Synchronized C?H activations at proximate dinuclear Pd2+ sites on silicotungstate for oxidative C?C coupling

Article abstract of DOI:10.1021/acscatal.0c05207

Carbon?carbon (C?C) coupling is critically important in organic synthesis. Direct C?C coupling to replace two C?H bonds is preferred over coupling of two prefunctionalized C?intermediates but is synthetically challenging. While such coupling is feasible through homogeneous catalysis, the mechanism regarding how the two C?H bonds are activated and coupled by heterogeneous active metal atoms remains not well understood. This work demonstrates the need for a proximate metal?metal dimer site to facilitate heterogeneously catalyzed C?C coupling reactions. We demonstrate that dinuclear Pd²⁺ sites in (Pd²⁺)₂-silicotungstate (Pd₂ST) catalyzed oxidative C?C coupling of 2-methylfuran by O₂ through synchronized double C?H activations under ambient conditions, selectively producing 5,5′-dimethyl-2,2′-bifuran (DMBF). Mononuclear Pd²⁺ ions in Pd H ST and H ST are not active. The ¹³C NMR and DRIFT spectroscopies of adsorbed ¹³CO, combined with DFT and theoretical ¹³C NMR calculations, determined that dinuclear Pd²⁺ ions are separated by ～3.5 ? on Pd₂ST and 3.1 ? in the Pd^2+-C(=O)-Pd²⁺ complex. XRD and TEM are used to confirm that the most active Pd₂ST/SiO₂ catalyst has near monolayer dispersion. ²⁹Si MAS NMR is used to confirm the presence of the silicotungstate structure after calcination. The original silicotungstate Keggin structure is maintained after the Pd₂ST/SiO₂ is calcined.

Full text of DOI:10.1021/acscatal.0c05207

pubs.acs.org/acscatalysis
Research Article
Synchronized C−H Activations at Proximate Dinuclear Pd2+ Sites on
Silicotungstate for Oxidative C−C Coupling
Kishore Ramineni, Kairui Liu, Cheng Zhang, Xuke Chen, Guangjin Hou, Pan Gao, Ravi Balaga,
Mahender Reddy Marri, Peifang Yan, Xian Guan, Zhi Xia, Michael J. Janik,* and Z. Conrad Zhang*
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ABSTRACT: Carbon−carbon (C−C) coupling is critically
important in organic synthesis. Direct C−C coupling to replace
two C−H bonds is preferred over coupling of two prefunctionalized
C−intermediates but is synthetically challenging. While such
coupling is feasible through homogeneous catalysis, the mechanism
regarding how the two C−H bonds are activated and coupled by
heterogeneous active metal atoms remains not well understood.
This work demonstrates the need for a proximate metal−metal
dimer site to facilitate heterogeneously catalyzed C−C coupling
reactions. We demonstrate that dinuclear Pd2+ sites in (Pd2+)2-
silicotungstate (Pd2ST) catalyzed oxidative C−C coupling of 2-
methylfuran by O2 through synchronized double C−H activations
under ambient conditions, selectively producing 5,5′-dimethyl-2,2′-
bifuran (DMBF). Mononuclear Pd2+ ions in Pd1H2ST and H4ST
are not active. The 13C NMR and DRIFT spectroscopies of adsorbed 13CO, combined with DFT and theoretical 13C NMR
calculations, determined that dinuclear Pd2+ ions are separated by ∼3.5 Å on Pd2ST and 3.1 Å in the Pd2+C(O)Pd2+
complex. XRD and TEM are used to confirm that the most active Pd2ST/SiO2 catalyst has near monolayer dispersion. 29Si MAS
NMR is used to confirm the presence of the silicotungstate structure after calcination. The original silicotungstate Keggin structure is
maintained after the Pd2ST/SiO2 is calcined.
KEYWORDS: biomass, hemicellulose, 2-methylfuran, palladium, C−C coupling, heteropoly acids, Diels−Alder reaction
coupling of arenes with high atomic efficiency is synthetically
challenging.20
1. INTRODUCTION
Hemicellulose is the third most abundant renewable carbon
resource, after cellulose and lignin, in lignocellulosic bio-
mass.1,2 The industrial use of hemicellulose has been limited
mainly for producing monofunctionalized furanics with five
carbons (F5Cs), such as furfural, furfural alcohol, 2-
methylfuran (2-MF), and their derivatives with a total global
market size of ∼6.5 × 105 tons per year.3 Unlike cellulose, the
weak amorphous structure of hemicellulose makes it
unattractive for material applications. Carbohydrates domi-
nated by xylose are the main sugar components of hemi-
cellulose.4 5-Hydroxymethylfurfural, a six-carbon furanic plat-
form molecule (F6C), and its derivatives have two functional
groups and are produced from cellulose for use in an array of
potential applications.5−9 The limited market for the
monofunctionalized F5Cs makes it imperative to expand
their product portfolio to unlock the value of renewable
hemicellulose.2,10−15 Converting F5C platform molecules
through C−C coupling of F5C molecules is an attractive
strategy to achieve this objective by producing bifunctionalized
chemicals.16−19 However, direct C−H activation for C−C
The synthesis of 5,5′-dimethyl-2,2′-bifuran (DMBF) in 72%
yield from 2-MF has been reported via the multi-step Gilman
reaction in which 2-MF, n-BuLi, and CuCl2 in a molar ratio of
1:1:1 were used sequentially in dry THF solution at −78 °C.21
In another process, Pd(OAc)2 in the presence of trifluoroacetic
acid (TFA) catalyzed the synthesis of DMBF in DMSO, with a
turnover number (TON) of 8.622. Without TFA, the DMBF
selectivity greatly suffered.22,23 In addition to the long-standing
challenges of product separation and catalyst reuse in the
homogenously catalyzed reactions, little insights regarding the
activation of two C−H bonds and the coupling mechanism are
available. Furthermore, the role of TFA as a strong acid for the
limited improvement in DMBF selectivity is not clear in
Received: November 28, 2020
Revised: February 6, 2021
Published: March 3, 2021
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whether it is catalytically involved. An efficient catalyst capable
of achieving oxidative C−C coupling from C−H sources
without the need for a strong acid will be highly desirable to
avoid corrosion issues.
In this work, we demonstrate a new heterogeneously
catalyzed C−C coupling process requiring synchronized
activation of two C−H bonds by a pair of Pd2+ ions in close
proximity as in (Pd2+)2-silicotungstate on silica (Pd2ST/SiO2,
see Table 1 for the catalyst nomenclature). The Pd2ST/SiO2
2.1.2. Preparation of Silica-Supported PdxSTA (x = 0, 0.5,
1, and 2) Catalysts. Silica-supported catalysts were prepared
by an impregnation method.27 The required quantity of
PdxSTA was dissolved in the minimum amount of water
followed by adding to the support under constant stirring for
12 h at 25 °C. The excess water was evaporated on a hot plate,
and the catalyst masses were dried in an oven in air at 120 °C
for 12 h. The catalysts were then calcined in air at 300 °C for 2
h. These catalysts were denoted as 40 PdxSTA/SiO2 (x = 0,
0.5, 1, and 2). The Pd2ST with different loadings on silica were
also prepared for comparison.
Table 1. Catalyst Nomenclature
A similar procedure was followed for the preparation of the
Pd2ST with different loadings on SiO2 calculated from eq 1.
These catalysts were denoted as y Pd2ST/SiO2 (where y = 10−
60).
catalyst name
description
H4ST (STA)
Pd0.5STA
Pd1STA
H4SiW12O40, Silicotungstic acid
Pd0.5H3SiW12O40
Pd1H2SiW12O40
Pd2ST
5 Pd/SiO2
Pd2SiW12O40
5 wt % Pd on SiO2
weight percentage (wt%) of Pd2ST (y)
weight of Pd2ST
total weight of the catalyst (Pd2ST + SiO2)
10 Pd2ST/SiO2
20 Pd2ST/SiO2
30 Pd2ST/SiO2
40 Pd2ST/SiO2
50 Pd2ST/SiO2
60 Pd2ST/SiO2
40 Pd0.5STA/SiO2 40 wt % Pd0.5H3SiW12O40 on SiO2
40 Pd1STA/SiO2
40 STA/SiO2
10 wt % Pd2SiW12O40 on SiO2
20 wt % Pd2SiW12O40 on SiO2
30 wt % Pd2SiW12O40 on SiO2
40 wt % Pd2SiW12O40 on SiO2
50 wt % Pd2SiW12O40 on SiO2
60 wt % Pd2SiW12O40 on SiO2
=
× 100
(1)
2.2. Catalyst Characterization. Powder X-ray diffraction
(XRD) patterns of the catalysts were recorded on a
PANalytical X’Pert powder X-ray diffractometer using Ni-
filtered Cu Kα radiation (λ = 1.5406 Å) with a scan speed of 2°
min−1 and a scan range of 10−80° at 40 kV and 30 mA.
