Microwave-Assisted Polycondensation Reactions of Anhydrides with

Microwave-Assisted Polycondensation Reactions of Anhydrides with
Faculty of Applied Sciences
Microwave-Assisted Polycondensation Reactions
of Anhydrides with Glycols
Master Thesis Project: P&E-2396
Name: Martijn Barmen ’t Loo
Student number: 1386611
Program: Chemical Engineering
Supervising Tutor: Prof.dr.ir. A.I.Stankiewicz
2nd reviewer: Dr.ir. G.Stefanidis
3rd reviewer: Prof.dr.ir. F.Kapteijn
Keywords: microwaves; polycondensation; anhydride; glycol; triflate catalyst;
Start date: 1-9-2010
End date: 6-7-2011
Acknowledgements
It is particularly pleasing to have the opportunity to acknowledge the contributions of Magdalena
Komorowska-Durka. It was pleasant having you as my daily supervisor. I really appreciated that you
were always available for discussion and thanks for all your good advices. I had fun working with you.
I want to thank Georgios Stefanidis for all the nice conversations about the microwave research. Your
enthusiasm about this subject was really encouraging and your opinion about the results was very
interesting and helpful.
Last but not least I want to thank my father Jaap Barmen ‘t Loo and Sarah Brassington for spending
time reading and correcting my report and of course my mother Jeanny Barmen ‘t Loo and my
girlfriend Nienke Seiger who always support me during my study.
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Summary
This graduation thesis describes the research focused on application of microwave irradiation to
synthesis of unsaturated polyesters. The idea of using microwaves for process intensification is that
the direct heating of the reactants with microwaves can lead to fast and homogeneous heating to
reduces the energy consumption and give each molecule the same processing experience.
Two poly-esterification reaction systems were studied: first was maleic- and phthalic anhydrides with
propylene glycol, and second maleic anhydride with hexamethylene glycol. Experiments were
conducted in different microwave applicators, as CEM Discover, CEM MARS and Sairem INTL, as well
as with conventional heating (oil bath and with an electric heating mantle).
The hypothesis was that microwaves can accelerate reaction rates, increase conversion, improve the
end product quality and quantity and save energy. Conversion, reaction time, molecular weight and
polymer structure, and the quantity of by-products (i.e. water) obtained from different microwave
applicators were compared with conventional heating. Comparison of power consumption of
microwave reactor systems with two different types of applicators, multimode cavity and internal
transmission line technology, to conventionally heated reactor was made and presented.
Different reaction vessel sizes (1 and 2 liters) were used to investigate whether this can improve the
removal of the by-product. The reaction was performed under nitrogen atmosphere or under
vacuum at the time when water is produced in order to increase water removal. Different catalysts
were tested to discover which catalyst has the best influence on the reaction, and to see, specifically,
the difference in performance between microwave heating and conventional heating. Forced cooling
of the reactor vessel was used so higher power levels could be applied without increasing the bulk
temperature in the reactor. Silicon carbide cylinders were used to increase locally the temperature
and to use the mixture as a heat sink.
The conclusion of this research is that the reaction kinetics of the poly-condensation reaction of
maleic and phthalic anhydrides with propylene glycol is not influenced by microwave irradiation.
Compared to conventional heating there were no significant differences in conversion, reaction time,
molecular weight and amount of by-product obtained. Therefore no reduction in energy
consumption was achieved. However based on quantitative analysis of distillate more light organic
components were found in the distillate by microwave irradiated reactions.
With the use of microwaves the temperature control can be more advantageous compared to
conventional heating. This can prevent overheating and thus thermal degradation of the end
product.
The catalysts that were used didn’t show better results, but the catalyst that give the best results for
the reaction of maleic anhydride with propylene glycol was p-toluenesulfonic acid. The triflate
catalysts that were used for this reaction resulted in degradation of the polymer. However for the
reaction of maleic anhydride with hexamethylene glycol the lanthanum triflate catalyst improved the
reaction system.
Analyzes of the polymer structure showed that with microwave irradiation of the reaction of maleic
anhydride with propylene glycol the maleate structured polymer is in favor. However further
research is needed to verify this conclusion.
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Table of Contents
Acknowledgements ................................................................................................................................. 2
Summary ................................................................................................................................................. 3
1.
Introduction ..................................................................................................................................... 6
2.
Theory.............................................................................................................................................. 8
2.1
3.
2.1.1
How Microwaves can Speed up Reaction Rates.............................................................. 9
2.1.2
Dielectric Properties ...................................................................................................... 10
2.2
Polymerization Reactions under Microwave Conditions ...................................................... 11
2.3
Poly-esterification Reaction of Maleic and Phthalic Anhydride with Propylene Glycol ........ 14
2.4
Poly-esterification Reaction of Maleic Anhydride with 1,6-Hexanediol or 1,4-Butanediol .. 15
2.5
Different Microwave Applicators .......................................................................................... 15
Experimental Section..................................................................................................................... 17
3.1
Materials................................................................................................................................ 17
3.2
Experiment ............................................................................................................................ 17
3.2.1
Microwave Discover ...................................................................................................... 17
3.2.2
Microwave MARS .......................................................................................................... 18
3.2.3
Microwave INTL ............................................................................................................. 19
3.3
4.
Analytical Measurements ...................................................................................................... 19
Results and Discussion .................................................................................................................. 21
4.1
Mono-mode Microwave Applicator ...................................................................................... 21
4.1.1
Different Catalyst........................................................................................................... 21
4.1.2
Vacuum .......................................................................................................................... 26
4.2
5.
The Fundamentals of Microwaves .......................................................................................... 8
Large Scale Microwave Applicators ....................................................................................... 29
4.2.1
Results of the Products and Distillate Obtained ........................................................... 29
4.2.2
Power Consumption ...................................................................................................... 33
Conclusion and Recommendations ............................................................................................... 37
References ............................................................................................................................................. 38
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APPENDIX .............................................................................................................................................. 40
Appendix I Reaction Mechanism and Equations of the Poly-esterification Reaction of Maleic and
Phthalic Anhydride with Propylene Glycol ........................................................................................ 41
Appendix II Tables of all Executed Experiments with Corresponding Reaction Conditions ............. 45
Appendix III 13C NMR Spectra ............................................................................................................ 46
Appendix IV GC-MS Spectra .............................................................................................................. 47
Appendix V GC Spectra and Extrapolations ...................................................................................... 48
Appendix VI Energy Balance Calculations ......................................................................................... 49
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1. Introduction
The demand for more efficient and more environmentally friendly methods of performing polyesterification reactions is growing. Poly-esterification is one of the major processes carried out in the
chemical industry and the products are used for producing cast items, for structural applications and
coatings. Fundamental knowledge about the poly-esterification reaction mechanisms is well
documented [1],[2]. It is known that poly-esterification is an equilibrium reaction and produces water
as a by-product. In order to shift the equilibrium, continuous removal of water will result in higher
yields. A disadvantage is that poly-esterification reactions have long reaction times and are
sometimes thermally sensitive. One solution to these problems can be to use microwave irradiation.
Microwaves are electromagnetic waves with frequencies between 0.3 GHz and 300 GHz. Polar
molecules are stimulated to move by the change of electric field applied by the microwaves. The
generated kinetic energy of the molecules is converted into heat. The idea of using microwaves for
process intensification is that the direct heating of the reactants with microwaves can lead to fast
and homogeneous heating to reduces the energy consumption and give each molecule the same
processing experience.
Recent research indicates that microwave synthesis performs reactions faster, delivers higher yields,
is more energy efficient, and produces cleaner products [3]. The application of microwave energy
began in 1960, and was used for the vulcanization of rubber, product drying and solvent extraction
applications. The use of microwaves to carry out organic synthesis started in 1980 and it can be seen
exponential growth since then. A considerable number of the papers that have been published about
the application of microwave irradiation are on polymerization reactions [4],[5],[6].
The fundamentals of polymerization with the use of microwave irradiation is described in the book
by Bagdal and Prociak [3]. Microwave equipment is described and methods by which temperature
can be monitored and controlled. Different examples of polymer reactions are also discussed. In the
paper by Komorowska et. al. [7] the temperature measurements techniques are compared and the
similarities between glass set-up in microwaves and conventional heating are explored. The
properties of polyester resins were examined by Legros et al. [8],[9]. The dielectric properties vary at
different frequencies and temperature. Variations in vessel size, the volume of heated materials and
power levels have been examined by Panzarella et al. [10].
However little literature was found on the reactions between maleic anhydride with propylene glycol
carried out under different microwave conditions [11]. This graduation thesis will therefore describe
the poly-esterification reactions of maleic and phthalic anhydrides with propylene glycol under
microwave irradiation to produce unsaturated polyesters. The main aim is to accelerate reaction
rates, to increase conversion, to improve the end product quality, to obtain linear maleate structured
polymer chains, and to save energy. Conversion, reaction time, molecular weight and polymer
structure, the quantity of by-products (i.e. water) and the energy consumption of different
microwave applicators were compared with conventional heating.
