Triptycene-based Porous Metal-Assisted Salphen Organic Frameworks

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Triptycene-based Porous Metal-Assisted Salphen Organic Frameworks – Influence of the Metal-Ions on Formation and Gas-Sorption Dennis Reinhard, Wen-Shan Zhang, Yana Vaynzof, Frank Rominger, Rasmus R. Schroeder, and Michael Mastalerz Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00614 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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Chemistry of Materials

Triptycene-based Porous Metal-Assisted Salphen Organic Frameworks – Influence of the Metal-Ions on Formation and Gas-Sorption Dennis Reinharda, Wen-Shan Zhangb, Yana Vaynzofb,d, Frank Rominger,a Rasmus R. Schröderb,c, Michael Mastalerz*a,b. a

Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany.

b

Centre for Advanced Materials, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany. c

Cryo Electron Microscopy, BioQuant, Universitätsklinikum Heidelberg, Im Neuenheimer Feld 267, 69120 Heidelberg, Germany. d

Kirchhoff-Institut für Physik, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany.

ABSTRACT: Porous organic polymers (POPs) are chemically and thermally robust materials and have been often investigated for their gas sorption properties. From the related field of metal-organic frameworks (MOFs) it is known that open ligation sites at metal centers can enhance the performance of gas-sorption significantly, especially the selectivity towards one gas of a binary mixture, such as CO2/N2 or CO2/CH4. POPs that contain metal centers are rarer. One possibility to introduce metals into POPs is by the synthesis of metal-assisted salphen organic frameworks (MaSOFs), where the framework development is associated with the formation of the metal salphen pockets. Based on a hexakissalicylaldehyde, a variety of three-dimensional isostructural porous MaSOFs with different metal ions (Zn2+, Ni2+, Cu2+, Pd2+, Pt2+) are introduced. All compounds show a very similar pore-structure and comparable specific surface areas, which makes these MaSOFs ideal candidates to study the influence of the nature of the incorporated metal center on gas sorption selectivity. Due to the environmental importance, the adsorption of CO2 in comparison to N2 and CH4 was extensively studied. Depending on the metal ions, the heat of adsorption was different as well as the Henry and IAST selectivities. The CuMaSOF100 for instance shows a high Qst of 31.2 kJ/mol for CO2 and an uptake of 14.9 wt% at 1 bar and 273 K. The IAST selectivity of CO2/N2 for a 80/20 mixture is with SIAST = 52 very high for a metal containing POP and even comparable to some of the best performing MOFs. The MaSOFs are stable even in boiling water. This as well as the simple synthesis, makes them potential good candidates for CO2 removal of binary mixtures.

INTRODUCTION The remarkable interest in porous materials is motivated by the large range of potential applications made possible by their carefully designed properties.1 Specifically, one of the most frequently investigated properties of porous materials is their interaction with various gases, driven by the demand for new approaches towards gas storage or gas separation by selective sorption of the adsorbate in the material pores.2-4 From metal-organic frameworks (MOFs)5-6 it is known that for porous structures containing metal centers with open ligation sites, these have a significant effect on the adsorbent-adsorbate interaction and hence on the selectivity of gas sorption. The most prominent MOF scaffold that is isostructurally independent of the incorporated metal ions is MOF-74 (or CPO-27) and clear effects of the open-metal sites and trends relat-

ed to the nature of the incorporated metal ions on gas sorption properties have been reported for such MOFs with various metal ions.7-10 In contrast to most MOFs and COFs (covalent organic frameworks; derived by reversible reactions),4, 11-17 threedimensional porous organic polymers (POPs) are chemically and thermally more stable due to the intrinsically more robust covalent C-C bonds that are formed during the synthesis of the network material.4, 18-21 Although a large variety of organic networks have been synthesized, reports of structures with incorporated metal ions, metal atoms or well-defined metal-clusters are still limited.22-40 Most of such networks are based either on building blocks that contain the metal ion, such as phthalocyanines and porphyrins,22, 26, 29-30, 35, 38-42 or by post-

