Introduction

Water is a vital element for satisfying current and future societal requirements1,2. With the increasing expansion of industrial activities, the contamination of limited water supplies with organic and inorganic pollutants is a highly controversial issue on a global scale3. Synthetic dyes are one of the biggest water pollutants that have catastrophic and devastating influences on all of Earth’s living resources4. The direct discharge of cationic dye MB as the most abundant dye pollutant (from printing, paper, rubber, food, pharmaceutical, plastic, textile, cosmetic, and leather industries) resulting acute health problems such as contact dermatitis, hypertension, respiratory diseases, gastritis, and cancer5,6,7,8. Therefore, it is extremely necessary to develop a sensitive, simple, and loyal solution for the effective elimination of MB from freshwater. The investigations on the removal of MB indicate that the adsorption techniques are preferable to other methods such as electrochemical9, coagulation10, membrane separation11, and degradation12 due to their ease of operation, no secondary pollution, low economic cost, and high cost-efficiency13,14. However, an excellent adsorption process depends on excellent adsorbents with the ability to outer-surface modification, reusability, and good chemical resistance15.

Farahani et al. prepared the AF-GO multicomponent system by functionalizing the graphene oxide nanosheet with 3-aminopropyl-trimethoxysilane through post-synthetic modification. The AF-GO showed good performance as adsorbent in the removal of anionic dyes from wastewater16. In a study that was administered by Mahmoodi et al., silica nanoparticles (SSN) were applied for the removal of cationic dyes (Basic Violet 16 (BV16), Basic Red 46 (BR46), and Basic Red 18 (BR18), B). The maximum dye adsorption capacity of SSN for BV16, BR46, and BR18 was 416, 88, and 98 mg/g, respectively17. In another study, Mokhtari et al. prepared aminated nanoporous PAN (ANPAN) for dye removal. ANPAN was produced via solvent casting/porogen leaching procedure of nanoporous polyacrylonitrile/calcium carbonate (PAN/CaCO3) and then modification of the prepared nanofiber with triethylenetetriamine (TETA)18.

In the field of nanomaterials research Metal-organic frameworks (MOFs), often referred to as “wonder materials,” have attracted great attention due to their unique set of features19. The salient advantageous features such as huge surface area, crystallinity, high porosity, fine-tuning of structure and pores, and especially the ease of modification by various modifiers make MOFs superior to other materials in environmental and catalytic applications20,21. The structural flexibility of MOFs is one of the reasons for their superiority over other rigid porous crystalline frameworks such as zeolites22. Post-synthetic introduction of active sites in the structure of MOFs using diverse functional groups enhances their features, such as thermal and chemical stability, for specific purposes23.

Tetrahydrobenzo [b]pyrans are crucial fragments of heterocyclic organic compounds with biological activity24. These compounds with various pharmacological features such as anti-Alzheimer’s, diuretic, anti-anaphylactic activities, antileukemic, vasodilator, cardiotonic, hepatoprotective, antihypertensive, vasodilatory activities, and anticancer have received considerable attention25.

Among the wide spectrum MOFs, UiO-66-NH2, as a zirconium-based MOF, has received unprecedented attention within the area of reticular chemistry since its invention. UiO-66-NH2 framework possesses numerous potential advantages, including high penetrable surface area, flexibility in functionalization, high chemical stability, thermal stability, surface stability in aqueous conditions, wide range pH stability, ease of regeneration, and high adsorption capacities, which make its as a wonderful support and precursor for the construction of multifunctional nanocomposites in the catalytic and environmental fields26,27,28.

In a solid-liquid system, magnetic separation technology is a notable advantage. In this paper, Fe3O4 magnetic nanoparticles were decorated on the surface of functionalized metal organic framework (MOF) to obtain a novel hybrid magnetic nanocomposite. A highly stable Zr-based MOF, UiO-66-NH2, was functionalized with glutaraldehyde and adenosine to introduce abundant active sites on the surface. Next, the Fe3O4 nanoparticle was immobilized on the surface of Zr-MOF-glutaraldehyde-Adn to obtain a multifunctional nanocomposite named Zr-MOF-glutaraldehyde-Adn@Fe3O4. The adsorption capacities of the prepared nanocomposite for the removal of MB from aqueous media were thoroughly judged through adsorption tests. Moreover, the catalytic activity of the designed system was evaluated in an organic reaction as a heterogeneous catalyst for the synthesis of tetrahydrobenzo[b]pyrans derivatives. The novelty of this work is that the designed Zr-MOF-glutaraldehyde-Adn@Fe3O4 nanocomposite offers the well-defined the specified surface, chemical, and physical features of Zr-MOF, and the benefits of the presence of glutaraldehyde and adenosine functional groups, along with the magnetic separation performance of Fe3O4 nanoparticles, all in a single sustainable nanocomposite.

Experimental

Materials and instruments

All materials were commercially acquired from Merck and Fluka companies and used without any purification.

