Crystal Engineering of Phenylenebis(azanetriyl))tetrabenzoate Based

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Crystal engineering of phenylenebis(azanetriyl))tetrabenzoate based metal-organic frameworks for gas storage applications. Philipp Müller, Volodymyr Bon, Irena Senkovska, Juergen Getzschmann, Manfred S. Weiss, and Stefan Kaskel Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00184 • Publication Date (Web): 17 Apr 2017 Downloaded from http://pubs.acs.org on April 18, 2017

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Crystal Growth & Design

Crystal engineering of phenylenebis(azanetriyl))tetrabenzoate based metalorganic frameworks for gas storage applications Philipp Müllera), Volodymyr Bon a), Irena Senkovska a*), Jürgen Getzschmann a), Manfred S. Weiss,b) and Stefan Kaskel a)* a)

Department of Inorganic Chemistry, Technische Universität Dresden, Bergstraße 66, 01062

Dresden, Germany b)

Makromolekulare Kristallographie, Helmholtz-Zentrum Berlin für Materialien und Energie

BESSY-II, Albert-Einstein-Straße 15, 12489 Berlin, Germany

The

metal-organic

framework

Cu4(mpbatb)2

(mpbatb

-

4,4′,4″,4‴-(1,3-

phenylenebis(azanetriyl))tetrabenzoate), also known as DUT-71 (DUT – Dresden University of Technology) was functionalized via post synthetic cross-linking of the copper paddle wheels by linear salen derivatives and dabco (1,4-diazabicyclo[2.2.2]octane). This results in a series of porous MOFs, denoted as DUT-117(M) (M – Cu, Ni, Pd). Beside the significant improvement of the framework robustness, the influence of the metal coordinated by the salen ligand, on the gas adsorption capacity (hydrogen 77 K, methane 298 K, and carbon dioxide 298 K) was

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investigated. In this series DUT-117(Ni) stands out as the best material for adsorptive methane storage with a high working capacity of 171 cm3·cm-3 between 5 and 65 bar.

INTRODUCTION The storage of energy carrier gases such as H2 and CH4, as well as capture of environmentally relevant gases such as CO2, is of high importance.1–3 Physical adsorption of gases in porous media is a promising technology because of high storage density, fast kinetics, and reversibility of the adsorption/desorption processes. Metal-organic frameworks (MOFs) have emerged as a new class of versatile porous materials and are intensively studied regarding potential applications in the field of gas technology (e.g. capture, separation, storage).4–8 In the last decades significant scientific effort has been undertaken to extend the understanding of factors influencing gas storage capability of porous materials. A large part of this research involves also theoretical work investigating the effects of surface area, density of open metal sites, pore volume, and pore size on the adsorption properties of the MOF materials.9,10 In the meantime, it is widely accepted, that the density of open metal sites affects the affinity of the MOF to certain gases at lower coverages, while the high specific surface area, large pore volume and optimal pore size are beneficial for enhancing storage capacities in moderate to high pressure range. Certain pore size heterogeneity is needed to enhance the storage capacity at high pressures. For example for methane the small micropores reaches a saturation at about 25 bar and larger pores are essential to increase the adsorbed amount at higher pressure.11 MOFs are ideal modular materials, allowing to be modified in conformity with theoretical predictions, to achieve high storage density.12–16 The modularity of MOFs also allows, to some extent, the correlation between the distinct functional groups and gas adsorption properties. In


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this contribution, a MOF (DUT-71) based on copper paddle wheels and tetratopic ligand (mbpatb =

4,4′,4″,4‴-(1,3-phenylenebis(azanetriyl)tetrabenzoate)

was

systematically

tuned

post-

synthetically and the effect of the variations on storage performance was investigated for methane, hydrogen, and carbon dioxide as adsorptives. The structure of DUT-71 consists of two types of cages (Fig. 1): a larger one of 21.4 x 11.0 and a smaller cage of 22.7 x 5.2 Å in diameter (the distances are measured from Cu atom center to Cu atom center considering the van der Waals radii of the atoms). The structure contains a solvent-accessible void of 76.2% of the unit cell volume, but unfortunately, the framework collapse during desolvation could not be avoided even by using advanced activation techniques. Nevertheless, the post synthetic cross-linking strategy was previously shown to be very successful to achieve the robust framework with the specific surface area of 2700 m2/g and pore volume of 1.1 cm3/g in case of DUT-95 derivative.17 Therefore, several variations of the cross linker were evaluated to study the influence on the adsorption behavior of different gases.

