Solvent- and pH-Dependent Formation of Four Zinc Porous

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Solvent- and pH-Dependent Formation of Four Zinc Porous Coordination Polymers: Framework Isomerism and Gas Separation Jingui Duan, Yang Wang, Haifei Cao, Baishu Zheng, and Rongfei ZHOU Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01433 • Publication Date (Web): 19 Oct 2018 Downloaded from http://pubs.acs.org on October 22, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Crystal Growth & Design

Solvent- and pH-Dependent Formation of Four Zinc Porous

Coordination

Polymers:

Framework

Isomerism and Gas Separation Yang Wang†, Haifei Cao†, Baishu Zheng‡, Rongfei Zhou†, Jingui Duan†*

†State

Key Laboratory of Materials-Oriented Chemical Engineering, College of

Chemical Engineering, Nanjing Tech University, Nanjing 210009, China. Email: [email protected]

‡

School of Chemistry and Chemical Engineering, Hunan University of Science

and Technology, Xiangtan 411201, China.

KEYWORDS: Porous coordination polymer, Framework isomerism, Solvent system, pH, CO2 separation ABSTRACT.

Four

new

zinc

porous

coordination

polymer

(PCP)

were

synthesized from a linear ligand of [1,1'-biphenyl]-3,3',5,5'-tetracarboxylic acid

ACS Paragon Plus Environment

1

Crystal Growth & Design 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

(H4L)

and

zinc

nitrate

by

the

strategy

of

Page 2 of 39

high

throughput

solvothermal

syntheses. The structural difference of them arises from different pH of the medium, as well as the composition of the solvent system. Very interesting is that although NTU-34, -35 and -37 possess same 3, 3, 4-connections, they have completely different porous frameworks, along with varied cavities size and shape. In addition, given their same framework formula [Zn2L], these three PCPs can be considered to be a new group of framework isomers. Further, the coordination between distorted Zn4O clusters and linear ligands with partial intermolecular π···π interaction formed the other porous framework (NTU-36), which has a new topology with 3, 3, 3, 6-connection and point symbol of {4.62}4{42.5.64.88}2{5.102}{52.8}).

More

importantly,

gas

adsorption

and

breakthrough experiments revealed that the microporous nature enabled NTU-36 to selective remove CO2 from its CH4 or N2 mixtures under flowing conditions at ambient temperature.

INTRODUCTION


2

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As a kind of new crystalline solids, PCPs offer great promise in applications of gas storage, separations, catalysis, conduction, and drug delivery, since their highly tunable properties make themselves excellent rivals to other porous materials1-4. Following the recent development, the concept of secondary building unit (SBU) has been established as a synthetic module for the predesign and construction of robust frameworks5. For example, the coordination between Zn4O cluster and a linear carboxylate ligand preferred to form the porous framework with pcu topology6. Even for a longer ligand, the pcu framework remains to be formed, but the interpenetration occurs7, 8. For more popular Cu paddlewheel, it is mostly limited to obtaining only a single framework with no further exploration of multiple frameworks from a single carboxylate ligand. Despite these, a few exceptions were reported by Zhou and Fröba group, respectively9,

10.

In other words, there are still mixed opinion as to

whether a real pre-design, unlike the organic synthesis, in their synthesis can be applied. Therefore, in light of the SBU strategy, the crystallization process of PCP makes the predictability of the final network to be a difficult task11,


12.

3


Page 4 of 39

Accordingly, given the selected ligand and metal ion, structural diversity of the generated PCPs is related to the external environment, such as the crystallization method, reaction temperature, pH, solvent, reaction time, template agents and et. al.13-16. Among them, solvent system was generally believed as one of the most significant factors.17 This is because, by acting as a solvent, a template or a second ligand, the coordination reaction speed, the coordination configuration, and/or the coordination geometry may be influenced by different solvents molecules18,

19.

In addition, bearing the same building blocks, the

coordination isomerism was occasionally found in PCP area20-22, and now it is attracting more interest as it remarkably important in pharmacy and catalysis. This is because the active site of a isomer is often known to have crucial influence on specific property, but not for the other isomer23. Thus, the rational realization of coordinated isomerism, especially for the porous framework, would be an interesting and also a difficult task up to date24.

