Mode selection adsorption

Let us consider the separation of polymethylmethacrylate (PMMA) on a nonmodified silica column as an example. In THE (medium polar eluent) the PMMA eludes in size exclusion mode because the dipoles of the methylmethacrylate (MMA) are masked by the dipoles of the THE. Using the nonpolar toluene as the eluent on the same column, the separation is governed by adsorption because the dipoles of the carbonyl group in the PMMA will interact with the dipoles on the surface of the stationary phase. The separation of PMMA in the critical mode of adsorption can be achieved by selecting an appropriate THF/toluene mixture as the eluent. In this case all PMMA samples... 

Both GC and LC may be operated in one of several modes. The principal modes currently used for large-scale separations are elution, selective adsorption or desorption, and simulated countercurrent chromatography. In addition, reaction and separation can be combined in a single column with unique advantages. Elution is the most used and best developed form of the technique and is described first. 

The three main modes of chromatographic operation are elution chromatography, selective adsorption/desorption, and simulated countercurrent chromatography. Of these, elution chromatography, used as a cyclic batch process, was the first to be developed for large-scale separations. 

Affinity chromatography is conducted in the selective-adsorption mode, whereas ion-exchange chromatography is usually carried out in the selective-desorption mode. The... 

LIQUID CHROMATOGRAPHY. An analytical method based on separation of the components of a mixture in solution by selective adsorption. All systems include a moving solvent, a means of producing solvent motion (such us gravity or a pump I, a means ol sample introduction, a fractionating column, and a detector. Innovations in functional systems provide the analytical capability for operating in three separation modes (1) liquid-liquid partition in which separations depend on relative solubilities of sample components in two immiscible solvents (one of which is usually water) 12) liquid-solid adsorption where the differences in polarities nf sample components and their relative adsorption on an active surface determine tile degree ol separation (2) molecular size separations which depend on the effective molecular size of sample components ill solution. 

The secondary ring carbon atoms are preferentially oxidized with respect to that of the methyl group. A simple statistical calculation demonstrates that they are 6.5 times more Teactive. This behaviour is opposite to the gas phase photocatalytic oxidation of toluene (ref. 4), which produces only traces of benzaldehyde, whereas the aromatic ring withstands oxidation, at least in pure gas or liquid organic phase and in the absence of water. The above selectivities seem to be correlated to steric factors governing the mode of adsorption of methylcyclohexane on the surface of titania. 

The presence of an aromatic ring in the molecule favours the oxidation in the allylic position. By constrast with the selectivity pattern of photochemistry (see Fig. 1), the formation of 1-tetralone would be due to the mode of adsorption of tetralin at the surface of titania and/or to the mode of attack of photoactivated oxygen. 

Since many different modes of adsorption and rearrangement of molecule fragments are possible on catalyst surfaces, many reactions can occur in parallel. Therefore these catalysts often have a low selectivity. Selectivity is increased by the use of promotors or catalyst modifiers that create particular surface sites and increase selectivity by suppressing undesired reaction paths. 

Recently, Berweger and Raschke considered the polar phonon mode selection rules for TERS for the determination of nanocrystallographic information from solids using TERS as discussed in Sect. 4.2 [93]. This is a very different situation given that the location and orientation of the hot spot relative to the sample and excitation are well characterized and could be extended to the study of molecular adsorption on metals and other surfaces in the future. 

The selective partial hydrogenation of symmetric cyclic diketones was accomplished by interrupting the reaction after the consumption of one equivalent of hydrogen, which indicates a stronger mode of adsorption for the diketone than for the ketol product. 2 The hydrogenation of 1,4-cyclohexanedione (30) to the ketol (Eqn. 18.23) was catalyzed by nickel, copper, palladium, platinum, iridium, and ruthenium. Iridium was the most active and selective of these catalysts. Hydrogenations run over this catalyst in iso-propanol at 20°C and 6 atmospheres... 

As in the case of hydrogenolysis of cyclopentane, the change in selectivity observed in pulse and flow systems for the 1-5 dehydrocyclization of n-heptane to ethylcyclopentane and 1,2-dimethylcyclopentane, for instance, was interpreted by two modes of adsorption involving five or seven carbon atoms in contact with the surface (775) (Fig. 3). 

The product distribution in the t-butylation of phenol with isobutanol may be explained based on the nature of acid-base strengths and the mode of adsorption of phenol. It has been reported in the literature that phenol is adsorbed horizontally on acid-catalysts like AI2O3 and a vertical mode of adsorption is proved on basic catalysts like MgO [16,17], The horizontal adsorption of phenol results in O-alkylation and also C-alkylation at ortho and para positions which are close to the surface of the catalyst whereas in the vertical adosrption mode, only the ortho selectivity is observed. The extent of C-alkylation depends on the strength of the acid site. However, a combined participation of acid-base properties is also reported in the methylation of phenol over hydrotalcites [18]. Hence, a scheme depicting the correlation between the acid-base properties of the catalysts with the product distribution in the t-butylation of phenol is shown below as. 

In catalysis of organic reactions one of the most inq)ortant requirements is the selectivity, broadly understood as chemo-, regio- and enantioselectivity [1], The specific feature of many organic systems is that complex molecules possess several functional groups which adsorb differently on the catalyst surface and give a wide variety of products [2]. The selectivity towards a desired product can be controlled by the choice of catalyst, solvent and reaction conditions. The mode of adsorption is in some cases detrimental for the selectivity towards a particular product [2]. 

The analysis of experimental data revealed a correlation between the hydrodynamic mode of a tubular turbulent device and the interphase tension in the flow of the two-phase liquid-gas reaction system (Figure 2.52). This correlation confirms that the addition of surfactants is a reasonable solution for a reaction system with an interphase boundary. It leads to a decrease of bubble size and mass exchange intensification in the gas-liquid flow of fast chemical processes. In addition, the liquid-phase longitudinal mixing rate increases and the hydrodynamic mode of a process approaches perfect mixing conditions. Fast chemical processes, in two-phase systems, require consideration of the selective adsorption of feedstock reactants and reaction products on to the interphase boundary, and a change of the hydrodynamic motion structure of the continuous phase. A change in the work required to form the new surface is a typical phenomenon for all types of multiphase systems and depends on... 

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