Adsorbates amines

An alternative approach to increase the oxidation rate is the use of alkaline solutions, because bases enhance the reactivity of L-sorbose and weaken the adsorption strength of 2-KLG. Unfortunately, the rate enhancement at higher pH is accompanied by a drop in selectivity due to the poor stability of 2-KLG in alkaline solutions. To circumvent this problem, we have modified the platinum catalysts by adsorbed tertiary amines and carried out the oxidation in neutral aqueous solution [57], This allowed to enhance the rate without increasing the pH of the bulk liquid, which leads to detrimental product decomposition. Small quantities of amines (molar ratio of amine sorbose = 1 1700, and amine Pts = 0.1) are sufficient for modification. Using amines of pKa a 10 for modification, resulted in a considerable rate enhancement (up to a factor of 4.6) with only a moderate loss of selectivity to 2-KLG. The rate enhancement caused by the adsorbed amines is mainly determined by their basicity (pKa). In contrast, the selectivity of the oxidation was found to depend strongly on the structure of the amine. 

Changes of pH are about 3 pH units to the alkaline side for adsorbed acids and to the acidic side for adsorbed amines have been reported [17]. 

CH bonds a to an amino group Anodic methoxylation of unsymmetrical tertiary amines takes place preferentially at the methyl group (Fig. 6) [28-30]. Substitution at a CH bond a to an amino group proceeds by oxidation of the amino group to a radical cation, followed by deprotonation at the adjacent CH bond to a radical. This is oxidized to a cation, which undergoes solvolysis, in this case, methanolysis. The regioselectivity has been explained by assuming that an adsorbed amine from which the intermediate cation is formed is as distant as possible from the anode... 

The corrosion process can be inhibited by the addition of phosphate or polyphosphate ions [344], inorganic inhibitors as, for example, chromate ions [336], adsorbed alcohols [345], adsorbed amines, competing with anions for adsorption sites [339,] as well as saturated linear aliphatic mono-carboxylate anions, CH3(CH2)n-2COO , n = 7 — 11, [24]. In the latter case, the formation of the passive layer requires Pb oxidation to Pb + by dissolved oxygen and then precipitation of hardly soluble lead carboxylate on the metal surface. The corrosion protection can also be related to the hydrophobic character of carboxylate anions, which reduce the wetting of the metal surface. 

A,2V-dimethylaniline was adsorbed under vacuum at 20° C after pretreating the samples in vacuum (p = 2.10 6 torr) for 15 hours at 100°, 200°, 350°, 450°, 550°, 650°, and 750°C. Precautions were taken to prevent the adsorption of water impurities with amine vapors. Concentration of adsorbed amine amounted to about 15 molecules per 10 unit cells of the zeolites. 

Differential spectra of diffused reflection around 230-700 nm were measured with a two-beam recording spectrophotometer with corrected zero line it was constructed in our laboratory to measure the spectra of adsorbed species. Spectra were recorded before and after irradiation with a PRK-2 mercury lamp for 15 minutes of the zeolite sample containing adsorbed amine. 

The evaluation of the acid properties of calcined materials was based on the assumption that n-butylamine molecules interact with all acid sites, and the total acidity of the sample studied can be determined from the maximum amount adsorbed. Shown in Fig. 2 are thermogravimetric curves for n-butylamine thermodesorption, which were used to evaluate the amount of medium and strong acid sites for both samples. Thermodesorption of n-butylamine from CeMCM-41 exhibits three distinct ranges (i) desorption of physically adsorbed amine bellow 230 °C (ii) desorption of n-butylamine from medium acid sites at 230 - 410 °C (0.25 mmol/g), and (iii) its desorption from strong acid sites at 410 - 590 °C (0.21 mmol/g). However, only one weight loss was observed for pure silica MCM-41 due to thermodesorption of physically adsorbed amine, indicating negligible acidity of this material. 

Silica, alumina, silica-alumina 13C-NMR spectra of adsorbed amines 75b... 

