A Synthesis of Metal-Organic Frameworks Using Terephthalic Acid Obtained from PET Bottles with Dye Adsorption for Application in Experimental Chemistry Classes

doi:10.21203/rs.3.rs-4688613/v1

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A Synthesis of Metal-Organic Frameworks Using Terephthalic Acid Obtained from PET Bottles with Dye Adsorption for Application in Experimental Chemistry Classes

https://doi.org/10.21203/rs.3.rs-4688613/v1

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In this study, we synthesized Metal Organic Frameworks (MOFs) using five transition metals from the 3d series: iron, cobalt, nickel, copper, and zinc. The ligands employed were Terephthalic Acid, also known as H₂TPA (benzene 1,4-dicarboxylate acid), recovered from PET bottles. The process of obtaining H₂TPA involved alkaline hydrolysis of PET bottle fragments in a 10 mol·L⁻¹ NaOH solution (40% m/w) placed under reflux. Once H₂TPA was obtained, it could be mixed with transition metal acetates. When these mixtures were combined in N,N’-dimethylformamide (DMF) solutions, the formation of metallic MOFs was observed. Some characteristics of the MOFs, particularly their adsorption capacity, were demonstrated through UV-Vis absorption spectroscopy using solutions of common laboratory acid-base indicator dyes such as Methylene Blue (NB), Methyl Orange (MO), Nile Blue (NB), and Malachite Green (MG). This type of simple experiment can be applied in undergraduate Chemistry classes, involving synthesis, characterization, and applications of MOFs compounds extensively documented in scientific literature. Furthermore, the use of the H₂TPA ligand through chemical recycling adds an environmentally friendly aspect to the experiments, resulting in low-cost, easily executable procedures that align with green chemistry principles.

Metal Organic Frameworks

PET Bottles

Terephthalic Acid

Dye Adsorption

In the 18th century, the accidental discovery of Prussian Blue pigment intrigued scientists for over 200 years due to its unknown structure. This mystery was finally unraveled in 1979 with the advent of Single Crystal X-ray Diffraction (SCXRD) The structure was elucidated using Alfred Werner's Coordination Theory from 1893, with the compound Fe₄[Fe(CN)₆]₃ identified as a coordination polymer.[1] Werner introduced the term "complexes" to explain the formation of molecules through the coordinated bonding of transition metals to ligand species. The coordination sphere of the complex was defined as composed of the number of ligands that electronically saturated the metal. Subsequent studies determined the coordination sphere, coordination number, and geometry, taking into consideration the oxidation state of the central metal.

In turn, metal-organic frameworks (MOFs) gained prominence around the 1990s, primarily through the work of Omar Yaghi and collaborators, who synthesized the first MOFs with transition metals Co²⁺, Ni²⁺, and Zn²⁺ using the organic molecule acid-benzene-1,3,5-tricarboxylic acid (trimesic acid) as a ligand.[2] In 1999, the same author published a work describing the synthesis of a framework with permanent microporosity. The synthesis involved Zn²⁺ and the organic molecule acid-benzene-1,4-dicarboxylic acid, resulting in the well-known MOF-5.[3] The single crystal X-ray diffraction technique enabled the elucidation of structures, giving rise to a new field where ligands of different natures and virtually all metals from the s, p, especially d and f blocks could be applied.

MOFs (Metal-Organic Frameworks) are polymeric compounds formed through Lewis acid-base covalent bonding between a metal cation and multidentate organic ligands capable of bridging different metal centers. These ligands often contain coordinating functional groups such as carboxylates, pyridines, or imidazoles, leading to the creation of three-dimensional networks based on the arrangement of the metal coordination spheres.[4]

In some structures, the presence of Secondary Building Units (SBUs) is observed. SBUs are molecular clusters that preserve the metal's coordination geometry, causing the ligand's coordination modes to extend regularly. This imparts a certain structural rigidity to the network while providing high thermodynamic stability. In MOF-5, the SBU consists of a molecular octahedron µ-4-oxo [M₄O(O₂CR)₆], with an O²⁻ connected to 4 tetrahedral Zn²⁺ ions (c). On the other hand, in the MOF Cu-BDC, the SBU is called a paddle-wheel [M₂(O₂CR)₄L₂], where L = axial ligand and CR = Carboxylate RCOO⁻ (a). Both are present in the structures of zinc[5] and copper[6] acetates. Based on these characteristics, the International Union of Pure and Applied Chemistry (IUPAC) has agreed to classify these compounds as "a coordination network with organic ligands containing potentially empty cavities."