Different reaction vessel sizes were used to investigate whether this can improve the removal of the
by-product. The reaction was performed with a lower pressure at the time water is produced in
order to increase water removal [12]. Different catalysts were tested to discover which catalyst has
the best influence on the reaction, and to see, specifically, the difference in performance between
microwave heating and conventional heating [2],[13]. Forced cooling of the reactor vessel was used
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so higher power levels could be applied without increasing the bulk temperature in the reactor [14].
Silicon carbide cylinders were used to increase locally the temperature and to use the mixture as a
heat sink.
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2. Theory
2.1
The Fundamentals of Microwaves
Microwaves are electromagnetic energy
in the lower frequency range, between
0.3 GHz to 300 GHz (Fig. 1).
Electromagnetic waves consist of a
magnetic and an electric field (Fig. 2).
Only the electric field influences the
molecules. By rapidly changing the
electric field with microwaves, the
molecules try to align themselves which
results in the rotation of the molecules.
The coupling ability of a molecule is
related to its dielectric properties (this is
discussed in more detail in chapter 2.1.2).
This kinetic energy is transferred into heat by:
Figure 1 The electromagnetic spectrum
(2.1)
Where is the molecule weight,
the temperature.
the velocity of the molecule,
the Boltzmann constant and
Different materials possess different
properties
which
influence
the
interaction with microwaves. The type of
materials can be classed in three main
categories: materials that reflect
microwaves (electrical conductors: e.g.
metals, graphite), materials that let
microwaves penetrate through without
absorption, in the case of good insulators
(e.g. quartz glass, porcelain, ceramics)
and materials that can absorb
microwaves (i.e. polar materials) [3].
Materials that absorb microwaves do
this by means of a dielectric mechanism.
There are two main dielectric mechanisms which explain: how molecules respond to microwaves and
how they convert microwave energy into heat. One is achieved by ionic polarization. This applies to
systems with free ions or ionic species. The ions are electrically charged and move when an electric
field is applied. If the electric field is changing rapidly the ions orientate, this is what causes the
instantaneous superheating. At higher frequencies the ionic movement is increased and so the
temperature can rise. At some point the frequency becomes so high, that the electric field changes
too fast for the ions to align with the field due to ion inertia. The second mechanism involves dipole
rotation whereby molecules try to align themselves in the oscillating electric field applied by
Figure 2 An electromagnetic wave
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microwaves (Fig. 3). The movement of
the molecules and the friction
between them convert the absorbed
energy into heat. How effectively the
molecules couple with the changing
electric field is dependent on the
polarity of the molecules [15].
Microwaves do not affect the chemical
structure of a molecule. The typical
range of energy that is needed to
break a molecular bond is 80-120 Figure 3 Dipolerotation
kcal/mole, the energy of a microwave
photon is 0.037 kcal/mole. The effect of microwaves on a heated reaction system is purely kinetic
[16].
The advantage of using microwave techniques is that the microwaves directly couple with the
reactants (polar chemicals) without heating the whole system. This might lead to energy saving.
Moreover, the process does not depend on the thermal conductivity of the vessel material, as in the
case of conventional heating, but it creates instantaneous local hotspots. Therefore microwave
heating offers good reaction control, because when the microwave is turned off, only latent heat
remains.
2.1.1 How Microwaves can Speed up Reaction Rates
Reaction rates are dependent on the probability of collisions when the geometry between molecules
is proper ( ), and the probability of those colliding molecules, that have the minimum amount of
energy required to overcome the activation energy barrier ( ), in order to react. From equation
(2.2) the probability ( ) can be expressed according to the Arrhenius equation:
(
)
(2.2)
The exponent is temperature-dependent whereas the pre-exponential factor is partly dependent on
temperature [17]. An increase in temperature results in an increase in reaction rate. The primary
reason for the acceleration of chemical reactions by microwaves is the high instantaneous rise in the
kinetic energy of a molecule above the normal bulk temperature. It would be expected that so-called
“non-equilibrium local heating” can lead to an increment in reaction rate of 10 – 1000 fold [18].
Creating this “non-equilibrium local heating” can in principle never occur by means of conventional
heating. That is why application of microwave heating to chemical reactions is an object of interest.
A non-polar solvent, which is not affected (heated) by microwaves, can be used as a heat sink to pull
thermal heat away from the bulk. This makes it possible to operate microwaves at higher power
levels, thus increasing reaction rates without the danger of the product being thermally degraded.
The advantage of using microwave processes is that energy transfer is very fast, which in turn leads
to faster reaction rates, higher yields, a cleaner product, and even new reaction pathways.
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There has been some speculation that microwave irradiation could lead to non-thermal effects such
as lowering the activation energy of the reaction and geometrical alignment. It has been suggested
that the increase in the polarity of the reaction system from the ground state towards the transition
state can result in an acceleration of the reaction due to a stronger interaction occurring between
microwaves and the reagents during the reaction. For polar mechanisms, where the transition state
is more stable than the ground state, the activation energy could decrease, thus making the reaction
feasible [3]. Another explanation for the non-thermal effects might be the change in the preexponential factor in the Arrhenius equation. This phenomenon may occur because of changes in the
transport properties, and increases molecular agitation [19].
2.1.2 Dielectric Properties
The coupling with microwaves strongly depends on the polarity of a molecule. The polarity depends
on different parameters such as the dielectric constant, dipole moment, dielectric loss, and tangent
delta.
The dielectric constant, sometimes called relative electric permeability, is the ability to store electric
charges. This is a ratio of the electric permeability of the material to the electric permeability of the
evacuated capacitor. It is expressed by the following equation:
(2.3)
Where is the dielectric constant,
is the capacitance of the capacitor within vacuum and
capacitance of the capacitor of the material.
the
These values are dependent on the temperature of the heated material and frequency of the
microwave field.
The dipole moment is the product of the distance between the centers of charge in the molecules
and the charge by:
(2.4)
Where
is the dipole moment,
the charge and the distance between charges.
Molecules with a large dipole moment have large dielectric constants. This is dependent on the
dipole rotation (the ability of a molecule to align with a changing electric field).
The dielectric loss ( ) is the ability to transfer the microwaves absorbed into heat in the material.
In order to know how efficiently microwaves are converted into thermal energy, at a specific
temperature and frequency, the tangent delta can be calculated by:
(2.5)
This value gives information about the penetration depth of the microwaves. If the loss factor is high,
much of the radiation is absorbed and transferred into heat by the first layer of the material. The
microwaves will only penetrate a short distance and only the outer layer is heated up (skin effect). By
using another frequency, where the loss factor is not at the maximum, the microwave can penetrate
more deeply into the material and therefore can heat more homogenously. The penetration depth
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can be defined as the distance from the sample surface where the absorbed power is 1/e (=0.368) of
the absorbed power at the surface. After this distance it can be ignored that the material is heated
up by the microwaves. The penetration depth ( ) can be calculated by:
√
Where
(2.6)
is the length of an electromagnetic wave.
This means that heating with microwaves is a volumetric heating method in comparison with
conventional heating, which only heats a surface. As a consequence of the penetration depth of the
microwave, the shape and size of the reactor vessel, become important parameters, which can result
in homogeneous or non-homogeneous heating of the material [3, 16]. Stirring also affects how a
material is heated.
2.2
Polymerization Reactions under Microwave Conditions
The use of microwave heating has been intensively studied for different polymerization reactions.
The direct heating of the reactants with microwaves can lead to fast and homogeneous heating
resulting in higher conversions and less by-products and energy reduction. Under microwave
conditions some polymerization reaction can be performed which are impossible under conventional
heating (thermodynamic controlled reactions) [3, 16]. It is due to polymerization reactions that
temperature plays an important role; because this reaction is slow a high temperature is desired to
increase the reaction rate, but on the other hand a too high temperature results in degradation of
the product. In conventional heating systems, heat is transferred from a hot surface to the reaction
mixture. This results in a non-homogeneous temperature distribution in the reactor. To avoid
thermal degradation the temperature of the hot surface is never higher than the desired
temperature, so the mean temperature in the reactor is lower; however, this is not the case with
microwave heating. As mentioned earlier, microwave heating is volumetric. Microwaves are
converted into heat by dipole polarization mechanisms and these phenomena occur in a threedimensional space. Consequently heat is distributed homogeneously in this volume. Therefore the
mean temperature at microwaves conditions is higher than the mean temperature reached with
conventional heating [19].
Pielichowski et al. claim that with microwave irradiation, the poly-esterification of maleic and
phthalic anhydrides with epichlorohydrin and ethylene glycol catalyzed by lithium chloride, the
reaction time can be reduced by a factor of two. Because of the exothermic effect of the reaction
between epichlorohydrin and anhydrides the temperature control is an important factor, and it was
found that the possibility of controlling the temperature could be better done with microwaves.
Effective temperature control can prevent overheating and gelation of the reaction mixture. In
addition, the differences between multi- and single-mode cavities were described and they show
that with a multi-mode cavity, a higher average molecular weight was obtained when synthesizing
the unsaturated poly-ester [11]. Wolff et al. presents the results of the poly-condensation reaction of
maleic and phthalic anhydride with epichlorohydrin and ethylene glycol catalyzed by lithium chloride.