The 29Si magic-angle spinning (MAS) NMR spectra of solid
samples were acquired on an AVANCE III 600M wide bore
solid-state NMR spectrometer operating at 1H and 29Si Larmor
frequencies of 600.13 and 119.22 MHz, respectively. Single-
pulse 29Si spectra with high-power proton decoupling were
acquired with a 4 mm probe with sample spinning at 8 kHz.
The π/4 excitation and a recycle delay of 10 s were used during
acquisition, and the chemical shifts were calibrated using
kaolinite (−91.5 ppm), a second reference to tetramethylsilane
(TMS).
The transmission electron micrograph (TEM) images were
collected on a JEM-2100 transmission electron microscope.
The acceleration voltage was 200 kV. Samples for TEM were
prepared by placing droplets of a suspension of the sample in
ethanol on an ultrathin carbon film supported on a Cu grid.
Laser Raman spectra of the samples were collected with a
Horiba-Jobin Yvon LabRam-HR spectrometer equipped with a
confocal microscope, 2400/900 grooves per mm gratings, and
a notch filter. The visible laser excitation at 532 nm (visible/
green) was used. The scattered photons were directed and
focused onto a single-stage monochromator and measured
with a UV-sensitive LN2-cooled CCD detector.
Temperature-programmed desorption of ammonia (TPD of
NH3) was carried out on a MICROMERITICS AutoChem II
2920 automated catalyst characterization system. In a typical
experiment, about 0.1 g of the sample was pretreated at 300 °C
for 1 h by passing helium (99.9%, 50 mL min−1) through. After
pretreatment, the sample was saturated with anhydrous
ammonia (10% NH3 balance He) at 100 °C for 1 h and
subsequently flushed with He at the same temperature to
remove physisorbed ammonia. Then, the TPD analysis was
carried out from ambient temperature to 800 °C at a heating
rate of 10 °C min−1 and the desorbed NH3 was measured
using a gas chromatograph equipped with a thermal
conductivity detector.
40 wt % Pd1H2SiW12O40 on SiO2
40 wt % H4SiW12O40 on SiO2
40 M2ST/SiO2
40 wt % M2SiW12O40 on SiO2 where M = Cu2+, Fe2+,
Co2+, Ni2+, and Ru2+
catalyst uniquely enabled oxidative C−C coupling of 2-MF
with O2 at 25 °C in a dimethyl sulfoxide (DMSO) solvent,
producing DMBF in 95% yield. The byproduct (5%) was
identified as 1-(5-methylfuran-2-yl)pent-2-ene-1,4-dione, a
ring-opening derivative following the C−C coupling. We
further show that DMBF is readily converted to 4,4′-
dimethylbiphenyl (DMBP) by a tandem Diels−Alder reaction.
DMBP as a molecular analogue of petroleum-derived para-
xylene offers a renewable platform for high-performance
materials. The high C−C coupling efficiency demonstrated
in this work avoids costly substrates such as for the Suzuki
reaction, which requires stoichiometric amounts of expensive
arylboronic acid and arylbromide precursor molecules.24,25
2. EXPERIMENTAL SECTION
2.1. Catalyst Preparation. 2.1.1. Preparation of Palla-
dium-Exchanged Silicotungustic Acid Catalysts. Palladium-
exchanged silicotungstic acid (STA) catalysts with a varying
content of palladium in the secondary structure of STA were
prepared according to the reported procedure.26 STA (5 g,
1.73 mmol) was dissolved in 20 mL of deionized water and
stirred vigorously at room temperature. Then, the required
amount of aqueous solution of PdSO4 (palladium (II) sulfate
dihydrate) was added dropwise to the above solution. The
palladium content, x in PdxH4−2xSiW12O40 (x = 0.5, 1, and 2)
was adjusted using the required amount of palladium. The
resulted mixture was aged overnight at 25 °C, and the excess
water was removed on a rotavapor. The solid mass obtained
was dried at 120 °C for 12 h. The prepared catalysts with a
varying Pd content in PdxH4−2xSiW12O40 (x = 0.5, 1, and 2) are
denoted as Pd0.5STA, Pd1STA, and Pd2ST, where the number
indicates the number of Pd ions exchanged per formula unit
(Table 1).
Brunauer−Emmett−Teller (BET) surface area was meas-
ured with a commercial BET unit (ASAP 2020 Micromeritics
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volumetric adsorption analyzer) using N2 adsorption at 77 K.
Before the N2 adsorption, samples were treated under vacuum
for 6 h at 300 °C. The surface area of the sample was
calculated from the nitrogen isotherm using the BET method.
Fourier transform infrared (FT-IR) spectra of catalysts were
taken on a Thermo Fisher Nicolet iS50Adv FTIR spectrometer
using the KBr disc method. The nature of the acid sites
(Bronsted and Lewis) of the catalysts was determined by FT-
IR spectroscopy with chemisorbed pyridine. The pyridine
adsorption studies were carried out in the diffuse reflectance
infrared Fourier transform (DRIFT) mode. Prior to the
pyridine adsorption, catalysts were degassed under vacuum at
200 °C for 1 h followed by pyridine adsorption at 200 °C and
subsequent evacuation at 120 °C to remove physisorbed
pyridine. After cooling the sample to room temperature, FT-IR
spectra of the pyridine-adsorbed samples were recorded. The
elemental analysis of the samples was measured by a
PerkinElmer OPTIMA 8000 ICP spectrometer, inductively
coupled plasma mass spectrometer (ICP-MS).
Products were analyzed with an Agilent 7890 GC/5975 MS
using a HP-5 column (30 m × 0.32 mm i.d. × 0.25 μm film
thickness, Agilent Technologies, Inc.). The GC injector was
kept at 250 °C, the inject split ratio was 50:1, and the GC oven
was programmed with the following temperature regime: hold
at 50 °C for 5 min, heated to 275 °C at 10 °C/min. The
products were quantified by GC-FID with a calibration curve
after separation. Experiments were carried out in triplicates,
and the average values are reported.
LANL2DZ basis set was employed for W and Pd atoms with
effective core potentials (ECPs) for their core electrons. For
the 13C NMR calculation, the GIAO method was used, and the
13C NMR chemical shift was referred to Si(CH3)4. For the 13
C
NMR of Si(CH3)4, cc-pVDZ basis sets were employed for Si,
O, and C atoms.
2.3.1. Model Construction. The four protons of STA
(H4SiW12O40) were replaced by two charge compensating Pd
atoms (Pd2SiW12O40, Pd2ST). The position of Pd atoms on
the isolated ST Keggin unit was varied, and their optimal
position determined. We also explored the possibility of charge
compensating Pd atoms bridging between multiple Keggin
units in a two-dimensional periodic structure with an
optimized 9.9 × 9.9 × 20 Å unit cell.31
The oxidative coupling cycle can occur through initial 2-
methylfuran or oxygen adsorption, and therefore, the redox
cycle has the possibility of occurring through different Pd
oxidation states. We determined that reaction energies were
most favorable without including reduction to structures
including less than 40 O atoms (Pd atoms in formal +2
states). Therefore, reaction chemistry is discussed as initiating
through O2 adsorption. Reaction energies were computed for
all elementary steps in the reaction cycle. For each adsorbed
intermediate structure, we explored numerous possible
adsorption configurations and report only the most stable
found. Due to the wide assortment of possible configurations
of the two charge-compensating Pd atoms and O, H, and
C5OHx adsorbates, we cannot assure that we have identified
the global minimum energy structure. We report reaction
energetics to demonstrate that there is a viable reaction path
for 2-methylfuran oxidative coupling on Pd2ST and illustrate
the importance of Pd−Pd proximity in facilitating C−C
coupling.
All products were identified by NMR spectroscopy in
deuterated chloroform (CDCl3) using a Bruker AVANCE III
400 spectrometer instrument. Chemical shifts are reported in
parts per million (ppm) downfield from TMS.
