Zeolite membrane

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A zeolite membrane is a synthetic membrane made of crystalline aluminosilicate materials, typically aluminum, silicon, and oxygen with positive counterions such as Na+ and Ca2+ within the structure. Zeolite membranes serve as a low energy separation method. They have recently drawn interest due to their high chemical and thermal stability, [1] and their high selectivity. Currently zeolites have seen applications in gas separation, membrane reactors, water desalination, and solid state batteries. [2] Currently zeolite membranes have yet to be widely implemented commercially due to key issues including low flux, high cost of production, and defects in the crystal structure.

Contents

Production methods

There are several methods used for the formation of Zeolite membranes.

The In Situ method involves Zeolite membranes being formed on microporous supports of various materials, typically aluminum oxide or stainless steel. These supports are then immersed in a solution of Aluminum and Silicon at a specific stoichiometric ratio. Other factors of this solution can affect the formation of the zeolite membrane including: pH, Ionic Strength, temperature, and the addition of structure-determining reagents . Upon heating the solution, the crystals of the membrane begin to grow on the supports.

In 2012, a “seeding method” was developed to produce zeolite membranes. In this case, the support is seeded with preformed zeolite crystals, before immersing it in the solution. These crystals allow for the formation of thinner membranes that typically contain fewer defects by growing the membranes off of existing structures. [3]

Properties

Zeolite membranes drew initial interest as a separation method due to their high thermal and chemical stabilities. The crystal structure of zeolite membranes also creates a uniform pore size of approximately .3-1.3 nm in diameter. The crystal structure of zeolites also leads to the presence of several defects, which can often create gaps in the structure larger than these pores. The presence of defects can make these membranes far less effective, and it is difficult to produce defect free zeolite membranes. [4]

There are several mechanisms of transport that govern the separation of molecules by zeolite membranes. The main mechanisms for separation by zeolite membranes are molecular sieving, diffusion, and adsorption. Molecular sieving involves the rejection of any molecules of a size greater than the pore size of the membrane. This is a relatively simple sieving process which can separate out very large molecules. Adsorption involves molecules passing through the pores of the membrane being adsorbed onto the membrane surface. Adsorption properties of the membranes can be changed by adjusting various structural properties of the membrane. [5]

Surface diffusion is a process in which molecules adsorb to the pore wall of the membrane, and are slowly transported through the pores. During surface diffusion, molecules that are adsorbed at a higher rate can begin to block the membrane pores from other, less adsorbed, molecules. Surface diffusion can account for the high selectivity of certain molecules such as hydrogen by zeolite membranes. [6] Surface diffusion typically plays a larger role in the transport of molecules at lower temperatures.

Knudsen diffusion also contributes to the varying selectivity of zeolite membranes towards different molecules. Knudsen diffusion takes place when molecules are momentarily adsorbed to the pore wall and are then reflected off the surface in a random direction. This random motion allows for separation of molecules based on their velocities. Graham's law for diffusion dictates that lighter molecules will have a higher average velocity than heavier molecules, thus resulting in an increased flux with respect to lighter molecules. These differences in flux can be used to separate different molecules using zeolite membranes. [3]

Applications

Gas separation

Zeolite membranes have seen the most promise in regards to gas separation applications. The ability of zeolite membranes to adsorb certain molecules to its surface under varying conditions allows for researchers to perform highly selective separations. Adsorbed molecules block diffusion pores, and prevent the diffusion of other molecules through these pores. Zeolites typically adsorb carbon dioxide at the highest rate, lending themselves to use in carbon dioxide capture and separation. Diffusion selectivity governs the separation of molecules in zeolite membranes at higher temperatures. Diffusion selectivity allows for the quicker diffusion of smaller molecules through the membrane and slower diffusion of large molecules through the membrane's pores. [6]

The natural gas industry has seen the introduction of zeolite membranes for the separation of methane, carbon dioxide, and hydrogen gasses. Zeolites provide the advantage of thermal stability and higher selectivity when compared to polymer membranes that have typically been used for these purposes. [7] There still needs to be improvement in the production of zeolite membranes, particularly regarding the cost, before they see widespread use.

