A catalytic aldol condensation system enables one pot conversion of biomass saccharides to biofuel intermediates

Article abstract of DOI:10.1039/c7gc00362e

Producing bio-intermediates from lignocellulosic biomass with minimal process steps has a far-reaching impact on the biofuel industry. We studied the metal chloride catalyzed aldol condensation of furfural with acetone under conditions compatible with the upstream polysaccharide conversions to furfurals. In situ far infrared spectroscopy (FIR) was applied to guide the screening of aldol condensation catalysts based on the distinguishing characteristics of metal chlorides in their coordination chemistries with carbonyl-containing compounds. NiCl₂, CoCl₂, CrCl₃, VCl₃, FeCl₃, and CuCl₂ were selected as the potential catalysts in this study. The FIR results further helped to rationalize the excellent catalytic performance of VCl₃ in furfural condensation with acetone, with 94.7% yield of biofuel intermediates (C8, C13) in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl) solvent. Remarkably, addition of ethanol facilitated the acetal pathway of the condensation reaction, which dramatically increased the desired product selectivity over the furfural pathway. Most significantly, we demonstrate for the first time that VCl₃ catalyzed aldol condensation in acidic medium is fully compatible with upstream polysaccharide hydrolysis to monosaccharide and the subsequent monosaccharide isomerization and dehydration to furfurals. Our preliminary results showed that a 44% yield of biofuel intermediates (C8, C13) can be obtained in one-pot conversion of xylose catalyzed by paired metal chlorides, CrCl₂ and VCl₃. A number of prior works have shown that the biofuel intermediates derived from the one-pot reaction of this work can be readily hydrogenated to biofuels.

Full text of DOI:10.1039/c7gc00362e

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ARTICLE
A Catalytic Aldol Condensation System Enables One Pot
Conversion of Biomass Saccharides to Biofuel Intermediates
Received 00th January 20xx,
Accepted 00th January 20xx
a, b
a
a
*a
Huixiang Li † , Zhanwei Xu , Peifang Yan and Z. Conrad Zhang
DOI: 10.1039/x0xx00000x
Producing bio-intermediates from lignocellulosic biomass with minimal process steps has far-reaching impact for the
biofuel industry. We studied metal chloride catalyzed aldol condensation of furfural with acetone in conditions compatible
with the upstream polysaccharide conversions to furfurals. In-situ far infrared spectroscopy (FIR) was applied to guide the
screening of aldol condensation catalysts based on the distinguishing characteristics of metal chlorides in their
www.rsc.org/
2 2 3 3 3 2
coordination chemistries with carbonyl-containing compounds. NiCl , CoCl , CrCl , VCl , FeCl , and CuCl were selected as
the potential catalysts in this study. The FIR results further helped to rationalize the excellent catalytic performance of VCl
3
in furfural condensation with acetone with 94.7% yield to biofuel intermediates (C8, C13) in 1-butyl-3-methylimidazolium
chloride ([BMIM]Cl) solvent. Remarkably, addition of ethanol facilitated the Acetal pathway of the condensation reaction
which dramatically increased the desired product selectivity over the Furfural pathway. Most significantly, we
3
demonstrate for the first time that VCl catalyzed aldol condensation in the acidic medium is fully compatible with the
upstream polysaccharide hydrolysis to monosaccharide and the subsequent monosaccharide isomerization and
dehydration to furfurals. Our preliminary results showed that 44% yield of biofuel intermediates (C8, C13) can be obtained
2 3
in one-pot conversion of xylose catalyzed by paired metal chlorides, CrCl and VCl . A number of prior works have shown
that the biofuel intermediates derived from the one-pot reaction of this work can be readily hydrogenated to biofuels.
the bio-intermediates remains faced with some serious challenges
8
, 9
Introduction
due to poor oil stability . Dissolution of raw solid biomass in a
solvent and the subsequent reactions in converting cellulosic
biomass to biochemicals and biofuels are an important strategy for
Fossil fuels based on petroleum, coal, and natural gas provides
more than 75% of the world’s energy today. However, these
nonrenewable resources are increasingly depleted, and the heavy
utilizing of fossil resources causes harmful impacts to the
1
0-13
the establishment of bio-refineries
. The objective of this work
was to enable one-pot production of readily processible biofuel
intermediates in high yield from cellulosic biomass by addressing
the compatibility of aldol condensation with the upstream
polysaccharide conversions.
