Composite molecular sieve membrane and preparation method and application thereof

Abstract

The present application relates to the technical field of membrane separation materials, in particular to a composite molecular sieve membrane and a preparation method and application thereof. The composite molecular sieve membrane comprises: a porous silicon carbide ceramic support; a silicon carbide-based gradient transition layer formed on the surface of the porous silicon carbide ceramic support; and a zeolite molecular sieve separation layer formed on the silicon carbide-based gradient transition layer. The present application solves the problems of insufficient chemical stability in a strong acid environment, low interface bonding strength, poor long-term operation stability and the like of the existing membrane materials.

Description

Technical Field

[0001] This invention relates to the field of membrane separation materials technology, specifically to a composite molecular sieve membrane, its preparation method, and its application. Background Technology

[0002] Pervaporation membrane reactor technology, by coupling the reaction and membrane separation in situ, can effectively disrupt the equilibrium of esterification reactions, representing an advanced technological direction for improving reaction efficiency. The core of this technology lies in developing a high-performance dehydration membrane capable of long-term stable operation within the reaction system.

[0003] Currently, in the field of esterification dehydration, the main membrane materials used fall into two categories: organic polymer membranes and inorganic membranes. While organic polymer membranes (such as polyvinyl alcohol membranes and polyethersulfone membranes) are simple to prepare and have low cost, they suffer from poor chemical stability in strong acid environments, easily swelling, degrading, or even dissolving, leading to a rapid decline in membrane separation performance. For example, in a concentrated sulfuric acid-catalyzed esterification reaction environment, ordinary organic polymer membranes can lose their separation function within hours.

[0004] Inorganic membranes, especially molecular sieve membranes, have attracted widespread attention in recent years due to their excellent chemical stability and separation selectivity. In existing technologies, researchers mainly use porous ceramics (such as alumina and mullite) as supports and load hydrophilic molecular sieve membranes (such as MOR-type zeolites and CHA-type zeolites) onto their surfaces for esterification dehydration. These methods have demonstrated the feasibility of separation under mild or short-term conditions.

[0005] However, with the increasing demands for continuous and long-term stable operation of chemical processes, existing technologies still have the following problems:

[0006] First, the chemical stability of the support material is insufficient. Traditional ceramic supports (such as mullite, chemical formula 3Al2O3·2SiO2) belong to the aluminosilicate system, and their surface is rich in Al-O bonds and Si-O-Al bonds. They are prone to chemical corrosion in strong acid environments, which leads to changes in the pore structure of the support and affects the mechanical strength and separation performance of the membrane.

[0007] Second, the interfacial bonding strength is insufficient. The bond between the molecular sieve separation layer and the ceramic support mainly relies on physical adhesion or weak chemical bonding. Under the long-term coupling of thermal stress, chemical stress and mechanical stress, the separation layer is prone to peeling or detachment, leading to membrane failure.

[0008] Third, the long-term stability is difficult to meet industrial requirements. Under harsh conditions of strong acid, high temperature and organic solvents, existing molecular sieve membranes can usually only operate stably for tens to hundreds of hours, which is difficult to meet the needs of industrial equipment for long-term, maintenance-free operation.

[0009] Therefore, there is an urgent need to develop a new membrane material system that has higher chemical inertness from the material itself and can achieve stress management through innovative structural design, so as to meet the stringent requirements for membrane life and reliability under extreme working conditions. Summary of the Invention

[0010] In view of the shortcomings of the prior art described above, the purpose of this invention is to provide an ultra-high stability silicon carbide-based composite molecular sieve membrane for strong acid esterification and dehydration, its preparation method and application, to solve the problems of insufficient chemical stability, low interfacial bonding strength and poor long-term operational stability of existing membrane materials under strong acid environment.

[0011] To achieve the above objectives, the present invention provides the following technical solution:

[0012] The present invention provides a composite molecular sieve membrane, the composite molecular sieve membrane comprising: a porous silicon carbide ceramic support; a silicon carbide-based gradient transition layer formed on the surface of the porous silicon carbide ceramic support; and a zeolite molecular sieve separation layer formed on the silicon carbide-based gradient transition layer.

[0013] A second aspect of the present invention provides a method for preparing a composite molecular sieve membrane, the method comprising the following steps:

[0014] 1) A precursor solution of polycarbosilane or allyl hydrogenated polycarbosilane is coated on the surface of a porous silicon carbide ceramic support to form a coating. The coating is then subjected to pyrolysis treatment to form a silicon carbide-based gradient transition layer on the surface of the porous silicon carbide ceramic support.

