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Cooperative Sieving and Functionalization of Zr MOFs through Insertion and Post-Modification of Auxiliary Linkers Liangliang Zhang, Shuai Yuan, Weidong Fan, Jian-dong Pang, Fugang Li, Bingbing Guo, Peng Zhang, Daofeng Sun, and Hong-Cai Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05091 • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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ACS Applied Materials & Interfaces

Cooperative Sieving and Functionalization of Zr MOFs through Insertion and Post-Modification of Auxiliary Linkers Liangliang Zhang,†,§,∥ Shuai Yuan,‡,∥ Weidong Fan,† Jiandong Pang,‡ Fugang Li, † Bingbing Guo,† Peng Zhang,‡ Daofeng Sun†,* and Hong-Cai Zhou‡,*

† School

of Maters Science and Engineering, China University of Petroleum (East

China), Qingdao, Shandong 266580, China

‡ Department

of Chemistry, Texas A&M University, College Station, Texas 77843-3255,

United States

§ Xi'an

Institute of Flexible Electronics, Northwestern Polytechnical University(NPU),

Xi'an 710072, China

ACS Paragon Plus Environment

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KEYWORDS: zirconium-based MOFs, auxiliary linker, post-modification, cooperative catalysis, sieving, CO2 cycloaddition


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ABSTRACT : A major goal of metal–organic framework (MOF) research is to control the structures and functions of materials in accordance with their specific applications. However, due to the flexible coordination modes between metal ions and organic linkers in MOFs, it is still challenging to rationally assemble a framework with deliberate structures and desired functional groups. Sometimes, two or more phases coexist in a one-pot reaction, making it difficulty in separation. To this end, sieving and purification of MOF mixtures becomes vital for the following application. Herein, we demonstrate that the formation of zirconium-based MOFs (Zr-MOFs) can be regulated in a wider twodimensional scale by thermodynamics using auxiliary linkers. The auxiliary linkers favor the formation of the targeted Zr-MOF by selectively binding to its coordination vacancies and therefore increasing its formation enthalpy to achieve the sieving of MOF mixture. Furthermore, the resulting mixed-linkers MOFs not only maintain porosities, but also contain the installed auxiliary linkers as chemical handles to further incorporate functional groups, providing the possibility of introduction of active sites through postmodification. Finally, this synthetic strategy was applied to assemble a cooperative catalytic system in a MOF platform for CO2 cycloaddition with epoxides. To the best of


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our knowledge, this is the first example of sieving and functionalization of MOFs through insertion and post-modification of auxiliary linkers.

INTRODUCTION

As a new class of highly porous crystalline materials, metal-organic frameworks (MOFs), or porous coordination networks (PCNs), have attracted considerable research interest over the last two decades owing to their potential applications in gas storage, separation, catalysis, sensing, light harvesting, and catalysis.1-6 MOFs epitomize unlimited structural tunability owing to the diversity of metal nodes and organic linkers.7-9 The flexible coordination modes between metal cations and organic linkers further increase the structural abundance of MOFs.10, 11 Taking a simple linear BDC linker as an example, different MOFs, including MIL-53, MIL-68, MIL-88, and MIL-101, can be isolated depending on the structures of metal nodes and the connectivity of the linkers (BDC = terephthalate, MIL = Materials of Institute Lavoisier).12-16 Among the numerous MOFs, zirconium-based MOFs (Zr-MOFs) are especially attractive because of their superior chemical stability and structural tunability. The Zr6-cluster tolerates a variation


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of connection numbers from 4 to 12, which enriches the structures of Zr-MOFs. 17-21 The structural abundance of Zr-MOFs has significantly expanded their applications, but also poses a challenge to their synthesis as two or more “accompanying” phases can be formed simultaneously in a one-pot reaction. For instance, two Zr-MOFs, PCN-222 and PCN-224, were synthesized under similar conditions, with a slight difference in the amount of benzoic acid as modulating reagent.17, obtained with NU-901 as impurity.23,

24

22

Similarly, NU-1000 was often

Indeed, the solvothermal reaction process is

essentially a black box. This motivates us to understand MOF formation mechanisms and develop a method to take control of MOF formation.

