In-Situ Directional Construction Of TS-1 Zeolite Membranes And Study On Catalytic Performance Of Epoxidation Of Chloropropene

In the 21st century,realizing the comprehensive development of industrial catalysis in green,economic and safe is not only the ultimate goal of researchers,but also responds to the core concept of"green chemistry"and the strategic requirements of sustainable development.Developing new process routes with high atomic economy and designing catalysts with high activity and low pollution are important measures to achieve the above goals.TS-1 titanium silicalite zeolite is a derivative of pure silicalite zeolite,which is widely used in selective oxidation of industrial organic molecules due to its unique pore structure and catalytic activity of skeleton titanium species.As a typical representative of"green economy"catalyst,TS-1 has excellent catalytic activity and selectivity under mild conditions,which makes it show great advantages over traditional catalysts in epoxidation of olefins,hydroxylation of phenols and oximation of ketones.However,TS-1 zeolite with high catalytic activity is mostly raw powder with particle size of 100～300 nm,so it is difficult and costly to recover catalyst after reaction in slurry reactor.When a fixed bed reactor is used,the binder used in the traditional molding method will cover a large number of zeolite surfaces,thus reducing the reaction activity.Therefore,it is very important to keep high-efficiency catalysis while TS-1 zeolite is formed.In recent years,TS-1 molecular sieve membranes grown on the surface of inorganic nonmetallic materials can be used for the oxidation reaction of industrial organic molecules.This immobilized molecular sieve membrane has become a breakthrough to realize the coupling of reaction and separation at the level of catalyst design.However,TS-1 molecular sieve membranes synthesized at present are mostly formed by cross-linking disordered grains,which limits the transport of substances to a certain extent,resulting in low catalytic activity.In addition,the large-scale and rapid preparation of molecular sieve membranes is the basis of industrialization,so the selection of membrane immobilization technology should have the characteristics of easy scale-up synthesis and controllable membrane microstructure.In a word,it is necessary to deeply understand the influence of TS-1 molecular sieve membrane microstructure on diffusion,mass transfer and reaction activity,and seamlessly connect it with large-scale rapid immobilization technology to realize the design of high-activity immobilized oriented TS-1 molecular sieve membrane catalyst.In this paper,a simple in-situ hydrothermal crystallization method was adopted.Firstly,the influence of substrate surface properties on the directional construction of TS-1 molecular sieve membrane was investigated.The research shows that titanium dioxide is loaded on the surface of 304 stainless steel by dip coating method,and the hydroxyl group on the surface promotes the adhesion of crystal nucleus,which increases CPO_（0k0）index by 7.01%.Roasting temperature will affect the crystal form,roughness and hydrophilicity of the surface of titanium dioxide oxide layer,thus having a corresponding impact on the orientation of the membrane layer.On the basis of planar substrate research,the surface of 2 mm alumina beads is also loaded with oxide layer to isolate the substrate.Although the molecular sieve membrane obtained has a certain degree of orientation,its compactness is poor.By modifying the surface of the oxide layer with TBOT modification solution,more hydroxyl groups can be anchored,thus enhancing the compactness of the membrane.In addition,on the basis of single-layer spherical membrane,the effective control of membrane thickness is realized by the method of gradual crystallizationOn the basis of the study of substrate surface properties on membrane orientation,the influence of precursor chemical environment on the regulation of b-axis orientation membranes was investigated.It is found that the content of template and water will seriously affect the nucleation and growth process of molecular sieve,thus affecting the grain size of TS-1 and the crosslinking of the membrane.The orientation of the membrane is hardly affected by the concentration of titanium source in the system,so the amount of titanium source can be increased as much as possible without exceeding the highest proportion of skeleton titanium atoms.The ratio of optimized precursor solution is:n（TEOS）:n（TBOT）:n（TPAOH）:n（H₂O）=1:0.010:0.15:40;The crystallization temperature and time are 180°C and 72 h,respectively.B-oriented dense TS-1 molecular sieve spherical membrane was successfully prepared by comprehensively investigating the effects of substrate surface properties and precursor chemical conditions on membrane growth.This study broke through the limitation of membranes orientation growth on curved substrate,and realized the orientation construction of b-oriented TS-1 molecular sieve membranes on spherical substrate for the first time.This material can be used as a new type of immobilized TS-1 molecular sieve membranes catalyst with the functions of improving mass transfer and easy recovery,thus laying a foundation for its application in traditional reactors.The new immobilized oriented membrane catalyst was applied in a fixed bed reactor to study its catalytic efficiency for the epoxidation of chloropropene.It was found that the catalytic efficiency of zeolite membrane increased gradually with the increase of membrane thickness and titanium species content.After optimizing the microstructure and active sites of the membrane,the conversion rate of hydrogen peroxide reached 63.55%,and the selectivity of epichlorohydrin reached 95.79%.On the basis of the catalyst,the process conditions such as raw material ratio,reaction temperature and liquid hourly space velocity（LHSV）were optimized.The optimized reaction conditions were as follows:ingredient ratio:n（ACL）:n（H₂O₂）=2:1,reaction temperature:45°C,LHSV:0.6 h^-1.Under the optimized process conditions,the conversion of hydrogen peroxide and the selectivity of epichlorohydrin were75.88%and 94.28%,respectively.