Recyclable montmorillonite-supported thiazolium ionic liquids for high-yielding and solvent-free upgrading of furfural and 5-hydroxymethylfurfural to C10 and C12 furoins

Article abstract of DOI:10.1039/c6ra14594a

Six Na⁺/montmorillonite (MMT)-supported thiazolium (TM) ionic liquids (ILs), MMT-[TM], which are precatalysts to N-heterocyclic carbene (NHC) catalysts, have been synthesized and employed to catalyze the highly selective condensation of furfural (FF) and 5-hydroxymethylfurfural (HMF) into C₁₀ and C₁₂ furoins, respectively. Among them, five supported [TM] salts carrying the electron-donating group exhibit excellent catalytic activity. Thus, supported NHC catalysts, generated in situ from activation of MMT-[TM] with Et₃N, afford C₁₀ and C₁₂ furoins in 97-99% and 97-98% yield, respectively. This condensation reaction is also carried out under solvent-free conditions, and the catalysts can be readily recycled, showing no noticeable loss in their catalytic activity over four consecutive recycles. Thus, these desired features possessed by the current catalyst system have enabled the development of the greener and more environmentally benign process for the upgrading of FF and HMF.

Full text of DOI:10.1039/c6ra14594a

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Recyclable montmorillonite-supported thiazolium ionic liquids
for high-yielding and solvent-free upgrading of furfural and
5
-hydroxymethylfurfural to C and C furoins
10
12
a
∗a
a
a
b
Bing Yan, Hongjun Zang, Yi Jiang, Songbai Yu, and Eugene Y.ꢀX. Chen
a
State Key Laboratory of Hollow Fiber Membrane Materials and Processes,
Department of Environmental and Chemistry Engineering, Tianjin Polytechnic
University, 300387 Tianjin, China.
b
Department of Chemistry, Colorado State University, Fort Collins, Colorado,
8
0523–1872, United States
a
∗
Eꢀmail addresses: zanghongjun@tjpu.edu.cn or chemhong@126.com ( H. J. Zang)
Corresponding author. Tel.: +86 22 2401 9762; fax: +86 22 2452 8055.
1

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ABSTRACT
+
Six Na /montmorillonite (MMT) – supported thiazolium (TM) ionic liquids (ILs),
MMTꢀ[TM], which are precatalysts to Nꢀheterocyclic carbene (NHC) catalysts, have
been synthesized and employed to catalyze highly selective of furfural (FF) and
5
ꢀhydroxymethylfurfural (HMF) into C10 and C12 furoins, respectively. Among them,
five supported [TM] salts carrying the electronꢀdonating group exhibit excellent
catalytic activity. Thus, supported NHC catalysts, generated in situ from activation of
3
MMTꢀ[TM] with Et N, afford C10 and C12 furoins in 97%ꢀ99% and 97%ꢀ98% yield,
respectively. This condensation reaction is also carried out under solventꢀfree
conditions, and the catalysts can be readily recycled, showing no noticeable loss in
their catalytic activity over four consecutive recycles. Thus, these desired features
possessed by the current catalyst system have enabled the development of the greener
and more environmentally benign process for the upgrading of FF and HMF.
+
Keywords: Na ꢀmontmorillonite, furfural (FF), selfꢀcondensation coupling,
Nꢀheterocyclic carbene (NHC), 5ꢀhydroxymethylfurfural (HMF)
2

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1
. Introduction
With increasing depletion of nonrenewable resources and increased societal
awareness of environmental protection, both the scientific community and industrial
entity pay increasing attention to the research needs for sustainable energy and
1
ꢀ4
renewable chemicals. Annually renewable biomass resources have emerged as the
5
most abundant and sustainable resources to potentially meet such needs.
Furaldehydes, particularly furfural (FF) and 5ꢀhydroxymethylfurfual (HMF) are
considered to be two of the most valueꢀadded biomass building blocks or platform
6
chemicals by the U.S. Department of Energy. FF and HMF can be converted into a
7
ꢀ12
variety of chemicals and fuels.
However, these FF and HMF derivatives possess
low carbon numbers, low energy density and high volatility, unsatisfactory as fuel
1
3,14
additives.
In addition, FF has been a largeꢀscale production in industry, as well as
HMF has been studied extensively as part of major efforts in developing
technologically and economically feasible routes for liquid fuels, so it has more
practical significance for their transformation and utilization. Therefore, upgrading of
FF and HMF into higherꢀenergyꢀdensity fuel intermediates via CꢀC bond formation or
15ꢀ18
chain extension is of particular interest.
However, such furaldehydes can not
undergo selfꢀaldol condensation due to lack of αꢀhydrogen. In this context, Dumesic
and coꢀworkers utilized crossꢀaldol condensation of HMF with acetone, followed by
1
9
9
hydrodeoxygenation process to produce C ꢀC15 liquid alkane fuels.
Direct coupling of FF and HMF into the corresponding C10 and C12 furoins has
been made possible via benzoinꢀtype umpolung condensation catalyzed by organic
3