EXPERIMENTAL SECTION Nitrogen physisorption isotherms at -196 °C, carbon dioxide physisorption isotherms at -78°C and 0 °C, methane physisorption isotherms at -162 °C were measured using a BELSORP-max apparatus (MicrotracBEL Corp.). BET surface areas were calculated from the range of nitrogen adsorption isotherm identified through the consistency criteria (Fig. S25 – S28). Hydrogen physisorption isotherms at -196 °C up to 110 bar were measured using volumetric BELSORPHP apparatus (MicrotracBEL Corp.). High pressure methane and carbon dioxide adsorption at 25 °C was studied using a magnetic suspension balance (Rubotherm Co.). High purity gases were


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used (N2: 99.999%, H2: 99.999%, CH4: 99.5%, CO2: 99.995%). The total gas uptake (Ntotal) was calculated as: Ntotal = Nexcess + ρbulkVpore, where ρbulk is equals to the density of compressed gases at the measurement temperature and pressure, and Vpore was obtained from the N2 sorption isotherm at -196 °C. Thermogravimetric analyses (TGA) were carried out in air atmosphere using a Netzsch STA 409 thermal analyzer. Elemental analysis (C, H, N) was performed with a Hekatech EA 3000 Euro Vector CHNS analyzer. All powder X-ray diffraction patterns were measured in a range of 2° - 50° 2Θ on STOE STADI P (STOE, Darmstadt, Germany), equipped with a Cu-Kα1 radiation (λ = 0.15405 nm) and gas-filled 0D detector. MOFs synthesis: DUT-71 [Cu4(mpbatb)2·4H2O] and DUT-95 [Cu4(mpbatb)2(bpta)0.5(dabco)2.5] were synthetized according to the procedure reported previously.17 Synthesis of DUT-117(M) [M4(mpbatb)2(Cu(salen))0.5(dabco)x] (M = Cu, Ni, Pd): To the washed crystals of DUT-71, corresponding M(salen) solution was added. Cu(salen) solution: 48 mg (0.1 mmol) of Cu(H2Salen)(NO3)2 dissolved in 40 mL DMF; Ni(salen) solution: 50 mg (0.108 mmol) of Ni(H2Salen)(NO3)2 dissolved in a mixture of 5 mL DMF and 7 mL DMSO; Pd(salen) solution: 53 mg (0.122 mmol) Pd(salen) dissolved in 30 mL DMF. The mixture was heated at 80 °C for 7 days giving Cu4(mpbatb)2(M(salen))0.5 (DUT-116(M)). After cooling to room temperature, the supernatant was replaced by a fresh mixture of DMF and ethanol (1:1). To the washed DUT-116(M) crystals, dabco (57.4 mg, 0.51 mmol) dissolved in a mixture of DMF and ethanol (1:1, 6 mL) was added and the mixture was heated at 80 °C for further 7 days. After cooling to room temperature, the supernatant was replaced by a fresh mixture of DMF and ethanol (1:1).


