Mesoporous Silica Encapsulated Metal-Organic Frameworks for Heterogeneous Catalysis

Abstract

Encapsulation of metal-organic frameworks (MOFs) with a shell of mesoporous silica can boost overall performance of MOFs, offering versatility for synthetic architecture of integrated nanocatalysts with reactor-like features. Recently, Yu et al. report a green approach to make yolk-shell nanoreactors. Encapsulation of metal-organic frameworks (MOFs) with a shell of mesoporous silica can boost overall performance of MOFs, offering versatility for synthetic architecture of integrated nanocatalysts with reactor-like features. Recently, Yu et al. report a green approach to make yolk-shell nanoreactors. Over the past two decades, metal-organic frameworks (MOFs) have shown tremendous potential for a wide range of applications across diverse technological fields.1Furukawa H. Cordova K.E. O’Keeffe M. Yaghi O.M. The chemistry and applications of metal-organic frameworks.Science. 2013; 341: 1230444Crossref PubMed Scopus (7222) Google Scholar Although this class of functional materials possess many advantageous characteristics, high chemical vulnerability, low thermal stability, and weak mechanical strength are the major drawbacks that hamper their practical applications.2Li Z. Zeng H.C. Armored MOFs: enforcing soft microporous MOF nanocrystals with hard mesoporous silica.J. Am. Chem. Soc. 2014; 136: 5631-5639Crossref PubMed Scopus (118) Google Scholar To widen their scope of applicability and enhance their performance under harsh working environments, encapsulating microporous MOFs with a thin shell of mesoporous silica (mSiO2) has been proven to be an effective way for two major families of MOFs: isoreticular MOFs (IRMOFs) with carboxylate linkers and zeolitic imidazolate frameworks (ZIFs) with imidazolate bridging ligands coordinating to metal cations.2Li Z. Zeng H.C. Armored MOFs: enforcing soft microporous MOF nanocrystals with hard mesoporous silica.J. Am. Chem. Soc. 2014; 136: 5631-5639Crossref PubMed Scopus (118) Google Scholar Through the design and synthesis of [email protected]mSiO2 nanocomposites, a huge class of integrated nanocatalysts with interior voids and hollow spaces has been prepared or derived to meet an increasing demand of heterogeneous catalysis.3Li B. Zeng H.C. Architecture and preparation of hollow catalytic devices.Adv. Mater. 2019; 31: e1801104Crossref Scopus (65) Google Scholar In a broader sense, integrated nanocatalysts with this type of hollow configuration can be viewed as nanoreactors because they can bring in additional advantage of confinement effects, and thus increase reactant conversion and product selectivity, especially for those involving multiple reactions or tandem reactions, when the porosity and functionality of shell can be further engineered.3Li B. Zeng H.C. Architecture and preparation of hollow catalytic devices.Adv. Mater. 2019; 31: e1801104Crossref Scopus (65) Google Scholar In this issue of Matter, Yu et al. have reported a new development of [email protected]mSiO2 yolk-shell nanostructure, in which MIL-101(Cr) nanocrystals serve as the yolk and mSiO2 as the protecting shell.4Bao S. Li J. Guan B. Jia M. Terasaki O. Yu J. A Green Selective Water-Etching Approach to [email protected] SiO2 Yolk-Shell Nanoreactors with Enhanced Catalytic Stabilities.Matter. 2020; 3 (this issue): 498-508Abstract Full Text Full Text PDF Scopus (17) Google Scholar The synthetic route they investigated is depicted in Scheme 1. Briefly, MIL-101(Cr) nanocrystals are first wrapped with a shell of mSiO2 using silica source tetraethyl orthosilicate (TEOS) and colloidal template cetyltrimethyl-ammonium bromide (CTAB). Extracting CTAB is then performed by adding NH4NO3 to the intermediate product ([email protected]mSiO2-CS with CTAB template). Finally, water is used as an etchant to partially remove both MIL-101(Cr) core and mSiO2 shell, leading to the formation of final product [email protected]mSiO2-YS. It is important to recognize that instead of using conventional chemical etchants such as alkalis or acids, this work reports a cost-effective way as the etching process can be carried out simply with water solvent. It is hypothesized that this green etching strategy is likely to be a dynamic equilibrium process between the decomposition and regrowth of MIL-101(Cr). The [email protected]mSiO2-YS has also been applied in catalytic cycloaddition of CO2 to styrene oxide with tetrabutylammonium bromide as a co-catalyst.4Bao S. Li J. Guan B. Jia M. Terasaki O. Yu J. A Green Selective Water-Etching Approach to [email protected] SiO2 Yolk-Shell Nanoreactors with Enhanced Catalytic Stabilities.Matter. 