Molecular sieve membranes are a new type of porous inorganic material developed in the last decade by cross-linking molecular sieve grains.
Did you know that a molecular sieve membrane can separate molecules based on their size and shape? It is based on the dual principle of preferential adsorption and molecular sieving. The separation is achieved by selective adsorption, depending on the size or polarity of the molecules of the mixed system, through molecules of similar size but different polarity.
Today, molecular sieve membranes are commonly used in membrane permeation evaporation liquid separation, membrane-catalyzed reactions, separation of aromatic isomers, alkanes, and olefins, meteorological separation, separation of biochemical products, and environmental protection.
Let’s learn how to make a molecular sieve membrane with our detailed guide!
Our guide includes step-by-step instructions for the preparation, coating, and characterization of molecular sieve membranes, as well as the effects of different synthesis conditions.
Molecular Sieve Membrane Synthesis Processes
In the preparation of molecular sieve membranes, a synthetic liquid is firstly applied to the surface of the carrier according to a certain molar composition, then under certain synthesis conditions, such as temperature and speed, the crystal particles are nucleated and cross-linked by further crystallization on the surface of the carrier, thus forming a homogeneous and continuous membrane layer. Common methods of preparation include in situ hydrothermal synthesis, secondary growth, microwave-assisted growth, and vapor phase conversion.
1.In-Situ Hydrothermal Synthesis
This method consists of mixing silicon, aluminum, templating agent, and water in a certain molar ratio to form a synthesis solution, which is then transferred to an autoclave containing a PTFE liner and crystallized at a certain temperature for a certain period of time to allow the crystals to nucleate and grow in situ on the carrier, thus forming a zeolite molecular sieve membrane layer on the carrier to obtain a zeolite molecular sieve membrane.
The disadvantage of the in situ hydrothermal synthesis method is that the carrier is placed directly in the synthesis solution, the nucleation process of the molecular sieve is stochastic and the crystallization time is long. In addition, the method is difficult to operate and sensitive to the requirements of the synthesis conditions, slight deviations from the optimized conditions can adversely affect the film formation and performance of the molecular sieve film.
Advantages are the simplicity of the process, the high mechanical strength of the synthesized molecular sieve membrane, the ability to prepare highly oriented molecular sieve membrane, more suitable for large-scale production, and is currently the most commonly used synthesis method.
2. Crystalline Growth Method
The crystalline growth method is also known as the secondary growth method. The method is to coat the surface of the carrier with crystalline seeds in advance by some physical method, then put the carrier with pre-coated crystalline seeds in an oven to dry, take it out and put the carrier in a PTFE liner, pour in the molecular sieve synthesis solution to submerge the carrier, then put it into an autoclave, and finally crystallize under certain hydrothermal conditions to form a molecular sieve film layer on the surface of the carrier.
This method preintroduces a layer of molecular sieve crystal seeds on the carrier by means of physical coating, providing a nucleation center for the growth of molecular sieve crystals, allowing time for crystal nucleation and crystallization reactions, facilitating the growth of molecular sieve grains in a selective orientation, and making it easy to obtain dense and homogeneous, high-performance molecular sieve films with few defects. It can also effectively control the crystal orientation and can synthesize molecular sieve membranes without a templating agent.
It was found that the secondary growth method provides better control over the microstructure such as thickness and orientation of the membrane, higher reproducibility and a wider range of hydrothermal synthesis conditions than in situ synthesis, resulting in continuous, dense molecular sieve membrane layers.
3. Microwave-assisted Synthesis Method
Microwave is a type of electromagnetic radiation with a frequency between 0.3 and 300 GHz and is widely used in various research fields such as biology and medicine. Microwave-assisted synthesis of molecular sieve membranes is also one of the applications of microwave technology.
This method is a way of using microwave radiation as a reaction heat source for the synthesis of molecular sieve membranes. Compared with conventional hydrothermal synthesis, microwave-assisted synthesis uses the properties of microwave heating to facilitate nucleation, accelerate the rate of crystallization, shorten the synthesis time of molecular sieve membranes, and effectively inhibit the formation of heterocysts.
