Simultaneous switching of two different CO2-switchable amines in the same solution.

Most CO2-responsive systems operate by using a base in water that is expected to be mostly deprotonated when under an atmosphere of air and mostly protonated under an atmosphere of CO2. This concept has led to the development of many different CO2-responsive materials such as solvents, polymers, surfactants, and solutes. As CO2-responsive materials research continues, more complex systems may be developed, including systems containing two different bases with different basicities. Understanding the influence each base has on the protonation equilibrium of the other base is important for designing systems in which effective deprotonation and protonation occur. This article presents a model that can predict the solution pH and the % protonation of two different bases at various concentrations under air and CO2. Experimental data was collected to demonstrate the successful simultaneous switching of two amines and to evaluate the accuracy of the predictive model. The simultaneous switching of two different CO2-switchable amines in the same solution was determined to be possible but only if the amine concentrations and basicities are within certain ranges, and only if the pKaH values of the two bases differ by no more than 3 units.

CO2-Switchable colloids.

The development and design of CO2-switchable colloidal particles is described. A presentation of the principles of CO2 switching, especially as they apply to colloids, is followed by recent progress in the preparation of several types of colloidal particles (polymer nanoparticles, metal-organic frameworks (MOFs), quantum dots, graphene, cellulose nanocrystals, carbon nanotubes) for various applications (Pickering stabilizers, catalysts, latexes), and our perspective on future opportunities.

Adaptive Catalysts for the Selective Hydrogenation of Bicyclic Heteroaromatics using Ruthenium Nanoparticles on a CO2 -Responsive Support.

Ruthenium nanoparticles (NPs) immobilized on an amine-functionalized polymer-grafted silica support act as adaptive catalysts for the hydrogenation of bicyclic heteroaromatics. Whereas full hydrogenation of benzofuran and quinoline derivatives is achieved under pure H2 , introducing CO2 into the H2 gas phase leads to an effective shutdown of the arene hydrogenation while preserving the activity for the hydrogenation of the heteroaromatic part. The selectivity switch originates from the generation of ammonium formate species on the surface of the materials by catalytic hydrogenation of CO2 . The CO2 hydrogenation is fully reversible, resulting in a robust and rapid switch between the two states of the catalyst adapting its performance in response to the feed gas composition. A variety of benzofuran and quinoline derivatives were hydrogenated to fully or partially saturated products in high selectivity and yields simply by altering the composition of the feed gas from H2 to H2 /CO2 . The adaptive catalytic system thus provides controlled access to valuable products using a single catalyst rather than two specific and distinct catalysts with static reactivity.

Influence of a CO2-switchable additive on the surface and foaming properties of a cationic non-switchable surfactant.

Switchable materials in general and CO2-switchable materials in particular are of great interest in environmental research. The replacement of common non-switchable materials (solutions, solvents, surfactants, etc.) with their switchable counterparts has a great potential to make processes more environmentally friendly by enhancing reusability and circularity and thus reducing energy costs and material consumption. Inspired by this, the present work deals with the surface and foaming properties of aqueous solutions of a non-switchable surfactant in presence of a CO2-switchable additive. A 1 : 1 and a 1 : 5 (molar ratios) mixture of the non-switchable surfactant C14TAB (tetradecyltrimethylammonium bromide) and the CO2-switchable additive TMBDA (N,N,N,N-tetramethyl-1,4-butanediamine) were investigated. It was found that surface properties, foamability, and foam stability can be changed by switching the additive with CO2 as a trigger. This observation can be explained by the fact that TMBDA is surface active in its unprotonated, i.e. neutral form, which disturbs the tight packing of the surfactant molecules on the surface. As a consequence, foams generated with surfactant solutions containing the neutral TMBDA are less stable than their TMBDA-free counterparts. On the other hand, the switched diprotonated additive is a 2 : 1 electrolyte with hardly any surface activity and thus does not affect surface and foam properties.

