Study of H₂O₂/Cu²⁺ Catalyzed Oxidation Process of Maltodextrin

Abstract

In this study, oxidized maltodextrins with a high concentration of carboxyl groups were produced using CuSO₄ as a catalyst and H₂O₂ as an eco-friendly oxidant. Infrared spectroscopy, proton-nuclear magnetic resonance spectroscopy, and thermogravimetric analysis were utilized to examine the structure and properties of oxidized maltodextrins. The reaction conditions were optimized in terms of oxidant content, catalyst content, temperature, pH, and reaction time. The prepared oxidized maltodextrin had a carboxyl group content of 105% under the conditions of 200% molar H₂O₂, 1% molar catalyst, 55 °C, initial pH = 9.7, and 2 h reaction time. In comparison to the commonly used sodium hypochlorite oxidation process, the carboxyl group content was increased by 58%.

1. Introduction

Acute heart failure has serious consequences for health and can even result in death in some cases. Iron deficiency is one of the main causes of this disease, and it must be supplemented with adequate amounts of iron to alleviate the disease [1]. Inorganic small-molecule iron compounds, as a solution, can release iron quickly but are toxic; however, large-molecule encapsulated iron compounds can induce antibody formation, resulting in allergic reactions [2]. Ferric carboxymaltose has emerged as an ideal intravenous iron preparation thus far [3,4,5,6]. Current research on ferric carboxymaltose is primarily focused on its clinical application [7,8,9], with little investigation into its preparation process.

The oxidative modification of maltodextrin is a key step in the preparation of ferric carboxymaltose, and sodium hypochlorite (NaClO) is typically used as an oxidizing agent in the literature [10,11]. Maltodextrin, as a hydrolysis product of starch, has a low molecular weight and solubility facilitate, which facilitates the preparation of products with a high degree of oxidation [12]. There are very few studies on the oxidation of maltodextrins, but in starch oxidation systems, hydrogen peroxide oxidizes starch more quickly than NaClO due to its stronger oxidizing properties [13]. In addition, NaClO use has the potential to produce harmful byproducts derived from chlorine. It is consistent with the principles of green chemistry that hydrogen peroxide (H₂O₂), a clean oxidant, yields water as a reduction product [14]. H₂O₂ possesses strong activity in the presence of a CuSO₄ catalyst [15]. In this study, H₂O₂ and CuSO₄ were used as an oxidant and catalysts, respectively, for the preparation of oxidized maltodextrins with high carboxyl content. Systematically, the influence of reaction conditions such as oxidant and catalyst amounts, temperature, pH value, and reaction time on the activity of the H₂O₂/Cu²⁺ catalytic oxidation system was investigated. By using infrared spectroscopy (FT-IR) and proton-nuclear magnetic resonance spectroscopy (NMR), the chemical composition and structure of oxidized maltodextrins were investigated. TGA was performed to investigate the effect of the H₂O₂/Cu²⁺ oxidation system on the degree of oxidation of maltodextrins and its conformational relationships to lay the foundation for future research on the synthesis of ferric carboxymaltose.

2. Results and Discussion

2.1. Infrared Spectroscopy

Figure 1 displays the IR and ¹H NMR spectra of maltodextrin before and after oxidation. The broad absorption band at 3386 and 2936 cm⁻¹ are ascribed to the stretching vibrations of -OH and -CH₂ groups on the sugar ring, respectively [16,17]. In contrast to maltodextrin, oxidized maltodextrin exhibited a new characteristic peak at 1737 cm⁻¹ belonging to the C=O stretching generated by the oxidation of hydroxyl groups to carbonyl and carboxyl groups [18,19,20]. In conclusion, FT-IR spectra show that the maltodextrin hydroxyl group has been oxidized to C=O groups, such as carbonyl and carboxyl groups.

