Studies on the pathways of the heterogeneous catalytic decarbonylation of lactic acid to acetaldehyde

Abstract

We conducted studies on the pathways of the heterogeneous catalytic decarbonylation of lactic acid (LA) to acetaldehyde (AD) using unsupported NaNO₃ and amorphous silica-alumina (SiO₂-Al₂O₃)-based materials by catalytic testing, IR spectroscopic monitoring, temperature-programmed ammonia desorption and the IR spectroscopy of pyridine adsorption. A combination of IR monitoring and catalytic studies illustrated that the catalytic performance was dependent on the content of the in situ-generated lactates and that the interaction of lactates with SiO₂-Al₂O₃ enabled their stabilization against easy dehydration to acrylates and subsequent acrylate polymerization. A combination of IR monitoring, surface acid property and catalytic studies indicated that the AD yield and selectivity to AD increased with decreasing surface acidity. The finding that the ratio of the selectivity to AD to selectivity to acrylic acid almost did not change with varying intrinsic surface acidity indicated that both LA decarbonylation and dehydration were lactate dependent and the catalytic process of LA decarbonylation did not depend on the intrinsic surface acid sites. A probable mechanism of the heterogeneous catalytic LA decarbonylation was proposed, in which the lactates acted as the catalytic active species and physisorbed LA acted both as the Brønsted acid site assisting in the catalysis and as a product intermediate. Besides, the reactivity of LA with SiO₂-Al₂O₃ with regards to the catalytic active species and reaction pathways was discussed.