Ultramicroporous Lonsdaleite Topology MOF with High Propane Uptake and Propane/Methane Selectivity for Propane Capture from Simulated Natural Gas

Propane (C3H8) is a widely used fuel gas. Metal–organic framework (MOF) physisorbents that are C3H8 selective offer the potential to significantly reduce the energy footprint for capturing C3H8 from natural gas, where C3H8 is typically present as a minor component. Here we report the C3H8 recovery performance of a previously unreported lonsdaleite, lon, topology MOF, a chiral metal–organic material, [Ni(S-IEDC)(bipy)(SCN)]n, CMOM-7. CMOM-7 was prepared from three low-cost precursors: Ni(SCN)2, S-indoline-2-carboxylic acid (S-IDECH), and 4,4′-bipyridine (bipy), and its structure was determined by single crystal X-ray crystallography. Pure gas adsorption isotherms revealed that CMOM-7 exhibited high C3H8 uptake (2.71 mmol g–1) at 0.05 bar, an indication of a higher affinity for C3H8 than both C2H6 and CH4. Dynamic column breakthrough experiments afforded high purity C3H8 capture from a gas mixture comprising C3H8/C2H6/CH4 (v/v/v = 5/10/85). Despite the dilute C3H8 stream, CMOM-7 registered a high dynamic uptake of C3H8 and a breakthrough time difference between C3H8 and C2H6 of 79.5 min g–1, superior to those of previous MOF physisorbents studied under the same flow rate. Analysis of crystallographic data and Grand Canonical Monte Carlo simulations provides insight into the two C3H8 binding sites in CMOM-7, both of which are driven by C–H···π and hydrogen bonding interactions.

P ropane (C 3 H 8 ) is a widely utilized hydrocarbon that is a gas under ambient conditions.As a combustible and highly flammable fuel gas, 1−8 compression of C 3 H 8 is key to its storage and delivery as liquified petroleum gas. 1,2hanks to its thermodynamic properties, C 3 H 8 is also recognized as an eco-friendly refrigerant. 3,4Further, C 3 H 8 is involved in production of high-value chemicals, polypropylene, and polyethylene. 5,6In addition, in semiconductor manufacturing, C 3 H 8 is a precursor gas for silicon carbide deposition. 7,8 3 H 8 is generally produced by either of two routes: 1) extraction from petroleum refining; 2) extraction from natural gas (NG).NG is a mixture of hydrocarbons, primarily composed of methane (CH 4 ) and smaller amounts of ethane (C 2 H 6 ), C 3 H 8 , and other gases. 1,9−11 Low C 3 H 8 concentration in NG pipelines is desirable as it mitigates condensation reactions. 12Scheme 1 illustrates the preparation of C 3 H 8 and its utility as a commodity chemical.The global C 3 H 8 market reached 174.3 million tons in 2022, with a 2028 market forecast as high as 223.1 million tons. 13Despite this surging demand, energy-intensive distillation remains state-of-the-art to obtain C 3 H 8 from petroleum refining products and NG. 14,15verall, distillation processes account for 45−55% of the energy footprint of the chemical industry, a sector that consumes ca.15% of global energy consumption. 16−30 In the context of NG purification, molar mass and polarizability gradually increase from CH 4 to C 2 H 6 to C 3 H 8 (Table S3). 31he kinetic diameters of C 2 H 6 (4.44 Å) and C 3 H 8 (4.3 Å) are near-identical, both higher than that of CH 4 (3.76 Å).A CH 4 / C 2 H 6 /C 3 H 8 (85:10:5, v/v) ternary mixture is generally regarded as a suitable composition for studying the NG purification potential of a sorbent.−41 Developing an MOM with both high selectivity and high dynamic uptake for C 3 H 8 is a challenge that we address herein.
−46 These CMOMs are modular as they are composed of divalent metal cations, e.g., Co 2+ or Zn 2+ , that form RBBs involving mandelate or related anions cross-linked by 4,4′-bipyridine (bipy) linkers.These structures can form cationic bnn or dia porous coordination networks with extra-framework counteranions.In this study, we introduce a new CMOM variant which contains a coordinated thiocyanate, and report its gas sorption properties and separation performance.

