Direct Visualization of Supramolecular Binding and Separation of Light Hydrocarbons in MFM-300(In).

The purification of light olefins is one of the most important chemical separations globally and consumes large amounts of energy. Porous materials have the capability to improve the efficiency of this process by acting as solid, regenerable adsorbents. However, to develop translational systems, the underlying mechanisms of adsorption in porous materials must be fully understood. Herein, we report the adsorption and dynamic separation of C2 and C3 hydrocarbons in the metal-organic framework MFM-300(In), which exhibits excellent performance in the separation of mixtures of ethane/ethylene and propyne/propylene. Unusually selective adsorption of ethane over ethylene at low pressure is observed, resulting in selective retention of ethane from a mixture of ethylene/ethane, thus demonstrating its potential for a one-step purification of ethylene (purity > 99.9%). In situ neutron powder diffraction and inelastic neutron scattering reveal the preferred adsorption domains and host-guest binding dynamics of adsorption of C2 and C3 hydrocarbons in MFM-300(In).

Introduction

Light olefins, primarily ethylene (C2H4) and propylene (C3H6), are the cornerstone of petrochemical industries for the production of polymers and various fine chemicals.[1] The current global ethylene and propylene production is around 200 million tons per year.[1] These short-chain alkenes are produced typically by the steam cracking of feedstocks derived from crude oil, such as naphtha, which is a liquid mixture of short and medium, typically comprising C5–C12 chain hydrocarbons.[2,3] Steam cracking of naphtha produces a mixture of products, which must be separated prior to use. Most commonly, post-cracking separation is performed using cryogenic distillation operating at high pressure and low temperature (as low as −160 °C). This is thus an incredibly energy-intensive process, consuming around a third of the overall energy used in the process of ethylene production.[3] The development of energy-efficient alternatives to cryogenic distillation can effectively reduce energy consumption as well as emissions.[4] A possible strategy involves the use of porous materials to adsorb selectively a single component from gas mixtures (e.g., alkynes and alkanes) while allowing other components to pass through. The binding of gas molecules in these materials is based often upon multiple, weak, long-range supramolecular interactions, which facilitate the removal of adsorbed species and regeneration of sorbents via either temperature swing or pressure swing desorption. This can operate potentially at ambient conditions and thus carries a relatively low energy penalty. Several porous adsorbents have been proposed for this application, such as ion-exchange resins,[5] zeolites,[6,7] and, most notably, metal–organic framework (MOF) materials.[4,8−11]

Over the past 2 decades, MOFs have been studied widely for their applications in gas separations.[12] The ability to tune the pore size and the chemical environment of MOFs makes them excellent candidates for separating molecules with similar physical properties, such as light hydrocarbons. Several MOF materials have been proposed for application in hydrocarbon separation. These utilize different strategies including the use of open metal sites,[13−15] specific gate opening effects,[16−18] and kinetic size exclusion.[19−23] Recently, an interesting computational study on the effects of pore size on the selectivity of ethane/ethylene has been reported.[24] It was found that for a given adsorbent, separation could be controlled by altering the size of the pore along one dimension while maintaining the overall pore chemistry and structure. The purification of olefins from C2 or C3 hydrocarbon streams is considered one of the most challenging and important processes in the petrochemical industry. For the production of polymer-grade C2H4 from C2H4-selective adsorbents, an additional desorption step for the release of adsorbed C2H4 molecules is required, which adds additional energy costs via the application of vacuum and/or heating.[25] In contrast, C2H6-selective adsorbents have clear advantages in the practical separation of C2H6/C2H4 owing to the direct production of polymer-grade C2H4 in one step by selective retention of C2H6.[4,26] However, C2H6-selective adsorbents are far less common than C2H4-selective materials.[9,23,27,28]

