Deactivation behavior of alkali-metal zeolites in the dehydration of lactic acid to acrylic acid

• Acidic deposits, such as lactic acid and its open-chain esters, cause deactivation.
• Fast deactivation occurs when the acidic deposits form stable clusters.
• Lattice polarity and adsorption capacity are key properties influencing deactivation.
• Na-ZSM-5 zeolite allows dehydrating lactic acid without deactivation.

A series of FAU, MFI, and MOR Na-zeolites with different nSi/nAl ratios and varying exchanged cations Li+, Na+, K+ and Cs+ are investigated as catalysts in the gas phase dehydration of lactic acid to acrylic acid. The conversions and selectivities as well as their time dependence prove to be greatly affected by the structure, the type of alkali-metal and the acidic/basic nature of the zeolite. Analyses of the catalysts after the reaction show a possible reason for the deactivation, viz. the blocking of the active sites due to the formation of deposits of clusters containing carboxyl groups. The deactivation rate is high if large adsorptive clusters can be formed and stabilized by intra- and inter-molecular hydrogen bonds. The conversion of lactic acid into acrylic acid without catalyst deactivation is possible if adsorptive accumulation is effectively excluded, e.g. when using Na-ZSM-5 zeolite.

Figure optionsDownload high-quality image (88 K)Download as PowerPoint slide