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Effects of Reaction Parameters on the Glycidyl Etherification of Bark Extractives during Bio-epoxy Resin Synthesis Pei-Yu Kuo, Luizmar de Assis Barros, Mohini Sain, Jimi S.Y. Tjong, and Ning Yan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.5b01098 • Publication Date (Web): 21 Jan 2016 Downloaded from http://pubs.acs.org on February 1, 2016

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ACS Sustainable Chemistry & Engineering

Effects of Reaction Parameters on the Glycidyl Etherification of Bark Extractives during Bioepoxy Resin Synthesis Pei-Yu Kuo[a], Luizmar de Assis Barros[a,b], Mohini Sain[a,c,d], Jimi S.Y. Tjong[a] and Ning Yan[a,c]* a

Faculty of Forestry, University of Toronto, 33 Willcocks Street, Toronto, ON, Canada, M5S 3B3

b

Department of Wood Chemistry, University Federal Rural Do Rio de Janeiro. Rodovia BR 465-Km7 Campus, Universitário, Seropédica RJ, Brazil, 23851-970

c

Department of Chemical Engineering and Applied Chemistry, University of Toronto, 200 College Street, Toronto, ON, Canada, M5S 3E5

d

Centre of Advanced Chemistry, King Abdulaziz University, Jeddah, 21589, Kingdom of Saudi Arabia

*Corresponding author: (NY) Tel.: + 1(416)946-8070, E-mail: [email protected], Fax: + 1 (416) 978-3834

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KEYWORDS: Bio-based Epoxy; Bark Extractives; Glycidylation; Epichlorohydrin; Reaction Parameters; ABSTRACT: This study outlines the influence of a series of reaction conditions on the yield and reactivity of the glycidyl etherification reaction of the bark extractive-based bioepoxy monomer (E-epoxy). To maximize the yield and epoxy content, the glycidylation reaction was examined with various substrates, solvents, catalysts, time periods, reaction temperatures, and sodium hydroxide/hydroxyl (NaOH/OHV) ratios. Spray-dried bark extractives were used as substrates due to their higher hydroxyl group content and lower molecular weight compared to the oven-dried bark extractives. A dioxane/water combination was selected from among four solvents based on the yield and epoxy equivalent weight of the final product, and tetrabutylammonium hydroxide was chosen as a ring-opening catalyst due to its effect of suppressing hydrolysis. Furthermore, a response surface methodology was applied to find the optimal reaction time, reaction temperature and NaOH/OHV ratio of the E-epoxy monomer. The maximum extent of conversion with minimum epoxy equivalent weight was achieved after 4.5 hours with NaOH/OHV ratios of 3.4 at 80 °C. This work identifies the effects of reaction parameters on the yield and reactivity of E-epoxy and sheds new light on the glycidylation reaction between epichlorohydrin and renewable biomass.

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Introduction Recent developments in bio-thermoset polymers suggest that renewable resources can partially or fully replace petroleum-based polymers with comparable properties at reasonable costs and with lower ecological impact1-3. Among various types of biothermoset polymers, bio-epoxy resins attract considerable attentions from both academia and industry since the cost of conventional epoxy resins is often higher than other thermosets such as polyester and vinylesters. In addition, these environmentally friendly bio-polymers can reduce the usage of bisphenol A (BPA), which is the compound used to synthesize 90% of the world's production of petroleum-based epoxy resins3. The health concerns of BPA remain a subject for debate4-5, however, there is a strong interest to replace BPA with natural resources. Various renewable materials have been explored, including liquefied wood6-7, cashew nutshell liquid8, lignin9-11, furan12-13, vanillin14-15 flavonoids16-17, and tree bark18. Although many studies have reported promising results, there remains little systematic research on synthetic routes19 for these bio-based epoxy resins. Currently, the most common synthesis method of phenol-type epoxy is to etherify the hydroxyl groups using epichlorohydrin (ECH) as shown in Scheme 1. The mechanism as a whole is an SN2 reaction, and is known as the glycidyl etherification synthesis. Phenolic hydroxyl groups act as nucleophiles to attack the alpha carbon on ECH. In contrast to BPA, the natural phenolic hydroxyl groups have asymmetric structures and different acidity on their hydroxyl groups, which results in their own distinctive reactivity. For example, in a standard synthetic condition, BPA usually has 85-95% yield20, which is higher than gallic 3



