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Development of deep hydrodesulfurization catalysts
Fuel Processing Technology 61 Ž1999. 89–101 www.elsevier.comrlocaterfuproc
Development of deep hydrodesulfurization catalysts I. CoMo and NiMo catalysts tested with žsubstituted / dibenzothiophene W.R.A.M. Robinson, J.A.R. van Veen ) , V.H.J. de Beer, R.A. van Santen Laboratory for Inorganic Chemistry and Catalysis, EindhoÕen UniÕersity of Technology, P.O. Box 513, 5600 MB EindhoÕen, Netherlands
Abstract The applicability of CoMo and NiMo based catalysts for the deep hydrodesulfurization ŽHDS. of diesel fuel was evaluated from gas phase experiments with model reactants. The reactivity of the latter decreased in the order DBT Ždibenzothiophene. ) 4MDBT Ž4-methyl dibenzothiophene. 4 4E6MDBT Ž4-ethyl, 6-methyl dibenzothiophene., caused by the increase in steric hindrance in the direct hydrogenolysis route. An easier Žindependent. reaction pathway was possible on catalysts with increased hydrogenation activity: when NiMo was supported on amorphous silica-alumina ŽASA. instead of alumina, the activity for 4MDBT and especially 4E6MDBT HDS was enhanced. The activity for 4E6MDBT HDS in the presence of 0.2% H 2 S was considered as indicative for the performance under deep desulfurization conditions: NiMorASA had the highest activity, followed by a CoMoractivated carbon catalyst and an optimised CoMorAl 2 O 3 co-impregnation catalyst. All three catalysts were slightly more active than a commercial CoMorAl 2 O 3 catalyst. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Hydrodesulfurization; Diesel fuel; Dibenzothiophene; 4-Ethyl, 6-methyl dibenzothiophene; 4Methyl dibenzothiophene
1. Introduction The aim of the integrated JOULE project JOU2-CT93-0409 was to develop new catalysts for the deep desulfurization of diesel fuel. In this framework, we have studied the performance of conventional CoMo and NiMo catalysts, in which attention was )
Corresponding author
0378-3820r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 Ž 9 9 . 0 0 0 3 3 - 8
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focused on the influence of preparation technique, MrMo ratio ŽM s Co, Mo., and support choice on the catalytic activity. Catalyst preparation was performed by co-impregnation, sequential impregnation and a complexing technique. The MrMo ratio was varied by choosing a fixed Mo loading while applying different amounts of ‘promoter’ metal. As supports, in addition to the obvious alumina, amorphous silica-alumina ŽASA. and activated carbon were applied. The latter support sometimes gives rise to surprisingly high activities w1x, while the acid properties of ASA can be useful in the desulfurization process Žvide infra.. Catalyst performance was evaluated in gas phase desulfurization reactions with a number of model reactants; the latter were preferred to real feedstock in order to avoid possible complications arising from the presence of aromatics and N-containing compounds. From the results, the performance under actual deep desulfurization conditions may be deduced. Principal model reactant is 4-ethyl, 6-methyl dibenzothiophene Ž4E6MDBT., which is strongly related to the most refractory compounds in current fuels w2,3x. The model compounds 4-methyl dibenzothiophene Ž4MDBT., dibenzothiophene ŽDBT. and thiophene are progressively less refractory; their desulfurization rates thus provide important comparison material. Fig. 1 illustrates several possible desulfurization pathways for 4E6MDBT. Direct hydrogenolysis of 4E6MDBT Žpath 1. to MEBP is sterically hindered, as the sub-
Fig. 1. Possible pathways for 4E6MDBT HDS: Ž1. direct hydrogenolysis, Ž2. via hydrogenation, Ž3. via dealkylation, Ž4. via C–C bond scission.
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stituents interfere with the coordination of the reactant to the surface via the sulfur atom. Route Ž2. involves hydrogenation of Žone of. the aromatic rings; as these rings become more flexible, the steric hindrance is greatly relieved w4,5x. The products of this pathway are MECHB’s. If the catalyst can Žpartially. dealkylate the 4E6MDBT molecule Ž3., either 4MDBT or DBT is formed, which desulfurize faster than 4E6MDBT over most catalysts w5,6x. Catalysts which possess activity for rupturing the C–C bond in the thiophenic ring produce diphenylsulfide-like molecules Ž4., which can desulfurize with comparative ease w5,6x. It is attempted to enhance the deep desulfurization activity by promoting the pathways Ž2., Ž3., and Ž4.. The ASA support may be especially effective in this respect: because of its modestly acidic properties, dealkylation Žor at least isomerization. of the substituents in hindering positions, as well as C–C bond rupture, may be feasible without appreciable formation of coke.
2. Experimental 2.1. Catalyst preparation The following supports were used in catalyst preparation Ž0.25–0.50 mm particles.: activated carbon ŽNorit RX3 Extra: pore volume ŽP.V.. 1.0 mlrg, surface area ŽS.A.. 1200 m2rg., g-Al 2 O 3 ŽKetjen 001-1.5E: P.V. 0.60 mlrg, S.A. 276 m2rg., amorphous silica-alumina ŽASA. Žex Shell: SirAl ratio 0.70 atrat, P.V. 0.72 mlrg, S.A. 389 m2rg.. A series of MorAl 2 O 3 catalysts with variable Mo loading was prepared by pore volume impregnation of the support with ammonium heptamolybdate ŽAHM, ŽNH 4 . 6Mo 7 O 24 ; Merck. solutions. After drying overnight at 383 K, calcination took place at 723 K in static air for 2 h. Based on the results of DBT hydrodesulfurization ŽHDS. experiments with the latter catalysts, an optimum Mo loading of 10 wt.% Žrelative to the bare support. for the bimetallic catalysts was determined. CoMorAl 2 O 3 catalysts with a fixed Mo loading of 10 wt.% and a variable CorMo Žatomic. ratio were prepared by a co-impregnation technique using an aqueous ammoniacal CoMo solution; the dryingrcalcination procedure was analogous to that used for MorAl 2 O 3 . CoMorASA catalysts with the same loadings were prepared following the same technique. A CoMorC catalyst Ž1.6 wt.% Co, 8 wt.% Mo. was prepared according to the so-called NTA-technique w7x: impregnation took place with an aqueous solution containing AHM, CoŽNO 3 . 2 and NTA ŽNitrilo Triacetic Acid., followed by drying. A commercial CoMorAl 2 O 3 catalyst ŽC444; 3 wt.% Co, 9.5 wt.% Mo. was supplied by Shell and served as reference. Because of their more promising properties, attention was paid to the optimization of the preparation of NiMorAl 2 O 3 catalysts. Three different techniques were used to prepare series with a fixed Mo loading of 10 wt.% ŽNTA series: 8 wt.%. and a variable NirMo ratio: U Co-impregnation: impregnation with aqueous AHM and NiŽNO 3 . 2 solution, followed by drying and calcination Žas with CoMorAl 2 O 3 ..
