Bubble sizes in agitated water-hydrophilic organic solvents for heterogeneous catalytic reactions

We recently reported that the rate of heterogeneous catalytic hydrogenation of 2-butene-1,4-diol using water, 2-propanol, and mixtures of the two as the solvent under mass transfer controlled conditions is a maximum at similar to 5% 2-propanol, which also corresponds to the minimum bubble size. Here, bubble sizes are reported for other possible miscible water/hydrophilic solvents, along with some data with miscible hydrophobic/hydrophilic organic liquids. Air has been dispersed in water, methanol, 1-propanol, 2-propanol, MEK, tert-butanol, and n-butyric acid, as well as in miscible mixtures of the two in a closed reactor containing 1% air, stirred with a Rushton turbine at 1200 rpm (29.5 W/kg). In all cases, a very marked minimum bubble size was found at low concentrations of the hydrophilic organic. This concentration depends on the particular organic and corresponds well with its maximum surface excess. This study is the first in which the surface excess has been related to bubble behavior for mixtures of water and a wide selection of different potential hydrophilic solvents. With miscible hydrophobic/hydrophilic organics, a minimum bubble size was not found. This work should help solvent selection for heterogeneous catalytic reactions working in the mass transfer controlled regime.