Since their discovery in the late 1970s, mesoporous silica have attracted quite a bit of attention. This is mainly due to their unique properties such as ordered pore structures, very high specific surface areas, and their possible synthesis in a wide range of morphologies such as spheres rods, discs, powders, etc. Unlike traditional porous silica, mesoporous silica exhibit exceptionally ordered pores. This arises from the nanotemplating approach applied during synthesis of these materials. Over the past 30 years, a plethora of mesoporous silica (SBA 15, SBA 16, MCM 41, MCM 48, etc.) with a wide range of pore geometries (hexagonal, cubic, etc.) and particle morphologies such as discs, spheres, rods, etc. have been synthesised. Figure 1 shows some of the morphologies of mesoporous silica (MS) and porous silica spheres (PSS). The porous silica spheres are spherical particles with quasi ordered porosity. Recent developments in the manufacturing of mesoporous silica, allowing large scale production (up to kg scale), has facilitated their move from laboratory based research to more advanced application driven research.
MS with uniform and tailorable pore dimensions with high surface areas are currently being employed in a number of applications. These include their use in waste water remediation, indoor air cleaning, catalysis, bio-catalysis, drug delivery, CO2 capture, bioanalytical sample preparation, pervaporation membrane improvement. MS are also used as templates for controlling the aspect ratio of quantum-confined nanoparticles and nanowires. Table 1 summarizes some of the application areas and thematic areas of PSS and MS.
Figure 1.Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images showing varying morphologies and pore structures of porous silica spheres (PSS) (A&B) and mesoporous silica (MS) (C&D).
Because of their unique optical, electrical, and mechanical properties, the nanoscale structures of semiconductor wires are expected to play a vital role in emerging future technologies. MS that contain unidirectional arrays of pores, typically 2-15 nm in diameter, running throughout the material have been successfully exploited as templates for semiconductor materials formed from the gas phase. Recently, a novel supercritical fluid solution-phase based approach to produce silicon nanowires within the pores of mesoporous silica was also reported.1
Figure 2.X-ray diffraction (XRD) and TEM image of Si wires grown inside the pores of MS. The XRD and TEM images of the MS before (TOP) and after (BOTTOM) its pores were filled with silicon nanowires. The high packing density of the silicon “mesowires” in the silica matrix makes them ideally suited for the formation of quantum structures.
Metal ions are the most significant environmental pollutants found in wastewater and long-term exposure to solvated metal ions and their effects on human health and natural ecosystems is a major concern. The metals of primary interest include chromium, nickel, manganese, iron and various heavy metals. Several methods, such as precipitation, coagulation/flocculation, ion exchange, reverse osmosis, complexation/sequestration, electrochemical operation and biological treatment are employed for removal of metal ions from waste. However, these methods suffer from limitations rising from high operating and energy costs. Sorbents such as activated charcoal, zeolites and clays have also been used as wastewater treatment systems. However, disadvantages of these materials include relatively low and variable loading capacities and small metal ion-binding constants. Owing to their large surface areas (typically 200–1000 cm2 g−1), large pore volumes and ease of recycling/regeneration, the ordered MS are becoming established sorptives for metals ions. In fact, the functionalization of mesoporous silica with various chelating agents (or metal ion-specific ligands) allows selective removal of metal ions from aqueous or organic systems at larger uptakes. Table 2 shows the sorption capacities for the mono-functionalised (amine or thiol) and bi-functionalized (amine and thiol) SBA-15.2
Phosphorus (as phosphate ions) is an important element that is widely used in agriculture as a fertilizer and in industry as a detergent. However, the use of phosphorus causes eutrophication when it is released into aquatic environments. Several Environmental Protection Agencies worldwide have identified eutrophication resulting from excess phosphorus input as the greatest threat to fresh water supply. Eutrophication leads to fish deaths and the degradation of habitat with loss of plant and animal species. Ordered MS have been proven to be a good adsorbent for metal ions and amino acids. Recently there has been significant focus on using MS functionalized with selective ligands for metal ion adsorption. Also, MS doped with transition metals have been shown to be promising for phosphate adsorption. Figure 3 outlines several doped mesoporous silica and their phosphate removal efficiencies from model aqueous solutions. In all cases, the doped SBA-15 material exhibit high adsorption efficiency.3
Figure 3.Phosphate removal from waste water using titanium, iron, zirconium and aluminium doped SBA-15.