No differences in reaction time, nor improvement of the properties of the product were obtained by
microwave heating [20]. Velmathi et al. describes the co-polymerization of ethylene isophthalate
cyclic dimer and bis(2-hydroxyethel) terephthalate by microwave irradiation. Here larger molecular
weights were obtained in a shorter time compared to conventional heating, but unfortunately the
- 11 -
authors do not give an explanation of the possible mechanisms. Further studies to elucidate the
effect of microwave irradiation are expected [21]. The technical limitation of poly-condensation
reactions is the chemical equilibrium of the esterification step which produces water as a by-product.
Good coupling of microwaves with water results in quicker evaporation, and through that
mechanism equilibrium is driven to the polymer and thus improves the yield. This was reported by
Velmathi et al. for the poly-condensation of butan-1,4-diol with succinic acid catalyzed by 1,3dichloro-1,1,3,3-tetrabutyldistannoxane. Higher conversions were reached in a shorter time under
microwave irradiation [22]. In addition the increase in the reaction rate of succinic acid an sebacic
acid with 1,4-butanediol catalyzed by tin(II) chloride might be attributed to the effective interaction
of microwaves with water, thus shifting the equilibrium to the polymer side [23].
As mentioned earlier, non-thermal microwave effects can effect these polymerization reactions due
to specific heating of polar intermediates which lead to modified selectivities. Polar components can
be more reactive under microwave irradiation and therefore other reactions may be possible
compared to thermal heating [24]. Jermolovicius et al. claim that microwaves have influence on the
pre-exponential factor of the Arrhenius equation (eq. 2.2) for the poly-condensation reaction with
maleic anhydride with 2-ethylhexanol-1 and catalyzed by p-toluene sulfonic acid. This means that the
molecular alignment and agitation is different compared to conventional heating. They suppress the
reaction equations into a single general equation where the global order of this equation is lowered
by use of microwave irradiation [19]. On the other hand Gutmann et al. reported the contrary.
Different reactions were performed in a Pyrex glass-vessel (which is transparent for microwaves) and
in a silicon carbide vessel (which interacts well with microwaves and heat can be delivered to the
reaction mixture by the conventional way, but blocks the electromagnetic waves going inside the
vessel). With the use of the silicon carbide vessel no microwave effects whatsoever are possible. For
the reactor content no difference was detected, even when these reactions were performed under
conventional heating such as an oil bath. It should be stated however that previous papers of some
complex reactions claimed that there was a positive microwave effect. Special attention must be paid
to the way a reaction is carried out, and how conventional experiments are compared to microwave
experiments. In addition attention has to be paid to temperature measurement, shape/size of the
reactor vessel and the way of stirring [25], [26]. Still these non-thermal effects are a controversial
topic.
Another useful effect of microwave heating is that with microwaves, solid material can be heated
above the boiling point of the liquid in heterogeneous reaction systems without any observation of
boiling. On the solid surface reaction can go faster because of the locally higher temperature [27].
This can be done, for example, with so called “Passive Heating Elements” (PHE), like silica carbide
cylinders. The PHE interact well with microwaves. It is expected that on the surface of the PHE the
temperature is much higher in comparison with the bulk and so reaction rates might be locally
higher. The PHE are robust and do not interact with the reactants itself. After reaction the PHE can
be easily separated from the product and reused [28], [29].
Over the course of producing resins the dielectric loss are changing which is due to the formation of
long and/or cross-linked chains. Formation of longer chains causes an increase in viscosity and a
decrease in the molecular motion. Therefore the absorption of microwaves is reduced by progressing
of the poly-condensation reaction [3]. This results in lower conversion of microwave energy into heat
[12].
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As already mentioned, the shape and size of the reaction vessel is important for the penetration of
the microwaves, but the geometry of the vessel also influences the efficiency of the microwave cavity
and determines the amount of absorbed and reflected microwaves generated by the magnetron. The
efficiency increases in the case of a multi-mode cavity by increasing the size of the reaction vessel,
because more microwaves reach the reactor vessel and are converted into heat [7]. Nakamura et al.
also present results of energy savings by scaling up the process. The energy consumption per mole of
substrate consumed for the poly-condensation reaction of lactic acid catalyzed by tin(II) chloride and
p-toluenesulfonic acid is less when the reactor volume is scaled-up [12]. The size of the surface area
of the reaction mixture can determines the rate of vaporization. A larger surface area makes it easier
for water to escape the liquid phase and enter the vapor phase. This results in a shift in the
equilibrium reaction towards the polymer side.
In order to promote the removal of water during a poly-condensation reaction, a reduction of
pressure can be used at the moment water is produced. The equilibrium of the poly-condensation
can be shifted to the polymer side [23]. Attention must be paid to the fact that microwave plasma is
formed at pressures lower than 3000 Pa. This can cause degradation of the reactants, or even worse,
have a negative influence on the end products [12].
By using a catalyst the polymerization reaction can be speeded up. An acid catalyst promotes the
esterification, cis-trans isomerization and double bond saturation steps. Some typical acid catalysts
used for poly-esterification in industry are zinc acetate, p-toluenesulfonic acid, and titanium
benzenesulfonate [2]. Velmathi et al. reported the results of different catalysts used for the polycondensation reaction of succinic acid and sebacic acid with 1,4-butanediol. It appeared that tin(II)
chloride and p-toluenesulfonic acid were the most effective catalysts. The advantage of tin(II)
chloride is that is a relatively cheap metal catalyst and is not considered to be toxic [23]. pToluenesulfonic acid is a commercially available, cheap non-metal catalyst which is also not
considered to be toxic [30].
Heterogeneous metal catalysts promote the reaction in microwaves by creating local hot-spots. This
phenomenon is similar to previous explained formation of hotspots with PHE. The temperature
might be higher at the surface of the catalyst particle and reaction might run faster. It is expected
that with several hotspots around each catalyst particles, the global reaction rate could increase,
while the bulk temperature is kept the same because the reaction mixture acts as a heat sink. A
disadvantage of using (heavy) metals is that they are toxic.
The catalyst trifluoromethanesulfonate (triflate) is an environmentally-friendly catalyst while these
catalysts can also perform poly-condensation reactions under mild conditions. A triflate is an
extremely stable polyatomic ion and an extremely strong acid [13]. One of the advantages of this
type of catalyst is that it can be recovered by extraction with water, and reused. In contrary to Lewis
acids, triflates are not deactivated or decomposed by protic substances including carboxylic acids,
alcohols and water. Direct esterification of carboxylic acids and glycols catalyzed by triflates under
mild conditions, temperatures between 35 – 180 °C, were published by Takasu et al. [31], [32],
[33],[34]. Kricheldorf et al. reported results of aliphatic poly-esterification reactions with different
triflates and glycols. It was found that different metal triflates give different and sometimes
unexpected results. The catalytic effects of those metal triflates should be studied in more detail
[35], [36].
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2.3
Poly-esterification Reaction of Maleic and Phthalic Anhydride with
Propylene Glycol
In this report the poly-esterification reaction of maleic anhydride and phthalic anhydride with
propylene glycol is described. This reaction consists of four main steps:
-
Ring opening of the anhydrides
Poly-condensation
Isomerization
Saturation
The ring opening reaction takes place at a temperatures between 60 – 80 °C and is a fast exothermic
reaction (ΔH = -40 kJ/mole). The poly-condensation, isomerization and saturation steps take place at
temperatures above 140 °C [37].
The order of a poly-esterification reaction increases during the progression of the reaction from the
first order with respect to the carboxylic acid at the start of the second order, with respect to the
acid at the end. This increase is caused by the change in physical properties of the reaction mixture.
For the polymerization of phthalic anhydride the esterification reaction takes place only because
after the ring-opening reaction this becomes a phthalic acid and has the properties of a saturated
acid. Only isomerization and saturation reactions of the double bonds take place for the
polymerization of maleic anhydride. The side reactions of the carboxylic acids, with double bonds,
undergo cis-trans isomerization. The isomerization reaction is acid catalyzed and second order with
respect to the carboxylic acid. Cross-linking of the polymer, by the saturation reaction, is also acid
catalyzed and first order with respect to carboxylic acid and the glycol. This is also known as the
Ordelt reaction. These saturated acid molecules can still undergo esterification. Long reaction times
favor the formation of cross-linked and branched polymer chains which are a dominating by-product.
It appeared that all the reaction steps were acid catalyzed, and because of the presence of maleic
and phthalic acid the poly-esterification is self-catalyzed.
Salmi et al. describes a reaction mechanism for the poly-esterification of maleic and phthalic
anhydride with propylene glycol. Reaction rate constants of the different reaction steps are set
equal. Reaction constants and Arrhenius parameter, as pre-exponential factor and activation energy,
can be found in the paper [1], [2], [38], [39]. Shah et al. published an improved model for the
reaction with maleic anhydride with propylene glycol. A different definition of the chemical
equilibrium concentration of the carboxylic acid is given and the vaporization of water and glycol was
adjusted; furthermore the effect of the acid catalyzed reactions differs in the way that only the
forward reaction is considered. Arrhenius law parameters are recalculated. The dynamic model is
tested with the experimental results of Salmi et al (1994) and proved to be valid [40].
The description of the reaction mechanism and equations can be found in Appendix I.
