Nanoporous materials

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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. [1]

Contents

Most nanoporous materials can be classified as bulk materials or membranes. Activated carbon and zeolites are two examples of bulk nanoporous materials, while cell membranes can be thought of as nanoporous membranes. [2] A porous medium or a porous material is a material containing pores (voids). The skeletal portion of the material is often called the "matrix" or "frame". The pores are typically filled with a fluid (liquid or gas).

  1. There are many natural nanoporous materials, but artificial materials can also be manufactured. One method of doing so is to combine polymers with different melting points, so that upon heating one polymer degrades. A nanoporous material with consistently sized pores has the property of letting only certain substances pass through, while blocking others. [2]

Classifications

Classification By Size

The term nanomaterials covers diverse forms of materials with various applications. According to IUPAC porous materials are subdivided into 3 categories: [3]

These categories conflict with the classical definition of nanoporous materials, as they have pore diameters between 1 and 100 nm. [1] This range covers all the classifications listed above. However, for the sake of simplicity, scientists choose to use the term nanomaterials and list its associated diameter instead. [1]

Microporous and mesoporous materials are distinguished as separate material classes owing to the distinct applications afforded by the pores sizes in these materials. Confusingly, the term microporous is used to describe materials with smaller pores sizes than materials commonly referred to simply as nanoporous. More correctly, microporous materials are better understood as a subset of nanoporous materials, namely materials that exhibit pore diameters smaller than 2 nm. [6] Having pore diameters with length scales of molecules, such materials enable applications that require molecular selectivity such as filtration and separation membranes. Mesoporous materials, referring generally to materials with average pore diameters in the range 2-50 nm are interesting as catalyst support materials and adsorbents owing to their high surface area to volume ratios.

Sometimes classifying by size becomes difficult as there could be porous materials that have various diameters. For example, microporous materials may have a few pores with 2 to 50 nm diameter due to random grain packing. [3] These specifics must be taken into consideration when categorizing by pore size.

Classification By Network Materials

In addition to classification by size, nanoporous materials can be further classified into organic and inorganic network materials. [3] A network material is the structure 'hosts' the pores and is where the medium (gas or liquid) interacts with the substrate. [3] While there are plenty of inorganic nanoporous membranes, there are few organic ones due to issues with stability. [7]

Organic

Organic nanoporous materials are polymers made from elements such as boron, carbon, nitrogen, and oxygen. [8] These materials are usually microporous although mesoporous/microporous structures do exist. [8] These include covalent organic frameworks (COFs), covalent triazine frameworks, polymers of intrinsic microporosity (PIMs), hyper cross-linked polymers (HCPs), and conjugated microporous polymers (CMPs). [8] Each of these has different structures and manufacturing steps. In general, to create organic nanoporous materials, a monomer with greater than 2 branches (i.e. covalent bonds) is dissolved in a solvent. After additional monomers are added and polymerization occurs, the solvent is removed and the remaining structure is considered a nanoporous material. [8]

Organic nanoporous materials can be further classified into crystalline and amorphous networks. [8] Crystalline networks are materials that have a well-defined pore sizes. The pore sizes are so well defined that simply by changing the monomer, one can obtain different pore sizes. [8] COFs are an example of such crystalline structure. In contrast, amorphous nanoporous materials have a distribution of pore sizes and are usually disordered. An example is PIMs. Both categories have various uses in gas sorption and catalysis reactions. [8]

Inorganic

Inorganic nanoporous materials are porous materials that include the use of oxide-type, carbon, binary, and pure metal materials. Examples include zeolites, nanoporous alumina, and titania nanotubes. [3] Zeolites are crystalline hydrated tectoaluminosilicates. This material is a combination of alkali/alkali earth metals, alumina, and silica hydrates. These are used for ion-exchange beds [9] and for water purification. [10] Nanoporous alumina is a biocompatible material widely used in various dental and orthopedic implants. [11] Titania nanotubes are also used in orthopedics but are special as they can form a titanium oxide layer upon exposure to oxygen. [12] Because the surface of the material is oxide-protected, this material has excellent biocompatibility with incredible mechanical strength. [12]

Applications

Gas Storage/Sensing

Gas storage is crucial for energy, medical, and environmental applications. Nanoporous materials enable a unique method of gas storage through adsorption. [13] When the substrate and gas interact with each other, the gas molecules can physio-adsorb or covalently bond with the nanoporous material, which is known as physical storage and chemical storage, respectively. [14] While one may store gases in the bulk phase, such as in a bottle, nanoporous materials enable higher storage density, which is attractive for energy applications. [13]

