Dehydration of AlPO₄-34 studied by variable-temperature NMR, XRD and first-principles calculations

Abstract

Complementary techniques elucidate water–framework interactions in different states of dehydration, two of which have not been observed before.

Proton Acidity and Proton Mobility in ECR‐40, a Silicoaluminophosphate that Violates Löwenstein's Rule

“…In hydrated AlPOs and related materials, water molecules may bond to framework Al atoms, leading to a change in the coordination number of Al to five or six. The formation of such “higher‐coordinated” Al atoms has been well established experimentally and also been observed in previous DFT studies . An inspection of the AIMD average structures and last frames does indeed show such a formation of Al−O[H 2 O] bonds.…”

Superior Performance of Microporous Aluminophosphate with LTA Topology in Solar‐Energy Storage and Heat Reallocation

“…The calculations within the density functional theory (DFT) have been performed primarily on AlPO 4 ‐34 and MIL‐160 since their intermediate structures have been very well resolved both experimentally and in previous DFT studies. While AlPO 4 ‐34 exhibits three phases of hydration, we focus on the first phase whose mechanism matches that of AlPO 4 ‐LTA and gives rise to the steep part of the adsorption isotherm. In this first phase of hydration, two out of six aluminum atoms of the crystallographic unit cell of AlPO 4 ‐34 get six‐coordinated because of four water molecules that attach to them, and additional six water molecules adsorb into each chabazite cage and form a strongly hydrogen‐bonded cluster in it.…”

“…AlPO 4 ‐34 was prepared according to the published procedure, whereas synthesis of MOF‐801 was performed by the modified procedure from Furukawa et al, with the same molar ratios of reaction components and synthesis conditions as described in the literature, but using ZrCl 4 (99%, Sigma‐Aldrich) instead of ZrOCl 2 .…”

“…The reason for the sudden water uptake in AlPO 4 -34 is in the quick formation of energetically very favorable ordered hydrogenbonded water clusters within the pores. [14] It is proposed that the formation of these clusters is facilitated by the tendency of the framework aluminum atoms to attract water molecules and to rapidly and reversibly change coordination environment from tetrahedral to octahedral.…”

“…Most importantly, AlPO 4 ‐34 adsorbed water much more suddenly, i.e., in a much narrower relative‐pressure interval, and could be more efficiently regenerated at low temperatures than MIL‐160 and MOF‐801. The reason for the sudden water uptake in AlPO 4 ‐34 is in the quick formation of energetically very favorable ordered hydrogen‐bonded water clusters within the pores . It is proposed that the formation of these clusters is facilitated by the tendency of the framework aluminum atoms to attract water molecules and to rapidly and reversibly change coordination environment from tetrahedral to octahedral.…”