From Macro- to Nanoscale: Finite Size Effects on Metal–Organic Framework Switchability

Abstract

Switchable metal–organic frameworks (MOFs) conquer advanced applications in energy storage, sensing, gas separation, catalysis, and biomedicine. They benefit from unique adsorption characteristics and responsive behavior leading to anomalously high separation selectivity and deliverable storage capacity.Nanostructuring provides an effective means to tailor the responsive behavior of switchable MOFs. The characteristic switching pressure depends on critical crystal dimensions.The understanding of critical size effects affecting stimulus-responsive cooperative transformations is in its infancy. Thermodynamic and kinetic factors govern size-dependent switchability. General models explaining size-related changes in responsivity require further methodological development. Advancement of computational and analytical tools is crucial to achieve a deeper understanding of surface, interfacial, and finite size effects. Switchable metal–organic frameworks (MOFs) standt out for potential applications in energy storage, separation, sensing, and catalysis. The understanding of MOF switchability mechanisms has progressed significantly over the past two decades. Nanostructuring is essential for the integration of such materials into thin films, hierarchical composites, and membranes and for biological applications. However, downsizing below critical dimensions causes dramatic changes in the dynamic behavior and responsiveness towards external stimuli. We discuss the most important experimental findings and derive general guidelines and hypotheses of relevance for the impact of crystal size on switchability. Understanding nanostructure thermodynamics and implications for the tailoring of dynamic porous systems requires an interdisciplinary approach, advanced physical characterization techniques, and new modeling strategies to cover a wider range of time and length scales. Switchable metal–organic frameworks (MOFs) standt out for potential applications in energy storage, separation, sensing, and catalysis. The understanding of MOF switchability mechanisms has progressed significantly over the past two decades. Nanostructuring is essential for the integration of such materials into thin films, hierarchical composites, and membranes and for biological applications. However, downsizing below critical dimensions causes dramatic changes in the dynamic behavior and responsiveness towards external stimuli. We discuss the most important experimental findings and derive general guidelines and hypotheses of relevance for the impact of crystal size on switchability. Understanding nanostructure thermodynamics and implications for the tailoring of dynamic porous systems requires an interdisciplinary approach, advanced physical characterization techniques, and new modeling strategies to cover a wider range of time and length scales. MOFs are among the most porous systems, with great potential in diverse applications such as energy storage, separation, air purification, and many more [1.Jiao L. et al.Metal–organic frameworks: structures and functional applications.Mater. Today. 2019; 27: 43-68Crossref Scopus (117) Google Scholar,2.Yuan S. et al.Stable metal–organic frameworks: design, synthesis, and applications.Adv. Mater. 2018; 30: 1704303Crossref PubMed Scopus (667) Google Scholar]. An outstanding feature, compared with other, traditional porous materials, is their ability to transform (switch) between different phases with well-defined crystalline structures triggered by external stimuli, often by guest inclusion [3.Krause S. et al.Chemistry of soft porous crystals–structural dynamics and gas adsorption properties.Angew. Chem. Int. Ed. 2020; 59: 15325-15341Crossref PubMed Scopus (0) Google Scholar,4.Schneemann A. et al.Flexible metal–organic frameworks.Chem. Soc. 