Chemistry of Metal-organic Frameworks Monitored by Advanced X-ray Diffraction and Scattering Techniques

The research on metal-organic frameworks (MOFs) experienced rapid progress in recent years due to their structure diversity and wide range of application opportunities. Continuous progress of X-ray and neutron diffraction methods enables more and more detailed insight into MOF’s structural features and significantly contributes to the understanding of their chemistry. Improved instrumentation and data processing in high-resolution X-ray diffraction methods enables the determination of new complex MOF crystal structures in powdered form. By the use of neutron diffraction techniques, a lot of knowledge about the interaction of guest molecules with crystalline framework has been gained in the past few years. Moreover, in-situ time-resolved studies by various diffraction and scattering techniques provided comprehensive information about crystallization kinetics, crystal growth mechanism and structural dynamics triggered by external physical or chemical stimuli. The review emphasizes most relevant advanced structural studies of MOFs based on powder X-ray and neutron scattering.


Introduction
Metal-organic frameworks (MOFs) are a class of rapidly developing class of nanoporous materials.They are composed of metal-based building units coordinated to organic bridging ligands to form a three-dimensional network with uniform pore system including channels and cages with the openings typically ranging from 3 to 20 Å. [1][2][3][4][5] In the last two decades, MOFs experienced remarkable progress in the structural engineering, characterization and application due to their enormous structural versatility and unique chemical and physical properties.One of the most important advantageous features of MOF is the possibility to construct its frameworks by combining selected ligand and coordination geometry of metal-based building blocks.][8][9][10][11] Specific three-dimensional connectivity of inorganic and organic units enables the formation of MOF architectures with low framework densities (from 0.2 g/cm 3 ), high void volumes (up to 90%) and unprecedented internal specific surface areas (6000 m 2 /g).3][14][15] Very intriguing feature of MOF structures represent the presence of coordinately unsaturated metal sites (CUS) which can either be part of the as-synthesized framework or can be generated by post-synthesis modification process.7][18] Unique structural property that MOF frameworks often possess is their flexibility.MOF structures can respond to different external physical or chemical stimuli in a controlled manner. 19Structure dynamics can be for instance triggered by adsorption/desorption processes, temperature, magnetic field, photochemical or mechanical stimuli.These phenomena enable MOFs implementation in sensor applications as well.
The in-depth studies of MOF's structural features are crucial to enhance the performance in adsorption, separation and catalysis or to provide additional functionalities and thus widening their application opportunities.The diffraction and scattering methods are the most widely used for such purposes and offer different insights into the structure-property relationship of MOFs.Moreover, the increasing demands to understand their crystallization and crystal growth mechanisms which would enable more controllable and rational design, induces the development of in-situ diffraction and combined-diffraction techniques.
High-resolution powder X-ray diffraction is most frequently used technique for structure evaluation of MOFs.Even though traditional single-crystal data (SC-XRD) are always preferred for the high-resolution crystal determination of MOFs, the sufficiently large and undamaged single crystals required for such measurements are much more rarely available than the microcrystalline materials.Many improvements in instrumentation and data processing have been achieved to approximate the quality of PXRD data towards SC-XRD structure analysis.Synchrotron radiation with high brilliance, tunable beam energy and flexibility of optical setup that can be available at beam-lines enables better diffraction data which and significantly improves structure analysis of MOFs.1][22][23] This enables not only quick receiving of the data required for crystal structure determination but also detection of short-living metastable phases, monitoring of structure dynamics and performing kinetic studies of MOF formation.Instrumentation progress naturally goes hand in hand with the data processing evolvement as well.There are few recent breakthrough improvements regarding XRPD data processing of MOF materials.For instance, charge-flipping algorithm employed for PXRD structure solution addresses the issue of Bragg reflection overlap by their spherical averaging. 24The advantage over the conventional Le-Bail procedure is that the input of symmetry and chemical composition is not required.Structure difference envelope density map analysis (DED) enables the study of guest molecule inclusion within the MOF crystalline framework. 25The procedure takes into an account the difference between the observed and calculated envelope densities generated from the series of most intense low index reflections.Another, very powerful method for studying materials with limited or no long-range order, which has been increasingly used for total scattering data processing in last decade, is pair distribution function (PDF) analysis. 26The method evaluates the probability of finding two atoms at defined interatomic distances using free Fourier-transform of PXRD data.Developing technique in the field of PXRD methodology, which also needs to be mentioned here, is microdiffraction.Sophisticated optic setup can provide bright and highly focused incident beam with the area below 1 μm 2 .Such focused beam in-creases the signal-to-noise ratio of diffraction patterns of microsized polycrystalline samples.The setup enables the evaluation of orientation, strain mapping, crystallite orientation or even crystal imperfections.
Small-angle X-ray scattering (SAXS) techniques are more and more used in the field of MOF science.The basics of SAXS differs from the conventional X-ray diffraction from the fact that collimated X-ray beam interacts with the structure species having much larger dimensions than the wavelength of radiation.Scattering angles which are detected on extended sample-to-detector distances are in a narrow region between 0.1-10 °.SAXS pattern expanded through the q min -q max region provides information about nanoparticle size, shape and porosity.When the scattering angles (or q regions) are expanded out of the mentioned region, other variations of SAXS are used.The measurements in the scattering region below 0.1 ° is referred as ultra-small-angle X-ray scattering (USAXS), whereas wide-angle X-ray scattering (WAXS) includes data measured above 10 °.The development of in-situ measurement approaches using micro-beam setup with the beam size down to 10 μm enables the determination of nucleation kinetics of MOFs on sub-millisecond time with the spatial distribution having high statistical accuracy and wide-scale hierarchical structure (inhomogeneity) at local region. 27Grazing-incidence X-ray small angle scattering (GI-SAXS) or more generally grazing-incidence X-ray diffraction (GIXD) also represents an advanced technique which can be used for the studies of MOF thin film growth and their surface properties.
Neutrons radiation is scattered on crystalline materials in a similar manner as X-rays, but give complementary information to XRD due to the different scattering properties.In contrast to XRD, where X-rays scatter on electrons, neutron scattering (NS) or neutron diffraction (ND) techniques are based on interaction on nucleus.Scattering powers are therefore not dependent on Z-values but are sensitive to each isotope.Hydrogen and deuterium atoms possess comparable or even stronger scattering power as heavier atoms.In addition hydrogen has a negative scattering length, whereas deuterium has a positive one.This makes the two isotopes well distinguishable.Information extracted from ND techniques is very useful for MOFs structure-property relationship investigations.In last two decades, MOF community has been intensively devoted to hydrogen adsorption due to MOF's high potential for hydrogen storage.9][30][31] Similarly, adsorption processes of other hydrogenous hosting molecules within MOFs were studied as well.Additionally, neutrons scatters on magnetic moments of nucleus, thus ND (particularly small angle neutron scattering or SANS) methods can be used for probing magnetic structure features of MOFs.Herein, the recent advances on structural-property investigations of rapidly growing field of metal-organic framework science with the focus on advanced elastic scattering and diffraction techniques are overviewed, including studies of crystallization, structural dynamics and guesthost interactions The emphasis are on the use of advanced powder diffraction and scattering approaches on the selected cases which most significantly contributed to the better knowledge of specific MOF physical-chemical properties.