- 14 -
Figure 4 The chemical structure of the polymer chain after reaction of maleic and phthalic anhydride with propylene
glycol[8]
Legros et al. presented data of the dielectric properties of the polymer shown in figure 4. The
dielectric properties are temperature and frequency dependant, as mentioned earlier. The keton
group in the molecule is, in the main, responsible for the dielectric properties. These groups are
known to be dipolar and coming from maleic and phthalic anhydrides. However the phenyl group in
the molecule, which comes from the phthalic anhydride, limits the dielectric properties. Therefore
the absorption of microwaves increases as phthalic anhydride concentration decreases. Reaction
conditions can be optimized with respect to the frequency so a better interaction of the microwaves
can take place. Increasing the frequency results in a decrease of the dielectric constant ( ) so a
deeper penetration of microwaves into the reactor is possible and an increase in dielectric loss ( ).
This results in a more efficient transfer of microwave energy into heat. It can be stimulated with the
use of more maleic anhydride and less phthalic anhydride[8], [9]. In this report no further attention is
paid to the optimization because the magnetron used only had one operating frequency, namely
2.45 GHz.
2.4
Poly-esterification Reaction of Maleic Anhydride with 1,6Hexanediol or 1,4-Butanediol
The reaction of maleic anhydride with 1,6-Hexanediol catalyzed by p-toluenesulfonic acid is
described by Larez et al. The effect of even and odd numbers of carbon atoms of linear glycols on the
reaction is discussed. It appears that an odd number gives a higher percentage of isomerization.
There is a strong tendency that an even number of carbon atoms of the glycol undergoes crosslinking. With overly high temperatures gelation could occur, and with odd ones this cross-linking
results in a white non-sticky rubber-like solid [41]. Kricheldorf et al. investigated the isomerization of
maleic anhydride with 1,6-hexanediol with different triflate catalysts and different temperatures.
Reactions conditions above 100 °C result in significant more side-reaction and cyclization, which
limits the chain growth reaction. A glycol with a shorter carbon chain, 1,4-butanediol, was also used,
which resulted in more cyclization. It seems that the poly-condensation reaction is not only a protoncatalyzed esterification, as previously described, but that the metal ions of the triflates plays an
important part of the poly-condensation mechanism [13]. Because of these promising results,
experiments under microwave irradiation were executed and discussed in this report.
2.5
Different Microwave Applicators
There are different methods to apply microwaves to the reactor mixture. Energy efficiency and the
usability for scaling-up for industrial practice are of main interest. In this report three different kinds
of microwave applicators were used. A mono-mode microwave applicator (Discover, CEM) was used
for small reaction vessel sizes (50 mL); in which the microwaves are directed precisely to the reactor
vessel. A multi-mode microwave applicator (MARS, CEM) was used for larger reactor vessels
- 15 -
(respectively 1 and 2 liters); meaning the microwave field in the cavity is non-homogeneous. There is
more scattering of microwaves on the wall and on the reactor vessel; which results in a decrease of
the energy efficiency. An internal transmission line (INTL, Sairem) technology was used for a larger
size of reactor vessel (2,2 L). In this case the microwaves are directed by an antenna into the reactor.
The advantage is that microwaves can be directed very selectively and with a high efficiency to the
reaction mixture. An INTL can be placed in a stainless steel reactor vessel, and for bigger vessels
multiple antennas can be placed to improve the distribution of the microwaves. The INTL technology
makes it possible to scale-up microwave reactors to industrial scale, with minimum requirements
necessary to change the equipment. A schematic view of the INTL is shown in figure 5. In this report
we focused on the energy efficiency of those three different types of microwave applicators.
Figure 5 Schematic view of a reactor with an INTL (1. Reactor vessel, 2.Outlet valve, 3.Connection for Distillation column,
4.Connection for reagents loading, 5.INTL, 6.Inlet and outlet for cooling medium, 7.Connection to insert a temperature
probe, 8. U-shaped waveguide)
- 16 -
3. Experimental Section
3.1
Materials
The following reactants were used in the reaction: Maleic anhydride [CAS: 108-31-6], Phthalic
anhydride [CAS: 85-44-9], Propylene Glycol [CAS: 57-55-6], and Hexamethylene Glycol [CAS: 629-118]. The catalysts which were tested are p-Toluenesulfonic acid [CAS: 104-15-4], Tin(II) chloride [CAS:
7772-99-8], Tin(II) chloride dehydrate [CAS:10025-69-1], Lanthanum(III)triflate [CAS: 52093-26-2],
Copper(II) triflate [CAS: 34946-82-2], and Scandium(III)triflate [CAS: 144026-79-9]. The following
chemicals were used to analyze the polymer product: Tetrahydrofuran [CAS: 109-99-9], Ethanol [CAS:
64-17-5], Potassium hydroxide [CAS: 1310-58-3], Phenolphthalein [CAS: 77-09-8] (indicator),
Potassium hydrogen phthalate [CAS: 877-24-7], and distilled water. All chemicals were supplied by
Sigma Aldrich Chemistry, and were used as delivered.
3.2
Experiment
The results from conducted experiments are described in three sub-chapters. The experiments
carried out in the microwave Discover (CEM Corp.) will be described first. In these experiments a
vessel of 50mL was used. In order to compare the results of the microwave experiments with
conventional heating an oil bath was used. Secondly, the experiments done in the microwave MARS
(CEM Corp.) will be described. Here vessels with a larger volume were used, 1 and 2 liters
respectively. For conventional heating a heating mantle was used. In the third part, the INTL will be
described. In Appendix II tables can be found of all the performed experiments and corresponding
reaction conditions.
3.2.1 Microwave Discover
The microwave heating was carried out in a Discover microwave (CEM Corp.), in which the
magnetron operates at a frequency of 2.45 GHz, with a maximum power of 600 W. The Discover
microwave has an open single-mode cavity which can be used to connect additional glassware e.g. a
condenser. The conventional experiments were carried out in an oil bath on a heating plate with a
maximum power of 630 W (IKA RET basic C). These experiments were performed in a 50 ml reaction
vessel. During the reaction the mixture was continuously stirred by a magnetic stirrer with a speed of
100 rpm for conventional heating, or in low stirring mode for the Discover magnetron.
The Discover magnetron is equipped with an infrared temperature sensor which measures the
temperature of the reactor wall. Based on this measurement, the power can be applied to the
applicator. However infrared temperature readout was not used because this caused a delay in time
compared with the temperature measured by fiber optic probes. For this reason fiber optic probes
were used to measure and control the temperature and power delivered to the reactor vessel. The
temperature was measured with fiber optic probes at the reactor wall and in the center of the
reactor vessel.
The reaction was carried out isothermally at 100 °C and 140 °C in an inert atmosphere of nitrogen
which was added to the liquid as a stripping agent to promote the removal of by-products (mainly
water). Volumetric flow rate of nitrogen was 0.144 L/min. The power delivered was between 7 – 50
W. Prior to the heating process all reactants and the catalyst were placed into the reactor vessel. The
catalyst dissolves in the liquid creating a homogeneous reaction.
- 17 -
The synthesis of unsaturated polyester from maleic
anhydride and phthalic anhydride with propylene
glycol consist of three steps. The first step of the
reaction is a ring opening reaction. This is an
exothermal reaction (ΔH=-40 kJ/mole) [37], which
takes place at 60 – 80 °C. This reaction is fast and
for this reason the temperature can quickly be
increased, until the desired end temperature,
without any consequences. The second step is the
esterification reaction, with water as a by-product.
Esterification reactions were carried out at higher
temperatures; at 100°C and 140°C respectively.
After the desired temperature had been reached, it
was kept isothermal. This reaction is slow
compared to the ring opening reaction and the
conditions are kept the same for 2.5 hours. The
esterification reaction is an equilibrium reaction. In
order to shift the equilibrium to the product side,
water has to be removed; hence the reason a small
Figure 6 Microwave Discover set-up
condenser kit was placed above the reactor vessel
to collect water, which was then trapped in a flask. The third step was so the double bond in maleic
anhydride and phthalic anhydride could be isomerized or saturated. Saturation of the double bond
causes cross-linking in the polymer [42]. Figure 6 shows a picture of the set-up of the microwave
heated experiment.
3.2.2 Microwave MARS
The MARS microwave operates at a frequency of
2.45 GHz and can deliver a maximum power of
1600 W. MARS has a multi-mode cavity so the
energy distribution is non-uniform. Polycondensation reaction was performed with 1,2propylene glycol, maleic anhydride and phthalic
anhydride with a ratio of 2.37 : 0.76 : 1. The sizes
of reactor vessels used were 1 and 2 liters
respectively; and on top of the vessel a stirring
shaft was placed. The reaction mixture was
continuously stirred by a stirring blade with 25
rpm for the 1 L vessel and 50 rpm for the 2 L
vessel. The shaft was connected to a condenser
and the resulting vapor was collected in a
graduated cylinder. The temperature was
measured by a fiber optic probe which was connected to Figure 7 MARS set-up
MARS. Based on this temperature measurement the
microwave power was delivered to the applicator to maintain isothermal conditions. The microwave
unit was connected to a computer where the reaction conditions were controlled and the
- 18 -
temperature profiles were recorded. The temperature on top, where the vapor leaves the reactor
vessel, was measured by a thermometer. The reaction was carried out isothermally at 160 °C, 180 °C
and 200 °C for 6,5 hours, in an inert atmosphere of nitrogen which was bubbled into the liquid by
tube - with a flow rate of 1.5 L/min. Prior to the heating process all reactants were placed into the
reactor vessel. In order to compare the results of the microwave MARS to conventional heating a
heating mantle (Kletti-Mohr, LabHEAT, KM-MPE, 700W) was used. The rest of the set-up was kept
the same to achieve comparable results. Samples were taken from the vessel at different time
intervals. Figure 7 shows a picture of the set-up of the MARS experiment.
