Synthesis of Magnetically Modified Copper-Based Metal–Organic Frameworks for Cellulose Hydrolysis

Abstract

This study was aimed at the synthesis, characterization, and cellulose hydrolysis potential of pristine and magnetically modified Cu-based metal–organic frameworks (MOFs; Cu-BDC and MNPs@Cu-BDC). The MNPs@Cu-BDC was synthesized through an encapsulation approach by coating the presynthesized Fe₃O₄ nanoparticles (MNPs). The materials were characterized using Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis, and Brunauer–Emmett–Teller techniques. The FT-IR analysis confirms the presence of the corresponding functional groups in the synthesized Fe₃O₄, Cu-BDC, and MNPs@Cu-BDC materials. The SEM image depicted that the magnetic MOF had a rough surface with regular octahedral and embedded spherical-shaped particles, unlike pristine Cu-BDC. The XRD pattern of MNPs@Cu-BDC revealed distinctive signals of Fe₃O₄ along with Cu-BDC diffraction patterns. The catalytic activity of the synthesized MNPs@Cu-BDC was analyzed and optimized by varying the catalyst dose (0–5 mg), temperature range (120–200 °C), and residence time (30–120 min). The prepared MOF exhibited high catalytic activity toward carboxyl methylcellulose, and it achieved a total reducing sugar yield of 95.7 mg/g at 160 °C for 120 min. The MNPs@Cu-BDC catalyst was stable enough until 300 °C. Additionally, the catalyst displayed good reusability, maintaining a production rate of 40.81 mg/g after five cycles. Hence, these results underscore the potential of MNPs@Cu-BDC as an effective catalyst for cellulose hydrolysis, with promising recyclability features.

1. Introduction

The development of renewable energy sources is crucial for reducing greenhouse gas emissions, combating climate change, decreasing dependence on finite fossil fuels, ensuring energy security and sustainability for future generations, and fostering innovation in clean energy technologies. , Among renewable sources, cellulose is a key component due to its abundance in nature and its potential for conversion into valuable products like reducing sugars for biofuel production.

Cellulose, a complex polysaccharide, contributes to the formation of a plant cell wall structure. It is a biopolymer made up of elongated chains of glucose units connected by β-1,4-glycosidic linkages. The crystalline structure of cellulose is a fundamental characteristic that influences its mechanical attributes, interaction with water, and susceptibility to chemicals. As a result, the hydrolysis of cellulose is extremely crucial for biofuel production because bacteria can only feed on glucose, not cellulose during the fermentation process. Steam explosion, acid hydrolysis, enzyme hydrolysis, and solid acid catalysts are the main cellulose conversion methods via hydrolysis. Among them, solid acid catalysts show promise in cellulose conversion, offering advantages over conventional acids in terms of separation and purification as well as accessibility and cost-effectiveness.

Metal–organic frameworks (MOFs) are solid acid catalysts possessing properties like tunable pore size, structural functionality, and high internal surface area. These properties make them promising candidates for cellulose hydrolysis. Recently, MOFs have been applied for conversion of cellulose to chemicals and fuels. For instance, Akiyama and colleagues reported a study of utilizing sulfonic acid-modified MIL-101­(Cr) for the transformation of cellulose into monosaccharides and disaccharides via hydrolysis. The research also confirmed the robustness of the MOF material during the catalytic process. Another study conducted by Wang and co-workers reported the transformation of cellulose into levulinic acid (LA) in water using UiO-66 catalysts loaded with different metal oxides and enhancing the catalysts’ acidity, which boosts the catalysts’ acid content, resulting in increased selectivity and yield of LA. The application of MOFs in biomass conversion to fermentable monomers is constrained due to the necessity for catalysts with higher surfaces and superior hydrothermal stability. This requirement arises from the hydrolysis process being conducted at elevated temperatures, which demands catalysts that can withstand such conditions effectively. Another challenge lies in the reusability of MOFs posthydrolysis, as their separation and reuse necessitate additional pretreatment steps such as centrifugation. The modification of MOFs with magnetic nanoparticles (MNPs) can overcome the challenges mentioned above.

