Liquid phase thermochemical energy conversion systems—An application of Diels-Alder chemistry

Thermochemical energy conversion at moderate or low temperature (> about 400°C) employing liquid phase components throughout a cycle is suggested as a promising concept for high-efficiency conversion of various energy sources (e.g. solar or industrial waste heat) to a convenient chemical form. In particular, we propose liquid phase Diels-Alder cycloaddition chemistry as an important class of reversible reactions for such low or moderate temperature thermochemical energy conversion systems. One of the important attributes of thermally driven Diels-Alder reactions is their concerted mechanism, with consequent high yields and efficiencies relative to liquid photochemical systems. Since the systems we propose involve organic species, with thermal stability concerns above about 400°C, it is important to demonstrate equilibrium shift capability for the highly energetic reactions sought. We have therefore carried out experimental studies with model liquid Diels-Alder systems that clearly demonstrate the degree of control over equilibrium available through substituent entropy effects. For example, Keq is unity at about 420°C (T*) for the anthracene/maleic anhydride system (in solvent) while a phenyl substituent on the anthracene or isopropyl substituent on the anhydride reduce T* to about 200°C at constant ΔH0. These results are of importance as regards subsequent systematic identification of Diels-Alder reactions having ideal thermochemical and physical properties. We also have developed a rapid NMR technique for qualitative screening of candidate reactions, and have applied this technique to the study of various bicyclic diene/fumaric acid ester systems. Our paper further points to the need for better understanding of the catalysis likely required for these liquid phase Diels-Alder reactions.