Crystal structures of Ziegler-Natta catalyst supports

The crystal structures of three MgCl(2)·nEtOH complexes with n=1.5, 2.8, and 3.3 have been fully determined. Such complexes are the fundamental precursors for Ziegler-Natta polymerization catalysts used to produce polyolefins on a multimillion-ton scale worldwide. The ab initio structure solution showed that the structure of MgCl(2)·nEtOH complexes with n=1.5 and 2.8 are based on ribbons of metal-centered octahedra, whereas for n=3.3 this chainlike arrangement breaks into a threadlike structure of isolated octahedra linked by hydrogen bonds. A clear correlation between catalyst performance and the crystal structure of precursors has been found, and reveals the fundamental role of the latter in determining catalyst properties. The direct knowledge of building blocks in the precursor structures will help to develop more accurate models for activated catalysts. These models will not require the arbitrary and oversimplified assumption of locating the catalyst active sites on selected cut surfaces of the α-MgCl(2) crystal lattice.     

从分子碎片到活性位：分子筛催化材料的原位、共振紫外拉曼光谱研究

在过去的近十年中,各种新型原位表征技术和反应器设计被应用于多相催化过程和催化材料的合成研究中,并获得了许多新认识．特别是最近几年,利用原位、共振拉曼光谱技术对分子筛合成关键物种检测、杂原子分子筛催化活性位的研究取得了一系列进展．这些技术的应用使得从分子水平认识复杂的多孔材料成为可能：从合成初期碎片基元检测、碎片相互连接的关键化学键到预组装类微孔结构;从高度隔离过渡金属中心到配位化学键断裂生成活性中间物种,再到完成催化反应循环．这为设计特定功能和结构的催化材料及高选择性的活性中心奠定了坚实的基础．     

给电子体在丙烯聚合MgCl2载体催化剂体系中的作用

制备了3种含有不同内给电子体(邻苯二甲酸二异丁酯,9,9-二甲氧基甲基-芴和1,1-双甲氧基甲基-环丁烷)的MgCl2负载型丙烯聚合齐格勒-纳塔(Z-N)催化剂,研究了给电子体结构与聚合性能之间的关系,用红外光谱剖析了催化剂及其相关化合物的结构,结果发现催化剂中的内给电子体直接与MgCl2配位,而没有与TiCl4结合.内给电子体的加入,降低了Z-N催化剂中钛的含量,提高催化丙烯聚合的活性,使聚合物的分子量分布变窄.聚合物立构规整度的变化强烈依赖于内给电子体的结构.     

Base‐Free Non‐Noble‐Metal‐Catalyzed Hydrogen Generation from Formic Acid: Scope and Mechanistic Insights

The iron‐catalyzed dehydrogenation of formic acid has been studied both experimentally and mechanistically. The most active catalysts were generated in situ from cationic Fe^II/Fe^III precursors and tris[2‐(diphenylphosphino)ethyl]phosphine ( 1 , PP₃). In contrast to most known noble‐metal catalysts used for this transformation, no additional base was necessary. The activity of the iron catalyst depended highly on the solvent used, the presence of halide ions, the water content, and the ligand‐to‐metal ratio. The optimal catalytic performance was achieved by using [FeH(PP₃)]BF₄/PP₃ in propylene carbonate in the presence of traces of water. With the exception of fluoride, the presence of halide ions in solution inhibited the catalytic activity. IR, Raman, UV/Vis, and EXAFS/XANES analyses gave detailed insights into the mechanism of hydrogen generation from formic acid at low temperature, supported by DFT calculations. In situ transmission FTIR measurements revealed the formation of an active iron formate species by the band observed at 1543 cm^?1, which could be correlated with the evolution of gas. This active species was deactivated in the presence of chloride ions due to the formation of a chloro species (UV/Vis, Raman, IR, and XAS). In addition, XAS measurements demonstrated the importance of the solvent for the coordination of the PP₃ ligand.     

Precursor effect on the property and catalytic behavior of Fe-TS-1 in butadiene epoxidation

The effect of iron precursor on the property and catalytic behavior of iron modified titanium silicalite molecular sieve (Fe-TS-1) catalysts in butadiene selective epoxidation has been studied. Three Fe-TS-1 catalysts were prepared, using iron nitrate, iron chloride and iron sulfate as precursors, which played an important role in adjusting the textural properties and chemical states of TS-1. Of the prepared Fe-TS-1 catalysts, those modified by iron nitrate (FN-TS-1) exhibited a significant enhanced performance in butadiene selective epoxidation compared to those derived from iron sulfate (FS-TS-1) or iron chloride (FC-TS-1) precursors. To obtain a deep understanding of their structure-performance relationship, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Temperature programmed desorption of NH₃ (NH₃-TPD), Diffuse reflectance UV–Vis spectra (DR UV–Vis), Fourier transformed infrared spectra (FT-IR) and thermal gravimetric analysis (TGA) were conducted to characterize Fe-TS-1 catalysts. Experimental results indicated that textural structures and acid sites of modified catalysts as well as the type of Fe species influenced by the precursors were all responsible for the activity and product distribution.     

