João Marreirosa

Rodrigo De Oliveira-Silvaa

Paul Iacomib

Philip L Llewellynb

Rob Ameloota

Dimitrios Sakellarioua

In situ measurement of full adsorption isotherms by NMR

Keywords: porous materials, physisorption, MOFs, low-field NMR relaxometry

Physisorption using gas or vapor probe molecules is the most common characterization technique for porous materials. The method provides textural information on the adsorbent as well as the affinity for a specific adsorbate, typically through equilibrium pressure measurements. Here, we demonstrate how low-field NMR can be used to measure full adsorption isotherms, and how by selectively measuring 1H spins of the adsorbed probe molecules, rather than those in the vapor phase, this 'NMRrelaxorption' technique provides insights about local dynamics beyond what can be learned from physisorption alone. The potential of this double-barreled approach was illustrated for a set of microporous metal-organic frameworks (MOFs). For methanol adsorption in ZIF-8 the method identifies multiple guest molecules populations assigned to MeOH clusters in the pore center, MeOH bound at cage windows and to MeOH adsorption on defect sites. For UiO-66(Zr) the sequential pore filling is demonstrated and accurate pore topologies are directly obtained, and for MIL-53(Al) structural phase transitions are accurately detected and linked with two populations of dimeric chemical species localized to specific positions in the framework.

Porous solids with a high specific surface area exhibit unique physical and chemical properties and are crucial in catalytic1 and separation processes.2-4 Typically, the ad-sorption characteristics of such materials are evaluated by recording their uptake of a probe molecule as a function of its partial pressure at a given temperature. [START_REF] Thommes | Phy-sisorption of Gases, with Special Reference to the Evalua-tion of Surface Area and Pore Size Distribution (IUPAC Technical Report)[END_REF] While such physisorption methods are commonplace and established, the underlying adsorbent-adsorbate interactions are inferred from the macroscopic pressure-loading curves ac-cording to necessarily approximating models. Nuclear magnetic resonance (NMR) probes matter at the atomic level and can provide information inside these materials but its integration is not trivial. Early studies used electro-magnets to study NMR relaxation as a function of tem-perature.6-8 Modern day high-field NMR spectroscopy can offer a much more detailed picture, down to the atomic scale, by directly probing the adsorbed phase in terms of structure and dynamics. To date, however, mainly single-point adsorption high-field NMR experiments have been performed, since full isotherm measurements are nontrivial to implement on superconducting NMR instru-ments,9-13 or otherwise deviate from the ideal conditions set as industry standards for isotherm acquisition (e.g. using gravimetric techniques, imaging, etc.). [START_REF] Beyea | Spatially Resolved Adsorption Isotherms of Thermally Po-larized Perfluorinated Gases in Yttria-Stabilized Tetragonal-Zirconia Polycrystal Ceramic Materials with NMR Imaging[END_REF][START_REF] Leisen | Sorption Isotherm Measurements by NMR[END_REF] Recent studies have attempted to circumvent these limitations with the use of low-field NMR for the study of sorbent materials. Nonetheless these remain limited to ex situ sample analysis and often require the use of intricate custom-made high-pressure gas dosing systems.18-20 Herein, we showcase the integration of an ultra-compact low-field NMR device with a commercial physisorption instrument. Relaxometric measurements of adsorbed molecules are used to monitor and elucidate adsorption inter-actions and dynamics as a function of partial pressure. A small size homogeneous Halbach-like permanent magnet (0.3 T, 1200 cm3, 4 kg) and a solenoidal coil were used for polarizing, exciting and detecting the NMR signal. Their integration with the commercial sorption analyser was seamless due to the negligible stray magnetic field of the magnet. It is notoriously difficult to record signals from vapors, or gases by NMR because of their low spin density, which has given rise to the use of gas hyperpolarization techniques (CITE 16+17). In our case however, the con-centration through adsorption in porous media, combined with a high-sensitivity multiple-pulse sequence, allow detection of the adsorbed guest phase. Approximately 20 mg of various adsorbents were loaded in a custom dead-end glass sample tube fully compatible with the commercial adsorption apparatus, so that no modifications are required to the activation and isotherm measurement protocol (Figure S1 in SI). The NMR measurements were performed simultaneously with the isotherm measurements, by averaging 32 Carr-Purcell-Meiboom-Gill21,22 (CPMG) scans every ~180 seconds. For all tested adsorbent-adsorbate combinations, natural-abundance 1H NMR measurements under thermal polarisation and standard pressures gave a sufficiently high signal while also measuring longitudinal (T1) and transverse (T2) nuclear relaxation times. Non-adsorbed guest molecules were not detected by NMR due to their low 1H spin density. The NMR signal intensity was obtained by integrating the first 10 Hahn echoes, corresponding to a signal duration of 2 ms. The inverse Laplace transform (ILT) was used to fit the data and extract relaxation time distributions. This mode of detection filters out NMR signals that have very short (< 200 μs) T2 times. Although chemical shifts were not resolved because of the low magnetic field, different guest populations could be distinguished based on differences in T1 and/or in T2. Moreover, 2D correlation NMR experiments between T1 and T2 at specific partial pressures enabled to estimate the adsorbent-adsorbate interaction energy parameter (esurf). [START_REF] D'agostino | Interpretation of NMR Relaxation as a Tool for Characteris-ing the Adsorption Strength of Liquids inside Porous Mate-rials[END_REF] Single-pore MOF -ZIF-8

The relaxorption technique was demonstrated for the physisorption of methanol (MeOH) in ZIF-8, a MOF consisting of Zn2+ ions and 2-methylimidazolate linkers. [START_REF] Park | Exceptional Chemical and Thermal Stability of Zeolitic Im-idazolate Frameworks[END_REF] The MeOH NMR signal as a function of partial pressure corresponds with both the adsorption isotherm (Figure 1a -white and red lines) and previously reported data.25-28 A previous computational study on this system predicted two sorption stages: initial trace adsorption on the pore walls, followed by adsorbate clustering in the center of the pore.28 In excellent accordance, the NMR relaxation measurements show a two-stage process with a minimal signal at p/p0 < 0.15, followed by a strong signal for partial pressures above 0.15. The low NMR signal intensity at low pressures can be at-tributed to the strong bonding of the small number of adsorbed molecules, resulting in a very short T2, less than 100 s, for accurate signal detection. Where the isotherm rises steeply (0.15 < p/p0 < 0.25), increasingly longer T2 values are observed. These changes correspond well to a shift of MeOH molecules from the pore wall sites towards the center of the pore.28 At even higher partial pressures (0.25 < p/p0; Qads > 9.6 mmol/g), T2 remains constant for the majority of the adsorbed molecules (T2 ~ 100 ms), but a low-intensity population with a shorter T2 about 10 ms is also detectable, suggesting a small fraction of strongly adsorbed molecules besides those clustered in the pore center.

