Chemisorption extraction

Adsorption and Desorption Adsorbents may be used to recover solutes from supercritical fluid extracts for example, activated carbon and polymeric sorbents may be used to recover caffeine from CO9. This approach may be used to improve the selectivity of a supercritical fluid extraction process. SCF extraction may be used to regenerate adsorbents such as activated carbon and to remove contaminants from soil. In many cases the chemisorption is sufficiently strong that regeneration with CO9 is limited, even if the pure solute is quite soluble in CO9. In some cases a cosolvent can be added to the SCF to displace the sorbate from the sorbent. Another approach is to use water at elevated or even supercritical temperatures to facilitate desorption. Many of the principles for desorption are also relevant to extraction of substances from other substrates such as natural products and polymers. 

For the charcoal, XAD, and PUF adsorbents discussed above, solvent extraction techniques have been developed for the removal and concentration of trapped analytes. Although thermal desorption has been used with Tenax-GC in some specialized air sampling situations [primarily with sampling volatile organic compounds (EPA, Method TO-17 )], this approach is not a viable alternative to solvent extraction for the charcoal, XAD, and PUF adsorbents. The polystyrene and PUF adsorbents are thermally unstable and the charcoal chemisorption bonding is more easily broken by... 

Arsenate is readily adsorbed to Fe, Mn and Al hydrous oxides similarly to phosphorus. Arsenate adsorption is primarily chemisorption onto positively charged oxides. Sorption decreases with increasing pH. Phosphate competes with arsenate sorption, while Cl, N03 and S04 do not significantly suppress arsenate sorption. Hydroxide is the most effective extractant for desorption of As species (arsenate) from oxide (goethite and amorphous Fe oxide) surfaces, while 0.5 M P04 is an extractant for arsenite desorption at low pH (Jackson and Miller, 2000). 

Chemisorption involving covalent bonds as well as bound residue formation is also excluded, which is defined as any organic carbon remaining after exhaustive extraction that results from degradation of parent molecules. 

Extraction process, 26 381 Extractive chemisorption, 37 80 Extra-framework titanium, 41 293 Extrinsic field effect, 27 26-48 catalysts for, 27 49 theory, 27 50, 51 Extrusion, 28 83... 

Tannins are polyphenols that occur only in vascular plants such as leaves, needles, barks heartwood, seeds and flowers. Tannins exist primarily in condensed and hydrolysable forms. Natural tannin extracts have been employed since the turn of the last century in leather industry. Recent development of their industrial uses as adhesives, flocculants, depressants, viscosity modifier agents and more recently as corrosion inhibitors reflects their importance as industrial raw materials. Electrochemical studies have shown that tannins extracted from the barks of mangrove trees are excellent corrosion inhibitors of steel at very low pH. The mechanism of inhibition at this pH was due to the chemisorption of tannin molecules while at higher pH, inhibition was achieved via formation of ferric-tannates [6]. 

Hirschler and Hudson (36/6), however, favor the opinion that Bronsted sites are exclusively responsible for the activity of silica-alumina. In studying the adsorption of perylene and of triphenylmethane, they concluded that carbonium ions are not formed by a hydride abstraction mechanism as claimed by Leftin (362). Instead, triphenylmethane is oxidized by chemisorbed oxygen to triphenylcarbinol in a photo-catalyzed reaction, followed by reaction with a Bronsted acid giving water and a triphenylmethyl carbonium ion. After treatment with anhydrous ammonia, the organic compound was recovered by extraction as triphenylcarbinol. About thirteen molecules of ammonia per assumed Lewis site were required to poison the chemisorption of trityl ions. The authors explain the selective inhibition of certain catalyzed reactions by alkali by assuming that only certain of the acidic protons will ion-exchange with alkali ions. 

To study the promotion mechanism of Pt wire/FSM-16 in the PROX reaction, the Pt nanowires were extracted by HF/EtOH treatment from FSM-16, and the wires were again deposited on the external surface of FSM-16 from the ethanol solution. We found that the resulting external Pt wire/FSM-16 catalyst gave low TOFs (>35) and lower CO selectivity (>30%) in the PROX reaction [32]. This implies that the encapsulation of Pt wires in the silica channels of FSM-16 is a key to promote the selective CO oxidation in the PROX reaction. Furthermore, from the structural characterization by XANES, XPS and IR in CO chemisorption... 

More recently (8), another series of H-mordenites, acid-extracted to a greater degree, was examined. For these samples after drying at 110°C, the major results were (a) there was no evidence of hydroxyl nests stable above 100°C, and (b) NH3 chemisorption at 250°C and 11 torr roughly corresponded to a stoichiometric ratio (1 1, 25%) with the total amount of aluminum remaining in the lattice. 

