Pore size tailoring

Post-synthetic modification (PSM) also can tune the pore size. Generally, it decreases the pore size in the framework since the pore can be blocked by functional groups or hybrid units. Akin to the pre-modification method, a change in length or width of the organic linker effects porosity and the nature of the framework. However, the influence made by modification is not limited to tuning the pore size of the framework. 

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Pore size tailoring in traditional zeolitic molecular sieve materials has mainly focused on post-synthesis modification. Various organic and inorganic species have been reacted with zeolite surfaces or deposited into the internal pore system with the intent of increasing shape-selectivity and/or altering catalytic activity. Most of this work has focused on reducing the pore size of the molecular sieve. This area has been reviewed by Szostak [37]. 

The large majority of activated alumina products are derived from activation of aluminum hydroxide, rehydrated alumina, or pseudoboehmite gel. Other commerical methods to produce specialty activated aluminas are roasting of aluminum chloride [7446-70-0], AIQ calcination of precursors such as ammonium alum [7784-25-0], AlH2NOgS2. Processing is tailored to optimize one or more of the product properties such as surface area, purity, pore size distribution, particle size, shape, or strength. 

As RO membranes become looser their salt rejection falls (see Section 31.8.1). Eventually a point is reached at which there is no rejection of salts, but the membrane still rejects particulates, colloids and very large molecules. The membrane pore size can be tailored to a nominal molecular weight cut-off. The resulting filtering process is called ultra-filtration. 

To overcome the limitations of natural zeolites a whole range of synthetic zeolites have been manufactured since the 1950s. These have tailored pore sizes and tuned acidities, as well as often incorporating other metal species. The basic synthesis involves mixing a source of silica, usually sodium silicate or colloidal Si02, with a source of alumina, often sodium aluminate, and a base such as sodium hydroxide. The mixture is heated at temperatures up to 200 °C under autogenous pressure for a period of a few days to a few weeks to allow crystallization of the zeolite. 

Vertical media with very high coercivities can be produced by plating into alumina pores [112], Some of these media are too hard to be easily written with present heads. Tailoring of the pore size can be used to obtain structures with the desired Hc [115, 116], however. Recording characteristics of disks have been determined [112-114, 116] such media show excellent promise as vertical recording media. In addition, structures with electrodeposited Fe in the pores were tested in life-tests at elevated temperatures and humidity and in corrosive atmospheres. They were found to perform satisfactorily. 

Porosimetry data can be graphed in a variety of ways and can be tailored to the purpose of the study. Plotting volume versus pore size will easily display the pore sizes observed in the sample. Pore size distributions can be calculated from the raw data and plotted to give the pore volume per unit radius interval. Other parameters can be calculated from porosimetry data, including average pore radius [40,48], surface area [7,39,40], pore surface area [6], particle size [40], and density [6,49]. 

Molecular sieves are synthetic zeolites that can be manufactured with extremely close control of pore size. Therefore, they can be tailored to suit specific applications. In addition to gas drying applications, molecular sieves are used for the separation of gases and vapors on the basis of molecular size and shape. Surface areas range from 350 to 1000 ft2/ft3. 

The particles are mechanically strong, have high surface area and feature pore sizes that can be easily tailored. In addition, manufacturers can customize the properties of the material for a particular application by modifying the surfaces with octyl, octadecyl, phenyl and other types of functional groups that elicit separation-enhancing interactions between the components of the solution being analysed and the particles in the column. 

Below the outer membrane is a filter, usually composed of an anionic polymer, e.g. based on salicylate. Its precise composition and dimensions (thickness, pore size, amount and type of plasticizer, fillers, etc.) are optimized in order to tailor the diffusion rates of material crossing the filter from the analyte solution toward the working electrode of the sensor. Ideally, some uncharged molecules, such as H2O2, will traverse the filter so fast that, in effect, the filter is invisible to... 

The pore size of the membrane could also be controlled independently of the porosity by altering the size of the salt particles (Fig. 5a). Membranes with high surface area/volume ratios were produced and the ratio was dependent on both salt weight fraction and particle size (Fig. 5b). In addition, the crystallinity of PLLA membranes can be tailored to that desired for each application. These characteristics are all desirable properties of a scaffold for organ regeneration. The major disadvantage of this technique is that it can only be used to produce thin wafers or membranes (up to 2 mm in thickness). A three-dimensional scaffold cannot be directly constructed. This problem may be circumvented however, by membrane lamination. 

The work of Mallouk et al. (39) offers an interesting extension of the microemulsion sol-gel technique. In this case, microemulsion-derived silica nanoparticles were used as templates for preparing ordered mesoporous polymers with tailored pore sizes. Utilizing the Triton N-101/cyclohexane/hexanol/water/ammonia microemulsion, monodisperse silica nanoparticles were first synthesized. The silica product... 

New way to synthesize MCM-41 and MCM-48 materials with tailored pore sizes... 

Keywords MCM-41, MCM-48, tailored pore size, decane/TMB, swelling agents, phase transition, thermal stability... 

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