Polymer-metal complex based on copper(II) acetate and polyvinyl alcohol: thermodynamic and catalytic properties published by Ural Federal University eISSN 2411-1414; chimicatechnoacta.ru ARTICLE 2022, vol. 9(3), No. 20229304 DOI: 10.15826/chimtech.2022.9.3.04 1 of 7 Polymer-metal complex based on copper(II) acetate and polyvinyl alcohol: thermodynamic and catalytic properties Kuralay S. Maksotova a* , Dariya Т. Kalikh a, Arnur T. Omirzakova b, Botagoz S. Bakirova a, Dina N. Akbayeva a a: Al-Farabi Kazakh National University, Almaty 050040, Kazakhstan b: Nazarbayev University, Nur-Sultan 010000, Kazakhstan * Corresponding author: maksotovak@yandex.kz This paper belongs to the CTFM'22 Special Issue: https://www.kaznu.kz/en/25415/page. Guest Editors: Prof. N. Uvarov and Prof. E. Aubakirov. © 2022, the Authors. This article is published in open access under the terms and conditions of the Creative Com- mons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). Abstract In this work we obtained a polymer-metal complex by mixing aqueous solution of copper(II) acetate with PVA at a certain ratio, pH of the solution and temperature. The composition of the complex compound was determined by potentiometric and conductometric titration. The possibility of a complex formation was proved by calculating thermo- dynamic characteristics. The stability constant of the polymer-metal complex was calculated on the basis of the modified Bjerrum’s method. The metal-polymer complex was synthesized in the ratio 1:2. IR spec- troscopy and scanning electron microscopy (SEM) confirmed the co- ordination of polymeric PVA ligand to copper and allowed evaluating the morphology and features of the complex surface. The catalytic ac- tivity of the synthesized compound was evaluated in the oxidation re- action of elemental phosphorus (P4) by oxygen in aqueous-organic me- dia under mild conditions. Quantitative analysis of phosphoric acid was made by photocolorimetric method. We found that the oxidation process of P4 in the presence of the complex Cu(PVA)2(OAc)2 in aque- ous-organic media is characterized with the maximum absorption rate, in comparison with Cu(OAc)2·H2O oxidation process with P4, and yields up to 97% of the products. The process of oxidation of yellow phosphorus by oxygen in the presence of the copper(II)-PVA complex proceeds through key reactions of two-electron reduction of the cata- lyst P4 with the formation of intermediate phosphorus-containing products P3+ and the stages of catalyst regeneration by oxygen. Twenty-electron oxidation of P4 to the phosphorus-containing P5+ products involves 10 two-electron redox reactions and a number of complexation or hydrolysis stages. Keywords copper(II) acetate polyvinyl alcohol polymer-metal complex thermodynamic characteristics catalysis oxidation white phosphorus Received: 25.06.22 Revised: 08.07.22 Accepted: 08.07.22 Available online: 26.07.22 1. Introduction The development of oxidation processes is essential in to- day’s chemistry and industry [1–4]. Many oxidative tech- niques have been known to exist in natural life, and a lot of them have been used in various applications the industry, from wastewater treatment to cellulose or lignin bleaching [5–8]. Among these applications, oxidizing processes in the detergent industry, called bleaching, are particularly pre- ferred for removing dyes [9–11]. In general, the stability and selectivity of homogeneous catalysts are strongly related to their molecular structure. Given the steric, electronic and conformational properties, suitable ligands must be designed for metal complexes that function as effective catalysts. These ligands must also be flexible against oxidation and be electron donors in order to achieve high oxidation states of