(262b) Evaluating the Greenness of Green Chemistry Via Traditional and Thermodynamic Life Cycle Assessment

Developments in Green Chemistry are expected to result in novel approaches that are more environmentally benign than traditional methods. Much of the research in green chemistry focuses on replacing toxic and hazardous substances such as solvents, catalysts and reaction media by alternatives that are non-toxic or reduce emissions. Unfortunately, most new developments in green chemistry are not evaluated for their ?greenness? via a systematic analysis of their environmental impact during their life cycle. Consequently, it is likely that many techniques that are claimed to be green may not really have a smaller environmental impact over their life cycle. Although LCA is a well developed approach, it is still best suited for evaluating mature technologies for which life cycle inventory data are available. Such data are very difficult to find for emerging technologies. This paper presents one of the first LCAs to compare green versus traditional chemistries. A traditional LCA is performed with data reported by chemists. This approach is not convenient because information about emissions and impact is rarely available from the laboratory scale processes. A novel thermodynamic approach that does not rely on information about emissions and their impact is also used. This approach relies on input-side information, which is usually more readily available, and seems to be well correlated with emissions and their impact.

These techniques are used to compare alternatives for the hydrogenation of benzene to form cyclohexane. The methods evaluated include the traditional industrial process, and newer green chemistries based on ionic liquids and water solvents. Ionic liquids are organic salts whose melting point is normally below about 100oC. Recent research about IL's and their applications is booming, partly because they are considered to be ?green solvents'. This impression of environmental friendliness is due to their extremely low vapor pressure, which makes them good substitutes to traditional industrial solvents, most of which are Volatile Organic Compounds (VOCs). Replacement would prevent the emission of VOCs, a major source of environmental pollution. One of applications of Ionic Liquids is to be used as solvents in hydrogenation reaction.

Hydrogenation is an important catalytic method in synthetic organic chemistry both on the laboratory and the production scale. The classical hydrogenation catalysts are heterogeneous catalysts. However, homogeneous hydrogenation catalysts in liquid phase have attracted considerable interest recently, and Ionic Liquids are good alternative reaction mediums. Besides the advantage that Ionic Liquids have no (or negligible) vapor pressure, they have some other benign properties: Ionic liquids provide a polar, non-nucleophilic environment which can stabilize the homogeneous hydrogenation catalyst and increase catalyst lifetime; comparing with water, which is also considered as one of ?Green Solvents?, Ionic liquids can dissolve more hydrogen, which leads to a higher reaction rate.

Although Ionic liquids seem like a promising solution in organic chemistry, there are still several issues that need to be explored for confirming their ?greenness?. It is critical to collect environmental, health data before encouraging the use of IL's as alternatives for organic solvents, as well as perform their Life Cycle Assessment to study the environmental impacts throughout a product's broader life cycle to ensure that the environmental impact is not just being shifted to other stages of the life cycle.

In this paper, cyclohexane production from benzene hydrogenation is selected as an example. A ?cradle to gate? Life Cycle Assessment is performed to compare three alternatives: synthesis in Ionic Liquid (1-methyl-3-butyl-imidazolium tetrafluoroborate), industrial production in a heterogeneous catalyst process, and synthesis in water. Preliminary results indicate that the ionic liquid process may be the worst, followed by the water process, and then the industrial process. This is because Ionic Liquids are complicated compounds, and a long life chain is required to synthesis them, so large amount of pollutions are produced during this process. Furthermore, separation processes to obtain products of the desired purity also has a large environmental impact. Current work is improving the accuracy of the preliminary results by using better data. A thermodynamic LCA is also being completed and its results will be presented. It seems that compared to the industrial process, manufacture of cyclohexane via ?green? solvents may not be environmentally preferable. Chemistries where IL's may outperform traditional processes from a life cycle point of view will be discussed.