Solvent-free selective hydrogenation of 2-butyne-1,4-diol over structured palladium catalyst

Hydrogenation of 2-butyne-1,4-diol catalyzed by Pd nanoparticles on activated carbon fibers (ACFs) was studied. The ACF support in the form of woven fabrics provides the basis for structured catalytic fixed beds. In the present study, this catalyst was integrated in the stirrer of an autoclave. Hydrogenations were carried out in pure, solvent-free butynediol at temperatures from 352 to 392 K and hydrogen pressures of 1-2 MPa. The results were compared to those obtained for aqueous solutions at different pH's and temperatures of 293-333 K. High selectivities of >= 98% toward 2-butene-1,4-diol at conversions of <= 90% were attained in both systems. Turnover frequencies had comparable values at the same temperature. The activation energy of 30 kJ/mol was determined from initial reaction rates. Because this value is identical for the aqueous solution and the solvent-free system, mass-transfer limitations can be excluded for both systems. Concentration-time profiles were quantitatively predicted assuming the Langmuir-Hinshelwood kinetics with weak hydrogen adsorption. Estimated kinetic parameters allow a coherent interpretation of the experimental observations. Catalyst reactivation and multiple reuses were demonstrated.