Eli Moens organokatalysator voor de aldolcondensatie Een kritische evaluatie van chitosan als ...

102 Pages·2017·5.93 MB·Dutch

by  Eli Moens

Een kritische evaluatie van chitosan als heterogene
organokatalysator voor de aldolcondensatie
Eli Moens
Promotoren: prof. dr. ir. Joris Thybaut, prof. dr. ir. Jeriffa De Clercq
Begeleiders: Anton De Vylder, dr. ir. Jeroen Lauwaert
Masterproef ingediend tot het behalen van de academische graad van
Master of Science in de industriële wetenschappen: chemie
Vakgroep Materialen, Textiel en Chemische Proceskunde
Voorzitter: prof. dr. Paul Kiekens
Faculteit Ingenieurswetenschappen en Architectuur
Academiejaar 2016-2017

FACULTY OF ENGINEERING AND ARCHITECTURE

Laboratory for Chemical Technology
Director: Prof. Dr. Ir. Guy B. Marin

Laboratory for Chemical Technology

Declaration concerning the accessibility of the master thesis

Undersigned,

Eli Moens

Graduated from Ghent University, academic year 2016-2017 and is author of the
master thesis with title:

Een kritische evaluatie van chitosan als heterogene organokatalysator voor de
aldolcondensatie

The author(s) gives (give) permission to make this master dissertation available for
consultation and to copy parts of this master dissertation for personal use.
In the case of any other use, the copyright terms have to be respected, in particular with
regard to the obligation to state expressly the source when quoting results from this master
dissertation.

2/06/2017

Laboratory for Chemical Technology • Technologiepark 914, B-9052 Gent • www.lct.ugent.be
Secretariat : T [phone] • F [phone] • [email protected]
Een kritische evaluatie van chitosan als heterogene
organokatalysator voor de aldolcondensatie
Eli Moens
Promotoren: prof. dr. ir. Joris Thybaut, prof. dr. ir. Jeriffa De Clercq
Begeleiders: Anton De Vylder, dr. ir. Jeroen Lauwaert
Masterproef ingediend tot het behalen van de academische graad van
Master of Science in de industriële wetenschappen: chemie
Vakgroep Materialen, Textiel en Chemische Proceskunde
Voorzitter: prof. dr. Paul Kiekens
Faculteit Ingenieurswetenschappen en Architectuur
Academiejaar 2016-2017
Voorwoord
Bij het naderen van het einde van masterproef wil ik graag iedereen bedanken voor de kansen,
ondersteuning en advies die ik gedurende mijn masterproef heb gekregen. Hiervoor gaat eerst mijn dank
uit naar de organisatie en het personeel van het Laboratory for Chemical Technology die mij de kans
gaven mijn master proef te starten aan het LCT.
Hierbij gaat ook mijn dank naar prof. Joris W. Thybaut voor het opvolgen van mijn thesis en de
tussentijdse evaluatie met de daarbij horende constructieve feedback.
Vervolgens zou ik ook in het bijzonder mijn begeleider Anton De Vylder hartelijk willen bedanken om
voor de begeleiding, opvolgen, ondersteuning en opportuniteiten die mij werden gegeven gedurende
mijn thesis. Mijn dank gaat dan ook naar de open en vriendschappelijke mentaliteit die ik heb ervaren
gedurende mijn thesis waarbij er ruimte was voor redevoering en het beargumenteren van eigen
standpunten.
Verder wil ik ook Jeroen Lauweart bedanken voor zijn expertise betreffende het onderwerp van de
thesis, de feedback bij het naderen van het einde van de thesis en het nalezen van mijn werk.
My gratitude goes also to Brigitte Devocht, Jonas van Belleghem and Luis Lozano Guerra for allowing
me to work on my thesis in their office, for their hospitality and the very pleasant atmosphere to work
in.
Verder wil ik mijn vrienden, waaronder in het bijzondere Koen Van Dael en Simon Maes voor de
veelvuldige koffie- en middagpauzes die we samen hebben doorgebracht, en mede thesisstudenten aan
het LCT bedanken voor de aangename sfeer die ik heb ervaren gedurende mijn masterproef.
Mijn dank gaat ook uit naar mijn familie en vrienden, en in het bijzondere naar mijn ouders voor het
opvolgen van mijn thesis en de kans die ze mij hebben gegeven om deze studies te volgen.
Als slot van mijn dankwoord wil ik graag iedereen, die ik niet vermeld heb, maar mij ook heeft geholpen
en bijgestaan gedurende het maken van mijn thesis bedanken voor de tijd en moeite die zij voor mij
hebben vrij gemaakt.
Abstract
Het doel van dit onderzoek was een kritische evaluatie van chitosan als katalysator in de
aldolcondensatie. Hierbij is er onderzocht of er een verschil in katalytische activiteit op te merken is
tussen chitosanpoeder, -hodrogels en -aerogels gedurende de katalyse in de aldolcondensatie. Er is geen
verschil waargenomen tussen de drie verschillende fysische vormen van chitosan op vlak van de
waargenomen intrinsieke katalytische activiteit. Het gunstig effect van de aanwezigheid van water in
het reactiemedium is waargenomen en aan de hand van de verkregen resultaten was het ook mogelijk
een reactiemechanisme uit te werken voor de aldolcondensatie van 4-nitrobenzaldehyde en aceton op
chitosan waarbij de inhibitie door iminevorming van de primaire amines in kaart is gebracht.
Sleutelwoorden: aldolcondensatie, chitosan, organokatalysator
A Critical Assessment of Chitosan as a
Heterogeneous Organocatalyst for the Aldol
Condensation
Eli Moens
Supervisors: prof. dr. ir. Jeriffa De Clercq, prof. dr. ir. Joris Thybaut
Mentors: dr. ir. Jeroen Lauweart, ir. Anton De Vylder
 of the desired products complex, energy intensive and waste
Abstract: In this work, chitosan powder, hydrogels and aerogels stream producing processes are required. Therefore, the use of
were tested for the aldol condensation reaction of acetone with 4- a heterogeneous catalyst should be, in the context of green
nitrobenzaldehyde, and their catalytic activity defined. No chemistry, more appropriate [8-10]. Hence, during this study,
difference has been observed for the three physical forms of the interest went to the potential of chitosan as a heterogeneous
chitosan in terms of catalytic activity. Based on the phenomena organocatalyst in de aldol condensation. Some of the main
observed during the experiments a modified reaction mechanism
advantages of chitosan as a green catalyst is his
of the aldol condensation of 4-nitrobenzaldehyde and acetone on
biodegradability, non-toxicity and relative low cost due to the
chitosan has been defined, that takes into account the inhibition of
fact that it is easily synthesized out of chitin [2, 11-17].
the primary amines with formation of imines.