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2
0
Nꢀheterocyclic carbene (NHC) catalysts. For example, NHCꢀcatalyzed direct
coupling of two HMF molecules form exclusively C12 5,5′ꢀdihydroxymethyl furoin
21
(DHMF), which has been converted into C10
ꢀ12 linear hydrocarbons. An integrated
catalytic process for conversion and upgrading of biomass feedstocks into DHMF and
22
subsequently into nꢀC12
H26 alkane fuel has also been developed. Benzoin
23ꢀ27
condensation of FF has been extensively investigated,
with the best furoin yield
(>99%) achieved most recently by the NHC derived from the acetate substituted
28
thiazolium.
While molecular NHC catalysts are highly effective for the selfꢀcoupling of FF
and HMF, they are difficult to recycle and reuse. In this context, there has been
increased interest in developing recyclable, supported NHC catalysts. For example,
crosslinked polymerꢀsupported NHCs were used as recyclable organocatalysts for the
29
benzoin condensation reaction, but the catalytic activity and recyclability of this
system were limited. Poly(4ꢀvinylimidazolium) iodides combined with bases were
found to be a more efficient and recyclable catalyst system for benzaldehyde
3
0
condensation reaction, but an analogous polymeric NHC catalyst, when employed
22
for HMF selfꢀcoupling, gave a low yield of DHMF (18%). Insoluble polystyrene
(PS) supported thiazolium (TM) salts, when combined with a base, produced furoin
23
with under 67% yields in the FF coupling reaction. Likewise, polymerꢀsupported
imidazolium salts, when employed for the benzoin condensation of aldehydes in the
3
1
presence of a base, gave acyloin products under 77% yields. Recently, PSꢀ and
silicaꢀsupported imidazolium or benzimidazolium ionic liquids (ILs), which generate
4

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the corresponding NHC catalysts upon in situ activation with a base, afforded HMF
24
and FF selfꢀcondensation products in 94% and 97% yield in THF, respectively.
Furthermore, montmorillonite (MMT) supported benzimidazolium, when activated
with NaH, was found to effectively catalyze the selfꢀcoupling FF in THF producing
2
5
furoin
ꢀbenzylꢀ5ꢀ(2ꢀhydroxyethyl)ꢀ4ꢀmethylthiazolium precatalyst produced C10 furoin in
86% yield from the selfꢀcoupling of FF in THF at 25 °C after 5 h, and the
recyclability of this catalyst system was poor, as evidenced by a large reduction in the
in
>96%
yield.
But
the
MMTꢀsupported
3
~
25
furoin yield to only 54.1% when the recycled catalyst was used.
As can be seen from the above overview, the supported [TM] ILs gave lower
yields and poor recyclability for selfꢀcondensation of FF, while the use of supported
[
TM] ILs for selfꢀcondensation of HMF into C12 furoin has not been reported. On the
other hand, we recently found large electronic modulation of [TM] ILs, when
converted to the discrete NHC catalysts, on their catalytic performance in the
28
umpolung coupling of FF and HMF into C10 and C12 furoins. Hence, we reasoned
that, by supporting [TM] ILs varying electronic properties (which tune the
nucleophilicity or activity of the corresponding NHC catalyst derived from in situ
deprotonation with a base) onto the inexpensive MMT support, one could potentially
discover highly effective MMTꢀsupported [TM] catalyst systems that are not only
highly active and efficient in achieving high yields of furoins from selfꢀcondensation
of both FF and HMF, but also are recyclable and do not require use of any solvent.
Accordingly, this contribution presents a full account of our study towards the above
5

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central objective.
2
2
. Experimental Section
.1 Materials and Methods
All anhydrous and oxygenꢀfree operations were carried out in a nitrogenꢀfilled
+
glovebox or under nitrogen atmosphere. Na /montmorillonite (MMT) with a cation
exchange capacity (CEC) of 120 meq. /100g and an Aspect Ratio of 300ꢀ500 was
supplied by Nanocor Company. HMF (97%) was purchased from Tianjin Heowns
Biochem LLC (Tianjin, China) and used as received. FF (Aladdin Chemistry Co., Ltd.)
was vacuumꢀdistilled prior to use. Tetrahydrofuran (THF) was refluxed over metallic
3
sodium with benzophenone and distilled under nitrogen atmosphere before use. Et N,
t
KO Bu, 1, 8ꢀdiazabicyclo [5.4.0] undecꢀ7ꢀene (DBU), dimethylformamide (DMF),
4
ꢀmethylꢀ5ꢀhydroxyethylthiazole, 4,5ꢀdimethylthiazole, 4ꢀmethylꢀ5ꢀvinylthiazole,
benzyl chloride, 2ꢀbromoethanol, allyl bromide, and methyl iodide were purchased
from Across Organics and used as received.
1
H NMR spectra were recorded on a Varian Inova 300 MHz NMR spectrometer.
Fourier transform infrared (FTꢀIR) measurements were recorded in the range of
ꢀ1
5
00ꢀ4000cm on a Bruker TENSOR37 FTꢀIR spectrometer using a KBr pellet
technique. Thermogravimetric analysis (TGA) was performed under air atmosphere at
a heating rate of 10 °C/min from 20 to 900 °C using SDTꢀQ600, TA Instruments.
Xꢀray diffraction (XRD) data were recorded on a D/MAXꢀ2500 automated
diffractometer. The d spacing was calculated to the Bragg equation (nλ = 2d sinθ).
Thiazolium (TM) bromides including 4,5ꢀdimethylꢀ3ꢀhydroxyethylthiazolium
6

