Dried chitosan-gels as organocatalysts for the production of biomass-derived platform chemicals

Article abstract of DOI:10.1016/j.apcata.2012.08.014

Aldol condensations between sugar-derived dehydrated aldehydes (e.g. furfural) and acetone have been proposed as a route to provide useful biomass-derived chemicals. In the quest of sustainable catalytic ways for such aldol condensations, this paper assesses the use of dried chitosan-gels as naturally-immobilized, readily available and non-hazardous amino-based organocatalysts. At room temperature chitosan dried gels are not suitable catalysts for the desired reaction. However, at higher temperatures (>90 °C) reaction proceeds efficiently either in solvent-free systems (with addition of catalytic amounts of water) or in water. The set-up of closed reactor set-ups (thermoshakers or microwave reactions) proved highly beneficial for the reaction outcome. Furthermore, chitosan dried gels were successfully re-used for a number of cycles. An efficient catalyst drying method (either lyophilization or scCO₂ drying) was crucial to achieve virtually full conversions in 4 h. After pertinent further process optimization, dried chitosan-gels may become very useful catalysts for their use in biomass-based reactions in biorefineries.

Full text of DOI:10.1016/j.apcata.2012.08.014

Applied Catalysis A: General 445–446 (2012) 180–186
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Dried chitosan-gels as organocatalysts for the production of biomass-derived
platform chemicals
a
a
b
b
Henning Kayser , Christoph R. Müller , Carlos A. García-González , Irina Smirnova ,
Walter Leitnera,c, Pablo Domínguez de María
Institut für Technische und Makromolekulare Chemie (ITMC), RWTH Aachen University, Worringerweg 1, 52074 Aachen, Germany
Institute of Thermal Separation Processes, Hamburg University of Technology, Eißendorferstraße 38, D-21073 Hamburg, Germany
Max-Planck-Institut für Kohlenforschung, 45470 Mülheim an der Ruhr, Germany
a,∗
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 18 May 2012
Received in revised form 13 August 2012
Accepted 14 August 2012
Available online 24 August 2012
Aldol condensations between sugar-derived dehydrated aldehydes (e.g. furfural) and acetone have been
proposed as a route to provide useful biomass-derived chemicals. In the quest of sustainable catalytic
ways for such aldol condensations, this paper assesses the use of dried chitosan-gels as naturally-
immobilized, readily available and non-hazardous amino-based organocatalysts. At room temperature
chitosan dried gels are not suitable catalysts for the desired reaction. However, at higher temperatures
◦
(
>90 C) reaction proceeds efficiently either in solvent-free systems (with addition of catalytic amounts
Keywords:
Chitosan
Aerogels
Furfural
Gasoline
Biofuels
of water) or in water. The set-up of closed reactor set-ups (thermoshakers or microwave reactions)
proved highly beneficial for the reaction outcome. Furthermore, chitosan dried gels were successfully
re-used for a number of cycles. An efficient catalyst drying method (either lyophilization or scCO2
drying) was crucial to achieve virtually full conversions in 4 h. After pertinent further process opti-
mization, dried chitosan-gels may become very useful catalysts for their use in biomass-based reactions
in biorefineries.
Biorefineries
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
form biomass-derived chemicals (e.g. alcohols, solvents, etc.), and
gasoline-like alkanes (Scheme 1) [14–16]. Likewise, fermentative
strategies for the production of analogous alkane-like chemicals
have been put forward as well, starting from sugars (often glucose)
as carbon source [13,17].
