Hydrodesullphurizaaion mechanism of thiophene and tetrahydrothiophene on a cobalt molybdenum catalyst

JOSE M. PAZOS A N D PAULINO ANDREU Cenrro de Petrdleo y Quimica, Instituto Venezolano de Zncestigaciones Cientijcas, Caracas, Vetzezuela '

Received June 6, 1979

J o s i M. P ~ z o s and PAULINO ANDREU. Can. J. Chem. 58.479 (1980). The hydrodesulphurization mechanism of thiophene and tetrahydrothiophene has been developed at high pressures and over a

broad range of temperature and contact time on a commercial CoMo-A1203 catalyst. The influence of the pretreatment on the catalyst activity and stability was also studied. The pretreatment with a mixture of H, and H2S was found to be the most convenient. It was found that the sulphur uptake of the fresh catalyst increases with temperature and that an excess of sulphur in the catalyst leads to an initial higher activity.

The thiophene reaction seems to occur simultaneously by two pathways: one consists of ring opening, and the second is yielding tetrahydrothiophene. The latter is the slowest step. The tetrahydrothiophene reacts faster than the thiophene and its reaction mechanism involves mainly the rupture of the C-S bond. However, thiophene was detected in small concentrations, showing the contribution of a second route for the tetrahydrothiophene hydrodesulphurization. Experiments carried out with benzene seem to indicate the existence of three different kinds of active sites in the catalyst: desulphurization, aromatics hydrogenation, and olefin saturation sites.

Jos i M. PAZOS et PAULINO AKDREU. Can. J . Chem. 58,479(1980). On a elabore le mecanisme d'hydrodesulfurisation du thiophene et du tetrahydrothiophkne a haute pression et sur une large

echelle de temperature et de temps de contact avec une catalyseur commercial mixte: CoMo-Al,03. On a egalement etudie la stabilite ainsi que I'influence d'un traiternent prealable sur l'activite du catalyseur. On a trouve que le traitement prealable, par un melange de H, et de H2S, est celui qui convient le mieux. On a constate que la fixation de soufre sur le catalyseurfrais augmente avec la temperature et que l'exces de soufre sur le catalyseur conduit a une activite initiale plus forte.

La reaction du thiophene semble se produire suivant deux voies: l'une consiste en une ouverture du cycle. et l'autre conduit au tetrahydrothiophene. La derniere voie est l'etape la plus lente. Le tetrahydrothiophene reagit plus vite que ie thiophene et le mecanisme de sareaction implique principalement la rupture de la liaison C-S. Cependant on adecele de faibles concentrations de thiophene indiq~rant parlal'existence d'une deuxieme fagon d'hydrodesulfuriser le tetrahydrothiophene. Les experiences conduites dans le benzene semblent indiquer l'existence de trois differentes sortes de sites actifs sur le catalyseur: un site de desulfurisation un d'hydrogenation aromatique et un de saturation des olefines.

[Traduit par le journal]

Introduction other hand, the mechanisms proposed are generally Numerous sulphur compounds have been used valid only at conditions far from those used indus-

to study different aspects of the hydrodesul- trially. Therefore, in this work we studied the

phurization of crude oil distillates. As the greater mechanism of th io~hene h~drOdesul~hurization

part of the sulphur in the crude is held in stable ring over a wide range pressures, temperatures, and

structures of the thiophene type, thiophene has contact times. The reaction of the intermediate

often been chosen as a model compound. since the tetrahydrothiophene and the influence of different

pioneering work ofPease and Keighton ( I ) , several pretreatments on the catalyst stability and activity papers have been published dealing mainly with the were a'so studied in mechanism of thiophene hydrodesulphurization. ExpesimenbB The most accepted reaction models imply two separated mechanisms. Either the C-S bond is A p i f ~ ~ ~ ~ ~ ~ ~ ~ p s " ~ ~ , " ~ ~ a m i e d in a bench scale using a ruptured, a condition leading to such intermediates flow microreactor with a fixed bed of catalyst. The whole system as mercaptanes and birtadiene (2-41, or ring sat- was made of stainless steel. Thereactor was 21 cmlong and 1 cm Uration occurs, which forms te~rahydrsthiophene id. The catalyst bed was always 3 cm3 placed at the centre of the

