STRUCTURE AND DYNAMICS OF MG1-XALX(OH)2(NO3)X•NH2O LAYERED DOUBLE HYDROXIDES

Layered double hydroxides (LDHs) are among a unique class of materials capable of anion exchange. Mg_1-xAl_x(OH)₂(NO₃)_x•nH₂O represents a type of LDH which demonstrates a significant long-term anion exchange capacity. We have used a multinuclear NMR approach to probe the interactions and dynamics of both water and exchanged species in the interlayer and the local environment of the metal hydroxide sheets. LDHs with varying Al content were synthesized and characterized using ²⁷Al, ²⁵Mg and ¹H MAS NMR. ²⁷Al and ²⁵Mg NMR were used to examine the effect of aluminum content on the local environment and distortions of the metal hydroxide sheets. ¹H MAS NMR was used to discriminate between various proton environments in the metal hydroxide layers and interlayers and to understand the degree and role of hydrogen bonding. ¹H-²⁷Al TRAPDOR was utilized to assign hydroxide resonances in the metal hydroxide layer. The pristine LDHs were successfully anion-exchanged with various phosphates, including pyrophosphate and triphosphate. ³¹P and ¹H-³¹P CPMAS NMR were used to elucidate the connectivity of the phosphates in the interlayer as well as the hydrogen bonding between the anions, interlayer water and metal hydroxide sheets. Pristine LDHs with various aluminum contents were deuterated and studied using static and ²H MAS NMR in order to identify the rigid and mobile species.