The experimental results surface that microwave heating accelerates the rate of NaA molecular sieve membrane synthesis. The synthesis time by microwave heating was reduced from 3hr to 15min. The crystalline species on the surface of the carrier not only promoted the formation of molecular sieve membranes but also inhibited the conversion of NaA molecular sieve membranes to other types of molecular sieve membranes. In addition, NaA molecular sieve membranes synthesized by microwave heating were more dense, thin, and homogeneous with higher permeability.
4.Vapour Phase Conversion Method
Vapor phase conversion is also a relatively new method for the synthesis of molecular sieve membranes.
The molecular sieve crystals without a templating agent are first coated on the surface of the carrier to form a dry adhesive layer, and then the carrier is placed on top of, but not in contact with, the synthesis solution.
Then the molecular sieve synthesis liquid configured in a certain molar ratio is poured into the PTFE liner, sealed, placed in an autoclave and crystallized for a period of time under certain conditions such as temperature.
The method uses steam transport to deliver the desired reaction material into the dry gel, which allows the dry gel to undergo further transformation.
Compared to the traditional hydrothermal synthesis method, this method has the advantage of less demand for organic templating agents, no need to separate the membrane tube nucleation synthesis solution, the amount of waste liquid produced is small and environmentally friendly. The disadvantages are the longer and more difficult crystallization time and the tendency to cause membrane defects.
In addition to the above common preparation methods, there are some less common preparation methods. Such as vapour phase methods, embedding methods, electrodeposition methods, carrier modified covalent bonding synthesis methods, ionic liquid synthesis methods, mobile system synthesis methods, etc.
Preparation Process Of Molecular Sieve Membrane
- Preparation Of Crystalline Seed Gum
Dissolve the KOH in deionized water, add a small amount of aluminum flakes, and stir continuously until the aluminum is completely dissolved to form a clarified solution, then add another solution consisting of a configuration of tetraethylammonium hydroxide, silica gel, and deionized water.
After stirring for 30 min, the obtained white gel mixture is transferred to a high-pressure stainless steel reactor with PTFE lining. The reactor is fixed in a homogeneous reactor and crystallized at 20RPM, 150°C for 96hr. The white gel obtained is the crystal seed gel of the molecular sieve membrane.
- Carrier Crystal Coating
Carrier pretreatment: Porous α-Al2O3 is the carrier material commonly used in the preparation of molecular sieve membrane, using sandpaper to polish the stains on the outer surface of the porous tubular α-Al2O3 carrier until the surface is smooth, and then cleaning with deionized water by ultrasonic shock to remove the excess impurities on the inner and outer surface of the carrier and in the carrier pore channel, put it in a constant temperature drying oven for 2hr and then take it out, and finally put it in a furnace to roast at 500℃ for 2hr.
Common dipping and coating method: The two ends of the pretreated tubular α-Al2O3 carrier are sealed with PTFE plugs, quickly submerged in the above-mentioned crystal seed glue solution, set for 30s and then slowly removed, then the membrane tube is placed in a constant temperature drying oven and dried at 80°C.
Dynamic impregnation coating method: The pre-treated tubular α-Al2O3 carrier is sealed at both ends with PTFE and fixed vertically in a stainless steel Teflon-lined kettle. The above-mentioned solution of crystalline gum is poured over the carrier to submerge it. The stainless steel reactor is then placed in a homogeneous reactor and coated at a certain speed for a certain time. After completion, the membrane tube is slowly removed and placed in a constant temperature drying oven and dried at 80°C.
What Is The Process Of Making A Molecular Sieve Membrane?
Firstly, Potassium hydroxide, aluminum flakes, tetraethylammonium hydroxide, silica sol, and distilled water are combined in a certain molar ratio to form the synthesis solution. The α-Al2O3 support body, pre-coated with crystalline species as described above, is then sealed at both ends with PTFE plugs and fixed vertically in a stainless steel PTFE-lined high-pressure kettle, slowly pouring in the synthesis solution to submerge the support body and sealing the kettle.
The membrane is crystallized dynamically under certain conditions for a certain period of time. After the crystallization is completed, the reactor is rapidly cooled with water, the membrane tube is removed, the tube is repeatedly rinsed with deionized water to neutral, dried at 80°C for 2hr, and finally placed in a muffle furnace for high-temperature roasting at 500°C to obtain a molecular sieve membrane.
