Figure 5.7 TEM micrographs of SBA-15 synthesized at different aging temperatures: (a) 40°C, (b) 60°C, and (c) 100°C.
50 nm
(c)
Figure 5.8 Schematic representation of micelle dehydration upon temperature increase.
Figure 5.9 Phase diagram of P123, EO20PO70EO20
[82].
The evolution of SBA-15 with synthesis temperature closely corresponds to this pattern.
In all syntheses, the first step was a day-long aging at 35°C, during which the presence of silica prompted the formation of a composite mesophase at surfactant concentrations at which
only an isotropic solution is present in the absence of silica (Figure 5.9) [82]. Ordered SBA-15 materials are the result of the restructuring of this precursor mesophase formed at a low temperature; if the precursor mesophase is not allowed to form and the synthesis temperature is immediately raised, the solid that forms will be disordered, and possess both a broad pore-size distribution and hystereses indicative of restrictions at the mouths of pores.
SBA-15 materials formed at the lowest temperatures are expected to fairly represent the properties of the precursor mesophase. The wall thicknesses of the materials formed at temperatures not higher than 60°C resulted in the smallest distance between hydrated micelles in water. This is strongly suggestive of a mechanism by which silica precursors impregnate the intermicellar space, in which case the observed ultramicroporosity would be templated by the protruding PEO chains. If silica walls should reproduce the topology of the hydration shells of the PEO chains in water, no microporous bridges should be expected between adjacent micelles at low temperatures.
When the precursor mesophase formed at 40°C is heated to temperatures higher than 80°C, its fine structure is not preserved; on the contrary, the size of the structural mesopores increases, the thickness of the walls decreases, the ultramicroporosity disappears, and a pore-bridging secondary porosity appears. The temperature at which these phenomena begin to take place is very close to the cloud point of the surfactant, which indicating a lower salvation of PEO chains. Interestingly, silica-embedded micelles behave similar to micelles in water, in that the disappearance of ultramicroporosity parallels the disappearance of PEO fingers. PEO chains become a less efficient template for silica, and this effect corresponds to a weaker interaction between PEO and silica. The reduced strength of the PEO-silica interaction at higher temperatures is evidenced by the increased ease with which surfactant can be extracted of surfactant; however, the weak interaction between PEO and silica destabilizes the corona of the PEO-templated ultramicroporous silica that surrounds the micelles of the precursor mesophase. A phase separation of PEO and silica takes place at the nanometer scale,
bringing about an increase in the size of the structural mesopores and a densification of the silica walls. It is interesting to note that the cell parameter of the hexagonal structure remains virtually constant upon hydrothermal treatment, just as the evolution of the precursor mesophase with increasing temperature took place via the redistribution of organic volume inside a close micrometer. The process likely takes place in a haphazard way, with the opening of local gaps in the walls at the origin of the secondary porosity, acting as a bridge between structural mesopores.
The opening of bridges between the structural mesopores of SBA-15 synthesized at temperatures greater than 80°C—and thus in the system’s non-ionic surfactant-silica-water—seems to parallel the connections between micelles at the cloud point in the system’s non-ionic surfactant-silica-water. Although the driving force is undoubtedly the same, the weaker interaction between the micelle and its solvent, as well as the differences between the two systems, provide information on the role of the inorganics in the formation of composite mesophases. The increase in mesopore size with temperature does not have a clear-cut equivalent in the water-surfactant system. The partial dehydration of the PEO chains could be expected to bring about a decrease in area/volume ratio among the micelles and an increase in the aggregation number of ionic surfactants.
Figure 5.10 Schematic representation of SBA-15 synthesized (a) between 40 and 60°C, showing micropores and no connection between mesopores, (b) around 100°C, showing micropores and connection between mesopores [78].
5.1.2 Synthesis of SBA-15 at low acid concentrations
Structural and textural control is especially important in the design of functional porous solids—especially for applications that involve selectively tuned adsorption and diffusion—and in host-guest interactions within elaborately nanostructured materials.
However, synthetically, most of these large-pore mesoporous silicas were prepared at high acid catalyst concentrations (i.e., around 1.5–2 M in water), under which the mesophase formation occurs through a kinetically controlled competitive assembly of organic and inorganic species into nanostructured domains. The fast kinetics of hydrolysis and the condensation of silica at high acid concentrations results in a rapid hybrid mesophase assembly, somehow limiting the possibilities vis-à-vis the true and detailed design of textural and structural properties. Co-condensation reaction kinetics are especially difficult to control in this respect. In addition, under high acid concentrations, only a narrower range of SiO2/triblock copolymer ratio is acceptable for synthesis, otherwise, a marked decrease in ordering occurs. To overcome problems linked to quick condensation and a too-rapid mesophase assembly, we have developed an alternative synthetic strategy that is based on a combined decrease in acid catalyst concentration, to facilitate the modulation of the structural parameters of mesostructured products [83]. In this way, the formation of the mesophase can be governed more thermodynamically as the acid concentration decreases, preventing a sudden inter-micellar condensation of silica. Herein, in diluted acidic conditions, we have successfully discovered the simple principle of balancing kinetic and thermodynamic effects in materials-synthesis processes. In this respect, SBA-15 possesses some special features, all of which will be discussed here.
