HDS of dibenzothiophene and vanadyl porphyrin HDP on bulk Fe-Mo mixed sulphides

Hydrotreatmentand Hydrocrackingof Oil Fractions B. Delmon,G.F.Fromentand P. Grange(Editors) 9 1999ElsevierScienceB.V.All rightsreserved.

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HDS o f D i b e n z o t h i o p h e n e a n d V a n a d y l P o r p h y r i n HDP o n b u l k Fe-Mo mixed sulphides. M.A. Luis 1,2,3, A. Rives 2, R. H u b a u t 2, B.P. E m b a i d 1, F. Gonzalez-Jimenez I a n d C. E. Scott 1,*. 1. Universidad Central de Venezuela, Centro de Catklisis Petr61eo y Petroquimica, Apartado Postal 47102, Los C h a g u a r a m o s , Caracas, Venezuela. 2. Universit6 des Sciences et Technologie de Lille, Laboratoire de Catalyse H6t6rog6ne et Homog6ne, URA CNRS n~ B&timent C3, 5 9 6 5 5 Villeneuve d'Ascq, France 3. Universidad de Carabobo, F a c u l t a d de Ciencias y Tenologia, Dpto de Quimica. * e-mail: [email protected]

Abstract. Bulk Fe-Mo mixed s u l p h i d e s were p r e p a r e d by h o m o g e n e o u s precipitation. The obtained solids were c h a r a c t e r i s e d by chemical analysis, X-ray diffraction, X-ray photoelectron spectroscopy, S7Fe M 6 s s b a u e r spectroscopy, a n d nitrogen a d s o r p t i o n (BET) for surface a r e a determinations. HDS of d i b e n z o t h o p h e n e (in a b a t c h reactor) a n d HDP of vanadyl octaethyl porphyrin, at high p r e s s u r e , were u s e d as catalytic tests. It was found t h a t Fe strongly p r o m o t e s Mo for both reactions, with a more m a r k e d synergy for the HDP t h a n for the HDS. S7Fe M 6 s s b a u e r spectroscopy suggests t h a t Fe exist in only one p h a s e , for the mixed Fe-Mo sulphides. This p h a s e could be the results of some Fe s u b s t i t u t i o n in the MoS2 s t r u c t u r e . For HDP the activity is m a x i m u m at a Fe(Fe+Mo) atomic ratio of 0.68, a n d increases linearly with the weighted average hyperfine field, which is related to c h a n g e s in the density of states of d electrons at the Fermi level, clearly suggesting t h a t synergy is related to a n electronic effect. For HDS of DBT the activity m a x i m u m is located at a Fe/(Fe+Mo) atomic ratio of 0.52.

1. INTRODUCTION There is no d o u b t t h a t h y d r o t r e a t m e n t will become increasingly i m p o r t a n t in years to come. The need for processing heavier p e t r o l e u m fractions into light distillates as a result of the decline in fuel oil d e m a n d , together with stricter e n v i r o n m e n t a l legislation regarding the m a x i m u m c o n t e n t of s u l p h u r , nitrogen a n d a r o m a t i c s in fuels, have m a d e h y d r o p r o c e s s i n g increasingly difficult. One possible way to overcome these difficulties is to develop new, more active a n d selective, or less expensive, h y d r o t r e a t i n g catalysts. Fe is