The production of chemicals, biofuels and commodities from
biomass is currently an increasingly important research topic,
involving interdisciplinary fields from biology to chemistry and
chemical engineering [1–4]. Among platform chemicals obtained
from biomass, furfural and 5-hydroxymethylfurfural are key
structures that can be achieved from the dehydration of the
sugar fractions (hemicellulose and cellulose, xylose and glucose,
respectively) of lignocellulosic materials [5,6]. In this respect, lig-
nocellulose treatment usually involves fractionation processes –
the so-called pre-treatment – aiming to separate the main wood
components (lignin, hemicellulose and cellulose) [3,4,7–9]. Later
on processes often undergo polysaccharide depolymerizations to
afford fermentable sugars [10,11], and eventually sugar dehy-
drations to afford aromatic aldehydes, e.g. furfural from xylose
dehydration [12]. Formed furfural can be subsequently condensed
with acetone, actually another chemical accessible by fermenta-
tion [13], and obtained condensates can be further hydrogenated to
The above-described condensation route may be easily
approached by classic base catalysis (e.g. NaOH). Yet, that option
would lead to the production of significant amounts of wastewa-
ter at large scale. To circumvent this, several organocatalytic routes
have been proposed, ranging from catalysis based on small organic
molecules (e.g. proline) [18], to immobilized piperazines [19]. In
this context, envisaging a huge biomass-processing load at practi-
cal scale, the quest of readily worldwide available, non-hazardous
and biodegradable catalysts would certainly be privileged. With
these considerations in mind, the focus of this work was put on chi-
tosan – a common waste stream from food industry – as a naturally
heterogeneous, immobilized organocatalyst. Several applications
of chitosan for the aldol-like C C bond forming reactions have
been recently published [20–25]. In one of these publications, the
condensation of furfural with acetone at room temperature with
excellent conversions was reported by using chitosan gels [22]. Yet,
very recently it was shown that these good results were actually
related to the presence of NaOH impurities which were introduced
∗ Corresponding author. Tel.: +49 241 8020468, fax: +49 241 8022177.
E-mail address: dominguez@itmc.rwth-aachen.de (P. Domínguez de María).
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.08.014

H. Kayser et al. / Applied Catalysis A: General 445–446 (2012) 180–186
181
Scheme 1. Synthetic concept for the production of biomass-derived products. Furfural and acetone may be used for the production of added-value platform chemicals
3,4,14–16].
[
during the formation of chitosan hydrogels – and remained as a
base within the organocatalyst – and not to a chitosan-catalyzed
organocatalytic reaction [26]. Actually, chitosan hydrogels resulted
rather poor catalysts for aldol condensations, with low or even
no conversions for many substrates, when NaOH was carefully
removed from them [26].
In this paper new reaction conditions for the efficient chitosan-
catalyzed condensation of furfural and acetone for the production
of precursors of gasoline-like fuels are disclosed, representing a
novel, clean and sustainable approach to afford these chemicals
within biorefineries.
2.3. General reaction conditions
Furfural (96.08 mg, 1 mmol) was mixed with 20 eq. acetone
(1161.6 mg; 20 mmol), 20 mg chitosan aerogel beads, and 10 mg
water. The reaction mixture was: (a) stirred in a microwave reac-
◦
tor (300 W) 4 h at 150 C; or (b) shaked in a thermoshaker for 12 h
◦
at 90 C. Analytics were carried out by GC and NMR. Isolated yields
were determined by adding MgSO4 to the filtrated reaction mix-
ture, removing the volatile components by reduced pressure and
isolating the products by flash chromatography on silica (ethyl
acetate:n-pentane 1:1). For the catalyst recycling experiments, the
used aerogel beads were washed with an excess of acetone and
dichloromethane, dried at room temperature and high vacuum
until their weight was constant and reused.
2
. Experimental
2.1. Chemicals
2.4. Analytical methods
All chemicals were purchased in reagent-grade from
Sigma–Aldrich and employed without any further purifica-
tion. Acetone was distilled and stored over molecular sieves
Conversion and selectivity were determined by GC analysis (FID,
◦
◦
◦
CP-WAX-52 column, 1.5 bar N , 50 C 5 min, 8 C/min until 180 C)
2
using 1-phenylethanol as internal standard. Textural properties
of the chitosan aerogels were determined by low-temperature
N2 adsorption–desorption analysis (Nova3000e). Specific surface
area (SBET) was determined by the BET (Brunauer–Emmett–Teller)
method. Pore volume (Vp) and mean pore diameter (dp) were esti-
mated using the BJH (Barrett–Joyner–Halenda) method. The IR
studies were performed a Bruker Alpha-R A220/D-01 ATR-IR spec-
trometer.
(
4 A˚ ). Water was purified by membrane filtering, reverse osmotic
deionization and UV-sterilization in a Werner Easypure system to
−
1
‘
milli-Q’-grade (18.2 Mꢀ cm ).