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An activation barrier (EA or ΔGǂ) governs the kinetics and potential windows for metastable states. Nucleation (for nucleation theory see Glossary and see supplemental information online for additional Glossary terms) of the new solid phase is an important characteristic of the solid–solid phase transition. Nevertheless, the fluid kinetics of adsorption and, potentially, fluid nucleation kinetics may also play a role. The integration of switchable MOFs into applications frequently requires downsizing to the submicron or even nanoscale (Figure 1) (e.g., in membranes [11.Rodenas T. et al.Metal–organic framework nanosheets in polymer composite materials for gas separation.Nat. Mater. 2015; 14: 48-55Crossref PubMed Scopus (1099) Google Scholar, 12.Hou Q. et al.Balancing the grain boundary structure and the framework flexibility through bimetallic metal–organic framework (MOF) membranes for gas separation.J. Am. Chem. 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(A) Gating (purple, macro) and its suppression by downscaling leading to a type I isotherm of rigid crystal (orange, nano), (B) breathing (purple, macro) and suppression (orange, nano), (C) negative gas adsorption (NGA) (purple, macro) and rigidification (orange, nano), (D) gate-shift hysteresis and widening, and (E) gate-opening emergence.View Large Image Figure ViewerDownload (PPT) The purple lines show the isotherm of the macrosized bulk crystals; the orange line is the result of downsizing to the submicron or nanoregime. (A) Gating (purple, macro) and its suppression by downscaling leading to a type I isotherm of rigid crystal (orange, nano), (B) breathing (purple, macro) and suppression (orange, nano), (C) negative gas adsorption (NGA) (purple, macro) and rigidification (orange, nano), (D) gate-shift hysteresis and widening, and (E) gate-opening emergence. In the following sections, we identify key experimental findings in this field and relate these to existing theories of size-dependent phase transitions. We outline the most important factors relevant to size-dependent switchability and derive a comprehensive scenario of relevant aspects. From this, we deduce future directions for simulation and experimental studies to advance the understanding of nucleation, interfacial effects, and surface layers influencing switchability. Nanostructures are essential for many applications (Figure 1 and Box 1). The increased outer active surface and mass transport in nanocrystals is advantageous for catalytic applications [38.Herbst A. et al.Brønsted instead of Lewis acidity in functionalized MIL-101Cr MOFs for efficient heterogeneous (nano-MOF) catalysis in the condensation reaction of aldehydes with alcohols.Inorg. 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Rev. 2009; 38: 1380-1399Crossref PubMed Scopus (1226) Google Scholar, 99.Bureekaew S. et al.Orbital directing effects in copper and zinc based paddle-wheel metal organic frameworks: the origin of flexibility.J. Mater. Chem. 2012; 22: 10249-10254Crossref Scopus (44) Google Scholar, 100.Evans J.D. et al.Origins of negative gas adsorption.Chem. 2016; 1: 873-886Abstract Full Text Full Text PDF Scopus (0) Google Scholar, 101.Sarkisov L. et al.On the flexibility of metal–organic frameworks.J. Am. Chem. Soc. 2014; 136: 2228-2231Crossref PubMed Scopus (143) Google Scholar, 102.Krause S. et al.A pressure-amplifying framework material with negative gas adsorption transitions.Nature. 2016; 532: 348Crossref PubMed Google Scholar, 103.Carrington E.J. et al.Solvent-switchable continuous-breathing behaviour in a diamondoid metal–organic framework and its influence on CO2 versus CH4 selectivity.Nat. Chem. 2017; 9: 882Crossref PubMed Scopus (158) Google Scholar, 104.Fairen-Jimenez D. et al.Opening the gate: framework flexibility in ZIF-8 explored by experiments and simulations.J. Am. Chem. Soc. 2011; 133: 8900-8902Crossref PubMed Scopus (639) Google Scholar]. Energy-efficient separation processes are investigated in academia [105.Du Y. et al.New high-and low-temperature phase changes of ZIF-7: elucidation and prediction of the thermodynamics of transitions.J. Am. Chem. Soc. 2015; 137: 13603-13611Crossref PubMed Scopus (30) Google Scholar] and industry [106.Adil K. et al.Gas/vapour separation using ultra-microporous metal–organic frameworks: insights into the structure/separation relationship.Chem. Soc. Rev. 2017; 46: 3402-3430Crossref PubMed Google Scholar,107.Sholl D.S. Lively R.P. Seven chemical separations to change the world.Nature. 