The FT-IR analysis was achieved using a TENSOR BRUKER 27 spectrometer in the range of 400–4000 cm−1. Brunauer–Emmett–Teller (BET) procedure was recorded on a BELSORP mini ΙΙ instrument. The images of field ‎emission scanning electron ‎microscope (FE-SEM) and EDX/MAP analysis were gathered by FE-SEM-TESCAN ‎MIRA3 instrument. X-ray diffraction patterns were gained using a ‎BRUKER D8-Focus Bragg − Brentano instrument. Thermogravimetric analysis (TGA) was conducted on a TGA-STA504 (BAHR) apparatus. The magnetic property was evaluated using a vibrating sample magnetometer (VSM, Meghnatis Daghigh Kavir Co., Kashan, Iran).

Preparation of Fe3O4 nanoparticles

Magnetic Fe3O4 nanoparticles were prepared by one-pot hydrothermal method29. Briefly, 80 mg of FeCl2.4H2O and 216 mg FeCl3.6H2O were dissolved in 40 mL of distilled water under N2. After 30 min, 1 mL solution of NH3.H2O was dropwise added to the mixture with vigorous mechanical agitation and kept on reacting for 40 min at 50 °C protected by N2. When it was cooled to room temperature, the obtained solid was collected by an external supermagnet and washed with double-distilled water several times to pH = 7.

Synthesis of Zr-MOF

Zr-MOF was synthesized according to previously described method30. In brief, ZrCl4 (0.12 g, 0.54 mmol) was dissolved in DMF: HCl (5:1) and sonicated for about 20 min. Then 2-aminoterephthalic acid (0.13 g, 0.75 mmol) and DMF (10 ml) was also sonicated for 20 min and two mixtures were then combined and sonicated again. The resultant solution was heated at 80 ᵒC for 24 h. After cooling to room temperature, the resulting solid was collected by centrifugation and respectively washed with DMF, water, and ethanol. At last, the Zr-MOF was heated in the oven at 100 ᵒC.

Synthesis of Zr-MOF-glutaraldehyde

To 200 mg of Zr-MOF in 40 ml DMF, 2 mL of glutaraldehyde was added. Then 0.1 mL hydrochloric acid was added into the mixture reaction and subsequently stirred for 24 h at 80 ᵒC. The achieved solid Zr-MOF-glutaraldehyde was filtered and washed with organic solvents (DMF, and EtOH) several times and dried at 70 ᵒC under vacuum for 12 h.

Synthesis of Zr-MOF-glutaraldehyde-Adn

For the preparation of Zr-MOF-glutaraldehyde-Adn, initially, 200 mg of ‎ Zr-MOF-glutaraldehyde was dissolved in 40 ml DMF, and 200 mg of adenosine was added. Then, 0.1 mL hydrochloric acid was added into the mixture reaction and subsequently stirred for 50 h at 80 ᵒC. The achieved solid Zr-MOF-glutaraldehyde-Adn was filtered and washed with DMF and EtOH several times and dried at 70 ᵒC under a vacuum for 12 h.

Synthesis of Zr-MOF-glutaraldehyde-Adn@Fe3O4

For the preparation of Zr-MOF-glutaraldehyde-Adn@Fe3O4, initially, 200 mg of ‎ Zr-MOF-glutaraldehyde-Adn was dissolved in 40 ml DMF, and 30 mg of Fe3O4 nanoparticles was added and stirred for 24 h at rt. The achieved solid Zr-MOF-glutaraldehyde-Adn@Fe3O4 was filtered and washed with DMF and EtOH several times and dried at 50 ᵒC under a vacuum for 12 h.

Adsorption experiment

The adsorption experiments were conducted using the following procedures. First, one stock solution of MB (250 mg/L) was prepared by dissolving 62 mg standard substances in 250 mL of deionized water. Other standard solutions of MB with different concentrations were prepared by diluting the stock solution individually. During the optimization of the adsorption condition of MB using Zr-MOF-glutaraldehyde-Adn@Fe3O4, the effects of different parameters such as the pH, catalyst dose, initial analyte concentration, and contact time were investigated by batch experiments. The adsorption experiments were controlled by using a shaker incubator and the residual concentration of MB at the end of the process was determined by applying UV-Vis spectrophotometry at the maximum absorption wavelength (λmax). The systematic errors for UV–vis spectrophotometer concentration measurements were ± 0.3%. The error bars resulting from both experimental and systematic errors were calculated and depicted in the figures.

The adsorption capacity qe (mg g−1) and percentage removal (R%) were calculated according to the following Eqs. (1) and (2):

$$\:{Q}_{e}=\frac{\left({C}_{0}-{C}_{e}\right)V}{m}\:$$
(1)
$$\:{R}_{e}=\frac{{C}_{0}-{C}_{e}}{{C}_{0}}*100$$
(2)

where C0 and Ce (mg/L) are the concentrations of methylene blue at the initial and equilibrium in the wastewater, respectively. V (L) and m (g) are also the volume of the contaminated solution and dosage of Zr-MOF-glutaraldehyde-Adn@Fe3O4, respectively.

