Features of supercritical CO2 in the delicate world of the nanopores
Graphical abstract
Introduction
The field of porous materials is, currently, at an exciting stage in its technological evolution. The research on ordered −including zeolites, zeotypes, metal-organic frameworks and mesoporous silica- and disordered −including ceramics, sintered metals and foamed polymers- porous solids [1] is among the most creative, fascinating and attractive fields of materials science. The supercritical fluid technology addressed the processing of porous matter from the beginning. Innovations in “porous materials and supercritical fluids” were compiled by A.I. Cooper in 2003 [2]. The basis of the developments of supercritical carbon dioxide (scCO2) methodologies in porous materials is two-fold: first, the solubility of scCO2 in polymers, with a pressure-dependent behavior, is substantial in comparison with conventional solvents; and second, the adsorptive behavior of scCO2 in inorganic porous systems is insignificant when compared to liquid fluids, which allows the one-step design of surface grafting and impregnation processes.
scCO2 technology applied to nanopores takes profit of the compressed CO2 gas-like viscosity, high diffusivity and null surface tension, so capillary stresses are suppressed, converting this fluid in a non-damaging solvent for those structures, facilitating their synthesis and modification. The use of scCO2 overcomes the limitations of diffusivity and mass transfer of conventional solvents and can transfer an effective amount of materials into very small pores. Most importantly, pore collapse can be avoided because the expansion of scCO2 directly as a gas does not give rise to a liquid-vapor interface. When the process is carried out from a liquid solution, the possibility of competition between solvent and solute molecules for the substrate adsorption sites often leads to the incorporation of both components into the internal surface of the porous system. Competition between the solvent and the solute for the substrate adsorption sites is reduced in scCO2 with respect to liquid solvents, since supercritical fluids are essentially not absorbed. The adsorption by micropores, called micropore filling, is distinguished from capillary condensation that is molecular adsorption by mesopores, the later not possible in supercritical fluids. Only microporous materials are slightly effective at adsorbing scCO2, as physical adsorption is enhanced by the overlapping of the molecule-surface interaction potentials from opposite pore walls. The null or little use of organic solvents, the straight preparation of dry products in confined autoclaves and the CO2 intrinsic sterility are of particular interest to produce different nanoporous systems, their stabilization and formulation.
The production of bulk polymeric porous materials, which can be visualized as sponge-like substances with disordered pores, has been deeply studied using scCO2 [3]. In these materials, the open porosity is not intrinsic; actually, it is generated during the supercritical treatment. An additional bulk key product developed using this technology is the aerogel, obtained from an organogel after a supercritical drying treatment, which ensures the characteristic properties of this mesoporous material [4]. Finally, pore densification by scCO2 of structural concrete is a process that also deserves attention, due to the overwhelming success of the cement industry in our society since Roman times [5]. For ordered porous materials, scCO2 has traditionally been used to modify the characteristics of their internal surface or empty volume. Microporous zeolites, with intrinsic uniform pores, are the most consumed substrates in industry [6]. However, certain limitations, such as zeolites small pore size and structural rigidity, have motivated the development of alternative ordered porous materials, such as mesoporous silica [7], flexible metal-organic frameworks (MOFs) [8] and hierarchical zeolites [9]. Those compounds can be modified using scCO2 solvent; moreover, MOFs can be prepared in scCO2 plus a cosolvent and/or in the presence of an ionic liquid [10]. Main applications of scCO2 in the field of inorganic meso and microporous solid substrates are related to adsorption (e.g., high-value non-volatile organics separation, impregnation for drug delivery, protective coating and surface functionalization and CO2 capture) and desorption (e.g., cleaning and drying, regeneration of sorbents and extraction) processes. Besides being used as a solvent, scCO2 can also play other primary roles, such as antisolvent, solute or reaction medium, which offer a unique flexibility as a surface engineering technology [11].
