• International Journal of Technology (IJTech)
  • Vol 15, No 2 (2024)

Study of Solvent and Catalyst in Diimide Biphasic Hydrogenation System of Natural Rubber

Study of Solvent and Catalyst in Diimide Biphasic Hydrogenation System of Natural Rubber

Title: Study of Solvent and Catalyst in Diimide Biphasic Hydrogenation System of Natural Rubber
Dody Andi Winarto, Chandra Liza, Mohamad Irfan Fathurrohman, Abdulhakim Masa, Mochamad Chalid

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Cite this article as:
Winarto, D.A., Liza, C., Fathurrohman, M.I., Masa, A., Chalid, M., 2024. Study of Solvent and Catalyst in Diimide Biphasic Hydrogenation System of Natural Rubber. International Journal of Technology. Volume 15(2), pp. 414-424

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Dody Andi Winarto 1. Green Polymer Technology Laboratory, Department of Metallurgical & Materials Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, West Java, Indonesia, 2. Badan
Chandra Liza National Research and Innovation Agency, Building #460, Puspiptek Area, South Tangerang 15314, Banten, Indonesia
Mohamad Irfan Fathurrohman Indonesian Rubber Research Institute, Jl. Salak no 1., Bogor 16151, West Java, Indonesia
Abdulhakim Masa Rubber Engineering & Technology Program, International College, Prince of Songkla University, Hat Yai, Songkhla, 90110, Thailand
Mochamad Chalid 1. Green Polymer Technology Laboratory, Department of Metallurgical & Materials Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Depok, Depok 16424, West Java, Indonesia, 2. Cente
Email to Corresponding Author

Abstract
Study of Solvent and Catalyst in Diimide Biphasic Hydrogenation System of Natural Rubber

In Indonesia, solid rubber is the most common form of natural rubber (NR) intermediates because it is easy to store and transport. However, hydrogenation, a process aimed at improving the quality of NR by addressing its vulnerability to environmental factors due to the presence of carbon double bonds, is typically carried out under relatively mild conditions using latex material. This study explores a biphasic hydrogenation approach using solid NR dissolved in a solvent and a hydrogen source (hydrazine hydrate and hydrogen peroxide) mixed in water. The choice of solvents, catalysts, and the water-to-solvent volume ratio were examined for their impact on hydrogenation. Characterization was conducted using Fourier Transform Infrared (FTIR) spectroscopy as a qualitative indicator of hydrogenation. Meanwhile, Nuclear Magnetic Resonance 1H-NMR spectroscopy is used to measure the degree of hydrogenation. The results indicate that partial hydrogenation was successful using toluene as a solvent and ethylene diamine tetra acetic acid diammonium copper (Cu-EDTA) as a catalyst, with variations in water volume influencing the degree of hydrogenation. A degree of hydrogenation of 7.69% was achieved using 15 mL of water. The thermal properties of hydrogenated NR remain comparable to the original material, with improved heat resistance. This biphasic hydrogenation method offers the potential to enhance the properties of NR in various applications.

Biphasic hydrogenation; Diimide; FTIR; Natural Rubber; NMR; Thermal analysis

Introduction

Natural rubber plays a vital role in various aspects of human life, with applications in fields such as medical equipment and industry (Kim et al., 2020; Saengdee, Phinyocheep, and Daniel, 2020; Inoue and Nishio, 2007). Despite its significance, natural rubber contains carbon double bonds (Cifriadi, Chalid, and Puspitasari, 2017; Piya-areetham, Prasassarakich, and Rempel, 2013) that make it less resistant to oxidation, heat, ozone, and other environmental factors (Ngudsuntear et al., 2022). However, this limitation can be overcome by chemically modifying it, potentially making it a substitute for the thermally resistant Ethylene Propylene Diene Monomer (EPDM) rubber (Taksapattanakul et al., 2019; Inoue and Nishio, 2007).

Hydrogenation can be employed to enhance the quality of natural rubber (Saengdee, Phinyocheep, and Daniel, 2020; Phinyocheep, 2014; Winters et al., 2002). In this process, solid natural rubber is dissolved in a solvent and then reacted with hydrogen gas, with the help of a catalyst (Ha et al., 2016; Ha et al., 2015; Mahittikul, Prasassarakich, Rempel, 2009; Inoue and Nishio, 2007). Notably, this process demands the use of organic solvents and containers capable of withstanding high pressure, necessitating strict safety precautions. Moreover, these processes involve expensive catalysts and recycling challenges. While hydrogenation can be achieved using aryl-sulfonyl hydrazide thermolysis at temperatures above 100°C, it is accompanied by chain degradation and side reactions.

