research update mechanical properties of metal organic frameworks influence of structure and chemical bonding

Research Update: Mechanical properties of metal-organic frameworks – Influence of structure and chemical bonding Wei Li, Sebastian Henke, , and Anthony K.. From the viewpoint of materia

Research Update: Mechanical properties of metal-organic frameworks – Influence of structure and chemical bonding

Wei Li, Sebastian Henke, , and Anthony K Cheetham

Citation: APL Mater 2, 123902 (2014); doi: 10.1063/1.4904966

View online: http://dx.doi.org/10.1063/1.4904966

View Table of Contents: http://aip.scitation.org/toc/apm/2/12

Published by the American Institute of Physics

Research Update: Mechanical properties of metal-organic frameworks – Influence of structure and chemical bonding

Wei Li,1, a , bSebastian Henke,1,2, a , band Anthony K Cheetham1

1Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS, United Kingdom

2Lehrstuhl für Anorganische Chemie II, Ruhr-Universität Bochum, 44801 Bochum, Germany

(Received 1 October 2014; accepted 11 December 2014; published online 30 December 2014)

Metal-organic frameworks (MOFs), a young family of functional materials, have been attracting considerable attention from the chemistry, materials science, and physics communities In the light of their potential applications in industry and technology, the fundamental mechanical properties of MOFs, which are of critical importance for manufacturing, processing, and performance, need to be addressed and understood It has been widely accepted that the framework topology, which describes the overall connectivity pattern of the MOF building units, is of vital impor-tance for the mechanical properties However, recent advances in the area of MOF mechanics reveal that chemistry plays a major role as well From the viewpoint of materials science, a deep understanding of the influence of chemical effects on MOF mechanics is not only highly desirable for the development of novel functional mate-rials with targeted mechanical response, but also for a better understanding of impor-tant properties such as structural flexibility and framework breathing The present work discusses the intrinsic connection between chemical effects and the mechanical behavior of MOFs through a number of prototypical examples C2014 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.[http://dx.doi.org/10.1063/1.4904966]

Metal-organic frameworks (MOFs) are a young class of extended solids, which are constructed viacoordination association between inorganic metal ions (or clusters) and organic linkers.15Due

to their promising applications in carbon capture,6gas separation,7catalysis,8,9sensing,10etc., the area of MOF research has become one of the fastest growing fields in materials science over the past

15 years In light of the imminence of their applications,11mechanical properties, which are critical

to the industrial manufacturing and processing, have to be taken into account The first work in the area was reported by Allendorf and co-workers in 2007,12and more extensive and systematic work has been carried out since then by the Cheetham group and other researchers.13 – 35 In 2011, Tan and Cheetham published a comprehensive review paper introducing the fundamental mechanical properties of MOFs and their intrinsic structure–property relationships.36

In general, MOFs exhibit more complex mechanical properties compared with traditional inorganic materials, such as zeolites and perovskites The variable chemical interactions within MOFs, ranging from strong coordination bonds to weaker dispersion forces and hydrogen bonding interactions, may result in significant structural flexibility in response to temperature, hydrostatic pressure, or uniaxial stress.22,24,37,38Some MOFs undergo reversible or irreversible amorphization when exposed to mechanical stress,39while others exhibit drastic pressure-induced phase transitions, associated with striking bond rearrangements.40,41Furthermore, the structural anisotropy of many MOFs often gives rise to large mechanical anisotropy and, correspondingly, unique mechanics For

a Electronic addresses: dearweili@gmail.com and shenke738@gmail.com

b W Li and S Henke contributed equally to this work.

2166-532X/2014/2(12)/123902/9 2, 123902-1 © Author(s) 2014

123902-2 Li, Henke, and Cheetham APL Mater 2, 123902 (2014)

instance, some MOFs have been reported to exhibit negative linear compressibility (NLC)42 – 44and massive positive or negative thermal expansion (PTE and NTE).45 – 48Considering that MOF research

