Deep removal of trace C₂H₂ and CO₂ from C₂H₄ by using customized potassium-exchange mordenite

Abstract

Adsorptive separation using porous materials is a promising approach for separating alkynes/olefins due to its energy efficiency, while the deep removal of trace amounts of C₂H₂ and CO₂ from C₂H₄ is still very challenging for a commercial adsorbent. Herein, we report a low-cost inorganic metal cation-mediated mordenite (MOR) zeolite with the specific location and distribution of K⁺ cations acting as a goalkeeper for accurately controlling diffusion channels, as evidence of the experimental and simulation results. Deep purification of C₂H₄ from ternary CO₂/C₂H₂/C₂H₄ mixtures was first realized on K–MOR with exceptional results, achieving a remarkable polymer-grade C₂H₄ productivity of 1742 L kg⁻¹ for the CO₂/C₂H₂/C₂H₄ mixture. Our approach which only involves adjusting the equilibrium ions, is both promising and cost-effective, and opens up new possibilities for the use of zeolites in the industrial light hydrocarbon adsorption and purification process.

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Introduction

Ethylene (C₂H₄) is an essential petrochemical intermediate and currently the world's most extensively consumed organic compound, with a production capacity that exceeded 200 million tons in 2021.¹ C₂H₄ is primarily obtained through steam cracking or thermal decomposition of either naphtha or ethane (C₂H₆), which inevitably leads to the presence of acetylene (C₂H₂) and carbon dioxide (CO₂) as downstream byproducts. Purification of C₂H₄ from ternary CO₂/C₂H₂/C₂H₄ mixtures is a crucial process to yield polymer-grade C₂H₄ (C₂H₄ > 99.9% purity and C₂H₂ < 40 ppm).² Currently, in the petrochemical industry, C₂H₂ is first removed through catalytic hydrogenation using noble-metal catalysts or solvent extraction using organic solvents, and CO₂ is subsequently separated from C₂H₄ by the organic amine adsorption method. These step-by-step purification processes are complex, environmentally unfriendly, and energy-intensive.³ In fact, the total energy used for the purification of ethylene and propene alone accounts for 0.3% of global energy consumption.⁴ This high energy footprint has led to extensive research aimed at developing more energy-efficient approaches for the purification of C₂H₄.

Physical adsorption is a promising approach that has the potential to significantly reduce the energy consumption of separation processes.^5–13 Selective adsorption relies on the specific interaction between olefins and chemically modified adsorbents.^14–18 Most adsorbents use kinetic or steric effects to separate alkynes from their corresponding olefins, which requires an additional CO₂ removal step. Currently, there are few materials capable of effectively separating C₂H₄ from CO₂/C₂H₂/C₂H₄ in a single step, and the majority of these materials are metal–organic frameworks (MOFs) or other porous materials that have been reported in research studies.^19–25 Our research focuses on exploring the potential applications of zeolites, which are highly versatile and functional materials. These materials feature robust frameworks, molecule-sized pores, large surface areas, and other tunable properties.^26–32 These unique characteristics make zeolites capable of creating suitable pore space and environment to accommodate specific guest molecules. Numerous studies revealed that the adsorption behaviours were governed not only by the valence of the cations and the composition of the skeleton but also by their ionic radii and locations.^33–37 These merits are responsible for the successful applications of zeolites in various industrial processes. The material of our study is the MOR zeolite, which has a unique channel structure and adsorption properties that have been extensively studied by scientists. Furthermore, there has been significant progress in exploring the potential of the MOR zeolite in carbon dioxide capture,^8,38,39 removal of nitrous oxide,⁴⁰ and olefin/paraffin separation.⁴¹

