Epoxidation of Fatty Acids and Vegetable Oils Assisted by Microwaves

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Epoxidation of fatty acids and vegetable oils assisted by microwaves catalysed by a cation exchange resin Adriana Freites Aguilera, Pasi Tolvanen, Shuyana Heredia, Marta Gonzalez Muñoz, Tina Samson, Adrien Oger, Antoine Verove, Kari Eränen, Sebastien Leveneur, Jyri-Pekka Mikkola, and Tapio Olavi Salmi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05293 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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Industrial & Engineering Chemistry Research

Epoxidation of fatty acids and vegetable oils assisted by microwaves catalysed by a cation exchange resin Adriana Freites Aguileraa, Pasi Tolvanena, Shuyana Herediaa, Marta González Muñoza, Tina Samsonb, Adrien Ogerb, Antoine Veroveb, Kari Eränena, Sebastien Leveneura,b*, Jyri-Pekka Mikkolaa,c, Tapio Salmia. a

Industrial Chemistry & Reaction Engineering, Department of Chemical Engineering, Johan

Gadolin Process Chemistry Centre, Åbo Akademi University, FI-20500,Åbo-Turku, Finland b

Laboratoire de Sécurité des Procédés Chimiques, Institut National des Sciences Appliquées

de Rouen, 76800 Saint-Étienne-du-Rouvray, France. Email: [email protected] c

Technical Chemistry, Department of Chemistry, Chemical-Biological Centre, Umeå

University, SE-90187 Umeå, Sweden

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TOC


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Abstract Epoxidation of oleic acid and cottonseed oil was conducted in a semi-batch reactor with in situ formed percarboxylic acid (parecetic acid or perpropionic acid), using hydrogen peroxide as an oxidizing agent and carboxylic acid (acetic acid or propionic acid) as oxygen carriers. Amberlite IR-120 was implemented as the catalyst. The system comprised of a loop reactor where the mixture was pumped through a single-mode cavity in which microwaves irradiation was introduced. A heat exchanger was integrated to the system to replace microwave heating to compare the results obtained upon microwave vs. conventional heating. The catalyst loading effect was studied, as well as the influence of microwave irradiation and the implementation of the SpinChem® rotating bed reactor (RBR) in hope to decrease the influence of the internal mass transfer. The application of microwave irradiation results in an improvement of the reaction yield in absence of catalyst. Keywords: Vegetable oils, Fatty acids, Epoxidation, Microwaves, Cation Exchange Resins



Highlights *Use of SpinChem® RBR improves the mass transfer for the epoxidation reaction. *In the absence of Amberlite IR-120 (heterogeneous catalyst), microwave irradiation improves the reaction rate compared to conventional heating. *In the presence of Amberlite IR-120, reaction rates under microwave and conventional heating were similar. *Epoxidation by peracetic acid is faster than by perpropionic acid. *Epoxidized cottonseed oil is more stable than epoxidized oleic acid towards ring-opening reactions.


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1. Introduction Extraction, processing and use of fossil fuels and their derivatives are activities which continuously cause irreversible harm to our planet.1-3 The protection of the environment is an issue of major importance and it is one of the biggest challenges of the 21st century. Over the past decades, it has become a top priority to develop environmentally friendly materials which can replace oil and gas as building blocks for chemicals.4 Developing bio-based materials from renewable sources while adopting environmentally friendly technologies is a high priority. Epoxidized oils are exemplary cases for these bio-based components. These chemically modified oils can be used as platform chemicals for developing biolubricants, plasticizers, polyols, glycols and non-isocyanate polyurethanes, among others.5 Epoxidation implies the replacement of a double bond between two carbon atoms by an oxygen atom to form an epoxy/oxirane ring. Epoxidized oils are nontoxic and can be derived from oils extracted from plants, seeds and wood. Plant oils are one of the many renewable biomass resources and they can be used to realize a wide range of components used in food, soap, candles, cosmetic products, drying oils for paint and varnishes as well as components in lubricants and biodiesel.6-7 According to the Statista© portal, the global production of vegetable oils has been constantly increasing for the past decade and during the years 2015-2016, 185.78 million metric tons were produced.8 Vegetable oils are composed of triglycerides which, in turn, consist of three fatty acids attached to a glycerol molecule. For this study, oleic acid (OA) was used as the model compound as a representative vegetable oil. OA represents one of the most common fatty acids present in vegetable oils that can be functionalized into epoxidized oil.



