Construction of a 3D Metal-Organic Framework and Its Composite for Water Remediation via Selective Adsorption and Photocatalytic Degradation of Hazardous Dye.

In this work, a new bimetallic Na(I)-Zn(II) metal-organic framework (MOF), formulated as [Na2Zn3(btc)2(μ-HCOO)2(μ-H2O)8] n (1) (H3btc = benzene tricarboxylic acid), and its composite (ZnO@1) have been successfully synthesized using solvothermal and mechanochemical solid grinding methods. 1 and ZnO@1 were characterized by diffraction [single-crystal X-ray diffraction (XRD) and powder XRD], spectroscopic (ultraviolet-visible diffuse reflectance spectroscopy and Fourier transform infrared spectroscopy), microscopic (transmission electron microscopy), and thermal (thermogravimetric analysis) methods. The surface area and porosity of 1 were determined using a Brunauer-Emmett-Teller analyzer. Single-crystal diffraction of 1 confirms that Na1 and Zn2 have octahedral coordination environments, whereas Zn1 has a tetrahedral coordination geometry. Topological simplification of 1 shows a 3,6-connected kgd net. Na(I)-Zn(II) MOF (1) is crystallized with slight porosity and exhibits good tendency toward the encapsulation of zinc oxide nanoparticles (ZnO NPs). The photocatalytic behaviors of 1 and its composite (ZnO@1) were investigated over MB dye under sunlight illumination with promising degradation efficiencies of 93.69% for 1 and 97.53% for ZnO@1 in 80 min.

Introduction

In the past years, the demand of commercialized dyes and their products has increased rapidly. The bulk production of a variety of dyes was mainly aimed at to fulfill the demand of huge population, but less attention was paid to their discharges and implications on water bodies.[1−3] Near about thousands of natural and organic synthetic dyes and dyestuff are being used in textile, paper, and leather industries for coloring the products and are discharged into water sources. Based on the World Bank reports, ∼17–20% contribution of water pollution is due to the textile and dyeing industries.[4−6] According to recent reports, it has been concluded that the total annual production of azo dyes (−N=N−) accounts for about 7 × 105 tons and ∼13–15% of this value is discarded into the water bodies.[7,8] These waste dyestuff materials are potentially toxic not only to human health but also to environments and aquatic life.[1,9,10] Slow degradation of such dyestuff by sunlight in the water system and further complexation with several essential metal ions may be hazardous for living systems.[11]

Metal–organic frameworks (MOFs) are a new class of crystalline hybrid materials, also famous as brilliant porous coordination materials that are composed of the organic linker and inorganic secondary building units (metal ions/metal nodes).[12−16] MOFs have witnessed fast development in inorganic as well as material chemistry due to their attractive properties, intriguing topologies, and structural features coupled with high surface area, thermal stability, and porosity. Moreover, the conventional semiconductor nature of MOFs is solely dependent on the photoactive organic ligands, which can be tuned by incorporating active metal ions.[17,18] MOFs have been exploited potentially in a wide range of research areas such as photocatalysis,[19−21] adsorption,[8,22−26] gas sorption,[26−30] sensing,[31−33] magnetism,[34−38] and so on. The dimensionality and topology of MOFs are also dependent on their mode of coordination assembly.[39,40] Zhang and co-workers synthesized a new MOF material (NENU-505) that has shown adsorption of cationic dye, and the adsorption capacity of NENU-505 was 33.5 mg/g at room temperature for methylene blue (MB).[41] Li and co-workers have also constructed a new Cu-MOF for the removal of MB dye with a 45.045 mg/g or 96% removal rate.[42a] Abedi and co-workers have also shown the photocatalytic activity of the Ni(II)/K-based heterometallic MOF toward the degradation of bromocresol green (BCG) dye under visible light, which led to 94% degradation.[42b]

MOFs play the most promising role in host–guest chemistry. MOFs that are well-designed and have suitable porosity are capable of hosting the nanoparticles (NPs). NPs are nanosized materials with a high surface area and good semiconductor properties. Some pioneering researchers have developed a variety of MOF-based NPs (NP@MOFs) via the solid grinding method.[43a−43d] They successfully established that the NPs can be encapsulated inside the pores of MOFs via the host–guest mechanism, exploiting various non-covalent interactions.[44] Encapsulation of MOFs with NPs could result in decreasing the band gap due to fast (e–/h+) recombination between MOF architectures and metal NPs.[45−47] The above observation inspired us to design and synthesize a new ZnO@1 composite via the solid grinding method, and it shows promising MB dye degradation efficiency over the MOF (1).