2020; 3 (this issue): 498-508Abstract Full Text Full Text PDF Scopus (17) Google Scholar It has been found that active species of this nanoreactor-like catalyst are only from Cr3+ Lewis acid sites of MIL-101(Cr) nanocrystals and both catalytic activity and chemical stability of MIL-101(Cr) are improved owing to fully exposed active yolk surfaces and permeable mSiO2 shell for the reactants and products. Furthermore, the protective role of the rigid mSiO2 shell has also been elucidated because it could slow down the decomposition of MIL-101(Cr) nanocrystals encapsulated during the cycloaddition reactions. The synthetic investigation of [email protected]mSiO2-YS is also expanded to include UiO-66 as a core material by adopting the same synthetic strategy, which results in a similar [email protected]mSiO2 yolk-shell nanostructure. The future work is planned to optimize the synthesis parameters and improve the uniformity of the nanoreactors, which will hopefully further enlarge the family of this kind of MOF-based yolk-shell nanocatalysts.4Bao S. Li J. Guan B. Jia M. Terasaki O. Yu J. A Green Selective Water-Etching Approach to [email protected] SiO2 Yolk-Shell Nanoreactors with Enhanced Catalytic Stabilities.Matter. 2020; 3 (this issue): 498-508Abstract Full Text Full Text PDF Scopus (17) Google Scholar In terms of heterogeneous catalysis, both external surface area and internal porosity of single-crystalline MOFs play a crucial role in their actual catalytic performance.3Li B. Zeng H.C. Architecture and preparation of hollow catalytic devices.Adv. Mater. 2019; 31: e1801104Crossref Scopus (65) Google Scholar The above etching transformation of core-shell construct to yolk-shell configuration apparently brings the benefit of easing transport processes of reacting molecules and diffusing reaction products,2Li Z. Zeng H.C. Armored MOFs: enforcing soft microporous MOF nanocrystals with hard mesoporous silica.J. Am. Chem. Soc. 2014; 136: 5631-5639Crossref PubMed Scopus (118) Google Scholar since the blockage of active sites between core and shell can be circumvented by creating an interfacial space between them and ensuring fully exposed surfaces of nanocrystal yolk (Step 3, Scheme 1).4Bao S. Li J. Guan B. Jia M. Terasaki O. Yu J. A Green Selective Water-Etching Approach to [email protected] SiO2 Yolk-Shell Nanoreactors with Enhanced Catalytic Stabilities.Matter. 2020; 3 (this issue): 498-508Abstract Full Text Full Text PDF Scopus (17) Google Scholar Furthermore, in order to ease the mass transfer within the same piece of MOF crystal, hierarchical porosity can be engineered within the single-crystalline MOFs. For example, inter-shell spaces have been generated within MIL-101(Cr) nanocrystals through a step-by-step approach that is based on growth inhomogeneity of MOFs.5Liu W. Huang J. Yang Q. Wang S. Sun X. Zhang W. Liu J. Huo F. Multi-shelled hollow metal-organic Frameworks.Angew. Chem. Int. Ed. 2017; 56: 5512-5516Crossref PubMed Scopus (178) Google Scholar In particular, multi-shelled, hollow MIL-101(Cr) nanocrystals have been prepared through the careful control of growth rate and subsequent etching, resulting in cavity spaces among different shells. Such multi-shelled, hollow MIL-101(Cr) nanocrystals have been investigated for catalytic styrene oxidation reaction, showing significant enhancement of catalytic activity.5Liu W. Huang J. Yang Q. Wang S. Sun X. Zhang W. Liu J. Huo F. Multi-shelled hollow metal-organic Frameworks.Angew. Chem. Int. Ed. 2017; 56: 5512-5516Crossref PubMed Scopus (178) Google Scholar In addition to the formation of inter-shell macroporosity, mesopores can also be created within the same shell of multi-shelled, hollow MOF materials.6Sun B. Zeng H.C. A shell-by-shell approach for synthesis of mesoporous multi-shelled hollow MOFs for catalytic applications.Part. Part. Syst. Charact. 2020; 37: 2000101Crossref Scopus (5) Google Scholar In principle, such an intra-shell mesoporosity allows a fast commutation for reactants and products among various inter-shell spaces within the multi-shelled, hollow MOFs. Very recently, for the first time, polycrystalline multi-shelled hollow spheres of ZIF-67 have been synthesized using a shell-by-shell soft-templating protocol, which demonstrates great potential to create hierarchical pore structures, including macro-, meso-, and micropores, within an assemblage of nanocrystallites of ZIF-67.6Sun B. Zeng H.C. A shell-by-shell approach for synthesis of mesoporous multi-shelled hollow MOFs for catalytic applications.Part. Part. Syst. Charact. 