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viable products. Research conducted over the previous 5 years (1997–present) will be emphasized. Needless to say, this focus will eliminate discussion of processes that contain only separations unit oper- ations (example: extractions and cleaning). This does not mean that such processes are unimportant—on the contrary, several have been commercialized, including extraction of caffeine from coffee beans and tea leaves, certain acids from hops and various components from spice plants [36,46]. In addition, CO2-based chromato- graphic instruments have been commercialized at both the analytical and preparative scale [47]. Clearly, a continuing challenge to the reader who is interested or actively involved in research involv- ing CO2 as a solvent is ‘Can the use of CO2 create new products, eliminate waste, save energy, and/or en- hance safety to the point where the costs of the product are reduced and a more sustainable process created?’ The new DuPont fluoropolymer facility may be the first example of this, as the use of CO2 has eliminated the need for fluorinated solvents, has made working with some of the monomers safer and produces prod- uct with better properties than the traditional emulsion process. In each of the following sections, recent research on various aspects of green chemistry using CO2 will be summarized. Whereas much of the published work in this area emanates from academic groups, it should be noted that some industrial concerns have also been quite active. Industry quite naturally tends to patent before they publish and consequently a patent search was conducted for the period 1996–2001 where find- ing the term ‘supercritical’ in either the patent title or abstract was employed as the criteria defining a ‘hit’. This search produced 450 hits for the time period in question. Well over half of these patents described inventions where CO2 is used as the solvent in natu- ral product extractions or cleaning. Of the remainder, academic inventors filed nearly half. In addition, a search using ‘CO2 or carbon dioxide’ in title or ab- stract (without supercritical) produced 1500 additional ‘hits’, although the vast majority of these did not in- volve use of CO2 as a solvent. For each of the sections on CO2-based research, a paragraph is appended that describes industrial activity (as described in patents) that is significant but not expressly mentioned in the main body of the section. Without question, the most active industrial entities (in producing US patents) on use of supercritical fluids in green chem- istry/processing during 1996–2001 were DuPont, Micell Inc. and Thomas Swan (UK). Not surprisingly, each of these companies also has supported major commercialization efforts in CO2 -based chemistry and processing (DuPont—polymerization of fluoropoly- mers in CO2 ; Micell—dry cleaning in CO2 ; Thomas Swan—hydrogenations and alkylations in CO2 ). All three have strong research ties to universities. 1.9. A note on cleaning using CO2 There has been substantial effort made by both the academic and industrial community to employ carbon dioxide in the cleaning of clothing, me- chanical parts and the surface of microelectronics components. Whereas this report will not explicitly address the state of the art in cleaning using CO2, it will evaluate several technological issues that are significant to the advancement of CO2-based clean- ing. For example, although carbon dioxide is not a particularly strong solvent (see Section 3.3), it will readily solubilize low molecular weight, volatile, non-polar compounds. If the ‘contamination’ to be removed using CO2 falls into this category, then no additional fundamental science is required, and the economics of the design and construction of the equipment will determine whether the technology is practiced. Breakthroughs in the design of high pres- sure cleaning equipment that could rapidly process individual parts would greatly help to promote use of CO2 as a cleaning solvent. CO2 is a weak solvent and hence, cleaning that requires the solubilization of polar, inorganic or high molecular weight material will require the use of CO2- soluble auxiliaries (surfactants, chelating agents). The discovery that certain fluorinated compounds are ‘CO2-philic’ during the early 1990s allow for rapid advancement in the design of such auxiliaries and a discussion of the design of such auxiliaries is included in this report. For the future, the design of CO2-philic auxiliaries must likely include non-fluorinated build- ing blocks, as fluorinated materials are expensive and some (the fluoroalkyl sulfonate family) are environ- mentally suspect [48]. For the case of microelectronics processing, clean- ing is accompanied by the need to perform chemistry E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 133PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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