Catalyst supports quartz

Silica is of particular importance because of its use as a stable catalyst support with low acidity and its relationship to zeolite catalysts, which will be discussed in chapter 4. Silicon is an abundant material in the earth s crust and occurs in various forms including silica. Silica is also polymorphous with the main forms being quartz, cristobalite and trydimite. The stable room temperature form is quartz (Si02). Recently, a new family of stable silica-based ceramics from chemically stabilized cristobalites has been described using electron microscopy (Gai et al 1993). We describe the synthesis and microstructures of these ceramic supports in chapters 3 and 5. 

As previously described silica, which is an important catalyst support, is polymorphous (figure 3.36). Forms of silica other than quartz (Si02), can be stabilized chemically for use as catalyst support materials. Here we describe EM studies of the chemical stabilization of the cristobalite form of silica. It can be used as a stable catalyst support. 

An alternative concept is the so-called direct absorption concept, which applies solar-receiver reactors. According to this concept a solar reformer was developed by DLR in the SOLASYS project (Tamme, 2003) based on earlier experiences in the projects SCR and CAESAR (Bauer, 1994). It allows the concentrated radiation to penetrate through a transparent aperture into the reformer, where it is absorbed directly by the irradiated absorber. The reaction gases pass through the absorber which serves simultaneously as a heat transfer unit and as support for the catalyst. The quartz window, used as aperture closure, enables the reformer to be operated under pressure. A schematic of the solar receiver is shown in Figure 3 (right). The reformer was operated up to 0.9 MPa and 780°C. The pilot reformer was tested at a power level of about 300 kW(th) at the solar tower of the Weizmann Institute of Science in Rehovot, Israel. In a follow-up project SOLREF (Moller, 2006), the operation conditions will be about 1.5 MPa and 950°C. 

The mass variation calculated from the variation of the quartz crystal vibration frequency for an iron phthalocyanine catalyst supported on caibon is presented in Fig. 43. In the potential domain over 0.5 V vs RHE, the mass increases during the positive going scan and decreases during the negative going scan. This confirms that the change in oxidation state of the central metal ion is accompanied by an adsorption-desorption process. Furthermore,... 

The catalysts were prepared by consecutive impregnation with aqueous solutions of Ru(N0)(N03)3 and Mg(N03)2. The support was an activated carbon (commercial one provided by ICASA, Spain, Sbet = 960.7 m g ) purified by treatment with HCl solution, to remove inorganic compounds. For comparative purposes, a ruthenium catalyst supported on a Y-AI2O3 (Puralox condea, Sbet = 191.9 m -g ) was also prepared by similar procedure. The impregnants were dried at 383 K and subsequently reduced. Before reaction and chemisorption measurements, samples were in situ reduced at 673 K for 2 h. Activity, selectivity and stability under reaction conditions were measured at atmospheric pressure in a fixed-bed quartz reactor kept at 823 K by cofeeding CH, CO2 and He as diluent. An equimolecular mixture of CH4 and CO2 (10% CH4, 10% CO2 and balance He) was adjusted by mass flow controllers (Brooks) and passed through the catalyst at a flow rate of 100 cm -min (space velocity = 1.2-10 h ). The effluents of the reactor were analysed by an on-line gas chromatograph with a thermal conductivity detector. 

The Ru supported catalysts (Ru content in % w/w) were also prepared by precipitation technique (8). The supports (Indian Catalysts Ltd.) were directly used. RuCb (S.D. Fine, India) was precipitated using Ammonium hydroxide (S.D. Fine, India). The catalysts were calcined at 393 K for 8 h and reduced in an activation fiimace using a silica-quartz tube at 573 K in H2 flow of 50 cm /min for 7h. The reduced catalysts were passivated under N2 flow of 30 cmVmin for Ih. The detailed specifications of these catalysts (supported Ni and Ru catalysts) are shown in Table 1. 

This chapter summarizes data about the application of chiral metal catalysts supported on optically active quartz crystals in hydrogenation and other reactions. Despite the low enantioselective efficiency of these catalysts, recent result show that almost 100% enantioselectivity results when they are involved in autocatalytic processes. 

Terent ev, A.P., Klabunovskii, E.I., Patrikeev, V.V. (1950) Asymmetric synthesis by means of catalysts supported on right and left quartz. Bold. Akad. NaukSSSR, 74, 947-950, Chem. Abstr. 45, 3798c (1951). 

Chiral solid catalysts usually have two functions, activation and control. The activating function ensures that the solid actually catalyzes a reaction (chemical catalysis), and the control function provides the stereochemical direction that yields the required enantiomer. Early studies were carried out with metallic catalysts supported on inherently chiral solids such as quartz, cellulose (Harada and Yoshida, 1970), and polypeptides (Akabori et al., 1956 Beamer et al., 1967), in which the metal provided the activating function and the support provided the control function. More recent emphasis has been on binding chiral molecules to nonchiral supports. 

The catalyst supporting medium in each layer consists of 1/2 inch by 1 inch milled quartz... 