CO2-Responsive Low Molecular Weight Polymer with High Osmotic Pressure as a Draw Solute for Forward Osmosis.

A key challenge in the development of forward osmosis (FO) technology is to identify a suitable draw solute that can generate a large osmotic pressure with favorable water flux while being easy to recover after the FO process with a minimum of energy expenditure. While the CO2- and thermo-responsive linear poly(N,N-dimethylallylamine) polymer (l-PDMAAm) has been reported as a promising draw agent for forward osmosis desalination, the draw solutions sufficiently concentrated to have high osmotic pressure were too viscous to be usable in industrial operations. We now compare the viscosities and osmotic pressures of solutions of these polymers at low and high molecular weights and with/without branching. The best combination of high osmotic pressures with low viscosity can be obtained by using low molecular weights rather than branching. Aqueous solutions of the synthesized polymer showed a high osmotic pressure of 170 bar under CO2 (πCO2) at 50 wt% loading, generating a high water flux against NaCl feed solutions in the FO process. Under air, however, the same polymer showed a low osmotic pressure and a cloud point between 26 and 33 °C (depending on concentration), which facilitates the recovery of the polymer after it has been used as a draw agent in the FO process upon removal of CO2 from the system.

Electrical Response of Poly(N-[3-(dimethylamino)Propyl] Methacrylamide) to CO2 at a Long Exposure Period.

Amine-functionalized polymers (AFPs) are able to react with carbon dioxide (CO2) and are therefore useful in CO2 capture and sensing. To develop AFP-based CO2 sensors, it is critical to examine their electrical responses to CO2 over long periods of time, so that the device can be used consistently for measuring CO2 concentration. To this end, we synthesized poly(N-[3-(dimethylamino)propyl] methacrylamide) (pDMAPMAm) by free radical polymerization and tested its ability to behave as a CO2-responsive polymer in a transducer. The electrical response of this polymer to CO2 upon long exposure times was measured in both the aqueous and solid phases. Direct current resistance measurement tests on pDMAPMAm films printed along with the silver electrodes in the presence of CO2 at various concentrations reveal a two-region electrical response. Upon continuous exposure to different CO2 flow rates (at a constant pressure of 0.2 MPa), the resistance first decreased over time, reaching a minimum, followed by a gradual increase with further exposure to CO2. A similar trend is observed when CO2 is introduced to an aqueous solution of pDMAPMAm. The in situ monitoring of pH suggests that the change in resistance of pDMAPMAm can be attributed to the protonation of tertiary amine groups in the presence of CO2. This two-region response of pDMAPMAm is based on a proton-hopping mechanism and a change in the number of free amines when pDMAPMAm is exposed to various levels of CO2.

CO2-Assisted asymmetric hydrogenation of prochiral allylamines.

A new methodology for the asymmetric hydrogenation of allylamines takes advantage of a reversible reaction between amines and carbon dioxide (CO2) to suppress unwanted side reactions. The effects of various parameters (pressure, time, solvent, and base additives) on the enantioselectivity and conversion of the reaction were studied. The homogeneously-catalyzed asymmetric hydrogenation of 2-arylprop-2-en-1-amine resulted in complete conversion and up to 82% enantiomeric excess (ee). Added base, if chosen carefully, improves the enantioselectivity and chemoselectivity of the overall reaction.

Selectivity control in hydrogenation through adaptive catalysis using ruthenium nanoparticles on a CO2-responsive support.

With the advent of renewable carbon resources, multifunctional catalysts are becoming essential to hydrogenate selectively biomass-derived substrates and intermediates. However, the development of adaptive catalytic systems, that is, with reversibly adjustable reactivity, able to cope with the intermittence of renewable resources remains a challenge. Here, we report the preparation of a catalytic system designed to respond adaptively to feed gas composition in hydrogenation reactions. Ruthenium nanoparticles immobilized on amine-functionalized polymer-grafted silica act as active and stable catalysts for the hydrogenation of biomass-derived furfural acetone and related substrates. Hydrogenation of the carbonyl group is selectively switched on or off if pure H2 or a H2/CO2 mixture is used, respectively. The formation of alkylammonium formate species by the catalytic reaction of CO2 and H2 at the amine-functionalized support has been identified as the most likely molecular trigger for the selectivity switch. As this reaction is fully reversible, the catalyst performance responds almost in real time to the feed gas composition.

