Adsorption of TiCl4 and electron donor on defective MgCl2 surfaces and propylene polymerization over Ziegler-Natta catalyst: A DFT study

The formations of defective MgCl₂ surfaces, and subsequent adsorption of Ti species and electron donor, as well as propylene polymerization over the Ziegler-Natta catalyst have been investigated using density functional theory (DFT) method. Twelve possible support models of regular and defective MgCl₂ (110) and (100) surfaces were built. The individual adsorptions of titanium chlorides as mononuclear or dinuclear, and ethyl benzoate (EB) as electron donor, on these models were evaluated. The analysis of energies presented the cases of EB adsorption were generally more stable than titanium chlorides on both surfaces. Thus, EB as internal electron donor mainly prevented TiCl₄ from coordinating on the MgCl₂ surfaces where mostly non-stereospecific active sites could be formed. Exceptionally, A5 the site model with terminal Cl-vacancy on the MgCl₂ support, presented stronger adsorption of TiCl₄ than that of EB on (110) surface. Since the TiCl₄ and ethyl benzoate (EB) would compete to adsorb on the support surface, it seems reasonable to assume that TiCl₄ might predominately occupy this site, which can act as the most plausible active site for propylene polymerization. The first insertion of propylene monomer into the A5 active site model showed that it exhibited good regioselectivity but poor stereospecificity in the absence of electron donor.     

Titanium Defective Sites in TS‐1: Structural Insights by Combining Spectroscopy and Simulation

Ti silicates, and in particular, titanium silicalite‐1 (TS‐1), are nowadays important catalysts for several partial oxidation reactions in the presence of aqueous H₂O₂ as an oxidant. Despite the numerous studies dealing with this material, some fundamental aspects are still unclear. In particular, the structure and the catalytic role of defective Ti sites, other than perfect tetrahedral sites recognized as the main active species, has not been quantitatively discussed in the literature. We assess the structural features of defective Ti sites on the basis of outcomes of electronic spectroscopies, as interpreted through quantum mechanical simulation. Strong evidence is disclosed to support the fact that the most common defective Ti sites, often reported in the TS‐1 literature, are monomeric Ti centers that are embedded in the zeolite framework, and which have a distorted octahedral local symmetry.     

Probing the Coordinative Unsaturation and Local Environment of Ti3+ Sites in an Activated High‐Yield Ziegler–Natta Catalyst

The typical activation of a fourth generation Ziegler–Natta catalyst TiCl₄/MgCl₂/phthalate with triethyl aluminum generates Ti³⁺ centers that are investigated by multi‐frequency continuous wave and pulse EPR methods. Two families of isolated, molecule‐like Ti³⁺ species have been identified. A comparison of the experimentally derived g tensors and ^35,37Cl hyperfine and nuclear‐quadrupole tensors with DFT‐computed values suggests that the dominant EPR‐active Ti³⁺ species is located on MgCl₂(110) surfaces (or equivalent MgCl₂ terminations with tetra‐coordinated Mg). O₂ reactivity tests show that a fraction of these Ti sites is chemically accessible, an important result in view of the search for the true catalyst active site in olefin polymerization.     

Ethylenebis(5‐chlorosalicylideneiminato)vanadium dichloride immobilized on MgCl2‐based supports as a highly effective precursor for ethylene polymerization

Ethylenebis(5‐chlorosalicylideneiminato)vanadium dichloride supported on MgCl₂(THF)₂ or on the same carrier modified by Et_nAlCl_3?n, where n = 1–3, was used in ethylene polymerization in the presence of MAO or a common alkylaluminium compounds as a cocatalyst. The support type alter vanadium loading and also change the characteristic of the catalytic active sites. Et₂AlCl is the best activator for a catalyst which has been immobilized on a nonmodified support, whereas the systems which contain a carrier which has been modified by an organoaluminium compound reveal the highest activity in conjunction with MAO. That difference, together with different temperature effects on polymerization efficiency (i.e., decrease and increase of catalytic activity for increasing temperatures, respectively) suggest the formation of different types of active sites in the catalytic systems supported on modified and nonmodified magnesium carrier. However, all supported precatalysts possess a long lifetime, still being active towards ethylene polymerization after 2 h. All the systems yield wide MWD polyethylene, while bimodal MWD is found for some part of analyzed samples. Polyethylene with bimodal particle size distribution is formed with the system which contain modified carriers at higher temperatures. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 3480–3489, 2009     

Enhancement of stereospecificity in isoprene polymerization with alkylaluminum–titanium chloride catalysts through use of carbon disulfide