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generally regarded as a poorer promoter t h a n Co or Ni to Mo(1), and for t h a t it has been less frequently used. However, the beneficial effect of Fe, in catalysts formulations, have been shown in previous work(2,3). Thus, w hen Fe is added to V in Fe-V mixed sulphides, a weak synergy is found for hydrodesulphurization (HDS) of thiophene(2), and toluene hydrogenation a n d vanadyl octaethyl porphyrin (VOOEP) hydrodeporphyrinization (HDP). Also, it h a s been proposed(4) t hat Fe-Mo catalysts can be as active as Co-Mo catalysts (but less t h a n Ni-Mo). Thus, it has been found t hat Fe can be an efficient Mo promoter for HDS of thiophene, provided t hat it is kept in the ferrous state during preparation and activation of the catalyst(4) . On the other hand, Ho et. al(5-7) have prepared and studied Fe u n s u p p o r t e d Mo, a n d Fe-Co-Mo and Fe-Ni-Mo catalysts formed from thermal decomposition of bis(diethylenetriamine) iron thiomolybdate. Fe-Mo sulphides were quite selective towards hydrodenitrogenation (HDN) relative to HDS. They also found th at it consisted of a single sulphided phase, which during activity testing is partially transformed into a mixture of Fe sulphide and a MoS2 like phase. They also showed(7) t h a t HDS and HDN activities of the Fe-Mo catalysts can be significantly increased by promotion with Ni or Co. In a preliminary work(8) we have d e m o n s t r a t e d t h a t a m a r k e d synergetic effect, for HDP of VOOEP, is present in bulk Fe-Mo mixed sulphides. M6ssbauer spectroscopy revealed the existence of two p h a s e s for the mixed sulphides, one of which was proposed to be responsible for the HDP activity. In the present work we carry out a detailed study of Fe-Mo mixed sulphides in hydrotreatment. The catalysts were characterised by chemical analysis, X-ray photoelectron a nd M6ssbauer spectroscopies, as well as surface area determinations (BET). Dibenzothiophene (DBT) HDS a n d VOOEP HDP, in a continuos flow system were also tested.

2. EXPERIMENTAL.

Bulk Fe-Mo mixed sulphides were prepared according to our previous reported method(2,3). Thus, an a q u e o u s solution of Fe(III) nitrate (MERCK > 99%) was slowly added with stirring to a solution of a m m o n i u m tetrathiomolybdate (STREM CHEMICAL) in 20 % v / v a m m o n i u m sulphide u n d e r nitrogen at m os pher e (the use of an inert at m osphere represents a change from our previous method). After filtering off the obtained solids were v a c u u m dried and sulphided in a H2S/H2 (15 % v/v) flow at 673 K for 4 h. The a m o u n t s of iron nitrated and a m m o n i u m tetrathyomolibdate (TTM) were worked out in order to have the desire Fe/(Fe+Mo) atomic ratio. Iron sulphide was prepared in the same m a n n e r but without any TTM in the solution, a n d m o l y b d e n u m sulphide was prepared by direct sulphidation of TTM. After the sulphidation step the Service Central D'Analyse of the CNRS (Vemaison, France) analysed the samples (for Fe, Mo and S). Surface area determinations (BET) were obtained by nitrogen adsorption, on the airexposed solids, in a Q u a n t a s o r b Instrument. X-ray photoelectron

