Amorphous aluminas thermal stability

The stability of MCM-41 is of great interest because, from the practical point of view, it is important to evaluate its potential application as a catalyst or adsorbent. It is known that purely-siliceous MCM-41 (designated here as PSM) has a high thermal stability in air and in oxygen containing low concentration (2.3 kPa) of water vapor at 700 °C for 2 h [1], However, the uniform mesoporous structure of PSM was found to be collapsed in hot water and aqueous solution due to silicate hydrolysis [2], limiting its applications associated with aqueous solutions. After MCM-41 samples were steamed in 100% water vapor at 750°C for 5 h. their surface areas were found to be lower than amorphous silica-alumina and no mesoporous structure could be identified by XRD measurement [3]. In addition, PSM showed poor stability in basic solution [4]. 

Mixtures of transition metal (Mo or W) sulphides dispersed on y-alumina supports are used in hydrotreatment processes to remove sulphur, nitrogen, oxygen and metals from oil fractions. The addition of phosphorus to these catalysts enhances the solubility and stability of molybdate and improves the thermal stability of the alumina support. Solid state P double-resonance NMR experiments ( P- AI REDOR and TRAPDOR) have been used to investigate the interaction between the impregnating phosphorus and the support surface (van Eck etal. 1995). The results showed that most of the phosphorus is in close contact with the aluminium, and that the layer of AIPO4 formed on the surface is not completely amorphous, but is slightly more ordered. 

Alumina, silica and alumina silica are well known and widely used catalysis or catalyst supports which is mainly due to their acidic properties, porous structure and g<)od thermal stability. Probably the best known example is the amorphous alumina silica catalytic cracking... 

Amorphous aluminas have moderate thermal stabdity in dry atmospheres, that is, up to temperatures of less than 700 °C. However, the thermal stabihty of amorphous alumina is reported to significantly increase with decreasing particle size (141). Upon hydrothermal treatment, they tend to convert easily into bayerite, Y-AI2O3, or TI-AI2O3 (110,131,142). Amorphous aluminas appear to have only moderate activity as acid—base catalysts (110,130). Surprisingly, they have been reported to have redox properties (143). In summary, amorphous aluminas have so far not shown high activity for any catalytic reaction, and because of their Hmited structural stability, they are also expected to have poor stability as catalysts. However, they can serve as reference materials in the catalysis field. 

Thermal stability of zeolites Crystalline zeolites are more resistive to heat than amorphous materials the main reason being the geometrical structure of the crystalline framework. However, the effects of silica/alumina ratio and level of cation exchange on thermal stability also cannot be denied. Commercial zeolites having a high (Si02/Al203) ratio can resist... 

Owing to its excellent thermal and mechanical stability and its rich chemistry, alumina is the most widely used support in catalysis. Although aluminium oxide exists in various structures, only three phases are of interest, namely the nonporous, crys-tallographically ordered a-Al203, and the porous amorphous t]- and y-Al203. The latter is also used as a catalyst by itself, for example in the production of elemental sulfur from H2S (the Claus process), the alkylation of phenol or the dehydration of formic acid. 

The introduction of zeolites in cracking catalysts combined with various non-zeolite matrix types (a.o. higher stability silica-alumina types) certainly complicates the picture of FCC hydrothermal deactivation. Letzsch et al [7] have shown that like amorphous catalysts the zeolite is more strongly deactivated hydrothermally than purely thermally. 

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