An interchangeable homogeneous ? heterogeneous catalyst system for furfural upgrading

Article abstract of DOI:10.1039/c5gc01648g

Intercalation of benzimidazolium cations [BI]⁺ into the nanogalleries of Na⁺/montmorillonite (MMT) clay leads to generation of recyclable supported precatalysts [BI]⁺/MMT, which, upon treatment with a base, catalyze furfural self-condensation coupling reaction into furoin in almost constant yields of >96% over the three cycles investigated. This catalyst system combines the best features of both homogeneous and heterogeneous catalyst systems, as it performs the homogeneous molecular catalysis by the discharged N-heterocyclic carbene catalyst in solution and then recovers the catalyst through in situ heterogenization after the reaction via re-intercalation of the charged precatalyst. The [^12,12BI]⁺/MMT catalyst system carrying two long-chain C₁₂ dodecyl substituents on the [BI] nitrogen atoms is particularly effective for achieving both high product yield and catalyst recyclability.

Full text of DOI:10.1039/c5gc01648g

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An interchangeable homogeneous ⇔
heterogeneous catalyst system for furfural
Cite this: DOI: 10.1039/c5gc01648g
upgrading†
Received 20th July 2015,
Accepted 12th August 2015
DOI: 10.1039/c5gc01648g
Lu Wang and Eugene Y.-X. Chen*
www.rsc.org/greenchem
+
Intercalation of benzimidazolium cations [BI] into the nano- catalyst release and recapture process can then be repeated for
+
galleries of Na /montmorillonite (MMT) clay leads to generation of catalyst recyclability.
+
recyclable supported precatalysts [BI] /MMT, which, upon treat-
Along the course of current global efforts in developing
ment with a base, catalyze furfural self-condensation coupling technologically and economically feasible routes for converting
reaction into furoin in almost constant yields of >96% over the nonfood lignocellulosic biomass into fuels, chemicals, and
2
three cycles investigated. This catalyst system combines the best materials, furaldehydes (FAs), particularly furfural and
features of both homogeneous and heterogeneous catalyst 5-hydroxylmethylfurfural (HMF) derived from dehydration of
systems, as it performs the homogeneous molecular catalysis by biorefinery carbohydrates, have emerged as two of the most
the discharged N-heterocyclic carbene catalyst in solution and promising value-added biomass building blocks or platform
3
then recovers the catalyst through in situ heterogenization after chemicals. However, hydrodeoxygenation of furfural and
the reaction via re-intercalation of the charged precatalyst. The HMF produces low carbon-number alkanes, unsatisfactory as
1
2,12
+
4
[
BI] /MMT catalyst system carrying two long-chain C12 dodecyl fuel additives. To produce high-quality liquid fuels, furfural
substituents on the [BI] nitrogen atoms is particularly effective for and HMF need to be upgraded into higher energy-density
5
achieving both high product yield and catalyst recyclability.
species through chain-extension reactions. As FAs such as
HMF cannot undergo self-aldol condensation for chain exten-
sion, cross-aldol condensation with acetone has been develo-
A number of approaches have been developed to bridge homo-
geneous and heterogeneous catalysis, pursuing the ultimate
goal of creating a catalytic system that can combine the best
features of both homogeneous catalysis for high activity,
efficiency and selectivity enabled by quantitative, discrete
molecular catalyst sites and heterogeneous catalysis for ease of
4,6
ped. A greener process has been developed for direct self-
condensation coupling of HMF by organic N-heterocyclic
carbene (NHC) catalysts, which reverse the polarity of the HMF
carbonyl (umpolung) and enable coupling of HMF into C12
,5′-dihydroxymethyl furoin (DHMF) with near quantitative
7
5
1
product separation and catalyst stability, recovery or recycling.
8,9
yield and 100% atom-economy.
This organocatalytic
Among which, heterogenization of homogeneous catalysts by
supporting a molecular catalyst onto an insoluble inorganic or
organic carrier is most common. Such heterogenized mole-
cular catalysts perform catalytic reactions on the surfaces of
insoluble carriers, often exhibiting different performances
than the original molecular catalysts in a truly homogeneous
environment. We hypothesized that a more desirable design
would be a supported catalyst system that can release the
molecular catalyst into solution for homogeneous catalysis on
demand and then recover the catalyst back onto the support
through in situ heterogenization after the reaction. Such a
benzoin-condensation-type HMF chain-extension method,
which bears the hallmarks of green chemistry (i.e., a solvent-
free, metal-free process with 100% atom-economy and quanti-
tative selectivity), also works well for other FAs such as furfural
8,10
and 5-methylfurfural.