How to Make Analytical Testing And Characterisation Of Molecular Sieve Membranes
- X-Ray Diffraction (XRD) is a common analytical technique that uses X-ray diffraction of crystal formation to analyze the internal structure of a substance based on the position of characteristic diffraction peaks. It allows qualitative studies of crystalline materials, such as determining whether a sample is crystalline, determining the crystallinity of a crystal, identifying different crystal types, and determining whether a crystal is oriented.
- Scanning surface electron microscopy (SEM for short) is a characterization tool for observing and analyzing the surface morphology of a sample by impinging a high-energy electron beam on the sample and using the interaction of electrons with the sample to release and reflect back atoms from the sample surface. Combining it with energy spectroscopy (EDS) allows for compositional analysis. SEM techniques are now used in a wide variety of fields.
- Gas chromatography (GC for short) is a commonly used technique for separation and analysis. It works on the principle of using an inert gas as a carrier gas and separating mixtures by multiple partitioning according to the difference in partition coefficients of different substances in the gas phase and stationary phase. This method is highly sensitive, fast in analysis, simple in operation, and has a wide range of applications, making it an efficient separation method.
Effects of Different Synthesis Conditions On The Preparation Of Molecular Sieve Membranes
The method of coating is directly related to the denseness and continuity of the molecular sieve membrane.
When the molecular sieve membrane is prepared by the dynamic coating method and crystallized at 20RPM, 150°C for 24hr, it has the characteristic peaks of molecular sieves without spurious peaks and is stronger than the sample prepared by the normal dip coating method.
The surface of the membranes has rod-shaped crystal particles stacked flat on the carrier in layers, with tight cross-linking between the crystals and no obvious voids, forming a molecular sieve layer with interacting crystal growth. The structure is regular, continuous, and dense, and the layer is thinner and more homogeneous than that prepared by the common impregnation and coating method.
The rotation of the reactor allows the crystalline gum solution to be continuously washed over the surface of the carrier during the dynamic dipping and coating of the tubular α-Al2O3 carrier in the high-pressure reactor so that the crystalline gum solution can evenly contact all parts of the carrier surface, and the uniform rotation enables each part of the tubular carrier surface to be evenly stressed, so that the crystal particles can be evenly arranged on the carrier surface in an orderly and consistent direction, resulting in a thinner and more uniform membrane layer.
Different dynamic dipping and coating conditions, such as coating time, frequency, and rotation speed, are important factors influencing the coating effect of the crystal seeds and the formation of the molecular sieve membrane.
The crystals were dynamically coated at 20RPM for 1min, 5min, and 10min and repeated three times to investigate the effect of the three different coating times on the molecular sieve membrane crystals.
When the dynamic dip coating is applied for 1 min, the crystalline seed particles on the tubular α-Al2O3 carrier are not yet completely covered, and although a small number of rod-shaped crystal particles showed a tendency to be neatly aligned, most of the rod-shaped particles are still staggered and cross-packed, with large gaps between the crystals.
When the coating time reaches 5min, the crystal seed particles are dense and uniformly distributed on the surface of the tubular porous α-Al2O3 carrier, with good flatness and no obvious defects.
When the coating time is extended to 10min, the crystal particles are evenly distributed and the cross-linking between the rod-shaped crystals is tight, which is basically the same as the state at 5min of coating.
Therefore, 5 min was chosen as the optimum coating time.
Effect of dynamic coating times on molecular sieve film formation
When the molecular sieve films are dynamically coated at 20RPM for 5min, once, twice, and three times and crystallized at 150°C for 24hr at 20RPM to investigate the effect of the number of coats on the film formation.
When dynamically coated once, the coated molecular sieve film layer is barely visible on the carrier. When coated twice dynamically, there is a film layer covering the surface of the carrier, but the boundary between the carrier and the film layer is still not obvious, and the film layer is not uniform in thickness and does not form a dense and continuous molecular sieve film layer. With three dynamic coats, a dense and flat molecular sieve layer is formed with a clear, uniform demarcation line with the carrier.
It follows that at least three dynamic dip-coatings are required to form a continuous dense molecular sieve layer. If the number of coats is not sufficient, the seed gel solution will not adhere uniformly to the surface of the carrier, making it difficult to form a uniform, continuous, and flat molecular sieve layer.
