Dichloromethane, adsorption

A comparison of active (using pumps) and passive (relying on diffusion) sampling techniques for the determination of nitrobenzene, benzene and aniline in air was mentioned in Section IV.A77. Several LLE methods for nitroaromatic compounds dissolved in water were evaluated. High recoveries were achieved with discontinuous or continuous extraction with dichloromethane, adsorption on a 1 1 1 mixture of Amberlite XAD-2, -4 and -8 resins and elution with dichloromethane445. 

PAH Silica gel Hexane, dichloromethane Adsorption (remove polar materials), Fractionation (split alkanes from PAH)... 

The transformation of dichloromethane was carried out over FAU zeolites under the following conditions flow reactor, temperatures between 220 and 450°C, feed constituted of air with 4% steam and 1000 ppm CH2CI2. With all zeolites (NaX, NaY, NaHFAU and HFAU(Y)), dichloromethane can be selectively transformed into formaldehyde plus chlorhydric acid. The zeolite activity was shown to be related to the heat of dichloromethane adsorption, NaX being more active than NaY and much more active than HFAU(Y) and than NaHFAU(Y) zeolites without accessible sodium cations. 

The effect of NaX deactivation on the heat of dichloromethane adsorption was determined. Fig. 3 shows that the very strong adsorption sites which were found over the... 

This difference is rather due to the large difference in the polarity of zeolites shown by dichloromethane adsorption. Indeed, the order in activity is the same as the order in heat of dichloromethane adsorption of the plateau in Figure 1. NaX (AH = -64 kJ.mol ) is more active than NaY (AH = -54 kJ.mof ) and much more active than HFAU (AH = -40 kJ.mof ). 

Adsorption column chromatography has been employed to separate the constituents of pyrethrum. Florisil and aluminum oxide have been used as adsorption columns to retain much of the pigmented materials. The pyrethroids may be caused to elute with several solvents. In our experience mixtures of hexane with ethyl acetate, methanol, ethyl ether, dichloromethane, or acetone have provided different elution patterns. 

In spite of these limitations, three examples of (salen)-metal complex adsorption have been described. In the first one, Jacobsen s complex (la-MnCl) was adsorbed on Al-MCM-41 [27] by impregnation with a solution of the complex in dichloromethane, an approach that prevents the possible cationic exchange. The results in the epoxidation of 1,2-dihydronaphthalene with aqueous NaOCl were comparable to those obtained in solution, with only a slight reduction in enantioselectivity (55% ee instead of 60% ee). However, recycling of this catalyst was not described. 

We then designed model studies by adsorbing cinchonidine from CCU solution onto a polycrystalline platinum disk, and then rinsing the platinum surface with a solvent. The fate of the adsorbed cinchonidine was monitored by reflection-absorption infrared spectroscopy (RAIRS) that probes the adsorbed cinchonidine on the surface. By trying 54 different solvents, we are able to identify two broad trends (Figure 17) [66]. For the first trend, the cinchonidine initially adsorbed at the CCR-Pt interface is not easily removed by the second solvent such as cyclohexane, n-pentane, n-hexane, carbon tetrachloride, carbon disulfide, toluene, benzene, ethyl ether, chlorobenzene, and formamide. For the second trend, the initially established adsorption-desorption equilibrium at the CCR-Pt interface is obviously perturbed by flushing the system with another solvent such as dichloromethane, ethyl acetate, methanol, ethanol, and acetic acid. These trends can already explain the above-mentioned observations made by catalysis researchers, in the sense that the perturbation of initially established adsorption-desorption equilibrium is related to the nature of the solvent. 

Figure 17. The effect of cyclohexane (A) and dichloromethane (B) solvents on the desorption of cinchonidine (abbreviated as CD) from platinum [66], In both cases, a clean platinum surface was first exposed to a cinchonidine solution in CC14 to allow for the adsorption of cinchonidine, and the platinum disk was then exposed to either cyclohexane or dichloromethane. In the case of cyclohexane, a total rinsing with 180 mL in several sequential flushings did not lead to significant change of the infrared spectra. On the other hand, with dichloromethane (B), one flush was sufficient to remove most of the adsorbate. [Reproduced by permission of the American Chemical Society from Ma, Z. Zaera, F. J. Phys. Chem. B 2005,109, 406-414.]...

Ethyl cellulose and cellulose triacetate have been shown to form hydrogen bonded associates with dichloromethane (10,30). If this is so, then cellulose triacetate-dichloromethane interaction will be favored over polystyrene-cellulose triacetate interaction and thus no adsorption should be expected. 

