Efficient ethylene purification by a robust ethane-trapping porous organic cage

doi:10.21203/rs.3.rs-98247/v1

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Efficient ethylene purification by a robust ethane-trapping porous organic cage

https://doi.org/10.21203/rs.3.rs-98247/v1

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published 17 Jun, 2021

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The removal of ethane (C₂H₆) from its analogous ethylene (C₂H₄) is of paramount importance in the petrochemical industry, but highly challenging due to their similar physicochemical properties. It is still remaining unexploited to utilize emerging porous organic cage (POC) materials for C₂H₄/C₂H₆ separation. Herein, we report the benchmark example of a truncated octahedral calix[4]resorcinarene-based POC adsorbent (CPOC-301), preferring to adsorb C₂H₆ than C₂H₄, and thus can be used as a robust absorbent to directly separate high-purity C₂H₄ from the C₂H₄/C₂H₆ mixture. Molecular modelling studies suggest the exceptional C₂H₆ selectivity is due to the suitable resorcin[4]arene cavities in CPOC-301, which form more multiple C–H···π hydrogen bonds with C₂H₆ than with C₂H₄ guests. This work provides a fresh avenue to utilize POC materials for highly selective separation of industrially important hydrocarbons.

Materials Chemistry

ethylene

porous organic cage

hydrocarbons

Ethylene (C₂H₄), the largest feedstock in petrochemical industries with a global production capacity of more than 170 million tons in 2018, exceeds any other chemical feedstock molecules for its wide application in the manufacture of polyethylene¹. The industrial separation of C₂H₄ from ethane (C₂H₆) is typically by cryogenic distillation at high pressure and very low temperature condition, with very high towers, because they have similar sizes and volatilities². Such process is extremely energy consuming, and thus exploration of other effective C₂H₄ separation methods at ambient condition are highly demanded. Selective adsorption by porous materials to produce high-purity C₂H₄ has been determined to be one of the most desired methods for its low energy consumption^3,4. Of particular recent interest in this region is the use of C₂H₆-selective porous materials enriched with nonpolar/inert surfaces (e.g., featuring aromatic or aliphatic moieties), because they prefer to capture more polarizable C₂H₆, and thus can directly produce high purity C₂H₄ in a single adsorption step, avoiding an additional high-energy-consuming desorption step^5,6. So far, most of the developed C₂H₆-selective adsorbents are metal − containing materials^6–10. However, the polar metal centers in these materials can interact with unsaturated C₂H₄ molecules via strong π-complexation interactions, and thus reduce their C₂H₄/C₂H₆ separation abilities¹¹. Thus, it is very necessary to develop other C₂H₆-selective adsorbents such as metal-free organic porous materials^12,13, because they cannot only avoid the abovementioned problem, but also are easier to construct inert surface. However, the research of such materials is still in the initial stage¹⁴.

Porous organic cages (POCs), as a new type of porous materials, are intrinsically porous given their hollow cavities^15–20. The discrete inherent nature of the POCs makes them possessing distinct advantages in solution processing, regeneration and post-synthesis modification^20–23. Since the first elegant work reported in 2009 by Cooper et al.²⁴, the number of POCs with different shapes, sizes and properties has increased substantially^25–35, but robust POCs with high surface areas are still few, which highly hinder their practical applications in gas storage and separation. In the past decade, although the Brunauer-Emmett-Teller (BET) surface areas of the POCs have increased from the initial 624 m² g^− 1 to 3758 m² g^− 1, the majority of POCs have BET values less than 1000 m²/g^36,37. As for gas separation, Cooper and coworkers first reported that imine-linked tetrahedral POC (CC3) exhibits unprecedented selectivity of 20.4 for rare gas Xe/Kr separations at low rare gas concentrations in 2014³⁸. Two years later, they showed that CC3 could also separate sulfur hexafluoride (SF₆), a much more potent greenhouse gas than carbon dioxide, from nitrogen (N₂) with the highest SF₆/N₂ selectivity reported for any material at ambient condition³⁹. The selectivity for the abovementioned two cases is arising from the precise size match between the Xe and SF₆ with the organic cage cavity than Kr and N₂, respectively. In 2019, their team utilized post modification synthesis method to modify the internal cavities of CC3 to produce hybrid cocrystal material (6ET-RCC3-R/CC3-S), which are excellent quantum sieves for hydrogen isotope separation with excellent deuterium/hydrogen selectivity (8.0)⁴⁰. In the same year, Zhang’s and Zaworotko’s groups presented a soft imide-based POC (NKPOC-1) with gate opening behavior and can highly efficiently separate binary or ternary C3 hydrocarbon mixtures⁴¹. Up to now, POC adsorbents have only been limited to the above gas separations, and thus there is still much room to explore new POC materials for gas separations applications, especially for industrially important gases.

