丙烯齐聚催化剂活性中心的电子顺磁共振考察

EPR STUDIES OF THE ACTIVE SITES OF CATALYST FOR PROPYLENE OLIGOMERIZATION

硅酸铝担载的镍催化剂是一类性能十分优越的丙烯齐聚催化剂。应用ＥＰＲ技术系统地考察了这种催化剂在抽空、氢还原及丙烯活化等预处理条件下活性组分、价态及配位方式的变化。发现在该催化剂上丙烯齐聚反应是一个复杂过程，存在诸多反应中间体（物种Ⅴ，Ⅵ，Ⅶ等）；Ｎｉ＋与丙烯分子作用形成的物种［Ｎｉ（Ｃ３Ｈ６）］＋导致丙烯齐聚反应诱导期的消失。

In order to investigate the active sites,oxidation and coordination states of silica alumina supported nickel(Ni SA)catalyst in propylene oligomerization,the EPR spectra of Ni SA catalyst were taken during the treatment of pre evacuation,hydrogen reduction,activation and reaction with propylene,regeneration of the used catalyst to monitor the changes of Ni SA catalysts.Results showed that pre evacuation was a mild reduction process in which Ni 2+ ions were partially reduced to Ni + under certain temperatures and that the optimum hydrogen reduction temperature of Ni SA catalyst was around 673K.It also indicated that the Ni + ions were the active sites in propylene oligomerization,but the appearance of induction period was merely due to the reduction of Ni 2+ ions to Ni + ions.While the pretreated Ni SA catalyst was exposed to propylene,the induction period of propylene oligomerization could be eliminated and simultaneously a set of new species(signal Ⅳ)would appeared in EPR spectra,whose g tensors were g 1=2 026,g 2=2 057,g 3=2 104.It is suggested that the species Ⅳ might be the precursor of the intermediates formed by the interaction of Ni + with a propylene molecule.It is its formation that results in the elimination of induction period.There appeared three new EPR signals of Ni SA catalyst during the reaction process,their g tensors were respectively: Signal Ⅴ g 1=2 040,g 2=2 074,g 3=2 127; Signal Ⅵ g 1=1 921,g 2=2 067,g 3=2 205; Signal Ⅶ g 1=1 874,g 2=2 022,g 3=2 351 They have been attributed to the following three species respectively:Species Ⅴ +(P=terminal alkylene produced by dimerization of propylene);Species Ⅵ[Ni(P)(C 3H 6)] +,Species Ⅶ [Ni R] + .These three species might be the active intermediates of propylene oligomerization.Experiment of regeneration showed that Ni + ions could be rather stable on Ni SA catalyst.