General procedure for one-pot synthesis of tetrahydrobenzo[b]pyrans derivatives

0.02 g of catalyst was added to a mixture of aldehyde (1 mmol), malononitrile (1.2 mmol), dimedone (1 mmol) and 5 ml of H2O in 10 ml round-bottom flask. Then, the resulting mixture was stirred at 50 ᵒC. The reaction progress was monitored using TLC. After completion of the reaction process, the obtained product was dissolved by the addition of hot ethanol to the mixture. The incorporated magnetic catalyst was separated from the product by the application of a 1.4 T magnetic field followed by pouring the reaction product onto crushed ice for 10 min. The resulting product was then filtered and recrystallized from 94% ethanol (5 ml) to afford tetrahydrobenzo[b]pyran derivatives.

Results and discussion

Figure 1 illustrates the sequential steps of the preparation of multifunctional Zr-MOF-glutaraldehyde-Adn@Fe3O4 nanocomposite. After the solvothermal synthesis of Zr-MOF, glutaraldehyde was introduced to the surface through the covalent interaction between the free NH2 within the framework and the carbonyl group of glutaraldehyde ligand. In the step of preparation of Zr-MOF-glutaraldehyde-Adn, with the reaction of Zr-MOF-glutaraldehyde and the adenosine bioligand, a large number of amine groups as active adsorption sites were created. Finally, prepared Fe3O4 nanoparticles were stabilized onto Zr-MOF-glutaraldehyde-Adn to obtain Zr-MOF-glutaraldehyde-Adn@Fe3O4. The schematic representation of Zr-MOF-glutaraldehyde-Adn@Fe3O4 is depicted in Fig. 2.

Fig. 1
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Schematic representation of the production of Zr-MOF-glutaraldehyde-Adn@Fe3O4.

Fig. 2
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Zr-MOF-glutaraldehyde-Adn@Fe3O4 structure.

The crystalline structure of Zr-MOF, Zr-MOF-glutaraldehyde, Zr-MOF-glutaraldehyde-Adn, and Zr-MOF-glutaraldehyde-Adn@Fe3O4 nanocatalyst was researched through XRD analysis (Fig. 3). The analysis indicated that the characteristic peaks of Zr-MOF were identified in the correct location and are similar to the reference pattern reported for Zr-MOF. As can be seen, the main peaks of both Zr-MOF-glutaraldehyde and Zr-MOF-glutaraldehyde-Adn are maintained upon the introduction of glutaraldehyde and adenosine through the PSM process to the surface of Zr-MOF framework. In the XRD pattern of CS-Fe3O4, the usual signals of Fe3O4 NPs were discovered at 2θ = 25.93◦, 30.99◦, 35.84◦, 43.63◦, 56.94◦, and 66.24◦, corresponding to the (220), (311), (400), (442), (511), and (440) Bragg reflections, respectively31. In addition, the main diffraction peaks of the Zr-MOF-glutaraldehyde-Adn were maintained, confirming that the impregnation of Fe3O4 nanoparticles within the 3D network of MOF did not destructive impact on the crystallinity and integrity of structure.

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XRD patterns of Zr-MOF, Zr-MOF-glutaraldehyde, Zr-MOF-glutaraldehyde-Adn, and Zr-MOF-glutaraldehyde-Adn@Fe3O4.

Figure 4 indicates the FT-IR spectra of Zr-MOF, Zr-MOF-glutaraldehyde, Zr-MOF-glutaraldehyde-Adn, and Zr-MOF-glutaraldehyde-Adn@Fe3O4 nanocomposite. In the spectrum of unmodified MOF, the primary amine group appeared as a bifurcated peak at 3433 cm−1. The characteristic peak at 1572 cm−1 should be assigned to the carboxyl groups on the 2-amino terephthalic acid ligand. The other functional groups of the aromatic ring of the 2-amino terephthalic acid were observed at 1433, 1385, 1256, and 1155 cm−1. Two peaks located at 670 and 769 cm−1, associating with the vibration frequencies of the Zr-O bonds32. These peaks can be detected in the spectra of all prepared nanomaterials. In the FT-IR spectrum of Zr-MOF-glutaraldehyde, a peak at 1622 cm−1 is attributed to the azomethine group (C = N) that created through condensation reaction between carbonyl group of glutaraldehyde ligand and free -NH2 of primary Zr-MOF. A new band at 638.12 cm−1, attributed to the F-O tensile vibration, is present in the FT-IR spectrum of Zr-MOF-glutaraldehyde-Adn. This comprehensive characterization of FT-IR analysis approved the presence of Fe3O4 nanoparticles in the structure of Zr-MOF-glutaraldehyde-Adn and collectively the successful synthesis of Zr-MOF-glutaraldehyde-Adn@Fe3O4.

