An efficient and durable hierarchically porous KLA/TiPO catalyst for vapor phase condensation of lactic acid to 2,3-pentanedione

Abstract

Sustainable production of 2,3-pentanedione from bio-lactic acid via a vapor condensation reaction over KLA/TiPO (KLA: potassium lactate) was investigated in this work. A KNO₃ precursor supported on the surface of TiPO was in situ converted to basic sites in a KLA/TiPO catalyst. KLA together with Ti⁴⁺(Lewis acidic site) make up the acid–base pairs in the KLA/TiPO catalyst, resulting in excellent activity for the condensation of lactic acid to 2,3-pentanedione. The loading amount of KNO₃ was shown to have an important influence on the catalytic performance, since the acid–base properties of the catalysts were found to vary with the addition of KNO₃. Reaction conditions such as lactic acid feed flow rate and lactic acid concentration were also discussed. Both lactic acid conversion and 2,3-pentanedione selectivity increased with elevated lactic acid feed flow rates, indicating the existence of an external diffusion resistance of the lactic acid reactant during the catalytic reactions. However, the lactic acid feed flow rate increased to 1.0 mL h⁻¹ (corresponding to LA liquid hourly space velocity (LHSV) = 2.6 H⁻¹), and the external diffusion resistance was efficiently eliminated. Enhancing the LA concentration improved the selectivity of 2,3-pentanedione, suggesting that the reaction order of the lactic acid molecule for lactic acid conversion to 2,3-pentanedione is higher than the other side reactions. Encouragingly, in retaining 30–45% of the lactic acid conversion, the condensation reaction with a 2,3-pentanedione selectivity of around 73% proceeded efficiently for at least 116 h on stream. The long-term stability of the present catalyst was found to be related to its hierarchical pores, which ameliorated the mass transfer effect of the reactant and product, except for the appropriate acid–base properties for lactic acid condensation to 2,3-pentanedione.