[0015] 2) A zeolite molecular sieve separation layer is grown on the surface of the silicon carbide-based gradient transition layer described in step 1) by hydrothermal synthesis.

[0016] A third aspect of the present invention provides the use of the composite molecular sieve membrane as described herein in pervaporation dehydration under a strong acid environment.

[0017] A fourth aspect of the present invention provides an in-situ dehydration system for strong esterification reactions, comprising a reactor and a pervaporation membrane separation unit, wherein the pervaporation membrane separation unit is provided with the composite molecular sieve membrane described in the present invention; the permeate side of the pervaporation membrane separation unit is connected to a vacuum system.

[0018] By adopting the aforementioned technical solution, the beneficial effects of the present invention are:

[0019] (1) Fundamental innovation in the material system. This invention is the first to introduce silicon carbide covalent ceramics as the load-bearing and corrosion-resistant framework of the entire membrane element, replacing traditional aluminosilicate ceramics. Silicon carbide is a strong covalent compound that is thermodynamically absolutely stable against most strong acids, including concentrated sulfuric acid, thus providing an incorrosion-resistant rigid framework for the entire membrane element. This is not a simple material replacement, but a leap in material category, resulting in an order-of-magnitude improvement in chemical stability.

[0020] (2) The underlying principles of the interface construction methods are different. This invention uses polymer-derived ceramic technology to construct the interface layer. This method achieves interface fusion through the pyrolysis and transformation of the precursor at the molecular scale. Its bonding mechanism (chemical bonding and structural gradient) is fundamentally different from traditional physical adhesion or simple chemical modification, realizing "stitching" rather than "adhesion" between heterogeneous materials. The surface of the silicon carbide-based gradient transition layer is rich in Si-OH functional groups, providing abundant nucleation sites for the subsequent growth of MOR molecular sieves and enhancing the bonding strength between the molecular sieve layer and the transition layer.

[0021] (3) Revolutionary extension of lifespan under extreme environments. The combination of the above two points enables the composite membrane of this invention to exhibit unprecedented stability in accelerated aging tests (90℃, 50 wt% H2SO4) under simulated concentrated sulfuric acid catalytic esterification reaction conditions. Experiments show that it can maintain stable operation of core separation performance for more than 500 hours without any visible structural damage, which lays the foundation for long-cycle, maintenance-free operation of industrial plants.

[0022] (4) Excellent separation performance. The composite membrane of this invention maintains high flux and high selectivity even in strong acid environments. For example, in the embodiments, under conditions of 90°C and 50wt% sulfuric acid, the initial water flux reaches 1.8 kg / (m³). 2 •h), separation factor >1200; after 500 hours of continuous operation, the water flux stabilized at 1.6 kg / (m³). 2 ·h), the separation factor remains above 1100, and the performance degradation rate is only 11%.

[0023] (5) Broad application prospects. The composite membrane of the present invention is not only suitable for esterification and dehydration catalyzed by concentrated sulfuric acid, but also for esterification reactions catalyzed by other strong acids such as methanesulfonic acid and p-toluenesulfonic acid, as well as other membrane separation processes that require strong acid stability, and has broad industrial application prospects.

[0024] The composite molecular sieve membrane of this invention has a clear three-layer structure, consisting of a zeolite molecular sieve separation layer, a silicon carbide-based gradient transition layer, and a porous silicon carbide ceramic support, from the outside to the inside. The interfaces between the layers are well-fused, with no visible cracks. The silicon carbide-based gradient transition layer has a thickness of 50 nm to 5 μm and is highly porous; the MOR-type zeolite molecular sieve separation layer has a thickness of approximately 2 μm to 10 μm and is continuous and dense. X-ray photoelectron spectroscopy analysis confirms that the surface of the transition layer is rich in Si-OH functional groups. Detailed Implementation

[0025] The following describes in detail the implementation of the composite molecular sieve membrane, its preparation method, and its applications provided by the present invention.

