On pathways of heterogeneous catalytic decarbonylation of lactic acid to acetaldehyde

Abstract

We have conducted studies on the pathways of the heterogeneously catalytic decarbonylation of lactic acid (LA) to acetaldehyde (AD) with unsupported NaNO3 and amorphous silica-alumina (SiO2-Al2O3)-based materials by catalytic testing, IR spectroscopic monitoring, ammonia temperature-programmed desorption and IR spectroscopy of pyridine adsorption. A combination of IR monitoring and catalytic studies illustrates that the catalytic performance is dependent on the content of in situ generated lactate salts and that the interaction of lactate with SiO2-Al2O3 enables stabilization of the lactates against easy lactate dehydration to acrylates and subsequent acrylate polymerization. A combination of IR monitoring, surface acid property and catalytic studies indicates that the AD yield and selectivity to AD increase with decreasing surface acidity considering lactate hydrolysis causing loss of the lactates and competing formation of PolyLA and coke. The finding that the ratio of selectivity to AD to selectivity to acrylic acid almost does not change with varying intrinsic surface acidity suggests that both the LA decarbonylation and dehydration are lactate dependent and that the catalytic process of the LA decarbonylation is irrespective of the intrinsic surface acid sites. A probable mechanism of the heterogeneously catalytic LA decarbonylation is proposed, in which the lactates act as the catalytic active species while physisorbed LA acts both as the Brønsted acid sites assisting in the catalysis and a product intermediate. Besides, the reactivity of LA with SiO2-Al2O3 with regards to the catalytic active species and reaction pathways is discussed.