a Laboratoire de Réactivité de Surface, UMR CNRS, Université P. et M. Curie, 4 Place Jussieu, 75252 Paris, Franceb Institut de Recherches sur la Catalyse, UPR CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne, France
Received 13 September 2002; received in revised form 4 December 2002; accepted 4 December 2002
Keywords: Hydrotreatment; Hydrogenation of aromatics; Ruthenium sulfide; Dealuminated Y zeolite; Acidic Y zeolite
1. Introduction
Due to environmental concern, the development ofcatalysts able to perform the hydrogenation of aromat-ics in the presence of sulfur is still an important objec-tive. For this purpose, two types of active phase can beenvisaged, a thio-resistant metallic phase or a sulfidephase. Much research has been focused on metal cat-
1 On leave for the DICP, 457 Zhongshan Road, Dalian 116023,China.
alysts and great progress has been made in the knowl-edge of these catalysts as well as in the enhancement oftheir reactivity in the presence of sulfur[1–3]. Never-theless, the activity of a metal phase decreases rapidlywhen the amount of sulfur increases and this limits itsutilization, even for petroleum cuts containing a lowsulfur concentration because the sulfur content can bevery high upon gas recycling. This is the reason whywe have been interested in the properties of sulfidecatalysts which are by nature much more resistant tothe presence of sulfur compounds than metals. In aseries of papers, we have presented the properties ofruthenium sulfide dispersed in a set of Y zeolites withvarious properties[4,5]. It was found that the activity
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for the hydrogenation of tetralin and toluene, carriedout in the presence of H2S (1.9%), varies widely de-pending on the nature of the zeolites. Ruthenium sul-fide catalysts are much more active when using anacidic HY and HYd (dealuminated) zeolite than whenusing the KY support. The magnitude of this sup-port effect is very high, circa a factor of 200 betweenthe most and less active catalyst. It should also bementioned that the hydrogenation properties of thesecatalysts are very high by comparison to that of atypical industrial sulfide catalyst, i.e. NiMo/Al2O3.The increase in activity for the hydrogenation of aro-matics was assigned to the electron-deficient charac-ter of the sulfide particles in the acidic zeolites asproposed for metal catalysts[6]. A superimposed ef-fect of the acidic sites on the adsorption of the aro-matic molecules was not excluded. The remarkableactivity of these systems was also observed with realfeed cuts. The initial activity was very high by com-parison to conventional catalysts but a strong deac-tivation leading to a low steady state activity wasobserved. This strong deactivation was attributed tothe strong Brønsted acidity of these catalysts, whichmight be related to cationic extra-framework aluminaspecies.
Due to the interest of these systems from thefundamental and applied points of view, we de-cided to further examine the properties of ruthe-nium sulfide dispersed in dealuminated zeolitesprepared by different methods which lead (or not)to extra-framework Al species. We did not con-tinue the study of non-dealuminated zeolites be-cause their stability in the presence of H2S is ratherpoor. Moreover, non-dealuminated Y zeolite couldnot be envisaged as a support for the hydrogena-tion of gas oils cuts due to the restricted access ofthe large molecules present in these petroleum cutsto the active phase when it is dispersed within thezeolite.
Two commercial dealuminated zeolites with vari-ous Si/Al ratios were utilized as supports for ruthe-nium sulfide and ruthenium metal active phases. Azeolite further dealuminated by (NH4)2SiF6 was pre-pared. This sample without extra-framework Al wasalso utilized for supporting ruthenium active phases.The hydrogenation of tetralin was chosen as a modelhydrogenation reaction of partially saturated aromaticcompounds.
