Development of porous metal-organic frameworks for H2 Storage, Selective CO2 Capture and Utilization

Abstract

Porous metal-organic frameworks (MOFs) are a new class of crystalline porous materials which exhibit the fascinating capability to form diverse structural architectures and novel properties. The high surface areas, tunable pore size, and functionality make MOFs a suitable candidate materials for H2 storage and selective CO2 capture applications. Several MOFs with promising H2 storage properties at low temperature (77 K) conditions have been reported in the literature. However, the observed H2 storage capacities at ambient conditions are still very low due to the weak interaction of H2 with the framework. Therefore, several strategies have been reported to enhance the interaction energy of H2 with MOFs. In our efforts to increase the interaction energy of H2 with MOFs, we have synthesized porous MOFs with pores functionalized with polar groups which can enhance the interaction of H2 with the frameworks. On the other hand, there is a growing interest in developing catalytic systems for selective capture and utilization of carbon dioxide as a C1 source for the synthesis of value-added chemicals. In this context, the cycloaddition of CO2 to epoxides to generate cyclic carbonates has attracted considerable interest as 100% atom economic process. However, the thermodynamic stability and kinetic inertness of CO2 pose a great challenge for its conversion at mild conditions. Literature study revealed that, an efficient heterogeneous catalyst for conversion of carbon dioxide to cyclic carbonates should exhibit high selective adsorption of CO2 and possess high density of Lewis/Brønsted acidic and basic sites which are essential to activate the epoxide/CO2. Keeping these points, we have designed porous MOFs composed of Lewis acidic metal centers and with the pores functionalized with basic functional groups. These MOFs exhibit selective adsorption of carbon dioxide and act as efficient, recyclable catalysts for conversion of CO2 to cyclic carbonates. The thesis has been divided into seven chapters as summarized below. Chapter 1: In this chapter, we present a brief introduction to porous materials, and porous MOFs in particular, history of MOFs, their preparation methods, and their applications for gas (H2/CO2) storage and heterogeneous catalysis. Chapter 2: In this chapter, we report synthesis of a new microporous Zn(II)-HFMOF, MOF1 from a rigid, partially fluorinated V-shaped dicarboxylate ligand, H2hfipbba and a bipyridine, 4bpdb spacer. Further, rapid synthesis of phase-pure sample of MOF1 has also been achieved using green synthetic approaches like solvent-free sonochemical and mechanochemical routes. MOF1 shows a microporous 3D framework structure constituted by 6-connected Zn(II) dimeric paddlewheel units with {44.610.8}-net topology. The 3D framework of MOF1 possesses 1D channels with pore dimension of 3.1 X 4.1 Å2 and exhibits an interesting gas (CO2/H2) uptake properties. The estimated isosteric heat of adsorption (Qst) values for CO2 and H2 were found to be 36.4 and 8.8 kJ/mol respectively, suggesting stronger interaction of the gas molecules with the framework owing to the proper matching of the pore size with the kinetic diameter of H2/CO2 gas molecules. Moreover, MOF1 exhibits high thermal and water stability, and its temperature-induced solid state conversion to ZnO@C NC has been accomplished. Further, the as-synthesized ZnO@C NC shows good photocatalytic activity for degradation of MB. Chapter 3: In this chapter, we have demonstrated the rapid synthesis of 3D Mn(II)-MOF from a rigid dicarboxylate ligand, H2NDC by employing solvent-assisted mechanochemical and sonochemical methods. The Mn(II)-MOF undergoes a reversible structural transformation upon desolvation and resolvation of coordinated DMA molecules. The desolvated framework possesses 1D channels decorated with unsaturated Mn(II) centers and exhibits interesting H2 and CO2 uptake properties with isosteric heat of adsorption (Qst) values of 11.8 and 29.2 kJ/mol for H2 and CO2 respectively. Remarkably, the estimated value of Qst of H2 is found to be highest amongst the Mn(II) based MOFs reported so far. Further, the Mn(II)-MOF undergoes temperature-induced solid state conversion to a phase pure MnO nanocrystals incorporated in carbon matrix, MnO@C nanocomposite. Chapter 4: In this chapter, we have successfully constructed two new microporous 2-fold interpenetrated, 3D Zn(II)-MOFs exhibiting high thermal and water stability by self-assembly of Zn(II) ions with a long-chain aromatic tetracarboxylate ligand, BINDI and N, N'- dipyridyl (bpa/bpe) spacers. Interestingly, owing to the presence of polar pore surface and optimum pore size, MOF1 shows selective and higher uptake of carbon dioxide over MOF2. The high stability in aqueous medium and selective adsorption property for CO2, make MOF1 a potential candidate for gas separation. Thus, the effect of ancillary ligands on the structure, gas adsorption properties of two Zn(II)-MOFs is demonstrated. Chapter 5: In this chapter, we report construction of a bifunctional, microporous, 2-fold interpenetrated, 3D pillar-layered framework of Zn(II) by solvothermal route using mixed ligands approach and structurally characterized. In spite of 2-fold interpenetration, MOF1 possesses large rectangular 1D channels decorated with pendent -NH2 groups. Owing to the presence of basic functionalized pore surface, MOF1 exhibits selective adsorption of CO2 over other (H2, N2 and Ar) gases with high value of heat of adsorption (Qst = 46.5 kJ/mol) which is further supported by theoretically calculated binding energy of 48.4 kJ/mol. Interestingly, the value of Qst observed for MOF1 is about 10 kJ/mol higher than that of the analogues MOF with BDC ligand, which establishes the critical role of -NH2 group for CO2 capture. To the best of our knowledge, MOF1 represents the first example of an interpenetrated Zn(II)-MOF exhibiting selective adsorption of CO2 as well as, sustainable and efficient aqueous-phase sensing of TNP investigated through combined experimental and theoretical studies. Chapter 6: In this chapter, we report synthesis of a new Co(II) based porous 3D MOF by utilizing H4TCPB, as a rigid aromatic tetratopic ligand. MOF1 possesses a 3D framework structure with large 1D channels of dimension 10.5 X 10.5 Å2. Interestingly, MOF1 exhibits selective adsorption of CO2 over other gases (N2, Ar, H2) owing to the stronger interaction of carbon dioxide with unsaturated Co(II) centers lined in the 1D channels of the framework. Further, the activated sample, MOF1' catalyzes cycloaddition of CO2 with epoxides to synthesize cyclic carbonates with high selectivity and yield at mild conditions of 1 bar of CO2. The MOF1 catalyst was recycled for five cycles without substantial loss in the catalytic activity. This work demonstrates the design of porous MOFs composed of highly reactive Lewis acidic Co(II) ions for selective capture and efficient conversion of CO2 into cyclic carbonates. Chapter 7: In this chapter, we report the synthesis of an interesting 3D double-walled, 2-fold interpenetrated polyhedral Cu(II)-MOF which features large open cages with dimensions of ~ 29.8 Å using solvothermal technique. Gas adsorption studies of Cu(II)-MOF show selective capture of CO2 over N2 with high Qst value of 38.5 kJ/mol, owing to the presence of high density of basic -NH functionalities. Interestingly, Cu(II)-MOF serves as an efficient, solvent and co-catalyst-free heterogeneous catalyst for the cycloaddition of epichlorohydrin with CO2 under mild conditions. The Cu(II)-MOF was reused for five consecutive cycles without substantial loss in catalytic activity. Hence, this work demonstrates the synthesis of Cu(II)-MOF composed of Lewis acidic and basic sites for catalytic conversion of CO2 to cyclic carbonates at co-catalyst-free mild conditions.