Th. Maschmeyer

Synthesis, characterization and catalytic testing of a 3-D mesoporous titanosilica, Ti–TUD-1

Abstract An active and stable mesoporous titanosilica, denoted as Ti–TUD-1, has been synthesized using a cheap, small and multifunctional chemical, triethanolamine (TEA), instead of surfactants. TEA not only is applied as a mesopore template and as a director of positioning Ti-sites onto the internal mesopore surface via the formation of titanium–TEA complexes, but also stabilizes titanium sources, which ensures the possibility of forming isolated tetrahedrally co-ordinated Ti-sites. Ti–TUD-1 features three-dimensionally randomly connected mesopores with walls of about 2.5–4 nm in thickness. Its surface area can reach about 700–1000 m 2 /g and its pore sizes are tunable. After calcination at 1000°C and boiling in water, it kept a high integrity, a high surface area and only a slight change in pore sizes occurred. UV–VIS spectra revealed that tetrahedrally co-ordinated Ti-sites are quite stable under thermal and hydrothermal treatments. It also showed about 5.6 times higher activity for cyclohexene epoxidation than the framework-substituted Ti–MCM-41 and similar activity compared to Ti-grafted MCM-41.

New insights into the structure of supported bimetallic nanocluster catalysts prepared from carbonylated precursors: a combined density functional theory and EXAFS study

Abstract An ensemble of Ru 12 Cu 4 C 2 bimetallic clusters derived from organometallic precursors and anchored within mesoporous silica is investigated with molecular mechanics (MM) and ab initio techniques. The computational study, guided by experiment, yields a representative cluster structure which is compared with EXAFS data. The comparison yields possible insights into the effects of cluster structure upon the thermal decomposition of the carbonyl ligands. Density functional theory (DFT) calculations suggest an unexpected structural modification of the silica-supported cluster upon removal of the attendent carbonyl ligands, in particular the displacement of the central carbido atoms.

A new templating method for three-dimensional mesoporenetworks

A new templating method using small, inexpensive non-surfactant chemicals has been developed facilitating the synthesis of hydrothermally stable foam-like mesopore networks with high surface areas.

Molecular-dynamics analysis of the diffusion of molecular hydrogen in all-silica sodalite

In order to investigate the technical feasibility of crystalline porous silicates as hydrogen storage materials, the self-diffusion of molecular hydrogen in all-silica sodalite is modeled using large-scale classical molecular-dynamics simulations employing full lattice flexibility. In the temperature range of 700-1200 K, the diffusion coefficient is found to range from 1.610(-10) to 1.810(-9) m(2)/s. The energy barrier for hydrogen diffusion is determined from the simulations allowing the application of transition state theory, which, together with the finding that the pre-exponential factor in the Arrhenius-type equation for the hopping rate is temperature-independent, enables extrapolation of our results to lower temperatures. Estimates based on mass penetration theory calculations indicate a promising hydrogen uptake rate at 573 K.

Highly reproducible high-flux silicalite-1 membranes: optimization of silicalite-1 membrane preparation

Abstract Silicalite-1 membranes were prepared on a TiO 2 coated porous stainless steel support. Different thicknesses of the membranes were achieved by changing the synthesis temperature. Increasing the crystallization temperature resulted in the formation of a monolith-type layer, which is close to a perfect microporous phase (without pores between crystals forming the layer). The silicalite-1 membranes were characterized by permeation measurements using single gases and a mixture of n -butane and i -butane in a Wicke–Kallenbach set-up. A direct relationship between the membrane thickness and the selectivity of n -butane to iso-butane was observed; the selectivity improved with an increase in the membrane thickness. The improvement in the selectivity was correlated with decreasing the intercrystalline spaces between the crystals forming the membrane. The best performing membranes were synthesized in the temperature range of 453–463 K. The competitive adsorption of the butanes at 303 K was governing the separation properties of the membranes. The selectivity for n -butane in a 50:50 n -butane/iso-butane mixture was as high as 55, and the flux equal to 2.75 mmol/m 2 per s (WK method at 101 kPa, 303 K). The ideal selectivities, calculated from the single component measurements towards n -butane, were between 33–48 and n -butane fluxes between 7–12 mmol/m 2 per s (WK method at 101 kPa, 303 K). Small variations in the selectivity performance of the membranes synthesized under the same conditions show that the optimized preparation method was highly reproducible.

