Sustainable production of acrylic acid: Rb+- and Cs+-exchanged Beta zeolite catalysts for catalytic gas-phase dehydration of lactic acid

Rb+- and Cs+-exchanged Beta zeolites (RbxNa1-x beta and CsxNa1-x beta) of varying exchange degrees (x = 0-1.00) were employed to catalyze the gas-phase dehydration of lactic acid (LA) for sustainable production of acrylic acid (AA) in a flow fixed-bed reactor at 360 degrees C, using an aqueous solution of LA (10 mol% or 35.7%) as the reaction feed at a weight hourly space velocity by LA of 2.1 h(-1). An appropriate window of the ion exchange degrees for highly selective AA production (>= 60 mol%) was determined for either series of the samples, i.e., x = 0.85-0.98 for the RbxNa1-x beta and x=0.71-0.90 for the CsxNa1-x beta samples. The best performing catalysts Rb0.95Na0.05 beta, and Cs0.81-0.90Na0.19-0.10 beta offered the highest AA selectivity (ca. 70 mol%) and yield (ca. 60-65 mol%) for reaction periods of longer than 10 h. Measurements of the surface acidity and basicity of the catalyst samples by temperature-programmed desorption of NH3 and CO2 showed that the highly selective catalysts in such widows should have both weakly acidic and weakly basic surface sites with suitably balanced acidity and basicity. The acid-catalyzed decarbonylation/decarboxylation and base-catalyzed condensation of LA, which lead respectively to formation of acetaldehyde and 2,3-pentanedione, always occurred as the competing reactions over the investigated catalysts. Observations on the catalyst selectivity changes for these competing reactions clearly demonstrate that the suitably balanced acidity and basicity at the catalyst surface is the key to the high selectivity for the desired dehydration reaction. (C) 2015 Elsevier B.V. All rights reserved.