2.3. Density Functional Theory Methods. The Vienna
ab initio simulation package (VASP) was used to determine
optimal structures of paired metal ions adsorbed on ST and to
investigate the reaction chemistry of oxidative C−C coupling
of 2-methylfuran.28,29 Because the dispersion of STA on SiO2
is a near monolayer, a molecular model of ST was used,
including a single Keggin unit in a 14 × 14 × 20 Å unit cell that
provides sufficient vacuum space between adjacent cells to
represent the isolated Keggin/adsorbate system.30 Periodic
density functional theory (DFT) calculations were used for
structural optimization. To describe electron−ion interactions,
the projector-augmented wave (PAW) method was used. Pd
(4p, 5s, 4d;16 electrons), W (5p, 6s, 5d; 12 electrons), Si (3s,
3p; 4 electrons), O (2s, 2p; 6 electrons), C (2s, 2p; 4
electrons), and H (1s, 1 electrons) are represented with PAW-
PBE core potentials. An energy cutoff of 450 eV for the plane
wave basis set was used.30,31 The Perdew−Burke−Ernzerhof
(PBE) generalized gradient exchange-correlation functional
was used for all VASP calculations.32 A small smearing of σ =
0.003 eV was used to assist in convergence. Gamma point
calculations were performed for the isolated ST systems. The
van der Waals (vdW) forces were calculated using the method
of Grimme et al. as implemented in VASP.33−36 Optimizations
were carried out until the atomic forces were below 0.02 eV/Å
for geometry optimizations. All optimizations used the
conjugate gradient method.
2.4. Catalytic Reaction Tests. 2.4.1. Oxidative C−C
Coupling of 2-Methylfuran. The oxidative C−C coupling of
2-methylfuran (2-MF) reaction was performed at 25 °C in a 10
mL Teflon autoclave batch reactor system. Typically, 1 mmol
of 2-MF was added to the batch reactor that contained 1.5 mL
of DMSO and the catalyst (5.2 mol % Pd to 2-MF). Prior to
the reaction, the batch reactor vessel was purged by N2 gas to
prevent contamination from moisture and impurities. The
reaction mixture was stirred at 25 °C under 1 MPa oxygen
pressure. Small aliquots (0.1 mL) of the samples were taken
from the vessel at different reaction times. The collected liquid
product was analyzed by a gas chromatograph (GC) with a
flame ionization detector (FID). The product was identified by
GC−MS. After completion of the reaction, the reaction
mixture was diluted with 10 mL of acetonitrile solvent, from
which the catalyst was recovered by centrifugation and
acetonitrile was removed on a rotavapor. To the remaining
concentrated reaction mixture, 5 mL of water was added, and
the resulting solution was extracted with hexane (20 mL × 4);
the combined organic layer was washed with saturated aqueous
brine solution, dried over (anhydrous) Na2SO4, and filtered,
which was then concentrated to remove hexane under vacuum.
1
The resulted product was confirmed by H NMR and 13C
NMR. Integration of the protons in the spectrum showed that
the main product is in 95%. The byproduct [1-(5-methylfuran-
2-yl) pent-2-ene-1,4-dione, >5%] was identified by GC−MS
For the 13C NMR chemical shift calculation, Gaussian 09
was used.37 Before the NMR calculation, the model structure
was first optimized using Gaussian 09 with a DFT B3LYP
theory, and two kinds of basis sets were used in this work. cc-
pVDZ basis sets were employed for Si, O, and C atoms, and
1
and confirmed by H and 13C NMRs.
2.4.2. Scale-Up and Catalyst Recyclability Test for
Oxidative C−C Coupling of 2-Methylfuran. The scale-up
and catalyst recyclability tests for the oxidative C−C coupling
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of 2-methylfuran (2-MF) reaction was performed at 25 °C in a
500 mL Teflon autoclave batch reactor system. In this typical
reaction, 50 mmol of 2-MF was added to the batch reactor that
contained 50 mL of DMSO and the catalyst (5.2 mol % Pd to
2-MF). Prior to the reaction, the batch reactor vessel was
purged by N2 gas to prevent contamination from moisture and
impurities. The reaction mixture was stirred at 25 °C under a 1
MPa oxygen pressure. The reaction was followed by collecting
the liquid product and performing analysis by GC with a FID.
After the reaction is completed, the reaction mixture was
diluted with 100 mL of acetonitrile, from which the catalyst
was recovered by centrifugation. The recovered catalyst was
dried in the oven at 120 °C for 12 h and used for the next cycle
without any further treatment. To the remaining concentrated
reaction mixture, 100 mL of water was added, and the resulting
solution was extracted with hexane (200 mL × 4); the
combined organic layer was washed with saturated aqueous
brine solution, dried over (anhydrous) Na2SO4, filtered, and
then concentrated by removing hexane under vacuum.
Table 2. BET Surface of Different Catalysts
catalyst
SiO2
SBET (m2/g)
124
114
94
81
77
10 Pd2ST/SiO2
20 Pd2ST/SiO2
30 Pd2ST/SiO2
40 Pd2ST/SiO2
50 Pd2ST/SiO2
60 Pd2ST/SiO2
50
40
The transition in surface coverage of supported transition-
metal oxide on a supporting oxide from below monolayer to
above monolayer is typically assessed by X-ray diffraction.38
A
distinction can be made as the highly dispersed metal oxide on
the support does not show an XRD pattern of its bulk oxide
structure at or under monolayer, but the characteristic
crystalline structure pattern of the supported transition-metal
oxide emerges rapidly at loadings above monolayer dispersion.
The XRD patterns of y Pd2ST/SiO2 with different loadings (y)
and unsupported Pd2ST are shown in Figure 1B. The XRD
patterns in Figure 1B show the transition from under to near
monolayer Pd2ST dispersion on the amorphous SiO2 support,
as evidenced by the lack of a significant change in the XRD
patterns at Pd2ST loadings of 10−40% (Figure 1B, diffracto-
grams b−e). The prominent appearance of the intense XRD
patterns corresponding to the crystalline arrangement of
Pd2ST Keggin structure at 50% Pd2ST loading (Figure 1B, f)
unambiguously indicates that the Pd2ST loading of 50% on the
SiO2 support has exceeded monolayer Pd2ST dispersion. At
60% Pd2ST loading, the peaks corresponding to the Keggin
crystalline structure were further increased (Figure 1B, g). In
addition, the formation of the clearly extended crystalline
structure at 50 and 60% Pd2ST loadings suggests that the
Pd2ST unit remained intact after calcination at 300 °C. The
metal cation-exchanged Keggin structure is stable at 400 °C.39
The BET surface area of the silica support is 124 m2/g.
Taking the molecular weight (MW) of 3087 g/mol for the
Keggin unit Pd2ST, the density of the Keggin unit normalized
against the silica surface area is near 0.15 μmol/m2 for the
supported Pd2ST on silica in this work. This Keggin unit
density matches well the calculated value for a silica supported
H3PW12O40 Keggin unit at near monolayer dispersion in the
work of Corma et al.40 (see the Supporting Information on the
basis of the calculations). In addition, the BET surface area
sharply decreased from 77 to 50 m2/g when the Pd2ST loading
was increased from 40 to 50% (Table 2), which is also
consistent with the transition from near monolayer dispersion
of Pd2ST on SiO2 to above monolayer with the formation of
more crystalline particles of Pd2ST.
2.4.3. Diels−Alder Cycloaddition of 5,5′-Dimethyl-2,2′-
bifuran (DMBF). The Diels−Alder cycloaddition reaction was
performed at different temperatures in a high-pressure batch
reactor system. Typically, 1 mmol of 5,5′-dimethyl-2,2′-bifuran
(DMBF) was added to the batch reactor that contained 5 mL
of 1,4-dioxane and 100 mg of 40 STA/SiO2. Prior to the
reaction, the batch reactor vessel was purged by N2 gas to
prevent contamination from air, moisture, and impurities.
Ethylene (purity 99.9%) was then introduced into the reactor
at 2.5 MPa pressures. The reaction mixture at different
temperatures was monitored by GC and the product was
identified by GC−MS, and the final DMBP product was
1
confirmed by H NMR and 13C NMR to be pure without
impurities.
The 2-MF conversion and DMBF selectivity are calculated
by eqs 2 and 3, respectively.
moles of 2‐MF consumed
conversion of 2‐MF =
moles of 2‐MF charged
(2)
moles of 2‐MF turned into DMBF
selectivity of DMBF =
moles of 2‐MF consumed
(3)
The turnover number (TON) of the catalyst is calculated by
eq 4
moles of the converted 2‐MF
TON =
moles of Pd in the catalyst
(4)
3. RESULTS AND DISCUSSION
A systematic study was focused on y Pd2ST/SiO2 catalysts (y is
wt % of Pd2ST loading, as specified in Table 1). The BET
surface area (Table 2) decreased with increasing Pd2ST
loading. The decrease became significant when the Pd2ST
loading exceeded 40 wt %.
