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use of acetone in place of carbon tetrachloride would likely not involve any changes to the equipment used in the process, while use of CO2 would most certainly require equipment re-design. One manifestation of a systematic approach to choosing alternative solvents based on environmental considerations is SAGE, the solvent alternative guide, a web-based interactive tool [55a]. Carbon dioxide is indeed one of the possible choices that might result from an interactive session on SAGE, depending upon inputs, but no economic cal- culations are performed. An excellent description of the industrial perspective on choosing solvents given both physical property and regulatory constraints may be found in Ref. [55b]. As shown above, current regulations affect applica- tion of CO2 by rendering some conventional solvents better or worse (from the cost of complying with cur- rent regulations) than carbon dioxide. In addition, it is possible to envision how future regulations might also affect the use of CO2 in green processing. Given that CO2 has been determined to play a role in global climate change, it is conceivable that the emission of CO2 to the atmosphere will be regulated in the future. Consequently, a number of companies have begun instituting ‘trading credits’ in CO2 emissions, primarily on an internal basis. In these systems, CO2 is assigned a ‘negative value’ and thus use of CO2 as a raw material allows one to theoretically reduce the cost of the process or product. If this practice becomes widespread (owing to future regulation on CO2 emis- sions) it will likely spur research and development on processes or products that consume CO2. Another area where future regulation could greatly impact the use of CO2 is if restrictions are placed on the use of various fluorinated materials. Certain fluorinated materials have been found to be highly CO2-soluble (see Section 2.4.1 and Section 3.3) and hence these materials have been applied in the design of highly CO2 -soluble auxiliaries (surfactants and chelating agents). To date, the expense of fluorinated compounds has greatly limited their use in commer- cial CO2 technology, yet there are applications areas (such as microelectronics) where the cost of fluori- nated compounds will not be an impediment to com- mercial use of CO2 processing. However, it has been reported recently that certain fluorinated surfactants persist in the environment, causing concern within the environmental and public health communities. The EPA has proposed a significant new use rule (SNUR) for perfluorooctanesulfonic acid and closely related compounds [48] requiring manufacturers to notify EPA at least 90 days before commencing the manu- facture or import of these materials for a significant new use. This may be expanded to include perfluori- nated carboxylic acids (and their precursors) as well. If the use of fluorinated compounds is restricted in the future, it could limit the use of CO2 in certain areas of application. Needless to say, design of non-fluorinated CO2 -philic compounds would therefore become a priority in advancing the state of the science. 2. Reactions using gases In the following sections, recent significant research and development on the use of CO2 as solvent (or raw material) to aid in the ‘greening’ of various classes of reaction or material processing will be discussed. In this section, the use of gaseous reactants (H2 , CO, O2 ) in CO2 will be described. 2.1. Hydrogenation Hydrogenation is widely used in industry at scales ranging from grams per year to tons per hour [56]. Hy- drogenation is conducted at large scale in either the gas or liquid phase; further, while gas phase reactions are performed over a solid catalyst (heterogeneous cataly- sis), liquid phase reactions are conducted in either two (homogeneous catalyst, liquid and gas each present) or three (heterogeneous catalyst, liquid and gas each present) phase modes. Finally, heterogeneous cataly- sis is conducted in batch, continuous slurry and fixed bed reactor configurations, although the latter is less common than the former two. Despite the broad range of potential reactor config- urations and reactions, we can, by examining the 12 principles of green chemistry described previously, make some general comments as to how the use of supercritical fluids (CO2 primarily) can enhance (and possibly detract from) the sustainability and eco- nomic viability of a hydrogenation process. We will restrict this discussion to those hydrogenations cur- rently carried out in the liquid phase—addition of a supercritical solvent to a gas-phase reaction will sim- ply dilute the reactant concentrations, reducing the E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 135PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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