March 24, 2014
These catalysts convert ethylene to hexene and octene. Linear low-density polyethylene (LLDPE) has been the fastest-growing polyethylene product for the past 20 years. The key comonomers used in the manufacture of LLDPE are 1-butene, 1-hexene, and 1-octene.
Almost the only sources of 1-hexene and 1-octene are linear α-olefin (LAO) plants. Exceptions include the Chevron Phillips Chemical catalyst system that selectively trimerizes ethylene to 1-hexene and Sasol’s technology for extracting the olefins from its synthetic fuels stream. More recently, Sasol developed a catalyst system that gives a mixture of 1-hexene and 1-octene; the company is now building a commercial plant in the United States.
LAO plants oligomerize ethylene to a broad range of even–carbon number α-olefins, from C4 to C20 and sometimes higher. The problem is that the demand for LLDPE is outpacing the growth rate for LAOs. This supply–demand imbalance puts stress on 1-hexene and 1-octene supplies for LLDPE manufacture.
P. M. Fritz and co-inventors disclose another catalyst system that can trimerize ethylene to 1-hexene or produce a 1-hexene–1-octene mixture. In the patent’s only example, a solution of a ligand, Ph2PN(i-Pr)PPhNH-i-Pr, in toluene is added to CrCl3(thf)3. This catalyst solution and a solution of AlEt3 promoter in toluene are transferred to a 300-mL pressure reactor. The reactor is sealed and pressurized to 30 bar with dry ethylene.
At 40 ºC, the C6 yield is 89%; selectivity to 1-hexene in the C6 cut is 100%. If the temperature is raised, the selectivity to 1-hexene decreases, and increasing amounts of butenes are coproduced.
The inventors state that by using different cocatalysts or by varying the ligand structure, the system can make a mixture of 1-hexene and 1-octene. Unfortunately, no data for this process are given. (Saudi Basic Industries Corp. [Riyadh, Saudi Arabia]; Linde AG [Munich]. US Patent 8,637,721, Jan. 28, 2014; Jeffrey S. Plotkin)