Porosity, organic template control

Scheme 1.1 How organic templates control porosity of materials (and consequences from a functional, reactivity viewpoint).

Use of organic templates In the current preparation of a new series of microporous crystals, organic templates are often used. Organic chemicals are also used in the preparation of pillared clays to control the pore sizes and the porosity as follows The interlayer cations of silicate layer are exchanged with positively charged sol particles, and then a part of the sols are exchanged with organic template cations such as octadecyl trimethyl ammonium (OTMA). 

Structures originated by molecular self-assembly are usually larger (on the order of several nanometers, yielding mesoporous materials, Figure 3.4) than those obtained from organic templates (typically microporous, pore size <2nm) [5], The large size of the mesopore (2-50 nm) facilitates the access of reactants to the interior of the solid. This allows for processing of bulky molecules that cannot access the narrower porosity of microporous materials, like zeolites. Control of the synthesis... 

These new membrane developments can benefit from recent progress in sol-gel science and technology, in particular the control of porosity through templating methods, the role of nanophase structures and also the synergetic effect of organic and inorganic... 

The synthesis of ordered micro- and mesoporous silica materials often relies on the use of organic templates that are saciified in order to evacuate the pores. Inspired by the synthesis of amorphous microporous sihca materials by Maier et al.[l] applicable in molecttlar shape selective catalysis [2] and controlled release [3, 4] in this work we continued the exploration of silica synthesis imder strongly acidic conditions. We observed the outcome of the synthesis to be strongly dependent of the Si-soiuce, the type of mineral acid and the synthesis temperature. In this paper the synthesis of silica materials with a porosity ranging from micro- to mesoporous is demonstrated. This approaeh of synthesis of silica with tunable nanopores not involving sacriftcial templates will be convenient for many applications. 

The use of templates to control the porosity of solids is not limited to small organic molecules. Alternative templates include dendrimers [16, 17], polymers [18], hard templates such as nanoparticle colloidal suspensions [19] and latex spheres [20] or even biological materials like butterfly wings [21], DNA [22] or viruses [23]. 

In the present section we comment further on the chemical modifications of these materials when the R group is chosen for the preparation of micro-and mesoporous silicas. From a general point of view, the control of the porosity of silica via organic molecular templating is an attractive topic connected to molecular recognition, catalysis, chemical sensing and selective adsorption, etc. Many attempts have been made to control the pore size distribution in sol-gel derived silica30,196. 

Transition-metal-ion complexes have been introduced as templates to direct [2-1-2] photodimerizations in the solid state. The photoreaction has been achieved in both discrete complexes and metal-organic frameworks (MOFs). MOFs are intriguing platforms to control the photodimerization owing to changes to bulk physical properties (e.g., porosity) that can occur in the porous frameworks. 

The difficulty in direct synthesis of mesoporous transition metal oxides by soft templating (surfactant micelles) arises from their air- and moisture-sensitive sol-gel chemistry [4,10,11]. On the other hand, mesoporous silica materials can be synthesized in nimierous different solvent systems (i.e., water or water-alcohol mixtures), various synthetic conditions (Le., acidic or basic, various concentration and temperature ranges), and in the presence of organic (Le., TMB) and inorganic additives (e.g., CT, SO, and NOs ) [12-15]. The flexibility in synthesis conditions allows one to synthesize mesoporous silica materials with tunable pore sizes (2-50 nm), mesostructures (Le., 2D Hexagonal, FCC, and BCC), bimodal porosity, and morphologies (Le., spheres, rods, ropes, and cubes) [12,14,16-19]. Such a control on the physicochemical parameters of mesoporous TM oxides is desired for enhanced catalytic, electronic, magnetic, and optical properties. Therefore, use... 

The transfer of a probe ion across nano-ITIES arrays has been characterized voltammetrically. For instance, static IT voltammetry at the nano-ITIES array templated by a y-alumina membrane was used to determine membrane porosity. In this work, a nanoscale interface was formed at the orifice of each nanopore filled with the aqueous solution of a probe ion (tetraethylammonium, TEA ) in contact with the external organic solution (Fig. 17a). Potential sweep rates were chosen such that the mass transport of the probe ion during the forward potential sweep was controlled by the linear diffusion confined within nanopores (Fig. 17b). The resultant peak current based on TEA transfer depends on the total area of the nanoscale... 

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