A propane-selective metal-organic framework for inverse selective adsorption propane/propylene separation
Shanqing YANG , Lulu WANG , Qiang ZHANG , Jiajia LI , Yilong LI , Tongliang HU
Olefins, such as ethylene (C2H4) and propylene (C3H6), are significant chemical raw materials and have high commercial value in the petrochemical industry[1]. Separating and purifying olefins from corresponding paraffin-containing mixtures, as one of "seven chemical separations to change the world"[2], is a daunting challenge. As one of the crucial olefin feedstocks, C3H6 is broadly used for various high-value-added chemicals production, such as polypropylene, acroleic acid, and isopropanol[3-4]. The worldwide yearly output of C3H6 is projected to be 160 million tons in 2030, second only to C2H4 production[5]. Currently, industrial C3H6 production mainly depends on the catalytic or/and thermal cracking of hydrocarbons, which yields propane (C3H8) as the main by-product[6]. The highly similar physiochemical properties of C3H6 and C3H8 such as molecular size (C3H6: 0.416 nm×0.465 nm×0.644 nm, C3H8: 0.402 nm×0.452 nm×0.661 nm, respectively), boiling point (C3H6: 225.4 K, C3H8: 231.1 K, respectively), and polarizability (C3H6: 62.6×10-25 cm3; C3H8: 62.9×10-25 cm3, respectively)[7-8], pose a huge challenge to obtain high purity C3H6 and exacerbate the energy consumption in the broadly utilized cryogenic distillation process. Therefore, developing alternative energy-efficient separation technology can save enormous energy costs and reduce carbon emissions.
The adsorptive separation method based on physical adsorption, depending on porous solid adsorbents, offers a promising solution with simple operation, being environmentally friendly, and high energy efficiency. Porous solid materials, ranging from carbon materials, zeolites, and hydrogen-bonded organic frameworks (HOFs) to metal-organic frameworks (MOFs), have been widely explored as the C3H8/C3H6 separation splitters[9-16]. As emerging organic-inorganic hybrid porous crystalline materials, MOFs, composed of metal ions or metal cluster nodes and organic linkers, featuring modular chemistry and pore engineering, have received great attention[17-24]. Benefit from their well-defined structures, tunable pore shapes/sizes, and customizable functionalities, some MOF materials have exhibited unprecedented adsorption and separation performance for light hydrocarbons, including but not limited to C2H2/C2H4, C2H2/CO2, C2H4/C2H6, C3H4/C3H6, C3H8/C3H6, C4 olefins[25-33]. In the context of C3H6 and C3H8 separation, C3H6 is capable of preferring adsorption within the framework through embedding open metal sites or/and highly polar functional sites because of the existence of an unsaturated double bond in C3H6 and a larger dipole moment for C3H6 than that of C3H8[3-6, 34-40]. However, since C3H8 is the main contaminant in C3H6, inverse C3H8-selective adsorbents are preferred for C3H6 purification because they could permit the direct isolation of C3H6 as a pure product without an extra desorption process, which could save approximately 40% energy compared with an intricate regeneration process using C3H6-selective adsorbents[41]. Hence, it is imperative to explore C3H8-selective adsorbents with higher polarizability, more complex configuration, and more C—H bonds of C3H8. Previous attempts have exhibited that a suitable pore environment that matches the more complex configuration of C3H8 could favor the preferential trapping of the paraffin. For example, ZnFPCP constructed by Zn salt and V-shaped fluorinated ligand (4, 4′-(hexafluoroisopropylidene)-bis(benzoic acid)) exhibited C3H8-selective adsorption performance and could one-step purification of C3H6 from C3H8/C3H6 mixture with different gas ratios and flows[42]. FDMOF-2 decorating trifluoromethyl groups showed C3H8-trap performance for C3H8/C3H6 separation[43]. In addition, paraffin-selective behavior, including ethane-selective and C3H8-selective, could also be found in some MOFs with fluorinated pore surface[44-46]. In this context, the fluorinated pore environment is in favor of preferring to adsorb paraffine, in which the functionalized pore space of F atoms for C—H···F interactions may have a key role in this paraffin/olefin separation challenge. Currently, MOFs exhibiting inverse C3H8/C3H6 selectivity are rare, and within this subset of porous solid materials, the costs of precursors are generally expensive, and the C3H8 adsorption capacity is scarce in prevailing adsorbents. Therefore, for the separation and purification of C3H6 from corresponding C3H8, it is strongly desired to develop C3H8-selective porous adsorbents with low cost, which can one-step purify C3H6 without an extra desorption process and represent an energy-efficient, operationally simple, and cost‑effective alternative technology.