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acid (68%)17 or catechin (48%)21. Furthermore, these bio-derived epoxy monomers may have their epoxy content decreased along with their benxodioxane derivatives19. For these reasons, it is important to select suitable reaction conditions for each bio-based phenolic compound for epoxy resin synthesis. Factors that can influence epoxy synthesis can be very diverse. Some key parameters are related to substrate and solvent type, catalyst type and amount, and reaction time and temperature. Bark extractives are used as the substrate for this study since our previous research18 has shown that bark extractives can be a good candidate to replace BPA. Two types of substrates with various hydroxyl values (OHV) and molecular weights were prepared using different drying techniques. For a typical glycidylation reaction, a wide range of solvents have been reported for both single-phase systems20, 22-28, and biphasic solvent systems20,14. The effects of solvent on the yield and reactivity could be further linked to its solute solubility and pKa value. Another important factor must be considered in the selection process is the environment, health, and safety impacts (EHS) of the solvents. A biphasic solvent system generally requires phase transfer catalysts (PTCs) to accelerate the ring-opening reaction for producing an epoxy monomer with low molecular weight, narrow polydispersity, high yield, and elevated purity19. To subsequently close the ring, sodium hydroxide (NaOH) is added to remove hydrogen and chlorine. Since the price of PTCs is higher than NaOH, the other long-established option to catalyze the reaction is adding twofold NaOH, which is commonly practiced in industrial scale synthesis. In addition, it is well known that numerical response variables (such as reaction time and temperature) should not be investigated independently. Interactions may exist among the variables and can render such univariate investigations meaningless. Response surface 4


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methodology (RSM) is one of the most widely used methods to optimize the multiple process parameters. According to the literature, the temperature was varied from 25-100 °C20, 22 for the reaction liquid-type epoxy monomers, the reaction time ranged from 0.5 to 20 hours at reflux, or 24 to 26 hours at room temperature3, 29, and the molar ratio between NaOH/OHV was in the range of 0.12 to 1020, 24, 30. The purpose of this study is to further investigate the reaction of extractive-based bioepoxy monomer (E-epoxy) and to identify an optimized set of synthetic conditions for Eepoxy monomer with maximized yield and reactivity. Six parameters (substrates, solvents, catalysts, time periods, temperatures, and NaOH/OHV ratios) were examined in this study. Two types of extractives (spray-dried and oven-dried) were compared to observe the effect of OHV and molecular weight on the yield and reactivity of E-epoxy. Four types of solvent systems (water/dichloromethane, methanol, dimethylformamide, dimethyl sulfoxide, and water/dioxane) were selected, and their target product structures were studied using nuclear magnetic resonance (13C NMR). Tetrabutylammonium hydroxide (TBAH) was used as a PTC instead of the twofold alkaline addition, and the synthesized products were analyzed using Fourier Transform Infrared Spectroscopy (FTIR) and 1H-NMR. In addition, three sets of reaction variables (2-10 hour reaction time, 60-100 °C reaction temperatures and 1.4-4.2 NaOH/OHV molar ratios) for glycidylation reaction were optimized based on the yield and reactivity of the resulting E-epoxy.

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Scheme 1 Mechanism of glycidyl etherification between phenolic compounds and ECH

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3,

29, 31

Experimental Section Materials Bark chips of mountain pine beetle infested lodgepole pine (Pinus contorta var. latifolia) were provided by FPInnovations. Epichlorohydrin (99%), CDCl3, tetrabutylammonium hydroxide (1.0 M in methanol), 2-Chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane (TMDP), chromium(III) acetylacetonate, cholesterol, phthalic anhydride, crystal violet, potassium acid phthalate (primary standard grade), tetraethylammonium bromide (anhydrous crystals), and perchloric acid (0.1 N in acetic acid) were purchased from Sigma-Aldrich, ON, Canada. NaOH (pellet), acetone (>99.5%), methanol (>99.8%), dioxane (99%), pyridine (99.8%), glacial acetic acid (>99.5%), dimethyl sulfoxide (reagent grade), and chloroform (reagent grade) were supplied by Caledon Laboratory Chemicals, ON, Canada. All chemicals were used as received without further purification. Methods Extraction Procedure

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Bark chips (400 g) were immersed in 1% (w/w) NaOH in water with a 1:10 (solid:liquid) ratio for 2 hours at 90 °C. Bark extractives were collected after filtration and the liquid was dried in an oven at 103 °C for 24 hours or by using a spray-drier operating in an inlet temperature of 150 °C and an outlet temperature of 80 °C with an air pressure set at 0.1 MPa. Synthesis Procedure The synthetic route followed has been reported in detail in our previous research18 and was adapted as circumstances required. Briefly, extractives were mixed with epichlorohydrin, solvent and TBAH in a three-neck round-bottom flask and the temperature of the mixture was raised to the reaction temperature between 40-100 °C depending on each experimental run. NaOH (pellets) was added slowly to the solution to catalyze the reaction. The synthesis proceeded for between 2 and 10 hours in a nitrogen atmosphere. The replication of each experimental condition is three Refining Procedure The reaction products were diluted with excess acetone, and filtered to remove salt byproducts. Phase separation was then conducted on the resulting solution using water and chloroform in a separatory funnel. The chloroform-miscible compounds were retained. The unreacted epichlorohydrin and residual solvents were removed using a rotary evaporator at 120 °C under reduced pressure. Characterization Hydroxyl Value (OHV) Determination

