Thiophene, model compounds

The utility of sulfur K-edge X-ray absorption spectroscopy for the determination and quantification of sulfur forms in nonvolatile hydrocarbons has been investigated. X-ray Absorption Near Edge Structure (XANES) spectra were obtained for a selected group of model compounds, for several petroleum asphaltene samples and for Rasa coal. For the model compounds the sulfur XANES was found to vary widely from compound to compound, and to provide a fingerprint for the form of sulfur involved. The use of third derivatives of the spectra enabled discrimination of mixtures of sulfide and thiophenic model compounds, and allowed approximate quantification of the amount of each component in the mixtures, in the asphaltene samples and the coal. These results represent the first demonstration that nonvolatile sulfide and thiophenic sulfur forms can be distinguished and approximately quantified by direct measurement. 

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. 

Several attempts have been made to investigate anaerobic pathways for desulfurization. Mixed cultures of sulfate-reducing bacteria were used to desulfurize model compounds, thiophenes [12], organosulfides [12,13], and petroleum preparations [14-16], Production of hydrogen sulfide and biphenyl from DBT has been demonstrated in other studies [17-22],... 

Ti-Beta catalyst was synthesized according to reference [8], Thiophene and model compound oxidation were carried out at 8O0C in a block heater with magnetic stirring (500 rpm) in multiple high pressure tubes. Individual samples were prepared of 0.1 mol 30 % H202 solution, 0.02 mol thiophene, 25 g heptane and 100 mg Ti-Beta catalyst were added at room temperature,, and heated to 80°C. The same oxidation conditions were also investigated without heptane solvent. The reaction was stopped at different... 

The oxidation of thiophene and its derivatives with H202 was studied using a Ti-Beta molecular sieve. The oxidation product is very dependent from the aromaticity of model compounds. The thiophene oxidation product was mostly sulfates and the benzothiophene oxidation product was benzothiophene sulfone. Oxidation of mono and di-alkyl thiophenes also produced sulfates and sulfones. The diffusivity and aromaticity of the relevant sulfur compounds, intermediates and stable product, as well as the proposed new mechanism of oxidation will be discussed. This proposed new reaction pathway is different from current literature, which reports the formation of sulfones as a stable oxidation product. 

NMR studies on 28 and 29 indicate that both the thiophene and furan rings rotate freely at room temperature (vide infra) and therefore, anomalies in the UV spectra of 28 and 29 should be attributed to the through-bond interaction between the Si-Si cr bonds and aromatic 77 bonds. This was further confirmed by photoelectron spectral studies. As shown in Table II, the lift of HOMO for 12 relative to the model compound was 0.4 eV, but those for 28 and 29 were 0.7 and 0.6 eV, respectively. Apparently, more effective through-bond interaction occurs for 28 and 29 (21). 

Catalysts are heterogeneous sulfided nickel (or cobalt) molybdenum compounds on a y-alumina. The reaction has been extensively studied with substrates such as thiophene (Figure 2.40) as the model compound mainly with the aims of improving the catalyst performance. The mechanism on the molecular level has not been established. In recent years the reaction has also attracted the interest of organometallic chemists who have tried to contribute to the mechanism by studying the reactions of organometallic complexes with thiophene [41], Many possible co-ordination modes for thiophene have been described. 

A Sulfur K Edge X-ray Absorption Near Edge Structure (XANES) Spectroscopy method has been developed for the direct determination and quantification of the forms of organically bound sulfur in nonvolatile petroleum and coal samples. XANES spectra were taken of a number of model compounds, mixtures of model compounds, heavy petroleum and coal samples. Analysis of the third derivatives of these spectra allowed approximate quantification of the sulfidic and thiophenic components of the model mixtures and of heavy petroleum and coal samples. These results are compared with those obtained by X-ray Photoelectron Spectroscopy (XPS). 

There are several potential sources of error. Both methods of analysis use a binary model mixture, composed of sulfidic and thiophenic components. Thickness effects in the XANES of these model systems would alter the calibrations. There may be contributions from species not adequately represented by a simple dibenzothiophene-dibenzylsulfide model. While the XPS data are represented by 163.3 eV and 164.1 eV components, the model compound data base is as yet limited and not sufficient for a definitive interpretation in terms of alkyl sulfide and thiophenic forms. Examination by both XPS and XANES of a wider variety of model compounds and multiple component model compound mixtures will better define the sulfur species represented by these quantification methods. 

This work has demonstrated that organically bound sulfur forms can be distinguished and in some manner quantified directly in model compound mixtures, and in petroleum and coal. The use of third derivatives of the XANES spectra was the critical factor in allowing this analysis. The tentative quantitative identifications of sulfur forms appear to be consistent with the chemical behavior of the petroleum and coal samples. XANES and XPS analyses of the same samples show the same trends in relative levels of sulfide and thiophenic forms, but with significant numerical differences. This reflects the fact that use of both XPS and XANES methods for quantitative determinations of sulfur forms are in an early development stage. Work is currently in progress to resolve issues of thickness effects for XANES spectra and to define the possible interferences from pyritic sulfur in both approaches. In addition these techniques are being extended to other nonvolatile and solid hydrocarbon materials. 

Unfortunately, thiophene has been the prime indicator of HDS activity for the majority of the reported correlations between catalytic activity and catalyst structure. This is not an appropriate model compound for determining how many active hydrogenation sites are present in the catalyst... 