For the production of epoxides from olefinic compounds, different epoxidation methods are available, depending on the nature of reactants and the catalyst. Typical methods are epoxidation with percarboxylic acids, epoxidation with organic and inorganic peroxides, epoxidation with halohydrines and epoxidation with molecular oxygen.5 In this study, epoxidation with percarboxylic acids was selected because of its easy availability, low price, high reactivity with vegetable oils and environmentally friendly production process.5,9-11 Hydrogen peroxide (HP) was selected as the oxidant and acetic acid (AA) or propionic acid (PA) as the oxygen carrier, according to the Prilezhaev principle. Peracetic acid (PAA) and perpropionic acid (PPA) were generated in situ in the reaction environment. The reaction system consists of an aqueous phase and an oil phase. A simplified reaction scheme is provided in Figure 1.


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Figure 1. Simplified mechanism for fatty acid epoxidation with acetic acid assisted by hydrogen peroxide.



In our previous study,12 epoxidation of oleic acid with peracetic acid was performed in the absence of added catalysts. Optimal reactant molar ratios were obtained and the influence of temperature, stirring speed and microwave heating was studied. A clear enhancement of the fatty acid conversion was observed when applying microwave irradiation on the reaction mixture. Moreover, to reduce the reaction times (from 24 hours to 6 hours) and to improve the epoxide yields, an acidic ion exchange resin (Amberlite IR-120) was introduced in this study. The role of the catalyst was to enhance the formation rate of percarboxylic acid significantly. Moreover, to approach the development of cleaner, safer, smaller and more energy efficient processes with products of higher quality, i.e. process intensification,13-16 microwave (MW) technology was used and combined with a special mixing device, SpinChem® Rotating Bed Reactor (RBR). The aim of the new mixing technology is to immobilize the catalyst and to minimize the mass transfer limitations by forced, centrifugal flow through the catalyst bed. This work is focused on finding a method to achieve higher yields of epoxidized oils, i.e. oleic acid and cottonseed oil, using less energy and shorter reaction times by varying the catalyst loading, testing microwave heating and new reactor technology, as well as to investigate the choice of the appropriate oxidant carrier (carboxylic acid).


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2. Material and methods 2.1 Materials Acetic acid (glacial) 100%, aqueous hydrogen peroxide 30%, potassium iodide and Hanus solution were obtained from Merck KGaA (Germany). Sodium hydroxide solution 0.2N, oleic acid 90%, cottonseed oil, tetraethylammonium bromide, sodium thiosulfate solution 0,1M, Chloroform 99% and Amberlite® IR-120 were procured from Sigma Aldrich (Germany, USA, France and China). The resin was flushed gently with distilled water to remove possible loose sulfonic acid groups prior applying to the reactor. This procedure also reduces the risk of discoloration of the epoxidized product. Ammonium cerium sulfate solution 0.1M and perchloric acid in anhydrous acetic acid 0.1M were acquired from VWR Chemicals (Belgium).

Sulfuric acid 95-97% was obtained from Avantor Performance

Materials BV (Belgium). Amberlite IR-120 is a gel type strongly acidic cation exchange resin of the sulfonated polystyrene type. 2.2 Experimental setup The reaction setup was constructed and it was operated as a batch system. The system comprised of a glass unbaffled reactor connected to a loop system with a microwave cavity (MW) and a heat exchanger. The reaction mixture was pumped from the reactor through the microwave cavity (in which the reaction mixture was irradiated) and recycled back to the reactor vessel. A heat exchanger was incorporated to the loop to be used as the energy source in experiments with conventional heating (CH); both equipments (MW cavity and heat exchanger) remained fixed to the system even when they were not used, in order to maintain the experimental setup unchanged. While the MW irradiation was applied, the heat exchanger remained off and vice versa. In this system, the MW frequency was fixed to 2.45 GHz to avoid interfering with radar and telecommunication activities; most domestic and commercial



microwave instruments operate at this frequency.17 The reaction medium flowed through a transparent glass tube, which allows microwaves to heat the reaction medium without any interference. The reactor vessel was surrounded by a jacket filled with a mixture of ethylene glycol for cooling or heating to control and maintain the reactor temperature constant. Stirring played a key role in the experiments, given that it enabled the generation of an emulsion in which the reaction took place. The Spinchem® RBR was placed inside the reactor, as represented in Figure 2A. This device consists of a hollow rotating bed device which contains the immobilized catalyst inside and works as the stirring element itself. In this mixing system, the reaction solution was suctioned from the bottom of the bed, percolated through the solid catalyst and was returned to the storage vessel. The design of SpinChem® RBR maximizes both axial mixing and convective heat transport.18


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A

B

Figure 2A. Schematic view of the SpinChem® (Figure reprinted with permission from ref 19, copyright 2017, American Chemical Society, Washington, DC) and more information can be found on SpinChem Technology website 20 Figure 2B. Schematic view of the reactor setup.



