Formation of Base Sites on Calcined Mg−Ga Hydrotalcite-like [Mg_1-xGa_x(OH)₂](CO₃)_x/2·mH₂O

Abstract

A series of Ga-substituted hydrotalcite-like materials (GaHTs), [Mg 1-x Ga x (OH) 2 ](CO 3 ) x/2 ‚mH 2 O where x ) 0.12-0.36, were prepared in order to examine their textural and basic properties upon different Mg/Ga contents and calcination temperature. Specific surface area increases from 46 to 156 m 2 g -1 (Mg/Ga ) 7.7) when calcining from 523 to 673 K as a consequence of dehydroxylation and decarbonation of the material. The total number of base sites increases upon higher calcination temperatures until reaching a maximum at 873 K and then decreases. CO 2 thermal programmed desorption indicates that different Mg/Ga relative contents have a considerable effect on the amount of base sites. For instance, a GaHT with Mg/Ga ) 7.7 exhibits 2.4 times more total base sites than that of Mg/Ga ) 2.9. Additionally, in the former, the generation of medium and strong base sites is considerably higher than in the latter. By means of CDCl 3 adsorption and FTIR, several types of base sites were identified. The base sites strength (pK a ) -1 to +19) and proton affinity (815-987 kJ mol -1 ) values were estimated considering the ν CD shift of adsorbed CDCl 3 and correlating it with the pK a of bases that form hydrogen-bonded complexes.

Activity of Basic Catalysts in the Meerwein–Ponndorf–Verley Reaction of Benzaldehyde with Ethanol

“…The results for the mixed oxides containing Mg (Figs. 2a and 2b) were similar to those reported by López-Salinas et al (14); the strongest peak was also that observed at the highest temperature, so, as in MgO-600, the strongest basic sites predominate over those of medium and low strength. The two calcium-containing solids (HT-Ca/Al-500 and CaO-900) possess much smaller numbers of basic sites than the other catalysts; CaO-900 is similar to Mg-600 in that it abounds with strong basic sites, whereas CaO/Al 2 O 3 contains weak sites predominantly.…”

Fluoride Removal from Aqueous Solutions by Magnesium, Nickel, and Cobalt Calcined Hydrotalcite‐like Compounds

“…Hydrotalcites-like compounds (HT) were synthesized considering the techniques reported by López-Salinas et al (24) and Hesiquio-Garduño et al (25) as follows: two mixed aqueous solutions, the first one composed of M(NO 3 ) 2 and Al(NO 3 ) 3 , and the second of KOH and K 2 CO 3 were added to hot deionized water (333 K) maintaining a M 2þ :Al 3þ ratio of 2:1. The pH during reaction was around 10 and it was controlled using KOH and K 2 CO 3 .…”

Local environment and composition of magnesium gallium layered double hydroxides determined from solid-state 1H and 71Ga NMR spectroscopy

“…Here we report the results of a combined 1 H and 71 Ga SSNMR study of magnesium gallium (MgGa) LDH's with nitrate intercalated prepared using co-precipitation [Mg1-xGax(NO3)x yH2O] [19,20]. Gallium has two NMR active isotopes, 69 Ga and 71 Ga, which are both quadrupolar nuclei (I = 3/2) with high Larmor frequencies of which the latter is generally preferred due to a smaller quadrupole moment.…”