Porous materials incorporating unsaturated metal sites (typically transition metals) can afford unique electrostatic binding sites for C2H4 or C3H6via π-complexation.[14,28] Although these materials show strong host–guest interactions accompanied by a high adsorption enthalpy compared with MOFs without open metal sites,[20,29] these materials often show limited stability, especially when exposed to humid conditions. In this context, the MFM-300 series of MOF materials[29−31] represents a useful practical example to examine the adsorption of hydrocarbons. The MFM-300 series is a group of isostructural MOFs which are composed of biphenyl-3,3′,5,5′-tetracarboxylate (L4–) linkers connected to [M(μ2-OH)2]∞ chains in a wine-rack mode. This family of MOFs differs from other MOF materials, which have been reported for separations of light hydrocarbons, in that they utilize hydroxyl groups as the primary binding sites of adsorbed gas molecules. This may be advantageous over the reported method of relatively strong binding to open metal sites[14,32] in that the binding energy for guest–hydroxyl interactions is significantly lower, thus making MFM-300 materials more readily regenerable and less susceptible to poisoning by moisture.

Here, we report the adsorption and breakthrough separation of C2 and C3 hydrocarbons in MFM-300(In), which exhibits an unusual selective adsorption of ethane at low pressure that is distinct from that observed for MFM-300(Al).[29] We also describe the direct visualization of the supramolecular binding of C2 and C3 hydrocarbons within the pore by a combination of in situ neutron powder diffraction (NPD) and inelastic neutron scattering (INS) experiments. Breakthrough experiments confirmed the efficient separation of equimolar mixtures of C2H6/C2H4 and C3H4/C3H6 by MFM-300(In) to produce high-purity ethylene and propylene (purity > 99.9%) at room temperature.

Experimental/Methods

Synthesis of MFM-300(In)

H4L (330 mg, 1.00 mmol) and In(NO3)3·5H2O (585 mg, 1.50 mmol) were mixed in a dimethylformamide (DMF)/MeCN mixture (30 mL, 2:1 v/v) with conc. HNO3 (1.0 mL) in a 250 mL glass pressure reactor. The vessel was sealed and heated at 80 °C for 48 h. The resultant flaky white precipitate was then washed with DMF and immersed in an excess of acetone for 3 days with frequent exchange of the solvent.[33] Yield: 347 mg (42% yield based upon solvent content from microanalysis).

Gas-Adsorption Isotherms and Breakthrough Experiments

Gravimetric isotherms (0–1000 mbar) were recorded at 273, 283, 293, 303, and 308 K (temperature-controlled water bath) for C2H2, C2H4, C2H6, C3H4, C3H6, and C3H8 and at 195 K (dry ice/acetone) for C2H2, C2H4, and C2H6. Data were collected using an IGA-003 system (Hiden Isochema, Warrington, UK) equipped with a turbomolecular pumping system. Acetone-exchanged samples were loaded into the system and degassed at 120 °C and 1 × 10–6 mbar for 20 h to give a dry, desolvated material of a typical mass of ca. 50 mg. Ultra-pure research-grade (99.99%) gases were purchased from Air Liquide or BOC and used as received. C2H2 was purified using dual-stage cold trap systems operated at 195 K (dry ice) and an activated carbon filter before introduction to the IGA system. Dynamic breakthrough experiments were conducted on a Hiden Isochema IGA-003 with ABR attachments and a Hiden Analytical mass spectrometer by using a fixed-bed tube packed with 750 mg of MFM-300(In) powder. The sample was heated at 120 °C under a flow of dry He for 12 h for activation and then cooled to room temperature (293 K). Single-component gas breakthrough experiments with an inlet gas flow rate of 2 mL min–1 diluted in a flow of He (a total flow rate of 20 mL min–1) were performed through a fixed-bed packed with MFM-300(In). For equimolar mixtures of hydrocarbons, the flow rate of 2.0 mL min–1/2.0 mL min–1 diluted in He (a total flow rate of 20 mL min–1) was applied. Dynamic breakthrough experiments for 1:99 mixtures of C2H2/C2H4, C2H2/C2H6, and C2H4/C2H6 were conducted at the rate of 0.2 mL min–1/19.8 mL min–1. All breakthrough experiments were conducted at a total flow of 20 mL min–1 at 293 K. The concentration of the hydrocarbon gas at the outlet was determined by mass spectrometry and compared with the inlet concentration C0, where C/C0 = 1 indicates complete breakthrough.

