Role of acidity in heterogeneous nickel catalysts for the oligomerization of light olefins, The

Abstract

Nickel supported on amorphous silica aluminas (ASAs) are promising catalysts for the oligomerization of light olefins to liquid fuels. They are readily prepared and utilized, possess high activity towards ethylene and propylene, and are made from earth-abundant, cost-effective, and commercially-available materials. The material properties of these catalysts were characterized by elemental analysis, TPR, NH3 TPD, pyridine and CO DRIFTS, XAS, XPS, XRD, 27Al MAS NMR, N2 physisorption, and SEM imaging. A representative dehydrated mixed alcohol stream containing a blend of ethylene and propylene in a 2.8:1 ratio was oligomerized using a fixed-bed reactor system at conditions of Treactor = 120 °C, Ptotal = 200 psig, and WHSV = 0.75-1.0. Conversion and productivity values were determined by on-line GC analysis. Aromatic-free olefins in the C4-C14 range were produced over the catalysts at rates of 95-125 g kgcat−1 h−1. Conversions values ranged between 7.2-11.9% and 29.8-34.0% for ethylene and propylene, respectively, with mass balances of 83-95%. The formation of active nickel centers for ethylene conversion required the presence of anionic, Brønsted acidic support sites. Catalytic activity was further correlated with formation of reducible, Lewis acidic nickel sites. A comparison of two ASA supports having different aluminum content found their interactions with nickel to be substantially different. The incorporation of nickel on the more aluminum rich ASA produced more reducible nickel species, introduced greater Lewis acidity, and had a lower impact on Brønsted acidity. Despite these differences in nickel speciation, the Ni 2p3/2 peak in XPS appeared at 856.0 eV on both supports, indicative of an electronic environment similar to Ni(OH)2. By sum of in situ XAS and CO DRIFTS experiments, the active sites are proposed to be 5-coordinate nickel species that coordinate two ethylene molecules by release of a hemilabile support interaction. These hemilabile bond exchanges occur by interaction between nickel, terminal silanols, and coordinatively-flexible aluminum sites in ASA. This conclusion is supported by the disappearance of free silanols upon binding of Ni cations by CO, perturbations in the EXAFS fits of nickel upon ethylene exposure without change in coordination number, and changes in the coordination of Al upon dehydration and nickel incorporation.