The effect of CS₂ on isoprene polymerization with triisobutylaluminum-titanium tetrachloride catalysts was studied at Al/Ti ratios of optimum (0.9) and higher values. In the absence of CS₂, appreciable amounts of low molecular weight oils (“extractables”) were formed at the expense of cis-1,4-polyisoprene with higher than optimum Al/Ti ratios. Small amounts of CS₂ were found to prevent extractables formation and allow attainment of higher yields of cis-1,4-polyisoprene. The optimum CS₂/Ti chloride molar ratio (0.1) was independent of the Al/Ti ratio of the catalyst. Polymer microstructure and dilute solution viscosity were unaffected by CS₂. The results support the theory that the catalyst surfaces hold two types of active sites: p-sites, which initiate polymerization, and o-sites, which lead to oligomerization. CS₂ appears to enhance polymerization by coordinating selectively at the o-sites. The predominance of oligomerization at the higher Al/Ti ratios was attributed to a destruction of p-sites by excess trialkyl-aluminum.     

Kinetics and Mechanism Comparison between Cr/Ti‐Based Bimetallic and Ti‐Based Monometallic Catalysts for Ethylene Polymerization

Two (SiO₂/MgR₂/MgCl₂)·TiCl_x model catalysts are made by refluxing TiCl₄ with 0.35 wt% Cr modified silica gel/alkyl Mg adducts or silica gel/alkyl Mg adducts, which are named as Cr/Ti‐based bimetallic Cat‐1 and Ti‐based monometallic Cat‐2, respectively. The kinetics, active center counting, morphology, and polymer characterizations are studied to disclose the effect of low loading Cr active sites on the Cr/Ti‐based bimetallic Cat‐1 polymerization under mild conditions. The activity of Cat‐1 is 120.4% higher than that of Cat‐2, with a 114.1% higher [C*]/[M] value. Morphology results show the Cat‐1 fragmentation in the first 3 min is highly accelerated, which helps to release buried clustered Ti sites. Differential scanning calorimetry results show that low‐temperature heat absorbing shoulder of polyethylene (PE) from Cat‐2 demonstrates the signal of low crystallinity polymer made by Cat‐2 during the first 60 s, verifying the fluffy polymer in morphology results. GPC results show PE from Cat‐1 has a higher M_w in the first 3 min while a lower M_w in the end. The Cat‐1, which release active sites faster, has a high M_w in the early time. Lower M_w in the 900 s attributes to the effect of relative lower M_w polymer made by Cr sites, compared with Cat‐2.     

Density Functional Theory Study of Hydrogen Adsorption in a Ti‐Decorated Mg‐Based Metal–Organic Framework‐74

The Ti‐binding energy and hydrogen adsorption energy of a Ti‐decorated Mg‐based metal–organic framework‐74 (Mg‐MOF‐74) were evaluated by using first‐principles calculations. Our results revealed that only three Ti adsorption sites were found to be stable. The adsorption site near the metal oxide unit is the most stable. To investigate the hydrogen‐adsorption properties of Ti‐functionalized Mg‐MOF‐74, the hydrogen‐binding energy was determined. For the most stable Ti adsorption site, we found that the hydrogen adsorption energy ranged from 0.26 to 0.48 eV H₂^?1. This is within the desirable range for practical hydrogen‐storage applications. Moreover, the hydrogen capacity was determined by using ab initio molecular dynamics simulations. Our results revealed that the hydrogen uptake by Ti‐decorated Mg‐MOF‐74 at temperatures of 77, 150, and 298 K and ambient pressure were 1.81, 1.74, and 1.29 H₂ wt %, respectively.     

Polymer‐supported half‐titanocene catalysts for the syndiospecific polymerization of styrene

Monocyclopendienyltitanium trichloride (CpTiCl₃) was supported on polymer carriers with different hydroxyl contents, and the supported catalysts were used for styrene polymerization. The supported catalysts exhibited high activity even at low Al/Ti ratios and increased the molecular weight of the products, indicating that polymer carriers could stabilize the active sites. The polymers prepared with unsupported and supported catalysts were extracted with boiling n‐butanone and characterized by carbon nuclear magnetic resonance (¹³C NMR) and differential scanning calorimetry. The polymers obtained by supported catalysts had a high fraction of boiling n‐butanone‐insoluble part and high melting temperatures, but ¹³C NMR results showed that syndiotacticity decreased compared with that of polymers prepared with an unsupported catalyst. ESR study on the supported catalysts confirmed that the active sites supported on the carrier dropped into the solution and formed active sites the same as those in the unsupported system when they reacted with methylaluminoxane. ¹³C NMR analysis showed that the polymerization mechanism of the supported active sites was an active‐site controlled mechanism instead of a chain‐end controlled mechanism of the unsupported active sites. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 127–135, 2000     

