[elearnica.ir]-effect of re loading on the structure activity and selectivity of re c cat

Applied Catalysis A: General 240 (2003) 151–160

Effect of Re loading on the structure, activityand selectivity of Re/C catalysts in hydrodenitrogenation and

hydrodesulphurisation of gas oil

N. Escalonaa, M. Yatesb, P. Ávilab, A. López Agudob,J.L. Garcıa Fierrob, J. Ojedaa, F.J. Gil-Llambıasa,∗

a Universidad de Santiago de Chile, Casilla 40, Correo 33, Santiago, Chileb Instituto de Catálisis y Petroleoquımica, CSIC, Cantoblanco, 28049 Madrid, Spain

Received 26 April 2002; received in revised form 26 July 2002; accepted 27 July 2002

Abstract

A series of Re-containing catalysts supported on activated carbon, with Re loading between 0.74 and 11.44 wt.% Re2O7, wasprepared by wet impregnation and tested in the simultaneous hydrodesulphurisation (HDS) and hydrodenitrogenation (HDN)of a commercial gas oil. Textural analysis, XRD, X-ray photoelectron spectroscopy (XPS) and surface acidity techniqueswere used for physicochemical characterisation of the catalysts. Increase in the Re concentration resulted in a rise in the HDSand HDN activity due to the formation of a monolayer structure of Re and the higher surface acidity. At Re concentrations>2.47 wt.% Re2O7 (0.076 Re atoms nm−2) the reduction in the catalytic activity was related to the loss in specific surfacearea (BET) due to reduction in the microporosity of the carbon support. The magnitude of the catalytic effect was differentfor HDS and HDN, and depended strongly on the Re content and reaction temperature. The apparent activation energies wereabout 116–156 kJ mol−1 for HDS and 24–30 kJ mol−1 for HDN. This led to a marked increase in the HDN/HDS selectivitywith decreasing temperature (values >3 at 325◦C), due to the large differences in the apparent activation energies of HDS andHDN found for all catalysts. A gradual increase in the HDN/HDS selectivity with increased Re loading was also found andrelated to the observed increase of catalyst acidity. The results are compared with those obtained for a series of Re/�-Al2O3

catalysts.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:Supported Re catalysts; Re sulphide; Hydrodenitrogenation (HDN); Hydrodesulphurisation (HDS)

1. Introduction

In the future, a new generation of catalysts fordeeper hydrodesulphurisation (HDS) and hydrodeni-trogenation (HDN) and major reduction of aromaticcontent in diesel fuel, more efficient than the con-ventional alumina-supported Co (or Ni)-Mo (or W)

∗ Corresponding author. Fax:+52-562-6812108.E-mail address:[email protected] (F.J. Gil-Llambıas).

catalysts, will be necessary to satisfy the demandfor cleaner transport fuels[1,2]. For this purpose,considerable efforts have been made in recent yearsto develop more active hydrotreating catalysts basedeither on new active phases or new and modifiedsupports[2–4].

Several systematic experimental studies of the cata-lytic properties of transition metal sulphides demon-strated that unsupported or carbon-supported Resulphide catalysts had a high activity for HDS[5–7]