For experiments carried out without the SpinChem® RBR, a six-blade radial stirrer connected to a turbine engine was implemented. A stirring rate of 1200rpm was used. A peristaltic pump was used to pump the mixture through the system to avoid a contact with metal materials that can trigger catalytic decomposition of hydrogen peroxide and peracetic acid.21 In addition, a reflux condenser was incorporated to prevent evaporation of volatile compounds, i.e., acetic acid. Thermocouples were located inside the reactor, before the preheater, after the preheater, before the microwave and after the microwave zone. The temperatures were displayed and recoded by the software Picolog®. The schematic configuration of the reactor system is displayed in Figure 2B.


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2.3 Reaction procedure Calculated amounts of the oil reactant (oleic acid or cottonseed oil) and carboxylic acid (acetic acid or propionic acid) were added into the reactor vessel under vigorous stirring to homogenize the mixture. To prevent a rapid reaction, hydrogen peroxide at room temperature was added last into the mixture. The moment when the first droplet of HP reached the mixture was considered as the time zero. The addition of hydrogen peroxide can be considered fast (less than one minute). After the last droplet of HP, the peristaltic pump was switched on and the mixture started to circulate through the loop system. The heating jacket was turned on to maintain the operation temperature at the desired level in the reactor. When applying conventional heating, the heat exchanger was switched on at least ten minutes before the experiment to achieve the anticipated reactor temperature. In case of using microwave heating, radiation and security measures were activated after the addition of HP. The temperature throughout the entire experiment was monitored with the software Picolog®. The temperatures throughout the loop system were maintained at the same levels for both cases: conventional and microwave heating. Samples (7 mL) were withdrawn from the reaction mixture by a plastic syringe into glass flasks (8 mL). After the experiment was completed and the last sample was withdrawn, the reactor was emptied and the system was flushed with water and ethanol until no oil residuals were

found.



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Table 1. Experimental matrix Carboxylic

Oil phase

DB:HP:CA

Catalyst load

SpinChem

T (°C)

Heating

acid

Additional water (mL)

AA

OA

1:2.3:2.4

12 %

X

40

CH

-

AA

OA

1:2.3:2.4

12 %

X

40

MW

-

AA

CSO

1:2.3:2.4

12 %

X

40

CH

-

AA

CSO

1:2.3:2.4

12 %

X

40

MW

-

AA

OA

1:2.3:2.5

-

X

40

MW

-

AA

OA

1:2.3:2.4

-

X

40

CH

-

AA

OA

1:2.3:2.4

3%

X

40

CH

-

PA

OA

1:2.3:2.4

-

X

40

CH

-

PA

OA

1:2.3:2.4

3%

X

40

CH

-

PA

OA

1:2.3:2.4

12 %

X

40

CH

-

AA

OA

1:2.3:2.4

12 %

X

40

CH

200

AA

OA

1:2.3:2.4

12 %

X

40

MW

200

AA

OA

1:2.3:2.4

12 %

X

40

CH

120

AA

OA

1:2.3:2.4

12 %

X

40

MW

120

AA

OA

1 : 4,8 : 2,4

12 %

x

50

CH

-

AA

OA

1 : 4,8 : 2,4

6%

x

50

CH

-

AA

OA

2 : 4,8 : 2,4

18 %

x

50

CH

-

AA

OA

1 : 4,8 : 2,4

-

-

50

CH

-

AA

OA

1 : 4,8 : 2,4

12 %

-

50

CH

-

AA

OA

1 : 4,8 : 2,4

-

X

50

CH

-

AA

OA

1 : 4,8 : 2,4

6%

X

50

CH

-

AA

OA

1 : 4,8 : 2,4

6%

X

50

MW

-

AA

OA

1 : 4,8 : 2,4

4%

X

50

CH

-

AA

OA

1 : 4,8 : 2,4

4%

X

50

MW

-

AA

OA

1 : 4,8 : 2,4

12 %

X

50

CH

-

All experiments were performed under the same stirring and pumping speed, 1200 rpm and a hydrogen peroxide flow-rate of 120 mL/min respectively.