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Elemental analysis for DUT-117(Cu): Cu4(C68O16N4H40)(C16O2N4H16Cu)0.5(C6N2H12)2.05(H2O)1.5 [Cu4(mpbatb)2(Cu(salen))0.5(dabco)2.05(H2O)1.5]: Calc. (%): C, 57.1; H, 4.1; N, 7.6. Found: C, 56.0; H, 3.9; N, 7.8. Elemental analysis for DUT-117(Ni): Cu4(C68O16N4H40)(C16O2N4H16Ni)0.5(C6N2H12)1.75(H2O)0.9 [Cu4(mpbatb)2(Ni(salen))0.5(dabco)1.75(H2O)0.9]: Calc. (%): C, 57.3; H, 3.9; N, 7.3. Found: C, 56.8; H, 4.1; N, 8.0. Elemental

analysis

for

DUT-117(Pd):

Cu4(C68O16N4H40)(C16O2N4H16Pd)0.5(C6N2H12)1.85(C3ONH7)1.1(H2O)1.5 [Cu4(mpbatb)2(Pd(salen))0.5(dabco)1.85(DMF)1.1(H2O)1.5]: Calc. (%): C, 55.9; H, 4.2; N, 7.8. Found: C, 55.7; H, 3.9; N, 8.1. Crystal structure data Crystal data for DUT-117(Cu) C91H76Cu4.50N11O17, Mr = 1881.55, tetragonal P4/mnc, a = 26.490(4) Å, c = 27.350(5) V = 19192(7) Å3, µ = 0.975 mm-1, Z = 4, Dc = 0.651 g cm-3, 11830 independent reflections observed, R1 = 0.0575 (I > 2σ(I)), wR2 = 0.2303 (all data), and GOF (I > 2σ(I)) = 1.158. Crystal data for DUT-117(Pd) C91H76Cu4N11O17Pd0.50, Mr = 1902.98, tetragonal P4/mnc, a = 26.470(4) Å, c = 27.420(5) V = 19212(7) Å3, µ = 0.957 mm-1, Z = 4, Dc = 0.658 g cm-3, 10836 independent reflections observed, R1 = 0.0559 (I > 2σ(I)), wR2 = 0.2073 (all data), and GOF (I > 2σ(I)) = 1.069. Crystal data for DUT-117(Ni) C91H76Cu4N11Ni0.50O17, Mr = 1879.14, tetragonal P4/mnc, a = 26.520(4) Å, c = 27.260(5) V = 19172 (7) Å3, µ = 0.964 mm-1, Z = 4, Dc = 0.651 g cm-3, 9362 independent reflections observed, R1 = 0.0528 (I > 2σ(I)), wR2 = 0.1967 (all data), and GOF (I > 2σ(I)) = 1.073.


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CCDC-1526350-1526354 contain the supplementary crystallographic data for DUT-116(Cu), DUT-116(Pd), DUT-117(Pd), DUT-116(Ni) and DUT-117(Ni). CCDC-1524167 contains the experimental data for DUT-117(Cu). These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

RESULTS AND DISSCUSION The enormous variability introduced by cross linking renders DUT-71 as an ideal modular platform for post synthetic insertion of metallated linkers to study factors influencing gas storage capability. The role of the cross-linker is fourfold: a) to stabilize the network during activation; b) to reduce the pore size; c) to create additional surface area per volume; d) to facilitate the metalation and insertion of additional open metal sites.

Scheme 1. Schematic representation of the materials investigated. DUT-71 was infiltrated with two types of N-neutral pillar ligands of different length, to achieve the maximum degree of cross linking (Scheme 1, Fig. 1 and 2), the strategy previously explored


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for DUT-95 (Fig. S2, S3 and S18).17 Because the successful crosslinking of larger pore could be achieved earlier using di(4-pyridyl)-1,2,4,5-tetrazine (bpta), it prompt us to use a salen derivative metalloligand with similar distance between the donor nitrogen atoms. Previously, salen derivatives have been also successfully used to design MOFs for selective adsorption and separation processes.18–21

Figure 1. Crystal structure of DUT-71 (a), DUT-116(M) (b), and DUT-117(M).