Membrane reactors

Zeolite membranes have also been used in membrane reactors, since their chemical and thermal stabilities allow them to withstand reaction conditions. Membrane reactors function by removing the product of a reaction as the reaction occurs. This removal shifts the equilibrium of the reaction to allow for the formation of more products, as outlined by Le Chatelier's principle creating a more efficient reaction process. The high selectivity of zeolite membranes allows for them to be used to remove products from a reactor at high rates. [8]

Water desalination

Zeolite membranes have recently been studied as an alternative for energy efficient water desalination. Currently water desalination is primarily done by Reverse Osmosis filtration which uses a dense polymeric membrane to purify the water. Zeolite membranes have been tested as an alternative water purification method, and are able to separate water from impurities. Zeolites have not been implemented for industrial water desalination purposes primarily due to their high cost when compared to traditional reverse osmosis membranes. [9]

Related Research Articles

<span class="mw-page-title-main">Filtration</span> Process that separates solids from fluids

Filtration is a physical separation process that separates solid matter and fluid from a mixture using a filter medium that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filter medium are described as oversize and the fluid that passes through is called the filtrate. Oversize particles may form a filter cake on top of the filter and may also block the filter lattice, preventing the fluid phase from crossing the filter, known as blinding. The size of the largest particles that can successfully pass through a filter is called the effective pore size of that filter. The separation of solid and fluid is imperfect; solids will be contaminated with some fluid and filtrate will contain fine particles. Filtration occurs both in nature and in engineered systems; there are biological, geological, and industrial forms. In everyday usage the verb "strain" is more often used; for example, using a colander to drain cooking water from cooked pasta.

<span class="mw-page-title-main">Zeolite</span> Microporous, aluminosilicate mineral

Zeolite is a family of several microporous, crystalline aluminosilicate materials commonly used as commercial adsorbents and catalysts. They mainly consist of silicon, aluminium, oxygen, and have the general formula Mn+
1/n(AlO
2)−
(SiO
2)
x･yH
2O where Mn+
1/n is either a metal ion or H+. These positive ions can be exchanged for others in a contacting electrolyte solution. H+
 exchanged zeolites are particularly useful as solid acid catalysts.

<span class="mw-page-title-main">Adsorption</span> Phenomenon of surface adhesion

Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the adsorbate on the surface of the adsorbent. This process differs from absorption, in which a fluid is dissolved by or permeates a liquid or solid. While adsorption does often precede absorption, which involves the transfer of the absorbate into the volume of the absorbent material, alternatively, adsorption is distinctly a surface phenomenon, wherein the adsorbate does not penetrate through the material surface and into the bulk of the adsorbent. The term sorption encompasses both adsorption and absorption, and desorption is the reverse of sorption.

Pervaporation is a processing method for the separation of mixtures of liquids by partial vaporization through a non-porous or porous membrane.

<span class="mw-page-title-main">Activated carbon</span> Form of carbon with an extremely high surface area

Activated carbon, also called activated charcoal, is a form of carbon commonly used to filter contaminants from water and air, among many other uses. It is processed (activated) to have small, low-volume pores that greatly increase the surface area available for adsorption or chemical reactions that can be thought of as a microscopic "sponge" structure. Activation is analogous to making popcorn from dried corn kernels: popcorn is light, fluffy, and its kernels have a high surface-area-to-volume ratio. Activated is sometimes replaced by active.

<span class="mw-page-title-main">Heterogeneous catalysis</span> Type of catalysis involving reactants & catalysts in different phases of matter

Heterogeneous catalysis is catalysis where the phase of catalysts differs from that of the reagents or products. The process contrasts with homogeneous catalysis where the reagents, products and catalyst exist in the same phase. Phase distinguishes between not only solid, liquid, and gas components, but also immiscible mixtures, or anywhere an interface is present.

Desorption is the physical process where adsorbed atoms or molecules are released from a surface into the surrounding vacuum or fluid. This occurs when a molecule gains enough energy to overcome the activation barrier and the binding energy that keep it attached to the surface.

<span class="mw-page-title-main">Membrane gas separation</span> Technology for splitting specific gases out of mixtures

Gas mixtures can be effectively separated by synthetic membranes made from polymers such as polyamide or cellulose acetate, or from ceramic materials.

An oxygen concentrator is a device that concentrates the oxygen from a gas supply by selectively removing nitrogen to supply an oxygen-enriched product gas stream. They are used industrially, to provide supplemental oxygen at high altitudes, and as medical devices for oxygen therapy.

