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Review

Progress in the Separation and Purification of Carbon Hydrocarbon Compounds Using MOFs and Molecular Sieves

by
Yousheng Zhou
1,
Peicheng Li
1,
Yifan Wang
1,
Qiyue Zhao
1 and
Hui Sun
1,2,3,*
1
School of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China
2
International Joint Research Center of Green Energy Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China
3
Ministry Key Laboratory of Oil and Gas Fine Chemicals, School of Chemical Engineering and Technology, Xinjiang University, Urumqi 830046, China
*
Author to whom correspondence should be addressed.
Separations 2023, 10(10), 543; https://doi.org/10.3390/separations10100543
Submission received: 19 September 2023 / Revised: 9 October 2023 / Accepted: 17 October 2023 / Published: 19 October 2023
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Abstract

:
Carbon hydrocarbon compounds, especially low-carbon hydrocarbons (C1–C3), are vital raw materials in the petrochemical industry, but their efficient separation has great challenges due to their similar molecular structures and properties. In contrast to traditional low-temperature distillation and absorption separation technologies, selective adsorption employing porous materials as adsorbent has the advantages of low energy consumption, high efficiency, and high selectivity, indicating broad application possibilities in the field of low-carbon hydrocarbon separation. In this paper, the recent progress in the separation and purification of hydrocarbon mixtures by means of the two kinds of porous materials (metal–organic frameworks and molecular sieves) that have been widely used in recent years is reviewed, including purification of methane and separation of ethylene/ethane, propylene/propane, and some high-carbon hydrocarbon isomers. The structure–activity relationships between their chemical composition, structural characteristics, and separation performance are discussed to understand the separation mechanism. In conclusion, the issues encountered in the application of metal–organic frameworks and molecular sieves in the separation of low-carbon hydrocarbons are discussed in light of the current context of “carbon neutrality”.

1. Introduction

Natural gas, oilfield associated gas, coalbed methane, biogas, and other low-carbon hydrocarbon mixtures (C1–C3) are high-quality energy and chemical raw materials [1,2]. Natural gas, particularly methane (CH4), plays an essential role in reducing energy consumption and preventing environmental deterioration and is predicted to become the primary source of global energy by 2030 [3,4]. Ethylene (C2H4) and propylene (C3H6) are in high demand as the primary raw materials for synthetic polymers such as polyethylene, polyvinyl chloride, and polypropylene. In 2021, the global production capacity of ethylene and propylene exceeded 214 million tons and 120 million tons, respectively [5,6]. However, due to the presence of impurities (CO2, various trace gases, etc.), the optimum purity (>99.5%) cannot be attained in the manufacturing and transportation of low-carbon hydrocarbons, which limits the economic benefits and corrodes pipes, thus producing environmental contamination [7,8]. Therefore, to improve long-term growth and application, it is crucial to develop effective and affordable low-carbon hydrocarbon separation and purification technology.
Table 1 shows that the physicochemical properties of low-carbon hydrocarbon molecules are remarkably similar, making separation challenging. In industry, high purity goods are frequently obtained at the expense of high equipment expenditures and energy usage (about 15% of world energy demand) [9,10]. Cryogenic distillation, a common separation procedure, primarily utilizes the difference in the boiling points of molecules to separate the target products. However, this approach relies on a large number of trays and on high reflux; this has the disadvantages of being expensive and consuming a lot of energy [11]. Furthermore, solvent absorption is a typical approach that relies on the solubility of gas in the absorbent for separation. However, the addition of various solvents complicates the separation process by triggering solvent regeneration and solvent loss issues [12].
In comparison to the energy-driven distillation process, adsorption separation technology consumes less than one third of the energy and is simple to operate. Meanwhile, material selectivity guidance-based structural control can achieve extraordinarily high separation precision [9,14,15]. Metal–organic frameworks (MOFs) and molecular sieves are the most extensively utilized adsorption and separation materials in the field of carbon hydrocarbon separation. The diversity of their topologies and the designability of their pore structures have aroused the scientific community’s interest. MOFs are porous organic–inorganic hybrid materials with large specific surface area, multi-metal sites, customizable structure, and numerous modification methods. They could be used to precisely identify tiny differences in structural properties, such as the polarity and unsaturation of low-carbon hydrocarbon molecules, which is now a widespread issue [16]. Because of their sturdy construction, simple preparation process, and low cost, molecular sieves are widely utilized in the petrochemical industry and they also exhibit outstanding performance in the separation of low-carbon hydrocarbons [17]. They each have their own set of advantages and disadvantages. MOFs, a novel multidisciplinary porous material, are projected to boost the separation selectivity and adsorption capacity of low-carbon hydrocarbons to new heights, with enormous promise and broad possibilities. However, the cost of MOFs is prohibitively high, and more research into practical applications is required. In comparison to MOFs, molecular sieves are more stable structurally and have lower costs for raw ingredients, making them more suitable for industrial development. However, the molecular sieve’s pore size distribution, specific surface area, and surface functionalization are constrained by its design.
This article reviews the latest progress of metal–organic frameworks and molecular sieves as selective adsorbents in the separation of carbon hydrocarbon compounds, especially low-carbon hydrocarbons, with the aim of providing guidance for the development of more stable, efficient, and low-cost adsorption materials in the future. It primarily consists of methane purification (methane and nitrogen and methane and carbon dioxide), ethylene and ethane separation, and propylene and propane separation. Adsorption and separation mechanisms are explored, as well as the structure–activity relationship between pore structure, chemical properties, separation selectivity, and adsorption capacity (Figure 1). In order to upgrade the adsorption separation system, the use of high-carbon hydrocarbon isomer separation, which differs from low-carbon hydrocarbon separation, is briefly introduced. Finally, the challenges and future development of these two adsorbents in the field of separation and purification of carbon hydrocarbon compounds are explored in the context of carbon neutrality.