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The OHV of bark extractives was determined by both ASTM D4274 Test method B and 31P NMR analysis32-33. The ASTM method measures the total hydroxyl amount of a sample, and the 31P NMR analysis indicates the types of the hydroxyl groups. 31

P Nuclear Magnetic Resonance (31P-NMR) 31

P NMR analysis was done on an Agilent NMR System 500 MHz spectrometer using a

5 mm oneNMR H/F33 probe with an acquisition time of 1.5 s, relaxation delay of 0.1 s, 30° pulse flipping angle, and 2,000 scans. Samples were pre-treated based on the phosphitylation

method

using

2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane

(TMDP). The first step was to develop a stock solution by mixing pyridine-deuterated chloroform (1:1.6, v/v), which was then used to prepare a relaxation solution using chromium (III) acetylacetonate (5 mg/mL) as well as a standard solution of cholesterol (5 mg/mL). Note that all the solutions were dried with molecular sieves before use. The

31

P

NMR solutions were produced by mixing the sample (15 mg), relaxation solution (0.1 mL), standard solution (0.1 mL), and stock solution (0.8 mL). Finally, TMDP (0.1 mL) was added, shaken vigorously, and then transferred to an NMR tube for analysis. All spectra were referenced to the (TMDP)2O peak at 132.2 ppm, which was formed from the reaction of 1 mol of water with 2 mol of TMDP and had a peak for excess TMDP (at 174.9 ppm) to ensure that all reactive species had been completely phosphitylated. The OHV was integrated relative to the cholesterol standard. Molecular Weight The molecular weights of bark extractives were determined using a size exclusion column (SEC) MCX 8-300 mm equipped with a Dionex DX600 ion chromatography and a UV detector at 280 nm. The column was packed with sulfonated styrene-divinylbenzene 8


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copolymer-network of 10 µm particles, and the separation range was from 100 Da to 70,000 Da. The mobile phase used was 0.1 M NaOH, at a flow rate of 1 mL/min. The calibration curve was generated by polystyrene sulfonate standard from 6,430 Da to 890 Da in 0.1 M NaOH. The concentration of standards and samples were 1 mg/mL and all the samples were filtered before conducting the experiments. Mn, Mw and polydispersity were calculated following ASTM D5296-11. Epoxy Equivalent Weight (EEW) The EEW of the synthesized resin was determined according to ASTM D1652-11. Epoxy resin (0.4-2.3 g) was placed in a 50 mL flask and dissolved in 10-15 mL of chloroform. The tetraethylammonium bromide reagent (10 mL) was added and the solution was mixed using a magnetic stirring bar. Crystal violet at 0.1 % w/w in glacial acetic acid was used as the indicator to determine the endpoints of titration. The solution was titrated with 0.1 N of perchloric acid in acetic acid. 1

H and 13C NMR 1

H and

13

C NMR spectra were obtained from an Agilent NMR System 500 MHz

spectrometer using a 5 mm Xsens Cold probe. Samples (50 mg) were dissolved in deuterated chloroform (CDCl3). All spectra were referenced to the 1H and

13

C signals of

TMSP at 0 ppm. The 1H spectrum was recorded at 25 °C after 64 scans. The pulse flipping angle was 45°, with 4.5 s acquisition time and 1 s relaxation delay time. The

13

C spectra

were recorded at 25 °C after 1,000 scans. The pulse flipping angle was 30°, with 2 s acquisition time and 1 s relaxation delay time. Fourier Transform Infrared Spectroscopy (FTIR)

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Samples were studied with a Bruker Tensor 27 spectrometer in an environmental chamber. Liquid samples were placed between two KBr crystals, while solid samples were mixed with KBr powder to form a pellet. All spectra were recorded over 4000-600 cm-1 at a resolution of 4 cm-1 with 32 scans.

