New Technology Research, For Ethylene Oligomerization Of Linear Alpha-olefins

AbstractLinear α-olefins are important petrochemical materials and in particular the C4~C24 α-olefins have extensive uses in many fields. α-Olefins are primarily obtained through the following two processes, cracking of paraffin and oligomerization of ethylene in industrial scale, in which the first process has eventually been replaced by the latter. The methods of ethylene oligomerization including Gulf,Ethyl and SHOP are widely used in current industries. But the developments of all these processes have been limited due to rather difficult conditions, especially high pressure (about 20MPa). Recently there has been a trend towards the studies of new processes of ethylene oligomerization employing various kinds of catalysts, which are operated under the moderate pressure. Particularly, oligomerization of light olefins in the presence of a medium pore shape selective crystalline zeolite catalyst has been investigated. Comparison with homogeneous catalysis, gas-solid heterogeneous reaction has some advantages, for examples, the reactions to be carried out under the lower pressure, the catalysts to be easily regenerated, and the products to be simply separated.The reactions of ethylene oligomerization in presence of catalysts supported metal on zeolite are divided into four stages of A, B, C and D in the paper. The stage A involves ethylene oligomerization over NaZSM-5 zeolite catalysts. The stage B describes the synthesis of a series of MZSM-5 catalysts by the method of ion exchange and the ethylene oligomerization over the MZSM-5 catalysts under the preferred operating conditions (280 ℃,2.5 MPa, and 0.5 WHSV). Some highly active catalysts are found after the reactions finished in stage B. The catalysts of MZSM-5P and MZSM-5L in stage C and D are prepared by inactivated treatments with 2,4,6-trimethylpyridine and o-phenanthroline, respectively, before the reactions of ethylene oligomerization were carried out under the conditions as above mentioned. The experiment result indicates that the activity of NaZSM-5 is very low. The activities of MZSM-5 catalysts are sharp increase and the main products are composed of aromatic compounds. It can be found that the MZSM-5P catalysts possess a very low activity. The MZSM-5L catalysts show good activities, high stabilities, and much more α-olefins selectivity in stage D.Test results demonstrate that the silica to alumina molar ratio of catalysts is low and the distribution of pore diameter is not uniform. These results could account for the reason why many aromatic compounds formed in above four stages. IR spectra of various kinds of catalysts present that active centra of catalysts are attributed to the sites of B and L acid located on the channels of zeolite.This study is a useful exploitation on the aspect of heterogeneous ethylene oligomerization for preparation of α-olefins. And it also provides some experiment data for us to further improve the reaction conditions.