Dependence of ethene dimerization activity and selectivity on NiI in ion-exchange vs. framework sites in SAPO-5 and SAPO-11 materials

Abstract

Dehydrated nickel containing silicoaluminophosphates SAPO-11 and SAPO-5 materials are catalytically active for ethene dimerization. There is a striking difference between the catalytic activity due to nickel(I) species in ion-exchange and in framework positions. Both the ethene dimerization activity and the selectivity for the formation of n-butenes is dependent on the location (ion-exchange vs. framework site) of the nickel(I) species and on the channel size (10-ring vs. 12-ring) of the supporting materials. EPR studies show an isolated nickel(I) species A (g_∥= 2.489 and g_⊥= 2.108) which transfers to species E (g₁= 2.680, g₂= 2.474 and g₃= 1.961) after ethene adsorption. This species has been assigned to an Ni^I–(C₂D₄)n complex. Subsequently, after heating to 353 K, species B1 (g₁= 2.720, g₂= 2.259 and g₃= 1.996), B2 (g₁= 2.363, g₂= 2.174 and g₃= 2.067) and B3 (g_∥= 2.052 and g_⊥= 2.007), depending of the type of SAPO material, are observed with simultaneous butene formation, detected by gas chromatography. Electron spin-echo modulation analysis allows the assignment of B1, B2 and B3 to Ni^I–C₄D₈ complexes with different geometries.