Modeling and MASNMR spectroscopic studies on molecular sieves. 2. The nature of water molecules in hydrated AlPO4-11 host lattice

Abstract

I. I lbl Figure 5. (a) The geometry of the side pocket which is a double 6-member ring. The access to this side pocket from the 10-member ring is shown by shading. (b) The schematic view of the channels along the c axis (box) and the perpendicular a axis. group signals. However, since the splitting occurs only in the signal of the methyl group (Figure 4), the fact that the terminal methyl group alone is present in a different environment is brought (Jut.In this context, it is relevant to note a unique feature of the structure of AlPO,-1 1, namely the presence of interconnected double 6-member side pockets. There is access to these side pockets from the 10-member channel. The geometry of the side pocket and the schematic view of channels of AlP04-1 1 from two perpendicular directions are shown in Figure 5a and 5b, respectively. The nonbonded distance between A13 and P3 is 5.30 A. When 0.86 A for the ionic radii of A13+ (0.51 A) and Ps+ (0.35 A) is left out, the free dimension across the 6-member ring aperture is 4.44 A. This is an unusually large aperture for a 6-member ring compared to those in other molecular sieves, and this provides a hint to explain the cause of the split in the signal of the methyl group. On heating, the weakly bound amine molecule may shift its interaction site, resulting in the movement of the -CH fragment of the terminal methyl group into the 6-member side pocket, and thus exists in a different environment. These results indicate that the template molecules are still chemically intact and they possess the freedom that allows the dynamic behavior inside the neutral AlP04-l 1 framework. The amines may have more than one favorable adsorption site, and the values of the adsorption energies are expected to be comparable, thus facilitating its migration from one to another favorable site. This observation is in correlation with the findings from our electronic structure calculation that there are three oxygen sites for adsorption of basic molecules with comparable adsorption energies (Table 11). ConclusionsThe following features are the outcome of the present study: 1. The favorable oxygen sites for the adsorption of amine molecules are 0 4 , 0 5 , and 05' as predicted by the electronic structure calculations. It is heartening to observe that the results are consistent with the findings from single crystal powder diffraction studies of as-synthesized MnAPO-11, that the diisopropylamine is hydrogen bonded to 0 4 and 05. 2.The occluded amine molecules in as-synthesized AlPO,-ll samples are chemically intact. At high temperatures, they have a dynamic freedom inside the pores as indicated by NMR studies.3. In deciding the role of these amine molecules as template molecules, the geometric factor plays a relatively insignificant role compared to the electronic factor because of the conformational flexibility and the possibility of a methyl group fragment projecting into side pockets. A recent paper1' reporting the synthesis of AlP04-1 1 using 1-methylimidazole as the template also supports the fact t...

Revisiting the Structure of Calcined and Hydrated AlPO‐11 with DFT‐Based Molecular Dynamics Simulations**

“…[49] In a very early application of computational chemistry techniques to AlPO molecular sieves, Prasad et al employed semiempirical calculations to study the adsorption of water in AlPO-11. [50] The most favourable adsorption configuration was found for H 2 O adsorbed at the Al3 site, consistent with a hydration of 1/5 of the Al sites observed in NMR experiments. A periodic density functional theory (DFT) study of this system was published in 2007 by Herréra-Perez et al [51] These authors compared the adsorption of water molecules at the three nonequivalent Al sites of the Ima2 structure, assuming a complete hydration of each site by one H 2 O per Al atom.…”

“…employed semiempirical calculations to study the adsorption of water in AlPO‐11. [50] The most favourable adsorption configuration was found for H 2 O adsorbed at the Al3 site, consistent with a hydration of 1/5 of the Al sites observed in NMR experiments. A periodic density functional theory (DFT) study of this system was published in 2007 by Herréra‐Perez et al.…”

Revisiting the structure of calcined and hydrated AlPO-11 with DFT-based molecular dynamics simulations

“…corresponds to an adsorbed amount of only 0.06 g(H2O) per g(AlPO-11), whereas adsorption isotherm measurements delivered a saturation uptake in the range of 0.15 g(H2O) per g(AlPO-11), which corresponds to ~20 H2O/u.c.. [49] In a very early application of computational chemistry techniques to AlPO molecular sieves, Prasad et al employed semiempirical calculations to study the adsorption of water in AlPO-11. [50] The most favourable adsorption configuration was found for H2O adsorbed at the Al3 site, consistent with a hydration of 1/5 of the Al sites observed in NMR experiments. A periodic density functional theory (DFT) study of this system was published in 2007 by Herréra-Perez et al [51] These authors compared the adsorption of water molecules at the three non-equivalent Al sites of the 𝐼𝑚𝑎2 structure, assuming a complete hydration of each site by one H2O per Al atom.…”

A 27Al NMR study of the interaction of water with AlPO4-11

“…The results can be reconciled with the calculations of Prasad et aZ. 3 The 27Al DOR NMR spectra of fully hydrated AlPO-11 at various magnetic fields are given in Fig. 1.…”