We are interested in the design and construction of PCPs with functional nanospace for energy related applications.25-30 Here, in continue our work, we


4

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report the syntheses of four new zinc PCPs based on a biphenyl tetracarboxylate ligand via the systematic tuning of involved solvent composition and medium pH (Scheme 1). Interestingly, with the same framework formula [Zn2L], NTU-34, -35 and -37 can be considered as a new group of supramolecular isomers, along with varied cavity size and shape. By the coordination between distorted Zn4O cluster and rectangle ligand, the generated NTU-36 possesses good porosity and a new 3, 3, 3, 6-c topology with point symbol of {4.62}4{42.5.64.88}2{5.102}{52.8}). Importantly, gas adsorption and breakthrough experiments showed that the microporous nature of NTU-36 enabled highly selective CO2 removal from its CH4 and N2 mixtures under flowing conditions.


5


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Scheme 1. Four Zinc Porous Coordination Polymers with Varied Topology and Underlying Isomers, Synthesized by Changing the Solvent system and medium pH.

EXPERIMENTAL SECTION

Materials and Measurements. All reagents and solvents were commercially available and used as received. General procedures and details of the adsorption experiments, spectroscopic techniques and structure details are provided in the Supporting Information.

Synthesis of NTU-34. Zinc(II) nitrate (30 mg), H4L (8 mg) and HNO3 (10 μL, 16 mol/L) were mixed within 1.3 mL of N,N’-Dimethylformamide (DMF) in a 10 mL glass container and tightly capped with a Teflon vial and heated at 80°C for two days. After cooling to room temperature, the colorless crystals were harvested and washed with DMF and ethanol for three times, respectively. Elemental analysis (EA) calcd (%) for evacuated NTU-34, [C16H6O8Zn2]: C, 42.05, H, 1.32; found: C, 41.65; H, 1.51.


6

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Synthesis of NTU-35. Zinc(II) nitrate (30 mg), H4L (8 mg) and HNO3 (30 μL, 16 mol/L) were mixed with 1.3 mL of DMF/H2O (12: 1) in a 10 mL glass container and tightly capped with a Teflon vial and heated at 80°C for two days. After cooling to room temperature, the colorless crystals were harvested and washed with DMF and ethanol for three times, respectively. EA calcd (%) for evacuated NTU-35, [C8H3O4Zn]: C, 42.05, H, 1.32; found: C, 41.72; H, 1.48.

Synthesis of NTU-36. Zinc(II) nitrate (30 mg), H4L (8 mg) and HNO3 (30 μL, 16 mol/L) were mixed with 1.3 mL of DMF/methanol/H2O (8: 4: 1) in a 10 mL glass container and tightly capped with a Teflon vial and heated at 80°C for two days. After cooling to room temperature, the colorless crystals were harvested and washed with DMF and ethanol for three times, respectively. EA calcd (%) for evacuated NTU-36, [C48H18O27Zn2]: C, 40.63, H, 1.28; found: C, 40.25; H, 1.39.


7


Page 8 of 39

Synthesis of NTU-37. Only one rod shaped crystal of NTU-37 co-existed with NTU-36 crystals in one bottle. However, in a pity, it can’t repeat again, despite of hundreds of tries with systemically turned conditions.

Crystallographic analysis. Single-crystal X-ray diffraction measurements were performed on a Bruker Smart Apex CCD diffractometer at 298 K using graphite monochromated Mo/Kα radiation (λ = 0.71073 Å). Data reduction were made with the Bruker Saint program. The structures were solved by direct methods and refined with full-matrix least squares technique using the SHELXTL package

31.

Non-hydrogen atoms were refined with anisotropic displacement

parameters during the final cycles. Organic hydrogen atoms were placed in calculated positions with isotropic displacement parameters set to 1.2 × Ueq of the attached atom. The unit cell includes a large region of disordered solvent molecules, which could not be modeled as discrete atomic sites. We employed PLATON/SQUEEZE32,

33

to calculate the diffraction contribution of the solvent

molecules and, thereby, to produce a set of solvent-free diffraction intensities; structures were then refined again using the data generated. CCDC 1859360-


8

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1859363 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Crystal data are summarized in Table 1.

Table 1. Crystal Data and Structure Refinement of the Four PCP Structures.