Crystallization and Crystal Growth Studies
The fundamental knowledge of the MOF crystallization mechanisms is highly important in order to optimize the morphological and crystal growth control, as well as the control over the morphology.The studies of MOF crystallization mechanism are mostly focused on the local structure of species which are present in the solutions prior to the appearance of nanocrystals using different spectroscopic techniques (XAS, NMR).On the other hand, the examination of crystal growth in over-all length scale is important as well to build up the whole picture of crystallization process.Naturally, the crystallization mechanisms and the dynamic of the MOF's crystal growth cannot be generalized but are rather specific for each system.Several systems have been thoroughly investigated by means of diffraction techniques to elucidate the mechanisms and crystal growth dynamic of metal-organic frameworks structures.The recent investigations of crystallization mechanisms on MOFs based on the diffraction techniques are summarized in the Table 1.
3][34][35] When using Zn-nitrate as metal precursor, the ZIF crystallizes through the metastable semicrystalline-to-crystalline transformation following the Avrami's kinetic regime in the excess of the ligand. 32In the presence of basic Zn-carbonate instead of nitrate precursor however, the coexistence of ZIF-8 nanocrystals and nanosized ZnO wurtzite phase in the early stages of crystallization was proved. 33Cravillon et al. studied the nucleation at early growth events (in a second timescale) of ZIF-8 and identified the formation and gradual disappearance of pre-nucleation clusters, suggesting their involvement in the formation and growth of nanocrystals (Figure 1a). 35The above mentioned findings were limited to the crystallization mechanisms in methanol.Low et al. suggested different mechanism in the presence of other solvents (water, dimethylformamide, ethanol, etc.) which includes the phase transformation from two-dimensional ZIF-L phase (Figure 1b). 36These examples demonstrate the diversity of crystallization mechanisms which strongly depend on the starting synthesis parameters.Such in-depth studies are often possible only with the use of complementary diffraction techniques.The advantage of MOF crystallization is that they can be formed mechanochemically with the absence or with only small amount of solvent.Kinetics and mechanisms of such crystallization obviously significantly differ from the solvothermal processes.However, monitoring of mechanochemical transformations represents a big challenge.A breakthrough in this field was made by the studies of ZIFs crystallization by in-situ high-energy X-ray diffraction, where metastable intermediates and interconversions of frameworks were determined. 37,38Mechanochemical methods are more and more used even for up-scaled MOF syntheses, due to the energy and environmental efficiency of the process.In-situ monitoring of mechanochemical MOFs formation by X-ray diffraction is still relatively unexplored field of diffraction.With more intense development, the technique will certainly help to optimize synthesis conditions in specific environment of many other MOF interesting systems.
One of the most useful tool applied for crystallization investigations is time-resolved in-situ energy-dispersive Xray diffraction (EDXRD), providing the advantage of high intensity white beam X-rays which allow non-destructive penetration through reaction vessels under elevated temperature and autogenous pressures.EDXRD experiments for the purposes of crystallization studies were used for several MOF systems: Zr-fumarate, 39 Zr-terephthalate (UiO-66), 40 In-imidazolate, 41 Al-terephthalates (CAU-1), 42,43 Li-tiophenedicarboxylate, 44 MIL-100(Mn) 45 and CPO-27(Co,Ni). 46dditionally, the crystallization investigation using EDXRD is exampled on the Cu-benzene-1,3,5-tricarbocylate (HKUST-1) under solvothermal conditions. 47The structure crystallizes from homogenous DMF/EtOH solutions after no detectable induction period (Figure 2).The monitoring of crystallization at different temperatures (85-125 °C) enabled the elucidation of typical Avrami kinetic model suggesting that mechanism of crystallization is mostly controlled by the formation of nucleation sites.The trend of increased solvothermal stability with the time of crystallization suggests that HKUST-1 is thermodynamically stable structure.Similar study was performed on Cubenzenetrisbenzoate (MOF-14) where the kinetics of the crystallization was fitted with Gualtieri model. 48In contrast with HKUST-1, the crystallized MOF-14 gradually decomposes to Cu 2 O at 130 °C.Even though both systems possess the same local coordination environment and ligand geometry, MOF-14 seem to be less stable indicating that the size of the ligand obviously governs the thermal stability.
Millange et al. used EDXRD technique for the investigation of Fe(III)-terephthalate (MIL-53) crystallization from clear DMF solutions as well. 47The formation of MIL-53(Fe) undergoes the formation of intermediate phase (MOF-235).This phase occurs with no induction period and completely transforms to MIL-53 at 150 °C after 30 min and has a longer lifetime at lower temperatures of crystallization (more than 6h at 100 °C).Since both phases do not have any similarities in building unit features, the solid-state rearrangement most likely occurs via dissolving and release of reactive species and final MIL-