2
.2. Catalyst preparation
Hydrogel beads were prepared by a modified protocol reported
in literature [27]: 2.5 g chitosan (from shrimp shells, >75% deacety-
lated) were dissolved in an aqueous acetic acid solution (5.5 mM;
Furfurylhydroxybutanone 1: 1H-NMR (300 MHz, CDCl3) ı 7.35
(dd, J = 1.7, 0.7 Hz, 1H), 6.31 (dd, J = 3.2, 1.8 Hz, 1H), 6.25 (d, J = 3.2 Hz,
1H), 5.14 (dd, J = 8.8, 3.5 Hz, 1H), 3.05 (dd, J = 17.6, 8.8 Hz, 1H), 2.90
(dd, J = 17.6, 3.6 Hz, 1H), 2.20 (s, 3H). 13C-NMR (75 MHz, CDCl3) ı
208.5, 155.0, 142.2, 110.3, 106.3, 63.8, 48.2, 30.7.
1
00 mL). The solution was drop-wise precipitated through a 2 mm
syringe into an aqueous NaOH solution (0.1 M; 300 mL). After 2 h,
the hydrogel beads were filtrated over sintered glass crucible and
washed with deionized water until constant pH was reached. The
resulting beads were stored in water overnight and washed with
1
Furfurylbutenone 2: H-NMR (400 MHz, DMSO) ı 7.85 (d,
J = 1.2 Hz, 1H), 7.43 (d, J = 16.2 Hz, 1H), 6.96 (d, J = 3.4 Hz, 1H), 6.64
(dd, J = 3.4, 1.8 Hz, 1H), 6.47 (d, J = 16.1 Hz, 1H), 2.28 (s, 3H). 13C-NMR
(101 MHz, DMSO) ı 197.4, 150.4, 146.0, 129.7, 124.0, 116.3, 112.9,
27.2.
2
–3 L deionized water before filtrating. Cryogel beads were pre-
◦
pared by freezing the hydrogel to −5 C and water was removed
−3
by sublimation (2 × 10 mbar) until constant weight (1–2 weeks).
For the aerogel beads preparation, a water-to-ethanol solvent
exchange (5 steps EtOH/water 100:0) was carried out prior to
supercritical drying. The resulting chitosan alcogel beads were
3
. Results and discussion
dried by extraction of the solvent with a continuous flow of scCO
2
Following an analogous (and simultaneous) approach to that
−1 ◦
(
2–4 Nl min ) at 40 C and 110–120 bar for 6 h using the supercrit-
recently reported in the literature [26], the first goal was to assess
the formation of chitosan dried gels, and the role of the remnant
NaOH in the observed catalysis. To this end, different catalyst drying
conditions were applied, and chitosans were tested in the model
reaction (Scheme 1). As a first step the catalytic activity at room
temperature of wet chitosan hydrogels being washed with water
ical equipment described in the literature [28]. Finally, the pressure
was released (depressurization) slowly within 45 min at constant
◦
temperature (40 C) until atmospheric pressure was reached. Vol-
ume shrinkage of the gel of 55% was obtained after supercritical
drying.

1
82
H. Kayser et al. / Applied Catalysis A: General 445–446 (2012) 180–186
Table 1
Influence of entrapped NaOH in aldol addition 1 and condensation 2 of furfural and acetone using chitosan hydrogels as catalysts, which were just washed with water until
◦
neutrality (pH 7 of washed water). Conditions: 2 h, 25 C, 400 mg chitosan hydrogel beads (wet weight, ca. 90% H2O); 1 mmol furfural; 5 mmol acetone; 3 mL ethanol, magnetic
1
stirring. Blank reaction: reaction without addition of chitosan hydrogel beads, only NaOH. Conversion determined by H-NMR.