2. Separation Mechanism of Low-Carbon Hydrocarbons

The internal diffusion of gas in adsorbent particles is the important stage in determining whether it can be adsorbed in the separation and purification of low-carbon hydrocarbons [18]. At the same time, the structural properties of low-carbon hydrocarbons differ in terms of molecular polarity, molecule size, and degree of matching with adsorbents. To summarize, the separation mechanism of low-carbon hydrocarbons can be divided into three types [11,19,20]: (i) molecular sieve effect, which is primarily determined by molecular size and the shape, size, and pore structure of the adsorbent matching degree, as well as the mismatch of molecules for exclusion; (ii) kinetic effect, which influences separation performance based on the varied diffusion speeds of distinct gas molecules in the adsorbent; and (iii) thermodynamic equilibrium effect, particular interaction between distinct gas molecules and adsorbents, better separation selectivity can be attained by this specific recognition process.
The metal–organic framework can be built and regulated using the aforesaid separation mechanism [21,22]: (i) controlling the pore volume, pore diameter, and other pore features in the action site space; (ii) introducing a metal component, creating a strong action site in the pore, and improving the interaction between the gas molecule and the adsorbent; and (iii) adding polar components to the adsorbent’s surface. Organic/inorganic ligands can be used to functionalize metal–organic frameworks and introduce polar groups onto the pore surface.
The effect of molecular gates can be used on a wide range of molecular sieves, including carbon and zeolite molecular sieves, and the pore size can be precisely adjusted using acid, alkali, light, heat treatment, ion exchange, or loading to become close to the target molecular size for separation. Furthermore, metal ions can be used to modify or functionalize molecular sieves, giving them the features and functionality of recognizing certain molecules [23,24,25,26].

3. Purification of Methane

Methane is abundantly dispersed in nature, and it is frequently employed as a fuel (such as coal gas, natural gas, and so on) in civil industry. Because of its low C/H ratio, methane can lower CO2 concentration after combustion, making it a promising clean energy source [27]. However, due to its complicated sources, it contains impurities including N2, CO2, trace gases, and other contaminants. These impurities not only lower the calorific value of methane, but also produce pipeline pollution during transportation, which has a major impact on the environment. As a result, efficient methane gas separation can improve the economic and social benefits of practical significance.

3.1. Adsorption Separation of Methane and Nitrogen

One of the major sources of methane production is coalbed methane, a type of unconventional natural gas that is stored in coal seams and is mostly made up of CH4, N2, and CO2 [28]. The methane content in the coal seam can be greatly reduced by the injection of massive volumes of air (70% nitrogen) during underground mining, making it unsuitable for use as a fuel and chemical [29]. Every year, between 20 and 29 billion cubic meters of coalbed methane is emitted into the atmosphere, resulting in significant energy waste [30]. Methane/nitrogen separation and purification can help to alleviate energy shortages to some extent.

3.1.1. Metal–Organic Frameworks for Methane/Nitrogen Separation

It is challenging to distinguish between CH4 (3.8 Å) and N2 (3.6 Å) because of their similar molecular sizes. However, compared to N2, the adsorption binding point distance between CH4 and MOFs is shorter and the adsorption affinity is larger. In this scenario, controlling the pore size of MOFs is a good method of separation. To produce high CH4/N2 separation selectivity and high CH4 uptake, researchers frequently altered the pore size and pore surface chemical property by adjusting the ligand size, length, and polarity [31]. Chang et al. [32] synthesized a series of meshed aluminum-based MOFs with varying ligand sizes and polarity. As shown in Table 2, although Al-FUM-Me was weaker in terms of ligand polarity than Al-FUM, the existence of tiny pore size allowed it to demonstrate rational adsorption selectivity that was higher than that of many zeolite and carbon adsorption materials (8.6). The data also suggested that channel size was critical for CH4 separation, and that CH4 uptake was connected to a synergistic effect of pore size and ligand polarity, not just one of them. Furthermore, breakthrough studies (Figure 2a,b) revealed that the adsorbent exhibited good regeneration and structural stability while entirely separating CH4 and N2, serving as a model for the development of future MOFs adsorbents. Wang et al. [33] created four nickel-based diamond ligand networks from pyridine carboxylic acid bifunctional ligands of varying lengths and functional groups. The CH4/N2 selectivity (15.8) and CH4 uptake (40.8 cm3/g) of Ni(ina)2 were significantly better than those reported in the literature, which provided a new standard for the adsorption and separation of MOFs. Although the pore size of the material was similar to that of Al-FUM-Me, there were four possible CH4 binding sites in the pore and several interactions (C-H···π) with the pyridine ring adjacent, which had stronger adsorption affinity for CH4 molecule. The comparison also demonstrated how MOFs could effectively increase the separation performance by combining the molecular sieve effect with the thermodynamic equilibrium effect. Additionally, altering or tweaking the MOFs’ ligands could improve the materials’ stability while also improving their adsorption capacity. Li et al. [34] used 1,4-NDC (1,4-naphthalene dicarboxylic acid) and ADC (9,10-anthracene dicarboxylic acid) to replace BDC ligand on DMOF, and introduced non-polar aromatic ring to shield Zn-O bond so as to change the moisture resistance of DMOF, so that it could maintain stability in humid environment, and also provided new ideas for designing efficient and stable adsorption materials in the future.

3.1.2. Molecular Sieves for Methane/Nitrogen Separation

ETS-4 molecular sieve and CHA zeolite are two extensively used commercial adsorbents in the separation and purification of methane, and their tunable eight-membered ring structure and matching pore size indicate good potential in CH4 and N2 separation [35]. However, their small pore size limits their adsorption capacity and selectivity to some extent, making separation performance difficult to demonstrate [36]. Yang et al. [37] created a “doughnut-shaped” meso-micro layered K-Chabazite (CHA-type molecular sieve) nanocrystal based on this. The CH4 uptake through the newly designed pore and diffusion channel was as high as 40.12 cm3/g, indicating good industrial potential. In contrast to conventional CH4/N2 separation adsorbents, our group developed a series of Sr-ETS-4 samples employing N2 as a potent adsorbent through heat treatment and ion exchange [38]. The findings of molecular modeling and adsorption isotherms (Figure 3a–c) demonstrated that the thermal stability, nitrogen adsorption rate, and adsorption capacity of Sr-ETS-4 all increase as the degree of Sr ion exchange rises. This adsorbent was more suited to treating natural gas that contains more CH4 than N2, which helped to increase economic benefits and offers fresh approaches to the separation and purification of methane. Carbon molecular sieve (CMS) is also a useful fundamental substance for separating CH4 and N2, due to its acid, alkali, and heat resistance. The modification strategy based on thermodynamic equilibrium effect can considerably improve the adsorption and separation performance of carbon molecular sieve [39]. Yang et al. [40] found that the CH4 uptake of poly ethylene imine (PEI) modified coal-based carbon molecular sieve (CMS-P-N) prepared in nitrogen could reach 6.76 mmol/g. Zhang et al. [41] used chemical vapor deposition with benzene as the deposition agent to create a carbon molecular sieve (CMS) from cobalt and nickel impregnated carbonaceous material precursors, and its CH4/N2 kinetic selectivity could reach 35.26. Although the effect of the aforementioned modification method was remarkable, as adsorption time increases, some micropores may be covered and blocked by deposited coke, resulting in poor adsorption performance, which was also an urgent problem to be solved in the future modification of carbon molecular sieves.

