CARBIDES AND NITRIDES OF TRANSITION ELEMENTS WITH CONTROLLED POROSITY

Abstract: The present invention relates to a preparation method of carbides and nitrides of transition elements in extruded shape and from their oxides, with controlled porous volume and distribution. The meso and macroporous are generated by polymeric organic molecules such as microcrystalline cellulose, which after being burned form a more opened porous structure. The molding process involves the obtention of intermediate compounds that are susceptible to peptization and later extrusion. The preparation of these compounds, with porous diameter greater than 50 angstroms, implies in a range of catalytic reactions applications since more opened porous structures allow the catalytic process of larger molecules. Besides, in this way, there are less problems related to heat and mass diffusion processes. Problems which appear when dealing with very active catalysts and/or very exothermic reactions.

Description: Descriptive Report of the Patent of Invention "CARBIDES AND NITRIDES OF TRANSITION ELEMENTS WITH CONTROLLED POROSITY" . The present invention relates to a preparation method of carbides and nitrides of transition elements in extruded shape from their oxides, leading to a controlled porous volume and distribution, destinated to catalytic use like in isomerization or hydrogenation reactions or in reactions used to gas generation like hydrazine decomposition in propulsion systems . Carbides and nitrides are interstitial alloys formed by incorporating carbon and nitrogen into the cristalline nets of the transition metals oxide. These alloys show metallic characteristics which give them catalytic activity similar to various metals, like iridium, platinum and paladium. The important characteristics of such compounds, besides being extremely hard, are corrosion resistance and high melting point. As direct consequence of these properties carbides and nitrides are used not only in catalysis, but also in cutting tools, structural parts, etc. The use in catalysis came from the development by Boudart team of a preparation method which resulted in carbides and nitrides of high specific area. This preparation method consists of a reduction of oxide of the transition metal, using gaseous amonia for that in the nitrides case or a hydrogen and methane mixture in the carbides case, and increasing the temperature of the precursor oxide in a uniform and smooth way. Although this preparation method produces materials with high specific area, the obtained porous structure can not be controlled, leading to a porous diameter not higher than 50 angstroms. The invention herein is different from the nowadays existing techniques by the possibility of obtaining carbides and nitrides of transition elements in the extruded shape with the intended porosity. The preparation method consists in obtaining the transition elements acids, tungstic for the tungsten and molybdic for the molybdenum, in a peptizable shape. These acids which were obtained in the colloidal shape when formed at low temperatures, in powder shape, are mixtured with microcrystalline cellulose, also in powder shape. After this phase the mixture is peptized using as a peptization agent an ammonium hydroxide dilluted solution. The obtained product is then extruded with the aid of an extruder. The thermal treatment of the extruded product involves drying at 60°C and after at 120°C, both during approximately 12 hours. After that a calcination at 600°C, during 3 hours, is made with the goal of transforming the acid in its corresponding oxide and to burn the cellulose remaining in the acid. The ellimination of this cellulose generates, in the resulting oxide, a porosity that can be controlled by concentrating this compound mixtured with the acid or by the ammonium hydroxide content used as the peptizing agent. After this phase the carbides and nitrides are obtained by heating these precursors in a uniform and smooth way in a gaseous amonia environment, for nitrides, or in a hydrogen and methane environment, for carbides. TIPICAL EXAMPLE Figure 1 shows the porous structure obtained by this method for the molybdenum carbide_case. The Y axis is the incremental specific volume (dV/dD) in cm /g, where V is the porous specific volume and D is the porous diameter. In X axis the porous diameter, in angstroms, is shown in a logarithmic scale. The mode 1 shown in figure as region 1 has coordinates X=40 and Y=0.08 and represents the porosity generated during the precursor oxide transformation in carbide or nitride. The mode 2, having as coordinates X=1000 and Y=0.15, represents the porosity generated by the burning of polymeric materials and their X and Y values depend on the content and kind of the employed polymer.

Claims: CLAIMS 1 - Carbides and nitrides of transition elements characterized by the obtention of these compounds with controlled porous volume and distribution, used to catalitic applications in_general. 2 - Carbides and nitrides of transition elements as in claim 1, characterized by the use of long chain materials susceptible to combustion, used for generation of meso and macroporous, such_as microcrystalline cellulose. 3 - Carbides and nitrides of transition elements as in claim 1, characterized by the use of amonium hydroxide as peptizing agent in the molding process extruded products obtention - of the carbides and nitrides precursors.

Inventors: BUGLI GUY, CRUZ GILBERTO MARQUES, DJEGA-MARIADASSOU GERALD, RODRIGUES JOSE AUGUSTO JORGE, BUGLI, GUY, CRUZ, GILBERTO, MARQUES, DJEGA-MARIADASSOU, GERALD, RODRIGUES, JOSE, AUGUSTO, JORGE
Applicants: BUGLI GUY, CRUZ GILBERTO MARQUES, DJEGA MARIADASSOU GERALD, INST NACIONAL DE PESQUISAS ESP, RODRIGUES JOSE AUGUSTO JORGE, BUGLI, GUY, CRUZ, GILBERTO, MARQUES, DJEGA-MARIADASSOU, GERALD, INSTITUTO NACIONAL DE PESQUISAS ESPACIAIS, RODRIGUES, JOSE, AUGUSTO, JORGE
Application year: 1996
Main classification: B01J35
IPC codes: B01J27, B01J35, B01J37, C01B21, C01B31