Time-Resolved, In Situ X-ray Diffraction Studies of Staging during Phosphonic Acid Intercalation into [LiAl₂(OH)₆]Cl·H₂O

Abstract

Intercalation of a range of phosphonic acids into [LiAl 2 (OH) 6 ]Cl‚H 2 O at pH 8 leads to cointercalation of both mono-and dianionic guest anions. Solid-state 31 P NMR data can be used to show that these materials exhibit a 31 P chemical shift that is intermediate between the values for the monoanionic and dianionic forms of the respective acids, suggesting that rapid proton exchange occurs between the intercalated anions. It is possible to calculate the relative amounts of mono-and dianionic species present using the observed averaged chemical shift of the intercalate phase. In the case of methylphosphonic acid, the ratio of mono-and dianionic species (MePO 3 H -/MePO 3 2-) is estimated to be 2.1. The intercalation of methyl-, ethyl-, phenyl-, and benzylphosphonic acids into [LiAl 2 (OH) 6 ]Cl‚H 2 O was studied using time-resolved, in situ energy-dispersive synchrotron X-ray diffraction. The rates of intercalation are significantly greater for methylphosphonic acid (MPA), ethylphosphonic acid (EPA), and benzylphosphonic acid (BPA) than for phenylphosphonic acid (PPA). The large BPA anions intercalate the most quickly, and MPA reacts more rapidly than EPA. Kinetic analyses of the rate data suggest that these are diffusion-controlled reactions. In situ X-ray diffraction experiments performed with slow addition of the guest have allowed observation of intermediate crystalline phases during the reactions with MPA and BPA. The intermediate phases can be indexed assuming a second-stage intercalation compound in which alternate interlayer regions are occupied by phosphonic acid and Clanions, respectively (Williams et al. Chem. Commun. 2003, 1816. The observation of these secondstage intermediates is very surprising, because staging in rigid layer lattices such as layered double hydroxides (LDHs) is highly unusual.

Anion Exchange Kinetics of Nanodimensional Layered Metal Hydroxides: Use of Isoconversional Analysis

“…There is a small induction period observed at low temperatures (30 and 40 °C) as seen in Figure 8. Induction periods have been observed for other anion exchange reactions 48 and may be due to restructuring of the host layers which results in a delayed or slow release of the Cn anions and subsequent delayed crystallization and/or growth of the guest phase. During the induction period, the chloride phase is not observed for a period of up to 5 min at 30 °C, and then starts to grow slowly with Bragg reflections having very low intensities in samples obtained for reaction times less than 20 min.…”

A critical assessment of the methods for intercalating anionic surfactants in layered double hydroxides

“…Apart from the conventional layer, which consists of M II and M III metal cations, a mono-and trivalent cation matrix layer may be prepared as well (Williams et al, 2004;Nayak et al, 1997). …”

“…True ion exchange is a topotactic reaction, implying that any layer-stacking defects in the precursor will also appear in the pillared LDH product (Williams et al, 2004;Dimotakis & Pinnavaia, 1990). Another proposed mechanism by which ion exchange occurs is a two-step process involving dissolution of the LDH and recrystallisation of the new LDH with the exchanged anions (Palmer et al, 2009).…”

Layered intercalated functional materials based on efficient utilization of magnesium resources in China

“…LDHs containing chloride and nitrate anions are often used as precursors for the ion-exchange method, because they are easy to prepare and the anions are readily exchanged by a wide range of inorganic and organic anions [37][38][39].…”