt. (d) EDX mapping of Re. Analyses of more particles are given in Figure S9. (e) TPR profiles
for Pt/C, ReOx/C, and as-prepared Pt-ReOx/C. (f) XRD diffractograms of activated carbon, Pt/C, ReOx/C, and as-prepared Pt-ReOx/C. Peaks
with blue circles at 26, 37, and 54° represent ReO2.
Figure 4. Changes in XPS spectra of (a) Pt 4f and (b) Re 4f in Pt-ReOx/C over reaction and regeneration. In situ 30 min and 6 h samples under
the general reaction conditions for mucic acid (1) conversion were prepared without air exposure. After 6 h of the reaction, the catalyst was
separated and washed under atmospheric conditions, followed by drying overnight at 120 °C (spent sample). The spent sample was regenerated at
230 °C under H2 and transferred to a glovebox without air exposure (in situ regenerated sample). This in situ sample was exposed to air and
reoxidized at 120 °C (regenerated sample). (c) Activity test of as-prepared, spent, in situ regenerated, and regenerated samples using diisopropyl L-
(+)-tartarate (8) as a substrate. Reaction conditions: 8 (0.5 mmol), catalyst (20 mg), isopropanol (10 mL), N2 (15 bar), 170 °C, 20 min. The
reaction with the in situ regenerated sample was carried out under inert conditions.
inactive for CTH at 170 °C. The addition of Pt to ReOx/C
showed a significant increase in CTH. These, in combination
with no activity of Pt/C for DODH, suggest a bifunctional
mechanism; DODH occurs on ReOx and CTH on Pt in the Pt-
ReOx/C catalyst. The reactivity of a physical mixture of ReOx/
C and Pt/C for the DODH-CTH reaction being similar to that
for Pt-ReOx/C supports the bifunctional mechanism (entry 10
in Table 2).
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Figure 5. (a) Hot filtration test of the Pt-ReOx/C catalyst. For the control test, two samples were collected at 20 and 60 min. For the hot filtration
test, the reaction solution was extracted from a pressure reactor equipped with a ceramic filtration system at 170 °C after 20 min reaction. The
filtrate was submitted to the same reaction conditions for an additional 40 min. Reaction conditions: 8 (4 mmol), Pt-ReOx/C (100 mg),
isopropanol (40 mL), N2 (15 bar), 170 °C, 20 min. (b) Cumulative CO uptake (μmol/g) on the as-prepared and spent (one cycle) Pt-ReOx/C
with different pretreatment conditions. (c) ICP analysis and (d) XRD spectra of Pt-ReOx/C for the recycling test.
a
Scheme 2. Proposed Bifunctional Mechanism for DODH-CTH Tandem Reaction of 1
a
B denotes a basic site.
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Scheme 3. Isotope Labeling Experiments for CTH of (A) Acetophenone (11) and (B) trans,trans-Muconic Acid (2-diacid) with
a
Isopropanol-2-d1
a
Reaction conditions: substrate (0.5 mmol), Pt-ReOx/C (15 mg, 4.5 wt % Re, 1.8 wt % Pt), i-PrOH-2-d1 (1 mL), N2 (15 bar), 150 °C, 6 h.