We propose that the formation of Zr-MOFs can be regulated by thermodynamics using auxiliary linkers. The simultaneous formation of two phases under the same synthetic conditions indicates similar formation energy of two MOFs. To obtain a pure phase, the energy difference between two “accompanying” MOFs needs to be enlarged. It is known that Zr6 clusters may have various connection numbers in different MOFs. Usually, Zr-MOFs with high-connected clusters are thermodynamically more favorable


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than low-connected ones, as the formation of Zr−carboxylate bonds is an exothermic process.25-27 Therefore, a Zr-MOF will be thermodynamically favored if an auxiliary linker is used to increase the connection number of Zr6 clusters. The auxiliary linkers as modulators should be able to selectively insert into coordination vacancies in the targeted MOF, which will increase the connection number of Zr6 cluster and raise the energy difference between “accompanying” MOF pairs. Auxiliary linkers installed in coordination vacancies will not change the scaffold structures of MOFs, but will act as chemical handles for post-synthetic modifications to introduce additional functional groups. Therefore, auxiliary linkers provide an additional opportunity to regulate the structures and functions of Zr-MOFs (Scheme 1).

Scheme 1. Thermodynamic regulated synthesis of Zr-MOFs using auxiliary linkers.


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To demonstrate the effectiveness of the proposed strategy, we successfully separated two “accompanying” MOF pairs, PCN-222/PCN-224 by adding different auxiliary linkers. This method was applied further to separate PCN-606/PCN-608 (isostrucural to NU-901/NU-1000),23,

24

28

MOF isomers. In addition, different functional

groups were introduced into Zr-MOFs by post-synthetic modification of auxiliary linkers, which provided a versatile platform for a vast number of applications such as cooperative catalysis.

RESULTS AND DISCUSSION

Regulating MOF Synthesis by Auxiliary Linkers.

To separate “accompanying” MOF pairs, auxiliary linkers with proper lengths were chosen which can selectively insert into coordinatively unsaturated sites in the targeted MOFs. PCN-222 and PCN-224 are both composed of coordinatively unsaturated Zr6 clusters, which can further coordinate with carboxylate linkers. Each pair of adjacent 8connected Zr6 clusters in PCN-222 can be connected along the c-axis by linear linkers to form 10-connected Zr6 clusters. Similarly, each pair of 6-connected Zr6 clusters in


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PCN-224 forms a “pocket” for the installation of linear carboxylate linkers to fulfill the 12connectivity. We observed that the pocket in PCN-222 could accommodate NDC whereas the pocket in PCN-224 could fit BPDC (NDC = 2,6-naphthalenedicarboxylate, BPDC = 4,4'-biphenyldicarboxylate). Therefore, NDC and BPDC were chosen as auxiliary linkers to induce the formation of PCN-222 and PCN-224 structures, respectively (Figure 1a). To demonstrate the effectiveness of our strategy in the design and synthesis of novel MOFs, PCN-606 (isostructural to NU-901) and PCN-608 (isostructural to NU-1000) are adopted. They are both composed of the tetratopic linker DTPC (DTPC = 2,2’-dihydroxybiphenyl-3,3’,5,5’-tetra(phenyl-4-carboxylate)).23, 24 PCN606 is a flexible structure with adjustable pocket sizes whereas PCN-608 has a pocket of 6.8 Å (Figure 1b). Therefore, TPDC, with a length of 15.4 Å, is expected to promote the formation of PCN-606 structure exclusively by coordinating with the unsaturated Zr6 clusters (TPDC = 2',5'-dimethyl-terphenyl-4,4''-dicarboxylate).

Modulated synthesis was carried out by adding different linear linkers to the reaction mixtures of ZrCl4, tetratopic linkers, and benzoic acid in DMF under


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solvothermal conditions. The obtained solids were analyzed by powder X-ray diffraction (PXRD) to determine the phase purity. As shown in Figure S1, the addition of auxiliary linkers dramatically changed the products of the reaction. For example, PCN-222 was obtained by adding 32 mM NDC to the reaction mixture whereas PCN-224 was isolated when 26 mM BPDC was used (Figure S1a). A similar modulating effect can be observed in the synthesis of PCN-606 by adding 15 mM TPDC (Figure S1b). For comparison, a mixture of two or more MOF products was obtained without auxiliary linkers. These results unambiguously show that auxiliary linkers can alter the phase selectivity of MOF synthesis, giving rise to targeted products.


