Preparation And Research Of Membrane Catalytic Reactor For Propane Dehydrogenation

As an important chemical synthesis material,Propylene has been widely used in chemical products,such as polypropylene,propylene oxide,acrylonitrile,cumene and acrylic acid.With the rapid development of the polymer and petrochemical industry,the demand for propylene is increasing.The main sources of propane are steam cracking,coal to olefins,catalytic cracking,propane dehydrogenation and olefin metathesis.Propane dehydrogenation,as a promising technology,can not only use resources to create higher economic benefits,but also alleviate the pressure of oil shortage and propylene shortage to a certain extent.Propane dehydrogenation includes two methods:oxidative catalytic dehydrogenation and direct catalytic dehydrogenation.Oxidative catalytic dehydrogenation can reach a good conversion rate at low temperature,but it is easy to be deeply oxidized,resulting in greatly reduced the selectivity of propylene.Direct catalytic dehydrogenation has a simple process,but Cr-based catalysts are likely to cause environmental pollution,while Pt-based catalysts is expensive and easy to deposit carbon,resulting in a sharp reduction of catalyst activity and low conversion of propane.In response to this situation,we have developed a membrane catalytic reaction technology based on direct catalytic dehydrogenation,which can remove the hydrogen generated in the reaction from the reaction area,so that the reaction and separation can proceed simultaneously,break the restriction of thermodynamic equilibrium.and improve the conversion rate of propane and the selectivity and yield of propylene.In this paper,the Co@Carbon membrane catalytic reactor with a reaction-separation combination is designed.On the one hand,carbon molecular sieve membrane（CMSM）is used as a selective hydrogen permeable membrane to achieve separation.On the other hand,ZIF-67 is sintered at high temperature as Co-based catalyst for the catalytic dehydrogenation of propane.Based on the design idea of the Co@Carbon membrane catalytic reactor combined with reaction-separation,this paper mainly carries out the following three aspects of work:1.Preparation of carbon molecular sieve membrane（CMSM）.CMSM has attracted much attention due to its high chemical and thermal stability.Polydopamine（PDA）layer has non-toxic,excellent adhesion,high carbon yield and similar to phenolic resin,so it is expected to be used in the preparation of CMS membranes.In this paper,an ultra-thin and high hydrogen permeance CMSM was prepared using PDA as the carbon precursor.After carbonization of the PDA layer at 700℃for 8hours in N2 atmosphere,a compact and ultra-thin CMSM with a thickness of only 80nm has been formed on the Al2O3 disk.The CMSM shows excellent gas separation performances:the H₂ permeance is 2.7×10^-6 mol·m^-2·s^-1·Pa^-1,which is much higher than that of the previously reported CMSMs,and ideal separation factors of H2/CO2,H2/CH4 and H2/C3H8 are 18.3,31.4 and 49.1,respectively,which are far exceed the corresponding Knudsen coefficients（4.7,2.8 and 4.7）.2.Preparation of ZIF-67 membrane.Based on the fact that ZIF-67 prefers to homogeneous nucleation and growth in the reaction solution rather than heterogeneous nucleation and growth on the substrate,it is diggicult to prepare the ZIF-67 membranes with high permselectivity.Inspired by the fact that homologous metal ions are helpful to be acted as nucleation centers to promote the preparation of continuous MOF membranes,we develop a novel and effective synthesis method for in-situ growth of compact ZIF-67 membranes by using Co²⁺ions chelated PIM-1（PIM=polymer of intrinsic microporosity）as interface layer between the ZIF-67membrane and theα-Al2O3 support.After solvothermal synthesis for 48 h at 120℃,a well-intergrown ZIF-67 membrane with a thickness of about 12μm can be formed on the Co²⁺@c PIM-1 modifiedα-Al₂O₃ disk.For single gas permeation at 25°C and 1bar,the ZIF-67 membrane shows a high H₂ permeance of 6.2×10^-7 mol·m^-2·s^-1·Pa^-1,with H2/CO2,H2/CH4 and H2/C3H8 ideal separation factors of 15.1,51.7 and 56.4,respectively.In addition,other compact and H2 permselective ZIF membranes,such as ZIF-7 and ZIF-8 membranes,can be consistently prepared on the Zn²⁺@c PIM-1modified substrates by using the same strategy.3.Preparation and catalytic performance of cobalt catalyst based on ZIF-67-9.Cobalt based MOF materials ZIF-67 and ZIF-9 are one of the most commonly used precursors for preparing high activity catalysts due to their high nitrogen/metal content and easy synthesis.Co/NC nanocomposite catalysts were prepared by pyrolysis of Co-based ZIF-67-9 composite powders with different ligand ratios and different temperatures for 5 h.By pyrolysis in N2 atmosphere,the Co atoms and ligands of ZIF-67-9 become porous Co nanoparticles and N-doped carbon,respectively,forming highly dispersed Co/NC nanocomposites.The porosity and function of the framework are controlled by adjusting the relative content of different ligands.The results show that the carbonization temperature has only a little effect on the propane dehydrogenation performance of the catalyst,while the ligand ratios have a greater effect.the Co/NC-700℃（ZIF-67-9=1:2）has the best catalytic performance,with the propane conversion and propylene selectivity are 10%and 90%,respectively.And,it can be applied in the follow-up research of Co@Carbon membrane catalytic reactor.