Preparation, Characterization And Application Of MgMAL(M=Co Or Fe) Hydrotalcite-Like-Compounds

Hydrotalcite, also named anionic clays, is a kind of pillared clay materials with broad applications because of its properties of exchangeable anion and adjustable pore size. In this paper MgCoAl-, MgFeAl-HTLcs were synthesized with introduction of Cobalt and Iron by a method of variable pH value, then the effect of factors on phase were studied in details and their physicochemical properties were characterized. The catalytic action of HTLcs was investigated by the oxidation of benzaldehyde to benzoic acid using H2O2 as oxidant. In addition, surface modification of ZnAl-HTLcs was carried out through surface active agent, and the effect of sodium stearate on phase and particle size of prepared HTLcs werestudied.During preparation of HTLcs, the titration curves of different nitrate solutions using NaOH as precipitator were considered at first, and feasible pH value was confirmed, then the effect of many factors on phases, such as composition of nitrate solutions, hydrothermal treatment, calcined temperature, and so on, were determined. The compositions of products were analysed by element analysis. The construction and the relative crystallinity of samples were detected by FT-IR and XRD. TG-DTA conclusions were performed to characterize their thermal decomposition. HTLcs crystal dispersibility and particle size at different situations were analyzed by particle size analyzer.Due to the difference of element Co and Fe, the conditions and properties of prepared HTLcs were also different. It was concluded that the single MgCoAl-HTLcs phase was obtained when it was hydrothermally treated at 110 C for 6h as pH=5.5~6.2 and 7.7-8.5, the typical XRD patterns of the hydrotalcite structure were sharp and symmetric with high crystallinity. But the results of element analysis showed that the two products had different metalcompositions but the same phase. FT-IR spectrum showed that patterns of M-O bond shifted to short-wave zone because of different Co quantity. From TG-DTA spectra MgCoAl-HTLcs could be changed to mixed metal oxides at lower temperature owing to the destroyed layer.MgFeAl-HTLcs would be got as hydrothermal treated for 6h at 110 C. The same phase with different composition could be obtained both in the range of 7.5~8.0 and 9.4~10.0 of pH value. According to the TG-DTA results, there were two endothemic peaks, which respectively belonged to the dehydration on the layers and the decomposition of nitrates and a dehydroxylation process. Due to the element Fe the particle size of MgFeAl-HTLcs was diminished.The oxidation of benzaldehyde to benzoic acid using H2O2 as oxidant was carried out to determine catalytic properties of prepared HTLcs. It proved that catalytic actions of MgCoAl-, MgFeAl-HTLcs on this reaction were very well. Because during a short time a majority of benzaldehyde had been changed to benzoic acid, and benzoic acid selectivity of 100% almost. Benzaldehyde conversion was attained to 86.3% when 5ml benzaldehyde reactedVIIto 9.5ml H2O2 for Ih with 1.4g MgCoAl-HTLcs as catalyst, while with MgFeAl-HTLcs conversion was 85.4% at the same condition. They both had higher catalytic activity and selectivity.Simultaneously, surface modification of ZnAl-HTLcs was made by surface active agent. Sodium stearate molecule was absorbed on HTLcs surface so as to prohibit congregation of HTLcs particles and to make particles disperse equably and minish size. The results showed that HTLcs construction wasn't been destroyed when sodium stearate weight was 0.11~1.5%, and HTLcs particle size was smaller. While using superfluous sodium stearate HTLcs phase would be breakaged. Different sodium stearate weights had effect on phase and thermal decomposition of samples, it would greatly help to in-depth study of preparation of nanometer powders.