[0026] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60~120 and 80~110 are listed for a specific parameter, it is also expected that ranges of 60~110 and 80~120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1~3, 1~4, 1~5, 2~3, 2~4, and 2~5. In this application, unless otherwise stated, the numerical range "a~b" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0~5" indicates that all real numbers between "0~5" have been listed in this article; "0~5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0027] Through extensive research, this invention discovered that by using silicon carbide covalently bonded ceramic as a support and constructing an interface fusion layer using polymer-derived ceramic technology, a robust bond between the molecular sieve separation layer and the support is achieved. Experiments show that after 500 hours of continuous operation at 90°C and 50 wt% sulfuric acid, the water flux of this composite membrane remains at 1.6 kg / (m²). 2 The separation factor remains above 1100, exhibiting excellent acid resistance and separation performance, making it suitable for dehydration in esterification reactions catalyzed by concentrated sulfuric acid, methanesulfonic acid, or p-toluenesulfonic acid. Based on this, the present invention was completed.

[0028] Composite molecular sieve membrane

[0029] The present invention provides a composite molecular sieve, the composite molecular sieve membrane comprising: a porous silicon carbide ceramic support; a silicon carbide-based gradient transition layer formed on the surface of the porous silicon carbide ceramic support; and a zeolite molecular sieve separation layer formed on the silicon carbide-based gradient transition layer.

[0030] In the composite molecular sieve provided by this invention, the average pore size of the porous silicon carbide ceramic support is 0.1 μm to 10 μm and any value between them or any two values, optionally 0.1 μm to 5 μm, 5 μm to 10 μm, 0.1 μm to 0.5 μm, 0.5 μm to 2 μm, 2 μm to 5 μm, 5 μm to 8 μm, or 8 μm to 10 μm. Preferably, the average pore size of the porous silicon carbide ceramic support is 0.5 μm to 2 μm.

[0031] Furthermore, the porosity of the porous silicon carbide ceramic support is 20% to 60% and any value between them, or a range between any two values, and can be selected as 20% to 40%, 40% to 50%, or 50% to 60%. Preferably, the porosity of the porous silicon carbide ceramic support is 40% to 50%.

[0032] In the composite molecular sieve provided by the present invention, the silicon carbide-based gradient transition layer is formed by coating a porous silicon carbide ceramic support with a polycarbosilane or allyl hydrogenated polycarbosilane precursor solution and then pyrolyzing it.

[0033] Further, the thickness of the silicon carbide-based gradient transition layer is 50 nm to 5 μm and any value between them or any two values, optionally 50 nm to 0.5 μm, 0.5 μm to 2 μm, or 2 μm to 5 μm. Preferably, the thickness of the silicon carbide-based gradient transition layer is 0.5 μm to 2 μm.

[0034] Furthermore, the surface of the silicon carbide-based gradient transition layer is rich in Si-OH functional groups.

[0035] In the composite molecular sieve provided by this invention, the silicon-to-aluminum molar ratio (SiO2 / Al2O3) of the zeolite molecular sieve separation layer is greater than 10. More preferably, the silicon-to-aluminum molar ratio is 20-100.

[0036] Furthermore, the thickness of the zeolite molecular sieve separation layer is 2μm~10μm and any value between them or any two values, and can be selected as 2μm~5μm, 5μm~10μm, 5μm~8μm, or 8μm~10μm.

[0037] Preparation methods of composite molecular sieve membranes

[0038] This invention also provides a method for preparing a composite molecular sieve membrane, the method comprising the following steps:

[0039] 1) A precursor solution of polycarbosilane or allyl hydrogenated polycarbosilane is coated on the surface of a porous silicon carbide ceramic support to form a coating. The coating is then subjected to pyrolysis treatment to form a silicon carbide-based gradient transition layer on the surface of the porous silicon carbide ceramic support.

[0040] 2) A zeolite molecular sieve separation layer is grown on the surface of the silicon carbide-based gradient transition layer described in step 1) by hydrothermal synthesis.

[0041] In the preparation method provided by this invention, step 1) involves coating a porous silicon carbide ceramic support with a precursor solution of polycarbosilane or allyl hydrogenated polycarbosilane to form a coating, and then subjecting the coating to pyrolysis treatment to form a silicon carbide-based gradient transition layer on the surface of the porous silicon carbide ceramic support. Specifically:

[0042] In step 1) of this invention, the mass concentration of polycarbosilane or allyl hydrogenated polycarbosilane in the precursor solution is 5% to 40% and any value between them or any two values, optionally 5% to 15%, 15% to 30%, or 30% to 40%. Preferably, the mass concentration of polycarbosilane or allyl hydrogenated polycarbosilane in the precursor solution is 15% to 30%.

[0043] In step 1) of the present invention, a precursor solution is coated on the surface of the porous silicon carbide ceramic support by at least one of the following methods: dip-coating, spin coating, or spray coating.