Results and Discussion Effect of Substrates Scheme 1 shows the phenolate ions act as nucleophiles to attack ECH, which transforms hydroxyl groups to ether groups. Since the reaction initiates from hydroxyl groups, it is necessary to measure the OHV of substrates. In addition, the reactivity of hydroxyl groups also depends on their concentration and acidity. Higher acidity and lower pKa values can promote the reaction rate. If the substrates do not contain sufficient hydroxyl groups, side reactions may take place between ECH and a second hydroxyl group, which results in higher chlorine content and more impurities. Therefore, an understanding at the molecular level of the reactivity of phenolic monomers toward glycidylation represents a crucial step in the development of bio-based epoxy monomers19. In order to understand the effect of hydroxyl groups of substrates on the synthesis, two types of alkaline bark extractives were prepared with different OHV and Mw by different drying processes. The most common drying method is oven drying, which is cost-effective and no investment on special equipment. This drying process usually continues for 24 to 48 hours, and it may decrease the reactivity of extracts. The spray-drying method can convert fluid into uniform spherical shape powders in a short time, and it is widely applied on heat sensitive materials such as milk or tea extracts. 10


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The OHV and molecular weights of bark extractives were measured using the standard esterification-phthalic method and SEC, respectively. Their results are shown in Table 1. The OHV of spray-dried extractives (SDE) was 9.29 mmol/g, while the OHV of oven-dried extractives (ODE) was only 7.66 mmol/g. SDE had significantly higher OHV than ODE (p < 0.01). In the literature, the OHV of oven-dried extractives varies from 367-440 mgKOH/g, equal to 6.54-7.84 mmol/g34-35 akin to our ODE result. The ODE may have a further crosslinking reaction between these hydroxy groups due to a long period of dehydration, which is consistent with the results from SEC. To provide insight into each type of hydroxy group present, a 31P NMR method was applied (Fig. 1). Table 1 OHV and molecular weight of two types of extractives. OHV (mmol/g)

Mn

Mw

PDI

Peak 1

Peak 2

ODE

7.66 ± 0.78

1,343

4,442

3.31

10,112

1,254

SDE

9.29 ± 0.90

1,196

3,979

3.32

9,115

1,010

The results from 31P NMR agreed with the investigation above that SDE had higher OHV than ODE. However, the detected concentration of hydroxy groups using

31

P NMR was

much lower than that using the esterification-phthalic method since the bark extractives were not fully soluble in the pyridine/CDCl3 solution. Based on the soluble fraction, five types of hydroxy groups are reported in Table 2, including aliphatic OH, condensed OH, guaiacyl OH, p-hydroxyphenyl OH, and carboxylic OH. ODE exhibited low concentrations of carboxylic acid hydroxy groups and aliphatic hydroxy groups, but its concentration of condensed OH was approximately 14 times higher compared to SDE. The high concentration of condensed OH was likely due to the prolonged dehydration process in the oven. In addition, these hydroxy groups have their own distinct reactivaties to ECH based 11



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on their pKa values and steric hindrance. The pKa value of aliphatic hydroxyl is between 16 and 18; aromatic hydroxyl is approximately 10; carbonyl hydroxyl is from 3.4 to 4.519, 36

. Thus, carbonyl hydroxyl (e.g., in resin acid or fatty acids) had the highest reactivity, the

aromatic hydroxyl (e.g., in degraded lignin or flavonoids) had moderate reactivity, and the aliphatic hydroxyl (e.g., in polysaccharides) had the lowest reactivity, which has a similar probability as water to react with ECH. Based on the above, SDE was assumed to be a better substrate than ODE.

Fig. 1 31P-NMR analysis of (a) ODE and (b) SDE. Table 2 OHV from bark extractives using 31P-NMR. OH (mmol/g)

COOH

Aliphatic Condensed Guaiacyl P-hydroxyl

Total

(mmol/g) (mmol/g)

ODE 0.180

0.286

0.015

0.010

0.262

0.753

SDE

0.020

0.012

0.022

0.449

1.754

1.251

To confirm that SDE was indeed a better substrate than ODE, both extractives were used to synthesize bio-epoxy monomers (Table 3). The yield of epoxy monomer from ODE was lower than that from SDE (p = 0.03) because the high molecular weight of the ODE-based 12


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monomer precipitated during the synthesis process. The average EEW value of the ODEbased monomer was slightly higher than that of SDE-based monomer; however, there was no statistical difference in the EEW values between the two types of drying technologies (p = 0.21). The EEW value indicates the concentration of epoxy groups on monomers, which represents its reactivity for the subsequent crosslinking reactions. A lower EEW value corresponds to a higher concentration of oxirane and higher reactivity. Table 3 Yield and EEW value of two types of E-epoxy monomers. Yield (%)

EEW (g/eq)

