Rational Design of a Multifunctional MOFs for Alkane‐Selective Gas Separation

Abstract

Efficient separation of light hydrocarbons—including the removal of CH₄ from N₂ and the purification of olefins such as C₂H₄ and C₃H₆ from their corresponding alkanes (C₂H₆ and C₃H₈)—is critical in natural gas upgrading, steam cracking, and downstream petrochemical production. Traditional adsorbents are tailored to specific mixtures, limiting their broader applicability. The development of multifunctional adsorbents that can efficiently operate across multiple gas separation systems represents a promising strategy to simplify material design and broaden industrial relevance. Herein, methyl‐functional groups are innovatively introduced into porous coordination polymers (PCPs), synthesizing PCP‐BDC‐M and PCP‐BDC‐DM with precisely tailored microporous structures. Notably, the dimethyl‐functionalized PCP‐BDC‐DM demonstrates superior multifunctional selectivity toward CH₄/N₂, C₂H₆/C₂H₄, and C₃H₈/C₃H₆ gas mixtures. Adsorption isotherms and Ideal Adsorbed Solution Theory (IAST) calculations reveal significantly higher alkane selectivity in PCP‐BDC‐DM compared to PCP‐BDC‐M and existing alkane‐selective adsorbents. Grand Canonical Monte Carlo (GCMC) simulations provide molecular‐level insight, confirming that methyl groups effectively enhance interactions between alkane molecules and the framework. Dynamic breakthrough experiments further validate PCP‐BDC‐DM's excellent practical separation capability and structural stability. This study offers valuable insights into designing advanced adsorbents for alkane‐selective gas separation.

The design of a dimethyl‐functionalized adsorbent provides insight into the simultaneous separation of CH₄/N₂, C₂H₆/C₂H_4, and C₃H₈/C₃H₆ mixtures.

1. Introduction

The efficient separation and purification of chemical mixtures, particularly involving light hydrocarbons, remain energy‐intensive and economically challenging processes in the chemical industry.^{[ 1 ]} Adsorptive separation techniques based on porous materials have emerged as low‐carbon alternatives that could potentially replace traditional thermally driven distillation methods.^{[ 2 ]} Among these, the separations involving methane/nitrogen (CH₄/N₂) and olefins (C₂H₄ and C₃H₆) from their corresponding alkanes (C₂H₆ and C₃H₈) represent crucial tasks across multiple sectors, including natural gas processing, olefin production, and downstream petrochemical manufacturing.^{[ 3} , ⁴ , ^{5 ]}

CH₄, the main component of natural gas, is a clean energy carrier with a high heating value and is widely used.^{[ 6} , ^{7 ]} Its consumption continues to grow, surpassing that of conventional fossil fuels.^{[ 8 ]} In response to the growing demand, coalbed methane (CBM), a typical form of unconventional natural gas, has been increasingly utilized as a supplementary source. However, CBM is often diluted with air during extraction, undermine, lowering CH₄ concentration, and limiting its direct use. Consequently, most of the low concentration CBM was vented to the air, exacerbating the greenhouse effect.^{[ 9 ]} Given that nitrogen is the primary impurity, developing alkane‐selective adsorbents for efficient CH₄/N₂ separation is crucial for its utilization. Olefins (C₂H₄ and C₃H₆), often polymer‐grade or even ultra‐high purity (99.99%), are required to produce advanced fine chemicals and polymers.^{[ 10} , ^{11 ]} However, in industrial olefin production processes, such as steam cracking, the desired products are typically produced alongside alkanes. Therefore, effective separation strategies are critical to isolate high‐purity olefins.^{[ 12 ]} According to the reports, alkane‐selective adsorption process could obtain olefin directly through a single adsorption step, significantly simplifying the separation process, so having superior advantages over olefin‐selective adsorbents.^{[ 13} , ¹⁴ , ¹⁵ , ¹⁶ , ¹⁷ , ^{18 ]}

