## 2 Supercritical Fluids Technology: Challenges and Limitations  
A supercritical fluid (SCF) is a compound, mixture, or element above its critical pressure (_Pc_) and critical temperature (_Tc_), but below the pressure required to condense into a solid [14]. Under such conditions, the densities of both liquid and gas phases become identical, and the distinction between them disappears. The properties of SCFs are frequently described as being intermediate between those of a gas and a liquid. The possibility to manipulate the physical properties of these solvents by simply changing the pressure or temperature is unique to supercritical systems, which show exceptional abilities for extraction, reaction, fractionation and analysis processes. They also determine their environmentally benign character because of the easy way in which they can be fully recovered and reused. However, the key feature of SCFs is their tunability as solvents in response to changes in pressure and temperature, particularly remarkable in the vicinity of the critical point [15]. Table 1 shows the critical parameters of some common fluids used in supercritical conditions. The density of SCFs is highly sensitive to both temperature and pressure; hence, all their density-dependent solvent properties (e.g., dielectric constant, relative permittivity, Hildebrand solubility parameter, _etc._) may be substantially modified by small changes in pressure or temperature. This provides a potential for controlling (bio)catalyzed reactions by precipitation of a given product, or purification by the selective precipitation of the products. Several criteria must be considered before selecting a given SCF as medium for a (bio)catalyzed reaction, including the critical parameters themselves, safety and cost issues. Supercritical carbon dioxide (scCO\({}_{2}\)) is the most popular SCF because of its relatively low critical parameters, low toxicity and non-flammability; furthermore, it is chemically inert under most conditions, has good solvent properties for non-polar solutes, and is considered as a green solvent. Carbon dioxide is clearly a \"greenhouse gas\"; however, it is massively produced as a by-product at industrial scale (e.g., in ammonia plants), and it is therefore cheap. Furthermore, at atmospheric pressure, CO\({}_{2}\) is gaseous, which means that simple depressurization is sufficient to separate solutes from scCO\({}_{2}\), after which it can be pressurized for reuse as a solvent. The solvent power of scCO\({}_{2}\) can be modified by increasing the bulk density or by adding a modifier (e.g., MeOH, acetone). Co-solvents can therefore be used to increase or reduce polarity, or to enhance the affinity for aromatic substances, although of course, the more co-solvent that is added, the further scCO\({}_{2}\) moves from being a truly green solvent. The main limitation for industrial application of SCFs is related with the cost of high-pressure equipment [15]. The high catalytic efficiency of porous chemical catalysts (e.g., zeolites) in SCFs has been widely reported, especially when their active sites are placed within the internal structure of the catalyst. In that case, the high mass transfer of reactants within a catalyst's porous structure is particularly important for increasing catalysis rates [16]. Additionally, many of the reports on SCFs catalysis mention that catalyst lifetime can be prolonged by eliminating undesired poisoning substances. As an example, the alkylation of benzene with ethene on Y-type zeolites (250-285 \({}^{\circ}\)C, 7-8 MPa) in scCO\({}_{2}\) revealed a much lower catalyst deactivation, higher reaction rates, and higher selectivity. The reduction of catalyst deactivation was attributed to the removal of polyaromatics, considered to be precursors of coke, from the surface of the zeolites by increased solubility and diffusion rates.