Calix[4]resorcinarenes, a subset of calixarenes, were derived from the acid-catalyzed condensation of resorcinol with various kinds of aldehydes^42–44. They possess electron-rich π cavities and eight upper-rim phenolic groups, and have been determined to be effective hosts for inclusion of a variety of guests range from small gases to large organic molecules^44–48. Notably, their upper rims can be easily functionalized, and this makes them good molecular building blocks for construction of self-assembled cages as well as porous polymers^49–55. Very recently, by using pre-designed concave-shaped tetraformylresorcin[4]arene (RC4ACHO) as secondary building blocks and different diamine as linkers, our group have systematically designed and synthesized several porous POCs with structural diversity from [2 + 4] dimeric lanterns, [3 + 6] trimeric triangular prisms, to [6 + 12] hexameric octahedra⁵⁶. However, utilization of calix[4]resorcinarene-based POCs as solid adsorbents for gas separation applications is still unexploited. Herein, we present that the robust and highly porous [6 + 12] octahedral calix[4]resorcinarene-based POC (CPOC-301) is an excellent C₂H₆-selective material, and can be used as a robust absorbent to directly produce high-purity C₂H₄ from C₂H₄/C₂H₆ mixture.

Crystal structure and characterization of CPOC-301

As shown in Fig. 1a, CPOC-301 was successfully prepared via self-assembly of RC4ACHO (1 equiv.) and p-phenylenediamine (2 equiv.) under mild condition. Single-crystal X-ray diffraction reveals that CPOC-301 has a truncated octahedron structure, with eight trigonal ports with an edge length of about 12 Å, and a large cavity with inner diameter and volume of 16.8 Å and 4270 Å³, respectively. The solid-state packing of CPOC-301 suggests that it possesses a one-dimensional channel, with a diameter of ~ 7 Å, viewed from the [001] direction (Fig. 1b). Notably, the quantitative tautomerization of the imines in CPOC-301 to their keto-enamine forms has been observed from ¹H NMR and FT-IR spectroscopy (Supplementary Figs. 1–3). Such a transformation makes CPOC-301 more stable even when it is stored in air for over half a year, as has been confirmed by FT-IR spectra as well as ¹H NMR studies (Supplementary Figs. 3 and 4). TGA suggests that CPOC-301 is thermally stable up to 300 ^oC under an N₂ atmosphere (Supplementary Fig. 5). PXRD reveals that the desolvated CPOC-301 retains its crystal packing (Supplementary Fig. 6), which is very uncommon in large POC systems.

Gas adsorption and separation performances

The channel-type structure of CPOC-301 can potentially lead to interesting solid-state gas sorption properties. The permanent porosity of the activated CPOC-301 was confirmed by N₂ gas sorption experiments at 77 K (Fig. 2). Its N₂ adsorption isotherm shows a typical type I curve with a small fraction of a type II adsorption behavior. The maximum N₂ adsorption is 670 cm³ g^-1, and the calculated BET of CPOC-301 is up to 1962 m² g^-1. The pore volume and the micropore volume of CPOC-301 are 1.03 cm³ g^-1 and 0.46 cm³ g^-1, respectively. The PSD profile exhibits a relatively sharp distribution of micropores from 1.59 nm to 1.77 nm (Fig. 2 inset), which accords well with the cavity size of CPOC-301 in the crystal structure.