SDE-based monomer

48.1 ± 6.9

338 ± 38.4

ODE-based monomer

41.2 ± 4.9

366 ± 37.5

The yield is estimated based on the Eq. (1). According to the measurements using the esterification-phthalic method, the OH concentration of SDE was 9.29 mmol/g. Assuming that all the hydroxyls react to ECH, which converts hydroxy groups to 2-methyloxirane groups, the yield can then be calculated as follows: Yield (%) = M/[B*(1+0.52)]*100……………………………………………….Eq. (1) Where M is the weight of E-epoxy monomer, B is the weight of dry bark extractives, and 0.52 is the stoichiometric amount of 2-methyloxirane for 1 g of bark extractives. All of the yields reported in this paper were calculated using the above equation. Effect of Solvents The function of a solvent is to dissolve reactants, assist in reaction processes, and mitigate EHS impacts. In this chapter, the solute was alkaline bark extractives, which required high polarity solvents to dissolve. Conventional BPA-based epoxy synthesis often adopts water/dichloromethane (DCM) as the solvent system, but chlorinated solvents have 13



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the concern of being potentially carcinogenic compounds. In addition, polar aprotic solvents seem preferable for the entire process because the reactions follow an SN2 mechanism, which requires aprotic solvents to avoid a solvation shell. The literature have indicated that many solvents have been applied as reaction media in epoxy resin synthesis, including methanol (MeOH)22, dimethylformamide(DMF)23, dimethyl sulfoxide (DMSO)20, hexane20, benzene20, water24, water/dioxane25-26, water/acetone27, water/ethanol27, watertoluene-butanol28, water/DCM20, and water/hexane20, 37. The properties of these solvents are listed in Table 4. The solubility of bark extractive followed the solvent polarity index as solvents with higher polarity can dissolve more extractives. Based on the solubility results, four solvent systems were selected to synthesize the E-epoxy monomer and the results are reported in Table 5. Please note that the polarity scale is an overall measure of solvent strength and is a composite of all types of solvent-solute interactions, except for dispersive interactions. Among these solvents, the water/DCM combination (1:1 vol. %) showed the lowest yield (6.6 %) and highest EEW value (4,647 g/eq), which were possibly caused by secondary reaction of the hydrolysis of epoxy groups. Although water, which had little EHS impact on the synthesis process, readily dissolves bark extractives and NaOH, the final product with water as solvent was inadequate. Furthermore, a few difficulties emerged during the refining processes. For instance, a thick emulsion layer of liquid-liquid separation was created, which slowed the extraction of E-epoxy monomer from the raw products to a 24 hour long process. In order to suppress the hydrolysis reactions, the use of organic solvents to synthesize bio-epoxy resins is a widely accepted option.

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Table 4 Chemical and physical properties of common solvents for epoxy monomer synthesis. Solvents

SPI*1

Ex. sol.

pKa

BP(°C)

Sol. in H2O

EHSRA*2

Ref.

Water

9.0

Exc.

15.8

100

NA

-

38

DMSO

7.2

Good

35.1

189

M

-

38

Methanol

6.6

Good

15.5

64.6

M

-

38

DMF

6.4

Good

-0.30*3

153

M

**

38

Acetone

5.4

Fair

19.7

56.2

M

-

38

Ethanol

5.2

Good

15.9

78.5

M

-

38

Dioxane

4.8

Fair

2.1*3

101.1

M

**

38

DCM

3.2

Poor

-

39.8

1.32

**

Benzene

2.7

Poor

43

80.1

0.18

**

39

Hexane

0.06

Poor

60.0

69.0

0.06

**

38

*1 SPI = Solvent Polarity Index; *2 EHSRA=EHS regulatory alerts; *3 was measured by its conjugated acid Two types of single-solvent systems were examined, including MeOH (polar protic) and DMSO (polar aprotic). The results showed an improvement of yield, which increased to 28.4 % and 58.8%, respectively. The EEW from MeOH system had the lowest (231 g/eq) among the four systems, and the EEW value from DMSO media was 949 g/eq. Although MeOH, as a polar protic solvent, can create a solvation shell around the extractives, the results from the MeOH system showed an increase in both the yield and epoxy content. However, the pKa value of MeOH is 15.5, so there is a possibility that MeOH can join the glycidyl etherification. From the 13C NMR results (Fig. 2), a significant chemical shift at 60 ppm was observed, which indicated that MeOH may have reacted with epichlorohydrin to produce 2-methoxyoxirane. To address this issue, DMSO was chosen to improve the yield 15



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of final products and avoid the side reactions between solvent and ECH. However, the boiling temperature of DMSO is 189 °C, which requires a high refining temperature (140 °C in this study). At high temperatures, further condensation reactions can occur and be promoted to form a high-molecular-weight E-epoxy monomer with elevated EEW values. Table 5 Yield and EEW value of bio-epoxy monomer synthesized in different media. Solvents

