A facile dual-template-directed successive assembly approach to hollow multi-shell mesoporous metal–organic framework particles

Hollow multi-shell mesoporous metal–organic framework (MOF) particles with accessible compartmentalization environments, plentiful heterogeneous interfaces, and abundant framework diversity are expected to hold great potential for catalysis, energy conversion, and biotechnology. However, their synthetic methodology has not yet been established. In this work, a facile dual-template-directed successive assembly approach has been developed for the preparation of monodisperse hollow multi-shell mesoporous MOF (UiO-66-NH2) particles through one-step selective etching of successively grown multi-layer MOFs with alternating two types of mesostructured layers. This strategy enables the preparation of hollow multi-shell mesoporous UiO-66-NH2 nanostructures with controllable shell numbers, accessible mesochannels, large pore volume, tunable shell thickness and chamber sizes. The methodology relies on creating multiple alternating layers of two different mesostructured MOFs via dual-template-directed successive assembly and their difference in framework stability upon chemical etching. Benefiting from the highly accessible Lewis acidic sites and the accumulation of reactants within the multi-compartment architecture, the resultant hollow multi-shell mesoporous UiO-66-NH2 particles exhibit enhanced catalytic activity for CO2 cycloaddition reaction. The dual-template-directed successive assembly strategy paves the way toward the rational construction of elaborate hierarchical MOF nanoarchitectures with specific physical and chemical features for different applications.

generates from the fusion of the two strategies without distinctive conception.Secondly, the strategy is lack universality and extensibility in more types of MOFs.Lastly, the selection of catalytic reaction is too ordinary to highlight the advantages of the structure.Therefore, at current stage, these findings are not sufficient to support its publication in Nature Communications.In addition, if the following questions are answered, it will help readers better understand this work.
1.The synthesis method of multi-shell mesoporous MOFs in the manuscript is at a relatively low temperature.Further clarification is needed on whether the crystallinity, defects, and stability of MOF crystals will be affected.
2. Although the effectiveness of individual templates has been studied, the proposed mechanism of the dual templates need further verification with more experiment or characterization .
3. The catalysis analysis is incomplete.For example, what are the catalytic active centers?Does the acidic defect site in MOFs have catalytic activity?Does etching affect the catalytic performance of MOF structures? 4. For MOFs as catalyst, most of reports can use the uniform micropore of MOFs for high selectivity.However, in this manuscript, author should do some catalytic reaction to demonstrate that the mesopore MOF shell enhance the moleculer diffusion as well as micropore can achieve the selectivity.
5. The strategy is lack universality and extensibility in more types of MOFs.you can expore more kinds of MOFs, such as HKUST, MIL, which can be used this strategy to structure multi-shell mesoporous MOFs.
6. What is the maximum number of layers that can be achieved using this strategy, seven layers?Nine layers?7. Can you design the space location of the micorpore shell and mesopore shell by this strategy?8.There are many errors in the manuscript.For instance, the metal-organic framework should be corrected as metal-organic framework.The CO2 and TiO2 should be corrected as CO2 and TiO2 in the references.

Reviewer #3 (Remarks to the Author):
This work developed an interesting synthetic strategy for multi-shell mesoporous UiO-66-NH2.The method can precisely control shell numbers and sizes of the multi-shell mesoporous MOFs, with the demonstration of structural enhancement of adsorption capacities and catalytic performance.This work will expand the toolbox for multi-shell MOFs construction and will be of general interest to the scientific community.Therefore, it is suitable for publication in Nature Communications after addressing several points.
1.In Figure 1a and Figure 3, the acetic acid etching treatment is the last step to form the multi-shells.Also in line 115-121, the authors describe the last step is selective etching in the formation process of UiO-66-NH2 MSMPs.However, there are no etching procedure in Method sections.Please correct the confusing parts.
2. I am curious about the generality of the synthetic strategy.Could the method apply to other UiO-66?Could the method apply to other MOFs?
3. If the coordination interaction of ODMB is the main reason to form disordered worm-like pores in UiO-66-NH2, is it possible to use long chain fatty acid to replace ODMB? 4. Can the authors provide evidences that the catalytic active sites in UiO-66-NH2 are Lewis acid sites? 5.Even though some mesoporous structures show similar phenomena, Can the authors explain why the mesoporous layers can facilitate the accumulation of organic molecules?
Reviewer #4 (Remarks to the Author): In this manuscript, Xu et al. developed a dual-template-directed assembly approach for the preparation of monodisperse hollow multi-shelled structures (HoMS) UiO-66-NH2.After multiple growth process, the formation of multi-shells with controllable shell numbers and tunable particle diameters could be achieved by chemical etching.The highly accessible Lewis acidic sites and favored mass transfer within the multi-shelled nanostructures, triple-shelled UiO-66-NH2 particles show the highest activity in CO2 cycloaddition reaction.This work provide some new attempts in the synthesis of HoMS materials.There are still ambiguous points that should be further clarified, the minor revision is needed.
1.The multi-shell mesoporous UiO-66-NH2 particles reported in this work featuring more than two individual shells with isolated internal cavities have been well defined and widely used as hollow multishelled structures (HoMS) in previous reports (Adv. Mater. 2019, 31, 1802874, Nat. Chem. Rev. 2020, 4, 159-168, and Angew. Chem. Int. Ed. 2023, e202302621).It is strongly recommended that the author revise the nomenclature of the materials to better describe their characteristics and maintain consistency with the terminology used in the field.
2. In the introduction, the authors claim "The UiO-66-NH2 MSMPs process controllable shell numbers (1-4), tunable particle diameters (90-600 nm)…" It seems that only single-shell structure could be observed in 90 nm sized UiO-66-NH2 samples.In other words, what is the minimum material size that can form a quadruple-shelled UiO-66-NH2 structure?3.According to the synthesis mechanism now proposed, how is the regulation of shell spacing and shell thickness realized?4. As the author proposes this as a synthetic strategy, it is important to determine whether this strategy is universal.Can other MOF monomers be used to verify its effectiveness?What is the key prerequisite for realizing the dual-template-directed assembly? 5. Since several sized pores exist in the as-synthesized material, the roles of the different sized pores should be clarified.
6. Related to question 4, can the enhancement mechanism of mass transfer process be verified with substrates containing different functional groups or substrates with different sizes? 7. What is the reason for the lack of catalytic performance data for quadruple-shelled UiO-66-NH2 samples?

