Peer review of: Radiation-resistant metal-organic framework enables efficient separation of krypton fission gas fr (2020)

Reviewers' comments: 
 
Reviewer #1 (Remarks to the Author): 
 
The authors found that among the series of SIFSIX-3 MOFs, SIFSIX-3-Cu is good for 85Kr removal 
from nuclear reprocessing plants because it is radioactively stable and can selectively separate and 
store 85Kr using a two-bed breakthrough method. The topic is very interesting and the founding is 
attractive. However, their materials are already known ones and their results do not provide sufficient 
scientific insight. 
 
Most of all, the authors did not provide any explanation why SIFSIX-3-Cu is more radioactively stable 
than the other SIFSIX-3-M materials. They should have provided suggestions or explanations for the 
reason by using proper calculations or further experimental characterizations. 
 
Secondly, they argued that their materials have high Kr/N2 selectivities than other benchmark 
adsorbents. But, they did not provide any comparison with other adsorbents. Moreover, I guess many 
other adsorbents can have good Kr/N2 selectivities due to the differences in the polarizabilities of two 
molecules. It would have been more meaningful if the radioactive stability of SIFSIX-3-Cu was 
compared with other benchmark adsorbents. 
 
For the above reasons, it is regrettable that I cannot recommend this paper to be published in a 
prestige journal like Nature Comm. 
 


Reviewer #2 (Remarks to the Author): 
 
This manuscript explores the possibility of using SIFSIX-based small pore MOFs for Kr separation from 
fission gas in the presence of Xe, CO2, N2 and O2. The authors identified that SIFSIX-Cu is not only 
stable enough under radiation, but also has the ability of capture and separation of Kr from the UNF 
off-gas if a two-bed arrangement is used. The work is important and novel. The results are 
interesting. The paper is well written. I recommend it be accepted after the authors take care of the 
following. 
 
It is not immediate clear from the main text if the PXRD patterns shown in Figure 2 are as-made or 
activated samples. But the SI indicates that the authors only examined as-made materials. I strongly 
believe that the authors should irradiate activated MOFs and investigate their stability as the phase 
behavior of as-made and activated MOFs can be very different. It is the activated MOFs that are used 
for adsorption. A recent paper (J. Phys. Chem. C 2019, 123, 17798−17807) shows that the metal 
environment in as-made and activated SIFSIX-Zn can be very different, which should be cited. 
 
I was also wondering if they looked at those unknown phases to see if they still adsorb Kr. 
 
Is there any evidence backing up the very last sentence of Results and Discussion section suggesting 
Kr adsorption is due to SiF62- pillars? 
 


Reviewer #3 (Remarks to the Author): 
 
Management of spent nuclear fuel is one of the important challenges in any nuclear industry because 
of high radio-toxicity associated with it. Various physico-chemical processes are framed for the 
extraction of valuable elements from nuclear wastes and containment of radioactive gases. However, 
all the chemicals or materials to be used for these processes must be radiation-resistant for them to 
work efficiently. Chosen chemicals or materials designed for extraction of valuable elements either 
through liquid-liquid extraction processes or trapping of various fission gasses by porous materials 
(e.g. Metal-organic frameworks) will not be of any use if the designed materials are not stable in 
presence of high radiation dose. Therefore, designing radiation-resistant efficient materials is very 
important for management of nuclear wastes. Among various fission gases, managing radioactive 
xenon (Xe) and krypton (Kr) is a great challenge because of inert nature of the noble gases, Xe and 
Kr. The present communication deals with performance evaluation of a family of ultra-micro-porous 
metal-organic framework materials, SIFSIX-3-M (M = Fe, Co, Ni, Cu, Zn) for Kr removal from used 
nuclear fuel. Among all the materials considered in this work, SIFSIX-3-Cu has been found to be the 
most stable one when subjected to gamma radiation. The most important outcome of the present 
study is to selectively adsorb Kr gas in the second bed. In my opinion, the proposed radiation-
resistant material, SIFSIX-3-Cu, for the purpose of selective adsorption of Xe and Kr seems to be 
quite efficient, which has been thoroughly investigated through various techniques including 
computational methods. The work presented in the manuscript is novel for two reasons: (1) selective 
adsorption of Kr and (2) Evaluation study of irradiation stability of MOF, and subsequent establishment 
of SIFSIX-3-Cu as a radiation-resistant MOF. It may be accepted for publication in Nature 
Communications after following points are addressed appropriately: 
 
Is it possible to do two-bed breakthrough experiments with different loadings of noble gases (other 
than 400 ppm of Xe and 40 ppm of Kr), keeping the Xe/Kr ratio same? This study will provide new 
insights. 
 
Irradiation induced phase change has been mentioned and the results are reported in the 
supplementary information for various radiation doses. One of the figures from the supplementary 
information (particularly the Figure S15 corresponding to 50 kGy, SIFSIX-3-Cu) may be moved to the 
main manuscript. The possibility of fragmentation through radiation degradation of a material cannot 
be ruled out; however, no such comment is made in the manuscript. 
 
Grimme’s D3 semiempirical method (available in VASP) is more accurate and should be used instead 
of D2 method. VASP version is not mentioned! 
 
In the supplementary information: I fail to understand the phrase, “the Xe•••F distances for the 
annealed position were measured to be 1.94, 1.96, 1.98, and 2.00 Å, while distances of 2.14, 2.18, 
2.22, and 2.25 Å were measured for the Kr•••F interaction.” What do you want to convey with the 
shorter Xe---F distances ? Also how Xe---F distance is shorter than the Kr---F distance? 
 
It has been demonstrated that a combination of weak intermolecular interactions at low loadings and 
strong guest–guest interactions at high loadings leading to self-aggregation of guest molecules within 
confined space are responsible for adsorption of gasses in MFM-300 metal-organic frameworks. This 
confinement induced adsorption is a comparatively new strategy and related papers may be cited : . 
Zhang et al, “Confinement of Iodine Molecules into Triple-Helical Chains within Robust Metal–Organic 
Frameworks”, J. Am. Chem. Soc. 2017, 139, 16289-16296; Srinivasu et al, “Confinement-Directed 
Adsorption of Noble Gases (Xe/Kr) in MFM-300(M)-Based Metal−Organic Framework Materials” J. 
Phys. Chem. C 2019, 123, 27531-27541.