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Supercritical fluids - green solvents for green chemistry?









The Royal Society, London, 6-9 Carlton House Terrace, London, SW1Y 5AG


Scientific discussion meeting organised by Professor Peter Licence and Professor Andrew Cooper

Elastomer o-ring exposed to supercritical CO2. Credit - Sir Martyn Poliakoff CBE FRS

Event details

Supercritical fluids (SCFs) are universally acknowledged to be strategically vital for the development of more sustainable (greener) chemistry.  SCFs are now being introduced as solvents for chemical synthesis, extraction and analysis, in both academia and industry. This meeting will focus on successes and innovations in this active field.

Abstracts and biographies of the organisers and speakers are available below. Papers from this meeting will be published in a future issue of Philosophical Transactions A.

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Schedule of talks

Session 1

4 talks Show detail Hide detail

Polymers and supercritical carbon dioxide

Professor Steve Howdle, University of Nottingham


They are exploiting the unique properties of scCO2 to synthesise new polymers and materials that could not easily be made using conventional solvents. One of the major attractions is that carbon dioxide is freely available, inexpensive and provides an environmentally acceptable, and more energy efficient route to new materials, particularly for medical and pharmaceutical applications.

They have utilised controlled/living radical (RAFT) routes and dispersion polymerisation to create microparticles formed of block co-polymers. These materials show both the expected and also unusual morphologies on the nanoscale that arise from phase separation of the block copolymers.

Additionally he will describe approaches to the preparation and synthesis of new renewably sourced polymers that could be of value in a wide range of everyday applications. In particular, they highlight the utilisation of enzyme mediated polymerisation where scCO2 could bring significant advantages.

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Properties of supercritical fluid systems and their application in green chemistry

Professor Buxing Han, Chinese Academy of Science, China


Efficient utilization of greener solvents, such as supercritical fluids (SCFs) and ionic liquids (ILs) has attracted more and more attention. Green solvents have many unusual properties for developing green and efficient chemical processes. In recent years, they have been very interested in pyhsicochemical properties of green solvents and their applications in green chemistry, which include mainly: 1) phase behavior and intermolecular molecular interaction in complex SCFs, ILs, CO2/IL systems; 2) effects of phase behavior and intermolecular interactions on the properties of chemical reactions in SC CO2, ILs and CO2/ILs; 3)colloid and interface science of green solvent systems, including chemical thermodynamics, microstructures, and functions. In this presentation, he would like to discuss some recent results.

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Supercritical carbon dioxide: green solvent for pharmaceutical applications

Professor Vladimir Popov, Russian Academy of Sciences, Russia


Bioresorbable composite powders, in which biologically active guest species are dispersed throughout non-toxic polymer particles of specific sizes, morphology and porosity, have widespread pharmaceutical applications as depots and vehicles for controlled drug release. Conventional methods for preparing such materials use either toxic organic solvents or substantial temperature perturbation which often leads to thermal and solvent induced degradation of active pharmaceutical ingredients (API) or changes in their molecular conformations. Here, we report our results on development of a “green” approach to controlled drug release system fabrication based on supercritical fluid API purification, micronisation and encapsulation into biodegradable aliphatic polyethers. The combination of gas-like and liquid-like properties makes supercritical carbon dioxide (sc-CO2) a unique processing medium for preparation of polymer materials containing bioactive species, primarily because no solvent residues would remain in the final product. The process is based upon the interaction of sc-CO2 with a broad range of amorphous polymers which leads to reduction of their glass transition temperature, plasticisation and viscosity lowering. This facilitates an efficient incorporation of both soluble and insoluble in sc-CO2 guest substances following by composite microparticle generation. The entire process can be carried out at near ambient temperatures and provides excellent control of polymer morphology. This approach is applicable to a wide range of API which was used in our experiments: wound healing plant extracts and peptides, antibiotics, non-steroid analgesics and antipsychotic drugs. Unambiguous advantages of this methodology comprise the absence of organic solvents over all fabrication steps and versatile control of API release characteristics by delicate tuning of the process parameters.