Correlations between acidity, surface structure, and catalytic activity of niobium oxide supported on zirconia

The development of the acidity and the relationship between acidity, catalytic activity, and the surface structure for niobium oxide supported on zirconia were investigated for a series of solids. The catalysts were active for 2-propanol dehydration only above a threshold in Nb loading. The acidity was studied by infrared spectroscopy of adsorbed 2,6-dimethylpyridine as a probe molecule, and the onset of activity was correlated with that of the formation of relatively strong Br?nsted acid sites. The variation in the abundance of these sites also correlated with the catalytic activity. Raman, IR, and UV spectroscopy results indicated that the active sites were related to polymeric Nb surface species. These results were compared to those previously reported for the WO(x)/ZrO(2) catalysts.     

Stoichiometric compounds of magnesium dichloride with ethanol for the supported Ziegler-Natta catalysis: first recognition and multidimensional MAS NMR study

Ethanol associates easily with MgCl(2) to form adducts of complex architecture, but until now available characterization methods have failed to identify the pure stoichiometric compounds and their structures. To remedy this, we set about applying homonuclear and heteronuclear 2D correlated solid-state NMR spectroscopy to identify the pure compounds and the ethanol-to-magnesium coordination pattern. High spinning speed and Lee-Goldburg sequences were able to reduce the hydrogen spin-diffusion and homonuclear coupling in the crystalline solid, thus achieving high resolution also in the hydrogen domain. On this basis, the pure adducts, of interest as catalyst supports for Ziegler-Natta polymerization, were isolated for the first time. Magnesium coordination sites with given numbers of ligands and their multiplicity in the crystal cells were determined in the new-found stoichiometric complexes. Variable temperature and 2D carbon-carbon exchange NMR, as well as relaxation times in the fast motion regime, revealed the disordering phenomena generated by ethanol dynamics in the crystal. Decoding the intriguing polymorphism of the precursors permits to trace the genealogy of tailored MgCl(2) titanate granules, active as highly productive catalysts for the stereospecific polymerization of olefins.     

Hydrogen evolution across nano-Schottky junctions at carbon supported MoS2 catalysts in biphasic liquid systems

The activities of a series of MoS(2)-based hydrogen evolution catalysts were studied by biphasic reactions monitored by UV/Vis spectroscopy. Carbon supported MoS(2) catalysts performed best due to an abundance of catalytic edge sites and strong electronic coupling of catalyst to support.     

STUDY ON 1-HEXENE POLYMERIZATION BASED ON ZIEGLER-NATTA CATALYSTS WITH DOPED SUPPORT

A series of Ti/Mg supported catalysts are prepared by using ball-milled mixtures of MgCl2-ethanol adducts and NaCl as supports, and 1-hexene polymerizations catalyzed by the novel catalysts are studied. It is found that the molecular weight distribution of poly(1-hexene) becomes apparently narrower when catalysts with doped supports are used, indicating that changing the structure of the support is an effective way to regulate the active center distribution of heterogeneous Ziegler-Natta catalyst.     

Cr/Ni-catalyzed vinylation of aldehydes: a mechanistic study on the catalytic roles of nickel and chromium

The roles of nickel and chromium catalysts in the coupling reaction of vinyl halides and aldehydes, the so-called Nozaki-Hiyama-Kishi (NHK) reaction, have been studied by UV/Vis spectroscopy, electrochemical, and spectroelectrochemical methods. Electrochemical studies revealed that nickel plays the central role in activating the vinyl halide by reductive cleavage, to form a rapidly decomposing vinyl-Ni species. The latter can, however, be stabilized in the presence of the Cr complex. The redox behavior of the Ni complexes in the presence of vinyl halide demonstrated that the vinyl halide activation results from interaction with a one-electron reduced nickel species [formally Ni(I) ], not necessarily with a Ni(0) species. It was furthermore shown by UV/Vis spectroscopy and spectroelectrochemical methods that low-valent nickel [Ni(0) ] results from the interaction of the Ni(II) catalyst with CrCl(2) .     