the active metal. Most of them are heat-sensitive substances and generally deterio- rate above 150 °C [12–14]. Furthermore, the consideration of steric, electronic and conformational properties is necessary for the design of suitable ligands for metal complexes that will serve as ef- fective catalysts. Under heat treatment, polymers such as polyvinyl alco- hol (PVA), polyvinyl chloride (PVC), polystyrene (PS), etc., which have saturated main molecular chains and side http://chimicatechnoacta.ru/ https://doi.org/10.15826/chimtech.2022.9.3.04 mailto:maksotovak@yandex.kz https://www.kaznu.kz/en/25415/page http://creativecommons.org/licenses/by/4.0/ https://orcid.org/0000-0001-8606-5005 https://orcid.org/0000-0001-9101-2418 https://crossmark.crossref.org/dialog/?doi=https://doi.org/10.15826/chimtech.2022.9.3.04&domain=pdf&date_stamp=2022-7-26 Chimica Techno Acta 2022, vol. 9(3), No. 20229304 ARTICLE 2 of 7 groups, can form conjugated structures by removing the side groups from the main one. Thermal degradation of pol- ymers creates systems with delocalized π-electrons, which can lead to optical and electronic improvements. Then, pol- ymer ligand synthesis and selective chelation of specific metal ions is an active research area [15]. Metal ions from polymer complexes have potential applications in electro- lytes [16], sensors [17], stabilizers [18] and semiconductors [19]. Polyvinyl alcohol is an important material, given its large-scale applications, such as biomaterials, biosensors, electrochemical sensors, membranes with selective permit- tivity, viscous media to control the crystallization process of salts, controlled monitoring of drugs or catalytic sys- tems, etc. Polyvinyl alcohol (PVA) is a non-toxic, non-car- cinogenic, biodegradable, biocompatible, water-soluble and inexpensive polymer. It could also be used for metal ions or salts in ecological composites [20]. PVA is a potential ma- terial that has a high dielectric strength, a good charge stor- age capacity and dopant-dependent electrical properties. It has a carbon-chain dorsal bone with hydroxyl groups at- tached to the methane carbons. OH groups can be a source of hydrogen bonds and can, therefore, help in the formation of polymer complexes. PVA has unique mechanical properties and exhibits both ionic and electronic conduction [21]. Despite certain achievements in the chemistry of ele- mental phosphorus (P4), insufficient attention has been paid to the oxidative reactions involving P4 in the catalytic regime, the description of their kinetics and mechanics, the identification of the nature of catalytically active interme- diates. Therefore, in this work, the optimal molar ratio of a complex compound based on copper(II) acetate and polyvi- nyl alcohol was studied. The possibility of a reaction of pol- ymer-metal complex formation was studied by calculating thermodynamic characteristics. The complex was tested as catalyst in yellow phosphorus oxidation in aqueous-organic media under mild conditions. 2. Experimental Copper(II) acetate Cu(OAc)2H2O, polyvinyl alcohol (molec- ular mass 30 000, Sigma Aldrich), hydrochloric acid, so- dium hydroxide, sodium chloride, toluene, distilled water were used without purification. Yellow phosphorus of the Shymkent Production Association “Phosphorus” (Kazakh- stan) was used, which was previously mechanically cleaned from the oxide film under water. The concentration of P4 in the obtained toluene solution (P4, mol/L) was determined by iodometric titration [22]. 