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0926-860X(02)00430-1

Downloaded from http://www.elearnica.ir

152 N. Escalona et al. / Applied Catalysis A: General 240 (2003) 151–160

and HDN reactions[8,9]. However, relatively few de-tailed studies on the catalytic HDS or HDN activitiesof supported Re-based catalysts have been reported[10–14]. In a previous study[15], we found that theRe/�-Al2O3 system exhibited a high selectivity forHDN reactions relative to HDS. This behaviour isunusual on conventional supported molybdenum ortungsten sulphide catalysts, which have generallyhigher HDS activity relative to HDN activity. Excep-tional properties for HDN have been reported onlywith catalysts of unconventional composition, such asunsupported Fe-Mo or Fe-W sulphide catalysts[16],certain noble metal sulphides supported on active car-bon[17] and Mo-Ir/�-Al2O3 sulphided catalysts[18].However, most of these catalysts gave HDN/HDS ra-tios generally<1, while on Re/�-Al2O3 catalysts theHDN/HDS ratios were between 1.5 and 2.5 at 325◦C,depending on Re content, and<1 at higher reactiontemperatures[15]. This exceptional HDS selectivityof Re/�-Al2O3 catalysts is a very promising resultfor improvement of catalysts for HDN, and deservesfurther research on other supports more inert than alu-mina, such as carbon. Metal sulphides supported oncarbon are known to have a higher intrinsic activity inHDS, HDN and hydrogenation (HYD) reactions thanthe corresponding�-Al2O3-supported ones[6,19].The high activity of carbon-supported Co-Mo cata-lysts has been usually explained in terms of a weakcatalyst–carbon interaction, which leads to a higherdegree of sulphidation and formation of the more ac-tive Co-Mo-S type II structure[19], but the role of car-bon is still not clear. Recently, Chianelli and Pecorarohave claimed that carbon stabilises MoS2 particles,keeping crystallites smaller and less stacked, leadingto a better dispersion on a carbon support[20]. In thatstudy, it was also suggested that the active surfacein the stabilised structure of the molybdenum sul-phide phase was carbided. In the case of Re sulphide

Table 1Composition and physical characteristics of oxidic catalysts

Catalyst Re loading(wt.% Re2O7)

Re loading(atoms nm−2)

SBET

(m2 g−1)Sext

(m2 g−1)Total pore volume(cm3 g−1)

Micro porediameter (nm)

Re(0.00)/C – – 817 44 0.637 0.865Re(0.024)/C 0.74 0.024 795 51 0.739 0.847Re(0.076)/C 2.47 0.076 819 51 0.776 0.941Re(0.135)/C 4.29 0.135 704 49 0.684 0.954Re(0.380)/C 11.44 0.380 584 47 0.593 0.954

catalysts, it has previously been reported that the thio-phene HDS activity increased in the order SiO2 <

�-Al 2O3 < C and the Re catalysts were 2–20 timesmore active than molybdenum ones[10]. Thus, cata-lysts based on Re/C could be an interesting alternativefor hydrotreating processes, especially for HDN.

In the present study, the performance of a series ofsulphided Re catalysts supported on activated carbonwas examined in the simultaneous HDS and HDN ofa commercial gas oil. Catalysts were characterised byseveral physicochemical techniques.

2. Experimental

2.1. Catalyst preparation

Re-based catalysts at various metal loadings (0.74–11.44 wt.% Re2O7) were prepared by wet impregna-tion of the activated carbon support (SBET 817 m2 g−1,total pore volume 0.637 cm3 g−1, particle size20–16 mesh) with aqueous solutions of the appro-priate concentrations of NH4ReO4 (Aldrich, p.a.)in a rotary evaporator. After impregnation, the sam-ples were dried at 110◦C for 12 h. The Re loadingwas expressed as atoms nm−2 of the support. TheRe content was determined by inductively coupledplasma-atomic emission spectroscopy (ICP-AES), ina Perkin-Elmer model Optima 3300 DV spectrometerusing the 221.426 nm Re emission line. The catalystslisted in Table 1are denoted by the number of metalatoms nm−2 of initial support area, e.g. Re(0.076)/Ccontains 0.076 atoms of Re nm−2 (2.47 wt.% Re2O7).

2.2. Catalyst characterisation

The specific surface areas, micro and mesopore vol-umes and pore size distributions of the samples weredetermined from analysis of N2 adsorption–desorption

