Partial Hydrogenation of 2-Methyl-3-butyn-2-ol over Pd/ZnO: Effect of Reduction Temperature on Alloy Formation and Catalytic Response

We have investigated the catalytic effect of the beta-phase PdZn alloy on selective gas-phase hydrogenation of 2-methyl-3-butyn-2-ol over Pd/ZnO using Pd/Al2O3 as a benchmark. Activation (in H-2) of Pd/ZnO to 973 K generated a beta-phase PdZn alloy (from X-ray diffraction). Five intermediate samples with a modified surface PdZn/Pd delta- mole ratio (based on X-ray photoelectron spectroscopy) but similar metal nanoparticle size (mean size = 6 nm from high-resolution scanning electron microscopy/scanning transmission electron microscopy) were generated by changing the final activation temperature between 403 and 973 K. Activation over the same temperature range increased metal nanoparticle size (6 -> 10 nm) and lowered the surface Pd delta- content in the Pd/Al2O3 samples. In each case, regardless of the activation temperature, greater selectivity to target 2-methyl-3-buten-2-ol (MBE) was observed over Pd/ZnO relative to Pd/Al2O3. Both catalysts delivered a similar activity/selectivity trend characterized by enhanced MBE selectivity and lower activity over the systems activated at a higher temperature. We associate this response with the formation of beta-PdZn alloy and metal encapsulation that impacts on surface Pd delta- in Pd/ZnO. Pd/ZnO outperformed an industrial Lindlar catalyst, Pd/CuO, and Au/Al2O3 in terms of selective transformation to MBE. Our results establish the beneficial effect of the PdZn alloy phase to promote the continuous production of commercially important alkenols.