2016; 532: 435-437Crossref PubMed Scopus (932) Google Scholar]. The majority of MOFs are rigid compounds. Switchable MOFs with ultrahigh selectivity are receiving increasing attention for challenging separations [108.Yu J. et al.CO2 capture and separations using MOFs: computational and experimental studies.Chem. Rev. 2017; 117: 9674-9754Crossref PubMed Scopus (462) Google Scholar, 109.Bao Z. et al.Potential of microporous metal–organic frameworks for separation of hydrocarbon mixtures.Energy Environ. Sci. 2016; 9: 3612-3641Crossref Google Scholar, 110.Bobbitt N.S. et al.Metal–organic frameworks for the removal of toxic industrial chemicals and chemical warfare agents.Chem. Soc. Rev. 2017; 46: 3357-3385Crossref PubMed Google Scholar]. Switchable-framework compounds react selectively to external stimuli triggering changes in density, permeance, optical absorption, and/or magnetism. The physical response renders them efficient for sensing [111.Yanai N. et al.Gas detection by structural variations of fluorescent guest molecules in a flexible porous coordination polymer.Nat. Mater. 2011; 10: 787-793Crossref PubMed Scopus (297) Google Scholar, 112.Zhao X. et al.Metal–organic frameworks for separation.Adv. Mater. 2018; 30: 1705189-1705223Crossref PubMed Scopus (323) Google Scholar, 113.Schneemann A. et al.Different breathing mechanisms in flexible pillared-layered metal–organic frameworks: impact of the metal center.J. Mater. Chem. 2018; 30: 1667-1676Crossref Scopus (43) Google Scholar, 114.Freund P. et al.Switchable conductive MOF–nanocarbon composite coatings as threshold sensing architectures.ACS Appl. Mater. Interfaces. 2017; 9: 43782-43789Crossref PubMed Scopus (26) Google Scholar, 115.Freund P. et al.MIL-53 (Al)/carbon films for CO2-sensing at high pressure.ACS Sustain. Chem. Eng. 2019; 7: 4012-4018Crossref Scopus (14) Google Scholar, 116.Dong X.-Y. et al.A flexible fluorescent SCC-MOF for switchable molecule identification and temperature display.J. Mater. Chem. 2018; 30: 2160-2167Crossref Scopus (69) Google Scholar]. Additional applications include selective catalysis [117.Das R.K. et al.Direct crystallographic observation of catalytic reactions inside the pores of a flexible coordination polymer.Chem. Eur. J. 2012; 18: 6866-6872Crossref PubMed Scopus (89) Google Scholar,118.Yuan S. et al.Flexible zirconium metal-organic frameworks as bioinspired switchable catalysts.Angew. Chem. Int. Ed. 2016; 55: 10776-10780Crossref PubMed Scopus (121) Google Scholar], proton-conductive materials [119.Yang F. et al.A flexible metal–organic framework with a high density of sulfonic acid sites for proton conduction.Nat. Energy. 2017; 2: 877-883Crossref Scopus (241) Google Scholar], and CO2 separation [103.Carrington E.J. et al.Solvent-switchable continuous-breathing behaviour in a diamondoid metal–organic framework and its influence on CO2 versus CH4 selectivity.Nat. Chem. 2017; 9: 882Crossref PubMed Scopus (158) Google Scholar,120.Taylor M.K. et al.Near-perfect CO2/CH4 selectivity achieved through reversible guest templating in the flexible metal–organic framework Co(bdp).J. Am. Chem. Soc. 2018; 140: 10324-10331Crossref PubMed Scopus (0) Google Scholar]. Dynamic MOFs, featuring repeatable structural transformations, are attractive for research and many important applications [98.Férey G. Serre C. Large breathing effects in three-dimensional porous hybrid matter: facts, analyses, rules and consequences.Chem. Soc. Rev. 2009; 38: 1380-1399Crossref PubMed Scopus (1226) Google Scholar, 99.Bureekaew S. et al.Orbital directing effects in copper and zinc based paddle-wheel metal organic frameworks: the origin of flexibility.J. Mater. Chem. 2012; 22: 10249-10254Crossref Scopus (44) Google Scholar, 100.Evans J.D. et al.Origins of negative gas adsorption.Chem. 2016; 1: 873-886Abstract Full Text Full Text PDF Scopus (0) Google Scholar, 101.Sarkisov L. et al.On the flexibility of metal–organic frameworks.J. Am. Chem. Soc. 2014; 136: 2228-2231Crossref PubMed Scopus (143) Google Scholar, 102.Krause S. et al.A pressure-amplifying framework material with negative gas adsorption transitions.Nature. 