Tuning the guest-induced spatiotemporal response of isostructural dynamic frameworks towards e ﬃ cient gas separation and storage †

Understanding and control of the spatiotemporal stimuli-responsiveness of ﬂ exible metal – organic frameworks are crucial for the development of novel adsorbents for gas storage and separation technologies. Herein, we report two isostructural pillared-layer dynamic frameworks di ﬀ ering only in one atom that bridge a benze-nocarboxylate linker. Through a synthetic approach, we switch the stepwise CO 2 -induced transformation into a continuous one. Our ﬁ ndings are proved byequilibrium and time-resolved in situ powder X-ray di ﬀ raction collected during CO 2 adsorption at 195 K. Finally, we use high-pressure single and multi-gas adsorption experiments to show the superiority of continuous breathing in CH 4 storage and CH 4 / CO 2 separation at 298 K. This report demonstrates that the desirable mechanism of ﬂ exible frameworks can be readily achieved through single-atom exchange enabling e ﬃ cient gas separation and storage.

Understanding and control of the spatiotemporal stimuliresponsiveness of flexible metal-organic frameworks are crucial for the development of novel adsorbents for gas storage and separation technologies.Herein, we report two isostructural pillared-layer dynamic frameworks differing only in one atom that bridge a benzenocarboxylate linker.Through a synthetic approach, we switch the stepwise CO 2 -induced transformation into a continuous one.Our findings are proved by equilibrium and time-resolved in situ powder Xray diffraction collected during CO 2 adsorption at 195 K. Finally, we use high-pressure single and multi-gas adsorption experiments to show the superiority of continuous breathing in CH 4 storage and CH 4 / CO 2 separation at 298 K.This report demonstrates that the desirable mechanism of flexible frameworks can be readily achieved through single-atom exchange enabling efficient gas separation and storage.
Flexible metal-organic frameworks (MOFs) are porous coordination polymers that adapt their structure in response to external stimuli [1][2][3][4][5] such as pressure, 6 temperature, 7 electrical eld 8 and light. 9Their nano-elasticity is responsible for a plethora of novel macroscopic phenomena, which do not occur in rigid adsorbents, including the shape memory effect, 10 negative gas adsorption, 11 self-accelerating adsorption 12 or swelling. 13Moreover, the spatiotemporal adaptability of the frameworks opens the door to various potential applications, inter alia, gas storage 14,15 and separation, 16,17 logical operation, 18 proton conductivity, 19 molecular recognition, 20 catalysis, 21 drug delivery 22 and water isotopologue separation. 23lvent removal from the nano-cavities of exible MOFs transforms their porous structure (open pore phaseop) into a non-porous or less porous structure (close pore phasecp) expected in some cases as the desolvation of hydrated MIL-53(Cr). 24Gas adsorption reverses these processes and the structural transformation occurs in a discontinuous (stepwise) manner.For example, ELM-11 when exposed to N 2 or Ar exhibits a one-step transformation described as gating (cp / op). 25 The same effect is observed in DUT-8(Ni); this pillared layer framework transforms from the non-porous cp phase into the op phase during the n-butane adsorption, 26 while the N 2 adsorption prole of CoBDP has several steps, which correspond to the different well-dened intermediate phases. 27On the other hand, MIL-53 breathes CO 2 which is represented as two distinguished steps on the isotherm. 25osseinsky and others made a very intriguing comparison of the conformational energy landscape of a three-dimensional chiral MOF with exible macromoleculeshuman hemoglobin. 28The authors determined nine different crystal structures and calculated their energetical minima using the DFT methodology.However, there are exible porous materials that have a continuous spectrum of substructures [29][30][31] represented by an innite set of numbers.MIL-88(Fe) reported by Férey should be considered an important example of such swelling behavior. 13,32The cell volume of MIL-88 strongly depends on the type of solvent in its cavities.On the other hand, in 2017 Brammer and co-workers have reported SHF-61 which continuously changes the unit cell volume during the time-dependent desolvation. 29However, in most cases, the limited number of advanced structural characterization does not fully reveal the complete phase transition pathway.