In recent years indoor air pollution has become a major concern due to its adverse effect on human health. These health effects of indoor air pollution are expected to become more significant as lifestyles are predicted to become even more sedentary. In 2001, the national human activity pattern survey found that in the United States people typically spend ~90% of their time indoors. Aldehydes in particular are known to have adverse health effects (eye and lung irritation), and formaldehyde and acrolein are suspected as carcinogens. Changes in building design and improved energy efficiency, along with increase in insulation and reduction in air exchange, have led to increasingly airtight buildings. Modern synthetic building materials, such as sealants, plastics and solvent-based coatings have further added to the problem and volatile organic compounds (VOCs), non-volatile organic compounds (NVOCs) and semi-volatile organic compounds (SVOCs) have become a concern as indoor pollutants.
Recent work has shown that porous silica spheres (PSS) can be used to efficiently trap various indoor air pollutants both in simulated and indoor environments. The adsorbent was tested at relatively high concentrations (500 ppb) and flow rates (10 L min-1). It was shown that PSS outperforms the commercially available Amberlite XAD-4 resin at trapping non-polar VOCs and it is significantly more efficient at trapping polar VOCs present in ambient air. PSS adsorbent was shown to trap 100% of the gas phase carbonyl compounds present in a simulation chamber experiment in the first 10 min of sampling, while the XAD-4 resin was shown to have various levels of efficiency ranging from 100 to 8% over the sampling period for the same group of carbonyl compounds.
Figure 4 shows a trapping efficiency plot for a selection of small carbonyls: acetone, butanal, pentanal and hexanal. The trapping efficiency of the PSS for each of these compounds is close to 100% after the first 10 min. The XAD-4 resin showed varying trapping efficiency values ranging from 100 to 8% after the first 10 min. However, the trapping efficiency for both sorbents decreased gradually with time due to the progressive saturation of the sorption surface with trapped species.4
Figure 4.Data showing the trapping efficiency of XAD-4 (dashed line) and PSS (solid line) for (a) acetone, (b) butanal, (c) pentanal and (d) hexanal.
Highly ordered zirconium and titanium doped hexagonal MS with Si/Zr and Si/Ti ratios of 40:1 and 80:1 have been used as solid acid catalysts for the methanolysis of styrene oxide in a single-mode microwave reactor. As catalyst Si/Zr material demonstrated excellent substrate conversion, high product selectivity and remained highly active for several reaction cycles (Figure 5). These experimental results clearly show that the zirconium doped mesoporous silica is an efficient catalyst for the liquid phase methanolysis of styrene oxide. The isolation of the products required a simple filtration/evaporation step. A typical reaction carried out with a 40:1 Si:Zr catalyst in a single-cavity microwave reactor operating at 105 W for 10 min afforded 100% conversion of styrene oxide. Recycling studies have shown that the catalyst remains highly efficient for at least five cycles with 95% conversion of styrene oxide after the catalyst was subjected to microwave irradiation while suspended in methanol.5
Figure 5.Transmission electron microscopy (TEM) image of 40.1 Si/Zr sample showing hexagonal ordering (A) and pore retention (B). Bottom image shows the percentage styrene oxide conversion vs Recyclability and total turnover number (TON) for 40:1 catalyst (bottom image)
It is well established that increasing the effective surface area of a poorly water-soluble drug in contact with the dissolution medium can enhance drug dissolution. This can be achieved by loading drugs onto ordered MS, which are characterized by high surface areas, large mesopore volumes, narrow mesopore size distributions (5–8 nm), and ordered unidirectional mesopore networks. These properties allow the homogeneous and reproducible drug-loading and release with the use of MS.
Figure 6.SEM/EDX images of physical mix of fenofibrate loaded SBA-15 (top image) and melt sample (bottom). Here the SEM images are on the left while the EDX one are on the right. Image A in the bottom part shows the release profiles of unprocessed fenofibrate, physical mix and melt samples whereas the release profile of impregnated, liquid and SC-CO2 processed samples are shown in image B.
The release of drug from the silica carrier is a key performance indicator to consider when employing OMMs (ordered mesoporous materials) for drug dissolution enhancement. The in vitro release of drug from drug–silica samples and the dissolution of the starting fenofibrate are shown in Figure 6. Using MS as a carrier material improved the drug dissolution rate for all processed samples.6
Immobilization of enzymes can confer a number of advantages such as enhanced stability, ease of recovery and re-use, and the ability of using the enzyme in non-aqueous solvents where the enzyme is insoluble. However, the main disadvantage of immobilization is that it usually lowers the activity of the enzyme and the process of immobilization can add significant costs to the process. In addition, immobilization methods tend to be non-specific and typically the process of immobilization of a specific enzyme on a support is optimized and developed on a case-by-case basis. Ideally, support materials for the immobilization of an enzyme should be mechanically and chemically stable, have high surface area, be easily made at low cost and display low non-specific protein adsorption properties. Also, the immobilization should occur in a manner which does not compromise the enzyme conformation or activity, while maintaining the diffusion of the substrate and product to and from the active site. MS has been widely used as supports for enzymes and significant efforts have been made to use them as supports for biocatalysis.