Keywords: aldol condensation, chitosan, organocatalyst
I. INTRODUCTION II. REACTION MECHANISM OF THE ALDOL CONDENSATION
ON CHITOSAN
Chitosan is ones of the most applied derivatives of chitin, the
Based on the investigation of the silanol-assisted aldol
second most abundant biopolymer after cellulose [1, 2]. Chitin
condensation on aminated silica [9] and following studies [9,
is the main component of the shells of crustaceans and is
18, 19] it was possible to elaborate a reaction mechanism
commercially exploited from of crab- and shrimp shells, which
(Scheme 2) for the aldol condensation of 4-nitrobenzaldehyde
are a waste stream in the food industry. Chitin is made out of
and acetone on chitosan. It should be noted that there is a
β-(1-4)-N-acetyl-D-glucosamine units which can be
difference in acidity between the hydroxyl groups on chitosan
deacetylated by treating the chitin with a basic solution at a
and a silanol group, which has a higher pK -value than a
elevated temperature (Scheme 1). When the degree of a
hydroxyl group bounded onto a carbon backbone structure.
deacetylation (DDA), which is the amount of D-glucosamine

units versus the total amount of structural units, becomes more
than 50% the D-glucosamine unit are the main structural unit
of the polymer chain and the resulting polymer chain is called
chitosan. Commercially available chitosan generally has a
DDA of 75% [1-7].

