S. Kannan et Rv. Jasra, Microwave assisted rapid crystallization of Mg-M(III) hydrotalcite where M(III) = Al, Fe or Cr, J MAT CHEM, 10(10), 2000, pp. 2311-2314
MgM(III)-carbonate hydrotalcites where M(III) = Al, Fe and Cr were rapidly
synthesized through an instantaneous addition method coupled with microwave
irradiation. The materials were characterized by powder X-ray diffraction
(XRD), thermogravimetric analysis, Al-27 MAS-NMR spectroscopy, transmission
electron microscopy (TEM) and BET specific surface area measurements. Micr
owave irradiation resulted in a well crystallized material after a short sy
nthesis time (12 min) compared to conventional coprecipitation and aging of
the gel (25.5 h). However, the extent of enhancement in crystallinity upon
irradiation depended on the nature of the trivalent metal ion. For the Mg-
Cr system, microwave irradiation enhanced the crystallinity of the desired
hydrotalcite-like phase at the expense of the impurity brucite phase, while
the opposite effect was observed with conventional aging. The charged natu
re of the hydrotalcite network and water molecules present in the interlaye
r space enables uniform and effective absorption of microwaves, favoring lo
ng range ordering.