2. Experimental
2.1. Catalysts
Two commercial HY zeolites, dealuminated bysteaming, with different Si/Al ratios, were utilized inthis study. The first one supplied by Union Carbide(reference LZY-82) has an overall Si/Al ratio of 3.2,the second one provided by PQ (reference PQ-13.6)has an overall Si/Al ratio of 13.6. A commercialNH4Y zeolite was used as reference. (NH4)2SiF6dealumination proceeds via the replacement of aframework Al by a Si atom:
(SiO)3–Al–O−M+ + (NH4)2SiF6
� (SiO)3Si–O+(NH4)2AlF5 + MF
The parent zeolite LZY-82, has been dealuminatedby (NH4)2SiF6 according to the following procedure:24 g of LZY-82 was introduced in a beaker contain-ing 250 ml of a 0.8 M solution of ammonium acetatestirred vigorously and heated up to 353 K. At thistemperature, 80 ml of a 0.4 M solution of (NH4)2SiF6was added slowly and the reactants were maintainedduring 4 h after the end of the addition at this tem-perature. Then, the sample was filtered, washed sixtimes at 348 K with distilled water and dried overnightat 383 K. The dealuminated samples is referred to asDLZY-82. Its overall Si/Al ratio is 7.3.
Supported Ru catalysts were prepared by means ofion exchange using LZY-82, DLZY-82 and PQ-13.6.Zeolite (10 g) was introduced in 500 ml of a 10−2 Maqueous solution of [Ru(NH3)6]Cl3 (provided byJohnson Matthey), at room temperature for 24 h. Thecatalysts were washed six times with water and thendried overnight at 373 K. These samples are referredto Ru/LZY-82, Ru/DLZY-82, Ru/PQ-13.6. The chem-ical compositions of the catalysts are given inTable 1.
A commercial NiMo/Al2O3 hydrotreating catalyst(surface area 240 m2 g−1) containing 9% of Mo and2.4% of Ni on�-Al2O3 was used as reference. Thiscatalyst was sulfided similarly as the RuY samples.
2.2. Characterization
X-ray diffraction studies were carried out using aSiemens D 500 diffractometer with Cu K� radiationand silicon was used as an internal standard. Theweight ratio of the zeolites and silicon was nearly
C. Sun et al. / Applied Catalysis A: General 245 (2003) 245–255 247
a Calculated usingEq. (1).b Calculated usingEq. (2).c Vacuum dried (dehydrated sample): after vacuum drying at room temperature, the samples still contain 5–15% of water. The value
in brackets is the percentage of Ru for a hypothetical sample from which all the remaining water has been removed.d Calculated usingEq. (3).
6. The samples were put into a hydrator which wasmaintained at 35 relative humidity by a saturated so-lution of a salt such as CaCl2·6H2O for at least 24 hbefore measurement. The unit cell parametera0 wascalculated from the average of at least ten reflectionsof known Miller Indices and with relative intensitiesabove 15%. The number of Al atoms per unit cell ofthe framework (NAl ) was determined according to thefollowing equation[7]:
NAl = 112.1(a0 − 24.222) (1)
whereNAl is the number of Al atoms per unit cell.The crystallinity of the solids was also determined
by XRD using NH4Y zeolite as reference as describedin [8].
N2-adsorption desorption isotherms were collectedwith a Micromeritics ASAP 2010 system at 77 K. Thesamples were outgassed at 523 K under a pressure of10−1 Pa for 5 h. The apparent surface area of the ze-olites was determined using the BET model and theexternal plus mesoporous surface area and microp-orous volume were calculated with thet-plot methodaccording to Lippens and de Boer[9] betweenP/P0 =0.1 and 0.3. This method allows to discriminate be-tween the surface area originating from pores smallerthan 20 Å (microporous surface area) and the surfacearea originating from larger pores and the externalsurface.
29Si MAS NMR spectra were recorded using aBruker MSL 400 spectrometer working at 79.49 MHz,with a pulse width of 2�s (π/6), a pulse delay of10 s, a spinning rate of 4 kHz and 1000–4000 scans.The 27Al MAS NMR experiments were run on aMSL 300 spectrometer at 78.2 MHz, with a pulsewidth of 1�s (π/18), a pulse delay of 0.2 s, a spin-
ning rate of 14 kHz and about 5000 scans.29Si shiftswere referenced to tetramethylsilane (TMS), while a1 M aqueous Al(NO3)3 solution was used as standardreference for27Al. All samples were dried at 303 Kin air for 6 h and were finally stored during 24 h oversaturated CaCl2 solution to equilibrate with watervapor before testing.
