Mesoporous structures surfactants

Since this initial work there has been a plethora of literature on mesoporous molecular sieves. In addition to the silica and aluminosilicate frameworks similar mesoporous structures of metal oxides now include the oxides of Fe, Ti, V, Sb, Zr, Mn, W and others. Templates have been expanded to include nonionic, neutral surfactants and block copolymers. Pore sizes have broadened to the macroscopic size, in excess of 40 nm in diameter. A recent detailed review of the mesoporous molecular sieves is given in ref [73]. Vartuli and Degnan have reported a Mobil M41S mesoporous-based catalyst in commercial use, but to date the application has not been publicly identified.[74]. 

Zeolitic structures with pore sizes of 2000 to 10000 pm are known as mesoporous solids, and can be formed by a method known as liquid crystal templating (LCT). The combination of a suitable cationic surfactant together with silicate anions form arrays of rod-like surfactant micelles (Figure 3.7) surrounded by a polymeric siliceous framework. On calcination the mesoporous structure is formed. 

It is found in this study that an adjustment of pH value of solution by acid (HF or HC1) to 10.5 is very important for the effective formation of uniform mesopores. However, the acid should be added into the mixture solution after the addition of surfactant otherwise, the formation of the ordered mesoporous structure would be affected. The explanation is that when acid is added to a mixture solution without surfactant, the pH value of system will reduce and subsequently influence the interaction between cationic surfactant and anionic silicate species in the mixture, leading to the poor polymerization of inorganic silicate species. In addition, when HF is used prior to the addition of surfactant, the formation of stable NajSiFg can deactivate the polymerization of silicate species, further terminating the growth of mesoporous framework. 

The BET specific surface area, mesopore volumes, and pore wall thickness of the calcined and water-treated samples are given in Table 3. BET surface area of the samples prepared with Cm surfactants were found to be less affected by hydrothermal treatment. When the samples synthesized without TPA+ subjected to hydrothermal treatment the sharp inflection in the isotherm became very broad indicating wide distribution of pores. In contrast, the mesopore distribution of the samples prepared with TPA was found to be less affected by hydrothermal treatment. For the samples prepared without TPA, the mesopore volume was found to decrease sharply and the pore diameter was broadened over a large range indicating loss of the mesopore structure. Addition of TPA was found to minimize the structural collapse and thereby helps to preserve the mesoporosity. 

Since the discovery of the M41S materials with regular mesopore structure by Mobils scientists [1], many researchers have reported on the synthetic method, characterization, and formation mechanism. Especially, the new concept of supramolecular templating of molecular aggregates of surfactants, proposed as a key step in the formation mechanism of these materials, has expanded the possibility of the formation of various mesoporous structures and gives us new synthetic tools to engineer porous materials [2],... 

These results suggest that interactions between silicate species and surfactant micelles are weak in the precursor solution. The absence of any organization in the system prior to precipitation seems to indicate that the most important step in the process is the formation of siliceous prepolymers. The interaction of these prepolymers with surfactants could be responsible for micelle growth and subsequent reorganization of the silica/micelle complexes into ordered mesoporous structures. Such a hypothesis might be confirmed by preliminary potentiometric measurements using a bromide ion-specific electrode the amount of free bromide anion increasing at pH around 11 when the polymerization of silica starts. 

X-ray absorption spectroscopy has proved the presence of rhenium dioxide within this nanostructure [12]. Extraction of the surfactant with various solvents remained inefficient since either the surfactant persists within the composite or the nanostructure is lost. Calcination at mild temperatures as low as 300-350°C in nitrogen atmosphere leads to a mass loss under these pyrolytic conditions of about 50% with only little loss of the nanostructure. Similar results are obtained when the composite is oxidatively treated in an oxygen plasma for not more than ten minutes. Physisorption measurements on the calcined or plasma treated samples show only very small surface areas, which cannot be assigned to a mesoporous structure. Right now we believe that residual carbon may introduce some pore blocking effects within the nanostructure preventing good access of the inner pore surfaces. 

Synthesis of solid state materials using surfactant molecules as template has been extensively used in this decade. Among the advantages of the use of amphiphilic molecules, the self-assembling property of the surfactants can provide an effective method for synthesising ceramic and composite materials with interesting characteristics, such as nanoscale control of morphology, and nano or mesopore structure with narrow and controllable size distribution [1-5]. 