One example of this application is hydrogen storage. With the onset of climate change, there is an increased interest in zero-emission vehicles, especially in fuel cell electric vehicles. [15] By storing hydrogen at high densities using porous materials, one can increase electric car mileage range. [13]

Another use case for nanoporous materials is as a substrate for gas sensors. For example, measuring the electrical resistivity of a porous metal can yield the exact concentration of an analyte species in gaseous form. [1] Since the resistivity of the substrate is proportional to the surface area of the porous media, using nanoporous materials will yield higher sensitivity in detecting trace gaseous species than their bulk counterparts. This is especially useful as nanoporous materials have a higher effective surface area normalized to the top-view surface area

Biological applications

Nanoporous materials are used in biological applications as well. Enzyme catalyzed reactions in biological applications are highly utilized for metabolism and processing large molecules. Nanoporous materials offer the opportunity to embed enzymes onto the porous substrate which enhances the lifetime of the reactions for long-term implants. [1] Another application is found in DNA sequencing. By coating an inorganic nanoporous membrane on an insulating material, nanopores can be utilized for single-molecule analysis. By threading DNA through these nanopores, one can read out the ionic current through the pore which can be correlated to one of four nucleotides. [16]

Related Research Articles

<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">Nanopore</span>

A nanopore is a pore of nanometer size. It may, for example, be created by a pore-forming protein or as a hole in synthetic materials such as silicon or graphene.

An artificial membrane, or synthetic membrane, is a synthetically created membrane which is usually intended for separation purposes in laboratory or in industry. Synthetic membranes have been successfully used for small and large-scale industrial processes since the middle of the twentieth century. A wide variety of synthetic membranes is known. They can be produced from organic materials such as polymers and liquids, as well as inorganic materials. Most commercially utilized synthetic membranes in industry are made of polymeric structures. They can be classified based on their surface chemistry, bulk structure, morphology, and production method. The chemical and physical properties of synthetic membranes and separated particles as well as separation driving force define a particular membrane separation process. The most commonly used driving forces of a membrane process in industry are pressure and concentration gradient. The respective membrane process is therefore known as filtration. Synthetic membranes utilized in a separation process can be of different geometry and flow configurations. They can also be categorized based on their application and separation regime. The best known synthetic membrane separation processes include water purification, reverse osmosis, dehydrogenation of natural gas, removal of cell particles by microfiltration and ultrafiltration, removal of microorganisms from dairy products, and dialysis.

<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.

<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">Mesoporous material</span> Material with pores between 2 and 50 nm

A mesoporous material is a nanoporous material containing pores with diameters between 2 and 50 nm, according to IUPAC nomenclature. For comparison, IUPAC defines microporous material as a material having pores smaller than 2 nm in diameter and macroporous material as a material having pores larger than 50 nm in diameter.

A microporous material is a material containing pores with diameters less than 2 nm. Examples of microporous materials include zeolites and metal-organic frameworks.

Mesoporous silicates are silicates with a special morphology.

<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).

<span class="mw-page-title-main">Mesoporous silica</span> Nano-scale porous silica compound

Mesoporous silica is a form of silica that is characterised by its mesoporous structure, that is, having pores that range from 2 nm to 50 nm in diameter. According to IUPAC's terminology, mesoporosity sits between microporous (<2 nm) and macroporous (>50 nm). Mesoporous silica is a relatively recent development in nanotechnology. The most common types of mesoporous nanoparticles are MCM-41 and SBA-15. Research continues on the particles, which have applications in catalysis, drug delivery and imaging. Mesoporous ordered silica films have been also obtained with different pore topologies.

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.

Covalent organic frameworks (COFs) are a class of porous polymers that form two- or three-dimensional structures through reactions between organic precursors resulting in strong, covalent bonds to afford porous, stable, and crystalline materials. COFs emerged as a field from the overarching domain of organic materials as researchers optimized both synthetic control and precursor selection. These improvements to coordination chemistry enabled non-porous and amorphous organic materials such as organic polymers to advance into the construction of porous, crystalline materials with rigid structures that granted exceptional material stability in a wide range of solvents and conditions. Through the development of reticular chemistry, precise synthetic control was achieved and resulted in ordered, nano-porous structures with highly preferential structural orientation and properties which could be synergistically enhanced and amplified. With judicious selection of COF secondary building units (SBUs), or precursors, the final structure could be predetermined, and modified with exceptional control enabling fine-tuning of emergent properties. This level of control facilitates the COF material to be designed, synthesized, and utilized in various applications, many times with metrics on scale or surpassing that of the current state-of-the-art approaches.

