Potential of Metal–Organic Frameworks for Separation\nof Xenon and Krypton

Abstract

ConspectusThe total world energy demand is predicted to\nrise significantly\nover the next few decades, primarily driven by the continuous growth\nof the developing world. With rapid depletion of nonrenewable traditional\nfossil fuels, which currently account for almost 86% of the worldwide\nenergy output, the search for viable alternative energy resources\nis becoming more important from a national security and economic development\nstandpoint. Nuclear energy, an emission-free, high-energy-density\nsource produced by means of controlled nuclear fission, is often considered\nas a clean, affordable alternative to fossil fuel. However, the successful\ninstallation of an efficient and economically viable industrial-scale\nprocess to properly sequester and mitigate the nuclear-fission-related,\nhighly radioactive waste (e.g., used nuclear fuel (UNF)) is a prerequisite\nfor any further development of nuclear energy in the near future.\nReprocessing of UNF is often considered to be a logical way to minimize\nthe volume of high-level radioactive waste, though the generation\nof volatile radionuclides during reprocessing raises a significant\nengineering challenge for its successful implementation. The volatile\nradionuclides include but are not limited to noble gases (predominately\nisotopes of Xe and Kr) and must be captured during the process to\navoid being released into the environment. Currently, energy-intensive\ncryogenic distillation is the primary means to capture and separate\nradioactive noble gas isotopes during UNF reprocessing. A similar\ncryogenic process is implemented during commercial production of noble\ngases though removal from air. In light of their high commercial values,\nparticularly in lighting and medical industries, and associated high\nproduction costs, alternate approaches for Xe/Kr capture and storage\nare of contemporary research interest. The proposed pathways for Xe/Kr\nremoval and capture can essentially be divided in two categories:\nselective absorption by dissolution in solvents and physisorption\non porous materials. Physisorption-based separation and adsorption\non highly functional porous materials are promising alternatives to\nthe energy-intensive cryogenic distillation process, where the adsorbents\nare characterized by high surface areas and thus high removal capacities\nand often can be chemically fine-tuned to enhance the adsorbate–adsorbent\ninteractions for optimum selectivity. Several traditional porous adsorbents\nsuch as zeolites and activated carbon have been tested for noble gas\ncapture but have shown low capacity, selectivity, and lack of modularity.\nMetal–organic frameworks (MOFs) or porous coordination polymers\n(PCPs) are an emerging class of solid-state adsorbents that can be\ntailor-made for applications ranging from gas adsorption and separation\nto catalysis and sensing. Herein we give a concise summary of the\nbackground and development of Xe/Kr separation technologies with a\nfocus on UNF reprocessing and the prospects of MOF-based adsorbents\nfor that particular application.