Tetrameric self-assembling of water-lean solvents enables carbamate anhydride-based CO2 capture chemistry

Carbon capture, utilization and storage is a key yet cost-intensive technology for the fight against climate change. Single-component water-lean solvents have emerged as promising materials for post-combustion CO2 capture, but little is known regarding their mechanism of action. Here we present a combined experimental and modelling study of single-component water-lean solvents, and we find that CO2 capture is accompanied by the self-assembly of reverse-micelle-like tetrameric clusters in solution. This spontaneous aggregation leads to stepwise cooperative capture phenomena with highly contrasting mechanistic and thermodynamic features. The emergence of well-defined supramolecular architectures displaying a hydrogen-bonded internal core, reminiscent of enzymatic active sites, enables the formation of CO2-containing molecular species such as carbamic acid, carbamic anhydride and alkoxy carbamic anhydrides. This system extends the scope of adducts and mechanisms observed during carbon capture. It opens the way to materials with a higher CO2 storage capacity and provides a means for carbamates to potentially act as initiators for future oligomerization or polymerization of CO2.

In Situ Raman Methodology for Online Analysis of CO2 and H2O Loadings in a Water-Lean Solvent for CO2 Capture

Carbon capture represents a key pathway to meeting climate change mitigation goals. Powerful next-generation solvent-based capture processes are under development by many researchers, but optimization and testing would be significantly aided by integrating in situ monitoring capability. Further, real-time water analysis in water-lean solvents offers the potential to maintain their water balance in operation. To explore data acquisition techniques in depth for this purpose, Raman spectra of CO2, H2O, and a single-component water-lean solvent, N-(2-ethoxyethyl)-3-morpholinopropan-1-amine (2-EEMPA) were collected at different CO2 and H2O concentrations using an in situ Raman cell. The quantification of CO2 and H2O loadings in 2-EEMPA was done by principal component regression and partial least squares methods with analysis of uncertainties. We conclude with discussions on how this simultaneous online analysis method to quantify CO2 and H2O loadings can be an important tool to enable the optimal efficiency of water-lean CO2 solvents while also maintaining the critical water balance under operating conditions relevant to post-combustion CO2 capture.

Enhancing CO2 Transport Across a PEEK-Ionene Membrane and Water-Lean Solvent Interface

Efficient direct air capture (DAC) of CO₂ will require strategies to deal with the relatively low concentration in the atmosphere. One such strategy is to employ the combination of a CO₂ -selective membrane coupled with a CO₂ capture solvent acting as a draw solution. Here, the interactions between a leading water-lean carbon-capture solvent, a polyether ether ketone (PEEK)-ionene membrane, CO₂ , and combinations were probed using advanced NMR techniques coupled with advanced simulations. We identify the speciation and dynamics of the solvent, membrane, and CO₂ , presenting spectroscopic evidence of CO₂ diffusion through benzylic regions within the PEEK-ionene membrane, not spaces in the ionic lattice as expected. Our results demonstrate that water-lean capture solvents provide a thermodynamic and kinetic funnel to draw CO₂ from the air through the membrane and into the bulk solvent, thus enhancing the performance of the membrane. The reaction between the carbon-capture solvent and CO₂ produces carbamic acid, disrupting interactions between the imidazolium (Im⁺ ) cations and the bistriflimide anions within the PEEK-ionene membrane, thereby creating structural changes through which CO₂ can diffuse more readily. Consequently, this restructuring results in CO₂ diffusion at the interface that is faster than CO₂ diffusion in the bulk carbon-capture solvent.

Next steps for solvent-based CO2 capture; integration of capture, conversion, and mineralisation

In this perspective, we detail how solvent-based carbon capture integrated with conversion can be an important element in a net-zero emission economy. Carbon capture and utilization (CCU) is a promising approach for at-scale production of green CO₂-derived fuels, chemicals and materials. The challenge is that CO₂-derived materials and products have yet to reach market competitiveness because costs are significantly higher than those from conventional means. We present here the key to making CO₂-derived products more efficiently and cheaper, integration of solvent-based CO₂ capture and conversion. We present the fundamentals and benefits of integration within a changing energy landscape (i.e., CO₂ from point source emissions transitioning to CO₂ from the atmosphere), and how integration could lead to lower costs and higher efficiency, but more importantly how CO₂ altered in solution can offer new reactive pathways to produce products that cannot be made today. We discuss how solvents are the key to integration, and how solvents can adapt to differing needs for capture, conversion and mineralisation in the near, intermediate and long term. We close with a brief outlook of this emerging field of study, and identify critical needs to achieve success, including establishing a green-premium for fuels, chemicals, and materials produced in this manner.