This statement could well be expanded to include studies describing the kinetics of model compounds. In reviewing the literature, one finds that there are almost as many kinetic representations as there are researchers and/or model compounds. Even the same authors have found it necessary to use different equations to describe the different responses to inhibitors for closely related sulfur species such as thiophene, benzothiophene, and dibenzothiophene (104, 122, 123). The inhibiting effect of H2S for the hydrogenation of a simple molecule, such as toluene, has been found to require extremely complex equations to adequately describe mathemati-... 

Correlation of Stanton, Jure, and Hicks Stanton et al. [19] have developed a combined model and separate models for furanes, tetrahydrofuranes (THFs), and thiophenes. Model development has been based on descriptor analysis with 209 training set compounds. A variety of different structural descriptors has been employed. A fit error of 4.9% for the combined data set, of 5.8% for the furan-THF subset, and of 3.8% for the thiophen subset has been reported for Tb. 

By far, the majority of definitive studies which have been made on this catalyst have used thiophene as a model compound for hydrogenolysis. Table IX summarizes some of the more important findings. We will not discuss all the individual papers because few of them attempt any quantia-tive (or even qualitative) correlations with catalyst properties and there is considerable disagreement between workers on the effects obtained. The more significant features to be pointed out are the following ... 

As complex as the desulfurization of thiophene might appear, projection of the kinetic picture to benzothiophene and dibenzothiophene, and to their derivatives, is even more complex. As has already been noted for bond energy data, kinetic data derived from model compounds cannot be expected to include contributions from the various steric effects that are a consequence of complex molecules containing three-dimensional structures. Indeed, such steric effects can lead to the requirement of additional catalyst and process parameters for sulfur removal (Isoda et al., 1996a, 1996b). 

This work has demonstrated for the first time that organically bound sulfide and thiophenic sulfur forms can be distinguished and in some manner quantified directly in model compound mixtures and in petroleum asphaltenes and coal. The use of the third derivative XANES spectra was the critical factor in allowing this analysis. 

The pyrolysis properties of four model compounds were examined. Their molecular structures are shown in Table II. The three and four-ring molecules, containing a benzo [b] thiophenic unit were synthesized by Dr. Cagniant (L.S.C.O., Metz University). A polycyclohexanesulfide is an aliphatic sulfide synthesized by Dr. N. Spassky (Laboratory of Macromolecular Chemistry, Paris VI University). Polymeric aromatic sulfide was represented by a polybenzosulfide provided by Philips Petroleum. 

Thiol Decomposition. As explained previously, the elimination of sulfur from benzothiophene occurs stepwise after the aromatic thiol (o-thiocresol) has formed and not in a concerted fashion from the thiophenic ring system. Extrapolation of this implies that thio-phenic sulfur in coal is eliminated by conversion to an aromatic thiol that subsequently undergoes desulfurization. Since the aromatic thiol is the apparent organosulfur species that undergoes desulfurization, it is of interest to understand the chemistry involving the elimination of sulfur from aromatic thiols. Thio-phenol was used as a model compound to examine reactions, primarily the thermal decomposition reactions that might lead to sulfur elimination. In the experiments with caustic and benzothiophene, the intermediate (o-thiocresol) most likely exists in the salt... 

Due to the high degree of conjugation, oh go(thiophene)s advanced as attractive candidates for molecular bridges. For instance, Sato et al. [34] constructed hexyl-sexithiophene and methoxy-terthiophene derivatives bearing two terminal ferrocenyl groups. These served as model compounds for molecular wires. In the hexyl-sexithiophene derivative, the resultant oxidized states spread over both the ferrocene and the sexithiophene moieties. Similarly, in the methoxy-terthiophene derivative, the oxidized species spreads over the entire molecule containing the terthiophene and the other ferrocene moiety. In both cases, CT between the terminal units is inferred as it is mediated via the oh go(thiophene)s. 

Some samples of the TS and P series have been chosen for tests using model compounds in order to evaluate the deactivation. Conversions obtained for toluene hydrogenation at 3509C, cyclohexane isomerization at 380 and 400°C and thiophene hydrodesulfurization at 220°C are reported in Table 1. 

Electronic states of the fused-silole-thiophene derivatives have been obtained by ab initio MO calculations on model compounds at the level of RHF/6-31G. Relative HOMO and LUMO energy levels derived from the MO calculations are given in Table 18 <2004OM5622>. 

Fig. 4. Peak shifts, relative to solid thiophene, from the IINS spectra (TFXA, ISIS) of the thiophene modes in the model compounds [Fe(CO)2(i/ -thiophene)( -C5H5)][BF4] (Fe-i , ), [Mn(CO)3()) -thiophene)] (Mn-i , ), [Cr(CO)3( -thiophene)] A), and thiophene adsorbed on a reduced and...

HDS activities were measured in a flow reactor system using thiophene as the model compound. Reaction rates for the HDS of thiophene to butene were determined at 325°C and atmospheric pressure. HDS testing used -60 mesh catalyst. 

Performance testing in trickle-flow operation invariably takes longer than in gas-phase tests with model compounds. Testing with model compounds such as thiophene can be an effective contribution to fast screening, e.g. for the effectiveness of catalyst functions in a defined context, but should not be taken beyond that. 

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