2.1. Synthesis of Cu(CH3COO)2 – PVA A solution of 2.0 g (0.01 mol) of Cu(OAc)2·H2O in 15 ml of distilled water was added to 15 ml of an aqueous solution of 0.88 g of PVA (0.02 mol). The resulting mixture was stirred by magnetic stirrer for 1 hour at ambient temperature until the polymer was completely dissolved and bound to Cu(II) ions. The synthesized light-green complex was dried and stored in air at room temperature. Yield: 3.15 g (98%). The process of complex formation between copper(II) ion and PVA was investigated by potentiometric and con- ductometric methods with several ionic strengths and tem- peratures. Potentiometric studies were carried out in ther- mostated conditions on an ionomer pX-150MI using silver chloride and glass electrodes. The accuracy of the pH meas- urement was 0.02 pH units. Conductometric studies were performed on a ConductivityMeter 13701/93 device (PHYWE) under thermostatically controlled conditions. The polymer-metal complex was obtained by mixing aqueous solution of copper(II) acetate with PVA at certain ratio, pH of the solution and temperature. The stability constant of the polymer-metal complex was calculated on the basis of the modified Bjerrum’s method. IR spectra of PVP and Cu(II)-PVA complex were recorded on a FT IR-4100 type A JASCO instrument in the range of 4000–450 cm–1. SEM images were taken on a JSM-6490LA Jeol instrument equipped with an X-ray dispersive energy detector (EDX) for elementary analysis (JEOL, Japan). IR spectra and SEM images were obtained in analytical laboratories at the Technical University of Kaiserslautern (TUK, Germany). Quantitative analysis of phosphoric acid was performed by photocolorimetric method on a spectrophotometer SPEKOL 1300 (ANALYTIK JENA, Germany). 2.2. Typical Reaction Procedure Oxidation of yellow phosphorus by oxygen was carried out on a temperature-controlled laboratory setup with inten- sively stirred up glass temperature-controlled reactor with negligible temperature gradient ‘‘a catalytic duck”, sup- plied by the potentiometric device and connected to the gas burette filled with oxygen. The laboratory experiments were made as follows. The reactor with a total volume of 150 mL was charged with the catalyst (1.07 mmol) under an oxygen atmosphere. The reactor and the gas burette were preheated to 60 °C. The temperature was maintained by the water circulating between the glass reactor and the heating devices. Then, in oxygen flow, a solution of P4 in toluene (1.07 mmol) was added to water (9 mL, 9:1 by volume), and an electric motor was switched on. During the catalytic re- action the rates of oxygen absorption were recorded in cer- tain intervals. The temperature was maintained with an ac- curacy of ±0.5 °C by means of the thermostat. After the ex- perimental runs, the reaction solutions were mixed to- gether and analyzed on a spectrophotometer. 3. Results and discussion 3.1. Potentiometric titration Figure 1 shows the potentiometric titration curve of Cu(OAc)2 – PVA complex. The mixing of solutions of poly- mer with salt is accompanied by a pH decrease, which is explained by the deprotonation of initially protonated PVA during the complexation. Chimica Techno Acta 2022, vol. 9(3), No. 20229304 ARTICLE 3 of 7 From the titration curve (Figure 1), the optimal molar ratio of the reacting components k (k=[Cu2+]/[PVA]=0.50) was found. It means that one central metal atom bonds with two mono-links of polymer ligands. 3.2. Conductometric titration In order to confirm the composition of the formed PVA-Cu2+ complex, the dependence of the conductivity corrected for the viscosity on the ratio of the initial component of the sys- tem was studied (Figure 2). The increase in electrical conductivity is due to the re- leased H+ ions during the reaction between PVA and cop- per(II) ions. As can be seen from Figure 2, the electrical conductivity of the solution with an increase in the molar content of metal ions passes through the inflection point. Based on the data obtained as a result of conducted