N. Escalona et al. / Applied Catalysis A: General 240 (2003) 151–160 153

isotherms at−196◦C using a Carlo Erba 1800 Sorp-tomatic apparatus. The samples were previously outgassed at 110◦C to a final vacuum of<0.05 Pa. TheBET method[21] was employed to determine the spe-cific surface areas, taking the area of the N2 moleculeas 0.162 nm2 and the thickness of the monolayer as0.354 nm. As all of the samples were microporous, thelinear region of the BET equation was taken in therange of relative pressures between 0.02 and 0.12 p/p◦.The adsorption isotherms were also used to calculatethe micropore volume and external surface area by at-plot analysis[22] using the equation of Halsey[23]to determine the thickness of the adsorbed layer ateach relative pressure value. The micropore size dis-tributions were determined by the method developedby Mikhail et al. applied to the correspondingt-plots[24]. The mesoporosity was determined from the dif-ference between the microporosity calculated from thet-plot and the volume adsorbed at a relative pressureof 0.96 on the desorption branch of the correspond-ing isotherms, equivalent to a pore diameter of 50 nm.Mercury intrusion porosimetry (MIP) analyses weredetermined on samples previously dried overnight at110◦C in a Fisons Pascal 140/240 apparatus. Startingfrom vacuum and raising the pressure to 200 MPa thistechnique covers the pore diameters of∼300�m downto 7.5 nm applying the Washburn equation. Summa-tion of the micro and mesopore volumes obtained fromN2 isotherms with the pore volumes in pores >50 nmdetermined by MIP lead to the total pore volume ofthe sample.

The chemical state and surface composition ofthe sulphided catalysts were studied by X-ray pho-toelectron spectroscopy (XPS). The XP spectra wererecorded in a VG Escalab 200R electron spectrome-ter equipped with a hemispherical electron analyserand a Mg K� (1253.6 eV) photon source. Energycorrections were performed employing the C line ofthe carbon support at 284.9 eV as internal reference.The catalyst samples for XPS were pre-sulphided exsitu with a mixture of 10% H2S/H2 at 350◦C for4 h. The samples were then cooled to room temper-ature, flushed with He and transferred into flaskscontaining iso-octane. The intensities of the peakswere estimated by calculating the integral of eachpeak after subtracting an S-shaped background andfitting the experimental curve to a combination ofGaussian/Lorentzian lines.

The X-ray diffraction (XRD) was carried out on aSiemens D5000 diffractometer using the Cu K� radi-ation (λ = 1.540598 Å) operating at 40 kV and 40 mAand scanning 2θ angles in the range from 10 to 50◦.

The surface acidity of oxided catalysts was mea-sured potentiometrically by titration withn-butylaminein acetonitrile using an Ag/AgCl electrode[25]. Acidstrength was estimated by the initial electrode poten-tial, Ei .

2.3. Activity measurements

As in a previous study[15], catalytic measure-ments for simultaneous HDS and HDN of gas oilwere carried out in a high-pressure continuous-flowmicro-reactor. The catalyst bed consisted of 1 g ofcatalyst diluted 1:1 (v/v) with SiC particles to op-timise hydrodynamics. The remaining space in thereactor was filled with SiC particles. Prior to reaction,the catalysts were sulphided with a 7 vol.% CS2/gasoil mixture at 350◦C and 2 MPa total pressure for4 h. The feed for HDS and HDN was a commercialgas oil, containing 470 ppm S and 190 ppm N. Theperformance of the catalysts was determined in thetemperature range 325–375◦C under standard con-ditions: 3 MPa total pressure 9 h−1 LHSV, 3600 h−1

GHSV, and H2/feed ratio of 400. Under these reactionconditions the catalysts were stable and the reactionwas not controlled by mass transfer phenomena[26].In all the experiments a stabilisation period of at least2 h was allowed before the first sample was collected.Total sulphur in the effluents was determined byiodometric titration of SO2 using a LECO analyser,and total nitrogen was analysed on a Antek 703C in-strument by chemiluminescence detection. HDS andHDN conversions were defined as percent of totalsulphur and nitrogen, respectively, removed from theinitial gas oil.