(5-7). both proposed models, the final products reactor. The top and the bottom of the reactor were filled with an inert ceramic material.

are a mixture of butenes, and hy- The reactant, thiophene or tetrahydrothiophene, was diluted drogen sulphide. in heptane and mixed with an excess of hydrogen (H,/solvent =

The authors consider that the thiophene reaction 2 mol/mol), just before the reactor. A gas-chromatograph was

is worth a further because there is no general connected in series with the reactor, so that the reactor effluents could be analysed continuously. The extent of the reaction was consensus about its reaction mechanism. On the followed by the weight conversion of the

'Correspondence address: Instituto TecnoI6gico Venezolano Materials del Petroieo (INTEVEP), Apdo. 74343, Caracas, Venezuela. A typical commercial CoMo-A1203 was used in ali the ex-

W8-4042/80/050479-06$01 .00/0 01980 National Research Council of CanadalConseil national de recherches du Canada

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480 CAN. J . CHEM. VOL. 58. 1980

periments. It was supplied by Akzo Chemie (Ketjenfine 124 - 1.5 E) as 1.5 mm diameter extrudates. The catalyst was used as particles of an equivalent diameter equal to 0.75 mm. The catalyst characteristics are 4% COO, 1276, Moo,, surface area = 250 in2 g-I, pore volume = 0.52 mL g ' .

The chemicals used were hydrogen (99% purity), thiophene from Aldrich (99% purity), tetrahydrothiophene from Eastman Kodak, heptane from Fluka (99% purity), and benzene from Merck (99% purity).

Anul.vsis The reaction products were analysed as they left the reactor

by a gas-chromatograph provided with a flame ionization de- tector. The chromatographic column was SE-30 over Varaport (400 x 0.318 cm od), which was used to obtain a preliminary group separation of C, gases, thiophene. tetrahydrothiophene, and heptane. The C, gases were resolved by using a 16% bis-2- methoxiethyladipate (730 x 0.318 cm od) plus 30% DC-200 (100 x 0.318 cm od) over Chromosorb column.

The products detected in the thiophene reaction were C , gases, tetrahydrothiophene, and unreacted thiophene. A typical gas distribution obtained at 290°C and 28 atm was isobutane (ca. 50wt%), 1-butene (ca. 30wt%), and trclns-2-butene (ca. 20 wt%). A more exhaustive analysis of the liquid products was carried out by using a GC-MS system. In addition to tetrahydrothiophene, small amounts of 2-methyl-2-propanethiol and traces of di-n-butylsulphide, 2-n-butylthiophene, and 2-n- butyltetrahydrothiophene were also detected.

The products observed in the tetrahydrothiophene reaction were C, hydrocarbons. thiophene, and unreacted tetrahydro- thiophene. The distribution ofgases and liquids was found to be similar to that of thiophene, but no trace of 2-n-butyl thiophene and 2-n-butyl- tetrahydrothiophene was detected.

Results Pretreatment of lhe Catalyst

A study of the influence of different pretreat- ments on the catalyst stability and activity was carried out initially. Fresh samples (A, B, C) of the same catalyst were submitted to different pre- treatments in the reactor. Sample A was exposed directly to the feedstock at reaction conditions; sample B was initially reduced with hydrogen at 300°C for 1 h and finally 2 h at 350"C, and sample C was presulphided with a mixture of H, + H2S (3.5% volume of H,S) for 1 h at 300°C and 2 h at 350°C. Figure 1 shows the activity of the three catalyst samples as a function of the hours on stream (HOS). It can be observed that catalyst A exibits the lowest initial activity due to the fact that it was present as an oxide. As HOS increases, the catalyst becomes sulphided with the H2S formed under reaction conditions. It took about 20 h for the catalyst to become stable. The prereduced catalyst B shows a higher initial activity than catalyst A and required only 6 h to be sulphided. Finally it reaches the same level of activity of catalyst A. Catalyst C, previously sulphided, shows the highest initial ac- tivity, and thus indicating the importance of the sulphided form for the catalyst activity.

