MODIFIED ZEOLITIC IMIDAZOLATE FRAMEWORKS: HYBRID MATERIALS FOR WASTEWATER REMEDIATION

Abstract

In 1995 Yaghi and Li reported a new class of mesoporous materials named as Metal- Organic Frameworks (MOFs), which are hybrid of organic and inorganic moieties. Structurally metal ions or metal clusters, joined by polydentate organic ligand or linker through co-ordinate bonds and other forces e.g. van der Waals forces, H-bonding and π-π stacking, have a key role and give rise to highly crystalline, porous 3-D network or framework having definite pore size and pore volume throughout the material. Depending upon the metal ion or organic linker, there may be a huge number of MOFs. Generally, carboxylate linkers are used to synthesize MOFs but some other types of ligands such as Schiff’s bases, nitrogen and sulphur containing polydentate aromatic ligands, and amino acids are also used. In the last decade, more than 20,000 MOFs have been synthesized and studied for various applications. MOFs possess extraordinarily high surface area (experimental value of up to 10450 m2 g-1 (ZIF-210), the theoretical limit of 14600 m2 g-1), ultrahigh porosity (up to 90% free volume), tunable pore size, and modifiable internal surface, surpassing numerous porous materials such as activated carbon, mesoporous metal oxides and zeolites. MOFs demonstrated impressive capabilities in applications where access surface area is crucial. Owing to their high stability, porous nature, large surface area, uniform pore size and ability to be functionalized, MOFs are excellent candidates for use in gas storage, gas separation, heterohenous catalysis, sensing, drug delivery and supercapacitor materials applications. Due to high surface area and pore volume, varios types of nanoparticles can be encapsulated into the matrix of the MOFs producing a core-shell heterostructure. Imidazole can lose a proton to form imidazolate ion which can combine with Zn2+ or Co2+. Analyzing the dense phases of Co(IM)2 and Zn(IM)2, whose structures are founded on networks of connected CoN4 or ZnN4 tetrahedra, it is identified that IM bridges end up making an angle of Metal-Inmidazole-Metal, around 145 °, which coincides with the preferred Si-O-Si angle, usually found in many inner portions of zeolites. This makes zeolitic imidazolate framworks (ZIFs) a good potential host for heterogeneous catalysis. Due to their distinguished optical, electronic properties and due to their quantum confinement effects and size-dependent photoemission, the control of the size and morphology of metal nanoparticles (MNPs), metal sulphide nanoparticles (MSx NPs) and metal oxide nanoparticles (MOx NPs) have been a primary focus. But the oxidation of metal sulphide nanoparticles and agglomeration of nanoparticles with time decrease their efficiencies and limit their application for more sustainable technologies. Highly chemically and thermally stable ZIFs can provide an excellent matrix to encapsulate or anchor these nanoparticles to overcome the corrosion and agglomeration problem. The rapid population growth and the increased speed of industrialization have caused a massive increase in demand for freshwater in the last few decades. Excessive fertilizer application and anthropogenic activities related to rapid urbanization, agricultural practices, textile and paper industries, and population expansion in many parts of the world have contributed to the degradation of water quality. Water pollution is mainly due to the release of untreated wastes of various industries into the water bodies. In the textile industry, most of the coloring dyes avoid traditional wastewater treatment systems and remain in the environment due to their high light, temperature, water, detergents, chemicals, and microbial attack stability. A number of the simple and advanced chemical, physical and biological processes such as sedimentation, disinfection, Coagulation-flocculation, filtration, biological process and advanced oxidation processes (AOPs) have been developed for wastewater treatment. AOPs used semiconductor nanoparticles such as TiO2, CdS, WO3, Fe3O4. SnO2 and ZnO as photocatalyst. Agglomeration of nanoparticles limits the efficiency of AOP process; to overcome this a number of porous materials like zeolite, CNT, layered double hydroxides (LDH), graphene oxide (GO), polydopamine have been