载体受限结晶调控负载型Z-N催化剂的活性中心分布及聚烯烃链结构

Supported Z-N catalysts are widely applied in industrial production of polyolefins. They have advantages of high activity, stability, adaption for continuous processes and low cost. However, their multisite nature leads to broad and uncontrolled chain structure distribution of the polyolefin product. Effective regulation of active center distribution of the catalysts has become the key point for optimizing the structure of polyolefin resins. The precursors of active centers are titanium chlorides adsorbed on various crystal cuts of the MgCl2 surface, and heterogeneity of the support surface determines the active center distribution. In this project, the formation of MgCl2 crystals and adsorption of TiCl4 are to be confined in small space, in order to reduce and regulate the heterogeneity of the supported catalyst. It is expected that such catalysts can produce polyolefin with narrow distribution of chain structure and better performances. The MgCl2 support with confined crystals will be prepared by adsorption of MgCl2 solution into the channels of mesoporous silica and reaction of the confined MgCl2 crystallites with TiCl4. The confinement effect of the ordered nanoscale channels may reduce the surface heterogeneity of the formed catalyst.

负载型Z-N催化剂广泛用于聚烯烃树脂的工业生产，其主要优点是高活性、稳定性、适应连续生产工艺和低成本，但其多活性中心的特点也造成聚烯烃的链结构分布宽且不合理，树脂性能不够理想。如何有效地调控这类催化剂的活性中心分布以实现聚烯烃结构的优化是克服其缺点的关键。已知这类催化剂的活性中心前体是负载于氯化镁载体不同晶面的氯化钛物种，载体表面结构的多样性决定了活性中心的多分散性。本课题拟通过使氯化镁在规整的纳米尺寸空间内结晶和载负氯化钛，达到调控和减少载体表面结构多分散性的效果，使所制备的催化剂含有较少种类的活性中心前体，用其合成得到链结构分布较窄的聚乙烯、聚丙烯及烯烃共聚物，实现聚烯烃材料的高性能化。拟采取的制备受限结晶氯化镁载体的路线为：将氯化镁溶液吸附到介孔分子筛的纳米孔道内并与四氯化钛反应，利用分子筛规整孔道的限制作用调节反应中原位生成的氯化镁微晶体的表面结构，实现对活性种结构的调控。

本项目的研究目标是：通过使负载型齐格勒-纳塔催化剂的氯化镁载体在规整的纳米尺寸空间内结晶和载负氯化钛，使所制备的催化剂含有较少种类的活性中心前体，以合成得到链结构分布较窄的聚乙烯、聚丙烯及烯烃共聚物，克服现有负载型钛系催化剂合成的聚烯烃组成分布过宽的缺点。对以介孔分子筛为惰性载体的TiCl4/MgCl2/MCM-41负载型齐格勒－纳塔催化剂的制备方法、催化剂结构及其催化乙烯-alpha-烯烃共聚反应的规律进行了系统的研究，初步证实了介孔分子筛的引入对活性载体氯化镁的微观结构产生了显著的影响，使催化剂的活性中心分布及主要活性中心种类的催化特性产生了显著的变化。与常规方法制备的TiCl4/MgCl2负载型齐格勒－纳塔催化剂相比，含介孔分子筛的双负载催化剂表现出聚合活性高、乙烯共聚物分子量高及组成分布较窄的特点。用这类新型催化剂预计能合成出溶剂可萃取物含量低、力学性能优异的线型低密度聚乙烯树脂，实现通用聚烯烃材料的高性能化。