Energy-intensive Separations Research Articles

RAFT unchained — Scalable manufacturing of thiocarbonyl chain transfer agents

Here, we report an efficient approach for the manufacturing of chain-transfer agents (CTA) for RAFT polymerization using non-volatile organic compounds (VOC) in one pot with only water-washing as a work-up. Although the RAFT process is scientifically mature, nearly all RAFT CTAs are still produced through multi-step syntheses involving material- and energy-intensive separations. Today’s commercially available RAFT agents are high-value specialty products available only from fine chemical vendors, limiting RAFT’s industrial success. The current work illustrates a single-step one-pot approach to synthesize ethyl (3-oxo-2-butyl) carbonotrithioate and O-ethyl S-(3-oxobutan-2-yl) carbonodithioate at up to 10 L scale in various non-VOC solvents with varied degrees of potential for cross-reactivity: diethyl succinate, tributyl acetyl citrate, castor oil glycidal ether, and glycerol triglycidyl ether. We studied the purity and yield of these CTAs using NMR and LC-MS, and then conducted rigorous polymerization and kinetic studies to evaluate the efficacy of these molecules in maintaining the RAFT equilibrium. We found that the use of these non-VOC solvents impedes neither the synthesis of these molecules nor the RAFT control, thus enabling scalable and low-impact pathways to RAFT CTA manufacturing.

Merging polymers of intrinsic microporosity and porous carbon-based zinc oxide composites in novel mixed matrix membranes for efficient gas separation

Mixed matrix membranes (MMMs) have demonstrated significant promise in energy-intensive gas separations by amalgamating the unique properties of fillers with the facile processability of polymers. However, achieving a simultaneous enhancement of permeability and selectivity remains a formidable challenge, due to the difficulty of achieving an optimal match between polymers and fillers. In this study, we incorporate a porous carbon-based zinc oxide composite (C@ZnO) into high-permeability polymers of intrinsic microporosity (PIMs) to fabricate MMMs. The dipole–dipole interaction between C@ZnO and PIMs ensures their exceptional compatibility, mitigating the formation of non-selective voids in the resulting MMMs. Concurrently, C@ZnO with abundant interconnected pores can provide additional low-resistance pathways for gas transport in MMMs. As a result, the CO2 permeability of the optimized C@ZnO/PIM-1 MMMs is elevated to 13,215 barrer, while the CO2/N2 and CO2/CH4 selectivity reached 21.5 and 14.4, respectively, substantially surpassing the 2008 Robeson upper bound. Additionally, molecular simulation results further corroborate that the augmented membrane gas selectivity is attributed to the superior CO2 affinity of C@ZnO. In summary, we believe that this work not only expands the application of MMMs for gas separation but also heralds a paradigm shift in the application of porous carbon materials.

Truly combining the advantages of polymeric and zeolite membranes for gas separations

Mixed-matrix membranes (MMMs) have been investigated to render energy-intensive separations more efficiently by combining the selectivity and permeability performance, robustness, and nonaging properties of the filler with the easy processing, handling, and scaling up of the polymer. However, truly combining all in one single material has proven very challenging. In this work, we filled a commercial polyimide with ultrahigh loadings of a high-aspect ratio, CO2-philic Na-SSZ-39 zeolite with a three-dimensional channel system that precisely separates gas molecules. By carefully designing both zeolite and MMM synthesis, we created a gas-percolation highway across a flexible and aging-resistant (more than 1 year) membrane. The combination of a CO2-CH4 mixed-gas selectivity of ~423 and a CO2 permeability of ~8300 Barrer outperformed all existing polymer-based membranes and even most zeolite-only membranes.