The fundamental challenge in purifying C₂H₄ from ternary CO₂/C₂H₂/C₂H₄ mixtures primarily arises from the physicochemical properties of the respective gas molecules, including their size, boiling point, electrostatic potential distribution (Fig. S8†), kinetic diameters (C₂H₂: 3.3 Å, C₂H₄: 4.2 Å, and CO₂: 3.3 Å), quadrupole moments (7.2 × 10⁻²⁶vs. 1.5 × 10⁻²⁶vs. 4.3 × 10⁻²⁶ esu cm²), and polarizabilities (39.3 × 10⁻²⁵vs. 42.52 × 10⁻²⁵vs. 29.11 × 10⁻²⁵ cm³). The physicochemical properties of C₂H₄ are intermediate between those of C₂H₂ and CO₂, making it challenging for most physisorbents to demonstrate high selectivity. Since larger quadrupole moments and more π electrons of C₂H₂, C₂H₄, and CO₂ gases enable a stronger interaction with cations or highly polar T atoms of the skeleton, deep purification of C₂H₄ from ternary CO₂/C₂H₂/C₂H₄ mixtures would require physisorbents to show significantly stronger affinity toward both C₂H₂ and CO₂ over C₂H₄. To overcome this challenge, a diffusion channel regulation strategy using classic MOR zeolites (Fig. 1) has been proposed. In terms of C₂H₂/C₂H₄ separation, porous materials can typically achieve preferential adsorption of C₂H₂ over C₂H₄ with relatively high selectivity through pore size fine-tuning, channel environment adjustments, or incorporation of new skeleton components such as heteroatoms in molecular sieves, new groups in MOF ligands and more ways. However, due to the opposite electrostatic potential distributions of CO₂ and C₂H₂, it is nearly impossible for a C₂H₂-selective zeolite to simultaneously increase both C₂H₂ and CO₂ adsorption capacities. Instead, selectively inhibiting the C₂H₄-affinity sites in zeolites seems to be a feasible strategy for enhancing C₂H₄ purification capability without sacrificing the original C₂H₂ and CO₂ adsorption capacity. This special approach not only reduces C₂H₄ co-adsorption and enhances the recovery during the adsorption process, but also reduces the energy consumption of regeneration. Additionally, the unique CO₂ adsorption confinement effect offered by the MOR zeolite can be leveraged to overcome this challenge.⁴²

Fig. 1 Diffusion channel regulation strategy for CO₂/C₂H₂/C₂H₄ separation.

It is a mechanism that is often neglected among the many common adsorption and separation mechanisms in zeolites; channel and pore accessibility is another crucial factor in determining adsorption capacity and selectivity. An illustrative example using Na–ZSM-5,⁴³ where DFT calculations were performed, revealed that only the Na⁺ site located in the 10 MR channel interacts with CO₂ as it is the only accessible site to CO₂. Conversely, the Na⁺ located in the 5 MR adjacent to the wall is inaccessible to CO₂ due to the kinetic diameter of CO₂ being larger than the corresponding pore size (approximately 1.5 Å) through which the cation is accessed.

Zeolites offer the unique capability to regulate the accessibility of their channels through the presence of different cations. To create efficient adsorbent materials customized for targeted separation or purification processes, zeolite products must undergo ion exchange with diverse alkali metals, and alkaline earth, transition, or rare earth elements.⁴⁴ This customizable process allows the development of the corresponding special adsorbents for different practical application environments. Our study deduced that strategically introducing appropriate cations into the inert channel space of MOR zeolites can effectively impede the diffusion of C₂H₄ into specific adsorption sites. This mechanism can be combined with the confinement effect of CO₂ in MOR to achieve simultaneous separation of C₂H₂/C₂H₄ and CO₂/C₂H₄ using a single adsorbent. In this study, we provide a highly feasible reference for deep purification of C₂H₄ from 1/1/98 CO₂/C₂H₂/C₂H₄ mixtures, only facilitated through employment of a common zeolite via a traditional and effortless ion exchange process with an impressive C₂H₄ purification productivity.

