Mesoporous lamellar

Figure 12.7 (a) Small-angle XRD of ZnO electrodeposited with 15 wt% EO20PO70EO20, indicating a well-ordered lamellar phase. TEM images confirming (b) lamellar structure and (c) disordered mesopores. (From Jaramillo, T.F., Ivanovskaya, A., and McFarland, E.W., J. Comb. Chem., 4, 17, 2002.)... 

In previous literature, the type B hysteresis was ascribed to a lamellar-like structure that commonly observed in the pillared materials.[13,14] Here its existence in our mesoporous materials is associated with some internal defects in the channels. To further understand such hysteresis behavior, we compared the microtomed ultra-thin sections TEM micrographs of these two samples. In Fig. 2A, B, we show the typical parallel channels of MCM-41 and the well-ordered hexagonal mesoporous in pure silica sample(I). However in Fig. 2 C, D, one can obviously find the aluminosilicate(II) possessing the normal well-aligned MCM-41 nanochannels with extensive voids interspersed. The white void parts were attributed to the structural defects. These structural defects are not the lamellar form but the irregularly shaped defects. The size of the defects is not uniform and distributes between 5.0-30.0 nm. nanometers. Therefore, these aluminosilicate mesoporous materials were composed of structural defects-within-well-ordered hexagonal nanochannels matrix. 

PCH materials offer new opportunities for the rational design of heterogeneous catalyst systems, because the pore size distributions are in the supermicropore to small mesopore range (14-25A) and chemical functionality (e.g., acidity) can be introduced by adjusting the composition of the layered silicate host. The approach to designing PCH materials is based on the use of intercalated quaternary ammonium cations and neutral amines as co-surfactants to direct the interlamellar hydrolysis and condensation polymerization of neutral inorganic precursor (for example, tetraethylorthosilicate, TEOS) within the galleries of an ionic lamellar solid. 

Two categories of mesoporous solids are of special interest M41S type materials and pillared or delaminated derivatives of layered zeolite precursors (pillared zeolites in short). The M41S family, first reported in early 1990 s [1], has been extensively studied [2,3]. These materials exhibit broad structural and compositional diversity coupled with relative ease of preparation, which provides new opportunities for applications as catalysts, sorption and support media. The second class owes its existence to the discovery that some zeolite crystallizations can produce a lamellar intermediate phase, structurally resembling zeolites but lacking complete 3-dimensional connectivity in the as-synthesized form [4]. The complete zeolite framework is obtained from such layered zeolite precursor as the layers become fused, e.g. upon calcination. The layers posses zeolitic characteristics such as strong acidity and microporosity. Consequently, mesoporous solids derived from layered zeolite precursors have potentially attractive characteristics different from M41S and the zeolite species... 

Determination of the phase purity of mesoporous molecular sieves (MMSs) [1,2] is important in synthesis, modification and application of these materials [3-7]. Many of the synthesis procedures reported so far involved various phase transformations [8-20] and thus the desired MMS product may be contaminated with some mesostructured impurities. One of the possible impurities is a lamellar phase, which readily forms under various synthesis conditions [1,8-25]. Because of its layered structure, the lamellar phase collapses upon calcination [1] and therefore constitutes a disordered impurity of calcined MMS samples. 

For mesostructured aluminophosphates with lamellar structure WISE spectra indicate that template surfactant species have very restricted mobility as the ll wide lines are very broad.3 This is also consistent with the trans-conformation of the aliphatic chains. Other representatives of mesoporous A1PO exhibited template species with much narrower H resonances.2... 

Aida and coworkers developed poly(pyrrole)-containing mesoporous silica films in both hexagonal and lamellar phases.34 The polypyrrole chains are highly constrained and insulated when incorporated within hexagonal mesoscopic channels and the possibility of the polarons recombining into bipolarons is significantly suppressed. In contrast, the two-dimensional lamellar phase affords spatial freedom for electron recombination. 

Depending on the conditions and chemical precursors used for mesoporous materials synthesis, different morphologies, such as hexagonal, cubic, or lamellar, and different pore sizes can be obtained (Fig. 9.2). Additionally, these materials can easily be functionalized with organic groups to produce a variety of hybrid inorganic-organic materials with new properties, which directly affect the functionality of the enzyme [91, 92]. 

Fig. 9.2 Lamellar, hexagonal, and cubic morphologies obtained by different synthesis conditions of mesoporous materials...

Fig. 10 X-Ray diffraction patterns of (a) hexagonal, (6) cubic and (c) lamellar forms of mesoporous calcium aluminoborate (from Ayyappan and Rao27).

Fig. 11 Phase transitions in mesoporous solids a-d, lamellar-hexagonal e-f, hexagonal-cubic. The circular objects around the surfactant assemblies are the metal-oxo species (from Neeraj and...

The lamellar form of mesoporous zirconium oxide was prepared using dodecylamine (DA) as the surfactant. In a typical synthesis zirconium isopropoxide (0.01 mol) was added to a solution of the dodecylamine (DA) (0.03 mol) in propan-l-ol... 

The lamellar—-hexagonal thermal transformation of mesoporous zirconia in the solid state was studied by heating the lamellar form at a fixed temperature for different periods of time. The phase composition of the sample subjected to heat treatment was estimated on the basis of the intensity of the (100) reflection, the ((-spacing showing only small changes in the thermal transformation (Fig. 2). The kinetics of the transformation was studied as a function of time at 373, 403 and 413 K. The transformation was also studied by heating the lamellar sample for a fixed period of 2 h at different temperatures in the range 360-430 K. 

The lamellar form of mesoporous zirconia, on contact with 0.87 M phosphoric add, first transforms to the hexagonal form. 

In conclusion, the present study of the kinetics of the lamellar to hexagonal transformation of mesoporous zirconia shows that a loss of surfactant molecules accompanies the transformation. Transformation to the cubic form seems to require that all the starting material be in the hexagonal form. The thermally induced lamellar— hexagonal transformation is associated with an activation energy comparable to the hydrogen bond energy. 

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