Quartz catalysis

Another complicating factor is that the surface of quartz is the only asymmetric factor in the metal-quartz catalysis and its specific areas in most cases were very small (only 44 cm /g " ). This coupled with the fact that the amount of metal deposited on quartz was rather high, so the extent of racemization of butanol during reaction would be high, which detracts from the effectiveness of the catalyst. Thus, quartz appeared not to be an effective chiral carrier for catalysis or adsorption in asymmetric experiments. Nevertheless, in general the data using quartz crystals are of interest and received positive evaluations in several publications... 

Evaluation of absolute configuration of molecules using data from "Quartz-catalysis ... 

Results received in the studies of quartz-catalysis allow one to suggest that the mechanism of action of quartz catalysts is a result of as5mimetrie adsorption on the surfaee of optically active quartz. The effeetiveness of asymmetric catalysis is determined by the ability of the catal54ic system to form the labile diastereomeric complex intermediate, whieh will be determined by the extent of spatial correspondenee of the configuration of the reacting molecule to the structure of the surface of the... 

Mardaleishvilli, Sin-Chou, and Smorodin-skaya [Kinetics and Catalysis, 8 (664), 1967] have studied the catalytic decomposition of ammonia on quartz. The following initial rate data were obtained by these investigators at 951 C... 

The benefit of the LbL technique is that the properties of the assemblies, such as thickness, composition, and function, can be tuned by varying the layer number, the species deposited, and the assembly conditions. Further, this technique can be readily transferred from planar substrates (e.g., silicon and quartz slides) [53,54] to three-dimensional substrates with various morphologies and structures, such as colloids [55] and biological cells [56]. Application of the LbL technique to colloids provides a simple and effective method to prepare core-shell particles, and hollow capsules, after removal of the sacrificial core template particles. The properties of the capsules prepared by the LbL procedure, such as diameter, shell thickness and permeability, can be readily adjusted through selection of the core size, the layer number, and the nature of the species deposited [57]. Such capsules are ideal candidates for applications in the areas of drug delivery, sensing, and catalysis [48-51,57]. 

The first reported attempts of what was then called "absolute or total asymmetric synthesis" with chiral solid catalysts used nature (naturally ) both as a model and as a challenge. Hypotheses of the origin of chirality on earth and early ideas on the nature of enzymes strongly influenced this period [15]. Two directions were tried First, chiral solids such as quartz and natural fibres were used as supports for metallic catalysts and second, existing heterogeneous catalysts were modified by the addition of naturally occuring chiral molecules. Both approaches were successful and even if the optical yields were, with few exceptions, very low or not even determined quantitatively the basic feasibility of heterogeneous enantioselective catalysis was established. 

Among some kinds of reactions which are slow on a relevant time scale and in particular environments are certain metal-ion oxidations, oxidation of sulfides, sulfate reduction, various metal ion polymerizations (e.g., vanadium, aluminum), aging of hydroxide and oxide precipitates, precipitation of metal-ion silicates and carbonates (e.g., dolomites), conversions among aluminosilicates (e.g., feldspar-kaolinite), and solution or precipitation of quartz (9). Some of these reactions can be accelerated greatly by biological catalysis (e.g., sulfate reduction, metal ion oxidations) (7). 

The library, on a quartz substrate, consisted of almost 50 catalysts (ca. 200 pg). Each catalyst was heated by C02-laser irradiation to between 300 and 400 °C before reaction. Then, a gas mixture of N2, C2H6 and 02 (molar ratio 5 4 1) was blown to each catalysts. A library consisting of Mo-V-Nb with 10% compositional increments per matrix element was prepared and screened. The binary catalysts of Mo-V-O show low conversion and ethylene selectivity. The presence of Nb increases both activity and selectivity dramatically. The superior catalysts found by the screening were scaled up and showed similar catalysis. 