The pores formed in metal-organic frameworks provide them with flexibility on a micro-mesoporous scale, enabling the insertion of other molecules, typically gases. The Brunauer-Emmett-Teller (BET) method describes multilayer adsorption and can define the size of the MOF interstice. Other characteristics typically associated with this type of material include high crystallinity, low density due to the presence of successive unoccupied areas, and stability under ambient conditions. Kitagawa and Ferrey conducted research in the field of porosity and the behavior of guest molecules in relation to the structure. [8]^,[9]

PET (Polyethylene Terephthalate) it is a thermoplastic polymer of the polyester type developed by British chemists John Rex Whinfield and James Tennant Dickson in 1941. Primarily used in the form of fibers for the textile industry since the early 1950s by the American company DuPont and in the manufacture of beverage packaging since the early 1970s. Since the 1990s, it has been used for the production of food packaging from recycled material. Its formation occurs through condensation polymerization between ethylene glycol (1,2-ethanediol) and acid-benzene-1,4-dicarboxylic. PET is globally used for the manufacturing of bottles due to its lightweight and unbreakable nature, making it suitable for storing carbonated beverages and replacing glass bottles. It is because of this cost-effectiveness that its production and recycling are so interesting and profitable. However, with the widespread use of PET bottles since the 1990s, a serious environmental problem has arisen: many of these bottles are improperly disposed of and end up in landfills, rivers, sewers, oceans, and forests. The issue is compounded by the fact that it can take up to 800 years for PET bottles to decompose.[10] Therefore, one way to harness this material is the extraction of terephthalic acid and its application in the synthesis of MOFs, aligning with the concept of Green Chemistry. One of the criteria categorizing a synthesis as "green" or technologically clean is the use of recycled raw material sources.[11].

In turn, dyes improperly discarded in effluents are also pollutants that trigger environmental impacts, such as blocking the sunlight necessary for the photosynthesis of plants and aquatic microorganisms, leading to a serious biological imbalance. Additionally, many of these dyes and/or their degradation products are toxic.[12]

This work aimed to develop a route for obtaining terephthalic acid through the depolymerization of post-consumer PET bottles, adhering to the principles of Green Chemistry. The goal was to synthesize Metal-Organic Frameworks (MOFs) with transition metals and observe and quantify their adsorption properties for molecules in dyes. The intention was to apply these MOFs in experimental chemistry classes at the higher education level.

General remarks

The colorless PET bottles came from commercial bottled water purchased on the market, washed with neutral soap, rinsed with distilled water at least three times and dried in air. The bottles were cut into small pieces (approximately 1 cm²).

The solvents N,N’-dimethylformamide (DMF), Chloridric Acid 37%, the base NaOH 98,5%, and the sals FeCl₃∙6H₂O, Co(Ac)₂∙4H₂O, Ni(Ac)₂∙4H₂O, Cu(Ac)₂∙H₂O, Zn(Ac)₂∙2H₂O were obtained from Sigma-Aldrich and used as acquired.

Powder X-ray Diffraction (PXRD) patterns were collected at 298 K on a Bruker D2 PHASER diffractometer equipped with a Lynxeye-2 detector using CuKα-radiation (λ = 1.5406 Å). Samples were lightly ground, placed on a zero background sample holder and scanned over a 2θ range of 5° to 80°. A step size of 0.02° to give a total of 3752 steps was used. A current of 40 mA and accelerating voltage of 30 kV were used to generate the X-rays.

Diffuse Reflectance UV-Vis Spectra (DRS) were obtained using a 2600i Shimadzu UV − Vis recording spectrophotometer with integrating sphere covered with Ba₂SO₄ on range 800 − 200 nm.

Fourier-transform Infrared (FTIR) spectra were measured on Shimadzu IR Affinity 1 FTIR spectrometer with KBr pellets, scanning from 4000 to 400 cm⁻¹ at room temperature.

UV-Vis Spectra were obtained using a UV-2600 Shimadzu UV − Vis recording spectrophotometer with cuvetts of 1 cm of optical path, in H₂O on range 800 − 200 nm at 25°C.

¹H and ¹³C{¹H} NMR spectra were obtained in DMSO-d₆ from Sigma-Aldrich at 25,0°C on an Agilent MR 500 Ultrashield spectrometer operating at 500.13 and 125.61 MHz, respectively.