Crystal Structure Determination
The understanding of the details of the MOF crystal structure is a prerequisite for the explanation of its chemical and/or physical behavior and the prediction of their applicable properties.Therefore, the crystal structure determination of the newly synthesized MOF product is the first step before further exploration of its physical or chemical properties.Structure determination from single-crystal data is rather a straightforward process however the dimensions and the quality of the formed MOF crystals are often insufficient for single-crystal X-ray analysis.Therefore, one must rely on the structure determination based onpowder X-ray diffraction data, which is much more challenging due to the loss of data caused by peak overlap.2][53][54][55][56][57][58][59][60][61][62] Herein, the more nonconventional strategies and improvement overcoming the challenges of the X-ray powder diffraction structure determination of MOFs will be overviewed.
The improved methodologies of data processing included the use of charge-flipping instead of direct methods to determine the structures 1,2,3-triazolates based on different divalent metal cations. 63With the topological approach, the direct space solution methods providing the information about the rigid components of the structure were successful to solve the structures of nickel(II) polypyrazolyl-based MOFs. 64The phenomena of isoreticularity of some MOF networks enabled the successful employment of this method to solve the MOF structures with the expanded ligands (IRMOF series). 65,66Takashima et al. suggested the possibility of using the conventional X-ray diffractometer data to explore the structure of isotopical frameworks with modified ligands by the analysis of the obtained electron density maps. 67n recent years, the progress regarding the diffraction method instrumentation was made as well.Volkringer et al. used microdiffraction setup with the microfocused beam of 1 μm to determine the crystal structure of the microsized Al-benzene-1,3,5-tricarboxylate (MIL-110) with weak scattering factors due to the presence of light elements and very low structure density. 68However, successful crystal structure determination based on the data obtained from microdiffraction setups is at present still in large extend limited by the loss of microbeam intensity and the loss of crystallinity under highly focused beams.On the other hand, the crystal structure of bimetallic [Ca(H 2 O) 6 ].
[CaGd(oxydiacetate) 3 } 2 ].4H 2 O was refined using approach of separate dataset extraction to avoid the loss of crystalline integrity due to the X-ray damaging. 69The special data processing for the MOF with heavy atoms and high symmetry produced the model with accuracy comparable with the one obtained from single-crystal-based data.Lau et al. developed a method for high-through put synchrotron radiation powder X-ray diffraction (HTP-SR-PXRD) data mining (PLUXter) and applied for the generation of library of Gd-based MOFs. 70lectron crystallography, combining the electron diffraction and high-resolution TEM imaging offers a promising way to overcome the disadvantages of powder diffraction techniques for the structure solution analysis.However, this approach becomes very challenging for investigations of beam-sensitive materials such as MOFs.Some attempts of solving the MOF structures have been successful by using 3-D rotation electron diffraction (RED) or automated diffraction tomography (ADT).Zn-benzimidazolate (ZIF-7) was used as a model structure to proof the feasibility of the RED method on MOFs performed at -90 K to avoid the sample damage. 71The Zn and N atoms could be positioned using RED data, whereas C atoms were additionally inserted according to geometry of the imidazole ligand.ADT was employed for structure solutions of Cu-benzene-1-3-5-tricarboxylate (HKUST-1), 72 Zn-BTDD (MFU-4, BTDD = bis(1H-1,2,3-triazolo-[4,5b],[4,5-i]dibenzo [1,4]dioxin), 58 Bi-benzenetrisbenzoate (CAU-7) 73 and Zr-terephthalate (UiO-66) 74 .These methodologies can only be applied for structure determination of very limited assortment of highly stable MOF systems with the strong support of complementary methods.