A spatiotemporal response of MOFs to external stimuli is of paramount importance for most of the exibility-related applications, however, at the moment, there is no clear understanding of all factors inuencing the phase transition kinetics, e.g.repeatability, size effects, sample "history" etc. 33 In a prospective review, Van Speybroeck and co-workers described the vision and pathways for in silico prediction of the spatiotemporal response and encouraged the community to use machine learning potential and coarse-grained model techniques in combination with enhanced sampling techniques in a ner phase space. 34erein, using an example of two nearly identical exible MOFs that differ only in one atom of the repeat unit, we show both continuous and discrete structural transformations.To shine light on the observed phenomena, we employed in situ PXRD measurements applied under equilibrium and out-ofequilibrium conditions in parallel to CO 2 adsorption.The former involves simultaneous measurement of PXRD patterns at the dened points of the CO 2 isotherm at 195 K.The latter consists of a kinetic study in which 100 PXRD patterns were collected per second upon the CO 2 pressure jump from vacuum to 60 kPa at 195 K.The last part of our investigation shows that continuous transformation is superior to the discrete one in terms of CO 2 /CH 4 separation and CH 4 storage at 298 K.
Before desolvation, the N,N ′ -dimethylformamide was exchanged for dichloromethane.Then both materials were activated under dynamic vacuum at 353 K. Comparison of the ex situ IR-ATR and PXRD of as-synthesized materials with the desolvated ones reveals considerable structural changes and indicates the exible nature of both compounds (Fig. S3 †), for example, signals at 2q of 5.78 and 5.76°for UAM-1O and UAM-1S, respectively, disappear.Furthermore, the OCO stretching region of IR-ATR spectra proves the reorganization of secondary building blocks.Due to the fracture of the crystals, we were unable to determine the closed structure of desolvated MOFs, however, this will be subject of further studies.Exposure of the collapsed phases to gaseous carbon dioxide at 195 K causes its adsorption characterized by singularities 15 on the recorded isotherms (Fig. 2).Firstly, both UAM-1X adsorb ∼20 cm 3 g −1 which practically does not affect their structures.However, exceeding a pressure of 0.31 bar (UAM-1O) and 0.01 bar (UAM-1S) triggered different single-atom-dependent structural transformations which increased CO 2 uptake to 155 cm 3 g −1 and 152 cm 3 g −1 at p = 0.99 bar, respectively.Based on these maximal uptakes and applying the Gurvich rule, we calculated that the experimental pore volume is equal to 0.27 cm 3 g −1 (UAM-1O) and 0.28 cm 3 g −1 (UAM-1S), which agrees with the theoretical values of 0.28 cm 3 g −1 and 0.27 cm 3 g −1 .
Equilibrium in situ PXRD collected for UAM-1O above 0.31 bar shows evidence for only two states, thus the UAM-1O phase transition is described as 1st order and the MOF phases could be represented by a set of two numbers.The case of UAM-1S is more complicated since the fully CO 2 saturated phase was preceded by many intermediate steps.This raises the fundamental question of whether the observed process is truly continuous or whether it is multistep adsorption as described previously by Long and co-workers. 27On the other hand, Carrington et al. have described 29 nine H 2 O desorption steps monitored by PXRD as continuous breathing.Herein, we prove for the rst time that the observed mechanism is continuous.For this purpose, we employed time-resolved synchrotron PXRD during CO 2 adsorption at 195 K (Fig. 2).Importantly, before crucial experiments, we exclude the inuence of crystal size dependency on adsorbate diffusion by subjecting desolvated phases to repeated adsorption-desorption stress.Scanning electron microscopy imaging conrms that both materials have comparable crystal sizes (Fig S4 †).
