Electrostatically Driven Selective Adsorption of Carbon Dioxide over Acetylene in an Ultramicroporous Material

Abstract

Separating acetylene from carbon dioxide is important but highly challenging owing to their similar physical properties and molecular dimensions. Herein, we report highly efficient electrostatically driven CO₂/C₂H₂ separation in an ultramicroporous cadmium nitroprusside (Cd-NP) with compact pore space and complementary electrostatic potential well fitting for CO₂, thus enabling molecular quadrupole moment recognition of CO₂ over C₂H₂. This material shows a high CO₂/C₂H₂ uptake ratio of 6.0 as well as remarkable CO₂/C₂H₂ selectivity of 85 under ambient conditions with modest CO₂ heat of adsorption. Neutron powder diffraction experiments and molecular simulations revealed that the electrostatic potential compatibility between pore structure and CO₂ allows it to be trapped in a head-on orientation towards the Cd center, whereas the diffusion of C₂H₂ is electrostatically forbidden. Dynamic breakthrough experiments have validated the separation performance of this compound for CO₂/C₂H₂ separation.

Introduction

Acetylene is the simplest alkyne, which is widely used as a fuel and an important raw material in the chemical industry.^[1] This unstable hydrocarbon can be converted to a variety of valuable products, such as vinyl compound, butynediol and acrylic acid. The acetylene molecule shows a linear shape with molecular dimensions of 3.3×3.3×5.7 ų that are very close to those of its counterpart, carbon dioxide (3.2×3.3×5.4 ų), which is a major impurity in the manufacture of crude acetylene by the partial combustion approach.^[2] Both C₂H₂ and CO₂ are in high similarity with each other not only on their molecular sizes and shapes, but also on the physical properties including boiling point, dipole moment and polarizability. In this context, there has been great interest in separation of CO₂ from C₂H₂ owing to its industrial relevance and scientific challenge. Given the fact that C₂H₂ can decompose explosively at gas pressure above 2 bar upon any intense heat or shockwave, the energy-efficient separation schemes through pressure or temperature swing adsorption processes are also of particularly challenging.

Extensive research efforts have been devoted to developing porous solids such as zeolites, carbon molecular sieves, and metal–organic frameworks (MOFs) for adsorptive separations.^[3] Depending on the different binding preference for both gas molecules, the adsorbents can be either C₂H₂-selective or CO₂-selective. Most relevant adsorbents, including zeolites^[4] and carbon molecular sieves,^[5] are C₂H₂-selective with modest selectivity, featuring certain functional sites to preferentially bind the relatively acidic and polarizable C₂H₂. The performance of the C₂H₂-selective adsorbents relies on the strength of adsorbent–adsorbate interactions, ranging from nonelectrostatic interactions^[6] to hydrogen bonding^[7] and π-complexation.^[8] By incorporating basic moieties like open oxygen and fluoride sites as hydrogen-bonding acceptors, some C₂H₂-selective adsorbents can even show exclusive C₂H₂ adsorption with high adsorption enthalpy of up to 50 kJmol⁻¹.^[9] For adsorbents featuring open metal sites, the coordinative-like interactions with π-electrons of C₂H₂ endow some MOFs to show remarkably higher C₂H₂ uptakes in contrast to CO₂, especially at low-pressure region. But those open metal sites in MOFs can also interact with electronegative O sites of CO₂ that make inverse contribution for C₂H₂-selective separation, resulting in modest selectivity.^[10]

On the contrary, adsorbents with selective adsorption of CO₂ over C₂H₂ are rarely reported, which is more challenging not only because CO₂ is less polarizable but because of the lack of a clear separation mechanism for exploring potential adsorbents. It should be mentioned that the CO₂-selective separation is a more efficient and feasible purification process, since pure C₂H₂ can be directly produced in a single adsorption stage.^[11] Several adsorbents with framework flexibility show CO₂-selective adsorption.^[12] But there are significant C₂H₂ co-adsorption for flexible adsorbents upon CO₂/C₂H₂ mixture owing to structural transformations like gate opening. Even for adsorbents with chemisorptive CO₂-binding sites, there is still a significant C₂H₂ adsorption with minor uptake difference comparing to CO₂.^[13] Motivated by the complementary electrostatic interaction for determining the binding orientation of a variety of biological molecular recognitions, like protein folding and substrate binding,^[14] the dramatically different charge distributions of both gas molecules (opposite quadrupole moments, −13.4×10⁻⁴⁰ Cm² for CO₂ and +20.5×10⁻⁴⁰ Cm² for C₂H₂) facilitate the applicability to exclusively recognize CO₂ or C₂H₂ through similar mechanism.^[15] However, in wide open pore space with size beyond those of both molecules, both CO₂ and C₂H₂ can get their own optimal binding configurations in different orientations, making it impossible to differentiate them by their molecular charge distributions. In this context, restricting the molecular orientations in the rigid confined space make the charge-distribution recognition become possible, whereas charge distribution on the pore surface complementary to CO₂ can ensure the CO₂-selective recognition. Ultramicroporous materials featuring compact pore space (close to the molecular size of CO₂ and C₂H₂) and positive charge distribution at both ends are thus targeted to maximize the inverse selectivity and sieving effect of CO₂ over C₂H₂.