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Figure 1 Structures of different MOF pairs with pockets to selectively accommodate different auxiliary linkers. (a) PCN-222 and PCN-224 with pockets for NDC and BPDC, respectively. (b) PCN-606 with pockets for TPDC.

To systematically study the effect of auxiliary linkers, high-throughput experiments were carried out by altering the linker ratios of the starting materials (Tables S1-S3). The concentration of benzoic acid as the modulating reagent was also tuned, as this is known to affect the phase purity. The effects of linker ratio and modulating reagent are


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summarized into phase diagrams (Figure 2). Without modulating linker, the formation of different MOF structures is mainly determined by the amount of benzoic acid. The formation of pure product often requires precise control of benzoic acid concentrations, which complicates the synthesis. For example, the synthesis of pure PCN-608 crystals requires 1.10 to 1.35 M benzoic acid whereas further increasing the amount of benzoic acid induces the formation of PCN-606 impurities. Worse still, PCN-224 will always exist as impurity in the synthesis of PCN-222 regardless of the acid amount. By tuning the concentration of benzoic acid exclusively, a 1D phase diagram is obtained that indicates a limited tunability via synthetic conditions (Figure 2a). The utilization of auxiliary linker adds another factor that strongly affects the products, leading to the formation of a 2D phase diagram. As shown in Figure 2b and 2c, PCN-222 and PCN-224 can be synthesized with a wide range of acid concentration by adding NDC and BPDC respectively. More importantly, pure PCN-222 can be obtained when NDC is added to the reaction system. Similarly, PCN-606 can be obtained under a wide range of synthetic conditions by adding TPDC (Figure 2d). Note that the auxiliary linker (such as BPDC and NDC) can potentially compete against the primary linker to form a Zr-MOF


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individually (such as UiO-67 and DUT-53).18, 29 However, the tetratopic primary linkers are more competitive than the ditopic auxiliary linkers, so that UiO-67 is formed only if a large excess of BPDC is added. Therefore, the formation of UiO-67 and DUT-53 impurities can be easily avoided.

The compositions of MOFs synthesized with different linker ratios were also subjected to

1H-NMR

digestion experiments (Figures S2 and S3). Generally, the

auxiliary linker contents in MOFs gradually escalate as their ratios in the starting materials increase, suggesting the partial occupation of auxiliary linkers. Once a pure phase is formed, the ratio of auxiliary linkers quickly levels off, indicating that auxiliary linkers have saturated the MOFs’ pockets. The saturated linker ratios are used to calculate the composition of these mixed-linker MOFs (Table S5). Based on their linker ratios,

these

MOFs

can

OH)4][(OH)2(H2O)2](TCPP)2(NDC) OH)4[(OH)5(H2O)5](TCPP)1.5(BPDC)2

be

formulated

as

(PCN-222-NDC), (PCN-224-BPDC),

[Zr6(µ3-O)4(µ3[Zr6(µ3-O)4](µ3-

and

[Zr6(µ3-O)4(µ3-

OH)4][(OH)2(H2O)2](DTPC)2(TPDC) (PCN-606-TPDC). Using modulating linkers, pure


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phases as opposed to MOF pairs can be selectively formed under a wide range of synthetic conditions, allowing for a better control of the formation processes.

Figure 2 Phase diagram showing the effect of auxiliary linkers. (a) Syntheses of PCN222/PCN-224 system, PCN-606/PCN-608 system, and PCN-206/PCN-207/PCN-208 system without auxiliary linkers; (b),(c) Modulated syntheses of PCN-222/PCN-224 system; (d) Modulated syntheses of PCN-606/PCN-608 system. Note that modulated syntheses PCN-222 using different amount of NDC will form mixed-linker MOFs with different linker contents. But they are all refered as PCN-222 phase for simplicity.


