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The separation of isotopes has always been a challenge because of their identical size, shape and thermodynamic properties. Nowadays, the extraction of deuterium is performed e.g. by the Girdler Sulfid process or cryogenic distillation, which lead to low separation factors (below 2.5) in combination with high energy costs. The standard way to produce helium-3 is to skim it as a byproduct of the radioactive tritium decay. In this thesis, two alternative approaches have been investigated for the separation of light isotopes, Quantum Sieving and Chemical Affinity Sieving . While Quantum Sieving is based on confinement in small pores, Chemical Affinity Sieving relies on strong adsorption sites. Both methods use the mass difference of the isotopes, which is related to their zero-point energy. The microporous metal-organic frameworks are excellent candidates for studying these quantum effects due to their well-defined pore structure and the possibility to introduce strong adsorption sites directly into the framework. The samples have been exposed to an isotope mixture and the adsorbed quantity of each isotope was detected by low-temperature thermal desorption spectroscopy (TDS). The ratio of the desorbed amount of the isotopes leads directly to the selectivity (separation factor). The selectivity is determined as a function of exposure time and temperature and exhibits the highest value of 25 observed for hydrogen isotopes at temperatures well above the boiling point of liquid nitrogen.
In this thesis we present our research on hydrogen isotope separation using metal-organic frameworks (MOFs). Deuterium is one of the two stable isotopes of hydrogen. Despite its wide range of application, currently there is no ideal industrial method that can separate deuterium in a fast and efficient fashion. MOFs are a class of porous materials consisting of metal ions or clusters connected by organic ligands. They have shown great potential in separating hydrogen isotopes via quantum sieving effect. In this thesis, we first provide background on two state-of-art MOFs, Co-MOF-74 and Cu(I)-MFU-4l. Then we elaborate on the statistical theory of selectivity, the mechanism of separation and the basic idea of mass spectrometry, which is the main analytical technique used in this project. We present temperature programmed desorption (TPD) spectra for both samples. Direct separation measurement is made with Co-MOF-74. We confirm that TPD spectra can predict the results of direct separation measurements. The TPD spectra of Cu(I)-MFU-4l predict a selectivity of approximately 6 at easily accessible temperatures (~260K). This shows the practicality of using Cu(I)-MFU-4l for hydrogen isotope separation. Preferential adsorption separation is also performed with Co-MOF-74. The extracted activation energy agrees to within 10% of literature predictions based on quantum zero point energy models.
In this thesis, we designed and built a gas flow-through system to study dynamic adsorption separation of hydrogen isotopes in metal-organic frameworks (MOFs). MOFs are porous, crystalline materials composed of metal complexes connected by organic linkers. They have been proposed as a cheaper, more energy efficient approach to hydrogen isotope separation than current industrial methods. We have previously found evidence of a zero-point energy-based separation mechanism for hydrogen isotopes in two MOFs: Co-MOF-74 and Cu(I)-MFU-4l. This mechanism, chemical affinity quantum sieving (CAQS), has been extensively studied under static equilibrium conditions. The system in this work was developed so that CAQS could be studied under dynamic conditions that more closely resemble those in industrial separation. Breakthrough analysis is an established technique for studying dynamic separation in porous materials. Generally, a breakthrough experiment involves flowing a gas mixture through a fixed bed of adsorbent material and measuring the composition of the effluent flow. In this work, a 1:1 mixture of common hydrogen and its isotope deuterium was flowed through 71 mg of Co-MOF-74 or 22 mg of Cu(I)-MFU-4l. A quadrupole mass spectrometer was used to monitor the composition of the effluent flow. We saw preferential adsorption of deuterium over common hydrogen in Co-MOF-74 at 77K and Cu(I)-MFU-4l at 170K, 140K, and 110K. This behavior was absent in Cu(I)-MFU-4l at 77K, a phenomenon that we would like to investigate further. Minimal adsorption occurred in both MOFs at room temperature, as expected. A selectivity of deuterium over common hydrogen was calculated for each temperature. These selectivities were approximately 30% lower than comparable literature values. Our goal is to make improvements to our system and methods to measure the selectivity more accurately and reproducibly. Notably, all measured selectivities were higher than the selectivity of the Girdler Sulfide method and cryogenic distillation, two industrial hydrogen isotope separation processes we are trying to improve on. This new system gives us the capability to study dynamic adsorption and kinetic separation of hydrogen isotopes in metal-organic frameworks going forward. We hope that our work will inform the development of efficient, environmentally sustainable separation processes.
Metal-Organic Frameworks, or MOFs, are an exciting class of nanoporous crystalline materials with applications that include hydrogen storage and hydrogen isotope separation. The dynamics of adsorbed molecular hydrogen in the prototypical material known as MOF-5 have previously been studied using infrared spectroscopy. However, the rovibrational spectrum of the isotopologues, HD and D2 were obscured due to overlap with the MOF peaks. Overtone infrared spectroscopy in conjunction with a diffuse reflectance geometry is used to observe the spectrum of H2, HD and D2. The overtone spectrum is shown to facilitate the identification of hydrogen peaks. Further, the spectrum of trapped H2 near the crystallographic metal site is greatly enhanced relative to other sites and displays a greater intensity relative to the fundamental spectrum than is seen in gas phase hydrogen. The ability of the MOF to catalyze ortho to para conversion of trapped species is also discussed.