To identify the active species of DODH and CTH, the
changes in oxidation states of Re and Pt during the reaction
were investigated. Under the general reaction conditions for
mucic acid conversion, the oxidation states of Re and Pt were
gradually reduced over the course of the reaction, as shown in
Figure 4 (see also Table S5). To avoid oxidation of Re during
the preparation of samples for XPS, the catalyst after a 30 min
or 6 h reaction was separated and transferred to the XPS
chamber without exposure to ambient atmosphere. In 30 min,
a significant amount of ReVII in the as-prepared sample was
reduced with the appearance of ReVI (4f7/2 peak at 43.3 eV)
and ReII (4f7/2 peak at 41.2 eV). Re in the in situ 30 min
sample consisted of ReVII (31%), ReVI (16%), ReIV (28%), and
ReII (25%). Further reduction of Re proceeded over 6 h,
making ReVI (42%), ReIV (23%), and metallic Re0 (35%). The
reaction profile in Figure 2 shows that the catalysts with the
reduced Re oxidation states also converted mucic acid through
DODH. It should be noted that DODH of the in situ
regenerated sample containing metallic Re0 and ReII as the
dominant species is shown to be inactive later in this work
(Figure 4c). These observations indicate that high oxidation
states of ReOx (ReVII, ReVI, and ReIV) are the species involved
in the DODH reaction. This is consistent with previously
suggested DODH mechanisms including ReVII/ReV or ReVI/
ReIV redox couples over heterogeneous ReOx-based cata-
lysts.33,35,36,49,50 Similarly, the reduction of Pt under the
reaction conditions resulted in 96% of metallic Pt0 and a small
amount of PtII (4%) after 6 h of reaction, indicating metallic
Pt0 as being the active species for CTH.
The reported DODH mechanism involves three steps:
formation of Re-diolate, reduction of Re species, and extrusion
of olefin.14,11,26,33,36,51−53 In the literature on heterogeneous
DODH catalysts, oxorhenium species in a nanocluster can be
the active species for DODH,33,35 while other authors
demonstrated isolated ReOx as the active species.49,50 Hot
filtration experiments were performed to investigate the
contribution of molecular species to the DODH reaction
(Figure 5a). After hot filtration at approximately 30%
conversion at 170 °C, an additional reaction with the filtrate
without the solid catalyst showed a negligible increase in the
conversion. The amount of leached Pt and Re in the hot
filtration solution is similar to that of the control test solution
filtered at room temperature after 20 min of reaction (2−3%
Re and 0.4−0.5% Pt of the original amount of metal added).
These results suggest that DODH proceeds mostly on the
heterogeneous surface, precluding the involvement of the
molecular species.33,35 ReVII and ReVI, designated as the active
species of DODH in the solid Pt-ReOx/C catalyst, form Re-
diolate by coordinating to 1 (Scheme 2). Isopropanol reduces
the rhenium diolato complex, affording acetone as a byproduct
(Table S4). The reduced oxorhenium diolate releases an
alkene (the reverse of a 3 + 2 dihydroxylation), closing the
catalytic cycle on rhenium. After the DODH cycle, the reduced
Re goes back to ReVII or ReVI. Two DODH cycles afford 2
from 1.
With regard to the mechanism of CTH over Pt, isotope
labeling experiments were conducted. Previously, both a
“monohydride mechanism” and “dihydride mechanism” have
been proposed for transition-metal-catalyzed CTH (Scheme
S2).54,55 A deuterium labeling experiment of acetophenone
(11) using deuterated isopropanol is a widely used method to
distinguish between the two possible CTH mechanisms.56,57
Pt-ReOx/C-catalyzed CTH of 11 with isopropanol-2-d1
showed 80% incorporation of deuterium at the α-C (Scheme
3A and Figure S11). This is indicative of a monohydride
mechanism. Only the α-H of the donor forms Pt−H, and this
hydride is transferred to the α-C of acetophenone exclusively
while the O−H hydrogen ends up on the carbonyl oxygen
(Scheme S2).58 However, CTH of 2-diacid with isopropanol-
2-d1 resulted in the incorporation of deuterium at both α-C
and at β-C of the molecule (Scheme 3B and Figure S12). This
indicates that the hydride (Pt−H) generated by the
monohydride mechanism can be transferred to both α-C and
β-C of 2-diacid (Scheme 2). Similarly, incorporation of metal
monohydride at α-C and β-C without regioselectivity was
observed in the literature in the hydrogenation of α,β-
unsaturated carboxylic acid.59,60 Moreover, it has been shown
that noble-metal heterogeneous catalysts with a base additive
transfer hydrogen from isopropanol to ketone or olefin through
a monohydride mechanism.28,61 In Pt-ReOx/C, the base sites
of the catalyst would act as the base additive, transferring the
O−H hydrogen in isopropanol to the olefin (Scheme 2). The
possible basic sites include basic sites of the activated carbon
support and different Pt species.62 It is reported that the basic
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Figure 6. Stability of Pt-ReOx/C for (a) DODH (8 to 9) and (b) CTH (9 to 10). Reaction conditions: substrate (4 mmol), Pt-ReOx/C (100 mg),
activated carbon (200 mg), isopropanol (40 mL), N2 (15 bar), 170 °C, 20 min. (c) ICP analysis of as-prepared Pt-ReOx/C, spent sample after 8
cycles in (a) and 12 cycles in (b). Regeneration conditions: 5% H2 in Ar, 230 °C, and 4 h, followed by reoxidation (air/120 °C/1 h).