is mainly dominated by synthetic chemists, a general introduction to the intrinsic links between chemistry and mechanics is of particular interest, as it will bridge the gap between chemistry and mechanical engineering In this mini-review, we will discuss the mechanical properties of MOFs from the viewpoint of structure and chemical bonding by presenting a number of prototypical examples to demonstrate the strong connection between chemistry and mechanical behavior Before focusing on how chemistry influences the mechanics of MOFs, we briefly discuss two other general factors First, the supramolecular mechanics of MOFs can be described and understood by analysis of their framework topology, which characterizes the motif of connectivity within a network; this is analogous to a scaffold in mechanical engineering.49For the mechanical properties, this is of major importance As an approximation, MOFs can be described by sim-ple structural models composed of vertices connected by edges Naturally, the connectivity of a framework, i.e., the number of edges connecting one vertex with others, determines the general mechanical properties of the framework Similar to classical mechanics used in civil engineering, higher connectivity often results in higher stiffness and hardness This has been demonstrated by density functional theory (DFT) calculations on a variety of prototypic cubic MOFs possessing

different topologies with distinct connectivity (coordination number) of their secondary building units (SBUs).50HKUST-1 (four connected square planar SBU), ZIF-8 (four connected tetrahedral SBU), and MOF-5 (six connected tetrahedral SBU) exhibit significantly lower Young’s, bulk and shear moduli than UiO-66 (twelve connected octahedral SBU)

The second factor that determines the mechanical behavior of MOFs is the framework geom-etry.51For a given topology, the geometry of the connections between nodes and links often depends

on the nature of the individual building units For example, a 2D 44 topology can either have a square-shaped (isotropic) or a rhomb-shaped (anisotropic) geometry, which could result in signifi-cantly different mechanical behavior in response to stress This has been well demonstrated by Ortiz

et al.via a computational investigation of the full elastic tensors of two geometrical isomers of the framework [Zn2(bdc)2(dabco)]n (DMOF = dabco-MOF; bdc = 1,4-benzenedicarboxylate, dabco

= 1,4-diazabicyclo[2.2.2]octane) DMOF has a winerack-like structure based on [Zn2(bdc)2]n lay-ers with 44 topology, which are pillared by dabco ligands to form a 3D porous framework.52 This porous material can feature a square-shaped (DMOFsquare) or a rhomb-shaped geometry (DMOFrhomb) depending on adsorbed guest molecules, while the topology (i.e., the connectivity

of the building units) of the network is preserved (Fig 1top).53 DFT calculations reveal marked mechanical differences between these two isomers (Fig.1bottom) DMOFsquarehas a more isotropic nature, displaying positive linear compressibility (PLC) with similar magnitude along all three crys-tallographic directions ( βx= 23 TPa−1, βy= 12 TPa−1, βz= 12 TPa−1) In contrast, the distorted DMOFrhombfeatures extreme PLC and NLC ( βx= −623TPa−1, βy = 23 TPa−1, βz= 1158 TPa−1) coupled with negative Poisson’s ratio The drastic differences obviously result from geometric ef-fects: DMOFrhombcan undergo a facile hinge motion upon compression, whereas DMOFsquareonly favors isotropic compression due to geometric frustration Other geometric factors (linker lengths, bond angles, etc.) can influence the framework mechanics in the similar way

Since variations in framework topology or geometry can often be achieved by simple alter-ations in chemical composition and synthetic conditions, chemical aspects encompass both the above effects and can influence mechanical properties of MOFs in a more comprehensive way The chemistry of the molecular building units (MBUs) determines the supramolecular structure of a MOF (i.e., topology and geometry), and thus also characterizes the framework mechanics In 2012, the concept of mechanical building units (XBUs), which allows for the qualitative estimation of a framework’s mechanical response based on the mechanical nature of the individual building blocks, was introduced by Ogborn et al.54 Based on strong (covalent or coordinative bonds) and weak chemical interactions (hydrogen bonds, dispersion forces), XBUs can be analyzed and their rigidity and flexibility estimated Some XBUs are prone to show stronger bending or flexing than others and, hence, the incorporation or exchange of XBUs with different mechanical nature will result in

different framework stiffness In the following paragraphs, we will discuss the critical importance of various chemical effects for MOF mechanics based on four prototypical examples, focusing on the

FIG 1 Top: Views on the crystal structures of the square-shaped and the rhomb-shaped geometric isomers of DMOF shown along the dabco axis Zn, O, N, and C atoms are shown in yellow, red, blue, and gray H atoms are omitted for clarity Bottom: 3D surface representation of the linear compressibility of DMOF square and DMOF rhomb PLC and NLC are shown in green and red The representation on the left (axis length is 30 TPa −1 ) is enlarged by a factor of 50 with respect to the right one (axis length is 1500 TPa −1 ) Adapted with permission from Ortiz et al., Phys Rev Lett 109(19), 195502 (2012) Copyright