Experimental

Synthesis of Na–MOR

All reagents used in the experiment were commercially sourced and used without any purification (Table S1†). Na–MOR (N–MOR) zeolites were synthesized using a template-free method. 0.9475 g NaOH and 0.5075 g NaAlO₂ were mixed with 23.53 g deionized water in a plastic beaker and stirred vigorously at room temperature for 30 minutes. Next, 14.18 g of 30 wt% silica sol solutions were added to the mixture and stirred for 4 hours until a homogeneous slurry was formed. The slurry was then transferred to a Teflon-lined stainless-steel autoclave and statically crystallized at 453 K for 3 days. The solid was washed with deionized water and air-dried. Through large-scale synthesis carried out in a 10 L reactor, we were able to produce over 1 kg of N–MOR in a single reaction.

Preparation of M–MOR

The M–MOR (M = NH₄⁺, Li⁺, Na⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) were prepared via ion-exchange (ionic radius: Mg²⁺ < Li⁺ < Na⁺ < Ca²⁺ < Sr²⁺ < K⁺ < Ba²⁺ < Cs⁺ and ionic hydrated radius: Cs⁺ < K⁺ < Na⁺ < Li⁺ < Ba²⁺ < Sr²⁺ < Ca²⁺ < Mg²⁺). As an example, the as-synthesized N–MOR was exchanged with NH₄Cl solution (1 mol L⁻¹) at a solid-to-liquid ratio of 1 g/50 mL under stirring (298 K, 4 h). The slurry was filtered, washed with deionized water three times, and dried overnight at 373 K; other M–MOR were prepared in the same way. In addition, attempts were made to directly synthesize K–MOR, but only amorphous products were obtained. During the synthesis process, NaOH acted as a crucial mineralizing and structure-guiding agent, playing an essential role in MOR crystallization. It is difficult to obtain K–MOR through a one-step method without any organic template.

Structure of MOR

X-ray diffraction (XRD) patterns were obtained using a Bruker D8 ADVANCE powder diffractometer equipped with a copper-sealed target (λ = 0.15418 nm) at room temperature. The measurements were conducted by scanning over the 4–40° (2θ) range at a rate of 7.55 ° min⁻¹. The Na, Si, Al, and K content of the zeolite samples was analyzed using an inductively coupled plasma optical emission spectrometer (Avio™ 200 ICP-OES). The original CIF structure model was sourced from the zeolite database of the Material Studio, and the zeolitic structure was considered rigid. The reported aluminum distribution was used for CIF file modeling,⁴⁵ and the parameters describing the interaction between the adsorbent and cation were taken from COMPASS II. The sorption simulations were carried out using the Material Studio software.

Single-component static adsorption

Isotherms were collected on a Micromeritics ASAP 2020 automatism isothermal adsorption instrument using a calculated amount of adsorbent placed in a sample tube. Prior to the measurements, all the samples were degassed to 10⁻⁷ bar at 473 K. Adsorption isotherms were fitted with a double-site Langmuir–Freundlich isotherm model with six parameters. Ideal adsorbed solution theory (IAST) was used to predict binary adsorption isotherms from the single-component adsorption isotherms of C₂H₂, C₂H₄, and CO₂. Isosteric heat adsorption (Q_st) values were calculated from isotherms measured at 298 K, 318 K and 338 K for C₂H₂, C₂H₄, and CO₂.

The porosity of the samples was characterized using N₂ sorption isotherms at 77 K on an ASAP 3020 automated isothermal adsorption instrument. The Brunauer–Emmett–Teller method was employed to estimate the specific surface areas (S_BET), and the micropore volume was determined simultaneously.

Adsorption kinetics and desorption behaviours

Single composition CO₂, C₂H₂, and C₂H₄ dynamic adsorption profiles at 298 K were measured on an intelligent gravimetric analyzer (IGA-100, HIDEN). About 80–120 mg of the sample was first loaded into the sample chamber and activated at 473 K under high vacuum, the pressure was increased and decreased with a ramp rate of 100 mbar min⁻¹ during the desorption process, and a fast response furnace was employed for heating.

