Stable catalysis of neutral silica-supported potassium lactate for vapour-phase dehydration of lactic acid to acrylic acid - critical role of the support.

Abstract

We have deeply investigated KNO3-derived silica-supported potassium lactate catalysts for the vapour-phase dehydration of lactic acid (LA) to acrylic acid (AA) by catalytic testing, IR spectroscopic monitoring, ammonia temperature-programmed desorption, isopropyl amine temperature-programmed desorption, IR spectroscopy of pyridine adsorption and thermogravimetric analysis (TGA). A combination of catalytic and acid property studies illustrates that the acidic KNO3/silica systems are not favourable for catalytic selectivity and stability for the production of AA whereas the neutral KNO3/silica systems favour catalytic selectivity and stability for the production of AA. A combination of catalytic and TGA studies indicates that the interaction between KNO3 and silica has a strong promotional effect on catalytic stability for the production of AA. A combination of IR monitoring and catalytic studies suggests that the effects of the surface acidity and the interactions between the potassium salt or base and silica on the catalytic performance are associated with the content and stability of potassium lactate as the catalytic active species. The catalyst stabilization and deactivation under LA dehydration conditions are discussed in detail. The neutral KNO3/silica systems enable the formation of potassium lactate to become dominant whereas the unsupported KNO3 and acidic KNO3/silica systems lead to the formation of poly(potassium acrylate), i.e. catalyst deactivation. This contribution includes for the first time that the dehydration of LA to AA proceeds smoothly with a neutral heterogeneous catalyst.