Cycle
NaOH added (mol%)
Selectivity to 1 (%)
Selectivity to 2 (%)
Conversion (%)
1
2
3
4
5
6
7
8
–
–
–
1
1
1
–
–
1
3
0
0
0
0
0
0
0
1
97
100
0
100
100
100
<1
100
6
0
100
93
100
<1
0
0
99
Blank
100
until neutrality (pH 7) was measured (Table 1). In the first cycle
full conversion could be reached with an almost exclusive forma-
tion of the condensate 2 (entry 1). Notably, recycling the used beads
showed a significant decrease of the activity (entry 2), leading to no
activity at all in the third cycle (entry 3). Assuming that entrapped
NaOH from the sol–gel-formation step could explain this behav-
ior, in the following three cycles external NaOH was added (entries
conditions applied (solvent-free, in (sea)water, etc.). Interestingly,
at room temperature such chitosan cryogels were actually active
in the aldol condensation of other different activated aromatic
aldehydes with cyclic ketones, albeit at longer reaction times
(up to 8 days) (Table 2). Diastereoselectivities were in the same
range as those reported in literature, where moderate-to-high
enantiomeric excesses (70–90%) were also reported for some of
these compounds [26].
4
–6). By adding base full conversion could be reached. By stopping
again the NaOH external addition (entries 7 and 8) no activity was
observed again. Thus, consistent with recently disclosed literature
Remarkably, the condensation between furfural and acetone
proceeded quite well at higher temperatures (when conducted
at >90 C). Both aldol product 1 and condensate 2 (Scheme 1)
◦
[
26], chitosan hydrogels are poor organocatalysts for catalyzing
C
C aldol reactions at room temperature.
After observing these results, dried chitosan gels, instead of
were efficiently produced, reaching up to ∼50–60% conversion in
6 h under these still non-optimized conditions (data not shown).
Likewise, condensation of furfural and cyclopentanone or cyclohex-
hydrogels, were assessed. To this end, chitosan hydrogel beads
were formed under less basic conditions (0.1 M NaOH) than those
reported in literature [27,29], and thoroughly washed until the
washing water was at pH 5 (identical to pH before washing). Beads
were then stored in water for several days until no pH variation
of water after contacting the beads was observed. Subsequently,
dried gel beads were obtained by using different drying methods
such as evaporation at different temperatures from different sol-
vents (xerogel), lyophilization (cryogel), or supercritical drying (e.g.
◦
anone proceeded at 80 C as well (without observed conversions
at r.t., see Table 2). Therefore, to our knowledge this is the first
evidence of an efficient actual chitosan-catalyzed aldol reaction of
furfural and acetone.
Subsequently, a more systematic study on the parameters that
may affect the reaction outcome was performed. Different reactor
set-ups were assessed: thermoshaker vessel (with ∼2 bar pres-
sure), microwave 300 W (∼2 bar pressure) and open flask refluxing
system (Table 3). Traces of self-acetone condensation and bis-
furfural acetone product were observed as well (determined by
GC–MS, <1%). Moreover, several control (blank) experiments were
set under these (still mild) reaction conditions, showing that no
reaction was proceeding in the absence of chitosan along these
reaction times (Table 3).
scCO ) (aerogel) [30]. The employed gel drying method will dramat-
2
ically influence the surface area and porous structure of the dried
material, where even the complete collapse of the chitosan sec-
ondary structure may take place. The simple solvent evaporation
to dry the hydrogel leads to a high shrinkage of the beads [27,29]. In
this paper, two approaches were followed. In one of them, chitosan
cryogel beads were obtained from water-containing hydrogels by
removing the water by lyophilization (−5 C; 10 mbar) (Fig. 1).
As a second strategy, the use of scCO2 to form aerogels was set-
up. Literature-known approaches to get aerogels always require a
solvent exchange prior to the supercritical drying step [27,29]. In
As observed (Table 3), the reaction needed to be conducted in
◦
−3
◦
a closed system – presumably to fully reach 90 C in the reaction
mixture – and thus in open refluxing systems almost no conver-
sions were observed. Thermoshaker closed vessels and microwave
reactors led to analogous results. Furthermore, it was observed
that adding catalytic amounts of water were crucial for the reac-
tion to occur (Table 4), with an enhancement in the reaction
rate with excess of water. The reaction proceeded also in sea-
water (as envisaged non-edible source of water for biorefineries)
[10], with higher aldol condensation bias, albeit at lower yields
(entry 8).
particular, due to the low affinity of water to scCO , for the super-
2
critical drying of chitosan gels with scCO2 a replacement of the
water contained in the pores of the hydrogel with a suitable liquid
solvent must be conducted, typically by exchanging water by a less
polar solvent. Otherwise water residues form carbonic acid under
these conditions may remain, dissolving again the chitosan beads
(
see Section 2).