2012 by the American Physical Society.

influence of XBUs as well as guest-framework interactions, to give the reader a concise introduction

to the intrinsic relationship between framework chemistry and mechanics

The first example shows how the mechanical properties of winerack frameworks can be influenced significantly via simple building block exchange.55 MIL-53 is constructed from one-dimensional M-OH-M pillars (M= Al, Cr, Fe, Ga, In) composed of corner sharing metal-oxo-octahedra, which are interconnected by linear bdc linkers to form a winerack-like framework Im-portantly, the framework is very compliant and allows hinging around the metal-carboxylate bonds Exchanging half of the M-OH-M pillars in MIL-53 with a 1,4,5,8-naphthalenetetracarboxylate re-sults in MIL-122, having the identical network topology as MIL-53 (Fig.2) In comparison to the flexible four-connected metal-carboxylate unit, the tetracarboxylate is more rigid and does not allow flexing around this network node.55

Consequently one half of the XBUs of the MIL-122 framework are non-compliant, while the others are the same as in MIL-53 As a consequence of the spatial arrangement of compliant inor-ganic vertices and non-compliant orinor-ganic vertices, the overall mechanical nature of MIL-122 differs significantly from MIL-53, resulting in considerably higher and more isotropic Young’s moduli for MIL-122 compared to the highly anisotropic elastic nature of MIL-53

Besides the exchange of flexible inorganic XBUs for rigid organic ones, a simple varia-tion of the metal centers of a framework can markedly influence the stiffness of the material Tan et al studied the elastic properties of a series of four isomorphous formate frameworks [dma][M(HCOO)3]n (dma= dimethyl ammonium, M = Mn, Co, Ni, Zn) with the ABX3-type perovskite architecture in which the A, B, and X sites are occupied by the dma cations, M2+,

and formate ligands, respectively The nanoindentation results revealed significant variations in the Young’s modulus (E) depending on the metal center (Fig 3).56 Interestingly, the trend in mechanical stiffness cannot be ascribed to the different ionic radii of the four metal ions Mn2 +and

Zn2+feature markedly different ionic radii (0.82 Å for Mn2 +, 0.75 Å for Zn2 +) but their Young’s

moduli along the (012) direction are nearly identical (E ≈ 19 GPa) The Co (E ≈ 22 GPa) and Ni (E ≈ 25 GPa) perovskite frameworks, however, are significantly stiffer The authors found that the

differences in framework stiffness correlate well with the ligand field stabilization energy (LFSE)

of the octahedrally coordinated divalent metal centers The LFSE of Mn2+(3d5) and Zn2+(3d10) is zero due to the half filled and fully filled 3d electron configurations, while those of Co2 +(3d7) and

123902-4 Li, Henke, and Cheetham APL Mater 2, 123902 (2014)

FIG 2 Center: Sketch of the compliant winerack-like MIL-53(Al) and the non-compliant winerack-like MIL-122(In) Right: Directional Young’s modulus of MIL-53(Al) (large pore form) and MIL-122(In) as a 3D surface representation (axes tick labels in GPa), reproduced with permission from Ortiz et al., J Chem Phys 138(17), 174703 (2013) Copyright 2013 by the American Institute of Physics.

Ni2 +(3d8) are 71.5 and 122.6 kJ mol−1, respectively Elastic deformation along the (012) direction includes distortions of the formate linker, the M-O-C bridging angle, and the MO6octahedra For the four isostructural frameworks, all of these contributions are expected to be identical, except for the MO6octahedra The high LFSE of Ni2+and Co2 +suggests a higher resistance against distortion

of the MO6octahedra, which thus gives rise to increased framework stiffness It is quite remarkable that these electronic effects can play a more important role for the mechanical properties of MOF materials than the metal ion radii Given that many MOF families include a series of isostructural

FIG 3 Left: Representative load-depth curves for [dma][M(HCOO) 3 ] n (M = Mn 2+, Co2 +, Ni2 +, Zn2 +) with the (012)

orientated facets measured by a Berkovich tip Inset: Young’s moduli as a function of indentation depth Right: Correlation

of the LFSE to the Young’s modulus of the [dma][M(HCOO) 3 ] n frameworks Reproduced with permission from Tan et al., Dalton Trans 41(14), 3949-3952 (2012) Copyright 2012 by the Royal Society of Chemistry.