Although a variety of amines, particularly trimethylamine and n-butylamine have widely been used as poisons in catalytic reactions and for surface acidity determinations (20), comparably few spectroscopic data of adsorbed amines are available. As with ammonia, coordinatively adsorbed amines held by co-ordinatively unsaturated cations have preferentially been found on pure oxides (176, 193-196), whereas the protonated species were additionally observed on the surfaces of silica-aluminas and zeolites (196-199). However, protonated species have also been detected on n-butylamine adsorption on alumina (196) and trimethylamine adsorption on anatase (176) due to the high basicity of these aliphatic amines. In addition, there is some evidence for dissociative adsorption of n-butylamine (196) and trimethylamine (221) on silica-alumina. Some amines undergo chemical transformations at higher temperatures (195, 200) and aromatic amines, such as diphenylamine, have been shown to produce cation radicals on silica-alumina (201, 201a). 

The films generated by the triethoxysilane-(n)propylamine solutions (at adsorbate/amine mole ratios of 4 or greater) differed in a number of ways from the films formed by any of the other adsorbate solutions ... 

DTA/TGA/DTG of the as-synthesized SAPO-31 (fig-2) indicates multiple step weight loss. The first weight loss (endothermic at 377 K) is due to the desorption of water and of dipropylamine. The second endothermic peak at 503 K represents the loss of the chemically adsorbed amines on weaker (Lewis) acid sites. The two remaining peaks(endothermic at 673... 

The use of various metal salts such as FeCla, ZnCla, HgBr , CoClj and Znij as coatings for the detection of mono-, di- and trimethylamines was investigated (14). The coated crystal was attached to a vacuum line, and its frequency was measured after the system was evacuated to 10 torr. The gas trapped in a 1-liter glass storage bottle at 200 torr was then allowed to react with the coating, and the reaction was evaluated at room temperature with respect to time and sensitivity. It was found that FeCla adsorbs amines 2-4 times more than the other salts, and adsorption was greatest for trimethylamine. The reported sensitivity is 369 Hz... 

It is not always easy to find direct correlations between the heats of adsorption and the catalytic behaviour (activity and/or selectivity). Various attempts at establishing such correlations have been reported in the literature. For instance, a comparison of microcalorimetric measurements with kinetic studies performed over acidic zeolites for methylamine disproportionation reactions, methanol dehydration, and reactions of methanol and dimethylether with methylamines, suggests that acid sites are required in these reactions for the strong adsorption of ammonia and methylamines, while weak adsorption sites are required to facilitate desorption of adsorbed amine species from the acid sites [147]. 

Anionic sulfonated polyacrylamide (PAMS) is also found to increase amine flotation of quartz. Although PAMS does not adsorb on the negatively charged quartz and cause no direct activation of amine adsorption, the polymer-surfactant electrostatic interaction can lead to the formation of complexes. This polymer-surfactant complex can reduce the armoring of bubbles and lead to flotation. The anionic polymer can also bridge the adsorbed amine to the amine on the bubble surface and enhance flotation under saturated adsorption conditions. The hydration effect of the polymer may also be responsible for the enhanced flotation in this case. 

A study by Comyn et al. [8] indicated that low (or no) cure took place in the interphase between an amine cured epoxy and aluminum because the amine was preferentially adsorbed onto the aluminum oxide on the aluminum. Garton et al. [9] showed that the acidic surface of a carbon fiber selectively adsorbed amine and catalyzed the reaction between the amine and an epoxy resin. Nigro and Ishida [10] found that homopolymerization of epoxy resin was catalyzed by a steel surface. Zukas et al. [11] discovered, in a model system of an amine cured epoxy resin and an activated aluminum oxide, a change in the relative rates of the reactions leading to crosslinking of the epoxy, so that the material in the interphase was structurally different from that in the bulk. 

Preservation of Lewis acidity through hydrolysis and formation of the final solid was confirmed by both C MAS NMR and FTIR of adsorbed amines. In C NMR, the chemical shifts of the a- and P-carbon peaks of the bound piperidine in... 

Broennimann, C., Bodnar, Z., Aeschimann, R., et al. (1996). Platinum Catalysts Modified by Adsorbed Amines A New Method of Enhancing Rate and Selectivity of L-sorbose Oxidation, J. Catal, 161, pp. 720-729. 

Cyclohexylamine, pyridine, and triethyl-amine were studied on mild steel in 0.5 M H2SO4 by electrochemical impedance spectroscopy and XPS. Comparison of N (Is) spectra between active and passive regions showed that adsorption is significant only in the passive region. Two chemically distinct adsorbed amine species were detected in the experiment. The nature of the metal surface can be determinative of the performance of the inhibitors. 

---