To better understand the different populations of adsorbed molecules at selected partial pressures, 2D T1 -T2 correlations were measured for longer periods of time in order to achieve a higher signal-to-noise ratio (Figure S3; experimental details in SI). At a partial pressure of 0.125 (Figure 1a, red square), the 2D T1 -T2 NMR correlation map (Figure 1b) displays three resolved peaks, corresponding to individual populations of 1H spins (A, B, and C). Population A is present throughout the isotherm, and its signal intensity grows at higher partial pressures, likely corresponding to hydrogen-bonded MeOH clusters in the pore center. Population B is dominant at low partial pressures and fades as more MeOH is adsorbed. Therefore, it is at-tributed to MeOH adsorbed at cage windows and pore walls.28 Since the population C has the lowest signal intensity and shortest T2, it likely corresponds to adsorption on defect sites, especially given that the used synthesis meth-od is prone to defect formation.29,30 The surface interaction parameter esurf =-T2/T1, originally introduced by D'Agostino et al.,23 is a quantitative estimator of the adsorbate-adsorbent interactions during adsorption. Applying this theory here, populations B and C have similar interaction strengths, in accordance with esurf as indicated by their distance from the diagonal (Figure 1b).23 Analysis of esurf shows a weakening of the host-guest interactions with in-creased vapor loading (Figure S23).

While the low magnetic field prevents NMR identification of different adsorbates by chemical shift analysis, the different interaction strengths of ethanol (EtOH) and MeOH with ZIF-8 result in distinct T2 relaxation times (Figure 1c). This difference enabled uptake monitoring of each component from a MeOH/EtOH vapor mixture (see Figure S8). In contrast to MeOH, the stronger host-guest interactions for EtOH result in adsorption exclusively on the pore walls, without migration towards the center, leading to shorter T2 values. This strong confinement results in a different profile between the NMR intensity and the isotherm (Figure S7) since wall-bound molecules relax faster, leading to a partial loss of the NMR signal at low partial pressures and highlighting the underlying sorption mechanism. Dual-pore MOF -UiO-66(Zr) UiO-66(Zr), a dual-pore MOF built up of zirconium clusters and terephthalate linkers31 was also studied by relaxorption. As for ZIF-8, ethanol adsorption revealed a difference between the isotherm and the NMR signal intensity in the low-pressure region (p/p0 < 0.1; Figure 2a, Figure S11). This difference is attributed to strong EtOH adsorption expected in the low partial pressure regime (0.05 < p/p0) due to hydrogen bonding with the inorganic clusters.12 This hypothesis is fully validated by the short average (about 300 s) transverse NMR relaxation times. At the lowest EtOH load-ing (2.94 mmol/g; p/p0 = 0.0095), an estimated 1.2 mmol/g of alcohol are cluster-bound and thus beyond the limits of NMR detection (Figure 2a, Figure S11). NMR sensitivity is progressively restored as more EtOH is adsorbed (Qads > 2.94 mmol/g; 0.0095 < p/p0), with an increase in T2 relaxation times, likely due to competing guest-guest interactions. This hypothesis is supported by the increasing isosteric enthalpy of adsorption as a function of loading (Figure S14). As a result, at partial pressures above 0.2, the NMR signal intensity and physisorption data correspond well (Figure 2a). 2D T1 -T2 correlation experiments reveal a bimodal T2 distribution, as might be expected for the dualpore net-work of UiO-66(Zr) (Figures 2c, S10-S13). At highest loading (Qads = 12.40 mmol/g; p/p0 = 0.90) the deconvoluted and integrated T2 signals designated T2T (tetrahedral pores, 38%) and T2O (octahedral pores, 62%), corresponded well to the volume fractions of the tetrahedral and octahedral pores in UiO-66(Zr) (SI section I-b; VT = 40% and VO = 60%). The changes to T2T and T2O, over the course of the isotherm, reveal partial occupancy of both tetrahedral and octahedral pores, with preferential loading of the former at low pressures (Qads = 4.46 mmol/g; p/p0 = 0.0242). Further EtOH adsorption results in the saturation of the tetrahedral pore, followed by filling of the octahedral pore. Prior studies on the adsorption of CO2 in UiO-66(Zr) found a similar pore loading preference.32,33 Furthermore, an inverse linear relation between the quantity of adsorbed EtOH and esurf is observed (Figure S14). Similarly as for ZIF-8, increased loading favors guest-guest interactions which compete with the framework-vapor interactions and this leads to a reduction of esurf. This hypothesis is further confirmed by the increase in isosteric enthalpy of adsorption as a function of loading. b) Evolution of the transverse relaxation times (T2) as a function of the loading. The physisorption isotherm (white line) is superimposed with the T2 distributions. The red squares are the loadings where the T1-T2 2D NMR experiments where per-formed. c) Three T1-T2 correlation maps for the different conformational states I, II, and III. The evolution of guest populations is observed with the loading as well as changes in T1 and T2. The white dotted line correspond to the lines T1/T2 equal to 1 (diagonal).