The combination of MNPs like Fe₃O₄ with MOFs forms magnetic framework composites (MFCs), blending the unique properties of MNPs like magnetic responsiveness, high surface area, biocompatibility, and versatile functionalization with those of MOFs. These composite materials offer enhanced advantages over individual components, facilitating easy adsorption and separation processes and providing a simple method to improve the recyclability of MOFs by minimizing material loss. These MFCs synthesized through different approaches depend on the intended application and desired properties of the composite. The use of these composites in the hydrolysis of cellulose offers not only reusability, but also, the presence of Fe₃O₄ improves the catalytic activity of pristine MOFs by providing additional active sites for catalytic reactions. The incorporation of Fe₃O₄ also improves additional active sites that boost the performances of pristine MOFs. These MNPs enhance catalytic activity through synergistic effects such as improved reactant adsorption, facilitated mass transfer, increased hydrothermal stability in unique catalytic pathways, and the stabilization of Lewis active sites within the MOF framework. Collectively, these interactions create a synergistic effect that significantly improves overall reactivity, enabling efficient catalysis across various reactions. ,

Hence, in this study, the copper-based MOF (Cu-BDC) is functionalized by Fe₃O₄ through an encapsulation approach first by coating Fe₃O₄ with polydopamine (PDA) to improve the compatibility with Cu-BDC for hydrolysis of cellulose to reducing sugars. To the best of our knowledge, Fe₃O₄@Cu-BDC (MMOF) was applied for the first time for the conversion of cellulose to reducing sugar. The unsaturated metal sites in the Cu-BDC framework, typically represented by Cu²⁺ ions, act as Lewis acids crucial for hydrolysis reactions. When Fe₃O₄ is incorporated into Cu-BDC, the acidity of the composite is enhanced, leading to increased conversion rates. This acidity boost is attributed to the additional metal provided by Fe₃O₄ in the composite, which contributes to the catalytic activity and efficiency of the material.

2. Result and Discussion

2.1. Synthesis and Characterization of MOFs

The pristine Cu-BDC and MNPs@Cu-BDC were successfully synthesized using a Teflon-lined stainless-steel solvothermal reactor at temperatures of 120 °C for 12 h and 130 °C for 24 h, respectively.

The SEM images of Cu-BDC and MNPs@Cu-BDC are observed in Figure . The Cu-BDC was octahedral in shape and has an average surface size of 33.61 μm. The surface of Cu-BDC was observed to be rough and heterogeneous (Figure a). The SEM of MNPs@Cu-BDC revealed some embedded spherical shapes on regular octahedral crystals (Figure b). The presence of spherical-shaped particles possibly originates from the existence of Fe₃O₄ in the MOF structure. When the Fe₃O₄ was impregnated on the Cu-BDC MOF, the average sizes were changed from 33.61 to 46.22 μm, which is consistent with the XRD result. In addition, some surface appearances were changed including particle sizes and distributions in the MNPs@Cu-BDC catalyst.

1.

SEM images of (a) Cu-BDC and (b) MNPs@Cu-BDC.

The crystallinity and phase composition of the MNPs@Cu-BDC composite were described from its powder XRD on Figure . The peaks at 2θ values of 10.036, 11.96, 13.45, 16.92, 20.36, 21.87, 24.67, 33.86, and 42.04° correspond to diffractions of (110), (001), (020), (2̅01), (220), (111), (311̅), (132), and (512̅) of Cu-BDC. The XRD analysis revealed sharp peaks in the MNPs@Cu-BDC composite, consistent with previously reported findings in the literature. The peaks at 30.48, 35.94, 43.62, 51.87, and 57.55° match the diffractions of (220), (311), (400), (422), and (511), respectively. The peaks were consistent with JCPDF no. 65-3107 and indicate the phase of Fe₃O₄ in the composite. The Debye–Scherrer equation calculates the crystallite sizes of Cu-BDC and MNPs@Cu-BDC as 12.1 and 16.3 nm, respectively.

$$\mathrm{d}=\frac{k\lambda }{\beta \cos \theta }$$ 1

where d is the crystallite size (nm), k is 0.9 (Scherrer constant), λ is 0.15406 nm (wavelength of X-ray sources), β is the fwhm (radians), and θ is the peak position.

2.

XRD of Cu-BDC theoretical and synthesized, Fe₃O₄, and MNPs@Cu-BDC expressed in arbitrary units (a.u.).

