Studies On Synthesis Of A Novel SiO₂-supported Inorganic And Organic Hybrid Chromium-based Catalyst And Its Ethylene Polymerization Behavior

Nowadays high grade high density polyethylene (HDPE) pipe material is widely used for the transport of water or gas etc. at high pressure in the field of architecture with an increasing application in the world plastic market. It is manufactured from ethylene/α-olefin (Usually1-butune,1-hexene and1-octene) copolymers using mostly two-stage polymerization processes. However, up to now the technology of the two-stage processes and the catalysts thereof is monopolized by several developed countries. In order to substitute the current two-stage process with high cost and energy consumption, it is a mainstream trend to develop an one-stage process with low cost and energy consumption. However, it still remained as a great challenge in both academic and industrial fields to prepare a novel catalyst system for the one-stage process. In the industrial field, two important polyethylene catalysts namely inorganic Phillips (chromium oxide supported on the silica gel) and organic S-2(Bis(triphenysilyl) chromate supported on the silica gel) catalysts show their unique features on ethylene polymerization and their polyethylene products. In this work, a novel SiO2-supported inorganic and organic hybrid chromium-based catalyst is synthesized using the residual surface hydroxyl groups in Phillips catalyst to further support bis(triphenysilyl) chromate (BC), which is eventually successfully applied to manufacture high grade HDPE pipe material for the one-stage process. The novel hybrid catalyst gets the merits from each catalyst system, especially in the aspect of short chain branches distribution (SCBD) of its copolymer. By the characterization of the catalysts, the investigation of ethylene homopolymerization and ethylene/1-hexene copolymerization behavior and the microstructures of resultant polymers, several vital factors such as the addition amount of BC and1-hexene comonomer, cocatalyst, calcination temperature, total chromium loading, polymerization temperature, pressure and hydrogen etc, which could influence the hybrid catalyst system, were systematically investigated.During the catalyst preparation, the proper addition weight amount of BC was found to be CrBC≤0.25wt%. The suitable calcination temperature and total chromium loading were≤600℃and≤0.5wt%, respectively.As for ethylene homopolymerization, the proper dosage of alkyl aluminium cocatalyst such as triethylaluminium etc could enhance the activity of hybrid catalyst namely HCat-2. Kinetic results suggested that there might exist two kinds of Cr active sites in the hybrid catalyst system in the presence of cocatalyst:One was fast formation and fast decay type (Site A1and Site A2), and the other was slow formation and slow decay type (Site B1and Site B2), which might be formed through the reducing of surface chromate species by ethylene monomer and alkyl-Al cocatalyst, respectively. With raising the polymerization temperature from30℃to90℃, the activity of HCat-2catalyst and the average molecular weight (MW) of its polymer first increased and then decreased. Raising the ethylene pressure from0.14MPa to0.7MPa greatly enhanced the activity of HCat-2catalyst. But adding hydrogen decreased the activity of the catalyst and the average MW of its polymer.In the ethylene/1-hexene copolymerization, with increasing1-hexene from0vol%to7vol%, the activity of HCat-2catalyst and1-hexene incorporation of its copolymers showed a decreasing and an increasing tendency, respectively. The average MW of its copolymer first increased and then decreased. Kinetic results showed that there might still exist the above two kinds of Cr active sites in the hybrid catalyst system in the presence of cocatalyst, which might be formed through the reducing of surface chromate species by ethylene monomer,1-hexene comonomer and alkyl-Al cocatalyst. By comparing different chromium catalysts, HCat-2catalyst showed much higher activity than S-2catalyst. Its copolymer had higher average MW, boarder molecular weight distribution (MWD) than that obtained from Phillips catalyst, and showed slightly higher1-hexene incorporation than those obtained from Phillips and S-2catalysts.In the aspect of SCBD characterization of resultant copolymers, firstly, a quick, effective and qualitative method for characterizing the SCBD of HDPE samples namely temperature rising elution fractionation cross successive self-nucleation annealing (TREF+SSA) was developed in this work, which needs low cost and short time consumption. By using this method, the SCBD of copolymers obtained from Phillips, S-2, HCat-2and Cat-M catalysts was compared. The Cat-M catalyst was prepared by mechanically mixing the Phillips and S-2catalysts. The results showed that the copolymer obtained from HCat-2catalyst showed similar relative short chain branches (SCBs) content in its highest temperature TREF fraction as those obtained from Phillips and Cat-M catalyst. The SCBs content was slightly higher than that in the highest temperature TREF fraction of the copolymer obtained from S-2catalyst. In contrast, it had the least relative SCBs content in the lowest temperature TREF fraction of the four copolymers. This SCBD might be beneficial for the long term performance of high grade HDPE pipe material. The result might be explained by a synergetic effect between the above Cr active sites namely Site A1and Site A2in the copolymerization with the hybrid catalyst. Thereafter, the SCBD of two industrial test HDPE pipe material samples (PE-1with bad long term performance and PE-2with good long term performance, both made by the hybrid catalyst through industrial test) was compared by two different methods. One was TREF cross13C-NMR (TREF+13C-NMR) as a relatively direct, quantitative method with high cost and long time consumption, and the other was TREF cross step crystallization (TREF+SC) as a quick, qualitative method. The results showed that the relative SCBs content in the highest temperature TREF fraction (corresponding to the highest MW part) of PE-2sample was higher than that of PE-1sample. In contrast, the opposite situation occurred in their lowest temperature TREF fraction (corresponding to the lowest MW part). The results demonstrated the accuracy and effectiveness of the TREF+SSA for characterizing the SCBD of HDPE samples. It also suggested that the higher relative SCBs content in the highest temperature TREF fraction (corresponding to the highest MW part) of HDPE sample, and the lower relative SCBs content in the lowest temperature TREF fraction (corresponding to the lowest MW part) were beneficial for the long term performance of high grade HDPE pipe material. Therefore, the novel hybrid catalyst had well-balanced properties including activity, copolymerization ability and microstructures of its polymer especially SCBD.Based on our results including the development of a novel catalyst in the bench scale and the investigation of structure-property relationship of the high grade HDPE pipe material, Qilu Branch Co., SINOPEC further accomplished the commercial development of the novel hybrid catalyst for producing PE100pipe material with the one-stage UNIPOL process. Until the end of our joint project (2007BAE50B04)(Dec.31,2010), more than twenty thousand tons of PE100pipe materials using the one-stage process were successfully manufactured by Qilu Branch Co., SINOPEC. This technic creates huge economic and social benefit, and also owns the possibility for a wide application in other UNIPOL processes.