Why could the CrOx/SiO2 and VOx/SiO2 catalysts show so different behaviors in ethylene polymerization? A theoretical approach

Abstract

Phillips catalyst (CrOx/SiO2) is one of the most famous industrial catalysts for ethylene polymerization, while VOx/SiO2 catalyst is a promising catalyst capable of making UHMWPE, which was recently developed in our group. These two catalysts were known to show completely deferent behaviors in ethylene polymerization in terms of activity, H2 response and copolymerization ability. However, a molecular insight into the mechanism is not well understood. Herein, a theoretical comparison between them via the propagation through insertion and termination through chain transfer reaction pathways is performed. The results show the transition states for olefins (ethylene, 1-butene and 1-hexene) insertion are energetically more stable in VOx/SiO2. However, the binding energies of the olefins by VOx/SiO2 is obviously lower, which results in the insertion barriers for VOx/SiO2 about 2–3 kcal mol−1 higher than those for CrOx/SiO2. As to the termination, CrOx/SiO2 is more probably to terminate via the BHE (β-H elimination to Cr center) pathway, which generally has lower barriers than those for BHT (β-H elimination to monomer) in the VOx/SiO2 system. Meanwhile, VOx/SiO2 is superior to CrOx/SiO2 in hydrogenolysis, because it is more accessible by the H2, and the reaction barrier is also much lower. The energy gap between the barriers for propagation and termination is generally smaller for CrOx/SiO2, implying its tendency to produce lower MW PE when compared with VOx/SiO2. Furthermore, for CrOx/SiO2, the insertion barriers between C2H4 and α-olefin are bigger and the promotion effect of the incorporated α-olefins on termination is more obvious, which might account for the lower incorporation efficiency for comonomer by CrOx/SiO2, despite of its lower insertion barriers.