The Research For The Adsorption-Desorption Behavior And Catalytic Performance Of Hydroxide-Based Layered Compounds

Hydroxide-based layered compounds consist of layered multi- and single-metal hydroxides, and the metals are changeable ?e.g., common metals, transition metals, rare-earth metals and so on?. On account of the structures and properties of hydroxide-based layered compounds, the virtues of two-dimensional layered nanomaterials and metal ions are simultaneously existed on them, so that they have been widely used in adsorption, separation, catalysis, biology, and optical, electric, magnetic fields. Based on two kinds of hydroxide-based layered compounds, layered double hydroxides ?LDHs? and layered rare-earth hydroxides ?LRHs?, we respectively studied their adsorptive property and catalytic performance and applied them in analytical fields. The specific research contents have been discussed as follows:?1? Carbonate intercalated-LDHs ?CO₃-LDHs? were used as adsorbents to adsorb anionic dyes ?e.g., methyl orange? through electrostatic attraction. Polyethyleneimine was applied as a desorbent for the desorption of methyl orange from the surface of CO₃-LDHS. It was observed that the introduction of polyethyleneimine in the methyl orange-adsorbed CO₃-LDHs could easily desorb methyl orange from LDHs in a wide range of pH values. The mechanism studies demonstrated that the desorption of methyl orange at low pH values ?<9.5? was ascribed to electrostatic attraction between protonated polyethyleneimine and negatively charged methyl orange; at high pH values ?>9.5?, it was attributed to the cooperative contributions of hydrogen bonding between the sulfonate group of methyl orange and amino groups of electroneutral polyethyleneimine, as well as anion-exchange between abundant hydroxyl anions and anionic methyl orange. The adsorption capacity of the reused CO₃-LDH adsorbent was about 80% after five cycles of adsorption-desorption regeneration, which was much higher than that conducted by 0.1 M NaOH solution. Polyethyleneimine could be regarded as a novel and promising desorbent to open up a new way to investigate the adsorption-desorption of anions on LDH surface.?2? A composite adsorbent was fabricated via co-intercalation of carbon source ?sodium citrate? and surfactant ?sodium dodecyl sulfate, SDS? into LDH interlayer and then in-situ hydrothermal synthesis of organic-modified LDH-graphene quantum dots ?GQDs? composite ??GQDs+SDS?-LDHs?. The adsorption performances of the resulting ?GQDs+SDS?-LDHs towards neutral organic compounds were evaluated for the removal of 2,4,6-trichlorophenol. The experimental results showed that the adsorbent exhibited high adsorption capability towards 2,4,6-trichlorophenol ?119 mg/g? and the adsorption behavior of this adsorbent fitted well with Langmuir isotherm and pseudo-second-order kinetic model. The mechanism studies for the improved adsorption capacity of ?GQDs+SDS?-LDHs towards 2,4,6-trichlorophenol were speculated to the cooperative contributions of hydrogen bonding and ?-? interaction between GQDs and 2,4,6-trichlorophenol, as well as hydrophobic interaction between hydrophobic chain of SDS and 2,4,6-trichlorophenol. This new adsorbent could not only adsorb other neutral organic compounds but also open up new possibilities in fabricating composite adsorbents based on organic-modified LDHs and GQDs.?3? Layered rare-earth hydroxides ?Y-NO₃-LRHs? and their calcined product of Y₂O₃ were applied as catalysts to investigate the effect of chemisorbed oxygen on cataluminescence selectivity. The experimental results demonstrated that Y-NO₃-LRHs showed a perfect cataluminescence selectivity towards ethyl ether, but Y₂O₃ could catalytically oxidize many kinds of volatile organic compounds ?VOCs? to produce the cataluminescence signals. The mechanism of cataluminescence selectivity was originated from different contents of chemisorbed oxygen between Y-NO₃-LRHs and Y₂O₃. For Y-NO₃-LRH catalyst which has less chemisorbed oxygen, only ethyl ether could generate the cataluminescence emissions when its electronic excited state intermediates of CH₃CHO ?CH₃CHO^*? returned to their ground states. But enough chemisorbed oxygen on Y₂O₃ can catalytically oxidize not only ethyl ether into CH₃CHO^* but also other VOCs into electronic excited state intermediates of CO2 ?CO2^*?. Therefore, Y-NO₃-LRHs and their calcined product of Y₂O₃ were successfully applied as catalyst models to deeply demonstrate that chemisorbed oxygen plays an important role on the cataluminescence selectivity.