Ti‐Based Catalysts and Photocatalysts: Characterization and Modeling

This perspective article aims to underline how cutting‐edge synchrotron radiation spectroscopies such as extended X‐ray absorption spectroscopy (EXAFS), X‐ray absorption near edge structure (XANES), high resolution fluorescence detected (HRFD) XANES, X‐ray emission spectroscopy (XES) and resonant inelastic X‐ray scattering (RIXS) have played a key role in the structural and electronic characterization of Ti‐based catalysts and photocatalysts, representing an important additional value to the outcomes of conventional laboratory spectroscopies (UV‐Vis, IR, Raman, EPR, NMR etc.). Selected examples are taken from the authors research activity in the last two decades, covering both band‐gap and shape engineered TiO₂ materials and microporous titanosilicates (ETS‐10, TS‐1 and Ti?AlPO‐5). The relevance of the state of the art simulation techniques as a support for experiments interpretation is underlined for all the reported examples.     

用于丙烯环氧化的 Ti/HMS 催化剂的失活与再生

 研究了以异丙苯过氧化氢为氧化剂选择氧化丙烯制环氧丙烷的 Ti/HMS 催化剂的失活原因. 采用 XRD (X 射线衍射), FT-IR (傅里叶变换红外光谱), UV-Vis (紫外-可见光谱) 和 TG (热重分析) 等技术表征了催化剂失活前后的结构和化学组成. 结果表明, 催化剂的失活主要是由于反应过程中所产生的重组分在催化剂表面吸附, 覆盖了活性中心所致; 并不是由于四配位的活性钛物种转化为六配位、八配位或者锐钛矿 TiO2 所致. 采用热异丙苯溶剂洗涤的方法可以部分恢复失活催化剂的活性, 但不能恢复到新鲜催化剂的水平; 采用低温烧炭方法能够使催化剂的活性恢复到新鲜催化剂的水平. 因此, Ti/HMS 催化剂的失活是可逆的, 采用合适的再生方法可以恢复其催化活性.     

Acidity, surface structure, and catalytic performance of WO(x) supported on monoclinic zirconia

The relationship between the acidity, catalytic activity, and surface structure for tungsten oxide supported on zirconia was investigated for a series of solids prepared by equilibrium adsorption on monoclinic zirconia. The catalysts were active for propanol dehydration only above a threshold in W loading. The acidity was studied by infrared spectroscopy of adsorbed probe molecules (2,6-dimethylpyridine and CO), and the onset of activity was correlated with that of the formation of relatively strong Br?nsted acid sites. The variation in the abundance of these sites correlated with the catalytic activity. Lewis sites were present but could not be directly associated with the activity. Raman, IR, and UV spectroscopy results indicated that the active sites were related to polymeric W surface species.     

Investigation of LiAlH4-THF formation by direct hydrogenation of catalyzed Al and LiH

The formation of LiAlH(4)-THF by direct hydrogenation of Al and LiH in tetrahydrofuran (THF) was investigated using spectroscopic and computational methods. The molecular structures and free energies of the various possible adducts (THF-AlH(3), THF-LiH and THF-LiAlH(4)) present in a LiAlH(4)/THF solution were calculated and the dominant species were determined to be contact ion pairs where three THF molecules coordinate the lithium. Raman and X-ray absorption spectroscopy were used to investigate the effect of different Ti precursors on the formation of Al-H species and LiAlH(4)-THF and determine the optimal reaction conditions. A unique sample stage was developed from a microfluidic cell to evaluate the catalysts in situ. The effectiveness of two types of catalysts, titanium chloride (TiCl(3)) and titanium butoxide (Ti(C(4)H(9)O)(4)), and the catalyst concentration were evaluated under similar reaction conditions. Both catalysts were effective at facilitating hydrogenation, although TiCl(3) was more effective over the first few cycles with the greatest kinetic enhancement achieved with a low concentration of around 0.15 mol%. These results were qualitatively supported by infrared spectroscopy, which indicated that although a small amount of Ti is necessary for disassociating H(2), excess surface Ti (>0.1 ML) hinders the formation of Al-H species.     

Studies on Ziegler-Natta catalysts. Part IV. Chemical nature of the active site

The determination of the number of sites active in the polymerization of ethylene on the surface of α-TiCl₃–Al(CH₃)₃ dry catalysts leads to the conclusion that this number is small in comparison to the total surface of the catalyst. Qualitatively this conclusion is also reached by two other independent methods. Infrared spectra of the catalyst before and after polymerization do not show a change in the type of bonds present in the surface. Electron microscopy proves that no active sites are formed on the basal plane of the α-TiCl₃ which constitutes 95% of the total surface. The results strongly favor the lateral faces of α-TiCl₃ as the preferred location of active centers. The lateral faces contain chlorine vacancies and incompletely coordinated titanium atoms. This must then be the essential conditions for the formation of active centers. The propagation of the polymer chain has been repeatedly shown to follow an insertion mechanism. The active site, therefore, necessarily contains a metal–carbon bond. The study of catalysts derived from TiCl₃CH₃ leads to the conclusion that a Ti? C bond on titanium of incomplete coordination is the active species in these cases. The alkylation of surface titanium atoms was proven to be an intermediate step in the catalyst formation from TiCl₃ and AlR₃. Survival of titanium–alkyl bonds on the lateral faces, where titanium atoms are incompletely coordinated explains best, in the light of our data, the activity of Ziegler-Natta catalysts. Coordination of aluminum alkyl compounds in or around the active center probably complicates the structure of the active centers.     