Results and Discussion

Structural Description and Topological Study of MOF 1

The crystal structure of [Na2Zn3(btc)2(μ-HCOO)2(μ-H2O)8] (1) was confirmed by single-crystal X-ray diffraction (XRD), and it was crystallized in the monoclinic system with the C2/c space group. The asymmetric unit of 1 consists of one benzenetricarboxylate ion (organic linker), four water molecules (three coordinated and one μ-H2O), two independent zinc metal ions, and one sodium ion (Figure a). The fully deprotonated (btc3–) ligand adopts both the mono and bidentate fashion to connect Zn(II) and Na(I) ions. However, the formate (HCOO–) adopts the bridging monodentate fashion with Zn(II) and Na(I) ions. Na1 and Zn2 have octahedral coordination polyhedra (CN = 6), and Zn1 has a tetrahedral coordination polyhedron (Figure b). As depicted in Figure c, MOF 1 shows the following bond lengths: Zn1–O2 = 1.9481 Å, Zn1–O8 = 1.9682 Å, Zn2–O5 = 2.0469 Å, Zn2–O12 = 2.1307 Å, Na1–O1 = 2.3579 Å, Na1–O5 = 2.4019 Å, Na1–O8 = 2.3280 Å, Na1–O12 = 2.4530 and Zn1–Na1 = 3.504 Å, and Zn2–Na1 = 3.413 Å, which are within the reported range. The linear chain of Zn(II) and Na(I) is further extended by carboxylates of btc3– ligands to generate a three-dimensional heterobimetallic architecture (Figure d).

Figure 1

(a) View of the molecular structure of 1, (b) polyhedral view of metal ions, (c) linear heterometallic chain of Zn(II) and Na(I) ions, and (d) 3D architecture.

The topological study of 1 shows the coordination formula to be A5K52M26M14,[48] where A is a metal [(Zn)3 and (Na)2], K5 is C9H3O6, M2 is CHO2, and M1 corresponds to water. The standard representation of valence-bonded 1 in the 3,4,5,5-c nodal net of the new topological type with the point symbol for the net is represented as follows: {3.6.7}2{32.4.52.6.82.92}2{32.4.52.62.72.8}2{32.82.92}(Figure S1). To understand the extended generation of different subnets from the underlying net that contains the edges of weight no less than a specified value and having applied the multilevel analysis following the order of subnets, we describe the packing of the structure on different levels of solid angle (Supporting Information, Table S1). At a higher solid angle, the coordination of 1 is zero-dimensional (0D). On decreasing the solid angle, we have found extended dimension packing of the structure connected. Formation of a 3,4,5,5-c net depends on the value of the solid angle (Ω), as shown in Figure a. The topology in a 3,6-connected net with the point symbol for net {43}2{46.66.83} is presented in Figure b,c.

Figure 2

(a) Order of the subnets that describe the packing of the structure on different levels of the solid angle, (b) topological type after rod–net representation of structure 1, and (c) structural fragments in the (100) direction.