utilized as a support material. In recent years, the researchers started exploring MOFs as the matrix for the support of metal oxide and metal sulphide semiconductor nanoparticles. The application of MOFs as support material is still limited and a small number of reports are available on the encapsulation of well-known metals such as Au, Ag, Pt and Pd into MOFs for hydrogenation; TiO2, ZnO, SnO2 and CdS have been encapsulated mainly into ZIF-8 for H2 production and dye’s degaradation, but many high performance semiconductor nanoparticles such as WO3, MnO2, CeO2, WS2 and SnS2 have not been encapsulated within ZIFs and their potential photocatalytic applications are still unexplored. In the literature available, primerly, ZIF-8 and ZIF-67 have been used as support materials, other highly porous and stable ZIFs such as ZIF-11, ZIF-12 and ZIF-7 are not yet exploited for their application as support or shell material in AOPs. The objective of the present study is to encapsulate/decorate metal nanoparticles (MNPs), metal sulphide nanoparticles (MSx NPs) and metal oxide nanoparticles (MOx NPs) into/onto ZIFs. As synthesized modified MOFs were thoroughly characterized by various techniques and their applications for catalytic reduction of aromatic nitrocompounds, organic colouring dyes and photocatalytic degradation of methylene blue (MB) and gram negative and gram positive bacteria have been investigated. After the degradation of MB, end products were analyzed by GC-MS to provide a possible degradation pathway. For the sake of simplicity, the work done in the present study has been described in the following chapters. Chapter 1: It deals with the general introduction of the water pollution, and its treatment methods, application of metal-organic frameworks, metal nanoparticles (MNPs), metal sulphide nanoparticles (MSx NPs) and metal oxide nanoparticles (MOx NPs) and their available composites for environmental remediation. Chapter 2: This chapter provides the descriptions of experimental techniques that have been used for synthesis and characterization of ZIFs composites. The synthesis of metal nanoparticles (MNPs), metal sulphide nanoparticles (MSx NPs), metal oxide nanoparticles (MOx NPs) and their ZIFs composites have been carried out at room temperature. The structural analysis was performed by powder x-ray diffraction (PXRD) and Fourier-transform infrared spectroscopy (FTIR). The morphological, topological and compositional study was done by field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The absorption, reflectance and emission study were done by UV-Vis diffuse reflectance spectroscopy (UV-DRS), photolumence (PL). Brunauer-Emmett-Teller (BET) surface area analyzer was employed to evaluate the surface area, pore size and pore volume of synthesized materials. The end products of the photocatalytic reaction were analyzed on gas chromatography-mass spectrometry (GC-MS). Further, the catalytic performance of the composites was monitored by UV-Vis spectrophotometer. Optical density (OD) study, MTT assay, disk diffusion assay, flow cytometry for reactive oxygen species (ROS) induction assay, fluorescence spectroscopic analysis, fluorescence and optical microscopic imaging were performed to study the antibacterial efficiency of CdSNPs@ZIF-8 composites. Chapter 3: In this chapter, multifunctional novel core-shell composites, CdSNPs@ZIF- 8 (abbreviated as NC-1, NC-2 and NC-3), have been synthesized by in situ encapsulation of different amounts of CdSNPs (150, 300, and 500 μL suspension of CdSNPs in methanol) in ZIF- 8 at room temperature. The composites were characterized by structural, compositional, morphological and topographical techniques. XPS and HRTEM indicated the encapsulation of CdSNPs within ZIF-8 crystal without disturbing the crystal order of ZIF-8. The average size of embedded CdSNPs is found to be 16.34 nm. CdSNPs@ZIF-8 showed potential to be used as an antibacterial agent against Gram-positive Staphylococcus aureus and Gram-negative green fluorescent protein-expressing Escherichia coli in aqueous medium, as evident by various biophysical experiments. Further, the composite has been used as an efficient photocatalyst for the degradation of organic pollutant, such as methylene blue (MB) dye in aqueous medium and found that the core-shell composite, CdSNPs@ZIF-8 (150 μL) (abbreviated