Reinvestigation of Dehydration and Dehydroxylation of Hydrotalcite-like Compounds through Combined TG-DTA-MS Analyses

Abstract

We reinvestigated dehydration and dehydroxylation of hydrotalcite and four other hydrotalcite-like compounds (HTlcs) with the combined thermogravimetry/differential thermal analysis/mass spectrometry (TG-DTA-MS) technique. The observations indicate that HTlcs undergo dehydration first and then dehydroxylation, followed by or overlapping with decomposition of interlayer anions. The detail deconvolution of MS curves of m/z = 18 reveals that both dehydration and dehydroxylation occurs in two consecutive steps. Dehydration of crystalline water molecules starts on the surface and edge in the first step, and then continues to the interlayer water. Both steps are influenced by the nature of the interlayer anions and layer cations. For the first time, we have assigned two dehydroxylation steps to two types of hydroxyl groups in the HT lattice, i.e. OH−(M(II)3) and OH−(M(II)2M(III)). The thermal stability of these two hydroxyl groups is principally determined by the nature of the cations. According to this research, the thermal stability of hydroxyl groups in HTlcs is OH-(Ca3) (∼480 °C) > OH−(Mg2Al) (∼410 °C) > OH−(Mg2Fe) (∼350 °C) ≈ OH−(Mg3) (300−370 °C) ≈ OH−(Ca2Al) (∼330 °C) > OH−(Ca2Fe) (∼290 °C) at a ramping rate of 2 °C/min.

Distribution of OH bond to metal-oxide in Mg3−Ca Fe-layered double hydroxide (x= 0–1.5): Its role in adsorption of selenate and chromate

“…5 could be attributed to dehydration (D-H 2 O) 25-250°C and dehydroxylation (D-OH) at 250-500°C. This assignment accorded well with the result in our previous works [19,21]. Fig.…”

“…However, the shoulder band was located at 3461 cm À1 in LDH 0 and LDH 0.3 . This change of m-OH bands at various x values is contributed to the strong affinity of hydroxyl group connected to Ca-O-Fe in the LDH structure [21]. On the other hand, there also observed metal-O vibrations at 572-578 cm À1 , splitting bands of CO …”

“…It was proposed that the temperature of OH loss followed the order: OH-(Ca 3 )(480°C) > OH-(Mg 2 Fe) (350°C) % OH-(Mg 3 ) (300-370°C) > OH-(Ca 2 Fe) (290°C) [21]. This suggests that the affinity of OH-Fe was stronger in MgFe-LDH structure than that in CaFe-LDH structure.…”

“…Our previous report showed that dehydroxylation temperature of OH bond to Ca was stronger than that to Mg [21]. This may lead to different removal behavior of oxyanion by the ternary LDH.…”

Supported Catalysts for CO2 Methanation: A Review

“…where M 2+ and M 3+ are any divalent and trivalent cations, A n− is an exchangeable interlayer anion (organic or inorganic), and the water molecules are either crystalline in the lattice or physisorbed on the surface [189].…”