Selection of Suitable Micellar Catalyst for 1,10-Phenanthroline Promoted Chromic Acid Oxidation of Formic Acid in Aqueous Media at Room Temperature

Abstract

In the present investigation, kinetic studies of oxidation of formic acid with and without catalyst and promoter in aqueous acid media were studied under the pseudo-first order conditions [formic acid]T ${\gg}[Cr(VI)]_T$ at room temperature. In the 1,10-phenanthroline (phen) promoted path, the cationic Cr(VI) phen complex is the main active oxidant species undergoes a nucleophilic attack by the substrate to form a ternary complex which subsequently experiences a redox decomposition through several steps leading to the products $CO_2$ and $H_2$ along with the Cr(III) phen complex. The anionic surfactant (i.e., sodium dodecyl sulfate, SDS) and neutral surfactant (i.e., Triton X-100, TX-100) act as catalyst and the reaction undergo simultaneously in both aqueous and micellar phase with an enhanced rate of oxidation in the micellar phase. Whereas the cationic surfactant (i.e., N-cetyl pyridinium chloride, CPC) acts as an inhibitor restricts the reaction to aqueous phase. The observed net enhancement of rate effects has been explained by considering the hydrophobic and electrostatic interaction between the surfactants and reactants. The neutral surfactant TX-100 has been observed as the suitable micellar catalyst for the phen promoted chromic acid oxidation of formic acid.

EXPERIMENTAL

1,10-Phenanthroline (AR, Merck, Mumbai, India), formic acid (AR, Qualigens, Mumbai, India), K2Cr2O7 (AR, BDH, India), H2SO4 (AR, Qualigens, Mumbai, India), sodium dodecyl sulfate (AR, SRL, Mumbai, India), TX-100 (AR, SRL, Mumbai, India) and all other chemicals used were of highest degree of purity available commercially. All the solutions were prepared in double distilled water. Solutions of the oxidant and reaction mixtures containing the known quantities of the substrate (s) (i.e., formic acid), promoter (1,10-phenanthroline) under the kinetic conditions [formic acid] >> [Cr(VI)]T. Acid and other necessary chemicals were separately thermostated (± 0.10 ℃). The reaction was initiated by the requisite amounts of the oxidant with the reaction mixture. Progress of the reaction was monitored by following the decay of Cr(VI) at 450 nm wavelength at different different time intervals with the UV-vis [UV-VIS-NIR-3600 (SHIMADZU)] spectrophotometer equipped with a temperature controller. Quartz cuvettes of path length 1 cm were used. The pseudo-first-order rate constants (kobs, s−1) were calculated from the slope of plots of ln(A450) versus time (t) which were linear.11 The scanned spectra and spectrum after completion of the reaction were recorded with a UV-vis spectrophotometer [UV-1800 and UV-VIS-NIR-3600 (SHIMADZU)]. A large excess (15-fold) of reductant was used in all kinetic runs. No interference was observed due to other species at 450 nm.

Surfactant micelles provide an unusual medium, which may affect the rate of reaction. One of the most important properties of micellar system is their ability to affect the rate of the chemical reaction. The effect of surfactants on reaction kinetics is called micellar catalysis involves several contributing factors. Notably, formic acid has been widely used as hydrogen source in liquid-phase transfer hydrogenation reactions of carbon dioxide in the presence of base such as amines under ambient conditions. The excess formic acid can be reoxidized to get the deserved product with further addition of chromic acid, maintaining the same reaction condition. Obviously, electrostatic attraction/repulsion plays an important role during the course of the reaction in the presence of ionic surfactants. All the surfactant concentrations used during each set of experiments are above their critical micelle concentration (CMC). The CMC values of all the three surfactants are known from earlier literature. So CMC values are very helpful to prepare the solutions of surfactants in particular concentration. Combination of TX- 100 micelle and phen promoter enhances the reaction almost 600 times faster compared to the uncatalyzed and unpromoted reaction. The mechanistic paths of uncatalyzed unpromoted and SDS catalyzed phen promoted chromic acid oxidation of glycerol have also been compared. SDS micelle in absence of phen enhances the oxidation rate compared to the TX-100 micelle. But in combined reaction mixture i.e., in presence of phen the reaction rate is highly accelerates in TX-100 micelle compared to SDS micelle. It is also noted that the cationic micelle CPC retards the reaction compared to the uncatatalyzed and unpromoted reaction. The Cr(VI)−phen complex, a cationic species has been found to act as the active oxidant in the phen-promoted chromic acid oxidation of formic acid. The generation of the final Cr(III) species after the completion of the reaction has been investigated by a series of spectral observation by UV-visible spectrophotometer. The present method of formic acid oxidation is simple, accurate, rapid, economical, and precise. In conclusion, it can be said that neutral micelle TX-100 is an efficient micellar catalyst for the phen promoted chromic acid oxidation of formic acid.