Development of achiral and chiral metal-organic frameworks for catalytic applications

Abstract

1.Introduction Metal-organic frameworks (MOFs), also known as porous coordination polymers (PCPs) are a new class of crystalline porous materials with infinite network structures obtained by connecting the metal ions (nodes) or clusters (SBUs) with organic spacers (linkers). MOFs have attracted an immense attention in the recent years due to their fascinating capability to form diverse structural architectures and also for their novel properties. The possibility to tune the pore size and introduce desired functionality makes MOFs an important class of porous materials for selective gas storage and heterogeneous catalytic applications. In this regard, recent studies have shown that modulation of the pores with basic functionalities has resulted MOFs with potential applications for selective storage of CO2. On the other hand, there is a growing interest in the utilization of CO2 as an abundant C1 source to synthesize value-added chemicals such as cyclic carbonates. However, the thermodynamic stability and kinetic inertness of CO2 pose a great challenge for its conversion under mild conditions. Therefore, it is highly desirable to develop efficient catalytic systems for selective capture and conversion of CO2 into useful organic compounds at mild conditions. In this direction, cycloaddition of carbon dioxide to epoxides to generate cyclic carbonates has attracted considerable attention owing to its 100% atom economic nature. Hence, extensive research efforts have been reported in literature on development of various heterogeneous catalysts including MOFs for catalytic conversion of CO2 to cyclic carbonates. However, most of the catalysts reported require drastic reaction conditions of high temperature and/or high pressure of CO2 for high yield generation of cyclic carbonates thereby increasing the cost of the reaction process. An efficient heterogeneous catalyst for conversion of CO2 to cyclic carbonates should possess high density of Lewis/Brønsted acidic or basic sites to activate the epoxide/CO2 along with high affinity for carbon dioxide. Further, most of the catalysts require the addition of a nucleophilic cocatalyst such as TBAB (tetrabutyl ammoninum bromide) which facilitate in ring opening of the epoxides for high yield generation of cyclic carbonates. On the other hand, the number of examples of MOFs known for cocatalyst-free conversion of CO2 are limited. Keeping these points in mind, the objectives of this these work was to develop MOFs composed of high density of Lewis acidic and basic sites for selective capture and conversion of CO2 into cyclic carbonates at mild conditions. The thesis has been divided into seven chapters as summarized below. Chapter 1: This chapter includes a brief introduction to metal-organic frameworks, their historical aspects, various synthesis techniques and potential applications in various fields of research. Chapter 2: In this chapter, we have successfully constructed four new Zn(II)/Cd(II)-organic networks using mixed ligand strategy by room temperature self-assembly. Owing to the presence of exposed basic –C=N- functional groups in the 1D channels, compound 4 exhibits high catalytic activity for Knoevenagel condensation of aromatic aldehydes with compounds containing active methylene groups over the compounds containing non-azine based organic linkers. This study demonstrates the influence of ancillary ligands on the structure, topology and catalytic properties of the resulting co-ordination polymers. Chapter 3: In this chapter, syntheses of a series of six new Ni(II)/Co(II) MOFs using mixed ligand systems and their selective CO2 storage and catalytic properties for cycloaddition of carbon dioxide has been described. Gas adsorption study revealed selective adsorption property of compounds 1-4 for CO2 over other (H2, N2 and Ar) gases. Owing to the generation of unsaturated Lewis acidic Ni(II)/Co(II) centers in the 1D channels, the activated samples of 1-4 act as efficient recyclable catalysts for heterogeneous cycloaddition of CO2 to epoxides resulting in cyclic carbonates with high yield and selectivity. More interestingly, the cycloaddition reactions of CO2 with bulky epoxides showed decrease in the conversion with increase in the alkyl chain length of the epoxides due to the confinement of pore size of the MOFs. Compounds 1-4 represent a rare examples of interpenetrated MOFs exhibiting selective storage and conversion of CO2 at mild conditions. Chapter 4: In this chapter, we have synthesized two new highly connected (20-c) lanthanide MOFs based on Sm3+/Gd3+ ions which show exceptional thermal and chemical stability. Further, the MOFs possess an interesting 3D honeycomb-like structure with large 1D hexagonal channels of dimension ~10.20 X 10.11 Å2 and exhibit selective adsorption of CO2. Further, the activated samples of MOF act as efficient recyclable catalysts for heterogeneous cycloaddition of CO2 with epoxides resulting cyclic carbonates with high yield and selectivity. Therefore, the synthesis of a rare examples of 20-c lanthanide MOFs exhibiting selective capture and conversion of CO2 at mild conditions is demonstrated. Chapter 5: In this chapter, rational design of two new 3D, dual-walled, 2-fold interpenetrated, porous polyhedral MOFs of Co(II)/Ni(II) featuring large open cages (~ 29 X 29 Å2) composed of high density of Lewis acidic metal ions and basic -NH- groups. Owing to the presence of a basic functionalized pore surface, MOF1/2 exhibits selective storage of CO2 with high value of heat of adsorption (Qst = 39.7 kJ/mol) which is further supported by a theoretically calculated binding energy of 41.17 kJ/mol. Owing to the presence of high density of metal ions and basic -NH- groups, MOF1/2 act as efficient, green, recyclable catalysts for the cycloaddition of CO2 with epoxides at mild conditions of 1 bar of CO2 and 80 °C for efficient synthesis of cyclic carbonates in the absence of a co-catalyst. Overall, this work demonstrates design of porous MOFs composed of high density of both Lewis acidic and basic sites for selective CO2 storage and co-catalyst-free conversion of CO2 to cylic carbonates. Chapter 6: In this chapter, we have successfully constructed four new homochiral Cd(II)-organic frameworks using a chiral flexible dicarboxylate ligand (L-glutamic acid) containing basic amine group along with bipyridine spacers by solvothermal method. Interestingly, synthesis carried out at 100 °C resulted the 3D framework, 1 which features large rectangular 1D channels with dimension of ~10.38 X 4.44 Å2 decorated with pendant -NH2 groups. Whereas, increasing the temperature of the reaction to 120°C led to a non-porous highly connected 3D framework, 2 in which the -NH2 group of L-glu ligand is coordinated to Cd(II) node. Thus the effect of temperature on the structural diversity of 1 and 2 has been demonstrated. Owing to the presence of free -NH2 groups in the 1D channels, compound 1 exhibits selective adsorption of CO2 and base catalysis for Henry (nitro-aldol) reaction.