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Better ways to deliver medications using supercritical fluids

Professor Ana Aguiar-Ricardo, Universidade Nova de Lisboa, Portugal


The integrated use of supercritical carbon dioxide (scCO2) and micro- and nano-technologies has enabled new sustainable strategies for the manufacturing of new medications. “Green” scCO2-based methodologies are well suited to improve synthesis and materials processing leading to assembly of three-dimensional multifunctional constructs. Novel routes are being explored to synthesize stimuli-responsive polymers, thus enabling the design of efficient platforms for sensing and delivery of therapeutic payloads.

By using scCO2, either as C1 feedstock or as solvent, simple, economic, efficient and clean routes can be designed to synthesize materials with unique properties such as PURE-type dendrimers, oxazoline- and aziridine based oligomers. These new biocompatible, biodegradable and water-soluble polymeric materials can be engineered into multifunctional constructs with antimicrobial activity, targeting moieties, labeling units, and/or efficiently loaded with therapeutics. In particular, the remarkable features of POxylated polyurea dendrimers as potential theranostic nanodevices will be highlighted. Also, it will be discussed how scCO2 - assisted spray drying can be implemented to develop engineered formulations comprising nano-inside-microparticles for pulmonary delivery.

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Session 2

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Fast time resolved spectroscopy in SCFs

Professor Michael George, University of Nottingham, UK


Supercritical fluids have many unique properties and photochemistry and photophysics of molecules has been widely exploited to elucidate these intriguing properties. Photochemistry is also an effective method to generate new and unusual compounds. In this lecture Professor George will review his work on photochemistry in supercritical fluids and focus on: (i) Organometallic Alkane Complexes, Noble Gas Complexes and C-H Activation: a key intermediate in catalytic C-H Activation by organometallic complexes reaction is formation of organometallic alkane complexes. The facile activation of methane is considered a ’holy grail’ for chemists. He has performed systematic studies using fast time-resolved IR spectroscopy (TRIR), which has generated long-lived alkane complexes, which has subsequently allowed the characterisation of these complexes’ nuclear magnetic resonance (NMR). The use of supercritical fluids allow the coordination and the activation of methane and other lighter alkanes to be probed which provides key insight into both the bonding of the alkane to the metal and this oxidative addition reaction. TRIR experiments in supercritical noble gas solvents allowed the characterisation of a range of organometallic noble gas complexes including a Re-Xe complex which has allowed the characterisation of organometallic Xe by NMR spectroscopy in liquefied xenon at low temperature. (ii) Photochemistry in Flow:  this aspect of the work at Nottingham work focuses on trying to use reactions of singlet oxygen (1O2) to ultimately be used an industrial setting. A particular attraction from the point of view of Green Chemistry is that both of the atoms of 1O2 add to the substrate giving 100% Atom Efficiency in its reactions.  Unfortunately, there are several barriers to the routine use of 1O2 including the current practice of carrying out the reactions in CCl4, a solvent which is no longer environmentally acceptable for large-scale processes.  The use of supercritical CO2, as a solvent for the reactions of 1O2 has a number of attractions, including non-flammability, non-toxicity and miscibility with O2 has produced a continuous process for photooxidation reactions using photogenerated singlet oxygen. Lastly Professor George will report the recent use of this approach for the synthesis of the anti-malarial drug artemisinin.

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Ionic liquids versus deep eutectic solvents

Professor Cornelis Peters, The Petroleum Institute, Abu Dhabi


The removal of CO2 from natural gas or the capturing of CO2 from flue gas, produced by post-combustion industries, became a big challenge due to the large volumes of CO2 and often the low CO2 concentration in the source gases. Most of the commercial absorption processes for CO2 use different types of alkanolamine solvents, e.g. monoethanolamine (MEA), diethanolamine (DEA) or N-methyldiethanolamine (MDEA). Despite their good performance as solvents for chemical absorption, amine technologies show several important drawbacks in terms of operational cost, solvent regeneration and the susceptibility of amines to undergo thermal or oxidative degradation. Moreover, the emissions of amines and its degradation products may cause serious damage to the environment and human health as well. For this reason, the search for alternative solvents for CO2 capturing became a prominent research activity in solvent design.

