Mesoporous Materials: Properties and Applications
Sandra Forbes
Product Manager
Introduction
Materials featuring specially engineered porosity on the nanoscale hold significant applications across various domains such as optics, catalysis, drug delivery systems, coatings, cosmetics, bio-separation, diagnostics, gas-separation, and nanotechnology. Nanoporous materials comprise either an amorphous or crystalline structure interspersed with void spaces, which can take the form of cylinders or cage-like structures. Broadly, these materials are categorized into three primary types: microporous, mesoporous, and macroporous.1
Microporous materials, including Metal-Organic Frameworks (MOFs), zeolites, carbons, and amorphous glasses, exhibit narrow pore size distributions typically ranging from 0.5 to 2 nanometers2. These materials are characterized by their high thermal stability and catalytic activity, making them valuable in cracking processes, as well as serving as ion exchange media, drying agents, and materials for gas separation. Among these, MOFs represent one of the fastest-growing classes of microporous solids.3 Conversely, zeolites and related crystalline molecular sieves have inherent limitations in pore size and accessibility due to the pore templates used during their synthesis.
On the other hand, macroporous materials, with pore sizes spanning from 50 to 1000 nanometers, such as porous polymer beads, offer easy access to their internal pores but compromise on selectivity. To address these shortcomings, mesoporous materials have been developed, featuring an intermediate pore size range of 2 to 50 nanometers.4
Mesoporous materials offer several pivotal advantages:
· Narrow pore size distributions and high surface areas (>500 m2/g).
· Framework/wall substitutions with various metal oxides (MO2) including silica, alumina, and titania
· Simple functionalization strategies with organics
· Biocompatibility and low toxicity.
Structural Properties and Characterization of Mesoporous Materials
Ordered mesoporous materials can be categorized based on their structural dimensions and pore geometry, encompassing either two-dimensional (2D) or three-dimensional (3D) cylindrical structures, and three-dimensional cage-type architectures. Cylindrical structures, exemplified by MCM-48, AMS-6 (Iad), MCM-41, SBA-15, and NFM-1 (p6mm), feature uniform pore diameters and demonstrate potential applications in catalysis, adsorption, and drug delivery systems. Conversely, cage-type mesocaged solids, such as FDU-1(Imm), SBA-1(Pmn), and AMS-8(Fdm), consist of spherical or ellipsoidal cages interconnected in three dimensions through smaller cage-connecting windows, enabling the control of mass transfer of active agents.
Routine characterization techniques, including powder X-ray diffraction, N2 adsorption/desorption analysis, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), are employed to investigate the structural and textural properties of mesoporous materials, like silica, as illustrated in Figure 1.
Figure 1. Scanning Electron Microscopy image of a typical mesoporous silica material.
Powder X-ray diffraction (XRD) is a widely employed technique for determining the crystallographic symmetry of material phases at both nano- and meso-scales. Identifying phases in mesoporous materials via powder X-ray diffraction can pose challenges, as the majority of peaks emerge at low angles and may overlap due to comparable short-range order. Figure 2 illustrates a representative outcome depicting the morphology and structural arrangement of pores within mesoporous silica. High-resolution transmission electron microscopy (HRTEM) enables a thorough analysis of pore order and symmetry at the mesoscale, making it indispensable for extracting detailed information at these scales.
Complementing the XRD analysis, gas adsorption serves as a method to achieve a comprehensive characterization of porous materials. By adsorbing gases at various relative pressures onto the porous solids, insights into properties such as surface area, pore volume, and pore size can be gained. Figure 3 displays a typical gas adsorption isotherm.
Figure 2. X-ray diffraction of a typical cubic mesoporous silica material.
Figure 3. Representative characterization of mesoporous silica nanoparticles by N2-Isotherm corresponding to a surface area 850 m2/g with a pore size of 3.8 nM.
