Energy-intensive Cryogenic Distillation Research Articles

Metal-organic frameworks as O2-selective adsorbents for air separations.

Oxygen is a critical gas in numerous industries and is produced globally on a gigatonne scale, primarily through energy-intensive cryogenic distillation of air. The realization of large-scale adsorption-based air separations could enable a significant reduction in associated worldwide energy consumption and would constitute an important component of broader efforts to combat climate change. Certain small-scale air separations are carried out using N2-selective adsorbents, although the low capacities, poor selectivities, and high regeneration energies associated with these materials limit the extent of their usage. In contrast, the realization of O2-selective adsorbents may facilitate more widespread adoption of adsorptive air separations, which could enable the decentralization of O2 production and utilization and advance new uses for O2. Here, we present a detailed evaluation of the potential of metal–organic frameworks (MOFs) to serve as O2-selective adsorbents for air separations. Drawing insights from biological and molecular systems that selectively bind O2, we survey the field of O2-selective MOFs, highlighting progress and identifying promising areas for future exploration. As a guide for further research, the importance of moving beyond the traditional evaluation of O2 adsorption enthalpy, ΔH, is emphasized, and the free energy of O2 adsorption, ΔG, is discussed as the key metric for understanding and predicting MOF performance under practical conditions. Based on a proof-of-concept assessment of O2 binding carried out for eight different MOFs using experimentally derived capacities and thermodynamic parameters, we identify two existing materials and one proposed framework with nearly optimal ΔG values for operation under user-defined conditions. While enhancements are still needed in other material properties, the insights from the assessments herein serve as a guide for future materials design and evaluation. Computational approaches based on density functional theory with periodic boundary conditions are also discussed as complementary to experimental efforts, and new predictions enable identification of additional promising MOF systems for investigation.

Nitrogen Rejection from Methane via a "Trapdoor" K-ZSM-25 Zeolite.

Nitrogen (N2) rejection from methane (CH4) is the most challenging step in natural gas processing because of the close similarity of their physical-chemical properties. For decades, efforts to find a functioning material that can selectively discriminate N2 had little outcome. Here, we report a molecular trapdoor zeolite K-ZSM-25 that has the largest unit cell among all zeolites, with the ability to capture N2 in favor of CH4 with a selectivity as high as 34. This zeolite was found to show a temperature-regulated gas adsorption wherein gas molecules' accessibility to the internal pores of the crystal is determined by the effect of the gas-cation interaction on the thermal oscillation of the "door-keeping" cation. N2 and CH4 molecules were differentiated by different admission-trigger temperatures. A mild working temperature range of 240-300 K was determined wherein N2 gas molecules were able to access the internal pores of K-ZSM-25 while CH4 was rejected. As confirmed by experimental, molecular dynamic, and ab initio density functional theory studies, the outstanding N2/CH4 selectivity is achieved within a specific temperature range where the thermal oscillation of door-blocking K+ provides enough space only for the relatively smaller molecule (N2) to diffuse into and through the zeolite supercages. Such temperature-regulated adsorption of the K-ZSM-25 trapdoor zeolite opens up a new approach for rejecting N2 from CH4 in the gas industry without deploying energy-intensive cryogenic distillation around 100 K.

Double-salt ionic liquid derived facilitated transport membranes for ethylene/ethane separation