The aim of this article is to cover areas where the unique properties of scCO2 are exploited to generate porous materials with characteristics difficult to obtain by other routes, highlighting the specific benefits associated with the use of this fluid in relation to composition, purity, physiochemical properties, porosity and effectiveness in chosen applications. Herein, some of the most prominent classes of disordered and ordered porous materials are analyzed in detail, from both a synthetic and applied point of views, by focusing in examples of supercritically produced porous materials in our laboratory during the last two decades.
Section snippets
Polymers
From the beginning, polymers precipitation, modification and synthesis had constituted some of the most active areas of research in supercritical fluid technology [12], [13], [14], which soon led to its utilization in the production of polymeric foams [15]. First studies were focused on the foaming of high-viscosity amorphous polymers, such as polystyrene (PS) or polyethylene (PE) and their blends [16], [17], which are some of the most widely used commodity polymers for insulation and packaging
General aspects of scale-up
Current manufacturing bottom-up methods of sophisticated nanostructured objects, including nanoporous materials, are not easily applied to mass-production, which severely hinder their widespread commercialization. On one hand, the physical routes, mostly based on rapid condensation, lead to products with low contamination levels, but they are not easily scaled up at a reasonable cost. On the other hand, chemical bulk approaches can provide large quantities of nanosized entities at relatively
Concluding remarks and perspectives
As the contents of this article show, the current stage of development of scCO2 in porous matter has reached a peak in the accumulation of experimental facts and their theoretical understanding. A large number of supercritical researchers are involved in this area of science. Regarding discoveries and applications found for the technology, we can all say that during last decades “we have seen things you people wouldn't believe… Some of them will be lost in time, like tears in rain” [108], but
Acknowledgements
Authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness, through the “Severo Ochoa” Programme for Centres of Excellence in R&D (SEV-2015-0496) and project CTQ2014-56324-CO2-P1; and by the Generalitat de Catalunya with project 2014SGR377. A.M.L.P. acknowledges the RyC 2012-11588.
References (109)
- et al.
Binary ionic liquid supercritical CO2 solvent mixtures for the synthesis of 3D metal-organic frameworks
Microp. Mesop. Mater.
(2016) - et al.
Supercritical carbon dioxide as a green solvent for processing polymer melts: processing aspects and applications
Prog. Polym. Sci.
(2006) - et al.
Solubilities and diffusion coefficients of carbon dioxide and nitrogen in polypropylene, high-density polyethylene, and polystyrene under high pressures and temperatures
Fluid Phase Equilib.
(1999) - et al.
Bio-based polymers, supercritical fluids and tissue engineering
Process Biochem.
(2015) Polymer miscibility phase separation, morphological modifications and polymorphic transformations in dense fluids
J. Supercrit. Fluids
(2009)- et al.
New challenges in polymer foaming: a review of extrusion processes assisted by supercritical carbon dioxide
Prog. Polym. Sci.
(2011) - et al.
Supercritical impregnation of polymers
Curr. Op. Solid St. Mater. Sci.
(2003) - et al.
Evaluation of drug delivery characteristics of microspheres of PMMA-PCL-cholesterol obtained by supercritical CO2 impregnation and by dissolution–evaporation techniques
J. Control. Release
(2004) - et al.
Impregnation of a biocompatible polymer aided by supercritical CO2: Evaluation of drug stability and drug–matrix interactions
J. Supercrit. Fluids
(2009) - et al.
Porous polymers and resins for biotechnological and biomedical applications
Rev. Mol. Biotechnol.
(2002)
Supercritical fluid technologies and tissue engineering scaffolds
Curr. Op. Solid St. Mater. Sci.
A new experimental system for combinatorial exploration of foaming of polymers in carbon dioxide: the gradient foaming of PMMA
J. Supercrit. Fluids
Extrusion assisted by supercritical CO2: A review on its application to biopolymers
J. Supercrit. Fluids
Production of controlled polymeric foams by supercritical CO2
J. Supercrit. Fluids
The effect of ethyl-lactate and ethyl-acetate plasticizers on PCL and PCL-HA composites foamed with supercritical CO2
J. Supercrit. Fluids
Bio-safe fabrication of PLA scaffolds for bone tissue engineering by combining phase separation, porogen leaching and scCO2 drying
J. Supercrit. Fluids
Solid-state foaming of biodegradable polyesters by means of supercritical CO2/ethyl lactate mixtures: towards designing advanced materials by means of sustainable processes
Eur. Polym. J.