A variety of alkane‐selective adsorbents have been reported to fulfill the above separation scenarios.^{[ 19} , ²⁰ , ²¹ , ²² , ²³ , ²⁴ , ²⁵ , ^{26 ]} Nevertheless, most reported adsorbents exhibit selectivity toward only one particular gas pair, which limits their versatility and necessitates case‐by‐case material development. In contrast, multifunctional alkane‐selective adsorbents provide broader applicability using a single material, which reduces material complexity and improves adaptability in changing industrial conditions. Despite these potential benefits, multifunctional alkane‐selective adsorbents remain rare, primarily due to the difficulty of achieving selectivity across gas pairs with different molecular sizes and polarizabilities.^{[ 27} , ^{28 ]} Therefore, the rational design of porous materials that can simultaneously exhibit high selectivity for CH₄/N₂, C₂H₆/C₂H₄, and C₃H₈/C₃H₆ represents a valuable yet challenging task.

To address this challenge, we designed and synthesized a series of methyl‐functionalized porous coordination polymers (PCPs) with the goal of achieving multifunctional alkane‐selective adsorption.^{[ 29} , ^{30 ]} Specifically, we constructed two isoreticular frameworks, [Co(bdc‐m)(dpg)]n (PCP‐BDC‐M; bdc‐m = 2‐Methyl‐1,4‐benzenedicarboxylic acid, dpg = meso‐a,b‐di(4‐pyridyl) glycol) and [Co(bdc‐dm)(dpg)]n (PCP‐BDC‐DM; bdc‐dm = 2,5‐dimethylterephthalic acid), by incorporating mono‐ and di‐methyl‐substituted terephthalic acid linkers into a 1D channel architecture. The introduction of methyl groups was intended to enhance framework–alkane interactions, thereby promoting alkane recognition across different gas pairs. And these PCPs exhibit remarkable separation performance in three representative hydrocarbon systems—CH₄/N₂, C₂H₆/C₂H₄, and C₃H₈/C₃H₆—demonstrating their potential as versatile, multifunctional adsorbents. Grand Canonical Monte Carlo (GCMC) simulations and dynamic breakthrough experiments were further employed to elucidate the adsorption mechanisms and validate practical separation capability. This work highlights a rational design strategy for constructing a single‐adsorbent capable of addressing multiple industrially relevant separations.

2. Results and Discussion

Methyl‐functionalized PCP‐BDC‐M and PCP‐BDC‐DM were successfully synthesized (Figure 1d). Structural confirmation was achieved through Rietveld refinement and preliminary verification via Le Bail fitting of powder X‐ray diffraction data (Figures S1 and S2 and Table S1, Supporting Information). The results indicate that PCP‐BDC‐M and PCP‐BDC‐DM are phase‐pure, with their structures shown in Figures S3 and S4 (Supporting Information) and 1d. In both structures, the Co center adopts an octahedral coordination geometry, being coordinated by two nitrogen atoms from the pyridine rings, two oxygen atoms from the hydroxyl groups of the dpg ligands, and two oxygen atoms from the carboxyl groups of the methyl‐substituted terephthalic acid ligands. This arrangement forms 1D straight channels with pore sizes of ≈5.4 × 3.5 and 4.9 × 3.1 Å, respectively (Figure 1a–c). Thermogravimetric analysis revealed that all solvent‐exchange samples can be fully desolvated at 393 K and exhibit high thermal stability (Figure S5, Supporting Information). N₂ adsorption isotherms at 77 K were measured, as the number of methyl groups increases, the adsorption capacity of N₂ gradually decreases, and the N₂ adsorption amount on PCP‐BDC‐DM is basically zero. (Figure S6, Supporting Information). In this way, CO₂ adsorption isotherms at 273 K were used to investigate the specific surface areas and porosities of the materials. As shown in Figure 1e, the Brunauer‐Emmett‐Teller specific surface areas of PCP‐BDC‐M and PCP‐BDC‐DM were calculated to be 248.18 and 186.49 m² g⁻¹, respectively, indicating a gradual decrease associated with the increased number of methyl groups. Furthermore, the pore‐size distributions, determined by the Horvath–Kawazoe method, revealed pore sizes of ≈4.4 Å for PCP‐BDC‐M and 4.2 Å for PCP‐BDC‐DM (Table S2, Supporting Information). This observation indicates that additional methyl substitution slightly reduces the effective pore dimensions. Contact angle test and water adsorption isotherms indicate the hydrophobic property of PCP‐BDC‐DM is excellent, providing a foundation for its subsequent industrial application. (Figure S7, Supporting Information).