The X-ray diffractograms in Figure 1A indicate the Keggin
structure was retained following exchange of H+ with Pd2+
ions. In the insert spectra, a shift near 2θ = 25.5° was observed
for the catalyst containing Pd in the secondary structure of
STA. Earlier reports showed that silver-exchanged salts of
heteropoly acids had the same cubic crystalline structure as
that of pure HPAs, but with a contracted unit cell.26
The transmission electron micrographs (Figure 2) show the
images of 30−50 wt % Pd2ST loadings on the SiO2: 30 Pd2ST/
SiO2, 40 Pd2ST/SiO2, and 50 Pd2ST/SiO2. In Figure 2a, the
silica support was not completely covered by Pd2ST when the
Pd2ST loading was 30%. At 40% Pd2ST loading, there was a
full overlay of Pd2ST on the surface of silica, but no obvious
overlap was observed between Pd2ST particles (Figure 2b).
The results of Figure 2a,b indicate the under monolayer
dispersion of Pd2ST when the loadings were less than 40%. At
50% Pd2ST loading, the overlap among Pd2ST particles
appeared (Figure 2c), indicating that the particles of Pd2ST
appeared above monolayer dispersion on the silica surface.
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Figure 1. (A) XRD (zoomed) of bulk STA and Pd2ST (insert: zoomed spectra at 2θ between 24 and 27). (B) XRD spectra of (a) SiO2, (b) 10
Pd2ST/SiO2, (c) 20 Pd2ST/SiO2, (d) 30 Pd2ST/SiO2, (e) 40 Pd2ST/SiO2, (f) 50 Pd2ST/SiO2, (g) 60 Pd2ST/SiO2, and (h) unsupported Pd2ST
catalysts.
Figure 3. 29Si magic-angle spinning (MAS) NMR spectra of different
solid samples.
1020 cm−1 (Si−O), 979 cm−1 (WO), 877 cm−1 (W−O−W,
inter-octahedral), and 778 cm−1 (W−O−W, intra-octahe-
dral).27 These characteristic bands are found in the spectra of
the Pd2ST catalyst, indicating that the Keggin structure
remained intact after Pd exchange. For the 40 Pd2ST/SiO2,
the main FT-IR bands observed at 1105 cm−1 (strong and
broad), 811 cm−1 (medium), and 472 cm−1 (strong) may be
ascribed to that of pure silica.27 It is difficult to distinguish the
Figure 2. TEM images of 30−50 wt % Pd2ST loadings on the SiO2:
characteristic Keggin ion band at 1020 cm−1 after impregnating
(a) 30 Pd2ST/SiO2, (b) 40 Pd2ST/SiO2, and (c) 50 Pd2ST/SiO2.
Pd2ST on SiO2 due to the strong absorption of silica in that
Figure 3 shows the 29Si MAS NMR spectra of the silica, 40
Pd2ST/SiO2, STA, and Pd2ST. 29Si MAS NMR spectra of pure
STA and Pd-exchanged form of STA (Pd2ST) show a single
peak at 84.8 ppm. 29Si MAS NMR spectra of 40 Pd2ST/SiO2
show three chemical shifts at −84.8, −101.8, and −111.7 ppm.
These signals result from Pd2ST (−84.8 ppm) and silica
(−101.8 and −111.7), indicating that, after Pd2+ exchange
followed by impregnation and calcination of 40 Pd2ST/SiO2,
the Keggin unit retains its original structure. There is no
spectral indication of other lacunary Keggin structures.41 The
pH at which the Pd2ST was prepared and was loaded onto
SiO2 may not favor the formation of other structures.42
region. The characteristic IR bands of the Keggin ion are
observed at 979 and 778 cm−1 (W−O−W, intra-octahedral),
indicating the existence of Pd2ST Keggin ions on the support.
However, the 877 cm−1 band associated with the inter-
octahedral W−O−W bond became considerably weak,
consistent with the dispersed Keggin type structural units
with reduced inter-unit bonding for the dispersed Pd2ST on
silica.
Raman spectra of pure STA, Pd2ST, and 40 wt % Pd2ST/
SiO2 are shown in Figure 4B. Pure STA exhibited a
characteristic sharp band at 997 cm−1 with a distant shoulder
at 970 cm−1 that was ascribed to symmetric and asymmetric
stretching modes of υ(WO), respectively. An additional
band was observed at 875 cm−1, which was assigned to the
asymmetric stretching mode of υ(W−O−W).26 The strong
band at 243 cm−1 corresponded to the W−O−W bending
The FT-IR spectra of STA, Pd2ST, and 40 Pd2ST/SiO2 are
shown in Figure 4A. The bulk STA spectra displayed
absorption features in the range of 1100−700 cm−1. The
pure STA showed the characteristic infrared absorptions at
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Figure 4. (A) FT-IR of STA, Pd2ST, and 40 Pd2ST/SiO2. (B) Raman of pure STA, Pd2ST, and 40 Pd2ST/SiO2.
a
Table 3. Results of Catalyst Screening for Oxidative C−C Coupling of 2-Methylfuran
d
d
d
entry
catalyst
reaction time (h)
oxidant (MPa)
2-MF conversion (%)
DMBF selectivity(%)
TON
1
2
3
4
5
6
7
8
9
10
11
12
13
10 Pd2ST/SiO2
20 Pd2ST/SiO2
30 Pd2ST/SiO2
40 Pd2ST/SiO2
50 Pd2ST/SiO2
60 Pd2ST/SiO2
Pd2ST bulk
40 Pd2ST/SiO2
40 Pd2ST/SiO2
40 Pd2ST/SiO2
5 Pd/SiO2
24
24
24
24
24
24
24
24
24
24
48
24
24
24
O2 (1 MPa)
O2 (1 MPa)
O2 (1 MPa)
O2 (1 MPa)
O2 (1 MPa)
O2 (1 MPa)
O2 (1 MPa)
O2 (0.1 MPa)
air (1 MPa)
N2 (1 MPa)
O2 (1 MPa)
O2 (1 MPa)
O2 (1 MPa)
O2 (1 MPa)
49
65
75
100
79
70
20
67
25
0
95
95
95
95
95
95
95
95
95
n/d
70
95
0
9.4
12.5
14.4
19.2
15.2
13.5
3.8
12.9
4.8
n/d
n/d
7.3
<5
38
<15
0
b
40 Pd2ST/SiO2
40 M2ST/SiO2
40 H4ST/SiO2
c
n/d
0
14
0
a
Reaction conditions: 2-MF (1 mmol) in DMSO solvent (1.5 mL), catalyst (5.2 mol % Pd with respect to 2-MF), 1 MPa O2 at 25 °C for 24 h. n/
b
c
d
d: not determined. DMF as the solvent. M = Cu2+, Fe2+, Co2+, Ni2+, and Ru2+ at 70 °C. Conversion of 2-MF, selectivity of DMBF, and TON
defined in eqs 2−4, respectively.
mode of the intact Keggin. All these characteristic bands
remained for the Pd2ST, indicating the intact Keggin structure
after the exchange of protons with palladium. For the 40
Pd2ST/SiO2, the Raman band characteristic of the Keggin
structure can also be observed, indicating that the Keggin
structure was also maintained on the support. The low intense
broad band is a result of fine dispersion and the interaction
between Pd2ST and the SiO2 surface.
The performances of y Pd2ST/SiO2 (y = 10−60 wt %) were
evaluated for C−C coupling of 2-MF by varying the amount of
catalyst to maintain a constant Pd loading of 5.2 mol % with
respect to the amount of 2-MF under 1 MPa of O2 at 25 °C
(entries 1−6, Table 3 with detailed reaction conditions). This
group of catalysts all showed high selectivity (95%) to the
coupled product DMBF, even at full 2-MF conversion (100%)
corresponding to a TON of 19.2 on the 40 Pd2ST/SiO2 (entry
4, Table 3). The maximum TON of 19.2 was reached with the
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40 Pd2ST/SiO2 when the reactant, 2-MF, was fully converted
as the TON was calculated by dividing the number of moles of
converted 2-MF by the number of moles of Pd on the catalyst.