In earlier work by Wall and Moore on the decomposition of mixtures of C2ll6 + C2D6 the products were followed by mass-spectrographic analysis. The investigators found some evidence of surface catalysis in packed vessels and found too that NO enhances CII4 production and—contrary to the findings of other workers—is rapidly consumed. Their results were similar for reactions in quartz, pyrex, and KCl-coated vessels. They also found that the H2 produced was an almost statistical mixture of H2, HI), and D2 at all stages of the NO-inhibited reaction except possibly the very... 

Fig. II. Cell for the measuiement of work function by the photoelectric method. A, quartz window B, cathode C, wire to anode him D, graded glass seals E, filament F, substance to be adsorbed G, cold trap J, outlet to pumps. From R. Suhrman, Advances in Catalysis, 7 (I9SS) 308.

The effects of different kinds of radiation have been briefly explored. Radiation less prone to produce displacements than o°Co y-rays, namely 50-kv X-rays, produced the effects of y-rays color, adsorptive capacity, and catalytic activity (69). Dosimetry is difficult with soft radiations but qualitatively the efficiencies of these X-rays and of y-rays were similar. Since even 100-kv X-rays do not produce displaced atoms in quartz (65), it is unlikely that 50-kv X-rays do in silica gel. This means that the centers responsible for color, adsorptive capacity, and catalysis do not require displacements, but merely the production of electrons and holes. Ultraviolet light more energetic than 3000 A was sufficient to increase the catalytic activity (69), even stronger... 

The desire to produce enantiomerically pure pharmaceuticals and other fine chemicals has advanced the field of asymmetric catalytic technologies. Since the independent discoveries of Knowles and Homer [1,2] the number of innovative asymmetric catalysis for hydrogenation and other reactions has mushroomed. Initially, nature was the sole provider of enantiomeric and diastereoisomeric compounds these form what is known as the chiral pool. This pool is comprised of relatively inexpensive, readily available, optically active natural products, such as carbohydrates, hydroxy acids, and amino acids, that can be used as starting materials for asymmetric synthesis [3,4]. Before 1968, early attempts to mimic nature s biocatalysis through noble metal asymmetric catalysis primarily focused on a heterogeneous catalyst that used chiral supports [5] such as quartz, natural fibers, and polypeptides. An alternative strategy was hydrogenation of substrates modified by a chiral auxiliary [6]. 

Basic Catalysis. The catalytic properties of alkali zeolites free of acidic sites have been investigated for the cracking of hexanes (25, 26). At 500 C K-Y zeolite cracks easily n-hexane and its isomers resulting in product distributions markedly different from those obtained over acidic zeolites or even by thermal cracking (pyrolysis). Free radical-type mechanism predominates on the zeolite surface. The relative rates of H atom abstraction (bimolecular) and B-scission (unimolecular) are greatly affected by the zeolite matrix. Zeolites also concentrate hydrocarbon reactants within the crystal, which enhances the rate of bimolecular reaction step. Comparison with silicalite (Al-free ZSM-5 zeolite) and quartz chips has been done in order to characterize the zeolitic effect. Silicalite behaves as inert quartz chips with no effect on the rate of H-abstraction step,... 

The structure of quartz has been extensively studied [7-10], Table 2 summarizes structural data for low quartz obtained with the Accelrys Catalysis 3.0.0 software. The continuous connection of oxygen tetrahedra is apparent from its structure illustrated in Figs. 4 and 5 [11,13],... 

The supported Ni catalysts (Ni content in % w/w) were prepared by the precipitation technique (7). The supports (United Catalysis India Ltd., India) were calcined at 773 K for 4 h prior to use. Ammonium carbonate (Loba Chemie, India) was used for precipitating Ni from Ni(N03)2.6H20 (S.D. Fine, India). The catalysts were calcined in a static air furnace at 773 K for 10 h and reduced in an activation furnace using silica-quartz tube at 773 K in H2 flow of 50 cm /min for 10 h. The reduced catalysts were passivated under N2 flow of 30 cmVmin for 2 h. 

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