Results and Discussion

Material and Characterization

MFM-300(In) was synthesized by following our previously reported method[33] (see Experimental/Methods for details). MFM-300(In) is composed of one-dimensional (1D) [In(OH)2O4]∞ chains bridged by tetracarboxylate ligands L4– to afford a porous framework structure with channels decorated with cis-μ2-OH groups (Figure a). The powder X-ray diffraction pattern and thermogravimetric curves confirm the high phase purity and thermal stability of the material (Figures S1 and S2). Using N2 sorption isotherm data at 77 K, desolvated MFM-300(In) is found to display a Brunauer–Emmett–Teller surface area of 1030 m2 g–1 and a pore volume of 0.43 cm3 g–1 with a pore size distribution centered at 6.8 Å (Figure S3).

Figure 1

(a) View of the infinite chain of [InO4(OH)2]∞ linked by tetracarboxylate ligands (In: green; C: gray; O: red; H: light yellow; hydrogen atoms on the ligands are omitted for clarity). Single-component adsorption isotherms for (b) C2 and (c) C3 hydrocarbons in MFM-300(In) at 293 K. (d) Analysis of IAST selectivity of C2H6/C2H4 for MFM-300(In) at 293 K and 1 bar. (e) Isosteric heats of adsorption (Qst) for C2 and C3 hydrocarbons in MFM-300(In). (f) Adsorption kinetics of C2 and C3 hydrocarbons of MFM-300(In) at 293 K (30–70 mbar).

Analysis of Gas-Adsorption Isotherms

C2 and C3 hydrocarbons show fully reversible uptake in MFM-300(In) with type I isotherms being observed between 195 and 303 K (Figures b,c, S4−S11). Single-component adsorption isotherms reveal that MFM-300(In) has a distinct binding affinity to C3H4 over C3H6 and C3H8 and to C2H6 over C2H4 over a wide range of temperatures from 273 to 303 K. The uptake of C3 hydrocarbons exhibits steep adsorption isotherms at low pressure, with C3H4, C3H6, and C3H8 reaching 73 to 87% of their total adsorption capacity at 1 bar at a pressure of 100 mbar; it is notable that these isotherms reach a plateau at 400 mbar at 293 K. The total adsorption capacity of these gases at 1 bar and 293 K follows the degree of unsaturation of the gas, with C3H4, C3H6, and C3H8 reaching 6.3, 5.4, and 4.8 mmol g–1, respectively, comparable with the highest values reported for MOF materials in the literature.[34,35]

The C2 hydrocarbons exhibit less steep adsorption profiles than the C3 analogues, reaching only 16 to 49% of their total capacity at 1 bar at 100 mbar at 293 K. Interestingly, the uptake at low pressure of the C2 hydrocarbons does not follow the degree of unsaturation as is observed in the isostructural MFM-300(Al).[29] Furthermore, analysis of the isotherms by ideal adsorbed solution theory (IAST) indicates that there is a distinct reversal of the selectivities of ethane and ethylene so that MFM-300(In) exhibits selectivity toward ethane at 293 K (Figure d) similar to that observed in MFM-300(VIII).[36] This is an unusual observation considering that the In(III), V(III), and Al(III) analogues of MFM-300 have identical pore chemistry and only differ in that MFM-300(In) and MFM-300(VIII) have a slightly larger pore diameter than MFM-300(Al).[29,33,36] As the uptakes of ethylene in MFM-300(M) (M = In, VIII, and Al) are similar (4.9, 6.0, and 4.3 mmol g–1, respectively), this phenomenon can be explained by MFM-300(In) having a greater affinity for ethane, which has a much greater uptake in the In(III) and V(III) analogues (5.1 and 7.1 mmol g–1) compared to Al(III) (∼0.8 mmol g–1) at 293 K and 1 bar. Interestingly, a recent computational study reported that a small change in the diameter of the channel was able to induce a large effect on the selectivity of ethane/ethylene.[24] This may shed light on the observed reversal of selectivity between the ethylene-selective MFM-300(Al) and the ethane-selective MFM-300(In) as the former has a smaller pore compared to the latter (∼6.0 and 6.8 Å, respectively, determined by analysis of N2 isotherms at 77 K).