The high surface area and high density of nonpolar calix[4]resorcinarene cavities in CPOC-301, which may favor the preferential adsorption of more polarizable C₂H₆ in preference to C₂H₄, prompted us to investigate the sorption of these gases. Single-component adsorption isotherms of CPOC-301 for C₂H₆ and C₂H₄ were measured at 273 K, 283 K and 293 K at 1 bar. Notably, CPOC-301 exhibits preferential adsorption of C₂H₆ (87 cm³ g^-1) to C₂H₄ (75 cm³ g^-1) at 293 K (Fig. 3a), and also the other two temperatures (Supplementary Figs. 7 and 8). The corresponding isosteric heat of adsorption (Q_st) at zero coverage for C₂H₆ and C₂H₄ was calculated to be 32.4 and 24.2 kJ mol^-1, respectively (Fig. 3b and Supplementary Figs. 9 and 10). Such a difference suggests that the host-guest interactions between CPOC-301 and C₂H₆ are much stronger than that of C₂H₄. Notably, the C₂H₆ uptake capacity of CPOC-301 at 293 K and 1 bar exceeds that of all the reported porous organic materials¹⁴, and most of the reported C₂H₆-selective MOF materials⁵.

Motivated by the high uptake capacity and C₂H₆-selective behavior of CPOC-301, the ideal adsorbed solution theory (IAST) is used to evaluate the separation selectivity of C₂H₄/C₂H₆ (50:50) (Supplementary Figs. 11–13). IAST calculation results show that the C₂H₆/C₂H₄ selectivity range is from 1.3 to 1.4 at 293 K. To evaluate the actual separation performance of CPOC-301, the experimental breakthrough studies were conducted in a packed column of the activated CPOC-301 sample under an equimolar C₂H₄/C₂H₆ mixture at ambient conditions. The breakthrough curves shown in Fig. 3c prove that CPOC-301 can efficiently realize the complete separation of C₂H₄ from C₂H₄/C₂H₆ mixtures. Notably, the C₂H₄ gas breaks through the adsorption bed first to yield an outflow of pure gas containing no detectable C₂H₆. Conversely, the C₂H₆ gas breaks through the column following a substantial time-lapse, because the C₂H₆ molecule is more preferentially adsorbed in CPOC-301 than C₂H₄. From the dynamic breakthrough experiment, the separation factor was calculated to be 1.3 for an equimolar mixture of C₂H₄/C₂H₆, which is consistent with the predicted IAST result. For practical industrial applications, the ideal adsorbent should also have good recycling performance. We performed multiple C₂H₄/C₂H₆ mixed-gas dynamic breakthrough experiments under similar operating conditions. The separation performance of C₂H₄/C₂H₆ does not obviously change within seven continuous cycles (Fig. 3d), and the NMR and PXRD data after breakthrough experiments (Supplementary Figs. 14 and 15), indicate that CPOC-301 is robust enough to be a promising candidate for C₂H₄ purification.

Separation mechanism

To further understand the role of CPOC-301 in the mechanism of selective C₂H₆/C₂H₄ adsorption, modelling studies based on first-principles dispersion-corrected density functional theory (DFT-D) calculations were performed. It was found that the primary adsorption sites for both C₂H₆ and C₂H₄ molecules are located at the calix[4]resorcinarene cavities. The lowest-energy gas binding configurations are shown in Figs. 4a and 4b. For clarity, only one adsorbed gas molecule at the calix[4]resorcinarene cavity site is shown, because the remaining five sites within CPOC-301 are crystallographically identical. The corresponding calculated static binding energies (ΔE, ΔE = E_POC+gas − E_POC−Egas) of C₂H₆ and C₂H₄ are around − 63.4 and − 56.5 kJ mol^-1, respectively. Such binding energy variation suggests the stronger host-guest interactions between C₂H₆ and calix[4]resorcinarene cavity than that of C₂H₄, which is in accord with our aforementioned experimental observation. This can be ascribed to the more polarizable C₂H₆ in “matches” better to the electron-rich cavity of calix[4]resorcinarene than the less polarizable C₂H₄ molecule. As a result, the C − H···π interactions existing between the C₂H₆ and calix[4]resorcinarene cavity are stronger than those of the C₂H₄ molecule. To further authenticate our result, the host-guest interactions between CPOC-301 and C₂H₂ (acetylene), another guest molecule of the C2 hydrocarbons family, has also been simulated (Fig. 4c). The calculated ΔE value of C₂H₂ is -51.7 kJ mol^-1, indicating that its host-guest interaction is weakest in the C2 hydrocarbons. This is consistent with the experimental Q_st results (20.2 kJ/mol for C₂H₂), and has been confirmed by C₂H₂/C₂H₄ breakthrough experiment (Fig. 5 and Supplementary Fig. 16).

