Ethylene selective oligomerization catalyst systems and method for ethylene oligomerization using the same

A catalyst for selective oligomerization of ethylene, comprising three components:

• a ligand, wherein the ligand comprises carbene groups of imidazole ring;
• a transition metal compound, wherein the transition metal compound contains chromium;
• and
• an activator, wherein the activator is a mixture of alkyl aluminum compound and alkyl aluminoxane compound removed of volatile components, in which the alkyl aluminum compound is triethyl aluminum and the alkyl aluminoxane compound is methyl aluminoxane removed of volatile components;
• wherein the ligand contains at least one group as shown in general formula I as shown below;
• wherein the main chain of the bridging group A is —(CH2)n—SiR′R″—(CH2)m(0≤n≤3, 0≤m≤3), in which R′ and R″ are each independently methyl, isopropyl, cyclohexanyl, cyclopentyl, phenyl, naphthyl or 2,6-diisopropyl phenyl, respectively;
• the group E is a linear or cyclic group containing a second heteroatom, and the group E is selected from the group consisting of diisopropyl phosphino, dicyclohexyl phosphino, dimethyl phosphino, diethyl phosphino, diphenyl phosphino, di-o-methylbenzene phosphino, di-o-ethylphenyl phosphino, di-o-isopropylphenyl phosphino, diphenyl phosphino, dinaphthyl phosphino, methoxy, ethoxy, phenoxy, isopropoxy, methyl mercapto, ethyl mercapto, phenylmercapto, and isopropyl Mercapto;
• R is a hydrocarbyl group.