Thiophene catalytic desulfurization

In conclusion, unlike heterogeneous processes with commercial HDS catalysts, single-site catalysts have been found to desulfurize thiophenes (T, BT, DBT) exclusively after these have been converted to saturated thiols or thiolates. No example of catalytic desulfurization of THT or DHBT by a single-site catalyst has ever been reported, although stoichiometric reactions assisted by both mononuclear and polynuclear complexes are known for THT and other cyclic thioethers [2 b, 32]. 

Replacement of one PEts ligand by a phosphite leads to new series of phosphine/phosphite or bis(phosphite) complexes the differences in ability to promote HDS were accounted for in terms of donor-acceptor properties of the ligands involved in each case. Thiophene substituted with groups other than alkyls (e.g., -Cl, -NO2, -OMe, -OAc) also form thiaplatinacycles, which are notable in that they promote catalytic desulfurization. These substituted thiophenes also produced the first thiapalladacycles by reaction with the corresponding Pd-phosphine complexes, as well as Ni analogs.3,6-Dimethylthieno[3,2- ]thiophene also forms a thiaplatinacycle by activation of a C(vinyl)-S bond upon reaction with [Pt(PEt3)4] 2,2 -bithiophene and l-methyl-2-(2-thienyl)pyrrole react similarly but also promote G-H activation. 

Catalytic hydrogenation of thiophene poses a problem since noble metal catalysts are poisoned, and Raney nickel causes desulfurization. Best catalysts proved to be cobalt polysulfide [425], dicobalt octacarbonyl [426], rhenium heptasulfide [5i] and rhenium heptaselenide [54]. The last two require high temperatures (230-260°, 250°) and high pressures (140, 322 atm) and give 70% and 100% of tetrahydrothiophene (thiophane, thiolene), respectively. 

Catalytic hydrodesulfurization (HDS) is a very important industrial process that involves removal of sulfur from crude oils by high-temperature ( 400°C) treatment with hydrogen over Co- or Ni-promoted Mo or W catalysts supported on alumina. In an attempt to determine the mechanism of this process, many transition metal complexes of thiophene, a sulfur-containing heterocycle that is particularly difficult to desulfurize, have been prepared and their reactivities studied in order to compare their behavior with those of the free thiophenes that give H2S and C4 hydrocarbons under HDS conditions (88ACR387). Thiophene can conceivably bind to the catalyst surface by either cr-donation via a sulfur electron pair or through a variety of -coordination modes involving the aromatic system... 

OATS [Olefinic Alkylation of Thiophenic Sulfur] A gasoline desulfurization process. Thiophenes and mercaptans are catalytically reacted with olefins to produce higher-boiling compounds that can more easily be removed by distillation prior to hydrodesulfurization. This minimizes hydrogen usage. The process uses a solid acid catalyst in a liquid-phase, fixed bed reactor. Developed by BPAmoco in 2000 and tested in Bavaria and Texas. First used commercially at the Bayernoil refinery, Neustadt, in 2001. The process won a European Environment Award in 2002. 

A progressive loss in catalytic activity occurs in the plant reactor from small amounts of thiophene present in the feed. The sulfur acts as an irreversible poison of the metallic catalyst, ft has been suggested that hydrogen sulfide is the sulfiding agent, and mechanistically this occurs from desulfurization of thiophene to hydrogen sulfide on the catalyst (4). 

Thus, it is clear that a-MOjC(OOl) and Ni2P(001) can dissociate thiophene easily. The key to establish a catalytic cycle for desulfurization is in the removal of the decomposition products of thiophene (C H fragments and S) from these surfaces. At a temperature of 450 K, it is possible to hydrogenate and remove the C H fragments present on the Ni2P(001) surface [15]. On the other hand, the removal of the S adatoms is significant only at temperatures above 600 K... 

This cubane cluster can be converted back to the original one by reaction with CO (30 atm) to form COS, thus completing a cyclic reaction. Although this pair of reactions indicates the possibility of constructing a catalytic cycle for the HDS of thiophene, the presence of free CO unfortunately inhibits the forward desulfurization reaction and therefore the process cannot be rendered catalytic [68]. The exact mechanism of thiophene HDS by use of these clusters in not clearly understood, mainly because the reaction is fast and no intermediates have been isolated or identified. Nevertheless, Curtis has reported extensive related mechanistic studies on the desulfurization of a variety of thiols and sulfides using the same metal cluster Cp 2Mo2Co2S3(CO)4 and as a result of that, the main reaction pathways for desulfurization of these simpler substrates are now well understood [69],... 

Until fairly recent times, most of the reported homogeneous HDS-related reactions were limited to stoichiometric model transformations in which thiophenes and related molecules could be activated by coordination to the metal centers so as to undergo C-S bond scission and eventually hydrogenolysis to the corresponding free thiol, or in some cases even complete desulfurization to yield hydrocarbons. This has represented a remarkable contribution to the understanding of many of the elementary steps involved in complex heterogeneous HDS schemes, but it had obvious limitations, and it offered little promise in terms of developing catalytic systems of any mechanistic or practical importance. 

The catalytic conversion of thiophenic substrates to the corresponding thiols (hydrogenolysis) (Eq. 3) is a reaction of much relevance in the HDS process as the thiols can subsequently be desulfurized over conventional HDS catalysts with greater efficiency and under milder reaction conditions than those required to ac-... 

In 1983, separation on silica gel impregnated with 5% PdCl2 were introduced. During fractionated elution, the sulfur compounds were displaced by diethylamine from the Pd complex. Problems resulted from the catalytic effects of PdCl2, manifested by the partial desulfurization of compounds with terminal thiophene rings at elevated temperatures. Therefore Andersson combined elution with a subsequent aminopropyl-silicate layer, in which the amino group competes with the PASHs and thus binds the Pd " ". 

Catalytic reactor studies of 2,3-dihydrothiophene (2,3-DHT) have shown it to undergo desulfurization this supports the proposal that it might be an intermediate in the HDS of thiophene " (Scheme 17). Because there is also substantial conversion of this partly hydrogenated thiophene to tetrahydrothiophene (THT), it is of interest to investigate the reactivity of 2,3-DHT with metal hydride complexes to see whether hydrogen is transferred from the metal to the unsaturated heterocycle. 

It is tempting to speculate whether the role of promoter HDS metals (Co, Ni) is the C-S activation of the thiophenes whereas the catalytic metal (group 6, Mo, W) would be responsible for the transfer of hydrogen to the activated substrate. This is a very likely pathway to be envisaged on metal sulfide surfaces where sulfides or thiolates bonded to two or more metal atoms are plentiful. However, this type of interaction is obviously unavailable in single-site complexes, and this could explain why ring opening is frequent but complete desulfurization is rare on mononuclear derivatives. 

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