Page - 92 - in Advanced Chemical Kinetics

renewables are intermittent energy sources, which is unfit with the continuous energy need from society. Another example is the direct use bioalcohols as H2 sources. It can be envisioned that the H2 can be stored directly in the renewables by using, e.g., bioethanol or glycerol as the resource materials. Besides the energy application, AAD has demonstrated its usefulness in a plethora of pre- parative systems. This type of chemistry focuses mainly on the transformation of organic functional groups and will as such not be covered in this review. Charman studied the fundamental principles of AAD with isopropanol as a model substrate and [RhCl6]3− as a catalyst in the 1960s [2], and Robinson made further advances in the 1970s [3, 4]. As such, the field of AAD has been active for more than 5 decades. Nevertheless, it can be argued that the area is still immature and much fundamental research is still imperative to take the technology towards methods feasible for commercial application. This review aims to contribute to that end by shedding light on the kinetics and stabilities of various AAD systems mainly developed in the last approximately 10 years. For brevity, focus will be on contributions that provide both catalyst activity and longevity investigations on reactions using isopropanol, ethanol, or methanol. 2. Secondary alcohols Secondary alcohols are notoriously easier to dehydrogenate than primary alcohols for several reasons. The resulting ketone from dehydrogenating a secondary alcohol is more stable than the corresponding aldehyde, both from a thermodynamic and kinetic perspective. In addi- tion, the aldehyde may easily react further reaching more oxidised functional groups, such as ester, carboxylic acid, or amide depending on the reaction conditions. 2.1. Isopropanol In 1967, Charman reported that a turnover frequency (TOF) of approximately 14 h−1 can be achieved by employing a mixture of 7.6 × 10−3 M (580 ppm) RuCl3, 9.4 × 10 −2 M LiCl and 5.5 × 10−2 M HCl in refluxing isopropanol [2]. A decade later, Robinson reported that a combination of 4.45 × 10−2 M (3400 ppm) of [Ru(OCOCF3)2(CO)(PPh3)2] and 12 equivalent trifluoroacetic acid in refluxing isopropanol led to an initial TOF of approximately 13 h−1 [3, 4]. After an additional 10 more years, Cole-Hamilton demonstrated that a TOF of 330 h−1 could be reached by using 1.96 × 10−4 M (15 ppm) RuH2(N2)(PPh3)3 and 1 M NaOH at 150°C [5]. The Charman and Robinson systems employ acidic environments, whereas the Cole-Hamilton system is alkaline. However, a direct comparison between the systems and the effect of the additive is hampered by the large reaction temperature and catalyst loading differences, where Cole-Hamilton uses a reaction temperature highly elevated and considerably less concentrated catalyst compared to the others. In general, the mechanisms were believed to involve an inner-sphere β-hydride elimination of the alcohol followed by proton-assisted H2 extrusion from the organometallic catalytic intermediate. The proton source would be the acid when present (Charman and Robinson systems); otherwise, the alcohol itself served as the proton donor, which concurrently formed Advanced Chemical Kinetics92