NTU-34

NTU-35

NTU-36

NTU-37

Empirical formula

C25H29N3O12Zn2

C8H4O5Zn

C48H23O29Zn6

C16H12O11Zn2

Formula weight

694.29

245.50

1456.01

511.04

Space group

P21/n

R-3m

Imma

P21

a /Å

14.3251(17)

18.797(2)

39.394(9)

10.042(11)

b /Å

13.0260(13)

18.797(2)

20.323(5)

16.470(18)

c /Å

17.896(2)

24.928(6)

22.578(5)

11.508(12)

α /°

90

90

90

90

β /°

95.832(1)

90

90

115.87

γ /°

90

120

90

90

V /Å3

3322.1(7)

7628(3)

18076(7)

1713(3)

Dcalc/gcm-3

1.388

0.962

1.070

0.991

Z

4

18

8

2

μ/mm-1

1.502

1.433

1.623

1.432

Θ rangeº

-17≤ h ≤17

-22≤ h ≤22

-46≤ h ≤46

-11≤ h ≤11

-15≤ k ≤15

-22≤ k ≤22

-24≤ k ≤24

-19≤ k ≤19

-21≤ l ≤21

-30≤ l ≤30

-26≤ l ≤26

-13≤ l ≤13


9


Page 10 of 39

Index ranges

1.9, 25.0

1.5, 25.5

1.0, 25.0

2.0, 25.0

R1

0.0530

0.0274

0.1328

0.1047

wR2a[I>2σ(I)]

0.1811

0.0914

0.3429

0.2377

S

0.94

1.07

1.306

1.02

R = Σ||Fo|-|Fc||/Σ|Fo|, wR = {Σ[w (|Fo|2 - |Fc|2)2]/Σ[w (|Fo|4)]}1/2 and w = 1/[σ2(Fo2) + (0.1452P)2] where P = (Fo2+2Fc2)/3 RESULTS AND DISCUSSION

Structure

Description

of

NTU-34.

A

solvothermal

reaction

of

H4L

and

Zn(NO3)2·6H2O in DMF at 80 °C affords colorless crystals with high yield. The single crystal X-ray analysis reveals that the [Zn2(L)(H2O)(DMF)2]·solvent (named as NTU-34) crystallize in P21/n space group with a = 14.3251(17) Å, b = 13.0260(13) and c = 17.896(2) Å. Thus, the asymmetric unit of NTU-34 includes two Zn2+ ions, one ligand, one coordinated water molecule and two coordinated DMF molecules (Figure 1a). Further structural analysis showed that both of the two isolated Zn2+ ions have octahedral geometry. However, Zn1 center was completed by two chelate carboxylate groups (η2) and two η1 carboxylate

oxygen

atoms,

while,

Zn2

center


was

bridged

by

three

η1

10

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carboxylate oxygen atoms, two coordinated DMF and a coordinated water molecule (Figure 1b). Due to the chelating and bridging mode (μ2-η2: η1), Zn1 and Zn2 were connected together to form a dimmer cluster with the distance of 3.330 Å. Meanwhile, each ligand binds to four separated zinc dimmers, forming the 3D porous network and 1D open channel along c-axis (6 × 8 Å2) (Figure 1c). Applying the topological analysis, the zinc dimmer can be rationalized as 4-connected node, while, the ligand can be simplified as a two 3-c linker. Therefore, the network can be envisioned as a tfi net with point symbol of {62.84}{62.8}2 (Figure 1d-e). Calculated by PLATON32, the total accessible volume of NTU-34 reached to 61.6 % (2045.1/3322.1 Å3) upon guest removal with a crystallographic density of 0.914 g·cm-3 (Figure 1f). MOFs with very similar structures to that of NTU-34 were also synthesized and reported by Tian and Geng group, respectively34,

35.


11


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Figure 1. X-ray crystal structure of NTU-34: Asymmetric unit (a); Coordination modes of zinc dimmer and H4L ligand (b); Packing view of the 3D framework with 1D channel along c-axis (c); Simplification of ligand (two 3-c node) and dimmers (4-c node) (d); Topology view of NTU-34 with tfi topology (e) and the view of 3D cross-linking tunnels by Connolly surface (inner surfaces: blue, outer surfaces: grey) (f).