2.1 Recovery of terephthalic acid from PET bottles

The ligand Terephthalic Acid (H₂TPA) used in MOF syntheses can be obtained from PET bottles through alkaline hydrolysis. For this, 20 g of PET bottles, cut into small pieces, were added to 100 mL of a 10 mol·L⁻¹ NaOH solution (40% w/v) and refluxed at 160°C for 8 hours. After the elapsed time, the formation of a white-colored suspension was observed, which was acidified with concentrated HCl until reaching pH ≤ 0. The formed precipitate was filtered, washed with water, and left to dry. The residual PET solid was separated, washed, and weighed, resulting in a mass of 0.345 g. Since PET (Molecular Weight = 192.18 g/mol) contains 86% in moles of H₂TPA, the yield of Terephtalic Acid obtained was 98%. This was characterized by ¹H and ¹³C NMR spectroscopy and FTIR. The other component of PET, ethylene glycol, is regenerated during this hydrolysis and is removed in the washing process due to its water solubility. ¹H NMR analysis reveals a minimum residue at 3.34 ppm in dmso-d₆, attributed to the hidrogens from methylene group (CH₂) of this molecule.[13]

2.2 Synthesis of MOFs using recovered Terephthalic Acid.

For each synthesis of M-TPA MOFs, 0.02 mol of Terephthalic Acid (H₂TPA) (3.30 g) and 0.01 mol of metal acetate salts were required (except for iron, which was used as chloride), along with 0.02 mol of Triethylamine (Me₃N) 98% (2.80 mL) and 100 mL of N,N'-dimethylformamide (DMF). Two different beakers were used. In beaker 1, the metal salt and 20 mL of DMF were added, left under stirring and heating until complete dissolution of the metal salt. In beaker 2, H₂TPA and 30 mL of DMF were added, also under stirring and heating, and Triethylamine was added to deprotonate and assist in the dissolution of the ligand. After complete dissolution of the compounds, the solution from beaker 2 was poured into beaker 1, quickly forming a suspension, which was stirred for an additional 15 minutes without heating. Then, the solid was vacuum-filtered, washed with 50 mL of DMF, and left to dry at room temperature for 7 days. These were characterized by UV-Vis Diffuse Reflectance Spectroscopy (DRS), Infrared Rpectroscopy (FTIR), and Powder X-Ray Diffraction (XRD).

2.3Adsorptivity test of dyes on MOFs

For this step, solutions of the dyes were used: a) 0.1 mg·L⁻¹ Methylene Blue (MB); b) 5.0 mg·L⁻¹ Methyl Orange (MO); c) 5.6 mg·L⁻¹ Nile Blue (NB) and d) 5.6 mg·L⁻¹ Malachite Green (MG). 500 mg of each MOF was added to test tubes, and then 5 mL of each dye solution was added to each tube. The tubes were then centrifuged for 5 minutes. It was observed that the MOFs settled at the bottom of the tubes and that their surfaces were saturated with the dye. Therefore, with the aid of a rod, the bottom part was mixed again with the solution, and the mixture was centrifuged again for the same duration. Finally, the absorbances of the solutions were measured in a UV-Vis spectrophotometer

3.1 H₂TPA (Terephtalic Acid)

The H₂TPA obtained through basic hydrolysis in NaOH initially forms terephtalate TPA²⁻, which is slightly soluble in water and can be lost during washing. Therefore, the reaction product needs to be acidified to form H₂TPA. This can be observed in the ¹H NMR as a singlet in the aromatic region (s, 8.03 ppm, 4H) and a broad band in the COOH region (s, 13.28 ppm, 2H) in DMSO-d₆. The spectrum does shows minimum residues of ethylene glycol at 3.34 ppm, confirming the successful hidrolysis. TPA²⁻ appears as a more unshielded signal compared to H₂TPA (s, 8.31 ppm, 4H) and does not have the COOH band. FTIR spectra can also differentiate the obtained products H₂TPA and TPA²⁻. (See spectra on SI for reference).