3.2. Adsorption Separation of Methane and Carbon Dioxide

Biogas and landfill gas are composed of 35–70% CH4 and 15–40% CO2 [42]. Decarbonization is required to achieve high purity CH4 (>97%). However, the decarbonization cost must account for 20–30% of the entire cost, which is incompatible with large-scale production and contradicts the promoted “double-carbon” strategy, which is not conducive to industry and energy structure adjustment. Exploring an efficient and cost-effective way to decarbonization is thus an urgent problem that must be addressed [43].

3.2.1. Metal–Organic Frameworks for Methane/Carbon Dioxide Separation

Designing a porous material with a diameter between CO2 and CH4 is one of the most successful ways to separate them since there is a subtle variation in their molecular dynamics diameters (CO2: 3.3 Å to CH4: 3.8 Å). Additionally, CO2 has a greater contact with the surface of MOFs due to its increased quadrupole moment [44,45]. Under these conditions, it is possible to successfully encourage the separation of CO2 and CH4 by improving the contact between the guest molecule and its host spine [46]. These techniques, including adding high-density metal open sites (OMSs), doping polar groups, and altering the acidity and polarity of porous environments to promote host–guest interaction have shown positive results based on the aforementioned molecular sieve effect and thermodynamic equilibrium [47]. Chen et al. [48] altered Ni-MOF-74 by adding -CH3, -NH2, -F, and -O-Li groups. DFT and GGA simulations demonstrated that the addition of the-O-Li group in LI-O-Ni-MOF-74 modified the electrostatic potential gradient distribution surrounding the material, which impacted the interaction between the material and CO2 with a high quadrupole moment. Furthermore, the microstructure study revealed that the specific surface area and pore volume of Li-O-Ni-MOF-74 were 162.03% and 86.42% greater, respectively, than those of Ni-MOF-74, demonstrating that Li-O-Ni-MOF-74 had a higher adsorption capability. Zhang et al. [49] created a nano cage-based Fe-MOF with open metal sites (OMSs) and ideal pore space, with a distance of 3.8 Å between the outer cage and the open window hole in the inner cage. Each cage was also linked to six adjacent cages along the a-axis direction (as shown in Figure 4), and its pore screening effect had unique recognizability for small molecule gases, resulting in good methane and carbon dioxide separation performance, with CO2/CH4 selectivity reaching 16.7. Zheng et al. [50] investigated the high porosity and excellent water stability of MOFs while developing the subject–object interaction, and a water-stable acyl amide functionalized MOF (HNUST-8) with these three features was created. The breakthrough experimental results showed that CH4 was detected in approximately 8 min and CO2 in about 22 min, demonstrating the practicability of CO2/CH4 adsorption and separation while maintaining MOF stability.

3.2.2. Molecular Sieves for Methane/Carbon Dioxide Separation

In the separation of methane and carbon dioxide, zeolite molecular sieves and carbon molecular sieves are widely used as adsorbents. While zeolite molecular sieves, such as 13X and zeolite 5A, demonstrate superior separation performance under low pressure conditions (below 0.2 MPa), carbon molecular sieves offer the advantage of lower regeneration energy consumption. Conversely, under high pressure conditions (above 0.2 MPa), carbon molecular sieves exhibit better enhancement of methane purity, making them more favorable for the separation of these two gases [51,52]. A key strategy for CH4 and CO2 separation using carbon molecular sieves is the regulation of pore structure [53]. Liang et al. [54] utilized coconut shell carbonization material as a raw material and prepared carbon molecular sieves through a steam activation process. The study revealed that under the most suitable conditions, the CO2/CH4 equilibrium adsorption separation coefficient could reach 10.27. However, as the activation reaction was intensified, the microporous walls were further burned, resulting in the formation of macropores and a decrease in CH4 adsorption. This phenomenon further emphasizes the significance of pore structure in the separation performance of carbon molecular sieves. Kaya et al. [55] produced carbon molecular sieves from defatted waste coffee grounds using ZnCl2 and benzene, resulting in a pore size distribution of approximately 0.343 nm. Orlando et al. [56] utilized polyurethane (PU) as a carbon source to produce activated carbon for the preparation of carbon molecular sieves, with pore sizes smaller than 0.37 nm, matching the separation of size for CO2 and CH4. These methods of preparing carbon molecular sieves through the recycling and utilization of waste materials not only provide innovative approaches for methane decarbonization but also hold significant implications for mitigating global carbon emissions. However, further research is required to investigate the stability of adsorbent materials and their practical applications before they can be considered suitable for large-scale industrial separation operations.

4. Adsorption Separation of Olefins and Alkanes

Olefins play a vital role as bulk raw materials in the production of various high-value-added chemicals, including polyethylene, polyvinyl chloride, and polypropylene. The global market value of ethylene and propylene is projected to reach USD 47.58 billion by 2023, with a continuous increase in demand [57]. However, the industrial production process of ethylene and propylene inevitably introduces impurities such as ethane and propane [58]. The separation of olefins and paraffins, characterized by their remarkably similar molecular structures and sizes, is widely recognized as one of the seven separations that have the potential to revolutionize the world. It is worth noting that the industrial separation of ethylene and ethane alone accounts for 0.3% of the world’s energy consumption [59]. Therefore, the development of suitable and efficient adsorbent separation materials holds immense strategic value and significance in driving energy-saving initiatives and achieving carbon peak and carbon neutrality [60].