Figure 3.5 shows the organized mesoporous structure of one of these materials. The wide variety of organic molecules with self-assembling properties allows for the synthesis of a myriad of solids with controlled porosity. Cationic surfactants, like the hexadecyltrimethylammonium used to synthesize MCM-41 [26], or three-block copolymers (hydrophilic-hydrophobic-hydrophilic) are two good examples. 

This new family of mesoporous silica and aluminosilicate compounds were obtained by the introduction of supramolecular assemblies. Micellar aggregates, rather than molecular species, were used as structure-directing agents. Then, the growth of inorganic or hybrid networks templated by structured surfactant assemblies permitted the construction of novel types of nanostructured materials in the mesoscopic scale (2-100 nm) [110,113,117],... 

Several conclusions may be drawn from these data. (1) Weakly ordered mesoporous structures with high surface areas were obtained after the surfactant template was removed from TEOS-treated phosphomolybdate salts by solvent extraction. (2) IR and XRD data confirm that the silicate was indeed incorporated in the salt structures and changed these structures. Finally the Mo-O-Si bond was formed. (3) In contrast to the formation of the Mo-O-Si, the Mo-O-Ti bond is difficult to form in the same reaction conditions. So we only obtained a silica-containing mesoporous PMA. A significant breakthrough may be the appearance of good catalytic activity with the synthesis of the porous phosphomolybdic acid. The applications of the material in catalytic reaction are also being studied in our future work. 

A series of experiments have been eonducted in an attempt to improve the thermal stability of the Ti-Zr mesoporous mixed oxide prepared Ifom inorganic precursors. The results show that the crystallinity of the Ti-Zr mesoporous mixed oxide depends strongly on the Ti/Zr molar ratio in the gel mixtures, and a better crystalline mesoporous structure can be obtained on the sample with the 1 1 Ti/Zr molar ratio. It is also found that surfactant content, addition of sulfate species and auxiliary organic additive such as DDA, TritonX-lOO, triethanolamine and ethanol would play a key role in tailoring the mesopore structures and improving thermal stability of the obtained materials. In addition, the mixing order of organic surfactants, i.e., CTAB and DDA, also affects the the crystallinity of the product. 

In this study, it is foimd that the effects of (CH3CH2)3N on the formation of mesopore structures and the effects of the PEG on the thermal stability of the precipitated powders are very interesting. The results show that the Ce02 powders prepared by the precipitation method are mesoporous with crystalline walls. The mesoporous structure can be maintained upon calcination up to 873K. The mesoporous Ce02 synthesized from the system in the presence of non-ionic surfactant PEG exhibits better thermal stability than that prepared using one template of (CH3CH2)3N only. 

Some important metal oxide materials that have used molecular and supramole-cular templates to direct structure formation are the zeolites and related semi-crystalline aluminosilicates. In this section we shall discuss the use of ammonium cations that direct formation of microporous zeolites and finish with some of the possibilities that exist with the use of surfactant systems and molecular aggregates to create mesoporous structure. Excellent books and reviews are suggested for additional reading into the detailed description of the art [58-60]. The intention of this section is to briefly introduce this area and describe the types of materials being produced using various imprinting techniques in metal oxide materials. 

Fig. 8.16. Supramolecular assembly of surfactants to create organised rod-like micelles can template the formation of metal oxides with well defined mesoporous structure, i.e. MCM-41 materials. Adapted from [66].

Figure 9.9 (a) High resolution TEM image of calcined MCM-41 showing the hexagonal mesoporous structure, (b) schematic diagram of how the mesopores are templated using a surfactant (reprinted with permission from [6] 2000 American Chemical Society). 

The three M41S family silica phases obtained with cationic surfactants in a basic medium represent the primary set of principal mesoporous structures. The observed phase transitions are attributed to changing surface curvature resulting from decreasing packing factor, g, which is related to the surfactant molecule dimensions and defined as [35] ... 

Among the many varieties of mesoporous materials several stand out as most extensively studied. The initial 3 structures [3,4] remain dominant although MCM-50 has attracted least attention and found little practical value. It has an unstable structure collapsing upon calcination but still exhibits considerable microporosity, which is intriguing and possibly deserving closer attention. However, if a post treatment by adding a reactive silica source, e.g. TEOS, to the as-synthesized MCM-50 is done prior to air calculations, then a uniform mesopore structure (after calcination to remove the surfactant) with retention of the lamellar-like XRD is observed. MCM-41 and MCM-48 type structures and SBA-15 are considered here as representative of the entire class and to reflect the expected range of characteristics and behavior. 

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