Thiophenes conversion

In this paper deactivation of the hydrodesulfurization (HDS) catalysts is examined using the results obtained for the thiophene conversion on the supported phosphormolybdenum heteropolyacid as a model catalyst. 

One can be seen that H2 S released from thiophene is picked up by catalyst depending on the preliminary treatment (Figure lb). It shows that the initial thiophene conversion is determined not only by interaction of catalyst with H2S. 

At the same time a decrease of the thiophene conversion can be related with blockage of some active species by hydrogen sulfide released during reaction and thiophene adsorbed too. The activity restoration when adsorbed components of reaction mixture are flowed by H2 or Ar is related to the reformation of the active species (Figure 2). The H2 S desorbtion (revealed in desorbed products along with butenes) leads to increase of the HDS activity and intensity of the Mos signals. The same result was also observed on the preliminarily reduced sample. 

Figure 2, Dependence of thiophene conversion and state of molybdenum on the reaction mixture composition (sample HPM0/S1O2 presulfided at 250°C),...

Figure 3. Thiophene conversion into hydrocarbons on sample HPMo/SiOa presulfided at 250°C-l and 500°C-2.

The study of the HDS activity of HPMo/SiOa as a model catalyst has shown that the reversible deactivation effect is connected with increasing of sulfur iones in Mo5 surrounding, blockage Mos species by S-compounds and deeper reduction of molybdenum. The sample containing only molybdenum sulfide exhibits about two times lower initial and steady thiophene conversion in comparison with partially sulfided HPMo. 

Figure 4. Effect of shot size on thiophene conversion...

The results in Table II show the effect of adding hexene to thiophene in singleshot experiments. Cyclohexene had a similar effect cyclohexane, n-hexane, and benzene had no effect, though all were shown to adsorb on the catalyst. Thus the effect of adding 3 moles of hexene per mole of thiophene was to lower thiophene conversion by 21% 1 mole per mole of thiophene cut conversion by only 4.5%, so that assuming that butene and hexene behave similarly, the 1 mole of butene formed from each mole of thiophene probably would not retard the reaction appreciably. 

The importance of H2S adsorption from the point of view of the over-all reaction is shown by its effect on butene hydrogenation and on thiophene conversion (Figure 2). These indicate that H2S adsorption competes for sites which are essential to the reaction. Since its effect on thiophene adsorption was to cut peak delay by only 10 to 30%, it may be that H2S competes for hydrogen adsorption... 

The gas-phase reaction of CF3 radicals with thiophene has been studied <2003CJC1477>. The GF3 radicals were generated by photolysis of CF3I or CF3COCF3. At thiophene conversions of less than 20%, mainly 2-trifluoromethyl-and 3-trifluoromethylthiophene were produced in the ratio 16 1. 

Increasing the reaction time lead to the formation of the dithione in only very low yield (5 %). Also, the synthesis of 40 has been reported to have been accomplished using the same procedure starting from phthaloyl chloride and 2,5-dimethyl thiophene. Conversion of this to the mono- and di-thiones has been reported to be unsuccessful. 

Thiophene conversion is known to be less than 100% since the amount of thiophene fed to the reactor over these 16 hour experiments exceeds the amount of sulfur needed to cover all active sites by a factor of five to seven. For Catalyst A , the thiophene conversion is initially projected to be about 90% at T =0 but it drops to around 60% at T = 1.6 (about 18 hours). After long exposure to thiophene, the front part of the reactor is essentially inactive. Above a value of T = 1.6, the rate of thiophene disappearance increases as the thiophene goes through the catalyst bed, resulting in a change for the shape of the curve. 

The value of for Catalyst C indicates that thiophene conversion will be much less than for Catalyst A , and Figure 4 shows that the conversion of thiophene is only about 50% on the fresh catalyst. In fact, the disappearance of thiophene and the active site concentration as a function of axial bed position for Catalyst C is nearly linear. Indeed, Figure 5 suggests a fairly uniform profile for active sites as a function of axial position. 