1
-3
environment . To meet the growing demand for bio-based fuels
and chemicals, lignocellulosic biomass as potential sustainable
source of feedstock has been broadly explored through many
4
different thermal and biological processes . However, the structure
of such biomass is rather recalcitrant and complex. Multiple process
steps are typically involved to convert biomass feedstock to
potential products . The multiple units operation will decrease
the energy efficiency and the product yield. In addition, the
increased waste by-products in solvents or in gas phase cause
environmental issues. Therefore, it is critical to reduce the number
of processing steps for the development of economically viable
biomass conversion technologies.
O
O
OH
O
O
O
FA
5
, 6
acid
acid
acid
Hemicellulose
Biomass
O
OH
+
base
OH
OH
O
O
xylose
furfural
HO
O
FAF
O
OH
OH
O
OH
O
O
acid
O
Cellulose
HO
O
base
OH
OH
glucose
5-HMF
HMF-acetone
The US Environmental Protection Agency (EPA) has recently
proposed that biofuel intermediates have far-reaching impact for
biodiesel industry . While direct pyrolysis of lignocellulosic biomass
7
Scheme 1. A general route of biomass conversion to diesel fuel range
multi-carbon precursors, FA, FAF, and HMF-acetone.
is a single step process in producing bio-intermediates, upgrading of
Scheme 1 depicts the general route of cellulosic biomass conversion
to biofuel intermediates. Typically, cellulosic feedstock is
hydrolyzed to yield monosaccharide, followed by selective catalytic
monosaccharide dehydration to produce platform compounds such
1
4-
as furfural and 5-hydroxymethylfurfrual (5-HMF) in acidic media
.
2
2
2 4 3
Brφnsted acids such as HCl, H SO , CH COOH, maleic acid are
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widely used as catalysts in cellulosic biomass hydrolysis. Our M-Cl bond absorbance decreased as a result of carbonyl
previous works based on the ionic liquid system have established coordination to the added
the hydrolysis of polysaccharides to monosaccharide catalyzed by
23
metal chlorides. In addition, Chromium (II, III) chlorides showed
excellent catalytic effect in selective conversion of monosaccharide
isomerization and dehydration to produce 5-HMF and furfural in
2
4
ionic liquid
,
such as 1-butyl-3-methylimidazolium chloride
(
[BMIM]Cl). To exploit the full potential of furfural and 5-HMF as
25, 26
platform chemicals
, their transformation by aldol condensation
with ketones such as acetone has been studied as a pathway to
produce fuel range intermediate molecules - 4-(2-furyl)-3-buten-2-
2
7-29
one (FA), and 1,5-bis-(2-furanyl)-1,4-pentadien-3-one (FAF)
Typically, aldol condensation is catalyzed by base catalysts
.
,
3
0-33
such as sodium, magnesium and calcium hydroxides and activated
dolomite catalyst. However, the base catalyzed condensation
reactions are incompatible with the upstream hydrolysis and
3
4
dehydration reactions in acid medium (Scheme 1). Therefore, it is
highly desirable to develop an efficient aldol condensation catalyst
compatible with one-pot conversion of biomass polysaccharides to
biofuel Intermediates in a single medium.
Results and Discussion
We explored cellulosic biomass conversion to biofuel intermediates
in a one-pot process in [BMIM]Cl by first focusing on screening and
characterization of metal chlorides as potential catalysts for aldol
condensation (Scheme 2) in conditions compatible with the
Fig. 1 Far infrared spectra of the MCl
x
/[BMIM]Cl/cyclohexanone system.
