Zeolite Y hydrolyses methyl methacrylate to methacrylic acid in the gas phase

Abstract

Poly(methyl methacrylate) (PMMA) depolymerizes to its monomer methyl methacrylate (MMA) below 400 °C but the accompanying byproducts have similar boiling points, which incurs excessive separation costs rendering the process economically challenging. Hydrolyzing MMA to methacrylic acid (MAA) is an economic alternative since its boiling point is 61 °C higher. Zeolites are among a class of heterogeneous catalyst with Brønsted acidity that protonate the ester carbonyl group in the much less common gas-phase route to acids. Here, for the first time, we report the hydrolysis kinetics of MMA to MAA in the gas phase over a Y/SiO2 zeolite with a 960 m2 g−1 surface area. MMA conversion in an 8 mm fixed bed reactor averaged 94 % above 200 °C and a yield of 85 %. Operating the reactor in excess water vapour shifts the equilibrium conversion towards the products. Surface reaction between adsorbed MMA and water is the rate controlling step. Heterogeneous supports control the hydrophobic properties, and thereby activity and stability. Introducing zeolites in the matrix of a silica gel facilitates access to all external zeolite active sites compared to spray-drying with Ludox where the SiO2 shell covers the external catalyst surface. Nuclear magnetic resonance spectroscopy with magic angle spinning (MAS NMR) and ultra-violet visible (UV–vis) detected Brønsted acidity. The reaction rate is first order in methacrylic acid and water and the equilibrium conversion is about 95 % above 200 °C. The apparent activation energy was 106 kJ mol−1.