Synthesis and structure of NTU-35. By replacing the solvent from DMF to DMF/H2O and also adding a few drop HNO3 in solvent, the other colorless and


12

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block crystal, termed as NTU-35, was obtained. Single-crystal X-ray diffraction analysis showed that NTU-35 crystallizes in relative higher space group of R-

3m. The asymmetric unit of NTU-35 includes half zinc atom,

Figure 2. X-ray crystal structure of NTU-35: Asymmetric unit (a); Coordination modes of zinc dimmer and H4L ligand (b); View of two kinds of alternative and open cages (c); Simplification of ligand (two 3-c node) and dimmer (4-c planner node) (d); Topology view of NTU-35 with fof topology (e) and the view of 3D


13


Page 14 of 39

cross-linking tunnels by Connolly surface (inner surfaces: blue, outer surfaces: grey) (f).

one-fourth of ligand and half coordinated water molecule (Figure 2a). Along with the coordinated terminal water molecule in axial position, each Zn atom coordinates four carboxylate oxygen atoms from four different ligands. In other words, each carboxylate adopts the classical mode of η2 to bridge the two zinc atoms, thus, leading to a different zinc dimmer, regulated as [Zn2(CO2)4] paddlewheel (Figure 2b). Unlike the distortion of the ligand in NTU-34 (18.99°), the two phenyl rings are co-planar by symmetry growth in NTU-35. In addition, the four coordinated carboxylate groups are also co-planar with the phenyls by a small dihedral angle of 8.41°. Therefore, like MOF-50536, MOF-505 analog37 and NOTT series38, the generated 3D framework also consists of two different types of alternative packed open cages (Figure 2c). The small cage is about 9 Å in diameter surrounded by 6 paddlewheel Zn2(CO2)4 clusters and 6 ligands, and the large elliptical cavity is about 14 × 8 Å2 encapsulated by 6 Zn2(CO2)4 clusters and 3 isophthalic acid moiety and 3 biphenyl moiety (the van der


14

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Waals radii was subtracted). Due to the open characteristic of the two cages, there exists two kinds of small 1D channel along c-axis. Moreover, along a and

b axis, there also exists a kind of small 1D channel. Topology analysis showed that the underlying network possesses the fof topology when simplifying the organic ligand to be the two 3-c linker and Zn2(CO2)4 paddlewheel to be a planar 4-c node, defined by carboxylate carbon atoms (Figure 2d-f). Calculated accessible pore volume of NTU-35 is as high as 64.0% (4883.2/7628.0 Å3).

Synthesis and structure of NTU-36. By adding a certain amount of ethanol in the synthesis system for NTU-35, a new colorless and block crystal was obtained, termed as NTU-36. Single-crystal X-ray diffraction analysis revealed that the space group of NTU-36 is Imma, where the asymmetric unit includes two Zn3O clusters, one and two half ligands and two coordinated water molecule. It is worth to note that there is a symmetry mirror along the defined plane by Zn1, Zn3 and O22. Thus, the classical Zn4O cluster arose from the symmetry operation of the Zn3O cluster (Figure 3a). Similarly, the Zn4O also connected by six carboxylate groups from six different ligands. However, due to


15


Page 16 of 39

the coordination distortion of the carboxylate group, a pair of ligands stacked together by face to face, forming a significant intermolecular π···π interaction (3.697 Å).

Figure 3. X-ray crystal structure of NTU-36: Asymmetric unit (a); Coordination modes of zinc and EBTC ligand (b); View of open channel along b- and c-axis (c-d); Simplification of ligand (two 3-c nodes) and dimmers (4-c planner node) (e); Topology view of NTU-36 with new topology (f) and the view of three


16

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dimensional cross-linking tunnels by Connolly surface (inner surfaces: blue, outer surfaces: grey) (g).

Therefore, like a single planner tetra-carboxylate ligand, such pair of ligands also connects four Zn4O clusters in a co-planer configuration (Figure 3b). The infinite coordination extension generates a kind of unique ribbon, which was further bridged by another two ligands in vertically to form the 3D framework (Figure S11). Obviously, there are two kinds of regular open channels with size of 5 and 9 Å along c-axis. In addition, the open channels can also be found along b-axis


17


Page 18 of 39

Figure 4. X-ray crystal structure of NTU-37: Asymmetric unit (a); Coordination modes of zinc and ligand (b); View of open channel along a- and c-axis (c-d); Simplification of ligand (two 3-c nodes) and dimmers (4-c planner node) (e); Topology view of NTU-37 with new topology (f) and the view of two dimensional cross-linking tunnels by Connolly surface (inner surfaces: blue, outer surfaces: grey) (g).

and a-axis (Figure 3c-d). Thus, the tunnels intersected with each other and formed a fully open framework. Moreover, NTU-36 shows 16 water coordinated


18

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Zn4O units per unit, corresponding to potential open metal sites. For better understanding the framework, the ligand and Zn4O were simplified as two 3-c linkers and 6-c nodes, respectively. Thus, NTU-36 features a new topology with point symbol of {4.62}4{42.5.64.88}2{5.102}{52.8}. Similarly, calculated solventaccessible pore volume from single crystal structure is 52.8% (Figure 3e-g).

