Catalysis of Organic..

Marincean et al. 433suggesting that water (a strong competitor) is not present in the vicinity of thecatalyst in alcohol solvent. Second, the presence of PG clearly reduces reaction rate -water may be ineffective in removing product PG from the reaction environment,whereas alcohols may preferentially displace PG from the carbon pore structure.Finally, water is a poor solvent for hydrogen, as hydrogen solubility in water is only20 to 30% that in simple alcohols (12).Full replacement of water with organic solvents leads to further increases in theGO conversion and PG selectivity in the order water < EtOH < i-PrOH < t-BuOH(Figure 4). As one can see from Table 3, GO conversion almost doubles going fromEtOH to t-BuOH, and yield of PG (Figure 5) increases with conversion. Yet PGselectivity only increases modestly, while at the same time the LA and EGselectivities decrease. Taken together these experimental observations suggest thatwhile the pathway for LA formation becomes less important when little water ispresent, presumably aldol reactions remain accessible. Glyceraldehyde, the firstintermediate of GO hydrogenolysis, is known to be very reactive towards aldolcondensation in basic media (13). The lower carbon balances may be due to the tothe reactivity of the carbonyl compounds formed as intermediates towardcondensation reactions, resulting in higher molecular weight unsaturated compounds,consistent with the appearance of color in the reaction samples.Table 3. GO Hydrogenolysis in Different Solvents aSelectivity (%)Exp Solvent Conv(%) PG LA EG C Balance (%)1 Water 22.9 60.9 26.6 8.1 99.02 EtOH/Water 34.4 57.1 15.5 10.4 94.13 i-PrOH/Water 59.8 54.9 7.5 9.2 83.14 t-BuOH/Water 57.6 61.5 7.6 8.0 86.85. PG/Water 10.8 n/a 46.2 9.2 95.26. EtOH 50.8 45.0 5.5 8.1 79.07. i-PrOH 66.7 69.3 3.1 8.9 87.58. t-BuOH b 89.9 58.2 1.2 5.6 68.69. 1,4-Dioxane b 54.5 22.8 0.0 14.4 65.8a Reactions carried out for 4-6h at 200ºC and 1000psig H 2 using 0.5g Ni/Re catalyst,0.1M KOH, and 1.0M GO. b 1.5 g water was added to the reaction solution toenhance KOH solubility.The experiment with 1,4-dioxane as solvent was characterized by poor solubilityof GO and KOH into the solvent, hence water was added to the initial mixture. Theabsence of the hydroxyl functionality in the solvent significantly decreased thesystem’s reactivity. The overall conversion and reactivity in this medium wasintermediate, but interesting is that (a) no LA was formed and (b) the selectivitytoward EG was substantially higher than in any other solvents.