4.1. Adsorption Separation of Ethylene and Ethane

Ethylene, frequently referred to as the industry’s “lifeblood” is essential to the petrochemical industry. In 2021, the rise of the new energy industry has further stimulated the ethylene demand market [61]. The two main industrial processes for producing ethylene at the moment are catalytic cracking of light hydrocarbons and steam cracking of naphtha [59]. However, these procedures are not entirely effective and frequently lead to the appearance of ethane impurities, which affect the processing of future polymer products and jeopardize the purity of ethylene. As a result, it is thought necessary to purify and separate ethylene and ethane.

4.1.1. Metal–Organic Frameworks for Ethylene/Ethane Separation

The efficacy of metal–organic frameworks and molecular sieves in separating ethylene and ethane from other adsorption materials is excellent, as demonstrated in Table 3. Metal–organic frameworks, backed by their high surface area and pore volume, ordered crystal structure, and tunable pore environment, demonstrate significant potential for gas storage and separation, particularly with the combination of high adsorption capacity and separation selectivity. The existence of a π-π double bond in ethylene, which ethane lacks, allows it to create strong connections with open metal cation sites despite their structural similarity. Unsaturated coordinating metals are frequently added to MOF materials to improve the separation selectivity of C2H4/C2H6 [62,63,64]. AgM-DS, a more stable Ag(I) π-complex adsorbent, was created by Yin et al. [65] by mixing AgNO3 with the traditional metal–organic framework (MOF) MIL-101 via a dual-solvent method (as shown in Figure 5). Because MIL-101 has a smaller pore volume than the AgNO3 solution produced by this method, capillary forces may induce it to bind to the hydrophilic nanocavities of MOFs, so limiting the metal sites. This leads to a C2H4 uptake that can reach 75.5 cm3/g. The pore structure of MOFs can be altered in order to improve the separation efficiency of C2H4/C2H6.
Bao et al. [66] produced a number of mixed metal–organic framework materials based on terephthalate esters, which deviates from the conventional research link between kinetic diameter and pore structure. The 3D-linked zigzag channels in M-terephthalate esters (M = Ni, Mg, and Co) have pore diameters that are appropriate for separating ethylene from ethane based on molecular cross-sectional size rather than kinetic diameter. IAST selectivity for C2H4/C2H6 can be up to 52. Additionally, the majority of adsorbents preferentially absorb ethylene, but the co-adsorption phenomena make the desorption process energy intensive. Hence, the creation of porous materials that emphasize the separation of ethane and demonstrate superior performance has significant potential and has emerged as a recent focal point in research. Zeng et al. [67] took into consideration the impact of humidity on the adsorbent and devised JNU-2, which showcased great separation performance even in humid environments while prioritizing C2H6 adsorption. Di et al. [70] fabricated a one-dimensional hexagonal nonpolar pore surface MOF material, FJI-H11-Me (des), comprised aromatic rings and alkyl groups (as depicted in Figure 6d). Breakthrough experimental results (Figure 6a–c) reveal that FJI-H11Me (des) sustains excellent separation performance under varying gas flow rates, temperatures, and relative humidity conditions. It can produce 99.95% pure polymer-grade C2H4 in a single step, opening up new perspectives and separation methods for ethylene and ethane in the future.

4.1.2. Molecular Sieves for Ethylene/Ethane Separation

In the separation of ethylene and ethane, zeolites primarily utilize thermodynamic equilibrium separation mechanisms (π-complexation) and zeolite sieving mechanisms (pore structure regulation) [26]. Among them, LTA-type and FAU-type zeolites have shown high separation performance [71,72]. In recent years, researchers have often employed ion exchange methods to modify the structural properties of zeolites for further improvement of olefin/paraffin separation. Ilonavan et al. [73] conducted ethylene adsorption tests using single-metal-modified zeolites (K-A, Na-A, and Ca-A) with different ion radii, demonstrating the influence of pore size on adsorption performance. Liu et al. [68] prepared Ag-Ca-4A samples by adjusting 4A zeolite with bimetallic ions. This sample combines thermodynamic equilibrium and zeolite mechanisms, exhibiting precise size matching and specific recognition sites, with a high C2H4 uptake of 3.7 mmol/g.
Our group used tandem fixed beds to link two ion-exchanged LTA zeolites for the separation of C3H6 and C2H4 from a four-component gas mixture (C2H4/C2H6/C3H6/C3H8) (as shown in Figure 7a). This connection method first sets up two adsorbers (filled with Ni-5A and Ag-Cd-4A, respectively) in series, and introduces a mixture of ethylene, propylene, and alkane to adsorb propylene and ethylene; Afterwards, these two adsorbers were set in parallel and desorption agents (CO2) were introduced to obtain propylene or ethylene single component products. The research group created Ag-Ca-4A for the separation of ethylene and ethane by using Cd2+ as a transition metal ion with a similar ion radius to Ca2+ but a greater affinity for olefins [74]. By adding a trace of Cd2+ to the 4A zeolite, the pore size rose from 3.95 Å in 4A to 4.05 Å in Cd-A. Furthermore, because Ag+ has a greater ion radius than Na+ (1.26 Å vs. 0.95 Å), the low concentration of Ag+ doping in Cd-A resulted in an enlarged pore size of 4.00 Å in the bimetallic-doped Ag-Cd-4A (as shown in Figure 7b). When combined with DFT computations, Ag+ causes C2H4 molecules to expand, allowing selective access into pore channels with diameters less than 4 Å, resulting in separation effects. As shown in Figure 7c, Ag-Cd-4A has greater Henry selectivity and adsorption capacity than previously reported adsorbents. In terms of kinetic separation, Bereciartua et al. [69] found a flexible pure silica zeolite called ITQ-55 with a heart-shaped cage architecture (as shown in Figure 7d), which has a smaller kinetic diameter than ethylene and ethane and demonstrates strong affinity with ethylene. According to AIMD simulations, the addition of C2H4 molecules results in the distortion of the pore windows, which range in size from 2.38 Å to 3.08 Å, leaving only ethylene free to “open” the ITQ-55 pore mouth and speed up diffusion. This method of separation avoids co-adsorption events while displaying the flexibility of the framework and the mechanism of diffusion kinetics. It has important ramifications for ethylene and ethane separation.