2016; 532: 348Crossref PubMed Google Scholar, 103.Carrington E.J. et al.Solvent-switchable continuous-breathing behaviour in a diamondoid metal–organic framework and its influence on CO2 versus CH4 selectivity.Nat. Chem. 2017; 9: 882Crossref PubMed Scopus (158) Google Scholar, 104.Fairen-Jimenez D. et al.Opening the gate: framework flexibility in ZIF-8 explored by experiments and simulations.J. Am. Chem. Soc. 2011; 133: 8900-8902Crossref PubMed Scopus (639) Google Scholar]. Energy-efficient separation processes are investigated in academia [105.Du Y. et al.New high-and low-temperature phase changes of ZIF-7: elucidation and prediction of the thermodynamics of transitions.J. Am. Chem. Soc. 2015; 137: 13603-13611Crossref PubMed Scopus (30) Google Scholar] and industry [106.Adil K. et al.Gas/vapour separation using ultra-microporous metal–organic frameworks: insights into the structure/separation relationship.Chem. Soc. Rev. 2017; 46: 3402-3430Crossref PubMed Google Scholar,107.Sholl D.S. Lively R.P. Seven chemical separations to change the world.Nature. 2016; 532: 435-437Crossref PubMed Scopus (932) Google Scholar]. The majority of MOFs are rigid compounds. Switchable MOFs with ultrahigh selectivity are receiving increasing attention for challenging separations [108.Yu J. et al.CO2 capture and separations using MOFs: computational and experimental studies.Chem. Rev. 2017; 117: 9674-9754Crossref PubMed Scopus (462) Google Scholar, 109.Bao Z. et al.Potential of microporous metal–organic frameworks for separation of hydrocarbon mixtures.Energy Environ. Sci. 2016; 9: 3612-3641Crossref Google Scholar, 110.Bobbitt N.S. et al.Metal–organic frameworks for the removal of toxic industrial chemicals and chemical warfare agents.Chem. Soc. Rev. 2017; 46: 3357-3385Crossref PubMed Google Scholar]. Switchable-framework compounds react selectively to external stimuli triggering changes in density, permeance, optical absorption, and/or magnetism. The physical response renders them efficient for sensing [111.Yanai N. et al.Gas detection by structural variations of fluorescent guest molecules in a flexible porous coordination polymer.Nat. Mater. 2011; 10: 787-793Crossref PubMed Scopus (297) Google Scholar, 112.Zhao X. et al.Metal–organic frameworks for separation.Adv. Mater. 2018; 30: 1705189-1705223Crossref PubMed Scopus (323) Google Scholar, 113.Schneemann A. et al.Different breathing mechanisms in flexible pillared-layered metal–organic frameworks: impact of the metal center.J. Mater. Chem. 2018; 30: 1667-1676Crossref Scopus (43) Google Scholar, 114.Freund P. et al.Switchable conductive MOF–nanocarbon composite coatings as threshold sensing architectures.ACS Appl. Mater. Interfaces. 2017; 9: 43782-43789Crossref PubMed Scopus (26) Google Scholar, 115.Freund P. et al.MIL-53 (Al)/carbon films for CO2-sensing at high pressure.ACS Sustain. Chem. Eng. 2019; 7: 4012-4018Crossref Scopus (14) Google Scholar, 116.Dong X.-Y. et al.A flexible fluorescent SCC-MOF for switchable molecule identification and temperature display.J. Mater. Chem. 2018; 30: 2160-2167Crossref Scopus (69) Google Scholar]. Additional applications include selective catalysis [117.Das R.K. et al.Direct crystallographic observation of catalytic reactions inside the pores of a flexible coordination polymer.Chem. Eur. J. 2012; 18: 6866-6872Crossref PubMed Scopus (89) Google Scholar,118.Yuan S. et al.Flexible zirconium metal-organic frameworks as bioinspired switchable catalysts.Angew. Chem. Int. Ed. 2016; 55: 10776-10780Crossref PubMed Scopus (121) Google Scholar], proton-conductive materials [119.Yang F. et al.A flexible metal–organic framework with a high density of sulfonic acid sites for proton conduction.Nat. Energy. 2017; 2: 877-883Crossref Scopus (241) Google Scholar], and CO2 separation [103.Carrington E.J. et al.Solvent-switchable continuous-breathing behaviour in a diamondoid metal–organic framework and its influence on CO2 versus CH4 selectivity.Nat. Chem. 2017; 9: 882Crossref PubMed Scopus (158) Google Scholar,120.Taylor M.K. et al.Near-perfect CO2/CH4 selectivity achieved through reversible guest templating in the flexible metal–organic framework Co(bdp).J. Am. Chem. Soc. 2018; 140: 10324-10331Crossref PubMed Scopus (0) Google Scholar]. Because nanostructured MOFs are needed in many applications, it is essential to understand the effec