FIG 4 Top: Representation of the crystal structures of di fferent NO 2 -DMOF ⊃guest derivatives viewed along the dabco axes.

Zn polyhedra, O, N, and C atoms are shown in yellow, red, blue, and gray The dabco-linker plane is shown in cyan Bottom: Directional Young’s modulus as a function of indentation depth for the di fferent guest containing forms of NO 2 -DMOF determined via nanoindentation Reproduced with permission from Henke et al., Chem Sci 5(6), 2392 (2014) Copyright

2014 by the Royal Society of Chemistry.

compounds based on different metal centers, it is, in principle, possible to fine tune the mechanical nature of these MOFs by selecting different transition metals with the desired electronic properties Many MOFs have pores, which can host a variety of guest molecules (e.g., solvents, gases) Though these guests are not part of the extended framework skeleton, it has been shown that they can affect the mechanical behavior (e.g., compressibility) of the respective MOF quite dras-tically.30 , 32We have recently demonstrated that this is particularly true for flexible and responsive MOFs, which are able to change their structure reversibly upon adsorption of different guests.57By analogy with the previously discussed [Zn2(bdc)2(dabco)]n framework, [Zn2(NO2-bdc)2(dabco)]n

(NO2-DMOF; NO2-bdc= 2-nitro-1,4-benzenedicarboxylate) undergoes distinct changes in network geometry upon exchange of guest molecules (Fig 4 top) As-prepared NO2DMOF hosts N ,N -dimethylformamide (DMF), which can be exchanged by immersion of the crystal in other solvents, for example, dimethylsulfoxide (DMSO), mesitylene, or toluene Therefore, the structure trans-forms in a single-crystal-to-single-crystal fashion from a rhomb-shaped network (DMF guests) to

a tetragonal network featuring bent (DMSO) or straight linkers (mesitylene, toluene) Notice that the guest-triggered structural response of NO2-DMOF is deemed to rely on weak guest-framework interactions and is mainly restricted to the two-dimensional [Zn2(NO2-bdc)2]n layers, while the stacking along the dabco axis is unaffected

Nanoindentation along the dabco-linker plane (shown in cyan in Fig.4) reveals large changes

of the directional Young’s modulus of NO2-DMOF depending on the adsorbed guest (Fig.4 bot-tom) The rhomb-shaped framework, NO2-DMOF⊃dmf, where the NO2-bdc linker is tilted by about 6◦to the indenter axis, features the lowest Young’s modulus of only 2.1(1) GPa, while the square-shaped tetragonal frameworks NO -DMOF⊃mesitylene and NO-DMOF⊃toluene, with the

123902-6 Li, Henke, and Cheetham APL Mater 2, 123902 (2014)

FIG 5 Crystal structures of MOFP-1 (left) and MOFP-2 (right) viewed normal to (101) Color scheme: Mn 2+, green, light

blue, or teal; O, red; C, gray or black; N, blue; H, white N–H· · ·O bonds are represented as dotted purple lines Hydrogen atoms of formates are omitted for clarity Note: The azetidinium in MOFP-2 is equally disordered at two positions Adapted with permission from Li et al., J Am Chem Soc 136(22), 7801-7804 (2014) Copyright 2014 by the American Chemical Society.

NO2-bdc linker aligned parallel to the indenter axis, have Young’s moduli of 6.1(2) and 6.5(2) GPa, respectively NO2-DMOF⊃dmso, having bent linkers and a tetragonal structure, features a modulus

of 3.7(2) GPa and lies in between the other two geometries It is noteworthy that the Young’s modulus along the direction parallel to the dabco axis varies only slightly with the guest content These results confirm that the adsorption of different guests can alter the stiffness of such flexible MOFs dramatically because their guest-responsive nature gives rise to different network geometries, which hence possess distinct mechanical properties These large variations of the elastic modulus also suggest significant variations in other mechanical properties (e.g., thermal expansion behavior, compressibility) of these guest-responsive flexible frameworks