Breakthrough experiments

The setup was designed to measure and analyze the adsorption and separation of gases in real-time, replicating practical conditions for industrial applications, and the outlet gas was detected using an Agilent GC490 gas chromatograph (Table S3†). As an example, the C₂H₂/C₂H₄ (1/99, v/v) breakthrough experiment using a dynamic gas breakthrough equipment set up at 298 K was conducted. The experiment was carried out using a stainless-steel column with dimensions of 10 mm inner diameter and 110 mm length. The column was packed with a sample weighing 3.68 g. Prior to each breakthrough experiment, the column was vacuumed at 473 K until no guest molecule was observed, and then purged with Ar at room temperature. The mixed gas (C₂H₂/C₂H₄ 1/99, v/v) was then introduced at a flow rate of 10 mL min⁻¹. The outlet gas from the column was monitored using gas chromatography (GC-490, Agilent) with a thermal conductivity detector (TCD). After completing the breakthrough experiment, the column was regenerated with an Ar flow at a temperature of 453 K and 573 K. The breakthrough experiments performed in our study included the C₂H₂/C₂H₄ (50/50, v/v), C₂H₂/C₂H₄/C₂H₆ (1/90/9, v/v/v), and CO₂/C₂H₂/C₂H₄ (1/1/98, v/v/v) breakthrough experiments under humid conditions, in addition to the C₂H₂/C₂H₄ (1/99, v/v) breakthrough experiment. These experiments were similar in their operating way and were designed to measure the adsorption and separation of mixed gases under various conditions.

The dynamic separation selectivity was calculated using where x_i and x_j are the mole fractions of components i and j, respectively, in the adsorbed phase, and y_i and y_j are the mole fractions of components i and j, respectively, in the gas phase.

Results and discussion

To minimize environmental impact and reduce costs, we utilized a green and classic hydrothermal synthesis route to synthesize MOR zeolites; this approach allowed us to avoid the use of toxic organic reagents and complex synthesis routes. The environmentally friendly and low-cost MOR zeolites served as the study object and parent material with a composition of Na₅[Al₅Si₄₃O₉₆]. MOR zeolites possess a unique channel system consisting of parallel linear channels. Specifically, the main channel is composed of a 12-membered ring (12 MR) (6.5 × 7.0 Å) and runs along the c-axis. In addition, another channel called a side pocket is present, which consists of an 8 MR (2.6 × 5.7 Å) and runs along the b-axis. Therefore, there are two distinct adsorption spaces available: the 12 MR main channel and the narrow, highly restricted 8 MR side pocket. The larger 12 MR main channel provides a larger adsorption space with weaker interactions with the framework and balancing cations, leading to fewer constraints on guest molecules. Diffusion and adsorption in the 8 MR side pocket, on the other hand, are dominated by the interaction between the guest molecule framework and balancing cations, resulting in significant differences in both thermodynamic and kinetic properties when compared to those in the main channel.

The crystalline structure of the MOR zeolite remained intact after ion exchange with NH₄⁺, Ca²⁺, Mg²⁺, and K⁺ ions, as evidenced by the presence of all reflections corresponding to the ion-exchanged zeolites. However, the XRD patterns of Cs–MOR and Ba–MOR exhibited significant differences, and electron microscopy images confirmed multiple structural breakages in Li-, Cs-, Sr-, and Ba–MOR samples (Fig. S1–S3†).

After analyzing the adsorption isotherms of C₂H₂, C₂H₂, and CO₂ on M–MOR (Fig. S9†), K–MOR and NH₄–MOR stand out as the candidate materials. Although NH₄–MOR demonstrated exceptional performance during the initial adsorption and desorption, its application was limited due to the instability of NH₄⁺ in MOR and weak interaction with the MOR skeleton. With an increase in the number of cycle test times, NH₄⁺ may convert to NH₃ and become detached from MOR during the degassing step, which caused a significant drawback for its practical application.