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Sequential impregnation: impregnation with aqueous AHM solution and with aqueous NiŽNO 3 . 2 solution, with drying and calcination after each step. U NTA-complex technique: impregnation with an aqueous solution containing AHM, NiŽNO 3 . 2 and NTA, followed by drying and calcination. A series of NiMorASA catalysts Ž10 wt.% Mo. was prepared by co-impregnation. In the notation of all binary catalysts, the CorMo or NirMo atomic ratio will be included in brackets. 2.2. HDS reactions with model compounds The thiophene HDS activity was measured in an atmospheric fixed-bed microflow reactor system. A catalyst sample Ž20 mg, diluted with 40 mg alumina. is loaded between quartz wool plugs in a quartz reactor. The catalyst is sulfided in a 10% H 2 SrH 2 gas mixture Ž60 mlrmin. for 2 h at 673 K. Next, the feed Ž5.4 mol% thiophene in H 2 . is passed over the catalyst Ž673 K. at 50 mlrmin. The products are analyzed by gas chromatography ŽHP 5890 equipped with a CP-Sil-5CB column and FID.. Typically, the activity after 3.5 h on stream is measured. A medium-pressure stainless steel microflow reactor system was used to test catalysts for gas phase HDS of DBT, 4-MDBT or 4E6MDBT. The reactant is dissolved ŽDBT: 1.0 wt.%; others: 0.5 wt.%. in n-decane, which solution is evaporated Ž0.1 mlrmin. into a hydrogen stream Ž0.5 Nlrmin.. The resulting reactant concentration in the gas phase is about 200 ppm for DBT and about 90 ppm for 4MDBT or 4E6MDBT. The reactor is loaded with 50 mg catalyst ŽDBT HDS: 20 mg., diluted with 5 g SiC. Prior to the reaction, the catalyst is treated in a 10% H 2 SrH 2 gas mixture Ž0.15 Nlrmin, 15 bar., raising the temperature with 6 Krmin Ž2 Krmin for NTA catalysts. to 673 K Ž2 h isothermal.. After the sulfidation period, the oven is cooled to the desired reaction temperature and the reactantrhydrogen flow Ž30 bar. is passed over the catalyst. If desired, the reactor feed gas can be enriched with 0.2–2% H 2 S by appropriate mixing with a H 2 SrH 2 stream. Typically, 7 h is allowed at each reaction condition for the catalyst to stabilize. The gas mixture from the reactor is analyzed on-line by GC ŽHP 5890 equipped with a CP-Sil-5CB column and FID. at 40–60 min intervals. In order to compare the HDS activities, for each model reaction a pseudo-first-order rate constant Ž k HD S . is calculated from the conversion ŽX. of the reactant, according to the formula: k HD S s y Ž Patm rPreact . ) Ž TreactrTatm . ) Ž FfeedrWcat . )ln Ž 1 y X . in which: Tatm , Patm s Temperature wKx and pressure wPax at ambient conditions, Treact , Preact s Temperature wKx and pressure wPax inside reactor, Ffeed s feed gas flow at ambient conditions wm3rsx, Wcat s catalyst mass wkgx. 3. Results 3.1. DBT and thiophene HDS Fig. 2 compares the DBT HDS activities at 573 K of the CoMorAl 2 O 3 and NiMorAl 2 O 3 catalysts as a function of their CorMo ŽNirMo. ratio. The activity of the
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Fig. 2. DBT HDS activities at 573 K of NiMorAl 2 O 3 prepared by co-impregnation Žv ., by sequential impregnation Ž^., and by the NTA technique ŽB., and of CoMorAl 2 O 3 prepared by co-impregnation Ž`..
Žco-impregnated. CoMorAl 2 O 3 catalysts appears to level off above CorMos 0.25. The co-impregnated NiMorAl 2 O 3 catalysts are clearly more active than the corresponding CoMo catalysts; moreover, a more complex, yet reproducible, correlation with the NirMo ratio was found. Comparing the differently prepared NiMorAl 2 O 3 catalysts, it can be seen that the NTA-technique resulted in more active catalysts than co-impregnation or sequential impregnation. All NiMorAl 2 O 3 and CoMorAl 2 O 3 catalyst series have also been tested in thiophene HDS at 673 K ŽFig. 3.. The activity of the NTA-prepared NiMorAl 2 O 3 catalysts is, again, increasing between NirMos 0.25 and 0.40, but in contrast to DBT HDS they are not superior to the other catalysts. The co-impregnated catalysts now show a much smoother trend, with the activity leveling off above NirMos 0.30. Also for the CoMorAl 2 O 3 catalysts, the trend is now quite different from the DBT results. For CoMo as well as NiMo catalysts, the use of ASA as support resulted in a lower DBT HDS activity compared with Al 2 O 3 ŽFig. 4.. On the other hand, activated carbon-supported CoMo catalysts can have a very high activity; Fig. 5 shows that the NTA-prepared catalyst CoMorC-NTA is far more active than CoMorAl 2 O 3 or CoMorASA. At 573 K, all catalysts produce BP and, in smaller quantities, CHB. To obtain information about their paths of formation, a DBT experiment was performed over a NiMorAl 2 O 3 catalyst at 553 K, in which conversions between 30 and 75% were realized by variation of the feed flow. In this conversion range, CHB and BP were produced in an almost constant ratio of 0.12:1. In a competitive experiment, replacing the DBT feed over the NiMorAl 2 O 3 catalyst by a DBTrBP Ž1:1. mixture hardly increased the CHB production. These findings suggest a mechanism in which BP and CHB are produced by parallel paths.