Transmission electron micrographs of the sulfidedcatalysts were recorded on a JEOL-JEM 100 CXIIapparatus, in order to determine the particle sizes ofruthenium sulfide particles. Ultramicrotomy was usedto obtain extensive electron-transparent regions and toascertain that the observed particles are really locatedin the porosity of the zeolitic supports. Samples wereprepared by embedding the catalyst in a polymer resinsubsequently cured at 343 K during 2 days. Ultrami-crotomed slices (about 70 nm thick) were cut from theembedded sample using a diamond knife. They werelaid on carbon-covered copper grids.
For infrared spectroscopy characterization, the sam-ples were pressed into self-supporting discs. The spec-tra were recorded using a Bruker Vector 22 spectrom-eter with a 4 cm−1 resolution and a DTGS detector.Before measurement, the samples were heated up to573 K (heating rate was 4 K min−1) under vacuum(2×10−3 Pa). The spectra were recorded at room tem-perature. They were normalized using a band charac-teristic of the zeolite framework located at 1870 cm−1
[10].
2.3. Catalytic tests
The catalytic properties were determined in a highpressure flow microreactor working in the gas phaseas described in[4]. Prior to the catalytic activity
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measurements, the catalysts were either reduced insitu at 648 K during 6 h (ramp 5 K min−1) under apressure of 4.5 MPa of H2, or sulfided at 673 K during4 h (ramp 5 K min−1) using a gas flow of 15% H2Sin H2 at atmospheric pressure. The samples are thendenoted RuM/LZY-82, RuS/LZY-82, etc. where Mstand for reduced and S for sulfided.
Pure tetralin was introduced by means of a saturator.The experimental conditions were the following:PH2:4.5 MPa;Ptetralin: 5.92 kPa;PH2S: 43.8 kPa or 0;T =523 K.
The experimental conditions for tetralin hydrogena-tion were chosen in order to avoid thermodynamicequilibrium, which may favor the dehydrogenation oftetralin to form naphthalene, and to obtain a relativelylow conversion (less than 10%) of tetralin to hydro-genated products. After a first period of deactivation,all the catalysts deactivate very slowly and approx-imately at the same rate. Consequently, the specificrates were measured after 18 h time on stream.
3. Results
3.1. Chemical analysis
The overall Si/Al ratio are reported inTable 1, aswell as the amount of incorporated Ru. Although thesame amount of Ru was introduced in the exchangesolution, the amount of Ru incorporated in the cata-lyst varies with the sample. Two main factors affectthe incorporation of Ru, (i) the amount of frameworkAl is directly related to the ion exchange capacity ofthe zeolite and (ii) the presence of extra-frameworkspecies that can block the access to these sites. Nev-ertheless the amounts of incorporated Ru are ratherclose whatever the nature of the support.
Table 2Physico-chemical characteristics of the zeolitic supports and of the corresponding catalysts
a Variation of the microporous surface area upon introduction of Ru.
3.2. N2-sorption
The values of the surface area and porous vol-ume of the supports and of the RuS/Y catalysts arereported inTable 2. (NH4)2SiF6 dealumination (sam-ple DLZY-82) results in a diminution of the surfacearea and porous volume of the sample. The decreaseof the surface area arises from a diminution of themicroporous surface of the support whereas the ex-ternal plus mesoporous surface remains constant oreven increases slightly. This could be due either toa collapse of the structure or to a partial blocking ofthe porosity with amorphous SiO2. The formation ofan amorphous phase is in agreement with previouslyreported results[11], which concluded that dealumi-nation with (NH4)2SiF6 leads to an amorphisation ofthe structure for the most dealuminated samples (dea-lumination >50%). The authors have attributed thisto a structure breakdown that occurs at high concen-trations of (NH4)2SiF6. The increase of external plusmesoporous surface area can be assigned to the for-mation of mesopores during the (NH4)2SiF6 dealumi-nation process. Moreover, this dealumination methodinduces a decrease of the stability of the zeoliticsupport during the subsequent steps of the catalystpreparation, that is insertion of Ru and/or sulfidation.The relative variation of microporous surface area isof about 30% for (NH4)2SiF6 dealumination, whereasit is of 1 or 10% for dealumination by steaming (seeTable 1). The presence of silanol nests could be re-sponsible for to the lower stability of the structure, asmentioned by Gao et al.[12].
