Microwave-hydrothermal treatment

Two series of HTlcs whose compositions correspond to NiioMg6iAl29-C03 and Rho.iNi5Mg66Al28.9-C03, respectively, were prepared by the coprecipitation method at constant pH [6]. The slurry obtained was subjected to two different aging treatments (i) vigorous stirring at room temperature (hereinafter called BO), and (ii) microwave-hydrothermal treatment (MWHT) at 125°C in a Milestone Ethos Plus microwave oven (hereinafter called SA). The solids were washed with distilled water imtil Na and N03 were totally absent in the washing liquids, and then dried at 40°C. The catalysts were prepared by the calcination of fresh samples at 900°C in air for 14 h. 

Carbon/silica adsorbents or carbosils have been prepared by both conventional and microwave heating. These materials were produced by pyrolysis of CH2CI2 on microporous silica gel surfaces for 30 min to 6 h at 550 °C. The resultant materials were hydrothermally treated with steam or liquid water using either a conventional autoclave or a microwave unit. As with the clay materials, hydrothermal treatment using microwave irradiation leads to a significant increase of surface area and total pore volume of the carbosils, as compared to conventional methods. 

The syntheses of mordenite zeolite crystals by microwave heating revealed that there is an accelerate transformation rate of mordenite crystals and also enhanced purity and surface areas of the crystals compared to conventional heating [llLl]. Highly crystalline and pure mordenite crystals were obtained after hydrothermal synthesis of 6 h at 190 °C by microwave heating, whereas prrre mordenite crystals could not be obtained even after hydrothermal treatment of 72 h at the same temperature and conventional heating. 

The result of microwave treatment is well-crystalline material in less time than necessary for conventional hydrothermal treatments. 

Fukui et al. [8] have utilized NaCl for synthesis of Phillipsite from fly ash by hydrothermal treatment with microwave heating. It has been reported that the crystallinity of Philhpsite can be increased by addition of NaCl in the hydrothermal reaction matrix with low concentration of NaOH, whereas no effects have been reported in case of high concentration of NaOH. However, microwave heating has been a favorable tool for the generation of Hydroxy-sodalite. It has been demonstrated that the substitution of NaCl by NaOH can reduce the rate of dissolution of aluminate and silicate ions from fly ash. As such, the rate of generation of aluminosilicate gel can be enhanced by employing microwave heating of the solution. 

Fukui, K., Katoh, M., Yamamoto, T., Yoshida, H. Utilization of NaCl for phillipsite synthesis from fly ash by hydrothermal treatment with microwave heating. Adv. Powdta TechnoL 20, 35 0 (2009)... 

The application of microwave (MW) irradiation as an energy source during hydrothermal treatment has been developed in recent years. The crystallinity degree reached is similar to that with hydrothermal treatment in conventional furnaces, but the solids show a smaller particle size and then a larger specific surface area. The process requires shorter periods of time, saving in energy consumption [119]. 

Grabowska et al. (2008) prepared a composite oxide ZnAl O by microwave-assisted hydrothermal treatment of a precursor mixture of lydroxides obtained by precipitation of aluminum and zinc nitrates. Various studies show that ZnAl O is nanosized and is a micro/mesoporous material with large a suifaee area (140 mVg). The gas phase catalytic methylation of 4-hydroxypyridine in the presence of the ZnAljO catalyst was performed in a continuous process at atmospherie pressure in the temperature range of 240-360 °C. A mixture of O- and N-alkylated products, namely 4-methoxypyridine and N-methyl-4-pyridone were obtained. The alkylation of 4-hydroxypyridine with methanol at 345 °C offered 87.6% selectivity towards N-methyl-4-pyiidone with about 89% 4-methoxypyridine conversioa... 

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