Catalysis of Organic..

214 Cavitating Ultrasound Hydrogenationcavitation during ultrasound treatment, and enable the chemical effects arising fromcavitating conditions to be studied. Since not all reaction liquid mixtures readilycavitate, this technique decreases the power threshold for cavitation making it ofgeneral use.The first system we investigate will be the inert dopant study just introduced.Three additional studies will also be discussed. The first examines cis to transisomerization of cis-2-buten-1-ol and cis-2-penten-1-ol on Pd black. Isomerization isimportant in edible oil partial hydrogenation, where it is desirable to partiallyhydrogenate a C18 cis multiple-olefin without isomerizing its unconverted doublebonds. We will see here that cavitating ultrasound processing reduces the amount ofisomerization of these cis olefins to their trans form. The second system we discusspertains to the use of H/D isotope substitution in hydrogenation to yield informationabout the mechanisms of reaction. Here the two substrates 3-buten-2-ol and 1,4-pentadien-3-ol were employed. We compared D-atom substitution to controlexperiments (e.g., H-atom processes) using D 2 O instead of water for alcoholsubstitution and/or D 2 instead of H 2 during hydrogenation. The final investigationmade is that of using transition state theory to explain the origin of olefin exchange.Experimental SectionMaterials and ApparatusThe experimental details of our approach have been given previously [9-12],therefore only the salient features are outlined here. The reagents were obtainedcommercially from the following vendors at the stated purities: cis-2-buten-1-ol,ChemSampCo, 97%; cis-2-penten-1-ol, ChemSampCo, 95%; 3-buten-2-ol and 1,4-pentadien-3-ol, Aldrich Chem. Co., 97+%). Unless otherwise stated, a substrateconcentration of 100 mM was employed. All experiments except for the last (nonequilibriumsystem) employed a Pd-black catalyst (Aldrich, 99.9% purity metalsbasis) with a N 2 BET surface area of 42 m 2 /g. The non-equilibrium investigationemployed Raney Nickel (W.R. Grace Co. type 2800). Each system discussed belowwill present the substrate and catalyst mass employed. Deionized water (18 MΩ-cm)was used as the solvent. In some systems D 2 O was used as the solvent (AldrichChem. Co., 99.9% atom purity). Hydrogenations were performed with H 2 or D 2 gas(A&L specialty gas, 99.99% purity) at a pressure of 6.5 atm (80 psig). Allcomponents used for the reaction apparatus are commercially available and havebeen described in detail previously [9,10,13]. Experiments were conductedisothermally at a temperature typically of 298 K with an uncertainty of ±2.5 Kcontrolled using a water-bath circulation unit. All the studies used a 20 kHz BransonUltrasonics digital model 450 sonifier II unit capable of delivering up to 420 W. Ajacketed Branson Ultrasonics reactor that screwed onto the horn assembly was usedto contain the reacting systems. Samples collected during an experiment wereanalyzed on a Hewlett-Packard GC/MS (5890 GC and 5972 MSD). Authenticstandards were employed in the calibration of mass area counts when available. Thecolumn selected for separation was typically a 30 meter, 0.5 micron film, DB-5MS