Regarding pressure in the adsorber of an adsorption heat pump with thin synthesized zeolite layers on heat exchangers

Abstract During the heating period of the adsorber in a water–zeolite adsorption heat pump, water desorbs from the zeolite. As a result, the adsorber pressure mainly increases because of the rise in vapor mass. Zeolite layers synthesized on metal supports in an adsorber have a low amount of zeolite with respect to the void volume, because they are thin. A small amount of zeolite has only a low adsorption capacity for the necessary pressure change. This communication demonstrates that, for thin zeolite layers with respect to the void volume of the adsorber, the pressure does not rise by following the isosteric line, but rises with a much smaller slope. Previously calculated heat pump performances with thin zeolite layers did not take into account this pressure effect which has a substantial and negative influence on the performance.

Two-ring vibrational modes on silica surfaces investigated via fully coordinated nanoclusters

Abstract Vibrational modes of rings containing two silicon atoms and two oxygen atoms, two-rings, as found on a variety of silica surfaces, are modelled with fully connected (SiO2)12 clusters containing no terminating groups. Such clusters naturally reflect the embedding of surface two-rings in a silica matrix without the need for large calculations of silica surface layers. The relatively small size of the clusters allows us to employ the recommended high levels of theory for vibrational calculations, but are sufficiently large to study a range of two-ring-containing conformations. The calculated spectra for the clusters display many peaks in the experimental window, with some in excellent frequency and intensity agreement with measured bands. The results are discussed with respect to the structural nature of the clusters and the possibility of collective two-ring surface modes.

Computational modeling of active sites in heterogeneous catalysts

Industrial catalysts often owe their remarkable activity, selectivity and reliability to many years of gradual improvement and optimization. Such research largely relies on physically/chemically motivated systematic variations of important parameters such as catalyst composition and working conditions, but often there is only moderate emphasis placed on the elucidation of the fundamental reasons for a catalysts success. This “trial-and-error” approach is chosen not because of any strong reluctance to discover a catalysts intrinsic workings but, rather, because modern catalysts are extremely complicated systems, making fundamental investigations expensive in time and money and with no guarantee of useful results. One reason for this is that, from the theoretical analysis point of view regarding such complex systems, it can appear almost impossible to distinguish the catalytically important and active aspects from the redundant and passive. The assumption that this division of role can be made, however, lies at the root of most intuitive ideas relating to catalytic activity. In this article, we aim to illustrate that by combining computational approaches with this conceptual division of role much benefit can be derived. Such considerations are based around the general concept of localized active sites, as distinguished from the supporting liquid or solid environments of the catalyst, which are assumed to be relatively inert but not without influence. We will show how this concept can be used as the starting point of modern computational modeling techniques, which can be applied at a number of different levels to heterogeneous catalytic systems, pointing the way to a more efficient approach to catalyst optimization and understanding than trial-and-error. Our account will culminate in one of the most computationally extensive descriptions of an active site yet achieved.

New routes to synthesis of reproducible, high-performance supported silicalite-1 membranes

Unsupported and supported silicalite-1 zeolite membranes were synthesised in a highly reproducible manner. The type of silica sources used in the syntheses determines the thickness and porosity of the layers. Tetraethylorthosilicate was found to be the best silica source for making dense, well-intergrown membranes without any visible macropores between the crystals that form the membranes. These synthetically optimised zeolite membranes are characterised by significantly higher n-butane fluxes compared to the highest values reported in the literature (at a given n-butane to iso-butane flux ratio).

Hydrogen storage and diffusion in 6-ring clathrasils

Abstract In this paper the influence of geometry of five different 6-ring clathrasils on the diffusion rate of hydrogen through these material is investigated in order to determine the most suitable clathrasil for the storage of hydrogen at ambient temperature and pressure by means of encapsulation. The diffusion is estimated via transition state theory and the energy barries are calculated by means of energy minimisations using empirical force fields. A special aspect herewith is that the zeolite framework is modelled in a fully flexible fashion. The calculations show that sodalite (SOD) has the highest hydrogen diffusion rate. SOD also has the highest available void space. A rough estimation for the maximum hydrogen storage capacity in all silica sodalite is 2.5 wt%.