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FT-IR spectra of Zr-MOF, Zr-MOF-glutaraldehyde, Zr-MOF-glutaraldehyde-Adn, and Zr-MOF-glutaraldehyde-Adn@Fe3O4.

To investigate the effects of post-synthetic modification on the surface area and pore volume of the MOF, the BET analysis was done. BET isotherm plots for all nanomaterials are shown in Fig. 5 and corresponding data are illustrated in Table 1. The Zr-MOF presented SBET of 1123 m2g–1, and pore volume of 0.04 cm3 g–1. The value of BET was decreased to 745 m2g–1 by functionalization of Zr-MOF with glutaraldehyde. After adding adenosine to the structure surface and forming Zr-MOF-glutaraldehyde-Adn, SBET of 610 m2g−1, and pore volume of 0,03 cm3g−1 was obtained. The BET surface area substantially decreased for Zr-MOF-glutaraldehyde-Adn@Fe3O4 to 120 m2g−1 after the stabilization of Fe3O4 nanoparticles.

Fig. 5
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(a) BET curves of Zr-MOF, Zr-MOF-glutaraldehyde, Zr-MOF-glutaraldehyde-Adn, and Zr-MOF-glutaraldehyde-Adn@Fe3O4, and (b) BJH plot of the materials.

Table 1 Textural properties of prepared samples.
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To study the thermal stability of the Zr-MOF, Zr-MOF-glutaraldehyde, Zr-MOF-glutaraldehyde-Adn, and Zr-MOF-glutaraldehyde-Adn@Fe3O4, TGA analysis was performed as depicted in Fig. 6. Four samples showed a slight mass loss at around 100 ᵒC, originating from the removal of H2O molecules and surface-adsorbed organic solvent. The TGA curve of unmodified Zr-MOF exhibits a weight drop at about 200 ᵒC which is related to the break of the bonds and release of organic groups. At a temperature of about 400 ᵒC the collapse of the lattice occurred and after that, zirconium oxide was created as the final product of degradation. Zr-MOF-glutaraldehyde shows a drastic weight loss at 350 ᵒC that is related to the elimination of glutaraldehyde ligand anchored on the surface of Zr-MOF during chemical reactions to form the Zr-MOF-glutaraldehyde-Adn. In contrast, the decomposition rate of Zr-MOF-glutaraldehyde-Adn was less dramatic than that of Zr-MOF-glutaraldehyde. The considerable increase in decomposition temperature and thermal stability of Zr-MOF-glutaraldehyde-Adn@Fe3O4 compared to Zr-MOF-glutaraldehyde, and Zr-MOF-glutaraldehyde-Adn is mainly due to the introduction of the Fe3O4 to the framework and formation of the Zr-MOF-glutaraldehyde-Adn@Fe3O4 nanocomposite.

Fig. 6
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The TGA curves of Zr-MOF, Zr-MOF-glutaraldehyde, Zr-MOF-glutaraldehyde-Adn, and Zr-MOF-glutaraldehyde-Adn@Fe3O4.

The surface morphology of Zr-MOF and Zr-MOF-glutaraldehyde-Adn@Fe3O4 were investigated by FE-SEM techniques (Fig. 7). The original Zr-MOF has morphological characteristics such as a symmetric crystal structure with high porosity and triangular-base pyramid morphology. In the case of Zr-MOF-glutaraldehyde-Adn@Fe3O4, the surface of the MMOF tended to be rougher and Fe3O4 nanoparticles were well stabilized throughout the surface with a homogenous distribution, spherical shape, and mono-dispersity. EDS analysis was done to identify the elemental presence in the Zr-MOF-glutaraldehyde-Adn@Fe3O4 nanocomposite. Fig.8. shows the distinct signals of carbon, nitrogen, oxygen, zirconium, and iron atoms with their respective weight percentages of 27.08, 2.48, 34.72, 24.06, and 11.11 Wt%. The presence of Fe signal is assigned to the presence of Fe3O4 nanoparticles. Parts a−e of Fig. 9 indicate the elemental mapping of C, N, O, Zr, and Fe atoms. On the basis of the results, the Fe3O4 nanoparticles were distributed uniformly throughout the Zr-MOF-glutaraldehyde-Adn surface.

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FE-SEM images (a) Zr-MOF, and (b) Zr-MOF-glutaraldehyde-Adn@Fe3O4.

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The EDS analysis of the Zr-MOF-glutaraldehyde-Adn@Fe3O4.

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The elemental mapping of Zr-MOF-glutaraldehyde-Adn@Fe3O4.

The magnetic feature of the Zr-MOF-glutaraldehyde-Adn@Fe3O4 nanocomposite was studied using the VSM analysis. As shown in Fig. 10, the magnetic saturation value was 34.76 emu/g, which is high enough for quick recovery from the solution after the process using an external magnet.

