Acetylene (C
2H
2), as a key raw petrochemical material, is widely applied in manufacturing worthy chemical commodities such as acetaldehyde, acetic acid, and 1,4-butynediol [
1–
3]. However, carbon dioxide (CO
2) is the inevitable by-product of the acetylene industrial production process, and the existence of CO
2 will cut down energy conversion efficiency [
4,
5]. Owing to the identical molecular kinetic diameter and similar physical properties between CO
2 and C
2H
2 (Table S1 in Supporting information) [
6–
8], obtaining high-purity acetylene (>99%) to manufacture valuable chemicals is relatively energy-intensive in the industry [
9,
10]. Consequently, developing energy-saving, cost-efficient, and environmentally friendly substitutive approaches is essential for C
2H
2 purification.
Porous physical adsorbents-based gas adsorptive separation can realize energy-efficient economy and operability, which has attracted the immense attention of researchers [
11–
13]. Metal–organic frameworks (MOFs) exhibit tunable pore structure, shape, and size [
14], which provide numerous opportunities to create unique conditions for gas adsorption and separation. Pillar–layered MOFs (PL-MOFs) constructed from two-dimensional (2D) layers and adjustable pillars have great potential for C
2H
2 and CO
2 separation
via delicate pore environment regulation [
15–
17]. To date, a pillar-layered strategy [
18,
19] utilizing the selection of different pillars with distinct chemical properties, has been applied in pore environment control of PL-MOFs to obtain the anticipatory pore environment for the optimization of C
2H
2/CO
2 separation [
18]. Previous studies have proven that tuning the pillar length and derivatization is capable of manipulating the pore size and pore environment for task-specific gas separation [
19,
20]. However, most of the pillars are confined to aromatic linkers due to the characteristics of structural stability and easy functionalization [
21]. Compared to aromatic linkers, aliphatic linkers possess desirable features such as steric effect, nonaromaticity, and saturated C—H groups [
22,
23], are expected to expand the kinds of MOF and conducive to create a low-polar pore chemical environment, which generates distinct host-guest interactions for distinguishing specific gas molecules [
21,
24,
25]. So far, the study on the kind of host-guest interactions and the mechanism of adsorption between gas and aliphatic linkers are still scarce.
In light of these considerations, based on the tunability of PL-MOFs, we chose Ni(btc)(pyz) (named
Ni-MOF-1; H
3btc = 1,3,5-benzene tricarboxylic acid; pyz = pyrazine) as a prototype MOF [
26], which has verified the ability to remove trace CO
2 from C
2H
2/CO
2 mixture [
27]. Inspired by abundant saturated C—H groups of the aliphatic ligand 1,4-diazabicyclo[2.2.2]octane (dabco), we speculate that tuning pillars from pyz to dabco may create an unusual aliphatic chemical environment, pore size, structural rigidity, and enhanced adsorption performance toward C
2H
2/CO
2 separation (
Scheme 1) [
25,
28]. In this regard, Ni(btc)(dabco) (termed
Ni-MOF-2) is successfully obtained based on reticular chemistry [
29]. As expected,
Ni-MOF-2 not only exhibits the relatively high C
2H
2 capacity over CO
2 and 3.5-fold strengthened selectivity but also realizes a complete splitting of equimolar C
2H
2/CO
2 mixture (
Scheme 1b). In addition, separation performance and regeneration stability for equimolar C
2H
2/CO
2 mixture of
Ni-MOF-2 were testified by the dynamic breakthrough experiments. Theoretical calculations reveal that host-guest interaction is converted from hydrogen bonding interaction to van der Waals (vdW) interaction and the saturated C—H groups of dabco serve as primary adsorption sites to interact with C
2H
2. The enhanced C
2H
2/CO
2 selectivity, low regeneration energy, and cyclic stability suggest that
Ni-MOF-2 is a promising porous adsorbent for C
2H
2/CO
2 separation.
Ni-MOF-1 and
Ni-MOF-2 are both obtained by the post-synthetic ligand exchange (PSLE) method [
26,
30,
31]. They are two isostructural PL-MOFs constructed from the same 2D layers and different pillars (
Figs. 1a–
e). In the 2D layers, each Ni
2+ ion coordinates with four O atoms from three btc ligands to form hexagon pores (Figs. S1 and S2 in Supporting information). Then, pyz and dabco as pillars (
Figs. 1b and
c) connect adjacent 2D layers (
Fig. 1a) stacking up and down to generate three-dimensional (3D) frameworks of
Ni-MOF with the same
hms topology (
Figs. 1d and
e, Table S2 in Supporting information).