3.3. 27Al NMR
27Al NMR spectra of the zeolitic supports areshown inFig. 1. One or two NMR peaks are observed
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Fig. 1. Comparison of27Al NMR spectra of the various zeolites.( ) Denotes the spinning side-bands.
depending on the sample. For the samples dealumi-nated by steaming (namely LZY-82 and PQ-13.6) twopeaks are observed: the first one located at 60 ppmis assigned to tetrahedral Al and the second one at0 ppm is attributed to octahedral Al and evidences thepresence of extra-framework species of octahedrallycoordinated Al. The percentage of octahedral speciesis higher on the LZY-82 than on the PQ-13.6 sample.For the DLZY-82 sample only one peak is observedat 60 ppm suggesting that no extra-framework Alspecies is present. Moreover, it is possible to quan-tify the amount of these extra-lattice species. Thecomparison of the Si/Al ratios determined by chemi-cal analysis and by29Si NMR suggests that there isabout 20% of EFAl in the two samples dealuminatedby steaming. Still the AlTd/AlOh as determined by
27Al NMR is higher for LZY-82 (about 25%) thanfor PQ-13.6 (about 10%). This could be due to thefact that in the latter sample part of the Al is presentas amorphous alumina, which is very difficult to de-tect using27Al NMR. Anyway, these results are inagreement with the literature: samples dealuminatedby steaming are known to contain EFAl, whereas thedealumination with (NH4)2SiF6 prevents the forma-tion of extra-framework Al.
3.4. 29Si NMR
The spectra of the four supports are presented inFig. 2. The allocation of29Si signal between the var-ious Q4(nAl) peaks reflects the dealumination of thesample. For NH4Y, four types of zeolitic Si can be ob-served, ranging from Q4(3Al) to Q4(0Al). Upon dea-lumination a lowering of the signal of Si surrounded
Fig. 2. Comparison of the29Si NMR spectra of the various zeolites.
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by two and three Al atoms is observed. For the mostdealuminated sample (PQ-13.6) only Q4(1Al) andQ4(0Al) are observed reflecting solely the presenceof isolated Al (two Al atoms are separated by a min-imum of two Si atoms). For the DLZY-82 sample, alarge signal at−107 ppm characterizes the presenceof an amorphous silica phase. Such a broad signalwas previously observed for zeolite dealuminatedwith (NH4)2SiF6 and having a low crystallinity[13].The intensity of this peak is about 15–20% of theoverall NMR signal. However, this value is probablyunderestimated since the relaxation time of Si atomsin amorphous silica is higher than in zeolites. Thepresence of this amorphous phase is consistent withthe amorphisation of the zeolitic structure during thedealumination, as detected by N2-sorption measure-ments.
The ratio of tetrahedral Si and Al atoms in the ze-olite framework can be directly calculated from theQ4(nAl) peak intensities in a29Si MAS NMR spec-trum according to Engelhardt and Michel[14].
Si
AlSi NMR
= 4Q4(0Al) + Q4(1Al) + Q4(2Al) + Q4(3Al)
Q4(1Al) + 2Q4(2Al) + 3Q4(3Al)(2)
where Q4(nAl) represents a Si atom surrounded bynnumber of Al and (4− n) Si.
The Si/AlSi NMR ratio of the samples are reported inTable 1. They will be discussed afterwards.
3.5. XRD
The X-ray power diffraction patterns show thatthe commercial samples present a high crystallinity.The Si/Al ratio determined by XRD, using thea0dependency with framework Al content, is given inTable 1 and will be discussed later. A large signalcentered at 2θ = 20◦ is observed for the samples dea-luminated with (NH4)2SiF6 indicating the presenceof amorphous silica. The crystallinity of the samplesis reported inTable 2. This parameter is higher than90% for all samples except for DLZY-82. This resultconfirms the partial collapse of the zeolitic structureduring treatment with (NH4)2SiF6, in agreement withthe smaller surface area of this sample.