conduc- tometric studies, it can be argued that the complexation process is accompanied by an increase in the electrical con- ductivity of the system at the ratios PVA-Cu2+=2:1. In the process of complexation of the PVA polymer lig- and, their hydrodynamic dimensions decrease (chelate ef- fect); protons are released, as evidenced by the experi- mental results. Thus, it can be assumed that the complex of the composition is formed in the PVA–Cu2+ system. Figure 1 Сurve of potentiometric titration of PVA (10–2 M) with cop- per salt Cu(OAc)2 (10 –2 M) (where V – titrant volume in mL, pH – рН of solution). Figure 2 Сurve of conductometric titration of PVA (10–2 M) with copper salt Cu(OAc)2 (10 –2 M) (where V – titrant volume in ml,  – specific electrical conductivity of solution in Sm/cm). 3.3. Modified Bjerrum’s method calculations The stability constant of the resulting polymer complex and the coordination number of copper(II) were calculated us- ing the modified Bjerrum’s method. In accordance with the known method, the potentiometric study was carried out at three values of the ionic strength of the solution: 0.01, 0.05, and 0.1 mol/L, and the polymer ligand solution was titrated with hydrochloric acid (HCl), depending on the nature of the complexing metal salt, with a change in the pH of the medium in the absence and presence of metal ion, as well as at several temperatures (25, 45, 70 °C). Figure 3 shows the pH value change in the absence and presence of metal ions during the experiment. It is clearly seen that the pH value in the presence of metal ions is higher than in exper- iment without metal ions. It signifies the formation of the complex and means that the system reacts in the acidic me- dium. Table 1 shows the values of the Bjerrum’s formation functions (n) corresponding to the coordination number of the metal complexing agent at three ionic strengths and at 70 °C. The data obtained indicate the formation of a copper polymer complex in which the coordination number of the metal is equal to two. 3.4. Thermodynamic parameters of the process The knowledge of the thermodynamic parameters (changes in Gibbs’ energy (∆rG0), enthalpy (∆rH0) and entropy (∆rS0)) of the studied process is necessary for the scientifically based choice of the optimal conditions for its implementa- tion in practice. Moreover, many researchers admit that the fundamental laws of thermodynamics, which were estab- lished for the systems consisting of low molecular weight compounds, can be applied to the systems involving macro- molecules [23]. The PVA-Cu(OAc)2 system is characterized with the neg- ative Gibbs’ energy, which indicates the spontaneous occur- rence of the studied process in the direction of the com- pound formation (Table 2). Figure 3 Curves of potentiometric titration of aqueous solutions of polyelectrolyte of PVA (10–2 M) (1) and PVA – Cu2+ (10–2 M) (2) by hydrochloric acid (10–2 M) and Lf=0.01, T = 25 °C. Chimica Techno Acta 2022, vol. 9(3), No. 20229304 ARTICLE 4 of 7 In the temperature range of 25–70 °C, the complexation process of PVA with Cu2+ ion is accompanied by the release of heat (exothermic process), as a result of which the strength and stability of the polymer-metal complex de- creases with temperature increasing. Thus, based on analysis of the results of potentiometric and conductometric analysis, the formation of copper(II)- PVA polymer complex and its composition were established. 