3.2 MOFs M-TPA

For the formation of MOFs, five metals from the 3d series were used, varying their electronic configurations: d⁵, d⁷, d⁸, d⁹, and d¹⁰. This can be clearly observed in the UV-Vis diffuse reflectance absorption spectra (Fig. 3), where the characteristic colors of each MOF are related to their absorptions, especially in the visible region. In the FTIR spectra of the M-TPA MOFs (Fig. 4), the δ-M-O stretching bands show slight variations: 751 cm⁻¹ for Fe-TPA, 753 cm⁻¹ for Co-TPA and Ni-TPA, 757 cm⁻¹ for Cu-TPA, and 760 cm⁻¹ for Zn-TPA. The symmetric and asymmetric stretching bands of the δ-C = O group of the carboxylate groups appear around 1600 cm⁻¹ as broad bands due to their overlapping. The MOFs were obtained as non-crystalline solids due to the fast and direct synthesis method, not allowing sufficient time for the crystallization process. Therefore, for their structural elucidations, Powder X-ray Diffraction (XRD) spectra were necessary (Fig. 5). The obtained products were compared with the resolved structures of M-TPA MOFs reported in the literature (except for Fe-TPA, which does not have a resolved crystalline structure).

First, the acetate salts of Copper and Zinc are organized in the form of SBUs, where these salts actually form clusters with formulas [Cu₂(Ac)₄(H₂O)₂][14] and [Zn₄O(Ac)₆][5] (Fig. 6). Thus, the use of terephthalic acid for the formation of M-TPA MOFs involves the substitution of Ac or CH₃COO⁻ groups with terephthalates (⁻OOC)-Ph-(COO⁻), maintaining the same connectivity of the carboxylate groups and extending the structure due to the two coordination points of carboxylate groups in TPA. Generally, MOFs containing SBUs have greater structural rigidity and increased stability against metal leaching in their structures caused by solvents, including water. Zinc and Copper MOFs with terephthalic acid are structures that contain SBUs, which provide rigidity to these structures and are known as MOF-5[3] and Cu-terephthalate[15]. It can be observed in the PXRD spectra (Fig. 5) that these compounds exhibit a high degree of crystallinity, with sharp, non-overlapping, and highly intense peaks.

The Co-TPA and Ni-TPA MOFs do not have SBUs in their structures, similar to their precursor acetate salts. Consequently, the networks of both MOFs are organized in a more random manner. In these structures, the carboxylate groups coordinate in such a way that each O atom of a COO⁻ group binds to a different metal center (µ²-η¹:η¹). In all M-TPA MOF structures, the coordination number (CN) remained unchanged, being 6 for Co, Ni, and Cu (with Cu forming a Cu-Cu dimer) with octahedral geometry, and 4 for tetrahedral Zn due to the presence of the central O atom in the Zn₄O cluster. As can be seen in Fig. 7.

3.3 Stability of MOFs in water

Firstly, the stability of the MOFs in water was evaluated due to the solubility of the dyes used. To this end, 500 mg of each MOF were added to 5 mL of water and the mixture was centrifuged for 10 minutes. After the allotted time, a pink-colored solution was observed in the tube containing the Co-TPA MOF, while a very faint green color was observed in the tube with the Ni-TPA MOF. All solutions were subjected to UV-Vis absorption measurements, which showed characteristic bands for the [Co(H₂O)₆]²⁺ and [Ni(H₂O)₆]²⁺ complexes, indicating the partial leaching of these metals from the frameworks into the solution. The other three MOFs did not show absorption in the UV-Vis region, demonstrating stability in aqueous solution (Fig. 8). This was expected, particularly for the Cu-TPA and Zn-TPA MOFs, due to the presence of SBUs in their structures.

3.4 Adsorption of dyes

First, the adsorption of the dyes Methylene Blue (MB) 0.1 mg·L⁻¹ and Methyl Orange (MO) 5 mg·L⁻¹ was measured, and an intense absorption profile was observed, especially for Methyl Orange. Following the same approach, the dyes Nile Blue (NB) and Malachite Green (MG) were used at concentrations of 5.6 mg·L⁻¹, which have lower absorbance compared to the previous ones. The structures of the four molecules are shown in Fig. 9.

Adsorption tests were conducted, and after the centrifugation process, a significant discoloration of the solutions was observed, varying according to the MOF used. In the solutions containing Fe-TPA, the discoloration was almost complete, regardless of the dye used, indicating its high adsorption capacity (Fig. 10).