FTIR, PXRD, TGA, and UV-DRS Analysis

The vibrational bands show the binding of carboxylate, formate (μ-HCOO–), and water (μ-H2O) with sodium and zinc metal ions. Herein, the vibration stretching frequency of ν(O–H) of the bridging water molecules is observed at 3360 cm–1. The medium absorption bands ∼2800–3127 cm–1 could be due to the hydrogen-bonded −OH network inside the MOF. The absorption frequencies at ∼2750 and 1340 cm–1 may be attributed to the stretching and bending vibrations of bridging formate (HCOO–) ions.[8,49] The symmetric and asymmetric vibrational bands at 1412 cm–1 [νs(COO–)] and 1595 cm–1[νas(COO–)] could be assigned to the carboxylate groups.[50−52] The absorption bands ν(Na–O) and ν(Zn–O) appeared at 561 and 466 cm–1, respectively. Similarly, ZnO@1 shows shifting in the characteristic absorption bands. The absorption band at 3450 cm–1 could be attributed to ν(O–H), and the disappearance of the band at 3127 cm–1 indicates that zinc oxide NPs (ZnO-NPs) are encapsulated inside the pores via hydrogen bonding and other weak interactions.[53] The vibrational absorption band at 475 cm–1 could be due to ZnO NPs[54] (Figure S2).

Powder XRD (PXRD) patterns were recorded to check the bulk phase purity or structural integrity of 1 and ZnO@1. Simulated and experimental patterns match well, which confirm the bulk purity of the synthesized MOF. ZnO@1 shows new patterns from 25 to 40°, which could be an indication of the embedding of zinc oxide NPs inside the pores of 1. The structural integrity and crystalline nature of the architecture were maintained during the experiments (Figure S3). Thermogravimetric analysis (TGA) was performed to confirm the thermal stabilities of 1 and its composite (ZnO@1). The decomposition of the framework of 1 started at ∼115 °C due to the elimination of coordinated water molecules. MOFs 1 and ZnO@1 are thermally stable upto ∼290 °C and ∼315 °C, respectively (Figure S4).

Ultraviolet–visible diffuse reflectance spectroscopy (UV-DRS) spectra of 1 and its composite were recorded between 250 and 800 nm to identify their optical behaviors. The energy band gap of 1 and its composite were estimated from the Tauc plot. The plot of (αhν)2 versus the energy (hν) gives the band gap as the x-axis intercept of an extrapolated tangential line.[9] The band gaps of 1 and its composite were estimated to be 3.27 and 3.15 eV (Figure a,b), respectively. Moreover, zinc oxide NPs show a band gap energy of 2.83 eV (Figure c).

Figure 3

Optical band gaps as estimated from the Tauc plot using UV-DRS spectra (a) 1, (b) ZnO@1, and (c) ZnO NPs.

BET and TEM Analyses

The specific surface area and porosity of 1 were determined using Brunauer–Emmett–Teller (BET) adsorption isotherms for N2 gas at 77 K and pore size distribution evaluated from the non-local density functional theory (NLDFT) equilibrium model.[55] Degassed MOF 1 exhibited slight porosity with variable pore volume distributions (Figure S5).

MOF 1 and ZnO@1 have different particle sizes and morphologies that were confirmed by the transmission electron microscopy (TEM) analysis (JEOL JEM-2100F). Figure a depicts particles of 1 to be in a fractured cage-like architecture, which get encapsulated with ZnO NPs.[43b,43c,56] To give better insights into ZnO@1, both low-magnification (Figure b) and high-magnification (Figure c) images were recorded.

Figure 4

TEM images of (a)MOF 1, (b) ZnO@1 at low magnification, and (c) ZnO@1 at high magnification.

Dye Adsorption Studies

For this purpose, crystals of 1 were soaked in the aqueous solution of MB and methyl orange (MO) dyes of 15 ppm concentration at room temperature for 12 h.[57a] The changes in concentrations of the MB and MO dyes in the solution were measured using a UV–visible (UV–vis) spectrophotometer. The color visual and UV–vis absorption spectra of MB and MO dye solution revealed that MO dye showed a negligible change in absorption intensity with its maximum absorption wavelength (464 nm) (Figure S6). Interestingly, the selective adsorptive binding of cationic MB dye molecules on the framework of 1 could be directed by the formate ions and carboxylate oxygen atoms of btc3– ligands through electrostatic, π–π, and hydrogen bonding interactions (Figure S7 and Supporting Information, Table S2).[8,57b,58] Therefore, the adsorption experiment proceeded on the selected MB dye only.