3. Results and discussion

3.1. Textural properties

The BET areas results of dried Re(x)/C catalysts arepresented inTable 1. The BET area was almostunchanged with Re incorporation of up to 0.076atoms nm−2, but then fell severely with higher


Fig. 1. Variation of the pore volume as a function of the Re content for Re(x)/C catalysts.

concentrations. This behaviour was repeated withboth the micro and total pore volumes, presentedin Fig. 1. The mesopore volumes inFig. 1 and ex-ternal areas inTable 1, were largely unchanged inthe studied concentration range. On the contrary, themacropore volume determined by mercury porosime-try rose to a maximum for the sample with 0.076Re atoms nm−2, then fell. The average microporediameter from the MP method displayed a shift towider pores as the Re content was increased, as canbe seen inFig. 2 and Table 1. This last trend wasin close agreement with BET area and pore volumesresults. For alumina-supported Re catalysts the BETarea did not change significantly with the Re contentsup to 0.97 atoms Re nm−2 because the Re is locatedonly in meso and macropores since�-Al2O3 has nomicroporosity[15].

These textural characterisation results demonstratethat for the incorporation of quantities of Re lowerthan about 0.076 atoms nm−2 the surface area andmicropore volume were not effected but the macroporevolumes increased, suggesting that most of the Re waslocated outside of the micropores. However, at higherconcentrations the reductions of the specific area andmicropore volume suggest that the Re filled the narrow

micropores, causing the observed shift in the averagemicropore size to wider diameters.

3.2. X-ray diffraction

The powder XRD patterns of dried Re(x)/C cata-lysts were recorded. Diffraction lines different fromthose of activated carbon were observed only in theXRD pattern of the Re(0.380)/C sample (not shownhere), which displayed clearly two diffraction lines oflow intensity at approximately 16.9 and 25.9◦, corre-sponding closely to the two most intense lines of bulkNH4ReO4. The position and broadening of these XRDlines suggests the presence of distorted NH4ReO4 ag-gregates in the Re(0.380)/C catalyst. These results in-dicate that Re species are highly dispersed formingeither amorphous or crystalline phase<4 nm at Reloadings≤0.135 atoms nm−2.

3.3. Catalyst acidity

Fig. 3 shows the total acidity of calcined Re(x)/Ccatalysts as a function of the Re content. It is evidentthat both the number of acid sites and their strengthincreased gradually on increasing the Re content. The


Fig. 2. Pore size distribution for Re(x)/C catalysts.

acidity results displayed a nearly linear increase inacid strength with Re incorporation up to 0.135 atomsnm−2 and then a slower rise. This suggested that upto this concentration the dispersion of the Re salt re-mained similar in all of the samples, but above thisvalue formation of aggregates or the accessibility toRe was reduced, due to it entering the narrow micro-pores. However, in the case of Re/�-Al2O3 catalysts,both acid strength and total acidity increased with Reloading similarly to the Re/C catalysts but over thewhole range of Re content, indicating that in that sys-tem the fraction of the�-Al2O3 surface covered by Regradually increased without formation of aggregates[15].

3.4. X-ray photoelectron spectra

The XPS of the Re 4f region of sulphided Re(x)/Ccatalysts are shown inFig. 4. Curve fitting of the spec-

Fig. 3. Effect of Re content on total acidity and strength of theacid sites of dried Re(x)/C catalysts. Acid strength estimated bythe initial electrode potential,Ei .

tra revealed two partially overlapping doublets, bothcontaining the Re 4f7/2 and 4f5/2 peaks.Table 2sum-marises the binding energies (BE) of the most intenseRe 4f7/2 component of each doublet, their relative pro-portion and the surface Re/C atomic ratios. The Re4f7/2 component of the most intense doublet remainedconstant, at about 41.7 eV over the whole range ofRe loading considered, and corresponds closely to thevalue reported for ReS2 [6,27,28]. The Re 4f7/2 peakof the less intense doublet, at about 45.3 eV can beassigned to Re(VI) and Re(IV) oxidation states[27].These observations indicate that the sulphidation of thesupported Re species was slightly incomplete, under

Table 2XPS binding energies (eV), surface atomic ratios and degree ofsulphidation of sulphided catalysts