A milder method to hydrogenate natural rubber is by utilizing latex under conditions of low temperature and atmospheric pressure (Cifriadi, Chalid, and Puspitasari, 2017; Veni and Ma'zam, 2010). This process involves the use of hydrazine hydrate as a hydrogen donor in combination with hydrogen peroxide. Several studies have proven the success of this approach, although some side reactions may cause cross-linking before vulcanization takes place (Cifriadi, Chalid, and Puspitasari, 2017; Hinchiranan, Prasassarakich, and Rempel, 2006).

Although latex offers certain advantages as a raw material, one of its main disadvantages lies in the need for stabilization during transportation to the processing site to avoid coagulation (Winarto et al., 2023). This stabilization process adds complexity and cost to the overall production process. Moreover, transporting latex poses a challenge as it requires a substantial amount of water due to its bulkiness and weight, leading to additional logistical issues and potential environmental concerns. In contrast, solid natural rubber obtained by coagulating latex is the primary intermediate product produced by the Indonesian rubber industry (Yuningtyas, Hakim, and Novianti, 2019), presenting a more manageable and widely used alternative.

The objective of this research is to utilize a biphasic hydrogenation method that involves dissolving solid natural rubber in a solvent. This method also includes the use of a hydrogen source, namely hydrazine hydrate and hydrogen peroxide, which is mixed in water. The goal is to take advantage of the abundance of solid rubber and enhance its quality. The success of the hydrogenation process was evaluated through Fourier Transform Infrared (FTIR) spectral analysis and Nuclear Magnetic Resonance (NMR) spectroscopy. Additionally, the thermal properties of the resulting products were analyzed using Differential scanning calorimetry (DSC) and Thermogravimetric analysis (TGA).

Experimental Methods

2.1. Materials

    The natural rubber crepe was obtained from the Research Center for Rubber Technology, Indonesia. Toluene, xylene, and n-hexane were obtained from Mallinckrodt Chemical. Hydrazine hydrate (80% solution in water) for synthesis, ethylene diamine tetraacetic acid diammonium copper (Cu-EDTA) salt solution 0.025 M EDTA-Cu (NH4)2 CAS-No 67989-88-2 Supelco and copper (II) sulfate p.a. anhydrous were products of Sigma-Aldrich, and hydrogen peroxide 35% was products of Merck. Aquadest was provided by Polymer Laboratory BRIN, which was buffer-added so that its pH was 9.

2.2. Biphasic hydrogenation of natural rubber

    Natural rubber crepe (0.5 g) was dissolved in 50 mL of the solvent. In a separate container, hydrazine hydrate and copper sulfate were dissolved in 20 mL of aquadest at pH 9 and then poured into a container where natural rubber dissolved. They were mixed and stirred using a magnetic stirrer for 10 minutes at 500 rpm. Then, 2.914 mL hydrogen peroxide was put into the mixture drop by drop. The reaction was stirred for 2 hours and occurred under biphasic conditions, with a concentration ratio [C=C]:[N2H4/H2O2] of 1:12. The solution was left for more than 2 hours until the water and solvent were separated. The solution from the solvent side was taken out and put into a 250 mL boiling flask. The solvent was removed by rotary evaporation. The remaining material was characterized later. The scheme of the process is shown in Figure 1 below.


Figure 1 Schematic process of hydrogenation of natural rubber

In this work, the solvent volume used was fixed, at 50 mL. Several solvents, rubber contents, types of catalysts, and acidities of the water used were varied. The experimental conditions are listed in Table 1.

Table 1 Experimental Condition

2.3. Characterization

2.3.1. Fourier Transform Infrared Spectroscopy

        An analysis of the IR spectrum was performed using the Agilent Cary 630 FTIR Spectrometer, specifically employing the Attenuated Total Reflectance (ATR) method. The spectral range was from 4000 cm-1 to 650 cm-1, with sample scans 32 and a resolution of 4 cm-1.

2.3.2.  Nuclear Magnetic Resonance Spectroscopy

        1H-NMR and 13C-NMR spectroscopy was analyzed using JNM-ECZ500RlS 1. The sample was diluted in CDCl3. Measurements were performed at 500 MHz. The 1H-NMR was done with a repetition time of 6.75 seconds, 24 scans, and 13C-NMR was done with a repetition time of 2.83 seconds, 1044 scans.