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Fig. 3. Thiophene HDS activities at 673 K of NiMorAl 2 O 3 prepared by co-impregnation Žv ., by sequential impregnation Ž^., by the NTA technique ŽB., and of CoMorAl 2 O 3 prepared by co-impregnation Ž`..
3.2. 4MDBT HDS Because of limited availability of the reactant, a selection was made from the DBT tested catalysts. In principle, from series with variable loading, the catalyst with the highest activity per gram of metal loading was selected. This resulted in the following
Fig. 4. DBT HDS activities at 573 K of NiMorASA Žv . and of CoMorASA Ž`. prepared by co-impregnation.
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Fig. 5. DBT HDS activities at 573 K without additional H 2 S.
CoMo catalysts: CoMoŽ0.25.rAl 2 O 3 , CoMoŽ0.30.rASA, CoMorC-NTA, and the CoMorAl 2 O 3 reference catalyst. The selected NiMo catalysts were NiMoŽ0.25.rAl 2 O 3 Žco-impregnated., NiMoŽ0.40.rAl 2 O 3-NTA, and NiMoŽ0.25.rASA. Fig. 6 compares the 4MDBT HDS activities at 573 K. CoMorASA and the CoMorAl 2 O 3 catalysts have the lowest activities; CoMorC, NiMorAl 2 O 3 and NiMorASA are clearly more active. This trend approximately matches the DBT HDS results, however, the activities are about 50% lower. The main reaction product is
Fig. 6. 4-MDBT HDS activities at 573 K without additional H 2 S and with 0.2% H 2 S.
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Fig. 7. 4MDBT HDS over CoMoŽ0.25.rAl 2 O 3 : products as a function of reaction temperature Žno H 2 S added..
always 3-methyl-biphenyl ŽMBP.; the methyl-cyclohexylbenzenes ŽMCHB’s. are formed in lesser amounts. The product selectivities of the catalysts are quite similar; Fig. 7 ŽCoMoŽ0.25.rAl 2 O 3 . gives a typical example. Upon addition of 0.2% H 2 S to the feed Žat 573 K., all catalysts show a considerably reduced activity ŽFig. 6., and the differences among the catalysts are small. The three NiMo catalysts were tested at higher temperatures as well ŽFig. 8.; at 623–648 K, NiMorASA clearly has the highest activity.
Fig. 8. 4MDBT HDS activities at 573 K Žno additional H 2 S. and at 573, 623, and 648 K with 0.2% H 2 S.
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Fig. 9. 4E6MDBT HDS activities at 573 and 623 K Žno additional H 2 S. and at 623 K with 0.2% H 2 S.
3.3. 4E6MDBT HDS All 4MDBT tested catalysts were selected for the 4E6MDBT HDS experiments with the exception of CoMoŽ0.30.rASA and NiMoŽ0.25.rAl 2 O 3 . In Fig. 9, the 4E6MDBT HDS activities of the catalysts at 573 K and 623 K are collected. As in 4MDBT HDS, CoMorC and NiMorASA are more active than both CoMorAl 2 O 3 catalysts; NiMorAl 2 O 3 on the other hand is now clearly inferior. For all catalysts the activity for
Fig. 10. 4E6MDBT HDS over CoMoŽ0.25.rAl 2 O 3 : products as a function of reaction temperature Žno H 2 S added..
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4E6MDBT HDS is much lower than for 4MDBT or DBT HDS, as can be seen by comparing Fig. 9 with Figs. 4 and 5. The product selectivities of the CoMo and NiMo catalysts are again quite comparable; Fig. 10 exemplifies the results for CoMoŽ0.25.rAl 2 O 3 . The main product at all temperatures is MEBP; the MECHB’s are formed in lesser amounts. Upon addition of 0.2% H 2 S to the feed Žat 623 K., the reduction of the activity of the CoMorAl 2 O 3 catalysts is quite small ŽFig. 9.; CoMorC and the two NiMo catalysts are more strongly affected. 4. Discussion 4.1. DBT and thiophene HDS The activity of co-impregnated CoMorAl 2 O 3 catalysts appeared to level off above CorMos 0.25. For analogously prepared NiMorAl 2 O 3 catalysts, however, the relation was strikingly different. Repetition of the activity measurements, as well as testing a new series of analogously prepared catalysts, resulted in essentially the same trend. A further study of the catalysts with HREM ŽHigh Resolution Electron Microscopy., XPS ŽX-ray Photoelectron Spectroscopy. and DOC ŽDynamic Oxygen Chemisorption. revealed no structural differences which could explain the observed trends in DBT HDS. It is generally assumed that in Al 2 O 3-supported NiMo catalysts there are at least two different NiMoS phases w8x. In co-impregnated catalysts, a so-called type I NiMoS phase has been reported which is not fully sulfided and strongly bonded to the support. Type II, on the other hand, is fully sulfided and has only weak interaction with the support. It has been reported that Type II has a much higher activity in thiophene HDS than Type I, but is slightly less active in DBT HDS w8x. The ‘smooth’ thiophene HDS activity curve we measured with co-impregnated NiMorAl 2 O 3 catalysts Žin which Type I is expected., however, does not appear to be a mirror image of the DBT HDS curve. For the other two NiMorAl 2 O 3 series, the thiophene HDS and DBT HDS results are more alike. The highest activity for NiMorAl 2 O 3 was realized when the complexing agent NTA was added during preparation. Two catalysts of this series ŽNirMos 0.50 and 0.70. were re-prepared, now omitting the calcination step, so as to obtain the so-called NiMoS-II phase. It was found, however, that the DBT HDS activity Žper Mo atom. was hardly different from co-impregnated catalysts. Possibly, the complexing of the Mo, and thus the formation of NiMoS-II, was incomplete due to the high NirMo ratio w9x. The effect of ASA on DBT HDS activity was slightly negative; possibly an overall loss of dispersion has occurred, because MoS 2 is known to spread less well on the silica part of the support. As found previously w1x, high dispersions are possible on activated carbon, resulting in a high activity. The results of the feed flow variation and the competitive experiment suggest a mechanism in which BP and CHB are produced by parallel paths; the former via direct hydrogenolysis Žpath 1 in Fig. 1., the latter via Žpartially. hydrogenated DBT, comparable with route Ž2. in Fig. 1. Hydrogenation of BP to CHB thus seems relatively unimportant; moreover, in the next contribution it is shown that BP hydrogenation is strongly impaired in the presence of strongly adsorbing S-containing molecules. It is