As mentioned above, closed reactors were required to perform
the reaction in an overheated solution, thus assuring effectively
catalyzed reactions. Subsequently, the temperature influence was
studied taking microwave reactors as model and reproducible set-
ups (Table 5). With increasing temperature, the condensation 2
is slightly favored, presumably due to faster water elimination at
higher temperatures (as consecutive reactions). Thus, under these
conditions formation of 2 might be mostly driven by dehydra-
tion of 1, and not from mechanistic Mannich-type organocatalytic
approaches [31].
As also confirmed recently in literature [26], well-dried chi-
tosan gels (lyophilized) did not catalyze the condensation of
furfural and acetone at room temperature in none of the reaction
To examine the dependence of the catalytic activity on the
chitosan source, and in particular the molecular weight, different
chitosan powders were screened (Table 6). Entries 3–5 were
Fig. 1. Production of dried and neutral chitosan aerogel beads.

H. Kayser et al. / Applied Catalysis A: General 445–446 (2012) 180–186
183
Table 2
Study of the chitosan-catalyzed aldol reaction between different aldehydes and ketones performed at room temperature. General conditions: 1 mmol aldehyde/aldol-acceptor;
1
2
0 mmol ketone/aldol-donor; 1.5 mL H2O; 20 mg chitosan dried gel beads (cryogels). Diastereoselectivity determined via H-NMR.
Aldehyde
Ketone
Yield
syn/anti
3
5
8
1
38:62
32:68
6
5
7
9
24:76
17:83
7
–
2
44:56
–
samples with a defined molecular weight range. Higher activities
were observed with chitosan with less than 100,000 connected
units. Presumably, the backbone allows a higher accessibility of the
amine functions. Remarkably, prepared catalyst beads, cryogels or
aerogels (entries 6–8) displayed a significantly higher conversion
in the testing reaction, compared to the original raw powder from
shrimps (entry 2). Especially, chitosan aerogels dried with scCO2
scCO2 (entries 7 and 8) showed a well-structured porous structure
3
(Vp = 0.86 cm /g; dp = 4 nm) with a high BET-specific surface area of
2
−1
ABET = 250–280 m g , susceptible of providing a chitosan-based
catalyst with more accessible surface amino functional groups
[27,33]. The excellent textural properties of the chitosan aerogels
may explain the full reaction conversion obtained under the testing
conditions, as well as the enhancement of selectivity towards 2,
presumably due to more accessible base amino groups, that might
trigger the consecutive reaction (alcohol dehydration). Therefore,
the best derivatives may result in a combination of the chitosan
size, together with the enhanced textural properties created by
the aerogel formation.
(
4
entries 7 and 8), reached virtually full reaction conversion (>95%,
h). The reason for this dissimilar behavior may stem from the low
surface areas of chitosan gels dried under evaporative drying and
2
−1
lyophilization (lower than 5 m g ), which renders diffusional
limitations to catalyze the reaction. In contrast, beads dried in
Table 3
◦
Chitosan-catalyzed reaction of furfural and acetone in different reaction set-ups (together with blank experiments without chitosan). Conditions: 90 C, furfural:acetone
(
<
1:20), 12 h reaction time; 20 mg chitosan dried gel beads (cryogels); 10 mg H2O. Traces of self-acetone condensation and bis-furfural acetone product were found (GC–MS,
1%). Conversion determined by 1H-NMR.
Reactor
Selectivity to 1 (%)
Selectivity to 2 (%)
Conversion (%)
Thermoshaker, closed vessel (∼2 bar)
Microwave 300 W (∼2 bar)
52
49
42
0
<1
0
48
51
58
0
<1
0
15
24
<1
0
<1
0
Open flask, refluxing
◦
Thermoshaker, blank, 60 h, 100
C
◦
Microwave, blank, 4 h, 150
C
◦
Reflux, blank, 12 h, 95
Reflux, blank, 95 h, 95
C
C
◦
a
n.d
n.d.