Structure Defects and Framework Voids
The study of the defects within the crystalline MOF frameworks is important since it can provide important information on crystal growth mechanisms, and their occurrence can significantly influence on MOF's performances due to the changes in diffusion properties, generation of additional accessible sorption or catalytic sites, establishment of hierarchical architectures, strains, etc. Heterogeneity is often deliberately generated within the MOF structures in order to enhance the sorption or catalytic performances.This can be achieved by using mixed ligands or mixed metal precursors in the starting reaction mixtures or by post-synthesis acid treatment. 75The insight on defects within the frameworks by diffraction methods however is limited due to their random occurrence.
Structural disorder within the MOF frameworks has been studied for ZIFs by peak shape analysis of powder diffraction data and single-crystal diffuse scattering. 76By high-resolution neutron diffraction it was shown that UiO-66(Zr) framework contains a significant amount of vacancies due to the missing ligands. 77The concentration of vacancies can be tuned by using the acetic acid modulator and thus manipulate the porosity properties of the MOF.Study of ligand vacancies were performed on UiO-66(Zr) and UiO-67(Zr) by synchrotron single-crystal X-ray diffraction of as well. 78Moreover, with the use of diffuse scattering, electron microscopy, anomalous X-ray diffraction and pair distribution function, Clife et al. showed on the case of UiO-66(Hf) that the defect nanoregions within the frameworks do not occur just in random manner, but are correlated between their selves and can even be controlled (Figure 4). 79The mesopore voids and defects generated by the metal-ligand-fragment co-assembly approach using ligands with various substituent groups within Cubased PCN-125, NU-125 and HKUST-1 frameworks was monitored by powder XRD data. 80,81The ordered vacancies within Zn-based pyrazole-carboxylates generated by metal and ligand elimination reactions were elucidated by PXRD analysis as well. 82As it is indicated by the described examples, the diffraction/scattering methodologies and data processing enable evaluation of inhomogeneity and irregularity within the crystal frameworks only to limited extend, this is, if the imperfection domains still shows some degree of ordering or correlation between them.In opposite cases specific spectroscopic methods are probably more appropriate (e.g solid-state NMR or XAS).