A further increase of Pd2ST loading above 40 wt % resulted in
decreased 2-MF conversion (entries 5 and 6, Table 3). The 2-
MF conversion using unsupported Pd2ST was only 20% (entry
7, Table 3) under the same conditions due to its low surface
area. The trend in 2-MF conversion with this series of catalysts
is in clear accordance with the dispersion of Pd2ST on the SiO2
support. The maximum 2-MF conversion with the 40 Pd2ST/
SiO2 may be attributed to the near monolayer dispersion of
Pd2ST on SiO2.38 The lower conversions with Pd2ST loadings
of <40 wt % may be due to more adsorbed reactant/product
on the exposed SiO2 surface. The conversion became lower
under reduced O2 pressure (0.1 MPa) (entry 8, Table 3). The
reactions were also carried out in air and under inert
conditions. The activity was low under air (entry 9, Table 3)
due to low O2 partial pressure, and no reaction was observed
under inert conditions (entry 10, Table 3). However, SiO2-
supported Pd (PdSO4, 5 wt %) was a poor catalyst in
converting 2-MF (<5% conversion) under the same conditions
(1 MPa O2, entry 11, Table 3).
DMSO is a suitable solvent for 2-MF conversion. 40 Pd2ST/
SiO2 in dimethylformamide (DMF) solvent showed lower 2-
MF conversion (entry 12, Table 3). Other metal ion-
exchanged STA (40 M2ST/SiO2), including Cu2+, Fe2+,
Co2+, Ni2+, and Ru2+ showed poor performances (entry 13,
Table 3), failing to produce DMBF even at temperatures up to
70 °C. The detailed reaction conditions are given in Table S1.
The metals Cu2+ and Ru2+ showed very low (15 and 10%,
respectively) conversions of 2-MF converted into furfural at 70
°C. All the metals (Cu2+, Fe2+, Co2+, Ni2+, and Ru2+) showed
no conversion at room temperature. As expected, in the
absence of Pd2+ as with the 40 H4ST/SiO2, no 2-MF
conversion was observed (entry 14, Table 3). Therefore, the
results suggest that the structural affiliation of Pd2+ with STA
in the highly dispersed Pd2ST on the SiO2 surface is
responsible for its unique catalytic performance in direct
oxidative C−C coupling of 2-MF. It is intriguing to note the
unchanged selectivity to DMBF at various conversion of 2-MF
on the Pd2ST/SiO2 catalysts (Table 3) and with a varying
degree of Pd exchange in PdxSTA/SiO2 (x = 0, 0.5, 1, and 2)
catalysts (Table 4). This selectivity is characteristic of the
dinuclear Pd-exchanged ST catalyst. As Pd1H2ST and H4ST
species are not active, the same dinuclear site accounts for the
selectivity in all of these catalysts. The protons do not
contribute to the catalysis of 2-MF conversion. Even for the
nominal Pd1H2ST catalyst prepared by the ion change of 1
mole of Pd2+ to 1 mole of STA, the formation of a fraction of
Pd2ST units due to uneven exchange is responsible for the 2-
MF conversion, although the amount of Pd2ST is much lower
(Table 4, Figure 5a,d). Therefore, the results indicate that the
single site nature of the proximate dinuclear Pd ions
determines the conversion and selectivity for the PdxSTA/
SiO2 (x = 0, 0.5, 1, and 2) catalysts. When this type of active
site does not exist in other metal ions exchanged STA, the
selectivity to DMBF is actually 0 at 2-MF conversions of up to
15% (entry 13, Table 3). While the Pd/SiO2 showed low 2-MF
conversion, it had a DMBF selectivity of 70% along with 30%
selectivity to furfural.
We further investigated the effect of the Pd2+ ion density in
the PdxH4−2xSiW12O40 structure on the catalytic C−C coupling
performance. We synthesized PdxH4−2xSiW12O40, abbreviated
as PdxSTA in subsequent discussions, with a varied Pd2+/H+
ratio (the catalyst preparation method in the Supporting
Information). The catalytic performances of the 40 PdxSTA/
SiO2 (x = 0, 0.5, 1, and 2) catalysts, at the fixed 2-MF amount
and Pd-to-2-MF ratio, are shown in Table 4. Corresponding to
the x values of 0.5, 1, and 2 in PdxSTA/SiO2, the 2-MF
conversion was increased from 0 at x = 0 to 10, 30, and 100%,
respectively, while the selectivity to DMBF was again nearly
unchanged at 95%. Strikingly, the TON increased exponen-
tially with the increased level of exchanged palladium (Table
4). The acidity of smaller x values in the PdxSTA/SiO2 showed
a little impact on the selectivity of DMBF in oxidative coupling
of 2-MF, consistent with the inactive nature of STA protons to
2-MF and its indistinguishable participation in the catalysis by
Pd2ST under the reaction conditions. This trend of C−C
coupling activity in relationship to the x value of the PdxSTA/
SiO2 catalyst suggests that a synergistic effect of Pd2+ ions at
the full-exchange level played a particularly impactful role in
catalyzing the C−C coupling.
We therefore further investigated the possible surface
structural features responsible for the synergy among Pd2+
ions on the 40 PdxSTA/SiO2 with varying x by diffuse
reflectance infrared Fourier transform (DRIFT) spectroscopy
of adsorbed CO. DRIFT spectroscopy of chemisorbed CO on
metal sites is one of the most sensitive techniques in
characterizing the dispersion of ionic and metallic sites in
supported metal catalysts. In literature, νCO frequencies for
CO on Pd have been assigned as follows: Pd3+−CO, 2215−
2195 cm−1; Pd2+−CO, 2190−2135 cm−1; Pd+−CO, 2140−
0
2120 cm−1; Pd0−CO (atop), 2115−2000 cm−1; Pd2 −CO
(double bridging associated with different Pd particle sizes and
supports), 2000−1900 cm−1; and Pd3 −CO (triple bridging)
0
below 1900 cm−1.43 Specific samples of Pd C(O)Pd
include that of Pd0C(O)Pd0 with bridging CO
44
absorption at 1956 and 1970 cm−1 and that Pd+C(
O)Pd+ with bridging CO absorption at 1950 cm−1.45 Except
for the absorption band in this range to indicate the bridging
CO bonded to two Pd atoms, the charge on the Pd atoms did
not appear to make a significant difference on the IR
absorption band. In other words, the CO IR absorption
band is not sensitive to the oxidation state of the Pd atoms.
In this work, the Pd(II) cations are introduced to the Keggin
unit by ion exchange of the charge compensating protons. In
the FT-IR spectral region of 1700−2400 cm−1 in Figure 5a, the
broad peaks common to the Pd0.5ST/SiO2, Pd1ST/SiO2, and
Pd2ST/SiO2 are attributed to the SiO2 support background.
Table 4. Effect of the Palladium Exchange Level in PdxSTA/
SiO2 on 2-MF C−C Coupling Activity Based on the Same
a
Total Pd and on the CO-DRIFT Peak Area at 1953 cm−1
2-MF
DMBF
peak area of
CO-DRIFT
entry
catalyst
conv (%) sel (%) TON
1
2
3
4
40 H4ST/SiO2
0
0
0
0
40 Pd0.5STA/SiO2
40 Pd1STA/SiO2
40 Pd2ST/SiO2
10
30
100
95
95
95
1.8
5.5
19.2
0.05
0.12
0.36
12
The 1953 cm−1 band is attributed to Pd C(O)Pd
a
Reaction conditions: 2-MF (1 mmol), all catalysts (except 40 H4ST/
(bridging) carbonyl species, which is noted as a strong band in
Figure 5a. The DRIFT spectroscopy of chemisorbed 13CO on
SiO2) used were based on 5.2 mol % Pd with respect to 2-MF, O2 (1
MPa) in DMSO (1.5 mL) at 25 °C, 24 h.
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Figure 5. (a) In situ DRIFT of CO adsorbed on 40 PdxSTA/SiO2 (x = 0, 0.5, 1, and 2) at 20 °C; no CO DRIFT signal was observed at x = 0
(insert: 13CO-DRIFT of 13CO adsorbed on 40 Pd2ST/SiO2). The samples were exposed to CO flow for 2 min at 20 °C and then purged with Ar
for 10 min. (b) Optimized DFT structures for STA and Pd2ST. (c) 13C NMR spectra of 13CO-treated 40 Pd2ST/SiO2 and the corresponding
calculated Pd2ST model. (d) Plot of TON vs CO-DRIFT peak area of respective catalysts (40 PdxSTA/SiO2). (e) Scale-up and reusability of the
catalyst (reaction conditions: 2-MF (50 mmol), 40 Pd2ST/SiO2 catalyst (5.2 mol % Pd to 2-MF), 50 mL of solvent (DMSO), pressure: 1 MPa O2,
25 °C, 24 h).