2020; 37: 2000101Crossref Scopus (5) Google Scholar It has been proven that these additional passages and pores present among the multiple shells of ZIF-67 will further boost catalytic performances of this type of hollow MOFs, when they are further loaded with noble metal nanoparticles.6Sun B. Zeng H.C. A shell-by-shell approach for synthesis of mesoporous multi-shelled hollow MOFs for catalytic applications.Part. Part. Syst. Charact. 2020; 37: 2000101Crossref Scopus (5) Google Scholar For instance, synthesis of α,β-unsaturated nitriles has been achieved with one-pot oxidation-Knoevenagel condensation cascade reaction in which gold nanoparticles and Lewis basic sites of ZIF-67 work together as dual catalysts in the above hollow catalytic device.6Sun B. Zeng H.C. A shell-by-shell approach for synthesis of mesoporous multi-shelled hollow MOFs for catalytic applications.Part. Part. Syst. Charact. 2020; 37: 2000101Crossref Scopus (5) Google Scholar Apart from the alternation of pore structure and enlargement of accessible surface areas of MOFs, intrinsic catalytic activity of MOFs (e.g., Lewis acidity and basicity) can also be further modified or enhanced. For example, functional groups and/or molecular moieties can be introduced to organic linkers while metal cations in the MOFs can be partially substituted during synthesis or via post-synthesis ion exchanges. Such strategies will make MOFs with tunable catalytic properties of performing cores or yolks to suit different chemical applications. Additionally, foreign catalytic components can also be extrinsically added into matrixes of MOFs, making them virtually a class of nanocomposite materials. In particular, a broad array of nanomaterials, including semiconductor and noble metal nanoparticles, have been imbedded in the central part of ZIF-8 single-crystalline crystals.7Lu G. Li S. Guo Z. Farha O.K. Hauser B.G. Qi X. Wang Y. Wang X. Han S. Liu X. et al.Imparting functionality to a metal-organic framework material by controlled nanoparticle encapsulation.Nat. Chem. 2012; 4: 310-316Crossref PubMed Scopus (1403) Google Scholar Moreover, such functional nanoparticles can be allocated specifically to a designated crystal plane of interest. Examples of this type have been reported for the insertion of single layer of gold or silver nanoparticles on the {100} and {110} crystallographic planes respectively.2Li Z. Zeng H.C. Armored MOFs: enforcing soft microporous MOF nanocrystals with hard mesoporous silica.J. Am. Chem. Soc. 2014; 136: 5631-5639Crossref PubMed Scopus (118) Google Scholar,8Li Z. Zeng H.C. Surface and bulk integrations of single-layered Au or Ag nanoparticles onto designated crystal Planes {110} or {100} of ZIF-8.Chem. Mater. 2013; 25: 1761-1768Crossref Scopus (101) Google Scholar With an alternate metal-insertion and epitaxial-growth process, noble metal nanoparticles with multiple layered arrangements have been precisely immobilized inside of ZIF-8 nanocrystals, producing [email protected]@ZIF-8, [email protected]@[email protected], [email protected]@[email protected]@ZIF-8, etc. (M = Au and Ag nanoparticles) nanocatalysts.8Li Z. Zeng H.C. Surface and bulk integrations of single-layered Au or Ag nanoparticles onto designated crystal Planes {110} or {100} of ZIF-8.Chem. Mater. 2013; 25: 1761-1768Crossref Scopus (101) Google Scholar This approach has also been used in the preparation of MIL-101-based nanocatalysts; a single-layer of Pt nanoparticles is sandwiched between the same type or different types of MIL-101 (Cr or Fe; [email protected]@MIL-101).9Zhao M. Yuan K. Wang Y. Li G. Guo J. Gu L. Hu W. Zhao H. Tang Z. Metal-organic frameworks as selectivity regulators for hydrogenation reactions.Nature. 2016; 539: 76-80Crossref PubMed Scopus (796) Google Scholar Excellent catalytic performance has been demonstrated with selective hydrogenation of different α,β-unsaturated aldehydes using these configured nanocatalysts.9Zhao M. Yuan K. Wang Y. Li G. Guo J. Gu L. Hu W. Zhao H. Tang Z. Metal-organic frameworks as selectivity regulators for hydrogenation reactions.Nature. 