3.2.3 Microwave INTL
The microwave INTL of Sairem, France was used as another microwave device. This was a stainless
steel reactor of 2.2 L with an Internal Transmission Line (INTL) where the microwaves were brought
in direct contact with the reactants. The magnetron operated at a frequency of 2.45 GHz with a
maximum power of 2000 W. Poly-condensation reaction was executed with 1,2-propylene glycol,
maleic anhydride and phthalic anhydride with a ratio of 2.37 : 0.76 : 1. By-products of the polycondensation
reaction
were
continuously removed from the reactor.
This was promoted by using nitrogen as
a stripping agent. Nitrogen was
constantly bubbled through the liquid
with a volumetric flow rate of 1.5 L/min.
A distillation vigreux column was
connected on top of the reactor.
Distillate was collected in a graduated
cylinder which was connected to the
top of the distillation column. The
reaction mixture was kept isothermal at
a temperature of 160 °C. Cooling liquid
was circulated in the cooling jacket and Figure 8 INTLset-up
the reaction mixture was continuously
stirred at a speed of 30 rpm. Figure 8 shows a picture of the set-up of the INTL experiment.
3.3
Analytical Measurements
The acid group content in the product was determined as a measure of conversion by titration
according to ISO 2114 method [43]. Adjustments were made on this method in that the polymer
sample, taken from the reaction product was weighed with an accuracy of 0.001 g, and dissolved in
60 mL tetrahydrofuran and 10 mL of distilled water instead of a toluene/ethanol solvent mixture.
With a 0.1 M potassium hydroxide in ethanol, the solution was titrated. One percent
phenolphthalein in ethanol was used as indicator. The end point was obtained when the solution
turned pink and this color change had persisted for 10 seconds. The Acid Value (AV) was expressed in
mg of KOH per gram of sample.
(3.1)
Where is mL of KOH solution titrated,
is in mole/L the molarity of the KOH solution,
sample weight in grams and 56.1 is the molar mass of KOH.
- 19 -
is the
The molecular weight of the polymer samples was determined by Gel Permeation Chromatography
(GPC) of Water Breeze at 254 nm wavelength with a dual column with a retention time of 30
minutes.
The polymer was characterized by Nuclear Magnetic Resonance (13C NMR) at 100 MHz. By a Bruker
Avance-400 NMT spectrometer, THF-d8 solvent was used at a temperature of 25 °C with an 5 mm
sample tube.
The reaction water removed from the reactor, which is called here the “distillate”, was analyzed by
Gas Chromatography-Mass Spectrometry (GC-MS). In order to extract the organics from the water
mixture, di-ethyl ether was used as solvent. The GC-MS measurements were carried out on a
Shimadzu QP-2010S with a Varian Factor Four VF-1ms 25 m x 0.25 mm x 0.4 μm column. Injection
was carried out at 250 °C with a split of 20. The column was held at 50 °C for 5 minutes and then was
heated with 10 °C/min to 230. Data was processed using Shimadzu GC-MS solutions software.
The quantity of organic in the distillate was measured by Gas Chromatography (GC). The GC
measurements were carried out on a Varian 430-GC with a Varian Capillary Column WCOT FUSED
Silica CP-Wax 58 (FFAP)-CB 50 m, 0.25 mm, 0.2 μm #CP7727. Injection was carried out with a set
point of 250 °C with a split ratio of 100. The column was held at 50 °C for 1 minutes and then was
heated with 5 °C/min to 120 °C and with 10 °C/min to 210 °C with a total retention time of 34 min.
Data was processed using Galaxie software.
- 20 -
4. Results and Discussion
In order to investigate the differences between microwave and conventional heating, several series
of experiments were performed. As mentioned in the experimental section, the types of experiments
are divided into three parts, namely experiments carried out in the mono-mode microwave Discover,
in the multi-mode microwave MARS and in the INTL microwave. The results will be presented in two
chapters, namely mono-mode microwave applicator and large scale microwave applicators.
Additionally in the mono-mode microwave applicator chapter (chapter 4.1) several reaction systems
with different anhydrides and glycols as well as different catalysts were studied.
The penetration depth of the microwaves can be calculated by the data supplied by Legros et al. [9],
however this data is as a result of lower temperatures than the reactions that are described in the
report, so this penetration depth is an estimation.
√
4.1
√
(4.1)
Mono-mode Microwave Applicator
4.1.1 Different Catalyst
In figure 9, 10 and 11 the results of the reactions with propylene glycol (PG) and maleic anhydride
(MA) are plotted for microwave Discover heated, and for oil bath heated, reactions. All other
reaction conditions were identical in order to obtain a fair comparison between the experiments.
Different catalysts were used to improve the reaction and/or microwave effects. The ratio of
reactants and catalysts was PG:MA:Cat as 1.2:1:0.011. All experiments were performed at a
temperature of 140 °C and kept at isothermal conditions for 2.5 hours.
Acid Value
[mg KOH/ g]
250
200
150
100
CH Oil bath
50
MW Disc.
0
Figure 9 Acid Value of PG+MA+Catalyst reactions (Ratio: 1.2:1, Temp.: 140 °C, time: 2.5 h, Cat.: 0.5 mole%)
In figure 9 Acid Values (AV) of the reaction between PG and MA are plotted for microwave Discover
heating and for oil bath heating. The small differences in the AV plot can be explained by small
analytical measurement errors, or differences in time the polymer was cooled down. In order to
- 21 -
check the reproducibility the experiments with PG+MA were duplicated. The results were the same
(considering measurement errors). Figure 9 shows no decrease of AV so therefore there was no
improvement in terms of conversion obtained for the microwave experiments, compared to
experiments done with conventional heating.
In figure 10, the time needed to get the first drop of distillated water in the collecting vessel is
plotted to indicate when the poly-condensation reaction was started. This figure shows that the time
until the poly-condensation reaction was started is rather influenced by catalysts, and not by the
heating method.
PG+MA+SnCl2(2H2O)
PG+MA+SnCl2
MW Disc.
PG+MA+p-TSA.H2O
CH Oil bath
PG+MA
0
2000
4000
6000
8000
10000
12000
time
[s]
Figure 10 The time that was needed to get the first drop of distillate in the collecting vessel of PG+MA+Catalyst reactions
(Ratio: 1.2:1, Temp.: 140 °C, time: 2.5 h, Cat.: 0.5 mole%)
In figure 11 the total amount of distillate is plotted for the different experiments. The flow of the
nitrogen seriously influences the collected amount of distillate and therefore attention must be paid
to the way nitrogen was provided into the liquid and the flow rate. When the AV is lower (fig. 9), and
the poly-condesation reaction is started earlier (fig. 10) the amounts of distillate is higher. As can be
concluded from the figures 9, 10 and 11 this is especially the case for the reaction with the catalyst pTSA.H2O. The differences between the amounts of collected distillate from both two heating
methods were not significant compared to the total volume of the system so one can conclude that
the heating method has no impact on the overall performance of the reactions.
- 22 -
Amount of Distilllate
[mL]
2,5
2
1,5
1
CH Oil bath
0,5
MW Disc.
0
Figure 11 Amount of distillate of PG+MA+Catalyst reactions (Ratio: 1.2:1, Temp.: 140 °C, time: 2.5 h, Cat.: 0.5 mole%)
With 13C NMR the molecular structure of the polymer was characterized. The reaction between
propylene glycol and maleic anhydride was used without any catalyst. In Appendix II the overall
spectra can be found. A difference between the two spectra was found in the 125 – 136 ppm region
where the peaks are responsible for the double bond of the maleic anhydride (fig. 12). A larger
figure is enclosed in Appendix II. It appears that with microwave heating a larger peak arises in the
maleate region. This could be caused by the alignment of the molecules by the electromagnetic field
of the microwaves. However further research is needed to verify this conclusion.
Figure 1213C NMR spectrum of CH=CH Group of PG+MA reactions (Ratio: 1.2:1, Temp.: 140 °C, time: 2.5 h)
- 23 -
Other techniques were used in order to improve the interaction between microwaves and reaction
mixture. Passive heating elements (PHE), like Silica carbide (SiC) cylinders, were used to create local
hot spots. 4 PHE were used in a 50 mL reactor. The surface to volume ratio was calculated by,
(4.2)
Dimensions of the PHE cylinders are: diameter 10 mm and height 8 mm.
Kremsner et al. reported that with the use of PHE most of the microwaves were absorbed by the PHE
itself. Microwave energy was transferred to heat and conducted to the reaction mixture. Since most
of the energy is absorbed by the PHE only a small amount of microwaves interact with the mixture.