o
a) NiCl ; b) CoCl ; c) CrCl ; d) VCl ; e) CuCl ; f) FeCl ; 100 C, The
2
2
3
3
2 3
upstream polysaccharide processing conditions. In our recent study,
in-situ far infrared (FIR) spectroscopy was employed to follow the
coordination chemistry of several classes of metal chlorides in the
spectrum in black and bold is the background without cyclohexanone
35, 36
process of glucose conversion to 5-HMF
. FIR was demonstrated
model compound, but the absorbance by Co-Cl, Ni-Cl, and Cr-Cl
were restored to different extent when cyclohexanone was
removed from the solvent by evaporation (as indicated by the
upward arrows in Fig 1. a, b, and c, respectively). The Cr-Cl
absorption peak was partially restored and the peaks of Co-Cl and
Ni-Cl were fully restored under the same conditions. For Fe-Cl, Cu-Cl,
and V-Cl, the FIR absorption peaks continued to decrease to
different extent even when cyclohexanone was evaporated
gradually (as indicated by the downward arrows in Figs. 1. d, e, and
f, respectively). Thus the coordination strength of metal chloride
with carbonyl can be summarized in the sequence based on the
variation of the intensity of M-Cl FIR absorption peaks in the MClx/
as a unique tool capable of differentiating the bond strength among
some metal chlorides in forming complexes with compounds having
a carbonyl group. In this work, for the first time, we extend in-situ
FIR as a tool to guide the screening of metal chlorides as potential
catalyst to catalyze aldol condensation reaction. We use the
coordination bond strength as the initial screening criteria for the
2 2 3 3
identification of aldol condensation catalyst. NiCl , CoCl , CrCl , VCl ,
CuCl , and FeCl were selected to investigate the coordination bond
2
3
strength of these metal ions with the carbonyl group of
cyclohexanone model compound using FIR spectroscopy (Fig. 1).
[
BMIM]Cl/ cyclohexanone system: NiCl
2 2 3
, CoCl (Very weak) < CrCl
O
(moderate) < VCl (Strong) < FeCl , CuCl (Very strong). To use the
O
O
3
3
2
O
O
MClx
O
O
O
coordination strength of metal chloride with carbonyl as the initial
screening criteria for aldol condensation catalyst, it must be
recognized that, for an ideal catalyst, the interaction between the
+
+
[
BMIM]Cl
FA
FAF
Scheme 2. Furfural condensation with acetone catalyzed with catalyst and the reagent should be moderate; very weak and very
metal chlorides - MCl
x
in [BMIM]Cl. cyclohexanone
strong represent both extremes of no activation for the aldol
condensation and quantitative metal chloride consumption,
respectively. Therefore, it seemed that CrCl
3 3
and VCl could be
suitable catalysts based on the FIR spectroscopy results.
-
1
An intense absorption band near 300 cm for the MClx/
BMIM]Cl system is ascribed to a stretch vibration of M-Cl
[
3
6
bonds (M = Ni, Co, Fe, Cr, V, Cu, x = 2, 3) . When
cyclohexanone was added to the MCl /[BMIM]Cl system, the
x
2
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3 3
Although the selectivity to FA and FAF products by CrCl and VCl
catalysts were the highest, there was a significant portion of
furfural converted to undetectable oligomeric products. We
hypothesized that (1) the coordination of the furfural C=O group
with CrCl and VCl was not productive for the desired condensation
3
3
reaction, (2) such coordination reduced the effective concentration
of CrCl and VCl for the activation of acetone, and (3) the free
3
3
furfural molecules tended to form undetectable oligomers.
Therefore, several additives, ethylene glycol dimethyl ether (GDE),
ethanol, methyl tert-butyl ether (MTBE), were evaluated to
suppress the oligomerization of furfural and to promote the activity
Fig. 2 The FA and FAF products were yield by metal chloride of furfural to condense with acetone (Fig. 3a). The addition of
catalyzed furfural condensation with acetone in [BMIM]Cl. ethanol was found to improve the performance of VCl for the aldol
Reaction conditions: Furfural 2.1 mmol, [BMIM]Cl (IL) 2.0 g, MCl
condensation reaction with the most drastic improvement, both in
3
x
o
0
.1 mmol, Acetone 10.4 mmol; 140 C; 90 min.
furfural conversion and in FA and FAF selectivity. The overall
selectivity of FA and FAF was elevated from 56% to 94.7% (FA in
4
0.1% and FAF in 54.6%) with the added ethanol (Fig. 3a). With
CrCl , however, the addition of ethanol didn’t show as dramatic
3
We then investigated the catalytic performance of these metal improvement in the catalytic performance as with VCl (Fig. 3b).