Efficient ethylene/ethane separation by zwitterionic deep eutectic solvent membranes

Zwitterionic deep eutectic solvent membranes (ZDESMs) hold great promise in molecular separations yet have rarely been reported. Herein, for the first time, ZDESMs are constructed by zwitterionic deep eutectic solvent (ZDES) and ethylene-transport carrier (silver salt) for ethylene/ethane separation. Experiments and simulations reveal that the ZDESMs possess structural nano-heterogeneity with discrete zwitterionic nanodomains and interconnected carrier channels, thus flexibly regulating the carrier spatial distribution as well as promoting carrier activity and stability synchronously. The ZDESMs acquires high ethylene permeability, ethylene/ethane selectivity at low carrier concentration, and long-term stability, which is superior to most of the-state-of-the-art ethylene/ethane separation membranes. The proposed ZDESMs expand the membrane family, and the nano-heterogeneity design and visualization of membrane structure will offer valuable guidance on developing advanced membranes for energy-intensive gas separations.

Adsorptive molecular sieving of linear over branched alkanes using trianglamine host macrocycles for sustainable separation processes

The separation of high-purity linear alkanes from a mixture of linear and branched alkanes is of great importance to the oil and gas industry for the improvement of the gasoline quality. Herein, we investigate the methylene-bridged trianglamines (M-TA) as supramolecular adsorbents for the selective separation of linear alkane isomers. This selectivity depends on the host–guest complexation arising from the size matching between the trianglamines cavity and the linear guest molecules. Moreover, M-TA are highly recyclable with almost no loss of efficiency showing a promising prospect in industrial applications. We believe that the tailored design of host supramolecular entities as smart and selective recognition units for energy-intensive separations will be the preferred future approch towards environmentally friendly and sustainable processes in industrial operations.

Deep Eutectic Solvent Membranes Designed by the Same-Anion Strategy for Highly Efficient Ethylene/Ethane Separation

Deep eutectic solvents (DESs) are a new generation of designer and green solvents and offer tremendous opportunities for separation science; however, the construction of high-permeance DES membranes (DESMs) with excellent stability for ethylene/ethane separation is still a challenge. In this study, by the same-anion strategy, a series of DESs with the CF3SO3– anion were designed for the first time and then combined with an ethylene transport carrier (AgCF3SO3) for the construction of DESMs for highly efficient ethylene/ethane separation. DESMs were facilely fabricated by impregnating the as-designed DESs and AgCF3SO3 carrier into the commercial poly(vinylidene fluoride) membrane, where the DESs not only exhibited good compatibility with AgCF3SO3 but also stimulated high carrier activity and afforded good carrier stability. The resultant DESMs displayed high ethylene permeability, ethylene/ethane selectivity, and excellent stability, especially the maximum permeability and selectivity reached up to 910 barrer and 83, thus being superior to most of the state-of-the-art ethylene/ethane separation membranes. Finally, the separation mechanism was revealed, and the regulation of hydrogen-bond and coordinative interactions within DESMs by the rational structural design accounted for excellent performances. This work extends the DES library and will accelerate the prosperity of DESMs for energy-intensive gas separations.

Customized facilitated transport membranes by mixed strategy for ethylene/ethane separation

Mixed strategy has been widely utilized to design novel membranes or tailor membrane structure, however the construction of facilitated transport membranes (FTMs) with mixed strategy has rarely been reported. In this work, eight kinds of mixed protic ionic liquid (MPIL) derived FTMs (MPIL-FTMs) were designed with mixed strategy for highly efficient ethylene/ethane separation by impregnating MPILs and ethylene transport carrier into porous support. The morphology of MPIL-FTMs were characterized by SEM and AFM, and their molecular interactions were investigated by 1H NMR, FTIR and DSC. The MPIL-FTMs exhibited long-term stability and various ethylene permeability-ethylene/ethane selectivity combinations, which indicated mixed strategy could easily customize the separation performances of MPIL-FTMs. Moreover, a fundamental law for mixed strategy was obtained that the two parent PILs with similar chemical structure and molecular interactions favored the synchronous improvement of C2H4 permeability and C2H4/C2H6 selectivity. For example, the C2H4 permeability of the [BA][Pyr][NO3] derived MPIL-FTM reached up to 561.0 Barrer with improved C2H4/C2H6 selectivity of 26.7, which were superior to most of reported membranes, thus breaking through the ubiquitous trade-off effect. This study proves the mixed strategy is a promising platform for the design of high-performance FTMs as well as their structure modification, and the marriage of mixed strategy with diversity of ILs creates enormous opportunities to construct advanced membranes for energy-intensive gas separations.

