Low Carbon Feedstocks

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The impact of CO2 emissions is one of the most difficult challenges to be addressed in the 21st century. Virtually all human activities require energy and products that currently rely heavily on cheap and abundant fossil resources. The chemical industry is no exception. It requires energy for running its processes, and feedstock - most often carbon feedstock, eventually embedded in most chemical products and materials - results in CO2 emissions. The transition towards a carbon-neutral chemical industry entails a transition of production processes towards low-carbon production by further exploiting energy and resource efficiency measures, increasingly by using alternative carbon feedstocks, i.e. renewable raw materials (biomass) and CO2, which can replace fossil feedstocks and leverage a lower overall carbon footprint.

The development of chemical and bio-based processes that utilise low-carbon feedstocks is essential to the world’s transition towards a low-carbon economy. As part of the transition towards a low-carbon economy, we must develop - and deploy at scale - a range of new chemical and bio-based processes that can utilise low-carbon feedstock in a sustainable way to meet the demand for chemicals and fuels now and into the future.

At the CoE, we are interested in synthesising novel organometallic compounds and then exploring the electronic and structural properties of these materials via cyclic voltammetry and crystallography.

Recent Examples

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A family of group 4 permethylpentalene complexes, Pn*MCpRX (M = Ti, Zr; CpR = Cp, CpMe, CptBu, CpnBu, CpMe3, Ind; X = Cl, Me) has been synthesised and fully characterised by multinuclear NMR spectroscopy, elemental analysis, and single-crystal X-ray diffraction studies. The effect of substitution around the Cp ligand was examined for ethylene polymerisation.

A series of group 4 complexes η5-Pn*(H)TiCl3, [η5-Pn*(H)ZrCl3]2 and [η5-Pn*(H)HfCl3]2 containing a η5-permethylpentalene ligand were prepared by the reaction of η5-Pn*(H)SnMe3 with the corresponding group 4 starting materials. The complexes inititiated ring opening polymerisation of L- and rac-lactide.

We describe the synthesis, structure and bonding of the first iridium and rhodium permethylpentalene complexes, syn-[M(CO)2]2(μ:η5:η5-Pn*) (M = Rh, Ir). In fact, [Ir(CO)2]2(μ:η5:η5-Pn*) is the first iridium pentalene complex. The novel 14 electron species η8-Pn*TiR2 (Pn* = C8Me6; R = Me, CH2Ph) have been synthesised and spectroscopically and structurally characterised. Subsequent reaction with CO2 leads to the activation and double insertion of CO2into both Ti–alkyl bonds to form the electronically saturated η8-Pn*Ti(κ2-O2CR)2 (R = Me, CH2Ph) complexes.

We report a series of anti-bimetallic transition metal complexes based on the permethylpentanyl (Pn*, C8Me6) ligand: anti-(MCpR5)2Pn* (where M = Fe, Co and Ni and R = H or Me). All anti-(MCpR5)Pn* structures have diamagnetic ground states. 


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A series of bis(peralkylindenyl)zirconocene and hafnocene complexes were synthesised and characterised by NMR spectroscopy, mass spectrometry and elemental analyses.

We report the synthesis of two zirconocenes, dimethylsilylbis(hexamethylindenyl) zirconium dichloride, rac-(SBI*)ZrCl2, and nbutyldimethylsilyl(hexamethylindenyl) zirconium trichloride, [(Ind*SiMe2nBu)Zr(μ-Cl)Cl2]2. The complexes were characterised by NMR spectroscopy and X-ray crystallography, and the bonding was evaluated using density functional theory. 

Unsymmetrical permethylindenyl bent metallocene complexes have been synthesised and reacted with inorganic solid supports to afford catalysts for the slurry phase polymerisation of ethylene. Those supported on solid polymethylaluminoxane were both highly active catalysts and afforded polymers with a desirable, low aggregation, “popcorn” morphology.

Permethylindenyl constrained geometry complexes of the type Me2SB(RN,I*)TiCl2({(η5-C9Me6)Me2Si(RN)}TiCl2; R = i-Pr, t-Bu,n-Bu, Ph, 4-t-BuPh and 4-n-BuPh). Reaction with solid polymethylaluminoxane (sMAO) yields solid catalysts active for the slurry phase polymerisation of ethylene. 

Solid polymethylaluminoxane (sMAO) supported ansa-bridged permethylindenyl zirconocenes Me2SB(CpR,I*)ZrX2 ({(η5-C9Me6)Me2Si(η5-C5H3R)}ZrX2; R = H, Me, and nBu; X = Cl, Br, Me, and CH2Ph) have been investigated as catalysts for the slurry-phase polymerization of ethylene in the presence of H2. The catalysts demonstrated remarkable stability to H2 both in a high-throughput screening system and in a 2 L batch reactor, with an almost constant ethylene uptake maintained throughout the polymerization runs. The catalysts demonstrated very high ethylene polymerization activities, almost 3 times higher than sMAO-(CpnBu)2ZrCl2 (industrial standard zirconocene catalyst) under the same conditions. The presence of small quantities of H2 (<1%) led to significant decreases in polymer molecular weights to produce commercially desirable polyethylene waxes (Mn < 10 kg mol–1) in a batch reactor.

The synthesis and characterisation of constrained geometry scandium and aluminium permethylindenyl complexes Me2SB(RN,I*)ScCl(THF), Me2SB(iPrN,I*)Sc(O-2,6-iPr-C6H3)(THF), Me2SB(iPrN,I*)Sc(O-2,4-tBu-C6H3)(THF),Me2SB(nBuN,I*)Sc(O-2,6-iPr-C6H3)(THF), Me2SB(PhN,I*)Sc(O-2,6-iPr-C6H3)(THF), Me2SB(tBuN,I*)AlCl(THF), Me2SB(tBuN,I*)Al(O-2,6-Me-C6H3)(THF) and Me2SB(tBuN,I*)Al(O-2,4-tBu-C6H3)(THF) are reported. Ring-opening polymerisation of L- and rac-lactide using all complexes show first-order dependence on monomer concentration and produced polylactide with unimodal molecular weight distribution. 


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A series of substituted phenyl mono-imido complexes of the type W(NR)Cl4(THF) (R = C6H5, 2,6-Me-C6H3, 3,5-Me-C6H3, 2,4,6-Me-C6H2, 4-OMe-C6H4, 2,6-F-C6H3 and 3,5-CF3-C6H3) have been synthesised and characterised. Reaction of these complexes with solid polymethylaluminoxane (sMAO) leads to immobilisation and in situmethylation of the chloride positions on the surface of the support. Reaction of W(NR)Cl4(THF) with trimethylaluminium (TMA) yields the trimethyl complexes W(NR)Me3Cl. 

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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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