The degree of crystallinity indicates the amount of crystalline material in a substance, influencing its properties. The crystallinity of Fe₃O₄, Cu-BDC, and MNPs@Cu-BDC was determined by dividing the area of crystalline peaks by the area of the crystalline pattern. The results are listed in Table below. The high crystallinity values of MNPs@Cu-BDC indicate that these materials have well-defined crystal structures that can contribute to their stability and potential applications in cellulose hydrolysis.

1. Analysis of Crystallinity of Fe₃O₄, Cu-BDC, and MNPs@Cu-BDC .

material	area of crystalline peaks	area of the crystalline pattern	crystallinity
Fe3O4	535.67	618.20	86.65%
Cu-BDC	5910.78	6258.87	94.44%
MNPs@Cu-BDC	5489.92	6255.13	87.76%

FT-IR analysis was conducted to identify the functional groups present in the synthesized Fe₃O₄, Cu-BDC, and MNPs@Cu-BDC materials. As shown in Figure a, the peak at 565 cm^–1 is ascribed to the stretching of Fe–O, which appears in all three spectra confirming the incorporation of Fe₃O₄ to Cu-BDC. The weak or absent N–H stretching in dopamine on PDA-coated Fe₃O₄ FT-IR spectra is due to dopamine’s interaction with Fe₃O₄ during coating. Dopamine polymerization to form the PDA layer may chemically bond with Fe₃O₄, altering the N–H vibrational modes and causing their reduction or disappearance in the spectrum. Compared to Fe₃O₄ and Fe₃O₄@PDA, the occurrence of −CC–, −CO, and −C–O– at 1500, 1603, and 1387 cm^–1, respectively, from MNPs@Cu-BDC indicates the successful deposition of Cu-BDC on the surface of PDA. The peaks observed at 467 and 753 cm^–1 were likely associated with the bending and stretching vibrations of Cu–O bonds, respectively. The −C–C– vibrational stretching at 1387 cm^–1 corresponds to the aromatic carbon of H₂BDC. Furthermore, the peaks at 936, 1500, and 1603 cm^–1 (Figure b) were the result of −C–O, −CC–, and −CO bonds of the H₂BDC, respectively, indicating the successful synthesis of Cu-BDC.

3.

FT-IR spectra of (a) Fe₃O₄, PDA-coated Fe₃O₄, and MNPs@Cu-BDC; (b) Cu-BDC and MNPs@Cu-BDC.

The BET surface areas of the prepared Cu-BDC and MNPs@Cu-BDC were analyzed to determine the specific area, pore volume, and pore size of the catalysts. The surface areas of Cu-BDC and MNPs@Cu-BDC were 108.12 and 104.11 m²/g, respectively. The surface area of the materials indicated the catalytic potential toward cellulose hydrolysis. In Table , a summary of the BET surface areas, pore volumes, and pore sizes is explained.

2. Summary of the BET Surface Area, Pore Volume, and Pore Size.

sample	BET surface area (m2/g)	pore volume (cm3/g)	pore size (nm)	references
Cu-BDC	108.12	0.0858	3.17	this work
MNPs@Cu-BDC	104.11	0.0804	3.09	this work
2-MBIA-Cu-BDC	17.02	0.0271	8.48
Cu-BDC	400

N₂ adsorption–desorption of MNPs@Cu-BDC exhibited a type III isotherm with the type H3 hysteresis loop according to the International Union of Pure and Applied Chemistry (IUPAC) (Figure ), indicative of a mesoporous material with a slit-shaped hysteresis loop at a higher relative pressure (p/p° > 0.8). This means that pores are wider inside and narrower at the opening and a significant decrease in pressure is required for them to escape. The absorption capacities of Cu-BDC and MNPs@Cu-BDC were 55.25 and 52.73 cm³/g, respectively, at standard pressure and temperature.

4.

N₂ adsorption–desorption of Cu-BDC and MNPs@Cu-BDC.