sites of a support material play a role similar to that of base
additives in the CTH reaction.63 More details about the basic
sites of Pt-ReOx/C catalyst for CTH reaction will be clarified
in our future work.
analyzed in Figure 4 and Table S5. Diisopropyl L-(+)-tartarate
(8) was used as a substrate to study the activity of the catalysts
for DODH. While the spent catalyst exhibited oxidation states
similar to those of the as-prepared Pt-ReOx/C catalyst, it
showed the reduced conversion of 8 due to the organic
deposits. During the regeneration step at 230 °C, high
oxidation states of ReOx in the spent sample were reduced
to metallic Re0 and ReII, as measured by in situ sampling and
analysis without exposure to air (in situ regenerated sample).
This is consistent with the low reduction starting temperature
(125 °C) in TPR spectra of Pt-ReOx/C (Figure 3e) and XPS
results of the reduced Pt-Re bimetallic catalysts in previous
reports.22,23 The negligible activity of the in situ regenerated
sample indicates that the low oxidation states of Re are not
active for DODH, in agreement with previous observa-
tions.33,49 The in situ regenerated sample was reoxidized by
exposure to air and drying at 120 °C for 1 h (regenerated
sample). The regenerated sample showed oxidation states and
activity for DODH similar to those of the as-prepared catalyst,
confirming that the regeneration at 230 °C effectively removes
the organic deposits. It should also be highlighted that the
reoxidation step is necessary after the regeneration to recover
high oxidation states of Re and reactivity for DODH.
The regeneration and reoxidation were employed with the
catalyst after the second run in Table 2. After regeneration at
230 °C for 2 h and reoxidation, the reactivity of Pt-ReOx/C for
the conversion of 1 was regained in the subsequent third and
fourth runs (entries 3 and 4 in Table 2). A longer regeneration
time (4 h) under H2 at 230 °C resulted in a similar
performance of the as-prepared catalyst (Table 2, entry 5).