Functional Mesoporous Silicas
Porous silicas belong to one of the most versatile categories of mesoporous materials, combining precisely defined pore sizes with the well-established biocompatibility of silica across various applications. A crucial characteristic of porous silica materials is their capacity to incorporate functional organic moieties (R) onto the silica walls. Following calcination, porous silica exhibits a high concentration of surface silanol (≡Si-OH) groups. The reaction of these silanols with diverse silanes introduces various functional groups (≡Si-R) onto the silica framework, enabling the conjugation of molecules of interest.5
Properties and Potential Applications of Functionalized Materials
· Encapsulation of pharmaceutical drugs, proteins, and other biological molecules
· Adsorbent for gases, ions, and molecules
· Loading of active sites for catalysis
· Loading of nanoparticles including iron oxide, gold, etc.
Applications of Nanoporous Materials in Research and Industry
Drug Delivery Systems
The primary obstacle in advancing drug delivery systems (DDS) lies in the reduction of drug efficacy prior to reaching its intended target, largely attributed to the body's excretion of the drug. Furthermore, it is imperative that the drug carrier remains non-toxic and inert throughout the duration of treatment. Given that the majority of biological molecules and pharmaceuticals measure in the range of a few nanometers, nanoporous silica, featuring pore sizes between 2 and 30 nm, holds significant importance for applications within the life sciences domain.6
Catalysis
In the field of catalysis, materials with high surface areas and nanoscale characteristics are employed to create highly selective catalysts, which minimize energy consumption and the production of waste or pollutants in industrial processes.7 Porous materials, including zeolites (microporous solids), are prevalent in industrial applications as catalysts and catalyst supports. However, when catalyzing reactions involving large molecules, the mass transfer becomes a limiting factor for zeolite structures. To enhance the diffusion of reactants to the catalytic sites, researchers have expanded the pore sizes into the mesoporous range.8 These highly selective catalysts have the potential to substantially reduce costs across numerous industries.
Diagnostics
Mesoporous materials are highly suited for diagnostic applications owing to their enhanced image contrast and robust chemical stability. Furthermore, the capacity to conjugate functional moieties within their pores opens up new avenues for multifaceted measurements and detections. The low toxicity of silica-based porous materials, coupled with their ability to accommodate a wide range of fluorescent markers, allows for the utilization of dyes and drugs to monitor the location and activity of therapeutic agents.
Adsorbents
The high surface area of nanoporous materials renders them suitable for use as adsorbents for a variety of gases, liquids, and toxic heavy metals. The adsorption capacity of these substances can be significantly enhanced by tailoring the surface properties (such as hydrophobicity, hydrophilicity, or functionality) of mesoporous silica materials. Mesoporous materials have been employed in numerous applications, including the removal of pollutants from water, the storage of gases like CO2, H2, O2, CH4, and H2S, the adsorptive separation of xylene, and the separation of biological and pharmaceutical compounds.
Chromatography
Mesoporous silica, characterized by its large pore volume, high surface area, and narrow pore-size distribution, is an excellent candidate for size exclusion chromatography. These materials have been suggested for use as supports or stationary phases in various chromatographic techniques, including size exclusion chromatography, capillary gas chromatography, proteomics separations, normal-phase High Pressure Liquid Chromatography (HPLC), and enantioselective HPLC.
Aladdin Mesoporous Materials
Aladdin provides a diverse selection of porous materials, encompassing nanoporous silica, nanoporous alumina, porous carbon, and functionalized nanoporous silica materials. Furthermore, fluorescently labeled nanoporous silica particles tailored specifically for diagnostic and pharmaceutical applications are now on offer.
References
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7. Taguchi A, Schüth F. 2005. Ordered mesoporous materials in catalysis. Microporous and Mesoporous Materials. 77(1):1-45. https://doi.org/10.1016/j.micromeso.2004.06.030
8. Sayari A. 1996. Catalysis by Crystalline Mesoporous Molecular Sieves. Chem. Mater.. 8(8):1840-1852. https://doi.org/10.1021/cm950585+
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