Enhancing CO₂ adsorption of a Zn-phosphonocarboxylate framework by pore space partitions

Abstract

Using structure-directing agents, pore space partitions of a Zn-phosphonocarboxylate framework have been achieved. Selective adsorption of CO₂ over N₂ has been greatly improved from ca. 9 : 1 to 94 : 1.

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Porous materials with selective adsorption of CO₂ are highly desired.¹ Metal–organic frameworks (MOFs) are considered to be ideal porous materials,² because of their tunable host–guest interactions,³ adjustable pore size,⁴ and infinite structural possibilities.⁵

To enhance CO₂ adsorption and separation, various methods have been proposed to modify MOFs, such as constructing MOFs with high surface areas,⁶ grafting polar groups (–OH, –NH₂, etc.) or exposing metal sites,⁷ adjusting pore sizes for size-dependent separation,⁸ and synthesizing flexible frameworks for kinetic separation.⁹ Recently, it has been realized that a high surface area and large pore volume may not necessarily lead to a high CO₂ adsorption under ambient conditions.¹⁰ How to efficiently utilize the pore space to improve CO₂ adsorption has therefore become a hot topic. A feasible method is pore space partition (PSP), proposed recently.^10a The PSP process can be viewed as building molecular-scale walls in a known structure to divide its original large space into segments. However, few examples are known to date¹⁰ due to the great challenge to locate components into MOFs during crystallization.

Multi-functional phosphonate-based MOFs are robust, porous materials.¹¹ In the synthesis of these MOFs, structure-directing agents (SDAs) are widely used. Some SDAs can be incorporated into the framework during crystallization, while some of them can not,¹² which provides us with a good platform to study PSP in MOFs. The purpose of this paper is to present such an example. The rutile-type Zn(II)-phosphonocarboxylate framework, {[Zn₃(pbdc)₂]·2H₃O}_n (H₄pbdc = 5-phosphonobenzene-1,3-dicarboxylic acid, ZnPC-2), is a 3D porous structure synthesized by using triethylamine (TEA) as a SDA (Fig. 1).^11c By using pyrrolidine (PYR) and piperidine (PIP) to replace TEA, two iso-reticular frameworks, {[Zn₃(pbdc)₂]·HPYR·H₃O·4H₂O}_n (HPYR@ZnPC-2) and {[Zn₃(pbdc)₂]·HPIP·H₃O·5H₂O}_n (HPIP@ZnPC-2), have been isolated respectively, in which the incorporated HPYP and HPIP serve as the molecular-scale walls, dividing the original channel structures into segments. Gas adsorption results show that the selective adsorption of CO₂ over N₂ has been improved from ca. 9 : 1 for ZnPC-2 to 94 : 1 for Hpip@ZnPC-2.

Fig. 1 A view of the 3D porous structure of ZnPC-2 constructed by 6-connected [Zn₃(PO₃)₂(COO)₄] (Zn₃-SBU) and 3-connected pbdc ligands.

3D porous iso-structures of ZnPC-2, HPYR@ZnPC-2 and HPIP@ZnPC-2 crystallize in the same tetragonal system, I4̄2d space group, which has a crystallographically imposed twofold symmetry (Table S1, Fig. S1, ESI†). Pure crystalline phase of ZnPC-2 was isolated when TEA was used as the SDA (Fig. S2 and S3, ESI†). The host framework is constructed by 6-connected Zn₃-SBUs and tritopic pbdc ligands, showing a (4·6²)₂(4²·6¹⁰·8³) topology (Fig. 1). Two types of channels (ca. 6 × 6 Å along the c axis and 6 × 4 Å along the a and b axes, Fig. 2) are interconnected with each other (Fig. 2a). When TEA was replaced by PYR and PIP in the synthetic procedure, HPYR@ZnPC-2 and HPIP@ZnPC-2 were successfully isolated as the pure crystalline phase respectively (Fig. 2, Fig. S2 and S3, ESI†). Because of the incorporation of SDAs, the cell volume is enlarged from 7785 ų for ZnPC-2 to 7898 and 7928 ų for HPYR@ZnPC-2 and HPIP@ZnPC-2, respectively (Table S1, ESI†). According to the single-crystal diffraction results, HPYR and HPIP are located in the small channels and are connected to the host framework by strong hydrogen bonding interactions. Their amino groups point towards the Zn₃-SBUs and are completely covered by the host framework (Fig. S4, ESI†). Notably, their alkyl groups protrude into the large channels (Fig. 2). If considering the large channel as an idealized rectangular channel with small windows on each side, the located organic molecules, therefore, can be regarded as a kind of molecular-scale wall, which divides large channel into uniform segments with a length of ca. 12 Å and an entrance size of ca. 5.3 Å for HPYR@ZnPC-2 and ca. 4.8 Å for HPIP@ZnPC-2 (Fig. 3, Fig. S5, ESI†). From ZnPC-2 to HPYR@ZnPC-2 and HPIP@ZnPC-2, the PSP has been achieved in the rutile-type phosphonate-based MOF.

Fig. 2 3D iso-recticular structures of ZnPC-2 (a), HPYR@ZnPC-2 (b) and HPIP@ZnPC-2 (c) (deep yellow: the 3D framework; violet: HPYR and blue: HPIP).

Fig. 3 A view of the channel structure of the rutile-type ZnPC-2 before and after PSP.

TG-MS analyses were then carried out to assess their thermal stability (Fig. S6, ESI†). The results show that in the temperature range 30–300 °C, about 8.4% and 13.8% weight loss was observed for HPYR@ZnPC-2 and HPIP@ZnPC-2 respectively, which can be attributed to the release of water molecules. The mass signals of pyrrolidine (b.p. 89 °C, F.W. 71.1) and piperidine (b.p. 106 °C, F.W. 85.1) were detected at ca. 460 °C after decomposition of the host framework, respectively. These results agree well with the above structural analyses that the guests of HPYR and HPIP are strongly bonded to the host framework by hydrogen bonding interactions.