5
a
Not determined.

1
84
H. Kayser et al. / Applied Catalysis A: General 445–446 (2012) 180–186
Table 4
◦
Influence of water in the chitosan-catalyzed C C bond formation between furfural and acetone. 12 h, 90 C, 20 mg chitosan dried gel beads (cryogel); 1 mmol furfural;
2
0 mmol acetone.
Entry
Water added (mg)
Selectivity to 1 (%)
Selectivity to 2 (%)
Conversion (%)
a
1
2
3
4
5
6
7
8
0
10
100
1000
1000
0
–
52
59
57
–
–
49
19
–
48
48
43
–
–
51
81
<1
15
16
17
<1
<1
24
11
a
a
a
a,d
b
b
10
1500
a,c
a
5
1
mL vial heated in thermoshaker at 600 rpm.
0 mL microwave reactor 300 W.
b
c
Seawater from Mediterranean sea.
d
Blank experiment without chitosan. Conversion determined by 1H-NMR.
Table 5
Influence of the temperature in the chitosan-catalyzed reaction between furfural and acetone. 10 mL microwave reactor 300 W; 12 h; 20 mg chitosan dried gels (cryogels);
1
0 mg H2O; 1 mmol furfural; 20 mmol acetone.
◦
Entry
Temperature ( C)
Selectivity to 1 (%)
Selectivity to 2 (%)
Conversion (%)
1
2
3
4
5
90
100
120
130
150
150
49
45
39
41
30
–
51
55
61
59
70
–
24
34
30
36
42
<1
a
6
a
Blank experiment without chitosan. Conversion determined by 1H-NMR.
Table 6
Probing different sources and different preparation methods of the chitosan catalyst in the crossed-aldol reaction in furfural and acetone at 150 C, 4 h; 1 mmol furfural;
0 mmol acetone; 100 mg chitosan dried gel beads; 10 mg H2O; 10 mL microwave reactor 300 W.
◦
2
Entry
Chitosan source
Selectivity to 1 (%)
Selectivity to 2 (%)
Conversion (%)
a
1
2
3
4
5
6
7
8
Low MW
Shrimp
MW 60–120 K
MW 110–115 K
MW 140–220 K
Lyophilized beads (25 g L
scCO2 dried beads (12.5 g L
scCO2 dried beads (25 g L
Blank experiment
38
42
40
33
43
32
7
62
58
60
67
57
68
93
85
–
49
67
47
48
27
84
97
96
<1
a
a
a
a
b
−1
)
c
−1
)
c
−1
)
15
–
9
a
Commercially available powder (Aldrich); MW: molecular weight distribution is given as supplied by the manufacturer.
Lyophilized beads from commercial shrimp powder. 25 g L is the chitosan concentration in the gelling process. For details see Section 2.
scCO2 dried beads from commercial shrimp powder. Given is the chitosan concentration in the gelling process. For drying process details see Section 2. Conversion
b
c
−1
determined by 1H-NMR.
Subsequently, the stability and recycling of chitosan dried gel
beads was assessed by setting repetitive batch experiments in the
two closed reactors, thermoshaker and microwave reactors (Fig. 2).
As observed, within the first cycles the yield increased reaching a
maximum in the third cycle. This might be an effect of the absorp-
tion behavior of the beads, and eventually some swelling of them
(
supported by the fact that the yield in the second cycle may
be the cumulative value of the first and second cycle. See below
for further studies on adsorption–desorption). Under these non-
optimized conditions dried gel beads of chitosan showed activity
after 10 reuses.
In order to understand the catalytic activity and possible degra-
dation mechanisms for the observed behavior in the performed
recycling experiments, ATR-IR measurements of the beads before
and after the reaction were recorded (Fig. 3). The spectra of the
used catalyst (after washing with acetone and drying) shows clearly
−1
−1
−1
−1
Fig. 2. Recycling of chitosan cryogel and aerogel beads in furfural and acetone
peaks at 756 cm , 883 cm , 933 cm and 1018 cm which are
related to vibrations from the furfural ring structure [32]. Further-
crossed-aldol reaction. 1 mmol furfural; 20 mmol acetone; 100 mg chitosan dried
◦
gel beads; 10 mg H2O; closed 5 mL Thermoshaker reactions: 90 C, 12 h, vial heated
−1
more, the amine band at 2889 cm decreases along the reaction
in thermoshaker at 600 rpm; Microwave reactions: 150 ◦C, 4 h; 10 mL microwave
−
1
cycles as well as a shouldered characteristic band [34] at 1645 cm
reactor 300 W.