3.6. Si/Al ratio
The Si/Al ratio of the three supports, evaluated usingchemical analysis, XRD and29Si NMR are reportedin Table 1. Si/AlCA reflects the overall Si/Al ratio,while Si/AlXRD and Si/AlSi NMR reflect the framework(Si/Alfw) atoms only. For the sample dealuminated bysteaming and the non-dealuminated zeolite Si/AlXRDand Si/AlSi NMR are in close agreement. As expected,the dealumination by steaming results in a Si/Alfw ra-tio that is higher than Si/AlCA, confirming the presenceof EFAl in the sample. For the (NH4)2SiF6 dealumi-nated sample, the Si/AlSi NMR and Si/AlXRD are lowerthan the Si/AlCA, which is in line with the absenceof EFAl evidenced by27Al NMR and the presence ofEFSi (extra-framework Si species) as detected by29SiNMR. However, for this sample Si/AlXRD differs fromSi/AlSi NMR. The presence of residual silanol groupscould account for this discrepancy: silanol nests woulddiminish the Si/AlSi NMR ratio, by overestimating theamount of framework Al (due to the overlapping ofthe peaks due to Si(nOAl) and Si(nOH) in the 29Sispectrum). Still, due to the dealumination method wehave chosen, the amount of silanol should be negligi-ble: the dealumination with (NH4)2SiF6 has, in com-parison with the steaming method, the advantage thatSi atoms can directly replace the extracted Al atoms.In that way no new hydroxyl groups should be formedduring dealumination and pre-existing defects couldeven be healed. Nevertheless, this might not be truefor high dealumination ratio, where the rate of Al re-moval from the framework is higher than the rate ofSi insertion as reported by Triantafillidis et al.[11].
3.7. Infrared spectroscopy
The infrared spectra of the activated samplesrecorded in the range 4000–3000 cm−1 are shown inFig. 3. Three to four bands are observed, the bandlocated at 3744 cm−1 is characteristic of the pres-ence of terminal silanol groups located at the meso-porous and/or external surface, whereas the 3627 and3550 cm−1 bands are assigned to bridging OH groupslocated in supercages and in sodalite or hexagonalprism cages, respectively. The band at 3670 cm−1,detected only on the Ru/LZY-82 sample, correspondsto OH groups connected to extra-framework speciesin the large cages[13]. The spectra of Ru/LZY-82
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Fig. 3. Comparison of the FTIR spectra of the various Ru zeolites.
and Ru/DLZY-82 are very similar except for the bandcharacteristic of extra-framework Al for Ru/LZY-82.For both samples, only a small amount of terminalsilanol groups is detected, and the bands characteris-tic of the zeolite cages are intense. On the contrary,for Ru/PQ-13.6 the intensities of these two bandsare very weak indicating a much higher degree ofdealumination in agreement with29Si NMR results.Moreover, the intensity of the terminal silanol bandis about twice as high for the others which couldbe associated with an increase of the external and/ormesoporous surface area. This is in agreement withthe physiosorption study, which evidences the pres-ence of a larger amount of small mesopores than forall the others samples (seeTable 2). The presenceof extra-framework Al (EFAl) is clearly detectedfor Ru/LZY-82 and not for Ru/PQ-13.6. The loweramount of EFAl for the latter sample, relatively tothe former, as ascertained by27Al NMR, can explain
this dissimilarity. In addition the IR spectra of theRuM/Y samples were recorded after reduction in pureH2 at 623 K according to the pretreatment proceduredescribed in[5]. No significant differences were ob-served except for Ru/PQ-13.6 and RuM/PQ-13.6 (seeFig. 3). The main difference between the two spectrais the presence, in the spectrum of RuM/PQ-13.6, ofa weak band at 3602 cm−1, which can be assignedto hydroxyl groups that are under the influence ofcationic extra-framework Al species[13]. These hy-droxyl groups disturbed by extra-framework Lewisspecies are, according to previously published results,strong Brønsted sites[15]. The reason why this bandwas not observed on the IR spectra of the vacuumtreated sample is probably the incomplete reductionof the Ru ions during the vacuum treatment, whichcould be left in ion exchange position on the mostacidic sites. No significant modifications are observedafter sulfidation.
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Fig. 4. TEM micrograph of sample RuS/DLZY-82.