Guest Molecule Interactions
Sorption of molecules (particularly gas molecules) on MOFs is one of the most frequently studied phenome-na among the MOF community.In terms of suitability for applications, the gas capture efficiency of MOFs is conditioned by their adsorption capacities, adsorption selectivity and the ability of gas storage at mild conditions.Whereas sorption capacity is mainly dependent on the pore properties (dimensions and shape), the selectivity is in large extent governed by host molecule-to-MOF framework interactions.The knowledge about the nature of the sorption sites within the framework provides an understanding of the interactions which are established upon adsorption and it is important to design the materials with optimal adsorption and separation performances. 83KUST-1 represents a proper platform for crystallographic studies of interactions of guest molecules to framework due to the presence of unsaturated Cu sites.The low pressure adsorption of H 2 on Cr-based HKUST-1 material was studied by neutron powder diffraction (NPD) and inelastic neutron scattering (INS). 84NPD was performed by deuterium loading of 0.5-3 D 2 per Cr 2+ site at 4 K. Surprisingly, the binding of D 2 with open Cr 2+ metal sites preferentially occurs only at higher deuterium loading (from 1.0-1.5 D 2 per Cr 2+ site), whereas at lower loadings D 2 remains located in the apertures of the octahedral cages.6][87][88] The importance of open metal sites for CH 4 adsorption was showed by Getzschmann et al. 89 CH 4 is adsorbed within HKUST-1 only via two preferential adsorption sites, as it was elucidated by NPD.First type of sites represents open Cu-sites providing strong Coulomb interactions and the second type are defined as 'pocket sites' (small cages and the openings to these cages) providing van der Waals interactions with the framework. 90The insight on the guest-framework interactions of the variety of solvent molecules incorporated within HKUST-1 was gained using in-situ SCXRD (Figure 5). 91Guests reside in the smallest pores accessible to them.The occupancy of the guest molecules within the pores is governed by competitive guest-guest and hydrogen-bonding interactions.
Hydrophilic guests interact preferentially with the open Cu sites of the framework.The number of coordinated guests is dependent on steric interactions between neighboring bonded guests and guest flexibility.Guest coordination at the Cu sites was found to have a significant effect on the framework structure.Preferred binding sites were investigated for the adsorption of noble gases within HKUST-1 as well. 92The diffusion of long-chain alkanes (C 9 -C 16 ) within the MIL-47(V) channels was investigated with the combination of quasi-elastic neutron scattering (QENS) measured at 300-370 K and molecular dynamic simulations (Figure 6).The diffusivities of the hosting molecules are significantly higher in comparison with zeolitic Silicalite-1 system and the diffusivity rates decrease in the non-monotonic manner with the increase of chain length. 93pecific CO 2 adsorption sites were investigated on the rigid MOF-74(Mg) which exhibits one of the highest CO 2 sorption capacities, where the population of a second CO 2 layer was evidenced by NPD. 94Preferential binding with open Ni-or Co-sites of biologically active H 2 S and NO gases within the Ni-based MOF-74 analogue was determined by XRPD. 95,96orption sites for H 2 were investigated by NPD within desolvated Cu-based NOTT-112 materials possessing  97 Surface adsorption of liquid-like H 2 at 50 K was monitored by NPD.H 2 molecules form a loosely bonded condensed state which is above the critical temperature.Short-range ordering of the H 2 molecule within the pores of MOF-205 was indicated resembling the liquid state in spite of the physical conditions where liquids should not exist. 98Inelastic neutron scattering (INS) was used to evaluate the binding sites for H 2 on DMA-rho-, Li-rho-and Mg-rho-ZMOFs. 99At low H 2 loadings, all materials show at least 4 specific binding sites for H 2 .Slightly stronger metal-to-H 2 interaction was found in the case of Li-rho-ZMOF material in comparison to Mg-analogue due to the more open tetrahedral geometry of Li + versus octahedral environment found around Mg 2+ and potentially higher electrostatic field in the cavities in the case if Li-rho-ZMOF.
With NPD investigations the CH 4 -to-framework primary interactions are associated to the organic ligands and the inorganic oxo-clusters in the cases of ZIF-8(Zn) and MOF-5(Zn) respectively.Methane molecules on these primary sites possess well-defined orientations.With higher methane loading, extra methane molecules populate the secondary sites and are confined in the framework. 100referred adsorption sites for xenon were investigated for MFU-4l material by X-ray powder diffraction measured at 110 and 150 K. 101 The reconstruction of the electron density distribution was performed using the maximum entropy method to localize the adsorbed Xe.At 110 K, Xe atoms occupy 8 atoms per large pore, while at 150 K the occupancy descends to 2 atoms per large pore.