Metal ions and cyanides have been employed as building blocks to construct robust ultramicroporous adsorbents due to their short length and strong basicity, as exemplified by Prussian blue^[16] and Hofmann-type compounds.^[17] As a member of the Prussian blue family, transition metal nitroprusside (NP) compounds, M^II[Fe(CN)₅NO], have been developed for small molecule adsorptions and various applications.^[18] In particular, cadmium nitroprusside Cd[Fe(CN)₅NO] (termed as Cd-NP) exhibits a narrow pore aperture of 3.2 c connecting quasi-discrete ellipsoidal cavities that are similar to the molecular shapes of both CO₂ and C₂H₂, and the open Cd sites at the pore extremity afford a complementary electrostatic potential field for CO₂ binding. Such compact pore space is applicable for studying the interaction contribution of quadrupole moments under restricted molecule orientations, which prompted our investigation on potential CO₂/C₂H₂ separation in Cd-NP. Herein, gas adsorption experiments show that CO₂ is dramatically captured in Cd-NP (58.0 cm³g⁻¹) with a significant preference over C₂H₂, giving a new benchmark CO₂/C₂H₂ selectivity of 85 at room temperature. Crystallography and modeling studies reveal that CO₂ molecules can be well oriented in the confined charge-matching cavity through multiple weak host–guest interactions. Therefore, Cd-NP can preferentially absorb CO₂ from the CO₂/C₂H₂ mixture under ambient condition, which is well supported by breakthrough experiments. Furthermore, the facile and low-cost synthesis of this material indicates its potential for scaleup synthesis and C₂H₂ purification.

Results and Discussion

In this structure, each Cd^II atom is coordinated by five nitrogen atoms from different cyanide groups, whereas the Fe^II atoms are octahedrally coordinated by the five carbon atoms of these cyanide groups and one nitrogen atom of a nitrosyl group, which piles up in an antiparallel fashion along b-axis, affording a three-dimensional (3D) network (Figure 1). Cd-NP consisted of a 3D pore system with a void ratio of 27.2%, in which the quasi-discrete cavities (size: 6.1×4.5×4.5 ų) are interconnected through narrow window surrounded by cyanide linkers and nitrosyl groups (aperture size: 3.2 Å; Figure S1). The cavity and aperture size matches well with the molecular dimensions of CO₂ and C₂H₂, which underlies the potential sieving effect towards these two gases. After the removal of guest water molecules, the Cd atoms are exposed to the cavity as unsaturated metal center. We further mapped the electrostatic potential (ESP) of pore surface in the cavity of Cd-NP by applying density functional theory (DFT) calculations (Figure 1b, see the Supporting Information), where strong positive potential α and β are found on the vertex of the ellipsoidal cavity, near Cd center and nitrogen atom in the nitrosyl group, respectively, as well as weak negative potential γ near the nitrogen atom of the cyanide groups surrounding the center of the ellipsoid. Apparently, the ESP of pore surface in Cd-NP is complementary to CO₂ molecule whereas is not compatible for C₂H₂ (Figures 1c and S19), which promotes us to investigate it for CO₂/C₂H₂ separation.

Figure 1.

a) Local coordination environments of the nitroprusside linker and cadmium atom. Top: Crystal structure of Cd-NP viewed along the b-axis. Bottom: Side view of the ellipsoidal cavity unit. Cd light green, Fe orange, C gray, N blue. b) Electrostatic potential (ESP) of Cd-NP mapped onto the Connolly surface with a probe radius of 1.2 Å. c) Molecular electrostatic potential (MEP) of the CO₂ and C₂H₂ molecule mapped onto the 0.015 e⁻Å⁻³ electron density isosurfaces. The gradation on the scale bar is in kcalmol⁻¹.

