Zirconium hydrocarbyl chemisorption on sulfated metal oxides: new supports, chemisorption pathways, and implications for catalysis

The sulfated metal oxides (SMOs) sulfated stannia (SnS), sulfated iron oxide (FeS), and sulfated titanium dioxide (TiS) have been synthesized and examined as support materials/cocatalysts/activators for molecule-based olefin polymerization and hydrogenation catalysis. (13)C CPMAS NMR spectroscopic analysis of Cp(2)Zr((13)CH(3))(2)/SMO chemisorption shows that cationic zirconocenium species are formed along with varying amounts of catalytically inactive micro-oxo (Cp(2)Zr(CH(3))O-surface) species, depending on the support material. Ethylene polymerization data with the supported catalysts show that polymerization activity is dependent on both precursor ligation [Zr(CH(2)Ph)(4) > (Me(5)Cp)ZrMe(3)] and the nature of the support (SnS > FeS > TiS). Poisoning studies were performed in conjunction with ethylene polymerization, mediated by (Me(5)Cp)ZrMe(3) supported on each SMO, and reveal that, for (Me(5)Cp)ZrMe(3)/SnS, 61 +/- 5% of the Zr sites are catalytically significant, while, for (Me(5)Cp)ZrMe(3)/FeS, this quantity is 22 +/- 2%, and for (Me(5)Cp)ZrMe(3)/TiS, 63 +/- 9%. These catalysts are also active for benzene hydrogenation and are separable from liquid-phase products using physical or, in the case of FeS, magnetic techniques.     

Adsorption, diffusion and catalysis of mesostructured zeolite HZSM-5

Adsorption and diffusion properties of n-octane in meso-structured HZSM-5 zeolites were studied by high precision intelligent gravimetric analysis (IGA) and ZLC technology between 293?K and 393?K. As expected, great increase in adsorption capacity and diffusion efficient of n-octane in the mesostructured HZSM-5 zeolites was observed compared with conventional HZSM-5. At the same time, the adsorption activation energy of n-octane in the mesostructured HZSM-5 zeolites was significantly decreased. The adsorption heats with low n-octane loading showed a clear decline with increase of mesoporosity in the zeolite samples. These results clearly indicate that introduction of mesopores into the zeolites offered a short diffusion path and high diffusion rate for reactants and products, which resulted in a high yield of fuel oil and an enhanced resistance against the catalyst deactivation in the reaction of methanol to gasoline.     

The use of thermal analysis in studying catalysis and the chemisorption process

The term catalysis is examined closely and shown in application to a solid phase to be an approximation in that often not only the texture of the catalyst changes but also the composition. However, the action of catalysis still remains perfectly defined in that it opens up a new lower energy path for the reaction considered. The utilisation of thermal analysis tecniques in conjunction with textural studies is shown to be a specially suitable method of investigating catalytic systems. An outline of the necessary thermodynamic and kinetic considerations in catalysis systems is given. Examples of studies on inert supports are given, namely alumina and silica. The occurrence of catalysis in the thermal decomposition of oxysalts is illustrated by reference to oxalates and permanganates. Finally, thermal analysis methods suitable for studying catalysis systems are outlined.     

On the role of d electrons in chemisorption and catalysis on transition metal surfaces

A new model is proposed for the role of the d electrons in chemisorption and catalysis on transition metal surfaces. In this model, the d electrons remain localized on the atoms and do not participate in forming dsp hybrid bonds with the adsorbate. However, electrons in doubly-occupied d orbitals can be promoted to anti-bonding or non-bonding valence orbitals. These additional electronic configurations help increase the binding energy of the adsorbate and help stabilize reaction intermediates. This effect is enhanced by spatial rotation of the singly-occupied d orbitals which become perpendicular to the adsorbate. The singly-occupied d orbitals are also able to recouple their spins during the reaction, allowing the reaction to proceed on otherwise forbidden reaction paths.     

Surface chemistry connecting heterogeneous catalysis,photocatalysis and plasmonic catalysis

正Chemical reactions catalyzed by solid catalysts have recently expanded rapidly from traditional heterogeneous catalytic reactions to photocatalytic reactions and further to plasmonic-catalytic reactions,however,the fundamental understanding of the commonalities and differences among heterogeneous catalysis,     

Hydrogen chemisorption on supported platinum, gold, and platinum-gold-alloy catalysts

Alloyed catalysts receive considerable attention, because of their unique catalytic properties; they often show higher selectivity, activity, and stability compared to the pure metal particles. To provide insights in the origins of these features, we report the structure and the interaction of hydrogen with each of the metals in an intimately mixed platinum-gold catalyst and compare these characteristics to those in the single metal particles. X-Ray absorption spectroscopy (XAS) and electron microscopy analysis showed that the structure of the mixed particle differed from the single metal particles. The interaction of platinum with hydrogen is stronger than the H-Au interaction and the adsorption sites were different. EXAFS analysis showed that the structure of the platinum clusters changes with increasing hydrogen coverage, observed as a relaxation of the contracted Pt-Pt distance and an increase in the Pt-Pt coordination number. No such changes were observed for gold clusters. Well-mixed PtAu-alloy clusters, with a bulk Au-to-Pt ratio of two, supported on SiO(2), adsorb hydrogen on both platinum and gold atoms, which indicates that gold cannot be regarded as an inert metal. The heat of adsorption on the platinum ensembles does not decrease upon alloying; the weakening of the overall hydrogen adsorption strength when alloying platinum with gold is an ensemble-size effect.     

