Special Issue "Supercritical Fluids"

Guest Editor
Dr. Iolanda De Marco
University of Salerno, Department of Chemical and Food Engineering, Via Ponte Don Melillo 1, 84084 Fisciano (SA), Italy
E-Mail:
Interests: supercritical carbon dioxide; supercritical antisolvent micronization; supercritical extraction; supercritical impregnation

Guest Editor
Prof. Dr. Ernesto Reverchon
University of Salerno, Department of Chemical and Food Engineering, Via Ponte Don Melillo 1, 84084 Fisciano (SA), Italy
E-Mail:
Interests: related to supercritical fluids applications; in details: extraction; fractionation; microparticles and nanoparticles generation; emulsions; membranes and scaffolds

Dear Colleagues,

This special issue is intended to give an overview of supercritical fluid (SCF) applications in materials science.
A supercritical fluid is any substance at a temperature and pressure above its critical point. It shows a gas-like diffusivity and a liquid-like density; therefore, it can substitute organic solvents in a wide range of processes. Carbon dioxide and water are the most commonly used supercritical fluids.

Several processes using supercritical fluids have been proposed and, in some cases, applied to industrial scale, like, for example:

• Supercritical extraction and fractionation for food and pharmaceutical products
• Micro and nanoparticles (single or composite) production in pharmaceutical, nutraceutical, cosmetic and biomedical fields
• Micro and nanoemulsions
• Polymer applications like membranes, foams and scaffolds production or polymers synthesis, modification and recycling
• Reactions in supercritical media
• Inorganic or organic aerogels impregnation
  Nanostructured inorganic and metallic materials processing for catalysis, electronics and optics

We expect contributions to this special issue prevalently in these areas with the aim to set the state of the art and to promote research and industrial development in this field.

Iolanda De Marco, Ph. D.
Prof. Dr. Ernesto Reverchon
Guest Editors

Submission

All manuscripts should be submitted to materials@mdpi.org with a copy to the Guest Editor. Manuscripts can be submitted until the deadline. Papers will be published continuously (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are refereed through a peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Materials is an international peer-reviewed Open Access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this Open Access journal is 800 CHF per accepted paper.

• supercritical carbon dioxide
• supercritical water
• microparticles
• nanoparticles
• emulsions
• supercritical reactions
• nanostructured materials

Feature Papers

Type of Paper: Review
Title: Hydrothermal Synthesis of Metal Oxide Nanoparticles in Supercritical Water
Authors: Hiromichi Hayashi and Yukiya Hakuta
Affiliation: National Institute of Advanced Industrial Science and Technologes,Nigatake-4-2-1, Miyagino-ku, Sendai, 983-8551, Japan; E-Mail: h-hayashi@aist.go.jp
Abstract: Hydrothermal synthesis in supercritical water has advantages for synthesis of multi metal oxide compounds because the reaction rate is enhanced more than 10³ times than that under the conventional hydrothermal conditions (< 473K) owing to the low dielectric constant(< 10) as well as products with high crystallinity. Especially, density of water can be varied with the temperature and pressure under supercritical conditions. The crystallite phase can be controlled with the density of water. Tetragonal barium titanate(BaTiO₃) can be obtained by the flow reaction method under supercritical water conditions when the density of water was smaller than 0.5g/cm³[1-3]. γ-Al₂O₃ nanoparticles were obtained at 683K or higher where the density of water is 0.25g/cm³ or lower. In addition, compared with the batch reaction system, the particle size can be reduced by the flow reaction system due to short reaction time. Potassium titanate nanowire can be achieved by the flow reaction system at few seconds.
In the case of batch reaction system, the advantage for synthesis of metal oxides in supercritical water is reducing alkaline concentration for the crystal growth. Potassium niobate is a kind of famous ferroelectric materials, which has promising applications in photocatalyst and non-linear optical device. However, high concentration of KOH (< 6M) is required for the conventional hydrothermal preparation of KNbO₃. We have demonstrated hydrothermal synthesis of potassium niobate powders with various subcritical and supercritical conditions (473-673 K) under low KOH concentration (0.1-0.5M). A single phase of K₄Nb₆O₁₇ was formed under subcritical conditions, while mixed phases of K₄Nb₆O₁₇ and KNbO₃ were obtained under supercritical water conditions where KNbO₃ was predominated as the heating duration was increased . Thus, single phase of KNbO₃ can be achieved by hydrothermal synthesis in supercritical water even under low KOH concentration. The hydrothermally synthesized K₄Nb₆O₁₇ was used for photocatalytic hydrogen evolution from water decomposition. The hydrogen evolution rate over Ni loaded K₄Nb₆O₁₇ was much higher in comparison with the Ni-loaded solid state synthesized K₄Nb₆O₁₇. In contrast, the orthorhombic form of KNbO₃ powder shows a strong second harmonic generation (SHG) activity of non-linear optical property which is similar intensity of the solid-state synthesized KNbO₃. Similar compounds such as potassium titanoniobate and potassium tantalite can be obtained in supercritical water even even under low KOH concentration.
The application for metal oxide nanoparticles are extending in many fields. For example, TiO₂, ZrO₂, and BaTiO₃ nanoparticles were dispersed in poly(bisphenol A carbonate) matrix to fabricate the high refractive index transparent polymer nanocomposites for optical materials. Another metal oxides nanoparticles will be reviewed applying for catalysts, phosphor so on.

