Investigating Metal Organic Frameworks (MOFs) Porosity Using Positron Annihilation Lifetime Spectroscopy (PALS)

Positron annihilation lifetime spectroscopy (PALS) is a widely acknowledged non-destructive tool employed to study the porosity of various materials including silica gel, polymers, zeolites, carbon, porous glass by using positrons to probe the materials. The positively charged positron forms a metastable ortho-positronium (o-Ps) when it interacts with an electron within the porous materials. The distinctive long lifetime of ortho-positronium within the materials offers insight into properties such as average pore sizes, pore size distribution, pore connectivity, and surface properties of these materials. Metal-organic frameworks (MOFs) are porous crystalline materials composed of organic and inorganic components. Their structural versatility has captured the interest of researchers, making them suitable for a wide range of applications. The detailed and precise characterization of textural properties, including surface area, pore size, pore size distributions, and pore volume, is crucial for the development of novel and advanced MOFs. This thorough understanding of textural properties of MOFs contributes to the advancement of diverse applications. Various characterization techniques including gas physisorption, X-ray Diffraction (XRD), Thermogravimetric Analysis (TGA), Powder X-ray Diffraction (PXRD), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Raman spectra, and Nuclear Magnetic Resonance (NMR) are utilized to characterize MOFs. Among these techniques the adsorption of inert gas below their critical temperature is widely employed for analysing the porosity of MOFs. However, the potential of PALS for MOFs characterization is still undergoing exploration and investigation. Although each method has their own advantages and limitations the combined use of these techniques provides a comprehensive understanding of MOFs enabling researchers to tailor their synthesis route and design for specific applications. This thesis is designed to characterise the porosity of various MOFs utilizing PALS and to assess the sensitivity of this method by comparing with the results obtained from gas physisorption and a computational tool, Zeo++. Two distinct research projects were conducted for this thesis. The initial project focused on UiO-66 MOF due to its remarkable framework stability even after introducing defects. An Ideal UiO-66 was first synthesized without defects, followed by the synthesis of defect-containing two UiO-66 series adding different amount of modulators. These two series are termed as Formic UiO-66 MOFs and Acetic UiO-66 MOFs and were synthesized solvothermally by adding formic acid and acetic acid, respectively. XRD characterization revealed the presence of metal-cluster defects (reo) in some of these MOFs. SEM images indicated an increase in crystal size with the addition of modulators (both formic and acetic acid), transitioning from intergrown to individual octahedral-shaped crystals. TGA analysis quantified defect percentages, showing an increase with modulator concentration, with Formic UiO-66 series having a higher defect concentration than Acetic UiO-66 series. The impact of defects on porosity was studied through nitrogen gas adsorption, revealing a systematic increase in defects leading to changes in porosity. PALS was applied for the first time to analyse the porosity of a series of UiO-66 MOFs with systematically increased defects. PALS analysis identified small intrinsic micropores (>4.0 Å) in all UiO-66 MOFs, absent in nitrogen gas adsorption. Pore size distributions (PSD) from PALS were constructed and compared with PSD from nitrogen gas adsorption and Zeo++ simulations, considering the number of missing linkers in each UiO-66 framework. The presence of intrinsic micropores in both Ideal and defect-containing UiO-66 was evident from Zeo++. The PSD from PALS exhibited an intriguing phenomenon, with the PSD of largest pore becoming narrower and more intense with increasing particle size, supporting the role of particle size in positronium diffusion out from crystals. The second project involved selection of eight distinct MOFs with diverse topologies, each possessing unique surface area, pore size, pore volume, pore size distribution, and shape. These MOFs were synthesized through solvothermal methods, and their textural properties were probed using nitrogen gas molecule and positronium. Comparison of pore sizes obtained from three methods (nitrogen gas adsorption, PALS, and Zeo++) revealed differences between the analytical techniques. PALS demonstrated its unique capability by detecting ultramicropores in MOFs that were inaccessible to nitrogen molecules due to the small size of pore windows. Surprisingly, the CAU-10H MOF, initially considered nonporous from nitrogen gas adsorption analysis, was found to exhibit porosity through PALS analysis, challenging previous assumptions. Despite the individual advantages of gas physisorption and PALS, the current study emphasized the necessity of their combined application in porosity analysis. This approach is crucial for interpreting data more accurately and overcoming the inherent limitations of each method. The research detailed in this thesis emphasizes the porosity characterization of metal-organic frameworks through the utilization of the potential of PALS