Making microporous nanometre-scale fibrous PLA aerogels with clean and reliable supercritical CO2 based approaches
Microp. Mesop. Mater.
Supercritical CO2 antisolvent precipitation of polymer networks of L-PLA, PMMA and PMMA/PCL blends for biomedical applications
Eur. Polym. J.
Composite fibrous biomaterials for tissue engineering obtained using a supercritical CO2 antisolvent process
Acta Biomater.
Electrospinning in compressed carbon dioxide: hollow or open-cell fiber formation with a single nozzle configuration
J. Supercrit. Fluids
Polysaccharide-based aerogels: promising biodegradable carriers for drug delivery systems
Carbohydr. Polym.
Supercritical drying of aerogels using CO2: Effect of extraction time on the end material textural properties
J. Supercrit. Fluids
Feasibility study of hydrophilic and hydrophobic silica aerogels as drug delivery systems
J. Non-Cryst. Solids
Hybrid aerogel preparations as drug delivery matrices for low water-solubility drugs
Inter. J. Pharm.
Nanostructured silica-based drug delivery vehicles for hydrophobic and moisture sensitive drugs
J. Supercrit. Fluids
The role of organic solvent on the preparation of chitosan scaffolds by supercritical assisted phase inversion
J. Supercrit. Fluids
A review of accelerated carbonation technology in the treatment of cement-based materials and sequestration of CO2
J. Hazard. Mater. B.
New insights on the use of supercritical carbon dioxide for the accelerated carbonation of cement pastes
J. Supercrit. Fluids
Interaction of bentonite with supercritically carbonated concrete
Appl. Clay Sci.
Processing of microporous VPI-5 molecular sieve by using supercritical CO2: stability and adsorption properties
Microp. Mesop. Mater.
Intensity dependent nonlinear absorption of pyrylium chromophores
Chem. Phys. Lett.
Preparation of trityl cations in faujasite micropores through supercritical CO2 impregnation
Microp. Mesop. Mater.
Impregnation of a triphenylpyrylium cation into zeolite cavities using supercritical CO2
J. Supercrit. Fluids
Deposition of Pd into mesoporous silica SBA-15 using supercritical carbon dioxide
J. Supercrit. Fluids
Grafting of trialkoxysilane on the surface of nanoparticles by conventional wet alcoholic and supercritical carbon dioxide deposition methods
J. Supercrit. Fluids
Preparation of silane-coated TiO2 nanoparticles in supercritical CO2
J. Colloid Interface Sci.
Sorption of tryalkoxysilane in low-cost porous silicates using a supercritical CO2 method
Microp. Mesop. Mater.
Formation of carbamic acid in organic solvents and in supercritical carbon dioxide
J. Supercrit. Fluids
Regenerable solid CO2 sorbents prepared by supercritical grafting of aminoalkoxysilane into low-cost mesoporous silica
J. Supercrit. Fluids
Encapsulation and co-precipitation processes with supercritical fluids: fundamentals and applications
J. Supercrit. Fluids
Precipitation of ultrafine organic crystals from the rapid expansion of supercritical solutions over a capillary and a frit nozzle
J. Supercrit. Fluids
Solvent effect on tolbutamide crystallization induced by compressed CO2 as antisolvent
J. Cryst. Growth
Production of hybrid lipid-based particles loaded with inorganic nanoparticles and active compounds for prolonged topical release
Int. J. Pharm.
Function-led design of new porous materials
Science
Porous materials and supercritical fluids
Adv. Mater.
Polymer synthesis and processing using supercritical carbon dioxide
J. Mater. Chem.
SiO2 aerogels
Impact of carbon dioxide on the immobilization potential of cemented wastes: chromium
Cem. Concr. Res.
Organic-guest/microporous-host composite materials obtained by diffusion from a supercritical solution
Adv. Mater.
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