Flexible MOF -MIL-53(Al) Structural flexibility is a guest-induced response exhibited by the MIL-53 family of materials, which exhibit structural phase transitions as a function of pore occupancy, also known as "breathing".34-36 During EtOH adsorption, MIL-53 undergoes a contraction from a large-pore (LP) to a narrow-pore (NP) state, followed by gradual re-opening at increasing pressures. [START_REF] Millange | Effect of the Nature of the Metal on the Breathing Steps in MOFs with Dynamic Frame-works[END_REF] The EtOH isotherm and NMR intensity reveal unprecedented joint information. The NMR signal shows discontinuities and abrupt intensity fluctuations where structural transitions occur due to changes in the dynamics of the guest molecules (Figure 3a). In the initial LP form, T2 relaxation times of 0.5 and 1 ms are observed. With increasing loading, the phase transition to the NP configuration shortens the former relaxation time to 0.2 ms, while increasing the latter to 3 ms. Upon re-opening of the pores, T2 values close to the initial value are measured -small T2 deviations arise from the fact the final LP state does not perfectly match the initial LP state of MIL-53(Al) at the beginning of the isotherm. Changes in molecular dynamics impact the relaxation times and explain the differences in NMR signal intensity in Figure 3a, in line with water adsorption in MIL-53(Cr).33 2D T1-T2 maps at specific EtOH loadings confirm the existence of loading-dependent bimodal T2 distributions correlated to a single T1, whose value fluctuates also as function of guest loading (Figure 3c). Deconvolution of the T2 relaxation times reveals two (and possibly three at higher loading) guest populations, ascribed to the different guest-host coordination modes originally reported by Bourelly et al.31 These T2 components are likely matched to EtOH monomer coordination at Al(OH) cluster level (shorter T2 values), as well as dimer and trimer coordinations (longer T2 values ~ 10 ms). The average surface interaction parameter esurf changes abruptly as a function of guest loading, contrary to the continuously declining profiles observed for UiO-66(Zr) and ZIF-8 (Figure S23). Non-breathing sorbents such as ZIF-8 and UiO-66(Zr), show a continuous lowering in esurf with increased guest loading. By comparison, flexible materials display more complex esurf profiles with considerable fluctuations of this parameter, either Δesurf > 0 or Δesurf < 0 corresponding to a phase transition or guest loading, respectively. In correspondence with crystallographic data, Δesurf from NMR measurements may be used as a descriptor for the MIL-53(Al) phase transition, offering a clear distinction between strict pore loading (Δesurf < 0), and structural trans-formations (Δesurf > 0).

Conclusion

Integration of physisorption and NMR relaxometry for in situ measurements, dubbed as "NMR-Relaxorption", was demonstrated on three case-studies of prototypical microporous materials. Even though this technology is at an early stage, it already offered complementary insights into host-guest, guest-guest interactions, i.e. pore filling (UiO-66) and structural heterogeneity (ZIF-8), and host/guest dynamics, i.e. host phase transition (MIL-53(Al)). While the NMR signal sensitivity remains one of the main obstacles of this technique, several options exist to overcome it, including the use of stronger magnetic fields, cryo-cooled detection, as well as extraneous highly polarized sources of atoms or molecules, which may allow the detection of physisorbed monolayers and finer details in future developments. Detection of species having very short T2 times, as well as determination of isotropic chemical shifts inside heterogeneous/solid materials remain the core challenge, but the use of strong radiofrequency pulses and low magnetic fields might enhance alternative options for averaging anisotropy such as magic angle field spinning.39,40 Ultimately, this methodology has the potential to become a low-cost general technique for the analysis of other material classes (e.g. zeolites, carbons, silicas, biopolymers, amongst others) for which structural heterogeneity, host/guest mobility, paramagnetism or phase transitions warrant further research.

Figure 1 -

 1 Figure 1 -Alcohol vapor adsorption on powdered ZIF-8 monitored by the NMR relaxorption technique. a) The adsorption isotherm (white line) and the NMR signal intensity (red line) are shown as a function of partial pressure. A 2D graph of the T2 relaxation time distribution (vertical axis) also as a function of the loading (same horizontal axis) is placed bellow these curves, for comparison. In the inset the porous structure of the ZIF-8 is represented. b) T1-T2 correlation map for MeOH loading of 1.8 mmol/g (p/p0 = 0.157). Three adsorbate populations are identified -A, B and C. c) T2 relaxation time distributions for EtOH (green) and MeOH (blue) in ZIF-8 (p/p0 = 0.8). The two curves show different average relaxation times leading to speciation.

Figure 2 -

 2 Figure 2 -Ethanol adsorption on powdered UiO-66(Zr). a) Superposition of the NMR signal intensity and the physisorption isotherm. b) Physisorption isotherm (white curve) superimposed with the T2 distributions as a function of partial pressure. The red squares outline the loadings where T1-T2 experiments were performed and their results are shown in c). The 1D projections of these 2D maps were deconvoluted in two components. The white dotted line correspond to the lines T1/T2 equal to 1 (diagonal).

Figure 3 -

 3 Figure 3 -Ethanol adsorption on powdered MIL-53(Al). a) Superposition of the NMR signal and physisorption isotherms. Low field NMR was able to identify the conformational transitions, from LP (I) to NP (II) and again to LP(III) by the changes in the intensity of signal due to the loading of EtOH molecules.

ASSOCIATED CONTENT Supporting Information. Full description of the experimental details including XRD patterns, TGA curves, sorption isotherms, NMR results and all modelling results. This material is available free of charge via the Internet at http://pubs.acs.org. ACKNOWLEDGMENT DS is grateful to the CEA for many years of continuous support. The authors would like to thank Prof. D. De Vos for stimulating discussions and access to the 3Flex sorption instrument, and Prof. J. Karger for stimulating discussions and useful literature references, and M. S. Usé for help with the assembling the PID temperature controller. ABBREVIATIONS NMR, nuclear magnetic resonance; p/p0, partial pressure; Qads, quantity adsorbed; NP, narrow pore; LP, large pore.

Funding Sources

This project was supported by the KU Leuven Starting Grant and by the Research Foundation Flanders (FWO) under Grant PorMedNMR -n° G0D5419N.