The isosteric enthalpy (Qst) and entropy (ΔS) of adsorption as a function of gas uptake were determined by fitting of the Van’t Hoff equation to the adsorption isotherms measured for each gas (Figures e, S12, and S13). The initial value of Qst for C2H2 is around 25 kJ mol–1, and the change is relatively steady throughout the uptake process. The value of Qst for C2H6 at near-zero coverage is 30 kJ mol–1, higher than that for both C2H4 and C2H2 over the entire range of loading, suggesting that MFM-300(In) exhibits a stronger binding affinity for C2H6 than C2H4 and C2H2. At the same time, Qst for C2H6 increases continuously from 25 to 33 kJ mol–1 with the increase of gas loading from 0.1 to 4.0 mmol g–1, demonstrating the presence of strong adsorbate–adsorbate intermolecular interactions at high surface coverage, reflecting potential cooperative binding. Similar behavior has been observed in other porous sorbents.[4,37−39] The values of Qst for C2H6 in MFM-300(In) are significantly higher than that for MFM-300(Al), reflecting the larger pores in the former due to the larger metal center and associated lattice parameters. This allows additional C2H6 molecules to be located at optimal sites within the pore of MFM-300(In) via intermolecular interactions. The values of Qst for C2H6 in MFM-300(In) are comparable with those of other reported C2H6-selective MOFs.[4,26] The adsorption enthalpy for C3 hydrocarbons (30–36 kJ mol–1) is relatively high at low loading compared with C2 hydrocarbons, and C3H4 shows a higher value for Qst compared with C3H6 and C3H8, confirming strong binding affinity of MFM-300(In) for C3H4. The adsorption kinetics for substrate uptake have been measured for MFM-300(In) (Figure f), and all gases exhibit rapid diffusion to reach adsorption equilibrium within 10 min. MFM-300(In) shows more rapid diffusion of C2H6 than C2H2 and C2H4, implying a kinetic selectivity for C2H6 over C2H4 and C2H2, which is beneficial for their separation under dynamic conditions. The high capacity and strong binding affinity of MFM-300(In) for C3H4 and C2H6 as well as the rapid adsorption kinetics suggest potential for the purification of mixtures of C3H4/C3H6 and C2H4/C2H6 by selective adsorption of C3H4 and C2H6, respectively.

Breakthrough Experiments

The promising static adsorption data encouraged us to assess further the separation performance of MFM-300(In) under dynamic flow conditions. First, single-component gas breakthrough experiments were conducted to evaluate the dynamic gas adsorption (Figures and S16). The dynamic adsorption capacities for each component were calculated by integrating the breakthrough curves to give dynamic uptakes of 1.4 mmol g–1 (C2H2), 1.0 mmol g–1 (C2H4), 1.6 mmol g–1 (C2H6), 4.4 mmol g–1 (C3H4), 3.5 mmol g–1 (C3H6), and 3.1 mmol g–1 (C3H8) upon saturation. These values are consistent with those obtained from static isotherm experiments.

Figure 2

Dynamic breakthrough plots for single-component (a) C2H4, (b) C2H6, (d) C3H4, and (e) C3H6 with an inlet target gas flow rate of 2.0 mL min–1 diluted in He (total flow rate: 20 mL min–1). Dynamic breakthrough plots for equimolar mixtures of (c) C2H6/C2H4 and (f) C3H4/C3H6 with an inlet gas flow rate of 2.0 mL min–1/2.0 mL min–1 diluted in He (total flow rate: 20 mL min–1) through a fixed-bed packed with MFM-300(In) at 293 K.