It follows from Fig. 1 that catalyst C shows also the highest activity, ca. 90%, while A and B give

FIG. 1. Influence of the pretreatment in the catalyst activity and stability. Reaction conditions: P = 42 atm, T = 310°C, 8 = 0.97 h, pH0 = 26.3 arm, pTu = 2.5 atm. Sample A. catalyst with- out pretreatment. Sample B, catalyst prereduced with H,. Sam- ple C, catalyst presulphided with H, + H,S.

88%. This is due to a higher sulphur content of catalyst C. The reasons for this higher sulphur con- centration may be explained as follows: ( a ) The higher M,S concentration in the reactor, since catalyst C was presulphided with M, + H,S. (b) The higher suiphiding temperature of catalyst C. The dependence of sulphur uptake with tempera- ture has been previously reported (8). ( c ) The re- duction of catalysts A and B before their sulphida- tion. The reduced catalysts are more refractory to sulphidation than the corresponding oxides (9). Therefore, their sulphiding should be lower.

A used catalyst was regenerated by heating it up

I I I I I I 2 4 6 8 10 12

HOS

FIG. 2. P~esulphiding effect on catalyst activity. Reaction conditions: P = 28 atm, T = 290°C, 8 = 0.56 h, pHu = 17.6 atm, pTu = 1.66 atm. O, fresh catalyst presulphided for 3 h; 0. catalyst sulphided for 7 h; 0, regenerated cztalysd presulphided for 3 h.

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PAZOS AND ANDREU

F I G . 3 . Thiophene product distribution as a function o f the contact time, 8. Reaction conditions: P = 28 atm, T = 2 W C , pHD = 17.6 atm. pTO = 1.66 atm.

in air for 10 h and subsequently presulphided again loo

under the same conditions with H, + H,S (4.5% volume H,S) for B h at 300°C and 2 h at 350°C. Its 80

initial activity was surprisingly higher than that of the fresh catalyst, as can be observed in Fig. 2. We reasoned that the regenerated catalyst still had ; some remaining sulphur and consequently, it 8 showed after sulphiding an excess of sulphur con- centration, which is responsible for the initial higher activity. In order to confirm this, the fresh

20 catalyst, already presulphided, was sulphided for 4 h more at 358°C with H, + H,S (3.5% volume H,S). This catalyst showed a behavisur similar to the regenerated catalyst, as can be observed in Fig. 7- ec 2. indicates again the 'gher FIG, 4, ThiOphene product distribution as a function o f the activity is probably due an excess of sulphur in temperature. Reaction conditions: P = 28 atm.8 = 0.56 h,pHo= the catalyst. 17.4 atm,pTO = 1.66atm

Product Distribudion of Thiopkl~ne Hydrodesldl- temperature (Pigs. 3 and 4, respectively). It can be phurizcation observed in both figures that tetrahydrothiophene

The product distribution obtained in the thio- is the main intermediate detected. The low tetrahy- phene reaction was studied over wide conversion drothiophene concentration, as shown in Fig. 3, ranges as a function of the contact time (8) and indicates that the thiophene reaction occurs by a

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482 CAN. J . CHEM. VOL. 58, 1980

consecutive reaction, where the first step is the hydrogenation of the thiophenic ring to form tetra- hydrothiophene. However, a deeper analysis of Fig. 3 shows that the thiophene hydrsdesul- phurization also takes place by rupturing of the C-§ bond: the yield curve of the C, hydrocarbons formed do not exhibit the typical induction period characteristic of the consecutive reactions. These two facts clearly support that the thiophene hydro- desulphurization occurs by parallel reactions.

The thiophene product distribution as a function of the temperature, Fig. 4, indicates that at T < 250°C, the conversion of thiophene is very small (less than 10 wt%) and that the predominant reac- tion is the hydrogenation of thiophene to tetra- hydrothiophene. At T > 250°C, the tetrahydrothio- phene formation goes through a maximum and then decreases because the tetrahydrothiophene reacts very fast to give C, hydrocarbons. On the other hand, at T > 25WC the contribution of the ring opening of the thiophene to give directly C, gases increases markedly.