Yield (%)

EEW (g/eq)

Water/DCM

6.6 ± 3.2

4,647 ± 743

MeOH

28.4 ± 8.2

231 ± 76

DMSO

58.8 ± 7.5

949 ± 46

Water/Dioxane

48.1 ± 6.9

338 ± 38

* All the synthesis were conducted for six hours with 4.2 NaOH/OHV ratios at 60 °C The last solvent system was water/dioxane combination (1:9 vol. %), which can dissolve more extractives than a single water phase. The yield of the final product was 48.1 % and the EEW value was 338 g/eq. Although its yield and EEW values were only second best among the four types of solvents, the advantage of using water/dioxane combination was its well-balanced properties, including its moderate boiling temperature (approximately 100 °C), no side reactions observed, and stability in a high pH environment. Furthermore, the concentration of oxirane peaks on E-epoxy monomer was more significant than any above solvent system according to the

13

C NMR results (Fig. 2). For these reasons, the

water/dioxane combination was chosen for this study and for the following experiment.

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Fig. 2 13C-NMR spectra of E-epoxy monomers synthesized in various solvent systems. Effect of Catalysts In Scheme 1, NaOH was used to promote two reactions: forming phenoxide ions and dehydrohalogenating the intermediates. The conventional approach is to add NaOH into the synthesis solution at two stages, which is superior to adding all of the NaOH at once initially. However, this method provides an average yield and moderate purity with nonnegligible amount of byproducts such as 1-chloro-3-aryloxypropan-2-ols. The remaining chlorine can be up to 30 wt% of the epoxy resins3. In addition, when these NaOH do not bond to form phenoxides or withdraw chlorines, these free nucleophiles at high temperatures can further stimulate side reactions. To increase the yield and prevent side reactions, PTCs were introduced to synthesize low-molecular-weight epoxy resins in 198040. Compared to the conventional approach, the PTC method reduces the risk of hydrolyzing desired products and saves the amount of ECH consumed by NaOH20. Although the PTC method seems promising in the process of epoxy synthesis, only a small number of studies focus on this field because the cost of PTC can be 17



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5-10 times higher than NaOH. Among the many types of PTC, quaternary ammonium salts (QAS) are the most widely used catalysts for epoxy synthesis. The mechanism of QAS is to shuttle OH between an alkaline aqueous phase and an immiscible organic phase by the quaternary ammonium cation as shown in Scheme 2.

Scheme 2 Mechanism of glycidyl etherification catalyzed by TBAH 14, 41 Even though water/dioxane was not a biphasic system, the results showed that adding TBAH can improve yield and increase the epoxy content as shown in Table 6. It is likely because TBAH can prevent side reactions such as oligomerization and hydrolysis. Our FTIR and NMR results showed that the concentration of hydroxy groups of bio-epoxy resins under PTC synthetic path was significantly lower than that under twofold NaOH addition reactions (Fig. 3). According to the ratios of two NMR peak areas (5.3 ppm/3.1 ppm), the concentration of secondary hydroxy groups was twice less in TBAH method than in twofold NaOH synthesis. Therefore, TBAH can effectively increase the yield of the Eepoxy monomer and decrease the possibility of hydrolysis reactions during synthesis. Table 6 Yield and EEW value of two types of epoxy monomers through two synthetic paths. Yield (%)

EEW (g/eq)

18


2°-OH / Epoxy ratio

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TBAH + NaOH

48.1 ± 6.9

338 ± 38

0.27

NaOH + NaOH

15.1 ± 4.8

642 ± 27

0.55

Fig. 3 Effect of catalyst types on functional groups of E-epoxy products observing by FTIR (left) and NMR (right): (a) TBAH/NaOH and (b) twofold NaOH. Numerical Variables — Reaction Time, Reaction Temperature, and NaOH/OHV Molar Ratio Continuous variables were analyzed using response surface methodology (RSM) due to their dependency on one another. Incorporating the Box–Behnken design (BBD), 17 samples were tested with three levels for each factor (NaOH/OHV molar ratio, reaction temperature, and reaction time), and the response parameters were the yield of the glycidylation reaction and epoxy equivalent weight of the final samples. Compared to other RSM designs, BBD requires only 17 experimental runs and avoids the extreme treatment combinations. The span of each synthetic parameter was selected based on accepted methods as reported in the literature. For reaction temperature, it is known that increasing temperature can accelerate the overall reaction rate; however, side reactions can be promoted 19