AUTHOR INFORMATION

Corresponding Author * Prof. Dimitrios Sakellariou (dimitrios.sakellariou@kuleuven.be) Present Addresses †Dr. Paul Iacomi -ICGM, CNRS / Université de Montpellier, France.

Author Contributions

The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript. / ‡These authors contributed equally.


Eder Amayuelas

Paul Iacomi

Arkaitz Fidalgo-Marijuan

Miren Begoña Bazán

Karmele Urtiaga

Gotzone Barandika

Luis Lezama

María Philip L Llewellyn

Isabel Arriortua

Miren Karmele Urtiaga

María Isabel

Multifunctionality of weak ferromagnetic porphyrin-based MOF: selective adsorption in liquid and gas phase

Introduction

Metal-Organic Frameworks (MOFs) have proven to be interesting materials due to their potential applications in multiple fields, such as catalysis, [1,2] biomedicine [3,4] gas storage and separation. [5,6] This is by cause of their unique structural and chemical features, which result from the combination of organic linkers coordinated to metal atoms or clusters, giving rise to open structures with potentially high pore volumes and surface areas, as well as unique opportunities for designing active sites, [7,8] stimuli responsiveness [9,10] or localized cooperative phenomena. [11] The emergence of MOFs provides an alternative in applications such as pollutant entrainment and gas storage, extending the other commonly used classes of porous materials of activated carbons and zeolites. [12][START_REF] Soares | Brazilian Journal of Chemical Engineering[END_REF][START_REF] Díaz | [END_REF] For this purpose, accessible porosity and high surface areas are crucial features, with record values reported in several MOFs over the last decade. [15][16][17] However, a large uptake is not the singular requirement, as there are other relevant properties necessary such as selectivity and regenerability. [18] Here MOFs hold special promise due to their ability to be designed through the judicious choice of metal centers and corresponding linkers in order to tune their local properties and tailor them for selective adsorption or separation. [19] Despite their attractive properties, MOFs have been held back from industrial applications largely because of their poor chemical and thermal stability. [20,21] Extensive efforts are being made, from the design to the preparation stages, in order to improve these weak points. [22,23] One of the possible approaches in achieving this objective is by using inorganic and organic building blocks of known stability, in order to synthesize inherently more stable MOFs. A good example of such stable organic building blocks are metalloporphyrins, multifunctional biomolecules which are found in complexes, playing essential biochemical, enzymatic, and photochemical functions, based on the special properties of the tetrapyrrolic macrocycle. [24] Metalloporphyrins are remarkable precursors in supramolecular chemistry, giving rise to a variety of functional materials because of their unique chemical, physical and biological properties. [25][26][27] In the wide horizons of MOFs, metalloporphyrin frameworks rouse intensive research interest, although they are still in their premature stage. Since Kosal et al. reported the first porphyrinbased MOF, PIZA-1, with a high selectivity to guest solvent molecules, [28] similar materials have reported great results in several applications areas. [29] Despite the fact that surface areas for porphyrin-based materials are average for a porous compound, the chemical features of the pyrrole ring, combined with their high stability, provide porphyrin-based MOFs optimal pore environments and surface affinity for selective adsorption and separation via thermodynamic [30] or kinetic means. [31] Moreover, the use of porphyrins as main ligands for MOFs offer the possibility to include differing open metal sites than those that coordinate between these units, [32] furthering the choices available to the skilled chemist to create multifunctional materials. The work herein focuses on the 3D porphyrin-based MOF [Ni5(H2TCPP)2O(H2O)4]•nS (where TCPP is meso-tetra(4carboxyphenyl)porphyrin and S is the solvent), previously reported by our group and further referred to as 1, [33] and its role as a selective adsorbent for several pollutants liquid or gas phase. To achieve this goal, structural thermal and magnetic characterization were first performed in order to set the basis for its adsorbent behaviour based on its structural features, followed by adsorption tests and kinetic measurements for organic dyes, and a comprehensive characterization of the gas adsorption performance using calorimetry experiments of several greenhouse gases and light hydrocarbons.

Materials and methods

Materials

The non-metallated meso-tetra(4-carboxyphenyl)porphyrin (H6TCPP), nickel(II) nitrate hexahydrate >98.5%, 1,2,4,5benzenetetracarboxylic acid 96% (H4bta) and the solvents N,Ndimethylformamide 99.8% (DMF) ethanol 96% and acetone 96% were purchased from Sigma-Aldrich Co. and used as received. The synthesis procedure was previously detailed [33] and is summarised in the ESI.

Physicochemical characterization

Thermal behaviour was studied in-situ using X-ray thermodiffractometry on a Bruker D8 Advance Vantec diffractometer (Cu-Kα radiation) equipped with a variabletemperature stage (Anton Paar HTK2000) with a Pt sample holder. The powder patterns were recorded in 2θ steps of 0.0333° in the 5-27° range, counting for 0.8 s per step. Temperature was increased from ambient conditions (25 °C) to 500 °C at 15 °C min -1 . Electron paramagnetic resonance (EPR) spectra were measured on polycrystalline samples with a Bruker ELEXSYS 500 spectrometer, operating at X band, equipped with a super-high-Q resonator ER-4123-SHQ. The magnetic field was calibrated by a NMR probe and the frequency inside the cavity was determined with an integrated MW-frequency counter. Magnetic susceptibility measurements were taken in the range of 4-300 K with a Quantum Design SQUID MPMS-7T magnetometer. Dye adsorption experiments were performed in an ethanol solution. The organic dyes selected as simulants were cationic methylene blue (MB), crystal violet (CV), anionic Congo red (CR), and neutral dimethyl yellow (DY) (structures available in Figure S4 in ESI). 10 mg of compound 1 was added into a 4 mL glass beaker containing 1 x 10 -4 mol•L -1 of dye ethanol solution. The mixture was then left at ambient conditions for 15 days. Sorption kinetics were investigated through UV/Vis spectrometry, measured on a CARY 5000 spectrophotometer (Varian Inc., Santa Clara, CA, USA). Calibration of solution concentration was carried out by means of absorption at selected wavelengths, as detailed in Figures S5-S8, ESI. Dye concentration throughout loading was monitored in-situ, in the absence of light at room temperature by soaking 10 mg of compound 1 in a Quartz SUPRASIL® cell with 4 mL of dyecontaminated ethanol (1 x 10 -4 mol•L -1 ) for MB and CV and (1 x10 -5 mol•L -1 ) for DY. Experimental duration was 11400 min for CV and DY, and 7200 min for MB. The quantity of dye adsorbed by compound 1, expressed in mmol per gram of 1 was calculated by the following equation. [34] 𝑄 𝑒𝑞 = 𝑉(𝐶 0 -𝐶 𝑒𝑞 ) 𝑚 (eq. 1)