To evaluate the feasibility of separation of C2 and C3 binary mixtures using a fixed bed packed with MFM-300(In), breakthrough experiments for equimolar mixtures of C2H2/C2H4, C2H2/C2H6, C2H6/C2H4, C3H4/C3H6, C3H4/C3H8, and C3H6/C3H8 were performed at 293 K and 1 atm. Clear separation of mixtures of C2H4/C2H6 and C3H6/C3H4 was obtained (Figure ). In the separation of C2H6/C2H4, the breakthrough of C2H4 was observed at 6 min g–1, while the retention time of C2H6 was 15 min g–1, consistent with the analysis of unusual adsorption selectivity and high value of Qst for C2H6. It is especially challenging to develop C2H6-selective adsorbents to enable one-step purification of C2H4 due to the common co-adsorption of C2H4 and C2H6.[27] The profile of the breakthrough curves (Figure c) indicates[40] strong competitive sorption of C2H6 over C2H4 in MFM-300(In), further confirming the high efficiency of MFM-300(In) for practical C2H6/C2H4 separation. In the case of C3H4/C3H6, the breakthrough curves indicate the sharp breakthrough of both gases with retention times of 35 and 48 min g–1 for C3H6 and C3H4, respectively. The apparent interval in the breakthrough time between C3H4 and C3H6 suggests that MFM-300(In) is effective for the separation of C3H4/C3H6, again consistent with the analysis of adsorption selectivity and thermodynamic data. The separation of mixtures of C2H6/C2H4 and C3H4/C3H6 by MFM-300(In) yields a productivity of 4.6 L/kg of C2H4 (purity > 99.9%) and of 16.3 L/kg of C3H6 (purity > 99.95%) at the outlet. The productivity of ethylene of MFM-300(In) is comparable with reported ethane-selective MOFs, such as IRMOF-8 (2.5 L/kg),[41] Cu(Qc)2 (4.3 L/kg),[42] and PCN-245 (5.8 L/kg)[43] (Table S3). Thus, the efficient purification of C2H6/C2H4 and C3H4/C3H6 to produce polymer-grade olefins under the above conditions has been achieved by MFM-300(In).

Studies of the Preferred Binding Sites and Supramolecular Interactions

In situ NPD data of MFM-300(In) as a function of gas loading with C2D2, C2D4, C2D6, C3D4, C3D6, or C3D8 were refined by the Rietveld method to determine the preferred binding domains for adsorbed gas molecules within the pore. The refinements reveal two similar binding sites for all the C2 hydrocarbons (Figure ), comparable to those observed in MFM-300(Al). Site I occupies a position adjacent to the bridging hydroxyl of the framework. Both unsaturated molecules show an OH···π interaction, with acetylene having a reduced HOH···C2 distance of 2.52(1) Å compared to 3.85(1) Å for ethylene, consistent with the increased polarizability of acetylene. For acetylene, this interaction is supplemented by π···π interactions between the guest molecules and the adjacent phenyl groups of the linker at distances of 3.83(1) and 4.04(1) Å. Ethylene does not have π-orbitals facing the phenyl groups of the linker and so instead exhibits electrostatic interactions between the D-centers of the ethylene and the π-system of the phenyl group at distances of between 2.92(1) and 4.40(1) Å. In contrast, ethane does not exhibit a perpendicular interaction with the bridging hydroxyl of the MOF. Instead, it displays O–H···C–D hydrogen interactions supplemented by interactions with the phenyl groups at distances between 2.65(2) and 4.18(2) Å. This mode of binding is also observed in the structure of ethane-loaded MFM-300(Al), which has a longer CC···OOH distance of 3.82(1) Å compared to 3.22(2) Å in the In(III) analogue. This shorter interaction distance in MFM-300(In) suggests that the improved adsorption of ethane in MFM-300(In) is due to the presence of stronger intermolecular interactions between adsorbed ethane molecules, consistent with the greater Qst values for ethane in MFM-300(In) over MFM-300(Al). The stronger van der Waals interactions in MFM-300(In) are likely due to the slightly larger pore size allowing the ethane molecules to orient themselves in a more favorable position compared to those in MFM-300(Al).