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incorporation into free binding pockets,43 such as catechol units.32, 34 In 2012, we introduced a new approach for the construction of porous network materials by a direct formation of metal salphens.44 Molecular salphens (also called salophens in the literature), as well as the related salens, are square planar tetradentate O,N,N,O-ligands that are able to form complexes with a large variety of metals.45-48 The corresponding metal-salphens are easily formed either by reacting the salphen with a metal ion source, or directly in a one-pot-reaction, where the imine-condensation of a salicyldialdehyde and phenylene diamine in the presence of the metal ions (often provided by using the corresponding acetates) takes place. Depending on the choice of the incorporated metal ions, these metal centers contain free ligation sites within the complexes which, for instance, have been used in a number of catalytic transformations, such as epoxidation of olefins or formation of cyclic carbonates from CO2 and diols.47, 49-51 We used a rigid tetrahedral tetrakis-salicyldialdehyde52 as a building block in a metal-assisted reaction with ortho-phenylene diamine to form instantaneously porous metal-salphen frameworks (MaSOFs).44 Based on the same organic building block, nickel- and zinc-based MaSOFs with comparable specific surface areas (Brunauer-EmmettTeller model, BET) of SABET = 647 m2g-1 and SABET = 630 m2g-1 for the Ni-MaSOF and the Zn-MaSOF, respectively and similar pore-size distribution maxima at 4.2 Å were synthesized. Most interestingly, depending on the type of incorporated metal ions the heat of adsorption for adsorbents were different, demonstrating the effect of the metal ion on interaction with the adsorbate molecules.44 The synthesis of other porous metal-salphen networks (for simplicity, we name them all MaSOFs) by other groups have been reported, with particular focus on application as heterogeneous catalysts.53-62 To the best of our knowledge, there exists only one report for a MaSOFs with different metals that has been studied in terms of gas sorption selectivity.63 Herein, we report the direct synthesis of a series of structurally comparable triptycene-based MaSOFs with various metal ions (Zn2+, Ni2+, Cu2+, Pd2+ and Pt2+) incorporated by direct methods. The influence of the metal ions on the polymer formation and gas sorption properties is discussed.

RESULTS & DISCUSSION All the MaSOFs described here are based on the hexakissalicylaldehyde 1, which was synthesized by a six-fold Duff-formylation of hexakis-4-anisyltriptycene 2,64 giving the protected hexakissalicylaldehyde 3 in 50% yield (Scheme 1). By BBr3-mediated demethylation, the hexakissalicylaldehyde 1 was isolated with 65% yield after purification by column chromatography. All compounds were fully characterized by common methods (NMR, MS, IR, UV/vis, elemental analysis) as well as single-crystal structure determination (2 and 3). For detailed synthetic protocols and characterization, please see Supporting Information (SI). Methylated salicylaldehyde 3 is crystal-

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lizing in the triclinic space group P-1 with two molecules in the unit cell. Most interesting is the spatial arrangement of the salicyladehyde units to each other, which are twisted about 42° to the central ring of the triptycene (Fig. 1). The peripheral methoxy groups form a nearly ideal trigonal prism with edge lengths of ~14 Å x 8 Å, building the construction unit for the MaSOFs

Scheme 1. Synthesis of hexakissalicylaldehyde precursor 1. Yields are given in parentheses.

Figure 1. Single-crystal X-ray structure of triptycene 3. The MaSOFs were synthesized by stirring hexakissalicylaldehyde 1 with slightly over-stoichiometric amounts of ortho-phenylene diamine and the metal acetate in N,Ndimethylformamide (DMF) at an elevated temperature overnight respectively for a few days (Scheme 2 and Table 1). First, zinc acetate dihydrate was used as metal source and reacted at 100 oC for 16 hours to get the Zn-MaSOF100 in 97% yield as a yellow-brownish powder. When comparing the IR spectrum of the Zn-MaSOF100 with that of the salicylaldehyde precursor 1, the stretching band for the aldehydic C=O bond at 1650 cm-1 was no longer detected (Fig. 2 and Table 1, see also SI). Instead, pronounced characteristic C=N stretching bands appeared at 1614 cm-1 (ZnMaSOF), which are comparable to IR bands of structurally related salphens.52 The material was also analyzed by Xray photoemission spectroscopy (XPS). The Zn2p doublet at binding energies of 1022 and 1045 eV is characteristic for zinc in a +2 oxidation state. The N1s spectrum consists of two singlets, a main peak at 399 eV and a smaller contribution at 400.4 eV. The low binding energy peak is associated with the MaSOF, while the higher binding energy peak is likely to arise from peripheral amino units. To get an insight into the amount of Zn2+ incorporated into the salphen binding pockets of the three-dimensional polymer, a thermogravimetric analysis (TGA) under oxidative conditions (air) was performed. The TGA showed a nearly linear plateau until 300 oC, followed by a rapid decrease of weight until 500 oC, reaching another plateau with no further weight loss up to 800 oC. The remaining weight is 17.5 wt%. Assuming that the remaining material is zinc oxide exclusively, this value is in good agreement