3.8. HRTEM
The dispersion of the RuS2 nanoparticles is not fullydocumented in this work, due to the difficulty to de-tect these particles with TEM. Well-dispersed RuS2nanoparticles of about 20 Å have however been de-tected on the RuS/DLZY-82 sample (seeFig. 4). Noparticles were detected on the other samples whichmeans that the RuS2 particle size is at least smaller
Table 3Hydrogenation of tetralin: rate of tetralin conversion
Test:T = 523 K; Ptot = 4.5 MPa;PH2S = 43.8 kPa;Ptet = 5.92 kPa.a Intrinsic rate: number of tetralin molecule (molec) converted per second and per atom (at) of Ru.
than 20 Å. It has been shown in previous work, thatthe dispersion of the ruthenium sulfide phase was notsignificantly modify in a series of Ru/Y catalysts withvarious Si/Al ratio, so that we would indeed expect asimilar dispersion of the active phase for all the cata-lysts. However, the density of Ru in the Ru/PQ-13.6sample is two–three times lower than in the two oth-ers samples (seeTable 1). This could result in a higherdispersion of the RuS2 phase for this sample.
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3.9. Hydrogenation of tetralin
The activities per gram of catalyst (specific ac-tivities) and per metal atom (intrinsic activities) arereported inTable 3. The RuS/Y zeolite catalysts haverather similar intrinsic activities. Their activities arevery high and roughly 10 times the activity (expressedper gram of catalyst) of an industrial hydrotreatingcatalyst, i.e. NiMo/Al2O3. However, some activityvariations were observed depending on the support,that could be assigned either to a change in the RuS2particle size or in the acidity strength of the ze-olitic support. The catalytic properties of the RuM/Ycatalysts in the presence of sulfur are all identicaland much lower than those of the sulfided catalystsin similar testing conditions. Metal based catalystspresent higher selectivities (30–50%) toward theformation of isomerisation products than supportedsulfides.
4. Discussion
All the RuS/Y samples display a high activity forthe tetralin hydrogenation, much higher than that ofthe conventional NiMo/Al2O3 catalyst. These resultsare in agreement with those previously obtained ona series of Y zeolites with various acidic properties,namely RuS/KY, RuS/HY, and dealuminated zeolitesRuS/HYd and RuS/KHYd [5]. However, in this pre-vious study, the catalytic and acidic properties variedwithin a wide range. In the present work, which ob-jective was mostly the study of the influence of theextra-framework aluminium species, the variations ofthe number and strength of the acid sites are muchlower. This results in a much smaller variation ofthe catalytic properties. As a matter of fact, two ofthe samples, RuS/LZY-82 and RuS/DLZY-82 haveidentical activity while RuS/PQ-13.6 is 25% moreactive.
Regarding the influence of the acid sites on the re-activity of the RuS2 nanoparticles, the number of acidsite in the vicinity of a ruthenium sulfide particle isa judicious factor. In order to allow an evaluation ofthis parameter, the following hypotheses have beenmade, (i) the RuS2 nanoparticles are located in thesupercages, far apart from one another, (ii) the distri-bution of the aluminium atoms in the framework is
statistical. The number of Al atoms in a supercage isdetermined by the following equation:
NAl supercage=NTd supercage
NTd unit cellNAl unit cell (3)
whereNTd supercageis the number of tetrahedral atomsper supercage (that is 192),NTd unit cell the numberof tetrahedral atoms per unit cell (that is 48) andNAl unit cell the number of Al atoms per unit cell (de-termined from Si/AlNMR).
The NAl supercagevalues are reported inTable 1.This calculation verifies that, even for the most dea-luminated sample, the number of acidic sites in closevicinity of the RuS2 particle is higher than one. Thisexplains why the number of acid sites is not thedetermining factor for the enhancement of the ac-tivity of ruthenium sulfide particles of the presentstudy.