2.2. Nuclear Magnetic Resonance Spectroscopy

As previously reported, the characteristic peaks observed between 4.62 and 5.46 ppm are attributed to the proton signals of C₂ (5.00 ppm), C₃ (5.41 ppm), and C₆ (4.62 ppm) hydroxyl groups, as reported in previous works [21,22]. The proton on C₁ (5.09 ppm) was the glycosidic bond at α-1,4, while the signal at 4.89 ppm was primarily associated with the proton on C₁ at α-1,6 branch point [23]. Proton signals for CH and CH₂ groups on the glucose unit were observed in the range of 3.37~3.62 ppm [23,24]. The ¹H-NMR spectrum of maltodextrin is compared with its product after oxidation by H₂O₂, showing the significantly reduced intensity of the hydroxyl proton signal peaks at C₂, C₃, C₆, and hydroxyl groups of maltodextrin reduced after oxidation. This indicates that the hydroxyl groups at C₂, C₃, and C₆ in the glucose unit are oxidized and converted to carbonyl or carboxylate. The peak at 8.25 ppm in the ¹H NMR spectrum was attributed to maltodextrin hydrolysis during the oxidation process, resulting in OH with a hemiacetal structure [25], and the peak at 1.06 ppm was attributed to CH₃ in the incomplete volatilized ethanol [26].

2.3. Thermal Stability of Oxidized Maltodextrin

TGA tests were carried out on raw maltodextrin and oxidized maltodextrin with varying carboxyl contents to monitor weight loss and product degradation with temperature, and the results are shown in Figure 2. The measured T_5% is listed in

Table 1. Thermogravimetric parameters of proto- and oxidized maltodextrin with varying carboxyl contents.

Sample	T5%	Tmax
Proto-maltodextrin	138.3	329.5
Oxidized maltodextrin (3%)	207.5	334.1
Oxidized maltodextrin (42%)	216.3	335.2
Oxidized maltodextrin (105%)	173.1	323.5

. The transition temperature of oxidized maltodextrin with 3% carboxylate was increased from 138.3 °C to 207.5 °C when compared to raw maltodextrin. Besides, the carboxylate content of oxidized starch was increased from 3% to 42%, with a slight increase in temperature measured. However, at the 105% of oxidation level, the temperature was decreased to 173.1 °C. The measured Tmax values from DTG curves are listed in Table 1. The samples before and after oxidation show a mass loss step at about 330 °C. At the low carboxyl content, Tmax exhibited a slight increase with increasing carboxyl content but decreased by 6.0 °C at 105% carboxyl content when compared to raw maltodextrin. H₂O₂/Cu²⁺ oxidation can improve the thermal stability of maltodextrin, but too high a carboxyl group content can cause instability in the thermal performance.

2.4. Effect of Molar Content of Catalyst on the Formation of Carboxyl Groups

H₂O₂ rapidly decomposes in the presence of a metal catalyst to produce hydroxyl radicals. These highly reactive radicals readily react with carbohydrates via the removal of hydrogen from the C-H group present in the sugar ring to form new radicals (R-CHOH), as shown in Figure 3 [27]. Furthermore, H₂O₂ is unstable and readily decomposes spontaneously into oxygen and water, which is accelerated by the presence of a Cu²⁺ catalyst [28].

The influence of catalyst molarity on the resulting carboxyl content is depicted in Figure 4. The findings demonstrated that the oxidation level could be greatly increased using CuSO₄ as a catalyst. At 0.1% catalyst, the carboxyl content was increased from 3.0% to 42.1%. Cu²⁺ forms hydroxyl radicals during the reaction, which oxidize the hydroxyl group in the glucose units to form carbonyl and carboxyl groups. However, the growth of the carboxyl group slowed at 1% catalyst, and the carboxyl content increased from 42.1% to 105.0%. The increased oxidation efficiency is due to H₂O₂’s increased production of hydroxyl radicals at low Cu²⁺. High concentrations of Cu²⁺ have been shown to accelerate H₂O₂’s decomposition [28], which in turn slows its reaction with maltodextrin molecules before it is converted to H₂O and O₂. It is considerably larger than the product with a 47% carboxyl group prepared by Huang et al. using NaClO oxidation [11]. Increasing catalyst concentration accelerates the rate of H₂O₂ decomposition, resulting in a gradual decrease in oxidation. Maltodextrin molecules are water-soluble and lack a crystalline structure [28], allowing for more reaction sites with hydroxyl radicals in the presence of sufficient oxidant and catalyst, subsequently generating a higher degree of oxidation. Additionally, as the catalyst content exceeds 0.1%, the high Cu²⁺ content in the modified maltodextrin can result in undesirable discoloration [29], which can hinder the subsequent preparation of ferric carboxymaltose. Thus, 0.1% of Cu²⁺ demonstrated the highest catalytic efficiency.