In the same way that non-directional guest-framework interactions can influence the mechan-ical behavior of flexible MOFs, strong and directional hydrogen bonding interactions can also have an impact on MOF mechanics Li et al reported that the mechanical properties of a fam-ily of structurally similar MOF perovskites, [gua][Mn(HCOO)3]n(MOFP-1, gua= guanidinium), and [aze][Mn(HCOO)3]n (MOFP-2, aze= azetidinium), are strikingly dependent on the different hydrogen bonding modes between the anionic manganese-formate framework and the organic ammonium cation hosted in the perovskite cavities.58As seen in Fig.5, MOFP-1 and MOFP-2 have similar pseudocubic unit cells, though they crystallize in different orthorhombic space groups Nanoindentation measurements were performed along the three pseudocubic axes of the two frameworks, namely, normal to the (010), (101), and (10-1) planes (Fig.6) Strikingly, the Young’s moduli of MOFP-1 (E ≈ 24-29 GPa) are about twice as high as those of MOFP-2 (E ≈ 12-13 GPa)

As both hybrid perovskites possess the identical anionic [Mn(HCOO)3]−framework, the substantial mechanical contrast must be attributed to their distinct hydrogen bonding modes As seen in Fig

5, each gua+ in MOFP-1 is tilted 56.7(4)◦ to the (101) plane, and cross-links three alternative edges of the pseudo-cubic unit cell via six hydrogen bonds to give strong bridging constraints

in three dimensions In MOFP-2, however, each aze+ is oriented normal to the (101) plane and

hydrogen-bonded to the two opposite formate ligands within the same (101) face by four N–H· · ·O bonds These four hydrogen bonds add only a few cross-linked constraints to one face of the pseudo-cubic unit cell As a result of the significantly different hydrogen bonding constraints in these two frameworks, MOFP-1 can resist much larger elastic and plastic deformation, hence giving higher Young’s moduli and hardnesses DFT calculations, high-pressure, and variable temperature single-crystal X-ray studies support the above conclusions For instance, the thermal expansion of the weakly hydrogen-bonded framework, MOFP-2, is about five times higher than those of the strongly hydrogen-bonded MOFP-1 In marked contrast to the significant mechanical modulation

FIG 6 Nanoindentation data for (010), (101), and (10-1) oriented facets of MOFP-1 and MOFP-2 single crystals measured using a Berkovich tip Left: representative P − h curves; Right: elastic moduli as a function of indentation depth Reproduced with permission from Li et al., J Am Chem Soc 136(22), 7801-7804 (2014) Copyright 2014 by the American Chemical Society.

based on the nature of the A-site cation (only ∼15% size difference between gua and aze) in the MOF perovskites,59 – 62 the ∼22% radius difference of the A-site metal ion in perovskite oxides, GdAlO3, and ScAlO3, leads to only ∼15% variation of the elastic modulus This work presents further evidences that the mechanical properties of MOFs can be fine tuned via guest-framework interactions through simple chemical modification.15

Another way to tune and enhance the mechanical properties of MOFs is the preparation of MOF-based composites to exploit the synergistic interactions between them and inorganic nano-materials This was demonstrated recently by Kumar et al., who have shown that it is possible to preserve the functionality of the MOFs while enhancing the mechanical properties considerably in MOF− boron nitride (BN) nanocomposites by growing MOF particles on the BN nanosheets.63This avenue offers exciting possibilities for discovering new materials that combine functionalities of several different constituents.64

In this mini-review, we have summarized some of the recent advances in MOF mechanics with the purpose of attracting more attention from the broad materials community As strain is coupled to all physical properties of a material due to its perturbation on the intrinsic structure, the connection between chemistry (including structure) and mechanics is far more apparent than might have been expected There are many interesting phenomena with respect to MOFs which can only be addressed properly by the combined approach of both chemistry and mechanics, such as mechanoluminescence,65 negative compressibility and thermal expansion,26magnetoelastic coupl-ing,66 and framework breathing and gate opening.67,68Moreover, the interplay between chemistry and mechanics of MOFs can offer great opportunities to realize optimal MOF performance, namely, combined functionality and processability, targeted for industrial applications Recent advances in mechanical testing techniques, especially the utilization of nanoindentation techniques that allow for probing mechanical properties of small volume materials,69 , 70 and increased computational power needed for precise calculations of such properties, are facilitating major advances in this interdisciplinary field We hope this mini-review will not only bridge the gap between the materials chemistry and materials engineering sub-communities of the MOF field, but also encourage more MOF chemists to collaborate with materials scientists and engineers to examine their materials from

a new perspective

W.L acknowledges the Engineering and Physical Sciences Research Council for providing financial support S.H thanks the Alexander von Humboldt Foundation for a Feodor Lynen Fellow-ship

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