Modifier-free Pd/acetylene black and Pd/activated carbon catalysts prepared via impregnation and microemulsion methods as alternatives to the Lindlar catalyst

To improve the selectivity of selective hydrogenation catalysts without the use of toxic and unstable modifiers, either the substrate-to-Pd ratio can be increased, or Pd nanoparticles can be sintered into aggregates. Unfortunately, these measures are at the expense of activity. In this study, carbon black and activated carbon are used as supports for the Pd nanoparticles to investigate if the trade-off can be shifted to higher activities. Pd species were distributed on these supports by two synthesis protocols: thermo-destabilization of microemulsions and incipient wetness impregnation with a sintering step at 623 K. The results of the long-term catalytic hydrogenation of 3-hexyn-1-ol over these catalysts showed that for both catalyst types, a high enough substrate-to-Pd ratio increases the selectivity of the cis-hexenol product. However, the kinetic behavior of the obtained catalysts differed, such that the Pd/acetylene black catalysts showed kinetic results comparable to those of the Lindlar catalyst, serving as a benchmark. Also, changing the particle size from 4.5 to 1.5 nm revealed the structure-activity relationships. For the Pd/activated carbon catalysts, the microemulsion-prepared catalyst showed proper kinetic behavior at relatively higher substrate-to-Pd ratios, but the impregnated catalyst exhibited lower rates with a lower selectivity for the cis-hexenol product.