Reversible Absorption of Volatile Organic Compounds by Switchable-Hydrophilicity Solvents: A Case Study of Toluene with N,N-Dimethylcyclohexylamine.

Absorption is one of the most important treatment technologies for the removal of volatile organic compounds (VOCs) from tail gases, yet the separation of the absorbents and VOCs remains challenging because of concerns related to environmental impact and large energy requirements. Herein, we explored an absorption and desorption process using N,N-dimethylcyclohexylamine (CyNMe2) as a representative switchable-hydrophilicity solvent (SHS) and toluene as a representative VOC. The results showed that in comparison to common absorbents, CyNMe2 exhibits excellent toluene absorption performance. Desorption efficiencies of toluene from CyNMe2 of up to 94% were achieved by bubbling CO2 at 25 °C, and separation efficiencies of CyNMe2 from water up to 90% were achieved by bubbling N2 at 60 °C. Even after five absorption-desorption cycles, the toluene absorption capacity of CyNMe2 was comparable with that of the fresh absorbent, suggesting that CyNMe2 retains its absorption capacity. We demonstrate an innovative and reversible remediation strategy of VOCs based on SHSs, and the results indicate that SHSs can be used as an alternative to common absorbents for the removal of VOCs to reduce environmental pollution and energy consumption.

Surface Modification of Cellulose Nanocrystals via RAFT Polymerization of CO2-Responsive Monomer-Tuning Hydrophobicity.

Cellulose nanocrystals (CNCs) were converted into a CO2-responsive composite nanomaterial by grafting poly(dimethylaminoethyl methacrylate) (PDMAEMA), poly(diethylaminoethyl methacrylate) (PDEAEMA), and poly(diisopropylaminoethyl methacrylate) (PDPAEMA) onto its surface using both grafting-to and grafting-from approaches. The zeta potential (ζ) of the graft-modified CNC could be reversibly switched by protonation/deprotonation of the tertiary amine groups simply by sparging with CO2 and N2, respectively. Depending on the grafting density and the molecular weight of the polymer grafts, CNC can form stable aqueous dispersions at either mildly acidic pH (under CO2) or mildly basic (under N2) conditions. Moreover, it was also determined that the CNC hydrophobicity, assessed using phase-shuttling experiments at different pH values, was also dependent on both the grafting density and molecular weight of the polymer grafts, thereby making it possible to easily tune CNC dispersibility and/or hydrophobicity.

Formylation of Amines by CO2 Hydrogenation Using Preformed Co(II)/Nickel(II) Complexes.

Preparation of formamides by CO2 hydrogenation requires an efficient catalyst and temperatures around 100 °C or higher, but most catalysts reported so far incorporate rare and toxic precious metals. Five cobalt(II) or nickel(II) complexes with dmpe or PNN (dmpe = 1,2-bis(dimethylphosphino)ethane; PNN = [(2-(di-tert-butylphosphinomethyl)-6-diethylaminomethyl)pyridine) have been evaluated as precatalysts for the hydrogenation of CO2 to prepare formamides from the corresponding secondary amines. The most active catalyst for these reactions was found to be [NiCl2(dmpe)] in DMSO, producing dimethylformamide (DMF) from CO2, H2, and dimethylamine in up to 6300 TON, the highest activity reported for this reaction with an abundant metal-phosphine complex.

Microsuspension Polymerization of Styrene Using Cellulose Nanocrystals as Pickering Emulsifiers: On the Evolution of Latex Particles.

