SAMs catalysts

A system has been constructed which allows combined studies of reaction kinetics and catalyst surface properties. Key elements of the system are a computer-controlled pilot plant with a plug flow reactor coupled In series to a minireactor which Is connected, via a high vacuum sample transfer system, to a surface analysis Instrument equipped with XFS, AES, SAM, and SIMS. When Interesting kinetic data are observed, the reaction Is stopped and the test sample Is transferred from the mlnlreactor to the surface analysis chamber. Unique features and problem areas of this new approach will be discussed. The power of the system will be Illustrated with a study of surface chemical changes of a Cu0/Zn0/Al203 catalyst during activation and methanol synthesis. Metallic Cu was Identified by XFS as the only Cu surface site during methanol synthesis. 

Advantages of silicon x-radiation include the access of aluminum and magnesium core level (Is) lines and the corresponding (KLL) Auger transitions for chemical state identification and improved quantitation, because these lines are at least 10 times more intense than the corresponding (2p) or (2s) lines. The construction of an off-axis reactor has produced a simple, versatile and inexpensive system easily adapted to any vacuum system. The role of AES and SAM in catalyst research will also be highlighted by examples. 

The application of surface analytical techniques, most notably X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), or its spatially resolved counterpart. Scanning Auger Microanalysis (SAM), is of great value in understanding the performance of a catalyst. However, the results obtained from any of these techniques are often difficult to interpret, especially when only one technique is used by itself. 

For the copper/aluminum catalyst analyses later described, the sample mount was transfered from the XPS system following treatment and analysis to an inert atmosphere dry box without air exposure. From there individual pellets were transferred to the SAM for subsequent analysis without air exposure. The reverse process was employed for the next reaction cycle. 

Relatively little work has been done on ORR catalysis by self-assembled mono-layers (SAMs) of metalloporphyrins. The advantages of this approach include a much better defined morphology, structure, and composition of the catalytic film, and the surface coverage, and the capacity to control the rate at which the electrons ate transferred from the electrode to the catalysts [CoUman et al., 2007b Hutchison et al., 1993]. These attributes are important for deriving the catal5d ic mechatfism. The use of optically transparent electrodes aUows characterization of the chemical... 

In a typical experimental arrangement, the injection block heater of the gas chromatograph is used to heat a short catalyst bed containing platinum, palladium, copper or nickel coated on a diatomaceous support. The catalyst bed can be the top portion of a packed column or a precolumn connected to a packed or open tubular column. Hydrogen carrier gas flows through the heated catalyst bed (220-350 0) and then into the column. The sam B is injected by... 

Crown ethers have been used successfully as phase-transfer catalysts for liquid-liquid and liquid-solid oxidation reactions. Sam and Simmons (1972) observed that potassium permanganate can be solubilized in benzene by dicyclohexyl-18-crown-6 to yield concentrations as high as 0.06 M. From... 

In many supported catalytic systems, it is nearly impossible to determine either the specific species, responsible for the observed catalytic activity, or the mechanistic pathway of the reaction. Using a defined SAM system in which careful molecular design is followed by controlled deposition into a solid-supported catalyst of known morphology, surface coverage, mode of binding and molecular orientation, allows direct correlation of an observed catalytic activity with the structure on the molecular scale. SAM and LB-systems allow detailed and meaningful studies of established surface bound catalysts to understand their behavior in heterogeneous... 

A Au-coated substrate is another model surface, to which many surface characterization methods can be applied. To achieve surface-initiated ATRP on Au-coated substrates, some haloester compounds with thiol or disulflde group were developed [80-84]. Self-assembled monolayers (SAM) of these compounds were successfully prepared on a Au-coated substrate and used for ATRP graft polymerization. Because of the limited thermal stability of the S - Au bond, the ATRP was carried out at a relatively low temperature, mostly at room temperature, by using a highly active catalyst system and water as a (co)solvent (water-accelerated ATRP). 

The detection of many analytes on unmodified electrodes occurs at high potentials, hence the need for modified electrodes. Even though different types of SAMs have been reported, only a few of them have been employed as catalysts for electrochemical reactions. Most work has concentrated on the characterization of SAMs, with limited studies devoted to detection. Table 3 summarizes the analytes which have been detected by employing NIR-absorbing MPcs (such as MnPc and TiPc) containing amino or alkylthio substituents. In Table 3, the electrodes are modified by SAMs, polymers, electrodeposion, and adsorption. 