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2.4 Experimental matrix The experimental matrix is displayed in Table 1. The number of the experiment, followed by the carboxylic acid used, either acetic acid (AA) or propionic acid (PA), respectively are listed in the table. Column two refers to the main reactant oleic acid (OA) or cottonseed oil (CSO), respectively. The molar ratio of the reactants is indicated, where DB denotes double bonds, CA is carboxylic acid and HP hydrogen peroxide, respectively. The catalyst loading on a dry basis, with respect to the oil mass is shown in the fourth column. ‘X’ informs whether the experiment was performed with the rotating bed reactor or with the six-blade conventional stirrer, “-“. The sixth and seventh columns give the reactor temperature and the type of heating applied, conventional heating (CH) or microwaves (MW). The last column indicates whether any additional water was added to the system to increase the aqueous-to-oil phase volume ratio. The change of the reactant ratio from experiment 14 onwards was made in order to implement optimal conditions found in our previous study.12 2.5 Analytical methods After being withdrawn, the samples were immediately cooled down under cold tap water and the phases were separated with a Pasteur pipette. The organic phase was washed with distilled water at least three times, to remove all the aqueous phase and to prevent any further reactions. To remove aqueous residuals in the organic phase, a small amount of molecular sieve beads (Sigma 5 A, 8-12 mesh) was added. The samples from the organic phase were stored in a freezer to be analysed within next days, whereas the aqueous-phase samples were analysed immediately after being withdrawn from the reactor.



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Aqueous-phase analysis To avoid the decomposition of hydrogen peroxide and peracetic acid, the samples were analysed within 30 minutes after being withdrawn from the reactor. The Greenspan and MacKellar method (1948)22 was applied for the analysis of hydrogen peroxide, peracetic acid and perpropionic acid, respectively. For the determination of acetic acid and propionic acid, an automatic titrator with a 0.2M sodium hydroxide solution was used. Organic-phase analysis The organic phase of each sample was immediately stored in a refrigerator after being withdrawn. For the analysis, samples were heated in an oven at 70°C just until they melted and had a high enough fluidity to be handled with a pipette. The amount of double bonds was described as Iodine Value (IV) and epoxy groups were expressed as relative fractional conversion of oxirane (RCO). The IV was determined with Hanus solution (Lubrizol, 2006) and the oxirane content was determined with Jay’s method (1964),23 in which HBr is formed in situ by a reaction of tetraethylammonium bromide (TEAB) and titrated with standard 0.1M perchloric acid solution. The oxirane oxygen (OO) content was determined with the following equation: (1)


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From the oxirane content values, the relative fractional conversion of oxirane (RCO) was estimated from the expression below: (2)

where

is the experimentally determined content of oxirane oxygen, and

is the

theoretical maximum oxirane oxygen content in 100 g of oil, which was determined to be 5.61% for oleic acid and 6.62% for cottonseed oil from the following expression: (3)

where

is the molar weight of iodine, 126.9 g/mol,

16.0 g/mol, and

is the molar weight of oxygen,

is the initial iodine value of oil sample.

Eq 2 also represents the selectivity of oxirane groups with respect to double bond groups.



3. Results and discussion 3.1 Catalyst loading The effect of applying a solid-acid catalyst (Amberlite IR-120) on the epoxidation of oleic acid was investigated by using four different catalyst amounts: 0wt-%, 6wt-%, 12wt-% and 18wt-%, respectively, loading on a dry basis. The catalyst load was restricted by the size of the SpinChem® chamber, where the maximum capacity of the catalyst was 20 g. Figure 3 shows the evolution of iodine value and RCO% versus time at different catalyst loading. The experiments were carried out at 50°C, at a molar ratio of OA:AA:HP = 1:2.4:4.8, at a stirring rate of 1200 rpm and at a recirculation pumping speed of 120 mL/min. The selection of the catalyst Amberlite IR-120 was based on the fact that it accelerates the rate determining step of the reaction, which is the formation of the percarboxylic acid through perhydrolysis.24-26 Amberlite IR-120 is a gel-type cation exchange resin with a styrenedivinyl benzene matrix, on which the sulphonic groups are attached. The bead-shaped resin particles have a diameter of 620-830 µm. According to Musante et. al (2000),27 only the small carboxylic acid molecules can enter into its gel-type structure, while the bulky epoxidized vegetable oil molecules are left outside. Thus, the oxirane ring can be protected from the attack of protons which are confined inside the gel matrix and, as a result, a further ring decomposition is prevented. In these experiments, the concentration of hydrogen peroxide (HP) decreased linearly during the reaction time. The content of HP diminished faster as the catalyst amount was increased. The concentration of acetic acid seems to remain constant, confirming the fact that is regenerated in the organic phase upon epoxidation and transferred back to the aqueous phase (Figure 1).