With this in mind, we explored a low-cost microporous MOF, Zn-tfbdc-dabco, which was synthesized by low-cost chemicals of Zn(NO3)2, tetrafluoroterephthalate (tfbdc), and 1, 4-diazabicyclo[2.2.2]octane (dabco) under a solvothermal method, for removing C3H8 from a C3H8/C3H6 mixture. More F atoms of the fluorinated ligand tfbdc line the pore channels, meeting the functionalized pore space. As expected, Zn-tfbdc-dabco can interact with C3H8, featuring a more complex configuration and multiple polarizable C—H through multiple C—H···F hydrogen bond interactions, thereby achieving the preferential adsorption of C3H8 over C3H6, as revealed by single-component gas sorption isotherms and isosteric enthalpy of adsorption. The inverse selective C3H8/C3H6 separation performance was also proved by ideal adsorption solution theory calculations and a breakthrough experiment. Moreover, computational modeling studies reveal that the inverse C3H8-selective behavior of Zn-tfbdc-dabco can be primarily attributed to the multiple host-guest interactions between the C3H8 molecule and the fluorinated framework. This study enriches the C3H8- selective MOFs for efficient separation of the C3H8/C3H6 mixture and provides a novel perspective to construct inverse-selective MOF adsorbents for challenging paraffin/olefin separation.
All solvents and reagents were purchased from suppliers and directly used without further purification as received. Powder X-ray diffraction (PXRD) patterns of the samples were collected on the Rigaku Miniflex 600 using Cu Kα (λ=0.154 3 nm) radiation at 40 kV and 15 mA from 3° to 45° (2θ range) with a step size of 0.02. The thermal gravimetric analysis (TGA) curve was recorded with the Rigaku standard thermogravimetry-differential thermal analysis (TG-DTA) analyzer from room temperature to 800 ℃ in argon atmosphere at a constant rate of 10 ℃·min-1.
Zn-tfbdc-dabco was successfully prepared according to the previously reported procedures with some modifications[47]. Zn(NO3)2·6H2O (119 mg, 0.40 mmol) and H2tfbdc (96 mg, 0.40 mmol) were dissolved in methanol (5 mL). After dabco (22 mg, 0.20 mmol) and N, N-dimethylformamide (DMF) (1.2 mL) were added to the mixture, it was stirred for 1 h at room temperature, and then a white precipitate was filtered off. The resulting solution was heated at 365 K for 12 h, and colorless rod-shaped crystals could be harvested.
The gas adsorption isotherms of CO2, C3H6, and C3H8 at various temperatures were collected on the Beishide BSD‑PM2 gas adsorption instrument [Beishide Instrument Technology (Beijing) Co., Ltd.]. Before all of the gas adsorption tests, the samples were activated at 368 K under vacuum for 12 h. The precise control of 298, 313, and 333 K was implemented by the BSD 3H-2000 of Beishide Instrument Technology, which contains a cycle system of water. The sample was degassed at 60 ℃ under high vacuum for 1 h to regenerate at every interval of two independent adsorption isotherms.
Breakthrough experiments for the C3H8/C3H6 mixture were performed in a home-built apparatus. Approximately 0.95 g of the activated sample was placed in an empty glass column with the inner dimensions of 5.0 mm×200 mm, and both ends were filled with silica glass wool. Before conducting breakthrough measurements, a helium gas flow of 20 mL·min-1 was introduced into the packed column under ambient conditions for 10 min to further purge the samples. Then the adsorption column was followed by the introduction of the C3H8/C3H6 (15∶1, V/V) mixture. The raw mixed gas flow rate was maintained at 1.0 mL·min-1, controlled by a mass flow controller. Outlet gas from the column was monitored using gas chromatography (GC-7800).