Figure 1.

Structure scheme of PCP‐BDC‐M and PCP‐BDC‐DM a), PXRD patterns b) and CO₂ sorption isotherms at 273 K and pore size distribution c).

To evaluate the adsorption performance of PCP‐BDC‐M and PCP‐BDC‐DM, adsorption isotherms for CH₄/N₂, C₂H₆/C₂H_4, and C₃H₈/C₃H₆ were measured. Both PCP‐BDC‐M and PCP‐BDC‐DM exhibited alkane‐selective adsorption behavior. The CH₄ and N₂ adsorption isotherms on PCP‐BDC‐M and PCP‐BDC‐DM at 273 and 298 K are shown in Figures 2a,d and S8 (Supporting Information). At pressures of 0.5 and 1 bar, PCP‐BDC‐M exhibited CH₄/N₂ uptake differences of 8.25 and 13.38 cm³ g⁻¹, respectively, while PCP‐BDC‐DM showed slightly greater differences of 9.74 and 15.43 cm³ g⁻¹ (Figures S9, Table S3, Supporting Information). Notably, at low pressure (0.1 bar), PCP‐BDC‐DM had a relatively low CH₄ uptake (0.98 cm³ g⁻¹), but adsorption significantly increased with rising pressure, reaching 19.15 cm³ g⁻¹ at 1 bar, surpassing PCP‐BDC‐M. This phenomenon suggests PCP‐BDC‐DM can desorb CH₄ at higher pressures, making it advantageous for efficient CH₄ recovery in pressure swing adsorption (PSA) processes. Ideal Adsorbed Solution Theory (IAST) calculations indicated PCP‐BDC‐DM's CH₄/N₂ selectivity^{[ 35 ]} of 9.3 at 298 K and 1 bar, significantly exceeding PCP‐BDC‐M (5.3) and conventional adsorbents (Figure 2g,h; Table S4, Supporting Information).

Figure 2.

CH₄/N₂ a, d), C₂H₆/C₂H₄ b, e) and C₃H₈/C₃H₆ c, f) sorption isotherms at 298 K; IAST selectivity of CH₄/N₂, C₂H₆/C₂H₄ and C₃H₈/C₃H₆ g, h) on PCP‐BDC‐M and PCP‐BDC‐DM; C₂H₆/C₂H₄ selectivity plotted against C₃H₈/C₃H₆ selectivity at 298 K and 1 bar for some benchmark adsorbents i).^{[ 29} , ³¹ , ³² , ³³ , ^{34 ]}

Turning to the C₂H₆/C₂H₄ separation, both PCP‐BDC‐M and PCP‐BDC‐DM show the higher adsorption capacity for C₂H₆ compared to C₂H₄ (Figure 2b,e; Figures S10 and S12, Supporting Information), demonstrating a clear preferential adsorption of C₂H₆. At 298 K and 1 bar, both PCP‐BDC‐M and PCP‐BDC‐DM showed a C₂H₆/C₂H₄ selectivity of 3.1, surpassing most reported ethane‐selective adsorbents and also the parent PCP‐BDC material (2.8) (Figure S11, Supporting Information).^{[ 36 ]}

