What Is Mesoporous Material?

According to the regulations of the International Union of Pure and Applied Chemistry (IUPAC), mesoporous materials refer to a class of porous materials with a pore size between 2-50nm. Mesoporous materials have the characteristics of extremely high specific surface area, regular and ordered pore structure, narrow pore size distribution, and continuously adjustable pore size, which makes it difficult for many microporous zeolite molecular sieves to adsorb and separate large molecules, especially Plays a role in catalytic reactions. Moreover, the ordered channels of this material can be used as "microreactors", in which nanometer-scale uniform and stable "guest" materials are assembled to become "host-guest materials". Due to the host-guest effect between the host and the object And the small size effect and quantum size effect that the guest material may have will make it expected to be widely used in electrode materials, optoelectronic devices, microelectronic technology, chemical sensors, nonlinear optical materials and other fields. Therefore, mesoporous materials have attracted international interest in multidisciplinary research fields such as physics, chemistry, biology, materials, and information since its birth. At present, it has become one of the hot frontier fields across the world.

Mesoporous materials

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According to the regulations of the International Union of Pure and Applied Chemistry (IUPAC), mesoporous materials refer to a class of porous materials with a pore size between 2-50nm. Mesoporous materials have the characteristics of extremely high specific surface area, regular and ordered pore structure, narrow pore size distribution, and continuously adjustable pore size, which makes it difficult for many microporous zeolite molecular sieves to adsorb and separate large molecules, especially Plays a role in catalytic reactions. Moreover, the ordered channels of this material can be used as "microreactors", in which nanometer-scale uniform and stable "guest" materials are assembled to become "host-guest materials". Due to the host-guest effect between the host and the object And the small size effect and quantum size effect that the guest material may have will make it promising for electrode materials,

According to the classification of chemical composition, mesoporous materials can generally be divided into two major categories: silicon-based and non-silicon-based.

1. Silicon-based mesoporous materials have narrow pore size distribution, regular channel structure, and mature technology, and have been studied a lot. Silicon-based materials can be used in the fields of catalysis, separation and purification, slow-release drug embedding, and gas sensing. Silicon-based materials can be divided into two categories based on pure silicon and doping other elements. It can be further refined and classified according to the types of doping elements and the number of different elements. The doping of heteroatoms can be regarded as the replacement of the original silicon atoms by heteroatoms. The introduction of different heteroatoms will bring many new properties to the material, such as changes in stability, changes in hydrophilic and hydrophobic properties, and catalytic activity. Change and more.

2. Non-silicon mesoporous materials mainly include transition metal oxides, phosphates and sulfides. Because they generally have a variable valence state, it is possible to open up new application fields for mesoporous materials and show application prospects that silicon-based mesoporous materials cannot reach. For example, silicon-aluminophosphate (SAPOs) formed by partial replacement of P by Si in aluminophosphate-based molecular sieve materials, and metal-substituted AIPOs (MAPOs) introduced with divalent metals in the architecture have been widely used Adsorption, catalyst loading, acid catalysis, oxidation catalysis (such as methanol olefinization, hydrocarbon oxidation) and other fields. Activated carbon with large internal surface area and high pore capacity has become the main industrial adsorbent due to its high adsorption capacity and its ability to adsorb different types of compounds from gas and liquid. In addition, the electric charge storage capacity of the electric double layer capacitor material made of mesoporous carbon is higher than the capacitance after the metal oxide particles are assembled, and it is much higher than the commercially available metal oxide electric double layer capacitors. Titanium dioxide-based mesoporous materials have the characteristics of strong photocatalytic activity and high catalyst loading capacity, and there have been many studies on their structural properties and characterization.

In general, mesoporous molecular sieve materials are a class of ordered porous materials formed by supramolecular self-assembly in the solvent phase of inorganic species constituting the molecular sieve framework in the solvent phase. The most commonly used synthesis method is hydrothermal synthesis. Others such as room temperature synthesis, microwave synthesis, wet glue baking, phase transition and synthesis in non-aqueous systems have also been reported in some circles. The main theoretical basis for selecting inorganic species is sol-gel chemistry, that is, the hydrolysis and polycondensation rates of raw materials are comparable, and the degree of polycondensation is increased after hydrothermal processes. According to the skeleton composition of the target mesoporous material, the inorganic species can be directly added inorganic salts, or organic metal oxides that can produce inorganic oligomers after hydrolysis, such as Si (OEt) 4, Al (i-OPr) 3 Wait.

