A solvent-assisted ligand exchange approach enables metal-organic frameworks with diverse and complex architectures

Unlike inorganic crystals, metal-organic frameworks do not have a well-developed nanostructure library, and establishing their appropriately diverse and complex architectures remains a major challenge. Here, we demonstrate a general route to control metal-organic framework structure by a solvent-assisted ligand exchange approach. Thirteen different types of metal-organic framework structures have been prepared successfully. To demonstrate a proof of concept application, we used the obtained metal-organic framework materials as precursors for synthesizing nanoporous carbons and investigated their electrochemical Na+ storage properties. Due to the unique architecture, the one-dimensional nanoporous carbon derived from double-shelled ZnCo bimetallic zeolitic imidazolate framework nanotubes exhibits high specific capacity as well as superior rate capability and cycling stability. Our study offers an avenue for the controllable preparation of well-designed meta-organic framework structures and their derivatives, which would further broaden the application opportunities of metal-organic framework materials.

As one of the main limitations for a broader application of MOF, controllable preparation of MOF with diverse and complex micro/nano-structures is still at a very early stage. In this paper, the authors developed a powerful synthesis strategy called "SALE" approach, and synthesized 13 different MOF structures including a kind of rare double-shelled nanotube. These structured MOF materials were carbonized to nanoporous carbon further, which exhibited excellent electrochemical property for Na+ storage. Overall, this work is very interesting and would add significant contribution to this filed, the manuscript is well-organized, and the authors have done relatively detailed investigations and discussions. Therefore, I strongly recommend the publication of this work in Nature Communications after a few clarifications. 1. It would be better to include FTIR spectra characterization for ZIFs after the SALE process. 2. The stability of coordination bonds for ZIF-7/ZIF-71 corresponded to pKa value of linkers, how about Zn-HMT/ZnCo-PPF-3 and ZnCo-BTC/ZnCo-MOF-74 ? 3. In recent publications MOF-derived single-atom catalysts often show 3D solid particle shape, do you think whether or not single-atom catalysts derived from the prepared MOF nanostructures by your approach have an advantage over them ?
Thank you very much for all your comments on our manuscript entitled "A Solvent-Assisted

Ligand Exchange Approach Enables Metal-Organic Frameworks with Diverse and Complex
Architectures" (Manuscript ID: NCOMMS-19-28948-T). Your insightful comments and suggestions on our manuscript are all valuable and very helpful for us to improve the quality of our paper. We have now revised the whole manuscript carefully. The changes are marked in red font in the revised manuscript and supplementary information. Our response to your comments point by point is listed below.

Referee 1
The manuscript "A Solvent-Assisted Ligand Exchange Approach Enables Metal-Organic Frameworks with Diverse and Complex Architectures" by Wu and coworkers is an excellent report on the formation of complex MOF structures. It is well-written and should be published.
Nevertheless, I have some small comments that should be addressed.

