MODIFIED METAL ORGANIC-FRAMEWORKS: MULTICORE- SHELL COMPOSITES AS ADSORBENTS AND PHOTOCATALYSTS

Abstract

The sustainable strategies/methods for degradation of colouring materials, particularly for the organic dyes, are the prime concern worldwide because the discharge of such dye containing effluents into water bodies makes them polluted to such an extent that aquatic lives are in danger due to decrease oxygen contents. Organic pollutants, for instance, methylene blue (MB), rhodamine-B (RB), congo red (CR) and 4-nitrophenol (4-NP) are the main contaminates in the effluents of textile and some other industries. They have been recognized as toxic, carcinogenic and mutagenic along with a very minimal biodegradability in ecosystems. Prior to discharge of these organic pollutants into water bodies, the adequate removal of pollutants must be ensured. Various methods have been employed for the waste water treatment such as physio-chemical and biological methods but they are inefficient. In last few decades, much efforts have been dedicated for degradation of dyes by using novel metal nanoparticles (Au, Ag and Pt etc.,) and semiconducting nanomaterials such as ZnO, SnO2, Fe2O3, and TiO2 having small band gap and chemical stability but there are the drawback of fast recombination rate of electron (e-) and hole (h+), and agglomeration of NPs reduces their efficiency which can be partially solved by their immobilization. Several efforts have been made to immobilize them by encapsulation into porous materials such as zeolite. Recently, core@shell nanoparticles or composites, involving metal organic frameworks as shell/host, have been reported to reduce the recombination rate of e- and h+, thereby, enhancing their photocatalytic efficiency. Metal organic frameworks (MOFs) have emerged as a hybrid inorganic-organic materials which are constructed through covalent bonding of metal ions with organic linker molecules. These materials have high porosity, tunable shapes and pore sizes, high surface area, and a wide range of applications such as heterogeneous catalysts, gas storage and chemical sensing. Besides, MOFs are very useful in drug release application on account of their fascinating hydrophilic-hydrophobic internal microenvironments. Zeolitic imidazole frameworks (ZIFs) are a kind of metal organic frameworks which consist of mainly transition metal ions (e.g. Co and Zn) and imidazole or its derivatives as linkers. In ZIFs, the metal centers are tetrahedrally coordinated with N-atom of imidazole ring. The topology of ZIFs resemble zeolite as metal-imidazole-metal (M-Im-M) bond angles are retained to be 145° as aluminosilicate zeolite (Si-O-Si bond angle =145°). Myriad numbers of ZIFs have been reported on account of their constitutional unit and porosity. Among them, ZIF-8 was found to be more chemically and thermally stable. ZIF-8 has larger pores 11.6 Å connected with small ii apertures 3.4 Å. Owing to its highly porous, chemical and thermal stability, ZIF-8 is considered to be very useful for various applications such as gas storage, adsorbent, drug delivery and host of catalyst in heterogeneous catalysis. Fabrication of ZIFs with metal oxides, novel metal nanoparticles and molecules has been an emerging challenge. And so, ZIF-8 has been selected for fabrication of metal oxides and novel metal nanoparticles. The present study describes the synthesis of metal oxides (MOx) and novel metal nanoparticles (MNPs) encapsulated within ZIFs. Further, MOx@ZIFs and MNPs@ZIFs have been used as photocatalysts for the waste water treatment and biomedical applications which are described in the successive six chapters. First Chapter presents a brief literature on ZIF-8 and its composites with novel metal nanoparticles (Au, Ag, Pd and Pt), metal oxides (ZnO, TiO2, SnO2 and Fe3O4) and molecules. The synthetic methods and probable applications of some important reported composites have also been presented. Second Chapter describes the make and purity of chemicals used in the present study. This chapter also highlights the specifications of various sophisticated instruments and details of procedures/methods used in all the chapters. Third Chapter includes the synthesis of multi-core-shell TiO2NPs@ZIF-8 composites by in situ encapsulation of different amounts of TiO2NPs i.e. 150, 300 and 500 μL suspension of TiO2NPs in methanol within ZIF-8 at ambient temperature. Encapsulation of TiO2NPs in ZIF-8 was confirmed by transmission electron microscopy and X-ray photoelectron spectroscopy. ZIF-8 and its core-shell composites have identical crystal structure and morphology as confirmed by powder X-ray diffraction analysis and scanning electron microscopy. The detailed photocatalytic degradation and adsorption studies of MB and RB were investigated by analyzing various factors, viz., loading amount of TiO2NPs in ZIF-8, amount of photocatalyst, pH and initial concentration of the dye. At higher pH (11.5−12.6), TiO2NPs@ZIF-8 composite exhibited higher (ca. 8 times) photocatalytic activity as compared to TiO2NPs and the optimum amount of TiO2NPs@ZIF-8 composite was 10 mg for the maximum photodegradation of 3.19 mg L-1 (93%) and 2.4 mg L-1 (57%) of MB and RB, respectively. After the degradation of both dyes (MB and RB), the degraded by-products were analyzed by GC-MS and the degradation path has also been proposed. Chapter four describes the synthesis of multi-functional and