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bromide
([HMTM]Br),
4ꢀmethylꢀ5ꢀhydroxyethylꢀ3ꢀallylthiazolium
bromide
([AHTM]Br) and 4,5ꢀdimethylꢀ3ꢀallylthiazolium bromide ([AMTM]Br) were
3
2
prepared
according
to
the
literature
procedures.
[TM]
iodide,
3
3
3
,4ꢀdimethylꢀ5ꢀvinylthiazolium iodide ([MVTM]I)
ꢀbenzylꢀ5ꢀ(2ꢀhydroxyethyl)ꢀ4ꢀmethylthiazolium chloride ([BHTM]Cl)
and [TM] chloride,
34
3
were
prepared using the literature methods.
OH
N
N
N
Cl
Br
Br
HO
S
HO
S
HO
S
[
BHTM]Cl
[HHTM]Br
[AHTM]Br
OH
N
N
N
Br
Br
I
S
S
S
[
HMTM]Br
[AMTM]Br
[MVTM]I
The structure of thiazolium ILs used in this study
2
.2 Synthesis of 4-methyl-3,5-dihydroxyethylthiazolium bromide ([HHTM]Br)
A 100 mL threeꢀnecked roundꢀbottom flask equipped with an oil bath and a
magnetic stirring was charged with 4ꢀmethylꢀ5ꢀhydroxyethylthiazole (4.30 g, 0.03
mol), 2ꢀbromoethanol (3.72 g, 0.03 mol) and acetonitrile (15 mL). The resulting
mixture was refluxed for 48 h, after which acetonitrile was removed under reduced
pressure, and then 15 ml of acetone was added. The solvent was removed and the
residue was washed with diethyl ether (10 mL) and dried for 24 h under vacuum to
give a crude product. The crude product was recrystallized from a mixed
ethanolꢀacetone solution, and then dried under vacuum to give the pure product (79 %
7

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yield) as a yellow solid.
1
2 2 2
H NMR (300 MHz, D O): δ 4.55 (t, J = 5.1 Hz, 2H, NCH CH ); 3.95 (t, J = 5.1
2 2
Hz, 2H, NCH ); 3.80 (t, J = 6.0 Hz, 2H, CH OH); 3.10 (t, J = 5.7 Hz, 2H,
13
CH CH OH); 2.46 (s, 3H, CH ). C NMR (100 MHz, DMSOꢀd ): δ 157.1, 141.7,
2
2
3
6
1
34.8, 59.6, 58.7, 55.1, 29.5, 11.6 ppm.
.3 Preparation of MMT-Supported [TM] via ion exchange of Na /MMT with
+
2
[TM]X
A typical experimental procedure, using [BHTM]Cl as an example, was
described as follows. MMT (1.0 g, 120 meq. /100g) was placed in a 250 mL
threeꢀnecked roundꢀbottom flask and 100 mL of distilled water was added. The
mixture was vigorously stirred under reflux and heated to 80 °C for 4 h to become a
swollen MMT slurry. In a beaker, [BHTM]Cl (0.65 g, 2.4 mmol) was dissolved in 20
mL of distilled water and then poured into the threeꢀnecked roundꢀbottom flask
containing the MMT slurry. The mixture was stirred at 80 °C for 5 h. After the
reaction, the mixture was cooled to room temperature, and then the solid product was
filtered and washed thoroughly with distilled water to remove the excess thiazolium
ionic liquid. The resulting product, MMTꢀ[BHTM], was dried at 70 °C under vacuum
overnight before analysis and use. The same procedure was employed to prepare other
MMTꢀsupported [TM] precatalysts. TGA analysis of the MMTꢀ[TM] revealed the
contents of MMTꢀ[AHTM], MMTꢀ[AMTM], MMTꢀ[BHTM], MMTꢀ[HHTM],
MMTꢀ[HMTM] and MMTꢀ[MVTM] in the resulting MMTꢀ[TM] precatalysts to be
0
.99, 1.04, 0.88, 0.98, 0.95, and 1.05 mmol/g, respectively.
8

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2
.4 Typical procedure for the self-coupling reaction of FF into C10 furoin by
MMTꢀ[TM] and base
In a typical experimental procedure, FF (0.77 g, 8.0 mmol), MMTꢀ[BHTM] (0.2
g, 0.16 mmol [BHTM], 2 mol % relative to FF) and Et N (45 µL, 0.32 mmol, 4
3
mol % relative to FF) were added into a 20 mL glass bottle in a glovebox. The glass
bottle was sealed, taken out of the glovebox, and then the reaction mixture was placed
in an oil bath at 80 °C for 3 h. After the reaction, the solid product was dissolved in
dichloromethane, filtered and then dichloromethane was distilled off on a rotary
evaporator. After drying under vacuum, the product was obtained as a yellow solid
(99 % yield). For the recycling experiments, after the completion of the reaction,
dichloromethane and 37% aqueous HCl solution (75 µL, 0.8 mmol) was added and
the catalyst was separated by filtration, then the catalyst was dried at 70 °C overnight
under vacuum. The recycled MMTꢀ[BHTM] was reused to catalyze the FF coupling
reaction under the same conditions. This process was repeated several times to
evaluate the recyclability of the MMTꢀ[BHTM] catalyst.
1
3
H NMR (300 MHz, CDCl ) for C10 furoin: δ 7.63, 7.39, 7.27, 6.55, 6.42, 6.37
(m, 6H, furan ring H), 5.82 (s, 1H, CHOH), 4.23 (br s, 1H, CHOH).
2
.5 Typical procedure for the self-coupling reaction of HMF into C12 furoin by
MMTꢀ[TM] and base
In a typical procedure, HMF (0.126 g, 1.0 mmol), MMTꢀ[BHTM] (0.25 g, 0.20
3
mmol [BHTM], 20 mol % relative to HMF) and Et N (55 µL, 0.40 mmol, 40 mol %
relative to FF) were added into a 20 mL glass bottle in a glovebox. The glass bottle
9