Catalyst Re 4f7/2 (Re/C) × 102

atomic ratioResulf /Retotal

Re(0.024)/C 41.4 (87), 45.5 (12 ) 0.117 0.77Re(0.076)/C 41.9 (91), 45.2 (9) 0.233 0.84Re(0.135)/C 41.9 (74), 45.2 (26) 0.434 0.80Re(0.380)/C 41.8 (83), 45.5 (17) 1.018 0.89


Fig. 4. X-ray photoelectron spectra of the Re 4f region of sulphidedRe(x)/C catalysts.

the sulphidation conditions used. A slight increase inthe degree of Re sulphidation with increasing Re con-tent for the Re/C catalysts (Table 2) reflects changesin Re dispersion.

The variation of the surface XPS Re/C atomic ra-tio as a function of the nominal Re content in thecatalysts is shown inFig. 5. The sulphided Re phaseappears to be monolayer-like dispersed up to about0.135 Re atoms nm−2. The observed deviation fromlinearity above 0.135 Re atoms nm−2 indicates achange in Re dispersion, the formation of multilayersor small three-dimensional ReS2 particles, in agree-ment with the change also observed for textural andacidity results of catalysts in the oxidic state. Thisview is supported by the XRD results which showpresence of NH4ReO4 crystallites at 0.380 Re atoms.In Re/�-Al2O3 catalysts the deviation from linearityoccurs >0.5 Re atoms nm−2. [15] These differencesbetween carbon- and�-Al2O3-supported catalysts

occurs as this last support has no micropores but hasa much higher external surface area (194 m2 g−1)and, therefore, more Re loading can be incorporatedbefore multilayer or small three-dimensional particleformation.

3.5. Activity and selectivity of catalysts

Figs. 6 and 7show the effect of the Re loadingon the activity for simultaneous HDS and HDN reac-tions, respectively, of gas oil over Re(x)/C catalysts,expressed as a function of conversion versus Re con-tent (atoms nm−2) and the reaction temperature. Thevariation of the activity with Re loading was simi-lar for both HDS and HDN reactions, although themagnitudes were different for both reactions. For bothHDS and HDN, the activity increased with increas-ing Re loading up to about 0.076 Re atoms nm−2,and then slightly decreased. Similar activity trendsfor thiophene HDS[10] and for gas oil HDS andHDN [15] were previously reported over Re/�-Al2O3catalysts.

These results for HDS and HDN activities could becompared and contrasted with those obtained from thetextural characterisation of the catalysts, the acidity ofthe materials and the Re/C ratio by XPS. Acidity re-sults displayed a nearly linear increase of acidity withRe incorporation up to 0.135 atoms nm−2 and then aslower rise. This behaviour indicated that up to 0.135atoms nm−2 the dispersion of the Re salt remainedsimilar in all of the samples, but above this value therewas a probable formation of aggregates or the acces-sibility to Re was reduced due to it entering the nar-row micropores. The ratio of Re/C by XPS showeda similar trend of an almost linear rise up to 0.135atoms nm−2 and then a slower increase. On the con-trary, the activity results for both HDN and HDS atthe three studied temperatures displayed a maximumfor the catalyst with 0.076 Re atoms nm−2. Therefore,this maximum in activity may be related to the tex-tural characteristics of this series of catalysts, whereit was noted that up 0.076 atoms nm−2 the specificarea, micropore and total pore volumes were at theirhighest. With further Re content, although the cata-lysts had higher acidities and Re still well dispersed(XPS results), the reduction in activity seems to beassociated with the fall in the pore volume and areabeing more significant. At contents up to 0.076 Re


Fig. 5. Relationship between the XPS Re/Al atom ratio and the nominal surface density of Re for Re/�-Al2O3 catalysts; the dashed straightline is the best-fit linear correlation at low Re loading.

atoms nm−2, as the concentration is very low, the Relocated inside the micropores is highly dispersed andaccessible to the reactant molecules, leading then toa linear increase in HDS and HDN activities. Above

Fig. 6. Gas oil HDS activity of Re(x)/C catalysts as a function of Re loading and reaction temperature.