Because of the problems encountered with the water system, the use of aliphatic alcohols, ie.g., methanol, ethanol, and isopropanol, as modifiers of the adsorption strength has been recommended (44. 45. 50. 51). Usually, between 0.01 and 0.5% (v/v) alcohol is added to the eluent. As an example, the k values for the benzyl alcohols on a silica column are in the same range when eluted with dichloromethane containing either 0.1% water (50% water-saturated) or 0.15% methanol or 0.3% isopropanol (45). The preparation and preservation of these alcohol-eluent mixtures is accompanied by problems similar to those discussed with water-modified eluents. Also, column equilibration is slow (44). The efficiency of columns operated with alcohol-modified eluents is generally lower than that of water-modulated eluent system. At some alcohol concentrations, distorted peaks with tailing or frontal asymmetry have been observed 44), but olhei workers using another silica could not verify this observa tion (61). 

In starting a residue analysis in foods, the choice of proper vials for sample preparation is very important. Available vials are made of either glass or polymeric materials such as polyethylene, polypropylene, or polytetrafluoroethylene. The choice of the proper material depends strongly on the physicochemical properties of the analyte. For a number of compounds that have the tendency to irreversible adsorption onto glass surfaces, the polymer-based vials are obviously the best choice. However, the surface of the polymer-based vials may contain phthalates or plasticizers that can dissolve in certain solvents and may interfere with the identification of analytes. When using dichloromethane, for example, phthalates may be the reason for the appearance of a series of unexpected peaks in the mass spectra of the samples. Plasticizers, on the other hand, fluoresce and may interfere with the detection of fluorescence analytes. Thus, for handling of troublesome analytes, use of vials made of polytetrafluoroethylene is recommended. This material does not contain any plasticizers or organic acids, can withstand temperatures up to 500 K, and lacks active sites that could adsorb polar compounds on its surface. 

The fractions are prepared for the Ames assay by dilution with water, adsorption of the organics on a Cl8 cleanup cartridge, elution of organics with 5 mL of dichloromethane, and exchange into DMSO. 

A simple assay for TMP in bovine tissue and plasma was used. The tissue sample was homogenized with alkaline phosphate buffer and partitioned with dichloromethane. The TMP was reextracted with sulphuric acid. The aqueous layer was mixed with phosphate buffer, and it was further cleaned by SPE according to the previously published assays (174). Sulphadoxine, administered together with TMP, was assayed using TLC densitometry. These assays were used for the study of adsorption and depletion of TMP and SDX from the healthy animals. It was observed that both TMP and SDX were readily adsorbed onto the tissues, but TMP was eliminated much faster than SDX (175). 

A solution of substrate (0.20 mmol), pyrene-dimethyltinhydride (1.2 equiv.), and 2,2 -azobisisobutyronitrile (AIBN, 0.1 equiv.) in dry degassed benzene (1-2 ml) was stirred under reflux for 1 h [thin-layer chromatography (TLC) monitoring] under inert atmosphere. The mixture was cooled to room temperature and evaporated. Methanol/dichloromethane (3 2, 4 ml) was added followed by the addition of activated carbon (800 mg). The suspension was stirred for 30 min [the adsorption of the pyrene core was monitored by ultraviolet (UV)]. After filtration, the activated carbon was washed with methanol and the combined filtrates were concentrated to yield the product in pure form. 

For decades such adsorption had been assumed to involve dipole interactions and interacting sites were termed "polar." It is quite clear in the above studies that dipoles in the polymers and in the solid surfaces do not contribute measurably to adsorption. Even from carbon tetrachloride, the solvent most favorable to adsorption, the amount of basic polymer (PMMA) that adsorbed onto basic calcium carbonate was only 2.5% of the amount that adsorbed on the same area of silica surface. Similarly, the amount of acidic polymer (CPVC) that adsorbed onto the acidic silica from any of the six solvents was less than 0.2% of the amount that adsorbed from carbon tetrachloride or dichloromethane onto the same area of basic calcium carbonate. It is concluded that adsorption of organic acids or bases from neutral organic solvents onto inorganic solids is governed entirely by acid-base interactions and is quite independent of dipole phenomena. It is therefore proposed that heats of adsorption are actually enthalpies of acid-base interaction and should therefore be subject to the Drago correlation ... 

Carbon/silica adsorbents with pure or functionalized carbon deposits, or functionalized silica surfaces, are of interest for many purposes, An improvement of the structural and adsorption characteristics of carbon deposits is desirable.1 Pyrocarbon deposits formed by carbonization of low-molecular organic precursors (dichloromethane, cyclohexene, alcohols, acetylacetone, acenaphthene, etc) at oxide surfaces typically possess a low inner specific... 

Dinitrophenylhydrazones (DNPHs) were applied to the GC analysis of keto acids. As with carbonyl compounds, they are prepared by reaction with 2,4-dinitrophenylhydrazine and are also used mainly for the preliminary isolation of keto acids. They can be isolated from a dilute aqueous sample by adsorption on activated carbon and selective desorption [178] hydrazones of aldehydes with a methyl formate-dichloromethane mixture and hydrazones of keto acids with a pyridine-water azeotropic mixture. Hydrazones of acids are released from their pyridine salts with methanol containing hydrogen chloride. After... 

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