Hydrothermal modification

Ruhrmann, G. von Pechmann, E. 1989. Structural and hydrothermal modification of the Gaertner uranium deposit, Key Lake, Saskatchewan, Canada. IAEA TECDOC-500, 363-377. 

Jacobs, H. and Delcour, J. A. (1998). Hydrothermal modifications of granular starch with retention of granular structure A review. J. Agric. Food Chem. 46, 2895-2905. 

Initial silica samples and prepared carbosils were subjected to hydrothermal modification (HTT). A sample (2 g) of silica gel or carbosil was placed in a quartz vessel in an autoclave containing 20 cm3 of water heated at 150°C or 200°C for 6 h. These conditions ensured a saturated water vapor pressure in the autoclave. After modification, the samples were dried at 200°C for 6 h. To investigate the pyrolysis effects of different precursors on the silica structure, pyrocarbon was removed by heating at 500°C for 24 h in air (labeled T). 

Table 1 presents the structural characteristics of adsorbents prepared with Si-40 and acenaphthene.15 Larger changes in the Si-40 structure are caused by hydrothermal treatment, despite a relatively low temperature (150°C), than by heating at 500°C. However in the case of carbon-silica adsorbents, both hydrothermal modification and high-temperature pyrolysis changes the pore structure to a large extent. 

R. Leboda, E. Mendyk, A. Gierak, and V. A. Tertykh, Hydrothermal modification of silica gels (xerogels) 1. Effect of treatment temperature on their porous structure, Colloids Surf. A 105, 181-189(1995). 

CPG does undergo hydrothermal modification and the observed changes depend on time and temperature of the modification process. Like in silica gel, the structural changes in hydrothermally treated CPGs are caused by the dissolution of Si02 in one place and its condensation in another. In CPG, silica dissolves in large pores and precipitates in small ones. As a result hydrothermal treatment the mean pore diameter in CPG increases. 

Figure 22. Scanning-microscopy photos of . ) CPG B) CPG which underwent hydrothermal modification at 300" C.

Leboda, R., Tertykh, V.A., Sidorchuk, V.V., and Skuhiszewska-Zieba, J. 1998b. Peculiarities of steam-phase hydrothermal modification of pyrogenic silicium dioxide. Colloids Surf. A Physicochem. Eng. Aspects 135 253-265. 

Jyothi, A. N. Sajeev, M. S. Sreekumar, J. Hydrothermal modifications of tropical tuber starches. 1. Effect of heat-moisture treatment on the physicochemical, rheological and gelatinization characteristics. Starch/Starke 62 28-40 (2010). 

Leboda, R. Charmas, B., and Sidorchuk, V.V., Physicochemical and technological aspects of the hydrothermal modification of complex sorbents and catalysts Modification of porous and crystalline structures, Adsorpt. Sci. Technol., 15(3), 189-214(1997). 

Stute R. 1992. Hydrothermal modification of starches The difference between tuinealing and heat/moisture treatment. Starch/Starke 44 205-214. 

In this review the topic of synthesis covers both the primary synthesis work as well as important secondary chemical modifications of molecular sieve crystals. The first is the source of new crystal forms, pore structures, and new chemical compositions, while the latter leads to changes in composition and chemical or catalytic properties, without substantial changes in the crystal framework. In chemical modifications, preference is given to chemical substitution over the thermal or hydrothermal modifications often described in the earlier literature. 

Leboda, R. and Mendyk, E. (1991) Hydrothermal modification of porous structure of silica adsorbents. Materials Chemistry and Physics, 27,189-212. 

Leboda, R., Mendyk, E., Gierak, A. and Tertykh, V. A. (1995) Hydrothermal modification of sihca gels (xerogels).2. Effect of the duration of treatment on their porous structure. Colloids and Surfaces a-Physicochemical and Engineering Aspects, 105,191-197. 

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