■ EXPERIMENTAL SECTION
Reagents (≥98% purity) and solvents were procured from commercial vendors and used without further purification, except for nickel(II) thiocyanate (Ni(SCN) 2 ), which was prepared by the reaction of nickel(II) carbonate (NiCO 3 ) and potassium thiocyanate (KSCN). 47Full details on synthesis procedures are given in the experimental section of the Supporting Information.
Synthesis and Solvent Exchange.A methanolic solution of Ni(SCN) 2 was added to an N,N-dimethylformamide (DMF) solution of bipy, and S-IDECH (molar ratios of Ni(SCN) 2 : bipy: S-IDECH = 1:1:1) in a 15 mL glass vial.Blue crystals of {[Ni(S-IDEC)(bipy)(SCN)](DMF) 1.5 } n , CMOM-7-DMF, were obtained after the vial was heated in an oven at 85 °C for 24 h.After soaking crystals of CMOM-7-DMF in methanol for 5 days with fresh solvent exchange every day, the crystals transformed into {[Ni(S-IDEC)(bipy Dynamic Column Breakthrough (DCB) Experiment.About 0.88 g of activated CMOM-7 was used as a packed fixed-bed in quartz tubing (Φ 6 × 400 mm, outer diameter = 8 mm) inside a dynamic column breakthrough instrument (Figures S22 and S23). 48The packed sample was purged under a 20 cm 3 min −1 flow of helium at 80 °C for 1 h prior to conducting each breakthrough experiment.The composition of the outlet gas during the breakthrough experiments was monitored by a Shimadzu Nexis GC-2030 gas chromatograph (GC) with a flame ionization detector (FID) and a thermal conductivity detector (TCD).
Adsorption kinetics for CH 4 , C 2 H 6 and C 3 H 8 were assessed by exposing CMOM-7 to 10 mL min −1 flows of CH 4 , C 2 H 6 and C 3 H 8 at 303 K and 1 bar, separately.CMOM-7 adsorbed 0.29 wt % (0.18 mmol g −1 ) of CH 4 in 5 min, and 3.67 wt % (1.22 mmol g −1 ) of C 2 H 6 in 10 min.10.88 wt % (2.48 mmol g −1 ) of C 3 H 8 was adsorbed in 20 min (Figure 3f).Consistent kinetic results over three consecutive sorption cycles were found (Figure S10).Adsorption kinetics for C 3 H 8 and C 2 H 6 exhibited similar slopes at low loading, suggesting that the separation performance is not driven by kinetics alone.Nevertheless, under an identical flow rate and other conditions, CMOM-7 adsorbed a higher amount of C 3 H 8 than C 2 H 6 .This suggests that the C 3 H 8 selectivity is driven by a combination of optimal thermodynamics and kinetics, a phenomenon we have previously observed in ultramicroporous sorbents that exhibit benchmark CO 2 and C 2 trace capture. 28,64,65o assess hydrolytic stability, crystals of CMOM-7-MeOH were soaked in water for a day (Figure S1).Water molecules exchanged with methanol molecules, affording the waterloaded structure {[Ni(S-IDEC)(bipy)(SCN)](H 2 O) 6.5 } n , CMOM-7-H 2 O, as determined by SCXRD.CMOM-7-H 2 O crystallized in space group C222 1 (Table S2).PXRD patterns (Figure S7) verified the stability in water and at 75% relative humidity (RH).CMOM-7 did not change structure after solvent exchange (Figure S3).TGA data revealed weight loss of 17.62 wt % at 63 °C (Figure S9).
To gain insight into the C 3 H 8 -selective nature of CMOM-7, we obtained its C 3 H 8 loaded structure using a C 3 H 8 loaded Schlenk bottle at 195 K for 12 h (bath of dry ice) which enabled CMOM-7 crystals to equilibrate.SCXRD study of the resulting phase allowed us to determine the crystal structure of the corresponding C 3 H 8 loaded phase, {[Ni(S-IDEC)(bipy)-(SCN)](C 3 H 8 ) 1.5 } n , CMOM-7-C 3 H 8 , which had crystallized in the orthorhombic spaced group C222 1 (Table S2).PXRD diffractograms (Figure S6) confirmed the phase purity and crystallinity.The asymmetric unit comprised 1.5 molecules of C 3 H 8 , with C 3 H 8 uptake at 273 K (3.44 mmol g −1 ) matching the composition of CMOM-7-C 3 H 8 (3.45 mmol g −1 ).Two C 3 H 8 molecules were observed in the asymmetric unit at sites I and II (Figure 4).Cambridge Structural Database (CSD) mining revealed that this is only the tenth example of ordered C 3 H 8 molecules determined by SCXRD (Figure S27 and Table S7).As shown in Figure 4a, C 3 H 8 molecules in binding site I are 2-fold disordered in a ratio of 1:1 around an inversion center. 66 5b).The C 2 H 6 binding energy was calculated to be 25.7 kJ mol −1 .For C 3 H 8 , two binding sites were identified, where the second site revealed two orientations (Figures 5c,d and  S31).Site-I interacts with the pyridine rings of bipy ligands through C−H•••π interactions (3.24−3.99Å, Figure 5c).Site-II interacts through the S atoms of SCN − forming H-bonds (3.72 Å), and the benzene rings of S-IDEC (3.08−3.99Å, Figure 5d).The C 3 H 8 binding energies with CMOM-7 in sites-I and II were determined to be 38.5 and 31.1 kJ mol −1 , respectively.The modeled binding sites were found to be in agreement with site-I and part-B of site-II in the SCXRD determined crystal structure of CMOM-7-C 3 H 8 (Figure 4).Moreover, the calculated binding energies fit the trend indicated by the experimental Q st trends (Figure 2c).
In this work, a novel lon topology ultramicroporous MOF, CMOM-7, is reported.The ultramicroporosity and pore chemistry of CMOM-7 was found to exhibit stronger affinity toward C 3 H 8 vs both C 2 H 6 and CH 4 , leading to high-purity C 3 H 8 production from a ternary C 3 H 8 /C 2 H 6 /CH 4 (v/v/v = 5:10:85) gas mixture.DCB experiments revealed high C 3 H 8 uptake (2.45 mmol g −1 ), high C 3 H 8 /C 2 H 6 selectivity (10.1) and a long breakthrough time difference, Δt (79.5 min g −1 ) between C 3 H 8 and C 2 H 6 .SCXRD analysis of the propane binding sites in CMOM-7 channels and complementary modeling studies of the C 3 H 8 binding sites indicate that multiple weak intermolecular interactions are the key to the observed separation performance.That CMOM-7 exhibited stronger intermolecular interactions with C 3 H 8 molecules over C 2 H 6 and CH 4 is key to its high C 3 H 8 selectivity even under ternary mixture compositions, in turn leading to the benchmark breakthrough time difference, Δt.In this context, CMOM-7 outperforms previously studied MOFs under equivalent experimental conditions.The approach taken herein, systematic crystal engineering of ultramicroporous coordination networks to fine-tune pore size, shape, and chemistry, suggests that more energy-efficient approaches to purification of commodity chemicals will ultimately come to fruition.