where Qeq (mmol•g -1 ) is the amount of adsorbed dye by compound 1, C0 (mmol•L -1 ) is the initial concentration of dye in the water, Ceq (mmol•L -1 ) is the equilibrium concentration of dye remaining in the water, V (L) is the volume of the aqueous solution, and m (g) is the weight of compound 1 used. IR spectra on pristine and dye-loaded 1 were collected on a JASCO FT/IR-6100 spectrometer at room temperature in the range of 4000-400 cm -1 , in dry KBr pellets (1% of sample). Nitrogen physisorption at 77 K was used to determine BET area. [START_REF] Rouquerol | Adsorption by Powders and Porous Solids -2nd Edition[END_REF] Around 100 mg of sample was activated under secondary vacuum to 150°C for 16 hours with experiments carried out on a BELmax apparatus (MicrotracBEL, Japan). BET area of the microporous solid was calculated using the rules devised by Rouquerol et al. [START_REF] Rouquerol | Adsorption by Powders and Porous Solids -2nd Edition[END_REF][START_REF] Rouquerol | [END_REF] Accessible micropore volume was determined using the t-plot method. Gas adsorption isotherms and enthalpies at 303 K were measured simultaneously using a Tian-Calvet type microcalorimeter coupled with a home-made manometric gas dosing system. [37] The analyte is introduced into the system using a step-by-step method and each dose is allowed to stabilize in a reference volume before being brought into contact with the adsorbent located in the microcalorimeter. The introduction of the adsorbate to the sample is accompanied by an exothermic thermal signal, measured by the thermopiles of the microcalorimeter. The peak in the calorimetric signal is integrated to obtain the total energy released during this adsorption step. At low coverage, the error in the signal can be estimated to around 0.2 kJ•mol -1 . Around 0.4 g of sample was outgassed at 150°C for 16 hours under secondary vacuum prior to each experiment. For each injection, equilibrium was assumed to have been reached after 90 minutes. This was confirmed by the return of the calorimetric signal to its baseline (<5 μW). All laboratory-grade gases were obtained from Air Liquide. All IAST multicomponent adsorption simulations were performed using pyGAPS. [38] ARTICLE Please do not adjust margins Please do not adjust margins

Results and Discussion

Thermal Analysis

The structure of 1 with the formula [Ni5(H2TCPP)2O(H2O)4]•nS was previously reported. [33] In short, 1 crystallizes in the C2/m space group, with Ni-tetrapyrrolic units in a square planar coordination environment linked by centrosymmetric trinuclear complexes of octahedrally coordinated Ni atoms, giving rise to a 3D structure with two interconnected channels (Figure 1). After synthesis, the voids are occupied by solvent as DMF and water molecules, which can be removed through heating above 150 °C. In order to further confirm the thermal stability of compound 1 following the previously reported thermogravimetric and differential scanning calorimetry (DSC) analysis, [33] we employed thermodiffractometry, recorded on a powdered sample collected immediately after synthesis. The diffraction spectra (Figure S1 in ESI) show a small peak which is removed at 50 ⁰C leading us to think about the presence of impurity easily removable at low temperatures. Compound 1 retains crystallinity after removal of the guest molecules, as evidenced by the presence of discrete peaks beyond 150 ⁰C. However, above this temperature, the characteristic reflections begin to shift, indicating changes in the structure. Unfortunately, the decrease in crystallinity at high temperatures prevented an indexing of the evolution of cell parameters. Nevertheless, comparing simulated XRD pattern of 1 and the pattern registered at 160 ⁰C in thermodiffractometry (Figure S2 in ESI) it can be observed that the most characteristic reflections of 1 experimental and simulated 1 clearly fit better than low temperature patterns. This suggests that 1 suffers slight changes in its structure with the removal of the coordinated DMF and water molecules, approaching to the structural model previously reported [33] at a higher temperature. Considering that 67% of the electron density was assigned to removable solvates during structure resolution, [33] such changes are not surprising. In addition, previous IR spectroscopy results, alongside high resolution TEM images of activated compound 1 confirm the framework backbone remains stable up to 250 °C, with a slight deviation of the lattice. [33] Above 350 ⁰C, the structure collapses, forming a residue which has been identified by X-ray powder diffraction as NiO [S. G. Fm-3m, a = 4.17 Å]. [39]

Magnetic analysis

Owing to the Ni II trinuclear moieties present in compound 1, magnetic susceptibility measurements and EPR spectroscopy were able to be used for further characterization. The thermal evolution of the m -1 and m curves is shown in Figure S3 in ESI. The molar magnetic susceptibility m, is inversely dependent on temperature throughout the studied range. Above 20 K the susceptibility follows a Curie-Weiss law [m=Cm/( -θ)], with values of the Curie and Curie-Weiss constants of 6.09 cm 3 •K•mol -1 and 3.8 K, respectively. The m value at room temperature is 6.17 cm 3 •K•mol -1 , close to expected for five isolated S=1 Ni II ions with only a second order orbital contribution to the magnetic moment (6.05 cm 3 •K•mol -1 , considering g=2.20). When the sample is cooled, the m product undergoes a continuous increase reaching a value of

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Please do not adjust margins 9.68 cm 3 •K•mol -1 at 2.2 K. This result and the small positive Weiss temperature indicate the existence of weak ferromagnetic exchange interactions in this compound in the whole measured range.