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was sealed, taken out of the glovebox, and then the reaction mixture was placed in an
oil bath at 120 °C for 3 h. After the reaction, the solid product was dissolved in
dichloromethane, filtered and then dichloromethane was distilled off on a rotary
evaporator. After drying under vacuum, the product was obtained as a white solid
(97 % yield).
1
H NMR (400 MHz, DMSOꢀd
OH), 4.46 (d, J = 5.8 Hz, 2 H, CH
t, J = 5.9 Hz, 1H, CH OH), 5.73 (d, J = 6.2 Hz, 1H, CHOH), 6.06 (d, J = 6.2 Hz, 1H,
CHOH), 6.22, 6.34, 6.50, 7.50 (d, 4H, furan ring H)
6
) for C12 furoin (DHMF): δ 4.31 (d, J = 5.7 Hz, 2
H, CH
2
2
OH), 5.18 (t, J = 5.7 Hz, 1H, CH OH), 5.50
2
(
2
.
3
3
. Results and discussion
.1 Synthesis and Characterization of Supported Precatalysts MMTꢀ[TM]
FT-IR analysis. As described in the Experimental section, a total of six
MMTꢀSupported [TM] precatalyst have been synthesized via ion exchange of
+
Na /MMT with [TM]X (X = Cl, Br, I) ILs (Scheme 1). To confirm if the [TM] cations
were successfully intercalated into the MMT nanogalleries or not, the resulting
MMTꢀ[TM] products were analyzed by FTꢀIR. As can be seen from Fig. 1, the FTꢀIR
+
ꢀ1
spectrum of Na ꢀMMT displays the peak at 3631 cm that can be attributed to the OH
+
ꢀ1
units in Na ꢀMMT. The peaks at 3431 and 1641 cm correspond to the –OH
+
stretching vibration of free H O onto the Na ꢀMMT structure and the bands at 1044
2
ꢀ1
and 917 cm can be collectively attributed to Si–O stretching vibrations; However,
ꢀ1
the FTꢀIR spectrum of MMTꢀ[TM] exhibit the characteristic peaks at 3098 cm , 1587
ꢀ1
ꢀ1
cm and 1457 cm were assigned to the =C–H, C=C and C=N stretching vibration
1
0

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modes of the thiazole ring, respectively. Furthermore, the characteristic absorption
ꢀ1
ꢀ1
bands at 2937 cm and 2886 cm , attributed to the aliphatic C–H stretching vibration
of methyl and methylene groups, were also observed. These results suggest that the
[
TM] ILs were successfully supported onto MMT.
R2
N
R2
Na
Na
Na
N
X
R1
S
S
R1
R1
S
Na
Na
N
R2 Na
NaX
a
R
1
R
2
X
IL
Cl [BHTM]Cl
OH Br [HHTM]Br
Br [AHTM]Br
OH Br [HMTM]Br
CH=CH Br [AMTM]Br
CH [MVTM]I
d spacing (Ǻ)
15.5
CH
2
2
CH
CH
2
2
OH
CH
2
Ph
1
3.6
CH
OH CH
2
CH
2
CH CH OH CH CH=CH
2
13.4
13.3
2
2
2
CH
CH
CH=CH
3
CH
2 2
CH
1
3.4
3
CH
2
2
13.2
2
3
I
a
The basal spacing of MMTꢀ[TM] calculated from XRD results (Fig. 2).
Scheme 1. Synthesis of six MMTꢀ[TM] precatalysts.
(
a)36313
917
1044
431
(
b)
c)
d)
e)
(
1641
(
(
(f)
(g)
2886
2937
aliphatic CꢀH
3
098
=C ꢀH
1587
C=N 1457
C=C
3500
3000
2500
2000
1500
1000
ꢀ1
Wavenumber (cm )
11

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Fig. 1 FTꢀIR spectra of MMT (a), MMTꢀ[MVTM] (b), MMTꢀ[AMTM] (c), MMTꢀ[AHTM] (d),
MMTꢀ[HHTM] (e), MMTꢀ[HMTM] (f), MMTꢀ[BHTM] (g).
XRD analysis. Power Xꢀray diffraction (XRD) was carried out to investigate the
expansion of the MMT layers, upon intercalation of [TM] cations into the nanogallery
+
of Na /MMT. According to the Bragg’s Equation: 2d sin θ = λ (d is the basal spacing;
25
θ is the Bragg peak angle; and λ is the Xꢀray wavelength), we calculated the basal
spacings of these samples. As shown in Fig. 2, the calculated basal spacing for the
MMT,
MMTꢀ[BHTM], MMTꢀ[HMTM], MMTꢀ[AHTM], MMTꢀ[HHTM],
MMTꢀ[AMTM, and MMTꢀ[MVTM] was 12.1, 15.5, 13.3, 13.4, 13.6, 13.4, and 13.2
Ǻ, respectively. The results showed that the basal spacing of the MMTꢀ[TM] is clearly
larger than that of MMT, especially for MMTꢀ[BHTM], due to the presence of the
+
larger benzyl group in [BHTM] cation. Therefore, these XRD results offered
additional evidence for the [TM] ILs intercalated into the MMT interlayer spacing and
showed correlation between the size of the [TM] cation and the [TM] intercalated
MMT interlayer basal spacing.
1
3.2 Å
（
（
g
）
13.4
Å
Å
f
）
1
3.6
Å
（
（
（
e
）
1
3.4
d）
13.3 Å
c
）
15.5 Å
（
b）
1
2.1 Å
（
a）
3
4
5
6
7
8
9
10
1
2
2
θ(°)