3
chlorides for furfural condensation with acetone. The results are in
excellent agreement with our hypothesis, as CrCl , and VCl showed
3
3
good catalytic results in furfural condensation with acetone (Fig. 2).
The yield of furfural condensation products in the presence of CuCl
FeCl , NiCl , CoCl were much lower than that in the presence of
CrCl and VCl . The desired function of the metal chloride is to
activate acetone to enol form which subsequently condensates with
2
,
3
2
2
3
3
3
7, 38
furfural
3 3
. Thus the good catalytic results of CrCl and VCl may
be attributed to the moderate coordination strength with acetone
by forming activated enol intermediate. In addition, the higher
furfural conversions catalyzed by CrCl
VCl , CuCl , and FeCl is likely due to the reversible coordination of
carbonyl in furfural with CrCl , NiCl , and CoCl . However, the
weakest coordination of NiCl and CoCl with carbonyl group
3 2 2
, NiCl , and CoCl than that by
3
2
3
3
2
2
2
2
Fig. 4 The effect of ethanol amount on furfural condensation with
acetone catalyzed by VCl in [BMIM]Cl; n is the mole ratio of
ethanol to furfural. Reaction conditions: Furfural 2.1 mmol,
resulted in the least activated acetone for the desired FA and FAF
condensation products, and was found to favor the formation of
undetectable oligomers of furfural molecules. The strongest
coordination of CuCl2 and FeCl3 with carbonyl group resulted in
their lowest furfural conversion as expected due to the fixation of
these two metal chlorides by a fraction of the carbonyl compounds.
The low selectivity to FA and FAF products with CuCl and FeCl also
3
o
[
BMIM]Cl 2.0 g, VCl3, 0.1 mmol, Acetone 10.4 mmol; 140 C, 1.5 h.
2
3
We further investigated the basis of the most favorable ethanol
promotion, and the optimum ethanol amount. The results are
shown in Fig. 4. 2-furaldehyde diethyl acetal (FDEA) as a new
product was detected. The amount of FDEA increased with the ratio
of ethanol but stayed in low concentration. FDEA may have been an
intermediate in the reaction system. Accordingly, the reaction
pathways of furfural condensation with acetone are proposed as
shown in Scheme 3. Without ethanol, furfural was converted to
undetectable compounds which are soluble in [BMIM]Cl based on
GC-MS analysis. In addition, furfural reacted with acetone and
produced FA. FA further condensed with furfural and produced FAF
suggests the formation of undetectable oligomers from furfural or
acetone molecules.
(
in Furfural path). With the addition of ethanol, firstly furfural
reacted with ethanol to form FDEA (acetal) as an intermediate,
which then condensed with acetone to form FA. FA further
condensed with FDEA to form FAF (in Acetal path). Thus ethanol
changed the reaction pathway from Furfural path to Acetal path.
We further compared the relative reactivities of Furfural path and
Acetal path (Fig. S1). Only 15% FA and 6% FAF were obtained in
Furfural path, as compared to 26% FA and 14% FAF in the Acetal
path. Obviously the reactivity of Acetal path to FA and FAF is higher
than that of Furfural path.
Fig. 3 (a). The effect of several additives on furfural condensation
with acetone catalyzed by VCl
Reacted for 3 h; (b) The effect of ethanol on CrCl
3
in [BMIM]Cl; Additive 22.0 mmol,
, VCl , FeCl in
3
3
3
aldol condensation reaction; Ethanol 12.5 mmol, Reacted for 1.5
h. Reaction conditions: Furfural 2.1 mmol, [BMIM]Cl (IL) 2.0 g,
o
0.1 mmol, Acetone 10.4 mmol; 140 C.
MCl
x
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O
O
V(III)
O
O
O
O
O
F
O
O
O
Acetal
O
O
r2
p2
V(III)
FAF
FAF
R
O
O
O
O
O
O
r1 Acetone
p1 Acetone
R
O
O
Furfural path
O
O
O
+
+
Furfural
-H
O
2
O
ethanol
Humins
Humins
O
F
Acetal
Furfural path
Acetal path
R
O
R
Acetal path
O
OEt
OEt
Scheme 3 The reaction pathways of furfural condensation with
acetone catalyzed by VCl in [BMIM]Cl with (Acetal Path) or
without ethanol (Furfural Path). F: furfural; A: acetone.