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The crystals of DUT-71 were soaked in the corresponding metalloligand solution (M(salen), M = Cu, Ni or Pd) at elevated temperature to afford DUT-116(M). Further treatment with dabco solution leads to the formation of DUT-117(M). Powder X-ray diffraction patterns (Fig. S4 – S9) and single crystal X-ray diffraction studies (Tab. S7 – S9) of materials after linker insertion demonstrate the successful incorporation of desired molecules into DUT-71 (Fig. 1). According to PXRD analysis DUT-117 is readily desolvated without loss of framework integrity and porosity. A small shift of the peak positions (Fig. 2, Fig. S2 – S17, Tab. S1 – S4) was observed after desolvation indicating small changes of the lattice parameters and some minor flexibility of the networks.

Figure 2. Powder X-ray diffraction patterns of DUT-117(Pd): calculated (1), as made (2), resolvated (3), desolvated (4).


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Indeed, Le Bail analysis of the X-ray powder diffraction data shows that the unit cell volume reduces only up to 5% upon activation (Tab. S5). Interestingly, for other DUT-71 derivatives, such as DUT-90, DUT-91, DUT-113, and DUT-114 no reflections shifts in PXRDs were observed, indicating a higher degree of rigidity of the latter. After analysis of crystal structures of all DUT-71 derivatives studied in this work, the conclusion could be drawn, that if the distance between the paddle wheels is less than 14 Å, the material is quite rigid against solvent removal. Longer Cu – Cu spacings induce some flexibility in the framework. Table 1: Lattice parameters of DUT-95 and DUT-117(M) in “as made”, activated and methane-loaded form obtained from Le Bail analysis.

a in Å

c in Å

Unit cell volume in Å3

DUT-95 as made

26.50203(191)

26.84974(237)

18858(2)

DUT-95 dry

26.80013(604)

26.96332(742)

19366(8)

DUT-117(Cu) as made

26.42022(26)

27.33182(36)

19078.3(3)

DUT-117(Cu) dry

26.60229(174)

25.55323(220)

18083(2)

DUT-117(Pd) as made

26.44991(31)

27.45505(43)

19207.4(4)

DUT-117(Pd) dry

26.75955(119)

25.67058(123)

18382(1)

1 bar CH4@DUT117(Pd)

26.67761(73)

25.42702(192)

18096.2(1.5)

60 bar CH4@DUT117(Pd)

26.69385(62)

27.67484(199)

19720.0(1.5)

DUT-117(Ni) as made

26.51028(43)

27.27620(51)

19169.5(5)

DUT-117(Ni) dry

26.75937(89)

25.65426(154)

18370(1)

a)

Volume change in %

2.7a

5.2a

4.3a

9.0b

4.2a

(Vdry – V as made)/Vas made; b) (V60 bar – V 1 bar)/V1 bar


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Despite the unit cell changes after activation, no hystereses were observed in nitrogen physisorption isotherms of DUT-95, DUT-117(Cu), DUT-117(Pd), and DUT-117(Ni) measured at 77 K and the MOFs show Type Ia isotherm shape (Fig. S24 - S28), according to the IUPAC classification.22 The high BET surface areas and pore volumes determined for materials containing metallated cross-linkers and dabco (Tab. 2) were in line with others DUT-71 derivatives reported earlier.17 Table 2: Textural properties of investigated compounds.

Material

BET surface area, m2·g-1

Pore volume, cm3·g-1

DUT-95

2700

1.07

DUT-117(Cu)

2637

1.04

DUT-117(Pd)

2737

1.07

DUT-117(Ni)

2939

1.15

The degree of substitution achieved in a post-synthetic modification approach can vary over a wide range. The ideal composition of the materials synthesized was expected to be Cu4(mbpatb)2(L)0.5(dabco)2.5 (L = cross linker). De facto, significant variations of dabco content were detected, most notably for DUT-95 and DUT-117. The effective ratios of metal, carboxylic ligand and both N-neutral ligands were calculated based on the results of elemental analysis and thermal analysis (Fig. S18 – S22, Tab. S5). According to these data, the volumetric storage capacity was calculated using the crystallographic density based on the effective composition. Hydrogen adsorption at 77K


