Partial hydrogenation of propyne over copper-based catalysts and comparison with nickel-based analogues

摘要

The partial hydrogenation of propyne was studied over copper-based catalysts derived from Cu-Al hydrotalcite and malachite precursors and compared with supported systems (Cu/Al_2O_3 and Cu/SiO_2). The as-synthesized samples and the materials derived from calcination and reduction were characterized by XRF, XRD, TGA, TEM, N_2 adsorption, H_2-TPR, XPS, and N_2O pulse chemisorption. Catalytic tests were carried out in a continuous flow-reactor at ambient pressure and 423-523 K using H_2:C_3H_4 ratios of 1-12 and were complemented by operando DRIFTS experiments. The propyne conversion and propene selectivity correlated with the copper dispersion, which varied with the type of precursor or support and the calcination and reduction temperatures. The highest exposed copper surface was attained on hydrotalcite-derived catalysts, which displayed C_3H_6 selectivity up to 80% at full C_3H_4 conversion and stable performance in long-run tests at T ≥ 473 K. Both activated Cu-Al hydrotalcites (this work) and Ni-Al hydrotalcites [S. Abelló, D. Verboekend, B. Bridier, J. Pérez-Ramírez, J. Catal. 259 (2008) 85] exhibited a relatively high alkene selectivity under optimal operation conditions, but they present a markedly distinctive catalytic behavior with respect to temperature and hydrogen-to-alkyne ratio. The product distribution was assigned through Density Functional Theory (DFT) simulations to the different stability of subsurface phases (carbides, hydrides) and the energies and barriers for the competing reaction mechanisms. The behavior of Cu in partial alkyne hydrogenation resembles that of Au nanoparticles, while Ni is closer to Pd.