To understand the adsorption phenomenon of 1, batch adsorption experiments were performed, which give the important insights into the adsorption efficiency, kinetics, and adsorption mechanism.[59,60a]Figure a depicts the UV–vis spectra of 1 before and after adsorption of the MB dye. MB was adsorbed to the maximum extent of 1 with an adsorption efficiency of 78% within 10 mins of the adsorption experiment. However, an equilibrium condition could be attained after achieving 85.2% adsorption efficiency; afterward, MOF 1 showed saturation due to the limitation of adsorption sites on it.[60b]

Figure 5

(a) UV–vis absorption spectra of 1 for MB dye uptake, (b) removal efficiency at variable pH values, (c) effect of temperature, and (d) effect of contact time.

To investigate how the MB dye is being adsorbed, we took photographs of cuvettes containing the dye solution after isolating MOF 1 from it. Additionally, the scavenging effect of the MB dye adsorption on 1 was confirmed by UV–vis characteristic peaks at 664 nm with a decreasing trend of absorption intensity at the same wavelength.[60c] To confirm the structural integrity of 1, we recorded PXRD data before and after adsorption of MB dye on it (Figure S3). There is no significant variation observed in diffraction patterns with a slight change in the peak intensity. Therefore, it is confirmed that 1 facilitates dye adsorption without losing its crystallinity after adsorption.[61a,61b]

Adsorption Kinetics

To analyze the adsorption kinetics, the experimental data were fitted with adsorption kinetic models, such as pseudo-first-order, pseudo-second-order, and intraparticle diffusion kinetic models.[61c] Several batches of experiments were carried out to investigate the rates of adsorption, which depend upon the mobility of the dye molecules onto the surface of 1.[61d] By using the models, the values of kinetic parameters and the correlation coefficient can be calculated with the help of the following equations, eqs –3, respectively.[61d]Here, the adsorption capacity of 1 is described by qe and q (mg/g) at the equilibrium and time. Rate constants are different for different orders of the reaction such as k1 for the pseudo-first-order model (min–1), k2 for the pseudo-second-order model (g mg–1 min–1), and k3 for the intraparticle diffusion model (mg g–1 min–1).[62,63]

The fitting results such as the adsorption capacities, rate constant, and correlation coefficient are shown in Table . The correlation coefficient (R2) values of 1 at different initial concentrations were all <0.998 through the pseudo-second-order model.[64a,64b] The good linear fitting relationship supported that the pseudo-second-order kinetic model was the fitting model for calculating the amount of MB in a short period (Figure ). The highest adsorption amount of MB could reach up to 290.7 mg/g, which was comparable to those of the reported MOF architecture (Table ).[64c]

Table 1

Kinetic Parameters of MB Dye Adsorption for 1

model	pseudo-first-order kinetics				pseudo-second-order kinetics				intraparticle diffusion
parameter	k1 (min–1)	qe (cal) (mg/g)	qe (exp) (mg/g)	R2	k2 (g mg–1 min–1)	qe (cal) (mg/g)	qe (exp) (mg/g)	R2	k3 (mg/g min–1/2)	R2
MB	0.36	140.30	120.28	0.895	0.065	280.50	290.7	0.998	34.6	0.860

Figure 6

Adsorption kinetic plots for MB dye adsorption on 1: (a) pseudo-first-order, (b) pseudo-second-order, and (c) intraparticle diffusion models.

Table 3

Adsorption and Photocatalytic Degradation of MB Dye over 1

adsorption					photocatalytic degradation
MOFs	qe (mg/g)	kinetics order	mechanism	references	catalyst (MOF)	degradation efficiency (%)	source	time (min)	references
UiO-66	69.8	pseudo-second-order	π–π stacking interaction	(59)	3D Cu based	50	sunlight	210	(60)
MIL-53(Al)–NH2	45.2	pseudo-second-order	π–π stacking interaction, H-bonding	(77)	3D Zn based	89	UV light	80	(84)
MIL-53(Al)	3.6	pseudo-second-order	π–π stacking interaction, H-bonding	(77)	3D Zn based	63	visible	50	(93)
SCNU-Z2	455.6	pseudo-second-order	cation-exchange process	(90)	3D Zn based	79	visible	90	(94)
3D Na(I)–Zn(II) MOF (1)	290.7	pseudo-second-order	π–π stacking interaction, H-bonding	this work	3D Na(I)–Zn(II) MOF (1)/ZnO@1	93.69/97.53	sunlight	80	this work