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MOFs as materials with very high porosity have ideal initial requirements to achieve high gravimetric capacities such as high specific surface area (as adsorption relies on surface upon which gas molecules can adsorb) and high pore volume. The primary disadvantage of using adsorbents are the relatively weak adsorption enthalpies of hydrogen (of about 4 - 10 kJ/mol) requiring essentially cryogenic temperatures and high pressures to achieve high storage capacities.23 The calculated optimum adsorption enthalpy for room temperature storage at moderate pressures is 15 kJ/mol.24 To enhance the storage capacity, the incorporation of strong adsorption sites (for example open metal sites) is considered as one of promising strategies.25 Also reduced pore size was found to have positive effects on the density of stored gas.26 Calculations based on carbon materials showed that slit pores with a width of 6 Å should have the highest hydrogen uptake at very low pressures because they exhibit the strongest interaction potential.27 The actual target is set by DOE and amount to 5.5 wt.% for gravimetric and 40 g·L-1 for volumetric uptake. These values are targeted for the moderate system operating temperatures (-40 °C - 50 °C).28 The highest total uptake among investigated materials of 6.46 wt.% and 41.6 g·L-1 was measured for DUT-95 at 42.7 bar (pressure of maximum in the excess adsorption isotherm) and -196 °C (Fig. 3, Tab. 3 and Fig. S30 - 33). The values for the metal doped materials DUT-117(M) are lower (Tab. 3), but the maximum in excess adsorption is reached at lower pressures of ca. 35 bars (Fig. 3b, S30 - S33). Obviously, the reduction of pore size (Fig. 4) and additional open metal sites in the DUT-117(M) structures positively affect the interaction strength between hydrogen and the network. According to Tsivion et al., the hydrogen molecule can be substantially polarized by metallated linker, but the polarizing ability is highly dependent on the geometry of the metal ion coordination site, where a strong electrostatic dipole or quadrupole moment is required.29 The reached values of hydrogen uptake correlate with the


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“Chahine’s rule”.4,30,31 Furthermore correlations of gravimetric and volumetric hydrogen uptake were in good agreements with theoretical calculations of Siegel et al.31

8 50

hydrogen uptake / wt%

7 6

40

5 30

4 3

20

2 10

1 0

hydrogen uptake / g·L-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 0

10

20

30

40

50 60 p / bar

70

80

90

Figure 3. Hydrogen high pressure total adsorption isotherms (77 K) of DUT-95 (circle), DUT117(Cu) (triangle), DUT-117(Pd) (square) and DUT-117(Ni) (diamonds). Filled symbols represent adsorption, open symbols represent the desorption points. Table 3: Hydrogen adsorption data at 77 K.

Material DUT-95 DUT-117(Cu) DUT-117(Pd) DUT-117(Ni)

Pressure, bar 42.7

Uptake, wt.% excess/ total 5.17 / 6.46

Uptake, g·L-1 excess / total 32.8 / 41.6

34.7

4.85 / 5.91

32.3 / 39.8

44.0

4.84 / 6.18

32.2 / 41.7

35.7

4.57 / 5.70

30.3 / 38.3

43.0

4.55 / 5.91

30.25 / 39.8

35.9

4.90 / 6.12

32.1 / 40.6

43.0

4.89 / 6.35

32.0 / 42.2


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Figure 4. Pore size distribution for DUT-71 (black), DUT-95 (red), and DUT-117 (blue) calculated by Zeo++ software. Adsorption of methane at 298 K Similar to the hydrogen storage, it is challenging to store methane at sufficient high densities. The DOE targets claim 0.5 g·g-1 for gravimetric capacity and 350 cm3 cm-3 for volumetric capacity at 65 bar and 298 K.28 Theoretical calculations show that the optimal pore diameters of adsorbents for storage at 35 bar are in the range 4–9 Å and the optimum pore diameter at 100 bar is 8 Å.32 It is also known, that presence of additional side groups and open metal sites have positive influence on the methane uptake.13,15,33–35 The DUT-71 has pores which are obviously too large for the optimal storage at moderate pressures. By incorporation of pillaring ligands, the pore size is two times reduced approaching the optimal value (Fig. 4). The introduction of salen based linker creates additional adsorption sites and metal centers in DUT-117 series).