The MWNTs are synthesized in a fixed bed CCVD reactor at 700°C. A quartz boat containing about 1 g of catalyst is placed in the centre of the reactor fed with a 1-2 l(STP)/min flow of a 50-50% mixture of nitrogen and ethylene. A typical reaction time is 20 min. The used catalyst is Fcx-COy supported on alumina, prepared by impregnation as described elsewhere [10]. The purification of the MWNTs proceeds in two steps (i) the sample is leached with concentrated fluoric acid in order to dissolve the catalyst support and the metallic particles (ii) an acidic KMn04 solution is used to selectively oxidize fte amorphous carbon. After these treatments, the sample is filtered and washed with distilled water, and dried for 48 h in a vacuum oven heated at 120 C. 

For benzene oxidation, V + Mo oxides are usually catalysts. Inert supports such as silica, alumina, alumdum, quartz, pumice, metallicaluminum, etc. have been tried. High-surface-area supports are observed to have a deleterious effect on the benzene oxidation to MA. A low-porosity-catalyst support may peel off, particularly so in a fluidized bed. Catalyst supports of medium porosity are employed. 

Catalytic converter Shell 304H stainless steel Catalyst support 321 or 304H stainless steel Catalyst bedding quartz rock (nonspalling 650 °C) Shell A516 Gr.70 Catalyst support cast iron Catalyst bedding ceramic saddles or rings... 

Asbestos Pumice Kieselguhr (infusorial earth) Bauxite/titanium dioxide Carbon Metal salts, e.g., MgSOt MgCl2 Quartz lumps Contact process. Deacon process, hydrogenation catalyst supports. Fat hardening, hydrogenation catalyst support. Dehydration reactions, catalyst support and cracking catalyst. Support for precious metals. Contact process olefin polymerization. Used as an inert support for catalysts and also as a physical support at the bottom of a catalyst bed. 

Phosphates are the principal catalysts used in polymerization units the commercially used catalysts are Hquid phosphoric acid, phosphoric acid on kieselguhr, copper pyrophosphate pellets, and phosphoric acid film on quartz. The last is the least active and has the disadvantage that carbonaceous deposits must occasionally be burned off the support. Compared to other processes, the one using Hquid phosphoric acid catalyst is far more responsive to attempts to raise production by increasing temperature. 

The catalyst for the in situ FTIR-transmission measurements was pressed into a self-supporting wafer (diameter 3 cm, weight 10 mg). The wafer was placed at the center of the quartz-made IR cell which was equipped with two NaCl windows. The NaCI window s were cooled with water flow, thus the catalyst could be heated to 1000 K in the cell. A thermocouple was set close to the sample wafer to detect the temperature of the catalyst. The cell was connected to a closed-gas-circulation system which was linked to a vacuum line. The gases used for adsorption and reaction experiments were O, (99.95%), 0, (isotope purity, 97.5%), H2 (99.999%), CH4 (99.99%) and CD4 (isotope purity, 99.9%). For the reaction, the gases were circulated by a circulation pump and the products w ere removed by using an appropriate cold trap (e.g. dry-ice ethanol trap). The IR measurements were carried out with a JASCO FT/IR-7000 sprectrometer. Most of the spectra were recorded w ith 4 cm resolution and 50 scans. 

The catalysts were tested for their CO oxidation activity in an automated microreactor apparatus. The catalysts were tested at space velocities of 7,000 -60,000 hr . A small quantity of catalyst (typically 0.1 - 0.5 g.) was supported on a frit in a quartz microreactor. The composition of the gases to the inlet of the reactor was controlled by mass flow controllers and was CO = 50 ppm, CO2 = 0, or 7,000 ppm, HjO = 40% relative humidity (at 25°C), balance air. These conditions are typical of conditions found in spacecraft cabin atmospheres. The temperature of the catalyst bed was measured with a thermocouple placed half way into the catalyst bed, and controlled using a temperature controller. The inlet and outlet CO/CO2 concentrations were measured by non-dispersive infrared (NDIR) monitors. 

For the Pd-silk catalyst,2 PdCl2 was deposited on silk and reduced to Pd° moderate enantioselectivities were obtained for the hydrogenation of a C=C bond (66% enantiomeric excess, ee, which is the difference between enantiomers divided by the sum of enantiomers), but the silk support presented two problems it tended to deteriorate with time on stream and it varied from source to source, so enantioselectivities were not reproducible (Scheme 3.2). On the other hand, deterioration was not a problem with the metal-quartz catalysts. 

Fig. 4. Configuration of a ceramic membrane reactor for partial oxidation of methane. The membrane tube, with an outside diameter of about 6.5 mm and a length of up to about 30 cm and a wall thickness of 0.25-1.20 mm, was prepared from an electronic/ionic conductor powder (Sr-Fe-Co-O) by a plastic extrusion technique. The quartz reactor supports the ceramic membrane tube through hot Pyrex seals. A Rh-containing reforming catalyst was located adjacent to the tube (57).

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