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with the expected amount (17.7 wt%), under the assumption of a defect free ideal network, where all formed salphen pockets are filled with metal ions. This high degree of metal content is noteworthy, considering that for previously described MaSOFs in which the salphen organic framework was generated prior to post-synthetic zinc ion incorporation, less than 72% of the salphen pockets were occupied (the ratio reported for other metals is between 63% for Ni2+ to 88% for Cu2+).61 Under the same reaction conditions (DMF at 100 oC), without the addition of a metal ion source, no precipitate was formed, suggesting that no corresponding three-dimensional network was produced, since it is expected that it would be as well non-soluble. Investigation of the reaction solution by mass spectrometry methods indeed revealed only the formation of very small condensed molecular species, consisting of usually no more than one or two triptycene units that have reacted by condensation with phenylene diamine in a different ratio (see Supporting Information). These results confirm that polymerization occurs exclusively during metal salphen formation and explain the high percentage of measured metal content in excellent agreement with the expected value for an ideal composition.

Scheme 2. Synthesis of MaSOFs at different temperatures. Yields are given in parentheses. Conditions for MaSOF100 series: DMF, 100 oC, 16 h. Conditions for the MaSOF50 series: DMF, 50 oC, 5d. a) In both cases Pt(dmso)2Cl2 was used as metal ion source in combination with Et3N for the Pt-MaSOFs. For experimental details, see supporting information.

a 1650

b 1614

c 1611 2000

1800

1600

1400 1200 wavenumber [cm-1]

1000

800

Figure 2. Comparison of FT-IR (ATR) spectra of (a) the hexakissalicylaldehyde 1, (b) Zn-MaSOF100 and (c) CuMaSOF100. For full IR spectra of all MaSOFs, see Supporting Information. Similarly, copper(II)- and nickel(II) acetates were reacted under the same conditions, resulting in Cu-MaSOF100 and Ni-MaSOF100 in 79% and 90% yields, respectively. Similar to the Zn-MaSOF100, the analytics confirmed that no or minute amounts of aldehyde residues left in these two MaSOFs, as can be determined by the imine stretching bands at 1610-1612 cm-1 for both compounds as well as by XPS (see Supporting Information). XPS also confirmed the +2 oxidation state of the metal ions with no additional species detected for Ni, and only a very minor contribution of CuOx in the case of Cu-MaSOF. The situation was different when palladium(II) acetate was used as metal ion source. The IR spectrum of the Pd-MaSOF100 clearly shows a distinct peak at 1652 cm-1, which is associated to the stretching band of the remaining salicylaldehyde units. The ratio of Pd(II) and nitrogen determined by XPS is almost 1:2, suggesting possibly that the polymer particles are rather small and that salicylaldehyde units are mainly found at the outer surface. Another possible explanation could be that the network is less dense in the interior with more defects (unreacted aldehyde units). However, this possibility should be reflected by a broader pore-size distribution and/or a breathing behavior in gas sorption, due to lower stiffness of the material, which is not the case, so that the first explanation is more reasonable. Unlike for the other MaSOFs, for the Pt-MaSOF100 Pt(dmso)2Cl2 was used as metal ion source, and not the acetate. In this case, the results were comparable to PdMaSOF100 with the corresponding IR-spectrum clearly showing a band at 1651 cm-1 for salicyaldehyde groups. Similarly, to the Pt-MaSOF, TGA under air and XPS revealed that almost all salphen pockets were filled with Pt2+; once again suggesting that the material contains the salicylaldehyde units most likely in the outer surface. The reaction temperature can also be lowered to 50 oC to form the corresponding M-MaSOF50 with M = Zn2+, Ni2+, Cu2+, Pd2+, and Pt2+. Overall, the IR spectra as well as TGA and elemental analysis revealed similar conversions and