Over the course of five sequential reactions with the same
catalyst sample, some Re and Pt was leached out; however, this
did not affect the yield of 5 at high conversion (Figure 5c). It
has been addressed that the recycling test with a single
measurement at high conversion in a batch reaction system
might not fully reflect the deactivation of the catalyst.67,68
Thus, the catalyst stability should be tested at kinetically
controlled conversion, which is shown later in this work. On
the basis of XPS and XRD analysis, the Pt-ReOx/C catalyst
maintained its oxidation states and was still well-dispersed after
3.5. Catalyst Recycling and Regeneration. One of the
main advantages of heterogeneous catalysts is the possibility of
their reuse.64 While supported ReOx catalysts tend to
deactivate due to leaching under DODH reaction condi-
tions,15,34 some stable and reusable solid ReOx catalysts have
been reported.16,33,35 The reusability of the Pt-ReOx/C catalyst
was investigated. The recovered catalyst was tested with new
substrate 1 and isopropanol. A lower conversion and selectivity
of 5 were observed (entry 2 in Table 2). However, ICP analysis
of the catalyst after one cycle showed a small leaching of Re
(4% of the original amount of Re added) and no leaching of Pt
(Figure 5c). Thus, active site loss into solution during the
reaction did not make a major contribution to the loss of
activity and selectivity of 5. This, as well as hot filtration
experiments, is a significant point in demonstrating that the
active catalyst is indeed heterogeneous and not homogeneous
from leaching. According to CO chemisorption data, the
strongly reduced CO uptake on the spent sample (1.6 μmol/g)
was compared to the value for the as-prepared catalyst (20
μmol/g, Figure 5b). These observations suggest that organic
deposits during the reaction result in deactivation and lower
activity and selectivity. Unidentified byproducts (5−15%)
might be responsible for the organic deposits. It is not
uncommon for heavy byproducts to form a carbonaceous
deposit on the surface of catalysts in the reactions of organic
compounds.65
To recover reactivity, the spent catalyst was regenerated
under an H2 atmosphere.66 The regeneration temperature was
chosen to remove the organic deposits on the catalysts and to
minimize methanation of the support. With regeneration at
230 °C, the CO uptake of Pt-ReOx/C was restored to 16
μmol/g, which is a clear indication of the removal of organic
deposits from the surfaces of the spent catalyst (Figure 5b). A
lower temperature H2 treatment (200 °C) did not remove
organic deposits effectively. The changes in the oxidation states
and the activity of Pt-ReOx/C over the regeneration step are
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a
Table 4. Substrate Scope of DODH-CTH Tandem Reaction of Sugar Acids and Alcohols over Pt-ReOx/C
a
Reaction conditions unless specified otherwise: Pt-ReOx/C (150 mg, 4.5 wt % Re, 1.8 wt % Pt), substrate (1.0 mmol), i-PrOH (40 mL), and N2
b
c
d
1
(15 bar). Calculated by H NMR. Substrate (0.5 mmol). Calculated by GC-FID.
multiple recycling tests (Figure 5d and Figure S14). After five
consecutive cycles, ReO2 peaks disappeared, possibly due to a
change to an amorphous phase over multiple reactions and
regenerations.
Interestingly, in comparison to the bifunctional bimetallic
Pt-ReOx/C catalyst, the physical mixture of Pt/C and ReOx/C
could not be regenerated. Much lower conversions in the
second and third cycles were observed (entry 11 in Table 2
and Figure S13). The recycled physical mixture after
regeneration exhibited similar oxidation states and metal
amounts but lower CO uptake in comparison to as-prepared
catalysts (Figure S14 and Tables S6 and S7). Thus, the lower
conversion is attributed to organic deposits on ReOx/C that
were not removed during regeneration. In the physical mixture,
ReOx alone might not uptake enough hydrogen to eliminate
organic deposits. In contrast, in Pt-ReOx/C, hydrogen spillover
from Pt enhances the regeneration of ReOx sites.47,69−71 Thus,
while the Pt−ReOx interface was not critical for the
bifunctional DODH/CTH reaction, it was critical for effective
regeneration of the catalyst.
3.6. Stability Test. The stability of Pt-ReOx/C was studied
by observing changes in its activity for DODH and CTH at
low conversion levels over multiple consecutive runs. The first
run for the conversion of 8 with Pt-ReOx/C showed 33%
conversion after 20 min of reaction at 170 °C, producing 9 as a
major product through DODH (Figure 6a). The conversion
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gradually decreased until the sixth cycle, where the conversion
was 12%. This deactivation was attributed to both organic
deposits on ReOx and active site leaching. The regeneration
including reoxidation increased the conversion up to 28% in
the seventh cycle, indicating organic deposits making a major
contribution to the observed deactivation in the first six cycles.
