Peer review of: Refinement of pore size at sub-angstrom precision in robust metal–organic frameworks for separa (2020)

Reviewers' comments: 

 

Reviewer #1 (Remarks to the Author): 

In this report, by refining the pore size at sub-angstrom resolution in robust MFM-300(M) (M = In, V, 
Fe, Al), precise discrimination and excellent chromatographic separation of three xylene isomers was 
achieved with exceptional separation factors at room temperature. HPLC separation, breakthrough 
curves and batch adsorption experiments were employed to determine the separation capacity. In 
addition, the underlying adsorption mechanism was revealed by the Synchrotron PXRD, terahertz 
spectroscopic study and MD modelling. The research is novel, detailed and well-organized. The 
reviewer would suggest publication of this paper after all the following problems are appropriately 
addressed. 

1. Generally, the papers related to the separation of xylene isomers will discuss the BTEX (benzene, 
toluene, ethylbenzene, xylenes). Compared with benzene and toluene, which can be easily 
separated by distillation, the ethylbenzene (E) is more difficult to separate from xylenes. Why you 
took B and T into consideration but excluded the E under the circumstance that E is also a by-
product of catalytic reforming of crude oil? 

2. Since porosity is critical for the adsorption process, all the texture properties should be concluded 
for these four MOFs? 

3. In page 10, line 260-263, the authors stated that MFM-300 sample was ground in a mortar and 
packed into a stainless steel column. It is well known that the particle size would impact the 
separation efficiency, so what are the size of these ground particles, or how can you ensure the 
uniformity of the particles? 

4. Although the authors stated that the structures of MFM-300 samples are rigorous, evidence such 
as comparisons of geometry of unit cell or pore size should be provided to clearly support this 
statement. 

 
Reviewer #2 (Remarks to the Author): 

In this manuscript authors have reported chromatographic separation of three xylene isomers at 
room temperature by using four previously reported isostructural compounds with very similar pore 
sizes. Experiments were performed very carefully, I don’t see any technical error and manuscript is 
also well written. Several experiments were done and performance of the compounds are 
comparable/better than in few cases than the reported other compounds but at the end authors 
could not find anything conclusive to design an excellent material for xylene separation, rather 
concluded as “it’s a very a complex balance of pore size, shape, and chemistry, coupled with 
cooperative supramolecular interactions within the pore interiors and kinetic molecular sieving 
effects.” Although details studies were done with plenty of data (which are mostly routine for such 
studies) but since no conclusive result was found and also compound are also previously reported, in 
my opinion the current manuscript is not suitable for Nat Commun. 

 

Reviewer #3 (Remarks to the Author): 
 
The article authored by Prof. Schröder and co-workers reports the discrimination of xylene isomers 
via refinement of the pore size in a series of porous metal-organic frameworks, MFM-300 (Fe, V, In, 
Al) at room temperature. The separation performances are confirmed by chromatography and 
ternary breakthrough experiments, and the separation factors and selectivity for these xylene 
isomers are calculated and compared with the reported MOFs and COFs. Besides, the guest loading 
structures are solved by the high-resolution X-ray diffraction to better understand the interaction 
between the host and guest. Considering the record-high selectivity of MX over PX as well as clear 
demonstration on discrimination mechanism, I would like to suggest acceptance after major revision 
as summarized below. 

1.Kinetic separation can only be realized when there is significant difference in diffusivity in the pore 
channel between solutes. From Figure S9, I did not see any difference in diffusion rate among three 
xylenes. Although the authors have performed molecular dynamics simulations, which are able to 
obtain the self-diffusivity, determination of diffusivity of xylenes are very necessary no matter by 
modelling or by experiments in order to confirm so-called kinetic effect originated from pore size 
refinement. 

2.It is not reasonable that all atoms in the MFM-300 (In) were fixed during the molecular dynamic 
(MD) simulation because of an apparent structural distortion of MFM-300 (In) on loading of p-xylene 
as mentioned in the manuscript. So, it might be no sense to compare the time of xylene molecules 
diffusing inside MFM-300 (In). 

3.From single-component adsorption isotherms of individual xylene, the Henry coefficients are not 
as good as separation factor determined by chromatography. I suggest it is better to measure the 
vapor adsorption isotherms to further confirm their separation performance and the uptake capacity 
of xylene isomers. 

4.What about the BET surface areas and pore size distribution of these four materials? Some basic 
characterizations should be included in the manuscript, such as TGA, BET surface area. 