4.2. Adsorption Separation of Propylene and Propane

One of the five main commodity resins, polypropylene, requires high purity (>99.5%) propylene as its synthesis raw ingredient. Propane, the primary contaminant in the synthesis of propylene, has a boiling point that is quite close to that of propylene (just 5.3 K apart) [9]. Traditional low-temperature distillation techniques need a large amount of energy, which is out of step with the dual-carbon strategy’s present trend. Therefore, it is critical to discover an alternate technique that is effective and energy saving.

4.2.1. Metal–Organic Frameworks for Propylene/Propane Separation

Pore volume and open metal sites are critical elements influencing MOF separation performance in propylene and propane separation. To improve separation performance, researchers frequently change MOFs from a thermodynamic or kinetic standpoint. The equilibrium-kinetics synergistic method has become a key research focus since Wang et al. [75] reported the high separation performance of Co-MOF-74 in propylene and propane in 2019. Yang et al. [76] created Cu BTC (BTC = benzene-1,3,5-tricarboxylate) by using particular π-Cu bonds to bind strongly with unsaturated metal sites. The C3H6/C3H8 adsorption selectivity is based on the ultra-microporous structure and can achieve 63.322. Ding et al. [77] reported a phosphoric anion-functionalized metal–organic framework, ZnAtzPO4 (Atz = 3-amino-1,2,4-triazolate), which exhibits a synergistic effect between equilibrium and kinetics for effective separation of propylene and propane (Figure 8a–c). The capture capacity for propylene remains almost unchanged after five cycles, indicating a lower-cost regeneration capability, which is of great significance for industrial applications.
Furthermore, MOFs with dynamic structures, characterized by unique “molecular gate” effects, have broad prospects for improving the separation selectivity of propylene and propane. These MOF materials can dynamically change their structures from closed non-microporous phases to open porous phases under external stimuli such as temperature and pressure. Zeng et al. [78] suggested a new screening technique (orthogonal array dynamic screening) and created a novel MOF material (JNU-3) based on the dynamic molecular sieve effect. The combination of adsorbent stiffness and dynamic molecular sieve structure, as well as interconnecting dynamic molecular pockets and one-dimensional channels, offers high separation capacity and quick adsorption–desorption kinetics. The adsorption capacity is 58.6 cm3/g at 1 bar pressure, and the IAST selectivity is 513, greatly above that of similar propylene and propane separation materials. Tan et al. [79] achieved molecule recognition and separation by inserting a temperature-sensitive OMe group into the narrow bottleneck of the ultra-microporous aluminum-based MOF CAU-10. With temperature changes, the effective pore size of the intelligent molecular gate can be continuously adjusted between 3.6 Å and 5.2 Å, covering the size range of commercially important gas molecules. The selectivity for C3H6/C3H8 reaches 174 at room temperature and pressure.

4.2.2. Molecular Sieves for Propylene/Propane Separation

Zeolites are currently showing promise in the separation of propylene and propane, such as LTA-type [80], FAU-type [81], and CHA-type [82]. Because of their simple and transparent structure, as well as their low cost, LTA-type zeolites have benefits in separation adsorption. Obtaining both high uptake and outstanding separation selectivity, on the other hand, is a prominent research area in the field of propylene and propane separation employing LTA-type zeolites [83,84]. Wang et al. [85] used three-dimensional electron diffraction to study the crystal structure of LTA zeolite with ordered silanols (OSs), as shown in Figure 9a. The findings revealed that OSs were weakly interacting sites with propylene molecules and are related to the zeolite’s elliptical eight-membered ring pore structure. This research sheds fresh light on the link between structure and performance at the atomic level, bringing novel insights for the creation of high-performance separation materials. Furthermore, ion exchange and surface chemical modification procedures are useful options for improving the selectivity and adsorption capacity of LTA-type zeolites. Zhou et al. [86] synthesized Ca-Ag-LTA by combining Ag+ and Ca2+ in an LTA-type zeolite. The study demonstrated that precise exchange patterns and metal cation ratios can give zeolites good pore sizes and distinctive recognition sites, resulting in a C3H6 uptake of 2.37 mmol/g. Mohammed et al. [87] employed ion exchange to assess the quasi-differential heat of adsorption at 303 K and up to 5 bar for the first time, and used several models to correlate isotherm data to calculate the adsorbent’s IAST selectivity for propylene. Figure 9b illustrates the experimental results, and the selectivity of CaNa-LTA for distinguishing propylene/propane IAST is infinitely close to 15 at zero pressure limit. By using methods such as molecular layer deposition (MLD) or atomic layer deposition (ALD), average pore mouth size of 5A zeolite was reduced to be between propane and propylene, therefore, favors propylene separation from propane both thermodynamically and kinetically achieving precise separation. Song et al. [88] created a composite molecular sieve adsorbent by depositing an ultra-thin, porous TiO2 coating on the outer surface of 5A molecular sieve, which increased the adsorption selectivity of C3H6/C3H8 substantially. Zhou et al. [89] coated zeolite 5A with n-octadecylphosphonic acid (ODPA), which increased the kinetic selectivity of C3H6/C3H8 substantially over untreated 5A.

5. Adsorptive Separation of Isomers in High-Carbon Hydrocarbons (C6–C8) Using Metal–Organic Frameworks and Molecular Sieves

Adsorption separation of low-carbon hydrocarbons (C1–C3) utilizing metal–organic frameworks and molecular sieves has made significant progress. The separation of C6–C8 hydrocarbon isomers, on the other hand, is still in its early stages of investigation. High-carbon hydrocarbons have various isomers, unlike low-carbon hydrocarbons; one current research focus in the field of high-carbon hydrocarbon separation is the recovery of isomers from straight-chain alkanes to suit market demands [90]. C6 (hexane), in particular, holds a key place in the petrochemical sector as a basic raw material in the gasoline industry. The presence of up to five isomers of C6 hydrocarbons limits the petrochemical industry’s future expansion to some extent [91]. Based on three methods of separation that have been thoroughly researched, research has revealed that metal–organic frameworks and molecular sieves can efficiently separate straight-chain alkanes. Furthermore, studying the adsorptive separation systems of high-carbon hydrocarbon isomers can help to improve the material systems for hydrocarbon adsorptive separation, allowing for the prediction and control of adsorbent separation performance and the attainment of optimal separation effects.