This supports that many benefits of using microwaves, like non-thermal effects, will be lost [28].
Adding of the PHE had no influence on the AV measured. Probably the surface to volume ratio was
too small to make significant difference in bulk and PHE temperatures.
Forced cooling was used for one set of experiments performed in the Discover. With forced cooling
outside the reactor vessel it is possible to deliver more microwave power into the reactor. Forced
cooling was carried out by air flow around the reactor vessel in the microwave Discover, thus making
it possible to use 5 times more power, compared with experiments performed at the same process
conditions without forced cooling. After analyzing the polymer sample it appears that the AV was 8 %
lower than without forced cooling. Acid Value results are presented in figure 13.
250
Acid Value
[mg KOH/ g]
200
150
PG+MA
100
50
0
PG+MA
4 SiC Bits
Forced Cooling
Figure 13 Acid Value of PG+MA reactions with SiC PHE and with Forced Cooling performed in magnetron (Ratio: 1.2:1,
Temp.: 140 °C, time: 2.5 h)
The reactions with Lanthanum(III)triflate (La(III)triflate), Copper(II) triflate (Cu(II)triflate),
Scandium(III)triflate (Sc(III)triflate) were stopped earlier, because longer reaction times degraded the
polymer totally. If the reaction of condensation polymerization is not controlled in order to prevent
cross-linking this gives “brown gunk” as an end product [44]. With the use of microwaves we failed to
control this and so ended up with degraded polymer. Velmathi et al. reported that poly-condensation
reactions caused by overly high temperatures or reaction times can produce a brown viscous, or a
- 24 -
yellow liquid respectively [22]. Probably this was the case. The AV of these experiments is shown in
figure 14, and unfortunately no conclusions can be drawn from these results.
300
Acid Value
[mg KOH/ g]
250
200
CH Oil bath
150
MW Disc.
100
50
0
PG+MA+La(III)triflate
PG+MA+Cu(II)triflate
PG+MA+Sc(III)triflate
Figure 14 Acid Value of PG+MA+Catalyst reactions (Ratio: 1.2:1, Temp.: 140 °C, time: <1 h, Cat.: 0.5 mole%)
Controlling the temperature with microwave irradiation can be more advantageous compared to
conventional heating. Microwaves heat up the liquid directly and as a consequence stopping the
microwave irradiation removes the heating source immediately. Therefore the temperature
overshoot is that a lower and steady state is reached earlier, see figure 15. This prevents overheating
and thermal degradation of the polymer like gelation [11].
ΔT=15
Temperature
[°C]
160
140
120
Microwave
100
Conventional (Oil bath)
80
60
0
500
1000
1500
2000
2500
3000
time
[s]
Figure 15 Temperature profile of PG+MA reactions (Ratio: 1.2:1)
The reactions were performed again under the same conditions, except propylene glycol (PG) was
replaced by hexamethylene glycol (HD). The choice of glycol influences the polymer chain growth
[20]. The reactions were performed with La(III)triflate and Sc(III)triflate because these catalysts were
- 25 -
the most promising according to earlier research [13]. The results are shown in figure 16. Too little
distillate was collected to quantify. Figure 16 shows no improvement of AV by using microwaves.
200
180
160
Acid Value
[mg KOH/ g]
140
120
CH Oil bath
100
MW Disc.
80
60
40
20
0
HD+MA
HD+MA+Sc(III) triflate
HD+MA+La(III) triflate
Figure 16 Acid Value of HG+MA+Catalyst reactions (Ratio: 1.2:1, Temp.: 100 °C, time: 2.5 h, Cat.: 0.5 mole%)
4.1.2 Vacuum
The influence of applying a vacuum on the system, for the microwave irradiated reaction and the
conventional heating, was investigated. At vacuum conditions, water is pulled out of the reaction
mixture (by-product of poly-condensation). The reaction was performed at a temperature of 100°C
and 140°C and this temperature was kept at an isotherm for 2.5 hours in an inert atmosphere of
nitrogen. After the temperature of liquid had reached the set temperature, the nitrogen injection
continued for one hour. At the moment the nitrogen stopped, a vacuum was applied which caused a
decrease in the reaction mixture temperature. This temperature drop was simultaneous with a
vigorous boiling of the reaction mixture, due to the endothermic nature of evaporation. To prevent
this drop in the temperature the microwave power was increased to stabilize the temperature. The
pressure was gradually decreased in one hour to 50 mbar, and was kept low at this level until the end
of the reaction. Lower pressures increases the risk of producing microwave plasma [12], therefore
the pressure was only reduced to 50 mbar. In figure 17 the AV is plotted for the reaction with PG and
MA in figure 18 the AV is plotted for HD and MA. It shows an improvement of the AV due to low
pressure conditions. These results were expected because with the removal of water the equilibrium
of the poly-condensation reaction is shifted towards the product side.
- 26 -
200
180
160
Acid Value
[mg KOH/ g]
140
120
CH Oil bath Nitrogen
100
CH Oil bath Vacuum
80
MW Disc. Nitrogen
60
MW Disc. Vacuum
40
20
0
PG+MA+PA
PG+MA
PG+MA+p-TSA.H2O
Figure 17 Acid Values of nitrogen and vacuum experiments (Temp.: 140 °C, time: 2.5 h, Cat.: 0.5 mole%)
200
180
160
Acid Value
[mg KOH/ g]
140
120
CH Oil bath Nitrogen
100
CH Oil bath Vacuum
80
MW Disc. Nitrogen
60
MW Disc. Vacuum
40
20
0
HD+MA
HD+MA+La(III) triflate
Figure 18 Acid Values of nitrogen and vacuum experiments of HD+MA+Catalyst reactions (Ratio: 1.2:1, Temp.: 100 °C,
time: 2.5 h, Cat.: 0.5 mole%)
Figure 19 (PG and MA) and figure 20 (HD and MA) showed a significant increase of the total amount
of distillated water between the non-vacuum and vacuum experiments (the total amount of
distillated water is the amount of distillate in the collecting flask and the amount in the cold trap
which was in front of the vacuum pump). Figure 19 and 20 shows that there is more water removed
from the vessel with a vacuum than from one without. However using microwaves resulted in a
larger amount of distillate that could be removed from the reactor vessel, compared to conventional
heating.
- 27 -
6
Amount of Distillate
[mL]
5
4
CH Oil bath Nitrogen
CH Oil bath Vacuum
3
MW Disc. Nitrogen
2
MW Disc. Vacuum
1
0
PG+MA+PA
PG+MA
PG+MA+p-TSA.H2O
Figure 19 Amount of distillate of nitrogen and vacuum experiments (Temp.: 140 °C, time: 2.5 h, Cat.: 0.5 mole%)
3
Amount of Distillate
[mL]
2,5
2
CH Oil bath Nitrogen
CH Oil bath Vacuum
1,5
MW Disc. Nitrogen
1
MW Disc. Vacuum
0,5
0
HD+MA
HD+MA+La(III) triflate
Figure 20 Amount of distillate of nitrogen and vacuum experiments of HD+MA+Catalyst reactions (Ratio: 1.2:1, Temp.:
100 °C, time: 2.5 h, Cat.: 0.5 mole%)
Because of the uniform heating, by using microwave irradiation, all the molecules experienced the
same reaction pathway so the molecular weight distribution could be influenced. The average
molecular weight (Mw) was measured by GPC. In figure 21 and 22 these results are plotted. The
average molecular weight is clearly not influenced by microwaves. As expected the Mw is influenced
by the vacuum; as can be seen earlier in figure 17 and 18, where the AV is lower the Mw is higher.
- 28 -
1400
1200
Mw
[g/ mole]
1000
CH Oil bath Nitrogen
800
CH Oil bath Vacuum
600
MW Disc. Nitrogen
400
MW Disc. Vacuum
200
0
PG+MA+PA
PG+MA
PG+MA+p-TSA
Figure 21 Average molecular weight (Temp.: 140 °C, time: 2.5 h, Cat.: 0.5 mole%)
3000
2500
Mw
[g/ mole]
2000
CH Oil bath Nitrogen
CH Oil bath Vacuum
1500
MW Disc. Nitrogen
1000
MW Disc. Vacuum
500
0
HD+MA
HD+MA+La(III)
Figure 22 Average molecular weight of HD+MA+Catalyst reactions (Ratio: 1.2:1, Temp.: 100 °C, time: 2.5 h, Cat.: 0.5
mole%)
4.2
Large Scale Microwave Applicators
In this paragraph large scale experimental results are presented. Experiments were done with the
use of microwave MARS. Different conditions were used to show how the poly-condensation
reaction behaved under microwave conditions. The results of the microwave experiments were
compared with conventional heating, making use of a heating mantle. Also the results were
compared with another microwave device, namely the Internal Transmission Line (INTL) microwave.
4.2.1 Results of the Products and Distillate Obtained
Equilibrium experiments were first performed in order to see if microwaves have an effect on the
reaction rate. All vapors were condensed and were refluxed back into the reactor vessel. As can be
seen in figure 23 microwaves have no effect on the reaction rate compared to conventional heating.