Synthesis, spectroscopy and catalysis of

Chromium acetyl acetonate [Cr(acac)3] complexes have been grafted onto the surface of two mesoporous crystalline materials; pure silica MCM-41 (SiMCM-41) and Al-containing silica MCM-41 with an Si:Al ratio of 27 (AlMCM-41). The materials were characterized with X-ray diffraction, N2 adsorption, thermogravimetrical analysis, diffuse reflectance spectroscopy in the UV-Vis-NIR region (DRS), electron spin resonance (ESR) and Fourier transform infrared spectroscopy. Hydrogen bonding between surface hydroxyls and the acetylacetonate (acac) ligands is the only type of interaction between [Cr(acac)3] complexes and SiMCM-41, while the deposition of [Cr(acac)3] onto the surface of AlMCM-41 takes place through either a ligand exchange reaction or a hydrogen-bonding mechanism. In the as-synthesized materials, Cr3+ is present as a surface species in pseudo-octahedral coordination. This species is characterized by high zero-field ESR parameters D and E, indicating a strong distortion from O(h), symmetry. After calcination, Cr3+ is almost completely oxidized to Cr6+, which is anchored onto the surface as dichromate, some chromate and traces of small amorphous Cr2O3 clusters and square pyramidal Cr5+ ions. These materials are active in the gas-phase and slurry-phase polymerization of ethylene at 100 degrees C. The polymerization activity is dependent on the Cr loading, precalcination temperature and the support characteristics: a 1 wt % [Cr(acac)3]-AlMCM-41 catalyst pretreated at high temperatures was found to be the most active material with a polymerization rate of 14000 g polyethylene per gram of Cr per hour. Combined DRS-ESR spectroscopies were used to monitor the reduction process of Cr(6+/5+) and the oxidation and coordination environment of Cr(n+) species during catalytic action. It will be shown that the polymer chains initially produced within the mesopores of the Cr-MCM-41 material form nanofibres of polyethylene with a length of several microns and a diameter of 50 to 100 nanometers. These nanofibres (partially) cover the outer surface of the MCM-41 material. The catalyst particles also gradually break up during ethylene polymerization resulting in the formation of crystalline and amorphous polyethylene with a low bulk density and a melt flow index between 0.56 and 1.38g per 10 min; this indicates the very high molecular weight of the polymer.     

Preparation, characterization, and catalysis of mecro-catalysts

Because catalysis by metals is a surface phenomenon, many technological catalysts contain small (typically nanometre-sired) supported metal particles with a large fraction of the atoms exposed. Many reactions, such as hydrocarbon hydrogenations, are structure-insensitive, proceeding at approximately the same rates on metal particles of various sizes provided that they are larger than 1 nm and show bulk-like metallic behavior. But the catalytic properties are not known when metal particles become so small that their sizes are indium clusters consisting of several indium atoms. Here the catalytic behavior of precisely defined clusters of just four and six indium atoms on solid supports is shown. It is found that the Ir4 and Ir6 clusters differ in catalytic activity both from each other and from metallic Ir particles.     

Oxygen chemisorption on silver catalysts

Silver dispersity in the catalysts studied has been investigated. Equilibrium values of oxygen adsorption on silver catalysts with different mean sizes of metal crystallites are found and compared.

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Toward the origin of life over feldspar surfaces: Adsorption of amino acids and catalysis of conformational interconversions

Feldspars are a group of tectosilicate minerals terminated with a full layer of hydroxyl groups and have been regarded as “hotbed” for birth of life. In this study, periodic density functional theory calculations are conducted to address two associated issues, as adsorption of amino acids and conformational interconversions over feldspar surfaces. Distribution of glycine isomers depends strongly on the number of interacted surface silanols (n), and canonical isomers exist exclusively for n ≤ 2 while zwitterions predominate for n = 3. Adsorption of amino acids is significantly affected by sidechains, especially those forming H-bonds with surfaces; however, sidechain contacts with feldspar surfaces disfavor zwitterion stabilization, and zwitterions become preferred without sidechain contacts. For all amino acids, conformational interconversions undergo four elementary steps, and proton transfer between two carboxylic-O atoms (step 2) is always rate-decisive. Step 2 is greatly accelerated with catalysis of surface silanols and activation barriers depend significantly on sidechains. All amino acids have moderate activation barriers and conformational interconversions (in both forward and reverse directions) proceed favorably at ambient conditions. Sidechains of amino acids exhibit influences through sidechain contacts with surfaces rather than as substituent effect.     

The catalysis, synthesis and characterization of ferrocenylchalcone

Eight kinds of ferrocenylchalcone derivatives were synthesized by the reaction of acetylferrocene and aromatic aldehyde using solid as catalyst in the experiment. The influrence of the catalysts, the raw material molar ratios and reaction times on the yield of ferrocenylchalcone derivatives are discussed. The optimal conditions of synthesis reaction have been selected: the catalyst was KF/Al₂O₃, n(acetyl ferrocene)/n(formal dehyde) = 1:1.2. The structures of the products were characterized by melting point test, IR, ¹HNMR, ¹³CNMR and elemental analysis.