Further, the catalysts’ thermal stability was evaluated through TGA measurements conducted under an airflow environment. This method was employed to determine the decomposition temperatures and assess the thermal stability of the compounds. Figure reveals the TGA curves of both Cu-BDC and MNPs@Cu-BDC. The graph shows a temperature range from 20 to 800 °C. It has two stages for both materials; the first stage occurs within the 110–285 °C temperature range, as a result of the loss of the DMF solvent with a weight loss of 22.12% for both Cu-BDC and MNPs@Cu-BDC while the second stage occurs from 300 to 345 °C with huge weight losses of 48.78 and 43.8% for Cu-BDC and MNPs@Cu-BDC, respectively, due to decomposition of the MOF support. From the TGA curve, the MNPs@Cu-BDC is stable up to 300 °C denoting that the structural stability of Cu-BDC is significantly enhanced through the incorporation of MNPs, which alter the electronic environment of copper, increasing bond stability and improving interfacial interactions.

5.

TGA curves of Cu-BDC and MNPs@Cu-BDC.

2.2. Catalytic Activity of MNPs@Cu-BDC

The hydrolysis catalytic potentials of pristine Cu-BDC and MNPs@Cu-BDC were investigated based on the ability to produce total reducing sugar (TRS) from the CMC polymer in a given reaction condition. As the concentration of produced reducing sugar increased the absorbance of the complex increased. This is due to the formation of colored 3-amino, 5-nitro salicylic acid, which has high absorbance at a specified wavelength (371 nm) (Scheme ).

1. Complex Formation between DNS and d-Glucose.

The quantification TRS was calculated based on the standard calibration curve of d-glucose having a linear regression equation. A series of known concentrations of d-glucose and their corresponding responses (y-values), the slope “m”, and intercept “b” of the linear regression equation were used to calculate yield TRS according to the regression equation form-fitted calibration curve (Figure a). The catalytic performance of the MNPs@Cu-BDC composite was evaluated in comparison to that of Cu-BDC and the hydrothermal catalytic reaction. It was observed that the composite exhibited superior catalytic activity when compared to Cu-BDC alone (Figure b) due to the synergistic effects of Fe₃O₄ nanoparticles and the Cu-BDC framework.

6.

Standard d-glucose. (a) Wide-spectrum d-glucose and (b) catalytic activity of the catalyst.

The Fe₃O₄ surface may act as Lewis acid sites, coordinating to the oxygen of the glycosidic bonds in CMC and increasing the electrophilicity of the anomeric carbon, making it more susceptible to bond heterolysis. On the other hand, Fe₃O₄ surfaces may release protons in the hydrothermal medium and generate localized acidic microenvironments. These protons can protonate the glycosidic oxygen, facilitating departure of the leaving group and formation of an oxocarbenium ion-like intermediate. The Fe₃O₄ components in the MNPs@Cu-BDC provide extra catalytic sites and increase the yield of TRS due to the synergetic effect of Cu-BDC and Fe₃O₄.

2.3. Parameter Optimization

2.3.1. Effect of the Catalyst

The impact of concentration was studied by comparing two conditions: in the presence and absence of the MNPs@Cu-BDC catalyst in the hydrothermal condition. When the MNPs@Cu-BDC catalyst was present, its catalytic activity influenced the reaction kinetics and outcomes. The TRS yield increased from 40.51 mg/g (without the catalyst) to 95.68 mg/g (with the catalyst) at the same temperature and time (Figure a). The catalyst enhances yield and selectivity by lowering the activation energy and improving the reaction rate and efficiency. ,

7.

Effect of concentration on TRS yield (a), effect of temperature on TRS yield (b), and effect of time on TRS yield (c).

2.3.2. Effect of Temperature

The temperature significantly influenced the yield of TRS. The temperature was assessed at five levels in the presence of MNPs@Cu-BDC. The TRS yield rises with increasing temperature up to 160 °C but then starts to decrease at 180 °C, with a further decline observed at 200 °C (Figure b). This is due to the thermal degradation of the substrate at higher temperatures, leading to a reduction in the desired product yield. ,

2.3.3. Effect of Time

The impact of the residence time on the yield was evaluated at four different time intervals. It was observed that the TRS yield tends to increase with time, reaching a peak at 120 min before slightly decreasing at 180 min (Figure c). A longer residence time typically enhances TRS production under mild conditions. The decrease in the TRS yield at longer residence times and higher temperatures during catalytic reactions can be attributed to the thermal degradation of sugars. As the reaction time and temperature increase, there is a higher probability of sugars breaking down into nonsugar compounds, reducing the overall yield of TRS available from catalytic hydrolysis. ,

