Page - 97 - in Advanced Chemical Kinetics

products in substantial amounts. The former product might explain the decrease in activity (TOF6h = 690 h −1) due to a reversible dehydrogenation/hydrogenation process. Moreover, a similar mechanistic rationale as depicted in Figure 3 was provided. A constant catalyst activity towards full conversion of ethanol to ethyl acetate can be achieved by adding a minute amount of NaiOPr [13] Hence, when using 50 ppm of the commercially available Ru-MACHO ([RuHCl(PNPPh)CO]) in refluxing ethanol for 46 h in the presence of 0.6 mol% NaOEt, a 77% yield of ethyl acetate (TON = 15,400) was obtained. This could be increased slightly to 81% when using 500 ppm catalyst loading and 1.3 mol% NaOEt. Interestingly, a yield of 70% was obtained when conducting the reaction at merely 70°C. Studies into the effect of additive composition were undertaken. This provided two main results. First, NaOEt was superior to KOEt, NaOH, K2CO3 and Cs2CO3. Second, an optimal NaOEt loading with respect to maximising the TOF was observed. Moreover, the 1:1 [RuH2(PPh3)3CO]/PNPiPr ligand combination showed similar activity to Ru-MACHO with TOF’s of the former of 1107 and the latter of 1134 h−1 when employing 25 ppm catalyst loading and 1.3 mol% NaOEt. Notably, with Ru-MACHO, the conversion rate is practically constant until 90% of the ethanol is used up, at which point the NaOEt is precipitating out of the reaction. This resulted in a TOF2h of 934 h−1 and TOF10h of 730 h−1 when using 50 ppm catalyst loading. Hence, it was concluded that the reverse hydrogenation process of ethyl acetate was occurring at a negligible level. Again, a similar mechanism to the one depicted in Figure 3 was suggested. However, as shown in Figure 4, this now involved the dehydrogenation of two different species. Hence, initially eth- anol is dehydrogenated into acetaldehyde, which then reacts with either an ethanol or an ethox- ide to generate a hemiacetal or anionic hemiacetal intermediate. This compound then undergoes the second dehydrogenation step, leading to another H2 molecule and the ethyl acetate product. Figure 4. Proposed mechanism for the Beller ethanol AAD to ethyl acetate system. Best result: TOF = 1137 h−1. TON = 15,400. Yield = 81%. Stable for more than 46 h. Catalyst Kinetics and Stability in Homogeneous Alcohol Acceptorless Dehydrogenation http://dx.doi.org/10.5772/intechopen.70654 97