The GCMC simulations were carried out for the adsorption of C3H6 and C3H8 in Zn-tfbdc-dabco using Sorption Tools in Materials Studio package. The skeleton of Zn-tfbdc-dabco and gas molecules was regarded as rigid bodies. The optimal adsorption sites were simulated under 298 K and 100 kPa by the fixed loading task and the Metropolis method. The atomic partial charges of the host skeleton of Zn-tfbdc-dabco and all gas molecules were obtained from the charge equilibration (QEq) method. The equilibration steps and the production steps were set to 5.0×106 and 1.0×107, respectively. The gas-skeleton interaction and the gas-gas interaction were characterized by the standard universal force field (UFF). The cut-off radius used for the Lennard-Jones interactions is 1.55 nm, and the long-range electrostatic interactions were considered by the Ewald summation method.
Zn-tfbdc-dabco was successfully prepared via the solvothermal method, which takes cheap zinc nitrate, H2tfbdc, and dabco as chemicals (Fig. 1a). In the structure of Zn-tfbdc-dabco, each Zn(Ⅱ) ion coordinates with four carboxylate oxygens from four different tfbdc ligands and one nitrogen atom from the dabco ligand. The dinuclear Zn(Ⅱ)-paddlewheel nodes are linked by tfbdc ligands to form two-dimensional square-grid layers. The neighboring two-dimensional layers are further bridged by dabco ligands in the apical locations of Zn-paddlewheels to form a three-dimensional pillar-framework (Fig. 1b-1d).
(a) Zn, C, N, O, F atoms are presented by light purple, gray, blue, red, and green, respectively.
The PXRD patterns revealed that the diffraction peaks of Zn-tfbdc-dabco were consistent with the simulated ones derived from crystalline data (Fig. 2a), verifying their bulk phase purity and good crystallinity. The framework of Zn-tfbdc-dabco could be retained under different organic solvents (Fig. 2b), revealing its good solvent stability. The TG study of Zn-tfbdc-dabco suggested that the thermal stability of its structure could be retained up to around 262 ℃ in an argon atmosphere (Fig. 2c). In addition, the variable temperature PXRD (VT-PXRD) was further performed to verify this material′s thermal stability (Fig. 2d), agreeing with the result of the TG curve.
The permanent porosity of Zn-tfbdc-dabco was confirmed by CO2 adsorption experiment at 195 K. CO2 gas adsorption isotherms exhibited type Ⅰ adsorption curves for the pillar-layer MOF, showing its microporous structure. The Brunauer-Emmett-Teller (BET) surface areas and pore volumes were 822 m2·g-1 and 0.61 cm3·g-1 for Zn-tfbdc-dabco, respectively. Pore size distributions (PSD) obtained by using Horvath‑ Kawazoe method for the CO2 adsorption isotherms showed pores of 0.8 nm for Zn-tfbdc-dabco.
Inspired by the unique pore environment of Zn-tfbdc-dabco, subsequently, the single component adsorption isotherms of C3H6 and C3H8 on Zn-tfbdc-dabco were measured up to 100 kPa at different temperatures (298, 313, and 333 K). As shown in Fig. 3b-3d, the C3H8 uptake was 120.67, 112.19, and 97.78 cm3·g-1 at 298, 313, and 333 K, respectively, which outperforms most reported C3H8-selective MOF materials, including PCP-IPA (50 cm3·g-1)[48], NUM-7 (67 cm3·g-1)[49], Ni(ADC)(TED)0.5 (52 cm3·g-1)[50], WOFOUR-1-Ni (22 cm3·g-1)[51] at 298 K and 100 kPa. And C3H6 adsorption was 123.90 cm3·g-1 at 298 K and 100 kPa, which was slightly higher than that of C3H8, and the C3H8-selective behavior was only exhibited at the low-pressure region. As expected, the adsorption capacity of both C3H8 and C3H6 decreased with the increase of temperature, and the C3H8-selective behavior was seen in the low-pressure region at any temperature tested, suggesting the stronger host-guest interaction between the C3H8 molecule and the framework, which was also seen from the slope of gas adsorption isotherms. Besides, the Zn-tfbdc-dabco exhibited good C3H8‑ selective adsorption during the whole pressure area at 313 and 333 K. Zn-tfbdc-dabco exhibited a good C3H8/C3H6 adsorption ratio with 97.40% (298 K), 100.64% (313 K), and 105.40% (333 K) at 100 kPa.
