Dissociation thiophene

A typical SSIMS spectrum of an organic molecule adsorbed on a surface is that of thiophene on ruthenium at 95 K, shown in Eig. 3.14 (from the study of Cocco and Tatarchuk [3.28]). Exposure was 0.5 Langmuir only (i.e. 5 x 10 torr s = 37 Pa s), and the principal positive ion peaks are those from ruthenium, consisting of a series of seven isotopic peaks around 102 amu. Ruthenium-thiophene complex fragments are, however, found at ca. 186 and 160 amu each has the same complicated isotopic pattern, indicating that interaction between the metal and the thiophene occurred even at 95 K. In addition, thiophene and protonated thiophene peaks are observed at 84 and 85 amu, respectively, with the implication that no dissociation of the thiophene had occurred. The smaller masses are those of hydrocarbon fragments of different chain length. 

Next we will adopt a kinetic scheme and see if it describes the data of Fig. 7.16. Several treatments of HDS kinetics are available in the literature. Here we use a simplified scheme in which thiophene (T) exclusively adsorbs on sulfur vacancies, denoted by A, and H2 adsorbs dissociatively on all the sites (indicated by ) to form butadiene (B) and H2S in a rate-determining surface reaction (we ignore the kineti-cally insignificant hydrogenation steps of butadiene) ... 

Thiophene is the typical model compound, which has been extensively studied for typifying gasoline HDS. Although, some results are not completely understood, a reaction network has been proposed by Van Parijs and Froment, to explain their own results, which were obtained in a comprehensive set of conditions. In this network, thiophene is hydrodesulfurized to give a mixture of -butenes, followed by further hydrogenation to butane. On the considered reaction conditions, tetrahydrothiophene and butadiene were not observed [43], The consistency between the functional forms of the rate equations for the HDS of benzothiophene and thiophene, based on the dissociative adsorption of hydrogen, were identical [43,44], suggesting equivalent mechanisms. 

The main dilference between thiophene and selenophene is that on flash photolysis in the former SH and not S2, while in the later Se2 and not SeH were detected in the photodecomposition products. This suggests that in selenophene the dissociation takes place at the Se site and that Se2 is formed as a result of the recombination of the Se atoms.24... 

The experiment was performed with a quench-and-look approach, whereby the model system was first heated to 673 K and exposed to low amounts of atomic hydrogen produced by dissociating H2 on a glowing tungsten filament. The flux of hydrogen was then terminated, and the clusters were exposed to thiophene at an elevated sample temperature of about 500 K. Subsequently, the model system was... 

Thermolysis of the ylide (15) in thiophene results in ready rearrangement to dimethyl thiophene-2-malonate (16). The same product is obtained if the thermolysis is carried out in the presence of 2-methylthiophene or cyclohexene, proving that the rearrangement occurs by an intramolecular process (78CC85). However, when 2,5-disubstituted thiophenium ylides are thermolyzed, dissociation to carbenoid species seems to occur. This reaction is further discussed in Section 3.14.2.9. 

The equilibrium constant for the dissociation of the cyanohydrin of 5-acetyl-2,3-dihydrobenzo[6]thiophene has been determined.435... 

In contrast to the adducts of many dienes with maleic anhydride, the above adducts are characteristically stable to heat some do not dissociate at temperatures as high as 300°. This property is apparently associated with the sulfone group, since reduction of the anthracene adduct (346) with lithium aluminum hydride affords the corresponding cyclic sulfide, which readily dissociates at 250° to give anthracene and benzo[6]thiophene.726... 

The oxidative polymerization has been proposed to proceed via a radical coupling that involves the coupling of neutral radicals or cation radicals. The former case corresponds to the oxidative polymerization of phenols and dithiols in which the neutral radical is formed by one-electron transfer after dissociation of a hydron from the monomer, or by the elimination of a hydron after the oxidation. The latter case takes place when the cation radical formed by one-electron oxidation exists as a stable species. The cation radicals then couple with each other, and the dimer is formed through solvent-catalyzed hydron elimination from the intermediate dication. Oxidative polymerization of pyrrole and thiophene uses this mechanism [57-62]. 

Similar polymer/Au nanoparticle multilayer thin films were made by Wu et al. in a study of pH-sensitive dissociation behavior of poly (3-thiophene acetic acid) (PTA A) and PAA in a LbL film (of 8 bilayers).50 Unlike the pure polymer LbL film, the Au nanoparticles-containing LbL films were difficult to be released from the substrate by varying the pH. It was suggested that the gold particles act as a cross-linker in between the multilayers, thus further enhancing the stability of the LbL films. 

More recently, De Plaen (60) studied the adsorption of thiophene in liquid phase on various metals of Group VIII (Ni, Rh, and Pt) under hydrogen pressure. It was observed that butane and sulfur are the only products of the dissociative adsorption, which can be written as follows ... 

Several catalysts with various degrees of palladium loading and metal dispersions were submitted to thiophene adsorption. The dissociatively... 

Figure 5 presents the formation of butane obtained on dispersed palladium catalysts from the adsorption of several sulfur compounds thiophene, thiophane, dibutylsulfide, butanethiol, dibutyldisulfide. Twogroups can be observed (1) thiols and disulfides that undergo a rather low-level dissociation in butane and (2) thiophene, thiophane, and dibutylsulfide, which are dissociated to a much larger extent. The dissociation of dibutylsulfide produces a quantity of butane twice that produced from thiophene and thiophane, indicating that the two bonds (sulfur — butyl radical) are broken altogether. 

From observation of the first parts of the butane formation curves (initial time), it appears that the dissociation rate of thiophene is the highest one. 

Figure 5 shows that the dissociative chemisorption of butanethiol is much lower than that of thiophene. But these results were obtained at a... 

Verna (62) studied the dissociative adsorption of thiophene on platinum, palladium, and rhodium dispersed on alumina. Figure 7 and Table II present the dissociative chemisorption of thiophene to butane on the three metals. The sulfur coverage of platinum is very low compared to the other metals. The sulfur coverage on palladium is about 2.5 times higher than on platinum such a value is similar to the one found by Mathieu and Primet (63). 

Fig. 7. Formation of butane as a function of time, during the dissociative chemisorption of thiophene on different metals.

Boitiaux et al. and Verna (61, 62) studied the dissociative chemisorption of thiophene in the presence of isoprene and of the corresponding methyl-butenes on palladium. Figure 8 records the results of a typical experiment carried out with a mixture of thiophene and 3 mol% isoprene in heptane. The evolution of isoprene, isopentenes, and isopentane is shown as a... 

Analogous experimental conditions (i.e. Cl, 0.1-1 Torr) allowed for the detection by tandem mass spectrometry of the collision complexes formed in the ion-molecule reactions of several aromatic radical cations M+ (M = C6H5X, X= Me, N02, Cl pyridine, furan, thiophene) and neutral iodoalkanes RI (R= n-Pr, 2-Pr, n-Bu, 2-Bu, etc.) en route to areni-um ions34,35. The collision complexes are covalently bonded species, namely nonisomeriz-ing iodonium radical positive ions 4 which dissociate to arenium ions 5 via reductive elimination of I (Scheme 7)34. 

Carbon disulfide At room temperature, only physisorption occurs on Pt film [2], while partly reversible adsorption takes place on Pd film [31 with 0.7 chemisorbed coverage. The adsorption is believed to be dissociative over this latter surface [3]. Hydrogen can be adsorbed over CS2-covered Pt film [2] but cannot over CS2-covered Pd film [3]. Thiophene At room temperature, chemisorption occurs through the lone pair of sulfur... 

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