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As expected, an increase in the catalyst amount resulted in an increase in the rate of the in situ peracetic acid formation. In the experiments conducted over 0wt-% and 6wt-% Amberlite, the peracetic acid content remained rather constant under 1wt-% during the course of the reaction, indicating that it is generated in the perhydrolysis step and consumed by the epoxidation step. However, for the experiment with 18wt-% Amberlite, it was observed that as complete conversion of oleic acid into epoxidized oleic acid was approached, the peracetic acid content started to increase and accumulate in the aqueous phase. This is due to the fact that the consumption of PAA for the epoxidation is decreasing with less available double bonds in the oil. Similarly, accumulation of PAA was also detected in the experiment with 12wt-% catalyst load. The concentrations of acetic acid, peracetic acid and hydrogen peroxide as a function of the reaction time can be found in the supporting information (Figures S1-S3). The iodine value decreased as the double bonds of the oleic acid were consumed. The results shown in Figure 3 revealed that the conversion of oleic acid becomes faster with an increasing catalyst amount. The formation of the epoxide product during the reaction time is illustrated in Figure 3, too.



Figure 3. Iodine value and relative percentage conversion to oxirane for four different catalyst loads, at 50°C.


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The oxirane formation rate became faster with an increasing catalyst amount. Epoxidation seemed to proceed more rapidly than ring opening, given that for all the experiments the tendency to increase the epoxy content was maintained. Previous studies revealed that acetic acid boosts ring-opening reactions, which was observed by the degradation of oxirane product with time and acetic acid consumption.12,28 In this case,the acetic acid concentration remained constant throughout the reaction (Figure S1), which confirms that it was not consumed in the ring-opening reactions. The results revealed that the catalyst amount affected both the conversion of the double bonds and the formation of the desired product. When comparing the results with the non-catalytic experiment, the efficiency of the heterogeneous catalyst was demonstrated by enhancement of the reaction rate; i.e. a higher yield was obtained within a shorter reaction time. 3.2 Rotating bed reactor First, two experiments were performed without solid catalyst at 1200rpm and 50°C. A sixblade stirrer was used in one experiment and the SpinChem® RBR was tested in the other one. Figure 4 reveals that the conversion of double bonds behaved very similarly at the beginning of the experiment but at the end, a higher conversion was achieved for the experiment by using the conventional stirrer. In terms of selectivity (eq 2), the formation of the oxirane groups with time behaved almost the same in both experiments.



Figure 4. Iodine value and relative percentage conversion to oxirane using SpinChem RBR and a conventional stirrer with no catalyst, at 50°C.


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A similar experiment was performed but using a 12wt-% loading of Amberlite IR-120. An experiment was carried out by using the conventional stirrer and the Amberlite floating freely in the reaction media, while another one was performed by using the SpinChem® RBR with the Amberlite placed inside the stirrer (Figure 2A). The kinetics of epoxidation for these two stirring system was similar (Figure 5).

Figure 5. Iodine value and relative percentage conversion to oxirane using SpinChem RBR and a conventional stirrer with 12wt-% catalyst loading, at 50°C.



In the absence of catalyst (Figure 4) and in the presence of catalyst (Figure 5), kinetics of epoxidation system, including conversion of double bonds and formation of oxirane groups, are similar by using conventional stirrer and SpinChem® RBR system. Moreover, the series of experiments performed for testing different catalyst loadings were conducted while implementing the RBR.


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Figure 6. Conversion of double bonds in six hours at 50°C at 1200rpm, with the molar ratio 1:4.8:2.4 OA:HP:AA, as a function of the catalyst mass.



Figure 6, experiments carried out at 50°C with the molar ratios 1:4.8:2.4 OA:HP:AA, reveals that the conversion of the double bonds increases with an approximate linear tendency with the catalyst amount (R2 is close to 1) within the catalyst mass range 0-21 gram. A linear dependency may indicate the absence of external mass transfer limitations. Moreover, the small deviation from a linear behaviour can be attributed to the fact that with the increase of the catalyst mass, the catalyst placed inside the SpinChem chamber is more compacted and the mass transfer is slightly influenced. In our previous study,12 it was found that the stirring speeds exceeding 450rpm would suppress the mass transfer limitations. This observation, in addition to the approximate linear behaviour of the double bond conversion with the catalyst amount, indicates that when the Spinchem® RBR is operated at a minimum stirring speed of 1200rpm, the reaction takes place in the kinetic regime, which means the reaction rate is governed by the rate limiting steps of the chemical reaction rather that the mass transfer.29 Experiments with the SpinChem under stirring speeds lower than 1200rpm could be performed to determine if the kinetic regime can be achieved at lower speeds. Additionally, the use of the Spinchem® RBR streamlines the experimental procedure, because no filtrations are needed at the end of the reaction to separate the solid catalyst from the liquid medium.30 In addition, the use of RBR might extend the lifespan of the catalyst, due to the fact that no mechanical wear of the particles occurs while upon classical stirred batch operations the particles are pushed against the reactor walls, caused by centrifuge forces of the stirring ( SpinChem Technology).20