There are many kinds of surfactants used for the synthesis of mesoporous molecular sieve materials, but according to the electrical properties of hydrophilic groups, they can be roughly divided into the following four categories: anionic, with negatively charged polar genes; cationic, with Positively charged polar genes; non-ionic, polar groups are not charged; amphoteric, with two hydrophilic groups, one positive and one negative, such as trimethylamine hydantoin CAPB (one end Is a positively charged quaternary amine group, the other end is a negatively charged shuttle group) and so on. The interfacial assembly force between the polar head of a surfactant and an inorganic species is a common feature in the formation of mesoporous molecular sieves in different synthetic systems. Diversification of synthetic routes can be achieved by changing the type of two-phase interface force (such as electrostatic, hydrogen bonding, or coordination) or adjusting its magnitude (such as adjusting the surface charge density of micelles-one can adjust the two-phase electrostatic attraction) ; Adjusting the reaction temperature can adjust the size of the hydrogen bonding force) to achieve. Different inorganic species and surfactants can form specific synthesis systems under different assembly effects, and they can be assembled into mesoporous molecular sieve materials with different structures, morphologies and pore sizes.

The synthesis of mesoporous materials began in 1990. Yanagisawa et al. Mixed the layered silicate material Kanemite with long-chain alkyltrimethylamine (ATMA) under alkaline conditions to undergo ion exchange to obtain a narrow pore size distribution. Three-dimensional mesoporous silicon oxide material. This was the earliest discovered silicon oxide mesoporous material, but because of its suboptimal structure, it did not attract scientists' attention at the time. It was not until 1992 that Kresge and Beck of Mobil reported that they successfully used cationic surfactants to synthesize new M41S series silicon oxide (aluminum) -based ordered mesoporous materials with adjustable pore diameters ranging from 1.5 to 10 nm for the template. The study of ordered mesoporous materials sounded the horn circle.

In 1994, Stucky et al. Synthesized a series of mesoporous materials containing a cage structure. Compared with the synthesis of M41S mesoporous materials, he synthesized them using a double-stranded surfactant under acidic conditions at room temperature or at a lower temperature for a short time. .

In 1995, Es1 appeared successively in chemical modification of mesoporous materials. Chemical modification of mesoporous materials includes doping of the framework and modification and functionalization of the surface of the channels. The doping of the skeleton mainly refers to the introduction of Al3 +, Ti4 +, B3 + and other heteroatoms into the skeleton of pure silicon-based mesoporous materials to give them acid, base centers or catalytic active sites. Mesoporous channel surface functionalization is the most extensive and effective method for preparing mesoporous host-guest composites. For example, the use of hydrophobic substances to improve the performance of the material to improve the hydrothermal stability and improve its gas adsorption performance; the use of catalytic properties of the substance to develop a catalyst suitable for specific chemical reactions; the use of ruthenium, sulfur The ether group-modified mesoporous material can adsorb Hg 2+ Pb 2 ten heavy metal ions.

The successful synthesis of ordered mesoporous films was first reported by Brinker et al. In 1997. The use of an acidic alcohol solution as the reaction medium and the Volatile Induced Self-Assembly (EISA) process can synthesize high-quality silicon oxide mesoporous films, which are the mesoporous materials in the fields of membrane separation and catalysis, microelectronics, sensors, and optoelectronic functional devices. Application opens up broad prospects.

In 1998, Zhao et al. Reported for the first time that a large-pore SBA-15 mesoporous material was synthesized using a non-ionic triblock copolymer. Due to its large pore size (5-30nm) and wall thickness (3.1-6.4nm), Its thermal and hydrothermal stability has been significantly improved, thereby broadening the application range of mesoporous materials. At present, the research reports based on SBA-15 mesoporous materials are the most in the field of mesoporous materials.

In 1999, Ryoo successfully copied other mesoporous materials with mesoporous materials as hard templates. He successively copied CMK-1, CMK-2, CMK-3 mesoporous carbon molecular sieve materials using MCM-48, SBA-1, SBA-15 as templates, and successfully synthesized precious metals, metal oxides, sulfides, etc. Non-silicon based mesoporous materials provide a practical route.