this content more Re enters the micropores and re-duces their accessibility or blocks the narrower ones,sterically inhibiting the diffusion of the reactants tothe active sites due to the large size of the gas oil


Fig. 7. Gas oil HDN activity of Re(x)/C catalysts as a function of Re loading and reaction temperature.

molecules. This change at 0.076 Re atoms in the Redispersion into the micropores was, however, appar-ently not reflected either by the XPS results becausethis technique is only sensitive to a surface layer, orin the acidity results because then-butylamine can notenter the narrow micropores. With�-Al2O3 supportedRe catalysts[15], the decrease in HDS and HDN ac-tivity occurred at higher Re concentrations than in thecarbon supported ones because the�-Al2O3 is not mi-croporous and, consequently, in this support the lossin activity is essentially due to the reduction in the Redispersion at much higher loadings.

The apparent activation energies (Eap) calculatedfrom Arrhenius plots are given inTable 3. The Eapwere in the range 116–156 kJ mol−1 for HDS and24–30 kJ mol−1 for HDN, and similar to the corre-sponding 138–158 and 25–33 kJ mol−1 values foundover Re/�-Al2O3 catalysts[15]. As with Re/�-Al2O3

Table 3Apparent activation energy (Eap) in the HDS and HDN of gas oil

Catalyst HDS (kJ mol−1) HDN (kJ mol−1)

Re(0.024)/C 137± 5 27 ± 8Re(0.076)/C 156± 10 30± 7Re(0.135)/C 123± 6 28 ± 2Re(0.380)/C 116± 1 24 ± 2

catalysts, since both HDS and HDN reactions weremeasured simultaneously, the low apparent activationenergy for HDN cannot be attributed to pore-diffusionlimitations but was probably caused by differences insorption capacities of the nitrogen and sulphur com-pounds on the catalysts.

In Figs. 6 and 7and Table 3, it was observedthat the Re(0.076)/C catalyst had the highest activ-ity in both HDS and HDN reactions and also thehighest apparent activation energy. This apparentlyparadoxical behaviour can be readily explained fromthe compensation relationship[29]. For Re/C cat-alysts the calculated crude isokinetic temperatures(Tiso) are 300± 8◦C and 295± 14◦C for HDS andHDN reactions, respectively. Thus, considering thatconversions were measured at reaction temperatureshigher than bothTiso, the most active catalyst musthave the highestEap. This trend has been reportedfor HDS in Co-Mo/�-Al2O3, Ni-Mo/�-Al2O3 [29]and W/�-Al2O3 catalysts[30], and recently in Ni-Recatalysts[31].

The fact that the apparent activation energies forHDS over Re(x)/C catalysts differ markedly fromthose for HDN indicates that the two reactions involvedifferent types of catalytic sites, as has generally beenestablished in the literature[32–34]. Higher activa-tion energies for HDS compared to those for HDN


Fig. 8. HDN/HDS selectivity of Re(x)/C catalysts as a function of Re loading and reaction temperature.

obviously means that the former reaction was rela-tively more favoured at higher reaction temperaturesthan the latter reaction and reflected in the changeof HDN/HDS selectivity. Accordingly,Fig. 8 showsthat the HDN/HDS selectivity increased with de-creasing reaction temperature, being<0.8 at 375◦Cand >3 at 325◦C for all the catalysts. This may alsoexplain the apparently contradictory results on theHDS and HDN activities of unsupported Re catalystspreviously reported in the literature[9,35], since theywere obtained under different reaction conditions,particularly temperature.

Another interesting result shown inFig. 8 is thatthe HDN/HDS selectivity increased gradually with in-creasing Re content. Since the Re content does notmodify the nature of the active sites for either reaction(catalyst hadEap almost constant for both reactions), itsuggests that Re content changes the relative concen-trations of the sites for HDS and HDN reactions, in-creasing relatively more those for HDN. This relativeincrease in the overall HDN activity does not seem tobe related to a change in the size and the stacking of theReS2 slabs since according to the XPS results (Fig. 3)Re dispersion only changed at very high Re loadingand then only moderately. However, the increase of theHDN/HDS selectivity was gradual throughout the Re

content range studied and followed a similar trend tothat observed for the catalyst acidity, both the numberof acid sites and their strength. Therefore, it was con-sidered that the observed increase in the HDN/HDSselectivity was more related to the increase in catalystacidity rather than to changes in the size and morphol-ogy of the ReS2 slabs. It is well known that the acidsites promote the C–N bond scission reaction[32,33].

