Bettina Kraushaar-Czarnetzki

Comparing synthesis routes to nano-crystalline zeolite ZSM-5

Aiming at the identification of a suitable synthesis route to zeolite ZSM-5 with acidic properties and crystal diameters of about 100 nm or less, we have evaluated four different preparation methods. Three of these have been adopted from the literature, and one was developed by ourselves. The products, all of which synthesised from mixtures with a fixed Si/Al ratio of 60, were inspected by means of X-ray diffraction, scanning electron microscopy, laser-Doppler anemometry and temperature-programmed desorption of ammonia. Two methods turned out to be successful and well reproducible. The first one comprises hydrothermal crystallisation from clear solutions under autogenous pressure and has been described previously by van Grieken et al. The other approach is based on the use of colloidal silicalite-1 seed crystals in an open-vessel crystallisation at atmospheric pressure. The crystal size distribution and the Si/Al ratio of the products can be well controlled. The products from both synthesis types could be transferred into the H-form by conventional means without causing collapse of the crystal structure.

Improved processing stability in the hydrogenation of dimethyl maleate to γ-butyrolactone, 1,4-butanediol and tetrahydrofuran

Abstract In the hydrogenation of dimethyl maleate (DMM) to γ-butyrolactone (GBL), 1,4-butanediol (BDO) and tetrahydrofuran (THF), the performance of the process can be negatively affected by (1) fouling and plugging of the unit through deposition of polymeric by-products and (2) activity drop of the Cu/ZnO-based catalysts through structural changes of the active copper phase at reaction conditions. On the basis of thermodynamic data, we calculated the feed compositions required to prevent condensation of reactants and subsequent formation of polyester deposits in the relevant temperature (453– 523 K ) and pressure (1– 7 MPa ) regime. The resulting critical values of the minimum H2/DMM ratios of the feed, when corrected for capillary effects, were found to be in excellent agreement with the limits as experienced in the processing experiments. Conditions for safe and stable gas-phase processing of DMM in a single stage can thus be predicted. The Cu/ZnO-based catalysts were improved by modifying the thermal pre-treatment. As compared to conventional materials of the same composition, they need less runtime to reach stationary performance levels, and their steady state activities are higher. The yield ratio of GBL to BDO can be adjusted through temperature and total pressure because the corresponding hydrogenation attains thermodynamic equilibrium. The subsequent dehydration to THF can be promoted by applying higher temperatures; however, selectivities to tetrahydrofuran remain low over typical Cu/ZnO catalysts unless additional acid sites are implemented.

Effect of Electrolyte Addition on the Colloidal Stability of Aqueous Zeolite Sols

Introduction. ‐ Handling of aqueous sols or suspensions of zeolite crystals is a frequently recurring operation in the manufacturing of zeolite-based catalysts or adsorbents. Starting with the zeolite synthesis in aqueous medium, the typical workup comprises repeated ion exchange in aqueous solutions and, finally, a wet shaping process during which bodies of suitable size, shape, strength, pore texture, and site distribution must be formed. Large agglomerates of crystals are beneficial, when zeolites must be recovered from their mother liquor or from ion-exchange solutions, whereas isolated zeolite single crystals, dispersed in the porous matrix of a shaped particle, are highly desirable when catalytic properties are to be optimized. These opposing requirements for efficient solid-liquid separations, on one hand, and optimum product quality, on the other hand, demand for means to control the particle size in a reversible manner. It is well-known that the stability of particles with respect to aggregation depends on the balance between attractive London-Van der Waals and repulsive electrostatic forces. The magnitude of the electrostatic repulsion as a function of the distance from the particle surface can be influenced. It depends on the ionic strength in the diffusive layer and on the surface potential (Nernst potential), which, in turn, can be altered by adjusting the pH value. While the Nernst potential is not experimentally accessible, the electrokinetic potential at the shear plane, the zeta (z) potential, can be monitored. Dispersions can be regarded as stable, when the zeta potential is higher than ca. j 30 mVj , whereas the particles tend to form aggregates near the isoelectric point (IEP), which is defined as the pH at which the zeta potential is zero. The measurement of the zeta potential, therefore, is a widely used tool to characterize the stability of disperse systems. Electrokinetic data of numerous materials, in particular, of inorganics are documented in the literature.

Numerical investigation of interfacial mass transfer in two phase flows using the VOF method

ABSTRACT A mass transfer model is developed using the volume-of-fluid (VOF) method with a piecewise linear interface calculation (PLIC) scheme in ANSYS FLUENT for a free-rising bubble. The mass flow rate is defined via the interface by Ficks law and added into the species equation as a source term in the liquid phase using the user-defined functions (UDFs) in ANSYS FLUENT. The interfacial concentration field for the mass flow rate is discretized by two numerical methods. One of them is based on the calculation of the discretization length between the centroid of the liquid volume and the interface using the liquid void fraction and interface normal vectors at the interface cells, while in the second method the discretization length is approximated using only the liquid void fraction at the interface cells. The influence of mesh size, schemes, and different Schmidt numbers on the mass transfer mechanism is numerically investigated for a free-rising bubble. Comparison of the developed mass transfer model with the theoretical results shows reasonable and consistent results with a smaller time-step size and with cell size.

Partial Oxidation of Isobutane and Isobutene to Methacrolein Over a Novel Mo–V–Nb(–Te) Mixed Oxide Catalyst

Aiming at a catalyst with improved catalytic properties in the oxidation of isobutane and isobutene to methacrolein and methacrylic acid, a novel MoVNb(Te) mixed oxide catalyst was synthesized. Rietveld analysis revealed that the crystal structure is well distinguishable from the known MoVTeNb mixed oxide catalyst used in the (amm-)oxidation of C3 hydrocarbons. In the conversion of isobutane, activity as well as selectivities to methacrolein and methacrylic acid were low, and the dehydrogenation of isobutane to isobutene was found to be rate determining. However, the novel catalyst is highly suitable for the partial oxidation of isobutene. While selectivity to methacrolein is comparable to that reported for other catalysts, the activity is superior, resulting in very high space-time-yields at temperatures up to 420 °C. The novel mixed oxide is a Mars van Krevelen-type catalyst. Excess oxygen in the feed is required to ensure optimum performance.