Tuning the base properties of Mg–Al hydrotalcite catalysts using their memory effect

Herein,the relationship between the structure and base properties of Mg–Al hydrotalcite catalysts was comprehensively investigated in relation to heat treatment and rehydration processes,which are well known as memory effects of hydrotalcite.The structure of Mg–Al hydrotalcites changed from layered double hydroxide(LDH)to mixed metal oxide and subsequently to a spinel structure during heat treatment,and it was returned from mixed metal oxide to a LDH structure by rehydration.Based on various characterizations,we successfully proposed a detailed mechanism of memory effect.We also confirmed that the Mg–Al hydrotalcites had weak or strong base sites and that their base properties could be systematically tuned by heat treatment and rehydration.The prepared Mg–Al hydrotalcites were applied to a model reaction,isomerization of glucose to fructose,as base catalysts.Among the catalysts tested,the rehydrated Mg–Al hydrotalcite efficiently produced fructose due to its appropriate base and structure properties.We finally concluded that the base sites of Mg–Al hydrotalcites can be designed as desired by heat treatment and rehydration.Moreover,through systematic design of the base sites of Mg–Al hydrotalcites,these can be promising catalysts for various heterogeneous reactions over base catalysts,giving excellent catalytic performances.

Reusable high performance of calcined Mg/Al hydrotalcite for the removal of Navy Blue and Yellow F3G dyes

Water pollution with dye chemicals from apparel industries is a serious problem in the world.Since most of dyes are potentially have toxic and carcinogenic effects on human,it is important to remove them before they are discharged to the environment.Among many methods available for dyes removal in water,adsorption is the easiest and economically feasible that has no major obstacle for practical applications.In the present study,we tested calcined Mg/Al hydrotalcite(Mg/Al CHT)prepared by coprecipitation technique as an adsorbent for the removal of Navy Blue(NB)and Yellow F3G(YF3G)dyes.Mg/Al CHT was characterized by using a Fourier transform infrared(FTIR)spectrometer,an X-ray diffractometer(XRD)and a scanning electron microscope(SEM).The results showed that Mg/Al CHT was highly effective as an adsorbent for the removal of NB and YF3G under mild-acidic condition(pH 4)with removal capacities(b)according to Langmuir isotherm model were 7.97×10^(–4) and 5.80×10^(–4)mol·g^(-1),respectively.Kinetics study showed that the adsorption of NB and YF3D on Mg/Al CHT followed pseudosecond order with rate constant(k_(p2))11.57×10^(3) and 11.75×103 g·mol^(-1)·min-1,respectively.The spent adsorbent can be easily regenerated by simply calcining it at 450℃ for 3 h.Adsorption test on the mixture of NB and YF3G showed that the adsorption capacity of Mg/Al CHT was eightfold higher than that of Mg/Al HT and the value was maintained with repeated use.