An Affordable and Easily Accessible Approach for Acrylonitrile Monomer Purification through a Simple Column Technique ()
1. Introduction
The end products of organic reactions are rarely pure. The starting material or the reactions’ byproducts frequently contaminate them. The technique used for purification is highly dependent on the nature of the compound and the types of byproducts that may be present. Besides, removing impurities improves the product’s quality and standard, whether organic compounds are acquired from nature or synthesized in laboratories [1] . Eliminating impurities or byproducts is termed purification, and the purified material should undergo quantitative and qualitative testing. Popular purification techniques include filtration, crystallization, fractional crystallization, sublimation, distillation (fractional, under reduced pressure, stream, or azeotropic), differential extraction, and chromatography [2] [3] [4] [5] .
Researchers are increasingly interested in altering polymers into added-value products, especially in energy storage and conversion. Copolymerization, grafting, and crosslinking are all ways to change some properties of the host polymer, and they all need monomers as one of the reactants [3] [6] . As is well known, stabilizers are added to liquid monomers like styrene, acrylonitrile, vinyl chloride-d3, methyl acrylate, acryloyl chloride, 2-chloroethyl vinyl ether, etc., in the form of 4-tert-butylcatechol (TBC), 35 - 45 ppm monomethyl ether hydroquinone (MeHQ), and 400 ppm phenothiazine (PTZ) to prevent the monomers from self-polymerizing or being started by light, heat, or air. Thus, inhibitors are needed to process, transport, and store unsaturated monomers like acrylates, vinyl chloride, butadiene, and styrene. However, it should usually be removed before polymerization. Otherwise, it will react too early with the free radical initiator, reducing its efficiency and concentration [7] .
Acrylonitrile (AN) and acrylic acid (AA) are examples of liquid monomers that can be purified by distillation at a constant temperature of 69.5˚C and 80˚C - 130˚C, respectively [8] [9] . However, the distillation method has some disadvantages, such as being unsafe, cumbersome, and time-consuming. In addition, the distilled monomer would likely be exposed to moisture if not used immediately after distillation [10] . Column chromatography is a modern method for separating, isolating, identifying, and purifying small amounts of organic compounds. This method is based on the fact that the parts of a mixture move at different rates through a porous medium (the stationary phase) when a solvent or gas (the mobile phase) is present. This project is about finding a fast, safe, cheap, and easy-to-use way to purify AN monomer. A simple column is used to solve these problems. This study looked into the performance of a simple column on AN purification attributed to the advantages of this technology, as described in Figure 1. The authors’ main goal is to conduct a study to investigate and assess the performance of the simple column technology for acrylonitrile monomer purification. This study selected AN as a monomer because it has unique properties like high filmability, transparency, optical clarity, adhesive characteristics [11] , and good grafting efficiency compared to vinyl monomers
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Figure 1. A schematic diagram of a simple column’s advantages versus the distillation technique’s drawbacks.
[6] [12] . To our knowledge, there has been no publication of this study. This work’s long-term viability depends on a simple, low-cost approach for purifying monomers. In addition, this technique will increase the use of purified monomers in many processes, particularly polymerization reactions.
2. Experimental Procedure
2.1. Materials
Acrylonitrile (C3H3N, ≥99% w/v, contains 35 - 45 ppm MeHQ as an inhibitor), acrylic acid (C3H4O2, 99% w/v, contains 200 ppm MeHQ as an inhibitor), basic aluminum oxide (Al2O3, 99.9% w/w), and potassium carbonate (K2CO3, 98% w/w) were purchased from Sigma-Aldrich (Zwijndrecht, Netherlands; St. Louis, Germany; and St. Louis, MO, USA, respectively). The chemical structures of AN, AA, and MeHQ are displayed in Figure 2.
2.2. Monomers Purification
The monomers were purified similarly to what was described in the MRG LabNotes [10] , but with a few minor changes. The monomer purification was carried out in two stages: 1) column preparation and 2) purification.