Carbide-derived carbon (CDC), also known as tunable nanoporous carbon, is the common term for carbon materials derived from carbide precursors, such as binary (e.g. SiC, TiC), or ternary carbides, also known as MAX phases (e.g., Ti2AlC, Ti3SiC2). CDCs have also been derived from polymer-derived ceramics such as Si-O-C or Ti-C, and carbonitrides, such as Si-N-C. CDCs can occur in various structures, ranging from amorphous to crystalline carbon, from sp2- to sp3-bonded, and from highly porous to fully dense. Among others, the following carbon structures have been derived from carbide precursors: micro- and mesoporous carbon, amorphous carbon, carbon nanotubes, onion-like carbon, nanocrystalline diamond, graphene, and graphite. Among carbon materials, microporous CDCs exhibit some of the highest reported specific surface areas (up to more than 3000 m2/g). By varying the type of the precursor and the CDC synthesis conditions, microporous and mesoporous structures with controllable average pore size and pore size distributions can be produced. Depending on the precursor and the synthesis conditions, the average pore size control can be applied at sub-Angstrom accuracy. This ability to precisely tune the size and shapes of pores makes CDCs attractive for selective sorption and storage of liquids and gases (e.g., hydrogen, methane, CO2) and the high electric conductivity and electrochemical stability allows these structures to be effectively implemented in electrical energy storage and capacitive water desalinization.

<span class="mw-page-title-main">Conjugated microporous polymer</span>

Conjugated microporous polymers (CMPs) are a sub-class of porous materials that are related to structures such as zeolites, metal-organic frameworks, and covalent organic frameworks, but are amorphous in nature, rather than crystalline. CMPs are also a sub-class of conjugated polymers and possess many of the same properties such as conductivity, mechanical rigidity, and insolubility. CMPs are created through the linking of building blocks in a π-conjugated fashion and possess 3-D networks. Conjugation extends through the system of CMPs and lends conductive properties to CMPs. Building blocks of CMPs are attractive in that the blocks possess broad diversity in the π units that can be used and allow for tuning and optimization of the skeleton and subsequently the properties of CMPs. Most building blocks have rigid components such as alkynes that cause the microporosity. CMPs have applications in gas storage, heterogeneous catalysis, light emitting, light harvesting, and electric energy storage.

Mesoporous organosilica are a type of silica containing organic groups that give rise to mesoporosity. They exhibit pore size ranging from 2 nm - 50 nm, depending on the organic substituents. In contrast, zeolites exhibit pore sizes less than a nanometer. PMOs have potential applications as catalysts, adsorbents, trapping agents, drug delivery agents, stationary phases in chromatography and chemical sensors.

<span class="mw-page-title-main">MCM-41</span>

MCM-41 is a mesoporous material with a hierarchical structure from a family of silicate and alumosilicate solids that were first developed by researchers at Mobil Oil Corporation and that can be used as catalysts or catalyst supports.

Polymers of intrinsic microporosity (PIMs) are a unique class of microporous material developed by research efforts led by Neil McKeown, Peter Budd, et al. PIMs contain a continuous network of interconnected intermolecular voids less than 2 nm in width. Classified as a porous organic polymer, PIMs generate porosity from their rigid and contorted macromolecular chains that do not efficiently pack in the solid state. PIMs are composed of a fused ring sequences interrupted by Spiro-centers or other sites of contortion along the backbone. Due to their fused ring structure PIMs cannot rotate freely along the polymer backbone, ensuring the macromolecular components conformation cannot rearrange and ensuring the highly contorted shape is fixed during synthesis.

<span class="mw-page-title-main">Kenneth Balkus</span> American Scientist

Kenneth J. Balkus, Jr. is an American chemist and materials scientist. He is professor of chemistry and former department chair at The University of Texas at Dallas. He is a Fellow of the American Chemical Society and a recipient of the ACS Doherty Award. His well known work is synthesis of zeolite UTD-1, the first high-silica zeolite to contain a one-dimensional, extra-large 14-ring pore system. Other notable work include rare-earth metal organic frameworks. He is editor to Journal of Porous Materials, Springer. He is also co-founder of DB Therapeutics, a company developing cancer therapies.