Ionic Liquids (ILs) attracted particular attention over the past decade because they can be designed by choosing the proper cation-anion combination to pursuit the best performance as solvents for a certain application. Together with their, in general, extremely low volatility, the so-called “Task Specific Ionic Liquids” show promising advantages for CO2 capture compared to the conventional solvents. However, the “green” character of ILs can be questioned because most of them are produced from fossil resources and their synthesis cannot be considered as being “green”. Moreover, the high production and purification cost do not make ILs technology competitive with traditional solvents.

To overcome some of the limitations of ILs, Deep Eutectic Solvents (DES) were recognized as potential alternatives. These low transition temperature mixtures (LTTMs) consist of at least one hydrogen bond donor (HBD) and one hydrogen bond acceptor (HBA) counterpart, resulting in the formation of a liquid mixture showing an unusual low freezing point. Due to the high hydrogen bonding interaction, some of the promising characteristics of ILs as solvents are shared by DESs. They often possess an extremely low volatility, and their properties can be adjusted by selecting the nature and ratio of the hydrogen bonding pairs. They can also be designed to show a wide liquid range, water-compatibility, non-flammability, non-toxicity, biocompatibility or biodegradability. Finally, they can be easily prepared from readily available starting materials and becoming a competitive solvent in terms of cost.

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High pressure liquid-liquid-vapor equilibrium for CO2-toluene-ionic liquid systems

Professor Joan Brennecke, University of Notre Dame, USA


One of the interesting applications of ionic liquids is in the liquid-liquid extraction of aromatics from aliphatics, nominally as an alternative to the conventional sulfolane process.  A variety of ionic liquids have been identified that provide both good selectivity and good capacity.  Of course, one must then remove the aromatic from the ionic liquid. One possibility, albeit energy intensive, is evaporation of the aromatic.  As an alternative, we have been investigating the possibility of using CO2 as an anti-solvent to induce a liquid-liquid phase split as a way of recovering and recycling the ionic liquid.  We present measurements of the solubility of CO2 in mixtures of toluene with four different ionic liquids at 25 °C and 40 °C and pressures up to the liquid-liquid phase split pressures.  The pressure required to cause the liquid-liquid phase split is lower at the lower temperature and when the toluene concentration in the initial mixture is higher. Of the four ionic liquids investigated, the one with the largest molar volume, trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide, requires the highest CO2 pressure to cause the phase split.  Initial measurements using an apparatus equipped with sampling of both liquid phases that are formed indicates virtually negligible ionic liquid in the toluene-rich phase.  Thus, high pressure CO2 appears to be a viable technique for aromatic recovery and ionic liquid recycling.

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Session 3

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Supercritical route for green materials

Professor Tadafumi Adschiri, Tohoku University, Japan


Green materials processing is a philosophy of chemical research and engineering to encourage the design of products and processes that minimize the use and generation of hazardous substances, which involves 1)  contribution of products to minimize environmental problems (CO2 emission, environmental cleaning catalyst etc.), 2) recycle of materials to resources, 3) holistic life cycle assessment of the materials, and 4) combined multiple technological and operational systems for reduction of energy and resources. Supercritical fluids technology is expected to contribute for new materials synthesis with the green sustainable chemistry route, especially for nanomaterials.

So far, variety of materials have been developed, including ceramics, metals and polymers, but recent needs in the industries are of multi-functions of ceramics/metals and polymers. For fabricating multi-functional materials, we proposed a new method to synthesize organic modified nanoparticles (NPs) in supercritical water. Since the organic molecules and metal salt aqueous solutions are miscible under the supercritical state, and water molecule works as an acid/base catalyst for the reactions, organic-inorganic conjugate nanoparticles can be synthesized under the condition. This synthesis method can control the exposed surface of NPs, which shows high catalytic activity of nano-catalysis; This promotes the bitumen or biomass waste decomposition (endothermic reaction) at lower temperature without coke formation. This gives rise to recover the waste heat and the waste treatment problems at the same time, namely solve the energy (CO2) problems.