Using the low acid concentration method, the thermodynamic and kinetic effects are both occurred simultaneously, during silica species polymerization [84]. Under these conditions, the phase behavior of the triblock copolymer in the presence of silica
1 2 3 4 5
Intensity (a.u.)
2
θ(degree)
Figure 5.11 XRD patterns for mesostructured silica, synthesized under 0.001 M acid concentration.
Figure 5.12 SEM image of mesostructured SBA-15, synthesized under 0.001 M acid concentration.
Broken
species can be enriched in water, since the slower silica condensation kinetics are slower. The powder XRD pattern of mesoporous silica synthesized under a 0.001 M acid concentration is depicted in Figure 5.11, illustrating the major role of acid concentration on the structure of the mesophase. The figure shows the three peaks, which can be indexed according to a hexagonal array of mesopores, that are indicative of the diffraction planes (100), (110), and (200). The unit cell size, calculated from the (100) reflection of the p6mm phase, was measured to be 9.6 nm for a SBA-15 material synthesized under lower acid conditions—a value smaller than that of SBA-15 procured through high-acid synthesis. Because the condensation rate was faster than the hydrolysis rate of the silica precursor, the silica chains were branched randomly and the linear silicate formed tightly under mildly acidic conditions [85, 86]. The noises of the XRD peaks became higher as the acidic concentrations decreased. This finding is reasonable, because the presence of OH– causes hydrolysis in preformed SBA-15 samples. SBA-15 is synthesized under strongly acidic conditions by the S0(H+)X–I+ route, and its frameworks are electrically neutral: (S0H+X–I0)
[87],
where S0 is a non-ionic surfactant, I+ is a protonated silanol group, and X– is the counteranion. These mesoporous silica samples—such as MCM-41—are synthesized under mild conditions, via the S+I– route; however, they have negatively charged frameworks. Both products exhibited an enlargement and broadening of the (100) reflections, along with poorly resolved higher-angle peaks, while the acidic concentration decreased. Figure 5.12 shows an SEM image of SBA-15 synthesized under 0.001 M HCl conditions. It was found that the SBA-15 particles were spheroidal. Glassy material is generally present when acidity is very low, and it acts like a cement between particles. The results can be rationalized if a synthesis mechanism is considered, which takes into account the polycondensation rate of the silica species [88] and the dynamic of the micelles [89]. The increased in the condensation rate with acidity affected the nucleation and formation rate of the particles. Under mildly acidic conditions, the reactions possessed a rapid depletion reaction in the solution and the nucleation was active for only a short time. Understrongly acidic conditions, however, the interactions between silica and surfactant were weak, owing to the greater hydration of the acid through the formation of hydrogen bonds. The formation of the mesoporous silica was derived into two steps. During the first step, the surfactant micelles more or less controlled the polycondensation of the acidity, in turn raising, the discreteness of the particles. In the second step, the weakened and less-organized material from the first step were embedded. We assume the period of time needed to build a given length of silica wall around the micelle in mild acidic conditions. Nevertheless, the ratio of the lifetime of the micelle to the growth time of the wall becomes much higher than unity, under strongly acidic conditions. In such cases, three paths can be considered. (1) In strongly acidic conditions, the lifetime is too short compared to the time needed to construct the silica wall, and sharp, fiber-like sharp materials are presented. (2) In medium acidic conditions, there is a shortening of the micelle lifetime and an increase in the structural disorder, and numerous sharp, rod-like materials are presented. (3) In mildly acidic conditions, the lifetime of the micelles is very long, compared to the time needed for the formation of the solid, and spheroidal and polyhedral particles are presented. Nitrogen adsorption isotherms for SBA-15 samples are shown in Figure 5.13. The two step desorption isotherm appeared under mildly acidic conditions (i.e., 0.001 M). The nitrogen adsorption-desorption isotherm possesses some special features, namely, “plugged mesopores.” The formation mechanism is discussed below.
0.0 0.2 0.4 0.6 0.8 1.0 Nopen
Nblock
Nmicro
SBA-15 0.001 M