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expected that in 4MDBT and 4E6MDBT HDS the same arguments are valid, resulting in separate paths for Žsubstituted. BP’s and CHB’s production. 4.2. 4MDBT HDS The activities in 4MDBT HDS are clearly lower than in DBT HDS, in agreement with the extant literature w2–4x. The general trend, however, remained unchanged: in Fig. 11 we see that — with the exception of the CoMorAl 2 O 3 reference catalyst — all 4MDBTrDBT activity ratios are comprised between 0.45 and 0.65. As to the selectivity trends, it is striking that in 4MDBT HDS, the ratio of hydrogenated products to Žmethyl.BP is much higher than in DBT HDS. This can be caused by the selective retardation of the direct hydrogenolysis path by steric hindrance, and possibly a greater ease of hydrogenation of substituted DBT w10x. Use of ASA as carrier clearly increases this selectivity; this may be caused by a modified MoS 2 morphology, resulting in a lower overall activity, but a higher fraction of hydrogenation sites w11x. NiMo catalysts appear to be more sensitive to H 2 S than CoMo catalysts ŽFig. 6.; upon addition of 0.2% H 2 S to the feed, the NiMo catalysts show a larger percentual decrease in activity. This activity decrease can be counterbalanced by a temperature increase in the order of 50–75 K ŽFig. 8.. The significantly higher H 2 S resistance of NiMorASA may be caused by its higher hydrogenation activity; possibly, the hydrogenation route is less influenced by H 2 S w10x, placing NiMorASA in an advantage compared with its alumina-supported counterparts. 4.3. 4E6MDBT HDS Fig. 11 clearly illustrates that, over CoMo and NiMo catalysts, 4E6MDBT HDS is a far more demanding reaction than DBT HDS, in agreement with existing literature
Fig. 11. Relative activities of the catalysts Žno additional H 2 S.: k HDS Ž4MDBT.r k HDS ŽDBT. and k HD S Ž4E6MDBT.r k HDS ŽDBT. at 573 K Žn.m.s not measured..
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w4–6x. Direct hydrogenolysis Žpath 1 in Fig. 1. is now severely retarded by steric hindrance, causing an overall decrease in desulfurization rate of at least an order of magnitude, while hydrogenation Žpath 2. yields a less retarded desulfurization pathway. The higher hydrogenationrhydrogenolysis ratio can be clearly observed: when comparing 4E6MDBT with DBT HDS Žat not too different conversions., the ratio of hydrogenated products to biphenyls is clearly higher in 4E6MDBT HDS with all catalysts studied. A similar result is obtained when comparing 4MDBT and 4E6MDBT HDS product spectra ŽFigs. 7 and 10. for CoMoŽ0.25.rAl 2 O 3 : at comparable conversion, the ratio of hydrogenated products to Žsubstituted. BP is higher in 4E6MDBT HDS. As argued above, the performance of a catalyst in the 4E6MDBT test is indicative for its performance under actual deep desulfurization conditions. It should, however, be realized that the H 2 S partial pressure in actual practice is considerably higher than the 0.2% used in our model reactions ŽFig. 9.. The co-impregnated CoMorAl 2 O 3 catalyst, the CoMorC-NTA catalyst and the NiMorASA catalyst, although quite promising in the DBT test ŽFig. 5., appear only slightly more active in 4E6MDBT HDS than the commercial CoMorAl 2 O 3 Žreference. catalyst; NiMorAl 2 O 3-NTA is even less active. It is therefore unlikely that CoMo- or NiMo-based catalysts are suitable for a new generation of deep desulfurization catalysts.
5. Conclusions The HDS activity levels of the CoMo and NiMo catalysts appeared quite dependent on the type of reactant. The observed order in reactivity was DBT ) 4MDBT4 4E6MDBT, caused by the increase in steric hindrance in the direct hydrogenolysis route in the opposite order. The activity for 4MDBT and especially 4E6MDBT HDS could be enhanced by increased hydrogenation activity, providing an easier Žindependent. reaction pathway. The latter was observed if NiMo was supported on amorphous silicaalumina ŽASA. instead of alumina. In preparing NiMorAl 2 O 3 , the NTA-complexing technique resulted in a higher DBT HDS activity than co-impregnation or sequential impregnation. Compared with alumina-supported catalysts, ASA-supported CoMo and NiMo catalysts were less active in DBT and 4MDBT HDS. The use of activated carbon resulted in more active CoMo catalysts in DBT, 4MDBT and 4E6MDBT HDS. Judging from the 4E6MDBT HDS results, it is not expected that CoMo and NiMo catalysts can be modified to highly active deep desulfurization catalysts.
References w1x J.A.R. van Veen, H.A. Colijn, P.A.J.M. Hendriks, A.F. de Vries, Europacat-I, Book of Abstracts, Montpellier, 1993, p. 279. w2x X. Ma, K. Sakanishi, I. Mochida, Ind. Eng. Chem. Res. 33 Ž1994. 218. w3x X. Ma, K. Sakanishi, I. Mochida, Ind. Eng. Chem. Res. 35 Ž1996. 248. w4x T. Kabe, A. Ishihara, Q. Zhang, Appl. Catal. A 97 Ž1993. L1. w5x M.V. Landau, D. Berger, M. Herskowitz, J. Catal. 159 Ž1996. 236.
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w6x M.V. Landau, Catal. Today 36 Ž1997. 393. w7x J.A.R. van Veen, E. Gerkema, A.M. van der Kraan, A. Knoester, J. Chem. Soc. Chem. Commun. Ž1987. 1684. w8x J.A.R. van Veen, H.A. Colijn, P.A.J.M. Hendriks, A.J. van Welsenes, Fuel Processing Technol. 35 Ž1993. 137. w9x L. Medici, R. Prins, Europacat-II, Book of Abstracts, Maastricht, 1995, p. 45. w10x V. Lamure-Meille, E. Schulz, M. Lemaire, M. Vrinat, Appl. Catal. A 131 Ž1996. 143. w11x M. Daage, R.R. Chianelli, J. Catal. 149 Ž1994. 414.