Effect of pH

pH is an important parameter influencing the adsorption process by changing the surface charge distribution of the adsorbent.[65−68,69a] To gain deep insights into the influence of pH on the adsorption process, we prepared a series of solution of 1 with pH values in the range ∼3.0–14.0. The pH values were adjusted using 0.1 N NaOH and 0.1 N HCl solution in an aqueous medium under ambient conditions.[69b] It is evident from Figure b that as we increase the pH from 3 to 7, the adsorption capacity of 1 increases significantly. At a low pH of 3, 1 achieved ∼20% adsorption efficiency due to the interaction of the H+ ions with the positive part of MB dye.[62] However, 85.2% adsorption efficiency is reached at pH = 7. Conversely, as we increase the pH range from neutral to alkaline, pH = 11, it can be seen that the adsorption capacity (∼42%) decreases significantly due to the negative part of MB interacting with the base forming NaCl.[63,69c,70,71a] The effect of pH on the uptake capacity has been shown with 1 being a better adsorbent at a pH of 7 with 85.2% removal efficiency of MB dye.

To identify the surface charge of the 1 adsorbent, the point of zero charge (PZC) (pHpzc) method was utilized.[71b] The PZC (pHpzc) value can be estimated from the plot between initial pH and ΔpH (initial pH – final pH), which is ∼6.7 in the case of MOF 1 (Figure ). It is observed[71c,71d] that the negative charge on the surface of the adsorbent gets accumulated with the increase in the pH, resulting in the adsorption of cationic dyes on its surface being favored.

Figure 7

PZC (pHPZC) plot for 1.

Effect of Temperature

The temperature of the solution phase is considered to be a significant parameter because it changes the adsorption capacity of the adsorbent.[72a,72b,73] The effect of temperature on the adsorption efficiency of 1 was recorded at different temperatures ranging from 20 to 60 °C (Figure c). The maximum adsorption efficiency (82%) of 1 was observed at room temperature (20 °C), and it started to decline with further increase in the temperature from 20 to 55 °C, which indicates an exothermic process.[74a,74b,75]

Effect of Contact Time

The contact time is also an important factor that influences the adsorption capacity of the adsorbents.[76a] At the start of the reaction time, t = 0 min, 1 shows a negligible amount of MB dye adsorption. However, ∼45% MB dye adsorption was achieved during the first 10 min of contact time. A rapid adsorption process was recorded during the first 60 min of adsorption. After this period, the equilibrium is reached with 82.5% adsorption efficiency, and beyond it, saturation is observed (Figure d).[76b,76c,77,78]

Photocatalytic Activity

Furthermore, for the complete remediation of unabsorbed MB dye from wastewater, the photocatalytic process has been proved to be an efficient method to degrade or mineralize the organic pollutants from wastewater under sunlight.[79,80] Therefore, we performed photocatalytic degradation of MB dye over 1 and ZnO@1 by using batch experiments that have shown a characteristic change in UV–vis absorbance upon sunlight illumination. UV–vis experiments have shown a gradual change in the intensity of the dye solution under photoirradiation, which indicates the successful photocatalytic degradation of dye molecules (Figure a,d). Photocatalytic degradation of MB was estimated from the absorption intensity versus irradiation time plot at λmax = ∼664 nm (Figure b,e). Dye degradation efficiencies over 1 and ZnO@1 were calculated using the following relation (eq ).[81]

Figure 8

(a,d) UV–vis spectra of the change in MB dye concentrations over 1 and ZnO@1, (b,e) photocatalytic degradation of MB dye with time in an aqueous solution under sunlight irradiation, and (c,f) kinetics of MB dye degradation, plot of −ln(C0/C) vs time (min).

