ABSTRACT Lactic acid is crucial for food production and energy generation in body. It also has been a key monomer of poly lactic acid, biodegradable plastic. As methoxy carbonylation of vinyl acetate (VAM) produces methyl‐2‐acetoxypropionate, and methyl lactate as intermediates for lactic acid, its kinetic investigations are important to understand the reaction behavior. In this paper a study was conducted on the kinetics of methoxyarbonylation for vinyl acetate in a semi‐batch slurry reactor across a temperature range of 363–383K, utilizing PdCl 2 (PPh 3 ) 2 catalyst. The effect of catalyst, methanol, vinyl acetate monomer (VAM) concentration and partial pressure of carbon monoxide (CO) on the reactants and products concentration was investigated with respect to time. Catalytic methoxycarbonylation of VAM produces methyl acetoxy propionate which further forms methyl lactate and methyl acetate through consecutive reaction. Methyl acetoxy propionate and methyl lactate thus formed can be converted to lactic acid through hydrolysis. The methoxycarbonylation conditions produce byproducts dimethyl acetate and methyl acetate through parallel reaction. Based on the initial rate data of formations of lactic acid precursors through methoxycarbonylation and consecutive reaction byproducts through parallel reaction, and trend behavior of initial rates with concentrations of reactants, different empirical rate equations were evaluated. The integral concentration‐time data was fitted to evaluate the kinetic parameters at various temperatures. General elimination method and thermodynamic principles were used to suggest an appropriate empirical rate equation as shown below. r 1 , r 2 , and r 3 are rates of methoxycarbonylation reaction (methyl‐2‐acetoxypropionate formation), consecutive reaction (methyl lactate formation) and parallel reaction (dimethyl acetal formation) respectively. k 1 , k 2 , and k 3 are equilibrium constants for the reactions r 1 , r 2 , and r 3 respectively. K B and K C are equilibrium constants for VAM and methanol C A, C B , C C , C D and C W are concentration of dissolved CO in the liquid phase, VAM, methanol, methyl‐2‐acetoxypropionate and catalyst respectively. For catalyst and CO, the methoxycarbonylation reaction rate was observed to be first order, while for VAM and methanol, it was first order with a tendency to zero order. Energy of activation needed for the catalytic reaction was 81.62 kJ/mol. For a catalytic reaction, selectivity towards the required product is desirable. Kinetic investigations can be utilized to achieve maximum selectivity towards the desired products at optimum reaction rate. The proposed model suggests that 95%–97% selectivity towards the combined carbonylated products (methyl‐2‐acetoxypropionate and methyl lactate) can be achieved.
Tonde et al. (Tue,) studied this question.