Regular Paper

Type of Paper: Review
Title: The Potential of Supercritical Fluids to Support the Integration of Biorefining
Authors: Muhammad Baig, Steve Bowra and Regina Santos
Affiliation: School of Chemical Engineering, College of Engineering and Physical Sciences, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK; E-Mail: r.c.d.santos@bham.ac.uk
Abstract: Biorefining of biomass to create a range of chemical intermediates, materials and biofuels that will impact on all commercial sectors is a widely voiced vision for the future. The ultimate configuration of biorefining is that it will integrate, in a continuous flow, multiple processes and transformations yielding multiple products. The development of integrated biorefining offers the opportunity to design the process according sustainability principles which eliminate the environmental footprint associated with current processing and manufacture. Central to the design of the integrated process is the choice of solvent/s. Supercritical fluids are widely acknowledged as being environmentally benign 'green' solvents, therefore have the potential to support the integration of biorefining. In this context will review the many applications of supercritical fluids from extraction, encapsulation to material production with the intention of demonstrating the unique properties that confirm advantages and disadvantages of supercritical fluid to support integrated biorefining. We will conclude the review by illustrating the further research required to realise the full potential of supercritical fluids in relation to bio-processing.

Type of Paper: Review
Title: Dense CO₂ as a Solute or Co-Solvent in Particle Formation Processes: A Review
Authors: Ana V.M.Nunes ¹ and Catarina M.M.Duarte ^1,2
Affiliations: ¹ Instituto de Biologia Experimental e Tecnológica, Apartado 12, 2781-901 Oeiras, Portugal; E-Mail: anunes@itqb.unl.pt (A.V.M.N.)
² Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Avenida da Republica, 2780-157 Oeiras, Portugal
Abstract: The application of dense gases in particle formation processes has attracted great attention due to documented advantages over conventional technologies. In particular the use of dense CO₂ as a solute in the process has been subject of many works and explored in a variety of different techniques such as PGSS (Particles from gas-saturated solutions®), DELOS (Depressurization of an Expanded Liquid Organic Solution), CPCSP (Continuous Powder Coating Spraying Process), SAA (Supercritical Fluid-Assisted Atomization), CAN-BD (Carbon dioxide Assisted Nebulization with a Bubble Dryer®). This article presents a review of the current available techniques in use in particle formation processes, focusing exclusively on those employing dense CO₂ as a solute or co-solvent during the process. Special emphasis is given to recent advances and improvements introduced in the processes, as well as the difficulties that have been overcome.