Study On The Construction Of Novel MOFs And Their Inorganic Composites And Their Photocatalytic Performance Of Carbon Dioxide

With the rapid development of global society and economy,human’s demand for energy has increased sharply.85%of the world’s energy comes from fossil fuels such as coal,oil,and natural gas.Excessive reliance on fossil fuels has led to an energy crisis.At the beginning of the 20th century,the worldwide burning of fossil fuels led to an increase in atmospheric CO₂concentration from 280 ppm to 400 ppm.Excessive emissions of CO₂ raised the atmospheric temperature,leading to global warming,which will bring about various environmental problems.Therefore,it is urgent to study the methods to reduce atmospheric CO₂concentration.In this case,converting CO₂ to chemical fuel is considered to be one of the most effective solutions.However,because linear CO₂ molecules are very stable,their complex conversion process and similar product potential lead to low catalytic efficiency and poor product selectivity.MOFs have the advantages of high crystallinity,permanent porosity,ultra-high specific surface area,adjustable pore size,fine-tuning functional sites and designed framework topology,which make MOFs show great application potential in gas adsorption and separation,catalysis,luminescence,biomedicine,proton conductivity,sensors and other aspects.In particular,MOFs have the following unique advantages as catalysts for photocatalytic or electrocatalytic reduction of CO₂:high porosity of MOFs provides free diffusion of gas molecules through MOF channels;The clear structure of MOFs provides a perfect platform to study the reaction process and explore the mechanism of photocatalysis or electrocatalysis at the molecular level.The good CO₂ adsorption capacity of MOFs leads to a high CO₂ concentration near the active site,which is an ideal condition for photocatalytic or electrocatalytic reduction of CO₂.Because of these advantages,MOFs have been successfully used in photocatalytic and electrocatalytic reduction of CO₂.Although MOFs perform well in many fields,the photocatalytic efficiency of MOFs is worse than that of inorganic semiconductors,mainly because of the low conductivity and charge separation efficiency,which can be effectively overcome by the composite construction of MOFs and inorganic semiconductor materials.Cu₂O is a p-type semiconductor with a band gap of about 2.2 e V and can be excited by visible light.It has the advantages of cheap and easy to obtain,simple preparation method,high catalytic property,good electromagnetic,good optical properties and morphology.For the study of Cu₂O,foreign as early as in the late 1970s has begun,most of the research focused on the antifouling ability of Cu₂O,the content has been relatively mature and perfect,its antifouling performance is basically equivalent to the organic tin antifouling coating.Cu₂O has good catalytic performance under visible light.Therefore,the combination of n-type porous MOFs and p-type semiconductor Cu₂O to construct p-n junction can effectively take advantage of the advantages of high adsorption performance of MOFs,good band adjustable performance,semiconductor conductivity and narrow band gap width,and improve the effective separation of semiconductor electron hole pairs by forming heterojunction.In this paper,conjugated 1,3,5-trimethyl-2,4,6-tricarboxyphenyl was selected as the main organic ligand,and nitrogen containing auxiliary ligand was introduced.Six MOFs were synthesized by solvothermal method and transition metal salts.The example of Co-MOF was selected,and its performance in the liquid phase of photocatalytic CO₂ reduction was explored.At the same time,MOFs and inorganic composite materials were designed and synthesized,and the performance of the composite in the photocatalytic CO₂ reduction gas solid phase was explored.1.Six cases of transition metal organic frame materials were successfully designed and synthesized by simple solvothermal method.The photocatalytic performance of CO₂reduction（liquid phase）was studied by selecting two-dimensional layered Co-MOF with unsaturated catalytic sites and stability.The results showed that when the photosensitizer was present,the addition amount of Co-MOF and Ru（bpy）₃Cl₂ was 5 mg,the catalytic effect was the best.Under the synergistical action of the two,the average rate was 3728μmol g^-1h^-1,and the CO selectivity was about 75%.2.The composite x CMC（CM is Co-MOF,C is Cu₂O,x is Co-MOF mass ratio,x=13wt%,23wt%,33wt%,43wt%）was prepared by oil bath stirring method.The results show that 33wt%CMC has a certain photocatalytic potential.Therefore,under the condition of 1 m L pure water without sacrificing agent in the gas solid phase,when the mass of catalyst is 20 mg,the catalytic product is only CO,the selectivity is about 100%,the catalytic effect of 4 h is13wt%CMC 5μmol g^-1,23wt%CMC 7.5μmol g^-1.33wt%CMC 15.3μmol g^-1,43wt%CMC5.7μmol g^-1,Cu₂O 1.6μmol g^-1,Co-MOF 0.017μmol g^-1,found that the best catalytic effect of 33wt%CMC for 4 h was about 9.5 times that of Cu₂O.900 times the Co-MOF.3.ZIF-67 and Cu₂O composite material was successfully designed and synthesized by a simple preparation method,named as x-ZIF-67（x is the mass of Cu₂O,x=10,15,20,25,mg）.A series of characterizations were made for the composites under four masses.The results show that 20-ZIF-67 has a certain photocatalytic potential.Therefore,in the gas-solid phase of 1 m L pure water without sacrificing agent,the catalytic product was only CH₄ when the mass of catalyst was 20 mg,the selectivity was about 100%,and the catalytic effect was 10-ZIF-67μmol g^-1,15-ZIF-67 0.5μmol g^-1,respectively,in four hours.20-ZIF-67 1.0μmol g^-1,25-ZIF-67 0.3μmol g^-1,Cu₂O 0.5μmol g^-1,and ZIF-67 0μmol g^-1.The results showed that the best catalytic effect of 20-ZIF-67 was about 2 times that of Cu₂O for 4 h.