It is well known that the hydrated ion radius of K⁺ is much smaller than that of Na⁺, which makes K⁺ ions easily exchange with Na⁺ ions in MOR. The BET and the micropore volume decrease regularly (Fig. 2), which can be can be attributed to the large ionic radius of the K⁺ ion and its specific occupying effect, which limits the part of the 8 MR side pocket and 8 MR space that the N₂ molecule can access and occupy. After undergoing K⁺ ion exchange, the diffusion channel of MOR was effectively regulated by K⁺ cations, leading to alterations in the channel-pore environment. C₂H₂ molecules were still able to access and occupy the side pocket of MOR, while C₂H₄ molecules were unable to efficiently utilize the adsorption site and space within the side pocket, resulting in a significant decrease in the amount of C₂H₄ adsorbed. Conversely, due to the intrinsic properties of MOR regarding CO₂ confinement in the 8 MR side pocket, the amount of CO₂ adsorbed remained relatively constant. Through calculations of the Q_st of C₂H₂, C₂H₄, and CO₂ adsorption, it was found that the Q_st of C₂H₂ and CO₂ were higher than that of C₂H₄ at both zero-coverage and for the whole process, indicating the strong interaction between K–MOR and CO₂/C₂H₂ molecules. Moreover, it was discovered that N–MOR displayed a significant attraction towards C₂H₂, C₂H₄, and CO₂, with Q_st values that were comparatively higher than those of K–MOR. Specifically, N–MOR exhibited a higher heat of adsorption for C₂H₄ than for CO₂, indicating a more significant interaction between N–MOR and C₂H₄ (Fig. S17 and S18†). In addition, IAST was employed to predict selectivity on M–MOR at 298 K, and the adsorption isotherms were fitted using the dual-site Langmuir–Freundlich adsorption model. K–MOR-1 (K–MOR) exhibited both the highest 1/99 C₂H₂/C₂H₄ (28.6) and 1/99 CO₂/C₂H₄ (9.21) IAST selectivity compared to other M–MOR and K–MOR-X (X = 0.01, 0.05, 0.1, 0.5, 1.5, and 2, X represent different ion-exchange concentrations) (Fig. S11–S16†). This exhilarating selectivity makes dynamic breakthrough experiments more desirable. Furthermore, K–MOR exhibited exceptional moisture and pH stability (Fig. S10†), which is advantageous for practical applications in the industry.

Fig. 2 C₂H₂, C₂H₄, and CO₂ adsorption isotherms of (a) N–MOR and (b) K–MOR. (c) IAST selectivity of K–MOR for mixtures of C₂H₂/C₂H₄ (1/99) and CO₂/C₂H₄ (1/99). (d) Isosteric heats of C₂H₂, C₂H₄, and CO₂ adsorption on K–MOR. (e) N₂ sorption isotherms at 77 K on K–MOR with different K salt concentrations. (f) Comparison of ion-exchange concentration, specific surface areas (S_BET) and micropore volume in K–MOR.

Adsorption kinetics and desorption behaviours (Fig. 3) are critical parameters to consider when evaluating the separation efficiency of a material for industrial application. A key finding of this research was the enhanced desorption kinetics of CO₂ and C₂H₄ on K–MOR, which became more easier compared to that on N–MOR. Notably, only vacuum conditions at room temperature were sufficient for achieving desorption on K–MOR, demonstrating a significant energy consumption advantage over N–MOR. In contrast, desorbing adsorbents for C₂H₂, C₂H₄, and CO₂ from N–MOR through vacuuming proved to be difficult and time-consuming, requiring additional heating operations and energy consumption. While a small amount of C₂H₂ heating was necessary to completely desorb from K–MOR, this result suggests that K–MOR retains its strong interaction force with C₂H₂ while interactions with C₂H₄ weakened. As for adsorption, the C₂H₂ fractional uptake Q_t/Q_e exhibited a relatively slow increase with time for both K–MOR and N–MOR during the adsorption process, possibly due to the confinement effect of MOR and the strong interaction of the cations in MOR to reach the adsorption equilibrium. As for C₂H₄, the equilibrium time of C₂H₄ for N–MOR was significantly longer than that of K–MOR, possibly attributed to the fact that C₂H₄ molecules are limited by K⁺ ions, which makes it difficult for the C₂H₄ molecules to access and be retained in the 8 MR side-pocket as easily as C₂H₂ and CO₂, and mostly only diffuse and adsorb in the space of the 12 MR of MOR, which accelerates the process of C₂H₄ reaching the adsorption equilibrium and also contributes to the desorption of C₂H₄ in K–MOR.

