Elemental Sulfur as a Catalyst Precursor for Gas-Liquid Heterogeneous Chlorination of Acetic Acid: Kinetics and Optimization for Enhanced Monochloroacetic Acid Selectivity and Productivity

Abstract

Monochloroacetic acid (MCA) is a pivotal intermediate in agrochemicals and pharmaceuticals, but its industrial synthesis via acetic acid chlorination faces challenges related to selectivity and reaction time. This study investigates the kinetics of gas-liquid heterogeneous acetic acid chlorination using elemental sulfur as a catalyst precursor to establish a scientific basis for process optimization. A consecutive-parallel reaction mechanism was proposed incorporating acetic acid consumption, acetyl chloride conversion, MCA formation, and dichloroacetic acid (DCA) formation. Kinetic parameters were determined at 353, 363, 373, and 383 K in a steel bubble column reactor with fixed initial sulfur concentration (1.92 mol/L) and Cl₂ space velocity (4.028 L·L⁻¹·h⁻¹). The activation energy for DCA formation (87.55 kJ·mol⁻¹) was substantially higher than that for MCA accumulation (52.40 kJ·mol⁻¹). Relative rate analysis revealed that k₃/k₄ decreases continuously from 1.83 at 353 K to 0.76 at 383 K, confirming that lower temperatures favor MCA selectivity. The proposed kinetic model showed excellent agreement with experimental data (R² > 0.98). Based on the kinetic analysis, three optimization strategies were derived: maintaining high acetic acid concentration, dynamic adjustment of Cl₂ feed rate, and implementation of a decreasing temperature-time profile. This work provides a scientific basis for optimizing industrial MCA synthesis using low-cost sulfur as a catalyst precursor.