O
OEt
-EtOH
V(III)
O
V(III)
3
O
Et
O
[BMIM]Cl
O
R
O
R
O
R
O
Et
O
Et
Scheme 4 The proposed reaction mechanism of furfural and FDEA
condensation with acetone catalyzed by VCl in [BMIM]Cl. R=H, 2-
furan-2-yl) vinyl. (Furfural path and Acetal path).
Thus it is plausible that ethanol protected furfural from
oligomerization and from coordinating with metal center directly by
forming the acetal FDEA. The activity of FDEA with activated
acetone is higher than that of furfual in the condensation reaction.
3
(
The dramatic improvement in the catalytic performance of VCl
compared to that of CrCl (Fig. 3b) may be attributed to the
stronger coordination strength of VCl with carbonyl in acetone
than that of CrCl
. We propose that activation through carbonyl helped largely improve the extraction efficiency. By conducting
3
as
3
Our recent work showed that proper choice of extraction solvent
3
3
coordination in acetone remains the rate controlling step for CrCl3 reaction in a two phase system having an extraction phase over the
catalyzed condensation, such that activating furfural to acetal FDEA reaction phase, continuous removal of formed products into the
3
does not help. Because VCl coordinates with carbonyl stronger
extraction led to the efficient recovery of the product and
than CrCl , making furfural activation the rate controlling step,
3
minimized humins formation, resulting in significantly improved
furfural activation by ethanol to acetal FDEA promoted the
42, 43.
ionic liquid recycle.
In this work, we demonstrated that the VCl
3
3
condensation with activated acetone. Although Fe(III) in FeCl also
catalyzed aldol condensation system was reused 4 times without
decreasing in its performance, by extracting the products with
coordinates with carbonyl strongly, it is non-selective as it also
coordinates strongly with hydroxyl groups. In fact, ethanol may be
considered a poison to FeCl
activity of FeCl further declined (Fig. 3b) with the addition of used of the ionic liquid and improved product yield could be
ethanol. The electronegativity values of metal ions are in the order expected by adopting a similar two phase system.
3
. Therefore, the already low catalytic diethyl ether after each run (Fig. S2). As a future work, recycled
3
2
+
2+
2+
3+
3+
of Co (2.706) < Ni (2.891) <Cu (2.952) < V (3.813) < Fe (3.835)
3
+
39
<
Cr (4.026) . The corresponding Lewis acid strength of the metal
ions is expected to follow the same order, in proportion to the
4
0
generalized electronegativity of metal ions
performance of metal chlorides for aldol condensation as
determined in this work increased in the order of FeCl < CrCl < VCl
in acetal path and NiCl , CoCl < CuCl , FeCl << VCl < CrCl in furfural
. The catalytic
3
3
3
2
2
2
3
3
3
path, at clear variance from the order of Lewis acid strength. Thus
the Lewis acidity does not account for the metal chloride catalyzed
aldol condensation. The superior performance of VCl
3
is due to its
moderately strong and selective coordination with carbonyl.
Fig. 5 The catalytic characteristics of paired metal chlorides for
The mechanisms of Furfural pathway and Acetal pathway may one-pot xylose conversion to biofuel intermediates. (a) M Cl
1
x,
proceed as that detailed in Scheme 4. In the Furfural pathway, the
M
2
Cl
y
, each 0.06 mmol; (b) The effect of molar ratio - m: n of
(m) and VCl (n) on the catalytic
coordination of the carbonyl group in acetone or FA with the V (III) bimetallic chlorides CrCl
2
3
reactivity. The total of the bimetallic chlorides is 0.10 mmol.
Reaction condition: Xylose 1.25 mmol, [BMIM]Cl (IL) 2.0 g,
ion center resulted in enol form and then condensed with furfural
to form the biofuel intermediates through dehydration. In Acetal
pathway, carbonyl group in acetone or FA formed enolate first.
