Nonperiodic Phases

In addition to the silicas with ordered mesopores, disordered mesoporous silicas, so-called wormlike phases, form another class of useful materials. These materials have disordered pores (Fig. 11), amorphous silica wall, and, importantly, uniform pore diameters. These materials include MSU [118, 119], TUD-1 [120], HMS [121, 122], KIT [123], Al-MMS [124, 125] series, etc.

MSU mesoporous phases were prepared using a nonionic templating method. For example, they can be synthesized by the hydrolysis of tetraethylorthosilicate in the presence of PEO surfactants, which act as the structure-directing (templating) agents [118]. Hydrogen bonding interactions between the hydrophilic surfaces of wormlike micelles and Si[(OC2H5)4_x][(OH)x] were assumed to be important. Uniform diameters ranging from 2.0 to 5.8 nm can be tuned by varying the size and structure of the surfactant molecules.

TUD-1 was made by using a small, inexpensive organic template, triethanolamine [120]. The disordering nature of the wormlike material leads to more flexible morphology of the particles. The tunable pore size distribution can be made in the range of 2.5 to 25 nm. Beta-TUD-1 is a new type of material with a corn-shell structure (i.e. a beta-type zeolite with particles size of about 50 to 60 nm is incorporated into TUD-1). The TUD-1 shell is around a few nanometers in thickness, and therefore the material has mesopore-controlled selectivity and zeolite-controlled catalytic activity [126].

Figure 11. TEM image of wormlike mesoporous TUD-1.

Hexagonal mesoporous silicas were prepared via a non-ionic pathway [121]. Typical XRD patterns of these materials only show one peak at low angle. However, there is a local hexagonal ordering. This is probably why the strange term "hexagonal disordered phases" is used.

KIT-1 was synthesized by an electrostatic templating route using sodium silicate, hexadecyltrimethylammonium chloride, and ethylenediaminetetraacetic acid tetrasodium salt. Its three-dimensional nature was proved by TEM when the pores were partially filled by Pt. Although the mesopores are completely disordered, the pore size of KIT-1 is uniform with the pore diameter of 3.4 nm and peak width smaller than 3 nm at half-height as indicated by N2 adsorption results. It is noted that the structure of KIT-1 has outstanding hydrothermal and thermal stabilities. For example, it was stable in boiling water for 2 days. Incorporation of Al into the KIT-1 framework has been achieved with the ratio of Si/Al down to 5. The doped materials were also found to have high hydrothermal stability and ion-exchange capacity [123].

Aluminosilicate mesoporous molecular sieves (designated Al-MMSX, where X is the ratio of Si/Al used in the synthesis gel) were prepared at room temperature using the primary amine hexadecylamine as organic templating surfactant. The real ratios of Si/Al in the products are slightly higher than X. The average pore diameters of the materials are in the range from ca. 2 to 2.5 nm determined using the BJH method. The materials should be wormlike indicated by

Table 1. Summary of structures of mesoporous molecular sieves.
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