5.1. Metal–Organic Frameworks for High-Carbon Hydrocarbons (C6–C8) Isomer Separation

In contrast to kinetic and thermodynamic equilibrium effects, the sieving effect of molecular sieves is a low-energy and highly selective method for efficiently separating linear, mono-branched, and di-branched hexane isomers utilizing metal–organic frameworks. Metal–organic frameworks can tailor pore volume, pore size, and other pore architectures via mixed ligand pore engineering by changing connections or ligands, resulting in efficient separation of gas molecules with greater kinetic diameters [92]. Yu et al. [93] used a mixed ligand method to create two novel CAU-10 series MOFs, CAU-10-H, and CAU-10-Br. These MOFs may tolerate n-hexane (n-Hex) and 3-methylpentane (3-MP) while rejecting 2,2-dimethylbutane (2,2-DMB) by ligand replacement and precision mixing, allowing for successful separation of mono-branched and di-branched hexane isomers. Guo et al. [94] designed a unique HIAM-410LI material with an FTW network using 12 Zr6 nodes and 4 tetracarboxylate linkers on HIAM-410. This MOF not only has high hydrothermal stability, but also enables pore structure customization for perfect separation of mono-branched and di-branched hexane isomers. Unlike Guo et al., Bhajan et al. [95] expanded the dimensionality of the connectors from 2D to 3D to adjust pore size with sub-angstrom precision, reducing the likelihood of low separating efficiency. due to connector rotation. By hybridizing two organic ligands, 2-methylimidazole (2-MeIM) and formaldehyde-2-methylimidazole (OHC-IM), into the metal–organic framework ZIFs, our team [96] successfully created a series of hybrid connector ZIF-8-90 samples (as shown in Figure 10).

5.2. Molecular Sieves for High-Carbon Hydrocarbons (C6–C8) Isomer Separation

In the petrochemical sector, 5A molecular sieves are frequently used as adsorbent materials for the separation of isomers of high-carbon hydrocarbons, especially for the isolation of n-alkanes from hydrocarbon mixtures [97]. Conventional approaches for creating 5A molecular sieves, however, have poor separation and diffusion performance. In light of this, Liu et al. [98] used TPOAC as a template to create a variety of multi-level porous molecular sieves containing micropores and mesopores. The diffusion rate of high-carbon n-alkanes (pentane and hexane) in these molecular sieves has increased by an order of magnitude in comparison to 5A molecular sieves made using standard techniques, according to experimental results, making it easier to separate them from isomers. Furthermore, a binder must be added during the synthesis process to provide the molecular sieve the requisite mechanical strength. However, the performance of adsorption can be adversely affected by the presence of a binder to some extent. Thus, through in-situ hydrothermal modification of premade silica gel precursors, our team created binder-free 5A molecular sieves (as depicted in Figure 11). Following comparisons with 5A molecular sieves that contain a binder, its saturation adsorption capacity for n-alkanes increased by 25% to 35%, breakthrough adsorption quantity increased by 115% to 130%, and the mass transfer zone’s length was greatly decreased [99,100]. Although molecular sieve materials have intrinsic stiffness limitations, little diversity, and less attractive adsorption performance improvement opportunities than MOFs, they are extensively utilized in industry due to their reliable hydrothermal performance and affordable price.

6. Perspectives

6.1. Challenges

(1)
Several effective adsorption separation materials have been created recently, although they have only been used in lab settings. Even if their hydrothermal stability is still subpar, their separation performance will be significantly compromised in humid and hot application scenarios. Furthermore, the use of organic solvents and precious metals for modification raises the cost of adsorbents, posing small environmental hazards. This makes large-scale manufacturing difficult. The research on the concept of carbon neutrality and the study on recyclability during the gas adsorption process still have certain gaps, nevertheless.
(2)
Even when achieving large adsorption capacity or high separation selectivity, it is currently not possible to accomplish both huge adsorption capacity and high separation selectivity in carbon hydrocarbon compounds separation and purification. More in-depth research should be performed on the transport mechanism and separation performance of molecules in MOFs and molecular sieves in order to improve material design, increase separation efficiency, design adsorbents with both a large adsorption capacity and high selectivity, and achieve more effective, energy-saving, and environmentally friendly low-carbon hydrocarbon separation.

6.2. Opportunities

(1)
Hierarchical combination design of different adsorbent materials
Currently, the majority of adsorbent materials are carbon and crystal materials, albeit separation performance is typically subpar and much of their research is focused on single separation scenarios. However, if we can combine different adsorbent material types into a single material at a time to generate multifunctional adsorbent materials, the separation performance might surprise improve. Furthermore, the drawbacks of a single adsorbent loading can be improved by carefully combining MOFs or molecular sieves with different functionalities on a single fixed bed.
(2)
Computer-aided data processing and simulation for screening adsorbents
Technology advancements today make it possible to build enormous computer databases with tens of thousands of distinct adsorbents, faithfully recreating their microstructure, including their pore size, structure, and other features. When employing big data analysis to further screen and foresee suitable adsorbent materials, the performance of adsorption separation may be enhanced by creatively combining the structure–effect link with theoretical calculations.

7. Conclusions

An eco-friendly, low-carbon economy has emerged as the main trend for future development under the current “carbon neutrality” policy. One of the essential processes for the petrochemical industry’s sustainable development and a major route to a low-carbon economy is the effective separation and purification of carbon hydrocarbon compounds. Adsorption separation is a cutting-edge technique for separating low-carbon hydrocarbons, and its benefits of low investment, low energy use, and high efficiency play a vital part in this process as well as in the transformation of the energy structure. This article discusses the most recent scientific advances in separation and purification of carbon hydrocarbon compounds utilizing two of the most prominent adsorbent materials, metal–organic frameworks and molecular sieves. And combine existing separation mechanisms (thermodynamic equilibrium effect, molecular sieve effect, and kinetic effect) to investigate the relationship between structural composition and properties, as well as separation performance and mechanism, in order to provide reference opinions for the design of new adsorption materials. We believe that with interdisciplinary collaboration, the separation performance of carbon hydrocarbon compounds can be improved in the near future.