<span class="mw-page-title-main">Molecular sieve</span> Filter material with homogeneously sized pores in the nanometer range

A molecular sieve is a material with pores of uniform size. These pore diameters are similar in size to small molecules, and thus large molecules cannot enter or be adsorbed, while smaller molecules can. As a mixture of molecules migrates through the stationary bed of porous, semi-solid substance referred to as a sieve, the components of the highest molecular weight leave the bed first, followed by successively smaller molecules. Some molecular sieves are used in size-exclusion chromatography, a separation technique that sorts molecules based on their size. Another important use is as a desiccant. Most of molecular sieves are aluminosilicate zeolites with Si/Al molar ratio less than 2, but there are also examples of activated charcoal and silica gel.

<span class="mw-page-title-main">Pressure swing adsorption</span> Method of gases separation using selective adsorption under pressure

Pressure swing adsorption (PSA) is a technique used to separate some gas species from a mixture of gases under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at near-ambient temperature and significantly differs from the cryogenic distillation commonly used to separate gases. Selective adsorbent materials are used as trapping material, preferentially adsorbing the target gas species at high pressure. The process then swings to low pressure to desorb the adsorbed gas.

<span class="mw-page-title-main">Thin-film composite membrane</span> Membrane used for water purification

Thin-film composite membranes are semipermeable membranes manufactured to provide selectivity with high permeability. Most TFC's are used in water purification or water desalination systems. They also have use in chemical applications such as gas separations, dehumidification, batteries and fuel cells. A TFC membrane can be considered a molecular sieve constructed in the form of a film from two or more layered materials. The additional layers provide structural strength and a low-defect surface to support a selective layer that is thin enough to be selective but not so thick that it causes low permeability.

<span class="mw-page-title-main">Nanoporous materials</span>

Nanoporous materials consist of a regular organic or inorganic bulk phase in which a porous structure is present. Nanoporous materials exhibit pore diameters that are most appropriately quantified using units of nanometers. The diameter of pores in nanoporous materials is thus typically 100 nanometers or smaller. Pores may be open or closed, and pore connectivity and void fraction vary considerably, as with other porous materials. Open pores are pores that connect to the surface of the material whereas closed pores are pockets of void space within a bulk material. Open pores are useful for molecular separation techniques, adsorption, and catalysis studies. Closed pores are mainly used in thermal insulators and for structural applications.

<span class="mw-page-title-main">Membrane reactor</span>

A membrane reactor is a physical device that combines a chemical conversion process with a membrane separation process to add reactants or remove products of the reaction.

<span class="mw-page-title-main">Metal–organic framework</span> Class of chemical substance

Metal–organic frameworks (MOFs) are a class of porous polymers consisting of metal clusters coordinated to organic ligands to form one-, two- or three-dimensional structures. The organic ligands included are sometimes referred to as "struts" or "linkers", one example being 1,4-benzenedicarboxylic acid (BDC).

Nanotube membranes are either a single, open-ended nanotube(CNT) or a film composed of an array of nanotubes that are oriented perpendicularly to the surface of an impermeable film matrix like the cells of a honeycomb. 'Impermeable' is essential here to distinguish nanotube membrane with traditional, well known porous membranes. Fluids and gas molecules may pass through the membrane en masse but only through the nanotubes. For instance, water molecules form ordered hydrogen bonds that act like chains as they pass through the CNTs. This results in an almost frictionless or atomically smooth interface between the nanotubes and water which relate to a "slip length" of the hydrophobic interface. Properties like the slip length that describe the non-continuum behavior of the water within the pore walls are disregarded in simple hydrodynamic systems and absent from the Hagen–Poiseuille equation. Molecular dynamic simulations better characterize the flow of water molecules through the carbon nanotubes with a varied form of the Hagen–Poiseuille equation that takes into account slip length.

<span class="mw-page-title-main">Zeolitic imidazolate framework</span> Class of metal-organic frameworks

Zeolitic imidazolate frameworks (ZIFs) are a class of metal-organic frameworks (MOFs) that are topologically isomorphic with zeolites. ZIFs are composed of tetrahedrally-coordinated transition metal ions connected by imidazolate linkers. Since the metal-imidazole-metal angle is similar to the 145° Si-O-Si angle in zeolites, ZIFs have zeolite-like topologies. As of 2010, 105 ZIF topologies have been reported in the literature. Due to their robust porosity, resistance to thermal changes, and chemical stability, ZIFs are being investigated for applications such as carbon dioxide capture.