The proposed mechanism for the
NHC-catalyzed self-coupling of FAs into furoins via the umpo-
7,8
lung condensation
is analogous to that proposed for the
11
benzoin condensation of benzaldehyde by Breslow.
Homogeneous benzoin condensation of furfural has been
7,8,10,12
extensively investigated,
with the best furoin yield
(
>99%) achieved most recently by the NHC derived from the
1
3
acetate substituted thiazolium salt. However, from an econo-
mic and green chemistry point of view, it is more desirable to
develop efficient and reusable heterogeneous NHC catalyst
systems for benzoin condensation and FA self-coupling reac-
tions. In this context, polymeric NHCs were developed for
Department of Chemistry, Colorado State University, Fort Collins, Colorado
80523-1872, USA. E-mail: eugene.chen@colostate.edu; Fax: (+1) 970-491-1801
†
Electronic supplementary information (ESI) available: Experimental details as
1
13
well as H and C NMR spectra of the intermediates and products. See DOI:
0.1039/c5gc01648g
9
,14
1
benzaldehyde condensation reaction.
Insoluble polymer
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supported thiazolium salts, when combined with a base, pro-
duced furoin with under 67% yields in the furfural self-coup-
1
5
ling reaction.
Likewise, polymer-supported imidazolium
salts, when employed for the benzoin condensation of alde-
hydes in the presence of base, gave the acyloin products under
1
6
7
7% yields.
As can be seen from the above brief overview, highly
efficient and recyclable supported catalyst systems for self-
coupling of FAs, particularly furfural and HMF, have not yet
been achieved. To address this challenge, we have developed a
novel interchangeable homogeneous ⇔ heterogeneous catalyst
+
system supported onto Na /montmorillonite (MMT) clay and
successfully applied it to catalyze highly efficient self-coupling
reaction of furfural into furoin, achieving essentially quantitat-
ive furoin yields over the three catalyst recovery-reuse cycles
performed.
Benzimidazolium [BI] salts carrying two different or
same alkyl groups on the nitrogen atoms were prepared in
high yields from double alkylation of benzimidazolide, derived
from deprotonation of benzimidazole with a base (NaH or
1
,12
+
12,12
+
Fig. 1 XRD profiles of MMT (a), [ BI] /MMT (b), and [
BI] /MMT (c).
+
17
1,12
+
12,12
+
that the basal spacings of MMT, [ BI] /MMT, and [
BI] /
MMT were 1.07, 1.63, and 2.35 nm, respectively. It is well
known that the basal spacing of the MMT clay treated with a
long-chain ammonium salt is largely dependent on the chain
length of the ammonium salt. Accordingly, [
carrying two dodecyl chains on the [BI] cation exhibited the
largest basal spacing (2.35 nm) of the series, followed by
BI] /MMT with only one dodecyl chain (1.63 nm) which,
nonetheless, had larger basal spacing than the parent MMT
with the smallest cation (Na ) of the series (1.07 nm). This
comparative analysis provided this key initial evidence that the
BI] cations were successfully intercalated
into the nanogalleries of MMT.
Additionally, FT-IR was used to further verify the intercala-
tion of the [BI] cations into the interlayers of MMT. As can be
seen from Fig. 2, FT-IR spectra of [ BI] /MMT and [
MMT exhibited the characteristic absorption bands at 1472
and 1570 cm , which corresponded to CvC and CvN
stretching vibrations of the benzimidazole moiety. Addition-
NaHCO
3
). More specifically, a stepwise procedure involving the
18
12,12
+
BI] /MMT
first reaction of benzimidazolide with MeI, followed by the
subsequent reaction of the resulting 3-methylbenzimidazole
with dodecyl bromide, led to the desired 1-dodecyl-3-methyl
+
1,12
+
[
1
,12
benzimidazolium bromide, [ BI]Br, in 94% overall yield. On
the other hand, a simpler one-pot procedure involving the
reaction of benzimidazolide with 3 equivalents of dodecyl
bromide was employed to prepare 1,3-didodecyl benzimidazo-
+
1,12
+
12,12
+
[
BI] and [
1
2,12
lium bromide, [
BI]Br, in 93% yield. Next, intercalation of
+
the [BI] cations into the nanogalleries of MMT proceeded
through a cation exchange reaction between the sodium
cations of MMT and [BI] cations of the [BI]Br salts, affording
the MMT-supported [BI] precatalysts,
BI] /MMT (Scheme 1). The analogous MMT-supported
-benzyl-5-(2-hydroxyethyl)-4-methylthiazolium [TM]
+
+
1,12
+
12,12
+
BI] /
+
1,12
+
[
BI] /MMT and
1
2,12
+
[
−1
+
3
pre-
+
catalyst, [TM] /MMT, included here as a comparative example
of this study, was prepared in the same manner.