Figure 3

Binding sites (site I, orange; site II, green) of (a) acetylene, (b) ethylene, (c) ethane, (d) propyne, (e) propylene, and (f) propane in MFM-300(In) obtained from NPD refinements (In: green; C: gray; O: red; H: light yellow; the [InO4(OH)2] moiety is shown in green octahedron). The e.s.d. values of the bond distances are typically within 0.05 Å.

Binding domains of the C3 hydrocarbons were also determined (Figures d–f, S25−S27). A single binding site was found for propyne involving interactions with the hydroxyl group of the framework and the methyl group with a HOH–CC distance of 3.26(6) Å. Two binding sites were found for both propylene and propane. The primary binding site displays an interaction between the methyl group of the gas molecule and the hydroxyl group of the MOF with HOH···CC distances of 3.37(1) Å for propylene and 2.72(2) Å for propane. The secondary binding sites for both propylene and propane lie more centrally in the pores of MFM-300(In). The π-orbitals of propylene site II interact with the methylene D-centers of site I via a T-shaped interaction at a HC–CC distance of 1.91(2) Å. Site II of propane interacts with site I via van der Waals interactions between the D-centers of the respective methyl groups at distances of 2.92(2) to 3.19(2) Å.

The INS spectra have been obtained for MFM-300(In) as a function of gas loading (Figures d−f, S28−S30). For the C2H2-loaded material, the peaks in the difference spectra at around 80 and 95 meV are assigned to symmetric and asymmetric C−H bend vibrational modes of adsorbed C2H2, respectively. The peak at around 115 meV is assigned to the wagging of the C2H2 molecules, as well as to out-of-plane wagging of the four aromatic C−H groups on two benzene rings adjacent to each C2H2 molecule (Figure a). The spectrum of ethylene-loaded MFM-300(In) reveals a broad peak at low energy transfer (below 25 meV), which is characteristic of almost free rotational motion around the C=C axis. A peak is observed at around 100 meV, assigned to the in-plane rocking mode of −CH2[34] in both the difference spectrum and that of the solid ethylene. In the spectra for ethane-loaded MFM-300(In), a broad peak is observed below 25 meV, corresponding to the almost free rotational mode around the C–C axis. Two peaks are also observed in the difference spectra which align with peaks in the spectrum of solid ethane. A peak at 37 meV is assigned to the −CH3 torsion, whereas the peak at 101 meV can be assigned to the CH3 rocking motion.[33] The relatively high intensity of the −CH3 rotational mode compared to that observed in MFM-300(Al) indicates that the larger pore of MFM-300(In) and the shorter HOH···C2D6 distance provide a greater degree of free rotation of the ethane molecule at site I, thus decreasing the entropic penalty for adsorption and driving further uptake of ethane in MFM-300(In) compared to MFM-300(Al).

Figure 4

Comparison of the INS spectra of bare MFM-300(In) and MFM-300(In) loaded with (a) C2H2, (b) C2H4, and (c) C2H6. For comparison, INS spectra of the condensed gas in the solid state are also included.

Conclusions

An understanding of the detailed effect that pore size has on the selectivity of ethane/ethylene is important to the design of improved materials for the separation of light hydrocarbons. The MFM-300 materials provide a useful platform to investigate changes in pore size while maintaining identical pore chemistry. This study has shown that the MFM-300(In) with slightly larger pores exhibits the opposite IAST selectivity of ethane/ethylene to MFM-300(Al). Analysis of Qst of these two gases along with NPD studies has revealed that the reason for the reversal is that the slightly larger pores of MFM-300(In) allow ethane to sit in a more favorable position, which allows for a greater degree of host–guest and guest–guest interactions, thus increasing the overall uptake. MFM-300(In) exhibits excellent separation of mixtures of C2H6/C2H4 and C3H4/C3H6 as demonstrated by dynamic breakthrough experiments, allowing the production of polymer-grade C2H4 and C3H6 (purity > 99.9%) in a one-step approach. The understanding of structure−property relationships will inform the design of future efficient sorbent materials for important separations of gas mixtures.