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Supercritical water oxidation (SCWO) for the destruction of hazardous wastes – better than incineration

Dr Bushra Al-Duri, University of Birmingham


Supercritical water oxidation (SCWO) is based on the complete miscibility of supercritical water (SCW) with organics including hydrocarbons; biopolymers; and all gases, rendering SCW a superior reaction medium for the destruction of a diverse range of chemically stable wastes existent in municipal, clinical and industrial aqueous effluents, otherwise treated by incineration. The process is highly exothermic, generating high-grade heat, which is recoverable for other uses like production of electricity, after energy integration within the system. This contribution presents the main activities and studies at the University of Birmingham – UK. The scope of work is to achieve high performance, while using ‘simple’ reactor design, via improving the reaction kinetics. Laboratory investigations focus on the destruction of aliphatic and aromatic Ncontaining compounds, due to nitrogen abundance in waste and its interesting chemistry.

To this end, investigations have been carried out in a continuous (plug flow) reactor bench scale rig. Two approaches have been adopted: (i) ‘Split-oxidant’ system, where oxidant is split and fed via two inlets at different ratios, and (ii) the cofuel approach where iso-propyl alcohol is premixed with the feedstock. Both approaches gave positive outcomes. Theoretically, kinetic investigations have been conducted on the aliphatic, aromatic and real waste with success. Also,  detailed simulation of SCW heat transfer thermodynamic properties has been investigated using COMSOL, PROII and Matlab. Based on a PATENT (in application), a £0.5m industrially funded project of SCWO process prototype will start next month, including detailed consideration of the various process aspects, with a view of commercialization.

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Synthesis of nanostructured materials using supercritical CO2

Professor Albertina Cabanas, Universidad Complutense de Madrid, Spain


Supercritical CO2 (scCO2) is emerging as an excellent medium to prepared and/or modify nanostructured materials. Beside the environmental benefits, its high diffusivity and low viscosity and surface tension favour the penetration of precursors dissolved in scCO2 into nanostructures such as nanopores and the impregnation of high surface area materials, while preserving the support structure.

In particular, scCO2 can be used as the solvent and/or reaction media to deposit metals or metal oxides on different supports. Depending on the methodology employed and the experimental conditions nanoparticles, nanowires or thin films can be obtained. The preparation of these nanostructured metal-composite materials has numerous applications in catalysis, microelectronics, gas separation, hydrogen storage, sensors, fuel cells. In this presentation, they give examples of the deposition of Pd, Pt, Ru and Ni nanoparticles into mesoporous silica, carbon and graphene sheets using scCO2. The materials prepared have been tested as heterogeneous catalysts in hydrogenation reactions showing advantages over commercial catalyst.

The surface modification of highly porous materials can be also carried out in scCO2 by reaction of soluble silanes. Using this approach, they have introduced amine and thiol groups on the surface of mesoporous silica. The amine modified materials are proposed as CO2 sorbents in carbon capture processes. The thiol modified materials can serve as metal adsorbents. These surface modified supports can be also used in metal deposition experiments. In comparison to the conventional process using toluene, the surface modification in scCO2 is faster and yields, in some cases, larger silane loads.

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Industrial scale production of nanomaterials using continuous hydrothermal synthesis

Professor Ed Lester, University of Nottingham


Work is currently underway to build a full scale continuous hydrothermal synthesis plant in the UK. The plant uses a counter current reactor, designed at Nottingham. The plant will be capable of producing up to 2000 tons/year (dry weight equivalent) of a range of nanomaterials, from metals, metal oxides, hydroxides, carbonates and sulphides, as well as some more unusual materials such as metal organic frameworks. Specific questions need to be answered in the development of this plant and the presentation will cover each area in some detail.

Life Cycle Analysis – how does this process compare with other processes for nanomaterial production?

Formulation – can the nanomaterials be formulated continuously during production to facilitate product collection?

Waste treatment – what is present in the waste water post production? The precursors tend to be simple inorganic (e.g. nitrates) or organic (e.g. formates) salts, so what happens to the counter ion?

Recycling – can this waste water be recycled and/or can it be discharged without chemical pretreatment? The weight loading from the outlet tends to be around 1% wt/wt or less so can the products can be collected easily and effectively to produce a clean waste stream?