as NC-1) (5 mg), exhibited higher photocatalytic activity (≈1.8 times) than CdSNPs for degradation of 90% of methylene blue (10 mL of 10 ppm) at pH ≥ 7 due to the synergetic effect. Various factors such as amount of catalyst, initial pH and concentration of dye solution have also been investigated during the photodegradation studies of MB. It is observed that in situ encapsulation of CdSNPs in ZIF-8 provides an easy executable measure for purification of wastewater, for the effective photocatalytic degradation of organic pollutants as well as to remove the bacterial contamination under sunlight. The NC-1 can be used as photocalyst up to 5 cycles without much change in its morphology and catalytic performance. The scavenger study experiment showed that free hydroxyl radical (•OH) is the major reactive oxygen species in the photocatalytic degradation process. Chapter 4: In this chapter, modified zeolitic imidazolate framework (ZIF-8) with encapsulated WO3 nanoparticles have yielded three multicore-shell nanocomposites by in situ encapsulation of 150 μL (abbreviated as WZ-1), 300 μL (abbreviated as WZ-2) and 500 μL (abbreviated as WZ-3) suspension of WO3NPs in methanol in ZIF-8 matrix at room temperature. They have been characterized by powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), Fourier transform-infra-red (FT-IR) spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Encapsulation of WO3NPs was evidenced by TEM, SAED and ultraviolet diffuse reflectance spectroscopy (UV-DRS). The composites have a lower band gap with higher light absorbance compared to ZIF-8 due to the shift in the micro-environment of ZIF-8 framework. Particle size of encapsulated WO3NPs within the matrix of ZIF-8 is 9.56 ± 1.29 nm. ZIF-8 and WO3NPs@ZIF-8 are highly thermally stable up to 397 °C, and in the range of 450-650 °C ZIF-8 shell gets decomposed and converted into oxide. Application of WO3NPs@ZIF-8 towards photodegradation of methylene blue (MB) dye has been investigated by varying the amount of catalyst (5, 10 and 15 mg) and dye concentration (5, 10 and 15 mg L-1) at different pH (3¬11). WZ-2 (10 mg) exhibits excellent photocatalytic activity; degrades 99.2% MB (5 mg L-1) at pH = 3. Further, WZ-2 can be reutilized up to five cycles with the almost same efficiency and remains unchanged. Therefore, it can be utilized as an efficient photocatalyst for the remediation of environmental pollution. Photoluminescence (PL) of ZIF-8 and WO3@ZIF-8 composites showed that WZ-2 has lowest PL intensity which means WZ-2 has maximun electron-hole separation time, a major factor for enhancd performance of WZ-2. A scavenger experiment was also performed and the results concluded that free hydroxyl radical (•OH) is major reactive oxygen species along with minor role of holes (h+).Chapter 5: In this chapter, a highly efficient method to convert various nitrophenols and nitroaniline to more useful aminophenols and aminoaniline and reduction of organic colouring dyes by using gold nanoparticles decorated ZIF-11 (Au/ZIF-11) composite has been described. Au/ZIF-11 composites were synthesized via direct reduction of HAuCl4 (0.1 to 0.3 mM) using NaBH4 in the suspension of the pre-synthesized ZIF-11. All three composites (abbreviated as AZ-1, AZ-2 and AZ-3) were well characterized by employing PXRD, XPS, FE-SEM, HRTEM, and surface area was determined by BET surface analyzer. The TEM images confirmed the formation of spherical and uniformly distributed gold nanoparticles on the surface of ZIF-11 microcrystals having particle size of 7.35±0.95 nm. The catalytic reduction by using AZ-2 ([catalyst] = 15.87 μg/mL for 2-nitrophenol (2-NP), 3-nitrophenol (3-NP), 4-nitrophenol (4-NP) and 4-nitroaniline (4-NANI); [catalyst] = 31.74 μg/mL for picric acid) of as mentioned nitroaromatic compounds was performed at 1.58×10-4 M concentration of reactants in the presence of 0.04 M NaBH4 at room temperature. The apparent rate constant k̅ app and activity coefficient k’ were calculated. The k̅ app was 1.90 × 10-2 s-1, 4.64 × 10-2 s-1, 3.24 × 10-2 s-1, 1.19 × 10-2 s-1 and 4.0 × 10-3 s-1 with k’ (k’ = k̅ app/M) value of 380.61 s-1g-1 928.83 s-1g-1, 647.59 s-1g-1, 239.49 s-1g-1 and 40.04 s-1g-1 for 2-NP, 3-NP, 4-NP, 4-nitroaniline and picric acid, respectively. The recyclability test showed that Au/ZIF-11 composites can be used up to 10 cycles