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In order to further investigate the formation mechanism of pure MOF modulated by auxiliary linkers, 1H-NMR was used to monitor the reaction process. As an example, samples of PCN-224-BPDC were digested at different time intervals. As depicted in Figure S4, the linker ratios between TCPP and BPDC keep constant when PCN-224BPDC was formed. It shows that the incorporation of auxiliary linkers is a one-pot reaction, which is reminiscent of the linker installation process reported by our group in which linkers were post-synthetically installed into the coordination vacancies of ZrMOFs.30

Structural Characterization

Single crystal X-ray diffraction (SC-XRD) experiments indicate that the auxiliary linkers are inserted into the coordination vacancies of the parent MOF structures, forming mixed-linker MOFs. Single crystals of PCN-224-BPDC and PCN-606-TPDC were obtained (Table S4). PCN-224-BPDC crystalizes in the cubic space group Im-3m. The incorporation of BPDC in PCN-224 does not alter the space group or lattice parameters because of the framework’s rigidity. The auxiliary linker BPDC can be


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clearly observed between each pair of clusters with an occupancy of 60%. PCN-606TPDC crystallizes in the orthorhombic space group Cmmm, which is the same as its parent structure PCN-606. The auxiliary linker TPDC was precisely refined in the crystal structure with occupancy of 100%. Each Zr6 cluster was coordinated to eight carboxylates from DTPC and two carboxylates from TPDC. Interestingly, the a-axis length increased from 18.78 Å (PCN-606) to 23.15 Å (PCN-606-TPDC) to accommodate the TPDC, as the length of TPDC is longer than the size of the pocket in PCN-606. The composition of PCN-222-NDC, and PCN-606-TPDC perfectly matched with the structural model derived from SC-XRD and PXRD data. For PCN-224-BPDC (BPDC occupancy of 60%), the ratios of auxiliary linkers are slightly lower than expected indicating that the coordinatively unsaturated sites are partially occupied (Table S5).

Topological analyses were further performed on these MOFs to clarify their structural changes after the insertion of auxiliary linkers (Figure 3). Obviously, TCPP and DTPC are 4-connected linkers that can be simplified into square nodes, and


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dicarboxylates as auxiliary linkers are regarded as linear connections. The Zr6 clusters act as 10- or 12-connected nodes depending on the MOF structures. In PCN-222-NDC, each Zr6 cluster is coordinated to eight TCPP linkers forming the (4, 8)-connected scaffold structure (PCN-222) featuring a csq topology (Figure 3a). Installation of NDC along c-axis further transformed PCN-222 into a (4, 10)-connected net with a point symbol of {32.42.52}2{38.416.58.613} determined by TOPOS 4.0. The PCN-224-BPDC is simplified into a (4, 12)-connected net with a point symbol of {312.418.524.612}2{34.42}3. Each Zr6 cluster was connected by six adjacent TCPP linkers forming the parent structure (PCN-224), which was further installed by six BPDC to fulfill the 12-connected nodes in PCN-224-BPDC structure (Figure 3b). PCN-606 exhibit the (4, 8)-connected scu topology. After the installation of TPDC along a-axis of PCN-606, a (4, 10)connected structure was obtained with a point symbol of {416.512.616.7}{44.52}2 (Figure 3c). Note that the occupancy of auxiliary linkers was regarded as 100 % in topological analysis for simplicity.


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Figure 3 Topological analyses of (a) PCN-222-NDC, (b) PCN-224-BPDC and (c) PCN606-TPDC

Auxiliary linkers have been widely used to generate new MOF structures or to split the pore spaces, which may significantly change the porosity of scaffold MOFs.32-37, 47 However, mixed-linker Zr-MOFs in this work maintained their porosities, as proven by the N2 sorption isotherms at 77 K. The total N2 uptakes, Brunauer-Emmett-Teller (BET) surface areas, and pore size distributions of the mixed-linker MOFs as well as their parent structures were compared (Table S6, Figures S5-S14). PCN-222-NDC exhibits a


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typical type IV isotherm with a steep increase at the point of P/P0=0.3, suggesting a mesoporous structure (Figure 4a). It shows high porosity with a N2 uptake of 850 cm3 g−1 (STP) and a BET surface area of 2150 m2 g−1, which is similar to that of PCN222 (N2 uptake of 900 cm3 g−1 and BET surface area of 2200 m2 g−1). The pore size distribution derived from the N2 sorption curve indicates that PCN-222-NDC maintained its hierarchically porous structure with two types of pores (diameters of 1.2 nm and 3.0 nm) assigned to triangular micro-channels and hexagonal meso-channels, respectively. This is also consistent with the structural model of PCN-222-NDC. PCN-224-BPDC shows a typical type-I isotherm, with a N2 uptake of 725 cm3 g−1 (STP) and a BET surface area of 2520 m2 g−1.This is slightly lower than that of PCN-224 due to the incorporation of BPDC occupying the pore space. The maximum pore size is estimated to be 1.6 nm, which is slightly smaller compared to that of PCN-224 (1.8 nm).

