Bernd Tesche

Taking Nanocasting One Step Further: Replicating CMK-3 as a Silica Material

The replication of nanoscale structures by a direct templating process has been used in recent years in several creative ways for the synthesis of carbon replicas of zeolites[1] or ordered mesoporous carbons, such as CMK-1[2] or SNU-1.[3] Such processes rely on the fact that an ordered pore system, provided by the zeolite or ordered mesoporous silica, can be filled with a carbon precursor which is pyrolyzed and the silica leached with NaOH or HF solution. However, the technique is difficult to apply to the synthesis of framework compositions other than carbon, since the leaching of the silica typically also affects the material which is filled into the silica pore system. This problem could possibly be circumvented by not using the silica as the mold, but to instead go one step further and use the mesoporous ordered carbons as templates, which could then easily be removed by combustion or other techniques, as suggested recently.[4] On the macroscale, that is, for the production of photonic crystals, similar approaches are well known, where latex spheres are used as templates which can be removed by calcinations.[5] Also carbon black has been used as a TMtemplate∫, for instance to synthesize mesoporous zeolite single crystals, in which the pores, however, are disordered.[6] In a first attempt to show the feasibility of using ordered mesoporous carbon to synthesize ordered mesoporous oxides, we decided to template mesostructured silica by using an ordered mesoporous carbon. Although this brings one only back to the starting point, that is, a mesoporous silica, it COMMUNICATIONS

Direct imaging of surface topology and pore system of ordered mesoporous silica (MCM-41, SBA-15, and KIT-6) and nanocast metal oxides by high resolution scanning electron microscopy

We report here a detailed study on the surface topology of well-known ordered mesoporous silica (SBA-15, MCM-41, and KIT-6) and a series of nanocast Co 3O 4, Co 3O 4/CoFe 2O 4 composites by high resolution scanning electron microscopy (HR-SEM). Images of the MCM-41 structure were obtained at a resolution of the pore size, as well as a real space image of the gyroid silica surface of KIT-6 for two different aging temperatures, clearly revealing the differences of the aging procedures. By using the low voltage HR-SEM technique with extremely high resolution, we could very clearly show the influence of the template properties on the structure of the nanocast metal oxides.

Alumina-supported CO hydrogenation catalysts prepared from molecular osmium and ruthenium clusters

Os catalysts on γ-Al2O3 supports have been prepared from Os complexes of varying nuclearity, namely H2OsCl6, Os3(CO)12, H4Os4(CO)12 and Os6(CO)18. Characterization by infrared and X-ray photoelectron spectroscopy and transmission electron microscopy provides evidence of the stabilization of well-defined ensembles of Os atoms on the support surface, the ensemble size being determined by the nuclearity of the cluster precursor. The catalysts prepared from H2OsCl6 have a dispersion comparable with that obtained with Os3(CO)12, although the presence of smaller ensembles and single atoms cannot be excluded. The ensemble size may influence activity and selectivity for C2 and higher hydrocarbons in CO hydrogenation at atmospheric pressure and temperatures between 530 and 610 K, but the results suggest that the heterogeneity of the Os species and the chlorine content of the support also influence the catalyst performance. Data obtained with a more highly dispersed Ru/Al2O3 catalyst prepared from Ru3(CO)12 provide the first quantitative comparison between Os and another Group VIII metal catalyst for CO hydrogenation. The Os was approximately one order of magnitude less active than the Ru catalyst, but it was more selective for formation of C2 and C3 hydrocarbons.

Characterization of Hydrophobic Sol-Gel Materials Containing Entrapped Lipases

The entrapment of lipases in hydrophobic sol-gel materials of RSi(OCH3)3 or mixtures of RSi(OCH3)3 and Si(OCH3)4 results in heterogeneous biocatalysts having dramatically enhanced enzyme activities as measured by the esterification of lauric acid by n-octanol in isooctane. These materials have been characterized by solid state NMR studies, revealing the degree of cross-linking. It is shown that this parameter generally does not correlate with relative enzyme activity. Likewise, the specific surface area or the pore size does not seem to be the decisive factor in determining the relative enzyme activities of the lipase-containing hybrid gels and the corresponding SiO2-gels obtained from Si(OCH3)4. Scanning electron microscopic studies (SEM) show that the hybrid gels all have a similar morphology. On the basis of these studies a model is proposed according to which most of the lipase enzyme is entrapped near the surface of the gel particles, where it is readily accessible by substrate molecules.

