Magnesia-based materials

How to Solve the Deactivation Problem. Solutions to the deactivation problem are difficult. The patent literature (42) has claims that either sodium, manganese or phosphorous added to alumina prevents deactivation by silica. In addition, removal of matrix silica from cracking catalyst formulations should prevent further deactivation because zeolitic silica, as we have shown, migrates more slowly. There is at least one patent relating to very high alumina matrix cracking catalysts (43). Another solution is to use more active SOx catalysts such as magnesia-based materials. 

Extensive research has been conducted on catalysts that promote the methane—sulfur reaction to carbon disulfide. Data are pubhshed for sihca gel (49), alurnina-based materials (50—59), magnesia (60,61), charcoal (62), various metal compounds (63,64), and metal salts, oxides, or sulfides (65—71). Eor a sihca gel catalyst the rate constant for temperatures of 500—700°C and various space velocities is (72)... 

Two of the materials were furnished by catalyst companies and are referred to as magnesia-based or magnesia and lanthanum-based or lanthanum. These latter materials are both known to contain cerium and alumina as well. 

Figure 17 shows a comparison of the fresh SO2 removal ability for these five major types of commercially available SOx catalysts. The materials were tested at 1350 F at various concentrations with a very low capacity cracking catalyst. The magnesia-based catalyst is much better than lanthanum-based catalyst followed by platinum or cerium on alumina and finally alumina alone. The reverse order in activity observed for the lanthanum-based and cerium additives, compared to the relative results given previously for lanthanum and cerium, was not investigated, but may be related to the presence of cerium on the lanthanum-based additive (27). 

Commercial catalysts vary in the degree to which they are regenerable at reactor temperatures as shown on Figure 18. The initial SO2 removal for all five materials was adjusted to an equal basis by varying the amount of additive used 0.8% magnesia-based, 3% lanthanum-based, 10% of both cerium/alumina and... 

Magnesia and lanthanum based materials are the most effective of the samples tested to date. 

Chemical composition silica-based and silica-alumina-based materials, chrome, magnesia, chrome-magnesia, spinel, SiC, materials containing carbon (more than 1% carbon or graphite), and special materials (containing other oxides or materials such as zircon, zirconia, Si3N4, etc.)... 

A number of other materials has been found to be unsatisfactory for practical use, although applicable to demonstration or experimental purposes. Thus, the oxides of iron and cobalt, although catalytically active, are too volatile, as is magnesia when used as a base material. Calcium oxide is too readily attacked by water vapor and carbon dioxide of the air when cold to be practicable. 

El-Jazairi, B. (1987) The properties of magnesia phosphate cement based materials for rapid repair of concrete. Proceedings 3rd International Conference on Structural... 

However, the relatively mediocre corrosion resistance of the magnesia-based refractories suggests that its high temperature bonding strength is not sufficient, and that application of this material to copper smelting systems would be effective only under relatively static conditions, such as the bottom of stationary anode furnaces. 

The integrated planar (IP) SOFC stack concept, for instance, applied by Rolls-Royce Fuel Cell Systems, uses an array of series-connected cells deposited by screen printing onto a flat support tube, a schematic cross section of which is shown Fig. 21.23. The support tube, an inert, porous ceramic, is fabricated from a magnesia magnesium aluminate spinel (MMA), whereby the desired coefficient of thermal expansion is adjusted by the ratio of the base materials [76]. 

Acid—Base Chemistry. Acetic acid dissociates in water, pK = 4.76 at 25°C. It is a mild acid which can be used for analysis of bases too weak to detect in water (26). It readily neutralizes the ordinary hydroxides of the alkaU metals and the alkaline earths to form the corresponding acetates. When the cmde material pyroligneous acid is neutralized with limestone or magnesia the commercial acetate of lime or acetate of magnesia is obtained (7). Acetic acid accepts protons only from the strongest acids such as nitric acid and sulfuric acid. Other acids exhibit very powerful, superacid properties in acetic acid solutions and are thus useful catalysts for esterifications of olefins and alcohols (27). Nitrations conducted in acetic acid solvent are effected because of the formation of the nitronium ion, NO Hexamethylenetetramine [100-97-0] may be nitrated in acetic acid solvent to yield the explosive cycl o trim ethyl en etrin itram in e [121 -82-4] also known as cyclonit or RDX. 

Nickel—beryllium casting alloys are readily air melted, in electric or induction furnaces. Melt surface protection is suppHed by a blanket of argon gas or an alumina-base slag cover. Furnace linings or cmcibles of magnesia are preferred, with zirconium siUcate or mullite also adequate. Sand, investment, ceramic, and permanent mold materials are appropriate for these alloys. Beryllium ia the composition is an effective deoxidizer and scavenger of sulfur and nitrogen. 

The raw materials needed to supply about ten million new automobiles a year do not impose a difficult problem except in the case of the noble metals. Present technology indicates that each car may need up to ten pounds of pellets, two pounds of monoliths, or two pounds of metal alloys. The refractory oxide support materials are usually a mixture of silica, alumina, magnesia, lithium oxide, and zirconium oxide. Fifty thousand tons of such materials a year do not raise serious problems (47). The base metal oxides requirement per car may be 0.1 to 1 lb per car, or up to five thousand tons a year. The current U.S. annual consumption of copper, manganese, and chromium is above a million tons per year, and the consumption of nickel and tungsten above a hundred thousand tons per year. The only important metals used at the low rate of five thousand tons per year are cobalt, vanadium, and the rare earths. 

A variety of material could be used as the basis for cracking catalyst, including synthetic silica-alumina, natural clay, or silica-magnesia. If these materials did not contain significant amounts of metals such as chromium or platinum that catalyzed the burning of carbon, the burning rate of the coke is independent of the base as shown in Fig. 7. 

Catalysts. - Group VIII metals, conventional base metal catalysts (Ni, Co, and Fe) as well as noble metal catalysts (Pt, Ru, Rh, Pd) are active for the SR reaction. These are usually dispersed on various oxide supports. y-Alumina is widely used but a-alumina, magnesium aluminate, calcium aluminate, ceria, magnesia, pervoskites, and zirconia are also used as support materials. The following sections discuss the base metal and noble metal catalysts in detail, focusing on liquid hydrocarbon SR for fuel cell applications. 

When alumina is combined with the silica, forming a natural clay, a much more compact and fusible compound is formed with the lime than when the silica is alone. Indeed, it has been observed as a general principle, that tire point of fusion is materially affected by the relation and number of bases the whole materials contain thus, a more liquid scoria is obtsined by the addition of a limestone containing magnesia than with a pure limestone. But experience is against the use of a magnesieu limestone, because it deteriorates the iron produced, while the purity of the metal iB the primary consideration. That which contains much silica should also be used sparingly, as silica combines with the iron and injures its quality, -The purest limestones are the most suitable for flux. Common marble is nearly a pnre carbonate of lime but is too rare and expensive to be used as a flux. 

---