Since the crystal structure of compound 1 shows the presence of two well-isolated Ni II ions in tetrapyrrolic units and a Ni II trinuclear cluster, we have fitted the magnetic properties of this compound to a model of a S=1 linear trimer with and additional paramagnetic contribution from two S=1 isolated centers (eq.

2)

.

m = 2m(mono) + m(tri) (eq. 2)
The first term is calculated assuming a purely paramagnetic behavior obeying a Curie-law (eq. 3):

m(mono)=(2Ngm 2  2 /3k) (eq. 3)
In the second term, the magnetic exchange in a centrosymmetric S=1 trimer is described by the Heisenberg Hamiltonian (eq. 4).

H=-2J

[S1S2+S2S3]-2J13(S1S3) (eq. 4)

where J and J13 represent the exchange coupling between neighboring and terminal ions, respectively. In the absence of a clear exchange pathway between non-neighboring ions, J13 is usually taken as zero in this type of systems. [40] Thus, the following analytical expression for the magnetic susceptibility of the trimer can be derived using the van Vleck equation (eq. 5):

m(tri)=(2Ngt 2  2 /3k)(3+42e 4x +15e 2x +18e -2x +3e - 6x )/(3+7e 4x +5e 2x +8e -2x +e -4x +3e -6x ) (eq. 5)

where x=J/kT.

The best least-squares fit (solid line in Figure S3 in ESI) was obtained with the following set of parameters: gm=2.15, gt=2.22, J/k= 6.1 K (4.2 cm -1 ), with a reliability factor R=4.5x10 -5 (R=[mT exp -mT cal ]2 / [mT exp ] 2 ). An additional proof of the ferromagnetic coupling present in this compound is provided by the EPR spectra registered at low temperature. It is well known that octahedrally coordinated Ni II ions are usually silent in X-band EPR spectroscopy due to the rather large zero-field splitting (ZFS) of the 3 A2 ground state. [START_REF] Carlin | Magnetochemistry[END_REF] However, compound 1 exhibits a broad intense EPR signal near zero-field at 4.2 K (Figure 2). The intensity of the absorption line decreases with increasing temperature following a Curie-Weiss law similar to that observed for the magnetic susceptibility (Figure 2), being =4 K. This behavior indicates that the observed EPR signal corresponds to an allowed transition (Ms=±1) within the S=3 ground state, with lower ZFS than the nickel ions inside the porphyrinic units. Above 150 K the signal becomes undetectable because of its low intensity and the short spin-lattice relaxation time of Ni II . The observed weak ferromagnetic interaction can be explained considering the angle of the oxygen bridges between two neighboring Ni II ions, which is close to 118⁰. It is worth mentioning that this is a higher value compared to typical values found in literature. [START_REF] Mukherjee | [END_REF] Figure 2. X-band EPR spectra of compound 1 registered at several temperatures. Inset shows the thermal evolution of the intensity of the EPR signal.

Dye adsorption

We now turn the investigating the potential of 1 for several applications. Organic dyes are prevalent in many industries, including medicine, textile, leather, printing and plastic. [43] and have emerged as soils and water pollutants, media in which they remain for long periods of time due to their stability. [44][45][46] Due to the promise of MOFs as porous materials for their capture, [47], [48] we investigated the dye adsorption properties of compound 1 on four of the most commonly used organic dyes (Figure 3). Typically, 10 mg of 1 were soaked in a vial with 4 mL of dyes in 1 x 10 -4 mol•L -1 ethanolic solution and left for 15 days. To the naked eye, the colour of the solutions changed to clearer after that period of time in the case of MB, CV and slightly in the case of DY. No changes were observable in the case of CR

This journal is © The Royal Society of Chemistry 20xx CrystEngComm, 2021, 00, 1-3 | 5

Please do not adjust margins Please do not adjust margins Dye adsorption was confirmed by IR spectroscopy for MB and CV (Figure S9 in ESI). For these loaded samples of compound 1, a widening of the band at 1580 cm -1 can be observed, which can be assigned to a greater presence of C=N bonds. Additionally, for samples loaded with MB, a band can be observed at 1150 cm -1 , associated with C-S bonds. The presence of these bands in the IR spectra confirms the successful immobilization of MB and CV into the porous structure. DY characteristic IR bands could not be determined due to the low amount of adsorbed dye, alongside the similar bond nature between DY and compound 1. [49] As represented in the adsorption histogram (Figure S10 In order to put these results in context, a comparison has been made with the adsorbed amounts by some known porous materials for cationic MB (as is the most reported in the field and the dye with the highest amount adsorbed by 1 under similar conditions to those of this work. Despite the fact that 1 is not among the highest adsorption capacity values reported for other well-known MOFs, this compound exhibits a remarkable adsorption capacity towards MB comparable to other porous materials, It is worth mentioning that the molecular dimensions of MB, CV and DY are in the size range of the square pore (Figure 1), whereas CR is much larger in width and length. The different kinetics of adsorption can be rationalised as the synergy among the dye shapes (Figure S4 in ESI), dimensions (Table S1 in ESI) and ionic strength, [56,57] likely conditioned by weakly ferromagnetic interactions in Ni II trinuclear complexes. Thus, cationic MB and CV were adsorbed by compound 1, while anionic CR and neutral DY adsorption were null or negligible. Adsorption of MB was higher and faster than for CV, due to the fact that MB molecular size is smaller than CV and can fit properly in the square pores of compound 1 (1.3 nm wide). This leads us to postulate that dye adsorption of compound 1 is more effective for cationic and small molecules, as in the case of methylene blue. This result clearly indicates that 1 could effectively and selectively adsorb dye molecules based on the charge and size-exclusion effect, which makes it a promising candidate for application in dye removal and fluid treatment.