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Fig. 2 XRD profiles of MMT (a), MMTꢀ[BHTM] (b), MMTꢀ[HMTM] (c), MMTꢀ[AHTM] (d),
MMTꢀ[HHTM] (e), MMTꢀ[AMTM] (f), MMTꢀ[MVTM] (g).
N adsorption – desorption. N adsorption – desorption measurements were
2
2
performed to obtain more information about the catalyst. Due to the fact that the
precatalysts MMTꢀ[TM] have the similar structures, therefore, we take the N
2
adsorption – desorption isotherm of MMT and MMTꢀ[BHTM] as an example (as
shown in Fig. 3). And the curves of poreꢀsize distribution evaluated from desorption
data by utilizing the BJH model are also shown in the inset of Fig. 3. Through
3
analysis we found that both samples showed a type IV isotherm with H hysteresis,
according to the IUPAC classification, which is characteristic of the micro and
mesopore structure of samples with slitꢀlike pores. BET surface area, pore volumes
and pore diameters for MMT and MMTꢀ[BHTM] were presented in Table 1. A
2
increase in BET surface area from MMT (14.068 m /g) to MMTꢀ[BHTM] (15.580
2
m /g), that can be interpreted as being due to the pore diameters of MMT is too small
(3.822 nm), leading to the organic macromolecules attached to the surface of MMT
layers, and it was also indicated that the ionic liquid may be successfully intercalated
to MMT layers.
1
3

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0
0
0
0
0
0
0
.06
.05
.04
.03
.02
.01
.00
6
5
4
3
2
1
0
0
0
0
0
0
0
(a)
(b)
3
4
5
6
7
8
Pore Diameter (nm)
(
a)
(
b)
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
Fig. 3 N
2
adsorption – desorption isotherms: (a) MMT ; (b) MMTꢀ[BHTM]. (NOTE: The
position of the MMT isotherm is shifted.)
Table 1. Pore structure parameters of MMT and MMTꢀ[BHTM] derived from the N adsorption –
2
desorption isotherms.
2
3
Sample
MMT
S
BET (m /g)
V
p
(cm /g)
Pore diameter (nm)
3.822
14.068
15.580
0.05566
0.07529
MMTꢀ[BHTM]
3.825
Elemental analysis (EA) results. The loading amount of ionic liquids on
+
Na ꢀMMT was determined by means of elemental analysis. As can be seen from the
+
Table 2, there is not have the elemental of C, H and N in the Na ꢀMMT, indicating
that the [TM] cations were successfully intercalated into the nanogalleries of MMT.
Furthermore, through the N%, we can calculated the loading amount of ionic liquids
on MMT. The loading amount of ionic liquids on MMT ranges from 0.88 to 1.03
mmol/g (By EA method). The loading amount of ionic liquids on MMT was also
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determined by TGA analysis. The corresponding results are summarized in Table 2.
And the loading amount of ILs measured by TGA is in good agreement with the
calculated one from the EA results. These values were used to calculate the loading of
24,25
catalyst for the selfꢀcoupling reactions of furfural and HMF.
Table 2. EA and TGA methods for the loading amount of different ionic liquids on MMT.
N
C
H
Amount of IL Amount of IL
Precatalysts
a
b
(
wt%) (wt%) (wt%)
(mmol/g)
0
(mmol/g)
MMT
0
0
0
0
MMTꢀ[BHTM]
1.23
13.71
13.26
9.39
10.64
7.98
9.94
8.82
1.42
1.37
1.38
1.39
1.14
1.24
1.05
0.88
0.85
0.97
0.99
0.95
1.02
1.03
0.88
ꢀ
MMTꢀ[BHTM]ꢀrecycle 1.19
MMTꢀ[HHTM]
MMTꢀ[AHTM]
MMTꢀ[HMTM]
MMTꢀ[AMTM]
MMTꢀ[MVTM]
1.37
1.38
1.33
1.45
0.98
0.99
0.95
1.04
1.47
1.05
a
+
The loading amount of ILs on Na ꢀMMT (C) was calculated according to the following
3
expressiom: C = the percentage of nitrogen × 1 g × 10 /nitrogen atomic weight.
b
Calculated by TGA method (see Fig. 4).
ICP analysis. ICP was carried out to investigate the composition of the prepared
catalysts. As shown in Table 3, we can note that the prepared catalyst contains five
metal elements (Na, Mg, Al, Si and Fe). Through the comparison of the MMT and
MMTꢀ[BHTM], we found that the content of all the five elements present a drastic
decrease, that can be interpreted as being due to the intercalated of the ionic liquid,
which lead to the decrease of the relative contents. However, the decrease in the
+
content of sodium was the most obvious, for the ionic liquids via ion exchange of Na
onto the MMT layer. In addition, though the EA (Table 2) and ICP (Table 3) analysis
of the MMTꢀ[BHTM] and MMTꢀ[BHTM]ꢀrecycle, we found that the N% have a
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slight decrease and the Na% have a slight increase, so we envisioned that, some
leaching problems of prepared catalysts are present in the reactions.
Table 3. The metal content of MMT and MMTꢀ[BHTM].
Sample
MMT
Na (%)
3.21
Mg (%)
1.65
Al (%)
12.35
10.03
10.01
Si (%)
29.65
23.98
23.97
Fe (%)
3.99
MMTꢀ[BHTM]
MMTꢀ[BHTM]ꢀrecycle
1.46
1.34
3.23
1.54
1.34
3.21
TGA analysis. TGA was performed on the prepared MMTꢀ[TM] to characterize
their thermal stability and contents of the intercalated [TM] cations. From Fig. 4, one
can note that the weight loss rate of MMTꢀ[TM] was much larger than that of MMT,
further indicating that the [TM] cations were successfully intercalated into the
nanogalleries of MMT (Owing to the fact that MMTꢀ[HHTM] and MMTꢀ[AHTM] as
well as MMTꢀ[MVTM] and MMTꢀ[AMTM] have the similar weight loss curve, so we
just draw one of them in the Fig. 4). More importantly, through the weight loss of the
samples, we can calculated that the contents of the intercalated [TM] cations in the
2
5
resulting MMTꢀTM precatalysts MMTꢀ[AHTM], MMTꢀ[AMTM], MMTꢀ[BHTM],
MMTꢀ[HHTM], MMTꢀ[HMTM], and MMTꢀ[MVTM] to be 0.99, 1.04, 0.88, 0.98,
0
.95, and 1.05 mmol/g, respectively.
The loading amount of ionic liquids on MMT was calculated according to the
following equations:
W1
W2
M2
1
C (mmol/g) =
X 1000
M1
C: loading amount of ionic liquids on MMT, W : residual amount of MMTꢀ[TM], W :
1
2
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residual amount of MMT, M
1
: relative molecular mass of ionic liquids, M
2
: relative
molecular mass of NaX (X: Cl, Br, I).
1
00
95
90
85
80
75
9
3.8%
8
1.8%
80.7%
78.6%
(
(
(
(
(
a)
b)
c)
d),(e)
f ),(g)
7
6.2%
0
200
400
600
800
1000
o
Temperature( C)
Fig. 4 TGA curves of MMT (a), MMTꢀ[BHTM] (b), MMTꢀ[HMTM] (c), MMTꢀ[AHTM] (d),
MMTꢀ[HHTM] (e), MMTꢀ[AMTM] (f), MMTꢀ[MVTM] (g).
3
.2 Catalytic activity of MMTꢀ[TM] for coupling reaction of FF to C furoin
10
As the above analyses showed six MMTꢀ[TM] precatalysts have been
successfully prepared, next we investigated their catalytic performances for the
selfꢀcoupling reaction of FF into C10 furoin. We envisioned that, following the
25
established protocol, activation of such precatalysts with a suitable base should
generate the corresponding neutral NHC catalyst in situ, which catalyzes the
selfꢀcoupling reaction; the NHC catalyst can then be readily recycled through its
24,25
reaction with HCl reforming the [TM]Cl precatalyst (Scheme 2).
To this end, we
varied MMTꢀ[TM] precatalyst structure (six total) and examined effects of reaction
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conditions (including catalyst loading, solvent, time, and temperature) on their
catalytic performances, the results of which are summarized in Table 4.
a
Table 4. Experimental Results of FF Coupling Reaction Catalyzed by MMTꢀ[TM] and Base
Base
mol %)
Temperature Time
Yield
(%)
Entry Pre-catalyst (mol %)
Solvent
b
(
(℃)
(h)
1
2
3
4
5
6
MMT, 0.2 g
Et
Et N, 1.2
Et N, 2.4
3
N, 2
80
80
3
3
3
3
3
3
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
0
MMTꢀ[BHTM], 0.6
MMTꢀ[BHTM], 1.2
MMTꢀ[BHTM], 1.2
MMTꢀ[BHTM], 2
MMTꢀ[BHTM], 2
3
18
70
75
99
98
80
3
Et
3
N, 4.8
80
Et N, 4
3
80
Et
3
N, 4
100
c
7
8
9
MMTꢀ[BHTM], 10
MMTꢀ[BHTM], 2
MMTꢀ[BHTM], 2
MMTꢀ[BHTM], 2
MMTꢀ[BHTM], 2
NaH, 10
25
60
80
80
80
5
3
1
3
3
THF
ꢀ
86
Et
3
3
N, 4
N, 4
94
94
85
91
Et
ꢀ
1
0
Et N, 4
3
THF
DMF
1
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
3
Et N, 4
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
MMTꢀ[BHTM], 2
MMTꢀ[BHTM], 2
MMTꢀ[HHTM], 2
MMTꢀ[HHTM], 2
MMTꢀ[AHTM], 2
MMTꢀ[HMTM], 2
MMTꢀ[AMTM], 2
DBU, 4
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
3
3
3
1
3
3
3
3
3
3
3
3
3
3
3
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
64
t
KO Bu, 4
82
98
94
97
98
97
60
80
99
97
96
97
97
82
Et N, 4
3
Et
Et
3
3
N, 4
N, 4
Et N, 4
3
Et
3
N, 4
MMTꢀ[MVTM], 1.2 Et
3
N, 2.4
MMTꢀ[MVTM], 2
[BHTM]Cl, 1
[HHTM]Br, 1
[AHTM]Br, 1
[HMTM]Br, 1
[AMTM]Br, 1
[MVTM]I, 1
Et
Et
3
3
N, 4
N, 2
Et N, 2
3
Et
Et
3
3
N, 2
N, 2
Et N, 2
3
3
Et N, 2
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8