Tunable Supramolecular Cavities Molecularly Homogenized in Polymer Membranes for Ultraefficient Precombustion CO2 Capture.

Processable molecular-sieving membranes are important materials for realizing energy-efficient precombustion CO2 capture during industrial-scale hydrogen production. However, the promising design of mixed matrix membranes (MMMs) that aims to integrate the molecular-sieving properties of nanoporous architectures with industrial processable polymers still faces performance and fabrication issues due to the formation of segregated nanofiller domains in their polymer matrices. Here, an unconventional nanocomposite membrane design is proposed using soluble organic macrocyclic cavitands (OMCs) with tunable open cavity sizes that not only mitigate the formation the discrete nanofiller phases but also deliver distinct molecular-sieving separations. The versatile organic-solvent solubility coupled with highly interactive functionalities of OMCs allows them to obtain molecularly homogeneous mixing with matrix polymers and form only one integral continuous phase crucial to the robust processability of polymers. A series of polybenzimidazole-based molecularly mixed composite membranes (MMCMs) are fabricated via the incorporation of a soluble and thermally stable OMC choice, sulfocalixarenes, with various cavity sizes. These membranes achieve outstanding high-temperature mixed-gas H2 /CO2 separation performances comparable with several state-of-the-art molecular-sieving membranes owing to effective size-sieving gas passages through the open or partially-intruded supramolecular cavities. The broadly tunable structures and functionalities of OMCs would make their MMCMs attractive for other energy-intensive molecular separations.

Via Separations raises $38 million

Via Separations, a membrane start-up in Watertown, Massachusetts, has raised $38 million in a series B financing round led by NGP Energy Technology Partners. Via, one of C&EN’s 10 Start-Ups to Watch in 2019, is developing graphene oxide separation materials. It is targeting industries in which filtration can replace energy-intensive thermal separations. One example is removing solids from the black liquor that results from papermaking. Via will use the money it raised to deploy its system in the pulp and paper sector and adapt it for chemical production.

Recent Progress on Polymers of Intrinsic Microporosity and Thermally Modified Analogue Materials for Membrane‐Based Fluid Separations

Polymers of Intrinsic Microporosity Recent progress on polymers of microporosity and their thermally-treated analogs for membranebased fluid separations are highlighted by Yingge Wang, Yu Han, Ingo Pinnau, and coworkers in article number 2100049. Challenges and opportunities are identified for these materials in energy-intensive separations such as CO2/CH4, C2H4/C2H6 and C3H6/C3H8 separations, as well as for thin-film composite membranes containing polymers of intrinsic microporosity (PIM) motifs in liquid applications.

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.

Pressure‐Responsive Two‐Dimensional Metal–Organic Framework Composite Membranes for CO2 Separation

The realization of regulating permeance and selectivity in membrane systems may allow effective relief of conventional energy-intensive separations. Here, pressure-responsive ultrathin membranes (~100 nm) fabricated by compositing flexible two-dimensional metal-organic framework nanosheets (MONs) with graphene oxide nanosheets for CO2 separation are reported. By controlling the gas permeation direction to leverage the pressure-responsive phase transition of the MONs, CO2-induced gate opening and closing behaviors are observed in the resultant membranes, which are accompanied with the sharp increase of CO2 permeance (from 173.8 to 1144 gas permeation units) as well as CO2/N2 and CO2/CH4 selectivities (from 4.1 to 22.8 and from 4 to 19.6, respectively). The flexible behaviors and separation mechanism are further elucidated by molecular dynamics simulations. This work establishes the relevance of structural transformation-based framework dynamics chemistry in smart membrane systems.

