Features of supercritical CO2 in the delicate world of the nanopores

https://doi.org/10.1016/j.supflu.2017.11.011Get rights and content

Highlights

  • ScCO2 processing of disordered porous materials (polymers, aerogels, concrete).

  • Reactive synthesis methods for metal-organic frameworks (MOFs) with scCO2.

  • Concepts for processing ordered porous materials (zeolites, mesoporous silica, MOFs) in scCO2.

  • Dense and open-pore MOFs synthetized in scCO2 through different building blocks: dipyridine and curcumin.

  • Applications in CO2 capture, pharmaceuticals, drug delivery, catalysis and fabrication.

Abstract

This contribution highlights the main characteristic that makes supercritical CO2 (scCO2) a highly interesting solvent to perform both physical processing and chemical reactions to build or modify delicate porous nanostructures. Historically, the most promising developments of the supercritical fluid technology in the field of porous materials have been foaming of polymers, processing and/or impregnation of aerogels, and surface modification of micro and mesoporous solids. More recently, the technology has evolved to the synthesis of porous materials by developing reactive processes in scCO2. One example is the synthesis of empty-pore three-dimensional metal-organic frameworks (MOFs). This paper reviews process concepts of supercritical fluid methods applied to porous compounds, giving examples of materials produced in our own laboratory. The processing of disordered (polymers, aerogels, concrete) and ordered (zeolites, mesoporous silica's, MOFs) porous materials is addressed. Perspectives of future development of the technology in pharmaceutical formulations and CO2 capture applications are given.

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.

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