To elaborate on the difference in the host-guest affinity between the framework and guest molecules of Zn-tfbdc-dabco, the coverage-dependent isosteric enthalpy of adsorption (Qst) for C3H8 and C3H6 was calculated by the virial method fitting collected adsorption isotherms at 313 and 333 K, which could quantitatively illustrate the strength of host-guest interactions. As depicted in Fig. 4a, the low-coverage Qst values of C3H8 and C3H6 are 29.0 and 28.4 kJ·mol-1, respectively, suggesting a stronger interaction between the C3H8 molecule and Zn-tfbdc-dabco framework than that for C3H6. Notably, the Qst value for C3H8 was lower than other reported C3H8-selective MOF adsorbents, such as Ni(ADC)(TED)0.5 (65.3 kJ·mol-1)[50], PCP-IPA (50.9 kJ·mol-1)[48], FDMOF-2 (34.6 kJ·mol-1)[43], MOF-801 (43.9 kJ·mol-1)[52], NUM-7 (40.0 kJ·mol-1)[49], HIAM-402 (34.5 kJ·mol-1)[53], BUT-10 (32.8 kJ·mol-1)[54], CPM-734c (31.5 kJ·mol-1)[55] (Fig. 4b). Such low enthalpy of adsorption for C3H8 implies the feasibility to easily regenerate the porous solid adsorbent under mild conditions, which may be beneficial for practical implementations. Moreover, the results of the isosteric heat of adsorption could further confirm the C3H8-selective behavior on Zn-tfbdc-dabco, which was in line with the performance of the experiment.
The obvious adsorption difference obtained from single-component gas adsorption data between C3H8 and C3H6 in Zn-tfbdc-dabco motivated us to investigate the separation of C3H8/C3H6 mixtures (10∶90 and 50∶50, V/V) owing to industrial relevance. Ideal adsorbed solution theory (IAST) was then employed to assess the adsorption selectivity toward C3H8/C3H6 mixtures at different conditions, which could quantitatively exhibit the separation potential of Zn-tfbdc-dabco for the removal of C3H8 substance from the C3H8/C3H6 mixture. As shown in Fig. 4c and 4d, the IAST selectivity values of Zn-tfbdc-dabco toward C3H8/C3H6 mixture were both 1.33 at 298 K, whereas these values are 1.42 (313 K) and 1.41 (333 K) for the 10∶90 mixture, 1.25 (313 K) and 1.25 (333 K) for the 50∶50 mixture. The IAST selectivity of Zn-tfbdc-dabco can be comparable with WOFOUR‑1‑Ni (1.6)[51], Ni(NDC)(TED)0.5 (1.3)[50], MOF-801 (1.1)[52], HIAM-402 (1.43)[53], Zn(BTFM)(DABCO)0.5 (1.57) and Cu(BTFM)(DABCO)0.5 (1.39)[46]. The obvious difference between the enthalpy of adsorption for C3H8 and C3H6, and IAST selectivity, implies the promising potential of Zn-tfbdc-dabco for the C3H8/C3H6 separation.
To further evaluate the separation feasibility of the C3H8/C3H6 mixture on the porous adsorbent of Zn-tfbdc-dabco, dynamic column breakthrough experiments were carried out at ambient conditions in a fixed-bed adsorption process. As shown in Fig. 5, a clear separation for the C3H8/C3H6 mixture could be accomplished by Zn-tfbdc-dabco at tested conditions in the pressure swing adsorption processes, whereby the C3H6 breakthrough occurred first. The results of the breakthrough experiment emphasized the separation capacity of Zn-tfbdc-dabco as a C3H8-selective candidate for one-step purification of C3H6 steam from the C3H8/C3H6 mixture.
c and c0 represent the effluent and initial gas concentrations, respectively.