- 29 -
CH Heating Mantle
300
MW MARS
250
Acid Value
[mg KOH/ g]
200
150
100
50
0
0
100
200
time
[min]
300
400
500
Figure 23 Total reflux of distillate till equilibrium is reached of PG+MA+PA reactions (Ration: 2.37:0.76:1, Temp.: 140°C,
time: 465 min., Vessel size: 2 L)
In figure 24 the AV’s in time are plotted where the by-products were constantly removed. It should
be noticed that the AV line of the conventional heated reaction is crossing the line of the microwave
heated reaction. With the microwave application, the reaction mixture reached the desired
temperature more quickly. That’s why the AV of the microwave experiment are below the
conventional AV’s, to ±100 min. After that the AV’s of the conventional experiment are lower. One
reason for this effect might be that the temperature at the wall of the reactor, with the conventional
heating, was higher than with the microwave heating. The temperature was measured in the center
of the vessel. This is a known phenomenon in conventional heating systems (as mentioned in chapter
2). So with a higher temperature the reaction at the wall was faster, and that might be an
explanation of a lower AV at the end (410 min).
250
CH Heating Mantle
MW MARS
Acid Value
[mg KOH/g]
200
150
100
50
0
0
50
100
150
200
250
300
350
400
450
time
[min]
Figure 24 Acid Value in time with continuous removal of water of PG+MA+PA reactions (Ration: 2.37:0.76:1, Temp.:
200°C, time: 410 min., Vessel size: 2 L)
- 30 -
The distillate that was collected from the reaction of PG+MA+PA was analyzed by GC-MS to see the
type of light organics present in the distillate, except water. GC-MS spectra can be found in Appendix
III. Propylene glycol was expected to be in the distillate because the reaction was performed above
the boiling point of the glycol. This explains why an excess of propylene glycol was used in order to
compensate this loss. Also 1,3-dioxolane, 1,3-dioxane and propionaldehyde were found in larger
quantities.
Figure 25 GC chromatogram of distillate from microwave and conventional heated reactions (Reaction: PG+MA+PA,
Ration: 2.37:0.76:1, Temp.: 200 °C, time: 410 min., Vessel size: 2 L)
Figure 26 Zoomed in version of GC chromatogram of distillate from microwave and conventional heated reactions
(Reaction: PG+MA+PA, Ration: 2.37:0.76:1, Temp.: 200 °C, time: 410 min., Vessel size: 2 L)
- 31 -
The small shift of the peaks were probably caused by a small pressure difference in the equipment. A
lot of equipment is running during the day and less overnight and this might cause the pressured
difference of the carrier gas which results in a difference in retention time for the same analysis
parameters. With GC the individual quantities of these components were determined. In figure 25
the GC spectrum is showed of the distillate obtained by conventional and microwave heated
experiments. Figure 26 zooms in on the dioxolane and propionaldehyde peaks. The corresponding
amounts can be found in table 1. It appears that more organic by-products were distilled off by
microwave heating. Probably those molecules interact well with microwaves. In Appendix IV the
extrapolations of the amounts from the different components can be found as well as the
corresponding GC chromatograms. In addition a magnified picture of figure 25 and 26 can be found
in Appendix IV.
Tabel1 Amount of organic components in distillate in Molar [mole/L] based on GC analysis (Reaction: PG+MA+PA,
Ration: 2.37:0.76:1, Temp.: 200 °C, time: 410 min., Vessel size: 2 L)
Component
Propionaldehyde
1,3-Dioxolane
Propylene Glycol
Microwave Heated
0.085
0.055
1.81
Conventional Heated
0.065
0.003
1.46
In order to see if the volume, and especially the surface area, of the vessel have an impact on the
amount of distillate collected, two reactor volumes were used, a 1 liter vessel and a 2 liter vessel. The
ratio of the surface of the reaction mixture to volume of the reactor vessel was calculated by
(4.3)
(4.4)
For a 1 liter vessel the surface is relatively larger than for a 2 liter vessel. In figure 34 the amount of
distillate that was collected is plotted. In the case of the 2 liter vessel twice the amount of reactants
were added, so twice the amount of distillate should be collected. However the expected amount of
distillate was less, and caused by the relative smaller surface area of the 2 liter vessel.
- 32 -
160
Amount of Distillate
[mL]
140
120
100
80
60
MW MARS 2L
40
MW MARS 1L
20
0
0
100
200
300
400
500
time
[min]
Figure 34 Amount of Distillate plot for the MARS experiment in a 2 and 1 liter vessel (reaction: PG+MA+PA, ration:
2.37:0.76:1, Temp.: 200°C)
4.2.2 Power Consumption
Now when we look at the power consumption of conventional heating and microwave heating we
see that the microwave was using 4,5 times more energy (figure 27). This was to be expected
because the efficiency of transferring electrical power to microwaves, and then to heat, is lower than
transferring electrical power directly into heat. The way microwaves are in beneficial is when they
speed up the reaction rate; which was not the case for this reaction, as showed earlier from the
results of the reactions performed at equilibrium conditions (figure 23).
250
Acid Value
[mg KOH/ g]
200
150
100
CH Heating Mantle
MW MARS
50
0
0
1
2
3
4
5
6
7
8
Power
[kWh]
Figure 27 Acid Value vs. Power consumption (Reaction: PG+MA+PA, Ration: 2.37:0.76:1, Temp.: 200°C, Vessel size: 2 L)
By delivering the microwave energy directly to the reactor vessel by an antenna (INTL), the overall
efficiency is higher because the reflection of microwaves is lowered. That is why the INTL microwave
gives a better efficiency compared to the (multi-mode) MARS device. As shown in figure 28 and 29
- 33 -
the percentage of reflected microwaves for the INTL is significantly lower than for MARS. The heat
losses to the surrounding of the INTL reactor vessel are caused by a cooling jacket. That’s why the
percentage of absorbed microwave power by the mixture is lower than by MARS. Despite that the
specifications of the cooling jacket indicate that control was feasible, no further attention was paid to
control this. In figure 30 the AV is plotted against the power consumed for the experiments with
microwaves. The INTL uses more energy than MARS to get the same AV, and this is caused by the
heat loss to the cooling jacket. If this loss was reduced it can be assumed that the amount of
consumed power would be less than that of MARS. The experiments were performed at 160 °C. The
MARS experiment was executed in a 2 L vessel.
The energy balance that was used for the calculations are:
(4.5)
Where
is the amount of energy that is lost by transferring electrical energy to microwave
energy, this was measured.
is the amount of energy which is applied to the microwave
cavity.
(4.6)
Where
is the amount of microwave energy lost in the cavity.
is the amount of
energy absorbed by the reaction mixture for reaction and phases transitions, this was measured by
the apparatus of the INTL. The same value, but scaled by volume, of
was used by the
calculations of the MARS energy balance.
is the amount of thermal energy lost to the
surrounding by the reactor vessel. For the INTL this is the amount of energy lost by the reactor vessel
to the cooling jacket. For MARS
was calculated as follows:
(4.7)
Where
is the total mass of the reaction mixture. The
is the specific heat of the polymer and
was measured (1.85 J/g. °C) and assumed to be constant.
For INTL
was calculated as,
(
)
(4.8)
Where
is the volumetric flow of the cooling liquid and was measured (45 L/h). is the density of
the cooling liquid (0.95 g/cm3 at 25 °C) and
is the specific heat of the cooling liquid (1.51 J/g. K)
- 34 -
12,45%
24,52%
27,25%
72,75%
34,16%
35,49%
33,78%
65,84%
19,61%
12,75%
Magnetron
Reflected
Magnetron
Reflected
Mixture
Surrounding
Mixture
Jacket
Figure 28 MARS power efficiency (Reaction: PG+MA+PA,
Ration: 2.37:0.76:1, Temp.: 160°C, Vessel size: 2 L)
Figure 29 INTL power efficiency (Reaction: PG+MA+PA,
Ration: 2.37:0.76:1, Temp.: 160°C, Vessel size: 2.2 L)
300
Acid Value
[mg KOH/ g]
250
200
150
MW MARS
100
MW INTL
50
0
0
1
2
3
4
5
6
7
8
Power
[kWh]
Figure 30 AV to Power consumption of MARS and INTL (Reaction: PG+MA+PA, Ration: 2.37:0.76:1, Temp.: 160°C)
In order to investigate the effect of the reflected power in the MARS multi-mode cavity, different
vessel sizes were used, respectively 1 and 2 liter. In figure 31 the AV’s against power consumption
are plotted. Here no difference can be noticed. In figure 32 and 33 the power consumption is plotted,
distributed over the following categories: reflected microwave energy in the cavity, amount of
energy absorbed by the reaction mixture and energy lost to the surrounding as heat. The reflected
microwave energy is much higher for the 1 L vessel then for the 2 L. This makes sense because the
smaller the volume, the more microwaves fail to reach the vessel and are lost in the cavity. It should
- 35 -
be noted that the amount of heat lost to the surrounding is larger for the 1 L vessel; this is because
the surface area to volume ratio of the spherical reactor vessel is larger for the smaller vessel so
relatively more heat is transferred to the outer surface area.