For UAM-1O, analogous to the static conditions, we observed only two states, with the phase transformation (cp / op) beginning approximately 7 s aer CO 2 exposure.In contrast, CO 2 injection into UAM-1S has an immediate effect on its structure, the transformation occurring as soon as the framework comes into contact with the gas but more importantly, an innitive number of intermediate phases are detectable.This is shown in the continuous smooth change of reection position from 2q = 8.4°to 8.0°but also other reections (Fig. 2c).This is clear evidence of a 2nd order transformation.As both frameworks are isostructural, the different transition mechanism is caused by the O or S atoms.In comparison to oxygen, sulfur has an additional electron shell thus is more easily polarizable and creates longer bonds.Crystal structures of UAM-1X reveal that the C-X bond length is 1.374 Å and 1.783 Å for UAM-1O and UAM-1S, respectively.These factors inuence the accessible range of vibrational modes between benzenocarboxylate rings in both xba 2− linkers.While in the case of UAM-1O, the C-O-C vibrational range has a narrower value, favoring the existence of two different states, the C-S-C range is higher.Therefore, a small increase in CO 2 pressure triggers small changes in the unit cell parameters of UAM-1S, resulting in continuous breathing.In the case of UAM-1O, there was limited oba 2− reorganization, restricting its ability to undergo gas-induced transformation.
Additionally, it has implications for the transformation pressure.Since UAM-1O exists only in closed or open states, the switching between these states requires overcoming a high energy barrier, reected on the isotherm as a "gate opening" pressure of 0.31 bar at 195 K. On the other hand, multiple intermediate phases with low energy barriers between them can be observed in UAM-1S, resulting in a transition at a very low pressure of 0.01 bar at 195 K.
Furthermore, we assessed the inuence of grinding on the adsorption properties of UAM-1O (Fig. S5 †).We found that the uptake in saturation slightly increased for the ground sample, from 152 cm 3 g −1 for the pristine material containing macrocrystals to 163 cm 3 g −1 for the microcrystalline sample.However, the isotherm exhibited a slightly different shape, with a steady increase in uptake in the rst stage, which ended with an even higher transformation pressure of 0.34 bar compared to the as-synthesized material.The larger uptake before the  S5; † l = 0.6199 Å, for consistency data were converted to l = 1.540599Å).To eliminate the influence of crystal size dependency, the samples underwent repeatable adsorption stress and subsequent SEM imaging prior to the in situ time-resolved measurements (Fig. S4 †).m pd : maximum pore diameter; p ws : pore window size; V pt , theoretical pore volume; and V pe , experimental pore volume for CO 2 adsorption calculated at ∼0.99 bar according to the Gurvich's rule.transformation pressure indicates that some grains open before others.All of this suggests the creation of unevenly distributed defects caused by mechanical force, which should be studied further, although it is beyond the scope of this investigation.
Understanding CO 2 adsorption mechanisms poses a second fundamental question "does continuous breathing affect the two important properties of novel porous materials: gas separation and delivery?"To answer this question, we investigated high pressure single and multi-component adsorption at 298 K (Fig. 3 and 4).
Methane was not taken up by UAM-1O, while CO 2 triggers the cp / op transformation at 45 bar, hindering the potential applicability of this material.CH 4 induced the transformation of UAM-1S at 7.4 bar, the total gas uptake being 90 cm 3 g −1 at 40 bar and the material released 80 cm 3 g −1 at 5.8 bar, making UAM-1S a moderate candidate as a CH 4 tank ller. 36t a CO 2 transition pressure of 7.5 bar, considerably lower than CH 4 (22.7 bar), UAM-1S adsorbs 109 cm 3 g −1 , while on desorption 98 cm 3 g −1 is released at 0.5 bar.This adsorptiondesorption window indicates the possibility of efficient CO 2 /CH 4 separation, but is based on considering only the separate adsorption of CH 4 and CO 2 .To answer the question of whether the excellent selectivity factor (S factor = 10; p CO2 = 7.5 bar and p CH4 = 7.5 bar, see eqn S(1) †) derived from single-adsorption isotherms would be maintained during co-adsorption, we conducted a series of co-adsorption measurements at various CO 2 /CH 4 ratios at 298 K (Fig. 4).