Intrinsically Porous Molecular Materials (IPMs) for Natural Gas and Benzene Derivatives Separations

ConspectusSeparating and purifying chemicals without heat would go a long way toward reducing the overall energy consumption and the harmful environmental footprint of the process. Molecular separation processes are critical for the production of raw materials, commodity chemicals, and specialty fuels. Over 50% of the energy used in the production of these materials is spent on separation and purification processes, which primarily includes vacuum and cryogenic distillations. Chemical manufacturers are now investigating modest thermal approaches, such as membranes and adsorbent materials, as they are more cognizant than ever of the need to save energy and prevent pollution. Porous materials, such as zeolites, metal-organic frameworks (MOFs), and covalent organic frameworks (COFs), have dominated the field of industrial separations as their high surface areas and robust pores make them ideal candidates for molecular separations of gases and hydrocarbons. Separation processes involving porous materials can save 70%-90% of energy costs compared to that of thermally driven distillations. However, most porous materials have low thermal, chemical, and moisture stability, in addition to limited solution processability, which tremendously constrain their broad industrial translation. Intrinsically porous molecular materials (IPMs) are a subclass of porous molecular materials that are comprised of molecular host macrocycles or cages that absorb guests in or around their intrinsic cavity. IPMs range from discrete porous molecules to assemblies with amorphous or highly crystalline structures that are held together by weak supramolecular interactions. Compared to the coordination or dynamic covalent bond-constructed porous frameworks, IPMs possess high thermal, chemical, and moisture stability and maintain their porosity under critical conditions. Moreover, the intrinsic porosity endows IPMs with excellent host-guest properties in solid, liquid (organic or aqueous), and gas states, which can be further utilized to construct diverse separation strategies, such as solid-gas adsorption, solid-liquid absorption, and liquid-liquid extraction. The diversity of host-guest interactions in the engineered IPMs affords a plethora of possibilities for the development of the ideal "molecular sieves". Herein, we present a different take on the applicability of intrinsically porous materials such as cyclodextrin (CD), cucurbiturils (CB), pillararene (P), trianglamines (T), and porous organic cages (POCs) that showed an impressive performance in gas purification and benzene derivatives separation. IPMs can be easily scaled up and are quite stable and solution processable that consequently facilitates a favorable technological transformation from the traditional energy-intensive separations. We will account for the main advances in molecular host-guest chemistry to design "on-demand" separation processes and also outline future challenges and opportunities for this promising technology.

A Polymorphic Azobenzene Cage for Energy-Efficient and Highly Selective p-Xylene Separation.

Developing the competence of molecular sorbents for energy-saving applications, such as C8 separations, requires efficient, stable, scalable, and easily recyclable materials that can readily transition to commercial implementation. Herein, we report an azobenzene-based cage for the selective separation of p-xylene isomer across a range of C8 isomers in both vapor and liquid states with selectivity that is higher than the reported all-organic sorbents. The crystal structure shows non-porous cages that are separated by p-xylene molecules through selective CH-π interactions between the azo bonds and the methyl hydrogen atoms of the xylene molecules. This cage is stable in solution and can be regenerated directly under vacuum to be used in multiple cycles. We envisage that this work will promote the investigation of the azo bond as well as guest-induced crystal-to-crystal phase transition in non-porous organic solids for energy-intensive separations.

Finely Tuned Submicroporous Thin-Film Molecular Sieve Membranes for Highly Efficient Fluid Separations.

Polymeric membranes with increasingly high permselective performances are gaining a significant role in lowering the energy burden and improving the environmental sustainability of complex chemical separations. However, the commercial deployment of newly designed materials with promising intrinsic properties for fluid separations has been stalled by challenges associated with fabrication and scale up of low-cost, high-performance, defect-free thin-film composite (TFC) membranes. Here, a facile method to fabricate next-generation TFC membranes using a bridged-bicyclic triptycene tetra-acyl chloride (Trip) building block with a large fraction of finely tuned structural submicroporosity (pore size < 4 Å) is demonstrated. The TFCs exhibit superb potential for removal of small (≈200 g mol-1 ) organic microcontaminants from organic solvent streams by showing both improved rejection and permeance in organic systems compared to current state-of-the-art commercial membranes. The TFCs also display unprecedented properties for desalination applications with performance located far above the current water permeance/sodium chloride rejection trendline. The strategy of using highly contorted triptycene building blocks with well-defined interconnected internal free volume elements establishes a scalable, generalized approach to fabricate highly selective, submicroporous TFC membranes for a wide variety of challenging energy-intensive fluid separations.

