Covalent Organic Framework-Supported Metallocene for Ethylene Polymerization

The loading of homogeneous catalysts with support can dramatically improve their performance in olefin polymerization. However, the challenge lies in the development of supported catalysts with well-defined pore structures and good compatibility to achieve high catalytic activity and product performance. Herein, we report the use of an emergent class of porous material-covalent organic framework material (COF) as a carrier to support metallocene catalyst-Cp2 ZrCl2 for ethylene polymerization. The COF-supported catalyst demonstrates a higher catalytic activity of 31.1×106  g mol-1  h-1 at 140 °C, compared with 11.2×106  g mol-1  h-1 for the homogenous one. The resulting polyethylene (PE) products possess higher weight-average molecular weight (Mw ) and narrower molecular weight distribution (Ð) after COF supporting, that is, Mw increases from 160 to 308 kDa and Ð drops from 3.3 to 2.2. The melting point (Tm ) is also increased by up to 5.2 °C. Moreover, the PE product possesses a characteristic filamentous microstructure and demonstrates an increased tensile strength from 19.0 to 30.7 MPa and elongation at break from 350 to 1400 % after catalyst loading. We believe that the use of COF carriers will facilitate the future development of supported catalysts for highly efficient olefin polymerization and high-performance polyolefins.

Cages versus Sheets: A Critical Comparison in the Size Range Expected for Methylaluminoxane (MAO)

New cage models (MeAlO)n (Me3 Al)m (n=16, m=6 or 7) isomeric with previously reported sheet models for the principle activator found in hydrolytic MAO (h-MAO) are compared at M06-2X and MN15 levels of theory using density functional theory with respect to their thermodynamic stability. Reactivity of the neutrals or corresponding anions with formula [(MeAlO)16 (Me3 Al)6 Me]- towards chlorination, and loss of Me3 Al is explored while reactivity of the neutrals towards formation of contact- and outer-sphere ion pairs from Cp2 ZrMe2 and Cp2 ZrMeCl is examined. The results suggest on balance that a cage model for this activator is less consistent with experiment than an isomeric sheet model, although the latter are more stable based on free energy.

MAO- and Borate-Free Activating Supports for Group 4 Metallocene and Post-Metallocene Catalysts of α-Olefin Polymerization and Oligomerization

Modern industry of advanced polyolefins extensively uses Group 4 metallocene and post-metallocene catalysts. High-throughput polyolefin technologies demand the use of heterogeneous catalysts with a given particle size and morphology, high thermal stability, and controlled productivity. Conventional Group 4 metal single-site heterogeneous catalysts require the use of high-cost methylalumoxane (MAO) or perfluoroaryl borate activators. However, a number of inorganic phases, containing highly acidic Lewis and Brønsted sites, are able to activate Group 4 metal pre-catalysts using low-cost and affordable alkylaluminums. In the present review, we gathered comprehensive information on MAO- and borate-free activating supports of different types and discussed the surface nature and chemistry of these phases, examples of their use in the polymerization of ethylene and α-olefins, and prospects of the further development for applications in the polyolefin industry.

The coordination chemistry of oxide and nanocarbon materials

Understanding how a ligand affects the steric and electronic properties of a metal is the cornerstone of the inorganic chemistry enterprise. What happens when the ligand is an extended surface? This question is central to the design and implementation of state-of-the-art functional materials containing transition metals. This perspective will describe how these two very different sets of extended surfaces can form well-defined coordination complexes with metals. In the Green formalism, functionalities on oxide surfaces react with inorganics to form species that contain X-type or LX-type interactions between the metal and the oxide. Carbon surfaces are neutral L-type ligands; this perspective focuses on carbons that donate six electrons to a metal. The nature of this interaction depends on the curvature, and thereby orbital overlap, between the metal and the extended π-system from the nanocarbon.

