Role of Surface Acidity in Formation and Performance of Active Ni Single Sites in Supported Catalysts for Butene Dimerization: A View inside by <i>Operando</i> EPR and <i>In Situ</i> FTIR Spectroscopy

Abstract

A series of supported Ni/SiO2–Al2O3 catalysts with the same Ni content but different ratios of Brønsted and Lewis acid sites resulting from different Al/Si ratios have been prepared by impregnation with Ni(Cp)2 and analyzed by operando electron paramagnetic resonance (EPR) during dimerization of n-butenes under industrially relevant conditions as well as by in situ Fourier transform infrared (FTIR) spectroscopy of adsorbed pyridine and CO to derive relations between the surface acidic properties, the nature of the Ni sites, and the catalytic performance. While EPR monitored the formation of different Ni+ single sites as well as ferromagnetic Ni clusters, it is silent for Ni2+. To compensate for this lack, FTIR spectroscopy of adsorbed CO was used to analyze the relative number and distribution of Ni single sites in both valence states of +1 and +2. This method was also used to analyze the relative number of Brønsted and Lewis acid sites exposed on the surface. Thus, a comprehensive picture on the formation of different Ni surface sites and their role for catalytic performance could be derived. It was found that Brønsted sites have both a positive and negative effect: (1) they stabilize active Ni single species while Lewis sites do not play a significant role for catalytic performance; (2) they promote undesired branching of C8 products. Thus, optimum catalysts should contain a maximum number of single Ni sites in the immediate vicinity of Brønsted surface sites, while an excess of the latter must be avoided to suppress isomerization.