2.3.3.  Differential Scanning Calorimetry

        Glass transition temperature Tg was performed using Perkin Elmer DSC8000. The sample was put into an aluminum crucible of about 10 milligrams. From RT, the sample was cooled to -100°C and then heated to 20°C at a heating/cooling rate of 10°C/min. Measurement was done in nitrogen condition (50mL/min).

2.3.4.  Thermogravimetry Analysis

        Thermogravimetry Analysis was performed using Netzsch TG 209 F3 Tarsus. The sample was put into an alumina crucible of about 20 milligrams. The temperature program was conducted from 30°C to 600 °C with a heating rate of 10°C/min. Measurement was done in nitrogen condition (50mL/min).

Results and Discussion

        Natural rubber (NR) possesses a predominantly cis-1,4-polyisoprene molecular structure (Widiyati and Poernomo, 2018; Phinyocheep, 2014) as depicted in Figure 2. It contains a tri-substituted C=C bond with two CH2 substituents and one CH3 substituent (Smith, 2022). With this structure, NR has FTIR spectra, as shown in Figure 3 (a). It contains both methyl and methylene groups; hence, there are three peaks between 2840 and 3000 cm-1. The unsaturated CH stretch of natural rubber is at 3022 cm-1, the C=C stretch is at 1664 cm-1, and the CH wag is seen at 830 cm-1. As a note, through this paper, we use the code PHNR as partially hydrogenated natural rubber, and it is shown the spectral difference between NR and PHNR in Figure 3.

Figure 2 Chemical structure of NR

Figure 3 FTIR spectra of NR (a) and PHNR (b)


    The chemical reactivity exhibited by the carbon-carbon double bond (C=C) within the isoprene repeating unit may be considered as C=C of alkenes. The hydrogenation reaction of natural rubber using diimide as a hydrogen donor occurs according to equation (1), which is a redox type of reaction (Taksapattanakul, 2016; Lin, 2005). R1HC=CHR2 denotes NR, and R1HCCHR2 denotes PHNR. It is expected that the C=C double bond will decrease, and the CC bonds and CH2 substituents will increase. This is reflected in the FTIR spectra of PHNR as depicted in Figure 3 (b), where the intensity of transmission of C=C stretch is at 1654 cm-1, and CH wag is at 834 cm-1 increase.

3.1.  Solvent screening for hydrogenation NR

    In biphasic hydrogenation, NR was dissolved in an organic solvent to increase the surface area of rubber particles. This, in turn, enhances the chances of the reactant encountering and reacting effectively. Therefore, dissolving NR is a crucial step in the process. It is important to mention that using a solvent with high solubility for the solid can significantly improve the reaction rate compared to using a solvent with low solubility for the reagent (Dyson and Jessop, 2016).
    The solubility of a material is determined by the interaction between its internal energy. The fundamental theory is based on cohesive energy and solubility parameters. In the context of polymers and solvents, their solubility is influenced by their chemical structures, with a general understanding that similarity in chemical structure promotes solubility. Additionally, the physical properties of the polymer, particularly for liquids that induce significant swelling, also play a role. For a liquid to be absorbed into a polymer, there must be adequate space or chain flexibility to accommodate it (Arnold, 2003).
      On this occasion, we employ the concept of the solubility parameter, denoted as  (Hansen, 2007). According to this approach, a solute is likely to dissolve in a solvent close to its own (Muhammad Adlim, 2021). Based on the data (Arnold, 2003; Hansen, 2007) of dispersion solubility parameter polar solubility parameter  and hydrogen solubility parameter of the material used in this experiment, the total solubility parameter of each material used in this experiment can be calculated. These are NR (polyisoprene) 17.94, toluene 18.16, hexane 14.90, and xylene 17.90. For comparison, the total solubility parameter of water is 47.91.
        NR was found to be soluble in all selected organic solvents. Researchers used xylene, toluene, and hexane to hydrogenate rubbers with a concentration between 1.00% and 2.00% (Samran et al., 2004; Phinyocheep, Pasiri, and Tavichai, 2003). However, based on the calculation of total solubility parameters, toluene and xylene were predicted to be the most suitable solvents for dissolving NR. Toluene has a boiling point of 111°C, and xylene has a boiling point of 138°C. Considering the ease of solvent evaporation for characterization purposes, toluene was selected as the baseline solvent for the experiment.
The hydrogenation of NR was also carried out using these three solvents. The FTIR spectra of the resulting product are depicted in Figure 4. The pattern of the spectra is almost the same. There are several NR’s specific peaks. Especially at 3022 cm-1, between 2840 and 3000 cm-1, 1654 cm-1, and 834 cm-1 as mentioned in Figure 3 (a). However, there is an increase in the intensity of transmission at wavenumbers 1654 cm-1 and 834 cm-1 using hexane (b) and toluene (c) as solvents. These indicators suggest that hydrogenation of NR has occurred. This has been confirmed by Cifriadi, Chalid, and Puspitasari (2017),  Mahittikul, Prasassarakich, Rempel (2009), Inoue and Nishio (2007), who also observed hydrogenation using FTIR spectra. However, in the case of xylene (d), the transmission intensity at wavenumber 1654 cm-1 remains low. This is probably due to the water content in the sample, while the FTIR measurement can be seen in a broad peak at the wavenumber of 3400 cm-1 and shifting the baseline of the spectra.