The effect of anions on copper ion catalysis is rarely reported. To investigate the effect of anion on Cu²⁺, CuCl₂ was substituted for CuSO₄ in oxidation experiments, and the carboxyl content was found to be 8.7%, whereas it was 42.3% when CuSO₄ was used under the same conditions. Sulfate is stable in solution and exhibited a minimal effect on Cu²⁺ catalysis. However, Cl⁻ will form a complex with Cu²⁺, inhibiting its catalytic activity [30]. Hence, CuSO₄ is considered a suitable catalyst.

2.5. Effect of Different Molar Contents of Oxidant on the Carboxyl Groups

Figure 5 shows the influence of oxidant content on the carboxyl content of the product. The formation of the carboxyl group increased proportionally from 3.8% to 42.1% as the oxidant molar content (calculated as the molar content of maltodextrin molecules in glucose units) increases from 30% to 200%. These results can be explained by the fact that the amount of catalyst is sufficient when the oxidant content increased from 30% to 200%. However, at the excessive oxidant content, the catalyst is insufficient to react with H₂O₂ to form hydroxyl radicals (HO·), resulting in a decreased reaction efficiency.

2.6. Effect of Different Reaction Temperatures on the Formation of Carboxyl Groups

The effect of reaction temperature on the carboxyl group concentration of the product is depicted in Figure 6. The carboxyl content was increased from 24.7% to 42.1% as the reaction temperature increased from 25 °C to 55 °C; however, when the temperature was raised to 65 °C, the carboxyl content remained nearly unchanged. The rate of hydrogen peroxide decomposition and hydroxyl radical formation is greatly affected by temperature [31]. The H₂O₂ activity gradually increased with temperature; however at 65 °C, the decomposition of H₂O₂ occurs, reducing oxidation efficiency.

2.7. Effect of the Duration of Reaction on the Carboxyl Groups

Figure 7 shows the effect of reaction duration on the carboxyl group content of the product. The contribution of the carboxyl group was increased with the increase in reaction time. As the reaction time increased from 30 to 120 min, the carboxyl content increased from 6.5% to 42.1%, with a significant increase in the degree of oxidation; further, the carboxyl content was measured to be 44.4% and 45.5% at the reaction time of 180 min and 300 min, respectively. Increasing the oxidant concentration at the outset of the reaction facilitates oxidation. The oxidant was consumed as the reaction progressed, resulting in a decrease in reaction efficiency. The optimal reaction time has been determined to be 120 min.

2.8. Effect of Different Initial pH Values on the Carboxyl Groups

Figure 8 shows the effect of initial pH on the carboxyl content of the product. The pH has a significant impact on the rate of decomposition of H₂O₂; its activity is reduced in acidic conditions. Increasing pH increases the hydrogen peroxide efficiency in generating hydroxyl radicals; however, it also accelerates the decomposition rate of hydrogen peroxide [31,32]. The created alkaline environment, with a pH between 9 and 10, optimized the activity of H₂O₂. At pH 10, H₂O₂ decomposed rapidly in the presence of a catalyst.

As a result, oxidized maltodextrin with a maximum carboxylate content of 105.0% was produced using 200% of oxidant, 1% of catalyst, an initial pH of 9.7, 55 °C, and a 120-min reaction time. The complexation site with Fe³⁺ can be slightly reduced when the carboxylate groups in oxidized maltodextrin are combined with more Cu²⁺. In comparison to NaClO, H₂O₂ is a clean oxidant that contains only 0.1% Cu²⁺ as a catalyst. It oxidized maltodextrin with 42.1% of carboxyl group content. It demonstrated superior thermal stability while it has sufficient complexation sites without generating harmful byproducts.

3. Materials and Methods

3.1. Materials

Food-grade Maltodextrin was acquired from Jinan Xuzhikun Chemical Co., Ltd. (Jinan, China); 30% H₂O₂, copper sulfate pentahydrate, and sodium hydroxide were acquired from Sinopharm Group Chemical Reagent Co. (Beijing, China). All of the reagents that were utilized in this experiment were of analytical quality.