[0044] When using the dip-lifting method, the lifting speed is 1mm / s to 5mm / s and any value between them or any two values, and can be selected as 1mm / s to 3mm / s or 3mm / s to 5mm / s.

[0045] In step 1) of this invention, the peak temperature of the pyrolysis treatment is 800℃~1200℃ and any value or any two values ​​therebetween, specifically 800℃~1000℃, 1000℃~1200℃, 800℃~900℃, 900℃~1000℃, 1000℃~1100℃, or 1100℃~1200℃. Preferably, the peak temperature of the pyrolysis treatment is 1000℃~1100℃. The heating rate is 1℃ / min~5℃ / min and any value or any two values ​​therebetween, specifically 1℃ / min~3℃ / min or 3℃ / min~5℃ / min. The holding time is 0.5 hours~3 hours and any value or any two values ​​therebetween, specifically 0.5 hours~1.5 hours, 1.5 hours~3 hours, 0.5 hours~2 hours or 2 hours~3 hours.

[0046] In step 1) of the present invention, the coating is subjected to pyrolysis treatment under an inert atmosphere, wherein the inert atmosphere is at least one of argon, nitrogen or helium.

[0047] In the preparation method provided by this invention, step 2) involves growing a zeolite molecular sieve separation layer on the surface of the silicon carbide-based gradient transition layer described in step 1) using a hydrothermal synthesis method. Specifically:

[0048] In step 2) of this invention, the hydrothermal synthesis temperature is 150℃~200℃ or any value between them, or any two values, and can be selected as 150℃~180℃ or 180℃~200℃. Preferably, the hydrothermal synthesis temperature is 170℃~180℃. The crystallization time is 24 hours~96 hours or any value between them, or any two values, and can be selected as 24 hours~48 hours, 48 ​​hours~72 hours, or 72 hours~96 hours. Preferably, the crystallization time is 48 hours~72 hours.

[0049] In step 2) of this invention, the molar composition of the synthesis liquid of the zeolite molecular sieve separation layer is: SiO2:Al2O3:Na2O:H2O = 1:0.02~0.05:0.15~0.25:30~40.

[0050]

use

[0051] The present invention also provides an application of composite molecular sieve membrane in pervaporation dehydration under strong acid conditions.

[0052] The composite molecular sieve membrane is used to selectively separate water from an esterification reaction mixture catalyzed by concentrated sulfuric acid, methanesulfonic acid, or p-toluenesulfonic acid, wherein the temperature of the esterification reaction mixture is 80°C to 150°C and the acid concentration is 5 wt% to 70 wt%. Preferably, the temperature of the esterification reaction mixture is 90°C to 120°C and the acid concentration is 30 wt% to 60 wt%.

[0053] In-situ dehydration system for strong esterification reactions

[0054] The present invention also provides an in-situ dehydration system for strong esterification reactions, the system comprising a reactor and a pervaporation membrane separation unit, wherein the pervaporation membrane separation unit is provided with the composite molecular sieve membrane described in the present invention; the permeate side of the pervaporation membrane separation unit is connected to a vacuum system.

[0055] In the system provided by the present invention, the permeation side of the pervaporation membrane separation unit is connected to a vacuum system, and the permeation side pressure is 1kPa~10kPa, which can be selected as 1kPa~5kPa, 5kPa~10kPa, 1kPa~3kPa, 3kPa~5kPa, 5kPa~8kPa, or 8kPa~10kPa.

[0056] The system provided by the present invention further includes a condensation device for condensing water vapor that permeates from the permeation side.

[0057] The beneficial effects of the present invention will be further illustrated below with reference to the embodiments.

[0058] To make the inventive objectives, technical solutions, and beneficial effects of this invention clearer, the invention is further described in detail below with reference to embodiments. However, it should be understood that the embodiments of this invention are merely for illustrative purposes and not for limiting the invention, and the embodiments are not limited to those given in the specification. Unless otherwise specified, specific experimental or operational conditions in the embodiments were prepared under conventional conditions or according to the conditions recommended by the material supplier.

[0059] Furthermore, it should be understood that the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps, does not preclude the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps, unless otherwise stated. It should also be understood that the combined connection relationship between one or more devices / apparatus mentioned in this invention does not preclude the existence of other devices / apparatus before or after the combined devices / apparatus, or the insertion of other devices / apparatus between these explicitly mentioned devices / apparatus, unless otherwise stated. Moreover, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and not for limiting the order of the method steps or limiting the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.