Ethylene/ethane separation is one of the key industrial gas separations which challenges the world because of their similar physical properties. Membrane separation technology holds great promise to replace or combine the currently energy-intensive cryogenic distillation to achieve effective ethylene purification. In this study, a series of double-salt ionic liquid derived facilitated transport membranes (DSIL-FTMs) were designed for the first time aiming at the highly effective ethylene/ethane separation, which were configured by incorporating ethylene transport carrier and double-salt ionic liquids (DSILs) into a robust Nylon support. DSILs are the liquid ionic compositions composing of more than one cation or anion, which offer an opportunity to enhance the design flexibility and structural regulation of the corresponding FTMs. The DSIL-FTMs exhibit good ethylene/ethane separation performance as well as high thermal stability and long-term durability, where the ethylene permeability is about 250 Barrer and ethylene/ethane selectivity reaches up to 67. The spectroscopic characterizations and structure-performance relationship investigation reveal that the excellent separation performances are attributed to the favorable rearrangement of cations and anions in the DSIL-FTMs, where the newly formed ion arrangement and cation-anion interactions of DSIL-FTMs favor the building of the compact membrane structure and obtainment of high carrier activity. This study opens a novel avenue to manipulate the physical-chemical properties of separation membrane by structural diversity of DSILs, and promotes the sustainable prosperity of FTMs for energy-intensive gas molecule separations.

Pore-Space Partition and Optimization for Propane-Selective High-Performance Propane/Propylene Separation.

The development of effective propane (C3H8)-selective adsorbents for the purification of propylene (C3H6) from C3H8/C3H6 mixture is a promising alternative to replace the energy-intensive cryogenic distillation. However, few materials possess the dual desirable features of propane selectivity and high uptake capacity. Here, we report a family of pore-space-partitioned crystalline porous materials (CPM) with remarkable C3H8 uptake capacity (up to 10.9 mmol/g) and the highly desirable yet uncommon C3H8 selectivity (up to 1.54 at 0.1 bar and 1.44 at 1 bar). The selectivity-capacity synergy endows them with record-performing C3H8/C3H6 separation potential (i.e., C3H6 recovered from the mixture). Moreover, these CPMs exhibit outstanding properties including high stability, low regeneration energy, and multimodular chemical and geometrical tunability within the same isoreticular framework. The high C3H8/C3H6 separation performance was further confirmed by the breakthrough experiments.

A Review on Polymer Precursors of Carbon Molecular Sieve Membranes for Olefin/Paraffin Separation.

Carbon molecular sieve (CMS) membranes have been developed to replace or support energy-intensive cryogenic distillation for olefin/paraffin separation. Olefin and paraffin have similar molecular properties, but can be separated effectively by a CMS membrane with a rigid, slit-like pore structure. A variety of polymer precursors can give rise to different outcomes in terms of the structure and performance of CMS membranes. Herein, for olefin/paraffin separation, the CMS membranes derived from a number of polymer precursors (such as polyimides, phenolic resin, and polymers of intrinsic microporosity, PIM) are introduced, and olefin/paraffin separation properties of those membranes are summarized. The effects from incorporation of inorganic materials into polymer precursors and from a pyrolysis process on the properties of CMS membranes are also reviewed. Finally, the prospects and future directions of CMS membranes for olefin/paraffin separation and aging issues are discussed.

Rigid Nanoporous Urea-Based Covalent Triazine Frameworks for C2/C1 and CO2/CH4 Gas Separation.

C2/C1 hydrocarbon separation is an important industrial process that relies on energy-intensive cryogenic distillation methods. The use of porous adsorbents to selectively separate these gases is a viable alternative. Highly stable covalent triazine frameworks (urea-CTFs) have been synthesized using 1,3-bis(4-cyanophenyl)urea. Urea-CTFs exhibited gas uptakes of C2H2 (3.86 mmol/g) and C2H4 (2.92 mmol/g) at 273 K and 1 bar and is selective over CH4. Breakthrough simulations show the potential of urea-CTFs for C2/C1 separation.

Elucidation of Diffusivity of Hydrogen Isotopes in Flexible MOFs by Quasi-Elastic Neutron Scattering.

