Page - 99 - in Advanced Chemical Kinetics

considerably more susceptible to the choice of metal than the PNP ligand, suggesting that two, or more, different mechanisms are operating. Finally, the low conversion obtained with complex 6 could be improved on by simply exchanging the P-phenyl substituents with isopropyl (13% with 6 versus 35% with 7). A mechanism akin to the depiction in Figure 4 was suggested. Furthermore, H2 extrusion from the hydrogenated form of 2 was proposed. Moreover, as Beller observed for isopropanol AAD [9], it was noted that decreasing the catalyst loading had a beneficial effect on the TOF. As such, the TOF24h was 375 h −1 with 100 ppm and 567 h−1 with 50 ppm. It was in this respect suggested that an associative/dissociative process was involved. Varying the loading of the osmium dimer in Figure 6 further corroborates such a process. Thus, when reducing the catalyst load- ing from 500 to 100 ppm, the TOF likewise increased approximately five-fold (56–275 h−1). This has the striking consequence that after 24-h reaction time, the 100 ppm loaded mixture afford 66% conversion, whereas the 500 ppm only provide 45%. In 2014, Beller demonstrated that bioethanol can be effectively converted to acetate by AAD [14]. The complex [RuHCl(PNPiPr)CO] provide the best catalyst turnover, and a TOF1h of 1770 h −1 is observed when employing 25 ppm catalyst loading in refluxing wet bioethanol containing 8 M NaOH. This result is similar to that found when employing dry ethanol [9] (1770 versus 1483 h−1) albeit at severely harsher conditions. The highly alkaline media was necessary to maintain the prod- uct in a deprotonated state, presumably to avoid catalyst deactivation by coordination of acetic acid to the catalyst. Moreover, a 70% yield was obtained within 20 h when using a 1:1 EtOH/H2O mix- ture. In addition, a long-term reaction with 10 ppm catalyst loading reached a TON 80,000 after 98 h. Overall, the results with ethanol clearly demonstrate that primary alcohols are notoriously more difficult to achieve high TOF with than with secondary congeners. Thus, when compar- ing state-of-the-art turnover frequencies of ethanol AAD (1770 h−1) [14] with that for isopropa- nol (14,145 h−1) [9], there is an order of magnitude difference in favour of the latter. Moreover, there is still a lack of studies into the mechanism of the various discrete catalytic steps. Shedding light on these would provide a deeper insight into the kinetic features and parameters of primary alcohol AAD by homogeneous catalysis. 3.2. Methanol In 1987, Cole-Hamilton demonstrated that MeOH can be dehydrogenated with a TOF of 7 h−1 by 1 × 10−3 M (43 ppm) [Rh(bipy)2]Cl in MeOH containing 5% (v/v) H2O and with 1.0 M NaOH at 120°C [11]. This was the year later improved to 37.3 h−1 by the same group by use of 1–5 × Figure 6. PNN osmium dimer by Gusev. 500 ppm: 45% conversion (24 h), 100 ppm: 66% conversion (24 h). Catalyst Kinetics and Stability in Homogeneous Alcohol Acceptorless Dehydrogenation http://dx.doi.org/10.5772/intechopen.70654 99