Microwave-hydrothermal processing of layered anion exchangers
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Microwave-hydrothermal processing of layered anion exchangers Sridhar Komarneni, Q. H. Li, and Rustum Roy Intercollege Materials Research Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802 (Received 28 June 1995; accepted 28 March 1996)
We have compared the microwave-hydrothermal (M-H) processing with conventional hydrothermal (C-H) processing in the preparation of two layered anion exchangers, i.e., Mg3 Al(OH)8 NO3 ? nH2 O and Ni12x Zn2x (OH)2 (CH3 COO)2x ? nH2 O. Both these phases can be crystallized more rapidly (an order of magnitude) under M-H processing compared to C-H processing. The above layered mixed basic salt of Ni and Zn was found to exhibit very high selectivity for PO4 ; (Kd 15, 000). Its order of selectivity for various anions in the presence of 0.1 N NaCl (ratio of Cl2 to anion in question is 100) increases as follows: PO4 ; @ SO4 . NO3 2 .
The term “microwave-hydrothermal” processing was first coined by us1 for reactions taking place in solutions that are heated above 100 ±C and under the influence of microwave field. Microwave processing of solid-state materials is widely studied.2–6 The use of microwaves in analytical chemistry to dissolve inorganic and organic samples for chemical analysis is also a common practice.7–10 During the last several years, a few groups have shown that chemical syntheses in solutions may be significantly accelerated using microwaves through dielectric heating effects. The microwave-assisted synthesis method has been shown to be applicable to organic reactions,11–16 organometallic syntheses,17–19 synthesis of intercalation compounds,20 and preparation of inorganic zeolites and molecular sieves.21–26 We, however, showed for the first time that microwaves under hydrothermal conditions can catalyze the synthesis of anhydrous ceramic oxides,1,27–32 hydroxylated phases,28,31 metal powders,28,33 and metalintercalated clays.34,35 We have also shown that novel phases1,28 can be synthesized by the M-H process. The acceleration of inorganic syntheses has been reported recently by others36–38 following our early work.1,27–33 In this communication we report the accelerated synthesis of two layered anion exchangers using microwavehydrothermal (M-H) processing, and we also report on the selective anion exchange properties of the novel layered Ni–Zn basic salt.39 All the M-H experiments were carried out with a commercially available microwave digestion system, MDS-2000, which is made by the CEM Corporation. Although this digestion system is meant for dissolution of inorganic and organic samples using 2.45 GHz microwave frequency, it can be used for syntheses.1,27–35 The system can operate at 0–100% full power (630 6 50 W) and is controlled by pressure to a maximum 1866
http://journals.cambridge.org
J. Mater. Res., Vol. 11, No. 8, Aug 1996
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of 200 psi. The synthesis reactions were carried out in double-walled digestion vessels (Fig. 1) which have an inner liner and cover made
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