The framework integrity of the three samples was confirmed by the PXRD patterns. Before the gas adsorption studies, all three samples were activated at 150 °C under vacuum for 8 h. Like most phosphonate-based MOFs,^11a there was no significant adsorption of N₂ at 77 K. This could be related to the presence of kinetic barriers in the phosphonate-based MOFs for N₂ at low temperature.¹³ The surface area was then calculated¹⁴ by a probe atom with a radius of 1.84 Å (Table S2, ESI†). The theoretically accessible surface area for ZnPC-2 is ca. 1429.28 Ų per cell (1497 m² g⁻¹), which is much higher than that of HPYR@ZnPC-2 (909.17 Ų per cell, 822 m² g⁻¹) and HPIP@ZnPC-2 (811.91 Ų per cell, 709 m² g⁻¹). The decrease in the surface area would be mainly related to the blocking of the small channels along the a and b axes.

CO₂ adsorption was then carried out. As illustrated in Fig. 4a, with the increase of pressure, the CO₂ adsorption in HPYR@ZnPC-2 and HPIP@ZnPC-2 was significantly higher than that of ZnPC-2. At 0.15 bar, the uptake amount of CO₂ is 31.1 mg g⁻¹ and 49.6 mg g⁻¹ for HPYR@ZnPC-2 and HPIP@ZnPC-2 respectively, much higher than that of ZnPC-2 (11.5 mg g⁻¹). The selectivity is then calculated based on the adsorption data of CO₂ at 0.15 bar and N₂ at 0.75 bar.^2e The selectivity is ca. 9 for ZnPC-2 and ca. 27 and 94 for HPYR@ZnPC-2 and HPIP@ZnPC-2, respectively (Table S3, ESI†). The uptake amount and selectivity for HPIP@ZnPC-2 are comparable to amino-functionalized MOFs by the post-exchange method¹⁵ and are the highest in phosphonate-based MOFs as far as we know. At 1 bar, the uptake amount of CO₂ for HPYR@ZnPC-2 and HPIP@ZnPC-2 is three times more than that of ZnPC-2 (24.3 mg g⁻¹). Furthermore, the desorption curve almost coincides with the adsorption curve (Fig. S7, ESI†); no marked hysteresis was observed. This suggests that there is no strong interaction between CO₂ and the framework, which is consistent with the characteristic of the structure that the amino groups are completely covered by the host framework. High pressure adsorption results show that even at 20 bar, the uptake amount of CO₂ on ZnPC-2 can only reach 42.7 mg g⁻¹ (Fig. 4c). In contrast, the uptake amount reaches 149.2 mg g⁻¹ on HPIP@ZnPC-2. Slight substeps in the adsorption isotherms for HPYR@ZnPC-2 and HPIP@ZnPC-2 are observed around 8 bar and 7 bar, respectively. These phenomena may be ascribed to further adsorption of CO₂ at the corners formed by the molecular-scale wall with the host framework. All these results illustrate that the adsorption amount and the selective adsorption ability of CO₂ have been greatly improved after the PSP.

Fig. 4 Adsorption isotherms of CO₂ (a) and N₂ (b) at 298 K (olive square: ZnPC-2, magneta sphere: HPYR@ZnPC-2, blue triangle: HPIP@ZnPC-2); (c) high pressure CO₂ adsorption isotherms of ZnPC-2, HPYR@ZnPC-2 and HPIP@ZnPC-2; (d) the CO₂ adsorption enthalpy at zero coverage of HPYR@ZnPC-2 and HPIP@ZnPC-2.

The isosteric heat of CO₂ adsorption was then calculated by the Virial method based on their adsorption isothermals at 298 K and 288 K (Fig. S8, ESI†). The enthalpies at zero coverage are ca. 36 and 32 kJ mol⁻¹ for HPYR@ZnPC-2 and HPIP@ZnPC-2, respectively (Fig. 4d), which are lower than some amino-functionalized MOFs (Table S3, ESI†). These results illustrate that this kind of MOF can be easily regenerated after adsorption of CO₂. To illustrate the reversible CO₂ adsorption from the gas mixture, selective adsorption was further carried out using HPIP@ZnPC-2 as the adsorbent on a TG apparatus at 305 K and atmospheric pressure (Fig. S9, ESI†). The gas cycling experiment was carried out using a flow of CO₂ in N₂ (V(CO₂)/V(N₂): 1 : 4), and followed by a flow of pure N₂ gas. A weight increase of ca. 0.45 wt% was quickly achieved after switching to mixed gases, and a quick weight loss was observed after switching to pure N₂ gas. The process can be cycled several times.

In summary, PSP has been realized in a Zn-phosphonocarboxylate framework by using appropriate SDAs for the first time. Gas adsorption results confirm that the uptake amount of CO₂ was greatly enhanced. Selective adsorption of CO₂ over N₂ on HPIP@ZnPC-2 is comparable to most reported carboxylate-based MOFs^2e and is the highest in phosphonate-based MOFs up to now.

This work was financially supported by NSFC (No. 21171042, No. 91027044, No. 21101031) and Shanghai Leading Academic Discipline Project (Project No. B108). The authors acknowledge helpful discussion from Prof. Qiwei Li in the Dept. Chem., Fudan University.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Details of measurements, synthesis, figures and tables are available. CCDC reference numbers 746203, 776925 and 894708. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c2cc37174j

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