2.3. Preparation of Columns
After thoroughly cleaning the work area within a fume hood, the column diameters were selected based on the quantity of monomers to be processed (approximately 100 mL). This led to the choice of columns with a diameter of roughly 4 cm. The cotton fibers were inserted into the stopcock at the base of the columns
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Figure 2. The chemical structures of (a) AN, (b) AA, and (c) MeHQ.
to act as effective filters. Subsequently, quartz sand was carefully poured over the cotton, ensuring that the sand level extended beyond the conical portion of the columns, reaching approximately 1.5 cm, to facilitate sample loading. Following this step, Al2O3 was meticulously added to the columns without inhaling the powder. This aluminum oxide layer was about 8 cm above the sand and acted as the stationary phase, removing impurities and MeHQ inhibitors from the samples by absorbing them. Lastly, oven-dried K2CO3 was added to the columns to a depth of about 3 cm to act as a barrier and stop the purified monomers from collecting when the impurified monomers in the columns got close to their top.
2.4. Purification Process
The requisite monomers were left at room temperature (RT) for two hours to prevent further water absorption via condensation. Subsequently, the monomers were meticulously introduced into the columns using glass funnels until the columns reached full capacity. Following this, the stopcocks were unsealed to enable a gentle, pressure-free flow of monomers through the columns, thereby preventing cloudiness. However, the AA monomer moved slowly through the column because the ‒COOH group of the AA reacted with the K2CO3, causing the column to bubble. In contrast, the AN monomer effortlessly descended to the column’s base. A small quantity of AN monomer was then collected in a clean vial to examine for Al2O3 contamination, revealing the AN monomer’s clarity. Subsequently, the purification continued, maintaining a continuous stream of AN monomer to prevent column desiccation. Purified AN monomer was collected in a securely sealed container once it became evident that impurities and inhibitors retained at the Al2O3 layer were not progressing further down the column (into the sand). Collection ceased when the impure monomer reached the upper portion of the K2CO3. Subsequently, the columns underwent thorough rinsing with multiple passes of acetone. For more testing, the purified AN was kept in a cold place with dry nitrogen to stop it from polymerizing at RT. A depiction of the experimental setup for the monomer purification procedure is illustrated in Figure 3.
2.5. Characterizations
2.5.1. Determining the Percentage Purity of the Purified AN
The purified monomer’s purity was calculated according to the mass of the purified chemical divided by the mass of the sample times 100 percent as follows:
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Figure 3. Experimental setup of monomer purification procedure.
where m1 and m2 are the mass of the purified monomer in the sample and the total mass of the impurified sample, respectively, the purity (99.8%) was estimated from the volume of monomer taken for purification (about 100 mL), the volume of the purified monomer (99.8 mL), and the density of acrylonitrile (0.81 g/cm3).
2.5.2. Fourier Transform Infrared (FTIR) Spectroscopy
Perkin-Elmer 2000 Frontiers Fourier-transform infrared (FTIR) (Waltham, MA, USA) was used to determine the chemical functional groups in the impurified and purified AN samples. The samples were analyzed using a spectral area ranging from 650 to 4000 cm−1, a scan resolution of 2 cm−1, and an accumulation of 32 scans at RT.
2.5.3. Nuclear Magnetic Resonance (NMR)
Fourier transform nuclear magnetic resonance (FT-NMR) at Bruker/Advance III HD 400 MHz (Bruker, Rheinstetten, Germany) with a standard-bore H-X probe and direct-polarization (DP) and cross-polarization (CP) models were used to do one-dimensional (1D) 1H-NMR and 13C-NMR. About 0.037 mL of AN samples were dissolved in 0.6 mL of deuterated chloroform (CDCl3) to study the proton and carbon environments of both purified and impurified AN at RT.
3. Results
3.1. FTIR Analysis
FTIR analysis was employed to elucidate the structural characteristics of AN after purification. The examination also facilitated the identification of alterations in distinctive bands associated with functional groups, contributing to a comprehensive understanding of structural modifications induced by the purification process through a simple column. The impact of purification on the FTIR spectrum of purified AN, in contrast to impurified AN, is depicted in Figure 4. Both spectra exhibited prominent peaks at 2230, 1736, 1614, and 967 cm−1, corresponding to the ‒C≡N, ‒C=C‒ stretching vibration, ‒C=N stretch resonance, and ‒C‒H wagging motion vibration of butadiene, respectively—considered as the principal functional groups in the AN structure [6] [13] [14] . Nevertheless, discernible variations in peak intensities were observed in the purified AN, attributed to the successful elimination of impurities during the purification process.