Synthesis and structure of NTU-37. Due to different morphology, the only one rod-shaped crystal was also observed during the synthesis of NTU-36. Single crystal X-ray analysis showed that this complex, termed as NTU-37, crystallizes in P21 space group with significant smaller cell parameters towards that of NTU-36 (Table 1 and Figure 4). The asymmetric unit of it consists of one ligand, two zinc ions and three coordinated water molecules (Figure 4a). As can be seen, the coordination geometry of Zn1 was also completed by two chelate carboxylate groups (η2) and two η1 carboxylate oxygen atoms, while, Zn2

center

was

bridged

three

η1 carboxylate oxygen atoms and three

coordinated water molecules. Similarly, the two Zn atoms here were also bridged by one carboxylate to form a dimmer cluster with the distance of 3.397


19


Page 20 of 39

Å, which is a little longer than the distance of Zn1 and Zn2 in NTU-34 (3.330 Å). In addition, each ligand was connected by four of such kind clusters (Figure 4b). However, the dihedral angle of the two phenyl rings in a ligand (62.21º) here is far larger than that of NTU-34 (8.41º) and also NTU-35 (0º). Therefore, the formed 3D porous network features the linked channels along a- and caxis, respectively, with size of 4.5 × 4.0 and 3.3 × 5.6 Å2 (Figure 4c-d). By using the same simplification mode, the network can be envisioned as a new 3, 3, 4-c net with point symbol of {83}2{86}. Similarly, calculated solventaccessible pore volume from single crystal structure is as high as 63% (Figure 4e-g).

Isomer analysis and influenced factors. Without thinking the coordinated and lattice solvent molecules, NTU-34, NTU-35 and NTU-37 possess the same framework formula [Zn2(L)]. Thus, they can be regarded as a new group of framework isomers. All nets are (3, 3, 4)-connections,


20

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Figure 5. Illustration of the linking mode of H4L ligand in NTU-34, NTU-35 and NTU-37 isomers.

the linking environment of the 3-c nodes of the ligands is different for the three PCPs. In contrast with the co-planar configuration of the ligand in NTU-35, the diphenyls in the structure of NTU-34 are left distorted (8.41), and the diphenyls of NTU-37 are right distorted (Figure 5). Therefore, the coordination mode of the ligand in NTU-35 (μ2-η1: η1 for all carboxylates) is significantly different with that of NTU-34 (μ2-η2: η1 and η1 for the upper two carboxylates, while μ2-η1: η1 for the nether two) and NTU-37 (η2 and μ2-η2: η1 for the right two carboxylates, while μ2-η1: η1 for the left two. Thus, a progression of framework isomers from tfi to fof and then to a new topology was observed.


21


Page 22 of 39

In addition, it is obvious that the synthesis parameter plays an important role in construction of such different networks. In order to rationalize the reason of the structure change, we systemically tuned the synthesis conditions for these PCPs. The observations from PXRD are summarized in Table 2 (Figure S1620). (1) NTU-34 can be obtained in DMF with varied wide range of pH (0 to 30 μL HNO3), whereas, when the amount of HNO3 reached to 50 μL, NTU-35 with pure phase was isolated. (2) When very small amount of water was added into the DMF system (DMF: H2O = 12:1), only NTU-35 can be obtained under relative fewer amount of HNO3

Table 2. Experimental Matrix of PCP Syntheses at 80 ºC, 48h.

DMF/H2

Solvent DMF

DMF/CH3CN/H2 DMF/THF/H2 DMF/MeOH/H2

O

O

O

O

(12:1)

(8:4:1)

(8:4:1)

(8:4:1)

?

?

?

?

pH

HNO

NTU0 μL

3

34


22

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NTU10 μL

NTU-36+NTUNTU-35

NTU-35

NTU-35

34

37

NTU30 μL

NTU-35

NTU-36

NTU-36

NTU-36

?