3.6. Comparison of selectivity/selectivity of carbonand alumina-supported catalysts

It is interesting to compare the activities and selec-tivities of these catalysts on carbon with those from aprevious study on�-Al2O3 [15]. At surface Re load-ings below about 0.2 atoms nm−2, Re(x)/C catalystshad higher HDS activity per gram of catalyst andalso per metal atom than equivalent Re(x)/�-Al2O3catalysts, and on carbon the intrinsic HDS activityper metal atom decreased abruptly with loading whileon alumina increased smoothly not shown here. Forthe same range of metal loading, similar trends werepreviously reported by Arnoldy et al.[10] for thethiophene HDS over carbon- and alumina-supportedRe catalysts. The behaviour for intrinsic HDN activ-ity showed similar trends as those observed for HDS


activity but with more pronounced differences and upto Re loadings of about 0.4 atoms nm−2.

Comparison of the HDN/HDS selectivties revealedthat the Re(x)/C catalysts were about twice as selec-tive for HDN than the Re(x)/�-Al2O3 catalysts overthe whole range of metal loading and reaction tem-perature studied. A similar effect was found for Mosupported on carbon in comparison to the conven-tional Ni-Mo/�-Al2O3 catalysts[36]. The increasedHDN selectivity of Re sulphide deposited on carboncompared to on alumina suggests that the carbonsupport causes a small additional effect on selectivitytowards the HDN reaction. In principle, the supportmight introduce structural and textural modificationsof the active phase[20] which were more favourableto HDN than HDS. Thus, recent results show that themorphology of MoS2 supported on carbon, due to itshigh dispersion and high fraction of corner sites, leadsto high HYD activities[37]. This higher HYD activ-ity of carbon-supported catalysts compared with thecorresponding alumina supported ones could explainthe increased HDN/HDS selectivity. The carbon couldalso take part directly in some of the reaction steps,for instance, its surface oxygenated acidic groups[38]participating in the cleavage of the C–N bond[32,33].Another possibility is that the increased HDN selec-tivity on carbon-supported catalysts could stem fromthe formation of a surface Re-carbide layer, similar tothat recently reported for Mo sulphide catalysts[20],since metal carbides have shown to be highly activefor HDN reactions[39].

Acknowledgements

Financial support from Projects 1990496-6 and1020043 FONDECYT (Chile), and from the ProgramCYTED, Subprogram V (Spain) is kindly acknowl-edged. N. Escalona gratefully acknowledges the fel-lowship from CONICYT (Chile). Empresa Nacionalde Petróleo Chile (ENAP).

References

[1] K.G. Knudsen, B.H. Cooper, H. Topsøe, Appl. Catal. A 189(1999) 205.

[2] P. Grange, X. Vanhaeren, Catal. Today 36 (1997) 375.[3] F. Luck, Bull. Soc. Chim. Belg. 100 (1991) 781.

[4] M. Breysse, J.L. Portefaix, M. Vrinat, Catal. Today 10 (1991)689.

[5] T. Pecoraro, R.R. Chianelli, J. Catal. 67 (1981) 430.[6] J.P.R. Vissers, C.K. Groot, E.M. van Oers, V.H.J. de Beer,

R. Prins, Bull. Soc. Chim. Belg. 93 (1984) 813.[7] M.J. Ledoux, O. Michaux, G. Agostini, J. Catal. 102 (1986)

275.[8] S. Eijsbouts, V.H.J. de Beer, R. Prins, J. Catal. 109 (1988)

217.[9] S. Eijsbouts, V.H.J. de Beer, R. Prins, J. Catal. 127 (1991)

619.[10] P. Arnoldy, E.M. van Oers, V.H.J. de Beer, J.A. Moulijn, R.