Syntheses of Silylene-Bridged Thiophene-Fused Cyclopentadienyl ansa-Metallocene Complexes for Preparing High-Performance Supported Catalyst

We synthesized a series of Me2Si-bridged ansa-zirconocene complexes coordinated by thiophene-fused cyclopentadienyl and fluorenyl ligands (Me2Si(2-R1-3-R2-4,5-Me2C7S)(2,7-R32C13H6))ZrMe2 (R1 = Me or H, R2 = H or Me, R3 = H, tBu, or Cl) for the subsequent preparation of supported catalysts. We determined that the fluorenyl ligand adopts an η3-binding mode in 9 (R1 = Me, R2 = H, R3 = H) by X-ray crystallography. Further, we synthesized a derivative 15 by substituting the fluorenyl ligand in 9 with a 2-methyl-4-(4-tert-butylphenyl)indenyl ligand, derivatives 20 and 23 by substituting the Me2Si bridge in 12 (R1 = Me, R2 = H, R3 = tBu) and 15 with a tBuO(CH2)6(Me)Si bridge, and the dinuclear congener 26 by connecting two complexes with a –(Me)Si(CH2)6Si(Me)– spacer. The silica-supported catalysts prepared using 12, 20, and 26 demonstrated up to two times higher productivity in ethylene/1-hexene copolymerization than that prepared with conventional (THI)ZrCl2 (21–26 vs. 12 kg-PE/g-(supported catalyst)), producing polymers with comparable molecular weight (Mw, 330–370 vs. 300 kDa), at a higher 1-hexene content (1.3 vs. 1.0 mol%) but a lower bulk density of polymer particles (0.35 vs. 0.42 g/mL).

Ionic Liquid-Modified Porous Organic Polymers as Efficient Metallocene Catalyst Supports

Porous organic polymers (POPs) are widely used in various areas such as adsorption, separation and catalysis. In the present work, ionic liquid-modified porous organic polymers (IL-POPs) synthesized by dispersion polymerization were applied to immobilize metallocene catalysts for olefin polymerization. The prepared IL-POPs were characterized by Fourier transform infrared spectrometer (FT-IR), nitrogen sorption porosimetry, X-ray photoelectron spectroscopy (XPS), thermal gravimetric analysis (TGA), inductively coupled plasma atomic emission spectrometer (ICP) and scanning electron microscope (SEM) analysis. The IL-POPs obtained pores with surface specific area (SSA) ranging from 16.9 m2/g to 561.8 m2/g, and total pore volume (TPV) ranging from 0.08 cm3/g to 0.71 cm3/g. The supported catalysts Zr/MAO@IL-POPs exhibit great activity (3700 kg PE/mol·Zr·bar·h) in ethylene polymerization, and the GPC-IR results show that the polyethylene has narrow molecular weight distribution (2.2 to 2.8). The DSC results show that the melting point of prepared polyethylene was as high as 138 °C, and the TREF analysis results indicate that they have similar chemical composition distribution with elution temperature at 100.5–100.7 °C.

Polymethylaluminoxane organic frameworks (sMAOF) – highly active supports for slurry phase ethylene polymerisation

A series of modified solid polymethylaluminoxane (sMAO) catalyst supports have been developed for slurry phase ethylene polymerisation, using aryl di-ol modifier groups.

Selective Trimerization of α-Olefins with Immobilized Chromium Catalyst for Lubricant Base Oils

The demand for poly(α-olefin)s (PAOs), which are high-performance group IV lubricant base oils, is increasingly high. PAOs are generally produced via the cationic oligomerization of 1-decene, wherein skeleton rearrangement inevitably occurs in the products. Hence, a transition-metal-based catalytic process that avoids rearrangement would be a valuable alternative for cationic oligomerization. In particular, transition-metal-catalyzed selective trimerization of α-olefins has the potential for success. In this study, (N,N′,N″-tridodecyltriazacyclohexane)CrCl3 complex was reacted with MAO-silica (MAO, methylaluminoxane) for the preparation of a supported catalyst, which exhibited superior performance in selective α-olefin trimerization compared to that of the corresponding homogeneous catalyst, enabling the preparation of α-olefin trimers at ~200 g scale. Following hydrogenation, the prepared 1-decene trimer (C30H62) exhibited better lubricant properties than those of commercial-grade PAO-4 (kinematic viscosity at 40 °C, 15.1 vs. 17.4 cSt; kinematic viscosity at 100 °C, 3.9 vs. 3.9 cSt; viscosity index, 161 vs. 123). Moreover, it was shown that 1-octene/1-dodecene mixed co-trimers (i.e., a mixture of C24H50, C28H58, C32H66, and C36H74), generated by the selective supported Cr catalyst, exhibited outstanding lubricant properties analogous to those observed for the 1-decene trimer (C30H62).