3.2. Oxidation

Maltodextrin (40 g) was initially dissolved in 60 mL of distilled water by heating and agitating in a 250 mL three-necked round bottom flask. After that, a certain amount of catalyst CuSO₄·5H₂O was added, and the pH was adjusted using NaOH solution. In addition, the temperature was raised to the reaction temperature. H₂O₂ was then slowly added drop by drop while the reaction was agitated for some time. The oxidized maltodextrin was precipitated with ethanol after the reaction was complete, and the final product was separated by centrifugation. The product was vacuum-dried at 50 °C, then ground and stored.

According to the literature [31,33], the effects of catalyst content (0, 0.1, 0.3, 0.5, 1.0, 1.5 and 2.0%), oxidant content (10, 30, 50, 100, 200 and 300%), reaction temperature (25, 35, 45, 55 and 65 °C), the duration of reaction (30, 60, 120, 180, and 300 min), and initial pH values (4.8, 5.7, 6.9, 7.7, 8.5, 9.7, and 10.5) on the degree of oxidation were systematically investigated.

In the study, the molar contents of CuSO₄·5H₂O and H₂O₂ were expressed in terms of glucose in maltodextrin molecules.

3.3. Determination of Carboxyl Group Content

The carboxyl group content was determined by titration with sodium hydroxide solution following the procedure described in the literature [29].

In a conical flask, 5 g of oxidized maltodextrin was weighed, and then 100 mL of distilled water was added. Next, phenolphthalein was added as an indicator, and the mixture was titrated with 0.1 mol·L⁻¹ NaOH standard solution. The volume consumed was V₁. The carboxyl content was calculated using Equation (1).

$$X=\frac{C_1\times V_1}{m/162}\times 100\%$$

(1)

where X is the carboxyl content; C₁ (mol·L⁻¹) is the concentration of NaOH standard solution; V₁ (L) represents the volume of 0.1 mol·L⁻¹ NaOH solution consumed during the titration; m (g) is the mass of maltodextrin.

3.4. Infrared Spectral Analysis (FTIR)

After being dried in a blast dryer, the raw and oxidized maltodextrin samples were then pressed into thin slices using the KBr pressing method. The Fourier transform infrared spectrometer, Model AVATAR370, Thermo Nicolet, Waltham, MA, USA, was used to measure the infrared spectra of the oxidized maltodextrin samples. The parameters for the experiment were as follows: the number of scanning waves was set at 400–4000 cm⁻¹, and the number of scanning times was set at 16.

3.5. Proton-Nuclear Magnetic Resonance Spectroscopy

DMSO was used as the solvent to perform the determination and analysis, 5–10 mg of samples were measured before being dissolved in 500 μL of DMSO. To describe the chemical structures of the raw and oxidized maltodextrin samples,¹H-NMR tests were carried out.

3.6. Thermogravimetric Testing (TGA)

Using a NETZSCH STA2500 thermal analyzer, 3–10 mg of dried samples were analyzed by thermogravimetric analysis (TGA) after being dried in a vacuum oven at 60 °C for 48 h. The experimental conditions included a N₂ atmosphere, a temperature rate of 10 °C/min, and a temperature-scanning range of 40 to 500 °C.

4. Conclusions

In conclusion, the preparation process parameters for the H₂O₂/Cu²⁺-catalyzed oxidation of maltodextrin were thoroughly examined in terms of the oxidant content, catalyst content, temperature, initial pH, and reaction time. The findings of FT-IR and NMR analysis showed that the hydroxyl group on the maltodextrin molecule was effectively converted to a carboxyl group via the H₂O₂/Cu²⁺-catalyzed oxidation process. The oxidation procedure ensured the efficiency of Cu²⁺ for maltodextrin oxidation by H₂O₂. At sufficient oxidant and catalyst levels, it was a safe and effective oxidant that could produce oxidized maltodextrin with extremely high carboxyl group content. The high carboxyl content of oxidized maltodextrin provides a strong foundation for optimizing the production of ferric carboxymaltose.