−1
ally, aliphatic C–H stretching vibrations at 2856 and 2930 cm
were also observed, accounting for the alkyl substituents in the
+
The prepared MMT-supported [BI] precatalysts were
1,12
+
12,12
+
[
BI] and [
BI] cations. Hence, these FT-IR results pro-
characterized by powder X-ray diffraction analysis (XRD),
Fourier transform infrared (FT-IR) spectroscopy, and thermo-
1
,12
+
gravimetric analysis (TGA). The XRD profiles of MMT, [ BI] /
1
2,12
+
MMT, and [
BI] /MMT are overlaid in Fig. 1 for comparison.
The basal spacings of these samples were calculated using the
Bragg’s Equation: 2d sin θ = λ, where d is the basal spacing; θ is
the Bragg peak angle; and λ is the X-ray wavelength (0.154 nm
in the current measurement). The calculated results showed
1,12
+
12,12
+
1,12
+
12,12
+
Scheme 1 Outlined synthesis of [ BI] /MMT and [
BI] /MMT.
Fig. 2 FT-IR spectra of MMT (a), [ BI] /MMT (b), and [
BI] /MMT (c).
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vided additional evidence for the intercalation of the [BI]
cations into the nanogalleries of MMT.
TGA and elemental analysis (EA) gave comparable results
on analyzing the azolium salt content in the inorganic
1
9
support. In this work, TGA measurements were carried out
+
+
to determine the contents of the [BI] and [TM] cations in the
Scheme 2 Schematic representation of the three processes involved in
resulting MMT-supported precatalysts. The TGA curves of the self-coupling of furfural by the MMT-supported catalysts.
1
,12
+
+
12,12
+
MMT, [ BI] /MMT, [TM] /MMT, and [
BI] /MMT are
1
,12
+
shown in Fig. 3, from which the contents of [ BI] and
BI] in the resulting MMT-supported precatalysts were cal-
culated to be 405.8 and 549.9 μmol g , respectively. One can
1
2,12
+
[
−
1
titative, selective, discrete molecular catalyst sites and hetero-
geneous catalysis for ease of product separation and catalyst
recovery or recycling.
Guided by this hypothesis, we examined catalytic perform-
ances in furfural self-coupling of the MMT-supported precata-
lysts in combination with NaH. While our initial screening
1
2,12
+
note that [
BI] with two long-chain alkyl substituents was
more favorably exchanged into the interlayers of MMT than
1
,12
+
[
BI] with only one such long-chain alkyl substituent. The
+
+
same analysis revealed the content of [TM] in the [TM] /MMT
−1
precatalyst to be 647 μmol g . These values were used to cal-
culate the loading of the NHC catalyst (which is generated in
situ upon treatment of the precatalyst with a base, vide infra)
relative to the furfural substrate.
showed that other bases such as Et N and 1,8-diaza-bicyclo-
3
[5.4.0]undec-7-ene (DBU) also worked to some certain extent,
the best-performing base was determined to be NaH in terms
of furoin yield and catalyst recyclability. Hence, subsequent
studies were focused on using NaH as the base, the results of
which are summarized in Table 1. The blank experiment
+
+
With the above three MMT-supported [TM] and [BI] pre-
catalysts in hand, next we investigated their catalytic perform-
ances in self-condensation reaction of furfural into furoin. We
envisioned that activation of the precatalyst through deproto-
nation of the C2-proton of the azolium salt with a suitable
base such as NaH should generate the corresponding NHC
(
Control-1) showed that MMT exhibited no catalytic activity
and thus functioned only as a support. In the presence of
+
1
0 mol% of [TM] /MMT and NaH, the self-coupling of furfural
in THF at 25 °C produced furoin in 85.8% yield after 5 h (run
–1). However, the recyclability of this catalyst system was
2
0
catalyst. Then the in situ generated neutral NHC catalyst
should be discharged into the solution phase as it is a neutral
species, while the sodium cations from NaH are intercalated
into the interlayers of MMT. Hence, the self-coupling catalysis
will actually occur in the solution phase behaving much like a
homogeneous catalyst, which converts furfural into furoin.