trends as described above for the reactions performed at 100 OC (see Table 1 and Supporting Information). Finally, for comparative reasons, a metal-free H2-SOF was synthesized in refluxing acetonitrile. In this case, similar to the Pd- and Pt-MaSOFs, pronounced bands of salicylaldehyde units could be detected by IR spectroscopy. All MaSOFs have been investigated by 13C MAS NMR spectroscopy (see Figs. S6-S10 in the Supporting information). Besides a number of intense peaks between 110 Zn-MaSOF100

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and 150 ppm for aromatic 13C nuclei, the most characteristic peak is always found at 58 ppm, which clearly can be assigned to the the triptycene bridgeheads. The peaks between 160 and 180 ppm are typical for the imine 13C nuclei of metal salphens. However, despite the good results by IR, which suggest no or minor aldehyde groups present in the samples, small peaks at approx. 200 ppm can be found in the corresponding 13C MAS spectra.

Zn-MaSOF50

1 µm Pd-MaSOF50

Pd-MaSOF100

400 nm

400 nm

400 nm Pt-MaSOF50

400 nm

400 nm Pt-MaSOF100

Cu-MaSOF50

Cu-MaSOF100

400 nm Ni-MaSOF50

Ni-MaSOF100

1 µm

400 nm

H2-SOF

400 nm

1 µm

Figure 3. SEM micrographs of all MaSOFs and the metal free H2-SOF synthesized from acetonitrile. Imaging of the MaSOFs by scanning electron microscopy (SEM) shows distinct structural features for each material depending on the incorporated metal (Fig. 3). These fea-

tures are found to be nearly identical for frameworks synthesized at 50 oC or 100 oC. The Zn-MaSOFs consist of needles, several hundreds of nm long, while Ni-MaSOFs


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rous three-dimensional organic polymers.4, 18, 21 For a more accurate comparison of the materials surface areas, we considered molar surface areas rather than specific surface areas, taking into account the influence of the atomic weight of the different metals. Indeed, the spread is now much smaller with molar surface areas ranging between 746 and 1111 m2mmol-1. Most intriguing, the analysis of the pore sizes by NL-DFT as well as QS-DFT67 showed that the pore structure is nearly identical for all compounds, with a sharp peak for a pore diameter of 5.7 Å and several very small broad peaks with pore diameters between 0.6 and 3.0 nm (see Figure 3 right). The micropore volume for all MaSOFs is approximately 70%.

are composed of agglomerated particles smaller than 100 nm. Cu-MaSOFs show more spherical agglomerates, whereas Pd- and Pt-MaSOFs can best be described as hybrids between flakes and nets in a layered structure. The metal-free network H2-SOF appears as µm long rods. Powder X-ray diffraction measurements show that no pronounced peaks could be found for any of the materials, in agreement with the expected amorphous nature of the materials, which is typical for three-dimensional polymeric structures made by irreversible reactions (for further details, see Supporting Information). All MaSOFs were investigated by nitrogen sorption at 77 K (see Fig. 4 and Table 1) and have been activated in an identical manner for a better comparison (180 oC, vacuum, o.n.). Isotherms of all compounds are type I isotherms65-66 that show, with the exception of Pd-MaSOF50, no significant hysteresis between the adsorption and desorption branches. The corresponding BET specific surface areas vary between 492 and 816 m2/g, which is typical for poMaSOF100 series

MaSOF50 series 400

Zn Ni Cu Pd Pt

200

1.0

100

0

0

0.2

0.4

0.6

0.8

1.0

Pt Pd Cu Ni Zn

200

100

0.0

QSDFT pore-size distribution

Zn Ni Cu Pd Pt

300

volume [cm3g-1]