Fig. 3 Kinetics of adsorption and desorption behaviours for CO₂, C₂H₂, and C₂H₄ on (a and b) N–MOR and (c and d) K–MOR.

The outstanding C₂H₂/C₂H₄ separation performance of K–MOR is further confirmed dynamically in a fixed bed (Fig. 4). The breakthrough curves are plotted with time (min g⁻¹), which is already adjusted for the dead volume obtained from the blank experiment. As expected, the C₂H₂/C₂H₄ mixture was effectively separated using K–MOR. C₂H₄ was the first to elute through the bed, and then the outlet gas quickly reached pure grade with no detectable C₂H₂ (below the detection limit of the experimental setup), whereas the solid adsorbent retained C₂H₂ for a remarkable time before the breakthrough of C₂H₂, which is in line with the sorption experiments. Through conservation calculations, the co-adsorbed C₂H₄ can be partially replaced by C₂H₂, which enhances the dynamic separation performance to a certain extent, and makes it possible to remove C₂H₂ from C₂H₄ without losing any valuable C₂H₄. The amount of polymer-grade C₂H₄ purified from the C₂H₂/C₂H₄ (1/99, v/v) mixture was as high as 2029 L kg⁻¹ (C₂H₂ < 10 ppm), the recovery rate was up to 97.61%, the amount of C₂H₂ adsorbed from the mixture was up to 20.35 cm³ g⁻¹, and a remarkable C₂H₂/C₂H₄ dynamic selectivity of 54.4 was achieved, compared with that of reported state-of-the-art sorbents (Table S4†). Moreover, regeneration of K–MOR under an Ar flow at 453 K demonstrated complete recovery of the adsorbed gas within 60 min g⁻¹ at 10 mL min⁻¹, which is significantly faster than the Ag-doped adsorbent (which can take up to thousands of minutes).⁴⁶ These results demonstrate that C₂H₂ removal can be effectively addressed under mild conditions (1–2 bar and 298 K), consistent with the realistic pressure of cracked gas steam leaving the thermal cracking furnace (ca. 1.55 bar).⁴⁷ The cycling C₂H₂/C₂H₄ (1/99, v/v) breakthrough tests were conducted on K–MOR under the aforementioned operating conditions, and the results showed the same retention time as the initial experiment (Fig. S21†).

Fig. 4 (a and b) C₂H₂/C₂H₄ (1/99, v/v) and (c and d) CO₂/C₂H₂/C₂H₄ (1/1/98, v/v/v) breakthrough experiments at 298 K. (e) Comparation of C₂H₂/C₂H₄ dynamic selectivity against C₂H₂ dynamic uptake under ambient conditions with a state-of-the art adsorbent. (f) Cycling breakthrough tests on K–MOR under humid conditions (RH = 11.5%).

Due to the MOR zeolite nature structure properties,⁴² K–MOR shows excellent CO₂ capture ability at extremely low CO₂ concentrations and room temperature as N–MOR. And the 1/1/98 CO₂/C₂H₂/C₂H₄ breakthrough experiment conducted on K–MOR showed that it is capable of capturing both CO₂ and C₂H₂ simultaneously, while producing highly purified polymer-grade C₂H₄ with a productivity of up to 1742 L kg⁻¹ (CO₂ < 100 ppm and C₂H₂ < 10 ppm). The dynamic uptake of C₂H₂ was up to 27.15 cm³ g⁻¹, while that of CO₂ was up to 32.61 cm³ g⁻¹ when adsorption equilibrium was reached. The desorption process revealed that the majority of adsorbed CO₂ was rapidly released during argon purging, with only a small portion requiring temperature desorption. Similarly, a fraction of adsorbed C₂H₂ was swiftly desorbed, while another portion required temperature desorption. In contrast, C₂H₄ showed significant replacement during the adsorption process, as confirmed by conservation calculations. During the desorption process, the majority of adsorbed C₂H₄ was directly purged and released at room temperature.

