HDS of thiophenes

C-S hydrogenolysis de hydrodesulfurization partial hydrogenation total hydrogenation 

Thiophene itself and HjS are known to inhibit the HDS reaction, and it is often thought that hydrogenation and hydrogenolysis occur at different active sites. It is also interesting to note that ring alkylation significantly affects the reactivity of thiophenes, but not in a straightforward manner for instance it has been found that the reactivity of methylated thiophenes varies in the order  

However, substitution by a third methyl group enhances the HDS rate showing that both steric and electronic effects are operative, and they may be acting upon different steps or upon more than one step, e.g. adsorption of the thiophene and/or the hydrogenation or hydrogenolysis step [19], 

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The product distribution of the HDS of thiophene over the Mo(lOO) surface is shown in Table III compared with that reported by Kolboe over a MoS catalyst (14). It is clear that the two are very similar ana that our catalyst mimics the MoS catalyst very closely in this respect. An Arrhenius plot fpigure 2) made in the temperature region mentioned above shows that butadiene is the only product whose rate of formation shows true Arrhenius type dependence and yields an activation energy of 14.4 kcal/mole. At high temperatures the rate of butane formation deviates even more sharply than that of the butenes and does so at lower temperatures (9). 

Figure 2. Arrhenius plot for the HDS of thiophene on the Mo(l00) surface. pCh ) = 780 Torr, P(Th) =2.5 Torr.

The sulfided catalysts were evacuated for 1 h before catalytic reactions. The reactions were carried out under mild conditions by using a circulation system (0.2 dm ) made of glass. The HDS of thiophene was conducted at 623 K and an initial pressure of 20 kPa (Hj/C H, = 36). The thiophene pressure was kept constant (0.54 kPa) during the reaction by holding a small amount of liquid thiophene kept at 273 K in the bottom of a U-tube in the reaction system. The products were analyzed by gas chromatography. The HDS activity was calculated from the amount of H S produced during the reaction. 

The hydrogenation (HYD) of butadiene was subsequently conducted at 473 K over the catalyst, that had been used for the HDS of thiophene for 1 h, after evacuation at 673 K for 1 h. The initial pressure was 14 kPa (HJ/C4H4 = 2). The HYD products were butenes with a small amount of butane. The catalytic activity was calculated on the basis of the total amount of the reaction products. 

Figure 1. Catalytic activities of MoSx/NaY CoSx/NaY(0), and MoSj/NaY (A) for the HDS of thiophene as a function of the metal content (metal atoms/SC).

Figure 9. Activities of CoSx-MoSx/NaY and CoSx/MoSx/NaY for the HDS of thiophene. The corresponding sum activities of the composite catalysts are also shown for comparison.

Reported Langmuir-ffinshelwood Parameters in the HDS of Thiophene Derivatives... 

Unfortunately, diaromatic hydrocarbons are not the only potential hydrocarbon inhibitors present in gas oils and diesel fuels. Triaromatic hydrocarbons are also present in significant amounts (see Fig. 2) (12). It is known that triaromatics, such as phenanthrene, are even stronger inhibitors than diaromatics for the HDS of thiophene compounds. Equilibrium adsorption constants for phenanthrene and naphthalene have been reported to be 65 and 11 atm-1, respectively (131). In Iranian gas oil, triaromatics have been reported to be present at about one-tenth the concentration of diaromatics (109). Thus, the contribution to inhibition of HDS reactions by triaromatics (XTri[Tri]) could be about the same as that from diaromatics, even though triaromatics are present in smaller amounts. 

The catalysts were tested in the dehydrogenation of tetrahydrothiophene (DHN of THT), the hydrodesulphurization of thiophene (HDS of thiophene) and the hydrogenation of biphenyl (HN of BP). The reactions were carried out in the vapor phase using dynamic flow microreactors equipped with an automatic online analysis. Reaction conditions are given in Table 1. 

The strong similarities between HDS and DHN results suggest that the type of catalytic sites involved in these two reactions are similar. This would mean that the routes for DHN of THT and HDS of thiophene include some identical reactional step(s), at least for the kinetically limitative one(s). Angelici and coworkers, in a detailed work [10] concerning the HDS of thiophene, THT, 2,3- and 2,5-dihydrothiophene showed that the dihydrothiophenes are much more reactive than the other molecules. On this basis and an additional study [11] on organometallic compounds, these authors have suggested that dihydrothiophenes are reaction intermediates and proposed a mechanistic pathway including this step. 