Kinetic-quantum-sieving-assisted H2 :D2 separation in flexible porous materials is more effective than the currently used energy-intensive cryogenic distillation and girdle-sulfide processes for isotope separation. It is believed that material flexibility results in a pore-breathing phenomenon under the influence of external stimuli, which helps in adjusting the pore size and gives rise to the optimum quantum-sieving phenomenon at each stage of gas separation. However, only a few studies have investigated kinetic-quantum-sieving-assisted isotope separation using flexible porous materials. In addition, no reports are available on the microscopic observation of isotopic molecular transportation during the separation process under dynamic transition. Here, the experimental observation of a significantly faster diffusion of deuterium than hydrogen in a flexible pore structure, even at high temperatures, through quasi-elastic neutron scattering, is reported. Unlike rigid structures, the extracted diffusion dynamics of hydrogen isotopes within flexible frameworks show that the diffusion difference between the isotopes increases with an increase in temperature. Owing to this unique inverse trend, a new strategy is suggested for achieving higher operating temperatures for efficient isotope separation utilizing a flexible metal-organic framework system.

Molecular Simulations on Tuning the Interlayer Spacing of Graphene Nanoslits for C4H6/C4H10 Separation

There are great challenges in developing efficient membranes to replace the currently energy-intensive cryogenic distillation processes for purifying C4H6 from C4H6/C4H10 mixtures due to their simi...

Dealuminated Cs-ZK-5 zeolite for propylene/propane separation

Developing new, more efficient adsorbents for propylene/propane separation is of urgent need, as the current energy-intensive cryogenic distillation method has to be replaced to achieve green chemistry goals. Here, we report that 973 K-steamed cesium ZK-5 zeolite with a framework Si/Al ratio of 16 (Cs-ZK-5-16-973) shows high propylene uptake and propylene/propane selectivity (1.9 mmol g−1 and 2.4, respectively) at 298 K and atmospheric pressure, together with fast adsorption kinetics, mainly due to its modified 8-ring window dimensions. More interestingly, no significant loss in uptake or selectivity was observed even after repeated adsorption–desorption cycling under vacuum-swing adsorption mode and dynamic breakthrough experiments, which may be a result of the lack of strong acid sites to induce propylene oligomerization at room temperature. This impressive regeneration is comparable with the behavior of the pure-silica small-pore zeolite ITQ-12, but much better than that of Na-A and Ca-A, rendering Cs-ZK-5-16-973 potentially useful for selective propylene/propane separation.

Efficient Trapping of Trace Acetylene from Ethylene in an Ultramicroporous Metal-Organic Framework: Synergistic Effect of High-Density Open Metal and Electronegative Sites.

Acetylene (C2 H2 ) removal from ethylene (C2 H4 ) is a crucial step in the production of polymer-grade C2 H4 but remains a daunting challenge because of the similar physicochemical properties of C2 H2 and C2 H4 . Currently energy-intensive cryogenic distillation processes are used to separate the two gases industrially. A robust ultramicroporous metal-organic framework (MOF), Ni3 (pzdc)2 (7 Hade)2 , is reported for efficient C2 H2 /C2 H4 separation. The MOF comprises hydrogen-bonded linked one-dimensional (1D) chains, and features high-density open metal sites (2.7 nm-3 ) and electronegative oxygen and nitrogen sites arranged on the pore surface as cooperative binding sites. Theoretical calculations, in situ powder X-ray diffraction and Fourier-transform infrared spectroscopy revealed a synergistic adsorption mechanism. The MOF possesses S-shaped 1D pore channels that efficiently trap trace C2 H2 at 0.01 bar with a high C2 H2 uptake of 60.6 cm3 cm-3 and C2 H2 /C2 H4 selectivity.