13
40 Pd2ST/SiO2 verified the Pd C(O)Pd, with the
13CO DRIFT peak at 1909 cm−1, as shown in the insert of
Figure 5a.46 No FT-IR peak was observed for CO adsorbed on
the isolated Pd ion. During the FT-IR experiment, there was
no direct experimental evidence for the reduction of the Pd
ions after introduced CO. Since the CO absorption band is
not sensitive to the oxidation state of Pd atoms as supported by
literature, it is likely that the Pd atoms remained 2+ state. It has
been reported that CO exposure at ambient temperature can
induce the agglomeration of the Pd0 atom.47 In the case of Pd
nanoparticles, linearly adsorbed CO on Pd0 would be expected
to exhibit intense bands.47 The absence of the linearly
adsorbed CO IR absorption rules out the formation of Pd0.
of the TON values against the peak areas of the 1953 cm−1
band of the 40 PdxSTA/SiO2 (x = 0, 0.5, 1 and 2) catalysts
(Figure 5d) shows a nearly linear relationship. Thus, the results
suggest that the oxidative C−C coupling activity is linearly
correlated to the paired palladium atoms in close proximity
(hereafter, we use Pd2) on ST. Accordingly, the mononuclear
Pd12+ and H+ sites did not appear to contribute to the coupling
of 2-MF.
The observation of the CO DRIFT band for the PdC(
O)Pd species and its linear relationship with the TON of
the 40 PdxSTA/SiO2 (x = 0, 0.5, 1 and 2) catalysts for the
oxidative C−C coupling of 2-MF inspired us to verify and
determine the detailed structure of the Pd2+C(O)Pd2+
species with 13C NMR spectroscopy. As shown in Figure 5c,
consistent with the prominent signal at δ =182.9 ppm, the
DFT calculation based on the Pd2+C(O)Pd2+ resulted
in the 13C NMR chemical shift at 186.4 ppm. Therefore, the
bridging 13CO may be unambiguously assigned to the Pd2+
C(O)Pd2+ species in 40 Pd2ST/SiO2. Both the DRIFT
and 13C NMR spectra of adsorbed CO reveal the dominant
presence of Pd2+ ion pairs that exponentially increase with the
Pd exchange level. The peak at 125.2 ppm (Figure 5c) is
ascribed to gaseous 13CO2 produced from the oxidation of
13CO by oxygen on the 40 Pd2ST/SiO2 catalyst.
Further characterization of the Pd2+ C(O)Pd2+ was
13
made by 13C NMR, which is presented and discussed below.
As the STA/SiO2 showed no catalytic activity in 2-MF
conversion, the active sites must be associated only with the Pd
ions. As the FT-IR absorption band corresponding to Pd
C(O)Pd is the only peak that grows with the Pd exchange
level, this only CO-adsorbed Pd site is the most plausible sites
responsible to the catalysis.
The signal intensity (Figure 5a) of the band at 1953 cm−1, as
measured by the integrated peak area (Table 4), increased
exponentially with increasing x in 40 PdxSTA/SiO2. The plot
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Density functional theory (DFT) calculations were used to
investigate the distribution of paired palladium atoms on the
silicotungastate (ST), and the importance of Pd−Pd proximity
to C−C coupling chemistry. For near monolayer dispersion of
ST on SiO2, an isolated molecular model of ST was used
(Figure 5b). Extensive structure screening showed that two
Pd2+ ions with the Pd−Pd distance of 3.52 Å on ST are the
most energetically favorable structure (Figure 5b; entry 1,
Table S2). Other structures with larger separation of the Pd2+
ions were energetically less favorable (entries 2−13, Table S2).
A CO bridging adsorption model was also optimized (Figure
5c), resulting in the corresponding 13C NMR chemical shift at
186.4 ppm, which is consistent with the experimental value.
The calculated distance between dinuclear Pd2+ ions in the
Pd2+C(O)Pd2+ species is 3.1 Å, a shortened distance
instigated by CO adsorption. We also investigated 2D periodic
structures with Pd atoms bridging Keggin units (entry 14,
Table S2). This structure provides saturated bonding to Pd
atoms, which are also locked between Keggin units, making the
Pd ions inaccessible to the large 2-methylfuran reactants. This
result is in line with the decreased activity observed
experimentally when the Pd2ST loading surpassed 40%. We
also used DFT to evaluate the elementary mechanism of the 2-
MF coupling reaction over the Pd−Pd proximate Pd2ST
model. The calculated structures, reaction energies, and
activation barriers (Figures S1 and S2) showed a viable
catalytic path for oxidative coupling of 2-MF on the proximate
Pd−Pd systems. Formation of co-adsorbed 2-methylfuryl
intermediates on the proximate Pd atoms leads to a decrease
in the Pd−Pd distance (Figure S2, 3.12 Å in intermediate VI,
2.95 Å in intermediate XIII, and 2.79 Å in the C−C coupling
transition state). C−C coupling is extremely favorable with
reaction energies downhill by ∼2 eV and a barrier of only 0.55
eV. Figure S2 shows the optimized structures of co-adsorbed 2-
methylfuryl species and the adsorbed product DMBF
molecule; these structures suggest the importance of the
proximate location of the two Pd atoms to the coupling
chemistry.
indicating that one of the rings of DMBF was opened to
form this byproduct after C−C coupling.
The DMBF can be further converted to 4,4′-dimethylbi-
phenyl (DMBP) by the tandem Diels−Alder reaction, followed
by dehydration over silicotungstic acid on silica (H4ST/SiO2,
see Table 1 for the nomenclature), similar to cycloaddition of
furanics with olefins to produce mono aromatics.48,49 The
success of directly producing DMBF from 2-MF as
demonstrated in this work creates a new synthetic route to
further expand the product portfolio directly to diaryl products
with high atom economy by avoiding the known Suzuki routes.
The Diels−Alder cycloaddition with ethylene at 2.5 MPa
successfully produced 2-methyl-5-(p-tolyl)furan (MTF) with
100% selectivity at the 20% conversion of DMBF at 180 °C
(entry 1, Table 5; Figure S7), and 100% selectivity to DMBP at
Table 5. 40 H4ST/SiO2-Catalyzed Diel−Alder
Cycloaddition
a
Reaction conditions: DMBF (1 mmol), catalyst (40 H4ST/SiO2, 100
b
mg), 1,4-dioxane (5 mL), ethylene gas (2.5 MPa). Reaction was
scaled up to 12 mmol
Although dinuclear Pd2+ species showed high yield in the
C−C coupling of 2-MF, other metal ions in 40 M2ST/SiO2
showed poor activity. DFT calculations showed that only Pd2+
ions favored the dinuclear distribution on ST (Tables S2 and
S3). For Cu2+, Fe2+, Co2+, and Ni2+, it is more favorable for two
M2+ ions to adsorb on different regions of the Keggin unit,
using two different four-oxygen coordination sites of ST
(entries 1−15, Table S3). This leads to a much longer distance
between the metal ions (>7.5 Å) with an effective energetic
repulsion if the two metal atoms are moved to a less stable,
proximate position. For Ru2+, there is no preference for
separate or paired adsorption (entries 16−18, Table S3).
We further scaled up the reaction and evaluated the catalyst
reusability in a 500 mL reactor. The catalyst was active for five
consecutive cycles with consistent selectivity. Because the
change in Pd contents from 2.40% in the fresh catalyst to
2.36% after repeated use of the catalyst was very small, the
decrease in conversions of 2-MF from 90 to 79% in five
consecutive tests (Figure 5e) without regeneration could be
mainly attributed to the handling loss of the amount of
catalyst. Product characterization was facilitated by this larger-
scale system. A byproduct (1-(5-methylfuran-2-yl)pent-2-ene-
1,4-dione) (<5%) was identified by GC−MS (Figure S4) and
confirmed by 1H and 13C NMR (Figures S5 and S6),
the 100% conversion of DMBF at 250 °C (entry 3, Table 5;
Figure S9). The two products were formed at an intermediate
temperature (entry 2, Table 5; Figure S8). Both products BMF
(oxidative C−C coupling of 2-MF) and DMBP (D-A reaction
of DMBF) were well characterized by 1H NMR and 13C NMR
(Figures S10−S13).
4. CONCLUSIONS
In summary, the synchronized double C−H activation by
dinuclear Pd2+ ions in close proximity in a Pd2ST unit is
identified as the crucial mechanism for the oxidative
homocoupling of 2-MF. The mononuclear Pd2+ and H+ sites
on STA were ineffective for the coupling reaction but did not
negatively affect the main reaction. The near monolayer
dispersion of Pd2ST on SiO2 resulted in the highest conversion
of 2-MF. The high DMBF yield from 2-MF using the solid
catalyst is a significant practical advantage, allowing catalyst
reuse as demonstrated for five cycles in scale-up tests.