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s p e c t r o s c o p y (XPS) m e a s u r e m e n t s were carried out in a n AEI ES 200B s p e c t r o m e t e r equipped with a n AI anode working at 300 W. The atomic composition of the e x a m i n e d s a m p l e s were d e t e r m i n e d from the integrated Fe2p a n d Mo3d with a b a c k g r o u n d linearly s u b t r a c t e d . X-ray diffraction p a t t e r n s were obtained on a Siemens D500 diffractometer equipped with a Cu anode. S7Fe M 6 s s b a u e r s p e c t r a were recorded at room t e m p e r a t u r e in a t r i a n g u l a r s y m m e t r i c mode. The s p e c t r a were c o m p u t e r fitted with a c o m p u t e r p r o g r a m supplied by P. Bonville(9) u s i n g a hyperfine field distribution method(HPFD) capable of fitting u p to 40 s u b - s p e c t r a per distribution, with the s a m e values of the Isomer Shift (IS), the Q u a d r u p o l e Splitting (QS) a n d the full width at the half m a x i m u m of the lines. The results are expressed in a h i s t o g r a m of the proportions of the a r e a s of the different s u b s p e c t r a contributing to the complete s p e c t r u m . H y d r o d e p o r p h y r i n i z a t i o n (HDP) of vanadyl octaethyl p o r p h y r i n (VOOEP) was carried out at 573 K, in a h i g h - p r e s s u r e (80 bar) c o n t i n u o s flow (liquidsolid-gas) system. A solution of VOOEP (3 10 .4 mol I-I) in decaline containing 2%(v/v) of dimethyldisulfide was u s e d as a liquid feed. Experimental details are given elsewhere(3). Dibenzothiophene (DBT) hydrodesulfurization was carried out in a 100 cm3 b a t c h reactor. The solids were p r e s u l p h i d e d in situ with flowing H2S/H2 (10% v/v) at 573 K for 4 h, a n d at a t m o s p h e r i c p r e s s u r e . Then, 70 cm 3 of a solution of DBT in decaline (2.71x10 -2 mol I-I) were a d d e d to the reactor by m e a n s of a internal device in order to avoid a n y contact of the catalyst with air. The reaction was carried out at 573 K a n d 60 bars. Initial activities were worked out from the conversion v e r s u s time plots.

3. RESULTS.

X-ray diffraction p a t t e r n s show t h a t the p u r e iron sulphide h a s the pyrrhotite s t r u c t u r e , in a g r e e m e n t with previous reports(2,3), a n d the p u r e m o l y b d e n u m sulphide c o r r e s p o n d to a r a t h e r a m o r p h o u s MoS2. For the mixed Fe-Mo catalysts the diffraction p a t t e r n s are not very conclusive. BET surface a r e a s a n d chemical a n a l y s e s are reported in table 1.

Table 1. Catalyst FeMo-0 FeMo-015 FeMo-030 FeMo-052 FeMo-068 FeMo-073 FeMo- 1

Fe/(Fe+Mo) {atomic) 0.00 0.15 0.30 0.52 0.68 0.73 1.00

Surface a r e a (m2.g -I) 33 43 32 44 49 39 6

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BET surface a r e a s are of the s a m e o r d e r (within the e x p e r i m e n t a l error) for all the Fe-Mo mixed s u l p h i d e s , as well as for the Mo sulphide. The s u r f a c e a r e a of the Fe s u l p h i d e s is the lowest c o m p a r e to o t h e r catalysts, a n d this in a g r e e m e n t with previous findings(2,3), a n d with the fact t h a t XRD s h o w s this solid to be the b e t t e r crystallised. Activities for HDS of DBT a n d HDP of VOOEP are s h o w n in figures 1 a n d 2, r e s p e c t i v e l y . . We see t h a t for b o t h r e a c t i o n s p u r e Mo s u l p h i d e is m o r e active t h a n Fe sulphide, however, the difference is more m a r k e d for the HDS of DBT t h a n for the HDP of VOOETP. On the o t h e r h a n d a synergetic effect is o b s e r v e d in e a c h case. The m a x i m u m activity is observed for a Fe/(Fe+Mo) ratio of 0.52 for the HDS of DBT, a n d for a ratio of 0.68 for the HDP of VOOETP. We see a two-fold a n d a three-fold i n c r e a s e in activity, for HDS a n d HDP respectively, in relation to p u r e MoS2.

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Figure 1. Specific activity for HDS of DBT

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Figure 2. Specific activity for HDP of VOOEP

Figure 3 s h o w s the fitted s p e c t r a t o g e t h e r with the c o r r e s p o n d i n g h i s t o g r a m on the right side. The H P F D ' s s h o w the s a m e s h a p e for all the catalysts, except for p u r e Fe sulphide. The HPFD is t a k e n b e t w e e n 0 a n d 300 K G a u s s fields. A r o u g h way to c h a r a c t e r i s e the d i s t r i b u t i o n is to c a l c u l a t e the weighted average of the HPFD (). It is clear t h a t the i n c r e a s e s as the iron c o n t e n t is i n c r e a s e d from .25 to .68 (see figure 4). The p a r a m e t e r s IS a n d QS are c o m m o n to all the Fe c o n t a i n i n g mixed catalyst. F r o m the s p e c t r a the values for IS (0.43+0.03 m m s-l), referred to ~-iron, a n d QS (0.03+0.01 m m s -I) c a n be w o r k e d out. This s u g g e s t s t h a t the Fe is p r o b a b l y