Fine-Tuning and Selective-Binding within an Anion-Functionalized Ultramicroporous Metal–Organic Framework for Efficient Olefin/Paraffin Separation

Olefin/paraffin separation is one of the intrinsically challenging tasks, mainly realized through the energy-intensive cryogenic distillation techniques. Here, we report the efficient separation of both propylene/propane (C3H6/C3H8) and ethylene/ethane (C2H4/C2H6) mixtures for the first time by a cheap and customized anion-functionalized metal-organic framework, ZU-36 (also termed as GeFSIX-3-M, M = Ni2+, Co2+), through molecular recognition and cross-section matching mechanism. Specifically, the pore window size of the fine-tuned ZU-36-Ni (4.42 × 4.42 Å2) decorated with high density of electronegative anions matches well with the cross section of C3H6 (4.16 × 4.65 Å2) and C2H4 (3.28 × 4.18 Å2), leading to efficient separation of C3H6/C3H8 and C2H4/C2H6. The high uptake capacity and separation selectivity of ZU-36-Ni were confirmed by adsorption isotherms and breakthrough tests. Binary dynamic breakthrough results showed that ZU-36-Ni can trap 1.35 mmol g-1 C3H6 and 1.08 mmol g-1 C2H4 from C3H6/C3H8 and C2H4/C2H6 mixtures, respectively. The separation selectivity for C3H6/C3H8 on ZU-36-Ni calculated from the breakthrough tests is 19, which sets a new benchmark for C3H6/C3H8 separation. The proposed cross-section matching mechanism together with proper selective binding affinity may serve as a guide for the design of effective porous materials for other important gas separation.

Modification of the pore environment in UiO-type metal-organic framework toward boosting the separation of propane/propylene

The separation of propane (C3H8) from propylene (C3H6) to obtain high purity C3H6 as a petrochemical raw material is an important, but challenging process. In the traditional industrial process, the separation mainly relies on energy-intensive cryogenic distillation. One alternative process that is currently becoming more attractive in the separation of C3H6 is adsorptive separation technology. Using porous materials, especially C3H8 selective adsorbents, high purity C3H6 can be directly obtained and thus, significantly simplifies the process. Herein, a series of functionalized ligands were introduced into UiO-type materials to finely regulate the C3H8 separation performance. Single-component adsorption isotherms reveal that BUT-10 has a distinct binding affinity for C3H8 over C3H6 over a wide range of temperature from 298 to 338 K and thus, achieved by far the highest C3H8 capture capacity (105 cm3 g−1 at 0.1 bar and 298 K) reported among the C3H8-selective materials reported to date. A combination of Grand Canonical Monte Carlo and density-function theory simulations demonstrate that the favorable pore environment provided by the multiple host-guest interactions toward C3H8 result in the higher C3H8 binding affinity observed in the separation process. Breakthrough experiments indicate that BUT-10 can selectively separate C3H8 from C3H8/C3H6 (1/1 and 1/15, v/v) mixtures and thus, directly produces high purity C3H6 (>99.99%) under ambient conditions.

Silver ZK-5 zeolites for selective ethylene/ethane separation

The development of efficient adsorbents for olefin/paraffin separation has attracted great attention, because an alternative energy-efficient technology is required to move away from the energy intensive cryogenic distillation processes. Here, we have prepared two silver-exchanged ZK-5 (Ag-ZK-5) zeolites with low (2.9) and high (3.7) Si/Al ratios, as well as various other cation-exchanged forms, for ethylene/ethane separation and compared their separation properties with those of zeolites Na-, Ca- and Ag-A with Si/Al = 1.0 and Ag-UZM-9 with Si/Al = 3.7. Among the zeolite adsorbents tested here, Ag-ZK-5 with Si/Al = 3.7 has the highest ethylene/ethane selectivity (2.54) at 298 K in vacuum-swing adsorption mode, together with fast adsorption kinetics. While the breakthrough results hint at its applicability in concentrated ethylene/ethane streams, the outstanding performance of this Ag-ZK-5 for ethylene/ethane separation appears to be due to a combination of its unique framework topology and composition (Al content) which can stabilize monovalent silver ions.