Reactivity Trends of Lewis Acidic Sites in Methylaluminoxane and Some of Its Modifications

The established model cluster (AlOMe)16(AlMe3)6 for methylaluminoxane (MAO) cocatalyst has been studied by density functional theory, aiming to rationalize the different behaviors of unmodified MAO and TMA-depleted MAO/BHT (TMA = trimethylaluminum; BHT = 2,6-di-tert-butyl-4-methylphenol), highlighted in previous experimental studies. The tendency of the three model Lewis acidic sites A-C to release neutral Al fragments (i.e., AlMe2R; R = Me or bht) or transient aluminum cations (i.e., [AlMeR]+) has been investigated both in the absence and in the presence of neutral N-donors. Sites C are most likely responsible for the activation capabilities of TMA-rich MAO, but TMA depletion destabilizes them, possibly inducing structural rearrangements. The remaining sites A and B, albeit of lower Lewis acidity, should be still able to release cationic Al fragments when TMA-depleted modified MAOs are treated with N-donors (e.g. [AlMe(bht)]+ from MAO/BHT). These findings provide tentative interpretations for earlier observations of donor-dependent ionization tendencies of MAO and MAO/BHT and how TMA depleted MAOs can still be potent activators.

Magnetization transfer from protons to quadrupolar nuclei in solid-state NMR using PRESTO or dipolar-mediated refocused INEPT methods

In solid-state NMR spectroscopy, the through-space transfer of magnetization from protons to quadrupolar nuclei is employed to probe proximities between those isotopes. Furthermore, such transfer, in conjunction with Dynamic Nuclear Polarization (DNP), can enhance the NMR sensitivity of quadrupolar nuclei, as it allows the transfer of DNP-enhanced ¹H polarization to surrounding nuclei. We compare here the performances of two approaches to achieve such transfer: PRESTO (Phase-shifted Recoupling Effects a Smooth Transfer of Order), which is currently the method of choice to achieve the magnetization transfer from protons to quadrupolar nuclei and which has been shown to supersede Cross-Polarization under Magic-Angle Spinning (MAS) for quadrupolar nuclei and D-RINEPT (Dipolar-mediated Refocused Insensitive Nuclei Enhanced by Polarization Transfer) using symmetry-based SR4₁² recoupling, which has already been employed to transfer the magnetization in the reverse way from half-integer quadrupolar spin to protons. We also test the PRESTO sequence with R16₇⁶ recoupling using 270₀90₁₈₀ composite π-pulses as inversion elements. This recoupling scheme, which has previously been proposed to reintroduce ¹H Chemical Shift Anisotropy (CSA) at high MAS frequencies with high robustness to rf-field inhomogeneity, has not so far been employed to reintroduce dipolar couplings with protons. These various techniques to transfer magnetization from protons to quadrupolar nuclei are analyzed using (i) an average Hamiltonian theory, (ii) numerical simulations of spin dynamics, and (iii) experimental ¹H → ²⁷Al and ¹H → ¹⁷O transfers in as-synthesized AlPO₄-14 and ¹⁷O-labelled fumed silica, respectively. The experiments and simulations are done at two magnetic fields (9.4 and 18.8 T) and several spinning speeds (15, 18-24 and 60 kHz). This analysis indicates that owing to its γ-encoded character, PRESTO yields the highest transfer efficiency at low magnetic fields and MAS frequencies, whereas owing to its higher robustness to rf-field inhomogeneity and chemical shifts, D-RINEPT is more sensitive at high fields and MAS frequencies, notably for protons exhibiting large offset or CSA, such as those involved in hydrogen bonds.