含Fe、Co、Ni的LDHs为催化剂前体生长碳纳米管

分别以具有相似Fe、Co、Ni含量的层状双金属氢氧化物（LDHs）为催化剂前体,用化学气相沉积的方法生长碳纳米管（CNTs）。 催化剂由LDHs焙烧还原得到。 通过X射线衍射（XRD）、扫描电子显微镜（SEM）、透射电子显微镜（TEM）及拉曼光谱（Raman）测试技术对LDHs及其焙烧产物的结构、CNTs的形貌和结构进行了研究。 结果表明,3种催化剂生长的CNTs均为多壁结构;其中Co催化剂活性较低,生长CNTs的管径较细、石墨化程度较高;Ni催化剂的活性较高,生长CNTs的密度较大、管壁较厚、石墨化程度较差;Fe催化剂的活性介于Co和Ni之间。 催化剂活性及CNTs的密度可以由生长CNTs的结构来解释。     

Monitoring surface metal oxide catalytic active sites with Raman spectroscopy

The molecular aspect of the Raman vibrational selection rules allows for the molecular structural and reactivity determinations of metal oxide catalytic active sites in all types of oxide catalyst systems (supported metal oxides, zeolites, layered hydroxides, polyoxometalates (POMs), bulk pure metal oxides, bulk mixed oxides and mixed oxide solid solutions). The molecular structural and reactivity determinations of metal oxide catalytic active sites are greatly facilitated by the use of isotopically labeled molecules. The ability of Raman spectroscopy to (1) operate in all phases (liquid, solid, gas and their mixtures), (2) operate over a very wide temperature (-273 to >1000 °C) and pressure (UHV to ?100 atm) range, and (3) provide molecular level information about metal oxides makes Raman spectroscopy the most informative characterization technique for understanding the molecular structure and surface chemistry of the catalytic active sites present in metal oxide heterogeneous catalysts. The recent use of hyphenated Raman spectroscopy instrumentation (e.g., Raman-IR, Raman-UV-vis, Raman-EPR) and the operando Raman spectroscopy methodology (e.g., Raman-MS and Raman-GC) is allowing for the establishment of direct structure-activity/selectivity relationships that will have a significant impact on catalysis science in this decade. Consequently, this critical review will show the growth in the use of Raman spectroscopy in heterogeneous catalysis research, for metal oxides as well as metals, is poised to continue to exponentially grow in the coming years (173 references).     

含Fe、Co、Ni的层状双金属氢氧化物为催化剂前体生长碳纳米管

分别以具有相似Fe、Co、Ni含量的层状双金属氢氧化物(LDHs)为催化剂前体,用化学气相沉积的方法生长碳纳米管(CNTs).催化剂由LDHs焙烧还原得到.通过X射线衍射(XRD)、扫描电子显微镜(SEM)、透射电子显微镜(TEM)及拉曼光谱(Raman)测试技术对LDHs及其焙烧产物的结构、CNTs的形貌和结构进行了研究.结果表明,3种催化剂生长的CNTs均为多壁结构;其中Co催化剂活性较低,生长CNTs的管径较细、石墨化程度较高;Ni催化剂的活性较高,生长CNTs的密度较大、管壁较厚、石墨化程度较差;Fe催化剂的活性介于Co和Ni之间.催化剂活性及CNTs的密度可以由生长CNTs的结构来解释.     

Stereospecific Cyclopolymerization with Group 4 Metallocenes

Homogeneous Ziegler-Natta catalysts are stereoselective cyclopolymerization catalysts for non-conjugated dienes. Cyclopolymerization of 1,5-hexadiene affords poly(methylene-l,3-cyclopentane) (PMCP), a polymer for which four structures of maximum order are possible. A variety of metallocene catalyst precursors have been investigated; the molecular weight and microstructure of the polymers are sensitive to the structure of the catalyst precursor as well as the reaction conditions. The selectivity for cyclization depends on reaction conditions; decreasing the olefin concentration and increasing the temperature of the reaction favor cyclization. The stereochemistry of cyclopolymers can also be controlled with appropriate choice of catalyst precursor. Diastereoselective cyclopolymerization of 1,5-hexadiene with achiral catalysts yields atactic trans-PMCP and cis-PMCP, depending on the catalyst precursor. Enantioselective cyclopolymerization with optically active catalysts yields optically active poly(methylenecyclopentane), a novel example of a polymer which is chiral by virtue of its main-chain stereochemistry.