We report a mechanistic study of the microsuspension polymerization of styrene stabilized by cellulose nanocrystals (CNCs) in its native form as well as graft-modified with copolymers of styrene and N-3-(dimethylamino)propyl methacrylamide (DMAPMAm) or N,N-(diethylamino)ethyl methacrylate (DEAEMA). Native CNCs and graft-modified CNCs were shown to form stable styrene emulsions with an average droplet diameter of 18-20 and 5-9 µm, respectively. Initiators of widely varying water solubilities [2,2'-azobisisobutyronitrile (AIBN), 2-2'-azobis(2,4-dimethylvaleronitrile) (Vazo-52), and lauroyl peroxide (LPO)] were employed for the polymerizations. The type of initiator and the type of CNC were shown to directly affect the microsuspension polymerization kinetics, particle size, and molecular weight distribution. Using AIBN and Vazo-52, submicron latex particles were observed in the final latex in addition to the desired 3-20 µm CNC-armored microsuspension particles. The resulting latex and microsuspension polystyrene particles were studied for their CNC coverage and surface charge. We found that the presence of CNCs in the aqueous phase did not lead to Pickering emulsion polymerization by heterogeneous nucleation.

Rh(i) and Ru(ii) phosphaamidine and phosphaguanidine (1,3-P,N) complexes and their activity for CO2 hydrogenation.

Phosphaamidine metal complexes Rh2Cl2[Ph2PC(Ph)[double bond, length as m-dash]NPh]2µ-CO (1), RuCl2[Ph2PC(Ph)[double bond, length as m-dash]N(Ph)]2 (2), [Rh{iPr2PC(Ph)[double bond, length as m-dash]NiPr}(COD)]BF4 (3), and RuCl2[iPr2PC(Ph)[double bond, length as m-dash]NiPr](DMSO)2 (4) are prepared by combining phosphaamidines Ph2P-C(Ph)[double bond, length as m-dash]NPh and iPr2P-C(Ph)[double bond, length as m-dash]NiPr (1,3-P,N) with their corresponding metal ions. Complexes 1 and 2 are stable in air while 3 and 4 are stable under inert conditions. For further comparison of structure and stability, a Ru(ii) phosphaguanidine complex, RuCl2[Me2NC(PPh2)[double bond, length as m-dash]NiPr](DMSO)2 (6) was prepared. Complex 6 is stable in air and in the presence of water. The structures of the phosphaamidine and phosphaguanidine complexes, determined using single crystal X-ray diffraction, revealed P,N bidentate coordination. While all five complexes have some activity as precatalysts, complex 6 was the most active.

Sodium Methyl Carbonate as an Effective C1 Synthon. Synthesis of Carboxylic Acids, Benzophenones, and Unsymmetrical Ketones.

Reported is the synthesis of carboxylic acids, symmetrical ketones, and unsymmetrical ketones with selectivity achieved by exploiting the differential reactivity of sodium methyl carbonate with Grignard and organolithium reagents.

CO2-Switchable-hydrophilicity membrane (CO2-SHM) triggered by electric potential: faster switching time along with efficient oil/water separation.

We report a membrane that can be reversibly switched between a hydrophilic state and a hydrophobic state simply by alternately bubbling CO2 into and passing electric potential (EP) through a solution in contact with the membrane. The prepared membrane could be effectively used for oil/water separation.

Characterizing the Effects of a "Switchable Water" Additive on the Aqueous Solubility of Small Molecules.

"Switchable water" is an aqueous solution containing a water-soluble amine additive that exhibits CO2 -switchable properties, such as large changes in ionic strength, by forming an ammonium bicarbonate salt. Switchable water has been used to reversibly "salt-out" organic compounds from water. This study explores the salting out of several compounds in switchable water when CO2 is present and also explores the solubility of small molecules in switchable water, compared to pure water, when CO2 is absent. The results show that organic compounds are generally more soluble in switchable water than pure water in the absence of CO2 , but less soluble in the presence of 1 atm CO2 . Exceptions include carboxylic acids and phenols which, presumably due to their acidity, are more soluble in switchable water than in pure water, even when CO2 is applied. Kirkwood-Buff solvation theory was applied to gain insights into the effects of the amine additive on the aqueous solubility of caffeine. Furthermore, the switchable properties of the additives allow for the preparation of switchable aqueous two-phase systems.