The precatalyst recovery is accomplished by quenching the
reaction with HCl to convert the neutral NHC back to the benz-
imidazolium salt, which is re-intercalated back into the nano-
galleries of MMT through cation exchange (Scheme 2). The
1
poor, as evidenced by a large reduction in the furoin yield to
only 54.1% using the recycled catalyst (run 1–2). The main
reason for this phenomenon could be attributed to the high
+
sensitivity of the [TM] derived NHC to moisture and oxidation
+
during the quenching process. Switching to the [BI] based
catalyst and using the current standard conditions (2.0 mmol
furfural, 10 mol% of the precatalyst and base relative to fur-
1
,12
+
fural, 3 mL THF, 25 °C, 6 h), the furoin yield by [ BI] /MMT
+ NaH was only moderate (76.0%, run 2). However, the catalyst
+
recycled MMT-supported [BI] precatalyst can then be reused
1
2,12
+
system based on [
BI] /MMT + NaH gave an excellent furoin
to catalyze the furfural self-coupling reaction. Overall, this
novel recyclable supported catalyst concept should combine
the best features of homogeneous catalysis for accessing quan-
yield of 96.6% (run 3–1), and this result was almost identical
to that obtained from homogeneous catalysis (Control-2). In
Table 1 Results of furfural self-coupling reaction catalyzed by MMT-
a
supported azolium salts in combination with a base
Run #
Precatalyst
MMT
Time (h)
Yield (%)
Control-1
5
5
5
6
6
6
6
6
0.0
85.8
54.1
76.0
97.1
96.6
96.1
96.1
+
1
1
2
–1
–2
[TM] /MMT
Recycled
1
1
1
,12
+
[
[
[
BI] /MMT
2,12
2,12
Control-2
BI]Br
+
3–1
3–2
3–3
BI] /MMT
Recycled
Recycled
a
+
+
Furfural, 2.00 mmol for [BI] based runs or 3.00 mmol for [TM]
based runs; MMT supported azolium salt (10 mol%); base (NaH)+,
1,12
+
+
Fig. 3 TGA curves of MMT (a), [ BI] /MMT (b), [TM] /MMT (c), and 10 mol%; THF, 3.0 mL; temperature, 25 °C; reaction time, 6 h for [BI]
12,12
+
+
[
BI] /MMT (d).
based runs or 5 h for [TM] based runs; nitrogen atmosphere.
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1
2,12
+
fact, a model reaction of [
BI] /MMT with NaH confirmed afford furoin in >96% yield, which remained essentially con-
that the majority of in situ generated NHC was released from stant over the three catalyst recovery-reuse cycles.
the interlayers of MMT (the TGA result of the recovered MMT-
supported catalyst after reacting with NaH was close to that of
pristine MMT, Fig. S6†). The observed much superior catalytic
Acknowledgements
1
2,12
+
performance by the NHC catalyst derived from [
BI] /MMT
1
,12
+
+
NaH, relative to that from [ BI] /MMT + NaH, could be This work was supported by Sekisui Chemical Co., Ltd. LW
attributed to the Wanzlick equilibrium, which describes the thanks Sekisui Chemical for a postdoctoral fellowship, and we
equilibrium of a tetraaminoethylene, formed by dimerization thank Mr. Yuji Eguchi for helpful discussions.
1
7,21
of carbenes, and its corresponding carbene.
[
Thus, the
BI] derived NHC possessing sterically less demanding
substituents may favor the formation of the carbene dimer,
1
,12
+
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1
2,12
+
the [
BI] derived NHC bearing sterically demanding substi-
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furoin. This supported catalyst system combines the best fea-
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ease of product separation and catalyst recovery or recycling.
This new concept has been demonstrated by the MMT-sup-
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upgrading molecular catalysis in the homogeneous, solution
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1
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