Surface science of single-site heterogeneous olefin polymerization catalysts

Well-Defined Ti Surface Sites in Ziegler-Natta Pre-Catalysts from 47/49Ti Solid-State Nuclear Magnetic Resonance Spectroscopy

Treatment of Ziegler-Natta (ZN) catalysts with BCl3 improves their activity by increasing the number of active sites. Here we show how 47/49Ti solid-state nuclear magnetic resonance (NMR) spectroscopy enables us to understand the electronic structure of the Ti surface sites present in such treated ZN pre-catalysts, prior to activation with alkyl aluminum. High-field (21.1 T) and low-temperature (∼100 K) NMR augmented by DFT modeling on the pre-catalyst and corresponding molecular analogues enables the detection of 47/49Ti NMR signatures and a molecular level understanding of the electronic structure of Ti surface sites. The associated Ti surface sites exhibit 49Ti NMR signatures (δiso, exp = -170 ppm; CQ, exp = 9.3 MHz; κ = 0.05) corresponding to well-defined fully chlorinated hexacoordinated Ti sites adsorbed on a distorted surface of the MgCl2 support, formed upon post-treatment with BCl3 and removal of the alkoxo ligands, paralleling the increased polymerization activity.

A strategy for high ethylene polymerization performance using titanium single-site catalysts

The synthesis of heterogeneous Ti(IV)-based catalysts for ethylene polymerization following surface organometallic chemistry concepts is described. The unique feature of this catalyst arises from the silica support, KCC-1700. It has (i) a 3D fibrous morphology that is essential to improve the diffusion of the reactants, and (ii) an aluminum-bound hydroxyl group, [(Si-O-Si)(Si-O-)2Al-OH] 2, used as an anchoring site. The [(Si-O-Si)(Si-O-)(Al-O-)TiNp3] 3 catalyst was obtained by reacting 2 with a tetrakis-(neopentyl) titanium TiNp4. The structure of 3 was fully characterized by FT-IR, advanced solid-state NMR spectroscopy [1H, 13C], elemental and gas-phase analysis (ICP-OES and CHNS analysis), and XPS. The benefits of combining these morphological (3D structure) and electronic properties of the support (aluminum plus titanium) were evidenced in ethylene polymerization. The results show a remarkable enhancement in the catalytic performance with the formation of HDPE. Notably, the resulting HDPE displays a molecular weight of 3 200 000 g mol-1 associated with a polydispersity index (PD) of 2.3. Moreover, the effect of the mesostructure (2D vs. 3D) was demonstrated in the catalytic activity for ethylene polymerization.

Alkali and Alkaline Earth Metal Complexes as Versatile Catalysts for Ring-Opening Polymerization of Cyclic Esters

Biodegradable polyesters such as poly(ϵ-caprolactone) (PCL) and poly(lactic acid) (PLA) have been considered for use in several areas, such as drug delivery devices, sutures, tissue engineering, and GBR membranes, due to its bio-renewability, biodegradability, and biocompatibility. Several synthetic techniques for the preparation of polyesters have been reported in the literature, amongst which the ring-opening polymerization (ROP) of cyclic esters is the most efficient. A convenient approach to access iso-selective PLAs is polymerization of racemic lactide (rac-LA), which shows excellent stereoregularity without the need for costly chiral auxiliaries or ligands. In this personal account, we review a series of methods that have been practiced to the synthesis of biodegradable polyesters from various cyclic monomers using alkali and alkaline earth metal complexes as efficient catalysts.

Pulsed Laser in Liquids Made Nanomaterials for Catalysis

Catalysis is essential to modern life and has a huge economic impact. The development of new catalysts critically depends on synthetic methods that enable the preparation of tailored nanomaterials. Pulsed laser in liquids synthesis can produce uniform, multicomponent, nonequilibrium nanomaterials with independently and precisely controlled properties, such as size, composition, morphology, defect density, and atomistic structure within the nanoparticle and at its surface. We cover the fundamentals, unique advantages, challenges, and experimental solutions of this powerful technique and review the state-of-the-art of laser-made electrocatalysts for water oxidation, oxygen reduction, hydrogen evolution, nitrogen reduction, carbon dioxide reduction, and organic oxidations, followed by laser-made nanomaterials for light-driven catalytic processes and heterogeneous catalysis of thermochemical processes. We also highlight laser-synthesized nanomaterials for which proposed catalytic applications exist. This review provides a practical guide to how the catalysis community can capitalize on pulsed laser in liquids synthesis to advance catalyst development, by leveraging the synergies of two fields of intensive research.

Fundamental aspects of heterogeneous Ziegler–Natta olefin polymerization catalysis: an experimental and computational overview

Using an internal/external donor containing MgCl₂-supported Ziegler–Natta catalyst is one of the important processes for polyolefin production.

A robust immobilization strategy in the nano-dispersed Ziegler–Natta catalyst: non-covalent O–Ti coordination

A robust non-covalent O–Ti bond facilitates the synthesis of polyethylene with a narrow distribution.

A new echelon of precision polypentenamers: highly isotactic branching on every five carbons

A systematic study shows that bulky allylic substituents on cyclopentene monomers affords a highly precise microstructure after ring-opening metathesis polymerization.

Immobilization of isolated FI catalyst on polyhedral oligomeric silsesquioxane-functionalized silica for the synthesis of weakly entangled polyethylene

POSS-functionalized silica can isolate the anchored FI catalysts and hinder chain overlap in the polymerization of ethylene.