Furthermore, this coupling pathway provides a new synthetic
method for coupled aryl products, as demonstrated for the
efficient conversion of DMBF to DMBP by using H4ST on
SiO2 with 100% DMBF conversion and 100% selectivity of
DMBP.
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Author Contributions
ASSOCIATED CONTENT
■
Z.C.Z. conceived and directed the research. K.R. conducted
the experiments of the catalyst preparation, C−C coupling, and
D-A reaction. K.L. calculated DFT for C−C coupling and
supported the scale-up of C−C coupling reaction, C.Z.
prepared different metal-exchanged catalysts, X.C. acquired
TEM graphs and prepared samples for NMR, G.H. designed
and P.G. conducted 13C and 29Si NMR experiments, R.B.
conducted py-FT-IR and NH3-TPD of catalysts, M.R.M did D-
A reaction, P.Y. performed infrared spectroscopy and ICP-MS
of catalysts, X.G. prepared catalysts, and Z.X. supported the
experiments. M.J.J. contributed in DFT calculation. All authors
discussed the data. Z.C.Z. wrote the manuscript with
contributions from R.K, K.L., C.Z., and M.J.J.
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c05207.
Characterization data, DFT calculation data, and
mechanistic studies (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
Michael J. Janik − EMS Energy Institute, PSU-DUT Joint
Center for Energy Research and Department of Chemical
Engineering, The Pennsylvania State University, University
Park, Pennsylvania 16802, United States; Phone: +1(814)
8639366; Email: mjj13@psu.edu
Z. Conrad Zhang − Dalian National Lab for Clean Energy,
State Key Laboratory of Catalysis, Dalian Institute of
Chemical physics, Chinese Academy of Sciences, 116023
Dalian, China; orcid.org/0000-0001-8292-1169;
Phone: +86(411)84379462; Email: zczhang@yahoo.com
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Wenhao Cui for measurement of TEM. This work
was supported by the “Global Expert” program of China and
National Natural Science Foundation of China (Grant
21932005, 21690084, and 21721004) and DICP I-201936.
Authors
Kishore Ramineni − Dalian National Lab for Clean Energy,
State Key Laboratory of Catalysis, Dalian Institute of
Chemical physics, Chinese Academy of Sciences, 116023
Dalian, China
Kairui Liu − Dalian National Lab for Clean Energy, State Key
Laboratory of Catalysis, Dalian Institute of Chemical physics,
Chinese Academy of Sciences, 116023 Dalian, China
Cheng Zhang − Department of Chemistry, Long Island
University (Post), Brookville, New York 11548, United
States; orcid.org/0000-0002-5281-5979
REFERENCES
■
(1) Sahu, R.; Dhepe, P. L. A one-pot method for the selective
conversion of hemicellulose from crop waste into c5 sugars and
furfural by using solid acid catalysts. ChemSusChem 2012, 5, 751−761.
(2) Zhu, S.; Wang, J.; Fan, W. Graphene-based catalysis for biomass
conversion. Catal. Sci. Technol. 2015, 5, 3845−3858.
(3) Furfural market to report impressive growth, revenue to surge to us$
1,200.4 million by 2022. Zion market research, https://www.
zionmarketresearch.com/news/furfural-market
Xuke Chen − Dalian National Lab for Clean Energy, State
Key Laboratory of Catalysis, Dalian Institute of Chemical
physics, Chinese Academy of Sciences, 116023 Dalian, China;
University of Chinese Academy of Sciences, Beijing 100049,
China
Guangjin Hou − Dalian National Lab for Clean Energy, State
Key Laboratory of Catalysis, Dalian Institute of Chemical
physics, Chinese Academy of Sciences, 116023 Dalian,
China; orcid.org/0000-0001-8216-863X
Pan Gao − Dalian National Lab for Clean Energy, State Key
Laboratory of Catalysis, Dalian Institute of Chemical physics,
Chinese Academy of Sciences, 116023 Dalian, China
Ravi Balaga − Dalian National Lab for Clean Energy, State
Key Laboratory of Catalysis, Dalian Institute of Chemical
physics, Chinese Academy of Sciences, 116023 Dalian, China
Mahender Reddy Marri − Dalian National Lab for Clean
Energy, State Key Laboratory of Catalysis, Dalian Institute of
Chemical physics, Chinese Academy of Sciences, 116023
Dalian, China
Peifang Yan − Dalian National Lab for Clean Energy, State
Key Laboratory of Catalysis, Dalian Institute of Chemical
physics, Chinese Academy of Sciences, 116023 Dalian, China
Xian Guan − Dalian National Lab for Clean Energy, State
Key Laboratory of Catalysis, Dalian Institute of Chemical
physics, Chinese Academy of Sciences, 116023 Dalian, China
Zhi Xia − Dalian National Lab for Clean Energy, State Key
Laboratory of Catalysis, Dalian Institute of Chemical physics,
Chinese Academy of Sciences, 116023 Dalian, China
(4) Luterbacher, J. S.; Martin Alonso, D.; Dumesic, J. A. Targeted
chemical upgrading of lignocellulosic biomass to platform molecules.
Green Chem. 2014, 16, 4816−4838.
(5) Zhao, H.; Holladay, J. E.; Brown, H.; Zhang, Z. C. Metal
chlorides in ionic liquid solvents convert sugars to 5-hydroxyme-
thylfurfural. Science 2007, 316, 1597−1600.
(6) Román-Leshkov, Y.; Barrett, C. J.; Liu, Z. Y.; Dumesic, J. A.
Production of dimethylfuran for liquid fuels from biomass-derived
carbohydrates. Nature 2007, 447, 982−985.
(7) Li, J.; Liu, L.; Liu, Y.; Li, M.; Zhu, Y.; Liu, H.; Kou, Y.; Zhang, J.;
Han, Y.; Ma, D. Direct conversion of cellulose using carbon monoxide
and water on a Pt−Mo2C/C catalyst. Energy Environ. Sci. 2014, 7,
393−398.
(8) Hilgert, J.; Meine, N.; Rinaldi, R.; Schuth, F. Mechanocatalytic
̈
depolymerization of cellulose combined with hydrogenolysis as a
highly efficient pathway to sugar alcohols. Energy Environ. Sci. 2013, 6,
92−96.
(9) Op de Beeck, B.; Dusselier, M.; Geboers, J.; Holsbeek, J.; Morré,
E.; Oswald, S.; Giebeler, L.; Sels, B. F. Direct catalytic conversion of
cellulose to liquid straight-chain alkanes. Energy Environ. Sci. 2015, 8,
230−240.
(10) Gurbuz, E. I.; Gallo, J. M. R.; Alonso, D. M.; Wettstein, S. G.;
̈ ̈
Lim, W. Y.; Dumesic, J. A. Conversion of hemicellulose into furfural
using solid acid catalysts in γ-valerolactone. Angew. Chem., Int. Ed.
2013, 52, 1270−1274.
(11) Bhaumik, P.; Dhepe, P. L. Efficient, Stable, and reusable
silicoaluminophosphate for the one-pot production of furfural from
hemicellulose. ACS Catal. 2013, 3, 2299−2303.
(12) Dhepe, P. L.; Sahu, R. A solid-acid-based process for the
conversion of hemicellulose. Green Chem. 2010, 12, 2153−2156.
(13) Gómez Bernal, H.; Bernazzani, L.; Raspolli Galletti, A. M.
Furfural from corn stover hemicelluloses. A mineral acid-free
approach. Green Chem. 2014, 16, 3734−3740.
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(14) Nakagawa, Y.; Tamura, M.; Tomishige, K. Catalytic reduction
of biomass-derived furanic compounds with hydrogen. ACS Catal.
2013, 3, 2655−2668.
Type Basis Sets for Use in Molecular Orbital Studies of Organic
Molecules. J. Chem. Phys. 1972, 56, 2257−2261.
̀
(36) Gillan, M. J.; Alfe, D.; Michaelides, A. Perspective: How good is
DFT for water? J. Chem. Phys. 2016, 144, 130901−130933.
(37) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson,
G. A.; Nakatsuji, H.; Li, X.; Caricato, M.; Marenich, A.; Bloino, J.;
Janesko, B. G.; Gomperts, R.; Mennucci, B.; Hratchian, H. P.; Ortiz, J.