Structural Dynamic
Dynamics of flexible frameworks are unique feature of MOF structures which can be exploited for various applications such as sensing, separation and adsorption.Structural dynamics can be triggered by external stimuli (inclusion and exchange of guest molecules or by pressure and temperature changes).Adsorption of gases at high pressures in some cases induces structural transition and significantly increases the porosity at certain pressure point (gate opening effect).Some MOFs exhibit extensive flexibility when exposed to a certain type of guest molecules and show reversible structural dynamics upon adsorption/desorption processes (breathing effect).Temperature change is another very common external stimulus that can trigger structural changes.In this case, structural dynamics are usually driven by the removal of solvents or dehydration upon heating.All these processes can be monitored and evaluated by different in-situ diffraction studies.4][105] The response of flexible MIL-53 frameworks upon guest molecule adsorption was studied by in-situ powder XRD on MIL-53(Cr) system where large breathing effect induced by CO 2 employed on MOF at different pressures was observed. 106ouck et al. studied structural response to the adsorption of several light gases (CH 4 , H 2 , N 2 , C 2 H 6 , C 3 H 8 ) on NH 2 -MIL-53(Al) using in-situ XRPD and observed breathing behavior only upon CO 2 sorption process. 107The breathing behavior was investigated by high-resolution XRD on Fe analogue of MIL-53 as well. 108CO 2 adsorption on the MIL-53(Fe) as a function of pressure undergoes three steps.Firstly the intermediate phase occurs at room temperature and 2 bars, followed by the transition to the narrow pore formed at 10 bars and finally rearrangement to the large pore form observed at 10 bars as well, but at 220 K.The crystal structures of the corresponding CO 2 loaded materials were successfully determined with the precisely located CO 2 molecules within the pores (Figure 7).MIL-53(Al) with extremely flexible framework was recently studied for D 2 gas adsorption effects by neutron powder diffraction between 4 and 77 K and up to 4.5 bar.Two distinct D 2 sites were found in the fully opened form.The kinetically trapped D 2 was evidenced within the closed MIL-53 channels upon desorption. 109Recently, the use of in-situ powder XRD and quasi-elastic neutron scattering studies revealed the extensive structure response upon CO 2 adsorption on various other MOF systems: [Zn 2 (BME-bdc) 2 (dabco)] n (BME-bdc = 2,5-bis(2-methoxyethoxy)-1,4-benzenedicarboxylate, dabco = 1,4-diazabicyclo[2.2.2]octane)where slow adsorption kinetic of CO 2 enabled the identification of the metastable intermediate, 110 V-based biphenyl-4,4'-dicarboxylate, 111 In-based biphenyl-3, 3',5, 5'-tetra-(phenyl-4-carboxyate) 112 and MIL-47(V). 113,114he in-situ XRD measurements coupled with adsorption equipment was used to perform the experiment of C 4 -isomers adsorption (n-butane, isobutane, 1-butene, isobutene) on 3∞[Cu 4 (μ 4 -O)(μ 2 -OH) 2 (Me 2 trz-pba) (Me 2 trzpba:4-(3,5-dimethyl-4H-1,2,4-triazol-4-yl)benzoate) at different pressures of and temperatures (283-343 K).The adsorption process is accompanied with phase transition of the material. 115he structural dynamic of MOFs can be induced by inclusion or removal of solvent molecules as well.One of the most known phenomena is an extensive structural response upon water removal and inclusion on MIL-53-type materials.For instance, the removal of water from the Cr and Al materials changes from closed pore form to large pore form involving the atomic displacement of >5 Å.However, such breathing behavior was not observed for MIL-53(Fe).Its structure expands in the presence of other solvent molecules.With the use of high-resolution X-ray powder diffraction data, Walton et al. suggested the presence of clusters of methanol located within the MIL-53(Fe) channels with considerable disorientation. 116The further insight on the nature of methanol-to-framework interactions was gained using inelastic neutron diffraction. 117The key reason for the large structure flexibility lies in the motions of terephthalate rings that result in their distortion and rocking motions about the bonds to the carboxylate groups.In the future, similar profound investigations of framework-to-water interactions will be certainly needed the systems with the potential for water sorption applications (heat storage or heat transfer).The structural changes upon guest-adsorption was investigated by in-situ powder diffraction on Zn-triazolate system, where high anisotropic structural flexing upon water or ethanol adsorption/desorption could be observed. 118The crystal-tocrystal transformations upon guest removal on Ca-MOFs (AEPF-1) based on the 4,4'-(hexafluoroisopropylidene) bis(benzoic acid) ligand and Ca-terephthalate (Ca(BDC) (DMF)(H 2 O) were investigated by XRPD. 119,120Structure response upon exposure to different solvents were investigated on pyrene-based JUC-118(Zn,Co). 121For the struc-tural evaluation of solvent within the MOF pores the difference envelope density (DED) method employed which is based on the difference of the observed and calculated structure envelope densities. 122The case studies of several MOF systems (HKUST-1, UiO-66, MAMS-4, PCN-200) proved that DED can be easily deduced from the routine powder XRD data.