The effect of dynamic coating speed on the formation of molecular sieve membrane
The effect of coating speed on the formation of the molecular sieve membrane is investigated by coating at 20RPM and 60RPM for 5min respectively, repeated three times and crystallizing at 150°C, 20RPM for 24hr.
According to the SEM diagram, a dense and continuous molecular sieve coating layer with clear demarcation from the carrier can be observed when coating is carried out at 20 RPM. When the coating speed reaches 60 RPM, the film layer on the surface of the carrier becomes thin and uneven.
This is probably because the coating speed is too fast, the adhesion between the crystal seed glue solution and the carrier becomes smaller, and the solution has a greater scouring force, which leads to the crystal seed glue solution not easily adhering to the carrier surface, thus the crystal seed particles cannot be well laid flat on the carrier surface, the crystal seed glue solution is unevenly coated and the film layer becomes thinner.
In summary, according to the SEM characterization of the molecular sieve membrane under different dynamic dip coating conditions, the best flatness and density of the molecular sieve membrane is obtained with three dynamic coatings at a coating time of 5 min and a coating speed of 20 RPM.
Effect of Crystallization Time On The Formation of Molecular Sieve Membrane
Crystallization time is an important condition affecting the formation of molecular sieve membranes. The length of the crystallization time plays a key role in the crystallinity, structure and surface morphology of the molecular sieve membrane.
The crystalline seeds were coated by the dynamic dip coating method and crystallized at 20 RPM at 150°C for 12hr, 24hr, 48hr, and 96hr respectively. The resulting molecular sieve membranes were characterized to investigate the effect of different crystallization times on the formation of the molecular sieve membranes.
XRD plots for crystallization times of 12hr, 24hr, 48hr and 96hr show that pure molecular sieve membranes can be synthesized in the time range of 12hr-96hr. However, as the crystallization time increases, the intensity of the characteristic peaks of the molecular sieve increases and then decreases, and the characteristic diffraction peaks of the molecular sieve are the weakest at 12hr, while the characteristic peaks of the molecular sieve reach the maximum at 24hr. Correspondingly, the intensity of the characteristic peaks of the above sample supports weakened and then increased.
As shown in the scanning electron micrographs of the molecular sieve films prepared at different crystallization times, when the crystallization time is 12hr, the crystals are irregularly rod-shaped, and there is a considerable amount of amorphous material on the surface of the membrane layer in addition to the molecular sieve grains, and the coverage of the molecular sieve membrane on the surface of the support body is not high, and a large number of intergranular pores exists.
When time is extended to 24hr, a molecular sieve membrane layer with a distinct rod-like structure is formed on the surface of the support, with a regular, dense and continuous structure, amorphous material on the surface of the membrane layer, and a high degree of crystallinity.
When the crystallization time is extended to 48hr, some of the rod-shaped crystals, which are tightly cross-linked together, disperse and accumulate on the surface of the membrane layer in a disorderly manner, resulting in gaps between the crystals and a deterioration of the flatness and density of the membrane layer.
When the crystallization time continues to be extended to 96hr, most of the crystals on the surface of the film layer are dispersed and some small crystals with rounded ends and different lengths appears, which are presumed to be formed by the ablative passivation of some of the edges of the original rod-shaped crystals.
However, no visible molecular sieve membrane layer is observed on the surface of the carrier at a crystallization time of 12hr.
When the crystallization time is extended to 24hr, a dense and homogeneous molecular sieve membrane with a clear demarcation from the support and a film thickness of about 10μm can be seen.
As the crystallization time continues to increase, some of the membranes begin to thin and become more inhomogeneous when the crystallization time is extended to 96hr.
In summary, the best crystallization time for the preparation of molecular sieve film by dynamic impregnation coating method at 150°C, 20RPM is 24hr. When shorter than 24hr, the time for molecular sieve grains to grow and nucleate is not enough, the crystallization of the membrane layer is not yet complete, there is a lot of amorphous material, more gaps between the crystals, and the membrane layer defects are larger. Longer than 24hr, the formed molecular sieve membrane layer in the strong alkali environment dissolve, so that the degree of cross-linking reduce, the membrane layer thinning or even disappear.
