Monolithic organic compounds

D. Badjic and N.M. Kostic, Behavior of organic compounds confined in monoliths of sol-gel silica glass. Effects of guest-host hydrogen bonding on uptake, release, and isomerization of the guest compounds. J. Mater. Chem. 11, 408 118 (2001). 

The sol-gel method has been extensively used for the preparation of n-metal oxides and organic compounds. The important examples are n-NiO, n-Mn02, n-W03 and n-Fe203 etc. which have homogeneous particles, pore sizes and densities. This method affords easy control over the stoichiometry and homogeneity which is not possible with conventional methods. Further, the materials with special shapes monoliths, fibers, films and powders of uniform and very small particle sizes can also be prepared. The most important attribute of NMs prepared by this method is that they also contain pores of similar dimensions. These pores may be filled with another phase to form a nanocomposite which has proved to be of significant use to the HEMs community [98]. 

As stated by Van der Sloot (1998), several factors can influence the release of contaminants from both granular and monolithic materials. These include major element chemistry, pH, redox status of the system, presence of complexants, humic substances or other dissolved organic compounds, liquid to solid ratio, and biological activity. 

Avila P, Bahamonde A, Blanco J, Sanchez B, Cardona AI, Romero M. Gas-phase photo-assisted mineralization of volatile organic compounds by monolithic titania catalysts. Appl Catal B Environ 1998 17 75-88. 

Monolith catalysts are used for the control of carbon monoxide and hydrocarbon (known as volatile organic compounds or VOCs) emissions from chemical plants and cogeneration facilities. In this case, square bricks are stacked on top of one another in a wall perpendicular to the flow of exhaust gases at the appropriate temperature location within the heat recovery boiler. The size of the brick can vary from 6 in (ceramic) to 21 ft (metal). Pt and Pd catalysts are used at operating temperatures between 600 and 1200°F. Cell sizes typically range between 100 and 400 cells per square inch. Typical pressure drop requirements for monoliths are less than 2 in of water. 

The dynamic adsorption behaviour of volatile organic compounds on activated carbon honeycomb monoliths... 

Due to its high photocatalytic activity towards the complete mineralisation of VOCs [7,8] titania in its anatase form is normally used. Using ceramic monoliths with high titania content (50%) the total oxidation of chlorinated organic compounds at low temperature has been demonstrated [9]. However, since the photons from natural light may only penetrate a few microns into the catalyst surface the use of a wash-coating technique, where only a thin active film of titania is applied to the ceramic or metallic support can be considered as an ideal technique to produce maintenance free photocatalytic reactors. 

The American 1970 Clean Air Act defined ambient air quality standards (NAAQS) in the United States for atmospheric ozone, NO, lead, carbon monoxide, sulfur oxides, and PM-10 (particulate matter less than 10 p.m). The strategy to reduce levels of lead, NOx, PM-10, and to some extent carbon monoxide was to control emissions from automobiles that included the phasing-out of leaded fuel. As previously noted, ozone is a product of the photochemical reaction of volatile organic compounds with NOx (photochemical smog), so the balance between organic compounds and NOx pollutants is important in meeting target ozone levels (e.g., 0.12 ppm). Emissions from stationary sources is an important factor, and limits have been set for them. Because of low pressure drop requirements, coated monolithic catalysts... 

Low pressure drop. Because of the shape of the void space through which the fluid flows, i.e., noncircular channels that are straight in the direction of the flow, the pressure drop across a BSR is comparable to that of a monolithic reactor. This feature is most profitable in processes operating at low pressure and high space velocities, such as catalytic removal of NO, SO, or volatile organic compounds (VOCs) from flue gases. The pressure drop can be manipulated by means of the voidage (see next item). 

Fuertes, A.B., Marban, G., and Nevskaia, D.M. (2003). Adsorption of volatile organic compounds by means of activated carbon fibre-based monoliths. Carbon, 41, 87-96. 

Montreuil and Shelef [81] studied the SCR reaction over ZSM-5/AI2O3 supported on a cordierite monolithic substrate. Various oxygenated hydrocarbons, such as methanol, ethanol, propanol, acetaldehyde, acetone, methyl ethyl ketone, and 1,4-dioxane were used as reductant at 755 K [81]. The reaction extent with the various organic compounds was compared with that obtained when using propene as reductant. Propene was the superior reductant. Methanol as a reductant showed a very low NO conversion, whereas propanol showed the highest activity, albeit lower than that of propene. Upon adding oxygen to the feed the difference between propanol and propene vanishes. 

There are two families of monolithic gels obtained from organic solutions. The first is basically identical to those obtained in aqueous medium. Similarities come from the mineral compound, which undergoes gelation the solvent and additives are organic compounds. The second refers to gels prepared from alkoxides in organic solvent. This kind of gel is the most popular and has been extensively reported in the literature. 

Nowadays, porous monoliths have found an extensive use in CEC of organic compounds, which represents a powerful separation tool, complementary to HPLC. CEC is a hybrid method in which the separation is performed through the phase distribution mechanisms of traditional HPLC (reversed-phase, ion exchange, etc.), while the flow of the mobile phase through column packing is affected by electroosmotic forces, as in electrophoresis. The coexistence of a stationary phase and an electric field permits separation not only of ions but also of neutral compounds, due to their different electrophoretic mobhity and different distribution between the mobile and the stationary phases. 

A small, 20 X 1 mm ID, monolithic rod, obtained by the polymerization of 80% tec/i-DVB in a mixture of dodecanol and toluene, was employed for the solid-phase extraction of polar organic compounds from aqueous solutions [415]. It was demonstrated again that by maintaining constant dilution of the monomer and changing only the weight percent ratio of toluene to dodecanol from 2 58 to 10 50, it is possible to increase the surface area of the sorbent from 188 to 348m /g and to reduce the mean pore diameter... 

Shim, W.G., Moon, H., and Lee, J.W. 2006. Performance evaluation of wash-coated MCM-48 monolith for adsorption of volatile organic compounds and water vapors. Micropor. Mesopor. Mat. 94 15-28. 

No conversion data are presented for gas inlet temperatures over 550 - 600 C, since this was the calcination temperature for all monolith catalysts. The catalysts presented here are therefore not particularly suitable as high-temperature combustion catalysts for hydrocarbons. However, they may have interesting properties for removal of volatile organic compounds at lower temperatures, which is now under investigation. 

Monolith materials may either be metallic or prepared from inorganic mixtures [117] or from organic compounds [118,119]. In this chapter, the term monolith or rigid rod will comprise cross-linked, organic materials that are characterized by a defined porosity and that support interactions/reactions with the surrounding liquid phase [120,121]. 

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