In 2003, Zhao et al. Proposed the concept of "acid-base pair". A series of non-silicon mesoporous materials were synthesized through the use of "self-adjusting" acidity in non-aqueous systems using inorganic precursors paired with acids and bases. This method solves the problem of how to find the metal sol precursor to a certain extent. It is a universal method for synthesizing multiple oxide mesoporous materials.

In 2004, Che et al. Reported that a chiral mesoporous material with a spiral channel was synthesized using an anionic chiral surfactant as a template. This mesoporous material with unique pore structure is expected to play a role in chiral molecular recognition, separation and catalysis.

Chemical industry

</ strong> The ordered mesoporous material has a large specific surface area, a relatively large pore size, and a regular pore structure, which can handle larger molecules or groups, and is a good shape-selective catalyst. Especially in catalyzing reactions involving large-volume molecules, ordered mesoporous materials show better catalytic activity than zeolite molecular sieves. Therefore, the use of ordered mesoporous materials opens up new horizons for catalytic cracking of heavy oil and residue. When the ordered mesoporous material is used directly as an acid-base catalyst, it can improve the coking on the solid acid catalyst and increase the diffusion rate of the product. The conversion rate can reach 90% and the selectivity of the product can reach 100%. In addition to direct acid catalysis, grafted materials can also be made by doping the framework of ordered mesoporous materials with transition elements with redox capabilities, rare earth elements, or supported redox catalysts. This kind of graft material has higher catalytic activity and shape selectivity, which is also the most active field for the development of mesoporous molecular sieve catalysts.

Ordered mesoporous materials can also be used in the field of polymer synthesis, especially nanoreactors for polymerization due to their large pore sizes. Because intracellular pore polymerization reduces the chance of double radical termination to a certain extent and prolongs the lifetime of free radicals, the molecular weight distribution of polymers obtained by polymerizing in the channels of ordered mesoporous materials is also narrower than the general free radical polymerization under corresponding conditions. By changing the amount of monomer and initiator, the molecular weight of the polymer can be controlled. And the active center can be typed in or introduced into the skeleton of the polymerization reactor to speed up the reaction process and increase the yield.

In the field of environmental treatment and protection, it is used to degrade organic waste, used for water purification and automobile tail gas conversion treatment. In the field of high-tech advanced materials, energy storage materials are used for the assembly of functional nano-objects in mesoporous materials, such as guest molecules with luminous properties, which are used to emit light and assemble photochemically active substances, allowing the use of mesoporous materials. With the advantages of surface area, new types of mesoporous structure optical materials that are superior to conventional optical materials are prepared, such as the ultrafast non-linear optical corresponding mesoporous composite films prepared by the Shi Jianlin Group of the Shanghai Institute of Ceramics, Chinese Academy of Sciences. Optical applications of mesoporous materials have been discussed in 2000 by Stucky GD and others. Conducting polymer polymerization in a uniform mesoporous channel, and then removing the mesoporous wall chemically, can form a conductive polymer material with a regular mesoporous channel structure. The nanometer mesoporous material regular channels are used as "microreactors" and Its carrier function synthesizes heterogeneous nanoparticles, or quantum wire composite assembly system has special advantages. Due to the small size effect and quantum effect caused by the limitation of the channel size and the regularization effect, it has been observed that this composite material can display special optical characteristics and electrical and magnetic properties. For example, the modified mesoporous zirconia material shows Special room-temperature photoluminescence phenomenon. These can be developed and researched for the application of mesopores and their composite materials in the fields of optical devices and micro sensors.

Ordered mesoporous materials are a branch of porous materials, and their rapid development also comes from practical applications in industries such as petrochemicals and fine chemicals. At the same time, we should also see that because the pore size of ordered mesoporous materials is in the range of 2-50nm, this provides a "reaction container", or "tool", for the preparation of new nanomaterials and nanocomposites. When M41S appeared in 1992, it coincided with the rapid development of nanotechnology, during which many nano-sized and nano-structured new materials were prepared, such as the research of carbon nanotubes. I think on the other hand, at the end of the 20th century, the development of nanotechnology led to the development of ordered mesoporous materials.

