Background
The molecular sieve membrane has excellent thermal stability, chemical stability and mechanical strength, and the separation performance is greatly superior to that of an organic membrane, so that the molecular sieve membrane is widely applied to the separation field. The mordenite membrane belongs to one of inorganic membranes, and is provided with 0.67nm by 0.70 nm large pore channels and 0.26 nm by 0.56 nm small pore channels in the directions parallel to a c axis and an a axis respectively.
The catalytic science, 2002, 23(2), 145-149 synthesizes a mordenite membrane on an alumina carrier by a hydrothermal synthesis method, the aperture of an alpha-alumina macroporous ceramic tube is 1 mu m, the pipeline of the ceramic tube has a one-dimensional structure and does not have three-dimensional connectivity, the particle size of the mordenite in the obtained zeolite membrane is larger and reaches 4-5 mu m, and the shape of the mordenite is a long block.
CN 201110077262.4 provides a method for preparing mordenite membrane, which comprises using mordenite as seed crystal, directly introducing ultra-thin uniform seed crystal layer on the surface of porous carrier tube, and hydrothermally crystallizing under fluoride ion condition to obtain mordenite membrane. The porous carrier tube is made of alpha-alumina tube, mullite tube or stainless steel tube, the average pore diameter of the tube is 1-5 μm, and the carrier tube is of one-dimensional structure and has no three-dimensional connectivity. The mordenite adopts seed crystals with two sizes, and the particle size of the mordenite is still larger than 1 μm in the finally formed film, and the mordenite is in a long block shape.
CN01132198.9 adopts ceramic, glass and stainless steel as carriers to prepare the mordenite membrane, the thickness of the obtained zeolite membrane is 30-40 μm, the grain size of the mordenite is 20-30 μm, and the zeolite is in a long block shape.
In the above several methods for preparing mordenite membranes, there are problems that the pore diameter of the carrier is too large, the macropores do not have three-dimensional connectivity, and the crystal grain shape of mordenite is long block, the crystal grain is too large or the membrane layer is thick, which is not favorable for the performance of the mordenite membrane in some fields.
Disclosure of Invention
Aiming at the defects of the prior art, the invention provides a mordenite membrane and a preparation method thereof. The mordenite membrane has a three-dimensional macroporous through structure, the membrane layer is continuous, the thickness of the membrane layer is not more than 1 mu m, and the zeolite forming the membrane layer is small-grain two-dimensional sheet mordenite, so that the mass transfer of materials is facilitated. The mordenite membrane of the present invention can be used in H2、N2、O2And the separation and purification fields of gas separation, alcohol water and dehydration of low-carbon hydrocarbons, and the like.
The mordenite membrane of the invention has the carrier of alumina, the alumina macropores are uniformly distributed and are three-dimensionally communicated, the aperture of the macropores of the alumina is 100-1000nm, and the porosity is 60-90%; the mordenite membrane is composed of mordenite with a two-dimensional sheet shape, the dimension of the mordenite in the long dimension direction is 20-700nm, preferably 100-500 nm, the dimension of the mordenite in the short dimension direction is 10-60nm, and the mordenite vertically grows on the inner surface of the alumina macropore in the long dimension direction; the thickness of the mordenite membrane is 20-1000nm, preferably 20-700nm, and more preferably 100-500 nm.
The preparation method of the mordenite membrane comprises the following steps:
preparation of three-dimensional through macroporous alumina
(1) Uniformly mixing an inorganic aluminum source, polyethylene glycol, low carbon alcohol and/or water, and then adding pyridine for uniform mixing to form gel; wherein the viscosity-average molecular weight of the polyethylene glycol is 2000-;
(2) slowly pressurizing the gel product obtained in the step (1) to 2-15MPa, aging for 1-72 hours at the temperature of 10-80 ℃, preferably slowly pressurizing to 2-8MPa, aging at the temperature of 25-60 ℃, and aging for 5-24 hours;
(3) after aging in the step (2), slowly releasing pressure to normal pressure, and then soaking the aged mixture for a certain time by using low carbon alcohol or a mixed solution of the low carbon alcohol and water;
(4) and (4) removing the liquid phase from the mixture obtained in the step (3), and drying and roasting to obtain the macroporous alumina precursor.