Reactor Performance – what are the fundamental limitations of the counter current reactor system in terms of mixing and fluid dynamics? As the Reynolds numbers increase with increasing flow rate, what happens to the mixing regime around the nozzle outlet?

Scale up design and sustainability– What does the plant look like and how can the energy demands be reduced through heat integration?

Scale out – can a system be built that allows multiple reactors to operate simultaneously? Does this allow different products at the same time?

To the authors knowledge this will be the largest continuous hydrothermal synthesis plant in the world, and should be completed by mid 2015. The plant itself should act as a demonstration facility for production at large scale and should hopefully help to provide a roadmap for future plants.

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Session 4

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Electrochemistry in supercritical fluids

Professor Phil Bartlett FRS, University of Southampton


Supercritical fluids are interesting but challenging solvents for electrochemistry.  They are challenging because they require working at elevated temperatures and pressures, and because the common supercritical fluids have low dielectric constants (<10). As a result it can be difficult to achieve sufficient dissociation of ions in solution to provide sufficient ionic conductivity for electrochemical studies.  Nevertheless supercritical fluids are attractive media for electrochemistry because their solvent properties can be tuned by varying the temperature and pressure, and because the lack of surface tension and high mass transport rates are attractive for the electrodeposition of high aspect ratio nanostructures.  In this lecture he reviews their work on electrochemistry in supercritical fluids and discuss the choice of suitable electrolytes, reagents and reference electrodes, and their progress in electrodeposition of nanostructures from supercritical fluids.

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Panta rhei - continuous-flow organometallic catalysis

Professor Walter Leitner, RWTH Aachen University, Germany


Continuous-flow catalysis based on molecularly-defined transition metal complexes in suitably supported state in combination with supercritical carbon dioxide (scCO2) as the mobile phase provides an attractive processing option for challenging catalytic transformations of low-volatility organic substrates, including asymmetric catalysis for the synthesis of chiral products.

The talk will illustrate how the design challenges at different time and length scales associated with this concept can be addressed in an integrated approach. Using an automated high-pressure continuous-flow setup enables isolation of the product directly in analytically pure form without the use of any organic co-solvent and with no detectable catalyst leaching. Phase behavior studies and high-pressure NMR facilitate the localization of optimum process parameters by quantification of substrate partitioning between the support matrix and scCO2. Fundamental insight into molecular interactions of metal complex, matrix, and support in working catalyst materials can be gained via spectroscopic studies, and labeling experiments. Finally, the rational development of dedicated chiral catalysts can open access to remaining “white spots” on the landscape of asymmetric transformations.

Examples for continuous-flow organometallic catalysis for the transformation of various functional groups including highly enantioselective synthese will be presented in the talk. Detailed comparison of batch-wise synthesis to continuous operation using stirred tank reactors or plug-flow tubular reactors will be critically discussed. Optimized continuous systems reach stable selectivities and productivities over extended periods of time with productivities corresponding to 1-10 kg L-1 h-1 space-time-yield and >100 kg product per gram catalyst, making such small and flexible units attractive for practical application.

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CO2-based solvents for extractions from biomass

Professor Philip Jessop, Queen's University, Canada


Many unusual solvents contain CO2. While the most obvious examples are supercritical CO2 and liquid CO2, other newer examples are CO2-expanded solvents and the three classes of CO2-triggered switchable solvents (switchable-polarity solvents, switchable-hydrophilicity solvents, and switchable water). After an introduction to each of these CO2-related solvents, including a comparison of their advantages and disadvantages, this presentation will focus on the use of such solvents for the extraction of valuable compounds and materials from biomass. Recent work in the author's group will be presented, such as the extraction of phenols from lignin pyrolysis oil, lipids from algae or oil seeds, and capsaicin from peppers.  Other applications include the production of natural rubber.  In each case, the extractions illustrate the advantages of the selected CO2-related solvents over others, but no solvent is appropriate for every application. However, the solvent best for the extraction is not the only way in which CO2 can be of assistance in biomass processing. New ways in which CO2 can assist in preparing biomass for extraction will also be presented.

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Supercritical fluids - green solvents for green chemistry? The Royal Society, London 6-9 Carlton House Terrace London SW1Y 5AG UK