Typically, the cracking gas contains some H₂O and the adsorbent operates at higher flow rates. Breakthrough experiments were carried out at different relative humidities and RH levels of 75%, 43.5%, and 11.5%. The separation performance of K–MOR was largely maintained, with the polymer-grade C₂H₄ production productivity reaching 1074 L kg⁻¹ under humid conditions (RH = 75%). Furthermore, breakthrough cycling experiments were conducted on 1/1/98 CO₂/C₂H₂/C₂H₄ with a 50 mL min⁻¹ flow (RH = 11.5%) (Fig. S22 and S23†), indicating that this material has great potential and feasibility for challenging separation of the C₂H₄ component in actual industrial cracked gas streams.

To explore the diffusion behaviours and adsorption density distribution of C₂H₂, C₂H₄, and CO₂ molecules in MOR, we integrate practical separation experiments with computer simulation techniques. While it is possible to determine the mean structure and chemical composition of a zeolite using established methods such as X-ray diffraction and elemental analysis, determining the specific location of aluminum atoms within the zeolite framework is challenging using conventional approaches. Although the determination of the mean structure and the chemical composition of a zeolite is well established, it is still very hard to get a direct determination of aluminum siting by the conventional methods. Many researchers have investigated the Al distribution in the MOR zeolite in both experimental^48–54 and simulated^45,54,55 ways, confirmed that Al atoms preferentially occupy tetrahedral positions in the four-membered ring (T₃ and T₄ sites) of the MOR zeolite structure and the proportion of Al atoms in the main channels relatively decreased with an increase in the Si/Al ratio of the MOR zeolite, and indicate that the order of occupancy of the sites obtained by many simulation methods is the same: T₃ > T₄ > T₁ > T₂. Our model was based on an up-to-date and reliable molecular simulation method reported by Jeffroy and co-workers⁴⁵ (Table S6†) to determine the Al distribution for the CIF file modification; this simulation result is not only suiting the case for the MOR zeolite, but also for several other materials studied by simulation belonging to the GOO and ANA families.^{45,49–52,54}

Adsorption properties of aluminosilicate zeolites are related to the location of cations; however, the location and distribution of aluminum atoms can strongly influence the cation location.^55–58 As a result of the atomic charge difference between silicon and aluminum atoms, the repulsive forces exerted on cations are expected to be smaller for aluminum atoms. This means that cations are likely to be located close to aluminum atoms, with the number of aluminum atoms close to a cation dependent on the Si/Al ratio. In general, a higher Si/Al ratio is expected to result in a smaller number of aluminum atoms surrounding a cation. However, the local Si/Al ratio around a cation can be lower than the global Si/Al ratio, and this can lead to the cation being more sensitive to the presence of aluminum atoms and being located closer to them. Understanding the relationship between aluminum atoms, cations, and Si/Al ratios is important in predicting and controlling the adsorption properties of zeolites for specific gas separation applications.