3.5. IR spectroscopy and SEM studies The process of the formation of the copper(II)-PVA complex is characterized by the negative value of the change in en- tropy, which is caused by the existence of donor-acceptor bond in the studied complex. This also indicates that the ratio between copper(II) ion and PVA is 1:2. To study the surface of the pure polymer and the polymer-metal com- plex, the scanning electron microscopy (SEM) method was used; the results of the study are presented in Figures 4 and 5. A comparison of microscopic images of the pure polymer and the resulting complex indicates the for- mation of porous spherulites of different sizes. The infrared spectrum was acquired for polyvinyl alco- hol and the complex copper(II) acetate – PVA (Figure 6). Table 1 Values of Bjerrum’s formation functions of the Cu(OAc)2 – PVA complex at 70 °C and I =0.01 mol/L. [LH+]104 (mol L–1)a pLb L106 (mol L-1)c Lc 10 3 (mol L-1)d ne 3.33 5.39 4.12 3.00 2.00 6.66 6.04 0.922 2.67 1.78 9.99 6.33 0.462 2.33 1.56 13.30 6.69 0.206 2.00 1.33 16.60 6.83 0.149 1.67 1.11 20.00 7.22 0.0597 1.34 0.89 23.30 9.42 0.000381 1.00 0.67 26.60 10.67 0.0000213 0.67 0.45 30.00 12.29 0.000000517 0.34 0.23 33.3 13.91 0.0000000124 0.014 0.01 a [LH+] – concentration of the protonated ligand groups; b pL or –lg[L] – concentration of the free ligands calculated on Henserson-Hasselbach equation pH = pKα + mlg[L]/[LH+], where m – the empirical coefficient considering interlink interaction of a polymeric chain; c [L] – concentration of the free ligands which are not involved in a complexing process; d [L]С – concentration of the ligand groups connected in a complex; e n – Bjerrum’s formation functions or average coordination num- ber of a metal ion. Table 2 Thermodynamic characteristics of the complexation. T, oC -∆rG o, kJ/mmola -∆rH o, kJ/mmolb -∆rS 0, kJ/mmolKc 25 0.32 1.6328 0.004405 45 0.23 1.0900 0.002704 70 0.16 1.3502 0.003470 a ∆rG o – Gibbs’s energy change of reaction; b ∆rH o – enthalpy change of reaction (heat effect); c ∆rS 0 – entropy change of reaction. By comparing these two IR spectra, a displacement of the band position νO–H is clearly seen. In the polyvinyl alco- hol infrared spectrum, the position of νO–H changes from 2390 to 2410 upon complexation with Cu(II), which can be seen in the infrared spectrum of the complex based on cop- per(II) acetate – polyvinyl alcohol, indicating its participa- tion in the formation of copper – polymer complex [20]. It gives strong indication of specific interactions between the ligand and metal ion. 3.6. Oxidation of yellow phosphorus (P4) cata- lyzed by Cu(PVA)2(OAc)2 complex under mild conditions The synthesized catalyst was used in the process of oxidation of phosphorus at room temperature in the presence of oxy- gen at atmospheric pressure: a) b) c) Figure 4 SEM micrographs of polyvinyl alcohol. Chimica Techno Acta 2022, vol. 9(3), No. 20229304 ARTICLE 5 of 7 a) b) c) Figure 5 SEM micrographs of copper(II) acetate-PVA complex. Figure 6 IR – spectra of PVA and PVA – Cu(OAc)2H2O. P4 + 12H2O + 5O2 → 4P(O)(OH)3 + 6H2O (1) The results of the oxidation reaction studies at 60 °C were presented in the mole ratio of the reagents [Cu(PVA)2(OAc)2]:[P4] = 1:1; 3:1; 6:1. During the interaction of yellow phosphorus with aqueous alkali solutions at 50oC due to poor solubility of phosphorus (S500 °C = ~3·10–3 g/L) a slow disproportiona- tion reaction takes place with the formation of hypophos- phite and PH3 [24]. Figure 7 shows the typical kinetic curves of P4 oxidation process in the solution of Cu(PVA)2(OAc)2–С7Н8–H2O. The re- action proceeds in an unsteady mode. Both the kinetic (W–) and the conversion (W–Q) curves go through maximum as in the case of non-modified Cu(OAc)2. The average duration of experiments was 130 minutes. The maximum oxygen absorp- tion rate was observed for the molar ratio of [Cu(OAc)2]:[Р4] and [Cu(PVA)2(OAc)2]:[Р4] of 6:1. The experiment of the oxidation process with P4 in the presence of the complex Cu(PVA)2(OAc)2 was characterized with the maximum absorption rate, in comparison with Cu(OAc)2·H2O. The reaction conditions and product yield of P4 oxida- tion by O2 with Cu(OAc)2·H2O and Cu(PVA)2(OAc)2 in aque- ous-organic solutions are presented in Table 3. 