C.J.J. den Ouden

Analysis of the formation of monodisperse populations by homogeneous nucleation

A mathematical analysis of the precipitation of monodisperse populations of particles, governed by homogeneous nucleation from solution, is presented. The model incorporates a “hydrolysis” reaction step, homogeneous nucleation of the population, and size-independent linear growth of the particles once formed. The population balance formalism is used to describe the particulate phase, and material balances are developed to account for changes in the solute concentrations in the precursor phase and the active phase. Any statistical parameters of the particulate population may be predicted. It is shown that the success of forming monodisperse populations depends on the magnitude of the interfacial energy of the solution/particulate system. It is also shown that separating the nucleation and growth steps of the process is not an essential feature in the formation of monodisperse populations; however, the length of time the solution concentration is above the nucleation threshold is important. The rate at which the intermediate active solute species is generated affects the induction time significantly, but has little to do with the success of forming a monodisperse particle population, at least in the cases shown here.

Fuel quality effects on particulate matter emissions from light-and heavy-duty diesel engines

This paper gives an update of Shell`s ongoing research on correlations between diesel fuel quality and particulate emissions in both heavy and light duty applications. An exhaust oxidation catalyst selectively decreases the particulate hydrocarbon fraction, leaving the fixed carbon fraction unaffected. This overall particulates reduction mechanism explains why particulate emissions from catalyst vehicles are less sensitive towards changes in fuel quality. An attempt has been made to explain the differences observed between particulate emissions from heavy- and light-duty engines. It is tentatively concluded that differences originate mainly from intrinsic differences between the heavy- and light-duty test cycles. 27 refs., 14 figs., 5 tabs.

COMPUTER SIMULATIONS IN ZEOLITE CATALYSIS RESEARCH

An overview is presented of computer simulations applied to zeolite catalysis research. It is shown that using (numerical) statistical methods one is able to explain features observed in Ni2+ ion exchange. Lattice energy minimisation calculations confirm the validity of the model used. Furthermore, Monte Carlo and molecular dynamics simulations offer opportunities to describe adsorption and mass transport phenomena at a molecular level which is of importance in studies on shape selective catalysis.

On the dynamics of organic-zeolite interactions: Tetramethylammonium in sodalite

The dynamics of tetramethylammonium (TMA) occluded in a β-cage of zeolite sodalite has been investigated in a comparative molecular dynamics (MD) and n.m.r. ( 13 C, 2 H) study. Concerning deuterium n.m.r., this is the first time that measurements on deuterated organics that are occluded during zeolite synthesis have been performed. The main objective was to understand the dynamics of organic-zeolite interactions on a molecular level, which may enhance to the insight into the role of the organic during zeolite synthesis. MD calculations as well as solid-state deuterium and crosspolarization magic angle spinning n.m.r. results indicate that TMA occluded in the sodalite β-cage has a considerable degree of rotational freedom. MD revealed this rotational freedom to be caused mainly by the dynamics of the zeolite lattice; the distance between diametrically opposed oxygen atoms in the β-cage was found to fluctuate by as much as 0.4 A.

Synthesis of High-Silica Sodalite from Aqueous Systems; A Combined Experimental and Model-Based Approach.

Abstract With trioxane as a template, high-Si sodalite (SOD; 20 Calorimetric studies have been performed to estimate the magnitude of this type of interfacial energy contribution to the stability of Si-rich zeolites. The results are compared with earlier calculations.

ION-Exchange of Ni2+ IONS in Siliceous ZSM-5

Ion exchange of Ni 2+ and K + ions in siliceous ZSM-5 with widely varying aluminium contents (20 Si/Al 250) has been studied. Whereas regardless of the aluminium content near-stoichiometric exchange can be achieved with the monovalent K + ions, only limited exchange is observed with the divalent Ni 2+ ions. With decreasing aluminium content the degree of Ni 2+ exchange drops markedly. These results are interpreted using a model which relates the position of the extra framework Ni 2+ ions to a specific configuration of the aluminium ions with regard to each other in the lattice.