NTU-36

NTU-36

NTU-36

CS

CS

CS

CS

34 NTU50 μL 35 100 CS μL Note: The concentration of HNO3 is 16 mol/L. The obtained unknown phase and clear solution were expressed by ? and CS, respectively. Only one NTU-37 crystal was obtained with NTU-36, thus, the corresponded PXRD was absent.

(10 and 30 μL). (3) When adding CH3CN in solvent system with ratio of DMF: CH3CN: H2O = 8:4:1 and 10 μL HNO3, pure NTU-35 was obtained. However, when more HNO3 (30 and 50 μL) was added, NTU-36 with good crystallinity appeared again. (4) Similar phenomenon was observed in the system of


23


Page 24 of 39

DMF/THF/H2O. (5) When adding methanol into the system, NTU-36 and NTU37 was found in one bottle. But, in a pity, this reaction could not be repeated even we did careful tuning of the synthesis parameters, emphasizing the complexity of the framework construction process. (6) At last, clear solutions were

found

in

all

systems

that

has

100

μL

HNO3.

Based

on

these

observations, we can conclude that the solvent system and pH regulate the environment that has significant influence for the self-assembly of Zn2+ and linking modes of H4L ligand. Although more theoretical and experimental efforts are required for finding the roles of solvent and pH in influencing the selfassembly of those structures, we can make a preliminary conclusion: suitable solvent system and matched pH can result in new balance of kinetic and thermodynamic of the final product. In addition, a number of examples of MOF synthesis show that each solvent system has a role in regulating the formation of unique coordination and crystal engineering. The selected solvents or solvent mixtures with varied solubility and polarity may participate in coordination with metal clusters or act as a structure directing agent for the crystal growth


24

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process. For instance, the extent and also the speed of the deprotonation of carboxylate ligands can be controlled by selecting suitable solvents system or by adjusting the pH of solvent medium.

PXRD Patterns and Thermal Stability Analyses. Phase purity of the bulk crystals is independently confirmed by powder X-ray diffraction (PXRD) (Figure S22-24). PXRD of the as-synthesized NTU-34 to -36 gives fully matched diffractions toward the simulated patterns from single-crystal data, reflecting good phase purity and well-defined structures. Thermal stability of those three PCPs was determined by thermal gravimetric analysis (TGA). As shown in Figure S25, the dehydration processes of NTU-34 started from 120°C. With further heating, it showed fast and continuous weight loss (from 100% to 66.5%) until 250°C, which can be assigned as the removal of one coordinated H2O, three lattice and coordinated DMF molecules in per unit (65.8%). Then, it showed a slow weight loss and decomposed after 420°C. For NTU-35, only a process of slow and continuous weight loss was found from beginning to 400°C. Decomposition of the framework occurred in range of 400 to 500°C. As


25


Page 26 of 39

for NTU-36, the initial weight loss (6.1%) occurred before 80°C, corresponding to 3 lattice methanol molecules (5.5%). Then, the second weight loss, started from 120°C to 220°C, increased to 15.2%, which may be assigned as the 6 H2O and 2 DMF molecules per unit (14.7%). Following the release of all guest molecules, the framework also decomposed in range of 400 to 500°C.

Gas adsorption and separation. To characterize the porosity of NTU-34 to -36, N2 (77K) and CO2 (195K) adsorption experiments were performed. As shown from Figure S27, very low N2 gas uptakes were found in all of them. However, in contrast, NTU-36 and -35 showed a certein amount of CO2 gas uptakes at 195K. Combine the PXRD of the activated samples, the lower gas uptakes may be expalined as the partial callpose of the activated frameworks. This phonomen is common for Zn-based porous frameworks. Despite these, the maximum CO2 adsorption of NTU-36 reached to 2.77 mmol/g (62.2 cm3/g). Given its higher framework density (1.07 g/cm3), the volumetric storage capacity, one of the important factors for feasible adsorbent, reached to 130.4 g/L at 195 K. Importantly, even at 273 K and 1 bar, the volumetric storage capacity of NTU-


26

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36 is as high as 51.7 g/L, which is higher than that of some important porous materials, such as MOF-5: 39.9 g/L39, and MOF-177: 50.7 g/L40, but far lower than that of the PCPs with amide group (Cu-TPBTM: 279.2 g/L41 and CuTDPAT, 344.8 g/L42). In contrast, the gas uptakes of N2 and CH4 of NTU-36 increased slowly accompanying with the pressure. This may be because of the relative weak interaction of these gases toward the framework compared with that of CO2. Therefore, NTU-36 may be a candidate for selective capture of CO2 from N2 and CH4 mixtures (Figure 6a).