with more than 90% conversion efficiency. Further, Au/ZIF-11 composites have also been investigated for the reduction of organic dyes (methylene blue (MB), methyl orange (MO), congo red (CR) and rhodamine B (RhB)) in the presence of sodium borohydride (NaBH4). The kapp was 7.88 × 10-3 s-1, 6.64 × 10-3 s-1, 2.56 × 10-3 s-1 and 9.22 × 10-4 s-1 with k’ value of 157.71 s-1g-1, 66.36 s-1g-1, 57.20 s-1g-1 and 9.22 s-1g-1 for MB, MO, CR and RhB, respectively. It has been observed that Au nanoparticles immobilized onto ZIF-11 may serve as an electronic relay system to transfer electrons donated by borohydride ions to the acceptor (nitro groups/organic dye). The Au/ZIF- 11 displayed the excellent ability for removal of hazardous nitroaromatic compounds and exhibited less catalytic activity for reduction of organic dyes, and thus can be selectively used for environmental remediation application. Chapter 6: A simple and robust process to immobilize noble metal nanoparticles as the catalyst is quite a demanding topic for heterogeneous catalysis. This chapter describes a simple and sustainable method for immobilizing silver nanoparticles on porous ZIF-7 by reducing silver nitrate in an alcoholic medium. Ag/ZIF-7 composite was comprehensively characterized by various techniques as discussed in chapters 3 and 4. The average particle size of Ag NPs was 6.24±1.74 nm with uniform distribution on to ZIF-7 matrix as support, which helped in reducing the agglomeration and consequently reducing the size of Ag nanoparticles having more numbersof catalytic active sites. As synthesized Ag/ZIF-7 composite demonstrated outstanding catalytic activity for the reduction of nitroaromatic compounds 2-NP, 3-NP, 4-NP 4-NANI and picric acid) and organic colouring dyes (MB, MO, CR and Rh B) in the presence of sodium borohydride (NaBH4) in aqueous medium under ambient condition. Among the nitroaromatic compounds studied, Ag/ZIF-7 displayed the highest catalytic efficiency for the reduction of 4-nitroaniline within 3 minutes at 1.587×10-4 M concentration, the apparent rate constant (k̅ app) was 2.64 × 10- 2 s‒1 with k’ (k’=k̅ app/M) value of 528.36. Similarly, composite exhibited maximum catalytic reduction efficiency towards methyl orange (MO), among organic dyes studied, at a concentration of 25 ppm within 4 minutes, having k̅ app value of 1.44 × 10-2 s‒1 and k’ = 287.78. The composite was easily recovered from the system by centrifugation and was reused up to 10 cycles for nitrophenol compounds with almost the same efficiency suggesting the high stability and reusability of Ag/ZIF-7 composite. Ag NPs anchored ZIF-7 composite acts as an antenna to facilitate the transfer of electrons from donor to acceptor whereas no reduction is taking place in absence of catalyst. Chapter 7: It outlines the conclusions and future perspectives of the work performed in this study. Multifunctional core-shell composites CdSPNs@ZIF-8 were successfully prepared by in situ encapsulation of different amounts of CdSNPs into ZIF-8. The composite (NC-1) exhibited higher photocatalytic activity for the degradation of methylene blue under UV-visible light irradiations as compared to CdS NPs and ZIF-8, and can be reused successfully after five cycles. Further, NC-1 exhibited the highest antibacterial activity as compared to NC-2 and NC- 3 against both GFP E. coli and S. aureus. WO3@ZIF-8 composites, synthesized by in situ encapsulation of WO3 in ZIF-8, demonstrated remarkable photocatalytic degradation (ca. 99.2%) in a wide range of pH and wide range of initial concentration of MB. Scavenger analysis has proved the major role of •OH and the minor role of h+ (ROS) in the photocatalytic degradation of MB. Further, this composite exhobits better photocatalytic activity towards MB as compared to CdS NPs@ZIF-8. Gold nanoparticles decorated ZIF-11 (Au/ZIF-11) composites have been prepared and AZ-2 composite displayed selectively the highest catalytic activity for the reduction of nitroaromatic compounds (2-NP, 3-NP, 4-NP, 4-NANI and picric acid) in the presence of 0.04 M NaBH4 at room temperature. Further, it has been observed that Au/ZIF-11 composite is less effective for reduction of organic dyes, and the %reduction follows the order: 3-NP ˃ 4-NP ˃ 2- NP ˃ 4-NANI ˃ picric acid for nitroaromatic compounds; for organic dyes, the order is MB > MO ˃ CR ˃ Rh B