Scheme 1. Deacetylation of chitin with the formation of chitosan
The aldol condensation is one of the most essential carbon-

carbon coupling reactions and plays an important role in the
Scheme 2. Proposed reaction mechanism for the cooperative catalysis
pharmaceutical, fine and bulk chemistry [8]. Here, the reaction
aldol condensation of 4-nitrobenzaldehyde with acetone by chitosan
is often catalyzed by a strong homogeneous base such as NaOH
or KOH [9]. Due to the fact that the catalyst is homogeneously As shown in Scheme 2 the reaction starts by the formation of
dissolved in the reaction mixture means that for the separation a hydrogen bridge with an acetone molecule (1) followed by a

E-mail: [email protected]
nucleophilic addition of the acetone onto the primary amine Hereby can also be said that the overall chemical composition
with formation of an alcohol (2). After dehydration of the of chitosan has not been altered during the synthesis of the
alcohol (3) and with the formation hydrogen bridge between a hydrogels and the aerogels. The slightly higher value for the
4-nitrobenzaldehyde molecule and a near hydroxyl group of the DDA of hydrogels was possible caused by CO and N present
2 2
chitosan backbone structure (4) an iminium-ion van be formed in the demineralized water in the hydrogels.
(5). Through hydration of the iminium-ion is transformed into
Table 1. DDA of chitosan powder, hydrogel and aerogel determined
a diol (6). By a proton transfer of the nearest hydroxyl group
by elemental-analysis
near the amine and by an electron shift the aldol product is
formed. Hereby the primary amine becomes available for Chitosan DDA Error
further catalysis of the reaction (7). To define the catalytic
activity of chitosan, chitosan powder, hydrogel and aerogel Powder 70,44 % 4,98 %
were tested. Previous studies [15-17, 19-21] have shown the
Hydrogel 77,94 % 4,71 %
beneficial effect of the presence of water in the aldol
condensation, and in the proposed reaction mechanism, it has Aerogel 72,01 % 3,55 %
recently become clear that water is needed for the regeneration

of the catalyst site. With this in mind, the influence of the
The macro porous structure of the aerogels has been visually
solvent on the aldol condensation on chitosan was also tested
confirmed through SEM-analysis. A SEM-image was taken of
by performing the reaction in different solvents.
a chitosan aerogel grain and one of the cross-section of the
grain (Figure 1). In both images, the presence of the macro
III. EXPERIMENTAL porous structure is clearly present.

A. Catalyst synthesis
Due to the fact that chitosan hydrogels and aerogels are not
commercially available, they had to be synthesized prior to the
experiments on catalytic activity. Based on information from
literature [13, 15, 21-23], and own research, the following
synthesis was developed. Five grams of low molecular weight
chitosan powder was homogeneous dissolved in 200 ml acetic-
acid solution (50mM). The resulting solution was then
dropwise added into a NaOH-solution (4 M) with a syringe and
needle (ø 0,08 mm). After 10 hours of hardening, the formed
Figure 1. SEM-image of a chitosan aerogel grain (a) and a close-up of
hydrogels were filtrated and washed with water. For the
the cross-section of the aerogel grain(b)
removal of the NaOH, entrapped inside the hydrogel, the
hydrogels were placed in double distilled water and gently
stirred at 60 rpm for 4 hours. This washing step has been C. Reaction conditions
repeated until the pH-value of the washing water was neutral. The experiments performed during this research were all
This washing step was then repeated an additional three times conducted under the same reaction conditions. The aldol
to assure no NaOH remained in the pores. The chitosan aerogel condensation reaction was carried out at a temperature of 55 °C
form has been obtained by freeze-drying the synthesized and the composition of the reaction mixture is displayed in the
hydrogels. Hereby water sublimates during the drying process following table (Table 2). It is important to note that the mass
while the macro porous structure of the initial hydrogel is kept ratio of 4-nitrobenzaldehyde to acetone is 1:100.
intact.

Table 2. Composition of the reaction mixture at start of the reaction
B. Catalyst characterization
Component Massfraction
For further investigation on the catalytic activity of the three
4-Nitrobenzaldehyde 0,45%
forms of chitosan the DDA of the used chitosan had to be
Acetone 44,69%
known. This is a necessity to be able to determine the effective
amount of active site which were initially present in the Methyl-4-nitrobenzoate 0,25%
reaction. For the determination of the DDA, elemental-analysis Solvent Water
has been applied. With the resulting nitrogen and carbon mass-
DMSO 54,62%
fractions, it was possible to calculate the DDA via equation 1.
Herein the assumption has been made that the chitosan polymer n-Hexane
chain only consist out of the acetyl-glucosamine and Chitosan 0,25 g
glucosamine units.

