Fluorinated Hydrotalcites: The Addition of Highly Electronegative Species in Layered Double Hydroxides To Tune Basicity

Abstract

Hydrotalcite-like compounds were synthesized by coprecipitation using a constant-pH method. Aluminum was introduced in the form of an octahedral aluminum fluorine complex during the synthesis. Controlling the pH during the synthesis ensured that aluminum fluorine did not decompose to tetrahedral species but remained in the octahedral (AlF(6))(3-) form to be incorporated into brucite-like sheets. The physicochemical, thermal, and spectroscopic characterizations showed significant modifications by fluorine introduction regarding structural, textural, and adsorption properties. The memory effect of fluorinated hydrotalcites differed from the memory effect commonly observed in fluorine-free hydrotalcites. Nitromethane adsorption showed that the basicity of hydrotalcite was greatly modified by the fluorine loading.

Fluorinated Cu/Zn/Al/Zr hydrotalcites derived nanocatalysts for CO2 hydrogenation to methanol

“…The first step of thermal decomposition of HTlcs at 373-473 K included two overlapped peaks, which can be assigned to the elimination of physically adsorbed water and the remove of the interlayer water molecules. It can be seen that the introduction of (AlF 6 ) 3À into the HTl structures increased the temperature of interlayer water loss, because fluorine can increase the strength of hydrogen bonds with hydrogen atoms from water [26]. Subsequently, the second weight loss at 473-623 K may ascribed to the removal of both the hydroxyl groups from the HTl network and carbonate anions from the interlayer anion, resulting in the collapse of the layered structure [33].…”

“…These results were related to the stability of (AlF 6 ) 3À species in solution. There exists an ionic equilibrium between (AlF 6 ) 3À and (AlF 4 ) À in aqueous aluminumsodium fluoride mixtures and only (AlF 6 ) 3À can react to be incorporated to hydrotalcite-like (HTl) structure under alkaline conditions as those used in a coprecipitation synthesis of hydrotalcite-like compounds (HTlcs) [26]. To accomplish the LeChatelier's principle, (AlF 6 ) 3À species were produced continuously during the synthesis.…”

“…It was to be noticed that the parameters c decreased with increase of fluorine content. Fluorine is able to form strong hydrogen bonds with hydrogen atoms from water, thus the decrease in c parameter can be due to the stronger attraction between the interlayer anions and the HTl layer [26]. Fig.…”

“…In the HTl structure, the M 3+ is surrounded by six oxygen atoms of the OH À anions as first neighbors and the fluorine ions can be directly introduced into HTl layers using a novel preparation method with partial substitution of the (Al(OH) 6 ) 3À octahedra by (AlF 6 ) 3À [26,27]. Therefore, fluorinated HTlcs can be easily obtained with a wide range in fluorine content.…”

Efficient KF loaded on MgCaAl hydrotalcite-like compounds in the transesterification of Jatropha curcas oil

“…Now, coming back to chemical shift at À182 ppm, it is closer to À190 ppm and it should be concluded that enriched fluorine AlF 6Àx O x species are formed. The fluorination of mixed oxide Mg(Al)O emerged from the thermal treatment of LDH was reported, but a better distribution of fluorine was obtained because of the method used by Lima et al [40]. In our case, the impregnation seems to be more local avoiding a high dispersion of fluorine into the structure of mixed oxide.…”