Ideal adsorbed solution theory (IAST) of Myers and Prausnitz43, a wellestablished method for describing gas mixture adsorption, was employed to evaluate the multi-component separation


27


Page 28 of 39

Figure 6. Gas adsorption isotherms (circle points) and the dual-site Langmuir– Freundlich fit lines (lines) of CO2, CH4 and N2 in NTU-36 at 273 K (a); Predicted CO2/CH4 and CO2/N2 selectivity for gas mixture (2:8) based on the isotherms at 273 K (b); Breakthrough curves of NTU-36 for mixtures of CO2/CH4 (c) and CO2/N2 (20:80) (d) at 273K.


28

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potential from experimental single-component gas isotherms44. As shown in Figure 6b, the predicted selectivity of CO2/N2 (87-695) is far higher than that of CO2/CH4 (13-26), which is consistent with the single gas isotherms. In addition, the gas selectivity of CO2/N2 and CO2/CH4, even at low pressure range (87 and 13), is all higher than 8, indicating high potential for feasible separation. To confirm this, breakthrough experiments on NTU-36 were performed under continuous and flowing condition45, which are more preferred separation process in industry46-48. The fully activated sample (0.455 g) was initially swept by He flow. After initial dosing of CO2/CH4 mixture, the concentration of 100% for CH4 and 0% for CO2 were detected by gas chromatography (GC) at 2.5 min and continued until the breakpoint at 4.5 min. When changing the feed gas to CO2/N2, the pure N2 gas was detected at 1.9 min and continued until the breakpoint at 4 min. More importantly, the fully identical breakthrough curves were obtained after 20 min He blowing at 298K, revealing a very facile and mild protocol for sample regeneration (Figure 6c-d).

CONCLUSIONS


29


In

Page 30 of 39

summary, by systemically tuning the synthesis parameters, three porous

supramolecular isomers and a new high porous framework were successfully prepared from zinc salts and a tetracarboxylate ligand. Due to the ligand distortion

and

varied

cluster

connection,

these

materials

exhibit

versatile

structural progression and topologies. In addition, breakthrough experiments showed that NTU-36 worked well for selective CO2 removal from its CH4 and N2 mixtures under flowing conditions at ambient temperature. Therefore, the solvent- and pH-dependent crystal engineering presents a prospective route for the construction of supramolecular isomers and other porous materials that have different pore environments.

ASSOCIATED CONTENT

Supporting Information. Crystal structures and topologies, PXRD patterns, TG analysis, crystallographic data and cif files, gas adsorption isotherms. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION


30

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Corresponding Author [email protected] (J.D.)

Author Contributions J. D. conceived the idea of the work. Y. W. and H. C. did the experiments and analysis the data. The manuscript was written and discussed by all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT We thank the financial support of the National Natural Science Foundation of China (21671102), Program of China Scholarships Council (201708320084), Natural Science Foundation of Jiangsu Province (BK20161538), Innovative Research Team Program by the Ministry of Education of China (IRT13070), Six talent peaks project in Jiangsu Province (JY-030).

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Page 38 of 39

Solvent- and pH-Dependent Formation of Four Zinc Porous

Coordination

Polymers:

Framework

Isomerism and Gas Separation Yang Wang†, Haifei Cao†, Baishu Zheng‡, Rongfei Zhou†, Jingui Duan†* †State

Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemical

Engineering, Nanjing Tech University, Nanjing 210009, China. Email: [email protected] ‡

School of Chemistry and Chemical Engineering, Hunan University of Science and

Technology, Xiangtan 411201, China.

Received xx x xxxx; Email:

Because

of

systemically

tuned

synthesis

parameters,

four

zinc

porous

coordination polymers were prepared from a tetracarboxylate ligand. Given the same framework formula and zinc dimmer, three of them can be considered to be a new group of porous isomers. Meanwhile, the coordination of partial


38

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coupled ligands and Zn4O clusters leads to the formation of the forth porous framework, which can selective remove CO2 from CH4 or N2 mixtures under flowing conditions at ambient temperature.


39

Solvent- and pH-Dependent Formation of Four Zinc Porous

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