CO2-to-Fuels Renewable Gasoline and Jet Fuel Can Soon Be Price Competitive with Fossil Fuels

Download : Download high-res image (69KB) Download : Download full-size image Rob McGinnis, PhD, is an inventor and entrepreneur. He is the founder and CEO of Prometheus, a company that is developing technology to remove carbon dioxide from the air and turn it into fuels. He previously founded Mattershift, where he developed large-scale carbon nanotube membranes. Rob was previously founder of Oasys Water, a company focused on developing forward osmosis technologies for water purification. Rob received his PhD in Engineering from Yale University.

Electropolymerization of Molecular-Sieving Polythiophene Membranes for H2 Separation.

Membrane technologies that do not rely on heat for industrial gas separation would lower global energy cost. While polymeric, inorganic, and mixed-matrix separation membranes have been rapidly developed, the bottleneck is balancing the processability, selectivity, and permeability. Reported here is a softness adjustment of rigid networks (SARs) strategy to produce flexible, stand-alone, and molecular-sieving membranes by electropolymerization. Here, 14 membranes were rationally designed and synthesized and their gas separation ability and mechanical performance were studied. The separation performance of the membranes for H2 /CO2 , H2 /N2 , and H2 /CH4 can exceed the Robeson upper bound, among which, H2 /CO2 separation selectivity reaches 50 with 626 Barrer of H2 permeability. The long-term and chemical stability tests demonstrate their potential for industrial applications. This simple, scalable, and cost-effective strategy holds promise for the design other polymers for key energy-intensive separations.

Publisher Correction: Mixed matrix formulations with MOF molecular sieving for key energy-intensive separations.

In the version of this Article originally published, the units of the y axis of Fig. 3b were incorrectly given as '106 cm2 s-1'; they should have been '10-8 cm2 s-1'. This has been corrected in the online versions of the Article.

Nanofiltration performance of conical and hourglass nanopores

Pressure-driven ion transport through conical and hourglass-shape (double conical) nanopores is investigated theoretically by solving the Poisson, Nernst-Planck and Navier-Stokes equations numerically. Due to the electrostatic asymmetry in conical nanopores, these latter exhibit nanofiltration rectification properties, i.e. they reject salts differently depending if the solution enters the nanopore from its base or its tip. The filtration rectification properties of conical nanopores result from two different phenomena, (i) the co-ion exclusion at the pore mouth and (ii) the sign and the magnitude of the (pressure-induced) electric field arising through the nanopores. Hourglass-shape nanopores exhibit improved separation performance compared with cylindrical and conical nanopores with identical average diameter, length and surface charge density. Notably, they allow target salt rejections to be reached with a smaller driving force than the other pore geometries, which makes them attractive candidates for the design of advanced nanofiltration membranes allowing less energy intensive separations.

Mixed matrix formulations with MOF molecular sieving for key energy-intensive separations.

Membrane-based separations can improve energy efficiency and reduce the environmental impacts associated with traditional approaches. Nevertheless, many challenges must be overcome to design membranes that can replace conventional gas separation processes. Here, we report on the incorporation of engineered submicrometre-sized metal-organic framework (MOF) crystals into polymers to form hybrid materials that successfully translate the excellent molecular sieving properties of face-centred cubic (fcu)-MOFs into the resultant membranes. We demonstrate, simultaneously, exceptionally enhanced separation performance in hybrid membranes for two challenging and economically important applications: the removal of CO2 and H2S from natural gas and the separation of butane isomers. Notably, the membrane molecular sieving properties demonstrate that the deliberately regulated and contracted MOF pore-aperture size can discriminate between molecular pairs. The improved performance results from precise control of the linkers delimiting the triangular window, which is the sole entrance to the fcu-MOF pore. This rational-design hybrid approach provides a general toolbox for enhancing the transport properties of advanced membranes bearing molecular sieve fillers with sub-nanometre-sized pore-apertures.