The catalytic activities of transition metal sulphides were classified in the DHN of THT, the HDS of thiophene and the HN of BP. The results clearly evidence strong similarities between the activities of the most active catalysts in the three reactions, which implies that the catalytic sites involved in each of these reactions are comparable. 

Moreover, this study points out the ability of sulphides to catalyze either reactions involving the conversion of sulphur containing molecules without desulphurization (DHN of THT) or desulphurization of these molecules (HDS of thiophene) as well as classical hydrogenation reactions (HN of BP). These properties make the sulphide catalysts interesting to be applied to organic chemistry, particularly in thiochemistry. 

Homogeneous modeling studies have recently contributed several mechanistic breakthroughs regarding most of the steps which may be involved in the metal-assisted HDS of thiophene to H2S and hydrocarbons. 

The negative influence of nitrogen compounds on the HDS of thiophene was quantified at 7 MPa and 633 K. The adsorption constants of most nitrogen compounds, determined with the aid of a Langmuir-Hinshelwood model, follow the same trend as their proton affinities in the gas phase (Table II) (77). 

Four laboratories have presented preliminary results on the hydrodcsulfurization (HDS) of thiophene catalyzed by a Co-Mo/y-AFOi catalyst of standard com-... 

The types of organosulfur compounds found in petroleum feedstocks are shown in Table Of them, the alkyl and aryl thiols (RSH), sulfides (RSR ), and disulfides (RSSR ) are the most rapidly desulfurized (equation 1). It is the broad class of thiophenes, stabilized by their aromatic character, that are most difficult to desulfurize and require relatively severe temperatures ( 400 °C) and H2 pressures ( 100 atm). Thus, it is the HDS of thiophenes that has been of greatest interest to inorganic and organometallic chemists. There are fewer model studies of the HDS of thiols, sulfides, and disulfides. Organometallic aspects of HDS and HDN have been summarized most comprehensively in a recent book by Sanchez-Delgado. A briefer overview has also been published. More specific reviews are cited in later sections of this chapter. 

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. 

The HDS of thiophene is not a first-order reaction. Hence nnmerical analysis is required. However, if the hydrogen concentration is constant, the reaction may be taken as pseudo-first order, with k-, replaced by a pseudo- lrst-ordei rate constant. The... 

The catalytic activities of fresh and spent catalysts ware deternined for the hydrodesulfurization (HDS) of thiophene in a gas-phase fixed-bed reactor at 400 °C. Following sulfidation in H2/H2S-the reported activities were obtained after 5 h on stream. The thiophene concentration was in the range of 4,9-7-1 vol % and the total pressuretl bar. 

C0/AI2O3 and C0-M0/AI2O3 catalysts after being used for high pressure (30 atm) hds of thiophen were examined by XPS and e.s.r. There were two forms of Co sulphided and reduced. There was more sulphided Co in the presence of Mo. The e.s.r. of sulphided C0/AI2O3 showed the presence of a Co sulphide more reduced than C09S8. The concentration of this reduced sulphide was much lower in the presence of Mo. A Mo -O species was also observed by e.s.r. 

The hds of thiophen (623-673 K, 1 atm) over sulphided M0O3 was followed as a function of time. Thiophen conversion and butane formation increased to a maximum and then decreased to a steady value (ca. 1 h), whereas butenes increased continuously to steady values. The reaction proceeded by two independent pathways adsorption of thiophen through S followed by hydrogenation to butane adsorption parallel to the catalyst surface followed by hydrogenation via butadiene and S elimination. 

The hds of thiophen, benzothiophen, and dibenzothiophen and their methyl-substituted derivatives were compared in pulse experiments (623-723 K) over a sulphided CoO(5.6)-Mo03(11.2)/Al203 catalyst. Reactivities at 1 atm pressure were roughly the same but at hi er pressures, reactivity decreased with the number of rings. For benzothiophen, methyl substituents did not affect the reactivities, but for dibenzothiophen methyl substituents in the 4- or 4 - and 6- positions caused a decrease of desulphurization rate. Aromatic ring hydrogenation was not a prerequisite of C-S scission, which was the slow step. 

The rates of hds of thiophen, benzothiophen, and polyaromatic thiophens were compared over a sulphided commercial C0O-M0O3/AI2O3 catalyst (573 K, 71 atm). Pseudo-first-order kinetics were obeyed. The mechanism of the reaction with thiophen (involving ring hydrogenation) was different from that of other compounds (S extmsion). The reactivity was not governed solely by the size of the ring compound interaction of the ir-electron system with the catalyst surface may be more important than the interaction of S except for thiophen. 

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