300

The gas sorption data for the H2-SOF clearly differ from all MaSOFs. The BET surface area for this material is 116 m2/g and the pores are less structured with a broad distribution in pore-size (see Supporting Information), with no significant micropore volume.

normalized d(S)r [m2/Å/g]

400

volume [cm3g-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


0.0

0.2

0.4

0.6

0.8

0.8 0.6 0.4 0.2 0.0

1.0

0

1

2

3

4

5

6

pore diameter [nm]

p/p0

p/p0

Figure 4. Comparison of nitrogen sorption isotherms at 77 K. Left: MaSOF100-series; middle: MaSOF50 series; right: QSDFT pore-size distribution. Table 1. Overview on experimental data and pore analysis of all MaSOF compounds. MaSOF

[cm ]

[m g ]

[m mmol ]

[m g ]

[%]

NLDFT

QSDFT

Zn100

97

1614

638

881

456

71

0.55

0.57

2 -1

2

-1

2 -1

dS(r)max [nm]

d

a

-1

SAMicro

c

ν(CH=N)

[%]

SABET

b

yield

e

Vmicro

[cm g ]

-1

[cm g ]

0.37

0.19

Vtotal 3

3

Zn50

91

1614

540

746

392

72

0.57

0.57

029

0.16

Ni100

90

1612

816

1111

522

64

0.57

0.57

g

0.52

0.22

Ni50

94

1612

754

1026

550

73

0.57

0.59

0.41

0.23

Cu100

79

1611

697

959

477

69

0.57

0.59

0.40

0.20

Cu50

89

1610

725

998

497

69

0.57

0.57

0.41

0.21

Pd100

85

1605

511

769

372

73

0.57

0.57

0.27

0.15

Pd50

89

1607

575

865

385

67

0.57

0.57

0.34

0.16

Pt100

94

1605

572

1013

449

78

0.55

0.57

0.26

0.18

Pt50

82

1605

492

871

344

70

0.57

0.57

0.25

0.14

H2-SOF

96

1618

116

139

0

0

1.38

1.92

0.17

0.00

a

f -1

Yields determined under the assumption of full conversion of the reactants towards the corresponding MaSOF compound b c and based on the molecular mass of the MaSOF formula units. SA = surface area; N2, 77 K. Micropore contribution to SA (td e f plot). dS(r)max = global maximum in pore size distribution. Pore volume based on QSDFT calculations. Micropore volume (tg plot). global maximum ≠ local maximum.



To the best of our knowledge, in contrast to other MaSOF materials, this very narrow pore size distribution with one type of micropore is exceptional, allowing a more rational comparison of the effect of the embedded metal centers on the gas sorption properties. Therefore, we investigated the sorption of H2, CO2, CH4 and N2 with a focus on CO2

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specifically due to its environmental relevance.3, 68-74 In general, the MaSOF50- and the MaSOF100-series gave comparable results in gas uptake, although the amounts of gases adsorbed are in most cases slightly higher for the materials synthesized at 100 oC.

Table 2. Left column: Gas uptakes of MaSOF compounds. All values are in mol·mol-1, a and wt% in parentheses. Maximum uptake at 1 bar. Right columns: Qst’s and gas selectivities MaSOF

H2 77 K

Zn100 Zn50 Ni100 Ni50 Cu100 Cu50 Pd100 Pd50 Pt100 Pt50 H2SOF a

CH4 273 K

CO2 263 K

273 K

N2 263 K

273 K

7.57

1.42

1.80

4.28

4.90

0.50

(1.10)

(1.65)

(2.08)

(13.6)

(15.6)

(1.30)

7.38

1.44

1.69

4.02

4.54

0.52

(1.08)

(1.67)

(1.96)

(12.8)

(14.5)

(1.22)

8.94

1.70

2.07

4.83

5.58

0.57

(1.32)

(2.00)

(2.43)

(15.6)

(18.0)

(1.52)

8.60

1.51

1.84

4.48

5.20

0.52

(1.27)

(1.77)

(2.17)

(14.5)

(16.8)

