Insights into the Structural Transformations of Aluminosilicate Inorganic Polymers: A Comprehensive Solid-State NMR Study

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 27Al MQ/MAS and {1H}-REDOR-27Al 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 [AlO4]− species (amorphous phase) is charge-balanced by Na+ counterions and exhibits considerable distribution of 27Al chemical shifts induced by the variation of Al–O–Si valence angles. In contrast, the minor fraction of [AlO4]− 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 [AlO4]− 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 1H-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 AlOH2+–H2O, and tight incorporation of proton fraction into the inorganic matrix.