The thiophene conversion data are consistent with the pseudo-first order reaction rate as can be observed in Fig. 5 .

Product Distribution of ~ e t r a h ~ d r o t h i o l ~ h e n e Hydrodesulphurizution

For a better understanding of the thiophene hydrodesulphurization mechanism, a further study of the tetrahydrothiophene hydrodesulphurization has been carried out under similar experimental conditions. Figures 4 and 7 show the product dis- tribution as a function of contact time and temper- ature respectively. These figures indicate the fol- lowing facts: ( a ) Thiophene, product of the dehy- drogenation of tetrahydrothiophene, is the main intermediate, although its concentration is very small (generally under 5 wt%). The presence of thiophene has been observed by other workers (10, 11). (6) Tetrahydrothiophene reacts faster than thiophene at similar operating conditions. This is in agreement with the fact that the hydrogenated compounds are more reactive than the cone- sponding aromatic compounds (12). ( c ) The mech- anism of tetrahydrothiophene hydrodesulphuriza- tion is similar to the thiophene reaction and can be expressed by parallel reactions. One reaction yields thiophene, and the other one is the rupture of the C-S bond of the tetrahydrothiophene. This second reaction is the predominant one.

The tetrahydrothiophene reaction can be ad- justed by the pseudo-first order kinetic equation as is shown in Fig. 5 .

I00

- 80 0 THIOPHENE + 1 A TETRAHYDROTHIOPHENE

8 60 - 0 f 40

P f .s 20 - 0

D

FIG. 5 . Pseudo first-order kinetic equation for thiophene and tetrahydrothiophene conversion. Reaction conditions: P =

28atm, T = 290°C. pH0 = 17.6atm. p,O = 1.66atm (S is thiophene and tetrahydrothiophene. respectively).

FIG. 6. Tetrahydrothiophene product distribution as a func- tion of the contact time. Reaction conditions: P = 28 atm, T =

290"C,pH0= 17.6atm,pTh0= 1.66atm.

Discussion The former results show that the thiophene

hydrodesulphurization is a complex reaction, which can be expressed as follows:

k , Yhiophene z====? Tetrahydrothiophene

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FIG. 7. Tetrahydrothiophene product distribution as a func- tion of the temperature. Reaction conditions: P = 28 atm, 0 = 0.56 h,pHO = 17.6 atm, pThO = 1.66 atm.

Thus the thiophene reaction occurs simultaneously by hydrogenation to tetrahydrothiophene and by a straight ring opening, which involves the rupture of the C-S bond.

The concentration of thiophene (T), tetra- hydrothiophene (Th), and C, hydrocarbons can be expressed by the following equations:

[I] T(8)=A1exp(a18)+A2exp(a28)

[2] Thj8) = A ,a, exp (a ,8) + A,@, exp (a29)

[3] C4(8) = A , -k A exp (a ,8) + A,a, exp (a28)

where Ai (wt%'o) and a i (h-I) are empirical constants and 8 is the contact time (h). These equations are the solutions of the rate expressions for the thio- phene network. Each reaction was assumed to be first order.

To evaluate the contribution of the different reactions to the thiophene network, the individual rate constants k i were estimated. Several methods are available to determine kinetic coefficients for complex reaction models (13). In the present work, the coefficients Ai and a i were determined from the experimental points of Fig. 3 with a non-linear least-squares program. From a i , the individual rate constants were estimated. The following values were obtained:

FIG. 8. Thiophene product distribution as a function of the total pressure. Reaction conditions: T = 290°C, 8 = 0,45 h.

The lines of Fig. 3 show the concentration profile of thiophene, tetrahydrothiophene, and C, predicted by eqs. [I] , [2], and [3]. There is a close agreement between these curves and the experimental points.

The values of the rate constants obtained indi- cate that the thiophene reaction occurs mainly through the rupture of the C-S bond. On the other hand, it is shown that tetrahydrothiophene has higher desulphurization activity than thiophene.