For both hydrothermal and MOF-hydrothermal hydrolytic processes of the CMC, the optimization parameter is summarized in Table . The table data show that the TRS yield increased from 11.98 to 40.51 mg/g and from 72.82 to 95.7 mg/g when the temperature was raised from 120 °C for 120 min to 160 °C for 120 min for both the hydrothermal and MNPs@Cu-BDC plus hydrothermal catalytic reactions, respectively, which is the maximum yield of TRS. The yield was quite higher than the previously reported TRS yield (26.65 mg/g) using diluted acid as a catalyst. This shows the effectiveness and efficiency of MNPs@Cu-BDC. However, as the temperature was further increased, there was a decrease in TRS yield to 25.86 mg/g for the hydrothermal process at 200 °C for 120 min, and in the presence of MNPs@Cu-BDC, a TRS yield increased to 89.73 mg/g at 200 °C for 60 min and then started to decline when the time reached 120 min. This means that at mild temperatures, the cellulose undergoes effective breakdown and release of sugars, leading to a high yield of TRS. However, at high temperatures, the cellulose might be subjected to degradation or the breakdown of the sugars to nonreducing sugar.

3. Summary of Optimization of TRS Yield.

run	MNPs@Cu-BDC (mg)	reaction temperature (°C)	reaction time (minutes)	TRS (mg/g)
1	0	120	30	4.81
2	0	120	60	7.58
3	0	120	120	11.98
4	5	120	30	58.33
5	5	120	60	66.17
6	5	120	120	72.82
7	0	160	30	8.72
8	0	160	60	19.36
9	0	160	120	40.51
10	5	160	30	68.12
11	5	160	60	80.21
12	5	160	120	95.70
13	0	200	30	21.54
14	0	200	60	24.31
15	0	200	120	25.86
16	5	200	30	84.37
17	5	200	60	89.73
18	5	200	120	81.56

Furthermore, the residence time has a great influence on the TRS yield. In the table below, as the time increased from 30 min at 120 °C to 120 min at 120 °C, the yield of TRS increased from 4.81 to 11.98 mg/g and 58.33 to 72.82 mg/g for hydrothermal and MNPs@Cu-BDC plus hydrothermal, respectively, and it is the same scenario for 160 °C with the change in time. At a higher temperature (200 °C), as the time increases from 60 to 120 min for hydrothermal, the yield increases from 24.31 to 25.86 mg/g, but for the MNPs@Cu-BDC, however, the yield decreases from 89.73 to 81.56 mg/g. This due to a high temperature and longer residence time causing degradation of the substrate and further reduction of reducing sugar to other nonreducing monomers.

The reduction in TRS might indicate the formation of byproducts such as hydroxymethylfurfural (HMF) and furfural, which can ultimately impact the overall yield of TRS. Nevertheless, our primary focus remains on optimizing the TRS yield through the use of MNPs@Cu-BDC as a catalyst. This catalyst shows promise for converting biomass into fermentable sugars using MNPs@MOFs as solid acid catalysts. While the TRS yield provides a general measure of hydrolysis efficiency, detailed analysis of individual sugar components and potential byproducts would offer a more comprehensive view of the reaction outcomes. Future work will aim to incorporate HPLC or NMR-based characterization to further elucidate the selectivity and performance of the catalytic system.

2.4. Reusability of MNPs@Cu-BDC

The recyclability of the MNPs@Cu-BDC was evaluated over five cycles, and the corresponding TRS yields. 5 mg of the MNPs@Cu-BDC catalyst was used repeatedly over five cycles.

The initial high TRS yield of 95.7 mg/g in the first cycle decreased to 80.88 mg/g in the second cycle, further dropping to 77.925 mg/g in the third cycle, 66.71 mg/g in the fourth cycle, and significantly lower at 40.81 mg/g in the fifth cycle. The decreasing trend in TRS yield over the cycles (Figure ) indicates a decline in the catalytic efficiency or stability of the MNPs@Cu-BDC catalyst with repeated use. This decline in performance could be attributed to factors such as catalyst deactivation, loss of active sites, or structural changes during the recycling process.

8.

Recyclability of MNPs@Cu-BDC.