Fig. 10
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Magnetization vs. applied magnetic field for Zr-MOF-glutaraldehyde-Adn@Fe3O4.

Adsorption study of methylene blue

Many manufacturers of cosmetics, paper, leather, textiles, rubber, plastics, and also chemical laboratories use MB extensively in their products33,34. MB is highly toxic and non-biodegradable and long-term contact with it can cause high blood pressure, mental disorders, vomiting, and anemia in humans and negative effects on the ecosystem35. Hence, the purification of contaminated water with MB is an inevitable prerequisite to creating a balance in the life cycle. In the adsorption process, the influence of pH (3–9), adsorbent dose (5–40 mg), the concentration of MB (10–40 mg/L), and treatment time (2–20 min) was evaluated at a mixing speed of 270 rpm.

Determination of the point of zero charge (pHpzc)

Based on the literature36, the point of zero charge (pHpzc) is described as the pH of the solution at which the adsorbent surface charge has zero value, i.e., the adsorbent has been equal amount of negative and positive surface sites. The surface charge is positive at pH < pHpzc and negative at pH > pHpzc. Thus, to understand the adsorption trend, the Zr-MOF-glutaraldehyde-Adn@Fe3O4 surface charge was measured by the solid addition method. For this purpose, a series of beakers, 10 mg of Zr-MOF-glutaraldehyde-Adn@Fe3O4 were introduced into 30 mL of 0.01 M NaCl solution. The initial pH (pH0) value of each sample was adjusted with 0.1 M HCl and 0.1 M NaOH to reach a certain pH in the range of 2 to 10. The samples were sealed and shaken for 24 h using an agitator at 500 rpm to reach the electrostatic balance between the adsorbent surface charge and the solution. A plot of change in pH (pHi-pHf) versus the initial pH allows for obtaining pHPZC (Fig. 11). The pHPZC (5.2) is the intersection point of the obtained curve with the horizontal axis.

Fig. 11
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The pHPZC curve of Zr-MOF-glutaraldehyde-Adn@Fe3O4.

pH

The pH of the working solution is a basic factor owing to its impact on the surface charge of the adsorbent and ionization process of the MB37. Hence, the effect of the change of the solution pH from 3 to 9, with a dose of adsorbent 10 mg for 10 mg/L MB solution and 10 min contact time for Zr-MOF-glutaraldehyde-Adn@Fe3O4 was investigated. In this experiment, to adjust the pH of dye solutions hydrochloric acid and sodium hydroxide were utilized. The results indicated that the removal of MB using Zr-MOF-glutaraldehyde-Adn@Fe3O4 was little at pH = 3. Under acidic conditions, competition occurs between the existing H3O+ ions and the cationic part of the dye MB in during of occupying the active sites on the adsorbent surface, which disrupts the adsorption process. According to Fig. 12a, by increasing the pH value from 3 to 7, the efficiency of the removal of MB increased considerably, and after that, the performance enhanced only a little, which is not meaningful. This is due to the fact that in the alkaline media, the sorbent attained negative surface charges, resulting in a stronger electrostatic attraction relative to the MB dye with cationic nature.

Fig. 12
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Effect of (a) pH, (b) sorbent dosage, (c) initial concentration, and (b) contact time.

Dose

The effect of adsorbent dosage on dye uptake was investigated by changing the amount of catalyst from 5 mg to 40 mg (pH = 7, dye concentration = 10 mg/L, and contact time = 10 min). Initially, the removal efficiency of MB using Zr-MOF-glutaraldehyde-Adn@Fe3O4 increased sharply with an increase in dosage from 5 to 20 mg, and then any increase in adsorbent dosage did not improve the uptake of MB at the same reaction conditions. The achievement of a higher removal percentage of MB at higher quantities of adsorbent is associated with the higher ratio of free sites on nanocomposite to analyte molecules (Fig. 12b).

Initial concentration

The removal percentage of MB using Zr-MOF-glutaraldehyde-Adn@Fe3O4 was reviewed based on the impact of initial concentration (pH = 7, dose = 10 mg, and contact time = 10 min). Adsorption studies were conducted at predetermined concentrations from 10 to 40 mg/L (Fig. 12c). Laboratory data showed that the removal percentage decreased with increasing analyte concentration, which could be due to steric repulsion between methylene blue molecules in the higher concentrations.

Time

The time needed to reach maximum adsorption and establish an equilibrium state is a prominent criterion in the process of evaluating adsorbent performance38. To explore the effect of the contact time on the adsorption of MB over Zr-MOF-glutaraldehyde-Adn@Fe3O4, adsorption experiments in five different time intervals (2–20 min) were done. The other influential parameters were set as follows: pH = 7, initial concentration = 10 mg/L, and dosage = 10 mg. It could be seen clearly, that the percentage of adsorption of MB by Zr-MOF-glutaraldehyde-Adn@Fe3O4 was 87% within 10 min, and reached 90% in less than 20 min. Beyond this time, the adsorption process by the biosorbent reached a steady state of equilibrium. This situation can be attributed to the occupation of accessible adsorption sites and more steric hindrance in the final stages of the absorption process (Fig. 12d).