Prins, Appl. Catal. 48 (1989) 241.[11] J. Räty, T.A. Pakkanen, Catal. Lett. 65 (2000) 175.[12] J. Quartararo, S. Mignard, S. Kasztelan, J. Catal. 192 (2000)

307.[13] E.W. Stern, J. Catal. 57 (1979) 390.[14] J. Shabtai, Q. Guohe, K. Balusami, N.K. Nag, F.E. Massoth,

J. Catal. 113 (1998) 206.[15] N. Escalona, J. Ojeda, R. Cid, G. Alvez, A. López Agudo,

J.L.G. Fierro, F.J. Gil Llambıas, Appl. Catal., in press.[16] T.C. Ho, A.J. Jacobson, R.R. Chianelli, C.R.F. Lund, J. Catal.

138 (1992) 353.[17] Z. Vıt, M. Zdrazil, J. Catal. 119 (1989) 1.[18] J. Cinibulk, Z. Vıt, Appl. Catal. 204 (2000) 107.[19] H. Topsøe, B.S. Clausen, F.E. Massoth, Hydrotreating cata-

lysts, in: J.R. Anderson, M. Boudart (Eds.), Catalysis Scienceand Technology, vol. 11, Springer, Berlin, 1996.

[20] R.R. Chianelli, T.A. Pecoraro, J. Catal. 198 (2001) 9.[21] S. Brunauer, P.H. Emmett, E. Teller, J. Am. Chem. Soc. 60

(1938) 309.[22] B.C. Lippens, B.G. Linsen, J. H de Boer, J. Catal. 3 (1964)

32.[23] J.H. Singleton, G.D. Halsey, Can. J. Chem. 33 (1955) 184.[24] R.S. Mikhail, S. Brunauer, E.E. Bodor, J. Colloid Interf. Sci.

26 (1968) 45.[25] R. Cid, G. Pechi, Appl. Catal. 14 (1985) 15.[26] F.J. Gil Llambıas, A. López Agudo, Anal. Quim. Suppl. 1

(1978) 1.[27] P. Clark, B. Dhandapani, S.T. Oyama, Appl. Catal. A 184

(1999) L175.[28] A.N. Startsev, V.N. Rodin, V.I. Zaikovskii, A.V. Kalinkin,

V.V. Kriventsov, D.L. Kochubei, Kinet. Catal. 38 (1997) 548.[29] L. Boussieres, C. Flores, J. Poblete, F.J. Gil Llambıas, Appl.

Catal. 23 (1986) 271.[30] F.J. Gil Llambıas, J. Salvatierra, L. Boussieres, M. Escudey,

Appl. Catal. 59 (1990) 185.[31] P. Baeza, J. Ojeda, N. Escalona, G. Alvez, R. Garcıa, R. Cid,

F.J. Gil Llambıas, Bol. Soc. Chil. Quim. 46 (2001) 415.[32] S.H. Yang, C.N. Satterfield, J. Catal. 81 (1983) 168.[33] G. Perot, Catal. Today 10 (1991) 447.[34] B. Delmon, Bull. Soc. Chim. Belg. 104 (1995) 173.[35] C.J.H. Jacobsen, E. Törnqvist, H. Topsøe, Catal. Lett. 63

(1999) 179.[36] Z. Vıt, Catal. Lett. 13 (1992) 131.[37] E.J.M. Hensen, P.J. Kooyman, Y. van der Meer, A.M. van der

Kraan, V.H.J. de Beer, J.A.R. van Veen, R.A. van Santen, J.Catal. 199 (2001) 224.

[38] G.J. McDougall, R.D. Hancock, Min. Sci. Eng. 12 (1980) 85.[39] R. Prins, Adv. Catal. 46 (2001) 399.

[elearnica.ir]-effect of re loading on the structure activity and selectivity of re c cat

Documents