Reply:
We thank you very much for your positive remarks on our work, we really appreciate it.
1. Is it possible to study the kinetics of ligand exchange/MOF transformation in more detail?
Maybe by watching the change in time in SEM and XRD after quenching?
Reply: The suggestion is highly helpful. We first tried to capture the states of transformation process in 1.2 M 2-methylimidazole (Hmim) ethanol solution, and found that the kinetics was too fast, although we realized that a core-shell structure would be an intermediate state before forming yolk-shell structure. In order to get more detailed information of the transformation as a function of time, 0.3 M Hmim was used to slow down the transformation kinetics, the nucleation of ZIF-8 nanocrystals was clearly seen on the surface of MOF-5, and ZIF-8 was gradually covering on the facets of MOF-5 nanocubes, resulting in the formation of yolk-shell ZIF-8@MOF-5 hybrid structure and finally the generation of ball-in-box structure. About the related discussions on the evolution, please kindly see Supplementary Figure Figure 9). We find out that a proper concentration range is needed not only to drive a rational kinetics for crystallizing ZIF-8, but also to offer steady ligand exchange for inheriting the shape of MOF-5 nanocubes. In addition, a higher concentration facilitates faster kinetics for phase separation in a shorter time, yielding a yolk-shell structure. The ligand concentration also determines the mimconcentration gradient along the vertical direction of shell wall, which would further affect mim -/Zn 2+ ratio inside the hollow interior and thus the evolution behavior of MOF-5 core. It is noted that in the case of ZIF-71-deirved double-shelled hollow ZIF-8, the higher mimconcentration would enable the sufficient mimligands inside the hollow interior for crystallizing ZIF-8, leading to the formation of secondary ZIF-8 shell (Pages 8-9 in the revised manuscript).
3. There some reports in literature the authors should take a look at (especially the first one): Reply: In our case, the occurrence of phase separation to be a yolk-shell-like structure is the necessary condition for preparing double-shelled structures, which is originated from the nonequivalent diffusion during the ligand exchange process. On the other hand, a certain molar ratio of diffused mim -/Zn 2+ (Co 2+ ) inside the hollow interior is needed to proceed the crystallization of secondary ZIF shell evolved from yolk mother MOFs. Otherwise, due to the trend of reducing nucleation energy and the relatively higher mim -/Zn 2+ ratio nearby the shell wall than in the center, the nucleation of ZIF prefers to continue on the inner wall of outer shell with the continuous dissolution of mother MOFs, resulting in the formation of single-shelled hollow structures (Page 8 in the revised manuscript). Of course, theoretically, there is another possibility of forming single-shelled structures that the phase separation never happens during the whole process, when the vacancies have enough time to diffuse into the center rather than accumulate at the phase interface (in our latest experiments, we observe such structure evolution from ZIF-71 to its derived single-shelled hollow ZIF-8, we never demonstrate the detailed results in this manuscript but in our following study). Considering the big significance of preparing 1D MOF nanostructures, to demonstrate it more clearly, we also add the discussions about the formation of single-/double-shelled nanotube structures in detail as shown in Pages 12-13 in the revised manuscript.
6. The evaporation of Zn species during carbonization can also have an effect on porosity and should be discussed.

Reply:
In the revised manuscript (Page 16), we have discussed the formation of micro-/meso-/macro-pores of ZIF-derived carbon during the carbonization process.
7. Small errors in writing: "nanowires was"; "peapod-like MOF structures have been reported before" (not been reported?) Reply: We have carefully checked and corrected the possible errors within the revised manuscript.

The authors should add page numbers to the SI and a table of contents.
Reply: Thanks. We have added the page numbers and a table of contents in Supplementary Information.

Referee 2
In this paper, the author reported new, facile and versatile synthesis strategies for the design of diverse and complex MOF architectures. By precisely adjusting the balance between the cleavage rate of old bonds and the formation rate of new bonds, 21 different MOF materials and 13 different types of MOF configurations ranging from 3D to 2D and 1D structures have been successfully prepared. In addition, after a carbonization treatment, MOF-derived nanoporous carbon was used for Na + ion storage. Considering the overall quality of the paper, I recommend its publication after minor revision.

Reply:
We thank you very much for your positive remarks on our work, we really appreciate it.
1. The author mentioned that "In order to demonstrate the advantages of MOF architectures, the structured MOFs are subsequently carbonized to fabricate nanoporous carbon for Na + ion storage" in the last paragraph of the introduction, however, it seems illogical. In view of the advantages of MOF architectures, why not the author uses the structured MOFs directly for Na + ion storage? After all, no introduction about the advantage of carbonization was mentioned in the preamble.
Reply: Thanks for the valuable comment. In fact, it has been reported that some MOF materials can be directly used for electrochemical applications, but there is a serious concern that MOFs like ZIFs often suffer from high ohmic resistance and poor stability when assembled into practical devices, leading to very low electrochemical performance. In contrast, MOF-derived nanoporous carbon by an optimized carbonization treatment can combine the excellent conductivity and stability of carbon with the porous feature and nano-architectures inherited from pristine MOF precursors, which is expected to exhibit high-efficiency performance. Hence, we have now added the related discussion on why to use MOF-derived carbon for Na + ion storage rather than pristine MOFs into the Introduction Part (Page 5 in the revised manuscript).
2. In line 132, is there something wrong with the "Hmim⇌H + +mim -". Please check it carefully.
Reply: Yes. 2-methylimidazole (Hmim) actually co-exists in the deprotonated state as a linker unit and in the neutral state as a stabilizing unit, therefore, a more scientific formula should be Hmim⇌ (1-x)H + +(1-x)mim -+xHmim, now we have corrected it (Page 7 in the revised manuscript).
3. In line 322, "CV curves of all ZIF-derived porous carbon are measured at various scan