V.; Izmaylov, A. F.; Sonnenberg, J. L.; Williams-Young, D.; Ding, F.;
Lipparini, F.; Egidi, F.; Goings, J.; Peng, B.; Petrone, A.; Henderson,
T.; Ranasinghe, D.; Zakrzewski, V. G.; Gao, J.; Rega, N.; Zheng, G.;
Liang, W.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.;
Throssell, K.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.;
Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V.
N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell,
A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Millam, J. M.;
Klene, M.; Adamo, C.; Cammi, R.; Ochterski, J. W.; Martin, R. L.;
Morokuma, K.; Farkas, O.; Foresman, J. B.; Fox, D. J. Gaussian 09.
Revision A. 1, Gaussian Inc.: Wallingford CT, 2009.
(15) Huang, K.; Brentzel, Z. J.; Barnett, K. J.; Dumesic, J. A.; Huber,
G. W.; Maravelias, C. T. Conversion of furfural to 1,5-pentanediol:
process synthesis and analysis. ACS Sustainable Chem. Eng. 2017, 5,
4699−4706.
(16) Corma, A.; de la Torre, O.; Renz, M.; Villandier, N. Production
of high-quality diesel from biomass waste products. Angew. Chem., Int.
Ed. 2011, 50, 2375−2378.
(17) Corma, A.; de la Torre, O.; Renz, M. Production of high quality
diesel from cellulose and hemicellulose by the sylvan process: catalysts
and process variables. Energy Environ. Sci. 2012, 5, 6328−6344.
(18) Li, G.; Li, N.; Wang, Z.; Li, C.; Wang, A.; Wang, X.; Cong, Y.;
Zhang, T. Synthesis of high-quality diesel with furfural and 2-
methylfuran from hemicellulose. ChemSusChem 2012, 5, 1958−1966.
(19) Wang, W.; Li, N.; Li, S.; Li, G.; Chen, F.; Sheng, X.; Wang, A.;
Wang, X.; Cong, Y.; Zhang, T. Synthesis of renewable diesel with 2-
methylfuran and angelica lactone derived from carbohydrates. Green
Chem. 2016, 18, 1218−1223.
(38) Xie, Y.-C.; Tang, Y.-Q. Spontaneous Monolayer Dispersion of
Oxides and Salts onto Surfaces of Supports: Applications to
Heterogeneous Catalysis. Adv. Catal. 1990, 37, 1−43.
(39) Newman, A. D.; Brown, D. R.; Siril, P.; Lee, A. F.; Wilson, K.
Structural studies of high dispersion H3PW12O40/SiO2 solid acid
catalysts. Phys. Chem. Chem. Phys. 2006, 8, 2893−2902.
(40) Blasco, T.; Corma, A.; Martínez, A.; Martíınez-Escolano, P.
Supported Heteropolyacid (HPW) Catalysts for the Continuous
Alkylation of Isobutane with 2-Butene: The Benefit of Using MCM-
41 with Larger Pore Diameters. J. Catal. 1998, 177, 306−313.
(41) Brahmkhatri, V.; Patel, A. Synthesis and Characterization of 12-
Tungstosilicic Acid Anchored to MCM-41 as well as Its Use as
Environmentally Benign Catalyst for Synthesis of Succinate and
Malonate Diesters. Ind. Eng. Chem. Res. 2011, 50, 13693−13702.
(42) da Silva, M. J.; de Oliveira, C. M. Catalysis by Keggin
heteropolyacid salts. Curr. Catal. 2018, 7, 26−34.
(43) Zheng, Y.; Kovarik, L.; Engelhard, M. H.; Wang, Y.; Wang, Y.;
Gao, F.; Szanyi, J. Low-temperature Pd/Zeolite passive NOx
adsorbers: structure, performance, and adsorption chemistry. J. Phys.
Chem. C 2017, 121, 15793−15803.
(44) Sheu, L. L.; Knozinger, H.; Sachtler, W. M. H. Characterization
by CO/FTIR spectroscopy of palladium supported on NaY zeolites. J.
Mol. Catal. 1989, 57, 61−79.
(20) Siba, M. P.; Manoj, K. S.; Vinod, G. L.; Rajamohanan, P. R.;
Balaraman, E. Reversed reactivity of anilines with alkynes in the
rhodium-catalysed C−H activation/carbonylation tandem. Nat.
Commun. 2015, 6, 1−10.
(21) Gadakh, S.; Shimon, L. J. W.; Gidron, O. Regioselective
transformation of long π-conjugated backbones: from oligofurans to
oligoarenes. Angew. Chem., Int. Ed. 2017, 56, 13601−13605.
(22) Li, N.-N.; Zhang, Y.-L.; Mao, S.; Gao, Y.-R.; Guo, D.-D.; Wang,
Y.-Q. Palladium-catalyzed C−H homocoupling of furans and
thiophenes using oxygen as the oxidant. Org. Lett. 2014, 16, 2732−
2735.
(23) Kozhevnikov, I. V. Oxidative coupling of furan derivatives to
bifurans catalyzed by palladium(ii). React. Kinet. Catal. Lett. 1976, 5,
415−419.
(24) Diederich, F.; Stang, P. J. Eds., Metal-catalyzed cross-coupling
reactions; Wiley-VCH: Weinheim, Germany, 1998.
(25) Gooßen, L. J.; Deng, G.; Levy, L. M. Synthesis of biaryls via
catalytic decarboxylative coupling. Science 2006, 313, 662−664.
(26) Venkateswara Rao, K. T.; Sai Prasad, P. S.; Lingaiah, N.
Solvent-free hydration of alkynes over a heterogeneous silver
exchanged silicotungstic acid catalyst. Green Chem. 2012, 14, 1507−
1514.
(27) Venkateswara Rao, K. T.; Haribabu, B.; Sai Prasad, P. S.;
Lingaiah, N. Vapor-phase selective aerobic oxidation of benzylamine
to dibenzylimine over silica-supported vanadium-substituted tung-
stophosphoric acid catalyst. Green Chem. 2013, 15, 837−846.
(45) Tessier, D.; Rakai, A.; Bozon-Verduraz, F. Spectroscopic study
of the interaction of carbon monoxide with cationic and metallic
palladium in palladium-alumina catalysts. J. Chem. Soc., Faraday Trans.
1992, 88, 741−749.
(46) Li, H.; Chen, G.; Yang, H.; Wang, X.; Liang, J.; Liu, P.; Chen,
M.; Zheng, N. Shape-controlled synthesis of surface-clean ultrathin
palladium nanosheets by simply mixing a dinuclear PdI carbonyl
chloride complex with H2O. Am. Ethnol. 2013, 125, 8526−8530.
(47) Zhang, Z.; Chen, H.; Sachtler, W. M. H. Migration and
coalescence of Pd carbonyl clusters in Zeolite-Y. J. Chem. Soc., Faraday
Trans. 1991, 87, 1413−1418.
(48) Williams, C. L.; Chang, C.-C.; Do, P.; Nikbin, N.; Caratzoulas,
S.; Vlachos, D. G.; Lobo, R. F.; Fan, W.; Dauenhauer, P. J.
Cycloaddition of Biomass-derived furans for catalytic production of
renewable p-xylene. ACS Catal. 2012, 2, 935−939.
(49) Wijaya, Y. P.; Winoto, H. P.; Park, Y.-K.; Suh, D. J.; Lee, H.;
Ha, J.-M.; Jae, J. Heteropolyacid catalysts for Diels-Alder cyclo-
addition of 2,5-dimethylfuran and ethylene to renewable p-xylene.
Catal. Today 2017, 293-294, 167−175.
(28) Kresse, G.; Furthmuller, J. Efficiency of ab-initio total energy
̈
calculations for metals and semiconductors using a plane-wave basis
set. Comput. Mater. Sci. 1996, 6, 15−50.
(29) Kresse, G.; Furthmuller, J. Efficient iterative schemes for ab
̈
initio total-energy calculations using a plane-wave basis set. Phys. Rev.
B 1996, 54, 11169−11186.
(30) Blöchl, P. E. Projector augmented-wave method. Phys. Rev. B:
Condens. Matter 1994, 50, 17953−17979.
(31) Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the
projector augmented-wave method. Phys. Rev. B: Condens. Matter
1999, 59, 1758−1775.
(32) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient
Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868.
(33) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and
accurate ab initio parametrization of density functional dispersion
correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010,
132, 154104−154119.
(34) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. Self-
consistent molecular orbital methods. XX. A basis set for correlated
wave functions. J. Chem. Phys. 1980, 72, 650−654.
(35) Hehre, W. J.; Ditchfield, R.; Pople, J. A. Self-Consistent
Molecular Orbital Methods. XII. Further Extensions of Gaussian-
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