Cu-BDC exhibits good thermal stability up to 250 °C in dry conditions; however, its Cu–O–C linkages are prone to hydrolytic cleavage in hydrothermal environments, which can lead to framework degradation. Encapsulation or integration with NPs, such as metallic, metal oxide, carbon-based, or polymeric NPs, improves these drawbacks. For instance, encapsulation of Pt nanoparticles in Zr-based MOF-808 enhanced structural robustness of the MOF. In the same manner, the polydopamine layer on nanoparticles (e.g., Au, Fe₃O₄, and Pd) enables heterogeneous MOF shell growth and formed robust NP@MOF hybrids.

3. Conclusions

The hydrolysis of cellulose into its monomer-reducing sugars is a critical step in biofuel production. Thus, hydrolysis using the as-prepared MNPs@Cu-BDC is one way to achieve it. Characterization techniques such as XRD, FT-IR, SEM, and BET were employed to confirm the successful formation of the MNPs@Cu-BDC composite. The BET surface area of the composite was found to be 104.11 m²/g, which is good enough for the catalytic hydrolysis of cellulose. The catalytic performance of MNPs@Cu-BDC was evaluated and optimized by using CMC as the substrate, and the yield of TRS was determined by using the DNS method with a UV–vis spectrophotometer. MNPs@Cu-BDC exhibits higher catalytic activity compared to pristine Cu-BDC MOFs, attributed to the composite’s bifunctionality, which arises from the synergetic effect of the Fe₃O₄ nanoparticles and the Cu-BDC framework in the composite. The maximum yield of TRS achieved 95.7 mg/g in the presence of MMOFs in the hydrothermal method, which is much greater than that of the simple hydrothermal method, having 40.51 mg/g.

Moreover, the encapsulation of Fe₃O₄ in the composite enhances the reusability of MNPs@Cu-BDC, as it retains catalytic activity even after five cycles, with a measured activity of 40.81 mg/g. This reusability surpasses that of simple hydrothermal catalytic conditions, indicating the effectiveness of the composite in sustaining its catalytic performance over multiple cycles. Thus, the synthesized MNPs@Cu-BDC is cost-effective and could be applied for large-scale applications.

4. Methods

4.1. Materials and Chemicals

All chemicals used in this study were of analytical quality and were employed without additional purification: iron­(III) chloride hexahydrate (FeCl₃·6H₂O) (99%, Alpha Chemika, India), sodium acetate anhydrous (C₂H₃NaO₂) (99%, Sisco Research Laboratories Pvt. Ltd., India), tris-sodium citrate (Na₃C₆H₅O₇·2H₂O) (98%, Chemicals Udyog-121001, India), polyethylene glycol (PEG 4000, HO­(C₂H₄O) _n ), (for synthesis, Loba Chemie Pvt. Ltd., India), d-glucose (C₆H₁₂O₆) (98%, Loba Chemie, India), ethylene glycol (EG, C₂H₆O₂) (99%, Research-Lab Fine Chemicals Industries, India), carboxymethyl cellulose (CMC, (C₆H₇O­(OH)_3–x (OCH₂COOH) _x ) _n ) (250,000 wt., 99%, ACS Chemicals, India), dopamine hydrochloride (C₈H₁₁NO₂·HCl) (≥97.5%, Sigma-Aldrich, Germany), benzene-1,4-dicarboxylic acid (H₂BDC, C₈H₆O₄) (98%, Sigma-Aldrich, USA), copper­(II) nitrate trihydrate (Cu­(NO₃)₂·3H₂O) (99%, Central Drug House (P) Ltd., India), N,N-dimethylformamide (HCON­(CH₃)₂) (99%, Loba Chemie Pvt. Ltd., India), ethanol (C₂H₅OH) (97%, Fine Chemical General Trading, Ethiopia), Tris buffer (C₁₄H₁₁NO₃) (99%, Alpha Chemika, India), 3,5-dinitro salicylic acid (C₇H₄N₂O₇) (DNS, >98%, Tokyo Chemical Industry Co., Ltd., Japan), sodium hydroxide (NaOH) (>97%, Central Drug House (P) Ltd., India), and potassium sodium tartrate tetrahydrate (OCOCH­(OH)­CH­(OH) COONa·4H₂O) (99%, Fujifilm Wako Pure Chemical Corporation PL, India).