In this perspective, we detail how solvent-based carbon capture integrated with conversion can be an important element in a net-zero emission economy.

Amphilic Water-Lean Carbon Capture Solvent Wetting Behavior through Decomposition by Stainless-Steel Interfaces

A combined experimental and theoretical study has been carried out on the wetting and reactivity of water-lean carbon capture solvents on the surface of common column packing materials. Paradoxically, these solvents are found to be equally able to wet hydrophobic and hydrophilic surfaces. The solvents are amphiphilic and can adapt to any interfacial environment, owing to their inherent heterogeneous (nonionic/ionic) molecular structure. Ab initio molecular dynamics indicates that these structures enable the formation of a strong adlayer on the surface of hydrophilic surfaces like oxidized steel which promotes solvent decomposition akin to hydrolysis from surface oxides and hydroxides. This decomposition passivates the surface, making it effectively hydrophobic, and the decomposed solvent promotes leaching of the iron into the bulk fluid. This study links the wetting behavior to the observed corrosion of the steels by decomposition of solvent at steel interfaces. The overall affect is strongly dependent on the chemical composition of the solvent in that amines are stable, whereas imines and alcohols are not. Moreover, plastic packing shows little to no solvent degradation, but an equal degree of wetting.

Integrated Capture and Conversion of CO2 to Methane Using a Water-lean, Post-Combustion CO2 Capture Solvent

Integrated carbon capture and conversion of CO₂ into materials (IC³ M) is an attractive solution to meet global energy demand, reduce our dependence on fossil fuels, and lower CO₂ emissions. Herein, using a water-lean post-combustion capture solvent, [N-(2-ethoxyethyl)-3-morpholinopropan-1-amine] (2-EEMPA), >90 % conversion of captured CO₂ to hydrocarbons, mostly methane, is achieved in the presence of a heterogenous Ru catalyst under relatively mild reaction conditions (170 °C and <15 bar H₂ pressure). The catalytic performance was better in 2-EEMPA than in aqueous 5 m monoethanol amine (MEA). Operando nuclear magnetic resonance (NMR) study showed in situ formation of N-formamide intermediate, which underwent further hydrogenation to form methane and other higher hydrocarbons. Technoeconomic analyses (TEA) showed that the proposed integrated process can potentially improve the thermal efficiency by 5 % and reduce the total capital investment and minimum synthetic natural gas (SNG) selling price by 32 % and 12 %, respectively, compared to the conventional Sabatier process, highlighting the energetic and economic benefits of integrated capture and conversion. Methane derived from CO₂ and renewable H₂ sources is an attractive fuel, and it has great potential as a renewable hydrogen carrier as an environmentally responsible carbon capture and utilization approach.

Catalytic coproduction of methanol and glycol in one pot from epoxide, CO2, and H2

An atom (100%) and energy-efficient approach to coproduce two commodity chemicals, methanol and glycol, has been demonstrated for the first time using H₂, CO₂, and epoxide as feeds. A basic medium used for CO₂ capture, polyethyleneimine (PEI₆₀₀), is shown to facilitate the formation of a key reaction intermediate, cyclic carbonates. Upon hydrogenation of cyclic carbonates in the presence of a homogenous Ru-PNP catalyst, a 1 : 1 mixture of methanol and glycol is produced. This approach has been demonstrated in one pot by adding all the required reactants directly or stepwise. The stepwise addition of reactants resulted in good yields (>95% for PG and 84% for methanol) and selectivity of products.

An atom (100%) and energy-efficient approach to coproduce two commodity chemicals, methanol and glycol, has been demonstrated for the first time using H₂, CO₂, and epoxide as feeds.

Subtle changes in hydrogen bond orientation result in glassification of carbon capture solvents

Water-lean CO₂ capture solvents show promise for more efficient and cost-effective CO₂ capture, although their long-term behavior in operation has yet to be well studied. New observations of extended structure solvent behavior show that some solvent formulations transform into a glass-like phase upon aging at operating temperatures after contact with CO₂. The glassification of a solvent would be detrimental to a carbon-capture process due to plugging of infrastructure, introducing a critical need to decipher the underlying principles of this phenomenon to prevent it from happening. We present the first integrated theoretical and experimental study to characterize the nano-structure of metastable and glassy states of an archetypal single-component alkanolguanidine carbon-capture solvent and assess how minute changes in atomic-level interactions convert the solvent between metastable and glass-like states. Small-angle neutron scattering and neutron diffraction coupled with small- and wide-angle X-ray scattering analysis demonstrate that minute structural changes in solution precipitae reversible aggregation of zwitterionic alkylcarbonate clusters in solution. Our findings indicate that our test system, an alkanolguanidine, exhibits a first-order phase transition, similar to a glass transition, at approximately 40 °C-close to the operating absorption temperature for post-combustion CO₂ capture processes. We anticipate that these phenomena are not specific to this system, but are present in other classes of colvents as well. We discuss how molecular-level interactions can have vast implications for solvent-based carbon-capture technologies, concluding that fortunately in this case, glassification of water-lean solvents can be avoided as long as the solvent is run above its glass transition temperature.