Metal-Organic Frameworks (MOFs) are crystalline compounds consisting of rigid organic molecules held together and organized by metal ions or clusters. Special interests in these materials arise from the fact that many are highly porous and can be used for storage of small molecules, for example H2 or CO2. Consequently, the materials are ideal candidates for a wide range of applications including gas storage, separation technologies and catalysis. Potential applications include the storage of hydrogen for fuel-cell cars, and the removal and storage of carbon dioxide in sustainable technical processes. MOFs offer the inorganic chemist and materials scientist a wide range of new synthetic possibilities and open the doors to new and exciting basic research. Metal-Organic Frameworks Materials provides a solid basis for the understanding of MOFs and insights into new inorganic materials structures and properties. The volume also reflects progress that has been made in recent years, presenting a wide range of new applications including state-of-the art developments in the promising technology for alternative fuels. The comprehensive volume investigates structures, symmetry, supramolecular chemistry, surface engineering, recognition, properties, and reactions. The content from this book will be added online to the Encyclopedia of Inorganic and Bioinorganic Chemistry: http://www.wileyonlinelibrary.com/ref/eibc
Advanced Structural Chemistry Discover the relationships between inorganic chemical synthesis, structure, and property with these comprehensive and insightful volumes Advanced Structural Chemistry: Tailoring Properties of Inorganic Materials and their Applications (3 Volume Set) offers readers the opportunity to discover the relationship between the structure and function of matter, develop efficient and precise synthesis methodology, and to understand the theoretical tools for new functional substances. Advanced Structural Chemistry clarifies the relationships between synthesis and structure, as well as structure and property, both of which are central to the creation of new materials with unique functions. In addition to subjects like the syntheses of metal-oxide clusters, metal-organic cages, and metal-organic frameworks with tailored optical, electric, ferroelectric, magnetic, adsorption, separation, and catalytic properties, the accomplished editor Rong Cao provides readers with information on a wide variety of topics, such as: Coordination-assembled metal-organic macrocycles and cages, including metallacycles and metallacages The structural chemistry of metal-oxo clusters, including the oxo clusters of transition metal, main group metal, and lanthanides Synthetic approaches, structural diversities, and biological aspects of molybdenum-based heterometallic sulfide clusters and coordination polymers Group 11-15 metal chalcogenides, including discrete chalcogenide clusters synthesized in ionic liquids The structures of metal-organic frameworks, including one-, two-, and three-dimensional MOFs Perfect for inorganic chemists, structural chemists, solid state chemists, material scientists, and solid state physicists, Advanced Structural Chemistry also belongs on the bookshelves of catalytic and industrial chemists who seek to improve their understanding of the structure and functions of inorganic materials.
This text discusses the synthesis, characterization, and application of metal-organic frameworks (MOFs) for the purpose of adsorbing gases. It provides details on the fundamentals of thermodynamics, mass transfer, and diffusion that are commonly required when evaluating MOF materials for gas separation and storage applications and includes a discussion of molecular simulation tools needed to examine gas adsorption in MOFs. Additionally, the work presents techniques that can be used to characterize MOFs after gas adsorption has occurred and provides guidance on the water stability of these materials. Lastly, applications of MOFs are considered with a discussion of how to measure the gas storage capacity of MOFs, a discussion of how to screen MOFs to for filtration applications, and a discussion of the use of MOFs to perform industrial separations, such as olefin/paraffin separations. Throughout the work, fundamental information, such as a discussion on the calculation of MOF surface area and description of adsorption phenomena in packed-beds, is balanced with a discussion of the results from research literature.
Separation of Isotopes of Biogenic Elements provides a detailed overview of this area of research covering all aspects from the value of isotope effects to their practical use (equilibrium single-stage isotope effect - kinetics and mass transfer – multiplication of the single-stage isotope separation factor - technological peculiarity of processes) with the purpose of extraction from the natural mixture of the enriched and highly concentrated isotopes. In contrast to traditional books on the theory of isotope separation, the theoretical part of the book describes separation in two-phase processes in counter-flow columns. The experimental part of the book presents systematic analysis of specialists in the field of isotope separation in counter-flow columns. This book will be of interest to scientists, engineers and technical workers engaged in isotope separation processes and isotope application in nuclear physics, medicine, agro-chemistry, biology and other areas. This book may also be used in teaching theory and practical aspects in courses on physical chemistry and Isotope separation of light elements by physicochemical methods.* summarises current state of isotope research, especially biogenic elements* covering all aspects from the value of isotope effects to their practical use* of interest to scientists, engineers and technical workers engaged in isotope separation processes and isotope application
The production of pure deuterium and the removal of tritium from nuclear waste are the key challenges in separation of light isotopes. Presently, the technological methods are extremely energy- and cost-intensive. Here we report the capture of heavy hydrogen isotopes from hydrogen gas by selective adsorption at Cu(I) sites in a metal-organic framework. At the strongly binding Cu(I) sites (32 kJ mol-1) nuclear quantum effects result in higher adsorption enthalpies of heavier isotopes. The capture mechanism takes place most efficiently at temperatures above 80 K, when an isotope exchange allows the preferential adsorption of heavy isotopologues from the gas phase. Large difference in adsorption enthalpy of 2.5 kJ mol-1 between D2 and H2 results in D2-over-H2 selectivity of 11 at 100 K, to the best of our knowledge the largest value known to date. Combination of thermal desorption spectroscopy, Raman measurements, inelastic neutron scattering and first principles calculations for H2/D2 mixtures allows the prediction of selectivities for tritium-containing isotopologues.