Preparation And Catalytic Performance Of Layered Double Hydroxides Based Heterogeneous Catalysts

Layered double hydroxides(LDHs)are a kind of 2D layered anions intercalated materials,which is constituted by the positively charged layers and the negatively charged anionic layers.LDHs has unique structural characteristics,such as highly ordered and uniformly dispersed metal elements in the layers,topological transformation of the layered structure during the calcination process,adjustable types and quantities of the intercalated anions,controllable size and thickness,etc,which endow them with adjustable properties and thereby a variety of LDHs-based catalytic materials.Topological transformation of LDHs can result in the formation of novel heterostructured metal oxides or supported metal catalysts.The interfical sites formed between the heterostructured metal oxides has been proved to be active sites for promoting charge separation and electrons transfer.Therefore,fabrication of heterostructured metal oxides with rich heterostructured interface by topological transformation of LDHs is an effecent strategy to improve catalytic activity.However,the formation and transformation of active sites at the interface of heterostructured metal oxides and their effects on catalytic performance have not been carefully investigated so far,to the best of our knowledge.In addition,with the controllability of the types and quantities of guest anions,functional anions with catalytic activity can be intercalated into the confined space of LDHs.Due to high dispersion,uniform dispersion and stable dispersion of active speces,the catalytic activity can be improved.However,the geometric configuration and charge of the host-guest interaction can generate significant influence on the development of the LDHs-based intercalated materials.Therefore,anions with large size or small charge density are difficult to be intercalated into the interlayer because of the limitation of the space and the weak interactions.Bearing the above problems and challenges in mind,we reported in this thesis the rational design and fabrication of new LDHs-based heterogeneous catalysts.On the one hand,based on the highly dispersed metal ions at the atomic level and topological transformation characteristics of LDHs,we can adjust the metal elements in the Co-based LDHs precursors and the calcination temperatures to fabricate heterostructured catalysts with heterostructured interface and defects for electrocatalytic oxygen evolution,photocatalytic methane activation reaction and alcohol-amine dehydrogenation coupling reaction.On the other hand,with the confinement effect of LDHs,the polyoxometalates(POMs)anions with large size can be intercalated into the confined space of LDHs by the layer modification method and the exfoliation/assembly method,respectively.The supramolecular assembly of POMs and LDHs can be realized,which leads to the improvement of the dispersion and stability of POMs,and then improves the catalytic activity.The study provides a strategy for the design and structural understanding of LDHs-based catalytic materials,and shows great potential for further application.The details are listed as follows:1.Heterostructured metal oxides of ZnO and Co3O4 with a large number of heterostructured interfaces between ZnO and Co3O4 were synthesized by topological transformation of ZnCo-LDH precursor.The formation and transformation of interface in the topological transformation process were studied in detail by various spectroscopic methods.It was demostrated that the interface and the electronic environment of ZnO/Co3O4 heterostructures can be regulated by changing the calcination temperature to further improve the oxygen evolution reaction(OER)of electrolytic water.The OER performance of different ZnO/Co3O4 heterostructures were evaluated,and the activity decreased with the increased temperature.In particular,ZnCo-200 showed the best performance with an over potential of 370 mV and a tafel slope of 54.7 mV/dec at current density of 10 mA cm-2.By combination of experiments and theoretical calculation,it is proved that the heterostructured interface between ZnO and C03O4 can reduce band gap,promote electron transfer and improve the oxygen evolution performance of electrolytic water.This study will deepen the understanding the topological transformation of LDHs at the molecular level,and open up a new way for controllable synthesis of excellent performance catalysts,fine regulation of heterostructured interface and understanding of structure-function relationship.2.In order to overcome the shortcomings of MgO photocatalyst in photocatalytic methane activation,such as weakly adsorption of H+,absorption of light in ultraviolet region and so on,the MgO-based photocatalytic heterostructured catalysts were prepared by topological transition MgCo-LDH as precursor.Under the full spectrum illumination,the selectivity of ethane could be reached the maximum(?42%)over MgCo-800 when the H2O dosage was 50 pL and the reaction time was 3 h.Various spectroscopic characterizations confirmed that MgO was highly dispersed by Co3O4,and metal element Co was doped into MgO to modify MgO-based photocatalysts,leading to better methane activity and ethane selectivity.3.The Co-based heterostructure catalysts with small particle size and highly exposed faceted Co3O4 nanosheets(MgCo-MMO,CoCo-MMO and ZnCo-MMO)were obtained by topological transformation of MgCo-LDH,CoCo-LDH and ZnCo-LDH precursors and exhibited excellent catalytic activity in alcohol-amine acceptorless dehydrogenative coupling reaction.The resulting ZnCo-MMO heterostructure exhibits outstanding performance in an acceptorless dehydrogenative coupling reaction with>99%yield of amine and the generation of clean energy hydrogen.The(112)faceted Co3O4 and the sites at the interface of Co-based heterostructure catalysts make synergistic effect to promote charge separation and electron transfer in the reaction process,which can reduce the reaction energy barrier and enhances the catalytic activity.4.In order to realize of cascade reaction for simplifing the synthetic pathway,reducing by-products and decreasing energy consumption,design and fabrication of multi-sites catalysts with space isolation to catalyze different reactions step-by-step is an effective method.The condensation product yield of 99%over Tris-LDH-Zn4(PW9)2 at 80 0C for 6 h can be achieved with benzyl alcohol and ethyl cyanoacetate as the reaction substrate,H2O2 as the oxidant,and acetonitrile as the solvent.Combining with the abundant surface alkaline sites of LDHs and the strong redox and fast electron transfer ability of POMs,a novel POMs intercalated LDHs bifunctional catalyst was designed and prepared for alcohol oxidation-Knoevenagel condensation scascade reaction.On the one hand,the electrostatic and hydrogen bonding effects of LDHs and POMs restrict the movement of POMs in the confined space,which can promote dispersion and stability of POMs and enhance the activity and selectivity of alcohol oxidation reaction.On the other hand,Knoevenagel condensation reaction occurred at the abundant base sites on the layers of LDHs.The synergistic effect of the POMs and LDHs enhances the activity and selectivity of cascade reaction.5.In order to improve the mass transfer efficiency of liquid-solid reaction system with POMs intercalated LDHs materials as catalysts constructed by exfoliation/assembly method,the flexible ionic liquids were covalently modified on the layers of LDHs to construct a new lanthanide-containing POMs intercalated ionic liquids modified LDHs catalyst.The catalyst can be applied to the deep oxidation desulfurization reaction,and the sulfur-containing substrate can be completely removed the sulfur-containing substrate(DBT)within 25 minutes over the Mg3Al-IL-EuW10 under the mild condition.The presence of ionic liquids significantly improves the mass transfer performance of substrates and promotes the accessibility of substrates and catalytic active sites,which makes the reaction more efficient.