Molecular-Level Overhaul of γ-Aminopropyl Aminosilicone/Triethylene Glycol Post-Combustion CO2 -Capture Solvents

Capturing carbon dioxide from post-combustion gas streams is an energy-intensive process that is required prior to either converting or sequestering CO₂ . Although a few commercial 1st and 2nd generation aqueous amine technologies have been proposed, the cost of capturing CO₂ with these technologies remains high. One approach to decrease costs of capture has been the development of water-lean solvents that aim to increase efficiency by reducing the water content in solution. Water-lean solvents, such as γ-aminopropyl aminosilicone/triethylene glycol (GAP/TEG), are promising technologies, with the potential to halve the parasitic load to a coal-fired power plant, albeit only if high solution viscosities and hydrolysis of the siloxane moieties can be mitigated. This study concerns an integrated multidisciplinary approach to overhaul the GAP/TEG solvent system at the molecular level to mitigate hydrolysis while also reducing viscosity. Cosolvents and diluents are found to have negligible effects on viscosity and are not needed. This finding allows for the design of single-component siloxane-free diamine derivatives with site-specific incorporation of selective chemical moieties for direct placement and orientation of hydrogen bonding to reduce viscosity. Ultimately, these new formulations are less susceptible to hydrolysis and exhibit up to a 98 % reduction in viscosity compared to the initial GAP/TEG formulation.

A novel high-temperature MAS probe with optimized temperature gradient across sample rotor for in-situ monitoring of high-temperature high-pressure chemical reactions

We present a novel nuclear magnetic resonance (NMR) probe design focused on optimizing the temperature gradient across the sample for high temperature magic angle spinning (MAS) experiments using standard rotors. Computational flow dynamics (CFD) simulations were used to assess and optimize the temperature gradient across the sample under MAS conditions. The chemical shift and linewidth of ²⁰⁷Pb direct polarization in lead nitrate were used to calibrate the sample temperature and temperature gradient, respectively. A temperature gradient of less than 3 °C across the sample was obtained by heating bearing gas flows and adjusting its temperature and flow rate during variable temperature (VT) experiments. A maximum temperature of 350 °C was achieved in this probe using a Varian 5 mm MAS rotor with standard Vespel drive tips and end caps. Time-resolved ¹³C and ¹H MAS NMR experiments were performed at 325 °C and 60 bar to monitor an in-situ mixed phase reverse water gas shift reaction, industrial synthesis of CH₃OH from a mixture of CO₂ and H₂ with a Cu/ZnO/Al₂O₃ catalyst, demonstrating the first in-situ NMR monitoring of a chemical system at temperatures higher than 250 °C in a pressurized environment. The combination of this high-temperature probe and high-pressure rotors will allow for in-situ NMR studies of a great variety of chemical reactions that are inaccessible to conventional NMR setup.

Assessing the impacts of dynamic soft-templates innate to switchable ionic liquids on nanoparticulate green rust crystalline structures

This experimental and theoretical study investigates how dynamic solvation environments in switchable ionic liquids regulate the composition of nanoparticulate green rust.

Mesoscopic Structure Facilitates Rapid CO2 Transport and Reactivity in CO2 Capture Solvents

Mass transfer coefficients of CO₂ are anomalously high in water-lean solvents as compared to aqueous amines. Such phenomena are intrinsic to the molecular and nanoscale structure of concentrated organic CO₂ capture solvents. To decipher the connections, we performed in situ liquid time-of-flight secondary ionization mass spectroscopy on a representative water-lean solvent, 1-((1,3-Dimethylimidazolidin-2-ylidene)amino)propan-2-ol (IPADM-2-BOL). Two-dimensional (2D) and three-dimensional (3D) chemical mapping of the solvent revealed that IPADM-2-BOL exhibited a heterogeneous molecular structure with regions of CO₂-free solvent coexisting with clusters of zwitterionic carbonate ions. Chemical mapping were consistent with molecular dynamic simulation results, indicating CO₂ diffusing through pockets and channels of unreacted solvent. The observed mesoscopic structure promotes and enhances the diffusion and reactivity of CO₂, likely prevalent in other water-lean solvents. This finding suggests that if the size, shape and orientation of the domains can be controlled, more efficient CO₂ capture solvents could be developed to enhance mass-transfer and uptake kinetics.