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simultaneously such as oligomerization and hydrolysis. Previous studies have reported that the reaction temperatures of liquid form epoxy monomers range from 25-100 °C, whereas in bio-epoxy synthesis20-21 the temperatures are between 30-104 °C24. Below or beyond this range, product yield and reactivity are both decreased. Generally, if the temperature is raised above 100 °C, the final products transform from liquid to solid form. In addition, increasing the temperature above 110 °C can cause thermal degradation for thermally sensitive compounds such as flavonoids42. For these reasons, we chose 40-100 °C as our reaction temperature range. According to the literature, the reported reaction time of conventional epoxy is in the range of 0.5-20 hours3 and for bio-epoxy synthesis the time ranges from 1.5-8 hours24. Most studies agree that the total amount of glycidyl substrate (monomers and oligomers) increases over time. In contrast, the influence of reaction time on the risk of hydrolyzing products is still uncertain22. In order to maximize the yield of monomer and avoid hydrolysis, time periods were selected based on a correlation between reaction time and reaction temperature, which was established based on the literature data24. In respect of the reaction temperature range, 2-10 hour reaction time periods were selected for this study. The last parameter considered in this study was the NaOH/OHV ratio, which was susceptible to having more significant effects than reaction time on yield and epoxy content. With insufficient NaOH, certain amounts of unreacted polyphenols will remain and decrease the yield. With excess NaOH, ECH can be hydrolyzed and wasted37. According to the stoichiometry, the mole ratio of NaOH to OHV on the substrate should be 1:1. However, some studies have shown that excess amount of base can accelerate the

20


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reaction20. In an ordinary bio-epoxy resins synthesis, the NaOH/OHV ratio is in the range of 0.12-4.56, 24. For this reason, 1.4-4.2 molar ratios were adopted for this study. Table 7 The B-B matrix and output responses. X1 Temperature X2 Time

X3 NaOH/Substrate

Y1 Yield

Y2 EEW

(°C)

(hour)

molar ratio

(%)

(g/eq)

60

2

2.8

10.5

351

60

6

1.4

15.2

899

60

6

4.2

34.2

382

60

10

2.8

21.2

317

80

2

1.4

22.5

449

80

2

4.2

51.0

398

80

6

2.8

54.5

348

80

6

2.8

51.0

398

80

6

2.8

54.0

468

80

6

2.8

57.5

444

80

6

2.8

59.0

368

80

10

1.4

38.9

693

80

10

4.2

50.5

1625

100

2

2.8

36.4

498

100

6

1.4

33.2

1045

100

6

4.2

64.0

2240

100

10

2.8

37.9

1771

The yield and the EEW values of the E-epoxy monomer based on B-B design are shown in Table 7. The ranges of yields and EEW values were 10.5-64.0 % and 317-2,240 g/eq, 21



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respectively. Although the yield can be up to 64 % based on the dry weight of spray-dried bark extractives, the yield of bio-epoxy resin based on the dry bark chips was only between 2-14 %. In addition, based on our previous research18, the average molecular weight of bioepoxy resins was 796 Da. Thus, the functionality per molecule was approximately 0.362.51. Prior to analyzing the influence of the parameters, the model adequacy was examined and results are provided in the supporting information. The responses were fitted with various functions such as linear, two-factor interaction (2FI), and quadratic models in order to find the best-fit model. Among the models, the quadratic model responded best to both the yield and EEW values according to their coefficients of determination (R2 = 0.96 and R2 = 0.98, respectively) and their p-values of lack of fit (p = 0.71 and p = 0.09, respectively). Since the model adequacy matched requirements at a 5% statistical significance level, the second step was to identify the data distribution by the normal probability plot of residuals (Fig. 4), which demonstrated that no further data transformation was required, since all of the data points were approximately linear. Other residual diagnoses are provided in the supporting information.

22


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(a)

(b)

Fig. 4 Normal probability plot of (a) yield and (b) EEW value of E-epoxy monomer. Thus, the response surface plots were generated to illustrate the influence of the continuous reaction parameters as shown in Fig. 5.We found that all three parameters were significant to the yield (p < 0.01) by ANOVA analysis. Reaction temperature and NaOH/OHV molar ratio had the highest effects on yield compared to reaction time. Temperature and time both followed a quadratic path, as shown in the 3D plots (Fig. 5). At higher temperatures, the yield decrease was likely due to further oligomer formation between E-epoxy monomers. Many studies have shown that oligomers will solidify when they have very high molecular weight37, 43. These oligomers can precipitate with salt to the bottom of a reaction flask, which might be left out of subsequent work up and decrease the total yield. In addition, the reactivity of the bio-epoxy resin was decreased due to the formed oligomers, which resulted in high EEW values (>1,000 g/eq). As opposed to time and temperature, the yield increased with an increasing NaOH/OHV molar ratio, which had a positive linear effect. It is well known that the rate determining step in a glycidylation reaction is reaction II in Scheme 144. More catalyst can promote the first equilibrium reaction, accelerate reaction II, and increase the total yield. A previous study24 also showed that the yield of a reaction generally increased with the increase of NaOH/OHV ratio and that the optimal NaOH/OHV ratio was 1.5. According to the ANOVA analysis, we found the final model to predict the yield is as follows: Yield (Y1) = 55.1 + 11.3X1 + 3.58X2 + 11.3X3– 16.4X12 – 12.2X22 …………Eq. (5)

