Abstract
Single-phase magnesium-aluminium layered double hydroxide (LDH) intercalated with dihydrogen phosphate was successfully produced by hydration of nanopowder of the respective mixed metal oxide (MMO) obtained using sol-gel based method followed by a two-step anion exchange hydroxide-to-chloride and chloride-to-phosphate. The MMO with the metal cation ratio of Mg/Al = 2:1 was prepared using the aqueous sol-gel method. Processes of the parent Mg2Al-OH LDH formation and the successive anion-exchanges, ОН− → Cl− and Cl− → H2PO4−, were considerably accelerated via the application of high-power (1.5 kW) ultrasound. The crystalline phases formed at all stages of the Mg2Al-H2PO4 LDH production were characterized using X-ray diffraction, scanning electron microscopy, scanning transmission electron microscopy, inductive coupled plasma optical emission spectroscopy, Fourier transform infrared spectroscopy, and thermogravimetric analysis. Based on the data of chemical analysis and the XRD data, the type of the intercalated phosphate anion was determined and the arrangement of this anion in the interlayer was modelled.
Highlights
In layered double hydroxides (LDHs), the positively charged mixed metal cation hydroxide layer alternate with charge-compensating interlayer of anions Am− coordinated by water molecules[1]
It was found that formation of LDH phase by hydration of Mg2Al(MMO) at room temperature is rather slow
In the second set of experiments conducted at 80 °C, a single-phase Mg2Al-OH LDH was obtained already after 2-h hydration of Mg2Al(MMO) (Fig. 1)
Summary
In layered double hydroxides (LDHs), the positively charged mixed metal cation hydroxide layer alternate with charge-compensating interlayer of anions Am− coordinated by water molecules[1]. We have developed an environment-friendly method for the fabrication of LDHs with more than one type of the MIII cation in the hydroxide layer[5,6,7] In this novel aqueous sol-gel processing route, LDHs are obtained as a result of the decomposition (calcination) of the precursor gels at 650oC followed by rehydration of the intermediate crystalline mixed metal oxide (MMO) powders in water. This method allows us to produce LDHs with different sets of cations, and with the desired MII/MIII ratio. It should be stressed here that in all the aforementioned cases, the applied sonication power was rather moderate (an order of 100 W) as compared with that employed in this work (1.5 kW)
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