Adsorbates aromatics

Yoshida has studied anodic oxidations in methanol containing cyanide to elucidate the electrode processes themselves.288 He finds that, under controlled potential ( 1.2 V), 2,5-dimethylfuran gives a methoxynitrile as well as a dimethoxy compound (Scheme 57). Cyanide competes for the primary cation radical but not for the secondary cations so that the product always contains at least one methoxy group. On a platinum electrode the cis-trans ratio in the methoxynitrile fraction is affected by the substrate concentration and by the addition of aromatic substances suggesting that adsorption on the electrode helps determine the stereochemistry. On a vitreous carbon electrode, which does not strongly adsorb aromatic species, the ratio always approaches the equilibrium value. 

Both mechanisms, termed the associative and dissociative v complex substitution mechanisms, proceed via n complex adsorbed aromatics (e.g., benzene) which occupy a position on the catalyst so that the plane of the ring is parallel to the catalyst surface [Eq. (7)] ... 

In the dissociative mechanism, the tt complex adsorbed aromatic reacts with a metal radical (active site) by a substitution process. During this reaction [Eq. (9)] the molecule rotates through 90°, and changes from its horizontally tt complex adsorbed position to a vertically cr-bonded chemisorbed state ... 

Under dehydrogenation conditions (385 °C ratio H2/HC = 4), an increase in the selectivity for aromatics with PtSn,(/Si02 catalyst has been observed. The increase in aromatic selectivity with tin content seems to be due to a geometric effect, favoring aromatic desorption. When the catalyst contains only small amounts of tin, an important poisoning by coke has been observed. As a consequence, it is possible that coke comes from adsorbed aromatic degradation. If aromatic formation starting from olefins had already and previously been proposed in the literature, their formation mechanism was still unknown. The coexistence of two possible dehydrocycHzation mechanisms has been proposed (Scheme 3.24). 

C. General Comments on the Spectra of Adsorbed Aromatic Molecules... 

Silica gel is an adsorbent for aromatics and has found use in extracting aromatics from refinery streams. Silica gel is manufactured amorphous silica that is extremely porous and has the property of selectively removing and holding certain chemical compounds from mixtures. For example, silica gel selectively removes aromatics from a petroleum fraction, and after the nonaromatic portion of the fraction is drained from the silica gel, the adsorbed aromatics are washed from the silica gel by a stripper (or desorbent). Depending on the kind of feedstock, xylene, kerosene, or pentane may be used as the desorbent. 

As has already been illustrated in Sect. 2.1, carbons with their graphitic surface structure often adsorb aromatic compounds well and thereby affect their reaction rates. In order to test what influence carbons would have on the solvolysis of an aromatic ester, Spiro and Mills [172] carried out exploratory experiments on the alkaline hydrolysis of benzyl acetate at 25°C... 

An immediate issue in the chemisorption of aromatic compounds is whether the phenyl ring is oriented parallel or perpendicular to the plane of the metal surface. In this regard, the intensity of the out-of-plane C-H bend (yen) relative to the in-plane C-H stretch (vch) provides a direct diagnostic indicator of adsorbed aromatic orientation. At specular... 

Non strippable additive coke, being adsorbed aromatics and polars (AC). 

The absence of a v(O-H) peak indicates either the absence of a phenolic O-H functional gronp and/or the imposition of a rigidly flat (t ) adsorbed-aromatic orientation in the latter configiuation. 

L.G., and Johansson, G. (1981) Catalytic-oxidation of reduced nicotonamide adenine-dinucleotide by graphite electrodes modified with adsorbed aromatics containing catechol functionalities. Analytical Chemistry, 53 (13). 1979-1982. 

Traces of octalins were always present during the hydrogenation of tetralin or naphthalene (3). Observed concentrations of octalins are listed in Table IV. The amounts were particularly small with palladium catalysts. One of the unique characteristics of palladium is its ability to adsorb and saturate olefins in the presence of aromatics or, conversely, its relative inability to adsorb and saturate aromatics in the presence of olefins. By way of contrast, some other metals, particularly ruthenium and rhodium, are more able to adsorb and saturate aromatics in the presence of olefins. Whatever the nature of the adsorbed state of naphthalene that leads to hydrogenation, one could imagine the possibility of two isomeric forms—one of which behaved more like an adsorbed olefin and the other more like an adsorbed aromatic ... 

The successive transfer of hydrogen atoms from the metal surface to the adsorbed aromatic ring and subsequently adsorbed intermediates, i. e. cyclohexa-diene and cyclohexene, finally results in the fully saturated ring system [7]. Although cyclohexene formation has been observed (cf. Section 8.6.3), no cyclohex-adienes have yet been detected as intermediates [8]. Several explanations have been proposed to explain why formation of a diene on the metal is by-passed,... 

The intracrystalline diffusivities of llZSM-5 zeolite were directly measured for several hydrocarbons at higher temperatures (373-773 K) by the constant volume method. High silicious HZSM-5 zeolite, which has no activity for reactions, was used as the adsorbent. Aromatics benzene, xylene-isomers ortho-, meta- and para-xylene) and toluene, and paraffins n-hexane, n-pentane, p-octane and iso-octane, were used as adsorbates. Intracrystalline diffusivities of aromatics markedly depended on the minimum size of the aromatics and that of paraffins depended on the carbon number (molecular weight of the paraffins). A method was developed for predicting diffusivity in terras of pore diameter and molecular properties of hydrocarbons. This method was found to well represent the experimental results. 

The configuration and value of an adsorbed aliphatic molecule or of an alkyl-substituted aromatic is more difficult to specify relative to the configuration of an adsorbed aromatic molecule. The flexibility of aliphatic molecules or of alkyl groups attached to aromatic rings permits a variety of possible configurations, including partial desorption of alkyl groups. Comparison of calculated and experimental values for alkyl... 

Recently the SERS spectra of on silver hydrosols adsorbed aromatic amino acids have also been studied The Raman spectra are enhanced from 1(K) to 2(K) times in the presence of silver colloids with primary sol particles 14 nm in size. The hydrosol-Phe interaction shows the strongest SERS spectrum. The frequency shifts between the NSRS- and SERS spectra are small (1-15 cm" ). The following SERS bands are characteristic for the aromatic amino acids Phe, 1005, 1034 and 1049cm" Trp, 762,1340 and 1377 cm - Tyr, 831,988,1169 and 1299 cm - His, 670 and 1310 cm -. ... 

While little use has been made of Raman spectroscopy, somewhat more has been done with Surface-Enhanced Raman Spectroscopy (SERS)," especially in the study of adsorbed aromatic hydrocarbons. ... 

In contrast to GC-MS, a wide range of tropane alkaloids, including A -oxides and calystegines, can be analyzed by HPLC without prior derivatization. Enantiomeric separation of atropine 5 and scopolamine 6 racemic mixtures has been also achieved [22]. A number of HPLC methods have been published for routine quantification of the major tropane alkaloids hyoscyamine 1 and scopolamine 6 in plant samples. The use of UV detectors however is limited to compounds with UV adsorbing (aromatic or other) functionality. This disadvantage, however, is easily overcome by using HPLC coupled with MS detectors [22]. A typical HPLC separation of hyoscyamine 1 and scopolamine 6 is presented on Fig. 7.2. 

Strongly adsorbed aromatic compounds, which cannot be desorbed even at high temperatures, probably involve an electron-donor-acceptor or charge-transfer mechanism. It has been shown that the surface of a carbon is more effectively used if non-electrostatic interactions are the driving force for the adsorption. This condition can be reached by controlling the surface chemistry of carbon, the pH and ionic strength of the solution. 

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