We have reported a pioneering work by using shape-persistent amine-linked CPOC-301 as POC adsorbent for C₂H₄ purification for the first time. Specifically, the highly porous CPOC-301 can efficiently trap C₂H₆ from C₂H₆/C₂H₄ mixture and therefore directly produce high-purity C₂H₄. The preferential interactions with C₂H₆ over C₂H₄ in CPOC-301, are due to the C₂H₆ form more multiple C–H···π hydrogen bonds with resorcin[4]arene cavities than C₂H₄ guests as indicated by the results of DFT-D calculations. This finding may shed some light on the design and synthesis of POCs based on supramolecular cavitands as “porous additives” in column and membrane separation applications for industrially important gases in the future. Efforts to explore these possibilities are ongoing.

Synthetic procedures of CPOC-301

All reagents and solvents are were purchased from Sinopharm Chemical Reagent Co., Ltd with analytical grade, and utilized as supplied without further puriﬁcation. CPOC-301 was synthesized according to the previously published procedure⁵⁶. Typical synthesis processes were as follows: 162 mg (0.20 mmol) RC4ACHO⁵⁷ and 43 mg (0.4 mmol) p-phenylenediamine were added into 5 mL of nitrobenzene and 15 mL of CHCl₃. The mixture was sealed in a 48 mL pressure vial, and heated to 65 ° C with stirring for 2 days, and afterward cooled down naturally. Red block single crystals of CPOC-301 with ~78% yield were obtained by the slow vapor diffusion of methanol into the abovementioned mixture.

Characterization of CPOC-301

The as-prepared CPOC-301 sample was characterized by the following available instruments in Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, China. Proton nuclear magnetic resonance (1H NMR) data were recorded at ambient temperature a Burker AVANCE 400 (400 MHz) for spectrometer. Fourier-transformed infrared spectroscopy (FT-IR) spectra were taken on a Magna 750 FT-IR spectrometer using KBr pellets in the 600−4000 cm^-1 region. High-resolution electrospray ionization mass spectrometry (ESI-TOF-MS) was recorded on a MaXis™ 4G instrument from Bruker. Thermal gravimetric analysis (TGA) was collected at a ramp rate of 10°C/min in dynamic nitrogen (N₂) flow within a temperature range of 30-900 °C with a NETZSCH STA 449C thermal analyzer. Powder X-ray diffraction (PXRD) pattern was performed on a Rigaku Mini 600 X-ray diffractometer for CuK_α radiation (λ = 0.154 Å), with a scan speed of 0.5°/min and a step size of 0.02° in 2θ.

Gas adsorption measurements

All the gas adsorption-desorption measurements of CPOC-301 were carried out by using automatic volumetric adsorption equipment (Micromeritics, ASAP2020). Pore size distribution (PSD) data was obtained from the N₂ sorption isotherm at 77 K based on the DFT model in the Micromeritics ASAP 2020 software package (assuming cylinder pore geometry). Prior to the measurements, the samples were degassed at 100 ^oC under dynamic vacuum (below 10 μmHg) for 10 h to remove the adsorbed impurities. The calculated pore volume and the micropore volume are based on the ASAP 2020 physisorption analyzer’s built-in software. The isosteric heat of sorption for C₂H₆ and C₂H₄ was calculated as a function of the gas uptake by comparing the adsorption isotherms at 273, 283 and 293 K. The data were modeled with a virial-type expression composed of parameters a_i and b_i (Equation 1), and the heat of adsorption (Q_st) was then calculated from the fitting parameters using Equation 2, where P is the pressure, N is the amount adsorbed, T is the temperature, R is the universal gas constant, and m and n determine the number of terms required to describe the isotherm adequately. The parameters were obtained from the fitting of the C₂H₆ and C₂H₄ adsorption isotherms. All isotherms were fitted with R²>0.999. In order to evaluate the separation performance for C₂H₆/C₂H₄, we used the Ideal Adsorbed Solution Theory (IAST) of Myers and Prausnitz⁵⁸ along with the pure component isotherm fits by dual-site Langmuir-Freundlich equation to determine the molar loadings in the mixture for specified partial pressures in the bulk gas phase (Equation 3). Where N is molar loading of species (mmol g^-1), A is saturation capacity of species (mmol g^-1), B is Langmuir constant (kPa^-c), C is Freundlich constant and P is bulk gas phase pressure of species (kPa). The adsorption selectivity based on IAST for mixed C₂H₆/C₂H₄ is defined by the following equation 4.