Ethylene polymerisation using solid catalysts based on layered double hydroxides

We report here the use of methylaluminoxane (MAO) modified aqueous miscible organic solvent treated (AMOST) layered double hydroxide, Mg₆Al₂(OH)₁₆CO₃·4H₂O (AMO-Mg₃Al-CO₃) as a catalyst support system for the slurry phase polymerisation of ethylene using immobilised metallocene and non-metallocene metal complexes.

Design strategies for the molecular level synthesis of supported catalysts

Supported catalysts, metal or oxide catalytic centers constructed on an underlying solid phase, are making an increasingly important contribution to heterogeneous catalysis. For example, in industry, supported catalysts are employed in selective oxidation, selective reduction, and polymerization reactions. Supported structures increase the thermal stability, dispersion, and surface area of the catalyst relative to the neat catalytic material. However, structural and mechanistic characterization of these catalysts presents a formidable challenge because traditional preparations typically afford complex mixtures of structures whose individual components cannot be isolated. As a result, the characterization of supported catalysts requires a combination of advanced spectroscopies for their characterization, unlike homogeneous catalysts, which have relatively uniform structures and can often be characterized using standard methods. Moreover, these advanced spectroscopic techniques only provide ensemble averages and therefore do not isolate the catalytic function of individual components within the mixture. New synthetic approaches are required to more controllably tailor supported catalyst structures. In this Account, we review advances in supported catalyst synthesis and characterization developed in our laboratories at Northwestern University. We first present an overview of traditional synthetic methods with a focus on supported vanadium oxide catalysts. We next describe approaches for the design and synthesis of supported polymerization and hydrogenation catalysts, using anchoring techniques which provide molecular catalyst structures with exceptional activity and high percentages of catalytically significant sites. We then highlight similar approaches for preparing supported metal oxide catalysts using atomic layer deposition and organometallic grafting. Throughout this Account, we describe the use of incisive spectroscopic techniques, including high-resolution solid state NMR, UV-visible diffuse reflectance (DRS), UV-Raman, and X-ray absorption spectroscopies to characterize supported catalysts. We demonstrate that it is possible to tailor and isolate defined surface species using a molecularly oriented approach. We anticipate that advances in catalyst design and synthesis will lead to a better understanding of catalyst structure and function and, thus, to advances in existing catalytic processes and the development of new technologies.

Probing the surfaces of heterogeneous catalysts by in situ IR spectroscopy

This critical review describes the reactivity of heterogeneous catalysts from the point of view of four simple, but essential for Chemistry, molecules (namely dihydrogen, carbon monoxide, nitrogen monoxide and ethylene) that are considered as probes or as reactants in combination with "in situ" controlled temperature and pressure Infrared spectroscopy. The fundamental properties of H(2), CO, NO and C(2)H(4) are shortly described in order to justify their different behaviour in respect of isolated sites in different environments, extended surfaces, clusters, crystalline or amorphous materials. The description is given by considering some "key studies" and trying to evidence similarities and differences among surfaces and probes (572 references).

The interfacial properties of MgCl(2) thin films grown on Ti(0001)

Photoelectron spectroscopy with synchrotron radiation (SRPES), temperature programmed desorption (TPD), low energy electron diffraction (LEED), and ion-scattering spectroscopy (ISS) were used in order to study the MgCl(2)/Ti(0001) interface. A clear hexagonal LEED pattern confirmed the presence of a quite large grain of Ti(0001) on the substrate while no new superstructure was formed after deposition of MgCl(2) either at room or at elevated temperatures. A series of high resolution spectra after step by step MgCl(2) deposition and gradual annealing indicated strong interaction between MgCl(2) and the substrate while ISS measurements showed that there is no migration of Ti atoms into the deposit layers. Additional quantities of deposited MgCl(2) grew stoichimetrically on top of the chemically active interface. Annealing at approximately 350 degrees C caused clustering of the MgCl(2) multilayer and TPD results showed that they desorbed stoichimetrically at temperatures between 360 and 380 degrees C. The interfacial TiCl(x)Mg(y) species dissociated by the disruption of the Cl-Mg bonds at temperatures higher than 400 degrees C and metallic Mg evaporated. The Cl atoms remained attached on the Ti surface but they did not form any ordered structure even after annealing at 730 degrees C. The present results indicate the occurrence of charge transfer at the Ti/MgCl(2) interface through the Cl ligands and provide valuable information for catalyst design.

Nascent crystallization of a growing chain on a catalyst surface: a nonequilibrium molecular dynamics simulation study

The growing chain molecular dynamics (GCMD) simulation method, a new nonequilibrium molecular dynamics code, is proposed to simulate the polymer chain aggregation behavior during polymerization on a catalyst surface. We found that the growing chain crystallizes on the surface in two stages: the nucleation stage and the crystal growth stage. In the first part of the nucleation period, the short polymerizing chain first absorbs on the surface and can be in either an ordered or disordered structure. Still in the nucleation period, when the chain reaches a degree of polymerization, about 100 bonds, the chain folds into a stable nucleus on the substrate with 3-5 stems. In the crystal growth stage where the polymerization also proceeds, we observed a stem elongation process in combination with a chain folding process. In the stem elongation step, the number of stems in the nucleus remains constant, and all the stems expand together to a length of ca. 5-25 ns. In the subsequent chain folding step, the stem length decreases about 20 bonds within a period of ca. 0.1-0.5 ns. During chain growth, the elongation process and the folding process occur in an alternating and repeated fashion. The crystallization mechanism of the polymerizing chain was discussed.