Adsorption isotherms

To deeply understand the adsorption behavior of MB using Zr-MOF-glutaraldehyde-Adn@Fe3O4 and determine the maximum adsorption, isotherm studies were performed. To study the MB dye-Zr-MOF-glutaraldehyde-Adn@Fe3O4 interaction different isotherm models of Freundlich, Langmuir, and Temkin models were applied. The Freundlich model is based on the heterogeneous multi-layer adsorption behavior, whereas the Langmuir model refers to single-layer adsorption on a homogeneous sorbent surface39. The Temkin model assumes that with increasing adsorbent surface coverage the heat of adsorption decreases linearly. In other words, the Temkin model reflects the targeting species–adsorbent interactions in relation to the heat of adsorption40. The linear forms of Freundlich, Langmuir, and Temkin isotherms41 are given in Eqs. (3), (4), and (5), respectively:

$$\:\text{ln}{q}_{e}=\text{ln}{K}_{F}+\frac{1}{{n}_{F}}\text{ln}{C}_{e}$$
(3)
$$\:\frac{{C}_{e}}{{q}_{e}}=\frac{1}{{q}_{max}{K}_{L}}+\frac{{C}_{e}}{{q}_{max}}\:$$
(4)
$$\:{q}_{e}=Bln\:{k}_{T}+Bln\:{c}_{e}\:$$
(5)

In Eqs. (3)–(5), Ceq is MB equilibrium concentration (mg L−1), KF and nF are a Freundlich constant, qe is the equilibrium adsorption capacity at a certain MB concentration (mg g−1), Qmax refers to maximum adsorption capacity (mg g−1), KL is Langmuir constant, B = RT/b; B is related to the heat of adsorption, T (K) is the absolute solution temperature, R is the universal gas constant, kT (L/mg) is the equilibrium binding constant. Figure 13 depicts the linear plot of Freundlich, Langmuir, and Temkin isotherms, and their derived parameters for the adsorption of MB on Zr-MOF-glutaraldehyde-Adn@Fe3O4 are presented in Tables 2 and 3. The value of the regression correlation coefficient (R2) can be a suitable criterion for selecting the best model. The findings indicated that the value of R2 obtained from the Langmuir isotherm model is much higher than that of the Freundlich and Temkin isotherm models, which demonstrates that the Langmuir model describes the adsorption process better. Hence, it can be concluded that the removal of MB using Zr-MOF-glutaraldehyde-Adn@Fe3O4 agreed with that of the Langmuir isotherm model and single-layer adsorption takes place at a homogeneous surface with identical active sites.

Fig. 13
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Adsorption isotherms plots for MB sorption on Zr-MOF-glutaraldehyde-Adn@Fe3O4 (a) Langmuir, (b) Freundlich, and (c) Temkin.

Table 2 Parameters of the Langmuir, Freundlich, and Temkin isotherm models for the adsorption of MB by Zr-MOF-glutaraldehyde-Adn@Fe3O4.
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Table 3 RL at different initial MB concentrations on Zr-MOF-glutaraldehyde-Adn@Fe3O4.
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Adsorption kinetics

Kinetic studies were carried out to explain the adsorption mechanism and also describe the efficiency of Zr-MOF-glutaraldehyde-Adn@Fe3O4 as a practical adsorbent in the industrial scale42. In this work, three classical kinetics models i.e., pseudo-zero, pseudo-first and pseudo-second-order were employed. The plot of the kinetic models for the adsorption of MB on Zr-MOF-glutaraldehyde-Adn@Fe3O4 nanocomposite is depicted in Fig. 14, and their derived parameters are illustrated in Table 4. According to data, the linear correlation of the pseudo-second-order model is remarkably better than that of the pseudo-first-order model. Moreover, the R2 value of the pseudo-second-order model is closer to 1 and the linear regression is better. This reveals that the adsorption process of MB by introduced adsorbent follows a pseudo-second-order model, and is the chemisorption type.

Fig. 14
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The plots of (a) pseudo-zero, (b) pseudo-first, and (c) pseudo-second for MB adsorption onto Zr-MOF-glutaraldehyde-Adn@Fe3O4.

