Ethene oligomerization on nickel microporous and mesoporous-supported catalysts: Investigation of the active sites

Nickel-containing mesoporous (using Al2O3 or SiO2-Al2O3 support) and microporous (using nano-or microcrystalline Beta zeolite support) catalysts were prepared by ion exchange and characterized by XRD, SEM, N-2-adsorption, MP-AES and FTIR. The samples were subjected to testing as ethene oligomerization catalysts at T = 120 degrees C, P-total = 29 bar, P-ethene = 11.6-25.1 bar. All catalysts were active for ethene oligomerization, and linear butenes were the main gaseous products. However, catalyst deactivation due to retained long-chain alkenes was observed. Ethene partial pressure variation experiments showed that the reaction order in ethene for butene formation was 1.5-1.7 for the mesoporous, and 2.0 for the microporous catalysts. Contact time variation experiments carried out with the microporous catalysts at Pethene= 18 bar showed that product selectivity was independent of ethene conversion, and suggested that 1-butene and 2-butenes are primary products. This result is consistent with the Cossee-Arlman mechanism. The nature of the active sites was investigated with FTIR spectroscopy with CO as probe molecule. The results point to Ni2+-counterions as the preeminent active sites, while we suggest that Ni-sites grafted on silanol groups and NiO particles are spectators.