Formation of Highly Active Ziegler-Natta Catalysts Clarified by a Multifaceted Characterization Approach
Formation of Highly Active Ziegler-Natta Catalysts Clarified by a Multifaceted Characterization Approach
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DOI：
10.1021/acscatal.1c03067
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发表时间：
2021-10-29
期刊：
影响因子：
12.9
通讯作者：
Taniike, Toshiaki
中科院分区：
文献类型：
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作者：
Piovano, Alessandro;Wada, Toru;Taniike, Toshiaki
虽然纳米级和缺陷的b-MgCl2的形成对Ziegler-Natta催化剂的性能至关重要，但由于表征的某些限制，该过程尚未得到充分的阐明。在这里，基于以x射线全散射和各种光谱为代表的多方面表征技术，结合化学成分分析和聚合测试，详细研究了Ziegler-Natta催化剂的纳米结构和活性表面的形成。在催化剂制备过程中，从Mg(OEt)(2)中提取固体样品进行分析。研究发现了几个有趣的结果。TiCl4的加入几乎自发地将Mg(OEt)(2)转化为MgCl2种子，主要暴露在{001}基表面，其尺寸小于2 nm；大量Ti以物理吸附的四倍配位TiClx(OEt)（4-x）的形式留在材料上。加热处理去除物理吸附的TiClx(OEt)（4-x）和/或将其转化为化学吸附的6倍配位TiClx(OEt)(4-x)，而随后添加的内部供体（这里是邻苯二甲酸二丁酯，DBP）促进了MgCl2种子的大量重建和生长，几乎与最终催化剂（约6 nm）相同，并暴露了与催化更相关的侧表面。DBP一部分吸附在MgCl2表面，另一部分与Ti位络合。在接下来的合成步骤中，该配合物仅被部分去除。第二次添加的TiCl4用6倍配位的TiCl4取代了化学吸附的TiClx(OEt)(4-x)，但它也引起了与DBP的副反应，形成了邻苯二甲酰氯。经三乙基铝（TEAl）活化后，在初始TiCl4加入后的整个制备过程中，每Ti对乙烯的活性几乎保持不变，而在加入供体之前，每Ti对丙烯的活性可以忽略不计，并在随后的制备步骤中急剧增加。进一步研究了TEAl活化样品的光谱，以确定负责催化的活性Ti物种，并监测TEAl反应后DBP的命运。多方面的表征方法使我们能够整合delta-MgCl2的形成、表面和吸附物质的信息，为我们提供了对工业催化剂制备方法中每一步意义的深刻见解，这种方法已经经过了很长时间的经验提炼。
Although the formation of nanosized and defective b-MgCl2 is essential for the performance of Ziegler-Natta catalysts, the process has not sufficiently been elucidated due to certain limitations in characterization. Here, the formation of nanostructures and active surfaces of Ziegler-Natta catalysts was investigated in detail based on a multifaceted set of characterization techniques represented by X-ray total scattering and various spectroscopies in correlation with chemical composition analysis and polymerization tests. Solid samples were extracted in the course of catalyst preparation from Mg(OEt)(2) and subjected to the analysis. Several interesting results were found. The addition of TiCl4 almost spontaneously converts Mg(OEt)(2) into MgCl2 seeds mainly exposing the {001} basal surface, whose dimensions are below 2 nm; a large Ti amount remains on the material as physisorbed 4-fold-coordinated TiClx(OEt)(4-x), species. The heating treatment removes the physisorbed TiClx(OEt)(4-x) and/or convert them into chemisorbed 6-fold-coordinated TiClx(OEt)(4-x), while the subsequent addition of an internal donor (here dibutyl phthalate, DBP) promotes a substantial reconstruction and growth of MgCl2 seeds to almost the same size as the final catalyst (ca. 6 nm), with the exposure of the more catalytically relevant lateral surfaces. DBP is in one part adsorbed on MgCl2 surfaces and in the other part complexed with Ti sites. This complex is only partially removed in the following steps of the synthesis. The second TiCl4 addition replaces the chemisorbed TiClx(OEt)(4-x) with 6-fold-coordinated TiCl4 species, but it also causes side reactions with DBP, as testified by the formation of phthaloyl chloride. After activation by triethylaluminum (TEAl), the activity per Ti for ethylene was almost constant throughout the whole preparation process after the initial TiCl4 addition, whereas the activity for propylene was negligible before the addition of the donor and increased dramatically in the subsequent steps of the preparation. This was further investigated based on spectroscopies for TEAl-activated samples in order to individuate the active Ti species responsible for the catalysis and to monitor the fate of DBP upon TEAl reaction. The multifaceted characterization approach allowed us to integrate information on the formation of delta-MgCl2, their surfaces, and adsorbed species, providing us with deep insights into the meaning of each step within an industrial catalyst preparation method that has been empirically refined over a long history.