Photocatalytic degradation efficiency (%)where the initial concentration is represented by C0 and the final concentration is represented by C at time t.

Overall degradation values of MB dye over 1 and ZnO@1 were observed to be 93.69 and 97.53%, respectively.

For 1, degradation values of MB over 10, 15, and 20 mg were obtained to be 27.83, 46.66, and 58.81% for the initial 10 min, respectively, and the overall percentage were 91.53, 93.26, and 93.69% within 80 min, respectively. Similarly, ZnO@1 has shown degradation of 81.60, 79.68, and 89.75% within the first 10 min and 95.92, 97.30, and 97.53% within 80 min, respectively.

Moreover, the degradation kinetics of MB dye was evaluated by using the Langmuir–Hinshelwood mechanism. Photocatalytic degradation of MB dye followed pseudo-first-order kineticswhere the initial dye concentration is C0 (mg/L) at time t = 0, the final dye concentration is C (mg/L) at time t = t (min), and k is the dye degradation rate for the catalyst of pseudo-first order, and on simplifying eq ,where, the rate of degradation –ln(C/C0) versus irradiation time shows linear fitting for MB dye as per eq . The apparent rate of the reaction was obtained using eq , which shows the degradation rate of the reaction for dye[82,83]where C0 is the initial dye concentration, kapp is the apparent adsorption constant for MB dye on the surface of the photocatalyst (L/mg), the maximum photocatalytic degradation rate (mg/L min) is represented by kr, and t1/2 is the half-life time of the reaction rate, which is calculated by using the following equation, eq (Table )[82]

Table 2

Apparent Rate Constant (kapp), Half-Life Time of the Reaction Rate (t1/2), and the Corresponding Correlation Factor (R2) for Degradation of MB Dye over the Catalyst (1 and ZnO@1), with Values Obtained from the Langmuir–Hinshelwood Rate Equation

	parameters
	1			ZnO@1
catalyst dose	kapp (min–1)	t1/2 (min–1)	R2	kapp (min–1)	t1/2 (min–1)	R2
10 mg	0.0187	37.48	0.9843	0.0262	26.46	0.9347
15 mg	0.0241	28.79	0.9903	0.0371	18.67	0.9788
20 mg	0.0280	24.75	0.9178	0.0433	16.02	0.9329

Linear fitting of the plot between −ln(C/C0) versus time gives a straight line with a slope parameter value which is equal to the apparent rate constant of pseudo-first-order reaction (kapp) (Figure c,f). The calculated values of kapp for 1 and ZnO@1 show that the reaction rate increase with the increasing catalyst dose for MB dye. The half-life time (t1/2) of reactions decreased with the increasing catalyst dose, which may be attributed to faster degradation of dye molecules at a 20 mg dose of 1 and ZnO@1. Moreover, to understand the better correlation between the catalyst dose and reaction rate, the correlation coefficient (R2) was considered. The R2 value of the 15 mg catalyst dose (0.9903 for 1 and 0.9787 for ZnO@1) proved its higher efficiency for degradation of MB dye. Therefore, the amount of the catalyst and concentration of dye play a pivotal role in the good heterogeneous recombination for photocatalytic degradation of dye molecules. It is observed that the formation of the electron–hole (e–/h+) heterojunction is closely related to the degree of availability of catalyst active sites, which results in successful photocatalysis under sunlight exposure. The tabulated MOFs have displayed good photocatalytic activity for degradation of MB dye under light sources (Table ).