Molecule design of effective C2H4/C2H6 separation membranes: From 2D nanoporous graphene to 3D AHT zeolite

It is of great importance and challenge to explore effective membranes with optimal pore structures from diverse porous materials to replace energy-intensive cryogenic distillation processes for C2H4/C2H6 separation because of their similar physical properties. In this work, we proposed a strategy to screen the ideal 3D zeolite membrane by matching the zeolite structure to the designed 2D nanopore. Here, we found out the ideal AHT zeolite membrane with elliptical windows similar to the designed graphene-embedded 22-crown-8 ether pore with ultrahigh selectivity of C2H4/C2H6 mixtures via molecular dynamics simulations. The results demonstrate that the AHT zeolite membrane only allows the transport of C2H4 based on molecular size exclusion, which is validated by density functional theory calculations. Furthermore, the permeation performance studies of pure components demonstrate that the AHT zeolite membrane exhibits a C2H4/C2H6 selectivity of around 100 and a C2H4 permeability of 958 Barrer, superior to most of reported membrane materials.

Bimetallic ions regulate pore size and chemistry of zeolites for selective adsorption of ethylene from ethane

The petrochemical industry currently accomplishes olefin/paraffin separation by energy-intensive cryogenic distillation at an enormous scale. We report a sequential Ca2+/Ag+ ion-exchanged zeolite that achieves nearly ideal molecular sieving of C2H4/C2H6 and superior C2H4 adsorption capacity. Sequential and partial ion exchange regulates the pore size in ±0.2 Å increments, ranging between 3.8 and 4.2 Å. The demonstrated C2H4 adsorption capacity of 3.7 mmol/g, under ambient conditions, is the highest among zeolite-based materials. Elaborated with DFT calculations, Ag+-induced the stretching of the C-H bond and reduction of H-C-H bond angle of the C2H4 molecule in confined pore, providing C2H4 with the molecular basis and favorable kinetics for selective admission to pore size even less than 4 Å. The strategy of using bimetallic ions to regulate pore aperture size and selective admission of gas molecules with favorable kinetics provides a general path to be extended to other analogous molecular separation processes.

Adsorptive separation of C2H6/C2H4 on metal-organic frameworks (MOFs) with pillared-layer structures

Adsorptive separation is a promising alternative to the energy-intensive cryogenic distillation process for separating the ethane/ethylene (C2H6/C2H4) mixtures. Herein, two pillared-layer metal-organic frameworks (MOFs), Ni(HBTC)(bipy) and Ni2(HBTC)2(bipy)0.6(dabco)1.4, are prepared as the C2H6-selective adsorbents for C2H4 purification from C2H6/C2H4 mixtures. The effect of the type of pillars on the framework structure, thermal and moisture stability, as well as the C2H6 and C2H4 adsorption propertiy, of the MOFs was studied. The use of the longer pillars (i.e. bipy versus dabco) to scafolld the Ni(HBTC) improved the pore size (5.5 Å versus 5.3 Å), specific surface area (1474 m2/g versus 1070 m2/g) and moisture stability in the relative humidity range of 0–90%, but compromised the thermal stability (267 °C versus 278 °C). Both MOFs were C2H6-selective, which was evidencd by the single component adsorption experiments using C2H6 and C2H4. The ideal adsorbed solution theory (IAST) selectivity for C2H6/C2H4 mixtures (1:1 and 1:15, v/v) is in the range of 1.4–1.7 at 25–50 °C and 0–1 bar. The preferential adsorption towards C2H6 over C2H4 on both MOFs is then explained by the isosteric heat of adsorption. Additionally, Ni(HBTC)(bipy) also shows the best capacity of up to 6.6 mmol/g for C2H6 adsorption in comparison with other C2H6-selective MOFs at 25 °C and 1 bar. Both MOFs showed the excellent recyclability, with the negligible reduction in the gas uptake observed during four cycles of adsorption/desorption tests. Besides, breakthrough experiments demonstrated that both MOFs can achieve efficient separation of an equimolar C2H6/C2H4 mixture. The findings suggest that Ni(HBTC)(bipy) and Ni2(HBTC)2(bipy)0.6(dabco)1.4 can be further considered as C2H6-trapping adsorbents for the C2H6/C2H4 separation applications in practice.

