Mesoporous Silica Applications

Background

Since their discovery in the late 1970s mesoporous silicas have attracted much attention due to their unique ordered pore structures, very high specific surface areas and range of morphologies that can be synthesised for examples spheres rods, discs and powders. Unlike traditional porous silicas, mesoporous silicas exhibit unusually ordered pores due to the nanotemplating approach applied during synthesis. Over the past 30 years a plethora of mesoporous silicas (SBA 15, SBA 16 MCM 41, 48 etc.) have been synthesised with a wide ranging pore geometries (Hexagonal, Cubic etc.) and particle morphologies e.g Discs, Spheres, Rods etc.

Figure 1 below illustrates some of the morphological qualities of mesoporous silica (MS) and porous silica spheres (PSS). PSS are spherical particles with quasi ordered porosity. Recent developments in manufacturing of mesoporous silica which allow production at a kg scale has allowed mesoporous silica to move from blue skies research laboratories into more advanced application driven research.

Mesoporous silicas with uniform and tailorable pore dimensions and high surface areas, are currently being employed in a number of advanced applications that include:

Application No.Application area Thematic AreaMaterial Type/Product
1Quantum  confined Nanowire HostElectronic/EnergySBA 15 , SBA 15 Large Pore, SBA 15 small pore
2Heavy metal Ion removal from waste WaterEnvironmentalSBA 15, SBA 16, MCM 48
3Phosphate removal from waste WaterEnvironmentalMetal Doped SBA 15 (PSBA15Ti)
4Removal of VOCs from Indoor airEnvironmental/HealthPSS
5Methanolysis of Styrene OxideOxideCatalysisMetal Doped SBA 15 (PSBA15Ti)
6Improving the bioavailability of poorly water soluble drug moleculesDrug Delivery/Pharmaceutical formulationSBA 15
7Enzyme Encapsulation for bio catalysisatalysisCatalysisSBA 15/PSS
8Phospholipid Extraction from biological matricesBio-Analysis/Sample PreparationMetal Doped SBA 15 (PSBA15Ti)
9Incorporation of Spherical Silica Particles in Pervaporation Membranes for the separation of Water from EthanolChemical SeparationPSS
10Direct Air Capture of CO2 by Physisorbent MaterialsEnergySBA 15
Application Area 1: Mesoporous Silica as Quantum Confirmed Nanowire Host

Nanoscale structures of semiconductor wires are expected to play a vital role as materials for both interconnects and emerging future technologies because of their unique optical, electrical, and mechanical properties.

Mesoporous Silicas that contain unidirectional arrays of pores, typically 2-15 nm in diameter, running throughout a material have been successfully exploited as templates for semiconductor materials formed from the gas phase. Recently, it was reported the use of a novel supercritical fluid solution-phase approach to produce silicon nanowires within the pores of mesoporous silica[1].

Figure 1 illustrates this work. TEM and XRD images of the MS before (TOP) and after (BOTTOM) shows the pore of MS 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.

XRD & TEM of Si Wires grown inside the Pores of Mesoporous silica

Figure 1 XRD & TEM of Si Wires grown inside the Pores of Mesoporous silica

Application area 2: Heavy metal Ion removal from waste Water using Modified Mesoporous Silica

Metal ions are the most significant environmental pollutants found in wastewater and long-term exposure to solvated metals ions and their effects on human health and natural ecosystems is of major concern. The metals of primary interest include chromium, nickel, manganese, iron and various heavy metals because of a combination of toxicity and exposure levels. Methods for metal ion removal include precipitation, coagulation/ flocculation, ion exchange, reverse osmosis, complexation/ sequestration, electrochemical operation and biological treatment. Some limitations of these are for example 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. Ordered mesoporous silicas (MS) are becoming established sorptives that may be exploited owing to their large surface areas (typically 200–1000cm2 g−1) and large pore volumes and ease to which they can be recycled/regenerated. Functionalising MS with various chelating agents (or metal ion-specific ligands) allows specific metal ion pollutants to be selectively removed from aqueous or organic systems with correspondingly large uptakes. Table 2 shows the sorption capacities for the mono-functionalised (amine or thiol) and bi-functionalised (amine and thiol) SBA 15 MS.[2]