Organometallic Zirconium Compounds in an Oxygen-Rich Coordination Environment: Synthesis and Structural Characterization of Tris(oxoimidazolyl)hydroboratozirconium Compounds

A series of tris(oxoimidazolyl)hydroborato ligands, which serve as L₂X [O₃] donors, have been employed to obtain organometallic zirconium compounds in an uncommon oxygen-rich coordination environment. For example, Cp[To^MeBenz]ZrCl₂ has been synthesized via the reaction of [To^MeBenz]Na with CpZrCl₃ and bears a structural resemblance to the bent metallocene dichloride derivative Cp₂ZrCl₂. In addition, the half-sandwich counterparts [To^MeBenz]ZrCl₃ and [To^But]ZrCl₃ have been obtained by metathesis of ZrCl₄ with [To^MeBenz]Na and [To^But]Na, respectively. The structurally related zirconium benzyl compounds [To^RBenz]Zr(CH₂Ph)₃ (R = Me, Bu^t, 1-Ad) have also been synthesized via the reactions of [To^RBenz]Tl with Zr(CH₂Ph)₄, and X-ray diffraction studies demonstrate that the benzyl ligands in these compounds are conformationally flexible and exhibit a large range of Zr-CH₂-Ph bond angles (94.7-131.7°). Protolytic cleavage of one of the benzyl ligands of [To^RBenz]Zr(CH₂Ph)₃ (R = Bu^t, 1-Ad) may be achieved by treatment with [PhNHMe₂][B(C₆F₅)₄] to generate {[To^RBenz]Zr(CH₂Ph)₂}[B(C₆F₅)₄], which are catalysts for the polymerization of ethylene. The molecular structure of the ether adduct, {[To^ButBenz]Zr(CH₂Ph)₂(OEt₂)}[B(C₆F₅)₄], has been determined by X-ray diffraction. In addition to the use of tris(oxoimidazolyl)hydroborato ligands, bis(oxoimidazolyl)hydroborato ligands have also been used to obtain zirconium benzyl compounds in oxygen-rich environments, namely, [Bo^MeBenz]₂Zr(CH₂Ph)₂ and [Bo^AdBenz]₂Zr(CH₂Ph)₂.

The Multifaceted Role of Methylaluminoxane in Metallocene-Based Olefin Polymerization Catalysis

In single-site olefin polymerization catalysis, a large excess of cocatalyst is often required for the generation of highly active catalysts, but the reason for this is unclear. In this work, fundamental insight into the multifaceted role of cocatalyst methylaluminoxane (MAO) in the activation, deactivation, and stabilization of group 4 metallocenes in the immobilized single-site olefin polymerization catalyst was gained. Employing probe molecule FT-IR spectroscopy, it was found that weak Lewis acid sites, inherent to the silica-supported MAO cocatalyst, are the main responsible species for the genesis of active metallocenes for olefin polymerization. These weak Lewis acid sites are the origin of AlMe₂⁺ groups. Deactivation of metallocenes is caused by the presence of silanol groups on the silica support. Interaction of the catalyst precursor with these silanol groups leads to the irreversible formation of inactive metallocenes. Importantly, a high concentration of MAO (14 wt% Al) on the silica support is necessary to keep the metallocenes immobilized, hence preventing metallocene leaching and consequent reactor fouling. Increasing the loading of the MAO cocatalyst leads to larger amounts of AlMe₂⁺, fewer silanol groups, and less metallocene leaching, which all result in higher olefin polymerization activity.

Preparation of monopodal and bipodal aluminum surface species by selective protonolysis of highly reactive [AlH3(NMe2Et)] on silica

The reaction of [AlH₃(NMe₂Et)] with silica treated at 200 °C leads to a well-defined bipodal aluminum hydride while with silica treated at 700 °C a mixture of mono- and bi-podal aluminum hydrides is obtained.

Avoiding leaching of silica supported metallocenes in slurry phase ethylene homopolymerization

Leaching of (n-BuCp)₂ZrCl₂/MAO catalyst from silica supports has been studied in slurry phase ethylene homopolymerisation by changing i) the polymerization protocol, ii) aluminum alkyl scavengers and iii) by adding butylated hydroxytoluene (BHT-H) along with aluminum alkyl scavengers.

17O MAS NMR studies of oxo-based olefin metathesis catalysts: a critical assessment of signal enhancement methods

We assessed the DFS parameters for robust and optimal signal enhancement in ¹⁷O NMR studies of silica-supported catalysts.

The design of a bipodal bis(pentafluorophenoxy)aluminate supported on silica as an activator for ethylene polymerization using surface organometallic chemistry

A well-defined solid activator for supported metallocene polymerization catalysts.