The adsorption isotherms of C₃H₈ and C₃H₆ on PCP‐BDC‐M at 298 K showed that, in the low‐pressure region (< 0.3 bar), C₃H₈ exhibits slightly higher uptake than C₃H₆. However, as the pressure approaches 1 bar, the adsorption of C₃H₈ (43.50 cm³ g⁻¹) becomes lower than that of C₃H₆ (44.23 cm³ g⁻¹), indicating the limited capability for preferential adsorption of C₃H₈ over C₃H₆ (Figure 2c,f). In contrast, PCP‐BDC‐DM shows a more pronounced difference in adsorption between C₃H₈ and C₃H₆ across the entire pressure range, indicating its potential for superior C₃H₈/C₃H₆ separation performance. At 298 K and 1 bar, the C₃H₈/C₃H₆ selectivity of PCP‐BDC‐DM reaches 3.1, which is significantly higher than the 2.3 observed for PCP‐BDC‐M (Figure 2h and Figure S13, Supporting Information), and outperforms most reported propane‐selective adsorbents. The above results demonstrate that increased methyl substitution enhances PCP‐BDC‐DM's multifunctional alkane selectivity, significantly improving performance for CH₄/N₂, C₂H₆/C₂H₄, and C₃H₈/C₃H₆ separation (Table S5 and Figure 2i).

To further investigate the interactions between the adsorbates and the adsorbents, the isosteric heats of adsorption for various gases were calculated using the Virial equation^{[ 37 ]} as shown in Figures S14–S25 (Supporting Information). Both PCP‐BDC‐M and PCP‐BDC‐DM exhibited higher adsorption heats for light alkanes than their counterparts. Specifically, PCP‐BDC‐DM had notably higher adsorption enthalpies (CH₄: 39.7 kJ mol⁻¹; C₂H₆: 40.4 kJ mol⁻¹; C₃H₈: 45.4 kJ mol⁻¹) compared to PCP‐BDC‐M (CH₄: 19.1 kJ mol⁻¹; C₂H₆: 34.5 kJ mol⁻¹; C₃H₈: 40.9 kJ mol⁻¹), indicating that dimethyl functionalization significantly enhances these interactions.

To gain molecular‐level insight into the adsorption behavior of CH₄, N₂, C₂H₆, C₂H₄, C₃H₈, and C₃H₆ within the channels of PCP‐BDC‐DM, Grand Canonical Monte Carlo (GCMC) simulations were performed, as illustrated in Figure 3 . The parallel alignment of terephthalic acid units, along with the presence of methyl groups, creates a tailored environment that favors interactions with the hydrogen atoms of alkanes. Specifically, the CH₄ molecule engages in multiple van der Waals interactions with phenyl rings, the methyl group, and uncoordinated oxygen atoms: two C─H···C (phenyl ring) interactions with distances of 2.991–3.009 Å, one C─H···C (methyl group) interaction at 2.926 Å, and one C─H···O hydrogen bond at 2.732 Å) (Figure 3a). In contrast, the N₂ molecule forms only two weak N···H (methyl) interactions (2.992–3.262 Å) with the framework (Figure 3b), indicating significantly weaker binding. Similarly, alkane exhibits denser and stronger interactions with the terephthalic acid units and methyl groups than olefins. Specifically, C₂H₆ and C₃H₈ molecules form multiple van der Waals interactions with the methyl groups (CH···C = 2.814–2.865 and 2.649–2.776 Å, respectively, Figure 3c,e). In contrast, C₂H₄ and C₃H₆ exhibit fewer and weaker interactions, mainly limited to C‐H···O hydrogen bond (3.237 and 2.734 Å) and C─H···C van der Waals contacts (2.936–2.990 and 2.666 Å), as shown in Figure 3d,f. These simulations affirm that methyl functionalities significantly enhance alkane binding, driving superior light alkane selectivity.

Figure 3.

GCMC calculated preferable binding sites for paraffins and olefins in PCP‐BDC‐DM. CH₄ a), N₂ b), C₂H₆ c), C₂H₄ d), C₃H₈ e), and C₃H₆ f) binding sites in PCP‐BDC‐DM. The closest contacts between framework atoms and the gas molecules are defined by the distances (in Å), and the distances include the Van der Waals radius of the atoms. (Framework: C, white; H, gray; N, blue; O, red; Co, light blue; Gas: C, white; H, gray).

