MFI zeolite membrane with all-silicon framework and hydrophilic pore wall as well as preparation method and application of MFI zeolite membrane

Abstract

The invention discloses an MFI zeolite membrane with an all-silicon framework and a hydrophilic pore wall as well as a preparation method and application of the MFI zeolite membrane. According to the MFI zeolite membrane, hydrophilic micromolecular organic matters are encapsulated in molecular sieve pore channels, so that on one hand, the pore size of the zeolite molecular sieve membrane can be regulated and controlled, and on the other hand, the hydrophilicity of permeation channels in the MFI membrane can be effectively improved. Compared with a conventional MFI zeolite molecular sieve membrane, the zeolite membrane has super-strong acid resistance, smaller average pore size and good hydrophilicity, so that the zeolite membrane can be suitable for efficient dehydration separation of organic matters, especially separation of water / organic matter mixed liquid in a strong acidic environment.

Description

Technical Field

[0001] The present invention belongs to the technical field of membrane separation, and specifically relates to the introduction of hydrophilic organic functional groups into the pores of the zeolite membrane to achieve directional regulation of the pore size and surface chemical properties of the zeolite membrane, thereby constructing a new MFI zeolite membrane with a full silicon skeleton and hydrophilic pore walls, and applying it to the efficient dehydration separation of organic matter. Background Art

[0002] Zeolite molecular sieve membranes are widely used in the efficient dehydration and separation of various organic substances due to their uniform microporous channels, good chemical stability and thermal stability. The Si / Al ratio in the zeolite molecular sieve membrane skeleton has an important influence on the dehydration performance. Generally speaking, the lower the Si / Al ratio in the molecular sieve membrane skeleton, the higher the dehydration flux and selectivity. However, the higher the Al content in the skeleton will lead to a decrease in the stability of the molecular sieve membrane, especially under harsh conditions such as strong acidic systems, the stability of the zeolite molecular sieve membrane will face important challenges. For example, among many zeolite molecular sieve membranes, type A zeolite molecular sieve membranes have an extremely low Si / Al ratio (1 / 1), which has extremely high flux and selectivity in the dehydration process of organic substances, but due to its poor stability, it is usually only suitable for dehydration separation of neutral solution systems.

[0003] For pure silicon zeolite molecular sieve membranes (such as MFI), since there is no Al atom in the framework, it has excellent thermal stability and chemical stability and can be used for dehydration separation under harsh conditions such as strong acid. However, since pure silicon zeolite membranes are hydrophobic, the membrane's permeation flux and separation selectivity for water are poor. For example, pure silicon MFI zeolite membranes show preferential permeability to ethanol in a water / ethanol binary system, so they are usually used to remove trace ethanol from water, but are not suitable for removing trace water from ethanol. Therefore, in order to achieve zeolite membranes with excellent acid resistance and high water permeation flux and separation selectivity, this requires that the zeolite membrane has not only a low framework Si / Al ratio but also a high hydrophilicity. However, in general, these two are difficult to obtain at the same time. The preparation of zeolite molecular sieve membranes with both ultra-high framework Si / Al ratio and high hydrophilicity is of great significance for the dehydration separation of organic matter under strong acid conditions. Summary of the invention

[0004] The purpose of the present invention is to encapsulate hydrophilic organic molecules in the pores of pure silicon MFI zeolite molecular sieve membrane, greatly improve the hydrophilicity in the membrane pores and simultaneously regulate the pore size of the membrane, so that the zeolite membrane simultaneously obtains excellent acid resistance, hydrophilicity and separation selectivity, and is applied to the efficient dehydration separation of organic matter under harsh conditions.

[0005] To achieve the above object, the technical solution of the present invention is as follows: A method for preparing an MFI zeolite membrane having an all-silicon framework and hydrophilic pore walls comprises the following steps: (1) Encapsulating the template into the pores of the MFI zeolite membrane through hydrothermal crystallization; (2) controllably decomposing the template agent in the pores of the MFI zeolite membrane obtained in step (1); The controllable partial decomposition of the template is achieved through an ozone low-temperature oxidation process, with an ozone concentration of 0.001%-50%, an oxidation temperature of 50-180°C, and an oxidation time of 1-120h.

[0006] Preferably, the ozone concentration is 1%-5%, more preferably 1%.

[0007] Preferably, the oxidation temperature is 80-150°C.

[0008] Preferably, the oxidation time is 20-100 h, more preferably 36-72 h.

[0009] Preferably, in step (1), the MFI zeolite membrane is prepared by guided synthesis in a synthesis solution to which an organic template is added.