Development of deep hydrodesulfurization catalysts I. CoMo and NiMo catalysts tested with žsubstituted / dibenzothiophene W.R.A.M. Robinson, J.A.R. van Veen ) , V.H.J. de Beer, R.A. van Santen Laboratory for Inorganic Chemistry and Catalysis, EindhoÕen UniÕersity of Technology, P.O. Box 513, 5600 MB EindhoÕen, Netherlands
Abstract The applicability of CoMo and NiMo based catalysts for the deep hydrodesulfurization ŽHDS. of diesel fuel was evaluated from gas phase experiments with model reactants. The reactivity of the latter decreased in the order DBT Ždibenzothiophene. ) 4MDBT Ž4-methyl dibenzothiophene. 4 4E6MDBT Ž4-ethyl, 6-methyl dibenzothiophene., caused by the increase in steric hindrance in the direct hydrogenolysis route. An easier Žindependent. reaction pathway was possible on catalysts with increased hydrogenation activity: when NiMo was supported on amorphous silica-alumina ŽASA. instead of alumina, the activity for 4MDBT and especially 4E6MDBT HDS was enhanced. The activity for 4E6MDBT HDS in the presence of 0.2% H 2 S was considered as indicative for the performance under deep desulfurization conditions: NiMorASA had the highest activity, followed by a CoMoractivated carbon catalyst and an optimised CoMorAl 2 O 3 co-impregnation catalyst. All three catalysts were slightly more active than a commercial CoMorAl 2 O 3 catalyst. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Hydrodesulfurization; Diesel fuel; Dibenzothiophene; 4-Ethyl, 6-methyl dibenzothiophene; 4Methyl dibenzothiophene
1. Introduction The aim of the integrated JOULE project JOU2-CT93-0409 was to develop new catalysts for the deep desulfurization of diesel fuel. In this framework, we have studied the performance of conventional CoMo and NiMo catalysts, in which attention was )
Corresponding author
0378-3820r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 3 8 2 0 Ž 9 9 . 0 0 0 3 3 - 8
90
W.R.A.M. Robinson et al.r Fuel Processing Technology 61 (1999) 89–101
focused on the influence of preparation technique, MrMo ratio ŽM s Co, Mo., and support choice on the catalytic activity. Catalyst preparation was performed by co-impregnation, sequential impregnation and a complexing technique. The MrMo ratio was varied by choosing a fixed Mo loading while applying different amounts of ‘promoter’ metal. As supports, in addition to the obvious alumina, amorphous silica-alumina ŽASA. and activated carbon were applied. The latter support sometimes gives rise to surprisingly high activities w1x, while the acid properties of ASA can be useful in the desulfurization process Žvide infra.. Catalyst performance was evaluated in gas phase desulfurization reactions with a number of model reactants; the latter were preferred to real feedstock in order to avoid possible complications arising from the presence of aromatics and N-containing compounds. From the results, the performance under actual deep desulfurization conditions may be deduced. Principal model reactant is 4-ethyl, 6-methyl dibenzothiophene Ž4E6MDBT., which is strongly related to the most refractory compounds in current fuels w2,3x. The model compounds 4-methyl dibenzothiophene Ž4MDBT., dibenzothiophene ŽDBT. and thiophene are progressively less refractory; their desulfurization rates thus provide important comparison material. Fig. 1 illustrates several possible desulfurization pathways for 4E6MDBT. Direct hydrogenolysis of 4E6MDBT Žpath 1. to MEBP is sterically hindered, as the sub-
Fig. 1. Possible pathways for 4E6MDBT HDS: Ž1. direct hydrogenolysis, Ž2. via hydrogenation, Ž3. via dealkylation, Ž4. via C–C bond scission.
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stituents interfere with the coordination of the reactant to the surface via the sulfur atom. Route Ž2. involves hydrogenation of Žone of. the aromatic rings; as these rings become more flexible, the steric hindrance is greatly relieved w4,5x. The products of this pathway are MECHB’s. If the catalyst can Žpartially. dealkylate the 4E6MDBT molecule Ž3., either 4MDBT or DBT is formed, which desulfurize faster than 4E6MDBT over most catalysts w5,6x. Catalysts which possess activity for rupturing the C–C bond in the thiophenic ring produce diphenylsulfide-like molecules Ž4., which can desulfurize with comparative ease w5,6x. It is attempted to enhance the deep desulfurization activity by promoting the pathways Ž2., Ž3., and Ž4.. The ASA support may be especially effective in this respect: because of its modestly acidic properties, dealkylation Žor at least isomerization. of the substituents in hindering positions, as well as C–C bond rupture, may be feasible without appreciable formation of coke.
2. Experimental 2.1. Catalyst preparation The following supports were used in catalyst preparation Ž0.25–0.50 mm particles.: activated carbon ŽNorit RX3 Extra: pore volume ŽP.V.. 1.0 mlrg, surface area ŽS.A.. 1200 m2rg., g-Al 2 O 3 ŽKetjen 001-1.5E: P.V. 0.60 mlrg, S.A. 276 m2rg., amorphous silica-alumina ŽASA. Žex Shell: SirAl ratio 0.70 atrat, P.V. 0.72 mlrg, S.A. 389 m2rg.. A series of MorAl 2 O 3 catalysts with variable Mo loading was prepared by pore volume impregnation of the support with ammonium heptamolybdate ŽAHM, ŽNH 4 . 6Mo 7 O 24 ; Merck. solutions. After drying overnight at 383 K, calcination took place at 723 K in static air for 2 h. Based on the results of DBT hydrodesulfurization ŽHDS. experiments with the latter catalysts, an optimum Mo loading of 10 wt.% Žrelative to the bare support. for the bimetallic catalysts was determined. CoMorAl 2 O 3 catalysts with a fixed Mo loading of 10 wt.% and a variable CorMo Žatomic. ratio were prepared by a co-impregnation technique using an aqueous ammoniacal CoMo solution; the dryingrcalcination procedure was analogous to that used for MorAl 2 O 3 . CoMorASA catalysts with the same loadings were prepared following the same technique. A CoMorC catalyst Ž1.6 wt.% Co, 8 wt.% Mo. was prepared according to the so-called NTA-technique w7x: impregnation took place with an aqueous solution containing AHM, CoŽNO 3 . 2 and NTA ŽNitrilo Triacetic Acid., followed by drying. A commercial CoMorAl 2 O 3 catalyst ŽC444; 3 wt.% Co, 9.5 wt.% Mo. was supplied by Shell and served as reference. Because of their more promising properties, attention was paid to the optimization of the preparation of NiMorAl 2 O 3 catalysts. Three different techniques were used to prepare series with a fixed Mo loading of 10 wt.% ŽNTA series: 8 wt.%. and a variable NirMo ratio: U Co-impregnation: impregnation with aqueous AHM and NiŽNO 3 . 2 solution, followed by drying and calcination Žas with CoMorAl 2 O 3 ..