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3.3 Microwave vs conventional heating In our previous study, it was found that the significant differences of the yields obtained in the case of conventional vs. microwave heating was due to the increment of the aqueous-tooil phase ratio and the high temperature in the cavity, followed by cooling in the reactor.12 For this reason, it was decided to conduct experiments with MW and CH with exact same temperature profiles throughout the reactor system. In this way, removing the heat from the system would be avoided31 and CH and MW could be compared under similar experimental conditions. The influence of the aqueous-to-oil phase ratio was studied later on (see water-tooil ratio section). Initially the experiments were carried out without any catalyst. The results shown in Figure 7 revealed higher reaction yields when applying MW and lower conversion of double bonds, which is an indication of supressed ring-opening reactions.



Figure 7. Iodine value and relative percentage conversion to oxirane for microwave and conventional heating in the absence of catalyst at 50°C.


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These results support the hypothesis of the selective heating effect of microwave heating:31 the aqueous phase acts as a MW-absorbent substance while the oil phase is transparent to microwaves.10,12 A higher temperature gradient between the phases is achieved in comparison to conventional heating, boosting reactions in the aqueous phase and possibly causing an improvement of the interfacial mass transfer, hence leading to an overall enhancement of the epoxidation rate. Additionally, as the temperature of the organic phase is lower than that of the aqueous phase, the solubility of acetic acid in the oil is lower. This decreases the probability of ring opening reactions which are well known to be triggered by higher temperatures and higher acid (proton) concentrations.28,32-33 Subsequently, new experiments were conducted in the presence of Amberlite IR-120 and with different catalyst loadings by duplicate, using conventional heating and microwave heating, respectively. The evolution of iodine value and epoxide content is depicted in Figs. 8A and 8B for 4wt-% and 12wt-% catalyst loadings, respectively.




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Figure 8A. Iodine value and relative percentage conversion to oxirane for microwave and conventional heating with 4wt-% catalyst load at 50°C. Figure 8B. Down: Iodine value and relative percentage conversion to oxirane for microwave and conventional heating with 12wt% catalyst load at 50°C.



For both cases, experiments performed with microwave heating resulted in very similar conversions and yields as with conventional heating. Same behaviour was observed for 6% catalyst load (Figure S5). Even though the yields upon non-catalytic epoxidation of oleic acid seemed to improve when microwave heating was applied, this does not seem to be the case when applying Amberlite IR-120. It was suggested by Chou and Chang (1986)24 that when performing epoxidation of oils, the slow step of the reaction is the perhydrolysis. In the case of microwave heating, its enhancing effect was attributed to the so-called selective heating, which produces an increase in the temperature of the aqueous phase, thus boosting the production of PAA. Moreover, in the case of Amberlite IR-120, it has been demonstrated by some authors34-35 that cation exchange resins have the ability to accelerate the rate of perhydrolysis. Thus, it can be inferred that the enhancing effect of the microwave heating observed previously for non-catalytic epoxidation, is now outran by the effect of the catalyst. 3.4 Acetic acid vs propionic acid Epoxidation of vegetable oils and free fatty acids is usually performed by using performic or peracetic acid. Performic acid is less stable than peracetic acid. Performic acid can undergo exothermic secondary reactions leading to gaseous products. By increasing the carbon chain of the percarboxylic acid, the risk of thermal runaway is diminished. Recently, it was demonstrated that the use of perpropionic acid for the epoxidation of vegetable oils and free fatty acids is thermally safer than the use of peracetic or performic acid.36


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The effect of carboxylic acid (acetic acid and propionic acid) on the epoxidation of oleic acid was investigated while using three different catalyst amounts: no catalyst, 3wt-% and 12wt-%, respectively. The catalyst load was restrained by the SpinChem® RBR chamber. The conversion of double bonds over time for epoxidation with several catalyst loadings is presented in Figure 9.

Figure 9. Conversion of double bonds in percentage for acetic acid and propionic acid with no catalyst, 6wt-% and 12wt-% loading, respectively at 40°C.