Noble Metal Nanoparticles Incorporated in Mesoporous Hosts

Noble metal nanoparticle containing [Me]x-MCM-41 were synthesized using surfactant stabilised palladium, iridium and rhodium nanoparticles in the synthesis gel. The materials were characterised with XRD, ICP AES, TA, TEM and nitrogen sorption, which showed that the nanoparticles were present inside the pores of MCM-41. The [Me]x-MCM-41 were found to be active and selective catalysts in the hydrogenation of cyclic olefins such as cyclohexene, cyclooctene, cyclododecene and norbornene.

Ethylene/hexene copolymerization with the heterogeneous catalyst system SiO2/MAO/rac‐Me2Si[2‐Me‐4‐Ph‐Ind]2ZrCl2: The filter effect

The main focus of this study is the ethylene/hexene copolymerization with the silica supported metallocene SiO2/MAO/rac-Me2Si[2-Me-4-Ph-Ind]2ZrCl2. Polymerizations were carried out in toluene at a reaction temperature of 40°C–60°C and the cocatalyst used was triisobutylaluminium (TIBA). The kinetics of the copolymerization reactions (reactivity ratios rE/H, monomer consumption during reaction) were investigated and molecular weights Mw, molecular weight distributions MWD and melting points Tm were determined. A schematic model for the blend formation observed was developed that based on a filtration effect of monomers by the copolymer shell around the catalyst pellet.

The particle-forming process of SiO2-supported metallocene catalysts

Homogeneous metallocene catalysts for the polyolefin production show compared with conventional Ziegler systems remarkable advantages, like the possibilities to regulate the microstructure leading to optimized polymer properties. However, for industrial application it is necessary to immobilize the metallocenes on a heterogeneous support. The parameters which now influence polymerization kinetics, polymer growth, polymer morphology, the decisive particle fragmentation are demonstrated and summarized in a physical and mathematical model.

Unusual ethylene polymerization results with metallocene catalysts supported on silica

With the development of methods to support metallocenes and methylaluminoxane cocatalysts on suitable carriers, it became possible to combine the specific advantages of homogeneous metallocene catalysis with those of heterogeneous Ziegler catalysts in olefin polymerization. By means of ethylene polymerization it could be shown that the method of supporting methylaluminoxane and metallocene on porous silica has a substantial influence on the progress of polymerization. In particular, fragmentation of catalyst particles during polymerization can be circumvented, maintaining the catalyst activity, if active catalyst sites are being generated on the particle surface only. A method of preparation for such newly designed supported metallocene catalysts is presented, where the active catalyst sites are located exclusively on the particle surface. Furthermore, the kinetics of ethylene polymerization and morphology properties prior to and after polymerization are discussed.

Development of a refined poly(propylene) growth model for silica supported metallocene catalyst systems

The poly(propylene) growth process on silica supported metallocene catalysts has been examined by means of electron microscopy and kinetic investigations. It was possible to extend the polymerization process and to identify different polymerization phases with different characteristic kinetic data by lowering the reaction temperature in the slurry phase. EDX analysis of microtome sections of a polymerized model system led to the development of a refined polymer growth model. It was found that the addition of activating hydrogen reduces the initial diffusion limitations as well as a prepolymerization step with 1-octene in the bulk phase.

Formation of a supported-metal catalyst by aggregation of rhodium complexes

Abstract Catalysts have been prepared incorporating mononuclear Rh bonded to pendant monoamine or phosphine groups linked to silica. H 2 at about 7 × 10 4 N/m 2 and 377 ° K caused reduction and aggregation of the metal on the monoamine-modified silica; aggregation took place similarly, but more rapidly, on the phosphine-modified silica. In flow-reactor experiments with ethylene hydrogenation, the metal aggregation was evidenced by increases in activity of each catalyst with time on stream. The Rh crystallites formed by aggregation in the presence of H 2 were characterized by transmission electron microscopy and found to be nearly uniform in size, being 16–17 and 12–15 A, respectively, for the amine- and phosphine-containing catalysts.