H. Kayser et al. / Applied Catalysis A: General 445–446 (2012) 180–186
185
◦
Fig. 3. IR-spectra of the dried chitosan gel beads before and after catalyzing the crossed-aldol reaction in furfural and acetone at 150 C, 4 h; 1 mmol furfural; 20 mmol
acetone; 10 mg chitosan dried gel beads (cryogels); 10 mg H2O; 10 mL microwave reactor 300 W.
appears, either belonging the aldehyde moiety (furfural) or its
formed imine (Fig. 3).
proceeding with different mechanistic pathways simultaneously
(Scheme 2).
These results provide evidence for the adsorption of fur-
fural onto the chitosan’s surface, and might explain the higher
activity of the catalyst after the first runs (Fig. 2). Both physi-
sorption and chemi-sorption (forming a Schiff base between the
free amine of the chitosan and the aldehyde moiety of the fur-
fural) might take place. The formation of this species may also be
an intermediate in the catalytic reaction cycle, if an organocat-
alytic “Mannich-type” proceeds [31]. Actually, chitosan might
provide a dual catalytic role, as a base and as an organocatalyst,
Moreover, BET measurements showed that during the reactions
the specific surface area of the chitosan aerogel beads decreased
2
−1
2
−1
from 250 m g
to 66 m g while the average pore radius of
about 100 A˚ stays constant (before and after 10 reaction cycles,
respectively). To further study these changes, adsorption experi-
ments under non-reactive conditions – chitosan in contact with
furfural in isopropanol as solvent – and desorption under reac-
tive conditions (addition of acetone) were conducted (Table 7).
As observed, it appears that during the first cycle virtually all
Scheme 2. Possible reaction pathways for the activation of acetone (Brønsted activation, blue, above); aldol-type activation (brown, middle) and furfural (Mannichtype
activation, red, below) in crossed aldol reaction.

1
86
H. Kayser et al. / Applied Catalysis A: General 445–446 (2012) 180–186
Table 7
[
2] F. Geilen, B. Engendahl, A. Harwardt, W. Marquardt, J. Klankermayer, W. Leitner,
Angewandte Chemie International Edition 49 (2010) 5510–5514.
Adsorption/desorption experiments of chitosan cryogel (100 mg, ca. 40 mol% refer-
ring to glucosamine units). 4 h; 150 C; 10 mL microwave reactor 300 W. Catalyst
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◦
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3
g i-PrOH, 10 mg H2O. Desorption, reactive: 20 mmol acetone added to washed
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furfural; 13% aldol product 1; 63% condensate 2.
[
5] A.A. Rosatela, S.P. Simeonov, R.F.M. Frade, C.A.M. Afonso, Green Chemistry 13
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(
Experiment
Acetone
Furfural
[mmol]
De-/adsorbed furfural (incl.
converted products)
[6] M.E. Zakrzewska, E. Bogel-Lukasik, R. Bogel-Lukasik, Chemical Reviews 111
(2011) 397–417.
[mmol]
[
7] P. Domínguez de María, T. vom Stein, P.M. Grande, W. Leitner, F. Sibilla,
EP11154705 (2011).
Adsorbtion, non-reactive
Desorption, reactive
–
20
1
–
39%
6%
[8] T. vom Stein, P.M. Grande, H. Kayser, F. Sibilla, W. Leitner, P. Domínguez de
María, Green Chemistry 13 (2011) 1772–1777.
[
9] C. Zetzl, K. Gairola, C. Kirsch, L. Perez-Cantu, I. Smirnova, Chemie Ingenieur
Technik 83 (2011) 1016–1025.
amino groups react with furfural (39% of adsorption using 40 mol%
catalyst, referred to glucosamine units, entry 1). By adding ace-
tone, furfural is partly leached (entry 2), providing an additional
support to the formation of a Schiff base of furfural and chi-
tosan as a catalytic intermediate (also products are observed,
Table 7). Remarkably, the formation of imino-chitosan structures
has been reported in the literature very recently by others as
well [34], providing a further evidence to the results observed
herein.