From our simulation result (Fig. 5), the K⁺ cations tend to be distributed near the center of the 8 MR side pocket which could adjust the connection channel of the 12 MR main channel and side pocket; the Na⁺ cations tend to be distributed in the 8 MR, which indicated that K–MOR could block some bigger molecules from diffusing into the 8 MR side pocket and the 8 MR of the MOR zeolite, and influence the adsorption environment of the 12 MR, providing special adsorption and separation properties to K–MOR. The high electro-positivity and large ionic size of K⁺ cations regulate the diffusion channel and alter the channel environment. As a result, C₂H₄ molecules cannot easily diffuse into the 8 MR side-pocket and 8 MR, and the majority of C₂H₄ can only compete with CO₂ and C₂H₂ for adsorption sites in the 12 MR. Additionally, K⁺ cations weaken the interaction between the host and the guest, which significantly reduces the adsorption capacity and strength of C₂H₄. This is consistent with the results of static adsorption performance and breakthrough experiments, and both C₂H₄ adsorption and desorption behaviours of K–MOR.

Fig. 5 (a) Aluminium distribution in different T sites (T₁, T₂, T₃, and T₄) of mordenite used for simulation, and the sphere diameter is proportional to the aluminium population in the corresponding site. The skeleton and cation distribution of (b) N–MOR and (c) K–MOR. Density distribution of C₂H₂ (purple), C₂H₄ (orange), CO₂ (blue) under the loading calculated from (d) C₂H₂/C₂H₄ (50/50, v/v), (e) C₂H₂/C₂H₄ (1/99, v/v) and (f) CO₂/C₂H₂/C₂H₄ (1/1/98, v/v/v) separation on K–MOR at 298 K.

To better understand the selective adsorption behaviours of C₂H₂ and CO₂ over C₂H₄, GCMC simulation was conducted using the sorption module of Material Studio to calculate the adsorption probability distribution of C₂H₂, C₂H₄, and CO₂ in N–MOR and K–MOR at 298 K and 1.0 bar. This simulation allows us to gain insights into the molecular-level mechanisms that govern the preferential adsorption of certain gases over others in these zeolite materials. To ensure a fair comparison, a super unit cell (222) containing 40 Al atoms based on the experimental Si/Al ratio was built, and 40 cations were used as the equilibrium cations. Sorption simulations were carried out for the same fixed loading of C₂H₂, C₂H₄, and CO₂ in N–MOR as in the blank control (Fig. S24†). The sorption calculation results based on the breakthrough experiment revealed that the density distribution of CO₂ and C₂H₂ was high in both the 8 MR side pocket and the 12 MR of K–MOR. The main potential adsorption site density of C₂H₄ was primarily distributed in the 12 MR of K–MOR and was rare in the 8 MR side pocket. These findings are consistent with our predicted adsorption behaviours and practical separation experiments, providing a better comprehension of the actual separation mechanism and guest molecular distribution in K–MOR.

Conclusion

The K–MOR zeolite was synthesized by a feasible and template-free green method. The special channel regulation of K⁺ cations poses a challenge for C₂H₄ to access and occupy the 8 MR side pocket as easily as C₂H₂ and CO₂, which reduces the accessible space for adsorption of C₂H₄, and meanwhile weakens the host–guest interaction. Breakthrough experiments reveal that K–MOR exhibits exceptional separation performance, achieving a remarkable polymer-grade C₂H₄ productivity of up to 2029 L kg⁻¹ for the separation of the 1/99 C₂H₂/C₂H₄ mixture and 1742 L kg⁻¹ for the 1/1/98 CO₂/C₂H₂/C₂H₄ mixture. Deep purification of C₂H₄ from CO₂/C₂H₂/C₂H₄ with high purity and productivity was achieved in this customized zeolite, while operating in an energy-efficient and cost-effective way. We believe that further research will lead to the discovery of more exceptional zeolites with potential applications in the adsorption and separation field, addressing some critical tasks in complex industrial processes.

Author contributions

All the experiments were conducted by H. C., B. W., B. Z., J. C., J. G., and X. S. with the supervision of W. Y., J. L., and L. L. All authors discussed the results and co-wrote the manuscript.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We acknowledge the financial support from the National Key Research and Development Program of China (2022YFB3806800, 2021YFA1500401, and 2021YFA1501202) and the National Natural Science Foundation of China (22090062, 22288101, U1967215, and 22178244).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3sc02147e

‡ These authors contributed equally.

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