J.H.C. van Hooff

Why are some titanium silicalite-1 samples active and others not?

Titanium silicalite-1 samples of different particle size are synthesized. X-ray diffraction (XRD), infrared (IR),29Si magic angle spinning nuclear magnetic resonance (MAS NMR), diffuse reflectance electron absorption spectrometry (DREAS), thermogravimetric analysis (TGA), pore volume and particle size measurements are used to characterize these samples. In the hydroxylation of phenol with hydrogen peroxide large differences in activity are found between different TS-1 samples. Using XRD, IR,29Si MAS NMR, and DREAS measurements it is impossible to differentiate between active and less active samples. Particle size determination (directly by scanning electron microscopy (SEM), Coulter Counter, and Malvern and indirectly by TGA and adsorption measurements) however can explain these differences. Smaller particles are more active than larger particles. From calculations of the Weisz modulus it can be concluded that large zeolite particle are not fully utilized because of pore diffusion limitations. The product distribution is also influenced by particle size.

Selective isomerisation of n-butene by crystalline aluminophosphates

The results presented in this paper demonstrate that Me-AlPO4-11s are active, selective and stable catalysts for the selective isomerisation ofn-butene to isobutene. A suggestion is given for an active site. The results support a monomolecular reaction mechanism. Absorption experiments ofn-butene on Me-AlPO4-11 and Me-AlPO4-5 are presented, indicating that under the reaction conditions butene polymerisation occurs on Me-AlPO4-5 leading to a rapid deactivation.

A test reaction for titanium silicalite catalysts

The catalytic hydroxylation of phenol to hydroquinone and catechol can be used for checking the quality of titanium silicate catalysts since the selectivity of these catalysts is strongly affected by the presence of small amounts of non-framework titania. Titanium silicates prepared by modification of [Al]ZSM-5 exhibits the same catalytic properties as hydrothermally synthesized TS-1 of high purity.

In situ measurements of the electrical conductivity of bismuth molybdate catalysts in operation for oxidative dehydrogenation of butene

Abstract In situ measurement of electrical conductivities were performed on MoO 3 , Bi 2 Mo 2 O 9 , and Bi 2 MoO 6 (koechlinite), the latter either as such or doped with small amounts of Mo or Bi; the reaction applied was the oxidative dehydrogenation of butene to butadiene. The reaction was performed in continuous flow or by pulsing butene over the catalyst, often followed by the addition of O 2 pulses. The catalysts were characterized by surface area, XRD, and XPS. The latter measurements showed that surface Bi Mo ratios were often different from those in the bulk; reduction by butene at temperatures around 673 K sometimes led to considerable changes in the surface Bi Mo ratio that ran parallel with changes in activity. Pulsing butene in a He carrier stream over the catalyst strongly increased the electrical conductivity while pulsing O 2 decreased it again; the conductivity is almost entirely located in the surface layers. The koechlinite-type catalysts, when heated in He, showed a considerably higher conductivity than the other catalysts. This is ascribed to a dissociation of O 2 presumably from the Bi 2 O 2 layers. Subsequent pulsing of butene first increased the conductivity but for catalysts with Bi Mo bulk ratios >1, the conductivities became constant while reduction continued. The first process is supposed to be connected with a stripping of the oxygen in the surface layer, the second with a migration of O 2− from bulk to surface and of electrons from surface to bulk. A model was presented for the surface band.

Chapter 7 Techniques of Zeolite Characterization

Publisher Summary This chapter discusses that before a zeolite can be used for a certain application it is necessary to characterize this zeolite, to see if it has the desired properties for that application. If not, another synthesis method should be used or the zeolite must be modified, to meet the specifications. Zeolite synthesis, modification, characterization and –application are thus strongly related. Not all zeolite properties are of the same importance for every application. This aspect explains importance for the main applications of three different zeolites. For example for the application of zeolite A in detergents particle size and morphology are extremely important while acidity and stability play a minor role. These characteristics, however, are just of crucial importance for the application of zeolite Y in cracking catalysts. For this reason, it discusses the different characterization methods in relation to the application for which the specific property is important.