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a
b
1
c
Furfural (0.768 g 8.00 mmol), nitrogen atmosphere. Determined by H NMR. Data point
25
taken from literature.
O
activation
R2
R
2
O
Et N
R2
3
N
N
Na
N
Na
Et NH
R1
R1
R = H, CH OH
3
2
S
S
S
R1
R1
catalysis
S
Et NH
3
N
Na
O
R2 Na
O
HCl
R
O
R
R2
recycle
OH
Scheme 2. Activation, catalysis, and recycling involved in the selfꢀcondensation coupling reaction
of FF and HMF by the MMTꢀ[TM]/Base catalyst system.
A control run using MMT alone showed no activity for the coupling reaction of
FF under the current conditions (entry 1, Table 4), thus confirming that MMT
functions only a catalyst support. We first focused on the MMTꢀ[BHTM] system and
optimized its catalytic performance (entries 2ꢀ13, Table 4). Under the same set of
conditions (2 mol% precatalyst loading, 4 mol% Et N, 80 °C, 3 h), the highest furoin
3
yield of 99% was obtained in the absence of solvent (entry 5), which outperformed
the reactions in THF (85%, entry 10) and DMF (91%, entry 11). Next, using the same
solventꢀfree conditions (80 °C, 3 h), the relative amount of precatalyst MMTꢀ[BHTM]
3
and base (Et N) was varied for the coupling reaction. When the mole ratio (relative to
furfural, mol%) of MMTꢀ[BHTM] and Et N was 0.6:1.2, 1.2:2.4, and 1.2:4.8, furoin
3
was achieved in 18 %, 70%, and 75 % yield, respectively (entries 2ꢀ4, Table 4).
Excitingly, when the mole ratio was changed to 2:4, essentially quantitative yield
(99%) of C10 furion was achieved (entry 5, Table 4). Through temperature variations
from 60 °C to 100 °C, we found the reaction carried out at 80 °C gave the best furoin
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yield (entry 5 vs. 6 and 8, Table 4). Shortening the reaction time from 3 h to 1 h
noticeably decreased the yield to 94% (entry 9, Table 4). Other common bases used to
generate
the
NHC
catalyst
for
coupling
reactions,
such
as
t
25,28
1
,8ꢀdiazabicyclo[5.4.0]undecꢀ7ꢀene (DBU) and KO Bu,
also worked to certain
t
extent, but they were much inferior to Et
3
N (64% by DBU, entry 12; 82% by KO Bu,
entry 13, Table 4). Overall, the optimal reaction conditions for achieving the highest
furoin yield (99%) from the FF selfꢀcoupling reaction were identified as follows: 2
3
mol % preꢀcatalyst, 4 mol % Et N, 80 °C, 3 h, no solvent. The current system also
outperformed considerably the previously reported system consisting of 10 mol%
MMTꢀ[BHTM], 10 mol% NaH, THF, 25 °C, 5 h, which gave a furoin yield of 86 %
2
5
(entry 7, Table 4).
Having established the optimal FF coupling reaction conditions for
MMTꢀ[BHTM], next we examined other five precatalysts intercalated into the
nanogalleries of MMT, including MMTꢀ[AHTM], MMTꢀ[AMTM], MMTꢀ[HHTM],
MMTꢀ[HMTM], and MMTꢀ[MVTM]. Under the above optimal conditions, all the
MMTꢀ[TM] + Et N catalyst systems produced C10 furoin in high yields (≥ 94%,
3
entries 14ꢀ18, Table 4), except for MMTꢀ[MVTM] (entries 19 and 20, Table 4). In a
comparative study, we also used these [TM] ILs without being supported on MMT for
the coupling reaction and found that the results were nearly identical to those obtained
from the supported MMTꢀ[TM] catalyst systems (entries 21ꢀ26, Table 4). Also
+
noteworthy is that [MVTM] , which contains an electronꢀwithdrawing vinyl group at
the 5ꢀpoisiton of the [TM] ring, consistently gave the lowest furoin yield in either
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0