Condensed-phase low temperature heterogeneous hydrogenation of CO2 to methanol

A low-temperature CH₃OH synthesis was achieved at 120–170 °C using tertiary amine and alcohol in the presence of a Cu/ZnO/Al₂O₃ catalyst by CO₂ hydrogenation.

Water-Lean Solvents for Post-Combustion CO2 Capture: Fundamentals, Uncertainties, Opportunities, and Outlook

This review is designed to foster the discussion regarding the viability of postcombustion CO₂ capture by water-lean solvents, by separating fact from fiction for both skeptics and advocates. We highlight the unique physical and thermodynamic properties of notable water-lean solvents, with a discussion of how such properties could translate to efficiency gains compared to aqueous amines. The scope of this review ranges from the purely fundamental molecular-level processes that govern solvent behavior to bench-scale testing, through process engineering and projections of process performance and cost. Key discussions of higher than expected CO₂ mass transfer, water tolerance, and compatibility with current infrastructure are presented along with current limitations and suggested areas where further solvent development is needed. We conclude with an outlook of the status of the field and assess the viability of water-lean solvents for postcombustion CO₂ capture.

Reinventing Design Principles for Developing Low-Viscosity Carbon Dioxide-Binding Organic Liquids for Flue Gas Clean Up

Anthropogenic CO₂ emissions from point sources (e.g., coal fired-power plants) account for the majority of the greenhouse gases in the atmosphere. Water-lean solvent systems such as CO₂ -binding organic liquids (CO₂ BOLs) are being developed to reduce the energy requirement for CO₂ capture. Many water-lean solvents such as CO₂ BOLs are currently limited by the high viscosities of concentrated electrolyte solvents, thus many of these solvents have yet to move toward commercialization. Conventional standard trial-and-error approaches for viscosity reduction, while effective, are time consuming and economically expensive. We rethink the metrics and design principles of low-viscosity CO₂ -capture solvents using a combined synthesis and computational modeling approach. We critically study the effects of viscosity reducing factors such as orientation of hydrogen bonding, introduction of higher degrees of freedom, and cation or anion charge solvation, and assess whether or how each factor affects viscosity of CO₂ BOL CO₂ capture solvents. Ultimately, we found that hydrogen bond orientation and strength is the predominant factor influencing the viscosity in CO₂ BOL solvents. With this knowledge, a new CO₂ BOL variant, 1-MEIPADM-2-BOL, was synthesized and tested, resulting in a solvent that is approximately 60 % less viscous at 25 mol % CO₂ loading than our base compound 1-IPADM-2-BOL. The insights gained from the current study redefine the fundamental concepts and understanding of what influences viscosity in concentrated organic CO₂ -capture solvents.

Two coexisting liquid phases in switchable ionic liquids

Switchable ionic liquids are attractive in gas capture, separations, and nanomaterial synthesis.

Improving the Molecular Ion Signal Intensity for In Situ Liquid SIMS Analysis

In situ liquid secondary ion mass spectrometry (SIMS) enabled by system for analysis at the liquid vacuum interface (SALVI) has proven to be a promising new tool to provide molecular information at solid-liquid and liquid-vacuum interfaces. However, the initial data showed that useful signals in positive ion spectra are too weak to be meaningful in most cases. In addition, it is difficult to obtain strong negative molecular ion signals when m/z>200. These two drawbacks have been the biggest obstacle towards practical use of this new analytical approach. In this study, we report that strong and reliable positive and negative molecular signals are achievable after optimizing the SIMS experimental conditions. Four model systems, including a 1,8-diazabicycloundec-7-ene (DBU)-base switchable ionic liquid, a live Shewanella oneidensis biofilm, a hydrated mammalian epithelia cell, and an electrolyte popularly used in Li ion batteries were studied. A signal enhancement of about two orders of magnitude was obtained in comparison with non-optimized conditions. Therefore, molecular ion signal intensity has become very acceptable for use of in situ liquid SIMS to study solid-liquid and liquid-vacuum interfaces. Graphical Abstract ᅟ.