Furthermore, the acidity strength of LZY-82 andof DLZY-82 are close as evidenced by the FTIRstudy and both materials contain a large number ofBrønsted sites in the sodalite cages and in the su-percages. PQ-13.6 zeolite presents a major differencewith the two others, which is pointed out by the29Si NMR spectroscopy. This zeolite contains onlyQ4(1Al) and Q4(0Al) Si sites, reflecting the presenceof isolated aluminium only (two Al atoms are sep-arated by minimum two Si atoms). The fact that allthe aluminium atoms are far apart from each other,should result in a higher strength of acidity for thissample. Indeed, PQ-13.6 contains a supplementarysort of acidic site as evidenced by the IR band at3602 cm−1. The presence of even a small amount ofthese strongly acidic sites could further modify theelectronic properties of the active phase or the adsorp-tion of the reactants and consequently the catalyticproperties. As mentioned above, the RuS/PQ-13.6is intrinsically more active than the other samplesin agreement with the fact that it possesses strongeracid sites. However, in the present study the densityof Ru atoms per unit of surface is much smaller forthe sample PQ-13.6 than for the two others. There-fore, we cannot discard the possibility that the RuS2particles for this sample are smaller than for the twoothers.
Samples RuS/LZY-82 and RuS/DLZY-82 haveidentical activity in spite of the fact that one of thesesamples contains extra-framework alumina, whereas
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the other one is free of these species. This showsthat, for this model reaction, the extra-frameworkspecies do not contribute significantly to the deac-tivation of the catalyst within 18 h time on stream.It is still possible that these species play a negativerole during gas oil hydrogenation. If this is the case,this problem can be overcome using (NH4)2SiF6 asdealuminating agent, because this technique evenallows to remove pre-existing extra-framework alu-minium species. However, the drawback of thismethod is the formation of EFSi species (as evi-denced by29Si NMR) which might also block theporosity.
The activities of the RuM/Y zeolites are much lowerthan the activities of the corresponding RuS/Y zeolitecatalysts in the conditions utilized here, i.e.PH2S =43.8 kPa. The higher activity of sulfide for aromaticshydrogenation by comparison to metal was alreadypointed out in the previous study[5]. This result is inagreement with the poisoning of noble metal nanopar-ticles in the presence of sulfur. However, the presenceof acid sites in the vicinity of the Ru nanoparticlesshould, as reported by Bartholomew et al.[16] con-tribute to their stabilization. In the present study, theactivity of all the catalysts in the presence of sulfur areidentical regardless of the Si/Al ratio, indicating that,even a high Si/Al ratio contribute to the thio-resistanceof the Ru phase. No effect of EFAl or of the num-ber and strength of the acid sites is detected, oppo-sitely to previously published results[2,3]. However,the activity of these catalysts being low under the ex-perimental conditions we have chosen, the absence ofany detectable effect of these parameters is only anindication that the effects of these parameter are notoverwhelming and might be masked by experimentalerror.
5. Conclusion
Using a wide variety of dealuminated zeolites withSi/Al ratios varying between 4.4 and 18.5 as supportsfor a ruthenium sulfide phase, highly active catalystsfor the hydrogenation of aromatics in presence of sul-fur were obtained. Using (NH4)2SiF6 as dealuminat-ing agent, a zeolite support without extra-frameworkAl species was obtained. The comparison of theproperties of ruthenium sulfide supported on sup-
ports with or without these species show that theirpresence does not affect the catalytic properties (for18 h on stream time). The highest catalytic activitieswere obtained with the most dealuminated supportswhich possess the lowest amount of acid sites butalso the strongest acid sites. It can be mentioned thatin any cases the number of acid sites is large enoughto ensure the presence of more than one acid site perRuS2 nanoparticle (assuming that the nanoparticlesare located in the supercage) which could explainwhy this parameter does not influence the activity.The strength of the acid sites might be the determin-ing factor in modifying the electronic properties ofthe ruthenium sulfide phase or the adsorption of thearomatic. Besides its strongest acidity, the most dea-luminated zeolite present also the highest surface areaand therefore the lowest Ru density. This means thatit cannot be excluded that the better activity of thiscatalyst is related to the better dispersion of the activeruthenium sulfide phase, which would in that case becomposed of tiny RuS2 particles (less than 1 nm).
Acknowledgements
This work has been carried out in the framework ofthe French-Chinese collaboration on catalysis (PICSand LFCC). P. Beaunier is gratefully acknowledgedfor the TEM micrographs.
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