Response to the reviewers' comments
Reviewer #1: Xu et al. introduced a straightforward method for fabricating monodisperse multi-shell mesoporous MOF (UiO-66-NH2) particles using a dual-template-directed successive assembly approach.This approach involves the selective etching of successively grown multi-layer MOFs that consist of alternating two types of mesostructured layers.By employing this strategy, it becomes possible to create multi-shell mesoporous UiO-66-NH2 nanostructures with precise control over the number of shells, accessible mesochannels, generous pore volume, and adjustable particle diameters and chamber sizes.The synthesis method presented in the study exhibits considerable appeal.Thus, I suggest its acceptance after some revisions.
Comment 1: A Table comparing the catalytic performance of the TSMPs with state-ofart catalysts in literature is suggested to be added in SI.

Response:
We are greatly thankful to the reviewer for the encouragement on our work and the valuable comments.We have compiled a table summarizing and comparing the performance of various MOF catalysts in CO2 cycloaddition reactions (Supplementary Table 1).The corresponding discussion has been added on Page 11, highlighted in yellow.Comment 3: The resolution of some pictures is too low, e.g. Figure 1i, j, k, Figure 5b,d, please update them.

Supplementary
Response: Per the suggestion of the reviewer, we have updated the related panels in Figure 1 and Figure 5.  Comment 4: How do the authors confirm the products?GC-MS data or NMR spectroscopy of the products are suggested to be given.
Response: Thank you for your valuable comment.The catalytic products of all epoxy compounds by CO2 cycloaddition are determined by 1 H nuclear magnetic resonance ( 1 H NMR) spectroscopy, and the conversion rates are calculated based on the peak area (Supplementary Figs.34-36).The corresponding discussion has been added on Page 10 and 11, highlighted in yellow.

Reviewer #2:
This manuscript reported a facile dual-template-directed successive assembly strategy to synthesize UiO-66-NH2 multi-shell mesoporous particles.The utilisation of dualtemplate are very distinctive, and the proposed mechanism in the manuscript is interesting.However, there are some issue to be address by authours.Firstly, the insufficient innovation in structural characteristics and synthesis methods.What is the different between this manuscript and some reported word in the design of the multishelled structure.F127 is the common template in synthesis of mesoporous material.
The strategy here is generates from the fusion of the two strategies without distinctive conception.Secondly, the strategy is lacking universality and extensibility in more types of MOFs.Lastly, the selection of catalytic reaction is too ordinary to highlight the advantages of the structure.Therefore, at current stage, these findings are not sufficient to support its publication in Nature Communications.In addition, if the following questions are answered, it will help readers better understand this work.