23



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Based on a confidence interval of 99%, the significant terms are X1, X2, X3, X12, and X22. Since the constants of Xn2 in yield model are negative, it means there is maximum extent of conversion. On the right side of Fig. 5, the influence of reaction conditions on EEW values is shown. Unlike the results for yield, those for EEW showed different trends at different levels. If we fixed two parameters at a relatively low level, the last parameter followed a quadratic path due to incomplete dehydrochlorination or propagation. As reported by other researcher21, the EEW value can be possibly increased by secondary reaction to form a benzo-dioxane type product. On the other hand, in an opposite situation, the last parameter showed a linear positive relationship. Previous studies have indicated that at lower temperatures, the effects of NaOH is not as significant as at high temperatures24. The final model to predict the EEW is as follows: EEW (Y2) = 405.2 + 450.63 X1 + 338.75X2 + 194.88X3 + 326.75X1X2 + 428X1X3 + 245.75 X2X3 + 339.65X12 + 396.65X32…………………………………………Eq. (6) Based on a confidence interval of 99%, all of the terms are significant except for X22. The constants of X12 and X32 are positive in the EEW model so there is a minimum of the value of EEW. To match the maximum point of yield and the minimum EEW value, we can reach an optimal point for both equations. The optimal conditions were 80°C, 4.5 hour and 3.4 NaOH/OHV molar ratios. To validate the accuracy of this regression equation, supplementary experiments were conducted under these optimal conditions. The results listed in Table 8 confirmed the accuracy of both models

24


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Fig. 5 Response surface plots of various parameters on the product yield and reactivity: (a) Effects of time and temperature, (b) Effects of temperature and NaOH/OHV ratio, and (c) Effects of time and NaOH/OHV ratio.

Table 8 Verification of the proposed optimal synthesis conditions. Yield (%)

EEW (g/eq) 25



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Predicated values

59.2

667

Experimental values

59.0 ± 1.8

594 ± 52

Conclusions In this study, we tested three categorical parameters (substrates, solvents, and catalysts) and three numerical parameters (reaction time periods, reaction temperatures, and NaOH/OHV ratios). Our results showed spray-dried extractives are a good substrate, with both high hydroxyl content (9.29 mmol/g) and low molecular weight (Mw = 3,979 Da). Among four types of solvent systems, a water/dioxane combination was selected for this study owing to its balanced properties on each perspective such as high yield (48.1 %), high reactivity (EEW = 338 g/eq), moderate boiling temperature (approximately 100°C), no side reactions observed, and stability in a high pH environment. To avoid hydrolysis reactions occurring, tetrabutylammonium hydroxide (TBAH) was added at the first stage as a ringopening catalyst. According to our 1H NMR results, the use of TBAH can significantly decrease the hydrolyzed by-products. To optimize the yield and reactivity, the experiment should be conducted for 4.5 hours with 3.4 NaOH/OHV ratios at 80 °C reaction temperature.

Acknowledgements The authors would like to acknowledge ORF-RE (Bark Biorefinery), NSERC-Strategic Network Plastic Manufacturing partners, the Canadian Foundation for Innovation, project number 19119, as well as the Ontario Research Fund for funding of the Centre for Spectroscopic Investigation of Complex Organic Molecules and Polymers for the financial 26


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support. Appreciation is extended to Andrew Paton and Lindsey Fiddes for their generous help and support.

References

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For Table of Contents Use Only Effects of Reaction Parameters on the Glycidyl Etherification of Bark Extractives during Bio-epoxy Resin Synthesis Pei-Yu Kuo, Luizmar de Assis Barros, Mohini Sain, Jimi S.Y. Tjong and Ning Yan Synopsis: Nearing a sustainable polymer industry: this study investigate six reaction parameters of the glycidylation reaction between epichlorohydrin and renewable bark extractives.

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Effects of Reaction Parameters on the Glycidyl ... - ACS Publications

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