ABSTRACT
Robert S. Disselkamp, Sarah M. Chajkowski, Kelly R. Boyles, Todd R. Hart,
and Charles H.F. Peden Institute for Interfacial Catalysis, Pacific Northwest National Laboratory,
Richland, WA 99352 USA
robert.disselkamp@pnl.gov Abstract Here we discuss results obtained as part of a three-year investigation at Pacific Northwest National Laboratory of ultrasound processing to effect selectivity and activity in the hydrogenation of water-soluble olefins on transition metal catalysts. We have shown previously that of the two regimes for ultrasound processing, highpower cavitating and high-power non-cavitating, only the former can effect product selectivity dramatically (>1000%) whereas the selectivity of the latter was comparable with those obtained in stirred/silent control experiments [R.S. Disselkamp, Y.-H. Chin, C.H.F. Peden, J. Catal., 227, 552 (2005)]. As a means of ensuring the benefits of cavitating ultrasound processing, we introduced the concept of employing inert dopants into the reacting solution. These inert dopants do not partake in solution chemistry but enable a more facile transition from high-power non-cavitating to cavitating conditions during sonication treatment. With cavitation processing conditions ensured, we discuss here results of isotopic H/D substitution for a variety of substrates and illustrate how such isotope dependent chemistries during substrate hydrogenation elucidate detailed mechanistic information about these reaction systems. Introduction Using ultrasound to enhance activity, and to a lesser extent to alter selectivity, in heterogeneous condensed phase reactions is well known [1-7], with the first paper on sonocatalysis having been published over 30 years ago [8]. In principle, there exists two separate domains for sonochemistry, these are non-cavitating and cavitating ultrasound regimes. For commercially available instruments, bath systems by virtue of their lower acoustic intensity are usually non-cavitating whereas probe (e.g., horn) systems can be either (high power) non-cavitating or cavitating. One objective of this paper is to contrast differences in a heterogeneous catalytic reaction for noncavitating and cavitating ultrasound compared to a control (stirred and silent) system. Only through “doping” our solution were we able to initiate the rapid onset of
cavitation during ultrasound treatment, and enable the chemical effects arising from cavitating conditions to be studied. Since not all reaction liquid mixtures readily cavitate, this technique decreases the power threshold for cavitation making it of general use.