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supported (80%) or unsupported (82%) form. Hence, its inferior catalytic performance
can be attributed to the electronꢀwithdrawing substitute that renders low
nucleophilicity of the corresponding NHC catalyst.
3
.3 Catalytic activity of MMT-[TM] for coupling reaction of HMF to C12 furoin
The supported MMTꢀ[TM] catalysts were also examined for their catatlytic
performances in catalyzing the selfꢀcoupling reaction of HMF to DHMF. Owing to the
fact that HMF has a CH OH group which could retard or deactivate the NHC catalyst,
the selfꢀcoupling reaction of HMF is much more challenging than the coupling of FF,
2
28
thus requiring higher catalyst loadings and more harsh reaction conditions. As can
be seen from Table 5, when 10 mol% of MMTꢀ[BHTM] and 20 % mol of Et N were
used at 120 °C for 3 h under solventꢀfree conditions, the yield of DHMF was only
3 % (entry 1, Table 5). Increasing the MMTꢀ[BHTM] and Et N loading to 20 mol%
3
5
3
and 40 mol%, respectively, enhanced the DHMF yield to 97% (entry 2, Table 5). On
the other hand, variations of other reaction conditions such as solvent (entry 3, Table
5
), time (entry 4, Table 5), temperature (entry 5, Table 5), and base (entry 6, Table 5)
all led to lower yields of DHMF. Finally, under the optimal conditions (20 mol%
MMTꢀ[TM], 40 mol% Et N, no solvent, 120 °C, 3 h) based on MMTꢀ[BHTM]), other
3
MMTꢀsupported ILs were shown to perform similarly, achieving high DHMF yields
+
of 97ꢀ98% (entries 7ꢀ10, Table 5). Again, [MVTM] ꢀbased catalyst carrying an
electronꢀwithdrawing vinyl group was the least effective, producing DHMF in low
yields of 30ꢀ78%, depending on the loading of the precatalyst and base as well as
temperature (entries 11ꢀ13, Table 5).
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a
Table 5. Experimental Results of HMF Coupling Reaction Catalyzed by MMTꢀ[TM] and Base
Base
mol %)
Temperature Time
Yield
(%)
Entry Pre-catalyst (mol %)
Solvent
b
(
(℃)
(h)
1
2
3
4
MMTꢀ[BHTM], 10
MMTꢀ[BHTM], 20
MMTꢀ[BHTM], 20
MMTꢀ[BHTM], 20
Et
3
N, 20
120
120
120
120
3
3
3
1
ꢀ
53
97
78
93
92
68
98
98
98
Et N, 40
ꢀ
DMF
ꢀ
3
Et
3
N, 40
Et N, 40
3
5
6
7
8
9
MMTꢀ[BHTM], 20
MMTꢀ[BHTM], 20
MMTꢀ[HHTM], 20
MMTꢀ[AHTM], 20
MMTꢀ[HMTM], 20
Et
DBU, 40
Et N, 40
Et N, 40
3
N, 40
80
3
3
3
3
3
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
120
120
120
120
3
3
Et
Et
Et
Et
Et
3
3
N, 40
N, 40
N, 20
N, 40
N, 40
1
0
MMTꢀ[AMTM], 20
MMTꢀ[MVTM], 10
MMTꢀ[MVTM], 20
MMTꢀ[MVTM], 20
120
120
120
80
3
3
3
3
ꢀ
ꢀ
ꢀ
ꢀ
97
32
78
70
1
1
2
3
3
3
3
1
1
a
b
1
HMF (0.126 g, 1.00 mmol), nitrogen atmosphere. Determined by H NMR spectra.
3
.4 Recycling experiments
To examine the recyclability of the current supported MMTꢀ[TM] catalyst system,
the betterꢀperforming precatalysts MMTꢀ[BHTM], MMTꢀ[HMTM], MMTꢀ[AHTM],
MMTꢀ[HHTM] and MMTꢀ[AMTM] were selected to catalyze the FF coupling
reaction under the optimal conditions (2 mol% preꢀcatalyst, 4 mol% Et N, 80 °C, 3 h,
3
no solvent) for four consecutive batch runs. At the end of each batch run, an aqueous
HCl solution was added to quench the reaction to convert the in situ formed NHC
+
catalyst back to the [TM] precatalyst, in which the [TM] is reꢀintercalated back into
the interlayers of MMT through cation exchange (Scheme 2). The recycled
2
2