4.2. Synthesis of Magnetically Modified MOFs

4.2.1. Synthesis of the Copper-Based Metal–Organic Framework (Cu-BDC)

Cu-BDC was synthesized by a solvothermal method as previously reported with slight modification. Typically, H₂BDC (6.0 mmol, 0.99678 g) and Cu (NO₃)₂·3H₂O (6.0 mmol, 1.454 g) were dissolved in 75 mL of a mixture of DMF:ethanol (50 mL:25 mL) ultrasonically for 15 min, and the mixture was transferred to a 100 mL Teflon-lined stainless-steel autoclave and maintained at 120 °C for 12 h. Subsequently, it was cooled to room temperature. The resulting product was then subjected to centrifugation and washed multiple times with DMF, distilled water, and ethanol. The MOF product obtained was dried in a vacuum oven at 60 °C for 12 h.

4.2.2. Synthesis of Fe₃O₄ and PDA-Coated Fe₃O₄

Fe₃O₄ nanoparticles were prepared using a solvothermal approach following a previously reported method. Briefly, a mixture of 1.3 g of FeCl₃·6H₂O, 2.0 g of PEG, 6.0 g of sodium acetate, and 1.0 g of sodium citrate was mixed in a round-bottom flask containing 80 mL of ethylene glycol solution. The solution was vigorously stirred at 100 °C until a homogeneous transparent solution was achieved. Subsequently, the suspension was transferred to a Teflon-lined stainless-steel autoclave and heated at 200 °C for 12 h. After cooling to room temperature, the resulting black product was collected using an external magnet, washed with distilled water and ethanol to eliminate any unreacted materials, and then dried at 50 °C in a vacuum oven for 12 h before further use.

Fe₃O₄ nanoparticles weighing 1.0 g were dispersed in 100 mL of Tris buffer (0.1M, pH 8.5) by using ultrasonication. Subsequently, 100 mg of dopamine chloride was introduced into the suspension. The mixture was stirred for 12 h, leading to the formation of polydopamine (PDA)-coated Fe₃O₄ magnetic nanoparticles, which were then retrieved using an external magnet. Finally, the product was dried at 50 °C in a vacuum oven for 12 h.

4.2.3. Synthesis of the Magnetic Copper-Based Metal–Organic Framework

MNPs@Cu-BDC were prepared through an encapsulation approach. In this approach, PDA was used as a buffer interface between Fe₃O₄ and Cu-BDC to facilitate the growth and enhance the compatibility of MOFs around the Fe₃O₄, resulting in the formation of MNPs@Cu-BDC. MNPs@Cu-BDC were synthesized with slight modifications based on a previously reported method. The ratio of the metal precursor, organic ligand, and Fe₃O₄ was optimized. , Briefly, Cu­(NO₃)₂·3H₂O (0.604 g, 2.5 mmol) and H₂BDC (0.415 g, 2.5 mmol) were dissolved in 40 mL of DMF, followed by the addition of 0.1 g of polydopamine-functionalized iron oxide (Fe₃O₄@PDA) to the solution. The mixture was then subjected to ultrasonication for 10 min to achieve a homogeneous mixture. Subsequently, the mixture was transferred to a 50 mL Teflon-lined autoclave and heated at 130 °C for 24 h in a preheated oven. The resulting products were collected using an external magnet and washed sequentially with DMF (3 times), water (3 times), and ethanol (2 times) to eliminate any unreacted materials. Finally, the product was dried at 60 °C for 12 h in a vacuum oven. ,

This article contains all the data created or examined during this investigation.

Ibrahim Nasser Ahmed: idea conception and planning of the study. Asfaw Zinabu Berhanu and Addisu Tamir Wassie: experimental work, result analysis, and preparation of the manuscript; Abera Merga Ariti: tables and graphics preparation; Yakob Godebo Godeto: instrumental analysis; Taju Sani, Ming-Hua Xie, Xiu-Li Yang, and Ibrahim Nasser Ahmed: supervision and manuscript editing. The final version of the manuscript was reviewed and approved by all authors.

This work was supported by the Addis Ababa Science and Technology University project grant code IG06/2021.

The authors declare no competing financial interest.