w% C - 6. w% N D. Experimental setup
DDA = (1-MMC MMN).100% (1)
2.m% N For the examination of the influence of the solvent on the
MMN
aldol condensation on chitosan the experiments were carried
Based on the results of the elemental-analysis, the out in a closed glass batch reactor setup showed in Figure 2.
assumption can be made that there is no difference between the The temperature was controlled by heating the oil bath to the
DDA of chitosan powder, hydrogel and aerogel (Table 1). required temperature.
B. Catalytic activity of chitosan

Before the catalytic activity of chitosan could be determined,
the progression of the reaction had to be evaluated to see if the
reaction circumstances were conform the conditions needed for
defining the catalytic activity. Therefore the decrease of 4-
nitrobenzaldehyde, increase in product and the difference
between both is plotted in Figure 5.
1,2
)
lo
m 1
m
(
Figure 2. Closed glass batch reactor for the investigation on the n 0,8
influence of the solvent on the aldol condensation
0,6
For the investigation on the catalytic activity of chitosan, a
more appropriate reactor was required. Therefore a Parr® batch 0,4
reactor (Figure 3) with a 300 ml reactor vessel was used. The
0,2
reaction temperature was kept constant during the reaction via
a thermocouple inside the reactor vessel an adjusted, if
0
necessary, by the heating and cooling mantle around the vessel. 0 60 120 180 t (min) 240
A stirrer inside the vessel kept the reaction mixture
homogeneous during the reaction and was set on 220 rpm. Increase in product Decrease of 4-nitrobenzaldehyde
Loss of 4-nitrobenzaldehyde
Figure 5. Evolution of the increase in product, the decrease of 4-
nitrobenzaldehyde and loss of 4-nitrobenzaldehyde during the reaction
Despite the fact that the reactions were conducted in a closed
reactor setup, there has been an observed loss of mass which
could not be attributed to the formation of the aldol product.
This loss reached a stable value after two hours. An analogous
trend could be noticed while looking at the evolution of the
formed product where the increase in amount of product was
the highest during the first two hours of the reaction. In
analogous research with chitosan as catalyst for the aldol
Figure 3. Parr batch reactor used for the catalytic experiments
condensation of furfural [20], binding of furfural onto chitosan
Analysis of the samples, taken during the reaction, were
with the formation of an imine was observed. Based on this
performed immediately after the sample was taken and
information, an FT-IR-spectrum was taken of chitosan powder
performed via HPLC-analysis.
before and after the reaction (Figure 6). In the spectrum of
chitosan after the reaction, the presence of 4-nitrobenzaldehyde
IV. RESULTS AND DISCUSSION becomes clearly visible due to the small new peaks between
700 cm-1 and 900 cm-1, which indicates the presence of C -H-
sp3
A. Influence of the solvent on the aldol condensation vibrations, and a new sharp peak at 1356 cm-1 which indicates
the presence of nitro-groups. The new peak at 1529 cm-1 can be
The experimental results regarding the influence of the
caused by the presence of C=C-stretching in aromatics. At 1605
solvent (Figure 4) confirms the fact that water is necessary for
cm-1 the formation of an extra new peak can be seen, and can
the reaction to occur. Therefore the experiments concerning the
be an indication for the presence of imines.
determination of the catalytic activity of chitosan were
conducted with water as solvent. 35
e
c
n
)% 15 ab 30
(0t( ) rosdA 25
o 10
rtin
4- 20
n
/ tcu 5 15
d
o
nrp 0 10
0 5000 10000 15000
Batchtime(mmol.s) 5
n-Hexane DMSO Acetone Water
0
600 1000 1400 cm-1 1800
Figure 4. Influence of the solvent on the aldol condensation on
Unused catalyst Spent catalyst
chitosan
Figure 6. FT-IR spectrum of chitosan before and after the aldol
condensation reaction of 4nitrobenzaldehyde and acetone
With these results, the assumption has been made that the Table 3. Experimental results for the catalytic activity of chitosan
mass loss of 4-nitrobenzaldehyde is due to imine formation on powder, hydrogel and aerogel
chitosan. Herein the primary amine becomes inhibited which
𝒓𝒂𝒕𝒆
results in a lower catalytic activity of chitosan during the
𝒎𝒎𝒐𝒍 Powder Hydrogel Aerogel
reaction. Due to the occurrence of inhibition, it was not possible [ ]
𝒎𝒎𝒐𝒍.𝒔
to define the intrinsic catalytic activity of the primary amines
present in chitosan. Nevertheless, in the context of the goals of
1 2,08 .10-5 1,86 .10-5 1,98 .10-5
this thesis, a formulation of the catalyst activity has been
proposed to define the catalytic activity in such way that the
resulting value can be used to define a hypothetical value for
2 8,32 .10-5 8,78 .10-5 7,37 .10-5
the intrinsic activity of the catalyst’s active sites. Therefore the
loss of active sites, due to imine-formation, was taken into
account during the reaction so that the amount of product
3 6,25 .10-5 6,30 .10-5 5,57 .10-5
formed, at a given time, per active site present during that time,
could be plotted in function of the reaction time. After applying