Figure 4 FTIR spectra of NR (a) compared with PHNR using (b) hexane, (c) toluene, and (d) xylene
3.2. Type of catalyst for hydrogenation NR

The possible reactions that occur between hydrazine and hydrogen peroxide are described in the following equation. Diimide (N2H2) is an intermediate product, formed due to the reaction between hydrazine and hydrogen peroxide, reaction (2). Diimide then reacts with the C=C double bond in natural rubber, reaction (3).

In the case of the presence of a C=C double bond in natural rubber and metal ions as catalysts, reaction (3) predominates. The use of this catalyst will increase the reaction for the formation of diimide, and Cu2+ is a suitable ion for use in hydrogenation using diimide (Mahittikul, Prasassarakich, Rempel, 2007).

Unlike hydrogenation with the latex system where copper ions are in 3 locations (water medium, polymer particle surface, and inside of the particles) (Lin, 2005), copper ions in the biphasic hydrogenation system exist solely within the water medium, and will only interact with the polymer when stirring is initiated. If this copper ion is only in the water medium, there will be no C=C reduction; therefore, stirring is very important. The reaction was run at 500 rpm.

The study compares the use of two catalysts, Cu-EDTA and CuSO4, in the biphasic hydrogenation of NR. A total of 20 mL of water was used in the experiment. Figure 5 displays the FTIR spectra of the hydrogenation product. Notably, an increase in transmission intensity at wavenumbers 1654 cm-1 and 834 cm-1, indicating the occurrence of hydrogenation of NR.

Figure 5 FTIR spectra of PHNR using (a) EDTA and (b) CuSO4
3.3.  Solvent of catalyst used in the hydrogenation of NR

In this experimental work, the catalyst is a substance dissolving in water. Therefore, the amount of water was also considered to be optimized by varying amounts of water in the biphasic hydrogenation system. In this series, EDTA is used as a catalyst. Variations of water amounts of 15, 20, and 25 mL were used for dissolving N2H4. The product of hydrogenation was analyzed using FTIR and presented in Figure 6. It is noted that there is an increase in the intensity of transmission of FTIR spectra at wavenumber 1654 cm-1 and 834 cm-1. This indicates that partial hydrogenation of NR occurred.
Figure 7. presents the 1H-NMR spectra of natural rubber (NR). The spectra exhibit three main characteristic peaks at 1.67 ppm, 2.03 ppm, and 5.2 ppm, corresponding to the proton signals from methyl and methylene groups in isoprene units and proton signals in proximity to the C=C bond.
      After analyzing the 1H-NMR spectra of hydrogenated natural rubber, we observed a decrease in intensity at the peak of 5.2 ppm. At the same time, a new peak appeared at 0.8 ppm, and there was a noticeable increase in signals at 1.1 ppm. These changes can be attributed to the presence of saturated –CH2 and –CH3 groups in the hydrogenated NR . The appearance of –CH2 was expected from the hydrogenation reaction and was also observed in the spectra FTIR in Figure 3 due to the reduction of C=C in NR. Based on these findings, the degree of hydrogenation resulting from the biphasic hydrogenation reaction of NR, with varying amounts of water used, is presented in Table 2. The degree of hydrogenation was calculated following the method presented by Taksapattanakul (Taksapattanakul, 2016).


