The Research Of Medium-High Temperature Mixed Conductive Ceramic Membrane And Catalytic Transformation Of Small Carbon-Based Molecules

Producing ethylene is the core work of petrochemical industry,because ethylene is one of the most important and most productive chemical intermediates in industrial chemistry.The traditional methods of producing ethylene,such as naphtha cracking and fluidized bed catalytic cracking,have undoubtedly worsened the environment and accelerated the oil consumption.Natural gas（mainly methane）has the advantages of large reserves,high calorific value and relatively cleaner.Whether in terms of strategic development of energy or diversity of resource utilization,it is extremely momentous and urgent to obtain high value-added chemicals from CH₄.In general,there are two ways to make ethylene from methane:1.Indirect conversion:Methane is partially oxidized to syngas,from which the target product can be obtained.2.Direct conversion:Methane reacts in one step to produce ethylene.Indirect conversion involves multi-step reaction,so the process is complex and the energy consumption is very high.For this reason,direct conversion has been widely concerned by researchers.Membrane reactors were first proposed by scientists in the 1960s.Using oxygen permeable membrane or hydrogen permeable membrane to directly convert methane to ethylene,not only simplifies the reaction device,shortens the reaction path,but also reduces the energy consumption of the reaction.This paper focuses on the preparation of asymmetric porous-dense ceramic oxygen and hydrogen permeable membranes.We also focused on their oxygen（hydrogen）permeability and their use as membrane reactors to realize direct conversion of methane to ethylene.The introduction part describes the development of oxygen and hydrogen permeable membranes,oxygen permeation mechanism,hydrogen permeation mechanism and membrane reactor research status.At the end of this chapter,the research ideas,main contents and innovative points of this paper are illustrated.In chapter 1,the preparation method of perovskite(La_0.8Sr_0.2)_1-xCr_0.5Fe_0.5O_3-δ（x=0,0.02,0.05,0.10）and Ba Ce_0.9Y_0.1Co_xO_3-δ（x=0,0.01,0.05,0.10）precursor powders was studied.The asymmetric membrane prepared by suspension coating method has a dense functional layer（about 100μm）and a porous layer（about 500μm）.We also controlled the non-stoichiometric ratio of the perovskite-type（ABO₃）ceramic membrane during the preparation process,which will resulted in the absence of the A position of LSCr F.Similarly,the doping of Co at the B site of BCY will also leads to the absence of the A site.After the ceramic membrane is treated with reducing atmosphere,the excess transition metals at the B site will be exsolved on the surface of the membrane in the form of metal nanoparticles.The metal nanoparticles can form the metal-oxide interface with the oxide substrate,which not only helps to activate the methane C-H bond,but also helps to enhance the coking resistance of the membrane.We carried out XRD tests on the oxidized and reduced samples of LSCr F and BCYC,and found that Fe and Co peaks appeared in the reduced XRD patterns.In XPS test,we found a large number of low-valent and zero-valent Fe and Co elements on the surface of the reduced sample.In SEM test,a large number of nanoparticles were found on the surface of the reduced sample.All these indicate that we have successfully constructed the metal-oxide interface on the surface of the ceramic oxygen permeation membranes and hydrogen permeation membranes.In chapter 2,the oxygen permeability and oxidative coupling of methane（OCM）performance of porous-dense(La_0.8Sr_0.2)_1-xCr_0.5Fe_0.5O_3-δ（x=0,0.02,0.05,0.10）were investigated.In the oxygen permeability test,high purity argon gas was passed on the porous side and air was passed on the dense side.We found that the oxygen permeability of LSCr F increased with the increase of temperature,which was in accordance with the general law of Wagner equation.(La_0.8Sr_0.2)_0.95Cr_0.5Fe_0.5O_3-δhas the best oxygen permeability,and its oxygen permeation flux can reach 0.47ml?min^-1?cm^-2at 1150℃.The oxygen permeation flux can be maintained at a full temperature gradient for more than50h.In the OCM test,high purity methane was passed on the porous side and air was passed on the dense side.The methane conversion increased with the increase of temperature.The C₂selectivity can also be gradually increased by regulating the methane feeding rate.At 1150℃,the methane conversion reached 28%and the C₂selectivity reached 40.2%.However,carbon deposition and by-products such as naphthalene,biphenyl and anthracene will occur when the reaction temperature is above 1100℃,which will limit the stability of OCM reaction.In chapter 3,the hydrogen permeability and dehydrogenative coupling of methane（DCM）performance of porous-dense Ba Ce_0.9Y_0.1Co_xO_3-δ（x=0,0.01,0.05,0.10）hydrogen permeation membranes were investigated.In the hydrogen permeability test,the H₂/Ar mixture was passed on the porous side and the high purity argon gas was passed on the dense side.We found that the hydrogen permeability of BCYC increased with the increase of temperature.When the temperature reaches 1100℃,the maximum hydrogen permeation flux reaches 0.205m L·min^-1·cm^-2.The stability of hydrogen permeation flux can maintain more than 80h under the full temperature gradient.In the DCM test,high purity methane was passed on the porous side and high purity argon was passed on the dense side.Similar to the case of oxygen permeable membrane,methane conversion rate increases with the increase of temperature.The C₂selectivity can also be gradually increased by regulating the methane feeding rate.At 1100℃,the methane conversion reached 24.8%and the C₂selectivity reached 45.6%.It is worth mentioning that,similar to the oxygen permeation membrane,Ba Ce_0.9Y_0.1Co_0.5O_3-δexhibits superior hydrogen permeation performance and DCM performance.It is also confirmed that the metal-oxide interface can significantly promote the activation of methane.It is also confirmed that the metal-oxide interface has a significant catalytic effect on methane to olefins.The fourth chapter is the summary and prospect of this work.