Table 4 The parameters of adsorption kinetics of methylene blue onto Zr-MOF-glutaraldehyde-Adn@Fe3O4 nanocomposites.
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Reusability/regeneration study

Reduction of the consumption of adsorbent, stability, and preservation of efficiency is a basic goal in the design and development of new adsorbents for scale-up applications43,44. The adsorbent stability was estimated using six cycles of trials. The experiments were conducted with the optimized operating parameters such as pH 7, catalyst dosage 10 mg, initial concentration of 10 mg/L, and contact time 15 min. The removal efficiency of fresh Zr-MOF-glutaraldehyde-Adn@Fe3O4 was 95.5%, which decreased to 94%, 91%, 88.5%, 83%, and 80.2% after each adsorption run of the catalyst against MB. It can be concluded that Zr-MOF-glutaraldehyde-Adn@Fe3O4 exhibits excellent reusability and stability in cycling runs for adsorption of MB molecules, as shown in Fig. 15a. Also, the recovered adsorbent was analyzed by taking an FT-IR spectrum, XRD pattern, and FESEM image (Fig. 15b-d).

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(a) Recycling experiments of Zr-MOF-glutaraldehyde-Adn@Fe3O4 sorbent in the removal of MB, (b) FTIR spectrum, (c) XRD pattern, and (d) FE-SEM image of Zr-MOF-glutaraldehyde-Adn@Fe3O4 after removal of MB.

Mechanism for adsorption of MB onto Zr-MOF-glutaraldehyde-Adn@Fe3O4 nanocomposite

According to the literature, multiple phenomena, such as π-π stacking interaction, pore-filling, electrostatic attraction force, and hydrogen bonding, can control the adsorption of MB onto Zr-MOF-glutaraldehyde-Adn@Fe3O4 nanocomposite (Fig. 16)45. The adsorption mechanisms of MB dye using the introduced adsorbent were confirmed by different analytical techniques.

Fig. 16
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The general mechanism for adsorption of MB on Zr-MOF-glutaraldehyde-Adn@Fe3O4.

The value of the zero point of charge (pHpzc) of the Zr-MOF-glutaraldehyde-Adn@Fe3O4 composite was equal to 5.2. At pH values smaller than the pHpzc, the surface of Zr-MOF-glutaraldehyde-Adn@Fe3O4 is positively charged, and at pH > pHpzc, the surface of Zr-MOF-glutaraldehyde-Adn@Fe3O4 will be negatively charged.

On the other hand, with the increase of pH, the -COOH and -OH groups of the adsorbent become deprotonated. Since MB is a positively charged chemical, a strong electrostatic interaction occurs between the -COO− and -O− groups of the Zr-MOF-glutaraldehyde-Adn@Fe3O4 and the nitrogen of MB.

The crystal structure of the adsorbent has aromatic rings and π electrons, which interacted with the six-carbon aromatic ring of MB molecules through π–π electron coupling. After the adsorption of MB using adsorbent, changes appeared in the FE-SEM image of the Zr-MOF-glutaraldehyde-Adn@Fe3O4 and roughness on the surface was reduced which can be due to the presence of MB in its surface (Fig. 15d).

Given that the average pore diameter of the Zr-MOF-glutaraldehyde-Adn@Fe3O4 is 1.68 nm, the removal of MB dye is possible through the penetration of MB molecules with a smaller diameter (1.38 nm) into the adsorbent pores46. Further, the pore-filling mechanism was approved by the reduction in surface area of Zr-MOF-glutaraldehyde-Adn@Fe3O4 after six cycles of adsorption of MB, as observed according to the BET analysis (Table 5).

Table 5 The porous properties of fresh adsorbent and recycled adsorbent after MB adsorption.
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The FTIR analysis demonstrated the different functional groups, such as –OH, C = O, C = C, CH2, C-O, C-H, C–C, and Fe–O exist in Zr-MOF-glutaraldehyde-Adn@Fe3O4. These groups can contribute to the formation of hydrogen bonds between nitrogen present in MB dye and hydrogen present on Zr-MOF-glutaraldehyde-Adn@Fe3O4.

Comparison

The adsorption performance of the present adsorbent in terms of qmax, equilibrium time, and recyclability were compared with other MOF and Fe3O4-based adsorbents. As the reports in Table 6 show, the Zr-MOF-glutaraldehyde-Adn@Fe3O4 has great performance and is well competitive with its counterparts.

Table 6 Comparison of different adsorbents for the adsorption MB.
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Catalytic behavior

The catalytic activity of Zr-MOF-glutaraldehyde-Adn@Fe3O4 was studied in one of the representative one-pot three-component reactions for synthesizing tetrahydrobenzo[b]pyran derivatives. A wide range of reactions were done under controlled conditions to optimize the influential parameters, such as solvent, temperature, and the dosage of the catalyst for performing the process in a productive manner (Table 7).

Table 7 Optimization experiments for the synthesis of tetrahydrobenzo[b]pyran derivatives with Zr-MOF-glutaraldehyde-Adn@Fe3O4 systema.
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To examine the impact of solvent, several protic and aprotic solvents were scrutinized. Poor yields were identified when EtOH, toluene, H2O, CH3CN, CHCl3, and THF were used as reaction media. H2O proved to be the most effective solvent for acquiring the highest product yield.

