Kinetic Studies On Ethylene Selective Oligomerization Over Cr-based Catalyst System

Catalytic oligomerization of ethylene into linear alpha olefins(α-LAOs)is an important industrial process.a-LAOs,especially 1-hexene and 1-octene,are very important chemical intermediates as monomers for linear low-density polyethylene(LLDPE).1-hexene and 1-octene have gained considerable attention,and the demand for 1-hexene and 1-octene is constantly increasing.Currently,ethylene trimerization has been successfully commercialized in 2003.However,the development of the ethylene tetramerization to 1-octene is still on a long way to go both in the academic research and industry application.Kinetic studies of ethylene oligomerization are crucial for understanding the catalyst performance in depth and for optimizing reactor design and reaction conditions.Manyik et al.,reported that the order to ethylene concentration is second order,however,Rensburg et al.,supported the first order to ethylene concentration.Thus,there is still much controversy,and the research on the kinetics of ethylene oligomerization could help resolve this.In this paper,the experimental kinetics of ethylene oligomerization over the Cr-based catalyst were investigated.The catalyst system comprising chromium triacetylacetonate(Cr(acac)3),bis(diphenylphosphino)tert-butylamine(Ph2PN(tBu)PPh2)as ligand,and methylaluminoxane(MAO)as cocatalyst.The influence of catalyst concentration,ethylene pressure and reaction temperature were systematically studied.The results showed that with the increase of ethylene pressure,the total selectivity of 1-hexene and 1-octene is almost constant,however,the selectivity of 1-octene increased.With the increase of temperature,the selectivity of 1-hexane increased,but the 1-octene decreased.The optimium ethylene pressure is 3.5 MPa in the range of 2.0-3.5 MPa.And the optimium temperature is 45 ℃ in the range of 30-55 ℃.And a detailed catalytic kinetic behavior and the kinetic simulation of this catalyst system were also established with Aspen Plus software.It could be found that the dependency of the reaction on catalyst concentration was the order of 1.16,and that on the ethylene concentration was the order of 1.74.The apparent activation energy of the reaction was 73.7 kJ/mol in the temperature range of 30-45 ℃ and in the starting pressure range of 2.0-3.5 MPa.The apparent reaction rate equation was:...