The 4% difference between the first and seventh cycle can be
explained by the leaching of Re over the multiple runs. ICP
analysis of the catalyst after eight cycles showed some leaching
of Re, attributed to the slightly reduced conversion of the
regenerated catalyst in comparison to the as-prepared catalyst
(Figure 6c). While ReOx in the Pt-ReOx/C catalyst can be
deactivated by both fouling and leaching during the DODH-
CTH reaction, the amount of Re leached out from the catalyst
is small (Figures 5c and 6c). As a result, the catalyst could be
reusable several times after regeneration without significantly
reduced reactivity.
Interestingly, the activity of Pt sites for CTH did not change
much (Figure 6b). Over 12 consecutive runs, the conversion of
9 was slightly reduced from 40% to 30% and maintained at
around 30% even with regeneration. The results indicate that
deactivation by organic deposits on Pt sites is negligible and a
small leaching of Pt sites could be a reason for the reduced
conversion (Figure 6c). Loss of Re over the 12 runs in Figure
6b is more pronounced than that over 8 runs in Figure 6a. This
is probably because the organic deposits on ReOx in Figure 6a
inhibit the leaching of Re.
This indicates that the conversion of 1 is hampered by
competition between DODH and esterification. D-Glucaric
acid lactone can be used as a substrate to produce 5.5,16 The
conversion of D-glucaric-1,4-lactone (14) through DODH-
CTH over Pt-ReOx/C gave 41% of 15 and 15% of 5. Ring
opening of 15 is required to increase yields of 5. In addition to
C6 sugar acids, the bifunctional catalyst successfully converted
L-(+)- tartaric acid (16), a C4 sugar acid, to a 98% yield of
succinic acid and its esters (17). The same reaction was also
applied to sugar alcohols. Glycerol (18) was converted to 1-
propanol (19) in 87% yield. D-Sorbitol (20) contains six OH
groups; thus, three DODH-CTH cycles produced n-hexane
(22). All of the NMR and GC spectra of the reaction solution
in this section are shown in Figures S18−S23.
4. CONCLUSION
Bifunctional Pt-ReOx/C catalysts were prepared and shown to
produce adipates 5 from the sugar diacid 1 through tandem
DODH-CTH reactions in high yield (77% isolated adipic
acid). Evidence was provided that the oxorhenium(VI and VII)
sites are responsible for DODH. The metallic Pt0 sites in Pt-
ReOx/C significantly enhanced the selectivity of adipates 5
througth CTH without affecting DODH, as long as the Pt
loading was maintained at or below 1.8 wt %. Isopropanol
works as a hydrogen donor as well as a solvent for both DODH
and CTH. Reusability and stability tests of the Pt-ReOx/C
catalyst indicate catalyst deactivation mostly due to organic
material deposition that can be removed by regeneration with
H2. With the regeneration and reoxidation steps, the recycled
Pt-ReOx/C catalyst afforded similar conversion and selectivity
of adipates up to five cycles in a 6 h reaction, where the
conversion is high (70−80%). However, in the recycling tests
at moderate conversion (30−40%), the conversion of diol was
not fully recovered with the regeneration and reoxidation due
to some rhenium leaching. Furthermore, DODH-CTH over
the bimetallic catalyst removed hydroxyl groups in various
sugar acids and alcohols. A bifunctional mechanism was
proposed on the basis of the reactivity with various substrates,
spectroscopic analysis, and isotope labeling experiments.