Catalytic Sites
Metal-organic frameworks have a great potential for heterogenous catalysis processes due to their unique structural features.With the removal of metal-coordinated solvents, the high concentration of unsaturated metal cations, and thus catalytically active sites, can be produce in a controlled manner.Active sites can also be encapsulated within the pores or attached through different post-synthesis modification procedures.In any case, the diffraction techniques offer a valuable tool to gain the information and understanding of functionalities for heterogenous catalysis processes of MOFs.
Multiwavelength anomalous X-ray dispersion (MAD) was used to determine the relative occupation of specific Mn 2+ metal sites exchanged with different cations (Fe 2+ , Cu 2+ , Zn 2+ ) on 1,3,5-benzenetristetrazolate-based MOFs (M 2+ MnBTT). 136The structure contains two metal sites with Cs and C4v symmetry from which only C4v sites are exchangeable (Figure 8).Refined occupancy difference (ROD) and integrated density difference (IDD) methods were employed to quantify the occupancy of C4v sites.According to MAD analysis, Cu 2+ and Zn 2+ are fully exchanged on C4v site whereas Fe 2+ exchanges in much lower extent (20%).
In order to enhance the catalytic performances of MOFs, various noble-metal nanoparticles were included within the pores of MOF systems.8][139][140][141][142][143] Recently, the method of high-energy Xray total scattering (XTS) was employed for the investiga-tion of low-concentration of nanoparticles within the matrix.The different amounts of Pd nanoparticles were immobilized within the MIL-101(Cr)-NH 2 material for Suzuki-Miyaura catalytic reactions. 144The distribution of Pd nanoparticles was determined by XTS.The method includes the extracting of pair distribution function (PDF) from Fourier-transformation of the total scattering intensities (including Bragg peaks and diffuse scattering).The obtained PDF describes the statistical distribution of all interatomic distances within the sample.

Amorphous MOFs
Amorphous metal organic-frameworks (aMOFs) lack the long-range ordering of building motifs but they still retain their original building blocks which are connected through organic ligands in more or less random manner.The development of aMOFs opened a new chapter in the field of MOF science, offering many exciting application opportunities.Aperiodic structure arrangements result in broad humps in their powder diffraction patterns caused by diffuse scattering.
6] The analysis of pair distribution function (PDF) calculated from Fouriertransform of X-ray total scattering patterns show that the amorphous structures are indistinguishable from one another.The differences in PDF were found comparing the a-MOFs prepared from ZIF-4(Zn) and ZIF-4(Co), due to the different scattering factors of Zn and Co. 147 Pressure induced amorphization of the ZIF-8 was monitored by in-situ X-ray powder diffraction. 148,149The structure undergoes irreversible transformation to continuous random network upon compression above 0.34 GPa.The attempt to structurally characterized I 2 -trapped aMOFs prepared from ballmilling of ZIF-8 and ZIF-69 was made by collecting X-ray total scattering data using Ag X-ray source (λ = 0.561 Å). 150 The decrease of the sharp diffraction peak (FSDP) with increased loading of I 2 in both aMOFs suggests retention of internal void structure upon guest inclusion.