The adsorptions of the dyes onto the MOFs were measured by UV-Vis spectra (Fig. 11) in relation to their initial concentrations and quantified by comparing them at their respective maximum absorption wavelengths (λ_max) according to formula 1.1. Overall, the Fe-TPA MOFs proved to be the most effective in adsorbing all dyes, with an efficiency exceeding 90%, as evidenced in graph 1. The Cu-TPA and Zn-TPA MOFs showed intense adsorptions for Methylene Blue and variable adsorptions for the other dyes. Additionally, the Ni-TPA and Co-TPA MOFs exhibited moderate adsorption for Methyl Orange and low stability, especially for Co-TPA, as observed in the absorption spectra indicating metal leaching. Overall, Nile Blue (NB) dye was also well adsorbed by most M-TPA MOFs.

(1.1) % Dye Removal = \(\:\frac{Abs\:Dye\:-\:Abs\:MOF}{Abs\:Dye}\) X 100%

Graph 1. Adsorption efficiency of dyes (in %) by their respective M-TPA MOFs.

The dye molecules in solution have the ability to penetrate the pores of the networks in solution. The adsorption process involves various interaction forces, such as electrostatic interactions between the positively charged metal and functional groups containing electronegative atoms such as N or O; coordination bonds and π-π stacking interactions between the aromatic rings of terephthalate ligands and the rings of dye molecules; and even hydrogen bonding. The Fe-TPA does not have a regular crystalline structure, thus it possesses active sites at the metal centers capable of interacting with the dye molecules, which would explain its high adsorption capacity. On the other hand, the Zn-TPA MOF, due to its high crystallinity, has its metal centers fully saturated with Zn-O bonds with the terephthalate ligand, resulting in reduced adsorption capacity. As for the Co-TPA, Ni-TPA, and Cu-TPA structures, these metal centers are coordinated to molecules of the solvent DMF or even H₂O, which can be easily exchanged by coordination bonds with dye molecules, explaining their intermediate adsorption capacities. Additionally, the size of the molecules and the pore size of each MOF can influence their adsorption capacity in relation to different dyes.[18]

In this study, we were able to synthesize MOFs with terephthalic acid, H₂TPA, directly recovered from PET bottles by basic hidrolysis, and using different transition metals from the 3d series, such as Fe, Co, Ni, Cu, and Zn, in a simple and straightforward manner. As an application, we observed and quantified the adsorption of four common laboratory dyes: Methylene Blue (MB), Methyl Orange (MO), Nile Blue (NB), and Malachite Green (MG). This allowed us to correlate their molecular structures with the structures of the synthesized M-TPA MOF networks, quantifying this adsorption and confirming the adsorption properties of these species, characteristic of this type of material due to its porous nature.

Conflicts of Interest

There are no conflicts to declare.

Author Contribution

Synthesis, characterization and methodology were conducted by J. M. R. Gonçalves. Photographs were obtained by J. V. Brandão. Final review was carried out by B. S. Lima-Neto.

Funding

This work was supported by the funding from Brazilian agencies Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) (No. 140012/2021-4), Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) and Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP).

Data Availability Statement

Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers CCDC 1132068 (1), 1313801 (2), 265092 (3), 638866 (4), 687690 (5), 1239799 (6). Copies of the data can be obtained free of charge via https://www.ccdc.cam.ac.uk/structures/.

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Schemes are available in the Supplementary Files section.

Graph 1 is available in the Supplementary Files section.

No competing interests reported.

SUPPLEMENTARYMATERIALManuscriptJMRGoncalvesJVBBSLN.docx
floatimage1.png
Graphical Abstract
Scheme1.png
Scheme 1 - Alkaline Hydrolysis of PET.
Scheme2.png
Scheme 2: Formation of a generic metal-organic framework from a metal salt and the ligand H₂TPA (metallic centers M are at the vertices, while the ligands are at the edges).
Graph1.png
Graph 1. Adsorption efficiency of dyes (in %) by their respective M-TPA MOFs.

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A Synthesis of Metal-Organic Frameworks Using Terephthalic Acid Obtained from PET Bottles with Dye Adsorption for Application in Experimental Chemistry Classes

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Abstract

Figures

1. Introduction

2. Experimental Procedure

2.1 Recovery of terephthalic acid from PET bottles

2.2 Synthesis of MOFs using recovered Terephthalic Acid.

2.3Adsorptivity test of dyes on MOFs

3. Discussion

3.1 H₂TPA (Terephtalic Acid)

3.2 MOFs M-TPA

3.3 Stability of MOFs in water

3.4 Adsorption of dyes

4. Conclusions

Declarations

References

Schemes

Graph 1

Additional Declarations

Supplementary Files

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