Diamines as switchable-hydrophilicity solvents with improved phase behaviour.

Removing solvents by distillation is not a sustainable process because it requires the use of volatile solvents and a high energy input. An alternative is to use a switchable-hydrophilicity solvent (SHS), which can be removed from products and recycled without any distillation step. SHSs are solvents that reversibly switch between hydrophilic and hydrophobic forms with the addition and removal of a trigger such as CO2. Monoamine SHSs can be separated from dissolved products by extraction into carbonated water, but the solvent removal is limited by the distribution coefficient of the SHS between the carbonated water phase and the product phase. In this article, the use of diamines as SHSs with improved distribution coefficients is explored. Several diamine SHSs are identified and their properties compared to those of monoamine SHSs. Comparisons include the pK aH (the pK a of the conjugate acid of a base) and log K ow (log of the octanol-water partition coefficient) requirements for amines to act as SHSs, distribution coefficients, removal from hydrophobic liquids, switching speeds, and risks to the environment and human health and safety.

A novel switchable-hydrophilicity, natural deep eutectic solvent (NaDES)-based system for bio-safe biorefinery.

A switchable-hydrophilicity solvent system, consisting of a fatty acid-based natural deep eutectic solvent (NaDES), complemented by a bio-friendly dilute amine solution, has been introduced. The potential of the most benign switchable solvent system has been characterised in microalgae biorefining according to the recently proposed 'Circular Extraction' scheme.

Fundamental properties and practical applications of ionic liquids: concluding remarks.

The Faraday Discussion on Ionic Liquids: From Fundamental Properties to Practical Applications took place in Cambridge in September 2017. Fundamental understanding of behaviour of liquids in the bulk and at surfaces was the primary emphasis of most of the talks, although applications were the motivation for the selection of many of the research projects. However, the conference almost entirely omitted discussion of the potential role of ionic liquids in green chemistry. Although initial claims of ionic liquids (ILs) being green were overstated, the search for green ionic liquids is still very much a worthwhile endeavour. Some confusion in the field has been caused by an overemphasis on the environmental impacts of ILs themselves, despite the fact that the manufacture of ILs causes greater impacts. Additional confusion has arisen from the mistaken use of the ready biodegradability test as an indicator for ultimate degradation. Because some ILs contain cores that are highly resistant to degradation, the ready biodegradability test can give a false positive result. The author offers suggestions as to how to tackle the problem of searching for greener ILs, including a greater focus on the impacts of the synthetic pathways of relevant ions. The final decision of whether an IL is green can only come from an application-specific life cycle assessment of a product or process using the IL compared to the same product/process using competing liquids.

Catalytic Formylation of Primary and Secondary Amines with CO2 and H2 Using Abundant-Metal Catalysts.

Catalytic hydrogenation of CO2 is an efficient and selective way to prepare formic acid derivatives, but most of the highly active catalysts used for this purpose require precious metals. In this study, in situ abundant-metal complexes have been evaluated as potential catalysts for CO2 hydrogenation to prepare formamides, including N-formylmorpholine, 2-ethylhexylformamide, and dimethylformamide, from the corresponding amines. From these initial screening results, the most active catalysts for these reactions were found to be MX2/dmpe in situ catalysts (M = Fe(II), Ni(II); X = Cl-, CH3CO2-, acac-; dmpe = 1,2-bis(dimethylphosphino)ethane) in DMSO. The optimal reaction conditions were found to be 100-135 °C and a total pressure of 100 bar. Morpholine was formylated with a TON value of up to 18000, which is the highest TON for the hydrogenation of CO2 to formamides using any abundant-metal-phosphine complex. With an appropriate selection of catalyst and reaction conditions, >90-98% conversion of amine to formamide could be achieved.