By mixing the aqueous solution of commercially available raw materials Cd(NO₃)₂ and sodium nitroprusside, the Cd-NP can be obtained under mild condition. The purity of the bulk products was validated by comparing the simulated PXRD patterns with the experimental one. The thermogravimetric analysis (TGA) indicated that Cd-NP lost all guest water molecules at around 100°C, with a weight loss of 8.5%, and exhibit good thermal stability up to 250°C (Figure S2). After removing the guest water molecules under 100°C and high vacuum, the framework of Cd-NP is conserved, as determined in powder X-ray diffraction (PXRD), which further supported the thermal stability of Cd-NP and allowed to examine its gas adsorption properties (Figure S3).

The Brunauer–Emmett–Teller (BET) surface area of activated Cd-NP was measured to be 305 m²g⁻¹ by CO₂ sorption experiment at 273 K (Figures S4 and S5). The experimental total pore volume is 0.13 cm³g⁻¹, consistent with the theoretical value from the crystal structure (0.14 cm³g⁻¹). The low pressure C₂H₂ and CO₂ gas adsorption isotherms of Cd-NP were collected at 298 K (Figure 2a). As expected, Cd-NP exhibited a large uptake of CO₂ (58.0 cm³g⁻¹, 2.59 mmolg⁻¹) at 1 bar and 298 K, corresponding to 0.88 CO₂ molecule per cavity. The adsorption amount of C₂H₂ at the same condition (9.7 cm³g⁻¹, 0.43 mmolg⁻¹) is significantly lower than that of CO₂, which gives an uptake ratio of 6.0, higher than those of most other CO₂ selective MOFs and comparable to [Mn(bdc)(dpe)] (6.4).^[12d] It should be mentioned there is no uptake increase for C₂H₂ at thermodynamically favorable temperatures namely 195 and 273 K (Figures S4 and S6). In this context, the minor C₂H₂ adsorption at ambient temperature is probably from surface adsorption of defective microcrystalline samples. Interestingly, larger molecules like C₂H₄ and C₂H₆ are completely excluded by the pore aperture, which could be attributed to kinetic diffusion issues as diameter of these molecules (4.2 Å and 4.4 Å, respectively) is obviously too large to permeate into the relatively rigid cavity (Figure S7). Single-site Langmuir–Freundlich equation was employed to fit the isotherms of CO₂ and C₂H₂ measured (Figure S9 and Table S1). Ideal adsorbed solution selectivity (IAST) theory was then utilized to evaluate the adsorption selectivity towards equimolar CO₂/C₂H₂ mixture at 1 bar and 298 K (Figure S8). The calculated selectivity (85) exceeded those of most MOFs, such as [Tm₂(OH-bdc)₂(μ₃-OH)₂(H₂O)₂] (18).^[11b] The high selectivity and uptake ratio both suggested the potential of Cd-NP for high sieving CO₂/C₂H₂ separation. The adsorption enthalpy (Q_st) of Cd-NP for CO₂ was calculated by virial fitting from the adsorption isotherms measured at 273 and 298 K. The calculated Q_st for CO₂ is 27.7 kJmol⁻¹ at near zero coverage, which keeps almost constant at higher coverage (Figures 2b and S11). Notably, the Q_st value for CO₂ is lower than those of typical inorganic porous adsorbents and other CO₂selective MOFs, such as zeolite 4A (33.5 kJmol⁻¹),^[4b] carbon molecular sieve (CMS) (28.4 kJmol⁻¹),^[5b] [Mn(bdc)(dpe)] (29.5 kJmol⁻¹),^[12d] SIFSIX-3-Ni (50.9 kJmol⁻¹),^[11a] Ionic crystal (38 kJmol⁻¹),^[12b] [Tm₂(OH-bdc)₂(μ₃-OH)₂(H₂O)₂] (45.2 kJmol⁻¹),^[11b] CD-MOF-1 (41.0 kJmol⁻¹) and CD-MOF-2 (67.2 kJmol⁻¹)^[13] (Figure 2c). Such low adsorption enthalpy for CO₂ supports the feasibility to easily regenerate this material under mild condition.

Figure 2.

a) CO₂ and C₂H₂ sorption isotherms for Cd-NP at 298 K. b) Coverage-dependent adsorption enthalpy of CO₂ calculated by the virial fitting method. c) Comparison of the zero-coverage heat of adsorption of Cd-NP with those of other materials for CO₂/C₂H₂ separation. d) Comparison of CO₂/C₂H₂ selectivity and uptake ratio between Cd-NP and other CO₂-selective materials at 298 K and 1 bar.