thermally stable SnO2NPs@ZIF-8 composites (NC1, NC2 and NC3) by a facile and sustainable approach involving in situ encapsulation of SnO2NPs (150, 300 and 500 μL suspension in methanol) within zeolitic imidazole framework at ambient temperature. The morphology and crystallinity of ZIF-8 remained unchanged upon the proper encapsulation of SnO2NPs in its matrix. Herein, iii for the first time, the antiviral potential of ZIF-8 and SnO2NPs@ZIF-8 composite was explored against chikungunya virus by investigating their cytotoxicity against Vero cell line employing MTT ((3-(4,5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide)) assay. The maximum non-toxic dose was found to be 0.04 mg mL–1 for ZIF-8 and SnO2NPs@ZIF-8, and 0.1 mg mL– 1 for SnO2NPs. Further, NC1 exhibited (based on plaque assay) reduction in viral load/titers up to >80% during post-treatment and >50% during pre-treatment, greater than that of ZIF-8 and SnO2NPs due to synergistic effect. Further, NC1 (10 mg) exhibited enhanced photocatalytic efficiency (≥96%) for degradation of MB (0.5×10-5 M) at pH ˃7.0. The probable mechanism for their anti-viral activity and photocatalytic activity has been discussed. Chapter five contains two sections. Section-A consists of synthesis of novel multicore- shell AgNPs@ZIF-11 (AZ1, AZ2 and AZ3) composites by in situ encapsulation of Ag nanoparticles (150, 300 and 500 μL suspension in methanol) in ZIF-11 (Zeolitic Imidazole Framework) at ambient temperature using binary solvent mixture (methanol and toluene). The lowering of band gap of ZIF-11 from 4.36 to 4.21 eV indicates the micro-environment of AgNPs within ZIF-11 framework. Particle size of encapsulated AgNPs within the matrix of ZIF-11 was found to be 11.76 ± 2.3 nm. ZIF-11 and AgNPs@ZIF-11 composites are highly thermally stable up to 500 °C under both air and nitrogen environments. Application of AgNPs@ZIF-11 (AZ1, AZ2 and AZ3) composites towards photodegradation of MB dye has been investigated by varying the amount of catalyst (5, 10 and 15 mg) and dye concentration (1.6, 3.19 and 6.38 mg L-1). AZ1 (10 mg) exhibits excellent photocatalytic activity; degrades 100% MB (1.6 mgL-1) at pH ≥ 7. AZ1 also exhibits potential efficiency (86%) for the conversion of 4-nitrophenol into 4-aminophenol. Further, AZ1 can be reutilized up to three cycles with 100% efficiency while under fourth and fifth cycle it can degrade 92.12% and 72.75% MB, respectively. The degraded by-products of MB dye have also been analyzed by GC-MS followed by the brief discussion of the mechanism. Section-B describes an auspicious approach towards the heterogeneous photocatalytic degradation of MB and CR using AgNPs@ZIF-8 composites. The facile synthesis of AgNPs encapsulated ZIF-8 via in situ synthesis of AgNPs and ZIF-8 has been demonstrated. AgNPs@ZIF-8 composite has been synthesized by incorporation of pre-synthesized ZIF-8 in situ synthesis of AgNPs and named as SZ while pre-synthesized AgNPs have been encapsulated within ZIF-8 in different amounts such as 150, 300 and 500 μL methanol suspensions in situ synthesis of ZIF-8 and named as SZ1 SZ2 and SZ3, respectively. All four synthesized SZ, SZ1 SZ2 and SZ3 composites have been characterized by various spectroscopic techniques. Owing to highly porous and unique properties of AgNPs@ZIF-8 iv composites, they exhibit remarkable photocatalytic activity for the degradation of MB and CR as compared to ZIF-8. Further, the degradation mechanism and degradation path way of both the dyes have been discussed. Chapter six comprises of two sections. Section-A describes the synthesis of ZnO-SnO2 nanocomposites in the various molar ratio i.e.1:1, 2:8, 4:6, 6:4 and 8:2 of Zn and Sn using solgel (ZS-11, ZS-28, ZS-46, ZS-64 and ZS-82) and grinding (ZS-A, ZS-B, ZS-C, ZS-D and ZSE) method and detailed characterization using various instrumental techniques. The synthesized ZnONPs looks like flower structure while SnO2NPs and ZnO-SnO2 nanocomposites have spherical morphology confirmed by SEM analysis. In addition, the formation ZnO-SnO2 was investigated by HR-TEM analysis. Further, the elemental composition and the close fitting of Zn 2p, Sn 3d, and O 1s of ZnO-SnO2 have been ensured by XPS analysis. In order to explore photocatalytic application, adsorption and photodegradation efficiency have been examined using a model dye, for instance, MB. It has been found that ZS-E (8:2 molar ratio of Zn:Sn synthesized by grinding method) nanocomposite exhibits maximum photodegradation efficiency which can degrade off 58.68% [MB] = 1.6 mg L-1 dye solution. Further, ZS-E nanocomposite has been optimized for its encapsulation within ZIF-8 due to its maximum photocatalytic response at pH ≥ 7 which is described in Section-B. In Section-B, ZnO-SnO2 nanocomposite (ZS-E) has been anchored (ZS@Z) and encapsulated (ZS@Z1;150 μL/ZS@Z2;300 μL/ZS@Z3;500 μL) within ZIF-8 framework successfully by employing solvothermal and bench method, respectively. The synthesized composites were also exploited as photocatalysts for the removal of MB. Further, it has been found that ZnO-SnO2 nanocomposite anchored on ZIF-8 composite (ZS@Z) exhibits 100% degradation of [MB] = 1.6 mg L-1 dye solution at pH ≥ 7 while ZS@Z1, ZS@Z2 and ZS@Z3 composites can degrade off 93.43, 97.43 and 91.93%, respectively. The possible mechanism has also been described. Chapter seven highlights the conclusions and future prospect of synthesized composites.