Figure 7.Activity profiles for cytochrome c immobilised on SBA-15 (Square), PSS (Triangle), and for aqueous cytochrome c (open circles).
The adsorption and activity of cytochrome c and lipase on two silica supports (SBA-15 and PSS) of the same average pore diameter but with different pore volumes and surface areas has been examined. On SBA-15 loadings of cytochrome c and Candida antarctica lipase B (CALB) of 15.6 and 2.04 mol g−1 were obtained in comparison to loadings of 0.94 and 6.7 mol g−1 on PPS respectively. These differences in loading can be ascribed to differences in the properties (pore volume, surface area and morphology) of the supports. The catalytic activity of cytochrome c was similar on both supports, while the activity of CALB was higher on SBA-15 in comparison to PPS (7.8 vs 4 s−1). These differences in activity for CALB likely arise from the pore morphology and the physical properties of the supports, which is the hypothesis that was supported by the higher recyclability obtained with SBA 15. The data indicates that the physical properties of mesoporous silica supports can significantly alter the activity and stability of enzymes.7
Food matrices are famous for being difficult to work with since they contain many unwanted or interfering compounds (i.e. phospholipids), and samples often require a clean-up and extraction prior to determination using liquid chromatography (LC) and/or mass spectroscopy. Recently doped MS (SBA-15 doped with Titanium) was used in dispersive solid phase extraction (dSPE) QuEChERS (Quick, Easy, Cheap, Effective, Rugged and Safe) based approach to sample preparation of liver tissues. Titanium doped SBA-15 has been used for preparation of biological samples prior to high performance liquid chromatography (HPLC) analysis and it has been shown to have preferential selectivity for phospholipids without removing the analyte of interest from the sample. It may be mentioned here that the doping of the titanium moiety into the silica framework offers significant advantages in terms of robustness and chemical stability over other metal silica hybrid materials where the metal moieties are attached to pure silica.
It has been shown that the recoveries from samples processed with the titanium doped mesoporous silica (SiTi) are higher than those processed with traditional C18 sorbent material. The data presented in Table 3 for C18 and SiTi(4%)-C18 sorbent materials suggest that SiTi(4%)-C18 is more effective than traditional C18 sorbent used in QuEChERS based method for anthelmintics sample preparation in d-SPE format.
Pervaporation is a membrane separation technology primarily used to dehydrate and recover solvents. It is also used to separate organic–organic mixtures. The pervaporation technology has significant advantage over other separation techniques as it can be used to effectively ‘break’ azeotropes of mixtures without any physical difficulties. The technique also does not suffer from the negative environmental impacts of techniques like azeotropic distillation. One method of improving membrane flux without significantly compromising selectivity is by inclusion of porous particles (e.g. zeolites and silica particles) into the polymer matrix. In porous ceramic–polymer membrane hybrids of this type the engineering of the particles (i.e. size, shape, monodispersity, pore size and surface chemistry) is of significant importance.
Figure 8.Membrane absorption vs. time: the upper image shows the absorption of 50:50 wt% solution of ethanol and water into selective membrane over time. The maxima in the graph represents point at which membrane becomes dissolute i.e. loses membrane form and becomes unusable. The picture A in the bottom part shows the surface image of 15 wt% silica loaded PVA membrane whereas the image B shows the surface image of 10 wt% silica loaded PVA membrane.
Recent work has shown that the incorporation of engineered PSS into polymer pervaporation membranes can be highly beneficial. For example, it has been demonstrated that incorporation of spherical and discreet sized monodispersed silica particles of 1.8–2μm diameter with a pore size of 1.8 nm into a poly (vinyl alcohol) [PVA] polymer produces composite pervaporation membrane that exhibits significant increases in both flux and selectivity (Figure 8). Unlike zeolitic systems, PSS can be very controllably engineered to give a wide range of pore sizes and chemistries and may provide new generations membranes for various pervaporation applications.8
Generation of carbon dioxide (CO2) poses a significant threat to global climate and it represents a topical challenge that requires urgent attention. In this context, the anthropogenic emissions of CO2 are a major concern as it is generated as an undesirable component of commodities like natural gas, biogas, etc. However, it is argued that sequestration of CO2 either from gas mixtures or directly from air could mitigate the risks associated with carbon emissions and various porous materials such as mesoporous silica, zeolites, metal organic frameworks have been examined for this purpose. Recent work has shown that amine functionalized SBA-15 out performs the benchmark materials like metal organic frameworks (MOFs) and zeolite in CO2 sequestration.9