[0060] In the following embodiments, unless otherwise specified, all the raw materials of the present invention are commercially available or prepared according to conventional methods in the art.

[0061] Example 1: Preparation of SiC-based composite molecular sieve membranes

[0062] Preparation of porous silicon carbide ceramic support:

[0063] SiC powder with an average particle size of 0.5 μm was mixed with a pore-forming agent (starch) and a binder (polyvinyl alcohol) at a mass ratio of 70:20:10. After kneading, extrusion molding, and drying, the mixture was sintered at 2150℃ under an argon atmosphere to obtain a porous silicon carbide ceramic tube support with an average pore size of 0.8 μm, a porosity of 45%, and dimensions of Φ12×8×100 mm.

[0064] Silicon carbide-based gradient transition layer:

[0065] Prepare a 25% (w / w) polycarbosilane (PCS) xylene solution. Immerse the cleaned porous silicon carbide ceramic tube support in the PCS xylene solution, pull it up at a speed of 2 mm / s, and dry it at room temperature for 12 hours.

[0066] The material was placed in a tube furnace and heated to 1050°C at a rate of 3°C / min under the protection of high-purity argon, and held at that temperature for 1.5 hours. It was then cooled to room temperature with the furnace. This process forms a porous silicon carbide-based gradient transition layer that is chemically compatible with the substrate, with a thickness of approximately 1.2 μm.

[0067] Hydrothermal synthesis of MOR zeolite molecular sieve separation layer:

[0068] A synthesis solution with a molar composition of SiO2:Al2O3:Na2O:H2O = 1:0.033:0.2:35 was prepared. Silica sol was used as the silicon source, and sodium aluminate as the aluminum source.

[0069] The obtained silicon carbide-based gradient transition layer was placed vertically in a high-pressure reactor, and the synthesis solution was added. The mixture was then crystallized at 175°C for 60 hours.

[0070] After removal, the membrane was washed three times with deionized water and dried at 80°C for 12 hours to obtain the composite molecular sieve membrane described in this invention (denoted as Membrane-S1).

[0071] Basic pervaporation performance test:

[0072] A pervaporation dehydration test was conducted at 90℃ using a 50wt% sulfuric acid aqueous solution as the feed, with the permeation side pressure maintained at 5 kPa. The test results are shown in Table 1.

[0073] Table 1: Basic pervaporation properties of Membrane-S1

[0074]

[0075] Example 2: Long-term stability test of composite molecular sieve membrane in simulated strong acidification environment

[0076] This Example 2 is used to evaluate the service life of the membrane of the present invention under extreme operating conditions.

[0077] Test apparatus and methods:

[0078] The Membrane-S1 prepared in Example 1 was packaged into a laboratory pervaporation membrane module.

[0079] The test solution was a high-acidity mixture simulating the later stage of concentrated sulfuric acid catalytic esterification reaction: 50 wt% sulfuric acid aqueous solution and ethyl acetate were mixed at a volume ratio of 1:1.

[0080] The membrane module was immersed in a test solution at 90°C, and a continuous pervaporation dehydration experiment was conducted with the membrane permeate side maintained at an absolute pressure of 5 kPa. The water flux (J) and water / ethyl acetate separation factor (α) of the membrane were measured every 24 hours.

[0081] Stability test results:

[0082] Initial performance: At 90℃, the initial water flux of the membrane to the test solution is 1.8 kg / (m²). 2 ·h), initial separation factor α > 1200.

[0083] Long-term operating performance: As shown in Table 2, the membrane exhibits excellent operational stability. After 500 hours of continuous operation, the water flux stabilizes at 1.6 kg / (m²). 2 (h) (attenuation rate of about 11%), the separation factor remains above 1100.

[0084] Table 2: Long-term stability test results of Membrane-S1 under 50wt% sulfuric acid and 90℃ conditions

[0085]

[0086] Post-shutdown inspection:

[0087] After the test, the membrane tube was removed for macroscopic and microscopic inspection. The membrane layer was intact, without blistering, cracking, or peeling. It was confirmed that after 500 hours of harsh environmental testing, the interface between the support, transition layer, and separation layer remained firm and clear, showing no signs of corrosion or separation. This demonstrates the extreme durability imparted to the membrane by the porous silicon carbide ceramic support and silicon carbide-based gradient transition layer structure of this invention.

[0088] Example 3: Dehydration performance of composite membrane in actual esterification reaction

[0089] This embodiment illustrates the effect of the membrane of the present invention in improving reaction efficiency in a practical reaction device.