It was observed that performing the reaction at 25 ᵒC afforded low yields, while 50 ᵒC afforded excellent yields. The catalyst showed slightly higher conversion at reflux temperature than at 50 ᵒC. However, to save energy, 50 ᵒC was chosen as the optimal temperature for further studies.

In the final section of the optimization experiments, the amount of catalyst was determined as the most effective factor in the progress of the reaction. Only 5% of the desired product was obtained in the absence of the catalyst after 7 h of reaction. The results indicated that the reaction efficiency was improved from 65% to 94% when the quantity of catalyst was increased from 10 to 20 mg. On further increasing the amount of catalyst to 30 mg, the reaction efficiency did not change significantly.

With the optimized condition in hand, the generality of the Zr-MOF-glutaraldehyde-Adn@Fe3O4 nanocatalyst in the preparation of tetrahydrobenzo[b]pyran derivatives were investigated with a wide range of substituted aldehydes (Table 8). The results demonstrated that aromatic aldehydes containing electron-accepting substituents react faster and produce high yields of corresponding products. In general, all the corresponding products were synthesized in good to excellent yields.

Table 8 Synthesis of tetrahydrobenzo[b]pyran derivatives catalyzed with Zr-MOF-glutaraldehyde-Adn@Fe3O4 systema.
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Reusability

The stability and the reusability of nanocatalyst are extremely crucial from the viewpoint of cost-effectiveness and eco-friendliness. Based on the results, Zr-MOF-glutaraldehyde-Adn@Fe3O4 was easily regenerated, and its performance was maintained over 98% in the synthesis of tetrahydrobenzo[b]pyran after at least 6 consecutive uses (Fig. 17a). As shown in Fig. 17b, the XRD spectra of the recovered catalyst were a good match with those of the fresh sample. Therefore, Zr-MOF-glutaraldehyde-Adn@Fe3O4 is a suitable catalyst for the synthesis of tetrahydrobenzo[b]pyran derivatives with excellent catalytic activity and good reproducibility.

Fig. 17
Fig. 17The alternative text for this image may have been generated using AI.
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(a) Recycling experiments of Zr-MOF-glutaraldehyde-Adn@Fe3O4 in the synthesis of tetrahydrobenzo[b]pyran derivatives, and (b) and (b) XRD patterns of Zr-MOF-glutaraldehyde-Adn@Fe3O4 before and after catalysis.

The suggested mechanism for the synthesis of tetrahydrobenzo[b]pyran derivatives using the designed nanocatalyst is illustrated in Fig. 18. Firstly, the Zr-MOF-glutaraldehyde-Adn@Fe3O4 nanocatalyst activates both malononitrile (1) and aldehyde (2). After, the Knoevenagel condensation reaction takes place between the methylene group of malononitrile and the carbonyl group of aldehyde to furnish the compound (3). At the same time, 1,3-cyclohexanedione is activated by the metal sites of the catalyst to create an intermediate enol 2, followed by the creation of intermediate 4 via Michael addition with intermediate 3. Subsequently, the target product is achieved through intramolecular cyclization and proton tautomerism of the intermediate 5.

Fig. 18
Fig. 18The alternative text for this image may have been generated using AI.
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The plausible mechanism for the synthesis of tetrahydrobenzo[b]pyran derivatives catalyzed with Zr-MOF-glutaraldehyde-Adn@Fe3O4 system.

Conclusion

The Zr-MOF-glutaraldehyde-Adn@Fe3O4 nanocomposite was synthesized and effectively applied for the removal of MB and the synthesis of tetrahydrobenzo [b]pyrans derivatives. The maximum MB removal efficiency was determined through the optimization of influential parameters. The best conditions for maximum removal of MB were obtained at pH 7, adsorbent dose of 10 mg, MB concentration of 10 mg/L, and time of 15 min. The results indicated that the removal of MB dye from aquatic media using Zr-MOF-glutaraldehyde-Adn@Fe3O4 followed the Langmuir isothermal (R2 = 0.99) and the pseudo-second-order (R2 = 0.99) models. Under the optimal adsorption conditions, the adsorption capacity of the Zr-MOF-glutaraldehyde-Adn@Fe3O4 nanocomposite was 62.5 mg/g. The catalytic activity of Zr-MOF-glutaraldehyde-Adn@Fe3O4 as an effective and stable nanocatalyst in the synthesis of tetrahydrobenzo[b]pyran derivatives was studied. Moreover, Zr-MOF-glutaraldehyde-Adn@Fe3O4 has excellent reusability, maintaining an adsorption efficiency of 80.2% for MB and catalytic activity of 84% for the synthesis of tetrahydrobenzo[b]pyran derivatives after six cycles. The findings demonstrated that all products were achieved with good to excellent yield under mild reaction conditions. This study can serve as a reference for further research about the design and development of new multifunctional systems with a variety of applications.