0 20 40 60 80 100 0,0 0,5 1,0 1,5 2,0 a) , min W O 2 x 1 0 2 , m o l/ (L ·m in ) 1:1 1:3 1:6 0 40 80 120 160 0,0 0,4 0,8 1,2 , min W O 2 x 1 0 2 , m o l/ (L ·m in ) b) 1:1 1:3 1:6 Figure 7 The kinetic curves (W–) of the oxidation process of P4 with oxygen in aqueous-organic medium in the presence of Cu(OAc)2·H2O (a) and Cu(PVA)2(OAc)2 (b) complex. Reaction con- ditions, mol/L: [P4] 1.07; [Н2О] 50; [C7H8] 0.94; 60 °C; [Cat]: 1 – 0.01; 2 – 0.03; 3 – 0.06. Chimica Techno Acta 2022, vol. 9(3), No. 20229304 ARTICLE 6 of 7 Table 3 Oxidative hydrolysis of yellow phosphorus with copper(II) acetate and copper(II) acetate – PVA at 60 °C. Runs Composition of solu- tion, mol/L T, °C Time, min Yield of products, % Cat Р4 H2O С7Н8 I II Cat Cu(OAc)2·H2O 1 1.07 1.07 50 0.94 60 120 21 40 2 3.21 1.07 50 0.94 60 120 27 47 3 6.42 1.07 50 0.94 60 120 33 55 Cat Cu(PVA)2(OAc)2 1 1.07 1.07 50 0.94 60 120 27 47 2 3.21 1.07 50 0.94 60 120 37 53 3 6.42 1.07 50 0.94 60 120 40 57 Note: I – (HO)2HPO; II – (HO)3PO. The Р4 molecule, its inorganic and organic derivatives are prone to two-electron oxidation in aqueous solutions: Р4→4Р+; Р+→Р3+; Р3+→Р5+. It is known that the products of two-electron oxidation of Р4 are stable compounds Р4(OR)2, Р4(OR)4, Р2(OR)4, Р(OR)3, Р(OR)5, while the products of one- electron oxidation of Р4 are unstable radicals [25]. They are strong two-electron reducing agents and impose the role of a two-electron oxidizer on Cu(II) complexes [26]. The standard reduction potentials of Cu(II) indicate that, de- pending on the redox partner, Cu(II) can be reduced to Cu(I) or Cu. The Cu(II) ion is prone to both single-electron (ЕCu(II)/Cu(I) = 0,538 V) and two-electron reduction (ЕCu(II)/Cu(0) = 0,337 V). 4. Conclusions In this study, the ratio of components in a complex com- pound based on copper(II) acetate and polyvinyl alcohol was determined by the potentiometric method. Two mono- links of polymers connect to one complex – forming metal ion. In addition, the results of the conducted conductomet- ric work also proved that the metal-ligand ratio is 1:2. Mi- crophotographs taken with SEM showed the formation of porous spherolites of various sizes. As a result of IR spec- troscopy, it was shown that the peak corresponding to the νO–H subgroup in the polymer-ligand shifted in a complex compound from 2390 to 2410 cm–1. The thermodynamic characteristics of the complex com- pound based on copper(II) acetate and polyvinyl alcohol were calculated, and it was found that the Gibbs’ energy value is a negative. The process of complex formation oc- curs spontaneously. The value of the enthalpy is also nega- tive, and with an increase in temperature, it is assumed that the reaction will shift in the opposite direction. The maximum oxygen absorption rate was observed in the case of the molar ratio [Cu(PVA)2(OAc)2]:[Р4] = 6:1 with yield of final products up to 97%. Supplementary materials No supplementary materials are available. Funding This research had no external funding. Acknowledgments None. Author contributions Conceptualization: D.N.A., B.S.B. Data curation: D.T.K., A.T.O. Investigation: K.S.M., D.T.K., A.T.O. Project administration: D.N.A. Writing – original draft: D.T.K., B.S.B. Writing – review & editing: K.S.M., D.N.A. Conflict of interest The authors declare no conflict of interest. Additional information Author IDs: Botagoz S. Bakirova, Scopus ID 57204585748; Dina N. Akbayeva, Scopus ID 6505789588. Websites: Al-Farabi Kazakh national University, https://www.kaznu.kz/en; Nazarbayev University, https://nu.edu.kz. References 1. 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