[0090] Construction of a reaction-dehydration coupling system:

[0091] A membrane reactor system was constructed, including a 2L jacketed reactor, a circulating pump, a heat exchanger, and a pervaporation membrane separator encapsulated with the Membrane-S1 membrane from Example 1. A condensation system and a vacuum pump were connected to the permeate side of the membrane, and the permeate side pressure was maintained at 5 kPa.

[0092] Esterification process:

[0093] 1.0 mol propionic acid, 1.2 mol n-butanol and 1.0 wt% p-toluenesulfonic acid were added to the reactor as catalysts.

[0094] Turn on the system and control the reaction temperature at 115℃. The water generated in the reaction is continuously removed in situ by the membrane and collected.

[0095] Table 3: Changes in propionic acid conversion rate over time in membrane reactor

[0096]

[0097] Table 3 shows the change in propionic acid conversion rate over time during the reaction. Due to the efficient removal of water, the reaction equilibrium was continuously disrupted. After 8 hours of reaction, the conversion rate of propionic acid reached 98.5%.

[0098] The system ran continuously for 120 hours, with continuous feeding and discharging. The membrane dehydration performance remained stable, and the reaction conversion rate was consistently maintained above 98%, demonstrating that the membrane of this invention is feasible and reliable in achieving efficient and continuous esterification production.

[0099] Example 4:

[0100] Keeping all other conditions unchanged from Example 1, and only changing the pyrolysis peak temperature of the silicon carbide-based gradient transition layer, composite molecular sieve membrane samples with pyrolysis temperatures of 800℃, 950℃, 1050℃, 1150℃, and 1200℃ were prepared. The performance test results of each sample are shown in Table 4.

[0101] Table 4: Effect of different pyrolysis temperatures on membrane performance

[0102]

[0103] As shown in Table 4, the 800℃ sample had incomplete pyrolysis, insufficient transition layer strength, and excessively high porosity. During the hydrothermal synthesis of the MOR layer, local dissolution occurred, resulting in an uneven final MOR layer. The separation factor (α ~ 520) and water flux were also low in the permeation test.

[0104] Sample at 950℃: The transition layer structure is basically stable and binds well with the MOR layer, showing good separation performance, but the throughput and selectivity are still lower than the optimal values.

[0105] The 1050℃ sample (Membrane-S1) exhibits a stable transition layer structure, well-developed pores, and a moderate content of Si-OH functional groups on the surface. It combines well with the MOR layer, demonstrating high throughput and high selectivity.

[0106] Sample at 1150℃: The transition layer has a high degree of crystallinity and a reduced number of active sites, resulting in slow growth of the MOR layer and slightly poor continuity, and a significant decrease in water flux.

[0107] Samples at 1200℃: The transition layer has excessive crystallinity, the surface active sites are significantly reduced, the growth of the MOR layer is difficult, the final separation layer has poor continuity, and both flux and selectivity decrease.

[0108] Therefore, the pyrolysis temperature significantly affects the transition layer structure, and thus determines the final membrane performance. 800°C to 1200°C is an effective process window, with 1000°C to 1100°C being the preferred range of this invention, which can optimally balance the chemical activity, structural stability, and porosity characteristics of the transition layer.

[0109] Example 5:

[0110] Keeping all other conditions unchanged from Example 1, only the mass concentration of the PCS precursor solution was changed to prepare PCS xylene solutions of 5%, 10%, 15%, 25%, 35%, and 40%, respectively, to prepare corresponding composite molecular sieve membranes. The performance test results of each sample are shown in Table 5.

[0111] Table 5: Effect of different precursor concentrations on membrane performance

[0112]

[0113] As can be seen from Table 5, when the PCS concentration is low (5%~10%), the resulting transition layer is thin and has excessively high porosity. Although the water flux is high, the separation factor is low because the transition layer does not provide sufficient support for MOR growth.

[0114] When the PCS concentration is in the range of 15% to 25%, the transition layer has a moderate thickness (0.68 to 1.20 μm) and a reasonable porosity (52% to 58%), which can effectively support the growth of the MOR layer without causing excessive resistance to mass transfer, thus exhibiting excellent comprehensive performance.

[0115] When the PCS concentration is too high (35%~40%), the transition layer becomes too thick and too dense, increasing the mass transfer resistance and causing a significant decrease in water flux.