Ni-MOF-1 and
Ni-MOF-2 show hexagonal-prism cages with pore apertures of 6.9 Å × 7.3 Å [
32] and 7.0 Å × 8.0 Å, respectively (
Figs. 1f and
g), which is consistent with pore size distribution (Fig. S7b in Supporting information). The high phase purity of
Ni-MOF-2 was verified by the bulk phase powder X-ray diffraction (PXRD) patterns (Fig S6 in Supporting information). The crystallinity of
Ni-MOF-2 can remain after being soaked in common organic solvents for 24 h (Fig. S8 in Supporting information). The whole structure and crystallinity of
Ni-MOF-2 survive up to 300 ℃, as verified by thermogravimetric analysis (TGA) (Fig. S9b in Supporting information) and variable temperature PXRD (VT-PXRD) (Fig. S10 in Supporting information). Compared to the thermal stability of
Ni-MOF-1, the thermal robustness of
Ni-MOF-2 is still preserved, implying the tiny impact of aliphatic pillars on the structural stability. The hexagonal-prism cages, chemical, and thermal stabilities of
Ni-MOF-2 pave the way for gas separation.
The activated
Ni-MOF-2 was prepared by heating at 150 ℃ for 12 h under a dynamic vacuum. Nitrogen (N
2) gas sorption isotherms at 77 K
Ni-MOF-2 show a typical type-Ⅰ sorption curve, indicating the microporous structure of
Ni-MOF-2. As shown in Fig. S7a (Supporting information), the saturated N
2 capacity of
Ni-MOF-2 increased from 282.4 cm
3/g [
32] in
Ni-MOF-1 to 351.6 cm
3/g, aligning with the pore size derived from the single crystal structure. The calculated Brunauer–Emmett–Teller (BET) surface area of
Ni-MOF-2 is 636.0 m
2/g, slightly lower than the value of
Ni-MOF-1 (686.0 m
2/g) (Table S3 in Supporting information) [
32]. Additionally, the effective free volume is also narrowed down from 52.8% in
Ni-MOF-1 to 44% in
Ni-MOF-2. The microporosity, high porosity, appropriate pore size, and additional saturated C—H groups of
Ni-MOF-2 exhibit the potential to address the challenge of C
2H
2/CO
2 separation.
To verify the effect of pillar modification on the adsorption of gas molecules, single-component adsorption isotherms of C
2H
2and CO
2 for
Ni-MOF were examined. As shown in
Figs. 2a and
b and Fig. S11 (Supporting information), at both 273 K and 298 K,
Ni-MOF-1 prefers adsorbing C
2H
2 at low pressure but the higher CO
2 capacity upon increasing the pressure. However, after pillar variation of
Ni-MOF-1, Ni-MOF-2 demonstrates a higher C
2H
2 adsorption capacity than CO
2 within the full pressure region, indicating the potential of
Ni-MOF-2 for separating equimolar C
2H
2/CO
2. Moreover, for the diminution of surface area and effective free volume, the saturated capacities of C
2H
2 and CO
2 on
Ni-MOF-2 at 298 K are reduced from 106.4 cm
3/g and 128.2 cm
3/g to 85.4 cm
3/g and 75.1 cm
3/g, respectively (
Fig. 2b and Table S3), demonstrating the greater reduction of CO
2 capacity and reversal of the interactions between frameworks and gas molecules after pore environment regulation. And the adsorption trend at 273 K of
Ni-MOF is the same (
Fig. 2a). Notably, the capacity of C
2H
2 of
Ni-MOF-2 at 298 K and 1 bar is higher than ZJU-74a (76.0 cm
3/g) [
33], NKMOF-1-Ni (61.0 cm
3/g) [
34], and Cu(bpy)NP (50.7 cm
3/g) [
35], comparable to CuSnF
6-dpds-
cds (80.9 cm
3/g) [
1], and CAU-10-pydc (88.3 cm
3/g) [
36], and lower than NCU-100 (102.4 cm
3/g) [
5], and HIAM-111 (108.9 cm
3/g) [
37] under the same condition (
Fig. 2e and Table S4 in Supporting information).
Subsequently, the ideal adsorbed solution theory (IAST) was employed to assess the adsorptive selectivity and evaluate the separation performance for equimolar C
2H
2/CO
2 mixture on
Ni-MOF-1 and
Ni-MOF-2. As depicted in
Fig. 2c, the equimolar C
2H
2/CO
2 selectivity of
Ni-MOF-1 is calculated as 1.0 and 1.3 at 273 K and 298 K and 100 kPa, respectively. Nevertheless, by virtue of switching pillars,
Ni-MOF-2 exhibits a relatively high selectivity of 3.3 and 4.5 for equimolar C
2H
2/CO
2 mixture, respectively, 3.3- and 3.5-fold higher than that of
Ni-MOF-1, respectively, implying the enhanced separation potential of
Ni-MOF-2 for equimolar C
2H
2/CO
2. Meanwhile, the equimolar C
2H
2/CO
2 selectivity of
Ni-MOF-2 at 298 K outperforms that of some reported MOFs for C
2H
2/CO
2, such as SNNU-98-Ni (3.3) [
38], HIAM-111 (2.4) [
37], and FJU-90 (4.3) (Table S4) [
39].
