Bubble Sizes in Agitated Water−Hydrophilic Organic Solvents for Heterogeneous Catalytic Reactions

Abstract

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 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. In catalytic hydrogenation, the rate of gas-liquid mass transfer may well be the rate-limiting step, especially with highly reactive heterogeneous catalysts giving a fast reaction on the catalyst surface. The rate of gas-liquid mass transfer is strongly dependent on the surface area between the gas and the liquid phase and, hence, on bubble size; the smaller the bubble size, the greater is the interfacial area and, therefore, the potential mass transfer rate. It has been observed that the composition of the mixed solvents strongly affects the rate and selectivity of heterogeneously catalyzed hydrogenation. 1 Other work has suggested that this strong effect may be due to one or more of the following: enhanced reactivity in the mixed solvents resulting from the relative solubility in the various solvents; beneficial competitive adsorption of the solvent at the reactive catalyst site; different levels of agglomeration of catalyst particles; or altered intermolecular interactions between the reactant and solvent molecules. 2 However, it might also be due to enhanced mass transfer, especially since it is known that small amounts of alcohol in water can dramatically reduce the bubble size compared to water. 3 Hence, recently, an investigation of the effect of the composition of the liquid phase and of hydrodynamic conditions on bubble size in a simulated “deadend” agitated stirred vessel, 4 i.e., one containing a fixed volume of gas without any gas feed, was undertaken using water, 2-propanol, and mixtures of the two. 5 In that work, 5 the same stirrer speed and agitator power input were used throughout. It showed that, in the single-component solvents, though the interfacial tensions are very different (water, U 72 mN m -1 ; 2-propanol, U 21 mN m -1 ), irregular, relatively large bubbles of similar sizes were observed with a wide size distribution. In the mixed aqueous/2-propanol solutions, and especially at the lower concentrations of 2-propanol