[0116] Therefore, the preferred mass concentration range of the precursor solution is 15% to 30%, with an optimal concentration of around 25%. Composite membranes with excellent performance can be prepared within this range.

[0117] Example 6:

[0118] This embodiment investigates the effect of the silicon-to-aluminum ratio (SiO2 / Al2O3) of the MOR molecular sieve separation layer on the membrane separation performance and acid resistance. MOR molecular sieve membranes with different SiO2 / Al2O3 ratios were prepared by adjusting the aluminum source content in the synthesis solution. Other preparation conditions were the same as in Example 1. The performance test results of each sample are shown in Table 6.

[0119] Table 6: Effect of different silicon-to-aluminum ratios on membrane performance

[0120]

[0121] As can be seen from Table 6, when the silica-alumina ratio is low (8~15), the aluminum content in the MOR molecular sieve framework is high and the hydrophilicity is strong, but the stability of the zeolite framework is poor in a strong acid environment, and the flux retention rate is low after 500 hours of testing.

[0122] When the silica-to-alumina ratio is in the range of 30 to 80, MOR molecular sieves exhibit both good hydrophilicity and excellent acid resistance, demonstrating optimal overall performance. In particular, when the silica-to-alumina ratio is 30, the separation factor is the highest (>1200), while maintaining good long-term stability.

[0123] When the silicon-to-aluminum ratio is too high (100), although the acid resistance is further improved, the hydrophilicity decreases slightly and the separation factor is reduced.

[0124] Therefore, the preferred silica-to-alumina ratio of the MOR molecular sieve separation layer is 20 to 100, and more preferably 30 to 50. Within this range, both separation selectivity and acid stability can be balanced.

[0125] Comparative Example 1:

[0126] A comparison membrane (denoted as Membrane-C) was prepared using mullite ceramic tubes (chemical formula 3Al2O3·2SiO2, average pore size 0.8 μm, porosity 45%) as a support and under the same MOR molecular sieve synthesis conditions.

[0127] Long-term stability tests were conducted on Membrane-S1 and Membrane-C under the same test conditions (90℃, 50wt% H2SO4), and the results are shown in Table 7.

[0128] Table 7: Performance Comparison of the Composite Membrane of the Present Invention with Traditional Composite Membranes

[0129]

[0130] As can be seen from Table 7, the initial properties of the two membranes are similar, but they show significant differences in long-term stability tests.

[0131] After approximately 380 hours of operation, the separation factor of the Membrane-C (mullite support) dropped below 650, the water flux decay rate reached 44.7%, and signs of membrane peeling appeared, indicating failure.

[0132] After 500 hours of operation, the Membrane-S1 (SiC support) maintained good performance, with a flux decay rate of only 11.1% and a separation factor remaining above 1100, without any structural damage.

[0133] Therefore, the present invention adopts a design of SiC support combined with PCS interface transition layer, which significantly improves the long-term stability of composite film and solves the problem that traditional aluminosilicate supports are prone to failure in strong acid environment.

[0134] Example 7: Application Tests in Different Strong Acid Systems

[0135] This embodiment tests the application performance of the composite membrane of the present invention in different strong acid catalytic esterification systems.

[0136] Testing System:

[0137] System A: 50wt% sulfuric acid catalytic system

[0138] System B: 40wt% methanesulfonic acid catalytic system

[0139] System C: 30wt% p-Toluenesulfonic acid catalytic system

[0140] Test conditions:

[0141] Temperature: 90℃

[0142] Solution composition: Acid-water solution / Ester (ethyl acetate) = 1:1 (volume ratio)

[0143] Osmotic pressure: 5 kPa

[0144] Table 8 shows the dehydration performance of Membrane-S1 in different strong acid systems.

[0145] Table 8: Dehydration performance of Membrane-S1 in different strong acid systems

[0146]

[0147] As shown in Table 8, Membrane-S1 in Example 1 exhibited excellent dehydration performance and long-term stability in three different strong acid systems. Although the acid strength and corrosive properties of different acids varied, the silicon carbide-based composite membrane maintained stable separation performance in all systems.

[0148] Therefore, the composite membrane of the present invention has wide applicability and can be used for dehydration in a variety of strong acid catalytic esterification systems, showing good prospects for industrial application.

[0149] In summary, this invention effectively overcomes the various shortcomings of the prior art and has high industrial application value.