2016; 539: 76-80Crossref PubMed Scopus (796) Google Scholar After the above functionally engineered cores or yolks are wrapped with a mesoporous silica shell, the mechanical strength and catalytic performance of MOFs can be further enhanced.2Li Z. Zeng H.C. Armored MOFs: enforcing soft microporous MOF nanocrystals with hard mesoporous silica.J. Am. Chem. Soc. 2014; 136: 5631-5639Crossref PubMed Scopus (118) Google Scholar It should be mentioned that the pore size of encapsulating silica is generally in nanometer regime (e.g., 2–4 nm). Therefore, such relatively large pores can be easily decorated with ultrafine nanocatalysts or anchored with organometallic catalysts, which can function as co-catalysts together with the pristine MOFs, modified MOFs, [email protected], and [email protected]@MOF (M = metal nanoparticles or any other catalytically active constituents) cores or yolks. Besides the direct engineering and functionalization of MOFs, the mesopores of mSiO2 shell can also be filled up with MOFs, and thus the porosity of the shell can be chosen according the opening window of a MOF material, resulting in the formation of MOFs-mSiO2 shell.2Li Z. Zeng H.C. Armored MOFs: enforcing soft microporous MOF nanocrystals with hard mesoporous silica.J. Am. Chem. Soc. 2014; 136: 5631-5639Crossref PubMed Scopus (118) Google Scholar It is expected that these nanocomposite shells will act as size-selective membranes, allowing the molecular screening of reactants and/or blocking of products. On the other hand, the MOF materials filled inside the mesoporous silica shell can also function as co-catalysts different from their MOF counterparts located in the central region. For this reason, the tailorability of this type of nanoreactors can be greatly raised. As a final note, the [email protected]mSiO2 themselves can also serve as solid precursors for fabricating metal-silicate based nanocatalysts through chemical transformation.10Zhan G. Zeng H.C. ZIF-67 derived nanoreactors for controlling product selectivity in CO2 hydrogenation.ACS Catal. 2017; 7: 7509-7519Crossref Scopus (72) Google Scholar This synthetic route has been utilized in making maze-like nanoreactors starting with [email protected]mSiO2, in which catalytic metal and alloy nanoparticles (i.e., Pt, Au, Cu, Ag, Pt0.5Cu0.5, and Au0.5Cu0.5) can be loaded on the thin layered cobalt silicate surfaces with high accessibility for entering reactants. Such maze-like low-density nanoreactors offer numerous entries and exits for the reactants and products and create intricately lengthened traveling paths within individual nanoreactors.10Zhan G. Zeng H.C. ZIF-67 derived nanoreactors for controlling product selectivity in CO2 hydrogenation.ACS Catal. 2017; 7: 7509-7519Crossref Scopus (72) Google Scholar Therefore, the overall retention time of the reactants inside the macroscopic catalyst bed can be prolonged effectively, and the catalyst performance can be improved accordingly.3Li B. Zeng H.C. Architecture and preparation of hollow catalytic devices.Adv. Mater. 2019; 31: e1801104Crossref Scopus (65) Google Scholar,10Zhan G. Zeng H.C. ZIF-67 derived nanoreactors for controlling product selectivity in CO2 hydrogenation.ACS Catal. 2017; 7: 7509-7519Crossref Scopus (72) Google Scholar Clearly, mesoporous silica encapsulated MOFs have received extensive attention in the important field of heterogeneous catalysis. With more research endeavors and new explorations, like the one reported by Yu et al. in the current issue,4Bao S. Li J. Guan B. Jia M. Terasaki O. Yu J. A Green Selective Water-Etching Approach to [email protected] SiO2 Yolk-Shell Nanoreactors with Enhanced Catalytic Stabilities.Matter. 2020; 3 (this issue): 498-508Abstract Full Text Full Text PDF Scopus (17) Google Scholar we have reason to anticipate that this class of nanocomposite catalysts and their derived nanoreactors will become more practical in the future, and they will eventually gain their applicability in the chemical industry as well as related sectors beyond. A Green Selective Water-Etching Approach to [email protected] SiO2 Yolk-Shell Nanoreactors with Enhanced Catalytic StabilitiesBao et al.MatterJuly 8, 2020In BriefTo improve the stability of MOF catalysts, a design of [email protected] SiO2 yolk-shell nanoreactors was developed via a mesoporous silica coating followed by selective water-etching strategy. Benefiting from their permeable mesoporous SiO2 shells, exposed active sites on MOF surfaces, and protective shells, the yolk-shell nanoreactors exhibit higher catalytic stability than bare MOF crystals in CO2 cycloaddition, with product yield remaining unchanged upon three cycles. Full-Text PDF Open Archive