Notably, the impurified AN spectrum manifested ‒C‒H stretching peaks for sp2 carbon above 3000 cm−1, indicative of CH stretch in the aromatic ring. Additionally, overtone absorption between 2000 and 1667 cm−1, attributed to para-substituted 4-methoxyphenol in MeHQ [15] , was identified in the impurified AN but not in the purified AN. Furthermore, alterations in absorption bands associated with ‒CH2 scissoring (δs ‒CH2), ‒CH2 rocking (ρ ‒CH2), ‒C‒C stretch, and ‒C‒H stretch within the range of 1450 to 650 cm−1—characteristic of the alkene structure—were changed after purification. These findings collectively suggest the successful removal of MeHQ, characterized by an aromatic ring with di-substituted benzene, as illustrated in Figure 2(b), following the purification of AN through a simple column.
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Figure 4. FTIR spectra of (a) impurified and (b) purified AN.
3.2. NMR Analysis
Proton and carbon nuclear magnetic resonance (1H-NMR and 13C-NMR, respectively) were also carried out to affirm the successful purification of the AN by simple column by studying the protons and carbon environment of the purified AN compared to the impurified sample. Figure 5(a) depicts three sets of protons
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Figure 5. 1H-NMR spectra of (a) impurified and (b) purified AN.
at δ 5.7 ppm, δ 6.1 ppm, and δ 6.2 ppm, denoting a doublet of Ha, Hb, and Hc in impurified AN. Besides, multiple peaks were detected at δ 7 - 8 ppm that belong to the aromatic protons of the inhibitor of MeHQ.
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Figure 6. 13C-NMR spectra of (a) impurified and (b) purified AN.
As reported by Jacobsen [16] , the benzene ring provides a general shift of δ 7 - 8 ppm in the proton spectrum since the ring has a strong electron-donating group, such as the methoxy group (‒OCH3), which allows the aromatic ring to be electron-rich relative to benzene, with aromatic protons shielded and shifted upfield from the generic value of δ 7.28 ppm. This result agreed with FTIR, which showed C‒H stretching peaks for the sp2 carbon at values greater than 3000 cm−1, corresponding to the ‒CH stretch in the aromatic ring. A strong peak was also observed at δ 2.5 ppm that may be related to ‒OCH3 of MeHQ, as shown in Figure 2(b). However, no peaks of the aromatic protons, or ‒OCH3, were detected in Figure 5(b). It should be noted, within Figure 5(a), that the unassigned peaks spanning the chemical shift range of δ 1 - 3 ppm, which lacked explicit identification, are attributed to impurities present in the sample. After the purification process, the absence of these peaks in the spectrum indicates their successful removal from the specimen.
The 13C-NMR spectrum of impurified AN, as displayed in Figure 6(a), exhibited three peaks at δ 107 ppm, δ 117 ppm, and δ 137 ppm for C1, C2, and C3 for AN. Peaks were also detected at δ 120 - 130 ppm, thought to be for aromatic carbons [15] . However, no peaks of these aromatic carbons were noticed in Figure 6(b), indicating the removal of the MeHQ from AN after purification. It should be pointed out that the peaks at δ 7.27 ppm and δ 77 in 1H-NMR and 13C-NMR ppm are associated with the CDCl3 solvent residue.
4. Conclusion
A simple column technique was employed to purify the acrylonitrile monomer, and it exhibited excellent performance, resulting in a purity level of 99.8%. Analytical techniques such as FTIR and NMR confirmed the efficacy of this column-based purification process for acrylonitrile. These results affirm that the purification successfully eliminated impurities and inhibitors, particularly compared to a sample without purification. Notably, it should be noted that this purification technique is best suited for monomers lacking acidic functional groups. In summary, the significance of this purification method should not be underestimated, as it holds substantial potential benefits across various applications, particularly in the realm of polymerization reactions.
Acknowledgements
The authors would like to thank Universiti Kebangsaan Malaysia (UKM) under grant UKM-GUP-2019-073, the government of Yemen, represented by Taiz University, and the Islamic Development Bank (IsDB) for funding the scholar project. Thankfulness also should be expressed to the School of Chemical Sciences, Faculty of Science and Technology (FST), UKM and Centre of Research and Instrumentation Management (CRIM), UKM, for providing characterization facilities. Special gratitude is extended to Türkiye Scholarships Bursları and Istanbul Medeniyet University (IMU) for their support of Dr. Rawdah Whba as a postdoctoral fellow in Turkey.