For rigid frameworks such as PCN-222 and PCN-224, the incorporation of auxiliary linkers generally reduces their pore volumes. However, auxiliary linkers can increase the porosity of flexible MOFs by expanding the pore sizes and rigidifying the framework


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structure. For example, the insertion of TPDC into PCN-606 opens up the MOF cavity along the a-axis resulting in an increased pore volume (Figure 4b). In addition, TPDC linkers also act as pillars to support the flexible framework, thereby avoiding MOF collapse during solvent removal. Therefore, PCN-606-TPDC shows higher total N2 uptake (592 cm3 g−1) and BET surface area (2165 m2 g−1) than that of PCN-606 (498 cm3 g−1, 1820 m2 g−1, respectively).

Figure 4 N2 sorption isotherms of (a) PCN-222/PCN-224 systems, (b) PCN-606/PCN608 systems synthesized with and without auxiliary linkers at 77 K and 1 bar.

Cooperative Catalytic System Built in PCN-224-BPDC.

Auxiliary linkers not only provide an efficient synthetic route to exert control over MOF formation, but also facilitate the functionalization of MOFs by modification of


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auxiliary linkers. The functionalization of TCPP is difficult because of complicated synthetic procedures and low yields. On the other hand, functionalization of linear linkers such as BPDC is much easier. The mixed-linker PCN-224-BPDC is an ideal platform to integrate the functionalities of linear linkers with the TCPP linker in a MOF platform, forming cooperative systems. As a proof of concept, quaternary ammonium bromides were tethered to BPDC linkers, which participated in conjunction with the Coporphyrin center as an integrated cooperative catalytic system for CO2 cycloaddition with epoxides.

The atom-economical cycloaddition reactions of CO2 with epoxides yield cyclic carbonates, which have wide applications in pharmaceutical and chemical industries.38 This reaction requires a Lewis-acidic site (such as a Co-porphyrin center) and a nucleophile (such as Br−) to promote the ring opening reaction of epoxides.39 For example, Co-porphyrins have been employed as catalysts in the presence of tetrabutylammonium bromide (TBAB) additives for the production of cyclic carbonates from CO2 and epoxides.40 In a typical catalytic system, the Co-porphyrin and TBAB are


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added into a homogenous reaction system. Immobilization of the Co-porphyrin and quaternary ammonium bromides simultaneously in a MOF platform offers several advantages over the homogeneous systems, such as easy catalyst separation and recovery, regeneration, and handling.

Figure 5 Schematic Representation of the Cooperative Catalytic System. Cooperative catalytic system for cycloaddition reactions of CO2 with epoxides constructed by incorporating different ammonium bromides on auxiliary linkers.

Intuitively, quaternary ammonium bromides can be attached to a Co-porphyrin based linker, which could then be constructed into a stable MOF such as PCN-224. However, most quaternary ammonium bromides cannot survive the synthesis of Zr-


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MOFs and the Br– tends to be exchanged by other anions. Therefore, Co-porphyrin and tetrabutylammonium bromide were integrated into a highly porous PCN-224-BPDC system

by

post-synthetic

modification.

PCN-224(Co)-BPDC-CH3

was

initially

synthesized using metalloporphyrin (Co-TCPP) and BPDC-CH3 with a pre-anchored CH3 group on BPDC as a chemical handle. Subsequent brominations and quaternary aminations were realized to tether ammonium bromides onto the BPDC linkers (Figure 5). The stable framework survived the multi-step modification reactions as proven by PXRD patterns (Figure S15). 1H-NMR digestion experiments show that 50% of the methyl groups were converted into ammonium bromides. Scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX) further proves the existence of Br in PCN-224(Co)-BPDC-CH2NBu3Br with a Zr/Br ratio of ~6:1 (Figure S16), which is in line with the

1H-NMR

data. The resulting MOF was tested as a catalyst for CO2

cycloaddition with epoxides.

Table 1. MOF catalyzed cycloaddition reactions of CO2 with epoxides.a


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Entry

R1

Catalyst

Conversion(%)b

1

Me

PCN-224-BPDC-CH2NBu3Br

51

2

Me

PCN-224(Co)-BPDC-CH3

Cooperative Sieving and Functionalization of Zr ... - ACS Publications

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