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Sequential impregnation: impregnation with aqueous AHM solution and with aqueous NiŽNO 3 . 2 solution, with drying and calcination after each step. U NTA-complex technique: impregnation with an aqueous solution containing AHM, NiŽNO 3 . 2 and NTA, followed by drying and calcination. A series of NiMorASA catalysts Ž10 wt.% Mo. was prepared by co-impregnation. In the notation of all binary catalysts, the CorMo or NirMo atomic ratio will be included in brackets. 2.2. HDS reactions with model compounds The thiophene HDS activity was measured in an atmospheric fixed-bed microflow reactor system. A catalyst sample Ž20 mg, diluted with 40 mg alumina. is loaded between quartz wool plugs in a quartz reactor. The catalyst is sulfided in a 10% H 2 SrH 2 gas mixture Ž60 mlrmin. for 2 h at 673 K. Next, the feed Ž5.4 mol% thiophene in H 2 . is passed over the catalyst Ž673 K. at 50 mlrmin. The products are analyzed by gas chromatography ŽHP 5890 equipped with a CP-Sil-5CB column and FID.. Typically, the activity after 3.5 h on stream is measured. A medium-pressure stainless steel microflow reactor system was used to test catalysts for gas phase HDS of DBT, 4-MDBT or 4E6MDBT. The reactant is dissolved ŽDBT: 1.0 wt.%; others: 0.5 wt.%. in n-decane, which solution is evaporated Ž0.1 mlrmin. into a hydrogen stream Ž0.5 Nlrmin.. The resulting reactant concentration in the gas phase is about 200 ppm for DBT and about 90 ppm for 4MDBT or 4E6MDBT. The reactor is loaded with 50 mg catalyst ŽDBT HDS: 20 mg., diluted with 5 g SiC. Prior to the reaction, the catalyst is treated in a 10% H 2 SrH 2 gas mixture Ž0.15 Nlrmin, 15 bar., raising the temperature with 6 Krmin Ž2 Krmin for NTA catalysts. to 673 K Ž2 h isothermal.. After the sulfidation period, the oven is cooled to the desired reaction temperature and the reactantrhydrogen flow Ž30 bar. is passed over the catalyst. If desired, the reactor feed gas can be enriched with 0.2–2% H 2 S by appropriate mixing with a H 2 SrH 2 stream. Typically, 7 h is allowed at each reaction condition for the catalyst to stabilize. The gas mixture from the reactor is analyzed on-line by GC ŽHP 5890 equipped with a CP-Sil-5CB column and FID. at 40–60 min intervals. In order to compare the HDS activities, for each model reaction a pseudo-first-order rate constant Ž k HD S . is calculated from the conversion ŽX. of the reactant, according to the formula: k HD S s y Ž Patm rPreact . ) Ž TreactrTatm . ) Ž FfeedrWcat . )ln Ž 1 y X . in which: Tatm , Patm s Temperature wKx and pressure wPax at ambient conditions, Treact , Preact s Temperature wKx and pressure wPax inside reactor, Ffeed s feed gas flow at ambient conditions wm3rsx, Wcat s catalyst mass wkgx. 3. Results 3.1. DBT and thiophene HDS Fig. 2 compares the DBT HDS activities at 573 K of the CoMorAl 2 O 3 and NiMorAl 2 O 3 catalysts as a function of their CorMo ŽNirMo. ratio. The activity of the
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Fig. 2. DBT HDS activities at 573 K of NiMorAl 2 O 3 prepared by co-impregnation Žv ., by sequential impregnation Ž^., and by the NTA technique ŽB., and of CoMorAl 2 O 3 prepared by co-impregnation Ž`..
Žco-impregnated. CoMorAl 2 O 3 catalysts appears to level off above CorMos 0.25. The co-impregnated NiMorAl 2 O 3 catalysts are clearly more active than the corresponding CoMo catalysts; moreover, a more complex, yet reproducible, correlation with the NirMo ratio was found. Comparing the differently prepared NiMorAl 2 O 3 catalysts, it can be seen that the NTA-technique resulted in more active catalysts than co-impregnation or sequential impregnation. All NiMorAl 2 O 3 and CoMorAl 2 O 3 catalyst series have also been tested in thiophene HDS at 673 K ŽFig. 3.. The activity of the NTA-prepared NiMorAl 2 O 3 catalysts is, again, increasing between NirMos 0.25 and 0.40, but in contrast to DBT HDS they are not superior to the other catalysts. The co-impregnated catalysts now show a much smoother trend, with the activity leveling off above NirMos 0.30. Also for the CoMorAl 2 O 3 catalysts, the trend is now quite different from the DBT results. For CoMo as well as NiMo catalysts, the use of ASA as support resulted in a lower DBT HDS activity compared with Al 2 O 3 ŽFig. 4.. On the other hand, activated carbon-supported CoMo catalysts can have a very high activity; Fig. 5 shows that the NTA-prepared catalyst CoMorC-NTA is far more active than CoMorAl 2 O 3 or CoMorASA. At 573 K, all catalysts produce BP and, in smaller quantities, CHB. To obtain information about their paths of formation, a DBT experiment was performed over a NiMorAl 2 O 3 catalyst at 553 K, in which conversions between 30 and 75% were realized by variation of the feed flow. In this conversion range, CHB and BP were produced in an almost constant ratio of 0.12:1. In a competitive experiment, replacing the DBT feed over the NiMorAl 2 O 3 catalyst by a DBTrBP Ž1:1. mixture hardly increased the CHB production. These findings suggest a mechanism in which BP and CHB are produced by parallel paths.