[0010] Preferably, the template agent described in step (1) is one or a combination of tetramethylammonium hydroxide, tetramethylammonium bromide, tetraethylammonium hydroxide, tetraethylammonium bromide, tetrapropylammonium hydroxide, tetrapropylammonium bromide, tetrabutylammonium hydroxide, tetrabutylammonium bromide, methylamine, ethylamine, propylamine, n-butylammonium, diethylamine, and triethylamine.

[0011] An MFI zeolite membrane with a full silicon skeleton and hydrophilic pore walls is prepared by any of the preparation methods described above.

[0012] The MFI zeolite membrane having an all-silica framework and hydrophilic pore walls is used for dehydration separation of water / organic mixtures.

[0013] Preferably, the dehydration separation of the water / organic mixture is carried out by pervaporation or steam permeation process.

[0014] Preferably, the organic matter is one or a combination of methanol, ethanol, isopropanol, n-butanol, tert-butanol, ethylene glycol, glycerol, formic acid, acetic acid, ether, acetonitrile, tetrahydrofuran, pyridine, ethylene glycol dimethyl ether, methyl tert-butyl ether, ethyl tert-butyl ether, N-methylpyrrolidone, and N,N-dimethylformamide.

[0015] Compared with the prior art, the present invention has the following beneficial effects: The present invention encapsulates hydrophilic organic matter in the pores of the MFI zeolite membrane to obtain an MFI zeolite membrane with a full silicon skeleton and hydrophilic pore walls. This new structure makes it have excellent acid resistance and hydrophilicity at the same time, and can regulate the pore size of the zeolite molecular sieve membrane at the sub-nanometer scale, solving the problem that conventional MFI zeolite membranes cannot have both hydrophilicity and acid resistance, and the problem that the pore size of zeolite molecular sieve membranes is difficult to regulate. This new MFI zeolite membrane can be used for efficient dehydration and separation of organic solvents under harsh conditions. DETAILED DESCRIPTION

[0016] In order to better understand the present invention, the present invention is described in detail below in conjunction with specific embodiments. It should be understood that the embodiments are only used to illustrate the principles and technical solutions of the present invention and are not intended to limit the scope of the present invention. For those skilled in the art, without departing from the spirit and scope of the present invention, various modifications and improvements can also be made, which should all be deemed to belong to the protection scope of the present invention.

[0017] Example 1 Tetraethyl orthosilicate (TEOS), tetrapropylammonium hydroxide (TPAOH) and water were fully mixed and stirred to obtain a synthetic solution with a molar ratio of 1TEOS: 0.2TPAOH: 100H2O. The α-Al2O3 carrier with a seed layer prepared in advance was placed vertically in the reactor, poured into the synthetic solution and sealed, and the reactor was placed in an oven at 150°C for crystallization for 24 hours. The MFI membrane in the reactor was taken out, washed with deionized water until neutral, and then the obtained MFI membrane was dried in an oven at 115°C. The dried MFI zeolite membrane was treated with 1% ozone at 150°C for 72 hours, and then used for pervaporation dehydration separation of 90wt% acetic acid aqueous solution. The results showed that the permeation flux of the membrane at room temperature was 0.157kg m -2 h -1 , the separation factor of water / acetic acid is 2359.

[0018] Example 2 The ozone treatment time of the MFI zeolite membrane was reduced from 72 h to 36 h, and other conditions remained the same as in Example 1. The activated MFI zeolite membrane was then used in the pervaporation dehydration separation of 90 wt % acetic acid aqueous solution. The results showed that the permeation flux at room temperature was 0.188 kg m -2 h -1 , the separation factor of water / acetic acid reached 4491. After the separation process was continued for 12 hours, the permeate flux was 0.163 kg m -2 h -1 , the separation factor of water / acetic acid is 4320, and the separation performance of the membrane remains stable.

[0019] Example 3 The ozone treatment temperature of the MFI zeolite membrane was reduced from 150°C to 80°C, and other conditions remained the same as in Example 1. The activated MFI zeolite membrane was then used in the pervaporation dehydration separation of 90 wt% acetic acid aqueous solution. The results showed that the permeation flux at room temperature was 0.083 kg m -2 h -1 , the separation factor of water / acetic acid reaches 5247.

[0020] Example 4 The activated ozone concentration of the MFI zeolite membrane was increased from 1% to 5%, and the other conditions remained the same as in Example 2. The activated MFI zeolite membrane was then used in the pervaporation dehydration separation of 90 wt % acetic acid aqueous solution. The results showed that the permeation flux at room temperature was 0.187 kg m -2 h -1 , the separation factor of water / acetic acid reaches 4551.