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MMTꢀ[TM] was reused to catalyze the FF coupling reaction of the next batch. The
results of these recycling experiments were summarized in Table 6, showing that the
C
10 furoin yield was kept nearly constant ranging from 96% to 99% throughout four
cycles. Overall, these supported catalysts are both highly effective and readily
recyclable.
Table 6. Recyclability test of MMTꢀ[TM]/Et N catalyst system for the FF selfꢀcoupling reaction
3
C
10 furoin yield (%)
Run
MMTꢀ[BHTM] MMTꢀ[HMTM] MMTꢀ[AHTM] MMTꢀ[HHTM] MMTꢀ[AMTM]
1
2
3
4
99
97
98
98
98
98
96
97
97
96
98
97
98
98
96
97
97
96
96
98
3
.5 The mechanism for the NHC-catalyzed self-coupling reactions of FF and
HMF into C10 and C12 furoins
In the present work, the mechanism of selfꢀcoupling reactions of FF and HMF
into C10 and C12 furoins catalyzed by the NHC catalyst was consistent with the
28
reported one. As displayed in Scheme 3, thiazolium cation I, in combination with a
suitable base, to form thiazolinꢀ2ꢀylidene NHC catalyst II. Nucleophilic attack of a
FA by II generates the carbeneꢀaldehyde zwitterionic adduct III, followed by proton
transfer, which affords the Breslow intermediate, IV. This diamino enol intermediate,
functions as an acyl anion equivalent and attacks a second FA molecule, forming
zwitterionic adduct V. After the proton transfer and the elimination of the furoin
24
product, the NHC catalyst II is regenerated. Finally, this catalyst was recovered by
quenching the reaction with HCl to convert the catalyst back to the precatalyst (as
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3

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shown in Scheme 2).
R2
N
R1
H
S
I
Base ꢀH
O
O
O
R
O
R
R2
R
N
O
HO
R = H, CH OH
R1
2
S
II
R
O
R
R2
R2
O
O
N
N
H
R1
R1
S
H
S
O
O
R
O
III
V
R
R2
O
N
O
O
R1
R
S
OH
IV
Scheme 3. Mechanism of selfꢀcoupling reactions of FF and HMF into C and C furoins
10
12
catalyzed by the NHC catalyst.
4
. Conclusions
In summary, through this study we have synthesized and characterized six
MMTꢀ[TM] supported precatalysts and employed them, in combination with a
suitable base, for the selfꢀcoupling reactions of FF and HMF into C10 and C12 furoins,
respectively. Our results showed that such MMTꢀ[TM] catalyst systems, particularly
+
[
TM] ILs carrying donating groups on the [TM] , upon activation with Et
3
N, exhibit
outstanding catalytic performance in both FF and HMF selfꢀcoupling reactions. For
the FF coupling reaction, optimized conditions (2 mol % preꢀcatalyst, 4 mol % Et N,
3
2
4

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no solvent, 80 °C, 3 h) led to formation of C10 furoin in essentially quantitative yield
97ꢀ99%). In the case of HMF coupling, use of 20 mol % precatalyst and 40 mol%
Et N at 120 °C for 3 h also led to near quantitative yield (97ꢀ98%) of C12 furion.
(
3
Significantly, this highꢀyielding, solventꢀfree coupling catalyst system is also
recyclable, thus affording essentially constant high furoin yield of 96% to 99%
throughout four catalyst recoveryꢀreuse cycles. Hence, these important features,
including the solventꢀfree process, the quantitative atomꢀeconomic and highꢀyielding
coupling reactions, an ease in product isolation and catalyst recovery, and excellent
recyclability of the catalyst, highlight the hallmarks of green chemistry possessed by
the current supported MMTꢀ[TM] catalyst system.
Acknowledgments
The authors would like to acknowledge the financial support from National
Natural Science Foundation of China (No.21406166). EYC thanks the US Department
of Energy grant DEꢀFG02ꢀ10ER16193 for support.
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