To clarify the problem of undesired crystallization of amorphous aluminosilicate inorganic polymers (AIPs) accompanied by the loss of mechanical properties, we synthesized two types of chemically identical AIPs that differ in durability under hydrothermal conditions. Whereas phase-stable AIPs remain amorphous, phase-unstable AIP systems undergo extensive crystallization with the formation of high fractions of chabasite and zeolite P. The application of 27 Al MQ/MAS and { 1 H}-REDOR-27 Al MQ/MAS NMR spectroscopies, combined with the recently developed biaxial shearing transformations, revealed a two-component character of the prepared AIPs. The prevailing fraction of [AlO 4 ] − species (amorphous phase) is charge-balanced by Na + counterions and exhibits considerable distribution of 27 Al chemical shifts induced by the variation of Al−O−Si valence angles. In contrast, the minor fraction of [AlO 4 ] − tetrahedra, which can be attributed to the partially ordered domains of aluminosilicate networks, shows a broad distribution of quadrupolar parameters that result from variability in the chemical nature of the counterions. A comparison of the prepared AIPs revealed that the partially ordered domains of the phase-unstable AIPs contained a considerably larger amount of [AlO 4 ] − species charge-balanced by H + . Therefore, we assume that the destabilization of AIPs is associated with the presence of bridging hydroxyl groups (Si−OH + −Al, Brønsted-acid sites) that induce breaking of Si−O−Al bonds. The resulting decrease in network density can induce a temporal release of extraframework Al species, their subsequent rearrangement, and the formation of a crystalline phase. The process of crystallization is supported by the higher mobility of proton species as revealed by a range 1 H-receptive MAS NMR experiments. In contrast, the factors that stabilize amorphous nature of aluminosilicate frameworks are the presence of four-coordinate extraframework Al species, such as Al(OH) 3 + or AlOH 2+ −H 2 O, and tight incorporation of proton fraction into the inorganic matrix.

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Synthetic hydrotalcite calcined at 350-550 °C was used for the removal of arsenate, chromate, and vanadate ions from water solutions. The initial anion concentrations were 0.002 mol l-1. The sorption isotherms were measured at 20 °C and at neutral pH. The Langmuir adsorption isotherm was used for the sorption capacity evaluation. The ability of the calcined hydrotalcite to remove the anions from solution decreased in the order of vanadate - arsenate - chromate. The hydrotalcite calcined at 450 °C exhibited the best sorption ability for all the anions. The sorbed anions were released by anion exchange in a carbonate-containing solution and the hydrotalcite after subsequent calcination was used again for the removal of anions. The repeating cycles calcination-rehydration-anion exchange gradually reduced the adsorption capacity of the hydrotalcite. The sorption capacity decreased by 50% after the first two cycles but it did not change significantly in the subsequent cycles. When chromate anions were adsorbed, the decrease in sorption capacity was not observed during repeated calcination-rehydration-anion exchange cycles. The change in the sorption capacity was influenced by the ability of calcined hydrotalcite to regenerate the layered crystal structure during the rehydration process.

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