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

• Published in: Catalysis today 2016-07, Vol.269, p.65-73
• Main Authors: Yan, Bo, Mahmood, Azhar, Liang, Yu, Xu, Bo-Qing
• Format: Article
• Language: English
• Subjects: Acid–base catalysis; Acrylic acid; Alkali ions; Beta zeolite; Catalytic dehydration; Lactic acid
• ISSN: 0920-5861; 1873-4308
• DOI: 10.1016/j.cattod.2015.10.030

•Gas-phase dehydration of lactic acid over Rb+- and Cs+-exchanged β zeolites is presented.•Effects of the exchange degrees for Rb+ and Cs+ on the catalyst behavior are uncovered.•Correlation between the catalyst performance and its acid–base property is given. Rb+- and Cs+-exchanged Beta zeolites (RbxNa1−xβ and CsxNa1−xβ) 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°C, using an aqueous solution of LA (10mol% or 35.7%) as the reaction feed at a weight hourly space velocity by LA of 2.1h−1. An appropriate window of the ion exchange degrees for highly selective AA production (≥60mol%) was determined for either series of the samples, i.e., x=0.85–0.98 for the RbxNa1−xβ and x=0.71–0.90 for the CsxNa1−xβ samples. The best performing catalysts Rb0.95Na0.05β, and Cs0.81–0.90Na0.19–0.10β offered the highest AA selectivity (ca. 70mol%) and yield (ca. 60–65mol%) for reaction periods of longer than 10h. 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.