In all the above cases, epoxidation in the presence of acetic acid resulted in a more rapid reaction and higher conversion compared to propionic acid. This evidences how acetic acid works as a more efficient oxygen carrier than propionic acid. According to Leveneur et al. (2009),37 for the perhydrolysis reaction catalysed by Amberlite IR-120, the apparent rate constant of the reaction with acetic acid is higher than that of propionic acid. Moreover, in a review article written by our group,38 it was proven that perpropionic acid is less reactive than peracetic acid. In a third study by Leveneur et al.,10 it was corroborated that in the case of epoxidation of oleic acid under MW and CH, higher yields were achieved in case of acetic acid compared to propionic acid.

3.5 Cottonseed oil vs oleic acid Epoxidation of cottonseed oil was conducted in order to move forwards from the model compound, oleic acid, to a plant oil composed mainly by triglycerides. Cottonseed oil represents a low cost and highly available stock of biomass. Moreover, cottonseed oil is rich in unsaturated fatty acids that are suitable for epoxidation.5 The composition of cottonseed oil is described in Table 2.39


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Table 2. Fatty acid constituents present in cottonseed oil. Fatty acid

% Composition

Myristic (14:0)

0.8

Palmitic (16:0)

24.4

Palmitoleic (16:1)

0.4

Stearic (18:0)

2.2

Oleic (18: 1)

17.2

Linoleic (18:2)

55.0

Linolenic (18:3)

0.3



A comparison between cottonseed oil and oleic acid epoxidation under microwave and conventional heating was done. The theoretical iodine value for cottonseed oil (CSO) is in the range of 105-112.40-43 Applying the same method as described in the experimental section, the IV from fresh oil from the bottle used in the experiments was determined to be 112. The consumption of the double bonds represented by the iodine value and the formation of the epoxide product during the reaction time are displayed in Figs. 10A and 10B, respectively.


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Figure 10A. Iodine value with time for oleic acid and cottonseed oil with MW and CH at 40°C. Figure 10B. Relative percentage conversion to oxirane with time for oleic acid and cottonseed oil with MW and CH at 40°C.



The iodine values decreased almost linearly with time (Figure 10). In case of oleic acid and cottonseed oil epoxidation, the conversion of double bonds proceeded almost in parallel. However, the conversion rate of double bond for oil is faster than that for oleic acid at the end of the reaction, due to the lowest iodine value for oleic acid compared to cottonseed oil. No signs of side reactions appeared when using cottonseed oil as a reactant; the epoxides formed seemed to be more stable than the ones formed from oleic acid. The formation of the cottonseed oil epoxide follows an upward linear trend until a maximum conversion at the end of the reaction time of 72.6% under conventional heating and 71.5% under microwave radiation. For oleic acid, a maximum RCO of 39.8% was reached under conventional heating and 51.2% under microwave irradiation. These values are significantly lower in comparison to the cottonseed oil conversion. It can be concluded that in the case of cottonseed oil it is easier to reach a maximum epoxide yield in comparison to oleic acid. Another observation of the cottonseed oil is that it gave rise to a minor apparent viscosity change during the reaction time. For the experiments with oleic acid, the viscosity of the organic phase visually increased notably during the reaction. Furthermore, the OA epoxidation samples were partially solidified at room temperature, a fact that has been attributed by other authors to the formation of ring opening products and in some cases dimers and trimers.44-46 In the case of CSO, it is probable the steric effects of the triglyceride complex which help in diminishing the polymerization reactions. This gives an evidence of a higher stability of cottonseed oil epoxides compared to oleic acid epoxide counterparts.


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3.6 Water-to-oil ratio Previously, when performing experiments under MW heating, increasing the aqueous-to-oil phase ratio seemed to have an improving effect on the epoxidation of oleic acid (Aguilera, 2016).12 According to Leveneur et al.,10 microwave exposure gives rise to an increased reaction rate when the continuous phase is the aqueous one.9 In this section, the influence of the aqueous-to-oil phase volume ratio was studied. Amberlite IR-120 was used as the catalyst. Six experiments were carried out, three under conventional heating and three under microwave irradiation. A set with a maximum water loading (The sign “+++” in Figure 11 means addition of 200mL of water), the second one with medium water loading (The sign “++” in Figure 11 means addition of 120mL of water) and the last one without any additional water (The sign “+” in Figure 11 means no addition of water), just including the water content present in the H2O2 30wt-% solution. Variations of the iodine values and epoxy-oxygen contents with time for epoxidation of oleic acid with peracetic acid and with different amounts of water are displayed in Figures 11A and 11B.



