| The biomass refinery of the future will require a wide range of catalytic conversions, largely aqueous phase reductions. This dissertation explores the use of a 5% ruthenium on carbon catalyst in hydrogenation and C-H activation studies of carboxylic acids, a significant class of biogenic feedstocks. Several two- and three-carbon organic acid substrates were studied to probe (a) the effects of alpha-substituents on hydrogenation and HID exchange reactions and (b) the relationship between these two processes. Substrates included the following groups: -H, -CH3, -OH, -NH3, -OCH 3, -N+H2CH3, -N+H(CH 3)2, and -N+(CH3)3. Most reactions were performed at 100°C or 130°C in D2O solution but ranged from 50°C to 150°C and 1000 psi H2 or D2. As expected, both catalyst and H2 gas are required to convert organic acids to the corresponding alcohols. Simple alkanoic acids (acetic, propanoic, and isobutyric) and betaine, ((CH3)3N +CH2COOH·Cl-), a quaternized glycine, are very unreactive. For the other methylated glycines, the rate of hydrogenation falls off by a factor of three with each additional methyl, and reduction requires acid, as found earlier by Jere. Use of water as reaction medium appears optimal; addition of organic co-solvents such as ethanol, ethylene glycol, THF, or ethyl lactate led to reduced hydrogenation rates. This apparent inhibition by organic additives may reflect their tendency to tie up catalytic sites via metal insertion into C-H bonds, especially those neighboring heteroatoms. Another feature of both the solvents and the substituents is their capacity for hydrogen bonding, which may play a role in modulating hydrogenation rates. Similarly, steric crowding may also affect the substrates' ability to access the surface in productive orientations. To explore this issue, activation of C-H bonds next door to amine N sites was probed via H/D exchange studies performed on the N-methylated glycine series.;Reactions run in D2O with either H2 or D2 and catalyst were effective at exchanging deuterium into the CH 2 and CH3 sites of the amino acids. The Ru catalyst also exchanges H (from H2) for D (from D2O) to form HOD and HD, independent of substrate. Analysis by internal standard-calibrated 1H NMR allowed complete isotopomer-specific inventory of H concentration in substrates and water. Simple sequential replacements of H by D were observed in the amino acids. Two kinetic models were developed to determine the rate constants for H/D exchange at both the methylene and methyl positions. Both included possible time delays due gas/liquid isotopic equilibration (k H) and/or a catalyst induction (kturn_on) period; ultimately the latter was discarded as redundant. The first model treated each isotope replacement step as a process unrelated to the others. Thus, between forward and reverse replacements in CH2 and CH3 sites and k H, eleven rate constants were freely varied in the fitting. The second model used a single rate constant for each site, further modified by primary and secondary isotope effect values, resulting in six values + kH for this fitting. The latter, more economical and interpretive model gave fits nearly as good as those from the eleven-variable one. Somewhat surprisingly, these analyses found H/D exchange rates to decrease in the following order: D2O > sarcosine (CH3NH2+CH 2CO2) > glycine (CH2 site) > N,N-dimethylglycine. |