Wherein, based on the total weight of the materials added in the step (1), the adding amount of water and/or low carbon alcohol is 10-60%, the adding amount of aluminum source is 15-45%, preferably 20-40%, and the adding amount of polyethylene glycol is 1-15%, preferably 3-7%.
In the above preparation steps, the pyridine and Al are3+Is 3.0 to 9.0, preferably 3.5 to 7.0.
The inorganic aluminum source is a water-soluble inorganic aluminum salt, can be one or more of aluminum chloride, aluminum nitrate and aluminum sulfate, and is more preferably aluminum chloride.
The lower alcohol is C5 or less alcohol, preferably one or more of methanol, ethanol, n-propanol and isopropanol, and more preferably ethanol and/or propanol.
The water and/or the lower alcohol may be mixed in any proportion.
The step (2) of pressurizing is to place the gel obtained in the step (1) in a closed pressure-resistant container, and then slowly introduce gas which does not physically or chemically react with the materials into the container, wherein the gas includes but is not limited to: one or more of air, inert gas, carbon dioxide, oxygen, and the like. Air is preferred for cost and safety reasons. During the pressing, the pressing rates were: not more than 0.5MPa/min, preferably 0.01-0.05 MPa/min. The pressure relief process in the step (4) should also be slow, and the pressure relief rate is not more than 1.0MPa/min, preferably not more than 0.5 MPa/min.
The lower alcohol in the step (3) is C5 or less, preferably one or more of methanol, ethanol, n-propanol and isopropanol, and preferably ethanol and/or propanol. When the mixed solution of the lower alcohol and the water is adopted in the step (3), the weight concentration of the lower alcohol is more than 60%. The lower alcohol used in the step (1) and the step (3) can be the same or different. The soaking time in the step (3) is 1 to 72 hours, preferably 24 to 48 hours.
The drying temperature in the step (4) is room temperature to 150 ℃, preferably 30 to 60 ℃, and the drying is carried out until no obvious liquid substance exists. The roasting conditions are as follows: 400-1200 ℃ for 1-24 hours, preferably 650-950 ℃ for 5-10 hours.
The macroporous alumina prepared by the method has the advantages that the diameter of macropores is 30-600nm, the macropores are uniformly distributed and are three-dimensionally communicated, and the porosity is 60% -90%.
Preparation of mordenite precursor liquid
Uniformly mixing a silicon source, an alkali source, N-methylpiperidine and water, carrying out closed hydrothermal treatment, cooling the material to room temperature, then adding an aluminum source into the material after the hydrothermal treatment, uniformly mixing, and aging to obtain the molecular sieve mother liquor.
The alkali source is alkali metal hydroxide, preferably sodium hydroxide; the silicon source is one or a composition of silica sol, silica gel, white carbon black, water glass, ethyl silicate-28, ethyl silicate-32 or ethyl silicate-40, diatomite and silicon alkoxide, and preferably one or more of silica sol, white carbon black, ethyl silicate-28, ethyl silicate-32 or ethyl silicate-40; the aluminum source is selected from one or a combination of sodium metaaluminate, aluminum nitrate, aluminum chloride, aluminum sulfate, aluminum oxide, aluminum hydroxide and organic aluminum alkoxide, and preferably one or more of sodium metaaluminate, aluminum isopropoxide and aluminum sec-butoxide; the mixing mode is any one of mechanical stirring, magnetic stirring or oscillation.
The alkali source, the aluminum source, the water, the silicon source and the N-methylpiperidine are calculated by the following substances, and the proportional relationship is satisfied: SiO 22/Al2O3The mol ratio of N-methylpiperidine to SiO is 20-802Molar ratio of 0.05-0.25, H2O/SiO2The molar ratio is 20-100, OH-/SiO2The molar ratio is 0.7-1.0.