Ag10Ti28‐Oxo Cluster Containing Single‐Atom Silver Sites: Atomic Structure and Synergistic Electronic Properties

Supported single‐atom catalysts have been emerging as promising materials in a variety of energy catalysis applications. However, studying the role of metal–support interactions at the molecular level remains a major challenge, primarily due to the lack of precise atomic structures. In this work, by replacing the frequently used TiO₂ support with its molecular analogue, titanium‐oxo cluster (TOC), we successfully produced a new kind of Ti‐O material doped with single silver sites. The as‐obtained Ag₁₀Ti₂₈ cluster, containing four exposed and six embedded Ag sites, is the largest noble‐metal‐doped Ti‐O cluster reported to date. Density functional theory (DFT) calculations show that the Ag₁₀Ti₂₈ core exhibits properties distinct from those of metallic Ag‐based materials. This Ti‐O material doped with single Ag sites presents a high ?_d and moderate CO binding capacity comparable to that of metallic Cu‐based catalysts, suggesting that it might display different catalytic performance from the common Ag‐based catalysts, for example, for CO₂ reduction. These results prove that the synergism of active surface metal atoms and the Ti‐O cluster support result in unique physical properties, which might open a new direction for single‐atom‐included catalysts.     

The syndiospecific polymerization of styrene in the presence of fluorine‐containing half‐sandwich metallocenes

The syndiospecific polymerization of styrene was investigated with the fluorine‐containing half‐sandwich complexes η⁵‐pentamethylcyclopentadienyl titanium bis(trifluoroacetate) dimer, η⁵‐octahydrofluorenyl titanium tristrifluoro‐acetate, η⁵‐octahydrofluorenyl titanium dimethoxymonotrifluoroacetate, and η⁵‐octahydrofluorenyl titanium tris(pentafluorobenzoate) in comparison to known chloride and methoxide complexes in the presence of relatively low amounts of methylalumoxane and triisobutylaluminum. After the selection of effective reaction conditions for a solvent‐free polymerization, the following orders of decreasing polymerization activity of the titanium complexes can be observed: for pentamethylcyclopentadienyl compounds, Cp*Ti(OMe)₃ > [Cp*Ti(OCOCF₃)₂]₂O ≈ Cp*TiCl₃, and for octahydrofluorenyl compounds, [656]Ti(OMe)₃ > [656]Ti(OCOC₆F₅)₃ > [656]Ti(OCH₃)₂(OCOCF₃) > [656]Ti (OCOCF₃)₃. The [656]Ti complexes, showing the highest polymerization conversions at 70 °C and in comparison with the Cp* Ti compounds, turned out to be highly efficient catalysts for the syndiospecific styrene polymerization. The fluorine‐containing Cp* and [656]Ti complexes lead to much higher molecular weights than the chloride and methoxide compounds because of a reduction in chain‐limiting transfer reactions. The introduction of only one fluorine‐containing ligand into the coordination sphere of the metal compound is obviously sufficient for a significant increase in molecular weight. The active polymerization sites of the [656]Ti complexes with methylalumoxane and triisobutylaluminum are extremely stable during storage at room temperature in regard to their polymerization activity. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2428–2439, 2000     

Mimicking Ziegler‐Natta Catalysts in Homogeneous Phase, 1. C2‐Symmetric Octahedral Zr(IV) Complexes with Tetradentate [ONNO]‐Type Ligands

Results of propene polymerization in the presence of two known octahedral C₂‐symmetric Zr complexes bearing tetradentate [ONNO]‐type ligands are reported for the first time. Depending on the steric hindrance at the active metal, isotactic site‐controlled or weakly syndiotactic chain‐end‐controlled polymers were obtained, in both cases via highly regioselective 1,2 (primary) monomer insertion. In this respect, the complexes mimic the behavior of the active Ti species on the surface of the heterogeneous Ziegler‐Natta catalysts of which they might represent good structural models.     