This mechanism has been proved to be valid over a broad range of pressures, as shown in Fig. 8. It can be observed that the tetrahydrothiophene con- centration increases with the total pressure. This can be an explanation of why tetrahydrothiophene has not been proposed as an intermediate by sev- eral authors who usually worked at atmospheric pressure (2-4).

In Fig. 9, we present a more complete picture of the mechanism of the thiophene reaction, based on the products observed by mass spectrometry at typical reaction conditions. Not only tetrahydro- thiophene and butenes, but also mercaptanes and sulphides have been detected as intermediates, which show the csmpIexity of this reaction. Wow- ever, they were found in small concentrations.

Finally, some experiments were carried out by substituting the solvent heptane by benzene in

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CAN. J . CHEM. VOL. 5 8 , 1980

~ - I - C ~ H I O ( ~ , + HzS(gi

t CH3-(CH&

\

CH3-(CH2I3 / S[d) + H2S(dl

CH,(CH2)3SH t fH3

HzC - CHZ 1 1 %- CH3-(CH2b-SH+CH3-C-SH I

HC- CH I ll CH3-C-H

HC, /CH I

CH2ZCH-CH2-CHqg, CH3 (0

C H ~ = C H - C H = C H - S H % C H ~ =CH-CH=CH2

-HzSidl

FIG. 9. Mechanism of thiophene HDS.

order to ellucidate the type of sites involved in the also wish to thank the contribution of F. Gonzalez thiophene reaction. The effect of changing the sol- to the mathematical calculations. vent from pure heptane to pure benzene through various ratios was to decrease the C4 from 5.7% 'a0 1. R, N. PEASE and W. B. KEIGHTOS. Ind. Eng. Chem. 25, 4.5% and the tetrahydrothiophene from 6.6% to 1012 (1933).

5.4y0, but the ratio letrahydrothiophene/G, re- 2. V. 1. KOMAREWSKY and E. A. KNACCS. Ind. Eng. Chem. 43, 1414(1951). mained 'Onstant at The benzene adsorb 3. P. J. OWENS and C. H. AMBERG. Adv. Chem. Ser. 33, 182

on hydrogenation sites affecting the tetrahydro- (1961). thiophene concentration but it should not be af- 4. J. M. J . C. LIPSCH and G. C. A. SCHUIT. J . Catal. 15. 179 fected to the same extent as the C, concentration. (1969).

~h~ results obtained seem to indicate that benzene 5. C. M. CAWLEY and C. C. HALL. J. Soc. Chem. Ind. 62, 116 (1943).

is On aromatic and 6. C. G. BERTIL HAMMAR. I ~ I Proceedings ofthe Third World desulphurization sites; therefore, it competes for Petroleum Congress, Sect. IV. The Hague. 1951. p. 295. the same adsorption sites of thiophene. Though 7. J . K R A U ~ and M. ZDRAZIL. React. Kinet. Catal. Lett. 6(4).

there is need for further evidence, we could suggest 475 8. F. E. MASSOTH. J. Catal. 36. 164(1975). the existence of three different types of active sites 9, Q, A, NPRA, Oil Gas J , 76(24), 74 (1978),

involved in h~drodesul~hurization reactions: lo. P. DESIKAN and C. H. AMBERG. Can. J . Chem. 42, 483 aromatics hydrogenation, olefines saturation, and (1964). desulphurization sites. 11. P. KIERAZ and C. KEMBALL. J. Catal. 4, 394 (1965).

12. D. R. K I L A ~ O W S K I . H. TEESWEN. V. H. J. DE BEER, B. C. AcbowPedgmenh GATES. G. C. A. SCHUIT. and H. KWART. J . Catal. 55, 129

(1978). The authors acknowledge the contribution of A . 13. D. M. H I M M E L B L A ~ . C. R. JONES, and K. B. BISCHOFF.

V. Sapre and B. C. Gates to the kinetic model: they Ind. Eng. Chem. Fundam. 6,539(1967).

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