Conclusion
In recent years progress in scattering and diffraction instrumentation and data processing enabled deeper and more detailed insight into structure determination and properties of MOFs.Better understanding of MOF chemistry consequently contributed to even faster and more intensive development of the structures with the desired properties by rational design.SAXS and SANS methods offered a lot of information about the nucleation kinetics and crystal growth mechanisms of MOFs.Improved highresolution XRD techniques in combination with new data processing approaches enabled the determination of very complex structures with extremely large unit cell volumes, which previously represented insurmountable obstacles.The investigations of the interaction of guest molecules with crystalline frameworks by neutron diffraction or scattering techniques offered the important insights into preferable sorption sites for specific hosting hydrogenous molecule.Structural dynamics of flexible structures induced by different external physical or chemical stimuli can be more accurately determined by time-resolved diffraction techniques and by employing combinational insitu methods.Topological disorder and structure defects, which found to play important role on different chemical properties of MOFs (i.e.structure stability, catalytic activity) can be recently successfully evaluated by pair distribution (PDF) analysis.Moreover, the effort to improve functionality or to design multifunctional MOF systems induced the investigation on multi-metal MOFs, which were successfully structurally described by MAD and PDF analysis.
With the continuous advance of diffraction and scattering capabilities, the monitoring of MOF's properties will become more and more routine and available to academic and industrial profiles.The future prospects lie on the improvement of instrumentation towards building multi-technique setups, further development of data processing which is prerequisite for successful interpretation of the measured experiments and generating even more powerful and more focused radiation sources.Furthermore, the use of electron diffraction together with electron microscopy imaging (ED, HR-TEM) for the crystal structure-property studies of MOFs is still a scarce area.Successful overcome of beam-damage issues would significantly contribute to easier determination of MOF microstructures.

Figure 1 .
Figure 1.(a) Time-resolved scattering patterns during ZIF-8 nanocrystal formation.Above: SAXS patterns for the first 150 s with the acquisition time interval of 1 s.Inset shows high-q region of selected SAXS patterns originating from the small clusters.Below: WAXS patterns measured between 1 and 800 s with the acquisition time interval of 1 s.Inset shows plot of the extent of crystallization a versus time t as produced from the integrated intensity of the 211 reflections in the WAXS patterns.35(b) Proposed scheme of ZIF-8 formation mechanism from layered phase shown along[100] direction (upper scheme) and [010] direction (lower scheme).36Reprinted with permission.Copyright American Chemical Society.

Figure 2 .
Figure 2. Time-resolved in-situ EDXRD data measured during the crystallization of the (a) copper carboxylate HKUST-1 at 125 °C and (b) MOF-14 at 130 °C.Insets: view of the structure of HKUST-1 and MOF-14 with five-coordinate Cu-based units as pink polyhedra. 47,48Reprinted with permission.Copyright Royal Society of Chemistry and John Wiley and Sons.

Figure 3 .
Figure 3.The sequence of events during the crystallization of terephthalate-based MOFs in different media: Low precursor concentrations (DMF); high precursor concentrations (H 2 O/DMF or H 2 O). 50Reprinted with permission.Copyright John Wiley and Sons.

Figure 7 .
Figure 7. MIL-53(Fe) structure perspective along the 1-D channels (upper images) and perpendicular to the channels (lower images) with determined crystallographic positions of CO 2 molecules within the channels with occupancies of (a) 0.22 CO 2 , (b) 0.63 CO 2 and (c) 2.72 CO 2 per formula with corresponding Rietveld plots. 108Reprinted with permission.Copyright Royal Society of Chemistry.

Figure 8 .
Figure 8. Exchangeable metal sites in M1M2BTT.Orange, red, green, blue, and gray spheres represent the C4v metal site, the partially occupied Cs metal site, Cl, N, and C atoms respectively.The pink sphere represents the substituting cation.136Reprinted with permission.Copyright American Chemical Society.