The discussion will be confined to pure NiO, since most work has been done with this catalyst. It appears from the 180 extraction results that at low pressures the primary act of chemisorption of N20 determines the number of active sites for each reaction, since x increases roughly as P o between 1 and 4.2 cm. (Table IV). At very low Pn2o, decomposition still takes place, although no appreciable pressure change occurs N2 appears in the gas phase and the 02 is retained by the catalyst. Again, simultaneous measurements (32) of the rate of the reaction and of the change of semiconductivity of a compressed pellet of the catalyst show that immediately on admission of N20 the resistance of the pellet begins to fall there is no trace of a brief initial increase in resistance such as would be expected if... 

There is a relationship between yield of the extract and the saturation sorption or imbibition of solvent that is independent of the rank coal or the particular amine solvent (Dryden, 1951). An adsorption isotherm for ethylenediamine vapor on an 82% carbon coal exhibited three main features (1) chemisorption up to 3 to 6% adsorbed, (2) a fairly normal sorption isotherm from the completion of chemisorption up to a relative pressure of at least 0.8, and (3) a steeply rising indefinite region near saturation that corresponded to observable dissolution of the coal. 

At sufficiently high temperatures, due to not too strong cohesion, the surface Pd atoms may acquire convenient positions to form a bond with reacting hydrocarbon molecule (189). This concept, called extractive chemisorption, was introduced by Burwell et al. (190, 191) as a possible cause of absence of steric hindrance in adsorption and reaction of some complex organic molecules. It was proposed that in chemisorption one or two metal atoms were displaced above the initial planar level, leading to increased bonding to the surface for low-dispersion catalysts. An extension of this concept to the problem of structure sensitivity allows one to explain several cases of the relatively mild (or absent) structure sensitivity in many reactions catalyzed by Pd catalysts. 

Radio frequency heating, 500 Steam stripping, 500 Vacuum extraction, 500 Aeration, 501 Bioremediation, 501 Soil flushing/washing, 502 Surfactant enhancements, 502 Cosolvents, 502 Electrokinetics, 503 Hydraulic and pneumatic fracturing, 503 Treatment walls, 505 Supercritical Water Oxidation, 507 Solid Solution Theory, 202 Solubility products, 48-53 Metal carbonates, 433-434 Metal hydroxides, 429-433 Metal sulfides, 437 Sorption, 167 See Adsorption Specific adsorption, 167 See Chemisorption Stem Layer, 152-154 Sulfate, 261... 

Due to the bonding selectivity of chemisorption, SAM of DNA can be prepared without significant amounts of impurities. Furthermore, molecular orientation within the layer is fairly well defined [19]. Owing to these characteristics, SAM films of DNA have been particularly useful in the determination of absolute yields and cross sections for specific damages. When extracting attenuation lengths (AL) or cross sections from electron-scattering experiments on thin molecular films, by far the most difficult parameter to determine and control, is the film thickness and its... 

In contrast to gas-phase chemistry, aqueous chemistry is performed batch-wise, and it is necessary to repeat the same experiment many times with a cycle of about 1 min, in order to obtain statistically significant results. The experiments are carried out either manually (liquid/liquid extraction or chemisorption) or with an auto-... 

Adsorptive purification of the type we are talking about here (sometimes called physisorptiori) involves a relatively weak interaction between the sorbate molecule and the surface active sites in the sorbent. The association depends mostly on van der Waals forces, which are primarily caused by electronic and/or electrostatic interactions between electron-rich or electron-poor regions of the sorbate molecule and receptive sites on the sorbent surface. Molecules bound by such weak forces can be removed by various solvent extraction techniques in contrast, molecules that actually form covalent bonds with the surface (chemisorption) usually cannot. 

The corresponding rules are valid both for classical (chemical) and electrochemical promotion and are intimately related to the previously discussed chemisorption promotional rules. Their extraction became possible due to easy continuous and systematic experimental O variation, which electrochemical promotion offers, but we present them in summary here since, as already noted, they are valid both for classical and for electrochemical promotion [14,129,130]. 

Mineral or particle surfaces are enriched with As due to several processes that are collectively referred to as sorption (Parks, 1990), but the chemical properties of surface-associated As have been difficult to study directly. Outer-sphere, or physisorption, describes weak, long range, attractive forces between the surface and sorbing As inner-sphere, or chemisorption, refers to the formation of chemical bonds between the surface and adsorbing As. Stronger adsorption is expected by the formation of a bidentate (two bond) adsorbed complex rather than a monodentate (1 bond) complex. Selective chemical extraction methods have been useful for empirical determination of the dominant chemical/mineralogical compartments retaining As in aquifer... 

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