Hydrothermal microwave heating

Metal-organic frameworks can be synthesized with a wide variety of metal cations and a large choice of functionalized organic linkers by use of various synthesis methods, such as solvothermal (hydrothermal), microwave heating, ultrasonic, mechanochemical, electrochemical synthesis, and the spray-drying. However, only the conventional solvothermal (hydrothermal) method is well studied for MOF synthesis. In a typical solvothermal synthesis, both organic and inorganic precursors are dissolved in solvent... 

Supported zeolite membranes have been prepared using numerous procedures [4] such as alignment of crystals in electrical fields, electroplating, self-assembly, growth on organic molecular layers, covalent linkages, hydrothermal synthesis (in situ and ex situ), hydrothermal method microwave heating assisted, dry gel method (vapor-phase transport method and steam-assisted crystallization), synthesis at the interface between two fluid phases, etc. 

Figure 4.43 Products by microwave heating at 180°C for 17 min from micro-emulsion compositions (A and C) and traditional hydrothermal conditions without microemulsion (E). Reproduced with permission from [148], Copyright (2003) American Chemical Society...

The nonanuclear complex [La3Nig(iminodiacetate)6(OH)6(H20),2 has been prepared from Ni(N03)2 6H20, iminodiacetic acid and La(N03)3-6H20 at 100 °C in water using microwave heating or under standard reflux conditions. The complex ion was obtained in 40% yield after 30 min when using microwave heating, but under conventional hydrothermal conditions the yield was only 28.5% after 3 days. 

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. 

In the view of the above section, it is clearly seen that coupling of hydrothermal method with microwave radiation is widely studied in the literature. In this section, we will shortly discuss some possible combination of microwave heating with other synthesis methods. 

Wang, Z., Zhu, J., Xu, W., Sui, J., Peng, H., and Tang, X. (2012) Microwave hydrothermal synthesis of perovskite BiFeOa nanoparticles an insight into the phase purity during the microwave heating process. Mater. Chem. Phys., 135, 330-333. 

Chent. Mater., 11, 882-895. (c) Rabeneau, A. (1985) The role of hydrothermal synthesis in preparative chemistry. Angew. Chem., Int. Ed., 24, 1026-1040. (d) Godinho, M., Ribeiro, C, Longo, E and Leite, E.R. (2008) Influence of microwave heating on the growth of gadolinium-doped cerium oxide nanorods. Cryst. Growth Des., 8, 384-386. 

Microwave-hydrothermal synthesis of tetragonal BaTiOs under various conditions. Mater. Chem, Phys., 97, 481-487. (b) Ma, Y., Vileno, E., Suib, S.L., and Dutta, P.K. (1997) Synthesis of tetragonal BaTiOs by microwave heating and conventional heating. Chem. Mater.,... 

Huang, A. and Yang, W. (2007) Hydrothermal synthesis of NaA zeolite membrane together with microwave heating and conventional heating. Materials Letters, 61, 5129-5132. 

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)... 

Iron oxide nanoparticles supported on almninosilicate catalysts were found to be efficient and easily recoverable materials in the aqueous selective oxidation of alcohols to their corresponding carbonyl compoimds using hydrogen peroxide under both conventional as well as microwave heating (Rajabi et al., 2013). A new and versatile approach to synthesize SnO nanociystals (ratile-type stmcture) using microwave-assisted hydrothermal method was reported by Mendes et al. (2012). They observed that there are substantial changes in optical absorbance of tin (IV) oxide nanoparticles. 

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