The leaching of rhenium species from solid ReOx/support
catalysts in the DODH reaction was studied in detail by Jentoft
et al.15 The proposed leaching mechanism during DODH
reaction formed a rhenium diolate by chelating the surface
rhenium with olefin or diol as a chelating ligand. The extent of
leaching depended on the solubility of the resulting rhenium
diolate complexes in the solvent. The leached rhenium-diolate
species were characterized by UV−vis spectroscopy, exhibiting
a peak at 493−499 nm while the perrhenate absorption was
around 230 nm.15 To investigate the rhenium species leached
from the Pt-ReOx/C catalyst, DODH of (R,R)-(+)-hydro-
benzoin was conducted and the supernatant was analyzed by
UV−vis and NMR spectroscopy. UV−vis spectra of the
supernatant showed a peak at around 507 nm, indicating the
possibility of rhenium-diolate as a leached complex (Figure
S16). This possibility was confirmed by 1H NMR results of the
supernatant. A broad peak at 5.62 ppm might be assigned to
the diolato ring protons (Figure S17b). MTO-catalyzed
DODH of (R,R)-(+)-hydrobenzoin formed MTO-diolate,
MeReVII(OCHPhCHPhO), characterized by 1H NMR (Figure
S17a) and our previous study.52 The MTO-diolate shows a
broad peak of the diolato ring protons at 5.75 ppm and a Re−
CH3 peak at 2.7 ppm. In addition, the MTO-diolate exhibited
a peak at around 476 nm in UV−vis spectra (Figure S16). The
different peak positions in NMR and UV−vis spectra might be
due to the different structures between MTO-diolate and the
rhenium diolate leached from Pt-ReOx/C. These results
suggest a rhenium-diolate intermediate as a possible leached
rhenium species. As addressed in previous studies, leaching by
chelation with the reactant and product can be mitigated by
choosing (1) an appropriate solvent that reduces the solubility
of rhenium-diolate species and (2) a different support material
that enhances the metal−support interaction.15,35
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c04158.
Catalyst screening, optimal reaction conditions, isolation
of adipic acid, isotope labeling experiment, carbon
balance, GC-TCD of the gas phase, characterization of
the leached rhenium species, NMR spectra of the
DODH-CTH reaction with various substrates, and
additional characterization data including STEM-EDX
and TEM images, XPS spectra, CO chemisorption, and
FT-IR spectra of pyridine of as-prepared, spent,
regenerated, and reduced Pt-ReOx/C (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Mahdi M. Abu-Omar − Department of Chemical Engineering
and Department of Chemistry and Biochemistry, University of
California Santa Barbara, Santa Barbara, California 93106,
United States; orcid.org/0000-0002-4412-1985;
Email: abuomar@chem.ucsb.edu
3.7. Substrate Scope. To explore the substrate scope for
Pt-ReOx/C, we studied various diols (Table 4). Diisopropyl
mucate (13) was converted to 5-diester in high yield (90%).
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Acid over Niobic Acid-Supported Pt Nanoparticles. Chem. Commun.
2019, 55 (55), 8013−8016.
Authors
Jun Hee Jang − Department of Chemical Engineering,
University of California Santa Barbara, Santa Barbara,
California 93106, United States; orcid.org/0000-0002-
1879-0544
Insoo Ro − Department of Chemical Engineering, University of
California Santa Barbara, Santa Barbara, California 93106,
United States; Department of Chemical and Biomolecular
Engineering, Seoul National University of Science and
Technology, Seoul 01811, Republic of Korea; orcid.org/
0000-0002-4704-142X
Phillip Christopher − Department of Chemical Engineering,
University of California Santa Barbara, Santa Barbara,
California 93106, United States; orcid.org/0000-0002-
4898-5510
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.0c04158
(16) Lin, J.; Song, H.; Shen, X.; Wang, B.; Xie, S.; Deng, W.; Wu, D.;
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Notes
The authors declare no competing financial interest.
(17) Wei, Z.; Sun, J.; Li, Y.; Datye, A. K.; Wang, Y. Bimetallic
Catalysts for Hydrogen Generation. Chem. Soc. Rev. 2012, 41 (24),
7994−8008.
ACKNOWLEDGMENTS
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This work was supported by the US Department of Energy,
Office of Science, Basic Energy Science, Award No. DE-
SC0019161 (M.M.A.-O.). The use of STEM, XRD, XPS, and
ICP equipment in the UCSB MRL was supported by the
MRSEC Program of the National Science Foundation under
Award No. DMR 1720256. Fan Zhang and Ali Chamas are
acknowledged for gas product analysis. We thank Jordan Finzel
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