Postsynthesis Processing

The final step in the synthesis of many (but not all) sol-gel materials is some postsynthesis treatment after or concomitant with drying, depending on the intended properties or uses. 

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V. POSTSYNTHESIS PROCESSING AND SAMPLE HANDLING A. Format Changes and Liquid Handling... 

Much of the discussion and focus of combinatorial chemistry is on the synthesis process itself. However, most practitioners in the field will attest that postsynthesis processing is far more time consuming than synthesis and that postsynthesis processing is often the rate-limiting step in an overall combinatorial chemistry project. Early design of synthesis apparatus paid scant attention to the format in which the products are delivered. For example, products delivered into test tubes may have to be individually moved to a concentrator to remove solvent, then individually moved to a liquid handler for dissolution and distribution to microtiter plates for bioassay. Each format change (change of vessel type or vessel carrier array) may require manual intervention and thus will slow the overall process. 

An alternative approach involves blending of conducting polymers with biomaterials of interest to form biocomposites. This, of course, ideally involves the TCP and biomolecule to be soluble in a compatible solvent that does not denature the biomolecule. Such examples are limited and usually involve polyanilines [26], as they are more postsynthesis processable. ICP polyanilines have, for example, been blended with collagen. Blending of poly(o-ethoxyaniline) and collagen resulted in formation of flexible free-standing semiconducting materials. 

The limitations in postsynthesis processability are due to the chain stiffness and interchain interactions that render these materials insoluble in common solvents. For example, polymers can become crosslinked, highly branched, or electrostatically crosslinked due to polaron/bipol-aron charge interactions. The chemical or ionic crosslinking renders the polymer intractable. However, several approaches have been adopted to facilitate solution processability including ... 

To improve the meso-structural order and stability of the mesoporous silica ropes, a postsynthesis ammonia hydrothermal treatment (at 100 °C) was invoked. As indicated by the XRD profile in Fig. 3A, 4-5, sharp features are readily observed in ammonia hydrothermal treated samples. Moreover, after the post-synthesis ammonia treatment, the sample also possesses a sharp capillary condensation at p/po 0.35(Fig. 3B) corresponding to a much narrower BJH pore size distribution of ca. 0.12 nm (at FWHM). In other words, the mesostructures are not only more uniform but also more stable when subjected to the post-synthesis treatment. The morphology of the silica ropes remained unchanged during the ammonia hydrothermal process. The mesostructures remain intact under hydrothermal at 100 °C in water even for extended reaction time (> 12 h). 

The third main step of combustion synthesis technologies is postsynthesis treatment. This step is optional, since not all products require additional processing after synthesis. Powder milling and sieving are used to yield powders with a desired particle size distribution. Annealing at elevated temperatures (800-1200°C) removes residual thermal stress in brittle products. The synthesized materials and articles may also be machined into specified shapes and surface finishes. 

The bulk chemistries, surface chemistries, and morphologies of silicon carbide whiskers vary widely depending on the type of process used, the stage of the process development, and the postsynthesis treatments practiced by the producer. The synthesis of the whiskers as described earlier is... 

Gravimetric measurements were executed by Pradhan et al. on SWNTs with different postsynthesis treatments. They reported hydrogen storage capacities at approximately 0.2 MPa ranging from approximately 1 to 6wt%, depending on the processing of the material. The sample was first oxidized in dry air to remove the amorphous carbon, then refluxed in HCl or HNO3 and finally annealed at different pressure. 

In conclusion, the epoxidation of propylene with bulky oxidants (such as cumene or TBHP) can be successfully achieved using titanium-containing mesoporous materials as catalysts. The catalytic chemistry of the active sites can be controlled via the synthesis conditions and postsynthesis modifications. The hydrophobicity of the catalyst is of great importance to achieve a highly selective catalyst. The Ti-MCM-41-based heterogeneous catalyst has demonstrated excellent performance in the commercial process for PO manufacture. 

Electrodeposition. In this process, a conductive substrate is placed in an electrolyte solution (typically aqueous) that contains a salt of the material of interest. When an electrical potential is apphed between the substrate and a counter electrode, redox chemistry takes place at the surface of the substrate which deposits material. Complex pulse trains and/or high-pulse frequencies are sometimes used to direct current flow and favor desired reactions. A postsynthesis calcination is often performed to reach a desired material phase. Electrodeposition is restricted to deposition of electrically conductive materials and produces polycrystaUine and amorphous films. This process is also appropriate for thin film surface treatment of PEC electrodes, such as electrocatalyst deposition. 

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