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, and their high selectivity. Currently zeolites have seen applications in gas separation, membrane reactors, water desalination, and solid state batteries. 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.

References

  1. 1 2 3 4 5 Lu, G. Q. Max; Zhao, Xiu Song (2004-11-22). Nanoporous Materials: Science And Engineering. World Scientific. ISBN   978-1-78326-179-6.
  2. 1 2 Holister, Paul. "Nanoporous Materials" (PDF). Cientifica. Retrieved 6 February 2013.
  3. 1 2 3 4 5 Polarz, Sebastian; Smarsly, Bernd (2002). "Nanoporous materials". Journal of Nanoscience and Nanotechnology. 2 (6): 581–612. doi:10.1166/jnn.2002.151. ISSN   1533-4880. PMID   12908422.
  4. ↑ "IUPAC Gold Book - microporous carbon".
  5. 1 2 "IUPAC Gold Book - macroporous polymer".
  6. ↑ Bispropylurea bridged polysilsesquioxane: A microporous MOFlike material for molecular recognition. Chemosphere (2021), 276, 130181
  7. ↑ Hentze, H.-P; Antonietti, M (2001-08-01). "Template synthesis of porous organic polymers". Current Opinion in Solid State and Materials Science. 5 (4): 343–353. doi:10.1016/S1359-0286(01)00008-0.
  8. 1 2 3 4 5 6 7 Dawson, Robert; Cooper, Andrew I.; Adams, Dave J. (2012-04-01). "Nanoporous organic polymer networks". Progress in Polymer Science. Topical Issue on Polymer Physics. 37 (4): 530–563. doi:10.1016/j.progpolymsci.2011.09.002. ISSN   0079-6700.
  9. ↑ Inglezakis, Vassilis J. (2005-01-01). "The concept of "capacity" in zeolite ion-exchange systems". Journal of Colloid and Interface Science. 281 (1): 68–79. doi:10.1016/j.jcis.2004.08.082. ISSN   0021-9797. PMID   15567382.
  10. ↑ Savage, Nora; Diallo, Mamadou S. (2005-10-01). "Nanomaterials and Water Purification: Opportunities and Challenges". Journal of Nanoparticle Research. 7 (4): 331–342. doi:10.1007/s11051-005-7523-5. ISSN   1572-896X. S2CID   136561598.
  11. ↑ Azevedo, C.; Tavernier, B.; Vignes, Jean Louis; Cenedese, Pierre; Dubot, Pierre (2008). "Design of Nanoporous Alumina Structure and Surface Properties for Dental Composite". Key Engineering Materials. 361–363: 809–812. doi:10.4028/www.scientific.net/KEM.361-363.809. ISSN   1662-9795. S2CID   137339587.
  12. 1 2 Gultepe, Evin; Nagesha, Dattatri; Sridhar, Srinivas; Amiji, Mansoor (March 2010). "Nanoporous inorganic membranes or coatings for sustained drug delivery in implantable devices". Advanced Drug Delivery Reviews. 62 (3): 305–315. doi:10.1016/j.addr.2009.11.003. ISSN   0169-409X. PMID   19922749.
  13. 1 2 3 Morris, Russell E.; Wheatley, Paul S. (2008). "Gas storage in nanoporous materials". Angewandte Chemie International Edition in English. 47 (27): 4966–4981. doi:10.1002/anie.200703934. ISSN   1521-3773. PMID   18459091.
  14. ↑ Broom, Darren P.; Thomas, K. Mark (May 2013). "Gas adsorption by nanoporous materials: Future applications and experimental challenges". MRS Bulletin. 38 (5): 412–421. doi:10.1557/mrs.2013.105. ISSN   0883-7694.
  15. ↑ Ross, D. K. (2006-08-03). "Hydrogen storage: The major technological barrier to the development of hydrogen fuel cell cars". Vacuum. The World Energy Crisis: Some Vacuum-based Solutions. 80 (10): 1084–1089. doi:10.1016/j.vacuum.2006.03.030. ISSN   0042-207X. S2CID   17891215.
  16. ↑ Derrington, Ian M.; Butler, Tom Z.; Collins, Marcus D.; Manrao, Elizabeth; Pavlenok, Mikhail; Niederweis, Michael; Gundlach, Jens H. (2010-09-14). "Nanopore DNA sequencing with MspA". Proceedings of the National Academy of Sciences. 107 (37): 16060–16065. doi: 10.1073/pnas.1001831107 . ISSN   0027-8424. PMC   2941267 . PMID   20798343.
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