Author Contributions

Conceptualization, H.S.; methodology, Y.Z. and Y.W.; writing—original draft preparation, Y.Z.; data curation, Y.Z. and P.L.; and writing—review and editing, Y.Z., P.L., Q.Z. and H.S. All authors have read and agreed to the published version of the manuscript.

Funding

This work is supported by the National Natural Science Foundation of China (Grant 22178109 and 21878097) and the Natural Science Foundation of Shanghai (Grant 21ZR1417700).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

H.S. acknowledges the institutional funds from the National Natural Science Foundation of China and the Natural Science Foundation of Shanghai.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Research abstract of adsorption separation materials in the field of low-carbon hydrocarbon purification and separation.
Figure 1. Research abstract of adsorption separation materials in the field of low-carbon hydrocarbon purification and separation.
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Separations 10 00543 g002
Figure 2. Breakthrough experimental results of Al-FUM-Me under real environmental conditions. (a) CH4/N2 (20/80) mixture with a humidity of 70% RH and (b) CH4/N2/O2 (20/64/16) mixture under ambient conditions. Republished from ref. [32], copyright (2022), with permission from ACS.
Figure 2. Breakthrough experimental results of Al-FUM-Me under real environmental conditions. (a) CH4/N2 (20/80) mixture with a humidity of 70% RH and (b) CH4/N2/O2 (20/64/16) mixture under ambient conditions. Republished from ref. [32], copyright (2022), with permission from ACS.
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Figure 3. Structural models of ETS-4 and simulated adsorption isotherms of nitrogen on Na-ETS-4 and Sr-ETS-4. (a) Na-ETS-4, (b) Sr-ETS-4, and (c) simulated adsorption isotherms of nitrogen on Na-ETS-4 and Sr-ETS-4 [38].
Figure 3. Structural models of ETS-4 and simulated adsorption isotherms of nitrogen on Na-ETS-4 and Sr-ETS-4. (a) Na-ETS-4, (b) Sr-ETS-4, and (c) simulated adsorption isotherms of nitrogen on Na-ETS-4 and Sr-ETS-4 [38].
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Separations 10 00543 g004
Figure 4. Schematic diagram of nanocage-based Fe–MOF structure. Republished from ref. [49], copyright (2020), with permission from ACS.
Figure 4. Schematic diagram of nanocage-based Fe–MOF structure. Republished from ref. [49], copyright (2020), with permission from ACS.
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Separations 10 00543 g005
Figure 5. Decoration of AgNO3 on MIL-101 using three methods: DS (upper route), wet-impregnation (middle route), and solid-phase grinding (lower route). Republished from ref. [65], copyright (2020), with permission from ACS.
Figure 5. Decoration of AgNO3 on MIL-101 using three methods: DS (upper route), wet-impregnation (middle route), and solid-phase grinding (lower route). Republished from ref. [65], copyright (2020), with permission from ACS.
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Separations 10 00543 g006
Figure 6. Experimental column breakthrough curves for a C2H6/C2H4 (v/v, 1/99) mixture in an absorber bed packed with FJI-H11-Me(des): (a) an experiment with FJI-H11-Me(des) under different gas flow rates, (b) the experiment of FJI-H11-Me(des) under different temperatures with gas flow rate of the 1 mL/min, (c) The experiment of FJI-H11-Me(des) under different relative humidity with gas flow rate of the 1 mL/min, and (d) FJI-H11-Me(des) one-dimensional hexagonal non-polar pore structure diagram. Republish from ref. [70], copyright (2022), with permission from Wiley.
Figure 6. Experimental column breakthrough curves for a C2H6/C2H4 (v/v, 1/99) mixture in an absorber bed packed with FJI-H11-Me(des): (a) an experiment with FJI-H11-Me(des) under different gas flow rates, (b) the experiment of FJI-H11-Me(des) under different temperatures with gas flow rate of the 1 mL/min, (c) The experiment of FJI-H11-Me(des) under different relative humidity with gas flow rate of the 1 mL/min, and (d) FJI-H11-Me(des) one-dimensional hexagonal non-polar pore structure diagram. Republish from ref. [70], copyright (2022), with permission from Wiley.
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Figure 7. Successively separating C3H6 and C2H4 from C2H4/C2H6/C3H6/C3H8 mixture and ITQ-55 cardioid cage topology: (a) graphical abstract; (b) pore size distributions of 4A, 0.005Ag-4A, 0.05Cd-4A, 0.0075Cd-4A, 0.01Cd-4A, and Ag/Cd-4A.; and (c) adsorption capacities and Henry selectivities of C2H4 and C3H6 on Ag/Cd-4A, and previously reported adsorption materials. Republished from ref. [74], copyright (2023), with permission from Elsevier. (d) ITQ-55 cardioid cage topology. Republish from ref. [69], copyright (2017), with permission from AAAS.
Figure 7. Successively separating C3H6 and C2H4 from C2H4/C2H6/C3H6/C3H8 mixture and ITQ-55 cardioid cage topology: (a) graphical abstract; (b) pore size distributions of 4A, 0.005Ag-4A, 0.05Cd-4A, 0.0075Cd-4A, 0.01Cd-4A, and Ag/Cd-4A.; and (c) adsorption capacities and Henry selectivities of C2H4 and C3H6 on Ag/Cd-4A, and previously reported adsorption materials. Republished from ref. [74], copyright (2023), with permission from Elsevier. (d) ITQ-55 cardioid cage topology. Republish from ref. [69], copyright (2017), with permission from AAAS.
