Catalysis of Organic..

Disselkamp et al. 215column. In a prior study we have shown that only synthetic chemistry occurs,namely that conservation of initial substrate concentration into well-defined productsresults, so that no sample “atomization” or other undesirable loss processes due toultrasound treatment occurs [13].Experimental ProcedureFor all experiments, 50 mL of water and catalyst were added to the reaction cell.For ultrasound-assisted, as well as stirred (blank) experiments, the catalyst wasreduced with hydrogen (80 psig) in water using non-cavitating ultrasound at anaverage power of 360 W (electrical; 90% amplitude) for at least one minute prior toreaction. The first sample for each experiment was taken for time equal to zerominutes and filtered through a 0.45 μm hydrophilic Millipore filter to removecatalyst powder into a capped vial for subsequent GC/MS analyses. During control(magnetically stirred/silent-MS) experiments, the reactor was connected to thesonifier probe assembly, purged and then pressurized with hydrogen (60 psig).Stirring was commenced and after a defined time interval, a (filtered) sample wascollected. This process was repeated throughout the course of reaction. Conversely,during ultrasound-assisted experiments (US), the reactor was connected, purged andthen pressurized with hydrogen (60 psig) as just described. The pressure chosen wasoptimal for cavitation. This is because the ability of a solution to achieve cavitationincreases with greater acoustic power delivered which is favored to a degree at largerstatic pressures. However, a competing effect is that cavitation ability is hindered bylarge static pressures. Hence only a narrow pressure range ensured cavitation, oftenonly realizable with inert dopants, occurred around 60 psig. The solution in the cellwas irradiated with ultrasound and samples collected after similarly defined timeintervals. In principle the hydrogenation pressure employed may affect chemicalselectivity, however for the sake of experimental brevity such investigations werenot made here. During non-cavitating sonication, an amplitude of 90% wasemployed, resulting in 360±15 W delivered from the power supply. For cavitatingultrasound, a dopant was used (see below) to cause cavitation within 7 seconds ofturning on the sonifier resulting in 190-280 W delivered to the convertor, here at>90% sonifier amplitude.Results and DiscussionInert DopantsThe hydrogenation of 3-buten-2-ol by our group [9] is an excellent choice forprobing the chemical selectivity and activity of ultrasound processing because itundergoes competing reaction pathways yielding two products. For all experiments,3.0±0.2 mg of Pd-black catalyst were used. The concentration of substrate was 100mM (33 M/g-catalyst based on initial concentrations). The four experimentalregimes of stirred (without dopant), non-cavitating high-power ultrasound, stirred(with 1-pentanol dopant), and cavitating ultrasound (with 1-pentanol) wereemployed. In the latter experiments, 50 mM of 1-pentanol was used as an inert