5.Figure 1, the position of xylene isomers in the channel of MFM-300 are obtained from the refined 
X-ray diffraction or simulated results? Figure 1e, the xylene molecules are out of the channel 
boundary. Please clarify. 
6.Page 5, the authors mentioned that there is no change in either crystallinity or porosity of these 
used four materials based on the PXRD data. In fact, porosity stability could not be supported by 
PXRD. At least the BET experiments should be conducted to further confirm the stability and 
porosity of these materials. 

7.Figure S5, the picture b is incomplete with a missing peak of m-xylene. In addition, as seen from 
the chromatograms with different flow rates (0.5 and 1.0 mL/min) on MFM-300 (In) shown in Figure 
a and b, the retention time of p-, m-, and o-xylene are almost the same. It is unreasonable. 

8.Page 10, please add the detail information about the constant flow rate and regular intervals in the 
HPLC experiments section. And the separation factor was calculated using the equation 1, the 
original figures of HPLC should be added in Supporting Information and the tri and tm for MFM-300 
materials should be listed, especially the tm of different sample. 

9.More detail information about static adsorption experiments on single-component adsorption 
isotherms of xylenes should be included in the experimental section. For example, Figure S9, how 
many samples are used for each batch for single-component liquid-phase adsorption? And what is 
the volume of the liquid used? Figure S10, what is the equilibrium time of each xylene isomers on 
these four materials for the single-component liquid-phase adsorption measurements? Additionally, 
the caption of y-axis of these four figures is mM/g, i.e., mmol/ml/g? It should be mmol/g. Please 
clarify. 

10.Figure 4b, the conditions for the selectivity data calculated for various MOFs are the same as the 
MFM-300? If not, the detail conditions need to be added. 

 


Reviewer #4 (Remarks to the Author): 

In this communication, Yang, Schröder, and co-workers report a study on the separation of xylene 
isomers with the metal-organic framework family MFM-300, including the Al, In, V, and Fe versions 
of the MOF. The authors have completed breakthrough and dynamic separation experiments finding 
differences in the separation ability of the materials depending on the MOF metal cation, which is 
explained by the changes in the pore size arising from the differences in the metal-oxygen 
coordination distances. 

While there are a significant number of works reported for the use of MOFs to separate xylene 
isomers, the performance of some of the materials here presented is highly optimal, particularly for 
the separation of m-xylene, making the results quite relevant. 

A recent review on the separation of xylenes with porous materials that should be cited is Ind. Eng. 
Chem. Res. 2017, 56, 14725−14753 (10.1021/acs.iecr.7b03127) 

- A major point of the report is focused on determining the binding sites and interaction between 
xylene isomers and frameworks, to account for the differences in selectivity. Thus, the authors have 
completed a structural analysis of the xylene loaded MOFs with the use of high-resolution powder X-
ray diffraction to locate the adsorbed molecules. This is also complemented with molecular dynamic 
simulations. 

The results of the Rietveld refinements show a nice fitting. However, the occupancy of the adsorbed 
xylene molecules in the provided CIFs is too high. Thus, the reported total amount of adsorbed 
xylene molecules ranges from 5.18 to 8 molecules per unit cell. While the molecules have partial 
occupancies, due to the space group symmetry, these numbers imply that xylene molecules would 
be bumping into each other inside the pores. Indeed, in the molecular dynamic simulations, the 
authors have included 8 molecules for a double volume supercell with twice c axis. The maximum 
loading amount should not be higher than 4 molecules per unit cell. In line with this, the resulting 
thermal parameters are very high, so it seems feasible from the refinement point of view that xylene 
molecules actually have a lower occupancy. 

I am also wondering if these numbers are in agreement with the xylene sorption quantification with 
other methods. Have the authors quantified the amount of adsorbed xylene with other methods 
(such as CHN) for the samples used for the PXRD analysis? 

In any case, the refinement of the occupancy factors of the adsorbed xylene molecules must be 
revised. 

- On the other hand, according to the crystal structures, there is presence of water molecules 
hydrogen bonded to the bridging hydroxyl groups in the channels of some of the materials (In, Al). I 
guess that these molecules could not be removed even though according to the experimental details 
the samples have been evacuated and activated before the xylene loading and subsequent X-ray 
measurements. I wonder how they are actually affecting the interaction of these frameworks with 
the adsorbed molecules. Moreover, their presence inevitably influences the accessible pore size. In 
any case, their presence has been overlooked from the discussion in the binding domains section, 
but they are at interaction distance with the located xylene molecules, in some cases as short as 2.0 
A, so the authors should at least comment on this fact.