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The methane adsorption isotherms of a series of synthesized compounds were measured gravimetrically, using magnetic suspension balance and total storage capacities were calculated taking into account the pore volume and density of compressed methane (see experimental section). DUT-95 and DUT-117(Ni) shows the highest capacity within the series with a total uptake up to 240 mg·g-1 up to 65 bar, whereas DUT-117(Cu) and DUT-117(Pd) score with lower values of 201 mg·g-1 and 229 mg·g-1 correspondingly (Fig. 5 and Fig. S34 – S37). Actual record holders in terms of gravimetric uptake are MOF-210,36 Al-soc-MOF-1,37 and DUT-4938 showing all an uptake of 410 mg/g at room temperature and 65 bar. To date, the highest volumetric methane storage and working capacities (at 65 bar and room temperature) were determined

for HKUST-1 (about 270 and 200 cm3 (STP)/cm3, respectively).39 The

effective release from the tank filled with HKUST-1 powder operated at room temperature between 65 and 5 bar is ca. 130 v/v. 40 Because the framework density is a crucial factor for calculation of volumetric storage capacity, the composition of the materials was estimated from elemental analysis in combination with thermogravimetric analysis. The framework of

starting

compound

DUT-71

has

a

crystallographic density of 0.515 g·cm-3. Further incorporation of the pillar ligands into the DUT-71 should increases the density of the framework. To show the influence of the stoichiometry deviation on the calculated volumetric uptake, the ideal (derived from ideal formula for fully pillared MOF: Cu4(mpbatb)2(L)0.5(dabco)2.5) and experimentally determined densities were compared. To calculate working capacities the differences in uptake between 5 and 65 bar were used.


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a) 250

0.25 200 0.20 150 0.15

methane uptake / a.u.

methane uptake / g·g-1

0.30

0.10

100 50

0.05 10

15

20

25

0.00 0

15

30

45

30

35 40 p / bar

45

50

55

60

methane uptake / cm3·cm-3

0

60 75 90 105 120 p / bar

b) 180

1.2 1.1 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

working capacity CH4 (5 - 65 bar) / cm3·cm-3

160 140 120 100 80 60 40 20 0 DUT-95

total pore volume / cm3·g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


DUT-117(Cu)DUT-117(Pd) DUT-117(Ni)

Figure 5. (a) Methane total adsorption isotherms (77 K) for DUT-95 (circles), DUT-117(Cu) (triangles), DUT-117(Pd) (squares) and DUT-117(Ni) (diamonds). Filled symbols represent adsorption, open symbols represent desorption points. (b) Relationship between the working capacity (5 to 65 bar) and the accessible pore volume. The best volumetric performance was achieved for DUT-117(Ni) with an uptake of 214 cm3·cm3

at 65 bar and working capacity of 171 cm3·cm-3. Despite offering additional metal sites, for


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DUT-117(Cu) and DUT-117(Pd) the working capacities are lower than that of DUT-95 (164 cm3·cm-3). DUT-95 contains a tetrazine based pillaring-ligand and can be considered as a reference system containing no additional metal atoms. The pillared DUT-71 derivatives have working capacities comparable to other MOFs, such as, PCN-14 (160 cm3·cm-3),41 NOTT-109 (170 cm3·cm-3),14 UTSA-80 (174 cm3·cm-3)42 or ZJU-36 (175 cm3·cm-3).33 Interestingly, high pressure methane adsorption isotherms of DUT-95 and DUT-117(Pd) show small hystereses between the adsorption and desorption branches in the pressure range 40-50 bar and 20-45 bar, correspondingly, while the adsorption/desorption isotherms for DUT-117(Cu) and DUT-117(Ni) are fully reversible (Fig. 5a). Since hysteretic behavior in methane high pressure adsorption is rear,43 the reproducibility of the phenomenon was proved by repeated adsorption experiment on DUT-95 (Fig. S38). In situ PXRD during high pressure methane adsorption on DUT-117(Pd) at 25° C Taking into account the “shrinkage” of the framework upon desolvation, the “opening” of the framework is expected during adsorption of gases. Therefore, in situ powder X-ray diffraction pattern were measured on the KMC-2 beamline (BESSY-II synchrotron, HZB) during high pressure methane adsorption on DUT-117(Pd) at 25 °C (Fig. 6).