Adsorption kinetics for C₂H₆, C₂H₄, C₃H₈, and C₃H₆ were investigated (Figure 4a; Figure S26, Supporting Information). While C₂H₆ and C₂H₄ exhibited comparable kinetics, PCP‐BDC‐DM revealed substantial kinetic differences for C₃H₈ versus C₃H₆. C₃H₈ rapidly reached equilibrium (≈10 min), whereas C₃H₆ took longer (≈20 min). In competitive adsorption, faster kinetics of C₃H₈ enable it to dominate available adsorption sites, enhancing selectivity and separation efficiency. To further assess the separation performance under practical conditions, dynamic breakthrough experiments were conducted using equimolar (v/v = 50/50) gas mixture of CH₄/N₂, C₂H₆/C₂H₄, and C₃H₈/C₃H₆. Excellent separation efficiency was observed in all cases. For CH₄/N₂ separation on PCP‐BDC‐DM, N₂ eluted first at ≈1.1 min g⁻¹, while CH₄ was retained until 2.6 min g⁻¹ (Figure 4b), achieving a clear separation. In the case of the C₂H₆/C₂H₄ mixture separation, C₂H₄ consistently elutes earlier than C₂H₆ on both PCP‐BDC‐M and PCP‐BDC‐DM (Figure 4c and Figure S27, Supporting Information). Notably, PCP‐BDC‐DM exhibited superior separation performance, with a retention time of up to 15.3 min g⁻¹ compared to 13.3 min g⁻¹ on PCP‐BDC‐M. During this period, ultra‐high purity (>99.99%) C₂H₄ can be collected. Considering that the C₃H₆ product streams are typically released at elevated temperatures,^{[ 38 ]} breakthrough experiments for C₃H₆/C₃H₈ mixture were carried at 318 K with a flow rate of 10 mL min⁻¹ to evaluate the real‐world separation performances of PCP‐BDC‐DM. In this system, C₃H₆ breaks through the column first, while C₃H₈ begins to elute only after 10 min g⁻¹ (Figure 4d), confirming the material's excellent separation performance. The stability of the PCP‐BDC‐DM was studied. Adsorption isotherms at 298 K on PCP‐BDC‐DM of another batch and adsorption cycles test on PCP‐BDC‐DM were measured, showing the repeatability of adsorption (Figures S28 and S29, Supporting Information). In order to study the acid‐base and hydrothermal stability, PCP‐BDC‐DM was immersed in solutions of different pH values and in water at 80 °C for 12 h. The results show that within the pH range of 3 to 11 and in hot water, the structure of PCP‐BDC‐DM remains basically unchanged, indicating its excellent stability (Figure S30, Supporting Information). Then, PXRD of PCP‐BDC‐DM after breakthrough tests and cycle tests further confirmed structural integrity and stability (Figure S31, Supporting Information).

Figure 4.

Adsorption kinetics of C₃H₈ and C₃H₆ on PCP‐BDC‐DM a), breakthrough curves of CH₄/N₂ (v/v = 50/50) at 15 mL min⁻¹ b), C₂H₆/C₂H₄ (v/v = 50/50) at 15 mL min⁻¹ and 298 K c), C₃H₈/C₃H₆ (v/v = 50/50) at 10 mL min⁻¹ and 318 K on PCP‐BDC‐DM.

3. Conclusion

This study systematically examined the adsorption separation performance of light hydrocarbons on methyl‐functionalized PCP series adsorbents. Results indicate that methyl groups effectively create additional alkane adsorption sites, significantly enhancing separation selectivity across CH₄/N₂, C₂H₆/C₂H₄, and C₃H₈/C₃H₆ mixtures and creating a multifunctional adsorbent. Specifically, PCP‐BDC‐DM exhibited optimal performance due to its dimethyl groups. GCMC simulations corroborated experimental findings, clarifying that methyl groups serve as potent alkane‐binding sites. Breakthrough experiments validated PCP‐BDC‐DM's practical utility, highlighting its comprehensive alkane separation capabilities. This work provides valuable insights and strategies for designing advanced alkane‐selective adsorbents.

Conflict of Interest

The authors declare no conflict of interest.

Supporting information

Supporting Information

Wang L., Liu Z., Wang Y., Liu J., Li J., and Yang J., “Rational Design of a Multifunctional MOFs for Alkane‐Selective Gas Separation.” Adv. Sci. 13, no. 2 (2026): e16118. 10.1002/advs.202516118

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.