Gas adsorption

All adsorption measurements were carried out with samples outgassed at 150 ℃ to ensure a complete removal of solvent from the pore network. The nitrogen accessible surface area at 77 K is found to be 388 m 2 •g -1 (Figure S13 in ESI), lower than crystallographically expected (over 1000 m 2 •g -1 assumed from the high void space in the lattice), indicating that some structural changes occur upon activation. Such effects are often encountered in porphyrin-based MOFs, with the surface area of the reference isostructural PIZA-1 [28] found to be three times lower than 1 after activation. As elucidated through thermal analysis, compound 1 shows subtle differences in its high temperature diffractogram (shifted characteristic peaks). The observed evolution likely accounts for some of the morphological surface changes, although we know from other techniques (FTIR and HRTEM) [33] that the framework remains stable. Ambient temperature (303 K) adsorption manometry was performed at high pressure with nine probes (N2, O2, CO, CO2, CH4, C2H6, C3H8, C3H6, C4H10), chosen for their differing physicochemical properties as well as for their potential industrial interest. Selected isotherms are presented in Figure 4 and 5. with the complete dataset of adsorption isotherms for each of the probes used available in Figures S16-S18 in ESI. All isotherms can be identified as Langmuir type I, with varying degrees of affinity to the framework due to the strength of the guest-host interaction. N2, O2 and CO isotherms are nearly linear, hinting to non-specific adsorption sites and weak attraction to the pore walls of 1. CO2, with its quadrupolar nature, has a much stronger affinity, reflected in the steep isotherm slope at low pressure. The slope is also observed to become sharper with the increase of the carbon number of the hydrocarbon series, as expected from the longer interacting chain of the probe molecule.

To further understand these interactions, adsorption microcalorimetry was employed. The surface properties of compound 1 can be investigated through direct measurement of the differential enthalpy of adsorption as a function of loading. While the recorded curve is a sum of all possible interactions within the system (both guest-host and guest-guest contributions) the shape of the curve can be still be rationalised.

In 1, it is expected that the Ni atoms coordinated at the centre of the porphyrin ligands may be able to act as sorption sites, interacting with adsorbed guests through the axial position, which should appear as a heterogenous profile in the enthalpy curve. The resulting experimental curves (with CO2, CH4, CO and C3H8 depicted in Figure 6 and other probes in Figures S16-S18 in ESI) do not suggest this to be the case, with the material displaying an unexpected surface homogeneity. Enthalpy profiles were either completely flat throughout the entire adsorption range or slowly decreasing as guest-guest interactions took over and further rearrangement of adsorbed molecules is required to completely fill the pores. [58] (35 kJ•mol -1 ) and HKUST-1 [59] (32 kJ•mol -1 ), yet lower than zeolite NaX (49 kJ•mol -1 ) or MIL-100(Fe) (39 kJ•mol -1 ), which are known to have specific CO2 adsorption sites. Carbon monoxide in particular, with its sorption enthalpy of 20 kJ•mol -1 , does not show evidence of porphyrin metal centre coordination. On the other hand, the initial adsorption enthalpy for methane is higher than prototypical MOFs and NaX zeolite, and comparable to Takeda 5A activated carbon. [59] This may suggest an effect from the framework itself, either through pore confinement or favourable stacking of molecules.

In terms of capacity, as assessed from uptake at 1 bar and 20 bar, 1 is seen to compare favourably to several conventional adsorbent families of industrial interest. These values are summarised in Table 1, alongside literature data for a microporous activated carbon molecular sieve, two zeolites and HKUST-1 MOF. It can be seen that 1 shows a lower-than-average capacity for CO2, around 1.2 mmol•g -1 at 1 bar, and up to 4 mmol•g -1 at 20 bar. In comparison, the carbon dioxide adsorption capacity for Takeda 5A and zeolite 13X are both higher, around 8 mmol•g -1 and 7.6 mmol•g -1 at 20 bar, respectively, while HKUST-1 is significantly higher at 13 mmol•g -

PCCP ARTICLE

This journal is © The Royal Society of Chemistry 20xx CrystEngComm, 2021, 00, 1-3 | 7

Please do not adjust margins Please do not adjust margins 1 . Other adsorbates follow similar trends as CO2, with uptakes on 1 roughly halved proportional to the microporous Takeda 5A. While the total uptake is lower than these reference materials, the more process relevant working capacity (WC) between these two pressures is seen to rival that of the zeolites at ~1.5 mmol•g -1 . Ideal adsorbed solution theory (IAST) calculations were carried out to assess the potential of 1 for mixture separation. The calculations had industrially relevant mixtures as model inputs:

(i) 10% CO2 in N2 as post-combustion flue gas simulant, (ii) an equimolar mixture of propylene and propane paraffin/olefin separation and (iii) a 1:1 ratio of carbon dioxide and methane as a common blend encountered in many natural gas deposits. The IAST-predicted selectivity is presented in Table 1 and compared to three relevant compounds. Selectivities for 1 are seen to be in the range of 19-19.5 for the CO2/N2 mixture, 3-4 for CO2/CH4, and slightly above unity for propane/propylene. For the former two mixtures, selectivities are comparable to those of the microporous carbon and the HKUST-1 MOF. However, they pale in comparison to those of Zeolite NaX, where strong CO2 interactions are known to efficiently sequester it from a mixture. Full composition range adsorbed-gas phase selectivity curves for each pair of gases chosen have also been generated, as shown in Figure S19 in ESI. It can be seen that the curves do not vary much with pressure, as previously suggested from the flat enthalpy profile for the mixture component gases. It should be noted that these selectivity values represent equilibrium results, with kinetic-based sieving likely able to play a role given the microporous nature of 1 and should be investigated in a further study. Overall, 1 is seen to most resemble the Takeda 5A activated carbon in its sorption behaviour, further motivating the general homogenous surface and flat enthalpy curves previously recorded.

Conclusions

In summary, the structural arrangement of the 3D porphyrinbased compound 1 exhibit crystallochemical features that result in several properties of interest, such us its weakly ferromagnetic interactions between Ni II ions and its permanent accessible surface area with open pores. As such, compound 1 has been tested and found to be a promising selective adsorbent for pollutants in solution and for energy-pertaining gases. For simulant dye pollutants in ethanol solution, our results confirm that the ferromagnetic compound 1 exhibits affinity for cationic dyes, as in the case of MB blue and CV. The channels in its crystal structure allow the entrance of guest molecules, albeit with possible steric hindrance for very large dyes. It is worth mentioning that 1 reduces the concentration of MB blue in ethanol more than 10 times in 15 days, a remarkable value comparable with other adsorbents in literature.