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Figure 6. PRXD patterns of DUT-117(Pd) measured in situ during high pressure methane adsorption between 1 and 60 bar (25 °C). Although, the data could not be collected at 2Θ below 10° due to the restrictions of the experimental setup, the variation in the position of (504) reflection is indicative for the changes in unit cell parameters (Fig. 6). In PXRD patterns, measured in adsorption branch at methane pressure from 1 to 45 bar, reflection (504) is observed at the same position, at 2Θ = 21.02°. Further pressure increase leads to the stepwise shift of the reflection towards 2Θ = 21.22°. The stepwise desorption of the methane reveals the opposite trend: the reflection moves back to 2Θ = 21.02° at 15 bar, pointing on the reversible structure transformation. PXRD patterns, measured at 1 and 60 bar were subjected to the Le Bail refinement, which allowed to extract the unit cell


17


parameters (Tab. 1). The values, obtained from in situ data are in a good agreement with values, obtained from refinement of “as made” and activated phases. This observation led to the assumption that the similar type of the framework dynamic occurs during the activation of the material and desorption of the gas. High pressure CO2 adsorption at 25 °C Since storage and capture of CO2 are discussed as technologies for the reduction of greenhouse gas emissions environment preservation, CO2 adsorption isotherms were measured at 25 °C up to

carbon dioxide uptake / g·g-1

50 bar (Fig. 7 and S39 to S42). 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0

300 250 200 150 100 50 0 0

5

carbon dioxide uptake / cm3cm-3

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10 15 20 25 30 35 40 45 50 p /bar

Figure 7. Carbon dioxide total adsorption isotherms (25 °C) of DUT-95 (circles), DUT-117(Cu) (triangles), DUT-117(Pd) (squares), and DUT-117(Ni) (diamonds). Filled symbols represent adsorption, open symbols represent desorption points. DUT-117(Ni) has again the highest gravimetric and volumetric uptake (1.045 g·g-1, 331 cm3·cm3

) followed by DUT-95 (1.029 g·g-1, 315 cm3·cm-3), DUT-117(Pd) (0.845 g·g-1, 272 cm3·cm-3),


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and DUT-117(Cu) with the lowest uptake (0.763 g·g-1, 246 cm3·cm-3). In contrast to methane physisorption no hystereses were observed. Low pressure CO2 (at -78 °C and 0 °C) and methane (-162 °C) adsorption The isosteric heat of adsorption would gain deeper insight into the nature of the adsorption sites and gas - framework interactions. Therefore the CO2 adsorption experiments at different temperatures were performed (Fig. S29) to calculate the heat of adsorption values using Van’t Hoff equation. Unfortunately, the calculation of the isosteric heat was not feasible, because the hystereses were observed between adsorption and desorption branches at -78 °C. It brought us to the conclusion that MOFs have pronounced flexible character during adsorption of CO2 at low temperatures and the flexibility degree is depending on the introduced pillaring ligand (Fig. S29). DUT-117(Cu) is the most rigid system, since only a small hysteretic loop was observed between adsorption and desorption branches. DUT-117(Pd) and DUT-117(Ni) show larger loops, and DUT-95 shows the highest degree of flexibility in the presence of CO2 at low temperature. Such metal dependent flexibility was also observed for other flexible MOFs such as DUT-8(M)44 and MIL-53.45–47 Thus, the compounds are further representatives of flexible MOFs, showing gas and temperature dependent flexibility.48–53 Since DUT-95 shows pronounced hysteresis during high pressure methane adsorption at 25 °C and low pressure CO2 (-78 °C) adsorption, the methane adsorption at -162 °C (boiling point of CH4) was also investigated. Interestingly, Type Ia isotherm and no hysteresis was observed in the methane adsorption isotherm of DUT-95 at low temperature by comparable uptake (Fig. 8).