Biomedical field

</ strong> In general, biological macromolecules such as proteins, enzymes, nucleic acids, etc., have a size of less than 10nm when their molecular mass is between about 1 million and 1 million, and viruses with a relative molecular mass of about 10 million have a size of about 30nm. The pore size of ordered mesoporous materials can be continuously adjusted in the range of 2-50nm and has no physiological toxicity, which makes it very suitable for the fixation and separation of enzymes and proteins. Experiments have found that ordered mesoporous materials such as glucose and maltose can successfully cure the enzyme and inhibit the leakage of the enzyme, and this method of enzyme immobilization can well retain the enzyme activity.

The emergence of biochips is a major development with the characteristics of the times in the high-tech field in recent years. It is a high-tech formed by the comprehensive cross of physics, microelectronics and molecular biology. The emergence of ordered mesoporous materials has made this technology a breakthrough. It can form continuous and firmly bonded membrane materials on different ordered mesoporous substrates. These membranes can directly perform cell / DNA separation for use in For building a microchip laboratory.

Direct embedding and controlled release of drugs are also good applications for ordered mesoporous materials. Ordered mesoporous materials have a large specific surface area and specific pore volume. Biomaterials such as porphyrin, pyridine, or immobilized proteins can be loaded into the pores of the material. Controlled release drugs can be modified by functional groups to improve the efficacy Persistence. The use of biological guidance can effectively and accurately hit targets such as cancer cells and lesions, and give full play to the efficacy of drugs.

Environment and energy

Ordered mesoporous materials as photocatalysts for the treatment of environmental pollutants are one of the hot topics in recent years. For example, mesoporous TiO2 has higher photocatalytic activity than nano-TiO2 (P25), because the high specific surface area of the mesoporous structure improves the contact with organic molecules and increases the water and hydroxyl groups adsorbed on the surface. Water and hydroxyl groups can be photo-excited with the catalyst surface The reaction of holes produces hydroxyl radicals, and hydroxyl radicals are strong oxidants that degrade organic substances. They can oxidize many difficult-to-degrade organic substances to inorganic substances such as CO2 and water. In addition, selective doping in ordered mesoporous materials can improve their photoactivity and increase the efficiency of visible light catalytic degradation of organic waste.

At present, the chlorine disinfection process widely used in domestic water has killed a variety of germs, but it has also produced a series of toxic organic substances such as chloroform, carbon tetrachloride, and chloroacetic acid. The serious "triple effect" (carcinogenic, Deformity, mutagenesis) has caused widespread concern in the international scientific and medical communities. By connecting -chloropropyltriethoxysilane to the inner wall of the ordered mesoporous material, a functional mesoporous molecular sieve CPS-HMS is obtained. The functional mesoporous molecular sieve removes traces of chloroform in water and other effects. Remarkably, the removal rate is as high as 97%. The concentration of chloroform in the treated water body is lower than the national standard or even lower than the drinking water standard.

Ordered mesoporous materials also have unique applications in the fields of separation and adsorption. In the temperature range of 20% -80%, ordered mesoporous materials have the characteristics of rapid desorption, and the range of humidity controlled by adsorption can be adjusted by the size of the pore size. Compared with traditional microporous adsorbents, ordered mesoporous materials have higher adsorption capacity for argon, nitrogen, volatile hydrocarbons and low concentrations of heavy metal ions. The use of ordered mesoporous materials can recover heavy metal ions such as lead, mercury, and other volatile organic pollutants and waste liquids without the need for a special adsorbent activation device. In addition, the ordered mesoporous materials can be quickly desorbed and reused, which makes them have good environmental protection and economic benefits.

Ordered mesoporous materials have spacious pores, and they can produce energy storage materials such as carbon or Pd in situ in their pores, increase the ease of handling and surface area of these energy storage materials, and slowly release energy to achieve energy storage effect.

At present, there are many scientific research institutions and units in China such as Suzhou University, Beijing University of Chemical Technology, Fudan University, Jilin University, Chinese Academy of Sciences, etc. engaged in the research and development of ordered mesoporous materials. It is believed that with the deepening of the research work, ordered mesoporous materials, like zeolite molecular sieves, are not far away from the industry as ordinary porous materials.