Anomalous water expulsion from carbon-based rods at high humidity

Three water adsorption-desorption mechanisms are common in inorganic materials: chemisorption, which can lead to the modification of the first coordination sphere; simple adsorption, which is reversible; and condensation, which is irreversible. Regardless of the sorption mechanism, all known materials exhibit an isotherm in which the quantity of water adsorbed increases with an increase in relative humidity. Here, we show that carbon-based rods can adsorb water at low humidity and spontaneously expel about half of the adsorbed water when the relative humidity exceeds a 50-80% threshold. The water expulsion is reversible, and is attributed to the interfacial forces between the confined rod surfaces. At wide rod spacings, a monolayer of water can form on the surface of the carbon-based rods, which subsequently leads to condensation in the confined space between adjacent rods. As the relative humidity increases, adjacent rods (confining surfaces) in the bundles are drawn closer together via capillary forces. At high relative humidity, and once the size of the confining surfaces has decreased to a critical length, a surface-induced evaporation phenomenon known as solvent cavitation occurs and water that had condensed inside the confined area is released as a vapour.

Dynamic Acid/Base Equilibrium in Single Component Switchable Ionic Liquids and Consequences on Viscosity

The deployment of transformational nonaqueous CO2-capture solvent systems is encumbered by high viscosities even at intermediate uptakes. Using single-molecule CO2 binding organic liquids as a prototypical example, we present key molecular features that control bulk viscosity. Fast CO2-uptake kinetics arise from close proximity of the alcohol and amine sites involved in CO2 binding in a concerted fashion, resulting in a Zwitterion containing both an alkyl-carbonate and a protonated amine. The population of internal hydrogen bonds between the two functional groups determines the solution viscosity. Unlike the ion pair interactions in ionic liquids, these observations are novel and specific to a hydrogen-bonding network that can be controlled by chemically tuning single molecule CO2 capture solvents. We present a molecular design strategy to reduce viscosity by shifting the proton transfer equilibrium toward a neutral acid/amine species, as opposed to the ubiquitously accepted zwitterionic state. The molecular design concepts proposed here are readily extensible to other CO2 capture technologies.

Measuring the Absorption Rate of CO2 in Nonaqueous CO2-Binding Organic Liquid Solvents with a Wetted-Wall Apparatus

The kinetics of the absorption of CO2 into two nonaqueous CO2-binding organic liquid (CO2 BOL) solvents were measured at T=35, 45, and 55 °C with a wetted-wall column. Selected CO2 loadings were run with a so-called "first-generation" CO2 BOL, comprising an independent base and alcohol, and a "second-generation" CO2 BOL, in which the base and alcohol were conjoined. Liquid-film mass-transfer coefficient (k'g ) values for both solvents were measured to be comparable to values for monoethanolamine and piperazine aqueous solvents under a comparable driving force, in spite of far higher solution viscosities. An inverse temperature dependence of the k'g value was also observed, which suggests that the physical solubility of CO2 in organic liquids may be making CO2 mass transfer faster than expected. Aspen Plus software was used to model the kinetic data and compare the CO2 absorption behavior of nonaqueous solvents with that of aqueous solvent platforms. This work continues our development of the CO2 BOL solvents. Previous work established the thermodynamic properties related to CO2 capture. The present paper quantitatively studies the kinetics of CO2 capture and develops a rate-based model.

Reversible nonpolar-to-polar solvent

Imagine a smart solvent that can be switched reversibly from a liquid with one set of properties to another that has very different properties, upon command. Here we create such a system, in which a non-ionic liquid (an alcohol and an amine base) converts to an ionic liquid (a salt in liquid form) upon exposure to an atmosphere of carbon dioxide, and then reverts back to its non-ionic form when exposed to nitrogen or argon gas. Such switchable solvents should facilitate organic syntheses and separations by eliminating the need to remove and replace solvents after each reaction step.

The Reaction of 1,8-Diazabicyclo[5.4.0]undec-7-ene (DBU) with Carbon Dioxide

Amidines have been reported to react with CO(2) to form a stable and isolable zwitterionic adduct but previous studies were performed in the presence of at least some water. However, spectroscopy of the reaction between DBU and CO(2) detects the rapid formation of the bicarbonate salt of DBU when wet DBU is exposed to CO(2) and does not indicate that an isolable zwitterionic adduct between DBU and CO(2) forms either in the presence or the absence of water.

Liquid poly(ethylene glycol) and supercritical carbon dioxide: a benign biphasic solvent system for use and recycling of homogeneous catalysts

Poly(ethylene glycol) (PEG), having a molecular weight of 900 or 1500, is a solid at room temperature but a nonvolatile liquid at 40 degrees C under CO2 pressure. Homogeneously catalyzed hydrogenation can be performed in the molten PEG, followed by extraction of the product by supercritical CO2. The catalyst-containing PEG phase which remains in the vessel can be reused for hydrogenation without addition of further catalyst or PEG.