Table II briefly summarizes the results of methanol conversion conducted in a pulse reactor using a variety of M -TSMs as catalysts (20). Each sam-...

When in situ dosing onto two different samples of Pt/silica (45, 48) and onto Cu/MgO was used (49), no evidence for spillover was found from NMR. Only one detailed study based on fully relaxed spectra led to observation of a non zero spillover (4T). In a Ru/Si02 catalyst, the silanol protons w ere exchanged for deuterons, the sam.ple was evacuated at 623 K, and a reference NMR spectrum w as taken at room, temperature. The sample was then exposed to 20 Torr of H2, an NMR spectrum was taken, and the difference with respect to the reference was calculated (line in Fig. 14). This represents the sum of reversible and irreversible hydrogen on the metal (resonating at -65 ppm) and spilled over on the support (at about 3 ppm). Then the sample was pumped out at room temperature for 10 min, and again a difference spectrum with the reference state was obtained (dashed line in Fig. 14) this represents irreversible hydrogen both on the support and on the metal. Similar in situ NMR techniques were used... 

Faller, J. W., Sams, D. W. I., Liu, X. Catalytic Asymmetric Synthesis of Homoallylic Alcohols Chiral Amplification and Chiral Poisoning in a Titanium/BINOL Catalyst System. J. Am. Chem. Soc. 1996,118, 1217-1218. 

GRACE Catalysts incorporating SAM matrix technology have exhibited commercially high activity (67-73 MAT) and low coke and gas selectivities with very high levels of Ni+V (8000 -12000 ppm)... 

The pore size distribution of the catalyst matrix is important for the catalytic performance. The optimal matrix pore size distribution will depend on a balance of mesopores and macropores depending on feedstock quality and reactor conditions (e.g. conventional vs. short contact time riser operation). SAM-technology catalysts (SPECTRA, RESIDCAT, ULTIMA) exhibit different pore size distributions that are matched to various types of feedstock and unit conditions. Figure 5 exhibits typical pore size distribution of SPECTRA-944, SPECTRA-444 and ULTIMA-444 catalysts. Since the only differentiating characteristic of these three catalysts is the matrix formulation, the pore size distribution variation is characteristic of the different matrix design ... 

The selectivity improvements of the SAM-200 containing ULTIMA catalysts are especially pronounced when the catalyst is metallated to simulate the equilibrium catalyst conditions in a high metals environment arising from processing of heavy feeds in the FCC Unit. Table 5 summarizes Riser Pilot Plant (DCR) results of a competitive Resid Catalyst versus ULTIMA-445 after Cyclic Metals Impregnation of the catalysts to 5000 ppm Ni+V. 

GRACE Davison has developed "Selective Active Matrix" catalysts (SAM-technology) based on structured Reactive Aluminas on Alumina-Sol binder. These matrices provide unique properties in cracking heavier and metals contaminated FCC feeds with minimum coke and gas yields. The choice of the specific SAM matrix formulation for any application will depend on feed quality and metals contamination as well as unit riser configuration. 

Both these catalysts are extremely suitable for "Short Contact Time" riser designs where high activity is desired via the combination of high concentration of RE-USY zeolite with Selective Active Matrices. Excellent commercial results with SAM matrix catalysts recently obtained in several "Short Contact Riser" FCC units in Europe have confirmed the advantages of this technology[5]. 

Studies on three different iron—sulfur enzyme systems which all require S-adenosylmethionine (SAM) — lysine 2,3-aminomutase, pyruvate-formate lyase, and anaerobic ribonucleotide reductase — have led to the identification of SAM as a major source of free radicals in living cells (for a recent review, see Atta et ak, 2010). As in the dehydratases, these systems have a [4Fe—4S] centre chelated by only three cysteines with one accessible coordination site. The cluster is active only in the reduced state [4Fe—4S] and appears to combine the two roles described previously, serving both as a ligand for substrate binding and as a redox catalyst (Figure 13.19). Their mechanism again requires that the exposed iron atom of the cluster shifts towards octahedral geometry as it binds... 

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