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Fig. 3. Thiophene HDS activities at 673 K of NiMorAl 2 O 3 prepared by co-impregnation Žv ., by sequential impregnation Ž^., by the NTA technique ŽB., and of CoMorAl 2 O 3 prepared by co-impregnation Ž`..
3.2. 4MDBT HDS Because of limited availability of the reactant, a selection was made from the DBT tested catalysts. In principle, from series with variable loading, the catalyst with the highest activity per gram of metal loading was selected. This resulted in the following
Fig. 4. DBT HDS activities at 573 K of NiMorASA Žv . and of CoMorASA Ž`. prepared by co-impregnation.
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Fig. 5. DBT HDS activities at 573 K without additional H 2 S.
CoMo catalysts: CoMoŽ0.25.rAl 2 O 3 , CoMoŽ0.30.rASA, CoMorC-NTA, and the CoMorAl 2 O 3 reference catalyst. The selected NiMo catalysts were NiMoŽ0.25.rAl 2 O 3 Žco-impregnated., NiMoŽ0.40.rAl 2 O 3-NTA, and NiMoŽ0.25.rASA. Fig. 6 compares the 4MDBT HDS activities at 573 K. CoMorASA and the CoMorAl 2 O 3 catalysts have the lowest activities; CoMorC, NiMorAl 2 O 3 and NiMorASA are clearly more active. This trend approximately matches the DBT HDS results, however, the activities are about 50% lower. The main reaction product is
Fig. 6. 4-MDBT HDS activities at 573 K without additional H 2 S and with 0.2% H 2 S.
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Fig. 7. 4MDBT HDS over CoMoŽ0.25.rAl 2 O 3 : products as a function of reaction temperature Žno H 2 S added..
always 3-methyl-biphenyl ŽMBP.; the methyl-cyclohexylbenzenes ŽMCHB’s. are formed in lesser amounts. The product selectivities of the catalysts are quite similar; Fig. 7 ŽCoMoŽ0.25.rAl 2 O 3 . gives a typical example. Upon addition of 0.2% H 2 S to the feed Žat 573 K., all catalysts show a considerably reduced activity ŽFig. 6., and the differences among the catalysts are small. The three NiMo catalysts were tested at higher temperatures as well ŽFig. 8.; at 623–648 K, NiMorASA clearly has the highest activity.
Fig. 8. 4MDBT HDS activities at 573 K Žno additional H 2 S. and at 573, 623, and 648 K with 0.2% H 2 S.
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Fig. 9. 4E6MDBT HDS activities at 573 and 623 K Žno additional H 2 S. and at 623 K with 0.2% H 2 S.
3.3. 4E6MDBT HDS All 4MDBT tested catalysts were selected for the 4E6MDBT HDS experiments with the exception of CoMoŽ0.30.rASA and NiMoŽ0.25.rAl 2 O 3 . In Fig. 9, the 4E6MDBT HDS activities of the catalysts at 573 K and 623 K are collected. As in 4MDBT HDS, CoMorC and NiMorASA are more active than both CoMorAl 2 O 3 catalysts; NiMorAl 2 O 3 on the other hand is now clearly inferior. For all catalysts the activity for
Fig. 10. 4E6MDBT HDS over CoMoŽ0.25.rAl 2 O 3 : products as a function of reaction temperature Žno H 2 S added..
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4E6MDBT HDS is much lower than for 4MDBT or DBT HDS, as can be seen by comparing Fig. 9 with Figs. 4 and 5. The product selectivities of the CoMo and NiMo catalysts are again quite comparable; Fig. 10 exemplifies the results for CoMoŽ0.25.rAl 2 O 3 . The main product at all temperatures is MEBP; the MECHB’s are formed in lesser amounts. Upon addition of 0.2% H 2 S to the feed Žat 623 K., the reduction of the activity of the CoMorAl 2 O 3 catalysts is quite small ŽFig. 9.; CoMorC and the two NiMo catalysts are more strongly affected. 4. Discussion 4.1. DBT and thiophene HDS The activity of co-impregnated CoMorAl 2 O 3 catalysts appeared to level off above CorMos 0.25. For analogously prepared NiMorAl 2 O 3 catalysts, however, the relation was strikingly different. Repetition of the activity measurements, as well as testing a new series of analogously prepared catalysts, resulted in essentially the same trend. A further study of the catalysts with HREM ŽHigh Resolution Electron Microscopy., XPS ŽX-ray Photoelectron Spectroscopy. and DOC ŽDynamic Oxygen Chemisorption. revealed no structural differences which could explain the observed trends in DBT HDS. It is generally assumed that in Al 2 O 3-supported NiMo catalysts there are at least two different NiMoS phases w8x. In co-impregnated catalysts, a so-called type I NiMoS phase has been reported which is not fully sulfided and strongly bonded to the support. Type II, on the other hand, is fully sulfided and has only weak interaction with the support. It has been reported that Type II has a much higher activity in thiophene HDS than Type I, but is slightly less active in DBT HDS w8x. The ‘smooth’ thiophene HDS activity curve we measured with co-impregnated NiMorAl 2 O 3 catalysts Žin which Type I is expected., however, does not appear to be a mirror image of the DBT HDS curve. For the other two NiMorAl 2 O 3 series, the thiophene HDS and DBT HDS results are more alike. The highest activity for NiMorAl 2 O 3 was realized when the complexing agent NTA was added during preparation. Two catalysts of this series ŽNirMos 0.50 and 0.70. were re-prepared, now omitting the calcination step, so as to obtain the so-called NiMoS-II phase. It was found, however, that the DBT HDS activity Žper Mo atom. was hardly different from co-impregnated catalysts. Possibly, the complexing of the Mo, and thus the formation of NiMoS-II, was incomplete due to the high NirMo ratio w9x. The effect of ASA on DBT HDS activity was slightly negative; possibly an overall loss of dispersion has occurred, because MoS 2 is known to spread less well on the silica part of the support. As found previously w1x, high dispersions are possible on activated carbon, resulting in a high activity. The results of the feed flow variation and the competitive experiment suggest a mechanism in which BP and CHB are produced by parallel paths; the former via direct hydrogenolysis Žpath 1 in Fig. 1., the latter via Žpartially. hydrogenated DBT, comparable with route Ž2. in Fig. 1. Hydrogenation of BP to CHB thus seems relatively unimportant; moreover, in the next contribution it is shown that BP hydrogenation is strongly impaired in the presence of strongly adsorbing S-containing molecules. It is