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700

gas uptake / cm3g-1

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600 500 400 300 200 100 0 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

p/p0

Figure 8. Carbon dioxide physisorption isotherms at -78 °C (circles) and methane physisorption isotherm at -162 °C (squares) of DUT-95. Filled symbols represent adsorption, open symbols represent desorption points.

CONCLUSIONS Summarizing, we could show that post-synthetic modification of MOFs by cross linkers is a powerful crystal engineering tool enabling simple and versatile functionalization of the frameworks. The detailed analysis of framework composition after modification points on different functionalization degrees, possibly leading to the some flexibility of the modified MOFs. The insertion of additional metals into DUT-71 was also possible in a post-synthetic way using salen derivatives. The investigated MOFs have high surface areas up to 2900 m2g-1 and pore volumes up to 1.15 cm3g-1, resulting in high adsorption capacities for hydrogen, methane and carbon dioxide. The best results in high pressure methane and carbon dioxide adsorption at 25 °C could be achieved for DUT-117(Ni). It shows a high volumetric working capacity up to


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171 cm3cm-3 (between 5 and 65 bar) for methane and can adsorb 331 cm3·cm-3 carbon dioxide at 50 bar. In contrary, the highest total uptake for hydrogen at -196 °C among investigated materials was measured for DUT-95 (6.46 wt.%, 41.6 g·L-1), a framework containing no additional metal sites. In the case of DUT-95 and DUT-117(Pd) a narrow hystereses in high pressure methane adsorption isotherms at 25 °C were observed. In situ powder X-ray diffraction measurements on DUT-117(Pd) at various methane loadings pressures at 25 °C proved the flexibility of the framework upon adsorption. The flexible behavior of the frameworks is strongly dependent on the adsorptive and the temperature of adsorption experiment. No indications of flexibility were observed in the methane adsorption isotherm recorded at -162 °C. In opposite, pronounced flexibility of DUT-95 upon adsorption of carbon dioxide at -78 °C is not reflected in the adsorption isotherm at 25 °C.

ASSOCIATED CONTENT Supporting Information. The following information is available free of charge: Synthesis of the linkers, PXRD patterns, details of

Le Bail analysis, thermogravimetric analysis data,

nitrogen physisorption isotherms, additional high pressure isotherms, carbon dioxide physisorption isotherms at different temperatures up to 1 bar, crystallographic data. AUTHOR INFORMATION Corresponding Authors * [email protected], [email protected];


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interests ACKNOWLEDGMENT We thank HZB for the allocation of synchrotron radiation beamtime as well as for financial support. Authors are thankful to Sven Grätz, En Zhang, Dr. Dirk Wallacher and Nico Grimm for performing in situ synchrotron PXRD experiments. V.B. gratefully acknowledges the German Federal Ministry of Education and Research (BMBF Project No 05K16D1) for financial support. ABBREVIATIONS MOFs, metal-organic frameworks; DUT, Dresden University of Technology; mbpatb, 4,4′,4″,4‴1,3-phenylenebis(azanetriyl)tetrabenzoate.

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For Table of Contents Use Only.

The influence of the metal (M) coordinated by the salen ligand in DUT-117(M) compounds (M – Cu, Ni, Pd) on the gas adsorption capacity for hydrogen (77 K), methane (298 K), and carbon dioxide (298 K) was investigated.


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Crystal Engineering of Phenylenebis(azanetriyl))tetrabenzoate Based

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