(1.35)

8.93

1.66

2.05

4.66

5.39

0.58

(1.31)

(1.94)

(2.40)

(14.9)

(17.3)

(1.49)

8.19

1.60

1.97

4.72

5.50

0.55

(1.20)

(1.85)

(2.30)

(15.1)

(17.6)

(1.43)

7.61

1.49

1.79

4.11

4.66

0.53

(1.02)

(1.59)

(1.90)

(12.0)

(13.6)

(1.19)

8.35

1.55

1.87

4.24

4.89

0.57

(1.12)

(1.65)

(1.98)

(12.4)

(14.3)

(1.24)

8.27

1.50

1.77

4.39

5.13

0.48

(0.94)

(1.36)

(1.61)

(10.9)

(12.8)

(1.00)

8.02

1.45

1.74

4.09

4.66

0.50

(0.91)

(1.31)

(1.58)

(10.2)

(11.6)

(0.98)

4.19

0.75

0.93

2.12

2.39

0.37

(0.71)

(1.01)

(1.25)

(7.84)

(8.84)

(0.78)

-1 b

Qst [kJ mol ]

Henry selectivities c (S) at 273 K

IAST selectivities d at 273 K

CH4

CO2

21.9

30.3

20

97

10.0

48

17.6

29.2

19

90

9.5

45

20.7

27.8

20

95

9.0

42

19.4

28.6

16

81

8.8

44

21.2

32.1

19

102

10.3

52

20.1

29.0

18

94

9.9

50

19.5

27.3

17

101

10.5

56

17.6

26.0

15

84

8.7

43

18.7

32.2

18

111

9.5

56

22.6

30.1

18

100

9.5

48

20.3

26.1

46

131

10.5

32

b

CO2/CH4 CO2/N2 CO2/CH4 CO2/N2

c

Mol adsorbate per mol MaSOF formula unit. Qst = isosteric heat of adsorption; values given for zero-coverage. Henry’s Law d selectivity of gas A over gas B. IAST selectivity for simulated CO2/CH4 (0.5:0.5) and CO2/N2 (0.2:0.8) mixtures at 0.1 bar.

All compounds adsorb at 77 K H2 (between 0.91 wt% for the Pt-MaSOF50 and 1.32 wt% for the Ni-MaSOF100), more than measured for the metal-free H2-SOF (0.71 wt%). However, all MaSOFs have also larger specific surface areas than the metal-free material (116 m2/g), which most likely is the main cause for this difference. At 273 K and 1 bar, reasonable amounts of CH4 is adsorbed by all MaSOFs, ranging between 1.31 wt% for the Pt-MaSOF50 and 2.0 wt% for the Ni-MaSOF100. The heat of adsorptions (Qst’s) vary from 17.6 for Pd-MaSOF50 to 22.6 kJ/mol

for Pt-MaSOF50. The latter is comparable to Ni2(dobdc), one of the best performing MOFs for methane storage under high pressure.75 It is far more interesting to study the effect of free coordination sites using gases such as CO2, which can interact with the Lewis-acidic metal centers of the salphens.50 As mentioned above, to discuss the influence of the metal ion centers, it is more reasonable to use the molar gas uptake as a figure of merit, rather than comparing the specific gas uptake. We find that the molar uptake values are all in the same range, e.g. for CO2 at 273 K between 4.09 mol·mol-1 for the Pt-MaSOF50 and


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4.83 mol·mol-1 for the Ni-MaSOF100, which corresponds to a maximum specific uptake of 15.6 wt%. Although not as pronounced as reported for the MOF-74 series,8 a clear trend depending on the incorporated metal ion can be seen. According to the Qst of CO2 at zero coverage, the largest binding energy is found to follow the order of: Pt100 (32.2 kJ/mol) ∼ Cu100 (32.1 kJ/mol) > Zn100 (30.3 kJ/mol) > Ni100 (27.8 kJ/mol) > Pd100 (27.3 kJ/mol). Comparing these values with those of another metal-salphen based series, the MsMOPs (Ni-MsMOP: -33.4 kJ/mol > PtMsMOP: -29.3 > Zn-MsMOP: -26.9) it can be concluded that the order of the series members is more or less inversed.63 We also compared the members of the MOF74series with the same metal ions incorporated (Ni-MOF74: -38.6 kJ/mol > Zn-MOF74: -26.8, Cu-MOF74: -24.0).8, 10 Again, the trend is not the same as we observe for our MaSOFs. 40

