Synthesis And Adsorborption Properties Of Organic-inorganic Hybrid Materials Based On Keggin-type Polyoxometalates

Polyoxometalates (POMs) are composed of transition metals (Mo, W, V, Nb or Ta)in high oxidation states and oxygen atoms. POMs are widely used in areas such asfunctional materials, catalytic and adsorption due to their characteristics such as strongacidity, structure diversity, so POMs have significant research values and promisingapplications. As one type of POMs, Keggin-type POMs are usually adopted asimportant building blocks because of their structure stability and suitable size. Threekinds of Keggin-type POMs and bipyridyl complex of nickel and cobalt were chosen asprecursors to synthesize six organic-inorganic compounds by chemical coprecipitationmethod. These compounds were characterized by elemental analysis, FTIR, TGA andBET. The adsorption abilities of all six compounds were investigated by removingrhodamine B (RhB) dye from aqueous solution, where contact time, pH, temperature,adsorbent dose, initial dye concentration and the reusability have been studied.Langmuir isotherm model was fitted well for these adsorption systems, and theiradsorption processes obey pseudo-second-order kinetics. The details were summarizedas follows:(1) The concepts, development and the actuality of coordination chemistry,polyoxometalates and ordinary adsorption materials were introduced.(2) The chemicals and instruments used in this paper were listed, and theexperimental methods researching the influences of different parameters on theadsorption process were iontroduced.(3) POMs frameworks [Ni(2,2′-bipy)3]3(PW12O40)2(1) and[Co(2,2′-bipy)3]3(PW12O40)2(2) were synthesized from reactions of phosphotungsticacid with [Ni(2,2′-bipy)3]Cl2and [Co(2,2′-bipy)3]Cl2by the facile chemicalcoprecipitation method. The FTIR analysis and element analysis suggests that thestructure of pyridine and polyoxometalate anions in these compounds remains intactthrough synthetic reaction. BET shows that the specific surface of these compoundswere considerable and the specific surface of [Ni(2,2′-bipy)3]3(PW12O40)2(1) reached 61.279m2/g. The TGA shows that all these compounds have good thermal stability andformed porous materials. The result of absorption experiment showed that thesecompounds performed great capability in adsorbing RhB and decolorized RhB greatly.(4)[Ni(2,2′-bipy)3]3(PMo12O40)2(3) and [Co(2,2′-bipy)3]3(PMo12O40)2(4) weresynthesized from the reactions of phosphomolybdic acid with [Ni(2,2′-bipy)3]Cl2and[Co(2,2′-bipy)3]Cl2. The FTIR and element analysis suggests that the structure ofpyridine and polyoxometalate anions in these compounds remains intact through thesynthetic reaction. The TGA shows that compound (3) has better thermal stability thancompound (4). Nitrogen adsorption characterization shows that the specific surfaces ofthese compounds are considerable. The result of absorption experiment shows thatthese compounds performed great capability in adsorprion process.(5) Silicotungstic acid,[Ni (2,2′-bipy)3] Cl2and [Co(2,2′-bipy)3]Cl2were used tosynthesis [Ni(2,2′-bipy)3]2SiW12O40(5) and [Co(2,2′-bipy)3]2SiW12O40(6). The FTIRand element analysis suggests that the structure of pyridine and polyoxometalate anionsin these compounds remains intact through the synthetic reaction. The TGA shows thatthe thermal stability of compound (6) is better than that of compound (5). The result ofabsorption experiment shows that the specific surfaces of these compounds areconsiderable, and compound (6) performed better capability in adsorbing RhB thancompound (5).