<span class="mw-page-title-main">Nitrogen generator</span>

Nitrogen generators and stations are stationary or mobile air-to-nitrogen production complexes.

Membrane technology encompasses the scientific processes used in the construction and application of membranes. Membranes are used to facilitate the transport or rejection of substances between mediums, and the mechanical separation of gas and liquid streams. In the simplest case, filtration is achieved when the pores of the membrane are smaller than the diameter of the undesired substance, such as a harmful microorganism. Membrane technology is commonly used in industries such as water treatment, chemical and metal processing, pharmaceuticals, biotechnology, the food industry, as well as the removal of environmental pollutants.

Michael Tsapatsis is an American chemical engineer and materials scientist. Tsapatsis is the 36th Bloomberg Distinguished Professor at Johns Hopkins University in the Department of Chemical and Biomolecular Engineering. Prior to this position he was the Amundson Chair (2008–present), professor (2003-present), and McKnight Presidential Endowed Chair (2017–present) in the department of chemical engineering and Materials Science at the University of Minnesota. Prior to his appointment at the University of Minnesota, Tsapatsis was an associate professor at the University of Massachusetts Amherst.

References

  1. ↑ Shehu, Habiba; Okon, Edidiong; Orakwe, Ifeyinwa; Gobina, Edward (2018-06-27). Design and Evaluation of Gas Transport through a Zeolite Membrane on an Alumina Support. IntechOpen. ISBN   978-1-78923-343-8.
  2. ↑ Algieri, Catia; Drioli, Enrico (2021-12-01). "Zeolite membranes: Synthesis and applications". Separation and Purification Technology. 278: 119295. doi:10.1016/j.seppur.2021.119295. ISSN   1383-5866.
  3. 1 2 Baker, Richard W. (2012). Membrane technology and applications (3rd ed.). Chichester, West Sussex: John Wiley & Sons. ISBN   978-1-118-35971-6. OCLC   785390224.
  4. ↑ Yu, Miao; Noble, Richard; Falconer, John (August 2, 2011). "Zeolite Membranes: Microstructure Characterization and Permeation Mechanisms". Accounts of Chemical Research. 44 (11): 1196–1206. doi:10.1021/ar200083e. PMID   21809809 . Retrieved April 19, 2023.
  5. ↑ Vaezi, Mohammad; Elyasi, Mahdi; Beiragh, Masoud; Babaluo (July 2019). "Transport Mechanism and Modeling of Microporous Zeolite Membranes". ResearchGate. Retrieved April 19, 2023.
  6. 1 2 Kosinov, Nikolay; Gascon, Jorge; Kapteijn, Freek; Hensen, Emiel J. M. (2016-02-01). "Recent developments in zeolite membranes for gas separation". Journal of Membrane Science. 499: 65–79. doi:10.1016/j.memsci.2015.10.049. ISSN   0376-7388.
  7. ↑ Sinaei Nobandegani, Mojtaba; Yu, Liang; Hedlund, Jonas (2022-10-15). "Zeolite membrane process for industrial CO2/CH4 separation". Chemical Engineering Journal. 446: 137223. doi: 10.1016/j.cej.2022.137223 . ISSN   1385-8947.
  8. ↑ Wenten, I. G.; Khoiruddin, K.; Mukti, R. R.; Rahmah, W.; Wang, Z.; Kawi, S. (2021-03-09). "Zeolite membrane reactors: from preparation to application in heterogeneous catalytic reactions". Reaction Chemistry & Engineering. 6 (3): 401–417. doi:10.1039/D0RE00388C. ISSN   2058-9883. S2CID   230566371.
  9. ↑ Fard, Ahmad; McKay, Gordon; Buekenhoudt, Anita; Salati, Huda; Motmans, Filip; Khraisheh, Marwan; Atieh, Muataz (January 2018). "Inorganic Membranes: Preparation and Application for Water Treatment and Desalination". Materials. 11 (1): 74. Bibcode:2018Mate...11...74K. doi: 10.3390/ma11010074 . PMC   5793572 . PMID   29304024.
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