Photocatalytic Mechanism for Dye Degradation

MOFs as a semiconducting catalyst can absorb photons and generate electron–hole pairs in conduction and valance bands.[83−85] Upon irradiation of sunlight, the organic linkers act as an electron-releasing agent via a conjugation system to metal ions.[86] The hopping of electrons from one place to another place triggers the redox mechanism. Herein, we assume that under the illumination of sunlight, the excited electrons from the ligand system could react with molecular oxygen, forming reactive oxygen species (ROS). Similarly, water molecules and their hydroxyl ions react with the holes and form reactive hydroxyl radicals (OH•).[85−87] Therefore, ROS and OH· radicals play a central role in the degradation of MB dye molecules (Figure ). According to some recent reports,[88−90] presumably, the lower band gap energy of ZnO@1 could facilitate the electron excitation at a lower energy and hence fast production of ROS and OH• radicals, which results in an enhanced rate of photodegradation of MB dye molecules.[91,92]

Figure 9

Plausible mechanism of photocatalytic degradation of MB dye over 1 and ZnO@1 catalysts. Hydrogen atoms are omitted for structural clarity of 1.

Conclusions

Here, we have summarized the synthesis of Na(I) and Zn(II)-based bimetallic MOF and its composite (ZnO@1) under solvothermal and mechanochemical solid grinding methods. Both the MOF 1 and its composite are thoroughly characterized by various spectroscopic, thermal, and microscopic techniques. MOF 1 proved to be a potential adsorbent toward selective removal of cationic MB dyes. The formate ions and carboxylate oxygen atoms of btc3– ligands are key components for the selective adsorptive binding of cationic MB dye molecules on the framework of 1 through several electrostatic and noncovalent interactions. 1 and its composite have exhibited promising photocatalytic degradation activity of MB dye under sunlight illumination.

Experimental Section

Materials

Benzenetricarboxylic acid (H3btc), zinc nitrate [Zn(NO3)·6H2O], sodium formate (HCOONa), and potassium nitrate were procured from Sigma-Aldrich. Sodium hydroxide, MB dye, and solvents were purchased from Fisher Scientific and used as received.

Instrumentation

A CE-440 elemental analyzer (Exeter Analytical Inc.) was used for elemental analysis of MOF 1. Photocatalytic degradation and adsorption studies were performed on a Thermo Scientific Evolution 201 UV–vis spectrophotometer with a standard path length (1 cm) of cuvettes at ambient temperature. Fourier transform infrared (FTIR) spectra of 1 and ZnO@1 were obtained using a Thermo Scientific NICOLET model (iS50). UV-DRS spectra were recorded on a PerkinElmer, Lambda 35 spectrophotometer instrument to estimate the optical assets of (1 and ZnO@1) photocatalysts. TGA curves of 1 and its composite were recorded on a Shimadzu TGA-50H instrument. For phase purity of 1 and ZnO@1, a Miniflexll X-ray diffractometer was used to record the PXRD pattern. To investigate the particle heterostructure and intimate contact morphology of 1 and its composite, a transmission electron microscopy (TEM) JEOL JEM-2100F instrument was utilized. A Quanta chrome Quadrasorb SI analyzer was used for BET analysis.

Single-Crystal Diffraction Details

The diffraction data of 1 were obtained on a Bruker SMART APEX CCD diffractometer and collected using monochromatic Mo Kα radiation (λ = 0.71073 Å) at 100(2) K. The linear absorption coefficients and the anomalous dispersion corrections were obtained from the International Tables for X-ray crystallography.[95] Using Olex2,[96] the structure was solved using the olex2. Solve[97] structure solution program using Charge Flipping and refined using the olex2. Refine[97] refinement package using Gauss–Newton minimization. All hydrogen atoms were located in different Fourier maps in the structures and refined isotropically. All non-H atoms were refined anisotropically. The crystal structural refinement data, details of the bond lengths, and bond angles of 1 are provided in Supporting Information, Tables S3–S5.

Synthesis of Complex 1

A mixture of 20 mg of benzenetricarboxylic acid (H3btc), 10 mg of sodium formate (HCOONa), and 100 mg of zinc nitrate [Zn(NO3)2·6H2O] was dissolved in 5 mL of the solvent mixture (2 mL of ethanol and 3 mL of H2O) and transferred into a Teflon autoclave. 0.5 mL of sodium hydroxide solution (1 mmol) was added into it and sealed tightly. The autoclave was placed in the hydrothermal oven for 72 h at 120 °C. After this period, we obtained a pale yellow colored solution, and it was filtered. The solution was left to allow slow evaporation. After 2–3 weeks, we observed light pale yellow cubic-like crystals (Scheme ). Melting point: 245 °C; elemental analysis (%): calcd for C20H24Na2O24Zn3 (890.57): C, 26.97%; H, 2.72%; found: C, 27.01%; H, 2.79%. IR (cm–1): 3359 (w), 3127 (w), 2800 (m), 2460 (w), 1586 (s), 1456 (s), 1379 (s), 1113 (w), 1017 (s), 880 (w), 820 (w), 768 (m), 716 (m), 561 (m), 466 (w).

