Support mullite

The formation of mullite at ca 1000 °C is supported by the XRD data shown in Figure 12. Thus, based on DTA, TGA and XRD, one might decide that phase pure mullite without carbon contamination is produced by pyrolysis of precursor to 1000 °C. This then would represent a processing goal for making mullite materials. Also, it appears to be similar to the SiC precursor results, where nanocrystalline, phase pure SiC is obtained... 

Typical substances that find wide use as high-area supports include silica gel and y-alumina, which can be obtained with surface areas in the range 100-800 m2/g. Materials used as low-area supports ( 1 m2/g) include a-alumina and mullite (alumina-silica). It is not easy to make general statements about the preparation of industrial catalysts because of the great variety of forms they take, but in many cases one can distinguish between the chemical operations in which the various components are assembled in the desired form, and the fabrication step in which they are made into the desired shape. The first step will be illustrated by a description of the method of preparing of silica gel and y-alumina support material [1]. 

SEM (H51) showed that the glass in one pfa cement was heavily etched after 7 days and that many of the particles up to 1-2 pm in size were consumed within 28 days. The crystalline phases appeared to be inert. A QXDA study of another pfa cement (D12) confirmed an earlier report (K44) that the ratio of mullite to quartz does not change on hydration, and supported the view that the crystalline phases in pfa do not react significantly within 1 year. In contrast, a TEM study (R42) of a 1-year-old specimen indicated that some, at least, of the quartz reacts at a rale comparable to that of the glass. 

Different supports are used, (see Section 10.6.4) with different geometry (discs or tubes), thickness, porosity, tortuosity, composition (alumina, stainless steel, silicon carbide, mullite, zirconia, titania, etc.), and symmetry or asymmetry in its stmcture. Tubular supports are preferable compared to flat supports because they are easier to scale-up (implemented as multichannel modules). However, in laboratory-scale synthesis, it is usually found that making good quality zeolite membranes on a tubular support is more difficult than on a porous plate. One obvious reason is the fact that the area is usually smaller in flat supports, which decreases the likelihood of defects. In Figure 10.1, two commercial tubular supports, one made of a-alumina (left side) and the other of stainless steel (right side) used in zeolite membrane synthesis, are shown. Both ends of the a-alumina support are glazed and both ends of the stainless steel support are welded with nonporous stainless steel to assure a correct sealing in the membrane module and prevent gas bypass. 

As a result of these reactions a mixture of C0-I-H2+C02-I-CH4+H20 is obtained which complies with thermodynamic predictions, and tend to effect complete equilibrium among all the components of the product gas. Conversions close to the equilibrium values can be achieved with considerable ease over supported Ni catalysts. To favour propane oxidation according to reaction (1) a selective catalytic material must be used. For practical purposes, nickel is usually impregnated on a suitable porous support which provides thermal stability at working temperatures [2]. But selectivity of a catalyst may depend on various other factors like composition, concentration of active component, physical and structural parameters. The effect of these parameters on the behaviour in propane oxidation of the Ni supported on mullite has been studied in our previous papers [3,4]. 

The activation energy for silicon diffusion during the formation of mullite from fused couples at 1,600 < T < 1,800°C [55] is in the range of 730 diffusion coefficients are much higher than those of silicon at temperatures above the mullite-silica eutectic [56],... 

Monolithic supports were obtained om CTI Company (Salindres, France). The chosen material is mullite, 3Al203-2Si02, and the monolith displays 7x7 square channels of 1 mm internal side length. The overall length of the decomposition chamber is 15 mm. The advantages of mullite are good thermal stability and mechanical properties. The monoliths are put for 1 h in a solution of nitric acid 1 mol.L then heated at 300°C for 1 h. 

Fig. 8. Regular blocks of mullite lattice fringes inside the zirconia grain interphase, (end of the triangles that are arrowed). Therefore, the HREM method can support, in this case, the idea of solid solubility of mullite inside the zirconia boundary [25]...

Kita et al. (2003) reported on a tubular-type PV and vapor permeation module with zeolite membranes for fuel EtOH production. They used two types of zeolite membranes (i) NaA-type zeolite membrane, which was grown on the surface of a porous cylindrical mullite support and (ii) T-type zeolite membrane, which was also grown hydrothermally on the mullite support. Both membranes were studied for the flux and the separation factor of PV and vapor permeation for water-alcohol mixtures at 50°C and 75°C. The membranes were selective for permeating water preferentially with the high permeation flux. The separation factor of the T-type zeolite membrane was slightly smaller than the NaA zeolite membrane. They also claimed that this can provide more energy-efficient concentration of the EtOH to fuel grade EtOH. 

The electrode system can take several forms. In one of these a suspended graphite rod sheathed in Mullite is used as the cathode support, the electrode itself consisting of a small horizontal molybdenum disc fixed to the lower end, which in operation is located two or three inches above the ceU base. The anode is a graphite annulus suspended from two graphite rods, a few inches above the molybdenum cathode. A molybdenum catch-plate rests on the base of the cell to collect thorium metal particles which fall from the cathode. This can be raised from the ceU and drained in the same way as the cathode itself, by removal of the ceU Ud. 

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