Strengthening Intraframework Interaction within Flexible MOFs Demonstrates Simultaneous Sieving Acetylene from Ethylene and Carbon Dioxide

Abstract Efficient separation of acetylene (C2H2)/ethylene (C2H4) and acetylene/carbon dioxide (CO2) by adsorption is an industrially promising process, but adsorbents capable of simultaneously capturing trace acetylene from ethylene and carbon dioxide are scarce. Herein, a gate‐opening effect on three isomorphous flexible metal–organic frameworks (MOFs) named Co(4‐DPDS)2MO4 (M = Cr, Mo, W; 4‐DPDS = 4,4‐dipyridyldisulfide) is modulated by anion pillars substitution. The shortest CrO4 2− strengthens intraframework hydrogen bonding and thus blocks structural transformation after activation, striking a good balance among working capacity, separation selectivity, and trace impurity removal of flexible MOFs out of nearly C2H2/C2H4 and C2H2/CO2 molecular sieving. The exceptional separation performance of Co(4‐DPDS)2CrO4 is confirmed by dynamic breakthrough experiments. It reveals the specific threshold pressures control in anion‐pillared flexible materials enabled elimination of the impurity leakage to realize high purity products through precise control of the intraframework interaction. The adsorption mechanism and multimode structural transformation property are revealed by both calculations and crystallography studies. This work demonstrates the feasibility of modulating flexibility for controlling gate‐opening effect, especially for some cases of significant aperture shrinkage after activation.

structure solution program using Intrinsic Phasing and refined with the SHELXL refinement package using Least Squares minimization. Detailed crystallographic data are summarized in Table S1. The thermal gravimetric analysis (TGA) was performed in TA-Q500 (TA Instruments) with heating rate of 10 C/min under N 2 circumstances from 50 to 800 °C. Adsorption-desorption isotherms of N 2 at 77 K, Ar at 87 K and CO 2 at 195 K were obtained on 3Flex-3MP adsorption apparatus (Micrometrics).

PXRD measurements for stability test of material
As-synthesized samples after being washed with deionized water, about 0.2 g for each batch, were immersed in 10 mL of aqueous solution of pH = 1 (HCl), and pH = 10 (NaOH) at room temperature for 24 h, a week and a month, respectively. The treated samples were washed with deionized water for several times and dried at room temperature before PXRD measurements.

Gas adsorption measurements
Single-component isotherms of C 2 H 2 , C 2 H 4 and CO 2 were measured up to 1 bar at 313, 298 and 273 K, on Micrometrics ASAP 2460 adsorption apparatus, respectively. The initial outgassing process was carried out under high vacuum (< 4 mmHg) at 100 °C for 24 h before adsorption measurements and about 120 mg sample was used for each gas adsorption measurement. The free space of the system was determined by using helium gas.

Calculation of adsorption selectivity
The dual-site Langmuir-Freundlich (DSLF) model is used to describe the gas adsorption isotherms.
[1] The model is well-defined as: where q A,sat and q B,sat are the saturated capacities of site A and site B, b A and b B are the affinity coefficients to the sites A and B, P is the pressure of the bulk gas at equilibrium with the adsorbed phase, q is the gas uptake amount of an adsorbent, and n A and n B represent the deviations from an ideal homogeneous surface. Prediction of the selectivity was established using ideal adsorption solution theory (IAST) [1] combined with DSLF for binary gas on porous materials. Herein, the adsorption selectivity is defined by: q 1 , and q 2 are the molar loadings in the adsorbed phase in equilibrium with the bulk gas phase with partial pressures p 1 and p 2 . The fitting parameters of the DSLF equation for C 2 H 2 , C 2 H 4 , and CO 2 at 298 K, respectively are presented in Supplementary Table 5.

Calculation of adsorption enthalpy Q st
The virial equation is used to fit the single-component isotherms and calculate the Q st , defined as: A virial-type expression of the above form was used to fit the combined isotherm data at 298 and 313 K. Where P (mmHg) is the pressure, N (mg/g) is the adsorbed amount of adsorbed gas, T (K) is the temperature, a i and b j are virial coefficients, m and n are the numbers of coefficients used to describe the isotherms. Q st (kJ/mol) is the coverage-dependent enthalpy of adsorption.

Breakthrough experiments
Breakthrough tests were conducted in a stainless-steel column (50 mm x 4.6 mm ID) manually packed according to different particle size and density of the sample powder.

Separation factor and dynamic capacity
The dynamic capture capacity of the gas is calculated as: Where is the adsorption capacity of the gas i, F is the total molar flow, C i is the concentration of the gas i entering the column and the time corresponding to the gas i, which is estimated from the breakthrough profile. The selectivity was then calculated according to the equation:

Density-functional theory calculations
First principles density-functional theory (DFT) calculations were performed using the Materials Studio's CASTEP code [2] . A semi-empirical addition of dispersive forces to conventional DFT was included in the calculation to account for van der Waals interactions. We use Vanderbilt-type ultrasoft pseudopotentials and generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) exchange correlation. A cut-off energy of 544 eV and 1 × 2 × 1 k-point mesh (generated using the Monkhorst-Pack scheme) were found to be enough for the total energy to converge within 0.01 meV·atom -1 . The structure of compounds (as-synthesized) was first optimized. The optimized structures are in good agreement with the experimentally determined crystal structures of the coordination networks. C 2 H 2 molecule was then separately introduced to various locations of the channel pore, followed by a full structural relaxed. To obtain the gas binding energy, a gas molecule placed in a supercell with the same cell dimensions was relaxed as a reference. The static binding energy (at T = 0 K) was then calculated using:    The activated crystal structure becomes slightly disordered.