Promoting Subordinate,Efficient Ruthenium Sites with Interstitial Silicon for Pt‐Like Electrocatalytic Activity

The fundamental understanding and rational manipulation of catalytic site preference at extended solid surfaces is crucial in the search for advanced catalysts. Herein we find that the Ru top sites at metallic ruthenium surface have efficient Pt‐like activity for the hydrogen evolution reaction (HER), but they are subordinate to their adjacent, less active Ru₃‐hollow sites due to the stronger hydrogen‐binding ability of the latter. We also present an interstitial incorporation strategy for the promotion of the Ru top sites from subordinate to dominant character, while maintaining Pt‐like catalytic activity. Our combined theoretical and experimental studies further identify intermetallic RuSi as a highly active, non‐Pt material for catalyzing the HER, because of its suitable electronic structure governed by a good balance of ligand and strain effects.     

Promoting Subordinate,Efficient Ruthenium Sites with Interstitial Silicon for Pt‐Like Electrocatalytic Activity

The fundamental understanding and rational manipulation of catalytic site preference at extended solid surfaces is crucial in the search for advanced catalysts. Herein we find that the Ru top sites at metallic ruthenium surface have efficient Pt‐like activity for the hydrogen evolution reaction (HER), but they are subordinate to their adjacent, less active Ru₃‐hollow sites due to the stronger hydrogen‐binding ability of the latter. We also present an interstitial incorporation strategy for the promotion of the Ru top sites from subordinate to dominant character, while maintaining Pt‐like catalytic activity. Our combined theoretical and experimental studies further identify intermetallic RuSi as a highly active, non‐Pt material for catalyzing the HER, because of its suitable electronic structure governed by a good balance of ligand and strain effects.     

Ethylene polymerization using novel titanium catalytic precursors bearing N,N‐dialkylcarbamato ligands

Polymerization catalysts based on N,N‐dialkylcarbamato complexes of titanium(IV) appear particularly interesting, because these novel catalytic precursors are rather cheap and easy to synthesize and handle. This contribution reports ethylene polymerization behavior of titanium(IV) complexes of general formula Ti(O₂CNR₂)₄ R = Me ( I ) and Et ( II ) and TiCl₂(O₂CNMe₂)₂ ( III ). These precursors in conjunction with methylaluminoxane resulted active catalysts for the polymerization of ethylene, affording high‐density polyethylene with limited branch content. The influence of the polymerization parameters was studied with particular reference to the type of catalyst components, solvent, temperature, monomer concentration, and Al/Ti ratio. The nature of the solvent appears crucial for catalytic performances: when toluene was replaced by chlorobenzene, a significant increase of the productivity was ascertained. The obtained polymers were characterized by DSC, size exclusion chromatography, FTIR, and NMR techniques. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Analysis of models for the ziegler-natta stereospecific polymerization on the basis of non-bonded interactions at the catalytic site—II: Edges,steps and reliefs on the surface of layered modifications of TiCl3

The computations performed in a previous paper for hypothetical structures of the site active in the polymerization of olefins at the (110) surfaces of TiCl₃-α are extended to edges, steps and reliefs on (111) surfaces of all the layered modifications of TiCl₃. Our computations suggest that edges, steps and reliefs of all the layered modifications of TiCl₃ should behave similarly in providing active sites; polymerization should occur on these more exposed sites because of the much lower activation energies than for plain surfaces. A unique general model emerges: lower steric repulsions appear to be implied for a coordination of the polymeric chain on the more hindered octahedral position at a titanium atom and this, in turn, causes a chiral orientation of the first carbon-carbon bond of the chain. This orientation may be the main factor in determining the stereospecificity.     

New multinuclear half-titanocene catalysts in the syndiospecific polymerization of styrene

The syndiospecific polymerization of styrene with a new class of multinuclear transition metal catalysts in the presence of methylalumoxane and triisobutylaluminum has been investigated. The new multinuclear catalysts [(η⁵-C₅Me₅)Ti]₄(μ-O)₆ and [(η⁵-C₁₃H₁₇)Ti]₄(μ-O)₆ were received by reaction of the corresponding mononuclear compounds with water and characterized by X-ray crystal structure analysis. The molecular structure of both complexes is tetrameric with six bridging oxygen atoms between the four titanium atoms, forming an adamantane-like cage structure with a substituted cyclopentadienyl ligand remaining η⁵-bonded to each titanium atom.The bulky [(η⁵-C₁₃H₁₇)Ti]₄(μ-O)₆ shows higher polymerization conversions than [(η⁵-C₅Me₅)Ti]₄(μ-O)₆. The polymerization activity is significantly increased by an enhancement of the MAO concentration after a short retardation period and levels off at MAO/[(η⁵-C₁₃H₁₇)Ti]₄(μ-O)₆ molar ratios above about 600. Triisobutylaluminum increases the polymerization yield to a maximum at a TIBA/[(η⁵-C₁₃H₁₇)Ti]₄(μ-O)₆ molar ratio of about 30-100, but considerably decreases it at higher molar ratios below the polymerization conversion reached without any additional aluminum alkyl. Both compounds affect molecular weight and molecular weight distribution without any influence on the stereospecificity of the different catalytic sites active in polymerization reactions.The new multinuclear transition metal catalysts reach about 30-50% of the polymerization activity of the mononuclear catalysts on a molar basis and show a remarkably high catalytic activity in complex-coordinative polymerizations even after storage in non-inert-atmosphere conditions. The active polymerization sites of the multinuclear catalysts are not as uniform as the active sites of the mononuclear catalysts are and provide polystyrenes of a slightly lower syndiospecificity, but do not significantly influence the weight average molecular weights.     

