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4.7. Inorganic chemistry Obviously, most inorganic compounds are not solu- ble in carbon dioxide and hence, inorganic chemistry performed in or with CO2 has been accomplished by finding ways around this seemingly intractable thermodynamic hurdle. The first inorganic chemistry performed in a supercritical organic solvent was prob- ably the work by Matson [270] at Battelle PNL in the late 1980s—here an emulsion was formed in a super- critical alkane and inorganic particles generated via a reaction at the micellar interface between an inorganic and an organic precursor (note that when Matson per- formed his study, it was not possible to form micelles in CO2!). Recently, several research groups have adopted the same strategy to create metal nanoparti- cles within micelles formed in carbon dioxide. Nat- urally, the great strides made during the 1990s in the identification and application of CO2-philes paved the way for this research. Both Fulton [271] and Roberts [272] have reported the formation of metals particles with diameters <20 nm by (a) creating an emulsion in CO2 where the aqueous cores of the micelles contain metal ions as well as water; and (b) adding a reduc- ing agent to the CO2, such that a reaction occurs at the micellar interface between ion and reducing agent to nucleate the particles. Particle growth then oc- curs through micelle–micelle collisions—Roberts has shown that one can control the particle growth rate via control over the degree to which the micelles can collide and exchange contents. Further, changing the physical properties of the compressible continuous phase can alter the micellar collision rate. An obvious question is ‘is this green chemistry?’ Because there is currently no sizeable industrial pro- cess for the manufacture of metal nanoparticles, this question is difficult to answer. Production of metal nanoparticles in a CO2 -continuous emulsion will likely be more environmentally friendly than the analogous reaction in an organic solvent. However, if such metal nanoparticles are ultimately applied com- mercially, there may also be other means by which to synthesize them, means that require no solvent at all. As can be seen by this and other such situations, it can be difficult to judge whether a process is green unless taken in context with competing processes—green seems not to be an absolute but rather a relative concept. 4.7.1. Inorganic chemistry: metal chelates Although separations will not expressly be covered in this report, the use of chelating agents for metal extraction should be noted. While many conventional chelating agents and their associated metal complexes are poorly soluble in carbon dioxide, concepts on the design of CO2-philic materials were applied very early to the design of CO2-soluble chelating agents [273], showing that fluorination improved solubility. On the other hand, tri-alkyl phosphates and tri-alkyl amines, known to bind several types of metals, have been shown to be miscible with CO2 at moderate pressures despite containing no fluorine. Various re- search groups [274] have demonstrated that one can extract metals (using the appropriate agent) from both solid and liquid matrices at high yields. It has also been shown that the phase behavior of the metal chelate can be substantially different from that of the agent (not surprising, since at the very least the molecular weight of the chelate is much greater than that of the agent). Finally, one of the first advances in the design of non-fluorous CO2-philes came about as a result of work by Siever’s group on chelating agent structure–solubility relationships [275]. It was shown that, in the case of copper--diketone complexes, the solubility of analogs containing branched alkyl groups was superior to fluorinated analogs. Again, we must pose the question, is the use of chelating agents in carbon dioxide green chem- istry/processing? The two most important cases for examination, that where metals are processed/purified for sale, and that where metals must be removed from solid or liquid matrices to remedy an environmental problem, will be examined here. Regarding the first case, both copper and precious metals (platinum groups metals; PGMs) are purified using solvent extraction. In the case of copper, sol- vent extraction and electrowinning (SX-EW) have captured ≈15–20% of the total amount of copper produced worldwide [276], replacing the significantly less green (owing to energy use and air emissions) conventional smelting process. In SX-EW, the metal is first extracted from the ore using sulfuric acid (along with substantial amounts of silver, lead, iron, zinc and arsenic, plus a wide variety of minor components) via heap leaching, where the acid is simply allowed to flow by gravity through an ore pile. This acidic solution is then contacted with an organic solvent E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 173PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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