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1.5.6. CO2 is a weak solvent (low polarizability per unit volume, low cohesive energy density) This is perhaps CO2’s greatest flaw, in that its inability to solvate compounds of interest (hence requiring uneconomically high process pressures) has greatly inhibited its commercial use. This is- sue will be discussed in more detail in Section 3.3. 1.5.7. CO2 poisons Ziegler-type polymerization catalysts CO2 will terminate olefin polymerizations that em- ploy classical Ziegler (titanium halide) catalysts, hence preventing such polymerizations from being carried out in carbon dioxide. 1.6. How we will approach our analysis Reaction schemes will be critiqued on their abil- ity to provide a more sustainable process compared to existing technology, using the 12 principles of green chemistry as a basis for judgments on sustainability. The basic principles of green chemistry have been outlined by Anastas and Warner [34] and are listed below: 1) Prevention (alter process schemes and chemical pathways to prevent the generation of waste, rather than remediate waste once formed). 2) Atom economy. 3) Less hazardous chemical synthesis. 4) Designing safer chemicals. 5) Safer solvents and auxiliaries (create and employ solvents and process aids that, if emitted to the en- vironment, exhibit a lower impact than currently used materials). 6) Design for energy efficiency. 7) Use of renewable feedstocks. 8) Reduce derivatives. 9) Catalysis (create catalysts that are more selective than current analogs and which therefore produce lower volumes of byproducts during reactions). 10) Design for degradation. 11) Real-time analysis for pollution prevention. 12) Inherently safer chemistry for accident preven- tion. If one examines the properties of CO2 and its many proposed applications, several common trends appear vis-à-vis the twelve principles shown above. CO2 has been proposed as a benign alternative to common or- ganic solvents, and hence principle (5) comes into play. If one assumes that some proportion of the or- ganic solvent that is employed in any chemical pro- cess will be emitted to the environment, then replace- ment of that solvent with CO2 is a mode of prevention (principle 1), as CO2 emissions are less problematic. The toxicity of CO2 is lower than for many organic solvents (principle 4) and is naturally abundant (prin- ciple 7). It should be noted that while use of CO2 is within the scope of several of the principles of green chemistry, improper or ill-considered process design could lead to egregious violation of some of the others. Indeed, if use of CO2 as solvent leads to higher energy con- sumption or an inherently unsafe process, then some of the 12 principles will be followed while others are violated. Judgment of the net benefit must be done on a case-by-case basis. Finally, the source of CO2 used in any process should be considered within the framework of the 12 principles of green chemistry. CO2 is naturally abundant, yet CO2 employed in an industrial process is typically not captured from the atmosphere. Car- bon dioxide is a byproduct (of sizeable volume) of the commercial ammonia process [13] and much of the commercially available CO2 is derived from this source (after purification). CO2 can also be captured from fermentation processes, yet this is not generally practiced commercially (owing to CO2’s low current value). Large deposits of CO2 exist naturally in the US; these are currently tapped for use in tertiary re- covery of petroleum in older fields in West Texas and Oklahoma [8]. Hence, if we examine the source of CO2, we can come to different conclusions of CO2’s worthiness as a benign solvent. If, for example, CO2 generated by the ammonia process is employed, then one could consider this as pollution prevention, as this CO2 would otherwise be emitted to the atmo- sphere. If we employ CO2 from natural deposits, this could be construed as ‘anti-sequestration’, as this CO2 would ordinarily remain underground. If CO2 could be captured from the atmosphere (or power plant flue gas) in an energy efficient and economic man- ner, then used in a process, this would likely be the best source with respect to the 12 principles of green chemistry. E.J. Beckman / J. of Supercritical Fluids 28 (2004) 121–191 127PDF Image | Supercritical and near-critical CO2 in green chemical synthesis and processing
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