Current Status of Microporous Metal-Organic Frameworks for Hydrocarbon Separations.

Separation of hydrocarbon mixtures into single components is a very important industrial process because all represent very important energy resources/raw chemicals in the petrochemical industry. The well-established industrial separation technology highly relies on the energy-intensive cryogenic distillation processes. The discovery of new materials capable of separating hydrocarbon mixtures by adsorbent-based separation technologies has the potential to provide more energy-efficient industrial processes with remarkable energy savings. Porous metal-organic frameworks (MOFs), also known as porous coordination polymers, represent a new class of porous materials that offer tremendous promise for hydrocarbon separations because of their easy tunability, designability, and functionality. A number of MOFs have been designed and synthesized to show excellent separation performance on various hydrocarbon separations. Here, we summarize and highlight some recent significant advances in the development of microporous MOFs for hydrocarbon separation applications.

Synthesis and modification of moisture-stable coordination pillared-layer metal-organic framework (CPL-MOF) CPL-2 for ethylene/ethane separation

Abstract Adsorptive separation of ethylene/ethane (C2H4/C2H6) binary mixture has growing interest in petrochemical industries compared to the conventional energy-intensive cryogenic distillation. Development of moisture-stable materials with high selectivity is of great importance to accomplish C2H4/C2H6 separation. Coordination pillared-layer metal-organic framework (CPL-MOF) CPL-2 was synthesised at room temperature, and then modified by silver ions impregnation to enhance the selectivity towards ethylene over ethane. The synthesised CPL-2 and Ag/CPL-2 MOFs have excellent moisture stability which was confirmed by the dynamic water vapour adsorption analysis under 90% relative humidity, showing no significant framework decomposition, even at 50 °C. The calculated selectivity based on gravimetric single-component gas adsorption experiments shows the significantly improved C2H4/C2H6 selectivity from 1.4 to 26.1 after loading 10 wt.% (theoretical) of silver ions on CPL-2. Breakthrough experiments for C2H4/C2H6 (1:1, v/v) mixture suggest that both CPL-2 and 10 wt.% Ag/CPL-2 can achieve the binary mixture separation, and 10 wt.% Ag/CPL-2 shows relatively better dynamic separation performance compared to parent CPL-2. The good adsorption selectivity and moisture stability allow CPL-MOF to be a class of promising porous materials for further exploitation in the separation of C2H4/C2H6 mixtures. Additionally, the method presented here can potentially be extended to other CPLs with different pore sizes for alkene/alkane separations.

Metal–Organic Frameworks with Metal–Catecholates for O2/N2Separation

Oxygen and nitrogen are widely produced feedstocks with diverse fields of applications but are primarily obtained via the energy-intensive cryogenic distillation of air. More energy-efficient proce...

Alternatives to Cryogenic Distillation: Advanced Porous Materials in Adsorptive Light Olefin/Paraffin Separations.

As primary feedstocks in the petrochemical industry, light olefins such as ethylene and propylene are mainly obtained from steam cracking of naphtha and short chain alkanes (ethane and propane). Due to their similar physical properties, the separations of olefins and paraffins-pivotal processes to meet the olefin purity requirement of downstream processing-are typically performed by highly energy-intensive cryogenic distillation at low temperatures and high pressures. To reduce the energy input and save costs, adsorptive olefin/paraffin separations have been proposed as promising techniques to complement or even replace cryogenic distillation, and growing efforts have been devoted to developing advanced adsorbents to fulfill this challenging task. In this Review, a holistic view of olefin/paraffin separations is first provided by summarizing how different processes have been established to leverage the differences between olefins and paraffins for effective separations. Subsequently, recent advances in the development of porous materials for adsorptive olefin/paraffin separations are highlighted with an emphasis on different separation mechanisms. Last, a perspective on possible directions to push the limit of the research in this field is presented.