Effect of Crystallization Temperature On The Formation of Molecular Sieve Membrane
The crystallization temperature is also a key influencing factor in the preparation of molecular sieve membranes, which are often selected depending on the membrane material. When prepared by dynamic impregnation coating method, crystallization was carried out at 90°C, 120°C, 150°C and 180°C respectively for 24hr to investigate the effect of different crystallization temperatures on molecular sieve membrane formation.
Analysis of the XRD plots of the samples tested shows that at lower temperatures the molecular sieve characteristic peaks are the weakest, while the support body characteristic peaks are the strongest. When the temperature was increased to 120°C, the molecular sieve peaks were slightly enhanced, while the support peaks were weakened. At 150°C, the molecular sieve peak is the strongest, and as the temperature continues to rise to 180°C, the molecular sieve peak is weaker than at 150°C, and correspondingly, the support body peak begins to increase.
When the temperature is 90 ℃, the molecular sieve grains have not been completely crystallized, the rod shape of the grain has not been fully formed, the molecular sieve membrane is not dense and continuous, and the defects are large.
When the temperature rises to 120°C, the molecular sieve grains are further crystallized, but there are gaps between the crystals and the flatness of the film layer is poor.
When raised to 150°C, the crystals are highly cross-linked and the surface of the membrane layer is flat. As the temperature continues to rise to 180°C, the molecular sieve grains begin to dissolve and some of the grains become rounded.
It has been demonstrated that the crystallization temperature has an effect on the crystallization rate of molecular sieve films. Typically, a suitable increase in crystallization temperature will result in a faster crystallization rate. If the temperature is too low, the crystallization rate of the molecular sieve is too slow, the crystallization is not complete within a certain crystallization time, and a continuous dense film layer is not formed.
In contrast, when the temperature is too high, the molecular sieve crystals dissolve in a strongly alkaline environment and the cross-linking between the crystals deteriorates.
Indeed, there is a connection between crystallization time and crystallization temperature, with temperature providing energy for the nucleation of crystals. A suitable increase in crystallization temperature increases the energy provided to the crystal, which to some extent speeds up the nucleation rate and shortens the crystallization time.
Effect of Rotational Speed On Molecular Sieve Film Formation
During the dynamic crystallization synthesis of molecular sieve films, the synthesis solution constantly flows on the carrier’s surface. As the rotation speed changes, the speed of the synthesis solution flowing on the surface of the tubular α-Al2O3 carrier in the reactor changes, thus affecting the film formation of the molecular sieve membrane.
When the rotation speeds are set to 0, 20, and 60 RPM, respectively, and the membrane are crystallized at 150°C for 24hr, the XRD patterns of the films obtained at the different rotation speeds shows that the samples synthesized at all three crystallization speeds had characteristic molecular sieve peaks, indicating that molecular sieve films could be synthesized at all three crystallization speeds.
The molecular sieve membranes synthesized at 20RPM have distinctive peaks and no spurious peaks are present. When the speed is increased to 60 RPM, the characteristic peaks of the molecular sieve membrane is weaker than at 20 RPM, while the characteristic peaks of the support increases.
The analytical reason for this may be that the acceleration of the rotational speed to 60RPM leads to a smaller viscosity of the synthetic liquid, which results in less adhesion between the synthetic liquid and the carrier, and the carrier surface does not have sufficient time to grow a dense molecular sieve film layer, resulting in a larger film defect.
Conclusion
It can be concluded that during the dynamic dip coating, the rotation of the reactor allows the crystalline gum solution to be continuously washed over the surface of the carrier, thus enabling the crystal particles to be arranged in an orderly and consistent direction on the surface of the carrier, forming a thinner and more uniform membrane layer.
A dynamic coating time of 5min and a coating speed of 20RPM for three times crystallization is the best condition, resulting in the best dense and flat molecular sieve membrane. The molecular sieve membrane synthesized by crystallization at 150°C for 24hr at 20 RPM is more crystalline, regular in structure, most dense and continuous, thin and homogeneous.
It’s important to note that the specific details of the zeolite membrane fabrication process can vary depending on the type of zeolite and the desired application. Additionally, expertise in materials synthesis and handling may be required to achieve high-quality zeolite membranes.
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