To understand the adsorption behavior of CO₂ and structurally visualize the host–guest interaction, high-resolution neutron powder diffraction (NPD) measurements were carried out to determine the binding site of CO₂ (see SI for details). From a sample loaded with CO₂ and equilibrated at room temperature and 1 bar, a single type of CO₂ binding configuration was identified. It is located at the ellipsoidal cavity (Figure 3a,b), with an occupancy of 0.712(6), as determined from Rietveld structural refinement. The CO₂ molecules are well confined within the narrow ellipsoidal cavity through Cd^δ+···O^δ− interactions (3.017(19) Å, Cd-O-C 123.4(9)°), accompanied by van der Waals (vdW) interactions with the two cyanide ligands symmetrically (N^δ−···C^δ+ 3.854(3) Å). Notably, the observed Cd···O(CO₂) distance is longer than those exhibited by other MOFs with open metal sites (for example, 2.27 Å for the Mg···O(CO₂) in Mg-MOF-74),^[19] which indicates the weak interactions and rationalizes the low adsorption heat. As shown in Figure 3c, the negative and positive potential regions of CO₂ molecules are mainly stabilized by the attractive interactions with positive potential region near Cd center and the negative potential region near cyanide groups, respectively, which validates the complementary electrostatically matching between Cd-NP and CO₂. In addition, comparing the bare Cd-NP structure to that of Cd-Np⊃CO₂, negligible unit cell expansion (ΔV/V_bare ≈1%) and conformation change are exhibited, revealing the rigidity of Cd-NP framework and its pore structure (Figure S12).

Figure 3.

a, b) Neutron diffraction crystal structure of Cd-NP⊃CO₂ with close contacts indicated. Guest molecules are highlighted as CPK models. c) Electrostatic potential (ESP) of Cd-NP⊃CO₂ mapped onto the 0.15 e⁻Å⁻³ electron density isosurface. The gradation on the scale bar is in kcalmol⁻¹. d) Electrostatically driven adsorption mechanism towards CO₂ and C₂H₂ molecules (see the Supporting Information for the CCDC deposition number).

We performed Grand Canonical Monte Carlo (GCMC) simulation to further investigate the C₂H₂ and CO₂ adsorption mechanism. The calculated CO₂ binding configuration is consistent with the experimental result from neutron powder diffraction data (Figure S13). The hypothetical structures indicated that both CO₂ and C₂H₂ were located in the ellipsoidal cavity with a head-on orientation towards cadmium sites due to the size confinement effect. In terms of electrostatic potential compatibility, the binding interactions between Cd-NP framework and CO₂ molecule (Cd^δ+···O^δ− 3.187 Å, N^δ−···C^δ+ 3.796 Å) is apparently allowed, whereas the C₂H₂ binding (Cd^δ+···H^δ+ 2.867 Å, N^δ−···C^δ− 3.767–3.910 Å) is electrostatically forbidden (Figures 3d and S15). The overall host–guest interactions gave a modest binding energy for CO₂ of approximately 26 kJmol⁻¹, in great agreement with the experimental one. In contrast, the hypothetical binding energy is only 7.5 kJmol⁻¹ after taking into account a significant electrostatic barrier of 24.9 kJmol⁻¹. Overall, suitable pore size and confinement effect could maximize the interactions with the preferentially absorbed guest molecule and optimize the sieving effect.

To evaluate the practical CO₂/C₂H₂ separation performances of Cd-NP, column breakthrough experiments were carried out, where CO₂/C₂H₂ (50/50 v/v) gas mixtures flowed over a packed column of activated Cd-NP at a flow rate of 2 mLmin⁻¹ at 298 K (Figure 4a). A complete separation of CO₂ from the mixture has been demonstrated at ambient condition. C₂H₂ was first eluted from the outlet with no detectable CO₂ found, showing a high purity of 99.9%, whereas CO₂ was retained in the column until the uptake capacity of Cd-NP got saturated, giving a dynamic C₂H₂ productivity of 2342 mmol per liter sorbent. Multiple cycling breakthrough experiments under the same condition showed the same retention time of CO₂ and C₂H₂ productivity, indicating the adsorption capability of Cd-NP is well retained under dynamic capturing (Figure 4b).

Figure 4.

Experimental column breakthrough curves for a) an equimolar CO₂/C₂H₂ mixture and b) cycling tests of the equimolar CO₂/C₂H₂ mixture (open symbols: C₂H₂, solid symbols: CO₂) in a column packed with Cd-NP at 298 K and 1 bar.

Conclusion

In summary, we have successfully realized inverse CO₂/C₂H₂ separation by an ultramicroporous metal nitroprusside compound Cd-NP. Basically, porous materials featuring size- or shape-matching pore structures do have the potential for high sieving gas separation. But for separating gases with identical molecular size and shape, the investigation on the electric properties of the pore surface and pore structure affords another feasible approach. Such molecular recognition and high separation selectivity are enabled by the pore sieving and confinement effect as well as electrostatically complementary pore surface, which will inspire future rational design of ultramicroporous materials for other challenging gas separation processes.