A comparison between conventional and ultrasound-mediated heterogeneous catalysis: hydrogenation of 3-buten-1-ol aqueous solutions

A power flow scheme applicable to probe-type ultrasound reactors is presented, that has been deduced from both theoretical estimates and experimental measurements employing a thermal insulated vessel. Under typical conditions for water at 1 atm pressure, 77% of the electrical power is converted into mechanical motion of the probe, that in turn is dissipated to both acoustic power (∼12%) and cavitational heating (∼88%). Approximately 92% of the mechanical power of the probe was converted into heat, with the remaining power presumably converted into audible acoustic and/or mechanical motion. In a second type of experiment performed here, heterogeneous catalysis experiments have been performed at 298 K in an isothermal (i.e., jacketed) reaction vessel comparing chemistry in conventional (e.g., thermal) versus ultrasound-assisted systems. Both product state distribution and reaction rate measurements have been performed for the hydrogenation (using hydrogen gas) of aqueous 3-buten-1-ol solutions employing Pd-black powder. Products from the heterogeneous catalysis include isomerization to cis- and trans-2-buten-1-ol, as well as hydrogenation to 1-butanol. A reaction scheme involving surface-bound alkyl-radical species, consistent with previous published work, is proposed to explain product formation. Based on the observed differences in cis- to trans-2-buten-1-ol ratios in conventional versus ultrasound experiments, employing untreated and prereduced catalysts, it has been determined that ultrasound creates catalyst site(s) enhancing the cis-to-trans 2-buten-1-ol ratio from 0.25 to 0.55. In addition, comparing the total isomerization to hydrogenation ratio (cis- plus trans-2-buten-1-ol to 1-butanol ratio), for ultrasound-assisted and conventional catalysis, reveal a ∼5-fold enhancement in isomerization relative to the more energetically favored hydrogenation due to the application of ultrasound. Finally, the product formation rates for 1-butanol, as well as isomerization plus hydrogenation, revealed that conventional and ultrasound experiments showed both a nonlinear dependence with applied ultrasound power and no differences between untreated and prereduced catalysts. The observed reaction rate enhancements were 1:36:183 for the conventional, 90 W ultrasound, and 190 W ultrasound experiments, respectively.