To compare the binding affinity of
Ni-MOF toward C
2H
2 and CO
2, the isosteric heats of adsorption (
Qst) were calculated (Fig. S13 in Supporting information). For
Ni-MOF-1,
Qst of C
2H
2 and CO
2 were calculated to be 38.6 kJ/mol and 32.8 kJ/mol at zero coverage, respectively (
Fig. 2d). And the
Qst of C
2H
2 on
Ni-MOF-2 is 42.2 kJ/mol, surpassing the
Qst of CO
2 on
Ni-MOF-2 (32.2 kJ/mol) (
Fig. 2d), exceeding that of ZNU-12 (36.1 kJ/mol) [
2], and MOF-303 (30.8 kJ/mol) [
36], inferior to SOFOUR-TEPE-Zn (45.6 kJ/mol) (
Fig. 2f and Table S4) [
8]. The appropriate
Qst of C
2H
2 facilitates regeneration using a low energy penalty. Increased adsorption disparity of C
2H
2 and CO
2, enhanced equimolar C
2H
2/CO
2 selectivity, and moderate isosteric heat of adsorption make
Ni-MOF-2 a prospective candidate for C
2H
2 purification.
To further validate the effect of pillar modification for equimolar C
2H
2/CO
2 separation, dynamic breakthrough experiments with different flow rates were conducted on
Ni-MOF-1 and
Ni-MOF-2 at 298 K and 1.0 bar. As shown in
Figs. 3a and
b, equimolar C
2H
2/CO
2 can be successfully separated on
Ni-MOF-2 with a total inlet flow rate of 1 and 2 mL/min, whereas
Ni-MOF-1 could not remove C
2H
2 from equimolar C
2H
2/CO
2 mixture. For
Ni-MOF-2 with a flow rate of 1 mL/min, CO
2 was first eluted through the column at 99.5 min/g, followed by C
2H
2 at 118.6 min/g (
Fig. 3a). When the flow rates increased to 2 mL/min and 3 mL/min,
Ni-MOF-2 still achieved a clean separation of equimolar C
2H
2/CO
2 mixture, and the breakthrough point of CO
2 reduced to 53.6 min/g and 34.4 min/g (
Figs. 3b and
c). And
Ni-MOF-2 can be regenerated by purging argon. The separation performance and regeneration of
Ni-MOF-2 can be retained with a flow rate of 3 mL/min, proved by cycling breakthrough tests and PXRD (
Fig. 3d and Fig. S17 in Supporting information). The practical separation performances and facile reproducibility pave the way for
Ni-MOF-2 as the available adsorbent to reduce the energy footprint.
To elucidate the improved separation performance of
Ni-MOF-1 and
Ni-MOF-2, grand canonical Monte Carlo (GCMC) simulations and density functional theory (DFT) calculations (
Fig. 4) were performed on
Ni-MOF-2. The aliphatic rings of btc ligands and saturated C—H groups of dabco ligands are the primary adsorption sites of C
2H
2 and CO
2 revealed by GCMC (Fig. S15 in Supporting information). As for DFT calculations, comparing with the C–H···O hydrogen bonding interactions as dominant host-guest interactions between
Ni-MOF-1 and C
2H
2 or CO
2 [
27], C–H···C vdW interactions and C–H···O hydrogen bonding interactions play the main role between frameworks and C
2H
2 and CO
2, respectively. As shown in
Figs. 4a and
b, after pillars regulation, in the site Ⅰ of
Ni-MOF-2, two C–H···O (H–O, 2.52–2.53 Å) hydrogen bonding interactions and four C–H···C (3.11–3.47 Å) vdW interactions between C
2H
2 and btc/dabco ligands were observed; as for CO
2, five C–H···O (H–O, 2.78–3.43 Å) hydrogen bonding interactions and four C–H···C (2.55–3.28 Å) vdW interactions were formed. In site Ⅱ, C
2H
2 was firmly captured by twelve C–H···C (H–C, 2.63–3.50 Å) vdW interactions; however, eight C–H···O (H–O, 2.82–3.30 Å) hydrogen bonding interactions were formed between
Ni-MOF-2 and CO
2 (
Figs. 4c and
d, Table S5 in Supporting information). The binding pattern of site Ⅲ is similar to site Ⅱ (
Figs. 4e and
f, Table S5). The type of host-guest interactions of C
2H
2 underwent the variation from hydrogen bonding interaction to vdW interaction after pillar modification, invariable for CO
2; and the number of host-guest interactions plainly clarifies the reason for the improved separation performance of
Ni-MOF-2 compared with prototype
Ni-MOF-1.
In summary, we reported two isostructural Ni-MOF-1 and Ni-MOF-2, whose molecular recognition abilities for C2H2 and CO2 are diverse. The challenge lies in equimolar C2H2/CO2 mixture separation in Ni-MOF-1 can be addressed via pillar variation from pyz to dabco by virtue of delicate pore environment regulation. Owing to supplemental vdW interactions with C2H2, pillar-modified Ni-MOF-2 achieved efficient separation of equimolar C2H2/CO2 and the 3.5-fold selectivity increased than that of Ni-MOF-1. The practical separation performance, cycle stability, and facile regeneration of Ni-MOF-2 were validated by cyclic experimental breakthrough tests. DFT calculations proved that the extra C—H groups of dabco contribute to more C–H···C vdW interactions between C2H2 and the framework, guaranteeing preferential adsorption of C2H2 over CO2 in Ni-MOF-2. This work sheds new light on delicate pore environment regulation for gas molecular recognition and suggests the potential for other challenging gas separations.
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