Meanwhile, the acetal FDEA was converted to an oxonium ion
o
Acetone 1.0 g, GDE 2.0 g, Ethanol 0.6 g. 160 C, 2 h.
3
intermediate catalyzed by VCl . The enol form of acetone or FA was
subsequently condensed with the oxonium ion intermediate to
form FA and FAF through dealcoholization (Scheme 4). The oxonium
ion intermediate is a strong electrophile , thus accounting for the
higher reactivity of Acetal pathway.
The results above indicate that the conditions at which the metal
chloride catalyzed aldol condensation are compatible with cellulosic
biomass conversion to biofuel intermediates in a one-pot process.
To verify the strategy of one-pot process, we then evaluated the
direct conversion of xylose in [BMIM]Cl with acetone to the biofuel
intermediates, FA and FAF, with paired metal chlorides as the
4
1
3 3 2 3 2
catalyst by the combination of VCl , CrCl , CrCl , FeCl , and CuCl .
4
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2 3
The combination of CrCl + VCl showed the highest FA yield, 44% ppm using tetramethylsilane (TMS, δ (ppm) = 0.00 ppm) as the
1
3
(
Fig. 5a).
The results are consistent with the superior performance of CrCl
isomerizing aldose to ketose which is essential for the formation of
internal standard. C NMR spectra were reported in ppm using
2
in CDCl as the internal standard.
3
2
4
furfural in the first step
condensation as discovered in this work. We further studied the
effect of the molar ratio of the metal chlorides, CrCl and VCl , on Acknowledgements
3
and that of VCl in catalyzing aldol
2
3
the catalytic performance (Fig. 5b). At the total metal chlorides in
0 mol% with respect to xylose, we achieved FA yield of 44% when
the molar ratio of CrCl to VCl was 1 to 4. It should be noted that
either CrCl only or VCl only catalyst produced less amount of FA
This research was supported by the CAS/SAFEA International
Partnership Program for Creative Research Teams. The national
science fund projects (Y731410602) and the Chinese Government
1
2
3
2
3
“
Thousand Talent” program funding.
and FAF (Fig. 5b).
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2
3
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5
coordination strength of metal chloride (NiCl
2 2 3 3
, CoCl , CrCl , VCl ,
6
7
FeCl , and CuCl with carbonyl) and provided insightful basis for the
3
2
screening of suitable catalysts. VCl3 catalyzed the condensation of
furfural with acetone with high selectivity to FA and FAF, consistent
with the FIR results. The insight gained from the FIR results led us to
the use of ethanol for dramatically improved performance of VCl3
by generating FDEA as an active acetal. The total selectivity of
biofuel intermediates reached 94.7% (FA in 40.1% and FAF in
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demonstrate one pot conversion of xylose to biofuel intermediates,
FA and FAF, catalyzed with paired metal chlorides. The biofuel
1
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2 3
and VCl (mole ratio
2
3
1
1
1
1
1
4
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3
7
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DOI: 10.1039/C7GC00362E
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., 2017, 00, 1-3
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Green Chemistry
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DOI: 10.1039/C7GC00362E
A Catalytic Aldol Condensation System Enables One Pot Conversion of
Biomass Polysaccharides to Biofuel Intermediates
Huixiang Li, Zhanwei Xu, Peifang Yan, and Z. Conrad Zhang*
O
O
O
FA
O
acid
acetone
xylose
acid
Biomass
+
catalyst: VCl
furfural additive: ethanol
3
O
O
FAF
O
Yield (FA + FAF) = 94.7%
Producing bio-intermediates from lignocellulosic biomass with minimal process steps
has far-reaching impact for the biofuel industry. We discovered an efficient catalytic
aldol condensation system that enables one pot conversion of biomass
polysaccharides to biofuel intermediates. In-situ far infrared spectroscopy
successfully assisted the screening of metal chlorides as potential catalysts by
studying the coordination chemistry of metal chlorides with carbonyl compounds. By
converting furfural to 2-furaldehyde diethyl acetal with ethanol, V(III) ion
coordinated moderately with acetone showed excellent performance in the
condensation of furfural with acetone, with 94.7% yield to FA and FAF.