Regarding gas adsorption, the various probes used in this work confirm its remarkable adsorption capacity, comparable with other known MOFs and activated carbons. In addition, adsorption enthalpies measured through calorimetry reveal a homogeneous surface chemistry of 1 in the adsorption process, which indicate no specific interactions with these probes. Finally, IAST calculations predict promising selectivity for the separation of CO2 in N2, C3H6 in C3H8 and CO2 in CH4. These characteristics make 1 a great example of multifunctionality of porphyrin-based MOFs and encourage the community to study further the structure versatility of these materials in order to discover their numerous applications.

Figure 1 .

 1 Figure 1. a) [100] view of 3D framework of compound 1 showing the 13.3 Å x 13.3 Å channels, and b) [112] view showing the second type of channels (23.7 Å x 8.8 Å) in crystal structure of 1. (Ni: green, C: grey, N: dark blue, O: red). H atoms have been omitted for clarity.

Figure 3 .

 3 Figure 3. Colour changes of dye solutions of methylene blue (MB), crystal violet (CV), anionic Congo red (CR), and neutral dimethyl yellow (DY). All photos were taken after 15 days since the addition of compound 1.

  in ESI), 1 mg of 1 can adsorb 85 mmol (25.58 mg) of MB, 2 mmol (0.45 mg) of DY, and 54 mmol (22.03 mg) of CV. No uptake amount was found for CR. The kinetics of adsorption studied via in-situ UV-Vis spectroscopy are presented in Figure S11 in ESI, with a two-step process visible. The first is very rapid, while the second one is much slower. It is noteworthy to specify that for MB and CV the adsorption process gradually continues after the measured time, while for DY the adsorption stabilizes after 24 h. Adsorption kinetics were adjusted to a first order model, being the best fitting model for this adsorption performance. Due to the presence of two kinetic regimes, adsorption rate constants were calculated for the first step where most of the adsorption has taken place. Thus, calculated adsorption rate constants for MB, CV, and DY are 1.2 x 10 -2 , 3.3 x 10 -3 and 7 x 10 -4 min -1 , respectively (Figure S12 in ESI).

Figure 4 .

 4 Figure 4. Gas adsorption isotherms for CO2, CH4, and N2 at 303 K.

Figure 5 .

 5 Figure 5. Gas adsorption isotherms for C1-C4 hydrocarbons at 303 K.

Figure 6 .

 6 Figure 6. Differential enthalpy of adsorption as a function of amount adsorbed for four adsorbents which have the potential to interact differently with the open metal site in 1: CO2, CH4, CO and C3H6

Table 1 .

 1 Comparison of capacities of adsorption of MB on different adsorbents

	Adsorbent	Dye	Qeq (mg•g -1 )	K1 (min -1 )/ K2 (g/mg•min)	Ref.
	1	MB	25.58	1.2 x 10 -2 / -	This work
	HKUST-1	MB	29,5	-/-	[50]
	MOF-235	MB	100	-/9.58x10 -5	[51]
	Amino-MIL-101 (Al)	MB	380	7x10 -2 /2.6x10 -3	[52]
	Chitosan-clay composite	MB	26.93	2.1x10 -3 /2.1x10 -2	[53]
	Graphene oxide	MB	39.92	1.4x10 -2 /9x10 -4	[54]
	Carbon nanotubes	MB	24	-/ -	[55]

Table 1 .

 1 Uptakes of CO2, N2 and CH4 at 1 bar and 20 bar for 1. Relevant reference adsorbents are included for comparison.

	Adsorbent	CO2 (mmol/g)	N2		CH4		Ref.
				(mmol/g)	(mmol/g)
	pressure (bar)	1	20	1	20	1	20
	1	1.2	3.7	0.2	1.4	0.5	2.4	This
								work
	HKUST-1	5	13	0.3	2.2	1.1	5	[60]
	Takeda 5A	2	8	0.3	2.6	0.9	4.7	[61]
	Zeolite 13X	5	7.6	0.3	2	0.9	3.8	[60]
	Zeolite NaX	4.7	6.5	/	/	0.5	3.6	[60]

Table 1 .

 1 IAST calculated selectivity for compound 1 for three common binary mixtures, at 1 bar and at 5 bar. Some reference materials are included for comparison.

	Adsorbent	Com. 1	Com. 2	%1	Selectivity	Selectivity	Ref.
					(1 bar)	(5 bar)
	1	CO2	N2	10	19.0	19.2	This
							work
	1	CO2	CH4	50	3.2	3.6	This
							work
	1	C3H6	C3H8	50	1.4	1.5	This
							work
	Takeda	CO2	N2	10	19	/	[59]
	5A
	Takeda	CO2	CH4	50	/	4	[59]
	5A
	HKUST-1	CO2	N2	10	19	/	[59]
	HKUST-1	CO2	CH4	50	/	7	[59]
	Zeolite	CO2	CH4	10	/	212	[59]
	NaX

Acknowledgements

The This work has been financially supported by the ''Ministerio de Economia, Industria y Competitividad" (MAT2016-76739R) (AEI/FEDER, EU) and the ''Gobierno Vasco'' (Basque University System Research Groups, IT-1290-19) and the University of the Basque Country (GIU18/197) which are gratefully acknowledge. This journal is © The Royal Society of Chemistry 20xx Please do not adjust margins Please do not adjust margins The technical and human support provided by SGIker (UPV/EHU) is gratefully acknowledged. The MADIREL authors received funding from the European Union's Horizon 2020 research and innovation program, under the Marie Sklodowska-Curie grant agreement No 641887 (project acronym: DEFNET).

Notes and references

Conflicts of interest

There are no conflicts to declare.