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p r e s e n t as only one p h a s e in the mixed Fe-Mo solids s t u d i e d (this is m o r e evident for c a t a l y s t s from 0.25 to 0.73).

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XPS shows that Mo is in the form of MoS2, and that some sulphate is present in the surface of all solids. Sulphate is certainly due to some oxidation during sample handling (even though care was taken to avoid contact with moisture). It is also observed, by XPS, that surface concentration of Mo and Fe are very close to bulk concentrations (see figure 5), indicating a good homogeneity for the solids prepared.

The selectivities for tetrahydro dibenzo thiophene (THDBT), hexahydro dibenzo thiophene (HHDBT), biphenyl (BiPh), cyclohexyl benzene(PhCy) a n d

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bicyclohexyl, for the HDS of DBT, are p r e s e n t e d in figure 6. An increase in the % of PhCy a n d BiCy a n d a decrease in the % of THDBT, for Fe promote catalysts,, in relation to MoS2(FeMo00), is observed, which is indicative of a higher h y d r o g e n a t i n g ability for the p r o m o t e d catalysts

4. DISCUSSION

The results obtained for HDP activities are c o n s i s t e n t with o u r previous reports (that is a synergetic effect is observed), but, the Fe(Fe+Mo) ratio for m a x i m u m activity is different (0,40 n o m i n a l in o u r previous p a p e r a n d 0.68 in this paper). More i m p o r t a n t are the differences in the M 6 s s b a u e r spectra, while before we identified two p h a s e s in the mixed Fe-Mo catalysts, now we see only one. However, the p h a s e identified in o u r previous work as a FeMoS active p h a s e for HDP is similar to the one p h a s e obtained in the catalysts p r e s e n t e d here. These differences could be due to differences in the p r e p a r a t i o n m e t h o d s , since before the catalysts were p r e p a r e d in air, while now they were p r e p a r e d u n d e r nitrogen a t m o s p h e r e . Obviously the u s e of an inert a t m o s p h e r e play a n i m p o r t a n t role in the type of solid obtained. Also, Ho et. ai(5-7) got only one p h a s e for b u l k Fe-Mo s u l p h i d e d catalyst, b u t according to the M 6 s s b a u e r p a r a m e t e r s their p h a s e is different to our. However, it is i m p o r t a n t to point out the good HDN activity p r e s e n t e d by this catalyst, a n d the good HDP activity we obtain with o u r preparation. On the other h a n d , the s h a p e of the M 6 s s b a u e r s p e c t r a (and their t h e r m a l behaviour, which will be reported elsewhere) are very similar to the one observed in the new Spin Density Waves (SDW) s y s t e m s (namely CuFeS2 a n d CuFeTe2), recently reported(10,11). In the aforementioned s y s t e m s Fe is a Fe 2§ in low spin state, so the magnetic properties observed are a t t r i b u t e d to the highly correlated d c o n d u c t i o n electron (itinerant antiferromagnetism). In the p r e s e n t case, the values for IS a n d QS justify the a t t r i b u t i o n of the spectra to Fe 2§ low spin, a n d the evolution of the at room t e m p e r a t u r e for the different relative c o n c e n t r a t i o n s of Fe a n d Mo are due to the c h a n g e s in the density of states of the d electrons at the Fermi level(12). It is clear from figure 5 t h a t there is a linear relation between the a n d the HDP activity. This correlation is less evident for the HDS of DBT, however, the two m a x i m u m s (for HDP a n d HDS) are very close. T h u s , one can consider t h a t we are in the p r e s e n c e of a n electronic effect (increase in the d electron density of states at the Fermi level). This could be a general explanation for the synergy found in different HDS catalysts. In fact, this new evidence allows u s to review previous proposition of electron t r a n s f e r in related systems(13-15). The electronic effect p r o p o s e d could not be, in view of the new evidence, a net electron t r a n s f e r from one metal to the other, b u t is r a t h e r a c h a n g e in the electron density of the bimetallic s y s t e m as a whole. This c h a n g e in electron density is the effect of the s u b s t i t u t i o n of one metal by a n o t h e r in the s a m e crystal s t r u c t u r e . A