■ ASSOCIATED CONTENT
* sı Supporting Information
7-H 2 O, interacting with the pore walls through O−H•••O and O−H•••S hydrogen bonds (H-bonds) involving the O atoms of S-IDEC and S atoms of SCN − ligands (Figure

Figure 4 .
Figure 4. Crystallographically determined binding site of C 3 H 8 molecules along the channel of CMOM-7 (a) and the intermolecular interactions between C 3 H 8 molecules and the host framework of CMOM-7 around site-I (b), site-II part A (c) and site-II part B (c).Atoms of C 3 H 8 in binding site-I are colored in orange, whereas atoms of C 3 H 8 in binding site-II part A and site-II part B are colored pink and dark yellow, respectively.The C−H•••π interactions are represented by blue dashed lines, and the H-bond is represented by a red dashed line.All distances are given in Angstrom units.
Site I molecules forms C−H•••π interactions with the pyridine rings of bipy linkers (3.24−3.91Å, Figure 4b).The molecule in binding site II is also 2-fold disordered, with occupancies of 0.68 and 0.32 for A and B, respectively.A interacts with the host framework through C−H•••π interactions on the pyridine ring of bipy (3.76−3.85Å) and the C�N bond of the SCN − ligands (3.11 Å, Figure 4c).B interacts with a) the S atoms of SCN − through H-bond interactions (3.71 Å); b) the edges of the pyridine rings of bipy; and c) phenyl rings of S-IDEC through C−H•••π interactions (3.22−3.99Å, Figure 4d).Grand Canonical Monte Carlo (GCMC) simulations and first-principles density functional theory (DFT) calculations were conducted to determine the binding sites and energies in CMOM-7. 67The CH 4 binding site was observed to have C− H•••π interactions with a) the edges of bipy-1 pyridine rings (2.89−3.25 Å); and b) the SC bond of SCN − (3.63 Å, Figure 5a), resulting in a CH 4 binding energy of 14.2 kJ mol −1 .The C 2 H 6 binding site featured stronger interactions with CMOM-7 through C−H•••π interactions with a) bipy pyridine rings (3.46−3.86Å) and b) the SC bonds of SCN − ligands (3.93 Å, Figure

Figure 5 .
Figure 5. Binding sites of CH 4 (a), C 2 H 6 (b) and C 3 H 8 (c, d) molecules along the ultramicroporous channel of CMOM-7, determined by GCMC calculations.CH 4 and C 2 H 6 molecules are colored mauve and light green, respectively, whereas the C 3 H 8 molecules residing in sites I and II are presented in orange and dark yellow, respectively.C−H•••π interactions are represented by blue dashed lines, whereas the H-bond is represented by a red dashed line.All distances are given Angstrom units.