Figure 6 FTIR spectra of NR (a) compared with PHNR using water (b) 15 mL, (c) 20 mL, and (d) 25 mL


Figure 7 1H-NMR spectra of NR (a) compared with PHNR using water (b) 15 mL
Table 2 Degree of hydrogenation of PHNR

 

Degree of hydrogenation %

NR

0

PHNR using Water 15 mL

7.69

PHNR using Water 20 mL

6.25

PHNR using Water 25 mL

5.96

Figure 8 (a) displays the 13C-NMR spectra of NR, featuring several specific peaks. Notably, the peak at 135.2 ppm correlates with the carbon atom at position 1, while the peak at 125.5 ppm corresponds to the carbon atom at position 2. Additionally, peaks at 23.4 ppm, 26.4 ppm, and 32.2 ppm are associated with the C of methyl and 2-methylene (carbon atoms at positions 4, 5, and 3).
On the other hand, Figure 8 (b) illustrates the 13C-NMR spectra of PHNR, revealing several new peaks (position ). These peaks are observed at 19.7 ppm, 25.6 ppm, 29.9 ppm, and 37.4 ppm, attributed to CH3, CH2, CH, and C(CH3)CH2 indicating that some reduction in the C=C double bond has occurred and PHNR was produced. This is in line with the results from other literature (Mahittikul, Prasassarakich, Rempel, 2006; Samran, 2005). This evidence provided by 13C-NMR spectra also aligns with FTIR spectra observed in Figure 3 and confirms that partial hydrogenation has occurred.

Figure 8 13C-NMR spectra of NR (a) compared with PHNR using water (b) 15 mL
3.4.  Thermal Properties of PHNR
 Figure 9 illustrates a thermogram of hydrogenated NR within the temperature range of -70°C to -40°C. The figure displays a notable change in heat flow between -70°C and -60°C, indicating the presence of the glass transition temperature (Tg) in the material. Tg represents the temperature at which the polymer's mechanical properties shift from a brittle material to an elastic material, resulting from changes in chain mobility. NR is elastic at room temperature but becomes brittle when the temperature falls below Tg. The Tg of NR is approximately -65°C. Interestingly, PHNR still exhibits a Tg around that temperature. Therefore, the application of hydrogenated NR is expected to remain in the same conditions as the initial NR.

Figure 9 DSC thermogram of NR (a) compared with PHNR using water (b) 15 mL, (c) 20 mL, and (d) 25 mL
 Meanwhile, Figure 10 displays the thermogram obtained from TGA analysis, which shows the degradation temperature as the temperature where the thermogram exhibits the minimum derivative. Table 3 summarizes the results, clearly indicating that the degradation temperature of hydrogenated NR increases with the amount of water used.

Figure 10 Thermogram of NR compared with PHNR using water 25 mL

Figure 11 Effect of water used in biphasic hydrogenation and Tg of the PHNR

Figure 11 demonstrates that the glass transition temperature (Tg) of hydrogenated natural rubber remains unchanged compared to that of natural rubber. However, the degradation temperature of hydrogenated natural rubber is observed to be higher than that of natural rubber. Consequently, it is anticipated that the application temperature of hydrogenated NR will remain the same while offering improved heat resistance, given its tendency for increased degradation temperature.
Table 3 Thermal characteristics of hydrogenated NR

 

Tg (°C)

Degradation temp (°C)

NR

-65.0

360.1

PHNR using Water 15 mL

-64.4

362.5

PHNR using Water 20 mL

-65.3

363.5

PHNR using Water 25 mL

-64.9

371.4

Conclusion

        The biphasic hydrogenation of natural rubber using hydrazine hydrate and hydrogen peroxide was successfully conducted. Hydrogenation was confirmed by 13C-NMR, and the degree of hydrogenation was calculated using 1H-NMR. The highest degree of hydrogenation of the product from the biphasic hydrogenation was 7.69%, achieved using 15 mL of water. The glass transition temperature (Tg) of the product was comparable to that of pre-hydrogenated natural rubber but improves its heat resistance. Nevertheless, the attained highest degree of hydrogenation (7.69%) may be considered relatively modest. Therefore, future research of biphasic hydrogenation should focus on exploring alternative reaction conditions to increase the degree of hydrogenation, as higher levels might be preferable for specific applications.

Acknowledgement

     We acknowledge the management of BRIN for providing funding for this research through the "Degree by Research" program, as per contract number KepKaLIPI No. 59/H/2020.

Supplementary Material
FilenameDescription
R1-MME-6683-20230925215508.docx Figure 3. Spectra FTIR
R1-MME-6683-20230925215536.docx Figure 4. Spectra FTIR
R1-MME-6683-20230925215608.docx Figure 5. Spectra FTIR
R1-MME-6683-20230925215641.docx Figure 6. Spectra FTIR
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