Functional Hybrid Materials

Solid polymethylaluminoxane supported catalyst for ethylene polymerisation 

An insoluble form of methylaluminoxane, also known as solid polymethylaluminoxane (sMAO), has been synthesised by the controlled hydrolysis of trimethylaluminum (TMA) with benzoic acid, followed by thermolysis. Characterization of sMAO by multinuclear NMR spectroscopy in solution and the solid state reveals an aluminoxane structure that features “free” and bound TMA and incorporation of a benzoate residue. Total X-ray scattering (or pair distribution function, PDF) measurements on sMAO allow comparisons to be made with simulated data for density functional theory (DFT) modeled structures of methylaluminoxane (MAO). Several TMA-bound (AlOMe)n cage and nanotubular structures with n > 10 are consistent with the experimental data. The measured Brunauer–Emmett–Teller (BET) surface area of sMAO ranges between 312 and 606 m2 g–1 and shows an N2 adsorption/desorption isotherm consistent with a nonporous material. sMAO can be utilized to support metallocene precatalysts in slurry-phase ethylene polymerization reactions. Metallocene precatalyst rac-ethylenebis(1-indenyl)-dichlorozirconium, rac-(EBI)ZrCl2, was immobilised on sMAO samples, to afford solids which showed very high polymerization activities in hexane, comparable to those of the respective homogeneous catalysts formed by treatment of the precatalysts with MAO. rac-(EBI)ZrCl2 immobilised on an sMAO containing an Al:O ratio of 1.2 gave the highest ethylene polymerisation activity.

 

 

 

Postsynthesis modification of solid polymethylaluminoxane (sMAO) with tris(pentafluorophenyl)borane or pentafluorophenol produces highly active metallocene supports “sMMAOs” for use in slurry-phase ethylene polymerisation. Characterization of the sMMAOs using elemental analysis, BET isotherm, SEM-EDX, diffuse FT-IR, and solid-state NMR spectroscopy reveals that the surface methyl groups are exchanged for C6F5 and C6F5O moieties respectively, giving a material with reduced aluminum content and a lower specific surface area than sMAO. Rac-(EBI)ZrCl2 immobilized on B(C6F5)3- and C6F5OH-modified sMAO displayed activity increases of 66% and 71% respectively for ethylene polymerisation compared to the same zirconocene catalyst precursor on unmodified sMAO. 

Physicochemical surface-structure studies of highly active slurry-phase ethylene polymerisation catalysts has been performed. Zirconocene complexes immobilised on solid polymethylaluminoxane (sMAO) (sMAO–Cp2ZrX2), have been investigated using SEM-EDX, diffuse reflectance FT-IR (DRIFT) and high field (21.1 T) solid state NMR (ssNMR) spectroscopy. The data suggest a common surface-bound cationic methylzirconocene is the catalytically active species. 91Zr solid sate NMR spectra of sMAO–Cp2ZrCl2 and sMAO–Cp2ZrMe2 are consistent with a common surface-bound Zr environment. However, variation of the σ-donor (X) groups on the metallocene precatalyst leads to significant differences in polymerisation activity. We report evidence for X group transfer from the precatalyst complex onto the surface of the aluminoxane support, which in the case of X = C6F5, results in a 38% increase in activity.

 

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Layered Double Hydroxides (LDHs) 

AMO-LDH

We report the synthesis and characterisation of a new family of layered double hydroxides entitled Aqueous Miscible Organic Layered Double Hydroxide (AMO-LDH). AMO-LDHs have the chemical composition [Mz+1−xM′y+x(OH)2]a+(Xn−)a/r·bH2O·c(AMO-solvent) wherein M and M′ are metal cations, z = 1 or 2; y = 3 or 4, 0 < x < 1, b = 0–10, c = 0–10, X is an anion, r is 1–3 and a = z(1 − x) + xy − 2. The role of the AMO-solvents such as acetone (A) or methanol (M) in the LDH synthesis is discussed. The distinguishing features between AMO, and conventional or commercial LDHs are investigated using X-ray diffraction, infrared spectroscopy, electron microscopy, thermal analysis, adsorption and powder density studies. These experiments show that AMO-LDHs are highly dispersed and exhibit significantly higher surface areas and lower powder densities than conventional or commercially available LDHs. AMO-LDHs can exhibit N2 BET surface areas in excess of 301 m2 g−1 compared to 13 m2 g−1 for the equivalent LDHs prepared by co-precipitation in water. The Zn2Al–borate LDH exhibits a pore volume of 2.15 cm3 g−1 which is 2534 times higher than the equivalent conventionally prepared LDH.

 

 

 

 

 

Unique Technical Advance (UTA)

  1. Rosette (flower-like) morphology
  2. High surface density, (100-430 m2 g–1)*
  3. High porosity,  (1.5-2.2 cm3 g–1) *
  4. Multi-pore size range
  5. Dispersible in hydrocarbon solvents
  6. Generally applicable across all LDHs
  7. Precursors to high surface area mixed metal oxides*
  8. Particles with OAN up to 350*

 

 

LDH@Core Hybrids

New 3D Hierarchical Structures

 

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LDH Nanosheets

Unique Technical Advance (UTA)

  1. Platelet morphology
  2. High Aspect ratio:  10-350
  3. Generally applicable across all LDHs
  4. Materials from waste

The O’Hare group at The University of Oxford has developed a novel method of synthesising carbon-capture compounds from wastewater by-products. The waste water by-product is struvite (MgNH4PO4•H2O) which is produced in plentiful supply during modern water treatment and for which there is currently no commercial use.

The O’Hare has developed a use for this struvite based on the production of layered double hydroxides (LDHs) one example of which is the mixed metal oxide Mg3Al-CO3 SLDO (struvite layered double oxide). This compound is of particular interest because of its enhanced ability to act as a carbon-capture material and be of significant help in the fight against climate change. There is more information on this under ‘Green Chemistry’ or in this article.

 

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