Novel Aqueous-Phase Process for Conversion of 3-Hydroxybutyric Acid to 1,3-Butanediol: Effects of Temperature, Hydrogen Pressure, and Nature of Ru-Supported Catalysts on Selective Hydrogenation

A novel aqueous-phase hydrogenation reaction was developed for the conversion of 3-hydroxybutyric acid (HBA) to 1,3-butanediol (1,3-BDO) over catalysts composed of Ru supported on TiO2, SiO2, and C. Analyses using XRD and Raman spectroscopy indicated strong metal–oxide interactions in the Ru/TiO2 samples, while weak interactions of Ru with SiO2 led to crystallization of the Ru species. Preliminary catalytic tests with a commercial Ru/C catalyst showed that higher H2 pressure and low temperature (130 °C) enabled selective hydrogenation of HBA to 1,3-BDO. Compared to the commercial catalyst, the synthesized Ru/TiO2 and Ru/SiO2 catalysts were more selective toward 1,3-BDO, achieving 38% selectivity with a catalyst composed of 2.0% Ru supported on TiO2. The SiO2-supported catalyst was significantly more active (62% HBA conversion at 150 °C) than the TiO2-supported counterpart but was also more prone to side reactions leading to 2-propanol and 2-butanol. Stability experiments with the 1,3-BDO product revealed that the Ru-based catalysts promoted side reactions originating from 1,3-BDO, especially over the SiO2-supported catalyst, which could explain the lower selectivity observed in the catalytic HBA hydrogenation. A mechanistic investigation indicated that 1,3-BDO could undergo dehydration to 2-butanol and dehydrogenation to 4-hydroxy-2-butanone, a possible intermediate in the formation of 2-propanol by a C–C cleavage pathway.