Figure 11A. Iodine value with time with MW and CH for three different volumes of additional water at 40°C for the epoxidation of oleic acid by peracetic acid. Figure 11B. Relative percentage conversion to oxirane with time with MW and CH for three different volumes of additional water at 40°C for the epoxidation of oleic acid by peracetic acid.


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The iodine value decreased linearly for all the cases except at the medium water loading with CH (++) (Figure 11), for which the gradient was more pronounced; only in this case the iodine value decreased to zero, which corresponds to the complete conversion of the double bonds. The reaction of the double bonds was slowest at the maximum dilution. Figure 11B indicates that the ring-opening reactions is more pronounced for the experiment at the maximum water loading and under microwave irradiation. Indeed, after 200 minutes of reaction the selectivity reached the value of 40% and after 500 minutes the selectivity decreased to 20%. This could be due to the fact that at high water content, MW absorption of the aqueous phase is higher than for lower water content; a higher temperature is then achieved, which can influence in the promotion of ring opening reactions.12 In both microwave heating and conventional heating experiments, the best yields were obtained in case of medium water content. MW irradiation seemed to promote epoxidation and prevent ring opening in all the experiments, except when the aqueous phase was much larger than the organic one (water +++), in which case a high degree of ring degradation was observed. In accordance to the work of Leveneur et al.,10 higher yields were achieved when using MW irradiation and higher aqueous-to-oil phase volume ratio. However, this tendency reverts when there is too much water in excess, i.e. experiments with the highest amount of water (+++). This can be attributed to the fact that when diminishing the concentration of the components in the aqueous phase, namely the peracid, the concentration in the organic phase also decreases, according to the partition coefficients.47



4. Conclusions Epoxidation of oleic acid and cottonseed oil was carried out with in situ formed percarboxylic acid, using hydrogen peroxide as the oxidizing agent under conventional and microwave heating. The use of the special mixing device, SpinChem® RBR was beneficial in terms of eliminating the mass transfer limitations, it enabled a simpler collection and recycling of the catalyst and minimised the mechanical wear of the solid catalyst, thus possibly prolonging the catalyst lifetime. Regarding the application of microwave irradiation, it improved the reaction yield in the absence of the catalyst. However, in the presence of Amberlite IR-120, the effect of the catalyst strongly overlapped the microwave effect. It was noticed that acetic acid was more efficient than propionic acid for the epoxidation. When comparing the model compound, oleic acid, with cottonseed oil, it was concluded that the cottonseed oil epoxide is more stable and better yields were achieved.


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ASSOCIATED CONTENT Supporting Information. Supporting information consists of additional information on: Effect of catalyst loading on kinetics and Comparison of microwave versus conventional heating on kinetics.



AUTHOR INFORMATION Corresponding Author Sébastien Leveneur*a,b 1

Normandie Univ, INSA Rouen, UNIROUEN, LSPC, EA4704, 76000 Rouen, France, E-mail

: [email protected] a

Industrial Chemistry & Reaction Engineering, Department of Chemical Engineering, Johan

Gadolin Process Chemistry Centre, Åbo Akademi University, FI-20500,Åbo-Turku, Finland b

Laboratoire de Sécurité des Procédés Chimiques, Institut National des Sciences Appliquées

de Rouen, 76800 Saint-Étienne-du-Rouvray, France. Email: [email protected]


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ACKNOWLEDGMENTS Financial support from Fortum Foundation and Åbo Akademi Forskningsinstitut is gratefully acknowledged as well as Spinchem AB for their co-operation with our research by donating their devices. This work is part of the activities at the Johan Gadolin Process Chemistry Centre (PCC), a centre of excellence financed by Åbo Akademi. The Bio4Energy programme (Sweden) is gratefully acknowledged.



ABBREVIATIONS Acetic acid CH

Conventional heating

CA

Carboxylic acid

CSO

Cottonseed oil

DB

Double bond

EP

Epoxyoleic acid Hydrogen peroxide

IV0

Initial iodine value

m

Mass, g Molar mass, g/mol

MW

Microwaves

OA

Oleic acid Experimentally determined oxirane oxygen, mol/100 g oil Theoretical maximum oxirane oxygen content, mol/100 g oil Propionic acid Peracetic acid Perpropionic acid

RBR

Rotating Bed Reactor

RCO%

Relative conversion to oxirane Temperature, °C Volume, L Weight percent

ρ

Density g/mL


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[]

Concentration, mol/L



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Epoxidation of Fatty Acids and Vegetable Oils Assisted by Microwaves

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