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mixing of the appropriate metal would c o n d u c e to a n o p t i m u m electron density of states of the d electrons in the c o n d u c t i o n band.

5. CONCLUSIONS.

A FeMoS p h a s e h a s been identified as the sole Fe containing p h a s e in b u l k Fe-Mo sulphided catalysts. This p h a s e could be the results of some Fe s u b s t i t u t i o n in the MoS2 structure. For HDP the activity is m a x i m u m at a Fe(Fe+Mo) atomic ratio of 0.68, a n d increases linearly with the weighted average hyperfine field , which is related to c h a n g e s in the density of d electrons at the Fermi level, clearly suggesting t h a t synergy is related to a n electronic effect. For HDS of DBT the activity m a x i m u m is located at a Fe/(Fe+Mo) atomic ratio of 0.52.

6. A C K N O W L E D G E M E N T S .

The a u t h o r s gratefully acknowledge the contribution m a d e by the F r e n c h Venezuelan PICS 324, a n d to CONICIT for its financial s u p p o r t t h r o u g h project G - 9 7 0 0 0 6 5 8 a n d BID-CONICIT QF15.

7. R E F E R E N C E S .

1. M. T e m a n , J. Catal., 104(1987)256. 2. C. E. Scott, B. P. Embaid, M. A. Luis, F. Gonzalez-Jimenez, L. Gengembre, R. H u b a u t a n d J. Grimblot, Bull. Soc. Chim. Belg., 104(1995)331. 3. C. E. Scott, B. P. Embaid, F. Gonzalez-Jimenez, R. H u b a u t a n d J. Grimblot, J. Catal., 166 (1997) 333. 4. J.L. Brito a n d A.L. Barbosa, J. Catal., 171(1997)467. 5. J.Y. Koo a n d T.C. Ho, Catl. Letters, 28(1994)99. 6. T.C. Ho, A. I. lacobson, R. R. Chianelli and C. R. F. Lund, J. Catal., 138 (1992) 351. 7. T. C. Ho, R. R. Chianelli and A. I. lacobson, Appl. Catal., 114(1994)127. 8. B. P. Embaid, M. A. Luis, C. E. Scott a n d F. Gonzalez-Jimenez, Hyperfine Interactions(C), 3(1998)96. 9. P. Bonville, S P E C / C E N Saclay, France. 10. A. Ribas, F. Gonzalez-Jimenez, L D'Onofrio, E. J a i m e s , M. Quintero a n d J. Gonzalez. Hyperfine Interactions 113( 1998)493. 11. F. Gonzalez-Jimenez, A. Ribas, E. J a i m e s , L D'Onofrio, M. Q u i n t e r o Quintero a n d J. Gonzalez. Phys B, In press. 12. P. C. H. Mitchell a n d C. E. Scott., Bull. Soc. Chim. Belg., 93(1984)619. 13. P. C. H. Mitchell a n d C. E. Scott, J. P. Bonnelle, J. G. Grimblot., J. Catal., 107(1987)482. 14. C. E. Scott, P. Betancourt, M. J. P6rez Zurita, C. Bolivar a n d J. Goldwasser., ApI~I. Catal. Submited.