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Figure 8. Schematic illustration of the crystal structure of ZnAtzPO4 and penetration curve of molar C3H6/C3H8 mixture of 298 K and 1.0 bar on ZnAtzPO4: (a) local environment and aperture size of the bottleneck-like pore window (Color code: C, gray −40%; H, gray −25%; N, light blue; Zn, yellow; P, aqua; O, pink), (b) cyclic breakthrough test of C3H6/C3H8 (50:50) mixture, and (c) five-cycle test experiment. Republish from Ref. [77], Copyright (2020), with permission from Wiley.
Figure 8. Schematic illustration of the crystal structure of ZnAtzPO4 and penetration curve of molar C3H6/C3H8 mixture of 298 K and 1.0 bar on ZnAtzPO4: (a) local environment and aperture size of the bottleneck-like pore window (Color code: C, gray −40%; H, gray −25%; N, light blue; Zn, yellow; P, aqua; O, pink), (b) cyclic breakthrough test of C3H6/C3H8 (50:50) mixture, and (c) five-cycle test experiment. Republish from Ref. [77], Copyright (2020), with permission from Wiley.
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Figure 9. IAST calculation chart of propylene with different adsorbents and structure of ordered silicon hydroxyl pure silicon zeolite under three-dimensional electron diffraction: (a) structure of ordered silicon hydroxyl pure silicon zeolite under three-dimensional electron diffraction. Republished from ref. [85], copyright (2023), with permission from ACS; (b) IAST calculations for the adsorption, on MgNa-LTA and (c) IAST calculations for the adsorption, on Si-LTA. Republished from ref. [87], copyright (2022), with permission from Elsevier.
Figure 9. IAST calculation chart of propylene with different adsorbents and structure of ordered silicon hydroxyl pure silicon zeolite under three-dimensional electron diffraction: (a) structure of ordered silicon hydroxyl pure silicon zeolite under three-dimensional electron diffraction. Republished from ref. [85], copyright (2023), with permission from ACS; (b) IAST calculations for the adsorption, on MgNa-LTA and (c) IAST calculations for the adsorption, on Si-LTA. Republished from ref. [87], copyright (2022), with permission from Elsevier.
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Figure 10. Structural schematic diagram of ZIF-8-90. Republished from ref. [96], copyright (2023), with permission from ACS.
Figure 10. Structural schematic diagram of ZIF-8-90. Republished from ref. [96], copyright (2023), with permission from ACS.
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Figure 11. Schematic diagram of unbonded 5A zeolite synthesis. Republished from ref. [99], copyright (2019), with permission from ACS.
Figure 11. Schematic diagram of unbonded 5A zeolite synthesis. Republished from ref. [99], copyright (2019), with permission from ACS.
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Table 1. Physical properties of C1~C3 light hydrocarbons [9,13].
Table 1. Physical properties of C1~C3 light hydrocarbons [9,13].
AdsorbateBoiling
Point (K)		Dynamic
Diameter (Å)		Dipole Moment (×1018/ ESU cm)Polarizability
(×10−25/cm3)
CH4111.63.8025.9
CO2194.63.302.76
N277.153.601.53
C2H6184.64.4044.3
C2H4169.44.2042.5
C3H8231.04.3–5.10.08462.9
C3H6225.54.70.36662.9
Table 2. Polarity and pore size of aluminum-based MOFs materials [32].
Table 2. Polarity and pore size of aluminum-based MOFs materials [32].
NameAl-BPDCAl-NDCAl-BDCAl-FUM-MeAl-FUM
Ligand StructureSeparations 10 00543 i001Separations 10 00543 i002Separations 10 00543 i003Separations 10 00543 i004Separations 10 00543 i005
Ligand Polarity
(kcal/mol)		10.68310.72911.46712.58514.291
Aperture (Å)11.59.28.25.06.3
CH4 Uptake
(cm3/g)		5.910.8615.9827.1920.44
CH4/N2
Selectivity		2.23.13.48.65.1
Table 3. Summary of some adsorption separation materials in C2H4 and C2H6 separation at 100 kPa.
Table 3. Summary of some adsorption separation materials in C2H4 and C2H6 separation at 100 kPa.
Adsorbents T (K)C2H4 Uptake (mmol/g)C2H6
Uptake (mmol/g)		SelectivityQst
(C2H4/C2H6,
kJ mol−1)		Ref.
C2H4/C2H6 C2H6/C2H4
MOFsUTSA-2802982.50.098>10,000-34.1/-[47]
Fe-MOF-743186.245.1913.6-45/25[47]
ZIF-72981.902.00-2.5-/27[47]
MUF-15 2984.154.69-1.96-/29.2[47]
Co-gallate2983.370.3152-44/-[66]
JNU-22983.624.11-1.6-/30[67]
ZeolitesDDR 3030.940.97-1.49-/25[62]
Silicalite-1 3051.842.00-2.90-/28.9[62]
Ag-Ca-4A2983.7-17,568--[68]
ITQ-553031.30 a0.80 b6.36--[69]
COFsCOF-1022981.731.90 1.48-/28.9[62]
COF-320 2981.792.35-1.52-/28.0[62]
CTF-DCTC-4002981.681.82-1.04-/22.7[62]
Carbon-Based AdsorbentsMC-S-Ag-32983.42.62.4--[17]
C-PDA-32985.16.61.83-22/22[17]
CuCl(8.0)/AC 3032.60.769.4--[17]
MGA-750–3 2985.687.02-2.0028.4/-[62]
FAU-ZTC3033.864.71-1.4825.0/-[62]
a. Pressure at 45 kPa; b. Pressure at 60 kPa.
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Zhou, Y.; Li, P.; Wang, Y.; Zhao, Q.; Sun, H. Progress in the Separation and Purification of Carbon Hydrocarbon Compounds Using MOFs and Molecular Sieves. Separations 2023, 10, 543. https://doi.org/10.3390/separations10100543

AMA Style

Zhou Y, Li P, Wang Y, Zhao Q, Sun H. Progress in the Separation and Purification of Carbon Hydrocarbon Compounds Using MOFs and Molecular Sieves. Separations. 2023; 10(10):543. https://doi.org/10.3390/separations10100543

Chicago/Turabian Style

Zhou, Yousheng, Peicheng Li, Yifan Wang, Qiyue Zhao, and Hui Sun. 2023. "Progress in the Separation and Purification of Carbon Hydrocarbon Compounds Using MOFs and Molecular Sieves" Separations 10, no. 10: 543. https://doi.org/10.3390/separations10100543

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