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expected that in 4MDBT and 4E6MDBT HDS the same arguments are valid, resulting in separate paths for Žsubstituted. BP’s and CHB’s production. 4.2. 4MDBT HDS The activities in 4MDBT HDS are clearly lower than in DBT HDS, in agreement with the extant literature w2–4x. The general trend, however, remained unchanged: in Fig. 11 we see that — with the exception of the CoMorAl 2 O 3 reference catalyst — all 4MDBTrDBT activity ratios are comprised between 0.45 and 0.65. As to the selectivity trends, it is striking that in 4MDBT HDS, the ratio of hydrogenated products to Žmethyl.BP is much higher than in DBT HDS. This can be caused by the selective retardation of the direct hydrogenolysis path by steric hindrance, and possibly a greater ease of hydrogenation of substituted DBT w10x. Use of ASA as carrier clearly increases this selectivity; this may be caused by a modified MoS 2 morphology, resulting in a lower overall activity, but a higher fraction of hydrogenation sites w11x. NiMo catalysts appear to be more sensitive to H 2 S than CoMo catalysts ŽFig. 6.; upon addition of 0.2% H 2 S to the feed, the NiMo catalysts show a larger percentual decrease in activity. This activity decrease can be counterbalanced by a temperature increase in the order of 50–75 K ŽFig. 8.. The significantly higher H 2 S resistance of NiMorASA may be caused by its higher hydrogenation activity; possibly, the hydrogenation route is less influenced by H 2 S w10x, placing NiMorASA in an advantage compared with its alumina-supported counterparts. 4.3. 4E6MDBT HDS Fig. 11 clearly illustrates that, over CoMo and NiMo catalysts, 4E6MDBT HDS is a far more demanding reaction than DBT HDS, in agreement with existing literature
Fig. 11. Relative activities of the catalysts Žno additional H 2 S.: k HDS Ž4MDBT.r k HDS ŽDBT. and k HD S Ž4E6MDBT.r k HDS ŽDBT. at 573 K Žn.m.s not measured..
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w4–6x. Direct hydrogenolysis Žpath 1 in Fig. 1. is now severely retarded by steric hindrance, causing an overall decrease in desulfurization rate of at least an order of magnitude, while hydrogenation Žpath 2. yields a less retarded desulfurization pathway. The higher hydrogenationrhydrogenolysis ratio can be clearly observed: when comparing 4E6MDBT with DBT HDS Žat not too different conversions., the ratio of hydrogenated products to biphenyls is clearly higher in 4E6MDBT HDS with all catalysts studied. A similar result is obtained when comparing 4MDBT and 4E6MDBT HDS product spectra ŽFigs. 7 and 10. for CoMoŽ0.25.rAl 2 O 3 : at comparable conversion, the ratio of hydrogenated products to Žsubstituted. BP is higher in 4E6MDBT HDS. As argued above, the performance of a catalyst in the 4E6MDBT test is indicative for its performance under actual deep desulfurization conditions. It should, however, be realized that the H 2 S partial pressure in actual practice is considerably higher than the 0.2% used in our model reactions ŽFig. 9.. The co-impregnated CoMorAl 2 O 3 catalyst, the CoMorC-NTA catalyst and the NiMorASA catalyst, although quite promising in the DBT test ŽFig. 5., appear only slightly more active in 4E6MDBT HDS than the commercial CoMorAl 2 O 3 Žreference. catalyst; NiMorAl 2 O 3-NTA is even less active. It is therefore unlikely that CoMo- or NiMo-based catalysts are suitable for a new generation of deep desulfurization catalysts.
5. Conclusions The HDS activity levels of the CoMo and NiMo catalysts appeared quite dependent on the type of reactant. The observed order in reactivity was DBT ) 4MDBT4 4E6MDBT, caused by the increase in steric hindrance in the direct hydrogenolysis route in the opposite order. The activity for 4MDBT and especially 4E6MDBT HDS could be enhanced by increased hydrogenation activity, providing an easier Žindependent. reaction pathway. The latter was observed if NiMo was supported on amorphous silicaalumina ŽASA. instead of alumina. In preparing NiMorAl 2 O 3 , the NTA-complexing technique resulted in a higher DBT HDS activity than co-impregnation or sequential impregnation. Compared with alumina-supported catalysts, ASA-supported CoMo and NiMo catalysts were less active in DBT and 4MDBT HDS. The use of activated carbon resulted in more active CoMo catalysts in DBT, 4MDBT and 4E6MDBT HDS. Judging from the 4E6MDBT HDS results, it is not expected that CoMo and NiMo catalysts can be modified to highly active deep desulfurization catalysts.
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w6x M.V. Landau, Catal. Today 36 Ž1997. 393. w7x J.A.R. van Veen, E. Gerkema, A.M. van der Kraan, A. Knoester, J. Chem. Soc. Chem. Commun. Ž1987. 1684. w8x J.A.R. van Veen, H.A. Colijn, P.A.J.M. Hendriks, A.J. van Welsenes, Fuel Processing Technol. 35 Ž1993. 137. w9x L. Medici, R. Prins, Europacat-II, Book of Abstracts, Maastricht, 1995, p. 45. w10x V. Lamure-Meille, E. Schulz, M. Lemaire, M. Vrinat, Appl. Catal. A 131 Ž1996. 143. w11x M. Daage, R.R. Chianelli, J. Catal. 149 Ž1994. 414.