In an atmosphere containing 75% v/v of CO 2 , the methane uptake reaches the maximum value of 6 cm 3 g −1 , while the maximum CO 2 uptake is 117 cm 3 g −1 at total pressure 22 bar.The calculated S factor for this point is 6.0.Enriching the composition of the gas mixture to an equimolar composition does not affect the ability of UAM-1S to exclusively adsorb CO 2 .Due to the considerable CO 2 uptake of 120 cm 3 g −1 , the real S factor increases to 33.6 for the last point.A further increase of CH 4 to 75% v/v caused a peculiar behavior of UAM-1S.At the two rst points of the isotherm, the CH 4 uptake is negligible, but then both gases compete for the adaptable nanoconned space of UAM-1S.A clear manifestation of this is the desorption of 4 cm 3 g −1 CO 2 at the last point (p total 32.6 bar).In our opinion, this competitive adsorption accompanied by structural changes needs to be studied in detail by theory and experiment.It is important to answer the questions as to what is the environment of each gas in such a complex system and how does the spatial adaptivity of UAM-1S changes during the adsorption of one component from the mixture.
Methane co-adsorption at the ratio of 25 : 75 has a minor impact on selectivity, dropping, for example, for the total pressure of 23.1 bar, from S factor = 9.7 to 7.9 at the last point.These values proved here that continuous structural transformation was superior in comparison to the discrete one as well as that UAM-1S has a comparable S factor to benchmark materials (Tables S2 and S3 †).We plan to assess next whether gradual changes can overcome the considerable problem of exible adsorbents, the slipping off phenomenon. 37n summary, we have characterized two exible thiazolothiazolate MOFs differing only by one atom in the linker, O or S. Upon desolvation, both materials transform into a closed phase, while the exposure of the cp phases to CO 2 rebuilds their porosity.Static in situ PXRD collected at specic pressures of CO 2 has shown that CO 2 induced transformation of UAM-1O (cp / op) occurs in two steps, while the analogous structure, UAM-1S, exhibits multiple intermediate steps between the cp and op phases.By the utilization of time-resolved powder X-ray diffraction, we have shown that UAM-1S transformation is gradual, with a formally innite number of intermediate states between the open and closed states.Furthermore, our ndings show the superiority of continuous transformation when applied to CO 2 /CH 4 separation and CH 4 storage.We consider that the obtained results represent important ndings in the chemistry of exible MOFs, ergo the following aspects of both frameworks should be studied and compared: (i) thermal management, (ii) detailed adsorption mechanisms, and their (iii) compatibility with mixed-matrix membranes.

Fig. 1
Fig. 1 Structural features of as-synthesized UAM-1X.(a) Coordination environment of the di-zinc unit and representation of the 2D layers of Zn 2 (xba) 2 ; for simplicity, shown only for UAM-1O.(b) Differences in pore geometry originates from different C-X-C angles: X = O or S; 115.2°and 101.4°, respectively.

Fig. 2
Fig. 2 Mechanistic understanding of CO 2 -driven structural transformation in UAM-1X (Otop row; Sbottom row): (a) CO 2 adsorption (full symbols) and desorption (open symbols) at 195 K juxtaposed with (b) corresponding in situ PXRD patterns collected at selected pressure (l = 1.540599Å); (c) in situ time-resolved PXRD during the CO 2 adsorption at 195 K (see Fig.S5; † l = 0.6199 Å, for consistency data were converted to l = 1.540599Å).To eliminate the influence of crystal size dependency, the samples underwent repeatable adsorption stress and subsequent SEM imaging prior to the in situ time-resolved measurements (Fig.S4 †).m pd : maximum pore diameter; p ws : pore window size; V pt , theoretical pore volume; and V pe , experimental pore volume for CO 2 adsorption calculated at ∼0.99 bar according to the Gurvich's rule.

Fig. 3
Fig.3The first indication that continuous transformation is superior to the discrete one in CH 4 separation and delivery processes: singlegas adsorption measurements for UAM-1X at 298 K; red CO 2 , black CH 4 , full symbols adsorption, open symbols desorption.Due to the remarkably high transformation pressure and the technical limitation that arose from it, no CO 2 desorption branch for UAM-1O was recorded.