Zn100 Ni100 Cu100 Pd100

35 30

Qst [kJ/mol]

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


Pt100

25 20 15 10 5 0 0

1

2

3

4

5

uptake [mol mol-1]

Figure 5. Heat of adsorption (Qst) of the MaSOF100 series for CO2. We note that a comparison of the metal-salphen materials with those of the MOF-74 series should be treated with care, because in MOF-74 the metals are pentacoordinated by oxo-based ligands, which has a tremendous influence on the free ligation site for a sixth ligand. This is different in metal-salphen based MaSOFs. Here, the metal ions are square-planar coordinated and in principle both faces of the metal-salphen plane are accessible to be attacked by CO2 molecules. Furthermore, one needs to take into account that depending on the metal used for the MaSOFs, in addition to polar effects of the metal centers (polarizability, electronic charge and van-der-Waals radius), a substantial amount of coordinative binding to CO2 of various strengths is influencing the heat of adsorption. For instance, it is known that zinc salphens act as Lewis acids to activate CO2 under higher pressure to react with epoxides to cyclocarbonates.50 In contrast, the corresponding Pd-, Ni- and Cu-salphens do not activate CO2 under same conditions. Despite this, Cu-salphens are known to be used as catalysts for epoxidations and thus can complex ligands in their axial positions.76 Pt-salphens

for instance show the tendency to form Pt-Pt dimers.77 Therefore, the measured values for the series most likely depend on these two basic contributions. In addition to that, all the values are derived from dynamic equilibrium measurements and therefore, the heat of adsorption is an average value for binding scenarios at different positions of the surface and not exclusively on the metal centers. However, more MaSOF materials need to be made to confirm this trend. The calculated Henry selectivities (SH) at zero coverage for CO2/N2 at 273 K of the MaSOFs follow nearly the same trend as the Qst’s for CO2 with the exception of the PdMaSOF100 found now between Cu- and Zn-MaSOF: Pt100 (SH =111) > Cu100 (102) ~ Pd100 (101) > Zn100 (97) > Ni100 (95). These values are substantially larger than for the metalsalphen based series of MsMOPs.63 Here, Ni-MsMOP was reported with a Henry selectivity of SH = 46 followed by the Zn-MsMOP (31) and finally the Pt-MsMOP (20). The Henry selectivities are also larger than for the most metalfree COFs70 and are in the same range as for some of the best performing amorphous three-dimensional polymers, such as BILP-2 (113)78 or PCNAD (112).79 It is worth mentioning that the metal-free H2-SOF shows an even higher Henry selectivity of SH =131, which is attributed to small and non-defined pores and not to a preferred interaction of the adsorbate and the adsorbents, because its Qst (26.1 kJ/mol) was found to be the lowest within this series. Despite the high value of SH, we note that not only the selectivity factors need to be considered for a prospective application, but also the overall amount (here 7.8 wt% vs. 15.6 wt% for Ni100-MaSOF) of uptake as well as the working capacities.72 More applicable are the IAST (Ideal Adsorbed Solutions Theory) values. These SIAST values were calculated for mixtures of 80/20 (N2/CO2) at 0.1 bar (see Table 2) and are found to be: Pt100 (56) ~ Pd100 (56) > Cu100 (52) > Zn100 (48) > Ni100 (42). These values are larger than those previously reported for Ni-MOF-74 (30)80, or BUT-11 (32)81, and comparable to Cu-TDPAT (58)82 but lower than for ZnMOF-74 (87),82 Mg-MOF-74 (148)83 or zeolites, such as Na-X (310).84 They are also substantially lower than those of systems that rely on kinetic sieving (>25000), although the amount of taken up CO2 is smaller in this case.85 We also determined the selectivities for a binary gas mixture of CO2/CH4 (50/50), which is a typical composition for a landfill gas. The Henry selectivities of all MaSOFs for such a mixture at 273 K are in the same range varying between SH = 15 for the Pd-MaSOF50 and SH = 20 for the Ni-MaSOF100 or Zn-MaSOF100. Again, the H2-SOF shows a higher SH = 46 than all MaSOFs, most likely for the same reason as discussed for CO2. In contrast to the Henry selectivities, the IAST selectivities of the best performing MaSOFs are in the same region as for H2-SOF (SIAST = 10.5). One of the major challenges for porous materials is their relative chemical instability with some of the best performing MOFs in terms of gas sorption being sensitive to humidity resulting in material degradation.86-88 Therefore,



