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 micro-crystalline Beta zeolite support) catalysts were prepared by ion exchange and characterized by XRD, SEM, N⁠2-adsorption, MP-AES and FTIR. The samples were subjected to testing as ethene oligomerization catalysts at T = 120 C, P⁠total = 29 bar, P⁠ethene = 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 P(Ethene) = 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.