Column Breakthrough Experiments

The mixed-gas breakthrough separation experiment was carried out using a home-built setup coupled with a mass spectrometer (Pfeiffer GSD320). For instance, in a typical breakthrough experiment for C₂H₄/C₂H₆/He (10:10:80, v/v/v) and C₂H₂/C₂H₄/He (10:10:80, v/v/v) gas mixtures, CPOC-301 powder (0.39 g) was packed into a custom-made stainless-steel column (3.0 mm I.D.×120 mm) with silica wool filling the void space. The packed column was heated at 100 ^oC for 12 h under a constant He flow (10 mL min^-1 at 298 K and 1 bar) to activate the sample. The flow of He was then turned off and a gas mixture of C₂H₄/C₂H₆/He (2 mL min^-1) was allowed to flow into the column. Outlet effluent from the column was continuously monitored using mass spectrometer. After the breakthrough experiment, the sample was regenerated in-situ in the column at 100 ^oC for 12 h. The complete breakthrough of C₂H₆ was indicated by the downstream gas composition reaching that of the feed gas. On the basis of the mass balance, the gas adsorption capacities can be determined as follows⁵⁹:

Where qi is the equilibrium adsorption capacity of gas i (mmol g^-1), C_i is the feed gas concentration, V is the volumetric feed flow rate (cm³ min^-1), t is the adsorption time (min), F₀ and F are the inlet and outlet gas molar flow rates, respectively, and m is the mass of the adsorbent (g). The separation factor (α) of the breakthrough experiment is determined as:

in which y_i is the molar fraction of gas i (i=A, B) in the gas mixture.

Binding energy calculations

The initial binding sites for C₂H₆, C₂H₄ and C₂H₂ were determined from simulated annealing calculations using Adsorption Locator Module in the Material Studio program package. The possible main adsorption sites for adsorbed C2 hydrocarbon molecules in CPOC-301 were investigated by the Dmol³ module in the Material Studio program package. The PBE-type exchange-correlation functional with a generalized gradient approximation and the Double Numerical plus polarization (DNP) basis sets that include a d-type polarization function on all non-hydrogen atoms and a p-type polarization function on all hydrogen atoms were employed for all calculations. The FINE quality mesh size was employed in the calculations. During the C2-Cage structure optimization, the atomic positions of the CPOC-301 were kept immobile and the single C2 hydrocarbon molecule was allowed to move during optimization. The adsorption energies were calculated in terms of equation:

ΔE=E_POC+gas−E_POC−E_gas

where E_POC+gas stands for the energy of the optimized adsorbate-Cage structure, and E_POC, and E_gas denote the energies of the bare Cage structure and the isolated C2 hydrocarbon molecule, respectively. According to this equation, more negative adsorption energy means more favorable binding.

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Conflict of interest

The authors state that they have no conflict of interest.

Acknowledgments

This work was financially supported by the National Key R&D Program of China (2017YFA0700102), the National Natural Science Foundation of China (22071244, 21707143), and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).

Author contributions

Daqiang Yuan and Kongzhao Su proposed the ideas and supervised the project. Kongzhao Su, and Shunfu Du prepared the cages and carried out the characterization of the cages. Wenjing Wang and Chunqing Ji performed the gas sorption and separation experiments. Daqiang Yuan, Kongzhao Su and Wenjing Wang analyzed the data and wrote the manuscript. All authors discussed the results and commented on the manuscript.

Additional information

Supplementary Information accompanies this paper at http://www.nature.com/naturecommunications.

Competing financial interests

The authors declare no competing financial interests.

There is NO Competing Interest.

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Efficient ethylene purification by a robust ethane-trapping porous organic cage

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Crystal structure and characterization of CPOC-301

Gas adsorption and separation performances

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