we tested the stability of Zn-MaSOF100, Ni-MaSOF100 and Cu-MaSOF100 in boiling water for three days. All compounds kept their porosity, although the Zn-MaSOF100 lost 34% of the specific surface area, which dropped from 638 m2/g to 420 m2/g. In contrast to the Zn-MaSOF100, the specific surface area of the Ni-MaSOF100 increased from 816 to 1093 m2/g and the specific surface area of the Cu-MaSOF100 remained nearly constant (608 as compared to 697 m2/g). Analysis of the materials by IR revealed in the case of the Zn-MaSOF100 a certain amount of degradation, because a shoulder at ṽ = 1651 cm-1 has evolved. In case of the Ni- and Cu-MASOFs this band is significantly less pronounced, which suggests that these materials are more stable, which is in good agreement to previous observations made on molecular salphens.89

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Supporting Information. Synthetic procedures and full characterization of all precursors and MaSOFs. NMR, FT-IR, MS spectra, TGA curves, PXRD of all MaSOFs and H2-SOF, single-crystal X-ray structure data of precursors. Gas sorption measurements and analytics of the isotherms of all MaSOFs and H2-SOF. XPS data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

CONCLUSION

The authors declare no competing financial interest.

To summarize, a new series of three-dimensionally connected metal-salphen networks (the MaSOFs) have been introduced based on a hexakissalicylaldehyde reaction with ortho-phenylene diamine and a metal ion source. Zn2+, Ni2+, Cu2+, Pd2+, and Pt2+ have been incorporated as metal ions. Under typical reaction conditions, it was found that the MaSOFs are exclusively formed in the presence of the metal ion source, otherwise a clear solution remained consisting of small molecular condensation products. As a consequence, the metal content of the resulting materials is close to that expected of an ideal network. All MaSOFs are porous with a very similar pore size distribution, which allowed the study of the role of the metal ions of these materials on gas sorption. Due to the importance of CO2-removal from gas mixtures, its gas sorption in comparison to N2 and CH4 was investigated more deeply. Amounts of up to 15.6 wt% of CO2 were taken up at 273 K, which is comparable to a large number of MOFs or metal-containing COFs. Depending on the metal ion, the heat of adsorption for CO2 varied approximately ∆Qst = 5 kJ/mol between 32.2 kJ/mol for the PtMaSOF100 and 27.3 kJ/mol for the Pd-MaSOF100. All MaSOFs showed high Henry as well as IAST selectivities for binary mixtures of CO2 and N2, which are substantially higher than for other metal-salphen based network materials, but also comparable to some of the record materials without metals, such as BILP-2 or PCNAD and some members of the MOF-74 series, such as the Cu- or the NiMOF-74. However, the compounds cannot compete with the Mg-MOF-74 or zeolite Na-X. Fortunately, it is known from salen and salphen chemistry, that a very large number of metal ions can be incorporated. This will allow us to explore more isostructural MASOFs with different metals than it is most likely possible for the MOF-74 series. Furthermore, by using different phenylenediamine precursors it is possible to optimize and fine tune the gas sorption parameters towards certain desired applications.90

ACKNOWLEDGMENT

ASSOCIATED CONTENT

The authors like to thank the DFG (Deutsche Forschungsgemeinschaft) for generous financial support. D. R. acknowledges Sven M. Elbert for helpful discussions on analysis of gas sorption data.

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Triptycene-based Porous Metal-Assisted Salphen Organic Frameworks

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