Cees Kwakernaak

On the Synergistic Catalytic Properties of Bimetallic Mesoporous Materials Containing Aluminum and Zirconium: The Prins Cyclisation of Citronellal

Bimetallic three-dimensional amorphous mesoporous materials, Al-Zr-TUD-1 materials, were synthesised by using a surfactant-free, one-pot procedure employing triethanolamine (TEA) as a complexing reagent. The amount of aluminium and zirconium was varied in order to study the effect of these metals on the Brønsted and Lewis acidity, as well as on the resulting catalytic activity of the material. The materials were characterised by various techniques, including elemental analysis, X-ray diffraction, high-resolution TEM, N2 physisorption, temperature-programmed desorption (TPD) of NH3, and 27Al MAS NMR, XPS and FT-IR spectroscopy using pyridine and CO as probe molecules. Al-Zr-TUD-1 materials are mesoporous with surface areas ranging from 700–900 m2 g−1, an average pore size of around 4 nm and a pore volume of around 0.70 cm3 g−1. The synthesised Al-Zr-TUD-1 materials were tested as catalyst materials in the Lewis acid catalysed Meerwein–Ponndorf–Verley reduction of 4-tert-butylcyclohexanone, the intermolecular Prins synthesis of nopol and in the intramolecular Prins cyclisation of citronellal. Although Al-Zr-TUD-1 catalysts possess a lower amount of acid sites than their monometallic counterparts, according to TPD of NH3, these materials outperformed those of the monometallic Al-TUD-1 as well as Zr-TUD-1 in the Prins cyclisation of citronellal. This proves the existence of synergistic properties of Al-Zr-TUD-1. Due to the intramolecular nature of the Prins cyclisation of citronellal, the hydrophilic surface of the catalyst as well as the presence of both Brønsted and Lewis acid sites synergy could be obtained with bimetallic Al-Zr-TUD-1. Besides spectroscopic investigation of the active sites of the catalyst material a thorough testing of the catalyst in different types of reactions is crucial in identifying its specific active sites.

High temperature oxidation behaviour of Ti2AlC ceramic at 1200°C

Abstract The oxidation process of Ti2AlC ceramics in dry synthetic air at 1200°C was monitored with thermogravimetry. The microstructural evolution of the oxide scale with time was characterised by X-ray diffractometry, scanning electron microscopy, X-ray microanalysis and electron back scattering diffraction. The oxide scale is comprised of a continuous α-Al2O3 inner layer with isolated coarse TiO2 particles on top. At the early oxidation stage (roughly less than 0.5 h), α-Al2O3 and TiO2 are the main reaction products, while at longer reaction times the oxidation only leads to the formation of α-Al2O3. The α-Al2O3 grains in the oxide scale grow in size upon high-temperature oxidation with the grain size being uniform throughout the thickness of the scale. As diffusion of oxygen along the grain boundaries dominates the oxide scale growth, the change in grain size affects the oxide scale growth kinetics. A simple oxide scale growth model, that takes into account this change in fast diffusion paths, describes the experimentally observed oxide scale growth kinetics perfectly.

Pyrococcus furiosus 4Fe-ferredoxin, chemisorbed on gold, exhibits gated reduction and ionic strength dependent dimerization.

Pyrococcus furiosus ferredoxin is a small metalloprotein that shuttles electrons between redox enzymes. In its native 4Fe–4S form the protein is highly thermostable. In addition to three cluster-ligating cysteines, two surface cysteine residues (C21 and C48) are present. We used the reactivity of these surface thiols to directly immobilize ferredoxin on a bare gold electrode, with an orientation in which the cluster is exposed to solution. Voltammetry, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) studies established the immobilization of the 4Fe form. Native and recombinant wild-type ferredoxins were compared with the C48S, C21S, and C21S/C48S mutants. The variants with one and two surface cysteines can be directly chemisorbed on bare gold. Cyclic voltammetry demonstrated that the reduction potentials are similar to those in solution. The interfacial electron transfer kinetics revealed that the reduction is gated by the interconversion between two oxidized species. AFM images showed that dimers are chemisorbed at low ionic strength, while monomers are present at high ionic strength. XPS spectra revealed the presence of S, Fe, C, N, and O at the surface, which are assigned to the corresponding atoms in the peptide and the cofactor. Analysis of the sulfur spectrum corroborates that both C21 and C48 form gold–thiolate bonds. Moreover, two inorganic sulfide and two iron species were identified, suggesting an inhomogeneous charge distribution in the 4Fe–4S cluster. In conclusion, P. furiosus ferredoxin can be directly and vectorially chemisorbed on gold with retention of its properties. This may provide a biocompatible electrode surface with docking sites for redox enzymes.