[
10] P.M. Grande, P. Domínguez de María, Bioresource Technology 104 (2012)
99–802.
7
[11] T. vom Stein, P.M. Grande, F. Sibilla, U. Commandeur, R. Fischer, W. Leitner, P.
Domínguez de María, Green Chemistry 12 (2010) 1844–1849.
[
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(2011) 1592–1594.
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4
[
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446–1450.
1
[
15] J. Julis, W. Leitner, Angewandte Chemie International Edition,
http://dx.doi.org/10.1002/anie.201203669.
[
[
[
16] J. Julis, M. Hölscher, W. Leitner, Green Chemistry 12 (2010) 1634–1639.
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4
. Conclusions
1
158–1161.
[19] S. Shanmuganathan, L. Greiner, P. Domínguez de María, Tetrahedron Letters 51
2010) 6670–6672.
Dried chitosan-gels are suitable non-hazardous catalysts for
(
condensation between furfural and acetone. Catalysts need activa-
[
20] N. Sudheesh, S.K. Sharma, M.D. Khokhar, R.S. Shukla, Journal of Molecular Catal-
ysis A: Chemical 339 (2011) 86–91.
◦
tion by setting the reactions at higher temperatures (>90 C) and in
closed reaction vessels. Under these novel conditions, efficient con-
densation is achieved, with successful catalyst recycling. Key-factor
is the gel drying process (catalyst preparation). Drying under scCO2
leads to aerogels with enhanced surface areas, and with outstand-
ing catalytic activities (full conversion in 4 h). After optimization,
chitosan might be a promising catalyst for biomass-based reactions
in mild and environmentally-friendly conditions.
[21] N. Sudheesh, S.K. Sharma, R.S. Shukla, Journal of Molecular Catalysis A: Chem-
ical 321 (2010) 77–82.
[
22] K. Rajender Reddy, K. Rajgopal, C.U. Maheswari, M. Lakshmi Kantam, New Jour-
nal of Chemistry 30 (2006) 1549–1552.
[23] A. Ricci, L. Bernardi, C. Gioia, S. Vierucci, M. Robitzer, F. Quignard, Chemical
Communications 46 (2010) 6288–6290.
[
24] D.J. Macquarrie, J.J.E. Hardy, Industrial and Engineering Chemistry Research 44
2005) 8499–8520.
[25] R. Valentin, K. Molvinger, F. Quignard, D. Brunel, New Journal of Chemistry 27
2003) 1690–1692.
26] D. Kühbeck, G. Saidulu, K. Rajender Reddy, D. Díaz Díaz, Green Chemistry 14
2012) 378–392.
(
(
[
[
[
[
[
[
[
[
[
Acknowledgements
(
27] F. Quignard, R. Valentin, F. Di Renzo, New Journal of Chemistry 32 (2008)
1300–1310.
This work was performed as part of the Cluster of Excel-
lence “Tailor-Made Fuels from Biomass”, which is funded by the
Excellence Initiative of the German Research Foundation to pro-
mote science and research at German universities. Mrs. Hannelore
Eschmann and Ms. Julia Wurlitzer are acknowledged for support in
analytical measurements (GC).
28] C.A. García-González, J.J. Uy, M. Alnaief, I. Smirnova, Carbohydrate Polymers 88
(
2012) 1378–1386.
29] C.A. García-González, M. Alnaief, I. Smirnova, Carbohydrate Polymers 86 (2011)
1425–1438.
30] C.A. García-González, M.C. Camino-Rey, M. Alnaief, C. Zetzl, I. Smirnova, Journal
of Supercritical Fluids 66 (2012) 297–306.
31] P. Domínguez de María, P. Bracco, L.F. Castelhano, G. Bargeman, ACS Catalysis
1
(2011) 70–75.
32] M. Rogojerov, G. Keresztury, B. Jordanov, Spectrochimica Acta, Part A: Molec-
ular and Biomolecular Spectroscopy 61 (2005) 1661–1670.
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8