Thiophene HDS was performed at 673 K in a microflow reactor with on-line gas chromatography (GC) analysis. The catalyst samples (200 mg) were pre-sulfided in situ using conditions described in the preparation section. The reaction mixture consisting of 4.0 mol% thiophene in H2 was fed through the reactor and was analyzed every 35 min (flow rate 50 ml min , 673 K, 1 bar). First order rate constants for thiophene conversion to hydrocarbons (Khds) and the consecutive hydrogenation of butene (knyo) were calculated as described elsewhere [8]. 

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 furan-thiophene conversion is optimum in the temperature range... 

Pd-based catalyst studies have focused on the influence of S-containing gases on reactions including thiophene hydrodesulphurization and methane oxidation. It appears that not only does the presence of sulfiir-containing gases decreases the effectiveness of the Pd-based catalysts studied, but the formation of Pd-sulfldes does as well [136, 137]. Eeuerriegel and coworkers demonstrated complete Pd-catalyst deactivation for the oxidation of methane with as low as 5.4 ppm of HaS present and as little as 0.08 ML of S completely poisoning the surface [137], while Vazquez and co-workers witnessed a 60% decrease in thiophene conversion for a sulfided catalyst as compared to a sulfide-free Pd catalyst [136]. 

It is of interest to compare in absolute values, the HDS activity and SV of two catalysts. (Data for calculation are taken from Ref. 65) For the 2 wt% Mo EV was 3—4 x 10 functioning site mg , the converted thiophene 3,5 X 10 mol/pulse. These values were 4 x 10 f site mg and 4,37 x 10 mol/pulse respectively for the Co(4 wt%) Mo (8wt%) catalyst, i.e., the ratio of V values was 11.76, and that of the thiophene conversions -12, 5. This seems to indicate the reliability of the applied method. It is seen from the data that the maximal density of sites with the higher H2 S release rate within the 0.3-0.4 Co/(Co- - Mo) ratio is typical for the CoMoS phase. At higher Co content, the CogSs phase formation hinders the formation of vacancies with the high sulfur release rate required for this. 

FIGURE 23.7 Effect of pressure upon thiophene conversion during hydrotreatment. Simulation temperature 553 K. (a) Thiophene conversion at network centerplane. (b) Fraction of pores on a percolating path to the pellet surface. The line is shown to guide the eye error bars represent the spread in thiophene conversion values resulting from simulations performed upon 5 separately generated pore networks. (From Wood, J., Gladden, L.F., and Keil, F.J., Chem. Eng. ScL, 57, 3047-3059, 2002. With permission.)... 

Similar kinetic experiments were carried out by Fatemi et al. [54] for the reaction of thiophene hydrogenolysis at the elevated temperature (170-190°C) and pressure (9.5 atm) that are close to conditions of industrial hydrotreating processes. They used n-heptane as a feed containing 3.24% of thiophene. Owing to the preferential condensation of n-heptane rather than thiophene, they did not observe a relationship between reaction rate and thiophene conversion, like the trend shown in Figure 23.9. So, the dependency of the reaction rate vs. temperamre was used (Figure 23.11). To represent their experimental data, Fatemi et al. [54] have used a model similar to (Equation (23.23) and Equation (23.24)). 

In contrast to toluene, the bromination of heteroaromatic compounds such as thiophene (Scheme 4.13) is very fast even at low temperatures ofO °C or below. Loeb and coworkers investigated the bromination of thiophene with regard to the control of multiple bromination employing a similar setup to that used for the bromination of toluene [26-28]. At a flxed bromine thiophene molar ratio of 1.0 the temperature was varied between 0 and 60 C, showing nearly no changes in the 1 1 ratio between one- and two-fold brominated products. The amount of three-fold substituted thiophene increased only slowly at elevated temperatures. At a fixed temperature of 50 °C, the authors varied systematically the molar ratio of bromine to thiophene from 1.0 to 5.0. The resultant product distribution obtained under conditions of complete thiophene conversion is shown in Figure 4.2. For 2,5-dibromothiophene (a relevant compound for the synthesis of OLE D materials), a selectivity of up to 80% could be achieved at a bromineithiophene molar ratio of 2.0. 

Figure 33 Effect of Co/(Co + Mo) ratio and H2 pressure on thiophene conversion (top) IMPa (bottom) 3MPa. ...

Figure 58 Thiophene conversion versus time - NiMoac presulfided, prereduced, x - M presulfided, A - M prereduced.

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