this correction onto the experimental results (Figure 7) the
When looking at the first defined catalytic activity for the
difference in slope has been greatly reduced. Yet the slope the
three chitosan catalysts, there is no difference between the three
linear fit in the second two hours of the reaction is still not the
forms, taking into account an experimental error of 10%. When
same as on the first two hours. The remaining decrease in the
looking at the catalytic activity calculated following the second
slope can be caused by the imine-formation out of the alcohol,
and third definition, a decrease of 25% can be noticed between
derived from the acetone binding onto the amine, when it is
the second and third defined catalytic activity for all three the
dehydrated (Scheme 3, 3’). It can also be suggested that the
catalysts. Chitosan powder and hydrogel show an equal activity
inhibition is more favored with a 4-nitrobenzaldehyde involved
and aerogel a lower activity with the second and third definition
than with only acetone bonded on the primary amine.
of reaction rate. However, this can be related to diffusion

limitations in the aerogels that are filled with air. This explains
nsetis evitca1,21 wrceahgtayiml ytesh,te s bwdyeh cicrleeh aittshoees aionnv esarleaorlpol egc,ae tlba elitysw tietche aenc tstihavemit yetr iaasn sls oietwhneet r.ao nTthdhe urss t,at bwthloee
n/ tcudorp0,8 caoctnicvliutys iboent wcaene nb ec hmitaodsea nt hpaot wthdeerre, hisy dnoro dgieflf earnedn caee riong ceal tianl ytthiec
0,6 aldol condensation of 4-nitrobenzaldehyde and acetone.

0,4
Additional experiments were conducted to verify the
reusability of chitosan hydrogels and aerogels and there results
0,2
are shown in Table 4. During the second run of the aerogel and
0 hydrogel catalyst, no transient regime was observed.
0 60 120 180 t (min) 240 Additionally, the catalyst shows an activity which is equivalent
to activity measured in during the stable regime of the first run.
Without correction (0h - 2h) With correction (0h-2h)
This confirms the hypothesis that there is an equilibrium
Without correction (2h - 4h) With correction (2h - 4h)
presence between the formation of imines and the formation of
the intermediate which reacts further with formation of the
Figure 7. Catalytic activity of chitosan during the reaction
aldol product.
For the study on the catalytic activity of chitosan and the
Table 4. Experimental determined catalytic activity of chitosan
difference between chitosan powder, hydrogel and aerogel,
aerogels with respect to the initial amount of active sites
three different values were calculated (Table 3) which have
been defined as followed;
𝒓𝒂𝒕𝒆

𝒎𝒎𝒐𝒍 1 1 1
1. The amount of product formed, per time unit, [ ] R1 R2 (0h-2h) R2 (2h – 4h)
𝒎𝒎𝒐𝒍.𝒔
divided by the initial amount of catalytic active sites
present in the reaction mixture during the stable
Hydrogel 1,86 .10-5 1,68 .10-5 1,64 .10-5
regime of the reaction (2h – 4h)

2. The amount of product formed, per time unit,
Aerogel 1,98 .10-5 1,66 .10-5 1,76 .10-5
divided by the effective amount of catalytic active
sites, per time unit, present in the reaction mixture
during the transient regime of the reaction (0h – 2h).

3. The amount of product formed, per time unit,
divided by the effective amount of catalytic active
sites, per time unit, present in the reaction mixture
during the stable regime of the reaction (2h – 4h).