250
Acid Value
[mg KOH/ g]
200
150
100
MW MARS 2L
MW MARS 1L
50
0
0
1
2
3
4
5
6
7
8
Power
[kWh]
Figure 31 AV to Power plot for the MARS experiment in a 2 and 1 liter vessel (Reaction: PG+MA+PA, Ration: 2.37:0.76:1,
Temp.: 200°C)
12,16
15,38
16,68
31,17
56,67
67,95
Reflected
Mixture
Surrounding
Reflected
Figure 32 Power efficiency [%] of MARS 2 liter vessel at 200 °C
- 36 -
Mixture
Surrounding
Figure33 Power efficiency [%] of MARS 1 liter vessel at
200 °C
5. Conclusion and Recommendations
The hypothesis was that microwaves can accelerate reaction rates, increase conversion, improve the
end product quality and quantity and save energy by microwave irradiation of the poly-condensation
reaction of maleic anhydride, phthalic anhydride with propylene glycol. Conversion, reaction time,
molecular weight and polymer structure, the quantity of by-products (i.e. water) and the energy
consumption of different microwave applicators were compared with conventional heating.
The conclusion of this research is that the poly-condensation reaction of maleic and phthalic
anhydrides with propylene glycol is not influenced by microwave irradiation. Compared to
conventional heating there were no significant differences in conversion, reaction time, molecular
weight and amount of by-product obtained. Therefore no reduction in energy consumption was
achieved. However based on quantitative analysis of distillate more light organic components were
found in the distillate by microwave irradiated reactions.
With the use of microwaves it can be more advantageous to control the temperature compared to
conventional heating. This can prevent overheating and thus thermal degradation of the end
product.
The catalysts that were used didn’t show better results, but the catalyst that give the best results for
the reaction of maleic anhydride with propylene glycol was p-toluenesulfonic acid. The triflate
catalysts that were used for this reaction resulted in degradation of the polymer. However for the
reaction of maleic anhydride with hexamethylene glycol the lanthanum triflate catalyst improved the
reaction system.
Analyzes of the polymer structure showed that with microwave irradiation of the reaction of maleic
anhydride with propylene glycol the maleate structured polymer is in favor. However further
research is needed to verify this conclusion.
A recommendation for future research is to investigate in more detail the structure of the molecules
that were created by the isomerization reaction of maleic anhydride with propylene glycol, and to
see if microwaves have an effect on the alignment of the molecules to promote maleate structured
polymers.
In order to investigate the reduction of energy by using silica carbide PHE, more experimental data is
needed. Special attention must be paid to the ratio of the surface area of the PHE to volume of the
reactor vessel. With the application of a larger surface area it is also possible that the reaction rate
can increase.
Furthermore, the microwave frequency can play an important part in energy consumption.
Optimization can improve the interaction of molecules with microwaves in the reaction system.
Higher energy efficiency can be obtained together with a constant and uniform heating of the vessel.
- 37 -
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- 39 -
APPENDIX
- 40 -
Appendix I Reaction Mechanism and Equations of the Poly-esterification
Reaction of Maleic and Phthalic Anhydride with Propylene Glycol
- 41 -
The reaction mechanisms are given below with possible site reactions [1-2, 38, 42].
The abbreviations that are used: product (P), ester bridge (COOR), acid end group (COOH), alcohol
end group (OH)
Reaction mechanism
Overall reaction:
2
Ring opening (both are second order and irreversible)
→
(I.1)
→
(I.2)
→
(I.3)
Esterification (poly- condensation)
*For Phthalic anhydride only reaction (I.4) will occur. The other reactions can be omitted [1]
← →
(I.4)
← →
(I.5)
Isomerization reaction (cis-trans)
← →
(I.6)
← →
(I.7)
Saturation reaction (Ordelt reaction)
← →
(I.8)
- 42 -
← →
(I.9)
← →
(I.10)
← →
(I.11)
Esterification of saturated acid
← →
(I.12)
Reaction equations
The main contribution to the acid catalysis probably comes from maleic acid or phthalic acid because
these are the strongest acids. Denoted as
(
(
(
) (
)
)
)
(I.13)
q=7 [1]
(I.14)
(I.15)
(I.16)
(
)
(I.17)
(
)
(I.18)
(
)
(I.19)
(
)
(I.20)
(
)
(I.21)
(
)
(I.22)
(
)
(I.23)
(
)
(I.24)
(
)
- 43 -
(I.25)
Assume ring opening constants for maleic anhydride are equal (*the ring opening constants of maleic
and phthalic acid are NOT equal) [42]
Assume that all esterification, isomerization and saturation constants are equal [1].
Long reaction times clearly favor the formation of saturated ester groups, which cause branching and
cross linking in the polymer chains.
- 44 -
Appendix II Tables of all Executed Experiments with Corresponding
Reaction Conditions
Figure II.1 Experiments with corresponding reaction conditions of reaction performed with catalysts
Figure II.2 Experiments with corresponding reaction conditions of reaction performed under vacuum
Figure II.3 Experiments with corresponding reaction conditions of reaction performed on large scale
- 45 -
Appendix III 13C NMR Spectra
Figure III.1 Total 13C NMR spectrum of conventional heated reaction (Reaction: PG+MA, Ratio: 1.2:1,
Temp.: 140 °C, time: 2.5 h)
Figure III.2 163 – 171 ppm13C NMR spectrum of conventional heated reaction (Reaction: PG+MA,
Ratio: 1.2:1, Temp.: 140 °C, time: 2.5 h)
Figure III.3125 – 138ppm13C NMR spectrum of conventional heated reaction (Reaction: PG+MA,
Ratio: 1.2:1, Temp.: 140 °C, time: 2.5 h)
Figure III.4 63 – 78 ppm13C NMR spectrum of conventional heated reaction (Reaction: PG+MA, Ratio:
1.2:1, Temp.: 140 °C, time: 2.5 h)
Figure III.5 15 – 21 ppm13C NMR spectrum of conventional heated reaction (Reaction: PG+MA, Ratio:
1.2:1, Temp.: 140 °C, time: 2.5 h)
Figure III.6 Total 13C NMR spectrum of microwave heated reaction (Reaction: PG+MA, Ratio: 1.2:1,
Temp.: 140 °C, time: 2.5 h)
Figure III.7 163 – 171 ppm13C NMR spectrum of microwave heated reaction (Reaction: PG+MA, Ratio:
1.2:1, Temp.: 140 °C, time: 2.5 h)
Figure III.8 125 – 138 ppm13C NMR spectrum of microwave heated reaction (Reaction: PG+MA, Ratio:
1.2:1, Temp.: 140 °C, time: 2.5 h)
Figure III.9 63 – 78 ppm13C NMR spectrum of microwave heated reaction (Reaction: PG+MA, Ratio:
1.2:1, Temp.: 140 °C, time: 2.5 h)
Figure III.10 15 – 21 ppm13C NMR spectrum of microwave heated reaction (Reaction: PG+MA, Ratio:
1.2:1, Temp.: 140 °C, time: 2.5 h)
- 46 -
Appendix IV GC-MS Spectra
Figure IV.1 GC-MS spectrum of conventional heated reaction (Reaction: PG+MA+p-TSA, Ratio: 1.2:1,
Temp.: 140 °C, time: 2.5 h, Cat.: 0.5 mole%). Sample name: 35
Figure IV.2 GC-MS spectrum of microwave heated reaction (Reaction: PG+MA+p-TSA, Ratio: 1.2:1,
Temp.: 140 °C, time: 2.5 h, Cat.: 0.5 mole%). Sample name: 28
- 47 -
Appendix V GC Spectra and Extrapolations
Figure V.1 Extrapolation of GC chromatogram of Aldehyde
Figure V.2 Extrapolation of GC chromatogram of Dioxolane
Figure V.3 Extrapolation of GC chromatogram of Propylene Glycol
Figure V.4 GC chromatogram of distillate obtained (Reaction: PG+MA+PA, Ration: 2.37:0.76:1, Temp.:
200 °C, time: 410 min., Vessel size: 2 L)
Figure V.5 Magnified GC chromatogram of distillate obtained (Reaction: PG+MA+PA, Ration:
2.37:0.76:1, Temp.: 200 °C, time: 410 min., Vessel size: 2 L)
- 48 -
Appendix VI Energy Balance Calculations
Figure VI.1 Cp values of the polymer end product (Reaction: PG+MA+PA, Ration: 2.37:0.76:1, Temp.:
160 °C, time: 410 min., Vessel size: 2 L)
Figure VI.2 Energy balance (Reaction: PG+MA+PA, Ration: 2.37:0.76:1, Temp.: 160 °C, time: 410 min.,
Vessel size: 2 L)
Figure VI.3 Power consumption of MARS and INTL (Reaction: PG+MA+PA, Ration: 2.37:0.76:1, Temp.:
160 °C)
Figure VI.4 Energy balance (Reaction: PG+MA+PA, Ration: 2.37:0.76:1, Temp.: 200 °C, time: 410 min.,
Vessel size: 2 L)
Figure VI.5 Energy balance (Reaction: PG+MA+PA, Ration: 2.37:0.76:1, Temp.: 200 °C, time: 410 min.,
Vessel size: 1 L)
- 49 -
- 50 -
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