Based on the obvious difference in adsorption performance and host-guest affinity, to further in-depth understand the preferential adsorption of C3H8 over C3H6 by Zn-tfbdc-dabco, the grand canonical Monte Carlo (GCMC) simulations were conducted to determine the adsorption sites at 333 K and 100 kPa. As shown in Fig. 6a and 6b, it was observed that the preferential adsorption sites for C3H8 and C3H6 molecules were near the tfbdc ligands. As expected, the C3H8 guest molecule interacts with the surrounding benzene rings through multiple C—H···F interactions (0.272 6-0.398 9 nm) (Fig. 6a). In contrast, the host-guest interaction for the C3H6 guest molecule and framework is evident to be weaker with the less C—H···F hydrogen bonds (Fig. 6b). Overall, compared to the C3H6 molecule, the C3H8 molecule with more complex molecular configurations and larger molecule size formed multiple C—H···F interactions, resulting in the preferential adsorption of C3H8 over C3H6 in Zn-tfbdc-dabco. The different number and intensity of C—H···F interactions between host molecules and guest framework facilitate the C3H8-selective behavior to realize the inverse selective C3H8/C3H6 separation and further achieve the possibility of one-step purification of C3H6 from the corresponding C3H8-containing mixture.
Zn, C, N, O, and F atoms are presented by light purple, gray, blue, red, and green, respectively.
In summary, by the fluorinated pore surface, the inverse selective C3H8/C3H6 separation could be achieved on a robust, low-cost microporous pillar‑ layered MOF (Zn-tfbdc-dabco). Zn-tfbdc-dabco exhibited a good C3H8 adsorption capacity with 120.67, 112.19, and 97.78 cm3·g-1 at 298, 313, and 333 K, respectively, and afforded a high C3H8/C3H6 adsorption ratio with 97.40%, 100.64%, and 105.40% at 298, 313, and 333 K, respectively. In addition, a breakthrough experiment further proved the inverse selective C3H8/C3H6 separation performance. The fluorinated pore space plays a key role in this paraffin-selective behavior, which can provide customized binding sites for the paraffin molecules. With the advantages of low-cost precursors, good adsorption capacity, and low isosteric enthalpy of adsorption, Zn-tfbdc-dabco becomes a promising candidate for critical industrial implementations. This research provides a good example of a robust fluorinated MOF with low cost to achieve this inverse selective C3H8/C3H6 separation, realizing one-step separation and purification of the target C3H6 substance, and thus paves the route for the construction of the inverse-selective porous MOF materials to address the very active yet challenging gas separation domain.
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Figure 1 Structure description of Zn-tfbdc-dabco: (a) the framework formed by Zn-paddlewheel, dabco, and tfbdc ligands; three-dimensional framework viewed along the (b) a-axis, (c) b-axis, and (d) c-axis
(a) Zn, C, N, O, F atoms are presented by light purple, gray, blue, red, and green, respectively.
Figure 2 (a) Experimental PXRD patterns of the as-synthesized Zn-tfbdc-dabco sample and the simulated one from single crystal XRD data; (b) PXRD patterns of Zn-tfbdc-dabco soaked in different organic solvents for 24 h; (c) TG curve of Zn-tfbdc-dabco under an argon atmosphere; (d) VT-PXRD patterns of Zn-tfbdc-dabco under an argon atmosphere
Figure 3 (a) CO2 adsorption isotherms for Zn-tfbdc-dabco at 195 K and the pore size distribution (Inset); Gas adsorption isotherms of Zn-tfbdc-dabco at (b) 298 K, (c) 313 K, and (d) 333
Figure 4 (a) Qst of C3H8 and C3H6 in Zn-tfbdc-dabco; (b) Comparison of the zero-coverage heat of adsorption of Zn-tfbdc-dabco with those of other materials for the C3H8/C3H6 separation; IAST selectivity of C3H8/C3H6 mixture at (c) 50∶50 and (d) 10∶90 (V/V) ratios
Figure 5 Experimental column breakthrough curves of C3H8/C3H6 mixture on Zn-tfbdc-dabco at 333 K and 100 kPa
c and c0 represent the effluent and initial gas concentrations, respectively.
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