Response:
We are greatly thankful to the referee for the encouragement on our work and the excellent comments and questions.We hope the detailed discussion and relevant revision could address the issues raised by the reviewer and dispel the possible hesitation and misunderstanding from the reviewer.Herein, we would like to restate the novelty and importance of our work as follows: The multi-shell MOF particles have attracted special research interest due to their unique compartmentation environments, plentiful heterogeneous interfaces, and remarkable diversity of crystalline architectures.Although there are several important studies on the synthesis of specific MOF multi-shell particles with intrinsic microporous shells (Angew.Chem.Int. Ed. 2017, 56, 5512;Angew. Chem. Int. Ed. 2018, 57, 2110;Nat Commun 2017, 8, 14070), general method that can achieve various multishell MOF materials with mesoporous shells has not been reported yet.Furthermore, we present a novel dual-template-directed successive assembly mechanism, which markedly differs from the formation mechanism observed in previous reported multishell MOF particles or multi-shell mesoporous particles.Thanks to this unique growth mechanism, we are able to achieve multi-shell mesoporous MOF particles with controllable shell numbers, accessible mesochannels, tunable shell thickness and chamber sizes.To date, precise control over the structure of multi-shell mesoporous MOF particles has not been achieved yet.
Moreover, we have verified the universality of methodology and growth mechanism by successfully synthesizing hollow multi-shell mesoporous Hf-UiO-66-NH2 and MOF-801 particles.Following the similar growth mechanism mentioned above, inhomogeneous mesoporous Hf-UiO-66-NH2 and MOF-801 precursor particles could be formed by ODMB/F127-directed sequence assembly.By repeating such dualtemplate-directed assembly processes and subsequent selective etching, hollow doubleshell mesoporous Hf-UiO-66-NH2 and MOF-801 particles could be prepared .The corresponding discussion has been added on Page 9, highlighted in yellow.reaction.As the reaction time proceeds, ODMB templates gradually exhaust, and then F127-directed self-assembly starts by the epitaxial growth of mesoporous MOF layer with radically oriented mesochannels on the initially formed MOF cores.
To further validate our speculation, we progressively increase the amount of ODMB from 0.05 g to 0.1 g, and then to 0.2 g, while keeping the amount of F127 constant.Consequently, the diameter ratio of the interior core with disordered wormlike pores to the whole MOF particle varies accordingly, changing from 0.41 to 0.55, and eventually reaching 0.73 (Supplementary Fig. 16).This observation provides confirmation of ODMB/F127-directed sequence assembly mechanism.The corresponding discussion has been added on Page 7, highlighted in yellow.Response: Many thanks for the comment.The Lewis acid metal centers in MOFs can promote the activation of the epoxide ring, while the functional groups in the ligands can act as basic sites improving the CO2 affinity inside the pore (Chem.Soc.Rev. 2019, 48, 2783-2828, Inorganics 2021, 9, 81).A previous study reported the acidic defect site can provide additional catalytically active sites from undercoordinated zirconium nodes, leading to higher catalytic conversion in the cycloaddition reaction (J.Mater.Chem.A 2022, 10, 10051-10061).
To explore the effect of etching on the catalytic performance of MOF structures, we synthesized mesoporous MOF using F127 as the sole template (Fig. 2c) and evaluate the catalytic performance between etched and unetched samples.The conversion of glycidyl 2-methylphenyl ether can reach 74% in 10 h for the unetched samples and the etched samples show conversion of 77% in the same period.The results indicate that etching has a negligible effect on the catalytic performance.
Comment 4: For MOFs as catalyst, most of reports can use the uniform micropore of MOFs for high selectivity.However, in this manuscript, author should do some catalytic reaction to demonstrate that the mesopore MOF shell enhance the moleculer diffusion as well as micropore can achieve the selectivity.
Response: Many thanks for the comment.The inherent CO2 absorbability, the exposed Lewis acid metal sites, and the confinement of the pore size make the MOF a promising heterogeneous catalyst for the cycloaddition of CO2 with epoxides to form cyclic carbonates (ACS Catal. 2018, 8, 3194-3201;Chem. Mater. 2019, 31, 3, 1084-1091).MOF-catalyzed CO2 cycloaddition of small substrates was carried out within the framework, while large ones cannot easily enter into the porous framework for catalytic reactions.Thus, the synthesized MOFs could exhibit size-dependent selectivity to different substrates on account of the confinement of the pore diameter (J.Am.Chem.Soc. 2016Soc. , 138, 2142Soc. -2145)).
To demonstrate the enhanced molecular diffusion and the size-dependent selectivity of mesopore MOF shell, we further examine their performances in CO2 cycloaddition reactions with two different epoxides propylene oxide and 1,2epoxydodecane substituted with methyl and decyl groups, respectively.As expected, the 3S-mesoUiO-66-NH2 catalyst shows the higher conversion rates than microporous MOF catalyst (Supplementary Fig. 33).Distinct from the catalytic processes involving smaller substrate propylene oxide, the 3S-mesoUiO-66-NH2 catalyst exhibits an almost twofold increase in the conversion rate of 1,2-epoxydodecane with larger substituted functional groups compared to the microporous MOF catalyst.This result suggests that mesoporous multi-shell nanostructures can not only enhance the mass transfer of larger molecules but also maintain higher size-dependent selectivity toward smaller epoxides in catalytic CO2 cycloaddition.The corresponding discussion has been added on Page 10, highlighted in yellow.

Comment 2 :
The time-conversion curves of catalytic cycloaddition with the error bars should be provided.Response: Per the suggestion of the reviewer, we have provided the time-conversion curves of the catalytic reactions with error bars.The corresponding discussion has been added on Page 10, highlighted in yellow.

Supplementary
Fig. 16 TEM and magnified TEM images of dual-mesopore core-shell UiO-66-NH2 particles prepared with 0.05 g of F127 but different amount of ODMB: (a,b) 0.05 g, (c,d) 0.1 g, and (e,f) 0.2 g.Comment 3: The catalysis analysis is incomplete.For example, what are the catalytic active centers?Does the acidic defect site in MOFs have catalytic activity?Does etching affect the catalytic performance of MOF structures?