摘要
Phillips铬系催化剂(CrOx/SiO2)是一种重要的工业聚乙烯催化剂,生产了全世界总量40-50%的高密度聚乙烯以及少量线性低密度聚乙烯。与该催化剂在工业应用上取得的巨大成功相比,Phillips催化剂的学术研究尤其是对催化剂的活性中心以及聚合机理等的认识还停留在初级阶段。本文通过实验与基于密度泛函理论的分子模拟相结合的方法,考察了活性中心的配位环境以及表面改性对乙烯聚合行为的影响。主要内容如下：(1)通过基于密度泛函理论的分子模拟,我们考察了Phillips催化剂诱导期内甲醛脱附对活性位形成与转化的作用机理。通过计算结果发现,CrⅡOx, surface和甲醛分子之间的配位稳定,这主要是由于CrⅡ的高度配位不饱和以及甲醛上氧原子的孤对电子。由于甲醛的位阻效应,吸附两个甲醛分子的催化剂模型上没有任何反应可以发生。对于吸附一个甲醛分子的催化剂模型,乙烯配位后的第一步反应遵循金属环状机理生成铬金属五元环,接下来的反应倾向于开环形成一个CrⅣ-H中间产物继而二聚生成1-丁烯,或者形成铬金属四元环中间产物继而易位生成丙烯/乙烯。而在相对较高的温度和压力下,两个吸附的甲醛分子很可能从CrⅡOx. surface脱附,从而形成没有甲醛吸附的活性中心(nHCHO=0),该模型上乙烯吸附后的第一步反应通过金属环状机理形成铬金属五元环,后续反应不倾向于乙烯二聚和易位反应,而是环增长形成金属七元环,进而一步法消去生成1-己烯。同时我们还发现引发过程中存在的五自旋多重态和三自旋多重态之间的自旋交叉会对反应的能垒产生一定的影响。(2)通过基于密度泛函理论的分子模拟,我们探讨了硅胶载体表面氟改性和钛改性对Phillips催化剂诱导期内聚合行为的影响。研究发现氟改性和钛改性都会增强Cr的缺电子性,使得CrⅡOx. surface和甲醛的配位更为稳定。由于甲醛的位阻效应,吸附两分子甲醛的催化剂模型上没有反应发生。当催化剂模型上吸附一分子甲醛时,只能发生二聚和易位反应,且氟改性和钛改性的影响都很小。当催化剂模型上吸附的甲醛分子完全脱附后,可以进一步环增长生成铬金属七元环,并且氟改性和钛改性对这一步反应都有促进作用；氟改性对铬金属七元环进一步开环生成1-己烯是不利的,而钛改性对这一步反应则是有利的；硅胶表面氟改性和钛改性对三价烷基铬聚合活性中心上的链增长插入反应都是有利的。(3)通过将挥发性的CrO2Cl2在无水无氧和室温条件下负载到500或800℃预处理的硅胶上,然后在300℃的温度下加热并用CO还原,我们制备了两种二价的Phillips非均相模型催化剂。通过乙烯聚合实验发现,在同样的聚合条件下负载在800℃预处理硅胶上的催化剂表现出更高的乙烯聚合活性；CO吸附在CrⅡOx.surface上的傅里叶红外谱图及其分峰拟合结果表明,两种催化剂体系中可能都存在两种相对比例不同的活性位；通过将量子化学/经典力学分层计算运用到Phillips催化剂的研究上，我们建立了Cr°负载在β-方石英(100)晶面上的Phillips催化剂模型，而且发现该计算模型上吸附的CO红外振动频率与模型催化剂上吸附的CO红外表征结果基本一致；通过对高度水氧敏感CrⅡ的高分辨率X光吸收光谱谱图的解析,我们得到了Phillips催化剂二价活性中心的结构信息,并且发现不同预处理温度下得到的CrⅡOx,surface会和硅胶表面邻近的硅氧烷配体存在不同的配位,从而形成不同的乙烯聚合活性中心,进而阐明了不同焙烧温度引起的活性中心配位环境与聚合活性的关联。以上一系列的研究结果合理地解释了实验现象,同时对Phillips催化体系的活性中心和聚合机理有了更深入的理解。
Phillips CrOx/SiO2catalyst, one of the most important industrial catalysts for ethylene polymerization, is responsible for40-50%of the world production of high density polyethylene, as well as linear low density polyethylene. Compared with its commercial success, academic research still lags far behind, especially for the understandings of the active sites and polymerization mechanisms. In this study, heterogeneous models for Phillips catalyst have been established to investigate the correlation between the coordination environment of the active sites and polymerization behavior via the combination of experiments and theoretical simulation by density functional theory.
