Fate of monoterpenes in near-critical water and supercritical alcohols assisted by microwave irradiation

Abstract

The rearrangement of alpha- and beta-pinene was studied under microwave irradiation in near-critical water and supercritical lower aliphatic alcohols, with the aim of identifying the pathway of alpha- and beta-pinene isomerization. Generally, two pathways occur, pyrolysis on the one and acidolysis on the other hand, whereby acidolysis is predominant in the case of near-critical water and the second pathway is favored for experiments employing supercritical alcohols. The different behavior of these two structurally related solvents is attributed to the increased availability of protons if water is heated to 270 degrees C and 80 bar, thus enhancing the autoprotolysis of water. The application of alcohols instead furnished rearrangement products clearly attributed to thermal pyrolysis route by the formation of radical reaction intermediates.

Environmentally Benign Transformations of Monoterpenes and Monoterpenoids in Supercritical Fluids

“…Very similar results were obtained under the same conditions with b-pinene 2 [26]. The main reaction products were g-terpinene 10 (24%) and terpinolene 11 (23%); smaller amounts of limonene 3 (19%) and a-terpinene 12 (14%) were identified as well.…”

“…This ratio between the products formed via thermolysis or via the acid-catalyzed pathway can obviously be attributed to the relatively low temperature and to the increased acidity of near-critical water. An increase in concentration of a-pinene 1 in near-critical water from 1.7% to 3.3% for the reaction duration of 10 min resulted in a drop in conversion from 85% to 72% [26].…”

“…As opposed to the aforementioned studies performed in a flow-type reactor, the authors [26] investigated thermolysis of a-and b-pinenes 1 and 2 in aliphatic alcohols 3-Carene 9 Limonene 3 α-Pinene 1 SCHEME 3 Pathways for a-pinene isomerisation into limonene.…”

“…When thermolysis of pinenes was carried out in nearcritical water in an autoclave, 0.03 M NaCl solution was used instead of pure water in order to attain stable and reproducible reaction conditions [26]. Heating of pinenes 1 and 2 in near-critical water for 1 h at 543 K and 80 atm provided quantitative conversion in both cases.…”

Production of Chemicals in Supercritical Water

“…For the case of SCW treatment, direct liquefaction produces oil by treatment at low temperature close to critical point and with short reaction time. Studies on pyrolysis in SCW included work conducted by Szuppa et al [61], Pandey and Kim [77], Mohammed et al [78], Brebu and Vasile [65], Akhtar and Amin [66], Toor et al [71], Zhou et al [79], Shen et al [80], Rezende et al [81], Yong and Matsumura [21], and Kruse and Ebert [82].…”