Preparation Of Cu-SBA-15 Catalyst And Its Catalytic Performance For Removal Of Dissolved Oxygen In Reclaimed Water

In recent years,the rapid development of China’s economy and the surge of population have led to the increasing contradiction between supply and demand of water resources.Under the condition of the shortage of freshwater resources,it is particularly important to increase the reuse amount of reclaimed water.The number of wastewater purification plants in China is increasing,and the sewage treatment capacity is also improving.However,the main problem limiting the large-scale reuse of reclaimed water in China is the lack of a complete distribution system of reclaimed water,which leads to the blocked reuse of reclaimed water.Constructing a complete reclaimed water transmission and distribution system is not only a huge investment but also difficult in design and construction,so it is difficult to implement effectively.Northern cities in China generally have a complete central heating pipe network,which can cover most areas of the city,and there are nearly 6-8 months of idle periods every year.It is very feasible to use the idle heating pipe network to transport urban reclaimed water.However,because the reclaimed water contains a certain amount of dissolved oxygen(DO),DO can cause serious oxygen corrosion to the heating pipeline in the transportation process,which can not only greatly shorten the service life of the heating pipeline,but also cause safety accidents in serious cases.Therefore,it is necessary to choose appropriate methods to reduce DO content to the prescribed range.Traditional physical and chemical deoxygenation methods require high temperature or high pressure to achieve a better deoxygenation effect,but there are widespread problems such as large energy consumption,high cost,complex equipment,difficult operation,and so on.Compared with traditional methods,the catalytic reduction method is one of the most promising methods of deoxygenation,which can achieve high efficiency at medium and low temperatures.Carbohydrazide is a green and safe chemical deoxidizer used at present.It is a derivative of hydrazine and has the advantages of stable chemical properties,low toxicity,and strong reducibility.However,it reacts slowly with DO at medium and low temperatures.To improve the reaction rate of carbohydrazide with DO,it is necessary to prepare a cheap catalyst with high catalytic activity.In this study,the mesoporous molecular sieve SBA-15 was used as the catalyst carrier,and the cheap transition metals(Fe,Co,Ni,Cu,Zn,Mn)were loaded into the SBA-15 framework by the post-synthesis method to prepare Cu-SBA-15 catalyst,which was used to catalyze the removal of DO from the reclaimed water by carbohydrazide.The removal rate of DO was taken as the evaluation index of the catalyst activity,and the effect of preparation conditions on the catalytic performance of the catalyst was investigated.The effect of experimental conditions on the catalytic removal of DO was discussed,and the stability of the catalyst was investigated.XRD,FT-IR,SEM,EDS,and BET techniques were used to characterize and analyze the catalyst samples.The reaction mechanism of carbohydrazide catalyzed by the catalyst for the removal of DO from the water was explored,and the effects of reclaimed water and simulated water on the removal rate of DO were investigated.Based on the static experiment,the DO removal effect of the formed Cu-SBA-15 catalyst in the dynamic experiment was investigated.The main results and conclusions are as follows:(1)When pure silicon mesoporous molecular sieve SBA-15 was used to catalyze the carbohydrazide to remove DO from water,SBA-15 only showed very weak catalytic activity.Although the characterization results of XRD and FT-IR showed that the pure silicon SBA-15prepared in this experiment was a typical two-dimensional hexagonal phase with regular pore structure and high long-range ordering.However,because pure silicon SBA-15 has no active component and few lattice defects,it does not have excellent catalytic activity and cannot meet the requirements of practical use.(2)The transition metals Fe,Co,Ni,Cu,Zn,and Mn were selected as the active components,and the pure silicon mesoporous molecular sieve SBA-15 was used as the support to support the transition metals into the framework of SBA-15 to prepare the single metal-supported catalyst.By comparing the DO removal rate,the catalyst supported by active metal Cu showed better catalytic activity.The active metal load,ultrasonic time,calcination temperature,and calcination time in the preparation process of the catalyst were deeply investigated,and the best active metal load,ultrasonic time,calcination temperature,and calcination time were 20 min,60 min,550℃and 5 h,respectively.The catalyst Cu(20)-60-550-5-SBA-15 with the best catalytic activity was prepared.The catalytic reduction efficiency of the catalyst to remove DO can reach 99.60%after25 min at 45℃.(3)The catalyst Cu(20)-60-550-5-SBA-15 with the best catalytic activity was selected to optimize the conditions of reaction temperature,catalyst mass concentration,initial pH value of the solution,and mass concentration of carbohydrazide in the static experiment.Under the premise of ensuring high deoxygenation efficiency,considering the deoxygenation cost and operation cost,the final reaction temperature was 40℃,the mass concentration of catalyst was0.5 g/L,the initial pH of the solution was 8,the dosage of carbohydrazide was 3 times of the theoretical dosage,and the deoxygenation effect was excellent.When the reaction time was 20min,the DO removal rate could reach 99.29%.At the same time,the stability of the catalyst was investigated.The catalyst still had good catalytic performance after repeated use three times,and the DO removal rate remained at 86.08%.(4)XRD,SEM,EDS,and BET were used to characterize the catalyst samples.It was found that the active metal Cu was successfully loaded into the framework of the mesoporous molecular sieve SBA-15.The modification of metal loading caused a certain degree of damage to the framework structure,resulting in the increase of lattice defects and the decrease of the order of the pore structure.The specific surface area of Cu(20)-60-550-5-SBA-15 catalyst is 558.27 m~2/g,the average pore size is 5.09 nm,and the pore volume is 0.7970 m~3/g.Compared with the pore parameters of SBA-15,the average pore size is smaller,the specific surface area and pore volume are larger.By FT-IR characterization analysis,it can be seen that the loading of active metal Cu does not change the type of functional groups,but has a significant effect on the number of functional groups.The high catalytic activity of Cu(20)-60-550-5-SBA-15 is mainly attributed to its relatively regular pore structure and highly dispersed active components.(5)In this study,in the process of reducing DO using Cu(20)-60-550-5-SBA-15 catalyst and carbohydrazide,the chemical deoxygenation process plays a dominant role,and the effect of removing DO meets the requirements of heating pipe network for DO.According to the experimental results,the mechanism of catalytic reduction reaction was speculated.The catalytic reaction mechanism could be divided into three steps:under the action of Cu-SBA-15 catalyst,carbohydrazide was firstly decomposed into hydrazine and carbon dioxide,then hydrazine was decomposed into nitrogen and hydrogen atom,and finally,hydrogen atom reacted with DO in the solution rapidly.(6)When the dynamic experimental conditions were set as follows:the reaction solution temperature was 40℃,the hydraulic retention time was 40 min,the dosage of carbohydrazide was 3 times of the theoretical dosage,and the dosage of molding catalyst was 3 g/L.After 120min of deoxygenation reaction,the molded Cu-SBA-15 catalyst showed good catalytic activity,and the DO removal rate reached 99.13%,which provided the possibility for industrial application.