First of all, theoretical calculations have been adopted to achieve a basic understanding of the effect of formaldehyde desorption on the active site formation and transformation during the induction period of the Phillips CrⅡOx/SiO2catalyst by density functional theory. The coordination of formaldehyde on CrⅡOx. surface is stable due to the high coordinative unsaturation of CrⅡ and lone-paired electrons of O in formaldehyde. No reaction can be initiated over the cluster model coordinated with two formaldehyde molecules owing to steric hindrance. The first reaction over cluster models, on which either one or no formaldehyde molecule is adsorbed, follows the metallacyclic mechanism into chromacyclopentane. Subsequent dimerization to1-butene and metathesis to propylene/ethylene are more favorable over the cluster model adsorbed with one formaldehyde molecule. Only after a complete desorption of formaldehyde does further ring expansion to chromacycloheptane followed by1-hexene formation become preferential. Spin crossing from quintet diethylene-Cr" complex to triplet chromacyclopentane with a spin acceleration effect is revealed.
Secondly, theoretical calculations have been adopted to achieve a basic understanding of the effect of surface fluorination or titanium-modification of silica support on the ethylene initiation during the induction period of the Phillips CrⅡOx/SiO2catalyst adsorbed with different amounts of formaldehyde molecules. The electron deficiency of CrⅡOx. surface is enhanced and the coordination between formaldehyde and CrⅡOx. surface is more stable after surface fluorination or titanium-modification. It is demonstrated that no reaction can be initiated over cluster models adsorbed with two formaldehyde molecules on account of steric hindrance. For cluster models adsorbed with one formaldehyde molecule, ethylene dimerization to1-butene and metathesis to ethylene/propylene via the chromacyclobutane intermediate takes place and fluorination/titanium-modification of the silica support shows minor influence on both reactions. After a complete desorption of formaldehyde molecules, further ring expansion to chromacycloheptane occurs and the surface fluorination/titanium-modification of the silica support shows improvement on this process. Fluorination of the silica support is unfavorable to the ring-opening of chromacycloheptane to give1-hexene, while titanium-modification shows a positive effect to this process. It is also demonstrated that fluorination/titanium-modification shows positive effect on the chain propagation over the models of CrⅢ-alkyl active sites during chain propagation.
Finally, compositionally and structurally uniform heterogeneous Phillips model catalysts have been prepared via the ambient temperature reaction of CrO2Cl2with silica pretreated at either500or800℃, followed by mild heating and CO reduction at300℃to obtain the corresponding CrⅡOx surface sites. CrⅡ grafted onto silica pretreated at800℃shows higher polymerization activity than that of500℃at the same reaction condition. The resulting structures have been investigated by IR spectroscopy of adsorbed CO combined with X-ray absorption spectroscopy and compared with model structures predicted by ONIOM calculations (B3PW91*/BS1:B3PW91*/STO-3G). The IR spectra suggest that there are two distinct and non-interconverting CrⅡOx. surface sites whose relative amounts differ on the two types of catalysts. The CO frequencies for the two model structures,(=SiO)2CrⅡ(CO)2and (=SiO)2(=Si2O)CrⅡ(CO), are calculated via the cluster models cut from the (100) crystal face of β-cristobalite. The calculated results agree well with experimental data. The X-ray absorption spectra reveal that the CO-free CrⅡOx. surface sites are coordinated by additional siloxane ligands, which might influence their propensity to initiate ethylene polymerization. Experimental observations have been rationalized well and deeper understandings on the active sites and polymerization mechanisms of Phillips catalyst have been achieved by this study.
引文
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