[0021] Example 5 The activation conditions of the MFI zeolite membrane were the same as those in Example 2. The activated MFI zeolite membrane was then used in the pervaporation dehydration separation of a 90 wt % isopropanol aqueous solution. The results showed that the permeation flux at room temperature was 0.518 kg m -2 ·h -1 , the separation factor of water / isopropanol reaches 9991.

[0022] Comparative Example 1 The synthesis conditions of the MFI zeolite membrane were the same as those in Example 1, but the decomposition process of the template agent in the zeolite membrane was calcined at high temperature. The heating and cooling rates during the calcination process were set to 0.5°C / min, and the prepared MFI zeolite membrane was calcined at 500°C for 6h. The activated MFI zeolite membrane was subjected to pervaporation dehydration separation of 90wt% acetic acid aqueous solution. The results showed that the permeation flux and water content on the permeate side at room temperature were 0.31kg m -2 h -1 , the separation factor of water / acetic acid is only 40.6.

[0023] Comparative Example 2 The synthesis conditions of the MFI zeolite membrane were the same as those in Example 1, but the decomposition process of the template agent in the zeolite membrane was calcined at high temperature. The heating and cooling rates during the calcination process were set to 0.5°C / min, and the prepared MFI zeolite membrane was calcined at 500°C for 6h. The activated MFI zeolite membrane was subjected to pervaporation dehydration separation of 90wt% isopropanol aqueous solution. The results showed that the permeation flux at room temperature was 0.47kg m -2 h -1 , the separation factor of water / isopropanol is only 114.

Claims

1. A method for preparing an MFI zeolite membrane having an all-silicon framework and hydrophilic pore walls, characterized in that: The following steps are involved: (1) Encapsulating the template into the pores of the MFI zeolite membrane through hydrothermal crystallization; (2) controllably decomposing the template agent in the pores of the MFI zeolite membrane obtained in step (1); Among them, the controllable partial decomposition of the template is achieved through an ozone low-temperature oxidation process, with an ozone concentration of 0.001%-50%, an oxidation temperature of 50-180°C, and an oxidation time of 1-120h.

2. The method for preparing an MFI zeolite membrane having an all-silicon framework and hydrophilic pore walls according to claim 1, characterized in that: The ozone concentration is 1%-5%.

3. The method for preparing an MFI zeolite membrane having an all-silicon framework and hydrophilic pore walls according to claim 1, characterized in that: The oxidation temperature is 80-150°C.

4. The method for preparing an MFI zeolite membrane having an all-silicon framework and hydrophilic pore walls according to claim 1, characterized in that: The oxidation time is 20-100 hours.

5. The method for preparing an MFI zeolite membrane having an all-silicon framework and hydrophilic pore walls according to any one of claims 1 to 4, characterized in that: In step (1), the MFI zeolite membrane is prepared by directed synthesis in a synthesis solution to which an organic template is added.

6. A method for preparing an MFI zeolite membrane having an all-silicon framework and hydrophilic pore walls according to any one of claims 1 to 4, characterized in that: The template agent described in step (1) is one or a combination of tetramethylammonium hydroxide, tetramethylammonium bromide, tetraethylammonium hydroxide, tetraethylammonium bromide, tetrapropylammonium hydroxide, tetrapropylammonium bromide, tetrabutylammonium hydroxide, tetrabutylammonium bromide, methylamine, ethylamine, propylamine, n-butylammonium, diethylamine, and triethylamine.

7. An MFI zeolite membrane having an all-silicon skeleton and hydrophilic pore walls obtained by the preparation method according to any one of claims 1 to 6.

8. The MFI zeolite membrane with an all-silica framework and hydrophilic pore walls as claimed in claim 7 is used for dehydration separation of water / organic mixtures.

9. The MFI zeolite membrane with a full silicon framework and hydrophilic pore walls according to claim 8 is used for dehydration separation of water / organic mixtures, characterized in that: The dehydration separation of the water / organic mixture is carried out by pervaporation or steam permeation process.

10. The MFI zeolite membrane with a full silicon framework and hydrophilic pore walls according to claim 8 is used for dehydration separation of water / organic mixtures, characterized in that: The organic matter is one or a combination of methanol, ethanol, isopropanol, n-butanol, tert-butanol, ethylene glycol, glycerol, formic acid, acetic acid, ether, acetonitrile, tetrahydrofuran, pyridine, ethylene glycol dimethyl ether, methyl tert-butyl ether, ethyl tert-butyl ether, N-methylpyrrolidone, and N,N-dimethylformamide.