[0150] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any form or substance. It should be noted that those skilled in the art can make various improvements and additions without departing from the method of the present invention, and these improvements and additions should also be considered within the scope of protection of the present invention. Any modifications, alterations, and equivalent changes made by those skilled in the art based on the above-disclosed technical content without departing from the spirit and scope of the present invention are equivalent embodiments of the present invention. Furthermore, any modifications, alterations, and evolutions made to the above embodiments based on the essential technology of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A composite molecular sieve membrane, characterized in that, The composite molecular sieve membrane comprises: Porous silicon carbide ceramic support; A silicon carbide-based gradient transition layer is formed on the surface of the porous silicon carbide ceramic support; A zeolite molecular sieve separation layer formed on the silicon carbide-based gradient transition layer.

2. The composite molecular sieve membrane according to claim 1, characterized in that, The silicon carbide-based gradient transition layer is formed by coating a porous silicon carbide ceramic support with a polycarbosilane or allyl hydrogenated polycarbosilane precursor solution and then pyrolyzing it.

3. The composite molecular sieve membrane according to claim 1, characterized in that, It also includes one or more of the following conditions: A1) The average pore size of the porous silicon carbide ceramic support is 0.1 μm to 10 μm; A2) The porosity of the porous silicon carbide ceramic support is 20%~60%; A3) The thickness of the silicon carbide-based gradient transition layer is 50 nm to 5 μm; A4) The silicon-aluminum molar ratio (SiO2 / Al2O3) of the zeolite molecular sieve separation layer is greater than 10; A5) The thickness of the zeolite molecular sieve separation layer is 2μm~10μm; A6) The zeolite molecular sieve separation layer is a MOR-type zeolite molecular sieve separation layer.

4. The method for preparing the composite molecular sieve membrane according to any one of claims 1 to 3, characterized in that, The preparation method includes the following steps: 1) A precursor solution of polycarbosilane or allyl hydrogenated polycarbosilane is coated on the surface of a porous silicon carbide ceramic support to form a coating. The coating is then subjected to pyrolysis treatment to form a silicon carbide-based gradient transition layer on the surface of the porous silicon carbide ceramic support. 2) A zeolite molecular sieve separation layer is grown on the surface of the silicon carbide-based gradient transition layer described in step 1) by hydrothermal synthesis.

5. The method for preparing the composite molecular sieve membrane according to claim 4, characterized in that, It also includes one or more of the following conditions: B1) In step 1), the mass concentration of polycarbosilane or allyl hydrogenated polycarbosilane in the precursor solution is 5%~40%; B2) In step 1), a precursor solution is coated on the surface of the porous silicon carbide ceramic support by at least one of the following methods: dip-coating, spin coating or spray coating. B3) In step 1), the peak temperature of the pyrolysis treatment is 800℃~1200℃, the heating rate is 1℃ / min~5℃ / min, and the holding time is 0.5 hours~3 hours; In step 1) of B4), the coating is subjected to a pyrolysis treatment under an inert atmosphere, wherein the inert atmosphere is at least one of argon, nitrogen or helium.

6. The method for preparing the composite molecular sieve membrane according to claim 4, characterized in that, It also includes one or more of the following conditions: In step 2) of C1), the hydrothermal synthesis temperature is 150℃~200℃ and the crystallization time is 24 hours~96 hours; In step 2) of C2), the molar composition of the synthesis liquid of the zeolite molecular sieve separation layer is: SiO2:Al2O3:Na2O:H2O = 1:0.02~0.05:0.15~0.25:30~40.

7. The use of the composite molecular sieve membrane according to any one of claims 1 to 3 in pervaporation dehydration under strong acid conditions.

8. The use according to claim 7, characterized in that, The composite molecular sieve membrane is used to selectively separate water from an esterification reaction mixture catalyzed by concentrated sulfuric acid, methanesulfonic acid, or p-toluenesulfonic acid, wherein the temperature of the esterification reaction mixture is 80°C to 150°C and the acid concentration is 5 wt% to 70 wt%.

9. An in-situ dehydration system for strong esterification reactions, characterized in that, The system includes a reactor and a pervaporation membrane separation unit, wherein the pervaporation membrane separation unit is provided with the composite molecular sieve membrane as described in any one of claims 1 to 3; the permeate side of the pervaporation membrane separation unit is connected to a vacuum system.

10. The system according to claim 9, characterized in that, It also includes one or more of the following conditions: D1) The system also includes a condensation device for condensing water vapor that permeates from the permeation side; D2) The permeate vaporization membrane separation unit has a permeate side pressure of 1 kPa to 10 kPa.

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