Homogeneous redox catalysts based on heteropoly acid solutions: I. Pilot testing of a catalyst and methyl ethyl ketone synthesis

Abstract

The pilot experiments on methyl ethyl ketone (MEK) synthesis in 2005–2006 were aimed at developing a technology for highly selective n-butylene oxidation with oxygen into MEK in the presence of a new homogeneous catalyst. This catalyst was a chloride-free aqueous solution of Pd(II) + HPA-7′, where HPA-7′ is a vanadium-rich molybdovanadophosphoric acid of modified (non-Keggin) composition with the empirical formula H₁₂P₃Mo₁₈V₇O₈₅. For ensuring the explosion and fire safety of the pilot unit, the MEK synthesis process was carried out in two steps conducted in different reactors. The catalyst solution in the pilot plant circulated in a closed loop. The target reaction (step (1)) was performed at 60°C and an n-butylene pressure of 9 bar in a tubular plug-flow reactor 1 in the absence of O₂. The high oxidation potential of the HPA-7′ solution ensured a high rate of the reaction, which lasted <20 min. The separation of MEK from the reduced catalyst was carried out in a film evaporator (stripping column) at 100°C. Subsequent catalyst regeneration with atmospheric oxygen (step (2)) was performed in an original perfect-mixing reactor 2. At 160–190°C and a pressure of 20 bar ($P_{O_2 }$ = 4 bar), this step lasted 40–50 min. The regenerated catalyst was returned into reactor 1 to be involved in the next catalytic cycle. Thus, the catalytic oxidation of n-butylene with oxygen in the pilot plant was carried out in two steps, while MEK was drawn off continuously. The pilot tests demonstrated that the unsteady-state catalysis mode is appropriate for the two-step MEK synthesis. The Pd(II) + HPA-7′ catalyst developed has a near-optimum composition. An analysis of the drawbacks of the pilot plant design showed the ways of attaining the rated production capacity, 250 (kg MEK)/day. The results of carrying out the MEK synthesis process on the pilot scale will be taken into consideration in developing two-step technologies for the oxidation of other organic compounds with oxygen in the presence of an HPA solution.