Rapid ring‐opening polymerization of 1,4‐dioxan‐2‐one initiated by titanium alkoxides

Ring‐opening polymerization of 1,4‐dioxan‐2‐one in bulk was initiated by three titanium alkoxides, titanium dichlorodiisopropoxide (TiCl₂(OiPr)₂), titanium chlorotriisopropoxide (TiCl(OiPr)₃), and titanium tetraisopropoxide (Ti(OiPr)₄). The results indicate that the polymerization rate increased with number of OiPr groups in the initiator. High conversion of monomer (90%) and high molecular weight (11.9 × 10⁴ g/mol) of resulting polymer can be achieved in only 5 min at 60 °C with Ti(OiPr)₄ as an initiator. Analysis on nuclear magnetic resonance (NMR) spectra suggests the initiating sites for TiCl₂(OiPr)₂, TiCl(OiPr)₃, and Ti(OiPr)₄ to be 1.9, 2.6, and 3.8, respectively. Coordination‐insertion mechanism for the polymerization via cleavage of the acyl–oxygen bonds of the monomer was proved by NMR investigation. Kinetic studies indicate that polymerization initiated by Ti(OiPr)₄ followed a first‐order kinetics, with an apparent activation energy of 33.7 kJ/mol. It is noteworthy that this value is significantly lower than earlier reported values with other catalysts, namely La(OiPr)₃ (50.5 kJ/mol) and Sn(Oct)₂ (71.8 kJ/mol), which makes it an attractive catalyst for reactive extrusion polymerization. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

The CrOx/SiO2/Si(100) catalyst – a surface science approach to supported olefin polymerization catalysis

Depositing catalytically active particles onto flat, thin and oxidic support forms an attractive way to make supported catalyst suitable for surface science characterization. Here we show how this approach has been applied to the Phillips (CrO_x/SiO₂) ethylene polymerization catalyst. The model catalyst shows a respectable polymerization activity after thermal activation in dry air (calcination). Combining the molecular information from X‐ray Photoelectron Spectroscopy (XPS) and Secondary Ion Mass Spectrometry (SIMS) we can draw a molecular level of the activated catalyst that features exclusively monochromate species, which are anchored to the silica support via ester bonds with the surface silanol groups. These surface chromates form the active polymerization site upon contact with ethylene. Upon increasing calcination temperature we observe a decrease in chromium coverage as some of the surface chromate desorbs from the silica surface. Nevertheless, we also find an increasing polymerization activity of the model catalyst. We attribute this increase in catalytic activity to the isolation of the supported chromium, which prevents dimerization of the coordinatively unsaturated active site. Diluting the amount of chromium to 200 Cr‐atoms/nm² of silica surface enables the visualisation of polyethylene produced by a single active site.     

Configuration of MFI-type zeolite interacting with the probe molecule P(CH<Subscript>3</Subscript>)<Subscript>3</Subscript> – a theoretical study

With P(CH₃)₃ as the probe molecule adsorbed on titanium silicalite (TS-1) zeolite, the special and important role of T12 site in MFI-type zeolite was clearly elucidated. There are altogether three active sites present in TS-1 zeolite with Ti at the T12 site. Owing to the preferential adsorption of probe molecules on the first Brönsted acidic site, the Ti12 center will probably fail to show Lewis acidity. The ionic [HP(CH₃)₃]⁺ species can be stabilized by the first or second Brönsted acidic site, with the former energetically favored. The latter was formed through the transfer of the ionic [HP(CH₃)₃]⁺ species from the first to the second Brönsted acidic site.