The closed hydrothermal treatment conditions are as follows: sealing and hydrothermally treating at 90-120 deg.C for 1-24 hr. The aging conditions are as follows: aging at 30-80 deg.C for 0.5-10 hr, preferably 1-6 hr, more preferably 2-5 hr under stirring, more preferably ultrasonic dispersing and stirring; wherein the ultrasonic conditions are: the energy density of ultrasonic dispersion is 0.2-4kW/L, and the time of ultrasonic and stirring action is 2-5 hours; the stirring mode comprises mechanical stirring and/or magnetic stirring.
Preparation of mordenite membrane
Soaking the three-dimensional through macroporous alumina in a silane coupling agent solution for wetting, taking out and drying, placing in a mordenite precursor solution for sealing, heating for crystallization, carrying out solid-liquid separation after crystallization reaction is finished, and roasting the obtained solid phase to obtain the mordenite membrane taking the three-dimensional through macroporous alumina as a carrier.
The silane coupling agent is at least one of KH550, KH560, KH570, KH580, KH602, KH792 and KH 902. The drying temperature is 40-180 ℃, and the drying time is 1-12 hours. The crystallization conditions are as follows: the crystallization temperature is 140-. The roasting conditions are as follows: roasting at 500-600 ℃ for 2-10 hours.
In the process of preparing the alumina, the adopted low molecular weight polyethylene glycol can be stably dispersed in the alumina gel under a normal pressure system, the solid-liquid separation of the gel cannot be induced, and only a small amount of macropores can be generated by the decomposition of the polyethylene glycol through high-temperature roasting. According to the invention, the gel system is placed under a high-pressure condition, so that the original metastable state gel is subjected to degeneration under high pressure, the polyethylene glycol is changed into a precipitate phase from being stably dispersed in the gel, the whole system is triggered to carry out solid-liquid separation, and finally macropores are generated in the material. Because the dispersibility of the low molecular weight polyethylene glycol is far higher than that of the high molecular weight polyethylene glycol, the low molecular weight polyethylene glycol used in the invention can reach higher dispersibility in gel before high-pressure induction of solid-liquid separation, more polyethylene glycol small aggregates are generated, and the large number of small aggregates can finally generate more three-dimensionally communicated macroporous pore passages.
In the preparation of the mordenite precursor solution, the mordenite precursor solution is fully converted into the soluble ionic high-activity silicate by moderate silicon source hydrothermal treatment under higher alkalinity. Meanwhile, when the silicon source is hydrothermally treated, the aluminum source is isolated at a milder temperature, so that the crystallization of the silicon source can be avoided. Then in a hydrothermal synthesis mordenite system, N-methylpiperidine modulates the growth mode of a zeolite precursor, and stirring and ultrasonic dispersion are jointly used, so that the generation of small-grain flaky mordenite is caused. The N-methylpiperidine used in the invention has less dosage, belongs to a large amount of organic intermediates, can use industrial products and has lower cost.
The method comprises the steps of firstly obtaining three-dimensional through macroporous alumina by a phase separation technical means, then preparing a mordenite precursor by a special template agent and a preparation measure, and then carrying out surface modification on the three-dimensional through macroporous alumina to ensure that a mordenite precursor liquid can be uniformly attached to the inner surface of a large pore of alumina to grow in a hydrothermal crystallization stage, and controlling the shape, the size and the thickness of a mordenite crystal by adjusting reaction conditions, thereby obtaining the mordenite membrane which takes the three-dimensional through macroporous alumina as a carrier and has a flaky small crystal grain shape and a membrane layer thickness of not more than 1 mu m.
The mordenite membrane can be used in the purification fields of gas phase separation, organic phase and water phase separation and the like, such as alcohol-water separation, acetic acid dehydration, low-carbon hydrocarbon dehydration separation and the like.