Information processing during cardiac surgery: an event related potential study

The aim of this study was to investigate whether information processing persists during general anesthesia, and if so, to determine the relationship between the degree of cognitive processing measured during anesthesia and the presence or absence of intraoperative memories measured after anesthesia. Subjects were 12 patients, undergoing cardiac surgery with propofol/alfentanil anesthesia. During several periods of the operation, event related potentials (ERPs) to frequent and infrequent tones of different pitch were analyzed. After the operation, a word recognition task with ERP recording was administered to determine whether intraoperatively presented words would elicit a (covert) recognition reaction in the brain. ERP wave forms could be obtained during the intraoperative recording periods but differed substantially from those in the awake state. The presence of ERP components up to 500 msec after stimulus presentation suggests that auditory information processing continued during anesthesia up to a certain level of cognition. Intraoperative ERPs to frequent and infrequent tones were not different from each other implying that differences in pitch could not be detected. The postoperative results demonstrated evidence for intraoperative memories in 3 patients. For 2 of these 3 patients, low propofol levels as well as reliable ERPs with large amplitudes were found close to the moment of information presentation. The results emphasize the importance of combining intra- and postoperative measurements and suggest that late ERP components might be used as indicators of an increased risk of auditory perception.

A 69Ga and 71Ga MAS NMR study of the gallium analogue zeolite ZSM-5

69Ga and 71Ga magic-angle-spinning (MAS) NMR spectra of a series of gallium analogue zeolites H—(Ga)ZSM-5, with varying Si/Ga ratios, were obtained at 14.1 and 7.1 T. The quadrupole coupling constant e2qQ/h was deduced from the shift difference of the centre of gravity of the 69Ga and 71Ga resonance. An analysis of the linewidth as a function of the magnetic field shows that the width is not only determined by the second-order quadrupolar broadening, but also by a distribution of chemical shifts. The values obtained for the quadrupole coupling constant and chemical shift distribution are compared to those found for 27Al in H—(Al)ZSM-5. It is argued that the observed differences are related to the different NMR properties of the isotopes, and do not point to any structural changes of the ZSM-5 lattice.

Paramagnetic platinum and oxygen species on supported platinum

EPR was used to determine the paramagnetic platinum and oxygen species which are present at various stages in the genesis of a supported platinum catalyst and to determine the effect of the support on the formation of such species. Paramagnetic platinum centers, believed to be due to Pt 8+, are formed when Pt(NHa)4+2/A1203 is calcined in 02; for low metal contents a large fraction of the platinum is in the form of these species. Catalysts prepared by the same techniques with SiO2 as a support show no paramagnetie platinum signals. Adsorption of O2 on the Pt/A120a catalysts results in the formation of a 3-g 02- signal and a second 2-g signal due to O- and results in a corresponding reduction in the platinum signal. Again Pt/SiO2 did not show comparable behavior. The results are interpreted in terms of electron transfer to the AlcOa.

Distribution of cobalt in CoAPO-5 and CoAPO-11 studied by 31P NMR

Abstract A1PO-5 and A1PO-11 containing cobalt were studied by diffuse reflectance electronic absorption spectroscopy in combination with 31 P NMR. Quantitative 31 P NMR and relaxation measurements (Carr—Purcell—Gill—Meiboom procedure) were performed on calcined samples with increasing amounts of paramagnetic cobalt on lattice positions. The isomorphous substitution of aluminium by cobalt leads to NMR-invisible phosphorus (up to 30%); at least the first and third sphere (P) have to become NMR invisible to explain the observed losses for the lowest cobalt loadings. The amount of undetectable phosphorus, however, does not increase proportionally to the cobalt content, suggesting clustering of cobalt on lattice positions, as previously reported for CoAPO-50. The 31 P T 2 relaxation behaviour of NMR-detectable phosphorus is also influenced by framework cobalt. The measured T 2 decays were fitted with a biexponential Lorentzian function. Both the T 2 values, as well as the relative contributions, change with increasing amounts of lattice cobalt. Impregnated samples (containing extralattice cobalt) behave essentially like normal aluminophosphates. This is probably due to the poor dispersion of cobalt throughout the crystal in the impregnated samples.

Application of CoAPO-5 molecular sieves as heterogeneous catalysts in liquid phase oxidation of alkenes with dioxygen

Incorporation of divalent cobalt into the lattice of AIPO-5 yields a catalyst that can be used in the liquid phase epoxidation of (cyclo)alkenes at mild temperatures with dioxygen as oxidant and in the presence of a sacrificial aldehyde.