Scheme 1

Schematic Representation of Synthesis of 1

Synthesis of ZnO@1

Zinc oxide NPs (ZnO NPs) and ZnO@1 were synthesized using earlier reported procedures with slight modification.[43,47,98] We took 100 mg of MOF 1 and 10% (w/w) of zinc oxide NPs in an agate mortar and pestle and performed mechanochemical solid grinding for 2 h at ambient temperature. Following this, a few drops of ethanol in ground powder were added and subjected to drying under vacuum for 30 min. The obtained ZnO@1, as shown in Scheme , is thoroughly characterized. IR (cm–1): 3450 (s), 2850 (w), 2460(w), 1620 (s), 1477(s), 1376 (m), 1343 (m), 1190 (w), 1119 (s), 1017 (s), 840 (w), 768 (w), 725 (w), 622 (w).

Scheme 2

Schematic Representation of Encapsulation of NPs inside the Pores of the MOF

Batch Adsorption Process

We prepared a standard stock solution of 15 ppm [15 mg of the dye in 1 L of demineralized water (DMW)] MB dye. The crystalline sample of 1 was immersed in 30 mL of dye solution in a 100 mL conical flask and kept for 90 min at room temperature. Following this, we recorded UV–vis spectra between 450 and 750 nm wavelengths at a regular time interval (after each 10 min). The maximum absorption of the dye solution appears at a wavelength of λmax = 665 nm. Moreover, decreasing patterns were observed in the absorbance peak with time. Using eqs and 10,[82] the percentage removal efficiency and the adsorption capacity, respectively, of 1 can be estimatedwhere C0 (mg/L) and C (mg/L) represent the initial and at time t (min) equilibrium concentrations of the dye solution, respectively. Here, the sample volume is V (in mL) and “m” stands for the dose or mass (mg) of the adsorbent. We also explored the significance of parameters such as time, temperature, and pH on the adsorption process.

Photocatalytic Degradation of Dye

To check the photocatalytic activity, we took 10, 15, and 20 mg amounts of 1 and ZnO@1 dispersed in 30 mL of MB dye stock aqueous solution (15 ppm) in 50 mL separate beakers, and before using sunlight irradiation, the solution was magnetically stirred for 1 h to confirm the establishment of the equilibrium between adsorption and desorption processes after the suspended solution was formed. Following this, the solution was exposed to sunlight for photocatalytic degradation of MB dye molecules over 1 and ZnO@1, and their UV–vis spectra were continually recorded to confirm absorption behaviors. A continuous decrease in the intensity of the absorption edge confirmed the photocatalytic degradation of MB dye. The overall reaction was shown to follow pseudo-first-order kinetics for photocatalytic dye degradation and the rate constant of the reaction was calculated using eq .[98]where k is calculated from the plot of −ln(C/C0) versus time interval and C0 and C denote the concentrations of dye at time t = 0 and t = t (min), respectively.

PZC Study

To determine the surface charge of adsorbent 1 for the dye adsorption process, the pHPZC estimations were performed in both acidic and basic aqueous phases at different pH values. A series of 30 mL of (0.1 M) KNO3 aqueous solution were taken into 100 mL conical flasks, and their pH values were adjusted to values between 2 and 10 (2, 4, 6, 8, and 10) by adding 0.1 M HCl and 0.1 M NaOH. Afterward, 15 mg of adsorbent 1 was added in each conical flask for 24 h. Now, after the completion of this time period, the solutions were centrifuged, and their pH (final pH) values were measured.[91,92]