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

Abstract

* S Supporting Information CONSPECTUS: The total world energy demand is predicted to rise significantly over the next few decades, primarily driven by the continuous growth of the developing world. With rapid depletion of nonrenewable traditional fossil fuels, which currently account for almost 86% of the worldwide energy output, the search for viable alternative energy resources is becoming more important from a national security and economic development standpoint. Nuclear energy, an emission-free, high-energy-density source produced by means of controlled nuclear fission, is often considered as a clean, affordable alternative to fossil fuel. However, the successful installation of an efficient and economically viable industrial-scale process to properly sequester and mitigate the nuclear-fission-related, highly radioactive waste (e.g., used nuclear fuel (UNF)) is a prerequisite for any further development of nuclear energy in the near future. Reprocessing of UNF is often considered to be a logical way to minimize the volume of high-level radioactive waste, though the generation of volatile radionuclides during reprocessing raises a significant engineering challenge for its successful implementation. The volatile radionuclides include but are not limited to noble gases (predominately isotopes of Xe and Kr) and must be captured during the process to avoid being released into the environment. Currently, energy-intensive cryogenic distillation is the primary means to capture and separate radioactive noble gas isotopes during UNF reprocessing. A similar cryogenic process is implemented during commercial production of noble gases though removal from air. In light of their high commercial values, particularly in lighting and medical industries, and associated high production costs, alternate approaches for Xe/Kr capture and storage are of contemporary research interest. The proposed pathways for Xe/Kr removal and capture can essentially be divided in two categories: selective absorption by dissolution in solvents and physisorption on porous materials. Physisorption-based separation and adsorption on highly functional porous materials are promising alternatives to the energy-intensive cryogenic distillation process, where the adsorbents are characterized by high surface areas and thus high removal capacities and often can be chemically fine-tuned to enhance the adsorbate−adsorbent interactions for optimum selectivity. Several traditional porous adsorbents such as zeolites and activated carbon have been tested for noble gas capture but have shown low capacity, selectivity, and lack of modularity. Metal− organic frameworks (MOFs) or porous coordination polymers (PCPs) are an emerging class of solid-state adsorbents that can be tailor-made for applications ranging from gas adsorption and separation to catalysis and sensing. Herein we give a concise summary of the background and development of Xe/Kr separation technologies with a focus on UNF reprocessing and the prospects of MOF-based adsorbents for that particular appl...

Methodologies for evaluation of metal–organic frameworks in separation applications

“…This commensurate positioning also implies that the intracrystalline diffusivity of Xe will be signicantly lower than that of Kr. 29,33,34 The experimental breakthroughs reported by Wang et al 32 for 10/90 Xe/Kr mixtures in a bed packed with CoFormate are shown in Fig. 7a.…”

“…If intra-crystalline diffusional inuences are ignored and adsorption equilibrium is assumed to prevail at every position z, and any time t, we obtain the sharp breakthroughs represented by the dashed lines in Fig. 7b 29 For CoFormate the breakthrough simulations include intra-crystalline diffusion effects with the diffusivity values that are chosen to match the published breakthrough experimental data in Fig. 7.…”

“…7. 29 On the basis of the outlet gas compositions, we can determine the ppm Xe in the exiting gas as a function of the dimensionless time, s; see Fig. 8b.…”

What Are the Best Materials To Separate a Xenon/Krypton Mixture?

“…45,46 Xenon and krypton are also products of the nuclear fission of uranium and plutonium; 40 porous materials could be used to capture the radioactive xenon and krypton in the processing of used nuclear fuel. [47][48][49] Experiments regarding xenon and krypton adsorption 44,48,[50][51][52][53][54][55][56][57][58][59][60][61][62] suggest that it may be feasible to use nanoporous materials in an adsorption-based process to separate a Xe/Kr mixture.…”

Computational Screening of Porous Organic Molecules for Xenon/Krypton Separation

“…4 Currently, Xe is extracted directly from the air, where it is present as only a small component (0.086 ppm), 5 but the nuclear industry is a possible alternative source. 6 The separation of Xe from used nuclear fuel (UNF) would significantly lower the storage cost of the remaining 85 Kr, as well as providing a new source for Xe. However, the cryogenic distillation of Xe from air or used nuclear fuel is costly from both a financial and energetic perspective.…”